Sintered High Zirconium Refractory for Glass Industry

Glass melting pools require high temperature, erosion resistant, long life materials. Typical refractories commonly used in this high temperature application are fused cast zirconium corundum (AZS, 40% ZrO2) and zirconium oxide (>80%), which have low apparent porosity (<0.7%) and high bulk density. The microstructure of refractory bricks made by the fusion casting process varies between the surface and the center, and usually contains shrinkage pores. Sintered AZS refractories with high apparent porosity (~20%) are also used in glass processing. Refractory manufacturers, the performance of sintered high zirconium refractories was compared with fusion cast refractories.

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Preparation of Zirconia Refractory Materials

Specific process of preparing zirconia refractory materials: Mix deionized water (11.5%) and alumina in a mixer for 10 min to form a slurry. Then gradually add boron oxide and silica fume, and mix for 10 min before each addition. Add nitric acid (diluted to 50:50 with deionized water) to make the pH value of the slurry about 3.5. Add zirconium oxide to the slurry and stir continuously until the zirconium oxide is evenly distributed. Heat the mixed slurry from room temperature to 40-80℃ (oven) and dry it, then pass it through a 100-mesh sieve. The dried powder is uniaxially pressed into a strip sample, and isostatic pressing and extrusion molding can also be used. Sample firing system: 25-1000℃ heating rate 50℃/h. At 1000-1700℃, the heating rate is 25℃/h. Keep warm at 1700℃ for 6-48h. At 1700-1300℃, the cooling rate was controlled to be 50-200℃/h. The cooling rate from 1300 to 1000℃ was 25℃/h. The cooling rate from 25 to 1000℃ was 50℃/h.

Various properties (specific gravity, apparent porosity, flexural strength, XRD and thermal conductivity) of the sintered sample (BZR) were measured and compared with those of industrial fused-cast zirconia (Scimos CZ). Static erosion tests were conducted on the prepared sintered zirconia refractory and commercial fused-cast zirconia samples (Scimos CZ). The crucible with glass-cullet and the refractory sample were placed in a furnace respectively and preheated to 1660℃. After sufficient insulation for a period of time, the refractory sample was placed in the center of the crucible and kept at this temperature for 3 days. After the test, the corrosion loss of the sample was measured at different points. The resistance of the sintered sample at 1500℃ and 1600℃ was measured by the 4-wire method. The microstructures of BZR and Scimos CZ were observed by scanning electron microscope. The comparative experimental results are as follows:

• 1) After sintering, the sintered zirconia sample was white or slightly milky due to the presence of impurities. Fusion-cast refractory bricks are usually gray due to the use of graphite electrodes during the melting process.
• 2) The performance of the sample, its specific gravity, apparent porosity, flexural strength and thermal conductivity test results are comparable to those of commercial fused-cast zirconia refractory materials.
• 3) Static and dynamic erosion tests show that the corrosion resistance of sintered zirconia refractory materials is comparable to or slightly better than that of commercial fused-cast zirconia Scimos CZ.
• 4) Sintered zirconia refractory materials exhibit high resistivity at high temperatures (1500-1600 ℃), comparable to commercial fused-cast materials (Scimos CZ).
• 5) The microstructure of sintered zirconia shows that it is composed of fine zirconia particles with glass phases between the particles. The microstructure of the entire brick body is uniform. The fused-cast zirconia microstructure has large grains that vary in size from the surface (smaller) to the center (larger) of the brick body, which are formed during cooling. During crystallization, voids and/or pores form in the center of the specimen, and the glass phase formed in the center and on the surface may have different compositions.

Performance Characteristics of Sintered High Zirconium Refractory Materials for Glass Industry

By mixing nano-glassy precursors with zirconium oxide particles, high zirconium oxide refractory samples of the required size were prepared by pressing (or isostatic pressing, extrusion, etc.) molding process. Then the sintered zirconium oxide refractory (BZR) was prepared by controlling the sintering process. High zirconium oxide refractory has similar specific gravity and lower apparent porosity, similar flexural strength and thermal conductivity to commercial fused-cast refractory. X-ray diffraction patterns show that the main phases of the sample are zirconium oxide phase and glassy amorphous phase. Static and dynamic erosion tests show that BZR has excellent erosion resistance to different glasses at high temperature (1715°C). BZR also shows high resistivity (measured above 1500°C), which is comparable to fused-cast refractory.

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Zirconium-Containing Refractory Bricks

Zirconium-containing refractory bricks are refractory bricks made of zirconium oxide (ZrO2) and zircon (ZrSiO4) as raw materials. Zirconia bricks, zircon bricks, zircon mullite and zirconium corundum bricks all belong to this type of refractory bricks. According to different production processes, zirconium-containing refractory bricks are divided into sintered bricks, fused cast bricks and unfired bricks. Zirconium-containing refractory bricks have the characteristics of high melting point, low thermal conductivity, good chemical stability, especially good corrosion resistance to molten glass.

There are several phases in the Zr-O system that are non-stoichiometric solid solutions of oxygen in zirconium and oxides. The stable compound of zirconium and oxygen is dioxide ZrO2. The ion radius ratio in zirconium dioxide is 0.66, which is close to the boundary between the crystal coordination number 8 and 6. The zirconium cation size is large (0.082nm), and in order to achieve 8 coordination, the oxygen ions are as close as possible in the lattice. Therefore, ZrO2 exhibits abnormal coordination. When its coordination number is equal to 7, one of the oxygen atoms occupies the position between the two nodes of the AB lattice. A lattice -8 coordination and B lattice -6 coordination. At high temperature, the Zr-O bond length increases due to the thermal motion of ions at the lattice nodes. This is the result of the transition of oxygen ions from the unit space to the A or B position. At the same time, the 8-coordination of anion vacancies is achieved.

The properties of zirconium-containing refractory materials depend on the properties of ZrO2. The melting point of dense, stabilized zirconium oxide is 2677℃, and the service temperature reaches 2500℃. The bulk density fluctuates between 4.5 and 5.5g/cm3 due to the purity of the raw materials and the manufacturing method. The bulk density of dense zirconium oxide bricks can reach 5.75g/cm3. Sintered zirconium oxide products react chemically with liquid glass. Caustic solutions, carbonate solutions and acids (except concentrated H2SO4 and HF) do not react chemically with zirconium oxide. ZrO2 has high structural strength and has the ability to work at 2200~2450℃ as a lining hot surface.

Zirconia bricks have high mechanical strength, and the strength is maintained up to 1300~1500℃. The thermal conductivity of ZrO2 is much lower than that of all other oxide materials. This property of ZrO2 can be used as a high-temperature insulation layer, and the physical and chemical properties of zirconium-containing refractory bricks.

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.