[China Aluminum Network] With the development of China's economy, the demand for environmental protection has become increasingly stringent, and it has also presented a serious topic for the aluminum processing industry, mainly in the "three wastes" treatment of aluminum processing, especially Disposal of waste generated during smelting.
1. Composition of slag treatment contaminants The hot aluminum slag from the furnace encounters oxygen in the air and will continue to burn, generating large amounts of contaminants and a strong metal oxidation reaction. The pollutants in the slag treatment process can be divided into three categories: flue gas, dust, and waste heat.
The dust usually referred to refers to a mixture of smoke and dust. Therefore, there are gas and solid components in the smoke. The main components of the gaseous material are fluorides and sulfides generated when the residual slagging agent reacts with the hot aluminum slag.
The solid material is mainly residual nitrate, graphite powder, alumina powder and impurity dust during refining. The total amount of soot accounts for less than 2% of the slag amount, and most of them have a particle size of 1-10 μm and an overall density of ≈0.8.
The residual heat is mainly the self-heat released by the heat exchange of the hot aluminum slag with the cooling medium during the cooling process and the radiant heat to the surrounding atmosphere during the processing.
2. Environmental protection diffuses slag in the workshop to treat flue gas and deteriorates labor production conditions, which seriously affects the health of the workers. The slag treatment flue gas is not purified and diffused into the atmosphere, causing some damage to the ecology. Gases that are colorless, toxic, and irritating to odors such as silicon tetrafluoride (SiF4) produce smoke due to hydrolysis in humid air. Not only do they adversely affect the worker's body, the resulting smoke can also affect operators. The line of sight has a deep hidden danger to safety production.
In the flue gas purification process, not only the flue gas and dust are recovered, the environment is protected, and heat is collected to improve the working environment.
3. Dust treatment 3.1 Process composition Dust treatment purposes First, the dust in the flue gas is collected to a greater extent, so that the harmful gases are converted into harmless and stable substances, reaching the national emission standards. For dust treatment in the slag treatment process, dust sources and heat sources must be controlled. Among them, the dust concentration is about 10g-25g/m3, most of which have a particle size of 1-10μm, a density of about 0.8, and the temperature of soot is about 150-250°C.
The dust collection system consists of dust collecting flue, cyclone dust collector, pulse bag dust collector and centrifugal fan. The slag processing flue gas passes through a dust collection pipe to a cyclone dust collector to cool the mixed air and settle large particles of dust, and then passes through the dust collecting flue to enter the bag filter for dust removal. The purified flue gas is sent to the chimney by the flue gas exhauster ( As shown below).
3.2 Cyclone dust ash ash handling a feature is that the temperature is high, another feature is the concentration. Because the slag treatment system is semi-open, the degree of confinement cannot reach 100%. Therefore, the high-temperature flue gas generated during the slag treatment process and the external fresh air are introduced into the dust collection pipeline at the same time for preliminary mixing, and then introduced into the cyclone dust collector. Dust removal.
The cyclone dust collector has a low investment and good effect, and is suitable for purifying dust with large density and coarse particles. It is very suitable for handling the intermittent fumes produced by slag treatment.
When the gas enters the dust collector in the direction of the tangential line, the airflow will continuously rotate at a high speed along the circular inner wall. The particles in the gas will generate centrifugal force during high-speed rotation and will move around when they collide with the wall of the dust collector. Due to its own gravity, it descends into the dust collection chamber and achieves dust collection. The dedusting effect of the cyclone dust collector is obvious, and the particle size of 5-10 μm in the flue gas can be collected, and the efficiency reaches 90%. In addition, the flue gas and air can be fully mixed here, effectively reducing the flue gas temperature. Converting high-temperature flue gas into normal-temperature flue gas not only reduces the cost of the bag filter, but also reduces the daily maintenance work of the equipment.
3.3 Pulse bag filter After the flue gas was treated by the cyclone dust collector, large particles of dust were recovered and the temperature dropped below 150°C. However, the smoke still contains a large amount of fine particles of dust. To meet the national environmental protection requirements, the flue gas enters the bag filter for further dust removal. The bag dust collector has high dust collection efficiency and can collect particulates with particle sizes of 1-5 μm and more than 5 μm in the flue gas with an efficiency of 99%.
The bag dust collector is the main equipment for dry dust collection. The principle is to make the dust-containing gas pass through the filter bag to achieve the effect of dust collection. There are two commonly used:
(1)External system: There are several baghouses built into the dust collector. The surface of the baghouse has a filter cloth. When purifying, the exhaust gas enters the dust collector. The inside of the room is under negative pressure. After the dusty gas enters the dust collector, the particles are adsorbed. On the surface of the filter cloth, the purified gas was excluded from the baghouse. After a certain period of operation, dust accumulates on the filter cloth. Therefore, it is necessary to start the compressed air in a timely manner and perform reverse blowing. After the dust falls off, it enters the dust chamber.
(2) Internal bag filter: The principle is the same as that of the external system, except that the dusty gas enters the baghouse. The outside of the baghouse is under negative pressure. The gas passes through the filter cloth and dust accumulates in the baghouse. After a certain time After the vibration, the dust automatically falls off into the dust chamber to achieve the effect of dust removal.
The choice of parameters:
The choice of anti-condensation, filter wind speed and filter media are the key parameters for determining the structure of the dust collector.
Resolving bag condensation is one of the difficulties of pulse dust collectors. The dust generated from the slag treatment is mainly residual nitrate, alumina powder and impurity dust during refining, and has strong water absorption. Therefore, it is necessary to remove the water from the air source and control the water vapor content. When the temperature is lower than 80°C, the moisture contained in the flue gas will condense, which will make the dust bag damp, the dry surface will be dew condensation, compaction, resulting in increased system resistance, reduced dust removal efficiency and may cause the bag to break off . It is necessary to increase the heating and insulation measures to ensure that the dust chamber temperature is not lower than 80°C to avoid condensation in the dust collector.
Filtering wind speed is one of the key parameters to determine the structure of the dust collector. The flue gas produced by the slag treatment contains a large amount of fine soot and aluminum oxide powder, which is too high to filter the wind speed and is not conducive to the complete collection of dust. In view of this, on the basis of sufficient practice, it is determined that the filtration wind speed is about 0.6 to 1 m/min, which is more economical and applicable.
The choice of filter material is also one of the key parameters for determining the structure of the dust collector. According to the actual situation on the site and past experience in similar projects, the dust collector uses medium and high temperature filter materials. The following table compares the performance parameters of several commonly used filter media.
Comparison of filter performance parameters According to the actual situation, we choose glass fiber needle felt filter material.
4. Treatment of Exhaust Gas On the issue of treatment of gaseous pollutants and exhaust gases, there is currently no corporate governance. The main reason is that most of the harmful gases generated during the slag treatment process are neutral, and the characteristics of the aluminum slag processing to produce soot are not continuous. Sexual emissions, intermittent production of exhaust gas, emissions are limited, no further processing.
5. Summarize the above-mentioned environmental protection equipment and treatment process, and hope that companies can refer to the construction of dust removal equipment for slag treatment equipment.
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