Studies On Phase Equilibria In The Quinary System Li⁺,Na⁺,Mg²⁺//SO₄^2-,B₄O₇^2--H₂O And Its Relevant Subsystems At 273 K

In this thesis,isothermal evaporation method and isothermal solution equilibrium method were employed to obtain the phase equilibrium relationship of quinary system Li⁺,Na⁺,Mg²⁺//SO₄^2–,B₄O₇^2–-H₂O and its partial sub-systems at 273 K.On the base of the obtained data,the phase diagram,density-compositon diagram and water content diagram of each system were plotted.Besides,saturated solid phases of each system were separated and identified by X-ray powder diffraction.Compared standard lines with the obtained X-ray diffraction pattern,the solid phases were confirmed.The systems studied in this paper includes:the stable phase diagram and the metastable phase diagram of two ternary systems Na₂SO₄–Na₂B₄O₇–H₂O and Li₂B₄O₇–Na₂B₄O₇–H₂O,stable phase diagrams of two interactive quaternary system Li⁺,Na⁺//SO₄^2–,B₄O₇^2–-H₂O and Na⁺,Mg²⁺//SO₄^2–,B₄O₇^2–-H₂O,and stable phase diagram of a quinary system Li⁺,Na⁺,Mg²⁺//SO₄^2–,B₄O₇^2–-H₂O.The results show that the stable and metastable phase diagrams of the ternary system Na₂SO₄–Na₂B₄O₇–H₂O both belong to the hydrate type?,and no double salt or solid solution was formed.Both phase diagrams contained one invariant point,two univariate solubility curves,two saturated solid crystalline regions,which corrospond to Na₂B₄O₇·10H₂O and Na₂SO₄·10H₂O,respectively.The densities of the two systems both changed with the increase of the liquid mass fraction of Na₂SO₄ and reached the maximum at invariant point.Comparing the phase diagram of 273 K with that at other temperatures?288 K,298 K,323 K,348 K and 373 K?,it is found that the ternary system has a similar shape at different temperatures,the solubility of components increased with increasing temperature,and equilibrium solid phases Na₂B₄O₇·10H₂O and Na₂SO₄·10H₂O dewatered at high temperatures?323 K and 373 K respectively?,dehydrating to Na₂B₄O₇·5H₂O and anhydrous Na₂SO₄.The comparison between the metastable and stable phase diagrams of this system at 273 K shows that the position of the starting point have changed.The solubility of single salt Na₂B₄O₇ increases obviously in metastable state,while the saturated crystalline forms of the two salts remain unchanged,the invariant points of metastable and stable phase diagrams are similar.The stable and metastable phase diagrams of the ternary system Li₂B₄O₇–Na₂B₄O₇–H₂O both belong to the hydrate type?.Both of the phase diagrams consist of two univariate solubility curves,one invariant point and two solid-phase crystalline regions.However,there is a significant difference between the metastable and stable phase diagrams.When the stable equilibrium is reached,the equilibrium solid phases are Na₂B₄O₇·10H₂O and Li₂B₄O₇·3H₂O,while the equilibrium solid phases are Na₂B₄O₇·10H₂O and LiBO₂·8H₂O at metastable state.Compared with the stable phase diagram,the addition of Na₂B₄O₇ in the metastable system greatly affects the Li₂B₄O₇content in the liquid phase.As the sodium tetraborate content increases,the solubility curve of lithium tetraborate decreases rapidly,so Na₂B₄O₇ has salting-out effect on Li2B4O7 in this system.On the contrary,in the stable equilibrium,with the addition of sodium tetraborate,solubility curve of lithium tetraborate changes slightly.The stable phase diagram at 273 K was compared with that at 288 K and 298 K.The stable phase diagrams at three temperatures shows similarities and all of the equilibrium solid phases were Na₂B₄O₇·10H₂O and Li₂B₄O₇·3H₂O.The solubility of liquid components increased with increasing tempurature.At 273 K and 298 K,sodium tetraborate solubilized lithium tetraborate,while at 288 K,sodium tetraborate shows salting-out effect on lithium tetraborate.The interactive quaternary system Li⁺,Na⁺//SO₄^2–,B₄O₇^2–-H₂O is a complex system with double salt 3Na₂SO₄·Li₂SO₄·12H₂O.The phase diagram consists of three invariant points,seven unilateral solubility curves and five crystallization zones,which are Na₂SO₄·10H₂O,Na₂B₄O₇·10H₂O,Li₂SO₄·H₂O,Li₂B₄O₇·3H₂O and double salt3Na2SO4·Li2SO4·12H2O,respectively.Compared with the metastable phase diagram at273 K,those two phase diagrams have a similar shape and double salt3Na₂SO₄·Li₂SO₄·12H₂O are found.However,the solid phase of lithium tetraborate in the metastable state changes from Li₂B₄O₇·3H₂O to LiBO₂·8H₂O,and the area of each solid phases has changed.The interactive quaternary system Na⁺,Mg²⁺//SO₄^2–,B₄O₇^2–-H₂O consists of two invariant points,five univariate solubility curves and four crystallization zones,which are MgB₄O₇·9H₂O,MgSO₄·7H₂O,Na₂SO₄·10H₂O and Na₂B₄O₇·10H₂O.Comparing the phase diagram with that at 288 K,it is found that the 288 K phase diagram is composed of a double salt Na₂SO₄·MgSO₄·4H₂O,and that consists of three invariant points,seven solubility curves and five crystallization zones,it is a complex system at288 K.The quinary system Li⁺,Na⁺,Mg²⁺//SO₄^2–,B₄O₇^2–-H₂O has three invariant points in the phase diagram saturated with MgB₄O₇.Seven single variable saturated solution curves divide the phase diagram into five single crystalline regions which are Na₂B₄O₇·10H₂O,Li₂SO₄·H₂O,Na₂SO₄·10H₂O,MgB₄O₇·9H₂O and Li₂B₄O₇·3H₂O,respectively?saturated with MgB₄O₇?.The quinary system Li⁺,Na⁺,Mg²⁺//SO₄^2–,B₄O₇^2–-H₂O has double salt3Na₂SO₄·Li₂SO₄·12H₂O formed in the phase diagram saturated with Na₂B₄O₇.There are three invariant points,seven single variable saturated solution curves divide the phase diagram into five single crystalline regions which are MgSO4·7H2O,Li2SO4·H2O,MgB4O7·9H2O,Li2B4O7·3H2Oanddoublesalt3Na2SO4·Li2SO4·12H2O respectively?saturated with Na₂B₄O₇?.