Multivariate study of a novel hydrometallurgical route employing chloride/hypochlorite for leaching silver from printed circuit boards

Abstract

The rapidly expanding use of electronic products has led to the generation of very large quantities of waste electrical and electronic equipment (WEEE). This waste essentially consists of polymeric, ceramic, and metallic materials, with the metal fraction including noble and base metals, which may be toxic. It is therefore important to recycle printed circuit boards (PCBs) for both economic and environmental reasons, especially since this solid waste contains noble metals (gold and silver). Given this background, the aim of the present work was to develop a hydrometallurgical route for leaching of the silver present in computer PCBs, using a combination of sodium chloride and calcium hypochlorite as the lixiviants. Optimization was performed of the following independent variables: (i) pulp density, (ii) calcium hypochlorite concentration, (iii) sodium chloride concentration, and (iv) leaching time. Multivariate experimental designs were employed, consisting of a 24 full factorial design and response surface methodology with a spherical central composite design (CCD). The operational conditions were varied according to the mathematical equation obtained from the CCD, in order to find their best values which resulted in the maximum Ag extraction. The optimal conditions for the leaching of silver were a pulp density of 48 g L−1, calcium hypochlorite concentration of 165 g L−1, sodium chloride concentration of 65 g L−1, and extraction time of 200 min, resulting in 95.29 ± 0.72% extraction of silver. Silver was dissolved by forming complexes with chloride, being AgCl4 aq3− the predominant specie. In summary, the hydrometallurgical route employed in this work enabled satisfactory and environmentally favorable extraction of silver.