An investigation on the mechanism and reaction sequence of scheelite-magnesium mixtures during mechanical milling

The mechanism of mechanically-induced self-propagating reaction (MSR) of scheelite with magnesium during reactive ball milling was investigated using in-situ monitoring of the vial temperature, X-ray diffraction analysis and field emission scanning electron microscopy. Temperature variations of the vial versus milling time curves showed unusual behaviors in comparison to the reported patterns in literature. It is found that the main reason for these exceptional temperature behaviors is sublimation-deposition of magnesium. X-ray diffraction patterns of milled scheelite-magnesium mixtures revealed the formation of intermediate phases during MSR. The sequence of scheelite-magnesium reaction appeared as CaWO4→ Ca3WO6 → Ca2MgWO6 → WO3-x → W. Atomic force microscopy showed that the final product contains nanosize particles that are smaller than 50 nm.