Electrolytic Refining

Electrolytic refining purifies a crude metal by ELECTROLYSIS, carried out in an electrolytic cell containing an aqueous solution of a salt of the metal of interest as electrolyte. Thick rods of the IMPURE metal are used as the ANODE, and thin strips of already-PURE metal are used as the CATHODE. During electrolysis, metal at the anode dissolves (is oxidised) and passes into solution as metal ions, while an equivalent amount of metal ion from the solution is simultaneously reduced and deposited onto the cathode -- so metal is effectively transferred from the impure anode to the pure cathode through the solution, leaving impurities behind. Impurities in the anode that are LESS electropositive than the metal itself do not dissolve; they simply fall away and settle at the bottom of the cell as a sludge called ANODE MUD, which is periodically collected (and is often itself valuable, since it can be a source of precious-metal impurities). …