Influence of impurity components on the precipitation of calcium salts during the purification of phosphoric ac-id on anionite by the method of acid retardation

Abstract

The method of acid retardation in a sorbent is effective for the separation of mixtures of acids and salts. It is based on the difference in size of the molecules of acids and hydrated ion pairs of salt components which are formed in highly concentrated solutions. The method involves flowing a solution of a mixture of components with a common anion through a column with a gel anion exchanger in the same anionic form, which allows for the exclusion of the ion exchange. During the first stage of the process, the acid is retained in the sorbent nanopores, while the salt solution flows through the layer. This process continues until the acid retardation ceases and the acid appears at the exit of the column. Then the second stage is carried out: the acid is displaced by washing the layer in the column with water (a classical method) or with a special washing solution (a modified method). As a result, a solution of pure and highly concentrated acid is obtained at the outlet. This work analyses the results of an experiment aimed at the purification of industrial extract phosphoric acid (EPA) with a large amount of impurities by a modified method. In this method the dilute phosphoric acid solution used as a displacing solution prevents the precipitation of calcium salts. To study the effect of the washing solution on the precipitation process, a mathematical model of chemical equilibrium in a system of a limited number of EPA components was formulated. This model allows calculating the molecular and ionic composition of the mixture depending on the concentrations of the input substances, including the determination of the substance amount in the sediment. It was shown that in the case when only calcium salts and phosphoric acid are present in the initial solution the concentration of the washing solution of 0.1 mol/dm³ for phosphoric acid is sufficient to prevent precipitation. This effect is due to the formation of more soluble acid salts. If the solution contains salts of other metals, in particular aluminium and iron, according to the model's predictions, the mechanism of acid salt formation does not explain the absence of precipitation. This would require a washing solution with a concentration of at least 0.13 mol/dm³. The simulation's results suggest that other mechanisms are also involved in the prevention of sedimentation. One such mechanism may be the effect of isothermal supersaturation in the ion exchanger layer.