Acid waste water treatment technology for aluminum production

Acidic wastewater from aluminum production is mainly from the acid etching process in the oxidation plant, neutralization, oxidation process, spray pre-treatment oil removal, pickling and other processes, during which time it is rich in various harmful substances or heavy metal salts. The acid mass fraction is not as great, ranging from less than 1 percent in the low to more than 10 percent in the high. Alkaline wastewater mainly comes from oxidation workshop alkali etching, spray pretreatment alkaline washing and other processes. Some of the alkalis have a mass fraction of more than 5%, and some are less than 1 %. Wastewater also occurs from spraying and dyeing. Wastewater, in addition to being rich in acid and alkali, is often rich in oil, paint, fluoride salts, and other inorganic and organic matter.

The acid-alkali wastewater is highly corrosive and must be properly managed before it can be discharged. The general guidelines for the management of acid-alkaline wastewater are: 1 High-concentration acid-alkali wastewater, priority should be given back to the use of recovery, based on water quality, water quantity and different technical requirements, try to repeat the use: If repeated use is difficult, or the concentration is low, the amount of water is large, The concentrated method can be used to recover acid and alkali. 2 Low-concentration acid-alkali wastewater such as clean water in pickling tanks, and rinse water in caustic tanks should be neutralized.

Regarding the handling of neutralization, it is necessary to think first and foremost on the principle of waste disposal. For example, acid and alkali wastewaters are neutralized with each other or waste alkali (slag) is used to neutralize acidic wastewater, and waste acids are used to neutralize alkaline wastewater. In the absence of these conditions, neutralization agents can be selected for disposal.

According to the national standard GB8978-1996 "Water Pollutant Emission Limit" emission requirements are: COD level ≤ 60mg/L, secondary ≤ 120mg/L, suspended matter ≤ 100mg/L, fluoride ion F - ≤ 10mg/L, pH 6-9.

Modern wastewater disposal methods are mainly divided into physical disposal methods, chemical disposal methods and biological disposal methods.

1) Physical Disposal Method Wastewater treatment method that separates physical effects and recovers insoluble suspended contaminants (including oil film and oil beads) from wastewater. General use of sedimentation, filtration, centrifugal separation, air flotation, transpiration crystallization, anti-saturation and other methods. Contaminants such as suspended solids, colloids and oils are separated from the waste water, so that the waste water can be purified.

2) Chemical Disposal Method Dissociates and removes pollutants that are dissolved, colloidal, or converted into non-hazardous substances by chemical reactions and mass transfer effects. The general options are: neutralization, coagulation, oxidation recovery, extraction, stripping, blow-off, adsorption, ion exchange, and electrosweetening.

3) Bio-disposal method Through the metabolic effect of microorganisms, wastewater solutions, colloids, and organic matter, toxic substances, etc. in the form of fine suspensions are converted into wastewater treatment methods for stable and harmless substances. Biological disposal method is divided into two methods: aerobic treatment and anaerobic treatment. The aerobic treatment method is currently commonly used activated sludge, biological filters and oxidation ponds. The anaerobic treatment method, also called biological recovery treatment method, is mainly used for the disposal of high-concentration organic waste water and sludge, and the use of disposal equipment, mainly for the digester.

The intent of disposing of the sludge is to: 1 reduce the moisture content of the sludge and create conditions for disposal, application and transportation of the sludge. 2 eliminate harmful substances that pollute the environment. 3 Regain power and capital to achieve harm. Sludge disposal methods include sludge thickening, sludge digestion, sludge dewatering, and sludge drying. The intention of sludge concentration is to start the dewatering of sludge and reduce the volume of sludge to provide conditions for subsequent disposal. The intention of sludge dewatering is to further dewater the sludge to reduce the moisture content below 80%. Its methods include mechanical dehydration and natural dehydration. Mechanical dehydration method is divided into vacuum suction filtration, pressure filtration and centrifugation. Its advantages are high dehydration rate and small floor space, but the investment is more expensive. The natural drying method has low infrastructure and operating costs, but it has low dewatering power, large floor space, and poor sanitary environment. The purpose of sludge drying is to heat the sludge after dehydration to further reduce the moisture content and reduce the volume. Dry processing method commonly used rotary cylinder dryer. Its strengths are stable and reliable operation, but the larger footprint.