Views: 290 Author: Site Editor Publish Time: 2024-05-22 Origin: Site
Electroplating can change the surface properties of metal or non-metal products, such as corrosion resistance, conductivity, wear resistance, weldability, etc., and is widely used in machinery manufacturing, electronic and electrical, light industry, etc. The electroplating industry has become a heavily polluting industry due to the use of large amounts of strong acids, strong alkalis, solutions containing heavy metals, including toxic and harmful chemicals such as chromic anhydride, and the discharge of wastewater, waste gas, and waste residue that pollute the environment and endanger human health.
Table of contents（Click to go to where you want to see）
2. How is electroplating wastewater generated?
3. Properties of electroplating wastewater
4. Classification of electroplating wastewater
5. Hazards of electroplating wastewater
5.1 Cyanide
5.2 Hexavalent Chromium and Trivalent Chromium
5.9 Acids, bases and their salts
5.10 Other
6. How to treat electroplating wastewater
6.1 chemical method
6.1.1 Chemical reduction method
6.1.2 Electrochemical reduction method
6.1.3 Removal and recovery of heavy metal ions in wastewater by electrodeposition
6.1.4 Alkaline chlorination method
6.1.5 Neutralization method
6.1.6 Hydroxide precipitation method
6.2. Physical and chemical methods
6.2.1 Ion exchange method
6.2.2 Electrocoagulation
6.2.3 Adsorption method
6.2.4 Membrane separation technology
6.2.5 Supercritical water oxidation
6.3 Physical method
6.3.1 Evaporation concentration method
6.3.2 Crystallization
6.3.3 Air flotation method
Electroplating wastewater is the general term for plating liquid, rinsing wastewater, and releasing various waste liquids during the production process of electroplating plants. Electroplating wastewater varies depending on the electroplating factory's production process and coating type. The sources of electroplating wastewater can be roughly divided into pre-electroplating wastewater, plating rinse wastewater, post-treatment wastewater, and electroplating waste liquid.
Many electroplating factories produce wastewater during production and require wastewater treatment. However, before treating electroplating wastewater, it is necessary to fully understand the source and composition of electroplating wastewater and then design targeted electroplating wastewater treatment solutions. Let's take a look at the origin of electroplating wastewater:
The pre-treatment process mainly includes polishing, polishing, rolling, and sandblasting, and the chemical treatment mainly includes erosion, degreasing, and rust removal. Electrochemical treatment includes electrochemical corrosion and oil removal.
Conventionally, the primary source of heavy metal pollution in electroplating operations is the production of plating rinse water, while the main components of the electroplating solution in wastewater are complexing agents and metal salts. To improve the performance of the coating, some organic compounds need to be added to the plating solution. It can be seen that the rinsing wastewater of electroplated parts not only contains heavy metal ions but also contains a small amount of organic matter. Generally speaking, the ion types and emissions of heavy metals in plating rinse wastewater are affected by the physical state of electroplated parts, the management level of electroplating operations, and the formulation of the electroplating solution.
Post-plating treatment usually includes other unique surface treatments, such as passivation after rinsing and stripping poor coatings. During the post-treatment process, it is easy to produce a large amount of wastewater containing heavy metals such as niobium and copper, acid and alkali substances such as sodium carbonate, sulfuric acid, and phosphoric acid, as well as organic substances such as acetic acid and glycerin.
In electroplating operations, such as stripping, electroplating, passivation, etc., long-term accumulation and use of ordinary plating solutions can quickly produce a large amount of metal ions, or the additives may damage the quality of the passivation layer. Therefore, to control the impurities in the bath liquid, most projects will waste a part of it, and some factories will completely waste it. This waste liquid contains more heavy metal ions, increasing wastewater treatment's difficulty.
After understanding the source and composition of electroplating wastewater, combining various electroplating wastewater treatment processes is necessary according to the actual situation. Yosun Environmental Protection arranges professional engineering designers to develop detailed solutions to achieve zero discharge of electroplating wastewater and effectively improve wastewater utilization.
According to the different functional requirements of electroplating products, the bath liquid components of the process tanks vary.
Generally, in addition to the decorative protective layer, there is also electroplating to improve the hardness and wear resistance; Electroplating to enhance the electrical conductivity, magnetic permeability, and reflective properties of plated parts; electroplating to prevent local carburization and nitriding and to repair the size of parts—restorative plating, etc.
Due to the different functional requirements of plating parts, there are also many plating types, bath components, operating methods, process conditions, etc. Accordingly, the pollutants brought into the electroplating wastewater have become more complicated. However, the primary contaminants in wastewatesewageous metal ions; common ones include chromium, copper, nickel, lead, aluminum, gold, silver, cadmium, iron, etc., followed by acids and alkalis, such as sulfuric acid, hydrochloric acid, Nitric acid, phosphoric acid, sodium hydroxide, sodium carbonate, etc.; some electroplating solutions also use other substances such as pigments, most of which are organic substances. In addition, impurities such as grease, oil, oxide scale, and dust rinsed off during the pretreatment process of the plated substrate are also brought into the electroplating wastewater, making the composition of the electroplating wastewater more complex.
Electroplating wastewater is generally classified according to the primary pollutants in the wastewater, such as cyanide-containing wastewater, chromium-containing wastewater, acid-containing wastewater, etc. When the wastewater comprises more than one major pollutant, such as cyanide cadmium plating, which contains both cyanide and cadmium, it is generally still classified according to one of the pollutants. When the same plating method has several working hours, it is also classified according to Different plating processes, which are further divided into subcategories. For example, copper-containing wastewater is divided into pyrophosphate copper plating wastewater, copper sulfate copper plating wastewater, etc.
When several different types of plating wastewater contain the same primary pollutant, such as chromium plating and passivation wastewater, when they are mixed, they are collectively called chromium-containing wastewater. If the system is established by quality, they are chromium-plating and passivation wastewater, respectively. Generally, when wastewater of different plating types and different primary pollutants are mixed, it is collectively called electroplating mixed wastewater.
Main harmful substances in electroplating wastewater
There are two main ways in which electroplating wastewater pollutes the environment. One is the discharge of electroplating waste liquid with a small amount and high concentration, and the other is the discharge of electroplating wastewater with a relatively low concentration. If electroplating waste liquid is discharged directly without treatment, it will often cause severe pollution. Although the concentration of electroplating wastewater is low, the consequences of environmental pollution caused by the two are the same, and the former has not been taken seriously in the past. Since electroplating factories are dispersed over a wide area, compared with other industries, the pollution diffusion area is relatively large, although the amount of wastewater is relatively tiny. Therefore, the pollution caused by it is difficult to control.
In the past, there was insufficient understanding of the consequences of environmental pollution caused by electroplating wastewater. Due to poor management and lax control, some situations have occurred, such as heavy metal contamination of soil in some areas, causing accumulated heavy metals in crops and cyanide-containing wastewater discharged into rivers and lakes. , resulting in reduced fishery production and even massive fish deaths. At the end of the 20th century, the management of electroplating wastewater in various parts of my country has been standardized. However, in recent years, due to the strengthening of the management of other organic pollutants, insufficient emphasis has been placed on the management of electroplating heavy metal wastewater. In addition, since the water sources, soil, and groundwater contaminated by electroplating wastewater are difficult to purify in the short term, electroplating wastewater should still be strictly managed and adequately handled.
The hazards of some significant pollutants in electroplating wastewater are briefly described.
Cyanide is an extremely toxic, especially under acidic conditions; it turns into highly toxic hydrocyanic acid. Hydrocyanic acid, HCN, or hydrogen cyanide, is a colorless liquid with a unique smell of bitter almonds. Easily soluble in water, alcohol, and ether. It is easily dispersed evenly in the air and combustible in the air. When hydrogen cyanide content in the air reaches 5.6% to 12.8%, it is explosive. Hydrogen cyanide is a gas; its aqueous solution is called hydrocyanic acid.
Hydrocyanic acid is highly toxic. The toxicity of cyanide is mainly caused by the cyanide ions it releases in the body. Cyanide ions can quickly combine with the ferric ions in cytochrome oxidase in the body, inhibiting the enzyme's activity and preventing tissues from utilizing oxygen.
The harm of cyanide to the human body is divided into two aspects: acute poisoning and chronic effects. Acute poisoning caused by cyanide is divided into three levels: mild, moderate, and severe. Mild poisoning manifests as eye and upper respiratory tract irritation, with a bitter almond smell and numbness of the lips and throat, followed by nausea, vomiting, tremors, etc.; moderate poisoning manifests as sighing breathing, and the skin and mucous membranes are often bright red. Other symptoms worsen; severe poisoning manifests as loss of consciousness, tonic and paroxysmal convulsions, up to opisthotonus, decreased blood pressure, urinary and fecal incontinence, often accompanied by cerebral edema and respiratory failure.
Chromium is an element widely present in the environment and an essential trace element for the human body. There are three types of chromium compounds: divalent, trivalent, and hexavalent. Hexavalent chromium and its compounds are soluble in water and are the most toxic. Trivalent chromium and divalent chromium are very toxic. All chromium compounds are toxic. Hexavalent chromium is the most toxic, followed by trivalent chromium, and divalent chromium is the least toxic. Hexavalent chromium is almost 100 times more toxic than trivalent chromium.
Chromium can invade the human body through the digestive tract, respiratory tract, skin, and mucous membranes. The toxicity of chromium to the human body includes systemic poisoning and irritation to the skin and mucous membranes, causing dermatitis, eczema, tracheitis, and rhinitis, causing morbid reactions and carcinogenic effects. For example, hexavalent chromium compounds can induce lung cancer and nasopharyngeal cancer and are harmful to human health. The lethal dose is 5 g. When the mass concentration of hexavalent chromium in water is 0.1 mg/L, it can cause apparent pathological tissue changes in animals; when it is 0.45 mg/L, it can cause the accumulation of chromium in the body. The average value of urinary chromium in humans should be less than 0.01 ~ 0.015 mg/L.
Chromic anhydride is a chemical raw material widely used in the electroplating industry. It is mainly used in chromium plating, passivation, and depleting processes. It exists in two forms of hexavalent chromium, CrO42- and Cr2O72-, depending on the pH in the wastewater. . Generally, after chemical reduction treatment, chromium hydroxide is precipitated into the sludge. A small amount of trivalent chromium salts exist in the wastewater. Some hexavalent chromium ions are electrochemically reduced to trivalent chromium ions in the process tank. , and enters the rinsing water with the discharged liquid.
Therefore, chromium-containing wastewater is one of the primary sewage sources in electroplating. It is impossible to eliminate chromate in electroplating without thoroughly reforming the process. Actively adopting low chromic acid passivation and low chromic acid chromium plating will significantly reduce the concentration of chromium-containing wastewater, but it is still difficult to meet the national emission standards without treatment; therefore, the treatment of chromium-containing wastewater is still something that electroplating plants should pay attention to—one of the wastewater treatment projects.
Cadmium and its compounds are not essential for the human body and are harmful to fish, plants, etc. After the environment is contaminated by cadmium, it can accumulate in organisms and enter the human body through the food chain, causing chronic poisoning.
Cadmium forms cadmium sulfide protein in the human body reaches the whole body through the blood and is selectively accumulated in the kidneys and liver. Cadmium hinders bone growth and metabolism, causing bone porous, atrophy, deformation, etc. Chronic cadmium poisoning mainly affects the kidneys and can also cause anemia. Cadmium can cause chromosome aberrations in warm-blooded animals and humans. Cadmium has a long biological half-life in the human body, reaching 10 to 25 years so that it will accumulate in the body. Acute ingestion of large amounts of cadmium-containing items (such as water containing 16mg/L cadmium) can cause severe vomiting, nausea, and abdominal pain; if a large amount (150 g CdCl2) is ingested at one time, it may cause acute renal failure and liver damage even death.
The cadmium plating layer has many excellent properties, so it is widely used in many aspects, such as aerospace, aviation, instrumentation, etc. However, cadmium and its compounds are toxic. Once cadmium is discharged into the environment, the pollution caused by it is difficult to eliminate. People have been working hard to find other coatings to replace the cadmium coating in recent years, and significant progress has been made. Cadmium-containing wastewater generated by cadmium plating must be strictly controlled and treated to prevent cadmium and its compounds from polluting the environment.
Mercury is mercury, which is a liquid metal. The specific gravity is 13.6, the melting point is -39.3℃, and the boiling point is 357℃. Mercury evaporates at room temperature, and its vapor is colorless, odorless, and seven times heavier than air. Mercury and its compounds are very toxic, especially the organic compounds of mercury. Fish will be poisoned if they live in water containing 0.01 to 0.02 mg/L mercury; humans will die if they eat 0.1g of mercury. Mercury effortlessly combines with the sulfide groups of various proteins, and this combination is powerful and difficult to separate. Mercury can cause damage to the human digestive tract, mouth, kidneys, liver, etc. Mercury and its compounds can invade the human body through different routes, such as the respiratory tract, skin, or digestive tract. When mercury enters the human body, it accumulates in the liver, kidneys, brain, heart, bone marrow, and other parts, causing neurological poisoning and deep tissue lesions, causing fatigue, dizziness, tremors, bleeding gums, baldness, numbness of hands and feet, neurasthenia and other symptoms, and even mental disorders may occur, leading to crazy convulsions and death.
Mercury and its compounds are only used when mercury is polarized before silver plating. Since mercury is toxic, it has been gradually replaced by other processes, such as pre-silver plating, but some factories still use it. Mercury-containing wastewater must be strictly treated before discharge.
The deposition rate of lead entering the human body through the respiratory tract is about 40%. When it invades the human body, about 90% to 95% will form insoluble substances and be deposited in the bones. Lead harms most systems in the human body, especially the bone marrow, hematopoietic system, nervous system, and kidneys. Blood lead levels at high levels (approximately 80 μg/dL) can cause seizures, coma, and even death. Low levels of lead can damage the central nervous system, kidneys, and blood cells. Red blood cells and hypohemoglobin anemia are the main clinical manifestations of chronic low-level lead exposure. Chronic lead poisoning can also cause high blood pressure and kidney damage.
Copper is one of the trace elements necessary for life, but excessive copper harms the human body, animals, and plants. Acute copper poisoning is caused by accidental ingestion of excessive copper. For example, drinking a large amount of copper-contaminated beverages will cause symptoms of gastrointestinal poisoning, including a metallic taste in the mouth, upper abdominal pain, nausea, vomiting, or diarrhea; in severe cases, gastrointestinal mucosal ulcers may occur. , hemolysis, liver necrosis, kidney damage, and even death from hypotension and shock. The cause of poisoning is that absorption of excessive copper inhibits the activity of many enzymes, causing severe damage to cell membranes. Dermatitis and eczema can occur when the skin comes into contact with copper compounds, and skin necrosis can occur when exposed to high concentrations of copper compounds. When the copper content in the water reaches 0.01 mg/L, it has a significant inhibitory effect on the self-purification of the water body. If it exceeds five mg/L, it will produce a peculiar smell; if it exceeds 15 mg/L, it will not be drinkable. If copper-containing wastewater irrigates farmland, copper can be dissolved in the soil. Enriched and absorbed by crops, it will also cause poor growth of rice and barley and contaminate grain grains. Copper is also very toxic to aquatic life.
Copper is widely used in electroplating. The main copper salts in the plating solution are copper sulfate and copper pyrophosphate.
Although nickel and its salts have relatively low toxicity, as an element with biological effects, nickel can activate or inhibit a series of enzymes, such as arginase, carboxylase, etc., resulting in toxic effects. Animals eating nickel salt can cause stomatitis, gingivitis, and acute gastroenteritis and damage the heart muscle and liver. Experiments have shown that the lethal dose of nickel to rabbits is 7 to 8 mg/kg. Nickel and its compounds irritate human skin, mucous membranes, and respiratory tract and can cause dermatitis, tracheitis, and even pneumonia. It has been proven through animal experiments and human observation that nickel has an accumulation effect that accumulates most in the kidneys, spleen, and liver and can induce pharyngeal cancer and lung cancer.
Nickel is widely used in the electroplating industry, and nickel chloride or sulfate is mainly used as the main salt in the electroplating solution.
Zinc is one of the essential trace elements for the human body. Ordinary people absorb 10-15 mg of zinc from food every day. The liver is the storage place for zinc. Zinc combines with proteins in the liver to form zinc-thioprotein, which supplies the zinc necessary for the body's physiological reactions.
Zinc deficiency in the human body will cause many adverse symptoms. Accidental consumption of soluble zinc salts can corrode the mucous membrane of the digestive tract. Excessive zinc can cause symptoms of acute gastroenteritis, such as nausea, vomiting, abdominal pain, dizziness, general weakness, etc.
Zinc and its compounds are one of the most commonly used raw materials in electroplating. Zinc chloride, zinc oxide, zinc sulfate, etc., are the main salts in the plating solution.
Acid and alkali wastewater is highly corrosive and will corrode pipes and underground structures if directly discharged without treatment. After entering the water body, it will affect the water body's pH, destroy the water body's self-purification ability, and affect the growth of organisms and fishery production.
When the pH is 5 or 9, most fish migrate; when it is lower than 5, it harms ordinary fish and even causes death. If discharged into farmland as irrigation water, it will change the properties of the soil and endanger crops.
Acids and alkalis are widely used in the electroplating industry, most of which are used for pretreatment before plating, mainly acids such as sulfuric acid, hydrochloric acid, nitric acid and phosphoric acid, and alkalis such as sodium hydroxide and sodium carbonate; in addition, during wastewater treatment Substances such as acids, alkalis, and some salts are also added, so the salt content in the wastewater is also high.
There are also additives, brighteners, oil substances, aniline substances, fluorine inorganic compounds, sulfides, etc., in the electroplating wastewater. Although the content of these substances is not significant, they are often discharged beyond the standard due to operation, management, and other reasons. It pollutes the environment. Therefore, sufficient attention should be paid to it in the design.
The chemical treatment method adds various chemical reagents to wastewater to change the chemical properties and state of pollutants in sewage through chemical reactions, transforming them into harmless or low-harm substances and then removing them from the water body. Chemical methods are widely used in electroplating wastewater treatment. According to statistics, about 41% of electroplating plants in my country use chemical methods to treat wastewater. At present, the following chemical methods are commonly used to treat electroplating wastewater:
In electroplating wastewater treatment, chemical reduction is mainly used to treat wastewater with oxidative pollutants, such as chromium-containing wastewater. The most commonly used method is the ferrous sulfate-lime method. In addition, there are sulfur dioxide methods, sulfurous acid methods, iron filings methods, and hydrazine hydrate reduction methods. The advantages of this method are simple equipment, low investment, large processing capacity, and the ability to reduce highly toxic hexavalent chromium into less toxic trivalent chromium.
Electrochemical reduction method
The electrochemical reduction method uses the principle of electrolysis and can use porous electrodes or iron anodes to treat chromium-containing wastewater. When wastewater containing hexavalent chromium Cr(Ⅵ) passes through an electrolytic cell with iron as the anode and cathode, the anode continuously dissolves to produce ferrous ions. Under acidic conditions, Cr(Ⅵ) is reduced to trivalent chromium. At the same time, during the electrolysis process, A large amount of H+ is consumed, which gradually increases the pH of the wastewater. Currently, Cr3+ and Fe3+ form hydroxides and precipitate out of the solution. The operation and management of electrochemical chromium removal are simple, and the effect is stable and reliable.
Using electrolysis to reduce and deposit heavy metal ions in wastewater at the cathode is essential for recovering or removing heavy metal ions in wastewater treatment.
However, the concentration of heavy metal ions in wastewater is often relatively low (tens or hundreds of mg/L). Suppose it is electrolyzed in a standard plate electrode electrolytic cell. In that case, the concentration of metal ions can be reduced to the emission standard (often a few or a few tenths of mg/L). l) or less is difficult to achieve. This is because the concentration of metal ions is too low, and side reactions on the cathode, such as hydrogen evolution, are very obvious, resulting in very low current efficiency. Much research has been conducted recently regarding the electrolysis of low-concentration wastewater, and many new electrodes or electrolytic cells have appeared. These new electrodes, or electrolyzers, can be boiled down into two types:
●Enhance the relative movement between the electrolyte and the electrode to strengthen the mass transfer process and reduce concentration polarization;
●It strengthens the relative movement between the electrolyte and the electrode and greatly increases the electrode surface area.
An increase in the electrode surface area can reduce the current density of the electrode surface when the total current intensity is the same, such as the conductor particle fluidized bed electrolyzer, which belongs to the second type. When this electrolytic cell treats copper-containing wastewater, it recovers the copper in the wastewater as metallic copper. The copper content in the treated water is less than 1.0 mg/L, meeting the discharge requirements. The purity of the recovered metal is also high. In some cases, electrolysis is used to treat concentrated waste liquid, and the economic benefits are considerable.
The alkaline chlorination method uses the oxidation effect of chlorine-based oxidants in alkaline wastewater to oxidize and destroy cyanide in the wastewater to remove it.
Commonly used oxidants include sodium hypochlorite, bleaching powder, and liquid chlorine. This method is suitable for treating cyanide-containing wastewater, especially for sewage with a cyanide concentration lower than 250 mg/L, and the effect is better. The alkaline chlorination method is divided into "complete oxidation" and "acidic hydrolysis." Complete oxidation is now widely used, and its final products are non-toxic carbon dioxide and nitrogen, which completely solves the problem of cyanide pollution.
The neutralization method is mainly used to treat pickling wastewater from electroplating plants.
Commonly used techniques include chemical neutralization, filtration, drum neutralization methods, etc. For sulfuric acid-type acid wastewater, to solve the problem that the neutralization filter material is quickly surrounded by calcium sulfate and reduces the treatment effect, some electroplating plants use dolomite (the main component is magnesium carbonate) as the filter material in the drum or equipment; it can also be used Fluidized-bed filtration is used to solve the problem of calcium sulfate coating the filter material and low treatment effect.
Chemical precipitation method operation model
The solubility of many metal hydroxides in water is very low. The solubility products of heavy metal hydroxides such as copper, cadmium, chromium, and lead are generally minimal.
Therefore, hydroxide precipitation can remove these heavy metal ions from wastewater. Commonly used precipitants include lime, sodium carbonate, sodium hydroxide, etc. Since the precipitant used in this method has a wide range of sources and is relatively low in price, it is widely used in production practice.
The ion exchange method uses the exchangeable active groups (mainly H+, OH-) in the ion exchanger to exchange harmful components in the wastewater, thereby obtaining a concentrated solution of pollutants and purified water.
In the mid-1970s, some domestic companies first used ion exchange to treat chromium-containing wastewater, which not only eliminated chromium pollution, but also obtained chromic acid recovery liquid and a large amount of recyclable purified water.
In the late 1970s, a factory in Beijing applied the ion exchange method to the treatment of cyanide-containing wastewater, realizing the recovery and reuse of cyanide.
In the 1980s, more than 100 electroplating factories or workshops in Shenyang City alone used ion exchange to remove chromium. However, this method has high technical requirements, a large one-time investment, and there is residual chlorine in the recovered chromic acid, which affects recycling.
The ion exchange method was once widely used in my country's electroplating industry. At present, some domestic enterprises with better technical conditions are still using it.
Electrocoagulation concentrates on the neutralization reaction mechanism of chemical flocculation cations and surface charges, involving the reaction of water pollutants with strong electric fields and the oxidation and reduction reactions generated by electricity. Electroflocculation can remove more than 99% of heavy metal cations in water. , can also kill microorganisms in the water.
This process precipitates charged colloidal substances and significantly removes other ions, colloids, and emulsions. Electric flocculation technology can achieve standard effluent discharge from the original wastewater treatment process at a meager investment in wastewater treatment situations where the current technology cannot meet the requirements.
Heavy metal removal steps from electroplating wastewater
The adsorption method is an effective method for treating electroplating wastewater, and is mainly used to treat wastewater containing chromium, cyanide, etc. Currently commonly used adsorbents include activated carbon, activated alumina, diatomaceous earth, fly ash, etc. Since the 1970s in China, many units have carried out experimental research work, and some have been put into production.
The membrane separation process uses selective permeability membranes to separate pollutants in wastewater. The wastewater passes through the membrane medium under a particular driving force, and the wastewater components on the inlet side selectively pass through the membrane medium, thereby achieving the purpose of separation or purification.
The primary membrane water treatment technology applications are ultrafiltration, reverse osmosis, dialysis, and electrodialysis. Using membrane separation technology to treat electroplating wastewater can concentrate and recover specific functional components and recycle the permeate water. At the same time, corresponding membrane separation technology can be used according to different treatment requirements. In the 1970s, the reverse osmosis method was first used in China to recover and treat nickel plating rinse water, and later, it was applied to the treatment of copper plating, zinc plating, and other rinse solutions.
Electrodialysis is one of the most mature membrane technologies in research and development. It is mainly used to recover heavy metals from rinse water in the electroplating industry.
Electrodialysis is used to treat electroplating industrial wastewater. After treatment, the composition of the wastewater remains unchanged, which is beneficial for returning it to the tank for use. Wastewater containing Cu2+, Ni2+, Zn2+, Cr(VI), and other metal ions is suitable for electrodialysis treatment. Among them, nickel-containing wastewater treatment technology is the most mature, with complete sets of industrial equipment.
However, the electrodialysis method is used to treat electroplating wastewater, which requires many wastewater pretreatments and consumes a lot of electricity; the quality of the membrane needs to be improved to expand its application scope in industrial wastewater further.
The membrane separation method can be used as a purification technology and can recover metals. It has the advantage of high separation efficiency, so it is a promising separation technology. This method is especially suitable for the treatment of low-concentration electroplating waste liquid.
Research on supercritical water oxidation systems began in the 1970s. Pure substances have three phases: gas, liquid, and solid. When the system temperature and pressure reach a certain point, the densities of the gas and liquid phases are the same, and the two phases merge into a uniform phase.
This specific point is defined as the critical point of the material, and the corresponding temperature, pressure, and density are defined as the critical temperature, critical pressure, and critical density of the material. The critical temperature of water is 374.2°C, the critical pressure is 22.1 MPa, and the density is between gas and liquid.
Supercritical water (temperature higher than 374°C and pressure greater than 22.1 MPa) has unique properties that standard water does not have. Supercritical water can be completely miscible with organic matter, air/nitrogen/carbon dioxide, and other gases. Therefore, organic matter/air/water can be mixed under supercritical conditions during the supercritical water oxidation process. In a terse reaction residence time, more than 99% of the organic matter is rapidly oxidized into water, carbon dioxide, nitrogen, etc.
Under high temperatures and high pressure, the solubility of inorganic salts and heavy metals is extremely low, and they are easily separated and recycled during pressure reduction separation. The treated water can be reused, and no secondary pollutants will be produced during the treatment process.
The physical methods currently used in electroplating wastewater treatment mainly include evaporation concentration, crystallization, and membrane separation.
Evaporation concentration recovery is a treatment method that evaporates heavy metal electroplating wastewater to concentrate the heavy metals for recovery and reuse. It treats wastewater containing chromium, copper, silver, and nickel ions.
For wastewater containing low concentrations of heavy metal ions, direct application of the evaporation concentration recovery method consumes a lot of energy and is economically unreasonable. Generally speaking, applying evaporation and concentration to treat heavy metal wastewater in the electroplating industry is often combined with other methods. For example, using an atmospheric pressure evaporator and a countercurrent rinsing system to treat electroplating wastewater can achieve a closed-circuit cycle, which is a very successful combination.
In the 1980s, this method was widely used in my country, especially for the treatment of electroplating chromium-containing wastewater. The evaporation concentration method treats electroplating heavy metal wastewater. The process is mature and straightforward. It does not require chemical reagents and has no secondary pollution. It can recycle water or valuable heavy metals and has good environmental and economic benefits. However, due to high energy consumption and operating costs, The problem of impurities interfering with resource recovery remains to be studied, limiting its application.
Currently, the general evaporation concentration method is used as a unit in a combined process.
The crystallization method is a method in solid-liquid separation technology. It mainly uses the characteristic that salt substances can precipitate relatively pure crystalline salts in their supersaturated solutions to recover some metal salts as crystals. Generally, it is in the recycling of electroplating solutions such as cyanide, zinc plating, and nickel plating solutions.
Air flotation is a separation technology with high efficiency and fast separation speed, which starts from mineral processing. Since the 1970s, this technology has attracted the attention of environmental workers in wastewater treatment and has led to the rapid development of this technology.
The advantages of air flotation treatment of heavy metal wastewater are strong adaptability, continuous treatment, and easy automatic operation. It is especially suitable for treating heavy metal mixed wastewater with poor quality and significant changes. However, when air flotation is used to treat electroplating mixed wastewater, the mass concentration of heavy metal ions in the wastewater should generally not be greater than 30 to 40 mg/L.
The disadvantage of this method is that the problem of comprehensive utilization of scum must be solved. NH4+ and a large amount of surfactants are not allowed to be contained in the wastewater, otherwise the treatment effect will be affected.
Due to the wide variety of electroplating wastewater and the different compositions of wastewater from each factory, it isn't easy to achieve unified treatment methods.
Any governance method has its advantages and disadvantages. Using one method often fails to achieve the ideal governance effect. Therefore, two or more methods must be combined to complement each other to achieve the best technical and economic results.
For example, the ion exchange-ferrite method can better solve the secondary pollution problem of the ion exchange method; the electrolysis-ferrite method can solve the utilization problem of electroplating sludge; the chemical precipitation method and the air flotation method can be combined, it can enhance the removal effect of heavy metal ions; some people combine the ion exchange method and the reverse osmosis method to solve the problem of reuse of the regeneration solution.
Multiple combination technologies are still under development, with the main directions being multi-function, miniaturization, and control automation.
