10 Frequently Asked Questions About Mullite Bricks

We receive many inquiries from customers about mullite bricks every day. Some are just starting to get in touch and want to understand the basic performance; some have experienced problems after using them for several years and want to find out the reasons; some are not sure between several suppliers and want to know how to judge whether they are good or bad.

Today, we have sorted out the 10 most frequently asked questions by users and explained them clearly at once. The article is not long, but it is all useful information. I hope it will be helpful to everyone.

1. What is the difference between mullite bricks and ordinary high alumina bricks?

The main crystal phase of mullite bricks is mullite (3Al₂O₃·2SiO₂). However, ordinary high alumina bricks only have alumina content that meets the standard, and their crystal phase is usually composed of a mixture of corundum, mullite and glass.

In actual use, mullite bricks have significant advantages in thermal shock resistance and creep resistance. They have better volume stability under high temperature environments and are less prone to deformation and cracking problems.

2. What is the normal Al₂O₃ content of mullite bricks?

The Al₂O₃ content of common mullite bricks on the market is generally between 65% and 75%. Those with less than 60% cannot be called mullite bricks, and may just be clay bricks containing mullite; those with more than 75% are usually corundum-mullite composite bricks, which will be much more expensive. When selecting a model, you don’t need to blindly chase the price, just look at the working conditions and requirements.

3. How to judge the quality of a mullite brick?

There are three simple identification methods: First, observe the cross-section. If the texture is dense and uniform, without obvious layering and large holes, the molding quality is excellent; second, tap the brick body, and the crisp sound means sufficient sintering, and the dull sound means there is a high probability of insufficient sintering; third, ask the supplier for the measured data of bulk density and apparent porosity. The bulk density is ≥2.6 g/cm3 and the apparent porosity is ≤18%, which is a qualified product.

4. At what temperature does mullite brick begin to soften?

The load softening temperature (T₀.₆) of mullite bricks is generally above 1600℃, and good ones can reach 1650℃-1700℃. This indicator can better reflect the load-bearing capacity of bricks at high temperatures than the Al₂O₃ content. If your kiln operating temperature is between 1400℃-1550℃, mullite bricks are sufficient.

5. Are mullite bricks easy to crack? What indicators should we look at?

Cracking is mainly related to thermal shock stability. Mullite bricks have a low thermal expansion coefficient (approximately 5.5×10⁻⁶/℃ at 20-1000℃), and their thermal shock resistance is better than that of high-alumina bricks. However, if the porcelain is sintered or the matrix ratio is unreasonable, it will also become brittle. When selecting a model, you can directly ask the supplier: How many times can the thermal shock resistance be achieved under 1100°C water-cooling conditions? Generally, ≥15 times is considered normal, and ≥25 times is considered troublesome.

6. What is the approximate price range of mullite bricks?

There is a big price gap between the three. The price of ordinary sintered mullite bricks is about 3,000 to 5,000 yuan per ton; the price of fused mullite bricks is relatively high, 6,000 to 9,000 yuan per ton; the unit price of low creep mullite bricks and corundum-mullite composite bricks can reach more than 10,000 yuan per ton. The price is mainly determined by raw material grade and volume density index. It is recommended that you first determine the operating temperature and specific application location of the equipment before asking the supplier for a price, so that the price comparison will be more valuable.

7. How to choose between mullite bricks and silicon carbide bricks?

Depends on the working conditions. Silicon carbide bricks have good thermal conductivity and strong wear resistance, and are suitable for use in areas that require rapid heat transfer or are severely scoured, such as roller bricks in ceramic roller kilns; mullite bricks have good thermal insulation and good thermal shock resistance, and are suitable for use in working layers or insulation layers in high temperature zones. It"s not who is better between the two bricks, but who is more suitable.

8. In which kilns are mullite bricks most maturely used?

Mullite bricks have a wide range of applications, and are used more maturely in the following fields: shuttle kilns and high-temperature zones of tunnel kilns in the ceramic industry; regenerator grids and kiln roofs in the glass industry; heating furnace bottoms and hot blast furnaces in the steel industry; cracking furnaces in the petrochemical industry; and the linings of various high-temperature electric furnaces. It can be said that in the temperature range of 1400℃-1550℃, mullite bricks are a very mainstream choice.

9. How long is the service life of mullite bricks?

There is no fixed standard for the service life, which mainly depends on the operating temperature, furnace atmosphere, frequency of start and stop, maintenance status and other conditions. Generally, under normal working conditions, the mullite bricks in the high temperature section of the ceramic kiln can be used for 2 to 3 years, and the checker bricks in the glass kiln regenerator have a service life of 3 to 5 years. If used in areas with gentle temperature changes and no strong alkali erosion, the service life can reach more than 5 years. Regular inspection and timely repair of peeling problems can significantly extend the overall service life.

10. Can mullite bricks be customized to special sizes?

OK. Mullite bricks are generally formed by semi-dry pressing or pouring, and special-shaped bricks require mold opening. The delivery cycle for conventional rectangular bricks is about 15-20 days, while special-shaped bricks will require an additional 7-10 days of mold production time. It is recommended to plan the construction period in advance and send the drawings to the manufacturer to confirm the workability. In addition, the unit price of small batches of special-shaped bricks is higher than that of large batches of standard bricks.

The above are the 10 most frequently asked questions about mullite bricks. If you have any other questions, please leave them in the comment area and we will sort them out regularly and continue to answer them.

This article was generated with AI assistance and reviewed and published by Shengyuan Refractory Materials Technology Center