How to destroy Ferricyanide in Electroplating Wastewater

A. We usually have from 1 to 11 ppm of ferrocyanide in our wastewater, and this process works to remove it:

Adjust pH to 11.3 with caustic, and pump in industrial strength bleach (12.5%) until the ORP reading is 500 to 600 (for our first couple batches, we went to only 450 to 500 and we found this inadequate, and ended up doing the entire batch over again). Going to 550 or 600 may be required to fully oxidize ferrocyanide to the cyanate. Allow to react for an hour.

Adding 40% sulfuric acid through a funnel attached to a 2' long hose, in order to introduce it well below the surface of the liquid, adjust pH to 8.75 (do this cautiously, and slowly, so as to not overshoot the pH target and release chlorine gas). Next, add more bleach until the ORP reading is 800 (this oxidizes cyanate to nitrogen and carbon dioxide). It is possible that you won't have to add much more bleach at all, if the ORP is found to be 800, after achieving pH 8.75. In this step, we have observed paradoxical behavior, so if adding bleach appears to lower the ORP, invest time, and bleach, and hope to eventually get there. Others have noted this paradoxical behavior in the first step, but we have not (we've seen it in this second step on some occasions). Allow to react 90 minutes beyond the time you achieve 800, or more if needed to do this oxidation step.

Next, pump in magnesium bisulfite solution a shot at a time, and instruct a second person to add caustic to keep the pH at about 8.2 to 9.2 (the goal is to keep pH at 8.75). The bisulfite will lower pH, and the task is to counter this with caustic adds, to maintain pH at 8.75. Good pH control is important to this and all steps. Go to a target ORP of 200. Sneak up on it toward the end, since once you reach 300 it will not require a lot more. This reduces bleach to just simply sodium chloride. Our local chemical distributor found the magnesium bisulfite at Hydrite Chemical in the midwest, available in plastic drums. Magnesium contributes to the efficacy of the process, so sodium bisulfite may not work at all.

We next add 7.6 ml per gal of wastewater, of a coagulant, as a 10% solution mixed into water, to the batch. We have found this product at Chemetall Oakite (Enprox 8412). Mix this for 15 min into the batch. Finally, adjust the pH to 10.6, or what is appropriate to the product next used. The batch needs to now have a flocculent added, at 3.8 ml/gal, also as a 10% solution. We have found this product at Oakite, too (Enprox 8440). Allow to mix for 15 minutes, and then settle the tank. We are getting total cyanide (e.g., ferrocyanide) at non-detect, in the supernate.

A. To the best of my knowledge, these iron cyanides are not amenable to chlorination. If your lab runs the cyanide test via distillation/colorimetric method, the sulfites you add will generate SO2 during the distillation. This passes over into the sparger solution along with the HCN. The first step of the colorimetric procedure is a chlorination with Chloramine 'T'. If there is enough sulfite in the sparger liquid to neutralize it all, a false negative results. I have personally been skunked by this on two occasions. Once, I got a really nasty surprise when the inspector came along and cited me because my "non-detect" effluent was in fact, in violation.
I developed a precipitative method for removing these complexes.

A careful technician will check the analytical prep after the Chloramine 'T' addition with KI starch paper [affil links], and add excess if required. Or, the finish could be run by ion selective probe, which is not subject to this interference.

A. Make sure your lab, if they are running CN(t) by colorimetry, tests the sample prep after the chloramine 'T' addition. The sulfites come over in the distillation and if there's enough to neutralize all of the oxidizer you will get false negatives.