When it comes to refractory bricks, for those unfamiliar with the refractory materials industry, they may think that refractory bricks are only square-shaped and unaware that there are many types of refractory bricks. From the perspective of refractory brick materials, the most commonly used refractory bricks are: clay brick, high alumina brick, lightweight clay brick, lightweight high alumina brick, mullite lightweight brick, etc. Below, we will provide a detailed introduction to these commonly used refractory bricks.

01.Clay Brick

Clay brick is a refractory material with an Al2O3 content of 30% to 48%, made of clay clinker as aggregate and refractory clay (soft clay or semi-soft clay) as a binder. It is a widely used and high-yield refractory product.

The properties of clay products vary within a wide range due to differences in chemical composition and production processes. Clay products generally have the following properties:

(1) Chemical and mineral composition: Al2O3 content is 30% to 48%, SiO2 content is 50% to 65%, and a small amount of alkali metal, alkaline earth metal oxides, TiO2, Fe2O3, etc. The mineral composition generally includes mullite, orthoclase, quartz, and glassy phase.

(2) Refractoriness is generally between 1580°C and 1750°C, which increases with the increase of Al2O3/SiO2 ratio. When there is a high content of low-melting impurities, the refractoriness of the product significantly decreases.

(3) Load softening temperature is about 1250°C to 1450°C, with a wide range of variations. The starting deformation temperature is relatively low, about 200°C to 250°C lower than the 40% deformation temperature.

(4) Low linear expansion coefficient and low thermal conductivity.

(5) Good thermal shock resistance with a large fluctuation range. Generally, it can withstand more than 10 cycles of water quenching at 1100°C. This is related to the low linear expansion coefficient of clay-based products, insignificant crystal phase transformation effect, and plasticity at high temperatures.

(6) Resistance to chemical erosion: As it is weakly acidic, it has strong resistance to acidic slag erosion but weak resistance to alkaline substance erosion.

Clay products belong to acidic refractory products, and their acidity increases with the increase of SiO2 content. They have a certain resistance to acidic furnace slag erosion but poor resistance to alkaline slag erosion. Therefore, clay products are suitable for use as linings in acid kilns. They are also used in blast furnaces, hot blast stoves, glass kilns, carbon calciners, and other high-temperature kilns.

02. High Alumina Brick

The production process of high alumina products is similar to that of multi-clay refractory products. The specific production process of high alumina products should be determined based on the actual production conditions, raw material characteristics, product requirements, and production conditions. It involves strict grading of pre-crushed raw material blocks, particle material grading and storage, iron removal, mixing of clinker and bonding clay, fine grinding, etc.

Characteristics of high-alumina products:

Load softening temperature: The load softening temperature of ordinary high-alumina refractory products is generally above 1420-1550°C, higher than that of clay refractory products, and increases with an increase in Al2O3 content. When Al2O3 is less than 70%, the load softening temperature increases with an increase in the ratio of mullite phase and glass phase, and the quantity and properties of the liquid phase significantly affect the load softening temperature. Therefore, reducing impurity content in raw materials is beneficial for improving the load softening temperature and high-temperature creep resistance. When Al2O3 is greater than 70%, the increase in Al2O3 content does not significantly affect the load softening temperature.

Thermal shock resistance: The thermal shock resistance of ordinary high-alumina refractory products mainly depends on the chemical mineral composition and microstructure. Generally, it is inferior to clay products.

Chemical corrosion resistance: Ordinary high-alumina refractory products exhibit good resistance to acid or alkaline slag, metal liquid corrosion and oxidation, and reduction reactions. With an increase in Al2O3 content and a decrease in harmful impurity content, their resistance is enhanced.

High-alumina products are commonly used in blast furnaces, hot blast stoves, electric furnaces, and other industrial kilns.

High-alumina bricks are categorized based on their physical and chemical properties, including LZ-80, LZ-75, LZ-65, LZ-55, LZ-48, and several other grades.

03. Lightweight clay bricks

Lightweight clay bricks refer to lightweight refractory products with an Al2O3 content ranging from 30% to 46%. They are mainly made from clay clinker or lightweight clinker and plastic clay as the primary raw materials. They are usually produced using the combustible method but can also be formed through chemical or foam methods to create a porous structure. The ingredients are mixed with water to form plastic clay or mud, which is then extruded or cast and dried before being fired in an oxidizing atmosphere at 1250-1350°C. The typical bulk density of lightweight clay bricks is 0.75-1.20 g/cm3, with compressive strength ranging from 2.0-5.9 MPa. Lightweight clay bricks have a wide range of applications and are primarily used as insulation materials in various industrial kilns where they are not in contact with molten materials or exposed to corrosive gases. They can withstand temperatures ranging from 1200-1400°C. According to the Chinese standard (GB3994-1983), lightweight clay bricks are classified into ten grades based on bulk density, such as NG-1.5, NG-1.3a, NG-1.3b, NG-1.0, NG-0.9, NG-0.8, NG-0.7, NG-0.6, and others.

04.Lightweight High-Alumina Bricks

Lightweight high-alumina bricks generally refer to various lightweight refractory products with an Al2O3 content of over 45%. They are typically divided into two categories: one is ordinary lightweight high-alumina bricks made from natural high-alumina bauxite clinker, and the other is lightweight high-alumina products made from electrically fused or sintered alumina, where the main crystal phase is corundum, also known as lightweight corundum bricks.

Lightweight high-alumina bricks are commonly produced using the foam method. After grinding the clinker, it is mixed with binders (such as clay) and foaming agents to prepare a slurry. The slurry is then poured and molded before being fired at temperatures ranging from 1350 to 1500°C. The typical bulk density of lightweight high-alumina bricks is 0.4 to 1.35 g/cm3, with a porosity ranging from 66% to 73%. They have a compressive strength of 1.3 to 8.1 MPa and a thermal conductivity of 0.291 to 0.582 W/(m+K) at 350°C. Lightweight high-alumina bricks exhibit high refractoriness and excellent thermal shock resistance, making them commonly used as high-temperature insulation layers in kilns. They can be used as linings in furnaces and kilns that directly contact flames but should not be used in environments directly exposed to molten slag. Due to their good chemical stability in reducing atmospheres, lightweight high-alumina bricks are often used as insulation linings in kilns protected by gases such as hydrogen and carbon monoxide. The recommended operating temperature for lightweight high-alumina bricks is 1350 to 1500°C, while high-purity products can withstand temperatures up to 1650 to 1800°C. In China, lightweight high-alumina bricks are classified into several grades based on bulk density, such as LG-1.0, LG-0.9, LG-0.8, LG-0.7, LG-0.6, and others, according to the national standard.

05.Mullite Lightweight Bricks

Mullite lightweight bricks are a new type of thermal insulation refractory material that primarily consists of mullite (3Al2O3·2SiO2) as the main crystal phase and bonding phase, with an Al2O3 content ranging from 50% to 85%. Mullite exhibits excellent high-temperature mechanical properties and chemical stability. Mullite lightweight bricks possess advantageous properties such as high-temperature structural strength, low creep rate at high temperatures, low thermal expansion coefficient, strong resistance to chemical corrosion, and good thermal shock resistance. They can be used as linings in furnaces that directly contact flames and significantly improve energy efficiency. The recommended operating temperature for mullite lightweight bricks varies from 1350°C to 1700°C, depending on the alumina content and bulk density of the product.

The aforementioned descriptions cover commonly used types of refractory bricks. It is important to determine the specific usage environment of refractory bricks, as the performance and specifications may vary depending on the application conditions.