Production process of MgO-C bricks
The main raw materials for MgO-C bricks include fused magnesia or sintered magnesia, flake graphite, organic binders, and antioxidants.
Magnesia
Magnesia is the primary raw material for producing MgO-C bricks, available in fused magnesia and sintered magnesia forms. Compared with sintered magnesia, fused magnesia has advantages such as larger periclase crystal grains and higher bulk density, making it the preferred raw material for MgO-C brick production. For the production of ordinary magnesia-based refractories, the main requirements for magnesia raw materials are high-temperature strength and corrosion resistance, thus emphasis is placed on the purity of magnesia and the C/S ratio and B₂O₃ content in its chemical composition. With the development of the metallurgical industry and increasingly harsh smelting conditions, the magnesia used in MgO-C bricks for metallurgical equipment (converters, electric furnaces, ladles, etc.) requires not only specific chemical composition but also high density and large crystallite size in terms of microstructure.
Production process of magnesia carbon brickCarbon Source
Whether in traditional MgO-C bricks or widely used low-carbon MgO-C bricks, flake graphite is primarily used as the carbon source. Graphite is a key raw material for MgO-C bricks, mainly due to its excellent physical properties: ① non-wettability by slag; ② high thermal conductivity; ③ low thermal expansion. In addition, graphite does not undergo eutectic melting with refractories at high temperatures and has high refractoriness. The purity of graphite significantly affects the performance of MgO-C bricks; generally, graphite with a carbon content greater than 95%, preferably greater than 98%, is required.
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 produced by thermal decomposition or incomplete combustion of hydrocarbon compounds. It has fine particles (less than 1 μm), large specific surface area, carbon content of 90–99%, high purity, high electrical resistivity, high thermal stability, and relatively low thermal conductivity, belonging to the non-graphitizing carbon category. The addition of carbon black can effectively improve the spalling resistance of MgO-C bricks, increase residual carbon content, and enhance brick density.
Binder
Common binders used in MgO-C brick production include coal tar, coal tar pitch, petroleum pitch, as well as special carbonaceous resins, polyols, pitch-modified phenolic resins, synthetic resins, etc. The binders used can be classified into the following types:
Pitch-based materials. Coal tar pitch is a thermoplastic material with advantages such as strong affinity with graphite and magnesia, high residual carbon rate after carbonization, and low cost; it was widely used in the past. However, coal tar pitch contains carcinogenic aromatic hydrocarbons, especially high levels of benzopyrene. Due to increasing environmental awareness, the use of coal tar pitch is now decreasing.
Resin-based materials. Synthetic resin is produced by the reaction of phenol and formaldehyde. It can be mixed well with refractory particles at room temperature and has a high residual carbon rate after carbonization, making it the main binder currently used for MgO-C bricks. However, the glassy network structure formed after its carbonization is not ideal for the thermal shock resistance and oxidation resistance of the refractory material.
Modified materials based on pitch and resin. If the binder can form an镶嵌 structure and in-situ carbon fibers after carbonization, it can improve the high-temperature performance of the refractory material.
Antioxidants
To improve the oxidation resistance of MgO-C bricks, small amounts of additives are often added. Common additives include Si, Al, Mg, Al-Si, Al-Mg, Al-Mg-Ca, Si-Mg-Ca, SiC, B₄C, BN, and more recently reported Al-B-C and Al-SiC-C systems. The mechanism of additives can be roughly divided into two aspects: one is from a thermodynamic perspective, where at working temperatures, the additives or their reaction products with carbon have a greater affinity for oxygen than carbon does, thus being preferentially oxidized and protecting the carbon; the other is from a kinetic perspective, where the compounds formed by additives reacting with O₂, CO, or carbon change the microstructure of the carbon-containing refractory material, such as increasing density, blocking pores, and hindering the diffusion of oxygen and reaction products.
