Gold rush - advanced gold leaching
Separating gold from the surrounding rock and eliminating potentially harmful waste is important for gold recovery. Sonestie Janse van Rensburg, Senior Scientist from Mintek, South Africa, highlights the process of advanced gold leaching.
As available high-grade or non-refractory gold deposits become depleted, attention is increasingly focused on refractory or sulphide-associated orebodies, in which the gold is ‘locked up’. These require more specialist technology to recover the gold.
An advanced gold leach facility that supplements and extends the bottle roll technique for gold amenability testwork has been developed by Mintek, South Africa’s national minerals research institute. Understanding how variations in the controllable leach parameters affect the chemistry of the process will improve plant designs to meet environmental protection standards, without compromising gold recovery.
The advanced leach facility (ALF) consists of a semi-automated mini-plant. The facility serves the dual purpose of complementing the traditional gold amenability testwork by identifying the optimal gold leach parameters, while generating data regarding the environmentally harmful elements, such as cyanide, arsenic, and heavy metals, that are tracked in real time as the leach progresses.
Unlike the conventional bottle roll test-work, the ALF makes it possible to modify and measure the process parameters that can be realistically changed on an operating gold plant, such as pH, Eh, oxidant addition, pulp viscosity, cyanide concentration (cyanide staging), as well as the addition of supporting reagents such as lead nitrate.
Furthermore, instead of terminating a leach after gold dissolution, the leach can be continued into, for example, a cyanide destruction phase with the same amount of detailed data recorded. Almost all leach parameters can be adjusted at any time during operation. Specific changes can be made as the kinetic data becomes available, such as increasing agitation speed and aeration to improve dissolved oxygen concentration, or adding more cyanide to avoid free cyanide substrate limitation.
The ALF set-up consists of a three litre chemical resistant Polypropylene reactor fitted with over head stirrers that can be fitted with low or high shear blades. The stirrer shaft is mounted through three bearings, assuring precise alignment and stability in the case of high speed or shear agitation test-work. The specially designed air sparger is normally added to the bottom of the reactor, except in the case of anaerobic test-work. A small portion of filtered sample (approximately 150ml) is circulated using a peristaltic pump, filtering solution through a five-micrometre hydrophobic filter sock. The clean solution is contained in a 200ml sampling cell at a total retention time of about 20 minutes. Calibrated and automated instruments are connected to the sampling cell. Small volume samples can be removed from the cell for specific kinetic data generation.
Due to the small scale of this operation, high quality testing does not require large amounts of reagents and sample material, usually a 1.5kg ore sample is sufficient. The automated measurement and recording abilities of the instrumentation makes this technique less labour intensive. The most challenging aspect is to perform accurate interpretation where observations deviate significantly from the expected scenario. However, with time, the amount of data generated will contribute significantly to explaining the often complex interactions between the solid, liquid and gas phases.
Unlocking value, isolating waste
However, advanced leaching technology generates increased amounts of chemically complex waste products. With environmental pressures and regulations becoming more intense, topics such as cyanide management, water balance optimisation, the fate of arsenic, and minimisation of salt load in discharge to the aquatic environment, have become issues that require nearly as much attention as conventional optimisation targets.
Stricter environmental and sustainability legislation could have severe consequences, including regulatory fines, plant closures or high remediation costs.
The main aim of the ALF is to pro-actively identify upstream process modifications that avoid these problems, instead of the conservative reactive approach to ameliorating environmental problems once they become apparent.
Commissioning began as a R&D project to establish, optimise and validate the technique. This phase of the work has been completed and investigations are aimed at performing commercial amenability leach testing, while focusing on the effect of techniques, such as pre-oxidation and sulphur speciation, as well as identifying the cyanide consuming species on a more chemical level.
Long-term, the goal is to understand the leach kinetic chemistry of cyanidation, and arsenic and sulphur species liberation during the dissolution of precious metals in relation to the environmentally harmful substances. Ultimately, it will assist operations to design improved leach plants and leach parameters, so as to avoid generating excessive amounts of environmentally harmful waste products, while maintaining or improving gold recoveries.