Method of treating spent potliner material from aluminum reduction cells

A method of treating spent potliner material from aluminum reduction cells, which spent potliner material includes at least one material selected from the group consisting of fluoride compositions, cyanide compositions, iron compositions, calcium compositions, magnesium compositions, alumina, carbon, silica and sodium sulfate, said method comprising the steps of:

• (a) introducing said spent potliner material to an acid digester to produce a first gas component which includes at least one material selected from the group consisting of hydrogen fluoride gas and hydrogen cyanide gas, and a slurry component including at least one material selected from the group consisting of carbon, silica, alumina, sodium sulfate, iron sulfate, calcium sulfate and magnesium sulfate;
• (b) recovering said gas component from said digester and heating it to a temperature sufficient to decompose hydrogen cyanide which has been produced, and thereby produce a second gas component which is substantially free of cyanide and includes at least one offgas selected from the group consisting of CO.sub.2, H.sub.2 O, nitrogen and said HF gas;
• (c) combining said second gas component with water in a water scrubber to form hydrofluoric acid from HF included in the second gas component;
• (d) admixing said hydrofluoric acid with alumina trihydrate to form aluminum fluoride;
• (e) rinsing said slurry component with water and separating a first solid fraction including at least one of the group consisting of carbon, alumina and silica from a first liquid fraction;
• (f) adjusting said first liquid fraction to a first basic pH effective in forming an aluminate in solution while precipitating impurities containing elements selected from the group consisting of Ca, Fe and Mg and then filtering the impurities from liquid in the first liquid fraction to provide a filtered liquid fraction;
• (g) adjusting the filtered liquid fraction to a second pH effective in forming and precipitating alumina trihydrate from the filtered liquid;
• (h) admixing said first solid fraction with an alumina/silica mix to provide a mixture;
• and
• (i) subjecting said mixture to an elevated temperature in an oxygen-rich atmosphere to oxidize said carbon and vitrify said alumina and silica into refractory material.