Mining in focus
Amalgamation is a concentrating process in which
metallic gold is mixed with mercury, either in an
amalgamation drum or table.
With the advancement of
technology after the Industrial
Revolution, the means to evaluate
mineral properties along with their
metallurgical behaviour became
available. Experimentation with
various chemicals such as acids were
conducted to find ways to leach out
the gold metal and then recover
it from a liquid solution. Highly
corrosive acids such as hydrochloric
[20] MINING MIRROR APRIL 2018
and hydrofluoric acids were used
with fair success. However, they
are extremely dangerous to work
with and large concentrations
of the acid remain in the waste
material (tailings) produced, which
is difficult to treat and discard in an
environmentally safe manner.
In 1887, John Stewart MacArthur
developed the cyanide process
for gold extraction. During the
Gold processing has advanced to such an extent that it is now profitable to
re-mine old mine dumps. Johannesburg-based gold mining company DRDGold
operates its gold processing plant in the east of Johannesburg.
cyanidation process, a slurry of
ground ore is mixed with a sodium
cyanide solution in the presence
of activated carbon. The carbon
has a very high affinity for the
aurocyanide complex and adsorbs the
gold out of the solution, resulting
in very high gold loadings on the
carbon. At the end of the leach,
the loaded carbon is removed from
the slurry and the adsorbed gold is
stripped out at high temperature
and pressure with sodium hydroxide
and cyanide solutions to form a
high-value electrolyte solution.
Gold bullion is then recovered from
the electrolyte by electro-winning.
There are many variations, such
as carbon-in-leach (CIL), carbon-
in-pulp (CIP), resin-in-leach, and
heap leach, but the basic cyanidation
principles apply.
Ore that has limited sulphide
compositions or that has oxidised
over time, leaches extremely well.
Refractory ores, because of their
sulphidic composition, do not leach
well. Therefore, pre-treatment or