Effect of alkali metal–fluoride composite system in the blast furnace on the thermal properties and microstructure of coke

To address the challenges posed by fluorine and alkali metals (K, Na) in Bayan Obo iron ore during blast furnace ironmaking, this study systematically investigates the impact of alkali metal–fluoride composite systems on the thermal performance and microstructure of coke. An immersion method was used to simulate the vapour-phase adsorption and deposition of harmful elements on coke under simulated blast furnace conditions. The results show that alkali metal–fluoride composite systems significantly exacerbate coke deterioration. Coke reactivity index (CRI) exhibits nonlinear growth with increasing solution concentration (increase range: 12.54%–51.12%), while coke strength after reaction (CSR) decreases synchronously (decrease range: 1.73%–19.41%), following the influence order: K 2 CO 3 > Na 2 CO 3 > KF > HF > NaF. Phase analysis indicates that after gasification reactions, low-melting-point silicoaluminates such as KAlSiO 4 and NaAlSiO 4 are formed in coke immersed in K 2 CO 3 and Na 2 CO 3 solutions, leading to coke volume expansion, matrix cracking, and strength reduction. Scanning electron microscopy (SEM)–energy-dispersive spectroscopy (EDS) reveals the most severe pore evolution in the potassium system (porosity reaches 69.46% for 5% K 2 CO 3 ), followed by the sodium system, and the least in the fluoride system. Mechanistic studies demonstrate that alkali metals play a primary deteriorative role by catalysing gasification reactions, damaging carbon microcrystalline structures, and inducing pore connectivity, while fluorides enhance alkali metal migration through F − adsorption, with KF exhibiting the most significant synergistic effect. This study first reveals the quantitative relationship between migration ability and catalytic activity in the alkali metal–fluorine composite system. This research provides theoretical support for alkali load control in Bayan Obo iron ore blast furnaces and optimisation of coke blending schemes.