THE ACID MINE DRAINAGE LEGACY: RESOURCE RECOVERY AS A REMEDIAL OPTION

The two most common remedial options for acid mine drainage (AMD) emanating from legacy mine tunnels in the western U.S. have been mine plugging and treatment with lime neutralization. Mine plugging has high risks including: formation of a highly acidic mine pool; potential for uncontrolled leakage via faults, fissures, adits, and drillholes; corrosive reaction with wall rock and plug; and blowouts, with potentially devastating consequences downstream. Lime neutralization allows much greater control during remediation, but necessitates treatment until all the reacting sulfides are spent which could take hundreds to thousands of years. Neither of these options appear to be a permanent or sustainable solution to the problem. An alternate approach is to consider AMD as a potential resource rather than a waste. AMD typically has concentrations of metals high enough to be toxic to the environment but too low to recover economically by conventional means. Numerous techniques could be used to recover metals, sulfate, and water, if devised properly. They include chemical separation (oxidative precipitation, sulfide precipitation, solvent extraction), electrochemical separation (metal replacement such as Cu cementation, electrowinning, electrocoagulation, hydrolysis, electrodialysis), physicochemical separation (reverse osmosis, evaporative crystallization, freeze crystallization, ion exchange), microbiological separation (sulfate reduction, Fe reduction and oxidation), and combinations of these. Some of these are already in use at some sites. It is useful to classify AMD according to its composition (pH range, presence of metals in addition to Fe and Al, sulfate concentration range, etc.) to determine the most relevant potential products and effective technologies for their recovery. The four highest priority constituents to target in AMD with pH < 3 are Fe, Al, H⁺, and sulfate. As an example, separate Fe by oxidation and precipitation, then add lime to a targeted pH (~5) to remove Al, and finally remove gypsum with cation-anion exchange. If Cu and Zn are present, the sequence could be Cu cementation, followed by Fe oxidation and precipitation, then Zn-sulfide precipitation, ending with pH adjustment for Al removal. Careful consideration of AMD composition will dictate optimal recovery methods.