Soluble Silica Degradation in Mining Effluents Using Magnesium Hydroxide and Acidified Activated Carbon

High soluble silica (SiO2) concentration in mining effluents from processing plants of most mining companies in Ghana makes most mines noncompliant to the Environmental Protection Agency’s (EPA’s) soluble silica compliant limit of 20 mg/L. In this research work, magnesium hydroxide and acidified activated carbon have been used to degrade soluble silica in mining effluents to meet the EPA limit. Jar test were performed at three doses of magnesium hydroxide (Mg(OH)2) with concentrations (15-25 g/L) together with three weights of acidified activated carbon (5-10 g) were used at different initial pHs between 10-11. Characterization of the mine water effluent revealed that the silica, pH and conductivity were 124 mg/L, 10.01 and 1132 µS/cm respectively. This implied that soluble silica degradation greater than about 84% was required in other to meet the EPA limit. The outcome of the Jar test revealed that soluble silica degradation rate decreased with increasing Mg(OH)2 addition and pH from 15 g to 19 g and 10 – 10.5 respectively. Beyond the aforementioned Mg(OH)2 and pH range values, the degradation rate increased significantly. Soluble silica degradation rate increased with increasing the activated carbon under conditions of study. Whereas in the case of reaction time, soluble silica degradation remained approximately unchanged between 15 min and 27 min and subsequently increased afterwards, the degradation rate of soluble silica decreased with increasing stirring rate. Response surface methodology (RSM) technique was successfully used to develop models to optimize preparation conditions needed to degrade soluble silica in the mine effluent while maintaining the mine effluent conductivity below the EPA limit of 1500 µS/cm. The optimum conditions for high soluble silica degradation and low solution conductivity was identified to be Mg(OH)2 and activated carbon additions of 20.9 g and 5.0 g respectively, pH of 10.6, reaction time of approximately 44 min and stirring rate of 200 rpm. Under optimized conditions, silica degradation and solution conductivity of 96.0% and 1199.4 µS/cm was achieved experimentally. The cause for the high soluble silica degradation was ascribed to the precipitation of silicon phosphate emanating from reaction between magnesium phosphate and silica, adsorption of silica onto precipitated Mg(OH)2 and the formation of calcium silicate (CaH2SiO4). The current study shows that mine effluent containing soluble silica could be degraded while maintaining low solution conductivity by the use of magnesium hydroxide and acidified activated carbon to meet the EPA limit.