Aluminium Extraction PDF

_ @edubuzznotes 240 Chapter 6 Metallurgy 6. Silicate : Feldspar (K2O⋅Al2O3⋅6SiO2) China clay or Kaolin (Al2O3⋅2SiO2⋅2H2O) Bauxite is the ore most commonly used for extr...

_ @edubuzznotes 240 Chapter 6 Metallurgy 6. Silicate : Feldspar (K2O⋅Al2O3⋅6SiO2) China clay or Kaolin (Al2O3⋅2SiO2⋅2H2O) Bauxite is the ore most commonly used for extraction of aluminium. Bauxite is of two types 1. Red bauxite: Al 2O3 ⋅ 2H 2O + Fe2O3 (major impurity) + SiO2 + TiO2 2. White bauxite: Al 2O3 ⋅ 2 H 2O + SiO2 (major impurity) + Fe2O3 + TiO2 The main steps involved in the extraction of Al from bauxite are: 1. Benefciation of bauxite and preparation of pure alumina. 2. Electrolytic reduction of pure alumina. 3. Electrorefning of aluminium. Benefciation of bauxite Different processes adopted for benefciation of red and white bauxite are described below. Bayer’s process This is used for benefciation of red bauxite. Various stages of the process are depicted in the fow chart (Figure 6.24). Finely powdered bauxite Calcined Al2O3. 2H2O + Fe2O3 Calcined ore SiO2 + TiO2 FeO (if any)→Fe2O3 Removes organic (1) Digested with 45% NaOH matter solution at 150°C and 80 lb pressure for 8 hrs in an autoclave and finally filtered under hot condition Filtrate: NaOH (2) (i) Diluted with huge water Filtrate: Residue: together with NaAlO2 + Na2SiO3 Fe2O3, TiO2 freshly prepared Residue: + other insoluble Al(OH)3 as Si[OH]4 + Al[OH]3 impurities seeding agent Ignited or (ii) CO2 is at ~ 1100 °C passed Pure Al2O3 + SiO2(little) Figure 6.24 Flow chart for Bayer’s process for benefciation of bauxite. The reactions involved in the above process are: 1. In step 1: Al 2 O3 + 2OH − + 3H 2 O → 2[Al(OH)4 ]− SiO2 + 2 NaOH ® Na 2 SiO3 + H 2 O 2. In step 2: + 3H  [Al(OH)4 ] −     Al(OH)3 ↓   3+  Al + 3H 2 O OH − white Chapter-6.indd 240 2/19/2018 2:03:04 PM  _ @edubuzznotes 6.14 Extraction of Aluminium 241 Since Al(OH)3 is amphoteric in nature, it gets dissolved in alkali as well as in acid as shown in − r­ eactions above. Hence to get Al(OH)3 from [Al(OH)4] , the pH of the medium is to be reduced but it is to be taken care that the pH is not low enough to dissolve it further in the form of Al3+. The decrease in pH can be done by 1. adding large amount of water which increases the volume and decreases the pH. But without the ­seeding agent (which provides the nucleus of the precipitate) the precipitation is delayed. 2. acidifcation which reduces the pH of the medium. But here a strong acid is not recommended to avoid any further dissolution of Al(OH)3 in the form of Al3+. Hence weak acid like CO2 gas is passed which reacts as follows: CO2 + 2OH − → CO3 2 − + H 2 O CO2 + H 2 O → H 2 CO3 [weak enough to dissolve Al(OH)3 ] −  Al (OH )4   Al(OH)3 ↓ +OH − OH − ions are consumed in the above reaction and the formation of Al(OH)3 is favoured. Note: The Bayer’s process cannot be adopted for white bauxite because the major impurity SiO2 is also separated out along with Al­2O3 and fnally Al2O3 of inferior quality is formed. Hall’s process This process is adopted for benefciation of low grade red bauxite. Various stages of the process are depicted in the fow diagram in Figure 6.25. Residue: (1) Fused at 1100 °C SiO2 + Fe2O3, etc Finely divided bauxite ore together with Na2CO3 + little CaCO3 Filtrate: [Al(OH)4]− and finally extracted with water and filtered (2) CO2 passed and filtered Al2O3 (3) Ignited Residue: Filtrate: (pure alumina) at 1100 °C Al(OH)3 Na2CO3 solution Figure 6.25 Flow chart for Hall’s process for benefciation of low grade red bauxite. The reactions involved in the above process are: 1. In step 1: Al 2 O3 + Na 2 CO3 → 2 NaAlO2 + CO2 ↑ SiO2 + Na 2 CO3 → Na 2 SiO3 + CO2 ↑ Fe2 O3 + Na 2 CO3 → 2 NaFeO2 + CO2 ↑ CaO + SiO2 → CaSiO3 2. In step 2: 2 NaAlO2 + CO2 + 3H 2 O → 2 Al(OH)3 ↓ + Na 2 CO3 3. In step 3: ∆ 2 Al(OH)3 − 3H O → Al 2 O3 + 3H 2 O 2 Chapter 06.indd 241 3/21/2017 10:35:12 AM  _ @edubuzznotes 242 Chapter 6 Metallurgy Serpeck’s process This process is used for benefciation of white bauxite. Various steps of this process are depicted in the fow diagram in Figure 6.26. CO↑+ Si↑ Finely powdered (1) Heated bauxite + Coke to 1800 °C powder in a current AlN(s) residue + impurity (Fe2O3 + TiO2) of N2 gas (2) Hot and dilute NaOH solution is added and filtered Filtrate: NaOH/ Na2CO3 solution (3) Huge water Filtrate: Residue: or CO2 passed Na[Al(OH)4] solid impurity Al2O3 Ignited Residue: and filtered (Fe2O3 + TiO2) (pure at 1100 °C Al(OH)3 alumina) Figure 6.26 Flow chart for Serpeck’s process for benefciation of white bauxite. The reactions involved in the above process are: 1. In step 1: Al 2 O3 + 3C + N 2 ® 2 AlN(s) + 3CO ↑ SiO2 + 2C ® Si ↑ + 2CO ↑ 2. In step 2: AlN + NaOH + 3H 2 O ® Na[Al(OH)4 ] + NH 3 - 3. In step 3: H2O  Na[Al(OH)4 ]    Al(OH)3 ¯ + NaOH or -        [Al(OH) ] 4   Al(OH)3 ¯ + OH - CO2 + 2OH - ® CO3 2 - + H 2 O Electrolytic reduction of pure Al2O3 In electrolytic reduction of Al2O3, i.e. molten alumina (20 %)mixed, with cryolite (60%) and fuorspar (20 %) is taken in an iron tank with carbon lining that acts as the cathode. A graphite rod hanging from the top acts as the anode. A powdered coke layer is maintained at the top (Figure 6.27). + Graphite rod acts as anode Powdered − coke layer Iron tank Lining of carbon acts as cathode Molten Al Tap Electrolyte (molten alumina): molten alumina (20%) hole + cryolite (60%) and fluorspar (20%) Figure 6.27 Electrolytic reduction of alumina. Chapter 06.indd 242 3/21/2017 10:35:15 AM  _ @edubuzznotes 6.14 Extraction of Aluminium 243 On electrolysis molten Al is deposited at the cathode and since Al is heavier as compared to the electrolyte, it gets deposited at the bottom, while oxygen gas is liberated at the anode. 1. Reactions: Al 2 O3 → 2 Al 3 + + 3O2 − At the cathode: Al 3 + + 3e → Al Since Na and Ca are more electropositive than to Al, only Al3+ gets deposited at the cathode. At the anode: 2O2 − → O2 + 4e 2. Functions of fuorspar (CaF2) a. It reduces the melting temperature of the mixture to 900 °C from 2050 °C (m.p. of pure Al2O3) and saves on the fuel cost. b. It also improves the electrical conductivity of the melt as compared to molten Al2O3. 3. Functions of cryolite a. It also helps reduce the melting temperature of the mixture. b. It acts as solvent and helps dissolve Al2O3. Alternative theory for electrolytic reaction at the anode can explain the dissolution of Al2O3. AlF3.3NaF → Al 3 + + 3Na + + 6F − , Al 2 O3 → 2Al 3 + + 3O2 − and CaF2 → Ca 2 + + 2 F − Since the [F−] >>>[O2–], F − ions gets discharged at the anode frst which in turn react with Al2O3 and liberate O2 at the anode. 2F − → F2 + 2e 3F2 + Al 2 O3 → 2AlF3 + 23 O2 4. Function of the coke powder layer at the top: a. The oxygen liberated at the anode corrodes the anode surface reacting with graphite to produce CO and CO2 and fnally the anode cuts down (a) (b) (c) at the bottom and electrical connectivity is lost (Figure 6.28). Figure 6.28 Corrosion of graphite anode.   At the junction of liquid–solid–air interface, the energy available is maximum (this can be proved thermodynamically) and corrosion is maximum at this point. To prevent this corrosion, the coke powder (having large surface area for reaction) layer is kept at the top which reacts with liberated oxygen or oxygen from the air. b. The surface becomes rough unlike the shiny mirror like molten electrolyte; and the radiation loss of heat is also prevented. − Electrorefning of aluminium Impure aluminium mixed with copper melt Graphite rod is taken in an iron tank with graphite lining. Layer of pure Al The layer of pure Al acts as the cathode. The Impure Al (acts as cathode) ­graphite rods at the top are essential for electri- Electrolyte cal ­connection (Figure 6.29). Here the electrolyte is the molten mixture Graphite lining of cryolite and BaF2 saturated with Al2O3. BaF2 Al (impure) + Cu melt is added instead of CaF2 to adjust the density in such way that it exists as a separate middle layer. Similarly in impure Al, the Cu melt is deliber- + ately added to increase the density in such a way that it exists as a separate bottom layer. Figure 6.29 Electrorefning of aluminium. Chapter 06.indd 243 3/21/2017 10:35:19 AM  _ @edubuzznotes 244 Chapter 6 Metallurgy The reactions involved in the process are: At the anode: Al → Al 3 + + 3e Here the top surface of bottom layer acts as the anode and Al only enters into the electrolyte as Al3+ ° ° because E Al / Al 3+ > ECu / Cu 2+ At the cathode: Al 3 + + 3e → Al Here the bottom surface of the top layer (the pure Al melt) acts as the cathode and Al3+ enters as Al from the electrolyte. Note: (i) Impurities like Fe, Si and Cu remain intact in the bottom layer. (ii) When the thickness of the top layer is increased to certain limit, it is drained out into a separate container. 1. Pure Al2O3 is not used as the electrolyte because: a. The m.p. of Al2O3 is 2050 °C, hence the power consumption is very high. b. Al obtained at this temperature gets volatilized and the loss is very high. c. Aluminium is lighter than Al2O3, and foats at the top surface and volatilizes easily, as well as attacked by the oxygen liberated at the anode. 2. The aqueous solution of Al salt cannot be used as electrolyte because hydrogen is discharged at the cathode in preference to Al as the discharge potential of H+ is smaller compared to Al3+. For example, if Al (NO3)3 is used, then the reactions involved are Al(NO3 )3 → Al 3 + + 3NO3− H 2 O → H + + OH − At the cathode: 2 H + + 2e → H 2 (Al3+ does not discharge) At the anode: 4OH − → 2 H 2 O + O2 ↑ + 4e 3. Molten AlCl3 is not chosen as electrolyte because being covalent in nature, it is a poor conductor of electricity. It also sublimes easily. 6.15 | EXTRACTION OF LEAD The important ores of lead are: 1. Galena : (PbS) 2. Cerrusite : PbCO3 3. Anglesite : PbSO4 4. Crocoisite : PbCrO4 5. Lanarkite : (PbO.PbSO4) The ore used commercially for extraction of lead is galena. Depending upon the impurity content, lead can be extracted from galena by one of the following two processes. 1. Carbon reduction (when the impurity content is high enough). 2. Self reduction (when the impurity content is low). Carbon reduction The various stages involved in the carbon reduction process for extraction of lead are depicted in the fow diagram in Figure 6.30. Chapter 06.indd 244 3/21/2017 10:35:23 AM