Fate of pollutants post treatment of acid mine drainage with basic oxygen furnace slag: Validation of experimental results with a geochemical model

Abstract

Basic Oxygen Furnace (BOF) slag has been widely used for Acid Mine Drainage (AMD) treatment with its reaction chemistry and contaminants attenuation mechanisms barely reported. This pioneer study was therefore designed with the aim of evaluating the partitioning of inorganic contaminants post the interactions of acid mine drainage and BOF slag and explore the chemistry thereof. This will aid in situation where metal recovery and waste classifications need to be pursued. To fulfil the goals of the study, laboratory experiments were conducted at 60 min of shaking and 100 g: 1 L solids to liquids ratios (S/L). To validate the results, experimental results were complemented by pH Redox Equilibrium (in C language) (PHREEQC) geochemical model. The water chemistry and mineral phase changes during the interaction of BOF and Acid mine drainage (AMD) were evaluated. Reaction of BOF slag and AMD led to an increase in pH (≈10) and a drastic reduction in metal species (≥99%) and sulphate (≥75%). PHREEQC indicated that the removal of chemical species from AMD was dependent on pH. Furthermore, it indicated that Fe was mainly removed as Fe(OH)3, goethite, and jarosite whereas Al was removed as basaluminite, boehmite and jurbanite, Al(OH)3, gibbsite and diaspore. Al and Fe precipitated as iron (oxy)-hydroxides and aluminium (oxy)-hydroxides. Mn precipitated as rhodochrosite and manganite. Ca and sulphate were removed as gypsum. Mg was removed as brucite and dolomite. The results from experimental runs and geochemical modelling were in agreements hence proving that the obtained results are valid. As such, it can be concluded that BOF slag effectively neutralized AMD and significantly lowered the levels of inorganic contaminants to meet prescribed standards as stipulated by the water quality guideline. Gypsum and magnetite were also recovered from mine water treatment process using BOF slag hence depicting that there is commercial value from the treatment process.