Peroxide Bleaching of Eucalyptus CTMP Using Mg(OH) 2 as the Alkali Source. Z. He; Y. Ni University of New Brunswick, Canada

Transcription

1 Peroxide Bleaching of Eucalyptus CTMP Using Mg(OH) 2 as the Alkali Source Z. He; Y. Ni University of New Brunswick, Canada

2 Conventional Peroxide Bleaching of Mechanical Pulp Use NaOH as the alkali source High alkalinity, especially in the initial phase Peroxide decomposition and darkening reactions High COD load of the effluent High anionic trash production Decreased light scattering coefficient Decreased pulp yield

3 Mg(OH) 2 : An Alternative Alkali for Mild alkalinity Peroxide Bleaching Gradual release of HO - due to low solubility One unit weight of Mg(OH) 2 equals to 1.4 NaOH in the capacity of accepting protons.

4 Mg(OH) 2 - P M Bleaching Process MC #1 MC #2 PM DTPA/Na 2 S 2 O 4 Q Y Mg(OH) 2 H 2 O 2

5 P Mg(OH)2 Versus P NaOH P Mg(OH)2 P NaOH Lower ph Silicate decreased or eliminated Longer time required Less dissolved organics Lower COD Less anionic trash Higher ph Silicate needed Less sensitive to time More dissolved organics Higher COD More anionic trash

6 Objectives To compare the Mg(OH) 2 -based and NaOHbased peroxide bleaching of a eucalyptus CTMP, in terms of Pulp brightness COD Anionic trash Pulp yield

7 Experimental Eucalyptus CTMP from a Brazilian mill Chelation: 0.2% DTPA (as 100%), 3% consistency, ph 6.0, 70 C, 30 minutes. Bleaching conditions: 2.5%, 4.0%, 6.2% H 2 O 2 ; 1.5%-6.0% NaOH, or 0.5%-2.5% Mg(OH) 2 ; 16% (MC), 30% (HC) pulp consistency; C, 2-6 hours.

8 Medium-Consistency (MC) System

9 Brightness (% ISO) Brightness (% ISO) Effect of Peroxide and Alkali Charge on Brightness (MC System) % peroxide 4.0% peroxide 6.2% peroxide % peroxide 4.0% peroxide 6.2% peroxide NaOH charge (%) Mg(OH) 2 charge (%) Other bleaching conditions: 2.6% silicate, 0.13% Epsom salt (NaOH-based process), 16% pulp consistency, 80 C, 3 hours

10 Brightness (% ISO) Eucalyptus CTMP vs. Maple CTMP: Mg(OH) 2 -based Peroxide Bleaching Eucalyptus CTMP 66 Maple CTMP % peroxide, 1.0% Mg(OH)2 4.0% peroxide, 1.7% Mg(OH)2 6.2% peroxide, 2.5% Mg(OH)2 The eucalyptus CTMP is less bleachable in the Mg(OH) 2 - based process, compared with the maple CTMP.

11 Why Is the Eucalyptus CTMP Less Bleachable in the Mg(OH) 2 -based Process? Wood properties Extractives Lignin Pulping process Chemical pretreatment (alkali dosage)

12 Brightness (% ISO) Brightness (% ISO) Effect of Acetone Extraction Without acetone extraction With acetone extraction Mg Process Na Process Mg Process Na Process Peroxide dosage (%) Peroxide dosage (%) Other bleaching conditions: 2.6% silicate, 0.13% Epsom salt (Na process), 16% pulp consistency, 80 C, 3 hours (6 hours for the Mg process).

13 Alkaline Pre-treatment on Peroxide Bleaching of A TMP NaOH % Mg(OH) 2 % Silicate % DTPA (40%) % End ph Residual H 2 O 2, % Bright. % ISO Without alkali pre-treatment With alkali pre-treatment

14 filtrate conductivity (ms/cm) Filtrate conductivity (ms/cm) Conductivity of Bleach Filtrate (MC System) % peroxide 4.0% peroxide 6.2% peroxide % peroxide 4.0% peroxide 6.2% peroxide NaOH charge (%) Mg(OH) 2 charge (%) Other bleaching conditions: 2.6% silicate, 0.13% Epsom salt (NaOH-based process), 16% pulp consistency, 80 C, 3 hours

15 Anionic trash formed (meq/kg) Anionic trash formation (meq/kg) Anionic Trash Formation (MC System) % peroxide 4.00% peroxide 6.20% peroxide 5 2.5% peroxide 4.0% peroxide 6.2% peroxide NaOH charge (%) Mg(OH) 2 charge (%) Other bleaching conditions: 2.6% silicate, 0.13% Epsom salt (NaOH-based process), 16% pulp consistency, 80 C, 3 hours

16 Summary In the MC system The Na process was effective The Mg process gave ~ 5 units lower brightness than the Na process at the same peroxide dosage. But the Mg process produced less anionic trash and conductivity

17 High-Consistency (HC) System

18 Brightness (% ISO) Brightness (% ISO) Effect of Temperature and Time on Brightness (HC System) C, 2 hrs degree C, 3 hrs 80 degree C, 6 hrs 90 degree C, 6 hrs % Peroxide, 2.25% NaOH 4.0% peroxide, 3.0% NaOH 6.2% peroxide, 4.0% NaOH peroxide, 0.75% Mg(OH)2 4.0% peroxide, 1.0% Mg(OH)2 6.2% peroxide, 1.5% Mg(OH)2 NaOH-based Process Mg(OH) 2 -based process Other bleaching conditions: 2.6% silicate, 0.13% Epsom salt (NaOH-based process), 30% pulp consistency.

19 Comparison of the Bleaching Performance at Optimized Conditions (HC) Process NaOH-based (2.6% silicate, 0.13% Epsom salt, 80 C, 3 hrs) Mg(OH) 2 -based (2.6% silicate, 90 C, 6 hrs) H 2 O 2, % NaOH, % Mg(OH) 2, % Resid. H 2 O 2, % Bright., % ISO

20 For a very high-brightness CTMP Peroxide bleaching first Followed by the addition of OBA/FWA

21 Addition of Fluorescent Whitening Agent to Pulps from the Mg(OH) 2 -based Process FWA (Tinopal UP), % Brightness, % ISO Flu. comp., % ISO CIE whiteness, % L* a* b* Other bleaching conditions: 6.2 % H 2 O 2, 2.6% silicate, 30% pulp consistency, 90 C, 6 hours.

22 Total anionict trash formation (meq/gkg) Total Anionic Trash Formation in the HC System Na-based Mg-based Brightness of bleached pulp (% ISO) Cationic demand of filtrate measured at ph 7.0

23 Sources of Anionic Trash Polygalacturonic acids (pectic acids): polymer with uronic type of carboxylic groups (pka= ~3.4). Oxidized lignin: macromolecule with lignin-bound carboxylic groups (pka= ~5.4). Colloidal particles of extractive: aliphatic type of carboxylic groups (pka= ~ ).

24 De-methylation and Dissolution of Pectins COOCH 3 COO - * O OH O De-esterification * O OH O Dissolution Anionic trash O * O * OH n OH n Pectin Pectic acids pk a = ~3.4

25 Lignin Oxidation and Dissolution CH O Lignin HC O Lignin HC O Lignin C C C Oxidation Oxidation OCH 3 O COOH COOH OH O Dissolution COD Dissolution COD & Anionic Trash pka= ~5.4

26 Extractives Dissolved and dispersed extractives Example: resin acids, fatty acids COOH COO - Resin acids pka= ~

27 Cationic demand (meq/l) Anionic Trash Measured at Various ph Sodium PolyGalA Filtrate (alkaline treatment) Filtrate (peroxide bleaching) ph The measured amount of anionic trash is ph-dependent. But the cationic demand of pectic acids is ph-independent from 4.5 to 7.

28 Cationic demand (meq/l) Estimation of the Contribution of Pectic Acids, Lignin and Extractives to the Total Anionic Trash % H 2 O 2 Alkaline treatment lignin Extractives Pectic Acids ph of the DCS sample for CD titration The increase of cationic demand with ph is due to: Extractives Oxidized lignin

29 Hemicellulose type of anionic trash (meq/kg) Hemicellulose Type of Anionic Trash in Peroxide Bleaching of the Eucalyptus CTMP Na-based Mg-based Pulp yield loss during bleaching (%) Measured as the cationic demand of filtrate measured at ph 4.5

30 lignin and extractives types of anionic trash (meq/kg) Lignin and Extractives Types of Anionic Trash in Peroxide Bleaching of the Eucalyptus CTMP Na-based Mg-based Pulp yield loss (%) Difference of filtrate cationic demand measured between ph 7.0 and 4.5

31 Pulp yield loss (%) Comparison of Pulp Yield Loss at Various Brightness Levels Na-based process 0.5 Mg-based process Brightness (% ISO) Lower yield loss for the Mg- based process.

32 Pulp yield loss (%) Correlation of Pulp Yield Loss and COD y = x R 2 = COD of the bleaching effluent (kg/t) The same correlation for both the NaOHbased and Mg(OH) 2 -based processes

33 Dissolved solid in filtrate (g/l) Dissolved Solid (HC system) Na-Based Mg-based % peroxide 4.0% peroxide 6.2% peroxide

34 Filtrate conductivity (ms/cm) Filtrate Conductivity (HC System) Na-based Mg-based Peroxide charge (%)

35 Summary Under the optimized conditions, the Mg process in the HC system gives 1-2 units lower brightness than the Na process at the same peroxide dosages. The Mg process requires higher temperature and longer retention time to reach the brightness targets. At a similar brightness, the Mg process has 50-80% less anionic trash 40-50% less COD, 20-30% less loss of pulp yield 50-60% less dissolved solid 50-60% less conductivity

36 Conclusions The Mg(OH) 2 -based process has been successfully implemented in a number of mills in N.A. In comparison with the NaOH-based process, the Mg(OH) 2 -based process offers: Lower bleaching cost Similar or slightly lower brightness Higher light scattering coefficient Higher pulp yield Lower COD Less anionic trash

37 Acknowledgements Dr. Paulo Pavan, Melhoramentos Papéis Mark Wajer, Martin Marietta Magnesia Specialties