Chemistry Of Cyanidation
The cyanidation is based in that weak sollutions of alkaline cyanide have an action of prefferrential disolution for the gold and the silver contained in one mineral. The generally accepted reaction during various decades is the one proposed by Elsner:
(1) 4 Au + 8 CN + O2 + 2 H2O – 4 Au (CN) 2 + 4 OH
Recent investigations about the mechanism of cyanidation, nonetheless, they indicate that this reaction proceeds in two stages, in which peroxide of hydrogen as an intermediate product is formed:
(2) 2 Au + 4 CN + 2 H2O + O2 – 2 Au (CN) 2 + 2 OH + H2O2
The peroxide of hydrogen reacts partially as follows:
(3) 2 Au + 4 CN + H2O2 – 2 Au (CN) 2 + 2 OH
Adding the equasions 2 and 3, we obtain the classic equasion of Elsner.
The speed of dissolution of the gold is dependent of the concentration of NaCN, of the disposition of the oxygen and of the alkalinity of the sollution. The consumtion of the cyanide, generally is elevated due to the mechanical losses and in its interaction with cyanicide compounds. In the lixiviation in heap leaching of oxidated minerals, the decomposition of the cyanide by the CO2 can be as big as the one caused by the acid constituents of the mineral; this effect can be represented by the following reactions:
(4) NaCN + CO2 + H2O – HCN + NaHCO3
(5) 2 NaCN + H2SO4 – 2 HCN + Na2SO4
The decomposition of the cyanide by the CO2 as much as by the acid materials, is minimized using sufficient alkali, such as lime or caustic soda in the sollution of lixiviation to mantain the alkalinity in a range of pH of 9 to 11.
Some minerals of iron in alkaline solution are oxidated to form ferrous iron and, at the same time, ferrocyanides:
(6) Fe + + + 6 CN – Fe (CN) 6 -4
Many other complex ferric or ferrous salts are also formed, none of which is an effective solvent of the precious metal. What is more, simple sulphur ions derived from minerals which contain sulphur of iron, can insensibilize the surface of the gold, inhibiting the cyanidation.
The minerals of oxidated and sulphured copper are solluble in dilluted sollutions of alkaline cyanide in very varied proportions:
(7) 2 Cu + + + 7 CN + 2 OH – 2 Cu (CN)3 -2 + CNO + H2O
These cyanogenous compounds of copper are not good solvents of gold.
Minerals containing arsenic can also interfere with the cyanidation. The realgar (As2S2) and goldpigment (As2S3) react quickly with the solution of cyanide and inhibit the dissolution of the gold. The arsenopyrrite (FeAsS), nonetheless, is oxidated very slowly and has a very small adverse effect in the lixiviation. The estibine (Sb2S3) inhibits the cyanidation very strongly. The presence of ions of base metals such as Fe2 +, Ni2+, Cu2+, Zn2+ and Mn2+ in the solution, retard the cyanidation of the gold. In some cases the natural occurrence of carbonaceous materials in the minerals sedimentary type, act as absorvents of the dissolved gold. The woods of mining which are corbonized have similar properties which cause a premature precipitation of gold. Organic substances such as rotten wood, oil, grease and reactives of flotation prejudice the cyanidation by consumption of the dissolved oxygen in the solution of lixiviation and also inhibits subsequently the recuperation of gold of the solution of lixiviation by precipitation of gold with powder of zinc.
The activated carbon has the capacity of absorbing the cyanide complex of gold starting from the cyanized pulps and effluents not classified; in this form the stages of solid/liquid separation are eliminated, classification of aeration which are used in a decantation plant in countercurrent and precipitation of powder of zinc. Even though the activated carbon has been used in the recuperation of gold and silver starting from solutions of cyanide during many years, the mechanism of adsorption of gold over activated carbon is not yet totally explained. It is known that the gold is absorbed as a complex of gold cyanide, Au (CN)2.