Study on crystallization of calcium carbonate from calcium sulfate and ammonium carbonate in the presence of magnesium ions

Batch reactive crystallization of calcium carbonate (CaCO3) from ammonium carbonate ((NH4)(2)CO3) and calcium sulfate (CaSO4) was investigated in the presence of magnesium (Mg2+) ions. It was observed that Mg2+ ions partly inhibited the conversion of CaSO4 into CaCO3. When the content of Mg2+ was less than 2%, the reduction in conversion rate of CaSO4 was less than 2%, and the effect of Mg2+ ions could be ignored. Effect of impurity on crystallization kinetics of CaCO3, including the growth rate and nucleation rate, was investigated. The results revealed that when Mg2+ ions content was less than 1%, Mg2+ could promote the growth of CaCO3 and inhibit the nucleation process, which was favorable for the filtration of CaCO3.When the content of Mg2+ ions was greater than 1%, Mg2+ inhibited the growth of CaCO3, which resulted in explosion nucleation and led to a large number of particles in the solution, which was unfavorable for the filtration of CaCO3. Based on the Bransom model, the particle size distribution equations of CaCO3 were established. X-ray diffraction patterns and scanning electron microscopy images exhibited the existence of spherical vaterite of CaCO3 due to the reaction of CaSO4 with (NH4)(2)CO3 under the effect of Mg2+ ions, which was inconsistent with the results reported in the literatures.