Research On Purifying Phosphoric Acid With Melt Crystallization Process

Phosphoric acid is an important industrial raw material. Wet phosphoric acid must be purified before used in downstream industries. Compared with the extraction, crystallization without solvent, which can simplify the process and reduce energy consumption are environment friendly. So it is a potential method for purifying the wet process phosphoric acid. But the crystallization method is intermittent operation and difficult to meet the needs of large-scale industrialization. Therefore, how to make the continuous crystallization process is the key to purification of wet process phosphoric acid.For the problems in the process of continuous crystallization, using siphon to realize the transportation of slurry, and re-separation process to strengthen the countercurrent washing process.This paper research the density, viscosity, morphology of phosphoric acid crystal.Simulate the density and viscosity of phosphoric acid system by Aspen plus, and comparing with experimental data. Use Material Studio to predict the morphology of H3PO4O.5H2O. AE model was closest to the actual surface. By molecular dynamics calculation of impurity diffusion coefficient in phosphoric acid system, it was found diffusion coefficient of Cl- was largest. The radial distribution function of ions (RDF) can explain the micro effect of ions and water molecules in the system. Calculation of the influence of impurities on the crystal surface of H3PO4O.5H2O, the result shows that SO42- and Fe2 +have the greater impact on the surface of crystal.Secondly, by suspension melt crystallization, the crystal particles as smaller, uniform in distribution, convenient transportation, suitable for continuous operation. The impurity removal rate increases with increasing crystallization time in each mode. Suspension melt crystallization has the higher removal ratio of Fe3+ and F-. For the single-stage crystallization, multistage recrystallization is more effective in purifying crystal.Countercurrent washing process in the settling section of washing column, by statistics values, the settling rate is 6.61mm/s, it was similar to theoretical calculation 6.88mm/s under the condition of T=293.2K, dp=3mm. When the reflux ratio is larger, the purification effect is better. As the reflux ratio R=6, impurity removal rate can reach 60%.