The source of electroplating wastewater

The sources of electroplating wastewater are generally:

1. Plating cleaning water;

2. Waste electroplating solution;

3. Other wastewater, including flushing workshop floors, scrubbing electrode plates, condensate water from ventilation equipment, as well as various tank liquids and drainage caused by plating tank leakage or improper operation and management;

4. Equipment cooling water, cooling water is not contaminated during use except for temperature rise. The water quality and quantity of electroplating wastewater are related to factors such as electroplating production process conditions, production load, operation management, and water usage methods. The water quality of electroplating wastewater is complex and its components are difficult to control. It contains heavy metal ions such as chromium, cadmium, nickel, copper, zinc, gold, silver, and cyanide, some of which are highly toxic substances that can cause cancer, malformation, and mutation.

Types of Electroplating Wastewater

There are many types of electroplating wastewater, which are generally classified according to the elements of heavy metals. The following are some common types of electroplating wastewater:

1. Chromium containing wastewater: mainly comes from processes such as chromium plating, passivation, chemical chromium plating, anodizing treatment, etc. This type of wastewater contains metal ions such as hexavalent chromium, trivalent chromium, copper, iron, and sulfuric acid.

2. Nickel containing wastewater: mainly from the nickel plating process, the wastewater contains salts such as nickel sulfate, nickel chloride, boric acid, sodium sulfate, as well as some additives and brighteners.

3. Cyanide containing wastewater: mainly generated from processes such as galvanizing, copper plating, cadmium plating, gold and silver plating, alloy plating, etc. The wastewater contains complex metal ions of cyanide, free cyanide, sodium hydroxide, sodium carbonate and other salts, as well as some additives and brighteners.

In addition to the above-mentioned types of wastewater, electroplating wastewater can also be divided into copper containing wastewater, zinc containing wastewater, cadmium containing wastewater, etc. according to the types of pollutants; According to the classification of treatment processes, there are acidic and alkaline wastewater, organic wastewater (such as detergents, surfactants, etc.), oil wastewater, metal oxide wastewater, etc.

The types and components of electroplating wastewater are very complex, and the specific types and components may vary due to different electroplating processes, raw materials, additives, and other factors. When treating electroplating wastewater, it is necessary to choose appropriate treatment methods and processes based on the characteristics and treatment requirements of the wastewater to ensure effective treatment and standard discharge.

Treatment methods for electroplating wastewater

Currently, physical and chemical methods are generally used for treatment. There are many processing methods and many effective ones, but not many can achieve overall compliance.

The main source of zinc in electroplating and metal processing wastewater is the dragging solution of electroplating or pickling. The pollutants are transferred to the rinsing water through the metal rinsing process. The pickling process involves first immersing the metal (zinc or copper) in strong acid to remove surface oxides, and then immersing it in a brightener containing strong chromic acid for brightening treatment. The wastewater contains a large amount of hydrochloric acid, heavy metal ions such as zinc and copper, as well as organic brighteners, which are highly toxic. Some of them also contain highly toxic substances that can cause cancer, malformation, and mutation, posing great harm to humans. Therefore, electroplating wastewater must be carefully recycled and treated to eliminate or reduce its pollution to the environment.

The electroplating wastewater treatment equipment consists of a regulating tank, a dosing tank, a reduction tank, a neutralization reaction tank, a pH regulating tank, a flocculation tank, an inclined tube sedimentation tank, a box filter press, a clean water tank, an air flotation reaction, an activated carbon filter, etc.

There are several main methods.

1. Air flotation method

The air flotation method is to introduce air into the water to produce tiny bubbles. Because of the adhesion between bubbles and fine suspended solids, a flotation body is formed. The flotation effect of bubbles is used to float up to the water surface to form foam or scum, so that suspended substances in the water can be separated. According to the different ways of bubble generation, it can be divided into three categories: inflatable air flotation, dissolved air flotation, and electrolytic air flotation.

Air flotation is a new solid-liquid separation method that replaces sedimentation. In 1978, Shanghai Tongji University successfully applied air flotation for the treatment of electroplating heavy metal wastewater for the first time. Subsequently, due to the continuous processing, compact equipment, small footprint, and ease of automation, it was widely applied.

The air flotation solid-liquid separation technology has strong adaptability and can treat chromium plating wastewater, chromium containing passivation wastewater, and mixed wastewater. Not only can heavy metal hydroxides be removed, but other suspended solids, emulsified oil, surfactants, etc. can also be removed. The principle of using air flotation method to treat chromium plating wastewater is to undergo oxidation-reduction reaction between ferrous sulfate and hexavalent chromium under acidic conditions, and then produce flocs under alkaline conditions. Under the action of countless fine bubbles, the flocs are floated out of the water surface, making the water quality clearer.

2. Ion exchange method

The ion exchange method mainly uses the exchange ions in the ion exchange resin to exchange with certain ions in electroplating wastewater and remove them, so as to purify the wastewater.

The use of ion exchange technology to treat electroplating wastewater in China began experimental research in the 1960s. By the late 1970s, due to the urgent need to solve environmental pollution problems, this technology had greatly developed and has become one of the effective methods for treating electroplating wastewater and recovering certain metals. It is also an important link in achieving closed-loop circulation of electroplating wastewater for certain types of plating. However, the investment cost of using ion exchange method is high, and the system design and operation management are complex. It is difficult for ordinary small and medium-sized enterprises to adapt, and often due to poor maintenance and management, the expected results cannot be achieved. Therefore, the promotion and application are limited to a certain extent.

Currently, it is common in China to use ion exchange method to treat electroplating wastewater containing chromium and nickel, and there is already mature experience in design, operation, and management. After treatment, the water can meet the discharge standards and the effluent quality is good, so it can generally be recycled. After resin exchange adsorption saturation, the regenerated eluent can be reused in the plating tank after adjusting and purifying the electroplating process composition, basically achieving closed-loop circulation. In addition, ion exchange method can also be used to treat wastewater containing copper, zinc, gold, and other metals.

3. Electrolysis method

The electrolysis method mainly involves the oxidation and reduction reactions of harmful substances in wastewater at the positive and negative electrodes through the electrolysis process, converting them into harmless substances; Alternatively, chemical reactions can occur between electrode oxidation and reduction products and harmful substances in wastewater to generate insoluble precipitates, which can then be separated and removed or recovered through electrolysis reactions. In the 1960s, China began to use electrolysis to treat electroplating wastewater containing chromium. In the late 1970s, experimental research was conducted on wastewater containing silver, copper, and other metals to recover them, achieving good results.

The electrolysis method for treating electroplating wastewater is generally used in small and medium-sized factories. Its main characteristics are that it does not require the addition of treatment agents, 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 metal, it has good economic benefits for the recovery of precious metals. But when the amount of water processed is large, the electrolysis method consumes a lot of electricity and consumes a large amount of iron plates. At the same time, the separated sludge is not easy to dispose of like the chemical treatment method, so it is less commonly used.

4. Extraction method

Extraction method is a method of using a solvent that is insoluble in water but can dissolve a certain substance (called solute or extract) in water and adding it to wastewater to fully dissolve the solute in the solvent, thereby separating, removing or recovering a certain substance from wastewater. The extraction process includes three main steps: mixing, separation, and recovery.

The process flow of electroplating wastewater

The main steps include:

1. Pretreatment: first, remove the large particle impurities and suspended solids in the wastewater through coarse screening, and then adjust the pH of the wastewater to meet the requirements of the subsequent treatment process.

2. Sedimentation treatment: The wastewater is sent to a sedimentation tank and a certain amount of chemical agents (such as polymeric ferric chloride) are added to form precipitates of metal ions and organic matter in the wastewater. Separate the sediment from the wastewater by stirring and settling.

3. Flocculation treatment: The wastewater after sedimentation treatment is sent to the flocculation tank, and flocculants (such as polyaluminum chloride) are added. Vibration and stirring combine the small suspended solids in the wastewater with the flocculant to form flocculent substances, improving its settling speed.

4. Air flotation treatment: After flocculation treatment, air is injected into the wastewater through a jet device to combine bubbles and flocs in the wastewater to form floating and settling substances. The floating and settling substances in wastewater rise to the water surface, forming a layer of scum, which is then concentrated and removed by mechanical scrapers.

5. Biochemical treatment: The previous treatment process can remove most of the organic matter and suspended solids in the wastewater, but it may still contain a certain amount of organic matter and microorganisms, so biochemical treatment is necessary.

6. Plant treatment: In order to further improve water quality, wastewater may be treated with plants.

In addition, there are specific treatment methods for electroplating wastewater based on its composition. For example, electroplating wastewater containing heavy metals can be treated using the evaporation concentration method to concentrate and recycle the heavy metal wastewater. The activated carbon adsorption method is mainly used to treat wastewater containing chromium and cyanide. The electrolysis rule is applicable to the treatment of electroplating wastewater with high or single precious metal content.

Please note that the specific process flow may vary due to factors such as the composition, concentration, treatment requirements, and local environmental regulations of electroplating wastewater. In practical operation, it is necessary to choose appropriate treatment methods and process parameters based on specific situations to achieve the best treatment effect. Meanwhile, the sludge and waste generated during the electroplating wastewater treatment process also need to be properly treated to prevent secondary pollution to the environment.

Emission standards for electroplating wastewater

The discharge standards for electroplating wastewater are mainly based on the "Emission Standards for Electroplating Pollutants" (GB21900-2008).

This standard is applicable to the management of water and air pollutant emissions in existing electroplating enterprises, as well as to the environmental impact assessment, environmental protection facility design, completion environmental acceptance, and water and air pollutant emissions management after production of electroplating enterprise construction projects. This standard also applies to anodized surface treatment facilities, as well as legally permissible pollutant discharge behaviors.

The specific standards for the discharge of electroplating wastewater include but are not limited to the content standards for total chromium, hexavalent chromium, total nickel, total cadmium, total silver, total lead, total mercury, total copper, total zinc, total iron, total aluminum, etc., as well as the pH value and suspended solids content standards for the total discharge outlet of enterprise wastewater.