Role of PH on sulphate removal from circumneutral mine water using coal fly ash

Abstract

Fly ash (FA) is a coal combustion by-product. South Africa produces 27 MT of FA annually, of which a small amount (5%) is beneficially utilized while the rest is disposed in landfill, ash dumps or ponds. This waste product has a serious impact on the environment apart from the high costs associated with its storage. Acid mine drainage (AMD) is another environmental hazard and the detrimental effects of AMD on the environment are well researched and documented in the literature. Neutralization of AMD is being achieved in various ways that include chemical and biological techniques. Low cost and effective treatment methods are constantly being sought and FA treatment is one of the promising methods due its low cost and availability in large quantities. FA has been successfully employed to neutralize and remove toxic elements from AMD. Approximately 80-90 % of sulphate was removed when FA was used as an ameliorant while treating low-pH, Fe, Al and sulphate-rich AMD. In South Africa, many contaminated mine waters are near to neutral pH (i.e., neutral mine drainage, NMD) and are often rich in Ca and Mg and poor in Fe and Al content. Such waters have been treated with FA in this study and no significant removal of sulphates was observed at final pH of less than 10. However, treatment of the NMD to pH greater than 11 has seen more than 70 % sulphate removal.