Geochemical Transport Modelling of Long Exposed Acid Generating Tailings with a Monolayer Cover.

Mine tailings containing iron sulfides may generate acid mine drainage (AMD) when put in contact with oxygen and water. The resulting acidic effluent often contains high concentrations of dissolved metallic ions and sulfates which may migrate out of the impoundments. Considering that both oxygen and water are required for the oxidation reaction to occur, reclamation methods mainly tend to restrain the access of one of these components to the sulfide minerals.

The efficiency of monolayer covers for controlling acid mine drainage for two long exposed acid generating tailings impoundments is assessed using simulations conducted with the MIN3P finite volume model (Mayer et al., 2002). This code takes into account coupled groundwater flow, oxygen diffusion and multi-component reactive transport. The model is applied to simulate the experimental results obtained from large drainage columns tests conducted on partially oxidized tailings from two sites located in Quebec (Canada).

The columns (230 centimeters high, 15 centimeters in diameter) were filled with tailings covered with a single layer of relatively fine grained material, i.e. a widely graded till (for two columns) and a non acid generating tailings (for two others). Monthly wetting and drying cycles were applied using demineralized water added on the top of the columns. Water content, suction and oxygen concentrations in the columns were monitored during 18 drainage cycles. The effluent was sampled at the bottom of the columns for chemical analysis including pH, electrical conductivity, redox potential. The concentrations of sulfate, iron and 14 other elements (including copper, aluminum, calcium and magnesium) were determined using atomic absorption and spectrophotometry. XRD and ICP-AES analyses were also performed to complete the characterization and to obtain the parameters required for the simulations.

The MIN3P model was used to simulate water and oxygen fluxes as well as geochemical reactive transport in the columns. Weathering and depletion of iron sulfides during the oxidation process was simulated using the shrinking core model within MIN3P. Other reactions, such as indirect oxidation by Fe³⁺, are also being considered. Once the column models were calibrated using laboratory (and in situ) measurements, simulations were run to extrapolate results over space and time. Alternative reclamation methods for both sites were also analyzed. The main results from this investigation are presented and discussed in the presentation.