Electroplating Wastewater: Map Every Stream Before Choosing a Treatment Route

Four sources feeding one mixed sewer

Electroplating plants rarely discharge one recognisable waste. Other wastewater alone includes 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, meaning spills, leaks, drips and runoffs. Equipment cooling water is unpolluted apart from a temperature rise. Metal surface treatment covers cleaning before surface treatment, electroplating, passivation-film protection, machining and coating, with electroplating the main activity.

Following the process, the wastewater splits into post-treatment wastewater, spent plating solutions, spent stripping solutions and so on. Before any metal goes into a bath, parts pass through mechanical treatment such as grinding, polishing, sandblasting, barreling and brushing, then chemical treatment such as degreasing, derusting and etching, then electrochemical treatment such as electrochemical degreasing and electrochemical etching. Degreasing commonly uses alkaline compounds: NaOH, Na2CO3, Na3PO4 and Na2SiO3. Badly oiled parts may first see organic solvents such as kerosene, gasoline, acetone, toluene, trichloroethylene and carbon tetrachloride. To shift certain mineral oils, an emulsifier is added to the degreasing solution, typically OP emulsifier, AE emulsifier or triethanolamine oleate soap. The rinsing wastewater and spent solutions from this stage are therefore alkaline and often carry oils and other organic compounds. Pretreatment wastewater accounts for about 50% of the total and contains salts, free acid and organic compounds, with a composition that swings widely with plating type, pretreatment process and management level.

What actually carries the metals

Plating rinse water is the main source of heavy-metal pollution. Plating solutions are built from metal salts and complexing agents: various metal sulfates, chlorides and fluoroborates, plus cyanides, ammonium chloride, nitrilotriacetic acid, pyrophosphates and organic phosphonic acids. Additives go in to improve coating properties, coumarin, butynediol and thiourea as leveling agents, and saccharin, vanillin, benzylideneacetone, p-toluenesulfonamide and benzenesulfonic acid as brighteners. So rinse water carries heavy-metal ions and a little organic matter, and its volume and composition move with part geometry, bath formulation, rinsing method and operating discipline. Rinsing practice in particular drives the heavy-metal concentration, which in turn decides whether recovery is worthwhile.

Post-treatment adds more. Passivation after rinsing, stripping of defective coatings and special surface treatments generate large amounts of heavy-metal wastewater containing 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. Volume is unstable, so this stream is usually combined with mixed or acid-alkali wastewater for treatment. Bath liquors are the third problem. Over long use they accumulate metal ions, or coating and passivation quality degrades because additives are destroyed or effective components drift out of balance. Plants discard part of the liquor and replenish, or discard it entirely, and these liquors carry very high heavy-metal concentrations with widely varying impurity profiles, which is why treatment of them is so process-specific.

Domestic practice first divides the waste into three categories, with reduction used mainly to treat hexavalent chromium. Physicochemical methods dominate. Some approaches are effective but few achieve overall compliance; one that does is the DTCR heavy-metal ion capturing agent from Shaanxi Futianbao Company, which forms a macromolecular chelate with heavy-metal ions in the wastewater and, after flocculation, removes them to national standards.

Four separation routes, four cost profiles

Flotation introduces air into water to generate tiny bubbles. Bubbles adhere to fine suspended solids to form floatable agglomerates, and their buoyant rise carries the solids to the surface as foam or scum. Depending on how bubbles are generated, it is dispersed-air flotation, dissolved-air flotation or electrolytic flotation. In 1978, Tongji University in Shanghai first successfully applied flotation to electroplating heavy-metal wastewater, and the continuous process, compact equipment, small footprint and easy automation made it popular. It handles chrome-plating wastewater, chromium-containing passivation wastewater and mixed wastewater, removing heavy-metal hydroxides as well as suspended solids, emulsified oil and surfactants. On chrome-plating wastewater the sequence is redox between ferrous sulfate and hexavalent chromium under acidic conditions, floc formation under alkaline conditions, and flotation of the flocs by countless micro-bubbles.

Ion exchange swaps the exchange ions in resin for target ions in the water. Domestic experimental research began in the 1960s, and by the late 1970s, driven by urgent pollution pressure, the technology had developed greatly and become an effective means of treating electroplating wastewater and recovering metals, as well as a route to closed-loop circulation. The obstacle is cost: high capital, complex design and demanding operation, which ordinary small and medium enterprises struggle with, so poor maintenance often defeats it. It is now commonly used for chromium- and nickel-containing wastewater, where design and operating experience is mature and treated water is reusable. Once the resin saturates, the regenerant eluate can be adjusted and purified for reuse in the plating bath, which basically closes the loop. Copper, zinc and gold wastewaters are also treatable.

Electrolysis drives oxidation and reduction at the anode and cathode, converting harmful substances into harmless ones, or uses electrode reaction products to form insoluble precipitates, or recovers metals directly. It was used domestically from the 1960s for chromium-containing wastewater, and from the late 1970s for silver and copper with good recovery results. It suits medium and small plants: no treatment chemicals, simple flow, easy operation, small footprint, and high-purity recovered metal, which is attractive for precious metals. At large volumes, though, it consumes a great deal of electricity and many iron electrodes, and the sludge is as hard to dispose of as chemical sludge, so it is now less used.

Extraction adds a water-insoluble solvent that dissolves a target solute, then separates and recovers it in three steps: mixing, separation and recovery.

Cleaner production cuts the load first

Cleaner production means continuously adopting improved design, clean energy and raw materials, advanced process technology and equipment, better management and comprehensive utilization to reduce pollution at the source and cut pollutant generation across production, service and product use. In electroplating it starts before any bath. Mechanical polishing with wheels, belts or a barrel with abrasive removes burrs, scratches, welds and sand holes and generates no wastewater. Degreasing follows, using organic-solvent degreasing tanks followed by clean-water rinsing; wastewater here comes from the rinsing step and sits at pH 8.5-10. Acid etching then removes rust and oxide film and activates the surface; that rinse water carries large amounts of iron ions at pH 2-5.

In the plating stage itself, etched parts move through the electroplating tank, a recovery tank, a clean-water tank and a clean-water rinse. Rinse water 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. Streams are split accordingly. Cyanide wastewater is separated and put through two-stage cyanide destruction and pH adjustment before solid-liquid separation; chromium wastewater is separated, reduced, then neutralized and separated. Drying uses mechanical, natural and thermal energy and discharges nothing. Stripping of defective parts, by chemical immersion or anodic electrolysis, produces rinse water at pH 2-6 that must be pre-treated separately rather than sent directly to the mixed-treatment pool.

Two more sources deserve attention. The main source of zinc is the drag-out liquor from electroplating or pickling: metal is immersed in strong acid to remove oxides, then in a brightener containing strong chromic acid. That wastewater carries hydrochloric acid, zinc and copper ions and organic brighteners, is highly toxic, and in some cases contains carcinogenic, teratogenic and mutagenic substances. Pickling and derusting with hydrochloric and sulfuric acid also use inhibitors such as thiourea, sulfonated coal tar and hexamine benzidine, and the rinse water is highly acidic with heavy metals and a little additive.

The standard that closes the file

The electroplating discharge standard specifies limits, monitoring and surveillance requirements for water and air pollutants, and sets special discharge limits for water pollutants to guide process and pollution-control technology. From the date it takes effect, the relevant provisions of the Integrated Wastewater Discharge Standard (GB 8978-1996) and the Integrated Air Pollutant Discharge Standard (GB 16297-1996) no longer apply. Malodorous pollutants and environmental noise follow their own national standards, and solid waste identification, treatment and disposal follow the national solid-waste pollution control standards. On the reuse side, plants run trains from tap water through multimedia and activated-carbon filtration, dosing, security filtration and two-stage RO to a pure-water tank, or recover plating solution from rinse water via multimedia filtration, security filtration and ultrafiltration.