Converting Minerals to Metals - Mining and the Environment: From Ore to Metal - page 263

where
V

mass of liquid extractant (B entering, A leaving)

YB concentration entering with solvent (zero if fresh solvent)

YA concentration in solvent leaving

L mass of retained liquid on inert solids (A entering, B leaving)

XA concentration of solute in adhering entering liquid

XB concentration in adhering liquid leaving process

Not only does VAT leaching allow several stages of leaching, it enables the metallur-
gist to modify other parameters such as pressure or temperature to obtain an optimum
environment to leach metals from minerals. Two examples of hydrometallurgical plants
follow.

The Use of Cyanide Solution to Dissolve Gold

In the late nineteenth century amalgamation with mercury was the main method for
separating gold from its ore. By the 1880s, however, problems were being experienced in
the South African gold mines as workings deepened and the sulphide content in the ore
increased, markedly reducing the effectiveness of amalgamation. The cyanide process,
introduced in South Africa in 1880 (Kettel 1982), represented a vast improvement over
amalgamation and other earlier methods, and has been extensively used ever since. The
use of cyanide leaching of gold is a hydrometallurgical process, and is the most effective
way of extracting fine gold particles from ores. Cyanide (CN) is a compound of carbon
and nitrogen, two of the most common elements in the Earth's crust. It is produced natu-
rally in a number of micro-organisms, insects, and plants. Today it is a chemical manufac-
tured for use in a number of important industries. About 20% of all manufactured cyanide
is used in the form of sodium cyanide for mineral processing (TRI/Right-To-Know
Communications Handbook, Section 5).
The reason that cyanide is so widely used in gold mining is that it is one of the very few
chemical reagents that will dissolve gold in water, using only oxygen from the air as an
oxidizing agent. Other chemicals will work but only in much higher concentrations using
much stronger oxidizing agents, e.g. chlorine, nitric acid or hypochloride. At these higher
concentrations they can be more dangerous to handle than cyanide. In commercial mining,
cyanide has thus become the chemical of choice for the recovery of gold from ores. Used in
metal extraction since 1887 it is safely managed in gold recovery around the world. In 2000,
there were about 875 gold or gold and silver mining operations in the world. This number
does not include the contribution from base metal mines where some gold is recovered as a
by-product at the mine or the smelter. Of those 875 sites, 460 (i.e. 52%) used cyanide, of
which 15% were heap leaches and 37% used cyanidation in tank leaching. The remaining
48% used a variety of processes, primarily gravity separation and flotation to form a concen-
trate. These concentrates were then sent to a smelter for final processing (Mudder 2000).
In general, there are two basic types of cyanidation operations, tank leaching and heap
leaching. Tank leaching involves one of three distinct types of operations, Carbon-in-Pulp
(CIP), Carbon-in-Leach (CIL), and the Merrill Crowe Process. In Carbon-in-Pulp opera-
tions, the ore pulp is leached in an initial set of tanks with carbon adsorption occurring in
a second set of tanks. In Carbon-in-Leach operations, leaching and carbon recovery of the
gold values occur simultaneously in the same set of tanks. The Merrill Crowe process uses
zinc to remove the gold from solution and is generally used for ores that have high silver
to gold ratio.

The reason that cyanide is so
widely used in gold mining is that
it is one of the very few chemical
reagents that will dissolve gold
in water.