Where theworld gathers for metal finishing
Q&As since 1989
-----
Filter quickly blinded by sludge from cadmium stripping
Please see also --
• Topic 3212 "Stripping Cadmium plating and choosing a method"
Q. We are currently in the process of commissioning a new plating facility that includes cadmium plating. Our parts are salvaged parts used to rebuild large expensive systems. Thus, stripping the old plate is normally a part of our operation.
Our designer included an ammonium nitrate strip to strip our cadmium plate. The solution is currently set at 1 lb/gal ammonium nitrate in water at 90 °F. A test of this strip on cadmium plated carbon steel resulted in the generation of high volumes of red sludge. The sludge is an insoluble iron compound. The results of experiments indicate that the steel is being etched. The etch rate was determined to level out at about 0.05 mils/hr once the solution is saturated with the iron compound and the precipitation reaction starts to govern the etch rate. The sludge blinds our filters in very short order.
We have been told that our observations are uncommon. Previous thread 1622 stated a white precipitate. Does anyone else out there have similar observations? Is our ammonium nitrate formulation correct? Any info or suggestions?
Chas Valenza- Tobyhanna, Pennsylvania, USA
2003
A. I never saw the problem that you describe. We ran at room temp rather than 90 °F. Are you a WG or the high priced expert? What type of tank are you using? We never filtered our strip; would stir it in the AM and run it all day.
James Watts- Navarre, Florida
2003
A. The red sludge is not insoluble iron, it is cadmium. Cadmium has limited solubility in ammonium nitrate solution ~30 g/L at RT. Once you have stripped more than that, you will see it in the tank as a red solid. The sludge should not be viewed as evidence that you are etching steel. Ammonium nitrate solution should not etch steel -- if it does, there is something very wrong with your solution. The presence of the sludge is not detrimental to the stripping process; however, the more you strip the less active the strip solution becomes. It is possible to maintain the strip solution to a near constant strip rate by addition of fresh ammonium nitrate. The bath can have a very long life if you de-sludge it occasionally and avoid copper contamination. I would not bother trying to filter the bath continually.
Jon Barrows, MSF, EHSSC
GOAD Company
Independence, Missouri
2003
How would one go about treating this ammonium nitrate stripping waste? I am currently trying to find a waste treatment method for this same solution. Thanks!
Mike Gray- Jamestown, New York, US
July 16, 2008
this text gets replaced with bannerText
Q, A, or Comment on THIS thread -or- Start a NEW Thread