To improve the overall sustainability of MgO-based refractory production, a novel process to produce high purity MgO from calcined low-grade magnesite in ammonium chloride solution was developed. The process was designed on the basis of the phase equilibria of the NH4Cl-MgCl2-NH3-H2O system obtained using the Mixed Solvent Electrolyte model embedded in OLI software. The optimum calcination temperature of low-grade magnesite was determined to be 650 degrees C in terms of the conversion ratio of magnesium and calcium in the leaching experiments. An apparent activation energy of Mg extraction was 30.98 kJ/mol, which is slightly lower than that of Ca leaching. An empirical kinetic model of magnesium extraction was also developed to describe the effects of NH4Cl concentration, particle size of calcined magnesite, and solid-to-liquid ratio on the extent of extraction of magnesium. At leaching time of 10 min, the leachate with high Mg/Ca molar ratio was obtained. Then, MgO with a purity of 99.09% was produced through the decomposition of intermediate 4MgCO(3)Mg(OH)(2)4H(2)O. (c) 2015 American Institute of Chemical Engineers AIChE J, 61: 1933-1946, 2015