1 graphitization

Graphitization process is a process in which the carbonaceous material is heated to 2300~3 000 ℃ by making full use of resistance heat, so that the amorphous carbon with chaotic layer structure is transformed into the ordered stone ink crystal structure. The energy of graphite crystal structure transformation and atomic rearrangement comes from high temperature heat treatment. With the increase of heat treatment temperature, the graphite layer spacing gradually decreases, generally between 0.343 nm and 0.346 nm. The change is significant when the temperature reaches 2 500 ℃, and gradually slows down when the temperature reaches 3000 ℃, until the whole graphitization process is completed. Artificial graphite anode material is through graphitization high temperature treatment, the carbon structure is successfully transformed into graphite structure and has the corresponding function of lithium battery anode.

2.Main furnace type and process of graphitization of anode material

At present, the furnace types used in the process of anode material graphitization mainly include Acheson graphitization furnace, internal series graphitization furnace, box type graphitization furnace and continuous type graphitization furnace, among which the most commonly used is Acheson graphitization furnace, and a small number of internal series graphitization furnace is used. Box-type graphitization furnace and continuous graphitization furnace are new furnace types developed in recent years. Box-type graphitization furnace develops rapidly, mainly through Atchison furnace renovation and partial new construction. Continuous graphitization furnace is newly built and still in the process of testing, its furnace type and process are not fully mature, and it will take some time for it to be widely used.

Atchison furnace is to install the carbon anode material in the single hole (1 hole crucible) crucible, and then the crucible is loaded into the graphitization furnace and the resistance material is installed between the resistance, and the two sides and the top cover are loaded into the insulation material to complete the graphitization through electricity transmission. The inner series graphitization furnace is to install the carbon anode material in the porous crucible (9 hole crucible), and then the crucible is connected end to end in the graphite furnace through the series connection mode, and the two sides and the upper cover are loaded with insulation materials to complete graphitization through electricity transmission. Box-type graphitization furnace is to load the carbon negative material directly into the large box installed with carbon plate or graphite plate in advance, and add carbon or graphite cover plate as the resistance, the upper and both sides of the heat preservation material into the graphitization through electricity transmission. Continuous graphitization furnace is to continuously add carbon anode material into the graphitization furnace chamber, after high temperature graphitization cooling discharge.

3.Key technical points in different graphitization furnace process

The processing process of anode materials is mainly divided into two key links, granulation and graphitization, and both of them have high technical barriers. Anode materials through graphitization can significantly improve the specific capacity of anode materials, the first effect, specific surface area, compaction density, conductivity, chemical stability, such as performance index, so control and master good graphitization technology is an important approach to guarantee the quality of the anode materials, because of the box-type furnace and continuous graphitization furnace technology is not fully mature, The following focuses on the Atchison furnace and internal series graphitization furnace process points to do to introduce.

3.1 Loading of Acheson furnace and inner series furnace (crucible)

3.1.1 Volatiles collocation during furnace loading

When the temperature in the graphitization furnace rises to 200~1 000 ℃, a large number of volatiles will be discharged from the negative electrode in the furnace. If the volatiles cannot be discharged in time, it may lead to the accumulation of volatiles, which will cause the safety accident of the spraying furnace. When a large number of volatiles escape, volatiles combustion is not sufficient, will produce a large number of black smoke, resulting in environmental pollution or environmental accidents. Therefore, the following points should be paid attention to when loading the furnace:

(1) When installing the negative electrode furnace, it is necessary to carry out a reasonable collocation according to the volatile content level to avoid excessive concentration and concentration of high volatile parts in the process of power transmission;

(2) Appropriate air holes should be set on the top of the insulation material to play an effective escape;

(3) When designing the power supply curve, it is necessary to take full account of slowing down the curve appropriately in the concentrated discharge stage of volatiles, so that volatiles can be discharged slowly and fully burned;

(4) Reasonable selection of auxiliary materials, ensure the composition of auxiliary particle size, reduce the amount of 0~1 mm powder in the auxiliary materials, generally accounting for less than 10%.

3.1.2 Furnace resistance should be uniform when loading

When the negative electrode and resistance material are not evenly distributed in the furnace, the current will flow from the place with low resistance, and the phenomenon of bias current will occur, affecting the effect of graphitization of the whole furnace negative electrode. Therefore, the following points need to be paid attention to when loading the furnace:

(1) When loading the furnace, the resistance material should be discharged from the head of the furnace chamber to the tail long line of the furnace chamber to avoid concentration of small particles or large particles;

(2) the old and new crucible into the same furnace also need to be reasonable collocation, taboo new crucible with a layer, the old crucible with a layer of phenomenon;

(3) Avoid the resistance material exposed into the side wall material.

3.2 Acheson furnace and internal series furnace power supply

3.2.1 Basis for power curve formulation of anode material during power transmission

According to the different quality requirements of the cathode material, it can be divided into low temperature material (2 800 ℃), medium temperature material (2 950 ℃), high temperature material (3 000 ℃), but the graphitization high temperature treatment process is generally between 2 250 ℃ and 3 000 ℃, in order to make all positions in the furnace reach the required temperature, it is necessary to keep in the high temperature process for a period of time. In order to ensure the uniformity of temperature in the furnace, usually due to different furnace type, need to keep different time, general high temperature keep for 6~30 h, in the process of power transmission to prevent the furnace resistance rebound need to keep 3~6 h. The specific situation needs to be explored and formulated according to the following technical points.

(1) Select different heating curves according to the furnace core, anode material, resistance material, crucible, furnace loading amount, etc.;

(2) Different curves should be selected according to the volatiles of anode materials and resistance materials in the furnace. If the volatiles are high, a slower heating curve should be selected; otherwise, a faster one should be selected;

(3) When the ash content of the anode material and resistance material in the furnace is high or the anode material is relatively difficult to graphitization, the power transmission time should be appropriately extended.

3.2.2 Anode material power transmission process to prevent furnace injection accidents

Because the anode material is powdery material, volatile content is high and not easy to discharge, easy to produce arc and high volatile content caused by the furnace accident, the specific operation process should pay attention to the following matters:

(1) When the anode material is installed in acheson furnace, the resistance material should be ramped to avoid the arc caused by suspended resistance material between crucible during power transmission;

(2) the displacement change of the negative material of the inner series furnace is mainly reduced in the process of power transmission. Therefore, when the negative material is installed in the furnace, the stroke of the hydraulic cylinder should be calculated to ensure that there is a stroke and enough pressure in the process of power transmission, so as to avoid the arc spray furnace accident caused by the loss of pressure;

(3) Coarse particles and low volatile materials should be selected for both furnace types;

(4) In the process of power transmission, pay close attention to whether there is local heating in the furnace;

(5) In the process of power transmission, it is necessary to pay close attention to whether the furnace top and furnace wall have cross fire phenomenon;

(6) In the process of power transmission, it is necessary to pay close attention to whether there is a low roar in the furnace;

(7) It is necessary to pay close attention to whether there is a large fluctuation of current in the process of power transmission.

If (4)-(7) phenomenon occurs in the process of power transmission, power should be cut off in time to avoid the occurrence of furnace injection accident.

3.3 Cooling and baking

(1) In the process of graphitization cooling, the anode material can not be forced to cool by watering, but can be naturally cooled by grabbing the material layer by layer with grab bucket or suction device.

(2) Anode material crucible about 150 ℃ out of the best, early removal of crucible, due to high temperature, lead to anode material oxidation, specific surface area increases, will also lead to increased cost of crucible oxidation damage. Taking out the crucible too late will also make the cathode powder material oxidized, the specific surface area increases, the production cycle becomes longer and the cost increases.

(3) Under high temperature of graphitization at 3000 ℃, all elements except element C are vaporized and discharged. However, there will still be a small amount of impurities in the cooling process adsorption on the surface of the cathode, crucible surface will form a layer of rough hard shell, high ash, high volatile materials form more hard shell material. The selection of low ash and low volatile excipients is based on this reason.

(4) Hard shell material in the index and qualified anode material performance difference is great, so when taking out the crucible, it is necessary to knock off 1~5 mm thick hard shell material in advance for storage and storage separately, the qualified material with smooth surface is normally collected, put into the ton bag for storage and delivery to customers.