I’m looking for some help. I have a black oxide shop and as part of our process line we have a hydrochloric acid bath. From time to time, we unknowingly get Zinc from our customers. We have to send water samples from our rinse tanks to a lab for reporting our discharge to the city. We have been over our allowable limits many times. My question is, when Zinc is removed from steel parts in our hydrochloric bath, what happens to the Zinc? Could we be getting residual Zinc from our acid bath every time we go to rinse? The max limit allowable by the city is 3.5 ppm. Thank you!
Zn in HCl forms ZnCl
2 or zinc chloride. Its highly soluble in water, so should remain in solution in the bath. As long as you are reusing that acid from one part to another, the bath material you are rinsing would contain some amount of Zn.
With that 3.5 ppm limit, it would only take somewhere around 2.5 ounces of Zn in a 50 gallon bath to reach that limit in the bath itself. With only residuals from that bath making it into the rinse though, it’s tough to make any prediction on contamination limits for parts being processed in the acid bath.
If your water samples are giving consistent numbers, you could possibly scale up your bath volume to a point where the concentration is low enough that the residual rinse remains under the limits.
You might look at trying to precipitate residual metal ions from the rinse tank water, either as a continuous process, or at least just before discharging as waste water.
Zinc should precipitate out by raising the pH of the rinse water to about 9.0 to 9.5, by addition of NaOH (lye). Doing this as a continuous process would also eliminate the residual acids, not sure if you want that or not, but would give plenty of time for the ZnOH sludge to sink to the bottom of the tank.
Does the addition of NaOH do anything other than increase the pH? I have other high pH alkaline chemicals that could be used, if it’s just to raise the pH. This may explain why this is such a new problem for us. We used to use our alkaline hot soap bath much more than we do now, they share the same rinse bath. I used to think that was just to neutralize the pH, but now I see that there may be other benefits.
Adding NaOH reacts the ZnCl2 in solution to form Zn(OH)2 and salt (NaCl). So, the water you pour off is salty water, and the sludge goo is zinc hydroxide…which you would have to dispose of or try to sell to somebody…not sure it has any uses though, and the dry powder is an eye/lung irritant.
As far as other chemicals, yes probably. How well it would work would depend on the solubility of whatever the new zinc molecule is…i.e. the pH I gave is for reacting ZnCl2 to Zn(OH)2 which happens when you add lye (NaOH) to the mix, and get it to the right pH. If you know what the other alkaline chemicals are, I might be able to look up what the reactant solubility is.
You also have to be careful, some reactions might produce toxic gases…lye won’t and I think you would also be ok adding sodium carbonate (washing soda) or trisodium phosphate. These two species would (I think) be easier to dispose of than the zinc hydroxide, but the reactants needed might cost more overall. Easier to dispose of, because the reactants (zinc phosphate and zinc carbonate, respectively) would dry to a powder or cake, and they are relatively non-toxic, and potentially useful to somebody, i.e. you could get people to give you money to take it away if you made enough of it.
Thank you for your help! I really appreciate it. I’ll implement this and hopefully never see high levels of zinc on our water report again. Again, Thank you very much!
@btrueblood mentioned it but I’ll say it again because it can’t be stressed enough. Be fully aware of any reaction byproducts for whatever alkaline materials you intend to use. Toxic gases are a real and dangerous thing even at seemingly minuscule concentrations.