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 A combined NaOH-HCl-HF treatment process was used to purify flake graphite by 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 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 retained its flaky form， with lengths over 100 μm and relatively thin layers. After purification with HCl and HF， the flake structure of the graphite sample remained intact， with the edges of the layered structure unaffected by high-temperature heating. In XRD spectrum， its diffraction peaks for graphite carbon appeared at 2θ=26. 6°， 54. 8°， and 87. 3°， while its peak intensities and widths remained essentially unchanged compared to raw graphite. This indicated that the alkali-acid purification process did not alter the intrinsic structure of the graphite itself. When roast⁃ ing for 2. 5 hours at 450 ℃， 500 ℃， and 550 ℃， and with a hydrochloric acid（mL）to graphite（g）ratio of 2∶1， the NaOH-HCl method resulted in an average fixed carbon contents of 97. 35%， 97. 98%， and 97. 86%， respectively. If the roasting temperature was excessively high， NaOH would react with Al₂O₃， SiO₂， and other substances to form aluminosilicates， which had poor solubility and exhibited strong resistance to acid， making it difficult to dissolve through acid leaching. The average fixed carbon content of graphite increased with NaOH graphite ratio， reaching 97. 52%， 97. 55%， and 98. 13% at ratios of 0. 4∶1， 0. 5∶1， and 0. 6∶1， respectively. However， its fixed carbon content only improved marginally at ratios beyond 0. 4∶1. Therefore， considering cost and energy consumption， a mass ratio of 0. 4∶1 was recommended when roasting at 500 ℃. The fixed carbon content of graphite increased gradually with longer roasting time， reaching a maximum value at 2. 5 hours before gradually decreasing. At this point， the carbon content of graphite was 98. 26%. The fixed carbon content in graphite may decrease due to excessive roasting and minor graphite oxidation. The NaOH-HCl purification method was used to reduce SiO₂ content to 0. 26% and 0. 62%， Fe₂O₃ to 0. 07% and 0. 2%， Al₂O₃ to 0. 21% and 0. 15%， and CuO to 0. 001% and 0. 002%， respectively. The results indicated that the impurity content of graphite significantly decreased after purification. Average fixed carbon contents were 99. 91% and 99. 93% for NaOH graphite ratios of 0. 5∶1 and 0. 6∶1. However， considering factors such as cost and efficiency， it was determined that a mass ratio of 0. 5∶1 for NaOH and graphite was optimal， which met the ideal process conditions.

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

Get Citation:LIU Y Z， MENG F R， CUI X M， et al. Research on optimizing alkaline-acid method for purifying graphite［J］. China Powder Science and Technology，2024，30（3）：76−87.