Corundum is a crystalline form of alpha-alumina (Al₂O₃) and an important raw material for both abrasives and refractory applications. It belongs to the trigonal crystal system and is known for its extremely high hardness, second only to diamond.
Key physical properties include:
Density: 3.95–4.1 g/cm³
Mohs hardness: 9
Melting point: 2050°C
Boiling point: 3400–3700°C
Thermal conductivity (1000°C): 21.1 W/(m·K)
Linear expansion coefficient (20–1000°C): 8.0×10⁻⁶/°C
Elastic modulus: 36.3×10¹⁰ Pa
Corundum is chemically stable and highly resistant to acid and alkali corrosion, making it suitable for high-temperature and harsh industrial environments.
2. Classification of Corundum
Corundum can be divided into natural corundum and synthetic corundum.
Natural Corundum
Naturally occurring corundum includes gemstones:
Ruby (Cr-containing, red)
Sapphire (Ti-containing, blue)
Synthetic Corundum
Synthetic corundum is widely used in industry and includes:
Fused Corundum
Al₂O₃: ≥94%
Density: 3.65–4.05 g/cm³
Microhardness: 17640–24010 MPa
Sintered Corundum
Al₂O₃: ≥86%
Density: ~3.75 g/cm³
Microhardness: ~16660 MPa
Common industrial types include:
White corundum
Brown corundum
Dense fused corundum
Tabular corundum
Item
Brown Fused Alumina
Bauxite-based Corundum
Fused Corundum
Tabular Corundum
Sintered
Fused
Standard
Dense
Sintered
Dense
Al₂O₃ (%)
≥95.0
98.7
98.8
98.1
98.9
99.2
99.3
SiO₂ (%)
≤1.8
0.01
0.30
0.50
0.68
0.20
0.18
Fe₂O₃ (%)
≤1.0
0.12
0.31
0.30
0.06
0.10
0.10
TiO₂ (%)
≤3.5
0.01
0.08
0.20
0.10
–
–
CaO (%)
≤0.25
0.02
–
0.29
0.08
0.04
–
MgO (%)
≤0.22
0.02
–
0.14
0.10
0.06
–
K₂O (%)
–
0.02
0.10
0.05
0.01
–
–
Na₂O (%)
–
0.05
0.04
0.41
0.09
0.30
0.28
Color
Brown
Grey-white
Grey-white
White
Grey-black
–
–
True Density (g/cm³)
≥3.9
–
≥3.9
3.94
3.95
–
–
Bulk Density (g/cm³)
≥3.80
3.67
3.80
–
3.81
3.66
3.76
Apparent Porosity (%)
–
1.8
≤4.0
–
3.5
2.9
2.6
Refractoriness (°C)
≥1850
≥1850
≥1850
≥1850
≥1850
≥1850
≥1850
3. What is White Corundum (White Fused Alumina)?
White corundum is produced by melting high-purity alumina in an electric arc furnace at temperatures above 2000°C, followed by crushing, shaping, magnetic separation, and grading.
Key Characteristics
High purity and low impurity content
High hardness (higher than brown corundum)
Strong cutting ability
Excellent self-sharpening performance
Stable at high temperatures
Resistant to acid and alkali corrosion
Physical Properties
Bulk density: 1.53–1.99 g/cm³ (depending on grain size and distribution)
Applications
Grinding wheels (ceramic & resin bonded)
Sandblasting and surface preparation
Precision casting
Semiconductor and electronic polishing
High-grade refractory materials
4. Other Major Types of Corundum
Brown Fused Corundum
Produced from bauxite with carbon reduction:
Al₂O₃: ≥94.5%
Contains TiO₂, SiO₂, Fe₂O₃
Features:
Higher toughness
Good wear resistance
Applications:
Abrasives
Refractory castables and bricks
Dense Fused Corundum
Al₂O₃: ≥98%
Bulk density: >3.8 g/cm³
Apparent porosity: <4%
Advantages:
Excellent wear and corrosion resistance
High-temperature stability
Applications:
Blast furnace troughs
Slide gates and nozzles
Continuous casting systems
Tabular Corundum
Al₂O₃: ≥99%
Bulk density: ≥3.7 g/cm³
Na₂O: <0.3%
Features:
Large, well-developed α-Al₂O₃ crystals
Excellent thermal shock resistance
High volume stability
Applications:
High-performance refractory aggregates
Sintered Corundum
Produced at 1750–1900°C:
Al₂O₃: >99.3%
True density: >3.98 g/cm³
Apparent porosity: <5%
Advantage:
Better thermal shock resistance than fused corundum
Chrome Corundum
Cr₂O₃: 10–15%
Higher toughness than white corundum
Features:
High mechanical strength
Improved high-temperature performance
Zirconia Corundum
ZrO₂ content: 10–40%
Features:
High toughness and strength
Excellent thermal shock resistance
Strong resistance to molten metal corrosion
Applications:
Heavy-duty grinding wheels
Steel and alloy processing
Advanced refractory components
5. Industrial Applications of Corundum
Due to its outstanding properties, corundum is widely used in:
Corundum is a critical material in modern industry, combining extreme hardness, high-temperature resistance, and chemical stability. Among its types, white corundum, tabular corundum, and dense fused corundum are especially important for high-end applications.
Choosing the right type of corundum can significantly improve performance, durability, and efficiency in demanding industrial environments.
