Magnesia-carbon bricks appeared in the early 1970s. They were first used in ultra-high-power electric furnaces, and then in converters and external refining furnaces, achieving very good results. From this, people realized the role of the combination between graphite, carbon materials and high-temperature refractory oxides. Poor fracture toughness, high-temperature spalling, and poor slag resistance are the fatal shortcomings of high-temperature fired refractory products. The emergence of carbon-containing refractory products has overcome these weaknesses. In magnesia carbon bricks, magnesium oxide and graphite are wrapped around each other, and there is no so-called sintering in the traditional concept; graphite has the advantages of high thermal conductivity, low elastic modulus, small thermal expansion coefficient, and is not easily infiltrated by slag. Therefore, Due to the introduction of stone black, the fracture toughness and slag permeability resistance of furnace lining refractory products are essentially improved. The main feature of magnesia carbon bricks is the formation of carbon combinations on the microstructure. This combination is formed by coking and carbonization of organic binders at high temperatures.
Magnesia carbon brick is a kind of unfired product. Its physical and chemical indicators are: Mg070~85%, C10~20%, apparent porosity ≤3%, volume density 2.87g/cm3, compressive strength 40~50MPa, 1400℃ flexural resistance Strength 10~15MPa.
The process factors that affect the performance of magnesia carbon bricks mainly include raw materials, binders, additives, etc.
1. Magnesia
When the magnesia carbon bricks were first produced abroad, high-purity sintered magnesia was used. With in-depth research on the use process of magnesia carbon bricks, it was discovered that the following reaction occurs at high temperatures: MgO+C→Mgt+CO↑.
This reaction generally starts at 1650°C and intensifies when it reaches 1750°C. This is one of the important reasons for the loss of magnesia carbon bricks during use. It is also the reason why the loss of magnesia carbon bricks is significantly increased when used above 1700°C. Impurities such as SiO2 and Fe203 in magnesia promote the above reaction, so magnesia is expected to have higher purity.
Compared with sintered magnesia, fused magnesia has a more complete crystal structure and a more stable carbon reduction effect. In particular, these characteristics of large crystalline fused magnesia are more prominent, so the production of magnesia carbon bricks began to shift to use Fused magnesia. Considering the bonding state of carbon and the wettability of the binder, fused magnesite and sintered magnesia can also be mixed. my country’s magnesia carbon bricks basically use fused magnesia.
The results of using magnesia carbon bricks show that magnesia with high MgO content, large periclase phase crystal particles and a calcium to silicon ratio greater than 2 has the best effect in producing magnesia carbon bricks.
2. Graphite
Graphite is another basic component in magnesia carbon bricks. Graphite has good basic properties of refractory materials. The main physical and chemical indicators are: fixed carbon 85% to 98%, ash content 13% to 2% (main components Si02, A1203, etc.), relative density 2.09 to 2.23, melting point 3640K (volatile). Because graphite is very easily oxidized, it has not attracted people’s attention for a long time.
During the use of magnesia carbon bricks, there are three reasons for the oxidation of graphite:
(1) Oxidation of graphite by oxygen in the air;
(2) Oxidation of graphite by oxides in the slag;
(3) Oxidation of graphite by impurity oxides contained in graphite itself.
These oxides mainly refer to SiO2 and Fe203.
After the impurity oxides in magnesia carbon bricks react with graphite, the brick structure becomes loose, the air permeability increases, and the strength decreases. This is the internal cause of damage to magnesia carbon bricks. Therefore, graphite with high purity and large phosphorus flake crystals is mostly used in the production of magnesia carbon bricks.
3. Binders
Binders play a vital role in magnesia carbon bricks and other carbon-containing refractory products. There is no mutual miscibility relationship between graphite and refractory oxides, and it is impossible to sinter each other. They must be bonded and solidified by a bonding agent at room temperature. At high temperatures, the binding agent will coke and carbonize, forming a carbon bond with graphite. Generally, this binding agent refers to organic substances such as resins and asphalts. The binder forms about 3% carbon after high-temperature coking and carbonization. Although this amount is not much, it is the most active component in magnesia-carbon bricks or other carbon-containing products and has an important impact on the high-temperature performance of the products. The production process and product quality of magnesia carbon bricks or other carbon-containing products in my country are not stable enough. One of the important reasons is that the binder is unstable. Magnesia-carbon brick binders can be roughly divided into three types: phenolic resin, modified asphalt, and petroleum cracking by-products. Among them, phenolic resin has the best effect and the largest dosage.
4. Binding agent
Binders play a vital role in magnesia carbon bricks and other carbon-containing refractory products. There is no mutual miscibility relationship between graphite and refractory oxides, and it is impossible to sinter each other. They must be bonded and solidified by a bonding agent at room temperature. At high temperatures, the binding agent will coke and carbonize, forming a carbon bond with graphite. Generally, this binding agent refers to organic substances such as resins and asphalts. The binder forms about 3% carbon after high-temperature coking and carbonization. Although this amount is not much, it is the most active component in magnesia-carbon bricks or other carbon-containing products and has an important impact on the high-temperature performance of the products. The production process and product quality of magnesia carbon bricks or other carbon-containing products in my country are not stable enough. One of the important reasons is that the binder is unstable.
Magnesia-carbon brick binders can be roughly divided into three types: phenolic resin, modified asphalt, and petroleum cracking by-products. Among them, phenolic resin has the best effect and the largest dosage.
4. In the damage process of magnesia carbon bricks, the oxidation of graphite is one of the main reasons. Due to the loss of carbon due to oxidation, the brick structure is loose and the strength is reduced. The damage process follows the path of oxidation and carbon loss – structural looseness → erosion → erosion and dissolution. In order to improve the oxidation resistance of magnesia carbon bricks, a certain amount of additives can be added, including silicon powder, aluminum powder, FeSi alloy, CaSi alloy, SiC, Si3N4, B4C, etc. Another function of the additive is to “build a bridge” between refractory oxide and graphite, so that graphite and refractory oxide form a strong bond. This effect is due to the formation of a new mineral phase by the additive at a certain temperature.
China produces magnesia carbon bricks and other phosphorus-containing refractory products. The most commonly used additives are silicon powder, aluminum powder and SIC powder.
