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 CaSO 4 ·2H 2 O are by-products from acid mine drainage treatment processes. This study was carried out to explore the feasibility of separating Mg(OH) 2 from CaSO 4 ·2H 2 O 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 Ca 2+ , Mg 2+ and SO 4 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 -CaSO 4 ·2H 2 O sludge mixture was determined to be 99.34% Mg 2+ and 0.05% Ca 2+ in the aqueous phase when contacted with CO 2 at a temperature of 5°C and pressure of 150 kPa. The model predictions were in agreement with the experimental findings. Temperature and pressure have a significant impact on the dissolution of the mixed sludges when contacted with CO 2 . Keywords : Carbonation, gypsum, dissolution, reclamation, carbon dioxide, sludge disposal
Highlights
South Africa is experiencing large environmental threats and problems due to uncontrolled discharge of acid mine drainage (AMD) into public water courses
Lime is used for the removal of magnesium as Mg(OH)2 and further sulphate removal is achieved through gypsum crystallisation at pH 11
Surface carbonation of the suspension occurs from the surface area exposure of the liquid to carbon dioxide and this is enhanced by the vortexing action caused by the axial flow impeller
Summary
South Africa is experiencing large environmental threats and problems due to uncontrolled discharge of acid mine drainage (AMD) into public water courses. Limestone is used in the pre-treatment stage for the removal of acid and metals. Partial sulphate removal is achieved through gypsum crystallisation at pH 6. Lime is used for the removal of magnesium as Mg(OH) and further sulphate removal is achieved through gypsum crystallisation at pH 11. Separation of Mg(OH) from gypsum is needed for the CSIR-ABC process as described by Rukuni et al (2012a; 2012b). A mixed Mg(OH)2-CaSO4·2H2O sludge is produced in the second stage of the process. In order to exploit the potential usefulness of the sludge, it is important to separate individual compounds from one another, such as Mg(OH) from CaSO4·2H2O. Rukuni et al (2012a and 2012b) have studied the separation of CaCO3-BaSO4 and Mg(OH)2-BaSO4 sludge mixtures in contact with CO2
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