Inhibition of CaCO3 growth and synthesis of submicron particles by preferential adsorption of additive Ca2+ ions on fresh precipitates

Abstract

This study demonstrates a method to inhibit the growth of CaCO₃ and synthesize submicron particles in a chemical precipitation process under ambient and high supersaturation conditions. Equimolar CaCl₂ and Na₂CO₃ solutions were mixed in a model tubular reactor at a constant flow rate, and the precipitates were continuously dispersed in stirred 250 mL of 10 mM Ca(OH)₂ solution. This approach resulted in the synthesis of colloidally stable submicron CaCO₃ particles for a precipitant concentration ≤75 mM. Varying the precipitates’ retention time in the tubular reactor had no significant effects on the particle size and colloidal stability. Time-dependent changes in the mean size, crystal form, morphology and specific surface area of the synthesized particles were also studied. For a precipitant concentration of 75 mM, the particles were monodispersed and porous spindle-like scalenohedral crystals which gradually grew in all faces as more precipitates were fed into the Ca(OH)₂ solution. The mean hydrodynamic size of the particles was ∼850 nm at the 8th minute. However, in the absence of additive Ca²⁺ ions, the particles obtained at the 8th minute were polydisperse mixtures of vaterite and rhombohedral calcite particles greater than 4 μm in size. The results show that free additive Ca²⁺ ions are irreversibly adsorbed onto the particles as the precipitates dissolve and recrystallize into smaller crystals upon reaching the Ca(OH)₂ solution.