Acid Pickling and Phosphating Wastewater Treatment Equipment

　　Acid pickling and phosphating is a pre‑treatment process used prior to metal machining, designed to remove oil and rust, perform surface conditioning, and apply a phosphate conversion coating. The wastewater generated during this process originates from degreasing and phosphating operations in metal finishing. It primarily contains emulsified oils, degreasers, lubricants, surfactants, suspended solids, phosphate ions, as well as organic and inorganic contaminants such as acids and alkalis. The combined effluent exhibits high levels of organic matter, along with substantial amounts of phosphorus, grease, and suspended solids.

　　Acid Pickling and Phosphating Wastewater Treatment Process Flow

　　To ensure that pollutants in the production wastewater meet discharge standards, the project employs a treatment process comprising chemical precipitation, coagulation–air flotation, and activated carbon adsorption to treat acid‑pickling and phosphating effluent. The treated effluent consistently complies with the National First‑Class Wastewater Discharge Standard (GB 8978‑1998).

　　Our company’s acid‑pickling and phosphating wastewater treatment equipment, taking into account the characteristics of such effluents and the specific features of phosphating discharge, prioritizes the removal of PO4³⁻, petroleum‑derived compounds, and Zn²⁺. The process employs a sequence of chemical precipitation, coagulation–air flotation, and activated carbon adsorption to treat the acid‑pickling and phosphating wastewater, with the treatment flow diagram shown in the figure.

　　After being adjusted in an equalization tank, the phosphorus‑containing wastewater from the phosphating workshop is pumped to the first‑stage reactor, where NaOH is dosed to regulate the pH. The optimal pH for coagulation and sedimentation is maintained at 8.5–9.0. Following thorough mixing, Zn²⁺ in the wastewater reacts with NaOH to form Zn(OH)₂ precipitate. Subsequently, a coagulant—PAC—along with a small amount of polymeric flocculant (PAM) is added; after passing through the primary clarifier to remove Zn²⁺, the effluent flows into an intermediate tank. It is then pumped to the second‑stage reactor, where lime milk is dosed to raise the pH to 10–11. In the secondary clarifier, PAC and a minor dose of PAM are again introduced, enabling calcium ions to react with phosphate ions and precipitate as calcium phosphate, thereby removing phosphates. After removal of Zn²⁺ and phosphates, the wastewater undergoes air flotation and filtration to further reduce its COD. To ensure that the effluent pH meets regulatory standards, a pipeline mixer is incorporated to facilitate neutralization with H₂SO₄ prior to discharge.

　　The treatment process for acid‑pickling and phosphating wastewater primarily employs physicochemical methods, with the main objectives of neutralizing acids and alkalis in the effluent and removing substantial amounts of suspended solids, phosphate ions, metal ions, and other contaminants.

　　The commonly used treatment process is neutralization, coagulation, high-efficiency lamella sedimentation, and air flotation.

　　Description of the Acid Pickling and Phosphating Wastewater Treatment Process

　　1. Chemical removal of zinc

　　Chemical methods for zinc removal include the sulfide process and alkaline coagulation–precipitation, among others. This process employs alkaline coagulation–precipitation to remove zinc because the solubility product constant of Zn(OH)2 is Ksp = 7.1 × 10⁻¹⁸. Based on the precipitation–dissolution equilibrium of metal hydroxides M(OH)n and the ionic product of water Kw = [H⁺][OH⁻], the pH at which the hydroxide precipitates can be calculated as: pH = 14 − (1/n) × (log[Mⁿ⁺] − logKsp). Sodium hydroxide is added to adjust the pH, with the optimal pH for coagulation–precipitation maintained at 8.5–9.0. Under these conditions, Zn²⁺ reacts with NaOH to form Zn(OH)₂ precipitate, achieving complete removal.

　　2. Chemical precipitation of phosphorus using the lime process

　　Lime milk is dosed to adjust the pH to 10–11, causing most of the phosphate in the wastewater to precipitate and be removed. Simultaneously, the lime milk also serves to neutralize the effluent.

　　3. Coagulation–air flotation for phosphorus and oil removal

　　Adding a flocculant induces coagulation between residual phosphorus and other pollutants in the wastewater; the resulting flocs adhere to numerous fine bubbles generated by air flotation, and the buoyant force of these bubbles carries them to the water surface. While removing phosphorus, this process can also adsorb and eliminate oil droplets. With rapid bubble ascent and safe, reliable operation, it represents an economical and practical oil‑removal technology.

　　4. Phosphorus removal by activated carbon adsorption

　　Activated carbon is the most widely used adsorbent in water treatment, characterized by excellent adsorption performance and chemical stability, resistance to breakage, and low airflow resistance. It is commonly available in powdered and granular forms. This process leverages the enormous specific surface area of activated carbon to effectively adsorb trace residual phosphorus and other pollutants from wastewater.

　　5. Mixing and blending device

　　Both the primary and secondary reactors are equipped with agitators, driven by pulleys or gear reducers to progressively reduce the stirring speed within the tanks. This ensures optimal mixing at each stage of the reaction process, promoting more complete reactions and facilitating the subsequent sedimentation step. Additionally, in‑line static mixers are used to enhance the neutralization reaction when H₂SO₄ is dosed, enabling the effluent pH to consistently meet regulatory standards.

　　6. Sludge removal by gravity or water pressure

　　In the system, sludge and scum generated in the primary and secondary reactors as well as in the dissolved‑air flotation tank are periodically discharged into the sludge thickening tank by gravity or hydraulic pressure for concentration. After natural drying, the concentrated sludge is transported offsite for landfill disposal, while the supernatant from the thickening tank is returned to the equalization basin for physicochemical treatment.

　　7. Backwashing and Desorption

　　In the system, both the silica sand filter and the activated carbon adsorber require periodic backwashing to prevent clogging, with the resulting wastewater returned to the equalization tank. Additionally, to regenerate the activated carbon, a high-temperature thermal regeneration process is employed, using steam to desorb contaminants from the granular carbon in the activated carbon filter, thereby ensuring its continued adsorption performance.