| Amalgam/Amalgamation |
When gold, silver and a number of base metals, such as zinc or lead, come into contact with mercury they amalgamate to form an alloy known as amalgam.
The amalgamation of gold with mercury was the principal method of extracting gold from ore until the late nineteenth century, when it was largely replaced by cyanidation.
The sequence of extracting
alluvial gold
using mercury to form an amalgam
Amalgamation
was used, in particular, during the Californian and Australian gold
rushes. The ore was initially crushed in
water in stamp batteries until it was reduced
to fine particles, which then flowed, as a pulp, over thin sheets of copper
coated with mercury. The heavier gold particles settled on the mercury to form
an amalgam that could be heated in a retort, evaporating off the mercury and
leaving gold. The process, however, was hazardous both to workers and the environment.
The revival of amalgamation
in the alluvial gold workings of Brazil,
Venezuela and other countries since the 1980s has led to severe pollution problems.