Letter 13011

Sludge removal from Zinc Cyanide tank 

We have a problem with the level of sludge in our cyanide zinc plating tank (sodium cyanide/zinc/brighteners etc).

None of the local waste removal companies will touch the stuff, and I am wondering a) what it is (greyish sludge which I believe I've heard comes from the caustic soda in the solution) and b) how can we neutralise it or reduce it's toxicity to a level where we can have it dumped.

Also suggestions on easiest removal would be best. Only our acid tank is the same size as the plating tank, and for obvious reasons that will not be used. Filtration is also unlikely to be an option, as we're talking at least a foot of depth here :-( The only filter we have is on the acid zinc barrel line - unusable for the same reasons as the pickle tank.

Many thanks in advance for any help.

David Chord
ElectroZinv - Lower Hutt, New Zealand

David, you have a major problem. This sludge contains breakdown byproducts from plating, it is normal. It contains sodium carbonates, ferrocyanate (from cyanide, virtually insoluable in hot water), metallics i.e. iron, copper, chrome, etc. Probably have undissolved cyanide and caustic from additions. You cannot successfully filter this mess. When I was plating cyanide zinc, we had waste treatment tanks specifically for this sludge. We would slowly add small amounts to warm water, stong mechanical mixers, added copius amounts sodium hypochlorite, acids (toxic fumes), many tanks. Then bleed small amounts into our main reactor. It took months and much money. Perhaps someone on this site can help you. In the USA, only a small number of licensed haulers that can neutralize this stuff are around. It was still cheaper to have it hauled than process it ourselves.

Good luck,

Bill Hemp
- Grand Rapids, MI, USA

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