Predicting the neutralisation of acid mine drainage in anoxic olivine drains

The precipitation of gypsum within anoxic limestone drains subjected to very acidic and sulphate rich acid mine drainage is a problem that can be prevented by initial neutralisation of the drainage using magnesium-rich olivine. The applicability of this concept is limited to the low pH region (i.e. pH < 3) where gypsum precipitation is a potential problem and the olivine dissolution rate is sufficiently fast to be of practical importance. This paper presents a simple finite element model for predicting the performance of olivine drains with respect to the neutralisation of highly acidic solutions under anoxic conditions and low pH. The model is based on the kinetic rate expression for olivine dissolution and the sulphuric acid dissociation equilibrium. To test the model, a series of column experiments were conducted in which olivine sand (0.1–1 mm) was subjected to different solutions of dilute sulphuric acid (0.0025–0.01 M) that was pumped through the column at various flow rates. For practical purposes the synthetic solutions contained no iron. The steady-state effluent pH obtained from each experimental run was used to calculate the relative amount of neutralisation. The neutralisation process, as expressed by the relative neutralisation values, was adequately described by the simple prediction model.