An investigation is presented into the mechanisms involved in the leaching of inorganic components from oil shale mined at the Rundle deposit, Queensland, and retorted by the Lurgi-Ruhrgas process. The aim of the study was to develop a model w hich describes the generation and transport of the major components (i.e. Ca, Mg, Na, K, Cl and SO4 ) of the leachate as it moves through retorted shale under conditions of unsaturated flow at a temperature of 25oC.
A review of previous work on leachate generation from shale and other materials identified the most likely controlling mechanisms as dissolution, cation exchange, solution speciation, and hydrodynamic effects. Each of these phenomena was investigated independently, using experiments designed to characterise them in a way suitable for incorporation into a solute transport model.
Particle leaching kinetics were studied with extra-particle mass transfer effects being minimised by the use of spinning basket or shaker experiments. A comparison of the leaching kinetics of different particle sizes indicated no evidence of control by intraparticle diffusion. Other experiments were carried out using representative samples of particle size range < 1 µm - 3000 µm. The leaching kinetics of Mg, Na, K and Cl were rapid (≈ 90% dissolved w ithin one hour) and controlled by dissolution kinetics or limiting extra- particle mass transfer. This was consistent with the residence of most dissolvable salts being close to the e xternal sur face of the shale particles. However, leaching of CaSO4 salts was much slower and dependent on solution concentratio ns. This was an indication of control b y limiting extra-particle mass transfer with the "saturation deficit" of the solution with respect to CaSOhttps://espace.library.uq.edu.au/js/fckeditor/editor/images/spacer.gif being a concentration driving force.
The exchange properties of the retorted shale were studied in a series of batch experiments designed to investigate their variation in magnitude and selectivity with solution conditions. Shale was identified as a composite of materials with differing exchange properties. Within the carbonate buffered pH range of leachate (7-8.5 ), the cation exchange capacity (CEC) of the shale was only weakly pH dependent with values ranging between 21 and 26 meq/100 g. The higher values applied after the shale had been subject to weathering. No anion exchange capacity (AEC) was observed. Competition of cations for exchange sites was described using Gapon equilibrium expressions. The general order of adsorptive preference was:
Ca2+ > Mg 2+ > Na+ > K+
Although the selectivity coefficients characterising equilibrium between the different species were found to be functions of solution composition, average values were calculated for use in the solute transport model. ..............................