Study On Crystallization Process Of Potassium Dihydrogen Phosphate

The crystallization metastable zone properties of potassium dihydrogen phosphate aqueous solution was systematically studied.And the effects of PH value,cooling rate, variety and content of impurities on the shapes,sizes and size distribution of the crystals also were studied.They are of the instructional significance to the production of potassium dihydrogen phosphate.Firstly,the metastable zone widths of the potassium dihydrogen phosphate (KDP) aqueous solution at different temperatures, under different cooling rate and different agitating rate were investigated. The effect of Fe3+ on the metastable zone width was also studied. The metastable zone widths narrowed with the increase of temperature and widened with the increase of the cooling rate. The metastable zone widths narrowed with the increase of the agitating rate,and it varied 1.6～2℃under lower temperature and about 1℃under higher temperature.The metastable zone widths increased with the increase of the concentration of Fe3+.The mathematical relationship in the system of KDP between the metastable zone width and the concentration of Fe3+ was obtained. temperature rising. The job provides the basis for the industrial crystallization to select the suitable supersaturation.Next, the effects of PH value,cooling rate and the impurities such as F^-,Cl^-,SO₄^2-,Ca²⁺,Mg²⁺,NH₄⁺,Fe³⁺ and Al³⁺ on the crystallization process of potassium dihydrogen phosphate were studied. The experimental results show that all the factors but the NE₄⁺ resulted in the reduction of crystallization temperature. The effects of Fe³⁺ and Al³⁺ on the shapes are the most.The shapes are changed from prism to spiculate shape.And their effect principle are similar. The effect of F^-is less than them.The ratio of length to diameter of cryatal are obviously minished because of F^-The shape are changed from long prism to short prism.The cryatal is still long prism with the other condition but above factors.The reason that the crystal shape are changed is that impurities which are absorbed on the surface chang the growth rate of this face of crystal. When the pH value is more or less than 3.0, the output of crystals is evidently reduced.With the increase of pH value, the average size of crystals significantly increases. With the increasing of cooling rate, the average size of the crystal is evidently reduced.and the crystallization speed significantly increases.With the increase of dopant F^-, the average size is gradually reduced.And With the increase of dopant SO₄^2-, the average size first increases and then lowers. The change of Cl^-content has little impact on the average crystal size. With increasing of the Cl^-content, the crystal transparency rises. With increasing of the Ca²⁺ concentration, the average size of the crystal gradually descends, the output of crystals declines gradually, and the transparency of crystals descends by degrees. With increasing of the Mg²⁺ concentration, the crystal output gradually decreases. When the average size and the ratio of length to diameter of the crystal were studied, A critical concentration 0.3% occurred.When the concentration is lower, with increasing of the Mg²⁺ concentration, the average size and the ratio of length to diameter are reduced,and when the concentration is higher,the result is contrary. when the NH₄⁺ content is 0.75%, the output of crystals is the most.And when the NH₄⁺ content is 0.5%,the average size of crystal comes to the largest. With the increase of NH⁴⁺ concentration, their changes are slow. With the increase of dopant Fe³⁺,the ratio of length to diameter increases gradually,the obtained crystal amount decreases gradually,crystal bulk density is reduced gradually and water contained in the crystal increased.The effect rule of the dopant of Al³⁺ is similar to that of Fe³⁺.In the end, the associated effect of Fe³⁺ and F^-on the crystallization of potassium dihydrogen phosphate was studied.When the molar ratio of Fe³⁺ to F^-was less than 3×10^-4,the cubic prism crystal was obtained.When the molar ratio was from 0.0017 to 0.0034,the long prism crystal whose ratio of length to diameter was 2 was obtained. When the molar ratio reached 0.0034,if the contents of the two dopants were low, the ratio of length to diameter was still 2.With the increase of the contents,the mixed crystals with the prism and prick were obtained,and the ratio of length to diameter also increased.That of prism cryatals was 3 to 5,and that of pricd crystals was 7. When the molar ratio was more than 0.0034,with the increase of the molar ratio the crystals is spiculate and the ratio of length to diameter increased too.When the F^-content was reduced to 0,the crystal was spiculate and the ratio of length to diameter was the most(that is 12).With the increase of the molar ratio, the included water in the crystal increased.