Formation and aggregation of polymorphs in continuous precipitation. 2. Kinetics of CaCO₃ precipitation

Abstract

Experiments with the continuous precipitation of calcium carbonate are reported in this study. Particle size distributions were measured on-line and interpreted by means of the simultaneous nucleation and the transformation models developed in part 1 of this sequence. Three regimes of precipitation are reported in this study, governed by the supersaturation and liquid phase [Ca²⁺]/[CO₃²⁻] ionic ratio. These comprise of a heterogeneous regime involving calcite and vaterite nucleation at moderate [Ca²⁺]/[CO₃²⁻] ionic ratio in an unscaled reactor, homogeneous nucleation of vaterite followed by transformation to calcite in a scaled reactor at low and moderate [Ca²⁺]/[CO₃²⁻] ratio, and formation of stable vaterite at high [Ca²⁺]/[CO₃²⁻] ratio. The particle size distributions in the different regimes are well fitted by the applicable model from part 1 or earlier studies of single-species precipitation. Where appropriate, kinetic parameters obtained from the fits are correlated with supersaturation by power law models.