Witwatersrand basin acid mine drainage (AMD) softening via urea hydrolysis : Part 1 Experimental

Abstract

South Africa is a water scarce country with a semi-arid climate. The pollution of freshwater resources by the acid mine drainage (AMD), which is mostly acidic and hard, has exacerbated the problem. This research focuses on the treatment of AMD from the three goldfields (Eastern - EB, Central - CB and Western - WB basins) of the Witwatersrand basin, Gauteng, South Africa with the aim of recovering water (fit-for-purpose) and marketable mineral(s) for industrial use. In this preliminary study, the AMD water samples from these three basins were subjected to urea hydrolysis at varying urea concentrations (urea to total metal [U]/[M] ratios ranging from 3.3 to 10), constant reaction times (3 h) and temperature (80 °C). This resulted in the pH of the AMD water samples of the three basins (EB, CB and WB) being increased to an average of approximately 8.8, and the softening of water, with more than 98% removal of calcium. The sulphate removal ranged between 38 to 50%, while the method also displayed a positive impact in the reduction of magnesium (up to 34%) and the monovalent cations (sodium up to 33% and potassium up to 44%) from the polluted AMD water samples. In addition, calcium carbonate (aragonite polymorph), a mineral with vast industrial applications, was recovered in the process. The significance of the findings is that, firstly an environmentally benign method has been demonstrated to effectively soften and remove the AMD pollutants. Secondly, the AMD pollutants were converted to a valuable industrial product without generating any waste in the process.