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Alumina Hollow Microspheres

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Hollow microspheres, as dispersible particles, are widely used in refractory material production. In use, they act as aggregates in various heat-resistant materials. Due to their unique properties and structure, nanostructured Al2O3 microspheres are used as raw materials in a variety of products, such as filters, porous lightweight bricks, bubble alumina brick, and refractory products. Hollow Al2O3 microspheres can be used to prepare refractory materials with porous structures and low firing shrinkage. A promising development direction is the preparation of high-porosity ceramic materials through the compaction of nanostructured hollow microspheres.

Hollow Alumina Microspheres

Melted-blown alumina is a common method for preparing hollow alumina microspheres. This method involves melting industrial alumina at high temperature and then using compressed air or oxygen to blow the molten alumina into small spherical shapes, ultimately forming hollow alumina microspheres. The specific steps are as follows:

Industrial alumina containing more than 98% alumina is placed in an electric arc furnace for high-temperature melting. During the melting process, fluxes and stabilizers can be added to increase the melting temperature and stabilize the molten pool.

The molten alumina is then sprayed through nozzles while a pressure valve is opened, using compressed air or oxygen to ablate the liquid alumina into small spherical shapes.

During the blowing process, the flow rate and pressure of the nozzles need to be controlled to ensure that the size and shape of the hollow alumina microspheres are consistent. The blown hollow alumina microspheres need to be screened to remove broken or unsuitable microspheres.

Alumina Bubble Brick - Rongsheng Refractory
Alumina Bubble Brick

Characteristics of Hollow Alumina Spheres

Hollow alumina spheres are lightweight refractory bricks made from industrial alumina using an electro-melting and blowing method. bubble alumina brick, Lightweight refractory insulating bricks made from hollow alumina spheres can be used as linings in high-temperature furnaces in contact with flames.

  1. High operating temperature: Up to 1750 degrees Celsius or higher, with good thermal stability. Low reheat linear shrinkage rate, resulting in longer service life.
  2. Optimized structure and reduced furnace weight: Currently used high-temperature resistant materials are heavy bricks with a bulk density of 2.6-3.0 g/cm³. bubble alumina brick, Hollow alumina sphere bricks, however, have a bulk density of only 1.1-1.5 g/cm³. For the same cubic meter volume, using hollow alumina sphere bricks can reduce weight by 1.1-1.9 tons.
  3. Material savings: To achieve the same operating temperature, the price of heavy bricks is comparable to that of hollow alumina sphere bricks. Furthermore, a considerable amount of refractory insulation material is required. Using alumina hollow sphere bricks can save 1.1-1.9 tons of heavy bricks per cubic meter, and even more, 80% of refractory insulation materials can be saved.
  4. Energy Saving: Alumina hollow spheres have significant insulation properties and a low thermal conductivity, providing excellent insulation. This reduces heat loss, improves thermal efficiency, and thus saves energy. Energy savings can reach over 30%.

Production Process of Hollow Alumina Spheres

  1. Alumina raw materials are melted into a liquid state in a tilting electric arc furnace.
  2. The furnace is then tilted at a certain angle, allowing the molten liquid to flow out of the casting trough at a certain speed. The liquid flow is then dispersed by a high-speed airflow of 0.6-0.8 MPa through a flat nozzle at a 60°~90° angle to the flow stream, thus forming hollow alumina spheres.
  3. Hollow alumina spheres have high temperature resistance and good utilization efficiency. Due to their special structure, they can effectively reduce the weight inside the furnace. Hollow alumina spheres have significant heat insulation effects, thereby reducing heat loss and achieving energy saving. Before construction, hollow alumina spheres must be cleaned to achieve better adhesion. Clean water must be used for mixing, and the mixing time must be carefully controlled. The mixed material should be used immediately.

For high-quality bubble alumina bricks, please choose Rongsheng Refractory Materials Manufacturer. Contact Rongsheng for free samples and quotations.

 

Performance of Zirconia Sizing Nozzle Bricks

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Currently, most steel billets worldwide are produced using continuous casting technology, making the service life and performance of refractory materials for sizing nozzles crucial. Zircon bricks, zirconia sizing nozzles are widely used in tundishes for the continuous casting of small square billets. The performance of the nozzle directly affects the duration of continuous casting.

Zirconia Sizing Nozzle Bricks
Zirconia Sizing Nozzle Bricks

Sizing nozzles are the nozzle bricks in a sliding gate system used in steel mills when casting square billets. The sliding gate system took nearly 80 years from conception to industrialization, primarily due to inadequate material quality, despite its significant advantages over stopper rod systems. Zirconia nozzles exhibit good resistance to corrosion from molten steel and slag, and are typically made from high-purity zircon, CaO-stabilized ZrO2, and industrial-grade zirconium oxide.

The most critical components of a sliding gate system are the slide gate brick and the nozzle brick embedded within it. The system typically consists of upper and lower slide gate bricks and nozzle bricks. Casting begins when the upper and lower nozzle bricks align. The upper nozzle brick is mounted at the bottom of the ladle and remains stationary along with the upper slide gate brick, while the lower nozzle brick slides along with the lower slide gate brick, thus controlling the steel pouring process. The upper nozzle brick is immersed in molten steel for a long time and is relatively difficult to replace, requiring high-quality products and a long service life. The lower nozzle brick is mainly used to control the steel pouring speed and quality, and has lower service life requirements; its lifespan can reach eight hours or longer.

 

Acid-Resistant Bricks of Refractory Materials

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Acid-resistant bricks are primarily made from quartz, feldspar, and silicon dioxide, and are produced through high-temperature firing, resulting in a corrosion-resistant material. They are not easily oxidized at room temperature and can withstand alkaline media of any concentration, exhibiting excellent resistance to hydrochloric acid, nitric acid, and warm chloride solutions. However, they are not resistant to molten alkalis at high temperatures. Acid proof bricks are widely used in corrosion protection applications in chemical, petroleum, power, papermaking, metallurgy, chemical fiber, pharmaceutical, fruit juice, and electroplating industries. They also demonstrate high corrosion resistance in underground sewage systems.

Acid Proof Bricks - Rongsheng Refractory Bricks
Acid Proof Bricks

Types of Acid-Resistant Bricks

  1. Plain acid proof bricks

Plain acid-resistant bricks typically refer to floor anti-corrosion and acid-resistant bricks. These bricks usually have small particles on their surface, providing a certain degree of slip resistance. Therefore, they are often used in factory workshops. They offer excellent corrosion resistance, but cleaning them is relatively difficult.

  1. Semi-Glazed Acid-Resistant Bricks

Semi-glazed acid-resistant bricks combine the advantages of glazed and plain acid proof bricks. Their surface has a thin glaze, making it smooth to the touch, and also contains granular material. Therefore, they offer excellent slip resistance and are relatively easy to clean.

  1. Glazed Acid-Resistant Bricks

Glazed acid-resistant bricks have a relatively smooth surface. These bricks are coated with a thick layer of glaze. Therefore, they are very easy to clean and are often used in factories for walls, pool walls, linings, and other areas requiring relatively high cleanliness.

Rongsheng Anti-Acid Bricks
Rongsheng Anti-Acid Bricks

Parameters of Acid-Resistant Bricks

Acid-resistant bricks are widely used in pharmaceuticals, chemicals, papermaking, fertilizers, printing and dyeing, electroplating, nuclear power, food, and beverage industries due to their corrosion resistance, acid and alkali resistance, and high mechanical strength. To better utilize them, it is necessary to understand their various specifications.

  1. Acid and Alkali Resistance: ≥99.82%
  2. Water Absorption: ≤0.4%
  3. Flexural Strength: ≥21.8MPa
  4. Stain Resistance: Not lower than Level 4
  5. Freeze-thaw Resistance: No cracks or peeling after multiple cycles
  6. Resistance to Rapid Temperature Changes: No cracks, peeling, or damage after one test

Characteristics of Acid-Resistant Bricks

The main characteristics of acid proof bricks are as follows:

  1. Their main component is silicon dioxide, which, after high-temperature firing, becomes a large amount of andalusite. This material has a dense structure and low water absorption, and can withstand alkaline media of any concentration at room temperature. It is widely used in the construction of acid trenches, acid wells, and acid storage tanks.
  2. They are made primarily from quartz, feldspar, and clay, which are decomposed at high temperatures into corrosion-resistant materials. They possess high acid and alkali resistance, low water absorption, are not easily oxidized, and are not easily polluted. They are widely used in corrosion protection projects in petroleum, chemical, metallurgical, power, chemical fiber, papermaking, pharmaceutical, fertilizer, electroplating rooms, and for towers, pools, tanks, and troughs.
  3. Their water absorption rate is very low. Water absorption rate refers to the percentage of the product’s weight in water absorbed when the pores are saturated. In my country, bricks with a water absorption rate ≤0.5% are classified as porcelain bricks, and those with a water absorption rate >10% are classified as earthenware bricks.
  4. It has excellent thermal shock resistance. During production, it underwent 10 thermal shock tests without any cracking or splitting.
  5. It has excellent gloss, and gloss is one of the standards for judging the quality of polished tiles. Higher gloss indicates better product quality, and vice versa.

Applications of Acid-Resistant Bricks

Conventional acid-resistant ceramic tiles are mainly used in chemical, petroleum, and pharmaceutical applications. Rongsheng‘s acid proof bricks are used as linings for drying towers, absorption towers, and reactors in the metallurgical, chemical, and petrochemical industries. They are also used as lining bricks for corrosion-resistant pools, floors, and passageways. Contact Rongsheng for free samples and quotations.

 

Research on the Resistance of Alumina Silica Refractory Materials to Reducing Gas Erosion

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Alumina-silicon refractories are mainly composed of Al₂O₃ and SiO₂, with corundum and mullite as their primary phases. Corundum possesses advantages such as high mechanical strength, wear resistance, and high chemical stability. Mullite exhibits advantages such as high refractoriness, high load softening temperature, excellent thermal shock resistance, and strong resistance to chemical corrosion. Therefore, alumina silica fire brick possess excellent mechanical properties, high-temperature resistance, and corrosion resistance, making them widely used in high-temperature industrial equipment such as blast furnaces and converters.

With the development of hydrogen metallurgy technology, the hydrogen content in reducing gases will further increase, and the service environment of refractory materials will become more demanding. Therefore, the applicability and corrosion mechanisms of alumina-silicon refractories have become a research hotspot for scholars both domestically and internationally. This paper introduces the research results on the thermodynamic stability of refractory materials under reducing atmospheres, the influence of various factors on reaction rates, and the research on the corrosion mechanisms of alumina silica fire brick, aiming to provide a reference for extending the service life of refractories used in hydrogen-based shaft furnaces.

Alumina Silica Fire Brick
Alumina Silica Fire Brick

Thermodynamic Stability Studies

Many scholars have analyzed the potential reactions between various components of alumina silica fire brick and reducing gases, as well as the Gibbs free energies of these reactions, through thermodynamic calculations. Simultaneously, the thermodynamic software FactSage was used to simulate the thermodynamic equilibrium states of refractory material components with reducing gases at different temperatures, atmosphere compositions, and pressures, thus obtaining the order of thermodynamic stability of each component under a reducing atmosphere.

Some scholars have calculated the standard Gibbs free energies of the reactions of corundum, quartz, and mullite with H₂ at 1400℃, and calculated the equilibrium partial pressures of the gaseous products. These were used to determine the thermodynamic stability of the reactants, with the order being corundum > mullite > quartz. This indicates that corundum possesses good thermodynamic stability under a pure H₂ atmosphere. Subsequently, the partial pressures of gaseous products when typical components in brown fused alumina reach equilibrium with H₂ below 1400℃ were further calculated, yielding the following thermodynamic stability order for each component in the brown fused alumina raw material: Al₂O₃ > CaO > MgO > SiO₂ > TiO₂ > Fe₂O₃.

Some researchers have calculated the Gibbs free energy changes of possible chemical reactions between components such as Al₂O₃, MgO, CaO, ZrO₂, MA, CA₆, Fe₂O₃, Cr₂O₃, and SiO₂ and H₂ in refractory materials within the temperature range of 600–1600 K. The results show that the Gibbs free energy changes of the components Al₂O₃, MgO, CaO, ZrO₂, MA, and CA₆ with H₂ are all greater than zero. This indicates that the reaction is not easily initiated, and these components exhibit good thermodynamic stability in a pure H₂ atmosphere. Other impurities, such as Fe₂O₃ and Cr₂O₃, react more readily with H₂, exhibiting poor stability. Furthermore, at temperatures above 1200℃, SiO₂ reacts with H₂ to produce SiO and H₂O gases. Therefore, Fe₂O₃, Cr₂O₃, and SiO₂ are relatively unstable in reducing atmospheres, and the content of these components should be controlled in alumina silica fire brick manufactured under high-temperature reducing atmospheres.

Scholars have calculated the partial pressure of water vapor at equilibrium of the reduction reactions of refractory material components (MgO, CaO, Al₂O₃, CA₆, 3Al₂O₃·2SiO₂, SiO₂, TiO₂, Fe₂O₃) under different atmospheres [pure H₂ atmosphere, a mixed atmosphere of 75%(φ)H₂+25%(φ)CO] and different pressures (0.2~1 MPa) for different service environments inside hydrogen-based shaft furnaces. The results show that:

  • 1) Under a pure H₂ atmosphere, the main components of alumina silica fire brick can be reduced to elemental forms or lower valence compounds. Fe₂O₃ and TiO₂ are relatively easily reduced, and the equilibrium partial pressure of water vapor for 3Al₂O₃·2SiO₂ and SiO₂ is basically the same at different temperatures. The thermodynamic stability order of the components in the refractory material is: MgO > CaO > Al₂O₃ ≈ CA₆ > 3Al₂O₃·2SiO₂ ≈ SiO₂ > TiO₂ > Fe₂O₃.
  • 2) Under a mixed atmosphere, Al₂O₃, MgO, CaO, and CA₆ do not undergo reduction reactions. The stability of 3Al₂O₃·2SiO₂, SiO₂, TiO₂, and Fe₂O₃ is worse than under a pure H₂ atmosphere.
  • 3) Under different pressures, the partial pressure of water vapor is the same when all components reach equilibrium with the reducing gas.
  • Among the main components of aluminosilicate refractories, Al₂O₃ exhibits strong thermodynamic stability, while SiO₂, Cr₂O₃, Fe₂O₃, and TiO₂ show relatively weak stability under reducing atmospheres. Therefore, the contents of SiO₂, Cr₂O₃, Fe₂O₃, and TiO₂ in the material should be controlled under high-temperature and high-pressure reducing atmospheres.
Alumina Silica Fire Brick - Rongsheng Refractory
Alumina Silica Fire Brick

Factors Affecting the Reduction Reaction Rate

During service, refractory materials are subjected to the combined effects of hydrogen-rich gas erosion, thermal stress, and mechanical loads. Therefore, the factors influencing the reduction reaction in refractory materials are extremely complex. These include the diffusion and adsorption of reducing gases, chemical reactions, and the desorption of gaseous products. As the reaction continues, the refractory material itself undergoes changes, such as mass loss, chemical changes, and alterations in pore structure. Currently, research on the effects of reduction reactions mainly focuses on the chemical composition of the refractory material, temperature, and the composition of the reducing atmosphere.

Performance Characteristics of Aluminosilicate Refractory Components under Reducing Atmospheres

Simulation calculations using FactSage thermodynamic software revealed the thermodynamic stability order of various components in aluminosilicate refractories under reducing atmospheres, providing a theoretical basis for the rational selection and precise design of refractory materials. In the erosion tests of alumina silica fire brick, the factors affecting the reduction reaction rate mainly include the chemical composition of the refractory material, temperature, and the composition of the reducing atmosphere. Increasing the reduction temperature, increasing the CO concentration in the reducing gas, and having a higher SiO₂ content in the material components all increase the degree of reaction. Under high-temperature conditions, the phase composition and microstructure of aluminosilicate refractories undergo significant changes after being eroded by a reducing atmosphere, thus reducing their stability under reducing atmospheres. With the further development of low-carbon metallurgical technology, the increased volume ratio of H₂ in the reducing gas and the rise in temperature will make the service environment of aluminosilicate refractories even more severe. Therefore, to address the current problems in the resistance of aluminosilicate refractories to reducing gas erosion and improve their service life, research can be conducted in the following directions:

  • (1) Thermodynamic simulations of various gases (H₂, CO, CO₂, H₂O, and CH₄) should be performed, along with corresponding high-temperature simulated erosion tests to verify the stability of alumina silica fire brick under complex operating conditions.
  • (2) Refractory material design should be tailored to the service conditions of different parts of hydrogen-based shaft furnaces. Aluminosilicate refractories can improve their reduction resistance and inhibit CO disproportionation by improving chemical composition, controlling apparent porosity, reducing glass phase composition, and decreasing Fe₂O₃ content, thus extending their service life.
  • (3) Long-term kinetic studies should be conducted to quantify the reaction rate of aluminosilicate refractories under reducing atmospheres, identify the main factors affecting the reaction rate, and predict the service life of aluminosilicate refractories under service conditions.

 

Use of Alumina Silica Fire Bricks in Hydrogen Reduction Furnaces

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Hydrogen (H2) is a highly reducing gas and the lightest substance. Its molecular movement and diffusion speeds are extremely rapid, and it possesses strong permeability at both normal and high pressures. Due to H2’s strong thermal conductivity, the refractory materials used in the furnace’s heat-resistant structure must exhibit enhanced resistance to hydrogen reduction, lower porosity, and lower thermal conductivity.

Alumina Silica Fire Brick Used in Hydrogen Reduction Furnaces

In furnaces with low H₂ content and operating temperatures generally between 800°C and 1300°C, the lining refractory materials used are primarily alumina silica fire brick, primarily low-iron, high-alumina bricks and corundum bricks. The working layer primarily uses low-iron, high-alumina bricks and hollow corundum sphere castables, while the insulation layer primarily utilizes lightweight mullite bricks and low-iron insulating castables. This is primarily due to the performance characteristics of alumina-Si refractory materials that meet the requirements of actual working environments and have the potential to serve as refractory materials for furnace linings.

Use of Alumina Silica Fire Bricks
Use of Alumina Silica Fire Bricks in Hydrogen Reduction Furnaces

High-alumina bricks and corundum bricks are both alumina-silica fire bricks, which are based on alumina and silica. They offer a wide variety of products and a wide range of applications, representing a significant portion of refractory production. In addition to being made into fired or unfired bricks, aluminum-silicon refractories can also be made into cast products, as well as various grades of monolithic refractories, such as ramming materials, castables, and refractory coatings. The main advantages of aluminum-silicon refractories for their widespread application in high-temperature industries are:

  1. Excellent high-temperature resistance: Alumina silica fire brick has high refractoriness, excellent thermal shock resistance, and high-temperature chemical stability. They generally maintain good stability and strength in high-temperature environments and can withstand the extreme temperatures found in high-temperature furnaces.
  2. Good corrosion resistance: These refractories exhibit a certain degree of corrosion resistance against chemicals such as acids and alkalis, allowing them to operate stably for long periods in corrosive atmospheres.
  3. Good mechanical properties: These refractories generally possess excellent mechanical strength and durability, capable of withstanding the mechanical stresses and vibrations experienced during furnace operation.
  4. High adaptability: Aluminum-silicon refractories can be tailored to meet the specific refractory requirements of different industrial sectors through different formulations and preparation processes, demonstrating their high adaptability.
  5. Good cost-performance: Compared to other high-performance refractories, alumina silica fire bricks generally have a lower cost, providing a more economical solution.

To purchase high-quality alumina-silicon refractory bricks, please contact Rongsheng Refractory. Receive free samples and quotes.

 

Hardening and Curing of Rotary Kiln Lining Castables

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After the castable begins to set, the curing time must be at least 24 hours at room temperature. To some extent, extending the curing time to 48 hours or longer will help increase the strength of the concrete. For low-cement castables, the setting time is often longer.

Curing of Rotary Kiln Lining Castables

For areas using high-strength castables, such as coal injection pipes and kiln openings, the curing and demolding times should be appropriately extended. Before hardening, moisture evaporation should be prevented. Treatments such as covering the lining surface with a thin layer of plastic or grass matting, or applying mud and water for curing, can be employed. During this period, due to the low strength of the concrete, excessive mechanical forces that could damage the concrete should be avoided. After hardening, the castable should be allowed to dry for a period of 24 to 48 hours at an ambient temperature of 15 to 30°C. The lower the temperature, the longer the curing time. If the temperature falls below 10°C, consider increasing the temperature to improve curing conditions. Sodium silicate and phosphate castables should be cured in a dry environment, utilizing the dehydration of the sodium silicate to increase strength. Avoid watering during curing. Demolding time should be appropriately selected. Non-load-bearing formwork should be demolded when the castable strength is sufficient to prevent it from falling apart due to cold. Load-bearing castables should be demolded when the strength reaches 70%. The core mold with higher strength should be demoulded in time without causing damage to the concrete, so as to avoid difficulty in demoulding due to excessive strength of the castable.

Construction of Refractory Castables for Rotary Kiln
Construction of Refractory Castables for Rotary Kiln

Drying and Heating the Preheater and Precalciner Systems

After hardening or drying, the castable still contains residual physical and chemical water. This water must be removed by heating it to 300°C for gasification and dehydration. Due to the dense structure of the castable, the heating rate must be slow to avoid damage to the castable caused by stress due to rapid temperature increases and excessive evaporation of water. The kiln system’s drying and heating schedule may not always meet the drying requirements of the preheater and precalciner (the grate cooler, kiln head hood, and tertiary air duct meet the kiln system’s drying and heating schedule and are not listed separately). Therefore, the kiln system’s baking and heating schedule, described below, should be implemented in conjunction with the requirements of this section.

If the primary preheater fails to meet the drying requirements when the kiln system reaches 600°C (based on the kiln exhaust gas temperature), the kiln system’s holding time at 600°C should be extended. The final batch of refractory castables should be cured at approximately 25°C for at least 24 hours (for low-cement castables, the curing time should be extended to 48 hours, if appropriate). After the castable has achieved a certain strength, the formwork and supports are removed.

After drying for 24 hours, the curing process can begin. If the curing temperature is too low, the curing time will need to be extended. Based on the kiln exhaust gas temperature, increase the temperature at a rate of 15°C/hour until it reaches 200°C, holding it for 12 hours. Increase the temperature at a rate of 25°C/hour to 400°C, holding it for at least 6 hours. Increase the temperature to 600°C, holding it for at least 6 hours.

The following two conditions are necessary and sufficient for the bakeout of the decomposition furnace and preheater systems:

When the temperature of the refractory castable at the cyclone preheater pouring hole, on the side closest to the silicon cover plate, reaches 100°C, continue drying for at least 24 hours.

At the manhole door of the first-stage cyclone preheater, contact the flue gas with a clean piece of glass. If no moisture condensation on the glass is observed, hold it for 6 hours.

Curing and Curing Time for Refractory Castables

The curing and curing time for refractory castables are affected by a variety of factors. The following is a guide to common curing times:

Regular Curing Time

  • Formwork Curing: After pouring, the castable generally requires 24 hours of formwork curing before demolding.
  • Post-Demolding Curing: A 24-hour natural curing period is ideal after demolding. This allows the castable to reach a certain strength and avoid premature disturbance.

Adjustments for Special Situations

  • Low-cement castables: Curing requirements are higher, and curing time may need to be extended to 48 hours or even longer, depending on the material properties.
  • Winter Construction: When the ambient temperature is below 5°C, insulation measures (such as steam curing or electric heating) are required. Curing time may be extended to 48-72 hours to ensure sufficient curing of the castable.
  • High Temperature Environment: When the ambient temperature is above 30°C, moisture-retention curing is required to prevent rapid evaporation of moisture, which could affect strength. Curing time is generally no less than 24 hours.

Curing Precautions

  • During curing, maintain the ambient temperature between 15-35°C and avoid sudden temperature fluctuations.
  • Cement-bonded castables require moist curing, while waterglass or phosphate-bonded castables should be cured in a dry environment and avoid watering.
  • During curing, avoid striking or vibrating the castable to prevent cracking.

The curing and curing time for common refractory castables at room temperature (10-30°C) is generally around 48 hours, but the specific time varies depending on factors such as material type, ambient temperature, and construction process. During construction, strictly follow the material specifications and construction specifications to ensure that curing conditions meet standards and guarantee the performance and service life of the castable.

 

Indicators Need to Know When Buying Fused Cast Bricks for Glass Furnaces

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Glass kiln cast refractory bricks are the most common refractory bricks purchased in glass kilns. The main technical indicators for assessment include chemical composition, bulk density, glass phase seepage temperature and quantity, and foaming rate. Other general refractory material inspection items such as porosity, refractoriness, and load softening point are not very meaningful for actual use conditions. During the condensation process of the ingot, due to the effect of the temperature gradient, chemical phase composition segregation will form along the cross section. This is a rule of cast AZS products. The higher the ZrO2 content and the larger the ingot size, the greater the degree of segregation. The ingot is generally divided into three parts:

  • ① The surface rapid cooling zone: This zone has minimal deviation from the original design composition and features small, uniform crystals.
  • ② The intermediate dense zone: ZrO₂ is enriched in this zone, where primary ZrO₂ crystals appear.
  • ③ The internal shrinkage zone: This zone contains a large amount of liquid phase, allowing for the precipitation of large, stacked, and oriented (C+Z) particles.

Fused zirconium products are categorized by their chemical composition into: fused AZS, AZS re-sintered fused bricks, and fused zirconium mullite bricks.

Fused Cast AZS Brick
Fused Cast AZS Brick for Glass Kiln Lining

Fusion-cast bricks for glass furnaces

AZS fusion-cast bricks, AZS No. 41 fusion-cast bricks.

AZS re-sintered fused bricks, AZS re-sintered fused bricks, are primarily used for masonry of thermal equipment such as the floor, walls, and regenerator checker bricks of glass furnaces.

Zircon-mullite fusion-cast bricks, whose primary mineral phases are mullite, baddeleyite, corundum, and glass phases. Their chemical composition lies within the triangular region of the Al₂O₃-SiO₂-ZrO₂ ternary phase diagram (approximately 63% Al₂O₃, 21% SiO₂, and 21% ZrO₂), where stable compounds are formed. The melting point of the mullite solid solution in this region is 1830-1870°C.

The main raw materials for making zircon-mullite fusion-cast bricks are industrial alumina, natural zircon, and a small amount of sodium oxide (in the form of soda ash). Some methods use raw bauxite, soft clay, and zircon to press into rough blocks. After firing, the blocks are crushed to a suitable particle size. MgO, a mineralizer, is then added during casting. The Al₂O₃/SiO₂ ratio should ideally be between 2.2 and 3.2.

Zirconium-mullite fused-cast bricks are characterized by a dense crystal structure, a high refractoriness under load, and excellent thermal shock resistance. They offer high mechanical strength at both room and elevated temperatures, excellent wear resistance, good thermal conductivity, and excellent resistance to slag erosion. They are used effectively in applications such as the cooling tank walls of glass melting furnaces.

Erosion Mechanism of the Slurry Hole, a Key Part of a Glass Furnace

The slurry hole is a crucial component of a glass furnace. Melted, clarified, and homogenized molten glass flows through it as it enters the distribution and supply manifolds, undergoing further homogenization and cooling before entering the molding equipment for production. Every day, 360 to 400 tons of molten glass pass through this narrow passageway, causing significant erosion and wear. Furthermore, the slurry hole operates in a harsh environment with high ambient temperatures, making it difficult to replace any problems. Therefore, fused AZS41# bricks or dense chromium oxide bricks (for some glass products) are highly corrosion-resistant for slurry holes.

Fused AZS bricks exhibit an upward pitting tendency at the interface between the solid, liquid, and gas phases. The fused AZS bricks are the solid phase, the molten glass is the liquid phase, and any bubbles remaining in the molten glass are the gas phase. The slurry hole cover bricks erode more than any other area, eroding upward by 300 to 400 mm in just 5 to 6 years. When the slit hole cover bricks erode by 300-400mm, the depth of the molten glass becomes shallower, equivalent to a 300-400mm reduction in the original kiln pool depth. This inevitably impacts the kiln’s discharge capacity. To meet this capacity, the furnace temperature must be increased, which in turn increases energy consumption. After a few years, the furnace cover and other components of the furnace body burn out, thinning the furnace body and increasing internal temperatures, further increasing heat dissipation.

In summary, the structure of the slit hole affects the kiln’s lifespan, discharge capacity, and energy consumption. To address the mechanism of slit hole cover brick erosion, the contact surface between the slit hole cover brick and the molten glass can be designed with an inclined design (upward angle of approximately 18°). This prevents bubbles in the molten glass from adhering to the slit hole cover bricks, significantly slowing erosion and extending the life of the slit hole, and therefore the kiln.

 

Application of Clay Silicon Carbide Bricks in Ladle Lining

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To extend the lifespan of ladle linings, new materials must be specifically designed based on the challenges inherent in current ladle operations. These new materials must possess excellent thermal shock resistance, adequate resistance to desulfurizer erosion, and sufficient thermal insulation. Research and development primarily focus on aluminum-carbon materials, specifically those designed for the molten iron environment. In recent years, fired aluminum-silicon carbide carbon bricks have gained widespread adoption in major steel mills due to their excellent thermal shock and erosion resistance. However, because they contain carbon materials such as graphite, exposed areas like the slag line are susceptible to oxidation, becoming porous and susceptible to erosion, significantly reducing their erosion resistance. Furthermore, the presence of carbon increases thermal conductivity, leading to high furnace shell temperatures and significant heat loss. This can lead to both capping and bottoming, making treatment difficult and hindering production. The average temperature drop during ladle transportation at one steel mill is approximately 190°C, slightly higher than at other mills. Some mills have reduced this temperature drop by adding caps to their ladles, but this issue remains a significant problem in mills with open-top ladle operations and long turnover routes. Based on this situation, clay silicon carbide bricks with good matching to the ladle process and excellent related performance were developed. Their thermal shock resistance and thermal insulation performance are better than those of aluminum-chromium silicon carbide bricks and aluminum silicon carbide bricks.

Clay Silicon Carbide Bricks
Clay Silicon Carbide Bricks

Clay Silicon Carbide Bricks for Ladle Lining

The main raw materials for clay silicon carbide bricks are pyrotechnics, andalusite, Guangxi white mud, silicon carbide, and alumina powder. The antioxidant is metallic silicon, and the binder is sulfite pulp wastewater. The purity of the main raw material, pyrotechnics, is ≥43.0% w(Al2O3) and has a particle size of 3 μm to 0 mm. Andalusite has a w(Al2O3) content of ≥69.0% and a particle size of 3 mm to 0 mm. Guangxi mud has a w(Al2O3) content of ≥30.0% and a particle size of 200 mesh. Silicon carbide has a w(SiC) content of ≥90.0% and a particle size of 200 mesh. Alumina powder has a w(Al2O3) content of ≥99% and a particle size of 3 μm to 6 μm.

To extend the service life of the ladle lining, the first step is to improve the thermal shock resistance of the working layer bricks. To combat thermal stresses generated by rapid thermal cycling, a suitable lining structure should incorporate effective stress buffering and release mechanisms to prevent the propagation of thermal shock microcracks and the resulting delamination of the lining. Under thermal shock, crack initiation, growth, and propagation in porous refractories such as aluminum silicon carbide bricks are related to the fracture surface energy. To improve the thermal shock resistance of the material, a high elastic modulus and low strength should be ensured. The lower compressive strength of clay silicon carbide bricks compared to aluminum silicon carbide bricks improves their thermal shock resistance. Furthermore, due to their higher apparent porosity than aluminum silicon carbide bricks, the evenly distributed pores effectively buffer and release thermal stress, significantly reducing the occurrence of delamination cracks and delamination.

Under normal circumstances, due to the higher porosity of clay silicon carbide bricks compared to aluminum silicon carbide bricks, their looser structure reduces their corrosion resistance, making them more susceptible to desulfurization agents and slag. However, in actual use, due to its high porosity, the surface of the clay silicon carbide brick is prone to accretion of a layer of slag. When the ladle cools down after emptying, the slag solidifies within the voids, partially filling the pores. This slag layer blocks some of the pores and provides a protective barrier, ensuring that the clay silicon carbide brick also has good slag resistance. Furthermore, the slag layer protects the brick lining from direct impact of the high-temperature molten iron, providing a buffering effect and reducing damage to the brick lining caused by thermal shock stress.

The relatively high apparent porosity and low bulk density of clay silicon carbide bricks ensure their relatively good thermal insulation properties, with the lowest gas thermal conductivity. The evenly distributed pores in clay silicon carbide bricks reduce their thermal conductivity, minimizing the temperature drop caused by heat conduction at the bottom and opening of the ladle. This helps to reduce slag and capping at the bottom and opening of the ladle.

Practical Applications of Clay Silicon Carbide Bricks

When aluminum silicon carbide bricks were used in 150-ton hot metal ladles, their service life was as low as 220 heats. The lining of the ladles severely peeled due to thermal shock, resulting in an uneven surface and dense, narrow, layered cracks visible on the exposed bricks. After the clay silicon carbide bricks were put into use, the excellent thermal shock resistance significantly reduced the spalling of the brick lining, resulting in a smooth and orderly surface. Furthermore, their excellent thermal insulation properties alleviated the problem of bottom and cover buildup in the ladles, reducing the occurrence of blasting and mechanical stress damage to the lining.

Once the clay silicon carbide bricks were put into use, the average service life of the 150-ton hot metal ladle reached 350 heats, significantly extending its service life. Furthermore, because the clay silicon carbide bricks are cheaper per unit than aluminum silicon carbide bricks, and their low bulk density and light weight mean each bale weighs less, their service life is extended. As a result, the overall cost of using the 150-ton hot metal ladle has been significantly reduced since their introduction. Due to the excellent performance and significant cost reduction of clay silicon carbide bricks, they have completely replaced aluminum silicon carbide brick ladle linings in the 150-ton ladle system and are still in use today.

Based on the above analysis, the following conclusions are drawn:

  • (1) Thermal shock is the main factor leading to damage to the ladle lining. Clay silicon carbide bricks have excellent thermal shock resistance and can significantly reduce the layered cracks and layered peeling caused by rapid cooling and heating during the transportation and iron-bearing process of the 150-ton ladle.
  • (2) After being put into use, clay silicon carbide bricks can ensure relatively good slag resistance and meet the ladle lining’s corrosion resistance requirements for desulfurizers and molten iron slag.
  • (3) The thermal insulation effect of clay silicon carbide bricks is better than that of aluminum silicon carbide bricks, which improves the phenomenon of the bottom and mouth of the 150-ton ladle being solidified.

 

Effect of Aluminum Alloy Fused Casting Refractories Materials on Melt Quality

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The refractories used in the aluminum alloy casting process play a crucial role in the melt quality. This is because the refractories come into direct contact with the high-temperature aluminum alloy melt. Inferior or unsuitable refractories may chemically react with the melt, increasing impurities in the melt and affecting the purity and quality of the aluminum alloy. Selecting the right refractory is crucial to improving the quality of the aluminum alloy melt. Refractory selection should be tailored to the specific aluminum alloy melting conditions to ensure stable melt quality and excellent final product performance. The choice of refractory is crucial in the aluminum alloy casting process, as it directly affects the melt quality, as well as the mechanical properties and appearance of the final product. To further explore the impact of refractory materials on melt quality in aluminum alloy casting, the following key aspects are detailed.

Fused AZS Refractory Bricks for Glass Furnaces
Fused AZS Refractory Bricks

  1. Effect of Chemical Stability on Melt Purity

Aluminum alloy melts are highly chemically active at high temperatures and easily react with refractory materials. If the refractory material used has poor chemical stability, it is easily eroded and corroded by the aluminum alloy melt, producing reaction products. These products enter the melt, increasing metallic impurities and, in turn, affecting melt purity and product quality.

  • – Oxidation Reactions: Many common refractories, such as silicon oxide and iron oxide, undergo redox reactions in high-temperature aluminum melts, forming solid inclusions (such as aluminum oxide). These inclusions increase the impurity content in the melt and affect the mechanical properties of the aluminum alloy, especially in high-strength and high-toughness materials.
  • – Carbide and Nitride Formation: Some refractories may react with elements in the aluminum alloy melt at high temperatures to form carbides or nitrides. These compounds are insoluble in the aluminum matrix and ultimately form inclusions in the casting, reducing the purity of the material.

  1. Impact of Thermal Shock Stability on Durability

The temperature inside an aluminum alloy casting furnace fluctuates significantly, especially during the melting and pouring processes, when the furnace temperature can fluctuate dramatically. If the Fused Casting Refractories have poor thermal shock stability, it can crack or even spall due to uneven expansion.

  • – Impact of Cracking and Spalling: Once cracking or spalling occurs, the refractory fragments enter the aluminum melt, becoming insoluble foreign matter. These foreign matter are difficult to remove during the subsequent casting and solidification processes, forming hard inclusions in the casting. This leads to localized inconsistencies in the material’s mechanical properties and reduces the ductility and fatigue strength of the aluminum alloy.
  • – Thermal Expansion Coefficient of Refractory: Selecting refractory materials that match the thermal expansion coefficient of the aluminum alloy melt can help reduce thermal shock cracking, extend the refractory’s service life, and maintain a clean melt environment.

  1. Oxidation Resistance Controls Oxidation Inclusions in the Melt

Aluminum alloy melts readily react with oxygen to form alumina (Al2O3). Refractory materials with strong oxidation resistance can reduce oxygen contact with the melt and minimize the formation of alumina inclusions. This is particularly important because alumina inclusions are insoluble in aluminum alloys and significantly impact casting quality.

  • – Impact of Alumina Inclusions: Alumina inclusions not only increase the inclusion content in the melt but can also lead to localized degradation of mechanical properties in the casting, particularly tensile strength and fracture toughness. In demanding applications such as automotive and aviation, these inclusions can cause serious quality defects.
  • – Application of Anti-Oxidation Coatings: During the aluminum alloy casting process, an anti-oxidation coating can be applied to the refractory surface to reduce oxygen penetration and oxide formation. For example, coatings of magnesia, zirconia, or alumina can effectively improve the oxidation resistance of refractory materials.

  1. Impact of Sealing and Thermal Conductivity on Melt Temperature Control

The sealing and thermal conductivity of refractory materials directly impact the stability and uniformity of the melt temperature. Temperature control is crucial during the melting and pouring processes. Uneven temperatures can lead to uneven distribution of the aluminum alloy’s composition, thus affecting the alloy’s final mechanical properties.

  • – Impact of Temperature Fluctuations: Poor thermal conductivity of refractory materials can cause localized temperatures within the furnace to be excessively high or low. This, in turn, leads to uneven composition of the aluminum alloy melt, affecting the grain structure during solidification. This can result in localized variations in mechanical properties within the casting, such as hardness and strength.
  • – Thermal Insulation: A good refractory material should exhibit excellent thermal insulation, minimizing heat loss during the melting process and ensuring consistent melt temperature. Especially in large-scale casting, temperature uniformity is directly related to the quality and consistency of the casting.

  1. Wettability with the Aluminum Alloy Melt

The wettability of refractory materials with the aluminum melt is also a key factor affecting melt quality. If the wettability between the refractory and the melt is too strong, the aluminum alloy melt will react with the refractory, increasing the formation of metal oxides and other inclusions.

  • – Wettability Optimization: Low-wettability materials or coatings (such as boron nitride coatings) are often used to reduce the wetting between the aluminum melt and the Fused Casting Refractories, thereby reducing the formation of inclusions at the reaction interface. This helps improve melt purity and casting surface quality.

  1. Refractory Selection Recommendations

To ensure the quality of aluminum alloy melts, appropriate refractory materials should be selected based on the melting conditions. Commonly used refractory types include:

  • – Alumina-based refractories: High chemical stability, suitable for use with high-temperature aluminum melts.
  • – Silicon nitride-based refractories: Excellent oxidation resistance and thermal shock resistance, suitable for long-term use.
  • – Silicon carbide-based refractories: Excellent thermal conductivity, suitable for melting conditions requiring rapid heat transfer and high-temperature resistance.
  • – Magnesia and spinel refractories: High corrosion resistance, suitable for melting aluminum melts containing magnesium alloys.

The choice of refractory materials for aluminum alloy casting plays a decisive role in the melt purity, mechanical properties, and quality of the final product. High-quality refractory materials can reduce inclusions in the melt, control oxide formation, and ensure uniformity and stability of the melt temperature, thereby improving the overall quality of aluminum alloy castings. In actual production, refractory materials should be carefully selected and maintained according to different casting conditions to ensure that they can effectively protect the melt quality.

 

Why is it Necessary to Bake the Furnace After the Construction of Refractory Lining Materials?

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Furnace lining materials (such as refractory bricks and refractory castables) absorb significant amounts of moisture during construction. If heated directly without drying, this moisture rapidly evaporates, causing cracks, bulging, and even explosions in the lining. Furnace drying gradually evaporates this moisture by slowly heating it, protecting the refractory materials.

How is Water Removed During the Furnace Drying Process?

After refractory construction is completed, three main types of water exist in the system: free water, physically bound water, and chemically bound water.

During furnace drying, a large amount of free and bound water must be removed. This water evaporates significantly between 80°C and 150°C, after which the drainage rate slows. As the temperature continues to rise, around 300°C, the free and bound water are essentially completely removed. At 500°C to 600°C, the refractory material completes its crystal transformation. At around 1100°C, the sintered strength required for design is reached, and stress is completely released. To fully remove deep-seated free and bound water and complete crystal transformation, constant temperatures must be maintained for a period of time at different temperature ranges.

During furnace drying, the 80°C to 300°C stage is a critical phase. This is primarily because water transforms into water vapor at 110°C. During this phase, the amount of water removed is large and the rate of water removal is rapid. The rate of water removal significantly affects the refractory material. If moisture is removed too quickly, the remaining internal moisture will not diffuse as quickly as the surface evaporation rate. The internal moisture will also heat up and convert to vapor, causing expansion. This can cause cracking in the material, reduce bond strength, and weaken the material. Therefore, the insulation period should be appropriately extended. The ideal drying period is a constant drying rate, where the amount of moisture evaporating from the surface matches the amount of moisture removed from the interior.

When heating from 80°C to 600°C, the temperature should be increased slowly. Too rapid a temperature increase will cause the refractory surface to dry quickly, preventing the large amount of vaporized moisture from evaporating. This will generate destructive vapor pressure and cause cracks or fissures in the refractory. At 300°C, bound water and crystallization water convert to gas and are released through pores. Therefore, sufficient drying time is required to completely remove the crystallization water.

Typical Furnace Curve

The above describes the general principles of furnace curing. Because different refractory materials have different compositions and thicknesses, the furnace curing curve must be determined by the design company in conjunction with the refractory manufacturer. The figure below shows a typical furnace curing curve found online.

  1. First, slowly increase the temperature (20-30°C/hour) to 140°C and hold it at this temperature for 20 hours.
  2. Then, slowly increase the temperature (20-30°C/hour) to 350°C and hold it at this temperature for 10 hours.
  3. Then, slowly increase the temperature (20-30°C/hour) to 420°C and hold it at this temperature for 10 hours.
  4. Finally, slowly cool the temperature back to ambient temperature.

Furnace Drying in Waste-to-Energy Plants

Furnace drying is part of the furnace construction process. Generally, furnace construction is divided into three phases: construction and cold-state inspection, low- to medium-temperature drying, and high-temperature drying.

Construction and cold-state inspection, in simple terms, involve the construction of the furnace walls and refractory components. High-temperature drying refers to the drying of the furnace lining, which is performed simultaneously with the corrosion-resistant passivation treatment of the boiler’s steam-water system. This improves efficiency, saves fuel, and enhances economic efficiency.

The purpose of low- to medium-temperature drying is to remove moisture that cannot naturally separate from the castings, allowing them to further solidify and ensure the performance and quality of the furnace lining. This process virtually completely removes free water and most crystallized water, bringing the moisture content to less than 2.5%, a satisfactory standard.

Of course, a more important aspect of furnace drying is the inspection of the furnace walls after the relatively mild fire. Deformation, cracks, and collapse should be avoided to ensure they meet the performance requirements of the furnace under normal operating conditions.

The basic requirements for furnace drying are that the furnace is complete and has been naturally dried for seven days. While the temperature of a medium-low temperature furnace drying is not particularly high, it is still considered the first ignition. As a thermal equipment, the boiler must undergo its initial thermal testing. Therefore, requirements such as boiler insulation, drum water level, exhaust and drain piping, water and steam pipe hangers, sealed flue gas and air duct openings, pressurized water pressure control, lighting, fire protection, and safety precautions must be in place.

Next comes the preparation of furnace drying equipment and materials.

Before the advent of high-temperature flue gas furnace drying, the furnace drying process was typically accomplished using a combination of wood and fuel oil. Firewood was ignited at a defined location, and after the temperature reached a certain level, a starter oil burner was ignited to raise and maintain the required furnace temperature. Limited by the burning point of the firewood, the labor intensity involved, the high flame temperature of the starter oil burner, and the concentrated heat load, a slow and uniform furnace heating process was impossible. Structural constraints inevitably created “dead spots” within the boiler that were not accessible for drying, making furnace drying quality difficult to ensure.

Currently, most furnaces use light diesel fuel and utilize a furnace drying machine to generate hot flue gas outside the furnace. This hot flue gas serves as the heat source, and after entering the furnace, the furnace walls primarily absorb heat through convection. This prevents high-temperature radiation from the flame from damaging the furnace walls, ensuring uniform heat absorption.

This allows for a slow heating process for the lining material, adhering to the principle of slow and uniform moisture release from refractory and wear-resistant lining materials. Controlling the drying temperature and temperature rise rate promotes the smooth release of different forms of moisture. Ensure temperature control, avoid dead spots, and ensure even heating of the furnace walls.

During the furnace drying process, ensure that all furnace temperature monitoring data is recorded and archived. Ensure moisture drain holes are properly opened and sealed after the boiler is started. At all times, ensure that heating surface tubes are protected from overheating and overburning. Always inspect the smoke exhaust.

 

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