Phase Equilibria Of The Quinary System (Na⁺,NH₄⁺//Cl^-,SO₄^2-,HCO₃^--H₂O) And Its Subsystems At 273.15 K And 313.15 K

The combined alkali process is one of the leading production technologies of soda ash nowadays.Still,the separation process of the combined alkali process has sulfate enrichment in raw materials.Hence,the optimization of the separation process is essential for the theoretical guidance of the corresponding quinary system phase diagram.Therefore,the phase equilibria of the quinary system(Na⁺,NH₄⁺//Cl^-,SO₄^2-,HCO₃^-–H₂O)and its subsystems at 273.15 and 313.15 K have been studied using the isothermal dissolution method.The main research details and results are as follows:（1）Neither solid solution nor double salt in the quaternary system(Na⁺//Cl^-,SO₄^2-,HCO₃^-–H₂O)at 313.15 K was found.For the phase digram of this system,there is one invariant point（Na Cl+Na₂SO₄+Na HCO₃）,three univariant solubility curves and three crystallization regions,and the crystallization area was in order of Na HCO₃>Na₂SO₄>Na Cl.The water-phase diagram and the diagram of physicochemical properties versus the composition in the equilibrium liquid phases showed regular changes with the increase of Na₂SO₄.The calculated refractive index and density results agree well with the experimental data.（2）For the phase diagram of the reciprocal quaternary system(Na⁺,NH₄⁺//HCO₃^-,SO₄^2-–H₂O)at 273.15 K,there are three invariant points（NH₄HCO₃+（NH₄）₂SO₄+Na HCO₃,（NH₄）₂SO₄+Na₂SO₄·（NH₄）₂SO₄·4H₂O+Na HCO₃ and Na HCO₃+Na₂SO₄·（NH₄）₂SO₄·4H₂O+Na₂SO₄·10H₂O）,seven univariant solubility curves and five crystallization regions.The crystallization area of Na₂SO₄·10H₂O is the largest and easiest separate from the system.The dry-salt and water-phase diagrams were plotted,and the physicochemical properties versus the composition of the equilibrium liquid phases were also drawn,respectively.The physicochemical properties showed regular changes with the change of J（2Na⁺）.（3）For the phase diagram of the reciprocal quaternary system(Na⁺,NH₄⁺//HCO₃^-,SO₄^2-–H₂O)at 313.15 K,there are three invariant points（NH₄HCO₃+（NH₄）₂SO₄+Na HCO₃,（NH₄）₂SO₄+Na₂SO₄·（NH₄）₂SO₄·4H₂O+Na HCO₃ and Na₂SO₄+Na₂SO₄·（NH₄）₂SO₄·4H₂O+Na HCO₃）,seven univariant solubility curves and five crystallization regions.The crystallization area of Na HCO₃ is the largest and easiest separate from the system.The dry-salt and water-phase diagrams were plotted,and the physicochemical properties versus the composition of the equilibrium liquid phases were also drawn,respectively.The physicochemical properties showed regular changes with the change of J（2Na⁺）.（4）For the phase diagram of the quinary system(Na⁺,NH₄⁺//Cl^-,SO₄^2-,HCO₃^-–H₂O)all saturated with Na HCO₃ at 273.15 and 313.15 K,there are four invariant points,nine univariant solubility curves,and six crystallization regions.A comparison of the phase diagrams between 273.15 and 313.15 K shows that the crystallization form and area of the single salt change with the temperature change.In other words,with the increase in temperature,the crystal forms of sodium sulfate changed from Na₂SO₄·10H₂O to Na₂SO₄;the area of crystallization region of the double salt Na₂SO₄·（NH₄）₂SO₄·4H₂O is enlarged significantly while the area of crystallization region of NH₄HCO₃ is decreased.The dry-salt,water-phase,and sodium-phase diagrams were plotted,and the physicochemical properties versus the composition of the equilibrium liquid phases were also drawn,respectively.The physicochemical properties showed regular changes with the change of J(SO₄^2-).