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Need help removing rock like deposits in tank!

One of our baths consisting of nitric acid and ammonium bifluoride has a thick build-up on the side wall (white acidic crystal). Does anybody know how to dissolve it because the build up became very thick and very hard. We can not even break it out....If anyone could show me the chemical reaction that creates that rock, it would be great too. Thank you very much.

Vorakarn Mutsalklisana


I have seen this problem in several different aluminum processing applications, and in precious metal-ceramic dusts as well. I don't think the chemical reaction has much to do with it. Rather, the particular size, shape, and nature of the floc or precipitant allows gravity to slowly force the water out of the precipitated mass so it loses all mobility. If you collect a plastic bottle of the solution, it will form a rock at the bottom, but can be shaken until the precipitate goes back into solution. [Sorry, misleading wording. I should have said 'until the precipitate forms a slurry'].

If you have enough time, and the flexibility to deal with a large amount of water, I think you will discover that this "rock" can actually be washed away and allowed to reform in a waste receptacle of sort sort (perhaps on the paper of a continuous belt filter). That is, attach a pump to the drain on the belt filter, direct the pumped solution at the 'rock', and drain the tank to the belt filter.

As far as I know, the only way to avoid the problem is with continuous agitation.

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

My guess is that is your scale is white and very hard there is a good chance that it is aluminum fluoride.

I have seen this in an acid aluminum etching bath.

Terry Tomt


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