Every time you plug in a charging cable, you are relying on copper that had to be extracted from deep within the Earth's crust. Most metals do not exist in their pure form naturally; instead, they are found as compounds within an ore. The method we use to extract a metal from its ore depends entirely on its position in the reactivity series compared to carbon.
Carbon reduction is always the preferred method in industry whenever possible because it is significantly cheaper. It avoids the massive electrical costs associated with electrolysis, and the carbon itself (usually in the form of coke or charcoal) is an abundant, low-cost material.
Extracting metals using carbon is a redox reaction. This means that reduction and oxidation happen simultaneously.
During extraction, carbon acts as the reducing agent. It has a higher affinity for oxygen than the less reactive metals, so it 'steals' the oxygen from the metal oxide. The metal oxide is reduced to form the pure metal, while the carbon is oxidised to form carbon dioxide or carbon monoxide. These reactions are highly exothermic, often producing a visible red glow when performed in a laboratory setting (such as heating copper oxide with charcoal).
Iron is extracted in a blast furnace, which is a continuous process. Raw materials are fed into the top, and molten products are continuously tapped off at the bottom. The furnace is hottest at the base (around 2000°C) where hot air is blown in, and cooler at the top (around 1000°C).
The raw materials (the 'charge') added to the furnace are:
The extraction involves several key chemical reactions. First, coke burns in the hot air to provide heat (exothermic) and carbon dioxide:
This carbon dioxide then reacts with more coke to form carbon monoxide, the primary reducing agent:
Next, the carbon monoxide reduces the hematite to produce molten iron:
Finally, the limestone undergoes thermal decomposition and reacts with the sandy impurities to form a molten waste product called slag (calcium silicate). Slag is less dense than iron, so it floats on top of the molten iron and is tapped off separately.
Non-ferrous metals are those that do not contain significant amounts of iron. Examples include copper and zinc, which both sit below carbon in the reactivity series and can be extracted via smelting.
Copper Extraction: Copper is often found in sulfide ores (e.g., chalcocite). The extraction is a two-stage process:
Zinc Extraction: Zinc is extracted from zinc blende (zinc sulfide) using a very similar two-stage method:
When evaluating carbon extraction, chemists must weigh its chemical efficiency and low cost against significant environmental impacts.
To improve sustainability, alternative methods are increasingly used. Recycling metals preserves finite ore reserves and uses a fraction of the energy. For low-grade ores, bioleaching uses bacteria to produce a leachate solution containing metal ions. Alternatively, phytoextraction uses plants like Alpine Penny-cress to absorb metal ions from the soil; the plants are then burned to harvest a metal-rich ash. Both biological methods are environmentally friendly, but exceptionally slow.
Students often state that carbon can extract all metals. It cannot; carbon can only displace metals that are less reactive (below it) in the reactivity series.
In blast furnace equations, examiners often look for specific state symbols. Ensure you use (l) for both the molten iron and the molten slag at the bottom of the furnace.
During the extraction of zinc, the state symbol for zinc is (g) because the furnace temperature is higher than zinc's boiling point.
When a question asks you to 'Discuss' the environmental impact, you must explicitly state that CO2 causes global warming and SO2 causes acid rain to secure full marks.
If asked for an example of a plant used in phytoextraction, mentioning 'Alpine Penny-cress' will secure highly specific OCR marks.
Don't forget the first step in the blast furnace: the combustion of coke is vital because it provides the heat necessary for the other endothermic reactions to occur.
Ore
A rock or mineral that contains enough metal or metal compound to make it economically worthwhile to extract the metal.
Reactivity series
A list of metals arranged in order of their chemical reactivity, used to predict how they will react and how they can be extracted.
Oxidation
The gain of oxygen by a substance or the loss of electrons.
Reduction
The loss of oxygen from a substance or the gain of electrons.
Reducing agent
A substance that removes oxygen from another substance, becoming oxidised in the process.
Redox reaction
A chemical reaction in which both reduction and oxidation occur simultaneously.
Hematite
The main iron ore used in the blast furnace, consisting primarily of iron(III) oxide.
Coke
An impure, carbon-rich solid used as both a fuel and a reducing agent in the blast furnace.
Limestone
Calcium carbonate, added to the blast furnace to remove sandy impurities.
Thermal decomposition
The breakdown of a single compound into two or more simpler substances using heat.
Slag
The molten waste product (calcium silicate) formed in the blast furnace when limestone reacts with sandy impurities.
Non-ferrous
A metal or alloy that does not contain a significant amount of iron.
Smelting
The process of extracting a metal from its ore by heating and melting it, typically with carbon as a reducing agent.
Roasting
Heating a sulfide ore strongly in an excess of air to convert it into a metal oxide and sulfur dioxide.
Sustainability
Meeting the needs of the present population without compromising the ability of future generations to meet their own needs.
Bioleaching
The use of bacteria to slowly extract metals from low-grade ores, producing a solution of metal ions.
Leachate
The solution produced by bioleaching that contains dissolved metal ions.
Phytoextraction
The use of plants to absorb metal ions from contaminated soil; the plants are then harvested and burned to extract the metal.
Put your knowledge into practice — try past paper questions for Chemistry A
Ore
A rock or mineral that contains enough metal or metal compound to make it economically worthwhile to extract the metal.
Reactivity series
A list of metals arranged in order of their chemical reactivity, used to predict how they will react and how they can be extracted.
Oxidation
The gain of oxygen by a substance or the loss of electrons.
Reduction
The loss of oxygen from a substance or the gain of electrons.
Reducing agent
A substance that removes oxygen from another substance, becoming oxidised in the process.
Redox reaction
A chemical reaction in which both reduction and oxidation occur simultaneously.
Hematite
The main iron ore used in the blast furnace, consisting primarily of iron(III) oxide.
Coke
An impure, carbon-rich solid used as both a fuel and a reducing agent in the blast furnace.
Limestone
Calcium carbonate, added to the blast furnace to remove sandy impurities.
Thermal decomposition
The breakdown of a single compound into two or more simpler substances using heat.
Slag
The molten waste product (calcium silicate) formed in the blast furnace when limestone reacts with sandy impurities.
Non-ferrous
A metal or alloy that does not contain a significant amount of iron.
Smelting
The process of extracting a metal from its ore by heating and melting it, typically with carbon as a reducing agent.
Roasting
Heating a sulfide ore strongly in an excess of air to convert it into a metal oxide and sulfur dioxide.
Sustainability
Meeting the needs of the present population without compromising the ability of future generations to meet their own needs.
Bioleaching
The use of bacteria to slowly extract metals from low-grade ores, producing a solution of metal ions.
Leachate
The solution produced by bioleaching that contains dissolved metal ions.
Phytoextraction
The use of plants to absorb metal ions from contaminated soil; the plants are then harvested and burned to extract the metal.