The core characteristics of spinel bricks are determined by their raw materials. Based on the raw material composition, they are mainly classified into three categories: magnesia-alumina spinel bricks, magnesia-chrome spinel bricks, and magnesia hercynite bricks. In addition, there are a small number of spinel bricks with special compositions, such as magnesia-titanium spinel bricks and zinc-alumina spinel bricks.
Different types of spinel bricks exhibit significant differences in raw material composition, crystal structure, and performance characteristics, which in turn determine their suitability for different high-temperature industrial scenarios.
Classification of Spinel Bricks by Raw Materials and Their Core Differences
The matrix of spinel bricks is mainly composed of high-purity magnesia raw materials (such as fused magnesia and sintered magnesia). The spinel phase is formed by the reaction of different metal oxides (e.g., Al2O3, Cr2O3, Fe2O3) with MgO at high temperatures. The content of oxides that form the spinel phase in the raw materials directly determines the category and performance of the bricks.
| Classification | Core Raw Material Composition | Key Spinel Phase | Critical Raw Material Requirements |
| Magnesia-Alumina Spinel Bricks | – Fused magnesia (MgO ≥ 97%) – Sintered magnesia (MgO ≥ 95%) – Industrial alumina (Al₂O₃ ≥ 98%) – Calcined bauxite (Al₂O₃ ≥ 85%) | MgAl₂O₄ Magnesia-Alumina Spinel | – The Al₂O₃ content must be controlled (usually 5%-25%) to avoid reducing the thermal shock resistance of the bricks. – High purity of magnesia is required to minimize impurities such as SiO₂ and CaO. |
| Magnesia-Chrome Spinel Bricks | – Fused magnesia (MgO ≥ 96%) – Chromite ore (Cr₂O₃ ≥ 45%) – Fused magnesia-chrome sand | MgCr₂O₄ Magnesia-Chrome Spinel | – Chromite ore must have low silica (SiO₂ ≤ 2%) and low phosphorus (P₂O₅ ≤ 0.03%) to prevent the formation of low-melting glass phases. – The Cr₂O₃ content should be controlled (10%-30%) to balance corrosion resistance and environmental friendliness. |
| Magnesia Hercynite Bricks | – Sintered magnesia (MgO ≥ 94%) – Mill scale (Fe₂O₃ ≥ 70%) – High-iron bauxite (Fe₂O₃ ≥ 10%) | MgFe₂O₄ Magnesia hercynite | – The iron source must maintain the stability of Fe³⁺ to avoid the precipitation of Fe²⁺ at high temperatures, which would cause discoloration of the bricks. – The impurity (CaO) content in magnesia should be low to prevent the formation of low-melting substances (CaO·Fe₂O₃, melting point: 1216℃) with Fe₂O₃. |
Key Characteristics of Different Types of Spinel Bricks
The performance differences among the three types of spinel bricks stem from three core factors: the crystal structure of the spinel phase (e.g., MgAl2O4 has a face-centered cubic structure, while MgCr2O4 belongs to the isometric crystal system), the bonding state with the magnesia matrix, and the resistance to high-temperature melts and gases.
- Magnesia-Alumina Spinel Bricks (Most Widely Used)
Core Advantages
Excellent Thermal Shock Resistance: The MgAl2O4 spinel phase has a low thermal expansion coefficient and is uniformly distributed in the magnesia matrix. This enables it to relieve thermal stress caused by sudden temperature changes, and it is less prone to cracking after repeated rapid heating and cooling (e.g., water quenching at 1100°C).
Good Corrosion Resistance: It has low wettability to molten steel and slags (especially alkaline slags), and can resist the penetration of oxides such as FeO and MnO in slags, thus preventing brick spalling.
Environmental Friendliness and Non-Hazardous: It does not contain Cr⁶+(hexavalent chromium, a toxic substance that is easily soluble in water and carcinogenic), complying with the environmental requirements of modern industry.
Limitation: Its resistance to acidic slags (with high SiO2 content) is relatively weak. SiO2 tends to react with MgO to form 2MgO·SiO2, a low-melting substance (melting point: 1890°C), which causes softening of the bricks.
2. Magnesia-Chrome Spinel Bricks (Strong Corrosion Resistance but Non-Environmentally Friendly
Core Advantages
Exceptional Corrosion Resistance: The MgCr2O4 spinel exhibits high chemical inertness toward molten slags—especially high-temperature alloy slags (containing Cr, Ni) and cement kiln clinker. It can effectively resist the penetration of CaO, Al2O3, and Fe2O3 in slags, and is less likely to react with slags to form low-melting phases at high temperatures.
Excellent High-Temperature Compressive Strength: MgCr2O4 forms a solid solution with periclase (MgO), resulting in tight crystal bonding. Even above 1600°C, it maintains high room-temperature compressive strength (≥100 MPa) and high-temperature flexural strength (≥15 MPa at 1400°C).
Limitation
Environmental Risk: Cr3+ in the raw materials is prone to conversion into Cr6+ in high-temperature oxidizing environments (e.g., cement kiln preheaters, waste incinerators). Cr6+ is then discharged with flue gas or wastewater, posing hazards to the environment and human health. Some regions (e.g., the European Union) have restricted its use in environmentally sensitive scenarios.
3. Magnesia-hercynite Spinel Bricks (Low Cost with Focused Performance)
Core Advantages
Low Cost: The cost of spinel sources (mill scale, high-iron bauxite) is much lower than that of alumina and chromite ore. Additionally, the production process does not require excessively high temperatures (sintering temperature: 1600–1650°C, lower than the 1700–1750°C of magnesia-alumina spinel bricks), resulting in low overall costs.
Good Resistance to Reductive Corrosion: MgFe2O4 maintains high stability in reductive atmospheres (e.g., the slag line of blast furnaces in the iron and steel industry, non-ferrous metal smelting furnaces). It is not easily reduced by CO or H2, and can absorb FeO in slags to form stable solid solutions.
Limitations
Poor High-Temperature Dimensional Stability: MgFe2O4 is prone to crystal phase transformation at high temperatures (e.g., partial decomposition into MgO and Fe2O3 above 1450°C). This causes volume shrinkage or expansion of the bricks, affecting structural integrity.
Weak Erosion Resistance: The presence of Fe2O3 results in a slightly lower bulk density of the bricks (approximately 3.3 g/cm³, lower than the 3.5 g/cm³ of magnesia-alumina spinel bricks), leading to poor resistance against scouring by high-temperature gas flows or melts.
Main Application Scenarios of Different Types of Spinel Bricks
The selection of spinel bricks must be closely aligned with two core factors: operating conditions (temperature, atmosphere, slag type) and performance requirements (corrosion resistance, thermal shock resistance, cost).
| Classification | Core Application Fields | Specific Application Locations | Selection Rationale |
| Magnesia-Alumina Spinel Bricks | Iron & Steel, Non-Ferrous Metals, Glass | 1. Iron & Steel: Converter linings, LF furnace ladle linings, RH vacuum degassing furnace linings 2. Non-Ferrous Metals: Copper smelting converters, nickel-iron alloy furnace slag lines 3. Glass: Regenerator checkerworks of glass furnaces, melting tank sidewalls | Operating temp: 1500–1700°C. Requires resistance to alkaline slag erosion + repeated thermal shocks, with no Cr⁶⁺ environmental risk—matching mainstream industrial needs. |
| Magnesia-Chrome Spinel Bricks | Special Cement, Waste Incineration, Chemical Industry | 1. Cement: Burning zone and transition zone of new dry-process cement kilns (high-temperature, strong erosion + alkaline slag) 2. Waste Incineration: Waste incinerator furnaces (containing complex hazardous slag) 3. Chemical Industry: Sulfuric acid converter linings (high-temperature acidic atmosphere) | Operating conditions demand extreme corrosion resistance (complex slag composition, strong corrosivity), with acceptance of strict environmental controls (e.g., closed systems + Cr⁶⁺ treatment). |
| Magnesia-hercynite Spinel Bricks | Medium-Low Temperature Industrial Furnaces, Low-Cost Scenarios | 1. Iron & Steel: Combustion chambers of blast furnace hot stoves, medium-frequency induction furnace linings 2. Building Materials: Ceramic kiln car platforms, aggregates for refractory castables 3. Waste Heat Boilers: High-temperature flue gas duct linings | Operating temp: 1200–1500°C. Moderate requirements for thermal shock and erosion resistance; cost control is a priority, with no need for high purity or extreme environmental performance. |
Selection Recommendations for Different Spinel Bricks
“comprehensive performance + environmental friendliness” → choose magnesia-alumina spinel bricks (mainstream option);
“extreme corrosion resistance” and having the ability to manage environmental risks → choose magnesia-chrome spinel bricks (specialized scenarios);
“low cost” and operating under mild working conditions → choose magnesia-hercynite bricks (economic scenarios).
The control of impurities (e.g., SiO₂, CaO) in raw materials is critical for ensuring the performance of all types of spinel bricks. It directly affects whether low-melting phases are formed at high temperatures and whether structural spalling occurs.
