Artificial graphite processing methods

Artificial graphite needs to go through four major processes and more than ten minor processes. 

The aggregate of artificial graphite is divided into coal series, petroleum series and coal and petroleum mixed series, among which, coal series needle coke, petroleum series needle coke and petroleum coke are most widely used: generally speaking, needle coke is used as the raw material for the negative electrode with high specific capacity, and petroleum coke, which is cheaper, is used as the raw material for the negative electrode with ordinary specific capacity. Coal tar pitch is used as binder. Needle coke can be used to prepare ultra-high power graphite electrode, which has the advantages of low thermal expansion coefficient and high conductivity.

Artificial graphite is made of aggregate and binder by crushing, granulating, graphitizing and screening. The basic process flow is consistent, but the specific preparation process of each enterprise will have some differences.

The basic production process is as follows:

(1) Pretreatment

According to different products, graphite raw materials and coal tar pitch are mixed in different proportions, with the mixing ratio of 100: (5~20). The materials are transferred to the hopper through the vacuum feeder, and then put into the airflow mill by the hopper for airflow milling. Raw and auxiliary materials with the particle size of 5~10mm are ground to 5-10 microns. After airflow grinding, cyclone dust collector is used to collect the required particle size materials, and the dust collection rate is about 80%. The exhaust gas is filtered by the filter element filter and discharged. The dust removal efficiency is greater than 99%. The filter element is made of filter cloth with pores less than 0.2 μ m, which can intercept all dust above 0.2 μ m. The fan controls the whole system in a negative pressure state.

(2) Granulation

Granulation is divided into pyrolysis process and ball milling screening process.

Pyrolysis process: Put the intermediate material 1 into the reactor, replace the air in the reactor with N2, and make the reactor airtight. Under the pressure of 2.5Kg, conduct electric heating according to the temperature curve, stir at 200~300 ℃ for 1-3h, and then continue to heat to 400-500 ℃. The material with particle size of 10-20mm is obtained by stirring, and the material is discharged by cooling, that is, the intermediate material 2. The volatile gas in the reaction kettle is extracted by the fan, condensed by the condensation tank, the liquid is condensed in tar form, and the gaseous waste gas is led out by the fan, filtered by the activated carbon and then drained.

Ball grinding and screening process: Vacuum feeding, conveying intermediate material 2 to the ball mill for mechanical ball milling, and grinding 10~20mm materials into 6~10 micron materials. The powder produced by ball milling is transported to the screening machine through pipes for screening, and the materials under the screen are metered and packaged with an automatic packaging metering device to obtain intermediate material 3. The materials on the screen are transported to the ball mill by pipeline vacuum for ball milling again. The ball milling and screening are all conducted in a closed manner. The materials are transported in a vacuum. The gas is separated by air blowing and vibrating.  After gas material separation, the dusty waste gas is filtered by the filter element filter and discharged into the workshop.

(3) Cooperative graphitization

The graphite chemical process is processed in the form of cooperative processing, and the intermediate material 3 is entrusted to a nearby carbon plant for graphitization processing.

(4) Ball milling and screening

The graphitized materials are transported to the ball mill through vacuum for physical mixing and ball milling, sieved with 270 mesh molecular sieve, and inspected, measured, packaged and warehoused. The materials on the screen shall be sieved after further ball milling to meet the particle size requirements.

It can be seen that from the raw coke to the final lithium battery negative electrode materials, four major process steps (crushing, granulation, graphitization and screening) are required, which can be subdivided into more than ten small processes. The overall preparation process is very long.

Granulation and graphitization links processes the technical threshold

Among the four processes of artificial graphite, crushing and screening are relatively simple. Granulation and graphitization are the two main processes that reflect the technical threshold of the negative electrode industry and the production level of enterprises.

Granulation: The size, distribution and morphology of graphite particles affect the performance of negative electrode materials. In general, the smaller the particles, the better the magnification performance and cycle life, but the worse the first efficiency and compaction density, and vice versa. A reasonable particle size distribution (mixing large particles with small particles) can improve the specific capacity of the negative electrode; The morphology of the particles also has a great influence on the magnification and low-temperature properties.

Therefore, negative electrode enterprises need to have the ability to design and control the particle size and morphology to obtain the desired performance indicators.

Graphitization is another key step in the process of preparing artificial graphite, which is to change the arrangement of carbon atoms from a thermodynamically unstable "2-D disordered overlap" arrangement to a "3-D ordered overlap" arrangement.

The most commonly used graphitizing furnace in the production of artificial graphite is Acheson graphitization furnace. The problem of Acheson graphitization furnace is that it is easy to cause uneven temperature distribution and thermal stress. When the furnace temperature rises rapidly, obvious external heat and internal cooling may occur, resulting in cracked waste products. In recent years, another type of graphitizing furnace, the Lengthwise graphitization (LWG) furnace, has also had some problems. (For example, it is easy to cause the graphite powder on the crucible to fall off and introduce impurities and dust, and the volatilization of the product in the graphitization process is not easy to discharge, resulting in low purity)

In order to obtain better graphitization effect, negative electrode enterprises need to do well in the following aspects:

1. Master the method of loading resistance materials and materials into the furnace (horizontal, vertical, staggered and mixed charging, etc.), and adjust the distance between materials according to the different properties of resistance materials;

2. According to the different graphitization furnace capacity and product specifications, different power curves are used to control the heating and cooling rates in the graphitization process;

3. Under specific circumstances, the catalyst is added to the ingredients to improve the degree of graphitization, which is called "catalytic graphitization".

In addition, the selection and proportion of raw materials (petroleum coke, needle coke and binder) is also a core technology of negative electrode. For market analysis of artificial graphite products, you can further consult us.