The Future of Eucalyptus Pulp Bleaching Technology Tapani Vuorinen tapani.vuorinen@aalto.fi 7th International Colloquium on Eucalyptus Pulp May 26-29, 2015 Vitória, ES, Brazil
Outline • Aim • Advances in pulp bleaching technology • Chemistry of ECF bleaching • Outlook for the future • Conclusions
Aim of the talk To show the high potential and solution for a revolution in eucalyptus pulp bleaching practice from today’s best available ECF bleaching technology… …in fulfilling the ”more with less” challenge: less chemicals, less water, less energy, lower environmental impact, lower capex
Milestones • 1992-1995: Discovery of the importance of hexenuronic acid (HexA) in kraft pulps • 1995-: Early mill scale implementations of A-stage to remove HexA • 2000-: Large scale implementation of A- stage in hardwood (eucalyptus) pulp mills • 2000-: Increasing understanding on the chemistry of pulp bleaching • 2020-: Revolutionary pulp bleaching?
Advances in eucalyptus pulp ECF bleaching technology • Old (1990’s) – e.g. D-E OP -D-D • Modern BAT – e.g. A/D-E OP -D-P, D/A-E OP -D-P, A-E OP -D-P – Half less ClO 2 is needed (1.5-2 % as active chlorine dosage) – A hot acid stage with long retention time is added to remove HexA
Main targets of pulp bleaching • To reach high brightness – Low lignin chromophore content • To reach high brightness stability – Low residual lignin content – Low hexenuronic acid (HexA) content – Low carbonyl content • To remove most of chromophores, lignin and HexA without oxidizing polysaccharides
Chemistry of ECF pulp bleaching • Monitoring the bleaching result – Lignin content, HexA content – Brightness, viscosity, chlorate formation… – Oxidized structures, chromophores… • Knowledge on chemical reactions involved – Inorganic reactions – Reactions of lignin and HexA – Structure-reactivity relationships • Modeling and simulation of complex reaction systems – Visit Poster 6 by Anna Kalliola et al.
UV Raman spectra of bleached kraft pulps Kristiina Kellokoski M.Sc. Thesis 2013
General reaction scheme of chlorine dioxide bleaching
Oxidants in chlorine dioxide bleaching • Chlorine dioxide (ClO 2 ) – Oxidizes phenols and hydroxyquinones (2 equivalents ClO 2 per phenol or hydroxyquinone) – Produces ½ equivalents HOCl and ½ equivalents ClO 2 - • Chlorite (ClO 2 - ) – Oxidizes aldehydes (formed in situ ) forming equivalent amount of HOCl – Decomposes to chlorate (ClO 3 - ) and HOCl (catalysis by HOCl) – May react with HOCl to regenerate ClO 2 • Hypochlorous acid (HOCl) – Oxidizes HexA – Chlorinates and oxidizes lignin – Oxidizes cellulose and hemicelluloses
Stoichiometry of overall reactions • 2ArOH + HexA + 4ClO 2 2Ox Lig + Ox HexA + ClO 3 - + 3Cl - – In this simplified (but relevant) scheme HexA and lignin are oxidized in a 1:2 ratio! • Formation of HOCl in situ – 0.75-1 equivalents per added ClO 2 • Formation of chlorate (ClO 3 - ) – 0-0.25 equivalents per added ClO 2
Production of bleaching chemicals Hypochlorite: (1) NaCl + H 2 O → NaOCl + H 2 (electrolysis) Chlorate: (2) 3NaOCl → NaClO 3 + 2NaCl (3) NaCl + 3H 2 O → NaClO 3 + 3H 2 (electrolysis) Chlorine dioxide: (4) NaClO 3 + reductant → ClO 2 Hydrogen peroxide: (5) H 2 + O 2 → H 2 O 2 (catalytic)
Production of bleaching chemicals and their usage (share of oxidation power) 3 . NaCl 3 . NaOCl (eq. HOCl, Cl 2 ) HOCl, 40 % ≤ 30 % NaClO 3 ClO 2 HClO 2 , 20 % 40 %
Inefficiency in current ECF bleaching technology • Long retention times – huge bleaching towers • Excessive oxidation power needed in removal of residual lignin and HexA – 4-6 equivalents of oxidant per C 6 C 3 + HexA – 7-9 equivalents of oxidant per C 6 C 3 + HexA (HexA hydrolyzed by acid calculated out) ⇒ Target should be in doubling the efficiency!
Reactivity of HOCl • Electrophilic reactions – Primary oxidation of HexA - OK – Chlorination of lignin – reduces reactivity of residual lignin (by factor of 10 per each substitution) – Oxidation of lignin - OK – Oxidation of cellulose and hemicelluloses – may decrease brightness stability • Nucleophilic reactions – Secondary oxidation of HexA – consumes oxidant without promoting removal of HexA – Oxidation of chromophores - OK
Derivatives of HOCl (Cl 2 ) • General reaction (nucleophilic substitution on chlorine): Nu - + Cl δ + -Cl δ - Nu-Cl + Cl - Nucleophile Product Name H 2 O/HO - HOCl Hypochlorous acid ClO - Cl 2 O Chlorine monoxide ClO 2 - Cl 2 O 2 Dichlorine dioxide RCO 2 H RCO 2 Cl Acyl hypochlorite ROH ROCl Alkyl hypochlorite ArOH ArOCl Aryl hypochlorite R 3 N + Cl R 3 N Chloroammonium cation
Characteristics of R 3 N catalysis • The actve form, R 3 N + Cl is extremely reactive • R 3 N + Cl is not a nucleophile like HOCl is • We targeted at improving the selectivity of HOCl towards electrophilic reaction thus preventing e.g. overoxidation of HexA • We found that ozone becomes a highly selective oxidant after the catalytic treatment
UVRR spectra of eucalyptus kraft pulps in A-E OP -D-P bleaching sequence Leonardo Clavijo M.Sc. Thesis 2010
UVRR spectra of eucalyptus kraft pulps in H cat -Z-P bleaching sequence
Production of bleaching chemicals (sum reactions) Hypochlorite and peroxide: (6) NaCl + O 2 + H 2 O → NaOCl + H 2 O 2 Chlorate and peroxide: (7) NaCl + 3O 2 + 3H 2 O → NaClO 3 + 3H 2 O 2
Eucalyptus pulp bleaching sequence in the future? H cat -Z-P NaOCl/HOCl: 0.5-0.7 % (as Cl 2 ) Catalyst: 0.01 % O 3 : < 0.3 % H 2 O 2 : 0.25-0.35 % Total reaction time : ≤ 30 min Viscosity loss: < 50 ml/g
Conclusions • Eucalyptus pulp bleaching could and should be intensified from today’s BAT • In the future bleaching can be dramatically faster than today and consume much less chemicals • In addition to lower investment and production costs the new technology may better maintain beneficial fiber properties • Research and collaboration is still needed to overcome the remaining challenges and verify the technology on larger scale
Acknowledgements Long-term financers and idustrial collaborators: TEKES (Finnish Funding Agency for Technology and Innovation) Finnish Bioeconomy Cluster Ltd (Strategic Centre for Science, Technology and Innovation) Andritz, Metsä Fibre, Kemira, Stora Enso, UPM Research collaborators during the past years: Aalto University BOKU INPG Pagora Universidad de la República VTT
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