Fundamental factors influencing coke strength

Research Doctorate - Doctor of Philosophy (PhD)In an ironmaking blast furnace solid-gas reactions are important and the rate of iron production is strongly dependent on the permeability of the particulate bed. In this regards coke is probably the most important component in the bed as it remains a solid even at very high temperatures. All the iron-bearing particles in the bed have reacted and melted at these temperatures. Blast furnace productivity is believed to greatly depend on the properties of the coke particles in the bed - in particular its ability to degrade under the conditions experienced in a blast furnace. The suitability of coke for use in blast furnace is characterised by tests conducted on the charged coke under room temperature and also after reaction at high temperatures. At the BHP Billiton Newcastle Technology Centre a large fundamental research program commenced in 2004 to understand the relationship between coal blend properties and coke properties, and also between coke properties and blast furnace performance. The work reported in this thesis is part of the research program. The work uses coke samples manufactured in a pilot oven under carefully controlled conditions, 2D image analysis techniques developed to characters the fundamental structural properties of coke and proven methods to determine the material composition of coke or its petrographic composition. The work in this thesis is aimed at determining the relationship between coke fundamental properties (i.e., its structure and petrographic composition) and its contact strength since coke particles degrade in the blast furnace through inter-particle contact. Currently, coke strength is determined using tumble tests to provide a bulk measure of its resistance to degradation. In this study two techniques were applied to determine coke strength. The first involved macro-indentation to obtain a fundamental measure of coke contact strength. The second involved the uni-axial compression of a bed coke particles. At the inter-particle contact points fines are generated and the energy utilised in the generation of these fines was determined. The two tests are related in that a material which is penetrated more easily by an indenter will generate more fines because a larger volume of material has been fractured during the indentation process. Cokes from coking programs were used to assist in the development of a theoretical relationship or model. These cokes were termed model cokes because for each program the changes in coke properties were incremental - for example, cokes produced from a coal aged for varying periods formed cokes of similar petrographic compositions but with different structures. These model cokes were then characterised in terms if structural analysis and petrographic compositions, and contact strength and energy utilisation. This allows the influence of coke structure and petrographic composition on fundamental contact strength and energy utilised during inter-particle contact fracture to be studied. Many coke structural parameters are obtained using 2D image analysis. This work considered which of these parameters were relevant to coke contact strength. The study then explored the use of a single index to describe the petrographic composition of coke relevant to contact strength. A novel SP (structure-petrographic) index was then obtained by combining the petrographic index with those parameters found to be relevant to coke contact strength. The theoretical model which gave good fit to measured results for the model cokes incorporates porosity, pore size, mean wall thickness for the structural term, while the petrographic index takes into consideration the volumetric composition of the components that make up the coke wall and the micro-hardness of these components. The model was then tested against cokes of very different properties. Not surprisingly, there was deterioration in the coefficient of correlation but there is a still clear relationship between the SP index and measured contact strength and energy utilisation. These studies indicate that the SP index gives a meaningful measure of coke contact strength. The traditional coke tumble strength incorporates other factors such as fissuring and breaking by other mechanisms; it is, therefore, not surprising that there is no relationship between the SP index and tumble strength. The reactions that occur in a blast furnace greatly weaken the structure of coke, leading to accelerated degradation. At BHP Billiton there is a project underway to study the strength of reacted cokes. The macro-indentation test developed in this work was also used to measure the contact strength of some reacted cokes. Results were very promising and this test will be adopted in the study. Plans are also underway to use the uni-axial bed compression test and a modified SP index for reacted cokes