Graphitization Furnace Analysis: How Does 3000°C Transform Amorphous Carbon into Graphite?

Graphitization of carbon materials is a thermally activated process. According to experimental data:

• 1000-1500°C: Mainly removes volatiles and some impurities; carbon atoms remain largely unchanged.
• 1500-2000°C: Localized order begins to appear, but numerous crystal defects remain.
• 2000-2500°C: The crystal lattice grows significantly, but the interlayer spacing remains high (>0.338 nm), with a true density of approximately 2.15.
• 2500-3000°C: Carbon atoms gain sufficient kinetic energy, the interlayer spacing approaches the ideal graphite size of 0.3354 nm, and the true density exceeds 2.18.
• >3000°C: Graphite crystals develop further, but energy consumption increases dramatically, and the requirements for furnace materials are extremely high.

Therefore, for the industrial production of high-quality graphite petroleum coke, the actual furnace temperature must be stabilized within the range of 2800-3100°C. Below this range, the product can only be called "semi-graphitized" or "calcined coke," and its performance is far inferior to that of fully graphitized products.

Currently, there are two main types of furnaces used for producing graphite petroleum coke:

1. Acheson Furnace

Structure: A long, narrow resistance furnace filled with petroleum coke and resistance material (metallurgical coke granules), energized at both ends.

Principle: Current passing through the resistance material generates Joule heating, with the center temperature reaching 3000°C. The petroleum coke is placed in the central "furnace core" and is indirectly heated.

Characteristics:

Large furnace capacity (tens to hundreds of tons).

Poor temperature uniformity (temperature difference between edge and center can reach 200-300°C).

Long production cycle (heating + cooling requires 20-30 days).

High energy consumption.

Suitable for: Large-scale production of graphite petroleum coke with extremely high requirements for crystal development.

2. Internal Heating Series Graphitization Furnace (LWG Furnace)

Structure: Multiple petroleum coke cylinders are connected end-to-end and directly heated by electricity.

Principle: Current flows directly through the petroleum coke to be graphitized, generating its own heat; no resistivity material is required.

Features:

Extremely rapid heating (reaching 3000°C within hours).

Better temperature uniformity, higher product consistency.

Shorter cycle time (generally 3-5 days), energy consumption 20-30% lower than an Atchison furnace.

However, it requires high resistivity uniformity of the raw material.

Applications: Mid-to-high-end graphite petroleum coke, graphite for battery anodes.

Currently, most high-quality graphite petroleum coke in China uses modified Atchison furnaces or LWG furnaces; the latter is particularly suitable for export-grade products requiring narrow particle size and low carbon and sulfur content.

When petroleum coke is heated to 3000°C in a graphitization furnace, a series of irreversible transformations occur:

Carbon Atom Rearrangement

The disordered layer structure in amorphous carbon is broken, and carbon atoms migrate through diffusion, forming hexagonal ring planes and stacking layer by layer. The interlayer distance shrinks from 0.36 nm to 0.335 nm, and the crystal size increases from the nanometer scale to the micrometer scale. Impurity Volatilization and Removal

Sulfur: Most sulfides have boiling points below 2000°C, and almost all volatilize at 3000°C. The sulfur content of GPC can drop sharply from 0.5-1.0% to below 0.03-0.05%.

Nitrogen: Nitrogen atoms are released from the carbon ring as N₂, and the nitrogen content can be below 100 ppm.

Significantly Increased True Density

As the graphite lattice densifies, the true density jumps from 2.05-2.10 g/cm³ to 2.18-2.23 g/cm³. This is the most reliable indicator of the degree of graphitization-fully graphitized GPC should have a true density no lower than 2.18.

A Leap in Conductivity

After graphitization, the resistivity decreases by an order of magnitude (from 500-800 μΩ·m in calcined coke to 40-70 μΩ·m), making it the preferred carburizing agent for ladle refining and aluminum alloy smelting.

• Heating Curve: The temperature rise must be gradual, passing through the 1500-2000°C range. Otherwise, rapid release of internal gases (sulfur, nitrogen oxides) can lead to product cracking or structural loosening.
• Maximum Temperature Holding Time: Generally, holding at around 3000°C for 24-72 hours is required to ensure the entire furnace core, inside and out, reaches the graphitization temperature.
• Cooling Procedure: Excessive cooling will generate thermal stress, reducing particle strength. Natural cooling to below 500°C is usually required before unloading.
• Sampling and Testing: Samples should be taken from different parts of each furnace to test true density and resistivity, ensuring uniformity. Inferior graphitization furnaces are prone to producing "cooked on the outside, raw on the inside" products.

The graphitization furnace is the "furnace" in the production of graphite petroleum coke, transforming low-value petroleum coke into high-value-added functional materials. The high temperature of 3000°C not only endows GPC with excellent conductivity, high purity and high absorption rate, but also makes it irreplaceable in high-end casting, steel refining and lithium battery anode fields.