Weathering kinetics of waste rock from the Aitik copper mine, Sweden: scale dependent rate factors and pH controls in large column experiments

Geochemical processes in mining waste rock, including sulphide weathering, heavy metal release (Cu and Zn), and acidity consumption by carbonate and silicate mineral weathering have been investigated. We operated six large columns, each containing about 1.6 tons of waste rock, with unsaturated water flow for nearly 3 1/2 years. The column results illustrate two pH controls: (1) sulphide oxidation with calcite dissolution sustaining a neutral pH, and, (2) simultaneous sulphide oxidation and weathering of primary silicate minerals at pH 3–4. The column effluents either remained at near neutral pH or became acidic after a lag period of between 0.5 and 3 years. For columns developing acidic effluents, there was an associated mobilisation of copper with maximum copper concentrations as high as 500 ppm. The variable calcite content of the rock appears to be an important factor for controlling the effluent pH within the columns. Extrapolating these results to the field scale, suggests that the much slower than expected copper release at the site is due to fractions of waste rock with very low sulphide content, fractions with significant amounts of calcite remaining, and slower weathering rates due to lower temperature, larger particle size and lower oxygen content. In addition, immobile water in the waste rock is expected to accumulate weathering products rather than release them with the site drainage.