Umt»d States
Environmental Protection
Agency
industrial Environmental Research
Laboratory
Cincinnati OH 45268
November 1, 1976
Research and Development
&EPA
Environmental
Assessment:
Primary Aluminum
-------
October 1978
ENVIRONMENTAL ASSESSMENT OF THE
DOMESTIC PRIMARY ALUMINUM INDUSTRY
0 PREPUBLICATION COPY °
by
PEDCo Environmental, Inc.
11499 Chester Road
Cincinnati, Ohio 45246
Contract No. 68-03-2537,
Work Directive No. 1
EPA Technical Project Monitor
John 0. Burckle
Industrial Pollution Control Division
Industrial Environmental Research Laboratory
5555 Ridge Avenue
Cincinnati, Ohio 45268
U.S. ENVIRONMENTAL PROTECTION AGENCY
Industrial Environmental Research Laboratory
5555 Ridge Avenue
Cincinnati, Ohio 45268
-------
NOTICE
This document has been reviewed by the Office of Research and
Development, U.S. Environmental Protection Agency, and approved for
publication as an INTERIM report. Approval does not signify that the
contents necessarily reflect the views and policies of the Environmental
Protection Agency nor does mention of commercial products constitute
endorsement or recommendation for use. This report is being circulated
for comment on its technical accuracy and policy implications. Following
receipt of these comments, it 1s planned to make appropriate changes and
publish a final report. Publication of this Interim report is, therefore,
on a limited basis.
This INTERIM report is intended to provide state-of-the-art information
on methods to assess the load of pollutants on watercourses from nonpoint
sources. The "user" should be aware that some of the technical aspects
may be changed in the final report. Nevertheless, this interim report
does outline the type of methods that can be used to generate the
assessments and specifies the data which are required.
11
-------
PREFACE
The purpose of this study has been to assemble the data needed for a
comparative assessment of all discharges (air, water, and sol id waste) from
the production of primary aluminum in the United States. This "multimedia"
approach has resulted in a comprehensive overview that allows the environ-
mental problems of this industry to be addressed collectively, rather than
on an individual basis. The report outlines the processes used for the
production of aluminum, and the associated environmental discharges and
control strategies. It does not include economic analyses or detailed per-
formance assessments of pollution control devices and treatment technologies
as a principal objective of this effort has been to provide the background
information required to decide where such studies are most needed. The
report also identifies industrial processes and practices that could result
in more effective utilization of this country's raw materials and energy
resources. The data were assembled largely from the open literature, with
theoretical means used to fill gaps in the data, resolve inconsistencies,
and confirm literature findings. The results of previous single-media
investigations were assigned to the appropriate categories. Emphasis was
placed on fullest identification of all polluting components from readily
available information.
This report will be used as a reference by working groups within the
U.S. Environmental Protection Agency, as well as state and local control
agencies and individual researchers. It summarizes all data existing extant
in a format allowing continuous updating and further quantification as new
facts become available. The Metals and Inorganic Chemicals Branch of the
Industrial Pollution Control Division should be contacted for additional
information on this program.
iii
-------
ABSTRACT
This report presents the results of a multimedia (air, water, and solid
waste) study of the environmental impacts of the primary aluminum industry in
the United States. All production processes currently employed by the indus-
try are identified and described, and all pollutant effluents and environ-
mental effects from those processes are characterized. The various pollution
control systems in use or potentially applicable are described and evaluated
for domestic application. In addition, alternate production processes being
developed for use in this country are reviewed.
The domestic primary aluminum industry is relatively young; it is com-
posed of two bauxite mines, nine bauxite refineries, and 32 aluminum smelters.
The mines, both located in Arkansas, can supply only a fraction of demand. As
a result, the United States is largely dependent upon imported raw material
for the production of aluminum. The majority of bauxite reserves are located
in the less industrialized areas of the world, and levies imposed by the
producing countries have become the major factor in the cost of ore. The
refineries are located either near the mines or near natural gas supplies and
deep-water shipping ports. The smelters are also generally located near
sources of inexpensive hydroelectricity or fossil fuels. Aluminum production
requires large quantities of both electric and thermal energy; consequently,
the prospects of increasing energy costs and shortages of the petroleum
products used for fuel and raw materials at smelters are major problems
facing the industry.
Although there are polluting discharges at both mines and refineries, the
principal environmental control problem facing the industry occurs at smelters
during the reduction of alumina to aluminum metal. Impurities in the bauxite
ore are all removed during beneficiating and refining; however, particulate
matter and gaseous fluorides are released from the potlines. Organics are
also released either during electrolysis or during anode baking. Control of
these emissions is complicated by difficulties in effectively hooding certain
types of electrolytic cells and by the problems inherent in simultaneously
removing gaseous fluorides and particulate matter from a gas stream. Both wet
and dry control techniques are employed. The collected material requires
careful disposal to avoid simply transferring the fluoride problem to water or
land. Sulfur dioxide emissions from smelters are likely to become increas-
ingly significant because of a trend toward the use of higher-sulfur raw
materials for the manufacture of anodes.
iv
-------
Both industry and government are seeking solutions to the environmental,
energy, and raw material problems facing the primary aluminum industry. These
efforts include the development of nonbauxite sources of aluminum, alternative
production techniques that reduce energy consumption and environmental dis-
charges, and improved pollution control technologies.
-------
CONTENTS
Page
Preface iii
Abstract iv
Figures vii
Tables vii
Acknowledgment ix
1. Introduction 1
2. Primary Aluminum Industry 3
Industry description 3
Industry analysis 19
Process No. 1, mining 22
Process No. 2, ore treatment 29
Process No. 3, bauxite drying 30
Process No. 4, grinding, digestion, and 31
heat recovery
Process No. 5, liquor/mud separation 35
Process No. 6, waste alumina recovery 40
Process No. 7, A1(OH)3 precipitation 42
Process No. 8, spent liquor recovery 45
Process No. 9, calcination 47
Process No. 10, prebake cells 51
Process No. 11, HSS cells 63
Process No. 12, VSS cells 69
Process No. 13, casting 75
Process No. 14, paste preparation 78
Process No. 15, anode preparation 81
Process No. 16, anode cleaning 87
Process No. 17, cryolite recovery 88
3. Environmental Management 90
4. Emerging Technology 104
-------
FIGURES
Number Page
2-1 Location of U.S. primary aluminum industry 10
2-2 Flowsheet - the production of alumina from bauxite 20
2-3 Flowsheet - the production of aluminum metal from alumina 21
2-4 Prebake reduction cell 52
2-5 Horizontal stud Soderberg cell 64
2-6 Vertical stud Soderberg cell 70
TABLES
Number Page
2-1 U.S. Imports of Bauxite and Alumina 5
2-2 Distribution of End-Use Shipments of Alumina Products 7
2-3 Statistical Data for the U.S. Aluminum Industry in 1976 8
2-4 U.S. Bauxite Refineries 11
2-5 U.S. Primary Aluminum Smelters 12
2-6 Typical Composition of Bauxites Used for Aluminum Manfacture 23
2-7 Characteristic Analyses of Various Bauxites 23
2-8 Chemical Composition of Untreated Mine Water at Two Bauxite 25
Mines
2-9 Chemical Composition of Treated Mine Water at Two Bauxite 28
Mines
2-10 Possible Control Devices - Bauxite Grinding 33
2-11 Range of Chemical Analyses of Red Muds 36
2-12 Insoluble Solids in Red Mud from Jamaican Bauxite 37
2-13 Soluble Solids in Red Mud from Jamaican Bauxite 37
2-14 Unit Mud Production Rates for Various Bauxites 38
-------
Number Page
2-15 Relationship Between Precipitation of A1(OH)3 and Si02 43
2-16 Composition of a Good Alumina 48
2-17 Possible Control Devices - Calcination of Alumina Hydroxide 50
2-18 Particle Size Distribution of Uncontrolled Emissions from 55
Prebake Cells
2-19 Control Equipment on Center-Worked Prebake Potlines - 1974 58
2-20 Air Pollution Control Efficiencies - Prebake Potlines 59
2-21 Emission Factors for Control Devices on Prebake Cells 60
2-22 Particle Size Distribution of Uncontrolled Emissions from 66
HSS Cells
2-23 Air Pollution Control Removal Efficiencies - HSS Potlines 68
2-24 Emission Factors for Control Devices on HSS Cells 68
2-25 Air Pollution Control Efficiencies - VSS Potlines 73
2-26 Emission Factors for Control Devices on VSS Cells 74
2-27 Emission Factors for Paste Plant Materials Handling 80
2-28 Anode Baking Ring Furnace Emissions 84
2-29 Control Devices on Anode Bake Plants - 1974 85
2-30 Air Pollution Control Removal Efficiencies - Bake Furnace 86
2-31 Emission Factors for Control Devices on Anode Baking 86
3-1 Atmospheric Emissions from Bauxite Mines and Refineries 91
3-2 Atmospheric Emissions from Primary Aluminum Smelters 93
3-3 Wastewater Discharges from Primary Aluminum Production 98
3-4 Solid Wastes from Primary Aluminum Production 101
viii
-------
ACKNOWLEDGMENT
This report was prepared by PEDCo Environmental, Inc., under the direc-
tion of Mr. Timothy W. Devitt. The PEDCo project manager was Mr. Thomas K.
Corwin. Principal authors were Mr. Michael R. Bockstiegel, Mr. Corwin, Mr.
Hal M. Drake, and Mr. A. Christian Worrell III. Project Officer for the
Industrial Environmental Research Laboratory of the U.S. Environmental Pro-
tection Agency was Mr. John 0. Burckle.
ix
-------
SECTION 1
INTRODUCTION
This report presents the results of a comprehensive environmental assess-
ment of the primary aluminum industry in the United States. The report is
multimedia in scope; its purpose is to develop the background needed for a
comparative assessment of all discharges of the primary aluminum industry to
air, water, and land. An open literature survey and review of Environmental
Protection Agency (EPA) files identified all the processes employed in the
industry having a significant environmental impact. The report covers all
phases of production, from the mining, concentrating, and refining of bauxite
to the reduction of alumina to aluminum metal. Engineering studies of the
available data were undertaken to describe all sources of pollution from
existing and potential production processes. Potentially hazardous outputs
were singled out for special attention, and all toxic materials released to
the environment were identified. As a result of certain limitations on the
amount of information available, some important environmental questions have
been left unanswered. The report constitutes a comprehensive multimedia
environmental overview that identifies these information gaps and allows their
consideration in research and development aimed at more effective pollution
control.
As defined for this report, the domestic primary aluminum industry
consists of the facilities in this country that extend from the mining of
bauxite ore through the production of purified aluminum as marketable cast-
ings. Included are plants that partially separate the ore minerals, refiner-
ies that produce calcined alumina from bauxite, and smelters that reduce the
alumina to metal. The industry does not include operations that fabricate
aluminum into commercial products or that blend it to manufacture alloys.
The primary aluminum industry faces several major problems as it enters
the 1980's. United States bauxite reserves are inadequate to meet our needs.
As a result, domestic aluminum smelters are heavily dependent upon imports of
bauxite and alumina, and the producing countries have raised prices rapidly in
recent years. In addition, domestic' smelter capacity is inadequate to meet
expected demand in the near future, necessitating increased imports of alumi-
num metal. Another major problem facing the industry is the shortage of low-
cost energy. Aluminum production requires considerable amounts of both
electrical and thermal energy, and most facilities have been located to take
advantage of inexpensive sources of fossil fuels. The rapidly increasing
costs and potential shortages of these fuels, however, could significantly
affect the ability of the industry to produce a competitively priced product.
Pollution control must also be considered. The industry is relatively young;
most of its production facilities have been constructed since World War II.
Unfortunately, conventional production technologies release fluorides,
1
-------
organics, S02» and some heavy metals to the environment, and effective control
can be complicated and expensive.
Current research efforts by industry and government are being directed at
the problems that face the primary aluminum industry. This report encompasses
all known activities of the industry in the United States, and it can thus
assist in providing the perspective needed to decide where further research
efforts are needed.
-------
SECTION 2
PRIMARY ALUMINUM INDUSTRY
INDUSTRY DESCRIPTION
Although technologies for its large-scale production were not developed
until the late 19th century, aluminum is now used in virtually all segments of
the economy. The explosive growth of the industry that began during World War
II has largely subsided; however, aluminum is likely to continue to be a
growth metal in the foreseeable future. The primary aluminum industry is
composed of three distinct segments -- the mining and beneficiating of bauxite
ores, the refining of bauxite to produce alumina, and the reduction of alumina
to aluminum metal. The industry is distinguished from most other metal indus-
tries in that all purification takes place during refining, rather than during
the reduction to metal.
Worldwide, almost all bauxite mined is refined by the Bayer process,
patented in Germany in 1888. This process involves leaching with caustic at
high temperatures and pressure, followed by separation of the solution and
precipitation of alumina. There are two principal variations of the Bayer
process. The European Bayer is used for monohydrate ores and utilizes higher
temperatures, pressures, and caustic concentrations, and longer digestion
times. The American Bayer modification uses milder operating conditions
applicable to trihydrate ores. A variation of the American Bayer process adds
a step to reclaim additional alumina from high-silica bauxites. Aluminum
metal is produced from calcined alumina by electrolysis in a molten bath of
cryolite. Three types of electrolytic cells are in use -- prebake, horizontal
stud Soderberg (HSS), and vertical stud Soderberg (VSS) -- with the differ-
ences largely in anode configuration and method of alumina addition.
Raw Materials
Aluminum is the most abundant metallic element in the earth's crust,
although it is never found free in nature. The principal aluminum ore is
bauxite, a mixture of hydrated aluminum oxides. The principal minerals in
bauxite are gibbsite, a trihydrate, and the monohydrates boehmite and diaspore.
Common impurities are quartz, kaolinite clay, and iron oxides; essentially no
sulfur or potentially hazardous trace elements are present. Bauxite ores
range from stony materials to soft clay-like masses.
Ores are generally identified as one of three types, classified by the
degree of hydration of the aluminum minerals: (1)
0 Surinam - mainly trihydrate.
-------
0 Jamaican - a mixture of trihydrate and monohydrate.
0 European - monohydrate.
The preferred ores contain trihydrate, which is much more soluble in caustic
than monohydrate, and thus can be processed under milder conditions (2). A
low silica content is desirable since this element, in some chemical forms,
causes a loss of both aluminum and caustic as insoluble sodium aluminum sili-
cates. Most bauxite deposits in the United States are high in trihydrate
content, but contain considerable silica and clay.
Most bauxite reserves are located in tropical regions of the world, far
from industrialized areas of potential consumption. World reserves are esti-
mated at 22 billion metric tons, one-third of which are located in Guinea (3).
Jamaica is the world's largest bauxite producer, followed in order by Guinea,
the U.S.S.R., Surinam, Guyana, Greece, Hungary, and the United States (4). In
1974 11 of the major bauxite exporting countries formed the International
Bauxite Association in order to increase revenues and control operations in
member countries. The levies imposed on bauxite by the producing countries
are now the largest element in its cost (3).
In 1976, total domestic mine production of bauxite was about 2 million
metric tons; about 7 percent of this bauxite was used in the production of
abrasives, refractories, and chemicals, rather than for aluminum metal. Only
about 8 percent of aluminum produced in this country is from domestic mines
(3,5).
Consumption of bauxite in the United States reached a peak of 17.2
million metric tons in 1974; domestic reserves of 36 million metric tons are
inadequate to meet long-term demand (3). The domestic aluminum refineries now
rely heavily on imports of bauxite, especially from the Caribbean area, north-
eastern South America, and western Africa. Of the nine U.S. bauxite refin-
eries, seven process only imported ore (3). The trend, however, is toward
increased imports of alumina, a purified aluminum oxide, rather than bauxite.
About a third of the alumina used in U.S. aluminum smelters is imported,
primarily from Australia. The large reserves of bauxite in Australia, esti-
mated to be 4.1 billion metric tons, are not exported to the United States as
ore (3). Table 2-1 presents 1976 data for U.S. imports of both bauxite and
alumina.
Because of the dependence on foreign sources, considerable research and
development has been directed toward the use of alternate raw materials for
aluminum production. Possible alternatives include clays, feldspar rocks such
as anorthosite, other minerals, and coal wastes. High-alumina clays, con-
sisting mainly of kaolinite, are the most promising source. Pilot plant
evaluations were conducted during World War II, and research efforts have
recently been revived. However, dependence on foreign sources is likely to
continue for some time, until economical alternate processes are developed.
Other raw materials in addition to bauxite that are used for the pro-
duction of aluminum include caustic soda, lime, cryolite (NaJ\lFg), fluorspar
-------
Table 2-1. U.S. IMPORTS OF BAUXITE AND ALUMINA - 1976 (5)
(thousand metric tons)
Australia
Dominican Republic
Guinea
Guyana
Haiti
Jamaica
Surinam
Others
Total
Bauxite
-
517
3064
645
616
6284
1569
54
12749
Alumina
2795
-
-
13
-
626
213
35
3682
-------
(CaF2), sodium and aluminum fluorides, soda ash, other inorganic salts, chlor-
ine and inert gases, coke, and pitch.
Products
The principal product of the industry is aluminum metal. Gallium is
recovered at one location as a by-product of the refining of domestic bauxite.
Some bauxite deposits contain appreciable amounts of titanium, but economic
methods of extraction have not been developed (6).
Although aluminum metal is soft, its alloys have high strength-to-weight
ratios. Its principal applications take advantage of this weight saving or
its tendency to form an oxide surface that resists corrosion. Aluminum is the
most widely used nonferrous metal. As indicated in Table 2-2, the principal
markets are the building and construction, transportation equipment, and
container and packaging industries. Packaging and transportation are the
fastest-growing markets, but use of aluminum is increasing in many segments of
the economy. Copper, magnesium, titanium, and steel can substitute for many
aluminum applications, but often at higher costs or significant weight pen-
alties. The most promising substitutes are steel and wood in construction,
and steel, plastic, glass, and paper in the container market.
Primary domestic aluminum production in 1976 was 3.86 million metric
tons, with a value of $3.8 billion (7). Table 2-3 summarizes the general 1976
statistics of the industry. A peak production level of 4.45 million metric
tons was reached in 1974 (3). Over one-fourth of the aluminum industry's
capacity was closed during 1975. The industry has since rebounded from this
5-year low and was operating at over 90 percent of capacity by the end of 1976
(3). An annual rate of growth in production of 5.8 percent is estimated from
1976 through 1980 (3). Nevertheless, the United States will likely become
increasingly dependent on imports of aluminum metal unless domestic smelting
capacity is increased sharply.
Aluminum has been characterized historically by price stability, the
price ranging from 25 and 32 cents per pound in 1968 dollars between 1950 and
1968 (1). This stability was a result of the high degree of integration in
the industry, improvements in processing, and the long-term price stability of
electric power. Prices have climbed rapidly in recent years, however, and the
average price per pound of ingot in 1976 was 44.6 cents, up from 25.3 cents in
1973 (7).
Companies
Nearly half of the total world production capacity for bauxite, alumina,
and aluminum is controlled by six corporate groups or their subsidiaries --
Alcan Aluminum (Canada), Aluminum Company of America (Alcoa), Reynolds Metal,
Kaiser Aluminum and Chemical, Pe"chiney and Ugine (France), and Swiss Aluminum
(8). All of these groups are fully integrated from bauxite mining through
fabrication, and only Kaiser, of the three large American concerns, has signi-
ficant interests in areas other than aluminum. Twelve companies produce
-------
Table 2-2. DISTRIBUTION OF END-USE SHIPMENT OF ALUMINUM PRODUCTS
IN 1976 (7)
Building and construction
Transportation
Containers and packaging
Electrical
Consumer durables
Machinery and equipment
Other
Total to domestic users
Exports
Total
Quantity,
thousand metric tons
1341
1113
1165
602
407
410
298
5399
382
5781
Percent
23.2
19.2
20.2
10.4
8.1
7.1
5.2
93.4
6.6
100.0
-------
Table 2-3. STATISTICAL DATA FOR THE U.S. ALUMINUM INDUSTRY IN 1976 (5,7)
(thousand metric tons)
Primary production
Secondary recovery
Exports (crude and semicrude)
Imports for consumption (crude and semi-
crude)
Consumption, apparent
World production
Bauxite:
Mine production
Imports for consumption
Total consumption
Alumina:
Imports
Exports
3,856
1,048
439
679
5,527
12,491
1,989
12,749
14,738
3,682
1,050
8
-------
primary aluminum in the United States; five of these also produce alumina
domestically. Alcoa was the only domestic producer until World War II.
Smaller producers are generally more limited in their scope, although most own
fabricating plants in addition to bauxite refineries or other primary produc-
tion facilities. The industry has been characterized by aggressive marketing
techniques.
Figure 2-1 shows the location of the U.S. bauxite mines, refineries, and
primary aluminum smelters. Although bauxite was mined by eight companies at
12 locations in Arkansas, Alabama, and Georgia in 1976, the only production
used for aluminum occurred at two mines in Saline County, Arkansas. Total
1978 employment at all mines was about 300 to 500 (8).
Including one plant in the Virgin Islands, there were nine bauxite
refineries in the United States that produced alumina destined for aluminum
production. Table 2-4 lists these facilities and their estimated production
capacities, which vary with the type of bauxite feed. Two refineries are
located near the Arkansas mines; the six other plants in the continental
United States are on the Gulf Coast, where they have access to natural gas
supplies and deep-water ports. In general, domestic bauxite refineries are
older facilities, small by the standards of new refineries being built abroad.
No large additions are expected to domestic bauxite refining capacity in the
near future; minor expansions and additions to existing plants should account
for most increased capacity. About 7,000 to 8,000 people were employed at
bauxite refineries in 1978 (8).
The primary aluminum smelters in the United States are listed in Table
2-5, along with related information such as capacity, cell type, and raw
material source. The plants, which operate around the clock, 365 days a year,
range in production capacity up to 254,000 metric tons of aluminum per year.
Much larger smelters are feasible; a Canadian plant has an annual capacity of
412,000 metric tons, and one Russian facility can produce 500,000 metric tons
of aluminum per year (9). The industry is relatively young; only three plants
predate the rapid expansion of the industry during World War II. The industry
went through a second major expansion during the 1950's, and eight additional
plants have started within the last decade. The older smelters are electro-
chemical ly equivalent to the newer, although process modifications have
increased production efficiency and reduced polluting discharges (10). Nine
of the 11 plants opened since 1960 utilize prebake anodes, and one is testing
a chloride bath process. Recent plant expansions support the trend toward
prebake anodes: 99 percent of the total 324,000 metric tons added to existing
smelter capacity since 1973 has been at prebake facilities (9). Of the indus-
try's total annual capacity of 4.8 million metric tons, 68 percent is accounted
for by prebake plants, 20 percent by HSS plants, and 12 percent by VSS plants
(9). All the plants using HSS anodes are currently being modified with
Japanese reduction technology. About 23,000 to 26,000 people were employed at
primary smelters in 1978 (8).
Two new primary smelters are planned for completion soon. They are both
owned by Alumax, a joint venture of AMAX, Mitsui and Company, and Nippon
-------
a BAUXITE MINES
• ALUMINA REFINING PLANTS
oALUMINUM REDUCTION PLANTS
Figure 2-1. Location of U.S. primary aluminum industry.
-------
Table 2-4. U.S. BAUXITE REFINERIES (2,8)
Company
Al umi nun Company of
America
Kaiser Aluminum and
Chemical Corp.
Martin- Marietta
Aluminum, Inc.
Ormet Corp.
Reynolds Metal Co.
Location
Mobile, Alabama
Bauxite, Arkansas
Point Comfort, Texas
Baton Rouge,
Louisiana
Gramercy, Louisiana
St. Croix, Virgin
Islands
Burnside, Louisiana
Hurricane Creek,
Arkansas
Corpus Christi, Texas
Start-up
date
1938
1952
1959
1942
1960
1967
1958
1942
1953
Capacity
metric tons/year
467,000
177,000
630,000
484,000
378,000
215,000
284,000
397,000
655,000
Aluminum equivalent
11
-------
Table 2-5. U.S. PRIMARY ALUMINUM SMELTERS (9)
Company
Aluminum Company of
America
Anaconda Aluminum
Co.
Conalco, Inc.
(owned by Swiss Alumi-
num and Phelps Dodge)
Eastalco Aluminum Co.
(sub. of Howmetc)
Intalco Aluminum Corp.
(owned by Alumax and
Howmetc )
Location n
Evansville, IN
Massena, NY
Badin. NC
Alcoa, TN
Point Comfort,
TX
Rockdale. TX
Palestine, TX
Vancouver, HA
Wena tehee, WA
Sebree, KY
Columbia Falls,
MT
Lake Charles. LA
New Johnsonville,
TN
Frederick, MD
Ferndale. WA
Capacity
etric tons/yr
254,000
209,000
163,000
245,000
163,000
259,000
27,000
104,000
172.000
109,000
163.000
33.000
131.000
160,000
236.000
Cell
type3
PB
PB
PB
PB, VSS
VSS
PB
PB
PB
PB
VSS
PB
PB
PB
PB
Alumina,
source
ALA.AUS.JAM.SUR,
TX
JAM.SUR
ALA.JAM.SUR
ALA.JAM.SUR.TX
TX
ALA.TX
AUS
AUS.JAM
JAM
JAM
SUR
AUS, JAM
AUS. GR, JAM
AUS
Bauxite.
source
ARK, OR, GUI. JAM,
SUR
JAM. SUR
ARK, JAM, SUR
ARK, OR, GUI, JAM,
SUR
ARK, DR. GUI, JAM,
SUR
ARK, OR, GUI, JAM.
SUR
AUS
AUS.JAM
JAM
JAM
SUR
AUS
AUS, GR. JAM
AUS
Start-up
date
1960
1903
1916
1914
1949
1952
1976
1940
1952
1974
1955
1974
1963
1970
1966
Remarks
PB 84%, VSS 16* of capacity
Power capacity to expand
to 455,000 MTPY by 1988
Experimental chloride bath
process, expansion capability
to 272,000 MTPY if successful
Expanding to 163.000 MTPY (1979);
potential for 218,000 MTPY
Sumitomo/Alcoa pollution control
systems to be installed
Expanding to 46.000 MTPY (1978)
Expanding to 240.000 MTPY (1980)
ro
PB-Prebake, HSS-Horizontal stud Soderberg, VSS-Vertical stud Soderberg.
b ALA-Alabama, ARK-Arkansas, AUS-Australia, DR-Dominican Republic, GUI-Guinea, GUY-Guyana, HA-Haitl,
TX-Texas, Vl-Virgin Islands
c Howmet Corporation is owned 69% by Pgchiney Ugine Kuhlmann.
JAM-Jamaica, LA-Louisana, SUR-Surinam,
(continued)
-------
Table 2-5 (Continued). U.S. PRIMARY ALUMINUM SMELTERS
Company
Kaiser Aluminum &
Chemical Corp.
Martin Marietta
Aluminum, Inc.
National-Southwire
Aluminum Co.
Noranda Aluminum, Inc.
Ormet Corp.
(owned by Conalco and
Revere)
Revere Copper & Brass,
Inc.
Reynolds Metal Co.
Location
Chalmette, LA
Ravenswood, UV
Mead, MA
Tacoma, WA
The Dalles. OR
Goldendale. WA
Hawesville, KY
New Madrid, MO
Hannibal, OH
Scottsboro, AL
Listerhill, AL
Arkadelphia, AK
Jones Mills, AK
Massena, NY
Troutdale, OR
Longview, WA
Capacity
metric tons/yr
236,000
148.000
200,000
73,000
82,000
109,000
163.000
127,000
236,000
103,000
183,000
62,000
113,000
114,000
118,000
190.000
Cell
typea
HSS
PB
PB
HSS
VSS
VSS
PB
PB
PB
PB
HSS
PB. HSS
PB
HSS
PB
HSS
Alumina.
source
LA
LA
AUS
AUS
VI
VI
LA.TX
GUI, LA
LA
JAM
ARK
ARK
ARK
N.A.
AUS.JAM.SUR
JAM
Bauxiteb
source
JAM
JAM
AUS
AUS
GUI, GUY
GUI, GUY
ARK, DR. GUI, JAM.
SUR
GUI. JAM
SUR
JAM
ARK
ARK
ARK
N.A.
AUS.JAM.SUR
JAM
Start-up
date
1951
1957
1942
1942
1958
1972
1969
1971
1958
1971
1940
1954
1942
1953
1952
1941
Remarks
Dry scrubbing installed 1976
Capacity to expand to 193.000 MTPY
Dry scrubbing to be Installed
Dry scrubbing to be installed
Capacity to expand to 190,000 MTPY
Capacity to expand to 327,000 MTPY;
sale to Alcan set for July 1977
PB 25%, HSS 75% of capacity
"l
* PB-Prebake, HSS-Hor1zontal stud Soderberg, VSS-Vertlcal stud Soderberg.
b ALA-Alabama, ARK-Arkansas. AUS-Australia, DR-Domlnlcan Republic, GUI-Guinea, GUY-Guyana, HA-Haltl, JAM-Jama lea, LA-Lou1sana, SUR-Surinam,
TX-Texas, Vl-Virgln Islands
N.A. - Not available.
-------
Steel. One will be located in Umatillo, Oregon, and the other in Berkley
County, South Carolina, 20 miles north of Charleston; both will utilize pre-
bake anodes. The plans for the South Carolina plant include a requirement for
zero wastewater discharge, and the cost of pollution control equipment will
amount to one-eighth of the total capital expenditure. Three prebake plants
are undergoing expansion programs that will further increase the annual
capacity of the industry by 150,000 metric tons (9). Despite this, growth in
demand is expected to exceed production capacity for the next 5 to 6 years.
This could significantly affect the competitive impact of any price increases
caused by stringent pollution control requirements. Fifteen percent of the
smelter capacity of U.S. concerns is located abroad, and this trend is likely
to continue (11).
Energy
In 1971, only five industrial sectors consumed more energy than the
aluminum industry: blast furnaces and steel mills, petroleum refining, and
the manufacture of paper and allied products, olefins, and ammonia (11). The
aluminum reduction process accounts for 0.8 percent of all energy consumed by
manufacturing industries in the United States (12). In 1974, the annual
energy consumption of the primary aluminum industry was estimated at 80 to 100
billion kilowatt-hours (10). A total of from 13.8 to 18.7 kilowatt-hours is
required at all stages of processing to produce one kilogram of aluminum (13).
Electricity is the principal source of energy, but thermal power is required
for anode baking and casting.
Although bauxite refineries lose heat in the Bayer process, nearly all of
the energy required to produce aluminum is consumed chemically during the
electrolytic reduction process. The average energy consumption in Bayer
plants in the U.S. amounts to 300 kilowatt-hours of electricity and 3.2
million kilocalories of fuel per metric ton of alumina (11). Two-thirds of
the total cost of producing aluminum ingot from bauxite ore is incurred at the
smelter (1), where electric power is the second largest cost item following
the alumina itself (14). The availability of inexpensive hydroelectricity and
coal explains the location of many plants in the Pacific Northwest, along the
Mississippi and Ohio Rivers, and near the St. Lawrence River transportation
system. About 6.7 million kilocalories of fossil fuel is required at the
smelter per metric ton of aluminum product. All domestic smelters have
plants producing thermal power; most purchase electricity from utilities. New
smelters will require about 20 to 25 percent less electricity than existing
facilities because of process modifications (11). During 1972 to 1973, 53
percent of the total energy consumed in the industry was derived from petroleum
and natural gas, 36 percent from coal, 10 percent from hydroelectric power,
and 1 percent from nuclear power (15). Power curtailments due to drought
conditions forced significant production cutbacks in 1977 at some smelters
that rely on hydroelectricity. Natural gas shortages have also caused pro-
blems at some locations in recent years.
14
-------
A recent estimate (16) indicates that the energy penalties for total
compliance with environmental regulations expected to be in force by 1985
would be 1 to 2 percent when compared with the energy required to produce
aluminum in the absence of any regulation. Stricter regulation will cause
increased electrical usage because of more widespread use of dry alumina
absorption for control of atmospheric emissions; however, thermal energy
requirements may decrease because of the elimination of some cryolite recovery
plants at smelters where wet scrubbers would no longer be used.
Environmental Impacts
The principal waste stream at bauxite refineries consists of solid mud
residues separated from the caustic liquor used to digest the bauxite. Total
impoundment is the universal treatment method. One type of mud waste, called
"red mud," may be used in the manufacture of cement, to neutralize acidic
wastes, and as pigment. Another type, "brown mud," has been used in Arkansas
to neutralize acidic soils. Refineries produce some alkaline process waste-
water containing unrecovered aluminum. Although there are significant dif-
ferences in refinery size, the quality of wastewater does not vary greatly
throughout the industry. The principal air emission at refineries is dust
from calcinating furnaces, which is usually effectively controlled by electro-
static precipitators (ESP's) and at some locations also by fabric filters
(baghouses).
The principal environmental problem at smelters is air emissions from the
reduction cells and anode baking plants. At a poorly controlled smelter,
secondary emissions that escape the primary hoods can be several times as
large as those captured. Air emissions at smelters include organics from
anode baking facilities or Soderberg cells, as well as some SC^. The control
problem is complicated by the need for simultaneous removal of gaseous fluo-
rides and solid materials. Wet scrubbers are often used, but tars in the gas
stream resist wetting and can cause particle reentrainment. Dry scrubbers
using alumina adsorption have been installed at some locations.
The S02 emissions from smelters are not the result of a sulfur content in
the bauxite ores, but rather are caused by the consumption of anodes made of
petroleum coke and pitch. Release of S02 has not been considered a serious
problem in the past. Demand for low-sulfur fuel, however, has resulted in
petroleum refiners increasing the desulfurization of higher petroleum frac-
tions, thus increasing the sulfur content of residual products such as petro-
leum coke. This practice may result in significant increases in S02 emissions
from aluminum smelters in the future. The increased use of dry adsorption for
fluoride/particulate control will further aggravate the problem, as this
technology provides less effective S02 control than do wet scrubbers, and it
recycles captured sulfur to the cell with the fluoride values. Martin Mari-
etta Aluminum is installing Flakt SCL scrubbers on the potlines at its two VSS
smelters; soda ash will be used as tne alkali reagent. Installation is sche-
duled for completion by September 1978 (17).
15
-------
Wet scrubbers used for primary air pollution control of potlines are the
major source of contaminated wastewater at smelters. The effluent contains
significant quantities of fluorides removed from cell emissions. The waste-
water is generally treated with lime; at some plants the treated water is
recycled. The scrubber water may also be recycled after cryolite recovery.
After lime treatment, 117 kilograms of particulate per metric ton of aluminum,
containing 18 kilograms of fluorides, is discharged to lagoons (18).
Many potential solid wastes -- spent anode butts, spent potliners, and
potroom skimmings -- are generally recycled to the process, although there is
the possibility of contaminated runoff during storage of these materials. The
principal solid waste requiring disposal is thus the sludge generated by lime
treatment of scrubber water. The lagoons are typically dredged and the
sludge dumped on land, although at some plants new lagoons are built when the
old ones are filled to capacity. The lagoon deposits contain carbon, alumina,
fluorine, and calcium; most of the fluoride and calcium occur as calcium
fluoride. Lined or sealed sludge disposal areas are not used.
The quantity of sludge produced varies with the type of air pollution
controls and the degree of recycle practiced. An estimated 95 kilograms of
sludge is generated per metric ton of aluminum by lime treatment of scrubber
water; this figure can exceed 200 kilograms for once-through wastewater treat-
ment systems. Less than 20 kilograms of sludge per metric ton of aluminum is
produced by recovery of cryolite from potline scrubber water, while 125
kilograms is generated when cryolite is also recovered from spent potliners
(18).
About 189,000 metric tons of sludge, containing 30,000 metric tons of
fluoride, was produced as a result of primary aluminum production in 1977.
Some 212,000 metric tons of discarded potliners and potline skimmings, con-
taining 39,000 metric tons of fluoride and 203 metric tons of cyanide, was
disposed of on land. About 34,000 metric tons of dusts containing fluoride,
copper, and lead was also handled in this manner (18).
Outlook for the Future
The aluminum smelting industry is highly capital intensive, making rapid
technological change somewhat difficult. However, the industry has a high
degree of technical competence that will encourage acceptance of new develop-
ments. The major changes that can be expected in the next few years will be
additions to smelter capacity and installation of additional and improved
pollution control equipment. The trend at primary smelters is toward com-
puterized control of all operating conditions. Research is progressing in the
development of energy-saving technologies, new cathode leads using hard
metals such as titanium diboride, new cell lining materials, and substitutes
for the petroleum coke used for anodes. Alcoa is testing a new chloride bath
electrolysis process on a semi commercial scale. The process is based on
converting alumina to aluminum chloride and is said to require 30 percent less
energy than conventional Hall-Heroult electrolysis. A considerable amount of
research and development is directed toward economic recovery methods for
16
-------
nonbauxitic sources of alumina. Research also continues into more effective
dry absorption technologies to be used on atmospheric emissions from the
reduction cells and anode baking operations at smelters. Dry scrubbers
produce no water effluent and, in addition, the fluoride values are returned
to the reduction cells. Their use will be confined to treating the relatively
low volume primary gas stream, however, because of their high energy consump-
tion per volume of gas treated.
References
1. Stamper, John W. Aluminum. In: Mineral Facts and Problems. U.S.
Department of the Interior, Bureau of Mines, Washington, D.C., 1970.
2. Development Document for Proposed Effluent Limitations Guidelines
and New Source Performance Standards for the Bauxite Refining
Subcategory of the Aluminum Segment of the Nonferrous Metals Manu-
facturing Point Source Category. EPA-440/1-74-019. U.S. Environ-
mental Protection Agency, Washington, D.C., March 1974.
3. Commodity Data Summaries 1977. U.S. Department of the Interior,
Bureau of Mines, Washington, D.C., 1977.
4. Nonferrous Metal Data 1975. American Bureau of Metal Statistics,
Inc., New York, 1976.
5. Bauxite and Alumina in the Second Quarter of 1977. Mineral Industry
Surveys. U.S. Department of the Interior, Bureau of Mines, Wash-
ington, D.C., September 9, 1977.
6. Patterson, Sam H., and John R. Dyni. Aluminum and Bauxite. In:
United States Mineral Resources. Brobst and Pratt, Eds. Geological
Survey Professional Paper 820. U.S. Department of the Interior,
Washington, D.C., 1973.
7. Aluminum Industry in June 1977. Mineral Industry Surveys. U.S.
Department of the Interior, Bureau of Mines, Washington, D.C.,
September 28, 1977.
8. Aluminum Mineral Commodity Profiles, MCP-14. U.S. Department of
the Interior, Bureau of Mines, Washington, D.C. May 1978.
9. Primary Aluminum Plants, Worldwide. Part One. U.S. Department of
the Interior, Bureau of Mines, Denver, August 1977.
10. Development Document for Effluent Limitations Guidelines and New
Source Performance Standards for the Primary Aluminum Smelting
Subcategory of the Aluminum Segment of the Nonferrous Metals Manu-
facturing Point Source Category. EPA-440/1-74-019d. U.S. Environ-
mental Protection Agency, Washington, D.C., March 1974.
17
-------
11. Environmental Considerations of Selected Energy Conserving Manu-
facturing Process Options: Vol. VIII. Alumina/Aluminum Industry
Report. EPA-600/7-76-034h. U.S. Environmental Protection Agency,
Cincinnati, Ohio, December 1976.
12. Annual Survey of Manufacturers of Fuels and Electricity Consumption.
U.S. Department of Commerce, Bureau of the Census, Washington, D.C.,
1974.
13. Compilation of Air Pollutant Emission Factors, Second Edition.
AP-42. U.S. Environmental Protection Agency, Research Triangle
Park, North Carolina, April 1973.
14. Primary Aluminum. In: The Fabric Filter Manual. Ed. Charles
Billings. Mcllvaine Publishing Company, Inc., August 1977.
15. Stamper, John W., and Christine M. Monroe. Aluminum. In: Minerals
Yearbook 1974. U.S. Department of the Interior, Bureau of Mines,
Washington, D.C.
16. Energy Penalty of the Nonferrous Metals Industry. Part II: Primary
Smelting of Aluminum (Draft). Prepared by Arthur D. Little, Inc.,
for U.S. Environmental Protection Agency. EPA Contract No. 68-01-4381,
Work Area No. II, August 1977.
17. Air/Water Pollution Report. Vol. 16, No. 11. Business Publishers,
Inc., Silver Springs, Maryland, March 13, 1978.
18. Assessment of Industrial Waste Practices in the Metal Smelting and
Refining Industry. Vol. II, Primary and Secondary Nonferrous
Smelting and Refining. Prepared by Calspan Corporation for U.S.
Environmental Protection Agency. EPA Contract No. 68-01-2604.
18
-------
INDUSTRY ANALYSIS
The primary aluminum industry has been the subject of a number of indus-
trial and governmental studies in recent years. These reports have formed the
basis for the industry analysis that follows. Each individual production
operation, or process, is examined in detail to define its purpose and its
actual or potential effects on the environment. Only those processes opera-
ting in the primary aluminum industry in the United States are included. Each
process is examined in the following aspects:
1. Function
2. Input materials
3. Operating conditions
4. Utilities
5. Waste streams
6. Control technology
7. EPA classification code
8. References
The primary aluminum industry can be divided into three distinct segments:
(1) the mining and beneficiating of bauxite ore; (2) the refining of alumina
from bauxite; and (3) the reduction of alumina to aluminum metal. In contrast
to the terminology used in most nonferrous metallurgical industries, an
aluminum "refinery" processes the raw ore, while the "smelter" produces the
purified metal. Figure 2-2 is a flowsheet representing segments (1) and (2),
and Figure 2-3 represents segment (3). Processes, interrelationships, and
waste streams are indicated.
19
-------
.CAUSTIC
o
SCHUTIOK.
6*1 »0 IKS
DIGESTION.
MO KEAT
W COVER*
4
ro
o
won
LIQUM
«ECO««T
8
Figure 2-2. Flowsheet - the production of alumina from bauxite.
-------
E
PASTE
PUPAMTIC*
11
COtt
PITCH
-PACUM
-n»E cou
-ROD YOU ASSOeilCS
-CUIENTIIIC NATCIIIAL
-AlUMIHM
SODIUM
CAUSTIC
IIKI
CARBON OIOXIOE
f H»Tt«
9 All
9 SOLID
Figure 2-3. Flowsheet - the production of aluminum metal from alumina.
-------
PRIMARY ALUMINUM PRODUCTION PROCESS NO. 1
Mi n1ng
1. Function - Mining is the extraction of an aluminum ore from the earth and
partial separation from waste rock and clay. The only ore of aluminum pres-
ently mined in the United States is bauxite, a soft uncrystallized material
consisting primarily of aluminum oxides. During 1976, 12 domestic mines
produced bauxite, although production from 10 of these was used exclusively
for chemical and refractory alumina. Only two open pit mines located in
Arkansas produced bauxite that was used for aluminum production. A third open
pit mine is occasionally operated by one company and one underground mine has
been used in the past, although neither was operating at the end of 1976 (1).
Since bauxite forms by the weathering of feldspars and clays, most
deposits are nearly horizontal and close to the surface. Most is mined by
open pit methods utilizing draglines, shovels, and haulers. Stripping ratios
of as much as 13 meters of overburden to 1 meter of ore are considered min-
able; a 15-to-l ratio may be feasible. Pits of 30 meters in depth are common,
with 60 meters considered to be the economic limit. After stripping the over-
burden, the bauxite is usually loosened by blasting with low-strength explo-
sives; it is then excavated and transported to an ore treatment area (2).
About 15 million metric tons of material was handled at the 12 surface bauxite
mines in 1975, representing less than one percent of the total handled at all
U.S. metals mining operations (3). Ninety-one percent of all mining activity
was preceded by drilling and blasting (3).
Underground mining is costly, but may be an alternative to importing
expensive foreign ores. Initially, shafts are sunk to provide access to the
bauxite, and drifts are driven into the section to be mined. Ore is removed
from the deposits by means of a "continuous miner," a ripping-type machine
which cuts bauxite directly from the ore face and loads it into shuttle cars
for transport to the surface (2).
2. Input Materials - The principal input material is bauxite ore. Bauxite
was formed geologically from rock and soil by the leaching action of water in
a tropical rainforest environment. The mineral formed was a trihydrate alu-
minum oxide, A1(OH)3, known as gibbsite or hydrargillite. Later geologic
changes have caused part or all of the trihydrate mineral in some deposits to
lose water and to form monohydrate minerals of the form AIO(OH), known as
boehmite and diaspore. The trihydrate form is much more desirable for alumina
manufacture because it is more reactive in alkali than the monohydrates,
allowing the use of milder processing conditions when leaching the bauxite
(2). Most bauxites contain from 40 to 60 percent aluminum, as A1203- Arkan-
sas bauxite has 8 to 15 percent silica. Table 2-6 presents the compositional
range of bauxites normally used for aluminum production, and Table 2-7 pre-
sents specific analyses of bauxite ores commonly processed by domestic alu-
minum producers.
Explosives used in bauxite mining are almost always a mixture of ammonium
nitrate and fuel oil (AN-FO). At some mines, sodium nitrate is added to
22
-------
Table 2-6. TYPICAL COMPOSITION OF BAUXITES USED
FOR ALUMINUM MANUFACTURE (2)
Composition
Percent
^0, combined
A1203, total
Si03, free and combined
Fe203
Ti02
F, P205, V205, etc.
12 to 35
40 to 60
1 to 16
2 to 30
3 to 4
0.05 to 0.20
Table 2-7. CHARACTERISTIC ANALYSES OF VARIOUS BAUXITES (5)
Height percent
Al,0,, total
2 3
Si02
Fe2°3
Ti02
F
F2°5
V2°5
H20, combined
AUO-, tri hydrate
A120~, monohydrate
Jamaican
49.0
0.8
18.4
2.4
--
0.7
—
27.5
40-47
2-9
Surinam
59.8
3.8
2.7
2.4
--
0.06
0.04
31.2
59.6
0.2
Arkansas
48.7
15.3
6.5
2.1
0.2
--
—
25.8
34.1
14.6
Guiana
58.6
4.9
4.1
2.5
0.02
--
—
29.6
52.7
5.9
23
-------
slightly increase the power of the explosive (4). Quantities of explosives
consumed are not known.
3. Operating Conditions - Mining is conducted at ambient temperatures and
atmospheric pressure.
4. Utilities - Electrically operated equipment is used for drilling, haul-
ing, and operation of draglines and shovels in most mines. No quantitative
data are available. A small amount of water is needed for dust suppression.
5. Waste Streams - Dust from blasting and ore-loading operations at bauxite
open pit mines includes particulate matter of the same composition as the
input ore. Nitrogen compounds from AN-FO explosives are also present. Emis-
sions from open pit mining are about 0.25 kilogram per metric ton of ore and
emissions from underground mining are about 0.05 kilogram per metric ton of
ore (6,7). These fugitive emissions are localized and produce low level
impacts on the general environment.
The bauxite mining industry is reported to discharge about 57,000 cubic
meters of mine water daily (1). Nine dewatering is required at all bauxite
mines. The open pit mining technique is largely responsible for accumulation
of this water, with underground mines (when operating) not accounting for more
than a fraction of a percent of the total. The single underground mine that
has operated occasionally in recent years discharged an estimated 83 cubic
meters of water per day (8).
Two distinct classes of raw mine drainage are generated by the industry.
Wastewater can be acidic or alkaline, determined principally by the type of
rock or soil through which the drainage flows and to some extent by the mining
technique and amount of water present (2). Open pit drainage is usually
acidic, often with a pH of 2 to 4 created by oxidation of pyrite in the
overburden (2). The sulfuric acid and ferrous sulfate formed dissolve other
minerals, including those containing aluminum, calcium, manganese, and zinc
(2). Analytical data for raw mine water from two open pit mines is shown in
Table 2-8. Alkaline mine water is most often pumped from underground mines.
The alkalinity results from the leaching of lignitic clays located in over-
lying strata. The wastewater is not strongly alkaline; the pH is generally
around 7.5 (2).
The overburden produced by open pit mining is a significant source of
solid waste. Waste spoil is also produced by underground mining, but in
lesser quantities. In 1975, an average of about 4 tons of solid waste was
produced per ton of crude bauxite ore mined, resulting in a total of 12
million metric tons of solid waste from the 12 domestic bauxite mines (2).
6. Control Technology - The only control provided for fugitive dust is the
manual use of water sprays as needed.
Lime neutralization and impoundment are the only water treatments pres-
ently used at the two domestic mines that produce bauxite for the aluminum
industry. Acidic mine water and surface drainage pass through a number of old
mining pits and natural depressions before reaching the lime-neutralization
24
-------
Table 2-8.
CHEMICAL COMPOSITION OF UNTREATED MINE WATER AT TWO
BAUXITE MINES (7,8)
(Concentration,
PHC
Acidity
Alkalinity
Conductivity
TDS
TSS
Total Fe
Total Mn
Al
Ni
Zn
Sr
Fluoride
Sulfate
Mine A
range
2.8 to 3.0
250 to 397
0
1000
560 to 617
2
7.2 to 21.8
3.2 to 3.5
23.8 to 18.6
0.3 to 0.31
0.82 to 1.19
0.5
0.048 to 0.290
490 to 500
average3
2.9
324
0
1000
589
2
14.5
3.4
21.2
0.3
1.01
0.5
0.17
295
Mine B
b
average
2.9
240
0
2212
468
45
49.8
1.56
14.8
0.05
0.24
0.1
0.59
432
a Values based on two grab samples.
Values based on eight grab samples.
c Values in pH units.
Values in microhm/centimeter.
25
-------
facility. Suspended solids loadings are usually quite low prior to the
addition of lime to elevate the pH. Heavy metals precipitate as insoluble or
slightly soluble hydroxides and carbonates. Removal of the precipitated
metals is accomplished by settling (8).
Two variations of lime treatment are used at the minewater treatment
facilities, each of which achieves slightly different efficiencies of pol-
lutant removal. Control of pH appears to be the dominant factor in deter-
mining the concentration of dissolved metals in the final settling ponds (8).
Analytical data for the treated effluent from two open pit bauxite mines are
listed in Table 2-9. These treated wastewaters fall within the range of
compositions and characteristics of surface and ground waters in the area and
produce minimal impact on the receiving waters.
Solid waste generated by open pit mining is normally stored in heaps
adjacent to the pits. It may be used in refilling and recontouring mined-out
deposits where desirable. Natural revegetation proceeds very slowly because
of the acid conditions and general disruption of soils caused by stripping of
the overburden. The lack of vegetative cover accelerates the weathering of
the unconsolidated overburden and increases the acidity of area drainage
water.
7. EPA Source Classification Code - None.
8. References -
1. Personal communication with H. F. Kurtz, U.S. Dept. of Interior,
Bureau of Mines.
2. Development Document for Interim Final and Proposed Effluent Limita-
tions Guidelines and New Source Performance Standards for the Ore
Mining and Dressing Industry Point Source Category. EPA 440/1-75/
061. Group II, Vol. 1. U.S. Environmental Protection Agency,
Washington, D.C., October 1975.
3. Morning, John L. Mining and Quarrying Trends in the Metal and
Nonmetal Industries (Preprint). Minerals Yearbook 1975. U.S. Dept.
of Interior, Bureau of Mines, Washington, D.C.
4. Vachet, P. Aluminum and Aluminum Alloys. In: Encyclopedia of
Chemical Technology. Interscience Publishers, A Division of John
Wiley and Sons, Inc. New York, 1967.
5. Development Document for Proposed Effluent Limitations Guidelines
and New Source Performance Standards for the Bauxite Refining Sub-
category of the Aluminum Segment of the Nonferrous Metals Manu-
facturing Point Source Category, EPA 440/1- 73/019. U.S. Environ-
mental Protection Agency, Washington, D.C., October 1973.
6. Compilation of Air Pollution Emission Factors, Second Edition.
AP-42. U.S. Environmental Protection Agency, Research Triangle
Park, North Carolina, April 1973.
26
-------
7. Control Techniques for Emissions Containing Arsenic, Cadmium,
Copper, Selenium, and Zinc. Summary Report, Contract No. EHSD71-33.
U.S. Environmental Protection Agency, Durham, North Carolina.
January 31, 1973.
8. Development Document for Interim Final and Proposed Effluent Limita-
tions Guidelines and New Source Performance Standards for the Ore
Mining and Dressing Industry Point Source Category, EPA 440/1-75/061
Group II, Vol. II. U.S. Environmental Protection Agency, Washing-
ton, D.C., October 1975.
27
-------
Table 2-9. CHEMICAL COMPOSITION OF TREATED MINE WATER'AT TWO
BAUXITE. MINES (7,8)
(Concentration, mgA)
PHC
Acidity
Alkalinity
Conductivity
TDS
TSS
Total Fe
Total Mn
Al
Ni
Zn
Sr
Fluoride
Sulfate
Mine A
range
6.0 to 6.8
0 to 10
6 to 13
1000
807 to 838
1.2 to 4.0
0.14 to 0.20
2.25 to 3.37
0.33 to 0.80
0.18 to 0.19
0.07 to 0.09
1.74
0.03 to 0.67
500 to 581
average9
6.84
0.5
10
1000
823
3
0.2
2.8
0.6
0.2
0.08
1.74
0.35
541
Mine B
b
average
7.2
0
30
897
630
6.6
0.29
<0.02
0.12
<0.02
<0.02
-
0.56
343
Values based on two grab samples.
Values based on eight grab samples.
c Values in pH units.
Values in microhm/centimeter.
28
-------
PRIMARY ALUMINUM PRODUCTION PROCESS NO. 2
Ore Treatment
1. Function - Bauxite ores do not require the sophisticated beneficiation
techniques that are used with some other nonferrous ores. In addition,
impurities are often so finely dispersed in the ore that they cannot be
easily separated by physical means. The ore is normally simply crushed to a
size of about 5 centimeters in hammermills, gyratories, or jaw crushers. It
is then classified on screens to remove silica and clay and to further reduce
the overall particle sizes. Spiral concentrators and magnetic separators may
be used to remove iron minerals. This treatment simplifies handling and
removes part of the unwanted material (1,2).
2. Input Materials - Raw bauxite ore is the only input to this process.
Characteristics and chemical analyses of typical bauxites are given in Process
No. 1.
3. Operating Conditions - All concentrating operations are conducted at
ambient temperatures and atmospheric pressure.
4. Utilities - Electricity is used to operate equipment such as crushers,
vibrating screens, and conveyors. Water can be used to wash the crushed ore,
but it has been reported that this additional treatment is not practiced at
the operating U.S. mines.
5. Waste Streams - Dust emissions arise from crushing and conveying. From
0.5 to 4 kilograms of particulate is emitted per metric ton of feed (3). It
is estimated that particle size is largely in the coarser range. The impact
of these operations is very localized and confined largely to the operating
area.
There are no liquid or solid wastes generated by the ore treatment pro-
cess as it is now operated in this country.
6. Control Technology - Dust emissions from crushing and conveying are
usually controlled by baghouses. No quantitative data are presently available
on the efficiency of these controls.
7. EPA Source Classification Code - 3-03-000-01.
8. References -
1. Vachet, P. Aluminum and Aluminum Alloys. In: Encyclopedia of
Chemical Technology. Interscience Publishers, a Division of John
Wiley and Sons, Inc. New York, 1967.
2. Aluminum. Mineral Commodity Profiles, MCP-14. U.S. Department of
the Interior, Bureau of Mines. Washington, D.C. May 1978.
3. Sittig, Marshall. Environmental Sources and Emissions Handbook.
Noyes Data Corporation, Ridge Park, New Jersey, 1975.
29
-------
PRIMARY ALUMINUM PRODUCTION PROCESS NO. 3
Bauxite Drying
1. Function - Water is sometimes removed from wet bauxite ores to simplify
operations such as conveying and unloading. Minor quantities of organic
materials are also removed in this process.
Ore which has been stored in outdoor stockpiles, shipped in open barges,
or transported in open railroad cars can contain moisture levels in excess of
the 30 percent allowable in the refining process (1). Drying is normally
accomplished in direct-fired kilns, but other types of equipment would also be
suitable.
2. Input Materials - Bauxite ore containing more than 30 percent water is
the only input to this process. Imported ores are dried before shipment and
do not usually need this processing, unless accidentally wetted.
3. Operating Conditions - Ores are dried at elevated temperature and atmo-
spheric pressure. Specific operating temperature of the dryers has not been
reported.
4. Utilities - The most frequently used fuel for ore drying is natural gas,
although facilities for conversion to oil are usually provided. Electricity
is used for conveying materials and for mechanical operation of the dryer. No
data are presently available for utility consumption.
5. Waste Streams - Air emissions generated by the dryers consist of combus-
tion gases containing particulates of the same composition as the input ore.
Some naturally occurring organic materials are volatilized from the ore;
however, most of the volatiles are burned inside the kiln. Emission data
have not been reported.
There are no liquid or solid wastes associated with bauxite drying.
6. Control Technology - The types of control devices employed for particu-
late emissions from this process and the extent of their use is not known.
Baghouses or electrostatic precipitators would be suitable. The organic
materials in the concentrate are present only in small quantities and separate
treatment is probably unnecessary.
7. EPA Source Classification Code - None.
8. References -
1. Development Document for Proposed Effluent Limitations Guidelines
and New .Source Performance Standards for the Bauxite Refining Sub-
category of the Aluminum Segment of the Nonferrous Metals Manu-
facturing Point Source Category. EPA 440/1-73/019. U.S. Environ-
mental Protection Agency, Washington, D.C., October 1973.
30
-------
PRIMARY ALUMINUM PRODUCTION PROCESS NO. 4
Grinding, Digestion, and Heat Recovery
1. Function - Bauxite ore is ground and mixed with caustic solution and
processed in a digester, whose function is to hold the mixture at elevated
temperature and pressure for sufficient time to cause the aluminum values in
the ore to become soluble. The hot mixture is released from the digester into
a flash tank where a portion of the water in the mixture is converted into
steam.
Bauxite is ground to approximately 100 mesh to provide a large surface
area for attack by the alkaline solution (1). Ball mills or hammermills are
probably used, although the specific types of grinders being employed have not
been reported. Jamaican bauxite is of extremely small particle size as
mined, and it requires only the breaking of large lumps (2). The ground ore
is combined with caustic solution in large iron mixing tanks to bring the
overall concentration of the slurry to between 12 and 16 percent NaOH by
weight.
When heated to elevated temperature in the digester, aluminum oxides
react with caustic to form soluble sodium aluminate (3). The following re-
actions are involved:
Monohydrate ores --
AIO(OH) + NaOH + NaA102 + H20
Trihydrate ores —
A1(OH)3 + NaOH ->• NaA102 + 2H20
Other materials in the ore are not affected by the caustic solution, and they
remain as solids in suspension.
Digesters for bauxite processing are usually pressure vessels, or auto-
claves. They are sometimes surrounded with a steam jacket to supply heat but
are more often heated and pressurized by direct injection of live steam. The
process can be a batch operation, with the autoclaves alternately charged,
heated, and discharged; however, in most cases operation is continuous (2).
In either mode of operation, the digester is accompanied by a flash tank, a
vessel operating at lower pressure into which the digested slurry is dis-
charged. The flash tank provides space for release of the steam that forms
when the superheated water slurry is released to lower pressure.
Plug-flow "tube digesters" are used instead of autoclaves at a West
Germany refinery. The digester consists of an outer jacket for heat exchange
and an inner pipe through which the bauxite/caustic slurry flows. Hot slurry
flows out of the reactor to fill the inlet side of the jacket, and steam or a
fused-salt heat-exchange medium takes up the tail end. The tube digester is
used in conjunction with fluidized-bed calcination instead of a rotary kiln
31
-------
(see Process No. 9), and the combination is said to result in a 50 percent
savings in heating fuel over conventional Bayer processing (3).
2. Input Materials - The inputs to this process consist of dry, treated
bauxite ore and caustic soda. The caustic consists primarily of recycle
solution from Process No. 8. Fresh caustic is added as needed to maintain
proper composition. Most bauxite contains different compositions of the
monohydrate and trihydrate forms, and caustic solution concentrations must be
adjusted to optimize dissolution of the hydrated alumina. Monohydrate ores
require a caustic solution concentration of 200 to 300 grams of Na20 per liter
and trihydrate ores require 100 to 150 grams of Na20 per liter (4). At one
time, soda ash and lime were used to manufacture NaOH; however, 50 percent
electrolytic caustic soda is now used.
3. Operating Conditions- Grinding and mixing operations are conducted at
ambient temperature and atmospheric pressure. Temperature, pressure, and
retention time in the digester are determined by the composition of the ore
and the size of the ground particles. Monohydrate ores are usually digested
at 200° to 250°C and pressures of around 35 kilograms per square centimeter.
Trihydrate ores are usually digested at 120° to 140°C and at pressures of from
3.4 to 4.75 kilograms per square centimeter. The use of Jamaican bauxites
which contain boehmite requires higher pressure than domestic ores. Digestion
times vary from 0.5 to 1 hour for trihydrate ores and 2 to 8 hours for mono-
hydrate ores (5). The flash tank operates at approximately atmospheric pres-
sure, and temperatures of about 100°C. The German tube digester operates at
temperatures up to 300°C, with a retention time of only 1 to 5 minutes (3).
4. Utilities - Electricity is used for grinding, conveying, and general
operation of the mixing tanks. No data are available on actual consumption.
Steam at a pressure greater than the digester operating pressure is the
primary utility consumed in digestion. No quantities have been reported. The
flash tank releases a quantity of low-pressure steam, which is used in a
closed-loop heat recovery system (6). Any entrained materials that may be
present in the flashed steam are retained within the process. Electricity is
used to drive slurry pumps and auxiliary equipment.
5. Waste Streams - There are no emissions from the mixing step of this pro-
cess; all materials are routed to the digesters.
The bauxite grinding step generates particulates of the same composition
as the input ore. Approximately 3.0 kilograms of particulate emissions is
generated per metric ton of bauxite processed (7). No water or solid waste
streams are released.
The digestion step creates no waste materials.
6. Control Technology - The specific control devices now in use at U.S.
bauxite refining facilities are not well documented. Low-efficiency wet
scrubbers have been installed at a few plants, but they have not been effec-
tive. Table 2-10 lists potential control devices and controlled emission
factors for bauxite grinding. If a wet scrubbing system is used, liquid waste
32
-------
Table 2-10. POSSIBLE CONTROL DEVICES - BAUXITE GRINDING (8,9)
Controlled emission factor0
kg/metric ton bauxite
Spray tower
Floating-bed scrubber
Quench tower and spray screen
Electrostatic precipitator
0.90
0.85
0.50
0.060
Controlled emission factors are based on the average uncontrolled
factor and on observed collection efficiencies.
33
-------
will be generated which would be discarded into a mud lake (see Process No.
5). Use of a baghouse or electrostatic precipitator would produce a solid
waste stream that would likely be either recycled to process or sent to the
mud lake.
7. EPA Source Classification Code - 3-03-000-01 (grinding only).
8. References -
1. Shreve, R. Norris. Chemical Process Industries. Third Edition.
McGraw-Hill Book Company, New York, 1967.
2. Development Document for Proposed Effluent Limitations Guidelines
and New Source Performance Standards for the Bauxite Refining Sub-
category of the Aluminum Segment of the Nonferrous Metals Manu-
facturing Point Source Category. EPA 440/1-73/019. U.S. Environ-
mental Protection Agency, Washington, D.C., October 1973.
3. Chemical Engineering. Vol. 84, No. 11. May 23, 1977.
4. Vachet, P. Aluminum and Aluminum Alloys. In: Encyclopedia of
Chemical Technology. Interscience Publishers, a Division of John
Wiley and Sons, Inc. New York, 1967.
5. Stamper, John W. Aluminum. In: Mineral Facts and Problems. U.S.
Department of Interior, Bureau of Mines, Washington, D.C., 1970.
6. Hudson, L.K. Recent Changes in the Bayer Process. In: Extractive
Metallurgy of Alumina, Vol. I, Alumina. Gerard and Stroup, Ed.
Interscience Publishers, A Division of John Wiley and Sons, Inc.
New York, 1963.
7. Sittig, Marshall. Environmental Soi/rces and Emissions Handbook.
Noyes Data Corporation, Ridge Park, New Jersey, 1975.
8. Engineering and Cost Effectiveness Study of Fluoride Emissions
Control, Volume I. Prepared by TRW Systems and Resources Corp. for
U.S. Environmental Protection Agency, under Contract No. EHSD-71-
74. January 1972.
9. Particulate Pollutant System Study, Volume 1. Prepared by Midwest
Research Institute for U.S. Environmental Protection Agency. May
1971.
34
-------
PRIMARY ALUMINUM PRODUCTION PROCESS NO. 5
Liquor/Mud Separation
1. Function - The solution from the digesters, called "green liquor," is
separated from undissolved solids by a variety of thickeners, mud washers, and
clarifying filters. This can be a complex process since the particles may be
very fine, sometimes less than a micrometer in size. The final separation
stage is a continuous filter press which removes all insoluble solids from the
sodium aluminate liquor.
The complexity of the process varies with the source of the bauxite ore.
Surinam bauxite contains little residue, and only one stage of filtration is
needed. Arkansas bauxite may require up to seven stages of countercurrent de-
cantation and several stages of thickening.
The filtered liquor is pumped directly into the precipitators (Process
No. 7) for further processing. The "red mud" waste product is pumped to a
disposal site or in some cases to an additional alumina recovery step (Process
No. 6). The mud may require dilution with water to a consistency that may be
easily pumped (17-25 percent solids for a Caribbean ore).
2. Input Materials - The only input material is the slurry from the di-
gest ers(Proces~s~No7 4) , which contains soluble sodium aluminate, caustic, and
insoluble waste material.
3. Operating Conditions - These operations are carried out at approximately
100°C and atmospheric pressure (1).
4. Utilities - Water is used for diluting the sodium aluminate liquor and
washing the red mud residues. Part of the water is fresh, but most is re-
cycled from the heat exchangers and evaporators (Process No. 8). Electricity
is used to operate equipment and pumps.
5. Waste Streams - No air emissions are generated by this process.
The principal waste stream from most bauxite refineries is the "red mud"
slurry from this process. If the bauxite ore has a low silica content, the
slurry is discarded. Red mud, diluted with water before discharge, contains
between 17 and 25 percent insoluble solids. Table 2-11 presents analyses of
solids from three operating U.S. refineries. Table 2-12 presents a similar
analysis of solids formed from a typical Jamaican bauxite, and Table 2-13
shows the analysis of the soluble constituents in this waste. Bauxite waste
does not contain potentially hazardous trace elements such as those produced
in some other nonferrous metal industries.
The quantity of red muds to be washed varies with the origin of the
bauxite. Residue in Surinam bauxite is only a third of a ton per ton of
alumina product, while Arkansas bauxite residue can be as much as 2.5 tons per
ton of alumina (1). Other waste production data are given in Table 2-14.
35
-------
Table 2-11. RANGE OF CHEMICAL ANALYSES OF RED MUDS (1)
Component
Fe2°3
A1203
Si02
Ti02
CaO
Na20
Loss on
ignition
Weight percent
Alcoa
Mobile, Ala.
(Surinam)
30-40
16-20
11-14
10-11
5-6
6-8
10.7-11.4
Reynolds
Bauxite, Ark.
(Arkansas)
55-60
12-15
4-5
4-5
5-10
2
5-10
Reynolds
Corpus Christi , Texas
(Jamaica)
50-54
11-13
2.5-6
trace
6.5-8.5
1.5-5.0
10-13
36
-------
Table 2-12. INSOLUBLE SOLIDS IN RED MUD FROM JAMAICAN BAUXITE (1)
Consitutent
LiO
Si02
A1203
Fe203
P2°5
CaO
Na20
Ti02
Mn02
Miscellaneous
Percent
11.0
5.5
12.0
49.5
2.0
8.0
3.5
5.0
1.0
1.5
Note: Specific gravity =3.6
Table 2-13. SOLUBLE SOLIDS IN RED MUD FROM JAMAICAN BAUXITE (1)
Consitutent
Percent
A12°3
NaOH
Na2S04
NaCl
Na2C204
pH
BOD
COD
2.5 g/kg
3.7 g/kg
1.6 g/kg
0.4 g/kg
0.7 g/kg
0.1 g/kg
10.5
6 ppm
148 ppm
Note: Specific gravity = 1.0008
37
-------
Table 2-14. UNIT MUD PRODUCTION RATES FOR VARIOUS BAUXITES (2)'
Quantity of mud,
Volume of mud, m
Volume of mud, m
metric tons
W
3/yearb
Type of bauxite ore
Surinam
330
410
147,600
Jama i ca
1,000
1,248
449,280
Arkansas
2,000
2,496
994,560
Per 1000 metric tons of alumina production/day.
^
Density of settled mud taken at 1600 kg/m and solids at
50%.
38
-------
6. Control Technology - In the past, some bauxite refineries discharged red
mud as a waterborne slurry directly to a river. However, this waste stream
must now be contained, and large diked lakes ranging in size from 40 hectares
to 800 hectares are utilized for this purpose at domestic facilities. The
dikes, constructed of a stable sand/clay mixture, may be built initially to
their full height or built up continually to meet the rising lake level.
Total costs are comparable for the two methods. If the existing soil is
porous, the lake bottom may be lined with clay to reduce seepage. When the
dike is to be built up over time, the mud pipeline to the lake has a tapered
bottom, or "possum-belly," in which coarser sands tend to settle out. These
sands are extracted through release valves and used to form sand dunes that
gradually slope toward the lake and are built up as the level of the lake
rises. The main flow of red mud is deposited toward the center of the lake to
minimize erosion of these dunes. Bauxites with a coarser sand fraction, such
as those from Surinam, are most suitable for this approach. The dike height
may also be raised using settled mud dredged from the lake (1). At present it
is not economically feasible to process the low-silica red mud to recover its
iron content or for other uses such as treatment of gaseous or liquid efflu-
ents. However, total impoundment in this manner affords the opportunity for
reclamation if an economic recovery process can be developed. Vanadium,
gallium, and scandium have been recovered from red mud on a laboratory scale
(3).
7. EPA Source Classification Code - None.
8. References -
1. Development Document for Proposed Effluent Limitations Guidelines
and New Source Performance Standards for the Bauxite Refining Sub-
category of the Aluminum Segment of the Nonferrous Metals Manu-
facturing Point Source Category. EPA 440/1-73/019. U.S. Environ-
mental Protection Agency, Washington, D.C., October 1973.
2. Rushing, J.D. Aluminum Plant Tailings Storage, Paper No. A73058.
Metallurgical Society of the AIME, Chicago, 1973.
3. An Assessment of Technology for Possible Utilization of Bayer Pro-
cess Muds. EPA-600/2-76-301. U.S. Environmental Protection Agency,
Cincinnati, December 1976.
39
-------
PRIMARY ALUMINUM PRODUCTION PROCESS NO. 6
Waste Alumina Recovery
1. Function - When processing high-silica Arkansas bauxite, the red mud from
Process No. 5 contains alumina and caustic in quantities large enough to make
further recovery economical. This extra extraction is accomplished by mixing
the red mud with lime and soda ash, and then calcining the slurry in rotary
kilns. This pyrometallurgical process is a modification of the Sainte-Claire
Deville process, once commonly used for bauxite separation before the wide-
spread adoption of the Bayer process.
The important reactions in the recovery of additional alumina are the
conversion of silica to calcium silicate and of residual alumina to sodium
aluminate (1):
Si02 + CaO •*• CaSi03
A1203 + Na2C03 + 2NaAl02 + C02
The sintered products are leached in a caustic solution to produce additional
sodium aluminate solution. The resulting liquor is sent to the precipitators
(Process No. 7). Pressure filters concentrate the solid residue to between 40
and 50 percent solids. This material ("brown mud") has a composition, on a
dry basis, somewhat similar to that of Portland cement (2), and it is dis-
carded.
2. Input Materials - The input materials to the mixers are red muds from the
processing of high-silica Arkansas bauxites. Lime and soda ash are added in
calculated quantities.
3. Operating Conditions - Grinding and mixing are carried out at approxi-
mately 100°C, and calcining at 1100° to 1200°C (1). No data are available on
filtration temperatures. All processes operate at atmospheric pressure.
4. Utilities - Electricity is used to operate much of the equipment in this
process, including grinders, mixers, filters, and conveying pumps; it is also
used for the mechanical operation of the calciner. Water is used in the
grinding and leaching stage to slurry the calcined product. Calciners are
typically fired by natural gas, although facilities for conversion to oil are
usually provided. Specific utility consumption data have not been reported.
5. Waste Streams - The sintering process drives off water vapor, particu-
lates of lime and red muds, and products of fuel combustion. No data on
emission rates are reported.
The major waste stream is the brown muds generated in the pressure
filters.
6. Control Technology - The types of control devices used on the calciners
have not been reported. Baghouses, electrostatic precipitators, or scrubbers
40
-------
would all be suitable.
The best available control technology for disposal of the brown muds is
impoundment. This disposal method is the same as that described for red muds
in Process No. 5.
7. EPA Source Classification Code - None.
8. References -
1. Development Document for Proposed Effluent Limitations Guidelines
and New Source Performance Standards for the Bauxite Refining Sub-
category of the Aluminum Segment of the Nonferrous Metals Manu-
facturing Point Source Category. EPA 440/1-73/019. U.S. Environ-
mental Protection Agency, Washington, D.C., October 1973.
2. Stamper, John W. Aluminum. In: Mineral Facts and Problems. U.S.
Department of Interior, Bureau of Mines, Washington, D.C., 1970.
41
-------
PRIMARY ALUMINUM PRODUCTION PROCESS NO. 7
A1(OH)3 Precipitation
1. Function - In one of the principal processing steps at a Bayer plant, the
liquor from Process Nos. 5 and 6 is fed into very large cylindrical tanks in
which aluminum trihydrate precipitates from solution over a long period of
time. Precipitation is a hydrolysis of the sodium aluminate, and the basic
reaction that occurs is the reverse of the reaction that takes place during
bauxite digestion (1):
NaA102 + 2H20 -»• Al (OH)3 + NaOH
This precipitation is the essential reaction in the Bayer process.
Precipitation is carried out at 50°C to yield alumina trihydrate, the
stable solid phase at this temperature. Precipitation is slow and the liquor
has to be seeded with previously precipitated aluminum trihydrate and stirred.
The seed crystals range from 5 to 10 microns in diameter. Seeding serves to
support the precipitating hydrate, which settles in the form of small agglom-
erates. Precipitation is not carried beyond 50 percent; as shown in Table
2-15, coprecipitation of objectionable impurities greatly increases beyond
this value.
The precipitators are large holding tanks, usually very tall and covered
with open shed-type roofs. Each tank operates on a batch basis, although the
entire bank runs in a continuous fashion. A holding tank is filled, allowed
to stand for 50 to 80 hours, and then emptied. At any given time, some tanks
are being filled, some are holding liquor, and some are being emptied, thus
giving the overall process its continuity.
The precipitated aluminum trihydrate is separated from the spent caustic
liquor in classifiers. The resulting slurry is then washed, further thick-
ened, and filtered to remove the precipitated solids. Most of the solids are
sent to Process No. 9 for additional treatment, but some are recycled to the
precipitators for use as seed (1). Solids may also be sold in this form for
use in chemical manufacture.
The liquid, which contains caustic and residual sodium aluminate, is
further treated in Process No. 8.
2. Input Materials - Sodium aluminate liquor from Process No. 5 is the only
input.
3. Operating Conditions - Temperature is carefully controlled at a constant
50°C. Pressure is atmospheric.
4. Utilities - Electricity is used to operate slurry pumps for conveying
input and output materials and for mechanical operation of rotary filters. No
consumption data are currently available. Agitation in the precipitators may
42
-------
Table 2-15. RELATIONSHIP BETWEEN PRECIPITATION OF
A1(OH)3 AND Si02 (2)
Percent Al(OH).
precipitated
50
75
80
87
94
100
Corresponding percent
Si02 precipitated
4
8
12
25
80
100
43
-------
be accomplished mechanically or by the injection of compressed air which is
allowed to rise in the tanks.
5. Waste Streams - There is a possibility of fugitive atmospheric emissions
of the liquor due to the mechanical agitation or injection of compressed air
in the precipitators.
6. Control Technology - There are no controls applied to this process.
7. EPA Source Classification Code - None.
8. References -
1. Vachet, P. Aluminum and Aluminum Alloys. In: Encyclopedia of
Chemical Technology. Interscience Publishers, a Division of John
Wiley and Sons, Inc. New York, 1967.
2. Development Document for Proposed Effluent Limitations Guidelines
and New Source Performance Standards for the Bauxite Refining Sub-
category of the Aluminum Segment of the Nonferrous Metals Manu-
facturing Point Source Category. EPA 440/1-73/019. U.S. Environ-
mental Protection Agency, Washington, D.C., October 1973.
44
-------
PRIMARY ALUMINUM PRODUCTION PROCESS NO. 8
Spent Liquor Recovery
1. Function - Because the caustic solution has been diluted by the water
used to wash the muds and precipitated trihydrate, the spent liquor must be
concentrated in multistage evaporators before the solution is returned to
digestion (Process No. 4). These evaporators increase the caustic concentra-
tion to 128 grams per liter (1).
Recycle of the caustic solution results in the buildup of soluble con-
taminants, a problem common to most closed extraction circuits. These con-
taminants are principally sulfates and some sodium oxalate resulting from
traces of humic acid in the bauxite feed material. One method used to remove
impurities is an auxiliary "salting-out" evaporator in which part of the
solution is evaporated to a very low volume, causing the impurities to crys-
tallize. The crystals are then filtered from solution. Use of this procedure
permits closed-loop water recycle in a bauxite refinery. Alternatively, the
spent liquor may be sent to a holding tank in which the impurities are ad-
sorbed by, or precipitated onto, red mud. The impurities are then discarded
with the waste from Process No. 5; an overflow from the mud lake is usually
necessary.
Bauxite ores typically contain from 30 to 100 parts per million gallium
(2). The caustic solution gradually becomes enriched in this element as a
result of the recycling. When the concentration reaches 0.15 to 0.3 gram
Ga£03 Per liter, the solution may be used as raw material for the extraction
of gallium (3). One company in Arkansas recovers gallium from caustic solu-
tion received from a nearby bauxite refinery (4).
2. Input Materials - The only input to this process is dilute spent liquor
from Process No. 7. This liquor usually contains about 113 grams of NaOH per
liter.
3. Operating Conditions - The evaporators must operate at elevated tempera-
tures, although specific operating conditions have not been reported. The
initial evaporator stage operates at approximately atmospheric pressure and
the last stage will be under high vacuum.
4. Utilities - Steam is probably used as a heating source for the evapora-
tors. Large quantities of water are required if barometric condensers are
used on the evaporator effluent (1).
5. Waste Streams - No air emissions are generated by this process. The
vapors driven from the liquor in the first evaporator stages are condensed
against a cold process stream, and those from the last stages are condensed in
barometric condensers or in closed heat exchangers using noncontact cooling
water. All emissions from the evaporators are thus contained in the water
system of the refinery.
45
-------
There is a possibility of water pollution from the escape of caustic or
alumina into barometric condensers. Because of the large volume of water
associated with the condensers, the pollutant concentration is very low.
The principal solid waste is the crystals (calcium and sodium sulfate and
oxalate) removed from the caustic solution in the salting-out evaporator.
6. Control Technology - Carryover of caustic or alumina into the barometric
condenser cooling water is minimal if the evaporators are efficiently oper-
ated. Most plants carefully control this emission since it represents a loss
of reusable caustic. The water is discharged to receiving waters or the mud
lake. Conductivity meters monitor the effluent streams of the evaporators.
One technology used to control the waste generated in the salting-out
evaporator is treatment with lime and disposal of the solids in a landfill
where they are covered with soil to prevent leaching. The limed salts may
also be discarded in a mud lake. If the impurities are removed by adsorption
or precipitation onto red mud, they are discharged to the mud lake as a com-
bined waste.
7. EPA Source Classification Code - None.
8. References -
1. Development Document for Proposed Effluent Limitations Guidelines
and New Source Performance Standards for the Bauxite Refining Sub-
category of the Aluminum Segment of the Nonferrous Metals Manu-
facturing Point Source Category. EPA 440/1-73/019. U.S. Environ-
mental Protection Agency, Washington, D.C., October 1973.
2. De la Breteque, Pierre. Gallium and Gallium Compounds. In: Ency-
clopedia of Chemical Technology. Interscience Publishers, a Divi-
sion of John Wiley and Sons, Inc. New York, 1967.
3. Stamper, John W. Gallium. In: Mineral Facts and Problems, 1970.
U.S. Department of Interior, Bureau of Mines, Washington, D.C.,
1970.
4. Petkof, B. Gallium. In: Commodity Data Summaries, 1977. U.S.
Department of Interior, Bureau of Mines, Washington, D.C., 1977.
46
-------
PRIMARY ALUMINUM PRODUCTION PROCESS NO. 9
Calcination
1. Function - The hydrated alumina filtered from solution in Process No. 7
is fed into large rotary or fluid/flash furnaces for calcination. Heating
these solids removes moisture and converts the alumina into the inert alpha
crystalline or corundum phase needed for electrolytic production of aluminum.
As fed to the kilns, the alumina contains about 45 percent moisture; two-
thirds of this is water chemically combined in the hydrated material (1).
Calcination of hydrated alumina has been historically carried out in
rotary furnaces similar to cement kilns. During the 1960's, Alcoa developed a
system of fluid/flash calcination that not only reduces fuel requirements and
capital costs, but also yields a higher quality alumina product than rotary
kilns. In Alcoa's Mark III System, the moist filter cake first passes through
a drying section and then to the combustion zone of a cylindrical flash fur-
nace. The calcined alumina and combustion products exit the top of the fur-
nace to a cyclone where the solids fall into a fluid bed that provides control
of loss of ignition and surface area. The physical characteristics of the
alumina product are determined by the calcining temperature and retention
time. The calcined product is cooled in a series of heat exchangers and a
fluid-bed cooler (2).
The alumina obtained by calcination is a white powder consisting of
aggregates ranging in size from a few microns up to about 100 microns.
Monocrystals of anhydrous alumina at various stages of crystallization are
present. An alumina of good quality has the characteristics shown in Table
2-16. The only impurity which may appear in any appreciable amount is sodium.
The alumina is shipped to an electrolytic aluminum smelter for reduction to
aluminum metal.
2. Input Materials - Filter cakes from Process No. 7 are the only input to
this process.
3. Operating Conditions - The calcining kilns are operated at about 1200°C
and atmospheric pressure (1).
4. Utilities - The kilns are fired by natural gas, although facilities for
conversion to high-grade fuel oil are usually provided. The fuel requirement
for rotary kilns has been reported to be about 1170 kilocalories per kilogram
of alumina product, a figure that approaches the practical limit imposed by
this furnace design. The Alcoa calciner consumes 775 kilocalories of fuel per
kilogram of alumina, with about 55 kilocalories released by the exothermic
reaction and available for heating process water (2).
Electricity is used for conveying the filter cakes and for mechanical
operation of the dryer. Power consumption for the Alcoa system is 22 kilo-
watt-hours per metric ton of alumina product (3).
47
-------
Table 2-16. COMPOSITION OF A GOOD ALUMINA (3)
Composition
Percent
hLO, combined (loss on calcination)
H20, adsorbed (loss at 110°C)
Si00
T102
P2°5
V2°5
ZnO
Na20
0.05 to 0.15
0.20 to 0.50
0.005 to 0.015
0.005 to 0.020
0.004 to 0.005
<0.002
O.001
O.010
0.40 to 0.80
48
-------
5. Waste Streams - This process represents the main source of potential air
pollution in bauxite refining. For each ton of alumina produced, almost a ton
of water is discharged through the stack. This vapor carries with it approxi-
mately 100 kilograms of fine particles of alumina per metric ton of alumina
product (4). Combustion products are also included in this stream.
There are no liquid or solid wastes associated with this process.
6. Control Technology - The economic value of the alumina emissions from
calcining kilns is such that extensive controls have been employed to reduce
emissions to relatively small quantities. Typical control consists of a
combination of multicyclones followed by an electrostatic precipitator or bag-
house. Table 2-17 presents other possible control devices and their emission
factors.
7. EPA Source Classification Code - 3-03-002-01.
8. References -
1. Development Document for Proposed Effluent Limitations Guidelines
and New Source Performance Standards for the Bauxite Refining Sub-
category of the Aluminum Segment of the Nonferrous Metals Manu-
facturing Point Source Category. EPA 440/1-73/019. U.S. Environ-
mental Protection Agency, Washington, D.C., October 1973.
2. Alcoa Saves Energy on the Way to Aluminum with Fluid Flash Cal-
ciners. Engineering/Mining Journal. April 1974.
3. Vachet, P. Aluminum and Aluminum Alloys. In: Encyclopedia of
Chemical Technology. Interscience Publishers, a Division of John
Wiley and Sons, Inc. New York, 1967.
4. Sittig, Marshall. Environmental Sources and Emissions Handbook,
Noyes Data Corporation, Ridge Park, New Jersey, 1975.
5. Engineering and Cost Effectiveness Study of Fluoride Emissions
Control, Volume I. Prepared by TRW Systems and Resources Research
Corp. for U.S. Environmental Protection Agency under Contract No.
EHSD-71-74. January 1972.
6. Particulate Pollutant System Study, Volume 1. Prepared by Midwest
Research Institute for U.S. Environmental Protection Agency. May
1971.
49
-------
Table 2-17. POSSIBLE CONTROL DEVICES - CALCINATION OF
ALUMINA HYDROXIDE (4,5)
Spray tower
Floating-bed scrubber
Quench tower and spray screen
Electrostatic precipitator
Controlled emission factor
kg/metric ton alumina a
30.0
28.0
17.0
2.0
Controlled emission factors are based on the average uncontrolled
factor and on average observed collection efficiencies.
50
-------
PRIMARY ALUMINUM PRODUCTION PROCESS NO. 10
Prebake Cells
1. Function - Aluminum metal is produced by the reduction of alumina
in electrolytic cells, or pots. This takes place in aluminum smelters, most
often in prebake electrolytic cells. Cells employing Soderberg anodes are
also in use (Process Nos. 11 and 12). In all types of cells, alumina is
dissolved in molten cryolite, a double fluoride salt of sodium and aluminum,
and reduced to aluminum metal by direct-current electrolysis (the Hall-HeYoul t
Process). The oxygen released rises through the electrolyte and reacts with
the carbon anode to continually form carbon dioxide and carbon monoxide. The
emissions from prebake cells are more easily controlled than those from
Soderberg cells; in addition, larger production capacity per cell is possible
with the prebake technology.
The electrolytic cell is a steel container lined with refractory brick
and an inner liner of carbon. The outside dimensions of the pot vary from 1.8
x 5.5 meters to 4.3 x 12.8 meters or larger with a depth around 1 meter. The
pots are arranged in rows, called potlines, containing from 100 to 250 cells
electrically connected in series. The carbon liner constitutes the cathode of
the cell. The anode of the cell is also made of carbon, which is the only
material that can withstand the corrosive action of fluorides. Prebake cells
(Figure 2-4) utilize anodes that have been previously formed and graphitized
(Process No. 15). The anode assemblies (as many as 26 per cell) are attached
by clamps to the anode bus and can be individually or simultaneously adjusted
in height as they are consumed. These assemblies are usually installed in two
rows extending the length of the cell. When the anodes are mostly consumed,
the butts are removed and new anode assemblies are substituted. The carbon
cathode potliner becomes partially saturated with cryolite; it usually lasts
between 2 and 3 years before cracking, which contaminates the molten aluminum
with iron from the outer shelf of the cathode and necessitates potliner re-
placement (1).
The molten electrolyte bath in the cell consists of alumina, cryolite
(Na3AlFs), fluorspar (CaF^), and aluminum fluoride (A1F3). The aluminum
fluoride is added to combine with sodium, present in the alumina as an im-
purity, to form artificial cryolite. The addition of fluorspar lowers the
melting point of the bath, which is covered by a frozen crust of electrolyte
to diminish heat loss from the top of the cell and to protect the anode from
oxidation. The density of the aluminum (2.3 g/cm^) is slightly greater than
the density of the molten bath (2.1 g /cm*), and it therefore settles to the
bottom of the cell (2). It accumulates at a rate of about 230 to 800 kilo-
grams in 24 hours. As electrolysis proceeds, a small portion of the molten
aluminum is carried to the anode by circulation of the electrolyte. Here it
is oxidized to alumina while some carbon dioxide is reduced to carbon monox-
ide. The oxides of carbon are released to the atmosphere. Every 1 to 3 days,
aluminum is siphoned into either cast iron pots with airtight lids or large
thermally insulated steel crucibles, in which it is transferred to holding
furnaces for casting (Process No. 13). The aluminum may also be transported
51
-------
ALUMINA (ORE) BIN
r—ANODE BUS
ANODE ROD —
ClAMf H-JU
CRUST BREAKER
RISER BUS TO
NEXT CELL
& /
/
z
STEEL CRADLE
/STEEL CATHODE
COLLECTOR BAR
SIDE HOOD FOR
VENT CONTROL
ALUMINA
CRUST
CRYOLITE BATH
MOLTEN
ALUMINUM
^ALUMINA
INSULATION
Figure 2-4. Prebake reduction cell. (1)
CATHODE
RING BUS
FLOOR
52
-------
in a molten state to fabricating plants at some distance from the smelter.
The alumina concentration in the bath is usually about 2 to 5 percent,
but this decreases as electrolysis proceeds. When the concentration drops to
only 1.5 to 2.0 percent, it is believed that the bath fails to wet the carbon
anode sufficiently, causing a gas film to collect which raises the resistance
of the cell (2). This is called an "anode effect." The cell voltage difference
increases from its normal 4.5 to 4.8 volts to around 40 to 60 volts while the
amperage decreases slightly. The anode effect increases the power input to
the cell and raises the temperature of the electrolyte. The frozen electro-
lyte crust melts and the bath salts are more rapidly volatilized. This
greatly increases the amount of fluoride that escapes from the cell. Sampling
has shown a 27-fold increase in particulate fluoride and a 2.7-fold increase
in gaseous fluoride evolution during anode effects (2). To control anode
effects, the crust is broken before it melts and replacement alumina is added
to increase the cell concentration (3). The alumina may be charged to the
cell by breaking the crust either in the center of the cell or along the side,
with center-breaking allowing more efficient collection of the off-gases.
Aluminum reduction plants constructed since 1950 have cells ranging in
size from 65 to 150 kiloamperes, with most in the range of 80 to 100 kilo-
amperes. Prebake anode cells of 175 kiloamperes each are used in a recently
constructed Japanese smelter (4). Larger cells require less manpower per ton
of product, but the powerful magnetic fields in very large cells may result in
violent agitation, dispersing the aluminum in the bath and increasing the
possibility of reversing the reduction reaction. It is unlikely that cell
size will further increase in future plants, and the trend instead has been
toward longer potlines containing many cells (250 to 1000 in a typical plant).
2. Input Materials - Input materials consist of alumina, prebaked anodes,
electrolyte (natural or artificial cryolite, aluminum fluoride, and fluor-
spar), and replacement carbon liners for the reduction cell. The liners may
be made of coke or anthracitic coal. The composition range of the electrolyte
in an operating cell is 80 to 85 percent by weight cryolite, 5 to 7 percent
fluorspar, 5 to 7 percent aluminum fluoride, and 2 to 8 percent alumina. As
electrolysis proceeds and the alumina is reduced, additional alumina is added
to maintain the bath composition. Anodes are consumed at the rate of 500
kilograms per metric ton of aluminum product. Approximately 1950 kilograms of
alumina is consumed to produce 1 metric ton of aluminum (1).
3. Operating Conditions - The prebake cells are operated at a temperature
between 950° and 1000°C and atmospheric pressure (2).
4. Utilities - The electrical supply is direct current on the order of
several hundred volts. The prebake cells each utilize a voltage drop of
approximately 4.5 volts. Current efficiency ranges from 85 to 90 percent,
with the principal energy loss due to evaporation from the bath and reoxida-
tion of aluminum by carbon dioxide. Voltage efficiency is only 40 percent
because of electrical resistance, which forms heat that is lost by radiation
and as sensible heat in exhaust gases, tapped metal, and partially-consumed
electrodes. The resulting overall power efficiency is about 35 percent (3).
53
-------
The resistivity of the prebake anodes ranges from 5000 to 6000 microhms-
centimeter, and the current density at the anode is between 1.0 and 1.3
amperes per square centimeter (5). The electricity consumed can be as low as
13.2 megawatt-hours per metric ton of aluminum, with the generally accepted
range for all types of cells 13.6 to 22 megawatt-hours per metric ton (6).
5. Waste Streams - A gas stream is discharged from the carbon anodes. Prior
to contact with the air, it normally contains about 75 percent carbon dioxide
and 25 percent carbon monoxide (5), but the carbon monoxide content is ele-
vated when the cell temperature is abnormally high (2). The carbon monoxide
burns as it contacts oxygen, and measurements of the exit gas have shown that
less than 1 percent carbon monoxide by volume remains after combustion (7).
The quantities and composition of additional cell emissions vary ac-
cording to temperature and other operating conditions. Particulate components
in the emission stream that have been identified are alumina, carbon, cryo-
lite, aluminum fluoride, calcium fluoride, chiolite (Na5Al3F4), and iron
oxide. Gaseous constituents include carbon dioxide, carbon monoxide, sulfur
dioxide, hydrogen sulfide, carbonyl sulfide (COS), carbon disulfide (C$2),
silicon tetrafluoride (SiF/i), hydrogen fluoride, water vapor, and during anode
effects, fluorocarbons such as carbon tetrafluoride (CF4) and hexafluoroethane
(C2Fs)(2). Fluoride-bearing materials may be lost from the cell by vaporiza-
tion or entrainment of the electrolyte bath, direct hydrolysis of the bath to
HF by hydrogen (as water or hydrocarbon), direct fluoridation of the anode
carbon (anode effect), and direct entrainment of feed material (8).
Because of the variations resulting from changes in operation of the
cell, only ranges of emissions can be ascertained. Total particulates range
from 22.5 to 88.5 kilograms per metric ton of molten aluminum product, with a
weighted average of 47.2 kilograms reported by the industry. Gaseous fluo-
rides range from 8.1 to 17.4 kilograms per metric ton of molten aluminum
product, with a weighted average of 12.4 kilograms. Particulate fluorides
range from 4.7 to 14.8 kilograms per metric ton of molten aluminum product,
with a weighted average of 10.2 kilograms (2). The distinction between
gaseous and particulate evolution is not clear, as one can be readily converted
to the other (e.g., adsorption of HF onto alumina; hydrolysis of particulate
fluoride to HF by water vapor). Table 2-18 gives particulate size distribu-
tion; a significant proportion is very fine dust, ranging from about one
micron down to 0.05 micron or less. The quantity of sulfur oxides released
from consumption of the prebake anodes depends on the sulfur content of the
anode carbon. The S02 emissions may range from 15 to 50 kilograms per metric
ton of aluminum product (2").
There are no liquid waste streams directly generated by this process.
The solid wastes from prebake cells consist of spent anode butts, which
may be a source of fugitive emissions if they are left to cool in the aisle
next to the potline, and cathode potliners. About 53 kilograms of spent pot-
liners, containing 9 kilograms of fluorides, is produced per metric ton of
54
-------
Table 2-18. PARTICLE SIZE DISTRIBUTION OF
UNCONTROLLED EMISSIONS FROM PREBAKE CELLS (9)
Size range, ym
<1
1 to 5
5 to 10
10 to 20
20 to 44
>44
Particles within size range,
weight percent
35
25
8
5
5
22
55
-------
aluminum product (10). Residues, consisting almost entirely of bath material,
are skimmed from the hot metal after tapping. About 5.5 kilograms, containing
2.0 kilograms of fluorides, is generated per metric ton of aluminum (10).
6. Control Technology - It is necessary to closely control cell operating
conditions to minimize total potline atmospheric emissions. Reducing anode
effects by careful attention to alumina concentration can greatly reduce
overall fluoride emissions. With advanced process control and automatic crust
breakers, anode effects can be reduced to less than one effect per cell each
48 hours. Careful control of bath temperature is also important. If the cell
temperature becomes greater than 1000°C, the crust will melt, allowing in-
creased volatilization of the salts. Proper operation of cells requires
either highly trained cell operators, a highly-instrumented process control
system, or both. At some plants, computers monitor the pot operating con-
diting conditions, calculate charge requirements, and control the opening on
the pot charging door.
Collection of cell emissions may be either primary (cell hooding),
secondary (roof monitors), or both. Efficient collection is essential, as
even the best treatment system is ineffective if the majority of cell emis-
sions are not contained. Smelters with prebake potlines can achieve higher
overall control efficiencies than can those with Soderberg cells since hooding
systems for primary gas collection can be more efficiently applied. Adequate
fan draft is necessary to prevent a significant proportion of emissions from
bypassing the primary control system. One hundred percent collection effi-
ciency is not possible because the shields need to be opened for anode re-
placement and metal tapping. One installation, however, reports 95 to 97
percent hood efficiency, accomplished by doubling the hood air flow rate when
the shield is opened for working, tapping, or anode replacement (2). The air
volume through a prebake cell hooding system varies between 110 and 225 cubic
meters per minute per cell (9).
At most smelters, fugitive atmospheric emissions from the cells remain a
potentially serious problem. Ventilation air is allowed to enter the potroom
building through side doors or openings in basement walls; it sweeps across
the cells and exits from the building through roof monitors. Any gases es-
caping the hoods are also vented through the roof monitors, which may be
routed through low energy wet control devices such as spray screens.
Dry scrubbing techniques using alumina are the best approach currently in
use for controlling the atmospheric emissions from aluminum reduction cells.
Such systems have the advantage of simultaneously adsorbing gaseous fluorides
and mechanically capturing particulate matter. Several systems are in use
which differ slightly in the character of the alumina used, the methods of
obtaining gas contact with the adsorbent, and the designs of the mechanical
particulate separation devices. They are all based on chemisorption of gase-
ous fluorides by alumina particles. All dry scrubbing systems allow fluoride
values to be returned to the potlines without the need for further processing
and without producing liquid waste streams. At most smelters with dry scrub-
bers, all of the alumina feed to the potline is first routed through the con-
trol system.
56
-------
Dry alumina adsorption in fluid-bed scrubbers such as Alcoa's System 398
is the most widely used primary control system on prebake potlines. Alumina
is continuously fed to a reactor bed through which cell off-gases are drawn by
a fan, trapping virtually all of the fluoride by adsorption. A bag filter
mounted over the reactor captures fugitive alumina; it is periodically cleaned
with the particulate dropped back into the fluid bed. Such systems are capa-
ble of removing 98 percent of the particulates, 99 percent of the gaseous
fluorides, and 98.6 percent of the total fluorides (11).
Coated filter dry scrubbers such as Alcoa's System 173 and the Wheela-
brator Coated Filter System are used at some domestic plants. In these sys-
tems, finely ground reactive alumina is injected into the gas stream to form a
coating on the fabric filter bags that increases adsorption of gaseous fluo-
rides. Removal efficiencies are reported as 98 percent for particulate
matter, 90 percent for gaseous fluorides, and 93.4 percent for total fluorides
(11). Several smelters are currently replacing this type of system with
fluid-bed dry scrubbers. A variation of the coated filter that is also used
in aluminum smelters is the injected alumina dry scrubber. Although the
mechanical equipment is similar, injected scrubbers differ from coated filters
as to the point of alumina addition. Injection systems allow more time for
gas-solid contact. Air Industrie of France has developed an injected system
based on modular equipment that is installed at two domestic smelters. The
design includes a venturi reactor in which the alumina is injected, followed
by a fabric filter. The ratio of the filtering surface area to the filter
volume is twice as large as a conventional bag filter. Aluminum Company of
Canada (Alcan) has also developed an injected alumina dry scrubber. Removal
efficiencies for such systems in foreign applications are reported to be 98
percent for particulates, gaseous fluorides, and total fluorides (11).
A variety of less efficient air pollution control devices is also in use
on prebake potlines. Dry electrostatic precipitators can be used to reduce
particulate matter, followed by wet scrubbing to reduce volatile fluorides.
Multiple cyclones may also be used alone or in conjunction with electrostatic
precipitators to collect particulate matter prior to wet scrubbing.
In 1974, about 80 percent of the total prebake capacity was accounted for
by center-break charging, which is potentially more amenable to tight hooding
and effective control than side-break charging. Table 2-19 summarizes the
control strategies employed on center-break prebake potlines that year.
Nearly half of prebake production capacity accounted for by side-break charg-
ing did not control the primary gas stream leaving the cell, relying instead
on a roof monitor collection system with low-energy wet scrubbers. Although
low-pressure wet scrubbers collect gaseous fluorides and coarse particulate
with reasonable efficiency, nearly all of the submicron dust passes through
and is vented to the atmosphere. The remaining half of side-break capacity
utilized primary dry scrubbing with alumina (12). Table 2-20 lists control
efficiencies for primary and secondary controls currently used on prebake
potlines and those considered feasible. Emission factors for selected control
devices can be found in Table 2-21.
57
-------
Table 2-19. CONTROL EQUIPMENT ON CENTER-WORKED
PREBAKE POTLINES - 1974 (12)
Primary control
Dry scrubbing with alumina
Dry ESP - wet scrubber
Multiple cyclones - wet
spray tower
Wet scrubbers
None
Secondary
control
None
None
None
Wet spray
screen
None
Percent of
capacity
55
16
16
5
6
58
-------
Table 2-20. AIR POLLUTION REMOVAL EFFICIENCIES - PREBAKE POTLINES (1)
In current use
Primary control
Multiple cyclone
Fluid bed dry scrubber system
Coated filter dry scrubber
Injected alumina dry scrubber9
Dry electrostatic precipitator
Spray tower
Floating bed scrubber
Chamber scrubber
Vertical flow packed bed
Secondary (no primary)
Spray screen
Cross flow packed bed (0.9 m bed)
Applicable
Primary control
Cross flow packed bed (1.5 m bed)
Self-induced spray
Venturi
High pressure spray screen
Secondary (no primary)
Floating bed wet scrubber
Secondary (with primary)
Spray screen
Efficiencies derived
from reported data
Particulate HF
78
98 99
98 76-92
98 98
89-98
8Qb 89-98
80 98
85 88
85 66
45J; 93
84 99
Estimated efficiencies
Particulate HF
87 98
65 96
96 99
93 98
75 87-95
25 80
a Denotes foreign application.
b Fluoride particulate.
59
-------
Table 2-21. EMISSION FACTORS FOR CONTROL DEVICES ON PREBAKE CELLS (9):
(kg/metric ton of molten aluminum product)
Control device
Multiple cyclone3
Fluid-bed dry scrubber system
Coated filter dry scrubber
Dry electrostatic precipitator
Spray tower
Floating-bed scrubber
Chamber scrubber
Vertical flow packed bed
Dry alumina adsorption
Total ,
particulates
8.95
1.01
0.81
0.81 to 4.47C
8.1
8.1
6.1
6.1
0.81
Gaseous
fluorides
12.35
0.124
0.99 to 2.97C
12.35
0.247 to 1.36C
0.247
1.48
4.2
0.247
Particulate
fluorides
2.25
0.253
0.204
0.204 to 1.1 2C
2.04
2.04
1.53
1.53
0.204
Controlled emissions factors are based on average observed collection
efficiencies.
Includes particulate fluorides.
c Range of values observed.
60
-------
Liquid wastes from wet scrubbers are treated by either a once-through
lime treatment system or a treat-and-recycle system. In the once-through
system, wastewater containing 100 to 600 milligrams of soluble fluoride per
liter is mixed with a lime slurry to form insoluble calcium fluoride. The
mixture is then thickened, with the solids sent to an open landfill and the
clarified effluent (20-50 mg/1 fluoride) combined with other plant wastewater
and discharged. At some plants the sludges are left permanently in the
lagoons.
In the treat-and-recycle system, liquor from the primary and secondary
gas scrubbers is either transferred to cryolite recovery (Process No. 17) or
treated with calcium chloride. The liquor is then clarified and recycled to
the scrubbers. Fluorides are captured in the sludge from the clarifier
either as cryolite, which is salvaged and used in the potlines, or as calcium
fluoride, which is discarded. A bleed stream of liquor is necessary to keep
sodium sulfates from accumulating in the scrubber liquor. The volume of the
bleed stream depends on the sulfur content of the coke and pitch. The bleed
stream is diluted with other plant water streams and discharged.
Spent anode butts are cleaned as described in Process No. 16; they may be
stored for short periods of time before being sent to the carbon plant. The
spent cathode potliners contain carbon, aluminum, aluminum nitrides, cyanides,
and sodium and aluminum fluorides. They are frequently removed manually with
jackhammers, although at some plants the pot is removed from the potline and
dropped to dislodge the liner. The resulting chunks of carbon coated with
cryolite are either discarded in waste heaps or sent through cryolite recovery
(Process No. 17) on site or at an outside location and then discarded. Rec-
clamation of carbon from spent potliners has proved to be marginally econom-
ical at best. Potliners may be stored for months or even years prior to
cryolite recovery, although at some plants they are processed almost immedi-
ately. Rainwater percolating through discarded potliners dissolves cyanides
and fluorides, so adequate protection is required to prevent contamination of
runoff. The potliners are typically stored either on the ground or on con-
crete pads. At least one smelter uses a building designed to prevent leakage
for this purpose. PVC-lined pits are also employed. The majority of smelters
collect the runoff from potliner storage areas, although it is not always
treated prior to discharge. A few aluminum smelters are located in climates
where net evaporation exceeds rainfall, so simple impoundment is the only
control required. In other areas, collected runoff is treated by various
means including the use of chlorine and caustic soda to decompose the cyanide.
In some cases the collected runoff is used as makeup water for scrubbers.
Potline skimmings are generally returned directly to the pots. At some
plants the residues are reprocessed along with spent potliners.
7. EPA Source Classification Code - 3-03-001-01.
61
-------
8. References -
1. Development Document for Effluent Limitations Guidelines and New
Source Performance Standards for the Primary Aluminum Smelting
Subcategory of the Aluminum Segment of the Nonferrous Metals Manu-
facturing Point Source Category. EPA 440/l-74-019d. U.S. Environ-
mental Protection Agency, Washington, D.C., March 1974.
2. Singmaster and Breyer. Air Pollution Control in the Primary Alu-
minum Industry, Vol. I. New York, July 23, 1973.
3. Stamper, John W. Aluminum. In: Mineral Facts and Problems. U.S.
Department of Interior, Bureau of Mines, Washington, D.C. , 1970.
4. Aluminum Cell Modifications Cut Energy Use. Chemical Engineering
News. Vol. 53. August 4, 1975.
5. Vachet, P. Aluminum and Aluminum Alloys. In: Encyclopedia of
Chemical Technology. Interscience Publishers, a Division of John
Wiley and Sons, Inc. New York, 1967.
6. Environmental Considerations of Selected Energy Conserving Manu-
facturing Process Options: Vol. VIII. Alumina/Aluminum Industry
Report. U.S. Environmental Protection Agency, Cincinnati, Ohio
December 1976.
7. Background Information for Standards of Performance: Primary
Aluminum Industry. Vol. 1: Proposed Standards. EPA 450/2-74-020a.
U.S. Environmental Protection Agency, Research Triangle Park, North
Carolina, October 1974.
8. Wahnsiedler, W.E. et al. Factors Affecting Fluoride Evolution from
Hall-Heroult Smelting Cells. Paper No. LM 78-52. The Metallurgical
Society of AIME. New York.
9. Compilation of Air Pollution Emission Factors, Second Edition.
AP-42. U.S. Environmental Protection Agency, Research Triangle
Park, North Carolina, April 1973.
10. Assessment of Industrial Waste Practices in the Metal Smelting and
Refining Industry. Vol. II, Primary and Secondary Nonferrous
Smelting and Refining. Prepared by Calspan Corporation for U.S.
Environmental Protection Agency. EPA Contract No. 68-01-2604.
1977.
11. Primary Aluminum. In: The Fabric Filter Manual. Charles Billings,
Ed. Mcllvaine Publishing Co., August 1977.
12. Energy Penalty of the Nonferrous Metals Industry. Part II: Primary
Smelting of Aluminum (Draft). Prepared by Arthur D. Little, Inc.,
for U.S. Environmental Protection Agency. EPA Contract No. 68-
01-4381, Work Area No. II. August 1977.
62
-------
PRIMARY ALUMINUM PRODUCTION PROCESS NO. 11
HSS Cells
1. Function - Aluminum metal is produced by the reduction of alumina (A1203)
in electrolytic cells, or pots. One of the three cell types in use at domes-
tic smelters is the horizontal stud Soderberg (HSS), which like other cell
types (Process Nos. 10 and 12) is used for the reduction of alumina to alu-
minum and oxygen by direct-current electrolysis (Hall-HeYoult Process).
Energy consumption in Soderberg cells is greater than that of prebake cells
because of the less positive electrical connection between the anode and
current carrying studs. Soderberg plants require a smaller capital invest-
ment, however, because anode baking and rodding facilities are not required.
In spite of the lower capital cost, the trend in the industry is away from
Soderberg cells (1). All of the remaining domestic HSS capacity is presently
being modified using Japanese technology.
The difference between the HSS and the more commonly employed prebake
cells lies primarily in the anode construction. Figure 2-5 is a diagram of a
typical HSS cell. The single large anode is baked in the cell itself.
"Green" anode paste is fed periodically into the open top of a rectangular
steel compartment suspended above the furnace. The paste is rammed down the
casing, where it is gradually baked by the heat created by the electrical
resistance of the carbon, becoming a monolithic solid about 0.5 meter above
the surface of the bath (2). The process heat drives off the lower boiling
organics and fuses the new paste to the electrode. Additional paste is added
as the anode is consumed in the reduction process. Rows of steel or aluminum
studs in channels project horizontally into the paste and move with the anode.
These studs carry the electrical current for electrolysis, as well as support
the anode mass, and they are extracted as the anode is consumed and replaced
at higher levels. Periodic adjustment of the studs is required to maintain
interpole distances and adequate current efficiency. Anode adjustment by
means of powered jacks is normally made in conjunction with metal tapping
operations because the molten metal builds up at the same rate at which the
anode is consumed; however, more frequent adjustments may be required (2,3).
In large cells, the anode can be 6.5 meters long, 2 meters wide, and 1 meter
high, weighing up to 18 metric tons (2).
Further information on the general operation of electrolytic reduction
cells can be found in Process No. 10.
2. Input Materials - Input materials consist of alumina, anode paste from
Process No. 14, electrolyte (cryolite, aluminum fluoride, and fluorspar), and
the carbon potliner of the reduction cell. Charge composition and quantities
are the same as those used in the prebake cells (Process No. 10). Anode paste
is added at a rate of 500 kilograms per metric ton of aluminum product (4).
3. Operating Conditions - HSS cells are operated at a temperature between
950° and 1000°C and atmospheric pressure (2).
63
-------
AlUMINA HOMIRS
FASTI COMPARTMINT
COVIR
• IMOVAIll
CHANNI1S
PA1TI COMMRTMINT
CASINO
POT INCLOSURI
OOOI
ANOOI
STUDS
OAS AND FUMI
IVO1VINO
CATNOOI
COUICTOR IAI
Figure 2-5. Horizontal stud Soderberg (HSS) reduction cell (2)
64
-------
4. Utilities - Average electrical consumption in Hall-HeYoult aluminum
smelters is 15.6 megawatt-hours per metric ton of aluminum product. Power
consumption for Soderberg cells is approximately 1 megawatt-hour per metric
ton greater than for prebake cells (4,5). Resistivity of the anode is about
30 percent higher than that of a prebake anode and current density is lower,
between 0.7 and 0.9 ampere per square centimeter (3).
Further information on utility consumption can be found in Process No.
10.
5. Waste Streams - As with prebake cells, the atmospheric emissions from HSS
cells contain the following constituents:
0 Alumina ° Sulfur dioxide
0 Carbon ° Hydrogen sulfide
0 Cryolite ° Carbonyl sulfide
0 Aluminum fluoride ° Carbon disulfide
0 Calcium fluoride ° Silicon tetrafluoride
° Chiolite ° Hydrogen fluoride
° Iron oxide ° Water vapor
° Carbon dioxide ° Carbon tetrafluoride and hexafluoroethane
0 Carbon monoxide (during anode effects only)
Because the anode paste is baked during the process, additional hydrocarbons
(tar fogs) and S02 are released. The tar fogs are responsible for the bluish
haze characteristic of Soderberg reduction plants. These materials have been
reported to be 3 percent of the total volume of gases emitted from the anode
(2). Per metric ton of aluminum product, uncontrolled total particulate
emissions range from 46.8 to 52.0 kilograms, gaseous fluorides range from 12.6
to 14.4 kilograms, and particulate fluorides range from 7.2 to 8.1 kilograms
(2). Table 2-22 gives particulate size distribution for HSS cells. One
report shows SOp concentration in HSS plant off-gas to be 80 parts per million
(6).
No liquid wastes are directly generated by this process.
The only solid wastes generated are the spent potliners and potline
skinmings.
6. Control Technology - As with prebake cells, primary hood collection
efficiency is a key factor in the control of cell emissions. However, the
construction of the HSS cell prevents the installation of an integral collec-
tion device such as a gas skirt and complicates efficient collection of the
emissions. The hood is a suspended canopy with removable side panel closures,
or shields, which must be opened to add alumina or tap the metal. The hood is
not effective, and considerable cool air is drawn into the waste gas stream.
The air volume through the hooding system is maintained between 110 and 225
cubic meters per minute, but the waste gas is so cooled by dilution that self-
supporting combustion of the carbon monoxide and other combustible constitu-
ents cannot be maintained (2). Tars from the anode paste pass to the control
device, thereby increasing the difficulty of capturing polluting constituents
(1).
65
-------
Table 2-22. PARTICLE SIZE DISTRIBUTION OF UNCONTROLLED
EMISSIONS FROM HSS CELLS (6)
Size range, ym
<1
1 to 5
5 to 10
10 to 20
20 to 44
>44
Particles within size range,
weight percent
44
26
8
6
4
12
66
-------
All of the seven operating HSS potlines in the U.S. include controls of
cell emissions. Two use dry alumina scrubbing systems, two use wet electro-
static precipitators, two use wet scrubbers, and one uses a combination of wet
scrubbers and wet electrostatic precipitators. None of these potlines have
secondary air emission treatment. Dry adsorption systems represent the best
control technology as they allow the simultaneous capture of particulate
matter, organic tars, and gaseous fluorides. Further information on the
different types of dry scrubbing systems available can be found in Process No.
10. Wet control systems are less effective because tars present in the gas
stream resist wetting, resulting in particle reentrainment. Table 2-23 lists
efficiencies for various types of wet control devices currently in use or
considered applicable. Emission factors for three wet control devices are
presented in Table 2-24.
As described in Process No. 10, either a once-through or a recycle treat-
ment system may be used for the bleed stream from wet scrubbers. Cryolite may
also be recovered from the spent liquor (Process No. 17).
The spent potliners are either disposed of in waste heaps (see Process
No. 10) or sent through cryolite recovery (Process No. 17) prior to being
discarded. The potline skimmings are either returned directly to the pots or
reprocessed with the spent potliners.
7. EPA Source Classification Code - 3-03-001-02.
8. References -
1. Primary Aluminum. In: The Fabric Filter Manual. Charles Billings,
Ed. Mcllvaine Publishing Co., August 1977.
2. Singmaster and Breyer. Air Pollution Control in the Primary Alu-
minum Industry, Vol I. New York, July 23, 1973.
3. Vachet, P. Aluminum and Aluminum Alloys. In: Encyclopedia of
Chemical Technology. Interscience Publishers, a Division of John
Wiley and Sons, Inc. New York, 1967.
4. Development Document for Effluent Limitations Guidelines and New
Source Performance Standards for the Primary Aluminum Smelting
Subcategory of the Aluminum Segment of the Nonferrous Metals Manu-
facturing Point Source Category. EPA 440/1-74-019d. U.S. Environ-
mental Protection Agency, Washington, D.C., March 1974.
5. Environmental Considerations of Selected Energy Conserving Manu-
facturing Process Options: Vol. VIII. Alumina/Aluminum Industry
Report. U.S. Environmental Protection Agency, Cincinnati, Ohio,
December 1976.
6. Compilation of Air Pollution Emission Factors, Second Edition. AP-
42. U.S. Environmental Protection Agency, Research Triangle Park,
North Carolina, April 1973.
67
-------
Table 2-23. AIR POLLUTION CONTROL EFFICIENCIES -
HSS POTLINES (2)
In current use
Primary
Spray tower
Floating bed
Applicable
Primary
Wet electrostatic precipitator
Crossflow packed bed (1.5 m bed)
Self-induced spray
Spray screen
Chamber scrubber
Secondary
(with primary collection)
Spray screen
Efficiencies derived
from reported data
Particulate HF
63-803 91-93
78a 98
Estimated efficiencies
Particulate HF
98
81 a 98
62 96
75 93
94 94
25a 80
Fluoride particulate.
Table 2-24. EMISSION FACTORS FOR CONTROL DEVICES ON HSS CELLS (7)
(kg/metric ton of molten aluminum product)
Control device
Spray tower3
Floating-bed scrubber
Electrostatic precipitator
Total b
particulates
9.8 to 18. 2C
10.8
3.55
Gaseous
fluorides
0.93 to 1.195C
0.266
13.3
Particulate
fluorides
1.65 to 2.885C
0.1715
0.563
a Controlled emission factors are based on averaged observed collection
efficiencies.
Includes particulate fluorides.
c Range of values observed.
6O
O
-------
PRIMARY ALUMINUM PRODUCTION PROCESS NO. 12
VSS Cells
1. Function - Aluminum metal is produced by the reduction of alumina
in electrolytic cells, or pots. In addition to prebake cells (Process No. 10)
and HSS cells (Process No. 11), the vertical stud Soderberg (VSS) cell is also
in use in the U.S. industry. This technology is the least widely used cell
configuration, however, and is not likely to be considered for any new instal-
lations because of the problems inherent in controlling atmospheric emissions.
The main difference between the VSS and HSS cells lies in the manner in
which the current-carrying studs are inserted into the anode. The studs in
the VSS cell project vertically into the anode mass, as shown in Figure 2-6.
As with the HSS cell, the anode paste is rammed into the casing and the single
large anode is baked in the cell itself. The studs are extracted as the anode
is consumed, before they are exposed to molten aluminum. Anode size is
similar to the HSS anode (1).
Further information on the general operation of Soderberg electrolytic
reduction cells can be found in Process No. 11.
2. Input Materials - Input materials are the same as previously described
for HSS cells in Process No. 11.
3. Operating Conditions - The VSS cells are operated at temperatures between
950° and 1000°C and atmospheric pressure (1).
4. Utilities - Electrical consumption for VSS cells is equivalent to HSS
cells, averaging 15.6 megawatt-hours per metric ton of molten aluminum prod-
uct. This is about 1 megawatt-hour per metric ton greater than for prebake
cells (2,3).
5. Waste Streams - The air emissions generated by VSS cells are generally of
the same composition as HSS cells. Compared to prebake cells, they contain
additional hydrocarbon fumes and S02 from the baking of pitch binder in the
anode paste. Per metric ton of aluminum product, uncontrolled total particu-
late emissions are 39.2 kilograms, gaseous fluorides range from 10.0 to 17.4
kilograms, and particulate fluorides range from 2.8 to 27.7 kilograms (1).
VSS plants have been reported to emit the same quantity of S02 as HSS plants,
about 80 parts per million (4).
There are no liquid waste streams directly generated by this process.
The only solid wastes generated by VSS cells are the spent potliners and
the potline skimmings.
6. Control Technology - No U.S. smelters have installed a gas collection
hood above a VSS cell since electrical connections are immediately above the
cell and must be accessible for adjustments. In addition, hoods would disturb
the heat balance of the cell. Instead, skirts are installed between the anode
69
-------
ANODE »U$
ANODE tOD
STEEL ANODE
STUD
ANODE CASINO
OAS COLLECTING
SKIRT
MOLTEN
ELECTROLYTE
TO EMLUENT
COLLECTION
SYSTEM
BURNER
\ CARtON
HOCK
LINING
CRUST
ALUMINA -
MOLTEN ALUMINUM
CATHODE
FLOOR
/LEVEL
\_STEEl CATHODE
COLLECTION SAR
STEEL
CRADLE
CATHODE BUS
Figure 2-6. Vertical stud Soderberg (VSS) reduction cell (1).
70
-------
casing and the bath surface. The hood skirts are sealed to the electrolyte
crust by a blanket of alumina; however, a substantial part of the bath surface
lies outside the skirt. Of the portion of waste gas that is captured, there
is very low air dilution because the skirts are sealed to the crust. The
resulting small air volume, 11 to 17 cubic meters per minute, allows a suf-
ficiently high hydrocarbon concentration to permit combustion of the gases in
a burner (5). Combustion of the tars creates less interference with the
operation of subsequent control devices. Much of the carbon monoxide and 96.7
percent of the hydrocarbons (both gaseous and particulate) are converted to
carbon dioxide and water vapor. However, during crust breaking or charging
operations, the alumina blanket is dropped back into the electrolyte, breaking
the seal and allowing large quantities of gaseous and particulate emissions to
escape into the potroom. The problem of hooding VSS cells may have been solved
by Pe"chinery, however, which has successfully installed primary hoods on the
VSS pots at its St. Jeanne de Maurienne smelter in France. Total collection
efficiency is approximately 90 percent, whereas 80 percent is considered a
good overall efficiency for both primary and secondary systems at a conven-
tional VSS plant.
After combustion, the gas stream from the VSS cells is relatively low in
hydrocarbon content. Primary gas control is used on all domestic VSS pot-
lines. In 1974, 42 percent of production capacity used electrostatic pre-
cipitators for particulate removal followed by wet scrubbers to control fluo-
rides. The remainder of VSS capacity was equally divided between dry scrub-
bing with alumina and the combination of multiple cyclones and wet scrubbers
or wet electrostatic precipitators (6). Injected alumina dry scrubbing sys-
tems have been installed on VSS potlines at foreign smelters (5). Descrip-
tions of these control alternatives can be found in Process No. 10. One
domestic plant is currently replacing a wet scrubber system with the Alcoa 398
dry scrubbing system. Although considerable fugitive emissions escape from
VSS cells, only about 29 percent of the VSS production capacity had secondary
emissions control in 1974 (6). Table 2-25 lists reported efficiencies for
primary and secondary controls currently employed, and primary controls con-
sidered feasible but not in use at this time. Table 2-26 lists emission
factors for various control devices.
The treatment options for spent scrubber liquor are the same as those
described for Process No. 10. Either once-through or recycle treatment sys-
tems may be used. Cryolite may also be recovered (Process No. 17).
The spent potliners are either directly discarded in waste heaps (see
Process No. 10) or sent through cryolite recovery (Process No. 17) prior to
being discarded. The potline skimmings are either returned directly to the
pots or reprocessed with the spent potliners.
7. EPA Source Classification Code - 3-03-001-03.
8. References -
1. Singmaster and Breyer. Air Pollution Control in the Primary Alu-
minum Industry, Vol. I. New York, . July 23, 1973.
71
-------
2. Development Document for Effluent Limitations Guidelines and New
Source Performance Standards for the Primary Aluminum Smelting
Subcategory of the Aluminum Segment of the Nonferrous Metals Manu-
facturing Point Source Category. EPA 440/l-74-019d. U.S. Environ-
mental Protection Agency, Washington, D.C., March 1974.
3. Environmental Considerations of Selected Energy Conserving Manu-
facturing Process Options: Vol. VIII. Alumina/AJuminum Industry
Report. U.S. Environmental Protection Agency,"Cincinnati, Ohio,
December 1976.
4. Compilation of Air Pollution Emission Factors, Second Edition.
AP-42. U.S. Environmental Protection Agency, Research Triangle
Park, North Carolina, April 1973.
5. Primary Aluminum. In: The Fabric Filter Manual. Charles Billings,
Ed. Mcllvaine Publishing Co., Inc. August 1977.
6. Energy Penalty of the Nonferrous Metals Industry. Part II: Primary
Smelting of Aluminum (Draft). Prepared by Arthur D. Little, Inc.
for U.S. Environmental Protection Agency. EPA Contract No. 68-01-
4381, Work Area No. II. August 1977.
72
-------
Table 2-25.
AIR POLLUTION CONTROL EFFICIENCIES -
VSS POTLINES (1)
In current use
Primary
Burners
Multiple cyclones
Fluid-bed dry scrubber3
Injected alumina dry scrubber3
Dry electrostatic precipitatoi*
Wet electrostatic preci pita tor
Spray tower
Self-induced spray
Floating bed3
Sieve plate tower3
Venturi scrubber
Secondary
(with primary collection)
Spray screen
Applicable
Primary
High pressure spray screen
(3 stage)
Chamber scrubber
Efficiencies derived
from reported data
Particulate HF
_ _
40-50
98 99
98 b 98
98 b
90-99
75 b 99
b 99
78 97
96-97 99
96b 99
42b 88
Estimated efficiencies
Particulate HF
93 b 98
94 94
Crossflow packed bed (1.5 m bed) 87b 98
Denotes foreign application.
Fluoride particulate.
73
-------
Table 2-26. EMISSION FACTORS FOR CONTROL DEVICES ON VSS CELLS (4)
(kg/metric ton of molten aluminum product)
Control device
Spray tower3
Self -induced spray
Venturi scrubber
Wet electrostatic pre-
ci pita tor
Multiple cyclones
Dry alumina adsorption
Total .
Particulates
9.8
c
1.57
0.392 to 3.92d
1.96 to 2.35d
0.784
Gaseous
Fluorides
0.152
0.152
0.152
15.2
15.2
0.304
Particulate
Fluorides
1.325
c
0.212
0.053 to 0.53d
2.65 to 3.8d
0.105
Controlled emission factors are based on average observed collection
efficiencies.
Include particulate fluorides.
No information available.
Range of values observed.
74
-------
PRIMARY ALUMINUM PRODUCTION PROCESS NO. 13
Casting
1. Function - The casting process involves pouring molten aluminum into a
mold and cooling it with water. The molten aluminum metal from reduction
cells is siphoned into insulated crucibles, which are transferred to a casting
house and either poured directly into cast iron molds or into holding/al-
loying furnaces (1). Ingots may be produced continuously using casting
machines or casting wheels (2). Direct-contact cooling water is often used.
At some installations, the molten aluminum may be batch treated in
furnaces to remove oxide and gaseous impurities and active metals such as
sodium and magnesium. This operation is usually conducted in a reverberatory
holding furnace, although electrically heated furnaces are also used. The
process consists of adding a flux of chloride and fluoride salts and then bub-
bling chlorine gas, often mixed with an inert gas, through the molten mixture.
Chlorine reacts with impurities to form HC1 or metal chlorides. A dross forms
which floats on the molten aluminum. Prior to casting, the dross is removed
and discarded.
Other methods of degassing and purification have been developed that
cause less corrosion and create less air pollution. Degassing can be par-
tially accomplished mechanically by heavy stirring or gravity agitation. The
Alcoa 94 Process uses a granular filter bed for purification. The bed is made
of a refractory that is inert to molten aluminum. The Alcoa 181 Process uses
argon or another inert gas to simultaneously remove both inclusions and dis-
solved hydrogen. The Alcoa 469 Process uses two reactors in series, in which
the melt and a chlorine-inert gas mixture are contacted in countercurrent flow
through an Alcoa 94 Process filter bed. In the Alcoa 503 Process, which has
only been applied to a secondary smelter, a closed reaction chamber is used in
which chlorine or chloride salts can be made to react rapidly by agitation
(3). In British Aluminum Company's Fumeless In-Line Degassing (FILD) process,
the molten aluminum flows through a two-chambered crucible in which it is
sprayed with nitrogen gas to remove hydrogen. It then percolates through beds
of flux-coated and uncoated AlpOg pellets to remove inclusions and entrapped
flux (4).
Dynamic vacuum processes (stream degassing), similar to the BV process
used in steel production, are also employed at primary aluminum smelters. The
molten aluminum falls as droplets into a vacuum vessel, and dissolved elements
are removed by such mechanisms as diffusion, desorption, condensation, and
chemical reaction (5). The extent of use of vacuum treatment in the domestic
aluminum industry is not.reported.
After degassing and purification, the molten aluminum may be filtered
through fiberglass, rigid ceramic foam, or charcoal.
75
-------
2. Input Materials - The principal input to this process is molten aluminum
from the reduction cells. Chlorine, inert gases, and mixtures of sodium,
potassium, and calcium chlorides are used for purification and degassing.
Aluminum and magnesium chlorides may be used in some process variations. The
gases used include nitrogen, argon, and carbon monoxide. Chlorine may be
obtained by decomposition of hexachlorethane.
3. Operating Conditions - This process is carried out at elevated tempera-
ture and atmospheric pressure. Degassing media are pressurized for delivery
to the furnace.
4. Utilities - Electricity is used for operation of fluid pumps and mechan-
ical equipment. Water is used for direct cooling of ingots; the industry
average is 1620 liters per metric ton of aluminum. Holding furnaces are gas-
fired, with fuel consumption ranging from 0.4 to 3.0 million kilocalories per
metric ton of aluminum product (6,7).
5. Haste Streams - Minor fugitive emissions are released during pouring and
casting, but the principal atmospheric emission is created when chlorine or
chlorine compounds are used for fluxing. Furnace gases contain particles of
aluminum chloride which may hydrolyze to HC1 and A^Os in the presence of
moisture. Free chlorine is also released. Traces of fluorine may originate
with electrolyte impurities fluxed from the metal. Other gas constituents are
inert carriers, aluminum oxide, and products of combustion from the gas-fired
furnaces (2,8).
A liquid effluent is generated by direct water cooling of the aluminum
molds. This heated stream contains dissolved and suspended solids, oil, and
grease.
A solid waste is generated by this process, consisting of dross removed
from the molten aluminum prior to casting. Dross contains a high percentage
of water-soluble salts.
6. Control Technology - Domestic smelters control atmospheric emissions from
chlorine degassing operations. Control devices that are in use include wet
scrubbers, wet electrostatic precipitators, high-energy venturi scrubbers, and
coated baghouses. Alkaline liquors can be used to remove the acid gas in
scrubbers. The bleed stream from the scrubber may be alkaline or acidic
depending on operating conditions, and it will contain high levels of chloride
salts (usually NaCl) and hypochlorites. Few smelters control emissions from
casting.
Effective control of cast house direct-contact cooling water can be
achieved in a closed-loop system by means of a cooling tower with a bleed
stream to prevent the buildup of dissolved and suspended solids, oil, and
grease. Some smelters discharge the cooling water to the plant wastewater
system. The oil skimmed from the water is either sold, incinerated, or used
76
-------
for dust control on the smelter roads. One plant treats a 568 liters-per-
minute bleed stream in an aerated lagoon with a 15 day retention time, re-
ducing the amount of hydrocarbons present by 85 percent (1). In other cases
the cooling water is used as makeup water for secondary scrubbers at the
potlines.
Dross is usually held in small furnaces to reclaim entrained particles of
aluminum. The cleaned dross is often discarded in landfills, and secondary
pollution of water is possible. The salt mixture may also be sold to a re-
claimer or used for deicing of roads.
7. EPA Source Classification Code - None.
8. References -
1. Development Document for Effluent Limitations Guidelines and New
Source Performance Standards for the Primary Aluminum Smelting
Subcategory of the Aluminum Segment of the Nonferrous Metals Manu-
facturing Point Source Category. EPA 440/1-74-019d. U.S. Environ-
mental Protection Agency, Washington, D.C., March 1974.
2. Singmaster and Breyer. Air Pollution Control in the Primary Alu-
minum Industry, Vol. I. New York, July 23, 1973.
3. Hatch, John E. Chlorine Control by In-Line Fluxing Operations.
Presented at the 66th Annual Meeting of the Air Pollution Control
Association. Chicago, Illinois, June 24-28, 1973.
4. The Indian and Eastern Engineer. Vol. 119, No. 4. April 1977.
5. Van Wijk, G.W.M., and D.M. Ackermann. Stream Degassing of
Aluminum and Aluminum Alloys. Paper No. LM 78-41. The Metallurgical
Society of AIME. New York.
6. Environmental Considerations of Selected Energy Conserving Manu-
facturing Process Options: Vol. VIII. Alumina/Aluminum Industry
Report. U.S. Environmental Protection Agency, Cincinnati, Ohio,
December 1976.
7. Water Pollution Control in the Primary Nonferrous-Metals Industry,
Vol. II. Aluminum, Mercury, Gold, Silver, Molybdenum, and Tungsten,
EPA-R2-73-247b. U.S. Environmental Protection Agency, Washington,
D.C., September 1973.
8. Aluminum. In: The Fabric Filter Manual. Charles Billings, Ed.
Mcllvaine Publishing Co. August 1977.
77
-------
PRIMARY ALUMINUM PRODUCTION PROCESS NO. 14
Paste Preparation
1. Function - Aluminum smelters must include facilities to manufacture
electrodes for the electrolytic cells. In the paste preparation plant, or
"green mill," petroleum and pitch coke are mixed with a pitch binder to form a
paste which is used for Soderberg cell anodes, cathode linings, and green
anodes for prebake cells. Paste preparation includes the operations that
crush, grind, screen, and classify the coke. Carefully sized fractions are
then mixed with the binder in steam-jacketed mixing equipment.
In prebake anode preparation, the resulting hot thick paste is trans-
ferred to molds where green anode blocks are formed either in hydraulic
presses or by vibratory jolting (1). In Soderberg anode preparation, the
mixture is transferred directly to the potroom.
2. Input Materials - Raw materials for anode paste consist of high grade
coke (petroleum and pitch coke) and pitch. The anode raw materials are
usually specified to contain no more than 0.7 percent ash, 0.7 percent sulfur,
8 percent volatiles, 0.5 percent alkali, and 2 percent moisture. Pitch
ordinarily contains about 0.5 percent sulfur. Petroleum coke used at smelters
has been reported to contain anywhere from 1.5 to 10 percent sulfur with a
concentration of about 2 percent thought to be typical (2). The prebake anode
paste plant also utilizes spent anode butts received from the rodding room
(see Process No. 16).
Prebake carbon anode plants utilize pitch having a softening point in the
range of 90° to 120°C. Soderberg anode plants can use pitch with a softening
point of 55°C up to the harder pitch used in the prebake anodes (2).
3. Operating Conditions - All grinding and screening operations are con-
ducted at ambient temperature and atmospheric pressure. The mixing of coke
and pitch is conducted at elevated temperature and atmospheric pressure.
Exact temperatures employed have not been reported.
4. Utilities - Electricity is used for operation of pumps, grinders, screens,
conveyors, and other mechanical equipment. Liquid pitch handling systems
utilize steam, electricity, or high temperature heat transfer media (hydro-
carbon oils, glycols, or chlorinated biphenyls). Steam is used to heat the
coke and pitch mixers. Utility consumption rates have not been reported.
5. Waste Streams - The air emissions from these operations consist of coke
dust generated by materials handling. Approximately 5 kilograms of particu-
late is emitted per metric ton of aluminum product. Volatile hydrocarbon
fumes are generated to a limited extent by the paste mixing operation (1).
Anode butts returned to the carbon mix plant for regrinding and mixing with
pitch frequently are a source of fluoride emissions, since they usually con-
tain cryolite (see Process No. 16). Fluoride emissions are generally coarse
particulates and do not constitute a significant air pollution problem beyond
78
-------
the boundaries of the plant itself; however, they can be a source of secondary
water pollution.
No liquid or solid wastes are generated by this process.
6. Control Technology - Most primary aluminum smelters control atmospheric
emissions from their paste plants with baghouses. However, other controls are
in use, including electrostatic precipitators, multiple cyclones, and wet
scrubbers. A few prebake smelters utilize dry scrubbing systems with alumina
injection. Table 2-27 presents emission factors for several types of controls
used on materials handling emissions. Automated, enclosed, and ventilated
conveyors may be used to reduce fugitive emissions from coke transport.
7. EPA Source Classification Code - 3-03-001-99 for Paste Mix, 3-03-001-04
for Materials Handling.
8. References -
1. Singmaster and Breyer. Air Pollution Control in the Primary Alu-
minum Industry, Vol. I. New York, July 23, 1973.
2. Development Document for Effluent Limitations Guidelines and New
Source Performance Standards for the Primary Aluminum Smelting
Subcategory of the Aluminum Segment of'the Nonferrous Metals Manu-
facturing Point Source Category. EPA 440/l-74-019d. U.S. Environ-
mental Protection Agency, Washington, D.C., March 1974.
3. Sittig, Marshall. Environmental Sources and Emissions Handbook.
Noyes Data Corporation, Ridge Park, New Jersey, 1975.
79
-------
Table 2-27. EMISSION FACTORS FOR PASTE PLANT MATERIALS HANDLING (3)
(kg/metric ton of molten aluminum product)
Control device
Spray tower3
Floating-bed scrubber
Quench tower and
spray screen
Electrostatic pre-
ci pita tor
Total b
particulates
1.5
1.4
0.85
0.10
Gaseous
fluorides
Neg.c
Neg.
Neg.
Neg.
Particulate
fluorides
NAd
NA
NA
NA
Controlled emission factors are based on average observed collection
efficiencies.
Includes particulate fluorides.
c Negligible.
No information available.
80
-------
PRIMARY ALUMINUM PRODUCTION PROCESS NO. 15
Anode Preparation
1. Function - The green anodes to be used in prebake cells are delivered to
furnaces in which they are baked and graphitized to obtain thermal stability
and good electrical conductivity (1). Two types of anode furnaces are used in
the domestic industry, ring-type furnaces and tunnel kilns. At some plants,
the baking process is controlled automatically by computer.
In the ring-type furnace, anodes are packed into sunken pits with sur-
rounding interconnecting flues, with a blanket of coke or anthracite coal in
the space between the anode blocks and the walls of the pits. A layer of 25
to 30 centimeters of coke covers the anodes. The pits are fired with movable
manifold burners for about 40 hours. The flue system of the ring furnace is
arranged so that hot gases from the pits are drawn through the next section of
pits to preheat the anodes. The complete cycle of filling the pits, preheat-
ing, firing, cooling, and removing baked anodes takes approximately 28 days
(2).
The second type of furnace, the tunnel kiln, is a recent development that
is used at several installations. This kiln is an indirect-fired chamber in
which an inert atmosphere is maintained to prevent oxidation of the carbon
anodes. The green blocks pass through an air lock, a preheat zone, a firing
zone, and a cooling zone before being removed through a second air lock. The
gases from baking may be recycled to the fire box to burn the hydrocarbons as
fuel. This kiln has mechanical problems in design and operation, but has the
advantages of a shorter, more uniform baking cycle and reduced space require-
ments (2).
When thoroughly cooled, baked anodes are air blasted with fine coke or
brushed to remove fines and adhering material. They are then transferred to a
rodding room where rod yoke assemblies are connected. This part of the pro-
cess requires cementing material, usually molten iron. The rods, made of
steel and/or aluminum, serve both to support the anode in the cell and to
conduct electricity. The anode assemblies are usually sprayed with molten
aluminum to prevent unnecessary oxidation while in the pot (2).
2. Input Materials - The inputs to the baking process consist of pressed
green anodes and coke or anthracite coal. Rod yoke assemblies, cementing
material, and metallic aluminum are also used.
3. Operating Conditions - The baking furnaces operate at approximately
1200%C and atmospheric pressure. Rodding is conducted at ambient temperature
and atmospheric pressure, with the exception of the high temperature used to
melt the cementing material.
4. Utilities - The anode furnaces are fired by natural gas or oil; electric-
ity is used for operation of mechanical equipment. Fuel consumption is
81
-------
approximately 600,000 kilocalories per metric ton of aluminum product (3). No
data are available on specific electrical consumption. Compressed air is used
to clean the baked anodes. Gas-fired furnaces hold the aluminum used to coat
the anodes.
5. Waste Streams - Bake plant air emissions include products of fuel combus-
tion ,~EunTed~lmd~~u"nburned hydrocarbons from the heating and carbonizing of the
paste binder pitch, sulfur oxides from the carbon paste, fluoride from re-
cycled anode butts, and fine coke dust and other solids. Tar vapors are
formed from cracking, distillation, and oxidation of the pitch; they are
composed essentially of high boiling organic compounds (1). Composition of
SO? in anode bake plant waste gases has been reported to be 5 to 47 parts per
million (4). Table 2-28 indicates the range of pollutants generated (2). The
composition of the furnace off-gas varies to a high degree depending upon the .
baking procedure.
There are no liquid or solid waste streams from this process. All coke
or anthracite packing is recycled.
6. Control Technology - Some smelters rely on controlled firing to minimize
atmospheric emissions from anode baking and have no specific emission control
equipment. In other plants, spray towers are used to precool the gas stream
from the furnace and various control devices are then employed. Table 2-29
summarizes equipment in use in 1974; the bake plants at smelters representing
half the industry's capacity were uncontrolled at that time. Low-pressure-
drop wet scrubbers were once the most common primary control, and their bleed
streams contained tars, oils, and usually fluorides. Electrostatic precipita-
tors are generally not adequate to control fluoride emissions, although they
are still used at some plants. The use of electrostatic precipitators is
further limited because arcing can cause burning of tars and oils. Standard
baghouses are often unsuitable because they may be blinded by the tars and
oils. A few smelters have recently installed dry scrubbing systems using
alumina injection to remove tars prior to passing the gas through baghouses.
At least one plant uses calcined fluid petroleum coke in a dry scrubbing
system. Wet electrostatic precipitators and venturi scrubbers are also em-
ployed at a few plants. Table 2-30 presents estimated efficiencies for
various control devices and Table 2-31 presents emission factors for various
controls.
7. EPA Source Classification Code - 3-03-001-05.
8. References -
1. Development Document for Effluent Limitations Guidelines and New
Source Performance Standards for the Primary Aluminum Smelting
Subcategory of the Aluminum Segment of the Nonferrous Metals Manu-
facturing Point Source Category. EPA 440/1-74-019d. U.S. Environ-
mental Protection Agency, Washington, D.C., March 1974.
82
-------
2. Singmaster and Breyer. Air Pollution Control in the Primary Alu-
minum Industry, Vol. I. New York, July 23, 1973.
3. Environmental Considerations of Selected Energy Conserving Manu-
facturing Process Options, Vol. VIII, Alumina/Aluminum Industry
Report. EPA-600/7-76-034h. U.S. Environmental Protection Agency,
Cincinnati, Ohio, December 1976.
4. Energy Penalty of the Nonferrous Metals Industry. Part II: Primary
Smelting of Aluminum (Draft). Prepared by Arthur D. Little, Inc.
for U.S. Environmental Protection Agency. EPA Contract No. 68-01-
4381, Work Area No. II. August 1977.
5. Background Information for Standards of Performance: Primary
Aluminum Industry. Vol. 1: Proposed Standards. EPA 450/
2-74-020a. U.S. Environmental Protection Agency. Research Triangle
Park, North Carolina, October 1974.
83
-------
TABLE 2-28. ANODE BAKING RING FURNACE EMISSIONS3 (2)
Pollutant
Total solids
Hydrocarbons
Total F
Sulfur
Emissions
1.0 -
0.25 -
0.15 -
0.35 -
(g/kg
5.0
0.75
0.75
1.0
AD
For tunnel kiln baking, emission levels may be reduced
by factors of 0.01 in total solids and 0.02 in hydro-
carbon, fluorine, and sulfur.
84
-------
Table 2-29. CONTROL DEVICES ON ANODE BAKE PLANTS - 1974 (4)
(percent)
Dry scrubber
Dry ESP
Wet ESP
Venturi
scrubber
None
Center-
worked potlines
22
11
-
5
62
Side-
worked potlines
-
5
62
12
21
Total
17
14
14
7
52
85
-------
Table 2-30. AIR POLLUTION CONTROL REMOVAL EFFICIENCIES
ANODE BAKE FURNACE (2)
(percent)
In current use
Reported and vendor
estimated efficiencies
Dry electrostatic precipitator
Spray tower
Self-induced spray
Applicable
Incinerators
Wet electrostatic precipitator
Particulate HF
90
NA 96
98 96
Estimated efficiencies
Particulate
90
99
HF
Table 2-31. EMISSION FACTORS FOR CONTROL DEVICES ON
ANODE BAKING (6)
(kg/metric ton of molten aluminum product) (5)
Control device
Spray tower3
Dry electrostatic
precipitator
Self-induced spray
Total .
particulates
NAC
0.57
0.03
Gaseous
fluorides
0.0186
0.47
0.0186
Particulate
fluorides
Neg.d
Neg.
Neg.
Controlled emission factors are based on average observed collection
efficiencies.
Includes particulate fluorides.
No information available.
Negligible.
86
-------
PRIMARY ALUMINUM PRODUCTION PROCESS NO. 16
Anode Cleaning
1. Function - The spent anode butts from prebake cells are cleaned before
their carbon value is recovered. In the same rodding room used in anode
preparation (Process No. 15), the thimbles forming the connection between the
anode blocks and the current-carrying rod supports are cracked off and the rod
stubs are cleaned by grit blasting. The cleaned butts are then sent to anode
paste preparation (Process No. 14) (1).
2. Input Materials - Input materials consist of anode butts and grit.
3. Operating Conditions - All operations are carried out at ambient tempera-
ture and atmospheric pressure.
4. Utilities - Electricity is used for conveying and general equipment
operation. Compressed air is used as a carrying agent for grit blasting.
5. Waste Streams - Emissions to the atmosphere are shot-blast dust, aluminum
oxide, cryolite, and possibly other fluorides. About 5 kilograms of dust,
containing 0.5 kilograms of fluorides, is generated per metric ton of aluminum
product. Solubility tests have indicated that these dusts do not leach (2).
No liquid wastes are directly generated by this process.
Contaminated and worn-out grit constitutes a solid waste from this pro-
cess; it probably contains fluoride compounds.
6. Control Technology - Atmospheric emissions are controlled with fabric
filters and the collected dust and grit is discarded without specific control.
Transfer of the material to cryolite recovery is possible, but this treatment
has not been reported.
7. EPA Source Classification Code - None.
8. References -
1. Singmaster and Breyer. Air Pollution Control in the Primary Alu-
minum Industry, Vol. I. New York, July 23, 1973.
2. Assessment of Industrial Waste Practices in the Metal Smelting and
Refining Industry. Vol. II, Primary and Secondary Nonferrous
Smelting and Refining. Prepared by Calspan Corporation for U.S.
Environmental Protection Agency. EPA Contract No. 68-01-2604.
1977.
87
-------
PRIMARY ALUMINUM PRODUCTION PROCESS NO. 17
Cryolite Recovery
1. Function - Many smelter wastes contain recoverable quantities of fluoride
that can be reclaimed in a cryolite recovery plant. Cryolite (N33A1F5) is
the form in which the fluoride is recovered because it can then be returned to
the reduction cells. Bleed liquor from wet scrubbers, solids from wastewater
clarifiers, wash water from the cleaning of discarded potliners, and con-
taminated alumina pot insulation can all be treated by this process (1).
The treatment consists of reacting sodium aluminate with soluble fluo-
rides. The relatively insoluble double salt, cryolite, precipitates and is
recovered by a variety of methods. Several modifications of the process are
in use, differing in both equipment and procedures. An excess of sodium
aluminate is used in one approach, forming a precipitate that is a mixture of
cryolite and aluminum hydroxide. These solids are separated from the water
and calcined, and are then fed to selected electrolytic cells (bath cells) for
further purification. Excess aluminum containing the impurites is recovered
as the metal and the cryolite is tapped from the surface of the bath (1,2).
In other modifications, close chemical control is utilized and the precipitate
that forms is only cryolite. This precipitation takes place in an alkaline
solution to prevent aluminum hydroxide formation. With close control of pH
and the use of carbon dioxide for acidification, cryolite is obtained which is
sufficiently pure for direct use in the electrolytic cells without the use of
intermediate bath cells (1,2).
Many of the process modifications are designed both to recover fluorides
and to salvage contaminated alumina. In most instances, the sodium aluminate
used is obtained by digesting the alumina adhering to potliners or present in
pot insulation.
2. Input Materials - Potline scrubber liquor, alumina insulation, and
reduction cell liners are the main inputs to this process. Additional mate-
rials with an alumina or fluoride content, such as floor sweepings or spills,
may be processed at some smelters. Other input materials include sodium
aluminate, caustic soda, lime, and carbon dioxide.
3. Operating Conditions - Roasters and dryers operate at elevated tempera-
ture and atmospheric pressure while digesters operate at elevated temperature
and pressure. All other processes operate at ambient temperature and atmo-
spheric pressure. Specific operating conditions have not been reported.
4. Utilities - Natural gas is used to fire the roasters or dryers, although
facilities are probably provided for conversion to fuel oil. Steam is added
to the digester to supply heat and pressure, and water is used for soaking the
pot liners and washing the shells. Electricity is used for such operations as
crushing, grinding, conveying, and pump operation.
5. Waste Streams - Very little information has been released on the specific
details of the various process modifications or on their waste streams.
88
-------
Possible emissions of atmospheric pollutants include fuaitive losses from dry
materials handling and crushing and grinding operations. Products of combus-
tion are present in dryer or roaster off-gases.
Liquid wastes include water which has contacted the spent pot lining
during removal from the iron containers and rainwater runoff from the storage
yard. Either may contain fluorides, and storage yard runoff may also contain
cyanides. Following cryolite recovery, the scrubber liquor bleed stream can
contain up to 2 grams of fluorides per liter.
The only likely solid wastes generated by this process are undissolved
solids and cathode scrap.
6. Control Technology - The types of controls applied to the air emissions
from this process are not known.
The scrubber liquor bleed stream is typically controlled by reaction with
lime to remove additional fluoride as precipitated CaF2. It has been reported
that the fluoride content can be reduced to 60 milligrams per liter (2).
The undissolved solids and cathode scrap are transferred to waste heaps.
Process No. 10 contains a discussion of the problems involved with the storage
of these materials.
7. EPA Source Classification Code - 3-03-001-03.
8. References -
1. Singmaster and Breyer. Air Pollution Control in the Primary Alu-
minum Industry, Vol. I. New York, July 23, 1973.
2. Development Document for Effluent Limitations Guidelines and New
Source Performance Standards for the Primary Aluminum Smelting
Subcategory of the Aluminum Segment of the Nonferrous Metals Manu-
facturing Point Source Category. EPA 440/l-74-019d. U.S. Environ-
mental Protection Agency, Hashington, D.C., March 1974.
89
-------
SECTION 3
ENVIRONMENTAL MANAGEMENT
AIR MANAGEMENT
The discharge of dusts and gases to the atmosphere represents a serious
environmental control problem associated with the production of primary
aluminum. Many of the process operations produce air emissions, and in some
cases the control technologies applied to these sources are potential sources
of secondary wastewater and solid waste streams. Although few air emissions
other than fugitive dusts are created by the operation of a bauxite mine or
refinery, a potential exists for severe air pollution in the electrolytic
smelting process that converts alumina to aluminum metal. In contrast to the
processing of other nonferrous metals, ore constituents are not the source of
air pollution, since all impurities are removed from bauxite ore at the
refinery. The objectionable air pollutants are processing chemicals used at
the smelter.
Emission Sources
Table 3-1 lists the relatively minor air pollution control problems
associated with the mining and refining of bauxite. Mining activities in
the United States are limited, and fugitive emissions can be controlled by
water sprays. Proper blast design can partially reduce emissions, and open
pit mines provide sufficient natural ventilation to prevent dust conditions
from becoming unbearable.
The major source of atmospheric emissions at bauxite refineries is the
kiln in which aluminum trihydrate is calcined to produce alumina. In most
refineries, the drying process is not used, and only small amounts of dust
escape from ore grinding (Process No. 4). Only the few refineries that use
domestic ore employ the pyrometallurgical operation outlined in Process No.
6, and the escape of spray from the precipitation tanks (Process No. 7) is
minimal. Most refinery processes are chemical in natural and process materi-
als are completely enclosed. It is thus only during alumina calcination
(Process No. 9), in which large quantities of water are evaporated from a
powdered alumina product, that a potential exists for significant air pollu-
tion. However, the value of the product is so great that extensive controls
are employed to reduce emissions to only small quantities. The competitive
economics of the aluminum industry insures that continuous or large emissions
of alumina will not occur. Some loss, however, is evidenced by the quanti-
ties of inert, insoluble, white alumina powder in or near most bauxite refin-
eries.
90
-------
Table 3-1. ATMOSPHERIC EMISSIONS FROM BAUXITE MINES AND REFINERIES
Process
Emission
Characteristics
Control
1. Mining
2. Ore treatment
3. Bauxite drying
4. Grinding, diges-
tion, and heat
recovery
5. Liquor/mud separa-
tion
6. Waste alumina re-
covery
7. Al(OH).. precipita-
tion
8. Spent liquor re-
covery
9. Calcination
Participate from blasting and materials
handling (0.25 kg/metric ton ore [surface],
0.05 kg/metric ton ore [underground])
Particulate from crushing and conveying
(0.5-0.4 kg/metric ton ore)
Combustion gases from kilns
Particulate from grinder
None
Combustion gases from kilns
Possible fugitive particulate from
agitation in precipitators
None
Water vapor (1 kg/kg Al.O. product) and
combustion gases from kilns
Composition same as ore, with some nitrogen
compounds from explosives
Composition same as ore
Contain particulate of same composition as
ore; some organics
Composition same as ore
Contain water vapor and particulates of lime
and red muds
A1(OH)3, NaOH
Contain particulate Al,0, (1 kg/10 kg Al,0,
product) ' J * J
Water sprays
Baghouse
Unknown; baghouse or ESP's
suitable
Scrubbers; baghouses or
ESP's also suitable
Unknown; baghouses, ESP's
or scrubbers suitable
None
Multicyclones followed by
ESP or baghouse
-------
Air management is of most importance at aluminum smelters. As shown in
Table 3-2, every process in use at smelters releases air pollutants that may
include acidic gases and particulate matter containing compounds of fluoride.
The electrolytic cell itself is the most important source, in terms of quan-
tity of emissions, potential for damage, and complexity of control. Sulfur
oxides are also generated from the consumption of anodes containing sulfurous
petroleum coke and pitch. Except for S02, which is being evolved in small
but increasing quantities, the gaseous and particulate emissions are not
waste products, and there is an economic incentive to return all of these
materials to the processing equipment. The objectives of air management are
thus (1) to minimize emissions and (2) to capture escaped material and
return it to process. The objective is not disposal of unwanted material as
is the case with most other nonferrous smelting operations.
Potroom Air Management
The quantity and composition of atmospheric emissions from aluminum
reduction cells vary widely, both among different plants and over time for
an individual plant. Operating conditions such as temperature, bath ratio,
frequency of anode effects, and method of crust breaking all affect the
nature of these emissions. The three types of reduction cells in use at
domestic smelters -- prebake, horizontal stud Soderberg, and vertical stud
Soderberg -- differ slightly in the nature of their emissions and the types
of control strategy necessary. However, effective control of emissions from
all pot types requires three independent approaches:
0 Minimizing the production of atmospheric emissions;
0 ' Primary capture of the remaining emissions that do occur;
0 Secondary capture of the residual emissions that escape the primary
system.
These efforts are generally implemented separately, with the principal objec-
tive being achievement of minimum loss of fluorides. The basic assumption is
that efficient fluoride capture will result in effective control of dis-
charges of other materials.
Emission prevention, the most effective control strategy, requires
careful control of cell operating conditions. The phenomenon of anode
effects, which occurs when the alumina concentration in the electrolyte
drops, greatly increases fluorine emissions. At one time, cell operators
typically allowed anode effects to continue uncontrolled to help soften a
hard crust on the bath surface. This not only increased emissions, but also
wasted energy, and the practice has been largely discontinued. The automatic
crust breakers in use at many smelters can break through hard crusts, and a
carefully planned schedule of alumina charging can prevent the alumina con-
centration from falling to a point at which an anode effect begins.-
Improved process control is also effective in reducing emissions.
Operation of the cell at the lowest possible temperature needed to sustain
the reaction is important. Instrumentation can also be used to inform an
92
-------
Table 3-2. ATMOSPHERIC EMISSIONS FROM PRIMARY ALUMINUM SMELTERS
Process
Emission
Characteristics
Control
VO
CO
10. Prebake cells
11. HSS cells
12. VSS cells
13. Casting
14. Paste preparation
15. Anode preparation
16. Anode cleaning
17. Cryolite recovery
Mixed gas stream from cell
Mixed gas stream from cell
Mixed gas stream from cell
Combustion gas from flux furnace and
fugitive particulate from casting
and pouring
Coke dust from materials handling (5
kg/metric ton Al product); particulate
fluoride from anode grinding and mix-
ing; HC fume from mixing
Combustion gases from furnace
Particulate matter from air blasting
(5 kg/metric ton Al product)
Combustion gases from dryer and roaster;
fugitive particulate from materials
handling, crushing and grinding
Gas is primarily CO?, but also CO, SO?, HzS,
COS, CS2, S1F4, HF, H20, CF4, CpFg; contains
particulate Al?03, C, Na3AlF6, A1F3, CaF2,
NasAl3F4, Fe?03; total particulate fluoride
is 10.2 kg/metric ton product, gaseous
fluorides are 12.4 kg/metric ton Al product;
SOX are 15-50 kg/metric ton product
Gas composition similar to PB cell with addi-
tional HC and SO?; contains particulate of
composition similar to PB cell; gaseous
fluorides are 12.6-14.4 kg/metric ton Al pro-
duct; SO? concentration is 80 ppm
Gas composition similar to HSS cell with lower
HC content; contains particulate of composition
similar to PB cell; gaseous fluorides are 10.0-
17.4 kg/metric ton Al product; SO? concentration
is 80 ppm
Gas contains Inert carriers, free chlorine,
HC1, fluorine, and particulate
Contains HC, SO? (5-47 ppm), S03, fluorides,
other particulate matter, tar vapors
Contains shot-blast dust, Al-0,, Na-AlF,,
other fluorides(0.5 kg/metrit ton Al pr&duct)
Particulate (fugitive and in gas) contains
Al?03, Na3AlF6, other fluorides
Careful control of all operating
conditions essential; control
may be either primary (dry
alumina adsorption, multi-
cyclones/ESP/scrubber) second-
ary (roof monitor with scrub-
ber), or both
Careful control of cell oper-
ating conditions essential;
primary control only (dry
alumina adsorption, wet ESP,
scrubber)
Careful control of cell oper-
ating conditions essential; pri-
mary control (ESP/scrubber, dry
alumina adsorption, multicy-
clones/scrubber or ESP), with
some use of secondary control
(roof monitors with scrubbers)
Flux gas controlled by scrub-
bers, wet ESP's, Venturis,
coated baghouses (bleed streams
contain chloride salts and
hypochlorites); few control cast-
ing emissions (baghouses, ESP's
suitable)
Baghouses typical; also ESP's,
multicyclones, scrubbers, dry
alumina adsorption
Controlled firing; spray towers
with dry alumina adsorption, ESP,
wet ESP, or venturi (bleed
streams contain tars, oils, and
fluorides)
Baghouses
Unknown (dry alumina adsorption,
scrubber suitable)
-------
operator when alumina addition is needed, or to initiate the action if auto-
mated charging equipment is provided. In smelters in the United States, there
is an increasing use of digital computers. A single computer can monitor many
individual pots and provide logic calculations to maintain optimum temperature
and least power consumption. The computer-controlled potlines that have been
installed at some smelters operate almost free from anode effects. A fully
automated potline has not been demonstrated, but various degrees of computer
control in combination with mechanization have been developed. Alcoa's P-225
technology, which utilizes self-adjusting anodes, each under the control of a
process computer, is said to allow the recovery of 99 percent of the fluorides
generated by reduction. In addition to environmental considerations, automated
control systems can be profitable to the operating company. The power consump-
tion of a smelter can be reduced and the working life of a pot can be extended
by proper operation of the potline.
Effective control of potline emissions requires efficient collection of
the gas stream that evolves from the cells, so the second line of defense in
potroom air management is the primary hooding and collection system. All
aluminum smelters, except a few prebake plants, utilize primary control
systems; in some control schemes, particulate is initially segregated from the
gaseous component, while others remove acidic gases and dust concurrently.
Either wet scrubbers or dry alumina adsorption systems are used. Most smel-
ters with dry scrubbers tend toward the use of a "100-percent feed," in which
all the alumina input to the pots is first routed through the air pollution
control system. The control devices must operate with a fairly high pressure
drop, especially with dry systems. Operating costs of these units are in
proportion to the volume of gas handled rather than the volume of pollutants
captured. Best efficiency of overall pollution control and lowest operating
cost will therefore be obtained if the maximum quantity of pollutants is
captured by the primary system.
There are two general types of primary hooding systems employed on the
pots at domestic aluminum smelters. Vertical stud Soderberg cells do not
require side channel openings for pin replacement, so a primary hood may be
sealed to the surface of the electrolyte bath with a blanket of alumina. This
arrangement allows efficient collection of emissions within the diameter of
the hood as long as the blanket is unbroken. The hood, however, covers only
part of the bath surface, allowing only about 50 percent of the particulate
matter generated to be collected. In addition, the seal must be broken
during cell working, allowing additional gaseous and particulate emissions to
escape the primary control system into the potroom atmosphere. The second
type of primary collection system is found on prebake and HSS cells. The
design of these pots permits hooding, but the necessity of opening the hood
for metal tapping and (at prebake cells) for anode replacement results in
considerable dilution of the emissions. Crust breaking and charging operations
at prebake cells may be conducted at either the center or the side of the pot,
with side breaking less amenable to tight hooding. The design of the hoods on
newer prebake potline permits the area exposed during working operations to be
minimized, allowing a proportionately higher capture gas velocity and a
collection efficiency of 95 percent.
94
-------
Adequate maintenance of hood ductwork is necessary to seal the holes and
stop the unnecessary leaks that can prevent effective capture of cell emis-
sions. Dampers (if supplied) must be properly adjusted to maintain equal air
flows through each hood. Where dampers are not supplied, plate restrictions
may be needed to obtain accurate air distribution. Some smelter operators
have been able to justify the installation of automatic dampers to each pot
hood to double the rate of air flow during periods of cell working, alumina
charging, or anode replacement. Collection efficiencies of as high as 97 to
99 percent have been reported. Special hood extensions may have to be added
to some pots located near doorways or ventilation equipment to prevent drafts
that can cause pot emissions to escape the primary control system.
The final type of control system that may be employed is a secondary
collection system utilizing roof monitors on the tops of the potline buildings.
At some smelters where the hoods on the pots are not efficient, the secondary
emissions can be several times as large as those passing through the primary
system. In addition, at those prebake plants that do not have primary hood-
ing systems on their potlines, all pot emissions and room ventilation air
pass through a secondary control system.
A secondary control system may be either a wet scrubbing unit or a
fabric filter, depending on the design of the smelter and the efficiency with
which emission prevention and primary collection are applied. Low pressure
drop control devices are used to minimize fan motor power consumption because
of the large gas volumes handled. The pollutants removed from the gas require
additional treatment or handling, so the operating cost per kilogram of
pollutant captured is probably greater than would be found with an efficient
primary system.
Potline buildings have openings in the sides or floors to allow ventila-
tion air to sweep across the pots and exit through the roof monitors. This
design involves the possibility that air may leave the building through the
ventilation openings, reducing the collection efficiency of the secondary
system. The best management strategy is a tight building with the air inlet
louvers arranged so that drafts are not created by winds. Secondary fans can
be adjusted to avoid over-ventilation of the building; the air flow rate
should be sufficient to maintain a minimum quantity of air. Supplemental
fans may be necessary to provide even ventilation of all areas of the build-
ing.
Secondary air control systems may become increasingly important if the
trend continues toward an increase in sulfur content of the petroleum coke
and pitch used in anodes. Although smelters have traditionally used coke
with a sulfur content of about 2 percent, they may be forced to go to coke
containing as much as 5 percent sulfur within the next decade. Dry adsorption
primary systems are slightly less efficient for S02 control than wet scrub-
bers; in addition, they provide no permanent reduction in S02 content since
the captured sulfur value is re-released when the alumina from the scrubber
is fed to the electrolytic cell. Part of the primary system off-gas may
therefore require wet chemical scrubbing to obtain adequate S02 reduction.
Provisions to add S02 removal might be desirable in the design of any new
secondary scrubbing systems.
95
-------
A discussion of the specific control equipment used to remove pollutants
from aluminum potline emissions is provided in the Control Technology section
of Process No. 10.
Auxiliary Planet Air Management
At most aluminum smelters, the facilities to prepare carbon paste and
(at prebake plants) to clean and reclaim anode butts and manufacture new
anodes are separate; they are not connected to the main potline operations.
These auxiliary facilities are not continuous process plants, but are instead
comparable to production industries in which manual or automated machinery is
used to fabricate mechanical parts. The paste preparation plant involves
such operations as crushing, screening, calcining, grinding, and mixing.
Many of the process operations at the auxiliary facilities generate quanti-
ties of fugitive dust, usually containing carbon and fluoride compounds. A
few operations release smoke and hydrocarbon fumes, but not continuously or
at a uniform rate. The anode baking furnace is the only process that operates
in a relatively continuous fashion.
Two principal control strategies are employed for air emissions from the
auxiliary plants. The hot combustion gas from the anode baking furnace at
prebake plants is usually controlled as an independent stream, as described
in the Control Technology section of Process No. 15. The remaining pollu-
tants are captured by fans and hoods and are usually vented to the atmosphere
through separate control devices. Ventilation and combustion gas streams
receive combined treatment at a few plants.
Hot combustion gas streams from other auxiliary operations are either
mixed into the bake plant exhaust gases or vented to the atmosphere without
control. Indirect gas-fired aluminum holding furnaces can usually be directly
vented, whereas furnaces used to hold rodding material usually require emis-
sion control. A variety of control devices are used on degassing operations,
but few smelters control the emissions from casting operations.
Dust from specific plant sources can be minimized by use of enclosed
installations. Sandblast areas should be enclosed and provided with a sepa-
rate air recirculation system through a fabric filter. To reduce dust emis-
sions, at least one smelter transports all paste preparation coke in covered
conveyors.
Other specific sources of atmospheric emissions require hooding for
effective pollutant capture. The design of hoods to capture fugitive emis-
sions requires an integrated engineering study rather than a piece-by-piece
approach. Experience has shown that cross-currents of air past a hood can
seriously reduce its efficiency; hood design is thus not complete unless
provision is made for control of all air movements in the vicinity.
WATER MANAGEMENT
In addition to being the principal source of atmospheric emissions, the
smelter is also the most important source of polluted water discharges in the
primary aluminum industry. Bauxite mining activities in the United States
96
-------
are very limited, and conventional treatment is provided. A bauxite refinery
is a hydrometallurgical operation that recirculates large volumes of water.
A smelter uses very little direct process water; however, auxiliary water
applications can be heavily polluted, and secondary pollution from escaped
solids and gases can also be substantial. Table 3-3 summarizes the sources
of direct water wastes at all stages of primary aluminum processing.
Mine and Refinery Water Management
Two adjacent open pit mines in Arkansas have been responsible for all
the domestic bauxite production used for aluminum in recent years. The only
ore beneficiation operations at either mine are size reduction, crushing, and
grinding, so the only water use that occurs is for dust suppression and
utilities. Because the pits may cut through natural aquifers and the average
annual rainfall exceeds evaporation, mine dewatering is required to maintain
working condition in the lower level of the pits. The runoff from mines,
spoils storage areas, and other disturbed areas is characteristically acid.
Lime neutralization and precipitation are provided at the mines and, under
proper operating conditions, the acid discharge to the receiving stream may
be negligible. However, the treated water still contains some dissolved
constituents, primarily sulfates, and it is not suitable for use as makeup
water in the adjacent refineries. Not all wastewater discharges from the
mining areas are treated; at times, clearing and stripping operations or
potentially active mines may be located some distance from the central treat-
ment plant. The best management approach is demonstrated by one of the
mines, which has a mobile lime treatment plant to process quantities of
wastewater.
Zero discharge of water is approached by a few domestic bauxite refiner-
ies, even though all of these plants are located in regions of abundant
rainfall. During normal operation, a bauxite refinery has a negative water
balance, with less water entering from all sources (including normal rainfall)
than is discharged as vapor during the calcination of alumina. Closed-loop
recycle of water permits the salvage of almost all the caustic used in pro-
cessing and allows the mud lake to serve as one stage of mud washing. In
closed-loop operations, contaminating impurities are removed in special
"salting-out" evaporators, as described in Process No. 8.
A refinery water management program to achieve total recycle requires
individual study of each potential source of water discharge. Other than the
discharge of red mud slurry, the only large continuous stream is the water
from the barometric condensers of the caustic evaporators. At most plants
this water is recycled, sometimes after being cooled in an evaporation pond
or cooling tower. It can often be used for a specific purpose, such as one
stage of washing. Recycle has the advantage of directly salvaging any caustic
carryover.
The smaller waste streams at a refinery are less amenable to total
recycle. Cooling tower blow-down often contains algicides that prohibit its
introduction into process water, and boiler blow-down usually includes seques-
terant chemicals that will interfere with alumina precipitation. No practi-
cable method has been developed to treat blow-down waters, other than reverse
97
-------
Table 3-3. WASTEWATER DISCHARGES FROM PRIMARY ALUMINUM PRODUCTION
Process
1. Mining
2. Ore treatment
3. Bauxite drying
4. Grinding, digestion and
heat recovery
5. Liquor/mud separation
6. Waste alumina recovery
7. A1(OH)3 precipitation
8. Spent liquor recovery
9. Calcination
10. Prebake cells
11. HSS cells
12. VSS cells
13. Casting
14. Paste preparation
15. Anode preparation
16. Anode cleaning
17. Cryolite recovery
Discharge
Hinewater
None
None
None
None
None
None
Condenser cooling water
None
None direct; bleed stream
if wet scrubbers used
Same as prebake cells
Same as prebake cells
Direct-contact cooling
water
None
None
None
Pot washwater and rain-
fall runoff
Characteristics
Acid or alkaline depending on rock/soil type
and mining technique (usually acid)
May contain caustic or Al-0,
Contains soluble fluorides
Same as prebake cells
Same as prebake cells
Contains dissolved and suspended solids, oil,
and grease
Both contain fluorides; runoff also con-
tains cyanides
Treatment
Lime neutralization and impoundment
Reduced discharge by careful process
trol ; ultimate disposal to streams or
lake
Sent to cryolite recovery plant or
treated with lime (solids landfilled)
calcium fluoride (solids recycled to
line or landfilled)
Same as prebake cells
Same as prebake cells
con-
mud
or
pot-
Either closed-loop system with cooling
tower or discharge to plant wastewater
system
Unknown
CO
-------
osmosis followed by evaporation to dryness. The best disposal method is
apparently dilution; the best control strategy is to minimize the quantity and
pollutant concentration of these streams. Sanitary wastes are not considered
process waste streams, and they are often sent to local or municipal sewage
treatment facilities. Most other liquid waste streams, including leaks,
spills, plant washdown water, water softening slurries, and acid cleaning
wastes, are mixed into the mud waste and discharged to a mud lake. The water
is recycled after settling.
A refinery water management strategy must also consider rainfall on the
plant grounds, which can wash significant amounts of pollutants into neigh-
boring watercourses. Although a uniform policy does not exist, most refiner-
ies include facilities to collect and recycle the water that falls on con-
taminated portions of the plant during small rainstorms. However, some
bauxite refineries are located in a region where rainfall can be as much as 15
centimeters in less than 3 hours, so it is not technically feasible to capture
all the rainfall from the larger storms.
When total recycle is not practiced, the wastewater discharged from a
bauxite refinery contains turbidity and soidum salts; it also usually has a
high pH. Turbidity can be reduced by clarification and filtration, and the pH
corrected by neutralization with acid. However, there is no technology that
can be used to remove salts such as sodium sulfate or chloride, other than
total evaporation. Refinery discharges may also be too hot for environmen-
tally acceptable discharge to some streams.
Smelter Hater Management
The only direct process use of water in an aluminum smelter is for
cooling of the product ingots; however, many auxiliary uses of water
produce heavily contaminated waste streams. Smelter wastewater may contain
hydrocarbon oils and waxes, inert turbidity from alumina and carbon, and both
soluble and insoluble fluoride compounds.
The handling of wastewater at an aluminum smelter is not water manage-
ment as applied at a bauxite refinery, but rather an operational process to
provide adequate treatment to a number of differing waste streams. Manage-
ment consists of the appropriate recycle of the treated wastes. A large
fraction of the treated water can be recycled for makeup to scrubbers and
the cast house cooling system. If rainfall is also treated, there will
usually be an excess of water that will prevent the accumulation of excessive
sulfates and chlorides. Fresh water addition may be necessary at times,
however, to maintain an adequate quantity of discharge. There are five
principal wastewater sources at a smelter, each of which has distinctive
characteristics requiring.different degrees of treatment.
The cooling water from the cast house contains alumina turbidity and
hydrocarbon grease; however, its fluoride content is lower than some of the
smelter wastewater streams. It is often handled in a separate recycle system
that includes a cooling tower to remove excess heat. A bleed stream is
removed for treatment, which can consist of biological oxidation of the
hydrocarbons in an aerated lagoon, followed by pH adjustment and settling.
99
-------
The treated water is then either recycled or discarded.
The effluent from an anode bake plant scrubbing unit is heavily contami-
nated with oils and resins in addition to large quantities of carbon. This
wastewater stream is usually passed through a skimming chamber to remove much
of the oil. It can then be combined with the cast house bleed stream for
biological treatment, pH adjustment, and settling.
The overflow from a potline wet scrubbing system is acidic and heavily
loaded with aluminum and fluoride compounds. The most economical treatment
method for this stream is a cryolite recovery system (Process No. 17). As an
alternative, this wastewater can be treated with either lime, calcium chloride,
aluminum sulfate, or bone meal to precipitate the fluorides; after settling,
the water is recycled or discarded.
Plant drainwater and surface runoff from rainfall constitute another
major source of wastewater. It is assumed that any stream that has contacted
cryolite or potroom solids contains sufficient dissolved fluorides to require
treatment prior to discharge. Although cryolite is relatively insoluble,
contact of rainwater with finely-divided cryolite can produce concentrations
of fluoride ion as high as 200 parts per million.
A final wastewater stream arises from water leaching from discarded pot-
liners, which may contain dissolved cyanides. Treatment with chlorine
or ozone is usually required in addition to treatment for fluoride removal.
SOLID WASTE MANAGEMENT
The primary aluminum industry generates considerably less solid waste in
relation to product metal tonnage than is characteristic of other nonferrous
metals industries. Aluminum is such a plentiful element in the rocks of the
earth that only bauxite, the most concentrated of aluminum mineral deposits,
can be processed economically. The industry therefore discharges a relatively
small quantity of waste rock; only about a ton of solid waste is generated
during the production of one ton of aluminum, compared to about 30 tons of
waste for a ton of zinc and more than 100 tons for a ton of copper. However,
the absolute quantity of waste solids can be very large near refineries
because of the large production volumes.
Sources and Characteristics
As shown in Table 3-4, there are three types of solid wastes generated
by the primary production of aluminum. Bauxite mining results in general
disruption of the land and accumulation of overburden and waste spoil. The
excess minerals in the bauxite, which are separated at the refinery, are
discarded as a mud. The aluminum smelter also generates solid wastes, princi-
pally scrap man-made objects.
The United States contains no high-grade bauxite ores, and only the
central Arkansas deposit is marginally competitive with imported ore.
Reserves are limited, and mining wastes are thus restricted to only this one
100
-------
Table 3-4. SOLID WASTES FROM PRIMARY ALUMINUM PRODUCTION
Process
Waste
Characteristics
Disposal
1. Mining
2. Ore treatment
3. Bauxite drying
4. Grinding, digestion and
heat recovery
5. Liquor/mud separation
6. Waste alumina recovery
7. A1(OH)3 precipitation
8. Spent liquor recovery
9. Calcination
10. Prebake cells
11. HSS cells
12. VSS cells
13. Casting
14. Paste preparation
IS. Anode preparation
16. Anode cleaning
17. Cryolite recovery
Overburden and waste rock
None
None
None
"Red mud" (0.3-2.5 kg/kg Al-0,
product) J
"Brown mud"
None
Crystals from salting-out evaporator
None
Spent anode butts and potliners
Spent potliners
Spent potliners
Dross skimmed from molten aluminum
None
None
Contaminated grit
Undissolved sol Ids and cathode scrap
Landfill and recontour of mined-out land
Contains Fe,0,, Al.O,, S10-, T10-,
CaO. Na20 i 3 ' J i Z
Composition similar to portland
cement
Calcium and sodium sulfate and
oxalate
Potliners contain carbon, aluminum,
aluminum nitrides, cyanides, and
sodium and aluminum fluorides
Same as prebake cells
Same as prebake cells
Contains water-soluble salts
Contains carbon and fluorides
Contains carbon, aluminum, aluminum
nitrides, cyanides, and sodium and
aluminum fluorides
Discharged as slurry for impoundment tn
diked lake
Discharged as slurry for impoundment in
diked lake
Discharge to mud lake or landfill
Butts to anode preparation plant and pot-
liners to cryolite recovery or to waste
heaps
To cryolite recovery or waste heaps
To cryolite recovery or waste heaps
Landfill
Unknown
Waste heaps
-------
localized area. Overburden and spoil are now used principally to fill pits
from older workings. The entire area has poor, unconsolidated soils, and
drainage from the workings is muddy and occasionally acidic. However, because
of the limited nature of domestic mining, the aluminum industry will not
contribute significantly to the mining waste of this country unless competi-
tive technologies are developed to utilize other aluminum minerals.
Refinery muds constitute the largest tonnage of solid waste from the
primary aluminum industry. This material consists of the minerals present in
bauxite other than aluminum oxide, .principally aluminum silicates and stable
oxides of iron, silica, and titanium. Brown mud from plants processing
domestic ore also contains compounds of calcium. Although the soluble salts
usually leach from refinery muds, the bulk of the solid material is completely
inert and does not decompose or release trace elements into runoff water.
Bauxite itself is the residue of rock exposed to centuries of leaching by
warm, oxygenated water, so all potentially soluble trace elements have long
since leached away. The only unusual material reported from refinery muds is
the oxalate ion, which forms from the reaction of caustic with traces of
organic material in the bauxite. However, there is no indication that the
concentration of oxalates is high enough to constitute an environmental
hazard.
An additional solid waste is generated at some refineries, which consists
of soluble crystals from the "salting-out" evaporator (Process No. 8). This
waste represents an accumulation of minerals, primarily calcium and sodium
sulfate and oxalate, resulting from closed-loop recycle of process water. It
is usually discarded separately.
The solid waste problem at aluminum smelters is quite different from
that at bauxite refineries. Since alumina containing no impurity elements is
the only continuous input to the smelter, solid wastes are not generated as
an integral part of the processing operations. The principal solid wastes
are worn-out mechanical trash such as the butts of partially-used anodes and
the spent potliners and insulation from the rebuilding of electrolytic cells.
These materials are contaminated with processing chemicals, principally
fluorides, and with nitrides and cyanides from side reactions within the
cells.
An aluminum smelter also generates skimmings and drosses from the final
metal cleanup in the potroom and fluoride salts mixed with aluminum metal
and aluminum oxide. Although the quantity of skimmings and dross is rela-
tively small, it represents a substantial disposal problem since many of the
salts will leach into surfaces and ground waters.
An additional solid waste in most smelters consists of fugitive losses
of alumina, usually contaminated with fluoride-bearing substances. This
material originates from sweepings in potrooms and paste preparation plants,
grit from anode sandblasting, and accidental losses from materials handling.
These solids also require control because of their fluoride content.
102
-------
Waste Treatment
Mud waste from all domestic bauxite refineries is now controlled by total
impoundment. The solids are inert and have no current economic value. The
soluble salts leach into water and are either recycled to the process through
water reuse or else contribute to water.pollution from the refinery. Details
of the techniques in use are outlined in the Control Technology section of
Process No. 5. Brown mud from the processing of domestic bauxite is discarded
with similar techniques. Potential commercial uses of the muds would require
that they be dewatered sufficiently to allow the solids to be transported and
stored in a dry form. No process technologies have been developed that would
allow the muds to be dewatered at a cost less than the present cost of im-
poundment. Substantial modifications to the Bayer process would be needed to
produce muds of a coarser particle size that would settle and dewater more
easily.
Management of the worn-out and discarded materials at an aluminum smelter
consists of cleaning the waste of potentially harmful substances, as described
in Process Nos. 16 and 17, followed by either recycling or discarding the
cleaned solids. Alternatively, the articles can simply be discarded. The
waste heaps should be located in an area where all runoff water is collected
and treated; however, one of the principal environmental objections to the
procedures in use at some smelters is that such an area is not provided. If
a suitable drainage area is supplied, other environmental objections are that
contaminated potliners may be stored in the company yard for weeks awaiting
transfer to this area. Occasionally, the collection sump from the drainage
area simply overflows, allowing fluoride and cyanide salts to run into nearby
streams. Potliners may be processed in a cryolite recovery plant, but the
storage area for materials awaiting transfer to this plant may drain into an
open ditch. Anode butts may be stored for some time on an open dock, until
space for them is available in the cleaning room. Clear and definite solid
waste management procedures must be established and implemented to minimize
such unsound practices. The best management programs protect discarded
articles and contaminated alumina from exposure to the weather until effective
treatment is provided. At least one smelter maintains a special weatherproof
building in which contaminated material is stored.
Dross and skimmings are usually held at high temperature for a sufficient
period of time to permit alumina separation. The metal is recycled to the
process, and the remaining dross is either discarded or further processed.
Unless sealed landfills are used, the soluble chlorides and fluorides in these
materials will leach into .ground or surface water. Most of the fluoride can
be recovered as cryolite, as outlined in Process No. 17, either at the smelter
or by an outside processor. Alumina is then salvaged as an insoluble solid
for recycle, and the chloride salts are discarded in a liquid waste stream.
The demand for cryolite is less than the amount that could be produced in the
industry. However, some analysts believe that the value of the reclaimed
fluoride would more than pay for its recovery costs. More effective treatment
methods have not been developed.
103
-------
SECTION 4
EMERGING TECHNOLOGY
The research and development activities that are being conducted by
industry and government into primary aluminum production are taking several
directions. Our dependence on foreign supplies of bauxite has become in-
creasingly undesirable because of continually rising prices and the potential
for embargo. As aluminum is the most abundant metallic element in the earth's
crust, considerable research is therefore being directed toward its recovery
from nonbauxitic domestic materials. However, the use of such resources is
limited by both technical and economic considerations at this time. A second
problem area facing the aluminum industry is the high cost and potential
unavailability of the fossil fuels required for electrolytic reduction. As
the industry is highly energy intensive, research efforts are therefore being
directed toward the development of both alternative energy-saving production
technologies and more efficient energy utilization with present technologies.
The reduction of polluting discharges is the third major area being investi-
gated. Because research into energy conservation and pollution control is
being conducted primarily by the aluminum companies, details of much of this
work is proprietary in nature. This section summarizes certain of the more
promising research and development activities that have been described in the
literature. A brief description of the process and the status of the tech-
nology is given and, where available, data on energy requirements, environ-
mental residuals, and costs are also presented.
NONBAUXITIC SOURCES OF ALUMINUM
A variety of other rocks and minerals are being considered as alternative
sources of aluminum. Potential resources include alunite, aluminous shale and
slate, aluminum phosphate rock, dawsom'te, high-alumina clays, nepheline
syenite, anorthosite, saprolite, coal ash, and aluminum-bearing copper leach
solutions. Some of these materials are already used as sources of aluminum in
other countries, while others have been investigated in considerable detail
but have not been fully developed because of the continued availability of
bauxite. Nonbauxitic sources of aluminum in the U.S. are virtually inex-
haustible and could allow this country to become self-sufficient in aluminum;
however, their development depends largely upon the cost of imported bauxite.
In addition, there are technical problems, including the elimination of iron
and silica, preventing gelation of alkaline slurries, the need for inexpensive
acid-resistant construction materials, and the disposal or utilization of
solid wastes generated during processing (1). From 4 to 7 years would be
required for the development of an industrial-scale process for the production
of aluminum from nonbauxitic sources.
104
-------
The most promising alternative source of aluminum in the U.S. is high-
alumina clay. Kaolin and kaolinitic and refractory clays having alumina
contents of 25 to 35 percent are found in many parts of the country. The most
attractive deposits are those that would permit cheap surface mining on lands
already owned by aluminum companies or lands that could be acquired at com-
paratively low cost. Such areas include deposits associated with bauxite in
Arkansas, the extensive Georgia kaolin belt, and a belt including and ex-
tending northeast and southwest from the Andersonville district in Georgia.
High-alumina clays were evaluated in pilot plants during World War II and
were used as a source of aluminum in Germany and Japan at that time. More
recently, they have been investigated in Poland, the U.S.S.R., South Africa,
and other countries, as well as the United States (1).
The two other nonbauxitic sources of aluminum that have been investi-
gated recently in this country are alunite and anorthosite. Alunite
(KAl3(S04)o(OH)^) commonly occurs in veins, replacement bodies, and in dis-
seminated form in altered volcanic rocks. It contains 37 percent alumina and
is thought to be used as a source of aluminum in the U.S.S.R. and Mexico.
Alunite deposits in Utah have been investigated in recent years (1). Anor-
thosite is an igneous rock composed mostly of plagioclase fledspar containing
23 to 28 percent alumina. Large deposits are located in Wyoming, California,
and New York. The U.S. Bureau of Mines operated a pilot plant in Wyoming
during the 1950's. Another igneous rock, nepheline syenite, is mined for
aluminum production in the U.S.S.R. However, these deposits contain con-
siderably more alumina than the 18 to 20 percent found in domestic nepheline
syenite (1). Research has been carried on by industry and government into
various other nonbauxitic sources of aluminum, but results have not been
favorable.
The Energy Research and Development Administration and Alcoa have re-
cently announced a joint program to investigate the processing of low-grade
ore such as clay or anorthosite using carbothermic energy instead of elec-
tricity (2). An aluminum-silicon alloy would be produced by direct reduction
in a closed system, with by-product carbon monoxide collected as fuel or for
its feedstock value. Further details are not available.
Hydrochloric Acid Extraction/Induced Crystallization (3,4)
This process for producing alumina from clay dates from 1896 and is
currently being examined by the Bureau of Mines. Previous efforts to process
domestic clays failed to find an economical method of removing iron from the
leach liquor and were terminated, but recently developed solvent extraction
technology has eliminated this problem.
Process description—Kaolin clay is calcined for chemical activation,
removal of water, and destruction of organic material. Insoluble solids are
removed from the liquor and washed. The liquor and wash water are combined
and the iron is removed by solvent extraction, and the iron-free stream is
then vacuum-evaporated for concentration to the point of saturation.
Crystallization is accomplished with a hydrogen chloride gas treatment;
this gas is dissolved in the liquor, greatly reducing the solubility of the
105
-------
aluminum chloride and resulting in crystal formation. The separated crystals
are washed free of liquor and thermally decomposed to remove combined chloride
and produce reduction-grade alumina. Provisions are made for collection and
recycle of acid streams.
Energy requirements—Net energy requirement for this process is 6530
kilocalories per kilogram of alumina produced. Indirect heating for thermal
decomposition would allow use of coal as the main fuel.
Environmental residuals--Use of hydrogen chloride will necessitate
strict control measures to prevent environmental degradation. Solid wastes
from leaching will include silica, clay, and other minerals. Iron oxides and
metal sulfates will also comprise a solid waste. No liquid waste stream
should be produced; the problem of secondary water pollution will have to be
considered, however.
Costs—While quantitative data are lacking, capital and operating costs
for this process are expected to be the lowest among alternate technologies
investigated. Costs are expected to exceed those of conventional bauxite
processing, however.
Status of technology—A recent Bureau of Mines feasibility study has
resulted in a favorable evaluation from both environmental and economic view-
points.
Hydrochloric Acid Extraction/Evaporative Crystallization (3)
An alternative acid extraction process that uses evaporative crystalliza-
tion is also being developed by the U.S. Bureau of Mines.
Process description—This process is similar to the previously described
hydrochloric acid extraction/induced crystallization process except for the
method of crystallizing the aluminum chloride, decomposing the crystals, and
recycling the leach liquor. The strength of the hydrochloric acid leaching
solution is about 20 percent less than for the induced crystallization pro-
cess.
Two evaporative steps are utilized to concentrate the pregnant leach
liquor and produce aluminum chloride crystals. Hydrogen chloride gas is not
used. Crystals are separated by vacuum filtration and centrifuge; they are
then washed free of adhering liquor and thermally decomposed by direct contact
with combustion gases of clean-burning fuel. The combustion gases are treated
to recover acid, as are the crystals recovered from vacuum filtration.
Energy requirements-TEnergy usage for this process amounts to approxi-
mate 1 y~!T72lri
-------
thickener leach effluent will be approximately 50 percent water, with alumina,
silica, and iron oxides as the major solid components. This discharge could
amount to 6.7 tons per ton of alumina produced.
Costs—While operating and capital costs exceed those for the Bayer
process, they compare favorably with costs for most other emerging technol-
ogies for nonbauxitic alumina production.
Status of technology—A large pilot plant was operated by Anaconda
Company during the late 1950's and early 1960's. A recent Bureau of Mines
feasibility study, based on testing in a small integrated pilot plant, has
favorably evaluated this process on both technical and economic bases. Con-
tinued development is expected.
Nitric Acid Extraction (3,4)
Initial investigation of the use of nitric acid for alumina production
from clays was conducted in Norway early in this century. More recent de-
velopment work has been carried out in this country by the Idaho National
Engineering Laboratory, Arthur D. Little Company, and the Bureau of Mines.
Process description—Kaolin clay is calcined to chemically activate it
and to eliminate water and organic material. A nitric acid leach follows, in
which aluminum, alkalis, oxides, and iron are dissolved. Solids such as
silica are separated from the pregnant liquor in a thickening and washing
operation; the washing solution is combined with the leach liquor for subse-
quent solvent extraction of iron. After iron removal, the liquor is con-
centrated by vacuum evaporation and crystals of aluminum nitrate are formed by
cooling the super-saturated solution. The crystals are separated by centri-
fuge, melted, and sprayed as a solution into a heated fluidized bed of pre-
viously prepared alumina particles. Approximately 90 percent of the nitrate
is removed by hydrolysis in this step; the remainder is removed by decomposi-
tion in a second stage in which the particles are heated to a higher tempera-
ture in the presence of steam. Final alumina calcination involves trans-
formation to the alpha phase in an insulated silo.
Energy requirements—This process consumes considerably more energy than
the Bayer process. An estimate of requirements is 10,280 kilocalories per
kilogram of alumina produced.
Environmental residuals—A small amount of nitrogen oxides will be
produced during leaching; .a much larger amount will result from nitrate de-
composition. Waste from leaching and thickening would be considerable and
could amount to approximately 3.8 tons per ton of alumina produced; nearly 50
percent of this would be water. The major solid constituents are likely to be
acid insoluble clay, aluminum nitrate, and suspended solids. Areas of major
kaolin clay reserves, including Georgia, South Carolina, and Alabama, have
annual precipitation exceeding evapo-transpiration rates. This would imply a
potential for secondary water pollution.
Costs—Operating and capital costs for this process are high and are
expected to exceed those of many other alternate technologies. One estimate
107
-------
of capital costs for a plant with an annual production of 635,000 metric tons
of alumina is $322 million. Operating costs for a Georgia location would be
nearly twice that for an existing Bayer plant, although only about 3 percent
above that for a new Bayer plant operating under present conditions.
Status of technology—The nitric acid exchange process was investigated
during the 1960's in a nonintegrated pilot plant by Arthur D. Little Company.
It has also been tested in a small integrated pilot plant by the Bureau of
Mines, which has resulted in an unfavorable evaluation on technical and eco-
nomic bases.
Sulfurous Acid Extraction (3)
This process for the extraction of alumina from clay was developed in
Germany during World Wars I and II. More recent developmental work has been
carried out by the Bureau of Mines.
Process description--High-alumina clay is first calcined for chemical
activation and elimination of water and organics. The calcine is leached
under high pressure with a 20 percent sulfurous acid solution. This leach
continues for approximately 15 hours and results in dissolution of 75 to 80
percent of the contained alumina. Insolubles are filtered from the pregnant
liquor and washed before disposal. Combined liquor and washings are auto-
claved at 100°C and 3 atmospheres SO? pressure, causing precipitation of
sulfite. The slurry is thickened and autoclaved again, decomposing the basic
sulfite to a filterable alumina trihydrate. The trihydrate is thickened and
slurried to a modified Bayer process for final refining.
Energy requirements—The process has a net energy consumption of ap-
proximately 7835 kilocalories per kilogram of alumina produced. Coal may be
used to provide heat for clay calcination and steam generation. Bayer process
calcination of alumina will require a clean fuel, however.
Environmental residuals—The sulfur oxides resulting from use and pro-
duct io!T^f~TuT7uTbTi^~licTd~wTll require control. Acidic waste liquor from the
crude alumina trihydrate filtration step will require treatment. Solid
wastes will include red mud from the caustic purification step and solid
tailings from leaching.
Costs—Capital costs will be especially high for this process. Operating
costs are also expected to be greater than for many other alternative pro-
cesses.
Status of technology—A Bureau of Mines evaluation has indicated tech-
nical ~aliT^conom^~TTabTT>ties associated with this process as currently
developed.
Toth Aluminum Process (4)
The Toth Aluminum Corporation is developing a process for the coproduc-
tion of alumina and titanium dioxide by chlorinating clay in the presence of
carbon.
108
-------
Process description—Kaolin clay is crushed, screened to less than 0.65
centimeter, dried, and calcined. The calcine is fed to fluid-bed chlorination
reactors, where chlorine and carbon (petroleum coke or low-grade coal) are
added. The alumina, iron, silicon, and other ore constituents react to form
chlorides which are driven off as gases. The chlorides in the gas are sepa-
rated by scrubbing, fractional condensation, and distillation. Liquid alu-
minum chloride is produced in the distillation columns. This liquid is
vaporized and then oxidized in fluid-bed reactors to form alumina. Oxidizer
off-gas is recycled to the chlorinators where titanium tetrachloride and
silicon tetrachloride are produced. These gases are condensed and distilled
to separate aluminum chloride from the other components. The aluminum,
titanium, iron, and silicon chlorides are oxidized separately, producing alu-
mina and allowing the recovery of titanium dioxide (equivalent in value to
rutile) and chlorine.
Energy requirements—Total energy consumption is about twice that
required in a Bayer plant of comparable throughput.
Environmental residuals-Air emissions result from drying and calcination
as well as the condensing and scrubbing steps. It is assumed that all chlo-
rine emissions will be controlled because of the economics involved. Solids
wastes are expected to consist of inert material from the chlorinators,
discharge from the sodium chloride purge, and silica resulting from the
oxidation of silicon tetrachloride. These wastes should be disposed of in a
lagoon lined with an impervious elastomer membrane to prevent leaching.
Costs—Capital investment for this plant is estimated to be 83 percent of
that for a new Bayer plant. The estimated operating cost with credit for by-
product titania is approximately 80 percent of that for a new Bayer plant of
comparable throughput. Operating costs will exceed those for existing Bayer
plants, however.
Status of technology—This process is being developed by the Toth Alumi-
num Corporation on a small scale. Toth is actively seeking sponsors to build
an 80-metric-tons-per-day pilot plant.
Alunite Reduction Roasting/Modified Bayer Process (3)
Alunite has been considered as a raw material for alumina production for
many years and is being used in other parts of the world for this purpose.
The discovery of new alunite reserves in Utah in 1971 has resulted in renewed
emphasis upon the development of a practical technique for processing this
material.
Process description—.AIunite ore is crushed and ground in preparation for
a series of fluidized roasting operations. The initial roast is under oxi-
dizing conditions, using coal as fuel to remove water. The second roast
reduces much of the sulfate associated with the alumina; the third is again an
oxidizing roast for the conversion of any sulfides formed back to oxides.
Roasted ore is leached with a solution of recycled potassium sulfate and
potassium hydroxide to dissolve potassium and other sulfates already present
in the feed. The residue is washed and enters a modified Bayer process. This
109
-------
process uses a caustic leach and must be designed to extract alumina while
minimizing extraction of silica from the waste solids associated with the
alumina.
Energy requirements—Net energy requirement for this process is 7250
kilocalories per metric ton of alumina produced. The dehydrating/reduction
roast accounts for well over half of this figure.
Environmental residuals--Presence of sulfur in the ore necessitates the
use of S02 control for atmospheric emissions and suitable treatment of sul-
fates in liquid effluents. Impoundment is planned for all liquid wastes;
gases will be scrubbed.
Costs—Capital and operating costs are substantially higher than for
conventional bauxite processing. In addition, costs do not compare favorably .
with many other emerging nonbauxitic technologies.
Status of process—Because proven reserves of high-grade alunite are
limited, technical and economic problems associated with this process are not
expected to be overcome in the near future.
Anorthosite/Lime Sinter Process (3)
This process was first applied to anorthosite ores by the Bureau of Mines
in 1952. The lime sinter process had been previously applied to alumina-
containing clay as early as 1902.
. Process description—Crushed anorthosite and limestone are ground and
blended in a wet milling step. The slurry is classified and the minus-200-
mesh fraction is thickened, filtered, and partially dried. This material is
then mixed with filter cake from a desilication step and pelletized for feed
to sintering kilns where calcium aluminates and calcium silicates are pro-
duced. This sintered product is then cooled to 700°C and soaked to form beta-
dicalcium silicate. Rapid cooling to 25°C transforms or "dusts" beta-dical-
cium silicate to gamma-dicalcium silicate. This "dusted" portion of the
sinter is leached with soda ash to produce a sodium aluminate liquor. The
liquor is next desilicated and filtered. This reaction is seeded with product
filter cake. Precipitation of alumina trihydrate is accomplished by car-
bonating the liquor with washed sinter kiln gases in the presence of alumina
trihydrate. The precipitated product is classified, with fines recycled to
the carbonation step; the coarse fraction is filtered and fed to kilns for
conversion to alumina. Spent liquor is recycled.
Energy requirements—Net energy consumption for this process is 17,195
ki local ones per kilogram of alumina produced. Coal may be used'for sinter-
ing.
Environmental residuals—Emissions from coal-fired sintering kilns will
require controls for dust and SOp. Large quantities of waste leach residue
will be generated. These wastes will include entrained liquor and approxi-
mately 9000 metric tons dry weight of solids per day. Impoundment ponds would
be extremely large.
no
-------
Costs—Operating and capital costs would be higher than for Bayer opera-
tions or many alternate processes.
Status of techno!ogy--A Bureau of Mines evaluation indicates limited
potential for this process at present.
ALTERNATE REDUCTION TECHNOLOGIES
Because of the capital intensive nature of the industry, it will be dif-
ficult to justify closings of existing potlines unless new reduction tech-
nologies can be shown to offer significant potential cost savings. The major
problems that will be addressed in the foreseeable future will be methods for
reducing energy consumption and discharges of hazardous pollutants. Much of
this research is being conducted by the aluminum companies, and details are
proprietary. Alcoa is currently testing a reduction process that could result
in significant energy savings, and Sumitomo Chemical is operating modified
Soderberg cells that reduce both energy consumption and environmental dis-
charges. New cathode leads and substitutes for the petroleum coke used for
anodes are being investigated. Several aluminum companies and equipment
vendors are also developing more effective dry scrubbing technologies such as
those described in Process No. 10.
Alcoa Chloride Bath Process (4,5)
The aluminum chloride bath process represents a potentially significant
improvement over conventional Hall-He"roult electrolysis. The process is said
to result in a 30 percent reduction in energy requirements compared to exist-
ing plants. Oxygen is eliminated from the system, allowing the use of perma-
nent electrodes and eliminating expensive anode-baking facilities. Fluoride
emissions are avoided as no cryolite or other fluoride materials are used.
Process description--A1umina from conventional Bayer processing is
charged into the top stage of a two-stage fluid-bed reactor, while No. 6 fuel
oil is fed into the bottom bed. The oil is cracked and coked and 70 percent
of its carbon value is impregnated on the alumina. A portion of the cracked
off-gas is burned with air to provide heat for coking and deposition of carbon
on the alumina. The coated alumina is fed into a fluid-bed chlorination
system in which volatile aluminum chloride is formed. The noncondensible
gases containing the aluminum chloride, primarily carbon dioxide and carbon
monoxide, are subjected to high-temperature condensation to remove unreacted
alumina, sodium chloride, and some aluminum chloride. The resulting solid
mass is leached, with the recovered alumina dried, calcined, and returned to
the coking system. The condensate is separated and oxidized to recover chlo-
rine. The uncondensed gas passes through a second high-temperature condensa-
tion and then a final condensation at 65°C to solid or liquid aluminum chlo-
ride, the product feed to the electrolytic cells. Hydrogen chloride is ab-
sorbed from the remaining gas to produce a 35 percent hydrochloric acid by-
product.
The cell electrolyte contains 5 percent aluminum chloride and a mixture
of sodium chloride and lithium chloride. The cells operate at a slight posi-
111
-------
tive pressure to avoid air or water in-leakage; this eliminates anode oxida-
tion and permits the use of nonconsumable graphite electrodes. The cells
operate at about 700°C, considerably below Hall-HeYoult cells. It would be
possible to design cells with multiple sheet electrodes stacked one above
another, allowing one cell to become the equivalent of several conventional
cells.
Energy requirements--Electrical requirements are sharply reduced compared
to Hall-Hiroult cells because of the lower decomposition voltage and bath
resistivity. Estimated electrical consumption is 11.4 kilowatt-hours per
kilogram of aluminum product. This represents a savings of about 30 percent
over existing and 13 percent over new Hall-He"roult smelters. Fossil fuel
requirements are similar when the fuel value of the coke, pitch, and natural
gas in the Hall-He"roult process are considered.
Environmental residuals—There are no fluoride emissions from the chlo-
ride bath process, as cryolite or other fluoride compounds are not present in
the electrolyte. However, gas systems require tight control because of
possible chlorine emissions. Alcoa has installed a demonstration unit that
recovers volatile chlorides from fly ash; iron oxides are removed by magnetic
separation, with the nonmagnetic material chlorinated at high temperature.
The slight positive pressure in the electrolytic cells should result in some
fugitive emissions. Sulfur emissions from the coking operation require con-
trol similar to that provided for utility boilers. The exhaust from the HC1
absorption system can be cleaned in a caustic scrubbing system.
Water borne pollutants should be substantially similar to those from
Hall-HeYoult smelters, although fluoride would not be a factor. However,
small amounts of sodium and chloride would be present. Various sludges are
removed from the electrolyte; they are predominantly sodium aluminate con-
taminated with sodium chloride and lithium chloride.
Costs—Capital costs for the chloride bath process are expected to be
similar to those for a new Hall-He*roult smelter, while operating costs should
be about 5 percent less.
Status of technology—Alcoa is operating a 27,000 metric-tons-per-year
semi commercial plant utilizing this process. The plant can be expanded to
272,000 metric tons per year.
Sumitomo's Modified Soderberg Cell (6)
Sumitomo Chemical of Japan has developed modifications to both equipment
design and operating procedures at Soderberg reduction cells that reduce
energy requirements and polluting discharges. Cell stability is improved over
long-term operation, chiefly by minimizing voltage fluctuations; a consider-
able extension in cell life results. With about one-third of U.S. production
capacity (and one-half of world capacity) accounted for by Soderberg cells, a
large potential market exists for technologies such as Sumitomo's that can
both increase operating efficiency and reduce harmful emissions.
112
-------
Process^ descn'ption--The principal objective of the Sumitomo modifica-
tion s~iTlb~lmb^rhout~voltage fluctuations in the cell. Temperature-opti-
mization analysis permits the heat balance to be altered through the cathode,
allowing a given cathode material to be matched with the proper insulation
configuration. Most details of the technology are proprietary; however, the
modifications involve changes to anode material, design, and operating pro-
cedure, as well as mechanical changes in the alumina feed system.
Energy requirements--Direct-current energy demand is reduced 12 to 20
percent over conventional Soderberg cells.
Environmental residuals--Hydrocarbon emissions are reduced 50 percent by
changes in anode material, design, and operating procedure. Gaseous fluoride
emissions are reduced 40 percent by a revised system of crust breaking and
alumina charging.
Costs—The Sumitomo technology yields economic benefits by extending cell
life, thereby reducing both labor and material requirements. The working life
of cathodes has averaged 6 years, with some cells lasting more than 8 years;
this is double conventional cell life. The total material and labor costs of
relining cells and preparing them for operation, a significant part of the
production cost of aluminum, is substantially reduced because of the extended
cell life. Labor requirements are 0.7 to 0.8 man-day per metric ton of alumi-
num metal product, about half of conventional requirements.
Status of techno!ogy--Sumitomo has applied this technology to its three
smelters, which together supply one-fourth of Japan's primary aluminum market.
It is also installed at a Norwegian plant owned by Norsk Hydro. The modifica-
tions to the anode and alumina charging systems and operating procedures can
be made piecemeal at any time; the insulation may be replaced when a cell is
shut down for relining.
Refractory Hard Metal Cathodes (4)
The use of refractory hard metal cathodes has been investigated for the
last 15 to 18 years. Various refractory hard metals have been considered,
including zirconium and titanium carbides, borides, and some mixtures of these
metals. The principal interest now is in titanium diboride because of its
superior electrical conductivity, its ability to be wetted by molten aluminum
and cryolite in the cell, and the fact that in the pure state it is not cor-
roded by the fluorides in the electrolyte. These cathodes would reduce power
losses in the form of heat, and thereby produce more aluminum with less power.
Process description—The original concept was to replace the iron and
carbon with titanium diboride, eliminating the voltage drop between these
materials and also reducing the voltage drop between the cathode bus and the
aluminum pad. However, recent interest has shifted toward also replacing the
aluminum pad so that titanium diboride would provide connections between the
cathode bus and the electrolyte. Only a thin film of aluminum would form on
the titanium diboride because the aluminum produced would rapidly drain from
the cathode. The voltage drop would thus be reduced and the back reaction by
113
-------
which aluminum reacts with C02 to form CO and ^2^3 would be reduced or elim-
inated. The thin film of alumina would permit closing the distance between
the anode and cathode, reducing the voltage drop through the electrolyte and
permitting an increase in anode current density. The resulting increased
current flow and disassociation would allow more aluminum to be produced.
Production increases with corresponding current flow increases of 30 to 50
percent are believed to be possible.
Energy requirements--If the back reaction is substantially reduced, anode
consumption would be reduced and less petroleum coke and pitch would be
required. Anode consumption should be reduced about 20 percent. Total energy
consumption with titanium diboride cathodes should be about 80 percent of that
required in conventional cells. A power savings of 30 to 40 percent might be
achieved during periods of production curtailment.
Environmental residuals—Emissions would be similar to those from con-
vent ionaT~ceTTT!There woufd be higher C02 and lower CO concentrations due to
the reduced back reaction. The value of the titanium diboride cathode scrap
would permit recovery.
Costs--Most cost information concerning these cathodes is proprietary,
although the cost of fabricated diboride cathodes would be much higher than
those made from carbon. However, because of the increased production rate,
the capital cost for retrofitting titanium diboride cathodes could be only one
half that of adding new conventional cells to equal this increased output.
Operating costs are similar to conventional cells, although they become more
favorable when the increased capacity is considered.
Status of technology—Cathode life is the biggest problem with this
technology, as there have been problems due to spall ing and cracking. How-
ever, both Kauicki Berylco Industries and PPG Industries believe that 3-to 4-
year cathode life can be demonstrated. Widespread adoption of refractory hard
metal cathodes could have a dramatic effect on the aluminum industry.
References
1. Patterson, Sam H., and John R. Dyni. Aluminum and Bauxite. In:
United States Mineral Resources. Brobst and Pratt, Ed's. Geolog-
ical Survey Professional Paper 820. U.S. Department of Interior,
Washington, D.C., 1973.
2. American Metal Market. Vol. 85, No. 173. September 7, 1977.
3. Alumina Process Feasibility Study and Preliminary Pilot Plant Design
(Draft). U.S. Department of Interior, Bureau of Mines, Denver,
Colorado, September 1977.
4. Environmental Considerations of Selected Energy Conserving Manu-
facturing Process Options: Vol. VIII. Alumina/Aluminum Industry
Report. EPA-600/ 7-76-034h. U.S. Environmental Protection Agency,
Cincinnati, Ohio, December 1976.
114
-------
5. Primary Aluminum Plants, Worldwide. Part One. U.S. Department of
Interior, Bureau of Mines, Denver, Colorado, August 1977.
6. McAbee, Michael K. Aluminum Cell Modifications Cut Energy Use.
Chemical and Engineering News. Vol. 53. August 4, 1975.
115
------- |