1.5Extraction of Metal from Its Ore
Metals on Earth usually exist as compounds within minerals or ores, such as metal oxides, metal sulphides, and metal carbonates, mixed with rocks and dirt. Only unreactive metals like gold and silver exist freely as elements in nature. The method selected to extract a metal depends directly on its position in the reactivity series of metals relative to carbon.
Learning Objectives
- Explain the extraction of metal from its ore through the electrolysis process, specifically aluminium from bauxite.
- Explain metal extraction from its ore through the reduction process by carbon, specifically iron in a blast furnace.
- Explain metal extraction from its metal oxide using a more reactive metal (such as the thermite reaction).
- Relate the position of metals in the reactivity series to their suitable extraction methods.
Blast Furnace for Iron Extraction and Electrolytic Cell for Aluminium Extraction
Illustrate the structural setup, zones of temperature/reactions in a blast furnace, and electrode reactions in the Hall-Héroult aluminium electrolysis cell.
Very reactive metals (K, Na, Mg, Al) are extracted by electrolysis of molten compounds.
Moderately reactive metals (Zn, Fe, Sn, Pb) are extracted by reduction using carbon or carbon monoxide.
Less reactive metals (Cu, Hg) are extracted by direct heating in air.
Unreactive metals (Ag, Au) exist naturally as elements.
Aluminium Electrolysis Cell
Anode: Carbon (graphite) blocks.
Cathode: Carbon (graphite) lining of the steel tank.
Electrolyte: Molten mixture of \( \text{Al}_2\text{O}_3 \) and \( \text{Na}_3\text{AlF}_6 \).
Anode reaction: \( 2\text{O}^{2-}(\text{l}) \rightarrow \text{O}_2(\text{g}) + 4\text{e}^- \)
Cathode reaction: \( \text{Al}^{3+}(\text{l}) + 3\text{e}^- \rightarrow \text{Al}(\text{l}) \)
Blast Furnace Zones
Zone 1 (Bottom): \( \text{C}(\text{s}) + \text{O}_2(\text{g}) \rightarrow \text{CO}_2(\text{g}) \)
Zone 2 (Middle): \( \text{C}(\text{s}) + \text{CO}_2(\text{g}) \rightarrow 2\text{CO}(\text{g}) \)
Zone 3 (Upper): \( \text{Fe}_2\text{O}_3(\text{s}) + 3\text{CO}(\text{g}) \rightarrow 2\text{Fe}(\text{l}) + 3\text{CO}_2(\text{g}) \)
Slag formation: \( \text{CaO}(\text{s}) + \text{SiO}_2(\text{s}) \rightarrow \text{CaSiO}_3(\text{l}) \)
Brief Explanation
Understand the key ideas before exploring the extraction processes.
1. Metals Above Carbon
K, Na, Mg, Al cannot be reduced by carbon because carbon has a lower affinity for oxygen than these metals. Powerful electrical energy via electrolysis is required.
2. Metals Below Carbon
Zn, Fe, Sn, Pb can be reduced by heating with carbon or carbon monoxide, which removes oxygen to yield the free metal.
3. Blast Furnace Impurity Removal
Limestone is added to convert sandy acidic silicon(IV) oxide impurities into molten slag \( \text{CaSiO}_3 \) that floats above molten iron.
4. Thermite Reaction
Highly exothermic displacement reactions, such as the thermite reaction between aluminium and iron(III) oxide, generate molten iron locally without needing a giant blast furnace.
| Metal | Common Ore | Extraction Method |
|---|---|---|
| Aluminium | Bauxite (hydrated aluminium oxide, purified to \( \text{Al}_2\text{O}_3 \)) | Electrolysis of molten compound |
| Iron | Hematite (\( \text{Fe}_2\text{O}_3 \)), magnetite (\( \text{Fe}_3\text{O}_4 \)), iron pyrite (\( \text{FeS}_2 \)), siderite (\( \text{FeCO}_3 \)) | Reduction by carbon in a blast furnace |
| Copper, Mercury | Copper pyrite, cinnabar | Direct heating in air |
| Gold, Silver | Native elements | Exist naturally as elements |
Try These Questions First
Answer mentally, then check the explanation.
Why is cryolite added to aluminium oxide during the electrolytic extraction of aluminium?
In the extraction of iron in a blast furnace, what is the role of calcium carbonate (\( \text{CaCO}_3 \))?
Why can aluminium reduce iron(III) oxide in the thermite reaction, but carbon cannot reduce aluminium oxide?
Common Mistakes
Avoid these errors when explaining metal extraction.
Common Misconceptions
Compare each misconception with the correct fact.
Any metal can be extracted using carbon reduction if heated to a high enough temperature.
Metals above carbon in the reactivity series cannot be reduced by carbon because their bonds with oxygen are stronger than those carbon can form; they must be extracted by electrolysis.
Slag is an unwanted waste that has no useful function inside the furnace.
While slag is a byproduct, its presence floating as a liquid layer on top of molten iron protects the newly formed molten iron from being re-oxidised by incoming hot air blasts.
Aluminium is extracted from aqueous aluminium solutions by electrolysis.
Molten aluminium oxide must be used; if an aqueous solution were used, \( \text{H}^+ \) ions from water would be preferentially discharged at the cathode instead of \( \text{Al}^{3+} \) ions, producing hydrogen gas instead of aluminium.
Summary
Key facts to remember.
- Metal extraction is governed by the reactivity series of metals.
- Above Carbon (K to Al): Extracted by molten electrolysis. Cryolite lowers the melting point of alumina. Aluminium forms at the cathode, and oxygen at the carbon anode burns it to \( \text{CO}_2 \).
- Below Carbon (Zn to Pb): Extracted by reduction with carbon/\( \text{CO} \) in a blast furnace. Coke provides heat and forms \( \text{CO} \); limestone removes \( \text{SiO}_2 \) impurities as floating slag (\( \text{CaSiO}_3 \)).
- Thermite Reaction: A more reactive metal (e.g., \( \text{Al} \)) reduces a less reactive metal oxide (e.g., \( \text{Fe}_2\text{O}_3 \)) exothermically to yield molten metal.
- Environmental Responsibility: Bauxite purification creates toxic red sludge and electrolysis emits \( \text{CO}_2 \); recycling aluminium saves \( 95\% \) of extraction energy.
Quick Activity
Complete all parts before checking your score.
A. Objective Quiz
Why is cryolite (\( \text{Na}_3\text{AlF}_6 \)) added to molten aluminium oxide during the industrial extraction of aluminium?
Which chemical reaction occurs in Zone 2 of the blast furnace to produce the primary reducing agent for iron(III) oxide?
B. Fill in the Blanks
During the extraction of iron in a blast furnace, calcium oxide neutralises silicon(IV) oxide to form molten slag known chemically as ________.
In the thermite reaction used to weld railway tracks, aluminium powder reduces ________ oxide to produce molten iron.
C. Match the Substance or Zone with Its Role
Drag each substance on desktop. On mobile, tap a substance and then tap the matching box.
Answer Choices
Carbon anode in aluminium electrolysis Limestone in blast furnace Cryolite Molten slagRole / Description
Keywords
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