Electroplate

The electroplating industry is an indispensable link in the national economy, involving the fields of national defense, industry, and daily life. From a general perspective, it can be divided into metal electroplating and plastic electroplating for machine parts, achieving anti-corrosion, aesthetic, extended service life, and exterior decoration effects for workpieces.

The wastewater generated by electroplating is highly toxic and poses a threat to soil, animal and plant growth. Therefore, it is necessary to strictly treat wastewater that meets the discharge standards, and promote wastewater treatment and recycling in water deficient areas. From a technical production perspective, due to the need to add a certain amount of various chemicals in the electroplating production process and wastewater treatment process. After the treatment of electroplating wastewater, it can be recycled and reused. The reused water must be desalinated before it can be reused for production line water, without reducing the total salt content in the environment. The concentrated solution from resin exchange and reverse osmosis processes still returns to the ground.

1. Electroplating wastewater treatment process

The design of wastewater treatment process is determined based on the properties, components, and enterprise conditions of wastewater, as well as the requirements for water quality parameters after treatment, after comprehensive technical and economic comparison.

There are many processes for treating electroplating wastewater: resin exchange was popular in the 1970s, and electrolysis, chemical method, and air flotation were popular in the 1980s. According to our factory's 20 years of practice in electroplating wastewater treatment, resin exchange has its advantages in treating precious and rare metal ion wastewater and recovering precious and rare metals.

Electrolysis method: It has high energy consumption, high electricity and iron consumption, and is not suitable for producing too much sludge in high concentration chromium containing wastewater. At the same time, the treatment of cyanide containing wastewater is not ideal, so chemical methods are also needed for cyanide containing wastewater.

Chemical agents+air flotation method: Chemical agents are used for oxidation, reduction, neutralization, and air flotation is used for mud water separation. Due to the large proportion of electroplating sludge and the presence of various organic additives in wastewater, the air flotation separation is not complete in practical use, and the operation and management are inconvenient. By the late 1990s, the application of air flotation method was becoming less and less common.

Chemical agent+precipitation: This method is the earliest applied method, after more than 30 years of actual use comparison of different treatment processes. At present, it has returned to the earliest and most effective treatment process, and this method is mostly used in electroplating treatment abroad. However, after a long time of solid-liquid separation operation, sludge will flip up in the sedimentation tank, making it difficult to ensure stable and standard effluent.

The biological treatment process developed in recent years has high operating efficiency with a small amount of water and a single seed plating. Many large engineering applications are very unstable due to the difficulty in maintaining constant water quality and quantity. Microorganisms are difficult to adapt to changes in water temperature, variety, concentration of heavy metal ions, and pH value, resulting in a large number of microbial deaths in an instant, environmental pollution accidents, and difficult cultivation of bacteria.

This process is aimed at treating wastewater with different properties by adding different drugs for oxidation-reduction neutralization, and then using direct pressure filtration separation method to separate sludge. It has the advantages of low investment, convenient operation and management, stability and reliability, and low energy consumption.

Currently, many water scarce areas require the recycling and reuse of electroplating wastewater. On the basis of GB8978-1996 primary discharge pre-treatment, the water quality is deeply purified. The main recycled water contains a high salt content, accounting for 20% -23%, and must undergo desalination treatment. The process of coarse filtration, fine filtration, ultrafiltration, and reverse osmosis is used to meet the drinking water quality standards. This has a certain significance for the reuse of water resources, but concentrated pollutants such as chromium salts, accounting for 20% -23%, still return to the environment.

Five years ago, our factory used chemical agents, sedimentation, filtration, and ion exchange methods to treat high demand electroplating wastewater at Qianjiang Group, which requires heavy metals to be less than 0.05mg/l, which is 10 times better than the national discharge standard. Based on the current level of technology, reverse osmosis is more suitable.

Based on our company's practical experience in wastewater treatment engineering over the years, both the air flotation method and the sedimentation method can achieve solid-liquid separation under good reaction conditions with the addition of an appropriate amount of reagents. As long as solid-liquid separation is achieved and the separation is complete, stable, and reliable, and it is also suitable for high concentration wastewater treatment, timely and effective separation can be achieved. Neither the air flotation method nor the sedimentation solid-liquid separation method can meet the above conditions, This conclusion has been confirmed in previous projects conducted by our factory. For example, after centralized neutralization of the wastewater discharged by 85 small electroplating factories in Wenzhou Longwan Electroplating Base, the suspended solids account for about 50% of the total volume. When a sedimentation tank is used for separation, the water is clear in the first hour, and sludge is turned over in the second hour, causing the effluent to fail to meet the standard. To ensure continuous water discharge, continuous sludge discharge is required, with a sludge discharge volume of 50% of the inflow. This results in a large sludge treatment capacity, and these sludge needs to be dehydrated for treatment.

Based on years of engineering experience in our factory, the direct use of pressure filtration method for this high concentration wastewater can reduce investment in sedimentation tanks and ensure stable and standard treatment of wastewater with different concentrations. When the amount of sludge in the influent is small, the water output from pressure filtration is still large, and this method has been applied in multiple wastewater treatment projects. After more than two years of operation, the user feedback is good. Adopting direct pressure filtration can save investment by more than 10% and reduce floor area by 20%.

2. Brief description of electroplating production process and wastewater discharge

Most electroplating factories carry out comprehensive multi plating operations, involving multiple plating types such as chromium, nickel, zinc, copper, etc. From the types of plated parts, they can be divided into metal plated parts and plastic plated parts. Although the cyanide plating process is outdated and mostly eliminated, there are still many electroplating factories still using it.

The production process of a general electroplating factory is as follows: the electroplating production process mainly includes mechanical polishing (polishing or rolling) → degreasing → acid etching → electroplating → drying → qualified products entering the warehouse

Unqualified product stripping

2.1. Mechanical polishing (polishing or rolling) for plating pretreatment

The main method is to use special machinery to remove burrs, scratches, weld nodules, sand holes, etc. from the plated parts by using grinding wheels or belts in the machinery (or using a roller and abrasive to remove rust from certain plated parts), in order to improve the flatness of the plated parts and improve the quality of the plated parts. There is no wastewater discharge in this process.

2.2. Degreasing

The plated parts of metal products inevitably adhere to a layer of oil stains due to various processing and treatments. In order to ensure a solid connection between the coating and the substrate, it is necessary to remove the oil stains on the surface of the plated parts. There are many types of oil removal processes, mainly using organic solvents for oil removal. The process is as follows: polished parts → clean water washing → organic solvent oil removal tank → clean water tank → clean water washing. The wastewater in this process mainly comes from the clean water washing process, and the pH value of the water is between 8.5-10.

2.3 Etching

After degreasing, there is often a lot of rust and thick oxide film on the surface of the parts. In order to obtain a bright coating and better bond between the coating and the substrate, it is necessary to remove the rust and oxide film from the parts. After acid immersion, the surface of the parts can also be activated. The process is as follows:

Parts after oil removal → Sour water

Tank → Recovery Tank → Clear Water Tank → Clear Water Flushing

The wastewater from this section mainly comes from the clean water flushing process, which contains a large amount of iron ions with a pH value between 2 and 5.

2.4 Electroplating production process and water quality of each plating species

The production process generally includes: parts after etching treatment → electroplating tank → recycling tank → clean water tank → clean water flushing.

The wastewater from this section mainly comes from the clean water flushing process, which contains corresponding metal ions or cyanide. In the cyanide copper plating flushing water, there are cyanide and copper ions; Chromium plating rinse water contains hexavalent chromium; Nickel plating rinse water contains nickel ions, etc. The flushing water is treated by separating the effluent according to the different plating types. For example, the cyanide containing wastewater is separated and then subjected to secondary cyanide breaking and pH adjustment. After solid-liquid separation, it can meet the discharge standards; Chromium containing wastewater can be discharged up to standard after being separated and subjected to reduction reaction, neutralization, and solid-liquid separation.

2.5. Drying and warehousing

This process mainly relies on machinery, natural energy, and thermal energy to dry the moisture on the surface of the parts after electroplating and rinsing, in order to avoid rusting and damage to the oxide film. There is no wastewater discharge in this section of the process.

2.6. Deplating

There are two methods for plating removal: chemical impregnation and anode electrolysis, which are:

Unqualified plating parts → return plating tank → recycling tank → clean water tank → clean water flushing.

The pH of the wastewater in this section is between 2 and 6, and the wastewater mainly comes from the rinsing water after plating. The plating and rinsing water can enter the respective wastewater tanks for treatment, but cannot directly enter the wastewater mixing treatment tank. It should be pre treated separately before being discharged into the corresponding wastewater treatment tributary.