letter 4024

Separation of Free Acid from Metal Ions in Electropolishing solution

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I am working on a group project at Iowa State University and we are trying to remove iron ions from an electropolish bath. The bath is composed of sulfuric and phosphoric acid. We have looked into membrane technologies (Diffusion Dialysis, Filtration, Acid Sorption etc.) but the phosphate diffuses too slowly and separation and recovery of the acid are not possible this way. We also looked at Electrowinning, but the metal stream has to be concentrated for this to work. Right now, the acid bath is being discarded. It would be a lot more economical if we could recover the acid.

Also, could we use a different electropolish....one without phosphoric acid in it?? Then we could use the membrane technologies to separate. Help! We can't find a solution!!

Adriana Boettger
Iowa State University - Ames, IA

Well, if there were a practical solution, electropolishers would be using it already, wouldn't they? And when 'discarding' gets sufficiently expensive, an approach which is not practical today then will be.

But, yes, there are other process solutions available, including straight sulfuric acid. The best list of available solutions is in ASM's Metals Handbook, Vol. 5, Surface Engineering.

Ted Mooney, P.E.  finishing.com Brick, New Jersey

There is a technique, related to ion exchange, called "ion retardation". The acid is passed through a column of this material (Rohm & Haas makes a series of these resins under the name "Retardion") and it retains acid molecules by an ion pairing mechanism. The column effluent is reduced in acidity, and acid may be recovered by passing plain water through the column. This is something of a simplification - you'll have to research the details.

It might then be possible to remove the Fe from the acidity diminished column effluent by oxidizing it with air or hydrogen peroxide.

You are liable to find membrane technologies subject to membrane fouling problems because of the concentration of metal ions at the membrane solution interface, and the consequent formation of solid hydrolysis products. My personal opinion is that membrane tech for haz waste treatment will become practical when they reduce the price of the membrane to under 10 bucks. But, I'm hardly an expert on membranes.

Good luck with your project.

Dave Wichern - The Bronx, New York