The Effect of Functional Group Structure on the Elution of Metal Cyanide Complexes from Ion-Exchange Resins

Abstract

Previous investigations have established that the thiocyanate anion and the zinc cyanide complex are suitable eluants for the simultaneous recovery of metal cyanide complexes from ion-exchange resins. However, the effect of the ionic density of the resin and the stereochemistry of the functional group on metal recovery has never been systematically studied. The present study investigated the elution properties of five experimental resins that have the same type of resin matrix but contain different aliphatic amino functional groups. It is shown that ammonium thiocyanate is not a suitable eluant because iron and zinc are not completely eluted from any of the resins studied. It has been proposed that this is due to the precipitation of Zn2[Fe(CN)6] and possibly Fe4[Fe(CN)6]3 within the resin pores. It has been established that potassium thiocyanate at a pH greater than 12 can be used to recover more than 80% of copper, zinc, and iron within the first 10 bed volumes of eluant for most resins. The recovery of gold from each resin using potassium thiocyanate has been shown to be considerably slower than that of other metals, with approximately 60% recovered after 20 bed volumes of eluant. It has been found that the length of the alkyl chain of the functional group does not significantly affect the elution of metal cyanide complexes from the resins studied using the thiocyanate eluant. The elution of metals from all resins has been found to be superior when using a 0.5 M zinc cyanide eluant at a temperature of 50°C. More than 90% of copper, silver and iron were recovered from most resins within 4 bed volumes of eluant. However, the recovery of gold using a zinc cyanide eluant is dependent upon the ionic density of the resin and also the stereochemistry of the attached functional group. The results of this study show that zinc cyanide is not a suitable eluant for gold-selective ion-exchange resins.