Hexafluorosilicate Salts in Wet Phosphoric Acid Processes: Properties of X2SiF6-H2O with X = Na+, K+, or NH4+ in Aqueous Solutions at 353.15 K

摘要

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)(2)SiF6(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)(2)SiF6(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 m(max)(Na2SiF6(s)) = 0.25 mol.kg(-1); m(max)(K2SiF6(s)) = 0.30 mol.kg(-1); and m(max)((NH4)(2)SiF6(s)) = 0.22 mol.kg(-1). The thermodynamic characteristics, solubility products (ln K-sp degrees) and the standard molar Gibbs energy of dissolution Delta G(diss)degrees were also given. The solid phases obtained at saturated solution containing Na2SiF6, K2SiF6, or (NH4)(2)SiF6 were characterized by X-ray diffraction.