Influence of Admixture Impurities on the Crystal Quality of Inorganic Phosphorus Compound

In the industrial crystallization process, it is known that impurities in the mother liquor inhibit the crystal growth or decrease the crystal purity. It is important to investigate the influence of each impurity on the crystal growth and the crystal purity in the presence of admixture impurities assuming a real process. In this study, the influences of impurities on the crystal growth and the crystal purity have been simultaneously evaluated for the growth of crystalline particles in a real process solution in which admixture impurities are present. An operation method is proposed in order to increase the productivity while maintaining a certain crystal purity. The crystal growth rate is increased by masking Ca2+, an impurity that inhibits the crystal growth, EDTA to the real industrial process solution. This means that the initial supersaturation σi necessary to achieve a certain crystal growth rate can be lowered. Moreover, when the same crystal growth rate is established in two systems, the crystal purity in both systems is found to be very similar. It is found that Ca2+ has no influence on the crystal purity, even though Ca2+ inhibits the crystal growth rate. This study makes it clear that the driving force of the crystal growth, required for maintaining a certain crystal purity, can be lowered by preventing impurities from inhibiting the crystal growth. Therefore, the proposed operation method can be used in order to improve the productivity while maintaining a certain crystal purity when the crystalline particles are produced in the presence of admixture impurities.