Electroplating wastewater treatment process in electroplating plant

Waste water and liquid discharged from electroplating plants contain a large number of metal ions such as chromium, pickaxe, nickel, cyanogen, acid, alkali, and generally organic additives. Some metal ions exist as simple cations, some as acid radicals, and some as complex complexing ions. Electroplating wastewater treatment commonly used neutralization precipitation method, neutralization coagulation precipitation method, oxidation method, reduction method, barium salt method, ferrite method and other chemical methods. The chemical method has simple equipment, low investment and wide application, but the sludge often needs further treatment.

At present, there are chemical, biological, physicochemical and electrochemical methods for treating electroplating wastewater in China.

Chemical method

Chemical method relies on REDOX reaction or neutralization precipitation reaction to decompose toxic and harmful substances into non-toxic and harmless substances, or directly remove heavy metals from wastewater through precipitation or air flotation.

1. Precipitation method

(1) neutralization and precipitation method. In the wastewater containing heavy metals, alkali is added to neutralize the heavy metals, and the heavy metals are separated into water-insoluble hydroxide precipitation forms. Neutralization and sedimentation method is a common method to treat wastewater.

(2) sulfide precipitation method. Removal of heavy metal ions from wastewater by addition of sulfide to form sulfide precipitation. Compared with the neutralization and precipitation method, the advantages of sulfide precipitation method are as follows: the solubility of heavy metal sulfide is lower than its hydroxide solubility, the reaction pH value is between 7 and 9, the treated wastewater is generally not neutralized, the treatment effect is better. However, the disadvantages of sulfide precipitation method are: sulfide precipitation particles are small, easy to form colloid, sulfide precipitation residual in water, acid formation gas, may cause secondary pollution.

(3) chelation precipitation method. High molecular heavy metal precipitator (DTCR) rapidly reacts with Hg2+, Cd2+, Cu2+, Pb2+, Mn2+, Ni2+, Zn2+ and Cr3+ and other heavy metal ions in wastewater at room temperature to generate chelating salt of insoluble water, and then adds a small amount of organic or (and) inorganic flocculant to form flocculent precipitation, so as to achieve the purpose of capturing and removing heavy metal. DTCR series of agents can remove multiple heavy metal ions at the same time in the treatment of electroplating wastewater. It can also play a good role in the removal of heavy metal ions in the form of complex salts. The removal of colloidal heavy metals is not affected by coexisting salts.

2. Oxidation method

By adding oxidants, the toxic substances in electroplating wastewater are oxidized to non-toxic or low toxic substances, which are mainly used to treat CN-, Fe2+, Mn2+ low-price ions in wastewater, and various organic substances that cause color, smell and smell as well as pathogenic microorganisms. For example, when treating cyanide-containing wastewater, hypochlorite is commonly used to oxidize the cyanide-containing ions under alkaline conditions, making them decompose into low-toxic cyanate, and then further degrade into non-toxic carbon dioxide and nitrogen.

3. Chemical reduction

The most typical treatment of electroplating wastewater by chemical reduction is the treatment of wastewater containing chromium. The method is to add reducing agent in wastewater FeS04, NaHS03, Na2S03, S02 or iron powder, etc., make the Cr (Ⅵ) reduction of Cr (III), and then adding NaOH or lime precipitation separation. The advantages of this method are simple equipment, low investment and large treatment capacity, but the secondary pollution caused by sediment sludge should be prevented.

4. Neutralization method

Through the acid-base neutralization reaction, the acidity and alkalinity of electroplating wastewater are adjusted so that the acidity and alkalinity range of electroplating wastewater is neutral or close to neutral or suitable for the next step treatment.

5. Air flotation

As a new technology of electroplating wastewater treatment, air flotation is developed in recent years. Its basic principle is to use high pressure water pump pressure to several injection solvent tank, atmospheric pressure and water blending quickly dissolved air water, dissolved air water by dissolved air release into the pool, because suddenly reduced pressure, the air dissolved in water to form a large number of tiny air bubbles, and preliminary electroplating wastewater treatment of condensed material stick together, make its relative density less than water and float to the surface of the water become dross, so that the waste water purification.

Biological methods

Biological treatment is a new technique for treating electroplating wastewater. Some microbial metabolites can change the valence state of heavy metal ions in the wastewater. Meanwhile, the microbial flora itself has strong biological flocculation and electrostatic adsorption, which can adsorb metal ions and make heavy metals enter the sludge cake after solid-liquid separation, so that the wastewater can be discharged or reused according to the standard.

1. Biosorption method

Biological adsorbents are derivatives prepared by bodies or organisms that have the ability to separate metals from solutions. Biological adsorbents 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 morphology, the presence of other ions and chelates, and the pretreatment of adsorbents. Biosorption technology has certain advantages in the treatment of heavy metal pollution. Under low content conditions, biosorbent can selectively adsorb heavy metals, which are less affected by the interference of calcium and magnesium ions in aqueous solution. This method has high efficiency, no secondary pollution and can effectively recover some precious metals. However, the growth environment is not easy to control, often due to the change of water quality and a large number of poisoning and death.

2. Biological flocculation

Bioflocculation is a method to flocculate and precipitate by microorganisms or metabolites produced by microorganisms. Microbial flocculant is a kind of natural macromolecular material produced by microorganisms with high efficiency flocculation. Its main components are glycoprotein, mucopolysaccharide, cellulose, protein and nucleic acid. It has high electric charge or strong hydrophilicity and hydrophobicity, and can adsorb multiple colloidal particles with particles through ionic bond, hydrogen bond and van der Waals force at the same time, creating bridging phenomenon between particles, forming a network three-dimensional structure and settling down. At present, there are about a dozen kinds of biological flocculants that flocculate heavy metals. The amino and hydroxyl groups in the biological flocculants can form stable chelates and precipitate with Cu 2+, Hg2+, Ag+, Au2+ and other heavy metal ions. This method is safe, convenient and non-toxic, does not produce secondary pollution, has a wide range of flocculation, high flocculation activity, fast growth, extensive flocculation conditions, mostly not affected by ion strength, pH value and temperature, and is easy to achieve industrialization.

3. Biochemical method

Biochemical method is to remove soluble ions into insoluble compounds by direct chemical reaction between microorganisms and metal ions. Its advantages are: strong selectivity, adsorption capacity, do not use chemical agents. The metal content in the sludge is high, the secondary pollution is obviously reduced, and the heavy metal in the sludge is easy to recover, the recovery rate is high. However, the disadvantage is that the reaction efficiency of functional bacteria and metal ions in wastewater is not high, and the culture medium consumption of bacteria species is large, and the treatment cost is high.

Physical and chemical method

Physicochemical method is the use of ion exchange or membrane separation or adsorbent method to remove impurities contained in electroplating wastewater, which is widely used in industry and usually used in conjunction with other methods.

1. Ion exchange method

Ion exchange method is a method to separate harmful substances from wastewater by using ion exchange agent. The most commonly used exchange agent is ion exchange resin, which can be used repeatedly after acid and base regeneration after saturation. Ion exchange is achieved by ion exchange between the freely moving ions carried by the agent itself and the ions in the treated solution. In most cases, ions are adsorbed and then exchanged, which has the dual functions of adsorption and exchange. For waste water containing chromium and other heavy metal ion, anion exchange resin to remove Cr (VI) are available, and with cation exchange resin to remove Cr (Ⅲ), iron, copper plasma. It is generally used to treat waste water with low content of harmful substances, which has the advantages of recycling, chemical harm for profit and recycling water, 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 semi-permeable membrane as barrier layer and by means of selective osmosis of 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 clean production of electroplating. For comprehensive electroplating wastewater, after simple physical and chemical treatment, the membrane separation technology can reuse most of the water, and the recovery rate can reach 60% ~ 80%, reducing the total discharge of sewage and reducing the pollutants discharged into the water.

3. Evaporation concentration method

This method is a kind of treatment method that evaporates the electroplating wastewater so that the heavy metal wastewater can be concentrated and recycled, which is generally applicable to the electroplating wastewater containing chromium, copper, silver, nickel and other heavy metals. At present, it is generally used as an auxiliary treatment for other methods. It has the disadvantages of large energy consumption, high cost, large floor area and high operating cost.

4. Activated carbon adsorption method

Activated carbon adsorption is an economical and effective method for the treatment of electroplating wastewater. It is characterized by moderate treatment regulation, safe operation, and deep purification of the treated water can be reused. However, this method has some problems such as the complexity of activated carbon regeneration and the fact that the regenerated liquid cannot be directly used back to the plating bath.

Electrochemical method

1. Electrolysis

Electrolysis is the treatment or recovery of heavy metals by electrolysis, which is generally used in electroplating wastewater with high precious metal content or single. Electrolysis method dealing with Cr (VI), is to use iron to make electrodes, anode melting iron can produce ferrous ions in acidic conditions to Cr (VI) reduction of Cr (Ⅲ), Cr (Ⅵ) on the cathode direct reduction of Cr (Ⅲ), due to consume in the process of electrolytic hydrogen ions, water remaining hydroxyl ions from acidic to alkaline solution, and generate the chromium and iron hydroxide precipitation to remove chromium. Electrolysis can remove many kinds of metal ions at the same time, which has the advantages of good purification effect, small amount of sludge, small area, etc., but it consumes more electricity and steel, so it has been seldom used at present.

2. Galvanic cell method

Using granular carbon, cinder or other conductive inert material as cathode and iron scrap as anode, the conductive electrolyte in the waste water plays a conductive role to form a galvanic cell. In recent years, more and more attention has been paid to iron - carbon micro-electrolysis in the treatment of electroplating wastewater.

3. Electrodialysis

Electrodialysis is a kind of membrane separation technology. It is the negative and cation exchange membrane arranged alternately between the positive and negative electrodes, and separated by a special partition, under the action of electric field, with potential difference as the driving force, using the choice of ion exchange membrane permeability to separate electrolyte from solution, so as to achieve the concentration, desalination, refinement and purification of electroplating wastewater.

4. Electrocoagulation air flotation USES soluble anode (Fe, AI, etc.) materials to generate a large number of cations such as Fe2+, Fe3+, Al3+, etc., and generates Fe(OH)2, Fe(OH)3, AI(OH)3 and other sediments through flocculation 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 floating carriers to float together with flocculated wastes. A large number of flos float quickly under the abundant microbubbles to purify water.

The conventional treatment technology of electroplating wastewater in China has been relatively mature, and the modern biological treatment of electroplating wastewater is a promising wastewater treatment technology without secondary pollution. Membrane treatment technology will play an important role in electroplating wastewater treatment because of its high separation efficiency and recovery of heavy metals. At the same time, it is also the fundamental method to solve the pollution of electroplating wastewater by popularizing clean production technology, reducing the sewage discharge from every link of electroplating production and changing "passive treatment" to "active treatment".

In the treatment of electroplating wastewater, a single method is often difficult to meet the ideal requirements, and various combination methods will gradually replace the single treatment method. According to the development trends at home and abroad, it tends to be chemical method, supplemented by other combination processing technology and automatic detection control means. It is a developing direction to develop multifunctional combination processor. It can not only treat mixed electroplating wastewater with complex composition, but also miniaturize the process and equipment, thus saving floor space and engineering investment.