Technical Paper
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the purpose of this test the cool down rate does not really matter as it is the glassy phase that we are interested in.
As the AZS material cools down it will follow the blue line to about 1000°C. The slope between the alumina and zirconia are not exactly the same, but not vastly too different. This would imply that even if the AZS discs and alumina “crucible” were fixed to each other, stresses would be relatively low as the material cools down. At a 1000°C the glass phase between the AZS discs and the alumina could well be completely frozen resulting in a strong bond holding the two pieces together. However, between 1000°C and 900°C the Zirconia will expand significantly in comparison to the alumina that will steadily shrink. This massive difference in linear change will create very high stresses especially when the glass holding the pieces together is completely solidified and hard. A contributing factor that will increase the energy level is also the speed of cool down.
Figure 3: Top surface of the disc tested showing no signs of glassy phase exude
dissolved gasses which will also encourage the liquid phases to accumulate at the top of the sample.
Normally, the visual appearance of the exuded glass on the sample discs seems to be very viscous. The obvious reason why it tends to cause defects in the glass bath as it slowly runs down the sidewalls of the glass melter/ furnace superstructure ending up on the surface of the molten glass and causing glass quality defects. If the exuded glassy phase was very runny, it most likely would cause less glass quality defects as it will be quickly dissolved into the glass.
Considering our specific samples, it is clear that the surface of the samples were free of any glass phases (see Figures 3 and 4, also Figure 6 in the March 2017 issue), however, there was a major accumulation of white glass (and not transparent glass as is normally seen) on at the edges where it touched the alumina crucible. Taking all this into consideration, it is therefore possible that we are dealing with a different system than what we usually encounter and indicates that the composition of this glass was quite different to what is normally found.
During cool down the furnace is normally just switched off which, due to the high initial differential temperature, will cool the samples down much faster than the 50°C/hr rate that has been applied during heat-up. This is often found to cause excessive deterioration of the refractory linings in industrial units during cool down periods - a topic on its own. However, for
It is very unlikely that both sides of the disc that are in contact with the “crucible” will break at exactly the same time. The weakest side will break first causing a release of energy which will then flip the disc out of the “crucible”.
Why has this phenomena not occurred in the past? The main reason for it to happen this time was most likely due to the nature of the glass phase. Firstly it did not behave in a similar way than has been seen prior to this event. It accumulated at the contact points and froze completely before the turning point of the zirconia inversion.
Some years ago Monofrax introduced the optimised oxygen injection process to significantly reduce the amount of exudation of AZS materials. Is it possible that we have a new development to reduce exudation even further by evidence of samples jumping around in our furnace?
Figure 4: Bottom surface of the disc showing the lump of white glassy phase that accumulated at the contact point with the "crucible"
18 ENGINEER THE REFRACTORIES July 2017 Issue
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