In general, electrolytic refining is a process of purification of impure metal during which an electric current is passed through an electrolytic cell which contains impure metal as anode, a suitable electrolyte and pure metal as cathode.
In the electrolytic refining of Copper (electrolyte is CuSO4 solution + dil H2SO4 ;Pure Cu cathode and Impure Cu anode), metallic impurities having lower oxidation potential than Cu are deposited as __ sludge.