The Chemistry of Bleaching and Post-Color Formation in Kraft Pulps. Göran Gellerstedt

Transcription

1 The Chemistry of Bleaching and Post-Color Formation in Kraft Pulps Göran Gellerstedt

2 Content The structure of residual lignin The meaning of kappa number General aspects on bleaching The oxygen stage The chlorine dioxide stage The peroxide stage Post-yellowing of kraft pulps Conclusions

3 Content The structure of residual lignin The meaning of kappa number General aspects on bleaching The oxygen stage The chlorine dioxide stage The peroxide stage Post-yellowing of kraft pulps Conclusions

4 Degree of delignification for different wood species Pulp type Kappa No Lignin kappa Delign. degree Pine Birch Eucalyptus gl

5 G-units/S-units in white birch wood Morphological Differentiation Guaiacyl/Syringyl Lignin Fibre, S2-layer Vessel, S2-layer Ray parenchyma, S-layer 12 : : : 51 G-unit CH 3 Middle lamella (fibre-fibre) Middle lamella (fibre-vessel) 91 : 9 80 : 20 S-unit Lignin Middle lamella (fibre-ray) Middle lamella (ray-ray) 100 : 0 88 : 12 H 3 C CH 3 Ref. Saka and Goring, 1988

6 Kraft pulping of birch and E globulus respectively to similar kappa numbers Eucalyptus Birch

7 Possible reasons for the more rapid delignification in E globulus Higher frequency of b--4 structures. Probably not Less G-units. Yes, probably true Different chain lengths in lignin more phenolic end-groups in eucalypt. Not known but possible Different types of LCC. Yes, according to present knowledge

8 b--4 structures in wood and pulp based on thioacidolysis (Birch and Euc. gl) Degradation product, mmol/g of lignin S/G 2000 S/G G S Birch B pulp Euc E pulp Klason lignin, %:

9 b-ether cleavage in pulping; birch and eucalyptus globulus Birch: Gellerstedt et al, ISWPC 1989 Euc. gl.: Pinto et al, J Wood Chem Technol, 2002

10 Kinetic data for model compounds Phenolic units, 130 o C H Lignin HH 3 C Lignin H CH 3 H k = 6.6 x 10-3 min -1 CH 3 k = 12.1 x 10-3 min -1 H CH 3 H 3 C H CH 3 Non-phenolic units, 140 o C H Lignin HH 3 C Lignin H CH 3 H k = 1.5 x 10-3 min -1 CH 3 k = 1.2 x 10-3 min -1 Lignin CH 3 H 3 C Lignin CH 3 Ref. Kondo et al 1995

11 Peeling of linear lignin A low value of n gives a high number of phenolic endgroups and a faster lignin degradation/dissolution H S S n CH 3 H H CH 3 CH 3 Lignin CH 3 H H 3 C H H 3 C H 3 C CH 3 H H H 3 C H H CH 3 C 3 H H CH 3

12 Distribution of lignin in LCCs from spruce kraft pulp (Network = GlcMan-L-Xyl)

13 Chromatographic separation of pulp polymers in unbleached birch pulp Cellulose Lignin Hemicelluloses Ref. Karlsson, 1996

14 Distribution of lignin in LCCs from birch and E globulus kraft pulps Kappa Number ~16; Lignin-related Kappa = 4.0 and 5.7 respectively Soluble L-Xyl Insoluble L-Cell Isolated L-Xyl Isolated L-Cell

15 Content The structure of residual lignin The meaning of kappa number General aspects on bleaching The oxygen stage The chlorine dioxide stage The peroxide stage Post-yellowing of kraft pulps Conclusions

16 Contribution to kappa number in some selected pulps Pulp type/kappa number Lignin 1 HexA 1, 2 Non-lignin 1, 3 Pine kraft Pine soda/aq Birch kraft Eucalyptus kraft ) calculated in Kappa number units 2) HexA = Hexenuronic acid 3) Non-lignin = non-specified but oxidizable structures

17 Formation of hexenuronic acid on kraft pulping of E globulus 160 o C, AA=17%, sulf.=28%, L/W=4 Ref: C. Pascoal Neto et al, 1999

18 Non-lignin structures vs hemicelluloses Spruce kraft, kappa 26 Spruce kraft, kappa 16 Spruce prehydr. kraft Spruce sulfite Birch kraft Birch prehydr. kraft Birch sulfite Ref: S. Antonsson et al, 2003

19 Content The structure of residual lignin The meaning of kappa number General aspects on bleaching The oxygen stage The chlorine dioxide stage The peroxide stage Post-yellowing of kraft pulps Conclusions

20 Brightness development as a function of lignin-related kappa number

21 Successive changes in kappa number for a softwood kraft pulp Reduction in kappa number, % Lignin HexA ''non-lignin'' DE QP -40

22 Successive changes in kappa number for a birch kraft pulp Reduction in kappa number, % Lignin HexA ''non-lignin'' Q(P) Q(P) -120

23 Bleachability in P- and D-based bleaching respectively (softwood)

24 Consumption of bleaching chemicals (XE/kappa No) to reach +90% IS XE/kappa No Bleaching sequence Pine: D(P)(DQ)(P) Birch: D(P)D Euc. gl.: D(P)DP Pine Birch Euc. gl. Ref: F. Lundkvist, STFI-PF

25 Content The structure of residual lignin The meaning of kappa number General aspects on bleaching The oxygen stage The chlorine dioxide stage The peroxide stage Post-yellowing of kraft pulps Conclusions

26 The effect of alkali treatment, leaching, on kraft pulps Softwood kraft; 80 o C; 120 min; low cons.; lab. experiments Pulp Kappa No Reduction, % before after Industrial, after -stage Laboratory, unbleached Laboratory, unbleached Industrial eucalypt kraft; lignin extraction with/without oxygen -stage NaH, xygen, Temp., ph Kappa No Red., % kg/t kg/t o C unbl. -stage with oxygen without oxygen

27 Modes of lignin degradation in the oxygen stage L H CH 3 L 2 /H L + 2 or 2 Sidechain elimination Ring splitting CH 3 CH 3 L Demethoxylation H CH 3 to acidic groups and formation of methanol

28 Major carbohydrate reactions in the -stage resulting in cleavage of cellulose chains Cell CH 2 H H Cell 2 -ox. Cell H H Cell H 2 -ox. H - H - Cell-H Cell CH 2 H H Cell H H H Cell H - - Cell-H H 2 -ox. H CH 2 H H Cell H CH H Cell Further degradation H Alkali stable end-group of the HexA-type

29 Presence of glucuronic acid and glucuronaldehyde groups in kraft pulps Spruce/pine Relative Glucuronic acid/aldehyde unbl 2 D P Birch Relative Glucuronic acid/aldehyde unbl 2 Q1 P Blue = Glucuronic acid Red = Sum of glucuronaldehyde and glucuronic acid

30 Amount of lignin linked to glucan, xylan and glucomannan after the -stage Spruce/pine kraft pulp Glucan-LCC Glucomannan- LCC Xylan-LCC

31 Content The structure of residual lignin The meaning of kappa number General aspects on bleaching The oxygen stage The chlorine dioxide stage The peroxide stage Post-yellowing of kraft pulps Conclusions

32 Delignification response on D and D+C bleaching of kraft pulp with/without phenols Pine kraft pulp Ref. U. Germgård, Ph.D. Thesis 1982

33 The chemistry of chlorine dioxide oxidation of lignin L CH 3 +Cl 2 -Cl 2 L L CH 3 CH 3 +Cl 2 L Cl Ref: Ni et al, 1991 L +H 2 -HCl CH 3 +H 2 -HCl 2 ; -CH 3 H L CCH 3 CH

34 Formation of organic chlorine in a D- sequence (softwood) Lignin - H HCl/Cl 2 Lignin - Cl

35 Chemical changes in an A-stage, ph=3.5 Eucalyptus gr.; unbleached kappa 17.9; HexA = ~60 mmol/g = 85 o C = 95 o C Ref: Smedmark and Sjödahl, 2000

36 Efficiency of hexenuronic acid removal from E globulus pulp in a D-stage D 0 = ph 3, 60 o C, 45 min; D ht = ph 3, 90 o C, 240 min Stage HexA removal, Visc., Brightness, mmol/kg ml/g % IS Total Hydrol. x. - 0 (67) D D ht D 0 (E)D 1 D D ht (E)D 1 D ) riginal value in pulp after the -stage

37 Content The structure of residual lignin The meaning of kappa number General aspects on bleaching The oxygen stage The chlorine dioxide stage The peroxide stage Post-yellowing of kraft pulps Conclusions

38 Cleavage of a phenolic ether structure in lignin with peroxide CH 3 CH 2 H HC HC H Lignin H CH 3 CH 2 H HC CH Lignin Lignin CH 3 Lignin CH 3 CH 3 CH 2 H HC CH H Lignin H (or H 2 ) H 2 CH 3 CH 2 H HC HC H Lignin Lignin CH 3 Lignin CH 3

39 Alkaline peroxide oxidation of lignin, isolated from a kraft pulp after the - stage

40 Brightness after seems to determine the peroxide consumtion (softwood) IS brightness, % Total peroxide charge, kg/t Q 1 (P)Q 2 (P) 17::55,1 30::54,2 17::43,5 24::42,7

41 Content The structure of residual lignin The meaning of kappa number General aspects on bleaching The oxygen stage The chlorine dioxide stage The peroxide stage Post-yellowing of kraft pulps Conclusions

42 Yellowing - Influence of moisture Birch kraft; TCF-bleached; Temp: 60 o C

43 Yellowing Influence of temperature Birch kraft; TCF-bleached; Dryness: 92% Ref: C. Rööst, Södra

44 Post-color number for bleached kraft pulps from spruce, birch and eucalypt

45 Changes in the content of HexA, FA and FFA with ageing time E grandis; ECF-bleached

46 Correlation between brightness loss and the missing HexA

47 Reaction scheme for the formation of yellowing products in bleached kraft pulp Fe H H H HexA Xylan ageing, H 2 /H + HC FA FFA CH CH Xylan H RA H Fe-ions Further reactions to coloured products

48 Brightness of filter paper after ageing in the presence of oxidized sugars CH 2 H Ageing: 80 o C, 16 h, 80% Rh CH 3 H H Theander 1958 CH H H H H H + H + H H + others Added Compound None Cellobiose Glucuronic acid 3-keto-methyl-bglucoside Brightness, % Feather and Harris 1966 Reductic acid 19.4 Ref:. Theander 1988

49 Possible ways of reducing the yellowing tendency Prolonged D-stage at high temperature A specific A-stage Adjustment of temperature prior to baling Addition of radical scavanger

50 Conclusions The chemical reactions between pulp and various bleaching agents are well known The chemistry behind the high degree of yellowing of certain bleached kraft pulps is known The structure of residual fiber lignin after pulping is partially known The importance of condensation reactions in pulping is not known The changes occurring in pulping and bleaching of the wood polymers on the ultrastructural level are not known It is surprising that the lignin after pulping (2-3% in hardwoods) is so resistant towards all bleaching agents