Pore structure evolution during the coke graphitization process in a blast furnace

Pore structure is an important factor influencing coke strength, while the property of coke is essential to maintaining gas and liquid permeability in a blast furnace. Therefore, an in-depth understanding of the pore structure evolution during the graphitization process can reveal the coke size degradation behavior during its descent in a blast furnace. Coke graphitization was simulated at different heating temperatures from 1100 to 1600°C at intervals of 100°C. The quantitative evaluation of the coke pore structure with different graphitization degree was determined by vacuum drainage method and nitrogen adsorption method. Results show that the adsorption and desorption curves of graphitized coke have intersection points, and the two curves did not coincide, instead forming a “hysteresis loop.” Based on the hysteresis loop analysis, the porous structure of the graphitized coke mostly appeared in the shape of a “hair follicle.” Furthermore, with an increase in heating temperature, the apparent porosity, specific surface area, total pore volume, and amount of micropores showed good correlation and can divided into three stages: 1100–1200, 1200–1400, and 1400–1600°C. When the temperature was less than 1400°C, ash migration from the inner part mainly led to changes in the coke pore structure. When the temperature was greater than 1400°C, the pore structure evolution was mainly affected by the coke graphitization degree. The results of scanning electron microscopy, energy dispersive spectrometry, and ash content analyses also confirmed that the migration of the internal ash to the surface of the matrix during the graphitization process up to 1400°C contributed to these changes.