Mechanochemical activation of mixtures of wolframite (FeWO4) with carbon, studied by 57Fe Mössbauer spectroscopy

The effect of mechanochemical activation (high-energy grinding) on mixtures of natural FeWO4 with two forms of carbon has been investigated by X-ray powder diffraction and Mössbauer spectroscopy. The crystallinity of the mechanically activated mixtures, which have previously been found to exhibit enhanced reactivity as precursors for carbothermal production of tungsten carbide, is shown by X-ray powder diffraction to decrease on grinding, but without detectable formation of new crystalline phases. Mössbauer spectroscopy shows that the grinding process is accompanied by the appearance of an unexpected new singlet resonance, attributed to the formation of dilute FeC regions. The Mössbauer spectra also show that mechanochemical activation under the present conditions results in progressive oxidation of the Fe2+ in the wolframite to Fe3+; this oxidation is more marked in wolframite ground in the absence of carbon, suggesting that the latter exerts an oxygen-scavenging effect on the system. Similar results are obtained irrespective of whether the carbon source is activated carbon or graphite. The crystallization of iron tungsten carbides on subsequent heating of the ground mixtures in argon suggests that the regions identified by Mössbauer spectroscopy as containing iron–carbon interactions may also be associated with tungsten.