Thiosulfate Leaching of Gold

The recovery of gold using thiosulfate was first proposed early in the 1900s. In the Von Patera process, gold and silver ores were subjected to a chlorination roasting followed by leaching in a thiosulfate solution. In a thiosulfate leach liquor, the formation of gold and silver thiosulfate complexes proceeds via the catalytic oxidation of the zero-valent metal by a suitable soluble metal complex, acting as the primary oxidant. The sulphoxy anions initially present in an ammoniacal thiosulfate leach liquor are thiosulfate, sulphate, and sulphide from mineral sources. However, thiosulfate is metastable, which means it may be readily oxidized or reduced according to the initial solution potential. Reductive precipitation of gold from the pregnant leach liquor using inorganic zinc metal or organic acid is a common process in gold recovery; however, it is not very effective in the thiosulfate-ammonia system.