Pyrolysis of arsenopyrite for gold recovery by cyanidation

Gold in arsenopyrite or high-arsenic pyrites commonly exists as submicroscopic or “invisible” inclusions, making it refractory to cyanide leaching. Current practice favour an oxidising roast to liberate the gold for cyanidation because it is fast and energetically self supporting. However, the SO2 gas it produces can create a handling problem. One alternative we are investigating is pyrolysis under N2, CO2 and SO2 atmospheres. Instead of gaseous SO2, the process produces elemental sulfur and, if present in the ore, arsenic metal or solid arsenic oxide. Pyrolysis experiments were conducted on a gold-containing refractory arsenopyrite ore to determine the effect of temperature, time and atmosphere on the cyanidation characteristics of the pyrolysis product. In N2 and CO2, the ore behaved similarly, reaching maximum mass loss at 700°C and yielding maximum gold recoveries of 35 and 48 %, respectively. Pyrolysis under SO2 was faster, reaching maximum mass loss at 600°C and resulting in gold recoveries of up to 61%. The low gold recovery for N2 and CO2 pyrolysed products was due to the encapsulation of the exsolved gold particles by pyrrhotite as it recrystallised during the process. All the pyrolysis products, although composed mainly of pyrrhotite, did not seem to be highly active in cyanide solutions.