Detailed explanation of various process flow of graphite electrode

Raw materials: What are the raw materials used for carbon production？

In the production of carbon, the commonly used raw materials can be divided into solid carbonaceous materials and binders and impregnating agents.Solid carbonaceous materials include petroleum coke, pitch coke, metallurgical coke, anthracite, natural graphite, and graphite crushing, etc.; binders and impregnating agents include coal pitch, coal tar, anthracene oil, and synthetic resins.In addition, some auxiliary materials are used in production, such as quartz sand, metallurgical coke particles and coke powder.The production of some special carbon and graphite products (such as carbon fiber, activated carbon, pyrolytic carbon and pyrolytic graphite, glassy carbon) uses other special raw materials.

Calcination: What is calcination? Which raw materials need to be calcined？

The process of high-temperature (1200-1500°C) heat treatment of carbonaceous raw materials under the condition of air isolation is called calcination. Calcination is the first heat treatment process in carbon production. Calcination causes a series of changes in the structure and physical and chemical properties of various carbonaceous materials.

Both anthracite and petroleum coke contain a certain amount of volatile matter and need to be calcined. The coking temperature of pitch coke and metallurgical coke is relatively high (above 1000°C), which is equivalent to the temperature of the calciner in the carbon plant. It can no longer be calcined and only needs to dry the water. However, if the pitch coke and petroleum coke are mixed and used before calcination, they should be sent to the calciner together with petroleum coke for calcination. Natural graphite and carbon black do not need to be calcined.

Compression: What is the principle of extrusion molding？

The essence of the extrusion process is that the paste is pressed and plastically deformed into a blank with a certain shape and size after passing the die under a certain shape under pressure. The extrusion process is mainly the plastic deformation process of the paste.

The paste extrusion process is carried out in the material chamber (or paste cylinder) and the arc-shaped nozzle. The hot paste loaded into the material chamber is pushed by the main plunger at the rear. The gas in the paste is forced to be continuously eliminated, the paste is continuously dense, and the paste moves forward at the same time. When the paste moves in the cylindrical part of the feed chamber, the paste can be regarded as a steady flow, and the granular material layers move substantially in parallel.When the paste enters the squeezing nozzle part with circular arc deformation, the paste close to the wall of the nozzle is subjected to a large frictional resistance, the material layer begins to bend, and the different propulsion speeds occur inside the paste, the inner layer paste The material advances, leading to uneven density in the radial direction of the product, so internal stresses caused by the different flow rates of the inner and outer layers are generated in the extrusion block. Finally, the paste enters the linear deformation section and is squeezed out.

Roasting: What is roasting? What is the purpose of roasting？

Roasting is a heat treatment process in which the green product after molding is heated in a protective medium in a heating furnace under the condition of isolating air at a certain heating rate.
The purpose of roasting is to:
Excluding volatile matter：Products that use coal pitch as a binder generally emit about 10% of volatile matter after roasting.Therefore, the roasting yield is generally below 90%.
Binder coking： Raw products are calcined according to certain process conditions, coking the binder, forming a coke network between aggregate particles, and firmly connecting all aggregates with different particle sizes, so that the products have certain physical and chemical properties. Under the same conditions, the higher the coking rate, the better the quality. Generally, the coking residual carbon rate of medium-temperature asphalt is about 50%.
Fixed geometric form： During the firing process, the raw product softens and the binder migrates. As the temperature rises, a coking network is formed, making the product rigid. Therefore, even if the temperature increases, its shape will not change.
Reduced resistivity During the roasting process, due to the elimination of volatiles, pitch coking forms a coke grid, pitch decomposes and polymerizes, and generates large hexagonal carbon ring planar nets. The resistivity drops significantly. The resistivity of raw products is about 10000×10-6Ω.m, and it is reduced to 40--50×10-6Ω.m after firing, which is called good conductor.
The volume shrinks further. After roasting, the diameter of the product shrinks by about 1%, the length shrinks by about 2%, and the volume shrinks by 2-3%.

Impregnation: Why do you impregnate carbon products?

The porosity of the green product after compression molding is very low. However, after the roasting of raw products, part of the coal pitch decomposes into gas during the roasting process, and the other part is coked into pitch coke.The volume of generated pitch coke is much smaller than the volume originally occupied by coal pitch. Although it shrinks slightly during the roasting process, many irregular micropores with irregular pore sizes are still formed inside the product.For example, the total porosity of graphitized products is generally 25-32%, and the total porosity of carbon products is generally 16-25%. The existence of a large number of pores will inevitably have a certain impact on the physical and chemical properties of the product.Generally speaking, the pore size of graphitized products increases, the volume density decreases, the resistivity increases, the mechanical strength decreases, the oxidation rate at a certain temperature is accelerated, the corrosion resistance also deteriorates, and gases and liquids are more likely to penetrate.

Impregnation is a process that reduces product porosity, increases density, increases compressive strength, reduces finished product resistivity, and changes product physical and chemical properties.

Graphitization: What is graphitization? What is the purpose of graphitization?

Graphitization is to place the baked product in a graphitization furnace and heat it to a high temperature to transform the hexagonal carbon atom planar grid from disordered overlap in two-dimensional space to ordered overlap in three-dimensional space, and high-temperature heat treatment with graphite structure process.
the purpose is:
Improve the thermal and electrical conductivity of the product.
Improve the thermal shock resistance and chemical stability of the product.
Improve the lubricity and abrasion resistance of the product.
Eliminate impurities and increase product strength.

Mechanical processing: Why should carbon products be mechanically processed?

The need for plastic surgery
The carbon raw products with a certain size and shape are deformed and damaged indifferent degrees during the roasting and graphitization process. At the same time, some fillers are bonded on the surface, and they cannot be used without mechanical processing.
Therefore, the product must be shaped and processed into a specified geometric shape.
The need for use
According to the user's requirements for processing. For example, graphite electrodes for electric furnace steelmaking need to be connected and used, and threaded holes must be made at both ends of the product, and then the two electrodes are connected and used with special threaded joints.
Technological needs Some products are processed into special shapes and specifications according to the process requirements of users, and even require lower surface roughness.