Common clay is not a practical aluminum ore however much alumina it holds, because the reason is the extraction chemistry, not the aluminum content: clay is nearly half silica, which wrecks caustic digestion, and the acid route that avoids that needs at least 4 kg of hydrochloric acid and 1.89 kg of ammonia for every kilogram of metal before any electrolysis starts. Use bauxite if you can find it, and otherwise recycle scrap.
Part of Aluminum Smelting
Why clay is a poor aluminum ore, what the acid-leach route needs, and the reagent arithmetic per kilogram of metal.
Alumina Is There, But It Comes With Twice Its Weight Of Silica
The carrier says kaolin clay is 25-35% alumina, that it yields less than bauxite, and that it needs acid leaching first (hydrochloric acid dissolves the alumina, then ammonia precipitates it). Kaolinite is Al₂Si₂O₅(OH)₄. With recalled atomic masses (Al 26.98, Si 28.09, O 16.00, H 1.008) the formula mass is about 258.2. Alumina (101.96) is 101.96 / 258.2 = 39.5% of pure kaolinite, and silica (2 × 60.08 = 120.2) is 46.5%. Real clays are impure, hence the carrier’s 25-35%.
Why the caustic route fails. In the Bayer digestion of Simplified Bayer Process, silica from clay dissolves in the caustic and re-precipitates as a sodium aluminosilicate solid that takes both caustic and alumina with it; I recall bauxites with low silica are chosen for exactly this reason. The corpus does not give a limit, but the arithmetic is plain: pure kaolinite carries 1.18 kg of silica per kg of alumina (120.2 / 101.96).
Worked Example: The Acid Route Per Kilogram Of Metal
Alumina needed: 1.89 kg (Ore Processing).
- Clay: at 39.5% alumina (pure kaolinite), 1.89 / 0.395 = 4.8 kg; at the carrier’s 35% and 25%, 5.4 kg and 7.6 kg, all assuming 100% extraction.
- Hydrochloric acid: Al₂O₃ + 6HCl → 2AlCl₃ + 3H₂O. HCl is 36.46 g/mol, so 6 × 36.46 = 218.8 g per 101.96 g alumina, or 2.15 kg per kg alumina; for 1.89 kg that is 4.06 kg of pure HCl. The corpus gives village acid at 5-15%, up to 20-30% (Hydrochloric Acid). At 30% that is 4.06 / 0.30 = 13.5 kg of acid solution, and at 15%, 27 kg.
- Ammonia to precipitate: AlCl₃ + 3NH₃ + 3H₂O → Al(OH)₃ + 3NH₄Cl. 3 × 17.03 = 51.1 g per 78.0 g hydroxide, or 0.655 kg per kg; for 2.89 kg of hydroxide that is 1.89 kg of NH₃. The corpus mentions ammoniacal liquor from coal distillation (Coal Tar) but gives no strength.
These are floors. The ammonium chloride waste is large, and the hydroxide is then calcined and electrolysed exactly as for bauxite.
Other Problems The Corpus Does Not Cover
- Raw kaolin is hard to attack with acid; I recall it must be calcined first. Unsourced.
- Iron in the clay dissolves too and must be removed before precipitation.
- Acid-proof vessels (glass, clay, not iron; per the hydrochloric article).
Common Mistakes
- Believing “clay is everywhere” solves the ore problem. The reagents are the barrier.
- Feeding clay to the Bayer digester. It loses caustic and alumina to scale.
- Comparing clay to bauxite on alumina percentage alone.
Safety
Hydrochloric acid and ammonia
Both release irritating gas; work outdoors with the apparatus not sealed airtight, add acid to water, and keep ammonia away from acid spills, since they react violently. Wear eye protection and gloves.
What’s Next
- Learn next: Why Aluminum Matters - why scrap is the better feedstock
- Combine with: Precipitation and Calcination - the shared back end