Recovering resources from abandoned metal mine waters : an assessment of the potential options at passive treatment systems

Abstract

Remediation of metal-rich discharges from abandoned mines entails capture of metals within a treatment system and, typically, disposal of the waste. A preferable option would be to recover the metals for reuse. For many long-abandoned mines metal loads are often relatively small, albeit they often cause significant environmental pollution. Low-cost passive treatment systems, in which metals are retained in some form of treatment substrate, such as compost, are often preferred. This thesis investigates the amenability of such treatment systems to resource recovery. Two down-flow compost bioreactors, treating zinc-rich discharges, were the focus of the research: a pilot-scale unit at Nenthead, and a full-scale system at Force Crag, both in Cumbria, England. Laboratory investigations of the Nenthead substrate identified 7,900mg/kg zinc in the upper horizons of the substrate, and 2,400mg/kg in the lower horizons, after two years of operation. Acid leaching tests effectively de-contaminated the substrate with respect to zinc and cadmium. Complete recovery of zinc was observed after ≤30 hours across a range of acid leach tests, although 23-37 days were required before equivalent recovery was achieved by biological leaching. The Force Crag system removed >95% zinc over the first year of operation and, removal rates suggest that after 10 years of operation >20,000mg/kg zinc will have accumulated in the substrate. Substrate de-contamination could offer substantial life-cycle cost savings at passive treatment sites, especially by limiting volumes of material for disposal to landfill. Furthermore, recovery of metals has important implications for resource sustainability and circular economics. Other resource recovery options may exist at abandoned mine sites. At Force Crag 1.6kW of kinetic energy exists in flowing mine water, in addition to thermal energy which could be recovered for space heating applications. Recovering this energy would convert this site into a net-generator of power. Because of their often remote locations, renewable energy may be of particular value to off-grid facilities at some mine sites.