Anodic slimes formation in copper electrowinning

The slime formation in acidic electrolytes of copper with several metallic impurities has been studied. On Pb-Ca-Sn anode surface firstly the formation of PbSO4 takes place, then, it is transformed in PbO2, which covers the anode surface. It was experimentally established the formation of a manganese dioxide double layer at the anode. This layer was always composed of a thick external layer of non-adhering and easily removable scales, and of a thin internal layer, which adheres relatively well to the surface of the electrode. It was found that the manganese dioxide present in the slime is of different nature: a nonadhering layer produced by electrolysis (*b-MnO2,) on the PbO2, surface and a pure chemical precipitate in the solution (*b-MnO2). Lead sulfate was found on the (*b-MnO2, layer when iron was in the electrolyte. When arsenic or antimony was present in the electrolyte, the slime was lead sulfate and amorphous compounds of those ions. Slime of Chilean. electrowinning (EW) plants is also discussed.