Introduction

The metals which comprise the Platinum group are the following: Platinum, Palladium, Iridium, Osmium, Ruthenium and Rhodium.

Up to the year 1914 Russia produced over 90 per cent. of the world’s supply of platinum, the Republic of Colombia, South America, ranking next in importance with about 5 per cent. Owing, however, to the war and to the chaotic conditions brought about by the revolution, the output of Russia has considerably decreased, and although the Colombian production has been steadily increasing, the increased demand in connection with munition manufacture caused a somewhat serious shortage during the latter stages of the war, which was especially felt in the United States. As a result much exploratory work has recently been carried on in an endeavour to discover new deposits of importance; but up to the present, although several fresh occurrences have been brought to light, results have on the whole been disappointing.

Occurrences

Platinum usually occurs in nature as native metal alloyed with one or more of its allied metals in the form of very fine grains more or less flattened; sometimes in the form of irregular nuggets; and occasionally, though rarely, in small cubic crystals. It is sometimes coated with a black layer of iron oxide, which may be magnetic, in which case it is not easily recognizable. When unrefined it is referred to as “crude” platinum.

With the exception of a certain amount of the platinum metals, obtained from the refining of copper and gold bullion, particularly from the copper-nickel deposits of Sudbury, Ontario, about 99 per cent. of the whole supply is derived from alluvial deposits. Several occurrences of platinum in situ are known, but so far few are of commercial importance: however, in view of the indications of exhaustion shown by some of the placer deposits, notably in Russia, the exploration of primary ores is now receiving more attention and practical results have already been achieved in Russia and in Spain.

The mother rocks from which the deposits are derived, in the large majority of known cases, consist of basic and ultra-basic igneous rocks, including peridotites, pyroxenites and dunites. The two first are composed of iron magnesian silicates, pyroxene, augite and hornblende with olivine, chromite, ilmenite and magnetite: the dunites consist principally of olivine with some chromite. These rocks are often found to have undergone more or less alteration to serpentine. In addition, platinum has been found in quartz veins, notably at the Boss Mine, Nevada, and in a few known cases it has been derived from formations in schistose, or altered sedimentary rocks.

When present in serpentine, platinum is usually disseminated through the rock in fine particles. It seldom occurs in a lode-formation. In sedimentary rocks it usually occurs in sandstones. In cases where alluvial deposits have been derived from the basic igneous rocks, the associated minerals are usually chromite, magnetite, ilmenite, iridium and osmiridium. In sedimentary deposits the metal is commonly associated with quartz, copper, nickel, silver and palladium.

Platinum has been found in certain varieties of the copper ores tetrahedrite and bournonite. It has occasionally been located in shales and in coal, although not in recoverable quantities. In the latter case, in an Australian coal, it is associated with vanadium^[1] p. 992.

Several cases are known of platinum being present in meteorites, two well-authenticated instances having been reported from Mexico. Platinum has been shown to exist in meteoric iron from New South Wales.

Crude platinum, as recovered, contains from 70 to 90 per cent. of the metal, and, as mentioned above, is really an alloy of platinum with one or more of the allied metals, the chief impurities consisting principally of iron and copper.

The table on the next page gives the analyses of typical samples of crude platinum from the Urals, California, British Columbia, and other places.

Platinum also occurs in combination with arsenic in the mineral Sperrylite (PtAs₂), in the form of minute octahedral crystals. The colour of this mineral is tin white, its lustre is metallic and brilliant, its hardness varies from 6 to 7, and its specific gravity is 10·6. The mineral is brittle and breaks with a conchoidal fracture. It is very rare, and is interesting as being the only mineral of platinum known besides the native metal. It occurs associated with sulphide minerals of magmatic origin in gabbros and diabases, notably in the nickeliferous pyrites of Sudbury, Canada, and in the copper ores of the Rambler Mine, Laramie, Wyoming.

It is probable that the palladium, which is also found in these deposits, is similarly present in the form of an arsenide, but such a mineral has not yet been definitely proved to exist.

The following is an analysis of a sample of sperrylite: platinum, 54·47 per cent.; rhodium, 0·76 per cent.; palladium, trace; arsenic, 42·23 per cent.; antimony, 0·54 per cent.^[2] p. 69.

Properties of the Platinum Metals

Platinum.—The colour is white with a greyish tinge. When pure it is very malleable and ductile. Its coefficient of expansion is less than that of all other metals. Platinum fuses at about 1750° C., but the presence of impurities lowers the melting-point. Its specific gravity is 21·5, and its hardness is from 4 to 5. Its electric conductivity is low, being 13·4 at 0° C.^[3] p. 398.

Platinum is not acted upon by either nitric, sulphuric or hydrochloric acid, but is soluble in aqua regia (1 part of nitric to 2 parts of hydrochloric acid), or in other mixtures liberating chlorine, with the formation of platinum tetrachloride (PtCl₄). It is not acted upon by air or moisture, even at high temperatures. It is corroded by caustic alkalis, sulphides, sulphates, phosphates and arsenides, if heated in contact with them. It will not amalgamate with mercury unless sodium is present, in this respect differing from gold and silver.

Platinum, in a finely-divided condition, absorbs large quantities of hydrogen or other gases, which on occlusion become more active: hence its value as a catalytic agent.

Palladium.—This metal resembles platinum, but is sometimes fibrous, the colour being between that of platinum and of silver. It possesses a lower melting-point than platinum—about 1550° C. It is malleable, has a hardness of from 4·5 to 5, and a specific gravity of 11·5. As already stated, palladium, when found in copper ores, is probably present in combination with arsenic. Porpezite, a rare mineral, containing gold and up to 10 per cent. palladium, has been identified in gold-bearing veins in Brazil [see p. 54].

Palladium is produced from the refining of copper matte and of base gold bullion from Australia and elsewhere.

Iridium is a brilliant white brittle metal, with a specific gravity of 22·4, and hardness of 6 to 7. Its fusion point is very high—about 2200° C.—and under ordinary conditions it is not attacked by any acid. At 1100° C. it begins to oxidize to a purple oxide. Iridium usually occurs either in crude platinum, or alloyed with osmium, as iridosmine, or as native metal. The bulk of iridium is derived from the platinum placer deposits of the Urals, but the Californian metal is more valuable, on account of its better quality. It is also obtained in small amounts from copper bullion.

Osmium is a hard and brittle metal, bluish-grey in colour. Its specific gravity is 22·5, and it has a very high melting-point, in this respect being the most refractory of the group.

Iridosmine, or Osmiridium, an alloy of iridium and osmium, occurs as hexagonal crystals, or flattened grains of lighter colour than platinum. It may contain from 40 to 77 per cent. of iridium, and from 20 to 50 per cent. of osmium. If the iridium predominates, the alloy is called Nevyanskite, and Siserskite if the osmium content is high. It is distinguishable from platinum by the brittleness of the flakes. Siserskite gives off a pungent odour, if strongly heated, caused by the volatilization of osmium. Iridosmine has a hardness of 6 to 7, and a specific gravity of 19 to 21.

Ruthenium is a white metal, with a specific gravity of 12·1. It is scarcely acted upon by aqua regia. Ruthenium occurs mainly in small amounts in iridosmine. It is also found in the copper ores of Sudbury and other places. Laurite is a very rare sulphide of ruthenium (RuS₂), containing a small amount of osmium, which has only been recognized in the Borneo deposits.

Rhodium is a white metal resembling aluminium, with a specific gravity of 12·1, and a melting-point of about 2000° C. It is ductile and malleable at red heat. In addition to its occurrence in crude platinum, the metal is also contained in small quantities in the sperrylite found in the copper ores of Sudbury, Canada^[5] p. 779.

Colloidal Platinum.—This has only recently been detected in ores, and therefore its occurrence might not be detected qualitatively. By destroying its colloidal condition, however, its presence can be discovered in the ordinary ways^[4].

Metallurgical Treatment

Crude platinum can be refined either by dry or by wet methods, the following being brief outlines of the two processes:

By the wet method, the crude platinum is dissolved in aqua regia, with excess of hydrochloric acid. Evaporation is continued until the whole of the nitric acid is expelled. By addition of a solution of ammonium chloride, the platinum is then precipitated as ammonium platini-chloride. This precipitate is heated to redness, when chlorine and ammonium chloride are given off, and spongy platinum remains. The last is next granulated, after fusion by the oxy-hydrogen blowpipe in a small lime furnace. Platinum obtained by this method may contain small amounts of iridium, rhodium and palladium.

In the dry method, introduced by Delville and Debray, the crude platinum is smelted with galena in a small reverberatory furnace. A portion of the lead is reduced to the metallic state by the iron in the charge, and forms a fusible alloy with the platinum. The osmiridium present settles to the bottom of the furnace, and may thus be removed. Litharge is then thrown in to form more alloy, and some glass to act as a flux. The alloy is cupelled, and the residual platinum is then melted in a lime furnace with the oxy-hydrogen flame. The platinum thus obtained often contains iridium and rhodium.

A combination of these two methods is also sometimes employed^[3] p. 403.

The electrolytic process of gold-refining was introduced to treat platiniferous gold. In the gold chloride, or Wohlwill method, iridium and osmiridium are insoluble, and remain with the anode slime. Platinum and palladium, if nearly pure, are also insoluble, but when present alloyed with gold and silver, pass into the solution and remain there. In a hot bath it is stated that the platinum present should not exceed 50 gm. per litre. According to T. K. Rose, a cold bath containing only 20 gm. per litre causes a certain quantity of platinum to be deposited with the gold^[6].

H. F. Keller, in “Platinum, the Most Precious of Metals” (Journal of the Franklin Institute, November 1912) deals fully with the extraction and refining of platinum.

Platinum-Refining Agencies

The following contains a list of the principal firms engaged in the refining of platinum metals:

In England: Johnson, Matthey & Co., Ltd., Lees & Sanders, Warstone Smelting Works, Sheffield Smelting Works, Johnson & Sons. (This is the list of the Ministry of Munitions.)

In France: Legende et Cie., Compagnie Internationale du Platine, Lyon Allemand, Lecht Lyonnais, Henrique Marrett, Bonnen, Hesse Fils.

In Germany: W. C. Heraeus, G. Siebert, F. Eisennad & Co.

In the United States: Baker & Co., American Platinum Works (N.T.), Irvington Smelting and Refining Works, J. Bishop & Co., H. A. Wilson & Co., Belais & Cohn, Kastenhuber & Lehrfeld, Roessler & Hasslacher Chemical Co., Wildberg Bros., and others handling scrap.

According to Russian information about 25 per cent. of the Russian output before the war was refined in Germany. In Russia there are practically no platinum-refining facilities^[7].

The Uses of Platinum and its Allies

Platinum.—In the chemical industry platinum is largely used for catalyzers in the manufacture of sulphuric, acetic and nitric acids; for stills for the final concentration of sulphuric acid; and in the electro-chemical industry.

In the making of “contact” sulphuric acid a “contact mass” is charged into the chambers of the plant. This is formed by soaking asbestos, or anhydrous magnesium sulphate, with platinic chloride solution, and baking the mass to drive off the chlorine. The contact mass usually contains from 7 to 8 per cent. platinum, in a very finely-divided state. In the making of acetic acid from a mixture of air and alcohol vapour, platinized asbestos is used. For the catalyzer used in the conversion of ammonia into nitric acid a very fine-meshed platinum gauze is used; this is strengthened at its edges with platinum-iridium wire. One ounce of platinum is required for the production per annum of 25 tons of catalytic acid, or of 40 tons of nitric acid from ammonia.

In the finely-divided state all the other metals of the platinum group, especially palladium, have also the facility of absorbing great quantities of certain gases, and can be used as catalysts.

Owing to its high melting-point, and to the resistance to the action of acids at high temperatures, platinum is largely used for chemical ware in the form of crucibles, dishes, etc. Platinum crucibles are indispensable in the chemical analysis of rocks.

In the electrical industry platinum is largely used for contact points, in telegraph and telephone apparatus, in magneto-contacts, and in the construction of the thermo-couples of pyrometers. In the manufacture of jewellery, especially in the crown-setting of diamonds, platinum has been much used in the place of gold: alloyed with a little iridium it can be worked into delicate designs, which are durable. During the war, however, when platinum was largely wanted in the making of munitions, its use in jewellery manufacture was much restricted. Platinum was formerly largely used in dentistry. In photography potassium platino-chloride is required for producing platinotype prints. In the form of barium platino-cyanide it is used in X-ray photography as a coating for the projecting screen.

Platinum is required in the manufacture of certain parts of chronometers, theodolites and watches; also for standard weights and measures, and for various types of self-lighting lamps^[4] p. 561.

Palladium has its chief value as a substitute for platinum, in palladium-gold alloys, which are used extensively in dentistry, for jewellery and for chemical ware. It is also utilized in the manufacture of astronomical instruments and watches, also for plating metal ware^[1] p. 1002. The use of palladium as a catalyzer is well known.

Iridium, when pure, is of small value, being difficult to manipulate on account of its brittleness. It is principally used in alloy with platinum for hardening purposes. Jewellers’ platinum usually contains 10 per cent. iridium, and in the electrical industry an alloy composed of from 15 to 50 per cent. iridium is usually employed. Alloys with up to 10 per cent. of iridium are ductile and malleable, but with over that amount are hard and difficult to work.

The metal is used in the manufacture of fountain-pen points, for which purpose the grains require careful selecting^[8] p. 106; also for standard weights and for contact points. Iridium black, an oxide, is of value as a pigment for chinaware^[1] p. 1001.

Iridium is suitable for sharp surgical instruments, and gold needles with soldered iridium ends are employed for stitching wounds. It is also used in photography. Iridium is of greater scarcity than platinum, hence its greater value.

Osmium.—There is now little or no market for osmium. It was formerly in considerable use for the manufacture of incandescent lamps. Osmic acid is used for staining anatomical preparations in microscopic work. On account of the poisonous nature of its vapour the extraction of osmium is costly and dangerous.

Ruthenium is also of little or no commercial value. Both these metals possess the disadvantages of being brittle and easily oxidized.

Rhodium is of small commercial use. It is used principally in alloy with platinum. An alloy containing 10 per cent. rhodium is used for some thermo-couples of pyrometers, and in the making of laboratory utensils.

Platinum Alloys.—Platinum forms alloys with a number of metals, but only a few are of industrial importance^[3] p. 400. Platinum and iridium form a hard and elastic alloy, which is unaffected by air, and takes a high polish. Alloyed with 10 per cent. iridium platinum is used for one of the wires in thermo-couples of pyrometers; and with 10 to 20 per cent. iridium for making standard measures of length and weight.

Platinum and copper form various alloys. An alloy with 18·75 per cent. copper, called “coopers’ gold,” takes a high polish and closely resembles 18–carat gold.

An alloy of platinum and silver containing 66 per cent. silver is used as a standard of electrical resistance. An alloy containing 20 to 30 per cent. silver is used in dentistry.

Platinum alloys with lead, zinc and other metals at low temperatures; it is usually recovered from these alloys by cupellation.

Platinum alloys with steel in all proportions. With 10 per cent. platinum, rusting is prevented. A very elastic metal is produced by alloying platinum with from 5 to 10 per cent. gold.

The melting-point of silver is raised by alloying it with platinum, but its thermal conductivity is lowered.

Alloy Substitutes for Platinum

On account of the scarcity and high price of the platinum metals, much attention has lately been directed towards the discovery of suitable substitutes.

In the electrical industry an alloy of 3 parts of palladium and 2 parts of silver is in use, also an alloy of nickel and chromium. Platinite, an iron-nickel alloy, containing 46 per cent. nickel and 0·15 per cent. carbon, has the same coefficient of expansion as glass, and, when coated with copper, is used to replace the platinum connection wires of incandescent lamps. Tungsten is sometimes used for certain ignition devices. For cathodes an alloy of 90 per cent. gold and 10 per cent. copper can be used to replace platinum; the same alloy, if electrically coated with platinum, and then carefully polished and burnished, is suitable for platinum anodes.

For platinum chemical laboratory ware, there are several substitutes, such as fused quartz; various iron, chromium, and nickel-chromium alloys; palau, a gold-iridium alloy marketed in California; rhotanum, a general name for gold-palladium alloys containing from 60 to 90 per cent. of gold, which are suitable for most chemical purposes, except for use with hot concentrated nitric acid, and for electrolytic anodes; amaloy, which is a complex alloy containing nickel, chromium, tungsten, etc., highly resistant to corrosion and to cold nitric and sulphuric acids^[9] p. 600.

In the jewellery trade platinum has been replaced by an alloy of 90 per cent. palladium and 10 per cent. rhodium. For certain surgical work various stellite alloys, containing cobalt and chromium, and hardened by the addition of tungsten and molybdenum, are valuable substitutes for platinum, and are not affected by antiseptic solutions. In dental work pins are now made of tungsten coated with palladium^[10] p. 549. For most technical purposes an alloy of tungsten and nickel with gold or silver is used in Germany; it may be cast, rolled or forged, is acid-resisting, and capable of taking a high polish^[11]. White gold, another substitute for platinum, contains fine gold, from 75 to 85 per cent.; pure nickel, from 10 to 18 per cent.; and zinc, from 2 to 9 per cent. Illium, a chromium-nickel-copper alloy reported recently as the discovery of S. W. Parr, of Illinois, is a substitute for gold or platinum, costing only 25 cents per ounce. It is stated to have been a “50 per cent. standard of success.” The alloy withstands hot or cold, strong or diluted acid, can be both cast and machined, and is already used largely in the manufacture of calorimeter bombs^[12].

The results of researches made to discover substitutes for platinum, and undertaken by the National Dental Association of America, are described at length by F. A. Fahrenwald, in a paper read in January 1916 before the American Institute of Mining Engineers.

Scrap Platinum

A considerable amount of platinum in the form of old and worn articles is now collected for return to the refineries, where it is re-treated, and sold again as new metal. The trade in scrap platinum has been particularly active in the United States, official statistics showing that in 1916, 49,400 oz. of refined platinum were recovered.

World’s Output of Platinum

According to J. L. Howe, the estimated limits of the total world-production of crude platinum, up to January 1917, were as follows^[13]:

On the other hand, James M. Hill^[14] states that possibly 5,000,000 oz. was the total world’s production to June 1917, which he distributes according to the uses made of it as follows:

It is difficult to obtain exact figures of the annual production of crude platinum. This is particularly so in the case of Russia, where there appears to have been a tendency for private enterprises to keep their published outputs as low as possible, in order to avoid registration. The discrepancy between the official and actual figures of production in Russia is variously estimated at from 20 to 60 per cent.

The table on the next page is compiled from the sources considered most reliable.

Canada.—The recoveries of platinum at the works of the International Nickel Company in New Jersey for the years 1910–1912 were 258,666 and 497 oz. respectively, chiefly from Canadian matte.

Russia.—The actual productions of platinum as quoted in Mineral Industry in oz. were: 1910, 300,000; 1911, 280,000; 1912, 300,000; 1913, 275,000; 1914, 240,000; 1915, 124,000; 1916, 90,000; 1917, 50,000.

C. U.S.A. Mineral Resources, 1917, Geol. Surv.