The isothermal dissolution method
was applied for the phase equilibrium
experiment of the quaternary system K+, Rb+ //
Cl–, and borate–H2O at T = 323.2 K and p = 94.77 kPa. The solubilities
and refractive indices (n
D) of the system
were determined experimentally. The coexisting salts of the quaternary
invariant points were identified using the X-ray diffraction (XRD)
method. Based on the measured data, the stable phase diagram and the
figures of water content and refractive index (n
D) vs composition were plotted for this system at 323.2 K.
The results show that the system belongs to a complex type with the
formation of a solid solution [(K, Rb)Cl]. The stable phase diagram
consists of three quaternary invariant points, seven isothermal dissolution
curves, and five crystallization regions corresponding to KCl, RbCl,
RbB5O6(OH)4·2H2O,
K2B4O5(OH)4·2H2O, and [(K, Rb)Cl]. The size of the RbB5O6(OH)4·2H2O crystallization zone is the
largest among the coexisting salts in the quaternary system, which
means it is the most easily crystallized from the mixed solution containing
potassium, rubidium, chloride, and borate.
The phase equilibria of aqueous ternary systems NH4Cl
+ CaCl2 + H2O and NH4Cl + MgCl2 + H2O at 308.2 K were measured by the isothermal
dissolution method, and the solubility, density, and refractive index
were determined experimentally. The stable phase diagrams and the
diagrams of density/refractive index versus composition of these two
ternary systems are plotted. The results show that both these systems
at 308.2 K are complex systems, with the formation of double salts
2NH4Cl·CaCl2·3H2O and NH4Cl·MgCl2·6H2O, respectively.
The multitemperature phase diagrams of the ternary systems NH4Cl + CaCl2 + H2O at (273.2, 298.2, 308.2,
323.2, and 348.2 K) and NH4Cl + MgCl2 + H2O at (273.2, 298.2, 308.2, and 323.2 K) are compared, respectively.
The results show that within the temperature range discussed, the
double salt NH4Cl·MgCl2·6H2O is formed, and the double salt 2NH4Cl·CaCl2·3H2O is only formed at 308.2, 323.2, and
348.2 K. The crystallization areas of 2NH4Cl·CaCl2·3H2O and NH4Cl·MgCl2·6H2O gradually increase with increasing temperature.
Thus, the increasing temperature is conducive to the separation of
calcium or magnesium from these two systems in the form of 2NH4Cl·CaCl2·3H2O or NH4Cl·MgCl2·6H2O. The interaction relationship
between the ions of the system NH4Cl + CaCl2 + H2O becomes simpler at 273.2–298.2 K, which
is more beneficial to the precipitation of single salts. When the
temperature reaches 323.2 K, NH4Cl·MgCl2·6H2O precipitates into a more stable salt of commensurate
type. Meanwhile, the thermodynamic calculations of these two systems
at 308.2 K were fitted by the Pitzer–Simonson–Clegg
model, and the predicted values agree well with the experimental values.
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