Extracting metals from ores

Oxidation is the gain of oxygen; reduction is the loss of oxygen. These definitions describe what happens to a particular substance during a reaction.

When a metal reacts with oxygen to form a metal oxide, the metal gains oxygen and is oxidised. To obtain the metal again, oxygen must be removed from its oxide: the metal oxide is reduced.

Extraction of a metal from its ore involves reduction. For oxide ores, this is especially easy to picture: oxygen is taken away from the metal compound, leaving the metal. The oxygen does not disappear; it becomes part of another substance.

When carbon removes oxygen from a metal oxide, the metal oxide loses oxygen and is reduced, while the carbon gains oxygen and is oxidised. Carbon acts as the reducing agent, meaning the substance that causes the other substance to be reduced.

Why carbon works for iron but not aluminium

The reactivity series places metals in order of reactivity. Carbon is not a metal, but its position in the series gives a useful dividing line for extraction.

A metal below carbon is less reactive than carbon. Its oxide can be reduced by heating it with carbon. Iron is below carbon, so carbon-based reduction can be used to extract iron from iron oxide. Carbon is relatively cheap and can also provide heat when burned, making this an economical route. The detailed operation of a blast furnace is not needed here.

A metal above carbon is more reactive than carbon. Carbon cannot remove oxygen from its oxide, so heating with carbon will not extract it. Aluminium is above carbon and is extracted by electrolysis instead.

Electrolysis uses an electric current to break down an ionic compound. For extraction of these reactive metals, the compound must be molten, or dissolved in a molten electrolyte, so that its ions can move. Aluminium is obtained by electrolysis of aluminium oxide dissolved in a molten electrolyte. The metal ions are converted into metal: this is also reduction, although the extraction method is called electrolysis.

The choice is therefore not simply to use the cheapest process. The process must work chemically first. Carbon reduction is generally cheaper, but it cannot replace electrolysis for aluminium.

Biological routes for low-grade ores — Higher tier

A low-grade ore contains a relatively small proportion of the useful metal. Conventional extraction may cost too much for the amount recovered. Biological methods can recover metals such as copper from low-grade material and mining waste, with less need for extensive digging and movement of rock.

Bacterial extraction, also called bioleaching, uses bacteria to help break down ores. This produces an acidic solution called a leachate, containing dissolved metal ions. The solution contains ions, not pieces of pure metal. The metal must then be recovered, for example by displacement with a more reactive metal or by electrolysis.

Phytoextraction uses plants grown in soil containing the required metal. They absorb metal ions through their roots, and metal compounds become concentrated in parts of the plants. The plants are harvested, dried and burned. Their ash contains metal compounds from which the metal can then be recovered by further processing, such as displacement or electrolysis.

Weighing the benefits and limitations — Higher tier

Both biological methods can make low-grade material useful and reduce the environmental damage associated with conventional mining. They can also help recover metals from mining waste rather than leaving those metals unused.

However, both methods are slow. Plants need time to grow, and bacterial action takes time. Neither method produces pure metal in a single step, so further processing adds costs and may require electricity.

Bioleaching does not require high temperatures, reducing the energy needed for that stage. Its acidic, metal-containing solutions can nevertheless contaminate the environment and must be carefully contained and treated. Phytoextraction requires harvesting and burning the plants, so it is not an energy-free process either.

A biological route may therefore be worthwhile when the material has too little metal for conventional extraction to be economical and rapid production is not essential. Its lower mining impact must be balanced against its slow speed, further processing and any pollution-control costs.

• Below carbon — iron: oxide can be reduced using carbon-based reducing agents. Carbon is relatively cheap and can supply heat.
• Above carbon — aluminium: carbon cannot reduce its oxide; electrolysis is needed.
• Electrolysis uses large amounts of electricity, making it expensive. It also involves reduction.
• Choose a process that works chemically, then consider its cost.

Biological extraction — Higher tier

• Bacterial extraction: bacteria act on ore → metal ions in leachate → metal recovered by displacement or electrolysis.
• Phytoextraction: plants absorb metal ions → harvested and burned → metal compounds in ash → further extraction.
• Benefits: useful for low-grade material and mining waste; less conventional mining damage.
• Limitations: slow; further processing needed; bioleaching solutions may pollute; harvesting, burning and recovery still require resources.