Metals

Reflections and Exam tips

Metals and their extraction

Most metals in the Earth’s crust are found as oxide or carbonate compounds. Rocks that contain a high amount of a metal compound that makes it economical to extract are called metal ores. For example, Iron ore - haematite (iron oxide) and Aluminium ore - bauxite (aluminium oxide). A native metal is a metal that is found in the ground as the pure metal (such as gold).

Mining ores impacts on the local community

Reactivity series

Some metals react more vigorously with air, water and acids than others. The method of extraction depends on the metal’s reactivity.

Iron extraction

Reduction is the process of removing oxygen from an ore.
Iron is produced in a blast furnace where carbon (coke) displaces the oxygen in iron oxide.
Limestone is added to the blast furnace to remove any impurities (changes them into slag).

Raw materials: iron oxide, coke and limestone

Any metal higher in reactivity series can reduce (removal of oxygen) any metal lower down in reactivity series for example, iron can reduce copper oxide).

The iron from the blast furnance is called cast iron (96% iron + impurities) which makes it hard but brittle. To make it stronger it's mixed with other elements to form alloys of steel such as: carbon steels and stainless steel.

Aluminium extraction

Electro -lysis means using electric energy to break down an ionic cmpound.

Aluminium cannot be extracted by heating with carbon, because it is too reactive but is extracted using electrolysis.

Stages of aluminium electrolysis

Aluminium ore (bauxite) is heated (with cryolite to reduce the melting point) until it melts. Aluminium oxide has a higher melting point and cryolite helps reduce the energy cost as the melting point is lowered to around 900°C.
Electricity is passed through the molten ore, which splits the aluminium oxide into 2 charged particles (positive aluminium & negative oxide).
The negatively charged oxide move to the positively charged carbon anodes where they are oxidised. The oxygen produced reacts with the carbon anodes to produce carbon dioxide. Over a period the anodes must be replenished.

2O₂ - ——> O₂ + 4e^-
The positively charged aluminium moves to the negatively charged cathode where are reduced.

Al³⁺ +3e^- ——> Al

Aluminium sinks to the bottom and is collected off.

Properties of aluminium that make it useful for making drink cans,  saucepans, cooking foil, aircraft and bike frames:

1. High electrical and heat conductivity
2. Ductile (shaped into wires)
3. Resistant to corrosion.

Aluminium at the surface reacts with oxygen, (in the air) forming a thin layer of aluminium oxide around it which protects it from further corrosion.

Aluminium is ALLOYED with other elements to improve strength.

Copper extraction

Copper can be extracted using smelting and then purified using electrolysis.

Alternative methods to get low grade copper ore

Bioleaching

Bacteria to feed on low concentration of metal ions. A chemical process then forms a leachate.

Phytomining

Plants absorb small quantities of metal ions through their roots. The plants are burnt as a renewable form producing electricity. If the metal is copper, sulfuric acid is added to make copper sulphate.

In both phytomining and bioleaching, iron is added to displace the copper. Finally electrolysis is used to get pure copper metal.

Useful metals

Properties of transition metals:

1. Good conductors of heat and electricity
2. Strong
3. Malleable
4. Ductile

An alloy is a mixture of metals which locks the atoms in place making the metal harder.

Copper alloys e.g. bronze (copper + tin) and brass (copper + zinc). Brass is harder than bronze which is used for making musical instruments.

Aluminium alloys

Magnox - used in the building of nuclear power plants. 

Duralumin - aircraft and bike frames (lightweight, strong)

Metallic issues

The methods of extraction of mineral ores can have an adverse effect on the environment. For example, open cast mining can cause scarring of the landscape, create noise and air pollution, destroy habitats and leave eye sores on the landscape. The use of heavy polluting machinery can increase global warming issues as well as the washing of chemicals into ground water can destroy wildlife.