Mining in focus
An intricate system of mobile and
flexible equipment and conveyors
is required to move the ore from an
open pit to the leach pad.
Heap leaching has
become a popular
method of extracting
gold, writes Nicolaas
C. Steenkamp and
Leon Louw.
H
eap leaching is used to extract
gold, copper, silver, uranium,
and iodine. This method was
first employed to extract copper at the
Bluebird mine in the US in the early
1960s, and then by several gold mines in
the western parts of the US a few years
later. Since then, heap leaching has been
adopted successfully by many mines
across the world.
Heap leaching occurs after the
mining and crushing of low-grade
ores, usually in an opencast mine.
High-grade ores and ores not amenable
to cyanide leaching at coarse particle
sizes require further processing
to recover the gold values. These
processing methods can include further
grinding, concentration, pressure
oxidation, and roasting, which is used
to treat these ores to expose the gold
particles prior to cyanidation.
The crushed ore undergoes an
agglomeration stage, after which the
agglomerated ore is deposited onto
the heap leach pad. The heaped ore
is irrigated with a lixiviant (a liquid
medium) to dissolve the metals and
generate the leachate. The lixiviant
will depend on the target metal being
extracted.
The pad is compacted and then
lined with a high-density polyethylene
membrane, which prevents toxic
compounds and elements (such as
cyanide and the leachate solution) from
entering the groundwater system.
The leachate is collected in a pond or
tank, and it is referred to as a pregnant
or value-bearing solution. The solution
is then processed to recover the metals.
In gold operations, recovery is affected
through carbon adsorption or the
Merrill-Crowe process. The barren
solution, together with additional
lixiviant, is recycled back to the heap.
Heap leaching can take anything
from a couple of months to several
years. In the case of gold recovery, heap
leaching generally requires 60 to 90
days to leach the ore, compared to the
24 hours required by a conventional
agitated leach process. Gold recovery
is also usually only 70% compared
JUNE 2018 MINING MIRROR
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