Lightweight refractory bricks refer to refractory bricks with low thermal conductivity and low heat capacity, also known as thermal insulation refractory bricks. Lightweight refractory bricks generally have high porosity and low bulk density, and are also commonly called thermal insulating refractory bricks. Traditional lightweight refractory bricks have poor erosion resistance, strength and wear resistance. They are generally not used directly as working surface materials, but are placed behind the working surface as insulation layers. However, the closer the lightweight insulating refractory bricks are to the working surface, the better their thermal insulation effect. With the increasing requirements for energy conservation and emission reduction, the development and research of high-strength, high-temperature-resistant and erosion-resistant lightweight refractory bricks that can be used directly on the working surface have received widespread attention.
1. Classification of lightweight refractory bricks
Lightweight bricks, a key thermal insulation material in industrial kilns, are classified in a variety of ways, based on differences in chemical mineral composition, as well as different characteristics of use temperature, existence form and microstructure.
From the perspective of chemical mineral composition, the family of insulation refractory bricks includes alumina insulation bricks, high-alumina lightweight bricks, mullite lightweight insulation bricks, silica lightweight bricks, clay insulating refractory bricks, vermiculite lightweight bricks and diatomite lightweight bricks. Each material exhibits different refractory properties and applicable scenarios due to its unique chemical composition and mineral composition.
When considering the use temperature, the classification of lightweight refractory bricks is more refined. According to the temperature standard of reburning shrinkage rate not more than 1%~2%, we can divide it into low-temperature lightweight refractory bricks (use temperature below 600℃), medium-temperature lightweight refractory bricks (use temperature between 600 and 1200℃) and high-temperature lightweight refractory bricks (use temperature exceeds 1200℃). Among them, high-temperature lightweight refractory bricks have become an indispensable insulation material in industrial kilns due to their excellent thermal insulation properties.
In addition, the existence form of lightweight refractory bricks also provides another dimension for its classification. Powdered, shaped, fibrous and composite lightweight refractory bricks, these lightweight refractory bricks of various shapes not only meet the needs of different application scenarios, but also show the diversity and innovation of refractory technology.
Finally, from the perspective of microstructure, lightweight refractory bricks can also be divided into gas phase continuous structure type, solid phase continuous structure type and solid phase and gas phase continuous structure type. This classification method not only reveals the microstructural characteristics inside lightweight insulation refractory bricks, but also provides a scientific basis for our in-depth understanding and optimization of their performance.
2. Thermal insulation principle of lightweight refractory bricks
The basic principle followed in the preparation of lightweight refractory bricks is to reduce the thermal conductivity of the material. Since lightweight refractory bricks contain a large number of voids, the form of heat transfer through lightweight refractory bricks is heat transfer between the solid phase and the gas phase. The heat transfer form of the solid phase is mainly conduction, and the heat transfer form of the gas phase is more complicated: in the process of heat transfer from the high temperature zone to the inside of the insulation material, before encountering the pores, the effect of heat conduction occurs in the solid phase. After encountering the pores, the heat transfer pathways become two: continuing through the solid phase and transferring through the pores. For the part that continues to transfer through the solid phase, due to the change in the conduction direction, the distance of the heat conduction route is greatly increased, that is, the thermal resistance becomes larger; the heat transferred through the pores includes conduction, convection and radiation heat transfer through the gas. The specific conditions of the various heat transfer modes are as follows:
(1) Heat conduction: Under normal circumstances, the thermal conductivity of gas is very small. The gas in the pores of most lightweight refractory bricks is air. Table 9 shows the thermal conductivity of air at different temperatures. The thermal conductivity of air is much smaller than that of solid materials. Therefore, the heat transferred through the pores is very small.
(2) Convection heat transfer: Convection heat transfer mainly occurs through the flow of gas. Since the pores in most lightweight insulation refractory bricks are very small, the flow of air in the pores will be greatly restricted, the gas flow speed is very small, and the heat transferred is also very small. The smaller the pore size, the worse the fluidity of the air in the pores, and the smaller the amount of liquid transferred through convection. When the pore diameter is smaller than the free path of the gas molecules in the pore, the gas molecules stop moving and no heat is transferred through gas convection.
(3) Radiative heat transfer: Since the gas in the pores of most lightweight refractory bricks is air, and the gas molecules are mostly N2 and O2, they are all symmetrical diatomic molecular structures. The ability of these gas molecules to absorb and emit radiation is relatively poor. Therefore, the radiative heat transfer through the pores is mainly through the high-temperature wall of the pores to the low-temperature wall. But overall, the radiative heat transfer through the pores is not very large. It can be seen that the existence of pores is of great help to the thermal insulation ability of lightweight refractory bricks. In many cases, the design of thermal insulation materials is carried out around the introduction of pores.
