Influence of alkaline (Na, K) vapors on carbon and mineral behavior in blast furnace cokes

• Alkalization behavior of coke and corresponding influence were investigated.
• Potassium and sodium destruct coke texture with different degrees.
• Alkalization leads to the formation of additional alkalis-bearing phases.
• The destruction mechanism of alkalis vapors on coke was proposed.

A series of adsorption–alkalization experiments were conducted in a muffle furnace on two types of blast furnace cokes at 1300 °C in the presence of alkali vapors. Coke textures were found to peel off layer by layer after the alkalization process by potassium vapor, and macro fissures were observed for K/Coke ratios higher than 3/100. This phenomenon was not observed in the coke samples alkalized by sodium vapor. A number of additional potassium-bearing and sodium-bearing phases were detected with scanning electron microscope and energy dispersive spectrometer after the alkalization process. The formation of kalsilite or potassium aluminum silicate (KAlSiO4) and sodium alumina silicates (Na6Al4Si4O17) was confirmed through X-ray diffraction, however the formation of intercalation compounds that were expected to form in the alkalized coke samples could not be confirmed. The catalytic effect of sodium and potassium-bearing minerals appeared to be quite similar; the degradation of coke strength by sodium was however found to be stronger than that caused by potassium. The severe degradation of coke quality caused by alkali vapors was attributed to their strong influence on the coke carbon matrix, coke minerals, as well as their catalytic effect on the carbon gasification reaction.