What Types of Special Kiln Furniture are Used to Support Ceramic Blanks?

Sintering vessels are specialized kiln furniture used to support the fired ceramic blanks or to hold powders, such as cathode materials, magnetic powders, and high-purity ceramic materials, which are calcined and synthesized before undergoing heat treatment in roller kilns, pusher kilns, or tunnel kilns. Depending on the firing process of the user, this kiln furniture will be subjected to different heating conditions, and the material of these products depends on the type of sintered body and the heat treatment process.

Saggers/Crucibles

Saggers are used to hold powders (lithium-ion battery cathode materials, magnetic powders, high-purity ceramic powders) for heat treatment in roller kilns, pusher kilns, and tunnel kilns. They are generally formed using extrusion, machine pressing, casting, and isostatic pressing processes, with the appropriate forming process selected based on the composition and structure of the product. Widely used materials include cordierite-mullite, corundum-mullite, silicon carbide, and graphite. Their application is most prevalent in the synthesis of lithium-ion cathode materials.

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Cordierite-mullite saggers are widely used in the field of lithium-ion battery cathode materials due to their excellent thermal shock resistance and economic efficiency.

Aluminum-silicon saggers generally have a short lifespan due to the strong alkalinity and low melting point of lithium carbonate/lithium hydroxide, which are highly corrosive to acidic refractory materials.

Corundum saggers are mainly used for calcining some high-purity powders in environments with less severe thermal shock conditions and high operating temperatures. For example, the calcination of high-purity alumina powder requires the sagger to be fired at 1800℃, using alumina active powder with an Al2O3 content of 99.9wt% and low-sodium white corundum. The binder uses low-ash content (Ash ≤ 0.01wt%), ensuring effective impurity control throughout the raw material process, and achieving a low coefficient of thermal expansion through sufficient high-temperature firing.

Graphite and silicon carbide saggers possess high thermal conductivity, high-temperature resistance, and excellent thermal shock resistance. While their oxidation resistance is poor, they exhibit excellent resistance to alkaline corrosion under reducing atmospheres. Graphite saggers are commonly used as containers for loading materials during high-temperature sintering in reducing atmospheres, applied in lithium iron phosphate sintering and electromagnetic material sintering. Traditional graphite saggers are produced through machining, which is inefficient and costly. Silicon carbide saggers are also widely used in pharmaceuticals, fine chemicals, engineering metallurgy, and pickling industries.

Firing Plates

Firing plates must withstand the thrust of movement and the friction of loading and unloading products during service, and must not crack under thermal cycling. While meeting the thermal shock resistance requirements, improving the bending and crack resistance of the firing plate is crucial. The firing plate material must have excellent chemical inertness and not react with the products it supports. Firing plate materials include alumina, zirconium oxide, and composite materials, mainly used in electronic ceramics and special ceramics.

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Corundum firing plates refer to high-end kiln furniture with α-Al₂O₃ as the main crystalline phase. They possess excellent properties such as high strength, corrosion resistance, high temperature resistance, and wear resistance. They exhibit minimal deformation at high temperatures (>1650℃), but have high sintering temperatures and poor thermal shock stability. During the firing process of lead zirconate titanate piezoelectric ceramics, corundum firing plates face phenomena such as central warping, surface layered powdering, and peeling.

Zirconia is a corrosion-resistant, high-temperature refractory oxide. Zirconia sintering plates are frequently used as sintering pads in dielectric ceramics, powder metallurgy, chip capacitors, and ferrite magnetic materials to prevent parts from sticking together during sintering and to prevent the loss of electromagnetic properties of electronic components.

The firing temperature of conductive ceramics and device ceramics is generally between 1400 and 1650℃, and they are mostly corundum-mullite, corundum, or composite materials.

Barium titanate ceramics are a star product in the electronic ceramics industry. Sintering is a crucial step in its preparation process, determining the densification and microstructure of barium titanate; therefore, the quality of the sintering plate is particularly important. Because barium titanate has a low melting point (1625℃), high density, and is alkaline, traditional aluminosilicate kiln furniture easily reacts with it, leading to product contamination. Foreign kiln furniture manufacturers have applied plasma spraying technology to the sintering plate preparation process. The firing plate features a corundum-mullite interlayer and a zirconium oxide cladding coating, offering high thermal shock resistance and preventing reaction or adhesion with the fired chip-type multilayer ceramic capacitors.

With industrial development, the variety of products fired has increased, expanding the application areas of kiln furniture beyond ceramics to include fine chemicals and lithium-ion batteries. The performance requirements for kiln furniture materials, considering the characteristics of the fired products, are no longer limited to mechanical strength and thermal shock stability. Some applications also require excellent corrosion resistance and non-contamination of the fired products. Different corrosive media necessitate different requirements for kiln furniture materials. For example, in the production of lithium-ion battery cathode materials, kiln furniture materials must possess excellent thermal shock stability and resistance to alkaline lithium compounds. In the production of piezoelectric ceramics, kiln furniture materials must resist lead and its compounds.

Rongsheng High-Performance Corundum-Mullite Kiln Furniture

With the rapid development of science and technology and the economy, magnetic materials, functional ceramics, and electronic ceramics have been widely used in information, electronics, machinery, and chemical industries. This has also driven the rapid growth in demand for high-performance corundum-mullite kiln furniture materials.

Kiln furniture is a special type of refractory material, primarily serving to support and protect fired products. Product forms mainly include pushers, firing supports, and saggers. During service, kiln furniture is subjected to high temperatures, compression, friction, and thermal cycling, and its damage often manifests as fracture or deformation. Therefore, the key performance requirements for kiln furniture materials lie primarily in their high-temperature mechanical properties, which determine their performance, such as high-temperature flexural strength, high-temperature creep resistance, and thermal shock resistance. Improving these performance characteristics involves the design of the material’s microstructure and its resistance to damage under complex stress conditions under high-temperature loads. This requires in-depth research and development in areas such as raw material selection, matrix material treatment, process control, and thermal stress distribution.

Through rational selection of raw materials, optimized microstructure design, scientific product firing process, and stress distribution calculation, high-performance corundum-mullite kiln furniture has been developed. It can be widely used in the sintering of structural ceramics, electronic ceramics, and powder metallurgy parts.

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Technical Advantages of High-Performance Corundum-Mullite Kiln Furniture

1. Operating temperature up to 1750℃.
2. Strong creep resistance.
3. Meets the requirements of environments with strong thermal shock.

Applications of Corundum-Mullite Kiln Furniture

Corundum-mullite kiln furniture is an essential consumable refractory material in the ceramics industry, characterized by high demand and wide application. It is mainly used in the sintering of high-tech ceramics and powder metallurgy parts, such as electronic ceramics (MLCCs, varistors, magnetic materials, filters), Al2O3 structural ceramics (substrates, ceramic films, spark plugs, grinding media), ZrO2 structural ceramics (zirconia knives, mobile phone backplates, bearings, fiber optic ferrules), stainless steel, titanium alloys, etc.

High-performance corundum-mullite kiln furniture is used in pusher kilns to fire zirconia knives. High-performance corundum-mullite kiln furniture is used in pusher kilns to fire alumina ceramics.

The Influence of Binder on the Properties of Silicon Carbide Mortar

Silicon carbide fire mortar is used for the construction or bonding of silicon carbide bricks in blast furnaces, silicon carbide bricks in blast furnace cooling walls, silicon carbide bricks in aluminum electrolysis cells, and silicon carbide bricks in ceramic kilns. Currently, silicon carbide fire mortar mainly uses liquid phenolic resin as a binder, which is mixed evenly in a certain amount before construction. The overall structure after construction must have good structural stability and airtightness, and be able to withstand various physicochemical reactions at high temperatures, so that the equipment can operate safely and stably. During the production, transportation, and use of fire putty, pure phenolic resin is brittle, highly toxic, has a high curing temperature, slow speed, and is prone to cracking and has low wear resistance after curing. This results in low production efficiency, high energy and equipment consumption, and the need to dissolve and dilute it with flammable and explosive materials during the mixing process, which seriously pollutes the environment and causes adverse effects. To solve these problems, three binders were used in the experiment, and comparative tests were conducted on three aspects: cone penetration, bonding time, and bonding strength.

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Raw Materials and Proportions of Silicon Carbide Mortar Used in the Experiment

The main raw materials used in the experiment included: silicon carbide particles (0.5–0 mm) and fine powder (≤0.074 mm) with a w(SiC) content of 98.3%; clay fine powder with a w(Al₂O₃) content of 34.2% and a w(SiO₂) content of 48.3%; alumina sol solid fine powder with a w(Al₂O₃) content of 32.3%; Secar 71 cement fine powder; solid water glass fine powder with a modulus of 2.8; and additives.

The aggregate to fine powder was prepared according to a mass ratio of 40:60, and three different binders were added. The samples were numbered A, B, and C, where A represents alumina sol solid, B represents Secar 71 cement, and C represents solid water glass. The numbers correspond to the amount of binder added.

Sample Preparation and Performance Testing

According to the sample composition formula, the prepared materials were poured into a small mixer and dry-mixed for 1 minute. Then, 10% (w) water was added and mixed for 3 minutes, then an appropriate amount of water was added and mixed for another 3 minutes to form a slurry. The cone penetration, bonding time, and flexural bonding strength of each formulation sample were determined according to GB/T 22459—2008 (drying at 110℃ for 24 hours and charring at 1300℃ for 3 hours).

Performance of Silicon Carbide Firemortar with Different Binders

• (1) Silicon carbide firemortar using alumina sol as a binder exhibited the best cone penetration after 1 hour of standing. Silicon carbide firemortar using solid water glass as a binder showed the best cone penetration after 0.5 hours. However, silicon carbide firemortar using Secar 71 cement as a binder rarely achieved ideal cone penetration.
• (2) In silicon carbide firemortar, when using alumina sol and solid water glass as binders, the bonding time of samples with binder additions between 4% and 10% (w) was within the national standard requirements. However, when using Secar 71 cement as a binder, the bonding time requirement was only met when the binder addition was 2% (w).
• (3) Silicon carbide firemortar exhibited the best flexural bond strength after baking and heat treatment when the alumina sol solid addition was 8% (w). The flexural and bond strength properties were best when the solid water glass content was 6% (w) after drying and heat treatment. However, the strength of the Secar 71 cement sample after drying was too low, while the strength after heat treatment increased with the increase in content.

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What type of refractory mortar is used for laying corundum-silicon carbide composite bricks?

When laying corundum-silicon carbide composite bricks, it is essential to use refractory mortar of the same material, namely corundum-silicon carbide refractory mortar. This is because refractory mortars of the same material have identical performance characteristics and can resist the same furnace atmosphere under the same high temperatures or erosion conditions. There are various types of corundum-silicon carbide composite bricks, including precast bricks and sintered corundum-silicon carbide bricks.

Precast bricks do not require refractory mortar. This is because precast bricks are generally produced in small quantities and are cast according to specific brick shapes for specific locations, thus requiring no refractory mortar for laying.

Sintered corundum-silicon carbide composite bricks, however, are sintered at high temperatures and are generally used in areas of high furnace lining temperature and severe erosion and wear. The required quantity is larger than for precast bricks. Therefore, refractory mortar of the same material must be used for laying them, with the refractory mortar acting as a bonding material to seal the gaps between the bricks.

What phenomena occur with refractory mortar under high temperatures?

When preparing corundum-silicon carbide refractory mortar, the amount of silicon carbide added to the corundum-silicon carbide composite bricks must be considered. Generally, the proportion of silicon carbide added to the mortar should match the proportion added to the refractory bricks. This ensures that the performance is essentially the same for the same material. If too much silicon carbide is added, the bonding during construction will be poor, making construction impossible. Forcibly adding a binder to adjust the amount will affect the later strength. If an unbalanced proportion of silicon carbide is not used, or if refractory mortar without silicon carbide is used, the mortar will crack and fall off at high temperatures. Falling mortar will cause the brick layer to settle, leading to furnace shutdowns for maintenance and production delays.

Therefore, corundum-silicon carbide composite bricks must be laid with corundum-silicon carbide refractory mortar. This ensures that the material, atmosphere, operating temperature, and resistance to erosion and abrasion are identical.

Alumina-Silicate Refractory Materials for Rotary Kilns Used in Calcining Magnesia Materials

A certain company utilizes a rotary kiln to fire magnesia-based materials with an MgO content of approximately 30%. The kiln’s firing zone spans a length of 20 to 30 meters, with a firing temperature of around 1470°C and a shell surface temperature ranging from 300°C to 310°C. When using high-alumina bricks with a 65–75% alumina content, the service life of the rotary kiln lining is less than 20 days; with 80% high-alumina bricks, the service life extends to only about 30 days. The severe erosion of refractory materials necessitates frequent kiln maintenance, which significantly compromises both product quality and production efficiency. Consequently, the company is seeking non-magnesia refractory materials suitable for rotary kilns, aiming to ensure a service life of at least six months.

Refractory Materials for Rotary Kilns Used in Calcining Magnesian Materials

Application of the Original 80% High-Alumina Bricks in the Rotary Kiln

The characteristics of the 80% high-alumina bricks after 30 days of service were observed; from the surface of the used bricks, the primary manifestations were severe penetration and corrosion. An analysis was conducted on the physicochemical properties of the high-alumina bricks originally used in this rotary kiln. A static crucible method was employed to perform corrosion resistance tests on the 80% high-alumina bricks. Magnesian material was packed into the test specimens, and the temperature was raised at a specific heating rate to 1500°C. After holding at this temperature for 3 hours, the specimens were allowed to cool naturally within the furnace to room temperature, after which they were cut open along their central axis.

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Deterioration of the Original 80% High-Alumina Bricks in the Rotary Kiln

Analysis of the Deterioration Characteristics of the High-Alumina Bricks

Based on the physicochemical properties of the high-alumina bricks, it is evident that the material possesses high porosity, and its initial load-softening temperature is only 1418°C—significantly lower than the operating temperature of 1450–1470°C within the kiln’s firing zone. Consequently, during operation, the refractory material undergoes softening and deformation, leading to accelerated corrosion. Furthermore, an analysis of the bricks’ reheat linear change rate reveals that these 80% high-alumina bricks exhibit a tendency to shrink at high temperatures; this characteristic makes them highly susceptible to the widening of brick joints. Such widening compromises the structural integrity of the kiln shell and accelerates the deterioration of the refractory lining.

Corroborated by the characterization results of the corrosion resistance tests, the surfaces of the specimens appeared loose and porous following the reaction, exhibiting distinct signs of material penetration. This indicates that under high-temperature conditions, a liquid phase continuously forms and permeates through the brick joints, thereby expanding the surface area exposed to corrosion. This process accelerates the rate of deterioration, ultimately leading to a loss of structural integrity in the rotary kiln and a shortened service life.

Analysis of the Deterioration Mechanism of the High-Alumina Bricks

(1) Chemical Corrosion: This rotary kiln is utilized for the production of magnesian materials, which are primarily composed of MgO and SiO₂. Consequently, chemical reactions occur within a binary system dominated by MgO and SiO₂.

During the operation of the kiln, when the raw materials—predominantly MgO and SiO₂—come into contact with the Al-rich refractory lining (i.e., when Al₂O₃ is introduced into the MgO-SiO₂ system), a ternary MgO-SiO₂-Al₂O₃ system is formed. When 2MgO·SiO2 comes into contact with Al2O3, cordierite (2MgO·2Al2O3·5SiO2) begins to form at temperatures exceeding 1250°C; this newly formed cordierite subsequently decomposes and melts at 1460°C. Furthermore, within this ternary system, Al2O3 reacts not only with the MgO component to form MgAl2O4 (spinel) but also with the SiO2 component to form Al6SiO13 (mullite).

The formation of spinel is accompanied by a volume expansion of 5% to 8%. When combined with the liquid-phase melting induced by interfacial reactions, this process leads to the deterioration and damage of the refractory materials, ultimately compromising the overall service life of the kiln.

(2) Mechanical Stress. Rotary kilns are typically inclined at a gradient of 3% to 5%. During the production process, as the kiln rotates at a specific speed, the internal refractory lining is subjected to various forces: it experiences inertial forces that induce a tendency toward downward movement, while simultaneously enduring compressive stresses from the kiln shell as well as erosive wear caused by the flow of materials—all of which contribute to the degradation and failure of the refractory lining.

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Required Properties for Refractory Materials in Rotary Kilns

Based on the composition of the magnesia-based raw materials being calcined, the operating environment, and the firing temperature—and informed by an analysis of the failure mechanisms observed in high-alumina bricks previously used in such rotary kilns—the refractory materials intended for use in magnesia-calcining rotary kilns must possess specific properties. These include high density, high mechanical strength, and a high refractoriness under load (softening point). Furthermore, they must exhibit excellent resistance to chemical corrosion by the magnesia-based raw materials being processed, as well as favorable kiln lining-forming (coating) characteristics.

Material Selection

In response to client feedback regarding the poor spalling resistance and high cost of traditional magnesia-based refractories—which were deemed unsuitable for the current operational requirements of this specific rotary kiln—this study focused on selecting alternative refractory materials primarily based on the alumina-silica system. Specifically, samples of the following material types were selected for evaluation: mullite-andalusite bricks, high-alumina silicon carbide bricks, special high-alumina bricks, and corundum-spinel bricks. Subsequently, a static crucible test method was employed to analyze and compare their respective corrosion resistance capabilities. In this procedure, the magnesia-based raw material was packed into a cavity within each refractory sample. The samples were then heated at a controlled rate to a peak temperature of 1500°C, held at this temperature for 3 hours, and finally allowed to cool naturally within the furnace back to room temperature. Each sample was then sectioned along its central axis to facilitate a detailed examination of the corrosion resistance exhibited by the different refractory materials against the magnesia-based raw material.

Corrosion Resistance Results and Discussion

Based on the experimental data, the ranking of the materials in terms of corrosion resistance (from highest to lowest) is as follows: Sample #4 > Sample #2 > Sample #3 > Sample #1 > 80% High-Alumina Brick. The ranking in terms of penetration resistance (from highest to lowest) is: Sample #2 > Sample #1 > Sample #3 > 80% High-Alumina Brick > Sample #4.

• (1) Sample #1 consists of a mullite-andalusite composition. At high temperatures, the magnesia-based raw material reacted with the brick, resulting in the formation of a white, flocculent reaction layer on the brick’s surface. This material features a relatively high SiO₂ content; consequently, within the temperature range of 1200°C to 1300°C, an accelerated reaction occurs between the SiO₂, Al₂O₃, and MgO components, leading to the formation of cordierite. As the temperature continues to rise, the newly formed cordierite undergoes decomposition and melting at approximately 1450°C. This process releases a significant amount of supercooled liquid phase, which adheres to the surface of the raw material being processed, thereby causing corrosion of the refractory lining. While the overall corrosion resistance of this material is considered moderate, its high density endows it with excellent resistance to material penetration.
• (2) Sample #2 consists of high-alumina silicon carbide bricks. Following the incorporation of silicon carbide, the SiC on the surface oxidizes to form a dense, protective film of SiO2, which effectively prevents further erosion and infiltration of the brick by the magnesia-based materials. Consequently, this material exhibits superior resistance to both erosion and infiltration.
• (3) Sample #3 consists of special high-alumina bricks. The material surface appears white, and signs of erosion and infiltration are minimal; internally, this high-alumina brick develops a composite corundum-mullite phase structure. The mullite phase enhances the compactness of the structural network, thereby improving infiltration resistance. The corundum phase serves to resist erosion; specifically, when the refractory surface reacts with the magnesia-based materials at high temperatures, a molten phase is formed. Upon subsequent crystallization, this material adheres to the refractory surface, creating a structural layer resembling a kiln coating, which effectively prevents further erosive reactions of the refractory. Overall, this material is rated as having good resistance to both erosion and infiltration, while also offering the lowest cost.
• (4) Sample #4 consists of corundum-spinel bricks. While they demonstrate good resistance to erosion, their resistance to infiltration is poor, and their cost is high.

Based on a comprehensive evaluation of the erosion resistance and cost of the four materials, the high-alumina silicon carbide bricks and the special high-alumina bricks were identified as the preferred choices. However, when analyzed from the perspectives of thermal conductivity and cost-effectiveness, the high-alumina silicon carbide bricks possess both higher thermal conductivity and a higher price point than the special high-alumina bricks; therefore, the special high-alumina bricks were ultimately selected. The reaction characteristics observed between the molten magnesia-based materials and the special high-alumina bricks indicate that the latter possesses excellent capability for forming a protective kiln coating.

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Performance Optimization of Special High-Alumina Bricks

To better meet the operational demands of rotary kilns used for calcining magnesia-based materials, the specifications and manufacturing processes of special high-alumina bricks have undergone optimization. By optimizing the matrix A/S (Alumina-to-Silica) ratio, a composite corundum-mullite phase structure was generated; the mullite phase enhances the compactness of the structural network, thereby improving resistance to penetration, while the corundum phase serves to resist erosion. The newly optimized special high-alumina bricks exhibit excellent physicochemical properties, along with superior resistance to both erosion and penetration.

Static crucible tests were conducted to analyze the erosion resistance of the optimized special high-alumina bricks, demonstrating their excellent capability to withstand erosion.

Product Application Results

• (1) Monitoring of the special high-alumina bricks in actual customer operations revealed that the rotary kiln has operated continuously for 900 days, representing a significant extension of service life. During this period, no kiln shutdowns for maintenance were necessitated by issues such as erosion, spalling, or brick detachment. Production efficiency and product quality were successfully safeguarded, earning the product high acclaim from the client.
• (2) These special high-alumina bricks possess moderate thermal conductivity and excellent capability for forming a protective kiln coating (clinker coating). The surface temperature of the kiln shell in the firing zone remained at approximately 240°C—a reduction of about 60°C compared to previous operations. On one hand, this prevents plastic deformation of the kiln shell caused by excessive heat, thereby reducing safety risks associated with kiln operation. On the other hand, the reduction in heat loss from the kiln shell surface not only achieves the objective of energy conservation and cost reduction but also improves the working environment at the production site.
• (3) A microscopic analysis was performed on the special high-alumina bricks after 900 days of service. Observations of the reaction layer—magnified 56 times—combined with EDS (Energy-dispersive X-ray spectroscopy) scanning analysis, revealed that the magnesia-based charge material (predominantly composed of Mg and Si at a macroscopic level) had not induced any significant penetration reactions within the special high-alumina bricks.

Further high-magnification analysis of the reaction and erosion layers of the used bricks revealed an enrichment of K and Na elements on the surface of the refractory’s reaction layer. This enrichment resulted in the formation of a protective liquid-phase layer, which effectively prevented further erosion of the refractory material. When the reaction layer was magnified 300 times, the dynamic reaction processes occurring between the refractory material and the magnesia-based raw materials during kiln operation could be clearly observed. Measurements indicated that the thickness of the formed protective liquid-phase layer ranged from approximately 150 to 300 μm. The interior of the refractory material has not suffered erosion or damage from the magnesia-based raw materials; based on an assessment of the current performance of this specialized high-alumina brick, its service life is projected to exceed five years.