How to Judge the Quality of Silicon Carbide Bricks?

Silicon carbide bricks are refractory materials made of SiC as the main raw material. Contains 72% to 99% SiC. They are divided into clay-bonded, Si3N4-bonded, Sialon-bonded, β-SiC-bonded, Si2ON2-bonded, and recrystallized silicon carbide bricks. Silicon carbide bricks have high thermal conductivity and good wear resistance, thermal shock resistance, and corrosion resistance. They can be used for aluminum electrolytic cell linings, molten aluminum conduits and ceramic kiln furniture, the lower part of large and medium-sized blast furnaces, furnace waists and belly, aluminum refining furnace linings, zinc distillation tank linings, etc.

How to judge the quality of silicon carbide bricks?

When buying silicon carbide bricks, many users are inexperienced and cannot distinguish the quality of silicon carbide products when buying silicon carbide bricks and silicon carbide products. As a result, they often buy unqualified silicon carbide products, which greatly affects the service life of the kiln, increases the investment in refractory materials, and brings unnecessary losses to the company. So how to distinguish the quality of silicon carbide bricks?

high-quality silicon carbide bricks

First of all, the size specifications of silicon carbide bricks should be accurate. The specific gravity of silicon carbide bricks is generally between 2.45-2.6. Too small or too large a specific gravity indicates that the material itself is not pure. Other refractory materials must be added, which greatly affects the silicon carbide content and the heat transfer performance of silicon carbide. When buying silicon carbide bricks, check whether there are missing corners and cracks on the surface of the bricks. The missing corners and cracks should be acceptable within the national standard. If the national standard is seriously exceeded, the quality of the batch of products is unqualified.

Silicon carbide bricks are made of silicon carbide as the main raw material. SiC is covalently bonded and does not have the so-called sintering property. Instead, it relies on chemical reactions to generate new phases to achieve sintering, which is called reaction sintering. Silicon carbide bricks are divided into clay-bonded silicon carbide bricks, bonded silicon carbide bricks, silicon oxynitride-bonded silicon carbide bricks, silicon nitride-bonded silicon carbide bricks, Sialon-bonded silicon carbide bricks and recrystallized silicon carbide bricks according to different bonding methods.

1. Clay-bonded silicon carbide bricks are made of black silicon carbide as raw material, soft clay with good bonding properties, and pulp waste liquid as binder. Generally, clay is combined with 10% to 15%, and pulp waste liquid is combined with 3% to 5%. The large, medium and fine particles of silicon carbide are calculated based on the maximum bulk density. When mixing, first add silicon carbide particles and fine powder, then add clay fine powder, dry mix for 3 minutes, and then add 4% of pulp waste liquid (density is 1.26 to 1.28g/cm3) for mixing. After 12 hours of trapped material, the mud material passes through a 4mm aperture screen and is formed by a brick press. The volume density of the brick is 2.5 to 2.7g/cm3. The brick is naturally dried for 2 to 4 days and fired at 1400℃.

2. Silicon nitride-bonded silicon carbide bricks are made of silicon carbide and silicon powder as raw materials and fired by nitridation.

The silicon carbide raw material contains more than 97% SiC, and the silicon carbide particle ratio of the mud material is: coarse: medium: fine = 5:1:4. The silicon powder contains more than 98%, and more than 80% are less than 10μm, and the largest particle cannot exceed 20μm. After the silicon carbide particles and silicon powder are batched, mixed, formed, and dried, the bricks are placed in the nitriding furnace for firing. Nitrogen is introduced during the firing process, and the temperature, pressure, and atmosphere in the furnace must be strictly controlled. The process parameters are: the nitriding gas pressure is 0.02-0.04MPa, the O2 content in the furnace atmosphere is less than 0.01%, the final nitriding temperature is 1350-1450℃, and the total nitriding time varies with the shape and size of the product.

3. β-SiC bonded silicon carbide bricks, silicon carbide, silicon powder and carbon powder are matched, mixed, formed and fired in a reducing atmosphere at 1400℃. Most of them are fired by buried carbon. During the firing process, a β-SiC bonded silicon carbide brick with α-SiC as the skeleton and fine-grained β-SiC as the matrix is ​​produced. β-SiC is produced by the reaction of silicon powder and carbon powder during the firing process. This product usually contains a small amount of residual silicon and carbon.

4. Silicon oxynitride bonded silicon carbide bricks, the silicon powder in the ingredients is less than the ratio of Si3N4 bonded bricks, and after molding, it is fired in a N2-rich atmosphere (a certain O2 partial pressure is required) at a temperature of 1350-1400℃. The finished product is a silicon oxynitride bonded silicon carbide brick with α-SiC as the skeleton and Si2ON2 as the matrix. There is often a small amount of Si and Si3N4 in the matrix.

5. Sialon bonded silicon carbide bricks are made by adding Si3N4 and Al203 powder to silicon carbide of a certain particle size ratio, adding a binder and mixing, and then firing in a reducing atmosphere after forming. The result is a refractory product with α-SiC as the skeleton and Sialon as the matrix. Since the Sialon matrix exists between the particles, the product strength is improved and the thermal shock resistance is enhanced.

6. Recrystallized silicon carbide products are composed of 100% α-SiC without the presence of a second phase.

The raw material contains more than 99.5% SiC, adopts the particle grading with the maximum stacking density, is formed under high pressure, and is fired in an electric furnace isolated from air at a temperature above 2100°C. The electric furnace is required to be able to burn to 2500°C, and SiC evaporates and condenses, resulting in a self-bonding structure without shrinkage.

Silicon carbide bricks have high thermal conductivity and good wear resistance, thermal shock resistance, and corrosion resistance. They can be used for aluminum electrolytic cell linings, molten aluminum conduits and ceramic kiln furniture, the lower part of large and medium-sized blast furnaces, furnace waists and furnace belly, aluminum refining furnace linings, zinc distillation tank linings, etc.