Non-equilibrium Behaviors And Dynamics Mechanismof Salt-forming In Boiling Evaporation Process For The System Of K⁺、Mg²⁺//Cl^-、SO₄^2--H₂O At75℃

In the salt water system, the fact of the metastable phase diagram departing from the equilibrium phase diagram has been confirmed by the previous studies. And the research on the metastable phase diagram in high temperature is lack. In recent years, it has been proved by industry practice and the related research that the solid-liquid relationship of the seawater subtype system in evaporation process at high-temperature is seriously different from that in the equilibrium phase diagram, which can’t also be accurately described by the metastable phase diagram. Therefore we expanded the phase relationship research of the salt water system in the boiling evaporation state to the scope of non-equilibrium thermodynamics and studied its mechanism from the aspect of crystallization kinetics.The feed solution with certain composition of the quaternary system K+、Mg2+//Cl-、 SO42--H2O were boiling evaporated at the temperature of348±0.2K in an oil jacket glass tank by a heating agent of fixed temperature difference of45K, with constant stirring speed and average evaporation intensity of1.8-2.4g/(L· min)(water). Through monitoring the initial salt-forming point and the changes of solid-liquid phase composition with the evaporation process, we obtained the salt-forming regions of KCl, K2SO4and Leonite under the experimental conditions, and investigated the affectes of the crystal seed and operation conditions. The results show that:(1) In the experimental conditions, the salt-forming regions of KCl, K2SO4and Leonite are obviously different from that in the equilibrium phase diagram. The salt-forming region of KCl occupied the regions of K2SO4, Leonite, Langbeinite and kainite, whose area was extended from1019to1594. The area of K2SO4salt-forming region was reduced from4979to4690. Leonite salt-forming region expanded its boundary to the regions of K2SO4and Langbeinite in different degree, but was occupied by KC1region, whose area was extended from1736to2835.(2) On the co-saturated curve, due to the difference of salt formation rate, even if two salts precipitate at the same time, the liquid points didn’t move along with the co-saturated curve of the equilibrium phase diagram.(3) Under the non-equilibrium conditions, the salt-forming region of K2SO4will be smaller if no the existing of K2SO4seed. Leonite can stablely exist in its salt forming region in the experiment, whose transformation requires enough supersaturation or enough time.This study get the salt-forming regions and behaviors for three kinds of salts of the quaternary system K+、Mg2+//Cl-、SO42--H2O in the boiling evaporation process. Because these salt-forming regions depart from the equilibrium phase diagram, and can’t be well described by the metastable phase diagram, we define them as the salt-forming phase digram in the non-equilibrium state. This concept is significant to express the chemical nature of complex salt-water system and the phase relation of complex process scientifically.