Controlling factors and mechanism of reactive crystallization of calcium carbonate polymorphs from calcium hydroxide suspensions

The crystallization was carried out by adding the Na2CO3 solution to the Ca(OH)2 suspension, and the controlling factor and the mechanism of the crystallization of calcium carbonate polymorphs were investigated. The reaction between calcium hydroxide and sodium carbonate proceeded almost with the stoichiometric ratio when the addition rate of Na2CO3 solution was low; however, at fast addition rate calcium hydroxide solids remained even when an excess amount of sodium carbonate was added. This may be due to the limit of the dissolution rate of the calcium hydroxide. Calcite precipitates from the start of the reaction; however, aragonite begins to precipitate at the molar ratio of sodium carbonate to calcium chloride of around 0.3, and the composition of aragonite increases with time and attains a constant value. The alkaline solutions at pH larger than about 13.5 are considered to be advantageous for the nucleation of aragonite. The degree of the solubility of calcium hydroxide may contribute to this behavior. The crystallization of aragonite crystals was accelerated by decreasing the addition rate of sodium carbonate solution and at the fast addition rate calcite crystallized preferentially. On decreasing the addition rate, the amount of aragonite crystals with long needle-like morphology increased. With the increase of the solution volume and the stirring rate, the crystallization of aragonite was also accelerated and the amount of agglomerated fine particles of calcite decreased. These results indicate that if the concentration of calcium ions is at or near the solubility of Ca(OH)2, the low concentration of carbonate ion in the “diffusion field” around the droplets of sodium carbonate solutions (low local supersaturation) is advantageous for the crystallization of aragonite.