Passive biochemical treatment of ferriferous mine drainage: Lorraine mine site, Northern Quebec, Canada

摘要

The Lorraine mine site is located in the Temiscamingue region of the Quebec's province, Canada. The mine operated only for four years (1964-1968) for the extraction of Au, Ag, Cu, and Ni, but left behind around 600 000 t of tailings, disposed in a pond area of about 15.5 ha. The unsaturated zone of the tailings, located partially below the water table, has been exposed to the atmosphere for about 30 years, entailing sulfides oxidation and the generation of highly contaminated ferriferous (iron-rich) acid mine drainage (AMD).In 1998 Quebec's Ministry of Energy and Natural Resources initiated a rehabilitation program of the mine site to limit the AMD generation and environmental damages. The restoration plan mainly consisted of prevention measures of the further weathering of tailings using a multi-layer solid cover (cover with capillary barrier effects), as well as four passive treatment units, for the AMD flowing from the underneath of cover, using one limestone and three dolomite drains.Reclamation strategy proved effective, but the iron-rich AMD in the tailings pores is only progressively leached out over time. Hence, total Fe concentrations decreased from about 11 g/L, before the construction of the preventive cover, to around 2.5 g/L in 2015, while the pH increased from < 3 to 5-6 within only 12 years. However, in 2011 the replacement of one clogged dolomite drain by a three-unit passive biochemical treatment system (train) was necessary. The new train consisted of two biochemical reactors (filled with mixtures of limestone, poultry manure, compost, wood waste, and sand), separated by a wood ash unit. In addition, in 2012 some design changes, entailed by the very slow flow through the three-unit passive treatment system, were required.Yearly monitoring, during the warm seasons, was implemented and proved the efficiency of passive biochemical treatment system. Results showed that the pH increased from 5 to 6, and Fe and sulfate removal exceeds 70 % and 57% (from average initial concentrations of about 1.85 g/L and 4.7 g/L), respectively. Few more years would still be necessary for the complete leaching out and passive treatment of the ferriferous AMD.