Letter 31013

How to determine phosphate sludge for proper waste disposal  

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We are cleaning and zinc phosphating (250 mg/sq.ft) prior to e-coating on large and small parts in a conveyorized system. The resulting phosphating sludge is put through a continuous filter bed and the phos sludge cake is hauled away at a cost per metric ton. I am told that there are phosphates in the marketplace that produce sludges that are considered non-hazardous (containing less than 1000 ppm nitrates/nitrites consistently). Why would values in nitrates/nitrites change on the sludge? Is there a simple way to reduce the nitrates/nitrites if they have been detected in sludge. Or does everyone that has phos sludge haul it away as "hazardous". I don't understand why nitrites or nitrates would be considered hazardous in our environment to begin with. Are they not similar chemicals in our food processing industries?

Hellen Sencza
e-coatings applicator - Cambridge, Ontario, Canada

Zinc Phosphate processes that don't use nitrate or nitrites as accelerators will not have any of those components in the sludge. Several other accelerators exist such as chlorate, peroxide, and a few organic ones, all of which have advantages and disadvantages. I'm not up on the Canadian regs, but I know several shops down here that dispose of treated zinc phosphate sludge as a non-hazardous industrial waste.

Jeff Watson - Pearland, TX

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