Development of environmentally friendly coal based hydrophobic material as a replacement of mercury in small scale gold mining in Tanzania

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University of Dar es Salaam
The need for development of environmentally friendly material for application in gold recovery in small scale mining as a replacement of mercury was the motivation for this work. The use of mercury is associated with serious human health hazards such as destroying the central nervous system, brain cells, kidney damage, and causes embryonic abnormalities. In this work bituminous coal from Kiwira as the solid hydrophobic material was tested with three different types of liquid hydrophobic phases, bio-diesel, petro-diesel, and castor oil. Through mechanical agitation of coal and oil in a vessel at different conditions using radial flow impellers, it was possible to develop stable agglomerates capable of recovering gold from ores. The stability of agglomerates ranged from 82%, 84% and 88.6% for Petroleum-diesel, Biodiesel and Castor oil respectively. Most stable agglomerates were produced at oil/coal ratio of 0.3, agitation time of 20 minutes and 1000 rpm. No stable agglomerates were formed at speeds below 900 rpm due to tendency to form large agglomerates of low binding strength. A special coal-oil preconditioning invention based on initial agglomerate nucleation without using water increased the stability and the extent of agglomeration considerably without increase in oil utilization. Gold attachment tests were conducted by contacting agglomerates with gold bearing material at different conditions using axial flow impellers. The results revealed an increase in gold recovery with increase in agglomerate loading, surface area, contact time and the viscosity of the hydrophobic phase. The optimum recoveries ranged from 98.5% for castor oil while the two diesel phases, bio-diesel and petro-diesel attained close recoveries of 97.5% and 97.7% respectively. Microphotographs of feed ore showed changes in gold particle concentrations at different contact times in the slurry and revealed a first order reaction which obeyed the mass transfer equation Cin = Cout indicating possibility of gold recovery up to 100%. Using sections of gold loaded agglomerates observed under reflected light microscope it was possible to show gold selectivity during penetration into agglomerates and multilayer gold adsorption on the agglomerate surface and indicated the possibility of increasing gold recovery without increase in surface area of agglomerates. The recovery tests conducted using actual gold ores, showed sulphides as the main competing gangue. Using lime at pH 10 it was possible to suppress the attachment competition between gold and sulphide gangue. This work has generally shown a successful application of three liquid hydrophobic phases in developing stable agglomerates with coal, capable of attaining high gold recoveries from gold bearing materials and possibilities of suppressing the effect of competing gangue.
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Coal-oil agglomerates, Gold ores, Tanzania
Mlaki,A. E. L (2012) Development of environmentally friendly coal based hydrophobic material as a replacement of mercury in small scale gold mining in Tanzania, Doctoral dissertation, University of Dar es Salaam. (Available in print form)