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Influence of some phosphates on the rate of calcium sulfate dihydrate crystalistion in sodium chloride solution

N.S.Yehia, W.K.Saif Elyazal, A.M.Heneash, I.A.Ibrahem

Crystallization of calciumsulfate dihydrate (CaSO4.2H2O) in sodiumchloride solutions at different supersaturation (ä = 1.2–2.49), pH =3, ionic strength (I = 0.15 M) and at 25°C was studied. The influence of disodium hydrogen phosphate and sodium tripolyphosphate and disodium dihydrogen phosphate having very low concentrations (10-7mol dm-1) on the rate of crystallization at different supersaturation was investigated. The rate of crystallization was found to be dependent of the stirring rate suggesting diffusionmechanism. The addition of all additives retarded the rate of crystallization to an extent proportional to their amounts present. Furthermore, the retardation effect was enhanced as the supersaturation decreases. The results also revealed that the increase in both pH (3 – 10) and crystallization temperature (20 – 80 °C) brought about an increase in calciumsulfate crystallization rate.