Effects Of Main Raw Materials On The Performance Of Alumina-Magnesia-Carbon Bricks And Magnesia-Carbon Bricks

Carbon-based refractories are a new type of refractory material developed in the late 1970s. By taking advantage of graphite's high refractoriness and non-wettability, they greatly enhance the high-temperature performance and corrosion resistance of refractories. They exhibit excellent thermal shock stability, slag erosion resistance, spalling resistance, and high-temperature creep resistance.

Alumina-magnesia-carbon brick is a carbon-based refractory with Al₂O₃ as the matrix. It is prepared using premium high-alumina bauxite clinker as the main component, with an appropriate amount of high-purity magnesia, flake graphite, and binder added, followed by heat treatment at a temperature of 200–300 °C.

Magnesia-carbon brick is an unfired or lightly fired basic brick made from carbon materials and magnesia as raw materials, along with carbonaceous binders. First successfully developed in Japan, it was initially applied to hot-spot areas of high and ultra-high power electric arc furnaces and to tuyeres of bottom-blown converters, and was subsequently used throughout all parts of converters in Japan. Compared with tar-impregnated, fired magnesia-dolomite bricks, the service life of converters lined with magnesia-carbon bricks is increased by approximately 1.3 to 1.6 times.

While retaining the advantages of basic refractories, magnesia-carbon bricks incorporate carbonaceous binders, giving them superior properties such as high refractoriness, strong slag resistance, low thermal expansion, and high thermal conductivity. They significantly improve the overall performance of basic refractories and fundamentally overcome their inherent drawbacks, including poor spalling resistance and easy slag penetration.

The following illustrates the influence of the main raw materials of alumina-magnesia-carbon bricks and magnesia-carbon bricks on the finished products:

Super Grade Bauxite Clinker

The main mineral components of high-alumina bauxite raw materials in China are diaspore and kaolinite. Bauxite clinker is the product obtained after high-temperature calcination. The alumina content of super grade bauxite clinker is above 88.2%, and can reach up to 91.3%. Although alumina has excellent erosion resistance, pure alumina has a high coefficient of thermal expansion and poor spalling resistance. When pure alumina is used as the matrix, it is prone to slag penetration and melting loss, which exposes the aggregate and eventually leads to structural spalling.

High-Purity Magnesia

Magnesia is fully sintered from raw materials such as magnesite, brucite, and seawater-derived magnesium oxide at 1600–1900 °C. It is classified into sintered magnesia and seawater magnesia. Sintered magnesia is calcined from natural ores, while seawater magnesia is produced from magnesium oxide obtained from seawater.

Magnesia is mainly composed of magnesium oxide and also contains small amounts of SiO₂, CaO, Fe₂O₃, B₂O₃, and other components. Its color ranges from yellow to brown, with periclase as the main crystalline phase. The grain size is 0.02–0.05 mm, and the bulk density is 3.50–3.65 g/cm³. It exhibits excellent resistance to alkaline slag erosion.

The key technical indicators of high-purity magnesia include MgO content, CaO/SiO₂ ratio, microstructure, and bulk density. Magnesia with a high MgO content is dominated by periclase as the main crystalline phase with few impurities, and the refractory products prepared from it exhibit extremely high slag corrosion resistance.

The CaO/SiO₂ ratio determines the phase composition of the matrix in magnesia, which directly affects the bonding of periclase and the high-temperature performance of refractories. Generally, magnesia with a C/S ratio of 3–8 provides good high-temperature resistance; values outside this range can have adverse effects.

Microstructure is an important indicator that describes the grain size and bonding state of periclase. The grain size is normally required to be 80–150 μm. Bulk density is a key parameter reflecting the degree of sintering and compactness of magnesia. Magnesia with higher bulk density can better resist slag infiltration and improve the corrosion resistance of refractory products.

Fused Magnesia

Fused magnesia, also known as fused magnesium oxide, is produced by melting magnesite or sintered magnesia in an electric arc furnace at an ultra-high temperature of 2500 °C, followed by cooling and crushing. The purity of fused magnesium oxide depends on the purity of the raw materials used.

Its main crystalline phase is periclase. The periclase crystals formed from the molten liquid have a large grain size, dense structure, and a high degree of direct crystal bonding. Fused magnesia exhibits excellent water and slag resistance under atmospheric conditions, as well as superior high-temperature volume stability and chemical stability. It remains stable in an oxidizing atmosphere up to 2300 °C.

All performance characteristics of magnesia-carbon bricks are directly related to the properties of magnesia. To improve the properties of magnesia-carbon bricks, it is necessary to increase the MgO content to enhance the direct bonding of periclase in magnesia. Controlling the CaO/SiO₂ ratio can reduce the formation of silicate phases and minimize the isolation of periclase by these phases. Therefore, magnesium oxide content and the CaO/SiO₂ ratio are important indicators for evaluating magnesia, as shown in Table 1.

It can be seen from Table 1 that the appropriate CaO/SiO₂ ratio is either less than 1 or greater than 2. A high CaO/SiO₂ ratio helps enhance the stability of magnesia when it coexists with graphite at high temperatures.

Both the periclase grain size and the bulk density of magnesia have a significant effect on the corrosion resistance of magnesia-carbon bricks. Matsuo Akimitsu and other researchers prepared magnesia-carbon bricks using magnesia with different periclase grain sizes and measured their weight loss under a high-temperature reducing atmosphere. The results show that, for magnesia-carbon bricks made from magnesia with varying periclase grain sizes, larger periclase grains correspond to lower weight loss.

Therefore, in the production of high-performance magnesia-carbon bricks, fused magnesia with a CaO/SiO₂ ratio below 1 or above 2, high bulk density, and well-developed crystal morphology should be selected as raw materials.

Graphite

Graphite possesses excellent thermal conductivity and refractoriness, with a melting point as high as 3500 °C. It has a low thermal expansion coefficient (1.4×10⁻⁶ at 1000 °C) and outstanding thermal shock resistance against rapid heating and cooling. It is one of the few materials whose mechanical strength increases with rising temperature.

Graphite has a large wetting angle against slag and exhibits no eutectic reaction with Al₂O₃, SiC, or SiO₂, which helps prevent slag penetration into refractory products. Carbon in graphite can reduce iron oxide in molten slag to metallic iron, increasing slag viscosity and restricting the migration of slag components into the bricks, thereby reducing corrosion.

The main function of graphite in carbon-containing refractories is to effectively prevent slag from infiltrating the brick structure. It increases the wetting angle between the brick surface and slag, and reacts with MgO inside the brick to generate metallic magnesium and CO gas. The pressure of the CO gas hinders slag penetration. Meanwhile, the magnesium diffuses and volatilizes, then oxidizes on the brick surface to form a dense, impermeable MgO layer. This maintains a strong reducing atmosphere inside the brick, which not only reduces iron oxides in the slag but also increases slag viscosity and further inhibits slag infiltration.

Graphite with higher carbon content and larger flake size provides better corrosion and slag erosion resistance. There is a correlation between the SiO₂ content in graphite and the corrosion index of magnesia-carbon bricks, as shown in Figure 1.

It can be seen from Figure 1 that as the SiO₂ content increases, the corrosion index rises steadily, and the corrosion resistance gradually decreases. When the SiO₂ content in graphite exceeds 3%, the corrosion index of magnesia-carbon bricks rises sharply, accompanied by a drastic drop in corrosion resistance.

As the flake size of graphite used in magnesia-carbon bricks increases, oxidation resistance improves. When the flake graphite particle size exceeds 0.125 mm, the improvement in oxidation resistance slows, and the optimal graphite particle size is 0.125 mm.

Graphite is prone to oxidation, forming CO. Once oxidized, graphite loses its excellent overall properties, leading to reduced corrosion resistance in refractory materials. This is the main weakness of graphite and a significant cause of damage to carbon-containing refractory materials.

Binder

Although the binder content in finished products is relatively low, it is one of the key technical factors in the production of carbon-containing refractories. Binders directly affect the mixing and forming properties of the batch materials, as well as the microstructure of the final refractory products. During mixing and forming, the binder must have good wettability with both refractory aggregates and graphite, and an appropriate viscosity to ensure thorough mixing and high bulk density of the batch.

The main characteristics of a qualified binder are as follows:

Good wettability: favorable interaction with both magnesia and graphite.

Free of, or low in, ingredients harmful to human health.

Stable properties over time with negligible chemical reaction with aggregates.

High carbon residue during heating, and the carbonized polymer maintains excellent high-temperature strength.

Only with good wettability can the binder be evenly distributed over the surfaces of particles and graphite, forming a continuous network structure. After carbonization, this network forms a continuous carbon skeleton, improving the strength and corrosion resistance of the finished products.

The type of binder and the carbonization conditions directly affect the microstructure and properties of the bonding carbon. Different carbonization processes lead to significant structural differences in the resulting bonding carbon.

To ensure sufficient strength in green bricks, thermosetting phenolic resin is commonly used as the binder in the production of alumina-magnesia-carbon bricks and magnesia-carbon bricks. The selected resin should have proper viscosity, high carbon content, and a high carbon residue rate.

Phenolic resin, synthesized from phenol and formaldehyde, serves as a safer alternative to coal tar pitch, which contains benzopyrene. Depending on the reaction conditions, the products are either phenolic varnish resin or resol resin. Phenolic resin does not become thermoplastic upon heating, ensuring dimensional stability in the final products. Compared with graphite, the carbonized resin forms a banded lattice that accumulates into a layered structure, known as polymeric or glassy carbon.

During the high-temperature decomposition of phenolic resin, resol resin first decomposes into water, phenol, cresol, a small amount of xylenol, and formaldehyde, eventually forming polymeric carbon. Synthetic resin can be thoroughly mixed with refractory particles at room temperature without heating, producing a carbon residue similar to pitch, ranging from 50% to 70%.

The main disadvantages of liquid resol resin are limited storage stability, poor oxidation resistance due to inhomogeneity in the carbonized resin (which reduces corrosion resistance), and high sensitivity to thermomechanical stress. Adding easily oxidizable metal additives, such as aluminum, magnesium, and silicon, can help compensate for these deficiencies.

Additives

It is the presence of graphite that gives carbon composite refractories their excellent slag resistance and thermal shock stability. The main cause of failure in carbon composite refractories is the oxidation of graphite. Once graphite is oxidized, all its superior properties are lost. To improve the oxidation resistance of carbon composite refractories, a small amount of additives is commonly added, such as Si, Al, Mg, Zr, SiC, and BC.

The functional mechanism of additives can be analyzed from both thermodynamic and kinetic perspectives:

Thermodynamic perspective: At service temperatures, the affinity of additives-or the compounds formed by their reaction with carbon-for oxygen is higher than that of carbon itself. The additives are oxidized before carbon, thereby protecting the carbon matrix.

Kinetic perspective: Compounds generated by the reaction of additives with O₂, CO, or carbon can modify the microstructure of carbon composite refractories, for example, by increasing density, blocking pores, and hindering the diffusion of oxygen and reaction products.

Non-oxide additives incorporated in carbon-containing refractories generally serve the following functions:

(1)Reduce carbon monoxide to carbon, thereby inhibiting carbon consumption.

(2)Form carbides or oxides, improving the densification of the refractory.

(3)Promote further graphite crystallization.

(4)Reduce open porosity.

(5)Form a protective layer.

(6)Enhance high-temperature strength.