Author(s)

A. Aguiar, G. Xavier & A. Ladeira

Abstract

Acid mine drainage (AMD) is one of the main environmental issues faced by the mining industry. The acid mine water generally contains metals above the permissible discharging levels. Manganese is particularly present in this effluent and its removal is notoriously difficult due to its high solubility over a wide range of pH. While most of the metals precipitate at a pH below neutrality, the pH necessary for manganese precipitation is very high, above 10. Most systems that effectively remove this element from mine waters use the oxidation of Mn (II) followed by precipitation at an elevated pH. Precipitation consumes a great amount of lime, which implies high operational cost. Besides, the process generates a large amount of sludge containing metals which have to be disposed of. The objective of this study is to optimize the removal of manganese by using a laboratory prepared acid solution and an acid effluent from Poços de Caldas uranium mine (Brazil), in order to achieve the Mn permitted level for discharging (<1mg/L) and to reduce the amount of sludge generated. The precipitation process has been studied using lime, limestone and a nonconventional catalyst/adsorbent (MnO2 residue). The results obtained showed that both lime and limestone are effective in the removal of Mn in a pH higher than 10. However, there is a slight difference between the two reagents and lime shows a better performance. The volume of precipitate generated by the addition of lime was 50% smaller than that obtained when limestone was used. The use of the non-conventional material made the removal of almost 100% of the Mn possible as the concentration of manganese was reduced from 140mg/L to <1mg/L at a pH near neutrality (6.8 to 7.2). The final effluent complies with the recommended value for manganese discharge. Keywords: manganese, acid mine drainage, precipitation, manganese oxide.

Keywords

manganese, acid mine drainage, precipitation, manganese oxide