Zinc Treatment

Question:

Your March 2003, article, “Increased Floc,” prompted me to write regarding a similar problem we just started having. We have an alkaline zinc plating line with a conventional treatment system of pH adjust to 9.2, cationic polymer add, anionic polymer add, settling and final sand filter. Historically, our effluent zinc concentrations have been around 0.5 mg/liter or less, as compared to our limit of 1 mg/liter. Recently, we have experienced effluent zinc concentrations in the 2-4 mg/liter range. We have double and triple checked our treatment process to verify that the pH adjustment and polymer adds were correct. Our plating operators have confirmed that there has been no chemical change for the process. The wastewater coming out of the sand filter has the same clear look as it always had. Any suggestions? J.K.

Answer:

Based upon this information, it appears that something has indeed changed in your alkaline zinc process that is “tying” up the zinc and keeping it in a soluble form that your clarifier or sand filter is not removing. I recommend that you challenge your chemical suppliers to see if they have made a formulation change that added or increased the amount of chelating or complexing chemicals. If the chemical in question is not regulated by OSHA or is not on any of USEPA’s SARA chemical lists, then it is not required to be listed on the product’s Material Safety Data Sheet (MSDS).

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If you find that a formulation change has been made, you have two choices. First, you can require that your supplier go back to the original formulation, and if they are unwilling to meet your needs, find another supplier who can. Second, you can “live” with this new formulation and change your treatment chemistry.

You can try two simple chemistry changes. One is to work with your pretreatment chemical supplier to see if they have a cationic polymer/inorganic coagulant blend to replace your cationic polymer that would be effective in breaking the chelating or complexing bond and precipitating the zinc. If that is not promising or is extremely expensive due to very high dose rates, you should consider adding a sulfide, dithiocarbamate (DTC), or similar “metal scavenger” chemistry between your clarifier and sand filter; these “metal scavengers” will precipitate the zinc so it can be captured in the sand filter.

Hope these suggestions are helpful in getting your zinc back down