Magnesium-carbon bricks for oxidation prevention
Magnesia-carbon bricks are composite materials consisting of magnesia and carbon. Graphite plays a key role in inhibiting slag penetration and providing erosion resistance, while resin-bonded carbon provides the structural strength of the bricks. However, the greatest weakness of both resin-bonded carbon and graphite is their susceptibility to oxidation. Therefore, since the introduction of magnesia-carbon bricks, the development of anti-oxidants has remained a major focus of research.
There are two primary pathways for carbon oxidation in magnesia-carbon bricks: first, oxidation of carbon by gaseous components; and second, oxidation by oxidizing components in the slag or steel. The oxidizing components in the slag or steel are primarily (FexO) and [O], among others. This oxidation occurs alongside the corresponding liquid-phase penetration into the magnesia-carbon bricks, as shown in Equations (1) and (2):
FexO+C→Fe+CO(1)
MnO+C→Mn+CO(2)
Antioxidants are substances that prevent the oxidation of graphite by both gaseous and liquid phases. Currently, the antioxidants used in magnesium-carbon bricks are primarily classified as metallic or non-metallic. Metallic antioxidants mainly include Al, Si, and Al-Mg, while non-metallic antioxidants primarily consist of B4C, ZrB2, and SiC.
Among the metallic anti-oxidants, aluminum powder is the most widely used. At high temperatures, it first reacts with carbon to form Al₄C₃, which then reacts with CO(g) and other substances. The specific mechanism of action is as follows:
4Al+3C=Al4C3 (3)
2Al+3CO=Al2O3+3C (4)
Al4C3+6CO=2Al2O3+9C (5)
Al2O3+MgO=MgO·Al2O3(6)
As metallic Al or Al₄C₃ participate in the reaction, the partial pressure of oxygen in the brick decreases, thereby protecting graphite and other components. The anti-oxidation mechanism of metallic Si is similar.
Metallic Al exhibits superior anti-oxidation performance, primarily due to two factors: first, the reduction in the partial pressure of oxygen in the magnesia-carbon brick as shown in Equations (3) and (4); second, the volumetric expansion effect of the reaction in Equation (6), which densifies the structure of the magnesia-carbon brick. At the same time, Equations (3) and (6) also contribute to the high-temperature flexural strength of magnesium-carbon bricks, which is why magnesium-carbon bricks mostly use metallic Al powder as an anti-oxidant; however, since Equation (3) is accompanied by a significant volumetric effect, the addition of metallic Al in magnesium-carbon bricks is generally kept below 3%. Metallic silicon exhibits a relatively minor volumetric effect during the anti-oxidation process; however, the SiO₂ generated by its oxidation leads to the formation of compounds such as M₂S (2MgO·SiO₂), which degrades the material’s high-temperature performance.
In addition to reacting with carbon to form SiC, metallic silicon powder can also form whisker-like SiC fibers, thereby enhancing strength. Consequently, a composite of metallic aluminum powder and silicon powder is generally used as the anti-oxidant for magnesium-carbon bricks. When designing new slag-line magnesium-carbon bricks, the addition of both metallic Al powder and Si powder as anti-oxidants results in a longer service life compared to traditional slag-line magnesium-carbon bricks. This study examines and discusses the microstructure of magnesium-carbon bricks containing Al and Si, and combines thermodynamic analysis to elucidate the anti-oxidation mechanism.
Regarding other metallic anti-oxidants, Mg-Al alloys are commonly used. Zhang Jin and Zhu Boquan incorporated Mg-Al alloy powder into low-carbon magnesia-carbon bricks as an anti-oxidant. The mechanism of action of the Mg-Al alloy is similar to that of Al, while Mg also accelerates the formation of a secondary periclase layer, significantly improving the anti-oxidation properties of the magnesia-carbon bricks.
Compared to metallic anti-oxidants, there has been a significant amount of research on non-metallic anti-oxidants in recent years, and these have also demonstrated excellent anti-oxidation performance. Major non-metallic anti-oxidants include B4C, ZrB2, MgB2, TiN, and SiC; however, compared to other anti-oxidants, SiC is relatively less effective. Non-metallic anti-oxidants (using B4C and ZrB2 as examples) undergo the following reactions in magnesium-carbon bricks:
B4C+6CO=2B2O3+7C (7)
ZrB2+5CO=ZrO2+B2O3+5C (8)
The B₂O₃ produced by the reaction will react with MgO and other substances to form a sealing layer, thereby preventing further oxidation of the magnesium-carbon bricks.
By measuring the relationship between carbon mass loss and temperature (1300 and 1500°C) and time (2, 4, and 6 h), the oxidation resistance of MgO-C refractory samples containing 0%, 1%, and 3% by mass of various anti-oxidants (Al, Si, SiC, and B4C) was compared. It was concluded that at 1300°C and 1500°C, B₄C is the most effective anti-oxidant, performing significantly better than the other three at 1500°C in particular. This is attributed to the formation of an impermeable, dense Mg₃B₂O₆ layer on the brick surface. Although SiC can also improve the oxidation resistance of MgO-C bricks, its effectiveness is comparatively lower. Experimental methods such as thermogravimetric analysis and X-ray diffraction confirmed that B4C undergoes oxidation during the sintering process below 1000°C, resulting in the formation of 3MgO·B2O3, which is stable at high temperatures.
When MgB₂ and other compounds were used as anti-oxidants in magnesium-carbon refractories and fired under carbon-buried and air atmospheres, the results showed that their anti-oxidation performance was inferior to that of B₄C but superior to that of Al powder and Si powder. It was also noted that the optimal addition of MgB₂ in magnesium-carbon refractories is approximately 3% by mass. Two types of magnesium-carbon brick specimens were prepared: one without additives and one containing 2% carbon-containing TiN. The slag erosion test results indicate that the slag erosion resistance of the TiN-added samples is significantly better than that of the additive-free samples. The primary reason TiN enhances the slag erosion resistance of magnesium-carbon bricks is as follows: in the reaction layer, the oxidation product of TiN, TiO₂, reacts with CaO in the slag to form CaTiO₃ with a melting point of 1970°C; in the decarburization layer, TiO₂ formed by the oxidation of TiN reacts with C, CaO, and MgO to produce CaTiO₃ and 2MgO. TiO₂, TiC, and Ti(C,N) solid solutions are all high-melting-point mineral phases that increase the viscosity of the slag, reduce its penetration, and thereby improve the slag erosion resistance of the magnesium-carbon bricks. Furthermore, when TiN (2% by mass), aluminum powder (1% by mass), and B₄C (0.5% by mass) are used in combination, the high-temperature flexural strength, oxidation resistance, and slag erosion resistance of the magnesium-carbon bricks are all significantly improved.
In recent years, anti-oxidants for magnesium-carbon bricks have increasingly favored composites of metals and non-metals. This approach addresses the issue of poor oxidation resistance in single anti-oxidants within certain temperature ranges, thereby leveraging the performance advantages of each component. The combination of metallic anti-oxidants with B₄C or MgB₂ has improved both oxidation resistance and resistance to slag erosion.
By combining metal Al, metal Si, SiC, and B4C as anti-oxidants in various configurations and holding the samples at 1400°C for 2 hours, the analysis concluded that the Al-Si composite anti-oxidant performed best. At high temperatures, SiC oxidizes later than C, whereas B₄C oxidizes before C. Although the oxidation product B₂O₃ is in the liquid phase, which helps seal the material’s pores, its melting point is only 450°C. This causes its evaporation rate to gradually increase, ultimately reducing the oxidation resistance of B₄C-containing materials. In low-carbon magnesium-carbon bricks, 3% Al and 1% TiO₂ were introduced as additives. The bricks were subjected to carbon-buried heat treatment at 1000°C and 1300°C, with four groups compared: no anti-oxidant added, 3% Al added alone, 1% TiO₂ added alone, and a composite of 3% Al and 1% TiO₂ added. The results showed that the combined introduction of Al and TiO₂ additives prevented the formation of Al₄C₃, helping to address the issue of easy hydration in magnesium-carbon bricks after carbon-buried treatment with Al powder alone. This group exhibited the highest compressive strength and the thinnest oxide layer among the four groups.
Regarding anti-oxidants, despite years of research, they remain a primary area of study for magnesium-carbon bricks.