Electrogrind Waste

Question:

Our company currently electrogrinds the cut ends of small stainless tubes to deburr for further processing. The solution used is a nitrate salt with emulsifier to suspend the swarf. The 50 gal of solution is dumped to wastewater treatment when it contains about 3 g/liter iron and 0.5 g/liter nickel. How feasible would it be to remove the nickel and other metals via an electrowinning process? My goal is to reduce nickel and chrome by 90% prior to dumping to our batch wastewater treatment system where we pH adjust and precipitate metals using carbamamide (sulfide). Our limit on Ni concentration is 3.0 mg/liter before discharge to our city POTW and is being reduced to 0.5 mg/liter in 2 years. The electrogrinding process produces about 200 gal of waste solution per week. Thanks for your input. J.L.

Answer:

In my opinion, reduction of nickel via electrowinning may not be feasible for three reasons, although I would appreciate readers’ input about this. First, the high concentration of iron may interfere with the plating out process. Secondly, much of the nickel may be suspended solid matter rather than an ion and, therefore, would not plate out. Lastly, you would only be recovering, at best, about 1 lb of nickel a week, making electrowinning not very economically attractive.

I assume that this waste material is causing problems in your wastewater treatment system. If so, it is likely due to the emulsifier that is trying to keep contaminants in suspension in your wastewater treatment system in the same way it works in your process. There are a number of chemistries that could deactivate the emulsifier. Contact your chemical supplier. Also, you may want to set up a separate batch tank to pretreat the solution before sending it to wastewater treatment.