HIGH-SPEED DISPERSING BETWEEN TWO DEINKING LOOPS: ARE THERE OPTIMISATION POSSIBILITIES?

Transcription

1 HIGH-SPEED DISPERSING BETWEEN TWO DEINKING LOOPS: ARE THERE OPTIMISATION POSSIBILITIES? Benjamin Fabry and Bruno Carré Centre Technique du Papier Domaine Universitaire BP Grenoble Cedex 9 France ABSTRACT Since the end of the 8's, high-speed has became a basic treatment in multi-loop deinking process and is now included in all modern deinking facilities. Conventionally, this stage is implemented between the 2 deinking loops in order to improve the ink detachment and to reduce the speck contamination. Since the last decade, refining theory has been transferred to high-speed technology to give more scientific approach to this operation. In the same period and based on CTP pilot plant investigations, a simple and useful method based on fragmentation theory and volume energy approach was developed and is proposed in the present paper. This approach was then used for wood containing grade to determine the incidence of parameters including consistency, energy application and introduction of peroxide bleaching chemicals on the final properties of the deinked pulps (optical properties as well as morphological fibre characteristics) after post-flotation. Thanks to this new overall approach, a simple optimisation of high-speed is then proposed allowing to obtain the best deinked pulp properties with the lowest drawbacks associated to a reduced energy consumption. INTRODUCTION Early work on the dispersion of contaminants began in 1946 by a group of American companies with the intent of recycling kraft bitumen paper. One of the solution proposed consisted of high temperature treatment (15 C) in a device such as a disk refiner [1]. This first "thermo-mechanical treatment" can be viewed as the precursor of present units. Hot-dispersion in denking plant has started to be used since It consists of mechanical treatment at high temperature and high consistency with the use of appropriate techniques to transfer energy to pulp. Dispersion homogeneises the stock and residual ink particles which have not been detached from the fibres are so finely dispersed as to no longer appear as undesirable specks [2,3]. High-speed and low-speed kneading, the two most common technologies used for dispersion has also been proposed to simultaneously disperse the specks and as a high-consistency mixer for bleaching the pulp [4]. Several papers proposed the implementation of hot-dispersion between two flotation stages or between flotation and washing stages in 1989 [5-8]. Since this period, dispersion becomes a basic treatment in multi-loop deinking processes. The main application of treatment are: dispersion of contaminants (such as hot-melts, stickies, specks and residual ink), ink detachment prior to deinking, bleaching (thermal pre-treatment, mixing of chemicals, bleaching reactor), microbiological decontamination, changes in fibre properties. These various effects and application are well illustrated in [9]. Since the last decade, whereas more scientific approaches based on refining theory are now proposed (and will be described later on), CTP has developed an useful and simple method based on fragmentation theory. The principle of this approach will be described in the first part of this paper illustrated by two examples. The second part will focus on more practical examples to characterise the incidence of high-speed treatment when implemented between two deinking loops for wood containing grade. The parameters taken into account are energy application, consistency and introduction of peroxide bleaching chemicals. Finally some practical recommendations will be given in order to obtain the best deinked pulp properties with the lowest drawbacks associated with a reduction in energy consumption.

2 VOLUME ENERGY APPROACH Introduction to theoretical approach of and fragmentation The easiest way to characterize mechanical treatment is to consider the specific energy consumption (energy per unit of dry material expressed in kwh/t). This approach has the advantage to give directly economic consideration but it cannot be used for phenomena characterization. Since few years, more fundamental approaches are under investigation that include application of refining theory: - Brecht approach [1] assumes that refining treatment occurs at the edges of refiner bars with pulp response to refining being identical whatever the device considered. The specific edge load (SEL) is defined as the effective power input per total edge length per second (L B ). In 24, Rusinsky et al. [11,12] extended this approach to pyramidal plates. They assume that ink detachment occurs primarily at the edges of elements and that the magnitude of the force applied at the edge of high-speed disperser element is the critical factor determining ink detachment. - Miles and May approach [13] allows to distinguish the number of bar impacts and the specific energy per impact imposed during refining. In 21, Rusinsky et al. [14] applied this approach for high-speed disperser and observed that ink detachment is maximized at high specific energies per impact whereas reducing the number of impact imparted to a fibre minimized ink redeposition. Within the same period where refining theory was applied to, CTP investigated stage by an other approach that consist to consider volume energy consumption. This concept has been already successfully applied to have an overall and composite information for pulping phenomena [15]: the knowledge of volume energy consumption allowed determining defibering level whatever the pulper device (LC pulper, pulper, Drum pulper), the consistency and the pulping time. Based on this idea, the following equations can be written: Em Specific energy consumption (kwh/t) E Energy consumption (kwh) E E Em = and Ev = where M pulp Mass of o.d. pulp treated (T) M pulp V tot 3 Volume energy (kwh/m ) Ev 3 V Total volume of pulp treated (kwh/m ) tot M pulp By combining the two equations, Ev = Em Vtot If we suppose as a first approximation that the density of pulp suspension corresponds to water density (ρ=1 kg/m 3 =1 T/m 3 ), then Vtot Then, M tot = with ρ water M pulp M pulp Ev = Em = Vtot M tot M tot ρ water ρ water Total mass treated (pulp + Density of water water) That gives with the previous approximation Ev = Em Cm with Cm the mass consistency as a fraction (-) The different phenomena that are controlled during stage can be viewed as 'solid' fragmentation (speck fragmentation, ink particle fragmentation, ink detachment, etc.). The fragmentation phenomena can be described by two main parameters: - The forces involved during. The overall forces could be described by energy consideration even if we are not able to determine the energy applied to solid particles [16]. By using energy consideration, it should be possible to take into account both the intensity of the mechanical forces and the retention time inside the disperser. - The 'solid' particle strength. Cohesive forces that linked the 'solid' particle each other could describe this. The cohesive forces could be affected by external parameters such as temperature and physico-chemical parameters.

3 Raw material and methods During this part, exactly the same recovered paper composition is used. It corresponds to a 6% ONP/ 4% OMG mixture that has been artificially aged in an oven at 6 C during 3 days. This raw material has been pulped in pilot drum at 18% consistency during 25 minutes at 45 C. A conventional deinking chemistry was added at the beginning of this step:.7% NaOH (pure product),.7% Serfax MT9 soap (dry commercial product), 2.% silicate (commercial product) and.7% peroxide (pure product). After pulping stage, the pulp is submitted to drum coarse screening (φ 6 mm) followed by Kadant-Lamort CH3 screening (//.2). Thickening or not in vacuum filter and screw press are performed in order to cover a consistency range between 2.2 and 33%. The corresponding pulp pass then through low-speed-kneader or high-speed disperser with various applied specific energy at the same production (based on dried material). For each conditions, optical properties are measured on entire and hyperwashed pulp pads. In the following paragraphs, two examples of the interest in using volume energy approach is given. Results Ink fragmentation Direct measurement of ERIC just after treatment is not rigorous to determine ink fragmentation level. Indeed, a part of free ink particles are removed during thickening stage. In order to avoid this drawback, the water coming from the various thickening stages is reintroduced in the pulp after in order to make the comparison with the same ink content. By this way, changes in ERIC values can be attributed only to changes in ink particle size. These values are reported in the following figures. By considering the ERIC values on entire pulp as a function of specific energy consumption (Figure 1), it appears: - For a given mechanical treatment and a given consistency, the higher the specific energy consumption (and therefore the greater the intensity of the mechanical forces involved), the higher the ERIC values, i.e., the higher the ink fragmentation. - For a given specific energy consumption and a given mechanical treatment, the higher the consistency, the higher the ink fragmentation. In that case, the increase in consistency corresponds also to an increase in intensity of the mechanical forces. - For a given consistency and a given specific energy consumption, low-speed kneading induces higher ink fragmentation than high-speed. Note that no peroxide bleaching chemicals is added during lowspeed kneading. 25 Low 25 Low ERIC on entire pulp (ppm) Specific energy consumption (kwh/t) High-speed Dispersing Low-speed Kneading Medium High Medium High ERIC on entire pulp (ppm) Estimated volume energy Cm.Em (kwh/m 3 ) High-speed Dispersing Low-speed Kneading Medium High Medium High Figure 1 Ink fragmentation as a function of specific energy consumption for various conditions Figure 2 Ink fragmentation as a function of estimated volume energy consumption for various conditions By applying volume energy approach to describe ink fragmentation occurring into the two units, it appears that a characteristic curve can be obtained giving an overall approach for ink fragmentation (Figure 2). This overall approach is valid whatever the applied mechanical treatment (low-speed kneading or high-speed for various consistency and specific energy ranges). Besides, a power law can then describe ink fragmentation by a first order kinetic. The correlation between experimental and estimated data given by the power law is illustrated in Figure 3.

4 α Cm Em ERICentire pulp = ERIC ERIC e with ERIC ERIC obtained for infinite mechanical treatment ERIC ERIC ERIC at the inlet of mechanical treatment unit α kinetic constant Calculated data ERIC entire pulp = e Cm.Em r² =.88 Ink fragmentation modelling through volume energy approach ERIC entire pulp in ppm Cm : mass consistency expressed as fraction (-) ERIC and (ERIC -ERIC ) are mainly function of the initial ink content as well as the resistive strength of ink particles. These parameters depend also on thermal and physico-chemical parameters Ink detachment/ink redeposition After the screening sequence, the ERIC value on hyperwashed pulp is 285 ppm. Note that this relative high value can be attributed to the recovered paper conditioning (heated at 6 C during 3 days). For the highspeed treatment, it is possible to decrease the ERIC value from 285 to 21 ppm even if the recovered paper composition corresponds to condition where poor ink detachment is expected (artificial aging). This ink detachment improvement corresponds then to 2% ISO brightness gain on fibre fraction. By considering the ERIC values as a function of estimated volume energy (Figure 4), a characteristic curve is obtained: - For estimated volume energy lower than 2 kwh/m 3, the higher the volume energy, the lower the ERIC value on hyperwashed pulp that corresponds to conditions where ink detachment is predominant versus ink redeposition. 5 Em : specific energy consumption (kwh/t) Experimental data Figure 3 ERIC on hyperwashed pulp (ppm) Modelling of ink fragmentation in unit by first order kinetic High-speed Dispersing Low Medium High 1 Ink detachment and ink redeposition 5 Estimated volume energy Cm.Em (kwh/m 3 ) Figure 4 Ink fragmentation as a function of estimated volume energy consumption for various conditions - For estimated volume energy higher than 2 kwh/m 3, the higher the energy, the higher the ERIC value on hyperwashed pulp, i.e., ink redeposition is predominant versus ink detachment. A parallel with the incidence of pulping time on ink detachment can be brought to the fore. This competition between ink detachment and ink redeposition has been indeed reported as a function of pulping time [17]. Discussions The mechanical treatments performed in low-speed kneader or high-speed disperser are responsible for fragmentation phenomena such as ink fragmentation, ink detachment, speck fragmentation. The knowledge of specific energy consumption is not sufficient to describe them and only gives information from economical point of view. On the other hand, the knowledge of the estimated volume energy consumption given by the product between mass consistency and specific energy appears as a useful and easy approach. This approach is also present in Miles and May theory to describe the number of impact imparted to a fibre. The volume energy approach is then able to give an overall description of the treatment: a characteristic curve can be obtained whatever the consistency, the specific energy, and the unit. Examples were given in the previous paragraphs for the application of treatment just after pulping followed by a fine screening step. In the following paragraphs, this approach will be used when unit is implemented between two deinking loops as conventionally done in the present deinking lines.

5 APPLICATION TO HIGH-SPEED DISPERSING BETWEEN TWO DEINKING LOOPS Raw material and methods The recovered paper composition used for this set of trial corresponds to a 6% ONP/ 4% OMG mixture that has been artificially aged in an oven at 6 C during 3 days. This raw material blend corresponds then to - 6% ONP printed in coldest offset - 4% OMG representing 9.3% of heatset offset on LWC papers, 18.7% of heatset-offset on SC papers, 7% of rotogravure on LWC and 7% of rotogravure on SC papers. The general flowsheet of the trials is illustrated in Figure 5. The first deinking loop is common whatever the trials and is performed at pilot scale. Drum pulping is performed at 18% consistency during 2 minutes at 45 C with conventional chemistry (.7% NaOH, 2% silicate,.7% peroxide and.7% soap). Coarse screening is the performed in the screen part of the drum (φ 6 mm) followed by fine screening (// 2/1). Pilot flotation is then done in Verticell type flotation with an inlet consistency of 1.2%. Medium consistency thickening is performed in vacuum filter to reach about 22% consistency. For the trials corresponding to only MC, the steam is introduced before the pulp goes through the highspeed disperser illustrated in Figure 6. For the trials at higher consistency (), additional thickening in screw press is used to reach approximately 33% consistency. The pulp is then submitted to steam followed or not by peroxide bleaching liquor introduction (1% NaOH, 2.5% silicate, 1% peroxide) before going to high-speed disperser. In all the cases, post-flotation is performed before and after high-speed treatment in Voith 25L flotation cell (7 min, corresponding to 2% air ratio). During, various level of energy was applied by adjusting the gap between rotor and stator. Note that mechanical treatment performed in neutral condition can be representative of newsprint production and mechanical treatment in presence of peroxide bleaching liquor can be viewed as a reference for a deinking line producing magazine grade. For each sampling points, optical properties have been determined on pads from entire and hyperwashed pulps (Brightness, ERIC). Speck contamination has been measured on handsheet from hyperwashed pulp by SIMPALAB analyser (scanning method). Fibre morphological characteristics are determined by MorFi analyser before and after thickening, after steam introduction, after peroxide bleaching liquor introduction and after high-speed disperser treatments. Such analysis give us information related to fibre length (mean size and length distribution), fibre width (mean width and width distribution), coarseness, curl, kinked fibre content, broken ends, macro-fibrillation, fine elements. In addition, freeness was determined through Schopper Riegler ( SR). MC thickening in vacuum filter steam thickening in screw press steam Fine screening Kadant Lamort CH3 // 2/1 Pilot flotation Steam + chemical 1% NaOH 2.5% Silicate 1% Peroxide Figure 5 General flowsheet with the corresponding sampling Figure 6 CTP high-speed disperser

6 Incidence of thickening, steam introduction and peroxide bleaching chemicals In order to quantify the effect of thickening, steam introduction and/or introduction of peroxide bleaching chemicals and to separate the possible effect of such stages from the effect of high-speed disperser itself, controls have been performed at the inlet and outlet of each treatments. Incidence on optical properties According to the overall data collected, it has been observed that - Thickening and steam introduction does not induce significant effect on o Ink fragmentation, ink detachment/redeposition for the studied conditions o Brightness on both entire and hyperwashed pulp o Total speck contamination even if a slight speck fragmentation was observed during thickening in the screw press at the highest outlet consistencies. o Ink removal by flotation, nor on flotation yield. - Introduction of peroxide bleaching chemicals induces o No significant effect on ink fragmentation, on ink detachment/redeposition and on ink removal by flotation o An increase in brightness (+2% ISO on fibre fraction and +1.5% on post-floated pulp) due to chemical action of the bleaching liquor o A slightly lower speck contamination after post-flotation o An increase in post-flotation yield that can be related to the presence of silicate. Incidence on fibre morphological properties Thickening, steam introduction and/or bleaching do not induce significant effect on fibre length, fibre width, macrofibril content. Only curl and kink generation are sensitive to these treatments as illustrated in the two following figures: - During thickening in screw press, mechanical forces are applied on fibres (effect comparable to a lowspeed kneader, but with lower intensity) and disturb the fibre structure creating a kink. Note that these kinks can be viewed as weak point in fibre structure and application of more intense forces in subsequent process can be responsible for fibre cutting. Besides, as the action of screw press is similar to low-speed kneader, but with a lower extent, a slight increase in fibre curl is observed. - Introduction of steam and peroxide bleaching liquor are responsible for an increase in kink generation and in fibre curl. Softening of fibre due to heat or presence of alkali source could explain the increase in curl. An increase in temperature or the presence of alkali source could be also responsible for more intense disturbance of fibre structure creating some additional kinks Kinked fibre amount (%) Incidence of thickening consistency, steam and introduction of peroxide bleaching chemicals Without steam With steam Fibre curl (%) Incidence of thickening consistency, steam and introduction of peroxide bleaching chemicals Without steam With steam 5 (%) Figure 7 With steam + P chemicals Incidence of thickening, steam, peroxide bleaching on kinked fibre amount 6.5 (%) With steam + P chemicals Figure 8 Incidence of thickening, steam, peroxide bleaching on fibre curl

7 Incidence of high-speed on the efficiency of post-deinking As demonstrated previously, high-speed treatment is responsible for fragmentation phenomena that cannot be described by the knowledge of the specific energy applied that only gives us information on running costs. As shown previously, the knowledge of the estimated volume energy (given by the product between mass consistency expressed as a fraction and the specific energy consumption) corresponds to a promising approach to have an overall description of the mechanical phenomena involved in terms of optical properties. To simplify the analysis, this approach will be used in the next paragraphs for the analysis of the various properties. The data corresponding to kwh/m 3 corresponds to the pulp at the inlet of high-speed that is also submitted to post-flotation. Ink detachment and brightness on fibre fraction High-speed allows to detach ink particles from fibres corresponding to a decrease of ERIC value on hyperwashed pulp from 15 to 1 ppm depending on the high-speed parameters. In first approximation, a characteristic curve can describe this phenomenon as illustrated in Figure 9 (black symbol). Whatever the highspeed conditions: the higher the estimated volume energy, the lower the ERIC on hyperwashed pulp, i.e. the better the ink detachment. For the studied parameters, no ink redeposition is observed that can be related to the low ink content load in the high-speed disperser (45 ppm on whole pulp at inlet). The introduction of peroxide bleaching chemical is responsible for a slight ink detachment improvement, but this effect remains low in comparison to mechanical parameters. ERIC on hyperwashed pulp (ppm) Estimated volume energy Cm.Em (kwh/m 3 ) Brightness on hyperwashed pulp (% ISO) White symbol brightness Black symbol ERIC MC + P chemicals Figure 9 Ink detachment and brightness of fibre fraction for various conditions Besides, the characteristic curve seems to reach a plateau: application of energy higher than 1 kwh/m 3 does not induce significant improvement. Note that 1 kwh/m 3 represents 5 kwh/t and 33 kwh/t for respectively 2 and 3% consistency. The improvement in ink detachment is then responsible for an increase in brightness as illustrated in Figure 9 (white symbol). The brightness gain represents 2% ISO brightness if is performed without peroxide bleaching chemicals. On the other hand and as expected, the presence of bleaching liquor allows additional brightness gain for the same ink content on fibres (5% ISO brightness gain). Ink removal and brightness of floated pulp After the first deinking loop, the ERIC value on whole pulp was around 45 ppm. It corresponds to poor ink removal mainly attributed to the fact that 1% of the recovered papers have been artificially aged (very harsh conditions). As mentioned previously, steam introduction and/or introduction of peroxide bleaching liquor does not induce significant difference in terms of ERIC. However, as soon as post-flotation is implemented (without any high-speed treatment), the ink removal allows to reduce the ERIC to 28 ppm. When implementing high-speed treatment between the two deinking loops, an increase in estimated volume energy application is responsible for a decrease in ink removal efficiency illustrated in Figure 1 (black ERIC on floated pulp (ppm) Estimated volume energy Cm.Em (kwh/m 3 ) Brightness on floated pulp (% ISO) White symbol brightness Black symbol ERIC MC + P chemicals Figure 1 Ink content and brightness after flotation for various conditions

8 symbol) by an increase in ERIC on whole pulp. Even if ink detachment has been performed during high-speed stage (Figure 9), the ink fragmentation occurring during this stage is responsible for a decrease in ink removal by flotation. The relationship between ink fragmentation and ink removal by flotation is clearly illustrated in Figure 11. This negative relationship between ink fragmentation and ink removal has been already observed when increasing the pulping time and/or increasing the pulping consistency [18]. The decrease in ink removal efficiency has then direct consequence on brightness of floated pulp: the more intense the mechanical treatment (increase in Em.Cm), the lower the brightness of floated pulp (Figure 1 with white ERIC on floated pulp (ppm) Increase in estimated volume energy ERIC after high-speed (ppm) Ink content after flotation versus ink fragmentation during high-speed MC Figure 11 Ink removal versus ink fragmentation symbol). Besides, for the same high-speed intensity, the presence of peroxide bleaching chemical allows logically to improve the brightness of floated pulp: even if the ink content of floated pulp is the same, the bleaching action of peroxide on fibres (Figure 9) allows to improve brightness of the whole floated pulp. Final speck content Speck contamination after post-flotation is grandly determined by the initial speck contamination at the inlet of flotation cell as illustrated in Figure 13, i.e. after the condition. The other optical properties, speck contamination after post-flotation can be described by the estimated volume energy applied during high-speed as illustrated in Figure 12. Indeed, it is a direct consequence of a fragmentation phenomenon induced by the high-speed stage: the higher the estimated volume energy (through an increase in specific energy and/or in consistency), the lower the speck contamination. As for ink detachment, a plateau in the characteristic curve appears: for estimated volume energy higher than 2 kwh/m 3, any increase in applied energy does not induce significant gain in terms of final speck contamination. This 2 kwh/m 3 corresponds to 1 kwh/t and 66 kwh/t for respectively 2 and 3% consistency. Speck contamination - floated pulp (mm²/m²) Estimated volume energy Cm.Em (kwh/m 3 ) Figure 12 Incidence of highspeed on final speck content after postflotation MC Final speck content after flotation for various conditions Speck contamination - floated pulp (mm²/m²) Speck contamination after (mm²/m²) Figure 13 Incidence of highspeed on final speck content after postflotation MC Speck contamination after flotation vs speck contamination after

9 Incidence of high-speed on fibres properties Freeness Freeness expressed as Schopper Riegler ( SR) can also be related to the estimated volume energy consumption as represented in Figure 14: the higher the Cm.Em, the higher the SR. This characteristic curve is independent of mechanical and chemical parameters in a first approximation. For practical reason and as SR is directly related to the estimated volume energy, the following figures represent the fibre morphological characteristics as a function of SR (related to runnability on paper machine) instead of the estimated volume energy. Fibre length and width, fine generation Mechanical action induced by high-speed disperser can induce some fragmentation phenomena such as fibre cutting (reduction in fibre length) and fine generation as illustrated in Figure 15 where a direct correlation can be observed as a function of SR (directly related to the estimated volume energy consumption - Figure 14). As a characteristic curve can be obtained whatever the conditions (consistency, specific energy consumption, introduction of peroxide bleaching chemicals), it can be concluded that the observed phenomena are mainly due to fragmentation (mechanical aspect). Significant changes in these fibre morphological characteristics are only observed for the highest estimated volume energy (>2 kwh/m 3 ) that corresponds to specific energy consumption greater than conventional specific energy applied during high-speed treatment. SR Mean fibre length (mm) Estimated volume energy Em.Cm (kwh/m 3 ) Incidence of highspeed on freeness MC Figure 14 Schopper Rieggler for various conditions conventional Em range SR Fine content (based on area) % Black symbol fibre length White symbol Fine content MC Figure 15 Fibre length and fine content as a function of parameters The generation of fines during treatment has also been observed during industrial trials [19] for a deinking line producing market pulp and can have some consequences on the process yield if the mechanical treatment is applied before the deinking loop, particularly when washing is present where fines removal occurs. Note that this behaviour can also be encountered for flotation with a lower extent. Fibre curl and kinked fibre As mentioned previously, thickening, steam and introduction of peroxide bleaching chemicals are responsible for changes in kinked fibre amount and fibre curl. In order to characterize what is due to high-speed treatment from these effects, it is necessary to work in relative values to determine the incidence of treatment itself and in absolute value to determine the overall effects. Regarding kinked fibre amount, - Without introduction of peroxide chemicals, SR (and therefore the estimated volume energy) can describe the relative kinked fibre (Figure 16): an increase in SR is responsible for an increase in kinked fibre amount that is relevant of creation of weak point within fibre structure due to application of mechanical forces. Note that this effect is significant only for the highest energy applied that are out of range from conventional ones. On the other hand, in absolute value, the kinked fibre amount is mainly function of initial pre-treatment, i.e. the thickening stage in the screw press (Figure 17) - In presence of peroxide bleaching chemicals, the opposite tendency than previously is observed: an increase in SR (and therefore an increase in Cm.Em) is responsible for a decrease in kinked fibre amount.

10 The weak point that are created during thickening or due to the introduction of chemicals should be broken due to the application of the intense mechanical forces during high-speed. Relative kinked fibre amount (-) conventional Em range SR Kink development during treatment MC Figure 16 Relative kinked fibre amount as a function of freeness for various conditions Kinked fibre amount (%) SR Absolute kinked fibre amount (pre-treatment + ) MC Figure 17 Absolute kinked fibre amount as a function of freeness for various conditions Regarding fibre curl development that can have an interest for bulk property, - Without peroxide bleaching chemicals, mechanical treatment in the high-speed disperser does not induce fibre curl changes (Figure 18). However, by considering the absolute value (Figure 19), it appears clearly that this property is mainly function of the pre-treatment in the screw-press. Indeed, it is well known that low-speed kneader (action similar to screw-press but with more intense mechanical forces) is more adapted to develop fibre curl [2]. - In presence of peroxide bleaching chemicals, an increase in SR is responsible for a decrease in fibre curl. The gain observed after thickening, steam and peroxide bleaching chemicals is strongly reduced by considering the high-speed treatment (Figure 19). Relative fibre curl (-) conventional Em range SR Figure 18 Curl development during treatment MC Relative fibre curl as a function of freeness for various conditions Fibre curl (%) SR Absolute fibre curl (pre-treatment + ) MC Figure 19 Absolute fibre curl as a function of freeness for various conditions Discussions During this study, 6% ONP/4% OMG mixture has been 1% artificially aged in an oven at 6 C during 3 days that corresponds to very harsh conditions responsible for poor ink detachment, high speck content, high ink fragmentation and poor ink removal. This recovered paper blend has been subjected to conventional first deinking loop and then fed to high-speed stage at different consistencies, with and without the introduction of peroxide bleaching chemicals. The main results are summarized in Figure 2 as a function of estimated volume energy (product between specific energy consumption and mass consistency). This approach allows characterizing the different phenomena involved during including ink and speck fragmentation (and their removal by post-deinking), ink detachment and fibre degradation by a unique characteristic curve. The high-speed treatment allows improvement of ink detachment (ERIC on hyperwashed pulp from 15 to 1 ppm when increasing the estimated volume energy). However, there is no significant gain if estimated

11 volume energy is higher than 1 kwh/m 3 corresponding to 5 and 33 kwh/t for respectively 2 and 3% consistency. Even if ink detachment improvement can be obtained, the ink removal by flotation is not improved. After the first deinking loop, the ERIC value corresponds to 45 ppm. If additional stages of flotation are added (without treatment), it is possible to remove part of the ink particles and to reach 28 ppm. Whatever the applied parameters, an increase in ERIC after post-flotation is observed in comparison to the situation where post-deinking is directly implemented after thickening. Indeed, high-speed treatment is responsible for ink fragmentation that has a definitive negative impact on the final ERIC values after flotation (as well as the corresponding brightness). However, even if this degradation occurs, the final ERIC is lower than what can be observed after a first deinking loop if the estimated volume energy is lower than 4 kwh/m 3 that corresponds to 2 and 132 kwh/t for respectively 2 and 3% consistency (unconventional energy application). The main advantage of high-speed is related to speck fragmentation allowing to remove them by flotation (thanks to more suitable size) and to disperse them to a non-visible size range. An increase in estimated volume energy allows then reducing the speck contamination after post-flotation. However, no significant gain is observed for estimated volume energy greater than 2 kwh/m 3 corresponding to 1 and 66 kwh/t for respectively 2 and 3% consistency. This limit of 2 kwh/m 3 corresponds also to energy from which fibre degradation starts to be significant. The presence of peroxide bleaching chemicals does not induce significant changes in terms of ERIC, specks or ERIC on fibre or specks contamination Ink content after flotation Ink detachment Specks Estimated volume energy Cm.Em (kwh/m 3 ) ERIC after flotation on whole pulp White symbol ERIC on fibre fraction (ppm) Grey symbol ERIC on floated pulp (ppm) Black symbol Speck contamination after flotation (mm²/m²) MC + P chemicals Figure 2 Summary of the main effect of fibre characteristics in first approximation. The only advantage of the presence of peroxide bleaching chemicals is here the improvement in brightness of fibre fraction and of floated pulp. In order to control the stage, a reduction in applied energy is required in order to reduce the ink fragmentation and to reduce the negative impact on flotation efficiency. However, it is necessary to apply a sufficient energy level to induce ink detachment (but a stagnant value is reached at 1 kwh/m 3 ) as well as a reduction in speck contamination (stagnation levels commence at 2 kwh/m 3 ). Dispersing parameters should therefore take into account these antagonists phenomena according to the inlet of this stage: - If cleanliness is good after the first deinking loop, it is not necessary to put high energy level during. A net reduction of it should be recommended as the opposite tendency will increase the final ink content of floated pulp (and the corresponding brightness will decrease) - If cleanliness is 'poor' (generally due to raw material composition variations and/or summer effect), the energy to be applied must be adapted to the target of the mill in order to reduce the associated drawbacks (increase in ink content and decrease in brightness of floated pulp). In any case, the estimated volume energy should not exceed 2 kwh/m 3 as there is no significant gain in comparison to the additional energy applied. Note that 2 kwh/m 3 corresponds to 1 and 66 kwh/t for respectively 2 and 3% consistency - In any case, ERIC and speck measurements at the inlet (and/or at the outlet) of high-speed disperser are required for the regulation of running parameters. CONCLUSIONS AND PERSPECTIVES A mixture of 6% ONP / 4% OMG has been artificially aged in order to produce a pulp difficult to deinking. Various conditions have been applied after conventional first deinking loop. The parameters investigated were consistency, energy and introduction of peroxide bleaching chemicals. The knowledge of the estimated volume energy allows describing the fragmentation phenomena involved during high-speed such as ink and speck fragmentation, ink detachment and fibre degradation. Even if high speed allows ink detachment improvement, the ink particle fragmentation involves a decrease in quality of post-floated pulp. For this reason, a reduction in energy application must be applied in order to improve the final brightness of the deinked pulp. The main advantage of is then a reduction in size of specks particles but a plateau is reached for energy greater than 2 kwh/m 3 that corresponds to 1 and 66 kwh/t for respectively 2 and 3% consistencies. For

12 greater volume energy application, fibre degradation starts also to occur. The high-speed stage needs therefore to be optimised with the application of energy strictly required for speck fragmentation. Indeed, any energy application will be responsible for a degradation of the final optical pulp properties. ACKNOWLEDGEMENTS The authors wish to thank CTP and CTPi members as well as all the companies having supported this project. The authors wish also to acknowledge Pierre Crémon, Claude Guilmot, Max Guillet, Gilbert Chatel and Jean de Gracia for carrying out all the practical experiments. References 1. Higgins J.J. "Use of the asphalt dispersion system on waste paper", TAPPI Journal vol. 43 n 1, January 1959, p Ortner H. "Dispersion as a mean of quality improvement of deinked stocks", Paper Technology and Industry vol. 19 n 5, May-June 1978, p Lamort de Gail B., Hétier F. "Use of hot-dispersion system in treatment of secondary fibres" Paper Technology and Industry vol. 19 n 2, February 1978, p Cropper M.A. "Advances in deinking and its use in improving quality of newsprint" TAPPI Papermakers Conference, Atlanta, April 1976, p Ortner H., Fisher S. "Der Einsatz der Dispergierung zur Qualitätsverbesserung von deinkten Stoffen", EUCEPA Symposium, Ljubljana, October 1989, p Linck E., Matzke W., Siewert W. "Systemüberlegungen beim Deinking von Altpapier", EUCEPA Symposium, Ljubljana, October 1989, p Johansson O., Steffner S. "Field Experience gained in the new deinking plant at Hylte Bruks AB", EUCEPA Symposium, Ljubljana, October Galland G., Vernac Y., Bernard E., Alibaux P. "Amélioration de la blancheur des pâtes désencrées", Ljubljana, October Kumar S., Fabry B., Carré B., Cochaux A., Julien Saint Amand F., Galland G. "Past, present and future of dispersion and kneading", 8th CTP-PTS Advanced Training Course on Deinking, Grenoble, May Brecht W. "A method for the comparative evaluation of bar-equipped beating devices", Tappi Journal vol. 5 n 8, August 1967, p Rusinsky F., Zhao H., Bennington C.P.J. "Characterizing ink dispersion in newsprint deinking operations using specific energy load", 7 th PAPTAC Research Forum on Recycling, Québec, September 24, p Rusinsky F., Bennington C.P.J. "Toner particle comminution in office paper dispersion", 7 th PAPTAC Research Forum on Recycling, Québec, September 24, p Miles K.B., May W.D. "The flow of pulp in chip refiners", Journal of Pulp and Paper Science vol. 16 n 2, February 199, p Rusinsky F., Wang M-H., Bennington C.P.J. "Characterizing in newsprint deinking operations", 7 th PAPTAC Research Forum on Recycling, Magog, 1-4 October 21, p Fabry B., Carré B. "A new approach to characterize pulping processes for deinking", 11 th PTS/CTP Deinking Symposium, Leipzig, 27-3 April 24, Paper n Blazy P., Yvon D., Jdid E.A. "Fragmentation: Généralités-théorie", Technique de l'ingénieur, Génie des Procédés, Opérations Unitaires, vol. J4 17. Fabry B., Roux J-C., Carré B. "Characterization of friction during pulping: an interesting tool to achieve good deinking", Journal of Pulp and Paper Science vol. 27 n 8, August 21, p Fabry B., Carré B., Crémon P. " Pulping optimisation: effect of pulping parameters on defibering, ink detachment and ink removal", 6 th PAPTAC Research Forum on Recycling, Magog, 1-4 October 21, p Galland G., Carré B., Cochaux A., Fabry B., Julien Saint Amand F. "Improved recycling technology for high quality recycled pulps and new uses", Pulp Paper Conference, Helsinki, 5-7 June Mc Kinney R.W.J., Roberts M. "The effects of kneading and dispersion on fibre curl", TAPPI Recycling Symposium, Chicago, April 1997, p