CALFORNIA

MANUAL OF CYANIDATION

CHAPTER I

NOTES ON THE CHEMISTRY OF THE PROCESS

The usual reaction given for the dissolution of gold and metallic silver in cyanide solution is known as Elsner's equation.

2Au + 4KCN + 0 + H2O = 2KAu(CN)2 + 2KOH

Silver sulphide, the form in which silver most commonly occurs in its ores, involves a different set of reactions, which are usually expressed thus:

First stage, Ag2S + 4KCN

=

2KAg(CN)2 + K2S

This, being a reversible reaction, cannot proceed far before reaching equilibrium, unless the product K2S is removed out of the sphere of action. The potassium sulphide, however, happens to be very sensitive to oxidation so that a change rapidly takes place, probably in two directions,

(1) K,S + KCN +0+ H2O = KCNS + 2KOH

K,S+KCN

(2) 2K2S+ 202 + H2O = K2S2O3 + 2KOH

This thiosulphate would tend later to oxidize to sulphate,

K2S2O3 + 2KOH + 202

=

2K2SO4 + H2O

and perhaps also more sulphocyanate would be formed, K,S,O + KCN = K,SO3 + KCNS.

thus,

The action of cyanide on the haloid compounds of silver, e.g., horn silver, differs from those already given in that oxygen does not appear as an indispensable auxiliary:

AgCl + 2KCN = KAg(CN)2 + KCl

Reaction of Cyanide with Silver Sulphide. In connection with the reactions usually assumed to explain the action of cyanide on silver sulphide it is interesting to note that a solution of the double cyanide of potassium and silver in presence of an excess of potassium cyanide will tolerate the presence of an appreciable amount of soluble sulphide without precipitation of the silver. The reaction expressed by the equation

Ag2S+4KCN = K2S+2KAg(CN)2

is reversible, and its direction depends on the relative proportions of free cyanide and soluble sulphide, and the degree of their concentration. J. W. Sharwood in a letter to the Mining and Scientific Press (Sept. 20th, 1908) says

"The reaction has been more recently studied by Berthelot who found that in dilute solutions nearly 100 molecules of KCN were required to balance one molecule of K2S in order to retain silver in solution, instead of four molecules as the equation seems to indicate. To be exact, the equation given by Berthelot as representing the conditions of equilibrium is

96KCN + Ag2S = 2KAg(CN)2 + 92KCN + K2S

Now this proportion (96 mol. KCN to 1 mol. Ag2S) means that 96 × 65 parts of KCN are required to dissolve and hold in solution 2 × 108 parts of silver in the form of sulphide, or 28.9 to 1, if none of the sulphide is oxidized."

The above proportions, however, would appear to be different for different concentrations of cyanide because Sharwood states in another place that

"if we dissolve a fairly large amount of Ag2S in strong solution of KCN and dilute it shortly afterward more or less of the Ag2S will be reprecipitated proving that some of the sulphide radicle remained in the solution;"

and showing also that the degree of concentration of the cyanide as well as its amount materially affects its capacity of holding silver in solution against the opposing force of Na2S. Of course if the Na2S is removed out of the sphere of action either by oxida

tion or precipitation as an insoluble sulphide the equilibrium is disturbed and silver sulphide continues to dissolve.

In the Mining and Scientific Press of September 26th, 1914, Harai R. Layng advances an interesting theory that silver in the form of the sulphide goes into solution as a sulphocyanate dissolved in cyanide solution, with the formula AgCNS.KCN. While many might not be prepared to accept without reservation his statement, rather positively made, that silver "usually enters the solutions in this form" yet the possibility is worth considering, and may throw light on some of the obscure phenomena of the process. He does not give any equation to illustrate the reaction, but the following is suggested by the present writer. Ag2S+4KCN+O+H2O=AgCNS.KCN+KAg(CN)2+2KOH

The Function of Oxygen.-Oxygen appears to be an indispensable factor, either directly or indirectly, in the dissolution of gold and silver by cyanide solutions, except in the case of the haloid compounds of silver. Whether the action of oxygen in the dissolution of gold be a direct one, as illustrated by the Elsner equation, or an indirect one, in the sense of acting merely the part of a depolarizing agent, as maintained by Julian and Smart,1 will not affect the general statement that it is a necessary adjunct for the dissolution of the precious metal.

The most generally useful agent for this purpose is atmospheric oxygen, and in many instances sufficient oxygen is absorbed by the solutions in their circulation through the plant to accomplish all that is necessary. Ores that contain reducing constituents may need more oxygen than is obtained in this way, and the additional amount is most easily supplied in the case of percolation by draining dry between washes, and by the use of air lifts or centrifugal pumps, in agitation.

The addition of chemical oxidizing agents, such as potassium ferricyanide, permanganate, sodium peroxide, and ozone, has not been attended with much success, both on account of their cost, and also because of their tendency to oxidize the cyanide to

1 Cyaniding Gold and Silver Ores, page 71 (Second Edition).

cyanate (KCNO), which is useless for the purposes of the process. They have sometimes been found useful, however, for oxidizing a highly reducing pulp before adding cyanide, as in the case of slime which has stood in dams for long periods, undergoing partial oxidation with formation of ferrous compounds.

The only reagent of the oxidizing class that has attained to any importance commercially is bromocyanogen, BrCN, and that only in the raw treatment of telluride and mispickel gold ores.

Julian and Smart1 consider that the activity of this compound is not due to the liberation of cyanogen, though that probably occurs, but to a liberation of oxygen, in the sense of the equation. 2BrCN + KCN +4KOH = 2KBr + 2KCN +

KCNO + 2H2O + O

Reducing Agents.-Oxygen being necessary in almost every instance for the dissolution of gold and silver by cyanide, it follows that any substance that has the property of denuding the solution of its dissolved oxygen will retard or completely stop the action of the cyanide. It is a common practice to test working solutions for their "reducing power" by acidifying and titrating with standard permanganate. Such a test, however, is misleading in its bearing on the dissolving efficiency of a solution, because it includes the reducing effect of such substances as ferrocyanides and sulphocyanates which have not the power of absorbing oxygen from solutions and therefore do not retard the dissolving effect of the cyanide on account of their reducing character, in fact, they are not reducers when considered from that standpoint. If, then, an estimate of the detrimental reducers present be desired the ferrocyanides and sulphocyanates should be determined separately and their equivalent in terms of standard permanganate deducted from the "total reducing power," as already found.

permanganate = 0.001619 grm. KCNS

N

1 cc

10

1 Cyaniding Gold and Silver Ores, page 81 (Second Edition).