Removal of sulphates from South African mine water using coal fly ash

Abstract

This study evaluated SO4 2- removal from circumneutral mine water (CMW) collected from Middleburg coal mine using coal FA collected from Hendrina power station. The following parameters were investigated: the effect of the amount of FA, the effect of the final pH achieved during treatment, the effect of the initial pH of the mine water and the effect of Fe and Al on SO4 2- removal from mine water. The precipitation of ettringite at alkaline pH was evaluated to further reduce the SO4 2- concentration to below the DWAF limit for potable water. Removal of SO4 2- from mine water was found to be dependent on: the final pH achieved during treatment, the amount of FA used to treat the mine water and the presence of Fe and Al in the mine water. Treatment of CMW using different CMW:FA ratios; 5:1, 4:1, 3:1, and 2:1 resulted in 55, 60, 70 and 71 % SO4 2- removal respectively. Treatment of CMW to pH 8.98, 9.88, 10.21, 10.96, 11.77 and 12.35 resulted in 6, 19, 37, 45, 63 and 71 % SO4 2- removal respectively. When the CMW was modified by adding Fe and Al by mixing with Navigation coal mine AMD and treated to pH 10, 93 % SO4 2- removal was observed. Further studies were done to evaluate the effects of Fe and Al separately. Treatment of simulated Fe containing AMD (Fe-AMD) to pH 9.54, 10.2, 11.8, and 12.1 resulted in 47, 52, 65, and 68 % SO4 2- removal respectively. When Al containing AMD was treated to pH 9.46, 10.3, 11.5 and 12 percentage SO4 2- removal of 39, 51, 55 and 67 % was observed respectively.