Factors controlling the leaching of major and minor constituents from processed rundle oil shale

Abstract

The chemical characteristics of leachates from columns of processed oil shale from the Rundle resource, Queensland, were determined and the factors controlling the rate of release of the major and minor constitutents were delineated. The studies were carried out in two types of laboratory columns and a weathering column (lysimeter) exposed to prevailing weather conditions. The flow in the laboratory columns was maintained in an unsaturated mode in order to simulate the flow conditions found in the field. The major constitutents leached from Rundle spent shale are the sulphates and chlorides of Ca, Mg, K and Na. Most of these salts are eluted in the first few pore volumes of leachate. The only major constituents significantly constrained by solubility factors are Ca and SO 4, this being due to solution saturation with respect to gypsum (CaSO 4 · 2H 2O). Chemical equilibria with respect to the ion pairs CaSO 4 o and MgSO 4 o are important in determining the rate of release of Ca, Mg and SO 4. Ion exchange and sorption phenomena are significant in controlling the leaching rate of both the major cationic metal species and the minor (or trace) elements. Many of these minor elements are probably in the anionic form (e.g. As, Se and V), yet are still absorbed by the shale which exhibits no significant anion exchange capacity. Weathering reactions appear to play a significant role in the continued slow leaching of the major and minor species which is observed even after many pore volumes of liquid have eluted.