EFFECT OF LAST STAGE BLEACHING WITH PERACETIC ACID ON BRIGHTNESS DEVELOPMENT AND PROPERTIES OF EUCALYPTUS PULP

Transcription

1 4th International Colloquium on Eucalyptus Pulp, Concepcion, Chile May 4-6, 2009 EFFECT F LAST STAGE BLEACHING WITH PERACETIC ACID N BRIGHTNESS DEVELPMENT AND PRPERTIES F EUCALYPTUS PULP Denise Pires Barros, Vanessa Lopes Silva, Jorge L. Colodette Federal University of Viçosa, Viçosa, Brazil Hannu Hämäläinen R& D Manager, Kemira y, Vaasa, Finland

2 Introducion Among peracids, peracetic acid is the most studied; H3 C H C Peracetic acid (Paa) structure Since the end of the 40 s has been investigated as bleaching and delignification agent;

3 Introducion Paa Reactions H 3 C During pulp bleaching, peracids are consumed by: Spontaneous decomposition; Decomposition catalyzed by transitions metals; H Lignin/HexA oxidation reactions. H H RH H 3 C M n R CH + H + 3 CH 3 k' k'' H H + R - M n+1 RH k ' > k'' 2 CH 3 C 2 H + 2

4 Introducion Reaction with lignin The peracetic acid can react with lignin mainly two mechanism: Electrophilic addition and substitution (H + ) Nucleophilic reactions (CH 3 C - ) At acid ph, the peracetic acid is selective and stronger delignification agent (electrophile).

5 Introducion Reaction with lignin Reaction between lignin and Paa involves: 1. Electrophilic hydroxilation 2. Quinone formation 3. Lactones and muconic acid formation

6 Introducion Reaction with lignin Electrophilic hydroxylation occurs in active positions of aromatic ring. H CCH 3 H H - H + H CH 3 CH 3 CH 3 Creating a more reactive free phenolic hydroxyl group

7 Introducion Reaction with lignin Aromatic ring hydroxylation can lead to its demetoxilation through an intermediate hemiacetal. R R R - CH 3 C 2 H - CH 3 H H CH 3 H CH 3 H CCH 3

8 Introducion Reaction with lignin Final oxidation stages involve oxidative cleavage of quinone rings. R H CCH 3 R H CCH 3 - CH 3 C 2 H R + H 2 R CH CH

9 Introducion Reaction with lignin β-scission by electrophilic addition CH 3 CH 3 CH 3 HC C HC C CHH CH CH CHH -H 2 Paa CH 3 CH 3 CH 3 CH 3

10 Introducion Reaction with Hexenuronic acid - HexA Peracids can react with hexenuronic acids leading to hydroxylated compounds formation. H CH H Xylan Paa H H H CH H Xylan + H 2 H H H H 5 Paa 2 C HC 2 H

11 bjectives Establishing proper conditions to apply Paa as the last bleaching stage of ECF sequences; Evaluating the impact of last stage bleaching with Paa on bleached pulp refinability and physical-chemical properties.

12 xygen Delignified Eucalyptus Pulp Characteristics Pulp Characteristic Kraft 2 Brightness, % IS 55.7 Kappa Number 10.0 Viscosity, mpa.s HexA, mmol/kg 49.2 Metal content, ppm Fe Mn Cu

13 Conditions for last stage bleaching Paa: Paa: 1-5 kg/odt pulp (equilibrium Paa - 20% solution containing 4% H 2 2 ) Time: min End ph: ~5.0 Temperature: 75 ºC Reaction consistency: 9 % Pulp brightness before Paa stage: 86-88%IS

14 Reference Paa bleaching Bleaching sequences evaluated D(EP)DD D HT (EP)DD A/D(EP)DD D HT /Q(P) Z/EDP D(EP)D/Paa D HT (EP)D/Paa A/D(EP)D/Paa D HT /Q(P)Paa Z/ED/Paa

15 Washing or No washing between D 1 and Paa stages?

16 D(EP)D/Paa (no washing after D 1 ). D(EP)DPaa (washing after D 1 ). Results D(EP)D/Paa D(EP)DPaa Kappa Factor CH 3 C 3 H, kg/odt 1 1 The consumption of Paa is not significantly affected by the washing after D1 and is affected only slightly by reaction time ( min) and Paa dose (1-5 kg/tsa) Total Act. Cl, kg/odt Realtive Chemical Cost Kappa No HexA, mmol/kg Brightness, % IS Reversion, % IS Viscosity, mpa.s

17 Effect of Washing Between D 1 and Paa Absence of washing increase relative chemical costs slightly due to increased NaH demand; However, it was concluded that washing after D 1 was likely not necessary and therefore the optimization studies proceeded considering no washing.

18 Bleaching Performance: D HT (EP)DD vs D HT (EP)D/Paa

19 Results D HT (EP)DD D HT (EP)D/Paa* Kappa Factor CH 3 C 3 H, kg/odt - 1 Total Act. Cl, kg/odt Relative Chemical Cost Kappa No HexA, mmol/kg Brightness, % IS Reversion, % IS Viscosity, mpa.s *120 min reaction time.

20 D HT (EP)D/Paa Bleaching Paa optimum conditions were 1 kg/odt pulp and 120 min on the basis of final brightness and total relative chemical costs; Paa stage resulted in decreased total active chlorine demand but increase cost by 19%; Replacement ratio (1 kg Cl 2 /odt pulp:1 kg Paa /odt pulp).

21 Bleaching Performance: D HT /Q(P) vs D HT /Q(P)Paa

22 Results D HT /Q(P) D HT /Q(P)Paa* Kappa Factor CH 3 C 3 H, kg/odt - 5 Total Active Chlorine, kg/t Relative Chemical Cost Final Kappa No HexA, mmol/kg Brightness, % IS Reversion, % IS Viscosity, mpa.s *120 min reaction time.

23 D HT /Q(P)Paa Bleaching Paa optimum conditions were 5 kg/adt pulp and 120 min on the basis of final brightness and total relative chemical costs; Paa stage resulted in decreased relative chemical cost by 27%; Replacement ratio (4 kg H 2 2 /odt pulp:1 kg Paa /odt pulp).

24 Bleaching Performance: Z/ED(P) vs Z/ED/Paa

25 Results Z/ED(P) Z/ED/Paa* Kappa Factor CH 3 C 3 H, kg/odt Total Active Chlorine, kg/t Relative Chemical Cost Final Kappa No HexA, mmol/kg Brightness, % IS Reversion, % IS Viscosity, mpa.s *120 min reaction time.

26 Z/ED/Paa Bleaching Paa optimum conditions were 3.5 kg/odt pulp and 120 min on the basis of final brightness and total relative chemical costs; The Paa stage resulted in decreased total chlorine demand and the bleaching cost was slightly higher than reference; The Z/ED/Paa sequence requires only two washers; Replacement ratio (1.4 kgh 2 2 /odt pulp:1 kgpaa/odt pulp).

27 Effect of Paa stage on pulp refinability and strength

28 Paa bleaching under differents ph values Results D HT (EP)DD D HT (EP)D/Paa D HT (EP)D/Paa D HT (EP)D/Paa Paa Stage End ph Kappa Factor CH 3 C 3 H, kg/odt Total Act. Cl, kg/odt 30,3 28,6 28,6 28,6 Relative Chemical Cost Kappa No HexA, mmol/kg Brightness, % IS Reversion, % IS Viscosity, mpa.s

29 Effect of Paa stage on pulp refinability and strength The pulp refinability is improved when Paa bleaching ph is raised from 4 to 8.5. Tensile strength was also affected positively by raising Paa stage ph.

30 Effect of Paa stage on pulp refinability and strength The effects of bleaching ph on burst and tear strength were not significant but there was a slight trend for increasing burst and decreasing tear with increasing ph.

31 Effect of Paa stage on pulp refinability and strength The effects of bleaching ph on air resistance were not significant but there was a slight trend for increasing air resistance with increasing ph. Pulp opacity tended to decrease with increasing Paa bleaching stage ph.

32 Final Conclusion Peracetic acid tend to consume relatively fast when applied as the last stage of ECF bleaching sequences; The proper dose of Paa to be applied depends upon the sequence under investigation: D(EP)D/Paa, D HT (EP)D/Paa and A/D(EP)D/Paa - very low doses of Paa (1 kg/odt pulp); D/Q(P)Paa and Z/ED/Paa - demand higher Paa doses (3.5-5 kg/odt); The D HT /Q(P)Paa sequence and its D HT /Q(P) reference counterpart are both very expensive to run in comparison to the four stage type sequences;

33 Final Conclusion In general the Paa application as last bleaching stage caused slight decrease in pulp viscosity, kappa number and HexA content; The refinability and bonding strength properties of the pulps bleached with the sequences D HT (EP)DD and D HT (EP)D/Paa were quite similar when the ph of the last bleaching stage of both sequences were in the range 4-6.5; The pulp refinability and bonding strength properties are improved when Paa stage is run at ph 8.5, but Paa stage bleaching performance decreases.