Introduction to insulating bricks
Ceramic refractories are useful for the following functions:
1. Acting as a thermal barrier between the hot interior environment and the outer wall of the furnace
2. Withstanding physical stresses and preventing heat-induced erosion of the furnace wall
3. Protection against corrosion
4. Acting as thermal insulation
Refractories have many useful applications.
In the metallurgical and ceramic industries, refractories are used in the lining of furnaces, reactors, and other equipment that contains or transports hot materials such as metal and slag.
Refractories have other high temperature applications such as flame heaters, hydrogen reformers, primary and secondary ammonia reformers, cracking furnaces, boilers, catalytic cracking units, air-blast heaters, and sulfur processing furnaces.
This article reviews insulating fire bricks, a subset of thermal insulation bricks. These are shaped refractory products that have a service temperature above about 1000°C and a porosity of more than 45%.
Insulating brick is a relatively soft product made of refractory ceramic materials that simultaneously withstand high temperatures and have low thermal conductivity. The thermal conductivity of the final product depends on its chemical composition, density, and pore structure. Insulating brick is generally lightweight and can be shaped and cut with a hand saw or any other hand tool such as a chisel or drill.
Insulating bricks are ideal for forming into specific shapes, including curves and custom cavities. However, due to their high apparent porosity, loose structure, low strength, and poor resistance to various types of erosion and wear, they are mostly used in the thermal insulation layer of furnaces and other high-temperature thermal equipment.
In general, insulating bricks are not suitable for use in the main layer and structures under heavy loads or environments exposed to severe corrosion and abrasion.
Types of insulating bricks and raw materials for insulating bricks
Types of insulating bricks available in the market include aluminosilicate refractory bricks, silicate bricks or lightweight corundum (alumina) bricks and in general, the higher the alumina content and firing temperature of the insulating refractory brick, the higher its application temperature. The most common raw materials used for insulating bricks include clay, kaolin, kyanite, mullite, chamotte (light), sillimanite or andalusite.
Factors affecting the thermal conductivity of insulating bricks
Thermal conductivity is influenced by the constituent materials as well as the total porosity, pore shape and pore size distribution. The most common raw materials used for insulating bricks include clay, kaolin, kyanite, mullite, chamotte (light), sillimanite or andalusite.
Methods of creating porosity in insulating bricks
In insulating bricks, porosity is created by removing combustible materials and water. Common combustible materials include sawdust, straw, styrene bubbles, coke, and cellulose. In addition, foam or a foaming agent can be added to increase porosity.
How to produce insulating bricks
The raw materials and combustible materials are either mixed dry or with water. The amount of water required depends on the shaping process.
Different suppliers produce insulating bricks using different techniques, such as casting, slinging, extrusion or dry pressing. These different techniques can result in different insulating properties and qualities.
Production of insulating bricks by casting method
In the casting process of insulating bricks, soap or saponins are used as foam, while metal or carbide powders are used as foaming agents. Lightweight bricks with high porosity can be produced in this way. For the casting process, the bricks are cast directly into large molds, often made of plaster, with vibratory aids to help the slurry flow better.Hydrophilic raw materials in the slurry composition remove water or gel it, thereby helping to form the raw body. The gelling process can be accelerated by adding gypsum or cement to the mixture.Due to the large volume of water used in this method, the casting blanks can take a long time to dry. The casting method is used to form large volumes or more complex parts with small to medium volumes.
Production of insulating bricks using the slinging method
The “slinging” process is a continuous process in which lumps are hooked into large molds or onto a conveyor belt. The slinging process is a form of low-pressure extrusion of a wet clay mixture containing large amounts of flammable additives,With an additional processing step where the semi-extruded material is hung on a continuous belt to create more porosity before drying and firing. Medium density insulating refractory bricks can be produced in this way.
Production of insulating bricks by extrusion method
In the extrusion process, a moist clay mixture containing flammable additives is extruded through a die, and the extruded section is then cut into brick shapes and subsequently dried and fired. Dry pressing is usually done in one direction. The pressing method is suitable for producing bricks with higher density, so it is mostly used for high-density bricks.
Insulation bricks comparison table
The shaping process and pore-forming additives create the porous structures common in insulating refractory bricks.This results in a wide range of thermal conductivity within a product group, which in turn leads to variations in the capabilities of different types of insulating bricks to control energy loss from the furnace .
Class 23 IFB Production Process: Grease Casting Unit, Slinger, Extrusion, Dry Press/Cement Density
Kg/m3483611569520
ASTM C-155lb/ft330383633
Refractive Index (MOR)MPa10.70.91.2
ASTM C-93lb/in2145102131174
Cold Compressive Strength (CCS)
Mpa1.20.91.12
ASTM C-93lb/in2174131160290
The average value of the physical properties of four commercial grades of Class 23 insulating bricks, the differences in which reflect differences in the main production processes used by the manufacturers. In insulating bricks, low density is associated with low strength.
Cold compressive strength is measured as an indicator for brick classification.The “hot” properties of the material must be good enough to support walls and arches at the intended service temperature. Hot flexural strength or creep under load are also characteristics for classifying bricks for use at elevated temperatures.
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