The mineral composition, crystal structure and microstructure of a composite spinel made from magnesia and hercynite at elevated temperature were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive Xray analysis (EDAX). A composite spinel MgFexAl2-xO4 and MgFe2O4 were formed in the reaction between magnesia and hercynite, but MgO·Al2O3 was not found. The mechanism may be described as follows: hercynite is oxidized to form γ-Al2O3 and γ-Fe2O3. MgO reacts with γ-Al2O3 and γ-Fe2O3 to form MgO·Al2O3 and MgO·Fe2O3, respectively. The γ-Fe2O3 dissolves into MgO·Al2O3 or γ-Al2O3 dissolves into MgO·Fe2O3 to form the composite spinel. The composition and crystal structure of the composite spinel depend on the sintering temperature and hercynite content in the powder mixture of magnesia and hercynite. With an increase of the sintering temperature, the solubility of Fe2O3 in the composite spinel decreases, resulting in an increase in 2θ, and a decrease in the lattice parameter, plane interplanar spacing and lattice distortion of the composite spinel. When the hercynite content in the mixture of magnesia and hercynite powder increases from 5 wt% to 10 wt%, the solubility of Fe2O3 in the composite spinel increases, resulting in a decrease in 2θ, and an increase in the lattice parameter, plane interplanar spacing and lattice distortion of the composite spinel.

Keywords: Magnesia, Hercynite, Reaction, Sintering, Cement kiln.