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topic 6411

Regenerating/purifying iron phosphate bath


(2000)

Is there any common procedure for cleaning up an iron phosphate bath to avoid dumping it to waste periodically?

Thomas R Wallin
Illinois Environmental Protection Agency - Springfield, Illinois


(2000)

My response has more to do with a philosophy and with general experience in the finishing industry, rather than specific experience purifying iron phosphating. Nonetheless, for what it's worth ...

Iron phosphate processes are usually '3-stage' processes consisting of a single bath that does triple duty of cleaning, activating and phosphatizing, followed by two rinses.

Before the days of careful attention to resource conservation, this simplicity probably made sense. But any process that does double or triple duty is a high-strung and touchy chemistry that makes you pay hell if you try to stretch its life: You need to get the dirt out, and get the organics out, and get the reduced reagents which took part in the phosphatizing reaction out, and then restore the exact pH, exact phosphate balance, etc. You may not be able to do it in a laboratory let alone a real metal finishing shop.

Frankly, I think the shop might want to give consideration to revamping the line so it has a capable alkaline pre-cleaning station followed by a neutralizing acid activation dip and then a phosphatizing step. More along the lines of a '7-stage' zinc phosphatizing process. Complicated, yes, but then you have a reasonable shot at extending the life of the tanks because you don't have to worry about maintaining phosphate balance in the pre-cleaning station, you just get the oil and dirt out; and you don't have to worry about oil and dirt in the phosphating tank.

Ted Mooney, finishing.com Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey



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