Study On Phase Changes Of Salt-water Systems Containing Potassium And Nitrate At Low Temperature

Phase equilibria of water-salt systems are of great importance to guide the comprehensive utilization of seawater,salt lake,underground brine and wastewater.The current researches mainly focus on the stable and metastable phase equilibrium at room temperatures or high temperatures,the studies on phase equilibria at low temperatures are little.The low-temperature has a strong influence on the crystallization behavior and the solubilities of salts,especially for the sulfate-containing double salts.It is possible to ensure less double salts species and simplify the crystallization process of salts and more separation efficiency by operating at low temperature.Nitrate brine is found in Xinjiang Province of China,containing Na⁺,K⁺,Mg²⁺,Cl^-,NO₃^-,SO₄^2-,and belongs to a six-component system.The salt crystallization processes of the nitrate brine are extremely complex at normal or high temperature,and even cannot get the pure target product,due to the characteristics of multicomponent system and large numbers of complex salts.In addition,the winter is very cold and long in Xinjiang,it is of great significance to explore production processes at low temperatures.Because of the lack of relevant low-temperature phase diagrams as guidance,the work of exploration and development of brine is relatively blind,and make no progress.In this paper,the experimental study on phase equilibrium at 258.15 K in the system Na⁺,K⁺,Mg²⁺//Cl^-,NO₃^-,SO₄^2-–H₂O and its subsystems were investigated using the isothermal solution equilibrium method.According to the measured solubility and density data,different phase diagrams were constructed and precipitation laws of salts were analyzed;At the same time,the relationship between freezing point,eutectic point and composition of solution were studied using the cooling curves method,and graphical expression of ternary and quaternary systems were investigated.Combined with the phase diagrams,the law of salts crystallization from solutions containing Na⁺,K⁺,Mg²⁺,Cl^-,NO₃^-,SO₄^2-with different composition during cooling and evaporation was analyzed.These fundamental data could be valuable for promoting the research and further theoretical exploration for the development of nitrate brine or other related fields.The results are as follows:(1)The ternary systems Na⁺//Cl^-,NO₃^-–H₂O,Na⁺,Mg²⁺//NO₃^-–H₂O and Mg²⁺//Cl^-,NO₃^-–H₂O at 258.15 K all belong to simple cosaturated types without double salts formed.The phase diagrams consist of one invariant point,two univariant curves,and three crystallization fields.Double salt KCl·Mg Cl₂·6H₂O was found in the K⁺,Mg²⁺//Cl^-–H₂O system,in the phase diagram of which there are two invariant points,three univariant curves,and five crystallization fields.Temperature changes have a certain effect on the relative size of the crystallization areas of the salts,which provides design ideas in the process for production of potassium chloride or sodium nitrate from the brine.(2)Double salt KCl·Mg Cl₂·6H₂O was found in the homo-ion quaternary system Na⁺,K⁺,Mg²⁺//Cl^-–H₂O,there are two invariant points,five univariant curves,and four crystallization fields in this system.The crystallization fields decrease in the order of KCl?Na Cl·2H₂O?KCl·Mg Cl₂·6H₂O?Mg Cl₂·8H₂O.The reciprocal quaternary system Na⁺,Mg²⁺//Cl^-,NO₃^-–H₂O belongs to a simple system without double salt formed,the diagram has two invariant points,five univariant curves,and four crystallization fields.The crystallization flied of Na NO₃is the largest,and that of Mg Cl₂·8H₂O is the smallest,which indicates that the solubility of magnesium chloride is the largest,and sodium nitrate can crystallize out more easily from brine at low temperature.Compared with the phase diagram at 298.15 K,it is found that both the crystallized areas and forms of the salts all change greatly with a drop in temperature.(3)The quinary system Na⁺,K⁺,Mg²⁺//Cl^-,NO₃^-–H₂O saturated with Na Cl·2H₂O is of a complex type with the double salt KCl·Mg Cl₂·6H₂O formed at 258.15 K,which contains four invariant points,nine univariant curves,and six crystallization fields.The crystallization zone decreased in order of KCl,KNO₃,Na NO₃,KCl·Mg Cl₂·6H₂O,Mg Cl₂·8H₂O,Mg(NO₃)₂·6H₂O.The quinary Na⁺,Mg²⁺//Cl^-,NO₃^-,SO₄^2-–H₂O system saturated with Na Cl·2H₂O at 258.15 K contains three invariant points,seven univariant curves,and five crystallization fields corresponding to Na₂SO₄·10H₂O,Na NO₃,Mg Cl₂·8H₂O,Mg SO₄·7H₂O,and Mg(NO₃)₂·6H₂O.The crystallization areas of salts Na₂SO₄·10H₂O and Na NO₃occupy larger part,meaning they can be more easily separated from solution as compared with other coexisting salts in this system.Compared with the phase diagram at 298.15 K,the crystallization zones of double salts darapskite and astrakanite disappear,and the phase equilibrium relationship is greatly simplified with a drop in temperature.(4)The six-component system Na⁺,K⁺,Mg²⁺//Cl^-,NO₃^-,SO₄^2-–H₂O saturated with Na Cl·2H₂O at 258.15 K,the phase diagram of which can be drawn through a triangular prism,contains six invariant points and eight two-salt crystallization fields.The crystallization area of Na Cl·2H₂O+Na₂SO₄·10H₂O occupies the largest part because of its low solubility,which means Na₂SO₄·10H₂O is apt to crystallize out from brine at low temperature.Comparison with the diagram at 298.15 K shows that the species of complex salts are reduced from 6 to 1,and invariant points are lessened from25 to 6,which could simplify the phase diagram relationship.(5)The freezing(eutectic)temperature-solution composition diagrams of the systems Na⁺//Cl^-,SO₄^2-–H₂O,Na⁺//Cl^-,NO₃^-,SO₄^2-–H₂O and Na⁺,K⁺//Cl^-,NO₃^-–H₂O were constructed,which can present directly eutectic points/curves/regions of ice+single salt,ice+two salts,and ice+three salts.In addition,the freezing points of different compositions solution with water contents of 95%,85%and 75%in the two quaternary systems Na⁺//Cl^-,NO₃^-,SO₄^2-–H₂O and Na⁺,K⁺//Cl^-,NO₃^-–H₂O were studied and drawn in the freezing(eutectic)temperature-solution composition diagram.The extended UNIQUAC model was used to predict the freezing points.The calculated results are in agreement with the experimental ones,which verify the accuracy of the model for the freezing point calculation.(6)Through the cooling process of the brines with different characteristic compositions,it can be proved that the concentration of sulfate decrease with the temperature decreasing,and most of sulfate in brine can be removed by crystallizing Na₂SO₄·10H₂O resulting of few sulfate-containing double salt formed.The sample which has removed most of sulfate after cooling was used as evaporation raw material and was further evaporated naturally.The theoretical analysis based on the phase diagram of Na⁺,K⁺,Mg²⁺//Cl^-,NO₃^-–H₂O system at 298.15 K was in good agreement with the experimental results.Therefore,the treatment of the brine based on freezing and natural evaporation can not only separate mirabilite and sodium chloride,but also enrich potassium and nitrate in the brine,which is beneficial for production of potassium and nitrate.