| The phosphoric acid is a basic chemical material and is used widely in many fields. The traditional way (furnace process), which phosphate rock and coke are burned to phosphorous pentoxide and then hydrolyzed to phosphoric acid, is very energy consumption and environmental-unfriendly. Another production way (wet-process phosphoric acid) has advantages of energy saving and environment-friendly. However, because wet-process phosphoric acid contains many impurities, it can't meet the requirement for high purity grade application. Many methods such as solvent extraction, ion exchange, physical adsorption, electroosmosis, and crystallization, have been employed for the purification phosphoric acid. Among those technologies, crystallization possesses such advantages as high separation efficiency, low operation costs, environmental harmless and has been attracting more and more attention. In order to improve grade of phosphoric acid and decrease powder emission, it is necessary to purify wet-process phosphoric acid by means of crystallization method.In this study, the crystallization process has been studied systematically. And a phosphoric acid crystallization model has been established to provide basic data and theoretical foundation.The main contents and conclusions are listed as follows:(1) In this article the laser method is used to measure the solubility and metastable zone width (MZW) data of phosphoric acid. The results indicate that the solubility increases with the rising temperature and the role of impurities doped in phosphoric acid can decrease the value of solubility. There is a systematic difference of MZW between high phosphoric acid concentration and low area (78.56wt%-91.60wt%). At high concentration area, the MZW is higher than that of relatively low area. It exists a linear relationship between cooling rate log(-T) and metastable zone width logâ–³Tmax. An apparent secondary nucleation order of phosphoric acid crystallization of 3.3 has been obtained using modified regression method.(2) The effects of classic impurity ions on metastable zone width of phosphoric acid hemihydrate is as following:Al3+>Fe2+>Mg2+>Na+>SO42->F-. The change ofâ–³MZW added impurity ions isn't obvious at low concentration area. However, the influencing effect will be changed greatly in high ones and the value ofâ–³MZW becomes large. The more concentration of impurity ions doped in, the bigger movement ofâ–³MZW will be.(3) Using focused beam reflectance measurement (FBRM) technique, the crystal growth process of phosphoric acid is studied in cooling mode and a series of crystal length distributions (CLD) data is obtained. FBRM measurement results are presented in the form of CLD rather than traditional crystal size distributions (CSD). In this study, several experimental methods are applied to analysis on the relationship between CLD and CSD and get a conclusion that there is a clear accurate proportion among those data. The result reported in this paper is obtained by comparing with FBRM and practical case to get the accurate relationship between CLD and CSD. The method which hasn't been reported in literature is simple and reliable.(4) Phosphoric acid crystal growth acting on the same rate under different particle size cases deduces the conclusion of following size-independent growth (McCabe's AL law) during phosphoric acid crystallization process. According to this conclusion, the crystal growth kinetics and secondary nucleation kinetics of phosphoric acid are obtained. It can illustrate that the growth of phosphoric acid hemihydrate crystal is mainly controlled by diffusion process according to exponent fuction of 1.07.(5) By using the crystallization thermodynamis and kinetic data obtained above, the mathematical model of the crystallization process of phosphoric acid is established based on the population balance and the mass balance equations. The model is solved by a numerical method. According to the simulation results and comparing the experimental data, an optimum schedule for crystallization process is put forward.(6) Based on removal rate of impurity ions, the effect on phosphoric acid hemihydrate crystal of different process conditions is studied in detail. Finally, the optimum operation condition for amplified 10 times volume (5L) is determined by comprehensive consideration all sorts of factors. Directed by the optimum operation condition, both phosphoric acid doped signal ions and wet-process phosphoric acid are purified by crystallization process. The result shows that not only impurity ions concentration but also CSD is improved dramatically after crystallization treatment.Aim at difficulties of conventional approach at phosphoric acid crystallization process, a new way for converting CLD to CSD has been presented in this paper. Based on the new method, phosphoric acid crystallization process has been studied. The research provides the experimental foundation for the development and design of large-scale industrial equipment.
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