Article ID: M2016442
This study investigated the effect of dissolved impurities on the lightness, surface morphology, and current efficiency of deposited nickel during electrowinning. Nickel electrodeposition was performed at a current density of 300 A·m−2 and 7.2 × 105 C·m−2 of charge in an unagitated chloride solution containing Mn2+, Cr3+, and SO42− ion impurities with a pH from 1 to 3, at a temperature of 333 K. Solutions containing 10 g·dm−3 of Mn2+ resulted in a slight decrease in current efficiency for nickel deposition and smaller crystal sizes in the deposited nickel. The lightness of deposited nickel decreased for Mn2+ concentrations exceeding 1 g·dm−3. When nickel deposition was performed using a soluble nickel anode to prevent the formation of MnO2 at the anode, greater nickel lightness resulted than with an insoluble anode, suggesting that MnO2 produced by the insoluble anode caused decreased lightness of nickel. In solutions containing Cr3+, the current efficiency of nickel gradually decreased with increasing Cr3+ concentrations and significantly decreased at Cr3+ concentrations above 0.1 g·dm−3. The lightness of deposited Ni greatly decreased with increasing concentrations of Cr3+ above 0.001 g·dm−3. Formation of Cr(OH)3 at the cathode layer is presumed to suppress nickel deposition, resulting in some codeposition of NiO and Ni(OH)2 along with Ni, which causes the nickel current efficiency and lightness to decrease. Conversely, solutions containing SO42− resulted in a moderate decrease in nickel current efficiency at SO42− concentrations above 50 g·dm−3 and a significant decrease above 100 g·dm−3. The lightness of deposited nickel increased slightly at SO42− concentrations of 20 g·dm−3 and strongly increased above 20 g·dm−3. Because the overpotential for nickel deposition increases with the concentration of SO42−, the surface of the deposited nickel becomes smooth, resulting in greater lightness.
