Selection of carbon source and binder for magnesia-carbon bricks

Carbon-containing refractories emerged in the early 1980s as high-performance materials alongside advances in steelmaking technology. They are widely used in equipment and processes such as top-blown converters, ultra-high-power electric furnaces, ladle metallurgy, continuous casting, and molten iron pretreatment. Magnesium-carbon bricks, a carbon-containing refractory material developed in the 1970s, primarily consist of fused magnesia or sintered magnesia and carbon materials. They are formed by extrusion using organic binders.

Carbon source for magnesium-carbon bricks

Materials that can serve as carbon sources for magnesium-carbon bricks include petroleum coke, coal tar pitch, flake graphite, and carbon black. Among these, flake graphite is considered the best carbon raw material due to its intact crystal structure and high-temperature resistance.

Below is a brief introduction to the characteristics of flake graphite:

① Excellent high-temperature resistance, with a melting point reaching up to 3900°C and a boiling point typically at 4250°C. It withstands extreme heat exposure with minimal mass loss and volume expansion. Graphite strength increases after heat treatment and progressively grows with rising temperatures.

② Low electrical resistivity and thermal conductivity. While metals generally conduct electricity better than non-metals, graphite surpasses many metals in conductivity while generating minimal heat. Its thermal conductivity varies with temperature: high at room temperature but decreasing at elevated temperatures, even exhibiting insulating properties.

③ Excellent lubricity. Graphite's unique molecular structure allows easy sliding between layers, with friction coefficients decreasing as flake size increases.

④ Chemically stable: With over 80% carbon content, graphite remains largely inert to inorganic and organic reagents at room temperature.

⑤ High plasticity: Graphite exhibits excellent ductility and can be compressed into thin sheets.

⑥ Exceptional thermal stability: Graphite maintains structural integrity under rapid temperature fluctuations without fracturing due to expansion or contraction.

Magnesium Carbon Binder

Under specified production processes, products obtained by mixing raw materials such as magnesia, graphite, binders, and antioxidants without firing or through light calcination are termed magnesia-carbon bricks. Due to graphite's stable chemical properties, it cannot react directly with MgO at room temperature. A binder is required to facilitate their bonding, enabling the material to be mixed and formed. Consequently, the quality of the binder directly impacts the performance of magnesia-carbon bricks. We require the binder to effectively wet large-particle aggregates and graphite while possessing suitable viscosity. Furthermore, a binder with excellent wetting properties should distribute uniformly across particle and graphite surfaces. This ensures a continuous matrix structure and carbon skeleton after carbonization, both of which enhance the overall performance of the magnesia-carbon brick.

To adapt to varying carbonization conditions and positively influence material structure and performance, the binder for magnesium carbon bricks must meet the following criteria:

(1) Appropriate viscosity to effectively wet aggregates and graphite during mixing, without exhibiting aging after addition;

(2) Further self-condensation during heating and forming without significant volume expansion or contraction, ensuring product quality and preventing surface cracking;

(3) High coking value during high-temperature carbonization of pressed samples, resulting in high mechanical strength of the carbonized composite;

(4) Abundant resources, wide availability, and low cost.

Currently, substances usable as binders for refractory materials can be divided into three major categories:

(1) Bituminous materials, among which coal tar pitch is the most widely used. This is because coal tar pitch offers excellent impregnability, moderate viscosity, good fluidity without requiring high temperatures, high carbon residue after carbonization forming an anisotropic structure, and low cost. However, its drawbacks are significant: it contains toxic substances, particularly the potent carcinogen benzo[a]pyrene. As environmental concerns have grown, awareness of coal tar pitch's hazards has increased, leading to reduced usage.

(2) Resin-based binders: Phenolic resin is currently the most widely used binder. Produced through the condensation polymerization of phenol and formaldehyde, it exhibits excellent adhesion at room temperature and yields a carbon residue slightly lower than asphalt after carbonization. However, high-temperature treatment causes the resin to transform into a glassy state, increasing the brittleness of magnesia-carbon bricks and reducing the material's thermal stability and oxidation resistance.

(3) Asphalt-resin blends: Materials obtained by modifying asphalt and resin through blending. This novel binder addresses the respective shortcomings of each component, achieving complementary effects.

Magnesia-carbon bricks, through the correct selection of carbon source and binder, not only retain the excellent properties of carbon-containing refractories, but also improve their shortcomings. They are widely used in rotary electric furnace linings, furnace walls, and ladle slag lines, and are a highly efficient and energy-saving refractory material.