Purification Of Phosphoric Acid By Melt Suspension Crystallization

Phosphoric acid is a fundamental inorganic acid, high-purity phosphoric acid nowadays would find widespread application in the medical, food, water treatment, electronic industries and other fields. In today's market, it is mainly produced by the so-called furnace process and wet process. The wet-process of producing phosphoric acid is featured in low cost and low electric power consumption, but the product obtained contains many impurities such as iron, aluminum, magnesium, fluorine, and sulfate, while the product produced by furnace process is much purer, however, this process is energy intensive and has a larger environmental impact. Recently energy sources are in great deficiency, it is recommended that purification of wet process phosphoric acid take the place of furnace process. Several methods, such as, crystallization, ion exchange, electro-osmosis, sedimentation, concentration and solvent extraction, are employed for the purification of phosphoric acid in order to remove impurities from the acid. Among these technologies crystallization presents several advantages such as higher efficiency, low running cost, and less impact to environment, which is used for the purification of wet process phosphoric acid, so it is necessary to study this process in detail.Continuous melt suspension crystallization process has been suggested for the purification of the phosphoric acid in this study, which is conducted in a cascade of a crystallization section and a countercurrent solid-liquid separation performed in a purification column. Separation and purification principle of this equipment is studied through mathematical analyzing of this purification process, the optimization of operation conditions of the process and factors that should be concerned during the designation of crystallization equipment are also obtained.The main contents and conclusions are listed as follows:(1) Thermodynamic and physical properties of wet process phosphoric acid are studied in this paper, including density, viscosity and conductivity. Solubility of phosphoric acid increases with the increase of temperature; at the same concentration, density of phosphoric acid varies linearly with temperature; and the relationship between viscosity of phosphoric acid with temperature follows Arrhenius equation; while conductivity of phosphoric acid reaches a maximum as its concentration increases, and apogee of the parabola transfers to higher temperature.(2) Melt suspension crystallization process is operated on the batch mode, and its stability, capability of purification and the effects of various operation conditions on product are investigated. The equipment achieved a steady state after 11 h, the higher feed concentration, the less time needed to achieve the steady state, and the obtained product concentration is also higher, but when feed acid concentration exceeded 86%wt, the effectiveness of purification become less apparent, by analyzing data of laboratory experiments, removal rate of each impurity in the product reaches the maximum when feed concentration is between 85% to 86%wt. The higher stirring speed, the more amount of seed crystal added, the lower concentration of product and removal rate of each impurity.(3) When the equipment is operated continuously, dynamic characteristics of mother liquor concentration and suspension density in the stirrer are investigated, sizes and shape of crystals are also obtained with microscope under different operation conditions. With various feed concentration, mother liquer concentration in the stirrer achieves a reasonable steady state after 3 to 4 hrs, which is followed by a solid-phase in terms of the slurry density after 4 to 5 hrs, the higher feed concentration, the greater slurry density. Stirring speed has less effect on mother liquer concentration and slurry density than amount of seed crystal that is added, however, increasing stirring speed would cause serious second nucleation; mother liquor concentration decreases and suspension density increases as more seed crystal is added, however, too much seed crystal would be deleterious to the crystallization process, which lows crystal size, based on experimental data, the ideal amount of seed crystal is 0.15%wt of feed acid.(4) Melt suspension crystallization process is operated continuously, and its stability, capability of purification and the effects of various operation conditions on product are investigated in detail. The equipment achieves a steady state after 13h, concentration and purity of product increases and remains a constant value at last as reflux ratio increases. With reflux ratio arising, concentration of impurities in the product become lower, separation and purification capability of column is improved; high feed concentration is in favor of purifying product, when feed concentration varies largely, it also has a larger effect on the impurity concentration distribution along the column; a moving crystal bed is formed inside the crystallization column which is served as the purification section, the higher height of crystal bed, the less impurity of product; the stirring shaft in the column that provids the crystal bed with gentle agitation, avoids the formation of dead zones or preferential flows within the bed, however, further increase in stirring speed has no significant effect on the impurity concentration distributions. Among the previous four conditions, reflux ratio has the largest effect on the purification capability of the equipment.(5) In order to understand the mass transfer and heat transfer processes between two phases in the purification column, a mathematical model is developed by considering purification mechanism. After analyzing the column with mass conservation and energy conservation, a mathematical model of separation and purification in the column is obtained: impurity concentration y vary linearly with z on the logarithm coordinate, this line is called operation line, then experimental data under total reflux and continuous conditions are used to validate the mathematical model, and effects of operation conditions on the operation line are also investigated in detail. As feed concentration increases, operation line turns upward, purity of product increases; the higher reflux ratio, the bigger modulus of the slope of operation line, the purer of the product, but the lower yield of product; a certain height of crystal bed is crucial to the purification of column, the higher height of crystal bed, the purer of product, the better result of purification; there must be moderate stirring in the column which is in favor of purifying product.