The water quality of mine drainage predicted by hydro-chemical modeling is sensitive to the approach implemented when going from representative samples to the prediction model. In order to estimate the reactivity of solid samples, kinetic tests must be used. For this, several methods exist (e.g., humidity cells and on-site leach pads), but the international standard has been for many years the Humidity Test Cells (HTC) as described in the ASTM D5744-18. Nevertheless, this method does not intend to reproduce actual leachate from the field nor simulate site-specific conditions.
This work compares three approaches used to predict the quality of leachates produced by a 30 m high Waste-Rock Dump (WRD) under a hyper-arid climate (i.e., precipitation rates up to 5 mm/year) over a 500-year period. The first method corresponds to the direct HCT leaching rate scale-up. The other two methods correspond to the calibration of the abundance and reactive surface of the mineral assemblage responsible for the observed HCT leaching rates considering a batch reaction model and a 1D reactive transport column. This comparison provides insights to the sensitivity of the scaling method on the predictions of water quality over mine infrastructure.
This work concludes that long-term water quality predictions, using the direct HCT leaching rate scaling method, yields physicochemical conditions that resemble up-scaled dissolution of the most soluble minerals. However, under a hyper-arid climate, water infiltration rates can be comparable to kinetic reaction rates of minerals such as silicates, and, therefore, the underlying geochemical processes at WRD scale must consider reactions of fast- and slow-weathering minerals, and the precipitation of secondary minerals. In this regard, the calibration of the abundance and reactive surface of the mineral assemblage responsible for HCT elemental loading rates must be used for up-scaling purposes. This up-scaling method allows the implementation of mineral reactions acting at different time scales, such as sulfide oxidation, silicates hydrolysis, as well as secondary mineral dissolution and precipitation.
Since mineral weathering rates determined/calibrated from laboratory generally do not agree with those observed in the field, applying this up-scaling approach at multi-scale levels using 10-m high pile experiments, field barrels, laboratory columns and HTC, is highly recommended.
