Stopping Zinc Phosphate Sludge

In the September 2000, Painting Clinic, I answered a question from L.V. about zinc phosphate sludge.

In the September 2000, Painting Clinic, I answered a question from L.V. about zinc phosphate sludge. In it, he presented a list of procedures that were tried in an attempt to reduce the amount of sludge produced in his pretreatment line. Fortunately, folks other than my wife read the column. Furthermore, some are even willing to offer comments. L.S. wrote the comments in the answer below. The other readers and I thank him for reading the column and taking the time to write his comments. C.I.


In your Painting Clinic column in the September 2000, issue of Products Finishing, you answered a question about excessive sludging in a zinc phosphate tank. I would like to present you with another cause for excessive sludging and the solution.

I have found that continual, excessive sludging in phosphate coating tanks is caused by the reaction of the low pH phosphate coating liquid with calcium in the water. An analysis should show that the primary ingredient in sludge is calcium phosphate (Ca3PO4). Fortunately, calcium phosphate sludge is a non-hazardous solid waste and is easily disposed.

A temporary solution is to create a maintenance cycle where the phosphate coating liquid is periodically removed from the tank, the sludge chipped out then the phosphate coating liquid returned to the tank.

A permanent solution is to change water sources or to install water softening equipment that will remove the calcium from the make-up water. Even though calcium is the most likely cause of the problem, before anything is done, an analysis must be made of the sludge to be absolutely certain that it is indeed a build-up of calcium phosphate. I hope this information is useful to you and your correspondent. L.S.

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