Gold recovery from waste activated carbon - a comparison between mechano-chemical pretreatment and charcoal augmented oxidation
The use of activated carbon in recovering dissolved gold from solution has been embraced at virtually all gold processing plants worldwide. Technically, activated carbon may be added in the course of leaching, a process known as carbon-in-leach (CIL), or after leaching in the carbon-in-pulp (CIP) process. CIL has advantages when carbonaceous materials are present in the ore since it brings about competitive adsorption between the added carbon and the preg-robbers. Activated carbon may also be loaded into columns or up-flow contactors before the solution is passed through; it is referred to as carbon-in-column (CIC). The loaded carbon is then stripped and the gold recovered mainly by electrowinning. The carbon is reactivated, sized, and recycled. Activated carbon breaks into fragments during usage as a result of impact stresses against tank walls, screening, transportation, elution, and regeneration. Part of these fines (usually –20 mesh) may be retrieved as waste-activated carbon (WAC). A worldwide survey shows that the WAC may contain from about 0.11 to 0.14 kg of gold per tonne of carbon. With its relatively high gold content, WAC represents a potential source of extra revenue. The methods that have been proposed for pretreating and/or recovering gold from waste carbon may be generally classified as follows: (1) incineration, (2) electrochemical, and (3) competitive adsorption.
In this research, two methods that augment combustion of waste-activated carbon are evaluated. These are mechano-chemical activation before oxidation and direct combustion using charcoal as fuel. Mechano-chemical activation caused structural modifications of the waste carbon, which were measured by X-ray diffraction analysis. The quantitative changes in the crystallinity of the samples were determined by measuring the relative intensities at a specific lattice plane for the milled material (I) and an undisturbed reference standard (Io). The intensity ratio (I/Io), also known as the J-factor, decreased from a value of one for the undisturbed lattice to 0.56 after six hours vibration milling, which resulted in a reduction in the temperature for complete oxidation from above 700oC to 450oC.
Complete oxidation was also achieved at the same furnace temperature (450oC), for a combined sample of waste-activated carbon and 25% by weight of wood charcoal. Activated carbon does not contain sufficient volatile matter to sustain autogenous roasting. However, wood charcoal contains a good percentage of hydrocarbons and therefore readily sustains self-combustion and this augmented combustion of WAC at a low furnace temperature of 450oC. The ash content of the charcoal was 1.2% while that of the WAC was 11%. Therefore, the additional weight of ash due to the use of charcoal is not significant.
The mass loss of almost 89% during oxidation resulted in the production of ash that was very rich in gold (3.2 kg/t). Cyanidation of gold from the ash produced by both pretreatment processes was rapid. About 64% extraction was achieved within one hour for the vibration milled. Leaching progressed steadily and over 99% extraction was achieved after 20 hours. In a manner similar to that of the vibration-milled sample, about 53% of gold was extracted within one hour from the ash produced by the combined sample of WAC and wood charcoal, and over 98% extraction was achieved after 20 hours.
For industrial application, economic analysis shows that the better pretreatment option is charcoal-augmented combustion of waste-activated carbon. With this technique, mixing and size reduction can be done in ball mills, which are readily available and have a lower energy requirement. The gold extraction efficiency is equally high. After oxidation, the ash could be leached in an online reactor by intensive cyanidation and the pregnant solution sent directly for electrowinning due to its high gold value.