Alkaline Zinc Electroplating

Question:

The May 2003 letter from J.K. sounds familiar. I expect that his problem may be, as you pointed out, related to a chelator in the system. While it did not seem to affect removal of zinc from our plating wastewater, we saw copper in our effluent skyrocket when the final electrocleaning stage started using an additive with an amine compound. We have a similar process and treatment system. We do alkaline zinc electroplating and four colors of zinc chromate conversion coating. Our wastewater treatment process has two pH adjustment stages. In the first, we control pH to around 5 and add a blend of calcium chloride with a cationic polymer. In the second stage, we increase the pH to 8.5 - 9, followed by adding an anionic flocculant, settling and a Dyna-Sand filter. Since we got the chemistry and sand filter operating parameters dialed in last summer, our effluent zinc levels have been around 0.05 mg/l. The calcium chloride/polymer blend is admittedly more expensive than the ferric chloride we used to use, but the reduction in sludge volume of nearly 40% gave us overall cost savings of $2,100 in the first year. - D.L.

Answer:

Thanks, D.L., for sharing your similar experience with the readers of Pollution Control Clinic. Glad to see that the calcium chloride/cationic polymer blend worked for you. I followed up with J.K. on his efforts, and he reported that a calcium chloride/cationic polymer blend at an acidic pH followed by pH adjustment, settling and sand filter was unable to reduce their zinc to consistently below one mg/liter; looks like he will need a “metal scavenger” chemistry.

Just goes to show you that, just like metal finishing chemistry, the chemistry of treatment for metal finishing wastewaters is a mixture of science and art; what works at one facility does not necessarily mean it will work at another.