Development of an acid mine drainage treatment technology and recovery of minerals

Abstract

Acid mine drainage (AMD) is known to have negative environmental impacts due to low pH, high concentrations of toxic metals which can pollute freshwater sources. There is need for development of a technology which treats and recover beneficial minerals from AMD. Samples of AMD, water, sediments from the field and sludge from neutralization batch experiments were characterised for their chemical compositions using advanced research techniques. In designing the treatment method, hydrogeochemistry of decanting AMD from Mpumalanga was studied. Findings from this study revealed that decanting AMD continues to threaten the environment due low pH, high Electrical Conductivity (EC) and precipitation of secondary minerals. Simulated AMD prepared by choosing concentrations of worst-case scenario from Mpumalanga was used in batch experiment comparing flocculation and sonication techniques in treatment of AMD with calcined magnesite. Sonication technique was more effective in treatment of AMD because it raised pH faster and it enhanced precipitation of minerals than flocculation. Due to the superiority of sonication, it was used for recovery of metal oxides from AMD with calcined magnesite through sequential selective precipitation and recycling of metal enriched sludge over several cycles using filtration and pH adjustment. The technology enriched Fe -rich and Al-rich sludge up to cycle 3 while Mn-sludge up to cycle 2. Fe, Al, and Mn in product water were below the guidelines for irrigation water, while Ca, Mg, and SO42- remained at higher levels. The technology may not be used for recovery of Mn due to co-precipitation with Mg. Future studies recommends the treatment of product water to recover portable water and salts and upscaling the technology to pilot for industrial applications. The high purity of recovered metals suggests their potential use for industrial applications such as catalysts, batteries, fertilizer, and pigment manufacturing.