Investigation of carbonate dissolution for the separation of magnesium hydroxide and calcium sulphate in a magnesium hydroxide-calcium sulphate mixed sludge

Abstract

South Africa is experiencing a large environmental problem due to uncontrolled discharge of acid mine water into public water courses. The need for neutralisation and desalination of acid mine drainage is a significant issue in South Africa and the sludges that result from mine wastewater treatment usually contain elevated levels of mixed contaminants derived from those originally contained in the wastewater. A more reasonable approach to ultimate sludge disposal is to view the sludge as a resource that can be recycled or reused. Carbon dioxide and a sludge mixture consisting of Mg(OH)₂ and CaSO₄ ∙ 2H₂O are by-products from acid mine drainage treatment processes. This study was carried out to explore the feasibility of separating Mg(OH)₂ from CaSO₄ ∙2H₂O through dissolution of Mg(OH)₂ by accelerated carbonation in a pressurised, completely-mixed reactor. The effects of temperature and pressure, and of both together, on the dissolution of the sludge mixture with time were investigated. Parameters monitored included alkalinity, pH, conductivity and Ca²⁺, Mg²⁺ and SO₄^2- concentrations. OLI Analyser Studio Version 9.0 software was used for modelling predictions of chemical speciation of the mixtures. The optimum separation capacity for the Mg(OH)₂-CaSO₄ ∙ 2H₂O sludge mixture was determined to be 99.34% Mg²⁺ and 0.05% Ca²⁺ in the aqueous phase when contacted with CO2 at a temperature of 5°C and pressure of 150 kPa. The model predictionswere in agreement with the experimental findings. Temperature and pressure have a significant impact on the dissolution of the mixed sludges when contacted with CO₂.