A method of treating spent pot liner material arising from the electrolytic production of aluminium is described. The used pot lining is placed in an aqueous electrolyte solution, and an A.C. electrical current is applied to the solution, extracting contaminants from the pot liner material. Preferably the electrolyte solution is dilute hydrochloric acid. In an alternate form of the invention the process is conducted in two stages, the second stage using dilute hydrochloric acid as the electrolyte. In this case, the first stage uses alkaline salts leached from the spent pot liner as the electrolytes. Preferably, hydrogen peroxide is added to the electrolyte solution as an initiator. The A.C. electrical current can be applied continuously or intermittently, preferably at a current density of from 1 to 20 Amp/dm .