1、 Metal surface treatment

Metal surface treatment includes cleaning before surface treatment, electroplating, passivation film protection, mechanical processing, and coating coverage, mainly focusing on electroplating.

2、 Classification of Electroplating Wastewater

From the perspective of electroplating production process, electroplating wastewater can be divided into four categories: pre-treatment wastewater, plating rinsing wastewater, post-treatment wastewater, as well as waste plating solution and waste plating solution.

Chapter 2 Characteristics of Electroplating Wastewater

1、 Pre treatment wastewater

For metal substrate materials, the electroplating treatment process can be divided into:

1. Leveling the surface (including grinding, polishing, sandblasting, rolling, brushing, etc.)

2. Chemical treatment (including oil removal, rust removal, and erosion)

3. Electrochemical treatment (including electrochemical oil removal and electrochemical erosion, etc.)

Alkaline compounds such as NaOH, Na2CO3, Na3PO4, Na2SiO3, etc. are commonly used in the oil removal process. For parts with particularly severe oil pollution, organic solvents such as kerosene, gasoline, acetone, toluene, trichloroethylene, and carbon tetrachloride are sometimes used for oil removal, followed by chemical alkaline oil removal. To remove certain mineral oils, a certain amount of emulsifiers, such as OP emulsifier, AE emulsifier, triethanolamine oleic acid soap, etc., are usually added to the degreasing solution. Therefore, the cleaning wastewater and renewal waste liquid generated during the oil removal process are alkaline wastewater, often containing oil and other organic compounds.

Hydrochloric acid and sulfuric acid are commonly used for acid pickling and rust removal. In order to prevent corrosion of the plating substrate, certain corrosion inhibitors such as thiourea, sulfonated coal tar, and urotropine biphenylamine are often added. The cleaning water generated during the acid pickling and rust removal process generally has a high acidity, containing heavy metal ions and a small amount of organic additives.

Pre treatment wastewater is an important component of electroplating wastewater treatment, accounting for about 50% of the total electroplating wastewater. The wastewater contains certain salts, free acids, organic compounds, etc., and the composition changes greatly, depending on the plating type, pre-treatment process, and factory management level.

2、 Plating rinsing water

Plating rinse water is the main source of heavy metal pollution in electroplating operations. The main components of electroplating solution are metal salts and complexing agents, including various metal sulfates, chlorides, fluoroborate salts, as well as cyanide, ammonium chloride, aminotriacetic acid, pyrophosphate, organic phosphonic acid, etc. In addition, to improve the properties of the coating, certain organic compounds are often added to the plating solution, such as coumarin, butynediol, thiourea as leveling agents, and saccharin, vanillin, benzylidene acetone, p-toluenesulfonamide, benzenesulfonic acid, etc. as brighteners. Therefore, in addition to heavy metal ions, the rinsing wastewater of plated parts also contains a small amount of organic matter. The discharge amount of rinsing wastewater, as well as the types and concentrations of heavy metal ions, vary with many factors such as the physical shape of the plated parts, the formula of the plating solution, rinsing methods, and the level of electroplating operation management. Especially the rinsing process has a significant impact on the concentration of heavy metals in wastewater, directly affecting resource recovery and wastewater treatment efficiency.