Describe the methods generally used to extract metals.
Describe the methods generally used to extract metals.
The process of extracting pure metal from its ore is called metallurgy.
The main steps involved in it are as follows
1. Concentration or dressing or benefaction of ore: The process involves the removal of unwanted impurities (gangue) like sand, and earthly impurities from the ore. It can be done by the following methods.
(i) Hand-picking: The large stones and other earthly impurities are removed by handpicking.
(ii) Gravity separation or hydraulic washing: It is based on the difference in the densities of the metallic ore and gangue particles. It is used for the concentration of oxide ores of heavy metals like Iron , Tin , Silver, Gold and Platinum .
(iii) Froth floatation: This method is based on the preferential wetting properties of the ore with oil and gangue particles with water. The method is used for concentrating Sulphide ores.
(iv) Magnetic separation: It is based on the differences in the magnetic properties of the ore and gangue particles (one of them is attracted by the magnetic field and the other is not. It is used for the concentration of ores like Magnetite
2. Conversion of ore to oxide: Concentrated ore is converted into oxide by methods such as roasting, calcination, and leaching.
(I) Roasting: By heating concentrated sulphide ores in excess of air, corresponding metal oxides are formed
(ii) Calcination: The concentrated carbonate ore is heated in absence of air at a temperature which is insufficient to melt the ore, so as to convert it into the corresponding oxide.
(iii) Leaching: In this method, the powdered ore is treated with a reagent in which the ore is soluble but the impurities are insoluble. For example, the leaching of bauxite ore (Baeyer process).
3. Reduction of oxide to metal: To obtain crude or impure metal, the metal oxide is reduced by electrolytic reduction, reduction with powerful reducing agents, reduction with carbon, reduction by air or self-reduction
(i) Electrolytic reduction: Highly reactive metals like Potassium, Sodium, Calcium and Magnesium are extracted by electrolytic reduction of their molten chlorides or oxides.
(ii) Reduction with powerful reducing agents: Less electropositive metals like Chromium , Manganese , Nickel etc. are obtained by reducing their oxides with Carbon monoxide , Carbon monoxide Hydrogen, Sodium , Aluminium and Magnesium .
(iii) Reduction with carbon: The oxides of less electropositive metals like Lead , Iron , Zinc which does not form carbide are reduced by heating them with coke or coal. They can be reduced by Carbon monoxide also.
(iv) Reduction by air or self-reduction: The oxides of less reactive metals like Mercury and Copperare reduced to metal by heating their oxides in the air.
4. Refining or purification: The crude metals thus obtained are converted into pure metals by methods like liquation, distillation, oxidation, and electrolytic refining.
(i) Liquation: This method is used for refining metals with low melting points, e.g., Lead , Bismuth , Tin Impure metal is heated on a sloping hearth, keeping the temperature slightly above the melting point of the metal to be purified. Molten metal flows down leaving behind the impurities.
(ii) Distillation: Low boiling volatile metals like Zinc and Mercury are purified by this method. Crude metal is heated above its boiling point, to obtain the pure metal as distillate.
(iii) Oxidation: This method is used when impurities have a greater affinity for oxygen and undergo oxidation more readily than metal. Metals like Iron , Copper , etc. which contain impurities of Carbon , Silicon, Sulphur , Phosphorus etc. can easily be purified by this method.
(iv) Electrolytic refining: The method is used for the refining of metals like Copper, Silver ,Gold ,Zinc , Aluminium , etc. The impure metal is made into the anode, while the pure metal is made into the cathode. The electrolyte used is a salt solution of a metal which is to be refined.
The process of extracting pure metal from its ore is called metallurgy.
The main steps involved in it are as follows
1. Concentration or dressing or benefaction of ore: The process involves the removal of unwanted impurities (gangue) like sand, and earthly impurities from the ore. It can be done by the following methods.
(i) Hand-picking: The large stones and other earthly impurities are removed by handpicking.
(ii) Gravity separation or hydraulic washing: It is based on the difference in the densities of the metallic ore and gangue particles. It is used for the concentration of oxide ores of heavy metals like Iron , Tin , Silver, Gold and Platinum .
(iii) Froth floatation: This method is based on the preferential wetting properties of the ore with oil and gangue particles with water. The method is used for concentrating Sulphide ores.
(iv) Magnetic separation: It is based on the differences in the magnetic properties of the ore and gangue particles (one of them is attracted by the magnetic field and the other is not. It is used for the concentration of ores like Magnetite
2. Conversion of ore to oxide: Concentrated ore is converted into oxide by methods such as roasting, calcination, and leaching.
(I) Roasting: By heating concentrated sulphide ores in excess of air, corresponding metal oxides are formed
(ii) Calcination: The concentrated carbonate ore is heated in absence of air at a temperature which is insufficient to melt the ore, so as to convert it into the corresponding oxide.
(iii) Leaching: In this method, the powdered ore is treated with a reagent in which the ore is soluble but the impurities are insoluble. For example, the leaching of bauxite ore (Baeyer process).
3. Reduction of oxide to metal: To obtain crude or impure metal, the metal oxide is reduced by electrolytic reduction, reduction with powerful reducing agents, reduction with carbon, reduction by air or self-reduction
(i) Electrolytic reduction: Highly reactive metals like Potassium, Sodium, Calcium and Magnesium are extracted by electrolytic reduction of their molten chlorides or oxides.
(ii) Reduction with powerful reducing agents: Less electropositive metals like Chromium , Manganese , Nickel etc. are obtained by reducing their oxides with Carbon monoxide , Carbon monoxide Hydrogen, Sodium , Aluminium and Magnesium .
(iii) Reduction with carbon: The oxides of less electropositive metals like Lead , Iron , Zinc which does not form carbide are reduced by heating them with coke or coal. They can be reduced by Carbon monoxide also.
(iv) Reduction by air or self-reduction: The oxides of less reactive metals like Mercury and Copperare reduced to metal by heating their oxides in the air.
4. Refining or purification: The crude metals thus obtained are converted into pure metals by methods like liquation, distillation, oxidation, and electrolytic refining.
(i) Liquation: This method is used for refining metals with low melting points, e.g., Lead , Bismuth , Tin Impure metal is heated on a sloping hearth, keeping the temperature slightly above the melting point of the metal to be purified. Molten metal flows down leaving behind the impurities.
(ii) Distillation: Low boiling volatile metals like Zinc and Mercury are purified by this method. Crude metal is heated above its boiling point, to obtain the pure metal as distillate.
(iii) Oxidation: This method is used when impurities have a greater affinity for oxygen and undergo oxidation more readily than metal. Metals like Iron , Copper , etc. which contain impurities of Carbon , Silicon, Sulphur , Phosphorus etc. can easily be purified by this method.
(iv) Electrolytic refining: The method is used for the refining of metals like Copper, Silver ,Gold ,Zinc , Aluminium , etc. The impure metal is made into the anode, while the pure metal is made into the cathode. The electrolyte used is a salt solution of a metal which is to be refined.
