Microstructure of Metallurgical Cokes

Coke ovens are the second largest consumer of coal in Britain today, accounting for around ten percent of the UK's yearly coal output. Iron and steel making are the main users of coke (metallurgical coke), where it is of major importance in the production of millions of tons of steel per annum. It is generally accepted that coke fulfills three roles in the blast furnace. It provides a source of reducing gas, vital for the high temperature stages of iron oxide reduction. Second, it is the main source of heat in the furnace, to melt the iron and slag materials. Finally, it provides a matrix of permeable material, particularly in the lower regions of the furnace where it is the only solid constituent present, through which the gases may ascend and the molten materials percolate to hearth. The performance of coke in this third role is of vital importance in determining the output and fuel efficiency of the furnace. A lack of permeability will restrict blowing rates, lead to poor gas distribution within the shaft and will cause a considerable pressure drop within the furnace. The breakdown of coke, in the blast furnace, reduces the mean size and also decreases the voidage by altering the size distribution of the material. The present research has therefore been targeted at investigating the factors influencing the breakdown of metallurgical coke. Conditions in modern blast furnaces subject coke to severe chemical, thermal and mechanical stresses. In this context, it is believed that recirculating alkali vapour (in particular potassium) is a major influence in the degradation of metallurgical coke lumps. The microstructure of metallurgical cokes has been investigated using high resolution electron microscopy (HREM) and complementary techniques. The object of the study was to evaluate the role of recirculating alkali, present in blast furnaces, in the weakening and subsequent degradation of the coke. It is shown that the feed coke's microstructure (cokes charged to the furnace) markedly differs from samples removed from the high temperature regions of the furnace (ex-tuyere cokes). The feed cokes exhibited a turbostratic structure of short range order, whereas the ex-tuyere cokes show two distinct, additional microstructural species, viz. an intercalated material, retaining the turbostratic disorder and an extensively ordered carbon structure. It has also been found that the nature and distribution of the mineral matter, inherent in metallurgical cokes, is that of randomly dispersed alumino silicates. Successful replication of the ex-tuyere microstructural forms has been achieved by reaction of the feed cokes, with potassium carbonate, in laboratory experiments. It has been shown that both potassium vapour and thermal treatment cause localised ordering of the carbon structure. However, the greatest amount of structural ordering occurs in the cokes when these two major influencing factors are combined. It is postulated that the intercalated material formed is an intermediate in the changes in coke structure, from a turbostratic structure to a more ordered arrangement of the carbon layers. The observed structural changes appear to be independent of the inert carrier gas employed in the laboratory reaction of feed coke samples. It is believed that the mechanical weakening of metallurgical coke arises primarily from changes occurring at the molecular level which consequently alter the macrostructure of the material, leading to a decrease in the lump strength of metallurgical coke