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Publication of IWWA Hoosier Pipeline Magazine, Summer 1999
Copy Right© By Ann Bersbach

    Ann Bersbach

Zinc is a common metal, which can be easily removed from watestreams by hydroxide precipitation at pH 10.5. Complexed zinc is formed when zinc combines with certain agents, such as EDTA, ammonia, citrates, tartrates, quadrol, which may be found in cleaners or other chemicals added to improve processes. Complexed zinc is more difficult to remove from wastestreams because it tends to remain in solution rather than form precipitates and settles out. Complexed zinc is common in wastestreams from electroplating, immersion plating, etching and printed circuit board manufacturing.

Several steps should be taken to handle complexed zinc:

• The first step is prevention. Prevent complexed zinc formation. In most industries, all wastes reach a large equalization tank. Inside this tank, zinc, in a simple form, may combine with chelating agents found in cleaners and other chemicals used in some processes. It is advisable to know the chemical composition of the different solutions used in your processes so that you can segregate wastes or toxic components that may cause problems when combined with common metals.

• The second step is handling the segregated wastes. Cleaners and acids should be segregated from the wastestream and received in separate holding tanks. Cleaners have high pH and high chemical oxygen demand. Some spent cleaners may contain high concentrations of metals and should be sent into the treatment system gradually and in small increments to avoid upsetting the treatment process. Cleaners should be handled as follows: discharge gradually from the holding tank to combine with the rinse water in the equalization tank. A final tank, where the pH is adjusted to meet your permit limits should be the last step before the treated waste is discharged from your plant. Neutralization of cleaners with acid is not a practical procedure since it creates a tremendous amount of solids, which can, if released to the outside, become a problem. In addition, due to their jelly-like characteristics, these solids can be resistant to settling and difficult to dewater in the filter press. Acids from etching and stripping processes usually contain high concentration of zinc and other metals. These acids should be stored in a holding tank and treated gradually to prevent overloading the system with high concentration of metals beyond the system treating capacity.

Steps to treat complexed zinc:

• The first step in treating complexed zinc is to break down the bond between the zinc and the chemical it is combined with. Chemical reduction can accomplish this by using sulfuric acid to lower the pH to 2.0. A reducing agent, such as sodium bisulfite, is then added to bring the metal to a state that allows for the precipitation of the metal. Caustic soda is then added to bring the pH to 10.5 at which point metallic hydroxide precipitation will occur.

• Some complexed zinc will require increasing the pH to 12.0 or above to break the bond. Jar tests results will determine which procedure to follow.

• Organically complexed zinc will resist both methods of removal. Highly resistant forms of complexed zinc can be removed with the use of phosphoric acid. Phosphoric acid is extremely aggressive and combines rapidly with zinc to form zinc phosphate. Phosphoric acid will break the bond of the zinc with any chemical, organic or inorganic, on contact. This treatment is so effective that 2.5 gallons of phosphoric acid added to 8,000 gallons of wastewater, with a high concentration of zinc, caused the zinc concentration to drop to below 1.0 mg/L in the discharge. Another valuable observation is that the final copper concentration dropped to almost 0.00 mg/L.

The information published in this article is based on extensive research and experimentation conducted over a 14-month period.

More Information Removing Metals from Wastewater

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