As we have discussed previously on this blog, gold cyanidation can be a very effective means of extracting gold from mined ore. However, the process is ineffective for refractory gold ores – which are made up of sulfide minerals that block the cyanide solution from leaching the gold out of the ore. Luckily, the gold contained in refractory ores is not unsalvageable.
Gold’s resistance to oxidation is a critical point of most refining and extraction methods, and the roasting process is no exception. During roasting, refractory gold ore is treated with extremely hot air. This causes the sulfites to oxidize, separate from the ore, and become sulfur dioxide. Once all of the sulfides are oxidized, the remaining gold ore can be processed normally.
The roasting method has been around for hundreds of years. Prior to the 20th century, the process was done by burning wood on top of the ore. This would raise the temperature of the ore to the point where the ore’s sulfur content would become the primary source of fuel for the fire. Roasting with this method was extremely hazardous to the environment because of the large amounts of sulfur dioxide and other toxic compounds that were released. Luckily, the technique has been improved since then.