Determin A Tion And Graphics Expression Of Ice-salt Eutectic Points Of Multicomponent Salt-water Systems

The determination of freezing points and eutectic points of salt-water systems can not only provide a theoretical basis for the solid-liquid separation of salt-water systems at low temperature, but also play an important role in technical support and guarantee for the comprehensive utilization of brine resources. In this paper, the quinary system Na⁺,K⁺//Cl^-,SO₄^2-,NO₃^--H₂ O and its three quinary subsystems as well as three ternary subsystems were investigated. The freezing points, eutectic points of solutions and the crystallization regularities of salts have been studied by cooling method. According to experimental data, the eutectic point temperature–liquid dry salt composition diagrams, the dry salt projection diagrams and the eutectic point temperature projection diagrams for these systems were plotted. Besides, influence of cooling rate on the determination of the eutectic point temperature was studied under different solutions; The quaternary system Na⁺//Cl^-,SO₄^2-,NO₃^--H₂ O with different concentration characteristics in natural cooling process were studied by using the method in this paper.The results show:（1） The crystallization regularity of salts and the eutectic point temperatures can be obtained by measuring the temperature-time curves for multicomponent salt-water systems during the cooling process; the double salts existing under normal temperature, don’t appear at the eutectic point temperature in these systems, as a result, the phase relationships in these systems are simplified; The prism figure can be used to describe the relationships among the phases, freezing points、eutectic points and the compositions of liquids for the ternary systems,the quaternary homo-ion systems and interaction systems, the quinary systems.（2） For ternary systems Na⁺,K⁺//SO₄^2--H₂ O, K⁺//SO₄^2-,NO₃^--H₂ O and K⁺//SO₄^2-,Cl^--H₂ O, In the eutectic（freezing） point temperature–liquid composition diagrams（The triangle prism）, there are two eutectic curves with single salt and ice, one eutectic point with two salts and ice, one ice crystallizing region, two single-salt crystallizing region.（3） For the quaternary homo-ion systems K⁺//Cl^-,SO₄^2-,NO₃^--H₂ O and interaction systems Na⁺,K⁺//SO₄^2-,Cl^--H₂ O and Na⁺,K⁺//SO₄^2-,NO₃^--H₂ O, respectively, The combination of triangle prism, water projection diagram, the eutectic point temperature projection diagram, and dry salt projection diagram can be used to describe the relationships of eutectic point temperatures and liquid compositions. Respectively, there are three and four eutectic regions with single salt and ice, three and five eutectic curves with two salts and ice, one and two eutectic points with three salts and ice.（4） For the quinary system Na⁺,K⁺//Cl^-,SO₄^2-,NO₃^--H₂ O saturated with Na Cl·2H₂O, The combination of triangle prism, water projection diagram, chlorin projection diagram, the eutectic point temperature projection diagram, and dry salt projection diagram can be used to describe the relationships of eutectic point temperatures and liquid compositions. There are four eutectic regions of two salts and ice, five eutectic curves of two salts and ice, and two eutectic points of four salts and ice.（5）The cooling rate has little effect on the determination of the eutectic point temperature.（6） The crystallization regularities of different concentration solution in the Na⁺//Cl^-,SO₄^2-,NO₃^--H₂ O system are consistent with the conclusions in the literature.