Extraction of metals Revision Notes

The reactivity series ranks metals according to their tendency to lose electrons and form positive ions (cations). Metals higher in the series, such as potassium, sodium, calcium, and aluminium, are more reactive and form more stable compounds (ores). This stability means these metals are chemically bonded strongly to other elements, making them harder to extract.

Less reactive metals like copper, silver, and gold form less stable compounds or occur in native form, making extraction easier. The position in the reactivity series directly influences the choice of extraction method and the energy required.

• Reactivity series orders metals by reactivity: K > Na > Ca > Mg > Al > Zn > Fe > Pb > Cu > Ag > Au.
• Higher reactivity means stronger bonding in ores and greater difficulty in extraction.
• Lower reactivity metals often occur as native metals or in easily reduced ores.

Metals low in the reactivity series, such as copper, silver, and gold, can be extracted by simple heating or chemical reduction because their compounds are less stable.

Metals of intermediate reactivity, like iron and zinc, are commonly extracted by reduction with carbon (coke). Carbon acts as a reducing agent, removing oxygen from metal oxides to yield the metal.

Highly reactive metals, such as aluminium, calcium, sodium, and potassium, cannot be extracted by carbon reduction because their oxides are too stable. Instead, these metals require electrolysis of molten compounds to be extracted.

• Low reactivity metals: extraction by heating or chemical reduction.
• Intermediate reactivity metals: extraction by reduction with carbon.
• High reactivity metals: extraction by electrolysis of molten ores.

Iron, moderately reactive, is extracted from iron(III) oxide (Fe2O3) by reduction with carbon monoxide in a blast furnace.

The reaction is: Fe2O3(s) + 3CO(g) → 2Fe(l) + 3CO2(g)

Aluminium, highly reactive, is extracted by electrolysis of molten aluminium oxide (Al2O3) dissolved in cryolite to lower its melting point. Carbon electrodes are used, and the reactions at the electrodes are: At cathode: Al3+(l) + 3e− → Al(l) At anode: 2O2−(l) → O2(g) + 4e−

• Iron extraction uses carbon monoxide reduction at ~1500°C.
• Aluminium extraction requires electrolysis due to high reactivity.
• Electrolysis is energy-intensive but necessary for reactive metals.

Reduction of iron(III) oxide by carbon monoxide: Fe2O3(s) + 3CO(g) → 2Fe(l) + 3CO2(g)

Electrolysis of aluminium oxide: At cathode: Al3+(l) + 3e− → Al(l) (molten aluminium forms) At anode: 2O2−(l) → O2(g) + 4e− (oxygen gas bubbles observed)

Observations during extraction include the formation of molten metal, gas evolution (CO2 or O2), and slag formation to remove impurities.

• Iron extraction produces molten iron and carbon dioxide gas.
• Aluminium electrolysis produces molten aluminium and oxygen gas bubbles.
• Slag forms from impurities and floats on molten metal.

Consider sodium (Na) and iron (Fe). Sodium is higher in the reactivity series than iron. Sodium forms very stable compounds like sodium chloride or sodium oxide, which cannot be reduced by carbon.

Therefore, sodium extraction requires electrolysis of molten sodium chloride.

Iron oxide is less stable and can be reduced by carbon monoxide in a blast furnace, a less energy-intensive method than electrolysis.

• Sodium extraction: electrolysis of molten NaCl.
• Iron extraction: reduction of Fe2O3 by CO.
• Higher reactivity metals require more energy-intensive extraction.

In the blast furnace, coke, a form of carbon, is burnt in the presence of hot air to generate the high temperatures (around 1500°C) necessary for the extraction of iron. This combustion reaction produces carbon dioxide and releases heat.

The reaction is: C(s) + O2(g) → CO2(g) (combustion of carbon)

This exothermic reaction maintains the furnace temperature and produces carbon dioxide gas, which plays a further role in the extraction process. The coke also acts as a reducing agent later in the furnace.

• Coke is mainly carbon and burns in oxygen from hot air blast.
• Produces carbon dioxide and heat to maintain furnace temperature.
• Reaction: C + O2 → CO2 (no symbol equation required but useful for understanding).

The carbon dioxide formed in the upper part of the furnace reacts with more coke at high temperatures to form carbon monoxide. This reaction is endothermic and occurs at around 1000°C.

The carbon monoxide formed is the main reducing agent that converts iron(III) oxide to iron.

The reaction is: CO2(g) + C(s) → 2CO(g) (carbon dioxide reacts with coke to form carbon monoxide)

• CO2 reacts with excess coke to produce CO gas.
• CO is a more effective reducing agent than CO2.
• This reaction helps maintain a supply of CO for iron oxide reduction.

Hematite ore contains iron(III) oxide (Fe2O3). In the blast furnace, carbon monoxide reduces iron(III) oxide to molten iron by removing oxygen atoms.

This reduction occurs in the middle to lower parts of the furnace where temperatures are suitable for the reaction.

The reaction is: Fe2O3(s) + 3CO(g) → 2Fe(l) + 3CO2(g) (reduction of iron oxide to iron)

• CO removes oxygen from Fe2O3, producing iron and CO2.
• Molten iron collects at the bottom of the furnace.
• CO2 formed can cycle back to react with coke, sustaining CO production.

Limestone (calcium carbonate, CaCO3) is added to the blast furnace to remove impurities from the ore.

At high temperatures, calcium carbonate thermally decomposes to calcium oxide and carbon dioxide.

The reaction is: CaCO3(s) → CaO(s) + CO2(g) (thermal decomposition of limestone)

• Limestone decomposes at furnace temperatures (~900°C).
• Produces calcium oxide, which reacts with impurities.
• CO2 released can contribute to CO formation.

The calcium oxide produced reacts with silica (SiO2), an impurity present in the ore, to form calcium silicate (CaSiO3), known as slag.

Slag is molten and less dense than iron, so it floats on top and can be removed easily.

This process removes impurities and protects the iron from contamination.

• CaO reacts with SiO2 impurities to form slag (CaSiO3).
• Slag floats on molten iron and is removed separately.
• Slag formation improves iron purity and furnace efficiency.

Bauxite is the main ore from which aluminium is extracted. It is a mixture of aluminium oxides and hydroxides, primarily containing aluminium oxide (Al₂O₃) along with impurities such as iron oxides and silica.

Bauxite is mined from open pits and then purified to obtain aluminium oxide, also called alumina, which is the compound used in the extraction process.

The purification process removes impurities but is not covered in detail here.

• Bauxite contains aluminium oxide (Al₂O₃) and impurities.
• It is the main source of aluminium metal.
• Purification yields pure aluminium oxide for electrolysis.

Aluminium is extracted from aluminium oxide by electrolysis. Aluminium oxide has a very high melting point (over 2000°C), so it is dissolved in molten cryolite (Na₃AlF₆) to lower the melting point to about 900°C, making the process more energy-efficient.

An electric current is passed through the molten mixture, causing aluminium ions to gain electrons (reduction) at the cathode and oxygen ions to lose electrons (oxidation) at the anode.

• Aluminium oxide is dissolved in molten cryolite to lower melting point.
• Electrolysis involves passing electric current through molten mixture.
• Aluminium ions are reduced to aluminium metal at the cathode.
• Oxygen ions are oxidised to oxygen gas at the anode.

The electrolytic cell contains a carbon-lined steel container acting as the cathode (negative electrode). Carbon anodes (positive electrodes) are immersed in the molten mixture.

At the cathode, aluminium ions (Al³⁺) gain electrons to form liquid aluminium, which collects at the bottom of the cell and is periodically removed.

At the anode, oxide ions (O²⁻) lose electrons to form oxygen gas, which reacts with the carbon anode to produce carbon dioxide, causing the anodes to gradually wear away and require replacement.

• Cathode: carbon-lined container; anode: carbon blocks.
• Cathode reaction: Al³⁺ + 3e⁻ → Al (liquid)
• Anode reaction: 2O²⁻ → O₂ + 4e⁻
• Oxygen reacts with carbon anode: C + O₂ → CO₂
• Carbon anodes are consumed and replaced regularly.

The half-equations at the electrodes are: at the cathode, aluminium ions are reduced; at the anode, oxide ions are oxidised.

The overall reaction shows aluminium oxide decomposing into aluminium metal and oxygen gas, with carbon from the anode reacting to form carbon dioxide.

• Cathode (reduction): Al³⁺ + 3e⁻ → Al
• Anode (oxidation): 2O²⁻ → O₂ + 4e⁻
• Anode carbon reaction: C + O₂ → CO₂
• Overall reaction: 2Al₂O₃ + 3C → 4Al + 3CO₂

The electrolysis process requires a large amount of electrical energy, making it expensive.

Cryolite lowers the melting point of aluminium oxide, reducing energy consumption.

Carbon anodes must be replaced regularly due to reaction with oxygen gas producing CO₂, which gradually consumes the anodes.

• High electrical energy requirement.
• Cryolite reduces melting point and energy use.
• Carbon anodes wear out and need replacement.
• Molten aluminium is periodically tapped from the cell.

The extraction of iron from hematite (Fe2O3) in a blast furnace begins with the combustion of carbon (in the form of coke) in the presence of oxygen. This reaction provides the high temperatures required for the process and produces carbon dioxide (CO2).

The balanced symbol equation is: C(s) + O2(g) → CO2(g)

This exothermic reaction releases heat, raising the temperature inside the furnace to about 1500°C, necessary for subsequent reactions.

• Carbon (coke) burns in oxygen to form carbon dioxide.
• This reaction supplies heat for the blast furnace.
• Equation: C + O2 → CO2

The carbon dioxide produced reacts further with more carbon at high temperatures to form carbon monoxide (CO). This reaction is crucial because carbon monoxide acts as the reducing agent that converts iron ore into iron.

The balanced symbol equation is: C(s) + CO2(g) → 2CO(g)

Carbon monoxide is a more effective reducing agent than carbon dioxide and is essential for the reduction of iron(III) oxide.

• Carbon dioxide reacts with carbon to form carbon monoxide.
• Carbon monoxide reduces iron ore to iron.
• Equation: C + CO2 → 2CO

The main chemical reaction in the extraction of iron is the reduction of iron(III) oxide (hematite) by carbon monoxide. This reaction removes oxygen from the ore, producing molten iron and carbon dioxide.

The balanced symbol equation is: Fe2O3(s) + 3CO(g) → 2Fe(l) + 3CO2(g)

Iron produced is molten due to the high temperature and collects at the bottom of the blast furnace. Carbon dioxide formed is recycled in the furnace reactions.

• Iron(III) oxide is reduced to iron by carbon monoxide.
• Oxygen is removed from the ore, producing CO2.
• Equation: Fe2O3 + 3CO → 2Fe + 3CO2

Calcium carbonate (limestone) is added to the blast furnace to remove impurities. It thermally decomposes under heat to form calcium oxide (quicklime) and carbon dioxide.

The balanced symbol equation is: CaCO3(s) → CaO(s) + CO2(g)

This reaction occurs at temperatures above 825°C and provides calcium oxide for slag formation.

• Limestone decomposes to calcium oxide and carbon dioxide.
• Calcium oxide reacts with impurities to form slag.
• Equation: CaCO3 → CaO + CO2

Silicon dioxide (sand) is an impurity in the iron ore. Calcium oxide reacts with silicon dioxide to form calcium silicate (slag), which is molten and less dense than iron, allowing it to be removed easily.

The balanced symbol equation is: CaO(s) + SiO2(s) → CaSiO3(l)

Slag floats on top of molten iron and can be drained off, preventing impurities from contaminating the iron.

• Calcium oxide reacts with silicon dioxide to form slag (calcium silicate).
• Slag removes impurities from molten iron.
• Equation: CaO + SiO2 → CaSiO3

Aluminium is extracted from purified bauxite, which mainly consists of aluminium oxide (Al₂O₃). The extraction method used is electrolysis of molten aluminium oxide. Because aluminium is highly reactive, it cannot be extracted by reduction with carbon or other chemical reducing agents. Instead, electrolysis is used to separate aluminium metal from oxygen.

The process requires aluminium oxide to be molten to allow ions to move freely and conduct electricity. However, pure aluminium oxide has a very high melting point (about 2050°C), making direct electrolysis impractical due to the enormous energy required.

• Aluminium is extracted by electrolysis of molten aluminium oxide.
• High melting point of pure aluminium oxide makes direct electrolysis energy-intensive.

To reduce the melting point and improve the conductivity of the electrolyte, aluminium oxide is dissolved in molten cryolite (Na₃AlF₆). Cryolite acts as a solvent and lowers the melting point of the mixture to about 900°C, which significantly reduces energy consumption and cost.

Cryolite also increases the electrical conductivity of the electrolyte, allowing the electrolysis to proceed efficiently at lower temperatures.

• Cryolite lowers the melting point of aluminium oxide from ~2050°C to ~900°C.
• Cryolite increases the electrical conductivity of the electrolyte.
• Lower temperature reduces energy costs and equipment wear.

In the electrolytic cell, carbon anodes and cathodes are used. The carbon anodes serve as the positive electrodes where oxidation occurs. During electrolysis, oxygen ions (O²⁻) from aluminium oxide lose electrons at the anode and react with the carbon anode to form carbon dioxide gas.

This reaction consumes the carbon anodes, causing them to gradually wear away and require regular replacement to maintain the process.

The overall reaction at the anode is: 2O²⁻ → O₂ + 4e⁻, but the oxygen immediately reacts with the carbon anode: C + O₂ → CO₂.

• Carbon anodes are oxidised and consumed during electrolysis.
• Oxygen ions lose electrons and form oxygen gas at the anode.
• Oxygen reacts with carbon anode producing CO₂ gas.
• Anodes must be replaced regularly due to consumption.

During electrolysis, aluminium ions (Al³⁺) are reduced at the cathode (negative electrode) to form aluminium metal. Simultaneously, oxide ions (O²⁻) are oxidised at the anode (positive electrode) to form oxygen gas.

The ionic half-equations are: at the cathode, aluminium ions gain electrons to form aluminium atoms; at the anode, oxide ions lose electrons to form oxygen gas.

These half-equations are essential for understanding the electron transfer processes during extraction.

• Cathode reaction (reduction): Al³⁺ + 3e⁻ → Al (liquid aluminium)
• Anode reaction (oxidation): 2O²⁻ → O₂ + 4e⁻
• Oxygen gas reacts with carbon anode producing CO₂ gas.

The purified aluminium oxide is dissolved in molten cryolite to lower the melting point and increase conductivity. Electrolysis is carried out using carbon electrodes. Aluminium ions are reduced at the cathode to form molten aluminium, which collects at the bottom of the cell.

Oxide ions are oxidised at the carbon anode, producing oxygen gas that reacts with the carbon to form carbon dioxide, consuming the anode.

The process requires continuous replacement of carbon anodes and careful control of temperature and electrolyte composition to maintain efficiency.

• Aluminium oxide dissolved in molten cryolite for electrolysis.
• Aluminium metal forms at cathode and collects as molten aluminium.
• Oxygen forms at anode and reacts with carbon anode producing CO₂.
• Carbon anodes are consumed and replaced regularly.
• Process operates at about 900°C for efficiency.