The solution is separated from the ore by methods such as filtration and counter current decantation (CCD) and is then clarified in special filters, usually coated with diatomaceous earth to produce a clarified solution. Oxygen is then removed by passing through a vacuum deaeration column. Zinc dust is then added to the clarified, deaerated solution which precipitates the gold, zinc having a higher affinity for the cyanide ion than gold. Silver and copper are also precipitate if present.
The gold precipitate is then filtered out of the solution, mixed with fluxes and smelted to form crude and impure bars which are sent to a refinery to remove the copper and silver, the process used depending on the impurities in the gold.
The basic process was discovered and patented by Charles Washington Merrill around 1900, then later refined by Thomas B. Crowe, working for the Merrill Company.
In more recent years the EMEW technology has started to replace this process through the use of electrowinning
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