Nickel removal from an industrial effluent by electrocoagulation in semi-continuous operation: Hydrodynamic, kinetic and cost analysis

Abstract

In the nickel and cobalt production plant located in Punta-Gorda Cuba, an effluent liquor is obtained with several pollutants species of Ni(II)–NH3–CO2–SO2–H2O system. In previous studies the performance of electrocoagulation (EC) for the nickel removal from this effluent was demonstrated on a lab-scale and bench-scale. In this research, geometric, electrical and equilibrium parameters used as criteria for scaling-up the EC process were evaluated through hydrodynamic, kinetic and operating cost analysis. For this purpose, four semi-continuous stirred-tank electrochemical reactors of equal sized with 200 L useful volume were design. Based on the residence time distribution (RTD) curve, the hydrodynamic behavior of EC systems followed multi-branch tanks-in-series model. Experimental mean residence time was of 92 min and hydraulic efficiency 92% for a liquor flow rate 2 L∙min−1, due to the presence of short-circuit, internal recirculation and 3% dead zone. Then, at the operating parameters: nickel concentration 277 ± 52 mg L−1, current density 6.26 mA cm−2, pH 8.40 ± 0.28 and temperature 50 ± 2 °C, the maximum nickel removal was 97.8% with an operating cost $1.008 kg−1 Ni. Combination of RTD model and the conversion time kinetic model (CTV) was presented to predict the removal efficiency with successful outcomes, and the time for complete conversion was 23.3 min under control of chemical reaction. The precipitate presented a nickel concentration of 35.71 ± 2.13%, aluminum 6.05 ± 0.83%, true density 2836 ± 449 kg m−3 and typical layered structure. These results suggest the opportunity for the projection of an industrial production unit of double lamellar hydroxide and reduce the adverse environmental impact.