蒙脱石热处理产物的微结构变化研究

THE MICROSTRUCTURE OF MONTMORILLONITE AND ITS THERMAL TREATMENT PRODUCTS

本文对广东和平蒙脱石及其热处理产物进行了化学分析、差热和热重分析、X射线粉末衍射分析、红外吸收光谱分析、扫描电镜、原子力显微镜及魔角旋转核磁共振等研究。结果表明 ,蒙脱石在热处理温度为 1 2 6℃— 1 48℃时 ,主要脱出吸附水和层间水 ,这一脱水过程是可逆的。当热处理温度达到 659℃时 ,蒙脱石八面体片中的羟基开始脱失 ,但层状结构仍然保持 ,这种羟基的脱失过程对应着八面体片中 Al 向 Al 的转变。当温度达到 90 0℃时 ,蒙脱石的层状结构完全被破坏 ,并有新的矿物相 μ-堇青石产生。当温度为 1 2 0 0℃时 ,则出现方英石及莫来石相。当热处理温度达到 1 350℃时 ,方英石及莫来石的含量略有减少 ,并出现较多的含铁堇青石相。

The montmorillonite in Heping, Guangdong province has been studied by chemical analysis, DAT, TG, XRD, IR, AFM and MAS NMR. It is found that montmorillonite dehydrate adsorbed water and interlayer water when the temperature is between 126℃-148℃, this process is reversible. When thermal treatment temperature≤200℃, the CEC of montmorillonite maintains invariably. When the thermal treatment temperature reaches 300℃, the CEC of montmorillonite descends more than 30%. When the thermal treatment temperature surpasses 450℃, the CEC of montmorillonite descends rapidly, and the montmorillonite almost loses its CEC function. This behavior is consistent with a classical Hofmann-Klemen effect of dehydrated cations with little radius. Upon heating, dehydrated cations migrate toward the lacunae of the octahedral sheet through the hexagonal cavity formed by the arrangement of the silica tetrahedra, where they compensate the negative charge deficit of the clay layer. Meanwhile the cations with large radius can’t migrate toward the octahedral sheet because of their position resistance effect, but they can inset in the silica tetrahedra sheet to form covalent. In this case, The cations with large radius couldn’t be rehydrated. For this two reasons the layer charge and the CEC of montmorillonite descend sharply. When the thermal treatment temperature reaches 450℃, the migration of layer cations of montmorillonite finish basically, that made montmorillonite lose CEC function. When the thermal treatment temperature reaches 659℃, the hydroxyl in octahedra sheet begin dehydrating, but the layer structure is maintaining, this process is corresponding to Al(Ⅵ) turning to Al(Ⅳ) in octahedra sheet. After incandescing at 700℃, the surface of montmorillonite shows distorted and appears tiny dentation prominence. This is because of dehydration action made the flat layer structure of montmorillonite destroyed, and made the flat surface of montmorillonite roughly. When the temperature reaches 900℃, the layer structure of montmorillontite is destroyed, and the new mineral phase μ-cordierite is found. When the temperature reaches 1000℃, the μ-cordierite phase becomes more. In μ-cordierite phase, the Al is four coordination. In this temperature, there is melted phenomenon in many montmorillonite grains. When the temperature reaches 1200℃, the μ-cordierite phase loses stable, it decomposes cristobalite phase and mullite phase, meanwhile the recrystallization phenomenon in thermal treatment products is obvious. There is a small quantity Al Ⅵ signal in MAS NMR spectrum, this Al Ⅵ correspondence with the Al of mullite. When the temperature reaches 1350℃, the cristobalite and mullite phases reduce a few, and it appears more Fe-cordierite phase. There is correspondence to Fe-cordierite spectrum in XRD and MAS NMR.