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Etch rate drop after filtration

 

Dear Sir,

We use a phosphoric acid / ammonium bifluoride blend to etch aluminium. Recently we installed a filter to remove insoluble reaction products from the process tanks. We have found that the etch rate has dropped by approximately 25% after filtration even though the concentration of the process solutions have remained unchanged. This phenomena also occurred on a freshly made process solution when the filter was operated even though there were no insoluble to filter out. The filter is not used during the etch process but only when no components are within the tanks. Does anyone have a theoretical or practical explanation?

Regards,

Kevin Proudlock
Applied Chemicals - Smethwick, West Midlands, UK

I suspect you are titrating for total acid, but the main "etcher" in the mix in the ammonium bifluoride, and the ammonium fluoride is probably wasting itself eating something in the filter media. Take two aspirin and call me in the morning. Robert H Probert Robert H Probert Technical Services Garner, North Carolina It is possible that the solution reacts with the filter media. Have you checked the compatabilty of the material with these chemicals? Dado Macapagal - Toronto, CANADA

If the rate of reaction (etch rate) changed even though the concentrations remains the same, then this could mean that the filter media or something in it has a buffering effect on the solution. That substance may acting as a catalyst retarding the speed of reaction.

Dado Macapagal
- Toronto
 

Try leaching your filter media in a small amount of warm etch solution and see if the problem goes away.

Rick Richardson, MSF
Dayton, Ohio