Research On Melt Crystallization Preparation Process Of Hyperpure Phosphate Acid

Hyperpure phosphoric acid, which belongs to high end electronic chemical, has an immense demand in electronic industry and wide market prospect. In order to prepare high quality hyperpure phosphoric acid product, the system engineering investigation on melt crystallization preparing hyperpure phosphoric acid has been performed. By means of two different new crystallization technology-liquid film crystallization (LFC) and static multistage melt crystallization (SMC)-the hyperpure phosphoric acid product, which comes up to the multiplex international standards, has been successfully prepared.In this work, the density and viscosity of hyperpure phosphoric acid under common temperatures and concentrations had been accurately measured. The heat conductivity coefficient of phosphoric acid crystal layer under different temperatures and initial concentrations were also measured. In addition, the distribution coefficient of main ion impurities between crystal layer and liquid phase were determined, all these data have built a foundation for mathematical model.The macroscopic crystal layer growth and sweating kinetic data of LFC and SMC have been determined by intermittent volume method and direct measurement method, respectively. The crystal later growth kinetic equations were established with size-independent growth rate model; the sweating kinetic equations were established with the temperature gradient as driving force. In addition, we analyzed the resistance factors during sweating process and completed the sweating kinetics theory by introducing the paratmeters of layer structure.On the basis of predecessors'work, the liquid film crystallization dynamic model and static melt crystallization model have been established by heat balance calculation, boundary layer mass transfer theory and phase equilibrium. The models obtained good fitting results on two melt crystallization modes. We selected the seed ing temperature, material initial concentration, sprinkle density, cooling rate, cooling endpoint temperature and sweating endpoint temperature as the key technological parameters.On the seeping, which is distinctly important in melt crystallization research, the mathematical model which applied to melt crystallization crystal layer system had been established and a good fitting result had been obtained by introducing fractal porous media theory; the structure and characteristic parameters of crystal layer can also be obtained from the model. This mathematical model can be applied to the analysis of crystal layer structure and characteristic in other melt crystallization cases.Based on the above investigation of hyperpure phosphoric acid crystallization process, model establishment and simulation, crystallization and sweating kinetic researches, we had in-depth studied the LFC and SMC technology. The effects of operation conditions on the crystallization process and separation effect were investigated in detail by both experiments and model simulation, and the optimal operation strategy of LFC was established. In order to solve the problem of too much fine crystal in crystal layer and impurities parceled increasing with length, the static multi-step melt crystallization technology was developed. The hyperpure phosphoric acid can be obtained by this technology for the fine crystal had been effectively eliminated.On the basis of above research, we compared LFC and SMC on production capacity and separation efficiency with the method of elementary technical and economic index analysis. As a conclusion, we proposed the combining operation technology of two crystallization modes and optimized process scheme, which effectively increased the separation efficiency, yield and production capacity. In addition, this technology has achieved the zero release of liquid waste and low energy consumption, which believe to be a green-environment friendly technology. This combining melt crystallization technology has important significance for industrialization and can also be spread and applied to other hyperpure crystal product preparation with melt crystallization method, too.No same report to above study results have been published in literature up to date.