Effect of the deinking process on physical properties of various wood-free recovered paper furnishes

Transcription

1 T154 Effect of the deinking process on physical properties of various wood-free recovered paper furnishes By P. Huber, B. Carré, M. Petit-Conil, B. Fabry and S. Bhattacharjee Abstract: The aim of this study is to follow the evolution of DIP mechanical and along the deinking line, using wood-free recovered paper. An industrial two-loop deinking line is partially simulated on the CTP pilot plant. Thickening is found to be detrimental to both tensile and burst. Wet-stretch increase occurs mainly in the dispersion stage and is linked to fibre curl development. PY bleaching offers superior mechanical properties to YP. P. HUBER, M. PETIT-CONIL, ECOVERED PAPER needs thorough R deinking to be re-used for high-end applications, such as office or fine paper. Throughout the deinking line, fibres undergo various chemical and mechanical treatment that might affect their physical properties. The variations of fibre characteristics then influence optical and mechanical properties of deinked pulp (DIP). Moreover, variability in the recovered raw material is likely to modify DIP properties. In a previous study (Petit-Conil & Carré (2003), CTP research forum), the effect of raw material and deinking process on DIP optical and mechanical properties was assessed. Wood containing recovered papers was used, with a focus on the ratio of old newspaper/old magazine (ONP/OMG). A state-of-the-art two-loop deinking process was simulated on the CTP pilot plant (the second deinking loop being simulated at lab scale). It was found that the ONP/OMG ratio B. CARRÉ, B. FABRY, S. BHATTACHARJEE, M-Real New Thames Ltd., Kent, UK explained most of the DIP characteristics variations: mechanical pulp fibres helped maintain rigidity along the process and remaining filler content greatly affected DIP properties. In addition, the deinking process did not significantly affect intrinsic fibre properties. In the case of ONP/OMG deinking, Mahagaonkar et al. [1,2] show that physical properties of DIP were improved after flotation because of filler removal. Beneficial effect of chemical fibres from OMG fraction was also reported. McKinney and Roberts [3] studied the effect of kneading and dispersion on several DIP properties when deinking office grades (containing laser and xerographic papers). Fibre curl is induced by mechanical treatment performed at high-consistency. Therefore, high-consistency operations such as pulping, kneading, dispersion, thickening and bleaching are likely to increase curl. It has been reported that screw press thickening to 30 % solid content significantly increases fibre curl. As well, kneading and dispersion can have some effect on the curl (with kneading having the strongest effect). It is crucial to monitor fibre curl, as it has been shown to increase sheet properties such as bulk, wet web stretch and tear while lowering burst. Some mills have reported runnability problems when using wood-free DIP: more web wet draw is required, as wet stretch increases. It seems that the inner and outer chemical structure of fibre is not affected by the deinking process. Bouchard and Douek [4] show that cellulose crystallinity or molecular weight does not change after alkaline repulping followed by flotation. In addition, the hemicellulose fraction remains constant. Under those conditions, fibre curl is developed by mechanical actions on fibres only. It should be emphasized that the effect of bleaching was not studied. Pulp physical properties change as filler and contaminants are removed, but some variations are due to the mechanical action on fibres. The effect of recycling on pulp properties has been thoroughly investigated [5,6]. Hornification (resulting from loss of fibre swelling) explains most of the reported changes to beaten chemical pulps. Note that in those studies, recycling usually means multiple disintegration cycles after drying. However, little research has been done on the effect of unit operations in the deinking line on :7/8 (2006) PULP & PAPER CANADA

2 T155 DIP physical properties. The objective of this study is to analyse the variations of DIP properties when using wood-free recovered papers. The effect of the deinking process is assessed using a two-loop deinking line. In order to simulate variations in industrial recovered paper, four different raw materials are used, containing various proportions of recovered toner-printed paper, magazine, inkjet printed-paper and coated wood-free paper. The effect of bleaching strategies on DIP properties is studied through either peroxide-dithionite (P-Y) or dithionite-peroxide (Y-P) bleaching sequence. EXPERIMENTAL Deinking Process: The trials were conducted on the CTP recycling pilot plant with a two-loop deinking technology. The second loop was simulated at lab scale. The raw material is repulped in a helico pulper (Lamort, concentration of 17%, temperature of 45 C, 20 min, using 0.15 % of a non-ionic surfactant (Kemira)). The pulp is then coarse-screened, finescreened using 0.2 mm slots, floated (Lamort cell, 1% conc., T=45 C, 300% air ratio), washed on a Classiflux (continuous vibrating wire see [7,8]) and thickened in a screw press to about 35%. The peroxide liquor is introduced in a mixer with steam (1% caustic soda, 2.5% silicate, 1% peroxide and 0.15% DTPA, temperature =70 C and reaction time of 20 min in the retention screw). The pulp is then sent to dispersion (specific energy =90 kwh/t, T=70 C). The second deinking loop is performed at lab scale. The pulp is post-floated (Voith cell, 1% conc., 7 min, surfactant not required, T=45 C, 200% air ratio) and post-washed (Degussa diffusion washing cell, 1.2% pulp concentration, ratio washing water volume/pulp=4, water temperature= 15 C). Dithionite bleaching is done in a sealed glass reactor (1% HighBright (active product), 3% concentration, adjusted ph=7, 70 C, 1h). To sum up, the following deinking line is tested: [pulp.- flot.-wash.- screw press thick.- P bleachdisp.] -[flot-wash.-y bleach], Fig 1. Note that the washing filtrate was not recycled at the flotation cell that is representative of industrial practice in tissue and fine paper DIP lines, where waters are thoroughly clarified. The effect of several bleaching strategies is studied in a separate trial. After thickening, the pulp is submitted to either disp.-y-flot.-p, disp.-p-flot.-y or dispersion with pre-introduction of peroxide (disp.(p)-flot-y). The dispersion stage is performed on the pilot (same conditions as above), and the following stages (bleaching and post-flotation) are done at lab scale (same conditions as above, except Y retention time=45 min and P retention time=90 min). In the case of peroxide pre-introduction, the retention TABLE I. Composition of the studied furnishes. time after dispersion is done in the laboratory. The toner furnish only is tested (see below, Raw material). Pulp samples are collected along the deinking line (after screen, flotation, washing, thickening, dispersing, bleaching, post-flotation, post-washing and postbleaching). Pulp pads are tested for brightness (without UV), ERIC (whole and hyperwashed pulp) and ash content (475 C). Specks are tested on whole pulp handsheets (38 g/m 2 ), by image analysis (Simpatic sensor). Standard handsheets (75 g/m 2 ) are made to test bulk, burst, tear, rigidity (Kodak and Taber), tensile (dry and wet), zero-span (dry and wet), opacity, light absorption and scattering coefficients. High-consistency pulp samples (after thickening and dispersing) were disintegrated in hot water (70 C) for 5 min, before making pads or handsheets. It is known that fines loss during handsheet making reduces all sheet strength properties, except tear. However, according to Howard and Bichard [6], the trends are the same regardless of retained fines fraction. Anyway, few fine elements are left after post-washing. Fibre morphological properties are measured on a MORFI analyser. Flotation froths are also tested for ash content. Raw Material: Rolls of toner-printed paper (wood-free) were obtained from an OCE printer. The magazine furnish is a mixture of typical French magazines: 20% Télé 7 jours (SC roto), 20% VSD (LWC roto), 30% Le Nouvel Observateur (1/3SC+2/3LWC heatset offset) and 30% L Express (SC heatset offset). To simulate inkjet-recovered paper, a reference page is printed on a Xerox Exclusive paper (80 g/m 2 ) using an HP 2250 printer with ink cartridge HP No. 10/C4844A (black pigment-based). Coated wood-free paper corresponding to 2.08 recovered paper grades is also used. Four different furnishes are studied using a mixture of these raw materials, designated by their major raw material, Table I. RESULTS, DISCUSSION Efficiency of the Deinking Process: As expected, the main yield losses occur in the flotation and washing stages (unit operation yield=77 and 84 % respectively), Fig 2. Post-flotation and post-washing also contribute to lower the over-all yield of the deinking line (53 % on average). Residual peroxide after bleaching is in the range of 0.4 % (initial= 1%). % % % % Toner Magazine Inkjet 2.08 Toner Furnish Magazine Furnish Inkjet Furnish Furnish Brightness (% ISO) is mainly increased by flotation and washing, Fig. 3. Then every step in the second loop is beneficial to brightness. In the case of hyperwashed pulp, it can be seen from Fig. 4 that fibre brightness is increased by bleaching and dispersion, thanks to ink detachment. In addition, a minor brightness increase is observed after flotation, possibly due to ink detachment in the centrifugal pumps at the inlet of the flotation cell. As expected, flotation and post-flotation efficiently reduce ink concentration in the pulp, Fig. 5. Measurements on hyperwashed pulp, Fig. 6, show that preflotation, likely due to the shear pumps, and dispersion help reduce ink attached to the fibres. In the case of inkjet furnish, however, ink removal is difficult to obtain: flotation is less efficient as pigment ink particle are very small and less hydrophobic (Fig. 5). A thorough post-deinking loop is required to achieve acceptable residual ink values, where post-washing is essential. It is anticipated that inkjet removal would be poorer in an industrial case, as the laboratory washing procedure used here is very efficient. Hyperwashed ERIC after repulping (screen sampling point) is higher in the case of inkjet furnish (Fig. 6), indicating very fast ink redeposition, considering water-based ink is instantly detached. Pigment ink redeposition is also observed after thickening. Specks content (in surface) is drastically reduced after flotation and dispersion, Fig. 7. The effect of post-deinking is however limited, as many specks have been removed by the first loop, and also fragmented by dispersing below the 8-µm size limit for specks. The furnishes contain from 28% (inkjet) to 32% filler (for coated woodfree). Filler content is then considerably reduced by flotation and washing, down to 5-7% (Fig. 8). Filler content is eventually lowered to about 1% by post-washing. Filler content in the flotation froth is about 65% for all studied furnishes. Effect Of Deinking Process and Raw Material on DIP Physical Properties: Tensile index is improved after flotation and washing: filler removal enhances interfibre bonding and is therefore beneficial to tensile strength, Fig. 9. Then, the detrimental action of thickening is observed: the action of the screw press can be compared to that of a kneader (friction). The detrimental effect of thickening is antici- PULP & PAPER CANADA 107:7/8 (2006) 35

3 T156 FIG. 1. Deinking process (H=helico pulper, F=flotation, W=washing, SP+screw press, P=peroxide bleaching, Disp.=dispersion, Y=dithionite bleaching) FIG. 2. Evolution of total yield during the deinking line. FIG. 3. Evolution of brightness along he deinking line. FIG. 4. Evolution of hyperwashed pulp brightness along the deinking line. FIG. 5. Evolution of E.R.I.C. along the deinking line. pated to be less pronounced at industrial scale, because industrial screw presses are much larger than the pilot device used her. That scale effect when going from pilot to industrial screw press, has already been shown for ink behaviour [9]. For the high-speed disperser, the mechanical action is similar to that of a refiner (similar effect, but lower intensity). This kind of device, even if not optimized for the development of fibre properties, allows to improve the tensile properties of the pulp. After post-bleaching, the tensile index is comparable to initial values. These trends are similar for all furnishes. The same conclusions are applicable to burst index, Fig. 10. For a given furnish, a slight increase in FIG. 6. Evolution of hyperwashed pulp E.R.I.C. along the deinking line. bulk is observed after flotation and washing: this increase is attributed to a loss of filler in the flotation stage, Fig. 11. Then, bulk largely decreases after dispersion: that could reveal some fibre flexibility increase due to mechanical action on fibres. Eventually, the second deinking loop tends to develop bulk. The toner furnish is slightly bulkier than the others at :7/8 (2006) PULP & PAPER CANADA

4 T157 FIG. 7. Evolution of specks content along the deinking line. FIG. 8. Evolution of ash content along the deinking line. FIG. 9. Evolution of tensile index along the deinking line. FIG. 10. Evolution of burst index along the deinking line. FIG. 11. Evolution of bulk along the deinking line. FIG. 12. Evolution of tear index along the deinking line. the beginning. The over-all deinking process favours bulk development. Tear index steadily increases down to the thickening stage, Fig. 12. Dispersion has a small detrimental effect on tear index. Over-all, there is a net significant tear improvement after deinking. Again, comparable trends are observed for all furnishes. Web stretch (or failure strain) remains roughly constant along the deinking process, Fig. 13. The toner furnish has a higher stretch than the others. Wet stretch has a different behaviour: it increases after thickening, then mainly after dispersion, and eventually decreases along postdeinking, Fig. 14. No significant change of rigidity is observed along the deinking process (not shown). Effect of Deinking Process on Fibre Properties: Wet zero span tensile is related to fibre intrinsic axial strength, as rewetting the sample eliminates the contribution from inter-fibre bonding to the network strength. Wet zero span tensile is found to slightly decrease after dispersion stage, Fig. 15. The following deinking steps somewhat compensate for that reduction. It is concluded that the dispersion stage has a small effect on fibre strength. With the Morfi analyser, fibre curl is measured in a forced flow field: thus, the higher the fibre flexibility, the lower the curl. Fibre curl increases significantly after thickening and dispersion, then decreases after post-deinking, Fig. 16. Comparable trends are observed for the number of kinks per fibre, Fig. 17. Mechanical treat- PULP & PAPER CANADA 107:7/8 (2006) 37

5 T158 FIG. 13. Evolution of web stretch along the deinking line. FIG. 14. Evolution of wet web stretch along the deinking line. FIG. 15. Evolution of wet zero span along the deinking line. FIG. 16. Evolution of fibre curl along the deinking line. FIG. 17. Evolution of the number of kinks per fibre along the deinking line. FIG. 18. Wet web stretch vs. fibre curl. ment during thickening and dispersion affected the fibre flexibility and created weak points along the fibre (swelling of amorphous cellulose). Thickening creates some kinks onto the fibres that modify the flexibility behaviour (two rigid segments around the kink). McKinney and Roberts [3] also reported curl modification after thickening, kneading, dispersion and bleaching, most likely due to mechanical action imparted on fibres at high consistency. The decrease of curl along post-deinking is most likely due a latency effect [10]. On the other hand, it is not clear why the number of kinks is reduced after postdeinking Fibre curl has been shown to affect several mechanical properties. Most interesting is the strong relationship found between wet stretch and fibre curl, Fig. 18. That trend is valid for all furnishes. In the case of toner furnish, high fibre curl causes extensive wet stretch development. Wet stretch increases because of fibres being more rigid. Runnability problems related to increased web wet stretch have been reported by mills using wood-free DIP, as more web wet draw is required. It is suggested that these problems could partly be solved by limiting development of fibre curl during the thickening and dispersion stages (see also Figs 14, 16). Fibre length decreases slightly after thickening and dispersion, then increases again in post-deinking, Fig. 19. Moderate cutting action by the disperser is suspected which reduces mean fibre length and creates cellulosic fines. Those fine elements are then lost in the post-flotation stage, which explains the fibre length increase. (That hypothesis is supported by :7/8 (2006) PULP & PAPER CANADA

6 T159 FIG. 19. Evolution of fibre length along the deinking line. FIG. 20. Effect of bleaching strategies on. the corresponding yield decrease, Fig. 2.). By the end of the deinking process, fibre length is restored to its initial value. Effect of Bleaching Sequence on DIP Properties: Three bleaching strategies were studied: disp.-y-flot.-p, disp.-p-flot.-y or dispersion with pre-introduction of peroxide (disp.(p)-flot-y, already in practice in many deinking mills). The reference pulp is taken at the outlet of the screw press. In terms of, the brightness of entire pulp and opacity after the two deinking loops were slightly better when bleaching with PY than YP sequence, Fig. 20. Concerning mechanical properties, PY gives a slightly higher bulk than YP, Fig. 21. If the peroxide bleaching chemicals are introduced before the high-speed dispersing treatment, the bulk is decreased. The dry tensile index is higher when bleaching with PY than with YP. With the introduction of peroxide bleaching chemicals prior to the dispersing treatment, the tensile properties are reduced, but are still higher than those obtainable with a YP bleaching sequence. Unfortunately, all the studied bleaching sequences cause significant wet stretch increase. Both YP and PY are responsible for a wet stretch increase of about 30%, mostly due to dispersing, as suggested by previous trials reported in Fig. 14. However, in the case of PY bleaching, introduction of peroxide bleaching chemicals before dispersing, limits the wet-stretch increase to 15 %. It looks as if alkaline chemistry protects the fibre against mechanical degradation. It is suggested that alkali can lubricate the pulp or even smooth the flow regime of the pulp in the disperser, similarly to observed effect in the helico pulper [11]. With YP bleaching, a reduction in wet zero-span tensile is observed, suggesting that fibre strength is affected. On the other hand, the effect of PY bleaching on wet zero-span is slightly positive. The compromise between mechanical properties development, represented by tensile index, and wet-stretch increase is summarised in Fig. 22. The PY sequence is certainly the most interesting sequence in terms of mechanical properties (equivalent wet stretch and higher tensile index in comparison to YP). Besides, if high wet stretch is an issue (runnability problems on the PM), the introduction of peroxide chemicals before the high-speed disperser can be recommended. Therefore, the effective mechanical treatment imparted on fibres in highspeed dispersion seems to be strongly coupled with the type of chemistry used. CONCLUSION At the outlet of the deinking line, most fibres and pulp properties are similar to their initial values. Fibre curl and kinks are, however increased, as well as tear and wet-stretch. Along the deinking process, the effect of unit operations is nevertheless clearly visible. Flotation is generally beneficial to mechanical properties, because of ash removal. Thickening in the screw press (to 35%) is detrimental to tensile, burst, wet-stretch, fibre curl and creates kinks on the fibres. The high-speed dispersion stage plays the role of a refiner (with lower intensity), and is favourable to tensile, burst, while it causes fibre curl, creates kinks and considerably increases the wet-stretch. Wet web stretch is strongly linked to fibre curl. Machine runnability problems related to wet stretch could then be reduced by limiting fibre curl development in high-consistency treatments such as thickening, dispersion and possibly bleaching stages. Variation in the raw material (tonerprinted paper, magazine, inkjet printed paper or coated wood-free) is not found to change DIP mechanical properties. The toner furnish, however, has superior dry and wet stretch. By the end of the deinking process, do not vary much depending on type of initial furnish. However, inkjet-containing furnish (20%) is more difficult to deink than the others. The P-Y bleaching sequence offers superior optical and mechanical properties to the YP sequence. As an option to the PY sequence, introduction of peroxide bleaching chemicals prior to high-speed dispersion (i.e., [disp.(p)-flot.-y] sequence) can be an interesting compromise for both deinking efficiency and DIP mechanical properties: wet stretch increase is minimized compared to [disp.-p-flot.-y] sequence and mechanical properties slightly increase, altogether with acceptable final brightness and ERIC. It appears that the effects of mechanical and chemical treatment imparted on the fibres are strongly coupled. The effect of bleaching chemistry depends on the type of mixing associated with it. ACKNOWLEDGEMENT We thank Pierre CREMON, Claude GUIL- MOT, Alain BALME, Pascal DE LUCA, Jean DE GRACIA, Geneviève PROMON- ET and Max GUILLET for performing all the experiments. OCE printer is thanked for providing the toner-printed paper. Support from partners of the PHYSDIP project is gratefully acknowledged. The YP vs. PY bleaching pilot trials were financed by the EC through HPRI CT contract. LITERATURE 1. MAHAGAONKAR, M. and BANHAM, P. Effect of deinking on optical and physical properties of secondary fibre after pulping and flotation. Appita J. 48(6): (1995). 2. MAHAGAONKAR, M., STACK, K. BANHAM, P. The effects of coated magazines on deinking of newsprint after pulping and flotation. Proc., 1997 Recycling symposium, Chicago, , April (1997). 3. MCKINNEY, R.W.J., ROBERTS, M. The effects of kneading and dispersion on fibre curl. Proc., 1997 Recycling symposium, Chicago, , April (1997). 4. BOUCHARD, J., DOUEK, M. The effects of recycling on the chemical properties of pulps. Proc., 2nd Research Forum on Recycling, Ste-Adele, QC, 23-29, 5-7 Oct. (1993). 5. HOWARD, R.C. The effects of recycling on paper quality. JPPS 16(5):J143-J149 (1990). 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7 T160 FIG. 21. Effect of bleaching strategies on mechanical properties. FIG. 22. Compromise between dry tensile development and wet stretch increase, depending on bleaching strategy. 6. HOWARD, R.C., BICHARD, W. The basic effects of recycling on pulp properties. JPPS 18(4):J151-J159 (1992). 7. PERRIN, B., BIANCHIN, B., SAURET, G., French Pat July 25 (1980). 8. JULIEN SAINT AMAND, F., BERNARD, E., LEJE- UNE, P. Appareil de séparation et de récupération de particules en suspension: le Classiflux, Conf. Filtra 84, Paris (1984). 9. FABRY, B., CARRÉ, B., CREMON, P. Influence of shearing forces in thickener on deinking of various raw materials: evaluation on CTP pilot. 4th Round Table Meeting-Krict TAPPI-Daejon/Korea, April 3-4, (2003). 10. FINERAN, W.G., MESSMER, M. A proposed method for the measurement of deformation or latency imparted to secondary pulp fibers by the dispersion process. Proc., Recycling symposium, Chicago, , April (1997). 11. FABRY, B., CARRÉ, B. A new approach to characterize pulping stage in deinking. 11th PTS CTP Deinking Symposium, Leipzig, Germany, April (2004). Résumé: Le but de la présente étude est de suivre l évolution tout au long du circuit de désencrage des propriétés mécaniques et optiques des pâtes de vieux papiers sans bois. Un circuit de désencrage à deux boucles est partiellement simulé dans l usine pilote du CTP. Nous avons trouvé que l épaississage nuisait à la résistance à la traction et à l éclatement. L augmentation de l allongement à l étage de dispension, et est réliée au recroquevillement de la fibre (augmentation de la courbune). Les propriétés mécaniques obtenues par le blanchiment PY sont supérieures à celles du blanchiment YP. Reference: HUBER, P., CARRÉ, B., PETIT-CONIL, M., FABRY, B., BHATTACHARJEE, S. Effect of the deinking process on physical properties of various wood-free recovered paper furnishes. Pulp & Paper Canada 107(7/8): T (July/August, 2006). Paper presented at the 004 Recycling Conference in Quebec City, QC, on September 27 to 29, Not to be reproduced without permission of PAPTAC. Manuscript received on June 30, Revised manuscript approved for publication by the Review Panel on Dec 9, Keywords: DEINKED STOCK, MECHANICAL PROPERTIES, OPTICAL PROPERTIES, DEINKING, RECLAIMED FIBERS, FREE SHEET, THICKENING, BLEACHING :7/8 (2006) PULP & PAPER CANADA