Lab to field scale effects on contaminated neutral drainage prediction from the Tio mine waste rocks

The quality of mine drainage water is often predicted with laboratory prediction tests. However, laboratory and field results sometimes show significant differences in release rates or contaminant concentrations in drainage waters. Understanding the scale influence on the different factors that affect the geochemical behavior of mine wastes is necessary in order to extrapolate results from the laboratory to the field scale. The main objectives of the present study are to compare CND prediction results obtained at the laboratory (with columns, humidity cells, and weathering cells) and field scales (field test pads, waste rock pile), and to identify the main scale factors involved. This work focuses on CND-generating waste rocks from the Tio mine (Rio Tinto), in Québec, Canada, where Ni concentrations in the drainage waters of some of the waste rock piles sometimes exceed the regulated limits. Results obtained in the present study suggest that the liquid to solid ratio differences between the scales have a significant impact on the geochemical responses of the waste rocks. The results also demonstrate that the humidity cell and weathering cell tests exaggerate the dissolution of neutralizing minerals versus the sulfide oxidation rate for the waste rocks studied. This phenomenon most probably occurs because of the high liquid to solid ratios of these laboratory tests with regard to field conditions. On the other hand, the column test seems to accelerate the release rates versus the field, while the ratios of neutralization to oxidation rates relatively remain close to that in the field.