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

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 X 2 SiF 6 −H 2 O with X = Na + , K + or NH 4+ were determined at temperature T = 353.15 K. The measurements of the water activity were performed from dilution to saturation of Na 2 SiF 6 (aq), K 2 SiF 6 (aq), and (NH 4 ) 2 SiF 6 (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, Na 2 SiF 6 (s), K 2 SiF 6 (s) and (NH 4 ) 2 SiF 6 (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 (Na 2 SiF 6 (s)) = 0.25 mol•kg −1 ; m max (K 2 SiF 6 (s)) = 0.30 mol•kg −1 ; and m max ((NH 4 ) 2 SiF 6 (s)) = 0.22 mol•kg −1 . The thermodynamic characteristics, solubility products (ln K sp °) and the standard molar Gibbs energy of dissolution ΔG diss °were also given. The solid phases obtained at saturated solution containing Na 2 SiF 6 , K 2 SiF 6 , or (NH 4 ) 2 SiF 6 were characterized by X-ray diffraction.

Solubility of Mixed Ternary Systems of Hexafluoridosilicate Salts (M = Na+, K+, or NH4+) in Hexafluoridosilicic Acid Aqueous Solutions at T = 353.15 K

“…m s (calc) (eq ) is u ( m ) = 0.0001 mol kg –1 . For each ternary system, m 1 exp is the molality of H 2 [SiF 6 ] and m 2 exp is the molality of M 2 SiF 6 with M = Na + , K + , or NH 4 + . b Ref . …”

“…N is the number of experimental points. Each point is the average of 8–10 series of experiments. b Ref . c Ref . …”

“…The solubility is determined on thermodynamic considerations based on the approach of the ion-interaction model. − This has been evaluated taking into account the ternary systems as 1-2*1-2 charge type and the solubility of the salts in acid solutions. According to the measurements of the water activity and the ion-interaction parameters, binary , and ternary given in Table , the solute activity coefficients were evaluated for the aqueous solutions of hexafluoridosilicate salts and acid. The calculated solubility m s(tot) cal was determined by eq , using the experimental value of the water activity a w and the osmotic coefficients calculated from the mixing parameters (θ MN and ψ MNX ).…”

“…The experimental solubility in the ternary systems was determined, which is the same as that employed in the binary systems of M 2 [SiF 6 ]­(aq) with M = Na + , K + , or NH 4 + . For each ternary system, m 1 exp is the molality of H 2 [SiF 6 ] and m 2 exp is the molality of M 2 [SiF 6 ] with M = Na + , K + , or NH 4 + .…”

“…Their dissolution in acid aqueous solutions could be one of the solutions to overcome the problem of fouling during the industrial production of phosphoric acid. In our earlier work, the thermodynamic properties were determined for the binary systems of hexafluoridosilicate salts and acid in aqueous solutions at T = 353.15K, to understand the phenomena governing the process of phosphoric acid production in order to contribute for solving the problem of fouling.…”