Study On Suspension Crystallization Process Of Lanthanum Chloride

Lanthanum chloride,represented by lanthanides,is a very important kind of rare earth compounds.It is the source of matrix materials for agriculture,medicine,catalysis,optical function and other materials.In view of the problems existing in the domestic production process of lanthanum chloride,such as low purity,fine particles and uneven distribution,easy caking and poor fluidity,etc.In this paper,the nucleation and growth of lanthanum chloride crystals in the cooling crystallization process of aqueous solution were systematically studied,which provided the necessary theoretical basis for the design and amplification of lanthanum chloride crystallization separation and purification process.Firstly,the solubility of lanthanum chloride heptahydrate in water was determined by static equilibrium method.The solubility data were fitted and evaluated by Apelblat empirical equation and ?h equation,respectively.Meanwhile,the width of the metastable zone of lanthanum chloride cooling crystallization was measured by laser transmission method.Based on the deduction of primary nucleation,the primary homogeneous nucleation and primary heterogeneity of lanthanum chloride cooling crystallization were determined by combining the experimental results of the effects of operating parameters such as temperature,stirring and cooling rate on the width of metastable zone and nucleation series of lanthanum chloride cooling crystallization.The boundary point of phase nucleation is S0=1.81,and the important information such as interfacial tension and surface entropy factor are calculated based on the data of nucleation induction period.Furthermore,the experimental data of cooling crystallization kinetics of lanthanum chloride in water were measured by batch dynamic method,and the kinetic data were processed by moment transformation method.The kinetic equations of nucleation rate and growth rate were regressed as follows:B0-1.06×1014exp(-205523.82/RT)?C1.36MT1.13np0.29 G=1.79×104exp(-38677.93/RT)?C0.73.Meanwhile,the effects of supersaturation,suspension density,cooling rate and stirring rate on the nucleation rate and the crystal growth rate were analyzed by these model equations.The nucleation rate and growth rate of lanthanum chloride crystals increase with the increase of solution supersaturation,suspension density and stirring rate.It can be seen from the comparison of the indices of different variables in the two equations that supersaturation is the most important factor affecting crystal nucleation and growth.The indices of nucleation rate with supersaturation is greater than the index of growth rate.Therefore,when the supersaturation of the system increases,the nucleation rate of the crystal increases more rapidly.Finally,the batch and continuous cooling crystallization process of lanthanum chloride was studied in detail.The effects of batch operation parameters such as raw material concentration,cooling rate,stirring rate,residence time and seed crystal addition amount on the nucleation and growth of lanthanum chloride crystal were investigated,and the optimal process conditions for the crystallization process of lanthanum chloride were finally determined.Meanwhile,the effects of initial concentration,residence time,feed position,suspension density and state on the crystal size of lanthanum chloride crystals under continuous cooling crystallization were also investigated.It provides a basis for the design and amplification of crystallizers for the industrial production of lanthanum chloride.