Abstract:
A process for removing PCDD&#39;s and PCDF&#39;s from cellulose-containing fibers such as waste paper is provided. The process comprises contacting the fibers with supercritical or near supercritical carbon dioxide for a period of time sufficient to extract a substantial portion of the PCDD&#39;s and PCDF&#39;s without substantially damaging the fibers. Extraction efficiencies of greater than 95% have been achieved with the technique.

Description:
This is a divisional application of application Ser. No. 07/596,807, filed on Oct. 12, 1990 now U.S. Pat. No. 5,009,745. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to the use of supercritical or near supercritical carbon dioxide to remove polychlorinated dibenzo-p-dioxins, (referred to hereinafter as &#34;PCDD&#39;s&#34; or simply &#34;dioxins&#34;), and polychlorinated dibenzofurans, (referred to hereinafter as &#34;PCDF&#39;s&#34;) from secondary fibers. 
     As known in the art, secondary fibers comprise materials, usually cellulose-based, which have been used at least once in their intended primary use area but are, nevertheless, amenable to further processing and subsequent reuse. Waste paper, newsprint, ledger stock, packaging materials, cartons, boxes, computer printouts, telephone directories, corrugated boards, and the like represent suitable raw stock for conversion to secondary fibers. The pattern of reuse (i.e., use of the secondary fiber) may not always be similar to the us to which the primary (virgin) fiber was put. 
     Efficient management of solid wastes, of which cellulose-based materials constitute a significant part (e.g., waste paper, 40%; yard waste, 18%), has become an important societal theme. In recent years, efforts to recycle waste paper have intensified with the ever increasing concerns as to the rate of use of raw materials and the possible adverse environmental impact of common industrial processes. Novel screening systems and sophisticated flotation techniques have emerged which in large measure have successfully addressed the problem of deinking printed stock. New bleaching sequences which avoid the use of chlorine or chlorine compounds and rely solely upon hydrogen peroxide, dithionites, or formamidine sulfinic acid for attaining acceptable levels of brightness are also making their appearance. 
     One aspect of waste paper reuse, however, has remained a continuing concern. This area is the presence of small quantities of toxic compounds, in particular, PCDD&#39;s and PCDF&#39;s, in waste papers. 
     Bleached kraft fibers under a variety of guises (e.g., coated paper, ledger paper, etc.) are often present in substantial quantities in waste paper stock as purchased from commercial dealers. Kraft pulps, when bleached with sequences including an elemental chlorine stage, can contain small but detectable levels of PCDD&#39;s and PCDF&#39;s. The processing steps currently used to treat waste paper (e.g., pulping/screening/flotation/bleaching) are not effective in removing such compounds from stock containing chlorine-bleached fibers. 
     PCDD&#39;s and PCDF&#39;s are large groups of chloro-organic compounds which have become ubiquitous in industrial societies. The structures of these compounds are as follows, where in each case x+y=1-8: ##STR1## 
     Of the various possible isomers of these compounds, the following are reportedly the most toxic: 
     2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) 
     1,2,3,7,8-pentachlorodibenzo-p-dioxin (PCDD) 
     2,3,7,8-tetrachlorodibenzofuran (TCDF) 
     1,2,3,7,8-pentachlorodibenzofuran (PCDF) 
     2,3,4,7,8-pentachlorodibenzofuran (PCDF). 
     Also reported to be highly toxic are: 
     1,2,3,6,7,8-hexachlorodibenzo-p-dioxin (HCDD) 
     1,2,3,7,8,9-hexachlorodibenzo-p-dioxin (HCDD) 
     1,2,3,4,7,8-hexachlorodibenzo-p-dioxin (HCDD) 
     1,2,3,6,7,8-hexachlorodibenzofuran (HCDF) 
     1,2,3,7,8,9-hexachlorodibenzofuran (HCDF) 
     1,2,3,4,7,8-hexachlorodibenzofuran (HCDF) 
     2,3,4,6,7,8-hexachlorodibenzofuran (HCDF) 
     See Rappe et al., &#34;Analysis of Polychlorinated Dibenzofurans and Dioxins in Ecological Samples,&#34; in Chlorinated Dioxins and Dibenzofurans in the Total Environment II, Keith, L. H., Rappe, C. and Choudhary, G., eds., Butterworh Publishers, Boston, Mass. 1985, pages 125-126. 
     In the recent past, the issue of health hazards associated with PCDD&#39;s and PCDF&#39;s has received much attention in the news media. PCDD&#39;s and PCDF&#39;s are known to cause a temporary form of a skin ailment known as &#34;chlor-acne.&#34; Also, PCDD&#39;s and PCDF&#39;s (particularly 2,3,7,8-TCDD) have proved to be extremely toxic to certain animals in laboratory studies, in particular to guinea pigs (LD 50  =0.6-4.0 micrograms/kilogram). See, for example, Ottoboni, A., The Dose Makes the Poison, Vincente Books, Berkeley, Calif., 1984, and Dioxins, NTIS Report No. PB82-136847, Industrial Environmental Research 
     Cincinnati, Ohio, November, 1980, Section 6. 
     Because of this reported high level of toxicity to a common laboratory test animal (i.e., the guinea pig), there is a general concern as to the long-term effects of PCDD&#39;s and PCDF&#39;s on human physiology. Accordingly, there is an important need to remove or substantially reduce the content of PCDD&#39;s and PCDF&#39;s from waste paper stock as part of the recycling process. It is an object of the present invention to respond to this need. 
     Supercritical fluids, including supercritical carbon dioxide, have been proposed for use in various industrial and pollution control processes. A review of this work can be found in Eckert et al., &#34;Supercritical fluid processing&#34;, Environ. Sci. Technol., 1986, vol. 20, pp. 319-325. Among other things, these authors describe general applications of supercritical fluid technology to materials processing and pollution control. In particular, they discuss a study in which supercritical ethylene was used to remove trichlorophenol from soil as a model for the removal of dioxins and polychlorinated biphenyls (PCBs). No data are presented for dioxins. More particularly, the reference does not disclose or suggest removing dioxins from recycled paper using supercritical carbon dioxide. 
     Pang et al., &#34;Supercritical Extraction of Aromatic Hydrocarbon Solids and Tar and Bitumens&#34;, Ind. Eng. Chem. Process. Des. Dev., 1985, vol. 24, pp. 1027-1032 discuss the use of various supercritical fluids, including carbon dioxide, to extract organic materials from tar sands. The reference mentions the possibility of using supercritical extraction to remove hazardous materials such as PCBs and dioxin from soils. Again, no data are presented for dioxins and no suggestion is made that supercritical carbon dioxide can be used to remove dioxins from secondary fibers. Also, the data presented for tar sands shows that carbon dioxide extraction produced the poorest yield. 
     Other studies involving the use of supercritical fluids including carbon dioxide to remove hazardous organic materials from environmental solids such as soil can be found in Groves et al. &#34;State-of-the-art on the supercritical extraction of organics from hazardous wastes&#34;, CRC Critical Reviews in Environmental Control, 1985, vol. 15, pp. 237-274: Hawthorne et al., &#34;Extraction and Recovery of Polycyclic Aromatic Hydrocarbons from Environmental Solids Using Supercritical Fluids&#34;, Anal. Chem., 1987, vol. 59, pp. 1705-1708: Dooley et al., &#34;Supercritical Fluid Extraction and Catalytic Oxidation of Toxic Organics from Soils&#34;, EPA Report No. 600/9-87/018F, pp. 383-397: and Brady et al. &#34;Supercritical Extraction of Toxic Organics from Soils&#34;, Ind. Eng. Chem. Res., 1987, vol. 26, pp. 261-268. None of these studies discloses or suggests the use of supercritical carbon dioxide to remove PCDD&#39;s and PCDF&#39;s from secondary fibers. 
     Along these same lines, Modell U.S. Pat. Nos. 4,338,199 and 4,543,190 describe a process in which organic materials are oxidized in supercritical water. The &#39;199 patent includes a general statement that its process can be used to remove toxic chemicals from the wastes generated by a variety of industries including forest product wastes and paper and pulp mill wastes. No specific mention is made of dioxins. The &#39;190 patent describes the treatment of various chlorinated organics other than dioxins with supercritical water and states that conversion of these materials to chlorinated dibenzo-p-dioxins was not observed (see Example 6). The use of supercritical water to treat organic waste materials is also disclosed in PCT Patent Publication No. WO 81/00854, Modell et al. U.S. Pat. No. 4,113,446, Burleson U.S. Pat. No. 4,564,458, and Titmas U.S. Pat. No. 4,594,164. 
     A summary of experiments performed by Modar, Inc., using the Modell supercritical water process was published in Chemosphere--Chlorinated Dioxins and Related Compounds 1987, McNelis et al., editors, Pergamon Press, New York, 1989, Vol. 18, Nos. 1-6, page 50. As described therein, bench-scale tests were performed on soils and liquid wastes contaminated with chlorobenzenes and PCDD&#39;s. Supercritical water oxidation was found to remove 2,3,7,8-TCDD and chlorobenzenes from soil and to remove 2,3,7,8-TCDD, TCDD&#39;s and OCDD from liquid wastes. 
     Various uses of supercritical fluids in the processing of materials have been disclosed in the literature. For example, supercritical carbon dioxide has been used to remove tall oil and turpentine from coniferous woods in Fremont U.S. Pat. No. 4,308,200, to extract lignin from the black liquor produced by the Kraft process for pulp production in Avedesian, U.S. Pat. No. 4,493,797, to treat refinery sludges in European Patent Publication No. 314,223, to regenerate absorbents used in waste water treatment systems in Modell U.S. Pat. Nos. 4,061,566 and 4,147,624, to sterilize pharmaceuticals in Pilz et al. U.S. Pat. No. 4,263,253, to remove off-flavor materials from textured vegetable products in Sevenants, U.S. Pat. No. 4,675,198, to remove gamma-linolenic acid from fruit seeds in Traitler et al. U.S. Pat. No. 4,703,060, and to decaffeinate coffee in Katz U.S. Pat. No. 4,472,442; Toro et al. U.S. Pat. No. 4,728,525 and Kaleda et al. U.S. Pat. No. 4,767,634. See also, Friedrich U.S. Pat. No. 4,466,923; Lawson et al. U.S. Pat. No. 4,495,095: Myerson U.S. Pat. No. 4,550,198; Panzner et al. U.S. Pat. No. 4,554,170; Japikse et al. U.S. Pat. No. 4,647,466; Ritter and Campbell, &#34;The Effects of Supercritical Carbon Dioxide Extraction on Pine Wood Structure&#34;, Biotechnology and Bioengineering Symp., 1986, no. 17, pp. 179-182: Hatakeda et al., &#34;Extraction of Sugi (Cryptomeria japonica D. Don) with supercritical carbon dioxide&#34;, Nipon Kagaku Kaishi, 1987, no. 5, pp. 931-933; Shishikura et al., &#34;Concentration of Tocopherols from Soybean Sludge by Supercritical Fluid Extraction&#34;,  J. Jpn. Oil Chem. Soc., 1988, vol. 37, pp. 8-12; and Li and Kiran &#34;Interaction of Supercritical Fluids with Lignocellulosic Materials&#34;, Ind. Eng. Chem. Res., 1988, vol. 27, pp. 1301-1312. 
     Supercritical water or near supercritical water has been used to treat wood chips and black liquor from pulping in Modell, PCT Patent Publication No. WO 81/00855. See also Modell, M., &#34;Gasification and Liquefaction of Forest Products in Supercritical Water&#34;, Fundam. Thermochem. Biomass Convers., 1985, pp. 95-119; and West et al., &#34;Pyrolysis of 1,3-butanediol as a model reaction for wood liquefaction in supercritical water&#34;, Can. J. Chem. Eng., 1987, vol. 65, pp. 645-650. 
     In addition to their us in waste treatment and materials processing, supercritical fluids have been used in connection with various analytic procedures. For example, Suprex Publication No. TN-022, Suprex Corporation, Pittsburgh, Pa., 1989, mentions the use of supercritical carbon dioxide as part of an analytical procedure for assaying dioxins, but no mention is made of using supercritical carbon dioxide for the reduction of dioxins in cellulosic substrates. Similarly, Hawthorne et al., &#34;Directly coupled supercritical fluid extraction-gas chromatographic analysis of polycyclic aromatic hydrocarbons and polychlorinated biphenyls from environmental solids&#34;, J. Chromatogr., 1987, vol. 403, pp. 63-76, discuss the use of supercritical fluid extraction coupled to a gas chromatograph to analyze environmental solids, e.g., urban dust, for organic pollutants, specifically, polycyclic aromatic hydrocarbons. The extraction was performed using nitrous oxide as the supercritical fluid. Along similar lines, Schneiderman et al., &#34;Determination of anthraquinone in paper and wood using supercritical fluid extraction and high-performance liquid chromatography with electrochemical detection&#34;, J. Chromatogr., 1987, vol. 409, pp. 343-353, describe the combination of supercritical fluid extraction using carbon dioxide, high-performance liquid chromatography, and electrochemical detection to analyze Kraft paper and pine plywood sawdust for anthraquinone. 
     Significantly, none of these references in any way discloses or suggests that the problem of removing PCDD&#39;s and PCDF&#39;s from secondary fibers can be solved by treating the fibers with supercritical carbon dioxide. 
     Some references exist in the literature regarding attempts to decompose o destroy dioxins in a state of solution in liquid media (e.g., hexane) or in substances such as silica gel or clay via photolytic techniques, e.g., UV radiation. See Ottoboni, supra; Crosby, D. G., et al., Science, Vol. 173, Aug. 20, 1971, pages 173-174 Plimmer, J. R., Bull. Environm. Contam. Toxicol., Vol. 20, 1978, pages 87-92; Botre, Claudio, Adriana Memoil, and Franco Alhaique, Environmental Science and Technol., Vol. 12, No. 3, March 1978, pages 335-336; Crosby, D. G., et. al., Environmental Health Perspectives, September 1973, pages 259-266; Dulin, David, Howard Drossman, and Theodore Mill, Environ. Sci. Technol., Vol. 20, No. 1, 1986, pages 72-77: and Podoll, R. Thomas, Helen M. Jaber, and Theodore Mill, Environ. Sci. Technol., Vol. 20, No. 5, 1986, pages 490-492. The process has been shown to work to an extent but appears to be highly dependent upon the presence of a hydrogen donor solvent, the type and level of impurities present, and the substrate. Furthermore, the photoproduct resulting from irradiation of 2,3,7,8-TCDD has been reported to be trichloro- and dichloro-benzo-p-dioxins, which are less toxic than 2,3,7,8-TCDD but, nevertheless, are undesirable. 
     As with the art relating to supercritical fluid technology, these references do not address the secondary fiber problem and, in particular, do not suggest that this problem can be solved by extracting PCDD&#39;s and PCDF&#39;s from such fibers using supercritical carbon dioxide. 
     In considering the problem of removing PCDD&#39;s and PCDF&#39;s from secondary fibers, it is important to note various characteristics of secondary fibers and of PCDD&#39;s and PCDF&#39;s which make the removal process particularly difficult. For example, secondary fibers have a relatively high surface area per gram. As known in the art, high surface area materials are capable of strongly binding organic compounds, such as PCDD&#39;s and PCDF&#39;s. See Srinivasan et al., &#34;Binding of OCDD, 2,3,7,8-TCDD and HCB to Clay-Based Sorbents,&#34; in Chlorinated Dioxins and Dibenzofurans in Perspective, Rappe, C., Choudhary, G., and Keith, L. H., eds., Lewis Publishers, Inc., Chelsea, Mich., 1986, page 532. 
     Moreover, in an air or water system, PCDD&#39;s and PCDF&#39;s will adhere to solid sorbents rather than remaining free in solution. Thus, partition coefficients in the range of 2.8-67.1×10 3  have been reported for 2,3,7,8-TCDD for a variety of sorbents including hydroxy aluminum-clay and activated carbon. This compound also adheres quite well to glass in a water environment. See Srinivasan et al., supra at pages 531-537. 
     The combined effects of large surface areas and large partition coefficients makes the effective removal of PCDD&#39;s and PCDF&#39;s from secondary fibers difficult to achieve. It is to this challenge that the present invention is directed. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing state of the art, it is an object of the present invention to provide a method for removing PCDD&#39;s and PCDF&#39;s from secondary fibers. More particularly, it is an object of the invention to provide a method for reducing the level of PCDD&#39;s and PCDF&#39;s in secondary fibers by more than about 90% and preferably by more than about 95%. It is a specific object of the invention to remove one or more of the following PCDD and PCDF isomers from secondary fibers: 
     2,3,7,8-tetrachlorodibenzo-p-dioxin 
     1,2,3,7,8-pentachlorodibenzo-p-dioxin 
     2,3,7,8-tetrachlorodibenzofuran 
     1,2,3,7,8-pentachlorodibenzofuran 
     2,3,4,7,8-pentachlorodibenzofuran 
     1,2,3,6,7,8-hexachlorodibenzo-p-dioxin 
     1,2,3,7,8,9-hexachlorodibenzo-p-dioxin 
     1,2,3,4,7,8-hexachlorodibenzo-p-dioxin 
     1,2,3,6,7,8-hexachlorodibenzofuran 
     1,2,3,7,8,9-hexachlorodibenzofuran 
     1,2,3,4,7,8-hexachlorodibenzofuran 
     2,3,4,6,7,8-hexachlorodibenzofuran. 
     It is a further object of the invention to provide a method for removing PCDD&#39;s and PCDF&#39;s from secondary fibers which is cost-effective, simple to perform, and which will not deleteriously affect the desirable and marketable attributes of the fibers after treatment. 
     To achieve the foregoing and other objects, the invention provides a method for removing PCDD&#39;s and PCDF&#39;s from secondary fibers which comprises contacting the secondary fibers with supercritical or near supercritical carbon dioxide for a period of time at a temperature, pressure, and carbon dioxide flow rate such that 1) a substantial reduction in the level of PCDD&#39;s and PCDF&#39;s associated with the fibers is achieved, and 2) the properties of the fibers, e.g., their physical and chemical properties, are not substantially degraded. 
     In general terms, high levels of PCDD and PCDF extraction in combination with low levels of fiber degradation are achieved for pressures above about 60 atmospheres, preferably from about 73 atmospheres to about 300 atmospheres, and most preferably from about 80 atmospheres to about 200 atmospheres, temperatures above about 25° C., preferably from about 31° C. to about 150° C., and most preferably from about 31° C. to about 90° C., carbon dioxide flow rates in the range from about 0.01 standard liters/minute/gram of dry secondary fiber (slpm/gm) to about 10 slpm/gm, preferably from about 0.1 slpm/gm to about 1.0  slpm/gm, and processing periods of from about 1 minute to about 3 hours, preferably from about 3 minutes to about 1 hour, for the foregoing flow rates. In terms of solvent-to-feed ratios, the preferred range is from about 10 to about 200 grams of carbon dioxide per gram of dry secondary fiber. These ranges, of course, may vary depending upon the particular equipment used to perform detoxification and the particular characteristics of the secondary fibers being treated. The specific parameters for any particular application can be readily determined by person skilled in the art from the disclosure herein. 
     As demonstrated by the experimental data presented below, using processing conditions within the foregoing ranges, greater than 95% extraction was achieved for 2,3,7,8-TCDD, reportedly the most toxic of all of the PCDD&#39;s and PCDF&#39;s. In view of the strong adherence of this and related PCDD&#39;s and PCDF&#39;s to secondary fibers, the 95% extraction level achieved with supercritical carbon dioxide is considered to be unexpectedly high. 
     The accompanying drawings, which are incorporated in and constitute part of the specification, illustrate a preferred embodiment of the invention, and together with the description, serve to explain the principles of the invention. It is to be understood, of course, that both the drawings and the description are explanatory only and are not restrictive of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a pressure-temperature diagram illustrating carbon dioxide&#39;s supercritical fluid region. 
     FIG. 2 illustrates representative equipment which can be used in the practice of the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As discussed above, the present invention relates to the extraction of PCDD&#39;s and PCDF&#39;s from secondary fibers. 
     The invention can be applied to secondary fibers from a variety of sources including waste paper, newsprint, ledger stock, packaging materials, cartons, boxes, computer printouts, telephone directories, corrugated boards, and the like. In general, the major constituent of the fibers will be cellulose. The fibers can include other organic materials. Also, clays and other filler materials used in the paper manufacturing process can be present. Typically, the material being treated will include at least some fiber which was originally processed using a chlorine bleaching step and thus includes at least some PCDD&#39;s and PCDF&#39;s. 
     The extraction process of the invention is preferably applied to the secondary fibers prior to other steps in the reclamation (recycling) procedure, although, if desired, the process can be applied after such steps as de-inking and whitening of the secondary fibers have been performed. As used herein, the term &#34;secondary fibers&#34; is intended to include fibers at any stage of the reclamation process, i.e., from the raw waste material stage to the ready-for-the-second-use stage. 
     The extraction is performed using supercritical or near supercritical carbon dioxide. The three commonly encountered states or phases of matter are solid, liquid, and gas. Depending upon the ambient conditions (e.g., pressure/temperature), the three phases can exist singly or in equilibrium with other phases. The phase diagram of FIG. 1 illustrates the relationship between these phases for a pure substance, i.e., for pure carbon dioxide. The curves AB, AC, and AD represent the boundaries separating single phases. These curves are known as &#34;coexistence curves&#34; since the phases exist in equilibrium along these curves. Specifically, curve AB is the solid/gas equilibrium curve, curve AC is the liquid/gas equilibrium curve, and curve AD is the solid/liquid equilibrium curve. For simplicity, the curves are often termed the sublimation, vapor pressure, and melting curves. 
     Point A of FIG. 1 is known as the triple point where all three phases coexist in equilibrium. Point C, the end point of the vapor pressure curve, is known as the gas-liquid critical point where the distinction between gas and liquid disappears. The temperature and the pressure at the critical point are known as the critical temperature (T c ) and the critical pressure (P c ). No gas can be liquefied above its T c , no matter how great the pressure. The shaded region in FIG. 1 is known as the supercritical fluid state, and a substance existing in that region (i.e., a substance which has been heated above its T c  and compressed beyond its P c ) is termed a supercritical fluid (SCF). 
     In the supercritical state, substances behave partly as a gas and partly as a liquid. Their densities are liquid-like but in respect of diffusivity or penetration capability they resemble gases. The density of a supercritical fluid may be continuously changed without phase separation by changes in pressure and/or temperature. Density dependent properties such as solvent power also undergo corresponding changes. For example, when water is in the supercritical state (T c  =374° C. and P c  =218 atm), it tends to lose its hydrogen-bonding characteristics, its dielectric constant tends to suffer a dramatic decline, and it tends to become miscible with certain hydrocarbons such as benzene. 
     As discussed above, the present invention uses supercritical or near supercritical carbon dioxide to remove PCDD&#39;s and PCDF&#39;s from secondary fibers. The critical temperature and pressure for pure carbon dioxide are 31° C. and 72.9 atmospheres, respectively. Of course, as used in the present invention, carbon dioxide is not operating in a simple, pure state, but as part of a multi-component system. It is possible, therefore, that depending upon the particular system, the advantages of the invention can be achieved even though the pressures and/or the temperatures used are not above the critical values for pure carbon dioxide. It is in this sense that the phrase &#34;near supercritical&#34; is used herein. 
     FIG. 2 shows representative apparatus for practicing the invention in a batch mode. It should be noted that apparatus other than that shown in FIG. 2 can be used to practice the invention. In particular, the literature references and patent publications discussed above disclose various equipment configurations which can be used in extraction processes employing supercritical fluids. The use of such equipment with the present invention will be evident to persons of ordinary skill in the art from the disclosure herein. It should also be noted that the invention can be practiced in a flow-through mode, rather than a batch mode. 
     In the system of FIG. 2, carbon dioxide from supply 10 is fed through conduit 12 to compressor 14 which raises the pressure of the carbon dioxide above its critical pressure. The compressed carbon dioxide is then passed through conduit 16 where it is heated above its critical temperature by heating tape 18. 
     The supercritical carbon dioxide then enters extraction vessel 13 which has been previously loaded with the secondary fibers which are to be extracted. Vessel 13 will normally include heating means (not shown) to maintain the temperature of the carbon dioxide above its critical temperature. Vessel 13 will also be constructed to operate at pressures above the critical pressure of carbon dioxide. 
     As the supercritical carbon dioxide passes through vessel 13 it extracts PCDD&#39;s and PCDF&#39;s from the secondary fibers. The supercritical carbon dioxide with its load of chloro-organics leaves vessel 13 by means of conduit 20 and passes through pressure reduction valve 22 which reduces the pressure of the carbon dioxide. As a result, the PCDD&#39;s and PCDF&#39;s precipitate out of the carbon dioxide and are collected in collector 24. If desired, the PCDD&#39;s and PCDF&#39;s in the carbon dioxide stream can be captured on a solid support by passing the stream through, for example, a bed of activated carbon, clay, or the like. The PCDD&#39;s and PCDF&#39;s can subsequently be removed from the bed by, for example, heating the bed to a temperature sufficient to fully decompose the PCDD&#39;s and PCDF&#39;s. 
     After the PCDD&#39;s and PCDF&#39;s have been removed, the carbon dioxide stream leaves collector 24 through conduit 25 and passes through flow meter 26 and dry test meter 28. Dry test meter 28 is used to measure accumulative gas flow. Flow meter 26 is used to measure the flow rate of carbon dioxide through the system. As discussed above, the preferred flow rate is in the range of from about 0.01 to about 10 slpm/gm of dry secondary fiber. Based on the readings obtained from flow meter 26, pressure reduction valve 22 (or alternatively a separate flow controlling valve) is adjusted to achieved the desired flow rate. 
     As shown in FIG. 2, after passing through dry test meter 28, the carbon dioxide is vented to the 
     Alternatively, the carbon dioxide can be collected and recycled through compressor 14. 
     Without intending to limit it in any manner, the present invention will be further illustrated by the following example. 
     EXAMPLE 
     In overview, the experimental method used to demonstrate the ability of supercritical carbon dioxide to remove PCDD&#39;s and PCDF&#39;s from secondary fibers involved &#34;spiking&#34; a shredded waste paper sample with radioactively labelled 2,3,7,8-TCDD followed by extraction of the sample with supercritical carbon dioxide. The levels of TCDD before and after extraction constituted the measure of removal efficiency. 
     The waste paper used was a blend of white and colored ledger grades and coated sulfate. Both softwood and hardwood fibers were represented. The paper sample was shredded and then put through a Wiley mill for further comminution. 
     One hundred grams of the comminuted sample were suspended in 300 ml of hexane and the loosened particles, largely consisting of extraneous non-cellulosic matter (e.g., dirt particles), were skimmed off. The paper sample was then dried in a rotary evaporator. The sample was then added to 300  ml of hexane which contained 30 nanograms (ng) of radio-labelled TCDD 12 ( 13  C 12  2,3,7,8-TCDD). The mixture was stirred to ensure homogeneity. The &#34;spiked&#34; sample was then dried in a rotary evaporator. The entire procedure was repeated with a second 100 gm paper sample and the two samples were mixed. 
     The extraction was carried out using the apparatus shown in FIG. 2. Extraction vessel 13 had a capacity of 3.0 liters, was made of stainless steel and was electrically heated. The system was fitted with suitable instrumentation for measurement of temperature and pressure throughout the process. 
     A sample, approximately 77 gm in weight, of the spiked and dried secondary fibers was placed in the extraction vessel, sandwiched between layers of dioxin-free glass wool. Carbon dioxide was passed through the system at the rate of approximately 14.5 standard liters per minute (slpm) so as to give a flow rate per gram of approximately 0.2 slpm/gm. The pressure was maintained between 4800 and 5200 psia and the temperature between 67° C. and 75° C. As indicated above, T c  and P c  for CO 2  are 31° C. and 1072 psia, respectively. The extraction was carried out for a period of five hours. The overall solvent-to-feed ratio was 105 grams CO 2  per gram of fiber. 
     Samples of the extracted material and the spiked material prior to extraction (the &#34;control&#34;) were analyzed for  13  C 12  2,3,7,8-TCDD. The analysis was carried out using gas chromatography and high-resolution mass spectrometry. Five replicate analyses were performed. 
     The results of this experiment are shown in Table 1. As shown therein, supercritical carbon dioxide was very effective in removing TCDD from the &#34;spiked&#34; sample. In particular, the method of the invention achieved an extraction efficiency of approximately 97%. As discussed above, this high level of extraction is considered surprising in view of TCDD&#39;s high partition coefficient for solid organic materials and the high surface area of secondary fibers. 
     
                       TABLE 1______________________________________Calculation of Extraction Efficiencies of SCCO.sub.2in Removing 2,3,7,8-TCDD from Secondary Fibers             .sup.13 C.sub.12 (labeled)             2,3,7,8-TCDD (ppb)______________________________________before      a           .24extraction  b           .24       c           .23       d           .23       e           .22mean =              .232standard deviation =               .008after       a           .01extraction  b           .01       c           .01       d           ND       e           .01mean =              .008standard deviation =               .004______________________________________ ##STR2##