Magnesia carbon bricks were first developed in Japan as a refractory product. Practical trials began in electric furnaces in 1970, and after six years of testing, they were officially introduced. In 1977, Kawasaki Steel’s Chiba Works in Japan introduced the Q-POB converter, featuring resin-bonded, unfired MgO-C (graphite) refractory materials for the converter’s hearth and tuyere. This material achieved significant success, establishing a precedent for the use of graphite-containing composite refractories in converters. Subsequently, Western Europe developed pitch-bonded magnesia carbon bricks, which contain approximately 10% residual carbon and were successfully used in water-cooled electric furnaces, excluding hotspots, and also in converters. my country began researching magnesia carbon bricks in 1976. Experience with magnesia carbon bricks as converter linings confirmed their suitability for steelmaking, and to this day, magnesia carbon bricks continue to be widely used in converter steelmaking.

Properties of magnesia carbon bricks

Magnesia-carbon bricks are carbon-bonded alkaline bricks made from magnesia, carbon raw materials, an organic binder, and additives through mixing, high-pressure forming, and low-temperature treatment. By leveraging the advantages of both alkaline and carbon materials, these bricks possess excellent thermal shock resistance, spalling resistance, slag resistance, and high-temperature creep resistance, making them an ideal lining material for metallurgical furnaces and highly valued worldwide. Currently, my country can produce both standard and high-strength magnesia-carbon bricks, as well as functional products such as magnesia-carbon breathable bricks, which can essentially meet the needs of the metallurgical industry.

As furnace lining refractory materials, magnesia-carbon refractories effectively utilize the slag-erosion resistance of magnesia and the high thermal conductivity and low expansion of carbon, compensating for the primary drawback of magnesia: its poor spalling resistance. They offer the following key properties:

(1) High temperature resistance

MgO and C have no eutectic relationship at high temperature, and both have high melting points. The melting point of magnesium oxide is 2800℃, and the melting point of carbon is above 3000℃. Therefore, the magnesium-carbon refractory made by combining the two has a high melting point and good high temperature resistance.

(2) Strong resistance to alkaline slag corrosion

MgO itself has strong resistance to alkaline slag and high iron slag. The wetting angle of graphite to slag is large, and its wetting performance with molten slag is very poor.

(3) Good thermal shock stability

Among magnesium-carbon refractory materials, graphite has a high thermal conductivity, a very low thermal expansion coefficient and a small elastic modulus. Therefore, the thermal shock stability of this refractory material is good.

The performance of MgO-C bricks is mainly affected by the main and auxiliary raw materials, chemical composition, the relative content of each component and the structure of the mixture, but the type of binder, processing and molding technology also play an equally important role.

Production process of magnesia carbon bricks

Selection of raw materials: The raw materials for producing MgO-C bricks are mainly magnesia. Its technical requirements are high purity, few impurities, complete particle crystallization, uniform texture, low porosity, and high bulk density. Magnesia is divided into fused magnesia and sintered magnesia, but gradually replacing part of sintered magnesia with fused magnesia can significantly improve corrosion resistance. Carbon raw materials generally use natural flake graphite. It has no eutectic relationship with oxides such as MgO and does not form low-melting products. It has high thermal conductivity, low elastic modulus, low thermal expansion coefficient, and has the characteristics of non-wetting. Binder is the key material for producing MgO-C bricks. Its technical requirements are: (1) small wetting angle with carbon materials and good affinity; (2) high residual carbon rate; (3) low impurity and moisture content. Commonly used binders include phenolic resin, modified phenolic resin and tar pitch. At present, most of the MgO-C bricks are produced using synthetic phenolic resin. Admixtures primarily include single and multiple metal powders such as Al, Mg, Si, Al-Mg, Al-Si, and Al-Mg-Ca; pure carbon and composite alkalis such as B4C, Al8BC7, and Al4SiC4; and borides such as ZrB2, MgB, and CaB6.
Mixing Ratio: 50-60% 5-1 (3-1) mm magnesia; 10% <1 mm magnesia; 30-40% <0.074 mm magnesia + graphite + additives; and 5% resin (added externally).

Mixing: The order of adding materials is generally magnesia particles – binder – graphite – magnesia fine powder and additives. It is best to use mixing equipment equipped with a heater to ensure good resin flow and uniform distribution.

Molding: When using a hydraulic press to produce MgO-C bricks, a pressure of 115-200 MPa is generally required. The number of press cycles should be at least 15.

Heat Treatment: MgO-C bricks are generally treated at 200-250°C, with the following heating schedule: 50-60°C (maintaining temperature due to resin softening); 100-110°C (maintaining temperature due to solvent expulsion); and 200 or 250°C (maintaining temperature to ensure complete reaction).

It is clear that the production of MgO-C bricks, from raw materials to processes, has reached its limits. Therefore, improvements in the performance of existing MgO-C bricks will be slow and very limited. Furthermore, resources are limited. Because graphite is easily oxidized, high-temperature reactions between magnesium chloride and graphite are always present, especially in vacuum metallurgical environments.

The redox reactions of MgO-C bricks and the redox reactions between oxide inclusions and carbon are both determined by the nature of the material. Graphite’s susceptibility to oxidation is the dominant factor in determining this property. Therefore, artificial modification of graphite is an effective solution to this problem. Only in this way can the chemical compatibility of MgO-C bricks be further improved, thereby promoting their development.

Causes of damage to magnesia carbon bricks

The failure process of magnesia-carbon bricks: Magnesia-carbon bricks fail primarily due to oxidation of carbon within the brick, forming a decarburized layer. Furthermore, the significant difference in thermal expansion between magnesia and graphite at high temperatures (1.4% and 0.2%, respectively, at 1000°C) results in a loose structure and reduced strength. Slag erosion and mechanical scouring then gradually dissolve the magnesia particles within the brick, causing them to fall off layer by layer, ultimately destroying the brick. The failure process for magnesia-carbon bricks is: oxidation → decarburization → loosening → erosion → scouring → shedding → failure. Extensive research has demonstrated that above 1600°C, the following reactions are the primary causes of magnesia-carbon brick failure.

MgO(s)+C(s)→Mg(g)+CO(g) (1)

The damage of magnesia carbon bricks is first caused by the oxidation of carbon in the hot surface of the working lining, forming a thin decarburized layer. The oxidation of carbon is the result of continuous oxidation by iron oxides in the slag and O2 in the air, as well as oxides such as CO2 and SiO2, as well as the vaporization of carbon by MgO dissolved in the molten steel or in the bricks. Secondly, the high-temperature liquid slag penetrates into the pores of the decarburized layer or the cracks caused by the action of heat, reacting with the magnesium oxide in the bricks to form low-melting-point compounds, causing the surface layer of the bricks to undergo qualitative changes and weaken. Under the stress of strong slag stirring, mechanical scouring, etc., the bricks fall off layer by layer, resulting in the damage of magnesia carbon bricks. This cycle repeats itself, and the furnace lining becomes thinner layer by layer, and eventually the furnace needs to be repaired, repaired, or shut down.

01 Carbon Oxidation

The damage of magnesia carbon bricks is first caused by the oxidation of carbon in the bricks. The oxidation of carbon occurs through the following reactions:

Fe0+C→Fe+CO (2)

O2+2C→2CO (3)

CO2+C→2CO (4)

SiO2(s)+C(s)→SiO(g)+CO(g) (5)

MgO(s)+C(s)→Mg(g)+CO(g) (6)

Due to the oxidation of carbon, the carbon network structure in the bricks is destroyed, resulting in loose structure and reduced product strength. At the same time, the pores are increased, which also aggravates the erosion of the bricks by the slag.

02 The influence of pores

The pores in magnesia carbon bricks, especially the open pores, have an important influence on the damage of magnesia carbon bricks. During the use of magnesia carbon bricks, the oxidation damage of carbon is mainly promoted through the pores, which in turn aggravates the erosion of the brick lining by the slag, thus causing the damage of magnesia carbon bricks. Open pores within bricks draw in air during cooling. During reheating, oxygen in the air reacts with surrounding carbon to produce CO, which is then released. This process repeats, increasing porosity. Furthermore, the binder present in magnesia-carbon bricks is a significant factor in the formation of pores.

Phenolic resin is commonly used as a binder for magnesia-carbon bricks. Adding 3% to 4% phenolic resin results in a low porosity of approximately 3% after molding. However, during use, phenolic resin decomposes upon heating, producing gases such as H₂O, H₂, CH₄, CO, and CO₂ that evaporate and are then released. These evaporation pathways create pores, further increasing porosity. Consequently, oxygen in the air and oxides in the slag corrode the bricks through these pores, accelerating oxidation and damage to the carbon while exacerbating the reaction between the slag and the MgO in the bricks, damaging the magnesia-carbon bricks. This process repeats itself over and over again. Through the oxidation of carbon and the erosion of slag, on the one hand, the carbon network structure in the brick is destroyed, making the organizational structure loose and the high-temperature strength reduced. On the other hand, low-melting-point compounds are formed on the surface of the brick, weakening and deteriorating it. As a result, it falls off layer by layer under the stress of strong slag stirring, mechanical scouring, thermal shock, etc., causing damage to the magnesia carbon brick.