Properties of fluoride in wet phosphoric acid processes: Fluorosilicic acid in an aqueous solution of H2SiF6–H20 at temperatures ranging from 298.15 K to 353.15 K

• Water activities of aqueous solutions of fluorosilicic acid were measured at various temperatures from (298.15 to 353.15) K.
• The osmotic and activity coefficients of H2SiF6–H2O were determined at various temperatures.
• A construct model based on ion interaction was developed to calculate the excess thermodynamic properties.
• The proposed data help in understanding the role of fluorosilicic acid in the wet production process of phosphoric acid.

The properties of fluoride in wet phosphoric acid processes were of interest us to understand the processes and roles that fluorosilicic acid plays in phosphoric acid solutions. The literature data indicate that some deficiencies exist in the thermodynamic properties of hexafluorosilicic acid aqueous solutions. In this investigation, the water activity and osmotic coefficients of H2SiF6(aq) were determined in the temperature range from T = 298.15 to 353.15 K. The measurements of the water activity were performed at molalities from 0.10 to 3.00 mol kg−1 of H2SiF6(aq) using the hygrometric method. The modelling approach based on the Pitzer model was developed to determine the thermodynamic properties. From these measurements, the ion interaction parameters in binary solution were evaluated at different temperatures ranging from 298.15 K to 353.15 K and were used to predict the solute activity coefficients. The development of a model for solution behaviour was also employed to determine the excess molal Gibbs energy, enthalpy, entropy and heat capacity of the H2SiF6 aqueous solution within the temperature range from 298.15 K to 353.15 K.