General Principles and Processes of Isolation of Elements
General principles of Metallurgy
Metals occur in nature either in native state or in combined state. Minerals are the naturally occurring chemical substances which occur in the earth’s crust along with the impurities. Ores are those minerals from which metals can be extracted profitably. The scientific and technological process used for isolating the metals from its ores is known as metallurgy. Ore contains impurities such as sand, mica, earthy and rocky impurities, which are called gangue.
The metallurgical process (metallurgy) includes concentration of the ore, isolation of the metal from its concentrated ore and purification of the metal.
Concentration of ore (Ore dressing or Ore benefication) is the process of removal of gangue from ore. Concentration of ores can be done by Hydraulic washing, Magnetic separation, Froth floatation or Leaching. In Froth floatation, pine oils and xanthates are used as collectors and cresols and aniline are used as froth stabilizers. In case of ore containing ZnS and PbS, NaCN is used as depressant.
Leaching is a chemical method used for the concentration of ore.
The concentrated ore is first converted to metal oxide and then the metal oxides are reduced to free metal.
Conversion of the concentrated ore to a metal oxide is done either by calcination or by roasting.
Reduction of metal oxide to free metal is done by pyrometallurgy or electrolytic reduction or by using a suitable reducing agent such as coke, carbon monoxide or hydrogen.
Sometimes metals are obtained by the electrolysis of their molten salt. This method is based on electrochemical principles.
More reactive metals have large negative values of the electrode potential. So their reduction is difficult. If the difference of two EΘ values corresponds to a positive E, it leads to negative ΔGΘ, and then the less reactive metal will come out of the solution and the more reactive metal will go to the solution.
In thermal reduction or pyrometallurgy, it is necessary to know that which element will be most suitable as the reducing agent for a given metal oxide. It is decided by Gibbs free energy (G). A reaction will proceed if value of G is negative.
If reactants and products of two reactions are put together in a system and the net of the two possible reactions is negative, the overall reaction will be spontaneous. This is termed as the coupling of reaction.
Ellingham diagrams are the plots of change in the standard free energy with respect to the temperature for the formation of the oxides of various elements.
The metals obtained by any method is usually contaminated with some impurities and in order to get ultra pure metal, several techniques depending upon the differences in properties of metal and the impurity are used. This is called refining of metals. Metals can be refined by Distillation, Liquation, Electrolytic refining, Zone refining, Vapour Phase refining, Chromatography etc. Nickel is refined by Mond’s process while zirconium and titanium are refined by van Arkel Method.
Extraction of Aluminium
Aluminium is the most abundant metal in the Earth’s crust.
Ores of aluminium are Bauxite (Al2O3.2H2O), Feldspar (KAlSi3O8) etc. Aluminium is extracted from Bauxite. In leaching of Bauxite (Baeyer process), the powdered ore is treated with hot concentrated NaOH. Ore is converted into soluble sodium aluminate which is neutralised by passing CO2, as a result, Alumina (Al2O3 ) is precipitated.
Aluminium is obtained by the electrolysis of alumina by Hall Heroult process. In this method cryolite is used to lower the temperature. On electrolysis, aluminium is obtained at cathode which gets collected at the bottom of the tank; from where it is removed. The oxygen liberated at the anode reacts with the carbon (graphite) of anode producing carbon monoxide and carbon dioxide. Therefore, the graphite rods are gradually consumed and need to be replaced from time to time. The aluminium metal obtained by the electrolysis of the alumina is 99% pure. It is refined by Hoop’s electrolytic method.
Aluminium is used for various purposes like aluminium foils are used as wrappers for chocolates, aluminium wires are used as electricity conductors. The alloys of aluminium are light and very useful.
Extraction of Iron
Iron is the second most abundant metal (after aluminium) in Earth’s crust.
Iron mainly occurs in the combined state. Iron is extracted mainly from haematite ore.
The ore is crushed, sieved and concentrated by the magnetic separation method.
The concentrated ore is heated strongly in the presence of air (roasting) in the shallow kilns to remove moisture and impurities of sulphur, arsenic and phosphorus (in the form of their respective volatile oxides). The ore becomes porous after roasting. After roasting, smelting is carried out in the blast furnace to separate the metal from the impurities. Blast furnace is a tall cylindrical furnace made of steel lined inside with the fire bricks. It has three main parts, cup and cone arrangement; body, bosh and tuyers and hearth or crucible. In the blast furnace, coke is used as a reducing agent. Limestone acts as flux. The iron obtained from the blast furnace (after smelting) is called the pig iron. Pig iron is re-melted in a vertical furnace (cupola furnace). The molten pig iron is poured into moulds to get the cast iron. From the pig iron or cast iron, wrought iron is prepared by the puddling process in a reverberatory furnace lined with the haematite. The lining of the haematite acts as an oxidising agent so that the impurities present in the cast iron get oxidized. Different types of iron are used for different purposes like cast iron is used for casting stoves, railway sleepers, gutter pipes, toys, etc. Wrought iron is used in making anchors, wires, bolts, chains and agricultural implements.
Extraction of Zinc
Zinc does not occur in free state. In combined state, its important ores are Zinc blende (ZnS), Calamine, Zinc spar and Zincite (ZnO). Zinc is mainly extracted from zinc blende. The ore is crushed and finely powdered in ball mills and concentrated by the gravity process (levigation) and the froth floatation process. The ore is subjected to magnetic separation, if iron oxide is also present. The concentrated ore is roasted at about 1,200 K in the presence of the excess of air to get zinc oxide. When zinc oxide is reduced by the vertical retort process, vapours of zinc are obtained. These zinc vapours along with carbon monoxide are taken out from an exit and are condensed. The condensed zinc is known as zinc spelter. The impure zinc thus obtained can be purified either by fractional distillation method or by electrolytic refining method.
Zinc dust and granulated zinc are the two forms of zinc. Zinc is used for various purposes like, galvanising iron and as a constituent of many alloys.
Extraction of Copper
Copper occurs in the native state as well as in the combined state. Important ores of copper are copper pyrite (CuFeS2), copper glance (Cu2S), cuprite (Cu2O) etc. It can be seen in the Ellingham diagram that the oxide ore of copper (Cu2O) can be easily reduced to the metal by heating it with coke. However, most of the ores of copper are sulphide ores such as Cu2S and CuFeS2 and copper is extracted from them. Copper is extracted from copper pyrites by pyrometallurgy, involving the self-reduction process.
The sulphide ore is crushed, sieved and concentrated by the froth floatation process.
The concentrated ore is roasted on the hearth of a reverberatory furnace in a free supply of air. The roasted ore is mixed with coke and sand and the mixture is transferred to a blast furnace for smelting. After smelting copper matte, which contains Cu2S and FeS is obtained. The matte is transferred to a Bessemer converter for Bessemerisation. Cuprous sulphide and cuprous oxide react together to give metallic copper. The solidified copper obtained has blistered appearance due to liberated sulphur dioxide gas, so it is called blister copper. Blister copper is purified by Poling and then by Electrolytic refining. The impurities of silver, gold, tin etc. settle down as the anode mud.
Copper is used for various purposes, like copper wires are used in the electrical industry and for water and steam pipes. Copper is also used in the electroplating and electrotyping industry and in making several alloys.