Tank leaching, also called vat leaching, is a hydrometallurgical
method of extracting valuable material (usually metals) from ore. It involves
placing ore, usually after size reduction and classification, in large tanks or
vats containing a leaching solution. Often the vats are equipped with agitators
to keep the solids in suspension in the vats and improve the solid to liquid
contact. This is further assisted by the use of tank baffles to increase the
efficiency of agitation and prevent centrifuging of slurries in circular tanks.
Aside from chemical requirements, two key factors influencing extraction are
retention time and shear within the leach tanks. Retention time refers to the
total volumetric capacity of the leach tank(s) divided by the volumetric
throughput of the solid/liquid slurry. Retention time is commonly measured in
hours for precious metal recovery. A sequence of leach tanks is referred to as a
leach "train", and retention time is measured considering the total volume of
the leach train. Broadly speaking, shear refers to energy transfer into the
slurry from power input -- e.g. agitation. The degree of energy input in
conjunction with the tank configuration determines kinetics affecting solid
liquid contact.
After vat leaching, the leached solids and pregnant solution are usually
separated prior to further processing.
The method is commonly used to extract gold and silver from ore.
Dump leaching is an industrial process to extract precious metals and
copper from ores.
Dump leaching is similar to heap leaching, however, in the case of dump
leaching, ore is taken directly from the mine and stacked on the leach pad
without crushing; in the case of gold and silver, the dump is irrigated with a
dilute cyanide solution that percolates through the ore to dissolve the precious
metals. The solution containing gold and silver exits the base of
the dump, is collected, and precious metals extracted. The resultant barren
solution is recharged with additional cyanide and returned to the dump.
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