A New Approach to Quantification of Free MgO in Steelmaking Slag by using Solid-state 25Mg NMR Spectroscopy
Koji Kanehashi, Michihiro Aimoto
A new approach to quantifying free magnesium oxide (f-MgO) in steelmaking slag has been proposed by using solid-state25Mg nuclear magnetic resonance (NMR) spectroscopy. Although 25Mg (I= –5/2) signals in solid materials often suffer from the second-order quadrupolar broadening, a quite narrow and symmetric signal of MgO is obtained in the 25Mg NMR spectrum thanks to its highly symmetric Mg sites. By contrast, other Mg-containing compounds (e.g. magnesium hydroxide, diopside and ¬kermanite) are more or less affected by the second-order quadrupolar interaction, resulting in severe broad signals buried in the spectral noise. Therefore, the MgO peak can be only detected for steelmaking slag in the 25Mg NMR spectrum without any hindrance. It is significant to select the optimum internal standard (e.g. Mg2Si and MgS) showing the narrow 25Mg signal being free from the second-order quadrupolar broadening like MgO to obtain reliable data. Accuracy of f-MgO concentration obtained by the present method has been demonstrated by using the standard samples in which f-MgO concentration is known. The advantages of the present method to quantify f-MgO in steelmaking slag, no need to dissolve slag in any solvent and detection of surface f-MgO as well as inner f-MgO of slag particles, should outweigh the disadvantages of poor signal-to-noise ratio which is surmounted by using a larger sample rotor and/or a higher magnetic field. These ideas for quantification of f-MgO by NMR should be explicated to that of free calcium oxide (f-CaO) which is responsible for hydrated expansion of steelmaking slag as well.
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