Acid drainage neutralization and trace metals removal by a two-step system with carbonated rocks, Estado de Mexico, Mexico

The following study is carried out in a mine of Mexico where acid drainage is produced. At the site, two types of rock which contain calcite are present, graphitic phyllite and limestone, and their use is proposed in a treatment system. Acid drainage contains dissolved Al, Cd, Cu, Fe, Mn and Zn, as well as high concentration of sulphates. By means of shaking tests conducted using flasks, the efficiency of the proposed two-step treatment was evaluated, first using phyllite to remove Fe and Al and then limestone to remove Cu, Cd, Mn and Zn. During the tests performed using phyllite, a pH = 6.00–6.16 was reached, and 100% of Al, Fe and Cu was removed. During the second step, using limestone, a pH = 7.77–8.39 was achieved and 100% of Cd, 95–99% of Zn and 74–89% of Mn were removed. Sediment characterization indicates that Al and Fe precipitate as hydroxysulphates, Cu removal mainly occurs by sorption in iron hydroxysulphates, while Cd, Mn and Zn are mostly removed by adsorption in clay phases formed by alteration of phyllosilicates contained in rock.