Extractive Metallurgy
The process of extracting metals from their natural sources (ore) and preparing them for practical use is known as metallurgy.
Principles of extraction
The extraction of pure metal from its ore involves several physical and chemical methods. The suitability of method depends upon the nature of ore, the properties of metals and local conditions. The extractive metallurgy of a metal involves following operations :
The process of extracting metals from their natural sources (ore) and preparing them for practical use is known as metallurgy.
Principles of extraction
The extraction of pure metal from its ore involves several physical and chemical methods. The suitability of method depends upon the nature of ore, the properties of metals and local conditions. The extractive metallurgy of a metal involves following operations :
- Crushing of ores
- Concentration of ores
- Reduction
- Purification of ores
- Crushing of Ores:
Larger pieces of ores obtained from earth's crust are broken up into smaller pieces. This process is known as Pulverization. - Concentration of Ores:Presence of earthy matter, rock matter, sand, limestone etc. in ore are known as Gangue or Matrix. Removal of these impurities from ore is called concentration. Following methods are employed for concentration of ore.
- Gravity separation: This method of separation is based on the difference in specific gravities of the gangue particles and ore particles.
The powdered ore is agitated with water or washed with a running stream of water. Lighter impurities are washed away wuth floating water leaving behind heavier ores. Generally, oxide and carbonate ores are concentrated by this method. - Magnetic separation: This method is used when either ore or gangue has strong ferromagnetic nature.
Example: Wolframite (FeWO4) a magnetic ore is separated from the non-magnetic ore SnO2 by this method. - Froth Floatation process: This method is used for concentration of sulphide ores. This method is based on preferential wetting properties with the frothing agent and water. The powdered ore is added to water containing pine oil (frothing agent). A vigorous stream of air is now passed through, which thooroughly agitates the mixture of disposes the oil into colloidal sized particles. As a result of this the sulphide particles of the ore stick to the oil droplets and rise to the surface in the form of froth. Water wets the gangue particles which sink to the bottom. Some froth stablisers like ethyl xanthate, methyl xanthate etc. are added.
- Leaching: It involves the treatment of the ore with a suitable reagent so as to make it soluble while impurities remain insoluble method.
Example: Bauxite ore contains Fe2O3 and silica as impurities. When powdered ore is digested with an aqueous solution of Na OH at about 150oC under pressure, the alumina dissolves forming soluble sodium meta aluminate while Fe2O3 and silica remains in the insoluble part.
The concentrated ore is converted into a form which is soluble for reduction of metal.- Calcination: It involves heating of the ore below its fusion temperature in absence of air to produce a new compound having higher % oe metal as well as removing the moisture, organic matter and volatile impurities. Calcination is done in a reverberatory furnace and it makes ore porous.
CaCO3
CaO + CO2 
Al2O3.2H2O Al2O3 + 2H2O - Roasting: It involves heating of an ore in limited supply of air below its melting point, to reduce other chemical changes.
2CuFeS2 + O2 Cu2S + 2FeS + SO2
2FeS + 3O2 2FeO + 2SO2
2Cu2S + 3O2 2Cu2O + 2SO2
Roasting is usually done in reverberatory furnace or blast furnace.
- Calcination: It involves heating of the ore below its fusion temperature in absence of air to produce a new compound having higher % oe metal as well as removing the moisture, organic matter and volatile impurities. Calcination is done in a reverberatory furnace and it makes ore porous.
- Gravity separation: This method of separation is based on the difference in specific gravities of the gangue particles and ore particles.
- Reduction to free Metal
Some of the methods commonly used to get free metal from ore are given below:- Smelting : Reduction of the ore to the molten metals at a high temperature using powerful reducing agent such as C, H2, CO, water gas, Na, K, Mg, Al is called smelting.
- Carbon reduction process: The oxides of less electropositive metals are reduced by strongly heating them with coal or coke.
PbO + C Pb + CO
PbO + CO Pb + CO2
Fe2O3 + C 2 Fe + 3CO
Fe2O3 + 3CO 2Fe + CO2
To remove gangue still present in concentrated ore, certain substance are mixed with concentrated ore which combine with gangue to form Slag. Slag is usually lighter and floats on the surface of the molten metal. - Reduction of aluminium (Gold Schmidt alumino thermite): This method is employed in the case of those metals which have very high melting points and their reduction with carbon is not satisfactory. A mixture of concentrated ore and aluminium powder, commonly called as Thermite, is taken in a steel crucible. The reaction is started by the use of an ignition mixture of containing magnesium powder and barium peroxide.
Cr2O3 + 2Al 2Cr + Al2O3
3Mn3O4 + 8Al 9Mn + 4Al2O3
This reaction is highly exothermic. So, metal is obtained in molten state.
2Al + 3O2 Al2O3 + 1670 kJ
- Carbon reduction process: The oxides of less electropositive metals are reduced by strongly heating them with coal or coke.
- Self reduction process: The sulphide ores of less electropositive metals like Hg, Pb, Cu, etc. are heated in air to convert part of sulphide ore into oxide, which then reacts with the remaining sulphide to give the metal and sulphur dioxide.
2HgS + 3O2 2HgO + 2SO2
2HgO + HgS 3Hg + SO2 - Electrolytic reduction: The oxides of highly electropositive metals like Na, K, Ca, Mg etc. cannot be reduced by carbon at moderate temperature. These metals are thus obtained by the electrolysis of their oxides, hydroxides or chlorides in fused state. Sometimes, a small amount of some other salt is added to lower the fusion temperature or to increase the conductivity or both. The metal is deposited at cathode.
Example: Sodium is obtained by the electrolysis of fused NaCl. - Cyanide method: Silver and gold ore is dissolved in suitable chemical reagent followed by the extraction of metal by precipitation of the metal by a more electropositive metal.
- Smelting : Reduction of the ore to the molten metals at a high temperature using powerful reducing agent such as C, H2, CO, water gas, Na, K, Mg, Al is called smelting.
- Refining or Purification
The metal obtained after reduction process still contains some impurities which can be removed by applying following method.- Liquation process: This method is employed when impurities have higher MP than metal. The impure metal is placed on the sloping hearth of a furnace and gently heated. The metal melts and flows down leaving behind the impurities.
- Distillation method: Volatile metals (Hg, Zn, Cd) are easily purified by distillation. The impure metals is heated in a retort and vapours of volatile metals are collected and condensed in a receiver leaving behind non volatile impurities in the retort.
- Pyrometallurgical oxidation process: This process is used when impurities have greater affinity for oxygen than the metals itself. The oxidation is done by
- Cupellation
- Bessemerisation
- Poling: This process is used to purify the metal from its oxide. In this process, molten metal containing oxide of its impurity is stirred by logos of green wood to reduce oxide to metal eg Cu from CuO.
- Electrolytic refining: The impure metal is made anode while a thin sheet of pure metal acts as cathode. The electrolytic solution consists of an aqueous solution of salt or a complex of the metal. On passing the current, pure metal is deposited on cathode and equivalent amount of metal gets dissolved from anode. The soluble impurities pass into the solution and insoluble impurities collect below the anode as anode mud.
- Zone refining of fractional crystallization: The method is employed to get metals of very high purity. (Ge, Si, B, Ga, In). This method is based on the difference in solubility of impurities of molten and solid state of the metal. A movable heater is allowed to move across the impure metals rod from one end to the other end, The3 pure metal crystrallises while the impurities pass on to the adjacent melted zone.
- Van Arkel process: This method is employed to obtain ultra pure metals. The impure metal is converted into a volatile compound while the impurities are not affected. The volatile compound is then decomposed electrically to get the pure metal.
Ti(s) + 2l2(g) Til4(g)
Til4(g)
Ti + 2l2(g)
Other metals those can be purified this method are Zr, V, W, Hf etc. - Mond's Process: This method is employed for purification of nickel. Impure nickel is converted into volatile nickel carbonyl by reaction of CO at 60-80oC. Nickel carbonyl decomposes at 180oC to form pure nickel and CO.
Ni + 4CO
Ni(CO4) 
Ni +4CO
- Liquation process: This method is employed when impurities have higher MP than metal. The impure metal is placed on the sloping hearth of a furnace and gently heated. The metal melts and flows down leaving behind the impurities.
