Production of Aluminum: The Hall-Héroult Process
Dedicated September 17, 1997, at Oberlin College in Oberlin, Ohio, and November 2, 2001, at Alcoa Inc., in Pittsburgh, Pennsylvania.
On February 23, 1886, in his woodshed laboratory at the family home on East College Street, Charles Martin Hall succeeded in producing aluminum metal by passing an electric current through a solution of aluminum oxide in molten cryolite. Aluminum was a semiprecious metal before Hall’s discovery of this economical method to release it from its ore. His invention, brought into commercial-scale production by the Pittsburgh Reduction Company (now known as Alcoa), made this light, lustrous and non-rusting metal readily available and was the basis of the aluminum industry in North America.
- History of Aluminum
- Aluminum Team: Charles Martin Hall and Frank Fanning Jewett
- Hall’s Early Experiments with Aluminum
- Aluminum Production by Electrochemistry: The
- Commercialization of the Hall Process: The Pittsburgh Reduction Company
- How Aluminum Products are Produced
- Further Reading
- Landmark Designation and Acknowledgments
- Cite this Page
Before 1886, aluminum was a semiprecious metal comparable in price to silver. The third most abundant element in the earth’s crust—and its most plentiful metal—, aluminum is made from bauxite, a reddish-brown rock discovered in Les Baux, France, in 1821.
More than 7,000 years ago, Persians made their strongest pottery out of clay containing aluminum oxide. Three millennia later, ancient Egyptians were using other aluminum compounds in medicines, dyes and cosmetics. But because aluminum has a high affinity for oxygen and never occurs in its metallic form in nature, it proved difficult to isolate. In 1808, Sir Humphry Davy gave aluminum its name. In 1825, the Danish chemist Hans Christian Oersted finally produced a sample—albeit very impure—using heat and a potassium-based mixture. Over the next 20 years, Friedrich Wöhler, a German chemist, improved this process by using metallic potassium.
Although the element had been investigated by many European scientists, the only way to prepare the metal was by the complex and difficult process that culminated in reacting metallic sodium with aluminum chloride. When the Washington Monument was completed in 1884, a 6-pound pyramid of this costly aluminum was placed as an ornament at the very top. It also served as the tip of the lightning rod system, a practical application of the high electrical conductivity and corrosion resistance of this remarkable metal. However, economical methods were needed to wrest aluminum from its abundant minerals, which Henri Sainte-Claire Deville, the great French chemist, observed “could be found in every clay bank.”
Frank Fanning Jewett received his undergraduate education and did some graduate work in chemistry and mineralogy at Yale University. From 1873 to 1875, he continued his chemistry studies at the University of Gottingen in Germany. There he became well acquainted with current European science and became interested in the promise of aluminum. He met Professor Friedrich Wöhler, who had isolated aluminum as a metal in 1827 following H. C. Oersted's lead in 1825. Before Jewett returned to America in 1875 to become Oliver Wolcott Gibbs's private assistant at Harvard University, he obtained a sample of aluminum metal. In 1876, he was nominated by the president of Yale to teach science at the Imperial University in Tokyo, where he was one of a small group of Westerners. In 1880 at the age of 36, Jewett became the professor of chemistry and mineralogy at Oberlin College.
Charles Martin Hall first learned chemistry as a serious-minded youth in the town of Oberlin by reading an 1840s textbook he found on the shelves of his minister father's study. He also carried on experiments at home, the beginning of a lifelong enthusiasm for experimental work. An avid reader in many fields, he eagerly followed the popular invention literature in Scientific American. Hall was already intrigued by the romance of aluminum when, as a 16-year-old freshman at Oberlin College in the fall of 1880, he went to the chemistry laboratory to obtain some items for his home laboratory. There he met Professor Jewett.
Their association over the next five-and-one-half years led to the discovery of a practical process for making aluminum from its ore by an electric current. Within three more years, Hall was producing pure aluminum metal on an industrial scale. Aluminum, the curiosity, became a widely used material, and the younger man achieved his goal of a financially successful career in technology and industry.
Hall took his first formal course in chemistry as a junior in college. Earlier, with Jewett's guidance and encouragement, he had worked on aluminum chemistry and other projects in Jewett's laboratory and in his own laboratory at home. He heard Jewett lecture on the chemistry of aluminum, display his sample of the metal, and predict the fortune that awaited the person who devised an economical method for winning aluminum from its oxide ore. To a fellow student, Hall declared that he intended to be that person.
After many unsuccessful experiments with chemical methods of reducing aluminum ores to the metal, Jewett and Hall turned to electric current to provide the powerful reducing conditions that were needed. To obtain electricity in a college town in the 1880s, one had to assemble batteries. Hall and Jewett used Bunsen Grove cells, which consist of a large zinc metal electrode in a sulfuric acid solution that surrounds a porous ceramic cup containing a carbon rod immersed in concentrated nitric acid. Assembling enough of these cells to provide sufficient electrical energy for aluminum production was a large undertaking. The eventual laboratory process used about one pound of zinc electrodes, hand cast by Hall, to obtain one ounce of aluminum.
Hall did the first experiments with electricity in Jewett's laboratory during his senior year of 1884/85. He prepared aluminum fluoride from hazardous hydrofluoric acid in special lead vessels, and he passed a current through aluminum fluoride dissolved in water. Unfortunately, this system produced only unwanted hydrogen gas and aluminum hydroxide at the negative electrode.
After graduation, Hall continued the work in the woodshed behind his family's house. He experimented with molten fluoride salts as water-free solvents. He knew that the fluoride salts had the advantage over previously studied chloride salts of not absorbing water from the air. Hall was aware of Richard Grïtzel's success in obtaining magnesium metal by using an electric current in a magnesium chloride melt as reported in the Scientific American in 1885.
To work with molten fluoride salts, he needed a furnace capable of producing and sustaining higher temperatures than the coal-fired furnace of his earlier experiments. For this purpose, Hall adapted a second-hand, gasoline-fired stove to heat the interior of a clay-lined iron tube. Despite the higher temperature of this furnace, he was unable to melt calcium, aluminum or magnesium fluorides. Potassium and sodium fluorides melted but did not dissolve useful amounts of aluminum oxide.
Hall moved on to experiment with cryolite (sodium aluminum fluoride) as a solvent. He made cryolite, found that it would melt in his furnace, and showed that it would dissolve more than 25% by weight of aluminum oxide. The melting point of cryolite is 1000° C, an exceptionally high temperature for electrochemistry. He did this crucial experiment early in February 1886 and repeated it the next day for his sister Julia to witness.
Six days later, Hall first attempted to prepare aluminum metal by passing an electric current through a solution of aluminum oxide in molten cryolite. He immersed graphite rod electrodes into the fiery solution in a clay crucible and let the current run for a while. In Julia's presence, he poured the melt into a frying pan and broke apart the cooled mass but found no aluminum. There was only a grayish deposit on the negative electrode, a deposit that did not have the shiny metallic appearance of aluminum. After repeating this process several times, Hall realized that this deposit was probably silicon from silicates dissolved out of the clay crucible. If he had not been acquainted with the appearance of metallic aluminum from seeing Jewett's sample, Hall might have been slower to interpret this false result.
From a large graphite rod, Hall made a graphite crucible to line the clay crucible. He also lowered the melting point of the cryolite solution by adding aluminum fluoride. The first experiment with this new system was performed on February 23, 1886. The electric current ran for several hours, and once again he cooled the melt and broke it open in the presence of his three sisters and father. This time they found several small silvery globules, which he tested with hydrochloric acid. He took them to Jewett, who confirmed that they were aluminum.
On July 9, 1886, Hall applied for a patent. Meanwhile, Paul L.T. Héroult was granted a French patent on April 23, 1886, for a comparable process based on cryolite and aluminum oxide; he had also applied for a U.S. patent in May. This meant that Hall had to prove that he had made aluminum by the new method before the date of the French patent to obtain patent protection in the United States. Evidence from his family and Jewett, including two postmarked letters to his brother, George, helped to establish the priority of Hall's discovery in the United States in a ruling made by the Patent Examiner. Hall's patent rights were also upheld in two subsequent legal struggles with the Cowles Electric Smelting Co. of Cleveland, Ohio, which made copper/aluminum alloy.
How could it be that Paul Héroult in Paris, France, and Charles Hall in Oberlin, Ohio, made nearly simultaneous, yet independent discoveries of the same process of refining aluminum? Many factors seem to have contributed. Finding an economical process for refining aluminum was widely recognized as a prime target for inventors. Electrochemistry had begun to mature as an applied science. Large electricity-generating dynamos were being developed commercially. Interest had been aroused in the chemistry of fluorine-containing substances. Although Hall was working in a small U.S. college town, he had access to the latest in scientific thought with Jewett as his mentor. Proximity to Cleveland and its emerging technical industries, such as Standard Oil for gasoline, Brush Electric for large graphite rods, and Grasselli for chemicals, was also a contributing factor.
Hall, like Héroult, was a resourceful experimentalist, who not only devised a method of making aluminum metal, but made most of his apparatus and prepared many of his chemicals. Like Héroult, Hall had a burning desire to be a successful inventor and industrialist. In recognition of the contribution these two young men made to the development of this electrochemical process on both sides of the Atlantic, it is now called the Hall-Héroult process.
In the summer of 1888, a group of six industrialists led by Alfred E. Hunt, an MIT graduate involved in the metallurgical business in Pittsburgh, provided the financial backing that enabled Hall to found the Pittsburgh Reduction Company in 1888. Before that year was out, Hall and his first employee, Arthur Vining Davis, had produced the first commercial aluminum in a pilot plant on Smallman Street in Pittsburgh.
The process was soon simplified by using internal heating caused by electrical resistance in the reaction pots to achieve and maintain the molten state. Steam-driven Westinghouse dynamos provided the electricity. Further cost improvements resulted later from the use of hydroelectricity. As Hall improved his process, the price of aluminum ingots dropped from $4.86 per pound in 1888 to 78 cents per pound in 1893. Because manufacturers were reluctant to use an unfamiliar metal, the company developed prototype products such as the first cast aluminum tea kettle for use as sales tools.
The Pittsburgh Reduction Company became the Aluminum Company of America (Alcoa) in 1907, just before the patent rights ran out. At first aluminum was a solution in search of a problem, but gradually business grew as manufacturers grasped the benefits of this light yet strong metal, in applications ranging from aircraft and other modes of transportation to power lines for long-distance transmission of electricity, construction, food storage and decoration. In the mid-1930s, industrial designer Henry Dreyfuss predicted that “aluminum will play a large and significant part” in the “greatest period of redesign the world has known.” By the late 1930s, a pound of aluminum cost just 20 cents; its uses numbered more than 2,000.
In 1911 Hall became the fifth recipient of the Perkin Medal, which was awarded for “valuable work in applied chemistry” by the Society of Chemical Industry (American Section) with the support of the Electrochemical Society and the American Chemical Society. Paul Héroult attended the award ceremony in New York and made a graceful contribution to the speeches. Hall responded with equal warmth.
Upon Hall's death in late 1914, his holdings in Alcoa stock amounted to a sizeable fortune, most of which he bequeathed to educational institutions in this country and abroad.
Step 1: Mining bauxite
Four tons of bauxite produce one ton of aluminum—enough to make the cans for more than 60,000 soft drinks. Bauxite is formed over millions of years by chemical weathering of rocks containing aluminum silicates, producing an ore rich in aluminum oxide. Today, bauxite is mined primarily in Africa, Australia and the Caribbean.
Step 2: Refining alumina
The ore is ground and mixed with lime and caustic soda, then heated in high-pressure containers. The aluminum oxide is dissolved by the caustic soda, precipitated out of the solution, washed and heated to eliminate water. The resulting alumina is a white powder resembling sugar.
Step 3: Smelting into aluminum
An electrolytic reduction process known as smelting dissolves the alumina in a cryolite bath inside carbon-lined cells, or pots. A powerful electric current, which is passed through the bath, separates aluminum metal from the chemical solution and the metal is siphoned off. Smelting is the industrial-scale version of the process developed in 1886 by Charles Martin Hall in his woodshed laboratory.
Step 4: Fabricating aluminum products
Aluminum goes from the smelting pot into the furnace for mixing with other metals. These alloys have specific properties to meet specific uses. Fluxing purifies the metal, which is then poured into molds or cast into ingots. Fabrication may include forging, casting, rolling, drawing or extruding to create different finished products from automobiles to aircraft.
Step 5: Recycling aluminum
Recycling extends the life cycle of aluminum products, the most valuable material in the waste stream. Once used, aluminum products can be returned to recycling facilities to be melted down and fabricated into new aluminum products.
Every minute of every day, an average of more than 123,000 aluminum cans are recycled.
Since 1972, an estimated 660-plus billion beverage cans have been recycled—placed end-to-end, they could stretch to the moon nearly 300 times.
The average lifespan of an aluminum beverage can is six weeks, including the time it takes to be manufactured, filled, sold, recycled and remanufactured.
Recycling one aluminum can saves enough energy to keep a 100-watt bulb burning for almost four hours or provide enough power to a television for three hours.
Tossing an aluminum can wastes as much energy as pouring out half of that can’s volume of gasoline. If each person recycles one aluminum can each month, energy savings equal 1,750 to 3,500 gallons of gas.
American consumers and industry throw away enough aluminum to rebuild the entire U.S. commercial air fleet every three months.
Aluminum made up 1.5 percent of the total municipal solid waste stream in the United States in 1994. The overall rate for aluminum packaging was 55 percent.
In 1884, total United States aluminum production was 125 pounds. That year, a 100-ounce cast aluminum pyramid paced atop the Washington Monument represented twenty percent of this production.
Four tons of bauxite produces one ton of aluminum—enough to manufacture 60,000 beverage cans or spaceframes for seven full-size cars or 40,000 computer memory disks.
One pound of aluminum can replace twice that weight in steel in most applications.
Rubies, emeralds and sapphires consist mainly of crystalline aluminum oxide.
Manufacturers used Cold War technology to make Little League bats from aluminum.
Aluminum is light, strong, corrosion-resistant, nonmagnetic, nontoxic and naturally good looking.
(Facts produced in 2001.)
The American Chemical Society designated Charles Martin Hall’s discovery of a process for producing aluminum metal by electrochemistry as a National Historic Chemical Landmark at Oberlin College in Oberlin, Ohio, on September 17, 1997. The plaque commemorating the discovery reads:
On February 23, 1886, in his woodshed laboratory at the family home on East College Street, Charles Martin Hall succeeded in producing aluminum metal by passing an electric current through a solution of aluminum oxide in molten cryolite. Aluminum was a semiprecious metal before Hall’s discovery of this economical method to release it from its ore. His invention, which made this light, lustrous, and nonrusting metal readily available, was the basis of the aluminum industry in North America.
The American Chemical Society designated the commercialization of the Hall aluminum process as a National Historic Chemical Landmark at Alcoa Inc., in Pittsburgh, Pennsylvania, on November 2, 2001. The plaque commemorating the development reads:
In 1886 Charles Martin Hall invented an economical electrochemical process to release aluminum from its ore. Until then, this light, lustrous and non-rusting metal was rare and costly. A group of Pittsburgh investors, headed by metallurgist Alfred E. Hunt, agreed to support the commercialization of Hall's process and founded the Pittsburgh Reduction Company. In 1888 Hall, assisted by Arthur Vining Davis, began to produce aluminum in the company's pilot plant on Smallman Street. In 1907 the company became the Aluminum Company of America (Alcoa). Aluminum has since become part of everyday life with many uses — from teakettles in the early days, to aircraft, power lines, building materials, food packaging, and artwork.
Adapted for the internet from “Production of Aluminum Metal by Electrochemistry” (1997) and “Commercialization of Aluminum” (2001), produced by the National Historic Chemical Landmarks program of the American Chemical Society.
American Chemical Society National Historic Chemical Landmarks. Hall Process: Production and Commercialization of Aluminum. http://www.acs.org/content/acs/en/education/whatischemistry/landmarks/aluminumprocess.html (accessed Month Day, Year).