Technical Paper
REFRACTORIES ENGINEER
THE
Mullite Bonded Corundum Microstructure as supplied
Wt% Oxide
Na2 O
MgO Al2O3
SiO2 O
K2
CaO TiO2
Fe2O3 ZrO2
nd - not detected Figure 5. Frimul FX Back-scattered image DEPLETION OF SIO2 H2 can reduce SiO2 IN H2 REDUCING ENVIRONMENTS containing components in refractory compositions to
SiO(g) vapour phase from temperatures in excess of ~800°C; the reaction is as follows:-
SiO2 + H2 ¨ SiO(g) + H2 O The reaction affects all SiO2 containing materials and therefore in regard
to SRU hot face lining materials whether mullite or mullite bonded the phenomenon has to be assessed.
For high H2 mullite/mullite bonded materials will suffer SiO2 Whilst pure Al2 O3
concentrations (~13vol/%), it is acknowledged that all depletion.
is chemically the most resilient composition for H2
environments; in comparison with mullite it has poor thermo-mechanical stability and relatively high creep characteristics.
Samples of Frimul FX and a more traditional mullite bonded corundum material were placed on the working lining of an SRU (5 exposed faces) for a period of 2 years to investigate the interaction with H2
. The reaction
surface was examined by SEM in back scattered and elemental mapping modes, (Figures 7 and 8).
From Figures 6 and 7, the removal of SiO2 from the structure is clearly Figure 6. H2 Depletion of silica at front face (2 years exposure)
evidenced from the elemental mapping Si Ka1 images. The process could be isokinetic, that is the depletion of SiO2
1 - mullite
0.3 nd
77.9 21.8 nd nd nd nd nd
2 - mullite
nd nd
75.3 24.4 nd nd
0.3 nd nd
3 -
silicate bond
3.9 0.3
15.0 76.1 2.8 1.2 nd nd
0.9 4 -
silicate bond
3.6 nd
16.0 75.3 2.4 1.1 0.4 nd
1.2 5 -
nd nd
100 nd nd nd nd nd nd
6 - alumina alumina
nd nd
100 nd nd nd nd nd nd
continues at a constant rate without any barriers to the “speed” of the process.
If this is the case then Frimul FX is affected to a depth of ~1mm per annum which is similar to the traditional mullite bonded corundum compositions.
Following consideration by industry experts this affect is noted but not considered as significant due to acknowledgment that a 25 year campaign would potentially affect only 25mm of refractory; even in this hypothesis the load bearing stability portion of the refractory would not be compromised.
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16
November 2019 Issue
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