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19
2017
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01
Methods for treating copper ions in industrial wastewater
Copper plating is often used as an underlayer for nickel, tin, chromium, silver, and gold plating to improve the bonding strength between the substrate metal and the surface plating and the corrosion resistance of the plating. Therefore, copper-containing electroplating wastewater is very common in the electroplating industry, and this type of wastewater usually contains various heavy metals and complexing agents. Currently, the treatment of copper-containing electroplating wastewater mainly uses chemical methods, ion exchange methods, membrane separation methods, adsorption methods, and biological methods. 1 Chemical methods for treating copper-containing electroplating wastewater 1) Neutralization precipitation method Currently, chemical neutralization and coagulation precipitation methods are commonly used in China to treat copper-containing comprehensive electroplating wastewater. The neutralization of acids and alkalis in the wastewater is
Copper plating is often used as a base layer for nickel, tin, chromium, silver, and gold plating to improve the bonding strength between the base metal and the surface plating and the corrosion resistance of the plating. Therefore, copper-containing electroplating wastewater is very common in the electroplating industry, and this type of wastewater usually contains various heavy metals and complexing agents. Currently, the treatment of copper-containing electroplating wastewater mainly uses chemical methods, ion exchange methods, membrane separation methods, adsorption methods, and biological methods.
1 Chemical methods for treating copper-containing electroplating wastewater
1) Neutralization precipitation method
Currently, chemical neutralization and coagulation precipitation methods are commonly used in China to treat copper-containing comprehensive electroplating wastewater. While neutralizing the acids and alkalis in the wastewater, copper ions form copper hydroxide precipitates, which are then removed by a solid-liquid separation device.
For single copper-containing wastewater, copper ions can be precipitated and removed to meet the standard when the pH value is 6.92. Generally, when copper and iron coexist in electroplating wastewater, controlling the pH value at 8-9 can also meet the discharge standard. However, for mixed electroplating wastewater containing both copper and other heavy metals and complexing agents, the copper removal effect is not good, and often fails to meet the discharge standard. This is mainly because the essence of this method is to adjust the pH value of the wastewater, while the optimal pH values for precipitation of various metals are different, resulting in poor removal effect; moreover, if the wastewater contains complexing ions such as cyanide and ammonium, they form complexes with copper ions, making it difficult for copper ions to dissociate, so that copper ions cannot meet the discharge standard. In particular, after the treatment of copper-containing mixed wastewater containing cyanide, the concentration of copper ions and CN- is almost proportional. As long as CN- exists in the wastewater, the concentration of copper ions in the effluent will not meet the standard. This makes the effluent effect of using the neutralization precipitation method to treat copper-containing mixed wastewater poor, especially for the removal of copper.
2) Sulfide precipitation method
The sulfide precipitation method for treating copper-containing wastewater has great advantages, and can solve the problem of some weakly complexed heavy metals not meeting the standard. The solubility of copper sulfide is much lower than that of copper hydroxide, and the pH range of the reaction is wider. Sulfides can also precipitate some copper ion complexes, so there is no need for separate treatment. However, due to the small size and poor settling of sulfide precipitates, its application is limited. In addition, the presence of cyanide ions affects the precipitation of sulfides and can dissolve some sulfide precipitates.
3) Electrochemical method
The electrochemical method for treating copper-containing wastewater has the advantages of high efficiency, automatic control, and small amount of sludge. It can directly recover metallic copper from copper-containing electroplating wastewater, and the treatment is adaptable to a wide range of copper concentrations in wastewater, especially for wastewater with high concentrations (copper mass concentration greater than 1 g/L), it has certain economic benefits, but the current efficiency is low at low concentrations.
2 Ion exchange method for treating copper-containing electroplating wastewater
The ion exchange method is one of the main methods for treating copper-containing wastewater. Various ion exchange agents are constantly being developed. There are many types of ion exchange agents. Complexing agents have little effect on this method for treating copper-containing electroplating wastewater.
1) Ion exchange resin
Ion exchange resins have excellent copper removal effects. The treatment of high-concentration ammoniacal copper rinsing solution using resins has been reported; some factories also use weakly acidic cation exchange resins to treat acidic sulfate copper plating rinsing wastewater; some enterprises use strongly basic anion exchange resins to treat pyrophosphate copper plating wastewater, enabling partial water recycling. In addition, chelating resins have the advantages of good selectivity, large adsorption capacity, and fast speed, and the exchange speed is fast. However, due to the high price of these chelating resins, most of them are still in the experimental stage and are rarely used on a large scale in industry.
2) Ion exchange fiber
Ion exchange fiber is a new type of ion exchange material that has developed rapidly in recent years, and has also made great progress in the field of heavy metal wastewater treatment. Studies on the adsorption of copper in electroplating wastewater by modified polyacrylonitrile fibers have shown that after adsorption by modified polyacrylonitrile fibers, the content of copper ions in copper-containing electroplating wastewater is significantly lower than the national emission standard.
3 Membrane separation technology for treating copper-containing electroplating wastewater
Membrane methods for treating industrial wastewater generally use reverse osmosis, ultrafiltration, and a combination of the two. The key to membrane methods for treating industrial wastewater is to select the appropriate membrane according to the separation conditions. Many reports have been published on the treatment of copper-containing electroplating wastewater using reverse osmosis membrane separation technology. This method also has a good effect on the treatment of copper-containing complex electroplating wastewater, and some have been applied to industry and used in conjunction with other water treatment technologies to achieve good results.
4 Adsorption method for treating copper-containing electroplating wastewater
The adsorption method for treating copper-containing wastewater has many advantages and has become a research focus in water treatment. Many adsorbents with good performance have been developed, especially using industrial waste and agricultural residues as adsorbents, and modifying existing adsorbents to improve their adsorption performance. Zeolite and diatomite are inexpensive and widely used. Diatomite can adsorb more than 95% of copper ions; kyanite can achieve 100% adsorption of copper ions under appropriate conditions; coal ash and slag can be used as adsorbents to treat copper-containing electroplating wastewater, and 4A zeolite synthesized from coal ash can adsorb various heavy metals, and the adsorption effect on copper ions is very good.
Current research focuses on some plant and animal wastes as adsorbents. In order to increase the adsorption capacity and selectivity, modifications are carried out, and the adsorption effect of the modified adsorbents on copper ions is significantly improved. Rice husk modified with tartaric acid greatly improves the adsorption effect on copper ions, and the adsorption capacity of chicken feathers treated with alkali solution for copper ions is greatly improved, and the adsorption effect is very good. Using sawdust to adsorb copper ions in mixed electroplating wastewater has a better effect than treating copper in single wastewater.
5 Biological method for treating copper-containing electroplating wastewater
The biggest advantage of using biological methods to treat copper-containing wastewater is that microorganisms continuously proliferate during operation, and the amount of copper ions removed by biomass increases with the increase in biomass. Biological methods have many advantages in application, such as strong comprehensive treatment capabilities, effectively removing harmful metal ions such as copper, hexavalent chromium, nickel, zinc, cadmium, and lead from wastewater; simple and practical treatment methods; simple process control; small amount of sludge, significantly reducing secondary pollution. However, biological methods for treating copper-containing wastewater have drawbacks such as slow reproduction speed of functional bacteria and slow reaction rate, and difficulty in reusing treated water.
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