New insight into cleaner control of heavy metal anode slime from aqueous sulfate electrolytes containing Mn (Ⅱ): Preliminary characterization and mechanism analysis

In this work the characterization and initial nucleation-growth mechanisms of the anode slime in aqueous sulfate electrolytes containing Mn²⁺ were investigated in detail. Under laboratory conditions, this anode slime mainly consisted of manganese dioxides (in the form of α-MnO₂ and γ-MnO₂) with small amount of lead sulfate and oxides, determined by XRD, SEM and ICP-MS analysis. After the initial galvanostatic electrolysis, the results showed that the anode slime tended to generate onto the electrode surface at pH = 6.8 due to the rapid deposition of MnOOH solid intermediate, whereas more concentrated acid implied a positive role in favor of the generation of suspended anode slime via the diffusion of aqueous Mn³⁺. While deposited from a solution of 30 g/L Mn²⁺ in 0.1 mol/L H₂SO₄ onto a platinum substrate, the kinetic data was well described by progressive 3D nucleation model. Moreover, while deposited onto a lead-based anode surface, the kinetic data indicated to be close to follow the progressive 3D nucleation model whereas appeared the obvious deviation mainly due to the dissolution of the lead-based anode. While deposited at pH = 6.8, despite the corresponding nucleation-growth mechanism was observed to be unchanged, the most significant feature was the obvious appearance of the earlier onset of t_max as well as the larger response of current density from current–time transient curves attributed to the oxidation mechanism pathway preference of Mn³⁺ intermediate. Considering the potential in controlling and reducing the generation of anode slime via a cleaner way, deepening the understanding of the initial nucleation-growth process of anode slime contributes to providing a feasible control method in manganese electrowinning process.