Alternatives to cyanide for processing precious metal ores

摘要

It is important to recognise that hydrometallurgical treatment of gold ores predated the discovery of cyanidation. By the mid-1800s, many gold mines throughout the world had been deepened below historic water tables and encountered primary sulphides that responded poorly to amalgamation. Gravity concentration, followed by smelting, was effective on some ores, but not all. Various experimenters learned that sodium hypochlorite, sodium thiosulphate, bromine, and chlorine were effective leaching agents for some gold ores, and the commercial use of these reagents was common by 1880. The cyanidation of gold ores was patented in the UK by J. S. MacArthur and two brothers, W. and R. W. Forrest on October 19, 1887. A pilot-scale demonstration plant was built in 1888 in Ravenswood, Queensland, and the first commercial ore cyanidation plant was built at the Crown mine near Karangahake in Auckland Province, New Zealand. The first use of cyanide on the Witwatersrand was in 1890 at the Robinson mine. The first two plants in the US were built in 1891 at Mercur, Utah and Calumet, California. Due to development of the "Rand" and the adoption of cyanidation, South African gold production grew from 300 oz in 1890 to 300,000 oz in 1893. Between 1892 and 1905, US production increased from 1.7 Moz to 4.6 Moz, and this is significant because most of the growth resulted from the treatment of ores that were uneconomical with gravity concentration or amalgamation, and retreatment of tailings produced earlier by those techniques.Cyanidation was so economical and effective that it not only was applied extensively to new ores and tailings, but also rapidly replaced other leaching systems. Evolutionary improvements in practice continued for decades, along with revolutionary changes such as development by the US Bureau of Mines (USBM) in 1952 of a way f recovering gold from solution by carbon adsorption and caustic stripping. Agitated cyanidation of finely ground ore was the exclusive form of processing until heap leaching of crushed gold ore was proposed by the USBM in 1969. The first commercial heap leaching/carbon adsorption plant began operating at Cortez, Nevada in 1974 on rock which had been stockpiled because it was below the 0.08 oz/t gold cut-off grade for conventional milling. The US gold industry was quick to apply heap cyanidation to low-grade ores as gold prices began to rise during the mid-1970s, and the practice spread globally during the 1980s. Breadth of application had improved with development of agglomeration by the USBM in 1979. Cyanidation offers the advantages of:Requiring dilute solutions containing typically 300-1000 ppm (0.3-1 g/litre) sodium cyanideRequiring a pH range of 9.5-11.5 in which metals other than gold and silver are seldom mobilized to any extent, andRequiring simplified operating and control strategies.