Solvent extraction of silver from nitric acid solutions by calix[4]arene amide derivatives

Solvent extraction experiments were carried out to recover silver from nitric acid solutions using as the extractants calix[4]arene tetramide (LBC) and its thio-analogue (THIO) dissolved in dichloromethane. The stoichiometry of the formation of the silver-calixarene complex differs depending on the used extractant. It was shown that one silver ion reacts with a single molecule of LBC, while two silver ions are extracted with a single THIO molecule. Very high percent extractions (>90% with LBC and >99% with THIO) were achieved with both extractants by just adding surplus extractant (THIO: 0.6 mol/mol Ag+ and LBC: 1.4-1.5 mol/mol Ag+). Based on equilibrium data, the extraction isotherm relationship and the extraction constant Kex for each extractant were derived. The extraction results show that THIO is an efficient extractant extracting silver either from acid or from neutral solutions with the same efficiency. Nitric acid, in the concentration range from 0 to 0.1 mol dm−3, strongly affects the extraction capability of LBC in less acidic solutions, reducing the percent extraction down to 40%. LBC extracts both silver and sodium collectively and with the same efficiency. The selectivity coefficient has a value close to unity (β ≈ 1). THIO has demonstrated extremely high selectivity for silver over sodium in a mixture. Stripping of silver was performed by two-phase electrowinning from the loaded organic phase, thereby achieving a high stripping degree (50-90%) and reasonably high current efficiency (60-90%). The calixarenes did not change their extraction features during the electrowinning process and can be reused for extraction many times.