Burning aluminum

Burning aluminum in air

Unlike magnesium, single particles of aluminum do not ignite when heated in air or water vapor to 2100 K. Burning particles of magnesium were used to ignite aluminum. The latter were placed on the surface of the heating element, and aluminum particles were placed on the tip of the needle at a distance of 10–4 m above the former.

The ignition of aluminum particles during its ignition occurs in the vapor phase, and the intensity of the glow zone that appears around the particle increases slowly. Stationary combustion is characterized by the existence of a glow zone, which does not change its size until the metal is almost completely burned out. The ratio of the sizes of the glow zone and the particle is 1.6-1.9. In the glow zone, small droplets of oxide are formed, which merge upon collision.

The residue after combustion of the particle is a hollow shell that does not contain metal inside. The dependence of the burning time of a particle on its size is expressed by the formula (combustion is symmetrical).

Combustion of aluminum in water vapor

The ignition of aluminum in water vapor occurs heterogeneously. The hydrogen released during the reaction contributes to the destruction of the oxide film; while liquid aluminum oxide (or hydroxide) is sprayed in the form of droplets with a diameter of up to 10-15 microns. Such destruction of the oxide shell is periodically repeated. This indicates that a significant fraction of the metal burns on the surface of the particle.

At the beginning of combustion, the ratio r /r 0 is equal to 1.6-1.7. During combustion, the particle size decreases, and the ratio gsw/?o increases to 2.0-3.0. The burning rate of an aluminum particle in water vapor is almost 5 times greater than in air.

Combustion of aluminum-magnesium alloys

Combustion of aluminum-magnesium alloys in air

The ignition of particles of aluminum-magnesium alloys of variable composition in air, oxygen-argon mixtures, water vapor and carbon dioxide proceeds, as a rule, similar to the ignition of magnesium particles. The onset of ignition is preceded by oxidative reactions occurring on the surface.

The combustion of aluminum-magnesium alloys differs significantly from the combustion of both aluminum and magnesium and strongly depends on the ratio of components in the alloy and on the parameters of the oxidizing medium. The most important feature of the combustion of alloy particles is the two-stage process (Fig. 2.6). At the first stage, the particle is surrounded by a set of torches, which form an inhomogeneous glow zone of the reaction products. Comparing the nature and size of the glow zone surrounding the alloy particle during the first stage of combustion with the nature and size of the glow zone around the burning magnesium particle (see Fig. 2.4), we can conclude that at this stage it is mainly magnesium that burns out of the particle.

Rice. 2.6. Combustion of an alloy particle 30% A1 + 70% Mg at normal atmospheric pressure in a mixture containing 15% O by volume 2and 85% Ar:

1, 2 – magnesium burnout; 3-6 – aluminum burnout

A feature of the first stage of alloy combustion is the constancy of particle size and flame zone. This means that the liquid droplet of the alloy is enclosed within a solid oxide shell. Magnesium oxide predominates in the oxide film. Magnesium leaks out through film defects and burns in a vapor-phase diffusion flame.

At the end of the first stage, the course of heterogeneous reactions increases, as evidenced by the appearance of centers of bright luminescence on the surface of the particle. The heat released during heterogeneous reactions contributes to the heating of the particle to the melting point of the oxide and the beginning of the second stage of combustion.

At the second stage of combustion, the particle is surrounded by a homogeneous, brighter glow zone, which decreases as the metal burns out. The homogeneity and sphericity of the flame zone show that the oxide film on the surface of the particle is melted. Diffusion of the metal through the film is provided by the low diffusion resistance of the liquid oxide. The size of the flame zone significantly exceeds the size of the particle, which indicates the combustion of the metal in the vapor phase. Comparison of the nature of the second stage of combustion with the known pattern of aluminum combustion indicates a great similarity, probably, aluminum burns at this stage of the process. As it burns out, the size of the flame decreases, and, consequently, the size of the burning drop. The burnt particle glows for a long time.

Changing the size of the glow zone of a particle burning in accordance with the described mechanism is complex (Fig. 2.7). After ignition, the value r St. /r 0 quickly (in -0.1 ms) reaches its maximum value (section ab). Further, in the main time of the first stage of combustion, the ratio r sv/ r 0 remains constant (section bv). When magnesium burnout ends, r cv/ r 0 is reduced to a minimum (point G), and then, with the onset of aluminum combustion, it increases (section where). Last but not least aluminum burnout r St. /r 0 decreases monotonically (section de) to a final value corresponding to the size of the formed oxide.

Rice. 2.7.:

1 – alloy 30% Al + 70% Mg, air; 2 – alloy 30% A1 + 70% Mg, mixture 15% O2 + 85% Ar; 3 – alloy 50% A1 + 50% Mg, air

The mechanism and parameters of the combustion process of aluminum-magnesium alloys significantly depend on the composition of the alloy. With a decrease in the magnesium content in the alloy, the size of the glow zone during the first stage of combustion and the duration of this stage decrease. When the magnesium content in the alloy is less than 30%, the process remains two-stage, but becomes discontinuous. At the end of the first stage, the glow zone is reduced to the size of the particle itself, the combustion process stops, and aluminum burns out only after the particle re-ignites. Particles that do not re-ignite are hollow porous oxide shells containing droplets of unburned aluminum inside.

The dependence of the burning time of particles on their initial diameter is expressed by the following empirical formulas:

Combustion of aluminum-magnesium alloys in mixtures of oxygen with argon, in water vapor and in carbon dioxide.

The nature of combustion of particles of aluminum-magnesium alloys in oxygen-argon mixtures is the same as in air. With a decrease in the oxygen content, the size of the glow zone during magnesium burnout decreases markedly. The dependence of the burning time of the particles of the alloy 50% A1 + 50% Mg on the particle size and the oxygen content in the mixture in volume percent is expressed by the formula

The combustion of alloys in water vapor is significantly different (Fig. 2.8). The oxide film formed during the first stage is destroyed by hydrogen, and the particle takes the form of a coral. The aluminum remaining in the coral ignites only 1–10 ms after the end of the first stage. Such discontinuity of the process is typical for alloys of any composition.

Rice. 2.8. Combustion particles of aluminum-magnesium alloy (50:50) spherical(A) and wrong(b) forms in a medium of water vapor at normal atmospheric pressure:

1 – initial particle; 2 – particle before ignition; 3 – burnout of magnesium; 4 - aluminum burnout; 5 - coral formed after the particle

During the combustion of aluminum-magnesium alloys in carbon dioxide, only magnesium burns out of the particle, after which the combustion process stops.

Combustion of aluminum-magnesium alloys in a high-temperature flame

To study the process of combustion of metal particles at high temperatures, under a particle placed on the tip of a needle, a pressed tablet was burned from mixtures of ammonium perchlorate and urotropine, having calculated combustion temperatures of 2500, 2700, and 3100 K.

Combustion of particles of aluminum-magnesium alloys under these conditions occurs, as a rule, with an explosion. The presence of an explosion is characteristic of particles of all compositions. As a result of the explosion, a significant zone of luminescence is formed, which is a sign of the predominance of vapor-phase combustion. Photos of a burning particle at the beginning of combustion (Fig. 2.9, A) show that heterogeneous reactions occur on the entire surface of the oxide shell. Due to the heat of heterogeneous reactions, a rapid evaporation of the metal occurs (Fig. 2.9, b), contributing to the rupture of the oxide shell and splashing of the unevaporated drop (Fig. 2.9, V).

Rice. 2.9. Combustion of 95% Al alloy particles with 5% Mg in an oxidizing flame (temperature 2700 K):

A- the initial stage of combustion; b– stationary combustion; V- splitting up

According to B. G. Lrabey, S. E. Salibekov and Yu. V. Leninsky, the crushing of particles of aluminum-magnesium alloys is caused by a very large difference in the boiling points of magnesium and aluminum, as a result of which the boiling of magnesium when the particle is in the high temperature zone is explosive and leads to crushing of the remaining aluminum. The temperature of 2500 K is already sufficient for the occurrence of explosive combustion, which is quite natural, since this temperature exceeds the boiling point of both components.

• Arabey B. G., Salibekov S. E., Levinsky Yu. V. Some characteristics of ignition and combustion of metal dust // Powder metallurgy. 1964. No. 3. S. 109-118.

Dyldina Julia

The flame can have a different color, it all depends only on the metal salt that is added to it.

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MAOU secondary school No. 40

Subject

Flame coloring as one of the methods of analytical chemistry.

Dyldina Yudia,

9g class., MAOU secondary school No. 40

Supervisor:

Gurkina Svetlana Mikhailovna,

Biology and chemistry teacher.

Perm, 2015

1. Introduction.
2. Chapter 1 Analytical Chemistry.
3. Chapter 2 Methods of Analytical Chemistry.
4. Chapter 3 Flame Color Reactions.
5. Conclusion.

Introduction.

From early childhood, I was fascinated by the work of chemists. They seemed to be magicians who, having learned some hidden laws of nature, created the unknown. In the hands of these magicians, substances changed color, ignited, heated or cooled, exploded. When I came to chemistry class, the curtain began to rise, and I began to understand how chemical processes take place. The completed chemistry course was not enough for me, so I decided to work on the project. I wanted the topic I'm working on to be meaningful, help me prepare better for the chemistry exam, and satisfy my craving for beautiful and vivid reactions.

We study the coloring of a flame with metal ions in different colors in chemistry lessons when we go through alkali metals. When I became interested in this topic, it turned out that in this case, it was not fully disclosed. I decided to study it in more detail.

Target: With the help of this work, I want to learn how to determine the qualitative composition of some salts.

Tasks:

1. Get to know analytical chemistry.
2. Learn the methods of analytical chemistry and choose the most appropriate for my work.
3. Using the experiment to determine which metal is part of the salt.

Chapter 1.

Analytical chemistry.

Analytical chemistry -branch of chemistry that studies the chemical composition and partly the structure of substances.

The purpose of this science is to determine the chemical elements or groups of elements that make up substances.

The subject of its study is the improvement of existing and the development of new methods of analysis, the search for opportunities for their practical application, the study of the theoretical foundations of analytical methods.

Depending on the task of the methods, qualitative and quantitative analysis are distinguished.

1. Qualitative analysis - a set of chemical, physicochemical and physical methods used to detect elements, radicals and compounds that make up the analyzed substance or mixture of substances. In a qualitative analysis, one can use easily feasible, characteristic chemical reactions, in which the appearance or disappearance of coloring, the release or dissolution of a precipitate, the formation of gas, etc. are observed. Such reactions are called qualitative and with the help of them one can easily check the composition of a substance.

Qualitative analysis is most often carried out in aqueous solutions. It is based on ionic reactions and allows you to detect cations or anions of substances that are contained there. Robert Boyle is considered the founder of this analysis. He introduced this concept of chemical elements as the main parts of complex substances that cannot be decomposed, after which he systematized all the qualitative reactions known in his time.

1. Quantitative analysis - a set of chemical, physico-chemical and physical methods for determining the ratio of the components that make up

analyte. Based on the results of this, one can determine the equilibrium constants, solubility products, molecular and atomic masses. Such an analysis is more difficult to perform, since it requires a careful and more painstaking approach, otherwise the results can give high errors and the work will be reduced to zero.

Quantitative analysis is usually preceded by qualitative analysis.

Chapter 2

Methods of chemical analysis.

Methods of chemical analysis are divided into 3 groups.

1. Chemical Methodsbased on chemical reactions.

In this case, only such reactions can be used for analysis that are accompanied by a visual external effect, for example, a change in the color of the solution, evolution of gases, precipitation or dissolution of precipitates, etc. These external effects will serve as analytical signals in this case. The chemical changes that take place are called analytical reactions, and the substances that cause these reactions are called chemical reagents.

All chemical methods are divided into two groups:

1. The reaction is carried out in solution, the so-called "wet route".
2. A method of performing analysis with solids without the use of solvents, such a method is called the "dry path". It is divided into pyrochemical analysis and trituration analysis. Atpyrochemical analysis andThe substance under investigation is heated in the flame of a gas burner. In this case, volatile salts (chlorides, nitrates, carbonates) of a number of metals give the flame a certain color. Another method of pyrotechnical analysis is the production of colored pearls (glasses). To obtain pearls, salts and metal oxides are alloyed with sodium tetraborate (Na2 B4O7 "10H2O) or sodium ammonium hydrophosphate (NaNH4HP04 4H20) and the color of the resulting glasses (pearls) is observed.
3. Rubbing method was proposed in 1898 F. M. Flavitsky. A solid test substance is triturated with a solid reagent, and an external effect is observed. For example, cobalt salts with ammonium thiocyanate can give a blue color.

1. When analyzing by physical methodsstudy the physical properties of matter with the help of instruments, without resorting to chemical reactions. Physical methods include spectral analysis, luminescent, X-ray diffraction and other methods of analysis.
2. With the help of physico-chemical methodsstudy the physical phenomena that occur in chemical reactions. For example, in the colorimetric method, the color intensity is measured depending on the concentration of a substance, in the conductometric analysis, the change in the electrical conductivity of solutions is measured.

Chapter 3

Laboratory work.

Flame color reactions.

Target: To study the staining of the flame of an alcohol lamp with metal ions.

In my work, I decided to use the method of pyrotechnical analysis of flame staining with metal ions.

Test substances:metal salts (sodium fluoride, lithium chloride, copper sulfate, barium chloride, calcium chloride, strontium sulfate, magnesium chloride, lead sulfate).

Equipment: porcelain cups, ethyl alcohol, glass rod, concentrated hydrochloric acid.

To carry out the work, I made a solution of salt in ethyl alcohol, and then set it on fire. I spent my experience several times, at the last stage the best samples were selected, the field of which we made a video.

Conclusions:

Volatile salts of many metals color the flame in various colors characteristic of these metals. The color depends on the incandescent vapors of free metals, which are obtained as a result of the thermal decomposition of salts when they are introduced into the burner flame. In my case, these salts included sodium fluoride and lithium chloride, they gave bright saturated colors.

Conclusion.

Chemical analysis is used by a person in very many areas, while in chemistry lessons we get acquainted with only a small area of ​​\u200b\u200bthis complex science. The techniques that are used in pyrochemical analysis are used in qualitative analysis as a preliminary test in the analysis of a mixture of solids or as verification reactions. In the qualitative analysis of the reaction, the "dry" way plays only an auxiliary role, they are usually used as primary tests and verification reactions.

In addition, these reactions are used by humans in other industries, for example, in fireworks. As we know, fireworks are decorative lights of various colors and shapes, obtained by burning pyrotechnic compositions. So, a variety of combustible substances are added to the fireworks of pyrotechnics, among which non-metallic elements (silicon, boron, sulfur) are widely represented. During the oxidation of boron and silicon, a large amount of energy is released, but gas products are not formed, therefore these substances are used to make delayed-action fuses (to ignite other compounds at a certain time). Many mixtures include organic carbonaceous materials. For example, charcoal (used in black powder, fireworks shells) or sugar (smoke grenades). Reactive metals (aluminum, titanium, magnesium) are used, whose combustion at high temperature gives a bright light. This property of theirs began to be used to launch fireworks.

In the process of work, I realized how difficult and important it is to work with substances, not everything was fully successful, as I would like. As a rule, in chemistry lessons there is not enough practice work, thanks to which theoretical skills are worked out. The project helped me develop this skill. In addition, it was with great pleasure that I introduced my classmates to the results of my work. This helped them consolidate their theoretical knowledge.

It is easy to guess that the hue of the flame is determined by the chemicals that burn in it, in the event that exposure to high temperature releases individual atoms of the combustible substances, coloring the fire. To determine the effect of substances on the color of fire, various experiments were carried out, which we will discuss below.

Since ancient times, alchemists and scientists have tried to find out what substances burn, depending on the color that the flame acquired.

The flame of geysers and stoves available in all houses and apartments has a blue tint. Such a shade during combustion gives carbon, carbon monoxide. The yellow-orange color of the fire flame, which is bred in the forest, or household matches, is due to the high content of sodium salts in natural wood. Largely due to this - red. The flame of a gas stove burner will acquire the same color if you sprinkle it with ordinary table salt. When burning copper, the flame will be green. I think you have noticed that with a long wear of a ring or chain made of ordinary copper, not covered with a protective compound, the skin becomes green. The same thing happens during the combustion process. If the copper content is high, there is a very bright green fire, almost identical to white. This can be seen if copper shavings are poured onto a gas burner.

Many experiments have been carried out involving an ordinary gas burner and various minerals. Thus, their composition was determined. You need to take the mineral with tweezers and place it in the flame. The color that the fire takes on can indicate the various impurities present in the element. The flame of green color and its shades indicates the presence of copper, barium, molybdenum, antimony, phosphorus. Boron gives a blue-green color. Selenium gives the flame a blue tint. The flame turns red in the presence of strontium, lithium and calcium, in violet - potassium. Yellow-orange color is obtained during the combustion of sodium.

Studies of minerals to determine their composition are carried out using a Bunsen burner. The color of its flame is even and colorless; it does not interfere with the course of the experiment. Bunsen invented the burner in the middle of the 19th century.

He came up with a method that allows you to determine the composition of a substance by the shade of the flame. Scientists tried to conduct similar experiments before him, but they did not have a Bunsen burner, the colorless flame of which did not interfere with the experiment. He placed various elements on a wire of platinum in the fire of the burner, since when this metal is added, the flame does not color. At first glance, the method seems to be good; laborious chemical analysis can be dispensed with. It is enough just to bring the element to the fire and see what it consists of. But substances in their pure form can be found in nature extremely rarely. Usually they contain large amounts of various impurities that change the color of the flame.

Bunsen tried to isolate colors and shades by various methods. For example, using colored glasses. For example, if you look through blue glass, you will not see the yellow color in which the fire is painted when burning the most common sodium salts. Then the lilac or crimson hue of the desired element becomes discernible. But even such tricks led to the correct determination of the composition of a complex mineral in very rare cases. More than this technology could not achieve.

Nowadays, such a torch is used only for soldering.

Aluminum - combustible metal, atomic mass 26.98; density 2700 kg / m 3, melting point 660.1 ° C; boiling point 2486 °C; heat of combustion -31087 kJ/kg. Aluminum shavings and dust can ignite under the local action of low-calorie ignition sources (match flames, sparks, etc.). When aluminum powder, shavings, foil interact with moisture, aluminum oxide is formed and a large amount of heat is released, leading to their spontaneous combustion when accumulated in heaps. This process is facilitated by the contamination of these materials with oils. The release of free hydrogen during the interaction of aluminum dust with moisture facilitates its explosion. The temperature of self-ignition of a sample of aluminum dust with a dispersion of 27 microns is 520 °C; smoldering temperature 410 °C; lower concentration limit of flame propagation 40 g/m 3 ; maximum explosion pressure 1.3 MPa; pressure rise rate: average 24.1 MPa/s, maximum 68.6 MPa/s. The limiting oxygen concentration at which the ignition of the air suspension by an electric spark is excluded, 3% of the volume. The settled dust is a fire hazard. Self-ignition temperature 320 °C. Aluminum easily interacts at room temperature with aqueous solutions of alkalis and ammonia with the evolution of hydrogen. Mixing aluminum powder with an alkaline aqueous solution may cause an explosion. Reacts vigorously with many metalloids. Aluminum shavings burn, for example, in bromine, forming aluminum bromide. The interaction of aluminum with chlorine and bromine occurs at room temperature, with iodine - when heated. When heated, aluminum combines with sulfur. If aluminum powder is poured into boiling sulfur vapor, the aluminum ignites. Heavily ground aluminum reacts with halogenated hydrocarbons; a small amount of aluminum chloride (formed during this reaction) acts as a catalyst, speeding up the reaction, in some cases leading to an explosion. This phenomenon is observed when aluminum powder is heated with methyl chloride, carbon tetrachloride, a mixture of chloroform and carbon tetrachloride to a temperature of about 150 °C.

Aluminum in the form of a compact material does not interact with carbon tetrachloride. Mixing aluminum dust with some chlorinated hydrocarbons and alcohol causes the mixture to ignite spontaneously. A mixture of aluminum powder with copper oxide, silver oxide, lead oxide and especially lead dioxide burns with an explosion. A mixture of ammonium nitrate, aluminum powder with coal or nitro compounds is an explosive. Extinguishing media: dry sand, alumina, magnesite powder, asbestos blanket. The use of water and fire extinguishers is prohibited.

In its pure form, aluminum does not occur in nature, because it is very quickly oxidized by atmospheric oxygen with the formation of strong oxide films that protect the surface from further interaction.

As a structural material, not pure aluminum is usually used, but various alloys based on it, which are characterized by a combination of satisfactory strength, good ductility, very good weldability and corrosion resistance. In addition, these alloys are characterized by high vibration resistance.

chemical element of group III of the periodic system, atomic number 13, relative atomic mass 26.98. In nature, it is represented by only one stable nuclide 27 Al. Artificially obtained a number of radioactive isotopes of aluminum, the most long-lived 26 Al has a half-life of 720 thousand years. aluminum in nature. There is a lot of aluminum in the earth's crust: 8.6% by weight. It ranks first among all metals and third among other elements (after oxygen and silicon). There is twice as much aluminum as iron and 350 times as much as copper, zinc, chromium, tin and lead combined! As he wrote over 100 years ago in his classic textbook Fundamentals of Chemistry D.I. Mendeleev, of all metals, “aluminum is the most common in nature; it suffices to point out that it is part of the clay, so that the general distribution of aluminum in the earth's crust is clear. Aluminum, or the metal of alum (alumen), is therefore otherwise called clay, which is found in clay.

The most important mineral of aluminum is bauxite, a mixture of basic oxide AlO(OH) and hydroxide Al(OH)

3 . The largest deposits of bauxite are in Australia, Brazil, Guinea and Jamaica; industrial production is also carried out in other countries. Alunite (alum stone) is also rich in aluminum (Na,K) 2 SO 4 Al 2 (SO 4) 3 4Al (OH) 3, nepheline (Na, K) 2 O Al 2 O 3 2SiO 2 . In total, more than 250 minerals are known, which include aluminum; most of them are aluminosilicates, from which the earth's crust is mainly formed. When they weather, clay is formed, the basis of which is the mineral kaolinite Al 2 O 3 2SiO 2 2H 2 O. Iron impurities usually color the clay brown, but white clay kaolin is also found, which is used to make porcelain and faience products. see also BOXITES.

Occasionally there is an exceptionally hard (second only to diamond) mineral corundum crystalline oxide Al

2O3 , often colored by impurities in different colors. Its blue variety (an admixture of titanium and iron) is called a sapphire, the red one (an admixture of chromium) is called a ruby. Various impurities can color the so-called noble corundum also in green, yellow, orange, purple and other colors and shades.

Until recently, it was believed that aluminum, as a very active metal, cannot occur in nature in a free state, however, in 1978, native aluminum was discovered in the rocks of the Siberian Platform in the form of whiskers only 0.5 mm long (with filaments a few micrometers thick). In the lunar soil, delivered to Earth from the regions of the Seas of Crises and Abundance, it was also possible to detect native aluminum. It is assumed that metallic aluminum can be formed by condensation from the gas. It is known that when aluminum halides - chloride, bromide, fluoride are heated, they can evaporate more or less easily (for example, AlCl

3 sublimates already at 180°C). With a strong increase in temperature, aluminum halides decompose, passing into a state with a lower valency of the metal, for example, AlCl. When such a compound condenses with a decrease in temperature and the absence of oxygen, a disproportionation reaction occurs in the solid phase: part of the aluminum atoms is oxidized and goes into the usual trivalent state, and part is restored. Monovalent aluminum can be reduced only to metal: 3AlCl® 2Al + AlCl 3 . This assumption is also supported by the filamentous shape of native aluminum crystals. Typically, crystals of this structure are formed due to rapid growth from the gas phase. Probably, microscopic aluminum nuggets in the lunar soil were formed in a similar way.

The name aluminum comes from the Latin alumen (genus case aluminis). So called alum, double potassium-aluminum sulfate KAl(SO

4) 2 12H 2 O) , which was used as a mordant in the dyeing of fabrics. The Latin name probably goes back to the Greek "halme" brine, salt solution. It is curious that in England aluminum is aluminum, and in the USA it is aluminum.

In many popular books on chemistry, there is a legend that an inventor, whose name history has not preserved, brought to the emperor Tiberius, who ruled Rome in 1427 AD, a bowl made of a metal resembling silver in color, but lighter. This gift cost the master his life: Tiberius ordered to execute him and destroy the workshop, because he was afraid that the new metal could devalue the silver in the imperial treasury.

This legend is based on a story by Pliny the Elder, a Roman writer and scholar, author natural history encyclopedia of natural science knowledge of ancient times. According to Pliny, the new metal was obtained from "clay earth". But clay does contain aluminum.

Modern authors almost always make the reservation that this whole story is nothing more than a beautiful fairy tale. And this is not surprising: aluminum in rocks is extremely strongly bound to oxygen, and it takes a lot of energy to release it. Recently, however, new data have appeared on the fundamental possibility of obtaining metallic aluminum in antiquity. As shown by spectral analysis, the decorations on the tomb of the Chinese commander Zhou-Zhu, who died at the beginning of the 3rd century. AD, are made from an alloy that is 85% aluminium. Could the ancients have obtained free aluminum? All known methods (electrolysis, reduction with metallic sodium or potassium) are automatically eliminated. Could native aluminum be found in antiquity, such as, for example, nuggets of gold, silver, copper? This is also excluded: native aluminum is the rarest mineral that occurs in negligible quantities, so the ancient masters could not find and collect such nuggets in the right amount.

However, another explanation of Pliny's story is also possible. Aluminum can be recovered from ores not only with the help of electricity and alkali metals. There is a reducing agent available and widely used since ancient times - this is coal, with the help of which the oxides of many metals are reduced to free metals when heated. In the late 1970s, German chemists decided to test whether aluminum could have been made in antiquity by reduction with coal. They heated a mixture of clay with coal powder and common salt or potash (potassium carbonate) in a clay crucible to a red heat. Salt was obtained from sea water, and potash from plant ash, in order to use only those substances and methods that were available in antiquity. After some time, slag with aluminum balls floated on the surface of the crucible! The yield of metal was small

, but it is possible that it was in this way that the ancient metallurgists could obtain the "metal of the 20th century."aluminum properties. The color of pure aluminum resembles silver, it is a very light metal: its density is only 2.7 g/cm 3 . Lighter than aluminum are only alkali and alkaline earth metals (except barium), beryllium and magnesium. Aluminum also melts easily at 600°C (thin aluminum wire can be melted on an ordinary kitchen burner), but it boils only at 2452°CC. In terms of electrical conductivity, aluminum is in 4th place, second only to silver (it is in first place), copper and gold, which, given the low cost of aluminum, is of great practical importance. The thermal conductivity of metals changes in the same order. It is easy to verify the high thermal conductivity of aluminum by dipping an aluminum spoon into hot tea. And one more remarkable property of this metal: its smooth, shiny surface perfectly reflects light: from 80 to 93% in the visible region of the spectrum, depending on the wavelength. In the ultraviolet region, aluminum has no equal in this respect, and only in the red region is it slightly inferior to silver (in the ultraviolet, silver has a very low reflectivity).

Pure aluminum is a rather soft metal almost three times softer than copper, so even relatively thick aluminum plates and rods are easy to bend, but when aluminum forms alloys (there are a huge number of them), its hardness can increase tenfold.

The characteristic oxidation state of aluminum is +3, but due to the presence of unfilled 3 R- and 3

d -orbitals aluminum atoms can form additional donor-acceptor bonds. Therefore, the Al ion 3+ with a small radius is very prone to complex formation, forming a variety of cationic and anionic complexes: AlCl4 , AlF 6 3 , 3+ , Al(OH) 4 , Al(OH) 6 3 , AlH 4and many others. Complexes with organic compounds are also known.

The chemical activity of aluminum is very high; in the series of electrode potentials, it is immediately behind magnesium. At first glance, such a statement may seem strange: after all, an aluminum pan or spoon is quite stable in air, and does not collapse in boiling water. Aluminum, unlike iron, does not rust. It turns out that in air the metal is covered with a colorless, thin, but strong "armor" of oxide, which protects the metal from oxidation. So, if a thick aluminum wire or a plate 0.51 mm thick is introduced into the burner flame, the metal melts, but aluminum does not flow, as it remains in a bag of its oxide. If you deprive aluminum of the protective film or make it loose (for example, by immersion in a solution of mercury salts), aluminum will immediately show its true essence: already at room temperature it will begin to react vigorously with water with the evolution of hydrogen: 2Al + 6H

2 O ® 2Al(OH) 3 + 3H 2 . In air, aluminum devoid of a protective film turns into a loose oxide powder right before our eyes: 2Al + 3O 2 ® 2Al 2 O 3 . Aluminum is especially active in a finely divided state; aluminum dust, when blown into the flame, instantly burns out. If you mix aluminum dust with sodium peroxide on a ceramic plate and drop water on the mixture, aluminum also flares up and burns with a white flame.

The very high affinity of aluminum for oxygen allows it to “take away” oxygen from the oxides of a number of other metals, restoring them (aluminothermy method). The most famous example is the thermite mixture, during combustion of which so much heat is released that the resulting iron melts: 8Al + 3Fe

3 O 4 ® 4 Al 2 O 3 + 9Fe. This reaction was discovered in 1856 by N.N. Beketov. In this way, it is possible to reduce to metals Fe2 O 3 , CoO, NiO, MoO 3 , V 2 O 5 , SnO 2, CuO, a number of other oxides. When aluminum is reduced, Cr2 O 3 , Nb 2 O 5 , Ta 2 O 5 , SiO 2 , TiO 2 , ZrO 2 , B 2 O 3the heat of reaction is not sufficient to heat the reaction products above their melting point.

Aluminum readily dissolves in dilute mineral acids to form salts. Concentrated nitric acid, by oxidizing the aluminum surface, contributes to the thickening and hardening of the oxide film (the so-called metal passivation). Aluminum treated in this way does not react even with hydrochloric acid. With the help of electrochemical

anodic oxidation (anodizing) on ​​the surface of aluminum, you can create a thick film that is easy to paint in different colors.

The displacement of less active metals from salt solutions by aluminum is often hindered by a protective film on the aluminum surface. This film is rapidly destroyed by copper chloride, so the 3CuCl reaction proceeds easily.

2 + 2Al ® 2AlCl 3 + 3Cu, which is accompanied by strong heating. In strong alkali solutions, aluminum dissolves easily with the release of hydrogen: 2Al + 6NaOH + 6H 2 О ® 2Na 3 + 3H 2 (other anionic hydroxo complexes are also formed). The amphoteric nature of aluminum compounds is also manifested in the easy dissolution of its freshly precipitated oxide and hydroxide in alkalis. Crystalline oxide (corundum) is very resistant to acids and alkalis. When fused with alkalis, anhydrous aluminates are formed: Al 2 O 3 + 2NaOH ® 2NaAlO 2 + H 2 O. Magnesium aluminate Mg(AlO 2) 2 semi-precious stone spinel, usually colored with impurities in a wide variety of colors.

Aluminum reacts violently with halogens. If a thin aluminum wire is introduced into a test tube with 1 ml of bromine, then after a short time the aluminum ignites and burns with a bright flame. The reaction of a mixture of aluminum and iodine powders is initiated by a drop of water (water with iodine forms an acid that destroys the oxide film), after which a bright flame appears with clubs of purple iodine vapor. Aluminum halides in aqueous solutions are acidic due to hydrolysis: AlCl

3 + H 2 O Al(OH)Cl 2 + HCl. The reaction of aluminum with nitrogen occurs only above 800 ° C with the formation of AlN nitride, with sulfur at 200 ° C (Al sulfide is formed 2 S 3 ), with phosphorus at 500 ° C (AlP phosphide is formed). When boron is introduced into molten aluminum, borides of composition AlB 2 and AlB 12 refractory compounds resistant to acids. Hydride (AlH) x (x = 1.2) is formed only in vacuum at low temperatures in the reaction of atomic hydrogen with aluminum vapor. Hydride AlH, stable in the absence of moisture at room temperature 3 obtained in anhydrous ether solution: Al Cl 3 + LiH ® AlH 3 + 3LiCl. With an excess of LiH, salt-like lithium aluminum hydride LiAlH 4 a very strong reducing agent used in organic synthesis. It decomposes instantly with water: LiAlH 4 + 4H 2 O ® LiOH + Al(OH) 3 + 4H 2 . Getting aluminium. The documented discovery of aluminum occurred in 1825. For the first time this metal was received by a Danish physicist Hans Christian Oersted when he isolated it by the action of potassium amalgam on anhydrous aluminum chloride (obtained by passing chlorine through a hot mixture of aluminum oxide and coal). Having driven away the mercury, Oersted obtained aluminum, however, contaminated with impurities. In 1827, the German chemist Friedrich Wöhler obtained aluminum in powder form by reducing potassium hexafluoroaluminate: Na 3 AlF 6 + 3K ® Al + 3NaF + 3KF. Later, he managed to obtain aluminum in the form of shiny metal balls. In 1854, the French chemist Henri Etienne Saint-Clair Deville developed the first industrial method for producing aluminum by reducing a melt of sodium tetrachloroaluminate: NaAlCl 4 + 3Na ® Al + 4NaCl. However, aluminum continued to be an extremely rare and expensive metal; it was not much cheaper than gold and 1,500 times more expensive than iron (now only three times). From gold, aluminum and precious stones, a rattle was made in the 1850s for the son of the French Emperor Napoleon III. When in 1855 at the World Exhibition in Paris a large ingot of aluminum obtained by a new method was exhibited, it was looked at as a jewel. The upper part (in the form of a pyramid) of the Washington Monument in the US capital was made of precious aluminum. At that time, aluminum was not much cheaper than silver: in the USA, for example, in 1856 it was sold at a price of $ 12 per pound (454 g), and silver at $ 15. In the 1st volume of the famous Encyclopedic Dictionary of Brockhaus and Efron said that "aluminum is still used mainly for dressing ... luxury items." By that time, only 2.5 tons of metal were mined annually throughout the world. Only towards the end of the 19th century, when the electrolytic method for obtaining aluminum was developed, its annual production began to amount to thousands of tons, and in the 20th century. million tons. This made aluminum a widely available semi-precious metal.

The modern method of obtaining aluminum was discovered in 1886 by a young American researcher. Charles Martin Hall. He became interested in chemistry as a child. Having found his father's old chemistry textbook, he began to study it diligently, as well as to experiment, once even received a scolding from his mother for damaging the dinner tablecloth. And 10 years later, he made an outstanding discovery that glorified him throughout the world.

Having become a student at the age of 16, Hall heard from his teacher, F.F. Jewett, that if someone succeeds in developing a cheap way to obtain aluminum, then this person will not only provide a huge service to humanity, but also earn a huge fortune. Jewett knew what he was talking about: he had previously trained in Germany, worked for Wöhler, and discussed with him the problems of obtaining aluminum. With him to America, Jewett also brought a sample of a rare metal, which he showed to his students. Suddenly, Hall declared out loud: "I'll get this metal!"

Six years of hard work continued. Hall tried to obtain aluminum by various methods, but without success. Finally, he tried to extract this metal by electrolysis. At that time there were no power plants, the current had to be obtained using large home-made batteries from coal, zinc, nitric and sulfuric acids. Hall worked in a barn where he set up a small laboratory. He was assisted by his sister Julia, who was very interested in her brother's experiments. She kept all his letters and work journals, which allow literally day by day to trace the history of the discovery. Here is an excerpt from her memoirs:

“Charles was always in a good mood, and even on the worst days he was able to laugh at the fate of unlucky inventors. In times of failure, he found solace at our old piano. In his home laboratory he worked long hours without a break; and when he could leave the set for a while, he would rush through our long house to play a little ... I knew that playing with such

charm and feeling, he constantly thinks about his work. And the music helped him in this.

The hardest part was finding the electrolyte and protecting the aluminum from oxidation. After six months of exhausting labor, a few small silver balls finally appeared in the crucible. Hall immediately ran to his former teacher to report on his success. “Professor, I got it!” he exclaimed, holding out his hand: in the palm of his hand lay a dozen small aluminum balls. This happened on February 23, 1886. And exactly two months later, on April 23 of the same year, the Frenchman Paul Héroux took out a patent for a similar invention, which he made independently and almost simultaneously (two other coincidences are striking: both Hall and Héroux were born in 1863 and died in 1914).

Now the first balls of aluminum obtained by Hall are kept in the American Aluminum Company in Pittsburgh as a national relic, and in his college there is a monument to Hall, cast from aluminum. Subsequently, Jewett wrote: “My most important discovery was the discovery of man

. It was Charles M. Hall, who, at the age of 21, discovered a way to recover aluminum from ore, and thus made aluminum that wonderful metal that is now widely used throughout the world. Jewett's prophecy came true: Hall received wide recognition, became an honorary member of many scientific societies. But his personal life failed: the bride did not want to put up with the fact that her fiance spends all the time in the laboratory, and broke off the engagement. Hall found solace in his native college, where he worked for the rest of his life. As Charles's brother wrote, "College was his wife and children and everything his whole life." Hall also bequeathed most of his $5 million inheritance to the college. Hall died of leukemia at the age of 51.

Hall's method made it possible to obtain relatively inexpensive aluminum using electricity on a large scale. If from 1855 to 1890 only 200 tons of aluminum were obtained, then over the next decade, according to the Hall method, 28,000 tons of this metal were obtained all over the world! By 1930, the world annual production of aluminum had reached 300,000 tons. Now more than 15 million tons of aluminum are produced annually. Alumina solution (technical Al

2O3 ) in molten cryolite Na 3 AlF 6 , which is partly mined in the form of a mineral, and partly specially synthesized. Liquid aluminum accumulates at the bottom of the bath (cathode), oxygen is released on carbon anodes, which gradually burn out. At low voltage (about 4.5 V), electrolyzers draw huge currentsup to 250 000 A! For a day, one electrolyzer produces about a ton of aluminum. Production requires large amounts of electricity: 15,000 kilowatt-hours of electricity are spent to produce 1 ton of metal. This amount of electricity consumes a large 150-apartment building for a whole month. The production of aluminum is environmentally dangerous, as the atmospheric air is polluted with volatile fluorine compounds.The use of aluminium. Even D.I.Mendeleev wrote that "metal aluminum, having great lightness and strength and low variability in air, is very suitable for some products." Aluminum is one of the most common and cheapest metals. Without it, it is difficult to imagine modern life. No wonder aluminum is called the metal of the 20th century. It lends itself well to processing: forging, stamping, rolling, drawing, pressing. Pure aluminum is a fairly soft metal; it is used to make electrical wires, structural parts, food foil, kitchen utensils and "silver" paint. This beautiful and light metal is widely used in construction and aviation technology. Aluminum reflects light very well. Therefore, it is used for the manufacture of mirrors by metal deposition in a vacuum.

In aircraft and mechanical engineering, in the manufacture of building structures, much harder aluminum alloys are used. One of the most famous is an alloy of aluminum with copper and magnesium (duralumin, or simply "duralumin"; the name comes from the German city of Düren). This alloy, after hardening, acquires a special hardness and becomes about 7 times stronger than pure aluminum. At the same time, it is almost three times lighter than iron. It is obtained by alloying aluminum with small additions of copper, magnesium, manganese, silicon and iron. Silumins are widely used - cast alloys of aluminum with silicon. High-strength, cryogenic (frost-resistant) and heat-resistant alloys are also produced. Protective and decorative coatings are easily applied to products made of aluminum alloys. The lightness and strength of aluminum alloys were especially useful in aviation technology. For example, helicopter propellers are made from an alloy of aluminum, magnesium and silicon. Relatively cheap aluminum bronze (up to 11% Al) has high mechanical properties, it is stable in sea water and even in dilute hydrochloric acid. From aluminum bronze in the USSR from 1926 to 1957 coins were minted in denominations of 1, 2, 3 and 5 kopecks.

Currently, a quarter of all aluminum is used for construction needs, the same amount is consumed by transport engineering, approximately 17% is spent on packaging materials and cans, 10% in electrical engineering.

Aluminum also contains many combustible and explosive mixtures. Alumotol, a cast mixture of trinitrotoluene with aluminum powder, is one of the most powerful industrial explosives. Ammonal is an explosive substance consisting of ammonium nitrate, trinitrotoluene and aluminum powder. Incendiary compositions contain aluminum and an oxidizing agent nitrate, perchlorate. Pyrotechnic compositions "Zvezdochka" also contain powdered aluminum.

A mixture of aluminum powder with metal oxides (thermite) is used to obtain certain metals and alloys, for welding rails, in incendiary ammunition.

Aluminum has also found practical use as a rocket fuel. Complete combustion of 1 kg of aluminum requires almost four times less oxygen than 1 kg of kerosene. In addition, aluminum can be oxidized not only by free oxygen, but also by bound oxygen, which is part of water or carbon dioxide. During the "combustion" of aluminum in water, 8800 kJ are released per 1 kg of products; this is 1.8 times less than when the metal is burned in pure oxygen, but 1.3 times more than when it is burned in air. This means that plain water can be used instead of dangerous and expensive compounds as an oxidizing agent for such fuel. The idea of ​​using aluminum in

as a fuel back in 1924 was proposed by the domestic scientist and inventor F.A. Zander. According to his plan, aluminum elements of the spacecraft can be used as additional fuel. This bold project has not yet been practically implemented, but most of the currently known solid rocket propellants contain aluminum metal in the form of a finely divided powder. Adding 15% aluminum to the fuel can raise the temperature of the combustion products by a thousand degrees (from 2200 to 3200 K); the rate of exhaustion of combustion products from the engine nozzle also noticeably increases - the main energy indicator that determines the efficiency of rocket fuel. In this regard, only lithium, beryllium and magnesium can compete with aluminum, but they are all much more expensive than aluminum.

Aluminum compounds are also widely used. Aluminum oxide refractory and abrasive (emery) material, raw material for ceramic production. Laser materials, watch bearings, jewelry stones (artificial rubies) are also made from it. Calcined aluminum oxide is an adsorbent for cleaning gases and liquids and a catalyst for a number of organic reactions. Anhydrous aluminum chloride catalyst in organic synthesis (Friedel Crafts reaction), the starting material for obtaining high purity aluminum. Aluminum sulfate is used for water purification; reacting with the calcium bicarbonate contained in it:

Al 2 (SO 4) 3 + 3Ca (HCO 3) 2 ® 2AlO(OH) + 3CaSO 4 + 6CO 2 + 2H 2O, it forms flakes of oxide-hydroxide, which, when settling, capture and also sorb suspended impurities and even microorganisms in the water on the surface. In addition, aluminum sulfate is used as a mordant for dyeing fabrics, for tanning leather, preserving wood, and sizing paper. Calcium aluminate is a component of binders, including Portland cement. Yttrium aluminum garnet (YAG) YAlO 3 laser material. Aluminum nitride refractory material for electric furnaces. Synthetic zeolites (they belong to aluminosilicates) adsorbents in chromatography and catalysts. Organoaluminum compounds (for example, triethylaluminum) components of Ziegler catalysts Nattas, which are used for the synthesis of polymers, including high quality synthetic rubber.

Ilya Leenson

LITERATURE Tikhonov V.N. Analytical chemistry of aluminum. M., "Science", 1971
Popular library of chemical elements. M., "Science", 1983
Craig N.C. Charles Martin Hall and his Metall. J.Chem.Educ . 1986, vol. 63, No. 7
Kumar V., Milewski L. Charles Martin Hall and the Great Aluminum Revolution. J.Chem.Educ., 1987, vol. 64, no. 8