Understanding Cokes Performance in Blast Furnace Conditions- Coke Strength Extension

The performance of coke in the blast furnace ideally should include those properties of coke that reflect its resistance to degradation under the mechanical, chemical and thermal environment of the blast furnace. A previous collaboration between UNSW and Ruukki Works, Rahee, Finland, obtained matching feed coke and coke collected from the bottom of a medium sized blast furnace through a tuyere probe (ACARP project C17050 parts 1, 2 and 3). This extension to the project aims to take techniques used to understand the direct causal sources of mechanical failure in other model systems and apply them to both the feed metallurgical coke and the coke collected in the tuyere probe.

A series of five cokes were studied using the material technique of fractography which has been adapted by the authors of this report to the heterogeneous coke material. The series consisted of matched feed, bosh, raceway, birds nest and deadman coke lumps. The fractographic approach consists of breaking the coke, by compressing the coke in a Shimadzu universal tester. The fracture surface formed is then examined to develop an understanding of where the fracture occurred and the mechanism of fracturing. This technique has allowed us to successfully build a picture of the nature and amount of microstructural features in the coke. These were summarized using quantified "Radar graphs".

These radar graphs provide a unique failure profile for each of the Ruukki metallurgical cokes retrieved and tested. They provide both a visual and statistical representation of the failure mechanisms for each of the analysed cokes, and have a high degree of reproducibility not only because of the large number of fractured samples analysed but also because they are the physical representation of the actual cokes tested and how they endured the blast furnace environment. How this coke fails then is a direct result of this coke's formation and time in the blast furnace. Despite the inherent heterogeneity of coke, the growth of the external fissures formed in the large coke lumps and the resulting fracture faces have the main characteristics of the smaller lumps after failure because these fissures are a result of coke processing and its time in the blast furnace environment.

Better understanding of the weaknesses in the coke microstructure will allow us to utilise techniques such as root cause and effect analysis (RCEA) in combination with failure analysis methods which will allow us to identify the significant sources of weakness in the feed coke and other regions of the tuyere. A number of recommendations were made, including:

· Link the quantitative fractography results to the coal properties and processing of the coke to allow recommendations to be made on how to improve the performance of the coke in the blast furnace;

· Gain a better understanding of inert chemistry, for example through EDS analysis, including determination of the type of inerts in which nano slip films form. Fractographic analysis of cokes produced from controlled maceral mixes would assist with this;

· Perform root cause analysis of the failures to reduce the number of predictable failures;

· Study the effect of simulated blast furnace environments in the laboratory on coke breakage to better define the changes in microstructure and to identify features in feed coke that give good structural features in tuyere coke .