EcoFracSmart: A new stock-preparation process for testliner

EcoFracSmart: A new stock-preparation process for testliner Karl-Johan Grundström, Marco F. C. Lucisano 1, Bernt Bergström, Annika Bjärestrand STFI-Packforsk Noss Box 5604 Box 20 SE11486 Stockholm (Sweden) SE60102 Norrköping (Sweden) Eva Larsson Hans-Olof Lindh Roland Selin Owe Sänneskog Lyckeby Industrial Smurfit Kappa Lagamill JLR Pulping Systems Läckeby Water Degebergav. 60-20 Box 43 Svartviksslingan 11 Box1146 SE29191 Kristianstad SE28521 Markaryd SE16738 Bromma SE22105 Lund ABSTRACT This work summarizes a proof-of-concept for EcoFracSmart, a process technology toolbox for stock preparation of recovered fibres. We developed and demonstrated a new energy-efficient and flexible process for paper products based on a variety of recycled fibre sources, ranging from old corrugated containers (OCC) to household magazines, newsprint and office waste. The EcoSmart concept implies the extraction of impurities as early as possible in the repulping process, when particles are still large and easy to separate. Thereafter, hydrocyclone fractionation separates fibres in sub-streams with different quality, bonding properties and flexibility. Thereby energy intensive refining can be limited to the fibres that need it. The availability of more homogeneous fibre flows after fractionation allowed tuned additions of wet-end starch. We installed a multifunctional screen in a Swedish papermill and estimated that the energy consumption could be reduced by 35 40 kwh/t as electricity and at least 130 kwh/t as dispersion steam, when comparing with the conventional process line. Integration of both processes was tested at pilot scale. Trials showed that the new concept could produce testliner with properties that are technically and commercially relevant, e.g. high compression strength and burst index and excellent surface roughness. Hydrocyclone fractionation could isolate 60% of the fibre flow, which required no upgrade prior to papermaking. We concluded that the new concept, in conjunction with tuned addition of starch, has a large potential for stock preparation of recycled fibres. Its positive effects being particularly remarkable in respect to energy savings, simplified process layout and improved product quality. INTRODUCTION Recovered fibres are one of the main raw materials for modern papermaking: the European goal for paper recycling was 54.6% in 2005. A new and more ambitious strategic goal has been set to 66% by 2010, which translates into an additional 10.000.000 tonnes per year [1]. Traditionally, recovered fibres have been used for lower paper grades, but the increased recycling rates require that processes are developed to expand the applicability recycled raw materials to fields previously reserved to virgin fibres. This trend is also likely to imply that papermakers will have to cope with significantly more heterogeneous raw materials and recovered fibres. Moreover, today s industrial practice of using energy-intensive unit operations (such as dispersion and refining) to upgrade the properties of recovered fibres will have to be altered to meet the new environmental, technical and economical demands. Stock preparation systems in a secondary fibre line are composed of a sequence of unit operations with a two-fold aim: (i) fibres recovery to optimize the process yield and (ii) separation of contaminants to obtain the purity required for the final product. Once a fibre flow is extracted from the raw material, its papermaking properties are often upgraded by energy-intensive process steps, such as dispersion and refining [2-4]. The traditional strategy to balance yield and purity requirements has been to design processes with several stages of separation as well as recirculation 1 To whom correspondence should be addressed.

of large portions of the rejected materials. This leads to complex systems with high energy requirements. Additionally, the unit operations of stock preparation have secondary effects on the quality of the raw materials: fibres are affected by the mechanical action of pumps, screens and refiners, with consequent fibre development, fibre length reduction and fines generation. Whereas the fibre development has a direct positive influence on the strength properties of paper, the presence of fines influences the runnability of the papermachine, with reduced drainage efficiency and increased energy consumption in drying. This project aimed at providing a proof-of-concept of new process technology for stock preparation of recovered fibres. We have worked with two processes, which use modern separation equipment to optimize the fibre yield in a recovered fibre line. The governing idea in the EcoSmart-process is to separate impurities in the earliest process stages [5-6]. Here contaminants are still large in size and therefore easy to handle. This implies that the number and complexity of the subsequent cleaning and separation stages can be reduced. Moreover, the fibre quality is preserved because the number of fibre damaging operations is reduced. The process has two key components: a pulper and reject handling system and a multifunctional screen. The new pulper works both for defibration and as a screen at consistencies up to 19%. The design is such that a significant portion of the contaminants can be separated directly in the pulper, which implies a reduction of the demands on the following unit operations. The multifunctional screen is a modified pressure screen which combines four functions in one machine component: (i) defibration, (ii) separation of heavy contaminants, (iii) coarse screening and (iv) fine screening [6]. The main feature of this screen is an acceleration zone at the entrance to the screening area. In this area the screen performs a mild form of defibration based on shear forces and characterized both by lower fibre damage and decreased size reduction of the contaminants, which simplifies later removal. The separation of heavy contaminants is also carried out in the entrance section of the screen. Simultaneously with slushing, the fibre suspension is accelerated before entering the screening zone. The gentle mechanical handling of fibres preserves fibre length and minimizes the creation of fines. Additionally, the high consistency used (approximately 4%) reduces the demand of thickening equipment. A first assessment of the energy efficiency and performance of the proposed screening section showed reduced energy consumption by 16 33% over a conventional stock preparation line [4]. The difference increased to 77 87% or 210 kwh/t when the energy consumption in dispersion was accounted for. Fractionation is a unit operation aiming at separating fibres in two or more flows with different properties. The aim is either to eliminate impurities and undesired materials from the fibre flow or to act as a first stage of differentiated treatment or usage of the fractions (e.g. in multiply and stratified paper products). Fractionation can be performed in systems based on screens or hydrocyclones [7, 8]. In industrial fractionation using screening machinery, the fibre flow is split in two fractions depending on the fibres capacity to pass through the opening of a barrier, normally a plate or cylinder with holes or slots [9]. The effect of pressure screen fractionation on the physical properties of the fractions has not been conclusively investigated. Whereas some authors report little or no difference between the properties of the short fibre fraction and of the long fibre fraction [4], other sources present results suggesting a significant differentiation of the fibre properties [10]. However, it appears clear that fractionating a fibre furnish to limit energy-intensive refining operations to a substream has positive results on the energy economy of the process both in single-ply products and in multi-ply qualities [11-14]. Hydrocyclones are static machines in which the fibre flow is set in rapid rotation and separation takes place because of the centrifugal forces which move high density particles outwards, while light particles move inwards [8, 15]. In an ad-hoc designed hydrocyclone system, the split upon fractionation is not linked to the outer geometry of the fibres but rather to parameters such as fibre wall thickness, specific area and bonding ability with larger repercussions on the papermaking properties of the fractions [4, 16-18]. The governing hypothesis of our study is that hydrocyclone fractionation of recovered furnishes can be performed so that the fine fraction is ready for papermaking, with no additional unit operations. Although the coarse fraction contains both thick-wall fibres and sand, inorganic material and non-bonding fines, earlier laboratory-scale studies suggest that its properties can be upgraded by refining [4]. The availability of significantly more homogeneous fibre flows after fractionation opens for the possibility of tuned additions of paper chemicals to the system. In this investigation we selected and tested both commercial starch qualities and development products and we studied the response of the system to different dosage strategies.

This work relates to (i) a full scale installation of a new screening system in a Swedish recycled paper mill producing mainly testliner; and (ii) to pilot scale trials of hydrocyclone fractionation and separate handling of the fractions. EXPERIMENTAL This report presents a proof of concept of a new stock preparation process for secondary fibres from mixtures of OCC and consumer packaging. A multifunctional screen was installed in the stock preparation line of a Swedish recovered paper mill in parallel with the ordinary screening line. The process flow bifurcated after the screening operation: 1. The accept flow from the screen was transported in suspension to the EuroFEX pilot plant at STFI- Packforsk where it was fractionated with hydrocyclones. The coarse fraction was refined on a double-disc refiner and two-ply linerboard was produced with fine fraction in the top ply and mixed fine and coarse fraction in the bottom ply. We name this sequence hydrocyclone trial. 2. The accept flow from the screen passed through the stock preparation line at the papermill and testliner was produced according to the standard mill operation but without dispersion. Testliner was sent to the pilot plant where it was repulped, fractionated in a pressure screen and the long-fibre fraction was refined. Twoply linerboard was produced as in 1. with the short fibre fraction in the top ply and mixed short-fibre and long-fibre fraction in the bottom ply. We name this sequence reference trial. Screening - Full Scale Installation The new coarse screening concept was tested in the stock preparation system of a Swedish recycled paper mill producing approximately 100.000 t/y testliner. The original stock preparation system was designed and installed in the middle of the 1980 s (Figure 1). Water Press Pulper Dispersion Figure 1. Schematic flow sheet of the screening section before the installation of the new coarse screen [4]. We installed the new screen in a by-pass line and modified the flow connections to obtain reliable and safe runnability (Figure 2). No dispersion unit was necessary in the long-fibre line when the new screen was in use. The stock preparation system has machine refiners in front of the paper machine for both the top-ply and the bottom-ply. Cleaning was performed in the short circulation in the approach system of the paper machine. The production capacity of the new system was sufficient to feed one of the two papermachines in the mill. The screen installation was equipped with magnetic flow meters, pressure gauges and regulating valves, which were connected to the data acquisition and control system of the papermill. The rotor speed could be changed continuously by automatic frequency control. The velocity of the rotor tip was 24 m/s at 800 rpm. The number of revolutions and the electrical power could be sampled. Hydrocyclone Fractionation Pilot Trials The practice of fractionation is relatively common in the production of testliner from recovered fibres. Therefore, we compared hydrocyclone fractionation and refining of the coarse fraction (hydrocyclone trial, Figure 3) with the standard technology of pressure screen fractionation with refining of the long-fibre fraction (reference trial, Figure 4)

The focus of this series of experiments was on the balance between energy economy, selection of fibre source and product properties. Water To pilot plant Pulper Dump New Screen Chest To PM Reject Chest Figure 2. Schematic flow chart for the installation of the new coarse screen. The sampling position for the furnish used for pilot-scale fractionation trials is indicated. Raw Materials The raw material used in the paper mill was a mixture of OCC (Old Corrugated Containers) and consumer packages. The furnish contains significant quantities of plastic in addition to short fibres from hardwood and chemimechanical pulps. The raw material for the pilot trials was the accept flow from the multifunctional screen, which was transported in suspension. For the reference trial, we reslushed testliner that was produced at the papermill in parallel with pulp sampling for the hydrocyclone trial. We evaluated the effect of starch addition on the physical and surface properties of testliner. Here we tested a commercial product (starch A) and a novel experimental product (starch X), both from Lyckeby Industrial. The starch dosage program for the pilot trials had to be set considering the constraints in flow connections in the pilot system and the need for contact time and appropriate mixing between the furnish and the starch. Here we report results relative to starch addition to all fractions at the exit from the furnish storage tanks. As to the retention aids, we selected the same chemical systems as in the full scale papermill operation. Hydrocyclone Fractionation and Cleaning Hydrocyclone fractionation was performed in a two stage cascaded hydrocyclone system (Noss Radiclone, AM80-F, Figure 3). The coarse fraction from the second stage was fed into a two-stage cascaded cleaner system (Noss Radiclone, AM80-G). The system separated the feed flow in three fractions: (i) a fine fraction, extracted as accept flow from the first stage, dewatered over a disc filter, (ii) a coarse fraction, extracted as accept flow from the third stage, drained over a curved screen and (iii) a reject from the fourth stage. The split between the fractions was 60/40 and the fourth stage rejected 6% of the feed flow, containing large quantities of sand and heavy rejects. The fibre loss from the system was approximately 3% and consisted mainly of low quality fibre material. Screen fractionation Fractionation in the reference trial was performed with a Hooper PSV2100 pressure screen equipped with a MacroFlow screen basket with slot width 0.15 mm. The target split was 40:60 for the short fibre fraction and long fibre fraction respectively (Figure 4). When pressure screen fractionation was completed, the final value of the fraction split was 48:52 respectively for the short fibre fraction and long fibre fraction.

39 SR Q = 1400 l/min C = 0.66% P = 9.2 kg/min Q = 950 l/min C = 0.38% P = 3.6 kg/min Filtrate Fine fraction C = 3.3% P = 3.7 kg/min 72 SR Fractionating hydrocyclones Cleaners Coarse fraction C = 3.0% P = 2.5 kg/min 14 SR Q = 34 l/min Sand C = 1.3% Heavy reject P = 0.44 kg/min Figure 3. Schematic of the pilot scale fractionation, cleaning and drainage system. The illustration reports the final values of flow (Q), fibre concentration (C), production (P) and drainage resistance. P = 14 kg/min 34 SR Q = 700 l/min C = 0.9% C = 0.028% Short fibre fraction C = 3.9% P = 6 kg/min 59 SR Q = 1400 l/min C = 1.1% C = 0.39% Q = 700 l/min C = 1.3% Long fibre fraction C = 4.1% P = 8 kg/min 19 SR Figure 4. Schematic of the pilot scale screen fractionation and drainage system. The illustration reports the final values of flow (Q), fibre concentration (C), production (P) and drainage resistance. Refining The coarse fraction in the fractionation trial and the long fibre fraction in the reference trial were refined in a double disc machine refiner with fillings with specific edge length 231 km/s. The refiner was run at a constant specific edge load of 0.5 Ws/m. Two levels of net refining energy were used: 70 kwh/t and 120 kwh/t. Papermaking After stock preparation, we produced a two-ply linerboard on the FEX pilot papermachine, which has been described in [19]. The top ply (50 g/m 2 ) was formed in the twin-wire roll forming unit with a wrap angle of 77. The bottom ply (75 g/m 2 ) was formed on a fourdrinier table. The machine speed was kept constant at 350 m/min. The papermachine is equipped with three press nips: a double-felted roll press and two inverted single-felted shoe presses. The linear loads in the press nips were kept constant at 60 kn/m, 600 kn/m (tilt 1.25) and 800 kn/m (tilt 2.0). In a subset of the trial points we heated the third press nip to 180 C, thereby testing the effects of impulse technology [20]. Paper was reeled up in the wet state after the third press and dried off-line.

RESULTS The entire stock preparation process, consisting of a new screening system and hydrocyclone fractionation was tested with mixed industrial scale and pilot scale trials. We slushed OCC and consumer packages in a Swedish papermill equipped with the new multifunctional screen. Thereupon the accept flow was either transported to the fractionation pilot plant or pumped forward through the stock preparation line at the mill. In the former case (hydrocyclone trial), the furnish was fractionated and cleaned with hydrocyclones and the coarse fraction was refined before production of a two-ply testliner on a pilot papermachine. In the latter (reference trial), testliner was produced according to the standard mill routines but without dispersion. Testliner was shipped to our pilot plant in Stockholm where it was slushed, fractionated in a pressure screen and the long-fibre fraction was refined before forming a two-ply testliner. The papermaking trial was completed with a study of the effects of starch dosage. Furnish Characterization Standard laboratory handsheets formed with white water recirculation were used to characterize the physical properties of the furnish at different stages in the stock preparation processes. The raw material during the test runs consisted of a mixture of OCC (Old Corrugated Containers) and consumer packaging. New screening system The furnish was sampled at different positions in the stock preparation line during the test trials and the results compared with the conventional stock preparation line. We formed laboratory handsheets with the different furnishes and measured both strength properties and surface roughness (Table 1). A comparison of the new stock preparation system with the conventional one shows that the drainage resistance ( SR) and the content of impurities (Somerville 0.15 mm) at the machine chest were lower with the new screening section in operation. The removal of contaminants of the new screen was 60% at a fibre reject rate of 15%. Additionally, the machine operators reported that the papermachine runnability was excellent when the multifunctional screen was in operation. Handsheets made with pulp from the multifunctional screen had higher level of ash, which has a negative impact on the strength properties. As a rule of thumb, a 1% increase in ash content reduces the tensile index by 1 unit. One reason for the higher ash content was the absence of thickening stages after the coarse screen. In fact, the accept consistency from the screen was above 4% and no thickening was needed. Therefore, ashes that normally follow the filtrate water upon thickening were pumped forward in the process. The tensile strength properties and the burst index were somewhat lower when using the new screen. Yet, the system was consuming less energy for stock preparation. The z-directional properties, measured as Scott Bond showed on the contrary a slight improvement. The lower level of contaminants in the accept flow from the new screen was mirrored in the large improvement in surface roughness, measured according to Bendtsen. Pilot scale refining, fibre length, ashes and impurities In the following we present results of our pilot scale studies comparing a stock preparation strategy based on hydrocyclone fractionation with a more conventional approach based on pressure screens. Initially, it shall be noted that all measured properties were better for the feed pulp used in the reference trial than for the furnish of the hydrocyclone trial. This was expected since the raw material of the reference trial was repulped liner and it had already passed through the stock preparation system at the papermill. The average fibre length of the incoming furnish was approximately 1.4 mm. In both trials we split the furnish in two fractions: a long/coarse fibre fraction and in a short/fine fibre fraction. The former was refined at two levels of net refining energy: 70 kwh/t and 120 kwh/t. The refining operation was successful in both trials with little fibre shortening. The ash content is one important parameter to consider when evaluating the papermaking potential of a fibre furnish (Figure 5). Pilot scale fractionation and cleaning reduced the ash content from 11% to approximately 8%, whereas the commercial testliner samples had 9% ashes. Pressure screen fractionation enriched the short fibre fraction with ash. The low ash content in the long fibre fraction indicated that the filtrate from the curved screen was probably rich in ashes. On the other hand, the ash retention of the disc filter was higher.

Table 1. Strength properties, surface roughness and ash content for laboratory handsheet made of pulps before and after the installation of the multifunctional screen. Conventional stock preparation Multifunctional screen Before refiner Machine chest Screen accept Machine chest Drainage [ SR] 45 51 40 46 Fibre length [mm] 1.37 1.35 1.37 1.33 Contaminants [%] n.a. 1.17 1.09 0.7 Tensile index [Nm/g] 43.5 45.2 40.4 43.2 Stretch at break [%] 2.58 2.66 2.3 2.3 TEA-index [mj/g] 810 866 653 705 Burst index [kpag/m 2 ] 2.38 2.56 1.82 2.0 Scott-Bond [J/m 2 ] 265 294 270 333 Surface roughness 908 972 406 352 Bendtsen [ml/min] Ash content [%] 10.7 11.9 13.2 12.9 14 1,8 Ash content [%] 12 10 8 6 4 Impurities [%] 1,6 1,4 1,2 1,0 0,8 0,6 0,4 2 0,2 0 Feed Fine fraction Coarse fractionpaper samples 0,0 Feed Fine fraction Unrefined 70 kwh/t 120 kwh/t Coarse fraction Figure 5. Ash contents (900 C, left) and Somerville impurities (right) in testliner samples and in furnish fractions. The fine and coarse fractions were sampled after the disc filter and after the curved screen, respectively. Black = hydrocyclone trial; White = reference trial; Grey = commercial testliner sample. The Somerville impurities in the feed furnish for the fractionation trial were more that twice as high as in the reference trial (Figure 5). This is due to the fact that the repulped testliner had already passed the stock preparation system at the papermill. The pressure screen used in the reference trial separated impurities with high efficiency. The performance of the hydrocyclone system was also satisfactory. Yet a number of light plastic impurities and stickies were left in the accept flow. Refining of the coarse fraction reduced the frequency of non-defibrated material, thus reducing the Somerville count. A comparison of the physical properties of the fractionated furnishes confirmed that pressure screens and hydrocyclones fractionated fibres according to different properties, which is exemplified here with tensile index (Figure 6). Hydrocyclones generated a large difference in strength properties between a strong fine fraction (60% of the accepted mass) and a relatively weak coarse fraction (40% of the accepted mass). On the other hand, the difference in strength properties between the short-fibre fraction and the long-fibre fraction after the pressure screen was small or negligible. The pressure screen performed somewhat better at generating differences in Gurley air resistance and Bendtsen surface roughness between the long fibre fraction and the short fibre fraction (Figure 6). Yet, the difference between the fine fraction and the coarse fraction was larger for the hydrocyclone system. Refining the coarse fraction in the hydrocyclone trial upgraded its properties to a level that was comparable to or exceeded that of the fine fraction.

60 1000 Tensile index [Nm/g] 50 40 30 20 10 Surface roughness [ml/min] 800 600 400 200 0 Feed Fine fraction Unrefined 70 kwh/t 120 kwh/t Coarse fraction 0 Feed Fine fraction Unrefined 70 kwh/t 120 kwh/t Coarse fraction Figure 6. Tensile index of handsheets (left) and surface roughness (right) formed with white water recirculation. The fine and coarse fractions were sampled after the disc filter and after the coarse screen, respectively. Black = hydrocyclone trial; White = reference trial. Paper Properties Each of the pilot trials was concluded with a full day s papermaking at pilot scale. Here we produced a two-ply testliner mimicking one of the products of the papermill. The 50 g/m 2 top ply was formed on the twin-wire roll former with either fine fraction or short-fibre fraction. The 75 g/m 2 bottom ply contained either a mixture of fine and refined coarse fraction or a mixture of short-fibre and refined long-fibre fraction and it was formed on the fourdrinier table. The total ash content of the pilot testliner was relatively stable around 8% during both papermaking trials, which simplifies the direct comparison between the properties of the different trial points. The ash content in the commercial samples was 1% higher, which is expected to give some comparative disadvantage as to the strength properties of the commercial testliner. According to a rule of thumb, one percent unit in ash content influences the tensile and burst indices by 3% and the compression index by 2%. Values for the commercial testliner samples have been adjusted accordingly. The density of all trial points without impulse technology was rather constant between 720 kg/m 3 and 740 kg/m 3. Yet, increasing the temperature of the third press to 180 C increased the sheet density to approximately 760 kg/m 3. The compression strength and the burst strength are two parameters of high commercial interest for testliner. Hydrocyclone fractionation and refining of the coarse fraction with starch addition had higher SCT index than the reference (Figure 7). Addition of both starch qualities (A and X) gave some additional improvement in compression strength, yet no clear difference between the effect of the starch qualities could be observed. We observed no effect of impulse drying on the compression index. In all cases, the compression index for testliner from the pilot papermachine was very high with respect to the commercial testliner and to the characteristics of the furnish. As to burst index, the reference trial resulted in higher values than for the corresponding points in the hydrocyclone trial (Figure 8). Yet, this might partially be connected to the fact that the feed stock in the reference trial had higher burst strength. An appropriate value for the burst index of testliner II is 2.5 kpam 2 /g [21]. The addition of any of the two starch qualities studied increased the burst index of testliner with 4 5%. We could observe no clear difference between starch A and starch X. Three of the four impulse pressed testliners had higher burst strength than the corresponding qualities pressed at room temperature. All pilot samples had higher burst strength and compression strength than the commercial ones, even after the values of the latter had been adjusted to compensate for the different ash content. One reason for the difference between the pilot and the commercial testliner could be related to the consistency in the headboxes at pilot scale being lower. Headbox consistency has normally a significant effect on paper properties. The z-strength of the hydrocyclone samples, measured as Scott Bond, was always higher than the corresponding value for the reference samples (Figure 9). Addition of starch improved Scott Bond by as much as 10%. Impulse

pressing at 180 C was particularly effective on the samples of the hydrocyclone trial. All pilot samples had higher ScottBond than the commercial testliner. 30 Impulse drying 180 C 25 SCT index [Nm/g] 20 15 10 5 0 120 kwh/t 70 kwh/t 120 kwh/t 70 kwh/t 70 kwh/t 120 kwh/t No starch Starch X Starch A Commercial testliner Figure 7. SCT index of pilot and commercial testliner (adjusted for the different ash content). Energy data are relative to refining of the coarse/long-fibre fraction. Black = hydrocyclone trial; White = reference trial; Grey = commercial testliner sample. The surface roughness of all samples from the hydrocyclone trial was lower than for the corresponding points in the reference trial (Figure 10). Impulse pressing contributed to somewhat better surface roughness. This is a direct consequence of the fibre split in hydrocyclone fractionation, whereupon thin-walled flexible fibres accumulate in the fine fraction. 3,0 Impulse drying 180 C 2,5 Burst index [kpam 2 /g] 2,0 1,5 1,0 0,5 0,0 120 kwh/t 70 kwh/t 120 kwh/t 70 kwh/t 70 kwh/t 120 kwh/t No starch Starch X Starch A Commercial testliner Figure 8. Burst index of pilot and commercial testliner (adjusted for the different ash content). Energy data are relative to refining of the coarse/long-fibre fraction. Black = hydrocyclone trial; White = reference trial; Grey = commercial testliner sample. The commercial testliner had much higher Bendtsen roughness than the pilot samples. Yet, the differences in pressing and drying between the pilot scale experiments and the industrial conditions make direct comparisons of surface roughness virtually impossible.

500 Impulse drying 180 C 400 Scott Bond [J/m 2 ] 300 200 100 0 120 kwh/t 70 kwh/t 120 kwh/t 70 kwh/t 70 kwh/t 120 kwh/t No starch Starch X Starch A Commercial testliner Figure 9. Scott Bond of pilot and commercial testliner (adjusted for the different ash content). Energy data are relative to refining of the coarse/long-fibre fraction. Black = hydrocyclone trial; White = reference trial; Grey = commercial testliner sample. 1800 1600 Surface roughness [ml/min] 1400 1200 1000 800 600 400 Impulse drying 180 C 200 0 120 kwh/t 70 kwh/t 120 kwh/t 70 kwh/t 70 kwh/t 120 kwh/t No starch Starch X Starch A Commercial testliner Figure 10. Surface roughness (Bendtsen) of pilot and commercial testliner. Energy data are relative to refining of the coarse/long-fibre fraction. Black = hydrocyclone trial; White = reference trial; Grey = commercial testliner sample. Energy consumption The energy consumption of the industrial stock preparation line was registered both with the conventional system with dispersion and with the new system based on the multifunctional screen. Table 2 illustrates the electric power for the two alternative ways of running the stock preparation system. In the conventional system, the installed electric power of the two additional screening stages and of the dispersion unit was more than 500 kw. The largest electric power consumer was the dispersion unit, which also needs steam to heat the pulp suspension. The energy provided as steam was approximately 300 kwh/t for the heat-up of the suspension from 20 up to 80 C at 35 % consistency. Some other pieces of equipment, for instance the pump and mixer in the dispersion chest were switched off when the new screening system was in operation. All other process stages that were not affected by the two different ways of running the stock preparation system were disregarded in the energy analysis. The installed electric power was reduced by 300 kw, when the multifunctional screen was in operation.

Table 2. Installed electric power in different units with the conventional stock preparation system and with the multifunctional screen in operation. Conventional system [kw] Multifunctional screen [kw] Screen 1 121 - Screen 2 24 - Dispersion unit 375 - Multifunctional screen - 219 Total 520 219 Closing the dispersion unit eliminated the need for significant quantities of steam. Approximately 40% of the production was dispersed when producing testliner with the conventional stock preparation line. A steam saving corresponding to 250 300 kwh/t for 36.000 t/y (40% of the yearly production) implies a further saving of 10.000 MWh/y. Table 3 summarizes the energy consumption for the stock preparation of a recovered waste paper furnish for production of testliner and compares with an energy mapping of the conventional system [4]. The introduction of the new screen did not imply any increased energy demand upon refining. The runnability on the paper machine was maintained even without the dispersion of the top ply furnish. The results of the energy evaluation indicate that it was possible to reduce the energy consumption significantly when introducing the new multifunctional pressure screen. Table 3. Energy consumption in the stock preparation from pulping up to the completely refined pulp ready for papermaking at the mill. Conventional system [kwh/t] Multifunctional screen [kwh/t] Energy consumption exclusive steam in disperser 320 ± 40 280 ± 40 Energy consumption inclusive steam in disperser 450 ± 40 - The energy consumption in fractionation is primarily connected with pump energy. The installed power for the pumps used around the pilot fractionation system was 45 kw. The specific energy consumption [kwh/t] depends on both flow (Q [m 3 /s]) and concentration (C [kg/m 3 ]) according to: P e P = Q C The energy consumption for industrial fractionating hydrocyclone systems is generally 25 30 kwh/t. Since the furnish of both pilot scale trials had passed a cleaners system, the energy needed for cleaning has been disregarded in this comparison of the hydrocyclone fractionation concept with pressure screen fractionation. The energy needed for screen fractionation is the additive sum of the pump energy around the screen and of the energy for the operation of the screen. The pump effect was calculated according to: P = Q H g ρ µ where Q is the flow [m 3 /s], H is the pump head [m], g is the acceleration of gravity [m 2 /s], ρ is the density of the fluid [kg/m 3 ] and µ is the efficiency of the pump [-]. An effect-meter was installed on the screen motor. The measurement showed that the hydrocyclone operation required approximately 10 kwh/t less than the pressure screen. Yet the largest difference between the two processes lies in the difference in energy upon refining. In fact the split in the hydrocyclone process was 60:40 (fine fraction:coarse fraction), whereas the split with pressure screening was 48:52 (short fibre fraction:long fibre fraction). Thereby the energy required to refine the coarse/long fibre fraction was 16 24 kwh/t higher in the conventional process.

Table 4. Effective energy consumption in fractionation and refining in the hydrocyclone trial and reference trial. In the reference trial, the table does not include the effective energy consumption after screening in the stock preparation line. Effective energy consumption [kwh/t] Hydrocyclone system Fractionating hydrocyclones 30 Refining (70 kwh/t) 28 Refining (120 kwh/t) 48 Pressure-screen system Fractionation screen 40 Refining (70 kwh/t) 34 Refining (120 kwh/t) 62 DISCUSSION AND CONCLUSIONS This report summarizes the main results of a proof-of-concept for EcoFracSmart as a stock preparation process for secondary fibres. The project aimed at developing, applying and demonstrating a new energy-efficient and flexible process for paper products based on recycled fibres. The flexibility of the process allows its application to a variety of recycled fibre sources, ranging from old corrugated containers (OCC) to de-inked pulp (DIP) from household magazines, newsprint and office waste. The basic idea of the process is to extract impurities as early as possible in the stock preparation process, when impurity particles are still large and easy to separate. This implies a compact and energy efficient system with less machine components than traditional alternatives. Moreover the quality of the resulting pulp is higher because of a more careful fibre treatment. The strength of the hydrocyclone fractionation concept lies in its capacity of making optimal use of the potential of the fibre raw material, thereby reaching quality standards that are today in the domain of virgin fibres only. The process uses hydrocyclone fractionation to separate fibres in substreams with different potentials as to quality, bonding properties and flexibility. The fine fraction (earlywood fibres and bonding fines) has such a high quality that it can be used for papermaking without any further processing. Depending on its composition, the coarse fraction (latewood fibres, fillers and non-bonding fines) can be refined separately or separated from the process. Additionally, hydrocyclone fractionation of fibre raw materials opens for selective usage of paper chemicals specifically designed for their activity on fibre fractions with different nature. We installed one multifunctional screen of innovative design in the stock preparation line of a Swedish recovered paper mill. We compared the energy consumption for stock preparation for a system based on the new screen with the conventional stock preparation line. The former implied a more compact process solution with fewer process stages. We could show that both the electric power consumption and the total energy consumption were reduced when the multifunctional screen was in operation. In particular, the energy-intensive dispersion unit could be bypassed. We estimated that the installed power could be reduced by 300 kw as electricity and 1200 kw as dispersion steam. Papermachine runnability was considered satisfactory and the multifunctional screen produced an accept flow with low level of contaminants, which had positive effects on the surface roughness of the products. We observed a higher ash content when the multifunctional screen was operated, likely due to the new handling of process waters. This occurrence, together with the choice of a raw material mix of lower quality, contributed to somewhat reduced tensile and burst properties. Secondary fibres were repulped and screened in the new coarse screen and transported to STFI-Packforsk for continued processing based on a four-stage cascaded hydrocyclone system (fine fraction:coarse fraction = 60:40). The coarse fraction was refined and used for the bottom ply of a 125 g/m 2 linerboard, whereas the fine fraction was used directly in both the top ply and the bottom ply. The physical properties of the fine fraction were so high that no additional treatment was needed before pumping to the papermachine approach system. The properties of the coarse fraction could be upgraded with refining to values comparable with those of the fine fraction. The energy-consuming

refining operation could therefore be limited to 40% of the total accepted mass and act on a more homogeneous fibre mixture, which is likely to have positive effects on its energy efficiency. Testliner was produced at pilot scale with or without dosage of two starch qualities. For the purpose of comparison, commercial testliner from the papermill was repulped in the pilot system and fractionated in a pressure screen. The long-fibre fraction was refined and used for the bottom ply, whereas the shortfibre fraction was sent directly to both the top and the bottom plies. The results of this series of pilot trials show that our new stock preparation concept could be used to produce a twoply linerboard according to the specifications. The properties of the two-ply testliner produced with fractionated furnish were comparable to or better than those of the reference trial, in spite of the lower properties of the fibre furnish fed to the system. Indeed, most physical properties of the testliner were much higher than the corresponding values for commercial testliner. This suggests that the process as a whole can be used to produce testliner with properties that are technically and commercially relevant. In particular, we could produce testliner with high compression strength and burst index and excellent surface roughness. One of two starch qualities was added upon pumping from the storage towers. It was easier to adsorb starch on the furnish of the reference trial, then on the furnish fractions produced with hydrocyclones. In spite of the somewhat low degree of adsorption, both starch qualities resulted in strength improvements. The two starch qualities produced comparable improvements and the results offered no clear indication as to ranking between the two products. An interesting aspect which should be investigated with ad-hoc designed experiments is the strategy of starch addition to the individual fractions. It is likely that adding starch to a specific fibre flow could improve its efficiency if an appropriate fraction with low fines content could be used for starch addition. The comparison between the high values of most strength parameters measured in the pilot and the corresponding data for commercial testliner suggests that the process should have a very wide optimization window. In particular, it is likely that higher energy and process economy could be obtained if the split between the fine fraction and the coarse fraction could be shifted towards a somewhat coarser fine fraction comprising a larger percentage of the accepted mass. This could imply that a smaller portion of the total mass would require energy-intensive refining, with consequent energy savings. Additionally the coarse fibre fraction would be composed of a yet more homogeneous mixture of fibres, which might result in even higher energy efficiency. This study was performed using a four-stage cascaded hydrocyclone system. A full scale installation would probably require additional stages to optimize product quality and process economy. In particular, the fine fraction contains plastic particles in quantities that would justify the installation of inverted hydrocyclones for removal of light reject. This would also contribute to some drainage of the fine fraction. Additionally, a more precise investigation of the nature of the rejected material after the fourth hydrocyclone stage should be performed to determine whether it would be economically interesting to recover some of the fibre material that has been rejected in the present configuration. ACKNOWLEDGEMENTS The authors thank the partner companies in the EcoFracSmart project consortium. Financial support from STEM, The Swedish Energy Agency, is gratefully acknowledged. References 1. CEPI (2006) European declaration on paper recycling 2006-2010, CEPI and ERPA: Brussels. 2. Cathie, K. (1994) Secondary fibre treatment, Pira Reviews of Pulp and Paper Technology, Pira: Leatherhead. 3. McKinney, R. W. J. ed. (1995) Technology of paper recycling, Blackie Academic & Professional: Glasgow. 4. Grundström, K.-J. and Hagberg, M. (2003) Slutrapport Ny energieffektiv och kvalitetshöjande process för tillverkning av returfiberbaserade produkter, STFI: Stockholm (In Swedish). 5. Selin, R. (2000), Device for separating contaminants from pulp fibre suspensions, U.S.A. Patent 6,131,742. 6. Selin, R. (2004), Pulper for producing paper pulp from waste paper, International Patent Application No. 2004/0011490 A1.

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