Salt-Forming Regions of the Na+,Mg2+//Cl–,SO42––H2O System at 348.15 K in the Nonequilibrium State of Isothermal Boiling Evaporation

Industry evaporation processes are often operated at the compulsive nonequilibrium state at boiling temperature with a high evaporation intensity; meanwhile, the metastable phenomena for a complex salt–water system as Na+,Mg2+//Cl–,SO42––H2O are still typical in this case. The salt-forming regions in this condition are thus more complex and not always following the solubility diagram. However, the data of the metastable equilibria are lacking in high temperature, and the stability of metastable equilibria in the industry process attracts special attention. Therefore, to know more about the behaviors of the salt-forming region departing from the equilibrium phase area, the experiments of determining the salt-forming region of the Na+,Mg2+//Cl–,SO42––H2O system at 348 ± 0.2 K were carried out by the isothermal boiling evaporation method and with an evaporation intensity of (1.8 to 2.4) g·(L·min)−1 (water). The salt-forming regions were determined where the regions of halite, thenardite, and loeweite are enlarged, and they are 1.99, 1.67, and 1.35 times bigger, respectively, than those in solubility diagram, whereas vanthoffite and kieserite regions are reduced. Furthermore, the diagram composed of all salt-forming regions shows four one-salt stable regions and a complex interlaced zone called the conditional region, which gives information about the stability of the salt-forming region in the nonequilibria state. In addition, comparing the salt-forming region and the solubility region, the conditional region given in the isothermal diagram accounts for 37.6 % of the diagram's total area, and it was definitely divided into two-, three-, or four-salt regions where the salts precipitating may be one or another or together which depend more on the nonthermodynamic conditions, such as crystal seed, evaporation intensity, mechanical effects, and so forth.