Study On The Stable Phase Equilibria Of The Quinary System Li⁺,Rb⁺,Cs⁺,Mg²⁺//SO₄^2--H₂O At 298.2 K And 323.2 K

The salt lakes in the Qaidam Basin of Qinghai are rich in mineral resources,and contain great quantity rare metal elements such as Li,Rb and Cs,which have great development and utilization value.The study on the phase equilibrium of the quinary system Li+,Rb+,Cs+,Mg2+//SO42--H2O and its subsystems is the basic researches for the development of salt lake brine in Qaidam Basin.Based on the composition characteristics of the old brine in the Qaidam Basin salt lakes,the isothermal phase equilibrium of the quinary system Li+,Rb+,Cs+,Mg2+//SO42--H2O and its subsystems as the quaternary systems Li+,Rb+,Mg2+//SO42--H2O,Li+,Rb+,Cs+//SO42--H2O and ternary system Li+,Cs+//SO42--H2O were studied at 298.2 K and 323.2 K using an isothermal dissolution method.The solubility,density and refractive value were determined at research temperature.The solid phase was identified by X-ray powder diffraction method.According to the experimental data,the corresponding space diagrams,the planar projection diagrams,density vs composition diagrams,refractive value vs composition diagrams and water content diagrams were constructed.The ternary system Li+,Cs+//SO42--H2O belongs to complex system at 298.2 K and 323.2 K,with two double salts formed,those are 3Li2SO4·Cs2SO4·2H2O and Li2SO4·Cs2SO4.Its stable phase diagram consists of three cosaturated points,four uni variant curves,and four crystallization zones.The crystallization area of Li2SO4·Cs2SO4 is the largest and that of Cs2SO4 is the smallest.By comparing the two stable phase diagrams formed at different temperatures,it is found that the crystal forms of single salt and double salt are the same,whereas the size of the crystallization region of salts has changed.With the increase of temperature,the crystallization zone of 3Li2SO4 CS2SO4 2H20 increase obviously,while that of Li2SO4·H2O,Cs2SO4 and Li2SO4·Cs2SO4 decrease slightly.The quaternary system Li+,Rb+,Mg2+//SO42--H20 belongs to complex type at 298.2 K and 323.2 K,there are three double salts formed,their are 3Li2S04 Rb2S04 2H20,Li2S04 Rb2S04 and Rb2S04 MgS04 6H20.The stable phase diagram has four cosaturated points,six crystallization fileds and nine univariant curves.The crystallization area of Rb2SO4·MgSO4·6H2O is the largest and that of Rb2SO4 is the smallest.With the temperature increases from 298.2 K to 323.2 K,the crystal forms of the other single salts and double salt have no change,whereas the crystal form of MgSO4 changes from MgSO4·7H2O to MgSO4·6H2O,and meantime,the crystallization regions of the double salts Rb2SO4 MgSO4 6H2O,3Li2SO4·Rb2SO4·2H2O and the single salts Li2SO4·H2O and Rb2SO4 significantly increase.The quaternary system Li+,Rb+,Cs+//SO42-_H20 belongs to complex type at 298.2 K and 323.2 K,there are four double salts and a solid solution formed,which are 3Li2S·4 Rb2S·4·2120,Li2S·4 Rb2S·4,3Li2S·4 Cs2S·4·2H2O,Li2SO4 Cs2SO4 and solid solution[(Rb,Cs)2SO4].Its stable phase diagram has six cosaturated points,eight crystallization regions and thirteen univariant curves.The crystallization area of Li2SO4·CS2SO4 is the largest and that of Cs2SO4 is the smallest.With the temperature increases from 298.2 K to 323.2 K,the crystal forms of the other single salts and double salt have no change,but the size of the crystallization zone is different,and the crystallization regions of the single salt Li2SO4.H2O,Rb2SO4 and solid solution[(Rb,Cs)2SO4]is significantly reduced,the crystallization zone of 3Li2SO4·Cs2SO4·2H2O increase obviously.In the quinary system Li+,Rb+,Cs+,Mg2+//SO42--H20,Li,Rb,and Cs are alkali metal elements and belongs to the first main group,their atomic radii are similar to each other,which cause them very easy to form double salt or solid solution in an aqueous solution.At the same time,Mg is an alkaline earth metal element,has the similar chemically with Rb and Cs,and easy to form double salts among them.Thus the quinary system belongs to complex system at 298.2 K and 323.2 K,there are six double salts and a solid solution formed,which are 3Li2SO4·Rb2SO4·2H2O,3Li2SO4 Cs2SO4 2H2O,Li2SO4 Rb2SO4,Li2SO4 Cs2SO4,Rb2SO4 MgSO4 6H2O,Cs2SO4·MgSO4·6H2O and solid solution[(Rb,Cs)2SO4].Its stable phase diagram consists of eight cosaturated points,twenty-five univariant curves,and eleven crystallization zones at 298.2 K and 323.2 K.The crystallization area of Rb2SO4·MgSO4·6H2O is the largest and that of Cs2SO4 is the smallest.With the temperature increases from 298.2 K to 323.2 K,the crystal form of MgS·4 changes from MgS·4·7H2O to MgSO4·6H2O.