An ore is a rock that contains enough of a metal or a metal compound to make extracting the metal worthwhile.
Most metals are extracted from ores found in the Earth's crust. Ores are often metal oxides, although sulfide and carbonate ores are also common.
The extraction method used depends upon the metal's position in the reactivity series. In principle, any metal could be extracted from its compounds using electrolysis. However, large amounts of electrical energy are needed to do this, so electrolysis is expensive.
If a metal is less reactive than carbon, it can be extracted from its compounds by heating with carbon. Copper is an example of this. Copper mostly occurs as sulfide ores, which are heated in air to convert them to copper(II) oxide. Molten copper can be produced from copper(II) oxide by heating with carbon:
copper oxide + carbon → copper + carbon dioxide
2CuO(s) + C(s) → 2Cu(l) + CO2(g)
This table summarises the extraction methods used for different metals:
Although an unreactive metal is found as the uncombined element, chemical reactions are often needed to remove other elements that might contaminate it.
Iron is extracted from iron ore in a large container called a blast furnace. Iron ore - known as haematite - contains iron(III) oxide. This is added to the blast furnace along with coke, a form of carbon, and calcium carbonate in the form of limestone.
In the furnace, iron(III) oxide is reduced to molten iron when it reacts with carbon monoxide:
iron(III) oxide + carbon monoxide → iron + carbon dioxide
Fe2O3(s) + 3CO(g) → 2Fe(l) + 3CO2(g)
At the same time, limestone decomposes to form calcium oxide:
calcium carbonate → calcium oxide + carbon dioxide
CaCO3(s) → CaO(s) + CO2(g)
Calcium oxide is a metal oxide, and so it is a base. Calcium oxide reacts with silicon dioxide in the blast furnace. Silicon dioxide is an impurity present in the iron ore. This reaction is a neutralisation reaction because silicon dioxide is a non-metal oxide, so therefore can react as an acid:
calcium oxide + silicon dioxide → calcium silicate
CaO(s) + SiO2(s) → CaSiO3(l)
The product formed is often called 'slag'.
This method of extraction works because carbon is more reactive than iron, so it can displace iron from iron compounds. Extracting a metal by heating with carbon is cheaper than using electrolysis.
Write a balanced equation for the reaction between iron(III) oxide and carbon, forming molten iron and carbon dioxide.
2Fe2O3(s) + 3C(s) → 4Fe(l) + 3CO2(g)