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125 Years of Aluminum

October 16, 2013

Materials are so important to culture that archaeologists have named some periods of human activity by their premier materials. Thus, in the three-age system, we've lived in the Stone Age, which ended about 4000 BC when metalworking was practiced; the Bronze age, which started around 2500 BC; and the Iron Age, which started around 1000 BC.

Since gold is the least reactive metal, it occurs in nature as the metal, itself, so it was the first metal used by man. After gold, primitive man discovered ways to release other metals from their ores by heating. The ancients were able to smelt silver, mercury, copper, lead, tin, and finally, iron. Bronze, an alloy of copper and tin, was a stronger metal than copper, and it had sufficient properties to launch the Bronze Age.

Hesiod, Works and Days, lines 150-151
Hesiod, Works and Days, lines 150-151, scanned from the author's copy of ref. 1. The translation is "Their armor was of bronze, and their houses of bronze, and of bronze were their implements: there was no black iron."[1]

Iron smelting, the extraction of iron from its common ores, is much more difficult than the extraction of the other metals. The reduction of the common iron ore, hematite (Fe2O3 using carbon as a reducing agent is straightforward; vis.,
2Fe2O3 + 3C -> 4Fe + 3CO2
This so-called carbothermic reaction uses readily available materials, but it happens at higher temperatures than encountered for the smelting of the other metals known to the ancients.

Iron is a strong material, but iron rusts, so our ancestor metallurgists were always on the lookout for better metals. One of these is aluminum, which is known as aluminium outside the US. Although iron oxide is somewhat hard to reduce to pure iron, aluminum oxide is much, much harder to reduce. The following table illustrates this fact through the free energies of formation (ΔGf) of these two compounds at some representative temperatures.[2]

 OxideΔGf, 1000 K
kcal/mole		ΔGf, 1500 K
kcal/mole
 Fe2O3-134.082-104.585
 Al2O3-325.397-285.975

The most common aluminum ore is bauxite, which is a mixture of aluminum hydroxides. Since aluminum is a stronger reducing agent than carbon, a carbothermic reaction cannot be used to produce aluminum from any of its ores.

Electrochemistry lets you drive chemical reactions too difficult for thermal processing, but typical aqueous electrochemistry will not produce aluminum, since the aluminum will react with the water. Water, however, isn't the only solvent, and that's the key of the Hall–Héroult process for production of aluminum. This process was independently invented by the Frenchman, Paul Héroult, and the American chemist, Charles Martin Hall in 1886. Hall commercialized the process in 1888 by creating the Alcoa corporation, which celebrated its 125th anniversary on October first.[3-4]

The solvent used in the Hall–Héroult process is cryolite (Na3AlF6), which needs to be heated to about 1,000 °C (1,830 °F) before it's a liquid. Aluminium fluoride (AlF3) can be added to form a reduced melting point composition (see figure).

Portion of the cryolite-aluminum fluoride phase diagramCryolite-rich portion of the cryolite-aluminum fluoride phase diagram.

(Illustration by the author using Inkscape.)

In the Hall–Héroult process aluminum oxide is dissolved in molten cryolite and then electrolyzed with an applied voltage of up to five volts in an electrochemical cell with a carbon cathode and anode. Since the solvent temperature is above the melting point of aluminum (660°C), aluminum is deposited as a liquid at the cathode, and it sinks to the bottom of the cell.

Carbon dioxide (CO2) is produced at the anode, which is consumed, along with some hydrogen fluoride gas. The HF is neutralized to make sodium fluoride, and the CO2 is vented to the atmosphere; alas, as another contributor to global warming. The process is energy-intensive, since it takes about 13-15 kilowatt-hours of electricity to produce a kilogram of aluminum, which is about half the daily electrical consumption of an average US home.

Although the Hall–Héroult process allowed a relatively inexpensive method of making aluminum, aluminum had been available for quite some time before that, having been first isolated in 1827. Before Hall–Héroult, aluminum was prepared by a reduction of gaseous anhydrous aluminum chloride by liquid sodium metal at high temperature, the aluminum chloride being prepared from aluminum oxide, and the sodium prepared by electrolysis of sodium hydroxide; viz,
Al2Cl6(g) + 6Na(l) -> 2Al(s) + 6NaCl(s)
When the Washington Monument was built, it was topped with an aluminum apex, a solid pyramid about a foot high, weighing about 2.85 kg, fabricated in 1884. In 1884, aluminum was as valuable as silver, about a dollar an ounce ($25/ounce in today's money). The intended purpose of the apex was as a lightning rod for the monument. The chemical composition of the metal, as analyzed at the time, was 97.49-97.75% aluminum, with 1.70-1.90% iron, and 0.55-0.61% silicon.[5]

Fitting the aluminum apex on the Washington Monument, 1884Master Mechanic, P. H. McLaughlin, fitting the aluminum apex on the Washington Monument, 1884.

(Harper's Weekly illustration by S.H. Nealy, United States Library of Congress Prints and Photographs division digital ID cph.3b44599, via Wikimedia Commons.)

References:

  1. Hugh G. Evelyn-White, Trans., "Hesiod, The Homeric Hymns and Homerica," William Heinemann/The Macmillan Co., London/New York, 1914, pp. 12-13.
  2. L. B. Pankratz, "Thermodynamic Properties of Elements and Oxides," U. S. Bureau of Mines Bulletin 672, U. S. Government Printing Office (1982).
  3. Len Boselovic, "Alcoa celebrates 125 years, remembers roots," Pittsburgh Post-Gazette, October 1, 2013.
  4. Aluminum candles: Alcoa, Pittsburgh-born and bred, marks 125 years, Pittsburgh Post-Gazette, October 3, 2013.
  5. George J. Binczewski, "The Point of a Monument: A History of the Aluminum Cap of the Washington Monument," JOM, vol. 47, no. 11 (1995), pp. 20-25.