Acid dissolution of mineral matrix in Green River oil shale

The mineral matrix in Green River oil shale was partially removed by treatment with dilute HCl. The major ionic species in the solution from acid treatment (AT) were identified as Na⁺, Al³⁺, Fe²⁺, Mg²⁺, and Ca²⁺. The ion yields expected from reaction stochiometry, gravimetric analyses and comparison of calculated CO₂ yields with measured levels were consistent with the fact that Na⁺ and Al³⁺ originated primarily from analcite: Fe²⁺ and Mg²⁺ from dolomitic ankerite and Ca²⁺ from both dolomitic ankerite and calcite. Temperature and shale particle size were important parameters in the efficacy of AT. An increase in temperature and a decrease in particle size increased the rate of mineral dissolution. Fe²⁺ showed an anomalous trend in that the rate initially declined with increasing temperature after which it showed the usual increase with temperature. The kinetics of ion build-up in the solution from AT were analysed in detail for the case of Al³⁺. The Arrhenius expression was found to be valid only for finer particle sizes (e.g., ?35, +45 US mesh shale). A simple model is finally presented to account for the combined effect of temperature and shale particle size on mineral dissolution rates.