The importance of oxygen in the cyanidation process is well known and as with most leaching and chemical processes requiring oxygen, it is normally supplied by direct introduction of air, which is almost inexhaustible and also an inefficient source of oxygen. The addition of oxygen was studied in research laboratories and mining operations more than fifty years ago. Probably, the first industrial application of pure oxygen in gold dissolution in South Africa was in Consolidated Murchinson in 1983. The next year other gold operation, Rand Mines Crown Sand employed the new cyanidation process. With the time more operations such as Simmergo, Rogold and Southgo adopted the process. The addition of oxygen can be done into pre-conditioners and/or cyanidation tanks.
Considering the fact that the gold dissolution process follows Elsner’s equation, the effect of cyanide concentration on the cyanidation of gold has been found to have an optimum value of around 0.020% to 0.050% NaCN. Nevertheless, with excess of cyanide, a relatively small surface of gold exposed and if agitation is sufficiently intense, the controlling factor that influences the dissolution of gold is the dissolved oxygen concentration of the cyanide solution. In this way, the rate of dissolution of gold in 0.1% NaCN solution employing nitrogen, oxygen and mixtures of the two indicates that the best rate dissolution is obtained when there is not nitrogen. Consequently, the gold cyanidation rate is directly proportional to the oxygen content and higher dissolved oxygen level ensures a higher dissolution rate and a lower residual gold content.
The impurities present in gold ores are large consumers of cyanide and this represents an additional and costly use of cyanide. Basically, oxidation of these materials, if effected prior to the addition of cyanide solution can produce the reduction of overall cyanide consumption. Oxygen demand in gold containing slurries is due to impurities as well as gold dissolution reaction. Some of these impurities are organic matter, ferrous substances and pyrrhotite and can be the major part of oxygen demand during cyanidation. In this way, the oxygen requirement to satisfy the chemical oxygen demand of reclaimed materials has been shown to exceed the oxygen transfer capacities of several designs of mechanical agitators. In this way, the use of compressed air injected into these tanks is usually unable to meet the oxygen demand and seriously affects the agitation efficiency.
Work to date has shown that the use of pure oxygen in gold cyanidation can have important beneficial effects. The nature and level of benefit observed is influenced by several factors such as ore characteristics, leaching tank and control of the operation. Probably for individual applications it is not feasible to predict the extent of the effect of pure oxygen use without laboratory or pilot plant tests. Nevertheless, it is expected a decrease in the cyanidation time and cyanide consumption.