Studies On The Meta-stable Equilibrium Of The Quaternary System Na～+, K～+, Ca～(2+)//Cl～--H₂O At 15℃ And 35℃

Abundant of oil-field brine resources were discovered in the western of China,especially the oil-field brines in Qaidam basin, which contains high concentrations ofcalcium and potassium. In order to make full use of the local natural condition,research on the meta-stable phase chemistry of the feature oil-filed brine systems isvery significant to exploit the precious resources.In the thesis, the meta-stable equilibria of the quaternary system Na⁺, K⁺,Ca²⁺//Cl^-—H₂O and of the ternary system K⁺, Ca²⁺//Cl^-—H₂O at 15℃and 35℃respectively have been studied using the isothermal evaporation method. Solubilitiesand physico-chemical properties such as density, viscosity, refractive index,conductivity, and pH in the solutions of the mentioned systems were determinedexperimentally. According to the experimental data, the dry-salt diagram, water phasediagram, and the diagrams of physico-chemical properties via compositions in themeta-stable quatemary equilibrium system were plotted; the meta-stable equilibriumphase diagrams and the diagrams of density via composition in the temary systems atboth temperatures were also drawn.In the meta-stable phase diagrams of the temary system K⁺, Ca²⁺//Cl^-—H₂O at15℃and 35℃; there are all in one invariant point, two uninvariant curves, and twocrystallization zones corresponding to potassium chloride, antarcticite (CaCl₂·6H₂O)at 15℃and potassium chloride, calcium chloride tetrahydrate at 35℃. They belongto a simple eutectic type, and neither double salts nor solid solution were found. Thedensity value of the meta-stable equilibrium solution has a regular changes, andachieves one maximum values at the invariant point. The solubility of calciumchloride is higher than other coexisted single salt, and the its crystallized zone iscorrespondingly smaller than others; due to the solubility of potassium chloride isrelatively low, the crystallization zone is larger than the zone of potassium chloride in the ternary system at 15℃and 35℃. Compared the metastable diagrams in bothtemperatures, it is found that the solubilities of potassium chloride and calciumchloride are become larger along with the temperature increasing, and thecrystallization zones are become larger correspondingly, especially to calciumchloride. So it could be used to recover potassium or remove of calcium.In the meta-stable dry-salt diagram of the quaternary system Na⁺, K⁺,Ca²⁺//Cl^-—H₂O at 15℃and 35℃, there are both in one invariant point, threeunivariant curves, and three crystallization zones corresponding to potassium chloride,sodium chloride, calcium chloride hexahydrate (antarcticite) at 15℃and potassiumchloride, sodium chloride, calcium chloride tetrahydrate at 35℃, respectively. Neithersolid solution nor double salts were found in the meta-stable system. It can be furtherfound that the physico-chemical properties corresponding to density, viscosity, pH,electrical conductivity and refractive index in the meta-stable equilibrium solution arein regular changes with the changes of potassium chloride concentration. As to thequaternary system at 15℃and 35℃, the solubility of calcium chloride is the largest,therefore the crystallization zone is the smallest correspondingly. However, thesolubility of potassium chloride is the smallest as well as its crystallized zone is thelargest at 15℃, and the solubility of sodium chloride is the smaller so as to thecrystallized zone is the largest at 35℃. When comparison of the metastable phasediagrams at 15℃and 35℃, we found the crystallization zone of sodium chloride andcalcium chloride are become larger, and the crystallization zone of potassium chloridebecome smaller. It is obvious that potassium chloride has salting-in action for calciumchloride, on the contrary, sodium chloride has salting-out effect for potassiumchloride.Using the temperature-dependent equations, Pitzer parameters including singlesalt and mixture-ion-interaction actions were fitted. Average equilibrium constants ofequilibrium solids were evaluated using Pitzer modelling. The predictive solubilitiesof quaternary system were calculated with Pitzer model at 15℃and 35℃. Thepredictive results were in agreement with the experimental data essentially.Corresponding analysis and discussion of prediction results was demostrated.