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.
Minerals: The elements or compounds, which occur naturally in the earth’s crust, are known as minerals.
Ores: The minerals contain a very high percentage of a particular metal and the metal can be profitably extracted from it. These minerals are called ores.
Gangue: Ores mined from the earth are usually contaminated with large amounts of impurities such as soil, sand, etc., are called gangue.
Extraction of Metals:
All metals are extracted from its ore. Some metals are found in the earth’s crust in the free state. Some are found in the form of their compounds.
Metals Ores Formulla
Iron Haematite/magnetite Fe2O3/Fe3O4
Mercury Cinnabar HgS
Extraction of Metals acording to their reactivity.
(i) Highly reactive: Such as K, Na, Ca, Mg, Al like metals are extracted using Electrolysis.
(ii) Middle reactive :Such as Zn, Fe, Pb and Cu are extracted by Reduction using carbon.
(iii) Less reactive : Ag and Au are found in native state.
Contains of ores:
Ores mined from the earth are usually contaminated with large amounts
of impurities such as soil, sand, etc., called gangue. The impurities must
be removed from the ore prior to the extraction of the metal.
The processes used for removing the gangue from the ore are based on the differences between the physical or chemical properties of the gangue and the ore.
(i) Extracting Metals Low in the Activity Series:
Metals low in the activity series are very unreactive. Copper and silver are also found in the combined state as their sulphide or oxide ores. The oxides of these metals can be reduced to metals by heating alone. When ores are heated in the air, They are first converted into corresponding metals oxide. Further heating gives pure metals from its oxide.
2HgS(s) + 3O (g) →2HgO(s) + 2SO (g)
(ii) Extracting Metals in the Middle of the Activity Series:
The metals in the middle of the activity series such as iron, zinc, lead, copper, etc., are moderately reactive. These are usually present as sulphides or carbonates in nature. The metal sulphides and carbonates are converted into metal oxides. As It is easier to obtain a metal from its oxide, as compared to its sulphides and carbonates.
Converting into oxides:
(a) Roasting: The sulphide ores are converted into oxides by heating
strongly in the presence of excess air. This process is known as roasting.
2ZnS(s) + 3O (g) Heat→ 2ZnO(s) + 2SO (g)
(b) Calcination: The carbonate ores are changed into oxides by heating strongly in limited air. This process is known as calcination.
ZnCO (s) → ZnO(s) + CO (g)
The metal oxides are then reduced to the corresponding metals by using suitable reducing agents such as carbon. For example, when zinc oxide is heated with carbon, it is reduced to metallic zinc.
ZnO(s) + C(s) → Zn(s) + CO(g)
There is used reduction reaction to reduce Metals from its compound.
Using of displacement reaction:
Besides using carbon (coke) to reduce metal oxides to metals, sometimes displacement reactions can also be used. The highly reactive metals such as sodium, calcium, aluminium, etc., are used as reducing agents because they can displace metals of lower reactivity from their compounds. For example, when manganese dioxide is heated with aluminium powder, the following reaction takes place –
3MnO2(s) + 4Al(s) → 3Mn(l) + 2Al2O3(s) + Heat
Thermit Reaction: Some displacement reactions are highly exothermic. the amount of heat evolved is so large that the metals are produced in the molten state. such a reaction is known as thermit reaction.
use of thermit reaction:
The reaction of iron(III) oxide (Fe2O3) with aluminium is used to join railway tracks or cracked machine parts.
Fe2O3(s) + 2Al(s) → 2Fe(l) + Al2O3(s) + Heat
(iii) Extracting Metals towards the Top of the Activity Series:
The metals high up in the reactivity series are very reactive. These metals are obtained by electrolytic reduction. For example, 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).
Highly reactive metals have highly affinity for oxygen:
Highly reactive metals have more affinity for oxygen than carbon. So they can not be obtained from their compound by heating with carbon. For example, carbon cannot reduce the oxides of sodium, magnesium, calcium, aluminium, etc., to the respective metals.
Refining of Metals
The metals produced by various reduction processes described above are not very pure. They contain impurities, which must be removed to obtain pure metals. The most widely used method for refining impure metals is electrolytic refining.
Many metals, such as copper, zinc, tin, nickel, silver, gold, etc., are refined electrolytically. In this process, the impure metal is made the anode and a thin strip of pure metal is made the cathode.
A solution of the metal salt is used as an electrolyte. On passing the current through the electrolyte, the pure metal from the anode dissolves into the electrolyte. An equivalent amount of pure metal from the electrolyte is deposited on the cathode. The soluble impurities go into the solution, whereas, the insoluble impurities settle down at the bottom of the anode and are known as anode mud.