1.Magnesia sand, graphite selection
Magnesia sand as the main raw material has an important influence on the performance of magnesia carbon brick. Increasing the MgO content enhances the direct bonding of the periclase in the magnesia. Controlling C/S can reduce the amount of silicate and reduce the degree of division of the periclase by the silicate phase. Therefore, magnesium content and calcium to silicon ratio are important indicators for measuring magnesia (see Table 1)
|Phase combination and solidification temperature of different CaO / SiO2 in MgO-CaO-SiO2 system|
It can be seen from the above that it is suitable when the calcium to silicon ratio is less than 1 or greater than 2. Secondly, the high calcium to silicon ratio is beneficial to improve the stability of magnesium coexisting with graphite at high temperatures.In addition, the bulk density of magnesia, the size of the magnesite grains has a great influence on the corrosion resistance of magnesia carbon bricks. Song Shengzhao et al. made magnesia carbon bricks with different grain diameters of periclase into magnesia carbon bricks, and determined the high temperature. The weight loss results under the reducing atmosphere show that the larger the magnesite grains, the smaller the weight loss. Therefore, in the production of high-performance magnesia carbon bricks, CaO/SiO<1 or CaO/SiO> 2 should be selected, and fused magnesia with high bulk density and good crystal form should be used as the main raw material. The selection of 95% to 96% high-purity graphite is the most effective means to improve the durability of magnesia carbon bricks. However, it must be noted that high-purity graphite produces less liquid phase, and oxygen easily penetrates into the brick to oxidize and decarburize graphite. It is necessary to add antioxidants such as Al, Si, Al-Si, Al-Mg, SiC, BN, etc., which preferentially react with oxygen to form carbides and oxides, and expand in volume, block or fill the pores to densify the bricks, Thereby improving the oxidation resistance of the product.
2.Binding agent selection
(1)The choice of binder is the key to the production of magnesia carbon brick. The good binder should have the following conditions: (1) Good wettability, suitable viscosity and certain fluidity for MgO particles and graphite.
(2)The residual carbon ratio is high, and a continuous network carbon chain structure is formed after carbonization.
(3) Less water.
(4) The content of free phenol is small, and thermosetting phenolic resin is often used in production, and its physical and chemical indicators are as follows (see Table 2).
|Physical and chemical indicators of phenolic resin|
|Carbon residue||Solid content||Moisture||Free phenol||Viscosity||Ash||PH|
3.Metal antioxidant additive
In order to compensate for the shortcomings of graphite’s poor oxidation resistance, a certain amount of antioxidant should be added. By studying the action principle of anti-oxidation additives, the condensed phases SiC, Si, Si3N4, SiO2, C and Si2N2O, Mg, Al and B4C in Si, C, N, O and Mg, Al and B4C, respectively. Comparison of thermodynamic and kinetic analyses of BN and other reactions with the Mg-C system. Al and Si are additives for the production of magnesia bricks, which can significantly improve the oxidation resistance and high temperature flexural strength of the products. The principle is as follows: First, the affinity of Al and Si to oxygen is greater than C, which can inhibit the oxidation of C. Second, under the high temperature condition, the following different reactions occur under different atmospheres, different temperatures and different partial pressures:
4Al + 3C = Al4C3
2Al + 3CO = Al2O3 + 3C
Al4C3 + 6CO = 2Al2O3 + 9C
2Al + 3MgO = Al2O3 + 3Mg
Al2O3 + MgO = MgAl2O4
Si + C = SiC
Si + CO = SiO2 + 2C
SiC + CO = SiO + 2C
SiC + 2CO = SiO2 + C
These reactions are accompanied by volume expansion to close the pores, and magnesium-aluminum spinel is formed at a high temperature to form a partial ceramic binding phase, which improves strength and slag resistance, inhibits weight loss, and enhances oxidation resistance.