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What is Bauxite?
Bauxite is the primary raw material for producing alumina (Al₂O₃).
It is a sedimentary rock rich in aluminum minerals and is formed in tropical and subtropical climates through natural weathering processes.
Key Characteristics
• Found near the earth’s surface
• Mined by open-pit (surface) mining
• Reddish color due to iron oxide (Fe₂O₃)
• Quality depends mainly on Al₂O₃ content and silica level
Major Producing Countries
Australia (largest producer), Guinea, China, Brazil, India, Indonesia
Component | Typical Range
Al₂O₃: 30–60%
Fe₂O₃: 5–30%
SiO₂: 1–10%
TiO₂: 1–5%
Moisture: Variable
Bayer Process
Refining Bauxite into Aluminum Hydroxide (Al(OH)₃)
Process Steps
1. Crushing & Grinding
Bauxite is crushed and ground into fine particles
2. Digestion
Mixed with hot NaOH under high temperature & pressure (140–250°C)
Reaction: Al(OH)₃ + NaOH → NaAlO₂ + 2H₂O
3. Clarification
Impurities removed as red mud
4. Separation
Red mud is separated
5. Washing & Filtration
Al(OH)₃ is washed & filtered to produce alumina (Al₂O₃)
Key Points
• Produces high-purity alumina
• Caustic soda is recycled
• Red mud is the main by-product
• Most widely used alumina refining process
Calcination Process
Transforming Aluminum Hydroxide (Al(OH)₃) into Alumina (Al₂O₃)
1. Feed Material
Aluminum Hydroxide – Al(OH)₃ (from Bayer Process)
2. High-Temperature Heating
• Rotary kiln
• Fluidized bed calciner
• Temperature: 1000–1600°C
3. Chemical Reaction
2 Al(OH)₃ → Al₂O₃ + 3 H₂O
• Water vapor (H₂O) is released
• Structure changes to stable α-Al₂O₃
4. Final Product – Calcined Alumina (Al₂O₃)
Key Characteristics
• High purity (≥99% Al₂O₃)
• High melting point (~2050°C)
• Very hard and abrasive
Purpose
Remove chemically combined water and transform crystal structure
Chemical Properties
• High purity (typically ≥99% Al₂O₃)
• Chemically stable
• Excellent corrosion resistance
• Inert to most acids and alkalis
Thermal Properties
• Very high melting point (~2050°C)
• Excellent thermal stability
• Low thermal expansion
• Good thermal shock resistance (grade dependent)
Mechanical Properties
• Extremely hard (Mohs hardness ~9)
• High compressive strength
• Excellent wear and abrasion resistance
Electrical Properties
• Very good electrical insulation
• High dielectric strength
• Used in electrical ceramics and insulators
Physical Characteristics
• White fine powder
• Controlled particle size distribution
• Mainly α-phase (alpha-alumina)
• High density and low porosity (after sintering)