Thiosulfate leaching of gold - A review

The ammoniacal thiosulfate leaching process for gold and silver extraction has been reviewed in terms of leaching mechanism, thermodynamics, thiosulfa te stability, and gold recovery options. The application to different ore t ypes and process options have also been discussed. The thiosulfate leaching process it catalysed by copper and has several adv antages over the conventional cyanidation process. Thiosulfate leaching can be considered a non-toxic process, the gold dissolution rates can be faste r than conventional cyanidation and, due to the decreased interference of f oreign cations, high gold recoveries can be obtained from the thiosulfate l eaching 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 thi osulfate, 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, suitabl e Eh and pH conditions, thiosulfate leaching can be made practical. Generally the thiosulfate leaching conditions reported in the literature ar e severe with high reagent consumption. Further investigations are required on leaching under low reagent concentrations over extended periods where r eagent consumption is low. For high grade ores or refractory sulfide ores w here 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, activa ted carbon can be used to recovery gold from thiosulfate solutions. The use of resins to recover gold from solution appears to show some promise, howe ver 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 consider able potential as an effective and less hazardous procedure for gold and si lver extraction from auriferous ores. (C) 2001 Elsevier Science Ltd. All ri ghts reserved.