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A fully integrated process is provided for the recovery of valuable components from waste materials generated in electrolytic aluminum reduction systems. The waste materials, such as spent pot linings, channel and trench cleanings, floor sweepings and spent alumina from offgas purifying dry scrubber

A fully integrated process is provided for the recovery of valuable components from waste materials generated in electrolytic aluminum reduction systems. The waste materials, such as spent pot linings, channel and trench cleanings, floor sweepings and spent alumina from offgas purifying dry scrubbers, are combined, then pyrohydrolyzed at elevated temperature. Fluoridic values, such as NaF and HF can be recovered from the offgas generated by pyrohydrolysis, while alumina and Na.sub.2 O values, or if desired, sodium aluminate, is reclaimed from the solid residue of pyrohydrolysis.The fluoridic values from the pyrohydrolysis offgas can be used for the manufacture of both electrolytes for aluminum reduction cells and also for the production of anhydrous HF. The alumina from the pyrohydrolysis residue can be reclaimed by a Bayer process-type leach with a caustic solution and the recovered high purity alumina utilized, for example, as reduction cell feed and/or for scrubbing reduction cell offgases. If the solid residue of pyrohydrolysis contains significant amounts of sodium aluminate, this material can either be directly used for dry scrubbing cell offgases, or if desired, utilized for production of high purity alumina. SUBACKGROUND OF THE INVENTIONThis invention relates to a fully integrated system for the recovery of valuable components from spent materials generated in the electrolytic reduction of alumina to metallic aluminum with simultaneous improvement in the purity of aluminum produced in the reduction process.In the production of metallic aluminum by electrolysis of reduction-grade Al.sub.2 O.sub.3, the electrolysis is generally carried out in reduction cells or pot lines which are lined with a carbonaceous material. During the life of the cells, this carbon lining is gradually destroyed by penetration of bath materials into the lining, for example, metallic aluminum, cryolite and alumina. Also, due to the high temperatures employed in the electrolytic reduction process, gradual aging of the carbonaceous lining takes place. The combined result of penetration and aging can reach a stage where the further operation of the cell or cells reaches an economically prohibitive point and replacement of the carbonaceous lining becomes a must. The unusable or "spent" potlining is then removed and in most instances stockpiled. In large aluminum reduction facilities, this lining replacement is a continuous process and, consequently, the quantity of spent lining stockpiled increases from day to day.In aluminum reduction facilities, where metallic aluminum is produced by the electrolysis of Al.sub.2 O.sub.3 in the presence of a fluoridic electrolyte, such as cryolite (Na.sub.3 AlF.sub.6), the electrolysis results in offgases of high fluoride content. In addition to the fluoride content, the offgases generated in the reduction process contain gaseous and particulate impurities, for example, volatilized metallic compounds and carbon derivatives, together with solid matter and nonvolatile carbonaceous materials. The quantity of volatilized and solid carbon compounds in the offgases vary within wide limits depending on the type of anode used in the reduction system. Soderberg carbon anodes generate far more of these materials than prebaked carbon anodes.In order to protect the environment and to provide healthy operating conditions in the reduction facility, these offgases must undergo a purification process for the removal of harmful constituents. A common process for cleaning the offgases is to subject them to a dry scrubbing treatment which effectively removes essentially all of the environmentally harmful impurities from the offgases. In the dry scrubbing treatment of reduction offgases, alumina is usually employed as the scrubbing medium. The alumina readily absorbs the fluoridic components of the offgases and also captures the particulate impurities. It further removes harmful high molecular weight carbon derivatives. Consequently, the dry scrubbing of reduction cell offgases with alumina is an effective purification process resulting in purified off-gases containing only environmentally harmless components.While scrubbing of the offgases solves the environmental and health problems, it poses a serious disposal problem. The spent alumina from the scrubber system is heavily laden with impurities and cannot be directly employed as feed for reduction cells without introducing unacceptable alloying components in the metal to be produced and without seriously interfering with the efficient operation of the reduction cells. Since the alumina is spent, it cannot be used for further scrubbing without purification.In the production of metallic aluminum by the electrolytic reduction of Al.sub.2 O.sub.3 in a series of cells, a significant quantity of impure metal and contaminated aluminum oxide feed are also generated in the form of floor sweepings, channel and trench cleanings. These materials, due to their high impurity content,cannot be directly employed for making metallic aluminum of commercial purity and, in general, if not blended with pure feed materials, are considered as waste with no convenient way of disposal.Thus, from the above, it becomes clear that the producers of aluminum by the electrolytic process have major problems relative to the disposal of spent potlinings, exhausted alumina from the dry scrubbers, floor sweepings, channel and trench cleanings. These problems have been acutely recognized by operators of aluminum reduction facilities all over the world and partial solutions have been offered to overcome one or more of the problems associated with the generation of these spent materials.Several proposals have already been made to deal with the problems resulting from the accumulation of excessively large quantities of spent potlinings.Thus, in U.S. Pat. No. 3,151,934, it has been suggested that the spent potlinings be crushed, followed by extraction of fluoridic values and dissolution of metallic aluminum with a sodium hydroxide solution. The alkaline extract, after carbonation, is utilized for the preparation of synthetic cryolite, while the essentially fluoride-free carbon residue, is again contacted with an NaOH--Ca(OH).sub.2 solution. This treatment of the carbon residue or "black mud" removes any lithium present and then the black mud residue is disposed of. The treatment disclosed in this reference results in only a partial and expensive solution of the disposal problem; large quantities of black mud remain after the extraction treatments which cannot be utilized for any forseeable purpose.In U.S. Pat. No. 3,606,176, it has been suggested to crush the spent lining of reduction cells, followed by removal of the metallic aluminium content by mechanical screening. The residual crushed carbonaceous lining is then further reduced in size and subsequently slurried with salt water to allow separation of the bulk of the carbon fraction by flotation from cryolite, alumina and residual aluminum. Again, the carbon fraction is discarded and since this is the major portion of the spent lining, stockpiling with the corresponding problems has not been solved.Another process for treating spent potlinings is presented in U.S. Pat. No. 3,635,408. According to this reference, spent carbon lining is crushed, then treated with dry steam at a temperature insufficient to destroy the carbon. The steamed, carbonaceous material is then classified into coarse and fine fractions. The fine fraction is subjected to a chemical treatment for the recovery of its fluoridic values, together with the alumina and aluminum content, while the coarse fraction is utilizable for making new cell linings. However, if the coarse fraction resulted from cell linings of the monolithic type, the coarse fraction has an approximate carbon content of only 53%, the balance being fluorides, alumina and aluminum. This relatively high percentage of impurity content, when the coarse fraction

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