WWRC 91-15
Effects of a CO2 Pressure Process on the Solubilites
of Major and Trace Elements in Oil Shale Solid Wastes
Abstract
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 ~ 12.0. The CO2 treatment lowered the pH from 12.0 to ~ 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 flourite (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.
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