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

Effect of ultrasound power on HCl leaching kinetics of impurity removal of aphanitic graphite

This study aimed to investigate the effect of ultrasonic power and temperature on the impurity removal rate during conventional and ultrasonic-assisted leaching of aphanitic graphite. The results showed that the ash removal rate increased gradually (∼50 %) with the increase in ultrasonic power and temperature but deteriorated at high power and temperature. The unreacted shrinkage core model was found to fit the experimental results better than other models. The Arrhenius equation was used to calculate the finger front factor and activation energy under different ultrasonic power conditions. The ultrasonic leaching process was significantly influenced by temperature, and the enhancement of the leaching reaction rate constant by ultrasound was mainly reflected in the increase of the pre-exponential factor A. Ultrasound treatment improved the efficiency of impurity mineral removal by destroying the inert layer formed on the graphite surface, promoting particle fragmentation, and generating oxidation radicals. The poor reactivity of hydrochloric acid with quartz and some silicate minerals is a bottleneck limiting the further improvement of impurity removal efficiency in ultrasound-assisted aphanitic graphite. Finally, the study suggests that introducing fluoride salts may be a promising method for deep impurity removal in the ultrasound-assisted hydrochloric acid leaching process of aphanitic graphite.

Migration behavior of impurities during the purification of waste graphite powders

Metal-laden solid wastes (e.g., waste graphite powders) have attracted great attention owing to their hazardous effects on the surrounding soil and water. Additionally, the metal-bearing impurities also hinder the reutilization of waste graphite powders. Thus, it is necessary to remove these inorganic impurities and figure out the removal mechanism of impurities in the purification process. In this study, an alkaline roasting-water washing-acid leaching (AWA) method was used to upgrade the waste graphite powders, and the migration behavior of diverse impurities has been qualitatively and quantitatively investigated. A graphite product with high impurity removal efficiencies is attained under optimal conditions. The removal of impurities mainly follows three routes: (1) V-, P-, and S-bearing impurities were complete removed (some formed soluble salts during alkaline roasting, and the remainder was dissolved in acid); (2) most Al-, K-, and Si-bearing impurities were removed by alkaline roasting, with the remainder was dissolved in the acid-leaching process; and (3) Fe-, Mg-, Ti-, Ca-, and Zn-bearing impurities were decomposed at high temperature and reacted with alkali to form hydroxides or oxides, which was subsequently dissolved in acid. In addition, the treatment of the generated wastewater is also discussed. The uncovered migration mechanisms of diverse impurities would guide the purification and reutilization process of other metal-bearing solid wastes efficiently.

Recovery of Microcrystalline Graphite from Quartz Using Magnetic Seeding

Efficient beneficiation of microcrystalline graphite remains a challenge. Selective recovery of microcrystalline graphite from quartz using hydrophobized magnetite as magnetic seed is studied in this work. Magnetite was hydrophobized by the surface coating of sodium oleate. The hydrophobic agglomerates were then separated by magnetic separation. Sedimentation experiments were performed to study the adhesion of microcrystalline graphite and quartz to magnetite particles. The results showed that hydrophobized magnetite led to a higher microcrystalline graphite recovery than that of the original magnetite, due to the higher probability to bond with microcrystalline graphite. However, the hydrophobization of the magnetite surface had an insignificant effect on its interaction with quartz. The force analysis based on the extended Derjaguin-Landau-Verwey-Overbeek (EDLVO) theory indicated that the total attractive interaction between hydrophobized magnetite and microcrystalline graphite were obviously stronger than that between hydrophobized magnetite and quartz, resulting in the selective aggregation between hydrophobized magnetite and microcrystalline graphite.

Silicon as a ubiquitous contaminant in graphene derivatives with significant impact on device performance

Silicon-based impurities are ubiquitous in natural graphite. However, their role as a contaminant in exfoliated graphene and their influence on devices have been overlooked. Herein atomic resolution microscopy is used to highlight the existence of silicon-based contamination on various solution-processed graphene. We found these impurities are extremely persistent and thus utilising high purity graphite as a precursor is the only route to produce silicon-free graphene. These impurities are found to hamper the effective utilisation of graphene in whereby surface area is of paramount importance. When non-contaminated graphene is used to fabricate supercapacitor microelectrodes, a capacitance value closest to the predicted theoretical capacitance for graphene is obtained. We also demonstrate a versatile humidity sensor made from pure graphene oxide which achieves the highest sensitivity and the lowest limit of detection ever reported. Our findings constitute a vital milestone to achieve commercially viable and high performance graphene-based devices.

Electrochemical tuning of capacitive response of graphene oxide

The capacitance of graphene oxide can be maximized by precise control of the conditions of electrochemical reduction to balance the oxygen concentration and conductivity.

Cost-effective fabrication of graphene-like nanosheets from natural microcrystalline graphite minerals by liquid oxidation–reduction method

Graphene-like nanosheets were fabricated using natural microcrystalline graphite minerals (NMGM) instead of flake graphite (FG) using a liquid oxidation–reduction method.