Effect of Coke Properties on High-Temperature Strength and Hot Metal Reactivity Under Blast Furnace Conditions

This project focussed on investigating the differences in characteristics of cokes prepared from single coals and blends (binary and ternary) in terms of their behaviour for blast furnace ironmaking. Two single coal derived cokes (D and H) and two blended coal derived cokes (C1 and C2) were fabricated in the four compartment pilot-scale oven; these cokes were characterised in terms of gasification losses, room and high temperature mechanical strengths, mineralogical changes including extents of graphitisation under test conditions, and corrosion behaviour against molten iron and slag at different temperatures.

The results demonstrated that the weight losses after gasification had a close correlation to the rank and ash chemistry of the coals used in the preparation of the cokes with the ones completely and partially composed of lower rank components showing the greatest weight losses compared to cokes fabricated using high rank coal components. The ash chemistry and microtextural components resulting from the coals are known to influence the reactions leading to weight loss in gasification conditions. The weight loss during gasification was found to influence the mechanical strength degradation of the cokes such that the strengths decreased significantly for some after gasification. However, the high temperature strengths (1100°-1550°C) of the cokes showed no correlations with the presence of high or low-rank components or the gasification weight losses. The data showed that the strengths measured at high temperatures were higher than those observed at room temperature. The increased strengths at higher temperatures occur from the increased plastic deformation of the cokes at high temperature which arises from the graphitisation and increased ability to deform without sudden brittle failure.

The implications of this project are in terms of establishing the importance of the rank of the coals used in cokemaking and its impact on the high temperature behaviour of the cokes under blast furnace simulating conditions. The preparation of the cokes using coal blends is a very common practise in cokemaking and this project demonstrates that the presence of the low rank components, the associated differences in microtextural components, and the variations in ash chemistries could negatively impact on its high-temperature behaviour including weight losses during gasification and microstructural development and mineralogical changes during exposure to the high temperatures.

The results also show that the immersion testing method is highly suitable for producing reaction interfaces between the cokes and the metal for characterisation and also for recovering the majority of the coke sample after reaction for further analysis.

Results have revealed that the blending of coals has a major impact on the properties of the resulting coke blends and their performance at high temperatures compared to the cokes derived from the same precursor single coals. The results showed that the rank of the coal used in the blend can impact on the following key properties:

• Reactivity under blast furnace conditions;

• Room temperature strength;

• Extent of graphitisation;

• Corrosion performance against hot metal and slag.

The other important factor is the strength of the IMDC-RMDC interfaces that are formed and the diverse characteristics of the interfaces that are produced between the components when multiple coals are used in the blend design.

The ash content and its nature have been shown to impact on the high temperature properties and performance of the resultant cokes. These are important factors to consider for Australian coals for the future especially since the complexity of the coal geology is making it very difficult to wash the resources effectively to reduce the ash content to the required levels. There is growing focus on techniques to remove the ash via liquid phase formation by altering the ash fusion temperatures or via chemical treatments to ensure cokes with optimal ash characteristics and quality for metallurgical applications.