Research On The Separation And Purification Of Phosphoric Acid By Continuous Layer Melt Crystallization

Phosphoric acid (PA) is the raw material of many phosphates fine chemicals, and PA and its derivatives have been widely used in numerous fields with a good market prospect. The product PA produced by thermal or wet process need further purification to improve the purity. Layer melt crystallization is considered as a potential green, environmentally friendly and highly selective separation technology because of the following advantages:low energy consumption; no additional solvent; easy to perform on an industrial scale and so on. It is, indeed, appropriate for the separation and purification of PA. However, on the current laboratory researches and pilot scale applications, the processes of layer melt crystallization are generally conducted in a batch mode that the two most important stages, crystal layer growth and sweating processes, are accomplished on the cooled surface of the crystallizer in sequential order with complex operations and a long procedure. The operations are subject to many qualifications for the batch mode. For example, the cooling rate of the crystallization process should not be too large, resulting in crystal layer with a relatively high porosity and limited production capacity; the sweating temperature should not be sufficiently close to the melting point of the desired component (PA hemihydrate) to avoid the crystal layer break or fall with the following disadvantages:a long operation duration, a sharp reduction of separation efficiency with impurity entrapped in the crystal layer and a low yield of products and so on. The unfavorable production capacity and the complex processes of layer crystallization technology are not conductive to the industrial application for separation and purification of PA. Therefore, a continuous operation is one of the keys to successful industrialization of layer melt crystallization. The paper focuses on changing the traditional batch processes into a secondary-stage crystallization operation to realize the aim of continuous purification and separation of PA by layer melt crystallization.In this work, the processes of layer melt crystallization for PA have been studied systematically in advance:(1) The growth rate of crystal layer was accurately measured during the crystallization process under different initial concentrations or cooling rates; (2) The average thickness of crystal layer at the stage of crystallization equilibrium was investigated as a function of initial concentration and the crystallization temperature; (3) The effective crystal thermal conductivity was calculated by means of heat transfer coefficients. Meanwhile, the initial nucleation and the initial crystal layer adhering on the cooled surface were discussed comprehensively in order to prepare an ideal initial crystal layer. A initial crystal layer with a uniform thickness and completely covering on the cooled surface can be obtained by re-crystallization and will be easy to measure the average thickness in the experiments. What’s more, by adding a certain kind of impurity ions in the phosphoric acid, the variation of the distribution coefficients was obtained during the progressive freezing process to evaluate the separation efficiency. The morphology of crystal layer, sweating temperature and so on had been investigated during the sweating process.The temperature profiles in the crystallizer parts, crystal layer, melt, and the double jacketed crystallizer were obtained by the above experimental research. Then a proper mathematical model for crystal layer growth was developed by an energy balance at the moving crystal layer surface and several reasonable assumptions. It can be concluded that the growth rate of the crystal layer can be used to guide the experiment process and production. In addition, according to the model of sweating, the purification mechanism of PA hemihydrate has been discussed.Finally, a laboratory scale rotary drum melt crystallizer for PA had been fabricated for the use of the continuous process of layer melt crystallization. The preliminary investigations were carried out on the production capacity and the separation efficiency. Three operation modes with a good industrial prospect were proposed:multi-rotary-drum melt crystallization process, rotary-drum crystallization combined with the wet-sweating process, and rotary-drum crystallization associated with dry-sweating purification with a promising industrial application prospect.