Recycling of Used Aluminum Cans to Alum - Essay Example

Running head: RECYCLING ALUMINUM TO ALUM Recycling Of Used Aluminum Cans to Alum: A Study Affiliation Introduction Aluminum and its alloys find several applications today, right from soda cans to aircraft material. This is because aluminum is versatile and has many desirable properties: light weight, nontoxic, pleasing appearance, ability to take high polish, high electric and thermal conductivities, corrosion resistance, malleability, ductility, non-magnetic, and non-sparking. (Greenwood & Earnshaw, 1997) Aluminum combines with other metals to give alloys with high mechanical and tensile strengths. The abundance of its ore makes aluminum and its alloys easily available - Aluminum comprises 8.3% by weight of the earth’s crust, exceeded only by oxygen (45.5%) and silicon (25.7%).(Greenwood & Earnshaw, 1997) Apart from primary aluminum obtained from bauxite, secondary aluminum and aluminum compounds are obtained through recycling. Recycling aluminum prevents depletion of aluminum ore reserves. Compared with primary production, production of secondary aluminum requires a lower amount of energy. (Davis, 1993) One compound that can be obtained by chemical treatment of used aluminum is potash alum. Potash alum or common alum (potassium aluminum sulfate dodecahydrate, KAl(SO4)2.12H2O) is a double salt. The Al3+ ions in alum act as flocculating agent during water treatment and purification and also as a blood congealer in medicine. (Atkins & de Paula, 2006) To investigate a possible method for recycling used aluminum cans, potash alum was prepared from a piece of aluminum can. Feasibility of the recycling process was studied by calculating the percentage yield. Experiment Reagents Aluminum beverage can, aqueous potassium hydroxide (KOH (aq), 1.4M), aqueous sulfuric acid (H2SO4 (aq), 9.0 M), distilled water Equipment Balance, vacuum filtration apparatus, aspirator, ice bath, hot plate Precautions Gloves were used while handling aluminum. Heating of aluminum and KOH mixture was done in the hood. Procedure Collecting and weighing aluminum. The aluminum beverage can was washed. A 5×8 cm piece of aluminum was cut from the can. Paint from the outer surface of the can and plastic from the inner surface of the can were scraped off. The aluminum piece was cut into small squares. The aluminum squares were weighed and 0.996 g of aluminum was placed in a 250 mL beaker. Addition of potassium hydroxide. To the aluminum in the beaker, 50 mL of 1.4 M potassium hydroxide was added. The reaction mixture of aluminum and KOH was heated on a hot plate in the hood. Heating was stopped when the mixture turned grey. Vacuum filtering. Vacuum filtration apparatus was set up. The hot solution was vacuum filtered. The beaker used was rinsed with distilled water (5 mL) and this solution was also vacuum filtered. The filtrate obtained was cooled in an ice-water bath for a few minutes. Addition of sulfuric acid and crystallization of alum. To the cooled filtrate, 20 mL of 9.0 M sulfuric acid was added slowly with constant stirring. A clear solution was obtained. The beaker with clear solution was chilled in an ice-water bath. A drop of solution was placed at the end of a stirring rod. The liquid on the stirring rod was evaporated by blowing until a crystal was formed. This stirring rod with the seed crystal was then placed in the solution. The solution was left to stand for 15 minutes, by when crystallization took place. Vacuum filtration and drying of alum crystals. Vacuum filter apparatus was reassembled and the crystals were filtered. The beaker was rinsed with 12 mL cold ethanol (50%) and this rinsed solution was filtered. The rinsing was repeated. The crystals were then spread evenly over the filter paper. An aspirator was used for 10 minutes to dry the crystals. A clean beaker was weighed. The crystals were transferred to the beaker and the beaker was weighed again. Results Weight of aluminum used in the experiment = 0.996 g Volume of 1.4 M KOH used = 50 mL Volume of 9.0 M H2SO4 used = 20 mL Weight of the empty beaker = 95.77 g Weight of potassium aluminum sulfate dodecahydrate and beaker = 109.60 g Weight of potassium aluminum sulfate dodecahydrate obtained= 13.83 g Discussion An aluminum surface reacts with atmospheric oxygen to form an inert oxide layer, which makes the metal corrosion resistant. This resistance is enhanced by anodizing: immersing in sulfuric acid and connecting to the positive terminal of an electrode. (Greenwood & Earnshaw, 1997) 2Al + 3O2- - 6e-  Al2O3 Eq 0 In alkaline media, this oxide layer is destroyed. In the presence of aqueous sodium hydroxide or potassium hydroxide, aluminum dissolves to form the amphoteric tetrahydroxoaluminate(III) anion [Al(OH)4]- (aq) (Greenwood & Earnshaw, 1997): 2 Al (s) + 6H2O (l) + 2KOH (aq)  2K[Al(OH)4] (aq) + 3H2 (g) Eq 0 When sulfuric acid is first added to this solution, excess potassium hydroxide in the solution reacts to form potassium sulfate and water. 2KOH (aq) + H­2SO4 (aq)  K2SO4 (aq) + H2O (l) Eq 0 On further addition of sulfuric acid, the amphoteric tetrahydroxoaluminate(III) ions dissolve to form aluminum sulfate. (Greenwood & Earnshaw, 1997) 2K[Al(OH)4] (aq) + 4H2SO4 (aq)  K2SO4 (aq) + Al2(SO4)3 (aq) + 8H2O (l) Eq 0 On cooling this aqueous potassium sulfate and aluminum sulfate solution, alum crystallizes out. K2SO4 (aq) + Al2(SO4)3 (aq) + 24H2O (l)  2KAl(SO4)2.12H2O Eq 0 Based on this process, the reaction for preparing potassium aluminum sulfate dodecahydrate from aluminum is: 2 Al (s) + 2KOH (aq) + 4 H2SO4 + 22 H2O  2KAl(SO4)2.12H2O + 3H2 (g) Eq 0 Calculations Based on Equation 6, 2 moles of aluminum yield 2 moles of potassium aluminum sulfate dodecahydrate. As excess potassium hydroxide does not affect the yield of potassium aluminum sulfate dodecahydrate, aluminum is the limiting reagent. In the experiment, Weight of aluminum = 0.996 g Atomic mass of aluminum=26.9815 g Moles of aluminum = (0.996 g) ÷ (26.9815 g/mol) = 0.03691 mol Moles of alum expected = 0.03691 mol Molecular mass of alum = 474.39 g/mol Expected yield of alum = 0.03691 mol×474.39 g/mol=17.51 g Our experiment yielded 13.83 g of aluminum. So, the percentage yield is Percentage yield = (Yield ÷ Expected yield) ×100= (13.83 g÷ 17.51 g) ×100=79% Scope for Error Potential sources for error introduced in this experiment include: Presence of impurities in aluminum. Any impurities in the aluminum pieces obtained from the beverage cans could case a decrease in potential yield. Loss of aluminum during vacuum filtration. Inadequate rinsing of the beaker containing aqueous tetrahydroxoaluminate(III) ions could cause loss of aluminum during vacuum filtration. Adequate rinsing with distilled water can prevent this loss. Inadequate crystallization. If the crystallization of alum is not complete, some alum can be lost during filtration of crystals. The solution containing aqueous potassium sulfate and aluminum sulfate should be allowed to stand for at least 15 minutes to complete the crystallization process. Inadequate drying of alum crystals. During crystallization, alum crystals can retain up to 50% of water of crystallization. (Turner & Bache, 1835) If the crystals are not completely dried, this water of crystallization could cause an increase in weight of alum. The percentage yield could exceed 100%. Efficiency of aluminum recycling Modern aluminum recycling techniques allow large energy and emission savings while maintaining the material properties. (Green, 2007) Percentage yield. In this experiment, the number of moles of aluminum from beverage cans and the number of moles of alum obtained are equal. Under insulated conditions, if the aluminum scrap used is pure, this experiment should give a 100% yield. Cost of the operation. The reagents used for recycling are few – aqueous potassium hydroxide, aqueous sulfuric acid, and distilled water. The equipment used are not expensive. A small enterprise can produce alum from used aluminum cans with low capital cost. Production Time. Alum is produced from alum slate by roasting the ore. Purification involves frequent crystallization to remove the iron oxide, which adheres to the alum. (Turner & Bache, 1835) In the series of reactions used here, production time is less and the purification process is not required as the aluminum in used cans is already pure. Safety. Hydrogen gas is liberated during the addition of aqueous potassium hydroxide, which can be explosive. Waste Products. This series of reactions has few waste products – a small amount of water, potassium sulfate from oxidation of excess potassium hydroxide, and hydrogen gas. Apart from these, a few impurities may exist in the aluminum cans used. During recycling of used aluminum cans, waste products that may require special disposal are the exterior paints and inner plastic lining of the cans. Conclusion Recycling of used aluminum cans to common alum involves low operation cost, low production time, and high percentage yield. A small enterprise can find this recycling technique very profitable. Bibliography Atkins, P., & de Paula, J. (2006). Atkins’ Physical Chemistry, Eighth Edition. Oxford : Oxford University Press. Davis, J.R. (1993). Aluminum and Aluminum Alloys (Asm Specialty Handbook) (06610G). Kalamazoo: ASM International. < http://books.google.co.in/books?id=Lskj5k3PSIcC> Green, J.A.S. (2007). Aluminum Recycling and Processing for Energy Conservation and Sustainability. Kalamazoo: ASM International. < http://books.google.co.in/books?id=t-Jg-i0XlpcC> Greenwood, N.N., & Earnshaw, A. (1997). Chemistry of Elements, Second Edition. Oxford: Butterworth-Heinemann. Turner, E., & Bache, F. (1835). Elements of Chemistry. Desilver, Thomas & Co. Digitized 11 July 2007. < http://books.google.co.in/books?id=op0EAAAAYAAJ > Read More