Copper recovery from waste printed circuit boards by NH3−(NH4)2S2O8–CuSO4 slurry electrolysis

Abstract

The recovery of waste printed circuit boards (WPCBs) is always at the center of the issue of e-waste, and even “urban mining”. Slurry electrolysis has been certified as a new and efficient approach for directly recovering copper from WPCBs, although its current efficiency is not high. Therefore, an alkaline slurry electrolysis system with NH3−(NH4)2S2O8–CuSO4 was examined to improve the current efficiency because (NH4)2S2O8 produces SO4-· or ·OH in an alkaline environment. Factors such as the current density, concentration of NH3·H2O, (NH4)2S2O8, NaCl, and Cu2+, solid/liquid (S/L) ratio, and electrolysis time are discussed. The results showed that the NH3−(NH4)2S2O8–CuSO4 electrolyte could significantly impact the current efficiency and copper recovery rate. Under optimum conditions (4 mol/L ammonium hydroxide, 1 mol/L ammonium persulfate, 20 g/L copper ion(Ⅱ), 0.5 mol/L sodium chloride, S/L ratio of 30 g/L, 20 mA/cm2, and 3 h), the copper recovery rate was 90.10%, and the copper current density was significantly improved to 83.03%, which was considerably higher than that of the acidic slurry electrolysis system (approximately 70%). Because copper in the WPCBs and Cu2+ can form [Cu(NH3)2]+ with ammonia and ammonia persulfate, it is substantially easier to electrodeposit than [Cu(NH3)4]2+. Meanwhile, the purity of the obtained copper foil was > 99.986%, meeting the standard for commercial cathodic copper, and markedly higher than that of the acidic slurry electrolysis system. Thus, alkaline slurry electrolysis shows great potential for applications in the e-waste industry.