The Use of
Cyanide in Mining
The
use of cyanide as a chemical to dissolve gold from finely crushed ore is almost
universal in the gold mining industry. After leaching of the ore by a dilute solution
of sodium cyanide, the gold component is absorbed onto charcoal, filtered off,
and the excess cyanide solution is stored in large ‘tailings ponds’. Some mines chemically destroy the excess cyanide,
but often it is simply discharged in very dilute form under conditions specified
in a government licence. The rupture of
the tailings pond or seepage of liquid into the ground below the pond can release
the cyanide to the outside environment.
There
are a number of recent reports of alternative technologies for use in extracting
gold from the waste rock, none of which are in widespread use yet.
Although the routine handling of cyanide at gold mines around the world has generally been carried out with a high level of safety, some accidents have nevertheless occurred in recent years, affecting wildlife and river systems in the vicinity of the mine. National cyanide handling guidelines and regulations have been prepared in some countries, for instance the Best Practice Environmental Management in Mining (BPEM) booklet on “Cyanide Management” available from the Australian Minerals and Energy Environment Foundation (AMEEF), and the “Cyanide Management Guidelines” produced by the Department of Minerals and Energy - Western Australia .The International Council for Metals and the Environment (ICME) has produced an information booklet “The Management of Cyanide in Gold Extraction” for international use. The Journal “Mining Environmental Management” has focused on cyanide management in one of its issues.
A number of cyanide remediation methods exist some of which are employed at mining operations to reduce the cyanide content of tailings effluent. These are discussed in detail in a paper "Cyanide Remediation: Current and Past Technologies" by C.A. Young and T.S. Jordan from the Department of Metallurgical Engineering at Montana Tech (Adobe PDF - 532kB).
The Cyanide Extraction Process
The most common processes that use cyanide to extract gold are Merrill-Crowe recovery which uses zinc powder to precipitate the gold from solution, Carbon in Column (CIC), Carbon in Pulp (CIP), Carbon in Leach (CIL) a variation of CIP, and Heap Leaching. The CIP process is currently one of the most favoured in modern large scale mines, a general description of which is provided below.
A brief overview of the various gold recovery processes available, and in particular the processes using cyanide, can be found at the Denver Mineral Engineers Inc. website.
The Carbon in Pulp (CIP) Process
The
gold bearing ore is crushed, then finely ground to around 75 microns size, with
water added to form a slurry or 'pulp'. The pulp is dosed with cyanide,
and agitated through a series of tanks, in the process of leaching the gold from
the ore particles and into solution. This process is termed cyanidation.
On completion of cyanidation the pregnant pulp is transferred to the Carbon
In Pulp (CIP) process. Figure 1 below illustrates the CIP process from
the completion of cyanidation onwards.
The pregnant pulp is passed progressively
through a number of tanks in series (generally around 5 or 6). These are
mechanically stirred, or air agitated. Granulated carbon is passed through
the tanks countercurrent to the flow of the pulp. The granulated carbon
is formed from roasted coconut husks and is of a significantly larger particle
size than the ground ore particles. This allows the carbon to be pumped
countercurrent to the pulp through the use of screens sized to allow the finely
ground ore to flow through and the coarser carbon to be contained. As the
carbon passes through the tanks it collects increasing quantities of gold from
solution, termed loading. The loaded carbon is seperated from the pulp in
the final tank and transferred to the elution circuit. The barren
pulp is dewatered, to recycle water and cyanide for reuse in the process, and
the remainder, termed tailings, is transferred to the tailings dam. The
tailings will contain some cyanide.
Figure 1: Carbon in Pulp (CIP) gold recovery process. |
The loaded carbon is treated in the elution
circuit with a hot cyanide and caustic solution to remove the gold from the carbon.
The barren carbon is transferred to the reactivation circuit, then reused in the
CIP process. The cyanide and caustic solution is passed through a electrowinning
circuit where the gold is transferred to steel wool (or alternatively, it can
be precipitated using zinc dust). The barren cyanide and caustic solution
is recycled to the elution circuit. The gold plated steel wool is transferred
to the smelting circuit to produce gold bullion, an alloy of gold, silver
and other metals such as copper and iron.
Further information on the
CIP process can be found on the Mintek
website.