Performance of bauxite refinery residues for treating acid mine drainage

Novel application of alkaline waste materials from the alumina industry (Bayer precipitates) to both neutralize and remove dissolved metals from acid mine drainage (AMD) has been demonstrated. The hypothesis was that Bayer precipitates could not only economically treat a wide range of AMD compositions but also do this in an environmentally acceptable manner. Therefore, the aim was to synthesise a range of Bayer precipitates and test their remediation effectiveness with a variety of real AMD samples. Thermal activation of Bayer precipitate at 380 °C produced a material which required the least mass to neutralize AMD and removed greater amounts of problematic species such as manganese (e.g. 74 to 82.8% reduction for Bayer precipitate and 83.7 to 91.6% reduction for thermally activated Bayer precipitate). The presence of an amorphous mixed magnesium/aluminium oxide and lack of carbonate species in the thermally activated Bayer precipitate was proposed to explain the improved performance. The resultant sludge was composed of hydrotalcite and possibly calcium sulphate and calcium carbonate, with precise quantities dependent upon the AMD tested. Exposure of sludge to water promoted the dissolution of a fraction of the hydrotalcite which enhanced concentrations of Mg and Ca in solution; albeit, contact of the sludge with AMD resulted in further removal of dissolved metals and raised the pH of the AMD. In summary, this study has developed a new process using two industrial waste products to solve a critical environmental problem. Future research should investigate the commercial feasibility.