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Electroplating wastewater treatment

The main source of zinc in electroplating and metalworking wastewater is electroplated or pickled tow. The contaminants are transferred to the rinse water by a metal rinse process. The pickling process involves first immersing the metal (zinc or copper) in a strong acid to remove oxides from the surface, followed by immersion in a brightener containing strong chromic acid for brightness enhancement. The wastewater contains a large amount of heavy metal ions such as hydrochloric acid and zinc, copper, and organic brighteners, etc., which are highly toxic, and some contain highly toxic substances that are carcinogenic, teratogenic, and mutagenic, and are extremely harmful to humans. Therefore, the electroplating wastewater must be carefully recycled to eliminate or reduce its environmental pollution.
Electroplating wastewater treatment equipment consists of adjustment tank, dosing tank, reduction tank, neutralization reaction tank, pH adjustment tank, flocculation tank, inclined tube sedimentation tank, chamber filter press, clear water tank, air flotation reaction, activated carbon filter, etc. .
There are mainly the following methods.
1. Air floatation
       The air floatation method is to introduce air into the water to generate tiny bubbles. Due to the adhesion between the bubbles and the fine suspended matter, a flotation body is formed, and the floating effect of the bubbles is used to float up to the surface of the water to form foam or scum, thereby making the water The suspended matter is separated. According to the different ways of bubble generation, it can be divided into three types: inflatable air floatation, dissolved air flotation and electrolysis air floatation.
       The air floatation method is a new solid-liquid separation method instead of the precipitation method. In 1978, Shanghai Tongji University successfully applied the air floatation method to treat electroplating heavy metal wastewater. Subsequently, due to the continuous processing, the equipment is compact, the footprint is small, and it is easy to automate and has been widely used.
       The air floatation solid-liquid separation technology has strong adaptability and can treat chrome-plated wastewater, chromium-containing passivation wastewater and mixed wastewater. Not only heavy metal hydroxides can be removed, but other suspended solids, emulsified oils, surfactants, and the like can be removed. The principle of air floatation for treating chrome-plated wastewater is: under acidic conditions, ferrous sulfate and hexavalent chromium are subjected to redox reaction, then flocculation is produced under alkaline conditions, and flocs are floated under the action of numerous fine bubbles. The water surface makes the water clear.
2. Ion exchange method
       The ion exchange method is mainly a method in which an exchange ion in an ion exchange resin is exchanged with some ions in electroplating wastewater to remove it, and the wastewater is purified.
       Domestic treatment of electroplating wastewater by ion exchange technology began in the 1960s. By the end of the 1970s, this technology has been greatly developed due to the urgent need to solve environmental pollution problems. It has now become the treatment of electroplating wastewater and recycling. One of the effective means of certain metals is also an important part of the closed circuit cycle of some plating electroplating wastewater. However, the investment cost of using the ion exchange method is very high, and the system design and operation management are complicated. Generally, small and medium-sized enterprises are difficult to adapt, and often fail to achieve the expected results due to poor maintenance and management. Therefore, they have been promoted and applied. Certain restrictions.
       At present, domestic electroplating wastewater containing chromium and nickel is treated by ion exchange method, which has relatively mature experience in design, operation and management. After treatment, the water can reach the discharge standard, and the effluent water quality is good, and it can generally be recycled. The regenerated eluate after resin exchange adsorption saturation can be reused in the plating tank after being adjusted and purified by the electroplating process, and the closed loop is basically realized. In addition, the ion exchange method can also be used to treat wastewater containing copper, zinc, and gold.
3. Electrolysis
       The electrolysis method mainly causes the harmful substances in the wastewater to undergo oxidation and reduction reactions on the anodes and cathodes through the electrolysis process to be converted into harmless substances, or to chemically react with the harmful substances in the wastewater by using the electrode oxidation and reduction products to generate The water-insoluble precipitate is then separated or recovered by electrolytic reaction. In the 1960s, domestic electroplating of chromium-containing wastewater was started. In the late 1970s, experimental studies were carried out on wastewater containing silver and copper, and silver, copper and other metals were recovered, and good results were obtained.
       Electrolytic treatment of electroplating wastewater is generally used in small and medium-sized plants. Its main features are that it does not require the treatment of pharmaceuticals, the process is simple, the operation is convenient, and it takes up less production space. At the same time, due to the high purity of the recovered metals, it is very useful for recycling precious metals. Good economic benefits. However, when the amount of treated water is large, the electricity consumption of the electrolysis method is large, and the amount of iron plates consumed is also large, and the separated sludge is not easy to handle as the chemical treatment method, so it has been less used.
4. Extraction method
       The extraction method uses a solvent which is insoluble in water and can dissolve a substance (called a solute or an extract) in water, and is added to the waste water to dissolve the solute in the solvent, thereby separating or recovering a certain substance from the waste water. method. The extraction process consists of three main processes of mixing, separation and recovery.