Abstract

Objective To enhance the purification effect of natural graphite and reduce the environmental impact of excessive acid use，while meeting the continuous demand for high-quality graphite， a more environmentally friendly and efficient graphite purification process is required.

Methods The NaOH-HCl-HF combined treatment process was used to purify flake graphite， increasing its fixed carbon content and reducing impurities such as Si， Fe， Al， and Cu. The experiment investigated the influence of sodium hydroxide dosage and roasting temperature on the purification effect of the process. The study utilised scanning electron microscopy （SEM） to observe the morphological characteristics of graphite. X-ray fluorescence spectrometry （XRF） and inductively coupled plasma atomic emission spectrometry （ICP） were used to detect impurity content in the graphite before and after purification. Additionally， the crystal structure of the graphite and its ash was determined using X-ray diffraction （XRD）.

Results and Discussion The majority of the graphite structure exists in the form of flakes， which can reach lengths of over 100 μm and have a relatively thin thickness. Following purification with HCl and HF，the flake structure of the graphite sample remains unchanged， and the edges of the layered structure do not curl due to high-temperature heating. Following the purification treatment，the XRD spectrum of the flake graphite displays diffraction peaks of graphite carbon at 2θ=26.6°，54.8°，and 87.3°. The peak intensities and widths remain essentially unchanged compared to the raw graphite material. This indicates that the alkali-acid purification process does not alter the intrinsic structure of the graphite itself. When roasting for 2.5 hours，use roasting temperatures of 450 ℃，500 ℃，and 550 ℃ respectively，and a hydrochloric acid（mL）to graphite（g）ratio of 2∶1. Use the NaOH-HCl method to purify graphite， resulting in an average fixed carbon content of 97.35%，97.98%， and 97.86%，respectively. If the roasting temperature is too high，NaOH reacts with Al₂O₃，SiO₂， and other substances to form aluminosilicates with poor solubility. This aluminosilicate exhibits strong resistance to acid， making it difficult to dissolve through acid leaching. The average fixed carbon content of graphite increases with the mass ratio of NaOH to graphite， reaching 97.52%，97.55%， and 98.13% at ratios of 0.4∶1，0.5∶1， and 0.6∶1， respectively. However， increasing the mass ratio beyond 0.4 does not significantly improve the fixed carbon content of graphite. Based on cost and energy consumption， a mass ratio of NaOH to graphite of 0.4∶1 is recommended when roasting at 500 ℃. The fixed carbon content of graphite increases gradually as roasting time increases， reaching a maximum value at 2.5 hours before gradually decreasing. At this point， the carbon content of graphite is 98. 26%. The fixed carbon content in graphite may decrease due to excessive roasting time and oxidation of a small amount of graphite. It is important to note that the language used in this text is clear， objective， and value-neutral， adhering to the characteristics outlined in the assignment. The NaOH-HCl purification method was used to reduce the SiO₂ content to 0.26% and 0.62%，Fe₂O₃ content to 0.07% and 0.2%， Al₂O₃ content to 0.21% and 0.15%， and CuO content to 0.001% and 0.002%， respectively. The results indicate that the impurity content of graphite significantly decreases after purification. The average fixed carbon contents of graphite are 99.91% and 99.93% respectively when the mass ratios of NaOH and graphite are 0.5∶1 and 0.6∶1. However， based on factors such as cost and efficiency，it has been determined that a mass ratio of 0.5∶1 for NaOH and graphite is the standard that meets the ideal process conditions.

Keywords：graphite； alkali-acid process； alkaline roasting； fixed carbon； purification