Influence of citric acid on calcium sulfate dihydrate crystallization in aqueous media: Fid-Bau Portal

Influence of citric acid on calcium sulfate dihydrate crystallization in aqueous media

Titiz-Sargut, S. / Sayan, P. / Avci, B.

Effects of pH, temperature and different citric acid concentration on gypsum crystallization are studied. The results presented in this study indicate that pH did not affect the average particle size of gypsum at 20 deg C. On the contrary, the shape of gypsum changed from needle-like to thicker tabular form depending on pH. When the relationship between morphology and the crystal shape was considered, pH 3.5 was selected as the working pH. According to the SEM observations and crystal size distribution of gypsum crystals obtained in the presence of various citric acid concentrations at 20 deg C and pH 3.5, it was deduced that the temperature had also an important effect besides citric acid concentration. The results obtained from the experiments of which, the effect of temperature was investigated, showed that 65 deg C was an optimum working temperature for this study. Citric acid played an important role for gypsum morphology (size and shape) at pH 3.5 and 65 deg C. Especially, approximately 2500 ppm citric acid concentration was determined as a critical concentration in terms of average crystal size, crystal shape and filtration characteristics. The average particle size increased until 2500 ppm citric acid and then decreased rapidly with increasing citric acid concentration. Various crystal morphologies such as tabular, plate-like, double-taper leaf-like and flower-like, etc., were obtained depending on citric acid concentration. The change of crystal morphology influenced both filter cake resistance and filtration rate. Filter cake resistance and filtration rate reached minimum and maximum values respectively at a concentration of approximately 2500 ppm citric acid. Zeta potential values showed that citric acid adsorbed preferentially on the different crystal faces of gypsum.