固体 ~(25)Mg NMRによる製鋼スラグ中のフリーMgOの新規定量方法

摘要

製鉄プロセスで生成される製鋼スラグは，一般の土木•構造物材料等の成型体材料として広く使用されている。ここで，製鋼スラグ中に存在する精練過程で溶融せずに残存した未さい化の酸化ヵルシウム(CaO)および酸化マグネシゥム(MgO),あるいは溶融スラグが冷却され凝固する過程で晶出したCaOおよびMgOは，それぞれフリーライム(f-CaO)やフリーマグネシア(f-MgO)と呼ばれており，成型体中で水分と共存すると，水和反応によってそれぞれ水酸化ヵルシウム(Ca(OH)_2)および水酸化マグネシゥム(Mg(OH)_2)に変化し，その際に体積が2倍以上に膨張するため，成型体中に亀裂が発生し，粉化や破壊を引き起こす原因となっている。%A new approach to quantifying free magnesium oxide (f-MgO) in steelmaking slag has been proposed by using solid-state ~(25)Mg nuclear magnetic resonance (NMR) spectroscopy. Although ~(25)Mg (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 ~(25)Mg NMR spectrum thanks to its highly symmetric Mg sites. By contrast, other Mg-containing compounds (e.g. magnesium hydroxide, diopside and aekermanite) 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 ~(25)Mg NMR spectrum without any hindrance. It is significant to select the optimum internal standard (e.g. Mg_2Si and MgS) showing the narrow ~(25)Mg 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.