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Technical Paper


www.ireng.org


Figure 10: Reflected light micrographs of a MgO-C refractory: (a) low magnification image and (b) higher magnification image of the area in the circle. MgO(F): fused magnesia, MgO(S): sintered magnesia, C: graphite, Al2O3: corundum, SiC: silicon carbide, Si: silicon and Al: aluminum. [21]


Refractories MgO-C Brick


MgO-C brick have been the most studied of all of the carbon bearing refractories. Innumerable articles discuss the different additives and the resulting reactions. The type and amount of the additives selected for use for a particular application will depend on the environment’s severity, the prevailing failure mode and required vessel life. The most common additions are Al and Si, followed by B4C and other borides and, SiC. Figure 10 shows an as received MgO-C brick containing Al, Si and SiC [21].


Dense zone formation is one of the results of the use of metals as an additive. A spinel dense zone, the result of an Al addition to a MgO-C brick, is shown in Figure 11.


Slag


Dense zone


also benefit from the addition of Si and Al in the presence of microsilica. This combination has been shown to reduce the apparent porosity by decreasing the pore size and distribution [23].


Al2O3-MgO-C (AMC) Brick


The reactions that pertain to this discussion are the recrystallization of the MgO grains from reduction with C which can coat any grain within the refractory increasing its slag and erosion resistance and the formation of spinel resulting from the use of additives, in particular, Al and Si. In addition, the Al forms Al4C3 or AlN which act as part of the binder increasing the strength of the refractory when in use [24].


Al2O3-SiC-C (ASC) Refractories


Unaltered brick


These can be brick or castable. The SiC in these refractories acts mainly as a structural element although it will also behave like an antioxidant. For use as an antioxidant, Si can also be added. Al can be present in brick formulations, but can be a drawback if added to a castable as will be discussed in the next section. The primary application of these castables is in blast furnace troughs were they are exposed to thermal cycling and iron/slag attack. The SiC is highly abrasion resistant, has high thermal conductivity which minimizes the thermal cycling stresses and, in its antioxidant role, it closes off the pores to slag attack.


Disadvantages of Additions Figure 11: Microstructure of a dense zone. [22] Al2O3-C Refractories


Alumina graphite refractories generally have additions of Si, SiC and/or B4C, although others have also been reported. These additions improve the oxidation resistance of the refractory body following the same mechanisms as described in MgO-C refractories: Si reacts with CO, a gas, forming SiO which leads to the deposition of C. The SiO can further react with CO forming SiO2. These reactions are accompanied by a volume expansion sealing pores.


Slide gate plates, made out Al2O3-C, and, submerged entry nozzles often contain B4C in addition to Si. This enhances their strength at temperature (Fig. 7) and also positively affects their oxidation resistance. Slide gate plates


So far, the discussion has centered on the benefits accrued as a result of metal additions, but Al additions can also be detrimental. If Al is added to MgO-C brick it can contribute to its hydration [25] after exposure to temperature according to


Al4C3 + 12H2O → 4Al(OH)3 + 3CH4 (g) (13)


In addition, the gas formed as a result of this reaction can lead to excessive pressure within the brick which results in cracking due to stress relief.


Al addition to Al2O3-SiC-C monoliths is very common. Its purpose is to aid in the removal of water during the firing of the refractory. The mechanism is based on the generation of hydrogen gas soon after placement as shown


2Al + 3H2O → Al2O3 + 3H2 (g) (14)


For the gas to exit it forms channels in the setting refractory which, later on, are reused by the water/steam for the same purpose.


This reaction can also compromise the monolith’s integrity as excessive 22 ENGINEER THE REFRACTORIES


May 2016 Issue


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