1 / 33

Mechanical Pulping and Pulp Bleaching Chemistry

Mechanical Pulping and Pulp Bleaching Chemistry. Ian Suckling APPI May 2012. Agenda. Introduction Chemical reactions during mechanical pulping Sulfonation – chemimechanical processes Alkaline peroxide mechanical pulping (APMP process) Bleaching Wood & pulp chromophores

kirby
Télécharger la présentation

Mechanical Pulping and Pulp Bleaching Chemistry

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Mechanical Pulping and Pulp Bleaching Chemistry Ian Suckling APPI May 2012

  2. Agenda • Introduction • Chemical reactions during mechanical pulping • Sulfonation – chemimechanical processes • Alkaline peroxide mechanical pulping (APMP process) • Bleaching • Wood & pulp chromophores • Reductive brightening • Oxidative brightening • Effluents & air emissions • Reactions and behaviour of extractives covered by Dr Karen Stack

  3. Mechanical pulping • Separation and development of fibres by mechanical processes • Relatively high electrical energy demand • Main end uses: • Newsprint • Coated and uncoated magazine grades • Cartonboard • Tissue and absorbent grades

  4. Disc refining RMP: Refiner mechanical pulp TMP: Thermomechanical pulp PRMP: Pressurised refiner mechanical pulp CTMP: Chemithermomechanical pulp

  5. …or the fibre may be removed by forcing the log against the face of a rotating grindstone

  6. Different mechanical pulping processes

  7. Chemical changes • Only minor chemical changes occur • High pulp yield 90-98% vs 45-50% for chemical pulps • Pulps contain all/nearly all of components of wood raw material • ratios similar to those in starting wood • Amount of dissolved material increases with: • Temperature of treatment • Intensity of defibration • Addition of chemicals, e.g. sulfite, caustic

  8. Chemical reactions in mechanical pulping • Cellulose • Essentially unreactive • Hemicelluloses • Partial hydrolysis of acetyl groups → acetic acid/acetate • Partial dissolution of intact hemicelluloses • ~7-8% of galactoglucomannans dissolved in native form during TMP pulping of spruce. Can be purified by ultrafiltration • Dissolved hemicelluloses play a role in stabilisation of extractives • Demethylation of pectins & glucuronoxylans → methanol • Pectins also partially solubilised

  9. Chemical reactions in mechanical pulping - ctd • Lignin • Chromophore formation → pulp darkening • Lignin modification esp. in chemimechanical pulping • Dissolution of low molecular weight fractions • Extractives • Wood resins + lignans partially dispersed or dissolved • Dr Karen Stack to discuss in later lectures • Monoterpenes lost with steam/effluent (steam volatile)

  10. Dissolution during spruce CTMP pulping Konn J. et al. J. Pulp & Paper Sci. 28(12):395 (2002).

  11. CTMP: Chemithermomechanical pulping Chip presteaming Sulphite impregnation (~1-3%Na2S03) Cooking 3-10 min @ (125-140°C) Pressurised refining(1-4) bar

  12. OPCO: Inter- or post-stage sulphite treatment Addition of sulphite as an inter-, post-, or reject stage treatment (~6-10%Na2S03,30-60 min @ 140-160°C) Chip preheating Pressurised refining

  13. Sulfite-based chemimechanical processes • Sulfite charge varies from 1-3% Na2SO3 on od wood (CTMP) up to 10-20% on od wood (CMP) • Introduces sulfonate groups which swell fibre lignin • Also dissolution of low MWtsulfonated lignin fragments • Carbohydrates start to dissolve, esp. under more severe conditions • CTMP – 1-3% Na2SO3 on od wood • Optimum pH ~8 (initially 10-11) • Typically 120 – 135ºC for few minutes

  14. Sulfonation chemistry • Principally -sulfonation of phenolic units via quinone methide intermediate (neutral or mildly alkaline conditions) • More extensive degradation under more severe conditions • Also: • Some generation of new phenolic units • Solubilisation of sulfonated lignin fragments • Sulfonation of etherified units under severe conditions

  15. Effect of sulfonation on fibre Sulfonation reduces softening temperature Changes defibration mode → low shives, even at high freenesses

  16. Sulfonation – effect of wood species • Consistent with sulfonation of free phenolic units in lignin

  17. Fate of sulfite (Pilot plant trials) Pasco M and Suckling I. Appita J. 54(2):196 (2001).

  18. Alkaline peroxide mechanical pulping (APMP) • Alternative to sulfonation, particularly for low density hardwoods, e.g. aspen • Typically: • Pre-chelate to remove/de-activate transition metal ions • 1-5% hydrogen peroxide - brightening • 3-6% sodium hydroxide – strength development • 60-80ºC, ~ 1h • Increasing chemical charge: • increases bonding and sheet density • reduces scattering and refining energy to given freeness • Alkaline treatment causes: • carboxylation of lignin, modifying fibre and increasing fibre bonding • Softens lignin, giving easier fibre separation & less fines during refining

  19. Aspen APMP vs BCTMP

  20. Effect of caustic on handsheet density in APMP

  21. Formation of chromophores during mechanical pulping Specific light absorption coefficients of woods and pulps at 457 nm. Light adsorption spectra for (1) Norway spruce wood meal, (2) first stage refined pulp, (3) second stage refined pulp, (4) latency treated pulp and (5) finished paper furnish. Suckling, I. Res. Chem. Int. 21(3-5):275 (1995). Johansson, M. et al. NPPRJ. 15(4):282 (2000).

  22. Potential chromophores • Lignin dominant source for chromophores in mechanical pulps • Potential chromophore structures:

  23. Mechanical pulp bleaching • Aim is to remove chromophores without removing lignin, i.e. while retaining pulp yield • Main bleaching chemicals: • Alkaline hydrogen peroxide • In-refiner, e.g. APMP • Separate bleaching stage • Dithionite (hydrosulfide)

  24. Reductive vs oxidative bleaching • Comparison of bleach response of spruce groundwood to hydrogen peroxide and dithionite Suss, U. Pulp Bleaching Today, de Gruyter , Berlin (2010).

  25. Peroxide bleaching • Brightness increase up to 30 units ISO  1-4% H2O2 at pH 9-11 (2-3% NaOH)  50-70ºC, 120 minutes, 20-30% consistency  need tower system for high consistency • Attains high, stable brightness  relatively low brightness reversion  used for market mechanical pulps  must chelate metal ions to improve effectiveness (QP)

  26. Reactions of hydrogen peroxide • Active bleaching compound is perhydroxyl anion • Dominant reactions:

  27. Impact of caustic charge Suss, U. Pulp Bleaching Today, de Gruyter , Berlin (2010).

  28. Dithionite bleaching • Na2S2O5 (Y stage) • Limited brightness increase (~10 units ISO) 0.5-1.0% Na2S2O4 at pH 5-7  50-70ºC, 30-120 minutes, 3-10% consistency  can use storage tower (no post-washing)  used for brightness adjustment (eg, newsprint) • Reduced chromophores can be re-oxidised poorer brightness stability Need to exclude air from treatment, as oxygen generates acid (H2SO3 and H2SO4), which catalyses dithionite decomposition

  29. Reductive bleaching • Na2S2O4treatment also decreases light absorption by reducing: • quinones • quinonemethides • metals in catechol-metal complexes (Fe3+ → Fe2+) • – NaHSO3 also does this • Sulfur dioxide radical (▪SO2-) likely reactive species

  30. Comparison of bisulfite and dithionite

  31. Air emissions from TMP pulping • Same VOCs also in aqueous effluents, often at much higher levels (open water system unlike real mill) Suckling, I. et al. Appita J. Proc. 13th Int. Symp. Wood Fibre Pulping Chem. 2:91 Auckland (2005)

  32. Released wood components Typical amounts of dissolved and colloidal substance in 1% suspensions of mill spruce TMP. Lab bleaching with 3% H2O2. TOC-values were 214 and 288 mg/L respectively. Suss, U. Pulp Bleaching Today, de Gruyter, Berlin (2010).

  33. Impact of peroxide bleaching on effluent COD load (spruce groundwood) Suss, U. Pulp Bleaching Today, de Gruyter, Berlin (2010).

More Related