What is the best way to dispose of used ferric chloride PCB etchant?

Question

I know of at least one method, diluting it and slowly pouring into the drain, but it can still stain and eat away metals in the pipes a bit. Is there something else I could use to possibly just "neutralize" it? I think an alkaline metal nitrate would work (Li;Na;K) or some other salt. That or maybe even just try to get the pH as neutral as possible.

Note: PCB is an acronym electrical engineers/hobbyists etc used to stand for PC-board, a.k.a Copper-clad board.

I also now realize a nitrate salt, as well as many other salts, would only form complex ions/ligands. Especially since the used ferric chloride would already contain complex ions due to the copper dissolved off the board (copper and chlorine like to do that a lot). Although, even that might bring the pH closer to 7.

Answer 1

PCB enchant is Ferric Chloride (as you said), with a chemical formula of FeCl3...Iron (III) Chloride.

When you pour the Iron (III) Chloride on to a Copper-clad computer board with your circuit traced out on it, the enchant will eat away the undesired Copper by Oxidizing it to the very soluble CuCl2 and reducing the FeCl3 to FeCl2.

After you use your enchant, you will have a solution of FeCl2, CuCl2, and probably some remaining FeCl3.
All of these can be removed from solution by precipitating the metal ions out using a Hydroxide.

All of the Iron/Copper Chloride salts in your used etchant solution are quite soluble, but their Hydroxides are barely soluble.
if you add some Sodium Hydroxide ('lye', commonly found in drain cleaner), you can form the metal Hydroxide precipitates which can be filtered or decanted off, leaving a much milder solution behind.

FeCl3 (aq) + 3 NaOH (aq) --> Fe(OH)3 (s) + 3 NCl (aq)
FeCl2 (aq) + 2 NaOH (aq) --> Fe(OH)2 (s) + 2 NaCl (aq)
CuCl2 (aq) + 2 NaOH (aq) --> Cu(OH)2 (s) + 2 NaCl (aq)

Make sure and dilute the solution before you add the NaOH (preferably a NaOH solution). Dissolving NaOH in water is a very exothermic process, so the solution’s temperature will rapidly rise if you are not careful. It is also a very bad idea to get any NaOH in your eyes or on your skin, so be careful but when used responsibly NaOH is fine.

When the Hydroxide precipitates are heated, they will form the metal Oxide salts. So for example, by heating Cu(OH)2, you will form CuO. The heat generated by the above reactions might also be enough to convert the Hydroxides into Oxides, so that is certainly a possibility but disposing of Fe2O3, FeO (or the combination of the two, Fe3O4), and CuO should be no problem.

The remaining NaCl solution will be no problem disposing of down the drain, as would any excess NaOH which happens to remain.

Answer 2

Ferric Chloride Disposal

Answer 3

The most used form of the acronym PCB is polychlorinatedbiphenyls. If that is what you have, disposal by any of the ways you state would be illegal in the USA. Anything with PCB has to be dealt with as hazadous waste. If only ferric chloride, neutralization with caustic (very carefully) would be allowed. A metal nitrate will not do anything to help.

Answer 4

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