Effect of initial repulping ph on the deflaking rate of recovered papers

Transcription

1 Effect of initial repulping ph on the deflaking rate of recovered papers It greatly depends on the paper wet tensile strength By F. Brouillette, C, Daneault and G.M. Dorris Abstract: The effect of the initial repulping ph on the deflaking rate of ONP and OMG was analysed in term of the forces at work in a helical laboratory repulper. As the ph increased over 10.5, the reduction of the wet tensile strength of paper had the greatest effect on the deflaking rate. In the ph range studied, the volume concentration and the apparent viscosity of the pulp suspension had little effect on deflaking. Finally, it was determined that deflaking is very rapid, even at neutral ph, for a typical blend of paper used for newsprint production. R EPULPING IS A key operation in the deinking process whose main functions are to deflake paper and detach ink particles from the fibre surface while avoiding contaminants fragmentation. Depending on pulper operating conditions (energy, consistency, time, temperature and chemistry), many other phenomena may occur simultaneously to deflaking and ink detachment: release of non-fibrous material into the aqueous phase; dispersion and/or agglomeration of ink and sticky particles; yellowing and bleaching of fibres; redeposition of ink particles into the lumen [1]. All these phenomena, which have a significant impact on subsequent separation operations, will therefore affect the brightness and cleanliness of the deinked pulp as well as paper machine runnability and end product quality. The importance of repulping has always been recognized, but it is only recently that significant effort has been deployed to understand better the relative contributions of mechanical, chemical and thermal energies on the phenomena mentioned earlier. From the findings made on several aspects of deinking, [1-4], many recyclers question conventional practices for repulping time, pulp consistency and chemicals used in the alkaline deinking process. In this context, we investigated the effect of repulping ph on the deflaking rate and on the rate of release of non-fibrous material from several grades of paper found in deinked pulp for the production of newsprint. In this second article of a series on the role of the repulping ph, we report our observations on the effect of ph on deflaking. The trend toward the reduction of the ph during repulping is quite recent. Neutral deinking was first applied to mixed office waste because, when applied to bleached chemical fibres [5], ph has little effect on the detachment of toner particles, though some recent work indicates that for acidic ph, toner fragmentation is enhanced [6]. Near-neutral deinking is now spreading to wood-containing paper used for the production of newsprint [7,8]. This new applica- tion is motivated by the reduction of chemical costs, the possible decrease of dissolved substances and the reduction of sticky contaminants (macrostickies) fragmentation [9]. However, others have indicated some pitfalls of ph reduction like an increase in repulping time [10], a reduced rate of detachment of oil-based inks and a greater fragmentation of ink particles [11]. To facilitate implementation of this new technology, we have undertaken a series of studies to understand better the role of ph during repulping. The effect of the reduction of the initial repulper ph on the release of contaminants was studied in the first part [4]. This article presents the second part of this work. Kinetic Model of Deflaking: The importance of the various forces acting during the deflaking of paper in a pulper was first identified in the work of Paraskevas [12] and Holik [13]. Holik identified three types of forces: impact forces related to rotor-pulp interactions, shear forces related to the rheology of the pulp suspension and acceleration forces. He also pointed out the importance of paper wet strength and pulp suspension apparent viscosity on the deflaking rate. More recently, a shear factor related to the apparent viscosity of the pulp suspension was introduced by Fabry et al. [3] to explain changes in the deflaking rate and the level of ink fragmentation in a repulper. It is now well known that the flake content, F, measured as a function of pulping time, t, follows a first order kinetic: df = kf (1) dt where k is a first order rate constant. The value of k depends on operating conditions, and hence provides no insight on the forces at play during repulping. Bennington et al. [2] have proposed a model that defines the rate constant k in terms of measurable parameters. They showed that deflaking depends on the amount of contact between the rotor and the pulp suspension, on the strength of F. BROUILLETTE Pulp and Paper Research Center UQTR, Trois-Rivières, QC C. DANEAULT Canada Research Chair on Value-Added Paper UQTR, Trois-Rivières, QC G.M. DORRIS Paprican Pointe-Claire, QC Pulp & Paper Canada T :7 (2003) 35

2 TABLE I. Characteristics of paper samples. Basis Fibres weight & Fines Ash Other b Paper (g/m 2 ) (%) (%) (%) ONP LWC a 3.4 SCA a 1.9 a Kaolin clay, b Styrene-Butadiene Rubber (SBR) and other polymers [14]. TABLE II. Repulping conditions. Paper 100% ONP, LWC or SCA Consistency 8.5% (based on fibres) Filling time 30 s Repulping time 10 min Total volume 6.25 L Temperature 45 C Speed 780 rpm (13 rev./s) ph 5 to 12.7 FIG. 1. Helical laboratory pulper. FIG. 2. Initial repulper ph vs. NaOH charge at different mass concentrations (C m ). NaOH purity is ~100%. the paper and on the forces imparted by the rotor. Progress of deflaking was expressed as: df T M = k 0 C v BNG exp dt KF F (2) R where C v is the volume concentration of the pulp suspension, B the number of rotor vanes, N the rotor speed, G the surface swept by one rotor vane during one revolution, T M the wet tensile strength of the paper and F R the force imposed to the suspension by the rotor. k 0 is the intrinsic rate constant and K is a proportionality constant. The model has been validated by using several grades of paper at different mass concentrations. However, there is currently no theoretical explanation of the link between contact forces and tensile strength. Even though the importance of ph was identified, it was not studied in details. Based on the kinetic deflaking model of Bennington et al. [2], we investigated paper and pulp parameters that, by their sensitivity to ph changes, affect the deflaking rate of old newsprint and old magazines. Hence, our objective consists of studying the effect of the initial pulper ph on water absorption and wet tensile strength of paper and on pulp volume concentration. EXPERIMENTAL Repulping tests were done with three grades of paper: old newsprint (ONP), lightweight coated (LWC) paper and supercalendered A paper (SCA). These types of paper are commonly found in the conventional mix of old newsprint and old magazines used in the production of deinked pulp for newsprint production. Paper samples were obtained directly from the printer and stored at 4 C until they were used. The ONP was a 100% TMP furnish printed by the four-colour offset process, whereas the LWC and the SCA were printed by the heatset offset printing process. The fibre fraction of the LWC was a blend of stone groundwood (60%) and kraft pulp (40%), and the coating was a mixture of clay and SBR latex. The SCA was a 100% TMP furnish with a blend of clay and synthetic polymers. Characterization of Paper Samples: The characteristics of the three paper samples are reported in Table I. Fibre and ash content were determined by thermogravimetric analysis [14] using a Netzsch TG-209 thermal analyser. Basis weight was determined according to PAPTAC Standard Method D.3. Wet tensile strength (PAP- TAC Standard Method D.10) and water absorption (Cobb Method, PAPTAC Standard Method F.2) of paper were measured at different ph. The Cobb method was used instead of the more standard water retention value (WRV), because it allows the determination of the absorption rate and the total amount of water in the paper. To confirm further the values obtained by the Cobb method, water retention values were measured on some pulp samples. The Cobb method was found to give values about 8% higher than the WRV because of the much weaker (almost 10 times) drainage force imposed during the test. The volume concentration of the pulp (C v ) was calculated based on the proportion of each paper components and the water absorption of the paper [2]: V sf + V mf + V p C v = (3) V sf + V mf + V p + V fw and Am f V sf = V f + (4) ρgc f In Eq. 3 and 4, V sf, V mf, and V p represent the volume of swollen fibres, mineral filler and polymeric additives, respectively. V fw is the free water volume, A is the water absorption of the paper, m f is the fibre mass, ρ is the water density, G is the basis weight of the paper and C f is the fibre con :7 (2003) T 175 Pulp & Paper Canada

3 FIG. 4. Effect of initial repulper ph on deflaking rate (k). FIG. 3. Typical deflaking curves at (a) ph 5.5 and (b) ph FIG. 5. Absorption rate of water into paper. centration in the paper determined by TG analysis. Polymeric additives and mineral fillers volumes were calculated using density values of 1.0 g/ml for the polymeric fraction and 2.2 g/ml for the kaolin clay (mineral filler). The volume of air introduced in the pulp during repulping was ignored because it has been shown to be low at 8% mass consistency [15]. Repulping: Each paper was repulped separately in a helical laboratory pulper, (Fig. 1). Identical repulping conditions were used for each run, except for the ph that was varied between 5 and A detailed description of the repulping conditions is given in Table II. A fixed filling time of 30 s was used for each run. During this step, the ph of the dilution water was adjusted with sodium hydroxide (NaOH) and the paper was added to the pulper while using a low rotor speed of 180 rpm. A few seconds before the filling time had elapsed, the rotor speed was increased to 780 rpm. It is common for the mills to express sodium hydroxide charges as percentage, based on the dry weight of NaOH and paper. In this case, we preferred to use the ph scale in order to relate our observations to chemical phenomena involving changes in hydroxide concentration in the water phase. NaOH charges expressed in weight % do not allow such comparison because ph values associated to these charges would depend on the weight concentration of the pulp and the purity of the NaOH. Figure 2 shows the initial repulper ph obtained with different HCl or NaOH charges at various mass concentrations. In order to obtain shorter mixing times than Bennington et al. [2], we used a higher rotational speed (13 vs. 7.5 rev/s) and a smaller suspension volume (6.25 vs. 8 L). With this higher energy output, it was possible to get homogenous pulp samples after only 30 to 60 s of pulping. The flake content, determined at several repulping times, was measured according to the method proposed by TAPPI [16] using an hyperwasher equipped with a 10-mesh screen. The amount of energy transferred to the pulp was obtained by monitoring the power consumption of the repulper with an Ebro EB 1288 powermeter. Instantaneous power was given by the slope of the curve of the power consumption (kwh) vs. the repulping time (h). The power (P) used to move the pulp was calculated by subtracting the power required to run the empty repulper from the total power for each repulping trial. The force per unit length of rotor vane F R is then given by: P F R = (5) πd m NBH c where D m is the mean diameter of the rotor and H c is the height of rotor that is in contact with the pulp suspension. The type of rotor used in the repulping trials has three vanes. It has a mean diameter of m and a H c of 0,040 m for a suspension volume of 6.25 L. Because the moving pulp forms a cavity around the rotor, the Pulp & Paper Canada T :7 (2003) 37

4 FIG. 6. Water absorption in the ONP sample as a function of soaking time at ph values of 5.3 and FIG. 7. Effect of initial repulper ph on water absorption of fibres (soaking time = 3 min.). FIG. 8. Effect of initial repulper ph on volume concentration (C v ) of pulp. FIG. 9. Effect of initial repulper ph on wet tensile strength (T M ) of paper. height of the pulp is not uniform across the repulper and H c is lower than the actual maximum height of the pulp in the repulper. In consequence, only the part of the blade that was in contact with the pulp suspension was considered in the calculation of the contact area. RESULTS Deflaking Curves: Figure 3 shows typical deflaking curves obtained at repulping ph values of 5.5 (a) and 12.0 (b), respectively. The curves follow the exponential decay predicted by Eq. 1, but Fig. 3 (b) indicates that the initial flake content (at t = 0) is already well below 70% instead of 100% when the initial ph is at This indicates that paper is weakened rapidly enough at high ph to be deflaked in the filling step, when the rotor speed is only 180 rpm. This phenomenon was also noted by Fabry [17] for a 100% magazine pulp repulped at mass concentrations ranging from 8.5 to 13.5% and NaOH charges of 0 and 1%. He proposed to modify Eq. 1 to include an adjustable time TABLE III. Equivalent C m values at neutral ph. Equivalent C m Initial C v (neutral ph) repulper ph (%) (%) constant that brings the flake content at 100% at t = 0. Because use of this factor shifts the whole curve without changing the actual value of the rate constant, k was calculated here from Eq. 1. For all paper samples, Fig. 4 shows that a change of ph between 5 and 11 has no significant effect on the deflaking rate. However, over ph 11, the deflaking rate increases markedly. At all ph values, SCA and ONP are deflaked at similar rates whereas the rate for LWC is almost twice higher. Over ph 12.5, the deflaking rate is so high that the measurement of the flake content is no longer valid. Indeed, a k value larger than 0.05 s 1 corresponds to a flake content smaller than 5% after 60 seconds of pulping. Hence, we can consider that when k is higher than 0.05 s 1, paper is deflaked almost instantaneously. Effect of Initial Repulper ph: On the volume concentration of pulp (water absorption): As depicted in Fig. 5, the absorption rate of water into paper is similar in all paper samples and is independent of the ph for ONP and SCA, but not for :7 (2003) T 177 Pulp & Paper Canada

5 FIG. 10. Effect of initial repulper ph on the force per unit length of rotor vane (F R ). FIG. 11. Deflaking rate (k) vs. volume concentration (C v ) of pulp. FIG. 12. Deflaking rate (k) vs. wet tensile strength (T M ) of paper. FIG. 13. Deflaking rate (k) vs. C v exp( T M /F R ). LWC. Figure 6 indicates that over 80% of the water absorption takes place within one minute at 25 C. Because the pulper temperature is higher (45 C) than our test temperature, fluid viscosity will, in practice, be lower thus leading to an even faster absorption rate in industrial units. As shown in Fig. 6, the initial ph has a marked effect on the total amount of water absorbed in papers. It increases by 50% in going from ph 5.0 to Figure 7 compares the water absorption of the three papers at various phs. Though the filler content has been factored out, it is clear that on a fibre basis, ONP absorbs more water than SCA and LWC papers at all ph. The various types of fibres, the amount of fillers and the presence or not of polymeric additives are all responsible for these differences. At the same fibre concentration, the volume concentration depends principally on the volume of swollen fibres and on the free water in the pulper. Even though clay filler represents an important proportion of the SCA and the LWC papers, its contribution in term of volume is small because the density of kaolin is quite high and does not changes significantly when placed in contact with water. Figure 8 indicates that the volume concentration for the three paper samples is similar between ph 5.0 and 11.0, but it increases in all cases, by 4 to 5%, in going from a ph of 11.0 to However, in this ph range, the relative change is quite high (up to 20%). On the wet tensile strength of paper: In agreement with Holik [13], Fig. 9 shows that the initial repulper ph has a significant effect on the wet tensile strength of the paper, which in turn affects the deflaking rate (see Eq. 2). At neutral ph, the LWC and SCA papers have T M values of 180 and 280 N/m respectively, which is much lower than ONP (400 N/m). Between ph 11.0 and 12.5, all paper samples lose most of their strength. Old magazines are less affected than the ONP; they lose about 70% of their wet tensile strength versus more than 85% for the ONP. Clearly, the loss of wet tensile strength is related to the increase in the amount of water absorbed by fibres. It is likely that the higher proportion of fibres in the ONP leads to a more important loss of wet tensile strength when ph increases. On the force per unit length of rotor vane: According to Bennington et al. [2], the repulper operates in the laminar regime at medium to high consistency and high impeller speed. The force per unit length of rotor vane (F R ) in the laminar regime depends on suspension apparent viscosity and is given by: K L µd 2 m N F R = (6) πh c B Combining Eq. 5 and 6 and solving for P gives: P = (K L D 3 m N2 )µ (7) In Eqs. 6 and 7, K L is the impeller constant in a laminar flow and µ is the suspension apparent viscosity. Eq. 7 shows Pulp & Paper Canada T :7 (2003) 39

6 that, assuming a laminar flow, suspension viscosity is related to power by a linear relationship. Under the conditions used in our experiments, only the suspension apparent viscosity can vary. Hence a variation of P or F R would reveal a change in the suspension apparent viscosity. Figure 10 shows F R values calculated using Eq. 5 for ONP, LWC and SCA at ph values of 7.0 and 12.6, respectively. These values, which cover the whole ph range used in our experiments, are not significantly different at a confidence level of 95%. Hence, variations in suspension apparent viscosity could not be detected in that ph range for the three paper types studied at a constant fibre mass concentration. To further confirm this observation, we calculated the mass concentrations, at neutral ph, corresponding to a given volume concentration at an alkaline ph. For example, if we take ONP at ph 12.6 and C m = 8.4%, the equivalent value of C m at neutral ph is 10.3, an increase of almost two percentage points. Table III shows the equivalent C m values for ONP at several initial repulper ph values. These results indicate that the equivalent increase of C m caused by an increase of the initial repulper ph in the usual mill range (ph < 12.0) is less than one percentage point. The F R determinations in Fig. 10 suggest that this increase is not sufficient to change significantly the suspension apparent viscosity. Data in Figs 9 and 10 also indicate that F R is always higher than T M except for ONP at neutral ph. In theory, if F R is lower than T M, and even if the energy is completely transferred from the rotor vanes to the aqueous phase (e.g. K in Eq. 2 is equal to 1), the paper would not deflake. For ONP, T M (400 N/m) is sufficiently close to F R (324 N/m) to allow deflaking to occur. However, the total contact surface between the rotor and the pulp is ill defined here because of the uneven distribution of the pulp, especially at the top of the rotor. A small change of the rotor vanes surface in contact with the pulp suspension leads to a significant change in F R. DISCUSSION Model of Bennington et al.: The model of Bennington et al. [2] helps to understand how ph affects deflaking through a change of volume concentration, wet tensile strength or suspension apparent viscosity. If the repulper parameters are kept constant (rotor geometry, volume of pulp, rotational speed, mixing time, and temperature) then k in Eq. 2 may be reduced to: T M k = k 0 C v exp (8) KF R In our experiments, consistency (on a fibre basis) was held constant but the volume concentration of pulp changed with ph. According to Eq. 8, the deflaking rate is related linearly to the volume concentration and exponentially to the wet tensile strength and the inverse of the force per unit length of rotor vane. As we have shown before, for the papers used in our experiments, F R is not affected by ph at a constant mass concentration. Hence, it can be considered here as a constant. Figure 11 shows that the linear relationship between k and C v, predicted by Eq. 8, is observed for all papers and is quite similar for SCA and ONP. However deflaking rates are higher for LWC than for SCA and ONP at the same volume concentration. Figure 12 shows that the exponential relationship between k and T M, also predicted by Eq. 8, is observed for all papers. Even though correlation is good, each paper presents a different behaviour, as shown by their regression parameters, in particular at T M higher than 200 N/m (the ONP was unusually strong at neutral ph). The over-all correlation is better than for k vs. C v (r 2 = 0.619). Hence, wet tensile strength alone cannot fully explain the variations of the deflaking rate caused by changes of ph for all papers. However, when T M is lower than 200 N/m, high deflaking rates are reached for each paper samples, but the LWC seems to be more sensitive to wet tensile strength loss. Faster deflaking of LWC may tentatively be ascribed to the structural differences between the LWC paper and the two other samples. With the LWC paper, pigments are incorporated in a latex layer applied to both sides of the paper, whereas in the SCA paper they are distributed in the bulk of the paper (ONP contains no pigment). When the wet tensile strength is measured, an average value is obtained for the whole thickness of the paper. However, in the case of LWC, the wet strength is affected by the rate of release of the coating. The alkaline ph in the pulper promotes the detachment and dispersion of the LWC coating and leaves a thinner fibrous mat that is deflaked more rapidly than in the ONP and SCA for a similar wet tensile strength. This delamination phenomenon was revealed by visual inspection of the flakes produced by the LWC and the SCA papers at the same deflaking level. Whereas an important fraction of SCA flakes contains paper fragments with easily recognizable areas of colour and text after 60 s at ph 12.3, coloured areas and text have almost completely disappeared from the surface of the LWC flakes. We have seen so far that the individual relationships between the deflaking rate and the volume concentration, or the wet tensile strength can be predicted by Eq. 8 for all papers used in our experiments. Moreover, if we plot the graph of k vs. C v exp( T M /F R ), the correlation should be better than for each parameter alone. Figure 13 shows that the correlation between k and C v exp( T M /F R ) is good for each paper, but the over-all correlation is not better than for T M alone (r 2 = against 0.619). We can assume that the changes of C v (F R is constant) observed in going from ph 5.5 to 12.7 are not sufficient to affect the deflaking rate. Hence, the variation of T M with initial repulper ph has the most important effect, but other factors such as paper structure obviously affect the deflaking rate. Relation to Fibre Swelling: The changes of volume concentration and wet tensile strength can be related to the variation of the water absorption of fibres caused by ph changes. Katz, Liebergott and Scallan have shown that fibre swelling depends on the amount of acidic groups in the pulp [18], on the extent of acidic groups dissociation level, and on the nature and concentration of counter ions [19]. If the paper is made at a higher ph than the pk a of these acidic groups, then these groups are in their dissociated form. When the paper comes in contact with water in the repulper, the ion concentration is much higher within the cell wall than in the outer aqueous phase. This concentration gradient creates an osmotic pressure that attracts water into the cell wall and causes fibre swelling by the Donan effect. Katz et al. [20] have shown that treating mechanical pulp with sodium hydroxide causes the formation of new acidic groups in the pulp by hydrolysis of ester and lactone groups from the glucoronic units of hemicelluloses. The observed swelling is proportional to the amount of NaOH consumed by the pulp. Hence, an increase of the ph causes an increase of the water absorption of fibres and a decrease of the amount of free water in the repulper. Consequently, the volume concentration and the apparent viscosity of the pulp suspension are increased. The increase of fibre swelling causes then a concurrent reduction of the wet tensile strength of paper. As we have shown, all those parameters ultimately combine to promote a faster deflaking when the ph is higher than 11, but the effect of the volume concentration and pulp suspension apparent viscosity is significant only at ph values much higher than the usual mill repulper ph range. CONCLUSIONS We have shown that the effect of the initial repulper ph on the deflaking rate of ONP and OMG depends strongly on the paper wet tensile strength. In the ph range studied, the effect of the volume concentration (for a given mass concentration) and the apparent viscosity of the pulp suspension was very small. However, for a given ph, paper structure has also an important effect on the deflaking rate, and this factor can not be predicted properly by a change of wet tensile strength only. Finally, for paper samples having the :7 (2003) T 179 Pulp & Paper Canada

7 same wet tensile strength, the presence of mineral fillers or polymeric additives like SBR does not modify significantly the deflaking rate. If the deflaking aspect only is considered, we can conclude that deflaking is very rapid, even at neutral ph, for a typical blend of paper used for newsprint production. A reduction of the initial repulper ph could be detrimental only if a combination of two conditions occurs: the actual initial repulper ph is higher than 11 and the repulping time is a limiting factor. If not, the deflaking rate of all paper samples studied (ONP, LWC and SCA) is already sufficiently high to completely deflake the paper in about three minutes. In consequence, the intensity and time of mixing during the filling and emptying stages of a batch pulper are sufficient to ensure almost full deflaking, without an additional mixing time with the full paper charge. ACKNOWLEDGEMENTS The authors want to thank the Mechanical Wood-Pulps Network and Paprican for their financial support and also Natalie Pagé and Carlos Castro for their invaluable assistance. LITERATURE 1. BEN, Y., DAGENAIS, M., DORRIS, G.M. Irreversible ink deposition during repulping. Part I: Model deinking systems. JPPS 26(3): (2000). 2. BENNINGTON, C.P.J., SUI, O.S., SMITH, J.D. The effect of mechanical action on waste paper defibering and ink removal in repulping operations, JPPS 24(11): (1998). 3. FABRY, B., ROUX, J.-C., CARRÉ, B. Characterization of friction during pulping: an interesting tool to achieve good deinking. JPPS 27(8): (2001). 4. BROUILLETTE, F., DORRIS, G.M., DANEAULT, C. Effect of initial ph on the release of dissolved, colloidal and suspended solids during repulping of ONP and OMG. JPPS 28(2): (2001). 5. DORRIS, G.M., SAYEGH, N.N. The role of print layer thickness and cohesiveness on deinking of tonerprinted papers. Tappi J. 80(4): (1997). 6. AZEVEDO, M.A., DRELICH, J., MILLER, J.D. The effect of ph on pulping and flotation of mixed office wastepaper. JPPS 25(9): (1999). 7. SCHWINGER, K. The process Neutropur : Deinking in a neutral medium of mixtures of old newspapers and magazines. Wochenbl. Papierfabr. 123(19): (1995). 8. Marcus, A. Deinking survey WP Trade. Neutral deinking makes its debut. Pulp Pap. Int. 35(10):22-25 (1993). 9. ACKERMANN, C., PUTZ, H.-J., GÖTTSCHING, L. Effect of pulping conditions on deinking of wood-containing recovered paper grades. Pulp & Paper Can. 100(4): T109-T McKINNEY, R.W.J. technology of paper recycling. 1st ed. Glasgow: Blakie Academic & Professional, (1995). 11. RØRING, A., SANTOS, A. Pulper chemistry is a key to successful deinking. Preprints, 4th Research Forum on Recycling, PAPTAC, Quebec City, (1997). 12. PARASKEVAS, S High consistency repulping benefits & drawbacks. Proc., TAPPI Pulping Conference. Atlanta: TAPPI Press, (1983). 13. HOLIK, H. Toward a better understanding of the defibering process. Proc., TAPPI Engineering Conference. Atlanta: TAPPI Press, (1988). 14. DORRIS, G.M., BROUILLETTE, F., DANEAULT, C. Thermogravimetric Determination of Synthetic Polymers in Recycled Pulp Systems and Deposits. Preprints, 6th Research Forum on Recycling, PAP- TAC, Magog, QC, (2001). 15. DOSCH, J.B., SINGH, K.M., STENUF, T.J. Air content of medium and high consistency pulp slurries. Proc., TAPPI Engineering Conference. Atlanta: TAPPI Press, (1986). 16. Proposed Method Tappi T 270 pm-88 (1988). 17. FABRY, B. Etude de la rhéologie des suspensions fibreuses concentrées dans le but d améliorer le recyclage des papiers. PhD. Thesis. Institut National Polytechnique de Grenoble, Grenoble, (1999). 18. KATZ, S., LIEBERGOTT, N., SCALLAN, A.M. A mechanism for the alkali strengthening of mechanical pulps. Tappi J. 64(7): (1981). 19. SCALLAN, A.M. The effect of acidic groups on the swelling of pulps: a review. Tappi J. 66(11):73-75 (1983). 20. KATZ, S., SCALLAN, A.M. Ozone and caustic soda treatments of mechanical pulp. Tappi J. 66(1):85-87 (1983). Résumé: Nous avons déterminé l effet du ph initial de trituration sur le taux de défloconnage des vieux journaux et des vieilles revues en fonction des forces à l œuvre dans un triturateur de laboratoire. Lorsque le ph dépasse 10.5, la diminution de la résistance à l état humide du papier a la plus forte influence sur le taux de défloconnage. Dans la gamme de ph étudiée, la concentration volumique et la viscosité apparente de la suspension de pâte ont peu d effet sur le défloconnage. Finalement, il a été déterminé que, même à ph neutre, le défloconnage d un mélange typique de papier utilisé pour la production de papier journal est très rapide. Reference: BROUILLETTE, F., DANEAULT, C., DORRIS, G.M. Effect of initial repulping ph on the deflaking rate of recovered papers. Pulp & Paper Canada 104(7): T (July 2003). Paper presented at the 6th Research Forum on Recycling in Magog, QC, Canada. Not to be reproduced without permission of PAPTAC. Manuscript received August 9, Revised manuscript approved for publication by the Review Panel October 1, Keywords: WASTE PAPERS, RECYCLING, REPULPING, PH, DEFLAKING, NEWSPRINT, PULPERS. Pulp & Paper Canada T :7 (2003) 41