Synthesis of hercynite at very mild condition

Hercynite, Fe(II)Al2O4, is a double oxide with the spinel structure, found in rocks of various origin and composition. Due to its high hardness and thermal stability, it is exploited as an abrasive, in polishing/cutting tools and in refractories, such as in magnesia-hercynite refractory bricks. Hercynite is not easily obtained as a monophasic material, and its synthesis is usually a highly energy demanding process. In fact, the two main methods of preparation, electro-fusion and traditional solid state route from the composing oxides, need very high temperatures (1450-1650 °C). To make matter more complex, the presence of Fe2+ ions requires any heating process to be performed under oxygen-free conditions. In this work a method for obtaining hercynite at 490-600 °C is reported. The synthesis involves a sol-gel reaction, followed by dehydration of the gel and calcination of the solid under nitrogen or nitrogen/hydrogen atmosphere. Different iron precursors have been tested. Hercynite percentage in the products ranged from 80 (wt) % to 100 (wt) %, depending on the experimental parameters. The products have been characterized by X-ray powder diffraction, thermogravimetric and elemental analyses, and scanning electron microscopy investigations.