Almost universally the chemical recovery of gold from milled slurries uses the cyanide leach process, usually followed by counter current adsorption using in-pulp activated carbon. The process requires the alkaline slurry to be treated with dilute solutions of cyanide ions to enable aurocyanide complex formation. The process kinetics are generally considered to be diffusion dependent (less than 1st order) with a key factor being the provision of oxygen at the reaction site.
Cyanide leaching of gold encompasses a variety of oxygen assisted reactions involving dissolution in dilute cyanide solutions. A generally accepted mechanism is described by the Elsner equation.
4Au + 8NaCN + O2 + 2H2O = 4NaAu (CN)2 + 4NaOH
Elsner’s Equation shows oxygen as a requirement for the gold leach reaction – how much is still under debate. The above equation shows a cyanide to oxygen ratio of 8:1. Habashi et al propose a CN:O2 ratio of 6:1, eg. If the cyanide concentration is 150ppm, then the corresponding DO should be 25ppm.
Conventionally this has been effected by compressed air sparging of agitated pulp. While this is quite straightforward there may be interferences to contend with including:
- cyanide consumers
- oxygen consumers
- unintended adsorption media or preg-robbers
A typical gold leach may require at least two oxidation steps:
- pre-aeration to overcome the chemical oxygen and cyanide demand of mineral components such as pyrrhotite or other sulphides
- aeration to provide oxygen for the cyanidation leach reactions
Efficient aeration is required for both steps to optimise energy and reagents. Additionally the use of gaseous oxygen from on-site generators or bulk liquid provides distinct advantages, not least of which is an increase in the driving force for oxygen dissolution. In this case poor efficiency of mass transfer has direct consequences in losses of gas to atmosphere. The oxygen utilisation provided by tank sparge systems using air may be as low as 2-5%, by comparison with in-pipe oxygenation which can achieve 30-80%. Additionally the availability of oxygen in solution reduces any tendency to form ferrocyanide and thiocyanate. Benefits include reduced leach time, increased recovery, and reduced cyanide consumption. From an economic perspective air is obviously less costly than oxygen, but in view of the above factors the capital and operating costs of compressors outstrip those of using on-site oxygen generators and high efficiency reactors.