United States
Environmental Protection
Agency
Industrial Environmental Research
Laboratory
Research Triangle Park NC 27711
EPA-600/2-78-124
June 1978
Research and Development
Evaluation of
Emissions and
Control Techniques
for Reducing
Fluoride Emissions
from Gypsum
Ponds in the
Phosphoric Acid
Industry
-------
RESEARCH REPORTING SERIES
Research reports of the Off ice of Research and Development, U.S. Environmental Protec-
tion Agency, have been grouped into nine series. These nine broad ^eoorea were
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3. Ecological Research
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8. "Special" Reports
9. Miscellaneous Reports
This report has been assigned to the ENVIRONMENTAL PROTECTION TECHNOLOGY
series. This series describes research performed to develop and demonstrate instrumen-
tation, equipment and methodology to repair or prevent environmental degradation from
point and non-point sources of pollution. This work provides the new or improved tech-
nology required for the control and treatment of pollution sources to meet environmental
quality standards.
REVIEW NOTICE
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Protection Agency, and approved for publication. Approval
does not signify that the contents necessarily reflect the
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This document is available to the public through the National Technical Informa-
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-------
EPA-600/2-78-124
June 1978
Evaluation of Emissions and Control
Techniques for Reducing Fluoride Emissions
from Gypsum Ponds
in the Phosphoric Acid Industry
by
A.A. Linero and R.A. Baker
Environmental Science and Engineering, Inc.
P.O. Box 13454
Gainesville, Florida 32604
Contract No. 68-02-1330
Task No. 3
Program Element No. 1AB604
EPA Project Officer: Edward L Wooldridge
Industrial Environmental Research Laboratory
Office of Energy, Minerals, and Industry
Research Triangle Park, NC 27711
Prepared for
U.S. ENVIRONMENTAL PROTECTION AGENCY
Office of Research and Development
Washington, DC 20460
-------
TABLE OF CONTENTS
Page No.
v
Figures
viii
Tables
1.0 Industry Description
1.1 Process Description - Role of the gypsum pond .... 1
1 2 Size and Trends of the Phosphoric Acid ^ 4
Manufacturing Industry
1.3 Distribution of Gypsum Ponds
2'°
2.1 Process Description
2 2 Characterization of Sources of _ 49
Gypsum Pond Fluorides .............
.59
2 3 Section 2 Conclusions ................
...... 61
3.0 Gypsum Pond Chemistry ............
3.1 Source of Fluorine in Gypsum Ponds .........
3.2 Chemical Environment Within the Gypsum Pond ..... 64
CO
3.3 Development of a Model ...............
78
3 4 Discussion of the Simplified Gypsum Pond Model. . . .
... 79
3.5 Section 3 Conclusions .............
81
4.0 Determination of Fluoride Emission Factor ........
ftl
4 l Review of Cross and Ross1 Study ...........
88
4 2 Review of Tatera's Study .............
. . 97
4.3 Review of King's Study . . . • ..........
4 4 Comparison of Emission Factors _ 139
from the Literature ..............
iii
-------
5.0 Determination of Gaseous Fluoride Pa9e No<
Concentrations in the Vicinity of a Gypsum Pond 143
5.1 Description of a Typical Gypsum Pond 143
5.2 Ground Level Gaseous Fluoride Concentrations
in the Vicinity of a Gypsum Pond 146
5.3 Computer Model Sensitivity Analysis 150
6.0 Identification of Control Techniques . 153
6.1 Kidde Process
6.2 Swift Process
6.3 Liming of Cooling Ponds 179
6.4 Gypsum Transport by Conveyor Belt 189
6.5 Pretreatment of Ore by Calcining 191
6.6 Hemi/Dihydrate Process 193
7.0 Demonstration Costs 201
7.1 Verification of Predicted Fluoride Emission 201
7.2 Verification of the Swift Vapor Absorbtion System . . 209
Bibliography 216
IV
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LIST OF FIGURES
Page No.
V.I Wet Process for Production of Phosphoric Acid 2
1.2 Approximate Location of Major Wet Phosphoric Acid
Production Facilities 8
1.3 Approximate Locations of Gypsum Ponds in
Central Florida 9
1.4 Map Showing General Land Ownership Around Gypsum
Ponds in the Mulberry Area of Central Florida ....... 10
1.5 Map Showing General Land Ownership Around Gypsum
Ponds in Bartow-Fort Meade Area of Central Florida ..... H
?.l Phase Equilibrium Diagram for
System (Dalgran, p. 94) .................. 20
2.2 Typical Dihydrate Process with Vacuum Acid Concentration
and Fluosilicic Acid Recovery ............... 22
2.3 Hemihydrate-di hydrate Phosphoric Acid Process
(Takeuchi, p. 303) . .................... 22
2.4 Defluorinated Phosphoric Acid - Vacuum Process
(EPA 440/1-53/043) ..................... 30
2.5 Defluorinated Phosphoric Acid - Submerged
Combustion (EPA 440/1-75/043) ............... 30
2.6 Defluorinated Phosphoric Acid - Aeration Process
(EPA 440/1-75/043) . . ................... 31
2.7 Normal Superphosphate and Triple Superphosphate -
Run-of-Pile Process Schematic (Shrieve, p. 271) ...... 40
2.8 Granulated Triple Superphosphate (EPA 440/1 -74-011 -a) ... 41
2.9 Diammonium Phosphate Process Schematic (Shrieve, p. 227) . . 44
2 10 Defluorinated Phosphate Rock - Fluid Bed Process
Sthematic (EPA 440/1-74/043) ................ 46
2.11 Phosphate Fertilizer Complex - Fluoride and Water
Balance .......................... 58
3.1 Titration of Gypsum Pond Water with 1 N NaOH ........ 66
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Page No.
3.2 Buffer Capacity of Gypsum Pond Water ........... 67
3.3 Graphic Initial Description of Some Pertinent
Gypsum Pond Reactions ................... 59
3.4 Species Predominance Diagram for 0.4 M HF Solution .... 71
3.5 Effect of Liming on Fluoride Evolution from Gypsum
Pond Water (HEW, 1970) .................. 72
3.6 Major Gypsum Pond Equilibrium ............... 77
4.1 Location of A-Frame on Gypsum Pond ............ 83
4.2 Dimensions of A-Frame Used by Cross ............ 84
4.3 Drawing of Model Gypsum Pond and Water Bath Used
in Gypsum Pond Studies ............... ... 92
4.4 Schematic Diagram of Experimental Set-up Used in
Gypsum Pond Studies .................. . . 93
4.5 Tatera's Emission Factors for Process Water at
75°, 85°, 95°F ....................... 95
4.6 Tangential Flow Along a Sharp-Edge Semi-Infinite
Flat Plate With Mass Transfer Into Stream ......... 103
4.7 Fluorine Vapor Pressure Over Pond Water .......... 113
4.8 Fluoride Emission Rates for Ponds with Water
Containing 0.335 g moles/liter Fluorides, V = Wind
Speed at 16 Meters in Meters Per Second .......... 117
4.9 Fluoride Emission Pvates for Ponds with Water
Containing 0.628 g moles/liter Fluorides, Vie +
Wind Speed at 16 Meters in Meters Per Second ....... 118
4.10 Line Source Simulation of a Rectangular Area Source .... 123
4.11 Ambient Air Sampler Used in King's Study ......... 125
4.12 Pond 10 Plot Sketch .................... 128
4.13 Pond 20 Plot Sketch
4.14 Pond 10 - Measured Vs. Calculated Ambient Air
Concentrations at Samplers ................ 136
VI
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Page No.
4.15 Pond 20 - Measured Vs. Calculated Ambient Air
Concentrations at Samplers 137
5.1 Typical Gypsum Pond Servicing a 1,000 TPD P^ Plant .... 145
5.2 Isopleths of Calculated Ground-Level Fluoride X/Q
Ratios Downwind of a Hypothetical Gypsum Pond 149
5.3 Isopleths of Calculated Ground-Level Fluoride X/Q
Ratios Downwind of a Hypothetical Gypsum Pond 150
6.1 Two Pond System for Phosphoric Acid Plant 157
6.2 Kidde Closed-Loop Fluorine Recovery for a
1,000 TPD P205 Plant 163
6.3 Kidde Process for Recovery of A1F3 from
Waste (NH4)2SiF6 171
6.4 Swift Process for Fluorine Recovery at a
1,000 TPD P205 Plant 173
6.5 Fluoride Vapor Pressure Vs. pH I82
6.6 Proposed Single Liming System 184
6.7 >Bemi/Dihydrate Wet Process for Phosphoric Acid
Manufacture
198
7.1 Theoretical Ambient Concentration Profile as
a Function of Height 205
7.2 Ambient and Pond Emission Sampling Network -
Aerial View Z09
vn
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LIST OF TABLES
1.1 North American Consumption of Phosphoric Acid °'
2.1 Integration of Production in the Fertilizer Industry
(EPA-440/1 -74-011 -a) .............. _ _ 13
2.2 Typical Composition and Particle Size of Commercial
Grades of Florida Phosphate Rock (Legal, p. 21) ...... 16
2.3 Composition of Filtered Wet-Process Phosphoric Acids
(Slack, p. 656) ...................... 18
2.4 Effect of Concentration on Compostion of Wet-Process
Phosphoric Acids (Legal, p. 47) ........... ,5
2.5 Analyses of Wet-Process Acids and of Solids Obtain^ hw
Filtering the Acids (Hein, p. 695) .... .* . 35
2.6 Fluoride Content of Phosphate Rock (Teller, p. 745) . . eg
2.7 Distribution of Fluoride in Dihydrate Process ....... 5]
2.8 Fluoride Distribution in Phosphoric Acid Production
(Sanders, p. 765) „
2.9 Fluoride Distribution in Phosphoric Acid Production (Fox) . 55
2.10 Distribution of Fluorides from Florida Phosohate Rnrk
(Huffstutler, p. 728) ........... ....... 55
2.11 Phosphate Complex Water Usage (EPA 440/1 -74-011 -a) ... 55
2.12 Water Requirements for Phosphoric Acid Production
(Lutz, p. 195) ........................ 5?
3.1 Major Cation and Anion Concentrations in Gypsum Pond
Water. After ESE, 1974 .......... .
............ 62
3.2 Formation Constants of Al and Fe Fluoride Species ... 74
4.1 Results of Cross and Ross' Greenhouse Experiment on a
Gypsum Pond .........
............... 87
4.2 Analysis of Variance for King's Vapor Pressure Data . 115
4.3 Measured Background Ambient Fluoride Concentrations
and Simulation Model Estimates .......... 134
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Page No,
4.4 Comparison of Emission Factors Predicted by King and
Tatera at Various Temperatures ...............
5.1 Sensitivity Analysis for the Finite Line Source Model .... 153
6 1 Capital Investment and Operating Costs for Fluoride
Control of a 1000 TPD P205 Plant .............. I0*
6 2 Capital Investment Requirements for Kidde Process Producing
29,000 TRY (NH4)2SiF6 (1,000 TPD P205) ...........
6.3 Annuali zed Operating Costs for Kidde Process ........ 169
6 4 Fluorine Material Balance in a 1,000 TPD P205 Plant
Utilizing the Swift Vapor Absorption Process ........ "°
6.5 Capital Investment Requirements for the Swift Vapor
Absorption System ......................
179
6.6 Annuali zed Operating Costs for Swift Process ........
6.7 Effect of Liming on Fluoride Evolution From Gypsum-
Pond Water .........................
6 8 Fluorine Material Balance in a 1,000 TPD P205 Plant
Utilizing Liming as Fluoride Control Strategy ........
6.9 Capital Investment Requirements for Single Liming ^
System ...........................
189
6.10 Annual ized Operating Costs for Single Liming ........
6.11 Capital Investment Requirements for Dry Gypsum Conveyor ... 191
6.12 Capital Investment Requirements for Pre-Calcining
of Phosphate Ores ......................
201
6.13 Comparative Process Economics ................
7.1 Cost Breakdown for Fluoride Emission Determination
from Two Cooling Ponds ...................
7.2 Pilot Plant Program Costs for the Swift Vapor
Absorbtion System ......................
7 3 Estimated Costs for Demonstration of Swift Process
at a 500 TPD - ?2^S Facility ...... ...........
ix
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1.0 INDUSTRY DESCRIPTION
Phosphoric acid is the basic building block from which essentially all
mixed fertilizer used in the United States is manufactured. Over 75%
of this acid is manufactured by the wet process method. This consists
of digestion of phosphate rock with a strong acid, such as sulfuric or
nitric acid, to convert the phosphate from practically water insoluble
to a water soluble form. The acid used in this process is selected on
the basis of several factors including cost, simplicity of the process,
materials of construction and desired end products. In the United
States, sulfuric acid is used in nearly 99% of the total U.S. production
of phosphoric acid by the wet process. Nitric acid accounts for almost
all of the remainder.
1 • ! PROCESS DESCRIPTION - ROLE OF THE GYPSUM POND
The raw materials used in the sulfuric acid acidulation of phosphate
rock are ground phosphate rock, 93% sulfuric acid, and water. Phosphate
rock is mixed with the sulfuric acid after the acid has first been
diluted with water to a 55-70% H2S04 concentration. As shown in Figure l.l,
this mixing takes place in an attack vessel. This vessel is of sufficient
size to hold the raw materials mixture for several hours. The simplified
overall chemical reaction is represented by:
3 Ca3 (P04)2 (S) + 9 H2S04 (1) + 18 H20 (1) -» 6 H3P04 (1) + 9 CaS04'2 H20 (S)
In reality, phosphate rock is not the pure compound indicated, but rather
a fluorapitite material containing minor quantities of fluorine, iron,
-------
Phosphate
Rock
Sulfuric
Acid
Sulfuric
Acid Dilut.
Water
Attack
Vessel
Scrubber
Filtration
Gypsum
Pond
Evaporation
Gypsum
By-Product
Product
Acid
Figure 1.1 Wet Process for Production of Phosphoric Acid
-------
aluminum, silica and uranium. Of these the one presenting the most serious
overall process problem is fluorine.
Fluorine is evolved from the attack vessel and other plant equipment
as either the gas silicon tetrafluoride or hydrofluoric acid. Both of
these gases can be collected in a wet scrubber unit. Additional fluorine
remains in the by-product gypsum (CaS04) in a variety of fluorine com-
pounds.
Following the reaction in the digester, the mixture of phosphoric acid
and gypsum is pumped to a filter which mechanically separates the solid
qypsum from the phosphoric acid. The magnitude of the by-product gypsum
is best appreciated by the fact that the production of each pound of
pure phosphoric acid creates approximately 2.5 pounds of gypsum. The
gypsum is .generally sluiced with contaminated water from the plant to
a gypsum pond. The phosphoric acid separated from the gypsum is col-
lected for further processing.
The gypsum pond is an impoundment of from 65 to 670 hectares (160 to
1400 acres) in area. This impoundment serves two functions. One func-
tion is as a storage area for waste by-product gypsum. The second is
as an area for atmospheric evaporative cooling of the contaminated water
prior to its reuse in the various process units. This pond system
functions in a closed loop mode most of the time, releasing water for
treatment only during heavy rainfall.
-------
1.2 SjZE AMD TRENDS OF THE PHOSPHORIC ACID MANUFACTURING INDUSTRY
The manufacture of phosphoric acid is basic to the phosphate fertilizer
industry. The past and predicted North American consumption of phosphoric
acid is outlined in Table 1.1.
In the early 1960's, large wet process phosphoric acid plants were in the
200-300 tons P-0 per day range, with multiple pieces of equipment re-
quired to perform single unit operations such as acidulation and filtra-
tion. By 1965, single train phosphoric acid units and single unit opera-
tions equipment with capacities of 500 tons ?2Q§ per day became common-
place, followed with 900 tonsperday plant in 1967. Several plants now
being designed and constructed have capacities of 1000-1200 tons ?2Q$
per day.
Currently there are about 40 wet process phosphoric acid plants operating
in 15 states with capacities ranging from 45,000 to 440,000 tons PgOs
per year. The largest plants, which account for more than 50$ of the
nation's capacity are located near the phosphate rock sources in Florida.
There are a few located along the Gulf Coast and the Mississippi River
and isolated plants in North Carolina, Idaho, Utah, and California.
The North Carolina, Idaho, and Utah plants utilize locally mined rather
than Florida mined phosphate rock.
The construction of gypsum ponds is generally in proportion to the
number and size of new plants constructed. However, new gypsum ponds.-
may not be built in the future if the economics of the electric furnace
process become competitive with those of the wet process. In this
-------
YEAR
CONSUMPTION
(Millions of Metric Tons)
as H3P04
1965
1970
1975 (est)
1980 (est)
5.0
6.9
8.7
11.0
'65 - '70 Growth Rate = 7%
'70 - '80 Growth Rate = 5%
-------
process, the phosphate rock is reduced to phosphorus by the furnace and
is subsequently converted to phosphoric acid. This eliminates the use of
sulfuric acid in the process and the need for gypsum ponds. The major
factor in the economics of this process is the cost of electricity. The
Florida phosphate companies have been negotiating with the power companies
for lower rates in anticipation of expanding the electric furnace pro-
duction. If these new production methods can be employed, the industry
will go through a technological revolution.
1.3 DISTRIBUTION OF GYPSUM PONDS
Gypsum ponds are an important part of the wet phosphoric acid process.
They are generally diked areas where the gypsum by-product can settle
out and the surplus water is collected for reuse. Approximately one
acre foot of disposal volume is required per year for each daily ton of
^2^s produced by the plant.
The location of gypsum ponds thus depends upon the location of wet
phosphoric acid plants. These plants are generally located either near
the phosphate rock mine or near areas of large fertilizer use with
inexpensive bulk transporation facilities. Thus the plants are located
near mines in central and northern Florida; North Carolina; Idaho; and
Utah; near the coastal ports of Wilmington, Delaware; Pascagoula,
Mississippi; Houston, Texas; and New Orleans, Louisiana; along the
Mississippi River and its tributaries at St. Paul, Minnesota; St. Louis,
Missouri; Helena, Arkansas; Chicago, Illinois; Gary, Indiana; Tulsa,
Okalhoma; Joplin, Missouri; and several places in central Illinois;
-------
and along railroad routes from the western mines to central and southern
California. These locations are shown generally in Figure 1.2.
Generally, the ponds associated with the non-mining locations are in
heavy industry sections of cities, and as such are generally surrounded
by other heavy industries or transportation complexes. The mining lo-
cations are usually more remote from centers of population, but the worst
case situation is in central Florida.
The Florida plants account for well over half the United States capacity
for producing phosphoric acid by the wet process; thus the majority of
gypsum ponds may be found there. A more detailed map of the Polk-Hills-
borough County area of Florida, shown in Figure 1.3 reveals the location
of several gypsum ponds. These are generally some distance from the
centers of population; but because of the population growth in these
areas some ponds are rather close. Typically, these ponds are surrounded
by land held by various members of the phosphate industry. Where not
being mined, the land is used as raw pasture for cattle, with some
citrus groves. In most cases, the land can be classified as high scrub
land or low river swamp area.
In mined out areas there are recreation facilities, citrus groves and
improved pasture, as well as some housing in the most desirable areas.
A general land ownership map is shown in Figures 1.4 and 1.5. The average
population density in the areas outside the cities of Bartow and Fort
Meade is approximately 0.14* people per acre. The overall population
density for Polk County is 0.23* people per acre.
est. July 1, 1974 based on 1970 census.
7
-------
n
-------
Pasco
Hillsborough
County
Polk
County
and
Lake
Alfred
U inter
Haven
ke
• •
ul belfry
I •
Bradley
Peace
River
Fort Meade
Manatee
County
Hardee County
= Approximate Location of Gypsum Ponds
Figure 1.3. Approximate Locations of Gypsum Ponds in Central Florida.
-------
'.•• Mulberry
Land Owned By
(lining Corporations
Land Owned By Others
Centers of Population
Gypsum and Cooling
Pond Locations
Figure 1.4 Map Showing General Land Ownership Around Gypsum
Ponds in the Mulberry Area of Central Florida.
-------
Land Owned By
f-lining Corporations
Land Owned By Others
Centers of Population
Gypsum and Cooling
Pond Locations
Figure 1.5 Hap Showing General Land Ownership Around Gypsum Ponds in the
Bartow-Fort Meade Area of Central Florida.
-------
2.0 THE ROLE OF THE GYPSUM POND IN AN INTEGRATED PHOSPHORIC ACID/FERTILIZER COMPLEX
The gypsum pond is a vital unit process in the typical, highly integrated
phosphoric acid and fertilizer manufacturing complex. It serves not only the
wet process phosphoric acid facilities, but also the many fertilizer manufac-
turing processes which are dependent on phosphoric acid as a feed stock. The
gypsum pond is a source of cooling water, scrubber water, wash water, process
water and slurry water; a water pollution control and water conservation pro-
cess operation; and last, but by no means least, a disposal sink for waste
rock from the various phosphorus solubilizing operations.
Thus it is very difficult, if not impossible, to consider a gypsum pond
only in terms of phosphoric acid. The problem of studying the gypsum pond
is further compounded by the many different configurations of ponds used
by the industry.
Only three plants in the U.S. specialize in producing wet process phosphoric
acid. Forty-four plants (with a capacity greater than 120 tons P205
per day) were identified in the EPA Effluent Guidelines Study which manufac-
ture one or more fertilizer products in addition to phosphoric acid.
A breakdown of the product mix found throughout the industry is found in
Table 2.1. These products include the phosphate based Triple Superphosphate
(TSP) both run-of-pile and granulated; Superphosphoric Acid (SPA); and mon-
ammonium phosphate (MAP) and diammonium phosphate (DAP) as well as the nitrogen
based ammonia, urea, ammonium nitrate, and nitric acid. Each of the above manuf«<
turing processes utilizes gypsum pond waters to optimize the manufacturing
process efficiency and to control process generated air and water pollution.
Since the product mix at any given plant is primarily a function of market
12
-------
TABLE 2.1 Integration of Production
In The Fertilizer Industry
Source: .(EPA-440/l-74-011-a)
No. of
Companies Nlij U
22
2
2
3
3
3
'^ 6
1
7
3
1
1
3
1
2
13
5
3
1
16
1
2
1
4
1
1
1
21
1
2
2
1
1
1
3
7
1
1
1
1
TTo"
!_/
.?/
VI
N.
A.
S.
X
X
X X
X X
X X
X
X
X
X
X
X X
X
X X
X
X X
X X
X : X
X
X X
X
X
X X
X
X
X
X X
X X
Nof Identified individually In
of sulphuric a. Id fnrility a«
OnJv 10<» firms — Includes more
Urea
A. Nltiic arid
N. Amnonlun nitrate
A. Sulfurlr arid
N.A. A.N. S.A
X
X
I/
I/
!/
11
X
X
X X
X X
X
X X
X X
X X
X X
X
X X
XX 11
X X
X X
X
X
X
X
X
X
X
X
X
X
X
XX X
X X X
XX X
data used to develop this
Wet
X
X
X
X
X
X
•
X
X
X
X
X
X
X
X
X
X
A. P. TSP SPA
X
X
X
X
X
X X
X X
X XX
X
X
X
X
X X
X
X
X
X X
X X
X XX
X
X X
X
No. oi
Plants
22
2
2
12
9
3
1
6
6
3
14
9
4
4
6
3
4
26
15
9
3
64
3
8
4
20
7
4
S
4
6
8
S
4
5
IS
14
4
6
4
S
7
390
lint, but must assume existence
intermediate to vet acid production.
than one location of plant
operations for
some flrns.
Vet Wet phosphoric acid
A, P. Ammonium phosphate
TSP Triple Superphosphate
SPA Superphosphate, acid
13
-------
conditions, transportation costs, and geographical location, It 1s not
feasible to attempt to characterize all of the possible combinations In this
study. Instead the major manufacturing processes for each phosphate based
product will be discussed and the unit operations or other sources of Impact
on a gypsum pond for each process will be identified. These unit operations
will be characterized with respect to fluoride contribution to the gypsum
pond. A flow and mass balance for an integrated phosphate complex will be
developed.
2.\ PROCESS DESCRIPTIONS
2-1-1 Phosphoric Acid - Furnace Method
Phosphoric acid is produced by the furnace method. However, this pro-
cess is primarily oriented towards the recovery of elemental phos-
phorus and high quality technical grade phosphoric acid rather than
fertilizer and merchant grade acid. Furthermore, the furnace process
does not require the use of gypsum ponds. Consequently the furnace
method will not be discussed.
2•] - 2 Phosjihgrjc__Ac1d - Wet Process
The most common method of producing phosphoric acid is by a wet process
utilizing sulfuric acid. The use of this process is the reason for
the existence of the gypsum pond. It is general practice throughout
the industry to integrate one or more phosphate fertilizer manufacturing
14
-------
operations into the same complex with the wet process phosphoric plant.
Phosphate rock is a complex material; the principal mineral constituent,
fluorapatite, contains calcium, phosphate, fluoride, carbonate, and
many other elements or groups bound together in a crystal lattice.
Analysis of several typical grades of Florida rock is found in Table 2.2
When the rock is treated with a strong mineral acid, the apatite
lattice is destroyed and the phosphate constituent is solubilized
as phosphoric acid. The overall reaction with sulfuric acid is as
fol1ows:
1. The tricalcium phosphate constituent is converted to phos-
phoric acid and calcium sulfate.
Ca3(P04)2 + H2S04 -> 2H3P04 + 3CaS04 (2.1)
2. The calcium fluoride constituent of the fluorapatite reacts
with sulfuric acid to produce hydrogen fluoride and calcium
sulfate.
CaF2 + H2S04 •»• 2HF + CaS04 (2.2)
3. The calcium carbonate constituent is converted to carbon
dioxide and calcium sulfate.
CaC03 + H2S04 + C02 + CaS04 + H20 (2.3)
The entire reaction between the major constituents and sulfuric acid
is as follows:
Ca1Q(P04)6F2CaC03 + 11H2S04 + nH20(liq) -»• 6H3P04(aq) +
llCaS04nH20 + 2HF(aq) + H20 + C02 (2.4)
where n may equal 0, 1/2 or 2 depending on the form of the hydrated
CaS04, anhydrite, hemidydrate and dihydrate, respectively.
1
r.
-------
Table 2.2 Typical Composition and Particle Size of
Commercial Grades of Florida Phosphate Rock3.
(Legal, p. 21)
SPECIFIED BPL RANGE
BPL (by analysis)
P205 equiv.
H20
Fe2°3
A1203
Organic
Si Oo
co2
F2
CaO
so3
68/66
68.15
31.18
1.3
1.33
1.76
2.18
9.48
3.48
3.60
45.05
1.05
Screen
70/68
70.16
32.10
1.0
1.25
0.96
1.74
8.68
3.05
3.67
46.12
1.02
analysis
% larger or smaller
Specified sizeb
85% through 100 mesh
90% through 100 mesh
j*Dry basis.
All grades.
+50
1.5
1.0
+70
5.5
4.0
72/70
72.14
33.00
1.0
1.07
0.83
1.76
6.46
2.87
3.62
48.10
1.11
75/74
75.17
34.39
1.0
1.03
0.82
1.73
4.59
2.65
3.78
50.14
0.74
than indicated mesh
+100
14.0
10.0
+200
25.0
24.1
77/76
77.12
35.28
1.0
0.84
0.56
1.70
2.02
2.98
3.89
51.53
0.66
size
-200
75.0
75.9
16
-------
The excess fluoride present in the phosphate rock also reacts with
phosphoric acid and sulfuric acid for form HF.
CaF2 + 2H3P04 * Ca(H2P04)2 + 2HF (2.5)
CaF2 + H2S04 + H20 + CaS04 • 2H20 + 2HF (2.6)
The HF then reacts with excess silica:
Si02 + 6HF -> H S1F + 2H20 (2.7)
During the acid concentration steps, a considerable portion of the
fluosilicic acid dissociates to form HF and SiF4:
H2SiF6+ SiF4 + 2HF (2.8)
The fumes resulting from HgSiFg volatilization are scrubbed with
water, however, regenerating the H0SiFc:
£. O
3SiF4 + 2H20 -> 2H2SiF6 + Si02 (2.9)
Si02 + 6HF + H2SiF6 + 2H20 (2.10)
which is pumped to the ponds with the gypsum slurry.
The most common method of producing wet process phosphoric acid is
with the use of sulfuric acid in the dihydrate process. Typical anal-
ysis of this acid is found in Table 1.3. However, there are several
other processes which are worthy of mention because of the significant
effects their increased acceptance and use would have on the use of
gypsum ponds. These processes include the hemihydrate process, the
hemihydrate-dihydrate processes, the anhydrite processes, nitric acid
processes and hydrochloric acid processes. Only the sulfuric acid
processes will be discussed at this time, however, because they are
most commonly used in the United States. The nitric and hydrochloric
acid processes are used in Europe to produce directly complex fertilizers
rather than phosphoric acid.
17
-------
Table 2.3 Composition of Filtered Wet-Process Phosphoric Acids (Slack, p. 656)
CD
Acid composition, %
Rock source
Florida3
b
b
b
b
c
c,d
Western3
c
Tennessee0
P205
27.3
28.4
31.2
26.3
30.2
30.0
27.0-31.9
23.2
30.0
30.0
CaO
0.15
0.1
1.0
0.4
0.1
1.26
0.01-0.8
0.22
0.21
0.37
F
1.7
1.5
1.4
2.0
2.0
2.36
0.9-3.1
1.2
1.36
2.54
A1203
0.6
1.1
0.8
0.5
1.9
1.08
0.2-1.6
0.8
1.01
2.66
Fe203 MgO K20
1.1 0.28 0.03
2.0
1.7
1.1
1.1
0.86 0.06
0.8-2.4
0.6 0.33 0.05
0.42 0.05 0.06
2.27 0.07
ComDOsition of
Na20 Si02 503 suspended solids
0.08 1.2 B, A, C
3.9
0.2
1.1 1.0
1.6 3.1
0.01 1.21 3.72
0.2-0.6 0.4-4.6 A, C
0.13 1.0 A, C
0.01 0.74 2.63
0.43 0.1 1.49
3 Composition of clear, supernatant acid after cessation of precipitation; compounds identified in solids:
A=CaSOA 2H20; B= Ca4S04(AlF6)(SiF6)OH 12H?0; C=(Na, K)2SiF6.
b Compositions of acids include any suspended solid material in shipped acids.
Jj Data taken from Table II of Hill (7) and recalculated to 30% P2Q$ basis for comparison
" Composite analyses of 21 product acids.
-------
The basic objective of any method of preparing phosphoric acid from
phosphate rock and sulfuric acid is to obtain the highest concentration
of phosphoric acid possible with the maximum yield. The yield is
dependent on the completeness of the reaction of rock and sulfuric acid
and on the efficiency of the separation of phosphoric acid from
calcium sulfate. The quantity of wash water required to remove es-
sentially all the P205 from the calcium sulfate during filtration
determines the concentration. Consequently, the characteristics of
the calcium sulfate crystals formed during the reaction are the most
important fundamental factors in phosphoric acid production. The
temperature of the reaction mass and the phosphoric acid concentration
are the controlling factors governing the degree of hydration of the
calcium sulfate. The influence of temperature and concentration on
the state of hydration and the stability of the calcium sulfate has
been studied by many investigators and phase diagrams have been developed.
All the commercial wet processes for phosphoric acid are based on con-
ditions, as defined in these phase diagrams (Figure 2.1), that give
stable, filterable forms of calcium sulfate.
19
-------
ieo
SO
0.
t
zo
Tron*ition Di
•tilibrium Curve.
*-
Equilibrium Curve.
— — Hult-toorrv - L«J-ir«c.Uc«'» Curve.
Curve. From Solubilifq Data
^—•—— SanfoureHA'« Curve.
n Iff:
AnKqdrif* H/
, Un*f«|bl«.
Raqion Q
H , S+«bl«.
\\
\ \
\
V
R«qion
.
i M»<-«»t«ibl*
Un«tobl«L
10
ZO so 4O
50 <»0
Figure 2.1 Phase Equilibrium Diagram for CaSCL-P905-H20 System
(Dahlgren, p. 94) ^
20
-------
2.1.3 D1 hydrate Process
The di hydrate process produces 28-32% $2®$ phosphoric acid by acidu-
lating ground phosphate rock at a temperature of 160 to 185°F. These
conditions result in the formation of calcium sulfate in the dihy-
drate form.
The typical dihydrate process (Figure 2.2) used prior to the early 1960's,
and still in widespread use today, involves the reaction of sulfuric
acid and phosphate rock in a series or "train" of agitated reactor
tanks through which the slurry cascades by gravity. These trains may
consist of four or more tanks, and retention time in the system ranges
from 4 to 8 hours. The initial step consists of premixing sulfuric
acid with recycled weak phosphoric acid and recirculated phosphoric
acid slurry. Dry, ground rock is added to this mixture, in slightly
less than the stoichiometric proportion to maintain a slight excess
of sulfuric acid in the system. The slurry then flows through the
successive reactor tanks in which temperature and acid concentration
.are controlled to ensure completeness of reaction and good growth
of gypsum crystals to the size necessary for effecting good separation
of acid and gypsum in the filtration step. After leaving the last
stage reactor tank, the slurry is split into two streams in a
ratio of about 15 to 1. The larger stream is recirculated to
the premixing tank to be intermixed with the fresh sulfuric acid;
the smaller is pumped to a filter for separation and washing of the
gypsum crystals. Filter operation usually involves three continuous
stages. The product acid, containing 28-32% P^* is separated from
11
i. \
-------
um Porvd
7-7
«c*-iwle 'o^paoro
Pood W-«-*r- «^/ C5Mp*^,
Figure °.2 Typical Oihydrate Process With Vacuum Acid Concentration
and Fluosilicic Acid Recovery (Slack, p. 31)
90-ino°c
Figure 2.3 Hemihydrate - Dihydrate Phosphoric Acid Process
(Takeuchi, p. 303)
22
-------
the gypsum in the first stage, and the phosphoric acid remaining is
washed from the gypsum residue during the second and third stages.
Recycled gyp pond water is frequently used as the wash on the third
stage; the filtrate from this stage is circulated over the gypsum
in the second stage, where the acid concentration reaches about 20%
P205, and is then pumped back to the premixing tank. The washed
gypsum cake discharged from the third stage is usually reslurried
with water and pumped to a disposal area. The product acid from
the filter is concentrated to the desired strength in vacuum evap-
orators or submerged combustion concentrators.
Although the new plants constructed since 1960 employ the same basic
principles as those used in the old process, improved engineering de-
sign and materials of construction have decreased capital and operating
costs per unit of capacity and have improved overall operating efficiency.
In these plants (Figure 2.2) single-tank, multiple-compartment reactors
replace the multiple-tank train. Precise design of the tank interior
is important to assure complete reaction of the rock and sulfuric
acid and prevent the bypassing of unreacted rock. A series of agitators
and.baffles forces circulation of the slurry through prearranged paths
to give the same effect as the slurry recirculation and cascade ar-
rangement in the train system.
In a typical Florida operation 68 BPL (31.1% P205) rock (ground to
50-55% minus 200 mesh in a ball mill) is measured through gravimetric
feeders to the slurry in the first compartment of the reactor, where
23
-------
it is mixed with sulfuric acid (93%) and recirculated slurry. Good control of
raw material feeding and of slurry composition is essential to assure proper
operating conditions throughout the entire 676 hour residence time required
for reaction and gypsum crystallization. The rock:acid ratio, which must be
held within narrow limits, is controlled by the electronic proportioning of the
acid flow with the phosphate rock feed. The weight of solids in th& slurry
(termed pulp solids content, or sometimes slurry density), is controlled by
varying the amount of recycled acid. Upsets may occur which require wasting
of the slurry to the gypsum pond to regain the proper rockiacid ratio.
2.1.4 Hem ihydrate P roces s
The hemihydrate process produces phosphoric acid at 38-42$ P205 concentration
by acidulating the phosphate rock at temperatures of 90-135°C. The higher
concentration of the product acid reduces the number of evaporator stages
required for concentration to 54% merchant grade acid. This process has
been successfully demonstrated in Europe (AB Forenade), Japan (Taki) and the
U.S. (TVA), however, it has not been accepted by the industry due to
difficulties in controlling the operation.
2.1.5 Hemihydrate-dihydrate Process
The hemihydrate-dihydrate process involves the precipitation of calcium sulfate
in the hemihydrate form followed by recrystallization to dihydrate.
This basic process has been extensively developed in Japan and Europe
but has been evaluated only on the pilot plant scale in the U.S. This
process is claimed to have a high level of stability and reliability 1n plant
operations, 97-98.5% recovery of P205 from the phosphate rock (allegedly
higher than by the dihydrate process), and recovery of by-product gypsum (1.2
to 1.5 tons gypsum/ton phosphate rock) that is of high enough quality to
use in making gypsum plaster, gypsum wall board, portland cement and for
chemicals such as in the gypsum-ammonium sulfate process.
-------
The spent gypsum from the hemihydrate-dihydrate process is slurried to a
storage pond area where the by-product gypsum is recovered by either filtra-
tion or settling.
A flow sheet of a hemihydrate-dihydrate process is shown in Figure 2.3.
Operating features of the various process steps are as follows:
Decomposition - Raw phosphate rock is ground to 60 to 80% through 200 mesh
and transported to the rock hopper at the phosphoric acid plant. After passage
through a constant feed weigher, the rock is moved by a screw conveyor to the
premixing tank; active silica is also weighed and is fed by the same screw.
Sulfuric acid is measured by a flow meter, diluted to the desired concentration
(about 65%), and sent to the premixing tank where it is mixed with recycled
phosphoric acid of about 20% P205 content. Silica is added at this point
in the process to tie up HF as in SiF4. It is possible to recover the SiF4 as
a HoSiFg by-product, the temperature inside the premixing tank is maintained
between 194 and 212°F by the sensible heat of the mixed acid and the reaction
heat. Since mixing is rapid in the premixer, there is no opportunity for local
points of high temperature to persist long enough for anhydrite formation. Severe
foaming takes place during decomposition of the phosphate rock, requiring
the use of a defoaming agent or other means for controlling foam. No such
agent is required when calcined rock is used.
The slurry is thoroughly mixed in the premixing tank and then overflows into
the attacking tank where decomposition proceeds. The retention time required for
decomposition varies with the grade of phosphate rock but ordinarily about
2 hours is adequate.
In the course of decomposition, calcium sulfate produced by the decomposition should
be completely converted to hemihydrate. Should gypsum form in this step, gypsum
"crystals produced in the subsequent hydration step will not be uniform and the tota\
P205 content of the gypsum will increase, reducing P205 yield and adverselyaffectin
25
-------
the quality of the by-product gypsum. It is necessary, therefore,
to carefully control the concentration of phosphoric acid, the amount
of excess sulfuric acid, and the decomposition temperature. Off-gas
from the premixing and attacking tanks is collected in the duct and
washed in the Venturi scrubber to remove fluorine compounds. All the
tanks are cylindrical and equipped with agitators. Since corrosion Is
a more severe problem at the relatively high attack temperature used,
the tanks are lined with carbon bricks and with a rubber resistant to
high temperature. The agitators are also lined with such rubber.
Hydration (Recrystalization) - Slurry from the decomposition step
overflows into the hydration tank where it 1s mixed with cooled,
recycled dihydrate slurry, giving a temperature between 50 and 58°C.
The slurry recycles from the last hydration tank through the cooling
tower to the first hydration tank. The recycling also performs the
2+
function of maintaining uniform Ca concentration and concentrating
the slurry. Owing to the rapid increase in supersaturatlon, fine gypsum
crystals form in the recycling slurry and act as crystal seeds, making
it unnecessary to introduce seeds from outside the system.
The volume of recycled slurry is determined by the operating conditions,
but it is several times the volume of slurry fed to the filter. In
the hydration tanks hemihydrate recrystallizes gradually to dihydrate, and
at the outlet of the second tank the crystals are large enough for easy
filtration and washing. The degree of phosphate rock decomposition
is 99 to 99.5%.
The hydration tank and the cooling tower are rubber-lined, and the slurry
26
-------
pump is made of a special corrosion-resistant stainless steel.
Filtration - Any type of conventional filter can be used, such as tilting-
pan or traveling-pan filters. The gypsum cake fomed on the pans has
a thickness of 1.5-2.0 in. After counter-current washing it is removed'
from the pans in the form of a cake containing 20 to 25% water. The
product phosphoric acid (about 30% P205) is sent to the storage tank.
Dilute acid (about 20% P205) from the second stage of filtration is
recycled and mixed with the feed sulfuric add. The gypsum cake from
the filter is repulped with water and sent to the gypsum storage yard,
where it is separated from the water by means of settling or filtration.
It is sold to wallboard, plaster, and cement manufacturers.
2.1.6 Phosphate Rock Grinding and Drying,
Phosphate rock that has been mined and beneflciated is generally too
coarse to be used directly in acidulation to phosphoric acid. The rock
is, therefore, processed through equipment to mechanically reduce it to
the particle size required for optimum phosphoric acid plant process
efficiency.
Size reduction Is accomplished with ball, .roll or bowl mills. Phosphate
rock 1s fed into the mills and mechanically ground. After the rock enters
the,m1ll system, all flow through the sizing and reclamation circuits Is
by pneumatic means. Air Is constantly exhausted from the mill system
to prevent precipitation of moisture generated from the rock as a result
of.grlndlng. Normally, the exhaust air passes through a bag type air
cleaner to remove entrained rock partlcul.tes before discharge to the
atmosphere.
27
-------
Phosphate rock size reduction in all existing fertilizer plants 1s an
entirely dry processing circuit and does not directly Involve liquid
streams. Minor quantities of water are used for Indirect cooling of
lubricating oil and mechanical equipment such as bearings.
Rock grinding operations in the future will also utilize a wet grinding
circuit rather than the current dry grinding practice (Long, Slack).
This change is prompted by a combination of lower capital costs and the
elimination of the gas effluent streams associated with both the rock
drying and grinding operations. Use of this new technique will not
change the self-contained nature of the rock grinding circuit. There
will be no liquid effluents other than those mentioned 1n the dry grinding
process.
2.1.7 Acid Concentration and Defluorinated SuperphosphoHc Add
Wet-process phosphoric acid (30% P205) from the d1hydrate process Is not
directly suited for use in end products and must be concentrated by
evaporation. A concentration of 40 to 42% P205 is required 1n producing
ammonium phosphate (AP) and granulated triple superphosphate (GTSP) made
by the slurry process, and 52 to 53% P205 add is needed 1n making run-of-
pile (ROP) triple superphosphate and shipping-grade (merchant) add.
Wet acid produced by a hemihydrate process at 40 to 42% PgOs or by an
anhydrite process at 52 to 54% P205 1s directly suited for use In the
manufacture of such fertilizer products.
The term defluorinated phosphoric acid is a bit misleading to persons
associated with the fertilizer industry. The reason is that acid
defluorination is inherently included in the process of evaporating
23
-------
commercial wet process 54% P205 phosphoric add to the superphosphoHc
add (68-72% P205) concentration level. To fertilizer people therefore,
the principal U.S. defluorinated add process 1s better known as a super-
phosphoric acid unit.
Defluorinated phosphoric acid 1s used primarily as a raw material for pro-
duction of mixed fertilizer goods - both dry and liquid types. It 1s also
mixed with limestone in the manufacture of dicalcium phosphate for use as
an animal feed supplement. Approximately 67% of the estimated U.S. 835,000
annual tons P00n quantity of defluorinated add is used in fertilizer
c. b
manufacture and 33% in the production of dicalcium phosphate. The degree
of defluorination required to meet animal feed regulations is that the P
to F ratio be at least 100 to 1. (EPA 440/1-75/043)
The concentration and defluorination of phosphoric add 1s a difficult
operation, as the acid is corrosive. Scaling of equipment 1s also a
problem. Equipment designs historically have been of five types: (1) forced-
circulation vacuum (Figure 2.4),(2) thermal-siphon vacuum, (3) falling-
film vacuum, (4) submerged combustion (Figure 2.5) and (5) hot gas
evaporation. All five methods have been used commercially, but the forced-
circulation vacuum process with steam as the heat source is the most pop-
ular in modern phosphoric add plants.
,. • j*>. . • ••••"•
Another method of defluorlnatlng wet process phosphoric acid has come into
commercial use in the past few years (F1gure2,6). This process also uses
merchant grade wet process 54% phosphoric acid as the raw material. In
this process, silica is mixed with the phosphoric add to aid in the release
and volatilization of fluorine from the liquid. The mechanism for fluoride
29
-------
Figure 2.4 Defluorinated Phosphoric Acid - Vacuum Process (EPA 440/1-53/043)
Ac led
Pond
Water
A.r-
k°'F
Scrubber
Cooler
| tta» Fi
i
lt»r
To
A.. ,4
, V»
Acid
Cooler
Acid
Cool«.r
Alr-
Figure Z.5
Acid - Submerged Combustion
30
-------
JL I
-4
Fon
JL T
% PtO«
Pnovpharic. Acid
1
conda.n«afc
return
t
Pump
to
SHippino
Figure 2.6 Defluorinated Phosphoric Acid - Aeration Process
(EPA 440/1-75/043)
31
-------
removal from the acid is aeration (EPA 440/1-75/043).
2.1.8 Defluorinated Acid - Vacuum Type Evaporation
The vacuum type evaporation method for defluorination of wet process
phosphoric acid is essentially identical to the procedure and equipment
used to produce 54% P205 strength acid. Defluorination is performed 1n
vessels which use high pressure (450-550 psig) steam or externally heated
Dowtherm solutions as the heat energy source for evaporation of water
from the acid. These units affect evaporation by circulating add at
a high volume rate consecutively through a shell and tube heat exchanger
and a flash chamber under low absolute (vacuum) pressure conditions. In
the heat exchanger, steam or Dowtherm solution is applied to the shell
side and acid flows through tubes. Acid flow through the tubes 1s of the
wetted wall type rather than full tube flow. The flash chamber serves to
provide a large liquid surface area where water vapor Is released without
significant acid entrainment loss. Fluoride removal from the add occurs
concurrently with the water vapor release. Both of these gases pass to a
barometric condenser and are absorbed in the condenser water. Dependent
upon the quality of superphosphoric add being produced (e.g. 30% or
50-60% conversion to polyphosphates), either a single unit or a series of
two units may be used to accomplish the evaporation and/or defluorination
required. Fluosilicic acid may be easily recovered from this type
evaporation (Figure 2.2 and 2.4).
2.1.9 Defluorinated Acid - Submerged Combustion
A second method of phosphoric acid defluorination (Figure 2.5) is by the
direct contact of hot combustion gases with the acid. In this method a
combustion chamber fitted with one or more fuel oil or gas burners 1s
32
-------
mounted directly on top of an add containment chamber. Pressurized hot
gases from the fuel combustion are bubbled through the acid to an immersion
depth of up to approximately 46 cm (18 inches). Acid in the containment
chamber is maintained at a constant level by control of the low concen-
tration feed acid flow. Evaporated and defluorinated product acid from
the unit is continuous and is controlled by acid boiling point and/or
temperature.
Gases (evaporated water, stripped hydrogen fluoride and silcon tetrafluoride)
from the evaporation chamber flow to a series of gas cleaning and absorp-
tion equipment. First, entrained phosphoric acid is recovered from the
gas stream and re-introduced to the unit or to the phosphoric acid plant.
Following acid removal, the gases pass to a multi-stage direct contact
condenser system where a high percentage of the contaminants are removed
before exhaust to the atmosphere. Water can be used in all or only the
final stages of the condenser system as a condensing and scrubbing medium.
2J.10 Defluorinated Acid - Aeration
This method of defluorinating phosphoric acid is the most recent pro-
prietary method to come into commercial use (Figure 2.6). Relatively small
quantities of diatomaceous silica or spray dried silica gel with high sur-
face area characteristics are mixed with commercial 54% P205 phosphoric acid.
This silica material also serves to supply sufficient silica for conver-
sion of the minor quantity of hydrogen fluoride (HF) present in the impure
phosphoric acid to fluosilicic acid (H2SiF6). Fluosilicic acid at an
adequate temperature in turn breaks down to SiF4 and HF and by simple
aeration is stripped from the heated mixture. The gaseous effluent
stream is maintained above its dew point until it enters the gas scrubber.
33
-------
At this point the gas stream is contacted with water to remove contami-
nants before release to the atmosphere. Phosphoric add (54% P205) can
be defluorinated by this method to a weight ratio of P to F of 100
to 1 or better.
A major part of the impurities in filter acid is removed during concen-
tration. In Table 2.4 several analyses of acids obtained from operating
plants are given; they illustrate major reduction in impurity content
between filter acid and concentrated acid, particularly with respect to
fluorine and silica. The impurity-to-P205 ratio decreased significantly
for all impurities.
All concentrated acids contain solid material resulting from post-
precipitation of impurities. The compositions of concentrated acids
and the solids separated from them are given in Table 2.5. As the solids
in these acids were separated without washing, adhering phosphoric acid
is included in the analysis. For some of the acids, which presumably had
little or no clarification, the precipitates contained large amounts of
CaO, S04, F, and S102. indicating the presence of calcium sulfate and
fluosllicates. Others, however, had relatively low solIds content, and
Fe203 was an important constituent of the solids. The major precipitating
phase, forming in acids after concentration and clarification, has been
identified as a complex potassium-iron-alumina-phosphate compound.
Separated solids are either discharged to the gypsum pond or used as a
solids conditioner in the manufacture of granular superphosphate.
34
-------
Table 2.4 Effect of Concentration on Composition of Wet-Process
Phosphoric Acids (Legal, p. 47)
Kind of
rock
Concentrated acids
Florida
Unknown
Florida
Florida
Western
Western
Western
Florida
Florida
Florida
Filter acids
Florida
Florida
Florida
Florida
Unknown
P2°5
54.6
53.8
52.8
54.8
51.1
53.2
54.2
58.2
50.6
53.4
28.4
31.2
26.3
30.2
30.0
so3
3.6
2.8
2.7
3.4
1.9
1.5
1.0
2.3
3.0
1.5
3.9
0.2
1.0
3.1
1.8
i
CaO
0.3
0.1
0.1
0.5
0.1
0.1
1.3
0.0
0.1
0.1
0.1
1.0
0.4
0.1
0.3
by weight
Fe2°3
1.3
1.4
1.1
1.4
1.1
1.0
0.7
1.3
1.4
1.7
2.0
1.7
1.1
1.1
1.4
A1203
0.6
1.3
0.8
1.4
2.8
1.5
1.5
1.7
1.5
1.1
1.1
0.8
0.5
1.9
3.0
F
. .
0.7
0.4
0.9
1.4
0.6
1.1
0.6
0.8
0.9
1.5
1.4
2.0
2.0
2.4
S102
«-.
0.4
0.2
0.2
__
-_
0.6
0.2
. _ _
0.1
__
_ ••
1.1
1.6
--
MgO
__
0.4
0.5
0.5
__
0.8
0.5
0.7
-.»
0.5
__
__
__
__
--
Table 2.5 Analyses of Wet-Process Acids and of Solids Obtained
by Filtering the Acids (Hein, p. 695)
Acid
no.
1
2
3
4
Sampl e
Total acid
Solids
Total acid
Solids
Total acid
Solids
Total acid
Solids
P2°5
53.8
53.4
52.8
51.2
54.2
48.7
51.5
40.3
S°3
2.6
4.4
2.6
5.2
1.0
8.1
2.1
7.2
CaO
0.1
2.0
0.1
2.5
1.3
6.6
1.0
5.0
Fe2°3
1.4
8.6
1.1
4.1
0.7
3.4
2.1
3.6
Ut. %
A1203
0.8
2.3
0.8
0.9
1.5
2.3
1.1
3.0
F
0.5
3.4
0.4
2.1
1.1
5.0
1.8
8.1
S102
0.2
0.6
0.2
0.6
0.6
3.1
0.6
6.4
MgO
0.3
1.2
0.5
0.3
0.5
0.5
0.7
2.2
C
0.1
0.2
0.6
4.4
0.0
0.0
0.3
1.6
Solids
1.9
3.1
13.4
20.3
35
-------
2.1.11 Normal Superphosphate
Normal superphosphate was, for many years, by far the most popular phosphate
fertilizer. Since the mid-fifties, however, its popularity has been in a
sharp decline and only in the past few years has the rate of decline
started to moderate. The market share of this fertilizer has fallen from
68% in 1957 to 42% in 1965 and now appears to be leveling off at approxi-
mately 18%. The major reasons for this decline include such items as low
P205 content (20%) with the associated increased cost of transportation
per ton of nutrient and the trend to larger size plants.
Normal superphosphate can be manufactured in small inexpensive plants
with low production costs per ton of P205 since the CaS04 formed in the
acidulation of the rock is not separated from the final product. The
process is simple and easy to operate, requiring less sulfur per ton of
P205 than the production of phosphoric acid. The combination of low
investment and simplicity, together with recognition of the beneficial
fertilization effects of sulfur in the soil,assures that normal super-
phosphate production will not die out but sales will be limited to
an area in close proximity to the plant site.
The two raw materials used in the production of normal superphosphate
are 65-75% sulfuric acid and ground phosphate rock. Reaction between these
two materials is both highly exothermic and rapid (Figure 2.7). The basic
chemical reaction is shown by the following equation:
Ca3(P04)2 + 2H2S04+3H20 -> 2CaS04.2H20 + Ca^PO^O (2.11)
Phosphate Sulfuric Water Giypsum Normal Superphosphate
Rock Acid
36
-------
Ground phosphate rock (90% minus 100) is fed by a weigh feeder into a
double-conical mixer (TVA), where it is thoroughly mixed with metered
quantities of sulfuric acid. The sulfuric acid is diluted with water
in the cone. The heat of dilution serves to heat the sulfuric acid to
proper reaction temperature, and excess heat is dissipated by evapora-
tion of extra water added. The rate of water addition and acid concen-
tration may be varied to control the product moisture. The acid and
water are fed into the cone mixer tangentially to provide the necessary
mixing with the phosphate rock. The fresh superphosphate discharges from
the cone mixer to a pug mill, where additional mixing takes place and
reaction starts. From the pug mill the superphosphate drops onto the den
conveyor, which has very low travel speed to allow about one hour for
solidifying before reaching the cutter. The cutter slices the solid
mass of crude product so that it may be conveyed to pile storage for
"curing," or completion of the chemical reaction, which takes 10 to 20
days to reach an acceptable P205 availability as plant food. The contin-
uous den is enclosed so that HF and S1F4 fumes do not escape into the
working area. These fumes are scrubbed with water sprays to remove acid
and fluoride before being exhausted to the atmosphere.
The wet scrubbers are the only significant source of wastewater from
the process. When normal superphosphate is produced at an Integrated
fertilizer complex the scrubber will most likely utilize the gypsum pond
water.
2.1.12 Triple Superphosphate
Triple superphosphate (TSP), with its 46.0% - 48.5% P205 content, is a
high analysis phosphate fertilizer. As such, it provides transportation
37
-------
economy which has been instrumental in enlarging its share of the phos-
phate fertilizer market.
This product has in the 1950-1965 period taken over much of the market
lost by normal superphosphate and currently accounts for approximately
24% of the total phosphate fertilizer market. TSP's share of the market
for the near future is expected to remain relatively constant primarily
Due to the tremendous growth of the ammonium phosphates. TSP produc-
tion, unlike normal superphosphate, can be most economically produced
close to the phosphate rock source. In the U.S. this means that approx-
imately 83% of the total production if manufactured in Florida.
There are two principal types of TSP, Run-of-Pile (ROP) and Granular
Triple Superphosphate (GTSP). Physical characteristics and processing
conditions of the two materials are radically different. ROP material
is essentially a non-uniform pulverized material which creates difficult
air pollution problems in manufacture as well as difficult materials
handling problems in shipment. GTSP is a hard, uniform, pelletized granule
produced in process equipment which permits ready collection and treat-
ment of dust and fumes. Most new plants will be the GTSP type.
Both processes utilize the same raw materials, ground phosphate rock
and phosphoric acid. The basic chemical reaction is shown by the
following equation:
Ca3 (P04)2 + 4H3P04 + 3H20 > SCa^PO^ HgO (2.12)
Phosphate *- Phosphoric --Water Triple Superphosphate
Rock Acid (Monocalcium Phosphate)
38
-------
The wet scrubbers are the only significant sources of wastewater from
the process. When triple superphosphate is produced at an integrated
fertilizer complex the scrubber will most likely utilize the gypsum
pond water.
2,1.13 Run-of-Pile - TSP
The ROP process train is essentially identical to the normal superphosphate
process with the exception that phosphoric rather than sulfuric add is
used as the acidulating acid (Figure 2.7). Mixing of the 46-54% P^05
phosphoric acid and phosphate rock normally is done in a cone mixer. The
cone depends solely on the inertial energy of the acid for mixing power.
On discharge from the mixer the slurry quickly (15-30 sec) becomes plastic
and begins to solidify. Solidification with the evolution of the gas
(as HF and SiF^) takes place on a slow moving conveyor (den) enroute to
the curing area. The solidified material, because of the gas evolution
throughout the mass, takes on a honeycomb appearance. At the point of
discharge from the den the material passes through a rotary mechanical
cutter which breaks up the honeycombed ROP before it discharges onto
the storage (curing) pile. Curing occurs in the storage pile and takes
2-4 weeks before the ROP is ready to be reclaimed from storage, sized and
shipped.
2.1.14 Granular TSP
GTSP is produced quite differently (Figure 2.8). The phosphoric acid in
this process may be appreciably lower in concentration (40% P20g) than
the 46-54% P20g acid used in ROP manufacture. Forty percent P20g acid
and ground phosphate rock are mixed together in an agitated tank. The lower
strength acid maintains the resultant slurry in a fluid state and allows
39
-------
Rtc.uic.lt
Si-tlfurie Ac-.d
. \ I -
t me »+onc.
..
<3i-lp*um Fond
Den
CJ
>nve.oor
IJ
use* ^u If uric Ac.i
-------
5-. P.OB
Phosphor ic.
Acid
STREAM LEGEND
M«si n
Minor- Proce*»
Figure 2.8 Granulated Triple Superphosphate (EPA 440/1-74-01 la)
-------
the chemical reaction to proceed appreciably further toward completion
before It solidifies. After a mixing period of 1-2 hours the slurry
is distributed onto recycled dry GTSP material. Sludge from phosphoric
acid concentration and clarification is frequently added to the granu-
lator to recover ?2®5" This distribution and mixing with the dry GTSP
takes place in either a pug mill or rotating drum. Slurry wetted
GTSP granules then discharge onto a rotary drier where the chemical reac-
tion is accelerated and essentially completed by the drier heat while
excess water is being evaporated. Dried granules from the drier are
sized on vibrating screens. Over and under-size granules are separated
for use as recycle material. Product size granules are cooled and
conveyed to storage or shipped directly.
2.1.15 Ammonium Phosphates
Ammonium phosphate fertilizers include a variety of different formulations
which vary only in the amounts of nitrogen and phosphate present. The
most important ammonium phosphate fertilizers in use in the U.S. are:
Monoammonium Phosphates (MAP) D1ammonium Phosphates (DAP)
N P K N P K
TT - 4§~ - 0 T6~- 4l~- 0~
13-52-0 18-46-0
11 - 55 - 0
16-20-0
Diammonium phosphate formulations are produced In the largest tonnages
with DPA (18-46-0) being the most dominant.
The two primary raw materials used to produce ammonium phosphates are
ammonia and wet process phosphoric add. Sulfuric add 1s of secondary
importance when it is used in the production of the monoammonium phosphate
grade 16-20-0. As mentioned above, the various grades differ only in the
42
-------
amounts of nitrogen and phosphate present. It is primarily the nitrogen
that varies and this is accomplished by controlling the degree of
ammoniation during neutralization of the phosphoric acid. The chemical
reactions involved are indicated by the following equations:
H3P04 + NH3
Phosphoric Ammonia Monoammonium
Acid Phosphate
*H2S04 + 2NH^ » (NH4)2S04 (2.14)
Sulfuric Ammonia Ammonium
Acid Sulfate
*This reaction occurs only 1n the production of 16-20-0 and occurs
concurrently with the monoammonium phosphate reaction.
The processing steps are outlined in Figure?.9. Ammonia, either gaseous
or liquid, is reacted with 30-40% phosphoric acid in a cylindrical ammoniator
vessel which may or may not have mechanical agitation. The resultant slurry
is distributed onto dry recycled material which is used to control mois-
ture. Distribution and mixing takes place in either a pug mill or
rotating drum where granules are formed continuously. Wetted granules then
discharge into a rotary drier where the excess water is evaporated.
Dried granules are classified and a fraction separated for use as recycle
material. Product size granules are cooled and conveyed to storage or
shipped directly.
The wet scrubbers are the only significant sourcesof wastewater from'
the process. When ammonium phosphate is produced at an integrated
fertilizer complex the scrubber will most likely utilize the gypsum
pond water.
43
-------
Rec.L|c.le.
Figure 2.9 Dianroonium Phosphate Process Schematic (Shrieve, p 227)
-------
2.1.16 Defluorinated Phosphate Rock
Phosphate rock is used as an animal feed supplement in many regions, how-
ever, fluoride contaminants must be removed from the material prior to
this use. A rotary kiln or a fluidized bed reactor utilizing sodium or
silica fluxes are the most common processes in use. If this product is made
in a phosphoric acid complex, gypsum pond water would be used for wet
scrubber water (Figure 2.10).
The fluorapatite type of phosphate rock is the primary raw material. Phosphate
content of the rock is typically 35% P205. Other raw materials used in
lesser amounts but very critical to the process include sodium, containing
reagents, wet process phosphoric acid and silica. The quantity, point of
addition of these materials to the process, and how they are mixed with
i
the phosphate rock constitute some of the know-how involved to realize
a workable process and a consistent product quality. These raw materials
are added in specific quantitites or ratios dependent upon the feed phosphate
rock analysis.
The sodium containing reagent is commonly soda ash (sodium carbonate) which
has a 58% Na20 content or a 98% Na2C03 content. The wet phosphoric acid
reagent concentration used is 45-54% P205. Silica addition is in the form
of sand and is dependent on the silica present in the basic phosphate rock
feed. The point of addition and the material mixing techniques
(either as a physical mixture or agglomerated into nodular form)
are trade secrets. The above described mixture or charge is then fed into
either a rotary kiln or a fluid bed reactor. In the case of a fluid bed
reactor, it is desirable that the charge be nodular and dried prior to being
-------
Fluid iz.i
do*
Acid
D«
Fluid
Be-d
At- m o»phert
R.ec.Lfc.le v.
C on 4~orn i no t&d
w/atcr To
Pood
ond
Product
Figure 2.JO Defluorinated Phosphate Rock -Fluid Bed Process Schematic (ERA 440/1-76/043^
-------
fed into the reactor. This is to permit the fluid bed to classify different
sized particles and to minimize loss in the exhaust gas. Temperature control
and retention time are the process variables which require close control in
both the kiln and fluid bed reactor. Reaction temperatures are maintained
in the 1200-1370eC (2200-2700°F) range with the rotary kiln requiring the
upper portion of the range. Retention time ranges from 30 to 90 minutes with
the fluid bed reactor generally requiring the lesser time.
The state of the charge in the kiln or fluid bed reactor is highly dependent
upon the ratio of the raw materials added to the phosphate rock. This ratio
determines whether the fluorine is evolved in a minimal time period and in
sufficient quantity, and/or whether the charge fuses into an unmanageable mass
that solidifies in the unit. Another critical factor in these units is that
water vapor content be maintained at a sufficiently high percentage to effect
the required fluorine evolution. An equation representative of the chemical
i
reaction and fluorine release in the kilns and fluid bed reactors is:
Ca10F2(P04)6 \ + H20 + Si02 —*• 3Ca3(P04)2 + CaSi03 + 2HF (2.15)
Phosphate rock Water Silica Tricalcium Calcium Hydrogen
Phosphate Silicate Fluoride
From the kiln or fluid bed reactor,the defluorinated product is quickly
quenched with air or water. This is necessary to maintain the product in
the alpha rather than beta tricalcium phosphate form. The alpha form is the
high solubility material most desirable in the final product. From this
point the product is crushed and sized for storage or shipment.
(EPA 440/1-75/043),
47
-------
2.1.17 Fluosilicic Acid Recovery
Vapors from the evaporator/concentrators used to produce 54%-76% P205 acid
are by far the most attractive source of by-product fluorides from wet-process
phosphoric acid and other fertilizer process operations. Estimates of
fluoride evolution from this process operation rangg as high as 80%
of the total fluoride content of the input phosphate rock. The resultant
high concentrations of fluoride vapors make recovery of the fluosilicic
acid economically as well as technically feasible.
Significant quantities of fluoride are also evolved from other process
operations that are found in most phosphoric acid/fertilizer complexes,
including acidulation and filtration of phosphate rock to produce phosphoric
acid and acidulation of phosphate rock, curing, cutting, and storage processes
used to produce normal and triple superphosphate. These fluorides,
however, are not recovered because of their relatively low concentration
in the gas stream and resultant high costs and technical difficulties.
Fluosilicic acid recovery equipment is utilized primarily with the
vacuum type evaporator/concentrator. A typical unit is shown in Fig-
ure 2,2 as it would be used in the dihydrate process. It would also
be applicable to the vacuum process for defluorinated superphosphoric
acid production depicted in Figure 2..4.
48
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2.2 CHARACTERIZATION OF SOURCES OF GYPSUM POND FLUORIDES
2.2.1 Drying and Grinding of Phosphate Rock
This operation does not generate any wastewaters.
2.2.2 Calcining (Defluorinated Rock)
This process operation (Figure 2.10) uses wet scrubbers to control gaseous
emissions. Gypsum pond water is used in the scrubber if available. Up
to 65% of the fluoride originally in the rock can be evolved below fusion
temperature. At the fusion temperature (2500° to 2700°F) 87-90% of the
fluorine can be removed (Teller). A typical water flow rate through the
scrubber is 11,000 gal/ton rock (EPA 440/1-75/043). Gaseous concentra-
tions may range from 7.7-30.8 gr/cf.
2.2.3 Normal Superphosphate
Acidulation of phosphate rock with H2SO. evolves 11 to 42% of the fluorine
in the rock. Reaction temperatures are low (200-400°F rise) and SiF. is the
major compound evolved (Teller). Range of concentration over the den is
3.1-77.1 gr/cf..
2.2.4 Phosphoric Acid - Digestion and Filtration
In the production of wet-process phosphoric acid, both the type of rock
and the processing techniques affect the quantity of gaseous fluorides
evolved. Typical fluorine contents of the rock are as follows:
49
-------
Table 2.6 Fluoride Content of Phosphate Rock
(Teller, p. 745)
Source
Florida land pebble
Tennessee
Western U.S.
Morocco
Tunis
Christmas Island
Curacao Island
Nauru Island
USSR (Kola)
Ocean Island
%P2o5
30.5-32.6
27.9-32.4
27.5-32.5
35.1
27.6
39.5
38.6
38.9
40.3
40.3
%F
3.3-3.9
2.9-3.7
2.9-3.8
4.2
3.5
1.3
6.7
2.6
3.3
3.0
In the reaction of the rock with sulfuric acid to produce phosphoric add
plus gypsum, the fluorine originally present in the rock is distributed in
the precipitated gypsum, the phosphoric acid, and the exhaust gas (as SiF4).
The relative quantities of fluorine in the final products depend on the type
of rock and the operating conditions. The main factors appear to be:
1) the quantity of sodium or potassium salts present (which will
precipitate insoluble fluoride compounts);
2) the reaction temperature at which digestion occurs (increasing
temperature results in increasing gaseous emission);
3) the concentration of the product phosphoric acid;
4) acid:rock ratio
5) quantity of silica, aluminum and iron in the rock.
The effect of source and type of rock on distribution of the fluorine 1n the
dihydrate digestion process is as follows:
50
-------
Table 2.7 Distribution of Fluoride in Dihydrate Process
(Teller, p. 745)
Distribution of fluoride. % of total
Rock Source In acid In gypsum In exhaust gas
Nauru 23 75 2
Kola 73 15 12
Florida 52 45 3
Morocco 50 47 3
The effect of the type of process on fluoride concentration in the off-
gases from digestion depends on the quantity of diluent air. Digestion
of Florida rock will result in gaseous emission of 2 to 8 Ib. of fluoride
per ton of rock fed to the reactor when 26 to 30% P205 acid is produced.
With air cooling of the reactor, the fluoride concentration in the off-gases
will range from 0.6-1.2 gr/cf. If vacuum flash cooling is used, the
concentration will range from 3.1 to 7.7 gr of F" per cubic foot. A small
amount of solids, less then 0.8 gr/cf, is present in the effluent gas.
During acid filtration small amounts of SiF4 are released which must be
vented and scrubbed to meet the new air emission standards. These gases,
carrying 0.15 to 0.45 gr of F" per foot, are usually vented to the
digester-scrubber.
Equipment is available to reclaim the fluoride as H2SiF6 from the digestion
process exhaust gas, however, the economic and the operational feasibility
is marginal due to the relatively low amounts of fluoride present.
2.2.5 Concentration and Defluorination of Phosphoric Acid
Concentration of 32% to 54% P205 acid has been estimated to evolve 25% (Teller),
51
-------
35% (Fox), 42% (Sanders), and 80% (Forster) of the total fluoride input to
a wet-process acid plant. These authors are in agreement, however, that the
proper use of recovery units (such as in Figure 2.2) will permit recovery of
up to 90% of this fluoride as by-product H2SiFg. The unrecovered fluoride is
discharged to the gypsum pond.
2.2.6 Triple Superphosphate ROP
Fluorides are evolved In the ROP process in two steps (Figure 2.7). The first
stage is in the mixing cone, the transfer den, and the cutters where 31-33% of
the total fluoride available in the feed is evolved. The second is during the
2-4 week curing and storage period when 2.0-2.8% of the original fluoride is
released as S1F4. Teller also estimates that 67 to 74% of the fluoride is
retained in the product ROP-TSP.
Under ideal operating conditions wet scrubbers will capture 99+% of evolved
fluorides and transfer them to the gypsum pond cycle.
2.2.7 Granular Triple Superphosphate
Significant gaseous fluoride evolution also occurs in the manufacture of
granular triple superphosphate (Figure 2.8) along with production of dust.
The major gaseous evolution occurs 1n the reactor-dryer system but additional
evolution occurs in the product cooler and in the storage building.
The slurry resulting from reaction of the rock and 40% P205 acid is sprayed
on recycled product and dried. The fluoride evolution during this operation
is high, as much as 25 Ib/ton of P205> The gas effluent essentially is
SiF^, inasmuch as any HF formed immediately attacks the rock to form H3P04
and H2SiF6. The concentration of fluoride in the dryer effluent ranges from
52
-------
0.9 to 1.5 gr/cf in a gas flow of 60 to 200 ft3/min/daily ton of P^^
produced. (11 to 62 Ibs F/ton PgOg). The participate loading of this
stream is approximately 231 gr/cf. The effluent from the cooler contains
between 0.23-0.46 gr/cf of gaseous fluoride in the form of S1F4, and also
has a particulate loading of approximately 231 gr/cf. Granular TSP will also
evolve 2.0-2.8% of the original fluoride content as SiF4 while in storage for
a four week period. Wet scrubber water recirculated from the gypsum pond
will capture 99+% of the evolved fluoride and transport it to the gypsum pond.
2.2.8 Ammonium Phosphate
Gaseous fluorides are evolved from the ammonization of phosphoric acid,
the dryer, the cooler, and storage. Emissions of fluorides are relatively
low due to the fact that the feed phosphoric acid has already evolved about
75% of the fluoride originally held in the rock. Calculations based on
observations by Huffstutler indicate that about 33% of the remaining fluoride
in the phosphoric acid is evolved in this process.
2.2.9 Gypsum Pond and Phosphoric Acid Complex Fluoride Mass Balance
Typical fluoride distributions in the manufacture of wet process phosphoric
acid as derived from the literature are presented in Tables 2.8, 2.9 and 2.10.
Table 2.8 Fluoride Distribution in Phosphoric Acid Production (Sanders, p. 765,
~~~ % total Lb of fluoride
fluoride in per ton of
Processing stage phosphate rock
Production of filter
acid
Phosphate rock (32.57%
P205, 3.89% F) 100 239
Vapors from reactor
slurry 5.5 13
Gypsum filter cake 27.8 66
30% P205 filter acid 66.7 160
Concentration of filter acid
Vapors from concentrators 41.9 100
54% P20s concentrated acid 24.8 60
53
-------
Table 2.9 Fluoride Distribution in Phosphoric Acid Production (Fox)
% total fluoride
Processing stage in phosphate rock
Gypsum filter cake 46.2
Concentrated acid 13.3
Volatilized, total 40.5
In rock digestion 5.2
In acid concentration 35.3
Table 2.10 Distribution of Fluorides from Florida Phosphate Rock
(Huffstutler, p. 728)
Input To H3PO*
rock If/day p" % F
73,600 21,500
116,300 28,000
355,600 105,900
To gypsum
"input Ib/day F" % F "input
29 30,000 41
24 39,000 34
30 75,100 21
To pond
water
Ib/day F"
21 ,200
48,500
174,600
To
atmosphere
% F "input Ib/day F"
29 12
42 30
49 24
% F "input
0.016
0.026
0.007
A fluoride mass balance and a gypsum pond recycle flow balance for a
phosphate fertilizer complex has been synthesized based on available informa-
tion (Figure 2J1) • A phosphoric acid plant which produces 32% PgOs by the
dihydrate process 70 BPL Florida phosphate rock (32.57% P205, 3.89% F")
and concentrates it to 54% P205 in a two stage forced circulation vacuum
54
-------
WATER
3800-SOOO
«,«al/ton
01
Wc-t
Scrubb
RECYC.I-E VVATER -To V\/*»
RECYCLE MATER
2ZS -Z5O
l./ton
S7 Ik.. F/ *•«.!-> P»0
RECYCLE WATER
S.O -t-anft W-»*«.
P/to"
TOOO ^ol /tor. PZ0S
RECYCLE WATER
I./ton
RECYCLE WATER
IZOO-lSOO
./+OT.P.OS
P.O. • P.O.
• Rock.
RCCYCLC WATER
Its. F/1-on PE«>S
5 Ibs. F /1-on P«0B
Major Process
Figure 2.11 Phosphate Fertilizer Complex - Fluoride
•; and Water Balance
-------
evaporator is the heart of the complex. Fluosilidc acid, Market Grade
54% P205 acid, Granular Triple Superphosphate, Ammonium Phosphate, and
Superphosphate Acid are also produced at this complex.
Each of these processes utilizes the gypsum pond. The wastewater flow data
was primarily derived from EPA Effluent Guideline Study draft documents
for Fertilizer and Phosphate Manufacturing Point Source Categories. The
fluoride evolution rates have been derived from existing literature and
industry contacts.
The following criteria were used in the development of the fluoride mass
balance:
1} Acidulation/Filtration of phosphate rock evolves 5% of the original
fluoride content of the rock
2) Waste gypsum cake retains 45% of the original fluoride content of
the rock
3) Black acid (32% P20g) retains 50% of the original fluoride content
of the rock
4) Black acid (54% P205)retains 25% of the original fluoride content of
the rock
5) Merchant acid (54% P205) retains 24% of the original fluoride content
of the rock
6) Superphosphoric Acid (72% P00C) retains 2% of the original fluoride
i b
content of the rock
7) Ammonium phosphate retains 66% of the fluoride content of phosphoric
acid feed stock.
8) Triple superphosphate retains 66% of the fluoride content of phosphate
rock and phosphoric acid feed stock
56
-------
9) Fluosilicic acid recover reclaims 90% of fluoride evolved from
evaporati on/concentrati on.
2.2.10 Gypsum Pond and Phosphoric Acid Complex Water Balance
Contaminated gypsum pond water is used in all process equipment in the
phosphate subcategory except sulfuric acid manufacturing and rock
grinding. The water requirements of such major water using equipment as
barometric condensers, gypsum sluicing, gas scrubbing equipment, and heat
exchangers are all supplied by contaminated water. Each time the water
is reused, the contaminant level Is increased. However, build-up rates
of fluoride are not documented. While this contaminated water is a major
process effluent, it 1s not routinely discharged from the gypsum pond complex.
The following tables (2.11 and 2.12) list ranges of contaminated water
usage for each contact and non-contact process.
Table 2.11 Phosphate Complex Water Usage
(EPA 440/1-74-011-a)
Process
Sulfuric Acid
Rock Grinding
Wet Process Phosphoric Acid
Process Water
Sluice Water
NPK Process-Nitric Add
Addulation
Phosphoric Acid Concentration
Phosphoric Acid Clarification
Normal Superphosphate
Triple Superphosphate
Ammonium Phosphate
Rock Calcining
Defluorinated Phosphonic Acid
Vacuum Evaporation
Submerged Combustion
(Sypsum Pond
Recycle
gal /ton P20i;
None
None
3800-5000
7000-11000
240-540
550-570
225-250
225-250
158-250
1200-1500
11000
16900
4300
Fresh Water
Make-up
15-20
None
None
None
None
0.2-0.4
None
None
None
None.
None
57
-------
Table 2.12 indicates the water balance around a plant us.ing 94% H«SU/,.
C. T
without dilution, with vacuum cooling and evaporation of the product acid
to 54% P205 by stage-wise evaporation. A cooling water temperature of 90° F"
is assumed. These conditions represent a plant located in the southern
United States or in areas of equivalent climatic conditions.
Table 2.12 Water Requirements for Phosphoric Acid Production
(Lutz, p. 195)
U.S. gal/short
ton of P205
Contaminated water
Filter wash and repulp 7,000
Vacuum cooler condensers 7,500
Evaporator condensers 14,000
Scrubber water 1,000
Fresh water
Cooling water for sulfuric dilution
cooler 5,000
Vacuum pump water 300
Process water for HoS04 dilution 300
Sanitary and miscellaneous usage 300
It is immediately obvious that there are some discrepancies between flow
ranges assigned to scrubbers and the order of magnitude of fluoride loadings
on those same scrubbers in Figure2.11. For example, an estimated 111 Ib F~/ton
P205 evolves from the triple: superphosphate process, yet only 225-250 gal of
M
water/ton ?2°5 1s allocated to that process by the Effluent Guidelines
Contractor. However, for the acidulation/filtration process 3800-5000
gal/ton P?05 is allotted to remove a mere 12-16F~/ton p o_. At this time
fc 25
there are no better figures available to explain or pinpoint the cause for
such discrepancies. The numbers which have been used must be considered
to be order-of-magnitude,
53
-------
2.3 SECTION 2 CONCLUSIONS
1) The d1hydrate process 1s the most commonly used wet process for the manu-
facture of phosphoric add in the United States. It is common practice
within the phosphate industry to also manufacture one or more phosphate
based fertilizers in the phosphoric acid complex.
2) The existence of the gypsum pond is dependent on the use of the dihydrate
wet process to manufacture phosphoric acid. Secondary fertilizer
manufacturing processes within a fertilizer complex then utilize the
gypsum pond water in several contact and non-contact process operations.
3) Under normal operating conditions the evolution of fluorides from the
various unit processes will be in order-of-magn1tude agreement with
generation rates developed for the fluoride mass balance sheet
(Figure2.11). However, process upsets - such as wasting of acidulated
rock to restore the proper rock:acid ratio - may result in short-term
increases in fluoride loading to the gypsum pond which have not been
characterized.
4) General operating parameters which affect fluoride loading on the gypsum
pond for all phosphoric acid and phosphate fertilizer unit processes
are as follows:
a) Product specifications
b) Raw material specifications
c) Temperature of the reactions
d) Acid concentrations
e) Fluoride emission standards
f) Fluoride recovery (if any).
-------
A major operational parameter in phosphoric acid production is con-
trol of the crystaline form of calcium sulfate formed in the acidulation/
filtration process.
5) Fluosilic acid recovery on the evaporator/concentrator has been
demonstrated to remove approximately 23% of input rock fluorides from
the waste water which is discharged to the gypsum pond.
60
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3.0 GYPSUM POND CHEMISTRY
This section is to evaluate the potential for airborne fluorine emission
from gypsum ponds. This evaluation necessitates a fundamental under-
standing of the chemical interactions involving fluorine in the gypsum
pond environment. A theoretical and literature-based approach to this
problem is a first step in evaluating the potential for airborne emissions.
3.1 SOURCE OF FLUORINE IN GYPSUM PONDS
Figure 2.11 illustrates the sources of fluorine in a gypsum pond associated
with a phosphate rock processing facility. There are two sources: fluorine
released from the rock matrix during the acidulation process and fluorine
recovered by wet scrubbers from gaseous emissions at various points in
the process. Fluorine is released in the production of phosphoric acid
by the wet-process during acidification of the fluorapatite ore with sulfuric
acid:
Ca10(P04)6F2(s) + 10H2S04(aq) + 10nH20(l) > (3.1)
10CaS04 nH20(s) + 6H3P04(aq) + 2HF(aq)
where n may equal 0, 1/2, or 2 depending on the form of the hydrated
calcium sulfate (CaSO^). Some of the fluoride present in the phosphate
rock as calcium fluoride (CaF2) will react with phosphoric acid (1^04) and
sulfuric acid to form hydrofluoric acid (HF):
CaF2(s) + 2H3P04(aq) 7 Ca(H2P04)2(aq) + 2HF (aq) (3?2)
CaF2(s) + H2S04(aq) + H20(s) > CaS04«2H20(s) + 2HF(aq) (3.3)
The HF will then react with silicate (Si02) released from the rock matrix
to form fluosilicic acid:
61
-------
Table 3.1 Major cation and anion concentrations in gypsum pond
water. After ESE, 1974.
Cations
Ca++
Na+
A13+
Fe3+
Mg++
K+
H+
An ions
Fe
S04=
ci-
H?PO,"
concentration (mg/1)
2000
1600
500
300
240
200
pH - 1.4
8000
4800
200
1000
concentration (M)
0.05
0.07
0.018
0.005
0.01
0.005
0.04
0.42
0.05
0.006
0.02
62
-------
Si02(s) + 6HF(aq) - »H2SiF6(aq) + 2H20-(£) (3.4)
As a result of the elevated temperatures encountered in the acidulation,
concentration and other processing steps, a portion of the HF released
from the rock matrix is volatilized as hydrogen fluoride. Fluosilicic
acid (H2SiFg) is also dissociated into hydrogen fluoride and silicon
tetrafluoride (SiF^) at elevated temperatures:
+ 2HF-(g) (3t5)
Hydrogen fluoride and SiF4 are prevented from escaping from the facility
into the atmosphere in large quantities by the use of wet scrubbers employing
recycled gypsum pond water as the scrubbing medium. In the scrubbers,
hydrogen fluoride and SiF4 are removed from the air forming H2S1F6:
3SiF4(g) + 2H20(£) - -> 2H2SiF6(aq) t Si02(s) (3.6)
Si02(s) + 6HF(g) - >H2SiF6(aq) + 2H2Q.(£) (3.7)
If it is assumed that there exists a sufficient excess of silicate in the
rock, and this is not an unreasonable assumption for most natural minerals;
then the net result of all of the above reactions is that most fluorine
enters the gypsum pond in the form of HgSiFg and possibly CaF2. Smaller
quantities of HF(aq), Na2SiFe and K2SiFg will likely be present as well.
All chemical reactions result in an equilibrium among the reactartts
and the products. Each of the above equilibria, however, are balanced
far in the direction shown.
An attempt was made to evaluate the magnitude of possible airborne fluorine
emissions from the gypsum pond by mass balance calculations of the fluorine
entering the pond and leaving the pond. A mass balance is, however, imprac-
tical as a result of the orders of magnitude involved. To illustrate this,
63
-------
a 1000 ton per day P205 plant using ore containing 3.8 percent fluorine would
deliver approximately 140 tons of fluorine to the gypsum pond per day.
If 40 percent of that fluorine existed as CaF2, the daily flow of soluble
fluoride would be as large as 84 tons. Assuming a large pond (300 acres)
evolving HF at the rate of 4.7 Ib/acre/day (the highest rate suggested 1n
the available literature, King, 1969), l,410ibs of fluoride would be
emitted to the air each day. This emission rate represents only 0.83 percent
of the 84 tons entering the pond each day. Given the variability of pond
Influent composition and the uncertainties inherent in composite wastewater
sampling and analysis, an airborne emission rate of 4.7 Ib/acre would be
impossible to document by mass balance calculations even with a very large
number of samples. The problem would be further complicated should the
actual emission rate be less than 4.7 Ib/acre or the pond smaller than
300 acres.
3.2 CHEMICAL ENVIRONMENTAL WITHIN THE GYPSUM POND
Gypsum pond water is a complex mixture of various charged and uncharged
chemical species. Some of the major cations and anions present are listed
in Table 3.1 along with the approximate analyzed concentrations. Additional
minor elements such as Mn, Cu, Cr, Zn, Ba, U, and Sr elements are also present
in smaller quantities. H3P04, SiO,,, H2S1Ffi. CaF2 and a large number of
complexed chemical species will be present. Fluorine, designated
F in Table 3.1, will occur as the fluoride ion F~, as H2S1F6, HF/a )f
S1F4, HF2~ and complexed with metals such as FeFg, AlF**, A1F2+, AlFg,
A1F4", etc.
64
-------
Gypsum pond waters are of high ionic strength (I«l moles/1) as a result of
the large concentrations of charged species present. Ionic strength will
affect each of the equilibrium constants used in this evaluation to a consid-
erable extent. The present state of knowledge of gypsum pond chemistry is,
*
however, not sufficient to warrant the complication of the problem at this
point. In general, high ionic strenth will tend to oppose those reactions
that contribute multivalent ions to solution. Equilibria would tend to
be more balanced toward the uncharged or low-charged products than basic
equilibrium theory suggests.
The observed pH of gypsum pond waters is 1.5 to 2.0. At this pH, weak
acids will exist largely in the undissociated (protonated) state. Eighty
percent of the phosphate present should exist as H3?04 while approximately
20 percent is H2P04". H2SiFg and HF will both exist predominantly in
the undissociated condition.
A titration curve for a typical gypsum pond water is presented in Figure
3.1. From that titration curve, the buffer capacity (B » dCB/dpH, Stumm
and Morgan, 1970) of the solution as a function of pH has been calculated
and is illustrated in Figure 3.2. It is apparent that the system is highly
buffered both initially and In the vicinity of pH 7. The higher of
2- -
the two buffered pH regions appears to coincide with the HP04 /H2P04
buffering maximum. The engineering and economic significance of the
buffering capacity figure is obvious. Large amounts of lime (Ca(OH)2)
required to raise the pH initially to 3 and from 6 to 8, relative to the
amount needed to raise the pH of the pond water from 3 to 6.
65
-------
a.o
7.0
5.0
cn
I
Q-
3.O
z.o
I.O
«0
15
zs
•40
rneej,. NoOH
Figure 3.1 - T.tration of GUC.*um Pond */«ater with | N NJaOH.
-------
I
0.
A.O
3.0
2.0
I.O
Figure 3.2
- Por»a Wot*r.
67
-------
3.3 DEVELOPMENT OF A MODEL
A detailed model describing gypsum pond chemistry is beyond the current
state of knowledge. A simplified model can be developed by making quali-
tative assumptions concerning the nature of the chemical reactions in-
volved. Equilibrium equations alone, while of value, cannot constitute
a realistic model. The nature of the chemical kinetics involved is crucial
to the model. The only approach that can be taken at this point is to
consider all the major reactions involving the various fluoride species,
then to systematically eliminate those reactions which are not considered
to be of predominant importance.
3.3.1 An Initial Model
The initial model, which is actually a pictorial summary of possible reac-
tions involving the various fluoride species is presented in Figure 3.3.
The equilibrium that has generally been considered dominant in the gener-
ation of fluoride from gypsum ponds (King, 1969), H2SiF6(aq)^Z±2HF(aq)
+ SiF^(aq), is shown in the center of the diagram. This reaction proceeds
through several different routes as shown. Other reactions include the
precipitation of fluosilicic acid as the Na or K salt and the formation of
the fluoride ion, F~, by dissociation of HF. Fluoride will combine with
Al and Fe to form soluble fluoro complexes and with Ca, Mg and other cations
to form salts of low solubility. The Al and Fe must eventually be precipi-
tated; FeF3, A1F3, phosphate compounds or more complex mineral species will
likely constitute a significant portion of pond sediments. Volatilization
of either HF or of SiF^ are the potential sources of any airborne emissions
from the pond surface.
68
-------
en
10
t rr» O
S o ) «j -t I O n
AI,Fe
SiO,
Oomplexes
MCI
Sr*
Si
" HF
Si OK 1
-f- SiF.
No'
K*
CoF£
d i
n t
Figure 3.3 ~ Gr-opHio Initial Description of Some. pertinenV G4paam R»na ReociionS.
-------
Farrer and Rossotti (1964) suggest that HF, HF2" and F" are the only
fluoride species present in simple HF solutions of less than one molar
concentration. For a solution such as a gypsum pond was containing
0.4 M fluoride, the following equations relate the various species:
-] = [HF][F-] (3>g)
[HF] + 2[HF2-] + [F-] * 0.4 (3JO)
At 24°C, K, = 1.30 x 10"3 and <2 = 0.102 (Buslaeu and Gustyakove, 1965).
Simultaneous solution of these three fluoride equations in the pH range
of 1 to 8 gives species concentrations that are plotted in Figure 2.4. It
can be seen that HF is the predominant species for pH <2, and F" is pre-
dominant for pH >4. For pH between 2 and 4, all three species are present
in appreciable quantities. At pH =2.8, the system is approximately
equimolar in HF, HF2", and r.
The rationale for liming the gypsum ponds to control fluoride emissions is
apparent upon examination of Figure 3.4. An increase in pH results in the
dissociation of HF, making the fluoride ion available for reaction with and
reducing the concentration of volatile HF cations. The effect of liming
on the HF concentration in gypsum pond water is illustrated in Figure 3.5.
The dissociated fluoride can react with the calcium in the lime to form
CAF2, a relatively insoluble salt, which will precipitate removing a portion
of the fluoride from solution:
Ca^ + 2F-^T±CaF2> K = 2.5 x 1010. (3.11)
This equilibrium can be a key one in controlling fluoride concentration
in gypsum ponds without liming since gypsum, CaS04-2H20, is near saturation
in the pond waters providing excess calcium ions. In a similar manner,
70
-------
.45
.40
.35
.30
.Z'S
.13
.05
3.4
O.4- M HF Solution.
71
-------
9OOO
eooo
700O
C.OOO
-------
F- also forms insoluble precipitates with Mg, Sr and Ba which may be
impurities in the phosphate rock.
Al and Fe readily complex fluoride in solution as indicated by the magnitude
of the formation constants reported in Table 3.2. For a gypsum pond containing
0.019 M A13+ and 0.005 M. Fe3+, the formation of the trifluoro species would
consume 0.069 M F'. The [F"]/[HF] ratio, while small, is sufficiently large
to allow the complexation to proceed. Loss of HF from the solution through
formation of soluble complexes is not accompanied by a compensating shift
in the HF/H2SiF6 equilibrium as a result of inhibition by reaction of Si02
with any additional HF. The net effect of this complexation is to reduce
HF concentration. This is 16 percent of the total F^ in the pond.
It is reasonable to assume A1F3 and FeF3 will ultimately be incorporated
into the pond sediment in low pH waters either in the hydrated form or
as a more complex mineral association with gypsum. Al and Fe are continu-
ously released from the phosphate rock matrix and there are no known
volatile Al or Fe species, hence the sediment is the only possible final
disposition.
All of the chemical reactions discussed above tend to remove F" from
solution and eventually incorporate fluoride into the sediment. This
net transport of fluoride to the sediment affects all solution equilibria.
There exists one additional major pathway for transport of fluoride to
the pond sediment. Fluosilicic acid will react with Na"1" and K+ released
from the phosphate rock matrix to form sodium and potassium fluoro-
silicates which are of relatively low solubility (7.62 and 1.77 g/1 at
73
-------
Table 3.2 Formation constants of AT and Fe fluoride species.
Complex Formation Reaction Formation Constant
A13+ + F-^=± A1F++ KT = 1.4 x 106
A1F++ + F~ ^ x Al F2+ K2 » 1.1 x 105
A1F2+ + F\ ^ AlF3 K3 » 7.1 x 103
A13+ + 3F"k ^A1F3 K = K-iWl = 1>1 x 1C)15
K4 = 570
K5 = 42
K6 = 3.0
K = 1.2 x 1012
74
-------
25°C, respectively) and hence precipitate Into the pond sediments as
shown in Figure 3.3.
The chemical equilibria remaining to be discussed involve the complicated
interactions of HF, HF2", H2SiFg, SiF4 and S102. These reactions are
as follows:
H2SiF6(aq)^=>2HF(aq) + S1F4(aq) (3.12)
3SiF4(aq) + 2H20 HF2" (3.15)
3HF- + Si0(s) + 3H+
-------
King (1971) summarized Tatera's (1970) ionization constant data and extended
Tatera's curve to his gypsum pond water data. At 21°C,
EHF]2[S1F4]
K = CH2SIF6] = 3 x 10" . (3.20)
Applying this expression directly to a pond water containing 0.42 moles/
liter total FQ with the simplifying assumption that no other equilibria are
involved, an equilibrium fluorine distribution of 50 percent of the fluorine
as H2SiFg, 25 percent as HF, and 25 percent as SiF4 is calculated.
This calculation, however, ignores all the other reactions listed above.
Si02 plays a critical role in reducing the amount of HF in solution below
the simplistically calculated proportion. Reaction (3.14) has an equilibrium
constant of 2 x 1026 (Kirkland Othmer, 1964), indicating a strong balance
toward the H2SiFg rather than HF. Reaction (3.16) with Si02 also opposes
free HF in solution. The predominance diagram, Figure 3.4, suggests the
importance of the reaction of HF2" with Si02, since a considerable amount
of HF2" appears to be available at gypsum pond pH levels. Judge (1971)
concluded, in addition, that the reaction rate of Si02 with FH2" is approxi-
mately four to five times as fast as the reaction with HF.
Taking into consideration all of the above qualitative discussion of the
relative significance of the various reaction pathways illustrated in
Figure 3.3, a new figure (Figure 3.6) may be employed to illustrate
only those reaction pathways considered to be most significant in a gypsum
pond. This simplified model meets the criteria that all chemical species
entering the gypsum pond must either precipitate into the pond sediments
or volatilize and that waters in the pond are near saturation with respect
to many chemical species due to continuous recycling.
76
-------
c
o I u T i o n
Soluble.
F« on«* Al
HF
5iOz
HtSiF«.
3 e d i m a n t
Figure 3.S Major gypsum pond equilibrium
77
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3-4 DISCUSSION OF THE SIMPLIFIED GYPSUM POND MODEL
A qualitative overview of the simplified gypsum pond model, Figure 3.7,
indicates a number of pathways for transport of fluorine into the pond
sediments. Each of these pathways would have the effect of reducing the
amount of HF in solution and hence the amount of HF volatilized.
All the reactions shown in Figure 3.7 are temperature dependent and some
are PH dependent. For this first assumption, however, it may be assumed
PH and temperature in the pond do not vary to such an extent to affect the
order of magnitude of the various equilibria involved.
The single critical pathway that must now be discussed is volatilization
of HF. As an order of magnitude estimate, assume 25 percent of the F®
concentration of 0.42 M, the concentration of HF would be a maximum of 0.105 M.
This corresponds to an equilibrium concentration of 2 mg/1 HF in solution
or a 0.02 percent solution of HF. Referring to HEW (1970) Table D-4,
at 20°C, a 10 percent aqueous concentration of HF yields a vapor pressure
of 0.14 mm Hg, so the maximum vapor pressure of HF above the gypsum pond
may be expected to be 0.003 mm Hg under closed-system conditions. This
small vapor pressure would indicate a low rate of net transport of HF
into the atmosphere.
Additional thermodynamic analysis by Drs. Coots and Getzen (see O'Melia
et ah, 1975) suggests that HF (aq) may in fact be considerably lower
due to the CaF2 equilibrium. The value they suggest for the vapor pressure
of HF above the Gypsum pond is 0.00008 mm Hg. This is probably
a minimum since the possibility of formation of various calcium complexes
was not considered.
78
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3.5 SECTION 3 CONCLUSIONS
The development of the model of fluoride interactions presented in Figure
3.7 is based on little quantitative information. Available literature
is deficient in the quantitative information necessary to evaluate this
complex chemical solution. The model is based on several very qualitative
assumptions concerning the significance of particular chemical reactions.
Some reasonably detailed analytical data would be required to substantiate
the conclusions drawn. Nevertheless, there appears to be little to support
the contention that significant quantities of fluoride are emitted to the
atmosphere by volatilization from the gypsum pond surface. This results
from two conclusions: a) relatively little free HF and even less SiF.,
the only measurably volatile fluoride species, are present in the gypsum
pond waters, and b) HF a is of low volatility.
At least one of the assumptions used in arriving at this conclusion is
supported by evidence available to the authors. Laboratory analysis of
fluoride species in gypsum pond waters has indicated in excess of 80 percent
is 1n the form of fluosilicic acid, h^SiFg. One other empirical obser-
vation bearing on the qualitative conclusions reached is that recycled
gypsum pond water is used in the scrubbers which are designed to remove
HF which is volatilized during several process steps. If these recycled
waters have an HF concentration such that a significant quantity would
be likely to volatilize in the pond environment, it is highly unlikely
that these same waters would be effective in absorbing airborne HF in
the scrubbers.
Researchers who have measured airborne fluorine concentrations in the
vicinity of gypsum ponds have made no apparent attempt to differentiate
79
-------
between the participate and non-particulate forms. It is possible that
a large proportion of the fluorine detected near ponds results from
fugitive dust from gypsum piles and gypsum deposits surrounding the pond
and not as emissions from the pond surface.
80
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4.0 DETERMINATION OF FLUORIDE EMISSION FACTOR
To date, three studies have been conducted in an attempt to determine
the amounts of fluoride released to the atmosphere from gypsum settling
ponds. The first study by Frank L. Cross, Jr. was performed in 1967.
It consisted of measuring fluoride concentrations emitted from a small
A-frame structure placed on a 160 acre gypsum pond. The second
study conducted by B. S. Tatera in 1970 determined emission rates by
placing water from a gypsum pond in an experimental wind tunnel. The
third and final study determined fluoride emission rates by first
determining a mass transfer coefficient for water, based on many
experimental measurements from a large midwestern lake, and then
relating the mass transfer coefficient of fluorides to that of
water through existing mass transfer correlation. This study, by
W. R. King, was conducted in 1973.
Each of the above studies was critically examined for scientific validity
and consistency. In the following, each of these studies will be re-
viewed in the order presented above.
4.1 REVIEW OF CROSS AND ROSS' STUDY
The first known attempt to measure emissions from a gypsum pond was
that of Frank L. Cross, Jr. of the Manatee County Health Department,
Bradenton, Florida in 1967. This study was performed as a result of
public concern over the potential fluoride emissions from a nearby
fertilizer plant. Since results of fluoride emissions tests had indi-
cated that the major plant stacks were well within allowable fluoride
81
-------
kl
On Shore Fluoride
Sampler
Floating Greenhous
rr iuao my
including
f1uori de
..-„.-. -_-
offshore I
-. i I
Figure 4.1. Location of A-Frame on Gypsum Pond,
82
-------
X
o
*
^
6"
TOP
VIEW
33"
8" Diameter
Net Floor Area = 52.7 sq. ft.
'
6"
— Induced Draft Fan
5-7"
lO'll"
GREEN HOUSE DIMENSIONS
GYPSUM POND TESTS
Figure 4.2. Dimensions of A-Frame Used by Cross
83
-------
Missions levels set by the Florida State Board of Health, attention
was focused upon the gypsum pond as a potential source of ambient
fluorides. The study was conducted over a period of three months,
September through November, 1967.
4-1-l Experimental Design
A gypsum pond of ,60 acres 1» „„ was chosen. As Indicated In F1gure 4 ,
half of the area was used as an active gypsum p1le, so that the effec-
tive area of the liquid portion of the pond was 80 acres. A floating
greenhouse, shown In Figure 4.2, was placed between 225 feet and 300 feet
east of the shore. The greenhouse was a floating A-frame structure,
covered by transparent plastic and supported by six barrels. The bar-
rels kept the sides approximately 14 Inches above pond water level.
thus allowing for air Interchange between the greenhouse and the en-
vironment. An 8-1nch stack with an Induced draft fan was Installed
on the roof, providing a constant flow rate of ambient air through the
greenhouse. Ambient, 24-hour air samplers and a temperature recordlno
device were placed within the greenhouse. A similar ambjent fluor,de
sampler was located on the bank to provide comparison of ambient
fluoride levels with those monitored In the greenhouse. Cross felt
It was a requirement that the two measurements be approximately equal.
The reasons, however, are unclear and unsubstantiated.
The following parameters measured were fixed constants throughout
the experiment:
84
-------
A = Projected area of greenhouse over the pond water
y = 52.71 ft2
D = Diameter of stack = 8 inches
Vs = Velocity of air through the stack = 450 ft/min
Ap = Effective area of gypsum pond = 80 acres
The following independent variables were measured throughout the
experiment:
Tp = Temperature of gypsum pond
Tg = Temperature of greenhouse
Ta = Ambient temperature above pond
V = Wind speed
W = Wind direction
Dependent experimental variables were: .
/ygF"^
C_ = Concentration of fluorides in greenhouse air i-*=-i
Cos = Concentration of fluorides measured at on-shore sampler
E = Fluoride emission rate (Ib/acre-day) =
Cg x Q x /IP"* lb\ /43.560
A ^ 454 yg J I acre
4.1.2 Experimental Results
The data in Table 4.1 were generated from six different days of obser-
vations. As seen, the maximum value given is 0.161 Ib/acre-day. Cross,
however, states that this is a minimum emission rate, for unknown
reasons. Values as low as 0.04 Ib/acre-day were obtained and the mean
was 0.089 Ib/acre-day.
85
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4.1.3 Interpretation of Results
The first point to make concerning Cross1 study is that no attempt
was made to determine the parameters which influence gypsum pond emissions,
No parameters were varied throughout the experiment which entered the
emission calculation, except for the fluoride concentration measured
within the greenhouse. However, the intention of his experiment was
not to conduct an elaborate study, but to practically measure an emis-
sion rate of that pond during normal operating conditions.
Cross interprets the results to mean that all of the fluorides sampled
in the greenhouse were evolved from the projected area A . Actually,
the air entering the greenhouse contained fluorides which were trans-
ported into it as it moved across the pond before reaching the greenhouse.
During the testing period, the wind blew predominantly from the northeast.
As shown in Figure 4.1, air from the northeast would have traversed approx-
imately the same distance over the water on reaching the greenhouse sampler
as on reaching the on-shore sampler. Thus one would expect the fluoride
concentration in the greenhouse to be the same as that measured on shore
prpyidod the ajr f 1 ow Jbhj^ujh_^j^j^ the proper rate.
86
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Table 4.1. Results of Cross and Ross1 Greenhouse Experiment on a Gypsum Pond.
CO
Date
10/19/67
10/20/67
10/25/67
10/26/67
10/31/67
ll/ 1/67
<•«?
0.1374
0.1841
0.2240
0.3898
0.2653
0.0968
Cg ppb F"
6.3
8.4
10.2
17.8
12.1
4.4
CQS ppb F-
6.3
11.7
15.6
11.3
10.9
7.5
Q ft3/day
225,500
225,500
225,500
225,500
225,500
225,500
Ag ft/day
52.7
52.7
52.7
52.7
52.7
52.7
E lb
" acre-day
0.057
0.076
0.092
0.161
0.109
0.040
-------
Cross assigned an arbitrary area (Ag) and an arbitrary flow rate (Q) to
his system and to determine an emission rate multiplied the flow rate
by the fluoride concentration and divided by the projected area. Had
the area been twice as great, Q and Cg would have been approximately the
same as before and an emission rate equal to half of that obtained before
would have been measured. Similarly, had Q been doubled, Ag and Cg would
have remained unchanged and an emission factor twice as great would have
been obtained.
The conclusion is that fundamental errors caused by using an arbitrary
area and flow rate render the results meaningless. No judgments can
be made regarding accuracy of results since the method used is totally
invalid.
4.2 REVIEW OF TATERA'S STUDY
One of the major studies conducted to date was that of B. S. Tatera.
The purpose of his study was to determine which parameters affected
gypsum pond emissions. His investigations were submitted to the
University of Florida Department of Environmental Engineering as
a Ph.D. dissertation in 1970.
i
Tatera's study consisted of two phases. In the first phase, he measured
saturation vapor pressures of fluorides over liquid solutions of hydro-
fluoric acid (HF) or fluosilicic acid (H2SiF6) and distilled water or
gypsum pond water. In the second phase, Tatera conducted a laboratory
evaluation of the parameters that influence fluoride emissions from
gypsum ponds. These were determined to be fluoride ion concentration,
;amperature and air velocity over the water surface.
-------
4.2.1 Tatera's Vapor Pressure Studies
Tatera used a gas-saturation method for measuring fluoride vapor pres-
sures, which consisted of a bubbler (or series of bubblers) containing
the liquid solution. An inert gas was bubbled through the solution and
the gas stream saturated in fluorides according to the vapor pressure of
fluoride in equilibrium with a solution of a given concentration and temper-
ature. Tatera measured the vapor pressure of fluorides in this fashion for
the systems HF-H20, H2SiFg-H20, HF-gypsum pond water, and H2SiFg-gypsum pond
water. Concentrations were varied over the range 0.1M - 0.5M (2,000 to
10,000 ppm F"). Liquid temperatures for which data were gathered were
70, 85, and 100°F. In every case it was found that a linear relationship
existed between fluoride concentrations and fluoride vapor pressures
above the solution. Expressions derived for the system HF-H20 were:
Caf = 614.7 Csf at 70°F (4.1)
Caf = 817.0 Csf at 85°F (4.2)
Caf = 1,032.9 Csf at 100°F (4.3)
For the system H2SiFg-H20, the equations derived were:
Cflf = 48.0 Csf at 70°F (4.4)
Caf = 55.9 Csf at 85°F (4.5)
Cflf = 80.3 Csf at 100°F (4.6)
For the system H2SiFg-pond water, the equations derived were:
Cflf = 70.6 Csf at 70°F (4.7)
Caf = 80.7 Csf at 85°F (4.8)
Caf = 143.0 Csf at 100°F (4.9)
89
-------
For the system HF-pond water, the equations derived were:
Caf = 106.5 + 523.6 Cgf at 70°F (4.10)
Caf - 183.7 + 892.8 Csf at 85°F (4.11)
Caf = 223.6 + 1,177.8 Csf at 100°F (4.12)
where:
C,f - concentration of fluorides in air
Csf = molar concentration of fluorides in solution
The above correlations show that for similar fluoride concentrations
in a solution, fluorides in the form of H2SiFg exert a lower vapor
pressure than fluorides present as HF, by about a factor of 10. The
above correlations were not used in the determination of an emission
factor, but they do clearly show the dependency of fluoride vapor
pressures upon fluoride concentration in the water.
4.2.2 Tatera's Hind Tunnel Study
T!io second phase of Tatera's study was concerned with the determination of
a fluoride emission factor. His method involved the utilization of
a tank as shown in Figure 4.3, containing nypsum pond water over which
fresh air was passed as shown in Figure 4.4. The gypsum pond was con-
structed from black iron plate which formed a box, 2 feet wide, 6 feet
long and 1 foot high. It was placed within a gypsum pond water bath
4 feet wide, 8 feet long and 14 inches deep. Temperature control was
maintained by utilizing heating elements placed in the water bath.
Water was recirculated between the pond and bath at the rate of 68
gallons per hour.
90
-------
SCALE: 3/4" = T
> t
4'
v
2"
f
j
I
>
h
!'
i
1
;i !• i
1 ' '
1 ' 2,,X2,, 1 1 1
' / Supporters | 1 '
• K . , , i
it ii i
k &' >l
8' ,.,
TOP VIEW
t
(
~i ii ii i >
SIDE VIEW
Figure 4.3. Model Gypsun Pond and Hater Bath Used in Tatera's Gypsum Pond Studies,
-------
SCALE: 3/8" = V
Blower
-10'-
2'
1
TOP VIEW
Blower
- Sampling Port
•
T
t
SIDE VI
•2
• • • »D
ABC •!
1
1
1
_____(____ _
EW
Figure 4.4. Schematic Diagram of Experimental Setup Used in Tatera's Gypsum Pond Studies.
-------
A wind tunnel, 18 feet long, 2 feet wide, and 1 foot high ran from a
0.4 HP blower to and over the gypsum pond. Flow rate was controlled
by means of baffles placed within the ductwork. Instruments were
placed within the ductwork to accurately measure the average velocity
through the ductwork. Sampling ports were installed at the locations
shown in Figure ^..4. Fluoride concentrations at the end of the pond
were measured by utilizing a stainless steel sampling probe connected
to a series of impingers, filled with a fluoride absorbing solution
of sodium acetate. The air was then pulled through a wet test meter
/
by a vacuum pump.
Fluoride concentrations in the sampler were measured using an Orion
specific ion electrode for fluorides with a calomel reference electrode
and a Corning Model #10 pH and millivolt meter. Fluoride concentra-
tions were determined according to:
C = cs Vs Hf 4 lr,
where: Ca = concentration of fluorides in air sample
Cs = molar concentration of fluorides in sample 1 es j
Vs = volume of sample (L)
Mf = molecular weight of fluoride (jjj^s-j
Va = measured volume sampled at desired temperature (ft^)
Experiments were conducted in duplicate at four different velocities or
flow rates and at three different temperatures. An emission factor at' each
93
-------
of the twelve conditions was determined in accordance with:
r Ca x Q 43,560 ft2 . lb , 1,440 min
A acre A 4.54 x 10Q ug x 3ay (4.14)
where: E = fluoride emission rate (Ib/acre-day)
Q = air flow rate (ft3/min)
A = area of pond = 12 ft2
Results of the studies on process water are summarized in Figure 4.5,
The resulting linear correlations, through the origin, are, at the 95
percent confidence level:
E = (0.00816 ± 0.0015) V at 75°F (4.15)
E - (0.0103 ± 0.0042) V at 85°F (4.16)
E - (0.0306 ± O.C061) V at 95°F (4.17)
Where V = velocity measured at 0.1M height (ft/min)
4.2.3 Interpretation of Results
Tatera designed his experiment with some knowledge of the parameters
that affect emissions, i.e., wind velocity, temperature and concen-
tration. He developed vapor pressure data at various different liquid
fluoride concentrations (equations 4.1-4.12), which indicated higher
vapor pressures of fluorides when fluoride concentrations were increased
in the liquid phase. Generally these were linear relationships, in
accordance with Henry's Law:
Pf = HXf (4.13)
where: Pf = partial pressure of fluoride in vapor phase (nun Hg)
H = Henry's constant of proportionality (mm Hg" )
Xf = molar fraction of fluorides in liquid phase
94
-------
VO
cn
0
100
E = 0.00816V
o
5
4 oj
10
OL
o
o Process Water
-------
Perhaps the most nearly applicable equations are 4.7 through 4.9 which are
for the system H2SiF6 pond water. It is unfortunate that Tatera was not
able to incorporate these correlations into his emission factor. His
emission rates were therefore given as functions of velocity and temperature.
His technique appears experimentally sound and is a good practical in-
laboratory approach. His studies served as an estimate of fluoride emissions
from gypsum ponds. However, the results are not immediately applicable
to large scale systems, i.e., a real gypsum pond. As Tatera himself states,
the problem of relating what happens in a wind tunnel to what occurs in the
natural atmosphere is very difficult. Secondly, there are no generally
accepted hydrodynamic scaling criteria which allow the evaluation of the
suitability of prototypes in wind tunnels to actual concentrations.
One deficiency in Tatera's study is that the wind speed was measured at
0.1 meter above the surface. It can be readily appreciated that since
there is a well developed velocity profile in real systems, the velocities
measured at 0.1 meter above the surface should be readjusted to conform
with velocities that would be recorded by a met tower or ambient wind
measuring device. Therefore, when an emission rate is calculated at a
given velocity using equations 4.15 through 4.17, an erroneously high
emission rate will be obtained unless wind speeds are scaled down using
the appropriate velocity profile relationships. These correcions are
discussed in section 4.4 of this report.
96
-------
4.3 REVIEW OF KING'S STUDY
W. R. King conducted a study under U.S. Environmental Protection
Agency Grant No. R-800950 for the determination of fluoride emissions
from gypsum ponds. This probably represents the most complete study
of the problem to date.
4.3.1 Method Of Approach
The general approach used in King's study was to determine a mass
transfer rate for volatile fluorides into an air stream passing
over gypsum pond water. King, in accordance with common practice,
defined his emission rate according to the expression:
where
N = Fluoride transfer rate per unit surface
area from pond to atmosphere (g-moles F )
(hr-Nr )
Kf = Overall gas-side fluoride mass transfer
coefficient (g-moles F )
(hr-M* - mm ng)
P* = Partial pressure of fluoride at the gas
liquid interface in equilibrium with pond
water (mm Hg)
Pf = Partial pressure of fluorides in the atmosphere
above pond.
The term (Pf - Pf) is commonly known as the gradient or driving force
for mass transfer from a region of high concentration to a region of low
concentration, i.e. from the gas-liquid interface to the bulk air stream.
-------
Unfortunately at the beginning of the study, there was no information
available regarding Kf and unreliable information concerning P£ and
Pf. Thus a determination of an emission factor necessitated that in-
formation be developed and experiments be conducted to determine the real
values of the unknown quantities.
King's approach in the determination of an emission factor was:
1. Development of a correlation for predicting the mass
transfer coefficient (Kf) from existing data.
2. Measurement of the equilibrium vapor pressure (Pf) of
fluorides over samples of pond water.
3. Prediction of fluoride mass transfer rates by equation
(4.19).
Since King arrived at an emission factor by conducting laboratory
scale experiments in the determination of vapor pressures and by
utilizing mass transfer coefficients developed for another species
(v/ater), the applicability of the emission factor to an actual gypsum
pond required field verification. The procedure used by King to
accomplish this task involved:
1. Development of a computer simulation through the use
of a Gaussian dispersion model incorporating equation
4.19 to predict downwind fluoride concentrations.
2. Measurement of ambient fluoride levels downwind of
a gypsum pond.
98
-------
3. Comparison of measured and predicted downwind concentrations
resulting in the verification of the computer simulation
and it component parts, including the emission prediction
method.
4.3.2 Development of Mass Transfer Coefficient
Since no experimental mass transfer coefficient data was available
describing the evolution of fluorides from gypsum ponds, King
utilized a theoretical correlation developed from studies using short
flat plates and fitted the form of this correlation to data available
on mass transfer of water from a large lake. By use of a diffusivity
correction, results obtained for the water system were modified to
yield a correlation describing the mass transfer of fluorides from
a pond.
Some correlations for turbulent mass transfer systems have been
developed by use of analogies with similar heat transfer systems.
The main analogy relating heat and mass transport is that proposed
by Chi 1 ton & Col burn.
J* - Ju = 1/2 f = function of Reynolds Number
0 H
(4.20)
where
J^ = Re Pr
Nu = ill = Dimension less heat transfer Nusselt Number
Re = XVf> = Dimension less Reynolds Number
99
-------
pr r pti = Dimension less Prandtl Number
T~
h = Heat transfer coefficient
k = Thermal conductivity
y » Viscosity
X = Characteristic length
v = Velocity
C « Heat capacity at constant pressure
P * density of bulk stream
JD = Nu AB
Re ScT/3
NUAB s £*J* ~ Dimension less mass transport Nusselt Number
c DAB
Sc = u = Dimension!ess Schmidt Number
PDAB
-------
1/5
f = 0.072 (Re) ' (4.21)
This expression, however, was developed for, and directly applicable
to,a small flat plate under conditions of turbulent flow and a 1/7
power velocity distribution.
Upon substitution of equation 4.21 into 4.20 the following relation-
ship is obtained:
Nu AB = 0.072 Re"1/5 (4.22)
Re Sc 1/3
According to the definitions of the terms above, the following
relationship used by King is obtained:
Kx X = 0.036 (0VX)4/5 ( u ) 1/3 (4.23)
c DAB ( u ) ( pDfto\
Figure 4.6 indicates the system to which equation 4.23 is applicable.
A thin semi-infinite plate of a volatile substance, A, evaporates
under steady-state conditions into an unbounded gaseous stream of
A and B which approaches the plate tangentially in the X direction
with velocity V°°. Species B is present in the gaseous phase only.
At the Instant a differential element of moving gas contacts the
leading edge of the plate, the velocity of that element immediately
drops to zero, which is consistent with the concept of no-slip at
the boundary. Instantaneous deceleration requires Infinite force,
therefore, Tx, which is the stress on a plane of fluid moving 1n the
X direction parallel to the surface is equal to infinity at x = 0, y =0,
101
-------
o
ro
V.
Gas Properties of A-B
Laminar
Boundary
Layer
c
p
v
CP
\ .
Transition
Region
* Boundary layer below plate Is omitted.
v.
Turbulent Boundary
Layer* ( 0.99 V» )
Figure 4.6. Tangential Flow Distribution Along a Sharp-Edge Seni-Infinite
Flat Plate with Mass Transfer Into Stream.
-------
As the fluid element moves along the surface, a laminar region
builds up in which the flow is less than V« . Eventually, at a
length Reynolds Number ( pV «° X), Re = 10^, this layer becomes tur-
( y )
bulent in nature and it is in this region that equation.4.23 applies.
At this point, a few comments regarding the use of equations 4.20-4.23
are in order.
1. Equation 4.20 as applied to mass transfer has been shown
to be accurate in describing evaporation of water from
a circular vessel at the bottom of a wind tunnel. However,
these experiments were conducted in the laminar flow regime.
Although the analogy between heat and mass transfer is quite
good, some question remains about extending this experiment to
a large body of water.
2. Equation 4.21 has been verified for small flat plates and is
based on a 1/7 power velocity distribution.
3. The assumption is made that the evaporating species is a
pure component in the liquid phase. Applications involving
dilute solutions involve the possibility of large liquid
mass-transfer resistances.
4. The flat plate can operate at steady-state, in that at
any position, X, there is no change in any property with
103
-------
time, but it can never operate in fully developed flow
because the boundary layer continues to increase regardless
of how long the plate is.
In applying equations 4.20-4.23 to a gypsum pond, the following points
should be considered:
1. Equation 4.20 has not been shown to hold for mass transfer
from large bodies of water such as a gypsum pond. It is
basically useful in heat and mass transfer equipment design,
but not necessarily in a large system.
2. Equation 4.21 does not describe the relationship between
friction factor and Reynolds number for a pond. An expres-
sion, specific to that situation, would have to be experi-
mentally derived.
3. In a gypsum pond, King was interested in a dilute volatile
component, i.e., fluorides. The flat plate assumes a pure
component. A pond would be nearly pure water, but data
developed from it would not be easily transferred to apply
to fluorides by use of equations 4.20-4.23.
4. Air over the pond would be in fully developed turbulent flow due
to contact with a land surface prior to passing over water.
In view of the above, the flat plate analysis is judged to be inappli-
cable to the situation of evaporation of water from a gypsum pond
and less applicable in describing fluoride evolution from such ponds.
104
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Equations 4.20-4.23, while not directly applicable to a gypsum pond,
do, however, give insight into which variables affect mass transfer
coefficients. It can be seen from equations 4.20-4.23 that:
Kx = f (X, c, Dab, V, P, v, Cp) (4.24)
Since certain variables would be constant for a given system, the
effect of the other variables on Kx can be studied.
Kx = f (X, V) at constant (c, Dab, p, y, Cp) (4.25)
Based on his use of equations 4.20-4.23, King assumed the relationship
to be:
K¥ = aX-°-2 vO-8 (4.26)
/\
Verifying equation 4.26 or determining the correct functional relationship
in equation 4.25 will not yield information regarding the relationship
of K¥ to C, D . , p, y, or (L. Any inferences regarding the relationship
* ab p
between Kx and Dab from equations 4.20-4.23 need experimental verifica-
tion.
4.3.2.1 Mass Transfer for Coefficient From Evaporation Studies
King's analysis of the fluoride emission problem began by assuming
equations 4.23 and 4.26. His next step was to develop a mass transfer
coefficient for water evaporating in air, with the thought that in
doing so, he could infer information pertinent to the fluoride problem.
105
-------
An extensive amount of data had been collected for evaporation rates
of water from lakes and reservoirs. The study King chose to evaluate
was conducted by the U.S. Geological Survey at Lake Hefner, Oklahoma.
In this study, the U.S.G.S. compared different mass transfer equations
with measured evaporation rates. In analyzing the Lake Hefner study,
King used only those data that the U.S.G.S. personnel estimated had
potential errors in evaporation rates of less than 0.03 in/day (Class
A data). Of these 142 daily observations, King further eliminated
29 data points in which the humidity driving force was less than 1 mm
Hg, the evaporation rate was less than 0.02 in/day or where the signs
of the humidity driving force and evaporation rate did not agree. He
analyzed the remaining 113 data points and derived an expression re-
lating the average mass transfer coefficient of water to the velocity
of air above the water.
K = 0.432 V16°'82 (4.27)
where
K = gm - moles HoO
hr - M* mm flg
V-|6 = wind velocity measured at 16 meters above
the lake surface
Equation 4.27 indicates the hypothesis that K is proportional to velocity
raised to the 4/5 power is correct. No significant length effect was dis-
cerned. However, it should be noted that confirmation of the velocity
relationship does not yield information regarding the relationship of mass
transfer coefficients to all of the variables indicated in equation 4.24.
106
-------
Upon incorporating similar data from various evaporation pans with
the Lake Hefner data and setting the.velocity exponent equal to 0.8,
King obtained the following correlation which he considered the best
estimator of the overall, gas-side mass transfer coefficient for the
transfer of water from a pond or lake to the atmosphere.
K = 0.429 V]6 ' (4-28)
An analysis of variance was performed on the data which indicated that
at a given wind speed, the observed mass transfer coefficient would
lie between 0.67 and 1.50 of the value predicted by equation 4.28 with
95 percent confidence. Stated differently,
50%
Observed = predicted ± 33% k.29)
at the 95 percent level of confidence, for water evaporating from a
large body of water, such as Lake Hefner.
Harbeck (1062) analyzed evaporation data for Lakes Hefner and Mead and
several other smaller bodies of water. In his analysis, Harbeck found
that the mass transfer coefficient, N (at constant velocity), varied
with the square root of the surface area.
-.10
N - (A°<5) - (4.30)
The equation predicts that the mass transfer rate for a 100 acre pond
would be 17 percent greater than for a 2,300 acre lake (Lake Hefner).
In applying equation 4.28 to a pond, an appropriate correction factor
incorporating equation 4.30 would be in order. However, this factor
should be applied to each pond depending on its size and geometric con-
figuration.
107
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4.3.2.2 Derivation of Mass Transfer Coefficient
By writing equation 4.23 for the species water evaporating from a flat
plate into a stream of air, we have:
.
/
A similar expression for fluorides 1s;
X
- = 0.036
where:
Kaw = average overall gas-side mass transfer g mole
coefficient for water evaporating into air hr - M2 mmHg
*af = ayera9e overall gas-side mass transfer co- g mole
efficient for fluorides evaporating into air hr - M2 mmHg
Daw = binary diffusivity for a water cm2
vapor-air system £££"
Daf = binar^ diffusivity for a fluoride cm2
air system "^
By dividing equation 4.31 by 4.32, King obtained the following expression:
2/3
^aw
-------
even in a flat plate the above equation might not work when the two
species are evaporating simultaneously and one species is orders of
magnitude more dilute than the other.
As a result of equation 4,33, King felt that it was only necessary
to obtain diffusivity values for water and fluorides. The diffusivity
of water was readily available and by using standard approximation
methods, he calculated diffusivities for the assumed volatile species,
HF and SiF4.
It is important to know exactly which fluoride species is being evolved
from the pond water. King seems to have gone astray in his reasoning
at this point, for he proposes the following reaction occurring:
H2SiF6(1)"2HF(g) *SiF4 (9> (4'34>
He then calculates an "effective fluoride" diffusivity from the fol-
lowing equation:
D = 2 Da.HF + 4 D
It is difficult to understnacl what rationale King had for forr.iulatinq
this diffusivity since equation 4.3G implies that somehow a mechanism
is in effect which causes the molecules to diffuse more rapidly than
either species would independently. Although King was trying to relate
the diffusivity to molecular fluoride, this is not the correct approach
for the above mentioned reasons and for those reasons that follow.
According to Illarionov, the molar ratio of HF/SiF4 in the vapor phase
109
-------
over a 1.09 weight percent solution of H SiF (0.5 m) is about 12
2 6
Using this logic, it is proposed that an "effective diffusivity"
based on a weighted average be derived as follows:
n - 12 °a UF + 4 Da
-------
4.3.3 King's Vapor Pressure Studies
Having obtained what he considered to be an accurate mass transfer
coefficient, King proceeded to conduct equilibrium vapor pressure
studies on gypsum pond water in order to develop an emission rate
of the form indicated in equation 4.19 of this report,
King designed a system to conduct his vapor pressure studies that
would:
1. Use actual, undiluted pond water with bottoms slurry
present in order to maintain the various sol id/liquid
equilibria.
2. Provide information regarding the effect of temperature
upon the fluoride vapor pressure over the solution.
3. Give information from two different ponds for the pur-
poses of comparison.
By using an Othmar still, King was able to insure the establishment of
a vapor-liquid equilibrium and maintenance of the liquid-solid phase
equilibrium. His results are presented in Figure 4.7. Hhile the technique
is scientifically sound, two observations regarding the results can
be made:
1. There is a pronounced minimum between 80 and 90°F.
2. The precision of the measurements is very poor.
Ill .
-------
00
w
si
3
W)
CO
w
Crf
fl-
O
I
a
Ctf
8
1000
300
70
o
80 90
TEMPERATURE - F
* Upper line - Pond 10
O
Lower line - Pond 20
100
Figure 4.7. Fluorine Vapor Pressure Over Pond Water
Source: King, 1970
112
-------
3. A difference in vapor pressures exists between the
two ponds.
With regard to the first point, one would expect a continual rise over
the temperature range. It is very likely related to the transforma-
tions between the various fluoride species in solution (HF, HFg", F~,
A1FX, FeFx, h^SiFg, SiF^, etc.) as a result of the complex equilibria
occurring between them.
The second point is demonstrated in Table 4.2, which shows a variance
analysis for several of King's vapor pressure measurements. As this
Table shows, the 95 percent confidence limits for two ponds at two
different temperatures ranged from 33 to 60 percent. This is not
totally unexpected, however, given the low concentration of fluoride
being measured. It does reflect on the overall accuracy of the methods
King uses in his verification of emission estimates.
Both King and Tatera state that vapor pressure is not a strong function
of the fluoride level in pond water. However, at a temperature of
100°F, which is the most representative temperature of pond water, the
average fluoride vapor pressure for pond 20 (0.335 g moles/1 fluorides)
was about half of the value obtained from pond 10 (0.628 - 0.800 g moles/
1 fluorides).
113
-------
Table 4.2 Analysis of Variance for King's Vapor Pressure Data.
POND 10—0.628
Data
Temperature Points
90°F 9
100°F 8
— ' — ' —
gm moles F'/L
— — — ————___
mmHg x 10"6
(Mean)
327
591
"•
Standard
Deviation
67
178
--
95% C.L
(± 2a)
— — — — — .
41%
60%
•••
Temperature
90°F
100°F
POND 20—0.336
Points
7
9
gm moles F/L
mmHg x 10'6
(Mean)
410
477
Standard
Deviation
79
78
1 • i ..
95% C.L
(± 2a)
38%
33%
114
-------
The vapor pressure studies fall short on the following points:
1. No comparison was made between pH and vapor pressure.
2. No rigorous study was made between fluoride vapor
pressure and composition.
Due to the variability in chemical composition of gypsum ponds, the
vapor pressure studies should not be applied to all ponds. It is
immediately obvious that at 100°F emissions from pond 10 would be
greater than the amount emanating from pond 20.
4.3.4 Fluoride Emission Factor
King gives no algorithms to describe his fluoride emission factor.
The shape of the vapor pressure curves preclude this. He does present
families (Figures 4.8 and 4.9 are reproduced from King's report) of curves
from which an emission rate is calculated for pond 10 and pond 20. The
families of curves describe the relationship indicated in equation 4.19
as a function of temperature and wind speed.
If there were no errors introduced in deriving a fluoride mass transfer
coefficient from the water studies, the emission factor could, at best,
be accurate within - 90% with 95% confidence. This is based upon
the scatter in mass transfer data as well as vapor pressure data, as
stated previously. However, the method of calculating a fluoride mass
transfer coefficient by use of data from the evaporation of water is
an order of magnitude approximation. Therefore, it is concluded here
that King's emission factors are also order of magnitude approximations
and are not rigorously applicable to all gypsum ponds.
115
-------
10.0
£
u
-------
cn
i/t
0)
•o
•r-
s-
o
\ \ \\Ss
0.5
70
80 90 TOO
Average Pond Temperature (°F)
Figure 4.9, Fluoride Emission Rates for Ponds with Water Containing
0.628 g moles/liter Fluorides V,g = Wind Speed at 16
Meters in Meters Per Second.
(Source: King 1974).
117
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4.3.5 Field Verification of Fluoride Emission Factor
4.3.5.1 Verification/Calibration of Dispersion Models
Atmospheric dispersion modeling is an attempt to mathematically
simulate the transport, diffusion and transformation processes that
occur in the atmosphere. Models have been developed for elevated
and ground-level point sources, line sources and area sources.
In general, all dispersion models estimate ground-level concen-
trations of substances emitted into the atmosphere. The Gaussian
plume model is a theoretical treatment commonly used in dispersion
modeling. The basic formulation of the Gaussian equation assumes
that pround-level concentrations are inversely proportional to the
mean wind speed. Vertical and horizontal dispersion is simulated
by the use of standard deviations of plume concentration distribu-
tion for various stability categories, as determined from experi-
mental studies.
To be confident that dispersion model estimates are representative
of the real world, validation/calibration is necessary. Theore-
tical mathematical models have limitations which cause estimated
concentrations to be in error. The availability and accuracy of the
input data to the model and the accuracy of the mathematical algo-
rithm are the significant influences on the accuracy of the model
estimates. Similarly, calibration of the dispersion model is affect-
ed by the location, exposure and representativeness of the air
sampling sites and by the accuracy of the air quality data. In
validation or calibration, known pollutant emission rates, plume
118
-------
characteristics and meteorological data are combined to calculate
ground-level concentrations for specific time periods and locations,
The time periods and locations correspond to those for which am-
bient concentration measurements were obtained or are available.
The calculated and measured concentrations are compared and a
statistical relationship obtained (normally a linear regression
analysis). If the statistical relationship is significant the
model is calibrated and the relationship obtained is applied to all
future concentration estimates by the model,
4.3.5.2 Prediction of Fluoride Concentrations Downwind of Gypsum Ponds
Kinq developed a computer model whereby the simulation of dispersion
of fluoride emissions from gypsum ponds was attempted. Area source
emissions from the gypsum ponds were represented by a number of line
source emissions. The explicit form of the Gaussian plume equation
for a finite line source was utilized to describe dispersion of
emissions from the line sources. The Gaussian finite line source
equation is as follows:
_ 2g D Z2] Y rp2 1
X (X, 0, Z) = v/7ircrz U exp |_ 2 az J x J pl >J?Tr 6xp (-0.5 p2) dp (4.37)
where: x= concentration at (X, 0, Z)
X ^ downwind distance
Z = receptor height
q = line source emission rate per unit length
U = mean with wind speed
o, crz = horizontal and vertical dispersion parameters
119
-------
pl= Y'y
P2 " VCTy
Y!> y£ = limits of line source
The dispersion parameters, ay and az, are functions of downwind
distance as given by Turner (Turner, 1970).
Uncertainties enter into the Gaussian finite line source equation
through several parameters. These parameters include source emis-
sion rate (q), the dispersion parameters (ay and az) and the mean
wind speed (U). Accurate measurement of the mean wind direction is
also important in verification, since this establishes the source-
receptor relationship used in the model for concentration calcula-
tions. Proper verification/calibration of the dispersion model
requires an accurate knowledge of these parameters, and the use-
fulness of the model will depend upon the uncertainty associated
with these parameters.
Uncertainties in specifying the fluoride emission rate from the
pond contributes to errors in the model concentration estimates.
Since pollutant emission rate (q) enters directly into equation
18, the magnitude of the imposed error in the model due to Inac-
curacies would be equal to the error in the emission rate equation.
As described in Section 3.3.4, the emission rate equation is at
best accurate only within ± 90% at the 95% confidence level.
Another source of uncertainty in the dispersion model is in the
specification of the horizontal and vertical dispersion parameters,
ay and oz. King's use of ay and az is within the assumptions under
120
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which the parameters were experimentally determined, i.e,, sampling
time of about 10 minutes, height; the lowest several hundred meters
of the atmosphere, and the surface relatively open country. Even
so, for distances of travel up to a few hundred meters from the source
for all stabilities, oz may be expected to be correct only within
a factor of two (Turner, 1970).
King continuously recorded wind speed and direction with on-site
meteorological instruments in his field work. Type or manufacturer
of the instruments was not mentioned in King's work. Assuming that
the measurement instruments were properly calibrated and that they
meet Atomic Energy Commission guidelines for wind measuring devices,
they can be expected to measure wind speed within 0.5 mph and wind
direction within ± 5 degrees. With this degree of accuracy, the
wind speed measurements would be known within i 7 percent (at 7 mph
wind speed). These uncertainties in the wind speed and direction
specification in the dispersion model would add to the uncertainty
in concentration estimates by the model, and would affect the compar-
ison of those estimates with measured concentration values.
Errors are inherent in simulating an area source by a number of finite
line sources (refer to Figure 4.10). The simulation error lies in the
initial emission density or apportionment. Area source emissions are
initially emitted into a large volume of air since the emissions are
apportioned over an entire area. By dividing the area source into a
number of segments and concentrating each segment's emissions into line
source emissions in the simulation model, the emission density is altered
121
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Mean Wind
Direction
I
*O
X
I L
segment boundary
line source
inure 4.10. Line Source Simulation of a Rectangular Area Source.
122
-------
The magnitude of error introduced due to this technique is dependent
on the line source spacing used for the simulation, as well as the
source-receptor distance (see Section 5.3). A smaller line source
spacing will more closely simulate area source emissions, and there-
fore the model-simulated concentration pattern will closely resemble
that expected from a real area source, Receptors close to the line-
source simulated area source are affected to a greater extent than
those further away. In practice, iterations are commonly made a part
of the computer model in order to obtain an optimum spacing of line
sources, or a sensitivity analysis is performed. King does not men-
tion either of these procedures in his pond simulation model, wherein
he used a 10 meter line-source spacing. The effect of varying the
line-source spacing on concentration patterns is discussed in Section 5,3.
4.3.5.3 Ambient Fluoride Sampling Study
King used ambient samplers designed to measure HF concentrations in
the vicinity of ponds 10 and 20. Each station consisted of a sampler
with a capture element as shown in Figure 4.11, which was reproduced
from King's report. Absorbing medium was a 2 g/1 solution of Na^CO^
which flowed at a rate of 8-12 drops per minute along the inside
diameter of the capture element. Ambient air, entering at an inlet
1.67 meters above the ground, was continually drawn through 30 feet
of coiled tubing at a rate of 30 liters/minute, This allowed suf-
ficient liquid gas contact to effectively absorb 95 percent of ambient
123
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Air Inlet
1.67 N Above
Ground
500 ml-3 Neck
Flask Gas-Liquid
Separator
Rubber Tube
Connecter
Sampling Board -- 15
feet flat coll 6 mm
I.D. glass tubing
Sampler Post
Rotometer 30 L/m1n
Front View
Air Bleed
Valve
Rubber Tubing
Vacuum
Gauge
Vacuum
Pump
Figure 4.11. Ambient Air Sampler Used 1n
Study.
124
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air fluoride. The absorbing solution flowed into a liquid-gas separator
from which the sample was taken for lab analysis. The air stream was
pulled through a rotometer by a vacuum pump. The absorbing solution
was analyzed in the laboratory for its fluoride content in order to
determine ambient fluoride concentrations through the use of a fluoride
specific-ion electrode.
The use of the automatic "HF gas analyzer" is logical, since most likely
it is HF that is evolved from gypsum ponds. However, particulate fluor-
ides from the gypsum piles could cause positive interference by contributing
to the fluoride level measured by the analyzer. This possibility can
be readily appreciated since the disposal areas can be as high as 120
feet around the ponds. The original design of the fluoride sampler calls
for a vertical absorption tube. This allows the absorption of HF and,
in conditions of laminar flow, minimizes particulate-liquid contact. By
using coiled tubing, a cyclonic effect is induced which causes particu-
late matter to be scrubbed out of the gas stream, This could be averted
by the use of a teflon filter at the air inlet, but no indication that
such a filter was used was evident in King's report.
The exact errors that particulate matter would contribute are not pos-
sible to assess since particulate fluoride emission rates from the p-fles
would have to be known, as would any background concentrations in the
area. King did not address this problem in his experimental design or
analysis of results which precludes drawing any firm conclusions from
his field work regarding gaseous fluoride emission.
125
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4.3.5.4 Comparison of Measured and Predicted Fluoride Concentrations
Chapter 10 of King's work purports that significant correlations between
the simulation model predicted fluoride concentrations and measured
concentrations were obtained at the 95 percent confidence level for
both pond 10 and pond 20. However, these correlations were obtained
only after deletion of much of the data. Only 95 of 132 measurements
were used in the analysis. Justification for deletion of these data
in some cases does not seem to be valid, based on the information con-
tained in the report. However, the author was not contacted to discuss
the deletions.
In the analysis of pond 10, certain measured concentrations at Sampler
01 were not included in the analysis because the measured concentra-
tions were much higher than predicted concentrations calculated by the
simulation model. Figure 4.12 is a sketch of pond 10. These measured
concentrations occurred when the average wind speed was between 5 and
6.3 m/sec. Justification was based on the contention that the disper-
sion model could not accurately predict concentrations when high wind
speeds and short downwind distances existed.
A wind speed of 5 to 6.3 m/sec is well within the assumptions of the
Gaussian plume model and the conditions under which oy and oz were
derived (Turner, 1970). The model used by King should therefore be
as accurate at wind speeds of 5 to 6.3 m/sec as it is at wind speeds
of 1 to 5 m/sec. The data should not be rejected then on the basis
of model errors at wind speeds of from 5 to 6.3 m/sec.
126
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Study coordinate systen
Wind Speed and
Direction
corder
Active Gypsum Pile
Figure 4.12. Pond 10 Layout (After King)
127
-------
The dispersion parameters a and a used in the Gaussian plume model
are not applicable to the situation of short travel distances (less
than 100 meters) regardless of wind speed or stability (Turner, 1970;
Pasquill, 1974). Thus, if some data for Sampler 01 at pond 10 were
deleted for this reason, all data for Sampler 01 should be deleted
since the sampler was only 10 meters from the edge of the pond.
Finally, the measured concentration data rejected due to high wind
speeds at Sampler 01 show a significant correlation with calculated
concentrations at the 99 percent confidence level (r = 0.90, d.f. = 6).
However, calculated concentrations are consistently lower than measured
values. In spite of King's conclusions, the model seems to work well
for this situation, although corrections should be made in the model
so that measured concentrations .are,mot underestimated. A logical
place for this correction would be in the emission rate equation.
Several measured fluoride values collected at pond 10 were deleted
from analysis under the hypothesis that the active gypsum pile north-
east of the pond was contributing significant quantities of ambient
fluoride at the sampling locations (Samplers 01 and 02; see Figure 4.12).
The simulation model predicted concentrations well below the measured
values in most cases. The active gypsum pile could foreseeably con-
tribute to high fluoride values at the samplers from both gaseous
and particulate fluoride evolution.
128
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A study of the plot sketch of pond 10 and of the wind directions
presented in the Appendix of King's work for the usable measured
concentrations at pond 10 indicate that the mean wind direction must
be towards the stated direction in order for the gypsum pond to con-
tribute to fluoride levels at the samplers. The majority of these
usable measurements fell within the directions of -15 to 76 degrees
(based on the pond 10 coordinate system). The measurements not used
for pond 10 analysis were obtained with a mean wind direction of from
55 to 93 degrees, with only one measurement greater than 90 degree
wind direction. The pond 10 sketch indicates that these mean wind
directions are not directed from the active gypsum pile, but since
they are an average direction over a 60 minute period, wind flow over
the gypsum pile could have occurred a percentage of the time. King
does not indicate the percentage of the total time flow was actually
over the active gypsum pile, therefore the relative contribution or
effect of the gypsum pile on ambient levels at the samplers cannot
be estimated. It is also noted that for pond 20, the locations of
the gypsum pile, gypsum pond, and Sampler 01 (Figure 4.13) indicates
that particulate and gaseous fluoride from the pond 20 active gypsum
pile could contribute to fluoride concentrations at the samplers when-
ever the pond contributed to the measured concentration, i.e. when the
wind was directed from pond 20 towards Sampler 01.
Certain pond 20 data were also deleted in verification of the simu-
lation model. Sixteen (16) one-hour time periods at Sampler 02 were
removed from consideration because high fluoride values were measured
129
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Magnetic nor
ISO
-SCf
Figure 4.13. Pond 20 Layout (After King)
130
-------
during these time periods. Calculated values were much lower than
measured fluoride values. The explanation was given that some action
by the phosphate plant in the gypsum disposal area caused the high
ambient air concentrations. Sampler 01 displayed no similar increase.
However, due to the locations of the samplers, the gypsum pond and
the gypsum disposal area, it is likely that any activity in the dis-
posal area affecting Sampler 01 would also affect Sampler 02. It is
also likely that the gypsum pile would contribute to the fluoride
measured at the samplers when the wind was such that the pond would
also contribute. Since no upwind measurements were obtained, no
estimate of the background gaseous and particulate fluoride contri-
bution can be made.
Certain other data were also deleted from the pond 20 verification
when model calculated concentration estimates were higher than
measured values (see Appendix 15.5.4 of King's work). Justification
is given that the model could not satisfactorily predict concentrations
during conditions of strong solar insolation and low wind speed. This
is a possible explanation, but not the only explanation. The measured
1 and estimated concentration values were in good agreement (r = 0.73,
d.f. = 9; significant at 95 percent confidence level), and certainly
agreed within the experimental error and uncertainties inherent in the
simulation model. It is questionable, then, that these data should
not be used in the model verification.
King's conclusion that background sources of fluoride (i.e., sources
other than the pond) did not contribute significantly to measured
131
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fluoride values at the samplers is not substantiated by the data.
Only a limited number of background measurements were obtained:
six at pond 10 and three at pond 20. This limited data is not suf-
ficient to make any definitive conclusions on background concentrations.
In addition, since King concluded that background fluoride levels were
insignificant, no background measurements were made when the model
validation data was gathered. Thus, it is impossible to assess the
exact contributions of background fluoride to the total measured con-
centrations.
Pond 10 background estimates were obtained from one sampling station.
The measured background values ranged from 0.0 to 0.92 x 10"6 gm-moles/
M
m fluoride and averaged 0.36 x 10'6 gm-noles/m3 (Table 4.3). The aver-
age measured fluoride concentration obtained at pond 10 for the study was
3.12 x 10" gm-moles/m3 and concentrations ranged from 0.64 x 10"6 to
10.48 x 10"6 gm-moles/m3. These data indicate that background fluoride
could contribute on the average 12 percent (0.36 v 3.12) to measured
fluoride levels at pond 10, which is indeed significant.
Three purported background measurements were obtained at pond 20. All
of these measurements were made when calculations showed that the pond
contributed insignificantly to the fluoride concentration at the sampler,
i.e., when the average wind direction was not directed from pond to
sampler. However, the same sampling locations were used in measuring
the fluoride concentration when calculations with the simulation model
showed the pond contributed significantly to the fluoride at the sampler.
132
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Table 4.3. Measured background ambient fluoride
concentrations and simulation model
estimates.
Measured Fluoride
Concentration
Pond 10 o.O
0.0
0.0
0.92
1.06
0.68
0.20
Pond 20 0.53
0.37
Calculated Fluoride
Concentration Due
To Pond Emissions
(x 10-6g"moles/m3)
0.0
0.0
0.0
0.0
0.30
0.0
0.03
0.0
0.0
Background Fluoride
Concentration From
Unaccounted Sources
0.0
0.0
0.0
0.92
0.76
0.68
0.17
0.53
0.37
Thus, no upwind background measurements were made when the wind was
blowing from the pond toward the samplers. This is very disturbing
since the gypsum disposal area would be upwind of the pond and samplers
under these conditions, and could contribute to gaseous and particulate
fluoride levels at the sampling locations. It is also disturbing
since the author concluded for pond 10 that the active gypsum pile
there contributed significantly to fluoride levels near the pond. From
133
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these observations it must be concluded that background fluoride con-
centration could not be specified with any degree of confidence for
pond 20.
To obtain a better degree of confidence as to the performance of the
simulation model, all of the pond 10 and pond 20 measurements were
plotted on a scatter diagram of calculated and measured fluoride con-
centrations (Figure 4.14 and Figure 4.15). The pond 10 data, when
including all of the deleted observations, results in a significant
correlation (r = 0.85, d.f. = 69) at the 1 percent level of signifi-
cance. Thus, it can be stated with 99 percent confidence that a linear
relation does exist between the measured and calculated values. The
variation in the data due to uncertainties and errors in the simulation
model is equal to 1-r2. For pond 10, this unexplained variation accounts
for 28 percent of the total variation in the data. Linear regression
analysis of all of the pond 20 data resulted in a poor correlation
(r = 0.15, d.f. = 60). Thus, a linear relationship does not exist
between the measured and calculated concentrations for pond 20.
Ninety-eight (98) percent of the variation in the data cannot be explained.
134
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• Measurements used in data analysis
® Measurements not used in data analysis
0 Wind flow over gypsum pile data
© High wind speed data
data not used in verification
x
X
•J? »'
1 s^'
• S
1 1 1 1
1234
m
a
1 1 1 1
S * 7 I
*\
MEASURED x 10-6 g moles/m3
Figure 4.14. Pond 10 - Measured Vs. Calculated Ambient A1r Concen-
trations at Samplers.
135
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CO
CO
E
QJ
1 2
01
VO
I
o
o
UJ
Measurements used in data analysis
Strong solar radiation, low wind speed
High fluorine measurements ©I
MEASURED x 10'6 g moles/m3
>f Data not used in verification
Figure 4.15. Pond 20 - Measured Vs. Calculated Ambient Air Concentrations at Samplers.
-------
4.3.5.5 Verification of King's Simulation Model
The preceding discussion has alluded to many sources of uncertainty
and error associated with the application of King's simulation model
of gypsum pond emissions and dispersion. These uncertainties are
not uncommon in verification and calibration of other dispersion
models, and it is often difficult to obtain a good correlation between
observed and model calculated values. Very rarely is a one-to-one
correspondence between measured and predicted values obtained. As
sampling time decreases, however, it is expected that calculated
concentrations will more closely approach measured concentrations in
magnitude. This is true since the variability of the parameters de-
scribing the atmospheric phenomena is decreased for shorter sampling
times. In this respect, King's model is superior in that sampling
times were very short (10 minutes).
Contrary to King's statement that a simulation model is accurate only
if a one-to-one correspondence between measured and calculated values
is satisfied (calculated = 1.0 x measured), any linear relationship
between the two is accurate 1f the correlation is statistically signi-
ficant, or if the model predicts concentrations within some defined
confidence limits. Dispersion models typically overcalculate ob-
served concentrations. Overcalculatlon results from model assumptions
of steady-state conditions for emissions and meteorological parameters.
137
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Elevated point source models have been found to overcalculate concen-
trations by a factor of 2 to 10. This in itself, however, does not
invalidate the model.
Dispersion models cannot be used to validate parameters used as inputs
to the model. This is because our imprecise knowledge of atmospheric
phenomena does not insure that the dispersion of emissions into the
atmosphere is being correctly or completely simulated. In this respect,
verification of a dispersion model does not verify an emission factor,
for the emissions could be in error by a proportionate amount, and
verification of the model still obtained. (For instance, the slope of
the linear regression line would change). It is imperative that
before a dispersion model can be properly verified, the pollutant
emissions must be known.
In conclusion, the analysis of King's modeling work points up the
following major shortcomings:
1. Verification or calibration of a diffusion model for
fluorides does not substantiate an emission factor
for fluorides.
2. Several measured concentrations were deleted from
analysis but seemingly lacked justification for such
removal.
138
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3. Background concentrations of fluoride could not be esti-
mated based on the available data obtained during the study.
4. Since background concentrations were not available, the
contribution of fluoride from the gypsum ponds and that
from other sources of fluoride could not be delineated.
5. Based on 1-4 above, a reliable emission factor for
fluorides from gypsum ponds has not been substantiated.
4.4 COMPARISON OF EMISSION FACTORS FROM THE LTTFMTllpF
Since Cross1 and Ross1 study provided no useful data regarding an
emission factor, it will not be considered in this section. As stated
in Section 4.2.3, Tatera measured wind speed in a wind tunnel at a
point 0.1 meters above the water. It is important to note, first of
all, that the velocity distribution in the wind tunnel is such that
the velocity will approach zero at all four walls due to surface fric-
tion. As Tatera stated, there are no accepted hydrodynamic scaling
parameters which allow wind tunnel results to be applied directly in
large scale systems. The velocity profile above a pond generally
follows a logarithmic distribution. It is zero at the surface and
increases with height.
Nevertheless, in order to make a comparison of the two studies, a
relationship, empirically developed by King, was used to scale Tatera's
velocities to a height of 16 meters.
Vl zl
e*P [0-2909+ 0.06154 (L) - 0.01164 (V16)] (4.38)
139
-------
where:
V1 = wind speed at Z1 (0.1 meter)
V2 = wind speed at Z2 (16 meters)
L = measured lapse rate (between 2 and 16 meters)
V^ = wind speed at 16 meters
Comparisons of emission factors as determined by King and Tatera (incor-
porating the correction above) are presented in Table 4.4 for three tem-
peratures. Comparisons at 75°F and 85°F are not readily applicable
since most gypsum ponds operate at temperatures above 90°F. Typically,
process waters enter the pond at 115°F and are returned to the process
at 95°F. Unfortunately, Tatera obtained all of his data between 75°F
and 95°F, while King conducted vapor pressure studies between 70°F
and 100°F. It would have been far more useful to have obtained data
between 85°F and 115°F.
Tatera's emission estimates at 95°F are approximately a factor of 2
greater than those of King. In accordance with our previous discussion,
a judgement cannot be made as to the validity of either method of
estimation when applied to a particular gypsum pond. It can be argued
from purely physical chemical grounds that the emission rate should be
about one-fifth the values determined by King*. Thus, the best statement
* Special communication with Dr. Alonzo Coots of International Industrial
Consultants; based upon assumption of saturated calcium sulfate with
calcium fluoride controlling free fluoride levels found in gypsum ponds.
Attempts to verify these assumptions in the laboratory were confounded
by the complexity of the solution.
140
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Table 4.4. Comparison of Emission Factors Predicted by King
and Tatera at Various Temperatures.
Vlg M/Sec
V1 M/Sec
Ft/Mi n
75°F
King (Pond 10)
Tatera (Process Water)
85°F
King
Tatera
95°F
King
Tatera
EMISSION
1
0.25
50
0.96
0.41
0.75
0.52
0.92
1.5
FACTOR IN LB/ DAY-ACRE
2
0.54
106
1.6
0.86
1.3
1.1
1.6
3.2
4
1.2
238
2.9
1.9
2.2
2.4
2.8
7.3
141
-------
that can be made at this point is that the characteristic emission
factor appears to lie in the range of 0.1 to 10 Ib/acre-day and that
a careful field study would seem appropriate to determine the contri-
bution of volatile fluorides from the ponds to the atmospheric fluoride
levels around the ponds.
142
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5.0 DETERMINATION OF GASEOUS FLUORIDE CONCENTRATIONS IN THE VICINITY
OF A GYPSUM POND
In order to estimate concentrations it was necessary to assume
a typical gypsum pond, servicing a phosphoric acid plant, producing
1,000 TPD of PgOs- Secondly, since no accurate or reliable emission
rate is known, downwind x/Q isopleths were calculated. Concentration
•
isopleths can then be calculated as a function of an assumed emission
factor.
5.1 DESCRIPTION OF TYPICAL GYPSUM POND
The gypsum pond is an integral part of a typical phosphoric acid
plant. It serves as a receptacle for wastewater which during the
manufacturing process comes into direct contact with raw materials,
intermediate products, by-products, waste products or finished
products.
Gypsum pond water is recirculated to the phosphate processes and
used mainly in scrubbers, barometric condensers, and for slurrying
waste gypsum. After settling of gypsum solids in the ponds, the water
is allowed to cool by evaporation and'reused.
Figure 5.1 shows the configuration of the "typical" gypsum pond.
Process waters enter at about *15°F. The area around the entry
point would, therefore, be the area for maximum evaporation and
fluoride emission. The water cools by evaporation down to about
95°F which is the temperature at which it reenters the process.
A recirculation rate of 30,000 GPM would be expected.
143
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GYPSUM PILE
FERTILIZER
PROCESSES
FRESH
MAKE-UP
WATER
figure 5.1- Tj^fcal typsun fond Servicing A 1000 TPD-PzOs Pfttotf
-------
The average concentrations of species in gypsum pond water are
summarized below. These figures are based on the mean measurements
from thirteen ponds provided by The Fertilizer Institute.
Concentration (mg/1)
Al 170
Ca 1600
Cl 3500
F 5100
Fe 210
K 110
Mg 165
Na 930
NH3 - N 520
p 4500
S04 4400
Si 1560
PH 1.6
The important factors to be considered and the concentrations of
free fluoride ions' (F-), pH and temperature. The value for fluorides
given above are not free fluoride measurements, but are total soluble
fluoride. These three factors- should be sufficient to specify an
equilibrium vapor pressure for HF in the. pond water, which is directly
related to the emission factor.
A typical gypsum pond, handling both slurry and process water from
a 1.000TPD-P205 plant would have about 350 acres of wet area.
Water depth would be about 10 feet. Most likely it would be located
adjacent to the plant and surrounded by mined out land of sparse
vegetation or swamp. Assuming that the pond is used for both
gypsum settling and cooling, there would be a region where the
stream from the sluicing operation would join the pond. This area,
known as the gypsum flats,, is where the gypsum settles. It would
145
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constantly be worked by draglines which would remove settled wet
gypsum and transfer it onto an active gypsum pile to dry. The
gypsum pile would be about 80 feet high on about 150 acres adjacent to
the wet pond.
When rainfall exceeds evaporation, a certain amount of the water
would be discharged from the pond. Prior to entering a water
stream, it would be treated by double stage liming which would
raise the pH to 9.0. This would effectively remove fluoride as
fluorspar through a series of reactions. It would be necessary
to use this system on an intermittent basis for 3 months per year.
5.2 GROUND LEVEL GASEOUS FLUORIDE CONCENTRATIONS IN THE VICINITY OF A
GYPSUM POND — _
The Gaussian finite line source equation was used to estimate
gaseous fluoride concentrations downwind of a hypothetical gypsum
pond. A computer program was developed to calculate concentrations
due to a large number of spatially distributed line sources at a
number of downwind receptor points. The line source model was
chosen for the area source simulation because it is considered the
best available calculation technique.
A 350 acre rectangular pond, similar to the configuration shown in
Figure 5.1, was chosen for simulation. The pond's length was
taken to be twice its width. The pond was divided into a number
of equal sections, 10 meters wide. A line source was placed at the
center of each segment oriented cross-wing, and the segments emissions
concentrated into the line source contained therein.
146
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Estimations of ground-level fluoride concentrations were obtained for
two different meteorological situations (typical average day conditions
and worst conditions). Isopleths of concentration/emission rate
(x/Q) ratios were obtained for each situation (Figure 5.2 and Figure 5.3)
Average conditions consisted of stability class land a wind speed of
3 m/sec. Typical worst conditions consisted of stability class 6
and a wind speed of 2 m/sec. The x/Q value gives the normalized
ground-level concentration in relation to the emission rate. For
example, if the x/Q ratio is 1.0, the ground level concentration
will be equal to 1 ug/m3 if the emission rate is 1 Ib/acre/day. If
the emission rate is 10 Ib/acre/day and x/Q is equal to 1.0, the
3
ground-level concentration would be 10 yg/m .
The concentrations predicted by the simulation line source model
should not be construed to represent actual concentrations which
would exist downwind from a real gypsum pond. Verification and
calibration of the model is required before it can be confidently
used to predict concentrations near gypsum ponds. The greatest
utility in the present simulation is in the ground-level concen-
trations patterns generated by the mode. For meteorological conditions
similar to those used in obtaining Figures 5.2 and 5.3, the patterns
shown are expected to be similar to those actually occurring near
the pond. However, the magnitude of the concentrations may not be
those displayed.
The simulation assumes a typical gypsum pond as described previously,
and assumes a constant wind velocity, unaffected by topographic
147
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O.5
1.0
KILOMETERS
cx>
GYPSUM POND
I
Figure 5.2 Isopleths of Calculated Ground-Level Fluoride x/Q Ratios Downwind of a Hypothetical
Gypsum Pond. (Stability Class 6, Wind Speed = 2 m/sec.)
-------
0.5
1.O
KILOMETERS
GYPSUM POND
Figure 5 3 Isopleths of Calculated Ground-Level Fluoride x/Q Ratios Downwind of a Hypothetical
Gypsum Pond. (Stability Class 4, Wind Speed = 3 m/sec.)
-------
features such as the gypsum pile. It further assumes contributions
from only the gypsum ponds in gaseous form. Background levels would
have to be known and added, as would particulate or gaseous contri-
butions from the piles and plant. If it is true, however, that
gypsum ponds are the main sources of fluorides, then Figures 5.2 and
5.3 should be accurate in showing the relative dispersion of fluorides
around a gypsum pond.
5.3 COMPUTER MODEL SENSITIVITY ANALYSIS
In order to test the sensitivity of the model to changes in line source
spacing, the computer model was executed for a typical gypsum pond
configuration utilizing various meteorological conditions and downwind
receptor distances. Each of the ponds King studied were approximately
500m x 900m. Fluoride samples were located within about 100m of the
edge of the ponds. In his simulation model, King employed a 10m line
source spacing to calculate downwind fluoride concentrations.
For comparison purposes, a gypsum pond 500m x 840m was employed for
the sensitivity study. The mean wind direction was situated crosswind
(90°) of the 840m sides. Concentrations were calculated for receptor
distances of 10m, 25m, 50m and 150m, and for a range of meteorological
conditions. Calculations were made for both a 10m line source spacing
and a 5m spacing. The results are tabulated in Table 5.1. Examination
of this table indicates that the effects of line source spacing are
most critical at closer receptor distances, but are also dependent on
meteorological conditions. For the parameters listed in the table,
150
-------
Table 5.1 Sensitivity Analysis for the Finite Line Source Model
Fluoride concentration (pg/m3)
Line Source
Meteorological
Stabi-
lity Wind
1
3
4
4
5
Conditions
Speed (m/s)
3
5
1
10
3
Receptor
10
0.9
1.1
8.5
0.8
3.7
25
0.7
0.9
7.0
0.7
3.1
Spacing
Distance
0
0
5
0
2
50
.5
.7
.8
.6
.6
= 10m
(m)
150
0.4
0.5
4.5
0.5
2.1
Line Source
Receptor
10
0.8
1.0
8.1
0.8
3.6
25
0.7
0.8
6.8
0.7
3.0
Spacing
Distance
50
0.5
0.7
5.7
0.6
2.6
= 5m
(m)
150
0.4
0.5
4.5
0.5
2.1
-------
the largest difference between conditions calculated using 10m and
5m line source spacings is 0.4 yg/m3 (stability class 4 and wind speed
of 1 m/sec). This value represents only a 5 percent difference in the cal-
culated concentration. Based on the sensitivity analysis, it is
concluded that a 10m line source spacing, such as that employed by
King, will introduce a certain small error in the calculated concen-
trations, but that a 10m spacing results in sufficient accuracy for
model validation/calibration purposes.
152
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6.0 IDENTIFICATION OF CONTROL TECHNIQUES
Several control processes for reducing fluoride emissions from gypsum
ponds are evaluated in this section. All of the candidates examined are
unusual with respect to air pollution control equipment in that they are
indirectly applied to the air emissions. In general, air pollution con-
trol equipment includes mechanical collectors, bag filters, electrostatic
precipitators, and scrubbers. It is not surprising to find special pro-
blems associated with the processes considered herein due to the complex
chemistry involved in the pond waters, the large water volumes involved,
and the acidic nature of the fluorides themselves.
A search of the literature and discussions with members of the phosphate
industry and the EPA revealed six potential candidates. They are:
1. Kidde Process for treating barometric condenser and scrubber water.
2. Swift Process for treating barometric condenser and scrubber water.
3. Liming the gypsum pond water to raise the pH and produce insoluble
calcium fluoride.
4. Dry conveyance of the gypsum to waste stacks instead of sluicing with
water.
5. Calcining the rock prior to treatment in the process.
6. Changing the basic process to the Hemi/Dihydrate process for phosphate
production.
In addition to these six processes, consideration is given to complete
segregation of the gypsum and cooling ponds. This segregation is con-
sidered tantamount in applying any of the first four control options;
153
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therefore a brief discussion will follow concerning the benefits to be
derived from a two pond system.
Two Pond System in a Phosphoric Acid Plant
Phosphoric acid plants utilize a wide range of gypsum/cooling pond
arrangements. In most cases process and gypsum sluicing waters are
transported to a common pond allowing these waters, which are vastly
different in properties to mix, with the ultimate result that both
process and gypsum pond waters will be highly contaminated //ith acidic
P2°5' H2S04* as wel1 as H2SiF6.
In some cases, separate cooling and gypsum ponds are utilized. All
process waters except gypsum sluicing water are sent to evaporative
cooling ponds. Gypsum slurry is pumped from the filtration operation
to a gypsum pile where the gypsum settles. The supernatant water is
subsequently recycled through the cooling pond, thus contaminating it
with P205, H2S04, and fluorides from the filtered gypsum.
A third possible method involves employing a "Two Pond System" in which
the cooling pond can not be contaminated with waters from filtration. This
"=thod was discussed by Parish and Enriquez in a recent paper (1973). It
-: recommended as part of a two-step solution for reducing fluoride
ssions from gypsum and cooling ponds to be used simultaneously with the
S.ift Absorption Process".
154
-------
The reasons for separate ponds were given by Enriquez and Parish were:
1. The required size of the gypsum slurry pond is small (about 5 acres)
since no area is required for cooling. This water would be the
most contaminated and acidic water in the plant due to the presence
of P205, H2S04> iron and aluminum complexes, and fluorides from
the filtration operation.
2. The size of the pond required for the barometric condensers is
determined by the cooling duty requirements. This area is estimated
to be 0.1 acre/TPD P205 .
Since the cooling pond will receive condensed vapors from the flash
cooler and evaporators, it is possible that entrained phosphoric acid
could be present as a contaminant. This, however, may be minimized by
the addition of entrainment separators, the result being that the main
contaminant entering the cooling pond will be limited to fluorides.
Figure 6.1 schematically shows the proposed system. One pond would service
the filtration operation by receiving gypsum slurry. The othar pond would
receive waters from the fume scrubbers, flash coolers, and the evaporation
trains. The cooling pond will require about 200 acres for a 1,000 TPD P205
plant, while the gypsum pona will require approximately 5 acres.
The costs required to segregate the two ponds at an existing plant are
site specific, but are considered negligible compared to the other control
costs presented herein. Since each plant will have different problems
155
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Filter Wash Water
en
tn
Gypsum
Pond
(3 to 4 Acres)
Flash
Coolers
Fume
Scrubber
Figure 6.1. Two Pond System for Phosphoric Acid Plant.
-------
switching to a two pond system, costs have not been calculated. These
costs, however, are expected to be well below the cost of the fluoride
control equipment.
Judging the cost-effectiveness of each process was facilitated by using
the following criteria:
1. Ability of the process to lower gypsum and cooling pond emissions
(efficiency)
2. Total capital investment requirements
3. Cost per ton of P20s produced
4. Commercial availability
5. Demonstrated capability
Our findings regarding cost and efficiency are summarized in Table 6.1.
Examination of this table reveals a large range of control costs ($1.25
to $7.46 per ton P205) and efficiencies that warrant some discussion.
The Swift Process is, in this study's estimate, the most cost-effective
method for control of emissions from gypsum and cooling ponds when
employed in conjunction with a segregated two pond system. A saleable
product (H2SiF6) is recovered. The product is sold to large municipalities
for fluoridation of water at approximately $200/ton H2SiF6 (delivered)
and to aluminum companies for production of fluorosilicates and other
fluorine compounds used in processing aluminum at $60/ton H2SiF6
(FOB Plant). The economics of this process are considerably improved
if all of the fluosilicic acid produced is sold at the latter price.
157
-------
en
Co
Table 6.1. Capital Investment and Operating Costs For Fluoride Control of 1,000 TPD PoOn Plant.
2U5
Process
Kidde
Swift Absorption
Liming
Conveyor
Pre-Calcination
of Rock
Total Capital Annual ized Operating
Where Fluorine Removal Investment Costs
Applied Efficiency $MM Total , MM $/Ton P205 By-Product
Barometric
Condensers 95-98%a 2.57 2.31 $ 7.46 (NH,)9 SiF
42 6
Barometric
Condensers 90%b 1.30 0.17 1.25° H SiF
2 6
All Cooling b
Pond Water 90% 2 10 0 97 3J3 None
Gypsum Filter After
Acidulation a i.Q7 0.43 1 40 None
Crushed Phosphate
Rock b 29.81 7.57 24.42 None
a) Not calculated due to uncertainties in fluorides evolved from filter cake.
b) 90% removal of fluoride in rock is achieved. However, the fluorides evolved are scrubbed
and transferred to ponds.
c) A credit of $2.26/ton P205 is realized if all fluosilicic acid produced is sold at $60/ton
(100% HSiF).
-------
In this case a credit of $2.26/ton P205 may be realized. Although this
study did not conduct a market survey, discussions with several indi-
viduals involved in the production and marketing of phosphates and
aluminum lead the authors to believe the market for H2SiF6 can absorb
some expansion, primarily in the aluminum industry.
The second most cost-effective method for reducing emissions involves
liming the pond waters to a pH of about 4.0. This reduces emissions by
lowering the fluoride vapor pressure. Unfortunately, this technique does
not produce a saleable byproduct and is subject to serious limitations.
Perhaps the most severe of these is the increased potential for equipment
scaling caused by precipitation of silica. Current practice tends to
eliminate this problem which has plagued the industry in the past.
Additional problems arise from secondary impacts associated with the
mining of limestone necessary for this control. While it is outside
the scope of work for this contract to assess secondary environmental
impacts outside of the phosphate complex, we feel it significant that
approximately 37,200 tons per year of limestone will be required to line
the pond waters associated with a 1,000 TPD P205 complex.
The Kidde Process has the highest potential recovery of all candidate;
for fluorine in the weak phosphoric acid. Since it is believed that th^
fluorine from the barometric condensers is the primary source of emissions
from the cooling pond, this process receives the highest technical merit.
Unfortunately, the high annualized costs put this process at a severe
disadvantage when compared with its nearest competitor, the Swift vapor
absorbers. The high costs of the Kidde Process reflect the study's
159
-------
assessment that the process be limited to that required to control fluor-
ide emissions. Thus, the byproduct in this instance is ammonia silico-
fluoride and not aluminum fluoride, as Kidde suggests. The limited market
for the ammonia salt (ca. 2,000 TRY) renders this byproduct a waste
stream rather than a saleable product. Regardless, it is the opinion
of the authors that the increased process complexity and additional
capital investment required to produce aluminum fluoride is unwarranted
as a control scheme. Perhaps these are the very reasons why Kidde has
been unsuccessful in marketing this process.
The other three processes (dry conveyance of gypsum, pre-calcination
of the phosphate rock, and the hemi/dihydrate process) all suffer from
the disadvantage that a major process change or change in industry
practice is required. None of these three is currently used on a
large scale in the U.S.
The remainder of this section is devoted to a detailed assessment of
these six candidates regarding cost and effectiveness. In reviewing
this section, the reader is reminded that these estimates have been
derived from literature and other sources and not from detailed engi-
neering estimates.
160
-------
6.1 KIDDE PROCESS
6.1.1 Process Description
The Kidde Process, used to defluorinate wet-process phosphoric acid and
convert the extracted fluorine into various fluorine compounds, is a
patented process developed by Kidde Process Company, Pasadena, California.
Fluorine compounds that may be produced include aluminum fluoride, syn-
thetic cryolite (Na3AlF3), hydrogen fluoride, and synthetic fluorspar.
Figure 6.2 is a detailed process flow diagram of the Kidde Process for
a 1,000 TPD P205 facility. The reaction sequence is as follows. React-
ive silica is mixed with phosphoric acid as it is fed to the acid
concentrators. The silica has been dried to about 10 percent moisture
and is premixed with the raw 30 percent P205 acid in an agitated tank
with about 15 minutes retention time. The amount of silica required is
determined by the amount of fluorine in the raw acid. The resulting
slurry is fed to the concentrators through the usual control system. The
principal reaction at this point is:
Si°2(<0 + *HF(aq) SiF4(g) + 2H2° (6.1)
The overhead vapors from the acid concentrators are now condensed with an
aqueous ammonium bifluoride solution; the pH of this condensing solution
is approximately 4.0. As currently practiced, these vapors are condensed
with cooling water or weak fluosilicic acid with a pH of 1.5 to 2.0. Herein
lies a major advantage of the Kidde Process. Scrubbing with the higher pH
bifluoride solution produces condensed vapors with a lower fluoride vapor
161
-------
u-? 6.2. Kidde Closed-Loop Fluorine Recovery for a 1,000 TPD P205 Plant.
-------
pressure. At this point, the condensed SiF4 reacts with the bifluoride
as follows:
SiF4(g) + 2(NH4)F-HF(aq) (NH4)2SiF6(a + H2S1Fg(aq) (6.2)
In the next step of the Kidde Process, the condensed vapors from the
various stages of evaporation are combined. This combined stream is then
neutralized with ammonia which neutralizes the acid produced in the baro-
metric condensers (eq. 6.2). This proceeds according to the following
equation:
H2S1F6(aq) * 2NH3(aq)
Concentration of the resulting solution is then achieved in multiple
effect evaporators. The product, ammonium silicofluoride, is then stored
for shipment or further reaction, as follows..
•
The product ammonia silicofluoride can be reacted further to produce
additional products. An example of such a scheme is depicted in the
following reaction scheme:
+ 4NH3(aq) + 2H2° 6NH4F(aq)
NH4F'HF(aq) * NV (6-5)
6NH4F.HF(aq) + Al203-3H20(s) 2(NH4)3A1F6* + 6H20 (6.6)
900° F
(NH4)3AlF6(s) A1F3(S) + 3NH4Ft (6.7)
163
-------
This scheme is that recommended by Kidde to produce aluminum fluoride
which can be marketed to the aluminum industry. It has added advantages
in that the silica and ammonia are recovered; these can then be recycled
to the P205 plant for further use.
For the purposes of this study, reactions 6.4 through 6.7 are not incor-
porated in the fluoride recovery scheme. This is because the emission
reduction estimated in the following section is effected by reaction
6.1 through 6.3; i.e., removal of fluoride from recyclable process waters
is achieved at that point.
6-1-2 Process Applicability for Fluoride Emission Reduction
The applicability of this process as a candidate for reducing fluoride
emissions from gypsum ponds is due to the lowering of the fluoride vapor
pressure through neutralization with ammonia. An estimate of the emission
reduction potential is arrived at as follows. As currently practiced
with no fluorine recovery, the overhead vapors are condensed with
recycled cooling pond water; the pH of this solution is approximately
1.5 to 2.0. At this pH, the fluoride vapor pressure is approximately
13.8 x 1CT6 rrniHg (25 C) (HEW, 1970). In the Kidde Process, condensed
vapors are not sent to the cooling pond as in the above scheme. Instead,
the solution is concentrated and neutralized to a pH of 5.0. At this
pH, the fluoride vapor pressure is only 0.65 x 10'6 mmgHg (25 C). There-
fore, in the final condenser of the ammonium silicofluoride evaporation
train, the vapor pressure is approximately reduced by the following amount:
Approximate fluoride reduction = (13.8 - 0.65) x 100%/13.8 = 95.3%
164
-------
This is approximately the emissions reduction to be expected since the
ammonium silicofluoride evaporation train operates at this pH which is why
the fluorine content of the product streari is less than 0.2% F. (Figure 6.2).
6.1.3 Process Evaluation
In April, 1974, the Kidde Process Company submitted a technical and
economic study to Shahpur Chemical Co. Ltd. of a proposed fluorine-
recovery and aluminum fluoride-manufacturing installation at the company's
phosphate plant at Bandar Shahpur, Iran. The study predicts costs of
less than $10.00 per ton for extracted fluorine and less than $100.00
per ton for aluminum fluoride. The selling price for aluminum fluoride
at that time (1974) was over $300.00 per ton. However, no full-scale
plant actually exists today, or is currently under construction.
Adaption of a fluorine-recovery system to an existing wet-process phos-
phoric acid plant should present few problems. The major items of
equipment that must be added are the triple-effect evaporators for
ammonium silicofluoride concentration, the shell and tube heat exchangers
used for cooling the condensed overhead products, and a small cooling
tower for removing heat associated with the ammonium silicofluoride evap-
orators (see Figure 6.2).
The existing barometric condensers used in concentrating the P20g can
he used with the Kidde Process sincn the condonsing medium (ammonium
bifluoride) is less corrosive than the currently USPC! pnnri viaters.
165
-------
Additional benefits that should result from installation of the Kidde
Process include:
1. Reduction or elimination of liming for pond water discharged from
the plant's cooling ponds.
2. Improvement in the physical properties of the phosphoric acid due to
a reduction in its fluorine content.
3. Increased recovery of fluoride otherwise lost in the evaporation
stage.
Of these, the third was estimated in Section 6.1.2. The first two are
not easily quantifiable; i.e., it is not easy to assess cost benefits
associated with then. However, botli of these are benefits that sliould be
attainable with this system.
6•1•4 Process Economics
Estimated incremental capital investment requirements and annualized
operating costs for a 1,000 TPD phosphoric acid facility are summarized
in Tables 6.2 and 6.3, respectively.
The data were determined in the following manner. Equipment sizes rec-
commended by Kidde for a 560 TPD P205 plant were examined for reasonableness.
These sizes were then adjusted to process the flows corresponding to a
1,000 TPO plant. Costs for major equipment items were obtained from pub-
lished sources; standard engineering estinates were then used to arrive
at installed plant costs. As Table 6.2 shows, approximately 2.G million dol-
lars is required to install the equipment associated with this control process,
166
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Table 6.Z. Capital Investment Requirements for Kidde Process Producing
29,000 TRY (NH4)2SiF6 (1,000 TPD P205).
DIRECT COSTS
EQUIPMENT, E
PIPING
CONCRETE
STEEL
INSTRUMENTS
ELECTRICAL
INSULATION
PAINT
FIELD MATERIALS, M
DIRECT MATERIAL, E + M = M
DIRECT FIELD LABOR
SUB-TOTAL DIRECT COSTS
$ 652,000
297,300
33,250
20,200
66,500
13,000
32,000
3,000
465,250
1,114,250
410,000
1,524,250
INDIRECT COSTS
FREIGHT, INSURANCE AND TAXES
OTHER INDIRECTS (CONTRACTORS FEES, INTEREST DURING
CONSTRUCTION, ENGINEERING)
52,160
565,300
SUB-TOTAL INDIRECT COSTS
CONTINGENCY (20% DIRECT AND INDIRECT COSTS)
617,460
428.342
TOTAL CAPITAL INVESTMENT (TCI)
$ 2,570,052
167
-------
Table 6.3. Annualized Operating Costs for Kidde Process.
Quantity
Unit Cost Annual Cost
RAW MATERIALS
SILICA
AMMONIA
14,700 TPY
4,200 TPY
$ 35/TON
$190/TON
$
$
514,500
798,000
1,312,500
UTILITIES
ELECTRICITY
STEAM
COOLING TOWER MAKE-UP
OPERATING LABOR
OPERATING
SUPERVISION
MAINTENANCE
LABOR AND MATERIALS (L&M)
SUPPLIES
350 HP
37,500 LBS/HR
100 GPM
2 MEN/SHIFT
OF OPERATING LABOR
4% OF TCI
15% OF L & M
20 MILLS/KWH
$0.50/M LBS
0.20/M GAL
$ 5.50/HR
$ 35,500
139,500
8,930
$ 183,930
$ 102,300
15,345
$ 117,645
$ 102,950
15.450
$ 118,400
OVERHEAD
PLANT
PAYROLL
50% OPERATION & MAINTENANCE
20% OPERATING LABOR
$ 118,000
23.530
$ 141,530
FIXED COSTS
DEPRECIATION (5% OF TCI)
TAXES AND INSURANCE (2% OF TCI)
CAPITAL CHARGES (10% OF TCI)
TOTAL ANNUAL COSTS
COSTS PER TON OF P205
$ 128,685
51,475
257.370
$ 437,530
$ 2,311,535
$ 7.46
168
-------
Annualized operating costs for the Kidde Process were obtained using
estimates from Kidde's report and standard engineering estimates. A
310 day year was assumed to allow for equipment repairs and maintenance.
Examination of Table 6.3 shows a net annual operating cost of $7.46 per
ton P205. One will notice that no credit is given to the 29,000 TRY
of (NH4)2SiF6. This is explained by the fact that only one firm
(Agrico Chemicals) in the U.S. produces this chemical. This firm indi-
cated that they only market 2,000 TRY to commercial laundries where it
is used to neutralize residual alkalinity between the wash and rinse
cycles. Thus, even though this chemical has a current market value of
$340 per ton, credit cannot be assigned since a market does not exist.
The question now arises concerning the feasibility of extending this
process to include production of AlFj. Although a detailed assessment
was not made to determine the additional capital investment to do this
(i.e., produce A1F3), an estimate was made by pro-rating Kidde1s data.
This estimate indicates an additional investment of approximately $6.7
million to install a plant to process the (NH4)2 SiF6 to A1F3. This
plant would produce roughly 13,000 TRY of A1F3, recycle NH3 and Si02
to the fluorine recovery stages, and improve the economics of this con-
trol process significantly. A simplified flow diagram of this plant
is included in Figure 6.3.
The above figures indicate that approximately 9.3 million dollars would
be invested in a plant capable of servicing a 1,000 TPD P20S facility
169
-------
.._*•!
— _ — (*.{,,'
ci/^7 3.f ^_ _
Otrft -
; ££*STt"c'
-------
(310,000 TRY). The present capital investment requirements for a 1,000
TPD P205 complex is approximately 15 million dollars. This is mid-1975
battery limits costs for an existing plant. Thus, the total investment
required for the complete Kidde Process is of the same order of magnitude
as that required for the P205 complex itself.
Although the current market for A1F3 was not evaluated in this study,
one aluminum manufacturer that was contacted (Kaiser) indicated that
some market does exist for these chemicals used in producing aluminum.
In fact, this company operates a small facility in the central Florida
area where the sodium salt of fluosilicic acid is produced, shipped,
and then used to make cryolite, a flux used in producing aluminum.
6.2 SWIFT PROCESS
The Swift Process, was developed and patented by W. R. Parrish (patent
No. 3273713), assignor to Swift and Co., Chicago, 111. It involves the
removal of fluorine compounds from wet phosphoric acid manufacturing ir
a manner similar to the Kidde Process.
6.2.1 Process Description
Figure 6.4 presents a process flow diagram as well as a fluorine and
partial overall material balance for a wet process phosphoric acid plant
producing 1,000 tons/day• P20g. Evaporation from dilute (28.5% P 0 ) to
concentrated (52.5% P205) is effected by three evaporation units in
series. Vapors from each unit are scrubbed by an absorber which removes
90 percent of the fluorides evolved during each evaporation stage.
171
-------
TO SCRUBBERS AND CONDENSERS
FROM SCRUBBERS AND CONDE
COOLING
POND
43OOLB-F/HR
ATMOSPHERE
4SOLB-F/HR
REACTION/
FROM
cooLwe
POND
FROM
COOUN6
POND
5090LB-F/HR
2S24OO LB/HR
m
PHOSPHATE
MOCK
BAROMETRIC
CONDENSER
CONDENSER
CONDENSER
SCRUBBCR
I8»LB-F/>W
TOCOOUWPOND
I29LB-F/MR
TO COOLING
56LB-F/HR
TO COOLING POND
58OLB-F/HR
20SOOLB/HR
rrseoo LB/HR
196700 U/HR
62.5%
225200 LB/HR
4SOLB-F/HR
tSfOOLB/Mft
NOOOL»/H«
T.79%
I7MOLV/HR
15.5% HgSl^
HBdOLB/HR
Figure 6.4 _Sw1 ft Process for FT uori neRecovery at a 1000 TPD
Plant
-------
A scrubber with a 96 percent fluoride removal efficiency is utilized
for control of vapors generated in the acidulation reactor. Cooling
pond water is used as the absorbing medium and a stream of dilute (3.85%)
HgSIFg is continuously removed. This stream is introduced into a recir-
culation tank from which a solution of 7.75% H2SiF6 is drawn and pumped
into the Swift vapor absorber servicing the third evaporator. The
scrubbing liquor is introduced into the scrubbing unit through nozzles
in the form of a spray. After absorbing the volatile fluorine material,
the scrubbing liquor falls into a barometric leg, the end of which is
immersed in the recirculation tank. The temperature of the liquor being
recycled is maintained at a level which will ensure the absorption of
volatile fluorine compounds while minimizing water vapor condensation at
the operating pressure.
A solution of 7.75% HgSiFg is drawn from the recirculation tank servic-
ing the third unit and pumped into a recirculation tank servicing the
second unit. The operational descriptions of the Swift components
(absorber, recycle tank, and pump) servicing evaporation units 1 and 2
are identical to that described for evaporation unit 3. However, each
unit is operated at a different temperature, pressure, and recycle
rate, and a more concentrated stream of fluosilicic acid is drawn from
each recycle tank. The product (25% HgSiFJ is continuously removed
and transferred to storage tanks for shipment.
173
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6.2.2 Process Applicability for Fluoride Emission Reduction
The original purpose of the Swift Process was for the economic recovery
of byproduct fluorides and the realization of a return on investment.
In the original report on gypsum pond emissions (November, 1974), the
potential of the Swift Process as a means of controlling volatile
fluorides emanating from these ponds was discussed. Since that date
the original inventor, Parrish and his associate, Enriquez, have in
fact recommended this process as a pollution control strategy when
coupled with a two pond system. At least one phosphoric acid plant
presented this process as its solution to potential fluoride emissions
resulting from once-through river water for its process water require-
ments.
Table 6.4 indicates the level of fluoride recovery possible by the
installation of the Swift Process. Of particular interest is the fact
that fluorides entering the cooling pond are reduced by approximately
90 percent. Bearing in mind the fact that the gypsum pond is not
effected and that it comprises only about 5 wet acres, it is the author's
opinion that fluoride emissions should be reduced by 90 percent. The
basis for this conclusion comes from the work of Tatera, who correlated
fluoride concentrations in pond water with gaseous emissions.
6.2.3 Process Economics
A summary of capital costs is presented in Table 6.5. The primary pieces
of equipment are three sets of recirculation tanks, pumps, and absorbers.
174
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Table 6.4. Fluorine Material Balance in a 1,000 TPD P205 Plant
Utilizing the Swift Vapor Absorption Process.
With Swift Without Swift
In Phosphate Rock 10,000 Ib/hr 10,000 Ib/hr
Recovered
As 25% H2SiF6 3,855 Ib/hr 0
Not Recovered
Stays in gypsum 4,500 Ib/hr 4,500 Ib/hr
Lost at reactor 20 Ib/hr 20 Ib/hr
Stays in 52.5% P205 . 1,250 Ib/hr 1,250 Ib/hr
Lost to cooling pond 375 Ib/hr 4,230 Ib/hr
175
-------
Table 6.5. Capital Investment Requirements for the Swift Vapor
Absorption System.
Capital Investment (Installed Costs)
1. 2 Scrubbers handling 140,000 ft3/min each
0 $282,000 each $ 564,o0o
2. 1 Scrubber handling 180,000 ft3/min 362,000
3. 3 Rubber-lined recirculation tanks handling
1,050 gal each
(a $29,100 each 87,300
4. 3 Stainless steel recirculation pumps
(900 gpm @ 100 psi) $40,900 each 122,700
5. 4 Rubber-lined storage tanks 100,000
gal capacity @ $42,000 each i68tQQO
TOTAL CAPITAL INVESTMENT j 1,304,000
176
-------
The rest of the equipment shown in Figure 6.4 is required even without
the Swift Process. Storage facilities consisting of four rubber-lined
tanks are required to hold one week's supply of 25% H SiF produced.
2 6
Costs will vary greatly depending upon whether the plant is existing
or new. These costs were derived for retrofit to an existing plant.
The figures presented compare well with those quoted by a manufacturer
of $500,000 for the equipment servicing each evaporator of $1,500,000
total capital investment for three modules.
Annualized operating costs are presented in Table 6.6. Assuming no
income from the sale of fluosilicic acid, the increase in price of one
ton of P00_ would be $1.25. At the present market value of $60/ton
L 5
H SiF *, the Swift Vapor Absorption Process will operate at a profit
2 6
of $2.26/ton of PJ)_ if all fluosilicic acid produced is sold. It was
2 5
this potential profit which prompted several firms to utilize the Swift
Process on the basis of economics. As previously stated, a marketing
survey was not conducted. However, it is doubtful that all plants in the
U.S. could utilize the process and realize a profit on their venture.
Its use would have to be accompanied by a vigorous marketing effort and
possible price reduction in order to make by-product fluosilicic acid
a more attractive source of fluorides than Mexican fluorspar,
If gypsum and cooling pond are high emitters of fluorides and control
equipment is required, the Swift Process is an attractive low cost or
low profit abatement system.
*FOB Plant
177
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Table 6.6. Annualized Operating Costs for Swift Process.
Quantity
Unit Cost Annual Cost
UTILITIES
ELECTRICITY
OPERATING LABOR
OPERATING
SUPERVISION
MAINTENANCE
LABOR AND MATERIAL
SUPPLIES
OVERHEAD
PLANT
PAYROLL
FIXED COSTS
250 hp
1/2 MAN/SHIFT
15% OF OPERATING
4% OF CAPITAL INVESTMENT
15% OF LABOR AND MATERIAL
50% OPERATION & MAINTENANCE
20% OPERATING LABOR
20 mills/KWH $ 27,730
$ 5.50/HR
DEPRECIATION (5% OF CAPITAL INVESTMENT)
TAXES AND INSURANCE (2% OF CAPITAL)
CAPITAL CHARGES (10% OF CAPITAL)
TOTAL ANNUAL COST
COST PER TON OF P205
CREDIT
iai!4 TONS 100% H.SiFc
c o
ADJUSTED ANNUAL CREDIT
CREDIT PER TON P0
$ 60/TON
$ 24,000
3.600
$ 27,600
$ 52,160
7,840
$ 60,000
$ 43,800
5,520
$ 49,320
$ 65,200
26,080
130,400
$ 221,680
$ 386,330
$ 1.25
$1,086,870
$ 700,530
$2.26
178
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6.3 LIMING OF COOLING PONDS
Treatment of effluents by "double-liming", or two-stage lime neutraliza-
tion, has been practiced by wet-process phosphoric acid plants for
many years. The process simultaneously raises the pH while lowering the
concentration of soluble fluorides.
6.3.1 Process Description
Under the assumption that a two pond system will be used to segregate
gypsum sluicing water from other process water, cooling pond water will
contain fluorides and small amounts of P00C. Thus it is necessary only
f. 0
to add sufficient lime to precipitate out fluoride compounds as fluor-
spar (CaF2). Most fluorides from the scrubbers and barometric condensers
will be in the form of fluosilicic acid. Upon addition of sufficient
lime, fluorides will be precipitated according to:
H2S1F6(aq) + 3Ca°(s) + H2° 3CaF2(s) + 2H2° + S102(s) <6'8>
Holding the mixture in a quiescent area allows the particulate CaF2 to
settle.
Table 6.7 shows the reported effect that liming has on actual cooling
pond water. As this Table shows, a pH of 3.9, soluble fluorides are
only one percent of the value at a pH of 1.4. Figure 6.5 shows a graph
of fluoride vapor pressure versus pH from the data in Table 6.7. Use
of this figure allows one to calculate the potential emissions reductions
to be expected by liming the pond waters. Thus, at a pH of 3.9, the
179
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Table 6.7. Effect of Liming on Fluoride Evolution From Gypsum-Pond Water
PH
1.4
2.6
3.0
3.3
3.9
4.5
6.1
6.25
7.72
9.7
12.1
12.3
12.5
Soluble
fluoride,
ppm
8125
4000
NR
450
106
100
106
NR
NR
NR
NR
NR
16
Ca(OH)o,
Ib/gallon
0.116
0.145
0.156
0.157
0.160
0.192
0.193
0.207
0.213
0.222
0.246
0.346
Vapor pressure
of fluoride
@25°C, mmHg
13.8 x 10"6
6.22 x 10"6
NR
NR
NR
0.86 x 10"6
0.45 x 10'6
NR
NR
NR
NR
NR
NR
SOURCE: (HEW, 1970)
NR: Not Reported
180
-------
£3
3 8
V-i VO
5) O
U) •—I
2.0
1.0
• 9
.8
.7
.6
.5
w
.
PH
Figure 6."). Fluoride Vapor Pressure vs. pH (Source: HEW, 1971)
181
-------
fluoride vapor pressure obtained through interpolation of this figure
is 1.3 x 1(T6 mmHg. The expected emission reduction obtained in liming
from pH 1.4 to pH 3.9 is:
Reduction = (13.8 - 1.3)100%/13.8 = 90.6%
A pH of 3.9 can be achieved by single stage liming. Assuming that a
two pond system will be employed, it will be necessary to lime the
entire cooling pond contents from pH 1.4 to 3.9 only once and thereafter
to add sufficient lime to handle the theoretical amount of fluorides
entering the cooling pond. Thereafter the major acidic components, P 0
2 5
and H2$04, are confined to the gypsum sluicing water.
Figure 6.6 is a schematic diagram of a modern well-controlled single
stage liming system. Although not shown in this figure, a rotary kiln
for calcining limestone and the auxiliary equipment required for its
operation are included in the proposed scheme and cost estimates.
6-3.2 Fluoride Emission Reduction by Single Stage Liming
The reduction of fluoride emissions by liming is a consequence of the
reduction of fluoride vapor pressure upon the addition of lime as indi-
cated in Table 6.7. It is estimated that for a cooling pond having an
initial pH of 1.4, emissions can be reduced by 90 percent by the addition
of 0.157 pounds Ca(OH)2/gallon to the contents of the pond (Table 6.7).
182
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OUST COLLECTOR
LIME
FEEDER
FROM
BAROMETRIC
CONDENSERS
COOLING POND
Figure 6.6. Proposed Single Liming System
-------
Table 6.8 presents an overall fluoride material balance. The quantity
of fluorides entering the cooling pond is reduced by 90 percent. Since
the gypsum pond occupies only about 5 acres, and since fluorides present
in the gypsum are mostly solid CaF2> it is this study's estimate that
overall gaseous fluoride emissions can be reduced by 90 percent by liming
to a pH of 3.9 to 4.0.
Table 6.8
Fluorine Material Balance in a
1,000 TPD P20s Plant Utilizing Liming
as Fluoride Control Strategy
Basis: 10,000 Ibs/hr F
Item
Gypsum cake
Precipitated by liming
Vapors from reactor
c? c«y p^n
«/£. • \J lo r O **C
Cooling pond
With Liming
4,500
3,807
20
1,250
423
Without Liming
4,500
20
1,250
4,230
6.3.3 Process Considerations
Installation of single-liming systems at existing phosphoric acid
plants should pose few, if any, problems. Most plants already have
double-liming treatment systems for pond water overflow discharge.
184
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This study proposes use of a single-liming facility as shown in Figure
6.6. Lime requirements for a 1,000 IPS P205 plant are arrived at as
follows.
First, an initial, one-time dose of lime is required to brinq the
existing cooling pond water up to the required pH of 3.9 to affect
a 90 percent emissions reduction. Acidic constituents of the existing
pond requiring neutralization at this point are P205, l^SO^, >and ^SiFg.
The initial lime requirements are calculated based on the following
assumptions, applicable to a 1,000 TPD plant:
1. 300 acres of cooling pond waters at an average depth of 0.5 meters
(160 MM-Gal).
2. 0.157 Ibs Ca(OH)2 is required to treat each gallon (see Table 6.7)
Based on these assumptions, 9,530 tons of lime (as CaO) are initially
required to raise the pH to 3.9. Before this lime is added, the gypsum
and cooling ponds are segregated to ensure the integrity of the proposed
system.
The second phase of the liming program occurs once steady-state has been
achieved after the initial dosage has been applied. The quantities of
lime required during steady-state are determined from equation 6.8. The
lime requirements according to this equation are 1.47 Ibs CaO per Ib
fluorine (3 moles CaO/6 moles F). Based on data presented in Section 2.2
of this report, we estimate that approximately 91 Ibs F/ton PoOr will end
185
-------
up in the cooling pond waters. This includes fluorine scrubbed from the
reactor vapors and that removed in the barometric condensers. Based on
these data and assumptions, 135 Ibs CaO per ton P205 will be required
during steady-state operation (67.3 tons per day).
During the initial dosage two quantities of lime will be required—that
necessary to raise the pH and the daily steady-state requirements.
If we now assume a 40-day transient period to achieve steady-state,
the initial daily requirement will be 305 tons CaO/day (9530/40 + 67.3).
6.3.4 Process Economics
A summary of capital costs associated with liming is presented in Table
6.9. For the large quantities of lime required, it will be more econom-
ical for the plant to produce its own lime from limestone. Thus, included
in the capital costs are a kiln, limestone bins, and a scrubber for the
control of particulate emissions from the kiln. The other major expenses
are the chemical feed system as well as the initial lime requirement
which have been included into the total capital investment.
Annualized operating costs are presented in Table 6.10. The greatest
costs are those associated with fuel to calcine the limestone. Depre-
ciation and capital charges comprise approximately 33 percent of annualized
costs. Total annualized costs using this strategy will result in an
estimated cost increase of $3.13/ton
186
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Table 6.9. Capital Investment Requirements for Single Liming System.
Capital Investment (Installed Costs)
1. Lime kiln (81 x 140" with motor) $ 1,100,000
2. Lime kiln scrubber (includes fan, motor, ducting) 61,000
3. Limestone bins and conveyors 120,000
4. Chemical feed system (including slaker, pump,
storage, reaction vessel) 283,000
5. Miscellaneous (20% of equipment costs) 95,400
6. Initial lime requirements (9,530 tons
@ $47/ton) 447,910
TOTAL CAPITAL INVESTMENT $ 2,107,310
187
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Table 6.10. Annualized Operating Costs for Single Liming.
Quantity
Unit Cost Annual Cost
RAW MATERIALS
LIMESTONE
120 TPD
$ 4/TON $ 149,000
UTILITIES
ELECTRICTY
FUEL
OPERATING LABOR
OPERATING
SUPERVISION
50 hp
3.1 mm-BTU/Ton CaCO,
1 MAN/SHIFT
15% OPERATING
20 mills/KWH
$2/mm-BTU
$ 5.50/HR
$ 5,070
230,600
$ 235,670
$ 48,200
7.200
$ 55,400
MAINTENANCE
LABOR AND MATERIAL (L&M)
SUPPLIES
4% OF TCI
15% OF L & M
$ 81,624
11.884
$ 93,508
OVERHEAD
PLANT
PAYROLL
50% LABOR & MAINTENANCE
20% OPERATING LABOR
$ 74,454
11.080
85,534
FIXED COSTS
DEPRECIATION (5% OF TCI)
TAXES AND INSURANCE (2% OF TCI)
CAPITAL CHARGES (10% OF TCI)
TOTAL ANNUAL COST
$ 102,030
40,812
204.060
$ 346,902
$ 971.084
ANNUAL LIMING COST
$/TON CaO
$/TON P0
$ 46.62
$ 3.13
188
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6.4 GYPSUM TRANSPORT BY CONVEYOR BELT
Transporting the gypsum by-product to storage piles using dry conveyor
belt rather than slurry pipeline could possibly reduce the fluoride
emission from this segment of P205 production. The benefits (i.e.,
emissions reductions) are not as apparent as the annual costs are cal-
culable. This is because the gypsum filter cake contains some phosphoric
acid and between 18-35 percent free moisture. Thus the pH of this water will
most likely be low and the fluoride vapor pressure high. If it were pos-
sible to keep these solids dry once the free water evaporated, the emissions
would be negligible due to the very small vapor pressure of most solids
at ambient temperatures. However, operation of a gypsum stack does not
readily accomodate covering the stack to reduce emissions either with
dirt or some other impervious barrier.
Although several plants in Belgium are reported to be using a belt conveyor,
the authors have knowledge of only one dry-belt system in the U.S., in
California. Apparently it is economically justified in those cases where
the gypsum can be marketed for agricultural and other uses.
Capital investment requirements for a one-mile by 24-inch covered con-
veyor are presented in Table 6.11. These costs were estimated assuming
4.5 tons gypsum per ton of P205 with 25 percent free moisture included
(Slack, 1968). A six year life was assumed based on vendor estimates
of a useful life of 8 to 10 million tons transported.
189
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Table 6.11. Capital Investment Requirements for Dry Gypsum Conveyor.
Capital Costs
Belt conveyor—one mile @ $250/ft $ 1,056,000
Feeders including motors (2 each) 10,000
TOTAL CAPITAL INVESTMENT $ 1,066,000
Annualized Operating Costs
Depreciation (1°P_ = 16.7%) $ 178,000
b
Capital charges (10%) 106,600
Operating costs @ $ 0.08/ton-mile (vendor estimate) 148,800
TOTAL ANNUALIZED COSTS $ 433,400
$/TON P0 PRODUCED $ 1-40
190
-------
Annualized operating costs, also presented in Table 6.11, of approximately
$1.40 per ton of P205 were estimated. Although other measures would most
likely need incorporation for this system to be effective, the additional
costs were not estimated. These measures might include some form of
covering material on the dry gypsum stack and water treatment facilities
for surface runoff and seepage.
Although approximately 45 percent of the original fluorine in the rock
is retained in the gypsum, the potential emissions from this source are
most likely the smallest when the gypsum arid cooling ponds are separated.
This should be true even with commonly used slurry transport techniques
due to the small area requirements for the gypsum piles.
6.5 PRETREATMENT OF ORE BY CALCINING
6.5.1 Process Description
Precalcining the phosphate rock prior to accidulation with sulfuric acid
was considered as one candidate control process. Calcination involves
heating the rock in a rotary kiln or fluid bed reactor. Although direct
and indirect firing may be used, the latter method has some disadvantages
as discussed below. As currently practiced in producing animal feeds,
the rock is calcined by direct firing methods.
Defluorination of phosphate rock occurs in two regimes. In the first,
low-temperature regime, up to 66 percent of the fluorine can be removed
by heating the rock to approximately 2,000°F. Heating the rock to its
191
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fusion temperature, which ranges from 2,500°F to 2,700°F volatizes another
21 to 24 percent which increases the fluorine removal to approximately
90 percent. The prevalent gaseous species involved is HF, although SiF^
is present in smaller amounts, with the ratio HF:SiF4 increasing with
increasing temperature.
The off-gases from this reaction must be cooled and scrubbed to remove
particulates and fluorides before the gases are vented to the atmosphere.
Herein lies one of the chief disadvantages to this approach. First, the
gas volumes handled with direct firing are large (approximately 500 ACFM/
ton P205 at 2,000°F) and second, the fluorides scrubbed from the gases
must be disposed of, generally in the scrubber water cooling pond. As
currently practiced, cooling pond water is used to scrub the gases and is
neutralized before returning to the cooling pond, Thus, indirect-firing
only removes the fluorine from the rock to have it placed in the pond.
Indirect-firing in a rotary kiln reduces the quantity of off-gases that
must be handled. However, the temperatures that are obtained by this
method are limited to 1,200°F for stainless steels; use of special metals
will increase this to 2,000°F, but at a great additional expense.
As proposed herein, two direct-fired rotary kilns will treat 3,000 TPD
of phosphate rock. Prior to scrubbing, the hot (ca. 2,000°F) off-gases
will be cooled to 500°F by waste heat boilers which will produce approx-
imately 300,000 Ibs/hr low pressure (150 psi) steam. This last provision
will improve the economics somewhat by providing a steam credit.
192
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6.5.2 Process Economics
Capital investment and annualized operating costs are presented in Table
6.12. As this Table shows, the capital required to install kilns to treat
3,000 TPD of rock is roughly $30 million, nearly twice the investment of
the P20g plant itself. This is without any fluorine byproduct recovery.
Since scrubbing the gases only produces a one to two percent solution
of H-SiFg, additional equipment would be needed to concentrate this to
the 25 percent, which is the normal saleable concentration.
6.6 HEMI/PIHYDRATE PROCESS
The hemihydrate-dihydrate process for making wet-process phosphoric acid
has been in use in Japan for several years. Its primary advantages are
the higher overall yield of ?2®5 ancl ^e production of high quality
gypsum, suitable for gypsum plaster and gypsum boards. About 40 percent
of fluorides present in the rock are volatilized in the acidiculation
stages and may be recovered as fluosilicic acid.
6.6.1 Process Description
Although there are several variations of this process, they basically
differ from the common dihydrate process in that hemihydrate is formed
during decomposition of the phosphate rock and is subsequently recrystallized
as dihvdrate before filtration. The decomposition and recrystallization
steps are carried out in separate reaction vessels to allow optimum process
control. The resulting dihydrate crystals, as compared with those made in
the common dihydrate process, are larger and retain less P20e and other
impurities in the lattice.
193
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Table 6.12. Capital Investment Requirements for Pre-Calcinin§ of
Phosphate Ores.
Capital Costs
Rotary kilns: 2 - 16.5' x 600' direct fired
(installed costs, mid-1975) $ 28,400,000
Waste-heat boiler (2) 700,000
Gas-scrubbers (2) 710,000
TOTAL CAPITAL INVESTMENT $ 29,810,000
Annualized Operating Costs
Fuel $ 1,060,000
Electricity (1,400 hp @ $ 0.20/KWH) 175,000
Depreciation (10% TCI) 2,981,000
Maintenance (5% TCI) 1,490,500
Capital Charges (10% TCI) 2,981,000
TOTAL ANNUALIZED COSTS $ 8,687,500
Steam credit (300,000 Ibs/hr @ $ 0.50/M-lb) 1,116,000
NET ANNUAL OPERATING COSTS $ 7,571,500
$/TON P205 PRODUCED $ 24.42
194
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The key to the process is growing large, easily filterable crystals at
a rate fast enough to give economic operation, reducing the phosphate
substitution in the process. The diyhydrate is thus crystallized under
quiescent, carefully controlled conditions, thereby making it possible
to maintain a uniform excess of sulfuric acid throughout the slurry.
6.6.2 Effect of Hemi/Dihydrate Process on Fluoride Emissions
There is virtually no data available to allow a judgment to be made on
any fluoride emissions reduction which would result from employing the
Hemi/Diiiydr'ate process.
In order to make an estimate of potential fluoride emissions reductions,
the following will be assumed:
1. 45 percent of the fluorides initially present in the rock will remain
in the by-product gypsum.
2. 12 1/2 percent of the fluorides will be present in the product acid.
3. 0.2 percent of the fluorides will be released to the atmosphere.
4. 42.3 percent (or the remainder) of the fluorides will either be
recovered or end up in the cooling pond.
5. 30 to 40 percent of the fluorides are recoverable as fluosilicic-
acid (as claimed by the manufacturer).
Based on the above, it is estimated that 70 to 95 percent of the fluorides
which could contribute to cooling pond emissions can be recovered as by-
product H2SiF6.
195
-------
The common dihydrate process is based on a rather violent reaction which
produces local deficiencies of sulfuric acid, and thus leaves p205 in the
dihydrate lattice. As a result of the better control in the hemihydrate-
dihydrate process, loss of phosphate by lattice substitution is very low.
Recovery is thus improved and the by-product gypsum is more suitable for
use in building products.
Figure 6.7 presents a flow diagram of the Fisons Ltd. scheme for wet
process phosphoric acid manufacture. The notable differences with this
scheme from those employing dihydrate routes are that:
1. No evaporators are utilized since a 50 percent P205 product is obtained
in the reactors.
2. A much higher amount, which is estimated to be 30 to 40 percent of
all fluorides initially present in the rock, is evolved in the reac-
tion/filtration step. These vapors are sufficiently rich in fluorides
to allow the production of fluosilicic acid as a saleable by-product.
6.6.3 Process Economics
Since the hemihydrate-dihydrate process is an alternative to the exist-
ing U.S. wet phosphoric acid processes, the economics cannot be evaluated
in the same manner as the previous "add-on" control systems. In order to
use the hemihydrate-dihydrate process, the existing plant must be exten-
sively changed and markets found for the gypsum and fluosilicic acid
produced. The success of the process in Japan rests largely on the facts
that Japan must import all phosphate rock and most of its gypsum. Thus
196
-------
Cooling
Water
Process water
Process water
Reactor «l
Reactor«2
Filter Feed Tank
i Effluent
--' pumping Tcnk
50% PgOgproducTocid
Figure 6.7. Hani/Dihydrate Wet Process for Phosphoric Acid Manufacture.
Fisons Ltd. Scheme.
-------
the small added yield of PoOc in combination with the production of a
useful gypsum makes it most attractive to the Japanese.
In the United States, these factors are not present, and the relatively
low cost phosphate rock and gypsum do not create the favorable economics
here as in Japan. By way of comparison, however, a cost comparison
between the dihydrate and the hemihydrate-dihydrate process was develop-
ed by Fisons Ltd. for a European plant using Morocco phosphate rock and
built in 1971. This analysis, which is summarized in Table 6.13, indicates
the process is competitive for a new plant. Capital costs are also included
for a 1,000 TPD plant built in 1975.
The Hemi/Dihydrate routes are presently not practiced in this country.
Reasons given include:
1. Florida rock is not of sufficiently high quality to allow its use.
2. Problems are encountered in the filtration operation when using Florida
rock.
3. The relatively large amount of clay fines and other impurities make
this process difficult to control.
It is important to point out that, in fact, several Japanese firms use
Florida rock successfully. Sometimes calcining is practiced to improve
the quality of the rock. However, it is foreign firms which have the
experience in Hemi/Dihydrate acid production and no doubt hold many
198
-------
Table £.13. Comparative Process Economics*
(Basis: European plant (built In 1971) using Morocco rock to make 50,000 metric tons of P205/year as SOX phosphoric acid)
<£>
V£>
Extraction efficiency. X
(based on Measured cake losses)
Operating costs/ton
Phosphate rock, tons
Sulfuric Ic1d (100X), tons
Low-pressure stream, tons
Fresh Mater, tons
Cooling water, tons
Electricity, kwh
CAPITAL COSTS, $
Depreciation, Interest, taxes
and Insurance (201)
Maintenance (7X)
Operating .labor, men/shift
OVERALL PRODUCTION COST. I/METRIC TON
Typical D
-------
patents and rights associated with these schemes, It is therefore very
difficult at the present time to calculate cost comparisons for such plants
when built by American firms in' the United States.
200
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7.0 DEMONSTRATION COSTS
In this section demonstration costs for two items are considered.
First, a methodology for determining fluoride emissions from gypsum
ponds is discussed; costs for the proposed method are then pre-
sented. In the second portion of this section demonstration costs
are presented for determining the effectiveness of the control
method selected by this study as the most cost-effective candidate.
In summary, the costs for these items are:
Field verification of emission rate (2 ponds) $ 70,925
Demonstration of Swift system (1 pond)
a) pilot plant $295,760
b) modification of existing plant $274,475
As pointed out in section 7.2.2, the last item listed could be in
error by approximately $25,000. This is due primarily to the uncer-
tainties in estimating costs for pond segregation without site-
specific data.
7.1 VERIFICATION OF PREDICTED FLUORIDE EMISSIONS
Based on our findings concerning the emissions of fluoride from gypsum
ponds, it was concluded that no investigator had as yet established
experimentally the fluoride emission rate from gypsum ponds. While
King has conducted experiments in which ambient fluoride concentrations
were measured, he did not directly measure the emission rate. Even
201
-------
though his predicted concentrations correlated quite well with his
measured values, the mathematical dispersion model used in his predic-
tions is not exact and, as pointed out in Section 5.0 of this report,
might tend to overestimate.
For the above reasons, the following experimental program is proposed
which would 1) directly measure fluoride emissions as a function of
velocity and 2) verify the ambient ground-level fluoride concentrations
predicted in section 4.3 of this report.
Fluoride emissions would be measured as follows. All fluorides emit-
ted from the pond must pass through an imaginary vertical plane pass-
ing through the downwind dike of a gypsum cooling pond and extending the
length of the pond. If, at any height z^ above the dike, the fluoride
concentration and wind velocity are known, the mass flux at that point
can be determined. Mathematically stated:
E = VWAZC
where: W = width of pond
AZ = difference in height
C = fluoride concentration
By determining the flux at several heights above the dike, the total,
mass of fluorides passing through the plane is:
l (7.1,
where: C = 0
202
-------
One possible method for measuring concentrations at various heights
would be to construct towers at each end of the gypsum pond. A remote
long path infrared sensor could then establish the average concentra-
tion over the total length of the pond at different heights. The
concentration profile obtained from this operation could then be used,
together with the velocity profile, to calculate the fluoride emission
rate.
The above approach presents some practical problems that might be hard
to overcome. The biggest problem would be alignment of the remote IR
source with the detector. As the system got progressively higher up
the towers, the wind would most likely make it difficult, if not im-
possible, to keep the units aligned.
One advantage to the above approach is that by using a dispersive infra-
red detector, one could determine which fluoride compounds were being
emitted.
i
Another approach that could be taken to determine the emissions
is to install ambient fluoride samplers at different heights on
the towers. However, it is suggested that the towers now be located
at points equidistant from the ends of the pond (1/3L and 2/3L).
Figure 7.1 shows a theoretical concentration profile predicted by the
finite line source dispersion model discussed in Section 5.3 of this
report. As the curves in this figure indicate, the ambient concen-
tration 30 meters above ground level should be about ten and twenty-
five percent of ground-level concentrations for stability class four
203
-------
Height (m)
•o
fD
O
(D
O
o
fD
Stability Class 4
Stability Class 2
Figure 7.1 Theoretical Ambient Concentration Profile as a
Vunction of Height
-------
and two respectively. Thus, a 100 foot tower with five ambient samp-
lers equally spaced should be sufficient to establish the concentra-
tion profile.
Velocity profiles can be established with three cup-anemometers placed
at different heights, e.g., 2, 8 and 16 meters. Recorded wind speeds
can be used to establish the value of the power low exponent in equa-
tion 4.33 for each experiment conducted.
Once the concentration and velocity profiles are determined, the emis-
sion rate is calculated through equation 7.2.
To further verify King's and Tatera's equations for fluoride emission
rates, the following data should be collected for each experiment.
Ten centimeter wind velocity (Tatera), average temperature for the
pond water in each line source (Tatera and King), average liquid
fluoride concentration in each line source (vapor pressure) and am-
bient temperature and cloud cover (for stability).
To establish the accuracy of the ambient fluoride concentrations pre-
dicted in Section 5.0 of this report, a network of ambient samplers
should be placed downwind of the pond, e.g,at 100, 200 and 300 meters.
One sampler should also be placed upwind to establish the background
fluoride concentration.
7.1.1 Sampling Methods
It is important that an appropriate sampling method be used that will
ensure results that are as accurate as the state-of-the-art permits.
" 205
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For several years the State of Florida Department of Pollution Control
has been monitoring ambient fluoride levels in the vicinity of the
phosphate mining and processing area near Lakeland, Florida. The
sampling method used by this agency consists of a series of midget
impingers containing either sodium hydroxide or distilled water. This
method is under question, however, as one investigator (Sholtes, 1973)
found that in the concentration range of 10-100 ppm gaseous fluorides,
the collection efficiency decreased in direct proportion to the gas
concentration. Collection efficiencies as low as 30 percent were
measured in this range.
The ambient sampler used and evaluated by King was also tested by the
EPA (Baumgardner, CPL). This sampler consisted of a 15 foot length
of 6MM I.D. coiled glass tube coated with Na2C03. Ambient air is
drawn through this tube at approximately 30 LPM for one hour at which
time the glass tube is flushed with a buffer solution. The fluoride
content of the buffered Na2C03 solution can then be determined with
a specific ion electrode.
Evaluation of a commercial model of the above sampler by the EPA
indicated unusually large response times when the unit was calibrated
using an 8 foot length of small bore teflon tubing placed upstream
of the glass tubing. King reported a collection efficiency of 85 per-
cent for a 15 foot length of glass tubing and 95 percent for two 15
foot lengths connected in series which should be sufficient for the
purposes of the experiments proposed herein.
206
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7.1.2 Experimental Design
In order to gather sufficient data to verify the fluoride emission
estimates of King and of Tatera, it is proposed that two ponds be
tested, preferably at two different plants. Ponds of roughly rectangu-
lar shape should be utilized in order to minimize the number of as-
sumptions made in determining the fluoride flux.
Two telescoping meteorological towers, installed as shown in Figure 7.2
will house the ambient samplers and meteorological equipment. Prior
to performing each experiment, it will be necessary to measure the
pond water temperature at a minimum of two points in each line source,
as shown in Figure 7.2. One pond water sample from each line source
should be obtained for fluoride determination.
Each experiment should be approximately one hour long as longer time
periods might allow for larger variations in wind speed and direction.
This is important since the ground level wind direction should be approx-
imately normal to the finite line sources.
In order to verify ground level concentrations arid to determine the
fluoride species emitted from the ponds, it is proposed that a long
path remote sensing infrared detector be placed on top of the dike.
As the total path length of this instrument will be approximately
300-400 meters (pond width ), accurate determination will be possible.
7.1.3 Program Costs
In order to evaluate the fluoride emissions as a function of wind
velocity, it will be necessary to measure the emission rate at several
207
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•— o
0) •—
OJ
<_)
O)
A
A
A
A
A
C
* ' !* M4 m
D
A
A
A
A
A
E
C -
Ml /O *J
L/3 M
D
D
KEY: A - Liquid Temperature Measurement
B
C
D
E
- Upwind Ambient F, Sampler
Meteorological Tower
Downwind F^ Sampler
Remote Infrared Sensor
Figure 7.2 Ambient and Pond Emission Sampling Network - Aerial View.
208
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different velocities. As there will be no control over this variable,
it is anticipated that approximately one month will be needed for each
pond tested. Utilizing four men in the field, this will amount to
about 640 man-hours field testing for each pond.
Table 7.1 gives a breakdown of the estimated costs and man-hours for
a two-month field sampling program on the above basis (two ponds
tested).
7-2 VERIFICATION OF THE SWIFT VAPOR-ABSORPTION SYSTEM
Several experimental approaches were considered that would allow an actual
demonstration of fluoride emissions reduction upon the cooling pond. In
selecting an approach, the primary criterion considered useful in demon-
strating emissions reductions is the soluble fluoride level in the cooling
pond waters. As shown in Figure 6.5 of this report, the correlation
between pH, soluble fluorides and fluoride vapor pressure is such that
one can use this parameter in judging emissions reductions.
Two approaches at demonstration are considered. The first approach
involves construction of a pilot plant including acid attack system,
scrubbers, evaporators, gypsum and cooling ponds. This would conven-
iently be done near an existing plant where small quantities of steam,
electricity, and water can be obtained. The second approach would
involve segregation of the gypsum and cooling ponds at an existing
facility equipped with Swift vapor absorbers. After an equilibration
period the soluble fluoride content of the cooling water will be measured
209
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ro
o
Table 7.1 Cost Breakdown for Fluoride Emissions Determination from Two Cooling Ponds
Man-Hours
Engineer Technician
(=> S30/hr
-------
over a time period. When the fluoride content has reached a steady state
value, the system will be operated in that mode (i.e., Swift scrubbers
either on or off) until sufficient data has been gathered to allow con-
clusions to be drawn concerning operation in that mode. In the following
pages each approach will be discussed at length and program costs pre-
sented.
7.2.1. Pilot Plant Approach
This study estimates that a small pilot plant, approximately one ton
P2°5 Per day, should be sufficient to obtain results for demonstration
purposes. All unit-processes contained in a full-scale (500-1000 TPD)
facility should be incorporated with the exception of a sulfuric acid
plant. Sulfuric acid will be purchased in bulk for the experimental
program. A 10,000 ft cooling pond will be used to provide cooling for
the barometric condenser waters. This pond will be monitored daily for
fluorides. After approximately three months operation using the Swift
absorbers (and producing 20 percent H2SiF6), the absorbers will be turned
off and acid production stopped. The system will then be operated for
another three month period as before. Now the pond waters should be-
come more acidic and the fluoride levels increase. Comparison of the
data from each mode will then allow conclusions to be drawn concerning
the effectiveness of this system in reducing fluoride emissions.
Program costs for this approach are presented in Table 7.2 These costs
were arrived at by the following methods:
- Pilot plant construction costs, sans the sulfuric acid plant, were
estimated using a size exponent of 0.6 as suggested by Guthrie (1970)
211
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Table 7.2 Pilot Plant Program Costs for the Swift Vapor Absorption System
I. Plane Investment (installed 1975)
$150,000
II. Six Month Operating Costs
Quantity
Raw Materials
Phosphate rock
SuIfuric acid
Utilities
Electricity
Steam
550 tons
440 tons
720KWH/ton P205
4,0001b/ton
Pilot Plant Operation
Technician 3,120
Engineers 520
Unit Cost
$35/ton
$50/ton
$0.02/KWH
$0.50/M-lb
$20/hour
$30/hour
Laboratory Analyses
Fluoride
PH
Rock Samples 400 @ $7/sample
Water Samples 800 @ $5/sample
800 @ $3/sample
Data Analyses and Interpretation
Chemist4~0
Engineer/Scientist 80
Reports and Project Management
Engineer 200
$25/hour
$30/hour
$30/hour
Total Cost
$ 19,250
$ 22.000
$ 41,250
$ 2,640
$ 370
$ 3,020
$ 62,400
$ 15.600
$ 78,000
$ 2,800
$ 4,000
$ 2.400
I9,200
$ 1,000
$ 2,400
$ 3,400
$ 6,000
III. Miscellaneous Expenditures
A. Travel (2 man-trips P $250 each)
B. Telephone
C. Reports (60 copies @ 300 pages each)
D. Computer
E. Housing subsidence for on-site engineers
$ 500
$ 100
$ 900
$ 500
$ 2,000
IV, TOTAL Estimated Program Costs
$295,760
212
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Table 7.3 Estimated Costs for Demonstration of Swift Process at a
500 TPD - P205 Facility
I. Phase 1 Costs - Plant Selection
A. Labor (80 engineer hours @ $30/hour) $ 2 400
B. Non-labor (travel for site selection) $ 'SQQ
$ 2,900
II. Phase 2 Costs - Plant Modification
A. Engineering and supervision (200 hours) $ 6,000
D. Non-labor (travel and communication) $ 1,000
C. Subcontractor costs (dredging, dozing, piping, etc.) $25,000
D. H2SiF6 storage tanks (for additional $ 20,000
product storage one 50,000 gallon $ 52,000
rubber-lined tank)
III. Phase 3 Costs - Operation of Swift System
A. Labor
1) Engineering and supervision (260 hours) $ 7,800
2) Chemist (200 hours) (F", P?0c, etc.) $ 5 QOO
B. Non-labor ° ,
1) Travel (4,000 miles @ $0.20/mile) $ 800
2) Per diem (15 man-days @ $30/day) $ 450
3) Communications $
$ 12,350
IV. Phase 4 Costs - Operation without Swift System
A. Labor
1) Engineering (260 hours) $ 7 800
2) Chemist (200 hours for F~, P90R, etc.) $ 5*000
B. Non-labor c °
1) Subsidy for loss of H2SiFc to cooling
pond (25 TPD of 100% H2$iF6): 75% @ $60/ton $174,375
2) Travel (4,000 miles @ $OT20/mile) $ 800
3 Per diem (15 man-days @ $30/day) $ 450
4) Communications $ 100
$186,725
V. Phase 5 costs - Data Interpretation and Report Preparation
A. Labor
1) Engineer (160 hours @ $30/hour) $ 4 800
2) Chemist (80 hours 0 $25/hour) | 2'ooo
B. Non-labor * ^'UUU
1) Reproduction (60 copies - 300 pages @ $0.05/copy) $ QQQ
$ 7,700
Estimated Project Cost $274 4?5
213
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- Raw materials and utilities requirements were estimated using
guidelines presented by Slack (1968).
As shown, total program costs are estimated to be approximately $295.760
for a six month program.
7.2.2 Conversion of Existing Facility
The other approach considered in the report for demonstrating control
feasibility is as follows. An existing P205 plant, equipped with Swift
vapor absorbers and currently producing fluosilicic acid will be modi-
fied by completely segregating its pond systems. This will insure that
fluorine contained in the gypsum does not find its way to the cooling
pond waters. The plant will then operate in two modes. In the first,
fluosilicic acid will be produced 100 percent of the time the plant is
in operation. At present, our best estimates indicate the Swift absorbers
at most plants are not in operation at all times. This will probably
require purchase of additional storage facilities to handle the additional
fluosilicic acid produced during this phase of the experimental program.
Assuming this additional recovered fluosilicic acid requires five day
storage, one 50,000 gallon tank will be required. Once the facilities
are prepared, the plant will be operated in this mode for approximately
six months.
During the second operational phase of this program, the Swift absorbers
will not be operated. Instead, all of the unrecovered fluorine will be
condensed in the barometric condensers and go to the cooling pond. This
214
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should cause the fluorine content of the cooling waters to increase.
This increase will be measured until the steady-state concentration
Plateau is reached. At that time the plant will be run for an additional
period (approximately one month) to allow data collection and analyses.
During this phase it will be necessary to subsidize the P205 company
for loss of valuable HgSIFg. The current price of which is $60/ton on a
100 percent basis. For a 500 TPD - P205 plant, this will amount to approxi-
mately 25 TPD - H2SiF6 (100 percent).
Program costs for this approach are summarized in Table 7.3. The
most uncertain figure in this table is that required to separate the
two ponds (i.e. gypsum and cooling). For our purposes it is assumed
that approximately $25,000 will be required to affect separation.
However, the uncertainty in this figure is unknown but possibly large.
215
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BIBLIOGRAPHY
Atkin, Sydney, et .aU, Ind. Eng. Chem.. 53, 705 (1961).
Bjerrum, J.G. Schwarzenbach, and Sillen, L.G., Stability Constants of
Metal-Ion Complexes, with Solubility Products of Inorganic
bubstances7~Cnem.Soc.(London), Spc.Publ.No. 7 (1958).
Brosheer, J.C.; Lenfesty, F.A.; Elmore, Kelly L., Ind. and Eng. Chem.,
39, 423 (1947). a
Buslaeu, V.A and Gustyakoya, M.P. Zh. Neorqa. Khlm. 10, 1524 (-1965) In
John S. Judge, J. Electrochem. Soc., 118. 177? (1971).
Chilton, T.H. and Col burn, A.P., Ind. Eng. Chem., 26:1183 (1934).
Considine, D.M., Chemical and Process Technology Encyclopedia,
McGraw-Hill Co., New York, 1974.
Crosby, N.T., J. Appl. Chem.. 19, 100 (1969).
Cross, F.L. and Ross, R.W., J. Air Pollution Control Assoc.. JJJ, No. 1.
Dahlgren, Sven-Eric, "Chemistry of Wet Process Phosphoric Acid
Manufacture", Phosphoric Acid, edited by A.V. Slack, Vol I, Part I.
pp. 91-156,
Development Document for Effluent Limitations Guidelines and New Source
Performance Standards for the Phosphorus Derived Chemicals Segments
of the Phosphate Manufacturing Point Source Category.
Development Document for Interim Final Effluent Limitations Guidelines
and Proposed New Source Performance Standards for the Other Non-
Fertilizer Phosphate Chemicals. Segment of the Phosphate
Manufacturing Point Source Category, EPA sp. EPA 440/1-75/043,
Jan. 1975.
Development Document for Proposed Effluent Limitations Guidelines and New
Source Performance Standards for the Basic Fertilizer Chemicals
Segment of the Fertilizer Manufacturing Point Source Category, EPA,
sp. EPA 440/1-73-011, Nov. 1973.
Economic Analysis of Proposed Effluent Guidelines, EPA sp.
EPA 230-1-74-043, Sept. 1374.
EPA, EPA-440/l-74-006-a, Jan. 1974.
ESE, Inc., Personal Communication from J.C. Kutt to Jack Sosebee (1975).
216
-------
Rossotti' F'J'C-> J- Inorq- Nuclear Chem. 26, 1959
Forster, J.H. "Direct Contact Evaporation", Phosphoric Acid, edited by
A.V. Slack. Vol. I, Part II. pp. 579-607. -
Fox, E.J., Stinson, J.M., and Tarbutton, G., Superphosphate. U.S. Dept.
of Agriculture and Tenn. Valley Authority. 1964. Chapter 10.
P • uO / •
Getsinger, J.G., "Hemihydrate by the Foam Process", Phosphoric Acid
edited by A.V. Slack. Vol. I, Part I. p. 369. - -
Guthrie, K.M., "Capital and Operating Costs for 54 Chemical Programs "
Chem Eng.. June 14, 1979. 140-156.
Harbe<*» E-G'« Jr*' USGS Prof. Paper 272-E, U.S. Government Printing
Office, Washington D.C., 1962.
Hein, L.B., "Removal of Impurities (from Phosphoric Acid)," Phosphoric
Ac_[d, edited by A.V. Slack. Vol. I, Part II, pp. 687-708. -
Huffstutler, K.K., "Pollution Problems in Phosphoric Acid Production "
Phosphoric Acid, edited by A.V. Slack. Vol. I, Part II
pp. 727-737. '
Inorganic Fertilizer and Phosphate Mining Industries Water Pollution and
Control, EPA, 12020 FPD 09/71. Sept. 1971.
Judge, John S., J. Electrochem. Soc.. 118, 1772 (1971).
Kern Edward F. and Jones, T.R., Trans. Am. Electrochem. Soc. 49.
t / «3
King, W.R., and Farrell , J.K., Fluoride Emissions from Phosphoric Acid
Gypsum Ponds. EPA sp. EPA bb(J-^-M-(J9b. October,
Legal, C.C., and Myrick, O.D., "History and Status of Phosphoric Acid."
Phosphoric Acid, edited by A.V. Slack. Vol. I, Part I. pp. 14,
«it , 4U«
Long, Harold, Personal communication, Feb. 1975.
Lutz, W.A. and Pratt, D.J., "Principles of Design and Operation"
Phosphoric Add, edited by A.V. Slack. Vol. I Part I '
pp. 159-212.
Mesmer, R.E., and Baes, C.F., Jr., Inorg. Chem, 8, 6(1969).
Munter^Paul A.^Aepll. Otto T., Kossatz, Ruth A.. Ind. and Eng. Chem..
217
-------
Munter, Paul A., Aepli, Otto T., Kossatz, Ruth A., Ind. and Eng.
Chem.. 41, 1504 (1949). a~
Murakarrri, K., Hari, S., "Hemihydrate-Dihydrate Processes in Japan,"
Phosphoric Acid, edited by A.V. Slack. Vol. I, Part I. p. 287.
Pasquill, F. Atmospheric Diffusion. John Wiley & Sons, New York, 1974.
Sanders M.D., "Recovery of Fluorides as By-Products," Phosphoric Acid.
edited by A.V. Slack. Vol. I, Part II. pp. 765-778.
Schlichting. H. Boundary Layer Theory. McGraw-Hill, New York. (1955).
Lnapter 21. '
Shindo Kazuo S. Harada Y., "NKK Process," Phosphoric Acid. Edited by
A.V. Slack, Vol. I, Part I. pp. 328, 329^—^
Shireye. R.N. Chemical Process Industries. (16). McGraw-Hill Co., New
iorK, iyb/.
Sho1t"' £-s:> — T^' "Evaluation and Modification of Fluoride Sampling
and AnalyticaTllethods." EPA-650/2-73-007 (October, 1973).
Slm0nYork*U954)" -1uor1ne Chemi'strv- Vols. I-III, Academic Press, New
Acid, Vol. I, Part I,
Stumn, Werner, and Morgan, James J., Aquatic Chemistry.
Wiley-Interscience, New York (1970J:: - -
Takeuchi H. Tayama, I., "Mitsubishi Process," Phosphoric Acid. Edited
by A.V. Slack, Vol. I, Part I. p. 301. - -
* v£a.ramf.ters Which Inf1"e"ce Fluoride Emissions from Gypsum
Pn.u. Dissertation, University of Honda (1970). - -
Teller, A.J. and Reeve, David, "Scrubbing of Gaseous Effluent."
Phosphoric Acid. Edited by A.V. Slack, Vol. I, Part II, pp. 741-778.
Thiesenhusen, H., Gesundh-Ing. 53, 113-19, 1930.
' : Workbogk, ?f ...r^.on..smes- '-
h Service Publication, No. y99-AP-26, May 1970.
U.S. Department of Health, Education and Welfare, Atmospheric Emissions
from Wet-Process Phosphoric Acid Manufacture. National AIV -
Pollution control Administration, Raleigh, N.C. (1970).
Yasuda, T., Miyamoto, M. , "Nissan Process." Phosphoric Acid. Edited bv
A.V. Slack, Vol. I, Part I. pp. 307, 30?: — ^ - y
218
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TECHNICAL REPORT DATA .
(Ptvasc read Instructions on the reverse before completing)_
RtPORT NO.
EPA-600/2-78-124
2,
4. TITLE AND SUBTITLE Evaluation of Emissions and Control
Techniques for Reducing Fluoride Emissions from
Gypsum Ponds in the Phosphoric Acid Industry
'IEN
iCCES
REPORT DATE
June 1978
. PERFORMING ORGANIZATION CODE
7. AUTHOH(S)
A. A. Linero and R. A. Baker
. PERFORMING ORGAN
9. PERFORMING ORGANIZATION NAME AND ADDRESS
Environmental Science and Engineering, Inc.
P.O. Box 13454
Gainesville, Florida 32604
10. PROGRAM ELEMENT NO.
1AB604
11. CONTRACT/GRANT NO.
68-02-1330, Task 3
12. SPONSORING AGENCY NAME AND ADDRF.SS
EPA, Office of Research and Development
Industrial Environmental Research Laboratory
Research Triangle Park, NC 27711
13. TYPE OF REPORT AND
Task; 12/73-1/75
PERIOD COVERED
14. SPONSORING AGENCY CODE
EPA/600/13
,9. SUPPLEMENTARY NOTES IERL-RTP project officer Edward L. Wooldridge is no longer witn
EPA. For details contact R.A. Venezia, Mail Drop 62, 919/541-2547.
16. ABSTRACT
repor|. g^es results of a study of gaseous emissions from gypsum
disposal and cooling water ponds to determine their potential as sources of airborne
fluorides from the manufacture of phosphoric acid. A model of the chemistry within
the pond environment was developed. Previous emission rate studies were evaluated
with respect to assumptions, methodologies, and conclusions: none provided a suf-
ficient basis for determining a fluoride emission factor. The ponds, found in major
phosphate production areas (e.g. , Florida, North Carolina, and the Western U.S.),
usually cover several hundred acres. Process water enters a pond at over 100 F,
and leaves at lower temperatures after evaporative cooling. Ambient concentrations
near a typical pond were calculated by predict! ve modeling methods , assuming
emission rates of 0.1, 1, and 10 Ib per acre per day. At the higher rates, control
of fluoride emissions appears necessary, based on TLV criteria adjusted for the
general population. An analysis of possible control methods indicated that liming
is too expensive an alternative. Capital costs are too high for the complete Kidde
process. The most promising method appears to be the Swift process for fluoro-
silicic acid recovery, coupled with segregation of the cooling and gypsum pond
waters. The hemi/dihydrate process also appears to be promising. _ __
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.IDENTIFIERS/OPEN ENDED TERMS
Pollution
Gypsum
Ponds
Phosphoric Acids
Industrial Processes
Fluorides
Mathematical Models
Calcium Oxides
c. COSATI Held/Group
Pollution Control
Stationary Sources
Gypsum Ponds
Liming
Fluorosilicic Acid
Kidde Process
Swift Process
13B
08G
08H
07B
13H
12A
13. DISTRIBUTION STATEMENT
Unlimited
19. SECURITY CLASS (This:Report)
Unclassified
21. NO. OF P,
228
20. SECURITY CLASS (This page)
Unclassified
22. PRICE
EPA Form 2220-1 (9-73J
219
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