The mechanism of precipitation of chain-like calcite

Abstract

In this work a mechanism for the precipitation of chain-like calcite agglomerates is proposed and compared to those ones leading to nano- and microcalcite in the system Ca(OH) 2–CO 2–H 2O. The experiments were carried out by semicontinuous carbonation of a slaked lime (Ca(OH) 2) suspension at 20°C, 25°C and 30°C, fixing the suspension conductivity at 5 mS/cm. The processes at 20°C and 25°C were essentially similar except in the final stage. Basically, they both occurred via surface and were characterized by the initial precipitation of amorphous calcium carbonate followed by its crystallization into chain-like aggregates of calcite. These aggregates acted as an intermediate for the final products: spheroidal nanometric calcite at 20°C and submicrometric chain-like calcite agglomerates (similar to batons) at 25°C. On the other hand, at 30°C, the precipitation took place preferentially via solution, producing micrometric calcite of scalenohedral habit as final product. The change in the mechanism of precipitation from preferentially via surface (at 20°C and 25°C) to preferentially via solution (at 30°C) seems to be directly related to the availability of Ca(OH) 2 surface in the reaction mixture during the first steps of precipitation.