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ANNEXE 7
GENERAL DESCRIPTION OF CYANIDE CHEMISTRY IN GOLD
PLANTS
1
...

This slurry reports to agitated tanks where cyanide solution is added to leach gold from the
ore into an aqueous (dissolved) form
...
This carbon is then recovered from the slurry by
screening
...
Gold is then recovered from the eluate by electrowinning
and the electrowon gold is smelted into bars
...


2
...
J
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McArthur, a metallurgical chemist in Scotland, realised that
Elsner’s discovery of the solubility of gold in weak potassium cyanide solution might be of
value
...
In 1887
the first cyanidation patent was registered in Great Britain
...
Very few
metals are noble metals and noble metals are by nature unreactive
...
Leaching is the process through which cyanide
reacts with the gold in the ore to produce a gold-cyanide complex
...
It can be seen that both oxygen and cyanide are
necessary for this reaction to proceed
...
Other ore constituents (e
...
some sulphide minerals) in the
treated pulp are consumers of oxygen
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Some plants incorporate
the use of liquid oxygen or peroxide as sources of oxygen when necessary
...
If ignored, this
would destroy sodium cyanide by formation of hydrocyanic acid gas
...

The Effect of pH
pH refers to the alkalinity or acidity of the solution
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5
indicating neutrality
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5 and 11
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Generally the pH is maintained at 10
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This is essential 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
...


CYANIDE DETOXIFICATION
GENERAL
The SO2/AIR Cyanide Detoxification Process oxidises both free cyanide (CN-) and cyanide
weakly complexed with metals such as copper, zinc and nickel to cyanate (OCN-)
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The cyanide strongly
complexed with iron can be removed as an insoluble ferrocyanide salt by the process
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Temperature is ambient and the pH range is between 8
...
5
...
Sodium
hydroxide (NaOH) or lime is added to neutralise sulphuric acid generated in the process and
to maintain the desired pH level
...

The solution or slurry to be treated is contacted with the reagents in a simple mixing tank
...
The oxygen requirement for the process
is normally supplied by adding large volumes of air
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Retention times
vary depending on the solution composition being treated, but generally range from 1 to 3
hours
...
Dissolution of these precious metals in alkaline cyanide solution to form
stable complex cyanide anions depends on excess cyanide in solution
...
The oxidation of free cyanide to cyanate in the
Cyanide Destruction Process can be represented by the equation:

CN − + SO2 + O2 + H 2 O → OCN − + H 2 SO4

(1)

Reaction (1) requires dissolved copper to act as a catalyst
...
The theoretical SO2
requirement, according to reaction (1) above, is 2
...

However, the SO2 dosage is greater due to its consumption by other chemical reactions
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0 to 9
...

Temperature has little effect on the cyanide oxidation reaction, but the type of effluent,
solution or slurry, can have a large effect on the reaction (usually determined in laboratory
test work)
...
Owing to the low solubility of
oxygen in water, and its low transfer rate between gas phase and water, only a small
fraction of the oxygen added at the bottom of the reactor is actually transferred to the
solution
...
Insufficient
oxygen transfer will limit the destruction performance
...
Reactive sulphides such as pyrrhotite
can also react with cyanide to form thiocyanate, or produce partially oxidised species such
as thiosulphate
...

Weakly complexed cyanides of copper, zinc and nickel are dissociated as follows:

Cu (CN ) 3

2−

→ Cu + + 3CN −

(3)

Zn(CN ) 4

2−

→ Zn 2+ + 4CN −

(4)

Ni (CN ) 4

2−

→ Ni 2+ + 4CN −

(5)

Annexe 7 Page 3 of 4

Cuprous copper in solution is oxidised in the Cyanide Destruction Process to the cupric ion,
as follows:

2Cu + + SO2 + O2 + 2 H + → 2Cu 2+ + H 2 SO4

(6)

The liberated metal ions precipitate as metal hydroxides (below) and the liberated cyanide is
oxidised to cyanate, according to equation (1):

Cu 2+ + 2OH − → Cu (OH ) 2

(7)

Zn 2+ + 2OH − → Zn(OH ) 2

(8)

Ni 2+ + 2OH − → Ni (OH ) 2

(9)

The cyanate ion (OCN-) is unstable, and hydrolyses to ammonium and carbonate ions, as
follows:

OCN − + 2 H 2 O → CO3

2−

+ NH 4

+

(10)

This reaction is accelerated at low pH
...
The ammonium (NH4+) ion is in equilibrium with ammonia (NH3) with some
escaping the slurry as NH3 gas and some reporting to the solid phases present

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