Treatment of Wastewater from Zinc Phosphate Chemical Conversion Coating
Abstract
This article will discuss the potential treatment options for wastewater effluents generated from the zinc phosphate chemical conversion coating process. The article begins by describing the zinc phosphate process and the chemicals and waste products it generates. It then goes on to explain how these materials can be treated using several methods, including sludge processing, precipitation, coagulation, ion-exchange, and adsorption. Problems associated with wastewater discharges are also considered, such as contamination of the aquatic environment and systemic toxicity. The article then goes on to discuss some of the advantages and disadvantages of various treatments available. Finally, recommendations are made as to which treatment methods would be most appropriate for a particular industrial application.
Introduction
Zinc phosphate (Zn3 (PO4)2) is a chemical conversion coating process commonly used to provide corrosion protection for ferrous metal surfaces. It is made up of a bath solution consisting of zinc acetate or zinc chloride, orthophosphoric acid, and other additives. The solution is applied either as a single-application or a multi-stage process depending on the desired finish. During the process, the zinc salts react with the phosphoric acid to form an insoluble film of zinc phosphate that adheres to the surface.
The zinc phosphate process generates a variety of wastes, primarily in the form of sludge, precipitation liquid, ion-exchange regeneration wastes, and process slop. These materials must be disposed of properly to avoid environmental damage and health risks.
Sludge Processing
The sludge generated during zinc phosphate processes can be treated using a variety of methods. Physical de-watering is the most common approach, but other methods may also be employed. For example, biological treatment can be used to reduce organics in the sludge. Optimal treatment of the sludge before disposal depends on the type of sludge produced and other site-specific factors.
Precipitation
Another method for treating wastewater effluents from zinc phosphate processes is precipitation. This process involves adding chemicals, such as lime, iron (III) chloride, or aluminum sulfate, to the wastewater. The added chemicals cause suspended or dissolved metals to form insoluble compounds. This precipitate will settle at the bottom of the wastewater, allowing the supernatant to be discharged.
Coagulation
Coagulation is another common wastewater treatment method. This involves adding polyelectrolyte polymers to the wastewater. The polymers bind with suspended particles, causing them to clump together, or “flocculate”. This increases the effectiveness of settling, allowing more solids to be removed from the wastewater.
Ion-Exchange
Ion-exchange is a more specialized approach to wastewater treatment. This involves passing the wastewater through a resin which is capable of exchanging an ion with one that is of greater environmental concern. Many resins are available for this purpose, including those that can remove certain metals, such as zinc, from solution.
Adsorption
Adsorption is another method for removing contaminants from wastewater. This involves contacting the wastewater with another material, such as activated carbon, which is capable of binding with specific contaminants. The adsorbed material is then removed from the wastewater by either filtration or sedimentation.
Conclusion
The zinc phosphate process generates a variety of wastewater effluents that must be treated properly to avoid contamination of the environment and health risks. Several methods for treating these effluents are available, including physical de-watering, biological treatment, precipitation, coagulation, ion-exchange, and adsorption. Each has its own advantages and disadvantages, so the best method for a particular application must be determined on a case-by-case basis.
