Why do silver-rich slimes collect at the bottom of copper electrorefining tanks?

During the electrolytic refining of copper, silver-rich slimes collect at the bottom of the tank because they consist of insoluble impurities present in the copper anode.12

As an electric current is passed through the electrolytic cell, the copper from the anode dissolves into the solution to be deposited in pure form on the cathode. Because silver is insoluble under these specific conditions, it does not dissolve into the electrolyte along with the copper. Instead, these precious metal impurities detach from the anode and settle at the bottom of the cell as a sludge, commonly referred to as anode slimes. These slimes are highly valuable, as they contain the silver originally present in the copper concentrate at a significantly higher concentration, allowing for further processing to recover the metal.231