platinum group

▪ chemical element group

Introduction

      six metals, in order of increasing atomic weight, ruthenium (Ru), rhodium (Rh), palladium (Pd), osmium (Os), iridium (Ir), and platinum (Pt). The elements all possess a silvery white colour—except osmium, which is bluish white. The chemical behaviour of these metals is paradoxical in that they are highly resistant to attack by most chemical reagents yet, employed as catalysts, readily accelerate or control the rate of many oxidation, reduction, and hydrogenation reactions.

      Ruthenium and osmium crystallize in the hexagonal close-packed system, and the others have face-centred cubic structures. This is reflected in the greater hardness of ruthenium and osmium.

History

      Although platinum-containing gold artifacts have been dated as far back as 700 BC, the presence of this metal is more likely adventitious than deliberate. References to gray, dense pebbles associated with alluvial gold deposits were made by Jesuits in the 16th century. These pebbles could not be melted alone but would alloy with and adulterate gold to the extent that the gold bars would become brittle and impossible to refine. The pebbles became known as platina del Pinto—that is, granules of silvery material from the Pinto River, a tributary of the San Juan River in the Chocó region of Colombia.

      Malleable platinum, obtainable only upon purification to essentially pure metal, was first produced by the French physicist P.F. Chabaneau in 1789; it was fabricated into a chalice that was presented to Pope Pius VI. The discovery of palladium was claimed in 1802 by the English chemist William Wollaston (Wollaston, William Hyde), who named it for the asteroid Pallas. Wollaston subsequently claimed the discovery of another element present in platinum ore; this he called rhodium, after the rose colour of its salts. The discoveries of iridium (named after Iris, goddess of the rainbow, because of the variegated colour of its salts) and osmium (from the Greek word for “odour,” because of the chlorinelike odour of its volatile oxide) were claimed by the English chemist Smithson Tennant in 1803. The French chemists Hippolyte-Victor Collet-Descotils, Antoine-François Fourcroy, and Nicolas-Louis Vauquelin identified the two metals at about the same time. ruthenium, the last element to be isolated and identified, was given a name based on the Latinized word for Russia by the Russian chemist Karl Karlovich Klaus (Klaus, Karl Karlovich) in 1844.

      Unlike gold and silver, which could be readily isolated in a comparatively pure state by simple fire refining, the platinum metals require complex aqueous chemical processing for their isolation and identification. Because these techniques were not available until the turn of the 19th century, the identification and isolation of the platinum group lagged behind silver and gold by thousands of years. In addition, the high melting points of these metals limited their applications until researchers in Britain, France, Germany, and Russia devised methods for consolidating and working platinum into useful forms. The fashioning of platinum into fine jewelry began about 1900, but, while this application remains important even today, it was soon eclipsed by industrial uses. Palladium became a very desirable material for contact points in the relays of telephone and other wire communications systems, where it provided long life and a high level of reliability, and platinum, because of its resistance to spark erosion, was incorporated into spark plugs (spark plug) for combat aircraft during World War II.

      After the war the expansion of molecular conversion techniques in the refining of petroleum (petroleum refining) created a great demand for the catalytic (catalyst) properties of the platinum metals. This demand grew even more in the 1970s, when automotive emission standards in the United States and other countries led to the use of platinum metals in the catalytic conversion of exhaust gases.

Ores

      With the exception of small alluvial deposits of platinum, palladium, and iridosmine (an alloy of iridium and osmium), virtually no ores exist in which the major metal is from the platinum group. Platinum minerals are usually highly disseminated in sulfide ores, particularly the nickel mineral pentlandite [(Ni,Fe)₉S₈]. The most common platinum-group minerals include laurite (RuS₂), irarsite [(Ir,Ru,Rh,Pt)AsS], osmiridium (Ir,Os), cooperite (PtS), and braggite [(Pt,Pd)S].

      The world's largest deposit is the Bushveld Complex of South Africa. Other major deposits include the Sudbury deposit of Ontario, Can., and the Norilsk-Talnakh deposit of Siberia in Russia. Within the United States the largest deposit is the Stillwater Complex in Montana, but this is substantially smaller than the deposits cited above.

Mining and concentrating

      The major South African and Canadian deposits are exploited by underground mining. Virtually all platinum-group metals are recovered from copper or nickel sulfide minerals, which are concentrated by flotation separation. Smelting of the concentrate produces a matte that is leached of copper and nickel sulfides in an autoclave. The solid leach residue contains 15 to 20 percent platinum-group metals.

      Sometimes gravity separation is employed prior to flotation; this results in a concentrate containing up to 50 percent platinum metals, making smelting unnecessary.

Extraction and refining

      The separation chemistry of the platinum-group metals is among the most complex and challenging of metal separations. A brief description of procedures for isolating the platinum-group metals is set forth below, followed by descriptions of assaying and scrap-refining techniques.

Individual solubilization

      The classical procedure for separating the platinum metals begins with a mineral concentrate obtained as described above. This concentrate is leached with aqua regia, which dissolves the platinum and palladium and leaves the other metals as solids in the leach residue. The platinum is precipitated from solution with ammonium chloride, and the resulting crude platinum salt is recovered by filtration and then heated to decompose it to a powdered metallic form. The metal is redissolved in aqua regia, then reprecipitated with ammonium chloride and calcined to pure metal. The palladium, which remained in solution when the platinum was precipitated, is now precipitated by the addition of ammonia. After the palladium salts are recovered by filtration, they are redissolved and reprecipitated to form a pure salt, and this is converted to metallic form, usually by chemical reduction with formic acid.

      The residue left over from leaching the original mineral concentrate contains rhodium, iridium, ruthenium, and osmium. This is treated with molten sodium bisulfate to convert the rhodium to rhodium sulfate. The rhodium is then solubilized by water leaching, separated from the insolubles, and precipitated from solution by reduction with zinc powder. The crude rhodium metal product is converted to a soluble salt by treatment with chlorine and sodium chloride at high temperature, dissolved in water, precipitated with sodium nitrite, filtered, redissolved, and reprecipitated with ammonium chloride. This final precipitate is reduced to a pure metal powder.

      The residue from rhodium sulfate leaching is fused with alkali nitrate salts to convert ruthenium to soluble sodium ruthenate. After filtration, the solution of sodium ruthenate is treated with chlorine gas to distill off the ruthenium as the volatile compound ruthenium tetroxide. The ruthenium-bearing distillate is then treated with reducing agents to precipitate the ruthenium as a fine metal powder. Osmium is recovered in a similar fashion, although, unlike ruthenium, it can also be recovered by distillation from acidic solutions.

      The final residue is treated with sodium peroxide to convert iridium to a form soluble in hydrochloric acid, from which it can be precipitated with ammonium chloride and calcined to metal powder.

Simultaneous solubilization

      Simultaneous solubilization of all platinum metals can be accomplished by fusing the mineral concentrate obtained from copper and nickel sulfide ores with aluminum metal, dissolving the aluminum, and treating the residue with hydrochloric acid and chlorine. This dissolves all platinum-group metals, which are subsequently separated by solvent extraction. The individual metal solutions are then treated by conventional techniques to recover the various metals in a pure state.

Consolidation

      Irrespective of the chemical separation processes used, the platinum metals are recovered in a finely divided metallic powder form. They can be converted to massive metal form by electron-beam melting. The lower-melting-point metals palladium and platinum can be fused by induction melting techniques.

Refining from scrap (scrap metal)

      There is no universally applicable technique for reprocessing platinum-metals scrap. The chosen procedure depends on the various proportions of the platinum metals in the sample. For example, platinum or platinum-alloy scrap—such as laboratory ware, glass-furnace linings, and spinnerets used in synthetic-fibre manufacture—can be redissolved in aqua regia and recovered from solution by techniques discussed above. Alloys containing ruthenium and iridium are sometimes solubilized by alkaline fusion. Once the metal is dissolved, the process chemistry employed to recover it is similar to that discussed above.

      The bulk of platinum, palladium, and rhodium scrap is found in automotive catalytic converters. The catalyst is melted at a very high temperature with iron or copper to fuse the catalyst substrate and dissolve the platinum-group metals in the molten copper or iron. The alloy of copper and precious metals is leached to dissolve the copper or iron, leaving behind a platinum-palladium-rhodium concentrate, which is refined to pure metals with chemistry similar to that described above.

assaying

      Assaying ores and concentrates for platinum-group metals is extremely difficult, since the concentration of any particular metal may be less than one part per million. Initial concentration is achieved by fire assay. The assay bead is dissolved in aqua regia and the metals separated and concentrated, often by solvent extraction. Dissolution of the bead is complicated by the presence of iridium, rhodium, or ruthenium, so that special solubilizing techniques may be required. The concentrated solutions are analyzed by atomic absorption spectroscopy or photometric techniques. Arc spectroscopy is sometimes employed directly on the bead obtained from fire assay.

The metals and their alloys (alloy)

      The mechanical properties of the six platinum metals differ greatly. Platinum and palladium are rather soft and very ductile; these metals and most of their alloys can be worked hot or cold. Rhodium is initially worked hot, but cold-working can be done later with rather frequent annealing. Iridium can be worked hot, as can ruthenium, but with difficulty; neither metal can be cold-worked appreciably.

      Osmium is the hardest of the group and has the highest melting point, but its ready oxidation is a limitation. Iridium is the most corrosion-resistant of the platinum metals, while rhodium is valued for retaining its properties at high temperatures.

Structural applications

      Since pure annealed platinum is extremely soft, it is susceptible to scratching and marring. In order to improve hardness, it is alloyed with a variety of other elements. Platinum jewelry is very popular in Japan, where it is called hakkin, or “white gold.” Alloys for jewelry castings include 90-percent-platinum–10-percent-palladium, which is readily worked and brazed. Adding ruthenium to platinum-palladium alloys increases their hardness while maintaining their oxidation resistance. Alloys of platinum-palladium-copper are used in wrought products, since these alloys are harder than platinum-palladium alloys yet less costly.

      Crucibles used for single-crystal production in the semiconductor industry require both corrosion resistance and stability at high temperature. For this application, platinum, platinum-rhodium, and iridium are the best suited. Platinum-rhodium alloys are employed in the production of thermocouples (thermocouple) that are capable of measuring temperatures as high as 1,800° C (3,270° F). Palladium is used in both the pure and alloyed states for a variety of electrical applications (accounting for 50 percent of consumption) and for dental alloys (30 percent of consumption). Rhodium, ruthenium, and osmium are used rarely in the pure state but rather as alloying elements for the other platinum-group metals.

Catalysts (catalyst)

      Approximately 42 percent of all platinum produced in the Western world is employed as a catalyst. Of this, 90 percent is applied to automotive exhaust systems, where refractory pellets or honeycomb structures coated with platinum (as well as palladium and rhodium) promote the conversion of unburned hydrocarbons, carbon monoxide, and nitrogen oxides into water, carbon dioxide, and nitrogen.

      An alloy of platinum and 10 percent rhodium, formed into a gauze and operating at bright red heat, serves as the catalyst in the reaction between ammonia and air to produce nitrogen oxides and ultimately nitric acid. By feeding in some methane along with the ammonia mixture, hydrocyanic acid can be produced. In petroleum refining, platinum is distributed onto the surface of aluminum oxide pellets and placed into reactor vessels. There it catalyzes the reforming of long-chain naphtha molecules into branched-chain isoparaffins, which are desirable in the blending of high-octane gasoline.

electroplating

      All the platinum metals can be electroplated. Palladium is the easiest to process, and the plated metal is much harder than the wrought metal. The hardness of electrodeposited ruthenium makes it suitable for instruments where a low-pressure rubbing contact is required.

      Rhodium is the most commonly electroplated of the platinum metals because of the hardness and lustre of the electrodeposit. Although the cost of rhodium is greater than that of platinum, its lower density allows the use of a lesser weight of metal to obtain a deposit of comparable thickness.

Chemical compounds

      Organometallic complexes of platinum-group metals, such as alkylplatinum complexes, are employed as catalysts in olefin polymerization, the production of polypropylene and polyethylene, and the oxidation of ethylene to acetaldehyde.

      Platinum salts are finding increasing use in cancer chemotherapy as drugs marketed under the generic names carboplatin and cisplatin. Ruthenium oxide-coated electrodes are employed in the production of chlorine and sodium chlorate. Rhodium sulfate and rhodium phosphate are the compounds preferred for rhodium electroplating baths.

James Edward Hoffmann

Additional Reading

Comprehensive and up-to-date information on many aspects of metallurgy, individual metals, and alloys can be found in convenient reference-form arrangement in the following works: Metals Handbook, 9th ed., 17 vol. (1978–89), a massive and detailed source prepared under the direction of the American Society for Metals, with a 10th edition that began publication in 1990; Herman F. Mark et al. (eds.), Encyclopedia of Chemical Technology, 3rd ed., 31 vol. (1978–84), formerly known as Kirk-Othmer Encyclopedia of Chemical Technology, with a 4th edition begun in 1991; and its European counterpart, the first English-language edition of a monumental German work, Ullmann's Encyclopedia of Industrial Chemistry, 5th, completely rev. ed., edited by Wolfgang Gerhartz et al. (1985– ). Ed. Relevant volumes of Joseph William Mellor, A Comprehensive Treatise on Inorganic and Theoretical Chemistry, 16 vol. (1922–37, reprinted 1960–65), are an excellent source of descriptive chemistry and historical information. Information on the chemistry of the precious metals is scattered throughout the monumental Gmelins Handbuch der anorganischen chemie, 8th ed. (1924– ), with articles in German and English; since 1981 most of the articles have appeared in English, and the volumes now have English titles: Gmelin Handbook of Inorganic Chemistry (1981–89) and Gmelin Handbook of Inorganic and Organometallic Chemistry (1990– ). Linda S. Benner et al. (eds.), Precious Metals Science and Technology, trans. from Japanese (1991), is a comprehensive reference. F.E. Beamish, J.C. Van Loon, and Clyde L. Lewis, Analysis of Noble Metals: Overview and Selected Methods (1977); and F.E. Beamish, The Analytical Chemistry of the Noble Metals (1966), are useful texts for information on specific methods for the analysis of precious metals as found in a variety of rock matrices. Despite their ages, Ernest A. Smith, The Sampling and Assay of the Precious Metals, 2nd ed., rev. (1947, reprinted 1987); and Edward E. Bugbee, A Textbook of Fire Assaying, 3rd ed. (1940, reissued 1981), are an excellent place to begin the study and practice of fire assaying, which has changed very little since its beginning in antiquity. Louis J. Cabri (ed.), Platinum-Group Elements: Mineralogy, Geology, Recovery (1981), contains a useful review of the platinum metals chemistry, geology, and analytical chemistry.James Edward Hoffmann

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