Introduction

Electroplating Pollution: Electroplating employs electrochemical methods to deposit a thin protective coating (typically metallic) onto a prepared metal surface. Pretreatment (cleaning, degreasing, and other preparation stages), plating, rinsing, passivating, and drying are all part of the process. Depending on the metal surface to be plated, cleaning and pretreatment phases use various solvents (typically chlorinated hydrocarbons, which are prohibited) and surface stripping agents, such as caustic soda and a range of strong acids.

Degreasing does not require the use of halogenated hydrocarbons because water-based solutions are available. In the plating process, the cathode in an electrolytic bath is generally the item to be plated. Acid or alkaline plating solutions may contain complexing agents like cyanides.

Electroplating Pollution in the industry

Any or all of the electroplating chemicals (such as acidic solutions, hazardous metals, solvents, and cyanides) might end up in the wastewater, either by product rinsing or spillage and dumping of process baths. High quantities of volatile organic compounds (VOCs) and, in certain circumstances, volatile metal compounds, which may contain chromates, are produced by the solvents and vapours used in hot plating baths. When baths are not regenerated, around 30% of the solvents and degreasing chemicals used might be emitted as VOCs. Mixing cyanide with acidic wastewaters can result in deadly hydrogen cyanide gas, which must be avoided at all costs.

The entire effluent stream is varied (1 litre to 500 litres per square metre of surface plated). Still, it is frequently rich in heavy metals such as cadmium, chrome, lead, copper, zinc, nickel, and cyanides, fluorides, and oil and grease, all of which are process dependant. In addition, toxic organics like trichloroethylene and trichloroethane may be present in air emissions. Cleaning or replacing process tanks and wastewater treatment can result in large amounts of wet sludge containing high levels of hazardous organics or metals.

Controlling electroplating pollution

Plating entails various combinations of a wide range of processes, and there are several possibilities to improve on industry standards. Therefore, wherever practicable, the following enhancements should be made.

  • Zinc plating, which is high-quality and corrosion-resistant, can be used instead of cadmium plating. Wherever possible, use cyanide-free zinc plating processes. For example, use brilliant chloride, high-alkaline baths, or other options instead of cadmium plating. However, specific alternatives to cyanides may induce the discharge of heavy metals, which can pose difficulties with wastewater treatment.
  • Instead of hexavalent chrome, use trivalent chrome; acceptability of the finish modification must be encouraged.
  • Wherever possible, use water-based surface-cleaning chemicals instead of organic cleaning solutions, which can be hazardous in some cases.
  • When possible, regenerate acids and other process components.
  • Reduce drag out by emptying bath solutions from the plated component effectively, for example, by drilling drain holes in bucket-type parts if necessary.
  • Allow at least 10 to 20 seconds for dripping before rinsing.
  • While dripping, use fog spraying of components.
  • To prevent drag out, keep the baths’ density, viscosity, and temperature constant.
  • Place the rinse tanks before the recovery tanks (also yielding makeup for the process tanks). The recovery tank provides static rinsing with high drag-out recovery.

Rinsing System Water Consumption Minimization

It is feasible to create rinse systems that use 50–99 per cent less water than conventional systems. Testing is necessary to find the best way for each given procedure. However, some tried and accurate methods include:

  • Increasing rinsing efficiency by agitating rinse water or work items
  • Rinses with several countercurrents
  • Rinses with a spray (especially for barrel loads).

Targeting pollution loads

The amount of water used in each process is an essential factor to consider. Therefore, water-saving systems should be implemented. For example, water consumption of no more than 1.3 litres per square metre plated (l/m2) for rack plating and 10 l/m2 for drum plating should be reached if electroplating is frequently conducted on items with the available surface area in a production unit. The proposed pollution prevention and control techniques are capable of achieving the below-mentioned goal values.

  • Cadmium plating should be avoided if at all possible. When no other options are available, a maximum cadmium load in waste of 0.3 grammes per kilogramme of cadmium treated is suggested.
  • An air pollution control system, such as a carbon filter, must collect at least 90% of the solvent emissions to the air.
  • Solvents that deplete the ozone layer, such as chlorofluorocarbons and trichloroethane, are not allowed to be used in the procedure.

Treatment 

Because of the potentially hazardous reactions that might occur, waste streams must be separated. For example, strong acid and caustic reactions can cause corrosive liquids to boil and spill, and acids can react with cyanides to produce deadly hydrogen cyanide gas. Furthermore, concentrated streams that are separated are easier to treat.

  • Exhaust hoods and effective ventilation systems safeguard the working environment, but exhaust streams should be cleaned before venting to the atmosphere to decrease VOCs and heavy metals to tolerable levels. Before releasing, acid mists and vapours should be cleaned with water. VOC levels in vapours can be decreased in some circumstances by using carbon filters, which allow solvents to be reused, or by burning (and energy recovery) following cleaning, absorption, or other treatment procedures. 
  • Electroplating plants must have cyanide destruction, flow equalisation and neutralisation, and metals removal as a minimum. Individual design is required to suit the unique plant’s peculiarities. However, there are a few typical treatment processes. The option of sharing a common wastewater treatment facility should be examined for small facilities. The elimination of cyanide must take place before the other treatment steps. If hexavalent chrome is found in the effluent, it is generally processed with a reducing agent, such as sulphide, to decrease the chromium to a trivalent form. Equalisation, pH adjustment for precipitation, flocculation, and sedimentation/filtration are the major treatment procedures. The ideal pH for metal precipitation is generally between 8.5 and 11. However, this varies depending on the metal combination present. Significant amounts of oil and grease can reduce the efficacy of the metal precipitation process; as a result, the treatment choices and sequence are influenced by the quantity of oil and grease. The degreasing baths should be handled separately if possible. Flocculating chemicals are occasionally used to help with suspended particles filtration. Pilot testing and treatability studies may be required, and a final pH correction and further effluent polishing. Ion exchange, membrane filtration, and evaporation are all used in modern wastewater treatment systems to decrease harmful release and the amount of effluent that must be released. A closed system with a small bleed stream is possible with this design.
  • Treatment sludges typically include significant metals and should be treated as hazardous waste or delivered to a metals recovery facility. Metals can be recovered using electrolytic techniques. Sludges are often hardened, diluted with water, and stabilised using chemical agents (such as lime) before being disposed of in a controlled and permitted landfill. The high costs of appropriate sludge disposal are expected to become a stronger motivator for waste reduction in the future.