Bauxite is processed into alumina (aluminum oxide, Al₂O₃) through the Bayer process, which is then smelted to produce aluminum metal. Here’s a step-by-step breakdown of the process:
1. Mining Bauxite
– Bauxite is typically extracted via open-pit mining.
– The ore contains 30–60% aluminum hydroxide (gibbsite, boehmite, or diaspore), along with impurities like iron oxides, silica, and titanium dioxide.
2. Bayer Process (Alumina Refining)
The Bayer process converts bauxite into pure alumina:
# a. Crushing & Grinding
– Bauxite is crushed and ground into a fine powder to increase surface area for chemical reactions.
# b. Digestion
– The powdered bauxite is mixed with a hot (150–250°C) sodium hydroxide (NaOH) solution under high pressure.
– This dissolves the aluminum hydroxides while leaving impurities (like iron oxides and silica) as solid residues (red mud).
\[
\text{Al(OH)}_3 + \text{NaOH} → \text{NaAl(OH)}_4 \quad (\text{Sodium aluminate})
\]
# c. Clarification & Filtration
– The mixture is filtered to remove insoluble red mud waste.
– The remaining sodium aluminate solution is clarified.
# d. Precipitation
– The solution is cooled and seeded with fine alumina crystals to precipitate pure aluminum hydroxide (Al(OH)₃).
\[
\text{NaAl(OH)}_4 → \text{Al(OH)}_3↓ + \text{NaOH}
\]
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# e. Calcination
– The precipitated Al(OH)₃ is heated at ~1000°C to remove water, producing alumina (Al₂O₃).
\[
2\text{Al(OH)}_3 → \text{Al}_2\text{O}_3 + 3\text{H}_2\text{O}
\]
3. Hall-Héroult Process (Aluminum Smelting)
The alumina is then converted into aluminum metal via electrolysis:
# a. Electrolytic Reduction
– Alumina is dissolved in molten cryolite (Na₃AlF₆) at ~950°C in an electrolytic cell.
– A direct current passes through the cell, reducing alumina to aluminum metal at