PULP EXTRUSIONS INCORPORATING SLUDGE FROM THE PULP AND PAPER INDUSTRY C. Tim Scott Dan Klingenberg General Engineer Associate Professor USDA Forest Service Department of Chemical Engineering Forest Products Laboratory University of Wisconsin-Madison One Gifford Pinchot Drive 1415 Engineering Drive Madison, WI 53705 Madison, WI 53706 John Simonsen Associate Professor Department of Forest Products Forest Research Laboratory 105 Oregon State University Corvallis, OR 97331 Stefan Zauscher Assistant Professor Department of Paper Science and Engineering State University of New York One Forestry Drive Syracuse, NY ABSTRACT Pulp extrusion at ultra-high consistency ( 30% solids) has been shown to be a viable process for converting recovered paper, wastepaper, and papermill residues into solid sheets and profiles. We have previously shown that extrusion of ultra-high consistency pulps and model sludges requires the addition of a water-soluble polymer (WSP) to alter the rheological properties of the pulp such that an homogenous pulp paste can be formed and extruded. This paper discusses our attempts to evaluate the viability of extruding a variety of papermill sludges. Several sludges were acquired from various mills and classified by ash content, contaminant level, and biological activity. Various blends of sludge, newsprint (ONP), and WSP were prepared and extruded into continuous sheets. The sheets were then cut into strips for consolidation by press drying. These strips were further cut into coupons for tensile testing. The results showed that a variety of sludges could be successfully extruded with adequate tensile strength. However, the presence of contaminants (e.g., chips, staples, plastics, silica) and interfering chemistries posed some difftculties in processing these residues. INTRODUCTION The paper industry generates approximately 4 million tons of sludge each year, much of which contains a significant amount of high quality fiber [1]. However, the composition of sludges varies greatly from mill to mill and primarily depends on the pulping processes used. This fact poses a significant barrier to the utilization of sludge as a raw material in alternative processing technologies. Sludges that are derived from virgin pulping operations, for instance, contain considerable fiber content. However, they also can contain significant quantities of contaminants such as wood chips, shives, and grit. On the other hand, sludges derived from recycling mills generally contain less fiber and more filler but also contain a variety of contaminants, including plastics, staples, and grit. Unfortunately, it is difficult to characterize and quantify the vast diversity of papermill sludges that are generated. However, as more recycling operations are established and the demand for recycled paper increases, there will surely be an increase in the volume of sludge generated by the paper industry. Several methods have been devised to divert papermill sludge from landfills, including combustion and landspreading [1]. However, while any alternative method other than landfilling is desirable, none has yet been developed to produce a value-added product from sludge. The feasibility of extrusion to utilize these raw materials for the manufacture of panel and profile products, both structural and non-structural, has been demonstrated [2]. The mechanical properties depend on fiber type and fiber concentration of the feed material, the dispersing agent (WSP), and the additives used as well as the type of extrusion process employed. Moderately contaminated, high inorganic content materials have been extruded successfully. The mechanical properties of the densified and dried extrudates have been similar to those of wet-process hardboard. This project extended earlier work by investigating sludges from a variety of industrial sources. In the process of collecting papermill sludges for evaluation, we discovered that the task of characterizing sludge (i.e., solids and ash content, chemical composition, and fiber length distribution) would be much more difficult than we had anticipated. The diversity of methods used by mills to handle residuals and our inability to obtain information on sludge 2000 TAPPI International Environmental Conference & Exhibit / 203
characteristics directly from mills severely limited the scope of this study. We realized that analysis of sludge compositions would be limited to only those characteristics that had a direct impact on the rheology of the compositions for extrusion. These sludge characteristics are described in this paper. EXPERIMENTAL Acquisition and Characterization of Sludges Considerable effort was made to obtain and characterize a variety of mill sludges for this study. At the outset, three basic criteria were established to screen potentially useful sludges: (1) for handling purposes, the collected sludges needed to be completely non-hazardous, both in the collected state and after processing by extrusion; (2) sludges could not contain any biological activity that would accelerate degradation of fiber properties over time; and (3) sludges could not contain significant quantities of contaminants (e.g., staples, paper clips, wood chips. grit). These contaminants were considered to be damaging to the extrusion processing equipment. The following text describes the participating mills and observed characteristics of available sludges (as collected or received). Sludge properties are summarized in Table I. Mill A. Integrated mill for production of paperboard and printing grades, primarily from virgin fiber (kraft and groundwood). This mill incorporates two separate wastewater treatment facilities. Treatment plant A 1 produces a sludge residue accumulated from several combined effluent streams (e.g., digesters, pressure screens, paper machines). These streams are initially mixed then fed to a primary clarifier. Suspended solids are aerated and treated for secondary clarification. Solids from the primary and secondary clarifiers are combined, thickened, and dewatered in a belt press before removal to a landfill. Unfortunately, the residual solids (sludge) contain significant quantities of grit and fiber chips from the kraft digesters. They are also biologically active, rendering them unsuitable for our evaluation. Treatment plant A2 produces a residue that is strictly from groundwood production. The treatment process is similar to that in plant Al, but the fiber content of the sludge is much more consistent (no chips). However, this sludge is biologically active, rendering it unsuitable for our evaluation. Mill B. Recycling mill for production of linerboard and medium, primarily from recovered paper (OCC). Several screening stages and flotation units are integrated to separate usable fiber from non-fibrous materials. This mill is very selective in its choice of recycled grades for pulping, eliminating potential contaminants that would otherwise inhibit processing. Although the fiber quality appears good, the residue contains significant quantities of contaminants, including staples, foam beads, adhesives, tapes, and grit. Several residue streams are combined and dewatered to produce sludge. One of these streams appears to contain a high fiber fraction with few contaminants. An attempt was made to sample this stream before it was combined with other residuals. A large sample was collected and saved for evaluation. This mill provided no information on sludge composition. Mill C. Recycling mill for coated grades containing pre- and post-consumer wastepaper. Sludge was collected from the primary clarifier and sent to the Forest Products Laboratory (FPL) for evaluation. This sludge appeared to contain high levels of filler; the fibrous component was primarily short fibers. The composition of the sludge as received was stated to be 40% solids and 32% ash. This sludge was used in our trials. Mill D. Corrugating medium mill with primarily hardwood semichemical pulp furnish and some recycled kraft. Both primary and secondary sludges are produced in the wastewater treatment facility and dewatered to 30% solids for disposal. The mill has undertaken considerable effort to further separate this residue and recover a fraction suitable for landspreading. We obtained a sample of the separated material for evaluation. The mill also provided an extensive evaluation of sludge composition. Mill E. Kraft linerboard and Neutral Sulfite Semi-Chemical corrugating medium mill. Primary clarification of residuals is followed by secondary aeration. The mill provided extensive confidential documentation on residue composition. The sludge as received appeared to contain high levels of contaminants such as dregs, lime grits, and wood chips in addition to a strong biological essence (even though the material was to have been sampled from the primary stream). These factors rendered the sample unsuitable for evaluation. 204 / TAPPI Proceedings
Table 1. Selected properties of sludges acquired for evaluation Mill Furnish a Treatment Solids (%) Ash (%) A Kraft/ Primary+Secondary 2 2 B OCC/paperboard Screening 24 9.7 C Recovered paper Primary 40 60.3 D HWD Semichemical Primary 35 7.3 E SWD Kraft/Semichemical Primary 2 2 F Recovered paper Primary 38 58.3 G HWD/SWD Kraft Primary+Secondary 2 2 a HWD is hardwood, SWD softwood. Used no yes yes yes no yes no Mill F. Deinking mill for production of tissue and toweling from recycled paper. The sludge appeared to contain high levels of filler and a significant fraction of short fibers and was very similar to the sludge from Mill C. The Mill F sludge was used in our trials. Mill G. Kraft pulp mill for production of hardwood and softwood market pulps. Primary clarification and secondary aeration are used to process residues. The sludge as received contained numerous wood chips and possessed a strong biological essence. This sludge was not used in our study. Control Furnishes Initially, we proposed using a hardboard furnish (defibrator aspen + 15% softwood) as the base furnish. The sludges would then be added to this furnish at levels of 15%, 30%, and 45% for rheology and extrusion trials. As received, the hardboard furnish was very dry (55% solids) and had a high shive content and Canadian Standard Freeness of 790 ml. The furnish was further refined at 20% consistency in a 305mm single disk, laboratory atmospheric refiner with steam injection to reduce the shives and obtain a solids content of 25%. Very little reduction in freeness was obtained (750 ml), but the shive content was visably reduced. An old newsprint (ONP) furnish was prepared by pulping the local newspaper (without inserts) in a high consistency pulper at 15% consistency. The pulp was further dewatered in a bladder press to 27% solids and shredded to marblesized flocs. Ash content was determined to be 4.8%. ONP was used in previous extrusion trials and served as the back-up furnish for this study. An old magazine (OMG) furnish was prepared by pulping National Geographic magazines in a high consistency pulper at 15% consistency. This pulp was further dewatered in a bladder press to obtain a solids content of 33%. Ash content was determined to be 38.5%. OMG was used in a previous study [1] as a model sludge because of its high filler content and high percentage of hardwood (short) fibers. Extrusion Extruders are positive displacement pumps (similar to augers) that rely on the development of very high shear stresses to disperse and mix a feedstock composition and pump it through a die of predetermined cross-sectional shape. In this study, a 32-mm co-rotating twin-screw extruder was used for all trials. A 4 kg pulp/sludge blend (wet basis) was prepared for each extrusion trial. The barrel temperature was fixed at 50 C for all trials. A key to successful extrusion of pulps is the addition of a water-soluble polymer (WSP). These polymers must rapidly hydrate to reduce the viscosity of the pulp and promote dispersion of flocs in the feed section of the extruder. A homogeneous fiber paste is then formed that can be further dispersed in subsequent kneading zones and finally forced through a die. Previously [2-4], we have shown that very small amounts of a certain WSP (sodium-carboxymethylcellulose (CMC)) dramatically reduce the apparent viscosity of the pulps (bowlmixer torque) such that they can be extruded. We know that if the apparent viscosity is too high, the shear forces required to disperse the flocs will be too high and the extruder will jam. If the apparent viscosity is too low, the polymer will only coat the surface of the floc and dispersion will not occur. The judicious selection of WSP for pulp extrusion is therefore critical. 2000 TAPPI International Environmental Conference & Exhibit / 205
Our intent in selecting a typical hardboard furnish as a base pulp for extrusion trials was to produce an extruded board with properties comparable or superior to those of commercial hardboard. We planned to add sludges to the hardboard furnish in various ratios to determine their effect on properties. An attempt was made to characterize the rheological properties of the hardboard using CMC as the polymer. Various addition levels were evaluated and the hardboard furnish was compared with ONP (Figure 1). Unfortunately, the addition of CMC did not dramatically reduce the viscosity of the hardboard furnish even when 10% (dry basis) was added. Based on previous rheology and extrusion evaluations, we know that the apparent viscosity must be very low (at or near a bowlmixer torque of 1 N m) for trouble-free extrusion. Typically, 3% WSP or less should be used to obtain these torque levels. The addition of an OMG pulp in combination with 3% CMC was found to reduce the apparent viscosity to extrudable levels (Figure 2). The addition of filler was previously shown to reduce viscosity [4]. Based on the rheological determinations presented in Figures 1 and 2, we attempted to extrude blends of the hardboard furnish with sludge from Mill C. This sludge was chosen because of its similarity to OMG. A moderately aggressive screw configuration (three kneading zones) was selected for these trials. A 50.8-mm-wide, 6.3-mm-thick slit die was used to generate moderate backpressures in the range of 1000 to 1500kPa. Four trials were made: the first with the hardboard furnish only and the others with various addition levels of Mill C sludge (15 %, 30%, and 45%). All four trials used 6% CMC as the WSP. The steady-state screw torque and die pressures are shown in Figure 3. This figure clearly shows a dramatic reduction in the extrusion process conditions when Mill C sludge is added to the hardboard furnish. The blends of hardboard and Mill C sludge extruded well at very low torques and die pressures and with good dispersion after a single pass. However, the 100% hardboard furnish did not extrude easily. When a second pass was made to improve dispersion, the die pressure doubled and the extrudate no longer had a uniform paste-like consistency. We suspect that there was very little interaction between the CMC and the lignin-rich hardboard fibers. This rendered CMC ineffective in lubricating the fiber surfaces, increased viscosity, and prevented extrusion. We therefore abandoned the hardboard furnish altogether, since we would not be able to obtain a control condition, and switched to an ONP furnish as the basis for subsequent trials. With ONP as the base furnish, several blends with OMG and mill sludges were prepared for extrusion. The extrusion conditions were duplicated from previous trials. For trials with the ONP furnish, 3% CMC was used in all blends. Additionally, a wider (100 mm) and thinner (3 mm) slit die was used. Table II lists the extruded blends with the corresponding steady-state torque and die pressures for both passes through the extruder. The data show a large increase in die pressures compared to the trials with the hardboard/sludge blends. This increase was due to the more restrictive flow through the thinner die. Also, the addition of sludge increased both screw torque and die pressure. In the case of Mill D sludge, the increase in die pressure was significant. A second pass was made to improve dispersion. Both die pressure and screw torque increased on the second pass. All extruded blends appeared well dispersed and were saved for consolidation and measurement of physical properties. Consolidation Several 25-cm-long strips were collected at the die for each blend extruded. Since the extruder was run relatively cold (50 C), essentially no moisture was lost in the process. Therefore, a press-drying technique [5] was devised to consolidate the strips. A hydraulic hot press with a nitrogen accumulator was set up to apply a constant pressure to the strip as it dried. A 10-cm-long strip segment was placed between a nesting of screens (100 mesh) and pressed at 150 C until dry. Five of these segments were pressed per blend. Of these, three were pressed at 70 kpa and one at 350 kpa. A fifth segment was instrumented with three thermocouples to determine drying time at 70 kpa. When the temperature was observed to reach 140 C, the segment was considered dry. The drying times for all blends are listed in Table III. This press-drying technique is similar to that used to densify and dry hardboard. However, it quickly became evident that the extremely high pressures used in hardboard consolidation protocols [6], typically between 3,500 and 7,000 kpa, could not be achieved. Even at 350 kpa, there was excessive squeeze out of the pressed segment. Although density increased substantially between 70 and 350 kpa, nevertheless the targeted densities or that of high density hardboard could not be achieved (Table III). We suspect that this was due to the large reduction in viscosity imparted by the addition of the WSP. However, the presence of filler materials and other unknown WSPs may have contributed as well. 206 / TAPPI Proceedings
Table II. Extrusion conditions for blends of ONP with OMC and Mill sludges Extruded blend 100% ONP 85% ONP + 15% OMG 70% ONP + 30% OMG 55% ONP + 45% OMG Pressure (kpa) 1,100 1,100 1,100 970 Pass 1 Pass 2 Torque Mass flow Pressure Torque Mass flow (N m) (g/s) (kpa) (N m) g/s 13.6 10.2 1,310 19.0 12.4 12.2 9.2 1,310 14.9 10.7 14.9 12.0 1,100 13.6 14.9 15.1 1,175 14.9 85% ONP + 15% Mill C 1,380 17.6 1,590 20.3 12.2 70% ONP + 30% Mill C 1,175 16.3 1,380 17.6 12.2 85% ONP + 15% Mill F 1,311 19.0 1,520 20.3 12.5 70% ONP + 30% Mill F 1,242 14.9 1,660 19.0 70% ONP + 30% Mill B 1,104 13.6 1,175 14.9 13.6 70% ONP + 30% Mill D 1,590 19.0 1,795 20.3 11.7 Table III. Physical and mechanical properties of consolidated extrudate blends Pressed @ 150 C, 70 kpa Pressed @ 150 C, 350 kpa Drying Fail Fail Extruded blend time Density stress MOE Swelling Density stress MOE (s) (g/cc) (MPa) (GPa) (%) (g/cc) (MPa) (GPa) 100% ONP 350 0.58 18.5 3.14 73 0.62 25.2 3.96 85% ONP + 15% OMG 300 0.61 18.5 3.41 71 0.62 19.4 3.07 70% ONP + 30% OMG 275 0.66 17.8 3.60 64 0.72 25.1 4.45 55% ONP + 45% OMG 275 0.62 14.4 2.97 58 0.73 19.8 3.92 85% ONP + 15% Mill C 400 0.64 16.7 3.19 41 0.73 20.1 3.83 70% ONP + 30% Mill C 350 0.64 13.2 3.08 41 0.82 18.9 4.19 85% ONP + 15% Mill F 350 0.58 9.8 2.26 43 0.82 19 3.86 70% ONP + 30% Mill F 300 0.61 6.2 1.85 42 0.80 10.4 2.84 70% ONP + 30% Mill B 450 0.63 19.6 3.70 81 0.68 24.4 4.31 70% ONP + 30% Mill D 400 0.65 17.4 3.14 84 0.70 21.9 3.77 Hardboard [6] Tempered Standard 0.95 31.7 5.10 0.88 25.5 4.30 2000 TAPPI International Environmental Conference & Exhibit / 207
Properties After the segments were press-dried, they were cut into 15-mm-wide coupons for tensile testing. The strips were cut only in the flow direction; each strip yielded six test coupons. All coupons were allowed to equilibrate in a conditioned room at 23 C and 50% RH before testing. The thickness, length, and weight of each coupon were measured and used as input for the data acquisition system. A universal test machine was used to maintain a constant elongation rate of 3 mm/min as load and strain were acquired. Density, fail stress, and modulus of elasticity (MOE) were calculated and tabulated (Table III). Figure. 4 contrasts these values to minimum values typically associated with hardboard [6]. Thickness swell was also measured for the sludge compositions extruded and pressed-dried at 70 kpa. Selected coupons were allowed to soak in water for 24 h. They were then removed from the water and thickness was measured (Table III). Of particular interest is the association of swelling with amount of filler. Sludges from Mills C and F had significant amounts of filler materials (high ash) and low thickness swell. Conversely, sludges from Mills B and D had low amounts of filler (low ash) and high thickness swell. CONCLUSIONS At the outset of this study, we were very enthusiastic about the potential for utilizing papermill sludge as a raw material for pulp extrusion. All previous trials with various grades of recovered paper and wastepaper seemed to indicate that the pulp extrusion process would tolerate just about any paper-based furnish, even with high filler levels. However, as this study has shown, characterization of sludges for extrusion is not trivial, and each sludge is essentially unique to the process(s) from which it was derived. Unfortunately, our three basic criteria for screening sludges (biological activity, excessive contamination, and inertness) eliminated several prospective sludges that appeared to contain significant fiber fractions. Another unexpected barrier was the inability to use the hardboard furnish as our base pulp. Too much water-soluble polymer (WSP) was required to reduce hardboard viscosity for extrusion. This furnish was therefore abandoned and replaced by ONP. Of the four sludges used, all extruded easily in various compositions with ONP and 3% CMC. Screw torques and die pressures were low and not much higher than that of the ONP/OMG control blends. Sludges from Mills C and F had high filler contents (60% ash) and appeared similar to OMG (38% ash). Sludge from Mill B had a high fiber content (9.7% ash), but also contained minute levels of contaminants (staples, stickies, and foam beads). Sludge from Mill D was very consistent and contained a high percentage of fiber shives (7.3% ash). All extruded sludge compositions were consolidated by press drying and tested for tensile strength and thickness swell. Although densification was limited due to viscous flow under pressure, the tensile properties of the 70% ONP/ 30% Mill B sludge blend (pressed at 350 kpa) were similar to that of standard hardboard. The results of this study are encouraging. There is no doubt that some papermill residues can be blended and incorporated into extruded products rather than discarded. Several of the biologically active (secondary) sludges that were evaluated were very consistent and contained high fiber contents. If these sludges could be easily de-activated, so they would neither pose a biological hazard nor degrade fiber properties, they probably could be extruded. Also, several heavily contaminated sludges also have significant fractions of useable fiber. Unfortunately, these contaminants (metal, grit, and wood chips) could damage the extruder. Based on the results of this study and our observations at various mills and wastewater treatment centers, we believe that extruders can process paper-mill residues (primarily sludge) into value-added products. Even the most contaminated residuals have significant fractions of useable fiber. However, the challenge lies in extracting that fiber from the rest of the effluent stream. Unfortunately, most mills combine all of their effluent streams (e.g., digesters, pulpers, pressure screens) for wastewater treatment. If the residual streams that are the most consistent or contain the most useable fiber can be diverted and recovered, they could become a viable fiber resource for pulp extrusion. ACKNOWLEDGMENT The authors would like to thank the National Council of the Paper Industry for Air and Stream Improvement, Inc. (NCASI) for financial support for this study. 208 / TAPPI Proceedings
References 2000 TAPPI International Environmental Conference & Exhibit / 209
Figure 1. Apparent viscosity (torque vs. time) of hardboard and ONP pulps with CMC as measured with a bowlmixer rheometer. Figure 2. Apparent viscosity of hardboard and OMG blends with 3% CMC. 210 / TAPPI Proceedings
Figure 3. Extrusion conditions for trials with hardboard/sludge blends with 6% CMC. Figure 4. Tensile fail stress vs. modulus of elasticity for strips pressed at 350 kpa. 2000 TAPPI International Environmental Conference & Exhibit / 211