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Extraction and uses of metals
Some unreactive metals such as gold are often found as the uncombined element, but most metals are extracted from ores found in the Earth’s crust. An ore is a type of rock containing compounds from which metals can be extracted.
Ores are typically metal oxides or a compound of a metal that can be converted to an oxide. For example, the main aluminium ore is called bauxite which is aluminium oxide (Al2O3).
The metal is removed from the oxide in a process called reduction. In a reduction reaction, oxygen is removed to extract the metal from the oxide. Metal oxides are ionic compounds. The metal ions contained in the metal oxide are positively charged and must therefore gain electrons (be reduced) to form the neutral metal atoms.
The method used to extract the metal from the metal oxide is related to the position of the metal in the reactivity series and may also be determined by factors such as the cost of the energy needed and cost of reducing agent used.
Carbon extraction
Any metals which are lower than carbon in the reactivity series can be extracted using reduction where carbon is used as the reducing agent. Although other methods such as electrolysis could also be used for these metals, reduction using carbon is the cheapest method.
One common use of carbon reduction is the extraction of iron from iron(III) oxide (Fe2O3) or haematite. This method can be used to extract iron because carbon is above iron in the reactivity series and can therefore displace the iron from iron oxide. This process is completed inside a blast furnace where the high temperatures created provide the energy needed to melt the ore and break the ionic bonds holding the iron(III) oxide together. Materials involved in the reaction are:
- Iron ore
- Coke (carbon): this produces heat by burning in the air. It reacts to form carbon monoxide
- Limestone (calcium carbonate): this reacts with acidic impurities from the iron. The furnace is very hot causing the limestone to split into calcium oxide and carbon dioxide. Calcium oxide reacts with impure acidic oxides such as silicon dioxide. The product formed is calcium silicate which melts and flows to the bottom as a molten slag. This floats on top of the molten iron and can be tapped off separately
The iron(III) oxide reacts with the carbon in the hottest parts of the blast furnace. The carbon is oxidised to carbon dioxide and the iron(III) ions are reduced to iron metal atoms. The equation for this reaction is:
![\[2Fe_2O_{3(l)} + 3CO_{(g)} \rightarrow 4Fe_{(s)} + 3CO_{2(g)}\]](data:image/svg+xml;nitro-empty-id=NDUzOjM3OTY=-1;base64,PHN2ZyB2aWV3Qm94PSIwIDAgMzI4IDIxIiB3aWR0aD0iMzI4IiBoZWlnaHQ9IjIxIiB4bWxucz0iaHR0cDovL3d3dy53My5vcmcvMjAwMC9zdmciPjwvc3ZnPg== "Rendered by QuickLaTeX.com")
Carbon monoxide could also be used as a reducing agent instead of carbon.
The equation for this reaction is:
![\[Fe_2O_{3(l)} + 3CO_{(g)} \rightarrow 2Fe_{(s)} + 3CO_{2(g)}\]](data:image/svg+xml;nitro-empty-id=NDUzOjQyMDE=-1;base64,PHN2ZyB2aWV3Qm94PSIwIDAgMzE5IDIxIiB3aWR0aD0iMzE5IiBoZWlnaHQ9IjIxIiB4bWxucz0iaHR0cDovL3d3dy53My5vcmcvMjAwMC9zdmciPjwvc3ZnPg== "Rendered by QuickLaTeX.com")
Electrolysis
Metals which are above carbon in the reactivity series cannot be reduced using carbon, as the carbon would be less reactive than them. Metals such as aluminium and magnesium must be extracted through an alternative process known as electrolysis.
Electrolysis uses electricity to split ionic compounds into their individual elements and is an expensive process due to the amount of energy needed. Aluminium is commonly extracted through electrolysis.
The main ore from which aluminium is extracted is aluminium oxide or bauxite (Al2O3). Bauxite contains aluminium oxide along with impurities. Before the aluminium oxide can be electrolysed, it must be melted down so that the ions are free to move.
The melting point of bauxite is too high for it to just be melted down for electrolysis. To reduce the amount of energy needed to melt the bauxite, it is added to a substance known as cryolite. The addition of cryolite reduces the temperature needed to melt the ore.
The high amount of energy needed, the addition of cryolite, and the electricity needed for the electrolysis makes the extraction of aluminium a very costly process.
Once the aluminium oxide is molten the aluminium ions and oxide ions are free to move to the oppositely charged electrodes. The aluminium ions move towards the cathode where they gain electrons and are reduced to aluminium. The ionic half-equation for the reaction at the cathode is:
![\[Al^{3+} + 3c^- \rightarrow Al\]](data:image/svg+xml;nitro-empty-id=NDUzOjYwMTA=-1;base64,PHN2ZyB2aWV3Qm94PSIwIDAgMTQwIDIwIiB3aWR0aD0iMTQwIiBoZWlnaHQ9IjIwIiB4bWxucz0iaHR0cDovL3d3dy53My5vcmcvMjAwMC9zdmciPjwvc3ZnPg== "Rendered by QuickLaTeX.com")
The oxide ions move towards the anode, where they each lose two electrons forming oxygen atoms. The oxygen atoms then pair up to form diatomic molecules of oxygen gas, as shown by the ionic half-equation:
![\[2O^{2-} \rightarrow O_2 + 2e^-\]](data:image/svg+xml;nitro-empty-id=NDUzOjY0NzY=-1;base64,PHN2ZyB2aWV3Qm94PSIwIDAgMTQ4IDIxIiB3aWR0aD0iMTQ4IiBoZWlnaHQ9IjIxIiB4bWxucz0iaHR0cDovL3d3dy53My5vcmcvMjAwMC9zdmciPjwvc3ZnPg== "Rendered by QuickLaTeX.com")
The carbon anodes burn in the oxygen creating carbon dioxide and so need replacing frequently. This is another factor which increases the cost of aluminium extraction through electrolysis.
Metals which can only be extracted using electrolysis:
- Potassium
- Sodium
- Calcium
- Magnesium
- Aluminium
Metals which can be extracted by reduction:
- Zinc
- Iron
- Tin
- Copper
Uses of the metals
Each metal can be used for different purposes depending upon its properties. Some metals can be used on their own as the element and others can be combined with other elements to form alloys.
An alloy is a mixture of a metal with one or more other elements such as carbon or other metals. As we saw in the metallic bonding section, the atoms of carbon or other metals are different sizes to those of the pure metal. This difference in the sizes of the atoms disrupts the regular and layered arrangement of the metal ions in the metallic element and makes it more difficult for the layers of metal ions to slide over each other. This makes the metallic structure stronger. Alloys are therefore stronger and harder than the pure metals themselves.
- Aluminium – aluminium is soft and malleable and has a low density, making it very useful for products such as tin cans and kitchen foil. Aluminium also has high electrical conductivity and is lightweight, making it suitable for electrical transmission lines. As pure aluminium is not very strong, it is often combined with elements such as copper, manganese, magnesium and silicon to form alloys. Alloys of aluminium are stronger than aluminium itself but are still lightweight. This makes them ideal for car and aeroplane bodies and engine parts
- Copper – copper has excellent thermal and electrical conductivity. It is also ductile so can easily be drawn into wires. Most copper is used in electrical wires and heating pipes. Copper is also relatively unreactive so is often used in water pipes in central heating systems as it doesn’t rust. As copper is so unreactive, it is used as a coating for coins to prevent them from rusting
- Iron – pure iron is very soft due to the regular arrangement of its ions in the metallic structure. The layers of ions can easily slide over each other. As pure iron is too soft for many uses, other elements such as carbon are often added to increase its strength and make it useful
- Wrought iron has a very low carbon content (about 0.08%). It is relatively strong and corrosion resistant and is often used in structures such as garden gates or decorative statues.
- Cast iron contains a higher carbon content than wrought iron at over 2%. It is hard and brittle so can break if it is handled with heavy force. It is often used for kitchen pans, bathtubs and decorative items as it is corrosion resistant and hard.
- Steel – there are three types of steel which you must be aware of
- Low-carbon (mild) steel. This type of steel contains 0.25% carbon. It is strong but easily shaped and is commonly used to make vehicle body panels.
- High-carbon steel. This type of steel typically contains up to 2.5% carbon. It is hard and easily shaped so is often used to make cutting tools.
- Stainless steel. This type of steel contains iron, chromium and nickel. This alloy is strong and resistant to corrosion so is often used to make kitchen sinks and cutlery.
