Chemicals in Ore Processing — A Fifty-Year Review by RAYMOND E. BYLER1, Arthur D. Little, Inc.
Our increasing dependence on low grade ores poses the question, "How do we get the most metal out of the least ore?" The answer lies with the use of chemicals in ore processing I HE ability of the mining industry to meet the tremendous increase in d e m a n d for nonferrous metals developed over the past 50 years was m a d e possible only by major breakthroughs in the technology of processing ores and minerals. While m a n y improvements in physical methods of mineral concentration have been made—introduction of the Wilfley shaking table in 1905 completely revolutionized mineral dressing—but without the advances in chemical metallurgy, a famine of base metals would have immeasurably retarded our industrial growth. Most of the lower grade a n d complex mineral deposits o n which we are now dependent for nonferrous metals could not be worked profitably by the methods available at the turn of the century. T h e production of some metals a n d nonmetallic minerals, not then industrially used b u t today of great importance, has required application of chemical techniques entirely new to mineral processing. T w o major types of processes for treating ores, hydrometallurgical and flotation, are dependent on chemicals. These two processes account for most of the nonferrous mineral production, a n d for most of the approximately 2,000,000 tons of chemicals consumed annually in mining and processing ores in the U . S. Hydrometallurgy Dominating the metallurgical scene of 50 years ago, a new process— 1 Present address, Western Machinery Co., San Francisco, Calif.
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ANNIVERSARY FEATURE 50 A
cyanidation—was being adopted for treatment of gold ores throughout the world, replacing the centuriesold chlorine metallurgy. It marked a breakthrough which laid the foundation for m o d e r n hydrometallurgy. Cyanide plants became the proving ground in which the principles of leaching a continuous flow of ore pulp were evolved, a n d the prototypes of some unit process equipment now used extensively in chemical processing—the D o r r sand classifiers, gravity thickeners, various types of agitators, the Oliver rotary v a c u u m filter, a n d the M o o r e leaftype v a c u u m filter—were developed. T h e process created a large new market for sodium a n d calcium cyanide. By 1935 over 150 plants were in operation—a typical o p e r a tion treating 2000 tons of gold ore per day consumes annually 350 to 750 tons of calcium cyanide. Use of the process continued to grow until World W a r I I , since which time a n increasingly unfavorable economic climate for gold production has forced retraction in mining of gold ores, a n d cyanidation has shrunk proportionately. T h e feasibility of using a m m o n i a in large scale operations for leaching ores was demonstrated in 1917 by C. H . Benedict at Calumet, Mich., in a n 8000-ton-per-day closed-tank operation extracting fine native copper from gravity concentration tailings. I n the same year, a n 800-ton-per-day plant using a m m o n i a - a m m o n i u m carbonate as a solvent for nonsulfide copper minerals was p u t into operation at Kennecott, Alaska, to leach mill tailings. Although the leaching solutions contained from 6 to 1 1 % N H 3 , consumption was kept remarkably low (0.45 to 0.60 p o u n d of a m m o n i a per ton treated) by steaming the residues before discarding. These operations have been worked out, b u t interest in ammonia leaching has revived
INDUSTRIAL AND ENGINEERING CHEMISTRY
owing to the readiness with which a m m o n i a complexes copper, nickel, a n d cobalt, plus the fact that it is noncorrosive a n d is not wastefully consumed by gangue minerals in the ore. Pilot plant studies in 1941 by the Nicaro Nickel Co. confirmed the suitability of a m m o n i a - a m m o n i u m carbonate solution for leaching lateritic nickel ores, which could not be economically treated by sulfuric acid because of excessive acid consumption by serpentine minerals in the ore, a n d a 6500-ton-per-day plant was p u t in operation in 1943 a t Nicaro, C u b a . T h e ore is first roasted to reduce the nickel, a n d then leached with a solution containing 1 4 % NH» a n d 8 % C 0 2 . T h e net consumption of a m m o n i a is a b o u t 5.5 pounds per ton of ore. T h e leached solution is "boiled off" for recovery ( a n d recycling) of the a m m o n i a a n d the nickel precipitates simultaneously as basic carbonate. A m m o n i a leaching has been recently extended to direct t r e a t m e n t of nickel-copper-cobalt sulfide minerals. At elevated temperatures a n d pressure, a n d u n d e r oxidizing conditions, a m m o n i a reacts with the sulfide minerals to form a m m o n i u m sulfate, which complexes with the metals. T h e process was worked out by F . A. Forward (University of British Columbia) a n d installed in 1955 a t Fort Saskatchewan, Alberta, to treat 300 tons per day of flotation concentrates from the Sherritt Gordon M i n e . After dissolving t h e metals in pressure a u t o claves a t 175° F., the leach solutions are boiled off a n d copper precipitates as a sulfide; nickel a n d cobalt r e m a i n in solution as a m i n e sulfates. Hydrogen sulfide is used to m a k e u p any deficiency in sulfide ion to scavenge copper from the solution, p r e p a r a t o r y to precipitation of nickel a n d cobalt by hydrogen gas reduction, carried out in b a t c h a u t o -
attach to gas bubbles a n d float, while gangue minerals became wetted a n d sank. This gave rise to early separatory methods based on floating mineral particles agglomerated by bulk additions of oil, which achieved limited commercial use until 1906, when Sulman a n d Picard, in England, led the way to modern froth flotation by the discovery that only small quantites of oil were required if a large volume of air bubbles was whipped into the p u l p by mechanical means such as a revolving impeller, to create a froth. Sulfide and gangue minerals were m o r e completely separated by this procedure a n d after improvements in a p p a r a t u s developed by T . J. Hoover in 1910 provided better large scale operation, the process became firmly established. Since then, the c u m u l a tive efforts of m a n y experimenters have gradually developed precise chemical controls a n d greatly extended its usefulness. At first, various u n s a t u r a t e d oils (mineral, vegetable, a n d animal) were used both as "collectors" for the sulfide mineral particles a n d to provide a froth. Later (1912) Perkins found that soluble organic compounds containing nitrogen or sulfur radicals could be substituted for the oils but a real b r e a k t h r o u g h was achieved in 1925-26 when Keller introduced the xanthates, a n d Whitworth the dithiophosphates, as collectors. These compounds enable almost complete recovery of sulfide minerals. Progress in another direction was moving towards a selective or differential flotation, by which one kind of sulfide mineral in a complex ore is held in check while another kind is collected a n d floated off. Sheridan and Griswold in 1922 discovered that sodium cyanide would " d e p r e s s , " or hold in check, zinc and iron sulfide minerals while the associated lead sulfide mineral (galena) is separately floated off. Afterwards, addition of copper sulfate solution "activates" the zinc mineral, permitting its recovery. An increase in alkalinity of the pulp by addition of lime depresses or holds the iron sulfide (pyrite) in check during flotation of the zinc mineral. This differential process provided the first effective commercial method for utilizing large reserves of complex lead-zinc-iron minerals, not a m e n a b l e to other processing methods. By 1925, through dis-
covery of the action of carboxyl acids a n d their soaps as collectors for nonmetallic minerals, flotation - had been extended to the recovery of nonmetallic and earthy-type metallic minerals. T h e amines of fatty acids and various petroleum a n d sulfonated oils a r e also utilized as collectors in nonmetallic flotation. Now chemical control of flotation can be tailored to obtain m a n y complex separations of both metallic and nonmetallic minerals. More than 250 different chemicals are used as frothers, collectors, depressants, or activators in controlling flotation (Table I ) . Investigations into flotation p r o b lems are becoming less empirical and more scientific as understanding of the fundamentals increases, a n d the ultimate scope of the process is still beyond the horizon. T h e consumption of chemicals in processing ores is tied to metal production, but is increasing at a faster rate t h a n the production of metals, largely because of our increasing dependence on lower grade ores— more tons of ore must be processed per pound of metal produced. W e m a y look for chemical processing of northwestern clays for production of alumina—already a t the pilot plant stage—and further applications of flotation to processing of low grade nonmagnetic taconites a n d hematites. T h e beneficiation of large low grade manganese ore reserves by chemical processing is ready when needed. Production of some of the " n e w e r " metals now supplied commercially— zirconium, niobium, t a n t a l u m , uranium—calls for chemicals not previously used for treatment of minerals on a large scale. W e may, however, look for re-evaluation of present chemical metallurgy when metals now in short supply catch u p with d e m a n d .
ANNIVERSARY
FEATURE
Table I. Flotation Chemicals Commonly Used .Frothers Pine oil Cresylic acid Aliphatic alcohols Synthetics Methyl isobutyl carbinol Collectors Xanthates (dithiocarbonates) Dithiophosphates Thiocarbanilide Thionocarbamates (Cu) Fatty acids (oleic and linoleic) Fatty acid amines (tallow, coco, resin) Mineral oils (kerosine, fuel oil) Modifiers pH modifiers Lime Soda ash Sulfuric acid Sulfur dioxide Activators Copper sulfate (Zn) Sodium sulfide (Pb) Depressors Lime (Fe) Ferrocyanide (Cu) Sodium sulfite (Zn, Fe) Sulfur dioxide (Pb) Sodium cyanide (Zn) Zinc sulfate (Zn) Sodium dichromate (Pb) Sodium silicate (quartz) Tannic acid (calcite) Others Wetting agents Soluble gums Starch Synthetic polymers
Looking back over the great progress m a d e in chemical metallurgy leaves the conviction that as long as our technology requires metals, new a n d old; as long as economic pressures d e m a n d lower production costs; as long as chemists a n d metallurgists a r e inquisitive—we can Jook for continued improvements a n d expansion in the use of chemicals in ore processing.
Chemical Consumption in Flotation Plants Type of Ore Lead-copper-zinc Iron (oxide) Copper-lead-zinc Tungsten Manganese (oxide) Lead-zinc Copper-zinc-iron Nickel-copper Iron (jasper) Copper-molybdenum Copper Molybdenum
Tons Ore/Day 500 625 750 875 1,000 1,200 2,000 2,500 3,800 10,000 12,000 25,000
Frothers, Lb./Ton 0.07 ... 0.13 0.003 ... 0.10 0.26 0.05 0.15 0.20 0.13 0.91
Collectors, Modifiers, Lb./Ton Lb./Ton 0.70 7.75 5.70 2.70 0.17 6.23 0.28 20.80 184.00 16.00 0.15 3.30 0.34 11.36 0.15 1.57 1.08 ... 0.08 7.62 0.83 4.50 1.63 0.11
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Total, Lb./Ton 8.53 8.40 6.53 21.08 200.00 3.55 11.96 1.77 1.20 7.90 5.46 2.65
Chemicals, Tons/Year 765 945 880 3,305 36,000 765 4,320 800 825 14,200 11,800 12,000
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53 A
Chemical Consumption in Leaching Operations Ore, Tons/Day
Chemicals, T o n s / Y e a r
Acid leaching Copper ore
Uranium ore
Ammonia leaching Nickel ore (lateritic) Nickel-copper (flotation concentrates)
Carbonate leaching Uranium ore
leaching, u r a n i u m must be oxidized to the sexivalent state, and aeration plus chemical oxidants, either manganese dioxide (from 5 to 10 pounds per ton of ore) or sodium chlorate (from 2 to 4 pounds), is used. Acid consumption varies from about 50 to 200 pounds or more per ton of ore. U r a n i u m is recovered from the leach solution by ion exchange or solvent extraction. T h e impact of a new mining region on the chemical industry may be impressive. In the Blind River, Ontario, uranium district 11 new leaching plants have recently begun production, combined capacity is 34,000 tons per day and requirements at full capacity will be 800,000 tons of processing chemicals annually. Another milestone was marked by the first commercial use in 1952 of high temperature-high pressure sulfuric acid leaching for recovery of cobalt at the Calera refinery, Garfield, Utah. T h e plant has a rated capacity of 3,000,000 pounds per year. R a w complex cobalt-arsenic sulfide flotation concentrates are leached in continuous-flow type autoclaves at about 375° F. and 500 p.s.i. pressure. T h e sulfide sulfur oxidizes to produce the acid solvent. Soluble arsenic is removed by precipitation with iron powder, excess acidity is reduced with limestone, and the solution is neutralized with ammonia, which converts cobalt into the amine form. Hydrogen 52 A
10,000
3,000
6,500
H2SO< F e (scrap) H2SO( NaClOs Lime Glue Separan IX resin HNOs NaOH
NH S
300
NH 3 H 2 SO, H2S H,
500
Na 2 CO s NaOH
47,000 58,000 105,000 40,500 2,200 27,000 810 60 2,400 1,100 74,070 6,500 25,200 7,200 180 720 33,300
plus a catalyst was originally used to precipitate cobalt in batch autoclaves, but recently electrolytic reduction and refining have replaced hydrogen to obtain a purer product. A similar process is in use in Fredericktown, Mo., by National Lead. Currently, application of high temperature-high pressure sulfuric acid leaching for extraction of nickel (and a lesser amount of cobalt) from iron laterites is getting under way at M o a Bay, Cuba, where Freeport Sulphur is erecting a 6000-tonper-day plant. Unlike the lateritic ore treated at Nicaro, the Moa Bay ores contain only small amounts of serpentine and are suitable for direct sulfuric acid leaching. Effluent acidity will be adjusted with lime, and the nickel and cobalt precipitation with hydrogen sulfide. T h e product containing about 5 5 % nickel and 5 % cobalt will be shipped to the U. S. for separation and refining. Provisions for m a n u facture of the required chemicals at the mine site include a hydrogen plant to produce 1,500,000 feet per day, a hydrogen sulfide plant with a daily capacity of 160 tons (liquid), and a sulfuric acid plant designed for 1300 tons per day of 9 8 % acid. Acids other than sulfuric have not come into general use for direct treatment of ores. Hydrochloric acid is used on a small scale for upgrading tungsten concentrates to meet trade specifications by remov-
INDUSTRIAL AND ENGINEERING CHEMISTRY
ing soluble carbonate and phosphate minerals, and was used in batch leaching at high temperatures (220° F.) for removing impurities from tin concentrates at the Longhorn Tin Smelter, Texas, from 1942 until shut down recently. A determined effort was made from 1914 to 1925 to treat lead and silver ores by processes involving chloridizing, roasting, and acid brine leaching, but because of chemical difficulties and high costs processes of this type gave way to the cheaper and more effective flotation. Control of the physical state of ore pulps a n d colloidal slimes has long been a problem in acid leaching, but new synthetic flocculants (polyacrylamides and polyacrylonitriles) and water-soluble natural gums (guar gum) introduced in 1954 appear to be much more effective than glue, starch, and other additives, and are finding wide usage as new chemical tools in hydrometallurgy. Ion Exchange and Solvent Extraction. T h e recent introduction of ion exchange and solvent extraction techniques directly into the flow of large scale uranium ore processing operations is a long step toward reducing the spread between raw ore and refined metal. These methods are more selective than most direct chemical precipitation processes and yield leach products of substantially higher purity, important in reducing refining costs when using the less selective acid leach. Although markedly successful in uranium hydrometallurgy, and technically feasible for extraction of m a n y metals, use of these techniques in direct ore processing appears at present to be limited to recovery of metals having higher price tags. They are used for difficult separations such as niobium from tantalum, zirconium from hafnium, and removal of trace elements in purification of uranium and other metals. Flotation
By 1908 a revolutionary process for separation of the sulfide and nonsulfide minerals in an ore was in the making. Experimenters had been preoccupied with the property of oiled sulfide mineral particles dispersed in an aqueous phase to
claves at elevated t e m p e r a t u r e . Sodium carbonate has become increasingly i m p o r t a n t as a solvent in m o d e r n hydrometallurgy. Large tonnages of low-silica bauxite ore are leached by the soda-lime process for production of a l u m i n a , which more t h a n doubled following the K o r e a n conflict. A n o t h e r principal use, generated u n d e r A E C auspices, is for leaching u r a n i u m ores too high in c a r b o n a t e minerals for acid processing. From 15 to 20 pounds of sodium carbonate a n d a b o u t 20 pounds of caustic soda are consumed per ton of u r a n i u m ore. Extraction is speeded by moderately elevated t e m p e r a t u r e ; hence closed agitators or Pachucas are used. At Beaverlodge, Sask., pitchblende ore is leached at 230° F. a n d 80 p.s.i. in pressure autoclaves. At the beginning of o u r review period sulfuric acid leaching of copper ores was well established. T h e largest operations were at Rio T i n t o , Spain, where at one time there were 20,000,000 tons of ore in heaps, some u n d e r t r e a t m e n t for 30 years. Long weathering permitted atmospheric oxidation to aid dissolving. A somewhat similar operation, still practiced in the U . S., is leaching of copper oxide ore bodies in situ, without mining. Acid m i n e water, sprayed over the leach area, percolates t h r o u g h fractures or old
n» mwiiiintii'B'iHi
i
Chemically controlled
flotation—used
in processing many varieties o f ores
ents by electrolytic reduction. T h e c o m m o n hydrometallurgical process for t r e a t m e n t of zinc sulfide ores or concentrates involves roasting to zinc sulfate a n d then leaching with sulfuric acid. T h e leach solutions are purified a n d electrolyzed for reduction of the m e t a l , a n d spent electrolyte is recycled to supply acid to the leach. A large use of sulfuric acid has developed in leaching low-carbonate u r a n i u m ores. H o t dilute solution (140° F. a n d p H 1.0 or lower) is used, a n d leaching is in continuous-flow type agitators or P a c h u c a tanks. As in c a r b o n a t e
mined-out ore zones that are suitably porous. T h e process generates its own acid solvent t h r o u g h decomposition of iron a n d copper sulfide minerals within the ore. Solutions a r e collected in u n d e r g r o u n d tunnels a n d dissolved copper is recovered by the historic m e t h o d of precipitation on iron (cementation). Following discovery of large low grade surficial deposits of oxidized c o p p e r ores, acid leaching was begun in 1920 on a large scale in the U . S. a n d Chile. Crushed ore was charged to large open vats and series-leached for 6 to 8 days, copper being recovered from efflu-
••»
Battery of pressure leaching t a n k s — 1 5 0 0 ton per d a y uranium ore processing plant. pension
Air agitation keeps ore pulp in sus-
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