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WWRC 89-37
Application of a Geochemical Model to the Prediction of the Chemistry of Extracts from Non-Recarbonated and Recarbonated Spent Shale


The chemistry of extracts from spent shale disposal environments is controlled by a number of chemical processes including the formation of inorganic complexes and precipitation-dissolution reactions. Several geochemical codes such as EQ3/EQ6, GEOCHEM, MINTEQ, PHREEQE, and SOLMNEQ are available to predict these chemical processes. However, these codes lack data for solution species (association constants) and solid phases (solubility products) that are relevant to spent shale. The objectives of this study were (1) to alter the data base of a geochemical code by including data for different solution species and solid phases that are important to spent shale environments and (2) to predict the chemistry of extracts from non-recarbonated and recarbonated spent shales by evaluating the solid phases controlling the solubilities of major elements such as calcium and magnesium, and trace elements like flourine and molybdenum.

The geochemical code used for this study was the GEOCHEM model. The data base of this code was altered using updated association constants and solubility products for different elements that we, and others have developed.

Three spent shale samples (PPP3, PPP6, and Lurgi) were recarbonated under slightly elevated C02 pressure in a specially designed chamber. The non-recarbonated and recarbonated samples were subjected to solubility studies. The total elemental concentrations and pH of extracts were used as input to the GEOCHEM code to predict the chemistry. The model predicted that silicate phases (produced during the heating process) initially buffered the pH and controlled 2+ and Mg2+ concentrations in extracts. The recarbonation process in spent shales probably dissolves silicates, precipitates carbonates, and reduces the pH of extracts to approximately 9.0. The GEOCHEM model predicted that after recarbonation, the 2+ concentrations are controlled by calcite. The Mg2+ concentrations may be controlled by both clinoenstatite and magnesite. The F- concentrations in extracts from non-recarbonated and recarbonated spent shales are controlled by fluorite. Further, GEOCHEM predicted that powellite does not control the solubility of molybdenum in non-recarbonated recarbonated spent shale extracts.

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