Separation of Magnesium Hydroxide and Barium Sulphate from a Barium Sulphate-Magnesium Hydroxide Mixed Sludge by Carbonation: The Effect of Temperature

摘要

The solids that result from mine wastewater treatment usually contain elevated levels of contaminants that were originally contained in the wastewater. These must be carefully disposed or treated to avoid shifting of the original pollutants in the waste stream to the final disposal site where they may again become free to contaminate the environment. A more reasonable approach to ultimate solids disposal is to view the sludge as a resource that can be recycled or reused. In South Africa, reverse osmosis is already being used for desalination of mine water and huge sludge volumes are also produced. The Tshwane University of Technology-Magnesium-Barium-Oxide (TUT-MBO) process and its variations is an alternative technology that offers the benefit of lower cost as magnesium hydroxide, barium hydroxide and coal are the main process raw materials. In the first stage Mg(OH)2 is dosed to raise the pH of the acid mine drainage to 9 for removal of free acid, iron(II) oxidized to iron(III) and all other metals precipitated as metal hydroxides. In the second stage Ba(OH)2 is dosed for magnesium and sulphate removal as Mg(OH)2 and BaSO4 respectively. The resultant, mixed BaSO4/Mg(OH)2 sludge is treated in a thermal stage to produce BaS and MgO. The aim of this study was to separate magnesium hydroxide from barium sulphate, produced in the second stage of the TUT-MBO Process. Magnesium hydroxide is separated from barium sulphate through the dissolution of Mg(OH)2 with CO2 to Mg(HCO3)2. The results showed that: (a) By adding CO2 to a BaSO4/Mg(OH)2 sludge, selective dissolution of Mg(OH)2 occurred due to the relatively high solubility of Mg(HCO3)2 and the low solubility of BaSO4 and, (b) the solubility of Mg(HCO3)2 increased with decreasing temperatures and increasing pressures.