THERMAL PURIFICATION OF NATURAL GRAPHITE BY MEANS OF HIGH-SPEED HEATING
One of the reasons why the demand in high quality graphite has shown steady growth is the booming development of chemical power sources, wherein this material is utilized for anodes. In experts’ opinion, the current production dynamics of electric vehicles, electronic and energy storage devices can result in shortages of such strategic material in the near future. Therefore, sustainable mining, processing, application and recycling of high-quality graphite are the top priority for the most developed countries. Meanwhile, Ukraine features the expanding market of electric batteries and relevant graphite resources, which makes the issue highly important for the state.
Along with such existing technologies of graphite purification to battery grade as acid and chlorine treatment, the high-temperature electrothermal fluidized bed processing (EFB), developed by Thermal and Material Engineering Center (TMEC), deserves special consideration. The technology allows treating carbon powder at temperatures up to 3000C in continuous mode. It resolves purification and graphitization tasks consuming only electric power with possible integration of renewables. The last one can potentially result in the synthesis of high-quality almost «green» graphite with a low environmental footprint.
It should be noticed, that the EFB technology, except Li-batteries purpose, allows to resolve much wider tasks due to significant changes in lattice structure of carbon during treatment. Its potential includes capability of nuclear grade graphite production or graphitization of carbon black. As multiple studies by TMEC have shown, EFB operating parameters such as temperature and treatment duration dramatically influence performance of the technology like purification or graphitization level.
TMEC continuous its work on the improvement and development of the EFB technology, testing various carbon materials under simulated conditions. For this purpose, a 5 kW laboratory resistive furnace was developed. While processing carbon samples weighing up to 30 g, the reactor simulates the operating conditions that arise during fast heating to 3000°C in a fluidized bed under an inert gas atmosphere. A typical multistage purification cycle in laboratory conditions included the heating of the material for 0.5-1 min and holding it at a constant temperature for 3–30 min.
The laboratory study of the thermal purification of natural graphite from the Zavallevsky deposit was carried out. The composition of the processed samples was determined by elemental analysis using a spectrometer and by measuring the ash after combustion of the samples.
During heating up, the reduction of the non-volatile oxides in the mineral part is observed at the temperatures higher than 1000C. Reduction products transfer directly to the gaseous phase or transform into high-melting carbon compounds (carbides) which evaporate later at higher temperatures. The most noticeable reduction of mineral impurities occurs in the temperature range of 1400 to 2400C. Fe, Ca and Ti are elements that are hard to remove. As a result of the study, acceptable parameters of the operating modes were determined, in particular, the temperature of 2900–3100C and the processing time of 15 min, which ensure the manufacture of high-quality product with a carbon content of at least 99.97%.