Evolution of coke microstructure and metallurgical properties during graphitization in a blast furnace

The currently used low carbon operation in blast furnaces (BFs) leads to a thin coke layer and deterioration in gas and liquid permeability. Previous studies have reported that coke breeze, formed by the graphitization of coke, is one of the main influences on gas and liquid permeability in the high-temperature zone of a BF. To investigate the influence of the graphitization of BF coke on its metallurgical properties, the degree of coke graphitization under different heating temperatures from 1100 to 1500℃ was investigated. The dependence of the degree of graphitization on coke reactivity index (CRI) and coke strength after reaction (CSR) was also studied. The relationship between different degrees of graphitization in coke and alkali metals was clarified. Finally, the micro-morphology of coke was inspected and analyzed. Results show that with increasing temperature, the degree of coke graphitization intensifies. With every 100℃ rise in heating temperature, the degree of coke graphitization increases 1.8 times, the value of the spacing of layers (d₀₀₂) decreases by 2%, and the values of the layers diameter of microcrystalline structure (L_a) and the stack height (L_c) are improved by 3% and 15%, respectively. In addition, it is observed that the surface stomata of the coke decreases particularly the large pores, the mosaic structure decreases, the isotropic structure increases, and the degree of structural ordering increases. When the degree of graphitization intensifies, the CRI of the coke gradually decreases, the CSR gradually increases, the degree of coke degradation weakens, the formed large pores reduced, and the trend for pore wall damage is reduced. Moreover, the alkali metal plays a catalytic role in improving the CRI and decreasing the CSR. It is considered that the graphitized coke resists the destruction of the alkali metal and thus decreases the degree of coke degradation.