Introduction to circuit board wastewater
Printed circuit board (PCB), also known as printed circuit board, is an indispensable and important component in various electronic products. Printed circuit board is the largest industry in the electronic component industry, and it is widely used in various electronic devices such as mainframe computers, office and personal computers, household appliances, entertainment appliances and their auxiliary products. In recent years, the average growth rate of the world’s printed circuit board industry has reached 8.7%, and my country’s growth rate is as high as 14.4%.
In the production process of circuit boards, a variety of chemical materials with different properties are used, which constitutes the complexity of waste water and waste liquid generated in the production process. The waste water and waste liquid produced by different production processes contain pollutants of different properties, including heavy metal compounds, synthetic macromolecular organic compounds and various organic additives.
The main substances in the circuit board complex wastewater that can form complexes with heavy metals such as copper are EDTA, NH3, tartrate, citrate, CN, etc. These substances and copper will form relatively stable complex copper ions, affecting Copper removal.
Removal method of copper in complex copper wastewater
2.1 Sulfide precipitation method
S2- dissociated by sodium sulfide and Cu2+ form insoluble CuS with a small solubility product (KSP=6.3×10-36). Compared with Cu(NH3)42+ (its stability constant is 2.09×1013), the stability of CuS Therefore, the added S2- will compete for Cu2+ from Cu(NH3)42+, prompting Cu(NH3)42+ to break the complex and decompose, and finally reduce the concentration of copper ions in the wastewater, and complete the treatment of complex copper wastewater. Purify. In order to achieve a good copper removal effect, the addition amount of sodium sulfide should be slightly more than the theoretical calculation value; Fe2+ mainly plays a coagulation role, the purpose is to make the insoluble CuS fine particles agglomerate and increase, and accelerate the precipitation; the control of pH value is to meet the The coagulation reaction conditions of the coagulant; the length of the settling time has an impact on the effluent quality and economic factors.
2.2 Fenton oxidation method
The complexation process between the complexing agent and the metal ion is a process in which the complexing agent ligand replaces the water molecules around the metal ion (actually hydrated metal ion) to form a ligand and a coordination compound. In circuit board complexed copper wastewater, the stability of complexing agent is determined by the stability of metal ions and organic acid ligands. Fenton reagent is a strong oxidant, which can oxidatively destroy the chelate bond of Cu-EDTA, dissociate copper from a complex state to a free state, and complete the complex breaking process. Free copper can form copper hydroxide precipitation under alkaline conditions (pH=8). In order to speed up the sedimentation rate, the Fe2+ added for the second time mainly plays the role of coagulation. The purpose is to make the copper hydroxide coagulate and grow up. The agglomerated particles are further flocculated and enlarged, and the sedimentation speed is accelerated.
2.3 Coagulation method
By increasing the pH value of the wastewater, the copper ions in the wastewater can be precipitated by Cu(OH)2, but at this time the precipitate is in the state of fine suspended particles, which needs to be compressed by the coagulation reaction. The principle of bridging and other functions makes the fine pollutants agglomerate and grow up, and then precipitate and separate.
Comparative analysis and discussion
The sulfide precipitation method and the Fenton oxidation method can achieve the ideal treatment effect, but the coagulation method fails to discharge the complex copper wastewater up to the standard. According to the process conditions of sulfide precipitation method and Fenton oxidation method and the water quality characteristics of complex copper wastewater (Cu(NH3)42+ wastewater is alkaline, Cu-EDTA wastewater is acidic), in order to save the amount of acid and alkali for pH adjustment, Cu(NH3)42+ wastewater is alkaline. The sulfide precipitation method should be used for the (NH3)42+ complex wastewater, and the Fenton oxidation method should be used for the Cu-EDTA complex wastewater.
From the perspective of process operation management, the coagulation process is the simplest, followed by the sulfide precipitation method, and the Fenton oxidation method is the most complicated. It is difficult to control the amount of sodium sulfide added during the operation of the sulfide precipitation method. If the added amount is too small, the copper removal will not be complete; if it is too large, it is easy to produce odorous gas hydrogen sulfide, resulting in secondary pollution. Fenton oxidation of the Fenton oxidation method is the key link of the process, and the Fenton oxidation process conditions are strict, which also brings certain difficulties to the operation process.
In terms of treatment cost, the coagulation method has the lowest cost, followed by the sulfide precipitation method, and the Fenton oxidation method is the highest.
(1) Cu(NH3)42+ complex wastewater should use sulfide precipitation method. In this process, the appropriate addition amount of Na2S should be strictly controlled, so as not to cause incomplete copper removal due to insufficient addition of Na2S, or secondary pollution (generating odorous gas hydrogen sulfide) due to excessive addition of Na2S.
(2) The Fenton oxidation method should be used for the Cu-EDTA complex wastewater. The process operation is complicated and the cost is high, so the operation specification should be strictly enforced and the operation management should be strengthened to ensure the treatment effect and reduce the treatment cost.
(3) Although the coagulation method has the advantages of simple operation and low treatment cost, it is difficult to achieve the ideal treatment effect of complex copper wastewater by using this process alone.