Effect of CO2 gasification on the transformations of coke minerals at high temperatures

Mineral phases of three metallurgical cokes were characterized after reactions with CO2 at 1373 and 1773 K in a horizontal furnace for 2 h using X-ray diffraction (XRD) and scanning electron microscope/energy dispersive spectroscopy (SEM/EDS). After reaction at 1373 K, all cokes displayed an increased degree of mineral decomposition, dissemination within the carbon matrix, and inclusions in pores such that the intensity of transformation varied with coke types. As for the original cokes, XRD analysis of the reacted cokes indicated the presence of quartz, mullite, and high melting point aluminosilicate as the major refractory phases while pyrrhotite, triolite, iron oxides, and metallic iron phases were the main reactive phases. Framboidal sulfide appeared to diminish after CO2 reactions. Iron-spinel was the common reaction product while olivine was observed in only one coke. All minerals occurred in a wide variety of sizes and distributions such that fine minerals occurred mainly within or close to pores and were comprised of iron sulfide and Al2O3. At 1773 K, aggregates of Al-silicates decomposed further to form mullite and cristobalite while low melting point Al-silicates transformed to running slag in coke. Iron was observed to occur in many forms including metallic iron, silicates, and oxides but less frequently as sulfides and appeared to occur within or in close proximity to pores. At 1773 K, pore inclusions were the prominent features of the reacted cokes, particularly a high proportion of Al2O3 (corundum). Calcium was observed as silicates as well as thermally stable phosphate and sulfides. This study demonstrated that characteristics of coke minerals after reactions at standard coke strength after reaction (CSR) test temperatures of 1373 and 1773 K were significantly different in terms of phase distribution which would influence coke strength and reactivity accordingly. This study implies that coke properties measured at relatively lower temperatures in the conventional tests may not provide a true assessment of coke performance in an operating blast furnace.