Copper, zinc, and nickel recovery from printed circuit boards using an ammonia–ammonium sulphate system

The ammoniacal leaching has been considered very selective for copper compared with other leaching systems; however, there are interactions with zinc and nickel that also form stable ammonia complexes, implying higher consumption of ammonia to obtain high copper recoveries. This research aimed to clarify under which conditions copper, zinc, and nickel can be leached from some components from PCBs and followed the interactions among them. This study focuses on the copper, zinc, and nickel recoveries from connection pins and microchips using ammonia–ammonium sulphate leaching solutions. Factors that influence recovery as the stirring rate, L/S ratio, amount of ammonia and ammonium sulphate, and the type of oxidant were assessed. The results indicated hydrogen peroxide and oxygen are the most efficient oxidants. Metal recoveries of 90% were achieved with excess of oxidant and ammonia, relatively to the estimated stoichiometry. For efficient copper and zinc recoveries, two times the stoichiometric amount of ammonia was needed; four times the stoichiometry was needed for nickel as well as higher ammonium sulphate concentration. Metal recoveries depend strongly on the stability of ammonia complexes, and nickel had lower recovery under almost all conditions tested due to the lower stability of nickel ammonia complexes.