NaF-Na_(3)PO_(4)-H_(2)O体系相图与热力学模型

Phase Diagram and Thermodynamic Model of NaF-Na_(3)PO_(4)-H_(2)O System

NaF–Na_(3)PO_(4)–H_(2)O体系是稀土矿物加工的典型盐水体系,开发氟、磷回收工艺需要该体系的相图和热力学模型支持。利用等温溶解平衡法测定了273.15 K、298.15 K、323.15 K和348.15 K的固液相平衡数据,并基于eNRTL模型框架构建了该体系从最低共熔点到348.15 K的热力学模型。实验结果表明:NaF–Na_(3)PO_(4)–H_(2)O体系存在NaF、Na_(3)PO_(4)·12H_(2)O、Na_(3)PO_(4)·8H_(2)O和复盐NaF·2Na_(3)PO_(4)·19H_(2)O 4个固相物种;其中复盐在各温度均占据三元体系的主要相区,磷酸钠水合物的相区极窄,很难单独成盐,NaF相区相对较大并随温度升高而增大。热力学模型研究较好地表达了NaF–H_(2)O、Na_(3)PO_(4)–H_(2)O体系的多温离子活度系数、溶液渗透系数、二元体系相图,获得了盐水作用参数和6个固相的热力学数据;通过对三元体系多温相图数据的有效表达,获得了盐-盐作用参数和复盐的热力学数据,在此基础上推测了NaF–Na_(3)PO_(4)–H_(2)O体系相图结构,获得了难以实验测定的9个零变量点和9条单变量共饱和线,从而得到了全部固相的平衡相区,并给出了NaF–Na_(3)PO_(4)–H_(2)O体系完整相图,为更复杂体系的热力学表达以及工业应用提供了参考依据。

The NaF–Na_(3)PO_(4)–H_(2)O system is a typical brine system for processing rare earth minerals.The phase diagram and thermodynamic model of the system support the development of a fluorine and phosphorus recovery process.In this study,the solid-liquid phase equilibrium data at 273.15 K,298.15 K,323.15 K and 348.15 K were determined with the use of the isothermal dissolution equilibrium method,and a thermodynamic model of the system from the lowest cointegration point to 348.15 K was constructed based on the eNRTL model framework.The experimental results showed that the NaF–Na_(3)PO_(4)–H_(2)O system had four solid species,NaF,Na_(3)PO_(4)·12H_(2)O,Na_(3)PO_(4)·8H_(2)O and the double salt NaF·2Na_(3)PO_(4)·19H_(2)O;the double salt occupied the main phase zone of the ternary system at each temperature,the sodium phosphate hydrate had an extremely narrow phase zone which was difficult to salt alone,and the NaF phase zone was relatively large and increased with the increase of temperature.The thermodynamic model study better expressed the multi-temperature ionic activity coef-ficients,solution permeability coefficients and phase diagrams of the NaF–H_(2)O and Na_(3)PO_(4)–H_(2)O systems for the binary system,and obtained the brine interaction parameters and thermodynamic data for the six solid phases.It also expressed the multi-temperature phase diagram data for the ternary system,and obtained the brine-salt interaction parameters and thermodynamic data for the complex salts.On this basis,the structure of the phase diagram of the NaF–Na_(3)PO_(4)–H_(2)O system was predicted,and nine zero variation points and nine univariate co-saturation lines,which were difficult to determine experimentally,were also obtained,thus leading to a complete phase diagram of the NaF–Na_(3)PO_(4)–H_(2)O system,which has provided a basis for the thermodynamic expression of more complex systems,as well as industrial applications.