Evolution of Black Talc upon Thermal Treatment

Abstract

Black talc is a natural silicate clay mineral with a typical 2:1 layered structure, low electrical conductivity, large specific surface area, and high thermal stability. The world’s largest black talc mine, with known reserves of one billion tons, is located in China’s Jiangxi province. Due to the restriction of its color, the application of black talc is only limited to ceramic raw materials, coating filler, waterproof materials, and other low-end application industries. Thermal treatment is a common method of clay mineral modification. It is vital to examine the structural and physical changes of black talc during calcination in order to prepare black-talc-based composites and to broaden their applications. This work discusses the evolution of black talc upon thermal treatment (30–1000 °C) and the corresponding structural changes. The thermal stability of minerals was analyzed via thermogravimetric (TG) analysis and thermogravimetry–mass spectrometry (TG-MS). The decomposition of minerals during calcination consists of four processes: dehydration, organic carbon decomposition, dihydroxylation, and phase transformation. In situ FTIR and in situ XRD were employed to track changes in black talc in real time during thermal treatment. At 800 °C, black talc was found to begin to go through dihydroxylation, and the crystallinity index decreased significantly. The XRD pattern of samples at 950 °C (T950) showed the reflection of the enstatite structure, and the relative crystallinity index was 27.3%, indicating that the mineral had undergone phase transformation. In addition, the Brunauer–Emmet–Teller (BET), laser particle size analyzer, Zeta potential, scanning electron microscopy (SEM), and transmission electron microscopy (TEM) techniques were used to systematically characterize the physicochemical properties of minerals at different temperatures. The results show that black talc’s particle size and specific surface area increase with the calcination temperature. The surface charge changes, and more amorphous SiO2 and MgO appear, indicating that thermal treatment could induce structural changes and activate the surface of black talc.