Desulphurising high sulphur coal discards using an accelerated heap leach approach

Abstract

A large amount of waste rock, coal discards and coal fines result from the extraction and beneficiation of coal. These wastes may present a major environmental hazard. Exposure of their sulphidic mineral content, mainly pyrite, to moisture and oxygen causes acidic runoff or leachate from waste rock dumps, coal discard dumps and tailings facilities, referred to as acid rock drainage (ARD). Whereas sulphide can be removed from fine coal waste by physical separation, it is sufficiently unliberated in coal discards or waste rock for ready physical separation. This requires either further liberation followed by removal using separation (energy consuming and costly) or removal by reaction. Heap bioleaching, a technology predominantly used for the recovery of base metals from low grade metal sulphide-containing ores, provides a sustainable option for the accelerated reaction of the pyritic fraction within the coal discards, allowing its removal in solution under controlled conditions during the life of mine for the long-term prevention of ARD. Easily implementable, heap bioleaching may constitute an economically viable prevention alternative to traditional ARD treatment methods. Further, such accelerated sulphide removal by reaction is expected to produce an environmentally benign discard with value prospects. The desulphurization reactions are sustained by iron- and sulphur oxidising microorganisms. The iron oxidizers catalyse the regeneration of the leach agent Fe3+ from its Fe2+ form, and the sulphur-oxidising microorganisms produce sulphuric acid from elemental sulphur and sulphides. These maintain highly acidic conditions which support further Fe2+ to Fe3+ oxidation and ongoing oxidation and leaching of the pyrite.In this study, proof of concept experiments were performed in laboratory scale columns over 380 days, simulating coal discard heaps. Coal discards containing 10.4% by mass pyritic sulphur were used. Results obtained indicated more than 50% desulphurisation in 250 days with a terminal duration of ~600 days predicted. This confirms the potential of heap bioleaching as a viable strategy for the desulphurisation of high sulphur-containing coal discards during life of mine, with their potential re-purposing as saleable coal or risk-reduced disposal.