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.

Fused-Cast Z80 Bricks for Glass Furnaces

Properties of Fused-Cast Z80 Bricks. Linear Thermal Expansion Rate: The linear thermal expansion rate is one of the key parameters for evaluating the performance of refractory materials. It reflects the dimensional changes a material undergoes in response to temperature fluctuations. This parameter is crucial for the selection of furnace body materials in fused-cast furnaces, as it directly impacts the structural stability and crack resistance of the furnace.

A comparison of the linear thermal expansion rates of Fused-Cast Z80 bricks and Fused-Cast AZS41# bricks reveals that the rates for both materials are quite similar at 1400°C. However, the variation in the linear thermal expansion rate of Fused-Cast Z80 bricks across different temperature ranges is significantly smaller. Consequently, Fused-Cast Z80 bricks present a lower risk of cracking during heating cycles or rapid temperature changes (thermal shock). By selecting Fused-Cast Z80 bricks as the refractory material, it is possible to more effectively mitigate structural damage to glass furnaces caused by temperature fluctuations, thereby enhancing the safety and reliability of fused-cast furnaces throughout their operational cycles.

Bubble Generation Rate. The bubble generation rate of Fused-Cast Z80 bricks stands at 0% at 1300°C and 0.1% at 1500°C—levels considered extremely low. Fused-Cast Z80 bricks exhibit the lowest bubble generation rate among comparable materials, demonstrating excellent efficacy in suppressing bubble defects in glass products caused by the refractory lining. This superior performance is attributed to the uniform elemental composition and density distribution of Fused-Cast Z80 bricks; as they undergo no changes in their phase structure, they are able to consistently maintain an exceptionally low bubble generation rate.

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Physicochemical Properties of Fused-Cast Z80 Bricks

As a refractory material utilized in glass melting furnaces, the physicochemical properties of fused-cast Z80 bricks are of paramount importance. A comparative analysis against fused-cast AZS33#, AZS41#, and fused-cast 95% high-zirconia bricks reveals that the Z80 variety is characterized by a low impurity content, as well as high uniformity in apparent porosity and density. During service, the material exhibits no formation of glass phases, bubbles, or needle-like defects; furthermore, it possesses a relatively low coefficient of thermal expansion. Consequently, it demonstrates excellent resistance to corrosion, erosion, and thermal shock. Its comprehensive performance significantly surpasses that of fused-cast AZS33# and AZS41# bricks, placing it on par with fused-cast 95% high-zirconia bricks. However, given that its cost is lower than that of fused-cast 95% high-zirconia bricks, it offers an optimal balance between extending the service life of glass furnaces and managing investment costs.

Microstructure of Fused-Cast Z80 Bricks

The grain size of fused-cast Z80 bricks is significantly smaller than that of fused-cast 95% high-zirconia bricks—a characteristic that exerts multifaceted influences on the material’s overall performance.

First, a smaller grain size results in a higher density of grain boundaries. As key pathways for diffusion, grain boundaries effectively impede ion migration within high-temperature environments, thereby enhancing the material’s corrosion resistance and chemical stability. During the vitrification process of fly ash, fused-cast Z80 bricks demonstrate superior corrosion resistance, thereby minimizing material loss.

Second, a fine and uniformly distributed grain structure contributes to improved overall strength and toughness. Compared to materials with coarse grains, fine-grained materials exhibit reduced internal stress concentration and possess more tortuous crack propagation paths. Consequently, fused-cast Z80 bricks offer superior thermal shock stability and mechanical strength, resulting in a lower risk of fracture. In summary, the fine-grained microstructure of fused-cast Z80 bricks makes them an ideal choice for the refractory lining configurations of fly ash vitrification furnaces.

Fused-cast Z80 bricks possess exceptional resistance to thermal shock and exhibit favorable thermal penetration characteristics. During the furnace heat-up and commissioning process, fused-cast Z80 bricks remain free from cracking caused by thermal shock or linear thermal expansion variations; moreover, their efficient thermal penetration minimizes thermal shock-induced damage to the adjacent outer refractory layers.

Performance of Electrofused Z80/AZS Composite Bricks

Compared to electrofused AZS bricks, electrofused Z80 bricks exhibit superior resistance to corrosion and erosion, as well as excellent reheat performance; however, they come at a higher cost. Conversely, electrofused AZS bricks possess commendable mechanical strength and thermal stability. To extend furnace service life and reduce refractory material costs, a composite brick—comprising electrofused Z80 as the working face material and electrofused AZS as the backing support—has been developed. To ensure a robust bond between the two materials, a method combining physical structural design with the use of a high-temperature binder was adopted. During fabrication, mechanical interlocking was achieved through the precise control of interfacial roughness and geometry, while a high-temperature binder with a zirconia (ZrO₂) content of 90% by mass was selected to fill and reinforce the interface. Upon drying and curing at 200°C, this binder forms a strong bonding layer that enhances the structural integrity of the assembly and improves the composite material’s resistance to thermal shock and chemical attack.

In practical applications, the electrofused Z80/AZS composite brick has demonstrated excellent performance when compared against the electrofused AZS41# bricks and fused-cast 95# high-zirconia bricks produced by a renowned domestic manufacturer. The electrofused Z80/AZS composite brick offers several significant advantages in real-world usage:

• (1) It resolves the inherent challenge faced by single-material refractories—namely, the difficulty of simultaneously balancing manufacturing costs with performance requirements.
• (2) Through rational material pairing and optimized design, it enhances the cost-effectiveness of the refractory lining configuration, enabling users to achieve a longer furnace service life with a lower initial investment.
• (3) It reduces the frequency of maintenance and material replacements, thereby minimizing furnace downtime and generating substantial economic benefits for enterprises.

Requirements for Binders for Al2O3-SiC-C Bricks (Alumina Silicon Carbide Carbon Bricks)

Binders play a pivotal role in the production of Al₂O₃-SiC-C bricks (alumina-silicon carbide-carbon bricks), significantly influencing the mixing and forming properties of the green mix, as well as the microstructure of the final product. The primary requirements for binders used in Al₂O₃-SiC-C bricks are as follows:

• (1) They must exhibit excellent wettability with the Al₂O₃-based refractory aggregates and matrix materials.
• (2) They should contain no—or minimal—components that are harmful to human health.
• (3) The properties of the mixed batch should remain relatively stable over time, and the extent of chemical reaction with the aggregates should be minimal.
• (4) During the heating process of the product, the binder should maintain a high residual carbon yield; furthermore, the polymer structure formed after carbonization must possess excellent high-temperature strength.

Good wettability between the binder and the refractory aggregates and graphite—combined with appropriate viscosity—can significantly enhance the bulk density and mechanical strength of the final product. Moreover, excellent wettability with graphite facilitates the uniform dispersion of graphite particles throughout the product, ideally forming a continuous network structure. Upon carbonization, this network evolves into a continuous carbon-bonded skeleton, thereby substantially improving both the mechanical strength and high-temperature slag resistance of the product. Consequently, the selection of an appropriate binder is of paramount importance. In the manufacture of Al₂O₃-SiC-C bricks, phenolic resins and aluminum dihydrogen phosphate are frequently selected as the binders.

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Phenolic Resins

Phenolic resins are synthesized through a polycondensation reaction involving phenolic compounds (such as cresol, phenol, xylenol, and resorcinol) and aldehyde compounds (such as formaldehyde and furfural), catalyzed by acids or bases. In the refractory industry, phenolic resins are widely utilized in the production of carbon-containing refractories due to their high carbon yield, excellent bonding strength—which enhances product mechanical strength—low emission of harmful volatile organic compounds, and high thermal stability. Based on their thermal behavior and structural morphology, they are primarily classified into two categories: thermosetting phenolic resins and thermoplastic phenolic resins.

In the fabrication of Al2O3-SiC-C bricks, thermosetting phenolic resins are typically selected as the binder; they are generally added at a concentration of approximately 5%, resulting in refractory products with relatively low porosity after molding. When subjected to heat in a neutral or reducing atmosphere, phenolic resins undergo thermal decomposition, generating gaseous products such as CO2, CO, CH4, H2, and H2O.

Within the temperature range of 200°C to 1000°C, phenolic resins continuously decompose to generate gases. As these gases volatilize, they create open pores, thereby increasing the apparent porosity of the refractory material and negatively impacting the mechanical strength, oxidation resistance, and slag resistance of carbon-containing refractory products. Consequently, to ensure the optimal performance of refractory products, two key measures must be taken: firstly, selecting a resin binder characterized by low gas evolution and a high carbon yield; and secondly, determining and utilizing an appropriate addition level for the resin.

Aluminum Dihydrogen Phosphate

Aluminum phosphate is widely utilized in the preparation of refractory materials. One of its primary advantages as a binder for refractories is that the resulting bonded structure exhibits excellent properties at intermediate temperatures. Aluminum phosphate is typically synthesized through the reaction of aluminum hydroxide with phosphoric acid. Depending on the degree of neutralization during this reaction, three distinct products can be formed: aluminum dihydrogen phosphate [Al(H₂PO₄)₃], aluminum hydrogen phosphate [Al₂(HPO₄)₃], and aluminum orthophosphate [AlPO₄]. For refractory applications, aluminum dihydrogen phosphate is the preferred choice of binder; its primary mechanism involves the polymerization of reaction products upon heating to a specific temperature, thereby enhancing the intermediate-temperature performance of the refractory product. At room temperature, aluminum dihydrogen phosphate is water-soluble; however, when heated to a certain temperature, it decomposes into aluminum pyrophosphate and aluminum metaphosphate, simultaneously undergoing polymerization reactions.

The formation and polymerization of aluminum metaphosphate [Al(PO₃)₃]n generate strong adhesive forces, thereby imparting strength to the refractory material at intermediate temperatures. As the temperature continues to rise, the aluminum metaphosphate decomposes to yield AlPO₄ and P₂O₅. The P₂O₅ can further react with the Al₂O₃ present in the refractory material to form additional AlPO₄, thereby further enhancing the intermediate-temperature strength of the refractory product.

Below 500°C, the aluminum dihydrogen phosphate primarily undergoes a dehydration process as the temperature increases. As the free water within the refractory product is expelled, the product undergoes shrinkage. During this dehydration phase—prior to reaching 500°C—the overall volume of the product remains relatively stable; however, the porosity of the binder phase increases, while its bulk density decreases. At this stage—due to the gradual precipitation of AlPO₄ and the subsequent formation and polymerization of aluminum pyrophosphate and aluminum metaphosphate—the density of the bonded structure may decrease slightly, yet its mechanical strength increases significantly.

When the temperature exceeds 500°C, the dehydration process within the bonded structure diminishes, and the product’s weight loss becomes negligible. Prior to the onset of high-temperature ceramic bonding within the material, there are no significant changes observed in the product’s porosity or density.

For refractory specimens utilizing aluminum dihydrogen phosphate as a binder, the cold-state strength begins to decline once the temperature surpasses a turning point of approximately 500°C; the strength does not begin to rise again until high-temperature ceramic bonding is established within the interior of the specimen. In contrast to its cold strength, the hot strength of the specimen increased continuously, reaching a maximum value at a temperature of 900°C. This increase in hot strength with rising temperature is likely attributable to the formation of compounds such as AlPO4 and Al(PO3)3 during the heating process; furthermore, the thermal expansion of the material fills the pores, resulting in a denser structure.

Between 900°C and 1000°C, the material’s hot strength declined significantly. This decline may be attributed to two factors: on one hand, the P2O5—generated from the decomposition of aluminum phosphate and aluminum pyrophosphate within the product—begins to volatilize; on the other hand, it may be related to the crystallographic phase transformation of AlPO4. As the temperature rises, AlPO4 undergoes a series of phase transformations: the low-temperature quartz form (low-temperature berlinite) transforms into the high-temperature quartz form (high-temperature berlinite) at 586°C. The high-temperature berlinite form can further transform into the tridymite form at 815°C, and the tridymite form transforms into the cristobalite form at 1025°C. During these phase transformations, the various crystalline forms of AlPO4 undergo volume expansion or contraction; this disrupts the product’s structural integrity, induces cracking, and consequently reduces the product’s bonding strength. When the temperature exceeds 1000°C, aluminum dihydrogen phosphate completely decomposes to yield AlPO4 and P2O5 (note that AlPO4 itself does not undergo decomposition to form M2O3 and P2O5 until reaching 1760°C); once the P2O5 has volatilized, only AlPO4 remains. Calculations based on solid-solution formulas indicate that, at high temperatures, AlPO4 can react vigorously with SiO2 to produce mullite and P2O5.