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Assessment of In Situ High Temperature Strength of Cokes

Technical Market Support » Metallurgical Coal

Published: November 19Project Number: C25045

Get ReportAuthor: Pramod Koshy, Michael Drew, Xing Xing, Hannah Lomas, Wen-Fan Chen, Sushil Gupta, CC Sorrell | University of New South Wales, ANSTO

The research for this project was undertaken in three stages.

Stage I established a reliable and repeatable process for mechanical testing at high temperatures using facilities at ANSTO. Three cokes were tested in Stage I and II. In Stage I, high CSR coke samples displayed significantly greater strengths at higher temperatures in comparison to room temperature. This behaviour was attributed to increased plasticity of the cokes owing to mineralogical and microstructural transformations, including in situ graphitisation at high temperatures.

In Stage II, similar tests were done on two low CSR cokes. Researchers believed the popular NSC type test may have be underestimating the coking quality of at least some semi-hard coals which tend to display high cold strengths. It was thought that cokes made from semi-hard coals may display higher strengths in the temperature zones of a blast furnace, compared with the strengths measured at relatively lower temperatures in popular tests. The methodology and parameters for determining the high-temperature compression and creep behaviour of high-quality/high CSR cokes were established in stage I. In this stage the Researchers built upon the knowledge from stage I to evaluate the high-temperature creep compression behaviour of cokes of lower CSR or strength values.  

Stage III focussed on determining the effects of gasification under blast furnace simulating conditions on the strengths of the cokes at high temperature to determine the effects of changing mineralogical, chemical, and microstructural properties with temperature and gasification conditions on strength evolution.

In Stages I and II, Coke 1 (High CSR) and Cokes 2 and 3 (Low CSR 1 and 2) were annealed at 1100°C in argon and then compression tested at room temperature, 1400°C, 1550°C, and 1700°C to determine the changes in compression strengths and associated changes in coke properties. The results showed:

  • The compressive strengths were higher at higher temperatures (>1400°C) compared to room temperature (RT);
  • Cokes 2 and 3 (low CSR) showed higher strengths than Coke 1 (high CSR);
  • The increased load-bearing capacity was related to enhanced plasticity arising from the in situ graphitisation of cokes and resultant microstructural changes at high temperatures.

Coke 1 and Coke 2 were selected for work in Stage III owing to having similar ash contents. In Stage III, the cokes were gasified under conditions simulating the gas atmosphere in the blast furnace in the cohesive zone (up to 1400°C). Both cokes showed similar weight loss (~10%) after gasification possibly from ash-carbon reactions. Then the samples were and subjected to mechanical testing at room temperature and 1100°-1700°C.

Similar to previous stages, the cokes showed high strengths at higher temperature compared to room temperature. The highest strength was observed at 1100°C (figure on right); the strengths then decreased at 1400°C and then remained mostly constant for Coke 1, while for Coke 2, the strengths increased again with increasing temperatures from 1400°C. Graphitisation, SiC and Fe/Fe-Si formation were observed both mineralogically and microstructurally within both coke samples, with the graphitisation higher in Coke 1.

For tested samples at RT, Coke 1 showed a stronger RMDC compared to Coke 2 but at higher temperatures, RMDC and RMDC-IMDC interfaces were the major source of failure through crack propagation with increasing IMDC failure at the highest temperatures. Pore formation from gasification and roughening of edges from carbon loss were seen in gasified samples.

The strengths are seen to have a correlation with the CSR values at 1100°C after which there appears to be no correlation with Coke 2 showed higher strengths than Coke 1 at higher temperatures. This shows the potential for using low CSR cokes in blends in a blast furnace and also highlights their relative stability in the blast furnace conditions.

Implications

The work done in all three stages have allowed for improved understanding of the effect of temperature and gasification on the strength evolution of cokes at high temperature under blast furnace conditions. Overall, the cokes were subjected to the following conditions and then subjected to compression testing (the terminology is in parentheses):

  • Cokes annealed at 1100°C in argon (annealed cokes);
  • Cokes subjected to gasification in BF conditions (gasified cokes);
  • Cokes heated under compression tested conditions (heated cokes).

When subjected to gasification until 1400°C under conditions simulating the conditions within the blast furnace, the cokes showed similar weight losses. These would arise from ash-carbon reactions which would occur within the blast furnace under similar conditions. The ash contents and composition for the cokes were fairly similar and this would account for the similarity in terms of their observed weight losses. For both cokes, gasification was found to have a significant effect on the strength degradation between 1100°1400°C while at higher temperatures, gasification was not found to have a critical impact since the strengths were similar to the corresponding values for the cokes that were annealed in argon. At these conditions graphitization in the cokes is believed to have the stronger impact on the strength development. This behaviour at 1100°-1400°C represents the degradation in the granular zone and cohesive zone. In these regions, the coke has to show high strength to withstand the burden and also lower reactivity. Thus the CSR can be considered to be a measure of the strength and reactivity of the coke, with high CSR indicating higher strength and lower reactivity. Moreover, the CSR has a direct correlation with the rank of the cokes and indicates the potential for deterioration of the cokes. The direct relation between the CSR and the high-temperature strengths at 1100°C suggest that the CSR results are potentially valid for estimating the strengths of cokes under this condition. However at the higher temperatures of 1400°C in the cohesive zone, the temperature and subsequent graphitization start to minimise the impact of the CSR.

However, the lack of correlation (or generally a reversal of the correlation) of strengths vs. CSR at higher temperature suggests that the high temperature graphitization and associated microstructural evolution and mineral phase transformation have the defining effect on strength evolution at these temperatures. These show that temperature-derived phenomena have the stronger impact at these temperatures while gasification has the major impact on strength development at the lower temperatures. At temperatures of 1500°C and above, the strength requirements on the coke are reduced with the focus more on reactions with metal and slag, and thus the reduction in strength observed is an acceptable effect. Thus these other factors in addition to the CSR need to be considered at these high temperatures to understand the real degradation behaviour of cokes within the blast furnace.

Therefore, the work on single-cokes of varying CSR values demonstrates the potential for using coke blends of high and low quality cokes to show optimal strength and reactivity for blast furnace ironmaking. This leads on to the next phase of work on determining the strengths of blended cokes which will allow for developing more mechanistic correlations with strength variations at high temperature after gasification and coke and parent coal properties.

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