EPA-440ll-74-019-d
Development Document for Effluent Limitations Guidelines
and New Source Performance Standards for the
PRIMARY ALUMINUM SMELTING
Subcategory of the
Aluminum Segment of the
Nonferrous Metals Manufacturing
Point Source Category
MARCH 1974
•& U.S. ENVIRONMENTAL PROTECTION AGENCY
^
\ ^\l/^ ° Wasliington, D.C. 20460
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DEVELOPMENT DOCUMENT
for
EFFLUENT LIMITATIONS GUIDELINES
and
NEW SOURCE PERFORMANCE STANDARDS
for the
PRIMARY ALUMINUM SMELTING
SUBCATEGORY
of the
ALUMINUM SEGMENT
of the
NONFERROUS METALS MANUFACTURING
POINT SOURCE CATEGORY
Russell E. Train
Administrator
Roger Strelow
Acting Assistant Administrator for Air & Water Programs
Allen Cywin
Director, Effluent Guidelines Division
George S. Thompson, Jr.
Project Officer
March, 1974
Effluent Guidelines Division
Office of Air and Water Programs
U.S. Environmental Protection Agency
Washington, D.C. 20460
For sale by the Superintendent ol Documents, U.S. Government Printing Office, Washington, D.C. 2M02 • Price $1.80
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ABSTRACT
This document presents the findings of an extensive study of the
primary aluminum industry by the Environmental Protection Agency
for the purpose of developing effluent limitations guidelines and
standards of performance for the industry to implement Sections
304, 306, and 307 of the Federal Water Pollution Control Act, as
amended.
Effluent limitations guidelines contained herein for the primary
aluminum industry set forth the degree of effluent reduction
attainable through the application of the best practicable
control technology currently available and the degree of effluent
reduction attainable through the application of the best
available technology economically achievable which must be
achieved by existing point sources by July 1, 1977, and July 1,
1983, respectively. The standards of performance for new sources
contained herein set forth the degree of effluent reduction which
is achievable through the application of the best available
demonstrated control technology, processes, operating methods, or
other alternatives.
The data and recommendations developed in this document relate to
the production of primary aluminum by the electrolysis of
alumina. Water from wet scrubbers operated to control air
pollution is the major source of contaminated waste water from
this industry. Treatment of this water to precipitate fluorides
and to decrease the concentration of suspended solids and to
allow recycle of the treated water to the scrubbers represents
the best practicable control technology currently available for
existing point sources. Further lime treatment of bleed streams
and filtrates from such practice constitutes the best available
technology economically achievable. Alternate technologies for
achieving the limitations are available to some plants in
conversion from wet scrubbing to dry scrubbing or in total
impoundment of waste water. The best available demonstrated
control technology, processes, operating methods, or other
alternatives consists of dry scrubbing of potline air and the
control and treatment of other fluoride containing waste streams
by recycle and treatment of any necessary bleed stream by lime
precipitation.
Supportive data and rationale for development of the effluent
limitations guidelines and standards of performance are contained
in this document.
111
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CONTENTS
Section Page
I CONCLUSIONS 1
II RECOMMENDATIONS 3
Best Practicable Control Technology Currently
Available 3
Best Available Technology Economically
Achievable 4
New Source Performance Standards 4
III INTRODUCTION 7
Purpose and Authority 7
Summary of Methods Used for Development of the
Effluent Limitation Guidelines and Standards
of Performance 7
General Description of the Primary Aluminum
Industry 10
General Features of the Primary Aluminum
Facility 10
IV INDUSTRY CATEGORIZATION 15
Introduction 15
Objectives of Categorization 15
An Overview of the Interrelationship of Anode
Type, Process Technology, Air Pollution
Control, and Water Pollution Control 15
Aluminum Reduction Process Description 22
Water Usage in the Primary Aluminum Industry 29
Industry Categorization 30
V WASTE CHARACTERIZATION 37
Introduction 37
Sources of Waste Water 37
Effluent Loadings 40
Source of Waste Water from Developmental
Aluminum Reduction Processes 65
VI SELECTION OF POLLUTANT PARAMETERS 67
Selected Parameters 67
Rationale for the Selection of Pollutant
Parameters 67
Rationale for the Rejection of
Pollutant Parameters 70
VII CONTROL AND TREATMENT TECHNOLOGY 77
-Introduction 77
Control Technology 77
Treatment Technology g6
Summary of Waste Treatment Effectiveness 96
Control and Treatment Options qo
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VIII COSTS, ENERGY, AND NONWATER QUALITY ASPECTS 101
Introduction 101
Basis for Cost Estimation 101
Economics of Present Control Practice 102
Economics of Present Treatment Practice 105
Cost Effectiveness (Present Practice) 106
Costs of Additional Treatment Processes 109
Nonwater Quality Aspects 113
IX BEST PRACTICABLE CONTROL TECHNOLOGY CURRENTLY
AVAILABLE — EFFLUENT LIMITATIONS GUIDELINES 117
Introduction 117
Effluent Limitations 117
Identification of Best Practicable Control
Technology Currently Available 119
X BEST AVAILABLE TECHNOLOGY ECONOMICALLY
ACHIEVABLE -- EFFLUENT LIMITATIONS GUIDELINES 123
Introduction 123
Effluent Limitations 123
Identification of Best Available
Technology Economically Achievable. 124
XI NEW SOURCE PERFORMANCE STANDARDS 127
Introduction 127
Standards of Performance for New
Sources , 127
Rationale for Standards of Performance 128
Identification of Best Available Demonstrated
Control Technology, Processes, Operating
Methods, or Other Alternatives 129
XII ACKNOWLEDGEMENTS 131
XIII REFERENCES 133
XIV GLOSSARY 135
VI
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TABLES
Number
1 Summary of Features of Plants Visited 9
2 Matrix of the Characteristics of Primary
Aluminum Plants 31
3 Effluent Loading, kg pollutant/metric ton Al
(Ib pollutant/1,000 Ib Al) 42
4 Quantities of Selected Constituents in Water
Effluent from Selected Primary Aluminum Plants
in the U.S. 43
UA1-UK Concentrations of Selected Constituents
in Influent and Effluent Water, Primary Aluminum 44-62
5 Summary of Present and Potential Control and
Treatment Technologies 78
6 Effluent Levels Achieved by Various Treatment
Processes 97
7 Cost Data for Control and Treatment of Waste
Waters from Primary Aluminum Production 103
8 Costs of Various Alternatives for Fluoride
Removal 112
9 Energy Requirements and Solid Waste Production
for Various Water Effluent Control and Treatment
Technologies 115
10 Conversion Table 143
Vll
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FIGURES
Number
1 Locations of Aluminum Reduction Plants ll
2 Schematic Drawing of a Prebaked Anode Cell , 17
3 Schematic Drawing of a Horizontal Stud Soderberg
Aluminum Reduction Cell , 18
4 Schematic Drawing of a Vertical Stud Soderberg
Aluminum Reduction Cell 19
5 Process Diagram for the Electrolytic Production
of Aluminum 23
6 Schematic Composite Flow Diagram for Plants Using Wet
Scrubbing 39
7 Correlation of Plant Data on Suspended Solids,
Oil and Grease, and Fluoride Emissions 54
8 Diagram of Dry Gas-Scrubbing Process Elements 80
9 Process Schematic Recycle System for Fluoride
Removal 83
10 Process Schematic of Once-Through System for
Fluoride Removal 88
11 Flowsheet of Process to Remove Fluorides From
Waste Streams (Recycle Water Treatment) 89
12 Schematic Diagram of a Process to Remove
Fluoride by Alum Precipitation 91
13 Process to Remove Fluoride by Adsorption on
Activated Alumina 93
14 Reverse Osmosis Treatment of Fluoride Waste
water
15 Some Control and Treatment Options
17 Cost Effectiveness of Water Control and
Treatment to Remove Fluoride (Operating Cost)
.JS
99
16 Cost Effectiveness of Water Control and
Treatment to Remove Fluoride (Capital Cost) ,Q_
108
viii
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SECTION I
CONCLUSIONS
For the purpose of establishing effluent limitations guidelines
and standards of performance, the aluminum segment of the
nonferrous metals manufacturing point source category was divided
into three subcategories. This report deals with the primary
aluminum smelting subcategory.
Primary aluminum smelting is a single subcategory for the purpose
of establishing effluent limitations guidelines and standards of
performance. The consideration of other factors such as age and
size of the plant, processes employed, geographical location,
wastes generated, and waste water treatment and control
techniques employed, support this conclusion. The similarities
of the wastes produced by primary aluminum smelting operations
and the control techniques required to reduce the discharge of
pollutants further substantiate the treatment of primary aluminum
smelting as a single subcategory. However, guidelines for the
application of the effluent limitations and standards of
performance to specific facilities do take into account the
production level of the smelting facility.
Approximately one-third of the 31 primary aluminum plants are
currently operating with discharge levels of pollutants within
the July 1, 1977, effluent limitations contained herein. It is
concluded that the remainder of the industry can achieve these
levels by July 1, 1977, by the application of the best
practicable control technology currently available. Those plants
not presently achieving the July 1, 1977, limitations would
require an estimated capital investment of about $10/annual
metric ton ($9/annual short ton) and an increased operating cost
of about $4.6/metric ton ($4.2/short ton) in order to accomplish
the desired decrease in discharge of pollutants. It is estimated
that a further investment of $3.8/annual metric ton ($3.5/annual
short ton) and an additional operating cost of $1.13/metric ton
($l/short ton) would be required to decrease the discharge of
pollutants from the July 1, 1977, level to the July 1, 1983,
level.
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SECTION II
RECOMMENDATIONS
Best_Practicable_Control_Technologx
Currently Available
The effluent limitations for the primary aluminum smelting
subcategory to be achieved by July 1, 1977, and attainable
through the application of the best practicable control
technology currently available, are as follows:
Effluent Limitations
Effluent Average of daily
Characteristic Maximum for values for 30
~ any 1 day consecutive days
shall not exceed
Metric units (kilograms per 1,000 kg
of product)
Fluoride 2.0 1.0
TSS 3.0 1.5
pH Within the range 6.0 to 9.0
English units (pounds per 1,000 Ib
of product)
Fluoride 2.0 1.0
TSS 3.0 1.5
pH Within the range 6.0 to 9.0.
The best practicable control technology currently available for
the primary aluminum smelting subcategory is the treatment of wet
scrubber water and other fluoride-containing effluents to
precipitate the fluoride, followed by settling of the precipitate
and recycling of the clarified liquor to the wet scrubbers as a
means of controlling the volume of waste water discharged. Two
precipitation methods are currently available, cryolite
precipitation and precipitation with lime. This technology
achieves attendant reduction of the discharge of suspended
solids.
Alternate technologies for achieving the effluent limitations
include dry fume scrubbing and total impoundment.
The technology and rationale supporting these effluent
limitations are presented in Sections VII and IX.
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Best Available_Technologv^
Economically,Achievable
The effluent limitations to be achieved by July 1, 1983, by
application of the best available technology economically
achievable are as follows:
Effluent Limitations
Effluent Average of daily
Characteristic Maximum for values for 30
~ any 1 day consecutive days
shall not exceed
Metric units (kilograms per 1,000 kg
of product)
Fluoride 0.1 0.05
TSS .2 .1
pfi Within the range 6.0 to 9.0.
English units (pounds per 1,000 Ib
of product)
Fluoride 0.1 0.05
TSS .2 .1
pH Within the range 6.0 to 9.0.
The application of the best practicable control technology
currently available results in a relatively low volume, high
concentration bleed stream. The best available technology
economically achievable is lime treatment of such a bleed stream
to further reduce the discharge of fluoride. This technology
also achieves further reduction of the discharge of suspended
solids.
Alternate technologies for achieving the effluent limitations
include dry fume scrubbing and total impoundment.
The technology and rationale supporting these effluent
limitations are presented in Sections VII and X.
New Source Performance gtandards
The standards of performance for new sources attainable by the
application of the best available demonstrated control
technology, processes, operating methods, or other alternatives,
are as follows:
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Effluent Limitations
Effluent Average of daily
Characteristic Maximum for values for 30
any 1 day consecutive days
shall not exceed
Metric units (kilograms per 1,000 kg
of product)
Fluoride 0.05 0.025
TSS .1 .05
pH Within the range 6.0 to 9.0.
English units (pounds per 1,000 Ib
of product)
Fluoride 0.05 0.025
TSS .1 .05
pH Within the range 6.0 to 9.0.
The best available demonstrated control technology, processes,
operating methods, or other alternatives consists of dry
scrubbing of potline air, the control and treatment of fluoride-
containing waste streams by recycle and treatment of any
necessary bleed stream by lime precipitation. The technology and
rationale supporting these standards are presented in sections
VII and XI.
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SECTION III
INTRODUCTION
Purpose and Authorjty
Section 301(b) of the Act requires the achievement by not later
than July 1, 1977, of effluent limitations for point sources,
other than publicly owned treatment works, which are based on the
application of the best practicable control technology currently
available as defined by the Administrator pursuant to Section
304(b) of the Act.
Section 301(b) also requires the acievement by not later than
July 1, 1983, of effluent limitations for point sources, other
than publicly owned treatment works, which are based on the
application of the best available technology economically
achievable which will result in reasonable further progress
toward the goal of eliminating the discharge of all pollutants,
as determined in accordance with regulations issued by the
Administrator pursuant to Section 304(b) to the Act.
Section 306 of the Act requires the achievement by new sources of
a Federal standard of performance providing for the control of
the discharge of pollutants which reflects the greatest degree of
effluent reduction which the Administrator determines to be
achievable through the application of the best available
demonstrated control technology, processes, operating methods, or
other alternatives, including, where practicable, a standard
permitting no discharge of pollutants.
Section 304 (b) of the Act requires the Administrator to publish
within one year of enactment of the Act, regulations providing
guidelines for effluent limitations setting forth the degree of
effluent reduction attainable through the application of the best
practicable control technology currently available and the degree
of effluent reduction attainable through the application of the
best control measures and practices achievable, including
treatment techniques, process and procedure innovations,
operation methods and other alternatives. The regulations
contained herein set forth effluent limitations guidelines
pursuant to Section 304 (b) of the Act for the primary aluminum
smelting subcategory of the nonferrous metals category.
Summary of Methods Used for Development of the
Effluent Limitations Guidelines and Standards of Performance
The effluent limitations guidelines and standards of performance
contained herein were developed in the following manner. An
inventory was compiled of the primary aluminum smelting industry
with respect to process details, air pollution control systems,
waste water treatment methods, and ancillary operations. This
inventory provided an overview perspective from which to assess
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•the need for subcategorization of the industry, and a base from
which to evaluate current control and treatment practices.
General information was obtained on all 31 of the primary
aluminum plants operating in this country and detailed
information was compiled for 10 plants. Information was derived
from the following sources:
0 Applications to the Corp of Engineers for Permits
to Discharge under the Refuse Act Permit Program
were obtained for 14 plants. These contained data,
in varying degrees of detail, on the composition
and volumes of intake and effluent waters, waste
water treatment (in general terms), and daily
aluminum production rates.
0 Plant visits were made to 10 sites to obtain
detailed information on control and treatment
technologies and associated costs, identification
and concentration of waste water constituents, and
discharge volumes. Each of the plants visited
submitted a completed questionnaire, together with
flow diagrams of water use. The plants visited
included those which employ exemplary waste water
control or treatment as identified through the
discharge permit applications, through consultation
with the clean water subcommittee of the Aluminum
Association, and through primary aluminum company
representatives. Other plant-visit sites were
selected to be representative of various specific
industry practices. Table 1 summarizes the
features of the plants visited.
0 General information on the remaining plants was
obtained through telephone contacts with each
company.
0 Three of the plants were revisited for sampling and
analysis in order to verify the effluent data.
These three plants were selected because their
waste water practice represents the best waste
water treatment technology in use, which is
generally available to, and practicable for, the
entire aluminum industry. The field work included
the sampling of internal streams, in addition to
the outfall, in order to develop specific
information regarding unit operations within the
plants.
The data obtained were analyzed to identify the sources and
volumes of waste water produced, and the quantities of con-
stituents contained in the discharge. On the basis of this
analysis, the constituents of waste water which should be the
subject of effluent limitations and standards of performance were
identified.
The range of control and treatment technologies practiced by the
aluminum industry was identified from the industry profile and
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TABLE 1 . SUMMARY OF FEATURES OF PLANTS VISITED
Features Number of Plants
Anode Type
Prehaked 5
Horizontal Stud Soderberg 3
Vertical Stud Soderberg 2
Air Pollution Control
Wet 7
Wet and Dry 3
Plant Age
20-30 years 6
10-20 years 2
Less than 10 years 2
Plant Capacity
Less than 100,000 tons/year 2
100,000-200,000 tons/year 5
More than 200,000 tons/year 3
Scrubber Water Treatment
Cryolite precipitation with recycle 6
Lime precipitation with recycle 1
Lime precipitation - once through 2
None 1
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from the plant visits. In .addition, other technologies
applicable as primary aluminum plant waste water control and
treatment were identified. For each of the control or. treatment
technologies, the resultant effluent levels of waste water
constituents were determined and the limitations and problems
associated with each technology were identified. The nonwater
quality aspects of each technology were evaluated., Such aspects
include energy requirements , other types of pollution generated,
and the 'cost of application. From this information base, the
various alternatives available to the industry for reducing
pollutant discharges were identified.
All of the information thus developed was evaluated in order to
determine what levels of technology constitutes the best
practicable control technology currently available, the best
available technology economically achievable, and the best
available demonstrated control technology, processes, operating
methods, or other alternatives.
General Description of the Primary
Aluminum^Industrv
This document presents effluent limitation guidelines and
standards of performance for the primary aluminum smelting
industry, standard industrial classification (SIC) 333U. The
primary aluminum process is defined as the reduction of purified
aluminum oxide (alumina) to produce aluminum metal. A detailed
process description is presented in section IV of this document.
The large scale, economic production of primary aluminum became
possible when, in 1886, Charles Martin Hall and Paul Heroult
independently invented the electrolytic process. The Hall-
Heroult process has remained essentially unchanged since its
inception, except for equipment design modifications and
improvements in operating practice, and is employed in all
commercial United States production of primary aluminum. The
industry has developed rather recently, with the oldest plants
having been built in the early 1940's.
There are 31 aluminum reduction plants in the United States with
a total annual capacity of about U., 500, 000 metric tons (5,000,000
short tons) with about 60 percent of that capacity provided by
the three largest companies. The geographical distribution of
aluminum reduction plants is shown in Figure 1. The availability
of inexpensive electrical power is a major consideration in site
selection and accounts for the concentration of plants in the
Pacific Northwest and in the Tennessee Valley. The energy
consumed annually at full production is estimated to be in the
range of 80 to 100 billion kilowatt hours.
An overview oif the primary aluminum facility is presented in the
following paragraphs.
10
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MIDDLE...
...ATUANTIC......
WEST NORTH CENTRAL
EAST NORTH CENTRAL
A
WEST SOUTH--CENTRAL
Annual Capacities
+ - 0 to 100,000 tons
o - 101,000 to 150,000 tons
0 - 151,000 to 200,000 tons
A - >200,000 tons
Figure
1. Locations of aluminum reduction plants.
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The reduction of alumina to produce aluminum metal is carried out
in electrolytic cells, or pots, connected in series to form a
potline. The facility containing a number of potlines is
referred to as the potroom. The electrolysis takes place in a
molten bath composed principally of cryolite, a double fluoride
of sodium and aluminum. Alumina is added to the^ bath
periodically. As electrolysis proceeds, aluminum is deposited at
the cathode and oxygen is evolved at the carbon anode. The
oxygen reacts with the carbon to produce a mixture of CO and CO2,
and the anode is consumed.
*
Two methods of replacing the anodes are practiced. These are
referred to as the prebaked anode (intermittent replacement) and
the Soderberg anode (continuous replacement). For either system,
the anode preparation begins in the anode paste plant, where
petroleum coke and pitch are hot blended. For prebaked anodes,
the anode paste is pressed in molds, and the anodes are baked in
the anode bake plant. The baked anodes are used to replace
consumed anodes, and the anode butts are returned to the anode
preparation area. In the Soderberg anode system, the anode paste
is not baked initially, but is fed continuously, in the form of
briquettes, through a shell into the pot. As the paste
approaches the hot bath, the paste is baked in place to forjn the
anode. Soderberg anodes are supported in the sleeves by vertical
studs or by horizontal studs.
The continuous evolution of gaseous reaction products from the
aluminum reduction cell yields a large' volume of fume.
Ventilation systems are used to remove the fume from the potroom.
The ventilation air must be scrubbed to minimize air pollution;
both dry and wet scrubbing methods are used for this purpose.
Water from wet scrubbers, used for air pollution control on
potroom ventilation air, is the major source of waste water in
the primary aluminum industry.
The liquid aluminum produced is tapped periodically, and the
metal is cast in a separate casthouse facility. The molten metal
is degassed before casting by bubbling chlorine or a mixed gas
through the melt. The chlorine degassing procedure produces a
fume which must be scrubbed for air pollution control.
A few aluminum smelters have metal fabrication facilities, such
as rod mills, rolling mills, etc., on the primary reduction plant
site. Such metal fabrication operations are to be covered under
separate effluent limitations, and therefore, are not covered by
the effluent limitations derived in this document.
The cathode of the aluminum reduction cell is a carbon liner on
which the pool of molten aluminum rests. A service life of two
to three years is common. During service the cathode becomes
impregnated with bath materials and erodes, and is periodically
replaced. Water contacting spent cathodes has a significant
fluoride content due to leaching action. Spent cathodes are
either processed to recover fluoride values or retained in a
12
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storage area. Run-off from such storage areas is contaminated
with fluoride.
The potential sources of waste water from primary aluminum
smelting include: 1) wet scrubbers used on potline and potroom
ventilation air, on anode bake furnace flue gas, and on casthouse
gases; 2) cooling water used in casting, rectifiers and
fabrication, and 3) boiler blowdown. The effluent limitations
and standards of performance developed herein apply to all of the
waste water streams except those from aluminum fabrication and
boiler blowdown.
13
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SECTION IV
INDUSTRY CATEGORIZATION
Introduction
An overview of the interrelationships of several significant
factors, which could justify further categorization of the
primary aluminum industry, is presented in this section. A
detailed description of the aluminum reduction process is then
presented; the water uses and waste water sources are identified.
Finally, the rationale is developed for considering primary
aluminum smelting as a single subcategory for the purpose of
establishing effluent limitations and standards of performance.
Objectives of Categorization
The primary purpose of industry categorization is to allow the
development of quantitative effluent limitations and standards of
performance, which are uniformly applicable to a specific
subcategory. A number of factors have been considered as
potential bases for subcategorization. These factors were
examined to determine their effects on the quality or quantity of
waste water produced, on the feasibility of waste water
treatment, on the resulting effluent reduction, and on the cost
of treatment. After evaluating these factors, a determination
was made that the primary aluminum smelting subcategory should
not be further subdivided for the purpose of establishing
effluent limitations guidelines and standards of performance.
An Qyeryiew_Qf the InterrelationshipT of Anode^ Tvjgex
~ ~Progess Technologyt Air Pollution Control,
and_Water Pollution Control
In the development of effluent limitation guidelines for the
primary aluminum industry, consideration was given to the
interrelationships of the factors given above. The following
discussion is concerned with the various ways in which the
primary aluminum smelters have approached environmental control.
The purpose of this overview is to identify major factors;
details of various subjects are given in subsequent sections of
this document.
The specific factors which were considered are:
Anode Type
Prebake
Horizontal Stud Soderberg
Vertical Stud Soderberg
Air Pollution Control Method
15
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Hooding
Gas Cleaning
Dry Scrubbing
Wet Scrubbing
Once-through Water
Recycle Water
Anode Bake Furnace Gas (Prebake Anode Only)
Wet Scrubbing
Electrostatic Precipitators
Anode_Ty_2§
The mechanics of various anode types have been discussed in other
portions of this document and in the literature with the
significant differences as indicated in Figures 2, 3, and 4. The
principal advantage of the Soderberg type of cell is the absence
of a requirement for an anode baking furnace.
The factors of electrode type most pertinent to this study are
those related to air pollution control and include the efficiency
with which cells using the various anode types may be hooded, the
nature of emissions to the air associated with each anode type,
and the air pollution control devices applicable to each. Water
is not used directly in any of the types of anodes.
The major effect of differences in anode type on water usage and
streams are that for prebake anode plants, cell emissions (e.g.,
fluorides, SOx, COx, etc.) are separate from anode bake plant
emissions (e.g., tars and oils, etc.). In Soderberg-type
operations, all of these substances are emitted from the cell
area. Current practices with regard to control (and water usage)
are discussed below.
Hooding
The efficiency of hooding of cells is a factor which determines
the air pollution control measures required. in general, the
results of current practice are that if properly operated hoods
are sufficiently tight and efficient, air pollution control
devices may need to be applied only to primary pot gas to meet
atmospheric emissions standards. This gas may be characterized
as containing relatively high concentrations of pollutants and is
suitable for treatment by either dry or wet gas cleaning devices.
If hooding is of lower efficiency, emissions standards may
necessitate the treatment of pot room or secondary air. This air
may be characterized as containing relatively dilute
concentrations of pollutants, and the only practicable treatment
at this time, is by wet gas cleaning devices. • '
Dry Scrubbing
16
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Alumina Hopper
Molten Cryolite
Segmented Doors
Handle
Alumina
^ TO Primary
Control System
andle
\\\\\\\\x\\\\\\\\
Molten Aluminum
figure 2. Schematic drawing of prebaked anode cell.
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Alumina Hopper
oo
Carbon Anode
Alumina
To Primary
Control System
Hood Door
Anode Studs
Molten Aluminum
Molten Cryolite
Figure 3. Schematic drawing of a horizontal stud Soderberg aluminum reduction cell.
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vo
Carbon Anode
Skirt
Exposed
Cell Surface
Molten Cryolite
Molten Aluminum
Anode Studs
To Primary
Control System
Burner
Gas and
Tar Burning
Alumina
Figure 4. Schematic drawing of a vertical stud Soderberg aluminum reduction cell,
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Dry gas cleaning methods involve the use of dry alumina as an
adsorbent to remove pollutants from the pot gas. This technology
is discussed in detail elsewhere in this report as a method of
controlling (eliminating) a waste water stream. The salient
features of dry scrubbing are that the adsorbent (alumina)
subsequently is fed to the cells to be reduced to aluminum metal,
and that the recovery of fluoride values is virtually complete.
As mentioned above, dry scrubbing is applicable only to gas
streams with relatively high concentrations of pollutants (i.e.,
from cells with highly efficient hoods) .
Wet gas cleaning methods, as practiced in the industry, include
wet electrostatic precipitators, tower- type scrubbers, or spray
type scrubbers, alone or in combination, and with or without
demisting devices. All may be classed as low pressure drop
devices (i.e., 1-1C inches of water). No high energy venturi
type scrubbers are used in current practice. Wet scrubbing
devices may be applied to either relatively concentrated (pot) or
dilute (pot room) gases.
The scrubbing media are of paramount interest to this study and
may be described in terms of recirculating type systems or once-
through systems.
Anode__Bake_ Furnace Gas Scrubbers
In prebake anode plants, the anode bake furnace gases may be
controlled by electrostatic precipitators or most commonly by wet
scrubbers of the low pressure drop type. If wet scrubbers are
used, the waste waters contain tars, oils, SOx and GOx. If anode
materials are recycled from the electrolytic cells, the scrubber
waste waters will also contain fluorides.
Applications of electrostatic precipitators are relatively
limited because of hazards stemming from arcing and subsequent
burning of tars and oils in the precipitators. Gas cooling
sprays are generally applied, resulting in some waste water.
Such sprays are not designed to scrub fluorides, although some
incidental scrubbing action may occur. Hence, the dry
electrostatic precipitator is not always an adequate component to
meet fluoride air emission regulations. Baghouses are unsuited
to this purpose, because of the blinding action of the tars and
oils* Thus, wet scrubbers are in some cases the only adequate
air pollution control device for anode bake furnaces at this
time.
Current Practice
The current practices as determined during the effluent guide-
lines program are indicated by the following annotated citations
20
-------
of existing examples illustrative of the combinations of factors
under discussion:
A. (1) Plant A. Prebake Anode—totally dry scrubbing
on pot gas (zero water).
Anode Bake Plant—controlled firing.
(2) Plant C. Prebake Anode—wet scrubbing on pot
gas, once-through water; dry scrubbing
on some pot gas.
Anode Bake Plant—wet scrubbing.
(3) Plant D. Prebake Anode—wet scrubbing of secondary
air; scrubber water recycle with two stage
treatment before discharge.
Anode Bake Plant—wet scrubbing with
once-through water.
B. (1) Plant B. Vertical Stud Soderberg—wet scrubbing
of pot gas, total recycle of scrubber
water, bleed stream evaporated; dry
scrubbing planned.
C. (1) Plant J. Horizontal Stud Soderberg—wet scrubbing;
dry system on paste plant.
(2) Plant F. Horizontal Stud Soderberg—wet scrubbing
on pot gas, once-through water; dry scrub-
bing planned.
Some noteworthy factors in the above practices include further
variations of center-break and side-break technologies within the
prebake class of plants. The center-break variation, where cell
crusts are broken and alumina charged at spots along the center
of the cell, is potentially the most amenable to tight hooding
and dry scrubbing. The side-break technology is less amenable to
tight hooding, and thus may lead to a choice of wet scrubbing of
secondary air. Major emphasis is placed on the fact that the
anode configuration in side-break cells allows higher electrical
efficiency (6 kwhr/pound) relative to center-break cells (7-8
kwhr/pound).
The factor leading to the planned conversion of a vertical stud
Soderberg plant from wet scrubbing (but zero discharge of water)
to dry scrubbing was a need to meet a stack opacity standard
which was currently exceeded during pin changes.
Also, one horizontal stud Soderberg plant has a current
compliance program dependent on the installation of a dry
scrubbing system.
Summary
The major factors relating to discharge of water containing
pollutants have been identified as being dependent on industry's
approach to controlling both air pollution and water pollution.
The factors entering into the decision between wet or dry gas
cleaning systems have been shown to include cell geometry and
21
-------
electrical efficiency, air pollution standards, and/or water
pollution standards, but not to depend strictly on anode type, or
climate.
Aluminum Reductign^Prgcess_Descrip_tion
The basic elements in the electrolytic process for reducing
alumina to aluminum are shown schematically in Figure 5.
Individual plant practice may vary in specific detail from that
shown.
Raw Materials
The principal materials employed in the primary aluminum industry
include: alumina, cryolite, pitch, petroleum coke, and aluminum
fluoride. Very general approximations of the quantities of raw
materials used in the production of 1 kg (2.2 Ib) of aluminum
metal are:
2 kg (4.4 Ib) alumina
0.25 kg (0.55 Ib) pitch
C.5 kg (1.1 Ib) petroleum coke
C.C5 kg (0.11 Ib) cryolite
O.C4 kg (0.08 Ib) aluminum fluoride
C.6 kg (1.3 Ib) baked carbon •
22 kilowatt hours of electrical energy.
The Electrolytic Cell
The heart of the aluminum plant is the electrolytic cell, or pot,
which consists of a steel container lined with refractory brick
with an inner liner of carbon. The outside dimensions of the pot
may vary from 1.8x5.5 to 4.3x12.8 meters (6x18 to 14x42 feet) or
larger. Most cells are around one meter (three feet) in height.
The cells are arranged in rows, in an operating unit called a
potline, which may contain 100 to 250 cells electrically
connected in series. The electrical supply is direct current, on
the order of several hundred volts and 60,000 to 100,000 amperes.
The carbon liner on the bottom of the furnace is electrically
active and constitutes the cathode of the cell when covered with
molten aluminum. The anode of the cell is baked carbon. The
electrolyte consists of a mixture of cryolite, 80 to 85 percent
by weight, calcium fluoride, 5 to 7 percent, aluminum fluoride, 5
to 7 percent, and alumina, 2 to 8 percent. The composition of
the bath varies as electrolysis proceeds. Alumina is added to
the bath intermittently to maintain the concentration of
dissolved alumina within the desired range. The fused salt bath
usually is operated at a temperature of about 950°c.
Cells presently in use operate with current on the order of
100,000 amperes with a voltage drop across the cell of about 4.5
22
-------
Petroleum . Coke
Pitch
ANODE PASTE
HOT - BLENDING
COOLING
Soder
anode
Briqu
I Electrical Supply (Direct Current)
Alumina
"Cryolite
Calcium Fluoride
Aluminum Fluoride
Air
FUSED SALT
ELECTROLYTIC
CELL
Anthracite Pitch
MOLTEN ALUMINUM
To degassing and
casting
Aluminum (pig,
billet, ingot, rod)
Dry-Process Wet-Scrubber
Solids returned liquor to
to cell treatment
•Spent Potliners (to cryolite recovery
or disposal)
Figure 5. Process diagram for thei electrolytic production of aluminum.
23
-------
volts. The reaction in the aluminum reduction cell is not
completely understood (1), but results in the reduction of the
aluminum from the apparent trivalent state, assuming ionization
in the molten salt, to the liquid metal state at the cathode.
Oxygen, assumed present in the bath in the divalent state;
appears at the carbon anode and immediately reacts with the anode
and surrounding constituents to form a mixture of 75 percent
carbon dioxide and 25 percent carbon monoxide. This results in
the consumption of the carbon anode.
Thus, the operation of the electrolytic aluminum reduction cell
results in the continuous consumption of alumina and the carbon
anode, and the evolution of gaseous reaction products. The
aluminum is withdrawn intermittently from the bottom of the
molten bath at a rate of about 230 to 800 kilograms (500 to 1800
pounds) every 24 hours. The molten aluminum is collected in
ladles and cast into ingots or pigs as the final product of the
smelting process.
The continuous evolution of the gaseous reaction products from
the aluminum reduction cell yields a large volume of fume
consisting of carbon dioxide, carbon monoxide, as well as amounts
of volatile fluoride compounds, and sulfur oxides. A fine dust
also evolves from the cryolite, aluminum fluoride, alumina, and
carbonaceous materials used in the cell. The removal of this
fume from the working area, as well as the requirements for cell
cooling, involves extensive air quality control, which may extend
to the design of the plant building and hoods, ducts, dust
collectors, cyclones, and gas scrubbers. These dust and air
pollution control measures are outstanding characteristics of
aluminum reduction plants and account for a major use of water,
if wet gas cleaning methods are used.
Anodes
The operation of aluminum reduction cells results in the con-
tinuous consumption of anode material, about C.5 kg of anode per
kg of aluminum produced. This must be replaced either
continuously (Soderberg anodes) or intermittently (prebake
anodes). In either case, the thermal and electrical properties
of the anode are of primary importance for proper and economic
operation of a cell.
The raw materials for anodes (coke and pitch) must be prepared to
meet specifications by crushing, sizing, and blending. These
operations are conducted in the anode paste plant, which" is an
important adjunct to every aluminum smelter. The anode paste
consists typically of a mixture of high grade coke (pe-t-roleum and
pitch coke) and pitch or sometimes tar, although the latter is
seldom favored in American practice. Purity requirements"of the
aluminum product demand very low levels of ash, sulfur, alkali
and volatiles for anode raw materials. Maximum tolerance limits
vary, but maximum limits normally are below 0.7 percent ash n 7
24
-------
percent sulfur, 8 percent volatiles, 0.5 percent alkali, and 2
percent moisture.
The anode paste preparation plant involves, on the average, rough
crushing, screening, calcining, grinding, and mixing. For this
reason, extensive dust control equipment normally is included in
a paste plant. The principal difference in the paste preparation
plant for the two types of anodes used in the industry is in the
pitch handling system. Prebaked carbon anode plants utilize
pitch having a softening point in the range of 90 to 120 C (2CO
to 250 F). Soderberg anode plants can use pitches ranging from
soft (i.e., a softening point of 55 C (130 F)) to the harder
pitches used in the prebaked anodes. Two types of pitch handling
systems are used within the industry: solid pitch handling and
liquid pitch handling systems. Solid pitch is handled with
conventional conveyors, feeders, and automatic scales; this
frequently leads to considerable dust formation, which, if not
controlled, can result in air pollution problems. Liquid pitch
handling systems melt the pitch using either steam, electricity,
or high temperature heat transfer media (hydrocarbon oils,-
glyeols, or chlorinated biphenyls) for conventional transfer
using pumps and piping. Special precautions must be taken when
using liquid pitch handling systems to avoid toxic chlorinated
biphenyl vapors and ignition hazards (from hydrocarbon oils).
Prebaked Anode System. In the prebaked anode method the warm
paste is formed into anode blocks in a hydraulic press. The
anodes are baked and graphitized by a heating cycle that may, for
example, last 30 days with a maximum temperature of 1100 C (2000
F). The flue gases from the anode bake plant contain particulate
carbon, tar vapors, sulfur compounds, and the usual fuel
combustion products. Fluorine compounds also may be present
depending on anode stub recycle practice. The tars are formed
from cracking, distillation, and oxidation of the pitch used as a
binder and are composed essentially of high boiling organic
compounds. When the flue gases are treated by wet scrubbing or
electrostatic precipitation, the water effluent contains tars and
oils, sulfates, particulate matter, and in some cases, fluorides.
The baked anodes are supported in each cell by studs or rods,
which conduct the current to the molten bath. These studs are
anchored at the top of the anode by casting molten iron around
the anode cavity. Once in place, the anodes are individually
adjusted in height as they are consumed to maintain optimum
interpole separation distance.
Generally, prebaked anodes have smaller anode voltage losses than
Soderberg anodes; this , is ascribed to improved electrical
contacts between the current carrying studs and the anode in
prebaked anode systems. Overall cell voltage (including bus
losses) is lower on the average for prebaked systems than for the
Soderberg type system. This is reflected in power consumption
figures, being lower for prebaked anode systems by about 1 kwhr
per kilogram (2.2 pound) of aluminum. On the other hand, the
25
-------
manufacture of prebaked anodes requires higher initial capital
investment, as well as a higher labor demand.
Soderberg Anode Systems. In the case of the Soderberg continuous
anode, the anode paste is packed into a rectangular metal shell,
which is suspended above the electrolysis cell. in this paste,
the volume concentration of aggregate coke is of the order of 55
percent, and consists primarily of coarse and medium-size
fractions. As the anode paste descends through the anode shell,
it is gradually baked by the heat of the cell and the current
circulating through the partially baked mix. At a point
approximately 50 cm (20 inches) above the molten bath, the anode
mass becomes a fully baked, monolithic anode. The tars and oils
characteristic of anode baking are evolved at the cell, together
with the other fumes.
Two configurations presently are used in the industry to support
Soderberg anodes. One employs vertical rods or pins and is
referred to as the vertical spike (stud) Soderberg system (VSS);
the other uses horizontal pins, slanted at a slight angle, to
support the anode body and is called the horizontal spike (stud)
Soderberg system (HSS). In either system, periodic adjustment of
the position of the holding pins is required to maintain
interpole distances and adequate current efficiency. Because the
pool of molten aluminum builds up at about the same rate as the
anode is consumed, anode adjustment normally is made .in
conjunction with metal tapping operations, although more frequent
adjustments may be made to maintain a correct anode position. In
the vertical stud Soderberg modification, the pin adjustments are
made from above the cell, preventing the use of hoods directly
over the anode. In this case, the fumes escape to the pot room
air. However, a relatively tight fitting skirt surrounds the
lower zone between the anode form and the bath. The flammable
hydrocarbon compounds evolved in the final stages of baking are
sufficiently concentrated to be ignited by a burner as they,
along with the fumes from the bath, are removed from the cell to
air control ducts. In the horizontal stud Soderberg
modification, pin adjustments must be made from the side of the
cell and hooding is provided above the cell. This arrangement
allows more complete collection of cell gases, but the greater
volume of air required dilutes the hydrocarbon vapors so that
they cannot be burned satisfactorily.
Electrolyte
The electrolyte in aluminum reduction cells serves to dissolve
alumina, the raw material for aluminum reduction, and to provide
a molten bath with a melting temperature far lower than that
required to melt alumina, and low enough to prevent extensive
formation of aluminum carbides. The electrolyte must resist
chemical decomposition and must be free of oxidizing agents. Thp
primary consideration in electrolytes is, of course, to provide
an adequate medium for dissolution of alumina and subsequent
26
-------
transport of aluminum and oxygen ions to the electrodes for
reduction-oxidation reactions. In addition, fused electrolytes
should fulfill the following requirements:
o They should have a density, while in the molten
state, lower than that of molten aluminum.
o They should have adequate fluidity and low electrical
resistance at the operating temperatures.
o They should not be volatile at the operating
cell temperatures.
o They should contain no elements which will react
with aluminum and permanently impair product quality.
Natural or artificial cryolite, a double fluoride of sodium and
aluminum, meets these requirements and is universally used as the
major constituent in aluminum reduction cells. Other advantages
of cryolite are that it produces no slag or dross to be
eliminated from operating cells, and that it can be produced from
abundant and inexpensive supplies of fluorspar, aluminum
hydrates, and caustic soda.
Cryolite melts at about 1000°C. Addition of 5 to 15 percent of
alumina to cryolite lowers the melting temperature to values
below 940 C; further addition of alumina will cause rapid
increases in the melting point of the electrolyte and are to be
avoided. It is customary to add other salts to improve the
temperature, density, solubility, and resistance characteristics
of the electrolyte. For example, aluminum fluoride commonly is
added in modern practice to (a) maintain the aluminum ratio in
cryolite, (b) replace fluorine losses, (c) neutralize residual
sodium oxide present in the alumina feed, and (d) prevent sodium
contamination of the molten product. Other salts commonly used
include sodium fluoride, soda ash, fluorspar, calcium fluoride,
and, occasionally, sodium chloride. Use of these salts is a
matter of individual industrial practice and preference. In
general, these salts will affect the melting point, the
electrical conductivity, and the density of the electrolyte.
Additions of calcium fluoride, aluminum fluoride and alumina
affect the electrolyte resistivity. Consumption of these salts
varies with individual company practice, but usually ranges from
0.02 to 0.0'S kg per kg of aluminum produced. Cryolite and
alumina consumptions vary from approximately 0.03 to 0.05 kg of
cryolite per kg of aluminum metal, and about 2 kg of alumina per
kg of aluminum.
The alumina used in electrolytic cells is commonly of two types,
a semicoarse aggregate, sometimes containing agglomerated
material and called alumina sand, and a finer product called
flour alumina, which is used preferentially in Europe.
Molten electrolytes in industrial cells can be as deep as 36 cm
(1U inches), but the anode-cathode separation distance is only of
the order of 5 cm (2 inches). In normal cell operation, the
operating temperature of the bath is not sufficient to maintain
27
-------
all of the electrolyte in molten condition. This leads to the
formation of a frozen crust of cryolite at the surface of the
electrolyte which provides thermal insulation for the bath and
minimizes vaporization of bath components. The crust normally
supports a layer of alumina feed and provides a convenient method
for intermittent additions of alumina by breaking the frozen
cryolite crust.
Cathode Disposal Practice
In an operating primary cell, the pool of aluminum metal is the
cathode. This pool rests in a carbon container, formed of carbon
blocks and a rammed mix of anthracite and pitch. This carbon
container is a liner for the cast iron structure of the cell.
It is essential for purity of the product aluminum and the
structural integrity of the cell that the molten aluminum be
isolated from the iron shell. A service life of up to three
years may be attained for a properly installed liner in a well
managed cell, but an average life of between two and three years
is reported to be more common.
Upon failure of a liner, the cell is emptied, cooled, and removed
from the cell room to a working area. By mechanical drilling
and/or soaking in water, the shell is stripped of old lining
material, which may be processed through a wet cryolite facility
for recovery of fluoride values or simply set aside in a storage
yard,
Water which has contacted the spent pot-lining material, whether
it has been used deliberately in shell cleaning or it is run-off
from the storage yard, has a significant fluoride content. Such
waters ordinarily are joined with other plant streams for
treatment prior to discharge.
An estimation indicates that the accompanying solids disposal
problem is not large, amounting to about 1200 cubic meters (about
one acre foot) of liner waste per United States plant per year.
The following assumed values were used in this estimation: 4.5 x
106 metric tons per year (5 x 106 short tons per year) total US
aluminum production:
450 kg (1COO Ib) aluminum/cell/day
726 day average liner life
15 cm (6 inches) thick liner in 5.5 x 1.8 x
0.9 meter (18 x 6 x 3 foot) shell
30 plants
Ancillarv__Operations
Primary aluminum plants require various supportive activities
In addition to the cell room, anode paste plant, and anode bak<=-
28
-------
plant (in plants where prebaked anodes are employed), the primary
plant includes various air pollution control devices, a metal
casting facility, electric power generation or rectification, and
a sanitary treatment system. Some plants carry out a further
aluminum refining step; some practice rolling, drawing or other
metal fabrication operations.
Water Usage in the Primary Aluminum Industry
Primary aluminum smelters use water for sanitary purposes, boiler
feed, cooling circuits applied to d-c power equipment, metal
casting operations, fume scrubbing, furnace cooling in the anode
plant and miscellaneous equipment cooling. Cooling waters may be
circulated through a cooling tower, passed through several units
in series or used on a once-through basis, with various discharge
practices. The major contaminating use of water is in the fume
scrubbing operations (if wet systems are used) associated with
the anode plant, casting operations and the aluminum reduction
cells. Here practice varies from once- through methods to a
closed circuit with removal of constituents and recycling of
water.
The approaches to fume control include:
(1) No scrubbing
(2) Wet scrubbing using once-through water (dis-
charged with or without treatment)
(3) Wet scrubbing with recirculation of water
and reclamation of contained fluorides,
alumina, etc., often through precipitation
of cryolite
(4) The use of dry scrubbing systems using no
water, which allows reclamation of fume
components
Current economic and environmental pressures have brought much of
the industry to approaches (2), (3) and (4) above. The dry fume
scrubbing method is being installed in some of the plants
recently under construction, and has been or is being installed
to replace wet scrubbers in some of the older plants. The dry
fume scrubbing system reduces plant water requirements to
sanitary, boiler feed, and cooling needs. Thus, the potential
exists for the elimination of air pollution, a large decrease in
discharges of process-contaminated waste water, and the
reclamation and recycle of formerly wasted materials. However,
problems have existed in the application of dry scrubbing to
Soderberg potline fumes, caused by the hydrocarbons evolved
during anode baking. One company has a demonstration dry
scrubbing process on one of nine ESS potlines. The company plans
ultimately to convert to dry scrubbing at all of its plants.
Another company plans to convert its VSS plant to a dry scrubbing
process by 1975-76. The dry scrubbing of vertical stud Soderberg
cell gas is considered by the company to be a reliable measure
29
-------
since it has been demonstrated at overseas plants of U.S.
corporations.
A more detailed discussion of water usage and stream charac-
teristics is given in Section V of this document.
Industry Categorization
Industry Profile
In order to determine the role that various factors might play in
the consideration of potential subcategorization, a matrix of
basic information was compiled for . the industry. This
information, presented in Table 2, includes plant location,
production capacity, plant age, anode type, air pollution control
methods and water treatment methods. A summary of the
distribution of plants exhibiting each descriptive feature, as
determined from the information in Table 2, is given in the
following listing:
Feature No. of Plants
Current Production, metric tons/year
90,000 (100,000 T/yr), or less 6
90,000 to 180,000 (100,000 - 18
200,000 T/yr);
180,000 ( 200,000 T/yr), or more 7
Anode Type
Prebake 19
Horizontal Stud Soderberg 6
Vertical Stud Soderberg 4
Combination of:
Prebake and HSS 1
Prebake and VSS 1
Air Pollution Control Method
Primary, Potline Air
Wet Scrub, all or part 22
Dry Scrub, all or part 8
Secondary, Potroom Air
Wet Scrub 6
Anode Paste Plant
Wet Scrub 4
Dry Scrub 10
Anode Bake Plant
Wet Scrub 2
Dry Scrub 2
Casthouse
Wet Scrub 3
30
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TABLE 2. MATRIX OF THE CHARACTERISTICS OP PRIMARY ALUMINUM PLANTS
Anode Tvoe
Production
Rate 1973
Aluminum Company of America
Alcoa, Tennessee
Badin, North Carolina
Hassena, New York
Point Comfort, Texas
Rockdale, Texas
Vancouver, Washington
Warrick, Indiana
Wenatchee, Washington
Anaconda Aluminum Company
Columbia Falls, Montana
Sebree, Kentucky
Consolidated Aluminum Corp.
New Johnsonville, Tennessee
Eastalco Aluminum Company
Frederick, Maryland
Gulf Coast Aluminum Co.
Lake Charles, Louisiana
Mar tin -Marietta
The Dalles, Oregon
Goldendale, Washington
Intalco Aluminum Corp.
Ferndale, Washington
Kaiser Aluminum & Chem. Corp.
Chalmette, Louisiana
Mead, Washington
Ravenswood, West Virginia
Tacoma, Washington
National-Southwire Aluminum Co.
Hawesville, Kentucky
Noranda
New Madrid, Missouri
Ormet Corporation
Hannibal, Ohio
Revere
Scottsboro, Alabama
Reynolds
Arkadelphia, Arkansas
Jones Mills, Arkansas
Llsterhill, Alabama
Longview, Washington
Massena, New York
Corpus Chris ti, Texas
NOTE: MT - Metric ton
SI - Short ton
(a) 3 potlines have wet scrubbers,
(b) Conversion to dry scrubbing pi
(c) Cryolite filtrate treated with
(d) Demons trat'ion proprietary dry
(e) Zero discharge to navigable wa
1000 MT 1000 ST
249.5 275
90.7
113.4
158.8
249.5
204.5
158.8
163.3
109.0
127.0
77.1
31.8
240.4
235.9
149.7
150.6
163.3
63.5
217.7
101.6
61.7
113,4
- 183. -3 -
172.4
114.3
100.7
2 potlines
CaCl2.
scrubbing un
ters - plant
100
125
175
275
225
175
180
120
140
85
35
265 '
260
165
166
180 .
70
240
112
68
125
202
190
126
111 _
Soderbi
Age Vertical
23 202
9
31
23 X
21
26
21
X
10
3
2
7
22
30
16.
3
2
15
2
20
30
32/5
15
21
have dry scrubbing syfi
it on 1 of 9 potlines,
effluent is diverted
are Pre-
HorlE. Baked
801
X
X
X
X
X
X
x
X
X
X
X
X
x
x
X
X
75Z 25Z
X
X
X
X -
x
Air Pollution Control Methods
Anode Bake Anode Cast
Primary Secondary Plant Paste Plant House Other Water Effluent Treatment
Wet +
ESP Drv Hone None Baghouae
Drv . None Controlled Firing "
Dry ESP
+ Wet Scrub None Wet "
Wet None "
ESP + Wet
Scrub None None "
Drv None ESP "
Wet-DryOO None Wet flaghouse
Wet None None None
Wet Wet Wet
Wet Wet Drv Drv
Wet ESP Wet Wet
Drv Wet Wet
Wet*1* None None
Drv None None Bariums e
Wet None None Drv
Drv None None None
Wet None None
Wet Wet Dry Drv
Wet
Wet
Wet None Drv
.. .Het_
Wet None Drv
item.
to Sherwin Alumina plant for use as make-up water.
Lime - recycle
Lime - once throuzh
Recycle - no discharge
Lime - once through
None
Crvolite - Recycle (cj
Closed
None Svatem Settlina Basins
l°-Lime. 2°-None
Cryolite - Recvcle
None None
None None
None Drv Lime - Recvele Lanoon
Hone Drv
Llrae - Scrubber bleed
Crvolite - Recvcle
None
Cryolite - Recycle
Wet Crvolite - Recvcle
Cryolite - Recvcle
Cryolite - Recvrle(e)
-------
Feature No. of Plants
Treatment of Scrubber Liquor
Cryolite recovery, liquor recycle 8
Precipitation, settling of solids,
and recycle or discharge of liquor 15
Age of Plant
10 years, or less 8
10 to 25 years; 12
25 years, or more 8
Primary Aluminum Smelting as a Single Category
After review of the information compiled in Table 2, and con-
sideration of the various factors related to the application of
effluent limitations, the primary aluminum smelting industry is
considered as a single category, and effluent limitations and
standards of performance should apply uniformly.
Rationale. The conclusion that the primary aluminum industry be
considered as a single category is based upon the following
considerations: -
(1) All primary aluminum producers currently use
the Hall-Heroult process.
(2) The major difference in water use and waste
water generation lies in the use of wet or
dry potline fume scrubbers.
(3) The exemplary technologies for control and
treatment of aqueous fluoride discharges (i.e.,
precipitation of fluoride with removal of the
precipitate and recycle of the water) as
described in Section VII, can be applied to
fluoride-containing waters from any of the
sources common to primary aluminum plants.
In addition, these technologies produce a
concomitant reduction in suspended solids.
(4) Application of the identified best practicable control
technology currently available by all plants,
which use wet scrubbers, will result in a marked,
industry-wide reduction of pollutant emissions.
(5) Plants which employ dry fume scrubbing will be
able to meet the effluent limitations as established.
(6) Only about 12 percent of the aluminum plants
employ dry fume scrubbing for potline air or
anode bake plant flue gas; hence, a separate
category and separate effluent limitations
applicable to plants with dry scrubbing is not
warranted.
32
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Factors Considered in Categorization
Establishing a single subcategory for primary aluminum smelting
is based on the interrelationships among many factors. Those
factors are discussed briefly in the following paragraphs to
further set forth the rationale for considering primary aluminum
smelting as a single subcategory.
Process. All United States aluminum smelting is currently done
by the Hall-Heroult process. Since no significant modifications
are in commercial practice at this time, all discussion will
relate to this basic process.
For the future, other processes must be considered. However, in
their development, emphasis has been placed on the economics of
environmental control so that operation within effluent
guidelines can be expected.
Recently, the development of a new process was announced, in
which aluminum is produced by the electrolysis of aluminum
chloride. The process eliminates the use and consequent emission
of fluorides. Chlorine is recycled in the closed system. The
process is said to require 30 percent less energy than the Hall-
Heroult process. The company plans a 15,000 ton per year pilot
plant, which is expected to be operating in 1975 and could be
expanded to 30,000 tons per year. No performance data are
available for documentation of this development, and a technical
judgment regarding the ultimate impact of this technology cannot
be made at this time.
Development work is proceeding on a process for producing
aluminum by reduction of aluminum chloride with manganese. The
manganese chloride produced is converted to the oxide to recover
the chlorine for recycle and the oxide is then reduced and the
manganese is recycled. Advantages claimed for the process
include reduced capital and operating costs, and applicability to
many domestic aluminum-bearing minerals. A pilot plant is
planned for mid-1974 operation. No performance data are
available for this process and it has not been considered with
respect to the effluent limitations.
Anode Type. The type of anode employed by primary aluminum
smelters, prebaked, horizontal stud Soderberg, or vertical stud
Soderberg does not result in any significant differences in waste
effluent from the plant.
The air pollution control options had previously been determined,
in part, by the anode type. The option to choose dry gas
scrubbing is currently available for all three types of anode
configurations.
In those cases where the use of water is required, treatment
technology is available to achieve the limitations. Therefore,
subcategorization by anode type and/or existing air pollution
control systems is not necessary.
33
-------
Plant Size. A review of 31 aluminum reduction plants showed that
six plants have capacities of less than 90,000 metric tons
(100,000 short tons) per year, 18 plants have capacities between
90,000 and 180,000 metric tons (100,000 and 200,000 short tons)
per year, and seven plants have capacities greater than 180,000
metric tons (200,000 short tons) per year. No factors relating
to this distribution of plant size and pertaining to a given
plant's ability to achieve effluent limitations have been en-
countered. There is the possibility that economic constraints on
the smaller plants may become a significant factor. This point
must be evaluated further when the economic impact of effluent
limitations on the overall industry is considered.
Plant Age. Primary aluminum smelting is a relatively new
industry based on a single process. Therefore, the oldest plants
built in the early 1940's are electrochemically equivalent to
those built today; however, numerous modifications have been made
in process operation, which have resulted in greater production
efficiency and reduced pollutant emissions. As a result, neither
the level of constituents in effluent water nor the capability to
meet the limitations is related to plant age. Because of the
general uniformity of aluminum process technology, the
application of most environmental control methods and systems
that have been developed is dependent on factors other than age
(i.e., for the Hall process, the most recently developed unit
operations are used and these can be retrofitted independently of
plant age).
Product. Primary aluminum smelters produce aluminum metal and
various aluminum alloys. Some plants carry out an additional
refining step to produce higher purity aluminum and a few plants
also carry out rolling and wire-drawing operations. The refining
step is basically the same as the production operation and does
not represent a separate category. The fabrication operations
are to be covered under separate effluent guidelines; therefore,
fabrication is not established as a separate subcategory
Raw Materials. The basic raw material, alumina, is received in a
refined and purified form. Other raw materials which may be used
include cryolite, fluorspar, sodium fluoride, soda ash, aluminum
fluoride, and coke and pitch for anodes. Variations in raw
materials do not have a significant effect on the water treatment
methods employed; therefore, subcategories based on raw materials
are not warranted.
Plant Location. The option of selecting total impoundment of
effluent, with solar evaporation of water as a means of achieving
no process waste water pollutant discharge, is open to existing
plants in two areas of continental U.S. Plants located in south
Texas and the region east of the Cascade Mountains in Washington
Oregon, and Montana may expect water deficits of from twelve to
34
-------
thirty inches per year.(2,3) Adoption of this technology depends
on such other factors as:
(1) Local and short term rainfall-evaporation
balances.
(2) Cost and availability of land which can be
made suitable from topographic and soil
structure points of view.
(3) Nature and amounts of constituents in the
effluent.
(4) Wind stability of the dried residues .
(5) Integration of this technique with inplant
recovery of all possible reusable constituents
and water.
Since the areas where the climatic conditions are amenable to
total impoundment are limited, impoundment can not be cited as
technology available to the entire industry. A separate category
and separate effluent limitation based on geographical location
are not warranted.
Summary. The quality and quantity of waste water constituents
are similar throughout the primary aluminum industry and are not
influenced greatly by any of the factors considered above.
Likewise, the engineering feasibility of waste water treatment,
the resulting effluent reduction achievable, and the cost of the
applied controls and treatments are not significantly affected by
any of the factors considered. Therefore, the effluent
limitations may be applied uniformly to the primary aluminum
smelting industry as a single subcategory-
35
-------
SECTION V
WASTE CHARACTERIZATION
Introduction
The sources of waste water within the primary aluminum industry
are set forth in this section. The kinds and amounts of waste
water constituents are identified. The relationship between the
control and treatment technology applied and the resultant
effluent loadings is described.
Sources of Waste Water
A composite flow diagram of water use, treatment and disposal for
primary aluminum plants using wet scrubbing methods for air
pollution control is shown in Figure 6. In any specific plant,
the data will vary and a stream or unit illustrated may not
exist. Treatment of water at the source (1) depends upon the
quality required and varies from simple chlorination at well
heads for control of algae and bacteria to full clarification and
treatment of river intake water. Stream (2) is made potable and
the effluent is discharged through a sewage disposal unit.
Stream (3) is make-up water to a closed-loop cryolite recovery
stream (§) which may or may not include a potroom secondary air
scrubber. The combination of materials added during cryolite
recovery varies from plant to plant, as some cryolite recovery
systems are highly sophisticated (and proprietary) chemical
manufacturing facilities. Others are operated as byproduct
recovery or water treatment units with either disposal or
recycling of the solids. The bleed stream (9) is required to
limit the buildup of sulfates in the recovery loop. Some plants
do not practice cryolite recovery, in which case stream (9)
represents a once-through discharge. Stream (4), originating
from the casthouse furnace air scrubber is common at primary
aluminum smelters, but plans exist to eliminate the stream in
several plants by changes in degassing techniques to minimize
noxious fumes or by the installation of a dry system for
collection of alumina and occluded hydrogen chloride. Streams
(5) and (6) are not common since dry processes prevail; however,
where there is a liquid effluent, the carbon particulates are
usually settled in ponds. Segments of stream (7) are treated to
promote wetting and to inhibit corrosion and algae growth.
From this generalized picture, a number of potential sources of
waste water can be identified, including:
. o Wet scrubbers
Primary potline
Secondary potroom
Anode bake plant
Casthouse
37
-------
WATER SOURCE
1
ELECTRICAL
POWER UNIT
COOLING
J
CAST
HOUSE
COOLINR
J
ROD
MILL (
SETTLING
AGENTS
(TO
CRYOLITE
RECOVERY Oi
DISPOSAL)
CRYOLITE PRODUCT TO CELLS
ponds and/or disposal,recirculation, or impoundment.
Cling
Figure 6. Schanatic conpoBite flow diagram for plants using wet scrubbing.
In a specific plane any particular stream or unit miy not exist ss alternate
technology is applied.
-------
o Cooling water
Casting
Rectifiers
Fabrication
o Boiler blowdown
The constituents of the waste water from each of these sources
are identified in the following paragraphs.
Wet Scrubbers
Primary Potline Air Scrubbers. The wet scrubbers which collect
fumes and dust from the electrolytic cells are the source of most
of the waste water constituents from primary aluminum plants.
Carbon dioxide, carbon monoxide, and hydrogen fluoride are
generated in the overall cell reaction. In addition, cryolite
vaporized from the molten bath, sulfur oxides produced from
sulfur impurities in the anode, and dust from all materials
handled at the cell (i.e., alumina, cryolite, and fluorides of
calcium and aluminum) contribute to the scrubber liquid loading.
In those plants using a Soderberg anode system, in which the
anode paste mix is based at the cells, volatile hydrocarbons and
additional oxides of sulfur are also collected in the scrubber
liquor. The quantities of materials handled at the cells, as
well as the evolution of gas, are proportional to the quantity of
metal produced. Since the efficiency of scrubbers in receiving
water-soluble gases and dust from the primary air (that collected
from the cells) is uniformly high, above 96 percent, the quantity
of materials collected in the liquor is also proportional to the
production of aluminum metal.
Secondary^Potroom Air Scrubbers. Since some fumes and dust
escape from the cells7 some plants exhaust the potroom air from
the roof line through wet scrubbers. The constituents of this
scrubber liquor are similar to those from the primary air
scrubbers, but ordinarily constitute less than 10 percent of the
total amount. Because the large volume of air handled in
secondary scrubbers makes the capture efficiency relatively low,
most plant engineers prefer improved hooding at the cells over
secondary scrubbing of room air.
Anode Bake Plant Air Scrubbers. Primary aluminum smelters using
prebaked anodes have an associated anode bake plant. The flue
gas from the anode bake furnace is treated in wet scrubbers at
some installations. The resulting liquor contains acid, tars,
oils and sulfur oxides from the baking operation, and particulate
carbon. Fluorides may be present depending upon anode stub
recycle practice. Such a stream is not suitable for processing
through a recovery system which returns solids to the electrolyte
cells. Therefore, this stream is usually added to other effluent
streams, treated to promote settling, and diverted to ponds/ the
overflow from which is ordinarily mixed with other plant effluent
streams.
39
-------
Casthouse__Air Scrubbers. A third section of primary aluminum
plants which may employ wet scrubbing is the casthouse. Molten
aluminum from the cells is degassed by bubbling chlorine through
the melt. This batch operation is carried out in gas-fired
holding-alloying furnaces and is adjusted according to
specifications of the particular order being cast. If the off-
gas from the furnace is scrubbed, depending on the gas used, an
acidic liquor is produced containing dissolved chlorine,
chlorides, and suspended alumina. The quantities of these
constituents are quite variable depending on the extent of
degassing and time in the cycle. In one plant, degassing was
under way from one fourth to one third of the time.
Cooling^Water
Cooling water is used for aluminum casting, for electrical and
mechanical equipment, and in anode preparation. The usual
additive is chlorine for minimizing algae growth. The major
fraction of cooling water flows in closed systems. That portion
not recirculated is usually discharged without treatment.
Other Sources of^Waste_Water
In addition to the sources of waste water considered above,
general housekeeping and the manner of collection and disposal of
rain water run-off affects the total plant effluent. This
ordinarily includes the run-off from a used cathode storage or
disposal area. In addition, liquid and solid spills usually are
flushed into this system. Treatment varies widely from
reprocessing, through cryolite recovery, to simple discharge.
Effluent Loadings
The waste water from the several potential sources discussed
above usually are joined into a common plant outfall.
Quantitative waste water data were obtained from Corps of
Engineers Discharge Permit Applications and directly from a
number of companies. The original data are analytical
determinations of the concentrations of waste water constituents.
The concentrations can be converted to effluent loadings, in
kilograms of pollutant per metric ton of aluminum produced
(Ib/ton Al), by means of the following equation:
Effluent Loading = CFK/P kg/metric ton Al (Ib/ton Al), where:
C = concentration of pollutant in mg/1;
F = stream flow in cubic meters/day (gal/day);
P = production in metric tons Al/day (tons Al/day);
K = 10-3 (kg x l)/(mg x cubic meters) or 8.345 x 10-*
(Ib x I)/(ing x gal), the conversion factor required
to obtain the proper units.
40
-------
2 5 variation exists in the concentrations and flow en-
IE O_ 2red in primary aluminum plants. As an illustration of the
Q 3 5nt loadings which result from various arbitrary conditions,
o ^ :^x of flow rate versus concentration for a production rate
Q 0 p- 55 metric tons Al per day (500 tons Al per day) is given in
Q- 3 & 3, where the values are given in both metric and English
_ i ro c~
•o *-J o Data
->j ^r
K> ® ctual effluent loadings calculated from effluent concen-
Ł; n and flow data obtained for eleven companies are given in
< 4. The control and treatment technology practiced by each
^-.-.it. is as follows:
Anode
PI§DŁ Type Controller Treatment Applied
A PB Dry scrubbing
B VSS Lime/recycle
C PB Lime/once-through
D PB Cryolite/recycle
E PB Cryolite/recycle
F HSS None
G VSS Lime/once-through
H PB Cryolite/recycle
I HSS Cryolite/recycle
J HSS Cryolite/recycle
K PB Cryolite/recycle
The original data from which these effluent loadings were
calculated are presented in Table 4A through 4K. In those cases
where data were obtained for several separate discharges from a
single plant, separate tables are given for each pipe. The
effluent loading was calculated for each constituent from each
pipe and totalled to obtain the overall effluent loadings given
in Table 4.
The significance of the data given in Table 4 may be illustrated
by noting the effluent loadings for fluoride. Of the eight
plants reporting fluoride values, five (D, Hr I, J, K) are in the
range of 0.5 to 1 kg/metric ton Al (1 to 2 Ib/ton Al) and each of
the five plants practices cryolite precipitation and recycle.
Plant B uses lime precipitation with recycle to achieve 0.6
kg/metric , ton Al (1.2 Ibs/ton Al). Plants C and G use a once-
through lime precipitation and report effluent loadings of 5 to
10 kg/metric ton Al (10 to 20 Ib/ton Al) . Plant F practices no
water treatment and the effluent loading is 15 kg/metric ton Al
(30 Ib/ton Al) .
The practice of precipitating cryolite or calcium fluoride from
waste water is designed primarily to reduce fluoride emissions
and to recover fluoride values. However, the plant data show
that there is also an attendant reduction in the discharge of
41
-------
to
TABLE 3. EFFLUENT LOADING, kg pollutant/kkg Al
(Ib pollutant/ton Al)
For production rate, P = 455 kkg Al/day
(500 ton Al/day)
Flow Rate
in-Vday
(TO6 qal/day_l
37.58
(0.01)
189.3
(0.05)
378.5
(0.1)
757.1
(0.2)
1,136.
(0.3)
1,514.
(0.4)
1,893.
(0.5)
2,650.
(0.7)
3,785.
(i.o)
7,571.
(2.0)
11,360.
(3.0)
18,930.
(5.0)
26,500.
(7.0)
37,850.
(10.05
m3/min
jqal/min)
0.0261
(6.9)
0.1314
(34.7)
0.261
(69)
0.526
(139)
0.787
(208)
1.05
(278)
1.31
(347)
1.84
(486)
2.63
(694)
5.26
(1389)
7.88
(2083)
13.14
(3472)
18.40
(4861 )
26.29
(6944)
50
0.004
0.021
0.041
0.085
0.125
0.165
0.21
0.29
0.41
0.85
1.25
2.10
2.90
4.15
20
0.00165
0.0085
0.0165
0.0335
0.0500
0.065
0.085
0.115
0.165
0.335
0.50
0.85
1.15
1.65
Concentration, mg/1
10
0.00085
0.00465
0.0085
0.0165
0.025
0.0335
0.0415
0.060
0.085
0.165
0.250
0.415
0.60
0.85
5
0.00042
0.0021
0.0042
0.0085
0.0125
0.0165
0.021
0.029
0.042
0.085
0.125
0.210
0.290
0.415
2
0.00017
0.00085
0.00165
0.00335
0.005
0.0065
0.0085
0.0115
0.0165
0.0335
0.050
0.085
0.115
0,165
1
0.00009
0.00042
0.00085
0.00165
0.0025
0.0034
0.0042
0.0060
0.0085
0.0165
0.025
0.041
0.060
0.085
-------
TABLE 4. QUANTITIES OF SELECTED CONSTITUENTS IN WATER EFFLUENT FROM SELECTED PRIMARY ALUMINUM PLANTS IN THE U.S.
[kg/metric ton of Al produced (Ib/ton Al produced)]
Constituent
Alkalinity 0.01
Chemical Oxygen Demand 0.6
Total Solids 0.5
Dissolved Solids
Suspended Solids
Sulfate
Chloride
Cyanide
Fluoride
Aluminum
Calcium
Copper
Magnesium
Nickel
Sodium
Zinc
Oil and Grease
Phenol
0.45
0.55
—
~
—
—
—
—
—
—
—
0.1
—
A
N
(0.02)
(1.2)
(1.0)
(0.9)
(1-1)
—
—
—
—
—
—
—
—
—
(0.2)
—
B
C
GUJ N
3.5
—
1.1
4.2
50.3
0.6
—
—
—
—
— -
0.5
—
(7.1)
—
(2.2)
(8.4)
(100.6)
(1.2)
—
—
—
—
—
(1.0)
—
0.3
8.9
4.4
3.8
—
0.078
—
5.0
0.17
—
0.001
0.31
—
0>.47
0.015
0.45
—
(0.6)
(17.8)
(8.7)
(7.5)
—
(0.2)
—
(10.0)
(0.34)
(0.002)
(0.6)
—
(0.9)
(0.03)
(0.9)
—
D
G
1.1
0.8
10.8
10.5
0.38
4.6
1.5
—
0.34
0.03
—
—
—
2.1
0.0017
0.04
—
(2.1)
(1.7)
(21.7)
(21.0)
(0.8)
(9.2)
(2.9)
—
(0.7)
(0.07)
—
—
—
(4.3)
(0.003)
(0.08)
—
12.7
23.8
16.5
—
—
—
15.3
2.7
—
0.005
—
0.024
—
—
1.2
—
F
N(b)
(25.5)
(47.6)
(33.1)
—
(30.6)
(5.5)
—
(0.01)
—
(0.04)
—
—
(2.4)
—
-
—
9.3
14.0
—
—
9.8
1.3
3.7
0.002
0.1
0.017
1.5
0.003
0.23
0.067
G
N
-
—
(18.7)
(28.0)
—
(19.6)
(2.7)
(7.3)
(0.004)
(0.2)
(0.03)
(3.1)
-(0.007)
(0.47)
(0.13)
H
G
—
9.2
2.2
0.045
—
—
0.55
0.18
— ;
—
0.17
—
0.57
—
—
—
-
(18.5)
(4.5)
(0.09)
—
• —
(1.1)
(0.3)
—
—
(0.3)
—
(1.1)
—
—
—
2.6
0.65
16.8
10.9
5.9
1.3
3.8
0.001
0.35
0.005
1.2
0.001
0.45
0.00025
2.0
—
0.15
—
I
HU)
(5.3)
(1.3)
(33.7)
(21.9)
(11.9)
(2.6)
(7.6)
(0.002)
(0.7)
(0.01)
(2.3)
(0.002)
(0.9)
(0.0005)
(4.0)
—
(0.3)
—
J K
11.5
0.6
34.8
32.5
1.9
1.27
1.7
0.004
1.1
0.46
2.0
0.008
0.21
—
17.2
0.047
0.20
—
H
(23.)
(1.2)
(69.6)
(65.1)
(3.8)
(2.5)
(3.4)
(0.01)
(2.2)
(0.9)
(4.0)
(0.016)
(0.4)
(34.4)
(0.09)
(0.4)
—
N
1.8
1.6
0.22
5.0
0.7
0.001
0.96
0.076
—
0.0018
—
0.0016
3.3
0.0013
0.19
—
(3.5)
(3.1)
(0.4)
(9.8)
(1.4)
(0.003)
(1-9)
(0.15)
—
(0.004)
—
(0.003)
(6.7)
(0.003)
(0.4)
N = Net values. Concentration of each constituent in intake water subtracted from concentration in effluent
and the difference used to calculate values given.
G = Gross values. Data for intake water not available.
(a) Data reported as "Not to Exceed" so quantities are limits, not actually present.
(b) Does not include effluent from separate cryolite manufacturing facility operated on primary plant site.
(c) Zero discharge to navigable waters. Plant effluent is diverted to adjacent alumina refining plant for
use as make-up water.
-------
TABLE 4A1 . CONCENTRATIONS OF SELECTED CONSTITUENTS IN
INFLUENT AND EFFLUENT WATER, PRIMARY ALUMINUM
Plant A, Pipe 001, Volume 165,600 gpd
Influent Effluent Net
Concentration* Concentration * Concentration
Constituent mg/1 mg/1 mg/1
Alkalinity 23 27 4
Chemical Oxygen Demand 110 62 Neg
Total Solids 62 74 12
Dissolved Solids 41 60 19
Suspended Solids 10 11 1
Sulfate
Chloride 3.7 3.7 0
Cyanide
Fluoride
Aluminum
Calcium
Copper
Magnesium
Nickel
Sodium
Zinc 0.01 0.01 0
Oil & Grease Nil Nil 0
Phenol
* Source RAPP
44
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TABLE 4A2 . CONCENTRATIONS OF SELECTED CONSTITUENTS IN
INFLUENT AND EFFLUENT WATER, PRIMARY ALUMINUM
Plant A, Pipe 002, Volume 28,800 gpd
Influent Effluent Net
Concentration * Concentration * Concentration
Constituent mg/1 mg/1 mg/1
Alkalinity
Chemical Oxygen Demand
total Solids
Dissolved Solids
Suspended Solids
Sulfate
Chloride
Cyanide
Fluoride
Aluminum
Calcium
Copper
Magnesium
Nickel
Sodium
Zinc
Oil & Grease
Phenol
* Source RAPP
31
4
51
38
0
25
3.4
0.0
7.1
0.0
4.3
0.0
2.5
1.6
Nil
33
15
87
63
1
24
2.4
0.4
8.6
0.0003
4.3
0.020
1.8
15.4
Nil
2
11
36
25
1
Neg
Neg
0.4
1.5
-
-
0.02
N&g
13.8
—
45
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TABLE 4 A3. CONCENTRATIONS OF SELECTED CONSTITUENTS IN
INFLUENT AND EFFLUENT WATER, PRIMARY ALUMINUM
Plant A , Pipe 003 , Volume 360,000 gpd
Constituent
Alkalinity
Chemical Oxygen Demand
Total Solids
Dissolved Solids
Suspended Solids
Sulfate
Chloride
Cyanide
Fluoride
Aluminum
Calcium
Copper
Magnesium
Nickel
Sodium
Zinc
Oil & Grease
Phenol
Influent
Concentration
mg/1
3.4
Effluent
Concentration
mg/1
3.0
Net
Concentration
mg/1
31
4.1
51.
38.
0.0
31
111.
143
111
18.0
0
106.9
92
73.
18.
Neg
Nil
0.0
0.0
0.0
0.0
1.6
Nil
Nil
0.0005
0.040
0.0003
0.0002
18.
Nil
-
-
0.04
0.0003
0.0002
16.4
— ,
* Source-RAPP
46
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TABLE 4B1 . CONCENTRATIONS OF SELECTED CONSTITUENTS IN
INFLUENT AND'EFFLUENT WATER, PRIMARY ALUMINUM
Plant B, Pipe 1* , Volume 150,000 gpd
Influent Effluent Net
Concentration Concentration ** Concentration
Constituent mg/1 mg/1 mg/1
Alkalinity 100
Chemical Oxygen Demand
Total Solids
Dissolved Solids
Suspended Solids 20
Sulfate 50
Chloride 1000
Cyanide
Fluoride
Aluminum
Calcium
Copper
Magnesium
Nickel
Sodium
Zinc
Oil & Grease 10
Phenol
* Concentrations are reported "not to exceed"
** Source- Company Report
47
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TABLE 4B2 . CONCENTRATIONS OF SELECTED CONSTITUENTS IN
INFLUENT AND EFFLUENT WATER, PRIMARY ALUMINUM
PlantB , Pipe 3* , Volume 350,000 gpd
Influent Effluent Net
Concentration Concentration ** Concentration
Constituent mg/1 mg/1 mg/1
Alkalinity
Chemical Oxygen Demand 50
Total Solids
Dissolved Solids
Suspended Solids 20
Sulfate 100
Chloride 1000
Cyanide
Fluoride
Aluminum
Calcium
Copper
Magnesium
Nickel
Sodium
Zinc
Oil & Grease 10
Phenol
* Concentrations are reported "not to Exceed".
** Source-Company Report
48
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TABLE 4 B3. CONCENTRATIONS OF SELECTED CONSTITUENTS IN
INFLUENT AND EFFLUENT WATER, PRIMARY ALUMINUM
Plant B, Pipe 4* , Volume 150,OOOSPd
Influent Effluent Net
Concentration Concentration ** Concentratic
Constituent mg/1 mg/1 mg/1
Alkalinity 125
Chemical Oxygen Demand
Total Solids
Dissolved Solids
Suspended Solids 30
Sulfate 100
Chloride 1000
Cyanide
Fluoride
Aluminum
Calcium
Copper
Magnesium
Nickel
Sodium
Zinc
Oil & Grease 10
Phenol ,
* Concentrations are reported "not to exceed". See Til-if-.
** Source-Company Report
49
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TABLE 4 B4 . CONCENTRATIONS OF SELECTED CONSTITUENTS IN
INFLUENT AND EFFLUENT WATER, PRIMARY ALUMINUM
Plant B, Pipe 5* , Volume 150,000 gpd
Influent Effluent Net
Concentration Concentration ** Concentration
Constituent mg/1 mg/1 mg/1
Alkalinity
Chemical Oxygen Demand , 75
Total Solids
Dissolved Solids
Suspended Solids 20
Sulfate 50
Chloride 900
Cyanide
Fluoride
Aluminum
Calcium
Copper
Magnesium
Nickel
Sodium
Zinc
Oil & Grease 10
Phenol
* Concentrations are reported as "Not to exceed"
** Source-Company Report
50
-------
TABLE 4 C . CONCENTRATIONS OF SELECTED CONSTITUENTS IN
INFLUENT AND EFFLUENT WATER, PRIMARY ALUMINUM
Plant C , Pipe , Volume 18,800,000 gpd
Constituent
Alkalinity
Chemical Oxygen Demand
Total ,Solids
Dissolved Solids
Suspended Solids
Sulfate
Chloride
Cyanide
Fluoride
Aluminum
Calcium
Copper
Magnesium
Nickel
Sodium
Zinc
Oil & Grease
L
Phenol
Influent
Concentration *
mg/1
1.0
Effluent
Concentration *
mg/1
1.5
Net
Concentration
mg/1
50
10
95
92
3
7.5
12
152
113
27
Neg
2
57
21
24
0.5
0.16
0.033
30
0.004
0.005
0.005
1 '
0.047
0.0
32
1.13
41
0.011
0.007
0.004
4
0.146
2.9
31.8
1.1
11.
0.007
0.002
Neg
3
0.099
2.9
* Source-RAPP
51
-------
TABLE 4 D . CONCENTRATIONS OF SELECTED CONSTITUENTS IN
INFLUENT AND EFFLUENT WATER, PRIMARY ALUMINUM
Plant D, Pipe , Volume 1,220,000 gpd
Influent Effluent Net
Concentration* Concentration* Concentration
Constituent mg/1 mg/1 mg/1
Alkalinity 50
Chemical Oxygen Demand 39
Total Solids 511
Dissolved Solids 496
Suspended Solids 18
Sulfate 217
Chloride 69
Cyanide
Fluoride 16
Aluminum 1.6
Calcium
Copper
Magnesium
Nickel
Sodium 101
Zinc 0.08
Oil & Grease 2
Phenol
* Source-Company Report
52
-------
TABLE 4 Fl. CONCENTRATIONS OF SELECTED CONSTITUENTS IN
INFLUENT AND EFFLUENT WATER, PRIMARY ALUMINUM
Plant F, Pipe 2 , Volume 1,800,000 gpd
Influent Effluent Net
Concentration Concentration * Concentration
Constituent mg/1 mg/1 mg/1
Alkalinity
Chemical Oxygen Demand
Total Solids
Dissolved Solids 442
Suspended Solids 9
Sulfate 71
Chloride 78
Cyanide
Fluoride 55
Aluminum
Calcium
Copper
Magnesium
Nickel 0.07
Sodium
Zinc
Oil & Grease 5.2
Phenol
* Source-Company Report
53
-------
TABLE 4 F2. CONCENTRATIONS OF SELECTED CONSTITUENTS IN
INFLUENT AND EFFLUENT WATER, PRIMARY ALUMINUM
PlantF , Pipe 3 , Volume 17,000,000 gpd
Influent Effluent Net
Concentration Concentration* Concentration
Constituent mg/1 mg/1 mg/1
Alkalinity
Chemical Oxygen Demand 151
Total Solids
Dissolved Solids 396
Suspended Solids 116
Sulfate 117
Chloride 50
Cyanide
Fluoride 115
Aluminum 34
Calcium
Copper 0.074
Magnesium
Nickel 0.157
Sodium
Zinc
Oil & Grease 12
Phenol
* Source-Company Report
54
-------
, TABLE 4 G . CONCENTRATIONS OF SELECTED CONSTITUENTS IN
.INFLUENT AND EFFLUENT WATER, PRIMARY ALUMINUM
Plant G, Pipe , Volume, 20,000,000 gpd
Influent Effluent Net
Concentration * Concentration ** Concentration
Constituent mg/1 mg/1 mg/1
Alkalinity
Chemical Oxygen Demand
Total Solids
Dissolved Solids
Suspended Solids 2 30 28
Sulfate 20 62 42
Chloride
Cyanide
Fluoride 0.7 30 29.3
Aluminum 8 12 4
Calcium 10 21 11
Copper 0.004 0.01 0.006
Magnesium 6.7 7 0.3
Nickel 0.001 0.05 0.05
Sodium 13 17.6 4.6
Zinc 0.02 0.03 0.01
Oil & Grease 3.2 3.9 0.7
Phenol 0.001 0.2 0.199
* Source-RA^PP
** Company Report
55
-------
TABLE 4 HI. CONCENTRATIONS OF SELECTED CONSTITUENTS IN
INFLUENT AND EFFLUENT WATER, PRIMARY ALUMINUM
Plant H, Pipe 1 , Volume 430,000gpd
Influent Effluent Net
Concentration Concentration * Concentration
Constituent mg/1 mg/1 mg/1
Alkalinity
Chemical Oxygen Demand
Total Solids
Dissolved Solids 305
Suspended Solids 750
Sulfate
Chloride
Cyanide
Fluoride 24
Aluminum
Calcium
Copper
Magnesium 75
Nickel
Sodium
Zinc
Oil & Grease
Phenol
* Source-Company Report, Influent values not available.
56
-------
TABLE 4 H2 . CONCENTRATIONS OF SELECTED CONSTITUENTS IN
INFLUENT AND EFFLUENT WATER, PRIMARY ALUMINUM
Plant H, Pipe 2 , Volume 700,000 gpd
Influent Effluent Net
Concentration Concentration * Concentration
^rW Constituent :. «, mg/1 mg/1 mg/1
Alkalinity
Chemical Oxygen Demand
Total Solids
Dissolved Solids 933
Suspended Solids
Sulfate
Chloride
Cyanide
Fluoride 20
Aluminum
Calcium
Copper
Magnesium 70
Nickel
Sodium
Zinc
Oil & Grease
Phenol
* Source-Company Report. Influent Values not Available.
57
-------
TABLE 4 H3. CONCENTRATIONS OF SELECTED CONSTITUENTS IN
INFLUENT AND EFFLUENT WATER, PRIMARY ALUMINUM
Plant H, Pipe 3 , Volume 36,000 gpd
Influent Effluent Net
Concentration Concentration * , Concentration
Constituent mg/1 mg/1 mg/1
Alkalinity
Chemical Oxygen Demand
Total Solids
Dissolved Solids 7,730
Suspended Solids
Sulfate 900
Chloride
Cyanide
Fluoride 1,400
Aluminum 70
Calcium 120
Copper
Magnesium 100
Nickel
Sodium 2,500
Zinc
Oil & Grease
Phenol
* Source-Company Report. Influent Values not Available.
58
-------
TABLE 4 H4. CONCENTRATIONS OF 'SELECTED CONSTITUENTS IN
INFLUENT AND EFFLUENT WATER, PRIMARY ALUMINUM
Plant H, Pipe 4 , Volume 40Q,OOOgpd
Influent Effluent Net
Concentration Concentration * Concentration
Constituent mg/1 mg/1 mg/1
Alkalinity
Chemical Oxygen Demand
Total Solids
Dissolved Solids 920
Suspended Solids
Sulfate
Chloride
Cyanide
Fluoride 33.1
Aluminum 65
Calcium
Copper
Magnesium 60
Nickel
Sodium
Zinc
Oil & Grease
Phenol
* Source-Company Report. Influent Values not Available.
59
-------
TABLE 41. CONCENTRATIONS OF SELECTED CONSTITUENTS IN
INFLUENT AM) EFFLUENT WATER, PRIMARY ALUMINUM
Plant I, Pipe , Volume 1,720,000 gpd
Influent Effluent Net
Concentration * Concentration * Concentration
Constituent mg/1 mg/1 mg/1
Alkalinity
Chemical Oxygen Demand
Total Solids
/
Dissolved Solids
Suspended Solids
Sulfate
Chloride
Cyanide
Fluoride
Aluminum
Calcium
Copper
Magnesium
Nickel
Sodium
Zinc
Oil & Grease
Phenol
* Source-Company Report
59
2
335
322
9
24
104
0
0
0
32
0
11
0.01
52
0.04
4
174
30
1,065
797
268
80
270
0.05
15.
0.2
83
0.05
30
0.02
140
0.02
10
;-:Łii5
28
730
r 475
259
56
166
0.05
15
0.2
51
0.05
19
o.oK
88
Neg
6
60
-------
TABLE 4 J . CONCENTRATIONS OF SELECTED CONSTITUENTS IN
NV ^INFLUENT AND EFFLUENT WATER, PRIMARY ALUMINUM
Plant J, Pipe , Volume 13,700,000 gpd
: < Influent Effluent Net
., . , „, .fr.,. Concentration* Concentration** Concentration
Constituent mg/1 mg/1 mg/1
Alkalinity
Chemical Oxygen Demand
Total Solids
Dissolved Solids
Suspended Solids
Sulfate
Chloride
Cyanide
Fluoride
Aluminum
Calcium
Copper
Magnesium
Nickel
'••s
Sodium
Zinc
Oil & Grease
Phenol
* Source-RAPP
** Source-Company Report
162
6.4
278
263
15
0.1
14
0.01
0.2
0.078
44.8
0.004
12
16
0.036
0.2
270
12.
604
568
33
12
30
0.046
10.2
4.4
63.4
0.081
14
177
0.48
2.1
108
5.6
326
305 >€
v/.
18 '•''
11.9
f
16
0.036
10. *••<»
18.6
0.077
2
161
0.44
1.9
61
-------
TABLE 4 K . CONCENTRATIONS OF SELECTED CONSTITUENTS IN
INFLUENT AND EFFLUENT WATER, PRIMARY ALUMINUM
Plant K, Pipe , Vplume 3,760,000 gpd
Influent Effluent Net
Concentration * Concentration ** Concentration
Constituent mg/1 mg/1 mg/1
Alkalinity
Chemical Oxygen Demand
Total Solids
Dissolved Solids
Suspended Solids
Sulfate
Chloride
Cyanide
Fluoride
Aluminum
Calcium
Copper
Magnesium
Nickel
Sodium
Zinc
Oil & Grease
Phenol
* Source-RAPP
** Source-Company Data
120
260
230
30
5
45
0
0.6
1.5
35
0.02
8.
0
40
0.02
0
99
300
265
35
117
60.6
0.028
22
3.2
32
0.06
7.5
0.035
117
0.05
4.3
Neg
40 :
35
5
112
15.4
0.028
21.4 .
1.7
Neg
0.04
Neg
0.035
77
0.03
4.3
62
-------
suspended solids. This aspect is discussed in Section VII of
this document. This effect is shown graphically in Figure 7 in
which the effluent loading values for suspended solids from Table
H are plotted versus the fluoride effluent loading for several
plants. There is considerable scatter in the data resulting from
plant-to-plant variations in practice and from the fact that some
data represent net effluent values and others gross effluent
values. However, the correlation of suspended solids discharge
with fluoride discharge is apparent. Corresponding data for oil
and grease effluent versus fluoride effluent also is plotted in
Figure 7. Again, the correlation is apparent in spite of the
expected scatter. These data indicate that control and treatment
methods designed for the reduction of fluoride emissions result
in the reduction of suspended solids and oil and grease emissions
as well.
Verification Analysis
In order to verify the effluent loadings associated with
exemplary control and treatment practice, sampling and analysis
were carried out at three plants. The plants were selected as
exemplary representatives of various precipitation, and recycle
approaches to the control and treatment of waste water. These
plants were judged to be exemplary on the basis of the reported
effluent data. One plant chosen precipitates a high purity
cryolite and includes spent cathode reclamation in the circuit.
The second plant uses a simpler cryolite process, while the third
employs a calcium chloride precipitation of the cryolite
filtrate.
Company
Data
Verification
Analysis
Suspended solids, mg/1
Fluoride, mg/1
Oil and grease, mg/1
18
16
2
25
7
The agreement is considered good in spite of the brief period of
verification sampling. The concentration of each constituent is
well within the maximum range as reported by Plant D. It will be
noted that the above values are gross values. No subtraction of
influent concentrations was made.
The concentration data reported by Plant J and the average
concentration obtained in the verification analysis for samples
taken at ten consecutive shifts are listed in the following
tabulation:
Suspended solids, mg/1
Fluoride, mg/1'
Oil and grease, mg/1
Company
_Data
15.6
10.2
2.1
Verification
Analysis
15.8
1C.1
1.7
63
-------
giant Identification Letter
p
O in
00
^
H
KJ
p
O I*
H
p
• *
0 Ol
+^
oo
• ^.
H 0
N^ Ul
O Suspended Solids
O Oil and Grease
50
(100)
5
(10)
a
2
S'g
I
1
0.05-
(0.1)
. .5 5
(1.0) (10)
Fluoride Emissions, kg/metric ton
(Ib/ton)
30
(60)
Figure 7. Correlation of plant data on suspended solids,
oil and grease, and fluoride errissions.
64
-------
The correlation is good.
The results of the verification sampling at Plant K are compared
with plant data in the following tabulation. The verification
data are the averages of three 24-hour composite samples.
Company
Suspended solids, mg/1
Fluoride, mg/1
Oil and grease, mg/1
35
22
4.3
Verification
Analysis
44
10
4.1
As in the previous tabulations, the agreement is good.
Source of Waste Water from Developmental
Aluminum Reduction Processes
Pilot plant studies of the chloride electrolytic process for the
production of primary aluminum indicate that a wet gas scrubber
system will be used, which will have a discharge containing
chlorine and chlorides. The concentrations to be expected are
not known at this time. A blowdown from cooling towers also is
expected to be discharged.
65
-------
SECTION VI
SELECTION OF POLLUTANT PARAMETERS
Selected Parameters
The following waste water constituents are the significant
pollutants from the primary aluminum smelting subcategory:
Fluoride
Suspended solids
PH
The rationale for the selection of these constituents and for the
rejection of other constituents as pollutants is presented in the
following paragraphs.
Rationale for the Selection of Pollutant Parameters
Fluorides
As the most reactive non-metal, fluorine is never found free in
nature but as a constituent of fluorite or fluorspar, calcium
fluoride, in sedimentary rocks and also of cryolite, sodium
aluminum fluoride, in igneous rocks. Owing to their origin only
in certain types of rocks and only in a few regions, fluorides in
high concentrations are not a common constituent of natural
surface waters, but they may occur in detrimental concentrations
in ground waters.
Fluorides are used as insecticides, for disinfecting brewery
apparatus, as a flux in the manufacture of steel, for preserving
wood and mucilages, for the manufacture of glass and enamels, in
chemical industries, for water treatment, and for other uses.
Fluorides in sufficient quantity are toxic to humans, with doses
of 250 to 450 mg giving severe symptoms or causing death.
There are numerous articles describing the effects of fluoride-
bearing waters on dental enamel of children; these studies lead
to the generalization that water containing less than 0.9 to 1.0
mg/1 of fluoride will seldom cause mottled enamel in children,
and for adults, concentrations less than 3 or U mg/1 are not
likely to cause endemic cumulative fluorosis and skeletal
effects. Abundant literature is also available describing the
advantages of maintaining 0.8 to 1.5 mg/1 of fluoride ion in
drinking water to aid in the reduction of dental decay,
especially among children.
Chronic fluoride poisoning of livestock has been observed in
areas where water contained 10 to 15 mg/1 fluoride.
Concentrations of 30 - 50 mg/1 of fluoride in the total ration of
dairy cows is considered the upper safe limit. Fluoride from
waters apparently does not accumulate in soft tissue to a
significant degree and it is transferred to a very small extent
67
-------
into the milk and to a somewhat greater degree into eggs. Da?ta
for fresh water indicate that fluorides are toxic to fish at
concentrations higher than 1.5 mg/1.
Fluoride ion is one of the more significant pollutants associated
with the primary smelting of aluminum. Fluoride concentrations
currently range from 10 mg/1 in the effluent from well controlled
treatment plants to 30 mg/1, where less effective fluoride
control is applied to the waste water. The presence of fluorides
in the effluent stems primarily from wet scrubbing of gases for
air pollution control.
Total Suspended^Solids
Suspended solids include both organic and inorganic materials.
The inorganic components include sand, silt, and clay. The
organic fraction includes such materials as grease, oil, tar,
animal and vegetable fats, various fibers, sawdust, hair, and
various materials from sewers. These solids may settle out
rapidly and bottom deposits are often a mixture of both organic
and inorganic solids. They adversely affect fisheries by
covering the bottom of the stream or lake with a blanket of
material that destroys the fish-food bottom fauna or the spawning
ground of fish. Deposits containing organic materials may
deplete bottom oxygen supplies and produce hydrogen sulfide,
carbon dioxide, methane, and other noxious gases.
In raw water sources for domestic use, state and regional
agencies generally specify that suspended solids in streams shall
not be present in sufficient concentration to be objectionable or
to interfere with normal treatment processes. Suspended solids
in water may interfere with many industrial processes, and cause
foaming in boilers, or encrustations on equipment exposed to
water, especially as the temperature rises. Suspended solids are
undesirable in water for textile industries; paper and pulp;
beverages; dairy products; laundries; dyeing; photography;
cooling systems, and power plants. Suspended particles also
serve as a transport mechanism for pesticides and other
substances which are readily sorbed into or onto clay particles.
Solids may be suspended in water for a time, and then settle to
the bed of the stream or lake. These settleable solids
discharged with man's wastes may be inert, slowly biodegradable
materials, or rapidly decomposable substances. While in
suspension, they increase the turbidity of the water, reduce
light penetration and impair the photosynthetic activity of
aquatic plants.
Solids in suspension are aesthetically displeasing. When they
settle to form sludge deposits on the stream or lake bed, they
are often much more damaging to the life in water, and they
retain the capacity to displease the senses. Solids, when
transformed to sludge deposits, may do a variety of damaging
things, including blanketing the stream or lake bed and thereby
68
-------
destroying the living spaces for those benthic organisms that
would otherwise occupy the habitat. When of an organic and
therefore decomposable nature,, solids use a portion or all of the
dissolved oxygen available in the area. Organic materials also
serve as a seemingly inexhaustible food source for sludgeworms
and associated organisms.
Turbidity is principally a measure of the light absorbing
properties of suspended solids. It is frequently used as a
substitute method of quickly estimating the total suspended
solids when the concentration is relatively low.
Suspended solids present in the discharge from primary aluminum
plants have their origin in wet scrubbing of particulates from
gases and in the precipitation of solids from the waste water for
,fluoride control. Concentrations of suspended solids currently
range from 5 to 30 mg/1. Relatively unsophisticated methods are
available for the treatment of waste water to decrease the
suspended solids content. Suspended solids are included as a
pollutant subject to effluent limitations in order to assure that
treatment for fluoride control is followed by adequate settling
\of the resultant precipitates and that the discharge of
fluorides, among other suspended solids, is minimized.
pH, Acidity and_Alkalinity
Acidity and alkalinity are reciprocal terms. Acidity is produced
by substances that yield hydrogen ions upon hydrolysis and
alkalinity is produced by substances that yield hydroxyl ions.
The terms "total acidity" and "total alkalinity" are often used
to express the buffering capacity of a solution. Acidity in
natural waters is caused by carbon dioxide, mineral acids, weakly
dissociated acids, and the salts of strong acids and weak bases.
Alkalinity is caused by strong bases and the salts of strong
alkalies and weak acids.
The term pH is a logarithmic expression of the concentration of
hydrogen ions. At a pH of 7, the hydrogen and hydroxyl ion
concentrations are essentially equal and the water is neutral.
Lower pH values indicate acidity while higher values indicate
alkalinity. The relationship between pH and acidity or
alkalinity is not necessarily linear or direct.
Waters with a pH below 6.0 are corrosive to water works
structures, distribution lines, and household plumbing fixtures
and can thus add such constituents to drinking water as iron,
copper, zinc, cadmium and lead. The hydrogen ion concentration
can affect the "taste" of the water. At a low pH water tastes
"sour". The bactericidal effect of chlorine is weakened as the
pH increases, and it is advantageous to keep the pH close to 7.
This is very significant for providing safe drinking water.
Extremes of pH or rapid pH changes can exert stress conditions or
kill aquatic life outright. Dead fish, associated algal blooms.
69
-------
and foul stenches are aesthetic liabilities of any waterway.
Even moderate changes from "acceptable" criteria limits of pH are
deleterious to some species. The relative toxicity to aquatic
life of many materials is increased by changes in the water pH.
Metalocyanide complexes can increase a thousand-fold in toxicity
with a drop of 1.5 pH units. The availability of many nutrient
substances varies with the alkalinity and acidity. Ammonia is
more lethal with a higher pH.
The lacrimal fluid of the human eye has a pH of approximately 7.0
and a deviation of C.I pH unit from the norm may result in eye
irritation for the swimmer. Appreciable irritation will cause
severe pain.
Acid streams are produced in wet scrubbing of potline air and
casthouse and anode bake plant gases. Alkaline streams are
produced by cryolite recovery. Such streams are often mixed to
effect neutralization. In the event that these streams are not
sufficiently balanced stoichiometrically, additional
neutralization can be performed to maintain the discharge within
limits.
Rationale for the Rejection of Pollutant Parameters
Other waste water constituents identifiable with the primary
aluminum industry that are not the subject of effluent limita-
tions or standards of performance are as follows:
Oil and Grease
Cyanide
Dissolved Solids
Chloride
Sulfate
COD
Temperature
Trace Metals
Oil_and Grease
Oil and grease exhibit an oxygen demand. Oil emulsions may
adhere to the gills of fish or coat and destroy algae or other
plankton. Deposition of oil in the bottom sediments can serve to
exhibit normal benthic growths, thus interrupting the aquatic
food chain. Soluble and emulsified material ingested by fish may
taint the flavor of the fish flesh. Water soluble components may
exert toxic action on fish. Floating oil may reduce the re-
aeration of the water surface and in conjunction with emulsified
oil may interfere with photosynthesis. Water insoluble
components damage the plumage and coats of water animals and
fowls. Oil and grease in a water can result in the formation of
objectionable surface slicks preventing the full aesthetic
enjoyment of the water.
70
-------
Oil spills can damage the surface of boats and can destroy the
aesthetic characteristics of beaches and shorelines.
Volatile hydrocarbons are evolved during the anode baking process
and are collected in wet scrubbers employed for air pollution
control. The hydrocarbons associated with anode baking are
indefinite in composition and are referred to as "tars" in the
industry. Currently, the effluent concentrations range from 1 to
10 mg/1 of oil and grease. Oil and grease is not considered as a
significant pollutant, since data have shown that typical
concentrations of oil and grease found, in the effluents are too
small in magnitude to be significantly reduced by current
technology.
Cyanides in water derive their toxicity primarily from
undissolved hydrogen cyanide (HCN) rather than from the cyanide
ion (CN~). HCN dissociates in water into H+ and CN~ in a pH-
dependent reaction. At a pH of 7 or below, less than 1 percent
of the cyanide is present as CN~; at a pH of 8, 6.7 percent; at a
pH of 9, U2 percent; and at a pH of 10, 87 percent of the cyanide
is dissociated. The toxicity of cyanides is also increased by
increases in temperature and reductions in oxygen tensions. A
temperature rise of 10°C produced a two- to threefold increase in
the rate of the lethal action of cyanide.
Cyanide has been shown to be poisonous to humans, and amounts
over 18 ppm can have adverse effects. A single dose o'f about
50-60 mg is reported to be fatal.
Trout and other aquatic organisms are extremely sensitive to
cyanide. Amounts as small as .1 part per million can kill them.
Certain metals, such as nickel, may complex with cyanide to
reduce lethality especially at higher pH values, but zinc and
cadmium cyanide complexes are exceedingly toxic.
When fish are poisoned by cyanide, the gills become considerably
brighter in color than those of normal fish, owing to the
inhibition by cyanide of the oxidase responsible for oxygen
transfer from the blood to the tissues.
Cyanide is contained in the run-off from spent cathode storage
areas and is detectable in the effluent from some primary
aluminum plants. The reprocessing of spent cathodes for cryolite
recovery also results in cyanide discharges; The free cyanide
levels encountered in the plant surveys were low, ranging in
concentrations from 0.002 to 0.036 mg/1. Cyanide is not
considered as a significant pollutant, since data have shown that
typical concentrations of cyanide found in the effluents are too
small in magnitude to be significantly reduced by current
technology.
Dissolved Solids
71
-------
In natural waters the dissolved solids consist mainly of
carbonates, chlorides, sulfates, phosphates, and possibly
nitrates of calcium, magnesium, sodium, and potassium, with
traces of iron, manganese and other substances.
Many communities in the United States and in other countries use
water supplies containing 2000 to UOOO mg/1 of dissolved salts,
when no better water is available. Such waters are not
palatable, may not quench thirst, and may have a laxative action
on new users. Waters containing more than 4000 mg/1 of total
salts are generally considered unfit for human use, although in
hot climates such higher salt concentrations can be tolerated
whereas they could not be in temperate climates. Waters
containing 5000 mg/1 or more are reported to be bitter and act as
bladder and intestinal irritants. It is generally agreed that
the salt concentration of good, palatable water should not exceed
500 mg/1.
Limiting concentrations of dissolved solids for fresh-water fish
may range from 5,000 to 10,000 mg/1, according to species and
prior acclimatization. Some fish are adapted to living in more
saline waters, and a few species, of fresh-water forms have been
found in natural waters with a salt concentration of 15,000 to
20,000 mg/1. Fish can slowly become acclimatized to higher
salinities, but fish in waters of low salinity cannot survive
sudden exposure to high salinities, such as those resulting from
discharges .of oil-well brines. Dissolved solids may influence
the toxicity of heavy metals and organic compounds to fish and
other aquatic life, primarily because of the antagonistic effect
of hardness on metals.
Waters with total dissolved solids over 500 mg/1 have decreasing
utility as irrigation water. At 5,000 mg/1 water has little or
no value for irrigation.
Dissolved solids in industrial waters can cause foaming in
boilers and cause interference with cleanliness, color, or taste
of many finished products. High contents of dissolved solids
also tend to accelerate corrosion.
Specific conductance is a measure of the capacity of water to
convey an electric current. This property is related to the
total concentration of ionized substances in water and water
temperature. This property is frequently used as a substitute
method of quickly estimating the dissolved solids concentration.
Dissolved solids includes fluorides, chlorides, sulfates, and the
common cations, sodium, potassium, magnesium, and calcium. The
maximum concentration of dissolved solids reported by most plants
surveyed was less than 1000 mg/1. The present cost of treatment
-co reduce the level of dissolved solids is such that treatment of
dissolved solids is beyond the scope of the technologies defined
by best practicable or -best available.
72
-------
Chloride concentrations in discharged waste water range from 0 to
16 mg/1 in the primary aluminum industry. Conversion of chlorine
degassing of molten aluminum to other technologies will decrease
the observed levels. There is no suitable treatment currently
available for decreasing these levels further.
Sulfate
The sulfur impurities in various raw materials, such as pitch and
petroleum coke used in anode preparation, are converted to oxides
of sulfur which are collected in wet scrubbers as sulfates.
Sulfate concentrations range from 5 to 100 mg/1 in primary
aluminum plant effluents. Sulfate is partially removed by
fluoride treatment. As fluoride discharges are controlled,
sulfate levels will decrease.
Demand^ (COD}
The chemical oxygen demand is a measure of the quantity of the
oxidizable materials present in water and varies with water
composition, temperatures, and other factors.
Dissolved oxygen (DO) is a water quality constituent that, in
appropriate concentrations, is essential not only to keep
organisms living but also to sustain species reproduction, vigor,
and the development of populations. Organisms undergo stress at
reduced DO concentrations that make them less competitive and
able to sustain their species within the aquatic environment.
For example, reduced DO concentrations have been shown to
interfere with fish population through delayed hatching "of eggs,
reduced size and vigor of embryos, production of deformities in
young, interference with food digestion, acceleration of blood
clotting, decreased tolerance to certain toxicants, reduced food
efficiency and growth rate, and reduced maximum sustained
swimming speed. Fish food organisms are likewise affected
adversely in conditions with suppressed DO. Since all aerobic
aquatic organisms need a certain amount of oxygen, the
consequences of total lack of dissolved oxygen due to a high BOD
can kill all inhabitants of the affected area.
If a high COD is present, the quality of the water is usually
visually degraded by the presence of decomposing materials and
algae blooms due to the uptake of degraded materials that form
the foodstuffs of the algal populations.
A COD component associated with organic materials is present in
primary aluminum smelter discharges. Control of fluorides will
indirectly control oil and grease (see Figure 7) , which will, in
turn, control COD.
Temperature
73
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Temperature is one of the most important and influential water
quality characteristics. Temperature determines those species
that may be present; it activates the hatching of young,
regulates their activity, and stimulates or suppresses their
growth and development; it attracts, and may kill when the water
becomes too hot or becomes chilled too suddenly. Colder water
generally suppresses development. Warmer water generally
accelerates activity and may be a primary cause of aquatic plant
nuisances when other environmental factors are suitable.
Temperature is a prime regulator of natural processes within the
water environment. It governs physiological functions in
organisms and, acting directly or indirectly in combination with
other water quality constituents, it affects aquatic life with
each change. These effects include chemical reaction rates,
enzymatic functions, molecular movements, and molecular exchanges
between membranes within and between the physiological systems
and the organs of an animal.
Chemical reaction rates vary with temperature and generally
increase as the temperature is increased. The solubility of
gases in water varies with temperature. Dissolved oxygen is
decreased by the decay or decomposition of dissolved organic
substances and the decay rate increases as the temperature of the
water increases reaching a maximum at about 30°C (86°F), The
temperature of stream water, even during summer, is below the
optimum for pollution-associated bacteria. Increasing the water
temperature increases the bacterial multiplication rate when the
environment is favorable and the food supply is abundant.
Reproduction cycles may be changed significantly by increased
temperature because this function takes place under restricted
temperature ranges. Spawning may not occur at all because
temperatures are too high. Thus, a fish population may exist in
a heated area only by continued immigration. Disregarding the
decreased reproductive potential, water temperatures need not
reach lethal levels to decimate a species. Temperatures that
favor competitors, predators, parasites, and disease can destroy
a species at levels far below those that are lethal.
Fish food organisms are altered severely when temperatures
approach or exceed 90°F. Predominant algal species change,
primary production is decreased, and bottom associated organisms
may be depleted or altered drastically in numbers and
distribution. Increased water temperatures may cause aquatic
plant nuisances when other environmental factors are favorable.
Synergistic actions of pollutants are more severe at higher water
temperatures. Given amounts of domestic sewage, refinery wastes,
oils, tars, insecticides, detergents, and fertilizers more
rapidly deplete oxygen in water at higher temperatures, and the
respective toxicities are likewise increased.
When water temperatures increase, the predominant algal species
may change from diatoms to green algae, and finally at high
74
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temperatures to blue-green algae, because of species temperature
preferentials. Blue-green algae can cause serious odor problems.
The number and distribution of benthic organisms decreases as
water temperatures increase above 90°F, which is close to the
tolerance limit for the population. This could seriously affect
certain fish that depend on benthic organisms as a food source.
The cost of fish being attracted to heated water in winter months
may be considerable, due to fish mortalities that may result when
the fish return to the cooler water.
Rising temperatures stimulate the decomposition of sludge,
formation of sludge gas, multiplication of saprophytic bacteria
and fungi (particularly in the presence of organic wastes), and
the consumption of oxygen by putrefactive processes, thus
affecting the esthetic value of a water course.
In general, marine water temperatures do not change as rapidly or
range as widely as those of freshwaters. Marine and estuarine
fishes, therefore, are less tolerant of temperature variation.
Although this limited tolerance is greater in estuarine than in
open water marine species, temperature changes are more important
to those fishes in estuaries and bays than to those in open
marine areas, because of the nursery and replenishment functions
of the estuary that can be adversely affected by extreme
temperature changes.
Heat loads are comparatively small in the primary aluminum in-
dustry. Maximum temperature differentials of the discharge vary
with plant location. The control and treatment technologies
identified have associated retention times of various duration,
which will tend to control the temperatures of the outfall.
Trace Metals
Trace metals have not been included in the list of significant
pollutant parameters. Measurable quantities of zinc, copper, and
nickel are found in the effluents from primary aluminum smelters;
however, there are insufficient data available on which to base
effluent limitations and standards of performance.
75
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SECTION VII
CONTROL AND TREATMENT TECHNOLOGY
Introduction
The existing technologies for controlling waste water volume in
the primary smelting of aluminum include dry fume scrubbing, and
recycle of water to wet scrubbers after precipitation of
fluorides. Treatment methods for reducing pollutant concen-
trations include: cryolite precipitation, precipitation by lime
or alum, adsorption on activated alumina or hydroxylapatite, and
reverse osmosis.
As set forth in Section VI, the constituents of waste water from
primary aluminum smelters which are to be considered as pollutant
characteristics of major significance are fluoride, suspended
solids and pH. These pollutants originate from the operation of
wet scrubbers on the potline, pot room, anode bake furnace, and
from cryolite recovery. Minor sources of pollutants include:
casthouse wet scrubbers, anode paste plant wet scrubbers,
rectifier cooling, casthouse cooling, boiler blowdown, and rain
water run-off.
Current control and treatment practice varies throughout the
industry. Therefore, the steps required to be taken in order to
achieve the effluent limitations presented in this document will
vary depending upon the current status of each plant. A
generalized summary of the variation in current practice and
optional control and treatment modes applicable to each source of
waste water is given in Table 5. In the following paragraphs,
each of the technologies included in Table 5 is described, the
degree of effluent reduction achieved by each technology is
identified and finally, optional routes for achieving the
effluent limitations are identified.
Control Technology
In the context of this document, the term control technology
refers to any practice applied in order to reduce the volume of
waste water discharged. In the primary aluminum smelting
industry, the most significant reduction in discharge volume is
obtained by converting wet fume scrubbers to dry fume scrubbers
or by treating and recycling the water from wet scrubbers.
Dry^Scrubbing of Pot Line Gas
Identification. The dry scrubbing of pot line gas refers to the
use of~an air pollution control system for the removal of
pollutants contained in the gases from the electrolytic cell
77
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TABLE 5. SUMMARY OF PRESENT AND POTENTIAL CONTROL AND
TREATMENT TECHNOLOGIES
Wastewater
Source
Present Practice
Possible
Added Control
Possible
Added Treatment
Pot (primary)
wet scrubber
Discharge with-
out treatment
" Lime and settle
once-through
" Cryolite or line
ppnt. with recycle
Potroom (secondary) Discharge without
wet scrubber treatment
Lime and settle
once-through
Cast house
wet scrubber
Anode bake
plant wet
scrubber
Paste plant
wet scrubber
Cast house
cooling
Rectifier
Cooling
Rainfall runoff
Settle
Settle
Settle
Discharge with-
out treatment
Discharge with-
out treatment
Discharge with-
out treatment
Convert to
dry scrubbing
Install cryolite
or line pptn plus
recycle with bleed
Install recycle
with bleed
Install cryo-
lite or line pptn.
plus recycle
Install recycle
Convert to alter-
nate degassing
Recycle
Recycle
Close loop
Convert to
air-cooled recti-
fiers
Route to cryo-
lite recovery and
recycle
Install lime treat-
ment of bleed stream
Install alumina
adsorption
Install lime treat-
ment of bleed stream
Install alumina ad-
sorption
Flocculate and
aerate
Cooling tower
Cooling tower
78
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(pot) by contacting the gases with dry alumina. The pollutants
are sorbed by the media, subsequently collected as particulate
matter by fabric filtration. The system is applicable to gases
collected immediately above the pot line (i.e., pot line gas),
having relatively higher concentrations of constituents than does
pot room ventilation air.
The outstanding features of the system include the sorbtion of
emitted gases on alumina, the subsequent return of this media to
the pots to produce aluminum product, the associated return of
sorbed fluorine compounds to the pots, and the generally high
levels of collection efficiency for both gaseous fluorine
compounds and particulates (e.g., greater than 99 percent). This
process uses no water.
Process Description. The elements of the dry scrubbing process
(indicated in Figure 8) include hoods and ducts to collect and
deliver the gases from the pots to an operating unit; usually a
cyclone type device to separate coarse particulate; a reactor
section in which the gases are contacted with the alumina, and a
fabric filter, from which the gases are released to the
atmosphere. Associated equipment includes fans, alumina
delivery, storage, and baghouse auxiliary equipment.
Three commercial variations of the process exist, with differing
mechanisms, principally in the contactor stage. In one type of
dry scrubber, the contacting of gas and alumina is accomplished
in a fluidized bed, with the fabric filters, or a baghouse at the
top of the same chamber. In another design, the air at
relatively high velocity is blown upward through a venturi
throat, into which alumina is injected downward. The gases and
eluted solids are drawn from the column and thence to the
baghouse stage. In the third design, the collected gases are
drawn at high velocity through a horizontal duct with the alumina
being injected downward into the moving gases. In some cases the
gases may be passed through a cyclonic device, to remove the
larger particulates before the gas-alumina contacting stage.
Another variation of application includes the routing of the exit
gases from the baghouse to a wet scrubber to achieve further
cleaning, particularly of sulfur oxides. In one specific
application, associated with vertical stud Soderberg cells, the
particulates collected in the baghouse stage are, because of
hydrocarbon content, sent to storage or subjected to a special
treatment to remove the hydrocarbons and allow the alumina to be
charged to the pots without adverse effects on pot operation.
The dry scrubbing system is dependent on the phenomenon of
sorption of fluorine compounds on the surface of the alumina.
Highest sorption rates occur during the formation of the initial
monomolecular layer on the surface of the alumina. Thus,
operation of this system is strongly dependent on the surface
area of the alumina and the exposure or contact time. Sorption
decreases rapidly after the formation of the first monolayer.
79
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oo
o
Gases
Pots
To Pots
Alumina
Storage
Alv
1
mina
1
Contacting
1 * Reactor
1 1
1 1
1 I
1 1
L »«| Cyclone U- J
' i i
Baghouse
Sol
ids
1
T<
StJ
1
>
ick
1
Wet Gas
Scrubber
1
I
1
Waste
~~T Water
v •
•HV
1
Treatment I
1
To Storage
Figure 8. Diagram of dry gas spnalabing1 process elements.
(DASHED LINES INDICATE ALTERNATIVE ARRANGEMENTS)
-------
This factor leads to the practice wherein all the alumina feed is
first passed through the air pollution control system.
Applicability. As stated previously, there are three variations
of the dry scrubbing process available, from each of three
manufacturers; one in the United States, one in Canada, and one
in Europe. All three designs are in operation on the commercial
scale. In one case, the system has been operated for as long as
five years. To date, proven applications have been on prebake
anode and vertical stud Soderberg anode cells. No production
scale application to horizontal stud Soderberg type operations
exists in the United States, although a test unit serving several
pots of this type is being evaluated by one producer.
The applicability of any one of the specific systems to a
specific plant is influenced by the characteristics of the
alumina used at the plant. In general, the fluidized-bed design
is most compatible with "sandy" type alumina (i.e., 50 percent
-325 mesh material). Other designs have varying compatibilities
with different forms of alumina. The form of alumina available
to a given plant may be a constraint in the selection or
application of the dry scrubbing process, involving some tradeoff
in terms of the system selected or the sources of alumina.
Dry scrubbing control methods are being installed in the United
States on both new plants and existing plants. These dry
installations are serving as methods of achieving both air and
water pollution control. When the dry system is properly
operated with efficient hooding, relatively stringent atmospheric
emissions limits may be satisfied, without the use of water.
Thus, the dry scrubbing process is of major significance to water
pollution control at primary aluminum smelters.
Recycle of Water_from_Wet^Scrubbers
I^§Si4fiŁIii22« Water from wet scrubbers can be treated in
various ways~to remove impurities, so that the partially purified
water can be continuously returned to the wet scrubber. In the
case of primary potline and secondary potroom wet scrubbers, the
fluoride dissolved in the water can be precipitated and settled.
This treatment reduces the suspended solids and oil and grease
content at the same time.
Process Detail. In general, the method used to remove the
soluble fluoride values from the waste water is precipitation
either as cryolite or as calcium fluoride. In the first case,
sodium aluminate (or NaOH and hydrated alumina) is added. In the
second case, a lime slurry (or, in one case, CaC12) is used.
After precipitation, thickening of the slurry is accomplished in
clarifiers or thickeners.
81
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The treatment of wet scrubber liquors to recover cryolite is a
significant practice, because a sufficient quantity of fluoride
is removed to permit recycle of the treated liquor to the
scrubbers. The process also recovers the fluoride in a form
which usually can be returned to the aluminum electrolysis bath.
The value of the recovered cryolite represents a credit to the
treatment process. Full recycle cannot be achieved by this
treatment, because of the presence of sulfates in the liquor.
Sulfur impurities in the raw materials, principally in the
petroleum coke and pitch used in anode preparation, are converted
to sulfur oxides during electrolysis and are collected in the
scrubber water as sulfates. If 100 percent recycle of the liquor
were practiced the solubility of sodium sulfate would eventually
be exceeded. Therefore, a small bleed is maintained from the
scrubber liquor circuit to keep the sulfate concentration
sufficiently low to prevent precipitation of sodium sulfate.
This bleed stream is relatively low in volume, but high in
fluoride content; it represents the major portion of the fluoride
effluent from the entire plant. The actual volume of required
bleed is related to the sulfur content of the coke and pitch.
This sulfur value is expected to rise as the demand for low
sulfur fuel increases. Further treatment of this bleed stream is
not practiced in the industry at this time.
The recycle system consists of utilizing the clarified overflow
from the thickener tanks as the scrubbing medium. A schematic
diagram of the process is shown in Figure 9.
The liquor leaving the scrubber, containing about 1-2 g/liter
fluoride is reacted with sodium aluminate to form cryolite. This
stream is then sent to the thickener where suspended solids are
settled out. These suspended solids contain cryolite (about 30
percent by weight), carbon (5-15 percent), and other insoluble
solids which have been collected by the wet scrubbers. The
supernatent clarified liquor from the thickener is recycled to
the scrubbers.
The slurry from the bottom of the thickener tank, at a solids
concentration of from 200-500 g/liter, is then filtered to remove
some of the liquid, and the solid cake (about 60 percent solids)
is dried in a kiln or multiple hearth furnace. If the cryolite
is pure enough, it can be returned to the reduction pots at this
point; if not, it is landfilled. In some cases, the filtrate is
further treated to reduce the fluoride values by precipitation as
CaF2. In one case, this is done by the addition of CaCl2.
In general, suspended solids are removed in the thickening tanks
along with the fluoride precipitate. A recycle stream tends to
give better control than a once-through system (0.5 to 1.5
kg/metric ton (1 to 3 Ib/ton) of Al versus about 10 kg/metric ton
(20 Ib/ton)). It is apparent that these solids settle out faster
as the concentration of the suspension is increased. This is a
common phenomenon in solid-liquid separation, and the faster
settling is known as Type II settling. In this region, the
82
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MAKE UP WATER
;132 i
SCRUBBER
RECYCLE 3,785
(1,000)
FILTER
57
BLEED(15)
at (1 - 2 g/1 F)
REACTOR
SODIUM ALUMINATE
THICKENER
300 g/1 suspended solids
KILN
CRYOLITE (30%)
* Process rates: liters/min.
FILTRATE
Figure 9. Process schematic recycle system for fluoride removal.
C250 T/D Aluminum)
83
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particles coalesce and the resultant mass of particle settles at
a greater rate than the individual, unhindered particles.
There is some oil and grease in the waste water. These
hydrocarbons arise from the baking of the anode. At the present
time, no control techniques are employed to specifically remove
this pollutant, because of its relatively low concentration. The
data indicate that about one-half to two-thirds of the oil and
grease is adsorbed onto various precipitated solids. Thus, the
thickening operations can be considered as a means of control.
Some of the variations in current industrial
practice, and the fluoride levels in associated streams are
indicated below.
In a prebake anode plant, primary and secondary gas scrubber
liquors are treated with sodium aluminate to recover cryolite
with recycle of the liquor to the scrubber. A scrubber liquor
bleed stream (to control sulf ate content) , containing 2 g/1 F, is
diluted and discharged. The mixed plant discharge contains 20
mg/1 F, and is calculated as equivalent to an emission of 1.2 kg
F/metric ton (2.4 Ib/ton) of aluminum produced.
In a horizontal stud Soderberg plant, waste streams consist of
area run-off, potlining leaching liquor, and primary air scrubber
liquors, all of which are treated with HF and CO2_ to recover
cryolite, with recycle of the treated water to the scrubbers. A
scrubber liquor bleed stream with 2 g/1 F is diluted and
discharged as a mixed plant waste stream, containing 10.5 mg/1 F.
Total fluoride emission in water was calculated as 1.1 kg/metric
ton of aluminum produced (2.2 Ib/ton) .
The conclusions which can be drawn on the basis of the accumu-
lated data are as follows:
(1) Adequate means are available and are presently being
employed to reduce soluble fluoride emissions in waste
water to about 1 kg/metric ton (2 Ib/ton) of aluminum
produced and suspended solids to about 1.5 kg/metric ton
(3 Ib/ton) .
(2) The best means of control in present practice in the
aluminum industry is the precipitation of the fluoride
as cryolite or with lime and recycle of the clarified
liquid back to the scrubber. This practice is
considered to be the best practicable control technology
currently available. Alternate technology available to
some plants is dry fume scrubbing.
Cas_t.house Scrubber^ Water
There are, in practice, a number of variations in degassing
procedures that function as inprocess control techniques to
eliminate the use of water for wet scrubbing of fumes generated
during degassing of molten aluminum. Although the differences
84
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between the various techniques are of metallurgical significance,
the processes will be considered as a single class, since each
achieves the elimination of water use in casthouse scrubbers.
Degassing is an operation in which dissolved hydrogen and other
impurities are removed from molten aluminum prior to casting into
product form. The classical approach to degassing is the
bubbling of chlorine gas through the melt to react with and
remove the hydrogen as hydrogen chloride gas and other impurities
as chloride salts. Emissions to the air are normally controlled,
when necessary, by alkaline wet scrubbing. The raw waste water
stream produced may vary from acid to alkaline, depending on
operating conditions, with notable levels of dissolved salts,
usually sodium chloride.
The necessity for degassing requirements vary with product
specifications. Products, which must be especially high in
purity and free of pin holes caused by gas bubbles (e.g.,
aluminum foil), require stringent control of metal quality.
Certain alloy compositions or melting stock require reduction of
impurities to extraordinarily low levels to achieve specified
properties of strength, ductility, electrical conductivity, etc.
Thus, a number of alternative processes for controlling metal
purity have been developed., depending on product requirements.
Process, .Identification. The alternative approaches to degassing
include:
(1) Chlorine degassing with no air pollution control*
(2) Chlorine degassing with wet scrubbing of gases.
(3) Degassing with mixtures of chlorine and other gases.
(U) Degassing with inert (nitrogen or argon) gases,
(5) Filtration of the molten metal, using special
materials and conditions.
Only approach (2) involves contribution of pollutants to waste
water.
Noteworthy factors in the above list are that approaches (1) and
(2) imply the use of a stream of 130 percent chlorine. In the
last few years, environmental control efforts have resulted in
the development, and successful use, of gas mixtures, such as
chlorine plus an inert gas, or chlorine, carbon monoxide, and
nitrogen. In the latter case of mixed gases, gas burners or
controlled combustion gas generators are used to produce a gas of
carefully controlled composition.
In the case of degassing with an inert gas, a degree of
uncertainty exists with regard to the basic reactions in the
degassing process. The degassing process may depend to a degree
on the chemical reaction Of chlorine with hydrogen, followed by
evolution of hydrogen chloride gas bubbles. To some degree, the
degassing operation depends on the formation of gas bubble nuclei
85
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and interfaces, which furnish the basis for the simple physical
evolution of the hydrogen from its dissolved state in the metal.
Thus, the requirement for any specific concentration of chlorine
for degassing may be argued on the basis of metal impurity level,
product requirements, operating conditions, or other factors.
Applicability and Reliability. All of the above listed process
alternatives are in commercial use on a regular basis and have
been for sufficient time to be considered established practice in
one or more producing plants. There is no known evidence that
the alternatives are completely applicable to every plant.
Applicability of any one specific process to any one specific
plant must be determined on an individual basis.
Three of the processes listed above ( (3) , (4) and (5) ) , are
patented and accessible only under licensing agreements.
It is concluded that there are currently available alternative
process methods, which may be applied to achieve the elimination
of casthouse scrubber waste water.
Anode Bake Plant Scrubber^Water
At the present time, control of water from this source, such as
treatment followed by recycle to the scrubbers, is not practiced.
The solids produced by precipitation of fluoride from anode bake
plant scrubber water are not suitable for recycle to the smelter,
because of tar and oil contamination. The technology for lime
precipitation, described previously for water from potline wet
scrubbers, also could be applied to water from anode bake plant
wet scrubbers.
Dry electrostatic precipitators do not currently remove
significant, amounts of gaseous fluoride; thus, they may not be
suitable for anode bake plant furnaces in the future. Baghouses
are also unsuited for this application, because of blinding of
the bags caused by the tars and oils. One plant achieves an
acceptable air emission level without a wet scrubber on the
exhaust gases by exercising sophisticated control over the firing
of the anodes and by utilizing new flues in the exhaust circuit.
However, the company has reported that it has not been successful
in its efforts to apply this type of control at six other plants,
and wet scrubbing systems have been retained.
The conclusion reached is that control of water from anode bake
plant wet scrubbers can be approached through recycle, but that
technology for the elimination of wet scrubbers through the use
of dry devices or controlled firing is not adequately
demonstrated at this time.
Treatment Technology
86
-------
In the context of this document, the term treatment technology
refers to any practice applied to a waste water stream to reduce
the concentration of pollutants in the stream before discharge.
'. f.
Mater^From^Potline Wet Scrubbers
Treatment technology can be applied in once-^through systems or a
treatment method could be applied to the bleed stream or filtrate
from a recycled system in order to further reduce the fluoride
concentration and the suspended solids level.
' * : -
Once^Through System; The once-through system does not employ a
recycle loop. All of the scrubber water is treated, and then
discharged. A schematic diagram of the process is shown in
Figure 10. i-K; ,: =
In one prebake anode plant, scrubber water enters at a rate of 14
to 113 cubic meters/ton (4,000 to 30,000 gal/ton) of aluminum
produced, and fluorides and particulates are removed from the
effluent gas. The effluent, containing from ICO to 600 mg/1
soluble fluoride, is then contacted with a lime slurry. The
resulting suspension is thickened for about 5 hours, and is
usually aided by a polyelectrolyte coagulant. In general, the
solids from the thickener are sent to a landfill, and the
clarified effluent (20-50 mg/1 F) is combined with other waste
water from within the plant and discharged. Only lime is used
currently as. the precipitant in this process.
In a vertical stud Soderberg plant, the secondary air scrubber
water is diluted and discharged in mixed plant waste water at a
concentration of 20 mg/1 F. The primary gas scrubber liquor (2CO
to 500 mg/1 F) is limed and clarified to produce an overflow
containing 50 mg/1 F, which is combined with other streams before
discharge. Total emission of fluorides in water for this plant
was calculated as 10 kg/metric ton (20 Ib/ton) of aluminum metal
produced.
Treatment^of Recycle Bleed Streams. A process to remove fluoride
from the bleeci and filtrate streams obtained from a recycle
system can be depicted as shown in Figure.11.
The two streams are reacted with CaCl2 (or lime), and then enter
a clarifier where the suspended C&F2 is settled out. Based on
information provided by three aluminum companies, the following
assumptions are used for the design characteristics of the
process: ;
(1) The total flow to be treated is 0.106 cubic meters/
min (28 gpm).
(2) The input concentration of fluoride is 1 g/liter.
(3) Twice the stoichiometric amount of calcium is used.
(4) A residence time of 10 hours is used in the clarifier.
87
-------
15,000-114,000
(4,000 - 30,000)
oo
CO
SCRUBBER
LIME SLURRY
REACTOR
Flow Units:
liters/min.
f
THICKENER
EFFLUENT
(20 -50 mgF/1.)
SOLIDS TO DISPOSAL
Figure 10. Process schematic of once-through system for fluoride removal
(250 T/B aluminum)
-------
00
to
BLEED H2°
F~
FILTRATE
81,646
~ (180,000)
68
(150)
, _ 72,575
Hj.° * (160,000)
p-* „ 68
(150)
. „ 789
I CaC^- (1740)
1 -1842
1 H00 - AC"
1 2 (4060)
"1
THICKENERS
153,314
(338,000)
2.3
(5.1)
* Process rates are : Kg/da
(Ib/da)
^ 2676
(5900)
_ 134
(295)
Figure 11
. Flowsheet of process to remove fluorides from waste streams (recycle water treatment)
(250 T/D aluminum)
-------
(5) Output fluoride level is 30 mg/1.
On the basis of the above assumptions, this additional treatment
reduces the fluoride level from 1 kg/metric ton (2 Ib/ton) to
0.05 kg/ metric ton (0.1 Ib/ton). Also, it is estimated that the
additional settling time reduces the suspended particulate and
hydrocarbons in the effluent by 95 percent.
This secondary treatment of bleed and filtrate streams from a
recycle system is considered to be the best available technology
economically achievable.
Treatment of Dilute. Fluoride Streams. Typically, plants
utilizing a once-through treatment system yield a volume of waste
water of 14 to 14C cubic meters/metric ton (4,000-40,000 gal/ton)
of aluminum, having a concentration of 20-50 mg/1 soluble
fluoride. The fluoride discharged amounts to about 5 to 10
kg/metric ton (10-20 Ib/ton). There are several processes, not
in general practice in the primary aluminum industry, which could
be used to treat such dilute fluoride streams. These processes
are described below. For the purposes of design calculation, it
was assumed that the stream to be treated contains 35 mg/1
fluoride, and has a flow rate of 18,900 cubic meters per day (5
million gal/day). The model plant produces 225 metric tons (250
tons) of aluminum per day.
Aluminum gulfate ^Alum) . The addition of alum to a solution
containing the fluoride ion will remove the fluoride. Gulp and
Stoltenberg (4) showed that about 2/3 of the fluoride ion could be
removed by the addition of 500 ppm of alum, although the maximum
concentration of fluoride investigated was 6.0 ppm. Although the
quantity of alum necessary to treat the 35 mg/1 stream is
unknown, it is assumed that 1000 mg/1 would be adequate to remove
2/3 of the fluoride; that is, alum treatment would yield a stream
containing 12 mg/1 fluoride. From the data of Gulp and
Stoltenberg, there does not appear to be a large effect of alum
concentration on fractional removal of fluoride at different
initial fluoride concentrations.
A schematic drawing of a process to treat the dilute fluoride
stream from the once-through scrubbing system is shown in Figure
12. The alum is added and mixed with the stream in a tank,
providing a residence time of 2 minutes. The alum is then
allowed to flocculate for about 30 minutes. A period of 4 hours
is finally allotted for settling in a clarifier.
There is evidence to indicate that the pH of the waste water is
an important parameter for efficient fluoride removal by alum.
The data of Gulp and Stoltenberg indicate that the pH of the
treated stream should be above 6. In this case, the pH of the
stream coming from the thickener tank is fairly high, and should
present no problems with alum flocculation. One disadvantage of
this procedure is the disposal of a relatively large amount of
90
-------
vo
Alum ftoed
I 1B.900
(M.,600)
FtQCCOTJtlT
aunt
Alum - 18.900
(1*1.600)
r -
438
(965)
4 F- 220
(485)
Oat* In -
Figure 12. Schematic diagram of a process to remove fluoride by alum precipitation.
(250 T/D aluminum)
-------
sludge, about 18 metric tons/day (about 20 tons/day). One
advantage of this procedure is that a reduction in both suspended
solids and oil and grease should be effected. Very likely, some
oil adsorption on the alum floes will occur, and oil will be
removed along with the alum sludge. Likewise, it is expected
that particle-particle interactions will occur between the alum
floes and the suspended solids, enhancing the settling
characteristics of these solids.
on Activated Alumina. Activated alumina has been used
for some time in the treatment of municipal water supplies. In
this process, the waste water containing fluorides is passed
through a bed of activated alumina, which has an adsorption
capacity of about 0.022 kg of fluoride per liter of alumina
(0.286 Ib/cubic ft). Regeneration of the bed is accomplished by
either sulfuric acid (4 percent) or by sodium hydroxide (1
percent). The effluent from the adsorption bed contains about 2
ppm fluoride. Details of the pertinent experimental data have
been reported by Zabban and Jewett.(5) This technology is not
currently practiced for waste water treatment in the aluminum
industry.
A hypothetical process to treat the 18,900 cubic meters/day (5
million gal/day) stream of 35 ppm fluoride is shown in Figure 13.
Two alumina columns are used, operated alternately in an
adsorption mode and a regeneration mode. The regenerant solution
is H2SO4 at a concentration of 4 percent. About 16.5 kg of
sulfuric acid is required to regenerate the bed on which one kg
of fluoride has been adsorbed (or 16.5 Ib of acid/lb of
fluoride). The sulfuric acid-fluoride solution is then
neutralized with lime, resulting in the formation of calcium
fluoride and calcium sulfate. The final step is the settling of
the precipitates in a thickener tank.
One of the major disadvantages of this process from an environ-
mental standpoint is the discharge of a relatively large amount
of calcium sulfate, about 545 kg/day (about 1200 Ib/day) in the
water. Calcium sulfate may result in an increase in hardness of
the water.
It is unlikely that a significant removal of suspended solids
will occur with the activated alumina process. However, some
adsorption of the oil and grease probably will occur. It is not
known whether this characteristic will be eluted during
regeneration. If the oil and grease is not removed during
regeneration, the capacity of the bed could suffer.
Hy.droxylagatite. Hydroxylapatite (synthetic bone and bone char)
has been used to remove soluble fluoride.(6) The fluoride reacts
with the tri-calcium phosphate. Regeneration is accomplished by
caustic and phosphoric acid. This scheme is primarily a water
treatment process (initial fluoride content of about 13 ppm), and
its applicability to the 35 ppm stream in the present case is
unknown. Technical problems associated with this process are
92
-------
Feed
Regenerant Solution
\o
H90 - 18.9 x 10,
^ (41.6 x 10b)
F~ - 658
(1450)
Alumina
Column
17,840 «,
(630 ft3)
Adsorption
Cycle
HO - 18.9 x 10
1 (41.6 x 10b)
F~ - 38
(83.2)
H SO, - 23,900
1 * (52,600)
H_0 - 260,000
(573,000)
Alumina Column
17,840 SL
(630 ft3)
Regeneration Cycle
CaO -
8,000
(17,700)
F - 620
(1367)
H SO, - 10,840
(23,900)
Flows are in kg/day
(Ib/day)
CaSO, - 15,000
(32,000)
CaF, - 1200
(2700)
CaO - 900
(2,000)
CaSO,
- 95
(210)
- 20
(44)
Figure 13. Process to remove fluoride by adsorption on activated'alumina,
(250 T/D aluminum)
-------
high bed attrition and decreased efficiency in the presence of
chlorides.
Adsorption of oil and grease should occur on the bed; however,
whether oil and grease would be removed during subsequent
regeneration of the bed is unknown. There may also be some
removal of suspended solids by the process of filtration,
although a quantitative estimate of this removal is not possible
without experimental data.
Reverse Osmosis. Reverse osmosis (R.O.) is a process whereby a
waste~water stream is passed at pressures from 500-2000 psi over
a membrane, which tends to allow the water to permeate, but
rejects dissolved ionic salts. It should be possible, using
R.O., to produce an effluent which has only about 5 percent of
the fluoride content of the incoming water.
The fraction of the water which can permeate through the membrane
is of great importance in considering the applicability of this
process to dilute fluoride waste water. There are many
parameters which influence the fractional recovery of water, such
as level of dissolved solids, suspended solid content, solubility
relationships and equipment design. At least a 75 percent-
recovery should be obtained, resulting in a concentrate of about
135 ppm fluoride. This concentrated stream can be treated by
conventional lime precipitation.
The major technical problem, which can arise in the use of R.O.
for treatment of scrubber water, is the potential for fouling of
the membranes, due to the suspended solids and oil and grease
present in the stream. Before R.O. can be considered technically
feasible, experimental data would be necessary to establish the
severity of this problem. The presence of suspended solids may
preclude the use of the new hollow-fiber units which are more
durable in industrial applications. The oil and grease may tend
to form a relatively impermeable coating on the surface of the
membrane, with resulting elaborate and costly cleaning procedures
necessary. The widely used spiral-wound membranes are also quite
susceptible to plugging and would probably be unsatisfactory.
The tubular type membrane configuration is the most suitable
where plugging may be a problem.
A schematic diagram of a process to treat the effluent from the
once-through scrubbing system is shown in Figure 14. The feed is
pressurized, probably to about 5CO psi, and passed through the
reverse osmosis unit. Further treatment of the concentrate is
performed to reduce the fluoride content.
/
Anode Bake Furnace- Scrubber Water
The anode bake furnace flue gas contains particulate carbon, tar
vapors, sulfur compounds, and fuel-combustion products. Fluorine
compounds may be present, if anode stubs are recycled. The air
94
-------
vo
CaO - 1,873
•m j
*BBft i
H20 -
18.9 x 106,
(41.6 x 106>
1 -
658
(1^)
REVERSE
OSMOSIS
4.7 x 106
^ (10.4 x 106l
I -
i
635
<
(4.130)
r
MIXER
^
"
THICKK
HgO - 14.2 x 106,
Ł31.2 x 106)
, r- - 25 J
(55)
BJUt F - 136
VJ
CaT2- 1,012
(2,230)
[ CaO - 982
(2,165)
Data in - kg/da
(Ib/da)
Figure 14. Reverse osmosis treatment of fluoride waste water.
(250 T/D aluminum)
-------
pollution control applied to such flue gas includes no control,
dry systems, and wet systems. The wet systems may be either wet
electrostatic precipitators or wet scrubbers. If wet scrubbers
are used, the effluent from the scrubber contains tars and oil,
sulfates, particulate matter and, in some cases, fluorides. If
care is taken in the removal of fused cryolite from the anode
butts before reprocessing, fluoride emissions from the anode bake
plant would be greatly minimized, and hence fluoride
concentrations in the bake plant scrubber water would be
minimized.
Treatment of anode bake plant wet scrubber effluents consists, in
some instances, of settling the effluent in ponds after lime
treatment. After settling, the organic materials are skimmed
from the surface of the pond. Plants employing this practice
exhibit effluent loadings of oil and grease comparable to that
from other plants.
Casthpuse Cooling Water
Control of effluent water form the direct contact cooling of
ingots can be achieved by means of a cooling tower, with recycle
of the water. In this operation, a certain portion of the
cooling water must be bled from the circuit in order to prevent
the buildup of dissolved and suspended solids, as well as the oil
and grease. One plant treats this bleed stream (15C gpm) in an
aerated lagoon with a 15-day retention time, reducing the
hydrocarbon content by 85 percent.
Treatment of Cyanide-containing Streams
Cyanide is contained in the run-off water from spent cathode
storage areas and in the water circuit, if reprocessing of
cathodes is practiced. The values are low, ranging from net
concentrations of 0.002 to 0.036 ppm among the three plants
exhibiting a free cyanide discharge (Plant I reported a value of
0.05 ppm, but the effluent is not discharged to navibable
waters). The internal streams containing cyanide may be treated
with chlorine or hypochlorite to destroy the cyanide. No primary
aluminum plants currently treat cyanide specifically.
Summary of Waste Treatment_Effgctiveness
The data from the aluminum companies, as well as those data
calculated for different modes of water treatment, have been
summarized in Table 6. Several important points should be noted.
For water pollution control, a dry scrubbing system is best.
Better performance is a notable difference between the
once-through and recycle wet scrubber systems. The recycle
system is considerably more effective in the reduction of
fluorides and suspended solids. Effluent fluoride quantities are
96
-------
TABLE 6. EFFLUENT LEVELS ACHIEVED BY VARIOUS TREATMENT PROCESSES
'Typical values achieved or expected (see text)
Emission Level, kg/metric ton (Ib/ton)
Process
Dry Scrubbing
Wet Scrubbing
Wet Scrubbing
- Once Through
- Recycle
Recycle + Bleed & Filtrate
Treatment
Once Through +
Once Through +
Alumina
Once Through +
Once Through +
Alum
"Activated
Hydroxyl.apatite
Reverse Osmosis
Fluoride
0
5(10)
1(2)
0.05(0.1)
1(2)
0.25(0.5)
0.25(0.5)*
0.75(1.5)
Suspended
Solids
0
5(10)
1.5(3)
0.1(0.2)*
1(2)*
2.5(5)*
2.5(5)*
0.5(1)*
Hydrocarbons
0
0.5(1)
0.25(0.5)
0.01(0.02)*
0.05(0.1)*
0.25(0.5)*
0.05(0.1)*
0.05(0.1)*
* Estimate.
97
-------
about 5-10 kg/metric ton (10-2C lb/ ton) of aluminum, when a
once-through system is used, and 0.5 to 1 kg/metric ton (1-2
Ib/ton) of aluminum, when a recycle system is used.
It is both technically feasible and relatively simple to add a
further fluoride treatment process to the recycle system. This
is due primarily to the relatively high concentration of fluoride
(about 1 g/liter) and small flow volume in the effluent (bleed
and filtrate). By this technique, effluent values of about 0.05
kg/metric ton (0.1 Ib/ton) of aluminum can be obtained. Further
treatment of the once-through scrubber water is complicated by
the large volume of water, 19,000 to 38,000 cubic meters/day (5-
10 million gal/day), at low fluoride content ( 35 ppm). A
reduction of fluoride by four different methods has been
considered, and the best process appears to be fluoride removal
by sorption in an activated alumina bed.
Conclusions reached are that techniques are currently available
to reduce fluoride emissions to zero by use of a dry scrubbing
system on the potline off-gas, and to reduce fluoride emissions
to 1 kg/metric ton (2 Ib/ton) by a wet scrubber with recycle.
Control and Treatment Options
On the basis of the foregoing summary of the effectiveness of
various control and treatment technologies, some of the available
options by which an individual plant can achieve the effluent
limitations are summarized schematically in Figure 15. The
baseline is a plant using wet scrubbers for air pollution
control, with no treatment of the scrubber water prior to
discharge. The effluent fluoride loading for the baseline case
is about 15 kg per metric ton (30 lb per ton) of aluminum
produced. The effluent limitation (July 1, 1977) can be achieved
by the baseline plant by installing a cryolite-recovery system
with recycle and bleed. The effluent limitation (July 1, 1983)
then can be achieved by adding a lime treatment to the bleed
stream from the recycle scrubber circuit. Alternative options
open to a baseline plant include conversion of the wet scrubbing
system to a dry scrubbing system and retention of the wet
scrubbing system with provision for impoundment of the effluent.
Plants, currently practicing once-through lime and settle
treatment of water from wet scrubbers, have an effluent fluoride
loading in the range of 5 to 10 kg per metric ton (10 to 20
Ib/ton) of aluminum produced. Such plants can achieve the 1977
effluent limitation by adding a recycle system to the present
operation, or by adding additional treatment, such as adsorption
of fluoride on alumina. The 1983 effluent limitation can be
achieved by adding a lime treatment to the recycle bleed stream.
These plants also have the option of conversion to a dry
scrubbing system, or the employment of total impoundment.
Finally, plants now using cryolite recovery with recycle can,
with proper application of the technology and with good
housekeeping practice, achieve the 1977 effluent limitation. The
98
-------
Effluent
Loading
Baseline Case—Wet Scrubbing--Once-Through,
No Treatment of Scrubber Water
7.5
(15)-
1
.05
0
(2)
(0.1)
0
Lime and
Settle,
Once-Through
Cyrolite pptn
Plus Recycle
With Bleed
Total
Impoundment
Convert to
Dry Scrubbing
Adsorption
on Activated
Alumina
Lime Treatment of Bleed Stream
Level I
Level II
Level III-
Figure 15. Some control and treatment options,
99
-------
1983 effluent limitations then can be achieved by adding lime
treatment of the bleed stream.
The availability of various control and treatment technologies to
meet the effluent limitations provides flexibility to allow each
company to plan its future water pollution abatement program in
the manner most compatible with its existing pollution control
practices.
100
-------
SECTION VIII
COSTS, ENERGY, AND NONWATER QUALITY ASPECTS
Introduction
This section discusses the costs associated with the various
treatment strategies available to the primary aluminum smelting
industry to reduce the pollutant load in the effluents. In
addition, other nonwater quality aspects are discussed.
Basis for Cost Estimation
Data on capital investment and on operating costs for present
control practices were obtained from selected aluminum companies.
These data were modified in the following way to provide a common
basis.
(1) The capital investment reported was changed to
1971 dollars by the use of the Marshal and Steven's
Index (quarterly values of this index appear in
the publication Chemical Engineering, McGraw Hill).
(2) The operating cost was recalculated to reflect common
capitalized charges. To do this, the annual oper-
ating cost was calculated as follows:
Operating and maintenance - as reported by the
aluminum companies.
Depreciation - 5 percent of the 1971 capital.
Administrative overhead - 4 percent of operating
and maintenance.
Property tax and insurance - 0.8 percent of the
1971 capital.
Interest - 8 percent of the 1971 capital.
Other - as reported by aluminum companies.
The following procedure was used for estimating the capital and
operating costs of other processes, which could be applied to
water treatment.
Equipment costs were estimated from data in references (7) and
(8). The total capital investment was then calculated as this
cost plus:
Installation 50% of equipment
Piping 31% of equipment
Engineering 32% of equipment
Electrical Services 15% of equipment
Contractor's Fee 5% of equipment
Contingency 10% of equipment
The operating cost was calculated by estimating labor and raw
material requirements, and then adding the following items:
101
-------
Maintenance 5% of investment
Depreciation 5# of investment
Tax and Overhead 0.8% of investment
Interest 8% of investment
These additional capital and operating expenses were obtained
from values reported in reference (7) .
Economics_of_Present_Control_Practice
The economic data which will be discussed in this section are
summarized in Table 7. In order to present a total picture of
present practice with respect to control and treatment of water
in the primary aluminum industry, not all categories have cost
information. The following words have been used to denote the
reasons for the absence of cost information:
(a) Not used ^ no wet type pollution control device
is used.
(b) Untreated - a wet scrubber is used for air pollution
control, but the water is discharged untreated.
In addition, a dash indicates that insufficient information was
obtained to perform a cost estimate.
Where cost values are bracketed, this indicates that the cost was
calculated, either to put the costs on a common basis, or
calculated on equipment descriptions obtained. Also noted is
that Ł11 tons are metric throughout the following discussion.
Potline (Primary) Gas Scrubber Water
Essentially, there are two means to control the water effluent
from gas scrubbers on the potline: (a) use a dry scrubbing system
on the gases, which will reduce the water use to zero, (b)
recycle the scrubber water and precipitate the fluoride values
picked up in the scrubber. This latter technique, however, does
result in a small bleed stream of 10-20 gpm of effluent water.
The size of this stream must be varied to accommodate the build-
up of sulfates in the recycled stream. The cost data given below
were derived from reported analyses and costs, and are
independent of stream size, which may be characterized as being
on the order of 10-100 gpm.
The total capital cost of equipment, installation, and the
necessary plant facilities to provide dry scrubbing for a potline
have' been reported by several sources. In the present survey,
three companies quote a total investment for conversion to dry
scrubbing in the range of $36-$112 per annual ton of aluminum
produced. Nielsen and Kielback(9) report a capital cost of $30-
$40/annual ton, while Cook and Swany(lO) report a 1970 cost of
$60/annual ton for primary control of prebake plants, and
102
-------
TABLE 7. COST DATA FOR CONTROL AND TREATMENT OF WASTE WATERS FROM PRIMARY ALUMINUM PRODUCTION
o
CO
Level in Discharge
Plant
A
B
C
D
K
F-
G
B
I
J
K
Fluoride,
kg/ton
0
—
9
0.35
2.08
18.6
8.8 .
0.53
0.35*
1.06
1.20
Susp. Solid, Oil
kg/ ton
0.04
--
9
0.4
3.13
82**
10
2.65
6.2*
1.62
1.90
and Grease,
kg/ton
0.015
-
0.5
0.04
0.35
1.2
1.3
__
0.25*
0.22
0.24
Aluminum
Production,
ton/day
249
436
245
220
655
663-
218
598
284
455
259
Pot Line (Primary)
Capital Operating
$/annual ton $/ton
Dry scrubbing
..
12.5 1.24
(2.5)
Not used
Dry scrubbers
Untreated
2.32
6.06 1.45
(2.10)
12.4 1.0
(4.34)
--
3.03 4.33
(4.53)
Pot Room (Secondary)
Capital Operating
$/annual ton $/ton
Not used
Not used
Not used
23.2 7.06
(8.78)
5.4 3.15
(3.22)
Untreated
Untreated
Not used
Not used
Not used
Included with
pot line
Anode Bake
Capital Operating
$/annual ton $/ton
Not used
Soder.berg
0.53 0.10
1.04 0.20
0.38 0.07
Soder.berg
Soder.berg
Not used
Soderberg
Soderberg „
Not used
Cast House Cooling
Capital Operating
^/annual ton $/ton
(1.6) ' (0.4)
-
Untreated
Untreated
Untreated
Untreated
-
Untreated*
Untreated
(0.43) (0.06)
Rectifier
Cooling
Water
Air
Generators
Untreated
„
Air
--
Untreated
Untreated
Generators
Untreated
With cooling
water
NOTE: Ton li metric.
* Recycled to adjacent alumina plant.
** 65 of this from cryolite plant.
-------
$33/annual ton for vertical stud Soderberg. These costs include
both the collection system and the primary removal equipment. As
an average investment cost, a figure of $UO/annual ton is used in
the present study.
Operating cost data are relatively sparse because of the small
percentage (about 4 out of 31) of plants utilizing dry scrubbing.
Rush et al(ll) use an operating cost of $1C.20/ton for control of
prebake potline gases and a profit of $0.55/ton for vertical stud
Soderberg plants. The operating costs, all reflect the credit
calculated for recovered fluoride values. For the purposes of
this study, the value of $10.20/ ton, as representative of dry
scrubbing operating costs, was used, since more prebake plants
are in use than vertical stud Soderberg.
Cost information on the use of a recycle scrubber system has been
obtained from the aluminum companies surveyed. Referring to
Table 7, companies H, I, and K have a wet scrubber for primary
potline pollution control, which uses recycled solution for the
scrubbing operation. Capital costs vary from $3.03 to
$12.40/annual ton and, operating costs vary from $2.10 to
$4.53/ton. These costs include only the water control circuit,
namely, the chemical addition tank, thickener, cryolite recovery
equipment, and associated pumps, piping, etc. The operating
costs do not include any credit for recovered fluoride as, in
general, the cryolite is not of high enough quality to be
recycled to the potline. One company does calculate a credit
equal to about $2/ton of aluminum produced.
Gas^Scrubber Water
As shown in Table 7, only five of the 11 companies, surveyed for
cost information, practice air pollution control of potroom air.
Of these five, three utilize water control (D, E, and K) on this
circuit. Cost information was obtained from the two companies D
and E. Plant D reported a capital cost of $23.20/annual ton with
an operating cost of $8.80/ton. These costs include the recycle
water circuit only, consisting of chemical addition, thickening,
filtration, and kiln operation on the recovered cryolite, and a
treatment operation using CaCl2 to precipitate fluoride values
from the cryolite filtrate stream. No credit for recovered
cryolite is taken in operating costs.
In the second case (Plant E), costs reported are $5.40/annual ton
capital, and $3.15/ton operating. These costs again include only
the recycle water control circuit; however, in this case, no
treatment of the cryolite filtrate stream is done.
Anode_ B ake^Piant
At the present time, water control, by recycling water back
through the anode bake plant scrubber, is not practiced.
^However, there is one plant in which the bake plant is run at an
104
-------
acceptable air emission level with no wet scrubber on the exhaust
gases. This is done by exercising sophisticated control of the
firing of the anodes, and utilizing new flues in the exhaust
circuit. The company reported that it had not been successful in
its efforts to apply this type of control at six other plants.
Attendant with this type of control would be additional labor
requirements, and the necessity of proper flue condition.
However, no cost data have been obtained for this operation.
Costs are indirectly incurred in improved operating procedures or
plant improvements.
Casthouse Cooling
The method used to control effluent water from the casthouse
ingot cooling operation is the operation of a cooling tower. In
this operation, a certain proportion of the cooling water is bled
off in order to prevent the buildup of dissolved and suspended
solids, as well as oil and grease. One company (Plant A) treats
the bleed stream (150 gpm) in an aerated lagoon, with a 15-day
retention time, reducing the hydrocarbon content by 85 percent.
The cost of performing this operation, which includes the cooling
tower, lagoon and associated piping, engineering, services, etc.,
was calculated to be $1.60/annual ton capital and $0.40/ton
operating cost.
Rectifier Cooling Water
With the exception of heat, there are no process pollutants added
to water in use for rectifier cooling. There is, however, a
relatively large use of water (about 22,000 liters/ton of
aluminum (6600 gal/short ton)). The control measure, in practice
by industry, is to use air-cooled rectifiers. Cost data on
rectifier cooling were not obtained.
Economics of Present Treatment,. Practice
In this section, only those treatment processes applied to water
on a once-through basis are discussed. Although water treatment
is applied to techniques of water control by recycle, these were
discussed in the previous part.
Łotline_(Primary) Gas Scrubber_Water
Costs for treatment of potline scrubber water were obtained from
two companies (Plants C and G in Table 7). These costs include
only the water treatment in circuit, consisting of a mixing
chamber for the addition of the lime slurry, thickener tank, and
associated pumps, piping, etc.
Plant c reports a capital cost of $12.5/annual ton and operating
costs of $2.50/ton; the capital cost for Plant G is $2.32/annual
ton. It is noteworthy that Plant C treats about 3.5 times the
105
-------
volume of water as Plant G, 160,000 liters/ton (38,500 gal/short
ton) versus 52,000 liters/ton (12,500 gal/short ton). In
addition, included with the water treatment cost for Plant C is
the cost to treat anode bake furnace scrubber water, although
this probably amounts to less than 10 percent of the total cost
(on the basis of the proportion of flows).
Potroom (Secondary) Scrubber Water
Where wet scrubbing of potroom air (secondary control) is used,
those having a once-through system do not apply any treatment to
their effluent water. Of the four plants which do practice
secondary control, two utilize recycle water in the scrubber
(Plants D and E), and two do not treat the scrubber water.
Anode Bake Furnace Scrubber Water
Treatment costs have not been reported by the, three companies (C,
D, and E) practicing water treatment of the scrubber water from
the anode bake plant. The treatment method consists essentially
of ponding the effluent, and skimming the oil and tars from the
surface. Based on an estimate of the size of the ponds, costs
were calculated for each plant. These costs are shown in Table
7. Calculated residence times for Plants C, D, and E are 42
minutes, 21 hours, and 210 minutes, respectively. An analysis of
the scrubber water before and after the ponding treatment of
Plant D revealed that about 60 percent of the oil and grease and
suspended solids was removed.
Casthouse_Cogling^Water
One of the companies contacted (Plant K) employs a lagoon into
which all the water effluent, including that from the casthouse,
flows before being discharged from the plant. The estimated size
of this lagoon was 2 acres, and a cost was calculated to be
$0.43/annual ton capital and $0.06/ton operating for this
procedure. Data on the retention time and effectiveness of this
operation were not obtained.
Cost Effectiveness iPresent_PracticeJ_
Fluoride Effluent Control (Potline and Potroom)^
The cost data presented in Table 7 have been plotted against
total fluoride effluent from the plant. Figure 16 gives capital
costs and Figure 17 gives operating costs.
The figures indicate that the cost increases as the amount of
fluoride in the effluent stream decreases. The most expensive
option is the conversion of a wet scrubbing system on the potline
106
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55
O
H
H
CO
8
20
I
\
POINT PROCESS
1 DRY SCRUBBING
2 RECYCLE SECONDARY
3 RECYCLE PRIMARY
4 RECYCLE PRIMARY
5 RECYCLE SECONDARY
6 RECYCLE PRIMARY
7 ONCE THROUGH PRIMARY
8 ONCE THROUGH PRIMARY
9 UNTREATED
I
5 to
KgF/TON Al
Figure 16. Cost effectiveness of water control and
treatment to remove fluorides (capital COSt)
-------
10
o
oo
o
H
O
H
H
a
6
i06
POINT PROCESS
1 DRY SCRUBBING
2 RECYCLE SECONDARY
3 RECYCLE PRIMARY
4 RECYCLE PRIMARY
5 RECYCLE SECONDARY
6 RECYCLE PRIMARY
7 ONCE THROUGH PRIMARY
8 UNTREATED
PM
O
O
-4
/o
KgF/TON Al
Figure 17. Cost effectiveness of water control and
treatment to remove fluorides (operating cost)
-------
to a dry scrubbing one, although the water use would be zero. A
dry scrubbing system, however, has not been proven as technically
feasible for use on potroom secondary air.
The relatively high capital required for installation of a dry
scrubbing system actually applies only for those plants which
would be converting from a wet system. A wet scrubbing system
installed at a new facility costs about $38/annual ton, including
the cost of associated equipment. Thus, the difference in cost
between the two systems for a new plant would only be about
$2/annual ton.
The recycle of scrubbing water on both potline (primary) gas and
potroom (secondary) gas results in fluoride effluents of less
than 1 kg/ton (2 lb/short ton). An average cost for this means
of control is about $10/annual ton capital and $4.60/ton
operating.
The use of once-through water in the wet scrubbing system of
potlines, with lime treatment before discharge, results in
effluent fluoride levels of about 5 kg/ton (10 Ib/short ton).
Costs associated with this treatment process are $7.0/annual ton
capital and $2.50/ton operating.
The following conclusions can be made regarding the cost
effectiveness of fluoride control:
(1) The best cost-effective means of control for new plants
with a prebake or vertical stud Soderberg configuration is the
installation of a dry scrubbing system on the potline gaseous
effluents. Tight hoods should be provided, and the operation
conducted in such a manner as to minimize any potroom
contamination.
(2) The most cost-effective means of removing fluoride for
those plants with existing wet scrubber systems is the operation
of a recycle loop to the scrubber with cryolite or lime
precipitation. The difference in cost between this system and
the once-through system with lime treatment is relatively low.
Effluent fluoride amounts in the water from the recycle scrubber
operation are about 1/10 as high as those from gas scrubbers
operated on a once-through basis.
Suspended Solids Effluent Control (jPotline_and_PotroomJ_
Treatment to remove fluoride will tend to remove suspended
solids. In the dry system, there are no suspended solids. As a
wet system for fluoride control involves a settling operation of
CaF^, the suspended solids also will tend to settle. Therefore,
conclusions about cost effectiveness applicable to fluoride are
applicable to suspended solids control.
Costs of Additional Treatment Prgcesges
109
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As previously mentioned, dry scrubbing of the potline gas can
result in no discharge of pollutants, if a secondary wet
scrubbing system is not required. However, additional control
measures can be added to the wet scrubbing processes, which will
effect a reduction in the amount of pollutants discharged to the
waste streams. A technical discussion of these systems has been
given in Section VII of this document. The economics and cost
benefits associated with each of these processes are now dealt
with.
Potline and Potroom Scrubber Water Treatment
The choice of additional treatment schemes to be applied to
scrubber water effluent depends primarily on whether 5 recycle
system or once-through system is in use.
In a recycle system, additional control of fluorid ;s and
suspended solids can be affected by the lime or CaCl2_ t reatment
of the filtrate stream from the cryolite and the bleed stream
from the scrubber. The costs calculated for this treatnent are
$1.50/annual ton capital and $0.64/ton operating. Thes<: costs
include a mixing tank for chemical addition, a thicken< r tank,
pumps, piping services, etc. The costs are relativ*ly low
compared with other fluoride treatment processes, because of the
low volume of effluent to be treated, about 120 liters/mir ute (30
gpm) , and high concentration of fluoride, about 1,000 mg/1. It
is expected that this treatment would reduce suspended sclids by
a similar amount.
The addition of a treatment process to the water effluent from a
once-through potline and potroom scrubber after lime tieatment
(if practiced) is more costly than the previous treatr ent of
recycle effluents. In this case, large volumes of water vith low
concentration of fluorides and other pollutants are involved in
the treatment process.
For the purposes of cost calculations, a plant size of 227
tons/day (250 short tons/day) was taken. The flow rate of water
from the once-through scrubbing system was taken to be 83,300
liters/ton (20,000 gal/short ton) with a concentration of 35 mg/1
fluoride. The latter values represent averages found in the
aluminum industry.
Alum Treatment. The addition of an alum treatment would add
about Sll.O/annual ton capital and $8.40/ton operating. The
capital cost includes a mixing tank, flocculant tank, clarifier,
and pumps. The major equipment cost is the 37-meter (121-foot)
diameter clarifier, which accounts for 8H percent of the $372,000
equipment cost.
The major operating cost is expenditure for alum ($69/ton), which
represents about 69 percent of about $700,000/year.
110
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Activated Alumina. The costs associated with the use of an
activated alumina adsorption process are $2.7/annual ton capital
and $3.8/ton operating. Capital costs include two alumina
adsorption towers (one for adsorption, one for regeneration), a
mixer for the treatment of the spent regenerant solution, a
thickener, the initial charge of alumina, and associated pumps
and piping.
Regarding operating costs, the cost of the sulfuric acid used for
regeneration represents about 50 percent of the $313,0007 year
operating cost.
Hydroxvlagat.it e. The costs for the adsorption of fluoride on
bone char were taken directly, as reported by Wamsley and
Janes.(6) Capital costs were scaled up from 1947 values, and
depreciation, tax, and interest costs were added to the reported
operating costs. The costs obtained were $14.507 annual ton
capital and $14.5C/ton operating.
Reverse Osmosis. Costs associated with reverse osmosis treatment
are very sensitive to the nature of the dissolved constituents,
pH of the water, size of plant, pretreatment requirements, and
several other factors. For the present study, a typical
operating cost value of $0.26/1,000 liters ($1/1,000 gal)
calculated for several different reverse osmosis applications has
been assumed. Data on capital cost are too scattered to yield a
meaningful estimate. The operating cost per ton of aluminum was
calculated to be $22. This value includes the cost of reverse
osmosis, plus the cost necessary to treat the concentrated
effluent with lime.
Cost Effectiveness
The cost data developed in the foregoing paragraphs for
additional treatment of potline and potroom scrubber water are
summarized in Table 8, along with the estimated fluoride
discharge from the plant. The elements included in capital cost
and operating cost are those discussed on the first page of this
section. Several conclusions regarding cost effectiveness can be
drawn from the data:
(1) For new plants, a dry system is preferable. The cost
difference between a dry system and one with recycle and effluent
control would be negligible.
(2) For plants, which already have a recycle scrubber
operation on their potline or potroom gases, the addition of
further treatment of the two effluent streams is both inexpensive
and very effective.
(3) For plants utilizing a once-through scrubber system, a
conversion to the recycle mode yields the best cost benefit.
Although an activated alumina adsorption process added to the
once-through scrubber water costs approximately the same, about
111
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TABLE 8. COSTS OF VARIOUS ALTERNATIVES FOR FLUORIDE REMOVAL
Process Alternative
Dry scrubbing
Wet scrubbing--once-through
Wet scrubbing--recycle
Recycle with bleed and
filtrate treatment
Once-through and alum
treatment
Once-through and activated
alumina treatment
Once-through and hydroxyla-
patite treatment
Once-through and reverse
osmosis treatment
Discharge
Fluoride,
kg/ton
0
-5
1
0.05
1
0.25
0.25
0.8
Capital Cost,
$/ annual ton
40
7.4
10
11.5
18.3
9.7
21.9
Operating
Cost, $/ton
10.2
2.5
4.6
5.2
11
6.3
16.5
24.5
Note: ton - metric ton; values are 10 percent lower for short ton.
112
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five times the amount of pollutants would be discharged in the
water from the activated alumina system.
Nonwater Quality Aspects
Energy Requirements
Specific data on energy requirements were not available from most
of the plants surveyed. Data supplied by Plant D, which
practices cryolite recovery and recycle on secondary (potroom)
scrubber liquor, were as follows:
kg-cal/metric_tQn_Al Btu/ton_Al
Thermal Energy
Rotary kiln 75,000-151,200 300,000-600,000
Steam generation 25,200- 75,000 100,000-300,000
Total thermal energy 100,000^226,200 400,000-900,000
Electrical Energy
Pumps 41 kwhr/metric ton Al (37 kwhr/ton Al)
The total energy requirement expressed in terms of equivalent
electrical energy is 165 to 330 kwhr/metric ton (150 to 300 kwhr/
ton Al), which is 0.7 to 1.5 percent of the energy consumed by
the rest of the smelting operation. An estimate, supplied by
Plant E, was 13.3 kwhr for the electrical power required to
operate a similar cryolite recovery system, or a factor of three
lower than the corresponding estimate for Plant D. The
electrical requirements for the operation of other control and
treatment options described in this document are expected to be
of similar magnitude. Because the energy requirements of control
and treatment methods represent only a small fraction of the
total energy consumed in the primary aluminum industry, the
difference in energy requirements will not be a deciding factor
in the choice of control and treatment technology.
Solid Waste Production
A number of the control and treatment technologies identified in
this document produce solid waste as an adjunct to their
operation. An exception is the conversion of wet scrubbing
systems to dry scrubbing. This technology does not produce a
solid waste, but rather, allows the collected particulates and
gases to be returned to the electrolytic cell.
Limited data on the quantities of solid waste produced were
available. Plant D, which practices cryolite precipitation, must
dispose of about 30 kg/metric ton Al (60 Ib/ton Al) of solid
waste containing cryolite and carbon. Waste water treatment, by
addition of lime, produces a calcium fluoride sludge. Plant G
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reported the production of 25-30 kg sludge/metric ton Al (50-60
Ib sludge/ton Al) from this treatment. Plant C reported about 15
kg sludge/metric ton Al (30 Ib sludge/ton Al) from the same
treatment.
Summary
The energy requirements and solid waste production for the
various control and treatment technologies are summarized, for
purposes of comparsion, in Table 9. The values are calculated
from data supplied by various primary aluminum producers or are
estimated on the basis of assumed operating parameters. The
energy-use values are all calculated to include the energy
required by the scrubbing process, in addition to that required
by the subsequent treatment process, in order to provide a direct
comparison of wet scrubbing plus various treatments with dry
scrubbing. The data show that dry scrubbing compares favorably
with wet scrubbing plus recycle. In any case the energy
requirements are small when compared with the energy used by the
remainder of the primary aluminum process, which is about 22,000
kwhr/metric ton (20,000 kwhr/short ton).
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TABLE 9. ENERGY REQUIREMENTS AND SOLID WASTE PRODUCTION FOR VARIOUS
WATER EFFLUENT CONTROL AND TREATMENT TECHNOLOGIES
Process
Energy Use
Electrical,
kwhr/ton
Thermal,
Equivalent
kwhr/ton
Sludge
Production
kg/ton
Dry Scrubbing 233
Primary wet scrubbing 84
with recycle - Process A
Secondary wet scrubbing 394
with recycle
Primary wet scrubbing - 84
once through - Process B
Process A plus bleed and 85-395
filtrate treatment
Process B plus alum 100
treatment
Process B plus activated 100
alumina treatment
Process B plus hydroxy- 100
lapatite treatment
Process B plus reverse 546
osmosis treatment
0
200
200
200
0
73
76
40
77
123
110
60
Note: ton = metric ton; values are 10 percent lower for short ton.
115
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SECTION IX
BEST PRACTICABLE CONTROL TECHNOLOGY CURRENTLY
AVAILABLE—EFFLUENT LIMITATIONS GUIDELINES
Introduction
The effluent limitations, which must be achieved by July 1, 1977,
are to specify the degree of effluent reduction attainable
through the application of the best practicable control tech-
nology currently available. Best practicable control technology
currently available is based upon the average of the best
existing performance by plants of various sizes, ages, and unit
processes within the industrial category and/or subcategory-
This average is not based upon a broad range of plants within the
primary aluminum industry, but is based upon performance levels
achieved by exemplary plants.
Consideration also must be given to:
(a) The total cost of application of technology in relation to
the effluent reduction benefits to be achieved from such
application.
(b) The size and age of equipment and facilities involved.
(c) The processes employed.
(d) The engineering aspects of the application of various types
of control techniques.
(e) Process changes.
(f) Nonwater quality environmental impact (including energy
requirements).
The best practicable control technology currently available
emphasizes treatment facilities at the end of a manufacturing
process, but includes the control technologies within the process
itself, when the latter are considered to be normal practice
within an industry.
A further consideration is the degree of economic and engineering
reliability, which must be established for the technology to be
currently available. As a result of demonstration projects,
pilot plants and general use, there must exist a high degree of
confidence in the engineering and economic practicability of the
technology at the time of commencement of construction or instal-
lation of the control facilities.
Effluent Limitations
Based on the information contained in Sections III through VIII
of this document, the best practicable control technology
currently available for the primary aluminum smelting subcategory
is the removal of fluoride by precipitation and recycle of the
clarified liquor. The effluent limitations attainable through
the application of the best practicable control technology
currently available are as follows:
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Effluent Limitations
Effluent Average of daily
Characteristic Maximum for values for 30
any 1 day consecutive days
shall not exceed
Metric units (kilograms per 1,000 kg
of product)
Fluoride 2,0 1.0
TSS 3.0 1.5
pH Within the range 6.0 to 9.0.
English units (pounds per 1,000 Ib
of product)
Fluoride 2.0 1.0
TSS 3.0 1.5
pH Within the range 6.0 to 9.0.
These effluent limitations are based on the average effluent
loading values for six of the exemplary plants in the subcategory
as follows:
From Table 4 the following values (Ib/ton Al) are given for
effluent loadings:
2lSŁt Fluoride Suspended Solids
A — 1.1
D 0.7 0.8
H 1.1 4.5
I 0.7 11.9
J 2.2 i 3.8
K 1.9 ! 0.4
These six plants represent the best overall effluent levels of
those plants for which data are available. Of the fluoride
values, that for Plant D is questionable. The treatment system
is causing some difficulty, and the company reported a range of
values for fluoride of 0.4, 16, and 52 mg/1 for low, average and
high, respectively. The average value of 16 mg/1 was used to
derive the value 0.7 Ib. F/ton Al entered in Table 4. The
maximum value of 52 mg/1 would give an effluent loading of 2.2 Ib
F/ton Al. The data obtained during a second verification
sampling trip support the higher value. If the value 2.2 Ib
F/ton Al is taken for Plant D, the average of the 5 plants with
fluoride values reported is 1.8 Ib F/ton Al. This was rounded to
118
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2 Ib F/ton Al to obtain the effluent limitation. Note that all
five of these plants use cryolite precipitation with recycle.
The average of the six values for suspended solids is 3.7 Ib/ton
Al. Since Plant I sends its effluent to a companion plant, the
solids are probably not settled with care. Hence, the high value
for Plant I was given lesser weight, and the average value was
rounded down to 3 Ib/ton Al to arrive at the effluent limitation.
The effluent limitations described above and referred to as 30-
day average values are the maximum average of daily values for
any consectuive 30-day period. The single day maximum effluent
limitations were derived by comparing the maximum discharge and
average discharge values taken from Corps of Engineers Discharge
Permit Applications. The ratio of maximum fluoride discharge for
10 companies, for which such data were available, ranged from 1.1
to 10.7 with an average value of 2.8. When the highest value was
deleted, the average ratio was 1.75. For suspended solids, the
range of ratio values was 1.2 to 18.3, with an average of 3.4.
When the highest value was deleted, the average ratio was 1.78.
On the, basis of these data, the single day maximum effluent
limitations for each pollutant were established at a factor of
two greater than the 30-day average limitations.
Rationale for Effluent Limitations
The effluent limitations are based on the following
considerations:
(1) Achievement of the effluent limitations by all primary
aluminum plants will result in a marked, industry-wide
reduction in the discharge of pollutants.
(2) The effluent limitations are based on treatment and recycle
of wet scrubber water, as summarized in the next section.
However, alternate technologies have been identified, which
can also be employed to achieve the effluent limitations.
This flexibility of approach will allow each company to take
advantage of local conditions of climate, existing
facilities, staff experience, and other circumstances to
achieve the effluent limitations in a manner most compatible
with intermediate and long-range goals.
(3) The effluent limitations are realistic. Currently about one-
third of the primary aluminum plants are able to achieve the
effluent reductions.
Identification of Best Practicable
Control Technology Currently Available
The best practicable control technology currently available for
the primary aluminum industry is the treatment of wet scrubber
water and other fluoride-containing effluents to precipitate the
fluoride, followed by settling of the precipitate and recycling
119
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of the clarified liquor -to the wet scrubbers. Recycling will
control the volume of waste water discharged. Two precipitation
methods currently are available, cryolite precipitation and
precipitation with lime.
Precipitation of cryolite
The technology for cryolite precipitation is presented in Section
VII. To implement this technology requires:
(1) Segregation of fluoride-containing waste water for treatment
including: potline scrubber water, potroom scrubber water,
anode bake plant scrubber water, used cathode disposal liquor
or runoff from used cathode storage area, and storm water
runoff if contaminated with fluoride.
(2) Recycling clarified liquor after precipitation of cryolite.
Total recycle is not possible. A bleed from the system is
required to prevent sulfate build-up in the recycled liquor.
(3) Minimizing the volume of the bleed stream, so that the
quantity of pollutants discharged in the bleed stream does
not exceed the effluent limitations.
(U) Providing a holding pond or lagoon, if necessary, to
accomplish further settling of solids in the bleed stream.
Lime Precipitation
The technology for lime precipitation is presented in Section
VII. To implement this technology requires:
(1) Segregation of fluoride-containing waste waters as listed for
precipitation of cryolite.
(2) Recycling clarified liquor after precipitation of calcium
fluoride. Bleed as necessary to maintain the quality of the
recycle stream.
(3) Minimizing the volume of the bleed stream.
(4) Providing a holding pond or lagoon, if necessary, to minimize
the discharge of suspended solids.
Alternate Control Technology
Alternate control technology, which can be employed to achieve
the effluent limitations, includes dry fume scrubbing, total
impoundment, and reuse of effluent water by a companion
operation.
Dry Fume Scrubbing. The use of dry scrubbing of primary potline
gases eliminates~the major sources of water pollutants from the
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primary aluminum plant, and would achieve ths effluent
limitations.
Total Impoundment. At least one plant currently achieves no
discharge of waste water pollutants through total impoundment of
aqueous wastes. Such practice is indeed exemplary; however, it
may not be practicable for the industry in all geographic
locations at this time.
geuserof Effluent by a Companion Operation. One primary aluminum
plant currently achieves ho discharge of "pollutants by sending
all effluent water to the nearby plant for use as make-up water.
Again, this practice is exemplary. However, this is a unique
situation and the practice cannot be cited as currently
available.
Rationale for the Selection_of_Best Practicable Control
Technology Currently Available ~ ~ ~~ ~
The selection of best practicable control technology currently
available was based on the following considerations:
(1) The lowest unit effluent loadings for fluoride and suspended
solids are currently attained by plants using dry fume
scrubbing, total impoundment of effluent, reuse of effluent
water by a companion operation, or waste water treatment to
precipitate fluoride with recycle of water to control the
volume of water discharged. The first three alternatives are
limited in one way or another in their applicability, and are
not feasible for all plants in the primary aluminum smelting
subcategory at the present time. The fourth alternative was
selected as the control technology.
(2) The selected technology is capable of achieving significant
reductions in discharge of pollutants, as verified by
analysis of samples collected on-site at three plants
practicing variations of the indicated control technology.
(3) In addition to control of fluoride discharges, the selected
control technology also achieves reduction in the discharge
levels of suspended solids.
(U) This technology is compatible with all known industry
variations such as age and size of plant, processes employed,
plant location, and anode type. Thus, this technology could
be employed by any plant at the option of the company
management.
(5) This level of technology is practicable, because at least
one-third of the 31 plants currently practice some form of
precipitation plus recycle technology.
(6) The effluent reduction benefits balance the costs of this
technology. Cryolite recovered and returned to the aluminum
reduction process is a potential credit to the control
technology costs. One company has a market for calcium
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fluoride produced by lime precipitation. Based on the
information contained in Section VIII, it is concluded that
those plants not presently achieving the July 1, 1977,
limitations would require an estimated capital investment of
about $10/annual metric ton ($9/annual short ton), and an
increased operating cost of about $4.6/metric ton ($4.27
short ton), in order to achieve the effluent limitations.
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SECTION X
BEST AVAILABLE TECHNOLOGY ECONOMICALLY
ACHIEVABLE—EFFLUENT LIMITATIONS GUIDELINES
Introduction
The effluent limitations which must be achieved by July 1, 1983,
are to specify the degree of effluent reduction attainable
through the application of best available technology economically
achievable. This technology can be based on the very best
control and treatment technology employed by a specific point
source within the industry category and/or subcategory, or
technology that is readily transferable from one industry process
to another. A determination must be made as to the availability
of control measures and practices to eliminate the discharge of
pollutants, taking into account the cost of such elimination.
Consideration must also be given to:
(a) The age of the equipment and facilities involved.
(b) The process employed.
(c) The engineering aspects of the application of various types
of control technologies.
(d) Process changes.
(e) Cost of achieving the effluent reduction resulting from the
technology.
(f) Nonwater quality environmental impact (including energy
requirements) .
The best available technology economically achievable also
assesses the availability in all cases of inprocess controls as
well as the control or additional treatment techniques employed
at the end of a production process.
A further consideration is the availability of processes and
control technology at the pilot plant, semi-works, or other
levels, which have demonstrated both technological performances
and economic viability at a level sufficient to reasonably
justify investing in such facilities. Best available technology
economically achievable is the highest degree of control
technology that has been achieved, or has been demonstrated to be
capable of being designed for plant scale operation, up to and
including no discharge of pollutants. Best available technology
economically achievable may be characterized by some technical
risk with respect to performance and with respect to certainty of
costs, and thus may necessitate some industrially sponsored
development work prior to its application.
Effluent Limitations
Based upon the information contained in Sections III through VIII
of this report, a determination has been made that best available
technology economically achievable for the primary aluminum
smelting subcategory is lime treatment of the bleed stream from a
123
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fluoride precipitation and recycle system. The effluent
limitations attainable through the application of the best
available technology economically achievable are as follows:
Effluent Limitations
Effluent Average of daily
Characteristic Maximum for values for 30
any 1 day consecutive days
shall not exceed
Metric units (kilograms per 1,000 kg
of product)
Fluoride 0.1 0.05
TSS .2 .1
pH Within the range 6.0 to 9.0.
English units (pounds per 1,000 Ib
of product)
Fluoride 0.1 0.05
TSS .2 * .1
pH Within the range 6.0 to 9.0.
Rationale for_Effluent Limitations
The effluent limitations are based on the following
considerations:
(1) Achievement of the effluent limitations by all primary
aluminum plants will result in an additional 90-95 percent
reduction in pollutant discharges by 1983, relative to the
levels required by 1977.
(2) The effluent limitations are based on additional treatment of
wet scrubber water as summarized in the next section. It
represents a stepwise approach to near zero pollutant
discharge, the first step to be completed by 1977, and the
second step, in logical sequence, to be completed by 1983.
(3) Alternate technologies, such as dry scrubbing and total
impoundment, have been identified, which also can be employed
to achieve the effluent limitations. These alternative
technologies are options open to each company and provide for
a flexibility of approach to water pollution abatement.
Identification of Best Available^TechnQlogy^Economically__AchievabT_e
124
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The application of the best practicable control technology
currently available as described in Section VII and IX, results
in a relatively low volume, high concentration bleed stream. The
best available technology economically achievable is the lime
precipitation treatment of such a bleed stream to further reduce
the discharge of fluoride. Such techniques are described in
Section VII. To implement this technology requires:
(1) Restriction of the volume of fluoride-containing effluent to
be treated to approximately 5000 liters per metric ton of
aluminum (1200 gallons per short ton), and treating the
stream with lime or calcium chloride to reduce the fluoride
concentration to a final value of approximately 10 mg per
liter.
(2) Alternatively, volumes as high as 50,000 liters per metric
ton of aluminum (12,000 gallons per short ton) treated to a
final fluoride concentration of 1 mg per liter would achieve
the effluent limitations. Treatment to 1 mg per liter final
concentration would require processing by adsorption methods
which are not state-of-the-art methods in the primary
aluminum industry but which could be adapted from related
water conditioning applications.
Rationale for Selection of the Best Available
Technology Economically Achievable
The selection of the best available technology economically
achievable was based on the following considerations:
(1) Effluent loadings, substantially lower than those achieved by
the best practicable control technology currently available,
can be achieved by following such treatment with a second
stage precipitation of fluoride.
(2) While such second stage treatment is not practiced currently,
it represents similar technology applied to a smaller stream;
hence, the technology can be considered to be available.
(3) Based on information contained in Section VIII, those plants
already in compliance with the July 1, 1977, effluent
limitation, but not achieving the July 1, 1983, effluent
limitations, would have to invest an additional $3.8/ annual
metric ton ($3.5/annual short ton) and would require an
additional operating cost of about $1.13/metric ton
($1.0/short ton). The breakdown of these costs is as
follows:
125
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Capital Operating
$/annual metric $/metric
ton ton
Additional fluoride and suspended
solids treatment on scrubber water 1.5 0.6
Anode bake furnace scrubber water
treatment 0.7 0.13
Casthouse cooling water control and
treatment 1.6 0.4
TOTAL 3.8 1.13
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SECTION XT
NEW SOURCE PERFORMANCE STANDARDS
Introduction
The standards of performance which must be achieved by new
sources are to specify the degree of effluent reduction
attainable through the application of higher levels of pollution
control than those identified as best available technology
economically achievable for existing sources. The added
consideration for new sources is the degree of effluent reduction
attainable through the use of improved production processes
and/or treatment techniques. The term "new source" is defined by
the Act to mean "any source, the construction of which is
commenced after publication of proposed regulations prescribing a
standard of performance".
New source performance standards may be based on the best in-
plant and end-of-process technology identified. Additional
considerations applicable to new source performance standards
take into account techniques for reducing the level of effluent
by changing the production process itself or adopting alternative
processes, operating methods, or other alternatives. The end
result will be the identification of effluent standards, which
reflect levels of control achievable through the use of improved
production processes (as well as control technology), rather than
prescribing a particular type of process or technology, which
must be employed. A further determination must be made as to
whether a standard permitting no discharge of process waste water
pollutants is practicable.
Consideration must be given to:
(a) The type of process employed and process changes.
(b) Operating methods.
(c) Batch as opposed to continuous operations.
(d) Use of alternative raw materials and mixes of raw materials.
(e) Use of dry, rather than wet, processes (including
substitution of recoverable solvents for water).
(f) Recovery of pollutants as by-products.
Standards of performance are applicable to new sources in the
primary aluminum smelting subcategory.
Standards of Performance fgr_New_Sources
Based on the information contained in Sections III through VIII
of this report, the best available demonstrated control
technology, processes, operating methods, or other alternatives
for the primary aluminum smelting subcategory is the dry
scrubbing of potline air and the control and treatment of
fluoride-containing waste streams by recycle and treatment of any
necessary bleed stream by lime precipitation. The standards of
127
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performance attainable through the application of this technology
are as follows:
Effluent Limitations
Effluent Average of daily
Characteristic Maximum for values for 30
any 1 day consecutive days
shall not exceed
Metric units (kilograms per 1,000 kg
of product)
Fluoride 0.05 0.025
TSS .1 .05
pH Within the range 6.0 to 9.0.
English units (pounds per 1,000 Ib
of product)
Fluoride 0.05 0.025
TSS .1 .05
pH Within the range 6.0 to 9.0.
Rationale^ or Standards of Performance
The standards of performance for the primary aluminum smelting
subcategory are based on the following considerations:
(1) A new source has complete freedom of design, so that unit
processes can be chosen to minimize the use of water in the
plant.
(2) Dry fume scrubbing processes are avialable for air pollution
control of potline air. The use of such systems in the
design of a new plant will eliminate potline wet scrubbers as
a source of waste water contaminants.
(3) Even with dry scrubbing of potline air, certain water uses
will be required. There are no demonstrated dry scrubbing
systems at this time for anode bake plant flue gases, which
achieve acceptable control of fluoride emissions to the
atmosphere; thus, wet scrubbing may be required on anode bake
plants to meet air pollution control regulations. Casthouse
cooling water can be recycled through a cooling tower;
however, a bleed is required to prevent buildup of dissolved
and suspended solids.
(U) Water from anode bake plant wet scrubbers and the casthouse
cooling water bleed stream can be treated to minimize the
discharge of pollutants, but no discharge of process waste
water pollutants cannot be achieved by any demonstrated
control or treatment practice.
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(5) The standards of performance for new sources are lower for
• fluoride and suspended solids than those applicable to
existing sources by July 1, 1983, because the availability to
new sources of dry scrubbing potline air eliminates that
source of fluoride and suspended solids pollutants.
Identification_of Best Available
Demonstrated~Control Technology,
Processes^_O]peratincf Methods, or Other^Alternatives
As the primary smelting of aluminum requires no process water
directly, the principal area, where use of water can be minimized
in the design of a new plant, is the application of dry fume
scrubbing of potline air for air pollution control. Such methods
exhibit high collection efficiencies and the fluoride values
contained in the fume can be recovered in a form amenable to
recycle to the smelting process. Alternate technologies, which
may be employed in certain circumstances to achieve no discharge
of pollutants, are wet scrubbing for air pollution control with
total impoundment of the scrubber water or with total recycle of
the scrubber water.
Other alternative unit process designs, which have been
identified in currently operating plants, have included air-
cooled, solid state rectifiers, which eliminate both use and
discharge of rectifier cooling water, and a number of alternate
methods, of molten metal degassing techniques (identified in more
detail in section VII), which similarly eliminate both use and
discharge of casthouse scrubber waste water, while achieving
compliance with air pollution control regulations.
The treatment technology for fluoride and suspended solids
removal in waste water from anode bake plant wet scrubbers
consists of lime precipitation of the fluoride, followed by
settling of the solids and recycle of the clarified liquor to the
scrubbers, as required to control the volume of waste water
discharged. This technology is not currently practiced with high
effectiveness on water from anode bake plant wet scrubbers, but
is analogous to that presented in section VII for water from
potline wet scrubbers. The standards of performance require
restriction of the discharge volume to 835 liters per metric ton
of aluminum (200 gallons per short ton) at a final fluoride
concentration of 30 mg per liter, or equivalent combination of
fluoride level and volume. This treatment requirement can be
minimized by careful removal of fused cryolite and other bath
materials from the anode butts, before recycling them to the
anode preparation operation. Good quality control of that
operation will result in lower fluoride loads in the anode bake
plant scrubber water.
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SECTION XII
ACKNOWLEDGEMENTS
The Environmental Protection Agency would like to thank the staff
of the Battelle Memorial Institute, Columbus, under the direction
of Mr. John B. Hallowell, for their aid in the preparation of
this document.
The project officer, George S. Thompson, Jr., would like to thank
his associates in the Effluent Guidelines Division, namely Mr.
Allen Cywin, Mir. Ernst P. Hall and Mr. Walter J. Hunt for their
valuable suggestions and assistance.
Mr. Harry Thron, Effluent Guidelines Division, was responsible
for the proposed regulation and development document (October,
1973) for this industry.
The members of the working group/steering committee who
coordinated the internal EPA review are:
Mr. Walter J. Hunt, Chairman, Effluent Guidelines Division
Mr. Marshall Dick, Office of Research and Development
Mr. John Ciancia, National Environmental Research center,
Edison
Mr. Lew Felleisen, Region III
Mr. Swep Davis, pjEJLice of Planning and Evaluation
Mr. Taylor—Miller, Office of General Counsel.
Appreciation is also extended to the following trade associations
and corporations for assistance and cooperation provided in this
program:
The Aluminum Association, Clean Water Subcommittee
Aluminum Company of America
Eastalco
Kaiser Aluminum and Chemical Corporation
Martin - Marietta
Ormet Corporation
Reynolds Aluminum
Finally, many thanks are given to the hard working secretarial
staff of the Effluent Guidelines Division. In particular,
recognition is given to Ms. Linda,Rose, Ms. Kaye Starr, and Ms.
Nancy Zrubek.
131
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SECTION XIII
REFERENCES
(1) Kirk-othmer, EncYclopedia^of Chemical Technology, _ (2)
Interscience, 1963, p 941.
(2) Todd, D. K., (Editor), The Water Encyclopedia, Water
Information Center, Port Washington, N.Y., 1970, p 91.
(3) U.S. Naval Weather Service, 8,, May 1969, AD688472.
(4) Gulp, R. L., and Stoltenburg, H. A., "Fluoride Reduction
at La Crosse, Kansas", J. Amer. Water Works Assoc., .50,
423 (1958) .
(5) Zabban, W., and Jewett, H. W., "The Treatment of Fluoride
Wastes", Proc. 22nd Purdue Industrial Waste Conference,
pp 706-716 (1967).
(6) Wamsley, R., and Jones, W. E., "Fluoride Removal", Water
and Sewage Works, 94, 372 (1947).
(7) Peters, M. S., and Timmerhaus, K. D., Plant Design and
Economics for Chemical Engineers, 2nd Ed., MeGraw Hill
Book Company, New York, N.Y., 1968.
(8) Eckenfelder, W. W. Jr., Water.Quality Engineering for
Practicing Engineers, Barnes and Noble, Inc., New York,
N.Y., 1970.
(9) Nielsen, K., and Kielback, A. W., "Recent Developments in
Dry Scrubbing Technique", Proc. of Symp., 101st AIME
Meeting on Environmental Control, San Francisco, Calif.,
Feb. 20-24, 1972.
(10) Cook, G. C., and Swany, G. R., "Evolution of Fluoride
Recovery Processes Alcoa Smelters", Paper #A71-37,
Metallurgical Society of AIME.
(11) Rush, D., Russell, J. C., and Iverson, R. E., "Air
Pollution Abatement on Primary Aluminum Potlines:
Effectiveness and Cost", J. Air Pol. Control Assoc.,
23 (2) , 98 (1973) .
133
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SECTION XIV
GLOSSARY
The Federal Water Pollution Control Act Amendments of 1972.
Alumina
The pure granular oxide of aluminum, prepared from bauxite by the
Bayer process, and added periodically to the cells, as the source
of aluminum ions, for reduction to metal.
Ancillary Operations
Operations, which are often carried out at primary aluminum
plants but are not an essential part of the processing (for
example, rod, wire, or rolling operations, power generation,
etc.).
Anode
The positively charged carbon block supported from above and
extending into the electrolytic bath.
Anode Paste
The mixture of pitch and petroleum coke, from which anodes are
formed.
Anode Plant
Also referred to as the "carbon plant", this is the facility, in
which carbon for the anodes is received, comminuted, classified,
mixed with pitch, and formed into either anode blocks and baked
for prebake plants or into briquettes for delivery to soderberg
anodes at the cells.
Anode Shell
The metal form suspended above the electrolytic bath, in which
the anode paste is shaped as it is baked in moving into the hot
bath.
Anthracite
135
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A hard natural coal, low in volatile matter, which is ground,
mixed with pitch, and used in forming the cathodic lining of the
cells.
Bath
Or electrolytic bath, is a molten mixture of cryolite, calcium
fluoride, and alumina serving as the liquid medium for movement
of ions in the electrolytic process.
Best Available Technology Economically Achievable
Level of technology applicable to effluent limitations to be
achieved by July 1, 1983, for industrial discharges to surface
waters, as defined by Section 301 (b) (2) (A) of the Act.
Best Practicable Control Technology Currently Available
Level of technology applicable to effluent limitations to be
achieved by July 1, 1977, for industrial discharges to surface
waters, as defined by Section 301(b)(1)(A) of the Act.
Bi^gas
Mixtures of chlorine and nitrogen used in degassing primary
aluminum.
Capital^ Costs
Financial charges, which are computed as the cost of capital
times the capital expenditures for pollution control. The cost
of capital is based upon a weighted average of the separate costs
of debt and equity.
Casthouse
The facility at a primary aluminum plant which receives molten
metal from the cells, holds it in furnaces for degassing
(fluxing) and alloying and then casts the metal into pigs,
ingots, billets, rod, etc.
Category and Subcategory
Divisions of a particular industry, possessing different traits,
which affect waste water treatability and would require different
effluent limitations.
136
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Cathode
The negatively charged carbon cell lining, in which the molten
aluminum collects and becomes the actual cathode.
Center-break Technology
A system applicable to cells having two rows of prebaked anodes,
in which the crust of frozen bath is broken between the rows of
electrodes for addition of alumina and withdrawal of aluminum.
Clarifier
As used in this industry, the term refers to a unit which
provides for settling and removal of solids from a process
stream. See thickener.
Cryolite
A natural or synthetic chemical compound (3NaF.AlF3), which in
the molten state forms the major part of the electrolytic bath,
in which the alumina ore is dissolved.
Cyclone
A unit for removal of particulate matter from a gas stream by
centrifugal action in a vortex flow pattern. The principle is
also applied to cleaning of liquid flows.
Degassing (FluxingJ_
The removal of hydrogen and other impurities from molten primary
aluminum in a casthouse holding furnace by injecting chlorine gas
(often with nitrogen and carbon monoxide).
Depreciation
Accounting charges reflecting the deterioration of a capital
asset over its useful life.
Dry Scrubber
A unit in which fumes are removed from an air stream by sorption
on alumina particles. Filters for collection of alumina and
other solids is a part of this unit.
Dust Collector
137
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An air pollution control device for removing dust from air
streams. Filtration, electrostatic precipitation, or cyclonic
principles may be utilized, but the term usually infers a dry
system, not involving a water stream.
Effluent
The waste water discharged from a point source to navigable
waters.
Effluent Limitation
A maximum amount per unit of production of each specific
constituent of the effluent that is subject to limitation in the
discharge from a point source.
EffluentTLoading
The quantity or concentration of specified materials in the water
stream from a unit or plant.
(
Electrolytic Cell
The basic production unit for primary aluminum, consisting
essentially of a cast iron container (for example 8 ft wide x 18
ft long x 3 ft deep), the carbon cathode liner, the electrolytic
bath, and a carbon anode suspended from above.
Electrostatic Precipitator
A unit for removing particulate solids from a gas stream by
collecting the particles on electrically charged plates or wires.
The system may operate dry or the plates may be continuously
cleaned by a falling film of water.
Fluxing_j(T)egassing)
The removal of hydrogen and other impurities from molten primary
aluminum in a casthouse holding furnace by injecting chlorine gas
(often with nitrogen and carbon monoxide).
Hall/Heroult Process
An electrolytic process for primary production of aluminum in
which molten cryolite serves as the solvent for alumina. The
process was invented simultaneously by Hall in the United states
and Heroult in France in 1886.
138
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Shrouds at -the cells designed to promote capture of fumes and
dust by air withdrawal systems.
Horizontal Stud Soderberq (HSS) Plant (or Anode)
A facility for producing aluminum by the Hall/Herould process in
which the anode material is supported on spikes or studs, which
extend into the anode from the side, named for the inventor of
the continuous anode system.
A class of calcium hydroxy phosphate material prepared from bone
char.
Investment
The capital expenditures required to bring the treatment or
control technology into operation. These include the traditional
expenditures such as design; purchase of land and materials;
etc.; plus any additional expenses required to bring the
technology into operation, including expenditures to establish
related necessary solid waste disposal.
New Source
Any building, structure, facility, or installation from which
there is or may be a discharge of pollutants and whose con-
struction is commenced after the publication of the proposed
regulations.
Petroleum Coke
The carbon residue of petroleum refining used for making anodes.
Pitch
A class of thermoplastic carbonaceous residues, mostly from
petroleum refining, which is used as a hot-binder in making
anodes and pot liners.
Point Source
An individual plant, site or other location from which pollutants
enter navigable waters.
139
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|!Qllutant_Parameters
Those constituents of waste water determined to be detrimental
and, therefore, requiring control.
Pot
A common term for the electrolytic cell, also used to refer to
only the cast iron shell of that unit.
Pot_Gas
Gases (carbon monoxide, carbon dioxide, hydrogen fluoride), fumes
and dus.t arising at the cells during production of aluminum.
Pgtline
A row of from 100 to 250 electrolytic cells connected in series,
forming an electrical circuit.
Potliner
The brick and carbon structure used to separate the shell from
the molten aluminum and electrolytic bath in a cell. Also, the
material removed when the cell is taken out of service.
Potroom
The building housing a potline. Usually long and narrow to
provide ventilation along the line of pots.
Prebake^ Plant
A facility using anodes, which have been baked and graphitized
before installation in the Hall/Heroult electrolytic cell for
aluminum production.
Primary,_Air
That air stream drawn from around the cells in a primary aluminum
plant.
Primary Aluminum
Aluminum metal prepared from an ore, as distinguished from
processed scrap metal.
140
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Rectifier
A device which converts a-c into d-c by virtue of a charac-
teristic permitting appreciable flow of current in only one
direction.
Reverse Osmosis
A recovery process in which the more concentrated solution is put
under a pressure greater than the osmotic pressure to drive water
across the membrane to the dilute stream, while leaving behind
the dissolved salts.
Rod Mill (or Shopfr
A facility at some primary aluminum plants for casting aluminum
and forming rod usually about one-half inch in diameter.
Roddinc[_Plant
A facility for affixing support rods to baked anode blocks by
pouring molten iron around the rod in a cavity in the top of the
block.
Sanitary Water
The supply of purified water used for drinking, washing, and
usually for sewage transport and the continuation of such
effluents to disposal.
gcrubber Liquor
The liquid in which dust and fumes are captured in a wet
scrubber.
Secondary^ Air
Air in a potroom, containing those pollutants not captured in the
primary air hood system.
Side-break_TechnQlogy
A system in which the frozen crust of the bath is broken between
the electrode and edge of the cell for addition of alumina and
removal of aluminum.
Standard^of Performance
141
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A maximum weight discharged per unit of production for each
constituent that is subject to limitation and applicable to new
sources, as opposed to existing sources, which are subject to
effluent limitations.
Thickeners
A large tank for continuous settling and removal of sludge from a
process stream. Clarified liquid spills over the rim of the
tank.
Tri-Gas
Mixtures of chlorine, nitrogen, and carbon monoxide used in
degassing primary aluminum.
Vertical.Stud Soderberg JVSS) Plant. Ior_.Anode)
A facility for producing aluminum by the Hall/Heroult process in
which the anode material is supported on spikes or studs, which
extend into the anode from above, named for the inventor of the
continuous anode system.
Waste Water Constituents
Those materials which are carried by or dissolved in a water
stream for disposal.
Wet Scrubber
A unit in which dust and fumes are removed from a gas stream to a
liquid. Gas-liquid contact is promoted by jets, sprays, bubble
chambers, etc.
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TABLE 10
METRIC UNITS
CONVERSION TABLE
MULTIPLY (ENGLISH UNITS) by TO OBTAIN • (METRIC UNIT-S)
ENGLISH UNIT ABBREVIATION CONVERSION ABBREVIATION METRIC UNIT
acre ac
acre - feet ac ft
British Thermal
Unit BTU
British Thermal BTU/lb
Unit/pound
cubic feet/minute cfm
cubic feet/second cfs
cubic feet cu ft
cubic feet cu ft
cubic inches cu in
degree Fahrenheit °F
feet ft
gallon gal
gallon/minute gpm
horsepower hp
inches in
inches of mercury in Hg
pounds Ib
million gallons/day mgd
mile mi
pound/square inch psig
(gauge)
square feet sq ft
square inches sq in
tons (short) ton
yard yd
0.405
1233.5
0.252
0.555
0.028
IT?
0.028
28.32
16.39
0.555(°F-32)*
0.3048
3.785
0.0631
0.7457
2.54
0.03342
0.454
3,785
1.609
ha
cu m
kg cal
kg cal/kg
cu m/min
cu m/min
cu m
1
cu cm
°C .
m
1
I/sec
kw
cm
atm
kg
cu m/day
km
(0.06805 psig +l)*atm
0.0929
6.452
0.907 ~
0.9144
sq^ m
sq cm
kkg
hectares
cubic meters
kilogram-calories
kilogram calories/
.kilogram
cubic meters/minute
cubic meters/minute
cubic^meters
liters' ' " ';••
cubic centi'm'eters
degree Centigrade
meters
liters
•liters/second
killpwa;tt|p, *
centimeter^ > •
atmospheres
kilograms
cubic meters/day
kilometer
atmospheres
(absolute)7
square meters
square centimeters
metric tons',
(1000 .kilograms)
meters
* Actual conversion, not a multiplier
4U.S. GOVERNMENT PRINTING OFFICE:1974 546-318/346 1-3
143.
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