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Hard as rock sludge

We have an anodising plant for aluminium in Sweden. We strip the aluminium in a caustic soda ⇦liquid caustic soda in bulk on Amazon [affil link] solution. In the rinse we are having problems with the rock-like solids that build up on the walls of the tank. Does anyone have any suggestions for cleaning out this rinse tank? And what should I do with the "rock" I already gotten lose? Is there some way to dissolve it?

Jan Porath
Ahlins I Habo AB - Habo, Jpkg, Sweden

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I am confident that this 'rock' will easily dissolve in dilute hydrochloric acid if you can work up a protocol to do so safely.

But I have seen this stuff before. The aluminate particles look soft and fluffy as snowflakes, but let them sit a while and they drive out all the interstitial water and turn into the hardest darned stuff. When I was on one project I used to carry a small clear plastic bottle of this waste around to demo to the mixer manufacturers who wanted to quote. A rock covers the bottom quarter of the bottle, with liquid above. Turn it over and over and shake it for 10 minutes and it all gradually dissolves into snowflakes again; let it sit for 15 minutes and it's just a light fluffy soft sludge. Let it sit for 2 hours and it's might as well be marble again.

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