Refractories | Definition, Classifications, Types, Applications, and Manufacturing

Low-temperature Refractories: Also known as Ordinary refractories, these are refractories which withstand temperature up to around 600°C (1,112°F), such as refractory clay.

High-temperature Refractories: Also known as advanced refractories, these are refractories which withstand temperature from 1,500°C to 2,200°C (2,732°F to 3,992°F), such as chromates.

Ultra-high-temperature Refractories: Also known as super-grade refractories, these are refractories which withstand temperature beyond 2,200°C (3,992°F), such as zirconia.

• Fireclay Refractories.
• Silica Refractories.
• High Alumina Refractories.
• Magnesite Refractories.
• Zirconia Refractories.
• Refractory Castables.
• Insulating Refractories.
• Chromite.
• Magnesium.
• Bricks.

Fireclay Refractories:

Fireclay refractories primarily composed of clay minerals, with kaolinite being a common component. They are suitable for low-temperature applications like fireplaces and domestic heating systems.

Bricks are composed of silica, alumina, lime, iron oxide, and magnesia. They are shaped refractory units used for constructing linings in furnaces, kilns, and other high-temperature vessels.

Refractories properties

Refractories have high melting point, high heat resistance, good chemical stability, low thermal conductivity, and high wear resistance in some application.

Heat Resistance:

Refractories must withstand high temperatures without undergoing significant physical or chemical changes. The ability to resist thermal shock and maintain structural integrity at elevated temperatures is vital.

Chemical Stability:

Refractories should exhibit resistance to chemical reactions with the materials they come into contact with during use. This is crucial to prevent corrosion or erosion, especially in aggressive industrial environments.

Mechanical Strength:

Mechanical strength is essential to ensure that refractories can support the load and resist mechanical stress, abrasion, and wear. This property is particularly important in applications where refractories are subject to mechanical forces.

Porosity and Permeability:

Controlling porosity is crucial in refractories to regulate thermal conductivity and prevent the penetration of gases or liquids. Depending on the application, refractories may be designed to be dense or porous.

Thermal Conductivity:

The ability of a refractory material to conduct heat is a critical property. Lower thermal conductivity is often desirable, as it helps in maintaining temperature differentials and energy efficiency in high-temperature processes.

Thermal Expansion:

Refractories should exhibit minimal expansion or contraction when exposed to temperature variations. This property is vital to prevent cracking and maintain the structural integrity of the refractory lining.

Abrasion Resistance:

In applications where refractories are subject to abrasive forces, such as in furnaces with moving materials, resistance to abrasion becomes crucial to ensure a prolonged service life.

Cohesion and Adhesion:

The ability of refractories to maintain cohesion (internal strength) and adhesion (ability to adhere to the substrate) is essential for the durability and effectiveness of the refractory lining.

• Refractoriness Under Load (RUL): Is a measure of the ability of a refractory to withstand a specific load at high temperatures without deformation. It provides insights into the refractory's performance under working conditions.

Refractories applications

Refractories play a crucial role in various industrial applications, including steel industry, cement industry, petrochemical industry, glass industry, power generation, and non-ferrous metal industry.

Refractories are extensively used in the steel industry for lining the walls of furnaces, converters, and ladles. They provide insulation and resistance to the high temperatures generated during the production of steel. Different types of refractories, such as magnesite, chromite, and alumina-based materials, are chosen based on the specific requirements of each application.

Rotary kilns in cement plants operate at extremely high temperatures, and refractories are crucial for lining these kilns. The refractory lining protects the kiln shell from heat while also providing a surface that facilitates the proper formation of clinker. Alumina-silicate and magnesia-based refractories are commonly used in cement applications.

Refractories find applications in the petrochemical sector, especially in processes involving the production of chemicals, petroleum refining, and synthesis gas manufacturing. Alumina, silica, and zirconia-based refractories are chosen for their resistance to chemical corrosion and thermal stability.

Furnaces used in glass manufacturing operate at high temperatures, making refractories essential for lining the furnace walls. Refractory materials with good thermal shock resistance, such as silica bricks and zirconia-based materials, are preferred in the glass industry.

Boilers and gasifiers in power plants operate at elevated temperatures, demanding refractories for insulation and protection. Refractory materials such as fireclay, alumina, and silica-based products are chosen based on the specific conditions of each power generation unit.

Refractories are employed in the production of non-ferrous metals like aluminum, copper, and nickel. They are used in various applications, including smelting furnaces, anode baking furnaces, and refining vessels. Refractory materials with high thermal conductivity and resistance to metal corrosion are crucial in these processes.

Refractories manufacturing process

The manufacturing process of refractories involves several crucial steps :

1. Raw material selection.
2. Batching.
3. Forming.
4. Drying.
5. Firing (Sintering).
6. Cooling.
7. Finishing and quality control.

1. Raw Material Selection

The first step is the careful selection of raw materials based on the desired properties of the final refractory product. Common raw materials include alumina, magnesia, silica, chromite, zirconia, and various other minerals. The choice of materials depends on the application and the conditions the refractory will be exposed to.

The most important raw materials for manufacturing refractory materials are Alumina, silica, and magnesia.

2. Batching

Once the raw materials are selected, they are weighed and mixed in precise proportions. Batching is a critical step to ensure uniformity and consistency in the composition of the refractory mixture. The goal is to achieve the desired physical and chemical properties in the final product.

3. Forming

After batching, the refractory mix is formed into a specific shape. There are several forming methods, including:

4. Drying

The formed refractory shapes undergo a drying process to remove excess moisture. This is typically done gradually to prevent cracks or warping. Drying can occur in air, specialized drying chambers, or through controlled heating.

5. Firing (Sintering)

The dried refractory shapes are fired in kilns at high temperatures in a process called sintering. During firing, the refractory material undergoes chemical and physical transformations, becoming more dense and acquiring the desired properties. The firing temperature and duration are carefully controlled based on the type of refractory being produced.

6. Cooling

After sintering, the refractory products are slowly cooled to room temperature. Rapid cooling can lead to thermal shock and cracking, so controlled cooling is essential to ensure the integrity of the final product.

The finished refractory products may undergo additional processes such as grinding, cutting, or coating to meet specific dimensional and surface finish requirements. Quality control measures are implemented throughout the manufacturing process to ensure that the refractories meet the necessary standards and specifications.