EFFECTS OF A CO2 PRESSURE PROCESS ON THE SOLUBILITIES OF MAJOR AND TRACE-ELEMENTS IN OIL-SHALE SOLID-WASTES

Processing of oil shale at high temperatures produces a highly alkaline solid waste. The waste can be stabilized by a recarbonation process. In order to test a method for accelerating the recarbonation process, we exposed three moist oil shale solid waste (OSSW) samples to 5 psi CO2 pressure for 1 h. The treated and untreated samples were equilibrated with water for 7 days and the chemical composition of the aqueous extracts determined. Before CO2 treatment, the Ca2+ and Mg2+ concentrations appeared to be controlled by silicate phases present in the waste such as wollastonite (CaSiO3), forsterite (Mg2SiO4), and talc (Mg3Si4O10(OH)2), which buffered the pH at approximately 12.0. The CO2 treatment lowered the pH from 12.0 to approximately 9.0 through the formation of calcite. The Ca2+ concentrations from CO2-treated samples suggested a close approach to saturation with respect to calcite (CaCO3) whereas the Mg2+ concentrations appeared to be controlled by either magnesite (MgCO3) or possibly a silicate. The CO2 treatment generally decreased F and Mo concentrations in aqueous extracts. The F- concentration before and after CO2 treatment appeared to be controlled by fluorite (CaF2). Our results demonstrate that the CO2 pressure process is an effective means of reducing the pH and the concentrations of F and Mo in aqueous extracts from alkaline solid wastes.