Thermodynamic solid-liquid equilibrium model for mixed systems HF-NaF-H2O and HF-KF-H2O up to 2 m HF at T = 25oC

In this study we developed thermodynamic models for solid-liquid equilibrium for mixed systems HF-NaF-H₂O and HF-KF-H₂O up to 2 m HF at T = 25^oC. Models are developed on the basis of Pitzer ion interactions approach. The recommendations on mean activity coefficients (γ_±) have been used to construct the model for HF-H₂O system. The models for binary systems NaF-H₂O and KF-H₂O are taken from our previous study in the same journal [Donchev et al. ASN (2021) 8(2), 1-15]. To parameterize models for ternary systems we used the available solubility data, i.e. on the basis of solubility approach. The thermodynamic solubility products (as ln K^o_sp) of precipitating in mixed systems double salts [NaF.HF(cr), and KF.HF(cr)] have been determined on the basis of evaluated model ion interaction parameters and using experimental m(sat) solubility data. The model is in a good agreement with experimewnt solubility data in the systems HF–NaF–H₂O and HF–NaF–H₂O up to maximum HF molality of 2 mol·kg⁻¹. At higher acid molality the model predicts much higher solubility of double salts (NaF.HF(cr) and KF.HF(cr) for both mixed systems. It was concluded that not a correct raw data for binary HF–H₂O system used in parameterization are the main reason for concentration restriction of mixed HF models presented here.