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− LEACHING OF REFRACTORY GOLD ORES WITH AMMONIUM THIOSULPHATE: POWERFUL TECHNOLOGY FOR THE REPLACEMENT OF CYANIDE IN LARGE-SCAL

Abstract

Ammonium thiosulfate is a powerful viable technological and environmentally-friendly option for the leaching of precious metals, unlike the conventional cyanidation process. Thiosulfate is a non-toxic reagent, with a lower unit cost, compared with cyanide, and more commonly used as fertilizer in agricultural activities, helping to fix nitrogen in the soil. 

Ammonium thiosulfate is an alternative leaching agent used, instead of cyanide, in gold leaching. It is also specifically used for refractory minerals that are not adequate for the conventional cyanidation process, due to the presence of impurities (such as copper, arsenic, antimony, tellurium, and manganese), sulfides and preg-robbing minerals, which are the cause of the low gold recovery and high cyanide consumption. A gold ore associated with coal was assessed to confirm the optimal results of the innovative technology known as ATS. 

The main compounds of the thiosulfate leaching agent are ammonium thiosulfate, ammonium sulfate, cupric ion, and metabisulfite. Cupric ion is a catalyst used to dissolve gold. The copper concentration at a 0.5 Kg/Tm ratio provides an optimal gold extraction of 75.20% with a 8.60 Kg/Tm ratio of thiosulfate consumption at a treatment scale of 10 000 Tm in leach pads of 90%<2.5 inches granulometry in periods of 45 days leaching, and 81.20% gold extraction with 6.5 Kg/Tm thiosulfate consumption in clean/docile gold ores at a treatment scale of 20 000 Tm in leach pads of similar granulometry and leaching time. Regarding stirring tanks, gold extractions of 90.75% in refractory minerals were achieved in 120 hours of leaching at 80% <325 mesh granulometry, with a thiosulfate ratio of 7.4 Kg/ Tm, and, in the case of clean/docile minerals, gold extractions of 93.6% are achieved in 120 hours of treatment at 80% <325 mesh granulometry and thiosulfate rate of 6.3 Kg / Tm. Conventional leaching using cyanide in this type of refractory minerals does not exceed 10% of gold metallurgical recovery.

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