Abstract: Spent potliner from aluminum reduction cells includes carbon, inorganic brick and refractory material, and fluoride, sulfur, and cyanide compounds, and is considered hazardous. It is treated by introducing it into a vessel, and exposing it to the heat of a plasma torch at a temperature of at least 1000.degree. C. As a result, carbon is gasified and converted to combustible carbon monoxide or hydrocarbons, or to carbon dioxide; inorganic material is melted form slag; fluoride compounds are melted, vaporized, or reduced to gaseous HF; cyanide compounds are destroyed; and all other materials, including sulfur compounds, are either melted or gasified. As a result, the spent potliner is rendered non-hazardous, and the quantity of remaining slag has both its solid volume and mass substantially reduced by a factor of at least 1.5:1 in mass and at least 3:1 in volume relative to the input spent potliner.