Zinc is normally considered as a divalent cation that would be easily picked up by standard cation exchange resins, including DOWEX™ G 26 (H) Resin. While this is true, an interesting property of ionic zinc (like iron) is that in the presence of acid or brine, zinc forms multivalent anionic complexes (42KB PDF) that can be captured on anion exchange resins such as DOWEX 21K XLT Resin. Most other cations remain cationic under these conditions so they are not captured by an anion exchange resin, thus creating the opportunity for a new separation process. Zinc can be eluted from the resin by simply washing with water, reversing the equilibrium of the anionic complex formation:
DOWEX 21K XLT has been used by a geothermal power company to commercially mine 30,000 MT of zinc annually from geothermal brine. Classical zinc mining operations are generally very waste intensive; however, this new ion exchange process may enable geothermal power companies to operate the most environmentally benign zinc mining operations in the world.
Only the exceptional performance inherent to uniform particle technology could provide the processing edge that the process designers needed in order to meet their demanding cost/performance criterion. Very hot geothermal brine at a rate of more than 2.5 million pounds per minute (68,000 MT/hour) pass through DOWEX™ Anion Exchange Resin, where the zinc is captured and concentrated for further refining. The resin is eluted with plain water, eliminating the need for costly chemicals, storage, and permitting.
Removing zinc from water via ion exchange is common in chemical, petrochemical, oil refining, semiconductor manufacturing, and power utility plants.
In waters with low levels of dissolved salts, a strongly acidic cation exchange resin such as DOWEX™ G 26 (H) Resin is employed.
Weak acid cation exchange resins such as DOWEX MAC 3 Resin often provide high capacity and good multivalent ion selectivity.
In more brackish waters, however, chelating resins such as AMBERLITE™ IRC747 Resin may be required. Chelating resins have reactive units dispersed along their polymer matrix, each of which has multiple metal binding sites. Hence, chelating resins are selective for multivalent cations over monovalent cations like sodium and potassium.