The main raw materials of MgO-C bricks include fused magnesia or sintered magnesia, flake graphite, organic binders and antioxidants.
Magnesia is the main raw material for the production of MgO-C bricks. There are fused magnesia and sintered magnesia. Compared with sintered magnesia, fused magnesia has the advantages of coarse periclase crystal grains and large particle bulk density. It is the main raw material used in the production of magnesia-carbon bricks.
The production of ordinary magnesia refractory materials mainly requires high temperature strength and erosion resistance for magnesia raw materials. Therefore, attention should be paid to the purity of magnesia and the C/S ratio and B2O3 content in the chemical composition. With the development of the metallurgical industry, the smelting conditions are becoming more and more severe, and the magnesia used in the MgO-C bricks used in metallurgical equipment (converter, electric furnace, ladle, etc.), except for the chemical composition。 In terms of organizational structure, high density and large crystallinity are also required.
Whether in traditional MgO-C bricks or low-carbon MgO-C bricks used in large quantities. It mainly uses flake graphite as its carbon source. Graphite, as the main raw material for the production of MgO-C bricks, mainly benefits from its excellent physical properties:
① Non-wetting to slag.
②High thermal conductivity.
③ Low thermal expansion.
In addition, graphite and refractory do not eutectic at high temperature, and the refractoriness is high. The purity of graphite has a great influence on the performance of MgO-C bricks. Generally, graphite with a carbon content greater than 95%, preferably greater than 98%, is used.
In addition to graphite, carbon black is also commonly used in the production of magnesia-carbon bricks. Carbon black is a highly dispersed black powdery carbonaceous material obtained by thermal decomposition or incomplete combustion of hydrocarbon-like compounds. Carbon black has small particles (less than 1um), large surface area, carbon mass fraction of 90~99%, high purity, high powder resistivity, high thermal stability, and low thermal conductivity. It is difficult to graphitize carbon. The addition of carbon black can effectively improve the spalling resistance of MgO-C bricks, increase the amount of residual carbon, and increase the density of bricks.
The commonly used binders for the production of MgO-C bricks are coal tar, coal pitch and petroleum pitch, as well as special carbonaceous resins, polyols, pitch-modified phenolic resins, synthetic resins, etc. The binders used are of the following types:
1) Asphalt substances.
Tar pitch is a thermoplastic material, which has the characteristics of high affinity with graphite and magnesium oxide, high residual carbon rate after carbonization, and low cost. It has been widely used in the past. However, tar pitch contains carcinogenic aromatic compounds, especially benzoβ content; due to the strengthening of environmental awareness, the use of tar pitch is now decreasing.
2) Resin substances.
Synthetic resin is prepared by the reaction of phenol and formaldehyde. It can be well mixed with refractory particles at room temperature, and has a high residual carbon rate after carbonization. It is the main binder for the current production of MgO-C bricks. However, the glassy network structure formed after carbonization is not ideal for thermal shock resistance and oxidation resistance of refractory materials.
3) Substances obtained by modification on the basis of asphalt and resin.
If the binder is carbonized to form a mosaic structure and in-situ formation of carbon fiber mass, then this binder will improve the refractory material
In order to improve the oxidation resistance of MgO-C bricks, a small amount of additives are often added. Common additives are Si, Al, Mg, Al-Si, Al-Mg, Al-Mg-Ca, Si-Mg-Ca, SiC, B4C , BN and recently reported additives such as AI-B-C and AI-SiC-C [5-7]. The principle of action of additives can be roughly divided into two aspects:
On the one hand, it is from a thermodynamic point of view, that is, at the working temperature, the additive or the additive and carbon react to form other substances. They have a greater affinity for oxygen than carbon, are oxidized in preference to carbon. So as to play a role in protecting carbon.
On the other hand, from a kinetic point of view, the compounds generated by the reaction of additives with o2, CO or carbon change the microstructure of carbon composite refractories. Such as increasing the density, blocking pores, hindering the diffusion of oxygen and reaction products.