OCCURRENCE OF METALS
- The earth’s crust is the major source of metals. Seawater also contains some soluble salts such as sodium chloride, magnesium chloride, etc.
- The elements or compounds, which occur naturally in the earth’s crust, are known as minerals.
- Minerals they contain a very high percentage of a particular metal and the metal can be profitably extracted from it.
- These minerals are called ores.
- Note- The extraction of metals from their ores and then refining them for use is known as metallurgy.
EXTRACTION OF METALS
- Some metals are found in the earth’s crust in the Free State.
- For example, gold, silver, platinum and copper are found in the Free State.
- Some are found in the form of their compounds.
- Copper and silver are also found in the combined state as their sulphide or oxide ores.
- On the basis of reactivity, metals can divided into the three categories –
- Metals of low reactivity;
- Metals of medium reactivity;
- Metals of high reactivity
Steps are involved in the extraction of pure metal from ores.
- Steps are involved in the extraction of pure metal from Ores.
Extraction of metals with low reactivity
- Some Metals are very unreactive. The oxides of these metals can be reduced to metals by heating alone.
- For example-
- Cinnabar (HgS) is an ore of mercury. When it is heated in air, it is first converted into mercuric oxide (HgO) then reduced to mercury on further heating.
- Cupper from is Cu2S can obtained by heating in air
Extraction of metals with moderate reactivity
- Iron, zinc, lead, copper, are moderately reactive metals.
- These are usually present as sulphides or carbonates in nature.
- They are extracted by two different processes-
- conversion of sulphide ores into oxides by heating strongly in the presence of excess air this process is known as roasting.
- conversion of carbonates ores into oxides by heating strongly in limited air.
- This process is known as calcinations.
- After these two processes metal oxides are reduced to the corresponding metals by using suitable reducing agents such as carbon or by use of displacement reactions.
- These displacement reactions are highly exothermic during process large amount of heat is evolved due to which the metals are produced in the molten state.
- The reaction of iron (III) oxide (Fe2O3) with aluminium is used to join railway tracks or cracked machine parts. This reaction is known as the Thermit reaction.
Extraction of highly reactive metals
- Highly reactive metals cannot be obtained from their compounds by heating with carbon this is because these metals have more affinity for oxygen than carbon.
- For example, carbon cannot reduce the oxides of sodium, magnesium, calcium, aluminium, etc., to the respective metals.
- These metals are obtained by electrolytic reduction.
- Sodium, magnesium and calcium are obtained by the electrolysis of their molten chlorides.
- The metals are deposited at the cathode (the negatively charged electrode), whereas, chlorine is liberated at the anode (the positively charged electrode).