Analytical Chemistry Interdependence of 44
45
46
47
Ru
Rh
Pd
Ag
Ruthenium
Rhodium
Palladium
Silver
101.07
102.905
106.4
107.870
76
~ΎΊ
^78
^79
Os
lr
Pt
Osmium
Iridium
Platinum
Gold
190.2
192.2
195.09
196.967
The goal of the precious metals ana lyst is the accurate and rapid determi nation of silver, gold, and the plati num-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 pre cious metals occur requires that a va riety of classical and instrumental methods or combinations thereof be used in precious metals analysis. But before the details of these methods are presented, some background on the history, extractive metallurgy, and ap plications 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), palla dium (Pd), rhodium (Rh), iridium (Ir), ruthenium (Ru), and osmium (Os).
Au
The Pt-group elements are found to gether in a rectangular area of the pe riodic table of elements and are also found together in nature, with P t and Pd predominating in all known de posits. 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 to day's silver resources are associated with sulfides of copper, lead, and zinc, with which silver occurs in solid solu tion. The remainder is in vein depos its, with silver as the principal compo nent. For many centuries native silver was recovered from its ores by amalga mation with mercury, but cyanidation followed by precipitation is increas ingly used today. Silver is also recov
1020 A · ANALYTICAL CHEMISTRY, VOL. 56, NO. 9, AUGUST 1984
Classical
ered in the electrolytic refining of cop per concentrate and in the smelting of zinc concentrate. Recycling is another important source of silver, as it is for all the pre cious metals. Recycling of silver from coins or scrap frequently involves elec trolytic 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. Photog raphy provides the largest single mar ket for the metal, currently accounting for about 38% of the total. The second largest use is in electrical and elec tronic equipment, and the third is in sterling silverware and jewelry. One of the earliest forms of consumer protec tion was hallmarking, dating back 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 by the Phoenicians from 1200 to 650 B.C. The search for gold was undoubt edly one of the motivating forces lead ing 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 ex hausted; 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 © 1984 American Chemical Society
Report Silve Kallmann Ledoux & Company 359 Alfred Ave. Teaneck, N.J. 07666
of the Precious Metals and Instrumental Methods from the Rand in South Africa, which is mined at about a 10,000-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 metallurgy of gold in placer (alluvial or glacial) deposits is based on gravity methods, followed by amalgamation or cyanidation. Until recently, gold from lode and vein deposits was recovered by an amalgamation process, but due to environmental considerations this has been largely replaced by cyanidation followed by precipitation. In addition, significant amounts of gold are recovered from deposits of base metal sulfides, such as copper sulfide. It has been estimated that 95% of all the gold mined since King Solomon's reign in the 10th 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 + 2 or SO3 - 2 . Other important recycling processes 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
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 political, economic, and financial calamity. Gold is also widely used in the electronics industry and in dental alloys. But the largest use of gold by 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 U.S. recently because of the high price of gold. Pt-group elements. Comparatively new arrivals on the precious metals scene, the Pt-group elements can be traced back only to 1741, when metallurgist Charles Wood presented a specimen of a native platinum alloy to the Royal Society of London. At about the same time, Ullva, a Spanish scientist, described a mineral he discovered
in Colombia that he called platina 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 largely replaced the older methods. Recycling of platinum and palladium from automotive catalysts is still in a tentative stage, complicated by the presence of substantial amounts of
ANALYTICAL CHEMISTRY, VOL. 56, NO. 9, AUGUST 1984 · 1021 A
The melt is poured into a mold, and upon cooling, 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 U.S. federal emission control laws, which prohibit the use of leaded gasoline in vehicles with catalytic converters. Recycling of electronic scrap is also complex, since platinum and palladium are typically only present at levels of 5-10 ppm. Major uses of specific Pt-group metals are in automotive catalytic converters, jewelry, dentistry, electrical and electronics products, glass and glass fiber 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., electronic scrap), appropriate preliminary steps such as incineration, dissolution in acids, alloying, or matting must be taken. As an alternative or as a last resort a representative sample can be ensured by using a large sample weight. Chemical and Instrumental Analysis Fire assay. Perhaps the most important single analytical method in precious metals analysis is the fire assay, considered by many to involve as much art as science. The fire assay is an ancient technique. The Old Testament contains references to various features of the process, and cuneiform
tablets indicate that the Babylonians used fire assay in the 14th century B.C., when they suspected that gold sent by an Egyptian pharoah was impure. But fire 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 by 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 extracts 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 amounts 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, Al, and Te. Modern instrumental methods can now be used to extend the scope of fire assay. Thus, for the determination of gold at very low levels (
