JEE Main · 2005 · 08 May 2005 · Offline · General Principles and Processes of Isolation of Metals

In the electrolytic refining of copper the impurity metals sort themselves by how readily they oxidise. Those more reactive than copper dissolve into the electrolyte, but the noble metals, harder to oxidise than copper itself, simply do not go into solution; they fall away from the impure anode and gather beneath it as anode mud. Silver and gold are exactly these noble metals, so they make up the anode mud.