Effects of Kyanite-Mullite on the Properties of Ceramic Burner Bricks

Mullite-andalusite-cordierite bricks have excellent thermal shock resistance and are widely used in hot blast furnace ceramic burners. This paper conducts comparative tests on kyanite-mullite-andalusite bricks and mullite-andalusite-cordierite bricks to analyze the effect of kyanite-mullite on the performance of ceramic burner bricks.

Kyanite-Mullite on the Properties of Ceramic Burner Bricks

Kyanite-mullite is an artificially synthesized mullite raw material made of kyanite as raw material. The kyanite concentrate is crushed, wet-milled, filtered, squeezed into mud, and calcined at 1550℃-1600℃. The physical and chemical properties of kyanite-mullite are shown in Table 1, and the X-ray diffraction analysis spectrum is shown in Figure 1.

Table 1 Physical and chemical properties of kyanite-mullite

From Table 1 and Figure 1, it can be seen that kyanite-mullite is composed of 60% mullite phase, 30-40% glass phase and 3-5% quartz phase.

This experiment designed two sets of process ratios. Process No. 1 uses alumina-based mullite, garnet and cordierite as the main raw materials. Process No. 2 uses kyanite-based mullite and garnet as the main raw materials. A 50kg mixer was used for mixing, a 400 t press was used for molding, and a tunnel kiln was used for sintering. The test sample is shown in Figure 2, and the results of the physical and chemical properties test are shown in Table 2.

Table 2 Physical and chemical properties test results

It can be seen from Table 2 that the chemical compositions of processes No. 1 and No. 2 are similar. The porosity, bulk density, compressive strength and load softening temperature of process No. 2 are significantly better than those of process No. 1. The load softening temperature of process No. 2 is 68°C higher than that of sample No. 1. This is because after the temperature reaches 1460°C, cordierite is completely decomposed into mullite and glass phase. The raw materials of synthetic cordierite used in refractory materials have a high impurity content (in order to expand the temperature range of cordierite formation and promote sintering), so the high-temperature performance is general. Kyanite-based mullite has a low impurity content, a high main crystal phase content, and a good crystal shape. The high-temperature performance (including load softening temperature and creep rate) is much higher than other aluminum-silicon raw materials with the same aluminum content.

The number of thermal shock resistance (1100°C water cooling) of processes No. 1 and No. 2 is greater than 100 times, which meets the requirement of thermal shock performance of ceramic burner bricks > 100 times. The test bricks after 100 thermal shocks are shown in Figure 3. As can be seen from Figure 3, although both can complete 100 thermal shock tests without damage, the number, width and length of cracks in process 1 are significantly smaller than those in process 2. This shows that mullite-andalusite-cordierite bricks have better thermal shock resistance.

Analysis shows that kyanite-based mullite is a mullite-high silica glass composite material. Its well-developed mullite crystals, uniformly distributed network structure and high-viscosity silicon-rich glass phase all have an improving effect on the thermal shock resistance of the material. In particular, the presence of its high-viscosity glass phase can not only reduce the slip between crystals at high temperatures and improve the thermal mechanical properties of the material, but also inhibit crack extension.

The following conclusions were drawn through test data analysis:

1. The apparent porosity, bulk density, load softening temperature and creep rate of kyanite-mullite-andalusite bricks are better than those of mullite-andalusite-cordierite bricks.
2. The thermal shock of kyanite-mullite-andalusite bricks is greater than 100 times, which can meet the most demanding thermal shock requirements of hot blast furnace ceramic burners. However, mullite-andalusite-cordierite bricks have better thermal shock resistance.
3. Kyanite-mullite-andalusite bricks can replace mullite-andalusite-mullite-cordierite bricks and be used in hot blast furnace ceramic burners.

Semi-Cordierite Bricks and Kiln Furniture

Semi-cordierite bricks are a type of refractory material with cordierite (2MgO·2Al2O3·5SiO2) as the main component. Pure cordierite contains 13.7% magnesium oxide (MgO) and has the following characteristics: Low thermal expansion coefficient of 3×10-6/℃. Due to the low thermal expansion coefficient, cordierite materials generally have excellent thermal shock resistance.

Pure cordierite is an expensive material, so semi-cordierite materials with lower purity are often used as substitutes. Semi-cordierite materials also exhibit a lower thermal expansion coefficient.

Semi-cordierite materials are often used as kiln furniture, kiln car fixing blocks and ceramic parts for ceramic kilns. In some cases, alumina-silicate materials are also used for the same product applications. Table 7 lists the characteristics of some semi-cordierite products used for kiln furniture/kiln blocks. The maximum service temperature of these products is generally 1200℃ or higher.

Table 7 shows the relationship between magnesium oxide content and thermal expansion coefficient. When magnesium oxide content is very low, the thermal expansion coefficient is close to that of clay brick or mullite [about 6×10-6/℃].

Figures 5 and 6 show the microstructure of typical products of this type.

The microstructure of semi-cordierite material shows “crack-shaped” pores, which are typical of extruded clay products (Figure 5). It is not obvious from this micrograph that the size of the pyrophyllite aggregate particles is large, sometimes approaching 3 to 4 mm. In contrast, the microstructure of the pressed semi-cordierite material shows that the refractory aggregate particles are surrounded by a sintered clay matrix and the pores are mainly round (Figure 6). Many authorities believe that the presence of fine round pores improves thermal shock resistance. Therefore, a pressed product with a magnesium oxide content of about 0.9% (Table 7) may have similar properties to an extruded product with a magnesium oxide content of 3.0%.

The typical impurity in semi-cordierite bricks is cristobalite. When the refractory material is fired at a temperature of at least 1300°C, the “free” silicon dioxide (SiO2) or quartz in the raw material is converted to cristobalite. By reference to the MgO-Al2O3-SiO2 phase equilibrium diagram (not shown), it can be seen that free cristobalite is an equilibrium phase unless the composition of the product is exactly the same as pure cordierite. Too high a cristobalite content will cause “caking” and reduce the life of the kiln furniture/kiln blocks. Cristobalite is conveniently determined by X-ray diffraction (XRD) or thermal analysis (TA) techniques.

Application of Mullite and Its Composite Refractory Materials in Different Industries

Application of Mullite in the Ceramic Industry

Kiln furniture (saggers, shelves, push plates, etc.) is a tool used to space, support, cushion and protect the baked blanks in the industrial kiln during the baking process. The development of high-performance kiln furniture is of great significance for the firing of high-quality products. Since fused mullite has good thermal shock resistance, high-purity fused mullite is the best raw material for preparing high-quality kiln furniture.

1. Mullite-corundum kiln furniture

Mullite-corundum refractory material is one of the mainstream materials of kiln furniture at present. It has good high temperature strength, thermal shock resistance and chemical stability, and is particularly suitable for supporting soft magnetic (ferrite) materials and electrical insulating ceramics. Using M75 fused mullite and fused corundum as aggregates, aluminum glue, α-Al₂O₃ micropowder and SiO₂ micropowder as bonding matrix, mullite-corundum high temperature push plate with good thermal shock resistance was prepared. After 2 thermal shocks (1100℃⇌water cooling), the flexural strength retention rate was 78%, and no fracture occurred after 23 thermal shocks. The thermal shock resistance of corundum-mullite kiln furniture can be further improved. The mechanism of improved thermal shock resistance is: zircon decomposes into ZrO₂ and SiO₂ during the firing process. On the one hand, SiO₂ migrates outward from the ZrO₂ aggregate, and closed pores are generated at the position of SiO₂. On the other hand, the annular microcracks caused by thermal mismatch between ZrO₂ aggregates and the surrounding mullite matrix can disperse the stress generated during thermal shock cycles.

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2. Mullite-cordierite kiln furniture

Cordierite has a low thermal expansion coefficient (2.5×10⁻⁶℃⁻¹ from room temperature to 1000℃) and has excellent thermal shock resistance. The thermal expansion coefficient of cordierite is smaller than that of mullite. Due to the mismatch in the thermal expansion coefficients of the two, microcracks are easily formed at the interface between the two phases, which is beneficial to improve the thermal shock resistance of mullite-cordierite materials. The shelf is a special kiln furniture for supporting porcelain parts, and the material is mostly cordierite-mullite composite material. With M60 mullite and cordierite as the main raw materials and dextrin as a binder, a high-strength cordierite-mullite shelf is prepared, and the high-temperature flexural strength at 1200℃ can reach 17.7MPa. Because the skeleton of the material is composed of mullite and cordierite aggregates, the two are firmly connected by a “connecting bridge” composed of mullite, cordierite and low-aluminum high-silica glass phase, and this structure is conducive to improving the high-temperature mechanical properties of the material.

With the development of new energy vehicles, electronic mobile devices and energy storage fields, the demand for lithium batteries is increasing, and the kiln tools used for firing their positive electrode materials are also increasingly attracting attention.

3. Mullite-aluminum titanate kiln tools

Compared with cordierite, aluminum titanate has a lower thermal expansion coefficient (room temperature to 1000℃, 1.5×10⁻⁶℃⁻¹) and a higher melting point. It is currently the best material with high temperature resistance among low expansion materials. When mullite and aluminum titanate are used in combination, mullite-aluminum titanate kiln tools with good thermal shock resistance and high operating temperature can be obtained. Studies have shown that when mullite is added to the aluminum titanate matrix, the lattice stability of aluminum titanate can be improved, thereby preventing the decomposition of aluminum titanate. It was found that when mullite exists in aluminum titanate materials, the stability of aluminum titanate can reach 80%.

4. Mullite-silicon carbide kiln furniture

Silicon carbide has high strength, high thermal conductivity, wear resistance, chemical corrosion resistance and other properties. Therefore, silicon carbide kiln furniture has excellent thermal shock resistance, high wear resistance and strength at room temperature and high temperature. In order to improve the oxidation resistance of silicon carbide kiln furniture, Shi Jinxiong et al. prepared silicon carbide-mullite kiln furniture using silicon carbide, M70 sintered mullite and SiO₂ micropowder as raw materials.

Application of Mullite in Metallurgical Industry

The application of mullite in metallurgical industry is mainly reflected in steel smelting. Mullite bricks made of mullite as the main raw material have the characteristics of small thermal expansion coefficient, low creep rate, high high temperature strength, good thermal shock resistance and strong chemical corrosion resistance. It can be used for blast furnace, continuous casting, the dome of hot blast furnace and the middle and upper parts of the combustion chamber, etc., and can also be used as ceramic burner bricks. Amorphous refractory materials containing mullite also have good thermal shock resistance and mechanical properties, etc., and are used in blast furnaces, permanent linings of ladles, tundishes, ignition and insulation furnaces, hot metal desulfurization spray guns, etc.

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1. Mullite-corundum refractory materials

Mullite-corundum bricks have high density, low porosity, high strength and good resistance to molten iron and slag erosion. They are ideal refractory materials for blast furnace ceramic linings, furnace walls, tuyere and furnace bottom. Mullite-corundum castables can be used as lining materials for blast furnace hot blast furnaces and heating furnaces.

The purity of mullite raw materials affects the performance of mullite-corundum refractory bricks. Corundum-mullite bricks prepared with high-purity sintered mullite as raw materials have excellent resistance to alkali vapor and CO erosion. Suitable for use in high-temperature reducing atmosphere conditions such as hot blast furnaces. Compared with mullite-corundum castables prepared with fused mullite as aggregate, mullite-corundum castables prepared with microcrystalline mullite as aggregate have higher room temperature compressive strength, room temperature flexural strength, high temperature flexural strength, and better thermal shock resistance.

Introducing other refractory raw materials on the basis of mullite and corundum can further improve the performance of mullite-corundum refractory materials. Adding an appropriate amount of boron carbide to the silica sol combined mullite-corundum castable can further improve the room temperature flexural strength, room temperature compressive strength, high temperature flexural strength and thermal shock resistance of the castable.

2. Mullite-cordierite refractory materials

In order to meet the requirements of good thermal shock resistance, scouring resistance, high temperature resistance, erosion resistance, high load softening temperature and other requirements of refractory materials for ceramic burners, M70 sintered mullite, M70 fused mullite and cordierite are used as the main raw materials, and the mullite-cordierite ceramic burner composite bricks are prepared through the casting-centrifugal molding process.

The lining bricks of the coke oven door need to have good wear resistance, volume stability, carbon resistance and thermal shock resistance. With M70 sintered mullite and cordierite as the main raw materials and aluminate cement as the binder, a mullite-cordierite precast brick with low linear expansion rate, low porosity and good thermal shock resistance was prepared. The precast brick was applied to the door of a large coke oven. After one year of use, the surface of the precast brick was smooth and crack-free.

3. Mullite-alumina refractory materials

Casting materials made with mullite and bauxite as the main raw materials use less water and exhibit high mechanical strength over a wide temperature range.

In order to further improve the wear resistance and thermal shock resistance of mullite-alumina-based castables, a mullite-alumina-silicon carbide castable with excellent mechanical properties and wear resistance was prepared with M60 mullite, bauxite and silicon carbide as the main raw materials and calcium aluminate cement as the binder.

4. Mullite-magnesium aluminum spinel refractory

Magnesium aluminum spinel has excellent thermal shock resistance, corrosion resistance and wear resistance. Studies have found that the introduction of nano-magnesium aluminum spinel into mullite-based castables generates tension ring stress around the spinel and forms microcracks in the matrix due to the mismatch in thermal expansion coefficients between mullite and magnesium aluminum spinel. This is conducive to strengthening the internal structure of the material, thereby limiting the expansion of cracks. Therefore, the prepared mullite-magnesium aluminum spinel castable has good thermal shock resistance.

5. Mullite-silicon carbide refractory

Silicon carbide has high strength, high thermal conductivity, wear resistance and other properties, and can be combined with mullite to prepare mullite-silicon carbide castables with excellent performance. Mullite-silicon carbide wet jet castable. The wet jetting castable has good rheological properties, showing a pseudoplastic fluid with low yield value and apparent viscosity, suitable for pumping pipeline transportation, and can be used in kilns such as ladles, electric furnaces, blast furnaces, and torpedo tanks. Adding 6% (w) zircon to the mullite-silicon carbide castable can further improve the wear resistance of the mullite-silicon carbide castable, making it suitable for lining parts with strict requirements for thermal shock resistance and wear resistance.

6. Mullite-bonded corundum-silicon carbide refractory

In order to solve the problem of reduced high temperature performance of corundum-silicon carbide castables bonded with ordinary binders (such as calcium aluminate cement), a mullite-bonded corundum-silicon carbide cement-free castable was prepared using corundum and silicon carbide as aggregates, M70 mullite powder, Al₂O₃ micropowder and SiO₂ micropowder as bonding matrix. Because the mullite powder introduced into the matrix can form a network structure of columnar mullite inside the castable after sintering, the room-temperature flexural strength, room-temperature compressive strength, high-temperature flexural strength and thermal shock resistance of the corundum-silicon carbide castable are improved.

Development of the Application of Mullite and Its Composite Refractory Materials

Mullite and its composite refractory materials have been widely used as linings for various kilns in traditional fields such as metallurgy, ceramics, cement and petrochemicals. The future development trend of mullite in refractory materials is mainly reflected in the following aspects:

• (1) Expand the application field of mullite refractory materials. In addition to the traditional high-temperature industrial field, mullite refractory materials have emerged in aerospace, military and other fields. However, the application of mullite refractory materials in these fields is quite limited at present, and its application in aerospace, military and other fields will continue to be promoted in the future.
• (2) Promote the development of lightweight mullite. With the advancement of energy conservation and emission reduction, the development of lightweight and heat-insulating refractory materials is of great significance to reducing kiln heat loss and improving work efficiency. Refractory materials prepared with lightweight mullite not only have good high-temperature performance, but also have good thermal insulation effect. The use of lightweight heat-insulating refractory materials with good high-temperature performance can thin or even eliminate the working layer. Refractory castables made of lightweight mullite can solve the problem that lightweight refractory castables made of perlite, vermiculite or ceramsite have low use temperature and cannot be used as working linings in direct contact with flames at high temperatures. Therefore, the development of lightweight mullite with large porosity, low bulk density, low thermal conductivity and high strength plays a positive role in the development of lightweight insulating refractory materials with good high-temperature performance that can achieve energy saving and consumption reduction.
• (3) Strengthen the application of nano-mullite in refractory materials. In recent years, nanotechnology has been successfully applied in refractory materials and has continuously achieved breakthroughs in material performance. The use of nano-mullite powder can further improve the performance of refractory materials. In the future, nano-mullite will play an increasingly important role in refractory materials.

Thermal Insulation Properties of Low Thermal Conductivity Magnesium-Aluminum Spinel Bricks

The energy consumption of cement rotary kiln is very serious, especially when the refractory materials in the transition zone before and after the cement rotary kiln are not protected by the kiln skin, the refractory bricks are directly affected by material erosion, heat load and mechanical stress, and the use conditions are very harsh. At present, the transition zone of large cement rotary kilns generally uses Silicon carbide mullite bricks, silica-molybdenum red bricks or magnesia-alumina spinel bricks, and the service life basically meets the requirements of cement production. However, due to the large thermal conductivity of magnesia-alumina spinel bricks and silica-molybdenum bricks, Silicon carbide mullite red bricks, magnesia-alumina spinel bricks ≥3.0 W/m·K, Silicon carbide mullite bricks ≥2.8 W/m·K. As a result, the outer wall temperature of the cylinder in the transition zone before and after the operation of the cement rotary kiln is too high, averaging about 340℃ and up to 400℃. The higher outer wall temperature of the cylinder brings a series of problems, such as increasing the coal consumption per ton of cement clinker and increasing the emission of polluting gases. Severe cases also cause “red kilns”, affecting the safe operation of cement rotary kilns.

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Low Thermal Conductivity Magnesia-Alumina Spinel Bricks

In order to solve the problem of high thermal conductivity of magnesia-alumina spinel bricks, the sintering performance and microstructure of rare earth oxide-doped porous magnesia-alumina spinel refractory aggregates were studied. Porous magnesia-alumina spinel was prepared with industrial α-alumina micropowder and fused magnesia sand as the main raw materials, sodium dodecylbenzene sulfonate (SDBS) as the foaming agent, and dextrin as the binder. In addition, different mass fractions of Y2O3, Yb2O3, La2O3, and Sm2O3 were added respectively, and the samples were kept warm for 3 hours at 1600℃ for sintering. The bulk density and apparent porosity of the sintered samples were measured respectively, and the phase composition and microstructure were analyzed by XRD, SEM, EDS, and other characterization methods to reveal the mechanism of rare earth oxides promoting the sintering reaction process of porous magnesia-alumina spinel.

The introduction of rare earth oxides promotes the sintering of magnesium-aluminum spinel. The sample volume density and compressive strength reach the maximum when the Sm3O2 addition is 1.5wt%, which are 2.28g/cm³ and 50.5Mpa, respectively. A substitution solid solution is formed between the rare earth oxides and magnesium-aluminum spinel, which promotes the sintering densification of magnesium-aluminum spinel. Cation vacancies and lattice defects of magnesium-aluminum spinel are conducive to the development and growth of magnesium-aluminum spinel crystals. The thermal conductivity of the product made from this raw material is significantly lower than that of similar products.

The lightweighting of refractory materials is mainly achieved by introducing a certain amount of pores into the materials, which has a lower thermal conductivity without significantly reducing the strength of the refractory materials. It can save energy and reduce resource consumption in the preparation and service of raw materials, which is a direction of research and development of refractory materials. Therefore, the development of new low thermal conductivity magnesium-aluminum spinel lightweight refractory materials has important practical significance for energy saving and consumption reduction in the cement industry.

When synthesizing magnesium-aluminum spinel lightweight aggregate, adding yttrium oxide, ytterbium oxide, lanthanum oxide and samarium oxide can increase the volume density of the sample, reduce the apparent porosity, increase the compressive strength, and promote the formation of magnesium-aluminum spinel. Among them, the sample with samarium oxide added has better sintering performance. When the addition amount of Sm2O3 reaches 1.5 wt%, the sample has the lowest apparent porosity and the highest volume density of 2.28g/cm³ and 31.72%, respectively.

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Application of Low Thermal Conductivity Magnesia-Alumina Spinel Bricks

The low thermal conductivity multi-layer composite magnesia-alumina spinel bricks developed by the above technology are used in 24-32 meters, and the average temperature is 43℃ ​​lower than that of magnesia-alumina spinel bricks, and the service life reaches more than 12 months. Low thermal conductivity magnesia-alumina spinel bricks are not only recognized by domestic customers, but also have been used in many rotary kilns by overseas customers such as Mexican cement and Turkey, and have been highly praised.

In response to the high energy consumption and high carbon emissions in the cement industry, the low thermal conductivity multi-layer composite magnesia-alumina spinel bricks studied by Rongsheng Refractory Material Factory have the following advanced features: through the form of a three-layer composite structure, the working layer is optimized, the thermal conductivity of the working layer is reduced, the thermal shock resistance is improved, and the insulation layer is optimized to improve the strength of the insulation layer. Through the study of different insulation materials, zirconia reinforced alumina fiberboard with excellent high temperature resistance is selected as the insulation layer material. In the preparation process, innovative production processes are used to achieve synchronous molding and synchronous firing, simplify the process, reduce costs, and improve production efficiency. The thermal conductivity of low thermal conductivity magnesium-aluminum spinel brick is 2.4-2.5W/(m·K), which is much lower than the 3.0-3.3W/(m·K) of ordinary magnesium-aluminum spinel brick.

According to the actual application feedback from customers, low thermal conductivity multi-layer composite magnesium-aluminum spinel brick can effectively reduce the temperature of the transition zone cylinder of cement kiln, with significant energy saving and carbon reduction benefits. It provides a new direction for energy saving and emission reduction of cement rotary kiln.

Magnesium-Aluminum Spinel Brick for Lime Kiln

Magnesium-aluminum spinel brick for lime kiln is a high-performance refractory material designed for lime kiln. It is mainly composed of magnesium-aluminum spinel (MgAl2O4) generated by the reaction of magnesium and aluminum oxides at high temperature, and may contain a certain amount of other refractory oxides to enhance its performance. Magnesium-aluminum spinel has excellent high temperature resistance, corrosion resistance, thermal shock resistance and other characteristics, making it an ideal choice for key parts in lime kiln (such as burning zone, preheating zone, etc.).

Magnesium-aluminum spinel bricks for lime kilns have the following characteristics:

(1) High temperature resistance: Magnesium-aluminum spinel bricks have extremely high refractoriness and can withstand the high temperature environment in the lime kiln, ensuring the stable operation of the kiln.

(2) Corrosion resistance: In the reducing atmosphere of the lime kiln, magnesium-aluminum spinel bricks can resist the erosion of kiln slag and furnace gas, extending the service life.

(3) Thermal shock resistance: Magnesium-aluminum spinel bricks have good thermal shock resistance and can withstand the thermal stress caused by the rapid change of kiln temperature, preventing the brick body from cracking and falling off.

Magnesium-aluminum spinel bricks are mainly used in key areas such as the firing zone and preheating zone of the lime kiln. These areas have extremely high requirements for the performance of refractory materials. The use of magnesium-aluminum spinel bricks can significantly improve the operating efficiency and product quality of the lime kiln. Contact Rongsheng manufacturers for free samples and quotations.