- Email: [email protected]
Nonferrous Metallurgical Operations
Nonferrous Metallurgical O p e r a t i o n s Kenneth W, Nelson
I. Introduction IL Copper A. Mining, Milling, and Concentrating B. Smelting C. Refining D. Emissions and Controls III. Lead A. Mining, Milling, and Concentrating Β. Smelting C. Refining D. Emissions and Controls IV. Zinc A. Mining, Milling, and Concentrating B. Roasdng and Retordng C. Leaching and Electrolysis D. Emissions and Controls V. Aluminum A. Mining and Ore Treatment B. Electrolysis C. Emissions and Controls VI. Secondary Copper, Lead, Zinc, and Aluminum A. Sources B. Recovery Processes C. Emissions and Controls VII. Nonferrous Foundries A. Alloys and Operations B. Emissions and Conirols References
I.
171 173 173 174 175 175 179 179 179 180 181 182 182 183 184 184 186 186 186 187 188 188 188 189 189 189 189 190
Introduction
Nonferrous metals production has been important in the development of the science, if we may call it that, of air pollution. Smeldng processes attain high temperatures. Dusts, fumes, and gases are generated in the normal course of winning a virgin metal from its ores or concentrates. Most of the copper, lead, and zinc in the earth's crust occurs as sulfide minerals. The consdtuent sulfur is oxidized at one or more smeldng 171
172
KENNETH W.
NELSON
Steps and is evolved as sulfur dioxide. In the early days S O 2 was vented with combustion gases into flues and from stacks high enough only to provide adequate draft for furnaces. T o prevent operations from creat ing a smoke nuisance, smelters were established in isolated areas. As the scale of operations increased, however, and as lands near smelters were inhabited and cultivated by farmers, smelter smoke created problems. In 1905, Haywood of the U.S. Department of Agriculture examined plants grown near smelters and showed that foliage injured by eflluent gases had a higher sulfate content than noninjured samples (7). In 1915 the Selby Smelter Commission reported the results of its study of the eff'ects of smoke from the Selby smelter located on the northeast shore of San Francisco Bay (2). The study group measured the eff^ects of diff'erent concentrations of S O 2 on vegetation and clearly demonstrated that some growing plants are more sensitive to the gas than is man. The Commission's report is a classic in air pollution literature. In 1914 the American Smelting and Refining Co. established a de partment of agricultural investigations for systematic research into the eff^ects of smelter effluents and means of their control. Much of the first work on diff^erences in sensitivity of various species of plants was done by that department. In 1928 one of the members, M. D. Thomas, in vented an automatic instrument for detecting, measuring, and recording low concentrations of SO2. The Thomas Autometer is now widely used for monitoring. The earlier scientific contributions of O'Gara (5) and of Thomas' contemporaries and colleagues have also been noteworthy {4-6). Injury to crops and trees by S O 2 from smelters has been the basis for much protracted and costly litigation. Court-ordered shutdowns of California smelters prompted the establishment of the Selby Smelter Commission. Since then there have been numerous lawsuits by farmers and, in one instance, the United States was the complainant. It was claimed that smoke from the Consolidated Mining and Smelting Co. at Trail, British Columbia, was crossing the U.S.-Canadian border and causing damage to forests. The International Joint Commission began hearings on the matter in 1928. Not until 1941 did the Trail Smelter Arbitral Tribunal come to a final decision and set up a regime under which the smelter was required to operate. Some smelter owners have felt that a few farmers asked for damages allegedly caused by smoke when crop losses were actually caused by poor farming practices or unfavorable weather. Habitual claimants were referred to as "smoke farmers" and their actions forced smelting com panies to keep detailed records of S O 2 concentrations in potentially aff'ected farm areas and to inspect crop conditions regularly through the
37.
NONFERROUS METALLURGICAL OPERATIONS
173
growing s e a s o n . A typical program designed to protect the interests of the farmers as well as those of the company has been described by Davis (7). While S O 2 from the smeldng of copper, lead, and zinc has been the principal pollutant of interest in nonferrous metallurgy, gaseous and particulate fluorides from aluminum smelting also have been of concern. A significant difference between the two types of polludon is that the fluoride problems first came to attention because of adverse effects on grazing animals rather than effects on vegetation. Agate (8) reported that fluorosis was common among cattle and sheep grazing downwind from an aluminum plant operated near Fort William, Scotland.
II. A.
MINING, MILLING,
AND
Copper
CONCENTRATING
Both open-pit and underground mining are practiced (see also Chapter 35). Open-pit operations are conducted successfully with ores containing less than 1% Cu. Costs of operadng underground mines are higher, and such mines usually must have a somewhat better grade of ore. The greater propordon of copper ore from U.S. mines is taken from the low-grade, open-pit type. Copper ores are handled in tremendous quanddes. Mills processing 500 tons or more per hour are not unusual. Enormous jaw, gyratory, and cone crushers are used to reduce large boulders to pebble-sized pieces. An amusing bit of doggerel printed in a Tacoma newspaper 40 years ago tells the story (by "W. C. H." from the files of the late Paul H. Ray, Salt Lake City, Utah): The Joke Is on the Smelter
>
If the horses have the glanders, If the turkeys have the roup. If the deadly hawk is flying Into his chicken coop.
The farmer grabs his pencil He charges all to smoke, He swifdy sends his little (?) bill, And thinks it is a joke.
The farmer has his inning. The matter is no joke For he traces down his losses Direct to smelter smoke.
The water in the stream dries up. The south wind blasts his fields. His daughter has the whooping cough, His wheat it fails to yield.
The frost may blight the melons. The crows may get his corn, And the pigs may have the cholera. His cow a crumpled horn.
But the farmer's never troubled. He banks his wealth in town. He never feels the want of cash, Till the smelter closes down.
174
KENNETH W.
NELSON
Separation of the traces of copper-bearing minerals from the mass of waste is essential to economic recovery of the metal. Crushed ore is ground wet in ball or rod mills to produce a thin slurry. The slurry plus a variety of flotation reagents is piped to flotation cells where air is beaten into the mixture. A copper-rich froth is formed and skimmed off'. Solids in the froth are dewatered by settling and filtration and are shipped to a smelter. The concentrate contains 20-30% Cu. B.
SMELTING
Copper sulfide concentrates received at the smelter are normally roasted in multiple hearth roasters to remove moisture, to burn off' part of the contained sulfur, and to preheat the material before smeldng (Fig. 1). In recent years the roasdng step has been eliminated at many smelters. Roaster calcines or wet concentrates are charged into large reverberatory furnaces where copper in the form of oxides, sulfides, and sulfate is converted to cuprous sulfide. Some iron, calcium, magnesium, and aluminum—all as silicates—are removed in the form of a viscous slag.
Raw concentrates
Gases, volatile oxides
Fuel
and dust to dust recovery and stack or acid plant
Gases and dust
Flux and fettling material
Peverberatory
Fuel
to waste heat boilers slag to dump
Air
Gases to stack
Siliceous flux Miscellaneous
Converter
material high in copper
Blister copper
Air
to refinery
FiG. 1. Production of blister copper sulfide concentrates {8a).
37.
NONFERROUS METALLURGICAL OPERATIONS
175
Copper matte, a mixture of cuprous and ferrous sulfides, is tapped off into large ladles and transferred to converters. Siliceous fluxes are added to the matte in the converter and air is blown into the hot, molten mass through tuyeres. Iron and other impurities form a silicate slag on top of the denser cuprous sulfide. The slag is poured off^ at intervals and charged back to the reverbs. Further blowing of the converter oxidizes the sulfur—leaving free copper according to the equation: CugS + O 2 -> 2 Cu + S O 2
Copper is then poured off' and transferred to a holding furnace for deoxidizing and casdng as slabs of blister copper—about 98.5% pure; or as shaped anodes for shipment to a refinery. C.
REFINING
A small percentage of domestic copper leaves the smelter refined enough by special furnace treatment to be used directly in certain appli cations. The bulk of the copper is intended for electrical uses, however, and must be highly purified. Mere traces of certain impurities greatly reduce copper's electrical conductivity. Anodes of blister copper—so named because of its rough surface—are arranged alternately, face to face with thin sheets of pure copper in large tanks. The sheets are the cathodes of the multiple electrolytic cells to be formed in each tank. The tanks are nearly filled with a dilute solu tion of copper sulfate and sulfuric acid. A low voltage current passes between anodes and cathodes, causing solution of copper from the anodes and deposition on the cathodes. Trace metals present in the im pure anode either dissolve in the circulating electrolyte or setde to the bottoms of the tanks as a black sludge called anode mud or slimes. The slimes may contain selenium, tellurium, gold, silver, platinum, and palladium. All these elements may be associated with copper in its ores. D.
EMISSIONS AND
CONTROLS
Dust is a problem in underground mines in spite of wet collaring and drilling. Free silica in country rock may be appreciable and pose a health risk. Ventilation must be provided to keep dust concentrations down to safe levels. In most instances power ventilation is also necessary to re move nitrogen oxides and carbon monoxide from blasdng and to control heat and humidity. Although dust and blasdng gases are eventually discharged from the mine, there is no significant contribution to community air pollution. The concentrations of dust pardcles are low—of the order of 5 millions
176
KENNETH W.
NELSON
of particles per cubic foot of air. Blasting gases are absent except during the short period of blasdng and then are highly diluted by the large volume of vendladng air constandy being moved through the mine. Open-pit mining may create localized dusdness near operadng drills, power shovels, and other equipment. But it is standard practice to use water while drilling and to wet down ore and waste piles, when necessary, before loading the material into trucks or railway cars for transport from the mine. Roadways are sprinkled to reduce dusting. Blasdng, properly done, disperses surprisingly little dust in open-pit mining. Nitrogen dioxide may be produced from explosions of ammo nium nitrate—fuel oil (ANFO) or other ammonium nitrate combina tions. When the air in the pit is cool in relation to air at the brim of the pit, as on a summer morning, blasdng may create a visible cloud of NO2-N2O4. Viewed from a distance of several hundred feet, the cloud looks formidable. But tests have shown the nitrogen oxide concen tration to be less than 30 ppm. The gases dissipate in minutes. Ore-crushing generates considerable dust. Water is used to wet the ores but it must be applied judiciously to avoid difliculdes such as overwetting, build-ups of mud on conveyor belts and pulleys, and clogging of chutes. Furthermore, as ore passes through successive crushing stages new, dry surfaces are exposed and dust is readily abraded from them. Wetdng, therefore, must usually be supplemented by appropriate exhaust ventilation for good dust control. Because dust burdens in ventilating air may be heavy, effluents may be neighborhood nuisances unless effective collectors are incorporated in the exhaust systems. The flotation process itself creates no dust problems but waste mate rial, or tailings, issuing as a sandy slurry from the flotation plant may do so. The slurry is channeled into ponds from which water, free of setdeable solids, may be recovered. As the solids accumulate, a tailings d u m p covering many acres is formed. Surface drying of the d u m p and high winds may result in localized dust storms. Keeping the dump wet is an effecdve means of control. Planung and culdvation of ground cover and shrubs to act as windbreaks are also successful. The high temperatures attained in roasdng, smeldng, and converting cause volatilization of a number of the trace elements which may be present in copper ores and concentrates. The raw waste gases from these processes contain not only these fumes but also dust and S O 2 (Table I). Roasting drives off a portion of any arsenic, antimony, and lead as the oxides. More of those elements plus bismuth and some selenium and tellurium may be eliminated as fume in the reverberatory furnace. They also to some extent become incorporated into furnace slag. The stronger oxidizing conditions of converting effect almost com-
37.
NONFERROUS METALLURGICAL OPERATIONS
177
TABLE I EMISSIONS FROM COPPER ROASTERS, REVERBERATORY FURNACES, AND CONVERTERS"
Raw gas Waste gas (m"^) Roasters Reverberatory furnaces Converters
Dust content
S O 2 content
(gm/nrO
(%)
1300'^ 2000^
K5 4
2-8 -
10,000^
12
to 8
^ Clean Air Guide 2101, Kommission Reinhaltung der Luft, Verein Deutscher Ingenieure, VDI—Verlag (.mbH, Dusseldorf, West (Ger many. January, 1960. Per ton of concentrates. ^ Including extraneous air per ton of charge.
plete removal of the remaining volatile elements except selenium and tellurium. Nickel, cobalt, and the precious metals are also not voladlized significantly and remain dissolved in the crude copper. The value of the volatilized elements, as well as air pollution con siderations, dictates efficient collection of fumes and dusts from process off-gases. Balloon flues in which gases move at low velocities serve as gravity collectors of the larger particles and provide low resistance ducdng for the large volumes of combustion gases and vendladng air that must be moved. Cyclones may be used also. For collection of the finer particulates, electrostatic precipitators are most often used. Collec tion efficiencies up to 99.7% for copper dust and fume are attained by careful conditioning of flue gases. Cleaned hot flue gases are vented to the atmosphere via tall stacks for maximum dispersion and dilution of contained sulfur dioxide. A major proportion of the gas may be used to produce sulfuric acid. Not all of it is used for this purpose because the SO2 concentradon in gases available from some sources within the smelter may be too low for efficient catalydc oxidation to SO3 by presendy available commercial processes. To make sulfuric acid, SO2 concentration in the gas should be 4-8%. Utilization of SO2 reduces the potential air pollution problem asso ciated with copper smeldng; hence acid producdon would seem to be an obvious and simple soludon. The facts are that acid plants require heavy capital investment and substantial operating costs, which, for profitable operation of the recovery plant, must be returned by sale of the product.
178
KENNETH W. NELSON
There may not be a local market for acid. Distances to demand areas and consequent shipping charges may be prohibidve for smelter acid to compete with plants using Frasch sulfur as a source of SOg. Each copper smelter is unique in some ways, and the addition of an acid plant may or may not be economical. The idea of recovering sulfur dioxide from smelter gases is an attrac tive one, and a large number of processes have been described in the technical and patent literature. An excellent summary of them is given in a report by the Ontario Research Foundation (9). So far as major copper smelters are concerned, however, none of the processes, other than direct conversion to sulfuric acid, has yet proved practicable. Tall stacks, up to 828 feet, as at El Paso, Texas, are depended upon for dispersion and diludon of SOg to keep ground level concentrations to a minimum. The effectiveness of high smelter stacks was shown by Hill et al in 1944 {10) and has been emphasized recently by Smith {11). Monitoring stations at selected points around a smelter are helpful in carrying out a "Sea Captain" method of controlling S O 2 emissions. This technique makes use of measurements of meteorological conditions and an expert evaluadon of them. If conditions are unfavorable for adequate dispersion of gas, or are predicted to be unfavorable, SOg-emitdng operations are curtailed. Forecasdng is not perfect, however, and the detection of significant S O 2 at ground level by an automatic instrument may be the first indication that dispersion and dilution are not adequate. Telemetering of detector information to the smelter control center permits immediate curtailment action to be taken. If there are multiple source^ of S O 2 in a given area in the same direction from a detecting station, smelter control by monitoring is hampered. Electrolytic copper refining operations, by their nature, do not create significant air polludon. In tank houses traces of electrolyte mist are generated by splashing of liquid as it is circulated by gravity and pumps among the tanks. Repeated tests by the author have shown acid mists to be within the AC(;iH* limit of 1.0 mg H2SO4 per cubic meter of air. In one part of the copper refining process called electrolyte purifi cation there is the possibility of arsine evolution. T h e copper content of electrolyte becomes very low during purification, and hydrogen is produced at the cathode. If traces of arsenic are carried in the soludon, arsine, AsHg, will be evolved in sufficient amounts to be dangerous to men working in the vicinity. Hence, exhaust ventilation of purification tanks is essendal. Purification tanks are few in number compared to the total number of tanks in a refinery and so are easily isolated and vendlated. * American (vonference of (iovernmental Industrial Hygienists.
37.
NONFERROUS METALLURGICAL OPERATIONS
179
Treatment of slimes for recovery of silver, gold, selenium, tellurium, and traces of other elements usually entails fusion and oxidation in a furnace of appropriate size. Some selenium is voladlized during the process and is captured as the furnace gases pass successively through a wet scrubber and an electrostatic precipitator. The latter removes mists which escape the scrubber.
III. A.
MINING, MILLING,
AND
Lead
CONCENTRATING
Most lead ores are mined underground. Crushing, grinding, and concentrating follow the same general pattern as the processing of copper ores. Lead ores, however, are richer. The run of the mill con tains between 6 and 10% Pb. The lead mineral of greatest importance is galena, PbS. Traces of silver and other metals usually are present in lead ores and they accompany the lead in the ore concentrate. (See also Chapter 35.) B.
SMELTING
The sulfur content of lead concentrates is reduced by sintering them on Dwight-Lloyd sintering machines (Fig. 2). Moistened concentrateflux mix is fed to an endless belt of cast iron grate sections. The charge is ignited, burns under forced draft, and is finally discharged from the machine as grates flip over at the head pulley of the belt. The charge is now a loosely fused mass of material called sinter. It may be crushed, mixed with other materials such as flue dusts, and sintered a second dme, or it may be fed direcdy to the blast furnaces. In addition to eliminadng most of the sulfur from PbS concentrates, the sintering procedure prevents dust losses which would occur if concentrates were smelted direcdy. Also, it creates a more porous raw material to facilitate smeldng. A mixture of sinter, iron, and coke is charged into blast furnaces. Contact with free carbon and carbon monoxide at high temperatures reduces lead compounds to metallic lead. A mixture of molten lead and siliceous slag accumulates in the hearth of the furnace and is tapped off', either continuously or intermittendy. Gravity separation of the lead from the slag takes place in heavily insulated settlers. Slag is allowed to overflow from the top into slag pots for transport to the dump or to a fuming furnace for zinc recovery. As it is tapped periodically from the setders, the crude lead is at red heat and contains considerable amounts of dissolved impurides which become insoluble as the metal cools. Hence the hot lead is transferred
180
KENNETH W.
NELSON
Ore (1) Mine I (2) Crush (3) Concentrate if low grade (4) Rough roast (5) Bins or ore beds I (6) Sinter I (7) Smelt (usually in blast furnace) Gases I Cottrell or bag house
Base bullion
Slag I Waste
Dressing kettle Dross
1 Base bullion I Refining plant
Γ Flue dust
I
Cleaned gas
Work up values
Dross furnace Slag I Return to blast furnace
Matte-speiss I Ship to copper smelter
Base bullion I Return to drossing kettle
Gases To Cottrell or bag house
FK;. 2 . Usual treatment of" a sulfide lead ore (Sa).
to holding kettles for cooling and subsequent skimming of impurides from the surface. At this stage the metal is about 95 to 99% pure and is further refined on the premises or cast into blocks for shipment to a refinery. C.
REFINING
Lead bullion is purified by a number of different processes. The Parkes process consists, in broad outline, of heating under oxidizing conditions in a reverberatory furnace for removal of arsenic, antimony, and dn (softening), dissolving zinc in softened lead in kettles, cooling, and skimming of a silver-rich zinc crust (desilverizing), and removing dissolved zinc by vacuum disdllation (dezincing). If bismuth is present, it is removed by treatment with calcium and magnesium (debismuthizing). The Harris process employs treatment with molten sodium hydroxide and sodium nitrate as a subsdtute for furnace softening. Arsenic, andmony, and dn, if present, are separated as sodium salts. The Betts process is electrolydc and produces pure lead cathodes plus a slime containing impurities derived from the crude lead bullion.
37.
NONFERROUS METALLURGICAL OPERATIONS
181
An important part of lead refining is the recovery of silver and gold. Zinc crust collects both metals. The crust, containing a considerable percentage of entrained lead, is heated in graphite or clay retorts to distill off the zinc. The residual bullion is transferred to cupel furnaces for separation of lead. This is done by means of an air blast directed on the molten bullion. The litharge (PbO) produced is molten and is care fully decanted as it accumulates on the surface. Doré metal (Ag-Au) is the final product of cupellation and is tapped off for further treatment. D.
EMISSIONS AND
CONTROLS
Dust problems in the mining, milling, and concentradng of lead are the same as those oudined for copper. Hot gases from the lead concentrate sintering process carry dust and the oxide fume of volatile metals such as antimony and zinc (Table II). Also, some lead is volatilized and oxidized. Dust and fume are recovered from the gas stream by setding in large flues and by precipitation in Cottrell treaters or filtration in large baghouses. Collection efficiencies are up to 96% for precipitators and 99.5% for baghouses. Sulfur dioxide derived from sintering is not concentrated enough to be used directly by presently available commercial processes for sulfuric acid production. It is possible, however, by recirculation of gases or updraft sintering to build up to the SO2 concentration sufficiendy to permit economic production of H2SO4 or liquid sulfur dioxide from T A B L E 11 EMISSIONS FROM LEAD SINTERING, BLAST FURNACES, AND REVERBERATORY FURNACES"
Raw gas
Sintering Blast furnaces Reverberatory furnaces
Waste gas (nr-^)
Dust content (gm/nv^)
3000" 15,000-50,000^ 100-500^'
2-15 5-15 3-20
S O 2 content (%) 1.5-5 —
" Clean Air Guide 2285, Kommission Reinhaltung der Luft, Verein Deutscher Ingenieure, VDI—Verlag GmbH, Dusseldorf, West Germany, September, 1961. " Per ton of sinter. Per ton of coke. ^ Per ton of charge.
182
KENNETH
W.
NELSON
a portion of the gases. A practicable process for liquid SOg production at a lead smelter has been described by Fleming and Fitt (12). Because the sulfur proportion in galena, PbS, is only 13.4%, the total amount of by-product SOg theoretically available from lead smeldng is considerably less than from copper or zinc smelting for equivalent rates of metal production. A 5000-ton per month lead plant thus would have in theory only 670 tons of sulfur available for about 2000 tons of H2SO4 per month. Acid plants would therefore be relatively small and perhaps uneconomic in a particular set of circumstances. Low SO2 potendal, on the other hand, diminishes the dispersion problem and the necessity for SO2 recovery. Lead blast furnace gases, after cooling, are amenable to treatment in large baghouses having several chambers, each of which will contain hundreds of bags. A common size of bag used is 18 inches by 30 feet. Wool has been traditionally used for the bags, and the service lives in many cases have been remarkable; continuous service for 20 years has been recorded. Synthedc materials, including glass fibers, are com peting successfully with wool because of superior resistance to high temperatures. Lead refineries have baghouses for recovery of fume from softening furnaces and cupeling furnaces. Zine oxide fume released during distillation of zinc from zinc-silver skims may be captured by a local exhaust vendlation system and passed into the flue serving cupel furnaces.
IV. A.
M I N I N G , M I L L I N G , AND
Zinc
CONCENTRATING
The processes used for zinc are essentially the same as for copper and lead (see also Chapter 35). The bulk of zinc ore contains zinc as sphal erite, ZnS, which is separated as a concentrate from accompanying min erals by selective flotation. Concentrates contain about 60% Zn. In recent years, an important source of raw material for zinc metal has been zinc oxide from fuming furnaces. Zinc as an impurity in lead smeldng is recovered from lead blast furnace slag by headng the slag to high temperatures and blowing pulverized coal and air through it. Zinc is reduced, volatilized, reoxidized and is collected as ZnO in bag filter units. The baghouse product is passed through a rotary kiln to reduce the lead content by volatilization and to increase the density of the material for easier handling and shipping.
37.
B.
ROASTING
NONFERROUS
AND
METALLURGICAL
OPERATIONS
183
RETORTING
For efficient recovery of zinc, sulfur must be removed from concen trates to less than 2%. This is done by roasdng. Muldple hearth or Ropp roasdng may be followed by sintering; or double-pass sintering may be used alone (Fig. 3). The liberadon of zinc from roasted concentrates involves simple heating of a mixture of roast and coke breeze to about 1100 °C. Simul taneous reduction of zinc from the oxide to the metal and disdllation of the metal take place. Zinc vapor passes from the heated vessel into a condenser where it condenses to a liquid which is drained off at intervals into molds. Reducdon and disdlladon of zinc may be done as a batch process in banks of cylindrical retorts—the Belgian retort process—or in contin uously operadng vertical retorts. Gas is the preferred fuel, hence most smelters are located in natural gas fields. Electric distillation furnaces are used to a small extent. A spectroscopically pure zinc is produced by a condnuous fractional distillation process developed by the New Jersey Zinc Co. A process for simultaneous smeldng of roasted lead and zinc concen trates has been developed within the past decade by the Imperial Smelting Corp. T h e process makes use of carbon monoxide generated from coke to reduce lead and zinc oxides in a sealed shaft furnace. Lead bullion accumulates in the furnace bottom and acts as a collector for copper, silver, and gold. Zinc passes as a vapor out the top of the furnace into condensers in which a shower of molten lead is condnuously mainOre Concentrate Concentrates (alxDut 60% Zn)
I
Tails (waste or worked up for Cu and Pb)
Roast or roast-sinter SO2 + SO3 I Available for acid manufacture
ZnO (With l e s s than 2 % S)
Mix with coal and heat in retort Zinc vapor condense Dross and cast into standard slabs
Blue powder I Market or re-treat in next charge
Residue I Waste or further treatment by other methods
FIG. 3. Outline of zinc production from sulfide ore {8a).
184
KENNETH W.
NELSON
tained. Zinc vapor is condensed quickly to a liquid which dissolves in the molten lead. Outside the condenser the lead-zinc soludon is cooled and a 98% pure zinc floats to the surface and overflows into containers. The cooled lead is pumped back to the condensers (13). C.
L E A C H I N G AND
ELECTROLYSIS
Zinc of high purity may be produced from roasted concentrates from densified zinc oxide from fuming furnaces, or from impure metallic zinc, by solution in sulfuric acid, removal of impurities from the solution by appropriate chemical treatment, and finally electrolysis of the purified electrolyte. Electrolysis is done in tanks containing alternating anodes of lead and cathodes of aluminum. Pure zinc is deposited on the cathodes and later stripped from them by hand. The zinc is then melted in a small rever beratory furnace and cast into slabs or other forms for shipment. D.
EMISSIONS AND
CONTROLS
Dust, fume, and S O 2 are evolved from zinc concentrate roasdng or sintering (Table III). Particulates are caught in conventional baghouses or Cottrells. Sulfur dioxide attains concentrations of 6-7% in roaster gases and may be converted direcdy into sulfuric acid or vented from tall stacks. An interesdng and very successful process for recovery of zinc roaster S O 2 as well as of the more dilute S O 2 from lead roasting has been developed at Trail, Bridsh Columbia, by the Consolidated Mining and Smeldng Co. Inexpensive hydroelectric power for ammonia synthesis permits the absorption of S O 2 from the gas streams and the TABLE
III
EMISSIONS FROM ZINC SINTERING AND HORIZONTAL RETORTS"
Raw gas
Sintering Horizontal retorts
Waste gas (m^)
Dust content (gm/m^)
% Particles <10μ
S O 2 content (%)
4000^ 12,000-18,000^ 96,000-144,000^^
10 0.1-0.3
100 100
4.5-7
" Clean Air Guide 2284, Kommission Reinhaltung der Luft, Verein Deutscher In genieure, VDI—Verlag CimbH, Dusseldorf, West Germany. September, 1961. " Per ton of ZnS. ^ Waste flue gas per ton of Zn. ^ Condenser waste gas (including extraneous air) per ton of Zn.
37.
NONFERROUS METALLURGICAL
OPERATIONS
185
eventual production and marketing of ammonium sulfate as a fertilizer. Steps in the process are oudined in Fig. 4. In zinc disdllation by the retort process small holes are left in vapor condensers to vent gases from charged retorts as they are heated. When the temperature becomes high enough for zinc vapor to distill over, some vapor escapes from the hole and ignites spontaneously. The total Sullivan mine
I
- Trailings to dump
Concentrator
Zinc concentrate
Lead concentrate Dwight-Lloyd sintering machines
Lead sinter to blast furnace
Zinc calcine to zinc leaching
— Suspension roasters
t
Waste heat boilers
Humidifying flue
t
\
Cyclone treaters
Cottrell treaters
t
t Cooling towers Absorption towers
Cottrell hot treaters TaU gas i
t
TaU gas
Glover cooling towers
t
Aqua ammonia
Aqua ammonia Absorption towers Shriver filters i Acidifying tower
t . Eliminator tower _ Ammonium sulfate solution
Wash towers i MistCottrells Drying towers
Oslo crystallizers
Heat exchangers + Converters
f Heat exchangers I
Atmospheric cooler
t Absorbers i Dilution Storage i FIG. 4. Flow sheet of ammonia process93% at Sulfuric Trail, British Columbia (9). acid
186
KENNETH
W.
NELSON
zinc oxide fume so produced is appreciable. It is carried by convection currents up the fronts of the condenser banks and out to atmosphere with combustion gases through ridge ventilators of the furnace build ings. A disdnctive characteristic of the zinc retort plant is a flag of white ZnO fume. T h e actual fume concentrations are low and the cost of its collection would not be compensated for by the value of the recovered material. Leaching and electrolysis do not emit significant amounts of particu lates or gases. Tanks in which leaching and electrolyte purification are done are covered and ventilated to prevent worker exposure to possible toxic gases or mists. T h e electrolytic process itself does disperse some electrolyte mist because of gas evolution, but this is a problem only within the confines of the tank houses. V. A.
Aluminum
M I N I N G AND O R E T R E A T M E N T
Bauxite is the base ore for aluminum production. It is a hydrated oxide of aluminum associated with silicon, titanium, and iron, and contains 30-70% AI2O3. Most bauxite ore is purified by the Bayer process. T h e ore is dried, ground in ball mills, mixed with sodium hydroxide solution, and autoclaved for several hours to dissolve the AI2O3 as sodium alumínate, N a A 1 0 2 . By setding, dilution, and filtration, iron oxide, silica, and other insoluble impurities are removed. Aluminum hydroxide is precipitated from the diluted, cooled solution—the reaction being initiated or "seeded" by introduction of a small amount of freshly precipitated Al(OH)3. T h e precipitate is filtered, washed, and calcined to produce pure alumina, AI2O3. (See also Chapter 35.) B.
ELECTROLYSIS
Commercial recovery of aluminum from the oxide is accomplished by a unique electrolytic process discovered simultaneously in 1886 by Hall of this country and Heroult of France. Alumina is dissolved in a fused mixture of fluoride salts and dissociated electrically into metallic aluminum and oxygen. Fused natural or synthetic cryolite (3NaF-AlF3) with about 10% fluorspar and 5% dissolved alumina is contained in carbon-lined cells or pots. Heavy carbon anodes are immersed in the mixture to within about 2 inches of the cathode, a heel of molten alumi num covering the carbon lining. A heavy electric current between anode
37.
NONFERROUS METALLURGICAL OPERATIONS
187
and cathode reduces A I 2 O 3 to Al which accumulates and is drawn off at intervals into crucibles. Alumina is steadily fed onto the top of the molten electrolyte to replace that which has been decomposed. Crucible aluminum is skimmed of dross, has alloying metals added to it, and is charged into holding furnaces before being cast out as salable ingot. C.
EMISSIONS AND
CONTROLS
Calcining of aluminum hydroxide for the production of alumina en tails mechanical dust dispersion (Table IV). The valuable dust is re covered from kiln effluents by electrostatic precipitadon preceded by cyclone-type collectors. Oxygen liberated in electrolytic cells attacks carbon anodes, producing C O 2 and mechanically dispersing some carbon dust. Thermal and chem ical action in the cells also evolves some alumina dust, and both particu late and gaseous fluorides. The various effluents are collected by venti lation hoods over the cells. Aluminum plants using carbon anodes which have been baked before mounting in cells may have dry dust collectors to remove particulates from collected effluents, scrubbers for removal of gaseous fluorides, and stacks for final dispersion of scrubbed gases. Plants employing the Soderberg method of forming and baking carbon electrodes in place on the cells may omit dry collection and scrub all effluents direcdy. Tarry hydrocarbons from the baking carbon electrode mixture inter fere with the operation of dry collectors. Spillage of fluoride-containing waste gases from aluminum pot lines T A B L E IV EMISSIONS FROM CALCINING ALUMINUM HYDROXIDE AND FROM CLOSED ELECTROLYTIC CELLS USING SODERBERG ELECTRODES"
Raw gas
Calcining aluminum hydroxide Closed electrolytic cells using Soderberg electrodes
Waste gas (m^)
Dust content (gm/m^)
Fluorine content (gm/m^)
3000*
300-400
—
2000-3000^^
0.08-0.12
0.040
" Clean Air Guide 2286, Kommission Reinhaltung der Luft, Verein Deutscher In genieure, VDI—Verlag GmbH, Dusseldorf, West Ciermany. September, 1961. Per ton of AI2O3.
Including extraneous air. Waste gas per cell in cubic meters per hour.
188
KENNETH W.
NELSON
into the pot room and then out roof monitors has released sufficient fluorides at some plants to require the installation of scrubbers to wash the air leaving the roof monitors. Aluminum chloride or chlorine gas is used to treat aluminum in hold ing furnaces, to flux and to degas the molten metal. Aluminum chloride is voladle, subliming at 180 °C. It reacts with hot moist air to form a highly visible "smoke" cloud. Electrostatic precipitation, scrubbing, and condensadon of the voladlized salt are eff'ecdve means of control. VI. A.
Secondary Copper, Lead, Zinc, a n d Aluminum
SOURCES
Scrap provides an important source of metals for the market. Copper in substantial tonnage is recovered from electrical cable and automobile radiators. About 85% of the lead used in automobile batteries is collected by scrap dealers and eventually sold to secondary lead smelters. Zinc comes from galvanizing baths and die-cast scrap. Aluminum is recovered mosdy from industry-generated scrap, but aircraft assemblies and even pots and pans may be used. Economics governs the flow of material in and out of the secondary plant. B.
RECOVERY
PROCESSES
Scrap may be converted direcdy to usable metal by simple melting and casting into salable ingots. More often, scrap of similar composition is melted together and its composidon is adjusted by removal or addi tion of some constituent elements. When the right proportions of each are present, the charge is cast out. Brass, type metal, babbitts, and solders are produced in this way. Insulation is stripped or burned from electrical cable and the copper wire is melted and cast into anodes for direct conversion by electrolysis into refined copper. Lead battery plates are charged into reverberatory furnaces with coke and fluxes for smeldng. The product is an andmonial lead bullion which will be refined to pure lead or adjusted in composition for sale as an alloy. Zinc scrap is commonly refined by disdllation from retorts, the vapor being condensed to a liquid and cast into slabs, or condensed direcdy from the vapor state to a powder of controlled particle size. Aluminum alloys are melted in reverberatory furnaces and adjusted in composition by drossing, chlorination, and addition of alloying metals.
37.
C.
NONFERROUS METALLURGICAL OPERATIONS
EMISSIONS AND
189
CONTROLS
Emissions from secondary metal processes are similar to those from primary metallurgical operations except that little or no sulfur dioxide is evolved, and, in general, smaller quantities of metal oxide dusts and fumes are produced. There are no sulfides in secondary metal furnace charges as there are in primary smelting furnaces, and only small amounts of sulfur are used to ketde-refine lead-base alloys. Lead oxide is volatilized from secondary lead smelting. Zinc oxide fume is a by-product of zinc-alloy distillation and of brass furnace operation. Baghouses, more commonly, and electrostatic precipitators are successfully used for collection. Chlorination of molten aluminum in secondary refining furnaces produces aluminum chloride fume which creates a high-opacity white cloud when it comes in contact with moist air. Properly designed scrub bers or condensers are effective control devices. One successful installa tion has been described by Donoso {14).
VII. A.
A L L O Y S AND
Nonferrous Foundries
OPERATIONS
Brass, bronze, and zinc die-casdng metal are the principal alloys used. Brasses contain 60-65% copper; the other major constituent is zinc. Lead or dn or both may be present. Bronzes usually contain 85-90% copper with small amounts of alu minum, zinc, dn, manganese, silicon, and phosphorus—singly or in combination. Alloy names are misleading. Manganese bronze, for ex ample, may contain only 0.25% Mn. Zinc die-casting metal is about 95% high-grade zinc, 4% aluminum, plus traces of other metals. Copper base alloys are melted in rotary, reverberatory induction fur naces or in small crucibles and are poured at bright red heat into molds. After the casdngs have solidified they are shaken from molds, and finished. Zinc die-cast alloy is melted in kettles and poured or injected at a temperature of 775°-825 °F into permanent dies. B.
EMISSIONS AND
CONTROLS
Metal fume will be evolved during the melting and casting of alloys if the temperatures are high enough for volatile constituent elements
190
KENNETH W.
NELSON
to have appreciable vapor pressure, and if no intermetallic compounds are formed. For example, zinc is fairly voladle and will be vaporized from molten brass and oxidized, producing visible white ZnO fume. The higher the temperature and the greater percentage of zinc in the alloy, the more copious the fume. Copper in the brass, on the other hand, will not be volatilized significantly because its vapor pressure is very low at the meldng point of brass. Zinc die-casting alloy is used with no metal fume evolution. Shakeout of casdngs from sand molds is a dusty operation and may be conducted under hoods or over downdraft gratings. Dust loadings in ventilating air range from 0.25 to 1.0 grain per cubic foot. Collection of up to 97% of such material is effected with wet or dry centrifugal collectors, up to 99+% with,dry fabric type collectors (15), REFERENCES L J. K. Haywood, U.S. Dept. Agr., Bur. Chem. Bull. 8 9 (1905); U.S., Bur. Mines, Bull. 5 3 7 (1954) (abstr. No. 1231). 2. J. A. Holmes, E. C. Franklin, and R. A. Gould, U.S., Bur. Mines, Bull. 9 8 (1915). 3. P. J. O'Gara, Met. Chem. Eng. 1 7 , No. 12 (1917). 4. G. R. Hill and M. D. Thomas, Plant Physiol. 8 , 223 (1933). 5. M. D. Thomas, J. O. Ivie, J. N. Abersold, and R. H. Hendricks, Ind. Eng. Chem., Anal. Ed. 15, 287 (1943). 6. M. D. Thomas, R. H. Hendricks, and G. R. Hill, Proc. Ist Natl Air Pollution Symp., Pasadena, Calif., 1949 p. 142. 7. C. R. Davis, Pay Dirt, No. 326, p. 5, C. F. Willis, Phoenix, Arizona, 1966. 8. J. N. Agate et al. Med. Res. Council Memo. 2 2 (1949); quoted by P. D r i n k e r , / Roy. Inst. Public Health Hyg. 2 0 , 307 (1957). 8a. C. R. Hayward, "An Oudine of Metallurgical Practice," 3rd ed. Van Nostrand, Princeton, New Jersey, 1952. 9. Ontario Research Foundation, "The Removal of Sulphur Gases from Smelter Fumes." B.Johnson, Toronto, 1949. 10. G. R. Hill, M. D. Thomas, and J. N. Abersold, Proc. 9th Ann. Meeting Ind. Hyg. Found., Pittsburgh, 1944 p. 11. 11. M. F. Smith, Proc. 3rd Natl Conf Air Pollution, Washington, D.C, 1966 p. 151. 12. E. P. Fleming and T. C. Fitt, Ind. Eng. Chem. 4 2 , 2249 (1950). 13. P. J. Callahan and T. D. Parker, Chem. Eng. 7 4 , 159 (1967). 14. J. J. Donoso, Proc. 40th Ann. Conv. Smoke Prevent. Assoc. Am., Toronto, 1947 p. 39. 15. J. M. Kane, Am. Foundryman 19, 34 (1951).
GENERAL REFERENCE Additional pertinent material is in Chapter 6 of "Air Pollution Engineering Manual, Los Angeles APCD" (J. A. Danielson, ed.), PHS Publn. 999-AP-40, DHEW, Cincinnati (1967); and will be published in the National Air Pollution Control Administration's pub lications: "Control Technology for Particulate Air Pollutants," and "Control Technology for Sulfur Oxides Air Pollutants."
I. Introduction IL Copper A. Mining, Milling, and Concentrating B. Smelting C. Refining D. Emissions and Controls III. Lead A. Mining, Milling, and Concentrating Β. Smelting C. Refining D. Emissions and Controls IV. Zinc A. Mining, Milling, and Concentrating B. Roasdng and Retordng C. Leaching and Electrolysis D. Emissions and Controls V. Aluminum A. Mining and Ore Treatment B. Electrolysis C. Emissions and Controls VI. Secondary Copper, Lead, Zinc, and Aluminum A. Sources B. Recovery Processes C. Emissions and Controls VII. Nonferrous Foundries A. Alloys and Operations B. Emissions and Conirols References
I.
171 173 173 174 175 175 179 179 179 180 181 182 182 183 184 184 186 186 186 187 188 188 188 189 189 189 189 190
Introduction
Nonferrous metals production has been important in the development of the science, if we may call it that, of air pollution. Smeldng processes attain high temperatures. Dusts, fumes, and gases are generated in the normal course of winning a virgin metal from its ores or concentrates. Most of the copper, lead, and zinc in the earth's crust occurs as sulfide minerals. The consdtuent sulfur is oxidized at one or more smeldng 171
172
KENNETH W.
NELSON
Steps and is evolved as sulfur dioxide. In the early days S O 2 was vented with combustion gases into flues and from stacks high enough only to provide adequate draft for furnaces. T o prevent operations from creat ing a smoke nuisance, smelters were established in isolated areas. As the scale of operations increased, however, and as lands near smelters were inhabited and cultivated by farmers, smelter smoke created problems. In 1905, Haywood of the U.S. Department of Agriculture examined plants grown near smelters and showed that foliage injured by eflluent gases had a higher sulfate content than noninjured samples (7). In 1915 the Selby Smelter Commission reported the results of its study of the eff'ects of smoke from the Selby smelter located on the northeast shore of San Francisco Bay (2). The study group measured the eff^ects of diff'erent concentrations of S O 2 on vegetation and clearly demonstrated that some growing plants are more sensitive to the gas than is man. The Commission's report is a classic in air pollution literature. In 1914 the American Smelting and Refining Co. established a de partment of agricultural investigations for systematic research into the eff^ects of smelter effluents and means of their control. Much of the first work on diff^erences in sensitivity of various species of plants was done by that department. In 1928 one of the members, M. D. Thomas, in vented an automatic instrument for detecting, measuring, and recording low concentrations of SO2. The Thomas Autometer is now widely used for monitoring. The earlier scientific contributions of O'Gara (5) and of Thomas' contemporaries and colleagues have also been noteworthy {4-6). Injury to crops and trees by S O 2 from smelters has been the basis for much protracted and costly litigation. Court-ordered shutdowns of California smelters prompted the establishment of the Selby Smelter Commission. Since then there have been numerous lawsuits by farmers and, in one instance, the United States was the complainant. It was claimed that smoke from the Consolidated Mining and Smelting Co. at Trail, British Columbia, was crossing the U.S.-Canadian border and causing damage to forests. The International Joint Commission began hearings on the matter in 1928. Not until 1941 did the Trail Smelter Arbitral Tribunal come to a final decision and set up a regime under which the smelter was required to operate. Some smelter owners have felt that a few farmers asked for damages allegedly caused by smoke when crop losses were actually caused by poor farming practices or unfavorable weather. Habitual claimants were referred to as "smoke farmers" and their actions forced smelting com panies to keep detailed records of S O 2 concentrations in potentially aff'ected farm areas and to inspect crop conditions regularly through the
37.
NONFERROUS METALLURGICAL OPERATIONS
173
growing s e a s o n . A typical program designed to protect the interests of the farmers as well as those of the company has been described by Davis (7). While S O 2 from the smeldng of copper, lead, and zinc has been the principal pollutant of interest in nonferrous metallurgy, gaseous and particulate fluorides from aluminum smelting also have been of concern. A significant difference between the two types of polludon is that the fluoride problems first came to attention because of adverse effects on grazing animals rather than effects on vegetation. Agate (8) reported that fluorosis was common among cattle and sheep grazing downwind from an aluminum plant operated near Fort William, Scotland.
II. A.
MINING, MILLING,
AND
Copper
CONCENTRATING
Both open-pit and underground mining are practiced (see also Chapter 35). Open-pit operations are conducted successfully with ores containing less than 1% Cu. Costs of operadng underground mines are higher, and such mines usually must have a somewhat better grade of ore. The greater propordon of copper ore from U.S. mines is taken from the low-grade, open-pit type. Copper ores are handled in tremendous quanddes. Mills processing 500 tons or more per hour are not unusual. Enormous jaw, gyratory, and cone crushers are used to reduce large boulders to pebble-sized pieces. An amusing bit of doggerel printed in a Tacoma newspaper 40 years ago tells the story (by "W. C. H." from the files of the late Paul H. Ray, Salt Lake City, Utah): The Joke Is on the Smelter
>
If the horses have the glanders, If the turkeys have the roup. If the deadly hawk is flying Into his chicken coop.
The farmer grabs his pencil He charges all to smoke, He swifdy sends his little (?) bill, And thinks it is a joke.
The farmer has his inning. The matter is no joke For he traces down his losses Direct to smelter smoke.
The water in the stream dries up. The south wind blasts his fields. His daughter has the whooping cough, His wheat it fails to yield.
The frost may blight the melons. The crows may get his corn, And the pigs may have the cholera. His cow a crumpled horn.
But the farmer's never troubled. He banks his wealth in town. He never feels the want of cash, Till the smelter closes down.
174
KENNETH W.
NELSON
Separation of the traces of copper-bearing minerals from the mass of waste is essential to economic recovery of the metal. Crushed ore is ground wet in ball or rod mills to produce a thin slurry. The slurry plus a variety of flotation reagents is piped to flotation cells where air is beaten into the mixture. A copper-rich froth is formed and skimmed off'. Solids in the froth are dewatered by settling and filtration and are shipped to a smelter. The concentrate contains 20-30% Cu. B.
SMELTING
Copper sulfide concentrates received at the smelter are normally roasted in multiple hearth roasters to remove moisture, to burn off' part of the contained sulfur, and to preheat the material before smeldng (Fig. 1). In recent years the roasdng step has been eliminated at many smelters. Roaster calcines or wet concentrates are charged into large reverberatory furnaces where copper in the form of oxides, sulfides, and sulfate is converted to cuprous sulfide. Some iron, calcium, magnesium, and aluminum—all as silicates—are removed in the form of a viscous slag.
Raw concentrates
Gases, volatile oxides
Fuel
and dust to dust recovery and stack or acid plant
Gases and dust
Flux and fettling material
Peverberatory
Fuel
to waste heat boilers slag to dump
Air
Gases to stack
Siliceous flux Miscellaneous
Converter
material high in copper
Blister copper
Air
to refinery
FiG. 1. Production of blister copper sulfide concentrates {8a).
37.
NONFERROUS METALLURGICAL OPERATIONS
175
Copper matte, a mixture of cuprous and ferrous sulfides, is tapped off into large ladles and transferred to converters. Siliceous fluxes are added to the matte in the converter and air is blown into the hot, molten mass through tuyeres. Iron and other impurities form a silicate slag on top of the denser cuprous sulfide. The slag is poured off^ at intervals and charged back to the reverbs. Further blowing of the converter oxidizes the sulfur—leaving free copper according to the equation: CugS + O 2 -> 2 Cu + S O 2
Copper is then poured off' and transferred to a holding furnace for deoxidizing and casdng as slabs of blister copper—about 98.5% pure; or as shaped anodes for shipment to a refinery. C.
REFINING
A small percentage of domestic copper leaves the smelter refined enough by special furnace treatment to be used directly in certain appli cations. The bulk of the copper is intended for electrical uses, however, and must be highly purified. Mere traces of certain impurities greatly reduce copper's electrical conductivity. Anodes of blister copper—so named because of its rough surface—are arranged alternately, face to face with thin sheets of pure copper in large tanks. The sheets are the cathodes of the multiple electrolytic cells to be formed in each tank. The tanks are nearly filled with a dilute solu tion of copper sulfate and sulfuric acid. A low voltage current passes between anodes and cathodes, causing solution of copper from the anodes and deposition on the cathodes. Trace metals present in the im pure anode either dissolve in the circulating electrolyte or setde to the bottoms of the tanks as a black sludge called anode mud or slimes. The slimes may contain selenium, tellurium, gold, silver, platinum, and palladium. All these elements may be associated with copper in its ores. D.
EMISSIONS AND
CONTROLS
Dust is a problem in underground mines in spite of wet collaring and drilling. Free silica in country rock may be appreciable and pose a health risk. Ventilation must be provided to keep dust concentrations down to safe levels. In most instances power ventilation is also necessary to re move nitrogen oxides and carbon monoxide from blasdng and to control heat and humidity. Although dust and blasdng gases are eventually discharged from the mine, there is no significant contribution to community air pollution. The concentrations of dust pardcles are low—of the order of 5 millions
176
KENNETH W.
NELSON
of particles per cubic foot of air. Blasting gases are absent except during the short period of blasdng and then are highly diluted by the large volume of vendladng air constandy being moved through the mine. Open-pit mining may create localized dusdness near operadng drills, power shovels, and other equipment. But it is standard practice to use water while drilling and to wet down ore and waste piles, when necessary, before loading the material into trucks or railway cars for transport from the mine. Roadways are sprinkled to reduce dusting. Blasdng, properly done, disperses surprisingly little dust in open-pit mining. Nitrogen dioxide may be produced from explosions of ammo nium nitrate—fuel oil (ANFO) or other ammonium nitrate combina tions. When the air in the pit is cool in relation to air at the brim of the pit, as on a summer morning, blasdng may create a visible cloud of NO2-N2O4. Viewed from a distance of several hundred feet, the cloud looks formidable. But tests have shown the nitrogen oxide concen tration to be less than 30 ppm. The gases dissipate in minutes. Ore-crushing generates considerable dust. Water is used to wet the ores but it must be applied judiciously to avoid difliculdes such as overwetting, build-ups of mud on conveyor belts and pulleys, and clogging of chutes. Furthermore, as ore passes through successive crushing stages new, dry surfaces are exposed and dust is readily abraded from them. Wetdng, therefore, must usually be supplemented by appropriate exhaust ventilation for good dust control. Because dust burdens in ventilating air may be heavy, effluents may be neighborhood nuisances unless effective collectors are incorporated in the exhaust systems. The flotation process itself creates no dust problems but waste mate rial, or tailings, issuing as a sandy slurry from the flotation plant may do so. The slurry is channeled into ponds from which water, free of setdeable solids, may be recovered. As the solids accumulate, a tailings d u m p covering many acres is formed. Surface drying of the d u m p and high winds may result in localized dust storms. Keeping the dump wet is an effecdve means of control. Planung and culdvation of ground cover and shrubs to act as windbreaks are also successful. The high temperatures attained in roasdng, smeldng, and converting cause volatilization of a number of the trace elements which may be present in copper ores and concentrates. The raw waste gases from these processes contain not only these fumes but also dust and S O 2 (Table I). Roasting drives off a portion of any arsenic, antimony, and lead as the oxides. More of those elements plus bismuth and some selenium and tellurium may be eliminated as fume in the reverberatory furnace. They also to some extent become incorporated into furnace slag. The stronger oxidizing conditions of converting effect almost com-
37.
NONFERROUS METALLURGICAL OPERATIONS
177
TABLE I EMISSIONS FROM COPPER ROASTERS, REVERBERATORY FURNACES, AND CONVERTERS"
Raw gas Waste gas (m"^) Roasters Reverberatory furnaces Converters
Dust content
S O 2 content
(gm/nrO
(%)
1300'^ 2000^
K5 4
2-8 -
10,000^
12
to 8
^ Clean Air Guide 2101, Kommission Reinhaltung der Luft, Verein Deutscher Ingenieure, VDI—Verlag (.mbH, Dusseldorf, West (Ger many. January, 1960. Per ton of concentrates. ^ Including extraneous air per ton of charge.
plete removal of the remaining volatile elements except selenium and tellurium. Nickel, cobalt, and the precious metals are also not voladlized significantly and remain dissolved in the crude copper. The value of the volatilized elements, as well as air pollution con siderations, dictates efficient collection of fumes and dusts from process off-gases. Balloon flues in which gases move at low velocities serve as gravity collectors of the larger particles and provide low resistance ducdng for the large volumes of combustion gases and vendladng air that must be moved. Cyclones may be used also. For collection of the finer particulates, electrostatic precipitators are most often used. Collec tion efficiencies up to 99.7% for copper dust and fume are attained by careful conditioning of flue gases. Cleaned hot flue gases are vented to the atmosphere via tall stacks for maximum dispersion and dilution of contained sulfur dioxide. A major proportion of the gas may be used to produce sulfuric acid. Not all of it is used for this purpose because the SO2 concentradon in gases available from some sources within the smelter may be too low for efficient catalydc oxidation to SO3 by presendy available commercial processes. To make sulfuric acid, SO2 concentration in the gas should be 4-8%. Utilization of SO2 reduces the potential air pollution problem asso ciated with copper smeldng; hence acid producdon would seem to be an obvious and simple soludon. The facts are that acid plants require heavy capital investment and substantial operating costs, which, for profitable operation of the recovery plant, must be returned by sale of the product.
178
KENNETH W. NELSON
There may not be a local market for acid. Distances to demand areas and consequent shipping charges may be prohibidve for smelter acid to compete with plants using Frasch sulfur as a source of SOg. Each copper smelter is unique in some ways, and the addition of an acid plant may or may not be economical. The idea of recovering sulfur dioxide from smelter gases is an attrac tive one, and a large number of processes have been described in the technical and patent literature. An excellent summary of them is given in a report by the Ontario Research Foundation (9). So far as major copper smelters are concerned, however, none of the processes, other than direct conversion to sulfuric acid, has yet proved practicable. Tall stacks, up to 828 feet, as at El Paso, Texas, are depended upon for dispersion and diludon of SOg to keep ground level concentrations to a minimum. The effectiveness of high smelter stacks was shown by Hill et al in 1944 {10) and has been emphasized recently by Smith {11). Monitoring stations at selected points around a smelter are helpful in carrying out a "Sea Captain" method of controlling S O 2 emissions. This technique makes use of measurements of meteorological conditions and an expert evaluadon of them. If conditions are unfavorable for adequate dispersion of gas, or are predicted to be unfavorable, SOg-emitdng operations are curtailed. Forecasdng is not perfect, however, and the detection of significant S O 2 at ground level by an automatic instrument may be the first indication that dispersion and dilution are not adequate. Telemetering of detector information to the smelter control center permits immediate curtailment action to be taken. If there are multiple source^ of S O 2 in a given area in the same direction from a detecting station, smelter control by monitoring is hampered. Electrolytic copper refining operations, by their nature, do not create significant air polludon. In tank houses traces of electrolyte mist are generated by splashing of liquid as it is circulated by gravity and pumps among the tanks. Repeated tests by the author have shown acid mists to be within the AC(;iH* limit of 1.0 mg H2SO4 per cubic meter of air. In one part of the copper refining process called electrolyte purifi cation there is the possibility of arsine evolution. T h e copper content of electrolyte becomes very low during purification, and hydrogen is produced at the cathode. If traces of arsenic are carried in the soludon, arsine, AsHg, will be evolved in sufficient amounts to be dangerous to men working in the vicinity. Hence, exhaust ventilation of purification tanks is essendal. Purification tanks are few in number compared to the total number of tanks in a refinery and so are easily isolated and vendlated. * American (vonference of (iovernmental Industrial Hygienists.
37.
NONFERROUS METALLURGICAL OPERATIONS
179
Treatment of slimes for recovery of silver, gold, selenium, tellurium, and traces of other elements usually entails fusion and oxidation in a furnace of appropriate size. Some selenium is voladlized during the process and is captured as the furnace gases pass successively through a wet scrubber and an electrostatic precipitator. The latter removes mists which escape the scrubber.
III. A.
MINING, MILLING,
AND
Lead
CONCENTRATING
Most lead ores are mined underground. Crushing, grinding, and concentrating follow the same general pattern as the processing of copper ores. Lead ores, however, are richer. The run of the mill con tains between 6 and 10% Pb. The lead mineral of greatest importance is galena, PbS. Traces of silver and other metals usually are present in lead ores and they accompany the lead in the ore concentrate. (See also Chapter 35.) B.
SMELTING
The sulfur content of lead concentrates is reduced by sintering them on Dwight-Lloyd sintering machines (Fig. 2). Moistened concentrateflux mix is fed to an endless belt of cast iron grate sections. The charge is ignited, burns under forced draft, and is finally discharged from the machine as grates flip over at the head pulley of the belt. The charge is now a loosely fused mass of material called sinter. It may be crushed, mixed with other materials such as flue dusts, and sintered a second dme, or it may be fed direcdy to the blast furnaces. In addition to eliminadng most of the sulfur from PbS concentrates, the sintering procedure prevents dust losses which would occur if concentrates were smelted direcdy. Also, it creates a more porous raw material to facilitate smeldng. A mixture of sinter, iron, and coke is charged into blast furnaces. Contact with free carbon and carbon monoxide at high temperatures reduces lead compounds to metallic lead. A mixture of molten lead and siliceous slag accumulates in the hearth of the furnace and is tapped off', either continuously or intermittendy. Gravity separation of the lead from the slag takes place in heavily insulated settlers. Slag is allowed to overflow from the top into slag pots for transport to the dump or to a fuming furnace for zinc recovery. As it is tapped periodically from the setders, the crude lead is at red heat and contains considerable amounts of dissolved impurides which become insoluble as the metal cools. Hence the hot lead is transferred
180
KENNETH W.
NELSON
Ore (1) Mine I (2) Crush (3) Concentrate if low grade (4) Rough roast (5) Bins or ore beds I (6) Sinter I (7) Smelt (usually in blast furnace) Gases I Cottrell or bag house
Base bullion
Slag I Waste
Dressing kettle Dross
1 Base bullion I Refining plant
Γ Flue dust
I
Cleaned gas
Work up values
Dross furnace Slag I Return to blast furnace
Matte-speiss I Ship to copper smelter
Base bullion I Return to drossing kettle
Gases To Cottrell or bag house
FK;. 2 . Usual treatment of" a sulfide lead ore (Sa).
to holding kettles for cooling and subsequent skimming of impurides from the surface. At this stage the metal is about 95 to 99% pure and is further refined on the premises or cast into blocks for shipment to a refinery. C.
REFINING
Lead bullion is purified by a number of different processes. The Parkes process consists, in broad outline, of heating under oxidizing conditions in a reverberatory furnace for removal of arsenic, antimony, and dn (softening), dissolving zinc in softened lead in kettles, cooling, and skimming of a silver-rich zinc crust (desilverizing), and removing dissolved zinc by vacuum disdllation (dezincing). If bismuth is present, it is removed by treatment with calcium and magnesium (debismuthizing). The Harris process employs treatment with molten sodium hydroxide and sodium nitrate as a subsdtute for furnace softening. Arsenic, andmony, and dn, if present, are separated as sodium salts. The Betts process is electrolydc and produces pure lead cathodes plus a slime containing impurities derived from the crude lead bullion.
37.
NONFERROUS METALLURGICAL OPERATIONS
181
An important part of lead refining is the recovery of silver and gold. Zinc crust collects both metals. The crust, containing a considerable percentage of entrained lead, is heated in graphite or clay retorts to distill off the zinc. The residual bullion is transferred to cupel furnaces for separation of lead. This is done by means of an air blast directed on the molten bullion. The litharge (PbO) produced is molten and is care fully decanted as it accumulates on the surface. Doré metal (Ag-Au) is the final product of cupellation and is tapped off for further treatment. D.
EMISSIONS AND
CONTROLS
Dust problems in the mining, milling, and concentradng of lead are the same as those oudined for copper. Hot gases from the lead concentrate sintering process carry dust and the oxide fume of volatile metals such as antimony and zinc (Table II). Also, some lead is volatilized and oxidized. Dust and fume are recovered from the gas stream by setding in large flues and by precipitation in Cottrell treaters or filtration in large baghouses. Collection efficiencies are up to 96% for precipitators and 99.5% for baghouses. Sulfur dioxide derived from sintering is not concentrated enough to be used directly by presently available commercial processes for sulfuric acid production. It is possible, however, by recirculation of gases or updraft sintering to build up to the SO2 concentration sufficiendy to permit economic production of H2SO4 or liquid sulfur dioxide from T A B L E 11 EMISSIONS FROM LEAD SINTERING, BLAST FURNACES, AND REVERBERATORY FURNACES"
Raw gas
Sintering Blast furnaces Reverberatory furnaces
Waste gas (nr-^)
Dust content (gm/nv^)
3000" 15,000-50,000^ 100-500^'
2-15 5-15 3-20
S O 2 content (%) 1.5-5 —
" Clean Air Guide 2285, Kommission Reinhaltung der Luft, Verein Deutscher Ingenieure, VDI—Verlag GmbH, Dusseldorf, West Germany, September, 1961. " Per ton of sinter. Per ton of coke. ^ Per ton of charge.
182
KENNETH
W.
NELSON
a portion of the gases. A practicable process for liquid SOg production at a lead smelter has been described by Fleming and Fitt (12). Because the sulfur proportion in galena, PbS, is only 13.4%, the total amount of by-product SOg theoretically available from lead smeldng is considerably less than from copper or zinc smelting for equivalent rates of metal production. A 5000-ton per month lead plant thus would have in theory only 670 tons of sulfur available for about 2000 tons of H2SO4 per month. Acid plants would therefore be relatively small and perhaps uneconomic in a particular set of circumstances. Low SO2 potendal, on the other hand, diminishes the dispersion problem and the necessity for SO2 recovery. Lead blast furnace gases, after cooling, are amenable to treatment in large baghouses having several chambers, each of which will contain hundreds of bags. A common size of bag used is 18 inches by 30 feet. Wool has been traditionally used for the bags, and the service lives in many cases have been remarkable; continuous service for 20 years has been recorded. Synthedc materials, including glass fibers, are com peting successfully with wool because of superior resistance to high temperatures. Lead refineries have baghouses for recovery of fume from softening furnaces and cupeling furnaces. Zine oxide fume released during distillation of zinc from zinc-silver skims may be captured by a local exhaust vendlation system and passed into the flue serving cupel furnaces.
IV. A.
M I N I N G , M I L L I N G , AND
Zinc
CONCENTRATING
The processes used for zinc are essentially the same as for copper and lead (see also Chapter 35). The bulk of zinc ore contains zinc as sphal erite, ZnS, which is separated as a concentrate from accompanying min erals by selective flotation. Concentrates contain about 60% Zn. In recent years, an important source of raw material for zinc metal has been zinc oxide from fuming furnaces. Zinc as an impurity in lead smeldng is recovered from lead blast furnace slag by headng the slag to high temperatures and blowing pulverized coal and air through it. Zinc is reduced, volatilized, reoxidized and is collected as ZnO in bag filter units. The baghouse product is passed through a rotary kiln to reduce the lead content by volatilization and to increase the density of the material for easier handling and shipping.
37.
B.
ROASTING
NONFERROUS
AND
METALLURGICAL
OPERATIONS
183
RETORTING
For efficient recovery of zinc, sulfur must be removed from concen trates to less than 2%. This is done by roasdng. Muldple hearth or Ropp roasdng may be followed by sintering; or double-pass sintering may be used alone (Fig. 3). The liberadon of zinc from roasted concentrates involves simple heating of a mixture of roast and coke breeze to about 1100 °C. Simul taneous reduction of zinc from the oxide to the metal and disdllation of the metal take place. Zinc vapor passes from the heated vessel into a condenser where it condenses to a liquid which is drained off at intervals into molds. Reducdon and disdlladon of zinc may be done as a batch process in banks of cylindrical retorts—the Belgian retort process—or in contin uously operadng vertical retorts. Gas is the preferred fuel, hence most smelters are located in natural gas fields. Electric distillation furnaces are used to a small extent. A spectroscopically pure zinc is produced by a condnuous fractional distillation process developed by the New Jersey Zinc Co. A process for simultaneous smeldng of roasted lead and zinc concen trates has been developed within the past decade by the Imperial Smelting Corp. T h e process makes use of carbon monoxide generated from coke to reduce lead and zinc oxides in a sealed shaft furnace. Lead bullion accumulates in the furnace bottom and acts as a collector for copper, silver, and gold. Zinc passes as a vapor out the top of the furnace into condensers in which a shower of molten lead is condnuously mainOre Concentrate Concentrates (alxDut 60% Zn)
I
Tails (waste or worked up for Cu and Pb)
Roast or roast-sinter SO2 + SO3 I Available for acid manufacture
ZnO (With l e s s than 2 % S)
Mix with coal and heat in retort Zinc vapor condense Dross and cast into standard slabs
Blue powder I Market or re-treat in next charge
Residue I Waste or further treatment by other methods
FIG. 3. Outline of zinc production from sulfide ore {8a).
184
KENNETH W.
NELSON
tained. Zinc vapor is condensed quickly to a liquid which dissolves in the molten lead. Outside the condenser the lead-zinc soludon is cooled and a 98% pure zinc floats to the surface and overflows into containers. The cooled lead is pumped back to the condensers (13). C.
L E A C H I N G AND
ELECTROLYSIS
Zinc of high purity may be produced from roasted concentrates from densified zinc oxide from fuming furnaces, or from impure metallic zinc, by solution in sulfuric acid, removal of impurities from the solution by appropriate chemical treatment, and finally electrolysis of the purified electrolyte. Electrolysis is done in tanks containing alternating anodes of lead and cathodes of aluminum. Pure zinc is deposited on the cathodes and later stripped from them by hand. The zinc is then melted in a small rever beratory furnace and cast into slabs or other forms for shipment. D.
EMISSIONS AND
CONTROLS
Dust, fume, and S O 2 are evolved from zinc concentrate roasdng or sintering (Table III). Particulates are caught in conventional baghouses or Cottrells. Sulfur dioxide attains concentrations of 6-7% in roaster gases and may be converted direcdy into sulfuric acid or vented from tall stacks. An interesdng and very successful process for recovery of zinc roaster S O 2 as well as of the more dilute S O 2 from lead roasting has been developed at Trail, Bridsh Columbia, by the Consolidated Mining and Smeldng Co. Inexpensive hydroelectric power for ammonia synthesis permits the absorption of S O 2 from the gas streams and the TABLE
III
EMISSIONS FROM ZINC SINTERING AND HORIZONTAL RETORTS"
Raw gas
Sintering Horizontal retorts
Waste gas (m^)
Dust content (gm/m^)
% Particles <10μ
S O 2 content (%)
4000^ 12,000-18,000^ 96,000-144,000^^
10 0.1-0.3
100 100
4.5-7
" Clean Air Guide 2284, Kommission Reinhaltung der Luft, Verein Deutscher In genieure, VDI—Verlag CimbH, Dusseldorf, West Germany. September, 1961. " Per ton of ZnS. ^ Waste flue gas per ton of Zn. ^ Condenser waste gas (including extraneous air) per ton of Zn.
37.
NONFERROUS METALLURGICAL
OPERATIONS
185
eventual production and marketing of ammonium sulfate as a fertilizer. Steps in the process are oudined in Fig. 4. In zinc disdllation by the retort process small holes are left in vapor condensers to vent gases from charged retorts as they are heated. When the temperature becomes high enough for zinc vapor to distill over, some vapor escapes from the hole and ignites spontaneously. The total Sullivan mine
I
- Trailings to dump
Concentrator
Zinc concentrate
Lead concentrate Dwight-Lloyd sintering machines
Lead sinter to blast furnace
Zinc calcine to zinc leaching
— Suspension roasters
t
Waste heat boilers
Humidifying flue
t
\
Cyclone treaters
Cottrell treaters
t
t Cooling towers Absorption towers
Cottrell hot treaters TaU gas i
t
TaU gas
Glover cooling towers
t
Aqua ammonia
Aqua ammonia Absorption towers Shriver filters i Acidifying tower
t . Eliminator tower _ Ammonium sulfate solution
Wash towers i MistCottrells Drying towers
Oslo crystallizers
Heat exchangers + Converters
f Heat exchangers I
Atmospheric cooler
t Absorbers i Dilution Storage i FIG. 4. Flow sheet of ammonia process93% at Sulfuric Trail, British Columbia (9). acid
186
KENNETH
W.
NELSON
zinc oxide fume so produced is appreciable. It is carried by convection currents up the fronts of the condenser banks and out to atmosphere with combustion gases through ridge ventilators of the furnace build ings. A disdnctive characteristic of the zinc retort plant is a flag of white ZnO fume. T h e actual fume concentrations are low and the cost of its collection would not be compensated for by the value of the recovered material. Leaching and electrolysis do not emit significant amounts of particu lates or gases. Tanks in which leaching and electrolyte purification are done are covered and ventilated to prevent worker exposure to possible toxic gases or mists. T h e electrolytic process itself does disperse some electrolyte mist because of gas evolution, but this is a problem only within the confines of the tank houses. V. A.
Aluminum
M I N I N G AND O R E T R E A T M E N T
Bauxite is the base ore for aluminum production. It is a hydrated oxide of aluminum associated with silicon, titanium, and iron, and contains 30-70% AI2O3. Most bauxite ore is purified by the Bayer process. T h e ore is dried, ground in ball mills, mixed with sodium hydroxide solution, and autoclaved for several hours to dissolve the AI2O3 as sodium alumínate, N a A 1 0 2 . By setding, dilution, and filtration, iron oxide, silica, and other insoluble impurities are removed. Aluminum hydroxide is precipitated from the diluted, cooled solution—the reaction being initiated or "seeded" by introduction of a small amount of freshly precipitated Al(OH)3. T h e precipitate is filtered, washed, and calcined to produce pure alumina, AI2O3. (See also Chapter 35.) B.
ELECTROLYSIS
Commercial recovery of aluminum from the oxide is accomplished by a unique electrolytic process discovered simultaneously in 1886 by Hall of this country and Heroult of France. Alumina is dissolved in a fused mixture of fluoride salts and dissociated electrically into metallic aluminum and oxygen. Fused natural or synthetic cryolite (3NaF-AlF3) with about 10% fluorspar and 5% dissolved alumina is contained in carbon-lined cells or pots. Heavy carbon anodes are immersed in the mixture to within about 2 inches of the cathode, a heel of molten alumi num covering the carbon lining. A heavy electric current between anode
37.
NONFERROUS METALLURGICAL OPERATIONS
187
and cathode reduces A I 2 O 3 to Al which accumulates and is drawn off at intervals into crucibles. Alumina is steadily fed onto the top of the molten electrolyte to replace that which has been decomposed. Crucible aluminum is skimmed of dross, has alloying metals added to it, and is charged into holding furnaces before being cast out as salable ingot. C.
EMISSIONS AND
CONTROLS
Calcining of aluminum hydroxide for the production of alumina en tails mechanical dust dispersion (Table IV). The valuable dust is re covered from kiln effluents by electrostatic precipitadon preceded by cyclone-type collectors. Oxygen liberated in electrolytic cells attacks carbon anodes, producing C O 2 and mechanically dispersing some carbon dust. Thermal and chem ical action in the cells also evolves some alumina dust, and both particu late and gaseous fluorides. The various effluents are collected by venti lation hoods over the cells. Aluminum plants using carbon anodes which have been baked before mounting in cells may have dry dust collectors to remove particulates from collected effluents, scrubbers for removal of gaseous fluorides, and stacks for final dispersion of scrubbed gases. Plants employing the Soderberg method of forming and baking carbon electrodes in place on the cells may omit dry collection and scrub all effluents direcdy. Tarry hydrocarbons from the baking carbon electrode mixture inter fere with the operation of dry collectors. Spillage of fluoride-containing waste gases from aluminum pot lines T A B L E IV EMISSIONS FROM CALCINING ALUMINUM HYDROXIDE AND FROM CLOSED ELECTROLYTIC CELLS USING SODERBERG ELECTRODES"
Raw gas
Calcining aluminum hydroxide Closed electrolytic cells using Soderberg electrodes
Waste gas (m^)
Dust content (gm/m^)
Fluorine content (gm/m^)
3000*
300-400
—
2000-3000^^
0.08-0.12
0.040
" Clean Air Guide 2286, Kommission Reinhaltung der Luft, Verein Deutscher In genieure, VDI—Verlag GmbH, Dusseldorf, West Ciermany. September, 1961. Per ton of AI2O3.
Including extraneous air. Waste gas per cell in cubic meters per hour.
188
KENNETH W.
NELSON
into the pot room and then out roof monitors has released sufficient fluorides at some plants to require the installation of scrubbers to wash the air leaving the roof monitors. Aluminum chloride or chlorine gas is used to treat aluminum in hold ing furnaces, to flux and to degas the molten metal. Aluminum chloride is voladle, subliming at 180 °C. It reacts with hot moist air to form a highly visible "smoke" cloud. Electrostatic precipitation, scrubbing, and condensadon of the voladlized salt are eff'ecdve means of control. VI. A.
Secondary Copper, Lead, Zinc, a n d Aluminum
SOURCES
Scrap provides an important source of metals for the market. Copper in substantial tonnage is recovered from electrical cable and automobile radiators. About 85% of the lead used in automobile batteries is collected by scrap dealers and eventually sold to secondary lead smelters. Zinc comes from galvanizing baths and die-cast scrap. Aluminum is recovered mosdy from industry-generated scrap, but aircraft assemblies and even pots and pans may be used. Economics governs the flow of material in and out of the secondary plant. B.
RECOVERY
PROCESSES
Scrap may be converted direcdy to usable metal by simple melting and casting into salable ingots. More often, scrap of similar composition is melted together and its composidon is adjusted by removal or addi tion of some constituent elements. When the right proportions of each are present, the charge is cast out. Brass, type metal, babbitts, and solders are produced in this way. Insulation is stripped or burned from electrical cable and the copper wire is melted and cast into anodes for direct conversion by electrolysis into refined copper. Lead battery plates are charged into reverberatory furnaces with coke and fluxes for smeldng. The product is an andmonial lead bullion which will be refined to pure lead or adjusted in composition for sale as an alloy. Zinc scrap is commonly refined by disdllation from retorts, the vapor being condensed to a liquid and cast into slabs, or condensed direcdy from the vapor state to a powder of controlled particle size. Aluminum alloys are melted in reverberatory furnaces and adjusted in composition by drossing, chlorination, and addition of alloying metals.
37.
C.
NONFERROUS METALLURGICAL OPERATIONS
EMISSIONS AND
189
CONTROLS
Emissions from secondary metal processes are similar to those from primary metallurgical operations except that little or no sulfur dioxide is evolved, and, in general, smaller quantities of metal oxide dusts and fumes are produced. There are no sulfides in secondary metal furnace charges as there are in primary smelting furnaces, and only small amounts of sulfur are used to ketde-refine lead-base alloys. Lead oxide is volatilized from secondary lead smelting. Zinc oxide fume is a by-product of zinc-alloy distillation and of brass furnace operation. Baghouses, more commonly, and electrostatic precipitators are successfully used for collection. Chlorination of molten aluminum in secondary refining furnaces produces aluminum chloride fume which creates a high-opacity white cloud when it comes in contact with moist air. Properly designed scrub bers or condensers are effective control devices. One successful installa tion has been described by Donoso {14).
VII. A.
A L L O Y S AND
Nonferrous Foundries
OPERATIONS
Brass, bronze, and zinc die-casdng metal are the principal alloys used. Brasses contain 60-65% copper; the other major constituent is zinc. Lead or dn or both may be present. Bronzes usually contain 85-90% copper with small amounts of alu minum, zinc, dn, manganese, silicon, and phosphorus—singly or in combination. Alloy names are misleading. Manganese bronze, for ex ample, may contain only 0.25% Mn. Zinc die-casting metal is about 95% high-grade zinc, 4% aluminum, plus traces of other metals. Copper base alloys are melted in rotary, reverberatory induction fur naces or in small crucibles and are poured at bright red heat into molds. After the casdngs have solidified they are shaken from molds, and finished. Zinc die-cast alloy is melted in kettles and poured or injected at a temperature of 775°-825 °F into permanent dies. B.
EMISSIONS AND
CONTROLS
Metal fume will be evolved during the melting and casting of alloys if the temperatures are high enough for volatile constituent elements
190
KENNETH W.
NELSON
to have appreciable vapor pressure, and if no intermetallic compounds are formed. For example, zinc is fairly voladle and will be vaporized from molten brass and oxidized, producing visible white ZnO fume. The higher the temperature and the greater percentage of zinc in the alloy, the more copious the fume. Copper in the brass, on the other hand, will not be volatilized significantly because its vapor pressure is very low at the meldng point of brass. Zinc die-casting alloy is used with no metal fume evolution. Shakeout of casdngs from sand molds is a dusty operation and may be conducted under hoods or over downdraft gratings. Dust loadings in ventilating air range from 0.25 to 1.0 grain per cubic foot. Collection of up to 97% of such material is effected with wet or dry centrifugal collectors, up to 99+% with,dry fabric type collectors (15), REFERENCES L J. K. Haywood, U.S. Dept. Agr., Bur. Chem. Bull. 8 9 (1905); U.S., Bur. Mines, Bull. 5 3 7 (1954) (abstr. No. 1231). 2. J. A. Holmes, E. C. Franklin, and R. A. Gould, U.S., Bur. Mines, Bull. 9 8 (1915). 3. P. J. O'Gara, Met. Chem. Eng. 1 7 , No. 12 (1917). 4. G. R. Hill and M. D. Thomas, Plant Physiol. 8 , 223 (1933). 5. M. D. Thomas, J. O. Ivie, J. N. Abersold, and R. H. Hendricks, Ind. Eng. Chem., Anal. Ed. 15, 287 (1943). 6. M. D. Thomas, R. H. Hendricks, and G. R. Hill, Proc. Ist Natl Air Pollution Symp., Pasadena, Calif., 1949 p. 142. 7. C. R. Davis, Pay Dirt, No. 326, p. 5, C. F. Willis, Phoenix, Arizona, 1966. 8. J. N. Agate et al. Med. Res. Council Memo. 2 2 (1949); quoted by P. D r i n k e r , / Roy. Inst. Public Health Hyg. 2 0 , 307 (1957). 8a. C. R. Hayward, "An Oudine of Metallurgical Practice," 3rd ed. Van Nostrand, Princeton, New Jersey, 1952. 9. Ontario Research Foundation, "The Removal of Sulphur Gases from Smelter Fumes." B.Johnson, Toronto, 1949. 10. G. R. Hill, M. D. Thomas, and J. N. Abersold, Proc. 9th Ann. Meeting Ind. Hyg. Found., Pittsburgh, 1944 p. 11. 11. M. F. Smith, Proc. 3rd Natl Conf Air Pollution, Washington, D.C, 1966 p. 151. 12. E. P. Fleming and T. C. Fitt, Ind. Eng. Chem. 4 2 , 2249 (1950). 13. P. J. Callahan and T. D. Parker, Chem. Eng. 7 4 , 159 (1967). 14. J. J. Donoso, Proc. 40th Ann. Conv. Smoke Prevent. Assoc. Am., Toronto, 1947 p. 39. 15. J. M. Kane, Am. Foundryman 19, 34 (1951).
GENERAL REFERENCE Additional pertinent material is in Chapter 6 of "Air Pollution Engineering Manual, Los Angeles APCD" (J. A. Danielson, ed.), PHS Publn. 999-AP-40, DHEW, Cincinnati (1967); and will be published in the National Air Pollution Control Administration's pub lications: "Control Technology for Particulate Air Pollutants," and "Control Technology for Sulfur Oxides Air Pollutants."