Research On The Batch Cooling Crystallization Process Of Ammonium Sulfate And Regulation Of Crystal Size And Morphology

Ammonium sulfate is an important chemical byproduct and can be widely used in fertilizer and protein purification. To handle the existing problems in industrial production of ammonium sulfate, such as small size, nonuniform CSD, poor crystal shape. A systematic research was carried out and a novel method of regulating the size and morphology of ammonium sulfate was put forward.Firstly, the thermodynamic properties of ammonium sulfate in crystallization were investigated. A dynamic method was used to measure the solubility data of ammonium sulfate in different solvents. And the modified Apelblet equation and poly equation were used to fit the solubility data. The results show that both of the two equations can well correlate the relationship between solubility and temperature. Based on solubility data, the dissolution enthalpy and entropy and Gibbs free energy of ammonium sulfate was calculated by the van’t Hoff equation and modified Apelblet equation.Secondly, metastable zone width and nucleation kinetics of ammonium sulfate crystallization were studied. MSZW data of ammonium sulfate have been determined in the temperature range from 313.15 to 343.15K by direct method. The factors affecting MSZW of ammonium sulfate such as cooling rate, initial composition, stirring rate, presence of seed particles, solution volume, foreign ions and pH have been investigated in this paper. The self-consistent Nyvlt-like equation and the novel equation of three-dimensional nucleation theory were adopted to estimate the nucleation kinetics parameters of ammonium sulfate from the experimental MSZW data. It was observed that the MSZW of ammonium sulfate has an obvious decrease with the increase of initial composition, stirring rate, and the mass of seed particles, respectively. However, the MSZW becomes wider by enhancing the cooling rate and the diameter of particles. Besides, no obvious effect of working volume has been found on the MZSW. The nucleation order of ammonium sulfate-water system is about 4.5 and the activation energy is about 16.4kJ/mol.The cooling crystallization process of ammonium sulfate was optimized on the basis of thermodynamic and kinetic studies. The effects of various operating conditions, such as pH, cooling policy, stirring rate, solution initial composition seeding and batch time, on the cooling crystallization process was investigated. And the optimum operating conditions of ammonium sulfate during cooling crystallization process were established, products with larger size, uniform CSD, higher purity was obtained.Under the optimized conditions, uniformally large granular ammonium sulfate crystals with higher strength were obtained by adding micro-CaCO3 particles. The effect of CaCO3 particles on the size and morphological modification of (NH4)2SO4 crystals was studied. The results show that CaCO3 particles serve as heterogeneous nucleation catalyst for (NK4)2SO4, decreasing the growth time of (NH4)2SO4 (001) face. Moreover, CaCO3 particles surface may prefer to occupy the active growth sites of (NH4)2SO2 (001) face, suppressing the growth of (001) face.