OUTLINE HEXENURONIC ACID IN PULP BLEACHING AND HYDROLYSIS HEXENURONIC ACID REACTIVE COMPONENTS OF OXYGEN DELIGNIFIED KRAFT PULPS

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1 UTLINE EXENURNIC ACID IN PULP BLEACING AND YDRLYSIS EXENURNIC ACID exenuronic acid (exa) ydrolysis of exa exa in chlorine dioxide stages exa in ozone bleaching Impact of exa on peroxide bleaching exa in peracetic acid bleaching REACTIVE CMPNENTS F XYGEN DELIGNIFIED KRAFT PULPS CEMICAL NATURE F BLEACING STAGES Softwood pulp ardwood pulp Lignin ~ 2 % ~ 1 % Phenolic Non-phenolic Chromophoric exa < 0.5 % < 1 % Transition metals < 0.01 % < 0.01 % Polysaccharides ~ 97 % ~ 98 % Electrophilic: Lignin, exa Nucleophilic: Chromophores, carbonyl structures Radical: Phenolic lignin ther: Transition metals D (Cl 2, Cl), Z, Paa, A ( - ), D (Cl - ), P, Paa, D X, Q 1

2 FRMATIN F EXENURNIC ACID PRPERTIES F EXENURNIC ACID C 2 C 2 C 2 C 3 Xylan Xylan Xylan eat Brightness reversion Factors affecting: Wood species (xylan content) Cooking conditions (alkalinity, ion concentration) uncoloured xidized by permanganate Cation binding capacity CEMICAL REACTIVITY F EXENURNIC ACID YDRLYSIS F EXENURNIC ACID Chlorine ypochlorous acid Chlorine dioxide zone Peracids xygen Alkaline peroxide C 2 Xylan log (Rate constant, h -1 ) p 2

3 YDRLYSIS F EXENURNIC ACID YDRLYSIS F EXENURNIC ACID Furancarboxylic acids Consume KMn 4 Soluble in water Autocatalysis Autobuffering (p 3-4) Removal of metal ions (binding sites) A/Q very efficient for removal of Mn + C 2 90 % Yield loss (% on pulp) exa (hardwood kraft) exa (softwood kraft) ther (softwood kraft) ther (hardwood kraft) 10 % Reaction time (h) YDRLYSIS F EXENURNIC ACID 1200 REMVAL F LIGNIN AND EXENURNIC ACID IN AN ECF BLEACING STAGE Viscosity (ml/g) η = η 0 [1 - (κ 0 - κ)/ κ max ] k /k D E D E D Lignin exa Kappa number 3

4 PRDUCTS F EXENURNIC ACID IN ECF BLEACING STAGES (mmol/kg pulp) Cl Cl Cl 8.1 (DEDD) 9.4 (DEDD) 0.5 (A/DEDD) 0.3 (A/DEDD) 16.7 (DEDD) 10.5 (DEDD) AX 0.6 (A/DEDD) 0.0 (A/DEDD) PRDUCTS F EXENURNIC ACID IN ECF BLEACING STAGES igh p Low p Cl Cl Cl AX PRINCIPLE F A/D AND D/A/D PREBLEACING REACTIN RATE F CLRINE DIXIDE WIT A ARDWD KRAFT PULP AT 95 o C D A Phenolic lignin Non-phenolic lignin exa D(p 9-10) A D(p 3-4) Cl 2 igh p Cl - 2 Low p Cl 2 + Cl p Cl Cl 2 - After the fast reaction: - colour of Cl 2 disappeared - 60 % of active chlorine left A/D D/A/D Cl Time (s) 4

5 EFFECTS F TEMPERATURE AND TIME N A/D PREBLEACING (p 3.5 => 2.5) Temperature ( o C) Time (min) Kappa number Brightness (%) Viscosity (kg/m 3 ) EXENURNIC ACID AND CLRINE DIXIDE BLEACING exa increases chemical consumption exa increases AX exa and lignin react at ~ equal rates Generally A/D prebleaching is most cost-efficient kappa factor must be low and optimized little reaction with furancarboxylic acids no additional washing needed short retention time (D) Residual Cl 2 (%) ZNATIN F EXENURNIC ACID ZNATIN F EXENURNIC ACID ALTERNATIVE RUTE C 2 Xylan C 2 Xylan slow C 2 slow Xylan + (C 2 ) 2 + (C 2 ) 2 C 2 Xylan - C 2 Xylan 5

6 EXENURNIC ACID AND ZNE BLEACING exa increases chemical consumption exa forms oxalic acid calcium oxalate scalings exa forms reducing sugars additional chemical consumption in a subsequent P-stage Chemical reaction rates are diffusion controlled exa and lignin react at ~ equal rates furancarboxylic acids react rapidly washing is needed between A- and Z-stages EXENURNIC ACID AND PERXIDE BLEACING ydrogen peroxide does not react with exa Reducing sugars from Z-stage increase chemical consumption and decrease viscosity REACTINS F PERACETIC ACID WIT EXENURNIC ACID CMBINATIN F A- AND PAA-STAGES IN BLEACING F BIRC KRAFT PULP Ac C 2 C 2 2 Ac Xylan Xylan - C 2, Ac ydrolysis Xylan 4 Ac C C 2 Stage - PAA A (A)- PAA PAA (%) (mmol/kg) Kappa number exa (meq/kg) Brightness (% IS) Formic acid (mmol/kg)

7 REMVAL F LIGNIN AND EXENURNIC ACID IN ELECTRPILIC BLEACING EXENURNIC ACID AND PERACETIC ACID BLEACING Relative exa content (%) Paa-stage Z-stage D-stage exa consumes huge amounts of chemical exa reacts faster than lignin Peracetic acid bleaching is uneconomic if exa content is high Relative lignin content (%) SUMMARY Effects of hexenuronic acid brightness reversion consumption of bleaching chemicals chlorine dioxide ozone peroxide (when combined with ozone bleaching) peracetic acid calcium oxalate scalings ozone Selective hydrolysis of hexenuronic acid A-Z- A-D-, A/D- others possible 7