Extraction of Metals (Overview)

Sources of Metals

Metals are usually found in the Earth's crust combined with other elements as compounds called ores. A metal ore is a rock that contains enough metal compounds to make it economically worthwhile to extract the metal.

Common metal ores include:

• Haematite (iron oxide, Fe₂O₃) – source of iron
• Bauxite (aluminium oxide, Al₂O₃) – source of aluminium
• Chalcopyrite (copper iron sulfide, CuFeS₂) – source of copper
• Galena (lead sulfide, PbS) – source of lead

Mining involves digging the ore out of the ground. There are two main methods:

• Surface mining – removing soil and rock above the ore (used when ore is close to the surface)
• Underground mining – digging tunnels to reach ore deep underground

Methods of Extraction

The method used to extract a metal depends on its reactivity and the type of ore. The main methods are:

• Reduction with carbon – used for metals less reactive than carbon, such as iron
• Electrolysis – used for metals more reactive than carbon, such as aluminium and sodium. During electrolysis, metal ions are reduced at the cathode to form the pure metal, while oxygen is produced at the anode.
• Phytomining and bioleaching – newer, environmentally friendly methods that use plants or bacteria to extract metals from low-grade ores or waste

Reduction with carbon involves heating the metal ore with carbon (usually as coke). Carbon reacts with the oxygen in the metal oxide to form carbon dioxide, leaving the pure metal behind.

Electrolysis involves passing an electric current through molten or dissolved metal compounds to separate the metal from its ore.

Phytomining uses certain plants that absorb metal compounds from the soil. These plants are harvested and burned to produce ash containing metal compounds, which can then be processed.

Bioleaching uses bacteria to convert metal compounds in the ore into soluble forms, which can then be extracted from the solution.

Reactivity and Extraction

The reactivity series ranks metals from most reactive to least reactive. This series helps predict how metals can be extracted:

• Metals less reactive than carbon (e.g., iron, zinc, lead) can be extracted by reduction with carbon.
• Metals more reactive than carbon (e.g., aluminium, sodium, calcium) cannot be extracted by carbon reduction and require electrolysis.

This is because carbon can only remove oxygen from metal oxides if the metal is less reactive than carbon itself.

For example, iron oxide can be reduced by carbon:

$$\text{Fe}_2\text{O}_3 + 3\text{C} \rightarrow 2\text{Fe} + 3\text{CO}$$

But aluminium oxide requires electrolysis because aluminium is more reactive than carbon.

For instance, if you want to extract iron from haematite (Fe₂O₃), carbon monoxide (produced from heated coke) reduces the iron oxide to iron:

Carbon monoxide acts as a reducing agent by reacting with the oxygen in iron oxide to form carbon dioxide, leaving behind pure iron.

$$\text{Fe}_2\text{O}_3 + 3\text{CO} \rightarrow 2\text{Fe} + 3\text{CO}_2$$

Environmental and Economic Factors

Extracting metals requires energy and resources, so environmental and economic factors are important:

• Energy costs: Electrolysis uses a lot of electricity, making it expensive. Carbon reduction is cheaper but only works for less reactive metals.
• Environmental impact: Mining damages landscapes, causes habitat loss, and creates waste rock and tailings. Carbon reduction produces carbon dioxide, contributing to climate change. Additionally, mining can release sulfur dioxide causing acid rain and heavy metals that contaminate soil and water.
• Sustainability: New methods like phytomining and bioleaching aim to reduce environmental damage and use lower-grade ores or waste materials.

Choosing the extraction method balances cost, energy use, and environmental harm.

Learning Example

Calculate the mass of iron produced when 160 g of iron(III) oxide (Fe₂O₃) is reduced by carbon.

Step 1: Write the balanced equation:

$$\text{Fe}_2\text{O}_3 + 3\text{C} \rightarrow 2\text{Fe} + 3\text{CO}$$

Step 2: Calculate the molar mass of Fe₂O₃:

Fe = 56 g/mol, O = 16 g/mol

Molar mass = (2 × 56) + (3 × 16) = 112 + 48 = 160 g/mol

Step 3: Calculate moles of Fe₂O₃:

$\frac{160 \text{ g}}{160 \text{ g/mol}} = 1 \text{ mole}$

Step 4: From the equation, 1 mole Fe₂O₃ produces 2 moles Fe.

Step 5: Calculate mass of iron:

Mass = moles × molar mass = $2 \times 56 = 112 \text{ g}$

So, 160 g of iron oxide produces 112 g of iron.