Suitable for Aluminum Alloy Smelting – Unshaped Refractories Medium Temperature Low Cement Castable

The aluminum alloy melting process poses unique and demanding challenges to refractory materials, stemming from the physical and chemical properties of molten aluminum and its alloys. While the combustion chamber of an aluminum melting furnace can reach temperatures of approximately 1200°C, the furnace chamber area in direct contact with the molten aluminum typically only reaches temperatures of 700-800°C (the casting temperature for 6063 aluminum alloy is 720-740°C).

This means that the furnace lining material spends most of its time in a medium-temperature range, rather than a traditionally high-temperature state. In this temperature range, traditional refractories often experience a strength dip due to bonding phase transitions. For example, hydration products (such as CAH₁₀ and C₂AH₈) in cement-bonded Unshaped Refractories Castables begin to dehydrate at 300-400°C, losing their bonding properties while the ceramic bond is not yet fully formed, resulting in a significant drop in strength.

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Low-Cement Castables: Medium- and Low-Temperature Strength Properties

Low-cement castables exhibit unique strength properties in the medium- and low-temperature ranges, distinct from conventional castables. During heating, conventional aluminate cement unshaped refractories castables typically experience a decrease in strength (due to hydrate dehydration) followed by an increase (due to ceramic bonding), with a distinct strength dip occurring in the 800-1000°C range. Low-cement refractory castables, however, exhibit a significant increase in strength at medium temperatures, rather than a decrease in strength.

The research examples in the table below demonstrate that the hot flexural strength of low-cement castables at 800°C is significantly higher than that at room temperature. Low-cement castables made primarily of kyanite-mullite (M45 and M60) exhibit the greatest increase in hot flexural strength with increasing treatment temperature. Low-cement castables made primarily of high-alumina bauxite (M85) exhibit the second-highest increase. Conventional Unshaped Refractories castables using CA-50 cement as a binder exhibit a distinct strength dip after firing at 800°C.

Table: Changes in hot flexural strength after treatment at different temperatures (MPa)

The mechanism of this anomaly is that the dehydration of calcium aluminate hydrate in low-cement castables is slow and continuous, with minimal damage to the crystal structure. Simultaneously, the ultrafine powder begins to sinter at moderate temperatures, forming a preliminary ceramic bond.

Core Characteristics and Advantages of Low-Cement Castables

Low-Cement Castables (LCC) are a new generation of unshaped refractory materials developed in the 1980s. Compared to traditional aluminate cement Unshaped Refractories castables, their core characteristic lies in a significant reduction in the amount of calcium cement (typically from 12-20% to 3-8%). Furthermore, through the introduction of ultrafine powder technology and high-efficiency admixtures, they achieve a comprehensive performance optimization of high density, low porosity, and high strength.

The revolutionary breakthrough in low-cement castables stems from the application of ultrafine powder technology. Ultrafine powders (such as reactive SiO₂ powder and α-Al₂O₃ powder) with particle sizes less than 1.0μm can exceed 71%. These ultrafine particles possess an extremely high specific surface area and reactivity, effectively filling the gaps between aggregate particles and achieving the densest packing. It prevents particle size segregation, reduces porosity and pore diameter, ensures the fluidity of the mixture, and improves the density and bonding strength of the Unshaped Refractories castable. More importantly, the high specific surface area and reactivity of ultrafine powder significantly reduce sintering temperatures and promote sintering at medium and low temperatures.

Active SiO₂ ultrafine powder not only improves the fluidity of the castable but is also one of the most effective sintering accelerators. At temperatures above 900°C, SiO₂ ultrafine powder reacts with Al₂O₃ to form mullite (3Al₂O₃·2SiO₂), accompanied by a volume expansion of approximately 10.5%. This volume effect effectively offsets some of the volume shrinkage of the unshaped refractory castable, promoting strength improvement. Furthermore, the mullite phase forms at a relatively low temperature (beginning to form in large quantities at approximately 1000°C), and its needle-shaped or columnar crystal structure forms a cross-linked skeleton, significantly enhancing the material’s strength.

Ultrafine α-Al₂O₃ powder strengthens the material through a different mechanism. It promotes the formation of calcium hexaaluminate (CA₆) from calcium aluminate at high temperatures, along with smaller amounts of mullite, anorthite, CA, and CA₂. These minerals have large molar volumes, which prevent volume shrinkage. Furthermore, CA₆ crystals are small columnar and needle-shaped, while anorthite crystals are fine columnar. Together, they form a cross-linked structure of fine columnar and needle-shaped structures, resulting in a strong and dense structure that can reach strengths of around 100 MPa.

The setting and hardening mechanism of low-cement castables is also fundamentally different from that of traditional Unshaped Refractories castables. Traditional castables primarily derive their strength from hydration products (such as CAH₁₀ and C₂AH₈) produced by cement hydration. However, these hydrates dehydrate and decompose during heating, significantly reducing their strength at medium temperatures. Low-cement castables, on the other hand, rely primarily on a cohesive bonding mechanism: ultrafine powder particles form colloidal particles in water, which form a three-dimensional network structure through van der Waals forces and chemical bonds, tightly binding the aggregate particles together. Cement acts only as a delayed-acting setting accelerator. This cohesive mechanism ensures that the strength of low-cement castables does not decrease due to hydrate decomposition during heating, but instead continues to increase due to sintering.

By carefully controlling the type, particle size distribution, and additive amount of fine powder, low-cement castables achieve an ideal strength development curve within the operating temperature range of aluminum alloy smelting (700-900°C). This avoids the mid-temperature strength trough common in traditional Unshaped Refractories castables while providing sufficient high-temperature performance, perfectly adapting to the unique operating conditions of aluminum melting furnaces.

However, there are downsides. Low porosity and high densification also result in poor air permeability. During baking and heating, steam generated by internal moisture cannot be promptly dissipated, easily building up high pressure within the lining, causing it to spall or crack. Therefore, when using low-cement castables, a reasonable baking system and the addition of explosion-proof agents must be used.

Conclusion

From the “medium-temperature dilemma” of traditional Unshaped Refractories castables to the “precise breakthrough” of low-cement castables, the path to upgrading refractory materials is essentially a matter of precisely matching material properties with operating requirements. For the unique application of aluminum alloy smelting, low-cement castables restructure their strength formation mechanism through ultrafine powder technology. This not only addresses the strength limitations of the medium-temperature range, but also addresses the core requirements of high density and corrosion resistance. This makes them a key material support for the longevity and high efficiency of aluminum industry furnaces.

How to Improve the Performance Parameters of Zirconium Corundum AZS Bricks?

AZS Bricks, abbreviated as zirconia, are primarily composed of alumina and zirconium oxide, with small amounts of other additives. Their high alumina content and the addition of zirconium oxide impart unique properties. For example, a typical zirconia brick may contain approximately 70% alumina and 30% zirconium oxide. It may also contain trace amounts of additives such as borax and silica sol to improve sintering performance.

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Zirconium-aluminum bricks are primarily used in glass industry tank furnaces, including key areas such as the tank walls, vents, superstructures, hanging walls, and breast walls. In glass furnaces, zirconia-aluminum bricks can withstand temperatures exceeding 1600°C and the corrosive effects of molten glass, ensuring proper furnace operation and high-quality glass production. They are also used in high-temperature industries such as steel smelting furnaces, cement rotary kilns, and non-ferrous metal smelting.

Improving the Performance Parameters of Zirconia-Corundum Bricks

To maximize the performance parameters of Zirconia-Corundum bricks, various performance-enhancing additives are added during production. Nanomaterials exhibit unique size and surface effects, and the introduction of nanoscale additives into Zirconia-Corundum bricks can improve their performance. For example, the addition of nanoparticles such as nanoalumina and nanozirconia can form finer dispersions within the brick, hindering crack propagation and increasing the brick’s toughness and strength. Furthermore, these nanoadditives can synergize with the original components of Zirconia-Corundum bricks to enhance their thermal shock resistance and erosion resistance.

However, due to its high specific surface area and surface energy, nanoalumina is prone to agglomeration. If dispersion issues are not effectively addressed during production, agglomerated nanoalumina particles can form localized defects within the Zirconia-Corundum brick, reducing its overall performance. For example, agglomerates can become stress concentration points, making cracks more likely to form during use, impacting the brick’s strength and durability.

By modifying the nano-alumina, a carbon nanotube core-shell structure is grown in situ on the surface. This core-shell structure prevents direct contact between nano-alumina particles, thereby reducing the possibility of nano-alumina agglomeration. This allows the nano-alumina to be evenly distributed within the corundum monolithic refractory brick, forming a finer dispersed phase within the brick, hindering crack propagation and improving the brick’s toughness and strength.

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The technical solution is as follows:

Materials: 55-65 parts alumina powder, 25-35 parts zircon sand, 1-5 parts modified nano-alumina powder, 2-6 parts binder, 1-2 parts chromium oxide, 1-3 parts yttrium oxide, and 1-4 parts flux.

The alumina powder has a particle size of 1-1.5 mm and a purity >99%.

The zircon sand contains 65% zirconium oxide and 34% silicon dioxide, with a particle size of 0.05-2 mm.

The modified nano-alumina powder is nano-alumina modified with in-situ carbon nanotubes. A carbon nanotube core-shell structure is grown on the surface of the modified nano-alumina.

The binder is silica sol. The silica content is approximately 40%, the iron content is <0.01%, the sodium oxide content is <0.5%, the viscosity is 10-20 mPa·s, and the particle size is 20-70 nm.

The flux is borax with a purity of >95%, an iron content of ≤0.005%, and a particle size of 0.15mm.

The production steps are as follows:

Ingredients: Add alumina powder, zircon sand, modified nano-alumina powder, binder, chromium oxide, yttrium oxide, and flux to a mixer in the correct proportions.

Mixing: Stir the ingredients in the mixer at a speed of 120-180 rpm for 90-120 minutes to ensure thorough mixing.

Molding: The mixed ingredients are placed into a mold and molded using a friction press or hydraulic press. The molding pressure is 150-220 MPa, and the holding time is 30-90 seconds, until the bricks reach the desired shape.

Drying: The molded bricks are placed in a drying oven along with the mold to dry and remove moisture. The drying temperature is 110-140°C, and the drying time is 24-72 hours.

Sintering: The dried bricks are placed in an insulated sand box along with the mold and then sintered in a high-temperature kiln at a temperature of 1650-1800°C. The heating rate is 3-5°C/min, and the holding time is 4-8 hours.

Annealing: After the sintered zirconium-alumina refractory bricks and the mold are removed from the insulating flask, the mold is removed. The zirconium-alumina refractory bricks are placed back into the insulating flask for annealing for 24-48 hours. The cooling rate is 2-4°C/min.

Advantages: Nanomaterial modification technology and optimized production processes not only address the problem of nanoparticle agglomeration but also significantly improve the overall performance of the bricks, making them more suitable for high-temperature and highly corrosive working environments. The resulting zirconium-alumina refractory bricks offer high quality, a high forming rate, stable internal stress distribution, and resistance to high temperatures and corrosion.

Differences in Application Between Fused AZS and Fused Corundum Bricks

In float glass furnace design, the optimal combination is determined based on glass quality requirements, furnace service life, refractory properties, and cost. Different refractory types are used in different areas. Parts in contact with the molten glass, such as the tank walls and bottom, have a significant impact on furnace life and glass quality, and therefore require more stringent selection criteria.

With improvements in refractory quality and performance, and increasing demands for furnace lifecycles, fused AZS bricks are currently used for both the melting tank walls and bottom, as well as the neck tank walls and bottom. This is primarily to address chemical erosion caused by batch reactions and erosion from high-temperature molten glass. Fused AZS offers superior high-temperature erosion resistance compared to other refractory materials.

33# oxidation-process fused zirconia-corundum bricks can be used for the cooling tank walls and bottom, but high-quality float glass production lines typically use α-β fused corundum bricks, primarily for glass quality considerations. α-β fused corundum bricks have low porosity and exhibit excellent corrosion resistance at the 1350°C operating temperature of the cooling section, without any glassy phase precipitation. However, fused AZS bricks, due to their high content of SiO2 and alkaline oxides, are prone to glassy phase precipitation, and this volume change can lead to the formation of bubbles. Furthermore, after the zirconium corundum is dissolved by the molten glass, the corrosion-resistant ZrO2 material remains, forming microscopic dendrites. Discharge at the tip triggers an electrochemical reaction, ionizing and reducing the gas components in the glass to form bubbles. Therefore, based on glass quality requirements, kiln runner designs generally use α-β fused corundum bricks for the cooling section walls and floor paving.

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Applications of Fused AZS Bricks and Fused Corundum Bricks

1. Fused Zirconia Corundum

Grades include: AZS-33 (33% ZrO₂ content), AZS-36 (36% ZrO₂ content), and AZS-41 (41% ZrO₂ content).

The erosion resistance of fused zirconium corundum increases with increasing ZrO₂ content. 33# fused AZS bricks are suitable for the melting pool walls and the breast wall of the material processing area. Corners of the pool walls are more susceptible to erosion by the molten glass, so 36# fused AZS bricks or 41# AZS bricks with higher ZrO₂ content are the best choices.

The production process for fused zirconium corundum is divided into reduction and oxidation methods. Currently, the oxidation method is the primary shrinkage-free casting process. The oxidation melting method eliminates contamination caused by graphite electrodes in the melt, has a low carbon content, and can reduce the bubble content in the glass.

2. α-β Fused Corundum Bricks

Al2O3 content >98%. Made from high-purity alumina with a small amount of soda ash, it is melted at 2000-2200°C and produced using a shrinkage-free casting method.

The thermal expansion coefficient of fused corundum is 8.6, placing it in the lower-to-medium range for thermal shock resistance. Fused corundum bricks exhibit slightly poor high-temperature corrosion resistance, with resistance to molten glass decreasing rapidly at temperatures above 1600°C. However, at operating temperatures of 1350°C and below, they exhibit strong corrosion resistance and virtually no contamination of the molten glass. They are ideal materials for the cooling tank walls and floors of glass melting furnaces, as well as for float glass launders.

Alumina Bricks Properties

Different types of alumina bricks (such as β-alumina bricks and hollow alumina sphere bricks) may differ in their specific properties. For example, β-alumina bricks offer better resistance to alkali vapor corrosion, while hollow alumina sphere bricks excel at being lightweight and providing excellent thermal insulation. RS Alumina Bricks Manufacturer supplies high-quality alumina bricks. Contact RS Factory for free samples and quotes.

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Alumina Bricks Have the Following Properties

High-Temperature Resistance

Alumina bricks typically have a refractoriness exceeding 1900°C, maintaining stable physical and chemical properties in high-temperature environments. They are suitable for use in high-temperature industrial furnaces and melting furnaces, where they can withstand long-term high temperatures without deformation or damage.

High Mechanical Strength

They possess high compressive and flexural strengths, reaching approximately 250 MPa at room temperature and maintaining a strength of approximately 150 MPa at 1000°C. They can withstand mechanical and thermal stresses at high temperatures.

Good Chemical Stability

They are chemically stable and highly resistant to corrosion from acids, alkalis, salts, and other chemicals, especially at high temperatures. They are particularly resistant to corrosion from a variety of molten metals and chemicals. They are suitable for applications in chemical and metallurgical industries, where corrosion resistance is critical.

Low Thermal Conductivity

With low thermal conductivity, they offer excellent thermal insulation properties, effectively reducing heat transfer and lowering energy consumption. They are often used as insulation layers in high-temperature furnaces to improve energy efficiency.

Thermal Shock Resistance

High thermal shock resistance allows it to withstand rapid temperature changes without cracking or damage. Suitable for equipment subject to frequent startups and shutdowns or large temperature fluctuations.

High Purity and Low Impurity Content

High-purity alumina bricks (e.g., Al₂O₃ content ≥98%) have low impurity content and greater chemical stability, making them more resistant to chemical reactions and corrosion at high temperatures.

Good Electrical Insulation

With excellent electrical insulation properties, they can be used in the manufacture of high-temperature electrical insulation components, such as spark plug insulators and electronic component substrates.

Why are Alumina Ceramics Both Insulators and Conductors?

Common sense suggests that thermal insulation and thermal conductivity should be two distinct entities. For example, cotton is insulating and can be made into cotton-padded clothes, while iron is conductive and can be used in frying pans. The reverse is not true. However, in the real world of refractory materials, we see a different phenomenon: the same material can be used for seemingly opposite purposes: insulation and thermal conductivity. This is the case with alumina ceramics. Alumina ceramics can be made into insulating bricks for high-temperature kilns and heat sinks for electronic products like LED lights.

To answer this question, we need to consider two aspects.

First, as the question above suggests, the thermal conductivity of materials does vary. The most typical example is the difference in thermal conductivity at different temperatures. Take alumina, for example. As the temperature rises, its thermal conductivity decreases. At 1200°C, its thermal conductivity is only about half that at 400°C. However, alumina’s thermal conductivity is not insignificant: at room temperature, it’s 20-30 W/m•K. Even if this decreases by more than half, it still leaves about 10 W/m•K, which is higher than the thermal conductivity of many materials. Therefore, this small change seems insufficient to explain why alumina can both insulate and conduct heat. A more convincing explanation is needed.

Therefore, we need to consider the second, and most important, aspect. Alumina’s ability to both insulate and conduct heat stems from structural changes. In other words, the internal structure of alumina ceramic differs when used as an insulator and a conductor.

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When used as an insulator, alumina ceramic’s most significant structural characteristics are its porosity and low density. For example, when made into hollow alumina sphere bricks, the thermal conductivity of air is very low, and so is the thermal conductivity of hollow alumina sphere bricks. Some may ask, since air has a very low thermal conductivity, why bother incorporating air into the alumina material? This is because, while air has a low thermal conductivity, it cannot prevent thermal radiation. Just as the thermal conductivity of a vacuum is zero, heat from the sun still travels through it to Earth. Porous alumina blocks both heat conduction and radiation, effectively providing insulation and heat preservation. For example, a study reported that a type of alumina microporous ceramic has a density of only 0.6g/cm3, a porosity of 85%, and a thermal conductivity of only about 0.3W/m•K at 1200℃.

However, when alumina is made into thermally conductive ceramics, the requirements are completely different. The first requirement is high density—the higher the better. High density reduces pores, allowing the ceramic grains to bond tightly together, facilitating heat conduction. The second requirement is high purity. The higher the purity, the higher the thermal conductivity. For example, a ceramic with a 99% alumina content can achieve a thermal conductivity of ~26 W/m•K, while when the alumina content drops to 95%, the thermal conductivity drops to only ~20 W/m•K. This is because ceramics with low alumina content have a higher glass content, and glass has lower thermal conductivity, resulting in a lower overall thermal conductivity. Of course, cost is also a factor in practical applications. While high-purity alumina ceramics offer high thermal conductivity, they also come at a higher price. Therefore, alumina ceramics should be selected based on the product’s requirements, rather than simply pursuing high purity.

In addition to high purity and a dense structure, alumina ceramics used as heat sinks often have specific requirements for their external shape. For example, when making an LED heat sink, it often has a fin structure to increase the surface area and facilitate heat dissipation into the air, thereby achieving better heat dissipation effects.