Extraction Of Metals

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Notes

Metals and Ores

The Earth’s crust contains metals and metal compounds.
A **metal ore** is a rock that contains enough of the metal to make extraction worthwhile.
Many metal ores are **oxides** (e.g., hematite for iron, bauxite for aluminium).
Extraction is a **reduction** process because oxygen is removed from the ore.
Very unreactive metals like **gold** and **platinum** are found **native** (as uncombined elements).

Reactivity Series and Extraction Method

The extraction method depends on the metal's position in the **reactivity series**.
Metals **above carbon** must be extracted by **electrolysis** (e.g., aluminium).
Metals **below carbon** can be extracted by heating with **carbon** or **carbon monoxide** (e.g., iron).
Metals lower in the series are easier to extract than those higher up.
Electrolysis uses large amounts of electricity, making it expensive.

Extraction of Iron from Hematite – Blast Furnace

Iron is extracted from its ore **hematite** (Fe₂O₃) in a **blast furnace**.
Raw materials: iron ore, **coke** (impure carbon), and **limestone** (CaCO₃).
Hot air is blown into the bottom of the furnace.

Reactions in the Blast Furnace (Extended)

**Zone 1:** Coke burns: C + O₂ → CO₂ (exothermic, provides heat).
**Zone 2:** CO₂ + C → 2CO (carbon monoxide is the main reducing agent).
**Zone 3:** Fe₂O₃ + 3CO → 2Fe + 3CO₂ (reduction of iron ore).
Limestone decomposes: CaCO₃ → CaO + CO₂ (thermal decomposition).
CaO removes SiO₂ impurity: CaO + SiO₂ → CaSiO₃ (slag).
Molten iron is tapped from the bottom; slag floats on top and is removed separately.

Extraction of Aluminium from Bauxite – Electrolysis

Aluminium is extracted from **bauxite** (Al₂O₃) by **electrolysis**.
Aluminium is above carbon in the reactivity series, so carbon reduction is not possible.
Al₂O₃ is dissolved in molten **cryolite** to lower the melting point (from >2000°C to ~950°C).
The electrolysis cell has a **graphite** lining (cathode) and graphite anodes.
At cathode: Al³⁺ + 3e⁻ → Al (reduction).
At anode: 2O²⁻ → O₂ + 4e⁻ (oxidation).
Overall: 2Al₂O₃ → 4Al + 3O₂.
Oxygen reacts with the carbon anodes to form CO₂, so anodes wear away and must be replaced.

Key Points for Exams

Remember OIL RIG: **O**xidation **I**s **L**oss (of electrons), **R**eduction **I**s **G**ain (of electrons).
For iron extraction, know the three zones and the role of limestone.
For aluminium, explain why cryolite is used and why anodes need replacing.
Unreactive metals (Cu, Ag, Au) may be found native or extracted by simple heating.

Atomic structure of iron (Fe). Iron has 26 protons, 30 neutrons, and electron configuration 2,8,14,2.

Atomic structure of aluminium (Al). Aluminium has 13 protons, 14 neutrons, and electron configuration 2,8,3.

Practice questions

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1.Which of the following is the main ore of aluminium?
Easy
- ABauxite
- BHematite
- CMalachite
- DLimestone
2.Which of the following metals is found native (as the uncombined element) in the Earth's crust?
Easy
- AGold
- BIron
- CAluminium
- DZinc
3.What is the main reducing agent in the blast furnace for the extraction of iron?
Easy
- ACarbon monoxide
- BCarbon dioxide
- CCoke
- DLimestone
4.In the extraction of aluminium by electrolysis, why is aluminium oxide dissolved in molten cryolite?
Medium
- ATo lower the melting point of the mixture
- BTo provide a source of aluminium ions
- CTo act as a reducing agent
- DTo increase the conductivity of the solution
5.In the blast furnace, which substance is added to remove impurities such as silicon dioxide?
Medium
- ALimestone
- BCoke
- CIron ore
- DCarbon monoxide
6.Which of the following statements about transition elements is correct?
Medium
- ATheir compounds are highly coloured
- BThey have low melting points
- CTheir compounds are colourless
- DThey have only one oxidation state
7.The overall equation for the extraction of aluminium is 2Al2O3 → 4Al + 3O2. At the anode, oxygen reacts with the graphite electrodes. What is the main reason the anodes need to be replaced periodically?
Hard
- AThe graphite reacts with oxygen to form CO2, wearing away the anode
- BThe aluminium produced attacks the anode
- CThe cryolite dissolves the graphite
- DThe high temperature melts the graphite
8.In the blast furnace, which reaction is an example of thermal decomposition?
Hard
- ACaCO3 → CaO + CO2
- BC + O2 → CO2
- CCO2 + C → 2CO
- DFe2O3 + 3CO → 2Fe + 3CO2

Slides

Metals and Ores

The Earth's crust contains metals and metal compounds.
A metal ore is a rock that contains enough metal to make extraction worthwhile.
Many metal ores are oxides, e.g. hematite (iron ore) and bauxite (aluminium ore).
Extraction is a reduction process because oxygen is removed from the metal compound.

Why is the extraction of a metal from its oxide ore a reduction process?

Oxygen is added to the metal
Oxygen is removed from the metal compound
The metal gains electrons
The metal loses electrons

Reactivity Series and Extraction Method

The position of a metal in the reactivity series determines how it is extracted.
Metals above carbon (e.g. aluminium) must be extracted by electrolysis.
Metals below carbon (e.g. iron) can be extracted by heating with carbon or carbon monoxide.
Very unreactive metals (e.g. gold) are found native as uncombined elements.

Which of these statements about extraction methods are true?

Aluminium is extracted by electrolysis because it is above carbon in the reactivity series.
Iron is extracted by heating with carbon because it is below carbon in the reactivity series.
Gold is extracted by electrolysis.
Metals above carbon can be extracted by reduction with carbon.

Extraction of Iron: The Blast Furnace

Iron is extracted from hematite (Fe₂O₃) in a blast furnace.
Raw materials: iron ore, coke (carbon), and limestone are added at the top.
Hot air is blown into the bottom.
Coke burns to produce heat and carbon dioxide; carbon dioxide reacts with coke to form carbon monoxide, the main reducing agent.

In the blast furnace, the main reducing agent is ____.

Reactions in the Blast Furnace (Extended)

Zone 1: C + O₂ → CO₂ (exothermic, provides heat).
Zone 2: CO₂ + C → 2CO (reduction of CO₂ to CO).
Zone 3: Fe₂O₃ + 3CO → 2Fe + 3CO₂ (reduction of iron ore).
Limestone decomposes: CaCO₃ → CaO + CO₂.
CaO reacts with SiO₂ impurity to form slag: CaO + SiO₂ → CaSiO₃.

Which reaction in the blast furnace is exothermic and provides heat?

C + O₂ → CO₂
CO₂ + C → 2CO
Fe₂O₃ + 3CO → 2Fe + 3CO₂
CaCO₃ → CaO + CO₂

Extraction of Aluminium: Electrolysis

Aluminium is extracted from bauxite (Al₂O₃) by electrolysis.
Aluminium oxide is dissolved in molten cryolite to lower the melting point and reduce energy costs.
The mixture is electrolysed in a cell with graphite electrodes.
At the cathode (negative): Al³⁺ + 3e⁻ → Al (reduction).
At the anode (positive): 2O²⁻ → O₂ + 4e⁻ (oxidation).

Why is aluminium oxide dissolved in molten cryolite before electrolysis?

To increase the conductivity
To lower the melting point and save energy
To prevent oxidation of aluminium
To make the aluminium oxide more reactive

At the cathode, aluminium ions are reduced: Al³⁺ + 3e⁻ → ____.

Anode Reactions and Cost

At the anode, oxygen is produced: 2O²⁻ → O₂ + 4e⁻.
The oxygen reacts with the graphite anode to form CO₂, causing the anode to wear away and need regular replacement.
Electrolysis requires a large amount of electricity, making aluminium extraction expensive.

Which of these are true about aluminium extraction?

The anode is made of graphite and wears away over time.
Aluminium is extracted by reduction with carbon.
Cryolite is used to increase the melting point of aluminium oxide.
The overall reaction is 2Al₂O₃ → 4Al + 3O₂.

Summary: Extraction Methods

Metals above carbon (e.g. Al): extracted by electrolysis.
Metals below carbon (e.g. Fe): extracted by reduction with carbon/carbon monoxide.
Very unreactive metals (e.g. Au): found native.
Iron extraction uses a blast furnace; aluminium extraction uses electrolysis with cryolite.

Which metal is extracted by reduction with carbon?

Aluminium
Iron
Potassium
Gold

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