Modified pyrohydrolysis process for spent aluminum reduction cell linings

Claim 1

In the process of recovering valuable components from carbon and fluoride-containing aluminous spent and waste materials generated in electrolytic aluminum reduction systems by subjecting the spent and waste materials to pyrohydrolysis in the presence of steam in a fluidized bed reactor having a fluidized bed and a freeboard area above the bed at a temperature within the range of about 900° C. to about 1300° C., the improvement comprising the combination of steps of (a) combusting the carbon content of the spent and waste materials in the fluidized bed with an O 2 -containing stream having an O 2 content of at least about 90% by volume thus generating a hot offgas substantially free of nitrogen and containing HF and vaporized Na-containing compounds; (b) establishing an extended reaction zone in the freeboard area of the reactor by contacting the hot offgas with a relatively finely divided source of Al 2 O 3 having a particle size within the range of about 40 to about 500 microns whereby the Na-containing vapors are converted to HF and Na 2 O.xAl 2 O 3 ; (c) cooling the hot, HF-containing offgas by direct contact with an aqueous HF-containing stream and conducting the cooled offgas to a scrubber for the removal of its HF content by scrubbing with an aqueous medium, while generating a steam and inert gas containing scrubbed offgas; (d) recovering the aqueous HF from the scrubber and using at least a portion thereof for cooling hot HF-containing offgas generated in the fluidized bed reactor; (e) recovering an HF-containing side stream from the cooled HF-containing offgas prior to scrubbing of the offgas with the aqueous medium and employing the HF-containing side stream for the production of an AlF 3 product of at least about 85% by weight AlF 3 content in a fluidized bed reactor using as alumina source alumina trihydrate; and (f) recovering the AlF 3 product and steam from the AlF 3 reactor and recycling the steam to the pyrohydrolysis reactor.