1. Metal surface treatment includes cleaning before surface treatment, electroplating, passivation film protection, mechanical processing, and coating coverage, with electroplating being the main method. 2. Classification of electroplating wastewater Electroplating wastewater can be classified into pretreatment wastewater, plating rinse wastewater, post-treatment wastewater, and spent plating solution based on the electroplating production process ..

1. Metal surface treatment

Metal surface treatment includes cleaning before surface treatment, electroplating, passivation film protection, mechanical processing, and coating coverage, with electroplating being the main method.

II. Classification of electroplating wastewater

Based on the electroplating production process, electroplating wastewater can be categorized into four types: pretreatment wastewater, plating rinse wastewater, post-treatment wastewater, as well as waste plating solution and waste stripping solution.

Chapter 2: Characteristics of Electroplating Wastewater

1. Pre-treatment of wastewater

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

1. Leveling the surface (including polishing, buffing, sandblasting, barrel finishing, brushing, etc.)

2. Chemical treatment (including degreasing, rust removal, and etching, etc.)

3. Electrochemical treatment (including electrochemical degreasing and electrochemical etching, etc.)

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

Commonly used for pickling and rust removal are hydrochloric acid and sulfuric acid. To prevent corrosion of the plated substrate, certain corrosion inhibitors such as thiourea, sulfonated coal tar, and hexamethylenetetramine benzidine are often added. The cleaning water produced during the pickling and rust removal process generally has a high acidity and contains heavy metal ions and a small amount of organic additives.

Pretreatment 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., with significant variations in composition depending on the type of plating, pretreatment process, and factory management level.

II. Coating rinsing water

Rinsing water from plating operations is a primary source of heavy metal pollution. The main components of electroplating solutions are metal salts and complexing agents, including various metal sulfates, chlorides, fluoroborates, as well as cyanides, ammonium chloride, nitrilotriacetic acid, pyrophosphates, organic phosphonic acids, etc. In addition, to improve the properties of the plating layer, certain organic compounds are often added to the plating solution, such as coumarin and butynediol as leveling agents, and saccharin, vanillin, benzylidene acetone, p-toluenesulfonic acid, benzenesulfonic acid, etc. as brighteners. Therefore, the rinsing wastewater from plated parts contains not only heavy metal ions but also a small amount of organic matter. The discharge volume of rinsing wastewater and the types and concentrations of heavy metal ions vary with various factors such as the physical shape of the plated parts, the formulation of the electroplating solution, the rinsing method, and the level of management in electroplating operations. In particular, the rinsing process has a significant impact on the concentration of heavy metals in the wastewater, directly affecting resource recovery and wastewater treatment efficiency.

III. Post-plating treatment wastewater

Post-plating treatment mainly includes passivation after rinsing, stripping of defective coatings, and other special surface treatments. During the post-treatment process, a large amount of heavy metal wastewater is also generated. Generally, it often contains heavy metals such as Cr6+, Cu2+, Ni2+, Zn2+, and Fe2+; acid-base substances such as H2SO4, HCl, H3BO3, H3PO4, NaOH, and Na2CO3; and organic substances such as glycerin, nitrilotriacetic acid, hexamethylenetetramine, dye-proof salt S, and acetic acid.

In general, this type of post-treatment wastewater from plating is complex and variable, with unstable water volume, and it is generally treated in combination with mixed wastewater or acid-base wastewater.