Mineral Matter Reactions in Cokes

Some cokes appear to be particularly susceptible to weakening in the blast furnace. A mechanism which has been postulated to explain this is silica reduction by coke. Theoretically, silicates could react with the coke at high temperature to produce silicon carbide and carbon monoxide with the resultant loss of carbon contributing to the weakening of the coke. Moreover, this reaction would cause an increase in the surface area of the coke and hence increase its reactivity. However, evidence for this behaviour was indirect. Thus this project was initiated to ascertain the behaviour of quartz and silicates in coke with an emphasis on the role of the clay minerals. The objective of the project was to assess the role of mineral matter and mineral matter reactions in the degradation of coke through their influence upon coke reactivity and coke strength.

With advances in analytical technology it is now possible to obtain quantitative mineralogical data and, the case of coal, to also obtain quantitative data on mineral grain size, shape and association through the use of automated electron beam image analysis techniques. This new ability can allow relationships between the amount of minerals in a coke and its reactivity to be established for the first time.

Samples of five Australian coking coals were selected based upon quartz and clay mineral contents, mineral grain size and association as shown below:

· Coal A High kaolinite, moderate quartz, low illite

· Coal B High quartz, moderate kaolinite, low illite

· Coal C High kaolinite low quartz

· Coal D High illite, moderate quartz, kaolinite

· Coal E Moderate quartz, kaolinite, montmorillonite

Samples were also provided by BlueScope Steel of coal, feed coke, and tuyere coke samples from the bosh, deadman and raceway regions of the blast furnace.

The analytical work program conducted was as follows:

· Preparation of cokes by CSIRO

· Petrography of starting coals and cokes

· QEMSCAN of coals

· LTA and XRD of starting coals and cokes

· Coke reactivity tests [NSC and small scale]

· Petrography, LTA and XRD of reacted cokes.

· Petrographic and XRD examination of heat treated cokes.

This study indicates that the NSC reactivity test does not adequately reflect the behaviour of coke in the lower part of the blast furnace. Further investigation of the behaviour of coke in the lower part of the blast furnace is required with the objective of developing a laboratory test of coke reactivity which more accurately reflects blast furnace conditions.