Mining Methods Vary Widely - Mining and the Environment: From Ore to Metal - page 231

Heap Leaching

Heap leaching is a relatively common process used for the recovery of base metals and
precious metals (gold and silver) from amenable low grade ores or, occasionally, from pre-
viously processed tailings. Heap leaching is a single stage process in which the ore is in a
stationary heap and the solvent percolates through the heap. Some gold mining projects
use heap leaching for low grade ores with conventional vat leaching for higher grade ores.
At other projects where overall grades are lower, all ore may be treated by heap leaching.
Oxidizing bacteria may i rst be used to decompose sulphide minerals, thereby releasing
metals and facilitating the leaching process.
In the heap leaching process, ore is placed to form heaps on prepared leach pads. Leach
solution applied to the surface of the heaps by drip irrigation devices or sprinklers, perco-
lates through the ore and emerges from the base of the heap as 'pregnant solution'; this
solution is collected in drains which discharge to a pregnant solution pond, from where it is
pumped to recover the metals. After processing, the 'barren' solution is stored in a separate
barren solution pond from where it is pumped for re-use. Amenable ores are usually oxi-
dized. For gold and silver ores, weak cyanide solutions with a cyanide (CN-) concentration
typically in the range of 0.02% to 0.05% NaCN are used. The pH value is also important.
Most cyanide leaching is carried out at an alkaline pH of between 10 and 11, depending
upon lab testing of individual ores and the optimum leaching/chemical use rates.
Critical requirements in the leaching process are that the ore is crushed sufi ciently that
the process solution is able to contact all the target minerals; that percolation proceeds
evenly through the heap; and that percolation rates result in partially saturated condi-
tions. In general, these requirements are achieved by crushing to produce a range of par-
ticles in the gravel and coarse sand sizes which provide a permeability of 10 4 cm/sec, or
more. Ores containing excessive i nes (silt and clay sized particles) may be pre-prepared by
agglomeration to form larger particles that enable appropriate permeability to be achieved.
Following completion of leaching, the heap is rinsed by applying water or in areas of
higher precipitation, may be allowed to rinse naturally. Collection and treatment continue
until the quality of drainage from the heap achieves discharge standards. The rinsed heap
may be left in place for rehabilitation or removed to a separate i nal disposal site.
The main features of a heap leaching operation are:

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Ore preparation, involving one or more stages of crushing, and/or agglomeration, if
appropriate;

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Leach pads, which provide the foundation for the heap, and contain the solution by
means of a 'liner system' which may use clay, concrete, asphalt or a geomembrane such
as polyethylene or polyvinyl chloride (PVC);

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The heap itself, which can be placed by end-dumping using trucks, front end loaders,
or conveyors, the main considerations being to avoid damage to the liner and to avoid
segregation of particles;

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The solution application system which may involve sprinklers, or drippers to provide
application rates that produce partially saturated conditions, and avoid surface ponding;

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Solution collection system involving under-drains installed immediately above the pad
liner, either as a gravel layer or a series of closely spaced perforated pipes, draining to
the pregnant solution pond;

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Pregnant solution storage, comprising a pond, lined by clay or geo-membrane to mini-
mize seepage;

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Processing circuit for metal recovery, and

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Barren solution storage, also comprising a lined pond, together with reagent dosing
facilities.