In simple terms, heap leaching means the removal in solution of
valuable elements from a heap of material, and their recovery
from that solution. The heap of material can consist of essentially unprocessed
ore, sub-ore, low grade gangue and country rock, tailings from
other upgrading processes, or artificial materials such as slags or
furnace and flue products. The solvents can be solutions of acids (commonly sulphuric
acid because of cost) or of other chemicals such as cyanides.
Because of cost factors the solutions are aqueous; because of
this, leaching operations may be curtailed in cold-climate areas.
The guiding principle must surely be `large volume / low cost'. Since solutions are generally easier to manage than gases, there
is a strong incentive for replacing roasting of materials by
leaching, eg arsenical gold ores. Where sulphides are present in the material to be leached, these
may provide the necessary acid, with or without the aid of
bacteria, to dissolve and extract the recoverable metals. The mineralogical aspects of the operation relate to the nature
of the `ore' and gangue minerals present in the heap, their textural
relationships, and their effect on the process itself.