Improved Aspen Mechanical Pulp Through Coarse Grinding and Refining

United States Department of Agriculture Forest Service Forest Products Laboratory Research Paper FPL 443 Improved Aspen Mechanical Pulp Through Coarse Grinding and Refining By Dale C. Hedquist and James F. Laundrie

Abstract The Blandin Paper Company and the Forest Products Laboratory conducted a joint pilot scale study to determine the feasibility of refining coarse aspen groundwood (GW) pulp to increase the utilization of aspen and thereby lessen the demand for softwoods. The pulping rate of the grinder was nearly doubled by coarse grinding, and at comparable freenesses the quality of the unbleached refined coarse GW pulp was equal to that of typical aspen GW pulp from the mill. Alkaline bleaching and post refining of the refined coarse aspen GW pulp increased its quality to equal that of the bleached book grade GW pulp that contains about one part each of spruce, balsam fir, and aspen. KEYWORDS: Aspen, coarse grinding, groundwood pulp, peroxide bleaching, refining

United States Department of Agriculture Forest Service Forest Products Laboratory 1 Research Paper FPL 443 December 1983 Improved Aspen Mechanical Pulp Through Coarse Grinding and Refining Dale C. Hedquist, Product Development Specialist Blandin Paper Company, Grand Rapids, Minn. and James F. Laundrie, Project Leader Forest Products Laboratory Introduction Procedure The Blandin Paper Company and others (Britt 1964; Gavelin 1966) have long recognized that refined rejects from stone groundwood (GW) pulping produce a pulp superior to the original screened GW pulp of the same species. The strength properties of pulp from refined rejects are often similar to those of the superior thermomechanical (TM) pulps. Because of the higher energy demand required for producing TM pulps compared to conventional GW pulps (Pöyry 1977) interest is high in improving the quality of GW pulps without having a corresponding increase in energy consumption. The new pressurized GW process developed by Oy Tampella Ab (Karna 1980) and rapidly accepted by industry (Pulp and Pap. Int. 1981) is one way of doing this. Long before the development of pressurized GW pulping, however, De Montmorency (1958) and Hoholik (1958) showed that softwood GW pulps equal or superior in quality to conventional GW pulps could be made by coarse grinding to a high freeness followed by adequate refining. These were some early attempts at obtaining the improved properties of refined GW rejects. However, little incentive existed to commercialize the process because energy was inexpensive then, and the GW pulp quality was adequate for that time. Freshly cut aspen (Populus tremuloides Michx.) logs from northern Minnesota were used to produce GW pulps of 80, 140, 225, 340, and 450 Canadian Standard freeness (CSf). These pulps were made in a three-pocket, pilot scale grinder having a 1.37-m-diameter stone with a 0.20- m face. Descriptions of the stones used and the conditions and results of grinding are given in table 1. The three coarse GW pulps (225, 340, and 450 CSf) were each refined in a single pass through a 0.91-m-diameter, double rotating disk mill. The conditions and results of refining are also given in table 1. Both the grinder and the double rotating disk mill are at the Forest Products Laboratory. Post refining of the pulps was done in a 0.30-m-diameter, single rotating disk mill at the Blandin Paper Company. Bleaching and testing of the pulps and making and testing of handsheets were performed at the Blandin Paper Company. All tests were made according to TAPPI methods. The objectives of this pilot scale study were to increase the utilization of aspen by determining: (1) The quality of aspen GW pulp made by coarse grinding followed by refining; (2) The potentials for increased production and energy savings using this process; and (3) The effects of alkaline peroxide bleaching on the optical and physical properties of aspen pulps produced by this process. 1 Maintained at Madison, Wis.. in cooperation with the University of Wisconsin.

Results and Discussion Table 1. Conditions and results of coarse grinding 1 and refining 2 trials with aspen Conditions and Pulp identification results Control A B C D Grit size (pm) 3 246 246 4 540 540 540 Burr pattern (mm x mm) 305 x 38 152 x 38 152 x 38 152 x 38 152 x 38 Pressure (kpa) 241 172 124 207 345 Number of pockets used 3 3 3 2 2 Pit consistency (pct) 2.8 3.0 2.0 2.7 4.0 Freeness (Can. stand.) Grinder pit (ml) 80 140 225 340 450 After refining (ml) 79 127 159 141 Energy consumed per unit weight 5 Grinding (pct) 100 89 95 66 46 Grinding and refining (pct) 6 100 91 101 72 54 Grinder production rate 5 (pct) 100 110 172 195 203 1 Three-pocket grinder with a 1.37-m-diameter stone. Constant conditions used were 21.5-m/s peripheral speed, 0.15-m pit depth, and 77 C pit temperature. 2 One pass through a 0.91-m-diameter, double rotating disk mill with 36,314 plates. Constant conditions used were 18 2 pct feed consistency, 95 C feedwater temperature, and 9 1 pct refining consistency. 3 60 mesh, Tyler standard. 4 30 mesh, Tyler standard. 5 Based on the control trial 6 Post refining energy consumption not included Production Rate and Energy Consumption The results of the individual grinder and refiner trials are given in table 1. To establish a basis for comparison using the pilot scale equipment, a book grade (80 CSf) GW pulp was made from aspen as a control using a 246-µm grit stone. The production rate and energy consumed in producing this low freeness pulp were arbitrarily set at 100 percent, and all other pulps were compared to this control pulp. Changing the burr pattern, increasing stone sharpness, and decreasing the grinder pocket pressure from 241 to 172 kpa while using the 246-µm grit stone increased the production rate to 110 percent and decreased the energy consumption to 89 percent. However, the freeness only increased from 80 to 140 CSf. Based on these results, we decided that a coarser stone would be required to produce higher freeness pulps. With a 540-µm grit stone and a grinder pocket pressure of 124 kpa, a coarser pulp of 225 CSf was produced. The production rate increased to 172 percent and the energy consumption decreased to 95 percent. Refining this pulp to 127 CSf increased the total energy consumption to 101 percent, or slightly more than that required to produce the control pulp. Compared at equal production, however, this refined coarse GW pulp consumed only 58 percent of the energy consumed in producing the same amount of control pulp. Using grinder pocket pressures of 207 and 345 kpa and only two of the three pockets to prevent overloading of the grinder motor, coarser pulps of 340 and 450 CSf were produced. At 340 CSf, the grinder production rate increased to 195 percent while the energy consumption decreased to 66 percent. At 450 CSf, the production rate increased to 203 percent while the energy consumption decreased to only 46 percent. Refining the 340-CSf pulp to 159 CSf increased the energy consumption from 66 to 72 percent, and refining the 450-CSf pulp to 141 CSf increased the energy consumption from 46 to 54 percent. Unfortunately, the energy consumed in post refining of the refined coarse GW pulp was not measured, and we are therefore unable to draw any conclusions regarding total energy consumption. However, based on these results, we can conclude that the production rate of a conventional grinder can be greatly increased through coarse grinding followed by refining. Of course, these pilot scale results need to be confirmed through mill scale trials to determine the full potential of this process. 2

I Handsheet of Unbleached Pulps Shown in table 2 are the handsheet properties of a typical aspen GW pulp from the mill, the GW control pulp, and the refined coarse GW pulps. Although the control pulp and the refined coarse GW pulp made with a 246-µm grit stone (Pulp A) have a similar freeness, which is about 20 CSf lower than the mill pulp, the burst and tear indexes of handsheets made from the mill pulp are much higher. These results are a good indication that the procedures and equipment used to produce these pilot scale pulps were not the optimum for obtaining maximum strength. Conversely, the refined coarse GW pulps made using the 540-µm grit stone produced handsheets as strong as those from the control pulp despite their higher freenesses. Within the range of variables studied, the strongest handsheets were made from the coarse GW pulp that had a freeness of 225 CSf before refining (Pulp B). The handsheets from this pulp were similar in both strength and optical properties to the handsheets made from the typical aspen GW pulp from the mill. Post refining of the refined pulps made from the coarse GW pulps improved their strength properties considerably (table 3). After post refining, the burst index and breaking length of handsheets made from all the pilot scale pulps ground with the 540-µm grit stone exceeded those of the handsheets made from typical aspen GW pulp from the mill. Post refining also caused slight decreases in brightness and slight increases in opacity. The handsheets made from the coarse GW pulp of 225 CSf before refining were the strongest after post refining, so we selected this particular pulp for further alkaline bleaching studies. Handsheet of Bleached Pulps Three levels of sodium peroxide were applied to increase the brightness of the pulps (table 4). Handsheet properties were determined only after post refining to about 70 CSf. With only 0.5 percent sodium peroxide the brightness was increased from 60.5 to 68.9 percent, with 1.0 percent to 69.9 percent, and with 1.5 percent to 72.2 percent. Bleaching with up to 1.5 percent sodium peroxide did not appreciably increase the strength properties of the pulp handsheets. Table 2. Handsheet properties 1 of aspen pulps made by coarse grinding and refining Typical Pulp identification aspen groundwood pulp from the mill Control A B C D Freeness (Can. stand.) (ml) 104 80 79 127 159 141 Apparent density (kg/m 3 ) 386 408 410 388 377 382 Burst index (kpa m 2 /g) 0.69 0.52 0.56 0.63 0.49 0.52 Tear index (mn m 2 /g) 3.82 2.97 2.58 3.24 3.18 3.36 Breaking length (km) 2.32 2.23 2.39 2.45 2.36 2.11 Opacity (pct) 96.7 97.5 97.7 97.8 97.3 97.5 Brightness (pct) 62.3 66.8 61.2 61.9 59.9 59.4 Scattering coefficient (m 2 / kg) 60.2 73.2 64.1 67.0 59.3 61.4 Additional bleaches were made with increasing amounts of caustic soda combined with sodium peroxide. Handsheet properties were determined on the bleached pulps both prior to and after post refining, as shown in table 5. Dramatic increases in strength were obtained using combined caustic soda and sodium peroxide treatments. Also, most of these gains in strength properties were enhanced by post refining of the bleached pulps. As with the unbleached pulps, post refining also increased the opacity and decreased the brightness slightly while the scattering coefficient remained essentially unchanged. Overall, the quality of these combined caustic soda and sodium peroxide bleached pulps made from refined coarse aspen GW is comparable to that of the bleached book grade GW mill pulp that contains approximately one part each of spruce, balsam fir, and aspen. 3

Table 3. Handsheet properties 1 of post refined aspen pulps 2 made by coarse grinding and refining Typical Pulp identification aspen groundwood pulp from the mill Control A B C D Freeness (Can. stand.) (ml) 104 80 79 70 74 65 Apparent density (kg/m 3 ) 386 408 410 450 442 448 Burst index (kpa m 2 /g) 0.69 0.52 0.56 0.84 0.70 0.78 Tear index (mn m 2 /g) 3.82 2.97 2.58 3.16 3.01 2.87 Breaking length (km) 2.32 2.23 2.39 3.28 3.00 3.32 Opacity (pct) 96.7 97.5 97.7 98.1 98.2 98.3 Brightness (pct) 62.3 66.8 61.2 59.1 58.9 56.2 Scattering coefficient (m 2 / kg) 60.2 73.2 64.1 64.7 65.1 61.7 Table 4. Handsheet properties 1 of peroxide bleached aspen pulp 2 made by coarse grinding and refining Na 2 O 2 3 (pct) 0 0.5 1.0 1.5 Freeness (Can. stand.) (ml) 70 70 67 64 Apparent density (kg/m 3 ) 442 446 437 450 Burst index (kpa m 2 /g) 0.70 0.76 0.75 0.76 Tear index (mn m 2 /g) 3.48 3.40 4.00 3.75 Breaking length (km) 2.71 3.02 3.04 3.07 Opacity (pct) 98.2 96.0 95.7 94.5 Brightness (pct) 60.5 68.9 69.9 72.2 Scattering coefficient (m 2 /kg) 65.4 68.8 68.4 67.4 2 Pulp B after one pass through the 0.91-m-diameter disk mill Post refined after bleaching. 3 Based on dry weight of pulp. 2 Post refining was done in a 0.30-m-diameter, single rotating, disk refiner with 17,804A plates. Table 5. Handsheet properties 1 of combined caustic and peroxide treated aspen pulp 2 made by coarse grinding and refining Freeness (Can. stand.) (ml) Apparent density (kg/m 3 ) Burst index (kpa m 2 /g) Tear index (mn m 2 /g) Breaking length (km) Opacity (pct) Brightness (pct) Scattering coefficient (m 2 /kg) 0/0 108 5 88(63) 391 417(474) 0.63 0.76(1.03) 3.01 3.51(3.21) 2.42 3.01(3.87) 97.6 95.0(95.7) 59.9 69.2(66.5) 62.1 64.2(62.8) NaOH 3 (pct)/na 2 O 3 2 (pct) Typical mill book grade groundwood 2.5/1.5 3.0/1.5 3.5/1.5 4.0/2.0 pulp 4 90(72) 89(68) 437(461) 459(469) 0.85(0.96) 0.88(1.00) 3.51(3.57) 3.67(4.45) 3.21(3.73) 3.49(3.78) 94.7(95.7) 94.0(95.7) 69.7(63.6) 69.8(65.6) 62.9(58.7) 61.0(61.2) 89(72) 36 463(481) 450 1.02(1.14) 1.11 4.04(3.06) 3.46 3.68(4.29) 4.04 94.4(94.7) 97.4 69.3(67.3) 64.7 61.7(59.2) 73.2 2 Pulp B after one pass through the 0.91-m-diameter disk mill. 3 Based on dry weight of pulp. 4 Bleached composite pulp containing approximately one part each of spruce, balsam fir, and aspen. 5 Values in parentheses are for pulps post refined after combined caustic and peroxide treatment. 2.5-12/83 4

Conclusions Literature Cited 1. At nearly comparable freenesses, the quality of unbleached refined coarse GW pulp is about the same as that of typical aspen GW pulp from the mill. 2. The pulping rate of a grinder can be nearly doubled by producing coarse GW pulp. 3. Alkaline bleaching and post refining of the refined coarse aspen GW pulp can be used to increase pulp quality to equal that of the bleached book grade GW mill pulp that contains approximately one part each of spruce, balsam fir, and aspen. 4. The above conclusions need to be confirmed through mill scale trials. Britt, K. W. Handbook of pulp and paper technology. London: Reinhold Publishing Corp.; 1964: 264 p. De Montmorency, W. H. Pulp and Pap. Mag. Can. 59(C): 203-223 (convention issue); 1958. Gavelin, G. Science and technology of mechanical pulp manufacture. New York: Lockwood Publishing Co., Inc.; 1966: 160-168. Hoholik, F. S. Pulp and Pap. Mag. Can. 59(10): 141-146; 1958. Karna, A. Pap. jä Puu 62(1): 27-35; 1980. Pöyry, J. Pap. jä Puu 59(10): 604-608; 1977. Pulp and Pap. Int. 6: 48-51; 1981. Hedquist, Dale C.; Laundrie, James F. improved aspen mechanical pulp through coarse grinding and refining. USDA Forest Serv. Res. Pap. FPL 443. Madison, WI: Forest Products Laboratory; 1983. 5 p. The authors investigate the feasibility of refining coarse aspen groundwood pulp. At comparable freenesses, the quality of the unbleached refined coarse GW pulp was equal to that of typical aspen GW pulp from the mill. Alkaline bleaching and post refining of the refined coarse aspen GW pulp increased its quality to equal that of bleached book grade GW pulp that contains about one part each of spruce, balsam fir, and aspen. 5