The Study On Crystallization Process Of Trisodium Phosphate

Trisodium phosphate is a very important phosphate,which is widely used in many fields such as chemical engineering,textile,printing and dyeing etc.Trisodium phosphate exists mainly in two forms,dodecahydrate and anhydrous,and cooling crystallization method is generally used to purify and separate it.Therefore,in this paper a systematic study for the cooling crystallization process of trisodium phosphate was made,which mainly included trisodium phosphate crystallization thermodynamics,trisodium phosphate dodecahydrate dehydration mechanism,crystallization kinetics and crystallization process optimization.Crystallization thermodynamics plays a key role on trisodium phosphate crystallization process.The solubility of trisodium phosphate in water with different pH and variational temperatures was measured by laser monitoring technique.The results were correlated by Apelblat equation,which showed a good fit to the experimental data.Furthermore,with the solubility data the enthalpy and entropy of dissolution of trisodium phosphate with different pH were calculated.In addition,the supersolubility of trisodium phosphate with different pH was measured and the metastable zone was acquired.Trisodium phosphate dehydration process for its storage and transport in industry plays an important guiding role.The dehydration process of trisodium phosphate dodecahydrate(Na3PO4·12H2O)at 303.15 K to 413.15 K was investigated using thermogravimetry(TG),differential scanning calorimetry(DSC),power X-ray diffraction(PXRD)and scanning electron micrograph(SEM).It was found that 11 mol crystal water of trisodium phosphate dodecahydrate was dehydrated at 303.15 K to 393.15 K and the process included multi-step reactions.Activation energies of dehydration at different dehydration rate were calculated by Ozawa equation and Friedman equation,respectively and a good identity of results was reached.The process of dehydration consisted of four steps,which were different in dehydration mechanism.The PXRD and SEM figures of dehydration products at different temperature showed that it was channel-type inclusion hydrate and crystal structure has transformed during the process of dehydrationTrisodium phosphate crystallization kinetics rates of nucleation and growth are important factors for the crystalline product particle size distribution.According to the requirements of trisodium phosphate crystallization process,crystallization kinetics was analyzed using batch dynamic methods,and the equations of nucleation and growth rate of trisodium phosphate crystallization process were obtained using least square method.It provides an important theoretical support for industrial scale crystallization process simulation.On the basis of crystallization thermodynamics,dehydration process and crystallization kinetics studies of trisodium phosphate,crystallization process was optimized.The effects of stirring speed,cooling temperature and pH on the crystalline product yield,final particle size distribution and crystalline quality were investigated,followed by the determination of optimum conditions for trisodium phosphate crystallization process.