Most gold comes from load deposits, also called vein deposits. Load deposits require hard rock mining – the process of removing gold bearing rock by drilling and blasting.
After initial excavation the first stage in processing is to reduce the ore size by crushing. Following this the use of a mill further reduces the ore to a -5mm fraction and a water and cyanide solution is added before additional milling creates a pulp containing very fine material.
This pulp is delivered to large settling tanks where the suspended solids sink to the bottom of the tank while the water overflows a weir at the top of the tank. The wet solids are transferred to agitation tanks where air is added. The oxygen sets off a chemical reaction between the gold and the cyanide trapped in the ore, triggering the gold to dissolve and leach into the surrounding water. Drum filters then separate the water from the solids.
Zinc powder is then added to solidify the dissolved gold. To smelt into bars a chemical cocktail is prepared – manganese dioxide, fluoride, silica flour, borax and sodium nitrate. This mix – called flux – separates the gold from the impurities.
Smelting occurs at 1600 degrees centigrade. The smelter is rotated so that the contents heat evenly. Over two and a half hours the heavier gold eventually sinks to the bottom while the impurities, or ‘slag’ floats to the surface. The slag is poured out and a sample is taken to ensure it contains no gold – if it does it goes back in until it is gold free.
By now the gold has cooled slightly so it is reheated to 1600 degrees centigrade again before it is cast into bar shape moulds. The gold takes about four minutes to solidify then another hour to cool completely in a basin of cold water.
Gold bars are extracted from the moulds and cleaned of any slag residue. At this stage the gold is 80% pure. It is then further refined to 99.9% purity – the international gold standard.