I understand that an electrolytic metal recovery (EMR) process can remove up to 90% of metals from an industrial wastewater stream. What can be done about the remaining 10%? Is there another metal recovery/removal process that can be applied after the EMR process? W.L.
Electrolytic recovery in waste streams (also known as electrowinning) is a very good method to recover those metals and minimize further treatment. It essentially involves removal of metal ions from a plating solution and/or associated rinse by plating them out. Generally speaking, this process has a couple parameters that are not requirements, but which make the metal recovery more efficient.
The electrolytic metal recovery process is more efficient, the higher the concentration of metal in the waste stream. For instance, it would not work well with a flowing waste stream containing a relatively dilute amount of dissolved metals (or the relatively dilute 10% mentioned above). If using EMR, it is usually recommended to have a static, drag-out rinse following the tank containing the bulk of the metal loading (be it a plating or cleaning/etching tank). That tank will remove a large percentage of the metal from the parts and contain it in one volume. That tank can then be pumped offline and the EMR process applied to plate out the metals.
Another caution regarding EMR is “other ingredients.” This could be additional metal ions that will not plate in the conditions that the primary metal does, or it could be additives that interfere with the ability to plate out (oxidants, for example).
The process that I am aware of most often used in conjunction with EMR is ion exchange. The use of an ion exchange column is ideal for the removal of the lower concentration metal ions mentioned above, or the metal ions in a relatively dilute wastewater stream. Choice of resin is important to maximize removal.