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Technical Paper


Figure 23: Microstructure and EDS analysis of C5S26 sintered at 1650°C for 5 h. EDS analyses given in table 2.


Spot 1 2 3 4 5 6 7


Oxides Wt % Mole %


Al2 O3


MgO Al2


O3


Al2O3 CaO


Al2O3 CaO


MgO Al2


O3


MgO Al2


O3


MgO Al2


CaO O3


100 17.7 82.2 91.0 8.9


91.2 8.7


17.0 82.9 17.5 82.4 4.9


87.9 7.1


100


35.3 64.6 84.7 15.2 85.1 14.8 34.2 65.7 34.9 65.0 10.9 77.6 11.4


Intensity (count)


17,889 3,335 13,035 16,286 1,205


16,345 1,177 3,124


12,897 3,261


12,978 871


14,916 938


Table 4: EDS analysis of C5S26 sintered at 1650°C for 5 h


Possible Phase


Corundum MA spinel CA6 CA6 MA spinel MA spinel C2 M2 A14


Figure 26: Microstructure of A4S26 sintered at 1650°C for 5 h


5. Conclusion The thermodynamic analysis of the calcia containing system Al2


CaO by FactSage, and the calcia-free system Al2 O3


O3


-MgO- -MgO in contact with


iron oxide slag created during the oxygen lancing of purging plug, shows that the calcia-free system provides advantages with regard to melting temperature and the amount of liquid phase formed at a given temperature. A mixture of 50 wt% refractory matrix and 50 wt% iron oxide slag starts melting at 1665°C for the calcia-free system and at 1575°C for the calcia containing system. The calcia-free mixture has no liquid at 1600°C, but the calcia containing mixture has 28 wt% liquid phase at this temperature. It can be concluded, that the calcia-free system provides higher corrosion resistance against the iron oxide slag and would be advantageous in this regard when compared to the calcia containing system.


Cement bonded castables show clear differences when compared to hydratable alumina no-cement bonded castable with respect to mechanical properties and fracture behaviour. The strength as determined by CMoR, CCS, and HMoR at 1500°C is clearly higher for the cement bonded material. The fracture behaviour as tested with the wedge splitting method shows the no-cement castable to be more ductile and less brittle. The specific fracture energy and nominal notch tensile strength are lower and characteristic length lch


is higher. Also in the usual thermal shock resistance testing by


water quenching, the no-cement material shows lower relative strength loss after one and three quenching cycles.


The pre-firing temperature has a clear influence on the strength development and the fracture behaviour of the materials. High pre-firing temperatures of 1500°C or above enable strong sintering reactions especially in the cement bonded materials, leading to new phase formation in the matrix and different degrees of interlocking between aggregate grains and matrix. It is most pronounced for the spinel containing cement bonded material due to the ternary phase C2


M2 A14


growing from the matrix into the spinel


grains. This would explain the higher hot strength of spinel containing castables when compared to pure corundum castables. With regard to fracture behaviour and resistance against crack propagation, the higher porous, lower strength microstructure of the hydratable alumina castable retains its general characteristic of lower brittleness when compared to the cement bonded materials.


Figure 24: XRD analysis on C5S26 (pre-fired at 1650°C for 5 h)


From a technical point of view with regard to the application of purging plugs in steel ladles, both binder concepts have advantages and disadvantages. The established solution with spinel containing low and ultra-low cement castables clearly has higher hot strength, which is favourable for the required erosion resistance during intensive stirring. Regarding the spalling resistance due to thermal cycling, the no-cement system could provide advantages if the overall strength level is high enough for the application. Such hydratable alumina bonded materials may require pre-firing temperatures of beyond 1000°C (e.g. 1250°C) in order to prevent the low strength gap around 1000°C. Otherwise it may lead to cracking in the body of the purging plug, where temperatures are too low for real sintering reactions.


Figure 25: Microstructure of C5S0 sintered at 1650°C for 5 h


For cement bonded systems, it is not easy to make a judgement based on the current results if drying at temperatures around 400°C or pre-firing at e.g. 1600°C is the best overall solution for the performance of the purging plugs in operation. The fully established, highly sintered microstructure may have advantages for the erosion resistance, but it may be counterbalanced


July 2018 Issue ENGINEER THE REFRACTORIES 23


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