Before aluminum can be electrolysed, its ore must be refined to pure alumina: bauxite (40-60% alumina) is dissolved in hot caustic soda, the iron and silica residue is filtered off, and alumina is precipitated and calcined. Each kilogram of metal needs 1.89 kg of alumina, so at least 3.15-4.7 kg of bauxite; clay is a poorer ore and needs an acid route instead, with far more reagent per kilogram of metal.
Part of Aluminum Smelting
Turning bauxite or clay into pure alumina, the feedstock for electrolysis, and the quantities each route consumes.
Aluminum Is Locked In Oxide And The Ore Choice Decides The Route
The carrier says aluminum is never found as metal; it sits as alumina in bauxite (red-brown rock, 40-60% alumina) or clay minerals (kaolin, 25-35% alumina). Bauxite goes through the caustic route, the simplified Bayer process (Simplified Bayer Process, with its two hardest stages in Crushing and Digestion and Precipitation and Calcination). Clay goes through acid leaching (Using Common Clay).
Worked Example: How Much Ore Per Kilogram Of Metal
Alumina per kilogram of aluminum. Al₂O₃ has molar mass 2 × 26.98 + 3 × 16.00 = 101.96 g/mol (atomic masses recalled). The two aluminum atoms weigh 53.96 g, so aluminum is 53.96 / 101.96 = 0.529 of the oxide by mass. One kilogram of metal therefore needs 1 / 0.529 = 1.89 kg of alumina.
Ore per kilogram of metal, assuming every bit of the alumina is recovered:
- Bauxite at 60% alumina: 1.89 / 0.60 = 3.15 kg.
- Bauxite at 40% alumina: 1.89 / 0.40 = 4.7 kg.
- Clay at 35% alumina: 1.89 / 0.35 = 5.4 kg; at 25%: 1.89 / 0.25 = 7.6 kg.
Real recoveries are less than 100%, so the true figures are higher. The corpus gives no recovery.
Scale against the metal output. A cell making about 1 kg per hour (Hall-Héroult Electrolysis) consumes 1.89 kg of alumina an hour. In a day that is 24 × 1.89 = 45 kg of alumina, from at least 24 × 3.15 = 76 kg of the richest bauxite.
What You Need
- Bauxite or clay, assayed. The digestion itself is the best assay: digest a weighed sample, recover and weigh the aluminum hydroxide. This is my suggestion; the corpus gives no assay method.
- Caustic soda for bauxite (Caustic Soda).
- A pressure vessel rated for 150-200 °C digestion (Crushing and Digestion).
- Filters, settling tanks and a kiln reaching 1,050 °C.
- A disposal site for red mud. The residue is strongly alkaline, as I recall; keep it out of streams and wells.
Common Mistakes
- Planning ore supply from the metal target. Work back through the kilograms above.
- Using clay as if it were bauxite. See Using Common Clay.
- Ignoring the waste. The residue weight is of the same order as the ore.
Safety
Hot caustic under pressure
Digestion uses 25-30% caustic soda at 150-200 °C in a sealed vessel; the details and the stops are in Crushing and Digestion. Caustic burns skin and eyes at once, and hot caustic with steam is worse.
What’s Next
- Learn next: Simplified Bayer Process - the chemistry step by step
- Combine with: Why Aluminum Matters - why scrap usually comes first