In-depth Analysis Of The Anti-oxidation Properties Of Magnesium Carbon Bricks

Magnesia carbon brick is a composite material of magnesia sand and carbon, in which graphite is the key to inhibit slag penetration and erosion resistance, while resin carbon constructs the structural strength of magnesia carbon brick; however, no matter the resin carbon or graphite, its biggest weakness is easy to oxidize. Therefore, antioxidants have been the hotspot and focus of research after the appearance of magnesium carbon bricks.

There are two main ways to oxidize carbon in magnesium-carbon bricks, one is the oxidation of carbon by gas-phase components, and the other is the oxidation of oxidized components in slag or steel. The oxidized components in the slag or steel are mainly (Fe_xO) and [O], etc.; this oxidation occurs along with the infiltration of the corresponding liquid phase into the magnesium-carbon brick, as shown in Eqs. (1) and (2): the oxidation of carbon in the magnesium-carbon brick by the gas-phase components of the slag or steel is mainly by the oxidized components in the slag or steel, such as (Fe_xO) and [O].

Fe_xO+C→Fe+CO(1)

MnO+C→Mn+CO(2)

Antioxidant is to prevent the oxidation of graphite in the gas and liquid phases. Currently used in magnesium carbon bricks antioxidants are mainly metal and non-metal. Metallic antioxidants mainly include Al, Si, Al-Mg, etc., while non-metallic ones mainly include B₄C,ZrB₂,SiC, etc.

The most widely used metal antioxidant is metal Al powder, which firstly reacts with carbon to generate Al₄C₃ at high temperature, and Al₄C₃ reacts with CO(g), etc. The specific mechanism is as follows:

4Al+3C=Al₄C₃ (3)

2Al+3CO=Al₂O₃+3C(4)

Al₄C₃+6CO=2Al₂O₃+9C (5)

Al₂O₃+MgO=MgO·Al₂O₃(6)

With the participation of metal Al or Al₄C₃ in the reaction, the partial pressure of oxygen in the brick decreases and graphite, for example, is protected. The mechanism of oxidation protection of metal Si is similar.

The antioxidant effect of metal Al is better, which mainly comes from two sources, one, the reduction of oxygen partial pressure in magnesium-carbon bricks by Eqs. (3)~(4); and two, the volume expansion effect of Eq. (6) reaction, which densifies the structure of magnesium-carbon bricks. And at the same time, Eq. (3) and Eq. (6) also accomplish higher high-temperature flexural strength of magnesium-carbon bricks, which is the reason why most of the magnesium-carbon bricks adopt the metal Al powder as the antioxidant; however, because the reaction Eq. (3) is accompanied by a large volumetric effect, the amount of the metal Al added in magnesium-carbon bricks is generally less than 3%. The volume effect of metal Si in the antioxidant process is relatively small, but metal Si reduces the high temperature performance of the material due to the generation of M₂S(2MgO·SiO₂) and so on from the SiO₂ generated by oxidation.

Metal Si powder in addition to reacting with carbon to generate SiC, but also can form whiskers loaded SiC fiber, thus enhancing the strength, therefore, as an antioxidant of magnesium-carbon bricks, generally are metal Al powder and Si powder composite. In the design of new slag line magnesium carbon bricks respectively added metal Al powder and Si powder as antioxidant, its service life is higher than the original traditional slag line magnesium carbon bricks. From the microstructural point of view of adding Al, Si and other magnesium carbon bricks to observe and discuss, and with thermodynamic analysis of anti-oxidation mechanism.

Regarding other metal-based antioxidants, commonly used are Mg-Al alloys and so on. Zhang Jin and Zhu Boquan added Mg-Al alloy powder as an antioxidant in low carbon magnesium carbon bricks, the mechanism of action of Mg-Al alloy is similar to that of Al, while Mg also accelerates the formation of secondary magnesite layer, which significantly improves the antioxidant property of magnesium carbon bricks.

Compared with metal antioxidants, non-metallic antioxidants have been studied more in recent years, and also show very good antioxidant performance. Non-metallic antioxidants mainly include B₄C,ZrB₂,MgB₂,TiN,SiC etc. However, compared with other antioxidants, the effect of SiC is relatively poor. Non-metallic antioxidants (B₄C and ZrB₂ for example) in magnesium carbon bricks will occur in the following reaction:

B₄C+6CO=2B₂O₃+7C (7)

ZrB₂+5CO=ZrO₂+B₂O₃+5C (8)

The B₂O₃ generated by the reaction will react with MgO, etc. to generate a blocking layer, which in turn prevents the continued oxidation of the magnesium-carbon bricks.

The oxidation resistance of MgO-C refractory specimens with the addition of 0, 1% and 3% mass fractions of antioxidants (Al, Si, SiC and B₄C) was compared by determining the loss of carbon mass as a function of temperature (1300 and 1500 °C) and time (2, 4 and 6 h), and it was concluded that B₄C was the most effective antioxidant at 1300 °C and 1500 °C, and especially at The effect was much better than the other three at 1500°C, which was attributed to the formation of an impermeable and dense Mg₃B₂O₆ layer on the brick surface.SiC, although it could also improve the oxidation resistance of MgO-C bricks, was less effective in comparison. Experimental methods such as thermogravimetric analysis and X-ray diffraction were used to confirm that the oxidation of B₄C occurs during the firing process at temperatures lower than 1000°C, resulting in 3MgO-B₂O₃ that is stable at high temperatures.

The application of MgB₂ and others as antioxidants to magnesium carbon refractories, calcined under buried carbon and air atmosphere, respectively, showed that the antioxidant effect was second to B4C and superior to Al and Si powders, and it was pointed out that the reasonable addition mass fraction of MgB₂ in magnesium carbon refractories was about 3%. Two kinds of magnesium-carbon brick specimens without additives and with 2% of carbon-containing TiN were prepared. The results of the slag erosion test show that the slag erosion resistance of the specimen with TiN is significantly better than that of the specimen without additives.The main reason for TiN to improve the slag erosion resistance of magnesium-carbon bricks is that: the oxidation product TiO₂ of TiN in the reaction layer reacts with the CaO in the slag to generate CaTiO₃ with a melting point of 1970°C; the TiO₂ formed in the oxidation of TiN in the decarburization layer reacts with C, CaO, MgO CaTiO₃, 2MgO, TiO₂, TiC, Ti (C, N) solid solution, etc. are high melting point mineral phases, which increase the viscosity of slag and reduce the penetration of slag, thus improving the resistance of magnesium carbon bricks to slag erosion. Moreover, when TiN (mass fraction, 2%), aluminum powder (mass fraction, 1%) and B₄C (mass fraction, 0.5%) were used in the composite, the high-temperature flexural strength, oxidation resistance and slag erosion resistance of magnesium-carbon bricks were significantly increased and improved.

In recent years, the magnesium carbon brick antioxidant is more inclined to metal and non-metal composite, to solve the single antioxidant at a certain temperature range of antioxidant performance is not good, in order to play the respective performance advantages of antioxidants. Metal antioxidants and B₄C or MgB₂ composite, so that the antioxidant and slag erosion resistance are improved.

Metal Al, metal Si, SiC and B₄C were used as antioxidants in different combinations, and the specimens were held at 1400°C for 2h, and the results were analyzed to show that the use of Al-Si composite antioxidants was the most effective. At high temperature, SiC is oxidized after C, and although B₄C is oxidized before C, and the oxidation product B₂O₃ is liquid phase, which is conducive to plugging the material pores, but the melting point of B₂O₃ is only 450 ℃, which makes its evaporation speed gradually accelerated, and ultimately reduces the antioxidant performance of the B₄C-containing materials. Introducing 3% of Al and 1% of TiO₂ as additives in low carbon magnesium carbon bricks, buried charcoal heat treatment at 1000 ℃ and 1300, divided into no antioxidant, 3% of Al alone, 1% of TiO₂ alone, composite addition of 3% of Al and 1% of TiO₂4 groups to do comparisons. The results show that the composite introduction of Al, TiO₂ additives to avoid the generation of Al₄C₃, is conducive to the improvement of magnesium carbon bricks in the separate quoted Al powder buried charcoal treatment is easy to hydrate the problem, the compressive strength of the four groups of the highest, the thickness of the oxidation layer is the smallest.

In terms of antioxidant, although it has been studied for many years, antioxidant is still the main research direction of current magnesium carbon bricks.

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