Acid mine drainage treatment and metals recovery by means of selective precipitation using magnesium oxide (MgO): An experimental study

The treatment of acid mine drainage (AMD) by metals precipitation and neutralization releases a huge volume of highly polluted toxic sludge which can be valorized by the recovering of chemical species. In this study, the treatment of AMD and chemical species recovery by means of selective precipitation using magnesium oxide (MgO) was explored. One litre and half (1500 mL) of AMD from coal mine was used for the experiment. The pH of AMD water was gradually increased using an overhead stirrer and chemical species recovered at different pH gradients via a sequential precipitation technique. The results revealed that Fe(III) was recovered at pH 2.9–4 interval, Al at pH ≥ 4, Cu at pH ≥ 6.5, gypsum at pH 2.9–9.5 interval, Fe(II) at pH ≥ 7.5, Mn, Ni and Zn pH ≥ 8. The results further revealed significant removal of inorganic pollutants in the following order: Cu (100%) = Ni (100%) ≥ Fe (99.9%) ≥ Al (99%) ≥ Mn (98.6%) ≥ Zn (97.2%) ≥ SO₄^2─ (76%). Reduction of electrical conductivity (EC) and total dissolved solid (TDS) were also observed in the order of 82% and 85% respectively. Characterization studies were used to underpin and succinct the fate of chemical species of pure MgO and sludge recovered at different pH gradients. The product water obtained at pH 9.5 conformed to the prescribed standard for effluent discharge as prescribed by regulatory bodies.