Technical Market Support » Metallurgical Coal
Stage I of this project 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, namely Coke 1 (High Strength High CSR coke), Coke 2 (Low CSR 1, intermediate cold strengths), and Coke 3 (Low CSR 2, lower cold strength and lower CSR). In Stage I, high CSR coke (Coke 1) 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 (Coke 2 and Coke 3). These cokes showed similarities in terms of mineralogical data; however, their microtextural characteristics were significantly different, with Coke 3 showing an unexpectedly high volume of IMDC. In terms of mechanical properties, at room temperature, Coke 2 and Coke 3 fractured via expected brittle failure. The average strength for Coke 2 was slightly higher than that observed for Coke 3; these values were in a similar range to Coke 1. At high temperatures, both low CSR cokes (Coke 2 and Coke 3) showed significantly higher strengths in comparison to the values observed at room temperature. This behaviour was similar to what was observed for high CSR (Coke 1). However, the major difference was that these cokes showed a decrease or minor change in strength with increase in the test temperature from 1400°C to 1550°C, with a significant increase in strength with further increase to 1700°C. This is different from what was observed for Coke 1 (High CSR) which showed highest strengths at 1400°C and a decrease in strengths with further heat treatment temperature. The values were on average quite similar for both Coke 2 and Coke 3 (Low CSR) for the different test temperatures. With regard to the microstructural and mineralogical changes, there was again increased graphitisation for both cokes with increasing test temperatures, with the extent being greater for Coke 3 (Low CSR 2). The RMDC fractions showed increasing degradation with test temperature with the fracture dominated by cracking at the RMDC and RMDC-IMDC interface. IMDC degradation and cracking was also seen at the higher temperatures. In addition to graphitisation, SiC formation and reduced iron formation was observed both mineralogically and microstructurally within both coke samples.
Based on the figures above, it is quite clear that the strengths seen at high temperatures are quite independent of the CSR values. The two cokes with low CSR (Cokes 2 and 3) showed the highest strengths at both 1400°C and 1700°C. All three cokes showed similar values at 1550°C. These results are contradictory to expected trends and showcase the need for understanding the mechanisms of strength evolution of cokes at high temperatures.