Crystallization of aragonite in the causticizing reaction

Aragonite, a polymorph of CaCO3, is used in the paper industry as filler and pigment. To determine the reaction conditions used during causticization, the solubility of Ca(OH)(2) and CaCO3 in NaOH solution and the crystallization behavior of aragonite were investigated. The solubility of CaCO3 in a NaOH solution reached a maximum value with NaOH concentration. Decrease in the solubility after that value was due to the conversion of CaCO3 to Ca(OH)(2). In 1 mol/l of NaOH solution containing a concentration of Na2CO3 greater than 0.02 mol/l, CaCO3, appeared to be in a stable phase as compared to Ca(OH)(2), In the crystallization experiments, it was determined that reaction temperature strongly influenced the size, shape, and polymorph of CaCO3. The highest precipitation of aragonite occurred at 50 degreesC. At 50 degreesC, the percentage of aragonite in crystallized CaCO3 increased as the NaOH concentration in the mother liquor was increased. This result indicates that aragonite was predominantly precipitated in the presence of NaOH. (C) 2002 Elsevier Science B.V. All rights reserved.