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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.

 

Raw Material Composition of Refractory Materials

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The chemical composition of refractory materials is one of the most basic characteristics of refractory materials. Generally, the chemical composition of refractory materials is divided into two parts according to the content of the components and their functions:

  • (1) The basic components that account for the absolute majority and play a decisive role in their performance – the main components.
  • (2) The subordinate components that account for a small amount – the secondary components. The secondary components are impurities accompanying the raw materials or additives (additives) specially added during the production process to achieve a certain purpose.

Main Component

The main component is the component that constitutes the refractory matrix in the refractory material and is the basis of the characteristics of the refractory material. Its nature and quantity play a decisive role in the properties of the material. The main component can be an oxide or a non-oxide. Therefore, the refractory material can be composed of refractory oxides, or a refractory oxide and carbon or other non-oxides, or it can be composed entirely of refractory non-oxides. Oxide refractory materials can be divided into three categories according to the chemical properties of their main component oxides: acidic, neutral and alkaline.

  • (1) Acidic refractory materials. This type of material contains a considerable amount of free SiO₂. The most acidic refractory material is siliceous refractory material, which is composed of almost 94% to 97% free SiO₂. Clayey refractory materials have a relatively low content of free SiO₂ and are weakly acidic. Semi-siliceous refractory materials are in between.
  • (2) Neutral refractory materials. High-alumina refractories (with a mass fraction of Al2O3 above 45%) are acidic and tend to be neutral, while chromium refractories are alkaline and tend to be neutral.
  • (3) Alkaline refractories contain a considerable amount of MgO and CaO. Magnesia and dolomite refractories are strongly alkaline, while chromium-magnesium and forsterite refractories and spinel refractories are weakly alkaline.

This classification is of great significance for understanding the chemical properties of refractories and judging the chemical reactions between refractories and between refractories and contact materials during use.

Impurities

The raw materials for refractory materials are mostly natural minerals, so they often contain a certain amount of impurities. These impurities can reduce certain properties of the refractory. For example, the main component of magnesia refractories is MgO, while other oxides such as silicon oxide and iron oxide are impurities. The higher the impurity content, the greater the amount of liquid phase formed at high temperatures.

Impurities in refractory materials directly affect the material’s high-temperature properties, such as refractoriness, load deflection temperature, corrosion resistance, and high-temperature strength. On the other hand, impurities can lower the firing temperature of the product, promoting sintering.

At high temperatures, andalusite transforms into mullite and a free SiO2 glass phase. In the Al2O3-SiO2 system, mullite is chemically stable, so refractories containing andalusite are also chemically stable.

Additives

In the production or use of refractory materials, especially amorphous refractory materials, a small amount of additives is added to improve the physical properties, molding or construction performance (operation performance) and use performance of refractory materials. The amount of additives added varies with their properties and functions, ranging from a few ten-thousandths to a few percent of the total amount of refractory materials.

Additives are divided into the following categories according to their purpose and function:

  • (1) Changing rheological properties: including water reducers (dispersants), plasticizers, gelling agents, degumming agents, etc.
  • (2) Adjusting the setting and hardening speed: including accelerators, retarders, etc.
  • (3) Adjusting the internal structure: including foaming agents (air entraining agents), defoamers, shrinkage inhibitors, expansion agents, etc.
  • (4) Maintaining the construction performance of materials: including inhibitors (anti-swelling agents), preservatives, antifreeze agents, etc.
  • (5) Improving the use performance: including sintering aids, mineralizers, quick-drying agents, stabilizers, etc.

These added components, except those that can be burned off, remain in the material’s chemical composition.

Chemical composition analysis allows the purity and properties of a product or raw material to be determined based on the types and quantities of the components present. Phase diagrams can also be used to roughly estimate the product’s mineralogical composition and other relevant properties.

 

Application of High-Strength, Pressure and Erosion-Resistant Carbon Bricks in Large-Scale Submerged Arc Furnaces

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The lifespan of a submerged arc furnace lining is affected by a variety of factors, including the quality of the refractory materials, the level of masonry, the furnace drying process, and smelting operations. Some linings can last over 10 years, while others can last only a few months. This wide range in lining lifespan is primarily a significant test of the quality of the refractory materials. Carbon materials are the primary lining material for submerged arc furnaces. carbon blocks, carbon bricks uses. Among them, electric furnace carbon bricks, a special refractory material used to build the bottom and hearth of the submerged arc furnace, have a quality that directly determines the lifespan of the submerged arc furnace lining.

Carbon Bricks - Rongsheng Refractory Bricks
Carbon Bricks

Carbon Bricks for Submerged Arc Furnaces

With the rapid development of high-energy-consuming industries, submerged arc furnaces are becoming increasingly larger, placing increasing demands on the lifespan of furnace linings and furnace bottom safety. Simultaneously, incidents of molten iron penetrating the bottom are becoming more and more frequent, placing higher demands on upgrading furnace lining materials and improving their quality. Carbon bricks must operate at temperatures exceeding 2,000°C and are subject to long-term erosion, penetration, and corrosion from molten iron and furnace charge. When carbon bricks are damaged to a certain extent, preventing the furnace from operating normally, the furnace must be shut down and rebuilt. The service life of carbon bricks is the most important factor affecting the lifespan of an electric furnace. Therefore, improving the oxidation resistance, erosion resistance, and penetration resistance of carbon bricks in electric furnaces is particularly important.

Use of the First Generation of Carbon Bricks

In response to the special requirements of submerged arc furnaces for carbon bricks, and drawing on years of practical production experience, we have developed the first generation of carbon bricks – new high-density, low-porosity electric furnace carbon bricks. These high-density, low-porosity carbon bricks are made from raw materials with high thermal conductivity, low porosity, high bulk density, and strong antioxidant properties. The use of ultrafine powder (98% particle size below 0.045 mm) improves paste forming conditions and fills the micropores between large particles. Additives can reduce the micropores in the carbon bricks. By reselecting and adjusting raw materials, particle size, and mix ratios, and optimizing processes and equipment, we have finally produced electric furnace carbon bricks with a porosity of less than 15% and a compressive strength exceeding 40 MPa, meeting the production needs of submerged arc furnaces. These carbon bricks offer low porosity, high density, high compressive strength, and significantly enhanced resistance to oxidation, erosion, and penetration. The safety, reliability and service life of the furnace bottom of the electric arc furnace have been improved, and the average furnace life has been increased from less than 2 years to 4-5 years.

Microporous Carbon Brick from Rongsheng Factory
Microporous Carbon Brick from Rongsheng Factory

The Use of Second-Generation Carbon Bricks

As the industry continues to develop, first-generation carbon bricks, while maintaining a certain market share and influence, will face further upgrades to meet evolving market demands. Large-section carbon blocks have long been widely used in developed countries. However, due to limitations in equipment and process technology, most production is limited to small-section carbon bricks (400 mm × 400 mm), which cannot meet the demand for large-section carbon bricks in large-scale submerged arc furnaces. To meet the continuous development of the industry, through repeated exploration and experimentation, accumulated data, and summarized experience, the size of carbon blocks has been gradually increased from 400 mm × 500 mm, 400 mm × 600 mm, 400 mm × 800 mm, 600 mm × 800 mm, and 800 mm × 800 mm, ultimately achieving mass production of large-size carbon blocks of 800 mm × 800 mm × 4000 mm. The development of large-section carbon blocks involves more than simply increasing the size of existing blocks. Instead, it involves a comprehensive transformation and coordination of process technology, raw material preparation, molding equipment, roasting technology, and machining. While increasing the cross-section of the carbon blocks, the original physical and chemical properties are maintained.

The larger cross-section of these carbon blocks reduces gaps in the furnace floor masonry, thereby reducing the risk of molten iron burning through the furnace floor due to gaps. The larger cross-section also increases the weight of the individual carbon blocks, preventing them from drifting. The successful development and widespread use of the 800 mm × 800 mm large-section carbon blocks provide a strong guarantee for extending the life of large-scale submerged arc furnace linings.

Use of Third-Generation Carbon Bricks

Changes in Demand for Carbon Bricks for Submerged Arc Furnaces. Currently, newly built submerged arc furnaces both domestically and internationally are trending toward larger, more enclosed structures, higher power, and greater automation. Large-scale electric furnaces offer high thermal efficiency, high product quality, low unit investment, stable operation, and environmentally friendly performance. As submerged arc furnaces become larger, the corresponding furnace chamber size is also increasing. This places higher demands on the performance of the materials used to build the furnace.

First, the size of the carbon bricks used to build the furnace bottom is increasing, with the largest size reaching 800 mm × 800 mm × 3,700 mm.

As the cross-section and size of the carbon bricks continue to increase, the weight of each brick is also increasing. During the construction of large submerged arc furnaces, gaps can be reduced to prevent brick drift. This improves the utilization rate of fired blanks and reduces waste for manufacturers.

For a furnace with a diameter of 10 m, based on the design drawings for three sizes of carbon bricks: 400 mm × 400 mm × 1200 mm, 400 mm × 820 mm × 1200 mm, and 800 mm × 800 mm × 3700 mm, one layer of 800 mm × 800 mm × 3700 mm bricks is equivalent to two layers of the other two sizes. Based on the total length of gaps between 400 mm × 400 mm × 1200 mm bricks, replacing them with 400 mm × 820 mm × 1200 mm bricks can reduce the gap by 30.8%. Replacing them with 800 mm × 800 mm × 3700 mm bricks can reduce the gap by 72.1%. Taking the total length of gaps in a 400 mm × 820 mm × 1,200 mm carbon brick as a benchmark, replacing it with 800 mm × 800 mm × 3,700 mm carbon bricks can reduce the gap size by 59.7%.

At the same time, with the increasing size of submerged arc furnaces, the diameter and depth of the furnace chamber have increased. As the charge volume increases, the amount of molten iron loaded in the furnace increases, leading to higher production, which in turn increases the pressure on the carbon bricks at the furnace bottom. Due to the increased charge volume, larger electrode size, and higher furnace temperatures in large submerged arc furnaces, the eddy currents of the molten liquid during the smelting process intensify, increasing the impact on the carbon bricks.

As a manufacturer of submerged arc furnace lining materials, we are committed to upgrading our products to better meet customer needs in response to the challenges faced by downstream enterprises in their upgrades and development. For high-quality submerged arc furnace carbon bricks and blocks, please contact Rongsheng. Get free carbon brick pricing!

Application of Silicon Nitride Bonded Silicon Carbide Bricks in the Ceramic Industry

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Silicon nitride combined with silicon carbide bricks not only have the characteristics of high density, high strength, good thermal shock stability, high load softening point, good thermal conductivity, high resistance value, etc., but also have excellent resistance to melting erosion and oxidation resistance of cryolite, aluminum fluoride, sodium fluoride and calcium fluoride. It is mainly used in building sanitary ceramics, daily porcelain, electric porcelain, aluminum, copper, zinc smelting, steelmaking and rolling, ironmaking, metal heat treatment, environmental protection, and other fields.

Silicon Nitride Bonded Silicon Carbide Brick

Silicon nitride bonded silicon carbide brick is a kind of advanced refractory material with silicon carbide sand as the main raw material and silicon nitride as the bonding phase. Since SiC and Si3N4 are covalently bonded compounds, sintering is difficult, and reaction sintering is usually used in industrial production. SiC is used as aggregate, metallic silicon powder is added, and a reasonable particle grading, mixing system and forming process are selected. The dried green body is fired in a nitrogen atmosphere in a dedicated firing equipment. During the firing process, the generated Si3N4 (matrix bonding phase) is gray-white crystals. It can effectively combine SiC particles (aggregates) to form a spatial network structure, so that the brick has incomparable characteristics of other refractory materials. Silicon nitride bonded silicon carbide material is a special silicon carbide brick used in grinding wheels, ceramics, electric porcelain and other industries.

Silicon Nitride Combined with Silicon Carbide Bricks
Silicon Nitride Combined with Silicon Carbide Bricks

Superior Performance of Silicon Nitride Combined with Silicon Carbide Bricks

The high-temperature flexural strength of silicon nitride combined with silicon carbide is 4 to 8 times that of ordinary refractory materials. The thermal expansion coefficient is half of that of high-aluminum refractory materials. The thermal conductivity is 7 to 8 times that of general refractory materials, and the strength increases with the increase of temperature. When the temperature rises to 1400℃, the strength begins to decrease. But when the temperature drops to 1500℃, the room temperature flexural strength index is still maintained. The thermal conductivity of this material gradually decreases with the increase of temperature. It has the characteristics of high density, high strength, good thermal shock stability, high load softening point, good thermal conductivity, and high resistance value. And it has excellent resistance to melting erosion and oxidation resistance such as cryolite, aluminum fluoride, sodium fluoride and calcium fluoride. Its performance is as follows:

  • 1) Silicon nitride combined with silicon carbide bricks are hard, with a Mohs hardness of about 9. It is a high-hardness material among non-metallic materials, and its hardness is second only to diamond.
  • 2) Silicon nitride combined with silicon carbide bricks have high strength at room temperature, and maintain almost the same strength and hardness as at room temperature at high temperatures of 1200-1400℃. Depending on the use atmosphere, the maximum safe use temperature can reach 1650-1750℃.
  • 3) The thermal expansion coefficient is small, and the thermal conductivity is high compared to silicon carbide bricks. It is not easy to produce thermal stress, has good thermal shock stability, and has a long service life. It has strong high-temperature creep resistance, corrosion resistance, extreme cold and heat resistance, oxidation resistance, and is easy to make bricks with high dimensional accuracy that meet the requirements.
Silicon Nitride bonded Silicon Carbide Bricks
Silicon Nitride Bonded Silicon Carbide Bricks

Application of Silicon Nitride Combined with Silicon Carbide Bricks in the Ceramic Industry

Building Sanitary Ceramics Industry

At present, in the firing of ceramic products, roller kilns are mostly used to fire sanitary ceramics and wall and floor tiles, and the rollers of the roller kilns are basically made of silicon nitride combined with silicon carbide. The application of silicon nitride combined with silicon carbide rollers solves the problem of high firing temperature and heavy load, effectively reduces product energy consumption and improves product quality.

Daily Ceramics Industry

There are two ways to fire daily ceramics: one is to use a shuttle kiln with a sagger or similar kiln; the other is to use a roller kiln in the same way as building sanitary ceramics. The use of kiln tools made of silicon nitride combined with silicon carbide can significantly reduce the mass ratio of kiln tools to bricks, save energy and reduce consumption, and at the same time improve the quality and qualified rate of products. Using a roller kiln to fire daily ceramics, the use of silicon nitride combined with silicon carbide rollers increases the service life of the kiln.

Electrical Porcelain and Electronic Ceramics Industry

The electrical porcelain and electronic ceramics industry requires kiln furniture materials with high temperature, high strength, good thermal stability, long service life and reasonable price, which provides a broad market for the application of silicon nitride combined with silicon carbide kiln furniture.

Silicon nitride combined with silicon carbide has high density, high strength, good thermal shock stability, high load softening point, good thermal conductivity and high resistance. It also has excellent resistance to melting erosion and oxidation resistance of cryolite, aluminum fluoride, sodium fluoride, and calcium fluoride. It plays an increasingly irreplaceable role in building sanitary ceramics, daily ceramics, electrical porcelain, non-ferrous smelting, steelmaking and rolling, ironmaking, heat treatment, environmental protection and other industries. To buy high-quality silicon carbide bricks, please choose Rongsheng. Contact Rongsheng for free samples and quotes.

Does the Size of Refractory Particles have an Effect on the Performance of Refractory Ramming Materials?

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The size of refractory raw material particles has an impact on the performance of refractory ramming materials, especially the tensile force and impact resistance of the materials.

Granular Materials of Refractory Ramming Materials

The particles of refractory ramming materials are generally of different particle gradations of 0-5mm, and the particles are also 0-7mm, and will not exceed 8mm particles. Because the particles are small, the ramming will be more compact.

However, there are also 0-3 particles of refractory ramming materials. This is because some parts with too small gaps have special particle requirements. Otherwise, the 0-3 particles have low tensile force and insufficient impact resistance. In many cases, the 0-3 particle grading will not be used for process proportioning. For example, the particles of the baking-free ramming material used in the blast furnace slag skimmer will be larger. There are also refractory castable manufacturers that put the particles into 10mm, so that the impact resistance is strong during use, and it is also more wear-resistant.

Ramming Material Used for the Lining of the Furnaces
Ramming Material Used for the Lining of the Furnaces

Reasonable Particle Grading and Reasonable Process

Refractory materials themselves belong to applied disciplines, with applicability as the ultimate goal. If there is no special requirement under special circumstances, the particle grading between 0-8mm should be used as the proportion. Too large ramming is not dense, too small tension is not good and the impact resistance is not enough.

Reasonable particle grading and reasonable process ratio will produce products suitable for use. Therefore, the production of refractory ramming materials should be based on the use of different furnace linings, and reasonable particle grading is the most scientific.

According to the current market usage, 0-7mm particle grading is a more suitable ratio, and it is also a way to use the ratio combined with the particle grading of the ramming material. Although larger particles are OK, it is most suitable not to exceed 10mm, which is easy to construct.

In summary, the size of the particles does affect the performance of refractory ramming materials. Therefore, it is also a matter of attention in production and use. Although indicators are important, only those suitable for use are the best.

Silica Ramming Refractory Material for Intermediate Frequency Furnace
Silica Ramming Refractory Material for Intermediate Frequency Furnace

Why is Boric Acid Used in Refractory Ramming Materials?

Different refractory materials are suitable for smelting different metals or alloys. At present, medium frequency induction furnaces are widely used in metallurgy and foundry industries. During the use of induction furnaces, the impact, friction and electromagnetic stirring of the charge will aggravate the erosion of the furnace lining. The factors that cause the reduction of service life are:

Quartz sand refractory ramming material is widely used in the production of molten cast iron and cast iron alloys because of its high cost performance and good thermal shock stability, mechanical strength and resistance to acidic slag corrosion.

However, there are many types of quartz ores, and their important performance indicators such as impurity content and crystallinity are different. As a result, the service life of quartz refractory materials in medium frequency furnaces varies greatly.

Theoretically, quartz with low impurity content and high crystallinity has high mechanical strength and excellent thermal shock stability, and is particularly suitable as a lining material for large-capacity medium-frequency furnaces. However, the sintering performance of this quartz is poor, which will affect the service life of the lining.

Quartz ramming material is made of high-quality large-grained quartz sand as the main raw material, supplemented by a binder and a sintering promoter. During the production process, an important binder needs to be added, and boric acid is used as a sintering binder. During the sintering process of the furnace lining, boric acid is dehydrated and converted into boric oxide. At high temperatures, boric oxide plays the role of a high-temperature binder and promotes the sintering of the workpiece.

In addition, during the high-temperature use of quartz ramming material, quartz will react with boric acid B2O3 to form tridymite and cristobalite. Boric oxide can reduce the synthesis and sintering temperature of quartz without causing obvious harm to the material properties. Therefore, boric acid is an important raw material for quartz ramming material.

We know from a set of experiments that boric acid plays an important role in quartz ramming material. The main raw material is quartz sand, whose critical particle size is 5mm. Different particle gradings are carried out according to the maximum stacking density, and boric acid is added as a sintering agent. Then the test block is made and the total weight loss of quartz is observed to be about 3.0 when the temperature rises from 25℃ to 1500℃ in the furnace. When the temperature rises from 25℃ to 1000℃, the weight loss of quartz is about 2.2, and this part of the loss mainly comes from the evaporation of free water and crystal water. Due to the evaporation of free water, an endothermic peak appears at 80℃. Due to the transformation of β-quartz to α-quartz, there is an endothermic peak at 580℃. Due to the transformation of α-tridymite to α-quartz, a large endothermic peak appears at 1250℃. There are also thermal effects of other crystal transformations, but they are not very obvious, indicating that its phase change is easier to occur.

The linear change rate of the ramming material after sintering increases with the increase of the content of the sintering agent boric acid. The change rate of the ramming material sintered at 1100℃ generally shows an increasing trend, and the linear change of the ramming material sintered at 1600℃ first increases and then decreases. It is worth noting that the linear change rate of the ramming material sintered at 1100℃ is negative. The reason is that the sintering agent boric acid begins to form a liquid phase at this temperature, causing the sample to shrink. The linear change rate of the ramming material sintered at 1600℃ is positive, and the sample expands. The reason is that quartz has a transition from a low-temperature phase to a high-temperature phase at high temperature, accompanied by volume expansion. This phase change is irreversible, and the volume expansion still exists after the temperature is reduced.

Effect of the content of sintering agent boric acid on the compressive strength of the ramming material. With the increase of the boric acid content, the compressive strength of the ramming material shows a significant increasing trend. It can be seen from the B2O3-SiO2 phase diagram that after adding boric acid, a liquid phase appears in the ramming material at about 440℃, thereby promoting sintering and improving the strength of the ramming material. As the temperature rises, the amount of liquid phase increases, and the strength of the ramming material is bound to increase. However, due to the phase change of quartz at high temperature, the volume expansion associated with the phase change will partially offset the sintering effect of boric acid, making the influence of boric acid content on the strength of ramming material sintered at 1600℃ smaller. Boric acid mainly improves the medium-temperature sintering strength of the ramming material, while the role of the sintering promoter during high-temperature sintering is relatively small, and at this time it still mainly depends on quartz.

Forming Process of Refractory Materials

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  1. Semi-dry pressing

During the semi-dry pressing process, the loose materials do not have enough moisture and must be subjected to greater pressure. With the help of pressure, the blank particles are redistributed. Under the action of mechanical bonding force, electrostatic attraction and friction, the blank particles are tightly combined, elastic deformation and brittle deformation occur, air is discharged, and the blank particles are combined into products with a certain size, shape and strength.

The degree of action of the above forces depends on the particle shape, the physical and chemical properties of the blank and the surface state of the particles. For particles with complex shapes, mechanical bonding force plays a major role. For particles with simple shapes, friction and static electricity cause the main effects.

Within a certain range when other process conditions are the same, the pressure increases during pressing, the porosity of the blank decreases, the density increases, and the strength increases accordingly.

The blanks and bricks of refractory materials are three-phase systems composed of solid matter, water (or other binders in other states) and air. During the entire pressing process, because the amount of phase and liquid phase does not change, the amount of air in the blank is compressed and reduced, and the volume of the compressed blank is also reduced accordingly.

  1. Slurry injection molding method

Use powdered raw materials, select appropriate degumming agents (deflocculating agents) to evenly suspend them in the solution, adjust them into mud, pour them into a water-absorbent mold (generally a gypsum mold) to absorb water, and form a green body according to the shape of the model. This method is called slurry injection molding.

For materials that do not react with water, water is generally used as a suspension, and the moisture content of the mud is as high as 35~45%. For some materials that are easy to react with water, such as CaO, MgO, etc., organic matter can be used as a suspension, such as anhydrous alcohol. This method is suitable for the production of thin-walled hollow products. Such as thermocouple sleeves, high-temperature furnace tubes and crucibles. Various oxide products, from small to large pieces, have adopted this method.

For thin-walled hollow products, hollow casting is generally used. Inject mud into the model, and after a certain period of time, the raw materials are adsorbed on the model wall. When the specified thickness is absorbed, the internal mud is discharged. Dry to an appropriate degree and then demold, the gypsum mold can be used repeatedly. The mud used should have good fluidity, and the density is generally between 1.65~1.80g/cm2.

For the casting of thick and large products, solid casting is suitable. The mud used should be large, and the density should generally be above 1.8g/cm3. The thick mud has thixotropy, so it should be strongly mechanically stirred before casting to make it fluid. After casting, it can be solidified by standing for a period of time. Therefore, the plaster mold used does not need to have high water absorption.

  1. Plastic molding method

The moisture content of the blank used in the plastic method is generally above 16%. The prefabricated blank is put into the mud extruder, extruded into mud strips, and then cut. Then the blank is made into a blank according to the required size, and the blank is pressed by a press to make the blank have the specified size and shape.

The moisture content of the blank is related to different raw materials and products. For plastic clay materials, the moisture content can be appropriately reduced to 10~15%. The critical pressure of the mud extruder is related to the moisture content of the blank.

The plastic molding method is mostly used to prepare large products. Depending on the molding operation, it can be formed by hand, semi-mechanical (such as clamping hammer) or machine pressing.

With the improvement of refractory process technology and the development of molding machinery and equipment, the application of plastic molding methods in the refractory industry has a tendency to decrease.

  1. Vibration molding

When the material vibrates at a frequency of about 3,000 times per minute, the particles of the blank collide with each other, and dynamic friction replaces the static friction between the particles, and the blank becomes a fluid particle. Due to the energy input by the vibration, the particles have the ability to move in three dimensions inside the blank, so that the particles can be densely packed and filled in every corner of the model and the air can be squeezed out. Therefore, even under very small pressure, a high-density product can be obtained. When molding a variety of products, vibration molding can effectively replace heavy high-pressure brick presses. For example, crank lever brick presses and hydraulic presses can mold complex special-shaped and giant bricks that require manual molding or ramming molding, greatly improving labor productivity and reducing labor intensity. Vibration molding is also suitable for molding materials with a large difference in density and molding fragile brittle materials. Since the material particles are not destroyed during molding, it is suitable for molding easily hydrated materials, such as tar dolomite, tar magnesia, etc.

  1. Hot Pressing

Refractory products made by sintering require a long calcination time, and the porosity is still as high as 10~25%. Ceramic materials and products prepared by sintering still have a true porosity of 3~5% even under ideal conditions. Non-oxide ceramic materials, such as carbides and nitrides, have even greater porosity.

It is difficult to make very dense products. This is because during the sintering process, the gas pressure in the pores increases, offsetting the role of the interface energy as a driving force. On the other hand, closed pores can only be filled by diffusion of substances inside the crystal, but internal diffusion is much slower than interface diffusion. If the sintering process is carried out to the final stage and the product reaches an ideal dense state, there are two methods:

  • One is to use vacuum sintering to avoid gas accumulation in the pores.
  • The other is to apply pressure during sintering to ensure sufficient driving force.

The latter is called hot pressing.

Compared with the common sintering method, the advantage of hot pressing is that it can obtain special products with very high density, and its density value can almost reach the theoretical value. Adjusting the hot pressing conditions can control the grain formation, and hot pressing at high temperature is conducive to the contact and diffusion between particles. Thus, the sintering temperature is reduced (compared with the common sintering method).

  1. Hot Pressing Molding (Hot Pressing Grouting)

Hot pressing is one of the grouting methods and is a relatively new method for producing ceramic products and special refractory materials.

Hot pressing generally uses an organic binder as a dispersion medium and silicate mineral powder as a dispersed phase. At a certain temperature (70~85℃), it is prepared into a slurry and then molded into a metal model. This method is suitable for the production of small products with complex shapes and special requirements. It is also suitable for the production of materials with low plasticity, such as the molding of high-aluminum materials. Its semi-finished products have high mechanical strength and can be turned and drilled by machine tools. The plaster model and drying process can be omitted, the equipment is simple, and mechanization is easy to achieve.

  1. Electric Melting Method

The refractory raw materials are melted in an electric arc furnace, and then the melt is poured into a refractory casting mold for casting. Because the fluidity of the fluid must be good, the general pouring temperature must be between 1900~2000℃. The casting generates a stable crystal phase during the solidification process and forms a fine crystalline structure. After pouring, the casting mouth should be removed, and the casting should be slowly cooled for several days to prevent cracks from appearing during cooling. The casting block is finally processed on the surface to become a product. The products formed by this method are mainly fused zirconium mullite bricks, chrome corundum bricks, and fused quartz bricks. It is mainly used to build the bottom of the glass tank kiln. Most of the raw materials for producing rebonded bricks are also produced by this method.

  1. Isostatic Pressing

Isostatic pressing is a new forming technology developed later. This method mainly applies the Pascal principle to pressurize the liquid and evenly transfer its pressure to the powder through the rubber film, so it has the following characteristics.

  1. Formability. The pressurization is non-directional, and a green body with uniform density can be obtained, and there will be no layer density phenomenon during other mechanical pressing. It is easy to press into products with complex shapes.
  2. Sinterability. Since the green body has uniform density and shrinkage during firing is non-directional, it will not cause stress due to density difference and cause firing cracks. Due to the high density, the firing temperature is relatively reduced.

The main equipment consists of a high-pressure container and a high-pressure pump. During molding, the powdered material is placed in a rubber film or a plastic film, placed in an ultra-high-pressure container with a thick steel wall, and a high-pressure liquid is injected by a high-pressure pump. The working pressure is generally above 343MPa. This method only formed small products at the beginning, and has now developed to press large products.

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