The wastewater and waste liquid discharged from electroplating plants contain a large number of metal ions, such as chromium, pickaxe, nickel, cyanide, acid, alkali, and generally contain organic additives. Some metal ions exist in the form of simple cations, some in the form of acid radical anion, and some in the form of complex complex ions. Chemical methods such as neutralization sedimentation, neutralization coagulation sedimentation, oxidation, reduction, barium salt and ferrite are commonly used in the treatment of electroplating wastewater. Chemical method has simple equipment, less investment and wide application, but it often leaves sludge that needs further treatment.
The components of wastewater produced in electroplating production are very complex. In addition to cyanide (cn-) and acid-base, heavy metals are potentially harmful sewage in the electroplating industry. These substances seriously endanger the environment and human health. The main sources of electroplating wastewater are:
1. Plating cleaning water (the main source of waste water). In addition to heavy metal ions, the wastewater also contains a small amount of organic matter, which is low in content but large in quantity.
2. Discharge of washing water and waste plating solution after plating solution filtration. This part of wastewater is small in quantity, but high in content and pollution.
3. Waste liquid discharged by "running, emitting, dripping and leaking" caused by process operation, equipment and process flow.
4. Waste water from washing equipment, floors, etc.
The treatment of electroplating wastewater is widely valued at home and abroad, and a variety of treatment technologies have been developed to eliminate and reduce the discharge of pollutants through measures such as transforming toxic treatment into non-toxic and harmful treatment into harmless treatment, recycling precious metals, and water recycling. With the rapid development of electroplating industry and the increasing requirements of environmental protection, electroplating wastewater treatment has begun to enter the stage of cleaner production process, total amount control and the integration of circular economy. Resource recycling and closed-circuit circulation are the mainstream direction of development.
Chemical method
Chemical method relies on oxidation-reduction reaction or neutralization precipitation reaction to decompose toxic and harmful substances into non-toxic and harmless substances, or directly remove heavy metals from wastewater by precipitation or air flotation.
1. Precipitation method
(1) Neutralization precipitation method. Alkali is added to the wastewater containing heavy metals for neutralization reaction, so that the heavy metals can be separated in the form of precipitation of hydroxides insoluble in water. Neutralization precipitation method is a common method of wastewater treatment because of its simple operation.
(2) Sulfide precipitation method. Adding sulfide to make heavy metal ions in wastewater form sulfide precipitation and remove it. Compared with neutralization precipitation method, the advantages of sulfide precipitation method are: the solubility of heavy metal sulfide is lower than that of its hydroxide, and the reaction pH value is between 7 and 9. The treated wastewater generally does not need neutralization, and the treatment effect is better. However, the disadvantages of sulfide precipitation method are: sulfide precipitation particles are small and easy to form colloids. Sulfide precipitation remains in water and generates gas in case of acid, which may cause secondary pollution.
(3) Chelate precipitation method. Through the rapid reaction of high molecular heavy metal capture precipitator (DTCR) with heavy metal ions such as hg2+, cd2+, cu2+, pb2+, mn2+, ni2+, zn2+ and cr3+ in wastewater at room temperature, water insoluble chelate salt is generated, and then a small amount of organic or (and) inorganic flocculants are added to form flocculent sedimentation, so as to achieve the purpose of capturing and removing heavy metals. DTCR series reagents can remove a variety of heavy metal ions at the same time in the treatment of electroplating wastewater. It can also play a good removal effect in the case of heavy metal ions in the form of complex salts. The removal of colloidal heavy metals is not affected by coexisting salts, and has a good development prospect.
2. Oxidation method
By adding oxidants, toxic substances in electroplating wastewater are oxidized to non-toxic or low toxic substances, which are mainly used to treat cn-, fe2+, mn2+ low valence ions in wastewater and various organic substances and pathogenic microorganisms that cause chromaticity, vagueness and smell. For example, when treating cyanide containing wastewater, hypochlorite is often used to oxidize the cyanide ion in it under alkaline conditions, so that it can be decomposed into low toxic cyanate, and then further degraded into non-toxic carbon dioxide and nitrogen.
3. Chemical reduction method
Chemical reduction in electroplating wastewater treatment is a typical treatment of chromium containing wastewater. The method is to add reducing agents fes04, nahs03, Na2S03, S02 or iron powder into the wastewater to reduce Cr (Ⅵ) to Cr (III), and then add NaOH or lime milk for precipitation and separation. This method has the advantages of simple equipment, less investment and large treatment capacity, but it is necessary to prevent secondary pollution caused by sediment and sludge.
4. Neutralization method
Through acid-base neutralization reaction, adjust the pH of electroplating wastewater to make it neutral or close to neutral or the pH range suitable for next treatment. It is mainly used to treat the acid pickling wastewater of electroplating plant.
5. Air flotation method
Air flotation is a new process developed in recent years as a technology for the treatment of electroplating wastewater. Its basic principle is to use a high-pressure water pump to pressurize the water to several atmospheres and inject it into the dissolving tank, so that the gas and water are mixed into dissolved gas water. The dissolved gas water enters the pool through the dissolved gas releaser. Due to sudden decompression, the air dissolved in the water forms a large number of micro bubbles, which adhere to the condensate produced by the preliminary treatment of electroplating wastewater, making its relative density less than water and float to the water surface as scum for removal, so that the wastewater can be purified.
Biological method
Biological treatment is a new technology to treat electroplating wastewater. Some microbial metabolites can change the valence state of heavy metal ions in wastewater. At the same time, the microbial flora itself has strong biological flocculation and electrostatic adsorption, which can adsorb metal ions, so that heavy metals can enter the bacterial mud cake after solid-liquid separation, so that the wastewater can be discharged or reused up to the standard.
1. Biosorption method
Derivatives prepared by objects or organisms that have the ability to separate metals from solutions are called biosorbents. Biosorbents are mainly bacteria, algae and some extracts. The adsorption mechanism of heavy metals by microorganisms depends on many physical and chemical factors, such as light, temperature, pH value, heavy metal content and chemical form, other ions, the existence of chelating agents and the pretreatment of adsorbents. Biosorption technology has certain advantages in the treatment of heavy metal pollution. Under the condition of low content, biosorbent can selectively adsorb heavy metals, which is less affected by the interference of calcium and magnesium ions in aqueous solution. This method has high treatment efficiency, no secondary pollution, and can effectively recover some precious metals. However, the biological growth environment is not easy to control, and a large number of poisoning deaths are often caused by changes in water quality.
2. Biological flocculation
Bioflocculation is a decontamination method that uses microorganisms or metabolites produced by microorganisms to flocculate and precipitate. Microbial flocculants are natural macromolecules produced by microorganisms with high flocculation efficiency. Its main components are glycoproteins, mucopolysaccharides, cellulose, proteins and nucleic acids. It has high charge or strong hydrophilicity and hydrophobicity, and can adsorb multiple colloidal particles at the same time with particles through ionic bond, hydrogen bond and van der Waals force, resulting in bridging phenomenon between particles, forming a network three-dimensional structure and precipitation. At present, there are about a dozen varieties of bioflocculants that flocculate heavy metals. The amino and hydroxyl groups in bioflocculants can form stable chelates with heavy metal ions such as Cu 2+, hg2+, ag+, au2+ and precipitate. This method is safe, convenient, non-toxic, does not produce secondary pollution, has a wide range of flocculation, high flocculation activity, fast growth, extensive flocculation conditions, most of which are not affected by ionic strength, pH value and temperature, and is easy to realize industrialization.
3. Biochemical method
Biochemical method is to convert soluble ions into insoluble compounds and remove them through direct chemical reaction between microorganisms and metal ions. Its advantages are: strong selectivity, large adsorption capacity and no use of chemical agents. The metal content in the sludge is high, the secondary pollution is significantly reduced, and the heavy metals in the sludge are easy to recover, and the recovery rate is high. However, its disadvantage is that the reaction efficiency between functional bacteria and metal ions in wastewater is not high, and the consumption of culture medium for cultivating bacteria is large, and the treatment cost is high.
Physicochemical method
Physicochemical method is to remove the impurities contained in electroplating wastewater by ion exchange, membrane separation or adsorbent. It is widely used in industry and is usually used in combination with other methods.
1. Ion exchange method
Ion exchange method is a method that uses ion exchanger to separate harmful substances in wastewater. The commonly used exchanger is ion exchange resin, which can be reused after acid-base regeneration after saturation. Ion exchange is realized by the free moving ions carried by the exchanger itself and the ions in the treated solution through ion exchange. In most cases, ions are first adsorbed and then exchanged, which has dual effects of adsorption and exchange. For wastewater containing heavy metal ions such as chromium, anion exchange resin can be used to remove Cr (VI), and cation exchange resin can be used to remove Cr (III), iron and copper plasma. Generally used to treat wastewater with low content of harmful substances, it has the advantages of recycling, turning harm into benefit, recycling water and so on, but it has high technical requirements and large one-time investment.
2. Membrane separation method
Membrane separation refers to the separation of different components in the mixture under the action of energy, content or chemical potential difference by using the semi permeable membrane as the barrier layer and with the help of the selective permeation of the membrane. Membrane separation technology can be used to recover heavy metals and water resources from electroplating wastewater, reduce or eliminate its pollution to the environment, and realize cleaner production of electroplating. Membrane separation technology can be used to realize closed-circuit circulation of electroplating wastewater with high added value, such as gold, silver, nickel, copper, etc., and produce good economic benefits. For the comprehensive electroplating wastewater, after simple physical and chemical treatment, the membrane separation technology can be used to reuse most of the water, with a recovery rate of 60% - 80%, reducing the total discharge of sewage and reducing the pollutants discharged into the water body.
3. Evaporation concentration method
This method is a treatment method that evaporates the electroplating wastewater to concentrate and recycle the heavy metal wastewater. It is generally applicable to the treatment of electroplating wastewater containing chromium, copper, silver, nickel and other heavy metals. At present, it is generally used as an auxiliary means of other methods. It has the disadvantages of high energy consumption, high cost, large floor area and high operating costs.
4. Activated carbon adsorption method
Activated carbon adsorption is an economical and effective method to treat electroplating wastewater, which is mainly used for chromium and cyanide containing wastewater. It is characterized by mild treatment and regulation, safe operation, and the deeply purified treated water can be reused. However, this method has the problems that the regeneration of activated carbon is complex and the regeneration solution cannot be directly reused in the plating tank. The adsorption capacity is small, and it is not suitable for wastewater with high harmful content.
Electrochemical method
1. Electrolytic method
Electrolysis is the treatment or recovery of heavy metals by electrolysis, which is generally used in electroplating wastewater with high or single precious metal content. Electrolytic treatment of Cr (VI) uses iron as an electrode, and the ferrous ions produced by the continuous dissolution of the iron anode can reduce Cr (VI) to Cr (III) under acidic conditions, and Cr (VI) can be directly reduced to Cr (III) on the cathode. Due to the consumption of hydrogen ions in the electrolysis process, the remaining hydroxide ions in the water make the solution from acidic to alkaline, and generate chromium and iron hydroxide precipitation to remove chromium. Electrolysis can remove a variety of metal ions at the same time, and has the advantages of good purification effect, less sludge, small floor area and so on. However, it consumes more electric energy and steel, which has been less used at present.
2. Galvanic cell method
With granular carbon, cinder or other conductive inert substances as the cathode and iron filings as the anode, the conductive electrolyte in the wastewater plays a conductive role to form a primary battery. The purpose of wastewater treatment is achieved through the primary battery reaction. In recent years, iron carbon micro electrolysis technology has received more and more attention in the treatment of electroplating wastewater.
3. Electrodialysis
Electrodialysis technology is a kind of membrane separation technology. It is to arrange anion and cation exchange membranes alternately between the positive and negative electrodes, and separate them with special partitions. Under the action of electric field, with the potential difference as the driving force, the electrolyte is separated from the solution by using the selective permeability of ion exchange membrane, so as to realize the concentration, desalination, refining and purification of electroplating wastewater.
4. The electrocoagulation air flotation method uses soluble anode (Fe, AI, etc.) materials to generate a large number of cations such as fe2+, fe3+, al3+, and generates precipitates such as Fe (OH) 2, Fe (OH) 3, AI (OH) 3 through flocculation, so as to remove pollutants in water. At the same time, a large number of H2 microbubbles are generated on the cathode and a large number of O2 microbubbles are generated on the anode. These bubbles are used as air flotation carriers to float together with flocculated dirt. A large number of flocs float rapidly under the rich microbubbles, so as to achieve the purpose of purifying water quality.
The conventional treatment technology of electroplating wastewater in China has been relatively mature. Modern biological treatment of electroplating wastewater is a very promising wastewater treatment technology, and does not produce secondary pollution. The key is to use new technology for advanced treatment to further improve the effluent quality. Membrane treatment technology will play an important role in electroplating wastewater treatment in the future because of its high separation efficiency and the recovery of heavy metals. At the same time, through the promotion of cleaner production technology, we can reduce the sewage discharge from all links of electroplating production, and change "passive treatment" into "active treatment", which is also the fundamental method to solve the pollution of electroplating wastewater.
