Technical Paper


Much higher magnification shows the Al addition in a MgO-C brick coked in air to 1000o


after the temperature was increased to 1200o smooth and amorphous (Figure 3: A & B)


www.ireng.org antioxidants as it becomes a liquid which fills pores. At ~1000oC B4C will


C to form Al4C3 whiskers [14], which remained present even C. In the process they become


react with CO or N according to B4C (s) + 6CO or


→ 2B2O3 (l) + 7C (s) B4C (s) + 2N (g) → 4BN (s) + C (s) (10) (11)


When B4C is added to an MgO-C refractory it reacts with CO to form B2O3 which, in the presence of MgO, forms the low melting point 3MgO B2O3 phase.


The high temperature effect of Si as an antioxidant can be enhanced with the addition of a small amount of B4C. This is shown in Figure 5 for an Al2O3-SiO2-SiC-C castable containing 10% carbon [15].


(A)


Figure 5: Effect of adding Si to B4C on the rate of oxidation after firing for 3 hours in an oxidizing atmosphere [15]


(B)


Figure 3: MgO-C brick with Al addition coked in air: (A) after 1000o whiskers, (B) after 1200oC mostly amorphous whiskers [14]


C Al carbide


Under oxidizing conditions SiC can react to form SiO2 (eq. 5) which can be observed as a SiO2 rim surrounding SiC grains protecting them from further oxidation. Similarly Si can oxidize to SiO2 and surround its source metal (Figure 4).


Several borides have also been used to prevent oxidation. A commonly added one is ZrB2, which can be found in submerged entry nozzle slag line material. At temperatures around 650o to


C or greater it dissociates according ZrB2 (s) + 5 CO (g) → ZrO2 (s) + B2O3 (l) + 5 C (s)


Effect on Properties Porosity


The effect of metallic additions to the refractory’s porosity was determined early on. In general, the mechanism is the oxidation of the Al and/or Si which results in the in-situ formation of their carbides (Al4C, SiC). This leads to a finer pore structure thereby decreasing the refractory’s permeability. Among the studies, Sadrnezhaad et al.[7] reported experimental results from studying MgO-C refractories. They found that samples without antioxidants or those with only 1% Si or Al, all behaved in a similar manner. On the other hand, if the Si level was increased to 3% to 5%, and the refractory heated to between 1000°C to 1100°C, then the open porosity decreased as a result of the formation of silica containing phases. This led to a lowering of the ability of gas to diffuse through the refractory sample thereby decreasing its oxidation rate. At temperatures > 1500o apparent porosity increased again [8].


C, the


Figure 4: Micrograph of MgO-C brick with Si showing formation of an SiO2 rim covering SiC or unreacted Si [13]


C, but it is not a stable phase as, with increasing temperature, it will dissociate back to Al2O3 and SiO2 and the latter can further react to form SiC [8].


The presence of SiO2 and Al2O3 can lead to the formation of mullite at ~1500o


B4C, a common carbide 20 additive, does not behave like traditional ENGINEER THE REFRACTORIES May 2016 Issue


Another study showed the effect of varying the levels of antioxidants, Al and B4C, added to a MgO-C refractory. The results are shown in Figure 6 for tempered and coked porosity where T-12 had no antioxidants, while T-13, T-14 and T-15 had ever increasing amounts of Al and decreasing amounts of B4C while keeping the carbon, types and amount, and the MgO levels constant [16].


(12)


ZrB2 often contains some SiC. The presence of this SiC, or its addition, improves the ZrB2 oxidation resistance at temperatures above ~1100o


C


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36