A study of some structural features relevant to the degradation of metallurgical coke
Metallurgical coke for blast furnace and foundry use, and for some domestic boiler cokes are produced by carbonising coals in slot-type ovens. The coal is heated by heat transferred from the hot oven walls (usually 1250°C) so that a temperature gradient is set up within the oven charge. A layer of plastic coal is formed (350-500°C) which moves progressively towards the oven centre as carbonisation proceeds. Immediately after re-solidification, and again near 700°C, the semi-coke suffers shrinkage as the carbon structure reorganises The post re-solidification shrinkage is the more severe and results in large, horizontal and vertical primary fissures being formed which allow breakage of the charge into lumps as it is pushed from the oven. A large central fissure, formed when the two plastic layers meet, divides the charge into two. Further breakage occurs at primary fissures on handling the coke after pushing. Smaller secondary fissures present in the resultant coke lump are believed to be a result of the shrinkage near 700°C. Coke has three roles in the blast furnace: as a source of heat, a source of reducing agents and as a support for the burden. The latter function is particularly important at the bottom of the stack when coke is the only solid material present. The blast furnace output is dependent on the quantity of air that can be pushed up the stack. This depends on the permeability of the coke bed in the lower reaches of the stack. To maintain permeability, the coke should not suffer size degradation as it progresses down the stack; a mixture of large and small coke particles is disastrous. Thus coke quality tends to be assessed in terms of resistance to size degradation in a drum test. Size degradation occurs as a result of volumetric breakage at fissures and abrasion of surfaces. Brittle fracture theory suggests that volumetric breakage should occur as a result of the propagation of secondary fissures in coke lumps.