ACID MINE DRAINAGE AT MOUNT MORGAN, QUEENSLAND (AUSTRALIA): EXPERIMENTAL SIMULATION AND GEOCHEMICAL MODELLING OF BUFFERING REACTIONS

The Mount Morgan mine in Central Queensland, Australia, is a heavily impacted site with over 100 million tonnes of sulphide-rich waste rock accumulated during more than a century of gold and copper mining. Concern over the potential dispersal of acid mine drainage (AMD) into the surrounding environment prompted the construction of a water treatment plant in 2006. In this study we performed some column leaching experiments and developed a geochemical model to simulate the generation of AMD and reactions between the AMD and lime [CaO], the buffering agent used in the water treatment plant. Our results show that although the use of lime is effective in reducing acidity and contaminant levels in the water, extremely large amounts of metal-rich sludge will be generated, and the precipitation of large amounts of gypsum [CaSO4·2H2O] following the reaction between the lime and the AMD is likely to cause problems in the treatment plant and associated pipelines. The treatment process itself may have some negative consequences on the chemical stability of the rock waste: the reduction in acidity may lead to the dissolution of jarosite [KFe3(SO4)2(OH)6], a stable phase in the tailings under current pH conditions, and the release of contained metals into the system. We demonstrate that some relatively simple experiments and geochemical models can predict very accurately the outcomes of treatment/remediation strategies, but only if the geochemistry and the geology of the site are well known