Evolution of mineral phase in the chemical neutralization process of Mn dominated acid mine drainage under different aeration conditions

Abstract

Chemical neutralization is extensively used in the treatment of acid mine drainage, and the removal of manganese (Mn) is the limiting process for meeting the discharge standards. The formation of Mn-containing precipitates is the main removal mechanism in the neutralization process. This study compared the formation of Mn-containing precipitates under aeration and mechanical stirring conditions, and the effect of aeration rates was also studied. In the aeration-stirring group, the precipitate was composed of hausmannite (Mn3O4), bixbyite (Mn2O3) and rhodochrosite (MnCO3). In addition, Mn(OH)2 was detected in the mechanically stirred group. The results show that the Mn3O4 content in the precipitate increased significantly under aeration conditions and reached a maximum value of 71.9% when the aeration rate was 66.90 L/min. Its content decreased to 54.0% when the aeration rate was increased to 133.80 L/min. The MnCO3 content increased with an increase in the aeration rate and reached a maximum value of 27.3% at an aeration rate of 133.80 L/min. The scanning electron microscopy images revealed that the generated lamellar rhodochrosite (MnCO3) could wrap MnOx and hinder the formation of Mn3O4. This was also verified by the Mn average oxidized state (MnAOS) of the groups. Based on the experimental and calculation results, MnO2 was formed under acidic conditions, and Mn3O4 and Mn2O3 appeared with an increase in the pH level. Meanwhile, the Mn (III) and Mn (IV) oxides are intermediates that were converted into low valance manganese oxides through autocatalysis.