Technical Paper
temperature for 24hrs, then de-moulded and put directly into the furnace. The heating schedule for this explosion test was heating from 20 to 850°C at 50°C/hr; cooling from 850 to 20°C at 50°C/hr.
Figure 7 shows microsilica-gel bonded NCC without anti-explosion agent before and after the explosion resistance test. With no addition of drying agent, the 80kg block disintegrated during the test and parts of the block was completely pulverised.
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pore structure. By introducing EMSIL-DRY, the permeability and strength of the block seems to attain a good balance for removing the vapour during ebullition stage and therefore no explosion takes place even at a rapid heating rate of 50ºC/hr.
Figure 7: ~80kg block before (A) and after (B) explosion resistance test Figure 9: Temperature as a function of time
Figure 8 shows NCC-2 blocks containing EMSIL-DRY and Fiber-P2. The NCC-2 castable with EMSIL-DRY shows excellent explosion resistance and the ~80kg block was perfect after the explosion resistance test. The block with Fiber-P2 was split in two large parts and a ball-like crater was observed in the core. This indicates that high vapour pressure was generated in the centre while the outer surface was hard, like an “autoclave”. When the vapour pressure in the centre exceeded the mechanical strength, the block disintegrated. Certainly, the explosion resistance of NCC-2 with Fiber-P2 has been improved compared to the NCC without drying agent, but unfortunately not sufficiently to avoid damage.
Finally, an even larger block, 600x600x350mm (~400kg), of microsilica- gel bonded NCC-2, containing EMSIL-DRY, was produced for explosion resistance test. The block performed perfectly at a heating rate of 50ºC/hr, as shown in Figure 10.
Figure 10: ~400kg block with EMSIL-DRY after explosion testing at 850°C
3.4: Thermogravimetric analysis (TGA) and Scanning Electronic Microscopy (SEM) characterisation
Figure 8: ~80kg blocks after explosion resistance test
The temperature in the furnace and in the core of the NCC-2 blocks were recorded as a function of time, as shown in Figure 9. Under the rapid heat- up, the core temperature was lower than in the furnace. For example, the block with Fiber-P2 disintegrated at a core temperature of about 200ºC while the temperature in the furnace was 400ºC. At disintegration, the thermocouple was exposed to the furnace air, thus soon attaining the furnace temperature.
For the sample containing EMSIL-DRY, there was a small “break” around 200ºC even though no plateau as appeared in Figure 6 was observed. Prior to the “break” point the rate of the temperature increase in the core is slower than in the furnace air. After the “break”, a similar rate is observed even though the actual temperature in the core is lower. This indicates that an endothermic reaction is taking place at the “break” point, such as a massive evaporation. Obviously, the type of drying agent has strong influence on the rapid heat-up, which is related to the permeability and
To understand the mechanism of fast dry-out behaviour, a tabular alumina based microsilica-gel bonded NCC with and without EMSIL-DRY was
Figure 11: Drying rate as a function of sample temperature
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ENGINEER THE REFRACTORIES
September 2018 Issue
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