Let's Make a Mullite Matrix - Elkem

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$ageing of refractory castables; which interactions exist and ... - Elkem$

In this picture, the experimental set-up is shown. The sample is the cylinder placed Castables based on white fused alumina: between the two lightly yellow alumina discs on top of the set up. Above the sample is the furnace, which is lowered onto the sample and the load is regulated by a set-up with counter weights. It is seen that this sample has been tested to a temperature with significant subsidence, due to the slight drum shape the cylinder attains during testing with deformation. As is the case with hot-M.O.R. measurements, the results can be vastly different depending on thermal history, and unless pre-firing conditions are given, the interpretation of the test may in some cases be very difficult or close to meaningless. In the following section, the effect of microsilica and cement content on mullite formation will be presented for our model system based on white fused alumina. All aggregates are a high grade, white fused alumina. Together with calcined/reactive alumina, cement and microsilica, the castables were composed to have the same particle size distribution and water addition. To keep the particle size distribution as constant as possible, microsilica and reactive alumina with similar PSD were substituted for each other. More results, incl. recipes can be found in earlier papers 6, 8, 9 downloadable through www.refractories.elkem.com. The compositions are given in the Appendix, Tables 1 and 2, and are based on data from Ref. [8]. Some of the results (Figures 4, 7 and 8) are taken from earlier investigations (1997-99) and some (Figures 5, 6, 9, and 10) are recent (2007) remakes of these mixes. As some of the original ingredients are no longer available, these have been replaced by recent replacement materials. As the chemistry and PSD is very similar, it is considered justified to compare the new results (obtained in 2007) with the old data (from 1997-99). 8 B. Myhre and Aase M. Hundere: “Substitution of Reactive Alumina with Microsilica in Low Cement and Ultra Low Cement Castables. Part I: Properties Related to Installation and Demoulding” in Proc. UNITECR´97, New Orleans, USA, Nov. 4-7 1997, p. 43-52 9 Aa. M. Hundere and B. Myhre:” Substitution of Reactive Alumina with Microsilica in Low Cement and Ultra Low Cement Castables, Part II: The Effect of Temperature on Hot Properties.” Proc. UNITECR’97, New Orleans, USA, Nov. 4-7, 1997 p. 91-100 <strong>Elkem</strong> Materials – page 7
Low cement castables Hot M.O.R. (MPa) 30 25 20 15 10 5 0 1100 1200 1300 1400 1500 1600 Temperature (C) 8wt% MS 6wt% MS 4wt% MS 2wt% MS Figure 4: Hot-M.O.R. of low cement (6wt% cement), fused alumina-based castables as a function of temperature. Castables with different amounts of microsilica. 24 hours at temperature. q=0.25, max. particle size 4mm, 13 vol% water for casting (3.8- 4.2wt%) From Ref.[7] In Figure 4, results from a low-cement castable with 6wt% cement are shown for several different microsilica contents. We see that at 1400°C, strength increases as the microsilica addition increases. Particularly, levels of 6 and 8wt% microsilica seem beneficial to strength. It should be noted that mullite was not detected 7 at 1400°C unless 6 or 8% microsilica was used. With 6wt% microsilica only 1wt% mullite was found, which is in good correlation with the reactions and consequences described earlier in this paper. The peritectic liquid, being approximately 11wt% of the castable may, at 1400°C, be either highly viscous or even partially crystalline, which explains the strengthening effect seen at 1400°C. At 1500°C, the peritectic composition melts and starts attacking the mullite bond, with more or less total strength loss as result. In Figure 5, the effect of pre-firing of a low-cement castable on R.U.L results is seen. In this case we see that prefiring at high temperatures lowers the onset of “final subsidence”. Based on the findings of the sample prefired at 1000°C, one could be led to believe that the castable would tolerate a temperature of 1600°C. This is however difficult to understand if the hot- M.O.R. results shown in Figure 4 are taken into consideration, and even more if the phase diagram is consulted. The reason for this decline in refractoriness by the pre-firing, can be explained in the following manner: The mullite bond establishes so rapidly that the bond phase does not reach equilibrium. Then as equilibrium slowly commences, the bond phase is attacked and slowly dissolved by the lime-containing liquid; probably only partly, but at least sufficiently to allow a rapid subsidence at 1500°C. <strong>Elkem</strong> Materials – page 8
Page 1 and 2: Let´s Make a Mullite Matrix! Publi
Page 3 and 4: leading to softening and refractory
Page 5 and 6: Figure 3: The phase diagram Al2O3-S
Page 7: Examples: In this part of the paper
Page 11 and 12: microsilica, due to placing propert
Page 13 and 14: Hot M.O.R. (MPa) 30 25 20 15 10 5 0
Page 15 and 16: The characteristics of these castab
Page 17 and 18: hot-MOR [MPa] 35.00 30.00 25.00 20.
Page 19 and 20: Expansion [%] 2 1 0 -1 -2 0 500 100
Page 21: Table 3: Bauxite-based castable com

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