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[Original Articles] The processing of graphite electrode

[vc_row][vc_column][vc_column_text]Raw materials: What is the raw material used for carbon production?

In the production of carbon, the commonly used raw materials can be divided into solid carbon raw materials and binder and impregnating agents. Solid carbon raw material include petroleum coke, pitch coke, metallurgical coke, anthracite, natural graphite, and graphite crushing, etc.; binder and impregnants 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 some other special raw materials.

 

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

The process of heat treatment of carbon raw material at high temperature(1200-1500℃) 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 carbon 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℃), which is equivalent to the temperature of the calcination furnace 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.

 

Extrusion: What is the principle of extrusion?

The essence of extrusion process is that after the paste passes through the nozzle of a certain shape under pressure, it is compacted and plastic deformed into a blank with a certain shape and size. The extrusion molding process is mainly the plastic deformation process of paste.

 

Baking: What is baking? What is the purpose of baking?

Baking is a heat treatment process in which the raw products after molding is heated in protective medium in a heating furnace under the condition of isolationg air at a certain heating rate.

 

The purpose of baking:

  • Excluding volatile matter  Products using coal pitch as a binder, about 10% of volatile matter are generally discharged after baking. Therefore, the rate of baking products is generally below 90%.
  • Binder coking Raw products are baking 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 confitions, the higher the coking rate, the better the quality. Generally, the coking residual carbon rate of medium temperature asphalt is about 50%
  • Fixed geometric  During the baking 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.
  • Reducing resistivity During the baking 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, which is reduced to 40-50×10-6Ω·m after backing, which is called good conductor.
  • Further volume contraction After baking, 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 we impregnate carbon products?

The porosity of the raw product after compression molding is very low. However, after the baking of the raw product, part of the coal pitch is decomposed into gas during the baking 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 baking process, many irregular pores 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 process with graphite structure.

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 wear resistance of the product
  • Remove impurities and improve product strength

 

Machining: Why do carbon products need machining?

  • The need of shape

The carbon raw products with a certain size and shape are deformed and damaged in different degrees during the baking and graphitization process. At the same time, some fillers are bonded on the surface, and they cannot be used without mechanical processing, so the product must be shaped and processed into a specified geometric shape.

  • The need of using

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

  • The need of technologe

Some products are processed into special shapes and specifications according to the process requirements of users, and even require lower surface roughness.

 

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