Introduction To Magnesium Carbon Bricks And Causes Of Damage

Magnesia carbon brick is a type of refractory product first developed in Japan. In 1970, practical tests began on electric furnaces, and after six years of experimental work, the material was officially promoted for application. In 1977, Japan's Kawasaki Iron and Steel Company, Chiba Plant, introduced the Q-POB converter. Resin-bonded, non-fired MgO-C (graphite) refractory materials were selected for the converter bottom and tuyere area, achieving great success. This marked the beginning of the use of composite refractories containing graphite in converters.

Following this, Western Europe developed bitumen-bonded magnesia carbon bricks with approximately 10% carbon residue, which were successfully used in water-cooled electric furnaces in areas other than high-temperature hotspots, as well as in converters.

China began researching magnesia carbon bricks in 1976. Experience using these bricks as converter linings has confirmed their suitability for steelmaking. To this day, magnesia carbon bricks continue to be widely used in converter steelmaking.

Magnesia-carbon brick is a carbon-bonded alkaline brick formulated with magnesia, carbonaceous raw materials, organic binders, and additives. It is produced through mixing, high-pressure molding, and low-temperature treatment. The brick has excellent thermal shock resistance, spalling resistance, slag resistance, and high-temperature creep resistance. This is due to the combined advantages of both alkaline and carbon materials. It is considered an ideal lining material for metallurgical melting furnaces and is highly valued in many countries.

At present, our country can produce both ordinary and high-strength magnesia-carbon bricks, as well as magnesia-carbon permeable bricks and other functional products, which can basically meet the needs of the developing metallurgical industry.

As a furnace lining refractory, magnesia-carbon bricks make effective use of the slag erosion resistance of magnesia and the high thermal conductivity and low thermal expansion of carbon. This compensates for magnesia's greatest weakness-its poor resistance to spalling. These bricks mainly possess the following excellent properties:

(1) High-temperature resistance

MgO and C do not form a eutectic at high temperatures, and both have high melting points. The melting point of magnesium oxide is 2800 °C, and that of carbon is above 3000 °C. Therefore, magnesia-carbon refractories made from these materials have a high melting point and excellent high-temperature resistance.

(2) Strong resistance to alkaline slag corrosion

MgO itself has strong resistance to alkaline and high-iron slags. Additionally, graphite has a large wetting angle with slag, resulting in very poor slag wettability, which further enhances the corrosion resistance.

(3) Good thermal shock stability

In magnesia-carbon refractories, graphite provides high thermal conductivity, a very low coefficient of thermal expansion, and a low modulus of elasticity. As a result, these refractories exhibit excellent thermal shock stability.

The performance of MgO-C bricks is mainly affected by the primary and secondary raw materials, chemical composition, relative content of each component, and the structure of the mixture. However, the type of binder, as well as the processing and molding technologies, also play equally important roles.

Figure 1 shows the production process of an MgO-C brick.

1. Selection of Raw Materials:

The raw materials used in the production of MgO-C bricks mainly include magnesia. The technical requirements for magnesia are high purity, low impurity content, well-developed crystal structure, uniform texture, low porosity, and high bulk density. Magnesia is classified into fused magnesia and sintered magnesia. Gradually replacing part of the sintered magnesia with fused magnesia can significantly improve corrosion resistance.

Carbonaceous materials are generally selected from natural flake graphite. It has no eutectic relationship with MgO or other oxides and does not form low-melting-point compounds. It also has high thermal conductivity, a low modulus of elasticity, a low coefficient of thermal expansion, and is characterized by non-wettability.

The binder is a key material in the production of MgO-C bricks. Its technical requirements are:

(1) a small wetting angle with carbon materials and good affinity,

(2) high residual carbon yield,

(3) low impurity and moisture content.

Common binders include phenolic resin, modified phenolic resin, and tar pitch. Today, most MgO-C bricks are made using synthetic phenolic resin.

Additives mainly include elemental metals such as Al, Mg, and Si, as well as alloys like Al-Mg, Al-Si, and Al-Mg-Ca. Other additives include B₄C, Al₈BC₇, Al₄SiC₄ (carbon-based or composite additives with alkaline properties), and borides like ZrB₂, MgB₂, and CaB₆.

2. Ratio:

5–1 mm (or 3–1 mm) magnesia: 50–60%

<1 mm magnesia: 10%

<0.074 mm magnesia + graphite + additives: 30–40%

Resin: 5% (added additionally)

Mixing:

The general order of addition is: coarse magnesia particles → binder → graphite → fine magnesia powder and additive blend.

It is preferable to use mixing equipment with a heating device to improve resin fluidity and ensure uniform distribution.

3. Molding:

When using a hydraulic press to produce MgO-C bricks, a pressure of 115–200 MPa is typically required. The number of pressurizations should be no less than 15 times.

4. Heat Treatment:

MgO-C bricks are generally heat-treated at 200–250 °C. The heating schedule is as follows:

50–60 °C: maintain temperature to allow resin softening

100–110 °C: maintain temperature to allow solvent evaporation

200–250 °C: maintain temperature to ensure reaction completion

It is evident that the production of MgO-C bricks-from raw material selection to processing-has reached a limit state. Therefore, performance improvement of existing MgO-C bricks will likely be slow and limited. Additionally, raw material resources are also limited.

The susceptibility of graphite to oxidation causes continuous high-temperature reactions between MgO-C bricks and graphite, especially in vacuum metallurgical environments.

The redox reactions of MgO-C bricks and the reactions between oxide inclusions and carbon are determined by the inherent properties of the materials. The oxidative tendency of graphite plays a dominant role in this process. Therefore, artificially modifying graphite is an effective way to address this issue. Only through this approach can the chemical compatibility of MgO-C bricks be improved, thus promoting their further development.

Damage Process of Magnesia-Carbon Bricks:

The damage to magnesia-carbon bricks begins primarily with the oxidation of the carbon content inside the brick, which leads to the formation of a decarburization layer. In addition, the difference in thermal expansion rates between magnesia (MgO) and graphite at high temperatures (at 1000 °C, approximately 1.4% and 0.2%, respectively) causes loosening of the brick's structure and a reduction in strength. Subsequently, the brick is subjected to slag erosion, mechanical abrasion, and other stresses. Over time, the MgO particles in the brick are gradually melted and eroded layer by layer, eventually leading to complete failure.

The typical destruction sequence of magnesia-carbon bricks is as follows:

Oxidation → Decarburization → Structural Loosening → Erosion → Abrasion → Spalling → Failure

Extensive research has shown that at temperatures above 1600 °C, the following reaction is a major cause of magnesia-carbon brick damage:

MgO(s)+C(s)→Mg(g)+CO(g) (1)

Initially, carbon at the hot surface of the working lining undergoes oxidation, forming a thin decarburized layer. This oxidation occurs due to continuous exposure to oxidizing agents such as iron oxides in the slag, oxygen (O₂) in the air, carbon dioxide (CO₂), silicon dioxide (SiO₂), and other oxides. Additionally, carbon vaporization may occur due to dissolved species in the molten steel or MgO within the brick.

Secondly, high-temperature liquid slag penetrates the pores or cracks in the decarburized layer caused by thermal stress. It then reacts with MgO in the brick to form low-melting-point compounds, weakening the surface layer. Under intense stirring of molten steel, mechanical abrasion, and other physical forces, the weakened layers peel off progressively, ultimately leading to the failure of the magnesia-carbon bricks.

01Oxidation of carbon

Damage to magnesium carbon bricks is first and foremost the result of oxidation of the carbon in the bricks, which takes place 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 network structure of carbon in the brick is destroyed, which results in a loose organisational structure and a reduction in the strength of the product, as well as an increase in porosity, which also exacerbates the erosion of the brick by the slag.

02. The Effect of Porosity

The porosity of magnesia-carbon bricks, especially open pores, plays an important role in their degradation. During the service life of magnesia-carbon bricks, pores serve as channels that promote the oxidative destruction of carbon. This, in turn, accelerates slag erosion of the brick lining and ultimately leads to the damage of the magnesia-carbon bricks.

Open pores allow external air to be drawn into the brick during cooling. Upon reheating, the oxygen in the air reacts with the surrounding carbon to form CO gas. This cycle of cooling and reheating causes repeated oxidation and leads to an increase in porosity over time. Additionally, the binder used in magnesia-carbon bricks is a significant contributor to porosity formation.

Phenolic resin is commonly used as the binder in magnesia-carbon bricks. When 3%–4% phenolic resin is added, the initial porosity after molding is relatively low-around 3%. However, during use, the phenolic resin decomposes upon heating and releases gases such as H₂O, H₂, CH₄, CO, and CO₂. The evaporation of these gases creates pathways that form additional pores, thereby increasing the brick's overall porosity.

Through these pores, oxygen from the air and oxides from the slag can infiltrate the brick, promoting further oxidation of the carbon and intensifying the reaction between slag and MgO in the brick. This repeated process of carbon oxidation and slag erosion destroys the carbon network structure within the brick, resulting in structural loosening and reduced high-temperature strength.

At the same time, reactions at the brick surface form low-melting-point compounds, which weaken and degrade the surface. Under the effects of intense slag agitation, mechanical abrasion, thermal shock, and other stresses, the weakened layers peel off one by one-ultimately leading to the destruction of magnesia-carbon bricks.