Continuous ion exchange for hydrometallurgy: Purification of Ag(I)–NaCl from divalent metals with aminomethylphosphonic resin using counter-current and cross-current operation

• Ag–NaCl solution was successfully purified with continuous ion exchange.
• An amphoteric aminomethylphosphonium functional resin was used as stationary phase.
• Controlled partial neutralization of resin successfully implemented in the process
• Counter-current and cross-current operation modes were compared.
• Practically 100% yield and 100% purity were obtained for Ag in counter-current mode.

Continuous simulated moving bed (SMB) type ion exchange processes are usually operated in cross-current mode for metal separations whereas counter-current mode is commonly used for fine chemicals. The use of both process types was investigated for hydrometallurgical purification of Ag–NaCl solution from divalent impurity metals (Ca, Mg, Pb, Zn). An amphoteric aminomethylphosphonic functional ion exchange resin (Lewatit TP-260) was used as stationary phase. Also the influence of controlled partial neutralization of the resin on process performance was investigated. Successful separation was obtained with both counter-current and cross-current configurations. Ag product was collected in over 99% purity with practically 100% yield. All of the impurity metals were collected at the extract, and none of them migrated to the controlled partial neutralization zone. Column efficiency and the liquid to solid flow rate ratio were found to have strong influences on process performance. It was thus shown that counter-current simulated moving-bed process can be utilized for purification of precious metal solutions, especially when high yield is important. Remarkably improved performance is obtained by implementing a controlled partial neutralization procedure as an additional step within the continuous ion exchange unit.