GOLD 125 in crucibles in a furnace to more than 1000 C in the presence of silica, oxidizers, and fluxes The impurities oxidize and melt to form a separate silica-rich slag, which floats on top of the nearly pure molten gold The slag is skimmed off and the gold (containing more than 95% gold at this stage) is called dore Molten dore is further refined to in excess of 99% purity by injecting it with chlorine, which picks up the remaining metal impurities and removes them as volatile chlorides and solid chloride salts A final sequence of redissolution and electrowinning (or precipitation) improves the purity to more than 99.9% gold, which is then remelted and cast into gold bars for market When gold ores yield less than 80% of their gold after normal grinding and cyanidation, they are termed refractory – such ores typically contain gold locked up in sulphides, tellurides, and arsenides Such ores require more expensive (finer) grinding and sometimes roasting methods to free up the gold prior to cyanidation Roasting such ores in a furnace (to break the host phases down and release the gold) has environmental impacts because of the release of sulphur dioxide (which contributes to acid rain) and toxic arsenic gases Besides complicating the freeing up of gold during beneficiation, host phases can interfere with the effectiveness of cyanidation by having low solubility in the cyanide solutions, consuming oxygen or cyanide, or competing with gold to form aqueous cyanide complexes Other phases in the ore, such as coatings of carbon or silica, or the presence of clay minerals can inhibit the reactions and adsorb gold and other ionic constituents, causing gold recoveries to be less than ideal High-pressure and high-temperature oxidation in strong acids is sometimes used to process refractory ores, but this adds substantially to costs Other more recent (and less costly) methods include microbiological approaches, in which bacteria are used to break down refractory gold-bearing phases prior to cyanidation Species such as Thiobacillus ferrooxidans catalyze the oxidative destruction of pyrite and arsenopyrite (see Minerals: Sulphides), freeing up gold grains that are locked up in such minerals Cyanide Heap Leaching In many modern low-grade gold mines, particularly those of the western USA, the gold is finely disseminated in fractured near-surface rock that can be easily mined by open-pit methods and piled into heaps after only modest blasting and beneficiation The clever and inexpensive technique of cyanide heap leaching is used to extract gold very effectively from such ore heaps, avoiding the costs associated with grinding and smelting more traditional high-grade hard-rock ores The ore heaps are piled on top of thick plastic mats, and a sprinkler system is installed on top of the heaps (Figure 5) A cyanide solution is sprinkled on the Figure Heaps of low grade epithermal gold ore are piled on black plastic mats and sprinkled with sodium cyanide solution at the open pit Mesquite Mine, Imperial County, California, USA One set of steel pipes delivers fresh solution to the sprinkler systems atop the heaps, while the other set carries away ‘pregnant’ gold laden cyanide liquor that drains from the heaps and accumulates in the trenches