Dave Platt wrote:

In article .com,
Leon wrote:

I just flush it down the toilet. I think that ferric chloride is used
for water treatment and I can't see the small amount of copper doing
any harm by the time it has been diluted millions of times. It's only a
litre ot so every six months - it's negligible.

By the time you dump it in this way, you aren't dumping pure ferric
chloride by any means! You're dumping a residual amount of ferric
chloride, plus a bunch of copper chloride.

The waste-treatment authorities in this area are very much against
having soluble copper dumped into the sewage system. The copper
eventually ends up in San Francisco Bay (after having caused some
amount of interference to the biological sewage-digestion/treatment
system). Multiplied by a large number of potential sources, the
copper load can have an injurious effect on the Bay ecosystem... it's
toxic to algae and to many other natural microorganisms.

This problem can be prevented, quite easily, by simply titrating some
sodium hydroxide (lye) or sodium carbonate (washing soda, soda ash)
into the exhausted etchant. It may bubble a bit, with the bubbling
ceasing as the pH rises to the point of being mildly alkaline (pH of 7
to 8... use pH paper to monitor). The copper remaining in the
solution will precipitate out as an insoluble solid. Allow it to
settle and pour off the remaining liquid... MG says that the liquid
can now be poured down the toilet safely. The solid (copper hydroxide
or copper carbonate, I believe) should be disposed of according to
local requirements.

I've used the same precipitate-with-soda-ash trick when using sodium
monopersulphate as an etchant (it's surely work with the somewhat
faster-acting ammonium persulphate). It's quite striking in this
case... the clear blue-green copper-loaded etchant turns a milky
green, the precipitate settles out, and you're left with a clear
near-colorless liquid which contains little or no copper.

I've heard of people disposing of copper-loaded used etchant by mixing
in some Portland cement powder. This raises the pH, converts the
copper to an insoluble form, and then binds it into (in effect) solid
rock which can be disposed of in a landfill without causing
significant leaching of copper. The same trick could no doubt be used
to solidify the precipitated copper from a lye or soda-ash
neutralization of spent etchant.


I've also heard of putting steel wool into spent ferric chloride etchant,
which I think gets the copper out in a metallic form and probably replaces
it with iron. Whether that would be better I don't know.

Chris