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NORTH AMERICAN WEST COAST SOFTWOOD KRAFT PULP
KEY WORDS  Figure 1 To mimic soda carryover typical in a mill where incomplete brownstock washing occurs as is the case at Halsey, first-stage brownstock washer filtrate was used as a source of COD to add back to stock prior to undergoing oxygen delignification. The results of this analysis using 150 or 300 kg/ADt of equivalent COD addition are illustrated in Figure 2.  Figure 2 This work shows that some carryover of black liquor is actually helpful in achieving full delignification, probably due to its alkali value. However, excessive carry-over decreases the rate of delignification, likely because of oxidation of COD as opposed to lignin on the fiber. Based on these results, further optimization of oxygen delignification involved addition of 100-150 kg/ADt of COD to brownstock pulp as received. In an effort to further lower the Kappa number of pulp to a TCF bleaching sequence, two-phase oxygen delignification involving splitting of total alkali charge between two mixing steps was evaluated. The effect of increasing retention time in the first phase on delignification rate and resulting fiber strength is illustrated next in Figure 3.  Figure 3 This evaluation was done using pulp having a total COD content of 157 kg/ADt. Delignification conditions included total NaOH addition of 3.5% on A.D. pulp (two-thirds added in first phase, one third in second phase), starting oxygen pressure of 120 psi lowered to 90 psi in the first phase and oxygen pressure of 90 psi starting to 15 psi ending in the second phase. Temperature at 102 oC was maintained throughout the stage. As indicated, Kappa reduction is maximized at about 53.5% by retention in the first phase of twenty minutes followed by a second phase of delignification for sixty minutes. Based on the work, fiber strength expressed as zero-span tensile values appeared to be maintained during total delignfication. To improve delignification further using a two-phase reaction approach, 0.3% by weight hydrogen peroxide addition was evaluated prior to the second phase. Although hydrogen peroxide was known to extend delignification from earlier published work (3), it was not known whether it would be effective in this study due to the levels of COD carryover which were being evaluated. A positive effect however, with total carryover of 107 kg/ADt and total NaOH of 3.5% on A.D. pulp was seen as shown below in Figure 4.  Figure 4 In total, the O2 stage optimization work demonstrated that a) splitting the delignification reaction into two phases with a first phase retention time of 20 minutes and a second phase retention of 60 minutes and b) addition of a small amount of hydrogen peroxide prior to the second phase mixer could increase the percent delignification from 54% to about 58%. To prevent fiber strength loss, the work demonstrated that retention time in the first phase should be limited to about 20 minutes. TCF BLEACHING SEQUENCE OPTIMIZATION Large batches of (OpO) delignified brownstock pulp having a Kappa number of 12-14 were prepared for further evaluation to bleach pulp to a brightness of 87% ISO or higher and maintain fiber strength properties. Following some initial work, two TCF sequences were studied extensively and optimized stage by stage. These were (AQ)Z(PO)(ZQ)P and A[Px(PO)](ZQ)P where Px represents peroxymonosulphuric (Caro's) acid and both the (PO) and P stages were pressurized at elevated pressure. Figure 5 illustrates brightness through each stage of the two sequences compared to initial work. The sequence identified as "Initial" involved work on (O-O) delignified pulp and milder conditions in the (PO) stage.  Figure 5 Ozonation in the initial work was done at Econotech Services Ltd. at medium consistency using a flow-through reactor while that in the optimized work was done at medium consistency at Quantum Technology's Twinsburg, OH lab facilities using a Quantum® high-intensity mixer. All pressurized peroxide stages were also done in a Quantum high-intensity lab reactor. Complete conditions and results on a stage by stage basis can be found in the Appendix in Tables 1-3. A comparison of brightness gain between the two optimized sequences was made difficult because of difficulty in removing manganese from the pulp prior to (AQ)Z(PO)(ZQ)P bleaching since it was collected at a later date from the mill than pulp used in the other two sequences. The difference in manganese (Mn) level of pulp entering and leaving the metals removal stage is shown next in Table 4:
This difference in the ability to remove manganese from pulp sample to sample indicates one potential source of concern when operating a TCF sequence at a mill. Ultimately, further optimization of this stage involving increased temperature and time yielded more consistent results which would alleviate some of these fears. Figure 6 tracks Kappa reduction when comparing the same three TCF sequences. It can be seen that by adding a lignin activation stage involving either ozone or Caro's acid prior to the (PO) stage, Kappa number after the pressurized peroxide stage can be reduced by 2-3 units. Comparing these activation compounds on an active oxygen or AO basis, addition of Caro's acid involved 0.10% AO on A.D. pulp while that of ozone involved 0.15% AO on pulp. Through the combination of lignin activation and pressurized peroxide treatment, Kappa number for the optimized TCF sequences was reduced by 60-70% compared to that after the metals removal stage.  Figure 6 After (ZQ) bleaching in the third or fourth stage of the TCF sequence, Kappa number of pulp was lowered to about 1.5 for the two optimized cases versus 4.0 for the initial case. In all sequences, Kappa reduction after ozonation was calculated after performing a mild, alkaline extraction on the ozonated pulp. This Kappa reduction from ozone can be translated into an average drop of 76% after applying ozone in the Quantum mixer, but only an average drop of 42% after applying ozone in the flow-through reactor. The flow-through reactor was much less efficient than required for the work and was not utilized except in our initial studies. Measurements of fiber strength after PFI beating of final bleached pulp from the optimized bleaches indicated that there was significant reduction in breaking length occurring. After evaluating the data and literature with respect to ozone bleaching and the use of high-intensity laboratory mixers (4,5), it was decided to re-evaluate the two sequences by altering the laboratory procedure for chemical mixing. For this further work, a) (OpO) work was done in a peg mixer rather than a high-intensity mixer, b) ozone bleaching was done in a rotating glass reactor at room temperature and about 40% consistency and c) peroxide bleaching was done in glass beakers in a pressurized, high-intensity reactor with its rotor removed. TCF BLEACHING TO MAINTAIN FIBER STRENGTH Optimized results on fresh brownstock pulp at 29.0 initial Kappa that was delignified to 12.3 Kappa (57.4% reduction) resulted in pulp of final brightness 90-91% ISO via either an (AQ)Z(PO)(ZQ)P or A[(Px)(PO)](ZQ)P sequence. Chemical charges, conditions and measured data are found in Tables 1-3 in the Appendix. Key fiber strength properties at 400 CSF for (OpO) delignified and final bleached pulp from the two sequences compared to brownstock are plotted in Figure 7.  Figure 7 The results indicated that a 15% loss in tear strength occurs after (OpO) delignification, but that losses in burst and breaking length did not occur until the bleaching sequence. Overall, final bleached fiber lost 10-15% of its initial breaking length and burst strength and 35-40% of its initial tear strength. A further evaluation of bleaching optimization using fresh pulp at 31.2 Kappa (delignified to 14 Kappa to achieve 55% reduction ) after an (OpO) treatment, ozone prior to (PO) and lower charges of ozone and hydrogen peroxide in the final two stages to target a 87-88% ISO final brightness was made. This optimization work was successful in producing pulp of 87% ISO having a Kappa number of 1.2. Complete results can be found in Tables 1-3 in the Appendix. At 400 CSF, observed strength losses compared to brownstock pulp were as follows: breaking length - 8%, tear index - 24% and burst index - 7%. Further evaluation of the Caro's acid sequence was not undertaken as fiber strength data did not appear to favor this sequence, bleaching with an additional chemical would complicate plant operations and sequence chemical costs were high. COMPARISON OF FIBER STRENGTH , BLEACHING COSTS AND YIELD LOSSES TO ECF PROCESS Pulp strength properties of the final optimized TCF sequence were compared to those after an established laboratory bleached ECF sequence. Softwood kraft brownstock from the Halsey mill bleached to a similar brightness of about 87.5% ISO using an (OpO)D(EOP)(Dn)D sequence had superior strength retention as shown in Table 5 which follows:
For a producer of bleached softwood market pulp, this loss of competitiveness with respect to 7-10% lower fiber strength depending on the parameter being compared is an essential consideration when choosing a future bleaching sequence. In addition to fiber strength penalties observed, a detailed determination of operating chemical costs was performed. For this analysis, the amount of chemical applied at each stage was multiplied by the cost per unit for each chemical. Ozone cost per unit included power cost for ozone generation and power for auxiliary equipment, such as compression, and incremental mixing compared to the ECF sequence. A credit for recovered oxygen to be utilized in bleaching and delignification was also made. For chlorine dioxide, credit for recovered spent acid was made to the extent of current chemical makeup. The effect of bleach plant washer carryover and filtrate recycle was not made for either sequence. A comparison of TCF to ECF bleaching costs of the same pulps reviewed in Table 5, exclusive of identical costs attributed to (OpO) delignification, resulted in the TCF bleaching costs being 35-40% greater than ECF costs at similar entering Kappa number and an 87-87.5% ISO final brightness. Stage by stage chemical cost components for ECF compared to TCF bleaching are presented below in Figure 8.  Figure 8 Bleaching yield loss estimates were made by the determination of sequence filtrate COD as described in the literature. (6) The 87% ISO TCF sequence pulp resulted in a slightly lower total COD value compared to the 87.6% ISO ECF sequence pulp. Bleach yield for both sequences were about 95%. COD values doubled when a 91% ISO pulp was produced using the same TCF sequence, indicating substantial yield loss at elevated brightness levels. CONCLUSIONS This work demonstrates that TCF bleaching processes can be optimized to brighten 30 Kappa Douglas fir brownstock to market brightness levels. An optimized two-stage oxygen delignification strategy can be utilized that achieves a Kappa reduction level of 55% without fiber strength loss prior to bleaching measured as breaking length or burst index. Some loss in tear was evident. The need for two ozone stages is indicated to achieve market brightness at reasonable chemical application. The work further indicates that significant fiber strength and operating cost penalties would result during bleaching of this pulp utilizing the optimized TCF sequence compared to a standard ECF sequence, particularly to brightness levels above 87% ISO. Important considerations in designing a TCF sequence include: a) selective delignification before the bleach plant to lower the chemical demand and increase brightness ceiling, b) effective removal of transition metal ions prior to peroxide bleaching, c) utilization of increased pressure and/or temperature during peroxide bleaching to maximize stage effectiveness and d) limiting final brightness to 87-88% ISO to minimize losses in yield and fiber strength. Based on this study and investigations of commercial TCF facilities, production of TCF bleached pulp is not a feasible alternative to ECF bleaching for the Halsey mill. REFERENCES (1) Malinen, R. et al., "TCF Bleaching to High Brightness - Bleaching Sequences and Pulp Properties", Proceedings of 1994 CPPA/TAPPI/SPCI International Bleaching Conference, pp. 187-198. (2) Breed, D. et al., "Cost-Effective Retrofit of Existing Bleach Plants to ECF and TCF Bleached Pulp Production using a Novel Peroxide Bleaching Process", Proceedings of 1995 TAPPI Pulping Conference, pp. 779-788. (3) Parathasarathy, V.R. et al., "Hydrogen Peroxide Reinforced Oxygen Delignification of Southern (Loblolly) Pine Kraft Pulp and Short Sequence Bleaching", Proceedings of 1989 TAPPI Pulping Conference, pp. 539-547. (4) Fodor, R., "Evaluation of a Flow-Through Lab Scale Medium to High Consistency Ozone Reactor", Presented at 1996 CPPA Technical Section, Pacific Coast Branch Mini-Conference. (5) Murray, J.E. et al., "The Effect of Laboratory Mixing at Medium Pulp Concentration on Radiata Pine Kraft Pulp", Appita Vol. 48, No. 5, pp. 358-362. (6) Suss, H. and Kronis, J., "The Correlation of COD and Yield in Chemical Pulp Bleaching", Proceedings of 1998 TAPPI Breaking the Pulp Yield Barrier Symposium, pp. 153-162. APPENDIX Procedures for Bleaching and Testing of Critical Data Oxygen Delignification (OpO) Brownstock pulp was mixed in a plastic bag with the required amount of B.S. Washer filtrate, magnesium sulphate (7H2O-MgSO4), caustic soda and water and preheated to 90 oC in a microwave oven. After loading the reaction vessel [either a Quantum high-intensity mixer or a peg mixer], oxygen pressure was raised to 120 psig and the temperature in the reactor was brought up to 102 oC and held there for 20 minutes. Over the 20 minutes of delignification, pressure was gradually released until 90 psig was reached. The 90 psig of pressure was then released through a vent and the reactor was opened to add hydrogen peroxide and more caustic soda. The reaction vessel was then resealed, brought up to 90 psig by means of oxygen gas and held at 102 oC for an additional 60 minutes, during which time pressure in the vessel was gradually vented off until 30 psig was reached. Delignified pulp was then well washed. COD Measurement and Oxygen Delignification Stage Addition COD in the black liquor was analyzed in accordance with ALPHA "Standard Methods for the Examination of Water & Wastewater", Edition 18. Black liquor was then added back to the brownstock to reach the targeted COD carryover level. Acid/Chelation (AQ) Pulp was diluted to 10% consistency and soured to a pH of 2.5 using sulphuric acid before being heated to 80 oC in a microwave oven. After twenty minutes at temperature, DTPA was mixed into the acidified pulp and the pH of pulp was raised to 5.0 and the pulp was maintained at 80 oC for another 30 minutes before being washed. Procedures for Bleaching and Testing of Critical Data (cont'd.) Peracid (Px) Activation Chelated pulp was first mixed with caustic soda and then the required amount of Caro's (peroxymonsulphuric) acid was then added while mixing continued using a Hobart mixer. The resultant stock at 10% consistency was placed in sealed plastic bags and held in a constant temperature water bath set at 50 oC for 30 minutes. Pulp was removed and immediately sent on to a pressurized peroxide stage. Pressurized Peroxide (PO) or Pht The required bleaching chemicals including caustic soda, hydrogen peroxide, magnesium sulphate (7H2O-MgSO4) and organic stabilizer were first added to dilution water and then mixed into cold pulp using a Hobart mixer. The pulp was then placed in 250 ml. glass beakers and the entire beaker (up to four at once) were placed at the bottom of a Quantum high-intensity mixer which had its rotor removed. One inch of water was added at the bottom of the reactor creating in effect a "double boiler" (In earlier experiments, reactor bowl with rotor in place was used along with intermittent mixing). The Quantum mixer was then sealed and brought up to target temperature and pressure for the required amount of time. Ozone and Ozone/Chelation [Z or (ZQ)] Pulp was acidified, thickened in a centrifuge to 40% consistency and fluffed. It was thenloaded into a rotary glass vessel and the required amount of ozone gas was charged to the vessel. After three minutes the vessel was flushed with nitrogen for 20 minutes. Any unreacted ozone was collected in a potassium iodide trap. If chelation was performed, it was accomplished by diluting ozonated pulp to 10% consistency, adjusting the stock pH to 5.0 and adding 0.1% DTPA on pulp. (In earlier work, ozonation was done in either a flow-through reactor at Econotech Services - Vancouver, B.C. or in a Quantum reactor equipped with Ozone CaddyTM at Quantum Technologies - Twinsburg, OH.) Interstage Washing After a bleaching stage, pulp was diluted to 3% consistency using deionized water and then vacuum thickened to 12% consistency. Residual Chemical Determination Residual peroxide and ozone determinations were performed using iodimetric titration methods. Total alkali determination was performed using 0.1 N sulphuric acid and phenol red indicator. Micro Kappa Number Kappa number determinations as required were performed using TAPPI um-246. Brightness Measurement Brightness determinations were made using a Technidyne TB-1C reflectance meter in accordance with ISO procedures using 3.0 gram air dried handsheets. Metal Ion Analysis Metals analysis was performed using pulp that been completely digested in acid using an Inductively Coupled Plasma (ICP) procedure. |