In: TAPPI proceedings, 1987 pulping conference; 1987 November 1-5; Washington, DC. Atlanta, GA: TAPPI Press; 1987: 729-734. DISK SEPARATION: FIBER RECOVERY FROM RECYCLED NEWSPRINT PAPERMILL TAILINGS John H. Klungness Chemical Engineer USDA Forest Service Forest Products Laboratory One Gifford Pinchot Drive Madison, WI 53705-2398 ABSTRACT No single satisfactory industrial process exists for recovering fiber from wastepaper recycling mill tailing streams. This study was undertaken to determine the feasibility of using disk separation to recover recyclable fiber from a recycled newsprint mill screening system tailing stream. A secondary objective was to clean the fiber recovered from the tailings sufficiently to permit it to be fed forward to the papermill system. The study consisted of determining the first pass optimum disk operating conditions based on statistically designed experiments. Optimum operating conditions were defined as chose which gave the maximum net cash value based on contaminant removal, fiber recovery, and energy use. Disk separation removed 93%, of total contaminants including 98% of stickies in three passes, at first pass optimum conditions, from the original tailing sample. This separation resulted in reducing the total contaminant and stickie level of the recovered fiber to about that of the pulp fed forward to the papermill system. Approximately 72% of the original fiber was recovered. The recovered fiber length was nearly twice the length of the fiber normally sent to the paper machine system, with only about a quarter of the fines. KEYWORDS: Adhesives, contaminants, material separation, newsprint, recycling, residues, screenings, separation, wastepapers No single satisfactory industrial process exists for recovering fiber from wastepaper recycling mill tailing streams (1). These tailing streams contain valuable pulp fiber from which it is difficult to remove objectionable concaminants. Moreover, discarding such fiber causes environmental problems, added cost for disposal, or both (2, 3 ). The contaminants are typically- heterogeneous, thus requiring several individual separation processes for removal. Also some of the contaminants, stickies, are difficult to remove by existing processes without losing large amounts of fiber during their removal by screening and cleaning ( 1 ). Disk separation (4, 5, 6 ) has been shown to separate contaminants from pulp fibers based on differences in particle size and density, as do screens and centrifugal cleaners. In addition, the disk separator can separate particles based on wetting angle differences (7 ). Stickies, typically having about the same size and density as betted pulp fibers, are usually composed of synthetic adhesive compounds which have much greater wetting angles than pulp fibers. Thus, disk separation can more effectively remove stickie contaminants than screens and cleaners. In this study the feasibility of using disk separation to recover fiber from a recycled newsprint tailing stream was determined. A secondary objective was to clean the recovered fiber sufficiently to permit it to be fed forward to the paper machine system. Forward feeding of recovered fiber from tailings can be an effective strategy for fiber recycling ( 8 ). Optimum disk separation conditions were obtained by statistically designed experiments. RESULTS AND DISCUSSION Contaminant Removal Disk separation removed 93% of total contaminants including 98% of stickies in three passes from the original tailing sample. This separation resulted in reducing the total contaminant and stickie level of the fiber recovered from tailings (quaternary screen rejects) to about that of pulp fed forward to the papermill system. At first pass optimum operating conditions (Table 1), a single pass removed 53.6% of the total contaminants which included removal of 85.8% of the stickies. Optimum operating conditions were those which gave the maximum net cleaning value ($/ton) for the separation. The net cleaning value for separation was based on fiber recovery, contaminant removal, and energy use. This first pass reduced total contaminant content from 0.42% air dried weight to 0.21%, and the number of stickies per kilogram (kg) of ovendried feed from 260 to 82 (Table 2). The accepts from this first pass were not as low in contaminants and stickies as the primary screen accepts which are fed forward to the paper machine system. The primary screen accepts contained 0.05% total contaminants with 10 stickies per kg. The number of passes needed to adequately remove stickies from the fiber recovered from tailings so the fiber could be fed forward in the mill system was determined nest. The optimum conditions obtained for the fiber used for the first pass were also used for subsequent passes. Total contaminant after three passes of 1987 Pulping Conference / 729
disk separation was 0.04%. Also, the number of stickies had been reduced from the original 260 per kg to 11 per kg by the third pass. The size (crosssectional area) of the stickies remaining after three passes of disk Separation averaged 1.16 mm 2 compared to 0.72 mm 2 for the primary screen accepts. The contaminant levels after three passes compare favorably with the levels required for the primary screen accepts. Based on total contaminant and stickies removal, three passes were considered sufficient. If optimum operating conditions were significantly different after the first pass, operating at such conditions would only improve the results reported. In practice by operating at one set of conditions, the required number of disks for a specific application could be mounted on a single shaft and be fed in series to reclaim fiber. Recovered Fiber Recovered fiber from the mill tailings stream (quaternary screen rejects) after three passes of disk separation (third 730 / TAPPI Proceedings
pass accepts) had twice the length as fiber sent to the papermachine system and contained only about a quarter of the fines (Table 2). The increase in fiber length was due mainly to the design of the mill screening system. In the mill, the rejects of the primary screen were fed to a secondary screen, and the secondary rejects to a tertiary screen, etc. After four such screenings, the average length of the pulp fiber in the tailings stream (first pass feed to disk separator in Table 2) had increased to 1.9 mm from 1.0 mm in the primary screen accepts. The average length of the 90th percentile increased from 2.3 mm to 3.7 mm, and fines (<0.1 mm) decreased from 8.0% to 2.6%. After three passes of disk separation the average length of fiber recovered from the tailings increased from 1.7 mm after the first pass to 2.1 mm in the third pass accepts, the average length of the 90th percentile increased from 3.3 mm to 3.8 nun, and fines decreased from 3.6% to 1.9%. It should be noted that a visual inspection of handsheets from the primary screen accepts and the accepts from the third pass disk separation gave information not indicated by the previous tests. A number of small fiber bundles or nonfiberized paper particles were apparent in the third pass accepts from disk separation that were not present in the handsheets from the primary screen accepts handsheets. These particles appeared to be mainly coated paper. Thus the particles probably would not give such problems as stickies, and the particles would probably be removed by fine screening, cleaning, or further fiberizing, as is typically done with such particles. CONCLUSIONS AND RECOMMENDATIONS Disk separation can remove total contaminant and stickies from a mill tailing stream in three passes to a level comparable to that of pulp fiber which is fed to the papermill stock system. About 93% of the total contaminant and 98% of the original stickies were removed while recovering about 72% of the fiber. Fiber length of the recovered pulp fiber was, on average, about twice the length of the fiber normally sent to the papermachine 1987 Pulping Conference / 731
system, with only about a quarter of the fines (<0.1 mm). For ease of application, it is recommended that pulp slurry be fed in series to the required number of multiple disks mounted on a single shaft for reclaiming fiber from recycled mill tailing streams. The number of disks should be determined for each application. MATERIALS AND EXPERIMENTAL PROCEDURES Materials Pulp samples from the Southeastern Paper Manufacturing Company (SEPCO) newsprint mill in Dublin, GA, were dewatered to about 25% solids and sent to the Forest Products Laboratory (FPL). Samples were obtained from SEPCO for the accepts of the primary screen and rejects from the quaternary screen (tailing stream). A description of the millstock preparation has recently been published (9 ). Equipment and Operating Procedure The FPL-constructed experimental disk separator and operating procedure were used as recently described (6 ). Equipment consisted of a top-fed, motor-driven disk. Pulp slurry was fed directly to the spinning disk. The spray discharged from the disk shoulder was collected as the rejects. Spray discharged at the lip edge was collected as the accepts. Sample Testing The method in use at SEPCO for evaluating contaminants was used in this study. Approximately 0.454 kg (about 0.1 kg for the experiments to develop the model) of ovendried fiber was screened on a 0.002- mm (0.0015 mm was used for the model) slotted vibrating screen. The smaller fiber sample and smaller screen size was used in order to conserve fiber. The pulp slurry was fed to the screen at less than l% consistency. Screening was continued until the amount of material remaining on the screen stayed constant. About 10 minutes was used for screening 0.454-kg-sized samples. The material remaining on the screen was collected, dried, and weighed (22 C and 50% relative humidity). The contaminants were then mixed with 1.2 grams ovendried, unrefined bleached softwood kraft pulp in water and formed into a handsheet as described in TAPPI T205, excluding pressing. Both sides of the air-dried handsheets were then examined under a stereomicroscope using a laboratory pick to identify stickies. Stickies were defined as those which stretch or are tacky to the pick. Numbers and diameters of the stickies were then tallied using a stem leaf type tally sheet for each screening. The weight fraction of stickie and total contaminant were then calculated, assuming a density 732 / TAPPI Proceedings of 1400 kg/m 3 for the stickies (5 ). Fiber lengths and distributions were obtained by a Kajaani FS-100 fiber analyzer. Predicting Optimum Operating Conditions Because the separation process is not 100% effective in removing contaminants, incurs fiber loss to the reject stream, and requires energy to operate, the net cleaning value (V n) added by contaminant removal was determined as follows: where V n = net cleaning value, the value added by contaminant removal, dollars per metric ton ($/ton) of fiber feed (dry basis) ΔC= differential pulp costs (value of pulp needing no cleaning less the value of pulp needing cleaning), dollars per metric ton ($/ton) of fiber (dry basis)--nominal value of $110 per metric ton assumed R = Contaminant removal efficiency, percent total contaminant removal/ 100%. A = pulp fiber (dry basis) in accepts, metric tons F = pulp fiber (dry basis) in feed, metric tons V e = cost of energy consumed in the separation process (calculated at the rate of $0.017/megajoule), dollars per metric ton of fiber (dry basis) in the feed Total contaminant removal was used as this was reproducible with much smaller sample sizes than the test for stickies. Discussions with mill personnel indicated a general correlation between total contaminants and stickie level. Experimental Design Because of the interactive nature of the process variables, statistically designed experiments were used in determining optimum conditions. Statistical analysis of experimental results was also used, Specifically, standard response surface analysis (10) was used to determine desired optimum based on highest net cleaning value, V n (Eq. 1), acheived from results using a central composite design (10). Data from statistically designed experiments were collected for differing operating conditions. V values were n
next calculated for these conditions. Using these experimental V n values, response surface analysis (10) was then used to model how V n varied with disk speed, feed solids, and feed rate. The three disk separation process variables--disk speed, feed solids, and feed rate--are interactive in nature when determining optimum process conditions using Eq. 1. Different combinations of the three process variables can yield the same V n value (Figs. 1-3). As observed from the plots of predicted V n values, only one set of process conditions gives a maximum V n value. This set of conditions is taken as the optimized process conditions. However, there is a rather broad combination of process variables where near optimum results could be obtained. From a process control standpoint this is advantageous. Fig. 2 Net cleaning value (V n ) pre dicted for removal of contaminants from Southeast Paper Manufacturing Co. quaternary screen rejects. At optimum feed solids (%). (ML87 5370) Fig. 1 Net cleaning value (V n ) contours predicted for removal of contaminants from Southeast Paper Manufacturing Co. quaternary screen rejects. At optimum disk speed (r/s) (ML87 5372) Experimental Verificacion of Predicted Optimum Operating Conditions Confirmation of the accuracy of the model is obtained by comparing the experimentally determined and predicted V n values. Overall, the two values averaged within 8.9 of each other, which is considered close agreement for the model. Predicted optimum disk separation conditions are listed in Table 3. These conditions Fig. 3 Net cleaning value (V n ) contours predicted for removal of contaminants from Southeast Paper Manufacturing Co. quaternary screen rejects. At optimum feed rate (m 3 /h). (ML87 5371) 1987 Pulping Conference / 733
were then used experimentally as the optimum process conditions. The first pass, using the optimum conditions listed in Table 1, resulted in an experimentally obtained V n of 52.61 versus a predicted V n of 68.58. The difference between these two values was larger than the average difference of 8.9. Energy Energy used was the power required by the electric motor to drive the disk multiplied by the time of the experiment. All power values used in calculating V n were normalized to a power factor of one using the techniques described in (6 ). Literature Cited ACKNOWLEDGMENTS The author gratefully acknowledges the financial support of this study by Garden State Paper Company. I especially thank Frank W. Lorey of Garden State Paper Company and Thomas H. Eck of Southeast Paper Manufacturing Company for their advice and assistance. I also gratefully acknowledge the contributions of James W. Evans for statistical consul tations and evaluations, and Richard W. Shilts and Charles W. Polley for laboratory and pilot plant evaluations. The use of trade or firm names in this publication is for reader information and does nor: imply endorsement by the U.S. Department of Agriculture of any product or service. The Forest Products Laboratory is main tained in cooperation with the University of Wisconsin. This article was written 2nd prepared by U.S. Government employees on official time, and it is therefore in the public domain and not subject to copyright. 734 / TAPPI Proceedings Printed on recycled paper