Thermodynamics Research Center / ThermoML | Journal of Chemical and Engineering Data

ABSTRACT

During the production of phosphoric acid in wet processes, the acid and salts of fluorosilicates are formed in turn by inducing the problem of fouling. The knowledge of the physical chemical properties of hexafluorosilicates allows a better understanding of process governing complex equilibria in the mixture. The literature indicates that significant deficiencies exist in the study of these salts. In this work, the thermodynamic properties of hexafluorosilicate salts X2SiF6 H2O with X = Na+, K+ or NH4+ were determined at temperature T = 353.15 K. The measurements of the water activity were performed from dilution to saturation of Na2SiF6(aq), K2SiF6(aq), and (NH4)2SiF6(aq) using the hygrometric method. The osmotic coefficients were evaluated for these systems. From these measurements, the ion interaction parameters in binary solution were calculated at 353.15 K and used to predict the solute activity coefficients. The measurements of the binary systems were combined with reference pure water, Na2SiF6(s), K2SiF6(s) and (NH4)2SiF6(s) solubility data to develop a chemical model that evaluates solute and solvent activities from dilute to saturated solution concentration at 353.15 K up to mmax(Na2SiF6(s)) = 0.25 mol*kg 1; mmax(K2SiF6(s)) = 0.30 mol*kg 1; and mmax((NH4)2SiF6(s)) = 0.22 mol*kg 1. The thermodynamic characteristics, solubility products (lnKspdeg ) and the standard molar Gibbs energy of dissolution Gdissdeg were also given. The solid phases obtained at saturated solution containing Na2SiF6, K2SiF6, or (NH4)2SiF6 were characterized by X-ray diffraction.