Chromium crystal chemistry mullite–spinel refractory ceramics

A small amount of chromium oxide was added to a mullite–spinel refractory mixture to improve its thermal and mechanical properties. Two different compositions of mullite–spinel refractory were studied to define the crystal structures hosting the chromium cations, and the chromium solubility in spinel (MgAl2−xCrxO4) was determined. Powder X-ray diffraction (XRD), X-ray absorption near edge spectroscopy (XANES), and scanning electron microscopy (SEM) elemental distribution maps were used to determine the chromium crystal chemistry in the system. The observed maximum solubility of chromium in spinel was found at x = 1.2, but the presence of mullite in the mixture caused a strong decrease of this value. The chromium distribution among the crystal phases reflects the different reaction paths of the two samples: a stage involving spinel and melt drives all present chromium in the spinel, while a simultaneous crystallization of spinel–mullite distributes chromium cations between mullite, spinel, and secondary corundum.