The coprecipitation of cations with CaCO 3-III. The coprecipitation of Zn 2+ with calcite between 50 and 250° C

Abstract

The distribution of Zn and Ca between solution and calcite, in NaCl-free solutions and in concentrated Nad solutions has been measured at 50, 90, 167 and 250°C. Calcite was precipitated from aqueous solutions by the hydrolysis of a calcium-organic-acid-salt (trifluoroacetic, trichloroacetic or propionic acid). The value of the distribution coefficient, k zn c , in NaCl-free solutions at 167°C was found to be 57 ± 6. The value of k Zn c in runs in both 1 and 4 molar chloride solutions at 167°C tends to decrease with the percentage of calcium precipitated. This decrease in k zn c is thought to be due to some peculiarity of the surface of calcite during the early growth stage of calcite precipitation. If we exclude the contribution to the value of k zn c of the early growth stage of calcite precipitation, the value of k zn c in 1 molar chloride solution at 167°C is 1.3 and that of k zn c in 4 molar chloride solutions at 167°C is 0.11. The data concerning the variation of the value of k zn c with temperature are rather preliminary, but suggest strongly that k zn c increases rapidly toward lower temperature.