-------
TABLE 3. COMMON ORES MINED FOR THEIR ZINC CONTENT (5)
ZnO
ZnS04 . 7H20
ZnC03
Zn4Si207(OH)2
Zn2Si04
(Zn2Mn)0 Fe203
2ZnC03 3Zn(OH),
ZnS
(Fe,Zn)S
H20
Zincite
Goslarite
Smithsom'te (or calamine in Europe)
Hemimorphite (or calamine in America,
called electric calamine in Europe)
Willemite
Frank!inite
Hydrozincite r
Sphalerite, wurtzite
Marmatite
-------
Zinc ores are processed at the mine to form concentrates containing
typically 52 to 60 weight percent zinc, 30 to 33 weight percent sulfur, and 4
to 11 weight percent iron (7). Roasting at the plant lowers the sulfur con-
tent to about 2 percent. Other raw materials are required at the smelter for
producing metallic zinc. Coke or coal and sand along with inert materials are
required during pyrometallurgical sintering, in quantities depending upon the
specific concentrate properties and the desired pharacteristies of the sinter.
Coal or coke must also be added during reduction^ Exact quantitites are
variable, depending on the properties of the feed and type of reduction
furnace. In hydrometallurgical processing, sintering or pyrometallurgical
reduction is not required but sulfuric acid is required for leaching the
calcine. ;;
i _
The energy required for production of one metric ton of slab zinc is 8.8
to 16.4 million kilocalories depending on the process. In 1972, 9.6 billion
kilocalories were used in the manufacture of primary zinc slabs, an average of
15.3 million per metric ton (8). This represents a 9 percent increase in
efficiency of energy utilization over 1967, primarily due |to the closing of
inefficient horizontal retorts. ;
Products i
The principal products of the primary zinc industry are metallic zinc,
zinc oxide, and cadmium. Uses for these products are widely varied. Metallic
zinc is used for galvanizing, for making pigments and zinc compounds, for
alloying, and for grinding into zinc dust. Table 4 shows U.S. consumption of
slab zinc for 1978. Usage patterns in the U.S. differ from those in the rest
of the world in the heavy emphasis on zinc-base alloy castings, mainly for the
automotive industry. The primary product of most zinc companies is slab zinc,
which is produced in five grades and classified by its purity. These grades
are presented in Table 5. i
i
Zinc oxide is used in rubber, emollients, ceramics, and fluorescent
pigments, and in the manufacture of other chemicals. Metallic cadmium is used
in production of alloys, in corrosion-resistant plating for hardware, as a
counter electrode metal for selenium rectifiers, as neutron shielding rods in
nuclear reactors, in nickel-cadmium batteries, and in plastics and cadmium
compounds. Cadmium metal accounts for 60 to 70 percent of consumption, and
cadmium sulfide used for pigments for another 12 to 15 percent (9).
* \
Companies I
Capacities of the eight primary zinc smelters that use primary concen-
trate feed are listed in Table 6. The three pyrometallurgical plants range in
capacity from 57,000 to 227,000 metric tons per year. U.S. primary zinc
capacity is about equally divided between pyrometallurgical and electrolytic
processes. The single largest plant accounts for 35 percent of the total
domestic primary capacity. All of the primary smelters produce sulfuric acid
-------
TABLE 4. U.S. SLAB ZINC CONSUMPTION - (1.976) (9)
Galvanizing
Brass .and bronze product*
Zinc-base alloy
Rolled zinc products
Zinc oxide
Other
Total
Metric tons
342,893
150,817
387,403
27,088
35,405
35,287
1,028,893
Percent
38
15
38
3
3
3
100
TABLE 5. GRADES OF COMMERCIAL ZINC
Composition, percent weight
Special high grade
High grade
Intermediate
Brass special
Prime western
Zinc
99.990
99.90
99.5
99.0
98.0
Lead
0.003
0.07
0.20
0.6
1.6
Iron
0.003
0.02
0.03
0.03
0.05
Cadmi urn
0.003
0.03
0.40
0.50
0.50
-------
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-------
as a by-product. Four companies were responsible for over 80 percent of
primary cadmium production in 1978 (1). Salient statistics for the domestic
primary zinc industry in 1978 are presented in Table 7.
Environmental Impact
Uncontrolled atmospheric emissions from the primary zinc industry have
decreased in recent years because of the increase in electrolytic zinc re-
covery operations and the closing of several retort zinc smelters. In 1969,
zinc emissions to the atmosphere totalled 65 metric tons from mining and
milling, and 45,350 metric tons from metallurgical processing (13). Sulfur
dioxide is emitted from roasting and sintering operations, although the
roaster S02 emissions are normally collected to produce sulfuric acid.
Smelter solid wastes are usually recycled for by-product metal recovery.
Some sludges may require several months' storage before recycling, and others
may be disposed of as landfill. Sludges and solid wastes generated during
mining and concentration processes are disposed of at the mining site in
tailings ponds or as mine backfill. ,-.. :
Liquid effluents can be classified as noncontact and contact. Noncontact
water is used for cooling in heat exchangers. Contact, or process, wastewater
is produced in such operations as scrubbing of roaster gas and reduction
furnace gas, cooling of metal castings, cadmium production, and auxiliary air
pollution controls. The pollutants of concern are primarily,-z-inc and sulfates,
accompanied normally by such elements as cadmium,and lead, and small amounts
of arsenic and selenium.
Limitations on liquid effluent and atmospheric emissions from new sources
have been promulgated (10,11). The solid waste problem is currently under
investigation.
Bibliography
1.
Commodity Data Summaries, 1979. U.S. Department of Interior.
' Bureau of Mines. Washington, D.C. 1979.
2. Minerals Yearbook, 1976. U.S. Department of Interior. Bureau of
Mines. Washington, D.C. 1978.
3. Mineral Industry Surveys. Zinc Industry in July 1979. U.S. Depart-
ment of Interior. Bureau of Mines. Washington, D.C. 1979.
4 Deane, G.L., et al. Cadmium: Control Strategy Analysis. GCA
Corporation Report No. 6CA-TR-75-36-6. U.S. Environmental Protec-
tion Agency. Research Traingle Park, North Carolina. April 1976.
p. 157.
5. Smelting and Refining of Nonferrous Metals and Alloys. 1972 Census
of Manufacturers. Publication MC72(2)-33C Bureau of the Census.
U.S. Department of Commerce. 1972.
10
-------
TABLE 7. PRINCIPAL STATISTICS FOR THE PRIMARY ZINC
INDUSTRY IN THE UNITED STATES IN 1976 (9)
(metric tons)
Production:
Mine, recoverable zinc
Smelter, slab zinc
Imports:
Ores and concentrates
(dutiable zinc content)
Slab zinc
Consumption:
Slab zinc
Ores (recoverable zinc content)
Exports:
Slab zinc
302,569
441,472
118,003
617,840
1,050,<)85
89,959
723
11
-------
6.
7.
8.
9.
10.
11.
12.
13.
U.S. Department of the Interior, Bureau of Mines
Materials/A Monthly Survey, 55. July 1977.
Minerals and
K+il nth ' "r" a?d A:-Paul ThomPson- Zinc and Zinc Alloys. In:
Kirk-Othmer Encyclopedia of Chemical Technology. Interscience
Division of John Wiley and Sons, Inc. New York! 1967.
M-E-an(? D'H- Larson- Study of Industrial Uses of Energy
! E"Ylronmental Effects. EPA-450/3-74-044, U.S. Environ-
July 1974 6 9enCy' Research Tri ^gle Park, North -Carolina.
«'Rm P°llution Control in the Nonferrous-. Metals Industry
Noyes Data Corporation. Park Ridge, New Jersey.
Background Information for New Source Performance Standards-
Primary Copper, Lead, and Zinc Smelters. Office of Air and'waste
Management. U.S. Environmental Protection Agency. Research Triangle
Park, North Carolina. EPA-450/2-74-002a. October 1974. ld"ye
Development Document for Interim Final Effluent Limitations Guide-
lines and Proposed New Source Performance Standards for the Zinc
Segment of the Nonferrous Metals Manufacturing Point Source Cate-
in^;/,^3' Environrnental Protection Agency. Washington, D.C.
EPA-440/1 -75-032. November 1974.
American Bureau of Metal Statistics.
New York, New York. 1976.
Nonferrous Metal Data 1975.
W.E. Davis and Associates. National Inventory of Sources and
Emissions, Section V, Zinc. Report No. APTD-1139. National Technical
Information Service, U.S. Department of Commerce. Sprinqfield,
Virginia. May 1972. a
12
-------
INDUSTRY SEGMENT ANALYSIS
As with the copper and lead segments, the environmental impacts of the
zinc mining and smelting industries have received considerable attention in
recent years from both private and governmental organizations. This industry
segment analysis examines each production operation, to define its industrial
purpose and its potential and practice in affecting the quality of the
environment. Each process is examined as follows:
i
1. Function , :- f
2. Input Materials
3. Operating Conditions i
4. Utilities ,«
5. Waste Streams ;
6. Control Technology I
7. EPA Classification Code j
8. References . . i
The only processes included in this section are those that; are now
operating in the United States. Figure -2. is a flowsheet that shows these
processesj their interrelationships, and their major waste streams.
13
-------
v*L
CONCENTRATING
2
1
AJ
BUSTING
AGENTS
FLOTATION
CHEMICALS
- LIMESTONE
L CARBON
CONTACT
ACID PLANT
!
T WATER
Y AIR
YSOLID
Figure 2. Zinc industry flow sheet.
14
-------
>
SINTERING
7
h SAND
h COKE
- ZINC SULFATE
tfia
LEACHING
11
\l
A
I!/
cQ
VERTICAL
RETORTING
1
L COAL OR COKE
<&
ELECTRIC
RETORTING 9
L COKE
\&
OXIDIZING
FURNACE 1()
a
D .-
3
J .
A
₯g
A
BY PRODUCT
RECOVERY
:SAND
OTHER ZINC-BEARING
MATERIALS
^1
JO-
ELECTROLYSIS
13
A
SPEHT ELECTROIVTE
- INORGANIC
ADDITIVFS
L ALLOYING MATERIAL
"- FLUXES
-H2S04
-SPENT ELECTROLY1
-OTHER
INORGANIC
ADDITIVES
TO
- BY-PROOUCT RECOVERY
LEAD SHELTER
BAGHOUSE DUST
LIME
COAL OR COKE
H2S04 OR RETURN
ELECTROLYTE
SODIUH BICARBONATE
.HYDROGEN SULFIDE
Figure 2 (continued)
15
-------
PRIMARY ZINC PRODUCTION
PROCESS NO. 1
Mining
1. Function - Mining involves excavation and treatment of zinc ore deposits.
Most zinc is mined underground using open shrinkage, cut-and-fill, or square-
set stoping methods. In underground mines, walls and pillars are usually
left behind to support the overlying rock structure, unless the width of the
ore body is such that it can be supported and the entire ore body extracted.
A few mines, particularly in early stages of operation, use open pit methods,
which closely resemble those used in copper mining.
Mining operations consist of drilling, blasting, and removing the broken
rock. After removal from the mine, the ore is loaded onto trucks for trans-
portation to concentrating facilities. Mining operations range in size from
those handling several hundred metric tons of ore per day to complexes capable
of processing about six thousand metric tons per day. Some,large, single-
level, open-stage operations utilize "trackless" mining techniques, with
equipment mounted on crawler-type tread or pneumatic-tired vehicles. This
equipment facilitates movement and increases speed of operations, thus
increasing total output and reducing costs; with such equipment it is some-
times feasible to mine ore containing as little as 2 percent zinc.
2. Input Materials - Inputs to the mining process include the ore deposits
and the explosives used to remove them.
3. Operating Conditions - Ores are mined at atmospheric conditions. Condi-
tions depend upon type of mining, i.e., open pit or underground.
4. Utilities - Fuels and electricity are used for operating mining equipment
and transporting ore to concentrating facilities. One estimate for electrical
usage for mining ore in 1973 was 3.1 x 108 kilowatt-hours (1). With the 1973
mine production level of 434,000 metric tons (2), electrical usage was about
720 kilowatt-hours per metric ton of zinc moved. Unspecified amounts of
water are pumped into mines for machinery and hydraulic ba,ckfill operations.
5. Haste Streams - Zinc ores typically contain 3 to 11 percent zinc (3);
thus 10 to 40 tons of ore must be mined for every tori of zinc produced. Most
of this ore is mined for other metals as well. Fugitive dust emissions are
similar to those of other mining operations, amounting to about 0.1 kg per
metric ton of zinc mined (4). Cadmium emissions also occur during the mining
of zinc ore. Emissions due to wind loss from mine waste are estimated at 0.1
kilogram per metric ton of ore mined (5). Total emissions of cadmium to the
atmosphere were thus 240 metric tons for 1968 (5) and 220 metric tons for
1973 (6).
Mine waste water results from infiltration of ground water, water
pumped into the mine for machines and hydraulic backfill operations, and
infiltration of surface water. Quantities of pump out water are not necessarily
related to the quantity of ore mined. The water required to maintain opera-
tions may range from thousands of liters per day to 160 million liters per
day. This water contains such impurities as blasting agents, fuel, oil, and
16
-------
hydraulic fluid. Dissolved solids found in the wastewater are generally lead,
zinc, and associated minerals (7).
Conditions compatible with solubilization of certain metals, particularly
zinc, are associated with heavily fissured ore bodies. Although the minerals
being recovered are sulfides, a fissured ore body allows oxidation of the ore,
which increases the solubility of the minerals. j
The major solid wastes from mining operations result from removal of rock
to get to the ore. The discarded waste is of essentially the same composition
as raw ore, with lower metallic content. No quantitative or qualitative
estimate was found concerning these wastes. ,
6. Control Technology - Fugitive dusts from drilling and conveying can
be reduced by wetting and control systems (8).
!
In the zinc industry, mine water generated from natural drainage is
reused in mining and milling operations whenever possible. Discharge may
result because of an excess of precipitation, lack of a nearby milling facil-
ity, or inability to reuse all of the mine waste water at a particular mill.
*»
Small quantities of water are usually needed in the zinc flotation
process; mine water effluent is used at many facilities as mill process makeup
water. The mine water may pass through the process first, or it may be
conveyed to a tailings pond for later use with recycled process water. The
practice of combining mine water with mill water can disturb the overall water
balance unless the mill circuit is capable of handling the water volumes
generated without a resulting discharge. j
Acid mine water is neutralized by the addition of lime arid limestone.
Acid mine water containing solubilized metals may be effectively treated in
the mill tailings pond. The water may be further treated by lime clarifica-
tion and aeration.
The solid wastes from mining operations are disposed of in a pile or
pond. These wastes can be used as mine backfill.
7. EPA Source Classification Code - None
8. References -
1.
2.
3.
Dayton, S. The Quiet Revolution in the Wide World of Mineral
Processing. Engineering and Mining Journal. June 1975.
Commodity Data Summaries 1976. U.S. Department of Interior.
Bureau of Mines. Washington, D.C. 1976.
McMahon, A.D., et al. In 1973 Bureau of Mines
U.S. Department of the Interior. U.S. Government
1973.
17
Minerals Yearbook.
Printing Office.
-------
8.
National Inventory of Sources and Emissions - Section V, Zinc.
W.E. Doris and Associates. Report No. APTD - 1139. National
Technical Information Service, U.S. Department of Commerce.
Springfield, Virginia. May 1972.
W.E. Davis and Associates. National Inventory of Sources and
Emissions - Cadmium, Nickel, and Asbestos - 1968. Cadmium, Section
I. Report No. APTD - 1968. National Technical Information Service.
Springfield, Virginia. February 1970.
Deane, 6.L., et al. Cadmium: Control Strategy Analysis. GCA
Corporation Report No. 6CA-TR-75-36-6. U.S. Environmental Protec-
tion Agency. Research Triangle Park, North Carolina. April 1976.
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 Vol. 1. EPA
440/1-75/061. Effluent Guidelines Division Office of Water and
Hazardous Materials, U.S. Environmental Protection Agency,
Washington, D.C. October 1975.
An Investigation of the Best Systems of Emission Reduction for
Quarrying and Plant Process Facilities in the Crushed and Broken
Stone Industry. United States Environmental Protection Agency,
Office of Air Quality Planning and Standards, Emission Standards
and Engineering Divis;ion. Research Triangle Park, North Carolina.
Draft. April 1976.
18
-------
PRIMARY ZINC PRODUCTION
Concentrating
PROCESS NO. 2
1. Function - Concentrating consists of separating the desirable mineral
constituents in the ore from the unwanted impurities by various mechanical
processes. The ore from mining must be concentrated because mined sphalerite
is seldom pure enough to be reduced directly for zinc smelting.
The zinc-bearing ore is first crushed by standard jaw, gyratory, and
cone crushers to a size based on an economic balance between the recoverable
metal values and the cost of grinding (1). Size separation is accomplished
by vibrating or trommel screens and classifiers. Heavy-medium cones, jigs,
and tables separate the zinc minerals from a low specific-gravity gangue.
Classification and recycling between stages reduces the material to a particle
size appropriate for milling. The final milled product is typically 60
percent smaller than 325 mesh. ' j
After being transported to large bins for blending and storing, the ore
is pumped as an aqueous slurry to flotation cells, where It is conditioned by
additives and separated by froth flotation to recover the zinc sulfide and
sometimes lead or copper sulfides. In some cases, the ore is reconcentrated
by mechanical separation based on specific gravity differences prior to froth
flotation. Large mixers stir the solution, and the zinc-bearing minerals
separate and float to the surface where they are skimmed off,, Generally,
zinc sulfide flotations are run at a basic pH (usually 8.5 to 11), and the
slurry is periodically adjusted with hydrated lime, Ca(OH)2 (1). After
flotation, the underflow (tailings or gangue materials) is sent to a tailings
pond for treatment. j
Once separated, the metal concentrates are thickened in settling tanks
and the slurry is fed to vacuum drum filters, which reduce the moisture
content. Upon completion of the concentration process, the zinc content is
about 55 to 60 percent. Thermal drying in direct-fired rotary dryers may
further reduce the moisture content of the concentrates, which are then
transported to a storage site. Concentrate enters the dryer with about 11
percent moisture and leaves with about 3 percent moisture ;(2).
In certain western ores, notably those from Idaho, the lead and zinc are
too finely divided for satisfactory separation even by flotation. For such
ores, final separation involves sulfuric acid leaching at an electrolytic
zinc plant. Some foreign producers use the Imperial furnace to treat these
ores, but it is not used at any U.S. zinc smelters. Treatment of such ores
was the primary reason for development of electrolytic zinc recovery methods
in North America.
The quantity of zinc concentrate produced is about 1C
the zinc ore by weight. A typical analysis would be 52 to
30 to 33 percent sulfur,.4 to 11 percent iron, and lesser
cadmium, copper, and other elements (2). Table 8 lists some
elements of ore concentrate.
19
to 15 percent of
60 percent zinc,
quantities of lead,
of these
-------
TABLE 8. RANGE OF COMPOSITIONS OF ZINC CONCENTRATES (6,7,8)
Constituent
Pb
Zn
Au
Ag
Cu
As
Sb
Fe
Insolubles
CaO
S
Bi
Cd
Percent, weight
0.85-2.4
49.0-53.6
not iden.
not iden.
0.35
0.105-0.15
not iden.
5.5-13.0
3.4
not iden.
30.7-32.0
not iden.
0.24
20
-------
2- Input Materials - The quantity of ore required per ton of zinc concen-
trate produced varies with the zinc content of the ore. A common range is 5
to 10 tons of ore per ton of zinc concentrate (1). '
Hydrated lime is used to adjust the pH. Promoters or collectors are
added to the zinc sulfide to provide a coating that repels water and encour-
ages absorption of air. Frothers are added to produce a layer of foam on the
top of the flotation machine, and depressants are added to stop unwanted
minerals from floating. Table 9 lists commonly used reagents.
Additives are an estimated 1.9 kilogram of lime, 0.4 kilogram of copper
sulfate, 0.04 kilogram of Z-5 xanthate, and 0.02 kilogram of pine oil per
metric ton of concentrate (3). . i
3. Operating Conditions - Flotation machines are operated! at ambient temper-
atures and pressures. Flotations are generally run at elevated pH values of
8.5 to 11 (1).
i
4- Utilities - Electrical energy is consumed during milling operations. One
estimate for electrical usage in the milling of zinc ores in 1973 is 290
million killowatt-hours (4). At the 1973 mine production level of 434,000
metric tons (5), electrical usage was about 477 killowatt-hburs per metric ton
of zinc produced.
The water requirement ranges from 330 to 1,100 cubic meters per metric
ton of ore processed per day (1). Feed water for the mills is usually taken
from available mine waters. ;
5. Waste Streams - It is estimated that 1 kilogram of particulate is emitted
per metric ton of ore processed during crushing and grinding operations. The
composition is that of the raw ore fed to the process. After water is added
to form an ore-water slurry, particulate emissions are negligible. In plants
that incorporate an ore drying operation following concentration, dust emis-
sions occur as the hot air passes over the moving bed of concentrate. Oper-
ating factors affecting emissions are the process feed rate and moisture
content. I
Liquid waste streams from zinc mills vary in volume from 1000 to 16,000
cubic meters per day. In terms of volume of ore processed, liquid waste
streams from milling operations range from 330 to 1,100 cubic meters per
metric ton (1). Tailings slurry discharge is about 4 cubic;meters per metric
ton of ore processed. I
The raw wastewater from a lead/zinc flotation mill consists principally
of the water used in the flotation circuit, along with any housecleaning
water. The waste streams consist of the tailings streams (usually the under-
flow of the zinc rougher flotation cell), the overflow from,the concentrate
thickeners, and the filtrate from concentrate dewatering.
21
-------
TABLE 9. TYPICAL FLOTATION REAGENTS USED FOR ZINC
CONCENTRATION UNITS (1)
Reagent
Purpose
Methyl isobutyl-carbinol
Propylene glycol methyl ether
Long-chain aliphatic alcohols
Pine oil
Potassium amyl xanthate
Sodium isopropol xanthate
Sodium ethyl xanthate
Dixanthogen
Isopropyl ethyl thionocarbonate
Sodium diethyl-dithiophosphate
Zinc sulfate
Sodium cyanide
Copper sulfate
Sodium dichromate
Sulfur dioxide
Starch
Lime
Frother
Frother
Frother
Frother
Collector
Col 1ector
Collector
Collector
Collectors
Collectors
Zinc depressant
Zinc depressant
Zinc activant
Lead depressant
Lead depressant
Lead depressant
pH adjustment.
22
-------
The principal characteristics of the waste stream from mill operations
are as follows: ;
(1) Solids loadings of 25 to 50 percent (tailings).
(2) Unseparated minerals associated with the tails.,
(3) Fine particles of minerals, particularly if the thickener overflow
is not recirculatecl. ;
(4) Excess flotation reagents not associated with the mineral concen-
trates. ;
(5) Any spills of reagents that occur in the mill. '
It is very difficult analytically to detect the presence of excess
flotation reagents, particularly those that are organic. : The surfactant
parameters may give some indication of the presence of organic reagents, but
provide no definitive information.
A typical quantity of solid wastes is 0.9 to 1 metric ton per metric ton
of ore milled. Based on a 25 to 50 percent solids loading in the liquid
waste stream, solid wastes from flotation could range from 80 to 550 cubic
meters per metric ton (1). The main component of the waste is dolomite.
Table 10 gives raw and treated waste characteristics of five mills.
This summary does not include information for a mill usinjg total recycle and
one at which mill wastes are mixed with metal refining wastes in the tailings
pond. Feed water for the mills is usually drawn from available mine waters;
however, one mill uses water from a nearby lake. These data illustrate.the
wide variations caused by ore mineralogy, grinding practices, and reagents.
6- Control Technology - Particulate abatement equipment at the dryer can
capture dust and recirculate it into a storage bin for further use. This can
be accomplished with low-energy scrubbers and multicyclones. Cyclones,
operating on a dry principle, could remove many of the large particles for
reuse directly in the roaster without further processing.; Particulates
caught in the scrubbers must be dried before reuse in roasters or sintering
plants. i
'i
Lime precipitation is often used for the removal of heavy metals from
wastewater. This treatment yields reductions for several heavy metals
including copper, zinc, iron, manganese, and cadmium. i
i
Various techniques are employed to augment lime neutralization. Among
these are the secondary settling ponds, clarifier tanks, or the addition of
flocculating agents (such as polyelectro'lytes) to enhance removal of solids
and sludge before discharge. Readjustment of the pH after lime treatment can
be accomplished either by addition of su'lfuric acid or by recarbonation.
Sulfide precipitation may be necessary for further remova' of metals such as
cadmium and mercury.
23
-------
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Water separated from the concentrates is often recycled in the mill, but
it may be pumped to the tailings pond, where primary separation of solids
occurs. Usually, surface drainage from the area around the mill is also
collected and sent to the tailings pond for treatment, as is process water
from froth flotation. ;
Tailings pond water may be decanted after sufficient retention time. One
alternative to discharge, which reduces the output of effluent, is reuse of
the water in other facilities as either makeup water or process water. Usually,
some treatment is required before reuse of this decanted water. Treatments
include secondary settling, phosphate or lime addition, pH adjustment, floc-
culation, clarification, and filtration.
The most frequently used control technology is the use of a settling or
sedimentation pond system consisting of primary tailings pond and secondary
settling or "polishing" pond, with pH adjustment prior to discharge. Six lead
and zinc ore processing facilities now use this technology. Effluent concen-
trations are limited to the following average values (in milligrams per
liter): copper - 0.05, mercury - 0.001, lead - 0.02, and zinc - 0.5 (1).
Tailings from mine concentrator operations may present a serious water
pollution problem if adequate precautions are not taken. Coarse tailings may
be removed with a cyclone separator and pumped to the mine for backfilling
(1). The most effective means of control is impoundment with isolation of
disposal sites from surface flows. Techniques include the following (1):
(1) Construction of a clay or other type of liner beneath the planned
waste disposal area to prevent infiltration of surface water (pre-
cipitation) or water contained in the waste into the groundwater
system. I
(2) Compaction of waste material, to reduce infiltration.
(3) Maintenance of uniformly sized refuse to enhance good compaction
(which may require additional crushing). ,
(4) Construction of a clay liner over the material to minimize infil-
tration. This is usually followed by placement of top soil and
seeding to establish a vegetative cover for control of erosion and
runoff. ,
(5) Excavation of diversion ditches surrounding the refuse disposal site
to exclude surface runoff. These ditches can also be used to
collect seepage from refuse piles, with subsequent treatment if
necessary.
No data were found on the extent of application of these methods.
25
-------
7. EPA Source Classification Code - None
8. References -
1.
2.
3.
4.
5.
6.
7.
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 Vol. 1.
Publication EPA-440/1--75/061. Effluent Guidelines Division Office
of Water and Hazardous Materials, U.S. Environmental Protection
Agency. Washington, D.C. October 1975.
Field Surveillance and Enforcement Guide for Primary Metallurgical
Industries. EPA-450/3--73-002. Office of Air and Water Programs,
U.S. Environmental Protection Agency. Research Triangle Park,
North Carolina. December 1973. pp. 269-309.
Denver Equipment Company.
Colorado. 1962.
Mineral Processing Flowsheets. Denver,
Dayton, S. The Quiet Revolution in the Wide World of Mineral
Processing. Engineering and Mining Journal. June 1975.
Commodity Data Summaries 1976. U.S. Department of the Interior,
Bureau of Mines. Washington, D.C. 1976.
Water Pollution Control in the Primary Nonferrous - Metals In-
dustries. Volume 1, Copper, Zinc, and Lead Industries.
EPA-R2-73-247a. Office of Research and Development, U.S. Environ-
mental Protection Agency. Washington, D.C. September 1973.
Burgess, Robert P., Jr., and Donald H. Sargent. Technical and
Microeconomic Analysis of Arsenic and Its Compounds.
EPA-560/5-76-016. Office of Toxic Substances. U.S. Environmental
Protection Agency. Washington, D.C. April 12, 1976.
Katari, V., et al. Trace Pollutant Emissions from the Processing
of Metallic Ores. EPA-650/2-74-115. U.S. Environmental Protection
Agency, Office of Research and Development. Research Triangle
Park, North Carolina. October 1974.
26
-------
PRIMARY ZINC PRODUCTION
PROCESS NO. 3
Ferroalloy Production
1 Function - One U.S. zinc smelter recovers a ferro-manganese by-product
from zinc oxide ore by means of reduction, vaporization, arid oxidation in
Waelz kilns. The ore, from a New Jersey mine, is not amenable to flotation.
The zinc oxide fume produced in the kilns is collected and sintered in the
same manner as roasted sulfide concentrates. ;
The Waelz kilns in use are rotary kilns similar to those used in the
cement industry. Their sizes range from 3.0 meters in diameter by 42.7
meters long to 3.7 meters in diameter by 48.8 meters long.; Gas flow is
countercurrent to the solids flow, with the zinc oxide fume and kiln gases
withdrawn at the feed end. Automatic 10-compartment dust tube collectors
remove zinc oxide from the gas streams, which have been previously cooled by
dilution and radiation loss. Solid residues from the kiln:contain 20 to24
percent iron and approximately 10 percent manganese; they are processed in
open-arc electric furnaces to produce a nominal 20 percent manganese iron
(Spiegeleisen) (!) |
2 Input Materials - The charge to the Waelz kilns consists of the oxide
ores which are mixed with anthracite coal to raise the carbon content and
limestone to stiffen the charge. Feed to the kilns ranges: between 9 and 15
metric tons per hour (1). Various furnace residues and slags may also be
added. j
3 Operating Conditions - Waelz kilns operate at atmospheric .pressures (2).
Operating temperatures are about 1300°C in the kiln, with^bout 80 percent of
the kiln length kept above the boiling point of zinc. ;
i
4 Utilities - Heat generated by the oxidation reactions is usually suf-
ficient to sustain reduction. Additional heat if required for reduction of
the zinc can be supplied by the combustion of coal, oil, natural gas, or
electric furnace gas. No information is available as to the quantities of
additional heat that may be supplied by combustion of fuel.
5. Waste Streams - Unknown quantities of combustion products are emitted
from the kilns and vented to the atmosphere.
There are no liquid or solid wastes from the process!.
6. Control Technology - There are no known environmental controls on the
Waelz kilns used for ferroalloy production. {
7. EPA Source Classification Code - None i
27
-------
8. References -
1
2.
The New Jersey Zinc Company. Manufacturing Operations. Palmerton,
Pennsylvania.
Schlecten, A.W. and A. Paul Thompson. Zinc and Zinc Alloys. In-
Kirk-Othmer. Encyclopedia of Chemical Technology. Interscience
Division of John Wil«;y and Sons, Inc. New York. 1967.
28
-------
PRIMARY ZINC PRODUCTION
PROCESS NO. 4
Multiple-Hearth Roasting
1 Function - Multiple-hearth roasting is a high-temperature process that
removes sulfur from the concentrate and converts zinc to an impure zinc oxide
called calcine. Roasting may also be accomplished by suspension (process No.
5) or fluidized-bed (Process Mo. 6) units. In a multiple-hearth roaster, the
concentrates drop from hearth to hearth. As much as 20 percent of the cad-
mium present in the zinc concentrate may be vaporized (1). Any mercury
present is volatilized and enters the gas stream. Multiple-hearth roasters
are the oldest type of roaster in use in the United States.
At present, only two domestic primary zinc smelters use multiple-hearth
roasters. At one of these plants, the multiple-hearth roaster is used in con-
junction with fluidized-bed roasters. The deleaded product, termed partially
desulfurized concentrate," typically contains about 22 percent sulfur and is
used as feed to the fluidized-bed roasters. Mercury is recovered from the
flue dusts captured in the electrostatic precipitators used to clean the gas
stream from these roasters. j
In roasting, if enough sulfur is originally present as sulfide, the
operation becomes autogenous. For pyrometallurgical refining, zinc sulfate
must be removed. The following reactions occur during roasting:
2ZnS + 302
2S02 + 02
ZnO + SO,
2ZnO + 2SO,
2SO,
ZnSO,
In pyrometallurgical reduction only the oxide state is desired, whereas in
electrolytic reduction, small amounts of the sulfate state are acceptable
(3). j
Elimination of the remaining small percentages of sulfur requires a long
residence time. Hence a multiple-hearth-type roaster that eliminates all but
6 to 8 percent sulfur from up to 350 tons of copper concentrates per day can
roast only about 50 to 60 tons of zinc concentrate per day to about 2 percent
sulfur. '
The roaster consists of a brick-lined cylindrical steel column with 9
or more hearths. A motor-driven central shaft has two rabble arms attached
29
-------
for each hearth, as well as cooling pipes. The concentrates enter at the top
of the roaster and are first dried in an upper hearth. The central shaft
rotates slowly, raking the concentrates over the hearth with the rabble arms,
gradually moving them to the center and a drop hole to the next hearth. They
move across this second hearth to a slot near the outer edge, where they drop
to the next hearth. The concentrates continue down through the roaster in
this spiral fashion and are discharged at the bottom. Additional fuel must
be added to maintain combustion.
The low production rates are a major disadvantage of multiple-hearth
roasting. However, since less dust is carried away in the gas stream, more
volatile sulfides such as cadmium.are removed preferentially. This is help-
ful when cadmium is to be recovered from the flue dust, since there is less
zinc dust contamination (4).
Total residual sulfur in the calcine produced in multiple-hearth roasters
is 2.4 percent; 0.5 to 1.0 percent is present as sulfide and the balance as
sulfate (3).
day.
Feed capacities for multiple-hearth roasters average 180 metric tons per
2. Input Materials - Zinc concentrate is the input material for multiple-
hearth roasters. Approximately 2.4 tons of pure ZnS is required to produce 1
ton of pure ZnO. In practice, the quantity of raw materials required to
produce one unit of calcine varies depending on impurities in the concentrate,
efficiency of the dust-collecting devices on the roaster, and the type of
roaster used. JV
Sodium or zinc chloride may be added to combine with cadmium dust in
the roaster and facilitate removal of cadmium as a by-product after sin-
tering. Specific quantities have not been reported.
? Operating Conditions - Multiple-hearth roasters are unpressurized.
Average operating temperature is about 690°C (3); the lower hearths (sixth
through tenth from the top) are maintained at 950° to 980°C (2). Operating
time depends upon the type of roaster, composition of concentrate, and amount
of sulfur removal required.
4. Utilities - The reaction converting ZnS to ZnO is exothermic and is
self-sustaining after ignition. Gas, coal, or oil must be added initially to
bring the charge up to reaction temperature. About 280,000 kilocalories per
metric ton of ore are required for ignition. The primary fuel is natural
gas. In some multiple-hearth furnaces, when concentrations of less than 1 0
percent of sulfide sulfur are required, about 1.1 million kilocalories are
required per metric ton of feed (5). Some additional fuel is added to the
lower hearths to reduce the zinc sulfide content to as low as 0.5 to 1.0
percent.
Cooling water and air are also used to cool the furnace shaft
ity powers the rabble arms.
Electric-
30
-------
5. Waste Streams - In a zinc smelter the roasting process is typically
responsible for more than 90 percent of the potential S02! emissions; 93 to 97
percent of the sulfur in the feed is emitted as sulfur oxides. Concentrations
of S02 in the off-gas vary with the type of roaster operation. Off-gases
contain up to 6 to 7 percent sulfur, depending on the sulfur content of the
feed. The volume of off-gases ranges from 140 to 170 cubic meters per minute
(6) for furnaces currently in use (approximately 180 metric tons per day).
Typical S02 concentrations range from 4.5 to 6.5 percent ,(3). Oxygen,
"nitrogen, carbon dioxide, and water vapor are other components of the gas.
Particulate matter is also emitted. The amount and composition vary
depending on such operating conditions as air flow rate, particle size dis-
tribution, and equipment configuration. Particulate emissions consist of
fumes and dusts composed of the zinc concentrate elements in various combina-
tions. One plant has reported the following compositions: of flue dusts from
multiple-hearth, suspension, and fluid bed roasters: zinc, 54.0 percent;
lead, 1.4 percent; sulfur, 7,0 percent; cadmium, 0.41 percent; iron, 7.0
percent; copper, 0.40 percent; manganese, 0.21 percent; tin, 0.01 percent;
and mercury, 0.03 percent (2). Typical particulate emissions in the off-gas
range from 5 to 15 percent of the feed (5). Composition of the distilled
metal fumes, which constitute an appreciable portion of the waste gas parti-
culate carryover, depends primarily on the concentrate composition and opera-
ting conditions. The single zinc smelter using multiple-hearth roasters,
which also uses suspension and fluidized-bed roasters, recovers mercury
eliminated during roasting from the gas purification stream (2). At that
smelter, the roasted material is treated by flotation to separate a fraction
of waste rock prior to further processing. ;
I
Information on liquid and solid wastes is not available at this time.
6. Control Technology - Gases leaving the roaster are
"hot" Cottrells, operating at 200° to 220°C, and then to
routed directly to
the acid plant (2)
7. EPA Source Classification Code - Roasting/multiple-hearth 3-03-030-02.
8. References -
1. Development Document for Interim Final Effluent Limitations Guide-
lines and Proposed New Source Performance Standards for the Zinc
Segment of the Nonferrous Metals Manufacturing Point Source Cate-
gory. EPA-440/1-75/032. Effluent Guidelines Division Office of
Water and Hazardous Materials. U.S. Environmental Protection
Agency. Washington, D.C. November 1974. ;
!
2. Lund, R.E., et al. Josephtown Electrothermic Zinc Smelter of St.
Joe Minerals Corporation. AIME Symposium on Lead and Zinc, Vol.
II. 1970.
31
-------
3. Background Information for New Source Performance Standards:
Primary Copper, Lead, and Zinc Smelters. EPA-450/2-74-002a.
Office of Air and Waste Management. U.S. Environmental Protection
Agency. Research Triangle Park, North Carolina. October 1974.
4. Schlechten, A.W., and A. Paul Thompson. Zinc and Zinc Alloys. In:
Kirk-Othmer. Encyclopedia of Chemical Technology. Interscience
Division of John Wiley and Sons, Inc. New York. 1967.
5. Fejer, M.E., and D.H. Larson. Study of Industrial Uses of Energy
Relative to Environmental Effects. EPA-450/3-74-044. U.S. Environ-
mental Protection Agency. Research Triangle Park, North Carolina.
July 1974.
6. Field Surveillance and Enforcement Guide for Primary Metallurgical
Industries. EPA-450/3-73-002. Office of Air and Water Programs,
U.S. Environmental Protection Agency. Research Triangle Park,
North Carolina. December 1973.
32
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PRIMARY ZINC PRODUCTION
Suspension Roasting
PROCESS NO. 5
1. Function - Suspension, or flash, roasting is a process for rapid removal
of sulfur and conversion of zinc to calcine by allowing the concentrates to
fall through a heated oxidizing atmosphere or blowing them into a combustion
chamber. Roasting in suspension promotes better heat transfer than multiple-
hearth roasting and thereby increases reaction rates for desulfurization.
The chemical reactions occurring in the two processes are the same, and the
S02 stream produced during conversion of the zinc sulfides to calcine is
strong enough for sulfuric acid production. Removal of mercury and cadmium
is also similar.
i
Suspension roasting is similar to the burning of pulverized coal, in
that finely ground concentrates are suspended in a stream of air and sprayed
into a hot combustion chamber where they undergo instantaneous desulfuriza-
tion. In practice, a stream of hot air passes through the finely ground
concentrate to temporarily suspend the particles. The reaction usually
proceeds without the addition of fuel unless the sulfide content of the ore
is too low. I
The roaster consists of a refractory-lined cylindrical steel shell with
a large combustion space at the top and two to four hearths in the lower
portion, similar to those of a multiple hearth furnace. Because the feed
must be carefully sized, additional grinding may be needed for proper prepara-
tion. In more recent models of flash furnaces, concentrate is introduced
into the lower one or two hearths to dry before final grinding in an auxiliary
ball mill and introduction into the combustion chamber.
About 40 percent of the roasted product settles out on a collecting
hearth at the bottom of the combustion chamber. This coarser material is
likely to contain the most sulfur, so it is further desulfurized by being
rabbled across this hearth and another hearth immediately below before being
discharged from the roaster. Particulate collected in ducts and control
devices can be fed to these hearths to achieve further oxjdation and sulfate
decomposition or to obtain a more homogeneous product (I)1.
The remaining 60 percent of the product leaves the furnace with the gas
stream, passing first through a waste heat boiler and then to cyclones and an
electrostatic precipitator, where it is recovered. About 20 percent of the
suspended dust drops out in the boiler; the cyclones and precipitator remove
about 99.5 percent of the remainder (1). !
i
Total residual sulfur in the calcine is 2.6 percent,; with 0.1 to 5.0
percent as sulfide and the balance as sulfate (2). !
Feed capacities of older suspension roasters are about 90 metric tons
per day, whereas newer roasters can handle about 320 metric tons of con-
centrate per day (1).
33
-------
2. Input Materials - Zinc concentrate is the input material for suspension
roasters. Sodium or zinc chloride may be added to facilitate later removal of
cadmium and lead. Specific quantities for these inputs are not available.
3. Operating Conditions - Suspension roasters are unpressurized and operate
at an average temperature of 980°C (1). Off-gases exit the roaster at about
1000°C (3). Operating times vary, depending upon the same factors as in
multiple-hearth roasting.
4. Utilities - Natural gas, oil, or coal is used to bring the roaster feed
to ignition, after which exothermic oxidation of the sulfur maintains operat-
ing temperatures. About 280,000 kilocalories per metric ton of ore are
required for ignition (4). Air is also added to the suspension roaster.
Water is supplied to the waste heat boiler system, and relatively small
amounts of electricity are required for fans, pumps, and rabble arms.
5. Waste Streams - The S02 concentration in the off-gas from suspension
roasting is higher than that in multiple-hearth processes, averaging 10 to 13
percent (5). It also contains oxygen, nitrogen, carbon dioxide, and water
vapor. The higher S02 content increases the efficiency of sulfuric acid
production. Emission of particulates depends on operating conditions, averag-
ing about 6 percent of the feed. These emissions consist of dust and metal
fumes, depending on the concentrate composition and operating conditions. The
volume of off-gases ranges from 280 to 420 cubic meters per minute (5).
There are no process water or solid wastes. There is a boiler Slowdown
from the waste heat boiler. Solids are recycled to recover by-product metals.
6. Control Technology - The S02 stream is concentrated enough to allow
sulfuric acid production. The roaster off-gases are cooled to about 400°C by
heat transfer with waste heat boilers (3). The gas is then typically diluted
with air and humidified with water sprays before cleaning in an ESP. After
conditioning, gas volume is 475 to 700 cubic meters per minute, and the S02
concentration is 6 to 8 percent (6).
7. EPA Source Classification Code - None ;
8. References -
1. Schlechten, A.W., and A. Paul Thompson. Zinc and Zinc Alloys. In:
Kirk-Othmer. Encyclopedia of Chemical Technology. Interscience
Division of John Wiley and Sons, Inc. New York. 1967.
2. Background Information for New Source Performance Standards:
Primary Copper, Lead, and Zinc Smelters. EPA-450/2-74-002a.
Office of Air and Waste Management, U.S. Environmental Protection
Agency. Research Triangle Park, North Carolina. October .1974.
34
-------
3.
4.
5.
6.
Development Document for Interim Final Effluent Limitations Guide-
lines and Proposed New Source Performance Standards for the Zinc
Segment of the Nonferrous Metals Manufacturing Point Source Cate-
gory. EPA-440/1-75/032. Effluent Guidelines Division Office of
Water and Hazardous Materials. U.S. Environmental Protection
Agency. Washington, D.C. November 1974. 1
Study of Industrial Uses of Energy Relative to Environmental
Effects. EPA-450/3-74-044. U.S. Environmental Protection Agency.
Research Triangle Park, North Carolina. July 1974.
Field Surveillance and Enforcement Guide for Primary Metallurgical
Industries. EPA-450/3-73-002. Office of Air and Water Programs,
U.S. Environmental Protection Agency. Research Triangle Park,
North Carolina. December 1973.
Jones, H.R. Pollution Control in The Nonferrous
Noyes Data Corporation. Park Ridge, New Jersey
35
Metals Industry.
1972.
-------
PRIMARY ZINC PRODUCTION
PROCESS NO. 6
Fluidized-Bed Roasting
1. Function - The fluidized-bed roaster is the newest method for removing
sulfur and converting zinc to calcine. In this roaster, finely ground sul-
fide concentrates are suspended and oxidized in a bed supported on an air
column. As in the suspension roaster, the reaction rates for desulfurization
are more rapid than in the older multiple-hearth processes. The chemistry of
this process is the same as that in other roasters. This process also pro-
duces enough S02 for manufacture of sulfuric acid. Removal of mercury and
cadmium is similar to removal of other wastes.
The fluidized-bed roaster was originally designed for calcining arseno-
pyrite gold ores; several North American zinc smelters have adopted :it in
different forms for use in pyrometallurgical and electrolytic processes.
Designs differ primarily in whether the roasters are charged with a wet
slurry or a dry charge. One variation is fluid column roasting, which was
developed in this country by the New Jersey Zinc Company after being used
abroad. In this process, feed to the roasters is pelletized. The calcined
product is also pelletized, eliminating the need for further agglomeration by
sintering. Fluid column roasting operates at slightly higher temperatures
than fluidized, bed.
In the fluidized-bed process, no additional fuel is required after
ignition has been achieved. Operation of the system is continuous. The feed
enters the furnace and becomes fluidized, or suspended, in a bed supported on
an air column. Temperature control is achieved manually or automatically,
via water injection. Relatively low, uniform operating temperatures appear
to lessen the formation of ferrite. The temperatures in the roaster are high
enough to warrant the use of waste heat boilers to cool the off-gases.
The sulfur content of the charge is reduced from about 32 to
0.3 percent. Efficiency of the operation is maximum when 20 to 30 percent
excess air is supplied over stoichiometric requirements for oxidation of both
sulfur and metals of the charge (1). " '.
Dust carryover into the dust collecting system is somewhat less than in
flash roasting. Particulates emitted average 50 to 85 percent for fluidized
bed and only 17 to 18 percent for fluid column (2). Amounts vary with the
feed rate (and consequently with the air rate), and with the size of the
material being roasted.
The S0£ content of roaster gas is reported to be 7 to 12 percent. If
higher, it is diluted to about 7 percent before reaching the contact acid
plant. The theoretical maximum S0£ concentration achievable is 14.6 percent
when roasting 100 percent zinc sulfide concentrates to completion, unless the
air supply is enriched with oxygen (1).
Although the preferred method of regulating temperature within the bed
is by water injection with automatic thermocouple-operated control, water
36
-------
injection and slurry feeding can be eliminated when it is desirable to mini-
mize the water content of the gas.
The major advantage of the fluidization roaster is its ability to
process higher tonnages per furnace per unit time, because; of the increased
reaction rates for desulfurization. Fewer man-hours are required. Also,
like the suspension roaster, the fluidized-bed roaster can produce a calcine
with lower total sulfur content than the multiple-hearth processes; the exact
percentage elimination of sulfur during roasting is a function of the initial
sulfur content of the concentrate. !
Total residual sulfur is 2.6 percent, with 0.1 percent as sulfide and
the balance as sulfate (3).
(3).
Feed capacities can range up to 320 metric tons of concentrate per day
2. Input Materials - Zinc concentrate is the primary input material for
fluidized-bed roasting. As with other processes, sodium or zinc chloride may
be added to combine with cadmium dust, facilitating the later removal of
cadmium as a by-product. The feed is usually finely ground at one plant to
90 percent minus 0.044 millimeter (4). However, in some modifications, the
feed is pelletized to provide longer retention time in the> roaster (5).
3. Operating Conditions - Fluidized-bed roasters operate under a pressure
slightly lower than atmospheric through as much of the system as possible.
Operating temperatures average 1000°C. The temperature in fluid column
roasters is 1050°C. Operating times are variable, depending upon the same
factors as in other roasting processes. j
4. Utilities - Natural gas, oil, or coal is used to bring the roaster feed
up to reaction temperatures,, after which exothermic oxidation of the sulfur
maintains the temperature and operation is continuous. About 280,000 kilo-
calories per metric ton of ore is required for ignition (6). Cooling water
is added, as well as low-pressure air (19 to 21 kg/cm2) which is introduced
into the windbox for combustion and fluidization of the mix.
5. Waste Streams - Typical SO^ concentrations in the off-gas from fluidized-
bed roasters range from 7 to 12 percent, although the higher figure is more
common. Fluid column roasters average 11 to 12 percent. Exit gases also
contain oxygen, nitrogen, carbon dioxide, and water vapor.; The volume of
off-gas produced ranges from 170 to 280 cubic meters per minute (5). Temper-
atures are approximately 950°C.
As with the other processes, the amount and composition of particulates
and metal fumes depend on the concentrate composition and operating condi-
tions. ,
As with other roasting processes, solids are recycled to recover by-
product metals and there are no solid or process-water wastes.
37
-------
6. Control Technology - Emission controls are the same as for the flash
roasting process. The S02 stream is used for sulfuric acid production.
Since S02 control systems normally require clean gas streams, particulates
are captured by cyclones and ESP's before the gas stream enters the acid
plant and thus present no air pollution problems. Waste heat boilers cool
the gases to 400°C. ;
In one operation, roasting 127 metric tons of dry concentrates per day,
30 percent of the calcine left the roaster via the overflow pipe, 23, percent
was deposited in the waste heat boiler, 44 percent was captured by the
cyclones, and 3 percent entered the hot Cottrell electrostatic precipitator
with the flue gases (1). Flue dusts are not recirculated to the reattor,
being sufficiently low in sulficle sulfur. Roasters with pelletized feed
yield about 80 percent to the overflow and 20 percent carry-over as dust.
7. EPA Source Classification Code - None
8. References -
1. Schlechten, A. W., and A. Paul Thompson. Zinc and Zinc Alloys.
In: Kirk-Othmer. Encylcopedia of Chemical Technology. Inter-
science Division of John Wiley and Sons, Inc. New York. 1967.
2. Field Surveillance and Enforcement Guide for Primary Metallurgical
Industries. EPA-450/3-73-002. Office of Air and Water Programs,
U.S. Environmental Protection Agency. Research Triangle Park,
North Carolina. December 1973.
3. Background Information for New Source Performance Standards:
Primary Copper, Lead, and Zinc Smelters. EPA-450/2-74-002a.
Office of Air and Waste Management, U.S. Environmental Protection
Agency. Research Triangle Park, North Carolina. October 1974.
4. Development Document for Interim Final Effluent Limitations Guide-
lines and Proposed New Source Performance Standards for the Zinc
Segment of the Nonferrous Metals Manufacturing Point Source Cate-
gory. Battelle Columbus Laboratories. EPA Contract Environmental
Protection Agency. Research Triangle Park, North Carolina.
December 1973.
5. Van Den Neste, E. Metal!urgie-Hoboken-Overpelt's Zinc Electro-
winning Plant. CIM Bulletin. Sixth Annual Hydrometallurgical
Meeting. 1977.
6. Study of Industrial Uses of Energy Relative to Environmental
Effects. EPA-450/3-74-044. U.S. Environmental Protection Agency.
Research Triangle Park, North Carolina. July 1974.
38
-------
PRIMARY ZINC PRODUCTION
Contact Sulfuric Acid Plant
PROCESS NO. 7
1. Function - An acid plant catalytically oxidizes S0£ gas to sulfur tri-
oxide, and absorbs it in water to form sulfuric acid.
Contact sulfuric acid plants are continuous steady-state processing units
that are operated in other industries using S02 made by burning elemental
sulfur. They may be used with waste SOz streams if the gas is sufficiently
concentrated, is supplied at a reasonably uniform rate, and is free from
impurities. j
1
The heart of a sulfuric acid plant is a fixed bed of: vanadium pentoxide
or other special catalyst which oxidizes the S02. All other components of the
plant are auxiliary to this catalytic converter. The other components clean
and dry the stream of gas, mix the proper amount of oxygen with it (unless
sufficient oxygen is present), preheat the gas to reaction temperature, and
remove the heat produced by the oxidation.
The plant incorporates one or two absorbers to contact the gas with water
to form the acid. If only one absorber is provided, this'is described as a
single-contact sulfuric acid plant. If two are provided, the second is
placed between stages of the converter, and this is a double-contact plant.
The second absorber allows a larger proportion of the S02 to be converted into
acid, and thus removes more S02 from the gas stream if the initial concentra-
tion is high. |
2. Input Materials - Most contact sulfuric acid plants operate most effi-
ciently with a constant gas stream that contains 12 to 15!percent S02-
Performance almost as good can be achieved in plants that are designed for 7
to 10 percent S02 content. The ability of a plant to convert most of the S02
to sulfuric acid declines either as gas streams become weaker in S02 or as the
flow rate or concentration becomes less consistent. A concentration lower
than 4 percent S02 is extremely inefficient, since sufficient catalyst temper-
ature cannot be maintained (1). Certain modifications of the process, which
add heat by combustion of fuel, can provide better conversion at low S02
concentrations. }
The gas that enters the catalyst bed must be cleaned;of all particulate
matter, be almost completely dried, and contain no gases or fumes that act as
poisons to the catalyst. The acid plant is always supplied with special
scrubbers to remove final traces of objectionable materials.
form
Clean water is required to react with the $03 to
may be necessary to deionize the water in a special ion
order to avoid excessive corrosion or to meet acid quality
Steam condensate may also be used.
sulfuric acid. It
exchange system in
specifications.
39
-------
3. Operating Conditions - The catalyst bed operates properly only if temper-
atures are maintained between 450° and 475°C. Pressures do not usually
exceed 2 kilograms per square centimeter. The plants are usually not enclosed
in a building.
4. Utilities - Noncontact cooling water is required. At one plant producing
1500 metric tons of acid per day, about 12 million liters of water is required
each day (2).
A small amount of electricity is required for pumps and blowers. This
may be generated on-site in some cases, where recovery of waste heat is
maximized.
In certain patented modifications, heat from combustion of natural gas is
used to provide better efficiency at low S02 concentrations. Natural gas or
oil is also required to heat any acid plant to operating temperature following
a shutdown.
5. Waste Streams - Single-contact sulfuric acid plants using weak gas
streams can at best absorb only 96 to 98 percent of the S02 fed to them. The
remaining quantity passes through to the atmosphere. Efficiencies as low as
60 percent have been reported (3).
Double-contact acid plants provide a higher percentage of S02 removal if
they are fed gas with a higher S02 content. Efficiencies higher than 99
percent have been reported. Exit gas S02 concentration is still usually
within the same range as shown above, although one recently developed process
claims stack emissions of less than 0.005 percent S02 (4).
In sulfuric acid plants, it is difficult to prevent some loss of SOs, in
the form of a fine mist of sulfuric acid, with the absorber exit gases. This
is usually 0.02 to 0.04 kilogram of SOs Per metric ton of 100 percent acid
produced.
The scrubbing columns that clean the waste gas stream create off-grade
weak acid that cannot be sold. The amount is estimated as 4 to 8 liters for
each 10 cubic meters of gas treated (5).
In this industry, most particulate matter from gas cleaning equipment is
recycled to the metallurgical processes. The small quantities of particulate
removed by the acid scrubbing operations, however, are mixed with a stream of
weak sulfuric acid and cannot readily be recycled. They are discharged with
the acid plant blowdown.
In some sections of the country it is difficult to sell the product acid,
even for less than the cost of manufacture. Therefore, it may be less ex-
pensive to neutralize and discard the acid than to absorb the costs of ship-
ment to a distant user. Thus, the product acid can itself become a waste
stream.
40
-------
An acid plant does not produce solid wastes directly,: but the gypsum
formed in neutralization of acid can constitute a significant solid waste.
6 Control Technology - In this country the S02 in the tail gas from the
sulfuric acid plant is not controlled. When S02 emissions are large, the best
control may be to increase operating efficiency by adding additional catalyst
stages or by adding heating equipment to maintain proper catalyst temperature.
Changes in the metallurgical operations to produce a stream of higher S02
concentration at a more uniform rate are also good controls, if this is pos-
sible. Scrubbing of the weak S02 stream for final SOe abs-orption may also be
necessary. >
Mist eliminators in the form of packed columns or impingement metal
screens can minimize acid mist emissions. Manufacturers claim elimination of
all but 35 to 70 milligrams of mist per cubic meter of gas, and the units at
times perform .better. To prevent formation of plumes of mist during periods
of abnormal operations, however, electrostatic precipitators are often used.
Better regulation of feed rate and quality also minimizes;acid loss.
If volumes of strong acid must be neutralized, treatment with limestone
followed by more precise pH adjustment with lime, and discharge to a pond for
in-perpetuity storage of the resulting gypsum is the only tested and econom-
ical method of disposal. \
7. EPA Source Classification Code - None j
. - ^ i
8. References - ;
1. Jones, H.R. Pollution Control in the Nonferrous Metals Industry.
Noyes Data Corporation. Park Ridge, New Jersey. 1972.
3.
3.
4.
5.
Hallowell, J.B., et al. Water Pollution Control in the Primary
Nonferrous Metals Industry - Volume I. Copper,; Zinc, and Lead
Industries. EPA-R2-73-274a. U.S. Environmental Protection Agency.
Washington, D.C. September 1973. j
Confidential information from EPA. ,
Browder, T.J. Advancements and Improvements in the Sulfuric Acid
Industry. Tim J. Browder Co. San Marino, California.
Vanderqrift, A.E., L.J. Shannon, P.6. Gormena, jE.W. Lawless, E.E.
Sallee! and M. Reichel. Particulate PollutantiSystem Study - Mass
Emissions, Volumes 1, 2, and 3. PB-203 128 PB-203 522 and
PB-203 521. U.S. Environmental Protection Agency,, Durham, North
Carolina. May 1971. \
41
-------
PRIMARY ZINC PRODUCTION
PROCESS NO. 8
Sintering
1. Function - Sintering has two purposes: first., to volatilize lead and
cadmium impurities and discharge them into the off-gas stream where they can
be captured; and second, to agglomerate the charge into a hard, permeable
mass suitable for feed to a pyrometallurgical reduction system.
Dwight-Lloyd-type sinter machines are typically used in the zinc indus-
try. These are downdraft units in which grated pallets are joined to form a
continuous conveyor system. The feed is normally a mixture of calcine or
concentrates, recycled ground sinter, and the required amount of carbonaceous
fuel, which is pelletized and sized to assure a uniform, permeable bed for
sintering before it is fed to the machines and ignited. Different smelting
methods demand different properties of the sintered feed. Purity of the
final zinc product dictates the type of sinter needed.
The feed is dumped on one end of a moving conveyor and is ignited as it
enters the natural gas-fired ignition box. Combustion is sustained by sup-
plying air to the pellets. Temperature control is achieved by limiting the
coke and coal content and sulfur content of the sinter mix. Once oxidation
is started, it becomes self-sustaining. Air flow regulation provides addi-
tional temperature control. A rotating scalper shaves off the top layer of
the sinter bed just before the discharge end of the machine. This top layer
is the sinter product, with composition at one plant as shown in Table 11B
Estimates are that 80 to 90 percent of the cadmium and 70 to 80 percent of
the lead are removed from the sinter feed (2). Dust collected from this
circuit is greatly enriched in these impurities, and by recycling, levels are
built up high enough in the flue dust to permit economic recovery of cadmium
The lower portion of the bed, which was not removed by the scalper, is
discharged to a set of crushers. Coarser material may be separated on a
Vibrating screen. Oversized particles are returned to the sinter machine
while undersized material is incorporated with the feed mix. Usually 5 to 10
percent of the total feed appears as dust in the gas that is discharged (2)
These dusts become the input material to the cadmium recovery process.
Structural strength of the sinter must be considered, especially in
vertical reduction furnaces, since it must be able to support a great amount
of confining pressure from the overburdening charge. Mechanical collapse is
prevented by close chemical control of the nonvolatile ingredients and addi-
tion of silica to increase hardness and strength of the sinter mass. In
horizontal-reduction-type retorts, no longer in use in this county, a soft,
friable sinter was usually desirable. Structural strength was not needed
because of the lack of heavy overburdening pressures. Cadmium content of the
dust is usually 1 percent, allowing profitable cadmium recovery.
A process known as "sinter slicing" may be utilized in Dwight-Lloyd
machines where higher grades of zinc are sought or recovery of impurities is
profitable. The process is based on the fact that sintering or ignition is
not homogenous throughout the charge, but migrates downward, eventually
resulting in concentration of lead and cadmium sulfides at the bottom of the
42
-------
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sinter cake. The finished sinter is sliced off for further refining in zinc-
reduction retorts. The lower section of the sinter cake, with its high
cadmium content, passes on to the discharge end of the machine. After
crushing it is returned to the charge.
Dust collected in a baghouse still contains incompletely oxidized mate-
rials such as submicron-sized particles of cadmium metal. Temperatures are
controlled by CO gas burners and tempering air. The burned dust usually
contains 10 to 20 percent cadmium, 12 to 15 percent zinc, and 35 to ,45 percent
or more lead (4). The dust is sent to a lead smelter for further cadmium
concentration and then to an electrolytic plant for recovery of pure cadmium.
In sintering preroasted charges, it is reported that substitution of
fluosolid calcines for those made by older types of roasters has meant han-
dling a more finely divided charge. Such a charge consists of about 2.8
percent sulfur, of which about 2.4 percent is sulfate. These calcines are
reported to sinter very rapidly when mixed with 4 to 5 percent fine coal.
The product averages approximately 0.3 percent sulfur and 0.05 percent
cadmium (5).
In one pyrometallurgical zinc plant, a briquetting step is added in
preparing the charge for reduction. The sinter is ground, then mixed with
pulverized coal, clay, moisture., and a binder. The mixture is pressed into
small briquettes (about 0.7 kg) which are fed into a step-grade autogenous
coking furnace. The briquettes attain a strong structure which resists
disintegration as well as keeping the reductant and zinc oxide in close
contact. Heat is generated by burning volatile constituents of the charge
produced inside the furnace (6).
2. Input Materials - Specific quantities of input materials to a sinter
machine are dependent on properties of the concentrate and the type of reduc-
tion (i.e. retort or electric). The sinter mix typically contains calcine,
recycled sinter, coke or oil, arid sand or other inert ingredients. One plant
has reported using 22 kilograms of sand and 80 kilograms of coke breeze (not
including carbon in furnace residue and bag filter dust) per metric ton of
sinter produced (6). Moisture is added when the constituents are mixed,
although where available, zinc sulfate solutions from in-plant leaching
operations are used to moisten the feed for pelletizing, since this conserves
water and enhances zinc recovery.
In typical zinc sintering operations, charging capacities range from 220
to 550 metric tons per day; from 35 to 80 percent of the new feed is recycled
\'}
3. Operating Conditions - Typical operating parameters for zinc sintering
are 1040°C temperature and atmospheric pressure. The wind box fan operates
at a high negative pressure required to pull combustion air through the bed.
4. Utilities - The sinter mix usually contains 4 to 5 percent coke or coal
to supply enough heat for sintering (3). Natural gas is used to ignite the
mix. Electrical energy is required for operating the sinter machine. Fuel
consumption has been estimated to be about 280,000 kilocalories per metric
44
-------
ton of concentrate processed (8), although one plant reported use of 51,000
kilocalories per metric ton of sinter produced. Water (or zinc sulfate
solution, if available) is added to facilitate pelletizing. Air is supplied
to maintain combustion (6).
5. Haste Streams - The sintering operation is a source of air pollutants in
the form of parti culates and S02- The S()2 concentration in off-gas from the
sinter machine is very low, 0.1 to 2.4 percent by volume, which represents
only 1 to 5 percent of the sulfur originally present in the feed (3). When
zinc calcines are sintered, sulfur emissions from a sinter machine are pri-
marily determined by the sulfur content of the input calcine, although some
emissions result from the zinc sulfate liquor added to the sinter mix.
Typically, the resulting weak off-gas stream contains approximately 1000 ppm
SOg; the concentration, however, can range from 400 to 3000 ppm S02, depending
upon the total sulfur content; of the feed stock (7). >
All of the particulate matter is less than 10 microns. Solids loadings
range from 9 to 100 grams per cubic meter (9). Emissions1; consist of dusts
and metallic fumes of a composition similar to that of the calcine. Typical
chemical composition of particulates is 5 to 25 percent zinc, 30 to 55 percent
lead, 2 to 15 percent cadmium, and 8 to 13 percent sulfur (9). Other con-
stituents include copper, arsenic, antimony, bismuth, selenium, tellurium,
and tin. Cadmium and lead contents are especially high because about 90
percent of the cadmium and 70 to 80 percent of the lead is eliminated in the
sinter machine. The fumes condense and are collected with the dust. An
analysis of particulate emissions from a sinter machine is presented in Table
12. ;
In the briquetting step used at one plant, the coker combustion products
are released through an uncontrolled stack and contain sizable quantities of
metallic fume (10). Data on particulate emissions from this process are
presented in Table 13, ;
Gas rates vary widely. With a calcined feed, exhaust gas rates vary
from 42 to 72 cubic meters per square meter of grate. The gas rate with a
concentrate feed is 5.4 to 6.5 cubic meters per square meter (9). Tempera-
ture of combined exit sinter gases varies from 160°C to 380°C in different
plants (9). In addition to sulfur oxides, the gases contain air, water
vapor, carbon dioxide, and traces of other gases.
The sinter process includes no sources of process 1
wastes. Water that is added to the mix is vaporized and
stack, while all solids left unsintered are recycled.
iquid or solid
emitted through the
6. Control Technology - Currently no control methods ai
weak S02 stream from the sintering operation. Best ava-"
nology is application of chemical scrubbing techniques.
cooled by air dilution and water sprays in preparation-
e applied to the
avai'able control tech-
Exit gases may be
for gas cleaning.
45
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Ideal control for S02 emissions from sintering would be to eliminate as
much sulfur as technically possible during roasting. Based on the capability
of fluid bed and suspension roasters, a calcine averaging 1.5 percent total
sulfur could be produced, rather than the current typical calcine with
approximately 3 percent total sulfur. Roasting operations that reduce the
residual sulfur content in the calcine produce a corresponding decrease in
sulfur emissions to the atmosphere by sintering. Since additional coke or
coal is then needed to accomplish sintering, elimination of sulfur in the
calcine could increase costs.
Various combinations of settling flues, cyclones, ESP's, and baghouses
are used on sintering machines. Efficiencies range from 94 to 99 percent.
Table.12 includes comparative data for different control devices.
Sinter crushing and screening operations have enormous potential for
particulate emissions. These operations are hooded and ducted to control
devices. The exhaust gas is further cooled by dilution air and water sprays
to condition it for cleaning in dust collectors.
7. EPA Source Classification Code - 3-03-030-03
8. References -
1.
2.
3.
4.
5.
6.
Development Document for Interim Final Effluent Limitations Guide-
lines and Proposed New Source Performance Standards for the Zinc
Segment of the Nonferrous Metals Manufacturing Point Source Cate-
gory. EPA-440/1-75/032. Effluent Guidelines Division Office of
Water and Hazardous Materials. U.S. Environmental Protection
Agency. Washington, D.C. November 1974.
Field Surveillance and Enforcement Guide for Primary Metallurgical
Industries. EPA-450/3-73-002. Office of Air and Water Programs,
U.S. Environmental Protection Agency. Research Triangle Park,
North Carolina. December 1973.
Lund, R.E., et al.
Joe Minerals Corp.
1970.
Josephtown Electrothermic Zinc Smelter of St. ,
AIME Symopsium on Lead and Zinc, Vol. II.
Howe, H.E. Cadmium and Cadmium Alloys. In: Kirk-Othmer. Ency-
clopedia of Chemical Technology. Interscience Division of John
Wiley and Sons, Inc. New York. 1967.
Schlechten, A.W., and A. Paul Thompson. Zinc and Zinc Alloys. In
Kirk-Othmer. Encyclopedia of Chemical Technology. Interscience
Division of John Wiley and Sons, Inc. New York. 1967.
Development Document for Interim Final Effluent Limitations Guide-
lines and Proposed New Source Performance Standards for the Zinc
Segment of the Nonferrous Metals Manufacturing Point Source Cate-
gory. Battelle Columbus Laboratories. EPA Contract No.
68-01-1518. Draft data.
48
-------
7.
8.
9.
10.
McMahon, A.D., et al. The U.S. Zinc Industry: A: Historical
Perspective. Bureau of Mines Information Circular 8629. U.S.
Department of Interior.- 1974. ;
Fejer, M.E., and D.H. Larson. Study of Industrial Uses of Energy
Relative to Environmental Effects. EPA-450/3-74-044. U.S. Environ-
mental Protection Agency. Research Triangle Park, North Carolina.
July 1974. ;
Jones, H.R. Pollution Control in The Nonferrous Metals Industry.
Noyes Data Corporation. Park Ridge, New Jersey. 1972.
Jacko, Robert B., and David W. Nevendorf. Trace Metal Emission
Test Results from a Number of Industrial and Muncipial Point
Sources. Journal of the Air Pollution Control Association.
27(10):989-994. October 1977. '
49
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PRIMARY ZINC PRODUCTION
PROCESS NO. 9
Vertical Retorting
! Function - The vertical retort process is a continuous reduction/vola-
tilization method for producing high-purity zinc from zinc oxide by:reduction
with carbon at elevated temperatures in vertical silicon-carbide retorts. An
alternate retorting process is the electric retort (Process No. 10). The
older horizontal retorting process is no longer in use in the United States.
Because of the relatively low boiling point of zinc (906°C), reduction
and purification of zinc-bearing minerals can be accomplished to a greater
extent than with most minerals. Immediate separation from nonvolatile
impurities is possible. In fact, if the material is treated pyrometallurgi-
cally, there is virtually no alternative. Even at 857°C (the lowest tempera-
ture at which the oxide can be continuously reduced), the vapor pressure of
zinc is high enough to cause it to vaporize immediately upon reduction.
Balanced against the easy separation from nonvolatiles, however, are the
difficulties of condensing the vapor and the high volatility of several of
the most common impurities, especially cadmium and lead.
The substance most responsible for direct reduction of zinc commercially
is carbon monoxide. The zinc-reduction cycle consists qf the following
reactions:
ZnO + CO -» Zn(vapor) + C02 (actual reduction step) :
C0
2 CO (regeneration of CO)
Both reactions are reversible, and since the second is the slower of the two
at temperatures below about 1100°C, it controls the rate of reduction in most
commercial situations. Above 1100°C the rates of diffusion and heat transfer
predominate as rate-controlling factors. Both of the above reactions are
highly endothermic (1). The following additional reactions occur during
retorting:
Fe203 + 3C -> 2Fe + SCO
Fe203 + C -> 2FeO + CO
ZnS + Fe -» Zn + FeS
Zn + C02 * ZnO + CO (blue powder formation)
Zn2Si04
FeO + Si0
2C * 2Zn + 2CO
xFeO'SiO;2 (slag formation).
Carbon for reduction is normally provided by coal or coke, the choice
determined by the need for structural strength within the smelting column or,
if zinc oxide is the product, by the danger of contaminating the product with
soot. Carbon in excess of stoichiometric reduction requirements is normally
50
-------
provided to furnish extra reaction surface; this compensates for the slowness
of reduction of carbon dioxide by carbon. ;
The charge to a vertical retort must be in the form of a hard sinter or
briquette. The reduction fuel is mixed with ore and a temporary binder. The
briquettes are then coked in an autogeneous coking furnace in an operation
that also drives off volatiles which serve as fuel. Each unit has a capacity
of 90 to 108 metric tons of briquettes averaging about 40 percent zinc (1).
Although briquetting is an expensive operation, the ability to use
briquetted sinter feed could be turned into an advantage: since it permits the
use of a soft sinter produced either by coke-sintering or roast-sintering.
Such a process is being used in England by the Imperial Smelting Corporation,
Limited, but, with one exception, has not been adopted by domestic producers.
Vertical retorts are large, refractory-lined vessels with external gas
chambers. The furnaces consist of three major sections - the charge column,
the reflux section, and the combustion-heating chambers. The retort is
rectangular in general cross-section, about 0.3 meter wide, 1.8 to 2.4 meters
long, and 10.6 meters high, giving a capacity of about 7.25 metric tons of
zinc per retort per day. Walls are of silicon carbide to facilitate heat
transfer and minimize penetration by zinc vapor. Joints in the end walls are
packed with silicon carbide and graphite to permit differential expansion
upon heating. Production rates are reported at an average 195 to 215 kilo-
grams of metallic zinc per day for each square meter of long wall surface
when heated to 1300°C. Reported life of retorts is about 3 years (1).
i
Without intermediate cooling, coked briquettes are fed to the charging
extension at the top of each vertical retort. The charge is heated by gas in
chambers surrounding the retort sidewalls. Gases from the combustion chambers
are used to preheat incoming air for combustion by means; of recuperators.
The briquettes maintain their shape throughout the reduction operation. The
furnace has a vertical retort shaft which allows the charge, with the aid of
gravity, to pass downward through the combustion or heating zone of the
column; heat produced in the combustion chamber is transferred through the
refractory walls of the column to the charge. As the charge moves down
through the retort, the zinc: oxide decomposes to form zipc vapors and carbon
monoxide. Approximately 95 percent of the zinc vapor leaving the retort is
condensed to liquid zinc (2). The residue, containing approximately 10
percent zinc (3), is removed at the bottom through an automatically controlled
roll discharge mechanism into a quenching compartment, from which it is
removed for further treatment. I
During the passage of the briquettes through the retort, sufficient air
or exhaust combustion products are introduced at the base of the charge to
ensure that no zinc vapor moves concurrently with the charge and eventually
condenses on spent residue. The gaseous-reaction products formed in the
retort, which rise up through the charge column, are approximately 40 percent
zinc vapor, 45 percent carbon monoxide, 8 percent hydrogen, 7 percent nitro-
gen, and some carbon dioxide (4). These gases exit near the top of the
charge column, pass through a zinc condenser, and then to a .venturi scrubber.
51
-------
By means of a splash system, whereby a mechanically driven device fills the
condenser chamber with a rain of zinc droplets that fall back into a batch of
molten zinc, the zinc vapor from the retorts is condensed and collected with
excellent efficiency. Over 95 percent of the zinc vapor leaving the retort
is condensed to liquid zinc (5). The carbon monoxide is recycled to the
combustion zone.
When the zinc vapor is not cooled quickly enough from reduction tempera-
tures to condensation temperatures, some of the carbon monoxide present in
the mixture forms carbon dioxide. This carbon dioxide, and the small
amount already present in the v
-------
4. Utilities - Heating is done in vertical retort furnaces by combustion of
gas surrounding the retort side walls. One source estimates that 4,000 to
5,000 kiloca Tories per kilogram of zinc are required (6). Another source
reports energy efficiency to be about 10 percent, with typical energy consump
tion 5.0 million kilocalories per metric ton of zinc produced. One company
using vertical retorts finds' that 9.1 million kilocalories 'of coal and coke
per metric ton of zinc produced is required for the briquetting process.
However, this company claims an energy consumption by retorts of only 2.8
million kilocalories per metric ton of distilled zinc, for a total energy
consumption of 12.8 million kilocalories per metric ton (7). Energy require-
ments for refining are about 834,000 kilocalories per metric ton of zinc pro-
duced (8). ;
5. Waste Streams - Emissions are minor when compared with !other steps in
the smelting process such as roasting and sintering. SOg emissions average
less than 50 ppm (9). Flow rate for the carrier gas is 23,000 cubic meters
per metric ton of product, with 2.5 to 3.0 percent carbon dioxide (3).
Particulate emissions are evident only during charging for approximately
one minute. High-efficiency metal recovery is possible from these metal and
metal oxide fumes. Information on particulate emissions, including size
distribution data is presented in Table T4. Blue powder is! the principal
constituent of these emissions along with cadmium, copper, ;chromium, lead,
and iron. ;
The zinc and cdke content of the feed and the air flow rates are the
important process variables.
parameter.
Temperature is the most impotant process
The gas washing water contains zinc and metal oxides, .possibly hydro-
carbons, various particulates (as suspended solids), and the corresponding
products of hydrolysis. >
Residues are also generated in vertical retorting operations. Amounts
produced are around 1050 kilograms per metric ton of zinc produced (3). The
residues contain a variety of metals such as lead, copper, silver, gold,
nickel, germanium, gallium, arsenic, antimony, cadmium, zinc, indium, silicon,
iron, calcium, aluminum, magnesium, and manganese. ;
The germanium and gallium contents originally fourtd in the blends are,
in general, concentrated in the residues from the retorts. ; These residues
can be treated by dissolution in caustic soda, followed by ; treatment by
various methods, most important probably being the extraction of gallium
chloride with an organic solvent. There are many variations on these methods
to circumvent various impurity problems. This processing is riot done at any
of the primary zinc smelters. Two companies, one in Arkansas and one in
Oklahoma, account for the total domestic production of gallwrn, using residues
from z,inc and aluminum production. One refinery in Oklahoma produced all the
primary domestic germanium from zinc smelter residues in 1978 (11).
An analysis of some constituents in the residue from cine vertical
retort furnace is given in Table 15.
53
-------
OCD
i 1 1
CO ---
co
HH UJ
S O
LJU o;
=3 CJ3
o -z.
I I I I
fefe
o a:
LU
O LU
O >
O
UJ
CO
CM
O CM
CO i
co co
O r
«
O 0
to r-.
O CM
O O
o
o o
£ £
CM CM
A .V
LO
1"^
fm
O
O
o
o
CM
CO
o^
^o
r
0
co
CO
o
o
03
O
1
O
CO i
C3 CM
CM CM
O O
O O
O O
O O
co r^»
01 to
CO CTl
0 0
to p^
i LO
0 O
0 CD
CM CM
O r
0 0
CD CD
£ £
4->
(O (t)
4-0 S-
C T E
O -P O--N
i- O) -r- S-
W E to -C
t CJ) «r- C7)
E -^ c= \x-
LU |_U
54
-------
TABLE 15. ANALYSIS OF VERTICAL RETORT FURNACE RESIDUES (3)
Constituent
Cadmium
Chromium
Copper
Lead
Zinc
Concentration,
850
46
4,600
2,400
107,000
ppm
Blue powder production amounts to only about 3 percent of the zinc charged,
considerably less than in horizontal retorting (12). The residue also con-
tains less zinc. ;
In the redistillation system, no zinc vapors can escape since it is a
closed circuit. Solid residues can be reprocessed to recover zinc and other
metals. Waste gases are produced by the combustion; their composition
depends on the type of fuel used. i
\
6. Control Technology - Wet scrubbers are the available control method for
particulate emissions. All gases are exhausted from the furnace by means of
a venturi scrubber. The carbon monoxide from the zinc condensation chamber
is scrubbed with water sprays to remove entrained solids. The gas is then
used as part of the fuel for heating the retorts. Metallic zinc and zinc
oxide is recovered as blue powder residue from the scrubbing system and from
the condenser during periodic cleaning. The blue powder is'recycled.
Residues are either disposed of in open slag dumps or processed to
recover their metal values. The best control technology for the slag dump is
sealing the soil and routing runoff to a waste treatment lagoon.
7. EPA Source Classification Code - 3-03-030-05 !
8. References - j
1. Schlechtetr, A.W., and A. Paul Thompson. Zinc and Zinc Alloys. In:
Kirk-Othmer. Encyclopedia of Chemical Technology.; Interscience
Division of John Wiley and Sons, Inc. New York. ,1967.
2. McMahon, A.D., et al. The U.S. Zinc Industry: A Historical
Perspective. Bureau of Mines Information Circular 9629. United
States Department of the Interior. 1974. |
3. Calspan Corporation. Assessment of Industrial Waste Practices in
the Metal Smelting and Refining Industry - Volume ill, Primary and
Secondary Nonferrous Smelting and Refining. Draft. April 1975.
55
-------
4. Field Surveillance and Enforcement Guide for Primary Metallurgical
Industries. EPA-450/3-73-002. Office of Air and Water Programs,
U.S. Environmental Protection Agency. Research Triangle Park,
North Carolina. December 1973.
5. Development Document for Interim Final Effluent Limitations Guide-
lines and Proposed New Source Performance Standards for the Zinc
Segment of the Nonferrous Metals Manufacturing Point Source Cate-
gory. EPA-440/1-75/032. Effluent Guidelines Division Office of
Water and Hazardous Materials. U.S. Environmental Protection
Agency. Washington, D.C. November 1974.
6. Restricting Emission of Dust and Sulfur Dioxide in Zinc Smelters.
Association of the Metal Smelting and Refining Trade and Committee
on Zinc of the German Ore Smelting and Mining Society VDI No. 2284.
September 1961.
7. Battelle Columbus Laboratories. Development Document for Interim
Final Effluent Limitations Guidelines and Proposed New Source
Performance Standards for the Zinc Segment of the Nonferrous Metals
Manufacturing Point Source Category. EPA-68-01-1518. Draft data.
8. Fejer, M.E., and D.H, Larson. Study of Industrial Uses of Energy
Relative to Environmental Effects. EPA-450/3-74-044. U.S. Environ-
mental Protection Agency. Research Triangle Park, North Carolina.
July 1974.
9. Background Information for New Source Performance Standards:
Primary Copper, Lead, and Zinc Smelters. EPA-450/2-74-002a.
Office of Air and Waste Management. U.S. Environmental Protection
Agency. Research Triangle Park, North Carolina. October 1974.
10. Jacko, Robert B., and David W. Neuendorf. Trace Metal Particulate
Emission Test Results from a Number of Industrial and Municipal
Point Sources. Journal of the Air Pollution Control Association.
27(10):989-994. October 1977.
11. Commodity Data Summaries 1979. U.S. Department of the Interior,
Bureau of Mines. Washington, D.C. 1979.
12. Water Pollution Control in the Primary Nonferrous Metals Industry -
Vol. 1, Copper, Zinc, and Lead Industries. EPA-R2-73-247a.
Office of Research and Development, U.S. Environmental Protection
Agency. Washington, D.C. September 1973.
56
-------
PRIMARY ZINC PRODUCTION
Electric Retorting
PROCESS NO. 10
1. Function - Electric retorting is a continuous reductio'n/volatilization
process in which electricity supplies the energy needed to 'produce high-
purity zinc from zinc oxide by reduction with carbon at elevated temperatures
in a vertical cylindrical retort. Vertical retorting'(Process No. 9) is the
other reduction method in use at zinc smelters. This newest zinc-smelting
furnace was designed to overcome the difficulties involved n'n heating the
charge externally. As in the other retorting process, carbon monoxide is
used for direct reduction, producing zinc vapor. The carbon dioxide also
produced in this reaction is regenerated with carbon. The ^carbon for reduc-
tion is provided by coke. ;
There are several variations on the resistance-type ellectric furnace
available, including the electrothermic arc furnace (Sterling Process) and
the Imperial Smelting Furnace. However, the only electric retorts in use in
the United States are the electrothermic furnaces developed by St. Joe
Minerals Corporation, which began commercial operation in 1930 (1).
The St. Joe electrothermic furnaces are basically vertical, refractory-
lined cylinders. The largest furnaces now in use have an inside diameter of
1.5 meters and are 15 meters high, with a production capacity of about 90
metric tons per day (2). Graphite electrodes protrude into the shaft, and
the reaction heat is generated from the resistance of the furnace charges to
the current flow between the electrodes. Eight pairs of electrodes introduce
power into the furnace. Each top electrode has a mate near; the bottom.
i
I
Preheated coke and sinter,, along with miscellaneous mi'nor zinc-bearing
products such as blue powder, are fed continuously into the top of the
furnace from a rotary feeder. As in a vertical retort, gravity moves the
charge downward through the shaft. Unlike other retorting processes, an
unusually hard sinter is required to maintain strength and porosity in the
tall columns, even after most of the zinc content has been removed. Silica
is usually added to the sinter mix to increase its structural strength. The
coke serves as the principal electrical conductor, carrying the alternating
current between each top electrode and the bottom electrode on the opposite
side. The heat developed provides the energy required for smelting. The
zinc vapor and carbon monoxide produced pass from the main furnace to a vapor
ring, which provides a free space around the periphery of the charge for
removal of the gaseous mixture,, The gas then goes to a condenser, where zinc
is recovered by bubbling through a molten zinc bath. It was the development
of this Weaton-Najarian vacuum condenser that first made possible the produc-
tion of over 90 metric tons per day from a single unit. If necessary,
further refining, such as the liquation and redistillation jsteps described
for the other retorting processes, may be used.
The electrothermic furnace has a number of advantages over other pro-
cesses. First, the increased thermal efficiency (compared with external
heating methods) results in cost savings in fuel consumption. Larger quanti-
ties of charge can be treated, and the continuous operation is amenable to
57 . ;
-------
automation. The furnace can readily process secondary zinc scrap and zinc
residues. Because of special deleading by heat treatment in multiple-hearth
roasters followed by desulfurization in fluidized-bed roasters, electrothermic
furnaces emit practically no S0j> or particulates.
2. Input Materials - The feed consists of sinter, coke, and recycled zinc-
bearing products, such as blue powder. In addition, silica is usually added
to increase structural strength,, Particle size is controlled so as to provide
coke particles larger than sinter, thereby concentrating larger coke at the
axis of the furnace. In this way the maximum fraction of electric current is
directed along the axis, which becomes the region of maximum temperature,
minimizing both damage to the refractory walls by slagging and heat loss
through the walls. Quantities of coke required range from 0.5 to 0.8 ton per
ton of slab zinc produced (3).
3. Operating Conditions - The St. Joe electrothermic furnaces operate at
atmospheric pressure. Internal temperatures are 1400°C and higher at the
axis of. the furnace, 1200°C in the main body of the charge, and 900°C near
the wall. A vacuum of 15 to 25 centimeters mercury is applied to the outlet
of the condenser, causing the vapor/gas mixture to be drawn through it in
large bubbles (4). Water-cooled hairpin loops at the condenser cooling well
maintain a constant batch temperature of 480° to 500°C (2). Temperatures for
purification steps vary, since separation is based on differences in boiling
points.
4. Utilities - Electrothermic furnaces use electricity to supply the
energy for reduction. Current through each of the 30-centimeter diameter
graphite electrodes may range as high as 800 amperes. A furnace contains
eight individual single-phase circuits, each typically carrying 770 to 1190
kilowatts. The working range for such a circuit is 100 to 250 volts. The
overall power factor of transformers, bus system, and furnaces is from
90 to slightly over 95 percent. One plant using electrothermic furnaces
reported energy consumption averaging 2800 kilowatt-hours per metric ton
of metal. Power input per electrode circuit ranging up to 1255 kilowatts,
corresponding to a maximum of 10,000 kilowatts per furnace has been reported
(5). Energy consumption for subsequent refining steps is the same as that
used fn other retorting processes.
Another source reports 25 to 30 percent energy efficiency for electro-
thermic retorting, with the process consuming about 2.8 million kilocalories
per metric ton of zinc. With an additional 5.8 million kilocalories per
metric ton of zinc consumed as coke, a total of 8.6 million kilocaloHes per
metric ton of zinc is used. If the energy for electric generation is also
considered (assuming 33 percent efficiency), total energy consumption in-
creases to 14.2 million kilocalories per metric ton (6).
5. Waste Streams - As with other types of retorting, emissions are minor
relative to those from roasting or sintering.
58
-------
Particulate emissions consist primarily of metal and metal oxide fumes.
Participate emission levels for electrothermic retorting average about 10
kilograms per metric ton of zinc produced. As in the other.retorting pro-
cesses, blue powder is the principal constituent of these emissions, along
with cadmium, copper, chromium, lead, and iron. Also as with the other pro-
cesses, the important process variables are the zinc and coke content of the
feed and the air flow rates. Temperature, which is controlled by regulating
current flow to the electrodes,, is the most important procesjs parameter.
Temperature of the gases vented from the furnace vapor ring averages
850°C. Gas composition is approximately 45 percent zinc vapor and 45 percent
carbon monoxide; the balance is nitrogen, carbon dioxide, and hydrogen (5).
The gas washing water contains the same impurities as i'n vertical
retorting.
i
i
The residues generated during electrothermic reduction are similar in
composition and quantity to those from the other retorting processes.
Germanium and gallium are extracted from the residues by various methods at
other facilities. , I
Waste streams from further purification steps such as liquation and
redistillation are the same as described for vertical retorting.
6. Control Technology - High-velocity-impingment-type scrubbers are used to
clean gases from the condenser. The clean gas, containing 80 percent carbon
monoxide and having a heating value of 2200 kilocalories peri cubic meter,
furnishes fuel for smelter use (5). Some blue powder or uncondensed zinc and
zinc oxide is later recovered by settling the scrubber slurry in ponds. The
solids (75 to 80 percent zinc) are dried and briquetted for furnace feed.
Residue is removed from the furnace, preferably as discrete solid
particles. It goes to a reclamation plant, where residual coke and some
unreacted zinc are recovered and recycled. Besides permitting recovery
of residue containing enough zinc and carbon to make retreatment worthwhile,
a minimum of power is consumed in unproductive melting of re'sidue. At the
reclamation plant, where sand may be added to make a hard sinter, sufficient
ferrosilicon is present in some residues to warrant recovery as a by-product.
;
7. EPA Source Classification Code - None j
i
8. References - }
""" "'"' i
1. Background Information for New Source Performance Standards:
Primary Copper, Lead, and Zinc Smelters. EPA-450/2-74-002a.
Office of Air and Waste Management, U.S. Environmental Protection
Agency. Research Triangle Park, North Carolina. October 1974.
59
-------
2. Development Document for Interim Final Effluent Limitations Guide-
lines and Proposed New Source Performance Standards for the Zinc
Segment of the Nonferrous Metals Manufacturing Point Source Cate-
gory. EPA-440/1-75/032. Effluent Guidelines Division Office of
Water and Hazardous Materials. U.S. Environmental Protection
Agency. Washington, D.C. November 1974.
3. Calspan Corporation. Assessment of Industrial Waste Practices in
the Metal Smelting arid Refining Industry - Volume II, Primary and
Secondary Nonferrous Smelting and Refining. Draft. April 1975.
4. Schlechten, A.W., and A. Paul Thompson. Zinc and Zinc Alloys. In:
Kirk-Othmer. Encyclopedia of Chemical Technology. Volume 22.
Interscience Division of John Wiley and Sons, Inc. New York- 1967.
5. Lund, R.E., et al. Josephtown Electrothermic Zinc Smelter of St.
Joe Minerals Corporation. AIME Symposium on Lead and Zinc, Vol.
II. 1970.
6. Fejer, M.E., and Larson, D.H. Study of Industrial Uses of Energy
Relative to Environmental Effects. EPA-450/3-74-044. U.S. Environ-
mental Protection Agency. Research Triangle Park, North Carolina.
July 1974.
60
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PRIMARY ZINC PRODUCTION
Oxidizing Furnace
PROCESS NO. 11
1. Function - In the direct, or American, process for zinc oxide production,
zinc vapor from sintering is immediately oxidized without being condensed.
As with zinc metal production, zinc must first be produced in vapor form.
Only the direct method of producing zinc oxide is discussed because other
production methods, such as the French process, start with slab zinc, the
major product of the primary zinc industry.
!
The three types of furnaces used for this purpose in the U.S. are the
grate-type furnace, the rotary or Waelz kiln, and the electrothermic furnace.
In all three, the feed is reduced to form zinc vapor and subsequently the
vapor is oxidized and the product collected. Two of the main difficulties in
producing zinc metal, dilution of the zinc vapor and reoxidation by carbon
dioxide, are desirable in the production of zinc oxide.
In grate-type furnaces, the coal-sinter feed (usually as briquettes) is
spread over grates (traveling or stationary). Coal is spread first and
ignited, then the sinter is deposited on top of the fuel layer. Air is
forced through the bed to support combustion and furnish a reducing atmo-
sphere to liberate the zinc vapors. The zinc vapors are then ducted to a
combustion chamber, where oxidation occurs and the zinc oxiide product is
formed. . i
The Waelz kiln is a large diameter, long rotary kiln that can be used
for production of pure zinc oxide, although it is more commonly used for
pyrometallurgical concentration of residues of a rather mixed character.
Zinc-bearing material and solid fuel are continuously fed to the kiln, which
typically rotates 1 to 1.5 rpm (1). The additional heat for reduction of the
zinc is supplied by the flow of gaseous fuel through the kiln. The vaporized
zinc then is ducted to a combustion chamber, where air is admitted and the
vapor burned to form zinc oxide. Temperature control is vesry important,
since intimate contact must be maintained between the solid, zinc-bearing
part of the charge, the solid fuel in the charge, and the reducing atmosphere.
The St. Joe vertical electrothermic furnace may be modified for use as
either a metal or an oxide producer (2). Instead of a large "vapor ring" or
bulge in the furnace barrel midway between the upper and lower electrodes,
-the oxide furnace has openings at four levels between the electrodes, through
which evolved zinc vapor and carbon monoxide exit the charge. Preheated coke
and zinc-bearing sinter are continously fed to the furnace!' Coke serves as
the principal electrical conductor. Electricity introduced through the
electrodes develops the heat energy required for smelting, i Further details
on the electrothermic furnace are given in Process No. 9. i
i
Analysis of an American process zinc oxide from one plant is presented
in Table 16. ,
61
-------
TABLE 16. ZINC OXIDE IMPURITIES AND BRIGHTNESS (2)
Lead as PbO
Cadmium as CdO
Iron as Fe203
Manganese as MnO
+325 mesh screen residue
Brightness; Hunter D-40
0.009%
0.010%
0.015%
0.002%
<0.03% !
93.0 for large sizes
91.0 for fine sizes
The zinc oxide is filtered from the carrier gases in bag collectors. Soft
zinc oxide pellets may be formed by squeezing the oxide between two rubber
rolls to form pellet nuclei.
2. Input Materials - The mix for a grate-type furnace is typically 50
percent sinter briquettes and 50 percent coal (1). The preferred type of
coal is a fine-sized anthracite, used to minimize contamination of the vapor
stream with soot and to prevent caking and slagging. :
For the rotary kiln, the mix is usually 65 to 75 percent zinc-bearing
material and the remainder crushed coal or anthracite (to give about 25
percent carbon in the charge) plus a small amount of sand to stiffen the bed,
if desired (1).
The principal feed to the electrothermic furnace also consists! of
sinter and coke, but as much as 25 percent of the total zinc input is other
zinc-bearing materials. Nominal coke rate is 44 percent of the weight of
sinter (roughly equal volumes of coke and sinter) and approximately 300
percent of the stoichiometric carbon relative to the zinc in the sinter.
Other zinc-bearing materials are fed in the form of almond-shaped briquettes,
granules, 8 by 25 millimeter metallic screenings, and slab dross (2). High
product purity is achieved by using low-volatility cokes and sinter that is
low in impurities.
3. Operating Conditions - The grate-type furnace and rotary kiln operate at
atmospheric pressures. Temperatures in the grate-type bed are reported to
range from 1000°- to 1950°C (1);; operating temperatures for the rotary kilns
are about 1300°C.
Electrothermic furnaces also operate at atmospheric pressures. Operating
conditions are the same as in electrothermic furnaces for zinc metal produc-
tion, with temperatures of 120()°C at the vapor ring elevation, 1400°C in the
main smelting zone, 900flC near the walls, and 1300°C at the bottom electrode
elevation (1). Further details on operating conditions of electrothermic
furnaces are given in Process No. 9.
4. Utilities - No information is available regarding heat input per unit of
product output for grate-type or rotary kiln processes. The bed is ignited
by residual heat from the previous charge in grate-type furnaces, and gaseous
fuel supplies the heat for reduction in rotary kilns. Air is added in all
three furnaces in unspecified quantities.
62
-------
The main factor influencing production in electrothermic furnaces is the
quantity of electricity introduced. Josephtown's largest furnace operates at
10,000 kilowatts. Energy consumption averages 2800 kilowatt-hours per metric
ton of metal (2). j "
5. Waste Streams - Combustion products and some zinc oxide are emitted from
all three furnaces; again, quantities are unspecified. i
I
There are no liquid wastes from the process. I
i
A solid residue is produced, estimated at 350 kilograms per metric ton of zinc
oxide. This residue contains approximately 6.2 percent zinc and 0.09 percent
of other metals such as cadmium, chromium, copper, and lead (3). It may also
contain slag, coke, and globules of ferrosilicon. One analysis of waste
samples from oxide furnace residue revealed concentrations presented in Table
17. . | .
TABLE 17. SELECTED CONSTITUENTS OF OXIDIZING FURNACE RESIDUE (3)
Constituent
Cadmi urn
Chromium
Copper
Lead
Zinc
Concentration, p
10
17
810
68
62,000
Dm
Further details on waste streams from electrothermic furnaces are given in
Process No. 9.
6. Control Technology - Control techniques for direct zinc
are similar to those used in other reduction operations.
given in Process No's. 7, 8, and 9. Uncondensed zinc (blue
from the condenser is recovered by settling the water slurry
Solids can be dried and briquetted for furnace feed.
7. EPA Source Classification Code - None
8. References -
1.
2.
3.
Schlechten, A.M., arid A. Paul Thompson. Zinc and
Kirk-Othmer. Encyclopedia of Chemical Technology
Division of John Wiley and Sons, Inc. New York.
Lund, R.E., et al.
Joe Minerals Corp.
1970.
Josephtown Electrothermic Zinc
AIME Symposium on Lead and Zinc
oxide processes
rurther details are
powder) in gases
in ponds.
Zinc Alloys. In:
, Interscience
1967.
Smelter of St.
Volume II.
Calspan Corporation. Assessment of Industrial Waste Practices in
the Metal Smelting and Refining Industry - Volume II, Primary and
Secondary Nonferrous Smelting and Refining. Draft. April 1975.
63.
-------
PRIMARY ZINC PRODUCTION
PROCESS NO. 12
Leaching
1. Function - Electrolytic.product!on of zinc is an alternative to, pyro-
metallurgical processing. The first step is to separate zinc from gangue
minerals by leaching roasted calcine in recycled electrolyte solution. The
zinc dissolves, and the insoluble gangue is separated from the solution by
decantation, thickening, and filtration. The solution is purified (Process No.
12) and the waste solids are either discarded or, if their concentration
warrants, further processed to recover lead and precious metals (1,2).
Two general leaching methods can be employed, described as either a single
or a double leach. In a single leach, recycled electrolyte, which i$ a solu-
tion containing principally sulfuric acid, is brought only once into contact
with the calcine. Zinc oxide in the calcine reacts with sulfuric acid to form
soluble zinc sulfate and water. The single leach is not often practiced, how-
ever, since losses of sulfuric acid are excessive and recovery of zinc is poor.
Double leaching is used most often. In several variations, the calcine
is leached first in a solution that is neutral or slightly alkaline, then in
an acidic solution, with the liquid passing countercurrently to the flow of
calcine. In the neutral leach, the readily soluble sulfates from the calcine
dissolve, but only a portion of the zinc oxide enters into solution. The
second acidic leach solubilizes the remainder of the zinc oxide, but also
dissolves many impurities, especially iron. Recycle of the liquor to the first
neutral stage causes much of the iron to reprecipitate, so the neutral leach
acts also as an initial stage of solution purification. In some of the more
complex process variations, considerable overlap occurs between leaching and
purification steps, and the calcine may be subjected to as many as four
leaching operations in progressively stronger or hotter acids to bring as much
of the zinc as possible into solution.
The leaching process is conducted most often in a series of agitated
tanks. Batch operation is common, since it is thereby possible to compensate
for variations in calcine composition. A few smelters, especially those pro-
cessing ore of consistent quality, are equipped with continuous leaching
equipment. In all of the leaching operations, the pH, temperature, and
solution composition at each step are carefully regulated.
2. Input Materials - The principal input to the leaching process is calcined
zinc concentrate from roasting (Process No's. 4, 5, and 6). If a pelletized
feed is used in the roaster, the calcine must be ground prior to leaching. Air
classification and grinding is occasionally practiced also with unpelletized
material. One foreign plant grinds calcine to 95 percent through a 200-micron
screen (about 70 mesh) (3). Wet grinding with a portion of the leach liquor
is also practiced in some smelters.
The only other input specific to the leaching process is recycled spent
electrolyte from Process No. 13, containing about 200 grams of sulfuric acid
per liter (2).
64
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3. Operating Conditions - Batch leaching processes usually operate at atmos-
pheric pressure, whereas continuous processes may include pressurized steps
up to 2.5 kilograms per square centimeter (1). Most leach operations take
place around 50°C; exothermic chemical reactions maintain this temperature
without requiring additional heat energy. Some hot acid process variations
may employ temperatures up to 90°C(3). ;
4. Utilities - Electricity is required for pumping the solution and convey-
ing the calcine and final residue. No estimates of energy consumption for
the leaching process are given. In batch leaching, air at about 6.3 kilo-
grams per square centimeter must be available to clear out accumulating de-
posits of coarse material at the bottom of the tanks. In continuous leach-
ing, it is seldom necessary to use air at pressures higher than the normal
operating pressure of 1.4 to 2.5 kilograms per square centimeter (1). Con-
tinuous leaching requires a larger total volume of air per tank, because
agitation must be continuous. During agitation in either process, con-
sumption ranges from 2.8 to 4.2 cubic meters of air per minute (1).
5. Waste Streams - As in the closely associated purification process, emissions
of acid mist occur from the leach tanks. This waste is described more com-
pletely in Process No. 12. !
Leach solution may be cooled in open towers, as described for electrolyte
cooling in Process No. 13. Atmospheric mist may therefore be created from
these towers, as described in connection with the electrolysis process.
Except for leaks and spills, no liquid waste is produced from this pro-
cess, i .
After all leaching, the solid residue is filtered from solution and the
filter cake is rinsed with fresh water. This cake will contain all the lead
originally present in the concentrate, and also other acid-insoluble trace
elements such as indium, gold, and the platinum-group metals,! Other minerals
present will be silica, alumina, and silicates of iron, aluminum, and calcium.
The quantity and composition will vary with the characteristics of the ore
concentrate; one measurement reports a quantity of 360 kilograms per metric
ton of zinc produced (1). !
I
6. Control Technology - Control of acid mist emissions is described in con-
nection with the similar mists produced in Process Nos. 12 and 13.
The solid residue will frequently be sent to a lead smelter for recovery
of the lead and other reclaimable elements. Alternatively, the residue may
be batch-treated at the zinc smelter with cyanide for recovery of gold. In
other cases, the concentrations of recoverable metals may bejso small that
the residue will be discarded in a dump. Ore residue from ajdouble leach
process should be inert; however, as generally practiced, the residue will
also contain trace elements from Process No. 12. Single leach residues may
also contain zinc oxide or zinc ferrites. The potential of this waste for
secondary water pollution is unreported. ;
i
7. EPA Source Classification Code - None. I
65
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8. References -
Schlechten, A.W., and A. Paul Thompson. Zinc and Zinc Alloys. In:
Kirk-Othmer. Encyclopedia of Chemical Technology. Interscience
Division of John Wiley and Sons, Inc., New York. 1967.
Development Document for Interim Final Effluent Limitations Guide-
lines and Proposed New Source Performance Standards for the Zinc :
Segment of the Nonferrous Metals Manufacturing Point Source Category.
EPA-440/1-75-032. Effluent Guidelines Division, Office of Water and
Hazardous Materials. U.S. Environmental Protection Agency. Wash-
ington, D.C. November 1974.
E. Van Den Neste. Metallurgie Hoboken-Overpelt's Zinc Electrowinning
Plant. CIM Bulletin. August 1977.
66
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PRIMARY ZINC PRODUCTION
PROCESS NO. 13
Purifying ;.
-..''.' . ' t ' .
1. Function - The leaching of zinc calcine causes other elements in addition
to zinc to dissolve. Unless impurities are removed from the solution, they
wiM,either contaminate the zinc product or interfere with the proper operation
of the electrolysis process.. The solution from leaching is therefore purified
to remove metallic-ions that are more electropositive than zinc. .
Purification is usually conducted in large agitated tanks. A variety of
reagents is added in a sequence of steps that causes impurities to precipitate.
The precipitates are separated from the solution by filtration. The purifi-
cation techniques are among the most advanced applications of inorganic
solution chemistry in industrial use, and vary from one smelter to another.
Iron is often removed in conjunction with the leaching process (Process No.
12) by precipitation as a hydrated oxide (goethite) or a complex sulfate
(jarosite). Some of these processes, which are patented, also reduce the
concentration of arsenic and other elements. Almost all smelters then add
zinc dust, often in the form of "blue powder" from the pyrometallurgical pro-
duction of zinc. This addition causes a cementation reaction that precipitates
cadmium, copper', and several other elements. The final steps remove all but
trace quantities of a group of metals that includes arsenic,; antimony, cobalt,
germanium, nickel, and thallium. These metals severely interfere with
electrolytic deposition of zinc, and their final dissolved concentrations are
limited usually to less than 0.05 milligram per liter. i
2. Input Materials - The principal input is the filtered, acidic, mineral-
rich solution from Process No. 12. Reagents are mostly inorganic, primarily
finely divided metals such as zinc, arsenic, and antimony. Fresh sulfuric
acid may be added in small quantities, and lime may be used to remove excess
sodium carbonate or sodium hydroxide may be used for iron precipitation.
Organic materials such as l-nitroso-2-naphthol may be used to remove cobalt (1).
Inorganic salts such as copper sulfate may be added to catalyze or promote
some precipitation reactions. Except for zinc dust, the quantities of these
additives are small. \
3. Operating Conditions - The purification process takes place at tempera-
tures ranging from 40° to 85°C, and pressures ranging from atmospheric to 2.5
kilograms per square centimeter (2). The conditions at eaclrstep of pre-
cipitation are'carefully regulated. I
j
4. Utilities - Electricity is required to pump solutions and drive equipment
agitators. Steam and noncontact cooling water may be used to heat and cool
solutions. Quantities are not reported. \
1
5. Waste Streams - The atmospheric emission from this process consists of a
mist that develops from the ventilation of the leach and purification tanks.
Ventilation is necessary since side reactions can cause evolution of small
quantities of explosive hydrogen gas which must not be allowed to accumulate.
The mist contains sulfuric acid and smaller quantities of zinc, calcium, and
67
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arsenic (3). There is no estimate of the quantity of waste products,released
from this source.
Except for leaks and spills, no liquid waste is created by this process.
Precipitated solids consisting of impurity elements and excess reagent
metals are accumulated as cakes from pressure filters. The composition of the
cakes is variable, depending on the characteristics of the zinc concentrates
and the details of the processing. All are highly metalliferous.
In some process modifications, much of the iron, and part of the arsenic
and antimony originally present in the concentrate, are precipitated into and
discarded with the insoluble residue from Process No. 12.
6. Control Technology - Ventilation of leach and purification tanks is
usually controlled with impingement or centrifugal demisting equipment.
efficiency of these devices in this service has not been reported.
The disposition of most of the solid residues' has also not been reported.
With some zinc concentrates, a filter cake rich in copper is produced which is
sold to copper smelters. Some residues are recycled to the roaster or leach
tanks, or are separately treated to reclaim zinc and cadmium. Two companies
are reported to have refined indium from some of the residues (4). Filter
cakes rich in cobalt are apparently being stockpiled by a foreign refinery,
and similar disposition may be practiced by some U.S. refineries (2);. It is
not known whether any of these materials are being discarded.
7. EPA Source Classification Code - None.
8. References: !
The
1. Schlechten, A.W., and A. Paul Thompson. Zinc and Zinc Alloys. In:
Kirk-Dthmer. Encyclopedia of Chemical Technology. Interscience
Division of John Wiley and Sons, Inc. New York. 1967.
2. E. Van DeriNeste Metallurie-Hoboken-Overpelt's Zinc Electrowinning
Plant. CIM Bulletin. August 1977.
3. Privileged communication, EPA files.
4. Commodity Data Summaries 1979. U.S. Department of Interior, Bureau.
of Mines. Washington,, D.C. 1979.
68
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PRIMARY ZINC PRODUCTION
Electrolysis
PROCESS NO. 14 '
1. Function - In electrolysis, metallic zinc is recovered>from the purified
solution by passing current through an electrolyte solution, causing zinc metal
to deposit on a cathode. Electrolysis takes place in rectangular tanks, or
cells, each of which holds a number of closely spaced rectangular metal plates.
Alternate plates are made of lead containing 0.75 to 1.0 percent silver; these
are the anodes that are electrically connected to a positive potential. The
remaining plates are made of aluminum, and are connected with a negative
electrical potential. Purified electrolyte from Process No. 13 is circulated
slowly through the cells, and water in the electrolyte dissociates, releasing
oxygen gas at the anode. Electrode voltage is maintained sufficiently high so
hydrogen is not released at the cathode; instead, zinc ions absorb electrons
and deposit zinc metal. Hydrogen ions remain in solution, and thereby regen-
erate sulfuric acid for recycle to the leach process (l,2).i
Zinc smelters contain a large number of electrolytic colls, often several
hundred. They are most often made of concrete with a lead,iplastic, or
vitreous lining, and are electrically connected in series banks. A portion of
the electrical energy is converted into heat, which increases the temperature
of the electrolyte. Therefore, a portion of the electrolyte is continuously
circulated through cooling towers. These are usually open towers, in which
the electrolyte falls through a rising stream of air drawn through the tower
by fans. This method both cools the electrolyte and evaporates the excess
water (3). The cooled and concentrated electrolyte is repumped to the cells.
Every 24 to 48 hours, each cell is shut down and the zinc-coated cathodes
are removed, rinsed, and the zinc is mechanically stripped from the aluminum
plates. Stripping is accomplished manually in some smelters, while others use
specialized automated equipment. The aluminum cathodes arelthen chemically
cleaned, replaced in the cells, and the cell is restored to normal operation.
Stripped zinc is sent to Process No. 15 for melting and casting.
i
2. Input Materials - The principal input is purified electrolyte, which
is a water solution containing about 70 grams of zinc per liter and about 200
grams of sulfuric acid per liter (4). Barium hydroxide or manganese sulfate
may be added to the electrolyte in order to form insoluble coatings on the
cell anodes, thereby minimizing both anode corrosion and lead contamination of
product zinc. Colloidal materials are also usually added to prevent uneven de-
position of zinc on the cathodes; materials used include gluie, goulac, gum
arabic, and a mixture of agar-agar, sodium silicate, and cresylic acid (2).
3. Operating Conditions - Electrolytic cells operate at 30° to 35°C and
atmospheric pressure (4). ;
4. Utilities - Electrolysis consumes the major amount of energy in an
electrolytic zinc smelter. Most plants use direct current at a current
density of about 600 amperes per square meter of cathode surface. Voltage
drop is 3.3 to 3.5 volts per cell, and current efficiency is 85 to 94 per-
cent. Approximately 3300 kilowatt-hours of power are needed to electrolvze a
metric ton of cathode zinc (4).
I
69
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Electricity in much smaller quantity is used to operate pumps and fans.
5. Waste Streams - There are two sources of atmospheric emissions from this
process. Escape of oxygen at the cell anodes causes the formation of a mist
of approximately the same composition as the electrolyte that escapes into the
air of the cell room. Another mist of the same approximate composition is re-
leased from the atmospheric cooling towers. Cell emission has been estimated
to be 3.3 kilograms per ton of zinc produced (1). No estimate, of cooling tower
emissions has been reported.
If the ore concentrate contains quantities of sodium or halogen compounds,
a portion of spent, electrolyte must be routinely removed and discarded. This
is not known to be occurring regularly in U.S. smelters. In general, there is
no loss of electrolyte or other liquid waste other than leaks and spills.
A sludge accumulates in the cells which is periodically removed.
18 provides .reported analyses of this material.
TABLE 18. ANALYSES OF ANODE SLUDGES FROM
ELECTROLYTIC ZINC REFINING (5).
Table
Concentration, ppm
Fresh anode sludge
Old anode sludge (from dump)
Cd
12
1,400
Cr
10
8
Cu
85
1,900
Pb.
170,000
89,000
Zn
12,800
39,200
6. Control Technology - Cell rooms must be well ventilated to avoid accumu-
lation of oxygen. This ventilation also serves to remove mist from the room.
There are no reports of treatment being applied to this air stream, although
this is apparently the largest source of air pollution from the electrolytic
process (6).
Treatment of mist from the electrolyte cooling towers is also not docu-
mented. An opinion has been expressed that the concentration of these
emissions is so diluted by the large volumes of air that it is not a nuisance
to nearby residents (3).
Disposition of anode sludge is also unreported. The material is a rich
source of recoverable minerals, and is probably reprocessed or recycled, as
are the filter cakes from Process No. 13.
7. EPA Source Classification Code - 3-03-030-06.
8. References -
1. Background Information for New Source Performance Standards: Primary
Copper, Lead, and Zinc Smelters. EPA-405/2-74-002a. Office of
70
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2.
3.
4.
5.
Air and Waste Management, U.S. Environmental Protection Agency. Re-
search Triangle Park, North Carolina. October 1974.
Schlechten, A.W., and A. Paul Thompson. Zinc and Zinc Alloys. In:
Kirk-Othmer. Encyclopedia of Chemical Technology. ! Iritersclence
Division of John Wiley and Sons, Inc. New York. 1967.
E. Van DenNeste. Metal!urie-Hoboken-Overpelt's Zinc Electrowinning
Plant. CIM Bulletin. August 1977. \
Krupkowa, D., and A. Udrycki. New Electrolytic Zinc Plants. Chemical
Age of India. August 1975. ' ;
Calspan Corporation. Assessment of Industrial Waste Practices in the
Metal Smelting and Refining Industry - Volume II, Primary and Secon-
dary Nonferrous Smelting and Refining. Draft. April 1975.
6. Privileged communication, EPA files.
71
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PRIMARY ZINC PRODUCTION
PROCESS NO. 15
Melting and Casting
1. Function - The process involves melting and casting the zinc from an
electrolytic or pyrolytic plant into a marketable form. Pyrolytie zinc is
usually in a molten state whereas electrolytic stripped zinc sheets must
always be melted. Recent practice utilizes induction heating,, possibly
combined with a gas-fired furnace. The molten zinc is cast into 27-kilogram
slabs on an in-line casting machine. Some zinc is cast into 640- to 1100-
kilogram blocks in stationary molds (1).
Cathode zinc sheets from electrolytic plants are dried, melted, and cast
into various forms of slab zinc. Alloys of zinc are also prepared and cast.
Depending on market conditions, lead and other constituents may be added to a
relatively high grade of zinc to make a select grade for galvanizing. Zinc
dust is made at the plants for use in purification of solutions.
Because molten zinc exhibits a strong tendency to form dross, ammonium
chloride flux is usually added to the melting furnace to retard oxidation at
the surface and to collect any oxides formed. Ordinary slab zinc can be
melted in a reverberatory furnace with the formation of 1 percent or less of
dross; electrolytic cathodes lose 6 to 17 percent under these circumstances,
depending on the presence of glue, cobalt, and the like in the electrolyte
0).
2. Input Materials - Inputs are zinc, ammonium chloride flux, and various
alloying materials to meet special requirements. Quantities are not specified.
3. Operating Conditions - Melting and casting are at atmospheric pressure.
Zinc must be heated above its melting point (420°C) to form liquid zinc.
4. Utilities - Gas- or oil-fired melting pots are used for melting zinc.
Heat requirements are not specified. Water is used in some plants to cool the
molds rapidly, but usually does not come into contact with the metal.
5. Waste Streams - Some zinc oxides and chloride compounds are emitted by
the melting pots into the atmosphere; quantities are not given. To achieve
high overall melting efficiency, dross is skimmed from the melting pot so
that the globules of metal may be separated from thin oxide shells.
Casting cooling water generally contains suspended solids and oil and
greas.e in the form of metal oxides, mold washes, and lubricants from casting
equipment.
The only solid waste is the dross produced if the slab zinc is melted in
an electrolytic furnace.
6. Control Technology - Control of atmospheric emissions is with scrubbers
or fabric filters. Any processing to recover the zinc values from;the control
residue must deal with its chloride content. In electrolytic plants the
reaction is very sensitive to the deleterious effects of chloride.
72
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The dross is treated by liquation, centrifugal separation
fluxing, etc. Recovered metal may be used to make zinc dust
purification or it may be returned to the melting pots.
7. EPA Source Classification Code - None
8. References -
1. Schlechten, A.W., and A. Paul Thompson. Zinc and
Kirk-Othmer. Encyclopedia of Chemical Technology.
Division of John Wiley and Sons, Inc. New York.
Zinc Alloys. In:
Interscience
1967.
73
of metal,
for electrolytic
-------
PRIMARY ZINC PRODUCTION
PROCESSED. 16
Cadmium Leaching
1. Function - Leaching selectively dissolves as much cadmium as possible
from various cadmium-bearing dusts, fumes, and sludges from primary zinc
plants, precipitating lead and other impurities without precipitating any of
the dissolved cadmium. There is no separate primary cadmium industry in the
United States. Cadmium recovery processes use by-products of other opera-
tions, all involving zinc, in four major categories (1):
(1) Fumes and dusts from roasting and sintering of zinc concentrates.
(2) Recycled zinc metal containing cadmium.
(3) Dusts from smelting of lead-zinc or copper-lead-zinc ores.
(4) Purification sludge from electrolytic zinc plants.
The first and fourth of these categories are of concern here, since they
provide the inputs to the leaching process. The second category involves
recycled zinc metal, and is not relevant here; cadmium-bearing dusts from
lead and copper smelting, the third category above, are sent directly to
cadmium purification, Process No. 18.
During sintering, cadmium,, lead, and thallium chlorides form and are
drawn off as a fume to be recovered in ESP's. After collection, the fumes
and dusts are leached with dilute sulfuric acid and sodium chlorate to ensure
complete dissolution of cadmium sulfide. Cadmium goes into solution by the
following reaction:
CdO
CdSO,
H2°
Sodium chlorate is a strong oxidizing agent, added to prevent reduction of
any sulfur to sulfide and to prevent reprecipitation of cadmium or thallium
as sulfides. Cadmium and lead are converted to sulfates and chlorides. The
cadmium compounds remain in solution, but lead is almost completely converted
to insoluble lead sulfate. This is filtered out and sent to lead recovery
together with other insoluble materials such as quartz and silicates. The
lead residue may contain small but significant quantities of gold, silver,
and indium (2). More sulfuric acid is then added to the solution in order to
bring the concentration up to 10 percent and to raise the temperature.
Instead of a direct leach with sulfuric acid, in at least one plant the
dust is first roasted and then water leached. The sinter fume is heat- ,
treated in a four-hearth roaster, which selectively sulfates the cadmium and
makes about 90 percent of it water-soluble. The water leach that follows
produces relatively pure cadmium solutions containing about 40 grams per
liter cadmium and 10 grams per liter zinc. Addition of sodium bichromate to
this solution removes about 90 percent of the soluble lead. The residual
solids are batch treated with scrubber liquor and concentrated acid (3).
74
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Another element in the flue dusts from roasting and sintering
economically recoverable is thallium; this processing is not
zinc smelters. One refiner in Colorado is the sole domestic
thallium (4).
that is
done at primary
producer of
Another source of high-cadmium residues for leaching is 'electrolytic
zinc processing. In the purification stage, both copper and icadmium are
eliminated from solution by treating the electrolyte with powdered zinc.
Generally this is done in stages making possible rough separations such as
high-copper and high-cadmium precipitates. Copper sulfate and arsenic tri-
oxide may be added during these stages. The cadmium precipitate is high in
zinc and contains virtually all the cadmium originally present in the elec-
trolyte. This purification sludge constitutes the raw material for the
cadmium plant. It is oxidized, either by allowing it to stand exposed to air
or on a steaming platform. The acid-soluble cadmium is leached out by sul-
furic acid and spent electrolyte. Filtration to remove insoluble copper may
follow.
j
2- Input Materials - The fumes and dusts from roasting and sintering
processes and the purification sludge from electrolytic plant? are the
primary inputs to leaching. Copper sulfate and arsenic trioxide may be added
during pr&leaching purification stages. Dilute sulfuric acid" and/or spent
electrolyte are the leaching agents. The electrolyte contains 200 grams per
liter sulfuric acid and 65 grams per liter zinc. Sodium chlorate is added to
serve as an oxidizing agent, and in some plants water is used' to leach the
fumes and dusts. The residual solids from water leaching are1 batch-treated
with scrubber liquor bolstered with concentrated acid. Sodiuijn bichromate may
be added to remove soluble lead; one plant reported using 7.5i grams per
kilogram of cadmium.product. Sixty-five, grams of caustic per! kilogram of
cadmium produced are added (3). Quantities for other inputs a re not specified.
3. Operating Conditions - Cadmium leaching takes place at atmospheric
pressure. A temperature of 80°C is reached after the final addition of
sulfuric acid (1). j
4. Utilities - Electricity in unspecified quantities is used to pump
liquids. The water leaching process requires 1.3 cubic meters of natural gas
per kilogram of cadmium product to roast the dust, followed by the addition
of an unspecified quantity of water (3). ';
5. Waste Streams - No data were found on the quantities of residuals from
acid leaching, but they are probably very small. Typical analysis of this
residue at one plant is 32 percent lead, 8 percent zinc, 0.7 percent cadmium,
0.13 percent indium, 0.45 percent arsenic, 0,,30 percent copper, 0.23 percent
silver, and 4 grams per metric ton gold. The residue may also contain other
insolubles such as quartz and silicates (3). Water vapor is released from
purification and leacning. An exhaust gas temperature of 60°C from leaching
was reported at one electrolytic plant, which also reported exhaust gases
from prelcaching purification stages ranging from 60 to 90°C (3).
75
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6. Control Technology - Lead sulfate is filtered out from the solution
along with other insolubles. All residues are recycled. They are typically
high in lead content and are sent to a lead recovery plant.
7. EPA Classification Code - None
8. References - !
1. Calspan Corporation. Assessment of Industrial Waste Practices in
the Metal Smelting and Refining Industry - Volume II, Primary and
Secondary Nonferrous Smelting and Refining. Draft. April 1975.
2. Howe, H.E. Cadmium and Cadmium Alloys. In: Kirk-Othmer. Ency-
clopedia of Chemical Technology. Interscience Division of John
Wiley and Sons, Inc. New York. 1967.
3. Battelle Columbus Laboratories. Development Document for Interim
Final Effluent Limitations Guidelines and Proposed New Source
Performance Standards for the Zinc Segment of the Nonferrous Metals
Manufacturing Point Source Category. EPA-68-01-1518. .Draft data.
4. Commodity Data Summaries 1977. U.S. Department of Interior,
Bureau of Mines. Washington, D.C. 1977.
76
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PRIMARY ZINC PRODUCTION
Ca'dmium Precipitation
PROCESS NO. 17
1. Function - The purpose of precipitation is to treat the cadmium-zinc
sulfate solution from the leaching process with zinc dust to precipitate
cadmium as a metallic sponge and then to separate it from most of the zinc
dust while the solution is agitated. To avoid excess zinc contamination,
usually only 90 to 95 percent of the cadmium in solution is precipitated.
The initial cementation with zinc dust may result in a liquor containing a
residual 0.2 gram per liter of cadmium and 30 to 40 grams per liter of zinc.
To further decrease overall cadmium discharge, the stripped liquor is heated
to 40°C and recemerited with 1.6 times the stoichiometric amount of zinc to
reduce the liquor to 0.04 gram per liter of cadmium and 30 to 40 grams per
liter of zinc (1).
,t
The cadmium sponge is then filter-pressed. It contains about 69 percent
cadmium, 30 percent moisture, and small amounts of lead and zinc (2). It is
steam dried or dewatered in a centrifuge. The solution from filtration,
containing practically all of the zinc added and about 10 percent -of the
cadmium as chlorides and sulfates, is returned to the sintering operation.
For cadmium production at electrolytic plants, the leach liquor is first
filtered to remove insoluble copper introduced from the electrolyte. The
filtrate is then precipitated with zinc dust in two or three stages to mini-
mize zinc concentration in the cadmium sponge. Strontium carbonate may be
added in one of these stages. The sponge will contain about. 80 percent
cadmium and less than 5 percent zinc. Because the electrolysis step that
follows is not highly sensitive to the presence of impurities, the purifica-
tion step with zinc dust is usually adequate. If further purification is
desirable, cobalt may be precipitated with nitroso-2-naphthol or potassium
xanthate, and thallium precipitated with potassium chromate or dichromate.
The sponge is then oxidized again by steam drying to enhance the solubility
of cadmium and is leached in spent electrolyte and filtered. The filtrate
contains about 200 grams per liter of cadmium as sulfate and is; ready for
introduction into the electrolytic cells (1). :
2- Input Materials - The cadmium-zinc sulfate solution from leaching and
zinc dust are the principal inputs to precipitation. Strontium carbonate may
be added during purification in electrolytic processing. Impurities may be
removed by the addition of nitroso-2-naphthol, potassium xanthate, and
potassium chromate or dichromate. Quantities for these inputs are not speci-
fied.
3. Operating Conditions - Precipitation takes place at atmospheric pressure.
Temperatures range from ambient to 40°C (1).
4. Utilities - Electricity is used to pump liquids, and natural gas or oil
is used to heat the stripped liquor. Quantities are not cited.
5. Waste Streams - Water vapor is released from both purification and
precipitation operations at temperatures ranging from 45° to 60°C.
77
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The filtration solution from this process contains almost all of the
zinc added as well as about 10 percent of the cadmium as chlorides and
sulfates (2). The purification precipitates may contain arsenic and mercury,
both of which are relatively low in concentration in ores and concentrates.
One plant reported that a materials balance established 8 ppm mercury and
23,000 ppm arsenic in an iron precipitate from the cadmium process, the only
place where significant concentrations were found (3).
6. Control Technology - Much of the residual zinc and cadmium in the
filtration solution can be precipitated by lime treatment. It has been shown
that freshly precipitated cadmium hydroxide leaves approximately 1 milligram
per liter of cadmium in solution at pH 8; this is reduced to 0.1 milligram
per liter at pH 10. Even lower values of 0.002 milligram per liter have been
shown at pH 11 (2). Evidence has been presented that high levels of iron are
beneficial for removal of cadmium during lime precipitation. The resultant
light slurry goes to settling ponds, where the solids can be retained for
recycling. At one plant the'slurry is mixed with the neutralized roaster
scrubber liquor before being sent to a series of settling ponds.
7. EPA Source Classification Code - None '
8. References -
1. Howe, H.E. Cadmium and Cadmium Alloys. In: Kirk-Othmer. Ency-
clopedia of Chemical Technology. Interscience Division of John ,
Wiley and Sons, Inc. New York. 1967.
2. Calspan Corporation. Assessment of Industrial Waste Practices in
the Metal Smelting and Refining Industry - Volume II. Primary and
Secondary Nonferrous Smelting and Refining. Draft. April 1975.
3. Development Document for Interim Final Effluent Limitations Guide-
lines and Proposed New Source Performance Standards for the Zinc
Segment of the Nonferrous Metals Manufacturing Point Source Cate-
gory. EPA-440/1-715/032. Effluent Guidelines Division Office of
Water and Hazardous Materials. U.S. Environmental Protection
Agency. Washington, D.C. November 1974. -
78
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PRIMARY ZINC PRODUCTION
Cadmium Purification and Casting
1. Function - The purification and casting step purifies
sponge by melting it with a caustic flux, with distillation
redistillation, or by redissolving the sponge with sulfuric
ing the cadmium by electrolysis.
PROCESS NO. 18
the cadmium
and perhaps
acid and collect-
In pyrometallurgical processing, the dried cadmium sponge is first mixed
with coal or coke and lime. It is then transferred to a conventional hori-
zontal-type retort, where the cadmium is reduced and collected as molten
metal in a condenser. Occasionally, for ultra-purity, the metal is distilled
in graphite retorts. Thallium is removed by treatment with zinc ammonium
chloride or sodium dichromate. The metal may then be cast into a marketable
form or further purified by redistillation. Typical impurities in the pro-
duct cadmium are 0.01 percent zinc, 0.003 percent copper, 0.015 percent lead,
less than 0.001 percent thallium, less than 0.0005 percent tin, and less than
0.001 percent antimony (1). Cadmium recovery has been reported as 94 percent
from the feed to the leach plant: and 67 percent from the zinc concentrates.
i
Electrolytic processing of cadmium is carried out in banks of cells
similar to zinc cells. The sponge is first dissolved in dilute sulfuric acid
(return electrolyte). The anodes are lead. The cathodes of cadmium are 97
percent pure and represent 90 to 95 percent total recovery of cadmium from
ore to metal. Recovery in the electrolytic step is 96 percent from the
cadmium sponge. The stripped cathode metal is washed, dried, and melted
under a flux, such as caustic or rosin, and cast into various shapes. Total
depletion of cadmium from the solution is not carried out. When the ratio of
the^thallium to cadmium sulfate in the electrolyte reaches 1:10, the cadmium
cathodes must be removed and replaced with new insoluble cathodes. Contin-
uing electrolysis deposits an alloy containing 5 to 20 percent thallium.
These cathodes are then leached with steam and water to separate thallium
into the filtrate, leaving cadmium as a residue. Small amounts of cadmium in
solution are precipitated with sodium bicarbonate. Thallium is precipitated
with hydrogen sulfide, then dissolved in sulfuric acid. This sulfate solu-
tion can be electrolyzed for recovery of pure sponge thallium. The sponqe is
washed, pressed into blocks, melted, and cast. [
Processing of concentrated lead smelter baghouse dust in an electrolytic
cadmium plant is similar to processing of cadmium sponge. Since the dust is
generally higher in impurities than the sponge, some additional purification
is necessary. The dust is mixed with sulfuric acid and water to form a
paste, then calcined to a sulfated cake which is crushed and agitated with
spent electrolyte. Milk of lime may be added to neutralize the solution, and
sodium sulfide or impure cadmium sulfide is added to precipitate copper and
other metal impurities. After filtration to remove a lead cake, an agent
such as sodium chlorate oxidizes the iron and lime precipitates iron and
arsenic. Heating ensures complete precipitation. Precipitated thallium
chromate or dichromate is filtered off after the addition of a soluble
chromate or dichromate. Excess chromate remaining may be reduced by sodium
sulfide and precipitated by neutralizing with caustic. The filtered solution
79
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Is fed to the electrolytic plant. Where chlorine is present, irons high in
silicon are used as anodes instead of lead. The finished cathodes are
melted and cast (2). ;
2. Input Materials - Cadmium sponge from precipitation or lead smelter
baghouse dust are the principal inputs to purification and casting. In
pyrometallurgical processing, coal or coke and lime or sodium hydroxide are
added, and zinc ammonium chloride or sodium dichromate is used to remove
thallium. One plant reported using 65 grams of caustic per kilogram of
cadmium. In electrolytic processing, dilute sulfuric acid or return elec-
trolyte, a flux such as caustic or rosin, water, sodium bicarbonate, and
hydrogen sulfide are the inputs. The cell feed may run 100 to 200 grams
cadmium, 30 to 80 grams zinc, and 70 to 80 grams sulfuric acid per liter (1).
Processing of lead smelter baghouse dust may entail addition of sulfuric
acid, water, milk of lime, sodium sulfide or cadmium sulfide, sodium chlorate,
a soluble chromate or dichromate, and a caustic, added at different stages.
Quantities are not specified for any of these inputs.
3. Operating Conditions - Cadmium retorting furnaces are not pressurized.
Operating temperatures of 455°C have been reported for the gas melting and
ammonium chloride stages, and 790° to 910°C in the furnace itself. Electroly-
sis also takes place at atmospheric pressure, with the temperature held at
about 30°C (1). ;
4. Utilities - Electricity, oil, or natural gas is used for melting the
cadmium product. Air is brought into the furnace. Specific quantities were
not found. In electrolysis, current density may range from 140 to 360
amperes per square meter (2).
5. Waste Streams - Particulates are released at several stages in the
process. One plant reported the release of 20 kilograms per hour of ammonium
chloride from fluxing and 108 kilograms per hour of cadmium from the retorting
furnace (1).
Residues from retorting furnaces contain 1.5 to 6.0 percent cadmium,
together with varying amounts of zinc and lead. Filter cakes may also
include iron, arsenic, indium, mercury, and copper (2). ThalliunMs not
regarded as a serious impurity since it is removed during processing. A^
sample of a cadmium filter cake residue was obtained from one electrolytic
plant. After cadmium had been recovered from the flue dust, this waste
amounted to 514 kilograms per day. Analysis was as follows: cadmium - 280
ppm, chromium - 24 ppm, copper - 1150 ppm, lead - 21.5 percent, zinc - 3.9
ipercent, and thallium - .40 ppm. The filter cake amounts to 1.8 kilogram
per metric ton of zinc product (3).
6. Control Technology - Particulates can be controlled with fabric filter
systems. The filter cake derived from purification is returned to the
sintering operation. In some cases it is disposed of by open dumping or
transferred to unlined ponds for liming and settling. Sludge dredged from
the lagoons is stored on the ground for variable periods of time before
shipment to lead smelters. No plants are known to use lined lagoons or to
treat soil areas where dredged sludges are stored. In electrolytic pro-
80
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cessing, the spent electrolyte is returned.to various leacf-ing stages in the
circuit. In some cases the electrolytic cadmium process cycle is completely
closed with no discharge.
7. EPA Source Classification Code - None
8. References -
1. Battelle Columbus Laboratories. Development Document for Interim
Final Effluent Limitations Guidelines and Proposed New Source
Performance Standards for the Zinc Segment of the Nonferrous Metals
Manufacturing Point Source Category. EPA Contract No.-68-01-1518.
Draft data.
2. Howe, H.E. Cadmium and Cadmium Alloys. In: Kirk-Othmer. Ency-
clopedia of Chemical Technology, Interscience Division of John
Wiley and Sons, Inc. New York. 1967. j
3. Calspan Corporation. Assessment of Industrial Wa'ste Practices in
the Metal Smelting and Refining Industry - Volume II, Primary and
Secondary Nonferrous Smelting and Refining. Draft. April 1975.
81
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
REPORT NO.
EPA-600/2-80-169
3. RECIPIENT'S ACCESSION NO.
I. TITLE AND SUBTITLE
Industrial Process Profiles for Environmental Use;
Chapter 28 Primary Zinc Industry
5. REPORT DATE
July 1980 issuing date
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
Same as Below
8. PERFORMING ORGANIZATION REPORT NO.
9. PERFORMING ORGANIZATION NAME AND ADDRESS
PEDCo Environmental, Inc.
11499 Chester Road
Cincinnati, Ohio 45246
10. PROGRAM ELEMENT NO.
1AB604
11. CONTRACT/GRANT NO.
68-03-2577
12. SPONSORING AGENCY NAME AND ADDRESS
Industrial Environmental Research Laboratory
Office of Re'search and Development
U. S. Environmental Protection Agency
Cincinnati, Ohio 45268
13. TYPE OF REPORT AND PERIOD COVERED
One of Series
14. SPONSORING AGENCY CODE
EPA/600/12
1S. SUPPLEMENTARY NOTES
Project Officer: John 0. Burckle
16. ABSTRACT
The catalog of Industrial Process Profiles for Environmental Use was developed
is an aid in defining the environmental impacts of industrial activity in the United
states. Entries for each industry are in consistent format and form separate chap-
:ers of the study.
The primary zinc industry as defined for this study consists of mining, bene-
ficiation, smelting, and refining. A profile of the industry is given including
slant locations, capacities, and various statistics regarding production and con-
sumption of zinc, co-products, and by-products. The report summarizes the various
:ommercial routes practiced domestically for zinc production in a series of process
flow diagrams and detailed process descriptions. Each process description includes
available data regarding input materials, operating conditions, energy and utility
requirements, waste streams produced (air, water, and solid waste), and control
technology practices and problems.
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.lDENTIFIERS/OPEN ENDED TERMS
c. COSATI Field/Group
Sxhaust Emissions
Smelting
[race Elements
Pollution
Zinc Production
13B
18. DISTRIBUTION STATEMENT
Release to Public
19. SECURITY CLASS (ThisReport)
Unclassified
21. NO. OF PAGES
86
20. SECURITY CLASS (Thispage)
Unclassified
22. PRICE
EPA Form 2220-1 (Rev. 4-77) PREVIOUS EDITION is OBSOLETE
82
it U.S. GOVERNMENT PRINTING OFFICE: V980--657-165/00911
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