Raw Materials: What are the raw materials used in carbon production?

In carbon production, the raw materials commonly used can be categorized into two main types: solid carbonaceous materials and binders and impregnating agents. Solid carbonaceous materials include petroleum coke, pitch coke, metallurgical coke, anthracite coal, natural graphite, and graphite scrap, among others. Binders and impregnating agents encompass coal tar pitch, coal tar, anthracene oil, and synthetic resins, among others. In addition, certain auxiliary materials such as quartz sand, metallurgical coke pellets, and coke fines are also employed in the production process. For the manufacture of certain specialty carbons and graphite products (such as carbon fibers, activated carbon, pyrolytic carbon, pyrolytic graphite, and glassy carbon), other specialized raw materials are utilized.

Calcination: What is calcination? Which raw materials require calcination?

The process of thermally treating carbonaceous raw materials at high temperatures (1200–1500°C) under air-isolated conditions is called calcination. Calcination is the first thermal treatment step in carbon production, and it induces a series of structural and physicochemical changes in various carbonaceous raw materials.

Both anthracite coal and petroleum coke contain a certain amount of volatile matter and therefore need to be calcined. Pitch coke and metallurgical coke have relatively high coking temperatures—above 1,000°C—which are comparable to the temperatures in calcining furnaces used in carbon plants; thus, they do not require further calcination and can simply be dried to remove moisture. However, if pitch coke and petroleum coke are mixed before calcination, they should be fed into the calcining furnace together with the petroleum coke for calcination. Natural graphite and carbon black, on the other hand, do not need to be calcined at all.

Compression Molding: What is the principle behind extrusion molding?

The essence of the extrusion process is that, under pressure, the paste material passes through a die of a specific shape, undergoes compaction and plastic deformation, and thus becomes a blank with a predetermined shape and size. The extrusion molding process is primarily a process of plastic deformation of the paste material.

The extrusion process of the paste material takes place within the material chamber (also known as the paste cylinder) and the arc-shaped die. The hot paste loaded into the material chamber is pushed forward by the main plunger at the rear end, forcing the gases trapped within the paste to continuously escape. As a result, the paste becomes increasingly compacted while simultaneously advancing toward the front. When the paste moves through the cylindrical section of the material chamber, its flow can be considered steady, with each layer of particles moving essentially in parallel. However, as the paste enters the die section characterized by an arc-shaped deformation, the paste layers that closely adhere to the die walls encounter significant frictional resistance during their forward movement. This causes the particle layers to begin bending, resulting in varying velocities within the paste itself: the inner layers of paste advance more rapidly than the outer layers. Consequently, the final product exhibits radial density non-uniformity, giving rise to internal stresses within the extruded block due to the differing flow rates between the inner and outer layers. Finally, as the paste enters the straight-section deformation zone, it is extruded out of the die.

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

Baking is a heat treatment process in which green products after pressing are heated under protective atmospheres in a heating furnace, in an air-isolated environment, at a specified heating rate.

The purpose of roasting is:

(1) Volatile Matter Removal: Products made using coal tar pitch as a binder generally release about 10% of their volatile matter after roasting. Consequently, the yield of roasted products typically falls below 90%.

(2) The bonded coke-fired products are subjected to roasting under specific process conditions, causing the binder to coking and forming a network of coke between the aggregate particles. This network firmly bonds together aggregates of various particle sizes, endowing the product with certain physicochemical properties. Under the same conditions, the higher the coking rate, the better the product quality. Typically, medium-temperature pitch has a coking residual carbon content of around 50%.

(3) Fixed Geometric Form: During the firing process, green products undergo softening and binder migration. As the temperature rises, a carbonized network forms, causing the product to harden. Consequently, even if the temperature continues to increase, the product’s shape remains unchanged.

(4) Reduction in Resistivity: During the roasting process, due to the removal of volatile components, pitch undergoes coking to form a coke network. Simultaneously, the pitch undergoes decomposition and polymerization reactions, leading to the formation of large hexagonal carbon ring planar networks. As a result, the resistivity drops significantly. The resistivity of the green product is approximately 10,000 × 10⁻⁶ Ω·m; after roasting, it decreases to 40–50 × 10⁻⁶ Ω·m, qualifying as a good conductor.

(5) Further volume shrinkage: After firing, the diameter of the product shrinks by about 1%, the length shrinks by about 2%, and the overall volume shrinks by 2-3%.

Impregnation: Why is impregnation performed on carbon products?

After being formed under pressure, the green product exhibits very low porosity. However, after firing, part of the coal pitch decomposes into gases and escapes during the firing process, while the remaining portion undergoes coking to form pitch coke. The volume of the pitch coke formed is significantly smaller than the original volume occupied by the coal pitch. Although there is some shrinkage during firing, numerous irregular micropores of varying sizes still develop within the product. For example, the total porosity of graphitized products typically ranges from 25% to 32%, whereas that of carbon products generally falls between 16% and 25%. The presence of a large number of pores inevitably affects the physicochemical properties of the product. Generally speaking, as the porosity of graphitized products increases, their bulk density decreases, electrical resistivity rises, mechanical strength diminishes, the oxidation rate at certain temperatures accelerates, corrosion resistance deteriorates, and both gases and liquids become more easily permeable.

Impregnation is a process that reduces product porosity, increases density, enhances compressive strength, lowers the resistivity of the finished product, and alters the product’s physicochemical properties.

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

Graphitization is a high-temperature heat treatment process in which calcined products are heated to high temperatures within a graphitizing furnace under a protective atmosphere, causing the hexagonal carbon atom planes to transform from disordered, two-dimensional overlapping arrangements into ordered, three-dimensional overlapping arrangements, thereby acquiring a graphite structure.

Its purpose is:

(1) Enhance the thermal and electrical conductivity of the product.

(2) Enhance the product’s thermal shock resistance and chemical stability.

(3) Enhance the product’s lubricity and wear resistance.

(4) Remove impurities and enhance product strength.

Mechanical Processing: Why are carbon products subjected to mechanical processing?

(1) The need for plastic surgery

The carbon green products, which have been pressed into specific dimensions and shapes, undergo varying degrees of deformation and damage during the baking and graphitization processes. Moreover, their surfaces are often adhered to with some filling materials. If these products are not mechanically processed, they cannot be put into use. Therefore, it is essential to reshape them and machine them into the specified geometric forms.

(2) The need for use

Process according to the user’s requirements. For example, graphite electrodes used in electric arc furnace steelmaking need to be connected for use; therefore, threaded holes must be machined at both ends of the product, after which two electrodes can be joined together using specially designed threaded connectors.

(3) Requirements of the process

Some products need to be processed into special shapes and specifications according to the user’s operational requirements, and may even demand lower surface roughness.