What is the gold extraction process that uses mercury?

The gold extraction process that uses mercury is amalgamation.

In this method, mercury bonds readily with gold particles to form a gold–mercury amalgam. Miners first crush or wash ore so the gold is exposed, then bring it into contact with liquid mercury. The heavy amalgam can be separated from lighter waste material and later heated in a retort; the mercury vaporizes and the gold remains.

Amalgamation was historically important in placer mining and in processing finely divided gold that was difficult to recover mechanically. Mercury amalgamation also played a major role in silver extraction, including Mexico’s patio process, introduced in the 16th century. It is not the same as cyanidation, the solution-based process that became dominant in large-scale gold extraction from the late 19th century.

Mercury is highly toxic, especially when vapors are inhaled, and losses from mining have polluted waterways and soils. Large-scale operations in many developed countries phased out the method, although it remains common in some artisanal and small-scale mining regions.