Geochemical modelling of acid mine drainage in mill tailings Quantification of kinetic processes from laboratory to field scale

Sammanfattning: Assessment of the potentially acidic, heavy metal-ladenleachates that leave deposits of sulfide ore mill tailings andevaluation of various possible options for mill tailingremediation are scientific problems of increasing practicalimportance. High costs may be associated with the mill tailingremediation, not least after recent changes in Swedish andEuropean environmental legislation. This thesis presents amethodology for studying and quantifying geochemical processesthat contribute to generation of so-called acid mine drainage(AMD). The methodology builds from first principles regardinggeochemical processes, and is based on geochemicalcharacterisation of the mill tailings combined with explicitmodel quantification of the effect of factors, such astemperature, pH, and mineral (BET) surface area, that influencemineral weathering rates. Application of the modellingmethodology to a case study site, Impoundment 1, Kristineberg,northern Sweden, including quantification of slow processesthrough literature rate laws, successfully reproduced the pHand relative concentrations of major ions in the impoundmentgroundwater. Absolute concentrations of most major ions, withthe exception of Zn, were 1-2 orders of magnitude higher in themodel than in the field, which is consistent with the commonlyobserved scale dependence of mineral weathering rates; however,application of a single calibration factor, Xr=10-2, to all weathering rate expressions, sufficed toaccount for this apparent scale dependence.Subsequent laboratory determination of mineral weatheringrates in Impoundment 1 tailings indicated that rates for themajor minerals pyrite (FeS2) and aluminosilicates were in fact 1-2 orders ofmagnitude lower in the ~50-year-old tailings than ratesreported in the literature. Weathering rates of chalcopyrite(CuFeS2) and sphalerite (ZnS) were by contrast 1-3 ordersof magnitude greater than predicted by the literature rate lawsthat were used in the modelling study. While the mechanism ofZn release requires further investigation for improved forwardmodel prediction, the underestimation of Zn concentration inImpoundment 1 by the model was resolved. The laboratory studyfurthermore indicated that the weathering rates of most majorminerals exhibited the same dependence on pH, temperature andsurface area as reported in the literature, and therebysupported the use of literature rate laws for model assessmentof dominant geochemical processes in tailings deposits, onceallowance is made for lower rates in older tailingsmaterial.Analysis of the dominant geochemical processes in the modelof Impoundment 1 indicated that slow weathering ofaluminosilicate minerals provided the bulk of protonattenuation and, as a result, considerably affected the rate ofdepletion of fast-reacting pH-buffering minerals. Inclusion ofthe kinetics of aluminosilicate dissolution and of thefeedbacks between slow and fast processes is thus potentiallycrucial for prediction of pH and its long-term evolution. Thesensitivity of modelled groundwater composition and pH to ironredox reactions, such as may be accelerated by acidophilicbacteria, indicated that, while iron redox cycling was low atthe present case study site, quantification of microbialmediation of these reactions may be necessary for predictingAMD quality under other conditions. The laboratory studies alsoindicated that application of common sterilisation techniques,such as is necessary for study of relative contributions ofabiotic and biotic weathering processes, had little effect onthe long-term (>30 days) abiotic element release rates inthe tailings.This study suggests that within certain limits, which appearnarrower than currently recognised in industrial predictionpractices, it is possible to predict the weathering behaviourof major minerals, and hence proton release and attenuation, inbase metal tailings under field conditions.