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Effect of pH on Cyanidation Process (6 replies)

9 months ago
Amar 9 months ago

Apart from generating HCN, what are the other effects if pH in cyanidation process is low?

Dizzy Flores
9 months ago
Dizzy Flores 9 months ago

Running your CIL at a lower pH the causes the reaction to be more thorough rather than faster at lower pH conditions. What this means is- The lower pH may increase the number of free CN- ions and therefore as the concentration of dissolved CN ions is the main driver of dissolution rate of gold then the lower pH stands to drive the reaction faster or more thoroughly.

This increase in free CN ions at a lower pH may only be apparent as at higher pH conditions, you would have added a lot more alkaline and therefore be masking the CN ion content.

Another reason- A high pH would indicate heavy dosing of lime which will increase the tank viscosity which would keep the Carbon in suspension well but also slow the flow rate and perhaps inhibit the tank mixing due to slow viscous dead spots in the tank. Having Higher Densities is much more important.

9 months ago
OberstGruppen 9 months ago

As mentioned above yes, increase in free CN ions is also apparent at higher pH and this may result in the undesirable formation of cyanate (CNO-) through the oxidation of hydrogen cyanide & free cyanide. CNO doesn’t dissolve gold and thus reduces the free CN available for Au dissolution

Copper and other metal oxides present may also form metal cyanide complexes with excess free cyanide and be adsorbed onto the carbon.

As mentioned, high pH is a sign of lime overdose and apart from the effects of lime overdosing stated above, excess calcium coating may be observed on carbon surface hindering adsorption and this will be transferred to the EW circuit/smelting upon stripping. Another problem observed from lime overdosing in CIL is carbon interstage screen frequent blockages due to lime grits.

John Koenig
9 months ago
John Koenig 9 months ago

Hydrogen cyanide is a weak acid. At lower pH it tends to form un-dissociated HCN form. It means less free cyanide anions.
Then, could you clarify what form easier tends to convert into cyanate, hydrogen cyanide or cyanide anion.

Sandeep Bisht
9 months ago
Sandeep Bisht 9 months ago

Briefly, the concentrations of HCN & free cyanide are equal in slurry/solution at pH -9.4; at pH 70% of cyanide is lost as HCN gas; the conversion of free CN to cyanate (CNO-) requires strong oxidizing conditions (Inco process, per oxygen chemicals, ozonation etc) & is therefore unlikely under CIL/CIP aeration conditions and yes, pH >11 will affect slurry rheological characteristics, C suspension & C passivation or reduced adsorption.

Carmen Ibanz
9 months ago
Carmen Ibanz 9 months ago

Lower the pH - Increase HCN generation which will gas off from the leach/adsorption tanks - Increase risks of HCN poisoning to employees. For mine that is most pressing reason to ensure pH is kept high enough to prevent HCN generation.

9 months ago
Obergruppenfuhrer 9 months ago

CN- is required to leach gold and thus at decreasing pH's the quantity of CN- decreases due to the formation of HCN as pointed out above. Thus, the gold leach reaction can become cyanide limited if the CN- concentration gets too low - can compensate by having higher cyanide addition but also get higher HCN and hence HCN loss! Increase in oxygen reduction potential with decrease in pH may increase gold leach rate if the reaction is not diffusion limited; I'd however doubt this benefit would ever outweigh the HCN losses.

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