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. SU BACKGROUND OF THE INVENTION This 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 is used directly for making of new cell linings, will adversely affect the electrical and mechanical properties of the new cell lining and, consequently, will provide lower life expectancy, coupled with operating efficiencies below the desired parameters. If the coarse fraction results from spent linings made of the prebake type, the carbon content is higher and the undesirable impurity content is lower. However, the new linings made from this material will still perform below the desired values in terms of efficiency and life. Thus, it can be observed that although many efforts have been made to utilize the spent linings of reduction cells, these efforts only provided partial solutions to the existing problems which the ever-increasing piles of spent potlinings further emphasize. Regarding the spent alumina recovered from the dry scrubber systems, several processes have already been recommended for the treatment of this impurity-laden material. For example, German Pat. No. 970,919 (granted Nov. 13, 1958) has recommended the calcination of the spent alumina removed from the scrubber system. Calcination of this alumina in the presence of sodium carbonate below the sintering temperature of cryolite results in cryolite which can be recycled to the reduction cells as electrolyte. This cryolite would be a suitable substitute for either natural or synthetic cryolite generally employed for this purpose if it would be free of metallic impurities. However, the calcination employed to convert the spent alumina to cryolite can only remove some of the volatilizable impurities and perhaps carbon. It does not eliminate the metallic impurities, such as iron, silicon and phosphorus, and, consequently, by recycling it directly to the cell, the undesired impurity content in the produced metallic aluminum will constantly increase. This increase in impurity level significantly lowers the commercial value of the produced aluminum, apart from the deleterious effects caused by these impurities with regard to the cell lining life and efficiency of the electrolytic reduction process. More currently, it has been suggested in U.S. Pat. No. 4,006,066, that the spent alumina from dry scrubbers, which are appended to the electrolytic aluminum reduction cell system, can be purified by classifying the impurity-laden alumina to coarse and fine particle size fractions. The reason for this size separation is the fact that the major quantity of impurity from the reduction cell offgas is captured by the fine fraction of the alumina employed in the scrubber system. The coarse fraction will also capture impurities from the offgas; however, the impurity content of the coarse fraction is significantly smaller in proportion to its weight. Consequently, separation by size affords a preliminary purification and allows the return of the coarse fraction directly to the reduction cell as partial feed and also as partial fluoridic electrolyte replacement. This recycling of the coarse fraction, which can amount to up to about 80-85% of the alumina from the scrubber system, greatly assists in reducing the quantity of alumina to be purified before further utilization. The impurity content of this coarse fraction nevertheless still causes similar problems as described hereinbefore. The fine fraction from the classification contains the major amount of the impurities from the reduction cell offgases and this fraction, while smaller in percentage by weight than the coarse fraction, still presents a large quantity to be dealt with. The aforesaid U.S. reference pyrohydrolyzes this fine fraction in a special rotary kiln with water vapor and the resulting alumina product, which is free of fluorine, but still contains the other impurities as stated, is usable, for example, in the ceramic industry. This alumina, due to its high impurity content, cannot be returned to the electrolytic aluminum production system. Consequently, it constitutes a significant loss and affects the overall economy of aluminum production. Even more recently, in copending U.S. pat. application Ser. No. 709,025 (filed July 27, 1976), a process has been described which allows the separation of essentially all of the impurities from the spent alumina recovered from the dry scrubbers of aluminum reduction cell offgases. This is accomplished by slurrying the spent alumina with a solvent, followed by an ultrasonic treatment of the slurry. Although the ultrasonically treated, highly pure alumina can be recycled to the scrubber system or the cells after drying, the process, due to the large quantities of spent alumina to be treated, can create logistics problems and equipment constraints. Channel and trench cleanings and floor sweepings found in aluminum reduction facilities can contain a large percentage of metallic aluminum, together with cryolitic flux and aluminum oxide. Recovery of the aluminum values can be accomplished by screening or melting these materials in a furnace in the presence of a suitable flux. This operation requires special equipment and chemicals, not to mention the significant input of energy. This type of purification allows the recovery of metallic aluminum; however, both the cryolite and the alumina values become lost and in addition, the process poses disposal problems. From the above, it can be observed that there is a need for an integrated system which is capable of dealing with all of these spent materials with simultaneous recovery of all of the valuable components from these by-products of the electrolytic aluminum reduction process. The integrated system described hereinafter provides such a solution whereby all of the above-described spent materials can be fully utilized without affecting the purity of the metallic aluminum produced in the electrolytic reduction cells. BRIEF SUMMARY OF THE INVENTION An integrated process is provided for the recovery of valuable components from aluminum, carbon and fluoride-containing waste materials generated in electrolytic aluminum reduction systems. From these waste materials, which include spent potlining, spent alumina from dry scrubbers used for reduction cell offgas purification, channel and trench cleanings and floor sweepings, a feed is prepared for a pyrohydrolysis unit. Preparation may include comminution to less than about 6 mm particle size if the waste material is of greater size. If there are fine particles below about 1-2 mm in size, these are advantageously shaped prior to pyrohydrolysis. Also, if desired, sufficient carbon can be added to the feed to provide self-sustaining combustion in the pyrohydrolysis unit. Pyrohydrolysis of the feed is accomplished at about 1100.degree. to 1350.degree. C., while sufficient water is introduced into the pyrohydrolysis unit to obtain an offgas containing the fluoridic values from the feed. The offgas, after cooling, may be sequentially utilized for the production of NaF or an NaF-enriched alumina in controlled amount. Then either an AlF.sub.3 -enriched alumina or an HF solution is made. The solid clinker resulting from pyrohydrolysis is utilized for the production of high purity alumina and recovery of Na.sub.2 O values by treating it according to the Bayer process. If desired, sufficient basic sodium salts, such as Na.sub.2 CO.sub.3 and/or NaOH, is added to the feed or the hot clinker. In this instance, the clinker recovered from the pyrohydrolysis unit will contain a major amount of sodium aluminate. This sodium aluminate can either be used for the production of alumina or employed in dry scrubbers for the capture of impurities emanating from reduction cells. If desired, the sodium aluminate can be used for both of these purposes.