Treatment of automobile coating wastewater

Brief introduction of car coating process

Coating process is generally composed of three basic processes: surface treatment before painting, coating and drying.

The surface treatment before painting is the basis of painting process, which includes surface cleaning (derusting, degreasing, etc.) and phosphating treatment.

Degreasing is generally cleaned by hot alkaline solution and organic solvent. Alkaline solution is made of strong alkali, weak acid, polymerized alkaline salt (such as phosphate, silicate, etc.), surfactant (cationic or non-ionic) and other appropriate coordination.

Phosphating treatment is to form a non-metallic, non-conductive and porous phosphate film on the metal surface through chemical reaction, which can significantly improve the adhesion of the coating. Corrosion and water resistance. The body, carriage and other phosphating are generally treated by the film zinc salt fast phosphating. The main components of phosphating solution are zinc dihydrogen phosphate, oxidant (such as sodium nitrate), catalyst (such as sodium nitrite, sodium chlorate) and some additives (such as sodium tripolyphosphate, sodium fluoride). After phosphating treatment, it is usually washed for 2-3 times.

Coating refers to the operation of spreading the coating on the surface of the object to be coated. At present, most of the cationic water-soluble coatings are applied by cathodic electrophoresis. After electrophoresis, the ultrafiltration was used for 2-3 times of recovery water washing and then deionized water washing. For cars and light trucks with high decorative requirements, the electrostatic coating method is generally used to apply the intermediate coating; for the top coating, the cyanoaminoalkyd resin enamel is generally used, and the automatic spraying or electrostatic spraying is used.

Characteristics of automobile coating wastewater

Pollution source analysis

The wastewater produced in the coating process mainly includes pretreatment wastewater, electrophoretic coating wastewater and painting wastewater.

Pretreatment wastewater comes from degreasing, phosphating, surface adjustment and other processes of paint pretreatment, including emulsified oil, surfactant, phosphate, heavy metal ions (such as Zn2 +, filler (such as titanium dioxide), solvent, etc. The waste water of electrophoretic coating is produced in the cleaning process of floating paint and bath solution attached to the coating, generally including deionized water and ultrafiltration; its composition is the same as that of bath solution, including water-soluble resin (such as epoxy resin, phenolic resin, etc.), pigment (such as carbon black, iron oxide red, lead mercury, etc.), filler (such as titanium dioxide, talcum powder, etc.), cosolvent (such as triethanolamine, butanol, etc.) and a small amount Heavy metal ions.

The wet spray booth uses water to wash the air in the operation area of the spray booth, and the paint mist and organic solvent in the air are transferred to the water to form the spray paint waste water; the waste water contains a large number of paint mist particles, whose water quality is determined by the paint materials used (mainly nitro paint, amino paint, alkyd paint and epoxy paint), solvents (such as ethanol, acetone, lipids, benzene, etc.), and cosolvent.

2.2 characteristics of automobile coating wastewater

2.2.1 waste water has many types and complex components

There are many kinds of waste water (or waste liquid) discharged from automobile coating line. The quality (composition, concentration) of each kind of waste water varies with the materials used. There are many formulations of degreasing solution alone, and more types of coating (any coating is composed of resin, pigment, solvent, additive, etc.).

2.2.2 irregular discharge

In addition to the continuous overflow discharge of some washing water, most of the waste water or liquid waste from the coating line is discharged in a centralized way intermittently. See Table 1 for the drainage of body workshop in an automobile factory.

2.2.3 large changes in water quantity and quality

Due to the different composition and concentration of various waste water, and the irregular discharge, the water quantity and quality of automobile coating line change greatly and irregularly.