Weathering kinetics of sulphidic mining waste

Sammanfattning: The leaching of soluble metal contaminants from mining wasteis of great environmental concern, since it represents a threatto the surrounding aquatic environment. In this thesis,geochemical processes that contribute to the release of acidicmine drainage from waste rock are investigated. The Aitik site,which is currently Europe's largest copper mine, has been usedas a case study. Interpretations of field measurements,large-scale column experiments and laboratory-batch experimentsare included.A conceptual model has been developed to describe the waterrock interactions within sulphidic waste rock. The model canexplain the composition of the drainage water from the Aitikwaste rock dumps and is also consistent with results from batchand column experiments using Aitik rock. The model includes theweathering of sulphide, carbonate and primary silicate mineralsand theeffect of this weathering on the release of net acidityfrom the site. The relative rates of these processes determinethe pore water pH, which may vary locally within the dump. TwopH-control mechanisms have been observed: a rapid dissolutionof calcite maintaining a neutral pH in fresh waste rock, andsulphide and silicate mineral weathering at pH 3-4 afterdepletion of available calcite. The change between the two pHregimes had a major influence on the copper concentration,which increased from about 0.5ppm up to S00ppm in the drainagefrom laboratory columns.The experiments identified a scale-dependent mobilisation ofcopper (batch experiments: 1.5 x 10-1kg, column experiments: 1.5 x l03kg and the waste rock dump: 1.5 x l011kg). The lower release of copper in thefield-scale studies can be explained by slower sulphideoxidation and a greater retention of copper at neutral pH. Thecopper is almost insoluble at neutral pH but highly soluble atacid pH, and thus becomes mobile. Fractions of the waste rockdump with a lower sulphide content, a lower temperature, andlarger particle sizes are factors expected to contribute to aslower sulphide oxidation. Fractions of waste rock with aneutral pore water in which copper accumulates can be relatedto the strong variability in the sulphide and carbonate mineralcontent in the field scale.The long-term evolution of copper release from the sitedepends on the consumption and eventual depletion of sulphideminerals. The time to sulphide depletion increases stronglywith increasing particle size. The most intense sulphideoxidation and acidity release, due to the weathering of finematerials, was estimated to cover a period of several decades,with a gradual decrease over several centuries. The fractionsof the waste rock dump where acidic conditions do not developand where sulphidesare imbedded in rock-forming minerals(large particles) are expected to retain fractions of thecopper even after this period.KEYWORDS:Environmental assessment, geochemicalkinetics, geochemical modelling, the Aitik site, coppermobilisation, sulphide weathering, carbonate mineralweathering, silicate mineral weathering, batch experiments,column experiments, field observations.