Stable Phase Equilibria Of The Quinary System NH₄⁺,Mg²⁺,Ca²⁺,Sr²⁺//Cl^--H₂O And Its Partial Quaternary Subsystems At 298.2 K

In the process of utilization of strontium-bearing tailings,the salt-water system containing ammonium,strontium,calcium,magnesium,aluminum and other ions was formed after the addition of leaching agent.The salt interaction in the above systems is complex,and there are many forms of hydrated salt,double salt or solid solution,etc.,which makes it difficult to separate useful mineral elements from strontium-bearing tailings.Therefore,it is necessary to carry out the corresponding phase equilibria study of above-mentioned complex salt-water system.The phase equilibria of strontium containing quinary system NH₄⁺,Mg²⁺,Ca²⁺,Sr²⁺//Cl^--H₂O and its partial quaternary subsystems NH₄⁺,Mg²⁺,Ca²⁺//Cl^--H₂O,NH₄⁺,Ca²⁺,Sr²⁺//Cl^--H₂O,Mg²⁺,Ca²⁺,Sr²⁺,//Cl^--H₂O at 298.2 K were studied by using the isothemal dissolution equilibrium method.The solubilities,densities,and refractive indices of above-mentioned systems were determined experimentally,meanwhile,the X-ray diffraction method was applied to identify the composition of the equilibrated solid phase.Based on the experimental data,the stereo phase diagram,stable phase diagram,the water content diagram,density/refractive index vs composition diagram were constructed.There are two double salts 2Mg Cl₂·Ca Cl₂·12H₂O and NH₄Cl·Mg Cl₂·6H₂O formed in the quaternary system NH₄⁺,Mg²⁺,Ca²⁺//Cl^--H₂O at 298.2 K,no solid solution found.The system belongs to a complex type,and the corresponding phase diagram consists of three invariant points,seven isothermal dissolution curves,and five crystallization regions.Among the five regions in the phase diagram,the crystallization region of the salts decreases with the sequence of NH₄Cl,NH₄Cl·Mg Cl₂·6H₂O,Ca Cl₂·6H₂O,Mg Cl₂·6H₂O,and 2Mg Cl₂·Ca Cl₂·12H₂O,which indicate that the solubility of NH₄Cl is the lowest,and it is the easiest to reach the saturation and crystallize from the complex aqueous solution.The quaternary system NH₄⁺,Ca²⁺,Sr²⁺//Cl^--H₂O is a complex type at 298.2 K,with the solid solution[?Ca,Sr?Cl₂]·6H₂O and[?Sr,Ca?Cl₂]·6H₂O found,while there is no double salt formed.The stable phase diagram of quaternary system NH₄⁺,Ca²⁺,Sr²⁺//Cl^--H₂O at 298.2 K contains three invariant points,seven univariate curves,and five crystallization regions.Among the five regions in the phase diagram,the crystallization region of the salts decreases with the sequence of NH₄Cl,Sr Cl₂·6H₂O,[?Ca,Sr?Cl₂]·6H₂O,[?Sr,Ca?Cl₂]·6H₂O,Ca Cl₂·6H₂O,which indicate that the solubility of salt NH₄Cl is the lowest,while the solubility of Ca Cl₂ is largest.There are one double salt 2Mg Cl₂·Ca Cl₂·12H₂O,and solid solution[?Sr,Ca?Cl₂]·6H₂O,[?Ca,Sr?Cl₂]·6H₂O formed in the quaternary system Mg²⁺,Ca²⁺,Sr²⁺//Cl^--H₂O at 298.2 K.The system belongs to a complex type,and the phase diagram of the quaternary system consists of five invariant points,eleven isothermal dissolution curves,and seven crystallization regions corresponding to 2Mg Cl₂·Ca Cl₂·12H₂O,[?Sr,Ca?Cl₂]·6H₂O,[?Ca,Sr?Cl₂]·6H₂O,Sr Cl₂·2H₂O,Sr Cl₂·6H₂O,Mg Cl₂·6H₂O,and Ca Cl₂·6H₂O.Among the seven regions in the phase diagram,the crystallization region of the salt Sr Cl₂·6H₂O is the largest,the crystallization region of the salt Ca Cl₂·6H₂O is the smallest,which indicate that the solubility of Sr Cl₂ is the lowest,and it is the easiest to reach the saturation and crystallize from the complex aqueous solution.The stereo phase diagram of the quinary system NH₄⁺,Mg²⁺,Ca²⁺,Sr²⁺//Cl^--H₂O at 298.2 K is composed of nine invariant points,twenty-two univariate curves,and nine crystallization fields corresponding to five single salts NH₄Cl,Mg Cl₂·6H₂O,Ca Cl₂·6H₂O,Sr Cl₂·2H₂O,Sr Cl₂·6H₂O,two double salts 2Mg Cl₂·Ca Cl₂·12H₂O,NH₄Cl·Mg Cl₂·6H₂O,and solid solution[?Ca,Sr?Cl₂]·6H₂O,[?Sr,Ca?Cl₂]·6H₂O.The crystallization region of salt NH₄Cl is the largest,and it is easy to be separated from mixed solution.The phase diagram of the quinary system?saturated with NH₄Cl?contains three invariant points,seven univariate curves,and five crystallization fields.The phase diagram of the quinary system?saturated with Mg Cl₂?contains four invariant points,nine univariate curves,and six crystallization fields.