Mass transfer of reactive crystallization in synthesizing calcite nanocrystal

An experimental study on the mass transfer and reactive crystallization of calcite nanocrystal was carried out in a Na5P3O10–CaNa5P3O10–Ca(OH)2–CO2–H2O(OH)2–CO2–H2O multiphase system. According to the two-film theory and the two-steps crystal growth model, the kinetic characteristics in the system were investigated by the rate-comparison of the calcium hydroxyl dissolution, carbon dioxide absorption and calcium carbonate precipitation. The result indicated that the transfer resistance for the carbon dioxide absorption was negligible during the reactive crystallization, the rate-controlling step would shift from the crystallization of calcite to the dissolution of calcium hydroxyl at a turning time θcθc. When reaction time was less than θcθc, the calcite crystallization could be the rate-controlling step; while reaction time was larger than θcθc, the calcium hydroxyl dissolution became the rate-controlling step. The analysis of the relationship of Damköhler number Da   with surface integration effective factor ηη showed the crystallization kinetics would be affected by sodium tripolyphosphate. When [Na5P3O10]=0μM, the calcite crystal growth could be controlled by the growth unit transport from liquid bulk to the crystal boundary. With an increasing sodium tripolyphosphate concentration, the rate-controlling step could be shifted from the bulk diffusion to the surface-reaction of growth units.