How Do Main Raw Materials Affect The Performance Of Alumina-Magnesia-Carbon And Magnesia-Carbon Bricks?

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

Alumina-magnesia-carbon bricks are carbon-based refractories with Al₂O₃ as the matrix. They are mainly composed of high-quality bauxite clinker, with an appropriate amount of high-purity magnesia, flake graphite, and binder added, and are prepared by heat treatment at a temperature of 200–300℃.

Magnesia-carbon bricks are unburned or lightly burned basic bricks manufactured from carbon materials and magnesia, with carbonaceous binders. They were first successfully developed in Japan and initially applied to hot-spot areas of high- and ultra-high-power electric arc furnaces, as well as to the tuyeres of bottom-blown converters. Later, Japan applied magnesia-carbon bricks to all parts of converters. Compared with tar-impregnated fired magnesia-dolomite bricks, the service life of converters is increased by approximately 1.3 to 1.6 times.

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

The influence of the main raw materials of alumina-magnesia-carbon bricks and magnesia-carbon bricks on the finished products is as follows:

Special-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 special-grade bauxite clinker is above 88.2%, with a maximum of up to 91.3%.

Although alumina exhibits excellent corrosion resistance, pure alumina has a high thermal expansion coefficient and poor spalling resistance. When pure alumina is used as the matrix, it is prone to penetration and melting loss by molten slag, which exposes the aggregate and eventually causes structural spalling.

High-Purity Magnesia

Magnesia is fully sintered from raw materials such as magnesite, brucite, and seawater magnesia at 1600–1900℃. It is classified into dead-burned magnesia and seawater magnesia. Dead-burned magnesia is sintered from natural ores, while seawater magnesia is produced from magnesium oxide derived from seawater.

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

The key quality indicators of high-purity magnesia include MgO content, CaO/SiO₂ ratio, microstructure, and particle bulk density. Magnesia with high MgO content is dominated by periclase as the main crystalline phase with few impurity cements, enabling the produced refractory materials to possess extremely high slag corrosion resistance.

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

Microstructure is an important characteristic parameter reflecting the grain size and bonding state of periclase, with a typical required grain size of 80–150 μm. Bulk density is a critical index for evaluating the sintering degree and compactness of magnesia. Magnesia with higher bulk density can resist the penetration of molten slag and enhance the corrosion resistance of refractory products.

Table 1 shows that the appropriate CaO/SiO₂ ratio is less than 1 or greater than 2. A high CaO/SiO₂ ratio helps improve the stability of magnesia coexisting with graphite at high temperatures.

The grain size of periclase and the bulk density of magnesia have a significant influence on the corrosion resistance of magnesia-carbon bricks. Matsuo Shōsei and other researchers prepared magnesia-carbon bricks using magnesia with different periclase grain sizes and tested their weight loss under a high-temperature reducing atmosphere.

The results show that, among magnesia-carbon bricks made from magnesia with different periclase grain sizes, larger periclase grains correspond to lower weight loss.

It is therefore concluded that, for the production of high-performance magnesia-carbon bricks, fused magnesia with a CaO/SiO₂ ratio lower than 1 or higher than 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℃. It has a low thermal expansion coefficient (1.4×10⁻⁶ at 1000℃) and superior thermal shock resistance. It is one of the few materials whose strength increases with rising temperature.

Graphite has a considerably large wetting angle against molten slag and exhibits no eutectic relationship with Al₂O₃, SiC, or SiO₂, which enables it to hinder slag penetration into refractory products. Moreover, carbon can reduce iron oxide in molten slag to metallic iron, increasing slag viscosity and restraining the migration of slag components into the bricks, thereby mitigating corrosion.

The primary function of graphite in carbon-containing refractories is to prevent molten slag from penetrating the brick structure. It enlarges the wetting angle between the brick working surface and molten slag, and reacts with MgO inside the brick to generate metallic magnesium and CO gas simultaneously. The pressure of CO gas helps block slag penetration. Meanwhile, magnesium diffuses and volatilizes, then oxidizes on the brick working surface to form a dense, penetration-resistant 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 prevents slag infiltration.

Graphite with higher carbon content and larger flake size provides stronger 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 the corrosion index increases continuously, and the corrosion resistance gradually decreases with the rise in SiO₂ content in graphite. When the SiO₂ content in graphite exceeds 3%, the corrosion index of magnesia-carbon bricks rises sharply, accompanied by a dramatic drop in corrosion resistance.

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

Graphite is prone to oxidation, forming CO. Oxidized graphite loses its excellent comprehensive properties, resulting in a decline in the corrosion resistance of refractory materials. This is a critical weakness of graphite and an important cause of damage to carbon-containing refractory materials.

Binder

Although the binder content in finished products is not high, it is one of the key technical factors in the production of carbon-containing refractories. The binder directly affects the mixing and forming performance of the batch material, as well as the microstructure of the refractory products. During mixing and forming, the binder must have good wettability to the refractory aggregate and graphite, and proper viscosity to improve the mixing quality and bulk density of the batch.

The main characteristics of a qualified binder are as follows:

Good wettability: favorable wetting of both magnesia and graphite;

Safety: free of or low in ingredients harmful to the human body;

Stability: the properties of the mixed batch change little over time, with weak chemical reactivity toward aggregates;

High carbon yield: the binder should have a high carbon residue rate during the heating process, and the carbonized polymer should maintain excellent high-temperature strength.

Only with satisfactory wettability can the binder be evenly distributed on the surface of particles and graphite, forming a continuous network structure. After carbonization, this network forms a continuous carbon skeleton, enhancing the strength and corrosion resistance of refractory products.

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

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

Phenolic resin, a substitute for coal tar pitch rich in benzopyrene, is synthesized by the reaction of phenol and formaldehyde. Depending on the reaction conditions, the products are either phenolic varnish resin or resole phenolic resin. Since phenolic resin does not exhibit thermoplasticity upon heating, it ensures the dimensional accuracy of the final products. Compared with graphite, the carbonized product of resin has a banded lattice structure that stacks into layers, known as polymeric carbon or glassy carbon.

During high-temperature decomposition, resole phenolic resin first releases water (moisture in the resin), phenol, cresol, and small amounts of xylenol and formaldehyde, and finally forms polymeric carbon.

Synthetic resin can mix well with refractory particles at room temperature without heating, with a carbon residue rate similar to pitch, ranging from 50% to 70%.

The main disadvantages of liquid resole phenolic resin are limited storage stability, poor oxidation resistance due to the inhomogeneity of the carbonized resin, which reduces corrosion resistance, and high sensitivity to thermomechanical stress. Adding easily oxidizable metal additives such as Al, Mg, and Si into the mixture is an effective way to compensate for these deficiencies.

Additives

It is the presence of graphite that endows carbon-bonded composite refractories with excellent slag resistance and thermal shock stability. The main failure mode of carbon-bonded composite refractories is the oxidation of graphite. Once graphite is oxidized, all its superior properties are completely lost. To improve the oxidation resistance of carbon-bonded composite refractories, a small amount of additives is commonly incorporated, such as Si, Al, Mg, Zr, SiC, BC, and others.

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

Thermodynamic perspective: At service temperature, the additive itself, or the reaction products formed between the additives and carbon, has a higher affinity for oxygen than carbon. It is oxidized preferentially over 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-bonded composite refractories, for example by increasing densification, blocking pores, and hindering the diffusion of oxygen and reaction products.

(1)Non-oxide additives added to carbon-containing refractories generally serve the following functions:

(2)Reduce carbon monoxide back to carbon, thereby inhibiting the consumption rate of carbon;

(3)Form carbides or oxides to improve the densification of refractories;

(4)Promote graphite crystallization;

(5)Reduce open porosity;

(6)Form a protective layer;

(7)Enhance high-temperature strength.