Recovery of Gold and Silver from Ores by Hydrometallurgical Processing

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Abstract
The Bureau of Mines has played an important role in the revival of precious-metal mining in the Western United States. During the past 30 years, many techniques used by industry to recover gold and silver were pioneered and developed by the Bureau of Mines. In 1952, technology was developed to strip gold from granular activated carbon and allow the carbon to be reused. In 1969, the heap leaching concept was applied to low-grade gold and silver ores and mine wastes. Heap leaching in conjunction with carbon adsorption-desorption permits very low value material to be economically processed. In 1970, an oxidation treatment based on hypochlorite was developed to increase the recovery of gold from carbonaceous gold ores by cyanidation. In the early 1950's, attempts were made to employ carbon-in-pulp technology to recover gold from ores, but the fixed price of gold and increasing operational costs made cyanidation unprofitable. The Bureau of Mines and Homestake Mining Co. jointly operated a carbon-in-pulp pilot plant, and the technology was used in full-scale production at the Homestake Mine in 1972. In 1973, a pressure stripping method, which decreased the time needed for the stripping step, was developed to desorb gold and silver from carbon. An alcohol desorption method, developed in 1976, also decreased the carbon stripping time. In 1979, an agglomeration pretreatment was developed that permitted clayey precious metal ores to be heap-leached. Almost all precious-metal-mining companies use at least one of the above techniques in production facilities.

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