Electrorefining
The purification of a metal by dissolving it from an impure anode and depositing it in pure form at the cathode.
Unit
Dimensionless
In depth
The applied voltage is set just high enough to dissolve the target metal, so more reactive impurities stay in solution and less reactive ones fall to the bottom as anode mud. That mud is often valuable in its own right, containing precious metals.
Examples in the real world
Refining blister copper to 99.99% purity, with silver, gold and platinum recovered from the anode slime.