Carbon-based refractories are a new type of refractory material developed in the late 1970s. Utilizing the high refractoriness and non-wetting properties of graphite, they significantly improve the high-temperature performance and erosion resistance of refractory materials. They possess excellent thermal shock stability, slag erosion resistance, spalling resistance, and high-temperature creep resistance. Alumina-magnesia-carbon bricks are carbon-based refractories with Al2O3 as the matrix. They are made primarily from high-quality high-alumina bauxite clinker, with the addition of appropriate amounts of high-purity magnesia, flake graphite, and binders, and are heat-treated at 200-300℃.
The Influence of Main Raw Materials on the Finished Product of Alumina-Magnesium-Carbon Bricks
High-Grade Bauxite Clinker
The main mineral components of high-alumina bauxite raw materials in my country are gibbsite and kaolinite. Bauxite clinker is a product of high-temperature calcination. High-grade bauxite clinker has an Al2O3 content of over 88.2%, reaching up to 91.3%. Although Al2O3 has strong erosion resistance, pure Al2O3 has a large expansion coefficient and is not resistant to spalling. When pure Al2O3 is used as the matrix, the matrix is easily penetrated and melted by slag, exposing the aggregate and leading to structural spalling.
High-Purity Magnesia
Magnesia is obtained by fully sintering raw materials such as magnesite, brucite, and seawater magnesia at 1600-1900℃. Magnesia is divided into sintered magnesia and seawater magnesia. Sintered magnesia is made from natural ore, while seawater magnesia is made from seawater magnesia. Magnesia’s main component is magnesium oxide, with small amounts of SiO2, CaO, Fe2O3, B2O3, etc. Its color ranges from yellow to brown, with periclase as the main crystalline phase. The grain size is 0.02~0.05mm, and the density is 3.50~3.65g/cm3. It exhibits good resistance to alkaline slag erosion.
The main indicators of high-purity magnesia are MgO content, CaO/SiO2 ratio, microstructure, and particle bulk density. Magnesia with high MgO content has periclase as the main crystalline phase, fewer impurities and cementing materials, and the resulting refractory materials have extremely high erosion resistance. The CaO/SiO2 ratio determines the phase composition of the matrix in magnesia, directly affecting the bonding of periclase and the high-temperature performance of the refractory material. Generally, magnesia with a C/S ratio of 3-8 has better high-temperature resistance; exceeding this range can lead to adverse effects. Microstructure is an important means of characterizing the grain size and bonding state of periclase, typically requiring a grain size of 80~150μm. Bulk density is an important indicator of the degree of sintering and compactness of magnesia. Magnesia with higher bulk density can resist slag intrusion and improve the corrosion resistance of refractory products.
Fused Magnesia
Fused magnesia, also known as fused magnesia, is obtained by melting magnesite or sintered magnesia in an electric arc furnace at a high temperature of 2500℃, cooling, and then crushing. The purity of fused magnesia is determined by the purity of the raw materials. Its main crystalline phase is periclase, which crystallizes from the melt. Periaclase crystals are large, dense, and have a high degree of direct crystal contact. It exhibits good water and slag resistance in the atmosphere, good high-temperature volumetric and chemical stability, and remains stable in an oxidizing atmosphere at 2300℃.
Graphite
Graphite possesses excellent thermal conductivity and refractory properties, with a melting point as high as 3500℃. Graphite has a low coefficient of thermal expansion, 1.4 × 10⁻⁶ at 1000℃. It has high thermal conductivity and good resistance to rapid heating and cooling, and is one of the few materials whose strength increases with temperature. Graphite also has a relatively large wetting angle with slag, exhibiting no eutectic relationship with Al₂O₃, SiC, or SiO₂, thus preventing slag from penetrating into the product. Because carbon can reduce iron oxide in the molten slag to metallic iron, it increases the viscosity of the slag, reducing the migration of slag components into the brick and thus reducing erosion.
The main role of graphite in carbon-containing products is to effectively prevent slag from penetrating the brick structure. This is achieved by increasing the wetting angle between the working surface of the brick and the slag, and by reacting with MgO in the brick to reduce metallic magnesium while generating CO gas. The pressure of the gas prevents slag penetration, while magnesium diffuses, volatilizes, and oxidizes on the working surface of the brick, forming a dense, impermeable MgO layer. This creates a strong reducing state within the brick, reducing iron oxides in the slag and increasing slag viscosity, thus preventing slag from penetrating the brick. Graphite with stronger erosion and slag corrosion resistance has a higher carbon content and larger flakes. The SiO2 content in graphite has the following relationship with the erosion index of magnesia-carbon bricks:
As the SiO2 content increases, the erosion index increases continuously, and the erosion resistance decreases continuously. When the SiO2 content in graphite exceeds 3%, the erosion index of magnesia-carbon bricks increases sharply, and its erosion resistance decreases sharply. As the flake particle size of the graphite in magnesia-carbon bricks increases, its oxidation resistance increases. When the flake graphite particle size exceeds 0.125 mm, the increase in oxidation resistance slows down; the suitable graphite particle size is 0.125 mm. Because graphite is easily oxidized to form CO, oxidized graphite loses these excellent properties, which reduces the erosion resistance of refractory materials. This is a fatal weakness of graphite and an important reason for the damage of carbon-containing materials.
Binder
Although the binder content in the finished product is low, it is one of the key technologies in the production of carbon-containing products. The binder directly affects the mixing and molding performance of the blank, as well as the microstructure of the finished product. During mixing and molding, the binder needs to have good wettability with refractory aggregates and graphite, and a suitable viscosity to improve the mixing quality and bulk density of the blank.
The main characteristics of the binder are:
- (1) Good wettability: It has good wettability with both magnesia and graphite.
- (2) It contains little or no harmful components.
- (3) The properties of the mixed slurry do not change significantly over time, and the chemical reaction with the aggregate should be minimal.
- (4) During the heating process of the finished product, the binder should have a high residual carbon rate, and the carbonized polymer should have good high-temperature strength.
Only with good wettability can the binder be evenly distributed on the surface of the particles and graphite, forming a continuous network structure as much as possible. Carbonization is necessary to form a continuous carbon skeleton, improving the strength and corrosion resistance of the finished product.
The type of binder and carbonization conditions directly affect the microstructure and properties of the bound carbon. Different carbonization processes of the binder result in significant differences in the structure of the generated bound carbon. To ensure sufficient strength in the molded brick blank, thermosetting phenolic resin is typically used as a binder in the production of alumina-magnesia-carbon bricks. The resin used should have suitable viscosity, high carbon content, and high residual carbon rate. Phenolic resin, a substitute for benzo[a]pyrene-rich coal tar pitch, is produced by reacting phenol and formaldehyde. Depending on the reaction conditions, the reaction product is phenolic varnish resin or methyl phenolic resin. Because phenolic resin is not thermoplastic when heated, it ensures the dimensional accuracy of the final product. Compared to graphite, the carbonization products of resin have a banded lattice structure, which stacks to form a layered structure (polymerized carbon or glassy carbon). During the high-temperature decomposition process, phenolic resin first releases water (from primary phenolic resin), phenol, cresol, and small amounts of xylenephenol and formaldehyde, ultimately forming polymeric carbon.
The synthetic resin mixes well with refractory particles at room temperature without heating, and its char residue is similar to that of asphalt, being a liquid of 50%–70%. The main drawbacks of primary phenolic resin are limited stability, the homogeneity of its carbonization products leading to easy oxidation, reduced erosion resistance, and sensitivity to thermomechanical stress. Adding rapidly oxidizing metal additives such as Al, Mg, and Si to the mixture is intended to compensate for these shortcomings.
Additives
The presence of graphite is what gives carbon composite refractories their excellent slag resistance and thermal shock stability. Damage to carbon composite refractories is mainly due to graphite oxidation. Once graphite is oxidized, its advantages are completely lost. To improve the oxidation resistance of carbon composite refractories, small amounts of additives, such as Si, Al, Mg, Zr, SiC, and BC, are often added.
The working principle of additives will be analyzed from both thermodynamic and kinetic perspectives:
From a thermodynamic perspective, the working principle of additives is that at the operating temperature, the affinity of the additive or the product of the reaction between the additive and carbon for oxygen is greater than that of carbon for oxygen, thus preferentially protecting the carbon from oxidation.
From a kinetic perspective, the compounds generated by the reaction of additives with O2, CO, or carbon alter the microstructure of carbon composite refractories. This includes increasing density, blocking pores, and hindering the diffusion of oxygen and reaction products. Non-oxides added to carbon-containing refractories typically have the following effects:
- (1) By reducing carbon monoxide to carbon, the rate of carbon consumption is suppressed.
- (2) Formation of carbides or oxides, increasing the densification of refractory materials.
- (3) Further promotion of graphite crystallization.
- (4) Reduction of open porosity.
- (5) Formation of a protective layer.
- (6) Improvement of high-temperature strength.
