Thiosulfate leaching of gold—A review

Abstract

The ammoniacal thiosulfate leaching process for gold and silver extraction has been reviewed in terms of leaching mechanism, thermodynamics, thiosulfate stability, and gold recovery options. The application to different ore types and process options have also been discussed. The thiosulfate leaching process it catalysed by copper and has several advantages over the conventional cyanidation process. Thiosulfate leaching can be considered a non-toxic process, the gold dissolution rates can be faster than conventional cyanidation and, due to the decreased interference of foreign cations, high gold recoveries can be obtained from the thiosulfate leaching of complex and carbonaceous-type ores. In addition, thiosulfate can be cheaper than cyanide. The chemistry of the ammonia-thiosulfate - copper system is complicated due to the simultaneous presence of complexing ligands such as ammonia and thiosulfate, the Cu(II)Cu(I) redox couple and the stability of thiosulfate in solution. However, by maintaining suitable concentrations of thiosulfate, ammonia, copper and oxygen in the leach solution, and consequently, suitable Eh and pH conditions, thiosulfate leaching can be made practical. Generally the thiosulfate leaching conditions reported in the literature are severe with high reagent consumption. Further investigations are required on leaching under low reagent concentrations over extended periods where reagent consumption is low. For high grade ores or refractory sulfide ores where some pretreatment processing is required to liberate gold, the in-situ generation of thiosulfate should be investigated in more detail. This may lessen thiosulfate consumption and liberate more gold through the oxidation of host sulfide minerals. Cementation (or metal displacement), resins and to a limited extent, activated carbon can be used to recovery gold from thiosulfate solutions. The use of resins to recover gold from solution appears to show some promise, however more work is required to develop suitable elution and recovery methods, and greater selectivity over copper. While difficulties remain to be overcome, thiosulfate leaching has considerable potential as an effective and less hazardous procedure for gold and silver extraction from auriferous ores.