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In many industries, pickling baths are common. These are 10 cubic meter baths containing acid baths that metal components are dipped into prior to surface treatment. When the metals are dipped, some of their components are left in the acid bath and after some time he bath needs to be changed with new acid because the metals in the solution make the bath ineffective.

The old and metal contaminated acid is sent to destruction. But is it possible to remove the metals from the water and acid, to regenerate it?

Say we have a bath with 20% HNO3 in water and lots of metals. 20% HNO3 in water has a bp of 101°C which is slightly above water. If this was put through an evaporator, would both the HNO3 and the H2O evaporate and cooled down to a distillate that contains the water and acid? The metals won't evaporate of course and is left in the small amount of liquid that is left in the concentrate.

Is this possible? Also is it safe? I mean heating HNO3 creates nitrous gases, but if they're immediately cooled down and condensed, is it a problem?

Thanks in advance!

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  • $\begingroup$ Interesting spelling of "distillation" with the letter 'e' which presumably is only used in Swedish language. $\endgroup$
    – andselisk
    Feb 8, 2023 at 20:45
  • $\begingroup$ @andselisk Czech language uses "destilace" and I have always to self-check myself not to write "destillation" too. $\endgroup$
    – Poutnik
    Feb 8, 2023 at 20:55

2 Answers 2

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What is a piece of cake for $\pu{1 L}$ might become a nightmare for $\ce{10 000 L}$. So think twice…

$\ce{HNO3}$ forms an azeotrope with maximum of boiling point $\pu{120.5 ^\circ C}$ at $w(\ce{HNO3}) = 68\,\%$.

  • So the best would be to arrange the column distillation, with first going just water at $\pu{100 ^\circ C}$ with just traces of $\ce{HNO3}$ and $\ce{NO_x}$.
  • Then the column head temperature skips to $\pu{120.5 ^\circ C}$ at $w(\ce{HNO3}) = 68\,\%$.
  • A simple fractional distillation may be more effective timely and energetically, but on the other hand with big loses of $\ce{HNO3}$.

Another option is the simple or fractional distillation:

  • Initial fractions having roughly 1/3 $\ce{HNO3}$ concentration, compared to original $20\ \% $ solution.
  • From the phase diagram $\ce{HNO3-H2O}$, it visually looks like vapor above boiling liquid have roughly these compositions:
$w(\ce{HNO3},\text{l})[\%]$ $w(\ce{HNO3},\text{g})[\%]$
20 7
40 20
50 30
68 68
  • All depends on what is the target concentration. If we start with $w(\ce{HNO3})=20 \%$, using a single fraction, we would end with less than $20 \%$, as the composition of the solution being distilled would converge to $w = 68\ \%$.
  • Unless we use more fractions and drop too diluted first fractions.
  • The third theoretical option would be to distill near everything, so we would reach near the same concentration, but with the ugly rest to be cleaned up.
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  • $\begingroup$ Is it not just safer to leave it at 100 and let the HNO3 slowly trace its way over until there is nothing left (water nor HNO3)? The problem I see in a large scale would be not making a bomb... $\endgroup$ Feb 8, 2023 at 20:20
  • $\begingroup$ The trick seems to be to take the water off first. See reference in answer provided. Keep in mind everything from the 1st distillation can be saved. $\endgroup$ Feb 8, 2023 at 21:12
  • $\begingroup$ @Poutnik and what's left in the pot might go by a name worse than "bottoms". But more water could be added in to make sure the pot doesn't gel. Even 15% first pass might be workable. $\endgroup$ Feb 8, 2023 at 21:28
  • $\begingroup$ The answer extended, including deleted comment. $\endgroup$
    – Poutnik
    Feb 9, 2023 at 10:45
  • $\begingroup$ I like the third option the best. The resulting concentration of HNO3 in the distilled fraction is not important, its easy to just add a little concentrated hno3 to up the conc or dilute it with water. Whats left sent to destruction, but it is much cheaper than to keep buying new acid and having to send the entire batch to destruction once it is used. Still worried about the NOx though... You think it is a problem at 20% or less to begin with? What could happen? $\endgroup$ Feb 9, 2023 at 14:46
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Yes, this would seem feasible. Nitric acid forms an azeotrope with water and can be fractionally distilled to a concentration you desire after distillation from the "mother liquor".

Distillations in general are made safer when run under a hood. Certainly wear eye protection and, as nitric acid is also an oxidizer, appropriate distillation apparatus should be used$^1$. Inert gas such as N2 should used if there is any chance organics such as oils or grease may be in the pickling bath.

$^1$ Pyrex and Teflon sleeves

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  • $\begingroup$ What it costs you to heat the mixture to boiling point may be more than a new charge of nitric acid. $\endgroup$
    – Waylander
    Feb 8, 2023 at 18:47
  • $\begingroup$ True, but nitric acid is not inexpensive. Could be a good way for the new chemists to earn their keep in the plant. Run pilot scale first, and be aware of what is in the pickling bath beforehand. $\endgroup$ Feb 8, 2023 at 18:55
  • $\begingroup$ The destillation in question would in a large scale be run by waste heat, and using a packing material that expands the surface area of the pickling bath, this lowers the temperature needed to evaporate it. It is only in theory as of now, needs a lot of work but might work. $\endgroup$ Feb 8, 2023 at 20:14
  • $\begingroup$ @Evaporation123 show them it works lab scale first. I don't see any major scaling issues here. Vacuum distillation might help. Control azo by adjusting vacuum. Easier than heat. $\endgroup$ Feb 8, 2023 at 21:14
  • $\begingroup$ Easier than heat but electricity to keep vacuum going on a large scale is a lot, with waste heat is available in endless amounts in my case, so heat consumption is not a problem. $\endgroup$ Feb 9, 2023 at 14:43

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