My company uses a proprietary solution to electropolish stainless steel and incolloy components. The solution is approximately 65% weight phosphoric acid, balance water and additives. We maintain the solution at a maximum of 3% by weight dissolved metal content, per our supplier's recommendation. When the metal content reaches this value, we decant a portion of the working solution, replace with fresh solution and bleed the spent solution into our waste treatment system; decanting occurs about once every 3 months and is about 100 gal. I am seeking an alternative method of maintaining the electropolish solution. The ideal method would remove the dissolved metal from the solution, allowing the solution to continue to be used. As the solution has a high acid concentration, it is viscous. Our supplier is not aware of any method to achieve this. I would greatly appreciate any insight into this situation. T.K.
I know of three types of technologies that are proven to recover acids: adsorption, which is similiar to ion exchange; membrane nanofiltration; and diffusion dialysis. However, due to your very high acid concentration, none of these technologies would be very effective. Furthermore, due to your relatively low generation rate, these technologies would likely not be able to have an attractive financial payback even if the waste electropolish solution had a much lower acid content.
Because of the nickel and chrome content of the waste electropolish solution, you may want to investigate shipping it off-site for metal recovery. Check with your local or state waste exchange and/or contact suppliers of these materials at www.pfonline.com in the Suppliers database or in PF 2001 Directory and Technology Guide under "Refiners and Reclaimers, metals."