(311g) Behavior of Crystal Layer Growth in Dynamic Layer Melt Crystallization of Phosphoric Acid

Phosphoric acid (PA) and its derivatives have been widely used in numerous fields such as medical, food and electronic industries with stringent limit of impurity ion contents (e.g., Pb²⁺, K⁺, Mg²⁺,). Thus, PA produced by dry or wet process needs further purification to improve the purity by means of separation and purification techniques, such as solvent extraction, chemical precipitation, ion exchange and crystallization. Among these methods, layer melt crystallization is a technically proven method for the preparation of ultrapure substances, forming a crystalline layer on a cooled surface, and commonly further purifying the products by a sweating process to eliminate the impurity inclusions trapped in the crystal layer during its growth. This method is considered as a potential green, environmentally friendly and highly selective separation technology.

It is difficult to analyze the processes of layer melt crystallization quantitatively as the heat and mass transfer occurs simultaneously and changeably during the growth of crystal layer. Fortunately, the necessary thermodynamics properties of PA-H₂O have been measured in previous works, and these required properties can be used as input data in the mass and heat transfer. The growth rate and the effective thermal conductivity of PA hemihydrate during the crystal layer growth process were measured as a function of process conditions to characterize the manufacture efficiency. Besides, the impurity entrapment phenomenon was investigated in order to achieve a desired separation effect of layer melt crystallization for the purification of PA. A reliable model of dynamic layer melt crystallization was developed by the energy balance and some reasonable assumptions. The essence of this model is the description of heat transport during the crystal layer growth in a forced convective melt, showing the implicit relation between crystal layer growth and the forced convection.

During the experiments, the PA melt was mixed by forced convection caused by stirring to eliminate the temperature and concentration difference of the melt, and the variation of the temperatures of the whole process was recorded to have a deeper understanding of the crystallization. An assumed qualitative temperature profile in a cross section of the crystalline system was presented to enable the prediction of profiles of temperature in both the crystal layer and the melt conformed to the experimental data. The initial nucleation and basal crystal layer adhering on the cooled wall were studied as they have a profound and lasting effect on the crystallization process. During the course of crystallization, a recrystallization operation was used after the addition of seeds to guarantee a uniform cylindrical crystal layer around the cooling finger crystallizer. The experimental research on the crystal layer growth was focused on how the operational conditions affect the manufacture efficiency of PA hemihydrate product. Thus, the growth rate of crystal layer was investigated as a function of concentration and the degree of super-cooling. The formed crystalline PA hemihydrate layer is the mixture of the solid crystals and a small amount of mother liquor regarded as a complex, multi-phase, porous material. Then the effective thermal conductivity of PA hemihydrate was calculated by heat transfer coefficient method to characterize the overall heat transfer rate of crystal layer. The effective thermal conductivity of crystal layer of PA is thought to depend strongly on the physical structure affected by temperature and concentration.