Processing status of natural amorphous graphite

Natural amorphous graphite crystal is small, and each graphite particle is disorderly stacked by many amorphous, showing isotropy. It has important potential application value in the fields of cathode materials for lithium-ion power batteries. In recent years, the strategic position of natural amorphous graphite has been increasing and has become a hot spot in scientific research, industry, and finance. In this paper, the processing and application technology of natural amorphous graphite are summarized, and its development trend has been prospected.

Natural graphite is mainly divided into flake graphite and amorphous graphite. amorphous graphite has small crystals, and each particle is disorderly stacked by many amorphous s, showing isotropy to a certain extent. It has important potential application value in high-tech fields such as cathode materials of lithium-ion batteries (especially power batteries).

China is rich in natural amorphous graphite resources. Its reserves, output, and export volume rank first in the world, with high grade and good quality, which directly affects the international price of amorphous graphite. However, for a long time, there has been little research on amorphous graphite at home and abroad, resulting in the backward level of industrial technology, mainly raw ore and primary processed products, with low added value, resulting in the waste of a large number of valuable natural amorphous graphite resources. In recent years, with the attention of national management departments and relevant industries, the strategic position of natural amorphous graphite has been increasing. It has become the forefront of scientific and technological research and the investment hotspot of industry and finance.

In this paper, the processing status of natural amorphous graphite is summarized, and its development trend has been prospected.

1. Current situation of processing technology

1.1 Current situation of primary processing technology

Natural amorphous graphite generally has a high grade, and the fixed carbon content is generally 50% ~ 80% or even more than 90%. Therefore, it can be used only after simple processing. At present, the processing of amorphous graphite mainly includes three kinds of primary processing: grinding, screening, and ball pressing. Grinding is to crush amorphous graphite raw ore into a certain number of graphite powder through the Raymond machine. It is mainly used for casting demoulding, grounding resistance reduction, etc. Sieve particle is to screen out granular products with a certain particle size range after the coarse crushing of amorphous graphite raw ore, which is mainly used as a carburizing agent for steelmaking and casting. Ball pressing means that amorphous graphite raw ore is mixed with starch, caustic soda, and water in a certain proportion, and then processed into near-spherical products with certain strength through extrusion molding, drying, and other processes. This kind of product has the largest output and is mainly used as steelmaking warming agent, protective slag, and carburizing agent.

1.2 Current situation of deep processing technology

The deep processing of natural amorphous graphite mainly focuses on Purification and particle shaping modification.

1.2.1 Purification technology

1.2.1.1 Flotation purification technology

Graphite has good natural hydrophobicity. It can aggregate at the gas-liquid interface with the help of bubble buoyancy in the pulp system, so as to separate from impurity minerals and achieve the purpose of purification. However, natural amorphous graphite has poor floatability due to its small crystal and the dispersion and embedding of impurities in graphite particles. However, through multi-stage grinding, the graphite crystal can also be gradually dissociated to produce better floatability. Therefore, the flotation of amorphous graphite generally adopts multi-stage grinding, multiple separations, regrinding, and re separation of coarse concentrate and other processes. Many units and researchers have studied the flotation purification technology of natural amorphous graphite.

The existing research results show that the simple flotation method has certain limitations for the purification effect of amorphous graphite, it is difficult to obtain high-purity products, and it is not economical. It can only be used as the first step of amorphous graphite purification. At present, only China Building Materials Group Nanfang graphite Co., Ltd. has established an amorphous graphite flotation purification production line in the world, with an annual capacity of about 100000 t, which can purify 65% ~ 70% of the fixed carbon amorphous graphite to about 80% of the fixed carbon.

1.2.1.2 Acid-base purification technology

The acid-base method uses graphite to resist acid and alkali, and the main impurities (silicate, etc.) in graphite react with sodium hydroxide above 600 ℃ to form water-soluble products. Other impurities (such as iron oxide, etc.) are neutralized with hydrochloric acid after alkali melting to form water-soluble ferric chloride, which is removed by washing. This method is not only the main method of chemical purification but also a relatively mature process at present.

Liu Huaiqing and others studied the process of purifying amorphous graphite by acid-base method, which can purify amorphous graphite to more than 96% of fixed carbon content. Feng Qiming and others purified amorphous graphite with 85.60% fixed carbon to 97.86% by the acid-base method. Wang Yingwei and others purified amorphous graphite with 62.43% fixed carbon to 98.60% by the acid-base method. Li Xiaobo and others purified amorphous graphite with a fixed carbon content of 79.82% to 97.65% by the acid-base method. Jiang Fang and others purified amorphous graphite with a fixed carbon content of 85% and particle size of 0 ~ 0.048 mm to 98.79% by the acid-base method under the optimum process conditions. Liu Changqing and others purified amorphous graphite with a fixed carbon content of 82.67% to 93.42% by an acid-base method. Chen Hao and others purified amorphous graphite with a fixed carbon content of 86.0% to 99.898% by an acid-base method. Jiang Yingping and others purified amorphous graphite with 80.04% fixed carbon to more than 96% by pressurized alkali leaching atmospheric acid leaching process.

The acid-base process has strong adaptability, but there are some problems, such as large water consumption, low efficiency, large equipment investment, high production cost, and serious environmental pollution. Compared with the flotation method, the purity of the product obtained by the acid-base method is higher, up to 99%. It should be noted that the formation of silicic acid should be prevented during acid leaching after alkali fusion, because sodium metasilicate produced by sodium silicate in acid solution will be gradually condensed to form silica sol or silica gel when placed or changed, so it is difficult to remove it. Therefore, the acid-base method is more suitable for treating graphite with less mica. The alkali acid method has been applied to the further purification of flake graphite flotation concentrate in Shandong, Heilongjiang, and other parts of China. At present, there is no industrial production line to purify amorphous graphite by this method.

1.2.1.3 Purification technology by mixed acid method

According to the characteristics of acid corrosion resistance of graphite and that impurities in graphite can react with hydrofluoric acid or soluble fluoride salt to form water-soluble compounds or volatiles, amorphous graphite is soaked in a certain concentration of mixed acid containing hydrofluoric acid or soluble fluoride salt acid solution such as ammonium fluoride, then washed with water to remove impurities, dehydrated and dried to obtain products with high purity.

Kuang Jiacai et al. Purified amorphous graphite by ammonium fluoride hydrochloric acid system, and increased the content of fixed carbon from 83.08% to 99.47% under the optimum process conditions. Hongquan et al. Purified amorphous graphite with HF HCl mixed acid. The fixed carbon contents before and after purification were 87.8% and 99.1% respectively. Jiang Fang and others purified amorphous graphite with HCl HF mixed acid, and the fixed carbon content was increased to 99.36%. Xie Wei and others purified natural amorphous graphite by using the mixed system of hydrochloric acid and sodium fluoride, and the fixed carbon content increased from 83.08% to 98.37%. Duan Jiaqi et al. Purified amorphous graphite by ultrasonic mixed acid method, which can increase the fixed carbon content from 81.54% to 99.97%.

The advantages of the mixed acid method are high purification efficiency, high product purity, and low energy consumption. However, the hydrofluoric acid in the mixed acid is volatile, highly toxic, and highly corrosive. The production process must have strict safety protection and a wastewater treatment system. It has strict requirements for equipment and high cost and is not suitable for industrial large-scale production. At present, there is no industrial production line for purifying amorphous graphite by this method.

1.2.1.4 High-temperature purification technology

There are two technologies for purifying graphite by high-temperature method, chlorination roasting method, and pure high-temperature method.

The principle of the chlorination roasting method is to use the principle that the melting boiling point of SiO2, Al2O3, Fe2O3, Cao, and other impurities in graphite is high, while the melting boiling point of their chlorides is relatively low. Chlorine is introduced under a certain high temperature and atmosphere to make the impurities generate chlorides at a low temperature, gasify and escape, so as to achieve the purpose of purifying graphite.

The process factors affecting the purification effect include reaction temperature, reaction time, amount of reducing agent, Cl2 pressure and flow, grade and particle size composition of graphite raw materials, etc. Chlorination roasting method has the advantages of high purification efficiency, high product purity, and high recovery rate, but the process controllability is poor. At the same time, it has the disadvantages of high purification cost, serious equipment corrosion, and serious environmental pollution, which limits the popularization and application of this method. At present, the chlorination roasting method is still in the stage of exploration and development, and there is no industrial production line for purifying amorphous graphite by this method.

The pure high-temperature method is to use the characteristics of high-temperature resistance of graphite and the difference of melting boiling point between graphite and impurities to heat graphite to more than 2700 ℃ in a specific purification furnace under the protection of inert gas, so as to gasify and escape impurities, so as to achieve the purpose of impurity removal. Compared with other purification methods, the purity of the product purified by the pure high-temperature method can be increased to more than 99.99%. The disadvantage is that a specific high-temperature furnace is required, the production energy consumption is large, the investment and production cost is high, and there are high requirements for the purity of raw materials. The application field of the pure high-temperature method is relatively limited. Generally, only graphite products with strict purity requirements (such as spectral pure graphite for analytical instruments, nuclear graphite, biomedical graphite, etc.) are produced in small quantities. In recent years, Hunan Dingli Technology Co., Ltd. and Zhuzhou Vlad Technology Co., Ltd. have successively developed industrial-grade high-temperature continuous purification equipment, which can produce high-purity amorphous graphite with the fixed carbon content of more than 99.99%, with relatively low energy consumption and short production cycle. After further improving the service life and production stability of the furnace, it has a good market promotion and application prospect.

1.2.2 Particle shaping and modification technology

In order to improve the application value of natural amorphous graphite, in addition to its purification, it is necessary to adopt appropriate processes to improve its particle morphology and other properties.

He Ming et al. Studied the influence of the milling process on the structure and electrical properties of amorphous graphite. Smooth and smooth particles were obtained by high-energy ball milling and special crushing and classification technology. The irreversible capacity of shaped amorphous graphite was only 12% and the reversible capacity was 350 MAH / g. Hao Xiangyang and others studied the high-speed dynamic impact composite particle system of amorphous graphite particles. After shaping, the amorphous graphite particles are basically oval and spherical, and the length diameter ratio is 1.18. Shi Tao et al. Prepared particles smaller than l by stirring mill μ M and uniform particle size distribution of amorphous graphite ultrafine powder, with simple process and high efficiency. Uranus et al. Reshaped and modified amorphous graphite particles by wet stirring method, and then separated them into particle groups with narrow particle size distribution by the combination of gravity sedimentation and hydrocyclone to prepare natural amorphous graphite with high sphericity and narrow particle size distribution.

Deng Chengcai et al. Adopted the wet stirring mill technology to obtain the aspect ratio of 1.32, – 74 μ The yield of spherical amorphous graphite was 42.22%. Wang Jing and others studied the spheroidization process of amorphous graphite by using a complete set of QWJ airflow vortex pulverizers. Wang Cong and others first crushed the untreated amorphous graphite and then used the particle shaping system to make the particle surface nearly spherical. Finally, the centrifugal rotor classification system was used to reduce the particle size distribution, and the average particle size was about 9, 10, 15, and 20 μ M with uniform hemispherical shape and narrow particle size distribution.