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 (TM) 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 center dot 6H(2)O 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 center dot 6H(2)O, 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. (C) 2010 American Institute of Chemical Engineers AIChE J, 57: 1595-1606, 2011