Materials, Properties, and Applications of High-Alumina Bricks and Magnesia Bricks

2 days ago
3 min read

Material and Chemical Properties

1. High-alumina bricks belong to the aluminosilicate system, with an Al₂O₃ content of 45%–90%. The matrix is mainly corundum and mullite. Chemically neutral, they can withstand both weak acid and weak alkali corrosion.

2. Magnesia bricks belong to the magnesia system, with MgO ≥ 87%. The main crystalline phase is periclase. Alkaline materials exhibit extremely high inertness to strong alkali slag and iron corrosion, but readily react with acidic media to form low-melting-point calcium magnesium olivine. Therefore, direct contact with acidic refractory materials is strictly prohibited.

Key Physicochemical Indicators

1. Refractoriness: Magnesia bricks above 2000℃, high-alumina bricks 1700–1850℃.
2. Load softening temperature: Magnesia bricks 1800℃, high-alumina bricks vary from 1410–1600℃ depending on grade.
3. Bulk density: Magnesia bricks have a density of over 3.0 g·cm⁻³, making them dense and solid; high-alumina bricks have a density of 2.3–3.0 g·cm⁻³, increasing with increasing Al₂O₃ content.
4. Thermal conductivity: Magnesia bricks have high thermal conductivity, facilitating heat dissipation; high-alumina bricks have low thermal conductivity, which is beneficial for heat insulation.
5. Thermal shock resistance: Magnesia bricks have poor thermal shock resistance, cracking after only 3–5 water cooling cycles at 800 ℃; high-alumina bricks have excellent thermal shock resistance, with a resistance of 25–30 cycles under the same testing conditions.
6. Hydration resistance: Magnesia bricks react with water to form Mg(OH)₂, causing volume expansion and resulting in network cracks; storage and transportation must be protected against moisture, rain, and snow. High-alumina bricks do not have this problem and are safe for short-term outdoor storage.

Production Process

Magnesium bricks are made from high-purity magnesia sand (less than 5 mm thick), fired at 1600℃, allowing the periclase grains to bond directly, forming a high-melting-point skeleton. High-alumina bricks use high-quality bauxite as aggregate, sintered at around 1450℃, with corundum-mullite interleaved, balancing refractoriness and thermal shock resistance.

Applications

1. Alkaline corrosion zones: Magnesium bricks or magnesia-carbon bricks are the main lining material in steelmaking converters, electric furnace hot zones, and cement kiln firing zones.
2. Weak acid/weak alkali transition zones: High-alumina bricks are suitable for the decomposition zone of rotary kilns, the preheating zone of lime kilns, and the upper part of the regenerator in glass kilns.
3. Acid-alkali interface zones: Where an isolation zone needs to be built between the two types of bricks, high-alumina bricks are usually used as a buffer layer to prevent direct contact and reaction between magnesium bricks and silica bricks or clay bricks.

Economic Considerations

Under the same grade, magnesia bricks are approximately 2-3 times more expensive than high-alumina bricks. Adding the cost of moisture-proof packaging for transportation and storage, the overall cost is even higher. Therefore, when the temperature is ≤1400℃ and the corrosive medium is not a strong alkali, high-alumina bricks should be preferred, which can significantly reduce furnace construction costs.

How to Choose?

The selection logic for high-alumina bricks versus magnesia bricks can be simplified to "three considerations":

First, consider the chemical atmosphere: choose magnesia for strong alkalis, and alumina for weak acids and weak alkalis;
Second, consider the operating temperature: use magnesia for hot spots >1650℃, and alumina for medium and low temperatures <1600℃;
Third, consider thermal shock requirements: high-alumina bricks are preferable for frequent kiln start-ups and shutdowns.

Choosing the right one, not the most expensive one, achieves the best balance between lifespan and cost.