Magnesia-carbon bricks are refractory materials made from high-melting-point basic magnesium oxide (melting point 2800°C) and high-melting-point carbon materials that are resistant to slag wetting. Various non-oxide additives are incorporated, and the mixture is bonded using a carbon-based binder. Magnesia-carbon bricks are primarily used for the linings of converters, AC arc furnaces, and DC arc furnaces, as well as for the slag lines of steel ladles.
Typically, the melting loss of magnesium-carbon bricks occurs through the reaction between the magnesia on the working surface and the slag. The rate of melting loss depends not only on the properties of the magnesia itself but also on the size of the magnesia particles. Larger particles exhibit higher corrosion resistance, but they are also more likely to detach from the working surface of the magnesium-carbon brick and float into the slag; once this occurs, it accelerates the rate of brick deterioration.
Large magnesia particles exhibit greater absolute expansion than small ones. Furthermore, since the coefficient of thermal expansion of magnesia is much higher than that of graphite, the stress generated at the interface between large magnesia particles and graphite in MgO-C bricks is greater than that at the interface between small magnesia particles and graphite. Consequently, larger cracks form at these interfaces. This indicates that when the critical particle size of magnesia in MgO-C bricks is small, it helps mitigate thermal stress.
From the perspective of product performance, a smaller critical particle size reduces the open porosity of the product and decreases pore diameter, which helps improve the product’s oxidation resistance; however, it also increases internal friction between materials, making forming more difficult and resulting in lower density. Therefore, it is very difficult to generally determine the critical particle size of magnesia when producing MgO-C bricks. Typically, the critical particle size of magnesia must be determined based on the specific operating conditions of the MgO-C bricks. Generally, MgO-C bricks used in areas with large temperature gradients and severe thermal shock require a smaller critical particle size, whereas areas requiring high corrosion resistance require a larger critical particle size. To increase the bulk density of the product, manufacturers with lower-tonnage forming equipment may set the critical particle size slightly larger.
1. Fine-grained magnesia
To ensure uniform thermal expansion between the particles and the matrix in MgO-C bricks, a certain amount of fine magnesia powder must be incorporated into the matrix. This also helps the matrix retain a certain degree of structural integrity after oxidation.
However, if the fine magnesia powder is too fine, it will accelerate the reduction rate of MgO, thereby hastening the deterioration of the MgO-C bricks. Magnesia particles smaller than 0.01 mm react very easily with graphite; therefore, it is best to avoid incorporating such fine magnesia particles when producing MgO-C bricks. To obtain MgO-C bricks with excellent performance, the ratio of magnesia particles smaller than 0.074 mm to graphite should be less than 0.5. If this ratio exceeds 1, the porosity of the matrix will increase dramatically.
2. Amount of graphite added
The amount of graphite added should be determined in conjunction with the specific type of brick and its intended application. Generally, if the graphite content is less than 10%, it is difficult to form a continuous carbon network in the product, and the carbon cannot effectively fulfill its intended function; if the graphite content exceeds 20%, molding becomes difficult during production, cracks are likely to form, and the product is prone to oxidation. Therefore, the graphite content is typically maintained between 10% and 20%, with the specific amount selected based on the application. The fusion loss of MgO-C bricks is governed by two processes: the oxidation of graphite and the dissolution of MgO into the slag. Increasing the graphite content can reduce the rate of slag erosion but simultaneously increases damage caused by gas-phase and liquid-phase oxidation.
3. Compounding
Graphite is lightweight and tends to float to the top of the mixture during mixing, preventing it from coming into full contact with the other components in the formulation. High-speed mixers or planetary mixers are generally used. When producing MgO-C bricks, failure to pay attention to the order of material addition during mixing will affect the plasticity and moldability of the slurry, thereby impacting the yield and performance of the finished products.
The correct order of addition is: magnesia (coarse, medium) → binder → graphite → a mixture of fine magnesia powder and additives. The mixing time varies slightly depending on the type of mixing equipment. If the mixing time is too long, the graphite and fine powder surrounding the magnesia may detach, and the slurry may dry out due to the extensive evaporation of solvents in the binder; if it is too short, the mixture will be uneven and have poor plasticity, which is detrimental to molding.
4. Molding
Molding is a key method for increasing packing density and achieving a dense microstructure in the final product; therefore, high-pressure molding is required, and the pressing process must strictly follow the operating procedure of applying light pressure first, followed by heavy pressure, in multiple stages. In the production of MgO-C bricks, the density of the brick green is commonly used to control the molding process. Generally, the higher the tonnage of the press, the higher the density of the brick green, and the less binder required in the mixture (otherwise, the reduced distance between particles and the thinner liquid film would cause localized binder deficiency, resulting in an uneven product structure that affects performance and may also lead to elastic after-effects that cause the brick green to crack).
5. Hardening Treatment
MgO-C bricks bonded with phenolic resin can be heat-treated at temperatures ranging from 200 to 250°C. The resin can cure either directly (thermosetting resin) or indirectly (thermoplastic resin), giving the product high strength. The typical treatment time is 24 to 32 hours, during which the temperature must be maintained at 50 to 60°C to allow the resin to soften; at 100–110°C, the temperature must be maintained to allow for significant solvent evaporation; and at 200–250°C, the temperature must be maintained to allow the binder to undergo condensation and cure.
More details about magnesia carbon brick
What is magnesia carbon brick?
Magnesia carbon bricks are produced using high-purity sintered magnesia or fused magnesia combined with carbonaceous materials, primarily high-crystallinity graphite. A resin is used as a binder, and the mixture is shaped under high pressure and heat-treated.
What is the purpose of magnesia in brick?
Magnesia brick is used for permanent layers in steel making converters, AOD furnaces, and more. Magnesia brick is also used to line steel ladles and Basic Oxygen Furnaces (BOFs). Additionally, magnesia bricks are used in steelmaking furnaces, electric furnace bottom and wall, and high temperature tunnel kiln.
What is the melting point of magnesia carbon bricks?
2800 C
Magnesium-carbon bricks are made of high melting point alkaline oxide magnesia (melting point 2800 C) and high melting point carbon materials which are difficult to be soaked by slag, and various non-oxide additives are added.J
What are mag carbon bricks used for?
Magnesia carbon bricks, as one of the most important refractory materials for steelmaking applications, are widely used in electric arc furnaces (EAFs), ladles, and other high-temperature metallurgical equipment.
