Metal finishing Q&As since 1989

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Treated Cyanide waste affect on main waste stream

2007

We treat our cyanide containing waste water stream with caustic and chlorine to destroy the residual cyanide. However when we add this to our main waste stream (where the pH is low and ferric chloride [on eBay or Amazon] is added to help reduce the copper in the final discharge)for subsequent pH correction and flocculation (including the addition of magnesium hydroxide to make the final sludge acceptable to land fill) the system seems to stop flocculating.
Anyone seen this before? Any advice on how to overcome this problem.

Bob Lynch
plating company - Sydney, Australia

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Yes, Bob, overchlorination (residual chlorine) has been reported to inhibit flocculation many times in the literature. Quickest test: KI starch paper [affil links] turns blue to light purple if there is a reasonable amount of excess chlorine, very dark purple if there is too much, and actually turns white again if there is way too much.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
2007

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First of two simultaneous responses --

Ted,
Thanks for the advice. Just wondering if you have any figures of what would be indicated by the colour of the starch paper [affil links]. I have located a supplier in Australia that has graduated chlorine test strips but I am unsure of the range I should purchase. Ranges offered are 0-10, 0-50, 0-100 and 0-200 mg/l. Any additional advice appreciated.

Bob Lynch
plating company - Sydney, Australia
2007

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Second of two simultaneous responses -- 2007

What Ted says is perfectly true; it's the most common cause of this trouble. If you do the test that he describes, and it's positive, you can add some sodium metabisulfite to kill the excess chlorine. You could also try to tweak the ORP set-point in the cyanide destruct to reduce the excess chlorine, but that is riskier.

That being said:

1) I've never found ferric chloride[on eBay or Amazon]to be of any use in treating this kind of waste. ferrous sulphate ferrous sulphate [affil links] works better. Try that, but neutralize the excess chlorine first.

2) An aluminium based coagulant sometimes helps, especially if you're using some kind of precipitant. Plain alum, or polyaluminum chloride might help you.

3) Another trouble maker is cleaners and detergents, esp. bath dumps. I strongly suggest that people not put these through their system, but batch treat them instead.

4) If nothing else works, call your friendly polymer (floc) salesman and have him bring a variety of products to try. Sometimes, it's as simple as: you introduced a new process, or something else changed, and what always worked before doesn't anymore. You might have better luck with a different product.

Good luck!

Dave Wichern
Consultant - The Bronx, New York