Steelmaking is a multi-step process that involves melting scrap metals with electricity in an electric arc furnace (EAF). Graphite electrodes are essential to the steelmaking operation. Graphite is one of the most electrically conducting materials in existence and the only material that can withstand the extreme temperatures of the EAF. Graphite has many uses, including producing batteries, pencils and lubricants. Graphite electrodes used in steelmaking are typically made from petroleum coke, a carbon-rich byproduct of oil refining. However, it is expensive and time consuming to produce graphite from crude petroleum coke because of its low level of graphite purity.
In order to increase the amount of high-quality graphite available, scientists at NETL have developed technology that changes the crystalline structure of petroleum coke to generate high-purity synthetic graphite. The process, which combines iron-based catalysts and heat, is known as catalytic graphitization and can be performed on a wide range of petroleum coke grades. Graphite produced from the process is a critical raw material for making high-value products like batteries, cement and polymer composites.
Currently, the majority of petroleum coke in the United States is burned as fuel in power plants and cement kilns. This is problematic because the high sulfur content of petroleum coke limits its use as a fuel for power generation and causes problems in cement kilns, where it increases the set time and influences the quality of concrete. Additionally, the impurities present in petroleum coke such as heavy metals, nitrogen and sulfur can contaminate the metallurgical production process, resulting in defects in the finished steel product and reduced product lifespan.
Petroleum coke can be divided into three types based on its microscopic and macroscopic properties. The lowest value, fuel coke is formed from the bottom of the vacuum residues of delayed and fluid bed cokers, while the higher-quality anode coke and the highest quality needle coke are derived from the tops and sides of the coke drums of fluidized bed cokers. The calcined version of petroleum coke, GPC, is the most important form used in industrial applications such as graphite electrodes and anodes for the aluminum industry.
The morphology of each type of petroleum coke can be characterized by optical microscopy. Figure 3 shows a photomicrograph of CK-2, CK-3, and CK-1 coke from different petroleum production sources. Optical microscopy reveals that the CK-2 and CK-3 coke have a blocky, honeycomb-like structure while the CK-1 is an agglomerated mixture of spherical “shots.”
To obtain high-purity graphite from each type of coke, researchers perform a series of treatments on the CK-2 and CK-3 samples to remove the impurities and crystallineize the carbon. X-ray diffraction analysis of the resulting coke-derived graphite (CK-EEG) reveals that it has a layered structure identical to graphite, with a lattice constant of
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