Ion Exchange for Recovery of Precious Metals

ith the rapid rise that has occurred in the prices of precious metals during the past few years, recovery of even minute amounts of theseelements has become of greater interest to metal finishers. Although precious metals are recovered easily from concentrated solutions by reduction and precipitation or electrolysis, solutions containing a few parts per million aredifficult to handle by these methods. However, the use of ionexchange resins makes it possible to concentrate dilute solutions economically so that they can be subjected to common refining procedures to recover the metal. Also, the purity of the wastewater is improved to facilitate disposal. Ion exchange is well known for its use in water softening, where resins are used to replace the calcium and magnesium in “hard” water with sodium, thereby producing “soft” water. The resin used in this operation consists of a polymeric water-insoluble anion with a water-soluble cation attached to it (in this case, sodium). At low concentrations, the anion prefers large ions with higher valences and therefore readily exchanges the sodium ions for calcium and magnesium ions, according to the following equation: