A new method to recover NH4Cl from NH4Cl-rich aqueous solutions generated in the magnesia (MgO) production is developed on the basis of modeling the solid–liquid equilibrium (SLE) for the NH4Cl-MgCl2-H2O system with the Pitzer model embedded in Aspen Plus™ platform. The SLE values for the ternary system were determined from 278.15 to 348.15 K. The new standard-state chemical potentials of NH4Cl and MgCl2·6H2O were judicially obtained. The resulting equilibrium constants were used to determine new interaction parameters for the NH4Cl-H2O and MgCl2-H2O systems. These new parameters, together with the mixing parameters determined from correlating the experimental values, were used to correlate the equilibrium constant for NH4MgCl3·6H2O, which plays a key role in NH4Cl recovery. The results could extend SLE calculation for the NH4Cl-MgCl2-H2O system from 278.15 to 388.15 K, satisfying the process identification and simulation requirement involved in the recovery process. The phase-equilibrium diagram generated by modeling was illustrated to identify the process alternatives for recovering NH4Cl. The resulting course to recover NH4Cl by three fractional crystallization operations was finally proved feasible. © 2010 American Institute of Chemical Engineers AIChE J, 2011.