Study On Phase Equilibria In The Quinary System K⁺,Mg²⁺,Pb²⁺,Zn²⁺//Cl^--H₂O And Its Relevant Subsystem

In this work,isothermal dissolution equilibrium method was used to investigate the phase equilibria in the quinary system K+,Mg2+,Pb2+,Zn2+//Cl--H2 O and its quaternary subsystems as well as ternary subsystems at 373 K in geothermal brine.Based on determined solubilities of components and densities of saturated solutions,both phase diagrams and density composition diagrams were constructed.The equilibrium solid phases were identified by using the X-ray powder diffraction combined with Schreinemakers' method.The systems studied include nine ternary systems,four quaternary systems and one quinary system.Ternary systems contain NaCl-PbCl2-H2 O,MgCl2-PbCl2-H2 O,CaCl2-PbCl2-H2 O,PbCl2-ZnCl2-H2 O,KCl-PbCl2-H2 O,KCl-ZnCl2-H2 O,MgCl2-ZnCl2-H2 O,CaCl2-ZnCl2-H2 O and NaCl-ZnCl2-H2 O.Quaternary systems involve K+,Mg2+,Pb2+//Cl--H2 O,K+,Mg2+,Zn2+//Cl--H2 O,K+,Pb2+,Zn2+//Cl--H2 O and Mg2+,Pb2+,Zn2+//Cl--H2 O.The quinary system is K+,Mg2+,Pb2+,Zn2+//Cl--H2 O.Ternary systems NaCl-PbCl2-H2 O,MgCl2-PbCl2-H2 O,CaCl2-PbCl2-H2 O and PbCl2-ZnCl2-H2 O belong to simple co-saturated type.The solid phases were existed in saturated solution in the forms of single salt instead of double salt or solid solution.Phase diagrams of them were divided into four regions by two solution boundary curves intersected at an invariant point.These regions consist of unsaturated liquid phase area,two single salt crystallization areas and co-crystallization area of the two salts.However,ternary systems KCl-PbCl2-H2 O,KCl-ZnCl2-H2 O,MgCl2-ZnCl2-H2 O,CaCl2-ZnCl2-H2 O and NaCl-ZnCl2-H2 O is complex owing to double salts formed in their solid phases.They are KCl·2PbCl2,2KCl·ZnCl2,MgCl2·ZnCl2·5H2O,CaCl2·ZnCl2·2H2O and NaCl·ZnCl2,respectively.The diagrams of them are constituted of two invariant points and three univariant isothermal dissolution curves.These curves divideded the phase diagram into six regions corresponding to an unsaturated liquid phase area,two single salt crystallization areas,a double salt crystallization areas and two co-crystallization areas of the single salt and double salt.The phase diagram in the quaternary system K+,Mg2+,Pb2+//Cl--H2 O at 373 K consists of three invariant points,seven univariant isothermal dissolution curves and five solid phase crystalline regions corresponding to KCl,MgCl2·6H2O,KCl·MgCl2·6H2O(Car),KCl·2PbCl2 and PbCl2,respectively.The phase diagram in the quaternary system K+,Mg2+,Zn2+//Cl--H2 O at 373 K is made up of four invariant points,nine univariant isothermal dissolution curves and six solid phase crystalline regions corresponding to KCl,MgCl2·6H2O,KCl·MgCl2·6H2O(Car),2KCl·ZnCl2,MgCl2·ZnCl2·5H2O and ZnCl2.The phase diagram in the quaternary system K+,Pb2+,Zn2+//Cl--H2 O at 373 K is constituted of three invariant points,seven univariant isothermal dissolution curves and five solid phase crystalline regions corresponding to KCl,PbCl2,KCl·2PbCl2,2KCl·ZnCl2 and ZnCl2.The phase diagram in the quaternary system Mg2+,Pb2+,Zn2+//Cl--H2 O at 373 K contains two invariant points,five univariant isothermal dissolution curves and four solid phase crystalline regions corresponding to MgCl2·6H2O,PbCl2,MgCl2·ZnCl2·5H2O and ZnCl2.The phase diagram in the quinary system K+,Mg2+,Pb2+,Zn2+//Cl--H2O(saturated with PbCl2)at 373 K is complex.The existing form of salt in the equilibrium solid phase is consistent with all four quaternary subsystems.Compared with the quaternary subsystems,there is no special double salt and hydrate formed.Not only were three single salts PbCl2,ZnCl2 and MgCl2?6H2O found,but also three double salts KCl?MgCl2?6H2O,MgCl2?ZnCl2?5H2O and KCl?2PbCl2 were determined.The phase diagram of the projection with lead chloride consists of three invariant points,seven univariant isothermal dissolution curves and five crystallization areas corresponding to KCl?MgCl2?6H2O,MgCl2?ZnCl2?5H2O,KCl?2PbCl2,MgCl2?6H2O and ZnCl2,respectively.Among them,the crystallization zone of 2KCl?PbCl2 is the largest.While the crystalline zone of magnesium chloride hexahydrate is the smallest.The phase diagram in the quinary system K+,Mg2+,Pb2+,Zn2+//Cl--H2O(saturated with KCl)at 373 K is complex.Five equilibrium solid phases contained in the phase diagram were determined and they were KCl?MgCl2?6H2O,MgCl2?ZnCl2?5H2O,2KCl?ZnCl2,KCl?2PbCl2 and KCl.The phase diagram of the projection with potassium chloride includes two invariant points,five univariant isothermal dissolution curves and four crystallization areas corresponding to KCl?MgCl2?6H2O,MgCl2?ZnCl2?5H2O,2KCl?ZnCl2,KCl?2PbCl2,respectively.Among them,the crystallization zone of 2KCl?PbCl2 is the largest.While the crystalline zone of KCl?MgCl2?6H2O is the smallest.The phase diagram in the quinary system K+,Mg2+,Pb2+,Zn2+//Cl--H2O(saturated with MgCl2)at 373 K is complex.There were four equilibrium solid phases involved in the phase diagram and they were KCl?MgCl2?6H2O,MgCl2?ZnCl2?5H2O,PbCl2 and MgCl2?6H2O.The phase diagram of the projection with magnesium chloride is constituted of one invariant points,three univariant isothermal dissolution curves and three crystallization areas corresponding to KCl?MgCl2?6H2O,MgCl2?ZnCl2?5H2O and Pb Cl2,respectively.Among them,the crystallization zone of 2KCl?PbCl2 and PbCl2 are larger than MgCl2?ZnCl2?5H2O.The phase diagram in the quinary system K+,Mg2+,Pb2+,Zn2+//Cl--H2O(saturated with ZnCl2)at 373 K is complex.There were five equilibrium solid phases involved in the phase diagram and they were 2KCl?ZnCl2,KCl?2PbCl2,MgCl2?ZnCl2?5H2O,PbCl2 and ZnCl2.The phase diagram of the projection with zinc chloride is made up of two invariant point,five univariant isothermal dissolution curves and four crystallization areas corresponding to 2KCl?ZnCl2,KCl?2PbCl2,MgCl2?ZnCl2?5H2O and PbCl2,respectively.Among them,the crystallization zone of PbCl2 is largest,and the smallest one is MgCl2?ZnCl2?5H2O.