Magnesia carbon brick is a composite material of magnesia sand and carbon, among which graphite is the key to inhibit slag penetration and corrosion resistance, while resin carbon builds the structural strength of magnesite carbon brick; but both resin carbon and graphite have the biggest weakness of being easily oxidized.
There are two main ways of carbon oxidation in mgo 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 slag or steel are mainly (FexO) and [O], etc.; this oxidation occurs with the infiltration of the corresponding liquid phase into the magnesium carbon brick, as shown in formula (1) and formula (2):
FexO+C→Fe+CO(1)
MnO+C→Mn+CO(2)
Antioxidants are used to prevent the oxidation of graphite by gas phase and liquid phase. At present, the antioxidants used in magnesia carbon bricks are mainly metal and non-metal. Metal antioxidants mainly include Al, Si, Al-Mg, etc., while non-metal antioxidants mainly include B4C, ZrB2, SiC, etc.
Among metal antioxidants, the most widely used is metal Al powder, which first reacts with carbon at high temperature to form Al4C3, and Al4C3 reacts with CO(g) and the like. 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 metal Al or Al4C3 participates in the reaction, the oxygen partial pressure in the brick decreases, and graphite and the like are protected. The anti-oxidation mechanism of metal Si is similar.
The anti-oxidation effect of metal Al is relatively good, which mainly comes from two points. First, the reduction of oxygen partial pressure in magnesium carbon bricks by formula (3)~(4); second, the volume expansion effect of the reaction of formula (6) makes the structure of magnesium carbon bricks dense. At the same time, equations (3) and (6) also achieve the high high-temperature flexural strength of MgO-C bricks, which is why most MgO-C bricks use metal Al powder as an antioxidant; however, since reaction equation (3) is accompanied by a large volume effect, the amount of metal Al added to Magnesia carbon bricks is generally less than 3%. The volume effect of metal Si in the anti-oxidation process is relatively small, but metal Si generates M2S (2MgO·SiO2) due to oxidation of SiO2, which reduces the high-temperature performance of the material.
In addition to reacting with carbon to generate SiC, metal Si powder can also form whisker-like SiC fibers to enhance strength. Therefore, as an antioxidant for MgO-C bricks, metal Al powder and Si powder are generally used in combination. When designing a new slag line MgO-C brick, metal Al powder and Si powder are added as antioxidants, and their service life is longer than that of the original traditional slag line MgO-C bricks. From the perspective of microstructure, MgO-C bricks with added Al, Si, etc. are observed and discussed, and the anti-oxidation mechanism is analyzed in conjunction with thermodynamics.
Regarding other metal antioxidants, Mg-Al alloys are commonly used. Zhang Jin and Zhu Boquan added Mg-Al alloy powder as an antioxidant to low-carbon magnesium carbon bricks. The mechanism of action of Mg-Al alloy is similar to that of Al, and Mg also accelerates the formation of secondary periclase layer, significantly improving the oxidation resistance of magnesium carbon bricks.
Compared with metal antioxidants, non-metal antioxidants have been studied more in recent years and have also shown very good antioxidant properties. Non-metal antioxidants mainly include B4C, ZrB2, MgB2, TiN, SiC, etc., but compared with other antioxidants, the effect of SiC is relatively poor. Non-metal antioxidants (taking B4C and ZrB2 as examples) will undergo the following reactions in magnesium carbon bricks:
B4C+6CO=2B2O3+7C (7)
ZrB2+5CO=ZrO2+B2O3+5C (8)
The B2O3 generated by the reaction will react with MgO and others to form a blocking layer, thereby preventing the continued oxidation of magnesium carbon bricks.
By measuring the functional relationship between carbon mass loss and temperature (1300 and 1500℃) and time (2, 4 and 6h), the oxidation resistance of MgO-C refractory samples with 0, 1% and 3% antioxidants (Al, Si, SiC and B4C) added by mass fraction was compared. It is believed that B4C is the most effective antioxidant at 1300℃ and 1500℃, especially at 1500℃, the effect is much better than the other three, because an impermeable and dense Mg3B2O6 layer is formed on the surface of the brick. Although SiC can also improve the oxidation resistance of magnesia carbon bricks, the effect is worse in comparison. Experimental methods such as thermogravimetric analysis and X-ray diffraction confirmed that B4C oxidized during the firing process below 1000℃ to obtain 3MgO·B2O3 that is stable at high temperature.
MgB2 and other antioxidants were used in magnesia carbon refractory materials. They were calcined in carbon buried and air atmospheres. The results showed that the antioxidant effect was inferior to B4C and better than Al powder and Si powder. It was pointed out that the reasonable addition mass fraction of MgB2 in magnesia carbon refractory materials was about 3%. Two mgo-c brick samples without additives and with 2% carbon-containing TiN were prepared. The results of the slag erosion resistance test showed that the slag erosion resistance of the sample with TiN was significantly better than that of the sample without additives. The main reason why TiN improves the slag erosion resistance of magnesite carbon bricks is that the oxidation product TiO2 of TiN in the reaction layer reacts with CaO in the slag to form CaTiO3 with a melting point of 1970℃; TiO2 formed by oxidation of TiN in the decarburized layer reacts with C, CaO, and MgO to form CaTiO3 and 2MgO. TiO2, TiC, Ti(C, N) solid solution, etc. are all high melting point mineral phases, which increase the viscosity of the slag and reduce the penetration of the slag, thereby improving the slag erosion resistance of magnesium carbon bricks. Moreover, when TiN (mass fraction, 2%), aluminum powder (mass fraction, 1%) and B4C (mass fraction, 0.5%) are used in combination, the high temperature flexural strength, oxidation resistance and slag corrosion resistance of mgo-c bricks are significantly improved.
