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5i02 occurs in coking coals as pure quartz and also in complex aluminosilicates. At coking temperatures, pure 8i0 2 has several polymorphs (a-and J3-quartz and IHridymite) and transformations from one to another are accompanied by volume changes in the mineral matter that can lead to the formation of cavities and cracks in a coke matrix. These can weaken the coke physical strength and lead to a higher circulation of gases within the pieces. The grain size of the primary quartz crystals in a coal blend is also important for the estimation of coke strength: the larger the crystals that occur in a coal, the larger the cavities and cracks that will ensue (weaker coke). The occurrence of any given 8i02 polymorph indicates the temperature of its formation and can be used to correlate other changes in coke-forming compounds with certain temperatures. A high amount of free quartz in a coal blend can be considered a negative factor for coking. For a better understanding of the influence of bulk 8i02 on coke quality, mineralogical (phase) analysis and quartz grain size measurements on coking coals can be helpful. Under the blast furnace conditions, 8i0 2 polymorphs have no substantial influence on the coke physical strength.

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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 (Kand Na-, ϮCa-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 mm, and it reflects a trend towards reducing free surface energy. It was determined that total KϩNa drops from the parental particles to the new phases. Relative enrichment of K over Na (increase of K/Na at 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/Na at 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 (KϩNa) 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 NaϮCa-bearing aluminosilicates.