The role of microstrain on the thermostructural behaviour of industrial kaolin deformed by ball milling at low mechanical load

Abstract

The high-temperature structural behaviour of kaolin deformed by compaction and shear in a ball mill was investigated by extending the temperature range to 1500 °C. The deformation was induced for the first time with steps of low mechanical load through a specifically built planetary ball milling working in a controlled thermodynamic environment (25 °C and at a vacuum of 0.13 Pa). The investigated kaolin was made of about 65% of a well-ordered kaolinite, 10% of dickite and 25% of quartz. The mechanical energy transfer to the material was measured via the microstrain < ε 2> 1/2 because we wanted to investigate the details of the evolution of the kaolin modification as a function of the microstrain. At the very early stage of milling (up to 1 h), no strain was accumulated in the kaolinite structure which, however, presented lamination, layer flattening and texturing. Further milling induced a progressive reduction of the stacking layer coherence and an increase of the microstrain, in both cases as a non linear function of the deformation time. The thermo-structural behaviour of kaolin was investigated by TG-DTA in a helium atmosphere at 10 °C/min of heating rate. In the medium temperature range (400–1000 °C), the mechanical milling affected the dehydroxylation reaction of kaolinite by a significant anticipation of about 300 °C of the temperature range, which usually occurs between 400 and 800 °C. Other reactions related to the formation of a Si-spinel and other intermediate phases were observed. The mechanical deformation severely influenced of the high-temperature reactions related to mullite and cristobalite formation. All thermal reactions linearly correlate to the microstrain < ε 2> 1/2 accumulated in the kaolinite structure. The reported data are of particular usefulness in industrial applications involving grinding or milling of kaolin.