HRTEM analysis of carbon structure evolution during the formation of metallurgical coke and impacts on coke quality

The coke quality influences the performance of the blast furnace, and the coal properties and coke structure play important roles in affecting the ultimate coke quality. The evolution of the physical and chemical structures during the plastic layer formation is generally accompanied by the modification of the carbon structure, which influences the strength and reactivity of coke, e.g., mechanical strength (micum indices-M40/M25, M10) and hot strength (coke strength after reaction with CO₂-CSR and coke reactivity index-CRI). This paper reports the evolution and transformation of carbon structures during the coking processes of six Australian coking coals, aiming to draw correlations between the carbon structures of coke and coal properties as well as coke strength. Results indicate that cokes contain various proportions of turbostratic structures and graphitized structures derived from the transformation of carbon structures in terms of coal properties. After the chemical structural transformation of coals during the coking process, the heteroatoms are removed, and the carbon structures of the cokes consist of more ordered stacked structures. The cross-links and oxygen-containing functional groups in low-rank coals evolved to form more tortuous stacked structures. Cokes made from higher-rank coals formed fringes with longer average lengths and lower tortuosity in a more tightly stacked structure, leading to the formation of anisotropic structures. The carbon structure of coke was found to be closely correlated with the cold strength parameters.