Some of the latest investigations in P-RC APMP pulping of hardwood
Eric C Xu
Part 1: LCR at secondary refining
88th PAPTAC Annual Meeting, (pp. C1, Preprint), Montreal, Canada
Mixed hardwood from birch (82%) and maple (12%) was used in this study to investigate the potential of P-RC APMP process for this particular wood furnish, and low consistency refining at the secondary in this process. The results demonstrated that, a very wide range of pup properties comparable to, or better than, aspen market BCTMP pulps might be developed from this hardwood mixture using P-RC APMP process. Compared to high consistency refining, low consistency refining at the secondary is more efficient in reducing shives content, uses less energy, gives a higher light scattering, and produces a pulp that has similar tensile, bulk and intrinsic property, but a lower tear.
It is well known now in the industry [1-4] that alkali peroxide pre-treatment, compared to post bleaching, tends to use less overall energy, especially for hardwoods. Some later studies [5-8] showed that the alkali peroxide pre-treatment tended to produce a pulp with similar or higher bulk and light scattering. APMP, (alkali peroxide mechanical pulping), pulps from aspen tend to give better pulp intrinsic properties, (relationships between hand-sheet density and fibre bonding properties like tensile and burst) in comparison to BCTMP . In APMP pulping, it is important to optimize the ways of how the chemicals are applied and how the materials are processed in order to maximize the efficiency of the process [9-10]. In Part 1 of this series on recent developments of APMP technology  a new concept, P-RC (preconditioning followed by refiner chemical treatment), was discussed and a new chemical strategy was introduced to improve the efficiency of the process chemicals in pulp property development. By splitting the alkali peroxide chemicals between the chip impregnation stage(s) and the primary refiner, better chemical and energy efficiency, with improved light scattering, was observed in comparison to applying all the chemicals at the chip pre-treatment stage(s).
As Part 2 of this series, a blend of birch (82%) and maple (12% by o.d. weight) from the east coast of Canada was investigated using P-RC APMP process. There are quite abundant birch and maple woods in that region, and there is a growing interest in utilization of those woods for pulp and paper industry.
The second objective of this investigation was to study effects of using low consistency refining (LCR) in the second stage after high consistency retention from P-RC APMP process, or after the chemical treatments/reactions are completed. It has been observed from lab and mill experience that when refining energy consumption is reduced to a very low level in an alkali peroxide pre-treatment process, it becomes difficult to have a efficient narrow gap in conventional high consistency atmospheric refining. This in turn tends to cause a higher than normal amount of shives that pass through the plates without being properly refined. By reducing the refining consistency to low levels, (typically 5% or lower for LCR), it becomes possible to have a smaller plate gap and to provide a better opportunity for the shives to be refined.
Another reason for investigation of LCR is the potential energy savings in comparison to high consistency refining (HCR). By having LCR at secondary position in P-RC APMP process, it is expected that total refining energy consumption can be further reduced.
A blend of birch (82%, by oven dry weight) and maple (18%) from eastern Canada was used in this investigation. The wooden logs were debarked, chipped and screened at the Andritz Pilot Plant prior before further processing.
Two-stage impregnation was applied in all the trial runs. An Andritz Model 560GS Impressafiner with 4:1 compression ratio was used for all the chip impregnation/pre-treatment. The chips were first steamed for five minutes, and then pressed and treated with 0.3% dtpa. The dtpa-treated chips were steamed for five minutes, before being pressed again and impregnated with alkali peroxide chemicals consisting of various amounts of sodium hydroxide, hydrogen peroxide, silicate, dtpa and magnesium sulphate. The alkali peroxide treated chips were then allowed 30-45 minutes of retention, (at 50-60 0C), without steaming. Various amounts of alkali peroxide chemicals were again added to the eye of the primary refiner.
An Andritz Model 401 (36", 92-cm diameter) Atmospheric Double Disc refiner was used for all high consistency refining runs. An Andritz Twin-FloIIIB Dual Discharge refiner (20", or 51-cm, diameter) was used for all low consistency refining runs. Pulps from the primary were retained under cover for 30 minutes or longer, depending on the amount of chemicals used. The pulps were of approximately 20% consistency and 80 to 90 0C. After the high consistency retention, the pulp was either directly sent to the secondary refining stage at different energies to produce a curve, or diluted to 4.2% consistency for LCR and HCR (after dewatering) comparison.
(Refer to  for more information and discussions about process procedures for P-RC APMP pulping.)
Tappi standard methods were used for all pulp testing in this investigation.
Table 1: Chemical conditions for conventional P-RC APMP Series
Series I.D. 2.1% TA 4.4% TA 6.3% TA First stage impregnation % dtpa 0.3 0.3 0.3 Second stage impregnation % dtpa 1.0 1.7 4.6 % H2O2 1.1 1.2 3.1 % silicate 0.5 1.1 3.7 % MgSO4 0.05 0.1 0.1 % dtpa - - 0.2 At refiner eye % TA 1.1 2.7 1.7 % H2O2 2.7 3.6 2.1 % silicate 3.2 3.2 2.1 % MgSO4 0.1 0.2 0.1 % dtpa - - 0.2 Total % TA applied (consumed) 2.1 (2.1) 4.4 (4.3) 6.3 (5.9) % H2O2 applied (Consumed) 3.8 (2.7) 4.8 (3.8) 5.2 (4.7)
First stage impregnation
Second stage impregnation
At refiner eye
% TA applied (consumed)
% H2O2 applied (Consumed)
4. Results and discussion
4.1. Mainline conventional high consistency refining
Three P-RC APMP series were performed on this hardwood blend using a conventional high consistency double disc refiner (Andritz 401). Table 1 presents the main pertinent chemical conditions used. Total alkali (TA) in the table represents total alkalinity including contributions from the silicate used. The percentages of silicate and dtpa are based on standard commercial stock supplies, 38% of (Na2O + SiO2) for the silicate (41o Bé) and 48% solid liquor for the dtpa.
Figures 1 through 4 show graphic presentations of general trends for P-RC APMP pulping of this particular hardwood blend. Figure 1 shows that CSF(Canadian Standard Freeness)/SEC(specific energy consumption) relationship was less sensitive to total TA charge when TA was 4.4% or less. The energy, however, was reduced very quickly when the TA was increased to above 4.4% to 6.3%. As expected, increasing the TA charge increased tensile at a given freeness (Figure 2), but reduced bulk (Figure 3), and light scattering coefficient (LSC) (Figure 4).
Figure 1: P-RC APMP (82% Birch/18% Maple)
Figure 2: P-RC APMP (82% Birch/18% Maple)
Figure 3: P-RC APMP (82% Birch/18% Maple)
Figure 4: P-RC APMP (82% Birch/18% Maple)
Table 2: P-RC APMP pulp properties at 200 ml CSF (82% Birch/18% Maple)
% TA consumed 2.1 4.3 5.9 (Applied) (2.1) (4.4) (6.3) % H2O2 Consumed 2.7 3.8 4.7 (Applied) (3.8) (4.8) (5.2) SEC (kWh/odmt)3 2100 2050 1000 Bulk (cm3/g) 3.5 3.2 2.2 Tensile Index (N.m/g) 13 18 52 Tear Index (mN.m2/g) 1.9 2.7 7.0 Burst Index (kPa.m2/g) 0.5 0.65 2.8 % ISO Brightness 81.5 83 84 Light scattering coefficient (m2/kg) 57 54 35 % Pulmac Shives (0.10 mm) 0.08 0.10 1.2
% TA consumed
% H2O2 Consumed
Tensile Index (N.m/g)
Tear Index (mN.m2/g)
Burst Index (kPa.m2/g)
% ISO Brightness
Light scattering coefficient (m2/kg)
% Pulmac Shives (0.10 mm)
Table 2 compares overall pulp properties at 200 ml CSF from the three series. It shows that P-RC APMP is a very flexible process, and is able to produce a wide range of pulp properties by changing the chemical charges and the energy. In the present case, at 200 ml CSF, the tensile index varied from 13 N.m/g to 52 N.m/g as the TA consumption increased from 2.1 to 5.9%. A significant increase was also observed in tear and burst. As generally expected, bulk and light scattering decreased at a higher tensile, or TA consumption.
Note: a 30% lower energy is normally expected in a commercial operation.
It should be pointed out that, from our experience in the field, energy consumption in commercial operation is typically 25-35% lower than that at the lab for refiner pulping of hardwoods with or without chemical treatment. This difference should be applied when the specific energy numbers in this report are to be compared with any commercial operation. A fundamental trend between specific energy consumption and TA charge or pulp property development is, however, applicable in both lab and commercial operation. That is, increasing TA consumption decreases the energy consumption, bulk and light scattering coefficient (LSC), but increases fiber bonding strength properties, such as tensile, burst and others.
As for the shives content, for this particular wood furnish, it increased at a given freeness as the SEC demand decreased, which is different from the trend observed previously on other type of hardwoods . It appears then that for certain wood furnish, when SEC becomes very low, the gap between the refiner plates starts to increase, which allows for more fibre bundles to pass through. As in the present case, % shives increased from 0.08 to 1.2% as the SEC decreased from higher than 2000 to 1000 kWh/ODMT. (Note a double disc refiner was used in this investigation, which is known to have a larger gap and produce more shives than a single disc refiner .)
It should be pointed out that bleaching chemistry (or H2O2 efficiency) was not fully optimized (such as a combination of stabilizers and chemical distribution) for the different TA charges. Only one attempt, or series, was performed at each TA level. Despite these limitations, this hardwood blend of birch and maple appeared to respond well to the H2O2 bleaching using the P-RC APMP process. With 2.7% peroxide consumed, 81.5% ISO was achieved.
Figure 5 compares pulp intrinsic property development (tensile/density) from P-RC APMP of the mixed hardwoods and that from Canadian aspen market BCTMP. The P-RC pulps have a higher bulk than the BCTMP. The difference was larger at a higher tensile. (Pulp intrinsic property is a property that depends on the nature of the wood species and pulping/bleaching process used, but is independent of process variables, such as amount of chemicals and energy used [7,8,12] and how the pulps are refined, as shown in the later part of this investigation.). This observation consists with that reported earlier: APMP gives better intrinsic property than BCTMP for aspen  and birch APMP is better than aspen APMP .
Figure 5: Pulp intrinsic property
4.2. LCR vs HCR in secondary position
In the second part of this investigation, P-RC APMP pulps after high consistency inter-stage retention were diluted to 4.2% consistency and neutralized to pH 7 before further processing. A portion of this pulp slurry was refined at low consistency using the Twin-FloIIIB Dual Discharge refiner and the remainder was dewatered and high consistency refined using the 401 refiner. Main process conditions and pulp properties for the pulp before secondary refining were:
0.2% dtpa at first stage impregnation;
4.6% TA, 3.4% H2O2, 0.24% dtpa, 0.1% MgSO4 and 3.3% silicate at the second stage impregnation;
1.7% TA, 1.7% H2O2, 0.16% dtpa, 0.08% MgSO4 and 1.6% silicate at the eye of the primary refiner. (There were 0.62% H2O2 and 0.4% TA residuals, pH 8.8, after interstage HC retention.);
590 kWh/ODMT total SEC (including chip impregnation);
pulp properties: 538 ml CSF, 2.57 cm3/g bulk, 30.2 N.m/g tensile index, 5.8 mN.m2/g tear index, 85.1% ISO Brightness, 32.1 m2/g LSC, 10.5% Pulmac Shives (0.10 mm).
Table 3: Conditions for LCR and HCR Series
Series LCR HCR Consistency % 4.2 21 Plate Pattern (Durametal) 22TA013/014 36104 Flow Rate (L/min) 300 450 300 Throughput (ODMT/D) 18.2 27.7 18.3 10.3-38.1 Refiner Speed (RPM) 1100 1100 900 1200
Plate Pattern (Durametal)
Flow Rate (L/min)
Refiner Speed (RPM)
Figure 6: P-RC APMP LCR vs HCR
Three different LCR conditions (different flow rates and refining speeds) were investigated, and one standard HCR run was performed. Pertinent process information is provided in Table 3. Figure 6 shows that it took very little energy for LCR to reduce shives, which dropped from 10.5% to about 1% at less than 100 kWh/ODMT net (200-250 kwh/odmt gross that includes idle power). HCR, on the other hand, used much more energy, about 500 kWh/ODMT to reach 1% shives. Figure 7 shows the relationship between % shives and freeness. LCR, again, was much more efficient in reducing % shives at 150 ml SCF or higher. At 250 ml CSF, LCR had about 50% lower shive content than HCR. There, however, appeared to be a limit for how low the % shives could go in LCR refining, and it levelled off at about 1%. Further increase in SEC (Figure 6) above 100 kWh/ODMT (net) or reduction in freeness (Figure 7) did not show any significant changes in % shives.
Figure 7: P-RC APMP LCR vs HCR
Figure 8: P-RC APMP LCR vs HCR
As expected, LCR used much lower energy than HCR. Only less than 150 kWh/ODMT (net) was needed for LCR to reach 200 ml CSF, while more than 350 kWh/ODMT was required for HCR, Figure 8. The difference was even larger at a lower freeness. It should be noted that an increase of SEC in the LCR series was accompanied by an increase in intensity (edge load Ws/m) due to the one-stage refining power curves applied in this study. A refining intensity close to about 0.5 Ws/m (according to TAPPI standards) seems to mark a turning point in pulp quality development, as observed above and below. More works are needed to optimize LCR at low intensity and high SEC.
Figure 9. P-RC APMP LCR vs HCR
Figure 10. P-RC APMP LCR vs HCR
Figure 9 shows there was no difference in tensile development between LCR and HCR as the freeness varied from 540 to 100 ml where LCR appeared to reach a limit of 65 N.m/g tensile index.
Figure 11. P-RC APMP LCR vs HCR
Figure 12. P-RC APMP LCR vs HCR
There was also no difference in pulp intrinsic property development between LCR and HCR, Figure 10. This is a very interesting and important observation. The pulps from the different LCR and HCR runs all had different shives contents at a given tensile, as shown in Figure 11, but yet they all had the same bulk or density at any given tensile, suggesting the shives from the P-RC APMP process are very flexible (well chemically treated) or have a similar flexibility as other normal pulp fibers. More works, however, are needed to confirm this and to understand better the nature of the shives from P-RC APMP process and their effects in paper-making processes.
Figure 12 shows that LCR had a tendency to give the same or higher light scattering, when compared to HCR. The lowest intensity LCR refining series, "LCR (300 L/min, 1100 RPM)", had the best average LSC in a freeness range of 200-400 ml. These differences in LSC apparently were not, or at least not totally, caused by pulp fines content. As shown in Figure 13, some pulps from the LCR runs had similar or lower % p200 mesh fraction, and yet had higher light scattering, compared to HCR. The higher light scattering from LCR may be attributed, at least in part, to the increased middle fraction of the fiber distribution, as supported by Figure 14 where LCR consistently gave a lower %+28 mesh fraction than HCR. These results suggest that for hardwood P-RC APMP pulps, the middle fiber fraction plays an important role in pulp light scattering development.
Figure 13: P-RC APMP LCR vs HCR
Figure 14: P-RC APMP LCR vs HCR
A consequence of the lower %+28 mesh fraction from the LCR runs was a reduction in tear, as shown in Figure 15. The reduction was, however, not very significant for the low intensity series, about 10% loss at 250 ml CSF. Since fibres are often used in combination with hardwood for many paper-making processes to meet sheet tear requirement).
Table 4: LCR and HCR at 200ml CSF
Table 4 represents some of the main observations from this investigation, based on pulp freeness of 200 ml.
Figure 15: P-RC APMP LCR vs HCR
The results from this investigation demonstrated that using P-RC APMP process, a wide range of pup properties can be developed from hardwood blend of birch/maple. In comparison to HCR, LCR in the secondary position
P-RC APMP with LCR in the secondary has a good potential for the applications where energy savings, together with other benefits such as lower shives and higher light scattering, from LCR overweight its shortfall in tear development.
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