october 19, 2001

Element of the Day-- Aluminum

• rest of world adds an i : Aluminium

Aluminum is a familiar metal

• architectural: window and door frames
• consumer uses: beverage cans, foil
• can evaporate and coat plastics, mirrors, etc.
• reasonably strong (limits use)
• low density (valuable on aircraft, cars)
• ability to extrude-- complex cross section

Most Al use is as the metal

• often alloys, but 75-95% Aluminum
• limited uses of Aluminum Compounds

Most important exception

• Aluminum Oxide, Al₂O₃
• Chapter 6 also discussed aluminum salts in water cleanup
• also used in dye processes

• Aluminum metal was not isolated until 1827
  • extremely rare and expensive until 1890's
    • in 1852 $545 per pound
    • 1886 $11 per pound
• Napoleon had Gold and Aluminum place settings
  • Al was reserved for royalty and the very elite
• Cap on the Washington monument (1884)--
  • Aluminum $1/oz when 1 day's work=$1
  • nothing cheap for a young nations' pride and joy
  • symbolic of machine age, technology

Chemically

• Aluminum (compounds) very common
• 8% by weight of the earth's crust
  • almost all clays, many rocks, like granite are Aluminum species
  • useful ores are much more limited-- Bauxite

Chemically, Aluminum is very surprising

• It is a very reactive species
• If you put Al, Ca, Na, Zn and Fe in HCl (acid)
  • most reactive is Na, then Ca
    • (react with water)
  • then Al
  • then Zn
  • followed by Fe
• Yet Aluminum seems very inert
• Aluminum actually oxidizes rapidly
  • (5 minutes in air)
  • the oxide (Al₂O₃) sticks to the surface
  • the oxide layer is tough, glass-like
    • transparent
    • water and oxygen are blocked out
  • so further oxidation stops almost immediately
  • except for strong acids that dissolve the oxide
• aluminum tends to keep decades w/ little corrosion

Alumina or Aluminum Oxide

• white solid-- powder, granules
• very hard, great for cutting and polishing
  • best "sandpaper" is actually aluminum oxide
  • most grinding and cutting wheels are alumina
  • refractory-- melts at very high temperatures
    • can make bricks to line blast furnaces
    • can make ladles for molten metals
    • make holders for heating element in toasters
• will crystallize into larger clear crystals
  • colorless if pure
  • 1% of Cr, Ni, Fe, etc. will color strongly
  • called ruby (red), sapphire (blue)
  • natural and synthetic gemstones

Anodizing Aluminum

• we can actually make the surface coating thicker
  • put Al sheet into a NaOH solution
  • add second metal (electrode)
  • connect Al to one electrical terminal
    • still looks almost unchanged
    • perhaps a little hazy, less shiny
  • a. will absorb dye (can get quite dark or brilliant)
  • b. surface is really hard, won't scratch
    • metal surfaces slide better (windows)
  • c. small pieces are much stiffer
  • d. the metal becomes an electrical insulator

Big Part of the Al Story

how to convert an ore to metal?

• developed by Charles Martin Hall, 1886 after graduation from Oberlin College
  • simultaneously by Frenchman Paul L.T. Heroult

Charles Hall and Aluminum, web site

• need to do it electrically
  • Al⁺³ + 3e---> Al (metal)
• can't do it in water
  • H₂O --> H₂ + OH- instead
  • (this also means we can't electroplate aluminum)
• most Al compounds don't melt
  • when they do, nonionic liquid
  • discovered a good compound
    • cryolyte (it is an Al/Na/F compound)
    • melts at a modest temperature
    • will dissolve Al₂O₃
    • will create Al⁺³ and O^2- ions in solution
• key reaction Al⁺³ + 3e---> Al (metal)
• second electrode is graphite
  • C + 2O^2- ----> CO₂ + 2e

Cost

• Aluminum ore moderately inexpensive
• Heat to melt (modest cost)
• Electrical power for the chemistry (big)

• 100 amp line in my house (assume full use)
  • 110V so power is 110V x 100 amp =1.1 x 10⁴ watts
  • running for 1 hour that's 11 killowatt hr of energy
• to make 1 mole of Al metal , need 3 moles of electrons
  • 1 mole = 96,500 amp seconds
  • 9650 sec at 100 amps
    • about 3 hours
    • or 30 kilowats of energy
      • actually need about 10 volts
      • really 3 kilowatts
    • I pay about 5-10 cents /KWhr
    • so that's about $0.30 /pound to make Al

Al is a moderately expensive metal

• not to mine
• but to refine

• Al recycling is good business
  • cost is to collect and melt
  • avoid electrical costs
• Compare to Iron
  • ore costs less than shipping scrap metal
  • reducing ore to metal is smaller expense

University of Wisconsin's Element of the Day page for Al

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