(1) Clay-bonded silicon carbide bricks
Clay-bonded silicon carbide bricks are fired refractory products made primarily from silicon carbide with clay serving as the bonding agent. Characterized by high thermal conductivity, a low coefficient of thermal expansion, and excellent thermal shock and wear resistance, they represent the earliest type of silicon carbide-based brick developed.
These bricks utilize black silicon carbide as the raw material, with a chemical composition of: SiC 98.0%, free C 0.5%, Fe 0.2%, and free SiO₂ 0.6%. Soft clay with good bonding properties and pulp waste liquor serve as the binders. While historically high proportions of bonding clay (typically 10%–15%) were used, current practice favors a combination of 3% clay and 5% pulp waste liquor. Bricks with lower clay content exhibit thermal conductivity three to four times higher than those with higher clay content, as well as superior load-bearing softening temperatures and slag resistance. The manufacturing process for bricks with 3% clay content involves blending coarse, medium, and fine SiC particles to achieve maximum bulk density. After thorough dry mixing, the clay is added; following another 3 minutes of dry mixing, 4% pulp waste liquor (density 1.26–1.28 g/cm³) is introduced for wet mixing. The moisture content of the mixture is approximately 1.5%. After a 12-hour aging period, the mixture is passed through a 4 mm sieve and formed using a brick press. The bulk density of the green bricks ranges from 2.5 to 2.7 g/cm³. The bricks are air-dried for 2–4 days and then fired in a tunnel kiln at 1400°C.
These products are used for ceramic kiln furniture (such as batts, supports, and saggers), muffle furnace covers, linings for aluminum refining furnaces, distillation retorts for zinc distillation furnaces, and coolers for cement rotary kilns.
(2) Silicon nitride-bonded silicon carbide bricks
Silicon nitride-bonded silicon carbide bricks are refractory products manufactured from SiC and Si powders through a process of nitriding and firing. They are characterized by a silicon nitride (Si3N4) bonding phase. The Si3N4 exists as acicular (needle-like) or fibrous crystals between the silicon carbide (SiC) grains, making this an important type of advanced refractory material.
Utilizing the principle of reaction sintering, SiC green bodies containing silicon powder are sintered via a nitridation reaction; the firing process takes place in a sealed furnace with a nitrogen atmosphere, allowing for precise control. Furnace temperature, pressure, and atmosphere are strictly regulated. Key process parameters include: nitriding gas pressure of 0.02–0.04 MPa, furnace atmosphere oxygen content below 0.01%, and a final nitridation temperature of 1350–1450°C; the total nitridation time varies depending on the shape and size of the product.
Silicon nitride-bonded silicon carbide bricks exhibit excellent resistance to alkali erosion, resistance to wetting by molten cryolite, oxidation resistance, wear resistance, high thermal conductivity, thermal shock resistance, and extremely low electrical conductivity.
These bricks are primarily used as linings for the lower shaft of blast furnaces, offering a service life more than double that of traditional refractories. Additionally, they serve as linings for aluminum reduction cells and as ceramic kiln furniture-offering benefits such as energy savings, extended service life, and environmental friendliness-and are also used as sleeve bricks for water-cooled blast furnace tuyeres.
(3) Recrystallized Silicon Carbide Products
Recrystallized silicon carbide products are refractory materials made from silicon carbide raw materials, characterized by direct bonding between silicon carbide grains. They are distinguished by the absence of a secondary phase, consisting entirely of 100% α-SiC. The chemical composition of the raw materials used for manufacturing recrystallized silicon carbide is as follows: SiC 99.5%, free C 0.2%, Fe 0.08%, and free SiO2 0.08%.
Recrystallized silicon carbide products are primarily used as kiln furniture, offering advantages such as energy savings, increased effective kiln volume, shortened firing cycles, improved production efficiency, and high economic returns. They are also used for burner nozzles, ceramic radiant heating tubes, and component protection tubes (particularly in atmosphere furnaces).