Introduction to Electroplating Wastewater Treatment Methods
The sources of electroplating wastewater are generally:
(3) Other wastewater, including floor-washing water in the workshop, plate-brushing rinse water, ventilation-equipment condensate, and various tank liquors and drainages caused by tank leakage or improper operation and management (spills, leaks, drips, and runoffs);
(4) Equipment cooling water, which is not polluted during use except for a temperature rise.
(5) Metal surface treatment: includes cleaning before surface treatment, electroplating, passivation-film protection, machining, and coating, with electroplating as the main activity.
Based on the electroplating production process, electroplating wastewater can be divided into four categories: post-treatment wastewater, spent plating solutions, spent stripping solutions, etc.
For metal substrate materials, the electroplating process can be divided into:
1. (including grinding, polishing, sandblasting, barreling, brushing, etc.)
2. (including degreasing, derusting, and etching, etc.)
3. Electrochemical treatment (including electrochemical degreasing and electrochemical etching, etc.)
In degreasing, alkaline compounds such as NaOH, Na2CO3, Na3PO4, and Na2SiO3 are commonly used. For parts with severe oil contamination, organic solvents such as kerosene, gasoline, acetone, toluene, trichloroethylene, and carbon tetrachloride may sometimes be used for degreasing before chemical alkaline degreasing. To remove certain mineral oils, a certain amount of emulsifier-such as OP emulsifier, AE emulsifier, and triethanolamine oleate soap-is usually added to the degreasing solution. Therefore, the rinsing wastewater and spent solutions generated during degreasing are alkaline wastewaters, often containing oils and other organic compounds.
Electroplating wastewater treatment equipment consists of an equalization tank, dosing tank, reduction tank, neutralization reaction tank, pH adjustment tank, flocculation tank, inclined-tube sedimentation tank, chamber filter press, clear-water tank, flotation reactor, activated-carbon filter, etc.
Pretreatment wastewater is an important component of electroplating wastewater, accounting for about 50% of the total, and contains certain salts, free acid, and organic compounds. Its composition varies greatly with the plating type, pretreatment process, and the factory's management level.
Rinse water from plating is the main source of heavy-metal pollution in electroplating. The main components of plating solutions are metal salts and complexing agents, including various metal sulfates, chlorides, and fluoroborates, as well as cyanides, ammonium chloride, nitrilotriacetic acid, pyrophosphates, and organic phosphonic acids. In addition, organic compounds are often added to improve coating properties-such as coumarin, butynediol, and thiourea as leveling agents, and saccharin, vanillin, benzylideneacetone, p-toluenesulfonamide, and benzenesulfonic acid as brighteners. Therefore, plating rinse wastewater contains not only heavy-metal ions but also a small amount of organics. The discharge volume and the types and concentrations of heavy-metal ions vary with many factors such as the physical shape of the plated parts, the plating-solution formulation, the rinsing method, and the electroplating operation and management level. In particular, the rinsing process greatly affects the heavy-metal concentration in the wastewater, directly influencing resource recovery and treatment effectiveness.
Post-treatment of coatings mainly includes passivation after rinsing, stripping of defective coatings, and other special surface treatments. The post-treatment process also generates large amounts of heavy-metal wastewater. Generally, it often contains heavy metals such as Cr6+, Cu2+, Ni2+, Zn2+, and Fe2+; acidic and alkaline substances such as H2SO4, HCl, H3BO3, H3PO4, NaOH, and Na2CO3; and organic substances such as glycerin, nitrilotriacetic acid, hexamethylenetetramine, resist salt, and acetic acid. Overall, such post-treatment wastewater is complex and variable, with unstable volume, and is generally combined with mixed or acid-alkali wastewater for treatment.
Tank liquors commonly used in electroplating, passivation, and stripping accumulate many other metal ions over long use, or suffer degraded coating/passivation quality due to destruction of certain additives or imbalance of effective components. Therefore, to keep impurities within permissible process limits, many factories discard part of the tank liquor and replenish with fresh solution, while some discard the spent liquor entirely. These discarded liquors at various concentrations generally have very high heavy-metal ion concentrations and many accumulated impurities; not only do the pollutant types differ, but the main pollutant concentrations, other metallic impurity ion concentrations, and the solution medium often vary considerably. These differences determine the technical diversity and process-specificity of treating such wastewater.
Currently, domestic electroplating wastewater treatment first divides it into three categories:
Mainly, reduction is used to treat hexavalent chromium.
Currently, physicochemical methods are generally adopted. There are many treatment methods and quite a few are effective, yet few can achieve overall compliance. Some good examples exist, however-such as the DTCR heavy-metal ion capturing agent from Shaanxi Futianbao Company, which forms a macromolecular chelate with heavy-metal ions in the wastewater via DTCR, and after flocculation can effectively remove heavy-metal ions from electroplating wastewater and meet national standards.
In electroplating and metal-processing wastewater, the main source of zinc is the drag-out liquor from electroplating or pickling. Pollutants are transferred to the rinse water through the metal-rinsing process. The pickling step involves first immersing the metal (zinc or copper) in strong acid to remove surface oxides, then immersing it in a brightener containing strong chromic acid for brightening. This wastewater contains large amounts of hydrochloric acid and heavy-metal ions such as zinc and copper, as well as organic brighteners, and is highly toxic; some contain carcinogenic, teratogenic, and mutagenic highly toxic substances that are extremely harmful to humans. Therefore, electroplating wastewater must be seriously recovered and treated to eliminate or reduce its environmental pollution.
Hydrochloric acid and sulfuric acid are commonly used for pickling and derusting. To prevent corrosion of the plated substrate, certain inhibitors such as thiourea, sulfonated coal tar, and hexamine benzidine are often added. The rinse water generated during pickling and derusting generally has high acidity and contains heavy-metal ions and a small amount of organic additives.
The main methods are as follows.
The flotation method introduces air into water to generate tiny bubbles. Adhesion between bubbles and fine suspended solids forms floatable agglomerates; using the buoyant rise of the bubbles, they float to the surface to form foam or scum, thereby separating suspended matter from the water. According to the bubble-generation method, it is divided into dispersed-air flotation, dissolved-air flotation, and electrolytic flotation.
Flotation is a novel solid-liquid separation method that replaces sedimentation. In 1978, Tongji University in Shanghai first successfully applied flotation to treat electroplating heavy-metal wastewater. Subsequently, because of its continuous process, compact equipment, small footprint, and easy automation, it has been widely used.
The flotation solid-liquid separation technology is highly adaptable and can treat chrome-plating wastewater, chromium-containing passivation wastewater, and mixed wastewater. It can remove not only heavy-metal hydroxides but also other suspended solids, emulsified oil, and surfactants. The principle of flotation for treating chromium-plating wastewater is: under acidic conditions, ferrous sulfate and hexavalent chromium undergo redox; then under alkaline conditions flocs form; under the action of countless micro-bubbles the flocs float to the surface, clarifying the water.
The ion-exchange method mainly uses the exchange ions in ion-exchange resins to exchange with certain ions in electroplating wastewater, removing them and purifying the water.
Domestic experimental research on ion-exchange treatment of electroplating wastewater began in the 1960s. By the late 1970s, driven by an urgent need to solve environmental pollution, the technology developed greatly and has become one of the effective means for treating electroplating wastewater and recovering certain metals, as well as an important link for achieving closed-loop circulation of some plating wastewaters. However, the ion-exchange method requires high capital cost and complex system design and operation management; ordinary small and medium enterprises find it hard to adapt, and poor maintenance and management often fail to achieve the expected results, so its promotion and application are somewhat limited.
Currently, the ion-exchange method is commonly used domestically for chromium- and nickel-containing electroplating wastewater, with relatively mature experience in design, operation, and management. The treated water can meet discharge standards, with good quality and generally reusable. The regenerant eluate after the resin is saturated can, after composition adjustment and purification, be reused in the plating bath, basically achieving closed-loop circulation. In addition, the ion-exchange method can also treat copper-, zinc-, and gold-containing wastewaters.
The electrolysis method mainly causes harmful substances in wastewater to undergo oxidation and reduction respectively at the anode and cathode through the electrolysis process, converting them into harmless substances; or uses the oxidation and reduction products of the electrodes to react chemically with harmful substances in the wastewater, generating water-insoluble precipitates that are then separated, or recovers metals via the electrolysis reaction. Domestically, electrolysis was used to treat chromium-containing electroplating wastewater starting in the 1960s, and experimental research on silver- and copper-containing wastewater in the late 1970s recovered silver, copper, etc., with good results.
Electrolysis for electroplating wastewater is generally used in medium and small plants. Its main features are no need for treatment chemicals, simple flow, easy operation, and small footprint; moreover, because the recovered metals are of high purity, it offers good economic benefits for recovering precious metals. However, when the treatment volume is large, electrolysis consumes much electricity and many iron electrodes, and the separated sludge, like that from chemical treatment, is difficult to dispose of, so it is now less used.
The extraction method uses a solvent that is insoluble in water but can dissolve a certain substance (called solute or extract) in the water; the solvent is added to the wastewater so the solute fully dissolves in it, thereby separating, removing, or recovering the substance from the wastewater. The extraction operation consists of three main steps: mixing, separation, and recovery.
* Tap water -> water pump -> multimedia filter -> activated-carbon filter -> automatic dosing unit -> security filter -> high-pressure pump -> first-stage RO -> intermediate tank -> high-pressure pump -> second-stage RO -> pure-water tank -> pure-water pump (new process)
* Rinse water -> water tank -> water pump -> multimedia filter -> security filter -> ultrafiltration -> plating-solution recovery tank
* Rinse water -> water tank -> water pump -> multimedia filter -> security filter -> ultrafiltration -> plating-solution recovery tank -> high-pressure pump -> reverse osmosis -> cleaning-water tank
Most electroplating plants are comprehensive operations with multiple plating types, involving chromium, nickel, zinc, copper, etc., and can be divided by plated-part type into metal and plastic parts. Although the backward cyanide electroplating process has been largely eliminated, quite a few plants still use it.
The general production process of an electroplating plant is as follows: electroplating production mainly consists of mechanical polishing (grinding or barreling) -> degreasing -> acid etching -> electroplating -> drying -> qualified products into storage.
Cleaner Production in the Electroplating Industry
Cleaner production means continuously adopting measures such as improved design, use of clean energy and raw materials, advanced process technologies and equipment, better management, and comprehensive utilization, to reduce pollution at the source, improve resource-use efficiency, and reduce or avoid the generation and emission of pollutants during production, service, and product use, thereby mitigating or eliminating harm to human health and the environment.
Pre-treatment of plated parts: mechanical polishing (grinding or barreling)
Mainly uses special machinery with polishing wheels or belts (or, for some parts, a barrel with abrasive to remove rust) to remove burrs, scratches, welds, and sand holes from the plated parts, improving flatness and part quality. This step has no wastewater discharge.
Metal products inevitably adhere a layer of oil and grease after various processing and treatments. To ensure strong bonding between the coating and the substrate, the oil on the surface must be removed. There are many degreasing processes, mainly using organic-solvent degreasing, as follows:
Polished parts -> clean-water wash -> organic-solvent degreasing tank -> clean-water tank -> clean-water rinse
In this step, wastewater mainly comes from the clean-water rinsing process, with a pH of 8.5-10.
After degreasing, parts often have much rust and a relatively thick oxide film on the surface. To obtain a bright coating and better bonding with the substrate, the rust and oxide film must be removed; acid immersion also activates the part surface. The process is as follows:
Degreased parts -> acid tank -> recovery tank -> clean-water tank -> clean-water rinse
Wastewater in this stage mainly comes from the clean-water rinsing process and contains large amounts of iron ions, with a pH of 2-5.
Electroplating Production Process and Water Quality of Each Plating Type
Its production process is generally: etched parts -> electroplating tank -> recovery tank -> clean-water tank -> clean-water rinse.
Wastewater in this stage mainly comes from the clean-water rinsing process and contains the corresponding metal ions or cyanides-cyanide copper-plating rinse water contains cyanide and copper ions; chrome-plating rinse water contains hexavalent chromium; nickel-plating rinse water contains nickel ions, etc. Rinse water is split and treated according to plating type: cyanide wastewater is split and then undergoes two-stage cyanide destruction and pH adjustment before solid-liquid separation to meet discharge standards; chromium wastewater is split and then undergoes reduction, followed by neutralization and solid-liquid separation to meet discharge standards.
This step mainly uses mechanical, natural, and thermal energy to dry the water on the surface of electroplated and rinsed parts, preventing rust and damage to the oxide film. This step has no wastewater discharge.
The stripping process has two methods-chemical immersion and anodic electrolysis-with the process:
Defective parts -> stripping tank -> recovery tank -> clean-water tank -> clean-water rinse.
Wastewater in this stage has a pH of 2-6 and mainly comes from the rinse water after stripping. Stripping rinse water may enter its own wastewater pool for treatment, but must not enter the mixed-treatment pool directly; it should be pre-treated separately before being discharged into the corresponding wastewater treatment branch.
It specifies the discharge limits, monitoring, and surveillance 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 sets special discharge limits for water pollutants.
For malodorous pollutants and environmental noise emitted by electroplating enterprises, the corresponding national pollutant discharge standards apply; for the identification, treatment, and disposal of solid waste generated, the national solid-waste pollution control standards apply.
From the date this standard takes effect, the control of water and air pollutant emissions from electroplating enterprises shall follow this standard, and the relevant provisions of the 'Integrated Wastewater Discharge Standard' (GB 8978-1996) and the 'Integrated Air Pollutant Discharge Standard' (GB 16297-1996) shall no longer apply.
