Yasunori Nanri

Crystallization of aragonite in the causticizing reaction

Abstract Aragonite, a polymorph of CaCO 3 , 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 CaCO 3 in NaOH solution and the crystallization behavior of aragonite were investigated. The solubility of CaCO 3 in a NaOH solution reached a maximum value with NaOH concentration. Decrease in the solubility after that value was due to the conversion of CaCO 3 to Ca(OH) 2 . In 1 mol/l of NaOH solution containing a concentration of Na 2 CO 3 greater than 0.02 mol/l, CaCO 3 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 CaCO 3 . The highest precipitation of aragonite occurred at 50 °C. At 50 °C, the percentage of aragonite in crystallized CaCO 3 increased as the NaOH concentration in the mother liquor was increased. This result indicates that aragonite was predominantly precipitated in the presence of NaOH.

Effect of NaOH on aragonite precipitation in batch and continuous crystallization in causticizing reaction

Abstract In order to obtain CaCO 3 crystals with high aragonite content in the causticizing reaction in the paper industry, the effect of the NaOH addition at various temperatures was investigated using a batch operation. It is apparent that the NaOH addition increased aragonite composition in the crystal at all temperatures investigated. Then, aragonite crystallization was investigated using a continuous operation. In the continuous operation, it appeared that the dominant factor for the aragonite formation is the low molar ratio of Na 2 CO 3 to Ca(OH) 2 . The lower supersaturation and the higher ratio of [Ca 2+ ]/[CO 3 2− ] in the presence of NaOH are advantageous to precipitate aragonite in the causticizing reaction.