Synthesis of Amorphous Calcium Carbonate and Its Crystallization

Abstract

The synthetic conditions of amorphous calcium carbonate such as initial concentration of reactant, temperature and pH in the reaction of CaCl₂-Na₂CO₃-NaOH system, and its crystallization in water and NH₄Cl solution were investigated. Characteristics of the amorphous phase were determined by means of X-ray diffraction, thermal analysis (TG-DTA), scanning electron microscopic observation and measurement of solubility. The amorphous calcium carbonate (ACC) and amorphous basic calcium carbonate (ABCC) were precipitated by adding 0.1mol⋅dm^-3 CaCl₂ solution into the mixed solution of 0-3.0mol⋅dm^-3 NaOH and 0.1mol⋅dm^-3 Na₂CO₃, and then the precipitated colloidal phases were filtered and dried rapidly through dry air under reduced pressure. The stable phases of ACC and ABCC were formed in the region of pH 11-14 at 0°C but the formation regions of both amorphous phases were remarkably affected by pH in the mother liquor. Namely, ACC was formed at pH 11.0-13.5 and ABCC at pH 13.5-14.0. Their compositions were formulated as CaCO₃⋅1.5H₂O for the former and Ca₃(OH)_6X(CO₃)_3-3X⋅3YH₂O (0<X≤0.2, 1.5>Y≥1.2) for the latter. These results show that increase of pH contributed to the partial replacement of CO₃^2- of ACC by OH^-, which resulted in the formation of ABCC with different compositions. ACC was dissolved rapidly in water and its highest solubility was 1.8×10^-4mol Ca²⁺/100cm³ H₂O, i.e. ten times larger than that of a commercial calcium carbonate (calcite-type). The dissolution of ABCC was affected by the amount of OH^- contained in the random structure and the highest solubility of ABCC with composition Ca₃(OH)_0.72(CO₃)_2.64⋅4.14H₂O was 4.0×10^-4mol Ca²⁺/100cm³ H₂O after dipping in water for 10min. The control of crystal shape and modification of calcium carbonate were investigated by dipping spherical secondary-particles of ACC in water and 1mol⋅dm^-3 NH₄Cl aqueous solution, respectively. ACC was crystallized into rhombohedral-like calcite with about 4μm in size at 20°C, needle-like aragonite with length 1-3μm and diameter 0.2μm at 80°C when dipped in water and spherical vaterite with diameter 0.5-1.5μm in 1mol⋅dm^-3 NH₄Cl aqueous solution.