Here I present an Process EXAMPLE of Gold Extraction Cyanide in which the cyanidation feed consists of a pyrite concentrate floated after the selective flotation of a copper-gold concentrate. The pyrite concentrate is reground to 90% minus 325 mesh and aerated in a high-lime solution prior to cyanidation. It contains 99% sulphides of which 10 to 20 percent is pyrrhotite and 0.3 to 0.7% chalcopyrite, the remainder being mostly pyrite. The gold content is 0.10 to 0.13 oz. per ton, none of which is free; the gold particles are attached to pyrite, and in general they are less than 1 micron in size. Cyanidation is conducted under conditions of intense aeration for a period of 6 to 8 hours. Residues run about 0.04 to 0.06 oz. gold per ton. Cyanide consumption is about 2 lbs. NaCN per ton.

The conditions outlined by the authors for optimum gold dissolution and minimum cyanide consumption are: The pulp must be well aerated, the cyanide solution should be saturated with calcium sulphate, it should have at all times a minimum alkalinity of 1.0 lb. of CaO per ton, and the cyanide strength should not be more than 0.35 lb. NaCN per ton.

Here we compared Pachuca aeration with bottle aeration and agitation; they also investigated the effects of various sulphates in solution on gold extraction and cyanide consumption. Pachuca aeration gave better gold extraction and lower cyanide consumption than bottle agitation. With the former more copper was found in solution, and less thiocyanate was formed; with bottle agitation the reverse was the case, in addition to which copper cyanogen complexes were found in the filter cake. From this it was concluded that gold dissolution depends not only on the presence of free cyanide but also on the concentration of copper cyanide complexes in the cyanide solution. Optimum results were obtained when the weight of total NaCN by distillation to copper varied between 2.0 and 2.6 to 1. An analysis of a mill pregnant solution is shown here.

Mineral Processing engineers found that the addition of ammonium sulphate improved the results obtained in the Pachuca; the residue was reduced by 0.005 oz. of gold per ton and cyanide consumption by about 0.3 lb. per ton. The degree of improvement derived from ammonium sulphate increased as the pyrrhotite content of the cyanidation feed increased. Less thiocyanate was formed although there was not much difference in the amount of copper dissolved. Other sulphates were tested, and proved that the beneficial effect was due to the sulphate radical; calcium sulphate was as effective as ammonium sulphate. For this reason the solution resulting from the regrinding and preaeration of the cyanidation feed in high lime solution was found to he beneficial in the cyanide circuit as it was saturated with calcium sulphate. In addition it contained 2 to 3 lbs. per ton of thiosulphate and had an initial oxygen content of less than 2 mg. per litre. The thiosulphate was inert as regards to this EXAMPLE practice; at normal temperatures practically no reaction took place between thiosulphate and cyanide. The authors consider that the sulphate radical acts, to some extent, as a buffer against the oxygen demand of the pyrrhotite and pyrite and thus inhibits the reactions taking place between these minerals and cyanide in the presence of oxygen.

Suggestions that the copper cyanogen complex in the mill solutions at this EXAMPLE PLANT  is of the type Na2Cu(CN)3 and that this complex would be equal in dissolving effect to a cyanide solution containing about 0.004% NaCN. When the gold in an ore is very fine, as it is in the cyanidation feed at this EXAMPLE PLANT, and with no interfering factors, a solution of this type could be adequate for optimum extraction. Gold dissolution is possibly due to the relatively small amount of CN ions resulting from the dissociation of the sodium cuprocyanide. When other constituents of the ore besides gold are competing for the available CN ions, gold extractions would be affected adversely. By the reaction of such CN ions, to form thiocyanate for example, the ratio of total cyanide to copper would drop and the equilibrium of the sodium cuprocyanide would be affected. In order to restore equilibrium, either relatively insoluble copper cyanogen complexes such as NaCu(CN)2 or CuCN would be thrown out of solution, or more CN ions would have to be added to the solution. This would possibly explain the presence of cyanogen complexes in the filter cake when increased amounts of thiocyanate were present in solution, as they were with less intense aeration. As regards the decomposition of sulphide minerals such as pyrrhotite in cyanide solutions under various conditions of ‘ aeration, it was suggested that this could take place by two series of reactions simultaneously. The first, in the stages pyrrhotite, alkaline sulphide, thiosulphate and sulphate, while the second would be pyrrhotite, alkaline sulphide and thiocyanate; intense aeration favors the former series of reactions with the result that less cyanide is wasted as thiocyanate and, therefore, more is available for copper and gold dissolution. (These reactions will be discussed in more detail in the next section.) Thus, there will be a greater reservoir of CN ions to draw on and, therefore, less possibility of copper cyanogen complexes being thrown out of solution. The presence of sulphate radical also prevents to some extent the formation of thiocyanate but its effect is much less marked than that of intense aeration.