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Solubility Relationships and Mineral Transformations Associated with Recarbonation of Retorted Shales¹

ABSTRACT

Oil shales are heated at high temperatures to obtain oil. During this process carbonate minerals are destroyed by driving off CO₂(g) and large amounts of waste materials (retorted shale) are produced. The pH of such retorted shales often approaches 12.0. With time, the pH of these materials will drop by attaining equilibrium with calcite (CaCO₃) and atmospheric carbon dioxide (recarbonation). The objective of this paper was to examine solubility relationships and mineral transformations associated with recarbonation of retorted shales. Carbon dioxide gas (CO₂[g]) was bubbled through two shale samples, one retorted by Los Alamos National Laboratory (LANL) and another retorted by Lurgi Ruhrgas (Lurgi) to lower their pH. X-ray diffraction analysis was used to identify different mineral phases of retorted and recarbonated shales. The results show that recarbonation caused silicate minerals to dissolve, CaCO₃ to precipitate, and lowered pH from approximately 12.0 to 8.0. Solubility measurements showed that upon recarbonation Ca²⁺ activity for both shales approached equilibrium with CaCO₃ and measured CO₂(g). Initially Mg²⁺ activity appeared to be in equilibrium with either diopside (CaMg[SiO₃]₂) or antigorite (Mg_2.825Si₂O₅(OH)_3.65) and measured silica, but not with dolomite (CaMg[CO₃]₂) nor magnesite (MgCO₃). After 20 d of recarbonation for LANL, Mg²⁺ solubility decreased from 10^–2.78 M to 10^–3.15 M. The experimental findings in this study further suggest that bubbling CO₂(g) through retorted shales can reduce the concentrations of F and, in some cases Mo, in leachates.

Key Words: carbon dioxide • calcite • precipitation • leachate • groundwater contamination

¹ Funding was provided in part by U.S. Dep. of Energy Grant no. EV/4018-17.

² Graduate Research Assistant, Professor, Graduate Research Assistant, and Associate Professor, respectively, Dep. of Agronomy and Dep. of Range Science, Colorado State Univ., Ft. Collins, CO 80523.

Received for publication July 29, 1985.