Treatment of heavy metal ions in electroplating wastewater

With the progress of society and the improvement of people’s living standards, the requirements for goods are becoming more and more exquisite and beautiful. We can see that daily necessities, handicrafts and decorations treated by electroplating can be seen everywhere. The corrosion resistance and hardness of the electroplated metal are enhanced, and it can prevent abrasion, improve conductivity, smoothness and heat resistance, The appearance and glossiness of the workpiece after electroplating treatment are pleasing to the eye, but a certain amount of electroplating wastewater will be produced in the process of electroplating. The water quality is complex, and the composition is not easy to control. It contains chromium, cadmium, nickel, copper, zinc, gold, silver and other heavy metal ions and cyanide, and some substances are carcinogenic, teratogenic, toxic and other Mutagenic toxic substances must be treated before they can be discharged. However, due to the different types of electroplating workpieces, the fluctuation and complexity of water quantity and quality are large, so higher requirements are put forward for the treatment process.

Main sources and water quality of electroplating wastewater (taking nickel plating as an example).

The main process of electroplating is: workpiece, degreasing and oil removal, water washing, polishing, activation, nickel plating, water washing, copper plating, copper plating, water washing, nickel plating, water washing, drying.

Main treatment process of heavy metal ions in electroplating wastewater

Electroplating wastewater treatment processes generally include: physicochemical precipitation, ion exchange, membrane treatment process and evaporation treatment process. The following will give a brief introduction to these processes.

• Physicochemical precipitation method
  Physicochemical precipitation method is also called chemical precipitation method, the whole treatment process does not need complex equipment. Because the nickel containing wastewater from nickel plating process can be divided into electroless nickel plating and electroless nickel plating, it should be treated differently in the treatment process.
  Nickel plating wastewater can be treated by adding alkali directly to 11, adding PAC for coagulation reaction, PAM for flocculation reaction, and then precipitating the effluent. Generally, nickel can be less than 0.5mg/l. If the concentration of nickel containing wastewater is high, it is necessary to add a small amount of recapturer after adding alkali for chelation reaction to ensure the stability of nickel in the effluent.
  Because there are a lot of complexing agents in electroless nickel plating wastewater to form small complex nickel molecules, the direct addition of alkali, PAC and PAM can not make the precipitation reach the standard. It is necessary to first add a breaking agent to destroy the structure of complex bond, and then react by adding alkali and recapturer, and then coagulate and precipitate by adding PAC and PAM to remove nickel ions.
• Ion exchange method
  In the process of ion exchange, cations in water (such as Ni2 +, Na +, Ca2 +, K +, Mg2 +, Fe3 +) are exchanged with H + on cation exchange resin. The cations in water are transferred to resin, and H + on resin is exchanged to water. Generally, the resin needs to be regenerated after a period of use.
  Wedge wire type resin trap screen are required for this process.
• Membrane treatment process
  It is a separation method based on the principle of selective permeability of biofilm. The membrane used in separation is a kind of high molecular polymer designed and synthesized according to the needs, which can separate the mixed sample of liquid or gas. The selective separation of membrane can realize the separation, purification and concentration of different components of the material liquid. The process is a physical process, without phase change and additives. According to the different pore size, the membrane can be divided into microfiltration membrane, ultrafiltration membrane, ultrafiltration membrane, ultrafiltration membrane Nanofiltration membrane and reverse osmosis membrane. Now, due to the continuous improvement of wastewater treatment effluent requirements, membrane treatment technology is more and more widely used in the treatment of heavy metal wastewater.
• Evaporation treatment process
  It is mainly composed of preheating, evaporation, separation, condensation, crystallization and other parts. Preheating provides the heat needed for evaporation to the liquid and promotes the liquid to boil and vaporize; The evaporation chamber makes the gas-liquid two-phase completely separated. The vapor generated in the heating chamber carries a large amount of liquid foam, which can be separated from the vapor by self condensation or demister after reaching the evaporation chamber with large space. Generally, the condensed water contains few heavy metal ions, and most of them are crystallized into the residual liquid. According to the operating pressure, the evaporator can be divided into three types: normal pressure, pressure and pressure reduction.

The treatment process and effect are illustrated with examples

Taking an electroplating plant as an example, because the influent concentration of Ni2 +: 30mg / L, and it is chemical nickel, the plant is located in Taihu Lake Basin, and the emission requirements of heavy metal ions are relatively high, so it is necessary to achieve the requirement of zero emission. We adopt the process of two-stage physicochemical + ion exchange + ultrafiltration + two-stage ro + evaporation.

After two-stage physicochemical precipitation, the removal rate of nickel ion is more than 95%, and the effluent Ni2 + ≤ 1.5mg/l. Through the action of ultrafiltration and two-stage RO, the fresh water reuse rate is more than 80%, and the effluent Ni2 + ≤ 0.15mg/l. The concentrated water produced by RO enters the evaporation system, and the evaporated condensate is recycled to the production line together with the fresh water in front. The crystallization liquid is entrusted to a qualified unit outside for treatment. With this process, 97-98% of the water is recycled to the production line after treatment, and the rest is outsourced with the evaporation residue and dry sludge after pressure filtration, which basically meets the requirements of zero emission.