Phosphatizing wastewater is from metal surface preparation operations where the metal is either sprayed or dipped in a hot phosphatizing solution. The phosphatized metal surface is then ready for painting, bonding with plastic or rubber, cold extrusion or alone as a metal preservative.
Phosphatizing wastewater typically from rinsing and batch dumps is at a low pH of ~2.5, is hot (~110F), and contains a soluble form of phosphate. In order to remove this soluble form of phosphate it must first be made insoluble. The insoluble phosphate is then coagulated, flocculated and clarified by sedimentation.
The typical method to reduce and remove phosphate from wastewater is as follows:
Stage 1 Precipitation and Coagulation:
pH is raised from ~2.5 to 8.5 with the pH controller using caustic while adding a coagulant such as ferric chloride. Testing of the wastewater may confirm that a coagulant is not needed. A “pin floc” is developed indicating the phosphate is insoluble.
Stage 2 - Flash Mix:
The wastewater with it’s precipitated pin floc is introduced to the flash mix zone where a polymer flocculent is added. This stage maximizes flocculent dispersion throughout the coagulated wastewater.
Stage 3 - Flocculation:
The wastewater is now introduced to the slow mix zone to agglomerate the pin floc into larger rapid settling particles.
Clarifier, Inclined Plate:
The flocculated wastewater is introduced into the clarifier where the settling particles land on the inclined plates and are directed downward and into the sludge chamber. The clarified treated water then exits the top of the clarifier and flows downstream to sewer or further treatment if necessary.
Clarifier Sludge Handling:
The resulting clarifier waste sludge is periodically removed from the clarifier at a slow rate and sent to the sludge holding tank where it further thickens and accumulates a batch for disposal or processing in a filter press.
The thickened clarifier sludge is allowed to accumulate sufficiently to provide a full batch for the Filter Press. The filter press is pumped full of the sludge until it is full. The filter press is then emptied of the “cake” which is a semi solid of approximately 20-35 % solids. Sludge cake is high in phosphate and should be disposed of according to environmental regulations.
Phosphatizing shops are found in typically two categories, captive and independent shops. Some industries operate their own captive, in house plating operation while others outsource to an independent operation.
Typical industries include:
Plating shops, captive or independent
Automotive, trucks, motorcycles – Metal to rubber suspension and body parts
Can making plants – phosphatizing for corrosion resistance
Steel coil coating – large steel coils are phosphatized for corrosion resistance and painting preparation.
Metal forming such as conduit, nails, nuts, bolts and screws, electrical parts.
Aerospace – corrosion resistance surface prep
Appliance makers – phosphatizing is done to internal parts for corrosion resistance prior to painting
Computer component makers – phosphatizing is used as a dielectric surface prep