Kerala Plus Two Chemistry Notes Chapter 6 General Principle and Processes of Isolation of Elements<\/h2>\n
Metallurgy – the entire scientific and technological processes used for isolation of the metal from their ores. The extraction and isolation of metals from ores involve the following major steps.
\n1. Concentration of the ore
\n2. Isolation\/extraction of the metal from its concentrated ore and
\n3. Purification or refining of the metals<\/p>\n
Minerals:<\/span> Ores: Gangue : Occurrence of Metals :<\/span> Concentration (dressing or benefaction) of Ores:<\/span> Hydraulic Washing or Gravity Seperation:<\/span> Magnetic Separation:<\/span> Froth Floatation Method:<\/span> \u2018Depressants\u2019 are used to separate two sulphide ores. e.g. in case of an ore containing ZnS and PbS, the depressant used is NaCN. It selectively prevents ZnS from coming to the froth but allows PbS to come with the froth.<\/p>\n Leaching:<\/span> a) Leaching of Alumina from Bauxite: The aluminate solution is neutralised with CO2<\/sub> gas and hydrated Al2<\/sub>O3<\/sub> precipitated. At this stage, the slution is seeded with freshly prepared samples of hydrated Al2<\/sub>O3<\/sub> which induces the precipitation. The sodium silicate remains in the solution. The hydrated alumina is filtered, dried and heated to give back pureAl2<\/sub>O3<\/sub>. b) another example: Extraction of Crude Metal from Concentrated Ore:<\/span> ii) Roasting: Flux: b) Reduction of Oxide to the Metal: Thermodynamic Principles of Metallurgy:<\/span> Ellingham Diagram: When the value of \u2206G is negative in the equation \u2206G = \u2206H \u2014T\u2206S then the reaction will proceed. If \u2206S is positive, on increasing the temperature(T), the value of T\u2206S would increase (\u2206H < T\u2206S) and then AG will become -ve.<\/p>\n If the reactants and products of the coupled reaction (reduction of the metal oxide and oxidation of the reducing agent) are put together in a system and the net \u2206G of the two possible reacfions is -ve, the overall reaction will occur.<\/p>\n Applications of Ellingham Diagram<\/p>\n Limitations of Ellingham Diagram<\/p>\n a) Extraction of Iron from its Oxides: Two simpler reactions such as reduction of FeO and oxidation of coke(C) are are coupled in this process so that the Gibbs energy change of the net reaction is negative.<\/p>\n In Blast furnace, above 710 \u00b0C (983 K) coke (C) reduces FeO to Fe. At temperatures below 710 \u00b0C (983 K) CO reduces Fe3<\/sub>O4<\/sub> and Fe2<\/sub>O3<\/sub>to FeO. Hot air is blown from the bottom of the furnace and coke is burnt to give temperature up to 2200 K.<\/p>\n Reactions at lower temperature range (500 – 800 K) – Reactions athighertemperature range (900 -1500 K) – Lime stone is decomposed to CaO which removes silicate impurity of the ore as slag. Pig iron – iron obtained from the blast furnace which containes about 4% carbon and many impurities. Preparation of Wrought Iron:<\/span> Limestone is added as a flux and S, Si and P are oxidised and passed into the slag. The metal is \u2022 recovered and freed from the slag by passing through b) Extraction of Copper: The oxide can then be easily reduced to metallic copper using coke. This is because the Cu,Cu2<\/sub>O line is almost at the top in the Ellingham diagram. The ore is heated in a reverberatory furnace after mixing with silica. The iron oxide \u2018slags of as iron silicate and copper forms copper matte. This contains Cu2<\/sub>S and FeS. Matte is heated in silica lined converter. The remaining Fe is converted to FeSiO3<\/sub>. The remaining Cu2<\/sub>S and Cu2<\/sub>O undergoes self oxidation-reduction to form blister copper. The solidified copper obtained has blistered appearance due to the evolution of SO2<\/sub> and so it is called blister copper.<\/p>\n c. Extraction of Zinc from Zinc Oxide: The metal is distilled of and collected by rapid chilling.<\/p>\n Electrochemical Principles of Metallurgy:<\/span> More reactive metals have large negative E\u0398<\/sup> values. So their reduction is difficult. Sometimes a flux is added for making the molten mass more conducting.<\/p>\n Extraction of Aluminium (Hall-Heroult Process):<\/span> The electrode reactions are: Disadvantage: Refining:<\/span> a) Distillation: b) Liquation: c) Electrolytic Refining: Anode: During electrolysis Cu in the pure form istransfered from the anode to the cathode. Impurities deposit as anode mud which contains valuable elements like Sb, Se, Te, Ag, Au and Pt. Recovery of these elements meets the cost of refining.<\/p>\n Zn is also refined by electrolytic process.<\/p>\n d) Zone Refining: e) Vapour Phase Refining: Requirements for vapour phase refining: i) Mond Process for Refining Nickel: The nickel tetracarbonyl is heated at high temperature so that it is decomposed to give pure Ni.<\/p>\n ii) van Arkel Method for Refining Zr or Ti: The metal iodide is decomposed on a tungsten filament at 1800 K. The pure metal is deposited on the filament. Similarly, Ti can be purified. f) Chromatographic Methods: Uses of Aluminium, Copper, Zinc and Iron<\/span> 2. Copper: 3. Zinc: 4. Iron: Wrought Iron: Steel: We hope the Plus Two Chemistry Notes Chapter 6 General Principle and Processes of Isolation of Elements help you. If you have any query regarding Plus Two Chemistry Notes Chapter 6 General Principle and Processes of Isolation of Elements, drop a comment below and we will get back to you at the earliest.<\/p>\n","protected":false},"excerpt":{"rendered":" Plus Two Chemistry Notes Chapter 6 General Principle and Processes of Isolation of Elements is part of Plus Two Chemistry Notes. Here we have given Plus Two Chemistry Notes Chapter 6 General Principle and Processes of Isolation of Elements. Board SCERT, Kerala Text Book NCERT Based Class Plus Two Subject Chemistry Notes Chapter Chapter 6 […]<\/p>\n","protected":false},"author":7,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_genesis_hide_title":false,"_genesis_hide_breadcrumbs":false,"_genesis_hide_singular_image":false,"_genesis_hide_footer_widgets":false,"_genesis_custom_body_class":"","_genesis_custom_post_class":"","_genesis_layout":"","footnotes":""},"categories":[42728],"tags":[],"yoast_head":"\n
\nNaturally occuring chemical substance in the earth\u2019s crust obtainable by mining.<\/p>\n
\nminerals from which the metals are economically and profitably extracted. All ores are minerals but all minerals are not ores.<\/p>\n
\nearthly matter or unwanted materials present in ore.<\/p>\n
\nMetals are present in earth\u2019s crust as oxides, sulphides, carbonates etc.
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<\/p>\n
\nprocess of removal of gangue or matrix from the ore. The different process used are:<\/p>\n
\nIt is based on the differences in gravities. An upward stream of running water is used to wash the powdered ore. The lighter gangue particles are washed away and the heavier ores are left behind.<\/p>\n
\nIt is based on differences in magnetic properties of the ore components. It is carried out if either the ore or the gangue is capable of being attracted by the magnetic field. The powdered ore is carried on a conveyer belt which passes over a magnetic roller.<\/p>\n
\nused to separate sulphide ore from the gangue. Here a suspension of the powdered sulphide ore is agitated with collectors and froth stabilisers by passing a forceful current of air. The froth formed which carries the mineral particles is skimmed off and then dried.<\/p>\n
\na method of ore concentration by dissolving the ore in a suitable solvent.<\/p>\n
\nThe powdered bauxite ore is digested with concentrated solution of NaOH at 473-523 K and 35 – 36 bar pressure. The Al2<\/sub>O3<\/sub> is leached out as sodium aluminate leaving the impurities behind. But the impurity SiO2<\/sub> is also leached out as sodium silicate.
\nAl2<\/sub>O3<\/sub> (S)+ 2 NaOH(aq) +3H2<\/sub>O(l) \u2192 2Na[Al(OH)4<\/sub>](aq)<\/p>\n
\n2Na[Al(OH)4<\/sub>](aq) + CO2<\/sub>(g) \u2192 Al2<\/sub>O3<\/sub> + xH2<\/sub>O(s) + 2 NaHCO3<\/sub>(aq)<\/p>\n
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<\/p>\n
\nIn the metallurgy of silver and gold, the ore is treated with a dilute solution of NaCN or KCN. Later the metal ion in the solution is replaced by Zn metal, which acts as the reducing agent.
\n4M(s) + 8CN–<\/sup>(aq) + 2 H2<\/sub>O(aq) + O2<\/sub>(g) \u2192 4[M(CN)2<\/sub>]–<\/sup>(aq) + 4 OH–<\/sup>(aq) (M = Ag or Au)
\n2[M(CN)2<\/sub>]–<\/sup>(aq) + Zn(s) [Zn(CN)4<\/sub>]2-<\/sup>(aq) + 2 M(s)<\/p>\n
\nThe concentrated ore must be converted to oxide and then reduced to metal. It involves two steps.
\na) Conversion to Oxide
\ni) Calcination:
\nprocess in which the ore is heated strongly in the absence of air.
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<\/p>\n
\nprocess of heating the ore in a regular supply of air in a furnace at a temperature below the melting point of the metal.
\n2 ZnS + 3O2<\/sub> \u2192 2 ZnO + 2SO2<\/sub>
\n2PbS + 3O2<\/sub> \u2192 2PbO + 2SO2<\/sub>
\n2CU2<\/sub>S + 3O2<\/sub> \u2192 2Cu2<\/sub>O + 2SO2<\/sub><\/p>\n
\nsubstance which combines with gangue present in the ore and form easily fusible materials called the slag.
\nFlux + Gangue \u2192 Slag (fusible)
\nFeO + SiO2<\/sub> \u2192 FeSiO3<\/sub> (Slag)<\/p>\n
\nThe metal oxide is reduced by reducing agents (e.g. C, CO or even another metal) which combine with the oxygen of, the metal oxide.
\nMx<\/sub>Oy<\/sub> + yC \u2192 xM + yCO<\/p>\n
\nAll those metals which have more negative Gibbs energies of formation of their oxides can reduce the oxides of other metals whose Gibbs energies of formation are less negative.<\/p>\n
\ngraph of variation of \u2206r<\/sub>G\u0398<\/sup> vs. T for the formation of metal oxide from metals.
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<\/p>\n\n
\n
\nThe concentrated ore is mixed with lime stone and coke and fed into a Blast furnace from its top. Here the oxide is reduced to metal.
\nFeO(s) + C(s) \u2192 Fe(s\/ \\(\\ell \\)) + CO(g)<\/p>\n
\n3Fe2<\/sub>O3<\/sub> + CO \u2192 2Fe3<\/sub>O4<\/sub> + CO2<\/sub>
\nFe3<\/sub>O4<\/sub> + 4CO \u2192 3Fe + 4CO2<\/sub>
\nFe2<\/sub>O3<\/sub> + CO \u2192 2FeO + CO2<\/sub><\/p>\n
\nC + CO2<\/sub> \u2192 2CO
\nFeO + CO \u2192 Fe + CO2<\/sub><\/p>\n
\nCaCO3<\/sub> \u2192 CaO + CO2<\/sub>
\nCaO + SiO2<\/sub> \u2192 CaSiO3<\/sub><\/p>\n
\nCast iron – It contains 3% carbon.
\nWrought iron or malleable iron – purest form of commercial iron.<\/p>\n
\nIt is prepared from; cast iron by oxidising impurities in a reverberatory furnace lined with haematite, which oxidises C to CO.
\nFe2<\/sub>O3<\/sub> + 3C \u2192 2Fe + 3CO<\/p>\n
\nrollers.<\/p>\n
\nThe sulphide ore (Cu2<\/sub>S) is roasted to give oxide (Cu2<\/sub>O).
\n2Cu2<\/sub>S + 3O2<\/sub> \u2192 2Cu2<\/sub>O + 2SO2<\/sub><\/p>\n
\nCu2<\/sub>O + C \u2192 2Cu + CO<\/p>\n
\n2Cu2<\/sub>O + Cu2<\/sub>S \u2192 6 Cu + SO2<\/sub><\/p>\n
\nZnO is reduced to metallic Zn by heating with coke.
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<\/p>\n
\nThe metal ions in solution or molten state are reduced by electrolysis or adding some reducing element. For the reduction to be feasible E\u00ae should be positive so that \u2206G\u0398<\/sup> is negative (\u2206G\u0398<\/sup> = – nFE\u0398<\/sup>)- During electrolysis, the less reactive metal will come out of the solution and the more reactive metal will go to the solution, e.g.
\nCu2+<\/sup>(aq) + Fe(s) \u2192 Cu(s) + Fe2+<\/sup>(aq)<\/p>\n
\nPurified Al2<\/sub>O3<\/sub> is mixed with Na3<\/sub>AlF6<\/sub> or CaF2<\/sub> to lower the melting point of the mix and bring conductivity. The fused matrix is electrolysed. Steel cathode and graphite anode are used.
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<\/p>\n
\nAt cathode : Al3+<\/sup>(melt) + 3e–<\/sup> \u2192 Al(l)
\nAt anode : C(s) + O2-<\/sup>(melt) \u2192 CO(g) + 2e
\nC(s) + 2O2-<\/sup> (melt) \u2192 CO2<\/sub>(g) + 4e–<\/sup><\/p>\n
\nFor each kg of aluminium produced, about 0.5 kg of carbon anode is burnt away as CO and CO2<\/sub>.
\nThe overall reaction is,
\n2Al2<\/sub>O3<\/sub> + 3C \u2192 4Al + 3 CO2<\/sub><\/p>\n
\nFor obtaining high purity metal, several techniques are used.<\/p>\n
\nThe impure metal is evaporated to get pure metal, e.g. low boiling metals like Zn, Hg<\/p>\n
\nlow melting metals like tin and lead are made to flow on sloping surface and thus seperated from high melting impurities.<\/p>\n
\nAnode – impure metal, Cathode – strip of same metal in pure form, Electrolyte – soluble salt of the same metal. On electrolysis pure metal is deposited at the cathode.
\ne.g. Electrolytic refining of Cu.<\/p>\n
\nimpure Cu, Cathode: pure Cu strip, Electrolyte: acidified solution of CuSO4<\/sub><\/p>\n
\nThis method is based on the principle that the impurities are more soluble in the melt than in the solid state of the metal. A circular mobile heater is fixed atone end of a rod of the impure metal. The molten zone moves along with the heater. As the heater moves forward, the pure metal crystallises out of the melt. The process is repeated several times. At one end impurities get concentrated. This end is cut off. e.g., Ge, Si, B, Ga and In.<\/p>\n
\nthe metal is converted into its volatile compound. It is then decomposed to give pure metal.<\/p>\n
\n1. The metal should form a volatile compound with an available reagent.
\n2. The volatile compound should be easily decomposable, so that the recovery is easy.<\/p>\n
\nNickel is heated in a stream of CO forming a volatile complex, nickel tetracarbonyl.<\/p>\n
\nThe crude metal is heated in an evacuated vessel with l2<\/sub>. The metal iodide being more covalent, volatilises.
\nZr + 2l2<\/sub> \u2192 Zrl4<\/sub> (volatile)<\/p>\n
\nZrl4<\/sub> \u2192 Zr + 2l2<\/sub><\/p>\n
\nTi + 2l2<\/sub> \u2192 Til4<\/sub> (volatile)
\nTil4<\/sub> \u2192 Ti + 2 l2<\/sub><\/p>\n
\nbased on the principle that different components of a mixture are differently adsorbed on an adsorbent.The mixture containing different metal ions are added into the chromatographic column. Different components are adsorbed at different levels on the column. The adsorbed components are removed (eluted) by using suitable solvents (eluant). Column chromatography is very useful for purification of elements which are available in minute quantities, e.g. Inner transition metals are refined by this method.<\/p>\n
\n1. Aluminium:
\naluminium foils are used as wrappers for chocolates, fine dust of Al is used in paints and lacqures, in the extraction of Cr and Mn from thier oxides, as electricity conductors, for making alloys, e.g. Duralumin (Al + Mg), Alnico (Al + Ni + Co).<\/p>\n
\nfor making wires used in electrical industry, for making water pipes and steam pipes, for making alloys, e.g. brass (Cu + Zn), bronze (Cu + Sn)<\/p>\n
\nfor galvanising iron, in batteries, as constituent of many alloys, e.g. brass (Cu – 60%, Zn – 40%), german silver (Cu 25-30%, Zn-25-30%, Ni 40 – 50%), zinc dust is used as a reducing agent in the manufacture of dye-stuffs, paints etc.<\/p>\n
\nCast lron:
\nfor casting stoves, railway sleepers, gutter pipes, toys etc; in the manufacture of wrought iron and steel<\/p>\n
\nin making anchors, wires, bolts, chains and agricultural implements.<\/p>\n
\nNickel steel is used for making cables, automobiles and aeroplane parts, pendulum, measuring tapes; Chrome steel is used for cutting tools and crushing machines; Stainless steel is used for cycles, automobiles, utensils, pens etc.<\/p>\n