orderForm.noItems 3.2 Dry-out behaviour

To understand the drying behaviour, larger specimens, 300mm cubes of NCC-1(B) (~80kg) and 800x600x200mm blocks (~300kg), were produced and dried at 160ºC and 220ºC with various holding time. They were all intact after drying. The 300kg blocks released water much slower than the 75kg cubes as seen in Figure 3. When the holding time at 160°C was 10 hours, the 300kg block still contained more than 40% of the water. However, when keeping the block at 220°C for 10 hours, as much as 97% of the total water was removed.

Technical Paper

Figure 6: Temperature development as a function of time

approximately 5 hours the source of the endothermic process is depleted, and the temperature increases again and eventually attains the furnace temperature.

3.3: Explosion resistance Figure 4: Heat-up schedules

To look at various heat-up schedules, more industrial-scale blocks (300kg) were produced and dried using two different heat-up schedules, as shown in Figure 4. Schedule-A is holding at 160°C for 6 hours before continuing to 850°C at a rate of 75°C/hr. In schedule-B the block is kept at 220°C for 10 hours before continuing to 850°C at the rapid rate of 100°C/hr.

When the block was heated from 160°C to 850°C at a heating rate of 75ºC/hr, an increasing vapour pressure in the core of the block eventually exceeded the mechanical strength of the castable resulting in a complete disintegration of the block, as shown in Figure 5A. When a block was kept in the furnace at 220ºC for 10 hours, ~97% water was removed (Figure 3). The heating from 220 to 850ºC at a rate of 100ºC/hr caused no problem at all; a perfect block was produced (Figure 5B). This demonstrates that the removal of free water during the ebullition stage is the key to avoid/inhibit spalling and/or explosion (as indicated in Figure 1).

Microsilica-gel bonded NCCs (castable NCC-2 in Table 1) were used to further investigate the drying behaviour. Both lab-scale and industrial-scale explosion resistance tests were conducted. Table 3 shows the lab-scale explosion test results of both “wet” and “dried” samples tested according to Chinese Standard YB/T4117-2003. The samples were cured for 24hrs at room temperature and 100%RH before de-moulding. The freshly de- moulded samples are labelled “wet” and samples further dried for 24 hrs at 110°C are called “dried”.

Wet (20°C/24hrs)

Temp. (°C)

300 350 400 500 600 1000 1200 Without √ x


√ √ √ √ x √ √ √ √

√: passed; x: failed Table 3: Explosion resistance of microsilica-gel bonded NCC-2

Figure 5: 300kg blocks after rapid heat-up A) Drying schedule-A and B) Drying schedule-B

Figure 6 shows the temperature as a function of time for both the centre of the furnace and the core of the perfect 300 kg block. Surprisingly, a temperature plateau at ~180ºC was observed in the core of the block. This indicates that an endothermic process is on-going, e.g. boiling of water, decomposition of some hydrate or some other endothermic process. After

September 2018 Issue

All “dried” samples show excellent explosion resistance and pass the test at 1200°C. The good performance is attributed to a stable bond phase and the low amount of residual water in the bond phase. When the “wet” samples were tested, good explosion resistance was achieved for the microsilica- gel bonded NCC containing anti-explosion agents. Without anti-explosion agent, the specimens only survived the test at 300°C, and exploded at 350°C. The one with EMSIL-DRY exhibited the best explosion resistance. It passed the test at 500°C, whereas the one with Fiber-P2 exploded. This indicates that EMSIL-DRY causes the fastest dewatering of the NCC samples.

To further improve and understand the explosion resistance, a series of ~80kg (300mm) cubes were made. The cubes were cured at room

ENGINEER THE REFRACTORIES 19 √ √ Fiber-P2 √ √ √ x Dried (110°C/24hrs) Without



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