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Gold has been extracted from its ores since ancient times. The modern industry, however, was established about hundred years ago when the cyanidation process was invented in 1887. The process was applied successfully for a variety of ores and found wide-spread application in 1970’s for ores containing as low as 1 ppm gold. In the last decade new chemical beneficiation process have been developed to treat ores that are not directly amenable to cyanidation. These involve aqueous oxidation of sulfide minerals containing gold at high temperature and pressure in autoclaves lined with acid-resisting brick.