Phase Equilibria Of The Systems RbCl+CsCl+PEG+H₂O And Its Partial Subsystems At Multi-temperature

It is very difficult to separate Rb and Cs from the chloride type brine due to the similar properties and structures of RbCl and CsCl which makes a solid-solution formed easily between them.By adding polyethylene glycol（PEG）into the salt-water system,it causes a change in the properties of the solvent,and then the crystallization rule of salt in it may be changed,so it can be expected to work on the separation of mixing salts of rubidium and cesium.Thus,it will be of great significance to study the phase equilibria of the systems containing rubidium chloride（RbCl）,cesium chloride（CsCl）,polyethylene glycol（PEG）,and water.In this thesis,the phase equilibria of ternary systems RbCl+CsCl+H₂O,CsCl+PEG1000/4000/6000/8000/10000/20000+H₂O,and quaternary systems RbCl+CsCl+PEG1000/4000/8000+H₂O at 288.2,298.2,and 308.2 K were determined by isothermal dissolution method or turbidimetric method.Meanwhile,the discussion of effects of temperature and the molecular weight of PEG on phase equilibria relationship were given in the thesis.Both of Schreinemaker’s wet residue method and X-ray diffraction method were used to identify the equilibrated solid phase of ternary system RbCl+CsCl+H₂O at288.2,298.2 and 308.2 K,results show that besides a continuous variation solid solution[Cs_x,Rb_1-x]Cl,there are two single salts RbCl and CsCl formed simultaneously.The phase diagrams of the system RbCl+CsCl+H₂O at 288.2,298.2 and 308.2 K are all consist of two invariant points,three isothermal dissolution curves,and three crystallization zones corresponding to two single salts RbCl,CsCl,and one solid solution[Cs_x,Rb_1-x]Cl.The temperature-dependent correlations of single-salt parameters were obtained with a temperature range of 273.2 to 398.2 K.Pitzer mixed parametersθandψwere fitted in this work.The Pitzer model was applied to calculate the solubility of ternary systems RbCl+CsCl+H₂O at 288.2,298.2 and 308.2 K,the calculated solubility data agree well with experimental data.There is only solid-liquid equilibrium relationship（SLE）existing in ternary system CsCl+PEG1000+H₂O at 288.2 and 298.2 K,their phase diagrams consist of three parts,which are that:（1）area of unsaturated liquid phase（L）,（2）area of one saturated liquid phase with one equilibrium solid phase of CsCl（L+S）,（3）area of one saturated liquid phase with two equilibrium solid phases of CsCl and PEG（L+2S）.Compared with the phase diagrams of the system CsCl+PEG1000+H₂O at 288.2 and 298.2 K,the areas of liquid-liquid equilibrium（2L）and liquid-liquid-solid equilibrium（2L+S）with a solid of CsCl are added at 308.2 K,while area of one saturated liquid phase with two equilibrium solid phases（L+2S）dissapear.Both the solid-liquid and liquid-liquid equilibrium relationship coexist in the ternary systems CsCl+PEG1000+H₂O at 308.2K and CsCl+PEG4000/6000/8000/10000/20000+H₂O at 288.2,298.2,and 308.2 K,the corresponding phase diagrams were all divided into six parts:unsaturated liquid phase area（L）,area of liquid-liquid equilibrium（2L）,area of liquid-liquid-solid equilibrium（2L+S）with a solid phase of CsCl,two areas of one liquid phase with a solid phase of CsCl（L+S）,and area of one liquid phase with two solid phases of PEG and CsCl（L+2S）.For ternary system CsCl+PEG+H₂O at 288.2,298.2 and 308.2 K,with the temperature or molecular weight of polyethylene glycol increasing,it is more conducive to the formation of the liquid-liquid equilibrium region（2L）.At the same temperature,with the increase of molecular weight of polyethylene glycol,the area of liquid-liquid equilibrium（2L）,area of one liquid phase with one solid phase（L+S）,and area of one liquid phase with two solid phases（L+2S）of the system increase,while the unsaturated liquid region（L）decreases.When the molecular weight of polyethylene glycol is constant,with the increase of temperature,the unsaturated liquid region（L）and area of one liquid phase with one solid phase（L+S）increase,while the area of one liquid phase with two solid phases（L+2S）decreases,area of liquid-liquid-solid equilibrium（2L+S）changes slightly.Furthermore,the experimental binodal curves data were satisfactorily correlated with three non-linear expressionsln(w_PEG)（28）a（10）bw_Cs^0.5 _Cl（10）cw³_CsCl,w_{PE G}（28）a（10）bw_Cs^0.5 _Cl（10）cw_CsCl,andw_{PE G}（28）a（10）bw_Cs^0.5 _Cl（10）cw_CsCl（10）dw²_CsCl.The experimental tie-line data were related with Othmer-Tobias and Bancroft empirical equations successfully.Phase equilibria of quaternary systems RbCl+CsCl+PEG1000/4000/8000+H₂O at 288.2,298.2,and 308.2 K were stuided with the different value of R（the ratio of PEG in PEG+H₂O mixed solvents）,and different value of K(K（28）w_CsCl:w_RbCl).When the molecular weight of PEG is 1000,the comparison of phase diagrams of RbCl+CsCl+PEG+H₂O with R=0,0.2,0.4 shows that the solubility of CsCl and RbCl in mixed solvents decreased with the increase of R,and the crystallization regions of single salts RbCl and CsCl increased.When the molecular weight of PEG is 4000 or8000,the crystallization region of RbCl increase obviously in system RbCl+CsCl+PEG+H₂O at 288.2,298.2,and 308.2 K,and when the value of R=0.35～0.55,the solid solution[Rb,Cs]Cl transformed into RbCl with an introdution of PEG4000 or PEG8000 in the RbCl+CsCl+H₂O salt-water system.