Mass-transfer coefficients between an electrolyte solution and the wall of a hydrocyclone were experimentally determined by using the electrochemical technique. The local mass-transfer coefficients were measured as a function of the axial distance from the entrance changing the inlet volumetric flow rate and the underflow stream. An enhancement of mass-transfer was observed in comparison to that predicted for developing axial flow in an annular duct, being the cylindrical part more active than the conical one. The global mass-transfer coefficient in the cylindrical region depends on the Reynolds number raised to an exponent of 0.79, in accordance with the mass-transfer behavior of decaying annular swirl flow. The modified hydrocyclone was also examined for the removal of copper from dilute synthetic solutions. A diminution of copper concentration from 400 ppm to 40 ppm was achieved after 60 minutes of electrolysis in a recirculating system and the contaminant was recovered as a metal powder separated by the hydrocyclone. The space time yield and the specific energy consumption were 12.5 kg m(-3) h(-1) and 4.9 kWh kg(-1), respectively, with a cumulative current efficiency of 54%, showing that the modified hydrocyclone is a suitable reactor for the electrochemical treatment of effluents. (c) 2017 The Electrochemical Society. All rights reserved.