Basic knowledge of carbon - detailed explanation of carbon production raw materials and processes

Raw materials for carbon production
Among the commonly used carbon production raw materials, dry materials include petroleum coke, asphalt coke, metallurgical coke and anthracite, etc., and the binder is mainly coal asphalt.

1. Petroleum coke
Petroleum coke is the main raw material for the production of various carbon materials. It has a low ash content, generally less than 1%, and is prone to turning into graphite at high temperatures. The characteristics of petroleum coke have a great impact on the properties of carbon materials.

2. Asphalt coke
Asphalt coke is a solid product obtained after coking of coal asphalt, and the raw material for producing asphalt coke is medium-temperature asphalt or high-temperature asphalt. Medium-temperature asphalt is heat-treated at about 1100°C and a series of complex physical and chemical changes to form asphalt coke.

Performance of asphalt coke

The ash and sulfur content are low, generally below 0.5%, the volatile content is less than 0.8%, and the true density is about 2.03g/cm³.
It has a tight structure, a hard texture, uniform pores, and a dull gray appearance.
The oxygen content is high, because compressed air is passed through for oxidation during the production process, and a large number of oxygen bridge structures are formed in asphalt coke, which makes the mechanical strength of asphalt coke high, but it is not easy to turn into graphite.
The electrical and thermal conductivity is worse than that of petroleum coke.
The coefficient of thermal expansion is large.
Production application of asphalt coke

When producing graphite electrodes, in order to improve the mechanical strength of the product, 20% - 25% bituminous coke is generally added.
When producing pre-baked anodes, in order to improve the strength of the product, a certain proportion of asphalt coke is sometimes added.
When producing blast furnace charcoal blocks, a small amount of asphalt coke is also added to reduce the ash content of the product.

3. Anthracite
Coal in nature includes peat, lignite, bituminous coal and anthracite. The peat structure is loose, the hardness is low, and the calorific value is not high. With the deepening of coalification, coal with moderate metamorphism such as lignite and bituminous coal is gradually formed. Further deepening, anthracite with low organic matter content, tight structure, high strength and high calorific value is formed.

Properties of anthracite

Tight structure with few porosity.
The monomer has high mechanical strength and high hardness.
When crushed, it is easy to crack along the joint unit, and the surface of the particles is smooth and flat, so the bonding strength with the binder is small. In production, anthracite coal is generally not used alone as carbon raw material, but is used in combination with petroleum coke, asphalt coke, metallurgical coke and other coke.
Quality requirements for anthracite as carbon raw material Anthracite coal is the main raw material for the production of various carbon blocks (such as cathodes for aluminum) and electrode paste, but it can only be used to produce carbon materials if it meets the following quality requirements.

The ash content should be low. When producing blast furnace coal blocks, the anthracite ash content is required to be less than 8% and there is no gangue; The anthracite ash content used for electrode paste should be less than 10%.
The mechanical strength should be high, and the strength and block can be maintained for a long time.
Thermal stability should be good.
The sulfur content should be low. On the one hand, it is the requirements for the production of charcoal paste and charcoal blocks, and on the other hand, the sulfur content is large, and the coal briquettes are easy to explode, and some sulfur will escape during calcination, causing environmental pollution.

4. Metallurgical coke
Metallurgical coke is made from several kinds of bituminous coal with different volatile content and coking properties, which are distilled at high temperature of 1000 - 1200 °C in a coke oven. As a carbon raw material, it mainly has the following properties:

- High ash content, generally above 10%.

- High mechanical strength and hardness.

- Good thermal conductivity and high thermal stability.

- Compact structure with few pores, so it has strong oxidation resistance.

5. Coal asphalt
In carbon production, coal asphalt is commonly used as a binder because it meets the necessary conditions for carbon binders:

- Turns into a viscous liquid within the right temperature range.

- Good wettability to the main raw material.

- It is an organic matter with a low ash content.

- Performance does not change significantly under specified process conditions (time, temperature, pressure, etc.).

- Has a high coking value.

- For products that require graphitization, the binder is charcoal that easily turns into graphite after coking.

Carbon production process
1. Process characteristics of carbon material manufacturing
The process characteristics of carbon material manufacturing are determined by the properties of carbon itself:

- Under normal pressure, carbon sublimates at 3550±5°C, turning from carbon atoms into carbon molecular vapor without liquid phase. Moreover, carbon has high thermal stability and is difficult to heat and vibrate, so it cannot be prepared by heating and melting.

- Carbon is chemically stable, so it cannot be produced by chemical synthesis.

- Carbon oxidizes easily in air, so it needs to be heat-treated with air isolation during manufacturing.

Carbon material manufacturing is the process of heat treatment of carbon-containing organic gases, liquids or solids under the conditions of normal pressure, high temperature and air isolation, so that the remaining carbon after heat treatment changes from a two-dimensional disordered structure to an orderly layered structure with three-dimensional arrangement.

2. Carbon material production process
The basic process of carbon production includes: pretreatment of raw materials (pre-crushing, calcination); crushing, screening and grading of raw materials; batching granular and powdery raw materials; Add binder and mix; forming the paste after mixing; roasting the molded products; Graphitization of semi-finished products after roasting; For products that require high density and strength, the baked blank also needs to be impregnated and then roasted.