THERMODYNAMIC MODELING AND CALCULATION OF PHASE EQUILIBRIA IN TERNARY WATER-SALT SYSTEMS CONTAINING K⁺, NH₄⁺, SO₄^2- AND/OR NO₃^- IONS

Belova E.V., Knyazenko G.A., Kurdakova S.V.

Moscow State University

119991, Moscow, Leninskie Gory, 1

Complex fertilizers containing nitrogen, potassium, and sulfur are currently attracting increasing interest. Phase composition of such fertilizer is as important as chemical one, affecting how resulted product is hygroscopic, stable to detonation and effective in simultaneous release of macronutrients.

Phase equilibria in reciprocal K⁺, NH₄⁺|| SO₄^2-_, NO₃^- - H₂O system are complicated due to existence of double salts and solid solutions. Slightest changes in type of granulator and conditions will result in a loss of predictive force of empiric results of phase-equilibria study. Thermodynamic modeling allows to calculate phase composition at any component ratios and conditions within its predictive range. Thus, the goal of the work is to obtain all solubility products and develop thermodynamic models of liquid and solid solutions in 298 – 373 K temperature range at least up in the ternary subsystems of the reciprocal K⁺, NH₄⁺|| SO₄^2-_, NO₃^- - H₂O.

PSC model [1] was chosen for a liquid phase. Its parameters, along as solubility products of two double salts and (NH₄)₂SO₄, NH₄NO₃ are available for (NH₄)₂SO₄–NH₄NO₃–H₂O [1]. Solid solutions are absent in this system. PSC model parameters proposed in literature for K₂SO₄–H₂O [2] have too complex temperature dependence and give an ill extrapolation of luiquid phase properties to supersaturated solutions, which result in artifacts at high ionic strength in ternary systems. Therefore, these parameters were re-evaluated. New PSC parameters for KNO₃–H₂O were obtained. Ternary parameters for luiquid phase model were added to K₂SO₄–KNO₃–H₂O, solubility is adequate predicted.

The (NH₄)₂SO₄–K₂SO₄–H₂O and KNO₃–NH₄NO₃–H₂O systems require a solid phase model. Redlich-Kister model was chosen for sulfate and nitrate solid solutions. For sulfate solid solution, there is an isomorphous substitution for one structure, for nitrate solid solutions, there are three separate solid solutions. Phase equilibria were calculated without introducing new ternary parameters for luiquid phase model. Predicted liquidus and tie lines between solutions agree with experimental ones.

1. Clegg S. L. et al. Thermodynamic model of the system H⁺− NH₄⁺− SO₄^2-− NO₃^-− H₂O at tropospheric temperatures. // J. Phys. Chem. A. 1998. Vol. 102, Nr 12. P. 2137–2154. https://doi.org/10.1021/jp973042r

2. Li D. et al Phase diagrams and thermochemical modeling of salt lake brine systems. III. Li₂SO₄+H₂O, Na₂SO₄+H₂O, K₂SO₄+H₂O, MgSO₄+H₂O and CaSO₄+H₂O systems // Calphad. 2018. . Vol. 60. P. 163 – 176. https://doi.org/10.1016/j.calphad.2018.01.002

Work was financed on base of "Chemical thermodynamics and theoretical material study" 121031300039-1.