Effects of K, Na, KF and NaF gases on hot-state properties and microstructure of coke

Aiming at the degradation mechanism of coke induced by K, Na, KF and NaF gases during blast furnace smelting, gas adsorption experiments on coke were conducted via the gas-phase adsorption method. Combined with characterization techniques including XRD and SEM-EDS, the influence laws of K, Na, KF and NaF gases on the hot-state properties and microstructures of coke were systematically investigated. The results show that coke surface particles are exfoliated under the action of K gas, while powder-like spalling of coke surface is induced by Na gas. In contrast, the number of pores in coke is significantly increased by KF and NaF gases. After the adsorption of K, Na, KF and NaF gases by coke, the interplanar spacing of the(002) crystal plane of graphite microcrystals is enlarged, whereas the microcrystalline stacking height and microcrystalline layer size are reduced, which severely impairs the orderliness and integrity of the microcrystalline structure. The degradation degree of the aforementioned gases on the microcrystalline structure is ranked as follows: K>Na>KF>NaF. The variation of microcrystalline structure leads to an increase in coke porosity, which in turn elevates the coke reactivity index(CRI) and reduces the coke strength after reaction(CSR). Among these gases, the effects of K and Na gases on coke porosity are significantly stronger than those of KF and NaF gases. Specifically, the influence of K gas is greater than that of Na gas, and the impact of KF gas exceeds that of NaF gas. When coke is adsorbed with 1% NaF gas, its hot-state properties(with CRI≤25% and CSR≥65%) still meet the requirements for blast furnace smelting. However, after the adsorption of mass fraction of 1% K, Na, or KF gas, both CRI and CSR exceed the standard limits. Overall, the influence degree of various gases on the hot-state properties of coke follows the sequence: K>Na>KF>NaF. In addition, with the increase in gas adsorption capacity, the growth rate of CRI and the decline rate of CSR both exhibit a trend of marginal decrease.