Magnesium is a silvery white metal. Its most important physical property is a low density (p = 1.74 g / cm at 20 ° C). There are twelve electrons in the electron shell of the magnesium atom. Two electrons 3s located in the outer orbit can easily be torn off, which leads to the formation of the Mg’2 + ion, so magnesium is divalent in all known compounds.
Natural magnesium consists of a mixture of three stable isotopes. Moreover, the artificial isotope Mg with a half-decay of 10.2 h can be used as a radioactive indicator. Magnesium crystals have a compact hexagonal structure.
When magnesium is stored in dry air, an oxide film is formed on its surface, protecting the metal with a slight heating (up to 200 ° C) from further oxidation; In these conditions, the corrosion resistance of pure magnesium exceeds the resistance of low-carbon steel. However, in the humid air, its corrosion is greatly enhanced. It is practically not affected by kerosene, gasoline and mineral oils. However, it is not stable in aqueous solutions of salts (except fluorides) and is soluble in many mineral and organic acids.
Magnesium in the form of ingots or products is not flammable. The ignition of magnesium can occur only at a temperature close to its melting point (651 ° C) or after melting, if it is not isolated from the oxygen of the air. Covered with a flux, the metal can be heated and melted. Powdered magnesium or a thin magnesium tape easily lights up from the match and burns with a dazzling white flame. Magnesium is non-magnetic and does not spark at impacts or friction.
In its free form it does not occur, but it comes in the form of carbonates, silicates in many rocks, and is also dissolved in sea and lake water in the form of chlorides and sulfates.
Currently, magnesia, dolomite, carnallite, as well as sea water and waste from a number of industries are used to produce magnesium.
Natural mineral magnesite, in addition to magnesium carbonate, MgCO3 usually contains calcium carbonate, quartz, and also impurities of other minerals, including aluminum and iron oxides. For the production of magnesium, only pure caustic magnesite obtained by the reaction MgCO3 = MgO + CO2 is used when the natural magnesite is heated (roasted) to 700-900 ° C.
Dolomite is a rock, which is a double calcium and magnesium carbonate MgCO3-CaCO3. Dolomites usually contain impurities of quartz, calcite, gypsum, etc. The content and color of the impurities determine the color of the rock. Dolomite is widespread in nature and accounts for about 0.1% of all rocks that make up the earth’s crust. Dolomite, like magnesite, is preliminarily fired to obtain a mixture of oxides of MgO and CaO.
Carnallite is a natural magnesium chloride and potassium is a very hygroscopic crystalline substance, usually colored with impurities in pink, yellow or gray. Carnallite is subjected to hydrochemical treatment to separate bromine and some sodium chloride and potassium from it, resulting in the so-called artificial carnallite, which is used in the magnesium industry.
Inexhaustible reserves of magnesium in the form of bishofite MgCl2 * 6H, 0 in seawater; On average, 0.38% of MgCl2 is contained therein. In addition, in sea water there are magnesium compounds MgS04 (0.17%) and MgBr2 (0.01%).
Sea water is still rarely used to produce bischofite, as in many countries there are salt lakes, in which the content of magnesium chloride is much higher. In some ozepax of the Perekop group, for example, the content of magnesium chloride reaches 15% by the end of summer. In addition, a number of productions are now used as raw materials for the production of magnesium. In this case, especially, magnesium chloride, obtained by extracting titanium from its ores, is widely used.
The concept of an electrolytic process for the production of magnesium. In general, magnesium is produced by the electrolytic method, the most important stages of which are:
A) Preparation of pure anhydrous magnesium salts;
B) electrolysis of the melt of these salts
C) refining of magnesium.
Variants of the electrolytic method for the production of magnesium differ in the composition of the salts supplied to the electrolysis (carnallite, magnesium chloride, etc.), and by the method of obtaining these salts (chlorination of magnesite, dehydration of bischofite or carnallite). Chlorination of magnesite can be carried out similarly to chlorination of titanium oxide. Dehydration of carnallite is usually carried out in two stages: first by slow heating of natural carnallite in tubular furnaces, and then by melting the compound KCl * MgCl2 * H2O until the hydrated moisture is completely removed.
Electrolysis is carried out in molten chlorides of magnesium, potassium, sodium and calcium, since during the electrolysis of aqueous solutions of its salts, due to the negative magnesium potential, only hydrogen is released on the cathode.
Magnesium can be obtained by electrolysis of pure molten anhydrous magnesium chloride, but the high melting point, low electrical conductivity and other adverse properties of this salt force the use of electrolytes of more complex composition. It is more convenient to conduct electrolysis of carnallite, which usually contains sodium chloride as an impurity. This electrolyte has a lower melting point, a higher electrical conductivity and less dissolves magnesium. Therefore, when working with it, less energy is consumed.
Magnesium baths are connected in series in a series of 60-100 pcs. The number of baths in a series is determined by the voltage of a source of direct electric current; The tension of the bath, which depends on its design, the distance between poles, the composition of the electrolyte, varies between 5.5-7.5 V.
Maintenance of baths is to perform the following basic operations:
A) feeding with electrolyte;
B) temperature control;
C) extraction of magnesium;
D) removal of sludge.
Power baths with electrolyte. In the process of electrolysis there is a continuous decomposition of magnesium chloride, therefore fresh water is added to the bath periodically to fill fresh molten salt salts. It is most convenient to add anhydrous magnesium chloride to the electrolyte resulting from the reduction of titanium chloride by magnesium. In the case of an autonomous location of the magnesium plant bischofite must first be dehydrated. You can enter into the bath and anhydrous carnallite, but then you need to drain some of the electrolyte, since otherwise it will have an excess of potassium chloride. Potassium fertilizers are obtained from the spent electrolyte.
Temperature control. Electrolysis should proceed at a temperature of 690-720 ° C, with the lower limit desirable to adhere to when feeding the baths with magnesium chloride, and the top – when feeding carnallite. In the process of electrolysis, it is necessary to observe the temperature of the electrolyte, since a deviation from the norm, especially upward, considerably worsens the process parameters.
In magnesium baths, temperature regulation does not change the interpolar distance, as is customary in the electrolytic production of aluminum, but changes the composition, and with it the conductivity of the electrolyte. So, for example, to raise the temperature of the electrolyte, it is necessary to fill it with more pure magnesium chloride, which will increase the resistance of the electrolyte. Changes in temperature within 20-30 ° C can be achieved by varying the amount of aspirated gases from the cathode space of the bath.
In case of overheating of the electrolyte, the loading of solid sodium chloride is used; When the temperature drops excessively, for example, when the bath is turned off, the electrolyte is heated by alternating current, lowering the nichrome spirals into the cathode cells.
Extraction of magnesium from the cell. This is usually done at least once a day, using vacuum ladles. The bucket is preheated by the heating elements incorporated in it and then fed to the baths by a bridge crane. After creating a vacuum of 730-800 kPa in the cell of the bath, lower the suction pipe and open the valve. Metal and some of the electrolyte are sucked into the bucket. Then the valve is closed and the operation is repeated in the other cells of the bath.
Sludge removal. In the electrolyte with magnesium chloride, magnesium oxide also enters; In addition, hydrolysis of the electrolyte may occur to form magnesium oxide. It settles on the bottom of the cell, dragging along with other products and forming sludge. The sludge is removed once every two to three days, without allowing a significant accumulation of it on the bottom of the bath, as this sometimes leads to the closure of the anode with the cathode and worsens the conditions for the deposition of magnesium on the cathode.