www.ireng.org behave in contact with slags and different atmospheres.


The alumina systems did not undergo this type of development until the need for improved slide gate refractories became of importance, alumina- magnesia-carbon brick technology was developed, alumina-silicon carbide- carbon refractories were installed in blast furnace runners and torpedo cars, etc. The development of their addition technologies was based on the knowledge acquired previously.


Refractories: The Reactions


Yamaguchi [4] and Rymon-Lipinski’s [5], work provided the early theoretical basis for determining which phases were expected to be formed as a result of the addition of Al, Mg or Si metals to a MgO-C brick as a function of atmospheres and temperatures. Table 1 shows the expected reactions with Zones I through V representing the transition from the refractory’s hot face to its unaltered cold face.


Microscopy work, carried out on BOF brick samples, recovered after tear outs, showed the presence of many of the predicted phases (Figure 1) confirming the theoretical work [6].


Technical Paper found that the Si addition had mostly become SiC after exposure to ~1500o C


Similarly, the reactions of Al, Si, SiC and Si3N4 additions to an Al2O3-SiO2- SiC-C brick or castable were studied [8, 9, 10, 11]. The following reactions were observed after exposure to 1200o


C 4Al (s,l) + 3C (s) → Al4C3 (s)


The carbide can then react with either N or CO: Al4C3 (s) + 2N2 (g)


Al4C3 (s) + 6CO (g) → 2Al2O3 (s) + 9C (s) → 4AlN (s) + 3C (s) (7)


(8) (9)


Lower temperatures of formation have also been reported to lead to the formation on these phases [12]. In addition, at 1500o generated had reacted to become a spinel (MgO Al2O3).


C, almost all the Al2O3


Microscopy work carried out on laboratory mixed, pressed and fired MgO-C brick samples showed the sequence of reactions 7 and 9 to occur (Figure 2). The core or pore was assumed to initially have been filled with Al.


Figure 1: Typical mineralogical zones in used BOF brick [6]


Calculations of the standard free energy of reaction for Si and SiC in MgO-C brick have been carried out and are shown below [7].They confirm that these phases can be formed in the temperature ranges of interest.


Si(s) + C(s) → SiC(s) ΔG = -53943 + 7.29T


Si(s) + O2 (g) → SiO2(s) ΔG = -902070 + 173.64T


3Si(s) + 2N2 (g) → Si3N4(s) ΔG = -731118 + 320.31T


3SiC(s) + 2N2 (g) → Si3N4(s) + 3C(s) ΔG = -569289.00+298.44T


SiC(s) + 2CO (g) → SiO2(s) + 3C(s) ΔG = -732587+356.23T


SiO2(s) + 2MgO(s) → Mg2SiO4(s) ΔG = -67200+4.31T


298K (25C)<T<1686K (1413C) (1) 700K (427C)<T<1700K (1427C) (2) 298K (25C)<T<1686K (1413C) (3) 298K (25C)<T<1686K (1413C) (4) 700K (427C)<T<1700K (1427C) (5) 298K (25C)<T<1700K (1427C) (6) Additional reactions have been noted. In particular around 1200oC Si can


react with CO to form SiC and, SiC can also react with CO to form SiO and then SiO2 adding to the steps shown in equations 1 and 5. This SiO2 can subsequently react to become forsterite in the presence of MgO (eq. 6). Other reactions leading to forsterite are also possible.


Experimental evidence for equation 1 was provided by Chan et al. [8] who May 2016 Issue


Figure 2: Microstructures of MgO-C brick with Al as antioxidant, fired for 3hrs at 1200o


shell, (b) a solid Al4C3 grain covered by an Al2O3 shell, (c) an Al2O3 shell containing Al particles, enclosing a pore [13]


ENGINEER THE REFRACTORIES 19


C. (a) a pore surrounded by an Al4C3 core then covered by an Al2O3


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