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   I. Introduction
Electrolytic aluminum production process, emit large amounts of hydrogen fluoride gas and fluorine-containing dust and other harmful substances from the electrolytic cell, electrolytic production of alumina starting material has a strong adsorption capacity for hydrogen fluoride gas, a dry adsorption purification of exhaust gas from it, At present, most electrolysis production enterprises adopt purification and recovery methods, but the method has poor purification effect on carbon dioxide and sulfur dioxide during operation, and the alumina after the fluorine absorption is large, and the number of cycles of alumina in the adsorption process is large, resulting in alumina. The high crushing rate, the increase of impurities in the FE and SI, and the large power consumption of the aluminum electrolysis flue gas purification system.
   Second, dry purification system introduction
The flue gas dry purification technology is suitable for the flue gas treatment of various types of electrolytic cells in the aluminum electrolysis production industry. The basic principle of dry purification is to utilize the characteristic that alumina has strong adsorption capacity to gaseous hydrogen fluoride, so that the electrolysis flue gas is in full contact with the alumina, and the hydrogen fluoride gas in the flue gas is adsorbed on the surface of the alumina, and then the gas is carried out. Solid separation allows hydrogen fluoride to be purified.
The dry adsorption method is a pipeline method: before the total flue gas is introduced into the bag filter from the total flue gas, the fresh alumina and the circulating alumina are separately added to the exhaust pipe. During the full contact of the gas-solid two phases, hydrogen fluoride is adsorbed by the alumina. The added alumina and the dust taken out from the electrolytic cell with the flue gas are separated in the bag type dust collector and returned to the electrolytic cell for use, and the purified flue gas is sent to the chimney for emptying through the smoke exhauster. The discharge standards and discharge standards in the national standards are specified in Table 1.
Table 1 Flue gas emission standards for aluminum electrolysis plants
Location
Emissions
Emission Standards
Smoke
Fluoride (perfluoro)
≯1.0kg/t-Al
Dust
≯30mg/Nm 3
Sulfur dioxide
45m chimney ≯91kg/h
Factory skylight
Fluoride (perfluoro)
1.0kg/t-Al
Dust
≯30mg/Nm 3
Sulfur dioxide
/
Adsorption chemical reaction: During the adsorption process, a monomolecular layer adsorption compound is formed on the surface of the alumina, and each fluorinated fluorine molecule is adsorbed by each alumina molecule. According to X-ray diffraction, this surface compound is converted to AlF 3 molecules above 300 ° C. This process is characterized by fast and not easy desorption, and the reaction can be completed in 0.25 to 1.5 seconds.
Figure 1 dry purification flow chart
This reaction is the reverse process of high temperature hydrolysis of aluminum fluoride, and at a lower temperature, the reaction proceeds in the direction in which aluminum fluoride is formed.
Defects in dry purification:
(1) The defect of dry purification is the poor purification effect on carbon dioxide and sulfur dioxide;
(2) The alumina after the fluorine absorption is flying;
(3) The number of cycles of alumina in the adsorption process is high, resulting in high alumina breakage rate, and increased impurities in the FE and SI;
(4) The power consumption of the aluminum electrolysis flue gas purification system is large.
   Third, the optimization design of the dry purification system
(1) Basic principles
The raw material-alumina is produced by aluminum electrolysis as the adsorbent, and the fluoride (mainly hydrogen fluoride) in the flue gas is adsorbed under the set conditions (including the solid-gas ratio of the reaction section, the reaction time, the flue gas flow rate, etc.). Alumina is mixed with hydrogen fluoride to complete the adsorption of hydrogen fluoride in a very short time and achieve high purification efficiency.
Firstly, the first active high alumina is used to carry out the second hydrogen fluoride remaining in the flue gas by using the relatively low activity, the hydrogen fluoride-adsorbing alumina (also known as the fluorine-loaded alumina) and the aluminum-containing initial flue gas having a high fluorine concentration. The reaction is adsorbed to obtain a higher fluorine purification efficiency.
The reverse two-stage dry adsorption purification technology optimizes the dry adsorption mechanism, achieves the goal of obtaining a very high fluorine purification efficiency with a lower solid-gas ratio of the reaction section, thereby reducing the number of cycles of alumina in dry adsorption. It avoids the problem of high alumina breakage rate caused by multiple cycles, increased impurities brought into FE, SI, and high power consumption of aluminum electrolysis flue gas purification system.
Figure 2 Reverse two-stage dry adsorption purification flue gas configuration
(2) Technical key
In the dry adsorption purification of aluminum electrolysis flue gas, the reverse two-stage adsorption is firstly used to make the adsorption capacity of hydrogen fluoride fully exerted, and then the fresh ammonium oxide is added in the second stage to re-adsorb the remaining hydrogen fluoride in the flue gas, thereby increasing the purification process. "Drive power"; the method optimizes the dry adsorption mechanism, improves the fluorine load of alumina, and can effectively reduce the amount of adsorbent; in the selection of the dust collector of the aluminum electrolysis flue gas purification system, pay more attention to low resistance, Efficient, optimized after the selection of improved low-pressure pulse long bag dust collector and the use of aluminum electrolytic flue gas purification special filter material - JZL-D fine filter needle felt, its dust removal efficiency has reached the international advanced level.
(3) Equipment optimization and operation management after system optimization
1, the main equipment
DMD-II-340 low pressure pulse long bag dust collector
Y4-73NO20F smoke extractor (20X104m 3 /h·3500Pa)
B=260mm (combination) air chute (Q=16000kg/h)
Ф570mm (combination) body hoist
2, operation management
Under the set conditions, through the system debugging, the exhaust volume of each electrolyzer is balanced, and the flue gas purification system only needs one person to inspect every shift, which can ensure the stable and efficient operation of the purification system.
(4) Main technical indicators and conditions
1. The total solid-gas ratio of the reaction section is <35g/m 3 (the common solid-gas ratio at home and abroad is 50-60g/m 3 ).
Among them: one section is fluorine-loaded alumina, the solid-gas ratio is 25g/m 3 ; the second section is fresh alumina solid-gas ratio <10g/m 3 .
2, the total reaction time ≈ 1.0S
Among them: one segment is ≥0.5S, and the second segment is 0.5S.
3, fluorine purification efficiency ≥ 99%
Purification system chimney outlet fluorine concentration <1.0mg/m 3 (national standard is 9mg/m 3 )
Flux of tons of aluminum (factory skylight + chimney) <1.0kg/t-AL; tons of aluminum dust (factory skylight + chimney) <3.0kg/t-AL
(5) Supplementary measures
To improve adsorption efficiency, the following points should be met:
1. Alumina has a strong adsorption force. The analysis proves that the adsorption capacity of γ-Al 2 O 3 -based sand-like alumina is about ten times larger than that of α-Al 2 O 3 -based powdered alumina, because the γ-Al 2 O 3 crystal form is inferior. α-Al 2 O 3 is completely stable and has a large residual power, so the adsorption capacity is strong. At the same time, the particles of the sand-like alumina are coarse and loose in structure, the surface area is large, and the fluorine-carrying capacity is strong. Therefore, the reverse two-stage The dry adsorption purification method uses sand-like alumina as an adsorbent and requires a specific surface area of ​​more than 30 m 2 /g.
2. Alumina and flue gas must be in full contact. In order to improve the contact state of the gas-solid phase, alumina is present in the flue gas in the state of fluidization during the adsorption process, which promotes the continuous renewal of the gas-solid phase contact surface, reduces the diffusion resistance of the gas film, and increases the reaction speed. .
3. The higher the concentration of flue gas hydrogen fluoride, the better the adsorption process. Therefore, the concentration of smoke should be increased. In other words, the degree of sealing of the electrolytic cell should be increased to reduce air leakage into the gas collecting device to improve the adsorption efficiency.
   Fourth, the conclusion
The reverse two-stage dry adsorption and purification technology is used to purify the flue gas in the electrolysis workshop. The purification efficiency of fluorine and dust is high, and the investment is saved and there is no secondary pollution. All the purification indexes have reached the national advanced level and the purpose of effectively controlling pollution has been achieved. .