One of the problems associated with the Anglo elution procedure has been the occasional cementation of gold on the iron pipes and elution vessels. Recently, Kenna  has largely elucidated the reasons for this phenomenon, using electrochemical techniques. Initially, it was thought that iron dissolved to form ferrocyanides. In which case, Pourbaix diagrams indicate that geld should cement out over a wide range of pH and potentials. However, Kenna found that cementation occurred only at high temperatures when the pH was allowed to drop below 9 or rise above 14, and was accelerated by the presence of Cl” m the water. The mechanism depends on the corrosion of ircn to Iron oxides. Between pH 9-14 a passive layer of Fe(OHh prevents cementation, but outside this range, soluble iron species exist allowing the iron metal to reduce the gold from solution. Chloride ion interferes with this passive film giving pit corrosion. Thus for most operators, the simple solution is to rubber-line the columns.
Cementation versus eiectrowinning for gold recovery
Because Anglo elution is a batch process a multi-compartment flow-through cell has been designed to treat each batch of eluate. Gold plates onto the steel wool in the normal way but the several banks of steel wool cathodes ensure that the final eluant contains < 1 ppm gold. However although most gold plants in South Africa use these cells, it still takes much time and effort to finally recover the gold from the steel wool. In Australia this problem is being tackled in various ways. Baxter has shown that a pressurised eiectrowinning cell coupled to a pressure Zadra Unit efficiently electrowins gold at 130 degrees leading to high loadings of gold on the steel wool. Currently Costello is examining refining the gold from loaded stainless steel wool by making it the anode of a second cell and plating out gold foil on a steel cathode plate, whilst Biegler is developing a forced circulation cell to produce gold foil directly from dilute eluates.
Meanwhile, in South Africa, zinc cementation is often used to recover gold from batch eluates because it is quick and simple. Recent electrochemical studies have highlighted the important controlling factors and concluded that for this application there was no need for additives of Pb(N03>2 or to de-aerate solutions. Nevertheless a strict range of conditions was still required to avoid passivation or too high consumption of Zn.
Degradation of cyanide
The trend towards using carbon-in-leach, and high temperature elution procedures, has prompted a recent study at Murdoch University on the degradation of cyanide. The extent of the problem is indicated by the 60-70% cyanide loss during the Anglo elution process reported by Davidson. In summary, cyanide can be lost by a variety of mechanisms. For plant operators, cyanide decomposition by oxygen to cyanate and carbonate; and hydrolysis with water to ammonium formate and carbonate, are of most concern. Hydrolysis is important at the high temperatures used for elution.
Initial results from the laboratory show that half the cyanide is decomposed by hydrolysis in 8 hours at 100° whilst at 120 deg the rate is much faster.
Degradation by oxygen is catalysed by carbon and takes place during leaching. As shown the oxidation of cyanide with air and carbon takes place quite rapidly even at 18’C. Half the CN- is lost in 24 hrs; but in the presence of copper the rate is even faster. These results question the desirability of operating a carbon-in-leach circuit and rationalise the significant build-up of CaC03 on carbon as well as evolution of ammonia experienced by plant operators.