Noble Metals

gold, silver, platinum, and metals of the platinum group (iridium, osmium, palladium, rhodium, and ruthenium), which received their name owing chiefly to their high chemical stability and beautiful exterior appearance when made into artifacts. In addition, gold, silver and platinum have high plasticity, and metals of the platinum group are highly refractory. These qualities of the individual noble metals are combined in their alloys, which are widely used in technology.

Gold and silver have been known to man for several millennia; this is attested to by artifacts found in ancient burial sites and by primitive mines that have been preserved to this day. The chief centers of noble-metal mining in antiquity were Upper Egypt, Nubia, Spain, and Colchis (in the Caucasus); there is evidence of the extraction of noble metals in Central and South America and in Asia (India, the Altai, Kazakhstan, and China). Gold was being mined as early as the second or third millennium B.C. (the so-called Chud works) on the territory of Russia.

Noble metals were obtained from alluvial deposits by washing sand on boards covered with animal skins whose fur had been cut short (to catch gold particles) and also by using primitive troughs, pans, and buckets. Noble metals were extracted from ores by heating the rocks until they cracked, with subsequent crushing of the lumps in stone mortars, milling with millstones, and flushing. Sieves were used to separate the pieces by size. Interesting technological discoveries of that time include a method of separating silver and gold alloys by means of acids, separating gold and silver from lead alloys by cupellation (ancient Egypt), and extracting gold by amalgamation with mercury or by means of an oiled surface (ancientGreece). Cupellation was carried out in clay crucibles to which lead, salt, tin, and bran were added.

From the 11th through sixth centuries B.C, gold was mined in Spain in the valleys of the Tajo, Duero, Miño, and Guadiaro rivers. Between the sixth and fourth centuries B.C, the mining of ore beds and alluvial deposits of gold started in Transylvania and the West Carpathians. In the Middle Ages (up until the 18th century), primarily silver was mined, and the mining of gold decreased. In the 16th century the Spaniards began to exploit the noble metals in South America, beginning in Peru and Chile in 1532 and in New Grenada (present-day Colombia) in 1537. The mining of the “silver Hill” of Potosí in Bolivia began in 1545. Auriferous alluvial deposits were discovered in Brazil in 1577. By the mid-16th century, five times as much gold and silver was being mined in America as had been mined in Europe before the discovery of the New World.

In the first half of the 16th century, the Spanish colonizers turned their attention to a nonfusible, heavy, white metal found along with gold in the alluvial deposits of New Grenada. Because of its external resemblance to silver (Spanish plata), the Spaniards called it by the diminutive platina. Platinum was already known in antiquity; native ores were found together with gold and were called “white gold” (in Egypt, Spain, and Abyssinia), “frog’s gold” (Borneo), and so on. At first the Spaniards believed platinum to be a harmful impurity; therefore, a governmental decree was issued ordering that platinum be thrown into the sea. The first scientific description of platinum was made by Watson in 1741 in connection with the beginning of its mining on an industrial scale in Colombia in 1735.

In 1803, the English scientist W. H. Wollaston discovered palladium and rhodium; in 1804, the English scientist S. Tennant discovered iridium and osmium. In 1808, the Russian scientist A. Sniaditskii, while studying platinum ore brought from South America, extracted a new chemical element, which he called vestii. In 1844, K. K. Klaus, a professorat the University of Kazan, made a broad study of this element and named it ruthenium in honor of Russia. Metals of the platinum group are encountered in nature most frequently in polymetallic ores (copper-nickel), as well as in gold and platinum deposits.

The mining of noble metals in Russia began in the 17th century in the Trans-Baikal region with the underground mining of silver ores. The first written reference to the mining of alluvial gold in the Urals dates to 1669 (in the chronicle of the Dolmatovo monastery). One of the first gold deposits in Russia was discovered in Karelia in 1737 and exploitation started in 1745. The accepted date for the beginning of gold mining in the Urals is 1745, when E. Markov discovered the Berezovka ore bed. “New Siberian metal” (platinum) was discovered in 1819 in alluvial gold deposits in the Urals. Rich alluvial deposits of platinum and gold were discovered in 1824 on the eastern slopes of the Urals, and the first platinum mine in Russia and Europe was opened. Later, K. P. Golliakhovskii and others discovered the Isovka system of gold and platinum alluvial deposits, which became world-famous. In 1828, the Russian scientist V. V. Liubarskii published works about the first native platinum ore bed in the world, discovered near the Main Ural Ridge.

Until 1915, 95 percent of the platinum was mined from alluvial deposits; the rest was obtained by means of electrolytic refining of copper and gold.

In the 19th century many kinds of equipment—for example, the washing drum and the gold washer—were designed for the extraction of alluvial gold. The washing of ore was widely practiced in the Ural mines beginning in the first half of the 19th century. In the 1830’s, water for washing the alluvial ore was put through the ore under pressure. Further improvements in this technique led to the creation of water aprons, the prototypes of water monitors. In 1867, A. P. Chausov first achieved hydraulic mining of an alluvial deposit near Lake Baikal; later, in 1888, this technique was used by E. A. Cherkasov in the Chebalsuk River valley in the Abakan taiga. In the early 19th century, bucket dredgers were used to obtain gold and platinum from submerged alluvial deposits, and in 1870 a dredge was used for this purpose in New Zealand.

Since the second half of the 19th century, deep alluvial deposits in Russia have been exploited by underground techniques, and excavators and scrapers were introduced in the 1890’s.

In 1767, the Russian F. Bakunin made the first application of smelting of silver ores with the use of slag as a flux. The works of the Swedish chemist K. W. Scheele (1772) contained an observation about the transition of gold into a solution under the action of cyanide compounds. In 1843 the Russian scientist P. R. Bagration published a work on the dissolving of gold and silver in aqueous solutions of cyanide salts in the presence of oxygen and oxidizers, thereby laying the foundation for the hydrometallurgy of gold.

The purification and working of platinum were hindered bv its high melting point (1773.5° C). In the first half of the 19th century, A. A. Musin-Pushkin obtained ductile platinum by roasting it with amalgams. In 1827 the Russian scientists P. G. Sobolevskii and V. V. Liubarskii proposed a new method of purifying platinum, which laid the foundation for powder metallurgy. For the first time in the world, 800 kg of platinum were purified in a year—that is, the processing of platinum on a large scale was begun. In 1859, the French scientists H. E. Sainte-Claire Deville and A. Debret first smelted platinum in a kiln using an oxygen-hydrogen flame. The first work on electrolysis of gold dates to 1863, and this method was introduced industrially in the 1880’s.

In addition to amalgamation, the extraction of gold from ore by chlorination was first accomplished in Russia in 1886 at the Kochar’ Mine in the Urals. In 1896 the first Russian plant for extracting gold by cyaniding was built at the same mine. (The first such plant in the world was built in Johannesburg, South Africa, in 1890.) The cyanide process was soon applied in the extraction of silver.

In 1887–88 in England, J. S. MacArthur and the brothers R. and W. Forrest patented methods of obtaining gold from ore by dressing the ore with weak alkaline and cyanide solutions and precipitating the gold in these solutions with zinc shavings. In 1893 gold was precipitated by electrolysis, and in 1894, by powdered zinc. In the USSR, gold is extracted primarily from alluvial deposits; in other countries, about 90 percent of the gold is obtained from underground mines.

The dredging method is the most effective for obtaining noble metals from alluvial deposits; the scraping and bulldozing method and the hydraulic method are less economical. The underground working of alluvial deposits is 1.5 times as costly as the dredging method and is used in the USSR for the deep alluvial deposits in the valleys of the Lena and Kolyma rivers.

Silver is obtained chiefly from underground mines and is usually found in lead-zinc ore beds, which yield about 50 percent of the annual total; copper ores yield 15 percent, and gold ores yield 10 percent. About 25 percent of silver is obtained from silver lodes. A considerable percentage of the platinum metals is extracted from copper-nickel ores. Platinum and the metals of its group are smelted together with the copper and nickel and remain in the residue when the copper and nickel are purified by electrolysis.

The methods of hydrometallurgy, often combined with beneficiation, are widely used for the extraction of noble metals. Gravity preparation of noble metals permits the separation of large particles of metal. This technique is supplemented by cyaniding and amalgamation. Amalgamation received its first theoretical justification from the Soviet scientist I. N. Plaksin in 1927. Silver chloride is most effectively processed by cyaniding; silver sulfide ores are often cyanided after preliminary chlorinating calcination. Gold and silver are usually precipitated from cyanide solutions by metallic zinc; less commonly by coal and ion-exchange resins. Gold and silver may be extracted from ores by selective flotation. About 80 percent of silver is obtained chiefly by the techniques of pyrometallurgy, and the remainder by amalgamation and cyaniding.

Noble metals of high purity are obtained by affinage. The loss of gold in affinage does not exceed 0.06 percent (including losses in smelting), and the content of gold in the refined metal is usually not lower than 999.9 purity; the losses of platinum metals do not exceed 0.1 percent. Work is being done to intensify the cyaniding process (cyaniding under pressure or under an oxygen jet), nontoxic solvents for the recovery of noble metals are being sought, combined methods are being developed (for example, a flotation-hydrometallurgical method), organic reagents are being applied, and so on. Noble metals are effectively precipitated from cyanide solutions with the aid of ion-exchange resins, and bacteria are being successfully used to extract noble metals from ores (bacterial lixiviation).

While retaining their functions as currency metals (mainly gold), noble metals have also received wide industrial application.

In the electronics industry, noble metals are used to make highly reliable contacts (corrosion-resistant and stable under the action of the short-lived electrical arcs that form on the contacts). Contacts made of alloys of gold and silver, gold and platinum, or gold with silver and platinum are used in weak-current technology where there are low circuit pressures. Alloys of palladium and silver (with 60–5 percent palladium) are widely used in weak-current and mediumload equipment. Metal and ceramic contacts using silver as the conductor component appear promising. Magnetic alloys of noble metals with a high coercive force are used in making miniaturized electrical equipment. The resistors (potentiometers) for automatic apparatus and tensometers are made of noble-metal alloys (chiefly palladium and silver, occasionally palladium and other metals). They have a low temperature coefficient of electrical resistance, a low thermoelectric power when coupled with copper, high wear resistance, a high melting point, and immunity to oxidation.

In chemical machine building and laboratory technology, noble metals are used to make various corrosion-resistant apparatus, electric heaters, high-temperature kilns, and equipment for producing optical glass and glass fiber, thermocouples, resistance standards, and so on. The noble metals are used for this purpose either pure, as bimetals, or in alloys. Chemical reactors and their parts may be made entirely of noble metals or merely covered with a noble-metal foil. Platinum-covered apparatus is used in making pure chemical preparations and in the food industry.

When the chemical stability and refractoriness of platinum or palladium are insufficient, they are replaced by alloys of platinum and metals that increase those characteristics: iridium (5–20 percent), rhodium (3–10 percent), and ruthenium (2–10 percent). Examples of the application of noble metals in these areas of technology include the manufacture of boilers and basins for alkali smelting or for work with hydrochloric, acetic, and benzoic acids; and of autoclaves, distillers, flasks, and agitators.

In medicine, noble metals are used in instruments, parts, devices, artificial limbs, and various preparations, chiefly based on silver. Alloys of platinum with iridium, palladium, or gold are almost indispensable in making hypodermic needles. The most common medical compounds containing noble metals include silver nitrate and Piotargol (silver protein). Noble metals are used in radiation therapy (needles of radioactive gold to destroy malignant tumors) and in compounds that increase the defensive abilities of the organism.

In electronics, gold alloyed with germanium, indium, gallium, silicon, tin, or selenium is used to make contacts in semiconductor diodes and transistors.

In the photographic and cinematographic industries, noble metals in the form of salts are used to make light-sensitive materials (chiefly silver in the form of a bromide salt, the most important part of a light-sensitive emulsion); less frequently, salts of gold and platinum are used in toning pictures.

Noble-metal alloys are used in jewelry-making and applied decorative arts.

Used as coatings for other metals, noble metals protect them from corrosion or impart such qualities as reflectivity, color, and luster. Gold effectively reflects heat and light off the surface of rockets and space ships. An extremely thin layer of gold (1/60 micron) is sufficient to screen out the infrared radiation of space. The outer shell of satellites is covered with gold both for protection against external effects and to facilitate tracking the satellite. Some of the inner parts of the satellite, as well as the equipment compartment, are covered with gold to protect them from overheating and corrosion.

Noble metals are used in the manufacture of mirrors (silvering of glass with solutions or by vacuum-dusting with silver).

The casings of high-altitude aviation engines are covered inside and out with a very thin film of noble metals.

Noble metals cover the reflectors in infrared drying equipment, electronic contacts and components for conductors, radio apparatus, and equipment for X-ray therapy and radiotherapy. They are used as an anticorrosion covering for specialized tubes, fans, and containers. A wide assortment of auriferous pigments for painting metal, ceramics, and wood has been developed.

Antifriction alloys and solders based on noble metals have become widely used. Solders made with silver are significantly stronger than copper-zinc, lead, or tin solders; they are used to solder radiators, carburetors, filters, and so on.

Alloys of iridium with osmium and gold with platinum and palladium are used in compass needles and in “everlasting” pens.

The high catalytic qualities of some noble metals permit them to be used as catalysts: platinum is used in producing sulfuric and nitric acids; silver is used in the production of formaline. Radioactive gold can be substituted for the costlier platinum as a catalyst in the chemical and petroleum processing industries. Noble metals are also used to purify water.

REFERENCES

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Plaksin, I. N. Metallurgiia blagorodnykh metallov. Moscow, 1958.
Danilevskii, V. V. Russkoe zoloto. Moscow, 1959.
Buzlanov, G. F. Proizvodstvo i primenenie metallov platinovoi gruppy v promyshlennosti. Moscow, 1961.
Viazel’shchikov, V. P., and Z. N. Paritskii. Spravochnik po obrabotke zolotosoderzhashchikh rud i rossypei. Moscow, 1963. Analiz blagorodnykh metallov. Moscow, 1955.
Probootbiranie i analiz blagorodnykh metallov. Moscow, 1968.
Iordanov, Kh. V. Zapiski po metallurgiia na redkite metali. Sofia, 1959.
Silver. Princeton, 1967.

L. M. GEIMAN