Effect of additives including phosphonic acid on crystallization of calcium sulphate dihidrate

Crystallization of solid material from solution is one of the most frequently used unit operation and one of the oldest separetion and purification operations employed to produce a wide varety of materials in chemical industries. Models for the growth of crystals by condensation from the vapor are well established, but crystal growth and dissolution from solution is much more complicated. Dispite its almost universal applicability, the mechanism involved in many crystallization processes are still not completely understood. Calcium sulfate is important, in which they frequently precipitate from supersaturated solutions encountered in desalination processe, forming scale deposits. This scaling results in large reductions in heat transfer efficiency. The crystallization processes are influenced by a variety of factors such as the supersaturation, solution temperature, stirring rate and presence of impurities. Among the many factors affecting the process of crystallization impurities often exhibit the most pronounced effect.In this study, the effects of additives on the crystallization of calcium sulfate dihydrate (CaSO4.2H2O, gypsum) have been investigated in aqueous solutions. NaPC, EDTMP, HDTMP, ODTMP and DDTMP have been used as additive. The degree of inhibition of crystallization was measured as an increase in induction time and reduction in crystallization rate. The particle sizes and crystal morphology were determined with particle sizer (PS) and scanning electron microscope (SEM) devices.According to to experimental results, all additives tested in this study are effective on crystallization rate and crystal sizes and behaved as the inhibitor calcium sulfate dihydrate crystallization.