1、 Main characteristics of phosphating wastewater electroplating wastewater treatment equipment

Phosphating wastewater contains a large amount of phosphate, zinc ions, acidic and alkaline substances, and organic compounds. Depending on the production process, it sometimes also contains a certain amount of heavy metals such as nickel ions, copper ions, or lead ions, as well as surface active agents and other pollutants. The composition is complex and the treatment difficulty is high. At present, the treatment of such wastewater mainly relies on physical and chemical methods, which adopt a combination of processes such as distributed precipitation, air flotation, filtration, activated carbon adsorption, and membrane separation technology according to different treatment objects and purposes. When selecting the process, it is necessary to consider reducing the number of process steps, lowering engineering investment and operational difficulty, as well as considering the mutual interference between different pollution factors to reduce treatment effectiveness. The main pollutants in acid pickling and phosphating wastewater are phosphate and zinc ions, among which phosphate is also a difficult point in the treatment of this type of wastewater.

2、 The forms of existence of phosphating wastewater

Phosphate emissions are high, mainly in the form of inorganic salts such as zinc dihydrogen phosphate, as well as pollutants such as COD, petroleum, and suspended solids. The mainstream treatment process for phosphating wastewater in engineering practice is chemical precipitation/coagulation air flotation/sand filtration/activated carbon adsorption, which is complex, requires a large amount of chemicals, and has high operating costs.

3、 Processing technology

Phosphating treatment is a chemical reprocessing process applied to the surface of metal materials and their components. The surface of metal materials and their products treated with phosphating electroplating wastewater treatment equipment forms an immersive phosphate film layer, which has good bonding ability, wear resistance, and adhesion to coatings with the metal substrate. Therefore, industries such as machinery and steel use phosphating treatment technology to make protective layers for mechanical parts. The phosphating treatment process generally includes several steps such as alkaline washing to remove oil, hot water rinsing, cold water rinsing, acid washing to remove rust, secondary cold water rinsing, and phosphating. During the process, various types of wastewater are mainly generated, including alkaline washing emulsion wastewater, rinsing wastewater, acid washing waste liquid, and phosphating waste liquid.

The most commonly used methods for treating phosphorus containing wastewater are chemical precipitation and biological methods. Production treatment is mostly used to treat organic phosphorus wastewater. The phosphorus containing wastewater generated in electrolysis and chemical polishing mainly exists in the form of H2PO4- (pH=2-7) and HPO42- (pH=7-12), which can be treated by chemical precipitation method. The basic requirements of chemical precipitation method are:

1) After adding chemical precipitant, all phosphorus must be converted into insoluble phosphate;

2) The generated phosphate precipitate can be completely removed from wastewater by filtration, resulting in P ≤ 1mg/L in the wastewater.

4、 Process Introduction

1. Chemical method for zinc removal

Chemical methods for zinc removal include sulfide method and alkaline method coagulation precipitation. This process uses alkaline coagulation precipitation to remove zinc, because the solubility product constant Ksp of Zn (OH) 2 is 7.1 × 10-18. Based on the precipitation dissolution equilibrium of hydroxide M (OH) n and the ion product Kw of water=[H+] [OH -], the pH value that causes hydroxide precipitation can be calculated, which is pH=14-1/n × (log [Mn+] - logKsp). Add NaOH to adjust the pH value and control the optimal pH value for coagulation precipitation at 8.5~9.0. Under these conditions, Zn2+reacts with NaOH to form Zn (OH) 2 precipitate, which is completely removed.

2. Lime chemical precipitation for phosphorus removal

By adding lime milk clarification solution and adjusting the pH value to 10-11, the majority of phosphates in the wastewater can be precipitated and removed. Lime milk also serves as a neutralizing agent while removing phosphorus.

3. Coagulation air flotation for phosphorus and oil removal

Adding coagulants causes coagulation reactions between residual phosphorus and other pollutants in wastewater, resulting in flocs that attach to a large number of fine bubbles generated by air flotation, and are carried out of the water surface by the buoyancy of the bubbles. At the same time as phosphorus removal, it can also adsorb and remove oil droplets, with fast bubble floating speed and safe and reliable operation, making it an economical and practical oil removal technology.

4. Activated carbon adsorption for phosphorus removal

Activated carbon is the most commonly used adsorbent in water treatment, with good adsorption performance and chemical stability, not easy to break, low airflow resistance, commonly used in powder and granular forms. This process utilizes the huge specific surface area of activated carbon to fully adsorb trace residual phosphorus and other pollutants in wastewater.

5. Mixing and Mixing Device

Both primary and secondary reactors are equipped with stirring devices, using pulleys or reducers to gradually decrease the stirring speed in the tank, in order to meet the stirring and mixing needs of different stages of the entire reaction process, making the reaction more thorough and more conducive to the subsequent precipitation process. The pipeline mixer in the process makes it easier to carry out neutralization reaction when adding H2SO4, and can fully meet the pH value of the effluent.

6. Discharge of mud by gravity or water pressure

The sludge or floating slag generated in the primary and secondary reactors and air flotation tank in the system are regularly discharged into the sludge concentration tank for concentration by gravity or water pressure, and then transported for landfill after natural drying. The supernatant of the concentration tank is returned to the regulating tank for physical and chemical treatment.

7. Backwash and desorption

The silica sand filter and activated carbon adsorber in the system need to be backwashed regularly to prevent blockage, and the wastewater generated from them flows back to the regulating tank. In addition, to regenerate activated carbon, high-temperature heating regeneration method is used to activate it, and water vapor is used to desorb the particulate carbon in the activated carbon filter to ensure normal adsorption effect.