Copper recovery from waste printed circuit boards by NH₃-(NH₄)₂S₂O₈-Cu SO₄ slurry electrolysis

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)(2)S2O8-CuSO4 was examined to improve the current efficiency because (NH4)(2)S2O8 produces SO4- or OH in an alkaline environment. Factors such as the current density, concentration of NH3H2O, (NH4)(2)S2O8, NaCl, and Cu2+, solid/liquid (S/L) ratio, and electrolysis time are discussed. The results showed that the NH3-(NH4)(2)S2O8-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/cm(2), 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.
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