ALUMINUM: HISTORY, PROPERTIES, STRUCTURE, OBTAINING, USES - CHEMISTRY - 2025

The aluminum is a metallic element belonging to the (III A) group 13 of the periodic table and which is represented by the symbol A. This is a light metal with a low density and hardness. Due to its amphoteric properties, it has been classified by some scientists as a metalloid.

It is a ductile and very malleable metal, which is why it is used to manufacture wire, thin aluminum sheets, as well as any type of object or figure; for example, the famous cans with their alloys, or the aluminum foil with which food or desserts are wrapped.

Crumpled aluminum foil, one of the simplest and most everyday objects made with this metal. Source: Pexels.

Alum (a hydrated potassium aluminum sulfate) has been used by humans since ancient times in medicine, leather tanning, and as a mordant for staining fabrics. Thus, its minerals have been known forever.

However, aluminum as a metal was isolated very late, in 1825, by Øersted, which led to a scientific activity that allowed its industrial use. At that moment, aluminum was the metal with the highest production in the world, after iron.

Aluminum is found mainly in the upper part of the earth's crust, constituting 8% by weight of it. It corresponds to its third most abundant element, being surpassed by oxygen and silicon in its silica and silicate minerals.

Bauxite is an association of minerals, among which are: alumina (aluminum oxide), and metal oxides of iron, titanium and silicon. It represents the main natural resource for aluminum mining.

History

Alum

In Mesopotamia, 5000 years BC. C., They already made ceramics using clays that contained aluminum compounds. Meanwhile, 4000 ago, the Babylonians and Egyptians used aluminum in some chemical compounds.

The first written document related to alum was made by Herodotus, a Greek historian, in the 5th century BC. Alum was used as a mordant in the dyeing of fabrics and to protect the wood, with which the fortress doors were designed, from fires.

In the same way, Pliny "the Elder" in the 1st century refers to alum, today known as alum, as a substance used in medicine and mordant.

From the 16th century on, alum was used in leather tanning and as a paper sizing. This was a gelatinous substance that gave the paper consistency and allowed its use in writing.

In 1767, the Swiss chemist Torbern Bergman achieved the synthesis of alum. To do this, he heated the moonite with sulfuric acid, and then added potash to the solution.

Recognition in alumina

In 1782, the French chemist Antoine Lavoisier pointed out that alumina (Al ₂ O ₃) was an oxide of some element. This has such an affinity for oxygen that its separation was difficult. Therefore, Lavoisier then predicted the existence of aluminum.

Later, in 1807, the English chemist Sir Humphry Davy subjected alumina to electrolysis. However, the method he used produced an alloy of aluminum with potassium and sodium, so he could not isolate the metal.

Davy commented that alumina had a metallic base, which he initially designated as 'alumium', based on the Latin word 'alumen', the name used for alum. Davy later changed the name to "aluminum," the current English name.

In 1821, the German chemist Eilhard Mitscherlich managed to discover the correct formula for alumina: Al ₂ O ₃.

Isolation

That same year, French geologist Pierre Berthier discovered an aluminum mineral in a reddish clay rock deposit in France, in the Les Baux region. Berthier designated the mineral as bauxite. This mineral is currently the main source of aluminum.

In 1825, the Danish chemist Hans Christian Øersted produced a metal bar from a supposed aluminum. He described it as "a piece of metal that looks a bit like tin in color and shine." Øersted was able to achieve this by reducing the aluminum chloride, AlCl ₃, with a potassium amalgam.

It was thought, however, that the researcher did not obtain pure aluminum, but an alloy of aluminum and potassium.

In 1827, the German chemist Friedrich Wöehler managed to produce about 30 grams of an aluminum material. Then, after 18 years of investigative work, Wöehler in 1845 achieved the production of globules the size of a head of a pin, with a metallic luster and a grayish color.

Wöehler even described some properties of the metal, such as color, specific gravity, ductility and stability.

Industrial production

In 1855, the French chemist Henri Sainte-Claire Deville improved on Wöehler's method. For this, he used the reduction of aluminum chloride or sodium aluminum chloride with metallic sodium, using cryolite (Na ₃ AlF ₆) as flow.

This allowed the industrial production of aluminum in Rouen, France, and between 1855 and 1890 the production of 200 tons of aluminum was achieved.

In 1886, the French engineer Paul Héroult and the American student Charles Hall independently created a method for the production of aluminum. The method consists of the electrolytic reduction of aluminum oxide in molten cryolite, using a direct current.

The method was efficient, but it had the problem of its high electricity requirement, which made production more expensive. Héroult solved this problem by establishing his industry in Neuhausen (Switzerland), thus taking advantage of the Rhine Falls as generators of electricity.

Hall initially settled in Pittsburg (USA), but later relocated his industry near Niagara Falls.

Finally, in 1889 Karl Joseph Bayer created a method of producing alumina. This consists of heating the bauxite in a closed container with an alkaline solution. During the heating process, the alumina fraction is recovered in the saline solution.

Physical and chemical properties

Physical appearance

Aluminum metal bucket. Source: Carsten Niehaus

Silver gray solid with metallic luster (top image). It is a soft metal, but it hardens with small amounts of silicon and iron. In addition, it is characterized by being very ductile and malleable, since aluminum sheets with a thickness of up to 4 microns can be made.

Atomic weight

26,981 u

Atomic number (Z)

13

Melting point

660.32 ºC

Boiling point

2,470 ºC

Density

Ambient temperature: 2.70 g / mL

Melting point (liquid): 2.375 g / mL

Its density is considerably low compared to that of other metals. For that reason aluminum is quite light.

Heat of fusion

10.71 kJ / mol

Heat of vaporization

284 kJ / mol

Molar caloric capacity

24.20 J / (mol K)

Electronegativity

1.61 on the Pauling scale

Ionization energy

-First: 577.5 kJ / mol

-Second: 1,816.7 kJ / mol

-Third: 2,744.8 kJ / mol

Thermal expansion

23.1 µm / (mK) at 25 ºC

Thermal conductivity

237 W / (m K)

Aluminum has a thermal conductance three times that of steel.

Electrical resistivity

26.5 nΩ m at 20 ºC

Its electrical conductance is 2/3 of that of copper.

Magnetic order

Paramagnetic

Hardness

2.75 on the Mohs scale

Reactivity

Aluminum is resistant to corrosion because when exposed to air, the thin layer of Al ₂ O ₃ oxide that forms on its surface prevents oxidation from continuing inside the metal.

In acid solutions it reacts with water to form hydrogen; while in alkaline solutions it forms the aluminate ion (AlO ₂ ^-).

Dilute acids cannot dissolve it, but they can in the presence of concentrated hydrochloric acid. However, aluminum is resistant to concentrated nitric acid, although it is attacked by hydroxides to produce hydrogen and the aluminate ion.

Powdered aluminum is incinerated in the presence of oxygen and carbon dioxide to form aluminum oxide and aluminum carbide. It can be corroded by the chloride present in a sodium chloride solution. For this reason, the use of aluminum in pipes is not recommended.

Aluminum is oxidized by water at temperatures below 280 ºC.

2 Al (s) + 6 H ₂ O (g) => 2Al (OH) ₃ (s) + 3H ₂ (g) + heat

Structure and electronic configuration

Aluminum being a metallic element (with metalloid dyes for some), its Al atoms interact with each other thanks to the metallic bond. This non-directional force is governed by its valence electrons, which are scattered throughout the crystal in all its dimensions.

These valence electrons are the following, according to the electronic configuration of aluminum:

3s ² 3p ¹

Therefore, aluminum is a trivalent metal, as it has three valence electrons; two in the 3s orbital, and one in the 3p. These orbitals overlap to form 3s and 3p molecular orbitals, so close together that they end up forming conduction bands.

The s band is full, while the p band has a lot of vacancy for more electrons. That is why aluminum is a good conductor of electricity.

The metallic bond of aluminum, the radius of its atoms, and its electronic characteristics define an fcc crystal (face centered cubic, for its acronym in English). Such an FCC crystal is apparently the only known allotrope of aluminum, so it will surely withstand the high pressures operating on it.

Oxidation numbers

The electronic configuration of aluminum immediately indicates that it is capable of losing up to three electrons; that is, it has a high tendency to form the Al ³⁺ cation. When the existence of this cation is assumed in a compound derived from aluminum, it is said that it has an oxidation number of +3; as is well known, this is the most common for aluminum.

However, there are other possible but rare oxidation numbers for this metal; such as: -2 (Al ^2-), -1 (Al ^-), +1 (Al ⁺) and +2 (Al ²⁺).

In Al ₂ O ₃, for example, aluminum has an oxidation number of +3 (Al ₂ ³⁺ O ₃ ^2-); while in AlI and AlO, +1 (Al ⁺ F ^-) and +2 (Al ²⁺ O ^2-), respectively. However, under normal conditions or situations Al (III) or +3 is by far the most abundant oxidation number; since Al ³⁺ is isoelectronic to the neon noble gas.

That is why in school textbooks it is always assumed, and with good reason, that aluminum has +3 as the only number or oxidation state.

Where to find and obtaining

Aluminum is concentrated in the outer fringe of the earth's crust, being its third element, only surpassed by oxygen and silicon. Aluminum represents 8% by weight of the earth's crust.

It is found in igneous rocks, mainly: aluminosilicates, feldspars, feldspathoids and micas. Also in reddish clays, as is the case with bauxite.

- Bauxites

Bauxite mine. Source: User: VargaA

Bauxites are a mixture of minerals that contains hydrated alumina and impurities; such as iron and titanium oxides, and silica, with the following weight percentages:

-At ₂ O ₃ 35-60%

-Fe ₂ O ₃ 10-30%

-SiO ₂ 4-10%

-TiO ₂ 2-5%

-H ₂ O of constitution 12-30%.

Alumina is found in bauxite in hydrated form with two variants:

-monohydrates (Al ₂ O ₃ · H ₂ O), which have two crystallographic forms, boemite and diaspore

-Trihydrates (Al ₂ O ₃ · 3H ₂ O), represented by gibbsite.

Bauxite is the main source of aluminum and supplies most of the aluminum obtained from mining.

- Aluminum deposits

Of alteration

Mainly the bauxites formed by 40-50% of Al ₂ O ₃, 20% of Fe ₂ O ₃ and 3-10% of SiO ₂.

Hydrothermal

Alunite.

Magmatic

Aluminous rocks that have minerals such as syenites, nepheline and anorthites (20% of Al ₂ O ₃).

Metamorphic

Aluminum silicates (Andalusite, sillimanite and kyanite).

Detritics

Kaolin deposits and various clays (32% Al ₂ O ₃).

- Exploitation of bauxite

Bauxite is mined under the open sky. Once the rocks or clays that contain it are collected, they are crushed and ground in ball and bar mills, until obtaining particles of 2 mm in diameter. In these processes the treated material remains moistened.

In obtaining the alumina, the process created by Bayer in 1989 is followed. The ground bauxite is digested by the addition of sodium hydroxide, forming the sodium aluminate that is solubilized; while the pollutants iron, titanium and silicon oxides remain in suspension.

The contaminants are decanted and the alumina trihydrate is precipitated from the sodium aluminate by cooling and dilution. Subsequently, the trihydrated alumina is dried to give anhydrous alumina and water.

- Electrolysis of alumina

To obtain aluminum, alumina is subjected to electrolysis, usually following the method created by Hall-Héroult (1886). The process consists of reducing the molten alumina into cryolite.

The oxygen binds to the carbon anode and is released as carbon dioxide. Meanwhile, the released aluminum is deposited at the bottom of the electrolytic cell where it accumulates.

Alloys

Aluminum alloys are usually identified by four numbers.

1xxx

Code 1xxx corresponds to aluminum with 99% purity.

2xxx

Code 2xxx corresponds to the alloy of aluminum with copper. They are strong alloys that were used in aerospace vehicles, but they cracked from corrosion. These alloys are known as duralumin.

3xxx

The 3xxx code covers alloys in which manganese and a small amount of magnesium are added to aluminum. They are alloys very resistant to wear, being used the 3003 alloy in the elaboration of kitchen utensils, and the 3004 in beverage cans.

4xxx

The 4xxx code represents alloys in which silicon is added to aluminum, which lowers the melting point of the metal. This alloy is used in the manufacture of welding wires. Alloy 4043 is used in the welding of automobiles and structural elements.

5xxx

The 5xxx code covers alloys where magnesium is primarily added to aluminum.

They are strong alloys resistant to seawater corrosion, used to make pressure vessels and various marine applications. Alloy 5182 is used to make the lids of soda cans.

6xxx

The 6xxx code covers alloys in which silicon and magnesium are added to the alloy with aluminum. These alloys are castable, weldable and resistant to corrosion. The most common alloy in this series is used in architecture, bicycle frames, and the construction of the iPhone 6.

7xxx

The 7xxx code designates alloys in which zinc is added to aluminum. These alloys, also called Ergal, are resistant to breakage and are of great hardness, using alloys 7050 and 7075 in aircraft construction.

Risks

Direct exposure

Contact with aluminum powder can cause skin and eye irritation. Prolonged, high exposure to aluminum can cause flu-like symptoms, headache, fever, and chills; In addition, chest pain and tightness may occur.

Exposure to fine aluminum dust can cause lung scarring (pulmonary fibrosis), with symptoms of coughing and shortness of breath. OSHA established a limit of 5 mg / m ³ for exposure to aluminum dust in an 8-hour workday.

The biological tolerance value for occupational exposure to aluminum has been established at 50 µg / g of creatinine in urine. A diminishing performance in neuropsychological tests occurs when the aluminum concentration in the urine exceeds 100 µg / g of creatinine.

Breast cancer

Aluminum is used as aluminum hydrochloride in antiperspirant deodorants, having been linked to the development of breast cancer. However, this relationship has not been clearly established, among other things, because the skin absorption of aluminum hydrochloride is only 0.01%.

Neurotoxic effects

Aluminum is neurotoxic and in people with occupational exposure it has been linked to neurological diseases, which include Alzheimer's disease.

The brain of Alzheimer's patients has a high concentration of aluminum; but it is unknown whether it is the cause of the disease or a consequence of it.

The presence of neurotoxic effects has been determined in dialysis patients. In this procedure, aluminum salts were used as the phosphate binder, which produced high concentrations of aluminum in the blood (> 100 µg / L plasma).

The affected patients presented disorientation, memory problems and in advanced stages, dementia. The neurotoxicity of aluminum is explained because it is difficult to eliminate by the brain and affects its functioning.

Aluminum intake

Aluminum is present in many foods, especially tea, spices and, in general, vegetables. The European Food Safety Authority (EFSA) established a tolerance limit for the intake of aluminum in food of 1 mg / kg of body weight daily.

In 2008, the EFSA estimated that the daily intake of aluminum in food ranged between 3 and 10 mg per day, which is why it is concluded that it does not represent a risk to health; as well as the use of aluminum utensils to cook food.

Applications

- Like metal

Electrical

Aluminum is a good electrical conductor, which is why it is used in alloys in electrical transmission lines, motors, generators, transformers, and capacitors.

Building

Aluminum is used in the manufacture of door and window frames, partitions, fences, coatings, thermal insulators, ceilings, etc.

Transport

Aluminum is used in the manufacture of parts for automobiles, airplanes, trucks, bicycles, motorcycles, boats, spaceships, railroad cars, etc.

Containers

Aluminum cans for different varieties of food. Source: Pxhere.

Aluminum is used to make beverage cans, beer kegs, trays, etc.

Home

Aluminum buckets. Source: Pexels.

Aluminum is used to make kitchen utensils: pots, pans, pans and wrapping paper; in addition to furniture, lamps, etc.

Reflective power

Aluminum efficiently reflects radiant energy; from ultraviolet light to infrared radiation. The reflective power of aluminum in visible light is around 80%, which allows its use as a shade in lamps.

In addition, aluminum retains its silver reflective characteristic even in the form of a fine powder, so it can be used in the production of silver paints.

- Aluminum compounds

Alumina

It is used to make metallic aluminum, insulators and spark plugs. When alumina is heated, it develops a porous structure that absorbs water, being used to dry out gases and serve as a seat for the action of catalysts in various chemical reactions.

Aluminum sulfate

It is used in papermaking and as a surface filler. Aluminum sulfate serves to form potassium aluminum alum. This is the most widely used alum and with numerous applications; such as the manufacture of medicines, paints and mordant for the dyeing of fabrics.

Aluminum chloride

It is the most used catalyst in Friedel-Crafts reactions. These are synthetic organic reactions used in the preparation of aromatic ketones and anthraquinone. Hydrated Aluminum Chloride is used as a topical antiperspirant and deodorant.

Aluminum hydroxide

It is used to waterproof fabrics and the production of aluminates.

References

1. Shiver & Atkins. (2008). Inorganic chemistry. (Fourth edition). Mc Graw Hill.
2. Wikipedia. (2019). Aluminum. Recovered from: en.wikipedia.org
3. National Center for Biotechnology Information. (2019). Aluminum. PubChem Database. CID = 5359268. Recovered from: pubchem.ncbi.nlm.nih.gov/compound/Aluminum
4. The Editors of Encyclopaedia Britannica. (January 13, 2019). Aluminum. Encyclopædia Britannica. Recovered from: britannica.com
5. UC Rusal. (sf). Aluminum history. Recovered from: aluminumleader.com
6. Oviedo University. (2019). Aluminum metallurgy.. Recovered from: unioviedo.es
7. Helmenstine, Anne Marie, Ph.D. (February 6, 2019). Aluminum or Aluminum Alloys. Recovered from: thoughtco.com
8. Klotz, K., Weistenhöfer, W., Neff, F., Hartwig, A., van Thriel, C., & Drexler, H. (2017). The Health Effects of Aluminum Exposure. Deutsches Arzteblatt international, 114 (39), 653–659. doi: 10.3238 / arztebl.2017.0653
9. Elsevier. (2019). Aluminum Alloys. Recovered from: sciencedirect.com
10. Natalia GM (January 16, 2012). Aluminum availability in food. Recovered from: consumer.es