GOLD PROCESSING

preparation of the ore for use in various products. For thousands of years the word gold has connoted something of beauty or value. These images are derived from two properties of gold, its colour and its chemical stability. The colour of gold is due to the electronic structure of the gold atom, which absorbs electromagnetic radiation with wavelengths less than 5600 angstroms but reflects wavelengths greater than 5600 angstromsthe wavelength of yellow light. Gold's chemical stability is based on the relative instability of the compounds that it forms with oxygen and watera characteristic that allows gold to be refined from less noble metals by oxidizing the other metals and then separating them from the molten gold as a dross. However, gold is readily dissolved in a number of solvents, including oxidizing solutions of hydrochloric acid and dilute solutions of sodium cyanide. Gold readily dissolves in these solvents because of the formation of complex ions that are very stable. Gold (Au) melts at a temperature of 1,064 C (1,947 F). Its relatively high density (19.3 grams per cubic centimetre) has made it amenable to recovery by placer mining and gravity concentration techniques. With a face-centred cubic crystal structure, it is characterized by a softness or malleability that lends itself to being shaped into intricate structures without sophisticated metalworking equipment. This in turn has led to its application, from earliest times, to the fabrication of jewelry and decorative items. 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. (197889), 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. (197884), 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 ). The Editors of the Encyclopdia Britannica Relevant volumes of Joseph William Mellor, A Comprehensive Treatise on Inorganic and Theoretical Chemistry, 16 vol. (192237, reprinted 196065), are an excellent source of descriptive chemistry and historical information on the precious metals. 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 (198189) 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. Although much of the equipment described is outdated, the work by Thomas Kirke Rose and W.A.C. Newman, The Metallurgy of Gold, 7th ed., rev. (1937, reissued 1986), still contains a wealth of useful information about gold mining, extraction, refining, and chemistry. R.S. Salter, D.M. Wyslouzil, and G.W. McDonald (eds.), Proceedings of the International Symposium of Gold Metallurgy (1987), provides some information on contemporary research and current gold-recovery operations; it will appeal more to the reader skilled in the art than the casual reader. In the same vein, F.W. McQuiston, Jr., and R.S. Shoemaker, Gold and Silver Cyanidation Plant Practice, 2 vol. (197581), provides a wealth of information for the commercial operator, including actual flowsheets and operating data from a variety of gold and silver operations around the world. James Edward Hoffmann