Extraction of Aluminium from Bauxite
Section: 9. Metals | Syllabus: Cambridge AS Level Physics 9702
Introduction to Aluminium Extraction Aluminium is the most abundant metal in the Earth's crust, making up about 8% of it. Despite its abundance, aluminium is expensive to extract because it is very reactive and requires electrolysis.
Bauxite The main ore of aluminium, containing aluminium oxide (Al₂O₃) mixed with impurities like iron oxide and silicon dioxide. It is a reddish-brown rock. Why Not Carbon Reduction? Aluminium is above carbon in the reactivity series, so it cannot be extracted by reduction with carbon.
Carbon is not reactive enough to remove oxygen from aluminium oxide. Therefore, electrolysis is the only viable method. Overview of the Extraction Process Extracting aluminium from bauxite involves two main stages: Purification of bauxite: Separating aluminium oxide (Al₂O₃) from impurities Electrolysis: Breaking down pure aluminium oxide to obtain aluminium metal Stage Process Product 1 Purification of bauxite ore Pure aluminium oxide (Al₂O₃) 2 Electrolysis of molten aluminium oxide Pure aluminium metal (Al) Stage 1: Purification of Bauxite Raw bauxite contains impurities (mainly iron oxide and silicon dioxide) that must be removed before electrolysis.
This is done using the Bayer Process : Bauxite is crushed and ground into a powder Mixed with hot sodium hydroxide solution under pressure Aluminium oxide dissolves, impurities do not Impurities are filtered off Solution is cooled and pure aluminium oxide crystallizes out Pure aluminium oxide (Al₂O₃) is obtained - also called alumina Result This process produces pure aluminium oxide (alumina) which is a white powder.
This is the material that undergoes electrolysis. Stage 2: Electrolysis of Aluminium Oxide The Challenge: Aluminium oxide has a very high melting point (2050°C). Heating it to this temperature would be extremely expensive and energy-intensive.
The Solution: Using Cryolite Cryolite A compound (Na₃AlF₆) that is used as a solvent to dissolve aluminium oxide, lowering its melting point from 2050°C to about 950°C. Why Use Cryolite? Two key benefits: Lowers melting point: From 2050°C to ~950°C (saves energy and money) Increases conductivity: The molten mixture conducts electricity better Cryolite is not used up in the process - it acts as a solvent.
The Electrolysis Cell Interactive Diagram Detailed cross-section of an aluminium electrolysis cell showing: (1) Large rectangular steel container lined with graphite (cathode) at the bottom and sides.
(2) Multiple graphite anodes suspended from the top, dipping into the molten mixture. (3) Molten mixture of aluminium oxide dissolved in cryolite (white/gray) in the middle. (4) Molten aluminium (silvery liquid) pooling at the bottom on the cathode.
(5) Oxygen gas bubbles rising from anodes at the top. (6) Tap at bottom for removing molten aluminium periodically. (7) Labels showing: "Graphite anodes (+)" at top, "Molten Al₂O₃ in cryolite at 950°C" in middle, "Graphite cathode (-) lining" at bottom, "Molten aluminium (denser, sinks)" at very bottom.
(8) Arrows showing: electrons flowing from power supply, Al³⁺ ions moving to cathode, O²⁻ ions moving to anode. (9) Equations at each electrode. Use color coding: black for graphite, white/gray for molten electrolyte, silver for aluminium, blue arrows for electron flow.
Setup: Electrolyte: Molten aluminium oxide (Al₂O₃) dissolved in molten cryolite (Na₃AlF₆) at ~950°C Anode (positive electrode): Graphite (carbon) blocks suspended from the top Cathode (negative electrode): Graphite lining at the bottom and sides of the cell Product: Molten aluminium collects at the bottom (cathode) and is tapped off Why Graphite Electrodes?
Graphite is used because it: Conducts electricity Withstands high temperatures (950°C) Is relatively inexpensive However, graphite anodes react with oxygen and must be replaced regularly Reactions During Electrolysis Ionization of Aluminium Oxide: When molten, aluminium oxide breaks down into ions: Aluminium oxide ionizes: Al₂O₃(l) → 2Al³⁺(l) + 3O²⁻(l) Aluminium ions (positive) and oxide ions (negative) are free to move At the Cathode (Negative Electrode) - REDUCTION: Aluminium ions gain electrons: Al³⁺(l) + 3e⁻ → Al(l) Aluminium ions are reduced to aluminium metal Molten aluminium sinks to bottom (very dense) and is tapped off Remember Reduction occurs at the cathode - aluminium ions gain electrons to become aluminium atoms.
Think: Reduction Is Gain (of electrons). At the Anode (Positive Electrode) - OXIDATION: Oxide ions lose electrons: 2O²⁻(l) → O₂(g) + 4e⁻ Oxide ions are oxidized to oxygen gas Oxygen gas bubbles off at the anode Remember Oxidation occurs at the anode - oxide ions lose electrons .
Think: Oxidation Is Loss (of electrons). Overall Equation: Complete electrolysis reaction: 2Al₂O₃(l) → 4Al(l) + 3O₂(g) Aluminium oxide → Aluminium + Oxygen Problem: The Anodes Burn Away A major issue in aluminium extraction is that the graphite anodes react with oxygen produced during electrolysis: Graphite anode reacts with oxygen: C(s)…
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