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6A_Al_Extraction of metals.pptx

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6A_Al_Extraction of metals.pptx

  1. 1. Extraction of Aluminium 1
  2. 2. Aluminium • Important non-ferrous metal and is the second most useful metal. • Low density 2.54 g/cc • Low melting point 660 C • Can be subjected to rolling, extrusion, casting, forming and machining. • High specific strength. • Excellent corrosion resistance. • Lower strength and lower conductivity compared to copper, yet less expensive, more abundant and high specific strength. • Common alloying elements-Cu, Mg, Si, Mn, Zn 2
  3. 3. Natural occurrence of aluminum Most common minerals: • Gibbsite (Al2O3·3H2O) – theoretical concentration of Al2O3: 65.4% • Diaspore (Al2O3·H2O) – theoretical concentration of Al2O3: 85.4% Ores: • Bauxite: Mixture of gibbsite and diaspore along with Fe2O3, TiO2 and SiO2. • Typical composition of bauxite ores found in India: • Al2O3: 58-67% • TiO2: 5-10% • SiO2: 0.5-2% Mineral %Of the metal Chemical formula Bauxite 34.9% (Al) Al2O3 3H2O Gibbsite 34.6% (Al) Al(OH)3 Diaspore 85% (Al2O3) Al2O3H2O Cryolite 13%Al + 54.4%F Na3AlF6 Corundum 52.9% (Al) Al2O3
  4. 4. Natural occurrence of aluminum • Kashmir: ~ 75% alumina + ~ 5% silica (no TiO2) • Kashmir ores: not suitable for Bayer process due to the high concentration of SiO2. • Leaching results in the formation of a compound Na2O·Al2O3·3SiO2, which further reacts with other impurities to form an insoluble residue, “red mud” causing serious loss of alumina. • Hence ores with >5% SiO2 are considered unsuitable.
  5. 5. Extraction of aluminum • Tobe reduced from Al2O3 (thermodynamically very stable). • Carbothermic reduction? Not commercially feasible because.. • Extremely high temperatures up to 2000 °C arerequired. • Undesirable aluminum carbides form at this temperature. • High performance refractories are required, which are expensive and rare. • Aqueous electrolysis of aluminum salts? Not feasible because.. • Electrodeposition of aluminum ions requires higher potentials, but hydrogen evolution starts occurring at much lower voltages resulting in the electrolyte decomposition. • Electrolysis of Al2O3 dissolved in cryolite(3NaF·AlF3)? “Hall-Héroult process”
  6. 6. Extraction of aluminum • Electrolysis of Al2O3 dissolved in cryolite(3NaF·AlF3)? “Hall-Héroult process” • Al2O3 is obtained from Bayerprocess. • Hall-Héroult process consumes a large amount of energy. Hence a more recent innovation is: • Conversion of Al2O3 into AlCl3 (aluminum chloride), followed by electrolysis of fused salts of aluminum-/sodium-/lithium chlorides (energy consumption decreases by 30%). • Current scenario: high-grade aluminum resources are rapidly depleting. Hence low-grade ores such as clay are being sought after.
  7. 7. Bayer process • Aim: Toextract high-purity alumina (Al2O3) from bauxite. • Principle: Bauxite is leached with NaOH to form sodium aluminate [NaAlO2], from which aluminum hydroxide [Al(OH)3] is precipitated and later calcined to obtain pure alumina [Al2O3].
  8. 8. 8
  9. 9. Bayer process 1. Comminution: Grinding and crushing of bauxite ores to a very fine size in jaw crushers and hammer mills. • Silica concentrates in the finer fractions, while alumina gets enriched in the coarser fractions. • Partial beneficiation of the ore by screening to remove as much silica as possible. Jaw crusher Hammer mill
  10. 10. Bayer process 2. Ball milling with NaOH: Crushed bauxite is ground again with caustic soda (NaOH) solution in a ball mill. • This action results in the formation of a slurry. 3. Autoclaving of the slurry: alumina is dissolved in the NaOH slurry at ~ 220 °C and 5-25 atm pressure. • Impurities are left out as suspended solids in the solution.
  11. 11. Bayer process Leaching reactions during autoclaving: Al2O3·H2O + 2NaOH  2NaAlO2 + 2H2O …(1) Al2O3·3H2O + 2NaOH  2NaAlO2 + 4H2O …(2) Reaction (1) requires 3h at 230 °C. Reaction (2) requires 1h at 180 °C. Overall reaction time and temperatures are ~ 2.5h and 180-220 °C. Pressure (up to 25 atm) is applied in order to retain water in the liquid state. About 86-88% alumina is leached into the solution. ‘Pregnant liquor’
  12. 12. Bayer process 4. Settling and filtration: • The liquor is cooled to ~ 100 °C and completely depressurized. • Cooled liquor is sent to settling tanks: • Settling of red mud (ferric hydroxides, silica and some alumina) and clarification of the leach liquor. • Starch, as an additive, speeds up the settling ofred mud. • A series of washings, followed by filtration, is performed to remove the red mud completely. • Filtrate obtained: clear solution of sodium aluminate [NaAlO2].
  13. 13. Bayer process 5. Precipitation and thickening: • The heat is trapped by heat exchangers and the filtrate is cooled down to below critical temperature required for the precipitation of Al(OH)3. • Extremely fine, freshly prepared Al(OH)3 is added to the filtrate to accelerate the precipitation by introducing nucleation sites. 2H2O +NaAlO2→Al(OH)3 +NaOH • The precipitate [Al(OH)3] is separated in a series of thickeners. • Final product after precipitation and thickening: coarse hydroxide [Al(OH)3].
  14. 14. Bayer process Thickener tanks
  15. 15. Bayer process 6. Calcination: • The coarse hydroxide [Al(OH)3] is calcined in a rotary kiln to produce anhydrous alumina. 2Al(OH)3 → Al2O3+3H2O ~ 2-3.5 tonnes of bauxite ~ 0.08-0.12 tonnes of NaOH ~ 8-10 tonnes of steam ~ 0.18-0.2 tonnes of fuel oil 1 tonne of Al O 2 3
  16. 16. Factors affecting the Bayer process 1. Finer bauxite ore → higher surface area → faster reaction kinetics during leaching. (usual particle size ~ 150 µm) 2. Wet grinding is more efficient than dry grinding. 3. Wet grinding helps in saving time during the leaching/digestion stage.
  17. 17. Factors affecting the Bayer process 4. Digestion/leaching is temperature-sensitive according to the relation(Glastonbury1968): r =4.60 × 105A ·(CNaOH)1.73 e-23,850/(RT) r: initial extraction rate (g atom Al/second) A: surface area (of gibbsite) in cm2 CNaOH: Concentration of NaOH in mol L-1 R: gas constant T: absolute temperature • Large activation energy is required for the above reaction (23,850 cal mol-1). Hence temperature affects the kinetics significantly. The reaction at 100C is 150 time faster than that at 50C Al2O3·3H2O + 2NaOH 2NaAlO2 + 4H2O
  18. 18. Factors affecting the Bayer process 4. Too high temperatures: Higher pressures are to be applied to retain water in the liquid state. The solution may become unstable. • Insufficient temperatures: Al(OH)3 may precipitate leading to the loss of alumina and clogging of the filter. Hence, temperatures close to the boiling point of water are a minimum requirement. 5. Sensible heat • Hot pregnant liquor heat is recover by heat exchangers. • This uses to produce steam for the digesters
  19. 19. Factors affecting the Bayer process 6. During precipitation, the entire quantity of alumina is not allowed to precipitate: This may simultaneously lead to the precipitation of silica. Hence, some alumina is left in the liquor, which is recirculated to the digestion/leaching tank. 7. Calcination: • Drying of Al2O3 occurs at just 200 °C. But calcination of Al2O3 in rotary kilns is carried out at ~1400 °C. • Because the decomposition of Al(OH)3 produces monohydrate (Al2O3·H2O) and trihydrate (Al2O3·3H2O), in which water is chemically bound. These require temperatures of 800-1200 °C. • Addition of a small amount of CaF2 accelerates dehydration.
  20. 20. • https://www.youtube.com/watch?v=eGdXxFjqFsg&ab_channel=EmiratesGlobalAluminiu m 20
  21. 21. Hydrometallurgy of Aluminum • Bauxite: Al2O3.xH2O Gibbsite (x=3), and Diaspore (x=1) primary ore for Al impurities: SiO2 (must be less than 5%) Fe2O3 Aluminium oxide is very stable. Not possible pyro metallurgy Carbothermic reduction • Needs 20000C • Aluminum carbide formation – undesirable • High tamp refractories – highly expensive Aqueous Electrolysis of Al Salts • Decomposition and evolution H2 occurs – low voltage than Al 21
  22. 22. Bayer Process • Bayer process: bauxite (~ 50 % Al2O3) is concentrated to produce aluminum oxide. • Dissolve bauxite in strong base (NaOH) at high T(220C), P • To form soluble sodium aluminate • Al(OH)3 is calcined  Al2O3 Al2O3 dissolves [Al(H2O)2(OH)4] hydrated metal complex • Filter out solids Fe2O3, SiO2 do not dissolve • Lower the pH so that Al(OH)3(s) precipitates Takes advantage of the amphoteric nature of Al oxide. 22
  23. 23. 25 atm; 220 o C 1.5 % silica 100 o C below To cool calcined To produce 1ton of Al2O3 2.2-2.4 tons of bauxite 0.08-0.12 ton caustic soda 8-10 tons of steam 0.18-0.20 ton fuel oil 23
  24. 24. Fused salt electrolysis • Alumina dissolved in cryolite (3NaF.AlF3) is electrolyzed. • Alumina obtained by bayers process • Consumes large amounts of energy 6.51x106 kcal/ton • NaCl and LiCl fused salt : 30% less energy (Latest) • Hall: use purified Al2O3 in molten cryolite (Na3AlF6, melting point 1012C). Anode: C(s) + 2O2(l)  CO2(g) + 4e Cathode: 3e + Al3+(l)  Al(l) • The graphite rods are consumed in the reaction. • NaF and CaF2 are added – enhanced conductivity Electrometallurgy of Aluminum_ Hall Heroult process 24
  25. 25. Electrometallurgy of Al The Hall Heroult Process Anode: C(s) + 2O2-(l)  CO2(g) + 4e- Cathode: Al3+(l) + 3e-  Al(l) • Refractory lined steel box 5m long 2m wide, and 1m deep • Iron plate and iron bar which from the cathode • Anode is prebaked carbon electrodes 25
  26. 26. 26
  27. 27. Electrolytic refining of Al 27
  28. 28. Thank you 28

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