BAUXITE deposits generally occur at or near the surface and are usually covered with shallow layer of overburden. The ore is almost invariably mined by open pit methods. The usual practice is to thoroughly strip and clear the overburden, after which the ore body is blasted to permit excavation by mechanical shovels. Some deposits contain highly siliceous clays and ferruginous materials distri-buted intimately throughout the ore, and it is often necessary to beneficiate the bauxite to provide a desired quality of product.
A primary objective of the beneficiation process is
to reduce the silica content in the ore, as the latter
reacts with caustic soda during the digestion step to
precipitate out as insoluble sodium aluminium silicates,
e.g. 2Na2O.2Al203.3Si02.2 H20. This salt is eliminated
from the process along with the insoluble iron & titanium oxides in red mud, consequently entailing the loss of valuable alumina and caustic soda from the process
