Quantifying and treating contaminant discharges from the James Mine on New Zealand’s West Coast
Thesis DisciplineEnvironmental Sciences
Degree GrantorUniversity of Canterbury
Degree NameMaster of Science
New Zealand's West Coast is home to numerous unmitigated legacy mining complexes and galleries that regularly release acid mine drainage (AMD) into the downstream environment. In a typical setting, standard in-situ passive treatment technologies would be highly recommendable. However, a comparative examination between conventional technologies such as RAPs, SAPs or ALDs illustrate the inherent short-comings they exhibit when treating highly contaminated mine waters. The James Mine is a legacy coal mine that exhibits AMD with high acidity and metal loads. Furthermore, the mine’s topography and local high precipitation rates result in varying flow-rates representing significant challenges to treatment technologies.
The present research sought to identify and test an effective method to treat the James Mine effluent at its source and was based upon laboratory research and field experiments. Specifically, this study investigated the premise that in sites such as the James Mine, conventional treatment solutions would not function properly due to either site limitations or the high concentrations of trivalent metals. Moreover, these site parameters would invariably lead to a loss of porosity and a loss of reactivity as the armouring of the reactive media takes place. This research investigated the Dispersed Alkaline Substrate (DAS) technology as a solution to these problems. The DAS system works on the foundation of applying a fine-grained reactive substrate mixed with a coarse inert substrate. The fine-grained alkaline reagent retards passivation due to it dissolving almost entirely before the process can occur while also providing a significant reactive surface. The mixed media as a whole provides not only a means of dispersion for nuclei allowing for precipitates to form on the inert material but also provides for a large reactive surface from the fine-grained alkaline reagent.
Laboratory and field experiments demonstrated that the DAS was able to abate AMD consisting of high metal concentrations with pH ≈ 2.54 and an average acidity of 1349 mg/L as CaCO3. Peak performance of the field experiments showed a metal removal rate of 99 % (Fe, Al and Cu), 40 % (Mn), and 91 % (Zn and Ni) while increasing pH levels to > 6.40. Depth profiles provided chemical data that was used to create reactive transport models while hydraulic parameters were calculated during the experimental phase to determine hydraulic residence times (HRT). Each aspect assisted in identifying the observable changes in the chemistry of the AMD as it traversed through the system allowing for a more comprehensive analysis. It should be noted that the size of the DAS implemented was big enough to treat 2150 L/day of AMD effectively; however, the high acid load of the AMD quickly exhausted the neutralising capacity of the reactive substrate.
Chemical and mineralogical analyses demonstrated the effectiveness of targeted metal removal in the different stages of the DAS which was designed to ensure there would not be an overlap in precipitates. More notably, the analyses illustrated the metal removal mechanisms at play as the primary method of precipitation, co-precipitation or adsorption occurred as sulphates, (oxy)hydroxides, and carbonates. As suspected, Mn and the trace metals were the most difficult to remove, with removal likely occurring through adsorption onto the organic matter, the results of which can be concurred with the geochemical modelling results.
Overall, this study demonstrated that if the DAS was to be up-scaled to a full-scale system it could not only prove to be an effective means of treating heavily influenced trivalent mine waters but also divalent affected mine waters.