On
the basis of the comprehensive utilization of the Na–Mg
salt deposit, a new process to produce NaCl and ammonium carnallite
(MgCl2·NH4Cl·6H2O) by using
NH4Cl and the solution left after the preparation of potassium
salt as raw materials was proposed. For this, the phase equilibrium
of the quaternary system NaCl–MgCl2–NH4Cl–H2O at 25 and 0 °C was studied.
The solubilities of the quaternary system NaCl–MgCl2–NH4Cl–H2O were measured by an
isothermal method, and the corresponding phase diagrams were drawn
out. From the phase diagrams: there are four solid phase crystalline
zones, which correspond to NaCl, MgCl2·6H2O, NH4Cl, and MgCl2·NH4Cl·6H2O respectively. NaCl and MgCl2·NH4Cl·6H2O have larger crystalline zones at 25 °C
than at 0 °C, and NH4Cl has a smaller crystalline
zone at 25 °C than at 0 °C. It indicates that NaCl and MgCl2·NH4Cl·6H2O are easy to crystallize
out. On the basis of the analysis and calculation of the phase diagrams,
the appropriate conditions for the process were identified. In the
process, the NaCl could be prepared at 25 °C, and the ammonium
carnallite could crystallize out at 0 °C when NH4Cl
was added into the remaining solution. The solubilities of the studied
system were calculated based on the extended Pitzer model, and the
results showed that the calculated values were closed to the experimental
results.
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