Technical Paper
mullite can be detected in LCC-7 at 1500ºC, only corundum.
Based on both XRD and SEM characterization, it is clear why MSZ-7 shows the best thermo- mechanical properties at high temperatures and why LCC-7 collapses towards 1500°C (See Fig. 4 and Fig. 6). Obviously, if the microsilica content is too low or the cement content too high, mullite crystals will not form. As a consequence, instead of a strong and robust bond, liquid is formed which causes catastrophic lowering of hot-strength.
3.5 Explosion resistance of microsilica-gel bonded NCC
Drying behaviour of a microsilica-gel bonded bauxite NCC using SioxX- Zero cured at 20°C was reported in a recent paper10
. Industrial-scale
specimens, 800x600x200mm blocks (~300kgs), were produced and dried using two different rapid drying schedules. A block was heated according to a schedule holding at 160°C for 6hrs, then heated at a rate of 75°C/ hr from160°C to 850°C. During this rapid heat-up the water vapour pressure caused complete disintegration since the block still contained more than 40% of the total water after the holding period at 160°C. Another block was perfect after heating according to a schedule where it was kept at 220°C for 10hrs before continuing to 850°C at a rapid rate of 100°C/hr. Approximately 97% of the free water is removed at 220°C so this block was essentially dry when the heat-up started. Obviously, it is critical to remove the free-water before rapid heat-up.
In this paper, in order to further improve the drying behaviour of microsilica-gel bonded NCCs, the effects of a speciality fast-drying product (EMSIL-DRY) on explosion resistance have been studied using both laboratory-scale specimens and industrial-scale blocks. Table 2 shows the explosion test results of both “wet” and “dried” specimens of MSZ-7 with and without EMSIL-DRY tested according to Chinese Standard YB/T4117-2003. The specimens 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”.
As shown in Table 2, all “dried” samples showed excellent explosion resistance and passed 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 MSZ-7 with EMSIL-DRY, which passed the test at 450°C. Without EMSIL-DRY, the specimens only survived the test at 275°C. This indicates that EMSIL-DRY causes a much faster dewatering.
Industrial-scale explosion resistance testing were also carried out for MSZ-7 with and without EMSIL-DRY. The dimensions of the blocks were 600x600x400mm (~400kgs). The blocks were demoulded after 24hrs
curing at room temperature, and then put into the oven. A rapid heating schedule was used with heat-up from 20 to 850°C, at a rate of 50°C/hr, then cooling from 850 to 20°C at a rate of 50°C/hr. The block without EMSIL-DRY disintegrated whereas the one with EMSIL-DRY was perfect, as shown in Fig. 10.
This demonstrates that the explosion resistance/drying behaviour of microsilica-gel bonded NCC was significantly improved by adding EMSIL-DRY. EMSIL-DRY contributes to fast dewatering during firing and consequently improves explosion resistance. It indicates that rapid heating in an industrial environment is possible. The mechanism is further investigated, and more results will follow.
4 CONCLUSIONS
Based on our study comprising flowability, setting-behaviour, hot- properties, XRD and SEM characterisation of microsilica-gel bonded NCCs, ULCC and LCC, as well as explosion resistance of microsilica-gel bonded NCC with and without EMSIL-DRY, the following progress in microsilica-gel bonded NCC technology is reported:
• Compared to the silica-sol bond system, microsilica-gel bonded NCCs not only entail easier handling, storage and transportation thanks to the “all-in-the-bag” solution, but also exhibit improved setting behaviour, adequate green strength and slightly improved thermo- mechanical properties.
• Compared to a low cement bond system, microsilica-gel bonded NCCs with SioxX-Zero exhibits excellent hot-properties due to mullite formation.
• Mullite formation is essential to give excellent thermo-mechanical properties of microsilica-gel bonded NCCs. If the microsilica content is too low, or the cement content too high, mullite crystals will not form. Instead of a strong and robust bond, liquid forms which causes dramatic lowering of hot-strength.
REFRACTORIES ENGINEER
THE
Fig. 10: ~400kgs blocks of MSZ-7 after explosion testing at 850°C A) without and B) with EMSIL-DRY
√: passed; x: failed
Table 2: Explosion resistance of MSZ-7 with and without EMSIL-DRY
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September 2019 Issue
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