Zinc is most often found in plating and galvanizing operations. In plating shops, the zinc is often complexed with cyanide and the cyanide must be treated to free the zinc before precipitation can occur. Traditional cyanide destruct systems use sodium hypochlorite to oxidize the cyanide. Like copper, zinc can be precipitated as the hydroxide salt. Finally, it can be removed by ion exchange in methods similar to copper.
Precipitation of the insoluble hydroxide salt is the most common form of treatment. This salt is formed by adjusting the pH of the water to about 10-10.5 to form the precipitate. If cyanide is present, it must be pre-treated before entering the Hydro-Flo Technologies treatment system. When other complexing agents are present, Hydro-Flo engineers can design a treatment system using metal trapping chemistry.
Ion exchange can be used to remove zinc from wastewater. Hydro-Flo ion exchange systems are designed to treat plating rinse water with trace amounts of metals. The water is sent through cation and anion resin beds, along with activated carbon and/or media filtration to produce deionized water that can be returned to the process. The advantage offered by Hydro-Flo ion exchange systems is that the resin is regenerated on-site, eliminating the need for bottle haul off. Since the regenerate waste will contain any copper and other metals removed during treatment, a vacuum distillation system can be used to concentrate the regenerate even further to reduce the amount of liquid waste hauled away. The purified water from the vacuum distillation system can also be returned to the process.
The typical method to remove zinc from wastewater is as follows:
Stage 1 Precipitation:
pH is adjusted upward to a pH of 8-9.5 to the optimum chrome hydroxide precipitation point. Often, a coagulant such as ferric sulfate is added to enhance metal co-precipitation and the formation of “pin floc”.
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 accumulate in the sludge chamber. The clarified water then exits the clarifier and flows downstream to sewer or further treatment if necessary.
Clarifier Sludge Handling:
The accumulated sludge is periodically removed from the clarifier and sent to a sludge holding tank where it further thickens for disposal or dewatering.
Sludge dewatering is typically handled by a Filter Press. After processing a batch of "sludge" the filter press is emptied of “chrome cake” which is a semi solid of approximately 20-35 % solids. Chrome cake is high in chrome and sulfite and should be disposed of according to environmental regulations.
Plating 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 plating operation.
Typical industries include: