2005 Volume 45 Issue 1 Pages 1-7
Polished sections of tuyere coke were studied by optical and scanning electron microscopy and wavelength-dispersive analysis (WDS) to reveal structural and chemical changes occurring with mineral phases (K- and Na-, ±Ca-bearing aluminosilicates). The aluminosilicates in the tuyere coke form new compounds, which occur as particles with smooth outlines (spherules and irregular segregations), which are of larger size than their parental particles. The depth of the diffusion zone between particles of the parental mineral phases and new phases was found to be 5-20 μm, and it reflects a trend towards reducing free surface energy. It was determined that total K+Na drops from the parental particles to the new phases. Relative enrichment of K over Na (increase of K/Naat ratio) was traced from the parental mineral phases towards the final agglomerated phase, which may indicate an Na excess in the circulating gases and/or the carbon phase of the coke. On the other hand, the K/Naat ratio decreases during formation of spherules that point to loss of K. Micrometers-range WDS profiles across the spherules revealed variable alkali behavior, with some spherules characterized by a strong negative correlation of alkalis, indicating ordering of the alkali cations and the existence of K-Na substitution. The alkali total (K+Na) and balance (K/Na) changes observed during this study suggest that at least one point of alkali escape begins upon the release of these elements during the formation of spherules and irregular segregations of K- and Na±Ca-bearing aluminosilicates.