Petroleum coke is a solid by-product of oil refining and is a source of carbon for the manufacture of graphite electrodes and anodes in aluminium and steel industries. It is produced either as blocky sponge coke from delayed coker units or in a shot size form from fluid bed coker processes. The coke can be of fuel grade or anode grade, depending on the sulfur and metal content. The fuel-grade coke is used as a raw material for power generation and the anode-grade coke is further processed into calcined petroleum coke (CPC) or shaped products like anodes, graphite electrodes and regenerated coke.
The use of high-quality, low-sulfur PC for graphite electrodes is desirable because it reduces the environmental footprint of metallurgical industries. However, the conventional production process is energy intensive, requiring repeated roasting for desulfurization and energy-intensive graphitization in the Acheson furnace. To meet the demand for low-sulfur PC and to achieve China’s carbon peak emission goal, alternative routes towards graphitization with lower energy consumption are required.
Previous approaches to obtaining graphite from PC involve chemical treatments such as solvent extraction, oxidative desulfurization and hydrocarbon gas treatment. The chemical methods require expensive reactants and a high temperature to accelerate the graphitization reaction. However, the resulting graphite is not of high quality and cannot replace natural graphite in metallurgical applications.
Currently, the most common method for producing graphite is to produce it from natural graphite and petroleum coke via a smelting process. However, natural graphite is a finite resource, and much of it contains impurities that limit its use. Moreover, the smelting process is environmentally unfriendly because it emits considerable amounts of noxious gases such as SO2 and NOx.
The ECE process is a promising approach to obtain high-quality, low-sulfur graphite from petroleum coke. It consists of four steps: powder preparation, shape forming, baking and graphitization. During the ECE process, CK-b is compacted inside a hollow cellulose membrane, and a positive voltage is applied at the working electrode. This causes a deformation of the CK-b, and the resultant calcined coke-EEG is separated from the remainder of the coke by centrifuging. Raman spectroscopy of CK-b and CK-c before and after exfoliation reveals that the ECE process is capable of isolating primarily sp2 structures even from less crystalline precursor materials. Supplementary Fig. 15 shows the Raman spectra of CK-b, CK-c and CK-d. The ID/IG ratio of CK-d is slightly lower than that of CK-2a and CK-2c, but the difference is not significant. XRD of the parent cokes also shows that a significant amount of sp2 is retained during the ECE process. This is in agreement with the observation that a small fraction of sp2 forms when CK-b is exposed to high temperatures. It is possible that the high temperatures during the ECE process are sufficient to sinter the coke-EEG into a sp2-rich structure. However, further research is needed to verify this hypothesis.
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