CONVENTIONAL BLEACHING OF SOFTWOOD MECHANICAL PULP

Transcription

1 Experiences with Dithionite Based Additives (DBA) in (C)TMP in lab, pilot and mill scale synergies between high brightness, less specific energy consumption and development of pulp properties Martin Schachtl, Stefan Erren, Dieter Schönhaber, BASF SE, D Ludwigshafen, Germany Peter Dahlbom, Jon Henrik Steinsli, Norske Skog Skogn, N-7620 Skogn, Norway Lars Johansson, PFI-Paper and Fibre Research Institute, N-7491 Trondheim, Norway ABSTRACT The purpose of this paper is to report about our experiences with treating softwood mechanical pulp (MP) with DBA directly injected into the gap of high consistency (HC) refiners used as chemical reactors to improve the competitiveness and the ecological footprint of (C)TMP based products. After the description of the main factors and pros and cons of conventional oxidative and reductive bleaching of softwood MP the basic principles of the treatment of MP with DBA in the (C)TMP refiner are outlined. By means of exemplary results of trials in lab, pilot and mill scale as well as of productions the main advantages of DBA in (C)TMP are illustrated: efficient brightness gain, reduction of anionic trash, high wood yield, reduction of hard chelating agents, saving of refining energy and increase of strength properties. Using the example of Norske Skog Skogn it is demonstrated how the system works in practice in the production of high bright and standard newsprint made from normal and lower quality wood. Finally it is pointed out that to a certain extent DBAs can be tailored to the needs of the respective mill. INTRODUCTION Bleached TMP is used in many paper and board grades. One of the main advantages of MP is the very high resource efficiency: almost 100 % of the wood without the bark is gotten as fibers out of a tree. In comparison to chemical pulp - which works with wood yields below 50% - there are three main drawbacks: bleachability, minor strength properties and high electrical energy consumption for refining. Mechanical fibers in paper and board especially offer high opacity, uniform print image and high bulk. Besides the use for paper and board mechanical fibers are of interest as sustainable raw material for several future applications, e.g. for nano-ligno-cellulose produced from MP (1). Already in the 60s of the last century bleaching in refiners was tested (2). Through the decades many attempts were made in this area with different bleaching agents (3, 4). By adding sodium sulfite during chip impregnation strength properties have been increased and savings of refining energy together with small brightness gains have been achieved. Sulfonation of lignin components is seen as underlying basic mechanism of these results (5, 6). Since decades machine suppliers develop solutions to reduce the specific energy consumption (SEC) with constant or better fiber properties (7), also in combination with the addition of chemicals (8, 9). To achieve further substantial progress it is essential to gain a better picture about the mechanisms which are going on and prevailing in HC refining (10, 11). The purpose of this paper is to report about our experiences with treating softwood pulp with DBA directly injected into the gap of HC refiners used as chemical reactors to improve the competitiveness and the ecological footprint of (C)TMP based products. CONVENTIONAL BLEACHING OF SOFTWOOD MECHANICAL PULP The comparison of basic aspects of conventional reductive bleaching with hydrosulfite with conventional oxidative bleaching with hydrogen peroxide for softwood MP is shown in table 1. In some cases as first step oxidative and as second step reductive bleaching is used today (combined bleaching). In contrast to oxidative bleaching for reductive bleaching no bleach plant is necessary. Hydrosulfite is very reactive with chromophores and with other substances especially with oxygen. Therefore very good mixing of the hydrosulfite solution with the pulp is essential and usually ensured by dosing it directly in front of a medium consistency (MC) pump. Together with that ph and temperature are important parameters to gain good bleaching results.

2 Table 1: comparison of basic aspects of conventional reductive with oxidative bleaching of softwood mechanical pulp, COD - chemical oxygen demand BASIC PRINCIPLES OF THE TREATMENT OF MECHANICAL PULP WITH DBA IN THE (C)TMP REFINER The main reason to use a refiner for bleaching with hydrosulfite always was the good mixing inside. For a papermaker a TMP refiner is a mechanical device. This is true, but it is more: the refiner is a chemical reactor. The pressure is between 3 bar and 6 bar and could be even higher in the future. This means that there are temperatures of 120 C to 160 C and above which starts and accelerates chemical reactions which do not occur at all at conventional bleaching conditions. As it is common in chemical production as well as with many other chemical additives used in papermaking also with DBA in refining e.g. the composition, the dilution, the concentration of the chemical and the dosing position are essential factors for the whole process. The best results are achieved by dosing the DBA solution purely or together with the dilution water directly into the gap of the respective refiner, as illustrated in fig. 1. In the latter case the DBA solution should be dosed to the dilution water as short as possible in front of the injection into the refiner. By doing it the one or the other way the treatment is largely independent of the dilution water conditions like residual fiber content, ph or temperature. DBA Figure 1: dosing directly into the refiner gap with or without dilution water (Andritz, Valmet) Hydrosulfite which contains 88 % sodium dithionite is a very reactive reductive agent. Some possible side reactions of sodium dithionite at higher temperature and different ph values are shown in fig. 2. The DBAs, until now three of

3 them (DBA1, DBA2, DBA3), are developed to avoid these side reactions of dithionite and increase the selectivity for the reaction shown in green. This is necessary for different wood and water qualities as well as for different conditions in the refiners. By doing so deposits and corrosion, also in the condensate systems, are avoided too. Another criterion for the composition of the DBA used in the respective (C)TMP process is that the DBA should not require a special water quality or influence the conditions in the pulp line too much. Nevertheless the water quality is very crucial, e.g. it is not possible to produce high bright pulp and paper with dark waters in the cycles. A certain minimum water quality concerning brightness is necessary to produce paper with a certain brightness. Figure 2: some possible side reactions of sodium dithionite at higher temperature and different ph conditions All in all this means that the very reactive DBA and the new borne fibers with their fresh surface and easy accessible chromophore groups run into each other under very reactive conditions. The short residence time in the refiner in the range of seconds also helps to improve the selectivity of the reactions, i.e. to avoid back and side reactions. Both the theories about bleaching of MP as well as about softening of the wood in refining are described in (12). To achieve maximum brightness DBA has to be injected into all HC refiner stages where appreciable new surfaces are created in the respective pulp line, see fig. 3. Figure 3: multistage dosing to achieve maximum brightness gain The post dosing of standard hydrosulfite in the MC or low consistency (LC) part of the pulp line brings back the losses which occur in the refiners itself, which are caused by thermal yellowing, by long residence times on the way to the paper machine (PM) and by dilutions with dark waters. RESULTS IN LAB, PILOT AND MILL SCALE The following results are an extract of several series of lab trials for product development at the Technical University in Dresden/Germany, scale up trials at the Andritz pilot plant in Springfield/Ohio/USA and at KCL in Helsinki/Finland as well as of mill trials and long term productions of various paper mills with focus on Norske Skog Skogn. Efficient Brightness Gain, Reduction of Anionic Trash, High Wood Yield and Reduction of Hard Chelating Agents A CTMP mill changed from combined oxidative and reductive bleaching to pure reductive bleaching. The goal was reached that the COD was reduced enough to increase the production with constant high brightness without investing in the waste water treatment plant. In stable operation 6 kg/admt DBA2 were injected directly into the gaps of the

4 Valmet single disc refiners 1 st and 2 nd stage each together with the dilution water, see fig. 4. The chips were produced on site from spruce logs and were impregnated with 5 kg/admt of NaHSO 3. The refiners were operated at 3 bar. At several positions of the pulp line to the PM altogether 10 kg/admt also of DBA2 were dosed. With a standard hydrosulfite it would have been needed less but there was no possibility to dissolve a second powder in the mill at that time. The brightness has been increased from 59 % ISO of the NaHSO 3 impregnated pulp to 79 % ISO of the pulp at the outlet of the LC tower. Figure 4: CTMP mill: goal peroxide free mill reached with two stage dosing and post bleaching in MC/LC area of DBA2, fresh spruce, admt - air dry metric ton In another mill in stable production conditions the cationic demand of the TMP was cut by half by using 8 kg/admt DBA2 in the chip refiner instead of 15 kg/admt hydrogen peroxide together with 6 kg/admt water glass, 6 kg/admt caustic soda and 2 kg/admt chelating agent (EDTA) in a conventional peroxide process to bleach 85% spruce mixed with 15% pine from 54 % ISO to 68 % ISO at the outlet of the LC tower, see fig. 5: decrease from 250 mg PolyDADMAC/l to 125 mg PolyDADMAC/l. Still 2 kg EDTA/admt was used in this case. Figure 5: comparison of peroxide bleaching with treatment with DBA2: reduction of cationic demand to 50 % Caused by the alkaline conditions, which are necessary for peroxide bleaching a part of the fibers, i.e. of the wood, is dissolved. This decreases the wood yield, in some cases down to about 94 %. With the DBA process no or only a minor change is made concerning ph of the pulp and no wood is being dissolved. Thus the wood yield increases in the direction of close to 100 %. It is important to note that in such cases with a decisive reduction of anionic trash in the pulp the wetend chemicals system of the PM has to be adapted to the new situation. In general less chemicals are needed when you change from oxidative to reductive bleaching, e.g. the consumption of the fixing agent is reduced. Heavy metals like Fe, Mn, Cu and Co decompose peroxide. To avoid this hard chelating agents, e.g. DTPA, are added, e.g. at the chip impregnation or into the dilution water of the 1 st stage refiner. Depending on the heavy metal content of the wood this can be up to several kg/admt of pulp. For reductive bleaching chelating agents for this purpose are not necessary at all. In some cases a small dosage of a chelating agent is necessary to keep the heavy metals at a low oxidation stage to avoid yellowing along the pulp line to the PM. Saving of Refining Energy and Increase of Strength Properties Trials at the lab plant of Technical University Dresden indicated that with comparable brightness and freeness of ( ) ml CSF the SEC for fresh spruce was 8 % - 16 % less compared to TMP without adding chemicals, see fig. 6. Firstly 10 kg/admt DBA3 were added in stage 1 and in stage 2 each and secondly 10 kg/admt were added in the first and 20 kg/admt were added in the second stage. The pressure was 3 bar. The main saving was reached in the

5 1 st stage which has been proven in all pilot trials and mill productions. But also in the 2 nd stage the SEC was reduced which is also comparable to pilot and mill scale depending on the energy distribution of the two refining stages. Figure 6: SEC reduction by adding DBA3 in 1 st and 2 nd stage refining, Sprout Bauer, single disc 12, 1500 rpm, 3 bar, lab TU Dresden At KCL trials with a double disc used as single disc refiner RGP 44 with spruce saw mill chips showed 15 % SEC reduction in comparison to TMP at the same tensile index after dosage of DBA3 in various amounts in the 1 st and 2 nd stage altogether between 6 kg/odmt and 21 kg/odmt, see fig. 7. Figure 7: SEC reduction in comparison with TMP by adding DBA3 in 1 st and 2 nd stage refining, 3.5 bar/2.4 bar, Valmet RGP 44, 1500 rpm, KCL Helsinki, odmt oven dry metric ton In fig. 8 the results of the breaking length of a 100 % spruce pulp production (saw mill chips) for LWC base paper over several weeks is shown. The first stage is a single disc RTS refiner with 1800 rpm and the second is a single disc refiner with 1500 rpm. The dosage of DBA1 was (6-10) kg/admt into the first stage and about (2-4) kg/admt into the 2 nd stage together with the dilution water. The samples are taken out of the blow line of the 2 nd stage. Some time before the trial the plate set of the 1 st stage had to be changed. The influence of this change on the breaking length is much less than the one which results from the addition of DBA1 which is more than 500 m. During this trial also the tensile energy absorption has been increased between 15 % and 20 %.

6 Figure 8: improvement of pulp breaking length by addition of DBA1 in stage 1 and 2 in production of TMP for LWC base paper, single disc, Andritz 60 /62, 5.4 bar/4.4 bar, 1800 rpm/1500 rpm Results at Norske Skog Skogn The full scale study is conducted at the TMP 2B line at the mill to produce high bright and standard news, see fig. 9. This line has a production rate of 470 admt/day over 2 HC refiners and consists of the 1st stage Valmet CD 82 and of the 2 nd stage Valmet CD 76. The operating pressure is 5.0 bar. After chip washing the water is separated before it goes to a (possible) chip impregnation which is not used in this study. After the preheater the chips go into the first stage refiner R27 and afterwards directly into the second stage refiner R28. The raw material used is a mixture of round wood and saw mill chips from Norway spruce (Picea abies). NaHSO3 DBA DBA Figure 9: process overview: part of pulp line 2B at Norske Skog Skogn with dosing positions of NaHSO 3 and DBA into 1 st stage refiner R27 and into 2 nd stage refiner R28 DBA1 is dissolved in the mill s dissolving unit and pumped separately to the dilution water of the 1 st stage and of the 2 nd stage refiner. Alternatively sodium bisulfite solution (NaHSO 3 ) is dosed into the dilution water of the 1 st stage refiner. The dilution water is introduced into the entrance of the flat zones of the CD refiner gaps, see fig. 10. Pulp samples from blow lines after 1 st stage and 2 nd stage were taken continuously over 3 minutes, mixed well and analyzed with the following methods: Hot disintegration (ISO ), Canadian standard freeness (ISO , CSF), Rapid Köthen lab sheet (ISO , pressed and dried at room temperature), density (ISO 534), tensile index and elongation (ISO ), z-strength (Tappi T 541), tear index (ISO 1974), specific light scattering coefficient and light absorption coefficient at an effective wavelength of 457 nm (ISO 9416), brightness (ISO 2470), Bauer

7 McNett fractionation (SCAN-CM 6:05) and fiber length (L&W Fiber Tester Plus). Shives were measured in a Somerville laboratory screen (0.15 mm slots, Tappi T 275). dosage of dilution water DBA Figure 10: 1 st stage refiner R27: Valmet RGPA 82CD, 25 MW, 1500 rpm; 2 nd stage refiner R28: Valmet RGPA 76CD, 25MW, 1500 rpm; dilution water dosage together with DBA into both refiners Reference samples were taken with addition of NaHSO3 to the 1st stage refiner (6 kg/admt). To find optimum brightness, the dosing of DBA1 was done on several levels and split between 1st and 2nd stage refiner, see table 2 and figure 11. All samples were taken from the blow line of the 2 nd stage refiner R28. Table 2: effect of chemicals added to 1 st and 2 nd stage refiner on brightness of pulp taken from blow line stage 2 Chemical 1 st stage 2 nd stage brightness % ISO NaHSO 3 6 kg/admt DBA1 2 kg/admt DBA1 3 kg/admt DBA1 4 kg/admt DBA1 6 kg/admt DBA1 8 kg/admt DBA1 6 kg/admt 2 kg/admt 66.7 DBA1 5 kg/admt 3 kg/admt 68.1 DBA1 4 kg/admt 4 kg/admt 67.3

8 Figure 11: brightness after 2 nd refiner stage at freeness of about 140 ml CSF, basic brightness of TMP without chemicals: 58 % ISO Starting with a brightness of the unbleached TMP of 58 % ISO with an NaHSO 3 addition of 6 kg/admt into R % ISO is reached. Instead of NaHSO 3 DBA1 is added in R27 from 2 kg/admt to 8 kg/admt. The brightness increases from about 64 % ISO to about 67 % ISO. Then the 8 kg/admt DBA1 are distributed in several relations between dosage in R27 and R28. The best result with around 68 % ISO is achieved with dosing of 5 kg/admt in the 1 st and 3 kg/admt in the 2 nd stage. This dosage relation is kept constant during the following longer term trials to produce the high bright news. In fig. 12 the brightness along the pulp line from blow line of refiner stage 2 to the headbox of the PM is shown. At the point of time depicted the dosage of DBA1 in stage 1 was 5 kg/admt and in stage 2 it was 3 kg/admt for several hours. This resulted in a brightness of the pulp from blow line R28 of about 64 % ISO. With the addition of 4 kg/admt of standard hydrosulfite just before the pump in front of the MC tower a brightness of about 72 % ISO is reached at the inlet of this tower. This demonstrates that it is essential to combine the treatment of the pulp with DBA in the refiners with post bleaching with standard hydrosulfite dosed at appropriate positions of the pulp line before the PM. The darkening of the pulp between the dosage points is caused on the one hand by longer residence times in chests and by the quality of the dilution waters used in the process. To compensate these losses at the LC pump in front of the inlet of the LC tower 6 kg/admt hydrosulfite are dosed which leads to about 71 % ISO at the outlet of this tower. At this point of the process the pulp is delivered to the PM. To safeguard 68 + % ISO brightness continuously in the headbox another 3 kg/admt of standard hydrosulfite are added at the machine chest where a lot of water is used to dilute the pulp. Figure 12: brightness trend after refiner stage 2 to headbox of the PM: unbleached TMP: 58 % ISO, 5 kg/admt DBA1 in 1 st refiner and 3 kg/admt in 2 nd refiner After several hours the consumption of standard hydrosulfite moves from (4.0, 6.0, 3.0) kg/admt to (2.8, 6.0, 0.5) kg/admt to guarantee 68 + % ISO brightness at the headbox, see table 3. This demonstrates a very crucial aspect for such kind of trials as well as for PM production changes concerning brightness development and stability: depending on the water and pulp capacities of the pulp line and of the PM(s) water cycles you need a lot of time, not very seldom in the range of a day, to reach a stable production with high brightness. Table 3: dosages to safeguard 68 + % ISO brightness in the headbox of the PM brightness dosage DBA1 in post dosage remark headbox PM 1 st / 2 nd stage hydrosulfite > 68 % ISO (5 + 3) kg/admt 13.0 kg/admt start of process > 68 % ISO (5 + 3) kg/admt 9.3 kg/admt stable process

9 > 68 % ISO 0 kg/admt 24.0 kg/admt normal wood > 68 % ISO 0 kg/admt 38.0 kg/admt dark wood To reach 68 + % ISO at the headbox with normal wood without DBA in refiners 24 kg/admt of standard hydrosulfite are necessary additionally to the 6 kg/admt of NaHSO 3 dosed into the 1 st stage refiner R27. Seasonally the mill has to handle dark wood and then up to 38 kg/admt of standard hydrosulfite are needed together with the NaHSO 3 to get this brightness. The dosages of the standard hydrosulfite along the pulp line are not optimized until now. Also the water qualities used in the process can be rethought. All in all there is room for improvement in this respect. Together with the brightness the tensile index is increased with less specific energy consumption of the two refiner stages, see fig. 13. The injection of 6 kg/admt NaHSO 3 into the gap of the 1 st stage refiner leads to an increase of the tensile index without a significant reduction of SEC. 8 kg/admt DBA1 injection instead of NaHSO 3 at the same place results in the same tensile index and decreases the SEC by about 10%. Figure 13: tensile index after 2 nd refiner stage at comparable freeness of about 140 ml CSF This gives room for optimization in several directions: some mills increase the production rate, others change their mix of furnishes or reduce the grammage of the paper or whatever could be of interest in the relevant market. As mentioned earlier for mills it is necessary to handle different wood qualities to produce their full program of paper grades all over the year. In fig. 14 is shown how dark wood with a brightness of about 50 % reacts on the dosage of DBA1 into the 1 st stage refiner only. Already with 6 kg/admt about 65 % ISO are reached and 68 + can be secured in the headbox even with this kind of spruce with 50 % ISO unbleached brightness. Figure 14: brightness reaction of dark wood on dosage of DBA1 into the 1 st stage refiner As it was learnt by the trials for high bright paper shown above and from trials and productions in other paper mills there is a possibility for the mill to get rid of NaHSO 3 at all. To achieve this for the standard news besides getting the

10 brightness at the headbox of 59 % ISO certain values of tensile and z-strength are crucial to achieve. The results of these trials are shown in fig. 15. Instead of dosing 6 kg/admt NaHSO 3 into the 1 st stage 3 kg/admt DBA1 was injected there. The pulp samples were taken from the blow line of stage 2. Figure 15: tensile index and z-strength after 2 nd refiner stage at comparable freeness of about 140 ml CSF These results indicate that the necessary strength properties in this case can be achieved (tensile) or even outperformed (z-strength) with a low dosage of DBA1 into stage 1 only. Because of the learnings described above about post bleaching with standard hydrosulfite after treatment with DBA in the refiner it is expected that the brightness could be reached by less dosage of standard hydrosulfite when DBA is used instead to NaHSO 3. A long term trial will be carried out soon. Tailoring of DBAs Several trial series were conducted at the Andritz pilot plant in the US. The exemplary results shown in fig. 16 were obtained under the following conditions: first stage single disc refiner RTS 1800 rpm 36-1CP with 3.45 bar, second stage atmospheric double disc refiner model 401 (36 ) with 1200 rpm, 100% fresh spruce, dosing of 20 kg/odmt DBAs in 1 st stage only. The data are calculated by linear interpolation of the trial points. Figure 16: comparison of brightness and freeness of spruce pulp of TMP without and with dosing of 20 kg/odmt of different DBAs into the 1 st stage of two stage refining, each 36, 3.45 bar/1.0 bar, SEC 1800 kwh/odmt, pilot plant Andritz USA The brightness of the untreated TMP with an energy input in total of 1800 kwh/odmt is 46.9 % ISO. With 2 % DBA the three DBAs lead to pulps taken from the blow line of the 2 nd stage with three different brightness levels between 57.8 % ISO and 62.8 % ISO. As mentioned above for the full brightness development post bleaching with standard hydrosulfite is necessary. The CSF is also differing between the three DBAs. It is lowest with DBA3 and at the maximum with DBA1. These differences are triggered by different compositions of the blends of the DBAs, which can be adjusted to the respective conditions, like wood quality, ph, temperature, etc., and to the goals of the respective mill. E.g. it is possible to tailor a DBA more in the direction of brightness or more in the direction of energy saving.

11 CONCLUSIONS By injecting DBA directly into the gap of HC refiners at elevated pressures, as pure solution or together with the dilution water, chemical reactions are initiated which do not occur at the conditions prevailing under conventional reductive bleaching in pulp mills. This is true for high and low dosages of DBAs. Efficient brightness gains of up to 20 % ISO until now, a significant reduction of SEC in the range of 15 % and improvements of several strength properties like tensile and z-strength are observed. In comparison to oxidative bleaching the treatment of pulp with DBA in (C)TMP and post bleaching with standard hydrosulfite at the same brightness level lead to reduced chemical demand on the PM and to lower COD in the waste water plant. Thus the wood yield is increased. The production process is simple, e.g. a bleach plant is not necessary and the number and quantity of chemicals used in the mill are reduced. Therefore the fiber costs are decreased. All in all this leads to a more ecological production and to new degrees of freedom for the design of new wood containing paper grades - in existing and new pulp and paper plants. ACKNOWLEDGMENT The authors thank the staff of Technical University in Dresden, Andritz pilot plant in Springfield/Ohio, KCL pilot plant in Helsinki, A. Fredrikson Research & Consulting in Jyväskylä, PFI in Trondheim, Norske Skog Skogn, BASF in Ludwigshafen and the several not named paper mills who contributed so much to lift the knowledge about DBA in refining to a level ready to apply in practice. References 1. Osong, S.H., Norgren, S., Engstrand, P., Lundberg, M., Hansen, P., Development of nano-ligno-cellulose produced from mechanical pulp, International Mechanical Pulping Conference, Helsinki, Munroe, D.C., Theodorescu, G., Higher brightness mechanical pulps with sodium hydrosulfite/successful mill operation, TAPPI Pulping Conference, Melzer, J., Auhorn, W., Behandlung des Holzstoffs mit reduktiven Bleichchemikalien in Refinern, Wochenblatt für Papierfabrikation, 114, 1986, Nr. 8, pp Harrison R., Parrish T., Gibson A., Knapp C., Wajer M., Johnson D., Refiner bleaching with magnesium hydroxide (Mg(OH)2) and hydrogen peroxide, Tappi Journal, 7(9), p. 16, Richardson J., Sulphonation mechanisms in DWS and CTMP pulping, Appita J., 51(1), p. 39, Nelsson, E., Improved energy efficiency in mill scale production of mechanical pulp by increased wood softening and refining intensity, PhD, Mid Sweden University, Sundsvall, Vuorio, P., Ruola, V., Muhic, D., Bergquist, P., New refiner segment technology to produce high quality mechanical pulp with minimized energy consumption, International Mechanical Pulping Conference, Helsinki, Johansson, L., Hill, J., Gorski, D., Axelsson, P., Improvement of energy efficiency in TMP refining by selective wood disintegration and targeted application of chemicals, Nord. Pulp Pap. Res. J., 26(1), pp , Sabourin, M., Erren, S., Producing high brightness and low energy mechanical pulps using injected hydrosulfite formulations, International Mechanical Pulping Conference, Helsinki, Fredrikson, A., Salminen, L.I., Sirviö, J., Fiber pad phenomena of high consistency TMP refining, International Mechanical Pulping Conference, Helsinki, Ferritsius, R., Hill, J., Ferritsius, O., Karlström, A., On energy efficiency in chip refining, International Mechanical Pulping Conference, Helsinki, Lönnberg, B. (ed.), Papermaking Science and Technology, vol. 5, Mechanical Pulping, 2009

12 Na + O O Na + S S O O Experiences with Dithionite Based Additives (DBA) in (C)TMP in lab, pilot and mill scale - synergies between high brightness, less specific energy consumption and development of pulp properties Martin Schachtl, Stefan Erren, Dieter Schönhaber, BASF SE Peter Dahlbom, Jon Henrik Steinsli, Norske Skog Skogn Lars Johansson, PFI-Paper and Fibre Research Institute 1

13 outline HC refiner as chemical reactor for treatment of (C)TMP results in lab, pilot and mill scale efficient brightness gain and less generation of anionic trash high wood yield and reduction of hard chelating agents saving of specific energy consumption improvement of strength properties results at Norske Skog Skogn for high bright and standard news tailoring of DBAs conclusions 2

14 the refiner a chemical reactor For papermakers the refiner is a mechanical device high temperature high chemical concentration high consistency and intensive mixing short residence time For BASF as chemical company the refiner is a chemical reactor 3

15 high reactivity of sodium dithionite needs high selective reaction control High temperature alkaline Na 2 S 2 O 4 Na 2 SO 3 + Na 2 S Dithionite based Additives (DBA) - special development for refiner conditions! Unwanted side reactions are avoided Na 2 S 2 O 4 Na 2 SO 3 + Na 2 SO 4 High selective reaction control leads to highest efficiency and maximum results acid Na 2 S 2 O 4 Na 2 S 2 O 3 + Na 2 S 2 O 5 Na 2 S 2 O 4 SO 2 + S Na 2 S 2 O 4 SO 2 + H 2 S Unwanted side reactions are avoided Possible side reactions of Sodium Dithionite (exemplary) Dithionite based Additives (DBA) avoid corrosion and deposits 4

16 dosing as formula for success dosing directly into the refiner gap with or without dilution water DBA high reactive reductive additive and emerging fibers directly get in touch! multistage dosing to achieve maximum brightness gain DBA dosing in 1st refiner stage DBA dosing in 2nd refiner stage DBA dosing in reject refiner post bleaching with hydrosulfite in MC/LC-area 5

17 outline HC refiner as chemical reactor for treatment of (C)TMP results in lab, pilot and mill scale efficient brightness gain and less generation of anionic trash high wood yield and reduction of hard chelating agents saving of specific energy consumption improvement of strength properties results at Norske Skog Skogn for high bright and standard news tailoring of DBAs conclusions 6

18 efficient brightness gain and less generation of anionic trash CTMP, goal peroxide free mill, COD reduction, 100 % fresh spruce 1st refining stage dosage 6 kg DBA2/admt 2nd refining stage dosage 6 kg DBA2/admt MC/LC area dosage 10 kg DBA2/admt brightness gain 20 % ISO unbleached pulp 59 % ISO final brightness pulp 79 % ISO TMP mill, cationic demand reduction, 85 % fresh spruce, 15 % fresh pine brightness % ISO cationic demand mg PolyDADMAC/l cationic demand reduced down to 50% compared to oxidative bleaching with the same gain of brightness of 14 % ISO 53 TMP 1.5 % peroxide/admt 0.8% DBA2/admt 50 cationic demand ISO brightness 7

19 high wood yield and reduction of hard chelating agents caused by alkaline conditions necessary with peroxide bleaching part of the raw material wood is dissolved increase of COD and decrease of wood yield even down to 94 % reductive bleaching does not change the ph conditions of the pulp.no dissolving of wood.wood yield in direction of 100 % heavy metals which are very soluble under alkaline conditions decompose hydrogen peroxide a lot of hard chelating agents are necessary to protect it reductive bleaching is performed under neutral to slightly acid conditions. less amount of chelating agents is sufficient to keep the heavy metals at low oxidation stage to prevent yellowing of the pulp 8

20 saving of specific energy consumption KCL pilot plant Espoo Finland tensile index, Nm/g MWh/admt 2.0 MWh/ admt SEC, MWh/odmt TMP DBA3 15 % less SEC for the same tensile index 9

21 improvement of strength properties TMP mill printing papers, 100% spruce from saw mill breaking length in km plate set 2, with DBA1 plate set 2 w/o DBA1 plate set 1 w/o DBA1 Increase of > 500 m increasing breaking length freeness CSF in ml 10

22 outline HC refiner as chemical reactor for treatment of (C)TMP results in lab, pilot and mill scale efficient brightness gain and less generation of anionic trash high wood yield and reduction of hard chelating agents saving of specific energy consumption improvement of strength properties results at Norske Skog Skogn for high bright and standard news tailoring of DBAs conclusions 11

23 Skogn - process overview Norske Skog Skogn NaHSO3 DBA DBA 12

24 Skogn - dosing DBA into CD Refiner 82 /76 dosage of dilution water DBA 13

25 Skogn - effect of chemicals added in 1 st and 2 nd refiner stage kg DBA1 chemical 1 st stage 2 nd stage NaHSO 3 6 kg/admt DBA1 2 kg/admt brightness [% ISO] brightness, % ISO kg NaHSO 3 DBA1 3 kg/admt DBA1 4 kg/admt DBA1 6 kg/admt DBA1 8 kg/admt DBA1 6 kg/admt 2 kg/admt TMP DBA1 kg/admt DBA1 5 kg/admt 3 kg/admt 68.1 DBA1 4 kg/admt 4 kg/admt

26 Skogn - brightness trend with DBA1 from refiner stage 2 to headbox of PM brightness % ISO kg/admt hydrosulfite kg/admt hydrosulfite 3 kg/admt hydrosulfite (5+3) kg/admt DBA1 60 refiner stage 2 inlet MC tower outlet MC tower outlet latency chest blowline LC refiner inlet LC tower outlet LC tower machine chest mixing chest headbox PM 15

27 Skogn - stable production of high bright news: dosages for normal wood brightness headbox PM dosage DBA1 in 1 st / 2 nd stage post dosage hydrosulfite remark > 68 % ISO (5 + 3) kg/admt 13.0 kg/admt start of process > 68 % ISO (5 + 3) kg/admt 9.3 kg/admt stable process > 68 % ISO 0 kg/admt 24.0 kg/admt stable process 16

28 Skogn - room for cost reduction and design of new paper grades increase of strength and SEC reduction with DBA1 17

29 outline HC refiner as chemical reactor for treatment of (C)TMP results in lab, pilot and mill scale efficient brightness gain and less generation of anionic trash high wood yield and reduction of hard chelating agents saving of specific energy consumption improvement of strength properties results at Norske Skog Skogn for high bright and standard news tailoring of DBAs conclusions 18

30 Skogn - low dosage of DBA1 for standard news tensile Nm/g brightness % ISO TMP 6kg sod.-bisulfite/admt 3 kg DBA1/admt 20 tensile brightness dosage of 3 kg DBA1in 1 st stage refiner results in sufficient brightness and tensile strength 19

31 Skogn - low dosage of DBA1 for standard news z-strength kpa brightness % ISO TMP 6kg sod.-bisulfite/admt 3 kg DBA1/admt 20 z-strength brightness dosage of 3 kg DBA1in 1 st stage refiner results in sufficient brightness and z-strength 20

32 outline HC refiner as chemical reactor for treatment of (C)TMP results in lab, pilot and mill scale efficient brightness gain and less generation of anionic trash high wood yield and reduction of hard chelating agents saving of specific energy consumption improvement of strength properties results at Norske Skog Skogn for high bright and standard news tailoring of DBAs conclusions 21

33 tailoring of DBAs to the needs of the mills TMP 2% DBA1 2% DBA2 2% DBA brightness % ISO CSF ml example pilot plant Andritz: 20 kg of different DBAs/odmt result in different brightness and freeness values of a 100 % spruce pulp 22

34 conclusions the refiner a chemical reactor For papermakers the refiner is a mechanical device high temperature high chemical concentration high consistency and intensive mixing short residence time For BASF as chemical company the refiner is a chemical reactor 23

35 conclusions DBA in (C)TMP results in: brightness gain comparable to oxidative bleaching substantial lower energy demand high fines content and improved pulp properties higher wood yield compared to oxidative bleaching systems lower chemical demand at PM and in wastewater treatment more ecologically friendly production process enables to optimize production process and costs e.g. variation of fibermix, filler content grammage provides additional degrees of freedom to design paper grades 24

36 Na + O O Na + S S O O Experiences with Dithionite Based Additives (DBA) in (C)TMP in lab, pilot and mill scale - synergies between high brightness, less specific energy consumption and development of pulp properties Martin Schachtl, Stefan Erren, Dieter Schönhaber, BASF SE Peter Dahlbom, Jon Henrik Steinsli, Norske Skog Skogn Lars Johansson, PFI-Paper and Fibre Research Institute 25

37 TMP properties profile shows strengths and limitations Refiner mechanical pulp is used since a long time in paper industry. Improvements of efficiency and fiber properties by constructive modifications variations of pressure and temperature application of chemical additives (Sulfites) An Asplund Defibrator unit in the 1940s TMP strengths TMP limitations high opacity uniform print image high bulk level insufficient strength properties high electrical energy consumption bleaching step necessary 26

38 conventional reductive / oxidative bleaching of mechanical pulp Reductive bleaching (hydrosulfite) brightness increase up to 12 % ISO gentle bleaching conditions at slightly acid to neutral ph level marginal damages of fibers and fines no negative impact on fiber properties low generation of anionic trash/cod no additives necessary low conductivity level easy process control low cost of ownership Oxidative Bleaching (peroxide) brightness increase up to 20 % ISO and more aggressive bleaching conditions at alkaline ph level damages of fibers and dissolving of fines negative impact on fiber properties high generation of anionic trash/cod due to necessary additives high conductivity level complex process control high cost of ownership 27

39 Hydrosulfite: proven bleaching agent for mechanical pulp reductive bleaching is quick and effective with limited brightness gain intensive mixing is necessary ph-level und temperature are important reaction parameters Mid of the eighties also BASF investigated the addition of Sodium Dithionite (hydrosulfite) into the refiner Behandlung des Holzstoffs mit reduktiven Bleichemikalien in Refinern J. Melzer, W. Auhorn Wochenblatt für Papierproduktion Ausgabe 8,

40 saving of specific energy consumption Institute of Plant and Wood Chemistry TU Dresden SEC kwh/admt (8-16) % less SEC mainly in 1 st stage TMP (1+1) % DBA3 (2+1) % DBA3 1st stage 2nd stage total 29

41 Skogn - production of high bright news w/o DBA: dosages for normal and dark wood brightness headbox PM dosage DBA1 in 1 st / 2 nd stage post dosage hydrosulfite remark > 68 % ISO 0 kg/admt 24.0 kg/admt normal wood > 68 % ISO 0 kg/admt 38.0 kg/admt dark wood 30

42 Skogn - high bright news possible from dark wood with low dosage of DBA brightness % ISO DBA1 kg/admt 6 kg DBA1/admt into 1 st stage refiner increase brightness by 15 % ISO 31