1. Preparation of industrial grade alumina (purity 98-99.5%)
Bayer process:
Core process: Bauxite (containing Al ₂ O ∝ 40-60%) → Caustic soda leaching (150-250 ℃, 3-5 MPa) → Seed decomposition → Calcination (1100-1200 ℃).
Advantages: Low energy consumption (3.5-4.5 GJ/ton), suitable for processing high alumina silica ratio (A/S>7) ores, accounting for over 90% of global production capacity.
Sintering method:
Suitable for low alumina silica ratio ores (A/S=3-5), sodium aluminate is generated by adding lime sintering, followed by dissolution, desilication, carbonation decomposition, with high energy consumption (6-8 GJ/ton).
2. High purity alumina (purity ≥ 99.9%)
Chemical method:
Alcoholic salt hydrolysis method: Using aluminum isopropoxide as raw material, hydrolysis generates Al (OH) 3 precipitate, which is calcined to obtain 5N grade (99.999%) alumina with controllable particle size (50-200 nm), suitable for semiconductor substrates.
Aluminum ammonium carbonate thermal decomposition method: By controlling the pyrolysis temperature of (NH ₄) Al (OH) ₂ CO ∝ (800-1000 ℃), submicron level high-purity alumina is prepared with impurities (Fe, Si, Na) content<50 ppm.
Electrolytic refining method:
Using industrial alumina as raw material, purified by molten salt electrolysis (cryolite alumina system, 1000 ℃), the purity can reach 99.99%, but the cost is high, mainly used for aerospace components.
3. Nano alumina (particle size<100 nm)
Sol gel method: aluminum salt solution is hydrolyzed and condensed to form sol, which is then gel, dried and calcined, with narrow particle size distribution (CV<10%), suitable for polishing solution and coating materials.
Gas deposition method: AlCl ∝ reacts with O ₂ in the gas phase at high temperatures (1500-2000 ℃) to generate nano-sized α - Al ₂ O ∝ particles with a purity of>99.9% and excellent dispersibility.