Fixed carbon content and reaction mechanism of natural microcrystalline graphite purified by hydrochloric acid and sodium fluoride

In order to improve the fixed carbon content of microcrystalline graphite (MG) and broaden its application, this article discussed the fixed carbon content and purification reaction mechanism of natural MG by using hydrochloric acid and sodium fluoride. The influence of sodium fluoride amount, liquid-solid ratio, reaction temperature and reaction time were respectively investigated using MG as raw materials. Results indicate that the fixed carbon content of MG increased with the increasing amount of fluoride; with the increment of the liquid-solid ratio, reaction temperature, and reaction time. In all cases, the fixed carbon content significantly increased at first then slowed down its increasing rate. The fixed carbon content of MG can be increased from 83.08% to 98.37% when 13.5 g sodium fluoride amount and 30 g microcrystalline graphite reacted with 120.5 ml hydrochloric acid at 70 °C for 2.5 h. Hydrochloric acid can be used to convert the impurities such as metal compounds into soluble chloride. Hydrofluoric acid generated from sodium fluoride removes most of the complex silicate impurities, and then to be washed and filtered to achieve the separation of graphite. The crystal structure and morphology of MG will not be changed according to the SEM images and the XRD patterns. Results indicate that the hydrochloric acid and sodium fluoride method can enhance the fixed carbon content of MG to a higher level.