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A growth kinetic model has been developed from a rigorous thermodynamic perspective to describe the crystal growth rates of NH4Cl on the basis of the difference of chemical potentials of NH4Cl at solidliquid interface in aqueous NH4Cl, NH4Cl-NaCl, and NH4Cl-MgCl2 solutions. The solidliquid equilibrium and activity coefficient of NH4Cl are calculated by the newly developed accurate Pitzer model with aid of Aspen Plus(TM) platform. The predictions of the resulting model are in good agreement with the experimental data published in literature and determined in this work at 283.15333.15 K within the supersaturation up to 0.1. The kinetic model was subsequently used to analyze the effect of several operation variables, including temperature (283.15333.15 K), supersaturation (up to 0.1), and NaCl or MgCl2 concentration (0 similar to 2.5 mol kg-1), on the crystal growth rate of NH4Cl. The crystal growth rate of NH4Cl, with activation energy of 39 kJ mol-1, is strongly temperature-dependent and increases with increasing temperature in the three systems investigated. The advantage of MgCl2 over NaCl on the recovery of NH4Cl is theoretically and experimentally illustrated from the thermodynamic and kinetic perspectives with the aid of the established model. (c) 2011 American Institute of Chemical Engineers AIChE J, 2012

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