1．Pollutants and main sources in smelting and casting
The impact of smelting and casting processes on the environment is mainly reflected in the pollution of industrial waste gas. The main waste gas pollutants are: flue gas, sulfur dioxide (SO2), nitrogen oxides (NOx), aluminum compounds (Al2O3), production dust and HCI and a small amount of Cl2, etc.
The main pollutants of combustion waste gas emitted by aluminum melting furnaces are flue gas, SO2, and NOx. During the process of adding materials, stirring, and slagging during smelting, dust such as production aluminum compounds (Al2O3) is also produced. During the crushing, screening, and melting process of the covering agent, like the reverberatory furnace, it also emits production dust and combustion waste gas. In addition, due to the addition of tertiary waste (aluminum scraps) containing various oils, emulsions, graphite and other high pollutants, the content of pollutants such as smoke, SO2, NOx, etc. in the industrial waste gas emitted by the aluminum melting furnace increases instantly, aggravating the pollution of the atmospheric environment.
During the refining process, due to the oxidation of the aluminum surface and the use of solid flux, the furnace door intermittently emits aluminum compound dust, HCI and a small amount of Cl2.
Of course, in the aluminum alloy casting process, the water cooling technology used requires a large amount of cooling circulation. Due to long-term use, these waters contain some impurities and suspended matter, which will cause certain pollution to the water environment when discharged regularly.
2．Main sources of pollutants in plate and strip rolling
① Milling surface lubrication The lubricant for milling surface is mostly emulsion, with a concentration of 2%~10%. The emulsion should be scrapped and discharged after a period of recycling. Waste emulsion is a hazardous waste and will pollute the environment.
② Etching and washing The ingot etching and washing process is generally degreased with 10%~20% N2OH solution at a temperature of 60~80℃, rinsed with cold water, neutralized with 20%~30% nitric acid solution, and finally rinsed with hot water. The pH value of the rinse wastewater generated in the etching and washing process is generally between 6 and 8. However, acid and alkali waste liquids will emit acid mist and alkali mist during use, causing certain pollution to the atmospheric environment, and will be scrapped after a period of use (about 6 months) due to excessive impurity content. Waste acid and alkali liquids are hazardous wastes, and direct discharge has a significant pollution and corrosion effect on the water environment and drainage facilities.
③Hot rolling The most widely used cooling lubricant is emulsion, with a concentration of 2%~8% and a service life of 3~6 months. During rolling, the emulsion encounters high-temperature ingots and rollers, which will produce large oil mist and fume, which will cause certain pollution to the atmosphere. The emulsion is scrapped after a period of recycling, and direct discharge will pollute the water environment.
④Cold rolling The commonly used cooling lubricant is a mixture of base oil and 1%~10% additives. The base oil is mainly composed of saturated hydrocarbons, cycloalkanes, aromatic hydrocarbons and a small amount of olefins. The additives are mainly long-chain fatty acids, fats, and alcohols. A large amount of oil mist and fume (mainly the product of kerosene volatilization) will be generated during cold rolling, which will cause great pollution to the atmosphere. In addition, the rolling oil is scrapped after a certain period of use. The waste oil is a hazardous waste, and direct discharge also causes serious pollution to the environment.
⑤Annealing and quenching At present, electric furnaces are used for intermediate annealing and finished product annealing in domestic aluminum processing. Therefore, the industrial waste gas of the annealing furnace is generally not treated and directly discharged.
Quenching in an air quenching furnace generally does not have an impact on the environment. If quenching is done in a salt bath furnace (saltpeter), after quenching, the acid and alkali process waste gas and waste acid and alkali liquid generated by the etching process are directly discharged, which will cause certain pollution to the environment.
3. Main sources of pollutants in the rolling of coated aluminum coils
At present, in terms of technology, rolling oil is divided into two categories according to the different uses of coated aluminum coils. One is high-viscosity rolling oil for low-speed rolling, which is mainly composed of base oil (mostly No. 20 industrial oil) and additives (mostly turbine oil, high-speed engine oil, kerosene, etc.). The other is low-viscosity rolling oil for high-speed rolling, which is also composed of base oil and additives. Now, kerosene has been greatly improved as low-viscosity rolling oil. The additives are mainly alcohol, fat and fatty acid. Due to the friction between the roller and the foil during the rolling process, a large amount of aluminum powder is generated, which makes the rolling oil black and pollutes the rolling oil, so full-flow filtration is required. But it still needs to be scrapped after reaching a certain level. Direct discharge will cause serious pollution to the environment. Oil mist will also be generated during the rolling process, but the oil mist concentration is low, basically below 30mg•m-3. Therefore, the oil mist generated during the rolling of coated aluminum coils is generally not treated and directly discharged.
4. The main hazards of pollutants
As industrial wastewater is discharged into water bodies and industrial waste gas is discharged into the atmosphere, pollutants in wastewater and waste gas enter water bodies, soil and crops through various different pathways, and enter the human body through breathing and skin contact, causing harm and impact on the ecological environment and human health.
Oil floating on the water surface will isolate the air, reduce the dissolved oxygen in the water, adhere to the surface and respiratory system of aquatic organisms, and kill them. Oil deposited at the bottom of the water will produce highly toxic hydrogen sulfide after anaerobic decomposition.
When water is polluted by acid and alkali, its pH value will change. If the pH value exceeds the range of 6.5~8.5, it can affect the self-purification process of the water body and affect the survival of aquatic organisms.
The more serious the organic pollution, the more oxygen is consumed in the water, causing the dissolved oxygen content in the water to decrease, affecting the survival of fish, and also making the water body "eutrophic".
Sulfur dioxide is highly irritating and can cause pulmonary edema if inhaled in large quantities. Data from national survey agencies show that when the average concentration is above 50 mg•m-3, people who are exposed to more of this gas may suffer from rhinitis, tooth erosion, and systemic adverse reactions. In addition to the harm to human health, it has a great impact on buildings, metal materials, agriculture, forestry and aquatic products. Sulfur dioxide is also one of the main substances that form acid rain.
Among nitrogen oxides, nitrogen dioxide is the most toxic. It mainly irritates the human respiratory organs and can cause pulmonary edema. Continuous exposure to concentrations above 9 mg•m-3 can cause lung damage and bleeding. In addition to the harm to human health, nitrogen oxides react with hydrocarbon compounds to form photochemical smog. Dry deposition with sulfur dioxide will cause acidification of water bodies and soil, and is also one of the main substances that form acid rain.
Chlorine is an irritant gas that mainly damages the upper respiratory tract and bronchial mucosa of the human body, causing chronic and acute poisoning. High concentrations can cause pulmonary edema. When the concentration is 0.11~6.56mg•m3, the incidence of bronchitis, chronic rhinitis and pharyngitis in exposed personnel is high.
Hydrogen chloride is a highly irritating gas that mainly harms the human body through the respiratory tract and can cause chronic and acute poisoning. When the concentration exceeds 3.4~21mg.m3, most exposed personnel will have mucosal irritation and tooth erosion.
Long-term inhalation of aluminum compound dust by the human body can cause aluminum lung. When the concentration is above 4mg.m-3, most of the exposed personnel will have enhanced lung wrinkles.
