Effect of heating temperature on Li-fixation, layer charge and properties of fine fractions of bentonites

Abstract

The fine fractions (<2 µm) of four bentonites, in which the dominant minerals were Al-rich montmorillonites of various chemical compositions, have been Li-saturated. The admixtures identified in the fine fractions include pyrophyllite in Otay (USA), amorphous SiO2 in Ivancice (Czech Republic) and opal CT in Sarigus (Armenia) samples, while the Kriva Palanka sample (Macedonia) was a pure montmorillonite. The sample portions were heated at different temperatures (110 to 300°C) for 24 h to promote Li-fixation in the layers in order to reduce the negative charge on the layers in a systematic manner. Hence a series of materials with gradually decreasing charge, cation exchange capacity (CEC) and swelling ability were prepared. The CEC of the prepared samples was affected by the admixtures present and by the extent of Li+ for Ca2+ exchange in the starting material. The largest reduction in CEC (by 81%) was obtained for Otay montmorillonite, the mineral with the lowest tetrahedral and the highest octahedral charge, heated at 300°C. The extent of Li-fixation was very sensitive to the treatment temperature in the range 110–200°C. The development of uncharged layers with non-swelling interlayers occurred in all materials heated above 150°C. The shift of the OH-stretching band to higher wavenumber and a new band due to an AlMgLiOH stretching vibration in the infrared spectra confirmed the penetration of Li into the previously vacant octahedral sites.