Introduction
Refractory materials are those materials that are resistant to high temperatures or other severe conditions such as thermal shock, chemical attack, and abrasion. Refractory materials are usually made of refractory substances, such as alumina, magnesia and silica that have the ability to resist temperatures over 1500 degree Celsius for a considerable time period. Refractory materials must show sufficient thermomechanical and chemical stability to overcome the various environmental conditions within the furnace, such as high temperature flux, abrasive abrasion, and thermal shock.
Types of Refractory Materials
There are a variety of refractory materials in the market, including alumina, silica, magnesia, carbon-bonded as well as clay-bonded refractories, ceramic fiber refractories, castable and plastic refractories, and others.
1. Alumina Refractories
Alumina refractories are the most commonly used refractory materials containing high thermal stability and good performance in resisting chemical or slag attack. They are derived from industrial alumina, and made of synthetic corundum, mullite and other materials by sintering, molding and fusion. Alumina refractories are usually composed of more than 68% alumina. They are widely found in various thermal systems, such as coke-ovens, steel-making open hearth furnaces, ladles, and ore-fed furnaces.
2. Silica Refractories
Silica refractories are a kind of unshaped refractory material composed mainly of quartz sand, along with minor amounts of plastic clay and binding agents. They have low softening temperature and resistance to thermal shock as well as oxidative attack, and are widely used in glass melting furnaces, fire slag burning and gangue sintering systems.
3. Magnesia Refractories
Magnesia refractories are mainly composed of natural magnesia or magnesia-chrome, and formulated with magnesia-based clinkers, such as dead-burnt magnesite and caustic-calcined magnesite. They perform well in environments with higher temperatures and intensive chemical attack, and can endure extremely high temperatures of above 1500 degree Celsius, and are therefore widely used in high-temperature ladles, tundishes, and thermal layers in steel-making furnaces.
4. Carbon-Bonded and Clay-Bonded Refractories
Carbon-bonded and clay-bonded refractories are mainly composed of hard, refractory clinkers such as alumina, chromite, and magnesia with bonding agents such as tar, coal tar, asphalt or clay binder. Carbon-bonded refractories possess higher abrasion resistance and high thermal conductivity. Clay-bonded refractories are most commonly found in alkaline material systems, where their properties are inferior to the carbon-bonded type but cost less.
5. Ceramic Fiber Refractories
Ceramic fiber refractories are made from inorganic materials such as alumina, silica, mullite, and zircon, and are processed into fiber forms through professional techniques and made into special products. Ceramic fiber refractories have outstanding insulation performance, low thermal and mechanical shock resistance, high operating temperature, and durability and outstanding thermal stability. They are used in high-temperature access methods and insulation applications in furnaces and reaction vessels, for example, to coat the walls of the furnaces and prevent them from overheating.
6. Castable and Plastic Refractories
Castable and plastic refractories are composed of mixtures of different refractory materials, such as alumina, silicon carbide, zirconia and bauxite, formulated with cement and other plastic binders. They are characterized by good thermal shock resistance and easy construction. Castable and plastic refractories are often used in applications such as foundry furnace linings, radiation combination plates and combustion chamber linings.
Requirement of Materials
When selecting the many types of materials for the refractory composition of a given furnace, many factors need to be considered. These include the service temperature, thermal shock resistance, resistance to chemical corrosion and mechanical strength. The furnace designer must also ensure that the material can maintain its integrity for the expected service lifetime.
1. Thermal Stability
In order to ensure thermomechanical and chemical stability, the refractory materials must resist high temperature flux and abrasion for a certain time period, as well as being resistant to thermal shock and chemical attack from hostile environment conditions.
2. Thermal Shocks
In many cases, the refractory lining may suddenly contact with hot substances that can cause extreme thermal shock. Therefore, the refractory materials used must be able to withstand sudden temperature changes and prevent any damage to its structure.
3. Abrasion
The masonry bed used inside the furnace chamber and the lining of the furnace walls must be capable of resisting abrasive forces, which may come from material being filled, withdrawn or moved inside the chamber.
4. Slag Corrosion
Coking and glass production processes involve using materials mixed with acid components, which may cause severe chemical attack on the refractory lining. Therefore, the materials should be chosen to be corrosion resistant.
Conclusion
The choice of refractory material for lining a furnace depends largely on the system’s temperature, shape, and environment conditions. Various types of refractory materials, such as alumina, silica, magnesia, carbon-bonded and clay-bonded refractories, ceramic fiber refractories, castable and plastic refractories, have their respective performance and advantages, and the designer needs to consider all these factors when selecting the most suitable refractory material for the furnace.
