Best Insulation Bricks For Industrial Kilns: Types & Selection Guide

Heat transfer in thermal insulation refractories, such as insulation bricks, occurs via two pathways: solid-phase heat transfer and gas-phase heat transfer. At low temperatures, heat transfer is dominated by the solid phase, while gas-phase heat transfer becomes significant at high temperatures.

To reduce solid-phase thermal conduction, materials with low thermal conductivity should be selected, and the contact area between solid particles should be minimized. For instance, a more complex crystal structure increases phonon scattering and lowers thermal conductivity. Polycrystals exhibit poor structural integrity and regularity; combined with grain boundary impurities and lattice distortion, their thermal conductivity is lower than that of single crystals. Additionally, pores can scatter phonons, so an increase in porosity generally reduces thermal conductivity. Moreover, the use of spherical microparticles restricts heat transfer to the tiny contact areas between particles, which greatly lowers solid-phase thermal conduction.

Accordingly, the methods to improve the thermal insulation performance of insulation bricks are as follows:

1.Increase porosity;

2.Reduce pore size;

3.Enhance the complexity of multi-level microstructures;

4.Minimize solid-phase contact;

5.Add opacifiers to suppress radiative heat transfer.

Manufacturing Methods of Lightweight Insulation Bricks

1.Add lightweight aggregates, coat the aggregates with matrix materials, and then produce porous materials via cement bonding, chemical bonding, or ceramic bonding formed by sintering.

2.Mix combustible materials into the raw batch. The combustibles are completely burned out at high temperature, leaving in-situ pores that form porous materials.

3.Prepare a foamed slurry, solidify it with cement, and then conduct sintering to obtain porous materials.

The following are detailed application examples of the above processes for producing thermal insulation refractories, which help readers understand the application and construction techniques of such materials in terms of performance.

Fabrication of Insulation Bricks Using Alumina Hollow Balls

Alumina hollow ball bricks with excellent spalling resistance are prepared using alumina hollow balls, fused alumina powder, α-Al₂O₃ micropowder, SiO₂ micropowder, sillimanite, and clay powder as raw materials. The compositions of the raw materials are shown in Table 1.

It can be seen from Table 1 that alumina hollow balls serve as lightweight aggregates, providing high-temperature resistance and reducing bulk density. Alumina powder acts as an inert filler. Sillimanite functions as an expander, reducing firing shrinkage and enhancing thermal shock resistance. Alumina micropowder, silica micropowder, and clay powder serve as binders, lowering the sintering temperature, forming a mullite bonding phase, and improving both the high-temperature resistance and thermal shock resistance of the insulation bricks.

The brick mixture is prepared according to the designed batching formula. After mixing, shaping, and natural drying for 24 hours, the materials are further dried at 110 ℃ for 24 hours and fired at 1600 ℃ for 4 hours to produce the finished insulation bricks. When the sillimanite content is 12%, the bricks achieve the following properties: a bulk density of 1.48 g/cm³, compressive strength of 26 MPa, refractoriness under load of 1710 ℃, reheating linear change of +0.3% after heat treatment at 1600 ℃ for 2 hours, and thermal shock resistance of 17 cycles (from 1100 ℃ to water cooling). The prepared insulation bricks exhibit low bulk density, high strength, excellent high-temperature resistance, and superior thermal shock resistance.

Fabrication of Insulation Bricks with Lightweight Aggregates and Combustibles

Lightweight insulation bricks are produced by extrusion molding using polystyrene foam beads, expanded perlite powder, mullite fiber, clay, and silica sol as raw materials. The compositions of the raw materials are listed in Table 2.

It can be seen from Table 2 that expanded perlite acts as a lightweight raw material, further reducing bulk density and improving thermal insulation performance. Mullite fiber serves as a reinforcing agent to prevent shrinkage and cracking of the material during drying and sintering. Clay acts as a binder to form the matrix.

The adopted proportions are as follows: bonding clay 15%–30%, mullite fiber 3%–10%, expanded perlite 15%–28%, waste polystyrene foam beads 1.5%–3.5%, and silica sol 15%–25%.

The production process is described below. First, bonding clay, expanded perlite powder, and waste polystyrene foam beads are thoroughly mixed. Meanwhile, mullite fiber is uniformly dispersed into the binder at a controlled stirring speed, and water is added to form a slurry. Next, the powder mixture is blended with the slurry until a uniform viscous mass is obtained. After standing for a certain period, extrusion molding is carried out. The green bodies are dried at room temperature for two days or at 70 ℃ for several hours, followed by sintering to obtain the final products.

Fabrication of Anorthite Insulation Bricks via Conventional Foam Method

Anorthite-bonded mullite lightweight insulation refractories were developed using kyanite as the main raw material and white cement as the binder. The compositions of the raw materials are presented in Table 3.

This type of insulation material is a composite, with mullite as the primary crystalline phase and anorthite as the bonding phase. Anorthite has a melting point of only 1550 °C, yet it features a low coefficient of thermal expansion and low thermal conductivity. Accordingly, the composite insulation refractories combining anorthite and mullite integrate the low thermal conductivity of anorthite with the high service temperature of mullite. They also exhibit a lower sintering temperature, higher mechanical strength, and excellent thermal shock stability.

Anorthite-bonded mullite insulation materials are formed using the foam method. First, the foaming agent is prepared: rosin soap is synthesized from rosin and sodium hydroxide, and animal glue is then added to produce stable foam. Second, kyanite and white cement powder are mixed with water to form a slurry. Next, the foam is added and thoroughly stirred before casting for shaping. After that, curing, drying, and firing are carried out to obtain the green bodies. Finally, the finished products are obtained through cutting, precision grinding, and inspection. The foaming agent is the most critical factor in the foam forming process.

As shown in Table 4, mullite–anorthite insulation bricks exhibit excellent high-temperature resistance and superior thermal insulation performance, making them high-quality insulating refractories.