Adapted from: PAMP (Produits Artistiques de Métaux
Précieux), a gold, silver and platinum group metals refinery
based at Castel San Pietro, in Switzerland
The successful application of heap leaching to the extraction
of gold from low-grade deposits has been one of the main factors
in higher output since the 1970s, especially in the United States.
It is a low cost process that extracts a soluble precious metal
or copper compound by dissolving the metal content from the crushed
Ore is heaped onto open-air leach pads with a base of asphalt
or impervious plastic sheeting. A sprinkler system is then laid
along the top of the ore pile through which a solution of dilute
cyanide is sprayed. The cyanide percolates down through the heap
for several weeks, leaching out the gold. This solution, now
enriched with gold, drains off the bottom of the pad into what
is known as the "pregnant pond", from which it is pumped
to the recovery plant.
It should be noted that cyanide is extremely toxic and must
be handled with special care.
Heap leaching of gold was pioneered in the United States in
1973 at Placer Development's Cortez open pit in Nevada and proved
on a larger scale at Pegasus Gold's Zortman Landusky mine in
Montana. Although it is low cost, recovery rates average only
sixty to seventy per cent, significantly less than with conventional
milling. But it has enabled low-grade ores, which otherwise might
not be economically viable, to be processed. In the United States,
where heap leaching is used most extensively, half of all production
is won by this method.
Cyanide ions have a natural affinity for gold, which reacts with
the gold solid particles to make gold cyanide complexes which
are soluble in water. Cyanidation has been the principal method
of extracting gold from ore since the development of the MacArthur-Forrest
Process in 1887, which proved crucial in the development of the
South African gold mining industry.
The perfection of the cyanide process largely replaced amalgamation
with mercury that had previously been the main method of extracting
gold from ore. Cyanidation has also become crucial since 1970
in gold recovery from low grade deposits through heap leaching.
It should be noted that cyanide is extremely toxic and must be
handled with special care.
In the MacArthur Forrest Process the ore is crushed to a fine
powder and circulated through tanks containing a weak solution
of cyanide, which has form tetracyano gold complexes. The complexation
reaction results in dissolving the gold and the remaining rock
pulp is filtered off. Zinc dust added to the cyanide solution
to chemically reduce the gold oxidation state from III to zero.
The gold is now in the metal element state with zero charge
and appears as fine specks of gold to be precipitated out and
the precipitate is then refined.