Introduction to wastewater treatment in aluminum alloy surface treatment

     The above briefly introduces the clean production plan of the surface treatment part of aluminum alloy, but no matter what kind of clean production plan is adopted, there are also wastewater treatment problems. The related monographs on wastewater treatment have been introduced in detail, and only a few aspects are done here. Let me explain.

    The wastewater to be treated in the surface treatment of aluminum alloy can be roughly divided into common acid-base wastewater, chromium-containing wastewater, nickel-containing wastewater, dyeing wastewater and fluorine-containing wastewater.

    The dyeing wastewater comes from the dyeing process, and can be decolorized by the ozone method or by adding other oxidizing agents, and then sent to the acid-base wastewater pool for uniform neutralization and precipitation treatment.

    The fluorine-containing wastewater mainly comes from the acid texture etching process. The fluorine-containing wastewater is first filtered to precipitate, and then the calcium fluoride or calcium hydroxide is used to precipitate the fluorine to obtain a higher purity calcium fluoride. The calcium-treated wastewater can be used. Discharge into the acid and alkali wastewater pool for unified treatment.

    Nickel wastewater comes from electroless nickel plating, electroplating nickel and sealing. Nickel wastewater can be recovered by ion exchange method and then discharged into acid and alkali wastewater pool for neutralization and precipitation treatment. Separate electroless nickel plating wastewater requires additional treatment if the complexing agent is to be recycled. Readers can refer to the relevant materials.

    Due to the high toxicity of hexavalent chromium, the treatment is complicated, and the treatment method is discussed separately later.

    Ordinary acid-base wastewater is the sum of all wastewater except the above. If there is no electroplating nickel or electroless nickel plating in the production, the wastewater in the sealing process can also be shared with the common acid-base wastewater pool, but it is preferable to recover the nickel by ion exchange method and then discharge it into the acid-base wastewater tank. The separate nickel-containing wastewater can also be discharged into the acid-base wastewater tank for uniform treatment after the nickel is recovered.

    The treatment method of various metal ions in acid-base wastewater is mainly the selection of precipitant. If sodium hydroxide is used as the precipitant, the metal ions in the wastewater can be basically precipitated, and the amount of precipitation is small, but the surface treatment of aluminum alloy is small. For example, the concentration of sulfate ion and phosphate ion in the acid-base wastewater is high. Whether it is reused or discharged, a large amount of dilution is required, which inevitably results in the total amount of phosphate ions and sulfate ions discharged into the water body. The increase in the amount, especially the large amount of phosphate ions discharged into the water body, will make the water body eutrophic and bring secondary pollution to the environment. Of course, reverse osmosis can also be used to recover phosphates and sulfates in this case, but the cost of recycling needs to be carefully calculated. If calcium oxide is used as a precipitant, not only the metal impurities in the acid-base wastewater will precipitate, but also the phosphate ions and sulfate ions will precipitate, so that the solute concentration in the water is low, and the discharge requirement can be achieved without a large amount of dilution. It can also be used in the preparation of pre-treatment cleaning or pre-treatment for alkali etching, alkaline texture etching, etc. after ultrafiltration or reverse osmosis. However, after the calcium treatment, the hydraulic hardness is large, and the amount of precipitation is large (there is a large amount of calcium phosphate and calcium sulfate in the sediment). These two precipitation methods have their own advantages and disadvantages, and the enterprise needs to carefully calculate the comprehensive cost and adopt the society. The best and most cost effective solution is good. The comprehensive treatment of aluminum alloy surface treatment wastewater is shown in Figure 7-23 .

    If conditions permit, it can also be carried out by means of secondary precipitation, that is, first precipitation with sodium hydroxide to remove various metal ions in the wastewater, followed by filtration and second treatment of phosphate ions and sulfate ions with calcium agent. Secondary precipitation. Although the secondary precipitation is complicated and the construction cost of the wastewater treatment plant is increased, the precipitation can be classified, and at least the calcium sulfate and calcium phosphate precipitates are separated from other metal precipitates, which facilitates the classification management of the sediment. And waste utilization.

    Figure 7-23 Comprehensive treatment of aluminum alloy surface treatment wastewater

    In order to obtain a more detailed classification of precipitates, separate precipitation and filtration should be carried out according to the type of metal ions in the wastewater produced by each process. This will increase the initial input cost of wastewater treatment, and increase the difficulty and cost of management of various sediments. If these sediments can find a way to reuse, the profits generated can partially or completely offset the processing costs, and at the same time achieve a "zero emission" in a wide range, which is to promote the promotion of sustainable development strategies and Implementation is very important.