Pyrohydrolysis process for spent aluminum reduction cell linings

Claim 1

In the process of recovering fluoridic values from spent and waste materials generated in electrolytic aluminum reduction systems by subjecting the materials to pyrohydrolysis in a fluidized bed reactor at a temperature of at least about 1100° C., the reactor having a bed of fluidized spent and waste materials and a freeboard area extending from the surface of the bed to the top of the reactor, the improvement which comprises: (a) charging a relatively coarse charge of spent and waste materials to the fluidized bed, the charge having a particle size in the range of from about 2.0 mm to about 15.0 mm and a controlled Na to Al mole ratio of not less than about 0.7 and up to about 1.0; (b) fluidizing and pyrohydrolyzing the coarse charge to generate an offgas containing volatilized fluoridic values including Na-containing vapors; (c) establishing an extended reaction zone in the freeboard area of the reactor by charging a finely divided source of Al 2 O 3 to the reactor to contact the offgases and in a manner so as to obtain an excess of Al in the Na:Al mole ratio in the freeboard area whereby essentially all of the Na-containing vapors are converted to HF and Na 2 O.xAl 2 O 3 , the relatively finely divided source of Al 2 O 3 being characterized by an Al 2 O 3 content of at least about 25% by weight and a particle size in the range of about 40 to about 500 microns and at least a portion of the finely divided source of Al 2 O 3 being charged to the freeboard area; (d) recovering an offgas stream from the reactor containing entrained solids, separating the solids from the gas and recycling at least a portion of the solids to the fluidized bed reactor; and (e) recovering a solid clinker from the reactor having a controlled Na to Al mole ratio.