AnalyticalChemistryInterdependence of
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The goal of the precious metals analyst is the accurate and rapid determination of silver, gold, and the platinum-group elements in hundreds of different materials. Precious metal concentrations in these materials can vary from parts per million or lower to virtually 100%.Naturally, the wide range of sample types in which precious metals occur requires that a variety of classical and instrumental methods or combinations thereof be used in precious metals analysis. But before the details of these methods we presented, some background on the history, extractive metallurgy, and applications of the precious metals will help set the stage.
Profiles of the Precious Metals The precious metals include silver (Ag), gold (Au), and the platinumgroup elements-platinum (Pt), palladium (Pd), rhodium (Rh), iridium (111, ruthenium (Ru), and osmium (Os). 1020 I
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The Pt-group elements are found together in a rectangular area of the periodic table of elements and are also found together in nature, with Pt and Pd predominating in all known deposits. Silver. The Greeks called it argyreos (shining) and the Romans called it argentum, hence silver’s chemical symbol, Ag. Early in its history, in Egypt around 3500 B.C., its value was set at 40% that of gold. Persian and Chinese writings dating back to 2500 B.C. also contain references to silver. Approximately two-thirds of today’s silver resources are associated with sulfides of copper, lead, and zinc, with which silver occurs in solid solution. The remainder is in vein deposits, with silver as the principal component. For many centuries native silver was recovered from its ores by amalgamation with mercury, but cyanidation followed hy precipitation is increasingly used today. Silver is also recov-
ANALYTICAL CHEMISTRY, VOL. 56. NO. 9, AUGUST 198A
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ered in the electrolytic refining of copper concentrate and in the smelting of zinc concentrate. Recycling is another important source of silver, as it is for all the precious metals. Recycling of silver from coins or scrap frequently involves electrolytic deposition from a nitric acid medium. Spent photographic film,an important source of recycled silver, is burned to an ash that consists mainly of silver halides, which is then fused with sodium carbonate to yield pure silver. Silver is extremely important in various defense, transportation, and communications applications. Photography provides the largest single market for the metal, currently accounting for about 38% of the total. The second largest use is in electrical and electronic equipment, and the third is in sterling silverware and jewelry. One of the earliest forms of consumer protection was hallmarking, dating hack to English legislation in 1238, by which gold and silver articles were stamped by government officials to attest to their purity. Gold. Like silver, gold has been known since prehistoric times. It plays an important part in folk tales and legends, such as the story of the search by Jason and the Argonauts for the Golden Fleece. Gold is mentioned in Genesis and Exodus and was traded hy the Phoenicians from 1200 to 650 B.C. The search for gold was undoubtedly one of the motivating forces leading to the discovery of the New World by Columbus. Unlike silver, most gold occurs as native metal in lode and fissure veins, often in association with quartz. Most high-grade deposits have been exhausted; hence, most gold is mined at great depth. For instance, about half of the gold produced in 1983 came 0003-2700/84/A351-1020$01.50~0 0 1984 American Chemical Society
Remrt Silve Kallmann Ledoux 8 Company 359 Alfred Ave. Teaneck, N.J. 07666
of thePrecious Metals and Instrumentd Meth~ds from the Rand in South Africa, which is mined at about a 10,ooO-ft depth. The Homestake Mine in the Black Hills of South Dakota is also of considerable depth and has therefore been used recently for neutrino research. The extractive metaUurgy of gold in placer (alluvial or glacial) deposits is hased on gravity methods, followed by amalgamation or cyanidation. Until recently, gold from lode and vein deposita was recovered by an amalgamation process, hut due to environmental considerations this has been largely replaced by cyanidation followed hy precipitation. In addition, significant amounts of gold are recovered from deposits of hase metal sulfides, such as copper sulfide. It has been estimated that 95% of all the gold mined since King Solomon’s reign in the loth century B.C. is still around in one form or another, some of it undoubtedly recycled dozens of times. This was strikingly demonstrated in 1979 and 1980 when the price of gold and silver rose to unprecedented levels and millions of ounces of jewelry were turned in for refining. In one recycling process, gold scrap is melted with an excess of copper. The product is treated with nitric acid to remove silver, copper, zinc, and nickel, and the nitric acid insoluble residue (impure gold) is dissolved in aqua regia. After dilution and filtration, gold is precipitated with Fe+* or SOs-*. Other important recycling proc e s s for gold involve treatment of molten gold with chlorine for the removal of impurities; purification of gold by electrolysis; and recovery in copper smelting operations. The primary demand for gold comes from the investment world, in the form of coins, bullion, and medallions. Gold is a classic inflation hedge. For
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Krugerrands are widely held as a hedge against inflation; they contain exactly one ounce of gold
many people, it is also the ultimate refuge from politid, economic, and financial calamity. Gold is also widely used in the electronics industry and in dental alloys. But the largest use of gold hy far is in the jewelry industry. The gold content of jewelry is expressed by the karat scale, in which 100% gold is 24 karats. Commonly used alloys are 18, 14, and 9 karats. Hollow chains, lightweight pieces, thin electroplated gold layers, and low-karat jewelry have gained greater acceptance in the US. recently because of the high price of gold. Pt-group elements. Comparatively new arrivals on the precious metals scene, the Pt-group elements can he traced back only to 1741, when metallurgist Charles Wood presented a specimen of a native platinum alloy to the Royal Society of London. A t about the same time, UUva, a Spanish scientist, described a mineral he discovered
in Colombia that he calledplatina or “little silver,” to distinguish it from plata, real silver. Wollaston and Tennant identified palladium, rhodium, ruthenium, iridium, and osmium soon after the discovery of platinum. The Republic of South Africa, the Soviet Union, and Canada account for nearly all the world‘s newly mined (primary) Pt-group metals. In the past, recovery of the Pt-group elements has involved flotation or magnetic separation of the sulfide ore in which these elements are commonly found, followed by roasting, concentration, precipitation, distillation, and other separative steps. But various modifications of a solvent extraction process developed in South Africa have now lareelv reolaced the older methods. Recvcline of nlatinum and nalladium from aitoiotive catalysti is still in a tentative stage, complicated by the presence of substantial mounts of I
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The melt is poured into a mold, and upon eooling, the button containing the precious metals separates cleanly from the slag that holds the nonprecious-metal components of the sample
lead from large-scale flouting of US. federal emission control laws, which prohibit the use of leaded gasoline in vehicles with catalytic conveners. Recycling of electronic scrap is also complex, since platinum and palladium are tmicallv onlv_oresent . at levels of bldppm. Maior uses of swcific Pt-erouD metali are in automotive caLalytic converters, jewelry, dentistry, electrical and electronics products, glass and glass fiher manufacturing equipment, and in catalysts for the chemical and petroleum industries. ~
Sampling of Precious Metals A representative sample is a prerequisite for any meaningful analysis. In the case of homogeneous metals, alloys, and mixable powders, standard methods of sampling can be used. Unfortunately, because of the heterogeneity of many materials (e.g., electron. ic scrap), appropriate preliminary steps such &s incineration, dissolution in acids, alloying, or matting must he taken. As an alternative or as a last resort a representative sample can he ensured by using a large sample weight. Chemical and
tablets indicate that the Babylonians used fm m a y in the 14th century B.C., when they suspected that gold sent by an Egyptian pharoah was impure. But fme assay is still extensively used today for measuring the gold and silver content of ores, alloys, and reclaimed materials. In fire assay, the sample is mixed with a flux containing a large proportion of lead oxide (litharge) and varying quantities of sodium carbonate, potassium carbonate, borax, silica, potassium nitrate, and organic substances such as starch or flour. When this mixture is fused, the precious metals collect in the pool of lead created when the litharge is reduced hy flux and sample components. The base metal constituents of the sample end up in the slag.
The lead “button” obtained in the fusion step may be too large, or it may contain intefering impurities. If so, it is heated in a ceramic dish called a scorifier. Unlike fusion, scorification is carried out in an oxidizing environment, in which much of the lead is oxidized to form a glass or slag that further extracta impurities. After scorification (or if the scorification step is skipped), the lead button is placed in a porous cup called a cupel, and the lead is reoxidized to litharge and removed. A bead of precious metals, called a dore, remains and is weighed. To separate the gold from the silver, the dore is treated with dilute nitric acid or concentrated sulfuric acid, either of which dissolves the silver but not the gold. The resulting gold bead is washed, dried, and weighed. Silver is calculated “by difference” between the dore and gold bead weights. A venerable technique, the classical fire assay is still unsurpassed in its ability to extract milligram or even microgram mounts of gold and silver from even the most complex matrices. It has high selectivity,high sensitivity, and is readily adaptable to routine operations. Fire assay may also be used after wet chemical preseparations of large quantities of matrix elements such as Cu, Ni, Fe, Cr, Se, AI,and Te. Modem instrumental methods can now be used to extend the scope of fire assay. Thus, for the determination of gold at very low levels (
