May 27 2025
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.

Zircon Bricks for Glass Kilns
Zircon Bricks for Glass Kilns

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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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    Zircon Corundum Blocks for Glass Kiln

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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.

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        May 6 2025
        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

        ItemsKyanite-Mullite
        Al2O3   %46.43
        SiO2    %50.16
        Fe2O3   %0.64
        TiO2   %1.20
        CaO    %0.22
        MgO    %0.16
        K2O    %0.72
        Na2O   %0.1
        Bulk Density g/cm32.29
        Apparent Porosity  %1.3
        Mullite Phase  %60
        Glass Phase  %30-40

         

        Figure 1 X-ray diffraction analysis of kyanite-mullite
        Figure 1 X-ray diffraction analysis 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.

        Figure 2 Test sample diagram
        Figure 2 Test sample diagram

         

        Table 2 Physical and chemical properties test results

        Items1#2#
        Al2O3  %61.7361.22
        Fe2O3  %1.000.96
        Apparent Porosity %22.819.7
        Bulk Density g/cm32.322.42
        Compressive Strength MPa51.981.5
        Load Softening Temperature ℃

        T0.6/0.2MPa

        		14821548℃
        Creep Rate %

        0.2MPa

        		1250℃x50h

        -0.622

        		1350℃x50h

        -0.457

        Reburning Line Change Rate %

        1400℃×2h

        		+0.1、+0.1

        +0.2

        		+0.1、+0.2

        +0.1

        Thermal Shock Resistance

        1100℃, Water Cooling

        		＞100＞100

         

        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.

        Figure 3 Condition of test brick after thermal shock test
        Figure 3 Condition of test brick after thermal shock test

         

        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 the relationship between magnesium oxide content and thermal expansion coefficient

        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.

        Figure 5 Microstructure of semi-cordierite extruded into pyrophyllite
        Figure 5 Microstructure of semi-cordierite extruded into pyrophyllite

         

        Figure 6 Microstructure of clayey semi-clay pressed brick
        Figure 6 Microstructure of clayey semi-clay pressed brick

         

        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.

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