Q. I own a powder coating shop and process large amounts of both steel and aluminum. I have a three-stage wash with a multi-metal iron phosphate single-stage cleaner. I now have a customer who wants to bring in galvanized panels from China, and I’m concerned about the zinc going into my waste stream. Where could I find a solubility versus pH chart for zinc? B.C.
A. It’s a good thing you’re thinking ahead about this since zinc will be dissolved in the phosphating process. Most metals have some amount of solubility in the pH range you mentioned, which is typical for most pretreatment processes, including zinc and iron phosphate.
The pH versus hydroxide precipitation graph is quite common and isn’t exactly what you are looking for, but will give you an idea of the magnitude of the problem. Uncomplicated yet thorough coverage can be found at http://www.mtech.umd.edu/MTES/. This manuscript provides pH versus precipitation graphs for numerous metals including zinc.
Your ability to process galvanized steel will be dependent on your wastewater treatment system. I assume you’re not directly discharging the used cleaner/phosphatizer to your sewer, given the pH. However, beyond a pH adjustment and maintaining your grease and oil limits, the metals you currently process (steel and iron) likely do not have any discharge standards in the municipality where your business is.
It is likely that you will have to install more of a standard precipitation-based waste water treatment system to remove the zinc. If wastewater volumes are low enough, you may find alternate technologies like ion exchange or reverse osmosis to be a cost-effective alternative. If going with a standard waste water pretreatment system, discharge will not be that difficult since the optimal precipitation pH is about 8.5 to 9.0—similar to most other common metal hydroxides found in industrial practice.