Extraction of Metals from Ores' CARLE R. HAYWARD Professor of Process Metallurgy, Massachusetts Institute of Technology
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N INTRODUCING this subject, it will be of interest to glance rapidly over the principal metals and note the parts of the world from which they come. The following table is based on the year 1937 in which the figures are more normal than in the war years. TABLE 1 LEADING PRODUCERS OF COMMON METALS I N 1937 'Aluminum Antimony *Beryllium Bismuth *Cadmium 'Calcium Chromium Ore 'Cobalt 'Copper *Gold
1, United States; 2, Germany; 3, Russia. 1. China; 2, Mexico. 1, Germany; 2, France; 3, United States. 1, Peru; 2. Bolivia; 3, Mexico; 4, Canada. 1, United States; 2, Mexico; 3, Canada. 1, Germany; 2, United States; 3, Sweden. 1, Russia; 2, Rhodesia; 3, Turkey. 1. Belgian Congo; 2, Canada; 3, United states. 1, United States; 2, Chile; 3. Africa. 1, Transvael; 2. Russia; 3, United States;
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4. Canada.
1,United States; 2, Germany; 3, Russia. 1, United States; 2, Mexico; 3, Australia; 4, Canada. 1, Germany; 2, United States; 3, France. *Magnesium 1, Russia; 2, Africa; 3, India. Manganese Ore 1. Soain: 2. Italv: 'Mercunr - . 3.. United States. 1; united states. *~olybbdenum 1, Canada; 2, New Caledonia. 'Nickel 1. Russia; 2. Canada; 3, Colombia. Platinum Group 1, Mexico; 2, United States; 3, Peru. *Silver 1, Malaya; 2, Bolivia; 3, Africa. Tin 1, Burma; 2. China; 3. Malaya; 4. United Tungsten States. 1, Peru; 2, Africa; 3, United States. .Vanadium Ore 1, United States; 2, Belgium; 3, Canada; *Zinc 4, Germany. 'United States one of three highest producers. 'Iron 'Lead
to stock several years' supply of high-grade chromite. Both manganese and chromium are vital to the steel industry which must be maintained a t top efficihcy. Most of the world's nickel is produced in Canada, and in normal times is readily available to the United States. The nearest source of tin is Bolivia. Supplies of ore or metal could be easily stored if i t were thought desirable. MINERAL F O m S IN COMMERCIAL ORES
The method used for metal recovery depends not only on the metal itself but on the form in which i t exists in the earth's crust. The metallic minerals which make up our ores may be roughly divided into snlfides and non-sulfides. The latter, including oxides, carbonates, and silicates, are often called oxide minerals. A few elements occur in the metallic state. The following table lists the forms which predominate in the cornmercial ores of some of the common metals. TABLE2 METALS WHICH MAY OCCUR I N EARTH'S CRUST I N THE METALLIC STATE Gold* Silver Platinum*
copper Mercury Bismuth
COM.\ION .METALS OBTAINED COMMk:'RCIAI.LY, MAINLY FROM SULFIDE MIXEKALS Copper Lead Zinc Nickel COMMON
METALS
Antimony Mercury Bismuth Molybdenum OBTAINED
COMMERCIALLY,
A study of the above table shows the United States MAINLY FROM OXIDE MINERALS (OXIDES, CARBONATES, SILICATES, ETC.) to be among the leading producers of most of the important metals. Our principal deficiencies are in tin, Iron Chromium Tin Tungsten nickel, manganese, and chromium. In the cases of tin Aluminum Manganese and nickel, the United States production is negligible Magnesium Beryllium and there is little indication that this condition can * The principal form of occurrence of these metals. ever be improved. There are many low-grade manganese deposits which can be used in an emergency, It should be recognized that the above table is not but most metallurgists believe that in normal times i t inclusive but gives predominating forms. For instance, would be cheaper and wiser to continue the importation of high-grade ores. Many advocate a policy of stock- minor amounts of copper are obtained from ores where ing four or five years' supply of imported ores a t the copper is as metal or oxide, but most of the world's government expense for use when commerce is inter- production is from snlfides. rupted. The chromium situation is little better than CONCENTRATION OF MINJ3RAI.S the manganese situation and here also i t might be wise Iron ores are sufficiently abundant to supply the 'Address presented before the Fourth Summer Conference of the N.E.A.C.T. a t the University of New Hampshire, Durham, N. H., world's smelting furnaces with plenty of material running above 50 per cent iron, which readily lends itself August 12, 1942.
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to direct smelting. In the case of most of the common non-ferrous metals, however, the valuable minerals are so intermixed with minerals of no value, principally silicates, that the resulting ore is of very low grade. For example, the average copper content of the copper ores mined in the United States is under 1.35 per cent copper and the output of some of the largest mines is less than 1.0 per cent. The cost of smelting such lowgrade material would be prohibitive, but fortunately a method has been found to separaie the metallic minerals from waste rock and from each other by inexpensive methods, giving a product which can be further treated for metal recovery. One general method formerly used extensively took advantage of the relatively high specific gravity of metallic minerals. If a piece of silica and a piece of lead sulfide of the same size are suspended in water, a rising current of a certain speed will allow the sulfide to sink and the silica to rise. Various machines using thii and allied principles have been developed which succeeded in recovering fairly high percentages of the metallic minerals in an ore as a product suitable for smelting. A more recent and now more widely used method of concentration known as differential flotation utilizes some obscure and not completely understood chemical, physical, and electrochemical laws to produce excellent results. The ore must first be crushed, ground very fine, and placed in water containing small amounts of various organic and inorganic chemicals, one of which must aid in producing a froth when finely divided air is blown into the suspension or beaten in with propellers. By a suitable selection of reagents, the surfaces of certain minerals are rendered non-wettable and small air bubbles become attached to them, causing them to rise into a froth which retains them a t the surface of the water from which they may be skimmed off. By adding reagents successively a continuous operation may be worked out whereby an ore may be separated into several valuable mineral components and a worthless residue. Sulfide copper concentrates produced by differential flotation usually run 25 to 35 per cent copper, zinc concentrates more than 60 per cent zinc, and lead concentrates more than 60 per cent lead. All of these are satisfactorily and profitably treated by the relatively expensive smelting methods to make a final metal recovery. EXTRACTION OF XETAI.3 FROM HIGH-GRADE ORES OR CONCENTRATES
The fundamental operations used to produce metals from ores or concentrates are summarized in the following outline. The later comments will elaborate each item. 1. Oxide ores of iron, tin (concentrates), lead, and antimony. Fuse in contact with carbon (usually coke) and fluxes. 2. Sulfideconcentrates of lead, bismuth, antimony. Bum off sulfur, then fuse with carbon and fluxes.
3. Sulfide zinc concentrates. Burn off sulfur, then heat in clay retorts with carbou. Zinc distils and is condensed. 4. Sulfide mercury ore. Heat under oxidizing conditions. Sulfur removed as SOz, mercury as vapor which is condensed. 5. Gold ore. Dissolve metallic gold content in weak cyanide solution. Precipitate gold with zinc dust. 6. Sulfide copper concentrates. Fuse to form "matte" (Cu2S FeS). Blow air through molten matte in the presence of Si02. SO2 volatilizes; FeO unites with SiOi. to form slag, which is removed; Cu remains. 7. Oxide copper ores. Leach with dilute HzS04. Electrolyze solution to deposit copper. 8. Aluminum ore. Purify to produce AlzOa. Electrolyze in fused cryolite bath. Aluminum deposited and removed in molten state. 9. Tungsten, chromium, molybdenum coucentrates. Produce pure oxide. Reduce to metal without fusion by heating in reducing gas. Coalesce below melting point under high pressure. 10. Magnesium, calcium. Produce anhydrous chlorides. Electrolyze to deposit the metal. 11. Silver (and considerable gold) recovered as a by-product from lead and copper smelting and refining.
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1. Althoueh we have no nroof of the fact. it is orobable that the firs! mrtalc used hy primitive mm, 3 k l e from the small atnounti louml as meteoric iron and nativecol,ptr, were produced by3ccidentalreductioninGres. I t m d y be that oneof our iboriginal ancestresses used some lumps of azurite or malachite (beautiful blue and green carbonates of capper) to decorate the family fireplace. After cooking a hearty supper, the fireplace, filled with hot coals, was left overnight. The next morning a strange reddish material was found in the ashes which the man of the cave put t o good use. I t was metallic copper. Reduction of many oxides is fairly simple. Copper, Lead, and tin oxides were reduced to metals a t an early period in the world's history and the copper-tin alloys (bronze), since they melted a t a low temperature and had goad mechanical properties, were used for many purposes. The bronze age preceded the iron age because, although iron may be metallized by reducing a t a low temperature, i t was not easy for primitive man to fuse it. The first iron was agglomerated as a pasty mass full of slag. I n modern iron blast furnaces one hundred feet high, the oxide ore and coke move slowly downward, become heated, the iron metallized, or reduced, by reaction between the oxide and CO gas, and finally near the bottom of the shaft the iron is melted, sinks into the crucible, and on i t float most of the impurities as a molten silicate slag. These products are removed separately and the pig iron becomes raw material for numerous uses, principal of which is steel making. 2. I n the treatment of sulfide concentrates i t is merely necessary t o heat the material under oxidizing conditions t o remove sulfur a s SO2 and convert the metal t o oxide. This product may then be reduced t o metal by heating with carbon. The oxidation of sulfide concentrates is called roasting, and for best results a thorough knowledge of gas-solid equilibria is necessary. The reaction 2PbS f 3 0 ~ 2Pb0 f 2SOs is easy t o write, but i t is complicated by the auxiliary reaction 2 P b 0 f 250, Or +2PbS01. The latter will take place to some extent
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unless the temperature is above the point a t which PhSO. dissociates. T o promote complete desulfurizatiou it is desirable t o dilute or remove the sulfurous gases and operate a t a relatively high temperature. I n desulfurizing lead or zinc concentrates the usual procedure is t o use a machine which slowly moves grates over a suction box. The sulfides are fed continuously upon the grates and pass under a flame which ignites the surface of the charge. The suction below the grates then draws air down through the sulfides, causing rapid oxidation and the development of temperatures so high that the particles are fritted together, leaving a porous product very law in sulfur. I n the case of lead, this product makes an excellent blast furnace charge. In the case of zinc, the product is crushed for further treatment. The roasting of sulfide antimony ores is complicated by the fact that Sbr08is volatile a t low red heat. Excess air is necessary t o produce the non-volatile SblOs. Sintered lead ores are usually smelted by mixing with limestone and feeding with cake into a small shaft furnace, with cold air entering through pipes near the bottom. The coke furnishes beat by combustion and the hot carbon and carbon monoxide act as reducing agents t o convert PhO to Ph which, with the molten slap. is removed a t the bottom of the furnace. . Zinc boils a t 1700°F. (927T.) hut ZnO is not reduced by carbon until a slightly higher temperature. It is obvious, therefore, that when zinc oxide is reduced, the metal appears as a vapor, which must be condensed in some way. The common practice is t o mix the desulfurized concentrates with coal, place the mixture in cylindrical clay retorts closed a t one end with a clay condenser attached t o the other end. On heating to a temperature above the reducing point of ZnO the zinc vapor is formed and passes t o the condenser, where it is recovered in the molten state. 4. Mercury sulfide behaves in a peculiar way, due t o the fact that metallic mercury is not easily oxidized. When the are is heated under oxidizing conditions to a dull red heat the following On Hg SOz. Since the boiling reaction takes place: HgS point of mercury is 674'F., (357'C.) well below red heat, the mercury, SO2, and heating gases emerge from the furnace together. The low pressure of the mercury in this gas mixture causes difficulties in condensation and i t is necessary to pass the gases through a long line of condensing pipes followed by a water spray if complete condensation is t o he obtained. 5. Nearly all gold occurs in ores in the metallic state. Relatively coarse particles are recovered by amalgamating in mercury hut a larger amount is recovered by crushing the ore very fine and subjecting i t t o an aqueous solution carrying a few hundredths of a percent of NaCN. The reaction is 2Au 4NaCN 0 H ~ O - Z N ~ A U ( C N ) ~ 2NaOH. If zinc dust is added t o the solution, the gold is precipitated by the following reaction: NaAu(CN). 2NaCN Zn 2H90 NaZn(CN)a Au HI 2NaOH. The precipitated gold is recovered from this solution by filtering. 6. Pure comer .. oxide mav be readilv reduced t o oure metallic copper by heating with carbun. L'nlortunatrly, however, copper concentrates usually contain cumidrrable iron and this too wuuld be reduced to some extent and alloy wrth the copper. I n order to facilitate the separation of copper and iron, advantage is taken of the fact that copper has a much higher affinity for sulfur than has iron. I f a high-grade copper concentrate is fused, a double compound of iron and copper sulfides is formed. Sometimes part of the sulfur is roasted off before the concentrates are fused. The fused sulfides are olaced in a cvlindrical vessel iconvertcr) lined with maynesite brick. 'l'hib vr-*el re*ts on itr side. supportcd by rollcrs, and may be rotatcd through 360'. Silica is added to rhc bath and air is blown through the mixture cauring the following reactions:
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--
2FeS f 30% 2Fe0
+ SiOl
2Fe0
+ 2SOr
2FeO.SiOz
The operation is continued until all the iron sulfide is oxidized. then the resultant iron silicate is poured off, leaving CUSSin the converter. The blowing is then continued t o oxidize the remain-
ing sulfur and the operation is stopped when the copper starts t o oxidize. There is sufficient heat generated by the reactions to keep the charge molten without fuel. 7. Some low-grade copper carbonate ores cannot he economically concentrated. Where the quantity and nature of the ore justify the operation, it may be possible t o dissolve the copper selectively in a dilute HsS04solution. The largest plant in the United States using this operation puts the crushed ore in a succession of lead-lined concrete tanks, each holding 9000 tons of ore. The sulfate solution is pumped over the ore, filters through. and is then pumped t o the next tank in series until the acid is used up. Each day one tank is emptied and one filled. The resulting solution is passed through electrolytic cells containing lead anodes and copper cathodes, where the currene .deposits the copper and regenerates acid as follows:
This solution is returned to the leaching vats t o dissolve more comer. .. Zinc oxide rrjulring from roasting low-grade lint concentrates is surnerimes treated by the same principle, but instad of percolating the solution throurh the ore, it is necessary to agitate the fine ore in the solution. 8. Aluminum oxide will be reduced by carbon a t a high temperature. but unfortunately aluminum carbide is simultaneously formed. The only commercial method thus far developed for producing metallic aluminum is to dissolve AhOi in fused cryolite (NaaAIFSand pass a current through the bath from a carbon anode a t the top to a carbon cathode a t the bottom. The metallic aluminum produced is molten and heavier than the cryolite. I t therefore accumulates on the bottom of the cell until it is tapped out. Aluminum oxide is commonly known as alumina and the public, including newspaper reporters, fails to distinguish between alumina and aluminum. Various methods are available for producing pure alumina but, as stated above, only one for producing the metal. Various non-technical publications frequently announce excitedly the discovery of a new and supposedly cheap process for producing aluminum when they mean that someone has found another way for producing alumina. The cost of producing pure alumina by any known method is less than half the final cost of producing the metal. 9. Fern-alloys of tungsten, chromium, and molybdenum may be produced by simultaneous reduction of the oxides of these metals and iron. The resulting alloy has a melting point much lower than that of the pure metal and is satisfactory for use in the steel industry. The pure metals have extremely high melting ooints and tend to form carbides if reduced with carbon. I t is po,riblr, however, to purify the concentmtca by chemical mranr 1 0 produce pure oxides which mny be rcduccd a t relatively 10s. temperatures by heating in a hydrogen atmosphere, leaving a metallic powder. The tungsten filaments of incandescent lamps are made by compacting the powdered metal under high pressure heating t o a bright yellow temperature, swaging it t o a thin rod and drawing it t o wire. 10. The standard method for producing metallic magnesium or calcium is first t o make pure anhydrous chlorides and pass an electric current through them. The metals are deposited molten a t the cathode and chlorine gas evolved a t the anode. Magnesium boils a t 2025°F. (1107°C.) and various methods have been tried with moderate success to produce the metal by reduction and volatilization followed by condensation. The ease with which magnesium vapor ignites has proved a serious handicap to these methods and it remains t o he seen whether some volatilization method will ultimately prove superior t o the electrolytic method. 11. Bath silver and gold are frequently found in small amounts in copper and lead ores. When the ores are concentrated.. the nrecious metals are almost entirelv found in the concentrates and wlwn the concentratcs are smeltcd, the precious rnerals are found disolved in the haw metals. If a small amount of zinc is slirred itrto molten lead, it forms
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compounds with silver and gold which float to the surface and may be skimmed offand the precious metals recovered from the skimmings. Copper may be refined by electrolysis in an aqueous solution of sulfwic acid and capper sulfate. The impure copper is made the anode and pure copper the cathode, and the precious metals remain as a h e l v divided slime which settles to the bottom of the electrolytic tank and from which they may be recovered. A large amount of gold and silver is recovered by the above methods.
of copper and, since the largest use of copper is for electrical purposes, the high purity obtained by electrolytic refining is desirable. HANDLING SMELTER GASES
A visitor to a smelter is impressed by the large amounts of molten metal and slag which he sees, but gives little thought to the gases which he doesn't see. Yet the weight of gases is greater than the weight of FIRE R E F r n G metal and slag combined! An iron blast furnace may It has been impossible in such a brief space to give produce a thousand tons of pig irou in 24 hours. In more than the barest skeleton of metallurgical proce- the same time over three thousand tons of gaies will be dures. It will require some thought and imagination produced, and since these gases are combustible and to put meat on the skeleton. It will be noted that, are used for heating purposes, they must be freed of with few exceptions, crude metals are produced by a dust and passed through pipes to the points where reducing fusion which in practice utilizes the principles they are to be used. Gases from copper smelters contain a large quantity of selective reduction. Some oxides reduce more readily than others and i t is possible to produce a metal of sulfur compounds which, in most cases, is wasted. from one, whiie leaving others combined in a slag. The In order that they may be dissipated into the atmoslatter is usually a silicate, a combination of SiOz with phere in a way which will not injure vegetation, the one or more other oxides. Since these slags are nearly dust and volatilized compounds are first removed, after always lighter than metals a separation may be made which the gases are discharged through tall stacks 500700 feet or more in height. The great stack a t the in the liquid state by gravity, the slag floating on top. Unfortunately, i t is seldom possible to reduce one Anaconda smelter is not the tallest but bas an inside metal completely without a t the same time reducing diameter of 75 feet a t the bottom. The dust and fume from smelter gases are often some others. In the molten state the principal metal produced will usually dissolve other reduced metals to worked up for the recovery of valuable constituents. a certain extent, making subsequent refming necessary. SUMMARY One of the commonest methods used is selective oxidaIt will be ohsenwl from the foregoingdiscussion that tion in the molten state. ,, Phosphorus, silicon, and carbon are found in the pig a thoroueh knomledee of the fundamentals of chemistw. physics, and physical chemistry is necessary for successiron produced in the iron blast furnace. If pig iron is g put in an open-hearth furnace in the molten state or fully extracting metals from their ores and r e ~ i n the crude product. Coupled with this knowledge must be added solid and melted, the above impurities may be removed, or reduced to a fixed amount, by adding iron the ability to apply i t in commercial operations and ore as an oxidizing agent and limestone as a slag-forming the courage to enlarge and improve these operations in order to reduce the cost of production. constituent. A typical reaction is Si FezOa Fe To illustrate these points, visualize a copper mine a t FeO SO2. The silica forms slags of calcium and which the average copper content of the ore is slightly iron silicate, containing calcium phosphate and other impurities. It is customary to first dilute the im- less than one per cent, or 20 pounds per ton. With copper a t 12.5 cents per pound the value of the ore purities in the pig iron by adding steel s a a p to the would be $2.50 per ton. furnace charge. The only way in which this mine can operate profitArsenic, antimony, and tin may be removed from ably is to utilize every bit of technical and engineering molten lead by selective oxidation. They float on the skill it can find and work on a large scale, to cut oversurface as arsenate, antimonate, and stannate of lead. It is mining over 80,000 tons of ore per head costs. Bismuth, on the other hand, is less readiiy oxidized than day, every ton of which is finely ground and concenlead and, it present in appreciable amount, must be trated by flotation. The concentrate from this operaremoved by other methods. tion weighing about 3000 tons and containing between Other metals may be fire refined if they contain im25 and 30 per cent copper is roasted and smelted to purities which are more readily oxidized than themgive about 2500 tons of "matte" which is treated in a selves. converter to produce metallic copper. This is partly ELECTROLYTIC REFINING refined in a casting furnace which produces anodes for The most common example of electrolytic refining is electrolytic refining, and the pure copper from the that of copper, to which reference has already been latter operation is melted and cast into commercial made. The only satisfactory way to remove precious shapes. The fact that the product may be sold a t metals from copper is by electrolysis, and since most 12.5 cents per pound with a substantial profit is a copper contains appreciable amounts of precious metals credit to the knowledge, ability, and vision of all who the values recovered often more than pay for the participate in the operations. Equally creditable descriptions could be given of the operation. Furthermore, the presence o i G e i y small amounts of impurities lowers the electrical conductivity operations by which many other metals are produced.
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