Life cycle assessment of management/valorisation practices for metal-sludge from treatment of acid mine drainage

Abstract

The treatment of acid mine drainage in sulphide mining generates large amounts of metal-rich sludge whose management suppose an environmental quandary worldwide. Although traditional practices have focused on safe disposal in landfills, more environmentally friendly solutions may be adopted. The environmental performance of these solutions can be evaluated using tools like Life Cycle Analysis (LCA) that allows the estimation of the cumulative environmental impacts resulting from all stages in the product life cycle. The main goal of this study is, for the first time, to perform a LCA on different management strategies (i.e., i), encapsulation and disposal in landfill; ii) metal recovery using a chemical scheme; iii) pellets manufacturing for phosphorus removal from wastewaters; and iv) brick fabrication) for the metal-rich sludge generated from an active treatment plant. The manufacturing of pelletized material exhibited the lowest impacts (−35 REQs) due mainly to the environmental benefits of wastewater treatment. Despite the undoubtedly economic interest of recovering metals from the sludge, this route exhibited the highest environmental impacts (38 REQs), especially in the categories of fossil depletion (20,943 kg oil eq), marine (64 kg 1,4-dB eq) and freshwater ecotoxicity (50 kg 1,4-dB eq) due to the use of toluene and trybutylphosphate (TBP). The manufacture of bricks could be also an alternative route by obtaining economic value while exhibiting low environmental impacts (0.32 REQs), mainly on marine and freshwater ecotoxicity (0.7 kg 1,4-dB eq). These impacts can be notably reduced if waste materials are used replacing clays or using renewable energies. The encapsulation of these wastes is also a suitable option with a low environmental impact (0.46 REQs), but slightly greater than the brick manufacturing and with no economic return.