优化碱酸法提纯石墨的研究

Research on optimizing alkaline-acid method for graphite purification

【目的】优化天然石墨提纯效果,降低酸在提纯过程中的过多使用对环境造成的影响,满足各行各业对高品质石墨的需求,实现更环保、高效的石墨提纯。【方法】以鳞片石墨为原料,采取NaOH-HCl-HF联合处理的工艺对石墨进行提纯研究,提高石墨的固定碳含量(质量分数,下同),降低石墨中的主要杂质元素如Si、Fe、Al、Cu等的含量,详细考察NaOH的用量以及焙烧温度2个关键因素对该工艺提纯效果的影响;通过扫描电子显微镜(SEM)观察石墨形貌特征,X射线荧光光谱仪(XRF)和电感耦合等离子体原子发射光谱仪(ICP)测定提纯处理前后石墨的杂质含量,X射线衍射仪(XRD)确定石墨及其灰分的晶体结构。【结果】当焙烧温度为500℃、焙烧时间为2.5h,HCl的体积与石墨的质量比为2∶1,氢氟酸的体积与石墨的质量比为2∶1时,石墨的平均固定碳含量从原来的95.3%提高到99.93%;当NaOH与石墨的质量比分别为0.5∶1和0.6∶1时,石墨的平均固定碳含量为99.91%和99.93%;考虑到成本效益等因素,确定当NaOH与石墨的质量比设定为0.5∶1时为理想工艺条件;经过提纯处理后的石墨层结构并不会出现明显的变化,基本性能不变,提纯处理后的石墨相比于提纯处理前的石墨,杂质含量明显地降低。【结论】该碱酸工艺不仅能有效地去除石墨中的杂质,盐酸和氢氟酸的组合还可以显著地提升提纯效果,可有望应用在石墨提纯处理和新能源材料领域。

Objective To enhance the purification efficiency of natural graphite and reduce the environmental damage caused by excessive acid use,while meeting the increasing 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 morpho⁃logical characteristics of graphite.X-ray fluorescence spectrometry(XRF)and inductively coupled plasma atomic emission spectrometry(ICP)were used to detect impurity content in graphite before and after purification.Additionally,the crystal struc⁃ture of 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 100μm 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 graph⁃ite.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 tempera⁃ture 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, consid⁃ering cost and energy consumption, a mass ratio of 0. 4∶1 was recommended when roasting at 500 ℃. The fixed carbon contentof 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 excessiveroasting and minor graphite oxidation. The NaOH-HCl purification method was used to reduce SiO2 content to 0. 26% and0. 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 resultsindicated that the impurity content of graphite significantly decreased after purification. Average fixed carbon contents were99. 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.Conclusion The alkaline-acid process can effectively remove impurities in graphite and reduce the environmental harm causedby excessive use of hydrofluoric acid. Additionally, the combination of hydrochloric acid and hydrofluoric acid can significantlyimprove the purification effect. This method is expected to be used in the fields of graphite purification treatment and new energymaterials.