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HISTORY OF TECHNOLOGY, TECHNOLOGY, OBJECTS AROUND US
aluminum electrolysis. History of invention and production
Directory / The history of technology, technology, objects around us Modern life cannot be imagined without aluminium. This shiny light metal, an excellent conductor of electricity, has been widely used in various industries in recent decades. Meanwhile, it is known that aluminum does not occur in nature in free form, and until the XNUMXth century, science did not even know about its existence. Only in the last quarter of the XNUMXth century was the problem of industrial production of aluminum metal in free form solved. This was one of the biggest achievements of science and technology of this period, the significance of which we, perhaps, have not yet fully appreciated.
In terms of content in the earth's crust, aluminum ranks first among metals and third among other elements (after oxygen and silicon). The earth's crust is 8% aluminum (we note for comparison that the iron content in it is 8%, copper - 4%, and gold - 2%). However, this reactive metal cannot exist in a free state and is found only in the form of various and very diverse compounds. Their bulk is accounted for by aluminum oxide (Al0O003). Each of us has met this compound more than once - in everyday life it is called alumina, or simply clay. Clay is about a third of aluminum oxide and is a potential raw material for its production. The whole difficulty is to restore aluminum (take away oxygen from it). It is extremely difficult to achieve this chemically, since the bond between the two elements is very strong here. Already the first acquaintance with aluminum clearly demonstrated all the difficulties that scientists expected along the way. In 1825, the Danish physicist Hans Oersted was the first to obtain metallic aluminum in a free state from its oxide. To do this, Oersted first of all mixed alumina with coal, heated this mixture and passed chlorine through it. The result is aluminum chloride (AlCl3). At that time, it was already known that chemically more active metals were able to displace less active ones from their salts. Oersted subjected aluminum chloride to the action of potassium dissolved in mercury (potassium amalgam) and obtained aluminum amalgam (by rapidly heating aluminum chloride with potassium amalgam, potassium chloride was formed, while aluminum went into solution). Subjecting this mixture to distillation, Oersted isolated small ingots of aluminum. In a slightly different way, aluminum was obtained in 1827 by the German chemist Wöhler, who passed a vapor of aluminum chloride over metallic potassium (in this case, as in the Oersted reaction chemically, the more active potassium displaced aluminum and itself combined with chlorine). But both methods could not be used in industry, since very expensive potassium was used here to reduce aluminum. Later, the French physicist Saint-Clair-Deville developed another chemical process for obtaining aluminum, replacing potassium with cheaper, but still quite expensive sodium. (The essence of this method was that aluminum chloride was heated with sodium, which displaced aluminum from the salt, causing it to stand out in the form of small beads.) For several decades, aluminum was obtained in this way.
Investigating the properties of aluminum, Deville came to the conclusion that it could be of great importance for technology in the future. In his report to the French Academy of Sciences, he wrote: “This metal, white and shiny like silver, does not blacken in air, can be melted down, forged and drawn, and has a remarkable lightness, can be very useful if you can find a simple way to If we further recall that this metal is extremely common, that its ore is clay, then one can only wish that it finds wide application. The first aluminum ingots obtained by Deville were demonstrated at the world exhibition in Paris in 1855 and aroused the liveliest interest. In 1856, at the factory of the Tissier brothers in Rouen, Deville organized the first industrial enterprise for the production of aluminum. At the same time, the cost of 1 kg of aluminum was initially equal to 300 francs. A few years later, the selling price was reduced to 200 francs per 1 kg, but still it remained exceptionally high. Aluminum at that time was used as a semi-precious metal for the production of various trinkets, and in this form it even gained some popularity because of its white color and pleasant luster. However, as the chemical methods for extracting aluminum improved, its price fell over the years. For example, a plant in Albury (England) in the mid-80s. produced up to 250 kg of aluminum per day and sold it at a price of 30 shillings per kg, in other words, its price dropped 30 times over 25 years. Already in the middle of the 1854th century, some chemists pointed out that aluminum could be obtained by electrolysis. In XNUMX, Bunsen obtained aluminum by electrolysis of a molten aluminum chloride. Almost simultaneously with Bunsen, Deville received aluminum electrolytically. Deville's apparatus consisted of a porcelain crucible P inserted into a porous clay crucible H and provided with a lid D, which had a slit for the insertion of a platinum electrode K and a large opening for a porous earthen vessel R. In the latter was placed a carbon rod A, which was the positive electrode. The crucible and earthenware vessel were filled to the same level with molten double chloride of aluminum and sodium (double chloride was obtained by mixing two parts of dry aluminum chloride and common salt). After the electrodes were immersed, even at a low current, the decomposition of double chloride began in the melt, and metallic aluminum precipitated on the platinum plate. However, at that time it was impossible to even think about keeping the compounds in a molten state, using only heating during the passage of current. It was necessary to maintain the required temperature in another way from the outside. This circumstance, as well as the fact that electricity was very expensive in those years, prevented the spread of this method of aluminum production. The conditions for its distribution arose only after the appearance of powerful DC generators. In 1878, Siemens invented the electric arc furnace, primarily used in the smelting of iron. It consisted of a carbon or graphite crucible, which was one pole. The second pole was a carbon electrode located on top, which moved inside the crucible in a vertical plane to control the electrical regime. When filling the crucible with a charge, it was heated and melted either by an electric arc or due to the resistance of the charge itself when a current passed through it. No external heat sources were required for the Siemens furnace. The creation of this furnace was an important event not only for ferrous, but also for non-ferrous metallurgy. Now all the conditions for the electrolytic method of aluminum production were in place. It was up to the development of the process technology. Generally speaking, aluminum can be obtained directly from alumina, but the difficulty was that aluminum oxide is a very refractory compound, which becomes liquid at a temperature of about 2050 degrees. In order to heat alumina to this temperature and then maintain it during the reaction, a huge amount of electricity was required. At that time, this method seemed unreasonably expensive. Chemists were looking for a different way, trying to isolate aluminum from some other less refractory substance. In 1885, this problem was independently solved by the Frenchman Héroux and the American Hall. It is curious that both at the time when they made their outstanding discovery, was 22 years old (both of them were born in 1863). Eru, since the age of 15, after he got acquainted with Deville's book, constantly thought about aluminum. He developed the basic principles of electrolysis while still a student at the age of 20. In 1885, after the death of his father, Héroux inherited a small leather factory near Paris and immediately set to work. He bought a Gramma electric generator and first tried to decompose aqueous solutions of aluminum salts with an electric current. Having failed on this path, he decided to electrolyze molten cryolite - a mineral that includes aluminum (the chemical formula of cryolite is Na3AlF6). Eru began his experiments in an iron crucible, which served as the cathode, and the anode was a coal rod lowered into the melt. At first, nothing promised success. When the current was passed, the iron of the crucible reacted with cryolite, forming a fusible alloy. The crucible melted and its contents spilled out. Eru did not obtain any aluminum in this way. However, cryolite was a very tempting raw material, since it melted at a temperature of only 950 degrees. Eru came up with the idea that the melt of this mineral could be used to dissolve more refractory aluminum salts. It was a very fruitful idea. But what kind of salt to choose for experiments? Eru decided to start with one that had long served as a raw material for the chemical production of aluminum - with double aluminum chloride and sodium. And then, during the experiment, an error occurred, which led him to a remarkable discovery. After melting the cryolite and adding aluminum and sodium double chloride to it, Eru suddenly noticed that the carbon anode began to burn quickly. There could be only one explanation for this - during electrolysis, oxygen began to be released at the anode, which reacted with carbon. But where could oxygen come from? Eru carefully examined all the purchased reagents and then discovered that the double chloride decomposed under the influence of moisture and turned into alumina. Then everything that had happened became clear to him: aluminum oxide (alumina) dissolved in molten cryolite and the Al2O3 molecule decomposed into aluminum and oxygen ions. Further, in the course of electrolysis, negatively charged oxygen ions donated their electrons to the anode and were reduced to chemical oxygen. But in this case, what substance was reduced at the cathode? It could only be aluminum. Realizing this, Eru had already deliberately added alumina to the cryolite melt and thus obtained metal aluminum beads at the bottom of the crucible. Thus, a method of obtaining aluminum from alumina dissolved in cryolite, which is used to this day, was discovered. (Cryolite does not participate in a chemical reaction, its amount does not decrease during electrolysis - it is used here only as a solvent. The process proceeds as follows: alumina is periodically added to the cryolite melt in portions; as a result of electrolysis, oxygen is released at the anode, and aluminum at the cathode. ) Two months later, exactly the same method of aluminum production was discovered by the American Hall.
Eru received the first patent for his invention in April 1886. In it, he has not yet abandoned the external heating of the electrolyte bath to maintain the required temperature of the melt. But the very next year, he took out a second patent for a method for producing aluminum bronze, in which he refused external heating and wrote that "an electric current produces enough heat to keep the alumina in a molten state."
Since no one in France was interested in discovering it, Héroux left for Switzerland. In 1887, the Sons of Neger company signed a contract with him to implement his invention. Soon the Swiss Metallurgical Society was founded, which at the plant in Neuhausen launched the production of first aluminum bronze, and then pure aluminum. The industrial plant for the electrolysis of aluminum, as well as the entire production technology, was developed by Eru. The furnace was an iron box, isolated on the ground. The surface of the bath was covered from the inside with thick carbon plates, which were the negative electrode (cathode). From above, a positive electrode (anode) was lowered into the bath, which was a package of carbon rods. Electrolysis took place at a very strong current (about 4000 amperes), but at a low voltage (only 12-15 volts). A large current, as already mentioned in previous chapters, led to a significant increase in temperature. The cryolite quickly melted and an electrochemical reduction reaction began, during which aluminum metal was collected on the coal floor of the bath. Already in 1890, the plant in Neuhausen received over 40 tons of aluminum, and soon began to produce 450 tons of aluminum per year. The success of the Swiss inspired the French industrialists. In Paris, an electrical society was formed, which in 1889 offered Eru to become the director of the newly founded aluminum plant. A few years later, Héroux founded several more aluminum plants in different parts of France, where there was cheap electrical energy. Aluminum prices gradually fell dozens of times. Slowly but steadily, this wonderful metal began to win its place in human life, soon becoming as necessary as iron and copper, known from ancient times. Author: Ryzhov K.V.
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