Classification of thermal insulation refractories

Thermal insulation materials (also known as insulation materials) are materials or material composites with significant resistance to heat flow. Thermal insulation refractory materials generally have a porosity above 45%, low bulk density (no higher than 1.5g/cm3), and low thermal conductivity (≤1.0 w-m-1k-1). A wide range of insulation materials can generally be classified by material, temperature of use, form, and structure.

(1) Its internal organization and structure can be divided into three categories.

One is the porous granular category, the more common ones being expanded vermiculite and perlite.

The second is porous fibrous: the common ones are ultra-fine glass wool, asbestos, and mineral wool. These materials have good heat resistance and low thermal conductivity, usually made of inorganic fibers in a single or several composite fiber cloths or felt, with low thermal conductivity and good temperature resistance. However, glass and mineral wool specimens are prone to absorb water when applied and produce fiber dust in the air, which harms human health and seriously hinders their promotion and application. In the 1980s, fibrous insulation materials took a larger market share. They were mainly used for building wall insulation due to their excellent thermal and fire protection properties. Due to their many outstanding advantages, such as being lightweight, low thermal conductivity, and high heat capacity, inorganic fibers have been widely used in the thermal protection engineering of various types of advanced weapons. Currently, inorganic fibers that can be mass-produced include aluminosilicate, glass fibers, quartz, zirconium oxide, carbon, and silicon carbide fibers.

The third is the foam category: including inorganic, organic, and organic-inorganic hybrid type three types. The common inorganic types are foam glass and cement. In the 1980s, calcium silicate specimens were recognized as the best overall insulation material among rigid block materials due to low bulk density, low thermal conductivity, high refractoriness, and excellent mechanical properties. However, in the 1990s, pulp fibers gradually replaced them due to the presence of asbestos in their specimens. However, the pulp fibers are not resistant to high temperatures, making their high-temperature applications significantly compromised. The organic category includes phenolic foam, polyethylene foam, and polyurethane foam; these foam-type insulation materials have low bulk density, low thermal conductivity, and water resistance and are widely used in the insulation field.

(2) Low-temperature and high-temperature insulation materials are classified according to the use temperature.

The temperature range of low-temperature insulation materials is 600~900℃; the temperature range of medium-temperature insulation materials is 900~1200℃; the temperature of high-temperature insulation materials is generally higher than 1200℃.

(3) According to the bulk density, they can be divided into ultra-lightweight and lightweight bricks.

The bulk density of light bricks is 0.4~1.3g/cm3; the bulk density of ultra-light bricks is less than 0.4g/cm3.

(4) Classified by product shape.

One is the actual light refractory brick, including clay brick, silica, high alumina, and some pure oxide golden brick, etc.; the other is the indefinite soft refractory material, such as lightweight refractory concrete, etc.

High alumina thermal insulation bricks

(5) The raw materials can be divided into alumina, high alumina, silica, and magnesium thermal insulation refractory materials. Different materials of thermal insulation refractories have different use temperatures and related performance characteristics.

Alumina hollow ball thermal insulation bricks

(6) The material can be divided into three categories: organic heat insulation, inorganic heat insulation, and metal heat insulation.

The loss of heat dissipation on the surface of the furnace body and the failure of heat storage in the masonry in industrial furnaces is generally about 24~45% of the fuel consumption. When the structural material of the furnace body is made of lightweight bricks with low thermal fusion and small thermal conductivity, fuel consumption can be significantly reduced; at the same time, as the furnace can be rapidly heated up and cooled down, it can improve the production efficiency of the equipment; it can also reduce the weight of the furnace body, simplify the structure of the furnace body, improve the quality of the product, reduce the ambient temperature and improve the labor conditions.

Lightweight refractories have a loose structure and a large porosity within the material, which has the disadvantage of poor slag resistance. Slag can quickly invade the brick's porosity and break it, so it cannot be used in direct contact with liquid metal and slag; at the same time, due to its poor wear resistance, low mechanical strength, and poor thermal stability, it cannot be used as a load-bearing structure. It should not be used in contact with the furnace charge, which is easy to wear.

Due to the above disadvantages, light refractories are not used in industrial furnaces where they are in contact with the charge, in the hearth, in the slag, and hot air streams with high velocities and mechanical vibrations. Light refractories are mainly used as insulation or thermal insulation for furnaces. The main lightweight refractory products currently in use are zirconia hollow balls, alumina hollow balls, lightweight mullite bricks, lightweight, high alumina bricks, lightweight clay bricks, and refractory fibers are currently in everyday use as the current lightweight insulation materials.