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 = 25oC. 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 Kosp) 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.
