Characteristics and Treatment Solutions of Electroplating Wastewater
The sources of electroplating wastewater are generally:
(3) Other wastewater, including floor washing water, plate-brushing rinsing water, ventilation equipment condensate, and various tank liquids and drainage caused by "leakage, spillage, dripping, and escaping" due to tank leakage or improper operation and management;
(4) Equipment cooling water, which is not polluted except for the temperature increase during use.
(5) Metal surface treatment: Metal surface treatment includes cleaning before surface treatment, electroplating, passive film protection, machining, and coating, mainly electroplating.
From the electroplating production process, electroplating wastewater can be divided into post-treatment wastewater as well as waste plating solutions and waste stripping solutions, four categories in total.
For metal substrate materials, the electroplating can be divided into:
1. (including grinding, polishing, sand blasting, barreling, brushing, etc.)
2. (including degreasing, derusting, and etching, etc.)
3. Electrochemical treatment (including electrochemical degreasing and electrochemical etching, etc.)
Alkaline compounds such as NaOH, Na₂CO₃, Na₃PO₄, Na₂SiO₃ are commonly used in the degreasing process. For parts with particularly heavy oil stains, organic solvents such as kerosene, gasoline, acetone, toluene, trichloroethylene, and carbon tetrachloride are sometimes used for degreasing, followed by chemical alkaline degreasing. To remove certain mineral oils, emulsifiers such as OP emulsifier, AE emulsifier, and triethanolamine oleate soap are usually added to the degreasing solution. Therefore, the cleaning wastewater and renewal waste liquid generated during degreasing are both alkaline wastewater, often containing oils and other organic compounds.
Hydrochloric acid and sulfuric acid are commonly used for acid pickling and derusting; to prevent corrosion of the plated substrate, certain inhibitors such as thiourea, sulfonated coal tar, and hexamethylenetetramine are often added. The cleaning water generated during acid pickling and derusting generally has high acidity and contains heavy metal ions and a small amount of organic additives.
Pre-treatment wastewater is an important part of electroplating wastewater treatment, accounting for about 50% of the total electroplating wastewater. The wastewater contains certain salts, free acid, organic compounds, etc., and its composition varies greatly with the plating type, pre-treatment process, and factory management level.
Plating rinse water is the main source of heavy metal pollution in electroplating operations. The main components of the plating solution are metal salts and complexing agents, including various metal sulfates, chlorides, fluoroborates, as well as cyanides, ammonium chloride, nitrilotriacetic acid, pyrophosphates, organophosphonic acids, etc. In addition, certain organic compounds are often added to the plating solution to improve the plating properties, such as coumarin and butynediol as leveling agents, and saccharin, vanillin, benzylideneacetone, p-toluenesulfonamide, and benzenesulfonic acid as brighteners. Therefore, in addition to heavy metal ions, plating rinse wastewater also contains a small amount of organics. The discharge volume of rinse wastewater and the kinds and concentrations of heavy metal ions vary with many factors such as the physical shape of the plated parts, the plating solution formula, the rinsing method, and the electroplating operation management level. In particular, the rinsing process has a great influence on the concentration of heavy metals in the wastewater, directly affecting the recovery of resources and the treatment effect of the wastewater.
Post-plating treatment mainly includes passivation after rinsing, stripping of defective coatings, and other special surface treatments. A large amount of heavy metal wastewater is also generated during post-treatment. Generally, it often contains heavy metals such as Cr⁶⁺, Cu²⁺, Ni²⁺, Zn²⁺, Fe²⁺; acidic and alkaline substances such as H₂SO₄, HCl, H₃BO₃, H₃PO₄, NaOH, Na₂CO₃; and organic substances such as glycerin, nitrilotriacetic acid, hexamethylenetetramine, resist salt, and acetic acid. In general, this type of post-plating treatment wastewater is complex and variable, with unstable volume, and is generally combined with mixed wastewater or acid-base wastewater for treatment.
The tank liquids commonly used in electroplating operations such as plating, passivation, and stripping, after long-term use, accumulate many other metal ions, or due to the destruction of certain additives, or the disproportionation of certain effective components, affect the quality of the coating or passive film. Therefore, many factories discard part of the tank liquid to control these impurities within the permissible range of the process, supplementing with new solution; some factories discard all of these failed tank liquids. These discarded liquids of various concentrations generally have high concentrations of heavy metal ions and accumulated impurities. Not only do the kinds of pollutants differ, but the concentrations of main pollutants, other metal impurity ions, and the solution medium often vary greatly. These differences determine the diversity of treatment technologies and the specificity of the process for these wastewaters.
Currently, domestic electroplating wastewater treatment is mainly divided into 3 categories first,
mainly uses reduction to treat hexavalent chromium.
Currently, physical-chemical methods are generally used for treatment. There are many treatment methods, and many are effective, but few can achieve overall compliance. However, there are also those who do it well, such as Shaanxi Futianbao Company's DTCR—heavy metal ion capturing agent, which forms a macromolecular chelate with heavy metal ions in the wastewater through DTCR, and then flocculates to well remove heavy metal ions from electroplating wastewater and meet national standards.
The main source of zinc in electroplating and metal processing wastewater is the drag-out liquid from electroplating or acid pickling. Pollutants are transferred to the rinse water through the metal rinsing process. The acid pickling process includes 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. This wastewater contains a large amount of hydrochloric acid and heavy metal ions such as zinc and copper, as well as organic brighteners, and is highly toxic; some also contain carcinogenic, teratogenic, and mutagenic highly toxic substances, which are extremely harmful to humans. Therefore, electroplating wastewater must be seriously recovered and treated to eliminate or reduce its pollution to the environment.
Electroplating wastewater treatment equipment consists of a regulation tank, dosing tank, reduction tank, neutralization reaction tank, pH adjustment tank, flocculation tank, inclined tube sedimentation tank, chamber filter press, clean water tank, flotation reaction, activated carbon filter, etc.
There are mainly the following methods.
The flotation method is to pass air into water to generate tiny bubbles. Due to the adhesion between bubbles and fine suspended matter, a flotation body is formed. Using the buoyancy of the bubbles, it floats to the water surface to form foam or scum, thereby separating the suspended matter in the water. According to the different ways of generating bubbles, it can be divided into three types: aerated flotation, dissolved-air flotation, and electrolytic flotation.
The flotation method is a new type of solid-liquid separation method that replaces the sedimentation method. In 1978, Tongji University in Shanghai first successfully applied the flotation method to treat electroplating heavy metal wastewater. Subsequently, due to the continuous process, compact equipment, small footprint, and easy automation, it has been widely used.
The flotation solid-liquid separation technology has strong adaptability and can treat chromium-plating wastewater, chromium-containing passivation wastewater, and mixed wastewater. It can not only remove heavy metal hydroxides but also remove other suspended matter, emulsified oil, and surfactants. The principle of the flotation method for treating chromium-plating wastewater is: under acidic conditions, ferrous sulfate and hexavalent chromium undergo a redox reaction, then under alkaline conditions a floc is produced, and under the action of countless tiny bubbles the floc floats to the water surface, clarifying the water.
The ion exchange method mainly uses the exchange ions in the ion exchange resin to exchange with certain ions in the electroplating wastewater to remove them, thereby purifying the wastewater.
The domestic use of ion exchange technology to treat electroplating wastewater began with experimental research in the 1960s. By the late 1970s, due to the urgent need to solve environmental pollution problems, this technology developed greatly and has become one of the effective means for treating electroplating wastewater and recovering certain metals, and also an important link in making the electroplating wastewater of certain plating types achieve closed-loop circulation. However, the investment cost of the ion exchange method is very high, and the system design and operation management are relatively complex, making it difficult for general small and medium-sized enterprises to adapt; often due to poor maintenance and management, the expected effect is not achieved, so its promotion and application are subject to certain restrictions.
Currently, the domestic use of ion exchange to treat electroplating wastewater containing chromium, nickel, etc. is relatively common, and mature experience has been gained in design, operation, and management. The treated water can meet discharge standards, and the effluent quality is generally good and can be recycled. The regeneration eluate after the resin exchange is saturated with adsorption can be returned to the plating tank after composition adjustment and purification of the electroplating process, basically achieving closed-loop circulation. In addition, the ion exchange method can also be used to treat wastewater containing copper, zinc, gold, etc.
The electrolytic method mainly makes the harmful substances in the wastewater undergo oxidation and reduction reactions at the anode and cathode respectively through the electrolysis process, converting them into harmless substances; or uses the oxidation and reduction products of the electrodes to react chemically with the harmful substances in the wastewater, generating insoluble precipitates, which are then separated and removed, or recovers metals through the electrolysis reaction. Domestic use of the electrolysis method to treat electroplating chromium-containing wastewater began in the 1960s, and experimental research on wastewater containing silver, copper, etc. was carried out in the late 1970s, recovering metals such as silver and copper with good results.
The electrolytic method for treating electroplating wastewater is generally used in small and medium-sized factories. Its main feature is that no treatment chemicals need to be added, the process is simple, operation is convenient, and it occupies little production space. At the same time, because the recovered metal has high purity, it has good economic benefits for recovering precious metals. However, when the treatment volume is large, the power consumption of the electrolysis method is large, the consumption of iron electrode plates is also large, and the separated sludge is as difficult to dispose of as the chemical treatment method, so it is less used now.
The extraction method is to add a solvent that is insoluble in water but can dissolve a certain substance (called solute or extract) in the wastewater, so that the solute is fully dissolved in the solvent, thereby separating and removing or recovering a certain substance from the wastewater. The extraction operation process includes three main steps: mixing, separation, and recovery.
☆ Tap water ---- water pump ---- multi-media filter ---- activated carbon filter ---- automatic dosing device ---- security filter ---- high-pressure pump ---- primary reverse osmosis ---- intermediate tank ---- high-pressure pump ---- secondary reverse osmosis ---- pure water tank ---- pure water pump (new process)
☆ Rinse water ---- water tank ---- water pump ---- multi-media filter ---- security filter ---- ultrafiltration ---- electroplating solution recovery barrel
☆ Rinse water ---- water tank ---- water pump ---- multi-media filter ---- security filter ---- ultrafiltration ---- electroplating solution recovery barrel ---- high-pressure pump ---- reverse osmosis ---- cleaning water tank
Most electroplating plants are comprehensive multi-plating-type operations, involving chromium, nickel, zinc, copper, and other plating types. From the perspective of plated parts, they can be divided into metal plated parts and plastic plated parts. Although the cyanide electroplating process is backward and largely eliminated, many electroplating plants still use it.
The general production process of an electroplating plant is as follows: the electroplating production process is mainly mechanical polishing (grinding or barreling) → degreasing → acid etching → electroplating → drying → qualified products put into storage
Implementation of cleaner production in the electroplating industry
Cleaner production refers to the continuous adoption of measures such as improving design, using clean energy and raw materials, adopting advanced process technologies and equipment, improving management, and comprehensive utilization, to cut pollution at the source, improve resource utilization efficiency, and reduce or avoid the generation and emission of pollutants in the processes of production, service, and product use, so as to reduce or eliminate harm to human health and the environment.
Pre-treatment mechanical polishing of plated parts (grinding or barreling)
It mainly uses special machinery with grinding wheels or belts (or abrasives to remove rust from certain plated parts using a drum with abrasives) to remove burrs, scratches, weld beads, sand holes, etc. on the plated parts, so as to improve the flatness of the plated parts and their quality. This process has no wastewater discharge.
Metal product plated parts, after various processing and treatment, inevitably adhere to a layer of oil stains. To ensure a firm combination of the coating and the substrate, the oil stains on the surface of the plated parts must be removed. There are many degreasing processes, mainly using organic solvent degreasing, with the process as follows:
Polished parts → clean water wash → organic solvent degreasing tank → clean water tank → clean water rinse
From the date of implementation of this standard, the discharge control of water and air pollutants from electroplating enterprises shall be implemented in accordance with the provisions of this standard, and the relevant provisions in the "Integrated Wastewater Discharge Standard" (GB 8978-1996) and the "Integrated Atmospheric Pollutant Discharge Standard" (GB 16297-1996) shall no longer be implemented.
Parts after degreasing often have a lot of rust and relatively thick oxide film on the surface. To obtain a bright coating and better combine the coating with the substrate, the rust and oxide film on the parts must be removed. After acid immersion, the surface of the parts can also be activated. The process is as follows:
Parts after degreasing → acid water tank → recovery tank → clean water tank → clean water rinse
The wastewater in this section mainly comes from the clean water rinsing process, and the wastewater contains a large amount of iron ions, with a pH value between 2 and 5.
Electroplating production process and water quality of each plating type
Its production process is generally: parts after etching treatment → electroplating tank → recovery tank → clean water tank → clean water rinse.
The wastewater in this section mainly comes from the clean water rinsing process and contains the corresponding metal ions or cyanides. The rinsing water from cyanide copper plating contains cyanides and copper ions; the rinsing water from chromium plating contains hexavalent chromium; the rinsing water from nickel plating contains nickel ions, etc. The rinsing water is split and treated separately according to the plating type. For example, cyanide-containing wastewater is split and then undergoes two-stage cyanide destruction and pH adjustment, and after solid-liquid separation it can be discharged up to standard; chromium-containing wastewater is split and then undergoes a reduction reaction, then after neutralization and solid-liquid separation it can be discharged up to standard.
This process mainly uses mechanical and natural energy and heat energy to dry the moisture on the surface of the parts after electroplating rinsing, so as to avoid rust and damage to the oxide film. This process has no wastewater discharge.
The stripping process has two methods: chemical immersion and anodic electrolysis, with the process as follows:
Unqualified plated parts → stripping tank → recovery tank → clean water tank → clean water rinse.
The wastewater in this section has a pH between 2 and 6, and mainly comes from the rinse water after stripping. The stripping rinse water can enter its own wastewater pool for treatment, but cannot directly enter the mixed wastewater treatment pool; it should be pre-treated separately and then discharged into the corresponding wastewater treatment branch.
It stipulates the discharge limits, monitoring, and control requirements for water and air pollutants from electroplating enterprises. To promote regional economy and development, drive the adjustment of economic structure and the transformation of economic growth mode, and guide the development direction of electroplating production processes and pollution control technologies, this standard stipulates the special discharge limits for water pollutants.
The discharge of malodorous pollutants and environmental noise from electroplating enterprises shall apply to the corresponding national pollutant discharge standards; the identification, treatment, and disposal of solid waste shall apply to the national solid waste pollution control standards.
The wastewater in this section mainly comes from the clean water rinsing process, with a water quality pH value between 8.5 and 10.
