Sustainable Recovery of Gold using Biomass Wastes in an Alkaline-Acidic Hybrid System

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The gold processing industry extracts gold by leaching ore in cyanide solution followed by adsorption on activated carbon and electrowinning. However, both cyanidation and carbon adsorption have some major problems. Since cyanide is very toxic, gold plants are forced to destroy the cyanide in effluent that results in loss of expensive cyanide and increase in the overall operating cost. On the other hand, activated carbon, which is manufactured at 600-1200 oC, is expensive and it is not selective to gold only, hence further refining is necessary to produce pure gold. Also, during adsorption, gold is trapped inside the micro-pore of activated carbon that requires high temperature elution to reactivate the carbon. As a result, manufacturing and regenerating of activated carbon represents a major portion of the operating cost in a mining operation. Also, it releases greenhouse gases to the environment. To overcome these problems, in 2011, I started my NSERC Discovery research program to develop an innovative and sustainable gold recovery technology. In our process, gold was recovered from aqueous solution by biosorption on our adsorbents having novel functional groups. These adsorbents were prepared in a very simple process by crosslinking different biomass wastes with sulphuric acid at 100 oC. Biosorption is a new field in hydrometallurgical research, and since 2011, we made tremendous advancement in this area, generating 67 publications in high quality journals and in world-class conference proceedings. From this research, we found some remarkable results, such as: 1) unlike activated carbon, only gold is adsorbed on our novel bioadsorbents, hence no further treatment is necessary for impurity removal; and 2) when gold is adsorbed on our novel bioadsorbent, it is simultaneously reduced to metallic gold. These ground-breaking findings will make gold recovery much simpler, more cost-effective and energy efficient because no stripping, high temperature elution, and electrowinning are necessary. However, we observed that simultaneous adsorption-reduction of gold on our novel bio-adsorbents only takes place in acidic chloride media but not in alkaline cyanide solutions. Considering the urgent need to improve the gold cyanidation process, in the proposed research, I plan to recover gold using our novel bioadsorbents in an alkaline-acidic hybrid system where gold will be leached in an alkaline cyanide solution followed by acidification to recycle cyanide back to the leaching plant leaving behind gold in an acidic chloride media for simultaneous biosorption-bioreduction to produce pure gold. Our proposed biosorption process with recycling of toxic cyanide will ensure zero emission to the environment. This will eliminate the current practice of destruction of expensive cyanide in the effluent and result in huge cost savings for the industry and minimization of greenhouse gas emissions. Canadian mining companies will benefit from this innovative research.