EPA 440/1-73/019-a
DEVELOPMENT DOCUMENT FOR
PROPOSED 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
UNITED STATES ENVIRONMENTAL PROTECTION AGENCY
OCTOBER 1973
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Publication Notice
.s is a development document for proposed effluent
itations guidelines and new source performance
ndards. As such, this report is subject to changes
o alting from comments received during the period of
^ .ic comments of the proposed regulations. This
0 $ ment in its final form will be published at the
o o» mu the regulations for this industry are promulgated,
P
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DEVELOPMENT DOCUMENT
for
PROPOSED EFFLUENT LIMITATIONS GUIDELINES
and
NEW SOURCE PERFORMANCE STANDARDS
for the
PRIMARY ALUMINUM SMELTING
SUBCATEGORY
Of the
ALUMINUM SEGMENT
of the
NCNFERROUS METALS MANUFACTURING
CATEGORY
Russell E. Train
Administrator
Robert L. Sansom
Assistant Administrator for Air & Water Programs
Allen Cywin
Director, Effluent Guidelines Division
Harry M. Thron, Jr.
Project Officer
October, 1973
Effluent Guidelines Division
Office of Air and Water Programs
U.S. Environmental Protection Agency
Washington, D.C. 20460
1T.S. Environmental Protection
Rogl'-o 5 , L; >>rary (5PL-16)
230 S. Dearborn Street, Room 1670
Chicago, IL 60604
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ABSTRACT
This document presents the findings of an extensive study of the primary
aluminum industry by Battelle's Columbus Laboratories for 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 concentrations of suspended
solids and oil and grease 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 recommended limitations are available to some plants in
conversion from wet scrubbing to dry scrubbing or an total impoundment
of waste water. The best available demonstrated control technology,
processes, operating methods, or other alternatives consists of dry
scrubbing of potline air, the control and treatment of other fluoride-
containing waste streams by recycle and treatment of any necessary bleed
stream by lime precipitation and the treatment of other streams, as
required, for oil and grease removal.
Supportive data and rationale for development of the proposed effluent
limitations guidelines and standards of performance are contained in
this document.
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CONTENTS
Section
I CONCLUSIONS 1
II RECOMMENDATIONS 2
Best Practicable control Technology Currently
Available 2
Best Available Technology Economically
Achievable 3
New Source Performance Standards 3
III INTRODUCTION 5
Purpose and Authority 5
Approach Used in the Development of the 6
Effluent Limitation Guidelines and standards
of Performance
General Description of the Primary Aluminum 12
Industry
IV INDUSTRY CATEGORIZATION 20
Introduction 20
Objectives of Categorization 20
An Overview of the Interrelationship of Anode 20
Type, Process Technology, Air Pollution
Control, and water Pollution Control
Aluminum Reduction Process Description 27
Water Usage in the Primary Aluminum Industry 35
Industry Categorization 36
V WASTE CHARACTERIZATION 43
Introduction 43
Sources of Waste Water 43
Effluent Leadings 47
Source of Waste Water from Developmental 73
Aluminum Reduction Processes
VI SELECTION OF POLLUTANT PARAMETERS 74
Selected Parameters 74
Rationale for the Selection of Pollutant 74
Parameters
Rationale for the Rejection of 75
Pollutant Parameters
VII CONTROL AND TREATMENT TECHNOLOGY 77
Introduction 77
Control Technology 77
Treatment Technology gg
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Summary of Waste Treatment Effectiveness 98
Control and Treatment Options 101
Section
VIII COSTS, ENERGY, AND NONWATER QUALITY ASPECTS 104
Introduction 104
Basis for Cost Estimation 104
Economics of Present Control Practice 105
Economics of Present Treatment Practice 109
Cost Effectiveness (Present Practice) 110
Costs of Additional Treatment Processes 114
Nonwater Quality Aspects 116
IX BEST PRACTICAELE CONTROL TECHNOLOGY CURRENTLY 121
AVAILABLE, GUIDELINES, AND LIMITATIONS
Introduction 121
Recommended Effluent Limitations 122
Identification of Best Practicable Control 124
Technology Currently Available
Guidelines for the Application of the 127
Effluent Limitations
X BEST AVAILABLE TECHNOLOGY ECONOMICALLY 128
ACHIEVABLE, GUIDELINES AND LIMITATIONS
Introduction 128
Recommended Effluent Limitations 129
Identification of Best Available 130
Technology Economically Achievable
Guidelines for the Application of the 131
Effluent Limitations
XI NEW SOURCE PERFORMANCE STANDARDS 132
Introduction 132
Standards of Performance for New 133
Sources
Identification of Best Available Demonstrated 134
Control Technology, Processes, Operating
Methods, or Other Alternatives
Rationale for the Selection of the Best Available 135
Demonstrated control Technology
Guidelines for the Application of the Standards 135
of Performance
XII ACKNOWLEDGMEN1S 136
XIII REFERENCES 137
XIV GLOSSARY 138
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TABLES
Number
A Summary of Features of Plants Visited 13
1 Matrix of the Characteristics of Primary 37
Aluminum Plants
2 Effluent Loading, kg pollutant/metric ton Al 48
3 Effluent Loading, Ib pollutant/ton Al 49
U Quantities of Selected Constituents in Water 50
Effluent from Selected Primary Aluminum Plants
in the U.S.
4A1-4K Concentrations of Selected Constituents 51-69
in Influent and Effluent Water, Primary Aluminum
5 Summary of Present and Potential Control and 78
Treatment Technologies
6 Effluent Levels Achieved by Various Treatment 99
Processes
7 Cost Data for Control and Treatment of Waste 106
Waters from Primary Aluminum Production
8 Costs of Various Alternatives for Fluoride 117
Removal
9 Energy Requirements and Solid Waste Production 120
for Various Water Effluent Control and Treatment
Technologies
10 English/Metric Unit Conversion Table 146
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FIGURES
Number £§3§
A Wastewater Survey Questionnaire - Plant Visits 7
B Wastewater Questionnaire - Telephone Survey 14
1 Locations of Aluminum Reduction Plants 18
2 Schematic Drawing of a Prebaked Anode Cell 22
3 Schematic Drawing of a Horizontal Stud Soderberg 23
Aluminum Reduction Cell
4 Schematic Drawing of a Vertical Stud Soderberg 24
Aluminum Reduction Cell
5 Process Diagram for the Electrolytic Production 29
of Aluminum
6 Schematic Composite Flow Diagram for Plants Using Wet 44
Scrubbing
7 Correlation of Plant Data on Suspended Solids, 71
Oil and Grease, and Fluoride Emissions
8 Diagram of Dry Gas-Scrubbing Process Elements 80
9 Process Schematic Recycle System for Fluoride 84
Removal
10 Process Schematic of Once-Through System for 89
Fluoride Removal
11 Flowsheet of Process to Remove Fluorides From 90
Waste Streams (Recycle Water Treatment)
12 Schematic Diagram of a Process to Remove 93
Fluoride by Alum Precipitation
13 Process to Remove Fluoride by Adsorption on 94
Activated Alumina
14 Reverse Osmosis Treatment of Fluoride Waste 97
Water
15 Some Control and Treatment Options 102
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16 Cost Effectiveness of Water Control and
Treatment to Remove Fluoride (Capital Cost)
17 Cost Effectiveness of Water Control and
Treatment to Remove Fluoride (Operating Cost) 112
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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 recommended herein. It is concluded that the
remainder of the industry can achieve those levels by July 1, 1977, by
the application of the best practicable control technology currently
available. Those plants not presently achieving the recommended 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 $1.13/metric ton ($I/short ton) would be
required to decrease the discharge of pollutants from the July 1, 1977,
level to the recommended July 1, 1983, level.
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SECTION II
RECOMMENDATIONS
Best Practical le_Control Technology
Currently_Ayailable
The recommended 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 ja) _______
Effluent Single_paY_Maxiinun}Jb^ 30-Day Ayeragejc)
Ib/ton Al
Fluoride 24 12
Suspended Solids 3 6 1.5 3
Oil and Grease 0.5 1 0.25 0.5
Cyanide 0.01 0.02 0.005 0.01
pH 6-9
(a) Effluent limitations are defined as kilograms of
pollutant per metric ton of aluminum produced
or pounds of pollutant per short ton of aluminum produced.
(b) The single day maximum is the maximum value for any one day.
(c) The 30-day average is the maximum average of daily values for
any consecutive 30 days.
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 arid recycling of the
clarified liguor to the wet scrubbers as a means of controlling the
volume of waste water discharged. Two precipitcition metnods are
currently available, cryolite precipitation, and precipitation with
lime. This technology achieves attendant reduction of the discharge of
suspended solids and cil and grease.
Alternate technologies for achieving the recommended effluent
limitations include dry fume scrubbing and total impoundment.
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The technology and rationale supporting these recommendations are
presented in Sections VII and IX.
Best Available Technology
EConomica11y Achievable
The recommended effluent limitations to be achieved by July 1, 1983, by
application of the best available technology economically achievable are
as follows:
Ef f luent_Limitations_(a}
Effluent Single^BaY_MaximumJb]_ 30-Day_Ayerage_(cJ __
Characteristic
Fluoride 0.1 0.2 0.05 0.1
Suspended Solids 0.2 0.4 0.1 0.2
Oil and Grease 0.03 0.06 0.015 0.03
Cyanide 0.01 0.02 0.005 0.01
pH Range 6-9
(a) Effluent limitations are defined as kilograms of pollutant
per metric ton of aluminum produced or pounds of pollutant
per short ton of aluminum produced.
(b) The single day maximum is the maximum value for any one day.
(c) The 30-day average is the maximum average of daily values
for any consecutive 30 days.
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 and oil and grease.
Alternate technologies for achieving the recommended effluent
limitations include dry fume scrubbing and total impoundment.
The technology and rationale supporting these recommendations are
presented in Sections VII and X.
New_ Source Per formance^ Standards
The recommended 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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Standards of Performance(a)
Effluent Single Day Maximum(b) 30-Day Average(c)
Characteristic kg/kkq Al lb/tgn^Al j£S/]S]S3_£i Ib/ton Al
Fluoride 0.05 0.1 0.025 0.05
Suspended Solids 0.1 0.2 0.05 0.1
Oil and Grease 0.03 0.06 0.015 0.03
Cyanide 0.01 0.02 0.005 0.01
pH Range 6-9
(a) Standards of Performance are defined as kilograms of pollutant
per metric ton of aluminum produced or pounds of pollutant
per short ton of aluminum produced.
(b) The single day maximum is the maximum value for any one day.
(c) The 30-day average is the maximum average of daily values
for any consecutive 30 days.
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, and the treatment of casthouse cooling water and other
streams, as required, for oil and grease removal with a gravity
separator or aerated lagoon. The technology and rationale supporting
these recommendations are presented in sections VII and XI.
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SECTION III
INTRODUCTJON
o se an d .Authority
Section 304 (b) of the Federal Water Pollution Control Act, as amended in
1972, 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 achievement by not later than July 1, 1983, of
effluent limitations for point sources, other than nubliclyowned
treatment works, which are based on the application of the best
available technology economically achievable which will result in
reasonable further progress toward the national 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 proposed herein set forth effluent limitations guidelines
pursuant to Section 30 4 (b) of the Act for the primary aluminum smelting
segment of the aluminum subcategory of the nonferrous metals
manufacturing category of squrces.
Section 306 of the Act requires the Administrator, within one year after
a category of sources is included in a list published pursuant to
Section 306 (b) (1) (A) of the Act, to propose regulations establishing
Federal Standards of performances for new sources within such
categories. The Administrator published in the Federal Register of
January 16, 1973 (38 PR 1624) , a list of 27 source categories.
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Publication of the list constituted announcement of the Administrator's
intention of establishing, under Section 306, standards of performance
applicable to new sources within the primary aluminum segment of the
nonferrous metals industry which was included within the list published
January 16, 1973.
Limitatign^Guidelines and Standards^gf^Perfprmance
The effluent limitations guidelines and standards of performance
recommended 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 operation. This inventory provided an overview
perspective from which to assess 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
Questionaire together with flow diagrams of
water use.
A copy of the questionaire is shown in Figure A.
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, arid through primary
aluminum company representatives.
Other plant-visit sites were selected to be representative
of various specivic industry practices.
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PRIMARY ALUMINUM
Process Data Sheet
Date
Company
Plant
Basic Process, Hall/Herout
Process Variations
Anode Type and Configuration
Raw Materials
Plant Capacity ton Al/day
Age of Plant years
Air Pollution Controls in Use or Planned
Process Modifications Available Which Would Reduce Water Pollution
Other Notes:
FIGURE A. WASTEWATER SURVEY QUESTIONNAIRE—PLANT VISITS
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PRIMARY ALUMINUM
Other Data
What factors within the primary aluminum industry will influence
a specific plant's ability to meet effluent limitations:
1.
2.
3.
Other comments
FIGURE A. WASTEWATER SURVEY QUESTIONNAIRE—PLANT VISITS
(Continued)
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PRIMARY ALUMINUM
Wastewater Data Sheet
Date
Company
Plant
Attach a process flow diagram showing water and wastewater material
balances.
Pipe No.
Origin of Wastewater
Volume (Min/Ave/Max) gal/day
pH (Min/Ave/Max)
Temperature, °F, Winter (Min/Ave/Max)
Temperature, °F, Summer (Min/Ave/Max)
Wastewater Treatment(s)
FIGURE A. WASTEWATER SURVEY QUESTIONNAIRE—PLANT VISITS
(Continued)
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PRIMARY ALUMINUM
Costs of Waste Control and Wastcwnter Treatment
Date
Company
Plant
Method (Waste Control/Wastewater Treatment)
Control Capacity
For Waste Control -
For Wastewater Treatment -
Year Installed
Capital Costs
Hardware
Engineering
Installation
Annual Cost
Operating and Maintenance
Depreciation
Administrative Overhead
Property Tax, Insurance
Interest
Other, e.g. , water
analysis
Gross Annual Cost
Credits
Net Annual Cost
Tons Al Annual Capacity
gal/day treated
$/ton Annual Capacity
$/ton Annual Capacity
Impacts of this control method on other media
FIGURE A. WASTEWATER SURVEY QUESTIONNAIRE—PLANT VISITS
(Continued)
11
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Table A summarizes the features of the plants
visited.
° General information on the remaining plants was obtained
through telephone contacts with each company.
A copy of the questionnaire used in this telephone survey
is shown in Figure B.
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 constituents 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 from the
plant visits. In addition, other technologies applicable to primary
aluminum plant waste water control and treatment were identified. For
each of the control cr 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 including:
energy requirements, ether 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".
2§ne ral Description .of
Aluminum_Xndustry
This document presents recommended effluent limitation guidelines and
standards of performance for the primary aluminum smelting industry,
12
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TABLE A. SUMMARY OF FEATURES OF PLANTS VISITED
Features Number of Plants
Anode Type
Prebaked 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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Primary Aluminum
Date of Call Contact
Name and Location of Plant
Age of Plant
Production: T/year, T/day, No. of lines No. of Pots
Anode Type: PB , HSS , VSS
Fabrication at this site (rolling, drawing, etc.)
Air Pollution Control Methods
A. Primary (pot line):
B. Secondary (pot room):
C. Anode bake plant (PB only):
D. Anode paste plant
E. Cast house
F. Other
Present Water Treatment
A. Pot line wet scrubbers
1. Cryolite precipitation recycle
a. Recovery of cryolite
b. Discard cryolite how
c. Bleed from recycle
d. Bleed treatment
2. Lime precipitation recycle
a. Bleed from re-cycle
b. Bleed treatment
c. Sludge disposal
3. Other
FIGURE B. WASTEWATER QUESTIONNAIRE—TELEPHONE SURVEY
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B. Secondary air Scrubbers
1. Treatment
C. Anode bake plant scrubbers
1. Treatment
D. Anode paste plant scrubbers
1. Treatment
E. Cast bouse scrubbers
1. Treatment
F. Water from rod mill, drawing, etc.
1. Operation
a. Treatment
2. Operation
a. Treatment
G. Cooling water
1. Equipment cooled
2. Additives
3. Treatment or disposal
H. Boiler blow-down
1. Boiler water and additives
FIGURE B. WASTEWATER QUESTIONNAIRE—TELEPHONE SURVEY
(Continued)
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2. Treatment or disposal
I. Other
1. Source of wastewater
2. Treatment
Planned Wastewater Treatment Modification
FIGURE B. WASTEWATER QUESTIONNAIRE—TELEPHONE SURVEY
(Continued)
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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 in
the United States 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, 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 4,500,000 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.
General Features of the^Primajry Aluminum^Facility
A detailed process description is given in Section IV of this document.
An overview of the primary aluminum facility is presented in the
following paragraphs.
The reduction of alumina to produce aluminum metal is carriid 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 amolten bath composed principally of
cryolite, a double fluoride of sodium and aluminum. Alumina is added to
bath periodically. As electrolysis proceeds, aluminum is deposited at
the cathode and oxygen is evolved at the carbon anode. The oxygen
reacts with the carbcn to produce a mixture of CO and CO2 and the anode
is consumed.
Two methods of replacing the anodes are practiced which 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
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Pn
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paste is baked in place to form 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 which consists of carbon
dioxide, carbon monoxide, volatile fluoride compounds, sulfur oxides,
and fine dust evolved from the cryolite and other bath components.
Ventilation systems are used to remove the fume from the potroom. The
ventilation air must be scrubbed to minimize air pollution and 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
recommended in this document.
The cathode of the aluminum reduction cell is a carbon liner on which
the pool of molten aluminum rates. 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 have a significant fluoride content due to leaching
action. Spent cathodes are either processed to recover fluoride values
or retained in a storage area. Run-off from such storage areas is
contaminated with fluoride and cyanide.
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.
19
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SECTION IV
INDUSTRY CATgGORIZATION
Introduction
An overview of the interrelationships of several significant factors of
in the potential categorization of the primary aluminum industry is
presented in this section. A detailed description of the aluminum
reduction process are then presented and the water uses and waste water
sources are identified. Finally, the rationale is developed for con-
sidering primary aluminum smelting as a single subcategory for the
purpose of establishing effluent limitations and standards of
performance.
Cbj§ctives_of_Cate3or;ization
The primary purpose of industry categorization is to allow t he--
development of quantitative effluent limitations and standards of
performance which are uniformly applicable to a specific category,, or
snbcategory. A number of factors have been considered as potential
bctses for subca-t-egorizatiori. These factors were examined to determine
their effects on the quality or quantity of waste water produced, on t:l
-------
The specific factors which were considered are:
Anode Type
Prebake
Horizontal Stud Soderberg
Vertical Stud Soderberg
Air Pollution Control Method
Hooding
Gas Cleaning
Dry Scrubbing
Wet Scrubbing
Once-through Water
Recycle Water
Anode Bake Furnace Gas (Prebake Anode Only)
Wet Scrubbing
Electrostatic Erecipitators
Anode_Type
The mechanics of various anode types have been discussed in other
portions of this report and in the literature with the significant
differences as indicated in Figures 2, 3, and 4. A major factor
recognized by this study is that no Soderberg type plants have been
constructed recently, nor have any been predicted for future
construction by persons contacted during this study. The principal
advantage of this 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. It is obvious that water
is not used directly in any of the types of anodes.
21
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The major effect of differences in anode type on water usage and streams
are that for prebake ancde plants, oell emissions (e.g., fluorides, SOx,
COx, etc.) are separate from anode bake plant emissions (e.g., tars and
oils, etc.). In Soderfcerg-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 heeding of cells is a factor which determines the air
pollution control measures required. In general, the results of current
practice are that if (given proper operation) 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 con-
centrations 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
which may be characterized as containing relatively dilute
concentrations of pollutants, and the only practicable treatment is by
wet gascleaning devices.
Dry Scrubbing
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.
The total recycle cf emissions has associated with it the potential
problem of b,uild-up of trace metals and impurities in the product.
Wet_Scr_abbinc[
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 te classed as low pressure-drop devices, i.e., 1-10
inches of water. No high energy venturi type scrubbers are used in
current practice. toetscrubbing devices may be applied to either
relatively concentrated (pot) or dilute (pot room) gases.
25
-------
The scrubbing media are of paramount interest to this study and may be
described in terms cf recirculating type systems or once-through
systems.
Anode Bake Furnace Gas Scrubbers
1 * -—..»..i. —. ____.^» «_ *"—•*•
In prebake anode plants, the anode bake furnace gases may be controlled
by electrostatic precipitators or most commonly by wet scrubbers—again
of the "low" pressure-drop types. If wet scrubbers are used, the waste
waters contain tars, oils, SOx, COx as well as fluorides if anode
materials are recycled from the electrolytic cells.
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 generally are
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 emissions regulation. 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 devices for anode bake furnaces at this time.
Current Practice
The current practices as determined during the effluent guidelines
program are indicated by the following annotated citations of existing
examples illustrative of the combinations of the factors under
discussion:
A. (1) Plant A. Prebake Anode—totally dry scrubbing
en 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
26
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B. (1) Plant B
C. (1) Plant J.
(2) Plant F.
Vertical Stud soderberg—wet scrubbing
of pot gas - total recycle of scrubber
water * bleed stream evaporated - dry
scrubbing planned
Horizontal Stud Soderberg—wet scrubbing;
dry systems on paste plant
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 verticalstud 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.
It also may be noted that 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 gascleaning systems have
been shown to include cell geometry and electrical efficiency, air
pollution standards, and/or water pollution standards, trut not to depend
strictly on anode type, or climate.
Alurninum Deduction_Prgcegs Description
27
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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 details from that shown.
Raw MateriaIs
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
0.5 kg (1.1 Ib) petroleum coke
0.05 kg (0.11 Ib) cryolite
0.04 kg (0.08 Ib) aluminum fluoride
0.6 kg (1.3 Ib) baked carbon
22 kilowatt hours of electrical energy.
The Electrolytic Cel1
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 fro>m
1.8x5.5 to 4.3x12.8 meters (6x18 to 14x42 feet) or larger. Most cells
are around 1 meter (3 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 formed of baked carbon. The
electrolyte consists cf 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.
28
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Petroleum Coke
CRUSHING
AND
CLASSIFYING
Pitch
ANODE PASTE
HOT - BLENDING
PRESSING
BAKING
anodes
cathodes
BLENDING
T f
Anthracite Pitch
COOLING
I Soderberg
I anode
Briquettes
Electrical Supply (Direct Current)
Alumina
'Cryolite
Calcium Fluoride
Aluminum Fluoride
Air
FUSED SALT
ELECTROLYTIC
CELL
GASES, DUST
FUMES
GAS
SCRUBBING
MOLTEN ALUMINUM
To degassing and
casting
Aluminum (pig,
billet, ingot, rod)
FT
Dry-Process Wet-Scrubbi
Solids returned liquor to
to cell treatment
'Spent Potliners (to cryolite recover
or disposal)
FIGURE 5. PROCESS DIAGRAM FOR THE ELECTROLYTIC
PRODUCTION OF ALUMINUM
29 -
-------
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 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, which results in the consumption
of the carbon anode.
Thus, the operation cf 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, for example, by a
vacuum siphon device, at a rate of about 230 to 800 kilograms (500 to
1800 pounds) every 24 hours, although practice may vary. 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 which consists of
carbon dioxide and carbcn monoxide but also includes amounts of volatile
fluoride compounds, sulfur oxides, and fine dust evolved 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.
The operation of aluminum reduction cells results in the continuous
consumption of anode material, about 0.5 kg of anode per kg of aluminum
produced. This must te replaced either continuously (£3oderberg anodes)
or intermittently (prebaked 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 fcr anodes (coke and pitch) must be prepared to meet
specifications by crushing, sizing, and blending. These operations are
conducted in the so-called anode paste plant which is an important
adjunct to every aluminum smelter. The anode paste consists typically
of a mixture of high-grade coke (petroleum and pitch coke) with pitch
and sometimes tar, although the latter is seldom favored in American
practice. Purity requirements of the aluminum product demand very low
30
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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, 0.7 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 (200 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 con-
siderable 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, gyclcls, 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 and 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 to maintain
optimum interpole separation distance as they are consumed.
It is generally accepted that 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), also is
lower on the average for prebaked systems than for the Soderberg system.
31
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This is reflected in power consumption figures also are lower for
prebaked anode systems ty about 1 kwhr per kilogram (2.2 pound) of
aluminum. On the ether hand, the manufacture of prebaked anodes
requires higher initial capital investment as well as a higher labor
demand.
Soderberg_Anode_§Ystems. 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
verticalstud 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 burnei:
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 so
dilutes the hydrocarbon vapors that they cannot be burned
satisfactorily.
The electrolyte in aluminum reduction cells served to dissolve alumina
which is the raw material for aluminum reduction, and to provide a
molten bath with a melting temperature far lower than that required to
32
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melt alumina, and low enough to prevent extensive formation of aluminum
carbides. The electrolyte must resist chemical decomposition and be
free from oxidizing agents. The primary consideration in electrolytes
is, of course, to provide an adequate medium for dissolution of alumina
and subsequent 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 te 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 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 910 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 0.05 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.
33
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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 all of the electrolyte in
molten condition. This leads to 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 variously formed of carbon blocks
and a rammed mix of anthracite and pitch which forms a liner for the
cast iron structure of the cell.
It is essential fcr purity of the product aluminum arid the structural
integrity of the cell that the molten aluminum be isolated from the iron
shell. A service life cf 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 and carries most of the;
cyanide originating in a primary aluminum plant. Such waters ordinarily
are joined with ether 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 (1000 Ib) aluminum/cell/day
726 day average liner life
34
-------
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
A nc i 1 1 §. r y_Oj3er a t i o n s
Primary aluminum plants require various supportive activities. In
addition to the cell room, anode paste plant, and anode bake 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 and some
practice rolling, drawing or other metal fabrication operations.
W a £§£ Ug §9§_iD-i^le_ ££iffiS£Y_ 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
as 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 oncethrough
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 allow reclamation of fume
components
Current economic and environmental pressures have brought much of the
industry to states (2) , (3) and (4) above. The dry fume scrubbing
method is being installed in some of the plants recently under
35
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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 wastewater, 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 HSS
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
since it has been demonstrated at overseas plants of U.S. corporations.
A more detailed discussion of water usage and stream characteristics 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 1, includes plant location, production capacity, plant age,
anode type, air-polluticn control methods and water treatment methods.
A summary of the distribution of plants exhibiting each descriptive
feature as determined from the information in Table 1 is given in the
following listing:
Feature No. of_Plants
Current Production, jnetric tons/year
90,000 (100,000 T/yr) 6
90,000 to 180,000 (100,000 - 18
200,000 T/yr)
180,000 ( 200,000 T/yr) 7
Anode Type
Prebaked 19.4
Horizontal Stud Soderberg 7
Vertical Stud Soderberg 4.6
Air Pollution Control Method
36
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Air Pollution Control Method
Primary, Potline Air
Wet Scrub, all or part 22
Dry Scrub, all or part 8
Secondary, Potrocm Air
Wet scrub 6
Anode Paste Plant
Wet Scrub 4
Dry Scrub 10
Anode Bake Plant
Wet Scrub 2
Dry Scrub 2
Cast House
Wet Scrub 3
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 6
10 to 25 years 9
25 years 5
Pyimary_Aluminum_as.a^Single Category
After review of the information compiled in Table lr and consideration
of the various factors related to the application of effluent
limitations, it has teen concluded that the primary aluminum industry
should be considered as a single category, and that effluent limitations
and standards of performance be applied uniformly.
Rationale. The recommendation 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-Hercult process.
(2) The major difference in water use and waste-
water generation lies in the use of wet or
dry potline fume scrubbers.
38
-------
(3) Dry fume scrubbing is still developmental with
respect to horizontal-stud Soderberg installations; therefore
it is not feasible for all plants in the primary aluminum
smelting industry.
(U) The exemplary technologies for control and
treatment of aqueous fluoride discharges, i.e.,
precipitation cf 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 commcn to primary aluminum plants.
In addition, these technologies produce a
concomitant reduction in suspended solids and
oil and grease levels.
(5) Application cf the identified "best practicable
technology currently available" by all plants
which use wet scrubbers will result in a marked,
industry-wide reduction of pollutant emissions.
(6) Plants which employ dry fume scrubbing will be
able to meet the effluent limitations as established.
(7) 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.
Factors Considered in Categorization
The recommendation fcr establishing a single subcategory for primary
aluminum is based on the interrelationships among many factors. Those
factors are discussed briefly in the following paragraphs to further set
forth the rationale fcr considering primary aluminum smelting 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 economies of environmental
control so that operation within recommended effluent guidelines can be
expected.
39
-------
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 which 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 iranganese 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.
The type of anode employed by aluminum smelters, prebaked,
horizontal stud Soderberg, or vertical stud Soderberg does not result in
any significant differences in waste effluent from the plant.
However, the air pollution control options are determined in part by the
anode type. The option to choose dry gas-scrubbing is not available
currently for horizontal stud Soderberg pot lines, or for secondary air
pollution control.
In those cases where the use of water is required, treatment technology
is available to achieve the recommended limitations. Therefore,
subcategorization by anode type and/or existing air pollution control
systems is not necessary.
A review of 31 aluminum reduction plants showed that 6
plants have capacities of less than 90,000 metric tons (100,000 short
tons) per year, 16 plants have capacities between 90,000 and 180,000
metric tons (100,000 and 200,000 short tons) per year, and 9 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 encountered. 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.
40
-------
Elant_Ac[e. Primary aluminum smelting is a relatively new industry based
on a single process. Therefore, the earliest plants built in the early
1940's are electrochemically the same as those built today; however,
numerous modifications have been made in process operation which have
resulted in greater production efficienty and reduced pollutant
emissions. As a result, neither the level of constituents in effluent
water nor the capability to meet the recommended limitations is related
to plant age. Because cf 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 are able to be retrofitted independently of plant
age.
Product. 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 guide-
lines, therefore, fabrication is not established as a separate
subcategory
Raw Ma te rials. The basic raw material, alumina, is received in a
refined and purified form. Other raw materials which may be used
include cryolite, flucrspar, 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.
Tne option of selecting total impoundment of effluent
for solar evaporation of water as a means of achieving zero constituents
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 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
41
-------
(4) Wind stability of the dried residues
(5) Integration of this technique with in-plant
recovery of all possible reusable constituents
and water.
Since the areas where the climatic conditions are amenable to total
impoundment are limited, impoundment cannot be cited as technology
available to the entire industry. However, since any plant that, can
practice impoundment will be able to meet the recommended effluent
limitations. A separate category and separate effluent limitation based
on geographical location are not warranted.
Summary. Tne quality and quantity of waste water constituents are
similar throughout the primary aluminum industry and they 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 recommended effluent limitations may be
applied uniformly to the primary aluminum industry as a single
subcategory.
42
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SECTION V
WASTEDCHARACTERIZATION
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 and 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
(8) 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 seme cryolite recovery systems are highly
sophisticated (and proprietary) chemical manufacturing facilities, while
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 the once-through discharge. stream (U) through a casthouse
furnace air scrubber is common 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.
43
-------
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The chief effluent constituent is oil from rod mills or other
fabrication units, when present; thus the use of commercial oil
separators is common.
From this generalized picture a number of potential sources of waste
water can be identified, including:
o Wet scrubbers
Primary pctline
Secondary potroom
Anode bake plant
Cast house
o Cooling water
Casting
Rectifiers
Fabrication
o Boiler blcw-down.
The constituents of the waste water from each of these sources are
identified in the following paragraphs.
Wet_Scrubbers
2£im§£y._P2£line_AjLj:_Scrubbery. The wet scrubbers which collect fumes
and dust from the electrolytic cells are the source of most of the waste
water constituents frcm 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 also are 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_Sgrubbers. Since, even with the best hooding of
cells, fumes and dust escape, some plants exhaust the pot room 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 amounts.
45
-------
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 ever secondary scrubbing of room air.
Anode Bake Plant Air §££ubh>ers. 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, it usually is
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.
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, usually, in
modern practice, mixed with nitrogen and carbon monoxide. This batch
operation is carried cut 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 Waj:er
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 usually is
discharged without treatment.
Ot he r _ Sour ce s_ of_ W a s t e_ Vjater
In addition to the sources of waste water considered above, general
housekeeping and the manner of collection and disposal of rain run-off
affects the total plant effluent. This ordinarily includes the run-off
from a used cathode storage or disposal area which is the source of most
of the cyanide constituent in plant effluent. In addition, liquid and
solid spills usually are flushed into this system. Treatment varies
widely from reprocessing through cryolite recovery to simple discharge.
46
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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 from a number of companies directly. 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 ten of aluminum produced (Ib/ton
Al), by means of the following equation:
Effluent Loading = CFK/F 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 10~6
(Ib x l)/(mg x gal), the conversion factor required
to obtain the proper units.
A wide variation exists in the concentrations and flow encountered in
primary aluminum plants. As an illustration of the effluent loadings
which result from various arbitrary conditions, a matrix of flow rate
versus concentration for a production rate of 455 metric tons Al per day
(500 tons Al per day) is given in Tables 2 and 3, where the values are
given in metric and English units respectively.
Plant Data
The actual effluent loadings calculated from effluent concentration and
flow data obtained for eleven companies are given in Table 4. The
control and treatment technology practiced by each plant is as follows:
The original data froir vvhich 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.
47
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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
51
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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 KAPP
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
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Neg
0.4
1.5
-
-
0.02
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13.8
_
52
-------
31
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51.
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31
111.
143
111
18.0
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106.9
92
73.
18.
TABLE 4 A3. CONCENTRATIONS OF SELECTED CONSTITUENTS IN
INFLUENT AND EFFLUENT WATER, PRIMARY ALUMINUM
Plant A , Pipe 003, Volume 360,000 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 3.4 3.0 Neg
Cyanide
Fluoride Nil Nil
Aluminum 0.0 0.0005
Calcium
Copper 0.0 0.040 0.04
Magnesium
Nickel 0.0 0.0003 0.0003
Sodium
Zinc
Oil & Grease
Phenol ,
* Source-RAPP
0.0
1.6
Nil
0.0002
18.
Nil
0.0002
16.4
_
53
-------
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
54
-------
TABLE 4 B3. CONCENTRATIONS OF SELECTED CONSTITUENTS IN
INFLUENT AND EFFLUENT WATER, PRIMARY ALUMINUM
Plant B, Pipe 4* , Volume 150,000gpd
Influent Effluent Net
Concentration Concentration** Concentration
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". Ste -,1- if .
** Source-Company Report
55
-------
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
56
-------
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
57
-------
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
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
58
-------
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
59
-------
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
60
-------
TABLE 4 F2 . CONCENTRATIONS OF SELECTED CONSTITUENTS IN
INFLUENT AND EFFLUENT WATER, PRIMARY ALUMINUM
Plantp , 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
61
-------
TABUS 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
Sulfate
Chloride
Cyanide
Fluoride
Aluminum
Calcium
Copper
Magnesium
Nickel
Sodium
Zinc
Oil & Grease
Phenol ,
* Source-RAPP
** Company Report
2
20
0.7
8
10
0.004
6.7
0.001
13
0.02
3.2
0.001
30
62
30
12
21
0.01
7
0.05
17.6
0.03
3.9
0.2
28
42
29.3
4
11
0.006
0.3
0.05
4.6
0.01
0.7
0.199
62
-------
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.
63
-------
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
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.
64
-------
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 tng/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.
65
-------
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.
66
-------
TABLE 4 I . CONCENTRATIONS OF SELECTED CONSTITUENTS IN
INFLUENT AND 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
115
28
730
475
259
56
166
0.05
15
0.2
51
0.05
19
0.01
88
Neg
6
67
-------
TABLE 4 J . CONCENTRATIONS OF SELECTED CONSTITUENTS IN
INFLUENT AND EFFLUENT WATER, PRIMARY ALUMINUM
Plant J, Pipe , Volume 13,700,000 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 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
18
11.9
16
0.036
10.
18.6
0.077
2
161
0.44
1.9
68
-------
TABLE 4 K . CONCENTRATIONS OF SELECTED CONSTITUENTS IN
INFLUENT AND EFFLUENT WATER, PRIMARY ALUMINUM
Plant K, Pipe , Volume 3,760,000 gpd
Influent Effluent Net
Concentration * Concentration ** Concentration
Constituent mg/1 mg/1 tng/1
Alkalinity 120 99 Neg
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
260
230
30
5
45
0
0.6
1.5
35
0.02
8.
0
40
0.02
0
300
265
35
117
60.6
0.028
22
3.2
32
0.06
7.5
0.035
117
0.05
4.3
40
35
5
112
15.4
0.028
21.4
1.7
Neg
0.04
Neg
0.035
77
0.03
4.3
69
-------
Plant
Anode
Control or Treatment^Applied
A
B
C
D
E
F
G
H
I
J
K
PB
VSS
PB
PB
PB
HSS
VSS
PB
HSS
HSS
PB
Dry scrubbing
Lime/recycle
Li me/one e-through
Cryolite/recycle
Cryolite/recycle
None
Lime/once-through
Cryolite/recycle
Cryolite/recycle
Cryolite/recycle
Cryolite/recycle
The significance of the data given in Table 4 may be illustrated by
noting the effluent leadings for fluoride. Of the eight plants
reporting fluoride values, five (D, H, 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 suspended solids and of oil and
grease. This aspect is discussed in Section VIZ of this document. This
effect is shown graphically in Figure 7 in which the effluent loading
values for suspended solids from Table 4 are plotted versus the fluoride
effluent loading for several plants. There is considerable scatter in
the data resulting frcm plant-to-plant variations in practice and from
the fact that some data represent net effluent values; while others,
through lack of inlet water analytical data, represent gross effluent.
However, the correlation of suspended solids discharge with fluoride
discharge is apparent. The 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 reduc-
tion of fluoride emissions result in the reduction of suspended solids
and oil and grease emissions as well.
70
-------
Plant Identification Letter
g
4J
1-" C
•U O
01 -U
a
•a
o
m
fr
3
eo
O Suspended Solids
Q Oil and Grease
c
o
u c
4J O
c
n
A
0.OS-
CO. 1)
Fluoride Emissions, kg/metric ton
(Ib/ton)
FIGURE 7. CORRELATION OF PLANT DATA ON SUSPENDED SOLIDS, OIL AND GREASE,
AND FLUORIDE EMISSIONS
71
-------
¥§£ i figation Analysi s
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 Verification
Data Analysis
Jayerage)_
Suspended solids, mg/1 18 25
Fluoride, mg/1 16 7
Oil and grease, mg/1 2 4
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:
Company Verification
_Data __ ___ Analysis __
Suspended solids, mg/1 15.6 15.8
Fluoride, mg/1 10.2 10.1
Oil and grease, mg/1 2.1 1.7
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 Verification
Rej3ort_ __ Analysis __
Suspended sclids, mg/1 35 44
Fluoride, mg/1 22 10
Oil and grease, mg/1 4.3 4.1
72
-------
As in the previous tabulations, the agreement is good.
Source^cf Waste Water from Developmental
Aluminum Reduction^P£O.ce_sses
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.
73
-------
SECTION VI
SELECTION OF POLLUTANT PARAMETERS
Selected^ Parameterg
The following waste water constituents are the significant pollutants
from the primary aluminum smelting sufccategory:
Fluoride
Suspended solids
Oil and grease
pH
Free cyanide
The rationale for the selection of these constituents and for the
rejection of ether constituents as pollutants is presented in the
following paragraphs.
Rationale_for the Selection of_£ollutant_Paramgters
Fluoride
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-polluticn control.
Suspended Solids
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.
Oil and Grease
74
-------
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. These organic materials
can be determined analytically by hexane extraction which is the
standard method applied for the analysis of organic materials classed as
"oil and grease". Currently, the effluent concentrations range from 1
to 10 mg/1 of oil and grease. Treatment methods are available in that
fluoride control also achieves a decrease in the oil and grease levels.
This may be followed, if necessary, by oxidation in aerated lagoons.
Water used in contact cooling of castings may contain low concentrations
of wetting agents.
Acid streams are produced in wet scrubbing of potline air and casthouse
and anode bake plant gases. Alkaline streams are produced by cryolite
recovery or other types of 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 the recommended limits.
Free_Cy_anide
Cyanide is contained in the run-off from spent cathode storage areas and
is detectable in the effluent from seme 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.
Conventional treatment technology, alkaline chlorination, is available
to effectively destroy cyanide in industrial waste water.
Rationale fcr the Rejection of _Pollutant Parameters
Other waste water constituents identifiable with the primary aluminum
industry that are not the subject of effluent limitations or standards
of performance are as follows:
Total dissolved solids
Chloride
Sulfate
COD
Temperature
Trace Metals
Total Dissolved Solids
Total dissolved solids includes fluorides, chlorides, sulfates, and the
common cations, sodium, potassium, magnesium, and calcium. The maximum
75
-------
concentration of dissolved solids reported by most plants surveyed was
less than 1000 mg/1. It has been concluded that the present cost of
treatment to 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".
Chlgrjde
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 the 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.
Chemical_OxYgen_Demand __ {COD)_
A COD component associated with organic materials is present in primary
aluminum smelter discharges. Control of oils and grease (hexane
extractables) indirectly controls COD.
Temperature
Heat loads are comparatively small in the primary aluminum industry.
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.
76
-------
SECTION VII
CONTROL AND TREATMENT TECHNOLOGY
Intreduction
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 concentrations 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 pollutants of major
significance are fluoride, suspended solids, oil and grease, and free
cyanide. These pollutants in the discharge water originate from the
operation of wet scrubbers on the potline, pot room, and anode bake
furnace, and from cryolite recovery from potlinings where practiced.
Minor sources of pollutants include: cast house wet scrubbers, anode
paste plant wet scrubbers, rectifier cooling, cast house cooling, boiler
blowdown, and rainfall runoff.
Current control and treatment practice varies throughout the industry.
Therefore, the steps required to be taken in order to achieve the
effluent limitations recommended 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 recommended
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 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.
77
-------
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
-------
Dry Scrubbing of_Pot^Ga§
Identification. The dry scrubbing of pot gas refers to the use of a
system of air polluticn control by primary aluminum smelters for the
removal of pollutants from the gases evolved from the electrolytic cell
(pot) by contacting the gases with dry alumina to effect the sorption of
pollutants and subsequently collecting particulates by fabric
filtration. The system is applicable to gases collected immediately
above the pot, i.e., pot gas, having relatively higher concentrations of
constituents than does pot room ventilation air. The system is not
applicable to the latter because of the relatively dilute concentrations
of constituents.
The outstanding features of the system are the sorbtion of emitted gases
on alumina which is subsequently fed 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 (in
terms of gas delivered to the device). The process uses no water.
Process De scr ip_ti on. 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 located in a
courtyard between potline buildings, possibly a cyclone type device to
separate coarse particulates, a reactor section in which the gases are
contacted with the alumina, and a fabric filtration stage, from which
the gases are released to the atmosphere, usually through a stack.
Associated equipment includes fans, alumina delivery, storage, and
removal devices, and taghouse 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
result is extremely turbulent mixing of the solid and gas in the throat
and in the several-fcot high column above the throat. 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.
79
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Again, turbulent mixing and intimate contact of gases and solids occur,
with the gases subsequently drawn through a baghouse.
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 particuldtes 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 depends on the phenomenon of sorption of
fluorine compounds on the surface of the alumina. Highest sorption
rates, i.e., highest collection efficiency, is observed 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.
The above factors lead to practices where all the alumina input to the
pots, i.e., all the major raw material for the plant, is first put
through the air pollution control system. Thus, current practices tend
toward what is referred to as a "100 percent feed".
A££linability- As stated previously, there are three variations of the
dry scrubbing process available, froifi 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. This material is
readily fluidized. Published data on the evaluation of alumina for use
in the fluidized bed showed one type of "floury" alumina (50 percent
-325 mesh) not to fluidize and to be difficult to handle and feed in the
equipment, although mesh size is not the only factor which must be
considered. Other designs have varying compatibilities with different
forms of aluminas. The form of alumina available to a given plant may
be a constraint in the selection or application of the dry-scrubbing
81
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process, involving some tradeoff in terms of the system selected or the
sources of alumina.
In one existing prebake anode installation in the United States hooding
efficiencies are lower than necessary to achieve required collection
efficiencies and wet scrubbing is used to clean potroom air. collection
and treatment systems are separate.
One European installation uses the dry scrubbing process on pot gases
and wet scrubbing of pctroom air, but with routing of the exhaust gases
from the dry scrubber baghouse to the wet scrubber for removal of sulfur
oxide components.
Dry scrubbing control methods are being installed in the United States
on both new plants and existing plants, and serve 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.
Identification. 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; and in the second, a lime slurry (or in
one case CaCl2) is used. After precipitation, thickening of the slurry
is accomplished in clarifiers or thickeners.
The treatment of wet scrubber liquors to recover cryolite is a
significant practice because it removes a sufficient quantity of
fluoride to permit recycle of the treated liquor to the scrubbers, and
in the process recovers the fluoride in a form which usually can be
returned to the aluminum electrolysis bath. The value of the cryolite
thus recovered 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, prin-
cipally in the petroleum coke and pitch used in anode preparation, are
converted to sulfur oxides during electrolysis and are collected in the
82
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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 pre-
cipitation of sodium sulfate. This bleed stream is relatively low in
volume but high in fluoride content and it represents the major portion
of the fluoride effluent from the entire plant. The actual volume of
bleed required 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 (about 1-2 minutes' residence time) with sodium aluminate
forming 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 sclid cake (about 60 percent solids) is then dried
in a kiln or multiple hearth furnace. If the cryolite is pure enough,
it can be returned tc 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 CaC12.
In general, suspended sclids 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 particles coalesce and the resultant mass of particle
settles at a greater rate than the individual, unhindered particles.
83
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MAKE UP WATER
132 i
SCRUBBER
RECYCLE /3'7?5X
(1,000)
FILTER
FILTRATE
57 ,
BLEED(15)
at (1-2 g/1 F)
REACTOR
SODIUM ALUMINATE
THICKENER
300 g/1 suspended soUds
KILN
CRYOLITE (30%)
* Process rates: liters/min,
(gpm)
FIGURE 9. PROCESS SCHEMATIC RECYCLE SYSTEM FOR FLUORIDE REMOVAL
C250 T/D Aluminum)
84
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There is a certain quantity of oils and grease in the wastewater. These
hydrocarbons arise from the baking of the anode. At the present time,
no control techniques are employed to remove this oil specifically,
because of its relatively low concentration of about 20 ppm. The data
indicate that about one-half to two-thirds of the oil is adsorbed onto
various precipitated solids. Thus the thickening operations can be
considered as a means of control. One aluminum producer does have an
oil separator on a power plant effluent with a flow of 102 m_3/ day
(27,000 gpd) ; however, indications are that the concentration of oil in
this relatively large stream is 1,000-10,000 mg/1.
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 alurrinate to recover cryolite with recycle of the
liquor to the scrubber. A scrubber-liquor bleed stream (to control
sulfate 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 I. Total fluoride emission in water was calculated
as 1.1 kg /me trie ton of aluminum produced (2.2 Ib/ton) .
The conclusions which can be drawn on the basis of the accumulated 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 cf 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 te the best practicable control
technology currently available. Alternate tech-
nology available to some plants is dry fume scrubbing.
Cast House Scrubber Water
85
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There are in practice a number of variations degassing procedures that
function as in-process control techniques to eliminate the use of water
for wet scrubbing of fumes generated during degassing of molten
aluminum. Although the differences between the various techniques are
of metallurgical significance, the processes will be considered as a
single class, since they all achieve the elimination of water use in
cast house scrubbers.
Degassing is an operation in which dissolved hydrogen and other
impurities are removed from molten aluminum just before it is cast 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 the impurities as chloride salts. Emissions
to the air have been ordinarily 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 therefore 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
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 100 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
86
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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, there apparently is a degree
of uncertainty 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 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 geliabi_lj.ty. All of the above listed process
alternatives are in commercial use on a regular basis and have been for
sufficient times to be considered established practice in one or more
producing plants. There is no proof 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 te applied to achieve the elimination of cast house
scrubber waste water.
Anode Bake Plant Scrubber Wateg
At the present time, control of water from this source, by 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
contamination by tars and oils. 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 remove significant amounts of
gaseous fluoride, thus they will 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 with no wet scrubber
on the exhaust gases by exercicising sophisticated control over the
firing of the anodes and by utilizing new flues in the exhaust circuit.
However, the company reported that it has not been sucessful in its
-------
efforts to apply this type of control at six other plants and wet
scrubbing systems have had to be 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
In the context of this report 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.
Water From__Potline^Wet Scrubberg
Treatment technology can be applied in once-through systems, i.e.,
without recycle, 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, the suspended solids level, and the oil and
grease concentration.
.. The once-through system does not employ a recycle
loop; rather, all of the scrubber water is treated, and then discharged.
A schematic diagram of the process is shown in Figure 10.
In one prebake anode type plant, scrubber water enters at a rate of from
1U 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 100 to 600 mg/1 soluble fluoride is
then contacted with a lime slurry. The resulting suspension is
thickened for about 5 hours 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 froir within the plant and discharged. Only lime is
used currently as the precipitant in this process.
In a vertical stud Soderterg-type plant the secondary air scrubber water
is diluted and discharged in mixed plant wastewater at a concentration
of 20 mg/1 F. The primary gas scrubber liquor (200 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.
T£eatment_of_RecYcle_Eleed_Streams. A process to remove fluoride from
the bleedand filtrate streams obtained from a recycle system can be
depicted as shown in Figure 11.
88
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The two streams are reacted with CaCl^ (or lime) and then enter a
clarifier where the suspended CaF2 is settled out. Based on intormation
provided by three aluirinum companies, the following assumptions are used
for the design characteristics of the process:
...The total flow to be treated is 0.106 cubic meters/
min (28 gpm).
...The input concentration of fluoride is 1 g/liter.
...Twice the stoichiometric amount of calcium is used.
...A residence time of 10 hours is used in the clarifier.
...Output fluoride level is 30 mg/1.
On the basis of the above assumptions, the addition of this further
treatment reduces the fluoride level 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 110
cubic meters/metric ten (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 which are not in general practice in the primary
aluminum industry which could be used to treat such dilute fluoride
streams in order to further reduce the fluoride discharge levels. 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),
for a plant producing 225 metric tons (250 tons/day) of aluminum per
day.
Aluminum Sulfate (Alum). The addition of alum to a solution containing
the fluoride ion will remove the fluoride. The mechanism involved
probably is adsorption on the alum precipitate. 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
91
-------
treatment would yield a stream containing 12 mg/1 fluoride. From the
dajba 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 then is allowed to flocculate for about 30
minutes. A period of 4 hours is then allotted for settling in a
clarifier.
There is evidence to indicate that the pH of the waste water is an
important parameter fcr 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 sludge, about 18 metric tons/day (about 20
tons/day).
One advantage of this procedure to treat the waste from a once-through
scrubbing system 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 cccur, 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.
Adsorption 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 fcr 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 H2SO^ at a concentration
of 4 percent. It requires about 16.5 kg of sulfuric acid to regenerate
the bed on which cne 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.
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One of the major disadvantages of this process from an environmental
standpoint is the discharge of a relatively large amount of calcium
sulfate, about 545 kg/day (about 1200 Ib/day) in the water. This is due
to its fairly high solubility in water of about 0.21 percent. 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
oils and grease probably will occur. It is not known whether these oils
will be eluted during regeneration. If they are not removed during
regeneration, the capacity of the bed could suffer; whereas, if they can
be desorbed during regeneration, the oil and grease will be removed from
the water during precipitation of the calcium sulfate and calcium
fluoride sludge.
Hydroxylapatite. 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 high bed attrition and decreased
efficiency in the presence of chlorides.
Adsorption of oils and greases should occur on the bed, however, whether
these oils and greases would be removed during subsequent regeneration
of the bed is unknown. There also may be some removal of suspended
solids by the process of filtration, although a quantitiative 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, because of the necessity of additional
fluoride removal processes which must be applied to the concentrate.
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. It should be possible to
obtain at least a 75 percent recovery, resulting in a concentrate of
about 135 ppm fluoride. This concentrated stream can be treated by
conventional lime precipitation.
95
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The major technical problem which can arise in the use of R.O. to treat
the scrubber water is the potential for fouling of the membranes due to
the suspended solids and oils and greases 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 oils and greases
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 probably would be unsatisfactory also. 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 500 psi, and passed through the reverse
osmosis unit. Further treatment of the concentrate is performed to
reduce the fluoride content.
Anode^Bake^Furnace^Scrukber 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 pollution control
applied to such flue gas includes no control, dry syste:ms, 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 oils, sulfates, particulate matter and, in
some cases, fluorides". The source of the fluoride in the carbon anode
bake plant is the used anode butts recycled to the anode preparation
operation. Care in the removal of fused cryolite from the anode butts
before reprocessing wculd greatley minimize fluoride emissions from the
anode bake plant and hence minimize fluoride concentrations in the bake
plant scrubber water.
96
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Treatment of anode bake plant wet-scrubber effluents consists of
settling the effluent ir ponds, in some instances, after lime treatment.
After settling, the organic materials are skimmed from the surface of
the pond. Plants employing this practice exhibit effluent loading of
oil and grease comparable to that from other plants.
Cast House^Coolinq 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 cil and grease. One plant treats this bleed
stream (150 gpm) in an aerated lagoon with a 15-day retention time,
reducing the hydrocarbon content by 85 percent.
Treatment of ^Cyanide- contain ing gtrearns.
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). Treatment technology is available for reducing the
free cyanide content. The internal streams containing cyanide may be
treated with chlcrine cr hypochlorite to destroy the cyanide. No
primary aluminum plants currently treat cyanide specifically.
Summary of Waste Treatment Effectiveness
The data from the aluirinum companies as well as those data calculated
for different modes of v;ater treatment have been summarized in Table 6.
Several important points should be noted. For water pollution control,
a dry scrubbing systeir is best, when it can be used. It is fairly well
established that for plants committed to potroom air cleaning, i.e.,
secondary scrubbing, a dry scrubbing system cannot be used at this time.
In addition, the use cf dry scrubbing on the anode bake effluent is not
in practice at the present time.
There are notable differences between the two wet scrubber systems,
once-through and recycle. The recycle system is considerably more
effective in the reduction of fluorides, suspended solids and oils and
greases. Effluent fluoride quantities are about 5-10 kg/metric ton (10-
20 lb/ ton) of aluminum when a once-through system is used, and 0.5 to 1
kg/metric ton (1-2 Ib/tcn) of aluminum when a recycle system is used.
There are several reasons for the better performance of the recycle
system.
98
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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 - Once Through
Wet Scrubbing - Recycle
Recycle + Bleed & Filtrate
Treatment
Once Through + Alum ^
Once Through + Activated
Alumina
Once Through + Hydroxylapatite
Once Through + 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.
99
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There is strong evidence that the formation of the precipitate calcium
fluoride is a time-dependent phenomenon. The equilibrium concentration
of soluble fluoride in a slurry of calcium fluoride is about 10 ppm;
however, values of 20-50 ppm have teen observed in the effluent when
calcium fluoride is formed from lime.
Either the calcium fluoride forms and entraps liquor, or the calcium
fluoride forms about the lime particle as a "skin". In either case, the
reaction rate becomes diffusion controlled, and the higher the
concentration of fluoride ion, the greater will be the rate of mass
transfer of this ion into the reacting media. Thus, it is expected that
the precipitation reaction in the recycle liquor with 10-20 times the
fluoride concentration would be higher than that in the once-through
system.
100
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It is both technically feasible and relatively simple to add a further
fluoride treatment process to the recycle system. This is because the
effluent streams (bleed and filtrate) have a relatively high
concentration of fluoride (about 1 g/liter) and are also of small
volume. 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 it appears that the best process is fluoride
removal by sorption in an activated alumina bed.
It is concluded -that techniques are currently available to reduce
fluoride to zero by use of a dry scrubbing system on the potline, 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 recommended 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 Ib per ton) of
aluminum produced. The recommended effluent limitation (July 1, 1977)
can be achieved by the baseline plant by installing a cryolite-recovery
system with recycle and bleed. The recommended 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 case plant include: conversion of the wet-scrubbing
system to a dry-scrubbing system, or retention of the wet-scrubbing
system with provision fcr 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 now. The 1983 effluent
limitations then can be achieved by adding lime treatment of the bleed
stream.
101
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Effluent
Loading
Baseline Case—Wet Scrubbing—Once-through,
Mo Treatment of Scrubber Water
(30)
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 1
Level II
Level III
FIGURE 15. SOME CONTROL AND TREATMENT OPTIONS
102
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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.
103
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SECTION VIII
COSTS, ENERGY, AND NONWATEB QUALITYJASPECTS
IntrgductiQQ
This section deals with the costs associated with the various treatment
strategies available to the aluminum industry to reduce the pollutant
load in the water effluents. In addition, other nonwater quality
aspects are discussed.
Easis^fgr_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 put all on 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.
Regarding estimates for capital and operating costs of other processes
which could be applied to water treatment, the following procedure was
used: 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
104
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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:
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 contrcl and treatment of water in the primary aluminum
industry, not all categories have cost information-contained. 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 tc put the costs on a common basis, or calculated on
equipment descriptions obtained. It is also to be noted that all tons
are metric throughout the following discussion.
105
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gotline (Prirngry^ Gas Sgrubber Wat§r
Essentially there are two means to control the water effluent from gas
scrubbers on the pctline: (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, dees 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 $30-$UO/annual ton, while Cook and Swany(lO)
report a 1970 cost of $60/annual ton for primary control of prebake
plants, and $33/annual ton for vertical spike Soderberg. These costs
include both the collection system and the primary removal equipment.
As an average investment cost, a figure of $<40/annual ton is used in the
present study.
Operating cost data are relatively sparse because of the small
percentage (about 4 out of 33) of plants utilizing dry scrubbing. Rush
et al(ll) use an operating cost of $10.20/ton for a control of prebake
potline gases and a profit of $0.55/ton for vertical spike Soderberg-
type 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 because more prebake plants are in use than vertical-spike
Soderberg type.
Cost information on the use of a recycle scrubber system has been
obtained from the aluirinum 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 frcm $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.
Potroom (Sg c ondary )__ Gas_Scrubber^Water
107
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As can be seen in Table 7, only 5 of the 11 companies surveyed for costs
practice air pollution control of potroom air. Of these 5, 3 utilize
water control (Dr E, and K) on this circuit. Cost information was
obtained from the two companies D and E. Plant D reports 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/tcn operating. These costs again include only the
recycle water control circuit; however, in this case, no treatment of
the cryolite filtrate stream is done.
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 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.
Cast House Cooling
The method used to control effluent water from the cast house 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 the 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.
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
108
-------
ton) . The control measure in practice by industry is to use air-cooled
rectifiers. Cost data en rectifier coding were not obtained.
Economics, of Presept_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.
Hotline __ (Prjlmary^ Ga§ ^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 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) .
. Water
It has been found that 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, twc utilize recycle water in the scrubber (Plants D
and E) , and two do not treat the scrubber water.
Anode Bake Furnace ^Scrubber Via ter
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 cf the ponds, costs were calculated for each plant.
These costs are shown in Table 7. The higher cost oil separators are
associated with a longer residence time for the scrubber water, and con-
sequently probably result in a more effective separation. 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
109
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after the ponding treatment of Plant D was done, and it was found that
the pond did remove about 60 percent of the oils and greases and
suspended solids.
Cast House ^Cooling Water
One of the companies contacted (Plant K) provides a lagoon into which
all the water effluent, including that from the cast house, flows before
discharging 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-! f f££t iv enes s __ (P resent Practice)
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.
It is apparent from the figures 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 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 or for horizontal spike Soderberg potlines.
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 from scratch,
including the scrubber, fans, etc., costs about $38/anriual ton. Thus,
the difference in cost between the two systems for a new plant would
only be about $2/annual ton.
110
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The recycle mode of scrubbing water control on both potline (primary)
gases and potroom (secondary) gases does result in fluoride effluents
less than 1 kg/ton (2 Ib/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 at this time regarding the cost
effectiveness of fluoride control.
(1) The best cost-effective means of control for new plants with a
prebake or vertical-spike 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. Considering that 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 (Potline and Potroom)
Treatment to remove fluoride will tend to remove suspended solids. In
the dry system, any suspended solids will be caught in the collection
system. As a wet system for fluoride control involves a settling
operation of CaF2, the suspended solids also will tend to settle.
Therefore, conclusions about cost effectiveness applicable to fluoride
also are applicable tc suspended solids control.
Oils_andj3reases__( Potline)^
It appears that oils and greases emitted from the anode consumption in
the potline tend to te removed along with the suspended solids and
fluoride. This is evident on examining the effluent data from Plants C,
E, F, G, I, and J. As discussed previously in Section VII, perhaps
emulsion breaking and adsorption of oil onto the surface of the
precipitate is the reason for this. Thus, the same conclusions
regarding cost effectiveness for fluoride control can be applied to the
removal of oil and grease from the potline scrubber water.
113
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Qr e a se __ [ An od e^ B a ke__Plant^
There are too little data on the effectiveness or cost of the treatment
procedure of settling and skimming the anode bake plant scrubber water
to make definite conclusions regarding the cost effectiveness of this
procedure in the present study. The one piece of data obtained on the
effect of a lagoon on scrubber water from the anode bake plant does
indicate that the oil and grease content (as well as suspended solids)
can be reduced by 60 percent in a pond with a residence time of 21
hours. This residence time is relatively long; however, concentrations
of oils and greases are low (less than 10 mg/1) with incoming suspended
solids only at the 100 mg/1 level. Very likely further reduction of oil
and grease can be effected by Icnger residence times with
proportionately higher costs, although the exact relationship is not
known.
Oil and Grease _ (Cast House Cooling}
Again, there are not enough data to make definite conclusions about cost
effectiveness of control and treatment cf casthouse cooling water. The
one piece of data obtained implies that good reduction in oil and grease
(up to 95 percent) can be obtained using a cooling tower and aerated
lagoon treatment of the blowdown from the cooling tower at a moderate
cost of $1; 60/annual ton capital, $0.40/ton operating.
Heat From Rectifiers
At this point, it is difficult to say whether a transfer of the heat
output derived from the rectifiers from water into air represents a gain
or loss of effectiveness in relation to overall impact on the
environment. Nevertheless, the use of air-cooled rectifiers does
represent an effective water control measure.
Cgsts^of^AdditioQalJTreafanent^Processeg
As previously mentioned, dry scrubbing on the primary air system can
result in no discharge of pollutants, if a secondary wet scrubbing
system is not required. However, addi tional 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
report. The economics and cost-benefits associated with each of these
processes are now dealt with.
Po 11 ine^and_Potrogm_Scrubber Water_Treatment
114
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The choice of additional treatment schemes to be applied to scrubber-
water effluent depends primarily on whether a recycle system or once-
through system is in use.
In a recycle system, additional control of fluorides, suspended solids,
and oil and grease can be effected by the lime or CaCl2 treatment of the
filtrate stream from the cryolite and the bleed stream from the
scrubber. The costs calculated for this treatment are $1.50/annual ton
operating and $0.64/tcn operating. This cost includes a mixing tank for
chemical addition, a thickener tank, pumps, piping services, etc. The
costs are relatively low compared with other fluoride treatment
processes because of the low volume of effluent to be treated, about 120
liters/minute (30 gpm), and high concentration of fluoride, about 1,000
mg/1. It is expected that this treatment would reduce suspended solids
and oil and grease by a similar amount.
The addition of a treatment process to wa#er effluent from the once-
through potline and pctrcom scrubber after lime treatment (if practiced)
is more costly than the previous treatment of recycle effluents. In
this case, large volumes of water with 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_^Tj:eatment. The addition of an alum treatment would add about
$11.0/annual ton capital and $8.40/ton operating. The capital costs
includes a mixing tank, flocculant tank, darifier, and pumps. The
major equipment cost is the 37-meter (121-foot) diameter clarifier which
accounts for 84 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.
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 ccsts 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 $313,00O/ year operating
cost.
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3Y.d£Oxy_l apatite. The costs for the adsorption of fluoride en bone char
were taken directly as reported by Wair.sley 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 ten capital and $14.50/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
cf 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 po^room scrubber water are summarized in Table
8, along with estimated fluoride discharged from the plant. The
elements included in capital cost and operating cost are; those discussed
on the first page of this Section (VIII). 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 a recycle plus 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 plajits 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
scrutber water costs approximately the same, about five times the amount
of pollutants would be discharged in the water from the activated
alumina system.
Ngnwater Quality Aspects
Energy^Reguirements
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TABLE 8. COSTS OF VARIOUS ALTERNATIVES FOR FLUORIDE REMOVAL
Discharge
Fluoride, Capital Cost, Operating
Process Alternative kg/ton $/annual ton Cost, $/ton
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
0 40
5 7.4
1 10
0.05 11.5
1 18.3
0.25 9.7
0.25 21.9
0.8
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.
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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:
Btu/tonAl
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 alone 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, it is concluded that
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 net produce a solid waste, but rather, allows the
collected particulates and, gases to be returned to the electrolytic
cell.
Limited data on the quantiites 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 reported the production of 25-30 kg
sludge/metric ton Al (50-60 Ib sludge/ton Al) from this treatment.
Plant C reproted about 15 kg sludge/metric ton Al (30 Ib sludge/ton Al)
from the same treatment.
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Summary
The energy requirements and solid waste production for the various
control and treatment technologies are summarized for purposes of
comparison 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
tJiat required by the subsequent treatment process in order to provide a
direct comparison of wet scrubbing plus various treatments with dry
scrubbing. The data shew 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 rest 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
0
200
0
73
Secondary wet scrubbing
with recycle
Primary wet scrubbing -
once through - Process B
Process A plus bleed and
filtrate treatment
Process B plus alum
treatment
Process B plus activated
alumina treatment
Process B plus hydroxy-
lapatite treatment
Process B plus reverse
osmosis treatment
394 200
84
85-395 200
100
100
100
546
76
40
77
123
110
-
60
Note: ton = metric ton; values are 10 percent lower for short ton.
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SECTION IX
BEST PRACTICABLE CONTROL TECHNOLOGY CURRENTLY
AVAILAELE, GUIDELINES, AND LIMITATIONS
Introduction
The effluent limitations which must be achieved July 1, 1977, are to
specify the degree of effluent reduction attainable through the
application of the test practicable control technology 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
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time of commenceirent of construction or installation of the control
facilities.
Recommended Ef f luent_Limj.tations
Based on the information contained in Sections III through VIII of this
report, 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:
Ef f luent_I;imj.tatigns __ {a)
Effluent Single Day Maximum (b)^ 30-Day Average (c)^
Characteristic ]$
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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 irg/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. fr Ib F/ton Al. This was rounded to 2 Ib
F/ton Al to obtain the recommended 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 rounded down to 3 Ib/ton Al to
arrive at the recommended effluent limitation.
The five values for oil and grease average 0.28 Ib/ton Al. This value
was rounded up to 0.5 Ib/ton Al to arrive at the recommended effluent
limitation.
The cyanide effluent limitation is based upon data from plant J. This
plant reprocesses spent cathodes to recover cryolite values and can be
expected to have more cyanide in its waste water than plants which store
on site or otherwise dispose of spent cathodes. Data from plants I and
K, which do not process spent cathodes, indicate lower cyanide
discharges than plant J.
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 v/.'re 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. Data on the maximum and average oil and grease discharges
were available for only 3 companies. The ratio values were 1.1, 1.2,
and 3.2, with an average ratio of 1.8. 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.
Recommghded Ef flijentLimitations
The effluent limitations recommendations are based on the following
considerations :
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(1) Achievement of the recommended effluent limitations by all primary
aluminum plants will result in a marked, industry-wide reduction in
the discharge of pollutants.
(2) The recommended 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 also can
be employed to achieve the recommended 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 longrange goals.
(3) The recommended limitations are realistic, in that about one-third
of the primary aluminum plants currently are able to achieve the
recommended 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 tc precipitate the fluoride,
followed by settling cf 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 currently are
available: cryolite precipitation, and precipitation with lime.
Precipitation of_Cryolite
The technology for cr,yolite 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.
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(4) Providing a holding pond or lagoon, if necessary, to accomplish
further settling cf solids in the bleed stream, and providing
aeration of the lagccn to accomplish oxidation of oil and grease, if
necessary.
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.
(U) Providing a holding pond or lagoon if necessary to minimize the
dischare of suspended solids, and oil and grease.
Alternative_Control_Technolo
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Reuse_of_Ef^luent_by_a_Com£an_ion_O£eration. One primary aluminum
plant currently achieves zero 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
T§chnologY_CurgentlymAvailable
The selection of best practicable control technology currently available
was based on the following considerations:
(1) The lowest unit effluent loadings for fluoride, suspended solids and
oils and grease, are currently attained by plants using dry fume
scrubbing, total impoundment of effluent, reuse of effluent water by
a companion operation, or wastewater 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 recommended 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 and of oil and grease.
(4) 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) It is concluded that 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 fluoride
produced by lime precipitation. Based on the information contained
in Section VIII, it is concluded that those plants not presently
achieving the recommended July 1, 1977, limitations would require an
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estimated capital investment of about $10/annual metric ton
($9/annual short ton) and an increased operating cost of about
$4.6/metrie ton ($4.27 short ton) in order to achieve the effluent
limitations.
Guidelines for'the Application^of
the Effluent Limitations"^
The following guidelines are suggested for the application of the
recommended effluent limitations:
(1) The effluent limitations apply to the sum of all discharges from the
plant, with the exception of the discharge from sanitary waste
treatment and with the exception cited in the next paragraph.
(2) A limited number cf primary aluminum smelters have metal fabrication
facilities, such as rod mills, rolling mills, etc., on the primary
reduction plant site. The effluent limitations stated in this
document are not intended to apply to such metal fabrication
operations.
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SECTION X
BESTAVAILABLETECHNOLOGYECONOMICALLY
iQtroduction
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 eguipment 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 in-process 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. Although economic factors are
considered, the costs for this level cf control are intended to be top-
of-theline of current technology subject to limitations imposed by
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economic and engineering feasibility. However, 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.
Recommended_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 the lime treatment of the bleed stream from a fluoride
precipitation and recycle system. The effluent limitations attainable
through the application of the best available technology economically
achievable are as follows:
t ^ Effluent Limitations (a)
Effluent Single Day Maximum (b| 30-Day^ Average {c^
Characteristic ]$2/k]£3Al "^Ib/tonAl kg/kkgAl Ib/tonAl
Fluoride 0.1 0.2 0.05 0.1
Suspended Solids 0.2 0.4 0.1 0.2
Oil and Grease 0.03 0.06 0.015 0.03
Cyanide 0.01 0.02 0.005 0.01
pH Range 6-9
(a) Effluent limitations are defined as kilograms of pollutant per
metric ton of aluminum produced or pounds of pollutant per short ton
of aluminum produced.
(b) The single day maximum is the maximum value for any one day.
(c) The 30-day average is the maximum average of daily values for any
consecutive 30 days.
The effluent limitations recommendations are based on the following
considerations :
(1) Achievement of the recommended effluent limitations by all primary
aluminum plants will result in an additional 90-95% reduction in
pollutant discharges by 1983 relative to the levels recommended for
achievement by 1977.
(2) The recommended 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,
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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
recommended effluent limitations. These alternative technologies
are options open to each company which provide for a flexibility of
approach to water pollution abatement.
Identification of Best Ayailable_Technglogy-_EconoinicallY_Achievable
The application cf 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) t.r€;ated to a final fluoride
concentration of 1 mg per liter would achieve the recommended
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 relat€»d water conditioning
applications.
Ratignale^for Selection of the Best Ayailablg
T§ chnol o_2y_ Econom ical^y; ^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.
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(3) Based on information contained in Section VIII, it is concluded
that 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 $3.87 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:
Capital Operating
$/annual metric S/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
Cast-house cooling water control and
treatment 1..6 O.t^
TOTAL 3.8 1.13
Guidelines for the Application
of the_Effluent_Limitations "
The guidelines cited in Section IX for the application of the July 1,
1977, effluent limitations apply equally to the July 1, 1983, effluent
guidelines.
The concentration of pollutants in the outfall from a primary aluminum
plant achieving the recommended July 1, 1983 effluent limitations may be
below analytically detectable limits. Therefore, sampling to assure
compliance may be required on internal fluoride-containing streams
before dilution by other streams.
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SECTION XI
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 particulcir type of process or
technology which must be employed. A further determination must be made
as to whether a standard permitting no discharge of 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.
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Standards of Performance are applicable to new sources in the primary
aluminum smelting subcategory.
Standards of Performance for New Sources
Based on the information contained in Sections III through VIII of this
report, the best available demonstrated control technology, processes,
operating methods, cr other alternatives for the primary aluminum
smelting subcategory is the 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, and the
treatment of casthouse cooling water and other streams, as required, for
oil and grease removal with a gravity separator or aerated lagoon. The
standards of performance attainable through the application of this
technology are as follows:
Ef fluent_Limitations_(aJ
Effluent §iQ2i§_Day._Maximum_£b)_ 30 Day Average_(c)_._
Ch§.£acteristic JSH^JsiSS^l Ik/ton JS3/JsJS3_Al Ifc/ton
Fluoride 0.05 0.1 0.025 0.05
Suspended Solids 0.1 0.2 0.05 0.1
Oil and Grease 0.03 0.06 0.015 0.03
Cyanide 0.01 0.02 0.005 0.01
pH Range 6-9
(a) Effluent limitations are defined as kilograms of pollutant per
metric ton of aluirinum produced or pounds of pollutant per short ton
of aluminum produced,
(b) The single day maximum is the maximum value for any one day.
(c) The 30-day average is the maximum average of daily values for any
consecutive 30 days.
Rationale for_Recommended Standardg of^Performance
The recommendation of the standards of performance for the primary
aluminum smelting subcategory is based on the following considerations:
(1) A new source has complete freedom of design so that unit processes
can be chosen to irinimize 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.
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(3) Even with dry scrubbing of potline air, certain water uses will be
required. There are no demonstrated dry scrubbing systems 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. Cast
house cooling water can be recycled through a cooling tower,
however, a bleed is required to prevent buildup of dissolved and
suspended solids, as well as oil and grease.
(4) Water from anode bake plant wet scrubbers and the cast house cooling
water bleed streair 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.
(5) The recommended effluent limitations 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 for potline air eliminates that source of fluoride and
suspended solids pollutants. In a prebaked anode plant the oil and
grease levels will be unaffected by dry scrubbing of potline air.
Identificatior^of^Best Available
Cemcnstrated_Cgntrol^ Technology^
Processes j^Cserating 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 alternative methods of molten metal degassing
techniques (identified in more detail in Section VII) which similarly
eliminate both use and discharge of cast house 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 required for new sources
consists of lime precipitation of the fluoride followed by settling of
the solids and recycle of the clarified liquor to the scrubbers as
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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 recommended
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.
The treatment technology for oil and grease in the casthouse cooling
water bleed stream and, if required, in the treated effluent from anode
bake plant wet scrubber circuit, consists of providing a holding pond or
aerated lagoon to accomplish oxidation of the oil and grease.
Rationale for the Selection of the Best
Available Demonstrated Control Technology
The rationale for the selection of the technology cited in this section
is as follows:
(1) Dry fume scrubbing methods for potline air have been developed
and are currently in use within the primary aluminum industry.
(2) At the present state of development, dry scrubbing systems are
not universally applicable.
(3) A new source, in contrast to an existing source, has the
freedom to select primary smelting technology compatible with
dry fume scrubbing.
(4) The control and treatment technologies recommended for dealing
with the remaining water uses are available and can be
incorporated in the new plant design.
Guidelines for the Application of the
Standards of Performance^ ~*
The guidelines for the application of the effluent limitations appearing
in Sections IX and X also apply to the standards of performance for new
sources.
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SECTION XII
ACKNOWLEDGEMENTS
Individuals, companies, and associations assisted in the course of this
study. While published information has been used, the far greater input
to this report has been data and information provided in written form,,
conferences, plant trips, telephone conversations, etc, all requiring
personal attention by numerous people in the primary aluminum industry.
Particular appreciation is expressed to the personnel at the following
plants which were visited and to the corresponding corporate
headquarters:
Alcoa; Point Ccmfort, Texas
Alcoa; Wenatchee, Washington
Eastalco; Fredrick, Maryland
Intalco; Ferndale, Washington
Kaiser; Chalmette, Louisiana
Martin-Marietta; The Dalles, Oregon
Ormet; Hannibal, Ohio
Reynolds; Corpus Christi, Texas
Reynolds; Longview, Washington
Reynolds, Troutdale, Oregon
Acknowledgement is also made of the assistance of The Aluminum
Association and the Clean Water Subcommittee of that association.
Acknowledgement is also due for the assistance and direction provided by
those associated with the Effluent Guidelines Division program: Messrs..
Allen Cywin, Ernst Hall, Walter Hunt and Edward Dulaney. The technical
contributions of Mr. John Ciancia are especially acknowledged. Finally,
the assistance of Ms. Chris Miller, Ms. Nancy Zrubek and Ms. Kit
Krickenberger was invaluable in the timely preparation of this report.
136
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SECTION XIII
REFERENCES
(1) Kirk-Othmer, EncYclO2edia_of_Chemical_Technoloc[^, ^ (2)
Interscience, 1963, p 941." " ~"
(2) Toddr 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., McGraw Hill
Book Company, NevTYork, N.Y., 1968.
(8) Eckenfelder, W. W. Jr., Water Quality, Engineering for
p£££ticj.ncL_Enc[ineers, 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).
137
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SECTION XIV
GLOSSARY
Act
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
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.
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Anthracite
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 TechnglggY Economically Achievable
Level of technology applicable to effluent limitations to be achieved by
July 1, 1983r for industrial discharges to surface waters as defined by
Section 301 (b) (2) (A) of the Act.
Bestr Practicable^Cgntrol_/Technoloqy 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 weighed average of the separate costs of debt and equity.
Cast House
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 which possess different traits which
affect waste water treatability and would require different effluent
limitations.
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Cathode
The negatively charged carbon cell lining in which the molten aluminum
collects and becomes the actual cathode.
Center-break JTechnglogY
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.
Clarifieg
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
^^ ^B-^VWi XBK^H ^^
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 (Fluxing}
The removal of hydrogen and other impurities from molten primary
aluminum in a cast house 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 gcrubber
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.
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Dust^Cgllectgr
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.
Effluent^Loading
The quantity or concentration of specified materials in the water stream
from a unit or plant.
Electfolytic^cell
The basic production unit for primary aluminum, consisting essentially
of a cast ircn 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 Freeipitator
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 (Degassing)^
The removal of hydrogen and other impurities from molten primary
aluminum in a cast house holding furnace by injecting chlorine gas
(often with nitrogen and carbon monoxide).
Hall/Heroult Process
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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 ^.886.
Hoods
Shrouds at the cells designed to promote capture of fumes and dust by
air withdrawal systems.
Horizontal^Stud^Soderfcerg JHSS) Plant (orAnode}
A facility for producing aluminum by the Hall/Heroulcl 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.
HydrQXYlapatj.te
A class of calcium hydroxy phosphate material prepared from bone char.
Investment Costs
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 Sourge
Any building, structure, facility, or installation from which there is
or may be a discharge of pollutants and whose construction is commenced
after the publication of the proposed regulations.
t
Petroleum Coke
The carbon residue of petroleum refining used for making anodes.
Pitch
142
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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.
Pollutant Parameterg
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
dust arising at the cells during production of aluminum.
Potline
A row of from 100 to 250 electrolytic cells connected in series forming
an electrical circuit.
Pgtliner
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.
potropm
The building housing a potline. Usually long and narrow to provide
ventilation along the line of pots.
Prebake Plant
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A facility using anodes which have been baked and graphitized before
installation in the Hall/Heroult electrolytic cell for aluminum
production.
PrJmary 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.
Rectifier
A device which converts a-c into d-c by virtue of a characteristic
permitting appreciable flow of current in only one direction.
Reverse Qsmosj.s
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
A facility at some primary aluminum plants for casting aluminum arid
forming rod usually about one-half inch in diameter.
Rodding^ Plant
T
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.
SanitaryrWater
The supply of purified water used for drinking, washing, and usually for
sewage transport and the continuation of such effluents to disposal.
Scrubber^Liquor
The liquid in which dust and fumes are captured in a wet scrubber.
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Secondary Air
Air in a potroom. It contains those pollutants not captured in the
primary air hood system.
Side-break^Technolggy
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^gf^Performanee
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 liguid 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 (or. 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, names for the inventor of the continuous anode system.
ffastewater^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
CONVERSION TABLE
MULTIPLY (ENGLISH UNITS) by TO OBTAIN (METRIC UNITS)
ENGLISH UNIT ABBREVIATION CONVERSION ABBREVIATION METRIC UNIT
acre ac
acre - feet ac ft
British Thermal
Unit ?TU
British Thermal
Unit/pound BTU/lb
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 lb
million gallons/day mgd
mile mi
pound/square
inch (gauge) psig
square feet sq ft
square inches sq in
tons (short) t on
yard y d
0.405
1233.5
0.252
ha
cu m
kg cal
0.555
0.028
1.7
0.028
28.32
16.39
0.555(°F-32)1
0.3048
3.785
0.0631
0.7457
2.54
0.03342
0.454
3,785
1.609
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 +1)1 atm
0.0929 sq m
6.452 sq cm
0.907 kkg
0.9144 m
hectares
cubic meters
kilogram - calories
kilogram calories/kilogram
cubic meters/minute
cubic meters/minute
cubic meters
liters
cubic centimeters
degree Centigrade
meters
liters
liters/second
killowatts
centimeters
atmospheres
kilograms
cubic meters/day
kilometer
atmospheres (absolute)
square meters
square centimeters
metric tons (1000 kilograrr
meters
1 Actual conversion, not a multiplier
146
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