Converting Minerals to Metals - Mining and the Environment: From Ore to Metal - page 264

The discussion that follows includes material from Environment Australia (1998),
which provides an excellent synthesis of information, and to which the reader is referred
for further details.
Prior to dissolving or leaching of the gold, most types of ore must first be crushed and
ground to liberate or expose the individual grains of gold. The ground ore is mixed with
a solution of sodium cyanide in an alkaline solution. Typically, between 0.5 and 2 kg of
sodium cyanide is used for each tonne of gold-bearing ore. Both high alkalinity (typically
pH 10.3) and salinity are essential to ensure that most of the cyanide is present as CN ion,
minimizing the formation of hydrogen cyanide (HCN) gas, which is highly toxic.
The process in which gold and other metals are dissolved in cyanide solution is known
as cyanidation and the resulting solution is known as 'pregnant liquor'. Gold is dissolved
according to the following reactions:

Prior to dissolving or leaching of
the gold, most types of ore must
fi rst be crushed and ground to
liberate or expose the individual
grains of gold.

↔ 2Au(CN) 2 2OH (Bodlander's Equation)

(6.10)

H 2 O 2

2Au 4CN O 2 2H 2 O

8CN ↔ 4Au(CN) 2 4OH (Elsener's Equation) 4Au O 2 2H 2 O

(6.11)

Apart from the gold cyanide ions, cyanide forms a variety of complex ions with other
metals - for example:

Fe 2 6CN Fe(CN) 6 4

(6.12)

↔

The cyanide-metal complex ions differ considerably in their toxicities and stabilities, fac-
tors that are highly important in determining detoxification requirements.
In practice, the optimization of gold extraction is a highly complex process, requiring
close attention to cyanide concentration, pH, salinity, and sometimes oxygen. For optimum
leaching conditions, a pH value of somewhere between 9.5 and 11.0 should be maintained,
depending on the requirements of the ore being treated at the time. Generally the pH is
maintained at 10.5 by the addition of lime or sodium hydroxide to the slurry. This is essen-
tial to prevent the loss of sodium cyanide in solution to gaseous hydrogen cyanide (this
would obviously have safety implications) which would result in high cyanide consumption.
The time required to complete the cyanidation reaction varies depending on the gold
particle size, presence of other metals, cyanide concentration, oxygen concentration, and
amount of agitation provided. Optimum concentrations of reagents will vary, not only
from ore body to ore body, but within the same ore body in accordance with variations
in gold grades and the presence of other cyanide complexing metals. It is relatively com-
mon that 'spikes' in concentration of one or more base metals or the occurrence of some
carbonaceous substances, cause rapid depletion of cyanide, requiring addition of more cya-
nide solution to maintain gold extraction. Subsequently, when base metal concentrations
decline to normal levels, there may be an excess of cyanide in the pregnant leach solution.
For this reason, it is common practice to blend ores of differing composition, prior to the
crushing stage, in an attempt to maintain a feedstock of uniform grade so that variation in
cyanide consumption is minimized. In modern CIP and CIL plants, the concentration of
cyanide, pH, and salinity are monitored continually so that frequent adjustments can be
made, based on the reactions as they occur. However, it is unavoidable that variations in
the ore will cause variations in the consumption of cyanide which will be reflected in the
chemical quality of the pregnant liquor and, ultimately, the tailings liquor.
This text uses the example of the CIL process to illustrate ore processing and refining
using cyanide. In this process gold is extracted from the ore, after crushing and grinding,

The optimization of gold
extraction is a highly complex
process, requiring close attention
to cyanide concentration, pH,
salinity, and sometimes oxygen.