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Chemical pulp bleaching

Chemical pulp bleaching. Ian Suckling May 2012 APPI. Bleaching aim. Increase pulp brightness (whiteness) and brightness stability Without ruining fibre properties, e.g. strength At minimum cost (chemicals, Capex, energy) With minimal environmental impact A balancing act.

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Chemical pulp bleaching

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  1. Chemical pulp bleaching Ian Suckling May 2012 APPI

  2. Bleaching aim • Increase pulp brightness (whiteness) and brightness stability • Without ruining fibre properties, e.g. strength • At minimum cost (chemicals, Capex, energy) • With minimal environmental impact A balancing act

  3. Chemical pulp bleaching • Done by removingchromophores (light-absorbing structures) from fibre • most chromophores are lignin-derived • Generally done via multi-stage bleaching: • Earlier stages primarily for selective lignin removal • brownstock pulp contains 3 – 4.5% lignin • Latter stages in sequence primarily for brightening • removal of remaining chromophores/ light absorbing groups

  4. Bleaching chemicals

  5. Bleach sequences Historically CEHEH, CEH (semi-bleached) CEDED, (CD)EDED ECF (elemental chlorine free)  ODEODD, ODEOPDEPD, OODEopDnD (SW) • OOZEOD, O(DZ)EOD • OA(EOP)D(PO) - ECF lite • ODEopDP (HW) TCF (totally chlorine free) • OQ(OP)ZPZP • OQ(OP)Paa(PO) • O(ZQ)(PO)(PO) • OQ(EOP)AQ(PO/P) • OQOPP (HW)

  6. Pulp brightness • Measured as the reflectance of light at 457 nm (blue region) • Use MgO standard as 100% bright  ISO standard is reported in % ISO units • Typical pulp brightnesses • unbleached kraft = 20-30% ISO • bleached kraft = 85-90% ISO • unbleached TMP = 50-60% ISO • bleached TMP = 70-80% ISO

  7. Brightness fundamentals • incident light is reflected because it is not absorbed and it is scattered • i.e. brightness depends on: • amount of light scattered by the sheet - s • absorbance of pulp (i.e. level of chromophores) - k

  8. Light scattering • Light scattering due to changes in optical density/refractive index of materials light scatters at air-fibre interface collapsed fibres decrease scattering, as does improved bonding

  9. Light absorbance • Absorbance of pulp due to chromophores in pulp • Most of chromophores associated with lignin • carbohydrate-derived chromophores generally minor but can contribute to poor final pulp brightness & brightness stability (esp using TCF sequences) • Non-linear relation between chromophores & brightness Kubelka-Munkeqtns (B = brightness)

  10. Bleaching concepts • Effectiveness how efficiently a chemical achieves the target outcome • e.g. brightness/chemical consumption • Selectivity relative extent of a positive outcome versus a negative outcome brightness/cellulose degradation

  11. Oxygen delignification • O2oxidises lignin under alkaline conditions to degrade and make it water soluble • 1-3% NaOH to give pH 10-13 • 0.05-0.3% MgSO4 pressurise to 0.7 MPa (1% O2 consumed) 90-110oC, 60-120 minutes, 10-12% consistency • Wastewaters recycled in kraft recovery system • Minimise dilution

  12. Oxygen delignification - ctd • Limited delignification possible due to side reactions with cellulose (degrades pulp strength) • Softwoods: can lower kappa number by 40-60 %, i.e. 25-30 →10-15 kappa # • Hardwoods: lower kappa number by 25-40%, i.e. 18-21 → 14-10 kappa # • Hexeneuronic acids contribute to kappa number

  13. Oxygen delignification of hardwood pulp Effect of oxygen delignification on eucalyptus kraft pulp, Kappa 19. Conditions: 90ºC, 60 min, 0.5 MPa O2, 2% NaOH.

  14. Oxygen delignification

  15. 2-stage oxygen delignification

  16. Oxygen delignification • chemistry of oxygen, peroxide and ozone complex • reactions involving both charged & radical species 11.8 11.9 pKa 4.8

  17. O2 delignification - reactivity

  18. O2 delignification - phenolics • reaction of phenolic units: • ester hydrolysed to give diacid • enhanced lignin solubility in alkaline solution

  19. Reactions of carbohydrates with HO • Responsible for carbohydrate degradation and strength losses during O and P stages

  20. Reactions of carbohydrates with HO • a suggested mechanism

  21. Bleaching stages Aims: • to degrade lignin • to make lignin water-soluble by introducing phenols & and carboxylic acids • final chromophore removal • selective degradation of lignin in presence of high levels of carbohydrates w/out loss of yield/strength • more expensive that O2 or pulping

  22. CIO2-based bleaching • Multistage treatments to efficiently use expensive chemical  trade-off between operating and capital costs DEopD • DEop completes delignification (lignin removal) • final D or PD stages achieve final brightening (chromophore removal) • formed on-site from sodium chlorate 2NaClO3 + H2SO4 + SO2→ 2ClO2 + 2NaHSO4 4NaClO3 + H2SO4 + CH3OH → 4ClO2 + 2NaHSO4 + HCOOH + 3H2O NaClO3 + H2SO4 + NaCl→ ClO2 + ½ Cl2 + Na2SO4 + 3H2O

  23. Initial CIO2treatment (Do stage) • CIO2 charge based on incoming kappa number (lignin content)  at 15 kappa, need 1.5-2.0% CIO2  pH controlled to end pH of 2.5 – 3.5 40-90ºC, 30-60 minutes, 3-10% consistency • Lignin oxidised with little cellulose attack some oxidised lignin is soluble at pH 2.5, other must be extracted by alkali

  24. Chlorine dioxidespecies present • strong oxidant: ClO2 + 4H+ + 5e-→ Cl- + 2H2O

  25. Chlorine dioxidereactions • ClO2 reacts with lignin → ClO2- and HOCl • HOCl partially hydrolysed to Cl2 • HOCl and Cl2react with lignin → organo-chlorines • Cl2 + ClO2- → ClO2 • HOCl + ClO2- → ClO3- • ClO2- gives ClO2 + Cl- under the acidic conditions hypochlorous acid chlorite chlorine chlorite chlorate chloride

  26. Chlorine dioxidereaction of phenolic units • ClO2 predominantly attacks phenolic units or sidechain structures • but non-phenolic units slowly degraded • little chlorination of lignin

  27. Cleavage of ether groups by HOCl • Important to generate new phenolic units • Active species is chloronium ion Cl+

  28. Chlorine dioxidereaction of etherified units (slower) • Slow reaction with non-phenolic units • non-phenolics also degraded by rxn with HOCl and Cl2produced

  29. Reactions of chlorine (Cl2) with lignin • formed during chlorine dioxide bleaching • important as reacts with non-phenolic units

  30. Alkali extraction • E stage extracts oxidised lignin (generally acidic in nature)  1-3% NaOH (pH>11) • 65-85°C, 60-120 minutes, 10% consistency • Oxidiants added to E stage to boost delignification through further oxidation  O2 injected at 0.5% on pulp (Eo)  H2O2 added at 0.1-0.5% on pulp (Eop)  final kappa number = 1-4 (~1% lignin)

  31. Extraction stage chemistry • Key reaction = conversion of acidic groups in degraded lignin to water-soluble sodium salts • hydrolysis of esters → carboxylates • also hydrolysis of organically bound chlorine, especially after C stages

  32. Ozone (Z stage) • O3 is very active oxidant; attacks lignin as well as cellulose  used with alkaline extraction (ZE)  0.5-1.0% O3 on pulp (~10% O3 in O2)  gas mixed into 10% consistency pulp  pH 2.5-4.5, 40-60ºC, 5-20 minutes • Poor selectivity minimise charge to avoid strength loss • chelate metal ions with Q stage • radical decomposition products cause carbohydrate decomposition • Can be combined with D stage, e.g. OO(DZ)EopD

  33. Ozone System

  34. Ozone chemistry • very powerful oxidant • generally reacts as an electrophile, e.g. with double bonds and aromatic rings • stilbenes > styrenes > phenolics > muconic acids > nonphenolics >> aromatic aldehydes • unstable in aqueous solution, esp. at higher pH • products, O2, HO•, HOO•, H2O2, H20 • also decomposed by transition metals, e.g. Co, Fe

  35. Ozone chemistry - complex • Increased free & esterified unsaturated carboxylic acids and esters

  36. Final brightening • CIO2 degrades residual chromophores  1.0 -1.5% CIO2  pH controlled to ~4.0  60-80ºC, 120-240 minutes, 10% consistency • Two stages use less chemical but require more capital equipment interstage alkali treatment increases final D stage efficiency (DED or DnD)

  37. Complete Sequence

  38. Hydrogen peroxide (P stages) • H2O2 good at attacking chromophores so best used as final brightener  1-3% H2O2 at pH ~11 80-110ºC, 2-6 hours, 10% consistency • shorter reaction if pressurised (PO) • can be used to delignify under severe conditions • two competing reactions of the H2O2 • chromophore removal (brightening) • H2O2decomposition

  39. Hydrogen peroxide brightening • For both high yield pulps and final brightening of chemical pulps • Main active species is hydroperoxide anion (HOO-) • Main reaction is destruction of -carbonyls and conjugated double bonds in lignin sidechains

  40. Reactions of quinones

  41. Peroxide decomposition • Consequences: • unproductive loss of an expensive chemical - reduced efficiency • forms radicals, e.g. •OH, which attack carbohydrates – reduced selectivity • Decomposition by alkali - control pH • Thermal decomposition – practical upper limit to T

  42. Effect of temperature in Eop stage

  43. Peroxide decomposition • Transition metal ions such as Mn, Cu, Fe catalyse peroxide decomposition

  44. Transition metal removal • Organic sequestering agents bind with • metal ions to make water soluble  0.2-0.5% chelant (DTPA or EDTA)  50-80ºC, 15-60 minutes, ~10% cty  best results at pH 4-6 • Other approaches: • pretreatment with mild acids • addition of alkali-stable chelants (DTMPA) to P stages • use of silicate (mechanical pulping) • addition of Mg2+ (O2 delignification)

  45. Hexenuronic acid removal • Produced from uronic acids during kraft cooking • Contribute to pulp kappa number, especially for hardwoods • Removal of HexA units: • Hot acid (A) stage, e.g. H2SO4, pH 2.5, ~90ºC • Chlorine dioxide • Ozone

  46. Semi-bleaching • Kraft pulp bleached to only 60-70% ISO for use in newsprint  requires short sequence (CEH, HH) • Hypochlorite (H) from alkali plus chlorine  2NaOH + CI2 2NaOCI + H2O  OCI- attacks chromophores  brightener  pH ~ 10, 50-70ºC, ~60 minutes, 10% consistency

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