Controls on Zn Concentrations in Acidic and Neutral Mine Drainage from New Zealand’s Bituminous Coal and Epithermal Mineral Deposits

Abstract

Zinc is one of the most abundant transition metals in mine water from coal mines and epithermal metal mines in New Zealand. Zinc is commonly conservative in mine drainage systems and remains soluble at concentrations that exceed guideline values for protection of aquatic ecosystems. This study presents a compilation of Zn concentrations and geochemical data from acid and neutral mine drainage sites. Geochemical modelling using PhreeqC indicates that secondary Zn minerals are unlikely to influence Zn concentrations except where Zn exceeds 100 mg/L and mine drainage pH [ 7. Adsorption modelling using WHAM indicates that Zn is unlikely to be attenuated by adsorption onto Fe and Al precipitates that could form during neutralisation or oxidation of mine drainages, except when the ratio of precipitate to dissolved Zn is high (&100 mg/L precipitate and 0.1 mg/L Zn) and the pH [ 6. Instead, Zn concentration is controlled by the source minerals and their rate of dissolution and dilution, rather than precipitation of secondary Zn minerals or adsorption. At some abandoned mine sites, or mineral prospects where leachate testing has been completed, Zn (±acidity) is the main chemical parameter that must be removed before discharge to the aquatic environment. Our results and interpretations constrain options for water management at future mine sites in New Zealand where Zn is present to: active treatment, passive treatment using reducing systems, or passive treatment that incorporate CaO-bearing reagents to achieve a pH [ 7. Conventional limestone-based passive treatment systems are unlikely to remove Zn because pH [ 6 is seldom achieved in such systems in an acceptable reaction time. Active treatment is unlikely to be possible at sites established to clean up abandoned mines sites, and therefore passive treatment should include a reducing system to remove Zn in order to meet guidelines for protection of aquatic ecosystems.