Solubility of ammonium fluoride in aqueous sodium fluoride solutions of various concentrations from dilute to saturated at 298.15 K for fluorine recovery from wet-process phosphoric acid

Abstract

Recovery of fluorine from wet-process phosphoric acid (WPA) to produce ammonium fluoride (NH4F) and sodium fluoride (NaF) provides an environmentally-friendly route to purify WPA and produce high value-added fluoride salt. The solubility data and the related electrolyte thermodynamics are essential for a rational separation design of pure NH4F from aqueous NaF solution by crystallization. In this contribution, the solubilities of NH4F in aqueous NaF solutions of different concentrations from dilute to saturated were systematically measured at 298.15 K using a dynamic synthesis method. Experimental results show that the presence of NaF in the solution could significantly decrease the solubility of NH4F due to the common-ion effect which was decreased by 72.4% when the molality of NaF was only around 0.015 mol/kg. The data can be used to determine the yield of NH4F and liquid composition in the vacuum evaporative crystallization of NH4F. The mean activity coefficients of NH4F at saturation were determined from the experimental solubility data and then correlated with the extended Debye-Hükel equation and the Pitzer model. With four adjustable parameters, both models are able to accurately calculate the experimental data over a wide range of salt concentrations. The average relative deviations between the experimental and calculated logarithm of mean activity coefficients of NH4F from the two models are 1.43% and 2.29%, respectively. The binary and ternary ionic parameters for the Pitzer model are found to be concentration dependent and the correlations of them as functions of ionic strength are proposed.