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Reducing aluminum content in Phosphoric Acid

I have an anodizing plant which incorporates chemical polishing, this is made up of phosphoric acid (85%) nitric acid (4.5 - 5.5%)a little sulfuric and water (operating temp 100C). Ideally we would like to operate this bath with a maximum aluminum content of 15 to 20 grams/lit. Due to the cost of replenishment and waste treatment we have reduced our drag out and there for increased the aluminum content to 40 - 50 gpl. At these levels the brightness is reduced. We have in the past diluted a batch with water, let the aluminum settle out over a couple of weeks, decant and boil off the excess water to the correct S.G. level and re-use. I currently have a batch of 20 ton I need to treat and I hope there is an easier way to reduce the aluminum content. Cheers.

Michael Villante
anodizer - Melbourne, Victoria, Australia
2004

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Hi Michael,

Hope you remember me. I have been to Australia in 1990 with Oliver. I'm sure you remember.

Now to your problem with chemical polishing: The chemical polishing supplied from us works until 60-70 g/l AL with good results and it is made up without nitric acid! The use of nitric acid without expensive exhaust-systems and covered baths is not allowed in Germany so we invented the polishing without it. Due to the drag out the AL content will not increase over 70 g/l. The brightness is maybe a little not as good as with nitric acid but we have several satisfied customers using our product. I have discussed your problem with our laboratory staff and they said that there is no other way to reduce the AL in your material like you already do.

I'm looking forward to receive an answer from you!

Best regards,

Karsten Befeld
- Hamm, Germany
2004