Recycling Aluminum Chemically
Adapted for use at Collin College by A. El-Ashmawy and C. F. Jury © 1998
INTRODUCTION
Aluminum is the 3rd most abundant element in the earth’s crust. However, vast amounts of energy
are necessary for the production of aluminum from its ore sources. This is why many communities
have programs in place to recycle aluminum cans. The lifetime of an aluminum can on average is
about 100 years. The littering of cans along roadways are not only an eyesore but also a drain on
natural resources.
This experiment will demonstrate a chemical technique for recycling used aluminum. You will
prepare the compound potassium aluminum sulfate dodecahydrate, KAl(SO4)2 12 H2O, or Alum for
short, from elemental aluminum. Alum is used in the paper industry, in processing pickles, in dying
fabrics and in water purification as well as wastewater treatment. This technique can be used with
aluminum cans but only after removing the exterior paint and the interior plastic coating.
Aluminum is a reactive metal which reacts slowly with dilute acids. The rate of reaction is slowed by
an aluminum oxide layer which protects the surface of the metal. Alkaline (basic) solutions attack the
metal surface after dissolving the oxide layer. Excess base converts the aluminum to the
tetrahydroxoaluminate III ion. This ion is only stable in basic solution. Upon the slow addition of acid
(H2SO4), one of the hydroxide ions is removed, giving Al(OH)3 which forms a white flocculent
precipitate. Continued addition of acid dissolves the Al(OH)3 precipitate, eventually forming the
hydrated aluminum cation [Al(H2O)6]+3. One can also dissolve the Al(OH)3 precipitate by addition of
alkali reforming the [Al(OH)4)]- complex. This kind of behavior, a hydroxide which is soluble in either
acid or base is called amphoteric.
Alums are ionic compounds which crystallize from solutions containing sulfate anion, a monovalent
cation and a trivalent cation. Common cations used are K+, Na+, NH4+, Al+3, Cr+3, and Fe+3. Crystals
of alums may grow to considerable size given the right conditions. These crystals form as cubes or
octahedral, with the crystal lattice containing a total of 12 water molecules. Six water molecules are
bound tightly to the trivalent cation, and six water molecules are bound loosely to the monovalent
cation and sulfate anion.
REACTIONS
2 Al(s) + 2 KOH(aq) + 6 H2O(l)  2 K+(aq) + 2 [Al(OH)4]-(aq) + 3 H2(g)
2 K+(aq) + 2 [Al(OH)4]-(aq) + H2SO4  2 Al(OH)3 + 2 H2O(l) + H+(aq) + 2 K+(aq) + SO4-2(aq)
2 Al(OH)3 + 3 H2SO4  2 Al+3 + 3 SO4-2(aq) + 3 H2O(l)
K+(aq) + Al+3 + 2 SO4-2 + 12 H2O(l)  KAl(SO4)2 12 H2O(s)
OVERALL:
2 Al(s) + 2 KOH(aq) + 22 H2O(l) + 4 H2SO4  2 [KAl(SO4)2 12 H2O(s)] + 3 H2(g)
PROCEDURE (Work in a fume hood)
1. W eigh approximately 1g of Al metal & place in a 250mL beaker.
2. Add 25mL 2.8M Potassium Hydroxide (KOH) to the beaker. W hat happened?
3. After five minutes, place the beaker on a hot plate & heat the reaction mixture GENTLY.
4. If the liquid level in the beaker drops to half the original volume, add deionized water to maintain
the volume. Do not let the beaker dry out! W hat color is the reaction?
5. Heat the reaction until the reaction is complete. How will you know when this happens?
6. W hile the solution is HOT, it must be vacuum filtered to remove any undissolved materials. This is
done using an aspirator and Buchner funnel system. Be sure your apparatus is securely
clamped. Have the instructor check it before completing the filtration. What’s on the filter paper?
W hat color is the filtrate?
7. Rinse the reaction beaker 2x with 5mL portions of deionized water, pouring each rinse through the
filter.
8. Transfer the filtrate to a clean 250mL beaker. Rinse the filter flask with a 10mL portion of
deionized water. Add this to the filtrate.
9. Cool the solution, slowly and carefully.
10. W hen the solution is cool, W ITH STIRRING add 20mL 9.0M Sulfuric Acid (H2SO4). Any
observations?
11. If the solution is not clear, warm the solution GENTLY until it clears up.
What species does this solution contain? Can you name them? (Hint: See equations on previous
page)
12. Prepare an ice water bath by filling a 600mL beaker half full of crushed ice, just covered with
water.
13. Place the 250mL beaker containing the filtrate in the ice bath. Allow to cool for 15 minutes.
W hat is happening in the beaker?
14. W eigh a piece of filter paper. Clean and reassemble the vacuum filtration apparatus using the
weighed filter paper. Filter your mixture.
15. Prepare an ethanol-water rinse by mixing 12mL ethanol with 12mL deionized water. Cool the
rinse solution in the ice water bath for a few minutes. Rinse any solid which may be on the filter.
Apply vacuum suction until all crystals are dry.
16. Allow any remaining product to air dry.
17. W eigh the alum crystals.
18. Show product to your instructor, the follow their instructions for storing your product (if needed).
REPORT—Recycling Aluminum Chemically
Name: _____________________________ Class/Section: ______________
Data/ Observations
1. Mass of Al: _______________
2. Al + KOH Observations (what color?):
_________________________________________________________________________
5. Reaction is complete when _________________________________________________
6. (a) Vacuum filtration apparatus checked by instructor ________
(b) On Filter Paper: ______________________________________________________
(c) Color of Filtrate: ______________________________________________________
17. Mass of Alum: _______________
18. Alum shown to instructor: ________
Calculations (Show ALL work on the back of this sheet)
Mass of Al: _______________
Moles of Al: _______________
Mass of Alum produced: _______________
Moles of Alum produced: _______________
Theoretical yield of Alum: _______________
Percent yield of Alum: _______________