1. Production process (1) the aluminum electrolysis industry production process mechanism uses a Hall - Carvoeiro cryolite - alumina molten salt electrolysis. The so-called cryolite-alumina melt salt is a fluoride salt mainly composed of cryolite as a flux, and alumina is a multiphase electrolyte system composed of a molten material, namely Na 2 AIF 6 -A1 2 O 3 binary system and Na 3 The AIF 6 -AIF 3 -Al 2 O 3 ternary system is the basis of industrial electrolytes.
A liquid that is capable of conducting current and changing its composition as it passes through is called an electrolyte.
For many years, aluminum electrolytes have been dominated by cryolite, for the following reasons.
â‘ pure cryolite free deposition potential (discharge voltage) than aluminum metal corrected impurities (iron, silicon, copper, etc.), they do not bring impurities from the outside can be obtained for the electrolytic production of pure aluminum.
2 Cryolite can dissolve aluminum oxide well, and the solubility of alumina in cryolite solution is about 10% (mass) at an electrolysis temperature of 950-970 °C.
3 At the electrolysis temperature, the density of the cryolite-alumina melt is smaller than the density of the aluminum liquid of the same temperature, and it floats on the aluminum liquid to prevent oxidation of the aluminum, and at the same time, the electrolyte and the aluminum are well separated. Conducive to the electrolysis process, and simplify the structure of the electrolysis cell.
4 Cryolite has a certain conductivity, so that the voltage drop of the electrolyte layer is not too high.
5 Cryolite melt has a certain fluidity at the electrolysis temperature, the anode gas can be smoothly discharged from the electrolyte, and it is beneficial to the circulation of the electrolyte, so that the temperature and composition of the electrolyte are relatively uniform.
The solubility of 6 aluminum in cryolite melt is not large, which is a favorable factor for improving current efficiency.
7 Cryolite melt is very corrosive, but carbon material can resist the erosion of it. The construction of electrolytic cell with carbon material as the inner liner can basically meet the production requirements.
8 In the molten state, the cryolite is substantially non-absorbent and has low volatility, which will reduce material consumption and ensure relatively stable electrolyte composition.
Some of the above are the advantages of cryolite, and some of its disadvantages, such as the high melting point of pure cryolite (1008.5 ° C), poor electrical conductivity and corrosive, and the amount of alumina dissolved in them, etc. It leads to large electric energy consumption when producing molten aluminum by molten salt electrolysis, and the construction investment and production cost are high. Over the years, in order to overcome its shortcomings, it has been motivated to find new substances that can replace it, but it has not yet been successful. At the same time, the researchers also studied the use of additives such as calcium fluoride, magnesium fluoride, lithium fluoride, etc. To improve the properties of the cryolite-alumina melt. Therefore, the electrolyte used in the aluminum industry is far from being a simple binary system but a multi-element. The effects of the addition of calcium fluoride, magnesium fluoride and barium fluoride on the melting temperature of the electrolyte are listed in the table. [next]
Table 1 Effect of Additives on Electrolyte Melting Temperature | ||||
Electrolyte composition | Melting temperature / °C without additives | Melting temperature / °C when adding additives | Additive type | |
5% | 10% | |||
2.7N a · FAlF 3 + 5% Al 2 O 3 | 982 | 965 | 953 | CaF 2 |
950 | 920 | MgF 2 | ||
930 | 　 | LiF | ||
In the solid state, cryolite is denser than aluminum, but after melting, it is less dense than aluminum. Under the electrolysis temperature, the density change and the difference can make the electrolyte and aluminum liquid separate well. The aluminum liquid sinks at the bottom of the tank and is electrolyzed. The liquid floats on the upper layer, protecting the aluminum deposited at the cathode from oxidation.
The force that the liquid surface molecules or ions are attracted to the inner layer by the molecules or ions of the inner layer is called surface tension. There are four contact faces for different substances that produce surface tension in aluminum cells:
1 an interface between a molten electrolyte and a gas (anode gas and air);
2 an interface between the molten electrolyte and the solid carbon anode;
3 an interface between the molten electrolyte and the liquid metal;
4 The interface between the liquid metal and the solid carbon cathode.
In the electrolytic cell, there is a large surface tension at the interface between the metal and the electrolyte, and the metal and the carbon material, so that the aluminum cannot wet the carbon and cannot be absorbed by it. Since aluminum has a large surface tension at the electrolyte interface, it helps to reduce the dissolution loss of aluminum in the electrolyte. The surface tension of aluminum at the carbon interface is determined by its purity. Aluminum contains silicon, iron, and especially sodium can reduce its surface tension, thus promoting the penetration into the pores and cracks of the carbon block, thereby affecting the life of the electrolytic cell. . [next]
The raw materials used in the production of aluminum electrolysis in modern industry include cryolite, sodium fluoride, aluminum fluoride, calcium fluoride, magnesium fluoride and aluminum oxide. The electrolyte composed of cryolite and several other fluorides and aluminum oxide can conduct electricity well at around 950 ° C, and is only a carbon material which is resistant to such electrolyte corrosion and which is good in electrical conductivity and low in cost. Therefore, at present, the aluminum industry uses carbon anodes and carbon cathodes. The overall reaction equation for the electrolysis process is:
The electrolyte melt has a density of about 2.1 g/cm 3 at about 950 ° C, which is about 10% smaller than 2.3 g/cm 3 of the aluminum liquid at the same temperature, thereby ensuring stratification of the molten aluminum and the electrolyte melt during the electrolysis. In this melt, substantially no element which is more positive than aluminum is contained, so that the quality of the electrolytic product aluminum can be ensured. In addition, the cryolite-alumina melt does not substantially absorb water, and its vapor pressure is not high at the electrolysis temperature, and has a large stability.
When an electric current is passed through the electrolyte, the phenomenon in which these substances are separated on the electrode is called electrolysis, and the device containing the electrolyte is called an electrolytic cell. In normal production, a certain amount of metal aluminum is accumulated in the bottom of the electrolytic cell. The surface of this metal layer is the surface of the cathode, which is in direct contact with the electrolyte, so that some metal is dissolved in the electrolyte, and the aluminum metal in the cathode is dissolved in the electrolyte. It is the main reason for reducing current efficiency. The biggest factor affecting the solubility of aluminum in the electrolyte is temperature. The higher the temperature, the greater the dissolution loss of aluminum. According to multiple measurements on the aluminum electrolysis cell, the current efficiency is reduced by about 1% to 2% for every 10 °C increase in temperature. Therefore, the electrolytic cell strives to maintain low temperature operation, which is advantageous for improving current efficiency.
The anode effect is a special phenomenon in molten salt electrolysis. When the cryolite alumina is electrolyzed, when the alumina content in the electrolysis zone is reduced to 1% to 2%, an anode effect can occur on the anode. There are many explanations for the reasons for the occurrence of the anode effect, but the most important ones that can better explain the effect phenomenon are the wettability change theory and the electrode process change theory, and no detailed explanation is given here.
(2) Brief description of aluminum electrolysis production process The aluminum electrolysis production process mainly uses cryolite-alumina melt as electrolyte, carbon material as cathode and anode, direct current from anode, and electrolyte and aluminum layer. The cathode rod is derived, and the function of the direct current is to maintain the molten state of the cryolite, alumina and the like in a form of heat and to achieve an electrochemical reaction. The reaction results in the formation of carbon dioxide and carbon monoxide gas on the anode, and the liquid metal aluminum is precipitated on the cathode. As the electrolysis process progresses, the precipitated aluminum is accumulated and periodically taken out of the electrolytic cell, and the extracted aluminum is sent from the electrolysis plant to the casting department, and after being processed accordingly, it is cast into billets of various specifications. [next]
An electrolytic cell is a production unit, and a certain number of electrolytic cells are connected in series to form a series, one or several series forming an electrolytic workshop. The aluminum electrolysis production process is shown in Figure 1.
As can be seen from Figure 1, the raw materials used for electrolytic aluminum are alumina, anode paste or prebaked anode blocks (anode blocks), cryolite, aluminum fluoride and other fluorides. These materials are prepared in specialized workshops or factories. of.
In addition to carbon dioxide and carbon monoxide, the flue gas contains a small amount of hydrogen fluoride or other gases. Hydrogen fluoride is a harmful gas that affects the surrounding environment and should be disposed of properly. The current purification methods include wet method and dry method. Which method is more suitable, it should be determined according to the tank type and specific conditions, and the reclaimed cryolite or fluorine-containing alumina recovered during purification can be returned to the electrolytic tank.
After the installation, the main equipment for electrolytic aluminum production, the electrolysis tank, must be subjected to the roasting and start-up stage before it can be transferred to normal production. Generally, it can continue to be produced for 3-5 years, sometimes due to the early damage of the tank lining, which seriously affects the aluminum. When the production, quality or the risk of leaking occurs, the electrolytic cell will be shut down for maintenance, and after the repair, the furnace will be reheated and then resumed.
(3) Raw materials and quality requirements for electrolytic aluminum [next]
1 Alumina (A1 2 O 3 ) Alumina is an abbreviation for Al2O3, also known as aluminum oxide. It is extracted from aluminum ore. The sandstones currently used for the production of alumina include bauxite, nepheline, kyanite , alum and kaolin . At present, bauxite is mainly used as raw material for alumina production. The production process and product quality requirements are detailed in the alumina production process.
2 cryolite (Na 3 AIF 6 ) The molecular formula of cryolite can also be written as 3NaF.AlF 3 , the molar ratio of sodium fluoride and aluminum fluoride in cryolite The molecular ratio of cryolite is equal to 3, the relative density is 2.9, and its composition is 12.8% for Al, 32.8% for Na, and 54.4% for F. The cryolite molecular ratio greater than 3 is alkaline, less than 3 is acidic, and equal to 3 is neutral. There are four kinds of cryolite used in production: natural cryolite, into the cryolite, and the recovered flotation cryolite and regenerated cryolite.
a. Natural cryolite In the natural world, there are very few natural cryolite mines. The natural cryolite extracted from the mine generally contains about 80% pure cryolite. Therefore, it must be sorted to obtain the desired product. The composition of the cryolite after sorting is roughly: A1 accounts for 13% to 14%, F accounts for 53%-54%, Na accounts for 30%-32%, and there are also a small amount of impurities such as iron and silicon. With the development of the aluminum industry, due to the small amount of natural cryolite, the distribution is not universal, and the quality is poor, it can not meet the needs of electrolytic aluminum smelting. Therefore, we can only mass produce cryolite by using the method of input, China Aluminum Industry From the beginning, the use of cryolite to produce aluminum.
b. Into the ice crystals Generally known as synthetic cryolite, the industry is only referred to as cryolite, it is a white and slightly gray powder, soft texture, slightly sticky hands, can be folded into a group by hand, insoluble in water. The molecular ratio of synthetic cryolite for aluminum electrolysis is about 1.6-2.0. Due to the limitation of production method, the impurity content is higher than that of alumina. The product quality level of cryolite is shown in Table 2.
Table 2 Â Cryolite quality grade ( GB4291-84 ) | |||||||
grade | Chemical composition /% | H 2 O | |||||
F | Al | Na | SiO 2 +F 2 O 3 | SO 4 2- | P 2 O 5 | ||
Special grade | 53 | 13 | 31 | 0.25 | 0.8 | 0.02 | 0.5 |
First level | 53 | 13 | 31 | 0.4 | 1.2 | 0.05 | 0.8 |
Secondary | 53 | 13 | 31 | 0.5 | 1.5 | 0.05 | 1.3 |
C. When the flotation cryolite electrolysis cell is normally produced, a layer of carbon residue is often floated on the surface of the electrolyte. The removed carbon residue contains about 60%-70% electrolyte, which is usually selected from the carbon residue by flotation. A useful component of electrolysis is called flotation cryolite. Flotation cryolite can also be obtained by flotation of the waste carbon block under the tank.
d. Regeneration of cryolite The cryolite recovered during the purification of flue gas by the wet process is called regenerated cryolite, which often contains a small amount of alumina, other fluorides and carbon powder. It should be pointed out that the quality fluctuations of flotation cryolite and regenerated cryolite are large, and mass analysis is carried out before use, and often it is first added to the mother tank, and then purified and then transferred into the normal tank.
3 Aluminum fluoride (A1F 3 ) Aluminum fluoride is a white hard powder, slightly larger than the particles of alumina. Its fluidity is second only to alumina, it is volatile at the electrolysis temperature, and it is easily hydrolyzed by water. In the electrolysis process, the molecular ratio of the electrolyte is increased due to the volatilization and hydrolysis of aluminum fluoride. In order to maintain the prescribed molecular ratio, it is necessary to supplement the aluminum fluoride according to the analysis result to adjust the composition of the electrolyte. 3 is shown.
Table 3 Â Aluminum fluoride quality level ( GB4292-84 ) | |||||||
grade | Chemical composition /% | H 2 O | |||||
F | Al | Na | SiO 2 +F 2 O 3 | SO 4 2- | P 2 O 5 | ||
≥ | ≤ | ||||||
First level | 61 | 30 | 4 | 0.4 | 1.2 | 0.05 | 7 |
Secondary | 61 | 30 | 5 | 0.5 | 1.5 | 0.05 | 7 |
4 Sodium fluoride (NaF) Sodium fluoride is a white powder that is readily soluble in water. At the beginning of the operation of the electrolytic cell, because the carbon lining selectively absorbs sodium fluoride, the molecular ratio of the electrolyte drops sharply. At that time, more sodium fluoride is used to adjust the molecular ratio. In addition, since the molecular ratio of the synthetic cryolite used for grooving is too low, it is necessary to blend a large amount of sodium fluoride in order to adjust the molecular ratio required for the actuation, and the quality level of the sodium fluoride is shown in Table 4.
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