Magnesia Carbon Bricks Refractory

Conventional magnesia-carbon brick refractory , manufactured according to the cold-mixing process with a synthetic tar binder, harden and acquire the necessary strength as the tar is damaged, thus forming isotropic glassy carbon. The carbon does not exhibit thermoplasticity, which can provide timely relief of large amounts of stress during baking or handling of the lining. Magnesia-carbon bricks produced with asphalt binders have high high-temperature plasticity due to the anisotropic graphitized coke structure formed during the carbonization process of asphalt.

production process

The main raw materials of MgO–C bricks include fused magnesia or sintered magnesia, flake graphite, organic binders and antioxidants.

magnesia

Magnesia is the main raw material for the production of MgO–C bricks, and is divided into fused magnesia and sintered magnesia. Compared with sintered magnesia, fused magnesia has the advantages of coarse periclase crystal grains and high particle volume density, and is the main raw material used in the production of magnesia carbon brick refractory . The production of ordinary magnesia refractory materials requires magnesia raw materials to have high-temperature strength and corrosion resistance. Therefore, attention should be paid to the purity of magnesia and the C/S ratio and B2O3 content in its chemical composition. With the development of the metallurgical industry, smelting conditions are becoming increasingly demanding. The magnesia used in MgO–C bricks used in metallurgical equipment (converters, electric furnaces, ladles, etc.), in addition to chemical composition, also requires high density and high density in terms of organizational structure. Big crystal.

Whether in traditional MgO-C bricks or in widely used low-carbon MgO-C bricks, flake graphite is mainly used 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 of slag. ②High thermal conductivity. ③Low thermal expansion. In addition, graphite and refractory materials do not fuse at high temperatures and have high refractoriness. The purity of graphite has a great influence on the performance of MgO-C bricks. Generally, graphite with a carbon content of greater than 95%, preferably greater than 98%, should be used.

In addition to graphite, carbon black is also commonly used in the production of magnesia carbon brick refractory . Carbon black is a highly dispersed black powdery carbonaceous material produced by the thermal decomposition or incomplete combustion of hydrocarbons. The carbon black particles are small (less than 1 μm), the specific surface area is large, and the mass fraction of carbon is 90~ 99%, high purity, large powder resistivity, high thermal stability, low thermal conductivity, and is a 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 the bricks.

Commonly used binders in the production of MgO-C bricks include coal tar, coal pitch and petroleum pitch, as well as special carbonaceous resins, polyols, asphalt modified phenolic resins, synthetic resins, etc. The following types of binding agents are used:

1) Asphalt-like substances. Tar asphalt is a thermoplastic material with high affinity to graphite and magnesium oxide, high carbon residual rate after carbonization, and low cost. It has been widely used in the past; however, tar asphalt contains carcinogenic aromatic hydrocarbons, especially the benzo-β content. High; due to increased environmental awareness, the use of tar asphalt is now decreasing.

2) Resin substances. Synthetic resin is produced by the reaction of phenol and formaldehyde. It can be well mixed with refractory particles at room temperature. After carbonization, the carbon residue rate is high. It is the main binding agent currently used in the production of MgO-C bricks; however, it forms after carbonization The glassy network structure is not ideal for thermal shock resistance and oxidation resistance of refractory materials.

3) Substances modified based on asphalt and resin. If the binder can form a mosaic structure and form carbon fiber material in situ after carbonization, then this binder will improve the high-temperature performance of 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, and B4C. , BN and the recently reported Al-B-C and Al-SiC-C series additives [5–7]. The working principle of additives can be roughly divided into two aspects: on the one hand, from the thermodynamic point of view, that is, at the working temperature, additives or additives react with carbon to form other substances, and their affinity with oxygen is greater than the affinity between carbon and oxygen. , prior to carbon being oxidized to protect carbon; on the other hand, from a kinetic perspective, the compounds generated by the reaction of additives with O2, CO or carbon change the microstructure of carbon composite refractory materials, such as increasing the Density, blocking pores, hindering the diffusion of oxygen and reaction products, etc.

The refractory materials used in the early ladle slag lines were high-quality alkaline bricks such as directly bonded magnesia-chromium bricks and electrofusion bonded magnesia-chromium bricks. After MgO-C bricks were successfully used in converters, MgO-C bricks were also used in the refining ladle slag line and achieved good results.

Research shows that MgO-C bricks made from a mixture of fused magnesia and sintered magnesia, plus 15% phosphorus flake graphite and a small amount of magnesium-aluminum alloy as antioxidants, have good use effects and have a capacity of 100 tons. When used in the LF ladle slag line, compared with MgO-C bricks with a C content of 18% without antioxidants, the damage rate is reduced by 20-30%, and the average erosion rate is 1.2-1.3mm/furnace.