Calcium Carbonate Dissolution Rate in Limestone Contactors

The project summarized here investigated some of the parameters and relationships used to predict the performance of limestone contactors. The purpose of the project was to study the effect of limestone composition, especially the dolomite [CaMg(CO 3) 2 ] and impurity content of the stone, on the kinetics of carbonate mineral dissolution and to determine the effect of temperature on the rate of dissolution. The rate of dissolution was determined by using a rotating disk apparatus and samples of limestone of varied composition. The limestone samples included a white marble and a selection of sedi-mentary stones. The white marble contained a significant amount of silica (approximately 35%). The major mineral constituents of the sedimentary limestones ranged from approximately 100% calcite (CaCO 3) to essentially pure dolomite. The approximate iron (Fe) content of the stones ranged from 15 to 377 mg Fe/100g and the approximate aluminum content (Al) from 1 to 134 mg Al/100g. A heterogeneous reaction model for mineral dissolution effectively explained the results of the rotating disk experiments for all samples except the two with the highest dolomite content. The magnitude of the dissolution rate constant for fresh stone decreased by approximately 60% as the calcite content of the stone decreased from 0.92 to 0.09 g CaCO 3 /g stone. The rate of dissolution of stones with a high dolomite content may have been enhanced by the presence of small amounts of cal-cite. The rate of solubilization of magnesium (Mg) was negligible in all samples except the two with the highest dolomite content (93 and 100 mass percent dolomite). The overall dissolution rate constant decreased as the amount of calcium dissolved from the surface of the stone increased. For a given amount of calcium dissolved per unit area of stone surface, the magnitude of the percentage decrease in the dissolution rate constant increased as the iron and aluminum content of the stone increased. The effect of sample aging on the rate of dissolution was lowest when the weighted sum of the iron and aluminum content of the stone was less than about 10 mg/g. The weighted sum is equal to the aluminum content in mg Al/g plus 0.30 times the iron content in mg Fe/g. The presence of silica as the principal impurity in the white marble seemed to reduce the effective (cal-cite) surface area of the stone in proportion to the mass of silica in the sample, but …