Detailed explanation of environmental protection treatment of secondary aluminum exhaust gas pollutants
Recycled aluminum is an aluminum alloy or aluminum metal obtained by remelting and refining waste aluminum and waste aluminum alloy materials or aluminum-containing waste materials. It is an important source of metallic aluminum. Recycled aluminum mainly appears in the form of aluminum alloy. Recycled aluminum must be strictly tested before use. National non-ferrous metals can conduct secondary aluminum testing and issue corresponding testing reports.
1. The source of waste gas pollutants from recycled aluminum
my country's waste aluminum recycling mainly uses fire smelting, and the process that produces exhaust gas and may affect the environment is mainly the smelting process. In the smelting process, the main fuels used are coal, coke, heavy oil, diesel, coal gas, natural gas, etc. After the fuel is burned, the exhaust gas produced contains a large amount of soot and gas containing sulfur, carbon, phosphorus and nitrogen oxides. ; After the furnace charge (aluminum scrap) is heated, the oil and the mixed combustibles of the waste material will burn, and a large amount of sulfur, carbon and nitrogen oxides will also be produced; in the smelting process, in order to reduce burning loss, increase the recovery rate of aluminum and To ensure the quality of aluminum alloy, it is necessary to add a certain amount of covering agent, refining agent and degassing agent. These additives react with various impurities in the aluminum melt to produce a large amount of exhaust gas and smoke. Metal oxides and non-metal oxides may also contain harmful substances, which may pollute the environment.
2. Classification and composition of waste aluminum flue gas
1. Classification of waste gas in the scrap aluminum industry
According to the regulations of the environmental protection department, the waste gas produced by the industrial sector can be divided into two categories: waste gas containing particulate matter and waste gas containing gaseous pollutants. Due to the complex raw materials in the secondary aluminum industry, the above two types of waste gas exist in the production process, including both particulate waste and gaseous pollutants. Therefore, when the waste gas is treated, the dust and harmfulness in the waste gas must be considered comprehensively. The impact of gas on the environment.
2. The main components of smoke
Due to the different sources of raw materials in the aluminum scrap industry, different types of waste materials, different pollution levels, different fuels selected, different additive components, and different smelting technologies, the pollutants in the exhaust gas are also different. To sum up There can be the following:
(1) Granular wastes are mainly metal oxides and non-metal oxides produced in the smelting process, such as: oxides and chlorides of Mg, Zn, Ca, Al, Fe, Na, Mg, and K, as well as large amounts of carbon Ashes, etc., the above constitute the dust we often talk about.
(2) The main component of gaseous pollutants and waste gas. The secondary aluminum industry may have the following gaseous pollutants and exhaust gases: CO, CO2, NOX, SO2, HCl, HF, hydrocarbons, and volatile metal oxides or volatile metals, and there may be chlorine, etc., of course not All the ingredients are harmful, but most of them will have a greater impact on the environment, and some may endanger the lives of workers.
Three, control of smoke and dust
The purpose of smoke and dust treatment is to maximize the collection of dust in the flue gas, so that harmful gases can be converted into harmless and stable substances to meet national emission standards. Environmental governance must first be controlled from the source. For recycled aluminum, it is necessary to strengthen the pretreatment and classification of waste, try to make comprehensive use of various wastes, and reduce the amount of pollutants; second, try to choose pollution-free Additives; The third is to improve the smelting technology and reduce the amount of waste generated. Commonly used soot treatment technologies are as follows:
1. Dust collection room
Dust collection chamber is the simplest method in flue gas dust collection equipment. Its principle is that when the speed of the fluid suddenly slows down, the particles in it will settle down due to its own gravity. The flue gas produced by recycled aluminum flows at a certain speed in the flue. When the space of the flue suddenly increases, the speed of the airflow suddenly slows down. As the movement speed of the particles decreases, the particles are deposited under the action of their own gravity. There are many types of dust chambers.
The easiest way is to build it with bricks. If some baffles are built in the dust collection chamber, the dust collection effect of the particles can be improved, but in general, the dust collection effect of the dust collection chamber is not good.
2. Cyclone dust collector
Cyclone dust collector is a more suitable dust collection setting, with lower investment and better effect, and is used in many enterprises. The cyclone dust collector can also be called a centrifugal dust collector.
When the gas enters the dust collector in a tangential direction, the airflow will continuously rotate at a high speed in the dust collector in a certain direction. The particles in the gas will generate centrifugal force during the high-speed rotation and move around. When they collide in the dust collector When it is on the wall, it drops into the dust collection chamber due to its own gravity to achieve the dust collection effect. The effect of the cyclone dust collector is obvious, and it is used in many companies that produce dust.
3. Spray dust collector
The principle is to make the droplets of a certain solution capture the particles in the gas during the descending process. After the particles are wetted, they will adsorb other particles, and finally fall to the dust collection chamber due to their own gravity to be collected.
A number of nozzles are set in a cylinder to control the hole size, flow, pressure and angle of the nozzles, so that the sprayed liquid is misty, the gas rises from the lower end, and the particles are captured. The spray type dust collector is a simple Easy dust collection settings are widely used.
4. Popping Tower
Popping tower is also a commonly used dust collector, the effect is higher than the above several types of dust collectors, but the investment is relatively large and the operation is more difficult. The principle is to set a partition in the middle of a cylinder. There are countless small holes on the partition. During normal operation, there is a solution layer on the partition. The gas enters from the lower end of the tower and passes through the partition. In the liquid layer, dust is captured by the liquid, and clean gas is removed from the upper end of the liquid. When the gas pressure is equal to the pressure generated by the liquid on the partition, the gas-liquid two phases are still. When the gas pressure is greater than the water pressure, the gas flow passes through the liquid, but the gas pressure and the height of the water layer are designed by calculation. The pressure is too high to achieve the effect of dust collection. In actual production, the water surface is boiling.
5. Bag dust collector
Dry dust collection is mainly a bag filter. Its principle is to make the dust-laden gas pass through the filter bag to achieve the dust collection effect. There are two commonly used:
(1) External standard: There are multiple baghouses built in the dust collector, and the surface of the baghouse has filter cloth. During production, the exhaust gas enters the dust collector. The room is under negative pressure. After the dust-containing gas enters the dust collector, the particles are adsorbed On the surface of the filter cloth, the purified gas is removed from the baghouse. After running for a certain period of time, the dust on the filter cloth accumulates and the air permeability decreases, which affects the dust collection and exhaust effect. Therefore, it is necessary to remove the dust in time by starting compressed air and performing reverse air blowing. After the dust falls off Enter the dust room.
(2) Internal-standard baghouse dust collector: The principle is the same as the external standard, except that dust-containing gas enters the baghouse. The outside of the baghouse is under negative pressure. The gas passes through the filter cloth and the dust accumulates in the baghouse. After a certain period of time, Afterwards, after shaking, the dust will fall off into the dust chamber automatically, achieving the effect of dust removal.
The cost of bag dust collection is relatively high, but it is a kind of dust collector that is often used at present. When choosing a bag filter, you must pay attention to the temperature that the bag adapts to, generally below 250 degrees. In addition to the above equipment, there are more advanced dust collection equipment such as electric dust collection, but due to the high cost, no company in the secondary aluminum industry currently adopts it.
Fourth, the treatment technology of gas pollutants and waste gas
The components of gaseous pollutants and exhaust gas have already been introduced above. Regarding the treatment of this type of waste gas, the company has basically not done any treatment at present. The main reason is that there are many types of harmful gases produced by the secondary aluminum industry, and it is difficult to treat them separately. However, we can see that these gaseous pollutants and waste gas still have common features, that is, most of the substances in the gaseous pollutant waste gas are acidic, which provides convenience for the treatment of waste gas. Under normal circumstances, it can be effectively treated as long as weakly alkaline substances are added. Generally, Na2CO3 is added to neutralize, and finally meet the emission standards.
In the wet dust collection mentioned above, such as spray towers, foam towers, etc., water is generally added. If alkaline substances are added to the water, the harmful substances in the exhaust gas can be treated at the same time as the dust is collected.
Several commonly used environmental protection equipment are introduced above. I hope that companies can refer to them when constructing environmental protection equipment. At the same time, companies must take into account the characteristics of smoke and dust generated by recycled aluminum-not continuous exhaust gas, but intermittent exhaust gas generation. The above-mentioned bag filter, cyclone dust collector, spray tower, etc. are more suitable for the characteristics of the industry.