Upgrading Semi-Soft Coking Coal by Hydrothermal Treatment: Caking and structural properties
Date
2024-02
Authors
Journal Title
Journal ISSN
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Publisher
University of the Witwatersrand, Johannesburg
Abstract
Based on the current challenges faced by the metallurgical industry in South Africa in importing quality reductants, there is an urgent need to investigate a new approach to enhance the semi-soft coal available in the country. Importing prime coking coal has increased the steel price, resulting in many downstream operations involving steel closing down in the country. With a surplus of semi-soft coking coal in South Africa, this research sought to look into this kind of abundant coal to enhance its property as a reductant for blast furnace applications. For this reason, a study was conducted on two Southern African coals, Grootegeluk (semi-soft coking coal) and Moatize (higher quality coal). Both coals were individually hydrothermally treated and then blended at different ratios to further upgrade their metallurgical properties. The as-received Moatize coal showed properties that were more of prime coking coal with high total carbon content (76.50%), a crucible swelling index of nine, a maximum dilatation of 59% and volatile matter of 20.39%. It was a highly vitrinite coal with a vitrinite reflectance of 1.28%, a higher micropore volume than mesopore volume and a very low maximum fluidity of 24 dial divisions per minute (ddpm). According to the initial test, the Grootegeluk coal sample had a crucible swelling index of 5.5, a high volatile matter of 35.02% and a low vitrinite reflectance of 0.72%. In addition, the sample had a maximum dilatation of -10%, a maximum fluidity of 3ddpm and a higher mesopore volume than the Moatize coal. Hydrothermal treatment was conducted on the coal samples at numerous temperatures (100ºC to 200ºC), at various residence times (30 to 90 minutes) and at different coal masses (300 to 600grams (g)). According to the results, the optimal hydrothermal conditions were 200ºC, 90 minutes and 600g. Another hydrothermal treatment was performed at a higher temperature and residence time of 280ºC and 180 minutes. The same sample mass of 600g was used and the result showed no further improvement. The coal samples were then blended at various Grootegeluk/Moatize ratios (15% to 50% Grootegeluk), and further hydrothermal treatment tests were carried out based on the optimum conditions achieved. Both the hydrothermal test and the blending of the coal led to a coal with volatile matter ranging from 21.46% to 23.79%, which is a required specification for metallurgical application. The total carbon of the enhanced coal blend also ranged from 68.8% to 82.10%, with total sulphur below 1%. The mesopore-micropore ratio of the treated blend was higher than the individual coal samples, which is what is expected of a metallurgical coal. Based on these findings, coke was produced and analysed to identify a coke capable of withstanding blast furnace conditions. Using the particle reactivity index (PRI), proximate analysis and the pore size distribution, 90-(50% Grootegeluk+50% Moatize)-C product was identified as the coke with the least PRI and high fixed carbon. Further investigation showed that the blending and hydrothermal treatment affected the coal’s physiochemical, rheological and micro-molecular properties. The study has established that metallurgical properties of the locally mined semi-soft coking can be enhanced solely and when mixed with a hard coal. Even though the 90-(50%GG+50%M)-C did not meet the overall specifications required for use in the blast furnace, it was identified as a suitable reductant for other metallurgical applications.
Description
A dissertation submitted to the school of Chemical and Metallurgical Engineering, Faculty of Engineering and Built Environment, University of the Witwatersrand, Johannesburg. In fulfilment of the requirements for the degree of Master of Science in Engineering, in 2024.
Keywords
Coking coal, Reductant, Semi-soft coking coal, Hydrothermal treatment, Reactivity, Pore volume, South Africa, Southern African coals, Grootegeluk (semi-soft coking coal), Moatize (higher quality coal), UCTD
Citation
Ndumo, Jabulile. (2024). Upgrading Semi-Soft Coking Coal by Hydrothermal Treatment: Caking and structural properties. [Master's dissertation, University of the Witwatersrand, Johannesburg]. WIReDSpace. https://hdl.handle.net/10539/38882