Effect of Al3+, Zn2+ and Pb2+impurities upon metastable zone width of boric acid

Impurities in the crystallization medium affect the metastable zone width, induction time, crystal growth rate, nucleation rate, hygroscopic properties, crystal habit, agglomeration and flowability. The metastable zone width is key to industrial crystallization. Therefore, the effect of impurities on the metastable zone width should be determined. Boric acid is one of the main reagents used in the production of boron compounds. The crystal properties of boric acid directly affect the product quality. In this study, the effect of Al3+, Zn2+ and Pb2+ impurities on the metastable zone width of boric acid has been examined in the metastable zone width measurement system. In comparison with pure solution, the metastable zone width increased with Zn2+ additive at 10 ppm concentration. Also, with increasing Zn2+ concentration, metastable zone width narrowed. For 10 ppm Al3+ concentration, metastable zone width narrower, while the effect of increasing Al3+ concentration upon the metastable zone width more limited. At the presence Pb2+ additive at 25-75 ppm concentration, metastable zone width increased. for 100 ppm Pb2+ addition, metastable zone width values are about the same to the pure solution. For the pure and additive-doped solution, the dissolution of boric acid in aqueous solution is in the endothermic structure. The induction time of the pure boric acid solution is 50-60 minutes at low supersaturation values. it drops up to 20 minutes for high supersaturation values. In the presence of 100 ppm Zn2+ additive, the induction time increases more than twice as compared to the pure solution. In the presence of 500 ppm Zn2+, the induction time increases, although it is low value. Compared to the pure solution, in the presence of 1000 ppm Zn2+ additive, there are high induction times at low supersaturation values and lower induction times in high supersaturation values. For pure boric acid solution, the volume formation energy required for nuclei formation, in homogeneous nucleation, is decreases with the increase of supersaturation and also, in heterogeneous nucleation, it decreases with increasing supersaturation. In the same supersaturation values, the volume formation energy required for homogeneous nucleation is high in the presence of 100 and 500 ppm Zn2+ additives compared to the pure solution. In the presence of 1000 ppm Zn2+ additive, it is close to the value of the pure solution at low supersaturation, and lower at high supersaturation. The nucleation rates calculated in the presence of 100 and 500 ppm Zn2+ additive lower than the pure solution for the same supersaturation values.