"Rock hard scale problem in aluminum processing"

A. Hi Lyndon. We appended your inquiry to a generally similar thread where some ideas have been offered...
Regarding black oxide, EPI (Electrochemical Products Inc.) [a finishing.com supporting advertiser] has an article at
epi.com/blog/post/epi-how-to-clean-out-your-black-oxide-tank/
which suggests that some black oxide formulations can hold more iron, thereby limiting the problem.

This thread offer some ideas for the aluminum etch sludge, including addition of sodium gluconate.

It may be impractical for rinse tank drain lines, but if possible, keep recirculating the sludge and it will not attach and harden. Anything that keeps the sludge moving is key.

My own experience suggests that such sludges are NOT actually a scale; rather, when they first form they are rather light and fluffy well-hydrated precipitates. But when they are allowed to sit undisturbed, the heavier-than water sludge components quickly drive out all the interstitial water, leaving a rock. Take a plastic bottle full of the etch solution and I think you'll see it settle into a hard precipitate at the bottom and clear liquid above. Bu if you patiently invert the bottle many times, the 'rock' will be gradually washed back into solution. I have suggested on this thread a way that such sludge can be handled with a paper "gravity" filter, but no one has actually tried it to my knowledge :-)

Luck & Regards,

A. Hi Ryan. This is not an unusual problem. Read on for ideas on how to avoid it and how to deal with it. Mileage may vary, but I feel the best approach, if possible, is recirculation if that is possible even at a greatly reduced rate, and gradually 'washing' that aluminum precipitate onto your filter cartridges or plates.

Luck & Regards,

A. Hi Ryan. There have been suggestions about complexing agents on this thread, and hopefully another reader will have further suggestions; but sorry,I don't. I have seen this problem in aluminum capacitor manufacturing, and in precious metal slurries applied to catalytic converters, and in aluminum etching solutions, but I am not a chemist experienced in exactly what if any chemical reactions encourage it or will deter it.

My observation is that what starts as a light fluffy sludge gradually turns into a rock, not as a result of a continuing chemical reaction, but because -- for whatever reason -- these particular sludges slowly settle in a way that eventually pushes the water out, like in a winning game of Tetris :-)

I am 75% confident that your 2938 L tank can be cleared of that rock over the period of a couple of hours through the recirculating approach I sketched back in 2003, with a pressurized lance and using whatever filter you have available -- it's just that a paper gravity filter or paper vacuum filter is an ideal way to get rid of huge amounts of such sludge whereas it may take an unaffordable number of cartridges to capture it all.

Sorry I see no pics from you; please attach them to an e-mail to mooney@finishing.com.

Luck & Regards,

A. Dear Terry,

Look for aluminum fluoride is high 19.37. So you can use 5SSA 5 sulphosalycilic acid with log k=29.

Finishing.com honored Ken for his countless carefully
researched responses. He passed away May 14, 2015.
Rest in peace, Ken. Thank you for your hard work
which the finishing world continues to benefit from.

Thank you for the responses. I am concerned that sodium gluconate is made to work in alkaline pH region and not in acid as my case is. I will look into the salicylic acid for sure. Also I will research the gluconate.

A. It may be of only academic interest to you, or no interest, but this 'rock hard' sludge might only become rock hard when it's allowed to settle and squeeze out the interstitial water. If you can keep it moving, it may never harden. Even after it has hardened, though, I think you can remove it by getting it back into solution via patient washing, and then filter it out. I have seen a number of aluminum sludges/slurries that do this.

I haven't actually tried it, but I'll bet you can clean it out by using an automatic paper filter and recirculating water through the tank, directing the output at the scale

A. Hi Nuno. Any chance of hooking up an automatic paper filter, either for continuous use or for periodic cleanup per the sketch on this thread we appended your inquiry to?

Regards,

A. Hi Nuno. If you collect a plastic bottle full of your slurry, I think you'll discover that it settles out in a few hours to a rock hard sludge and a decantate ... but that if you turn the bottle over and back a hundred times or so the 'rock' gradually washes back into a slurry. If your aluminum 'scale' acts as I describe, I think it is possible to continuously filter the solution or, lacking that, to recirculate the decantate to wash the 'rock' back into a slurry for periodic filtration.

Automatic paper filter systems use disposable newsprint which unrolls onto a continuous belt of stainless steel mesh. So the fact that newspaper isn't particularly acid/caustic resistant shouldn't matter -- although the rest of the filter parts would have to hold up to the solution. I haven't tried it.

I am not a chemist and cannot comment on the complexing suggestions offered by Hadi, Ken, and Terry.

Regards,

Thanks Ted, your help was precious! I'm going to study and explore all these ideas. I'll keep you posted :-)
Hopefully with good news...

Kind Regards,