Carbon-containing refractory materials are high-performance refractory materials that emerged with the advancement of steel metallurgical technology in the early 1980s. They are widely used in top-blown converters, ultra-high-power electric furnaces, out-of-furnace refining, continuous ingot casting, and molten iron pretreatment. Equipment and technology. Magnesia-carbon bricks are a carbon-containing refractory material that emerged in the 1970s. Its main raw materials are fused magnesia or sintered magnesia and carbon materials, and are extruded by organic binders.
1. Carbon source
Materials that can be used as carbon sources for magnesia carbon bricks include petroleum coke, coal pitch, flake graphite, carbon black, etc. Among them, flake graphite is regarded as the best carbon raw material due to its complete crystal structure and high temperature resistance.
The following is a brief introduction to the characteristics of flake graphite:
① Good high temperature resistance, the melting point can reach up to 3900℃, the boiling point is generally 4250℃, it can withstand ultra-high temperature burning, and has minimal mass loss and volume expansion. The strength of graphite after heat treatment becomes larger, and as the temperature rises High intensity gradually becomes greater.
② The resistivity and thermal conductivity are low. Generally, the conductivity of metals is stronger than that of non-metals, but the conductivity of graphite exceeds that of many metals and rarely generates heat. Its thermal conductivity changes with temperature. The thermal conductivity is high at room temperature, but decreases at high temperatures, and even insulation phenomena may occur.
③Good lubricity. Due to the unique molecular structure of graphite, it slides directly between layers, and the larger the graphite sheet, the smaller the friction coefficient.
④Stable chemical properties. Since the carbon content is above 80%, it basically does not react with electrodeless organic reagents at room temperature.
⑤Plasticity: Graphite has good initial properties and can be pressed into thin sheets.
⑥High thermal stability. Graphite can maintain its integrity even under rapid changes in temperature, and will not expand or shrink and cause damage.
2. Magnesia carbon brick binder
Under the set production process, the product made by mixing magnesia, graphite, binder and antioxidant and other raw materials without burning or lightly burning is called magnesia carbon brick. Because the chemical properties of graphite are stable, it cannot react directly with MgO at room temperature. A binding agent is needed between them so that the material can be mixed and formed. Therefore, the quality of the binding agent directly affects its performance. We hope that the binder can well wet the large particle aggregate and graphite, and have a suitable viscosity. In addition, the binder with good wettability can be evenly distributed on the surface of the particles and graphite, so that it can have continuous properties after carbonization. The matrix structure and carbon skeleton are beneficial to improving its various properties.
In order to adapt to different carbonization conditions and have a beneficial impact on the structure and performance of the material, the magnesia carbon brick binder needs to meet the following conditions:
(1) Appropriate viscosity, which can well wet the aggregate and graphite during mixing, and does not cause aging after addition;
(2) It further condenses itself during heating and molding, and the volume does not expand or contract violently, ensuring the quality of the product and no cracks on the surface;
(3) The pressed samples have a higher coking value during the high-temperature carbonization process, and the combined body after carbonization has a higher mechanical strength;
(4) Resources are abundant and widely available at low prices.
Substances currently available as binders for refractory materials can be divided into three major categories:
(1) Asphalt, of which coal pitch is the most commonly used. This is because coal pitch has good impregnation, moderate viscosity, good fluidity without high temperature, high carbon residue rate after carbonization, formation of anisotropic structure, and low price. But its shortcomings are also obvious. It contains toxic substances, especially benzo[a]pyrene, which is highly carcinogenic. With the deterioration of the environment, people began to realize the dangers of coal pitch, thus reducing the use of coal pitch.
(2) Resins, phenolic resin is currently the most commonly used binding agent. It is obtained by the condensation polymerization reaction of phenol and formaldehyde. It has good adhesion at room temperature, and the carbon residue rate after carbonization is slightly lower than that of asphalt. However, the resin converted to glass after high temperature treatment increases the brittleness of magnesia carbon bricks and reduces the thermal stability and oxidation resistance of the material.
(3) Asphalt and resin mixtures, materials obtained by mixing and modifying asphalt and resin. This new binding agent can improve their respective defects and play a complementary role.
3. Conclusion
Through the correct selection of carbon sources and binders, magnesia-carbon bricks not only retain the excellent properties of carbon-containing refractory materials, but also improve the shortcomings of carbon-containing refractory materials. They are widely used in rotary electric furnace linings and electric furnace interior walls. It is a high-efficiency and energy-saving refractory material for parts such as the body and ladle slag line.