High Purity Graphite Research Articles

High efficiency purification of natural flake graphite by flotation combined with alkali-melting acid leaching: application in energy storage

The natural flake graphite (GO) with an initial fixed carbon content of 6.23% is purified using flotation combined with alkali-melting acid leaching to obtain the high purity graphite (PG3) for energy storage. The graphite concentrate (PG1) with fixed carbon content of 85.62% is obtained by the selective enrichment of GO particles based on the multi-stage flotation. The alkali-melting acid leaching purification method is used to further purify the PG1 to obtain the PG3 with the fixed carbon content of 99.93%. The fixed carbon content is increased by 93.7% through flotation combined with alkali solution and acid leaching. The purification process of GO is systematically evaluated and analyzed by the several characterization techniques. The impurities removal mechanisms are also analyzed, which indicates that silicate and quartz in GO react with sodium hydroxide to form soluble substances, resulted in impurities removal. Oxide in GO and hydroxide formed in the alkali-melting process are subsequently removed in acid leaching process. The obtained PG3 could be used in lithium ion battery with a higher reversible capacity of 387 mAh g−1 compared to the 345 mAh g−1 of commercial layered graphite, which has promising potential application in energy storage. The above analysis indicates that the low purity GO is converted into high purify PG3 for energy storage, realizing high quality utilization of GO.

An Electrochemical Approach to Critical Materials Via Molten Salts and Ionic Liquids

Both graphite and rare-earth materials are considered by the US Department of Energy to be critical materials. Due to their widespread use as anodes in batteries and magnets in wind turbines, the demand for high purity graphite and rare earth elements for battery and clean-energy applications is expected to increase for the foreseeable future. Herein, an electrochemical method for graphitization based on molten salts and an electrodeposition of rare earth elements based on ionic liquids, which require significantly reduced processing temperatures, will be presented. These electrochemical processes have great potential to reduce energy costs compared to existing processes.

Effective purification of graphite via low pulp density flotation-low temperature alkali roasting-acid leaching route: From laboratory-scale to pilot-scale

A typical flake graphite concentrate sampled from a graphite flotation plant was used to prepare high purity graphite. The carbon content of the concentrate was 85.6%, and silicate and iron-containing minerals were found principally in the concentrate. High-grade graphite concentrate with a carbon content of 95% was obtained in the flotation process with feed pulp of 5% solids by weight after regrinding by a low-speed ball milling system. A small amount of the flotation reagents such as caustic soda, water glass, kerosene, and pine oil were added. The resulting flotation concentrate was mixed with alkali and roasted at 500 °C for 90 min. During the alkali roasting, the silicate mineral was converted into the water- and acid-soluble materials. Afterward, the soluble materials and iron-containing minerals were dissolved by acid leaching. As a result, a product with a carbon content of 99.82% was prepared. According to the calculation results of Gibbs energy, the reactions between impurities and chemicals in alkali roasting and acid leaching processes were spontaneous. The purification route was very effective for high purity graphite production and has been successfully applied in the pilot-scale experiment.

Sustainable approach for reclamation of graphite from spent lithium-ion batteries

A scalable and facile regeneration route is utilized to recover the graphite from a spent lithium-ion battery (LIB). Eco-friendly organic acid is employed as a leaching-curing reagent for the present work. All the unwanted content of elements e.g. Ni, Co, Li, Cu and Al has been completely terminated from the graphite after the purification step without any additional calcination process. The optical, structural and electrochemical properties of as-reclaimed graphite have been studied by several analytical methods. Regenerated graphite is restored to its layered crystal structure along with expansion in the interlayer distance, and the same is confirmed from scanning electron microscopy and X-ray diffraction analysis respectively. Notably, high purity graphite is achieved and tested in its electrochemical storage property in supercapacitor (SC) applications. As an outcome, recreated graphite exhibits a maximum areal capacitance of 285 mF cm−2 at 5 mV s−1. The fabricated symmetric SC demonstrates the superior energy storage performance in terms of durability and higher capacitance (131 mF cm−2) with better capacity retention over several cycles. It is worth mentioning that this curing process is a facile route, consumes lower energy and eco-friendly methodology and thereby may have futuristic extent for the bench scale reclamation of graphite from spent LIBs.

Study on purification of flake graphite by heat activation and hydrofluoric acid

ABSTRACT Graphite is an allotropic type of carbon composed of hexagonally organised carbon atom layers in a planar condensed ring structure (graphene layers). In recent years, researchers have reported the purification of graphite using various separation techniques, leading to the development of many methods for purifying graphite to separate it from the ore mixture. However, the majority of these methods are time-consuming, detrimental to the environment, expensive, need much energy, and have rather difficult manufacturing conditions. Although graphite is an essential strategic resource, and China has the world’s largest deposits and output of graphite, China’s graphite processing technology is very low, restricting the area of application. Therefore, the purpose of this study is to investigate the heat-activated hydrofluoric acid method (HAH), which is a simple and quick protocol that is often used to purify flake graphite. The optimum reaction parameters for purifying graphite are also explored, including hydrofluoric acid (HF) concentration, activation temperature, activation time, and sulphuric acid concentration. The results showed that the HAH process could purify raw flake graphite up to 99 %. As a result, the purification method in this study is applicable to the production of high-purity graphite.

Analysis of Tool Wear and Roughness of Graphite Surfaces Machined Using MCD and NCD-Coated Ball Endmills.

The high-purity G5 graphite material is widely used for glass moulding and provides high hardness and brittleness because it is sintered to fine particles unlike other graphite materials. Hence, tool cutting of a G5 workpiece is performed by local fracture instead of plastic deformation of the machined surface. Although a diamond-coated tool with outstanding hardness is used to machine very hard graphite, the tool shows variability regarding the service life and machining performance depending on the grain size, even in the same machining environment. We investigated the wear and change trend of machined surface roughness considering microcrystalline diamond (MCD) and nanocrystalline diamond (NCD)-coated tools, which are generally used to machine graphite materials, and analysed their relation with coating. For rough machining, the MCD-coated tool, for which the delamination of coating occurred later, showed less wear and improved machined surface roughness. For precision machining, the NCD tool showed less tool wear rate relative to the cutting length, leading to a small difference in the machined surface roughness between the two tools. We conclude that if rough and precision machining processes are performed using the same cutting tool, the MCD-coated tool is advantageous in terms of service life, while the difference in roughness of the final machined surface between the tools is negligible.

Recycling of waste carbon residue from spent lithium-ion batteries via constant-pressure acid leaching

Waste carbon residue (WCR) was efficiently detoxicated and regenerated to high-purity graphite (PGC) used in lithium-ion batteries through the constant-pressure acid leaching technique. The leaching conditions were optimized by the combination of orthogonal and single-factor experiments. Results show that PGC with 99.5% purity is regenerated at temperature of 60 °C, initial acid concentration of 12%, leaching time of 180 min, and liquid-to-solid ratio of 25:1, satisfying the requirements of commercial graphite. Meanwhile, the sodium hydroxide precipitation process was designed to recover valuable components from leachate efficiently. Ni, Co, Mn, and Al recoveries reach 96.92%, 87.5%, 97.83%, and 92.17%, respectively, at pH=11. Moreover, the co-product NaF can be recovered with purity over 99% via evaporative crystallization. The loss rate of fluorine is less than 0.5%, thereby eliminating the pollution risk of fluorine to the environment. The proposed process shows considerable environmental and economic benefits.

Formation ability, thermal stability, and mechanical properties of the Zr50Cu34Al8Ag8 amorphous alloys prepared by different mold materials

In the present study, Zr50Cu34Al8Ag8 amorphous alloy rods with a diameter of 3 mm were prepared and the influence of the cooling capacity on the properties of cast samples was analyzed. To this end, three mold materials, including heat-resistant steel, copper, and high purity graphite were utilized. Then the amorphous structure, glass-forming ability, and thermal stability of the cast alloy were investigated using an X-ray diffractometer and differential scanning calorimeter. A universal testing machine was applied to investigate the mechanical properties of amorphous alloys with different mold materials. The fracture characteristics of the amorphous alloy were analyzed by scanning electron microscope, and the microstructures were analyzed using a high-resolution transmission electron microscope. The experimental results showed that among the studied cases, the highest glass-forming ability was achieved from the copper mold. Moreover, the glass transition and crystallization activation energies of the amorphous alloy obtained by suction casting of high-purity graphite mold were the largest, which were 525.8 kJ/mol and 334.0 kJ/mol, respectively, reflecting good thermal stability. Meanwhile, the performed analyses revealed that the fracture strength of the amorphous alloy and the corresponding strain were 1888.2 MPa and 3.55%, respectively, indicating that the cast sample has good mechanical properties. With the decrease of the cooling rate, the content of free volume in the amorphous alloy of high-purity graphite mold suction casting decreased, and the ordered structure was generated, which was the main reason for its high thermal stability and plastic deformation ability.

Analysis of Metal Elements Contained in Graphite Target Coated With Chinese Medicinal Material Nanoparticles Using LIBS

A nanoparticle-coated graphite target (NCGT) is presented to improve the analysis accuracy and stability of laser-induced breakdown spectroscopy (LIBS). A stable, relatively homogeneous, and close to optically thin laser-induced breakdown plasma was obtained by dispersing sample nanoparticles on a high-purity graphite substrate. Spectral structures dominated by the characteristic lines of carbon and the samples can greatly simplify spectral identification and avoid line interference. To maximize the analysis accuracy and stability, a series of experimental conditions were optimized step by step according to the spectral intensity and signal-to-noise ratio of the lines. Based on the final optimized conditions, the relative standard deviation values of Mg, Fe, and Sr elemental content in Chinese medicinal material (CMM) samples were reduced from 17.7, 16.6, 12.1% of the pressed target to 4.8, 9.5, and 4.5% of the NCGT, respectively. Comparisons with the inductively coupled plasma mass spectrometry (ICP-MS) results demonstrated that the present method has great potential for detection of LIBS.

A New Flexible Graphite Composite Electrical Grounding Material

Corrosion is an inevitable problem of metallic grounding grids. To propose an effective method avoiding the above corrosion, a new type of non-metallic grounding material was made in this paper. It is called flexible graphite composite grounding material, namely flexible graphite composite grounding material for short. It is made by high-purity flake graphite with a stranded wire hierarchical structure. And a series of experiments have been carried out to verify its feasibility as electrical grounding material, including material resistivity and its contact resistance measurement, temperature tolerance test, mechanical performance test, and scaled electrical grounding test of several typical electrical grounding grids. These experimental results all indicate that characteristics of the new flexible graphite composite grounding material can well meet practical electrical engineering requirements.

Microwave-induced self-ignition: An efficient approach for high purity graphite digestion and multitechnique halogen determination

In this work, high purity graphite, a high chemically stable material, was effectively digested using a single method allowing compatible solutions for the further multitechnique determination of halogens by: ion chromatography (F and Cl), inductively coupled plasma mass spectrometry (Cl, Br and I) and by ion selective electrode (only for F). The recent system using microwave-induced self-ignition (MISI) is based on the strong interaction between microwave radiation and graphite in a closed system pressurized with oxygen (Maxwell-Wagner effect). Carbon-based materials present intense and specific interfacial polarization when exposed to microwave electromagnetic field resulting in a fast heating rate. This effect associated to a pressurized oxygen system, provides a quick self-ignition of carbon-based materials and consequent combustion/digestion of organic matrices. Under optimized conditions, sample masses up to 600 mg were fully digested in a quartz vessel under 20 bar of oxygen pressure and using just a diluted solution (100 mmol L−1 NH4OH) for the quantitative absorption of all the analytes. MISI method was validated, and the accuracy (better than 94%) was evaluated by comparison of results obtained by pyrohydrolysis for two coal certified reference materials as well as with subsequent analytes determination by the three techniques: IC, ICP-MS and ISE. It is important to point out that no filter paper disks, electrical connections or other ignition aids are required as in the case of previous or classical combustion methods. Moreover, just a diluted absorbing solution was used resulting in negligible blanks and relatively low limits of detection. The digestion efficiency was higher than 99%, making the proposed method a suitable and powerful alternative for the quasi complete digestion of graphite and determination of halogens virtually free of interferences.

Purification mechanism of microcrystalline graphite and lithium storage properties of purified graphite

In order to improve the application value of natural microcrystalline graphite with carbon content of 49.5%, high-purity microcrystalline graphite was prepared by emulsifying kerosene flotation firstly, and then purifying hydrofluoric acid and hexafluorosilicic acid. Then the purified microcrystalline graphite was prepared for the lithium-ion battery anode material, its microstruture and electrochemical properties were analyzed, the purification mechanism and lithium storage mechanism were discussed. The research results show that carbon content of microcrystalline graphite after emulsified kerosene flotation and mixed acid purification are 93.5% and 99.0% respectively. After pickling, high-purity microcrystalline graphite shows the largest layer spacing, which is 0.351 5 nm and is 0.001 4 nm higher than that of natural microcrystalline graphite. Size disparity of acid washing sample is larger, layered structure is more obvious, cycle performance and magnification performance are better than those of floation sample. The pickled sample has the highest initial reversible specific capacity of 477.4 mAh/g, and the first Coulomb efficiency is 61.3%. Charge transfer impedance, interface impedance and SEI membrane impedance, and lithium ion diffusion impedance in electrode material are significantly lower than those of microcrystalline graphite after flotation.

Investigation of the waste heat recovery and pollutant emission reduction potential in graphitization furnace

The graphite furnaces, which are extensively used for high-purity graphite production, inevitably discharges a large amount of waste heat that requires significant recovery for purpose of the improved energy efficiency. The paper numerically analyzed the heat transfer performance of a graphitization furnace under the electrical-heating and natural-cooling period. The temperature-dependent properties and heat transfer coefficients were used to predict the waste heat recovery potential along with on-site measurements. The results show that during the graphitization production, the core furnace temperature can reach 3000 °C while the petroleum coke surface reaches 368 °C without the combustion of the volatiles in the coke. The heat dissipation on the top coke surface accounts for 48.5% of the total electricity input and 66.9% of the total heat dissipation from the furnace, respectively. The equivalent emission reduction of CO2, SO2 and NOx are 47.5 t, 7.4 t and 3.6 t respectively per annum. The investigation of the waste heat recovery potential for graphitization furnaces can provide a reference for the heat recovery equipment and operation for graphitization furnaces.

An improved liquid-liquid extraction technique to determine shale wettability

Quantifying the wettability of shales is very important for reservoir evaluation and petroleum production prediction. However, differences in wettability of the same sample can occur using conventional methods, such as contact angle measurement and spontaneous imbibition experiments, due to the heterogeneity of shales with comingling presence of μm-sized organic matter and minerals. In this study, an improved liquid-liquid extraction (LLE) technique using particles is presented to determine the wettability of four lacustrine shales, with high-purity graphite (hydrophobic) and quartz (hydrophilic) selected as the reference materials. The effectiveness of this improved technique is demonstrated by a good repeatability in shales, graphite and quartz. In addition, a simple model of a spherical particle at the two-phase liquid interface was presented to analyze the forces on the particle and explain the particle behavior. Several factors such as particle sizes, experimental procedure, and different liquids were found to affect the LLE results, especially for heterogeneous water-wet and zwittering shales. Experimental temperature is also a critical factor for LLE results. Although some factors have a negligible effect on the results of LLE tests for oil-wet materials, the use of uniform and consistent conditions is recommended in the LLE experiments.

A direct dc arc emission spectrometry determination of chlorine impurities in nuclear grade graphite

In the present study, the direct current arc atomic emission spectrometry (dc arc AES) method was proposed for direct determination of chlorine impurities in nuclear-grade graphites and graphite foils in the range of 1‐–100 ppm in order to meet nuclear power industry requirements. The procedure is based on near-infrared (NIR) AES approach for determination of non-metal impurities, contrary to the most common vacuum ultraviolet AES approach. Carbon dc arc in argon atmosphere was used as radiation source. This radiation source employs standard extra high purity 6 mm diameter standard shaped graphite electrodes and standard vertical spectroanalytic stand, but employs several improvements as well: special protective fused quartz tube, and specially built fiber optic refocuser. Registration of spectra was performed via a novel compact tunable spectrometer with linear complementary metal oxide semiconductor array (CMOS) detector. As the work in near-infrared spectral range is hampered by spectral interference factors, measures were taken to counter them. Due to specifics of the radiation source, the excitability of chlorine was studied both in air and argon in presence of carbon, as well as in its absence. Artificial standards for calibration of the spectrometer were prepared by mixing extra high purity NaCl and graphite powder and evaluated by two independent approaches: indirectly by dc arc AES determination of Na, and directly by water extraction and photometrical determination of Cl. The characteristics of evaporation and excitation in dc arc of chlorine from both standards and real nuclear graphite samples were studied due to the differences in the nature of chlorine source in them. The detection limit of was determined 0.8 μg/g. The whole cycle of measurement, including sample preparation is about 10 min.

Development of a chemical-free floatation technology for the purification of vein graphite and characterization of the products

A novel and simple flotation technique has been developed to prepare high-purity graphite from impure graphite. In this method, a suspension of pristine powdered graphite (PG) is dispersed and stirred in water without adding froth formers or supportive chemicals. This makes fine particles of graphite move upwards and float on water. X-ray diffraction (XRD) analysis reveals that the floated graphite (FG) has a lower c-axis parameter, indicating the removal of interlayer impurities. A notable increase in the intensity ratio of the D band to G band in the Raman spectra indicates that the FG has more edge defects due to their smaller crystallite sizes. Transmission electron microscopic (TEM) analysis shows the number of layers in FG has been reduced to 16 from 68 in PG. The absence of C=O vibration of Fourier Transformed Infrared (FT-IR) spectroscopy in treated and untreated samples suggests that their layers are not significantly oxidized. However, X-ray photoelectron spectroscopic (XPS) analysis shows the presence of C–O–C ether functionalities, possibly on edge planes. Further, the product has higher purity with increased carbon content. Therefore, the technique is helpful for the value enhancement of graphite, the reduction of the chemical cost of the conventional techniques, environmental friendliness, and improvement of its applications.

Purification effect of the methods used for the preparation of the ultra-high purity graphite

The purification effect of a high temperature and high temperature thermochemical purification methods on some of the major impurity elements (B, Mg, Al, Si, Ca, Ti, V, Mn, Fe and Ni) has been studied for comparison, and ultra-high purity graphite was prepared. The content of impurity elements in the purified graphite was determined using glow discharge mass spectrometry. From the results obtained from various impurity elements during the analysis of graphite, the high temperature purification method can appreciably decrease the content of impurity elements with the exception of boron and was inapplicable toward the preparation of high purity graphite with low-boron content. In comparison, the high temperature thermochemical purification method can significantly decrease the content of boron to <0.01 ppm and can be used to realize the preparation of ultra-high purity graphite. The relatively poor purification effect of the high temperature purification method on boron may be related to the high reactivity between boron and the graphite matrix, while the introduction of ecological fluorine will obstruct the reaction between boron and the graphite matrix, which is beneficial toward the removal of boron when the high temperature thermochemical purification method is used.

(Physical and Analytical Electrochemistry Division Max Bredig Award Address In Molten Salt and Ionic Liquid Chemistry) Electrochemical Graphitization via Molten Salts

Graphite is considered by the US Department of Energy to be a critical element. Due to its widespread use as anodes in batteries, the demand for high purity graphite for battery applications is expected to increase for the foreseeable future. Synthetic graphite, due to its high purity, is generally the preferred choice for battery applications. The current process for the synthesis of graphite is very energy intensive and requires heat treatment of carbon precursors to temperature up to 3300 K for successful graphitization. Herein, an electrochemical method for graphitization is reported, which requires significantly lower temperatures. The cathodic electrochemical polarization of amorphous carbons in molten salts at around 1100 K can transform amorphous carbons to highly graphitic carbon structures. This process is facilitated by molten salts and has the great potential to save significant energy compared to the current existing process.

A promising regeneration of waste carbon residue from spent Lithium-ion batteries via low-temperature fluorination roasting and water leaching

Recycling and reusing the Waste carbon residue (WCR) from spent lithium-ion batteries has substantial significance in environmental protection and economic growth. Nevertheless, research on the regeneration of WCR was seldom reported. Herein, a new environmentally friendly process of low-temperature fluorination roasting and water leaching technology were proposed to achieve optimal purification of WCR. And the transformation mechanism of water-soluble complex ions during fluorination roasting was investigated via DFTB indicating that the ionic bond of F and NH4 group in NH4F broken than form stable covalent bond with metal ions. This process showed that 99.59%, 99.54%, 99.82%, 96.38%, 98.41%, 97.28%, 99.23%, and 98.61% of cobalt, nickel, manganese, aluminum, silicon, iron, lithium, and sulfur in WCR could be removed under optimal conditions. Meanwhile, the high-Purity graphite carbon (PGC) products with purity of 99.98% were obtained. In particular, PGC in half-cell exhibited a stable specific capacity of 340.9 mAh/g and high ICE of 92.13%, and with a more than 96% capacity retention after 100 cycles at 1C, much better performance than WCR and similarly to commercial graphite. Moreover, the discharge capacity of PGC in full-cell was 2302.82 mAh at 0.2C rate with 87.3% ICE. The capacity retention maintained at 96% after 400 cycles at 1C/1C rate. Subsequently, the valuable components in leaching solution were recovered with ammonia as precipitant. The precipitation rates of Al, Co, Mn, Ni, and Si were 99.25%, 81.74%, 98.20%, 62.43% and 86.68%, respectively. Besides, exhaust gas was absorbed in fluorine-containing leachate for recovery of ammonium fluoride products by evaporation crystallization. This proposed process with circular economy and green chemistry characteristics is expected to be employed in the regeneration of WCR while recovering valuable components.

Uniformly dispersing GNPs for fabricating graphene-reinforced pure Ti matrix composites with enhanced strength and ductility

The uniform dispersion of graphene nanoplatelets (GNPs) in the titanium matrix is essential but challenging for preparing high-performance graphene-reinforced titanium matrix composites. This study conveniently dispersed GNPs uniformly in commercial pure titanium (CP Ti) powders by in-situ exfoliating GNPs from high-purity graphite balls via 3D vibration milling and fabricated graphene-reinforced titanium matrix composites (in-situ GNPs/Ti) with improved strength and ductility. Besides, the purchased GNPs were ex-situ introduced into CP Ti through 3D vibration milling as a comparison to investigate the effect of different carbon sources on the uniformity of GNPs distribution. Compared with CP Ti, the ultimate tensile strength, yield strength, elongation, and microhardness of the in-situ GNPs/Ti composite were improved by 42.8%, 20.2%, 12.6% and 9.6%, respectively. The improvements in mechanical properties were mainly attributed to the load transfer of the uniformly dispersed GNPs and the pinning effect of in-situ generated titanium carbide (TiC). Furthermore, it is benefited from considerable strengthening efficiency in this work, the high-purity graphite balls were verified to be a befitting carbon source for 3D vibration milling to introduce GNPs into composites. It could be generalized to fabricating other metal matrix composites reinforced with graphene.