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Difficulty: HardAluminium: Extraction, Amphoteric Properties, Alloys, and Compounds

Arrange the following sequential industrial steps of the Bayer process used in refining bauxite ore into pure alumina (aluminium oxide) in the correct chronological order from first to last.

  1. 1Digestion of crushed crude bauxite in hot, concentrated sodium hydroxide solution under high pressure to form sodium tetrahydroxoaluminate(III).
  2. 2Filtration of the mixture to separate insoluble impurities such as iron(III) oxide (red mud) from the clear filtrate.
  3. 3Seeding the filtrate with pure aluminium hydroxide crystals to precipitate hydrated aluminium hydroxide.
  4. 4High-temperature calcination of the washed precipitate to obtain anhydrous aluminium oxide.

Answer

The correct chronological order of the Bayer process is: Digestion of bauxite in concentrated NaOH →\rightarrow Filtration to remove red mud →\rightarrow Seeding to precipitate hydrated aluminium hydroxide →\rightarrow High-temperature calcination to yield pure anhydrous alumina.
The Bayer process refines crude bauxite into pure alumina through four distinct chemical phases: initial amphoteric dissolution in hot concentrated alkali (digestion), removal of solid iron oxide impurities via filtration (red mud removal), controlled crystallization of pure hydroxide (seeding/precipitation), and thermal decomposition (calcination) to produce anhydrous Al2O3\text{Al}_2\text{O}_3.

Step-by-Step Solution

1
Identify the chemical extraction reaction
Crude bauxite (Al2O3⋅xH2O\text{Al}_2\text{O}_3\cdot x\text{H}_2\text{O}) is treated with hot concentrated NaOH\text{NaOH} solution under pressure to dissolve aluminium amphoterically: Al2O3(s)+2NaOH(aq)+3H2O(l)→2NaAl(OH)4(aq)\text{Al}_2\text{O}_3(s) + 2\text{NaOH}(aq) + 3\text{H}_2\text{O}(l) \rightarrow 2\text{NaAl(OH)}_4(aq).
Aluminium oxide is amphoteric and forms soluble aluminate ions, whereas impurities like Fe2O3\text{Fe}_2\text{O}_3 do not react.
2
Separate insoluble residues
The dense, insoluble residue known as 'red mud' (containing Fe2O3\text{Fe}_2\text{O}_3, silica, and titania) is filtered out.
Filtration purifies the liquid stream so that subsequent precipitates are free from iron contamination.
3
Precipitate hydrated aluminium hydroxide
The clear sodium tetrahydroxoaluminate filtrate is cooled and seeded with pure Al(OH)3\text{Al(OH)}_3 crystals to induce precipitation: NaAl(OH)4(aq)→Al(OH)3(s)+NaOH(aq)\text{NaAl(OH)}_4(aq) \rightarrow \text{Al(OH)}_3(s) + \text{NaOH}(aq).
Cooling and seeding shifts the equilibrium back toward solid Al(OH)3\text{Al(OH)}_3 formation.
4
Dehydrate the precipitate via heating
The collected Al(OH)3\text{Al(OH)}_3 is washed and calcined in rotary kilns at 1000∘C1000^\circ\text{C}: 2Al(OH)3(s)→ΔAl2O3(s)+3H2O(g)2\text{Al(OH)}_3(s) \xrightarrow{\Delta} \text{Al}_2\text{O}_3(s) + 3\text{H}_2\text{O}(g).
Calcination removes all chemically bound water, producing pure dry alumina feed for the Hall-Héroult electrolytic cell.

Key Concept

Bayer Process for Bauxite Purification
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Topics

Aluminium: Extraction, Amphoteric Properties, Alloys, and Compounds

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