The Effects of Ash Minerals on Coke Reactivity at High Temperatures

This was the final stage of a larger study focused on coke behaviour at high temperatures, C29077: The Effects of Ash Minerals on Coke Reactivity at High Temperatures. The research furthers the understanding and impacts of slag contact with coke on the mineralogy and carbon structures within the coke, and its subsequent effects on coke dissolution in liquid iron.

Samples of three cokes were immersed in slags with compositions representative of blast furnace at 1500°C. These coke samples were then characterised to establish any changes in coke mineralogy and carbon structures. The main findings were that:

• The coke minerals were enriched with Ca after immersion in the slag. This was found to occur through the whole coke sample, although there was possibly more Ca-enrichment at the coke-slag interface.
• The main reactions between the coke and slag were the reduction of any Fe-oxides in the slag by carbon in the coke to form metallic Fe. SiC was found distributed through the coke samples, likely formed by the reduction of SiO₂ in the coke minerals rather than by reduction of SiO2 from the slag.
• Graphitisation of the coke occurred, driven by exposure of the coke to high temperatures. As there was little difference in the carbon structures between positions near and away from the coke-iron interface, the graphitisation of the coke was likely to be simply thermally driven, and not influenced by any interactions with the slag.
• Coke contact with the slag was not found to have an effect on the subsequent dissolution of the coke in liquid iron. The change in the composition of the coke minerals did not appear to have an effect on the minerals that formed at the coke-iron interface.
• A likely reduction of a small amount of (SiO₂) in the slag to [Si] in the liquid metal was found to have lowered the saturation value of [C] in the liquid metal, slightly decreasing final [C] in the slag-metal-coke samples.

In addition to the work on the coke-slag interactions, work was helped better understand coke dissolution. This work used coke analogues as well as coke samples, and through recharacterisation and analysis of samples from previous projects. This was done to overcome specific limitations in the results in C29077 Part II. One of the main findings from the current study was:

• The dissolution of the coke analogues closely replicated dissolution of the cokes. By using coke analogues, carbon structure, porosity and particle size were largely eliminated as variables from the dissolution tests, leaving coke mineralogy as the remaining variable. The differences between the dissolution rates of the cokes were most likely due to the differences in their mineralogy, and not by the differences in their carbon structures.

In addition to this finding, the further work on coke dissolution showed that:

• The dissolution of coke analogues with coke ash was found to closely replicate the dissolution of the corresponding cokes. The rate of dissolution of the cokes can be ranked in order of increasing dissolution rate.
• The kinetics of coke dissolution could be separated into a faster initial period and a slower later period.
• The was no simple correlation between the coverage of the coke-iron interface and the dissolution rate for the three cokes.

  THE FINAL REPORT FOR STAGE THREE, TWO AND ONE ARE AVAILABLE FROM THE ACARP WEBSITE AS A SINGLE DOWNLOAD