Hall-Héroult process
Encyclopedia
The Hall–Héroult process is the major industrial process for the production of aluminium
. It involves dissolving alumina in molten cryolite
, and electrolysing
the molten salt bath to obtain pure aluminium metal.
Aluminium cannot be produced by the electrolysis of an aluminium salt dissolved in water because of the high reactivity of aluminium with the protons of water and the subsequent formation of hydrogen. As in aqueous solution, protons (H+) are preferentially reduced to atomic hydrogen before Al3+ ions, the reduction of Al3+ is done by electrolysis of a molten aluminium salt. This is a water free medium, and hence, H+ reduction is avoided.
In the Hall–Héroult process alumina, Al2O3, is dissolved in an industrial carbon
-lined vat of molten cryolite
, Na3AlF6 (sodium hexafluoroaluminate
), called a "cell". Aluminium oxide
has a melting point of over 2000 °C (3,632 °F) while pure cryolite has a melting point of 1012 °C (1,853.6 °F). With a small percentage of alumina dissolved in it, cryolite has a melting point of about 1000 °C (1,832 °F). Some aluminium fluoride
, AlF3 is also added into the process to reduce the melting point of the cryolite-alumina mixture.
The molten mixture of cryolite, alumina, aluminium fluoride is then electrolyzed
by passing a direct electric current
through it. The electrochemical reaction causes liquid aluminium metal to be deposited at the cathode
as a precipitate, while the oxygen from the alumina combines with carbon from the anode
to produce carbon dioxide
, CO2. An electric potential
of three to five Volts is needed to drive the reaction, and the rate of production is proportional to the electric current
. An industrial-scale smelter typically consumes hundreds of thousands of Ampere
s for each cell.
The oxidation of the carbon anode reduces the required voltage across each cell, increasing the electrical efficiency, at a cost of continually replacing the carbon electrodes with new ones, and also the cost of releasing carbon dioxide into the atmosphere
. Hundreds of Hall-Heroult cells are usually connected electrically in series, and they are supplied with direct current
(DC) from a single set of rectifier
s that convert the alternating current
(AC) supplied to the factory
into direct current. The very high electric current is supplied to the cells through heavy, low electrical resistance
metal busbar
s made of pure aluminium or copper
. The cells are electrically heated to reach the operating temperature
with this current, and the anode regulator system varies the current passing through the cell by raising or lowering the anodes and changing the cell's resistance. If needed any cell can be bypassed by shunt
busbars.
The liquid aluminium is taken out with the help of a siphon
operating with a vacuum, in order to avoid having to use extremely high temperature valves and pumps. The liquid aluminium then may be transferred in batches or via a continuous hot flow line to a location where it is cast into aluminium ingot
s. The aluminium can either be cast into the form of final cast-aluminium products, or the ingots can be sent elsewhere such as a rolling mill for being pressed into sheets, or the a wire-drawing mill for producing aluminium wires and cables.
While solid cryolite is denser than solid aluminium at room temperature, the liquid aluminium product is denser
than the molten cryolite at temperatures around 1000 °C (1,832 °F), and the aluminium sinks to the bottom of the electrolytic cell, where it is periodically collected. The tops and sides of the cells are covered with layers of solid cryolite which also act as thermal insulation. The unavoidable electric resistance within each cell produces sufficient heat to keep the cryolite-alumina mixture molten.
With the percentage of aluminium dissolved in each cell being depleted by the electrolysis in the molten cryolite, additional alumina is continually dropped into the cells to maintain the required level of alumina. Whenever a solid crust forms across the surface of the molten cryolite-alumina, this crust is broken from time to time to allow the added alumina to fall into the molten cryolite and dissolve there.
The electrolysis process produces exhaust which escapes into the fume hood and is evacuated. The exhaust is primarily CO2 produced from the anode consumption and hydrogen fluoride
(HF) from the cryolite and flux
. HF is a highly corrosive and toxic gas, even etching
glass
surfaces. The gases are either treated or vented into the atmosphere; the former involving neutralization of the HF to its sodium salt, sodium fluoride
. The particulates are also captured and reused using electrostatic or bag filters. The remaining CO2 is usually vented into the atmosphere.
The very large electric current passing through the electrolytic cells generates a powerful magnetic field, and this can stir the molten aluminium with magneto-hydrodynamic
forces in properly-designed cells. The stirring of the molten aluminium in each cell typically increases its performance, but the purity of the aluminium is reduced, since it gets mixed with small amounts of cryolite and aluminium fluoride. If the cells are designed for no stirring, they can be operated with static pools of molten aluminium so that the impurities either rise to the top of the metallic aluminium, or else sink to the bottom, leaving high-purity aluminium in the middle.
Aluminium smelters are usually sited where inexpensive hydroelectric power is available. For some European smelters, the electric power
produced by hydroelectric plants in countries such as Norway
, Switzerland
, and Austria
is transmitted by high-voltage power line
s to such places as Denmark
, Sweden
, Germany
, and Italy
to be used by aluminium and magnesium
factories. Since aluminium factories require nearly-uniform supplies of electric current, they make the most of nearly-constant supplies of electric power, and these are also available close to many hydroelectric power plants. To give an example of such use of hydroelectric power, the three main regions for aluminium production in North America
have always been in the Tennessee River
Valley of the Southeastern United States
, the Columbia River
Valley of Washington and Oregon
, and the St. Lawrence River Valley of southeastern Canada
and the Northeastern United States
.
Many decades ago, before the existence of the Tennessee Valley Authority
, aluminium companies such as Alcoa
even built their own hydroelectric dams and powerhouses in the Appalachian Mountains
of North Carolina
and Tennessee
.
chemist Charles Martin Hall
and the Frenchman
Paul Héroult
. In 1888, Hall opened the first large-scale aluminium production plant in Pittsburgh
. It later became the Alcoa
corporation.
In 1997 the Hall–Héroult process was designated an ACS National Historical Chemical Landmark
in recognition of the importance of the commercialization of aluminium.
currently used in the industry. Today, there are two primary technologies using the Hall–Héroult process: Söderberg and prebake. Söderberg uses a continuously created anode
made by addition of pitch
to the top of the anode. The lost heat from the smelting operation is used to bake the pitch into the carbon form required for reaction with alumina. Prebake technology is named after its anodes, which are baked in very large gas-fired ovens at high temperature before being lowered by various heavy industrial lifting systems into the electrolytic solution. In both technologies, the anode
, attached to a very large electrical bus, is slowly used up by the process because the oxygen generated by the electrolytic process can oxidize the carbon anode. Prebake technology tends to be slightly more efficient, but is more labor intensive. Prebake technology is becoming preferred in the industry because of the various pollutant emissions related to the creation of the anode from liquid pitch.
s on Earth. However, it is rarely found in its elemental state
. Instead it is mainly found as the ore bauxite
or Aluminium oxide
, 23.
Prior to the Hall–Héroult process, elemental aluminium was made by heating ore along with elemental sodium
or potassium
in a vacuum
. The method was complicated and consumed materials that were in themselves expensive at that time. This meant the cost to produce the small amount of aluminum made in the early 19th century were very high.
Early aluminum was more costly than gold
or platinum
. Bars of aluminium were exhibited alongside the French crown jewels
at the Exposition Universelle of 1855
, and Emperor Napoleon III
of France was said to have reserved his few sets of aluminium dinner plates and eating utensils for his most honored guests.
Production costs using older methods did come down, however, when aluminum was selected as the material for the cap/lightning rod to sit atop the Washington Monument
in Washington, D.C.
, it was still more expensive than silver
.
New production based on the Hall–Héroult process, in combination with cheaper electric power
, helped make aluminium (and incidentally magnesium
) an inexpensive commodity.
This in turn helped make it possible for pioneers like Hugo Junkers
to utilize aluminium and aluminium-magnesium alloys to make items like metal airplanes by the thousands, or Howard Lund to make aluminum fishing boats.
Aluminium
Aluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances....
. It involves dissolving alumina in molten cryolite
Cryolite
Cryolite is an uncommon mineral identified with the once large deposit at Ivigtût on the west coast of Greenland, depleted by 1987....
, and electrolysing
Electrolysis
In chemistry and manufacturing, electrolysis is a method of using a direct electric current to drive an otherwise non-spontaneous chemical reaction...
the molten salt bath to obtain pure aluminium metal.
Process
In the Hall–Héroult process alumina, Al2O3, is dissolved in an industrial carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
-lined vat of molten cryolite
Cryolite
Cryolite is an uncommon mineral identified with the once large deposit at Ivigtût on the west coast of Greenland, depleted by 1987....
, Na3AlF6 (sodium hexafluoroaluminate
Sodium hexafluoroaluminate
Sodium hexafluoroaluminate is the inorganic compound with the formula Na3AlF6. This colourless solid, which occurs naturally as the mineral cryolite, is used extensively in the industrial production of aluminium.-History:...
), called a "cell". Aluminium oxide
Aluminium oxide
Aluminium oxide is an amphoteric oxide with the chemical formula 23. It is commonly referred to as alumina, or corundum in its crystalline form, as well as many other names, reflecting its widespread occurrence in nature and industry...
has a melting point of over 2000 °C (3,632 °F) while pure cryolite has a melting point of 1012 °C (1,853.6 °F). With a small percentage of alumina dissolved in it, cryolite has a melting point of about 1000 °C (1,832 °F). Some aluminium fluoride
Aluminium fluoride
Aluminium fluoride is an inorganic compound used primarily in the production of aluminium. This colourless solid can be prepared synthetically but also occurs in nature.-Production and occurrence:...
, AlF3 is also added into the process to reduce the melting point of the cryolite-alumina mixture.
The molten mixture of cryolite, alumina, aluminium fluoride is then electrolyzed
Electrolysis
In chemistry and manufacturing, electrolysis is a method of using a direct electric current to drive an otherwise non-spontaneous chemical reaction...
by passing a direct electric current
Electric current
Electric current is a flow of electric charge through a medium.This charge is typically carried by moving electrons in a conductor such as wire...
through it. The electrochemical reaction causes liquid aluminium metal to be deposited at the cathode
Cathode
A cathode is an electrode through which electric current flows out of a polarized electrical device. Mnemonic: CCD .Cathode polarity is not always negative...
as a precipitate, while the oxygen from the alumina combines with carbon from the anode
Anode
An anode is an electrode through which electric current flows into a polarized electrical device. Mnemonic: ACID ....
to produce carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
, CO2. An electric potential
Voltage
Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...
of three to five Volts is needed to drive the reaction, and the rate of production is proportional to the electric current
Electric current
Electric current is a flow of electric charge through a medium.This charge is typically carried by moving electrons in a conductor such as wire...
. An industrial-scale smelter typically consumes hundreds of thousands of Ampere
Ampere
The ampere , often shortened to amp, is the SI unit of electric current and is one of the seven SI base units. It is named after André-Marie Ampère , French mathematician and physicist, considered the father of electrodynamics...
s for each cell.
The oxidation of the carbon anode reduces the required voltage across each cell, increasing the electrical efficiency, at a cost of continually replacing the carbon electrodes with new ones, and also the cost of releasing carbon dioxide into the atmosphere
Atmosphere
An atmosphere is a layer of gases that may surround a material body of sufficient mass, and that is held in place by the gravity of the body. An atmosphere may be retained for a longer duration, if the gravity is high and the atmosphere's temperature is low...
. Hundreds of Hall-Heroult cells are usually connected electrically in series, and they are supplied with direct current
Direct current
Direct current is the unidirectional flow of electric charge. Direct current is produced by such sources as batteries, thermocouples, solar cells, and commutator-type electric machines of the dynamo type. Direct current may flow in a conductor such as a wire, but can also flow through...
(DC) from a single set of rectifier
Rectifier
A rectifier is an electrical device that converts alternating current , which periodically reverses direction, to direct current , which flows in only one direction. The process is known as rectification...
s that convert the alternating current
Alternating current
In alternating current the movement of electric charge periodically reverses direction. In direct current , the flow of electric charge is only in one direction....
(AC) supplied to the factory
Factory
A factory or manufacturing plant is an industrial building where laborers manufacture goods or supervise machines processing one product into another. Most modern factories have large warehouses or warehouse-like facilities that contain heavy equipment used for assembly line production...
into direct current. The very high electric current is supplied to the cells through heavy, low electrical resistance
Electrical resistance
The electrical resistance of an electrical element is the opposition to the passage of an electric current through that element; the inverse quantity is electrical conductance, the ease at which an electric current passes. Electrical resistance shares some conceptual parallels with the mechanical...
metal busbar
Busbar
In electrical power distribution, a bus bar is a strip of copper or aluminium that conducts electricity within a switchboard, distribution board, substation or other electrical apparatus....
s made of pure aluminium or copper
Copper
Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...
. The cells are electrically heated to reach the operating temperature
Operating temperature
An operating temperature is the temperature at which an electrical or mechanical device operates. The device will operate effectively within a specified temperature range which varies based on the device function and application context, and ranges from the minimum operating temperature to the...
with this current, and the anode regulator system varies the current passing through the cell by raising or lowering the anodes and changing the cell's resistance. If needed any cell can be bypassed by shunt
Shunt (electrical)
In electronics, a shunt is a device which allows electric current to pass around another point in the circuit. The term is also widely used in photovoltaics to describe an unwanted short circuit between the front and back surface contacts of a solar cell, usually caused by wafer damage.-Defective...
busbars.
The liquid aluminium is taken out with the help of a siphon
Siphon
The word siphon is sometimes used to refer to a wide variety of devices that involve the flow of liquids through tubes. But in the English language today, the word siphon usually refers to a tube in an inverted U shape which causes a liquid to flow uphill, above the surface of the reservoir,...
operating with a vacuum, in order to avoid having to use extremely high temperature valves and pumps. The liquid aluminium then may be transferred in batches or via a continuous hot flow line to a location where it is cast into aluminium ingot
Ingot
An ingot is a material, usually metal, that is cast into a shape suitable for further processing. Non-metallic and semiconductor materials prepared in bulk form may also be referred to as ingots, particularly when cast by mold based methods.-Uses:...
s. The aluminium can either be cast into the form of final cast-aluminium products, or the ingots can be sent elsewhere such as a rolling mill for being pressed into sheets, or the a wire-drawing mill for producing aluminium wires and cables.
While solid cryolite is denser than solid aluminium at room temperature, the liquid aluminium product is denser
Density
The mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...
than the molten cryolite at temperatures around 1000 °C (1,832 °F), and the aluminium sinks to the bottom of the electrolytic cell, where it is periodically collected. The tops and sides of the cells are covered with layers of solid cryolite which also act as thermal insulation. The unavoidable electric resistance within each cell produces sufficient heat to keep the cryolite-alumina mixture molten.
With the percentage of aluminium dissolved in each cell being depleted by the electrolysis in the molten cryolite, additional alumina is continually dropped into the cells to maintain the required level of alumina. Whenever a solid crust forms across the surface of the molten cryolite-alumina, this crust is broken from time to time to allow the added alumina to fall into the molten cryolite and dissolve there.
The electrolysis process produces exhaust which escapes into the fume hood and is evacuated. The exhaust is primarily CO2 produced from the anode consumption and hydrogen fluoride
Hydrogen fluoride
Hydrogen fluoride is a chemical compound with the formula HF. This colorless gas is the principal industrial source of fluorine, often in the aqueous form as hydrofluoric acid, and thus is the precursor to many important compounds including pharmaceuticals and polymers . HF is widely used in the...
(HF) from the cryolite and flux
Flux (metallurgy)
In metallurgy, a flux , is a chemical cleaning agent, flowing agent, or purifying agent. Fluxes may have more than one function at a time...
. HF is a highly corrosive and toxic gas, even etching
Etching
Etching is the process of using strong acid or mordant to cut into the unprotected parts of a metal surface to create a design in intaglio in the metal...
glass
Glass
Glass is an amorphous solid material. Glasses are typically brittle and optically transparent.The most familiar type of glass, used for centuries in windows and drinking vessels, is soda-lime glass, composed of about 75% silica plus Na2O, CaO, and several minor additives...
surfaces. The gases are either treated or vented into the atmosphere; the former involving neutralization of the HF to its sodium salt, sodium fluoride
Sodium fluoride
Sodium fluoride is an inorganic chemical compound with the formula NaF. A colorless solid, it is a source of the fluoride ion in diverse applications. Sodium fluoride is less expensive and less hygroscopic than the related salt potassium fluoride....
. The particulates are also captured and reused using electrostatic or bag filters. The remaining CO2 is usually vented into the atmosphere.
The very large electric current passing through the electrolytic cells generates a powerful magnetic field, and this can stir the molten aluminium with magneto-hydrodynamic
Magnetohydrodynamics
Magnetohydrodynamics is an academic discipline which studies the dynamics of electrically conducting fluids. Examples of such fluids include plasmas, liquid metals, and salt water or electrolytes...
forces in properly-designed cells. The stirring of the molten aluminium in each cell typically increases its performance, but the purity of the aluminium is reduced, since it gets mixed with small amounts of cryolite and aluminium fluoride. If the cells are designed for no stirring, they can be operated with static pools of molten aluminium so that the impurities either rise to the top of the metallic aluminium, or else sink to the bottom, leaving high-purity aluminium in the middle.
Aluminium smelters are usually sited where inexpensive hydroelectric power is available. For some European smelters, the electric power
Electric power
Electric power is the rate at which electric energy is transferred by an electric circuit. The SI unit of power is the watt.-Circuits:Electric power, like mechanical power, is represented by the letter P in electrical equations...
produced by hydroelectric plants in countries such as Norway
Norway
Norway , officially the Kingdom of Norway, is a Nordic unitary constitutional monarchy whose territory comprises the western portion of the Scandinavian Peninsula, Jan Mayen, and the Arctic archipelago of Svalbard and Bouvet Island. Norway has a total area of and a population of about 4.9 million...
, Switzerland
Switzerland
Switzerland name of one of the Swiss cantons. ; ; ; or ), in its full name the Swiss Confederation , is a federal republic consisting of 26 cantons, with Bern as the seat of the federal authorities. The country is situated in Western Europe,Or Central Europe depending on the definition....
, and Austria
Austria
Austria , officially the Republic of Austria , is a landlocked country of roughly 8.4 million people in Central Europe. It is bordered by the Czech Republic and Germany to the north, Slovakia and Hungary to the east, Slovenia and Italy to the south, and Switzerland and Liechtenstein to the...
is transmitted by high-voltage power line
Electric power transmission
Electric-power transmission is the bulk transfer of electrical energy, from generating power plants to Electrical substations located near demand centers...
s to such places as Denmark
Denmark
Denmark is a Scandinavian country in Northern Europe. The countries of Denmark and Greenland, as well as the Faroe Islands, constitute the Kingdom of Denmark . It is the southernmost of the Nordic countries, southwest of Sweden and south of Norway, and bordered to the south by Germany. Denmark...
, Sweden
Sweden
Sweden , officially the Kingdom of Sweden , is a Nordic country on the Scandinavian Peninsula in Northern Europe. Sweden borders with Norway and Finland and is connected to Denmark by a bridge-tunnel across the Öresund....
, Germany
Germany
Germany , officially the Federal Republic of Germany , is a federal parliamentary republic in Europe. The country consists of 16 states while the capital and largest city is Berlin. Germany covers an area of 357,021 km2 and has a largely temperate seasonal climate...
, and Italy
Italy
Italy , officially the Italian Republic languages]] under the European Charter for Regional or Minority Languages. In each of these, Italy's official name is as follows:;;;;;;;;), is a unitary parliamentary republic in South-Central Europe. To the north it borders France, Switzerland, Austria and...
to be used by aluminium and magnesium
Magnesium
Magnesium is a chemical element with the symbol Mg, atomic number 12, and common oxidation number +2. It is an alkaline earth metal and the eighth most abundant element in the Earth's crust and ninth in the known universe as a whole...
factories. Since aluminium factories require nearly-uniform supplies of electric current, they make the most of nearly-constant supplies of electric power, and these are also available close to many hydroelectric power plants. To give an example of such use of hydroelectric power, the three main regions for aluminium production in North America
North America
North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...
have always been in the Tennessee River
Tennessee River
The Tennessee River is the largest tributary of the Ohio River. It is approximately 652 miles long and is located in the southeastern United States in the Tennessee Valley. The river was once popularly known as the Cherokee River, among other names...
Valley of the Southeastern United States
Southeastern United States
The Southeastern United States, colloquially referred to as the Southeast, is the eastern portion of the Southern United States. It is one of the most populous regions in the United States of America....
, the Columbia River
Columbia River
The Columbia River is the largest river in the Pacific Northwest region of North America. The river rises in the Rocky Mountains of British Columbia, Canada, flows northwest and then south into the U.S. state of Washington, then turns west to form most of the border between Washington and the state...
Valley of Washington and Oregon
Oregon
Oregon is a state in the Pacific Northwest region of the United States. It is located on the Pacific coast, with Washington to the north, California to the south, Nevada on the southeast and Idaho to the east. The Columbia and Snake rivers delineate much of Oregon's northern and eastern...
, and the St. Lawrence River Valley of southeastern Canada
Canada
Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...
and the Northeastern United States
Northeastern United States
The Northeastern United States is a region of the United States as defined by the United States Census Bureau.-Composition:The region comprises nine states: the New England states of Connecticut, Maine, Massachusetts, New Hampshire, Rhode Island and Vermont; and the Mid-Atlantic states of New...
.
Many decades ago, before the existence of the Tennessee Valley Authority
Tennessee Valley Authority
The Tennessee Valley Authority is a federally owned corporation in the United States created by congressional charter in May 1933 to provide navigation, flood control, electricity generation, fertilizer manufacturing, and economic development in the Tennessee Valley, a region particularly affected...
, aluminium companies such as Alcoa
Alcoa
Alcoa Inc. is the world's third largest producer of aluminum, behind Rio Tinto Alcan and Rusal. From its operational headquarters in Pittsburgh, Pennsylvania, Alcoa conducts operations in 31 countries...
even built their own hydroelectric dams and powerhouses in the Appalachian Mountains
Appalachian Mountains
The Appalachian Mountains #Whether the stressed vowel is or ,#Whether the "ch" is pronounced as a fricative or an affricate , and#Whether the final vowel is the monophthong or the diphthong .), often called the Appalachians, are a system of mountains in eastern North America. The Appalachians...
of North Carolina
North Carolina
North Carolina is a state located in the southeastern United States. The state borders South Carolina and Georgia to the south, Tennessee to the west and Virginia to the north. North Carolina contains 100 counties. Its capital is Raleigh, and its largest city is Charlotte...
and Tennessee
Tennessee
Tennessee is a U.S. state located in the Southeastern United States. It has a population of 6,346,105, making it the nation's 17th-largest state by population, and covers , making it the 36th-largest by total land area...
.
History
The Hall–Héroult process was invented independently and almost simultaneously in 1886 by the AmericanUnited States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
chemist Charles Martin Hall
Charles Martin Hall
Charles Martin Hall was an American inventor, music enthusiast, and chemist. He is best known for his invention in 1886 of an inexpensive method for producing aluminium, which became the first metal to attain widespread use since the prehistoric discovery of iron.-Early years:Charles Martin Hall...
and the Frenchman
France
The French Republic , The French Republic , The French Republic , (commonly known as France , is a unitary semi-presidential republic in Western Europe with several overseas territories and islands located on other continents and in the Indian, Pacific, and Atlantic oceans. Metropolitan France...
Paul Héroult
Paul Héroult
The French scientist Paul Héroult was the inventor of the aluminium electrolysis and of the electric steel furnace. He lived in Thury-Harcourt, Normandy.Christian Bickert said of him...
. In 1888, Hall opened the first large-scale aluminium production plant in Pittsburgh
Pittsburgh, Pennsylvania
Pittsburgh is the second-largest city in the US Commonwealth of Pennsylvania and the county seat of Allegheny County. Regionally, it anchors the largest urban area of Appalachia and the Ohio River Valley, and nationally, it is the 22nd-largest urban area in the United States...
. It later became the Alcoa
Alcoa
Alcoa Inc. is the world's third largest producer of aluminum, behind Rio Tinto Alcan and Rusal. From its operational headquarters in Pittsburgh, Pennsylvania, Alcoa conducts operations in 31 countries...
corporation.
In 1997 the Hall–Héroult process was designated an ACS National Historical Chemical Landmark
ACS National Historical Chemical Landmarks
The National Historic Chemical Landmarks Program was launched by the American Chemical Society in 1992 and has recognized more than 60 landmarks to date. The program celebrates the centrality of chemistry...
in recognition of the importance of the commercialization of aluminium.
Development
The Hall–Héroult process is used all over the world and is the only method of aluminium smeltingSmelting
Smelting is a form of extractive metallurgy; its main use is to produce a metal from its ore. This includes iron extraction from iron ore, and copper extraction and other base metals from their ores...
currently used in the industry. Today, there are two primary technologies using the Hall–Héroult process: Söderberg and prebake. Söderberg uses a continuously created anode
Anode
An anode is an electrode through which electric current flows into a polarized electrical device. Mnemonic: ACID ....
made by addition of pitch
Pitch (resin)
Pitch is the name for any of a number of viscoelastic, solid polymers. Pitch can be made from petroleum products or plants. Petroleum-derived pitch is also called bitumen. Pitch produced from plants is also known as resin. Products made from plant resin are also known as rosin.Pitch was...
to the top of the anode. The lost heat from the smelting operation is used to bake the pitch into the carbon form required for reaction with alumina. Prebake technology is named after its anodes, which are baked in very large gas-fired ovens at high temperature before being lowered by various heavy industrial lifting systems into the electrolytic solution. In both technologies, the anode
Anode
An anode is an electrode through which electric current flows into a polarized electrical device. Mnemonic: ACID ....
, attached to a very large electrical bus, is slowly used up by the process because the oxygen generated by the electrolytic process can oxidize the carbon anode. Prebake technology tends to be slightly more efficient, but is more labor intensive. Prebake technology is becoming preferred in the industry because of the various pollutant emissions related to the creation of the anode from liquid pitch.
The legacy of the Hall–Héroult process
Aluminium is one of the most commonly occurring metallic elementMetal
A metal , is an element, compound, or alloy that is a good conductor of both electricity and heat. Metals are usually malleable and shiny, that is they reflect most of incident light...
s on Earth. However, it is rarely found in its elemental state
Elemental (disambiguation)
Elemental is the adjective form of the word element. In chemistry, it refers to matter composed of only one chemical element: graphite and diamond are both forms of elemental carbon....
. Instead it is mainly found as the ore bauxite
Bauxite
Bauxite is an aluminium ore and is the main source of aluminium. This form of rock consists mostly of the minerals gibbsite Al3, boehmite γ-AlO, and diaspore α-AlO, in a mixture with the two iron oxides goethite and hematite, the clay mineral kaolinite, and small amounts of anatase TiO2...
or Aluminium oxide
Aluminium oxide
Aluminium oxide is an amphoteric oxide with the chemical formula 23. It is commonly referred to as alumina, or corundum in its crystalline form, as well as many other names, reflecting its widespread occurrence in nature and industry...
, 23.
Prior to the Hall–Héroult process, elemental aluminium was made by heating ore along with elemental sodium
Sodium
Sodium is a chemical element with the symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals; its only stable isotope is 23Na. It is an abundant element that exists in numerous minerals, most commonly as sodium chloride...
or potassium
Potassium
Potassium is the chemical element with the symbol K and atomic number 19. Elemental potassium is a soft silvery-white alkali metal that oxidizes rapidly in air and is very reactive with water, generating sufficient heat to ignite the hydrogen emitted in the reaction.Potassium and sodium are...
in a vacuum
Vacuum
In everyday usage, vacuum is a volume of space that is essentially empty of matter, such that its gaseous pressure is much less than atmospheric pressure. The word comes from the Latin term for "empty". A perfect vacuum would be one with no particles in it at all, which is impossible to achieve in...
. The method was complicated and consumed materials that were in themselves expensive at that time. This meant the cost to produce the small amount of aluminum made in the early 19th century were very high.
Early aluminum was more costly than gold
Gold
Gold is a chemical element with the symbol Au and an atomic number of 79. Gold is a dense, soft, shiny, malleable and ductile metal. Pure gold has a bright yellow color and luster traditionally considered attractive, which it maintains without oxidizing in air or water. Chemically, gold is a...
or platinum
Platinum
Platinum is a chemical element with the chemical symbol Pt and an atomic number of 78. Its name is derived from the Spanish term platina del Pinto, which is literally translated into "little silver of the Pinto River." It is a dense, malleable, ductile, precious, gray-white transition metal...
. Bars of aluminium were exhibited alongside the French crown jewels
Crown jewels
Crown jewels are jewels or artifacts of the reigning royal family of their respective country. They belong to monarchs and are passed to the next sovereign to symbolize the right to rule. They may include crowns, sceptres, orbs, swords, rings, and other objects...
at the Exposition Universelle of 1855
Exposition Universelle (1855)
The Exposition Universelle of 1855 was an International Exhibition held on the Champs-Elysées in Paris from May 15 to November 15, 1855. Its full official title was the Exposition Universelle des produits de l'Agriculture, de l'Industrie et des Beaux-Arts de Paris 1855.The exposition was a major...
, and Emperor Napoleon III
Napoleon III of France
Louis-Napoléon Bonaparte was the President of the French Second Republic and as Napoleon III, the ruler of the Second French Empire. He was the nephew and heir of Napoleon I, christened as Charles Louis Napoléon Bonaparte...
of France was said to have reserved his few sets of aluminium dinner plates and eating utensils for his most honored guests.
Production costs using older methods did come down, however, when aluminum was selected as the material for the cap/lightning rod to sit atop the Washington Monument
Washington Monument
The Washington Monument is an obelisk near the west end of the National Mall in Washington, D.C., built to commemorate the first U.S. president, General George Washington...
in Washington, D.C.
Washington, D.C.
Washington, D.C., formally the District of Columbia and commonly referred to as Washington, "the District", or simply D.C., is the capital of the United States. On July 16, 1790, the United States Congress approved the creation of a permanent national capital as permitted by the U.S. Constitution....
, it was still more expensive than silver
Silver
Silver is a metallic chemical element with the chemical symbol Ag and atomic number 47. A soft, white, lustrous transition metal, it has the highest electrical conductivity of any element and the highest thermal conductivity of any metal...
.
New production based on the Hall–Héroult process, in combination with cheaper electric power
Electric power
Electric power is the rate at which electric energy is transferred by an electric circuit. The SI unit of power is the watt.-Circuits:Electric power, like mechanical power, is represented by the letter P in electrical equations...
, helped make aluminium (and incidentally magnesium
Magnesium
Magnesium is a chemical element with the symbol Mg, atomic number 12, and common oxidation number +2. It is an alkaline earth metal and the eighth most abundant element in the Earth's crust and ninth in the known universe as a whole...
) an inexpensive commodity.
This in turn helped make it possible for pioneers like Hugo Junkers
Hugo Junkers
Hugo Junkers was an innovative German engineer, as his many patents in varied areas show...
to utilize aluminium and aluminium-magnesium alloys to make items like metal airplanes by the thousands, or Howard Lund to make aluminum fishing boats.
See also
- Bayer processBayer processThe Bayer process is the principal industrial means of refining bauxite to produce alumina .Bauxite, the most important ore of aluminium, contains only 30–54% alumina, Al2O3, the rest being a mixture of silica, various iron oxides, and titanium dioxide. The alumina must be purified before it can...
- Aluminium: Production and refinement
- Aluminium smeltingAluminium smeltingAluminium smelting is the process of extracting aluminium from its oxide alumina, generally by the Hall-Héroult process. Alumina is extracted from the ore Bauxite by means of the Bayer process at an alumina refinery....