The electrolysis of aluminium
Modern life is unimaginable without aluminum. This brilliant light metal, perfect conductor of electricity, has received in the last few decades the most widely used in various industries. Meanwhile, it is known that aluminum in free form not found in nature, and up to XIX century science did not even know about its existence. Only in the last quarter of the XIX century, the problem of industrial production of aluminum metal in free form has been resolved. This was one of the greatest conquests of science and technology of this period, the value of which we may not yet evaluated until the end.
According to the content in the earth's crust aluminum ranks first among the metals and the third among other elements (after oxygen and silicon). The crust 8, consists of 8% aluminum (Note for comparison that the iron content in it - 4, 2% copper - 0, 003%, and gold - 0% 000005). However, the reactive metal may not exist in a free state, and occurs only in a very different and varied in their composition compounds. Their main weight falls on aluminum oxide (Al2O3). This compound, each of us have met more than once - in common parlance it is called alumina, or simply clay. Clay approximately one third consists of alumina and is a potential raw material for its manufacture. The difficulty is to recover aluminum (take away his oxygen). Chemical means to achieve this is extremely difficult, since the connection between the two elements is very strong. For the first acquaintance with aluminum clearly demonstrated all the difficulties that scientists expect along the way.
In 1825 Danish physicist Hans Oersted first managed to obtain aluminum metal in a free state from its oxide. To do this, first of all Oe alumina mixed with charcoal, the mixture was red-hot, and missed by her chlorine. The result is an aluminum chloride (AlCl3). While it was known that a chemically active metals can displace less active from their salts. Oersted subjected to the action of potassium aluminum chloride dissolved in mercury (potassium amalgam) and got aluminum amalgam (with the rapid heating of aluminum chloride with potassium amalgam formed potassium chloride, aluminum is left in the solution). Subjecting the mixture distillation, Oersted allocated small ingots of aluminum. Several other means of aluminum received in 1827 a German chemist Wohler, who missed a pair of aluminum chloride with potassium metal (in this case, as in the reaction of Oersted chemically more active potassium aluminum supplanted himself was connected with chlorine). But both methods can not be applied in industry, for the recovery of aluminum is used very expensive potassium.
Later, the French physicist-Saint-Clair Deville developed a different chemical process of producing aluminum, replacing potassium cheaper, but still quite expensive sodium. (The essence of this method lies in the fact that aluminum chloride was heated with sodium, which displaces the aluminum salt, making it stand out in the form of beads small.) For several decades, the aluminum was obtained in this way. Investigating the properties of aluminum, Deville came to the conclusion that he could in the future be of great significance to the art. In his report to the French Academy, he wrote Sciences: "The metal is white and shiny as silver, not blackened air, amenable to smelting, forging and drawing, has also remarkable ease, can be very useful if we could find a simple way to his preparation. If you continue to think that this metal is extremely common that it is clay ore, we can only wish that it is widely used. " The first aluminum ingots obtained Deville, were shown at the Paris World Exhibition in 1855 and called to his keen interest.
In 1856 the factory brothers Tissier in Rouen Deville organized the first industrial plant for production of aluminum. The cost of 1 kg of aluminum initially equal to 300 francs. After a few years managed to reduce the selling price to 200 francs per 1 kg, but still she remained exceptionally high. Aluminium at this time was used as a semi-precious metal for the production of various trinkets, and he has acquired in this form even some popularity because of its white color and a pleasant finish. However, with the improvement of methods of chemical separation of aluminum price has dropped over the years. For example, a plant in Albury (England) in the mid 80's. 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 has decreased 25 times in 30 years.
Already in the middle of the XIX century some chemists pointed out that aluminum can be produced by electrolysis. In 1854, Bunsen was aluminum by electrolysis of aluminum chloride melt.
Almost simultaneously with the Bunsen I got electrolytic aluminum by Deville. Deville The device consisted of a porcelain crucible the P, inserted in a porous clay pot with lid H and the D, where there was a gap to enter the platinum electrode K and a large opening for the porous clay vessel R. The last A carbon rod was placed, is a positive electrode. The crucible and earthen vessel filled to a level double the molten aluminum and sodium chloride (double chloride was prepared by mixing two parts dry aluminum chloride and sodium chloride). After immersing the electrode even with a small current in the melt began double decomposition chloride, and stood on a platinum plate aluminum metal. However, while it was impossible to think about how to keep the compound in a molten state, using only the heat 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 in those days was very expensive, prevent the spread of this method, the production of aluminum. Terms for distribution arose only after the emergence of powerful DC generators.
In 1878, Siemens invented the electric arc furnace, is mainly used in iron smelting. It consisted of a carbon or graphite crucible, is one pole. The second pole located on the top served as a carbon electrode, which was moved inside the crucible in a vertical plane for controlling the electric mode. When filling out the charge of the crucible is heated and melted by an electric arc or by resistance or the most charge by passing a current through it. No external heat source for ovens Siemens was required. The creation of this furnace has been an important event not only for the steel, but also for non-ferrous metallurgy.
Now all the conditions for the production of aluminum electrolytic method were available. The point was the development of the process technology. Generally, aluminum can be produced directly from alumina, but the difficulty was the fact that the alumina is very refractory compound which becomes liquid at a temperature of about 2050 degrees. In order to heat the clay to a temperature and then maintain it during the reaction it required an enormous amount of power. While this seemed to be unnecessarily expensive method. Chemists have searched for a way of trying to extract aluminum from some other less refractory material. In 1885, this task independently Era allowed the Frenchman and the American Hall.
Curiously, both at the time when they made their remarkable discovery, it was 22 years (both born in 1863). Era since the 15 years after he met with a book Deville, constantly thinking about aluminum. The basic principles of electrolysis, it has developed, while still a student, 20 years. In 1885, after the death of his father he inherited the Era of small leather factory near Paris, and immediately took up the experiments. He bought an electric Gram and at first tried to expand the electric shock aqueous solutions of aluminum salts. Having failed in this way, it decided to electrolysis of molten cryolite - a mineral which includes aluminum (chemical formula Na3AlF6 cryolite). The experiments started in the Iron Age of the crucible, which served as the cathode and the anode was dipped into the carbon rod melt. At first, no promises of success. When a current is passed iron crucible reacts with cryolite, forming a low-melting alloy. The crucible is melted, and its contents spilled out. No aluminum Era in this way is not received. However cryolite was a very attractive commodity, because the melted at a temperature of only 950 degrees. Eru had the idea that the melt of this mineral can be used for dissolving a refractory aluminum salts. It was a very fruitful idea. But what kind of salt to elect for experiments? Eru decided to start with the one that has long been used as raw material for chemical production of aluminum - with a double chloride of aluminum and sodium. And then during the experiment there was an error that led him to a remarkable discovery. Cryolite melt and adding a double aluminum and sodium chloride, Eru suddenly noticed that the carbon anode quickly began to burn. The explanation for this could be only one - during electrolysis at the anode became released oxygen reacts with carbon. But where could take oxygen? Era carefully studied all the purchased reagents and then discovered that the double chloride decomposed by moisture and turned into alumina. Then all became clear what had happened to him, aluminum oxide (alumina) dissolved in molten cryolite and Al2O3 molecule is split into aluminum and oxygen ions. Further, during the electrolysis, the negatively charged oxygen ions gave their electrons to the anode and oxygen is recovered into the chemical. But in this case, a material for the cathode recovered? They could only be aluminum. Realizing this, Era has deliberately added alumina to the molten cryolite and thus got to the bottom of the crucible blood oranges aluminum metal. Thus it was opened still applicable method for producing aluminum from alumina dissolved in cryolite. (Cryolite is not involved in chemical reactions, the amount not decreased during the electrolysis, - it is only used here as a solvent process is as follows: to melt the cryolite is added in portions alumina; by electrolysis at the anode, oxygen, and the cathode - aluminum.. ) In two months later, exactly the same way to the production of aluminum opened American Hall.
In his invention Era received the first patent in April 1886. In it, he has not abandoned the external heating bath with an electrolyte to maintain the desired temperature of the melt. But the following year he took a second patent for a method of producing aluminum bronze, which declined from an external heating and wrote that "the electric current produces a sufficient amount of heat to maintain alumina in a molten state."
Since no one in France is not interested in his discovery, Era went to Switzerland. In 1887, the company "Sons Neger" signed a contract with him on the implementation of his invention. It was soon established Swiss metallurgical company, which is the plant in the first Neygauzene launched production of aluminum bronze, and then to pure aluminum.
Industrial plants for the electrolysis of aluminum, as well as the entire production technology, has developed the Age. The oven was an iron box, isolated and fixed on the ground. The surface inside the bath was covered in thick carbon plate, which is a negative electrode (cathode). Above the bath lowered the positive electrode (anode), which was a package of carbon rods. Electrolysis occurs at very high current (about 4000 amps), but with a small voltage (of 12-15 volts). Most of the current strength, as already mentioned in previous chapters, resulted in a significant increase in temperature. It cryolite quickly melted and began electrochemical reduction reaction in which aluminum metal going in the coal floor bath.
Already in 1890 the factory in Neygauzene received more than 40 tons of aluminum, and soon began to produce 450 tons of aluminum per year. Swiss success inspired the French industrialists. In Paris, formed electrical company, which in 1889 proposed Era become director of the newly founded aluminum plant. A few years later founded Era in different parts of France, where there was a cheap electric energy, several aluminum smelters. Aluminum prices gradually fell dozens of times. Slowly but surely this wonderful metal has been gaining its place in human life, soon becoming as necessary as known since ancient times iron and copper.