Cellulose-based materials such as wastepaper make up ~40% of solid waste streams. In recent years, an impressive array of new technologies has been developed to address the problems associated with recycling these materials. New processes for de-inking printed stock use novel screening systems and sophisticated flotation techniques. Bleaching sequences without the use of chlorine or chlorine compounds are also under investigation in various research laboratories around the world. However, these technologies do not effectively address the problem of stickies, dioxins, and microbes in the wastepaper feedstock.
Another issue associated with wastepaper reuse is the probable presence of small quantities of dioxins and other chlorinated organic compounds. Kraft pulps may contain small but detectable levels of dioxins and related compounds, especially if their bleaching process used elemental chlorine. Bleached Kraft fibers under many guises (e.g., coated paper and ledger paper) are often present in substantial quantities in wastepaper purchased from commercial dealers. Clapp and co-authors (1) reported that many recycled fiber feedstocks contain up to 1200 ppm AOX (a broad group of halogenated organics, of which dioxin and dibenzofurans are a subset).
Additionally, cleanliness and sanitation issues arise when using postconsumer recycled paper products. It is desirable to inactivate any potentially pathogenic organisms in the feedstock in the event that bleaching steps such as ozonation (which is presumed to sanitize) are not used.
Dioxin extraction. We selected recycled paper representing both soft and hard wood fibers; it included white and colored ledger grades and coated sulfate (magazine) papers. We ground the papers using a Wiley Mill to particle sizes of <0.5mm and then spiked them with 232 ppt 13C-dioxin.
Supercritical CO2 solvent extraction conditions were 71 °C and 34.5 MPa. We measured extraction efficiencies using a 3-L batch extractor (depicted in Figure 1) and a supercritical CO2 solvent-to-feed weight ratio of 105. With high-resolution GC/MS, we analyzed the dioxin concentration in samples from the feed and the extracted product (five samples from each). Dioxin extraction efficiencies were up to 95%.
We performed similar experiments using supercritical propane at 125°C, 34.5 MPa, and a solvent-to-feed weight ratio of 30. In these experiments, however, we spiked the feed sample with 232 ppt 13C-dioxin and analyzed feed and extracted product samples for both native (unlabeled) and spiked (labeled) dioxin.
Supercritical propane removed 95% of the spiked dioxin and only 85% of the native dioxin. This is not surprising. It suggests that the native dioxin is more sterically trapped inside the fiber interstices or that the binding energy of the older, native dioxin is stronger than that of the spiked dioxin. This finding is in agreement with that of Hawthorne, as reviewed in Majors (2), who found that spiked naphthalene in sludge was extracted faster than naphthalene in aged sludge contaminated with polyaromatic hydrocarbons.
Stickies extraction. We ground paper fibers containing stickies -- ice cream cartons, paper pads with adhesive bindings, and book bindings -- to particle sizes of <0.5 mm in a Wiley Mill. In a procedure similar to that used for the experiments described earlier, we extracted the paper with CO2 at 60 °C, 34.5 MPa, and a solvent-to-feed ratio of 30. We determined the percentage of extractables in the feed and in the extracted product using standard Soxhlet extractions. The Soxhlet extraction solvents used were hexane, methylene chloride, acetone, and ethanol-benzene. Extraction efficiencies for stickies removal ranged from 55 to 79%, depending on the solvent used (Table 1).
We performed experiments similar to supercritical CO2 extraction with supercritical propane at 125 °C and 34.5 MPa and a solvent-to-feed ratio of 30. Stickies extraction efficiencies were 69-98%, depending on the Soxhlet extraction solvent used. Another experiment using supercritical propane at 125 °C and only 8.26 MPa gave stickies extraction efficiencies of 42-93% (Table 2).
IR spectra of Soxhlet extraction residues indicated that hexane did not remove stickies, only the natural wood extractives such as resins and fatty acids. All other solvents (and solvent blends) showed that styrene-butadiene, poly(vinyl acetate), and other representative stickies remained in their extracts.
Microbe inactivation. We performed similar experiments extracting recycled fibers with CO2 at 60 °C, 34.5 MPa (5000 psi), and a solvent-to-feed ratio of 30 (20-min exposure time). Analysis of feed samples and extracted product samples confirmed inactivation of all endogenous yeast and mold spores. This is not surprising, because CO2 is a waste product of these microbes (an organism's waste products are often toxic to that organism).
The plant design was for a 100 bone-dry ton per day recycled pulp and paper facility. We assumed that extractions were limited by the solubility of the contaminants and ignored matrix effects because of the limited amount of experimental data from the extraction experiments. We assumed a solvent loss rate of 1% per circulation, which is probably higher than an actual plant would run. Note that operating temperatures and pressures were not yet optimized for this analysis. We assumed that the use of an ethanol cosolvent at concentrations of 5% in CO2 would give an entrainer effect (in the form of a solubility multiplier) of 3. The validity of this assumption is debatable; however, it is not unusual to experience entrainer effects much higher than 3 in such systems, especially if the solutes have polar groups that would interact with the polar hydroxyl group on ethanol. We assumed that the cost of drying the fibers before using CO2 and ethanol as the solvent (to prevent azeotrope formation of ethanol-water) was relatively small.
The solute's solubility in propane was estimated to be 50 times greater than in CO2. This estimate takes into account two factors: The higher temperatures used in the propane extractions give a solute vapor pressure ~20 times higher than that under the CO2 conditions, and it was assumed that propane had an enhancement factor roughly two to three times higher than that for CO2 at the same reduced temperature.
We designed a semibatch process that used a continuous flow of supercritical fluid (SCF). In our design, the solids were loaded in parallel into several steel extraction vessels with high tensile strength, processed, and then unloaded. We reduced the pressure of the SCF stream in a separate blowdown vessel set at 5.2 MPa and recycled the SCF. A purge compressor removed residual SCF from the extraction vessels before they were opened, returning the residual SCF to the main compressor at 5.2 MPa. A granular activated carbon polishing step at the lower pressure provides the option to reduce contaminant levels in the SCF recycle stream.
A supercritical CO2 semibatch extraction facility, without the use of an entrainer, shows an initial capital investment of $74 million with an ongoing operating cost of $0.33/lb of bone-dry fibers (Figure 2). Adding a 5% ethanol entrainer allows us to reduce the solvent-to-feed ratio, decreasing overall process costs significantly and resulting in an initial capital investment of $40 million and an operating cost of $0.17/lb of bone-dry fibers.
When supercritical propane is used as a solvent, the batch process can be run at much lower pressures than when supercritical CO2 is used. Because propane is flammable, we designed the plant to flush the extraction vessels with nitrogen during unloading to prevent the formation of any explosive mixture and then flare the propane. We designed the plant so that the vessel was opened only when the mixture was beyond any possibility of explosion. When closing the vessel, the same process would be done in reverse to remove oxygen. This method had the lowest projected cost of the three methods discussed here; the initial capital investment would be $13 million, with operating costs of $0.067/lb of bone-dry fibers (Figure 3).
We stress that these promising results represent only the initial stages of a research program. Kimberly-Clark Corp. holds four U.S. patents related to this work (3-6). With technique refinements and imaginative modification of the basic supercritical solvents, extraction efficiencies and process economics will probably improve. Keeping this in mind, we recommend the following steps for further development work:
(1) Clapp, R. T.; Truemper, C. A.; Aziz, S.; Reschke, T. Tappi J. 1996, 79(3), 111.
(2) Majors, R. E. LC-GC 1995, 13(7), 542.
(3) Blaney, C. A.; Hossain, S. U. U.S. Patent 5 074 958, 1991.
(4) Blaney, C. A.; Hossain, S. U. U.S. Patent 5 009 746, 1991.
(5) Hossain, S. U.; Blaney, C. A. U.S. Patent 5 009 745, 1991.
(6) Hossain, S. U.; Blaney, C. A. U.S. Patent 5 213 660, 1993.
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