Investigation of scale inhibitors effect on calcium carbonate nucleation process

Calcium carbonate precipitation has been studied for more than a century. Nevertheless, the early stage of CaCO₃ nucleation process still attracts the attention of researchers. In this paper, a new method to investigate the effect of scale inhibitors on this nucleation is presented. It consists on the modeling of the electrical conductivity evolution during CaCO₃ precipitation by using a CO₂-degas-sing method called the fast controlled precipitation method (FCP). Tested inhibitors were sodium polyacrylate (RPI) and sodium-tripolyphosphate (STP). Results shown that nucleation process is governed by two main steps; in the first one ion pairs are formed by the association of Ca²⁺ and CO₃ 2– ions. The second one consists on the agglomeration of the formed ion pairs to form stable nuclei that can grow. Adding scale inhibitors slowed-down the nucleation rate and affects these two steps, especially the ion pair agglomeration; the ion pair formation and precipitation threshold requires a higher supersaturation coefficients showing that the inhibitor acts on the nucleation process by preventing the association of calcium and carbonate ions and disrupting the aggregation of the formed ion pairs. It was also shown that added chemicals inhibit the homogeneously formed CaCO₃ nuclei and affect the micro structure of the precipitate.