Mine solid residue and mine water treatment at laboratory and pilot scale by modified bauxite residue with gypsum, an effective and cheap source of iron and alkalinity
Résumé
Immobilisation of trace elements from mine residues, contaminated soils and water is effective with modified bauxite residue (MBR). The red mud from alumina production is washed and dried in press-filter, and reacted with gypsum to lower the pH from 10.5 to 8.5 (about the pH of limestone in water), and that non-hazardous waste reacts with elements, due to the reactive iron and aluminium oxyhydroxydes and the alkalinity content. Laboratory treatment of 5 mine residues, one sub-soil and 9 mine waters are presented (Figure 1, Table 1). Abatement is for most element and sample > 99%, excepted in some cases for Pb. A small plot treatment of mine residue and a pilot treatment of water were successful. For mine residue, the dose of input can be adjusted until the leachable fraction of trace elements is below the European inert landfill acceptance criteria. At that dose, with mineral fertilizers at agronomic rate for oligotrophic soils, normal plant growth of common species is obtained. The modified bauxite residue can decontaminate heavily polluted mine water (Figure 2), and the preliminary results from lysimeters (Figure 3) and speciation study indicate that the immobilization could be stable. Hypothesis for mechanisms of immobilization of elements are presented (Fe and Al: surface precipitation by effect of pH; Zn and Cd: complexation by oxyhydroxides of Fe and Al, electrostatic attraction is probably secondary; As: inner-sphere complex; Fe: oxidation and Mn: oxidation accelerated by a pH effect and a surface effect of Mn (II) to Mn (IV) and precipitation as MnO2).