The gold industry has been seeking alternative reagents for leaching gold from low-grade ores for the past 25 years. The cost and transport of cyanide, the poor recovery of gold from carbonaceous ores, the lack of selectivity of cyanide towards copper minerals and the cost of waste treatment are real factors which can make cyanide unattractive in some circumstances. However there are also perceived and real environmental concerns and pressures when dealing with cyanide solutions and waste streams in sensitive areas, which has an impact on the sustainability of the whole gold industry.
Compared to other industries like copper or oil, the gold industry has quite a good record. Few people realise that the gold mining industry only accounts for 13 per cent of total world cyanide consumption, and despite the widespread use and handling of cyanide in gold processing, the North American industry has recorded only three deaths attributed to cyanide over a period of 100 years (McNulty, 2001). What is remembered, are the 14 major incidents that have been recorded throughout the world over the past 25 years – ten involving tailings dams, two involving pipeline failures and two involving transportation accidents.
For this reason, the gold industry has been seriously looking for safer and cleaner reagents that not only meet chemical criteria but also toxicological and environmental criteria. Sparrow and Woodcock (1995) lists over 25 potential reagent types including polysulphides and various organics, but only a few attract serious attention. Alternative reagents that have been seriously considered for gold include chlorine, bromine, iodine, thiocyanate, thiourea, ammoniacal thiosulphate and ammoniacal copper cyanide. By necessity, all form strong complexes with gold enabling gold to be leached in a practical Eh range. Thiosulphate provides the strongest alternative complex although this does not compare closely to cyanide.
GOLD RECOVERY FROM THIOSULFATE LEACHING
Thiosulphate Gold Leaching Process
Comparison of Cyanide and Thiosulphate Leaching for Gold Production
Ammoniacal thiosulphate leaching of gold ore