Sulfur Recovery in a Pilot Biological Treatment System for Mine Affected Water
Rossouw, Sean Lloyd (1); Sheridan, Craig (1); van Hille, Robert (2)
1: Department of Chemical and Metallurgical Engineering, University of the Witwatersrand, South Africa; 2: The Moss Group, 13 Bell Crescent, Westlake Business Park, Westlake
Acid rock drainage (ARD) is a long-term consequence of mining and related activities, particularly with the local geology of South African mining regions. ARD impacts receiving water bodies, contributing acidity and sulphate salinity, as well as dissolved metals. This is already having an impact on the quality of water systems in the coal mining regions of South Africa.
Passive treatment options, typically involving biological processes, have been developed to remediate affected waters in conditions where capital and skills intensive active treatment options may not be suitable. Sulfate reducing bacteria (SRB) are a group of micro-organisms that use sulfate as a terminal electron acceptor, producing sulphide, and play a key role in reducing sulphate salinity during passive treatment. Sulfide is toxic and needs to be managed prior to discharge. Precipitation of unstable iron sulphides is often not a viable option. Sulfide remediation by partial oxidation to elemental sulfur has been identified as an attractive alternative and can be achieved in a floating sulfur biofilm.
A laboratory-scale treatment system consisting of a linear flow channel reactor, with an anaerobic section for sulfate reduction and a floating biofilm where partial sulfide oxidation takes place has been constructed to treat partially-neutralised mine water. The biofilm at the air liquid interface controls oxygen diffusion and allows for partial re-oxidation of the sulfide to elemental sulfur. The presence of this solid sulfur in the biofilm means it may be physically harvested. A pre-treatment unit contacts the ARD with waste biomass material to cause acid hydrolysis and liberate soluble organic components as an energy source for the subsequent biological processes. The final step is a constructed wetland to remove any residual organic compounds that could contribute to colour and odour. The system has been operating for several months, achieving neutralisation and partial sulfate removal.
The laboratory-scale system is being used to inform the configuration and operation conditions for a pilot plant, currently being commissioned at an active coal mine in the Mpumalanga area of South Africa. Critical performance factors identified to date include the amount of soluble organic component release during pre-treatment, the required contact time, the ideal ratio of these components to sulfate in the biological treatment reactor and the pH and redox conditions required for effective operation.