J.D. Lee Concise Inorganic Chemistry for JEE (Main Advanced) (iitjeebooks.com) export.pdf

248 Chapter 6 Metallurgy Cu 2 S + 3 O2 → Cu 2 O + SO2 ↑ 2 Cu 2 S + 2Cu 2 O → 6Cu(l) + SO2 ↑ and...

248 Chapter 6 Metallurgy Cu 2 S + 3 O2 → Cu 2 O + SO2 ↑ 2 Cu 2 S + 2Cu 2 O → 6Cu(l) + SO2 ↑ and Cu 2 S + 2O2 → Cu 2 SO4 Cu 2 S + Cu 2 SO4 → 4Cu + 2SO2 ↑ Thus the molten Cu obtained is poured into large container and allowed to cool and during cooling the dissolved SO2 comes up to the surface and forms blisters. It is known as blister copper. Refining of blister copper Blister Cu contains 2–3% impurity (mainly Fe, S and As). The steps involved in its refining are depicted in the flow diagram in Figure 6.34. (5) Poling Pure Blister Cu Purer copper Electrorefining copper Fe, S, As Removes SO2 (gas), (99.5% pure) Electrolyte : 15% CuSO + 4 (99.99%) As2O3 (volatile) 5% H2SO4 Cathode : Pure Cu strip and FeSiO3 (slag) Anode : Thick sheet of Cu (impure) Figure 6.34 Refining of blister copper. In the poling step, little Cu2O formed is reduced to metallic Cu by the reducing gases produced from charring of green wooden pole. The powdered anthracite (coke) spread on the top surface of the molten mass also helps produce a reducing environment. In the electrorefining step, the impurities like Fe, Ni, Zn get dissolved in the solution while Au, Ag and Pt are deposited as anode mud below the anode. 6.17 | EXTRACTION OF ZINC The various ores of zinc are: 1. Zinc blende : ZnS 2. Zincite : ZnO 3. Franklinite : ZnO.Fe2O3 4. Calamine : ZnCO3 5. Willemite : ZnSiO3 6. Electric calamine : ZnSiO3.ZnO.H2O The chief ore used for extraction of zinc is zinc blende, which is also known as ‘Black Jack’ due to the invariable association of galena (PbS) that is black in colour. Sometimes, calamine is also used to extract Zn by carbon reduction process. The flow diagram for extraction and refining of zinc from zinc blende is depicted in Figure 6.35. The changes and reactions involved at various stages of zinc extraction are: 1. In the froth floatation step: This is done in two steps to separate out PbS and ZnS depending upon their different floating characteristics. On addition of pine oil, PbS floats first and is removed. Then more pine oil is added and ZnS floats on the top. www.mediit.in  6.17 Extraction of Zinc 249 Finely divided (1) Froth floatation Concentrated zinc blende zinc blende (i) First with little pine oil + xanthate + little acid removes PbS. (2) Roasting (ii)Then more pine oil is added (i) Controlled current of air to float ZnS and silica/ (ii) Temp. = 850 °C to 900 °C silicates are removed Roasted ore ZnO + other impurities (3) Carbon Thermal Thermal reduction refining refining Spelter Zn Pure Zn Distillate + impurity (99.9 %) at 767 °C Zn and Cd at 920 °C Roasted ore (Pb, Cd, Fe) Removes Cd (distilled) + excess coke (vap)byproduct Temp. = 1300 °C to 1400 °C (4) Electrorefining Electrolyte: ZnSO4 + H2SO4 Cathode: Al sheet (acts as cathode) Anode: Impure Zn Pure Zn Figure 6.35 Flow chart for extraction and refining of zinc from zinc blende. 2. In the roasting step: During roasting, the temperature has to be above 850 °C and the air current must be controlled because below 850 °C and in excess of air, ZnS is converted into ZnSO4 which converts back to ZnS during carbon reduction of roasted ore. ZnS + 3 O2 850 above  °C → ZnO + SO2 ↑ 2 below ZnS + 2O2 850  °C → ZnSO4 during ZnSO4 + 4C carbon  reduction → ZnS + 4CO ↑ 3. In the smelting step: During smelting, excess coke is used to stop the production of CO2. Otherwise, Zn produced will be converted back to ZnO. Hence if any CO2 is produced, it is allowed to convert into CO by the reaction with coke. ZnO + C → Zn + CO 2ZnO + C  2Zn + CO2 (this reaction is reversible) CO2 + C → 2CO Temperature during smelting is kept above 1300 °C though the b.p. of Zn is 920 °C. The tempera- ture is kept much higher as compared to that required for vapourizing zinc from the furnace. This is done because the reaction of carbon dioxide with coke is highly endothermic and brings down the tem- perature to below 920 °C and the evaporation of Zn is affected. Hence the temperature is maintained at 1300 °C–1400 °C. 4. In the electrorefining step: For electrorefining of Zn (crude), Al sheet is used as cathode instead of pure Zn strip. This is because the electrolyte used is ZnSO4 + H2SO4(dil.), and in dil. H2SO4, Zn gets dissolved while Al does not. Zn + H 2 SO4 (dil) → ZnSO4 + H 2 ↑ Al + H 2 SO4 (dil) → No reaction 2 Al + 6 H 2 SO4 (conc.) → Al 2 (SO4 )3 + 3SO2 + H 2 O www.mediit.in 250 Chapter 6 Metallurgy Reactions at the electrode: ZnSO4 → Zn 2 + + SO4 2 − At the cathode: Zn 2 + + 2e → Zn At the anode: Zn → Zn 2 + + 2e Note: The H2SO4 is added in the electrolyte together with ZnSO4 to increase the over voltage of H+. This helps in the deposition of only Zn2+ at the cathode, otherwise H2 will be evolved at the cathode. 6.18 | EXTRACTION OF IRON The various ores of iron are: 1. Haematite : Fe2O3 2. Magnetite : Fe3O4 3. Brown haematite or limonite : Fe2O3.3H2O 4. Siderite or spathic iron ore : FeCO3 5. Iron pyrite : FeS2 The chief ore used for extraction of iron is haematite, while FeS2 is never used because iron obtained from this ore contains a lot of sulphur which makes it brittle and of no use. The flow diagram for extraction of iron from different ores is depicted in Figure 6.36 Magnetite Fe3O4 Roasted (1) Finely divided Concentrated (2) Roasting ore Haematite Fe2O3 ores are taken ore Fe2O3 Limonite Fe2O3.3H2O for gravity + Removes CO2, SO2, + separation. SiO2 SiO2 Siderite FeCO3 As2O3 and H2O Removes SiO2 + (small amount) (small amount) other silicates In (3) Smelting blast Roasted ore : 8 parts furnace Coke powder : 4 parts Lime stone : 1 parts Cast Fe - Scrap + Coke Molten metal Slag iron (Air is blasted) (Pig iron 4% C) (CaSiO3) (3% C) Figure 6.36 Flow chart for extraction of iron from haematite. Charge The reactions at various steps of the iron extraction process are : Cup and 1. In the roasting step: cone arrangement Fe3O4 → FeO + Fe2O3 400 °C FeCO3 → FeO + CO2 ↑ 660 °C 1 700 °C 2 FeO + O2 → Fe2O3 2 900–1000 °C Fe2O3.3H 2O → Fe2O3 + 3H 2O ↑ Bosch 1200–1300 °C Hence the final product of roasting is Fe2O3. Hot air Though there is no sulphide ore yet roasting is adopted here Hot air Tuyeres to convert all FeO present into Fe2O3. As Fe2O3 does not 1500°C (nozzles) form slag, this prevents the loss of FeO as slag (FeSiO3). Molten Hearth cast iron 2. In the smelting step: The various changes taking place Slag during smelting in the blast furnace are shown in Figure 6.37. The reactions involved are: Figure 6.37 Smelting in the blast furnace. www.mediit.in  6.18 Extraction of Iron 251 At 600–900 °C: Fe2 O3 + 3CO → 2 Fe + 3CO2 ↑  (Partially reduced) At 900–1000 °C: CaCO3 → CaO + CO2 ↑ CO2 + C → 2CO ↑ At 1000–1300 °C: Fe2 O3 + 3C → 2 Fe + 3CO ↑ CaO + SiO2 → CaSiO3 (slag) At 1500 °C (i.e. at the hearth): The coke powder crossing the line of tuyers does not have the scope of burning any more and reacts with MnO2 and SiO2 to produce impurities like Mn and Si as follows: MnO2 + 2C → Mn + 2CO SiO2 + 2C → Si + 2CO Ca3(PO4)2 present in the limestone reacts with SiO2 to produce slag and P2O5 is reduced by coke to produce P4 as impurity. Ca 3 (PO4 )2 + 3SiO2 → 3CaSiO3 + 2 P2 O5 2 P2 O5 + 10C → P4 + 10CO ↑ Finally the cast iron produced consists of impurities like Mn, Si, P, C and S. Purification of iron or preparation of wrought iron Wrought iron is the purest form of iron which con- Cast iron tains the total impurity less than 0.5%. The carbon content is 0.1 – 0.15% and other impurities (Mn, P, S, Mn, P, Si, C, S Puddling process Wrought Si) are less than 0.3%. The steps involved in prepara- taken in puddling Hot air is blasted iron furnace which is lined and stirred well tion of wrought iron are shown in the flow diagram with haematite Removes CO2, SO2 given in Figure 6.38. and slag The haematite lining has a special significance as it removes the impurity as well as produces iron at Figure 6.38 Flow chart for preparation of that place. wrought iron from cast iron. The various reactions taking place in the process are: S + O2 ® SO2 -; C + O2 ® CO2 - 3S + 2Fe2 O3 ® 4Fe + 3SO2 - 3Si + 2Fe2 O3 ® 4Fe + 3SiO2 3Mn + Fe2 O3 ® 2 Fe + 3MnO MnO + SiO2 ® MnSiO3 (slag) 3C + 2Fe2 O3 ® 2 Fe + 3CO - 4P + 5O2 ® P2 O5 ; Fe2 O3 + P2 O5 ® 2 FePO4 (slag) Byproducts of iron extraction 1. Slag: It consists of huge amount of CaSiO3 and little Al2(SiO3)3. These days it is used for making cement and now is known as slag cement. 2. Blast furnace gas: The composition of the blast furnace gas is 58% N2, 25% CO, 10.5% CO2 and 6.5% H2 and the rest are hydrocarbons. It contains a very large quantity of CO and the H2 which constitutes a good fuel. It is used for preheating the air used and for cooking purpose also. www.mediit.in 252 Chapter 6 Metallurgy Steel making Steel is made by removing most of the carbon and other impurities from pig iron. Composition of various steels depending upon percentage of carbon is given as follows %C Type of steel 0.15 – 0.3 Mild steel 0.3 – 0.6 Medium steel 0.6 – 0.8 High carbon steel 0.8 – 1.4 Tool steel The steel making process involves melting and oxidizing C, Si, Mn, S and P present in the pig iron so that these impurities are removed as gases or converted into slag. This is followed by addition of required additives (i.e. different elements) to iron to impart desired properties to steel. Different elements present in steel provide different properties as described below Element Properties imparted P above 0.05% Imparts low tensile strength and cold brittleness. Mn Imparts high hardness and increases tensile strength, e.g. rail road contains 13% Mn. Cr and Ni Imparts stainless characteristic by producing impervious coating of their oxides on the surface. N (above 0.01%) Makes steel brittle as well as difficult to weld C Improves hardness and strength. The addition of mixture of C and Mn into produced steel is known as ‘spiegeleisen’. C and Mn act as deoxidizers, remove any dissolved O2 and reduce FeO (if any). The excess carbon (if any) supplies the desired quality. Mn makes the steel hard and improves its tensile strength also. Various processes used for preparation of good quality steel are: FeO + Mn → MnO + Fe; FeO + C → Fe + CO ↑ 1. Puddling process: This process involves stirring of molten iron in reverberatory furnace by rods which are consumed in the process. 2. Bessemer and Thomas process 3. Siemens open hearth process 4. Basic oxygen process (also called Linz-Donawitz (L.D.) process). All these processes are developed to economize the production of steel from iron. In the Bessemer and Thomas process or Siemens open hearth process, impurities are oxidized by air; while in the L.D. process, pure O2 is used for the oxidation of impurities. This is because in the first two processes the molten metal takes up small amount of nitrogen from the air. In concentrations above 0.01%, nitrogen makes steel brittle and nitriding the surface makes the metal more difficult to weld. The use of O2 not only helps overcome these problems but also has the following advantages: 1. There is faster conversion, so a given plant can produce more in a day i.e. larger quantities can be handled in lesser time. For example, a 300 tonnes charge can be converted in 40 minutes compared to 6 tonnes in 20 minutes by the Bessemer process. 2. It gives a purer product and the surface is free from nitrides. The lining of the furnace is designed based on the impurities present in the cast iron: 1. If the cast iron contains Mn, but not P, S, Si, then the lining used is silica brick and the process is known as acid Bessemer process. 2. If the cast iron contains acidic impurities such as Si, S, P, a lining of calcined dolomite (CaO·MgO) or magnesia (MgO) is used and the process is called basic Bessemer process. In this process, the P2O5 formed from P combines with lime and forms basic slag [Ca3(PO4)2.CaO] which is known as Thomas slag. It is a valuable byproduct and sold as phosphate fertilizer. www.mediit.in