For example, Aluminium occurs in the earth's crust in the form of two well-known minerals, bauxite (Al2O3.2H2O) and clay (Al2O3 . 2SiO2 . 2H2O). At some places, minerals may contain a large percentage of metal whereas others may contain only a small percentage of the metal.
Aluminium Ore: Bauxite
The mineral from which metal can be conveniently and profitably extracted is called an ore. All ores are minerals but all the minerals are not ores.
For example, Copper occurs in nature in the form of several minerals like copper pyrites, copper glance (Cu2S), and cuprite (Cu2O).
But copper can be conveniently extracted from copper pyrites (CuFeS2). Therefore, the ore of copper is copper pyrites. Some common ores are listed below:
|Nature of Ore||Metal||Composition|
|Suphide Ores||Copper||Copper pyrites||CuFe2O2|
Since ores of many metals are oxides due to the high reactivity of oxygen and they are very abundant on the earth's crust.
The various processes involved in the extraction of metals from their ores and refining for use are known as metallurgy.
Various steps involved in the extraction of metals or metallurgical process :
1. Crushing and grinding of the ore.
2. Concentration of the ore or enrichment of the ore.
3. Extraction of metal from the concentrated ore.
4. Refining or purification of the impure metal.
Crushing of OresCrushed Ore converted into fine powder by Stamp Mill
The ores mined from the earth's crust contain a number of impurities, such as soil, sand, etc. called gangue and matrix. The process of removal of impurities (gangue) from the ore is called enrichment of ore or concentration of ore. Enrichment of ore is carried out by the following methods :
This method of concentration is usually applicable to oxides ore. For example ores of iron, tin, and lead are very heavy and therefore they are concentrated by this method.
The powdered ore is dropped over the moving belt at one end. At the other end, the magnetic portion of the ore is attracted by the magnetic roller and falls nearer to the roller while the non-magnetic impurities fall further off. For example, This method is used for the concentration of iron ores (Haematite)
Sample diagram of leaching process
For Example, Bauxite ore contains Fe2O3, SiO2, etc as impurities. Bayer's method is used to obtain pure aluminium oxide from bauxite ore. This process of chemical separation of aluminium by the chemical method is known as Bayer's process. This method involves the following steps.
(i) The finely powdered ore is treated with hot sodium hydroxide solution which reacts with Al2O3 present in bauxite ore to form sodium meta aluminate. (soluble in water).
(ii) The filtrate (containing NaAlO2 and sodium silicate (Na2SiO3)) is then stirred with a small amount of freshly prepared Al(OH)3.
The aluminium hydroxide (Al(OH)3) is added to induce the precipitation of Al(OH)3. It acts as a seeding agent and helps in quick precipitation.
(iii) The precipitate is separated by filtration. It is dried and heated to get pure aluminium oxide which is also known as alumina.
Extraction of the metal from the concentrated ore or enriched ore. The metal is extracted from the concentrated ore by the following steps :
The production of metal from concentrated ore mainly involves the reduction process. This can be usually done by two processes known as calcination and roasting process. The method depends upon the nature of the ore. A carbonate ore is converted into oxide by calcination while a sulphide ore is converted into oxide by roasting. These two methods are discussed briefly as below:
(i) To convert carbonates ores into metal oxide.
(ii) To remove water from the hydrated ores.
(iii) To remove volatile impurities from the ore.
For example, Calamine (ZnCO3) is the ore of zinc that calcined i.e. heated strongly in the absence of air to convert into zinc oxide. During calcination, carbon dioxide gas is expelled.
Similarly, in case carbonate ore of Fe, siderite (FeCO3), and ore of calcium and magnesium, are:
Calcination converts into oxide as:
Process of Calcination
1. (i) The sulphide ores undergo oxidation to form their oxides.
(ii) Moisture is used.
(iii) Volatile impurities are removed.
For example (i) Zinc blende (ZnS) is the ore of zinc that heated strongly in the presence of excess air to convert zinc oxide and sulphur dioxide gas is expelled.
2. Iron pyrite (FeS2) is converted into ferric oxide (Fe2O3) by roasting.
3. Galena (PbS) is converted into litharge (PbO) by roasting.
4. Cinnabar (HgS) is roasted to convert it directly into mercury.
Extraction of metal from metal oxide or conversion of metal oxide to metal.
The metal oxide formed after calcination or roasting is converted into metal by reduction. The method used for the reduction of metal oxide depends upon nature and chemical reactivity of the metal.
The different metals are extracted by different techniques. The different steps are discussed below:
The metals which are low in the activity series are very unreactive. The oxides of these metals can be reduced to metal by heating alone.
For example, Copper is obtained from its copper sulphide (Cu2S) are by this method. This involves the following steps :
(i) The concentrated copper sulphide is roasted in the air to form copper oxide.
2Cu2S(s) + 3SO2(g) 2Cu2O(s) + 2SO2(g)
(ii) Copper oxide is again heated with Cu2S to obtain reduced copper and SO2 gas is evolved.
2Cu2O(s) + Cu2S(g) 6Cu(s) + SO2(g)
Similarly, when cinnabar ore (HgS) is heated in air it is first gets converted into mercuric oxide (HgO), which is then reduced to mercury on further heating.
HgS(s) + 3O2(g) HgO(s) + SO2(g)
HgO(s) 2Hg(l) + O2(g)
The metals in the middle of the activity series such as iron, zinc, lead, etc are moderately reactive. There are usually present as sulphides or carbonates and which are must be converted to oxides by roasting or calcination.
The oxides of these metals can not be reduced by heating alone. Therefore, these metal oxides are reduced to free metal by using suitable reducing agents such as carbon, carbon monoxide, aluminium, sodium, or calcium. This is known as smelting.
Reduction with carbon :
When zinc oxide is heated with carbon, zinc metal is produced
Similarly, Iron and lead are obtained from their oxides by heating with carbon.
Fe2O3(s) + 3C(s) 2Fe(s) + 3CO(g)
PbO(s) + C(s) Pb(s) + CO(g)
Reduction with CO
Iron is obtained from ferric oxide by heating with CO.
Fe2O3(s) + 3CO(g) 2Fe(s) + 3CO2(g)
Reduction with Aluminium
Certain metal oxides are reduced by aluminium to metals. This method is known as aluminothermy or thermite process.
For example, Chromium, manganese, titanium, vanadium metal are obtained by the reduction of their oxides with Al powder. The following reaction takes place.
3MnO2(g) + 4Al(s) 3Mn(s) + 2Al2O3(s)
Cr2O3(g) + 2Al(s) 2Cr(s) + Al2O3(s)
These displacement reactions are highly exothermic, so, a large amount of heat is evolved and metals are produced in the molten state. In fact the reaction of iron(III) oxide (Fe2O3) with aluminium. The mixture of iron oxide and aluminium powder is called thermite is used to repair railway tracks or cracked machine parts. This reaction is known as a thermite reaction.
Fe2O3(g) + 2Al(s) 2Fe(l) + Al2O3(s) + Heat
Note: Al is an expensive metal and so, it is not used to reduce metals that are less expensive than aluminium.
Electrolytic reduction or reduction by electrolysis. The reactive metals (which are high up in the activity series) cannot be produced by any of the above methods. They are obtained by electrolytic reduction of their molten oxides or chlorides. During electrolysis, the cathode supplies electrons to metal ions for their reduction to the metal. The process of extracting metals by electrolysis process is called electrometallurgy.
For example :
(i) Aluminium oxide (Al2O3) is reduced to aluminium by the electrolysis of molten aluminium oxide.
Al2O3 → 2Al3+ + 3O2–
The aluminium ions present in aluminium oxide go to the cathode and are reduced there to aluminium atoms.
Al3+ + 3e- → Al
(ii) Sodium metal is obtained by the electrolysis of molten sodium chloride.
2NaCl 2Na+ + 2Cl–
2Cl– → Cl2 + 2e- (at anode)
2Na+ + 2e- → 2Na (at cathode)
2NaCl 2Na + Cl2
For example Impure iron (Pig or cast iron) is refined by oxidative refining method. Pig iron contains Carbon, Sulphur, phosphorous, silicon, and manganese as impurities. When a blast of air is blown over molten pig iron these impurities are oxidised to their oxides (CO2, SO2, P2O5, etc.) and get removed. The pure iron is left behind.
Similarly, Silver is refined by this method.
This is the most widely used method for the refining of impure metals. Many metals such as copper, zinc, tin, nickel, silver, gold, etc. are refined electrolytically. It is based upon the phenomenon of electrolysis. In this process, the impure metal is made of the anode and a thin strip of pure metal is the cathode. A solution of the metal salt is used as an electrolyte. On passing the electric current through the electrolyte, the pure metal from the anode dissolves into the electrolyte. An equivalent amount of pure metal gets 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.
For example: In the electrolytic refining of copper (The apparatus is set up as shown in the figure below)
Crude copper is made the anode, a thin sheet of pure copper is made the cathode. The electrolyte is a solution of copper sulphate containing a small amount of dilute H2SO4 acid. On passing the electric current copper dissolve from the anode into the electrolyte. An equivalent amount of copper is deposited at the cathode in the form of pure metal. The following reactions occur at the electrodes.
At anode :
At cathode :
The summary or flow sheet of different steps involved for three types of extraction of metals is given below :
Flowchart: Electrolytic Refining