HOW WEATHERED COAL REDUCES METALLURGICAL COKE QUALITY

It has long been known that the presence of weathered coal in a coking mix will negatively affect the properties of the resulting metallurgical coke, specifically coke stability (strength) and coking rate decrease whereas coke reactivity and coke breeze (undersized unusable coke) increase. However, the actual reasos for these effects is not well understood. For this study, coal mixes containing various amounts of weathered coal were coked in a commercial test oven. While an increase in coke porosity has often been cited as a reason for the degradation of coke properties, automated image analysis failed to show any increase in the porosity of the coke made with the weathered coal. But careful petrographic examination of the weathered coal particles showed that they are not incorporated into the coke cell as are normal inert particles. They are poorly bonded or not bonded at all to the coke cell walls, thus, they effectively act as pores. However, in a point count analysis (especially an automated image analysis) the weathered particles report as solid material giving an erroneously low estimate of porosity. Also, because the weathered coal particles are inert (they have lost their fluidity and thus do not melt in the coking process) they increase the total effective inert content of the coal mix. Published work done at the research labs of the Bethlehem Steel Corporation has shown that an increase in the effective inert content of a coal mix will lower coke quality. Thus, weathered coal causes an effective but not an actual increase in porosity as well as an increase in the effective inert content; these factors result in a decrease of coke quality. Both of these features are dramatically shown in SEM photomicrographs of the weathered particles appearing in voids and unattached to the coke cell walls. In these images the weathered particles appear similar to peanuts in their shells.