The essence of the process of aluminum production is the production of anhydrous, free of impurities alumina (alumina), followed by the production of metallic aluminum by electrolysis of dissolved alumina in cryolite.
In the modern aluminum industry, several methods for obtaining aluminum oxide are used; They can be divided into three groups.
The essence of electrothermal methods is the restoration of aluminum ore in the electric furnace; The impurities present in the ore are reduced to an elementary state and, by transferring them to the metal (siliceous cast iron), only aluminum oxide remains unreduced in the slag. Some partially unrecovered impurities also remain in the slag. The alumina obtained in this way can be used for making grinding wheels and other abrasive products, but for the production of high-quality aluminum such alumina is not suitable.
Acidic methods boil down to the fact that the aluminum ore is treated with an acid, for example, hydrochloric acid or sulfuric acid. The acid is reacted with aluminum oxide and the corresponding soluble salt (for example, aluminum chloride) is obtained. The main impurities (silica, calcium oxide, etc.) do not react with acids. However, a number of impurities (for example, iron oxides) interact with many acids, which creates great additional difficulties, since it is very difficult to completely separate iron salts from aluminum salts in solution. These methods are used little, but they have many patents both abroad and with us. And since the ore can be treated with acid only in acid-proof equipment, this adds to the cost and complicates the production of alumina.
Alkaline methods are used in most countries to produce pure alumina. The essence of alkaline methods is that aluminum ore is exposed to some alkali.
As a result of the interaction of aluminum oxide present in the ore, for example with caustic soda, under certain conditions, so-called sodium aluminates are formed. Aluminates of alkali metals are readily soluble in water. The bulk of the impurities present in the aluminum ore with alkalis does not interact and therefore remains in the undissolved state, and aluminum passes into the solution. But there are impurities that can interact with alkalis. The most important of them is silica. Release the solution from it is not easy.
However, alkaline methods are more economical than acid ones, because all operations can be carried out in steel and cast-iron equipment.
Bauxite and limestone are crushed and dosed with a soda solution in the following proportion: one mole of soda is added to one mole of A1203 and Fe203 and two moles of CaCOs are added to the mash per mole of silica
The resulting wet batch is finely ground in ball mills and it emerges from them in the form of a liquid pulp. After testing and some adjustment of its composition, the pulp is sent to slowly rotating tubular furnaces with a length of 80-120 m and a diameter of 2.5-3.5 m. The pulp is fed to the “cold” end of the furnace, where it meets with waste furnace gases having a temperature of the order of 300-400 ° C. As a result, moisture evaporates; Dried batch, gradually heated, moves to the hot zone, in which the temperature reaches 1200-1250 “C.
As the material is heated, complicated chemical processes take place in the charge. Many other processes take place in the sintering furnace, which lead to the formation of aluminates and ferrites of calcium, and some other complex compounds.
The reaction products are extracted from the furnace in the form of a so-called guardian (resembling a gray porous pebble) consisting mainly of sodium aluminate, sodium ferrite and calcium silicate.
The resulting speck is cooled, crushed and leached, the essence of which is the effect of weak soda solutions on the spec. As a result of leaching from the speck, sodium aluminate passes into the solution, and hydrolysis of sodium ferrites takes place. The resulting hydroxide of iron precipitates, and the solution is enriched with caustic soda. The resulting solution is separated from insoluble impurities by settling and filtration.
Along with these desirable reactions, reactions occur that complicate the production of pure alumina. So, for example, a certain amount of sodium silicates passes into the solution, which causes a special operation called desiliconization of the solution. The essence of this operation is a prolonged heating with mixing of aluminate solution and lime milk in strong closed cylindrical vessels with spherical bottoms – autoclaves – at a temperature of 150-180 ° C. As a result, a number of chemical processes occur.
After filtering the solution from the particles suspended in it, the pure aluminate solution is carbonized. The purpose of this operation is to separate from the solution pure aluminum hydroxide, which is not contaminated with other substances. This operation is carried out in cylindrical tanks with agitators – carbonizers, which are fed with carbon dioxide (usually purified furnace gases). Under the action of C02, the aluminate solution decomposes, a white precipitate of aluminum hydroxide precipitates out of it, which separates from the soda solution. The remaining soda solution after adding a certain amount of fresh soda is returned to the preparation of the charge for the next sintering, and alumina hydrate is calcined in tubular furnaces (similar to sintering furnaces) at 1200 ° C, resulting in an anhydrous, non-hygroscopic alumina, Subsequent electrolysis.
The main raw materials for the production of aluminum are aluminum ores: bauxites, nephelines, alunites, kaolins. The most important are bauxites.
Currently, the most widely used electrolysis cells, calculated for a current strength exceeding 100 kA, with pre-burned anodes or with an upper current supply to self-baking anodes. The production of aluminum in such an electrolyzer is carried out continuously for two to three years; The following basic operations are performed: monitoring the composition of the electrolyte, ensuring the timely loading of alumina and extraction of aluminum, monitoring the stress and servicing the self-baking anode system.
The process of electrolysis is reduced to the discharge of the ions A13 + and 02 +, of which alumina is composed, which is continuously consumed. Cryolite is not subjected to direct electrolysis and is consumed little, but due to its physical losses (evaporation, spills, etc.), as well as the interaction of its individual constituents with alumina impurities and the lining of the cell, it is necessary to systematically monitor its level in the bath (layer thickness 18-25 cm) and chemical composition.
Some plants introduce into the electrolyte small additives CaF2 and MgF2 to reduce the melting point of the electrolyte by several tens of degrees.
When there is little alumina in the electrolyte (less than 1%), an anodic effect occurs. Externally, it manifests itself in a rapid voltage jump on the electrolyzer from the usual 4.0-4.7 V to 30-50 V; In the area of the anode arcs appear, the electrolyte begins to overheat and bubble. To eliminate the anode effect, the crust of the electrolyte is pierced and, mixing, dissolve alumina in it (another portion of which is always poured into the crust of the electrolyte beforehand).
After dissolving the alumina in the electrolyte, the anode effect usually stops and the stress becomes normal. The anode effect during the production of aluminum plays both a positive and a negative role. On the one hand, he signals about the lack of alumina in the electrolyte and gives an opportunity to get an idea of the course of electrolysis, on the other hand – it leads to a surplus of electricity and a violation of the thermal equilibrium of the bath. At factories, they try to prevent the frequent occurrence of anode effects, introducing alumina prior to their appearance. In turn, the excess of alumina introduced into the electrolyte does not dissolve, settles to the bottom under the aluminum layer, impedes the normal course of electrolysis. Therefore, it is considered normal that one or two anode effects per day occur in the electrolysis cell.
Many researchers have studied the nature of the anode effect. Based on research conducted at the Moscow Institute of Non-Ferrous Metals and Gold under the guidance of Prof. A.I. Belyaev, it can be concluded that the cause of the anode effect is the different wettability of the carbon anode by the molten electrolyte, with different contents of oxides in it. When there is a significant amount of aluminum oxide in the electrolyte, the electrolyte well wets the anode carbon surface and therefore the resulting anode gases are easily removed from its surface without interfering with the passage of the electric current. With a decrease in the amount of alumina, the wettability by the electrolyte changes slowly and when the alumina content is less than 1%, the quantity becomes quality – the electrolyte ceases to wet the coal surface; As a result, a gas film forms between the electrolyte and the carbon anode, preventing the passage of electric current, which leads to a sharp increase in the voltage on the bath.
Aluminum is extracted from the cell, piercing the crust of the frozen electrolyte and lowering the steel tube to the bottom of the refractory lined with aluminum, through which aluminum is pumped into the vacuum bucket. On a modern aluminum bath designed for a current of 100 kA, about 700 kg of aluminum is produced per day, so the metal is extracted no more often than once a day (from less powerful baths once in two days).
As the aluminum is extracted, the anode is gradually lowered, while carefully adjusting the voltage and the pole-to-pole distance of the cell. Since the lower part of the anode burns and it gradually descends, it must be increased in the upper part. Anodic mass is systematically charged into the anode casing, which is coked on the hot cone of the anode due to the heat from the bath. Current-carrying steel pins are gradually lowered with the anode and, in order to avoid melting, they are alternately pulled from its body and raised to a higher level, and anodic mass 1 flows into the resulting cavity and is coked in it.
To produce 1 ton of primary aluminum by electrolysis, 15,000-17,000 kWh of electricity and almost 2 tons of alumina are consumed.
To remove nonmetallic inclusions (particles of coal, alumina, fluoride salts, etc.), aluminum extracted from the cells is often subjected to 10-15-minute chlorination in a bucket at a temperature of 750 ° C. Then aluminum is sent to large electric resistance furnaces, from which its semi-continuous casting is conducted into calibrated blanks for the production of pipes, wire and sheet. These same furnaces are used to produce many alloys on an aluminum base.
Primary aluminum produces 13 grades, which are divided into three groups: aluminum of special purity A999, four grades of high purity aluminum and eight grades of aluminum of technical purity. The primary metal is allowed to contain impurities from 0.15 to 1.0%, with the name of the mark indicating the degree of purity of the metal. Thus, the grade of aluminum of technical purity A8 means that it contains 99.8% of aluminum, and impurities (mainly silicon and iron) of only 0.2%. In high purity aluminum, the A99 grade is 99.99% aluminum and only 0.01% impurities.
In electrolysis baths, aluminum is obtained for technical purity. To produce higher grades of aluminum, its additional refining is required.