Solid-liquid Phase Equilibria Of The Quinary Oil-field Brine System(Li⁺,Na⁺,Ca²⁺//CL^-,borate-H₂O At T=288.15K

In the Qinghai-Tibet Plateau of China,the calcium chloride type oil-gas field brine with high concentrations of lithium,sodium,potassium,calcium and borate ions was found in the Nanyishan Section located in the Qaidam Basin.And the composition of the brine belongs to the complex six-component system(Li+,Na+,Ca2+,Mg2+//Cl-,borate—H2O).For the phase equilibrium of this complex system and qinary subsystems(Li+,Na+,Ca2+//Cl-,borate-H2O),quaternary system(Na+,Ca2+//Cl-,borate-H2O)at 288.15 K and their ternary system(Ca2+//Cl-,B6O102--H2O)at 288.15,298.15,308.15 and 323.15 K were researched by isothermal dissolution method.The solutions were summarized as following:As to the ternary system(Ca2+//Cl-,B6O102--H2O),it belongs to hydrate I type system,and neither double salt nor solid solution was generated at 288.15 K.There was one ternary invariable point,two univariant solubility curves,and two single-salt crystallization regions corresponding to CaC12·6H2O and CaB6O10·5H2O.The phase diagrams of this system at 298.15 K,308.15 K and 323.15 K were similar to that at 288.15 K.In all of the phase diagrams,no double salt or solid solution was found and belongs to hydrate I type system.The concentration of CaB6O10 is decreased with the increase of the content of CaCl2 in solution,and this result indicates that CaCl2 has a strong salting-out effect on CaB6O10.It was also found that CaB6O10·5H2O is stable solid phase,and it can not easily to convert into other forms of calcium borate.Moreover,it is obviously shown that the form of crystallization of calcium chloride changed from calcium chloride hexahydrate(CaCl2·6H2O)at 288.15 K into calcium chloride tetrahydrate(CaCl2·4H2O)at 298.15 K and then into more stable siniarite(CaCl2·2H2O)at 308.15 to 323.15 K with the increasing of temperature.It also indicated that the physicochemical properties including density,refractive index,and pH value all changed regularly with the concentration of CaCl2.Density and refractive index achieve the maximum values at the invariant point with the increase of the concentration of CaCl2.Comparing with the three phase diagrams at different temperatures,with the increasing of temperature the area of crystalline regions of CaCl2·nH2O(n=6,4,2)and CaB6O10·5H2O are gradually decreased and their solubilities are increased.In the dry-salt phase diagram of the quaternary system(Na+,Ca2+//Cl-,borate-H2O)at 288.15 K,there were two invariant points(NaCl + CaB6O10·5H2O + Na2B4O7·10H2O,and CaCl2·6H2O + CaB6O10·SH2O + Na2B4O7·10H2O),five univariant solubility isothermal curves and four crystallization regions.The physicochemical properties of the quaternary system(Na+,Ca2+//Cl-,borate—H2O)at 288.15 K change regularly with the composition.With B4O72-content increasing in liquid phase,the refractive index and density are increased obviously,and the pH value is reduced gradually.As to the dry-salt phase diagram of the quaternary system(Li+,Na+,Ca2+//Cl-,borate-H2O)at 288.15 K,there are in all four invariable points(NaCl + Na2B4O7·10H2O +Li2B4O7 3H2O + CaB6O10·5H2O,NaCl + LiCl H20 + Li2B4O7 3H2O + LiCl·CaC12·5H2O,NaCl + LiCl·CaCl2 5H2O + CaCl2·6H2O + CaB6O10 · 5H2O,F4:NaCl + LiCl CaCl2·5H2O +Li2B4O7·3H2O + CaB6O10·5H2O),six crystallization regions and nine univariant solubility curves.It was found that NaCl and CaCl2 had strong salting-out effect on borate.With Cl'content increasing,the refractive index and density are increased obviously,and the pH value reduced because of the increase of B4O72-content.Compared the different crystalline region in this phase diagram at 288.15 K,no solid solution was formed.However,the double salt(LiCl·CaCl2·5H2O)was formed in the part of this quinary system at 288.15 K.