Aluminum

Aluminum is a highly useful metal, second only to iron, because it is abundant, light in weight, non-toxic, malleable, and ductile, with a pleasant appearance.   It can easily be formed, machined, or cast, and it is highly resistant to corrosion because a thin, hard oxide film forms on the surface when aluminum is exposed to air.   This oxide can be thickened by anodizing.   Since the surface film is porous, the aluminum metal can be colored by the application of paints or dyes into the pores to achieve attractive finishes.   Consequently, aluminum is used extensively for kitchen appliances, door and window frames, venetian blinds, roofs, tools, decoration - wherever lightness, non-corrosion, and attractiveness are required.   Aluminum is non-sparking and non-magnetic, properties that enhance safety.   Diecast aluminum cylinder blocks are used for engines in motor vehicles.   Aluminum resists corrosion by many chemicals, which makes it useful in food-processing equipment and containers.   Airplanes and other structures, use considerable amounts of aluminum alloys, e.g., duraluminum (95% Al, 4% copper, 1% manganese + magnesium), so much so that aluminum ores and metal must be imported by U.S. industries   Although aluminum's electrical conductivity is only 60% of that of copper, its lighter weight makes it useful in electrical transmission lines.   In its pure form, aluminum is soft and weak, but these shortcomings can be improved by alloying it with other elements.   When aluminum is evaporated in a vacuum, it forms a highly reflective coating for light and heat from electromagnetic radiation. ^{Lide 4-3}

Aluminum powder is very reactive and is used as a strong reducing agent for removing oxygen in chemical processes.   In the Thermite Process, a large amount of heat is produced that reduces iron oxide to molten iron.   This process is used to weld steel to iron.   Aluminum oxide, alumina, [Al₂O₃] is found in corundum, an extremely hard, crystalline mineral and in combination with magnetite, an iron oxide, called emery.   Both materials are used as abrasives. Alumina has other uses:   It is used in water purification, glassmaking, steel alloys, textile waterproofing, ceramic coatings, cosmetics, refractory materials, electronics, and drying gases and liquids. ^{Considine 320}

Aluminum hydroxide [Al(OH)₃] is used as a mordant in dyeing.   In powdered form, it is used in column chromatography, in which a liquid mixture of compounds trickles down in a glass column through the powder, causing the compounds to separate at different levels.   Sodium aluminate, [NaAlO₂] is used in papermaking and in water and sewage purification as a flocculent.   Aluminum sulfate [Al₂(SO₄)₃•18H₂O] is a common source of aluminum hydroxide and used with sodium carbonate in foam fire extinguishers.   It forms double sulfates, called alums with other metals, e.g., potassium alum, K₂SO₄•Al₂(SO₄)₃•24H₂O.   Alums are used in dyeing and paper-making.   Chlorine gas is passed over aluminum foil to form a white solid, aluminum chloride, Al₂Cl₆, which is an important catalyst in the synthesis of aromatic compounds. ^{How 97-99}

Aluminum is the most abundant metal found on earth (8.1% of the crust), but not in free form because of its high reactivity.   It is the third most abundant element on earth, following silicon and oxygen.   The early Romans and Greeks used alum [potassium aluminum sulfate = KAl(SO₄)₂ •12H₂O] for a medical astringent and a dyeing mordant.   It was isolated by Wohler in 1827.   Bauxite, a mixture of hydrated alumina, Al₂O₃ • xH₂O and hydrated iron oxide, Fe₂O₃ • xH₂O, is the primary ore from which aluminum is extracted.   It contains from 30 to 60% alumina [aluminum oxide = Al₂O₃]. ^{Lide 4-3}

In 1886, Hall in the U.S. and Heroult in France obtained aluminum from alumina [aluminum oxide = Al₂O₃] dissolved in cryolite [sodium aluminum fluoride = Na₃AlF₆].   Before that time, aluminum cost $545 / lb in the U.S.   The Hall-Heroult Process reduced the cost to $11 / lb.   Eventually, the cost was 30¢ / lb. and as low as 15¢ / lb., indicating the considerable improvement in processing productivity and consequent availability to users.   Cryolite has since been replaced by a mixture of sodium, aluminum, and calcium.   The Bayer process is now used to obtain alumina from bauxite ore.   Following this process, the alumina is put through the Hall-Heroult refining process. ^{Lide 4-3}

The Bayer Process produces alumina (aluminum oxide) from bauxite.   The bauxite is dried, ground in ball mills, and mixed with sodium hydroxide [NaOH].   Lime [CaO] is added to control the phosphorus content and to improve the alumina solubility.   This mixture is pumped into a pressurized digester operating between 221 to 554°F.   After about 5 hours, a solution of sodium aluminate [NaAl₂OH] and insoluble red mud is cooled to 212%#176;F.   It is sent to a gravity separator or a wet cyclone to remove coarse sand.   A flocculent, such as starch, is added to increase the settling rate of the red mud.   The tank overflow contains a solution of alumina, which is clarified by filtration; then it is cooled, during which it becomes supersaturated with sodium aluminate.   Fine crystals of alumina trihydrate [Al₂O₃•3H₂O] are added to precipitate alumina trihydrate, which is washed and filtered.   The alumina trihydrate is calcined to produce a crystalline alumina, which is ready for electrolysis. ^{EPA 12.1-1}

Al₂O₃•3H₂O + 2NaOH ——> 2NaAl(OH)₄

2Al(OH)₃ ——> Al₂O₃ + 3H₂O

1. Using sodium aluminate from the Bayer Process:

NaAlO₂ + 2NaOH + 6HF ——> 3NaF•AlF₃ + 4H₂O

2. Substituting sodium carbonate for sodium hydroxide:

NaAlO₂ + Na₂ CO₃ + 6HF ——> 3NaF•AlF₃ + 3H₂O + CO₂

The crystalline alumina obtained from the Bayer Process is placed in shallow pots, which are steel shells lined with carbon.   Carbon electrodes extend into the pot as anode, while the carbon pot becomes the cathode.   Molten cryolite [Na₃AlF₆] is added as an electrolyte and a solvent for the alumina.   Electricity is passed through the mixture.   The chemical reaction is

alumina + carbon ——> aluminum + carbon dioxide

Aluminum is deposited at the cathode as molten metal below the surface of the cryolite bath.   The carbon anodes become depleted and must be replaced periodically.   The aluminum beneath the cryolite cover is tapped every 24 to 48 hours, using a vacuum siphon.   The released oxygen collects at the carbon anodes where it forms carbon dioxide, CO₂, and escapes.The aluminum is sent to a reverberatory holding furnace to be alloyed, fluxed, and degassed to remove trace impurities. ^{EPA 12.1-1}

Aluminum production requires large amounts of electricity.   About 20% of the aluminum comes from recycled aluminum products, which saves 95% of the energy required in production, thus lowering unit costs.

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In 1886, Hall and Heroult independently invent the the Hall-Heroult process for obtaining aluminum from alumina.

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