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Sulfite pulping

Sulfite pulping. Ian Suckling May 2012 APPI. Sulfite pulping processes. Sulfite pulping. equilibria in aqueous solution SO 2 + H 2 O ⇆ H 2 SO 3 (SO 2 .H 2 O) H 2 SO 3 ⇆ H + + HSO 3 - HSO 3 - ⇆ H + + SO 3 2- i.e. relative concentrations pH-dependent

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Sulfite pulping

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  1. Sulfite pulping Ian Suckling May 2012 APPI

  2. Sulfite pulping processes

  3. Sulfite pulping • equilibria in aqueous solution SO2 + H2O ⇆ H2SO3 (SO2.H2O) H2SO3⇆ H+ + HSO3- HSO3-⇆ H+ + SO32- • i.e. relative concentrations pH-dependent • liquors prepared by dissolving SO2 into corresponding base, e.g. SO2 + CaCO3 → CaHSO3+ CO2 • SO2 formed by burning sulfur (+ kraft TRS gases) H2SO3 SO32- HSO3-

  4. Sulfite pulping alternatives

  5. Acid sulfite pulping - conditions • pH 1 – 2 with variety of bases Ca2+, Mg2+, NH4+, Na+ • liquor typically ~6 wt% total SO2, 1-1.5% combined SO2 • Tmax typically 130 - 140°C, with slow heat-up • rate of heatup and Tmax depend on base used • total cook time 6 – 8 hours • Good impregnation vital for satisfactory cook • conditions depends on wood used • If insufficient base in chips, pH of cooking liquor drops • gives acid-catalysed condensation of lignin + decomposition of cooking liquor • result is chips with dark hard cores

  6. Acid sulfite pulping • Now largely replaced by kraft pulping • Relative to kraft pulping • recovery more difficult • fibre strength lower • cooks more difficult to control • more sensitive to wood species • higher yields • Still used for dissolving pulp production • low hemicellulose content & fibre strength less impt • lignosulfonates can be recovered ~ 1m tpa • Renewed interest as “biorefineries”

  7. Acid sulfite vs kraft

  8. Reactions during acid sulfite cooking • free sulfurous acid combines with lignin to produce relatively insoluble lignosulfonic acid • rate-limiting step at low pH (1-2) • in the presence of base, lignosulfonic salts are formed, which are much more stable • (acid) hydrolysis of lignin → new phenolic units + lowers molecular weight • acid-catalysed lignin condensation – need to minimise • hemicelluloses hydrolysed into soluble sugars

  9. Sulfonation of -ethers • both phenolic and etherified units sulfonated • SO3H/OCH3 ~ 0.5 in solubilised SW lignosulfonates • -ether links stable

  10. Condensation during acid sulfite pulping • condensation of carbonium ions can compete with sulfonation, esp. with increasing acidity, e.g. • must be minimised, as leads to lignin condensation

  11. Carbohydrate reactions • Acid-catalysed hydrolysis of hemicelluloses - eventually dissolve in cooking liquor and gradually hydrolysed to monosaccharides • Acetyl groups hydrolysed • Cellulose relatively stable except for a reduction in DP • only solubilised below ~45% yield • Spent liquor rich in monosaccharides + some sugar acids

  12. Sulfite pulping liquor Typical composition of Norway spruce and silver birch spent acid sulfite liquors (kg/ton pulp).

  13. Neutral sulfite semichemical pulping • uses 8-15% Na2SO3 + 2-5% Na2CO3 • AQ optional for hardwoods but usual for softwoods • accelerates pulping of radiata pine (3 hrs  1 hr) • pulping temperature 170-180°C • Na2SO3 : Na2CO3 depends on wood species, kappa required and mill Na/S balance if integrated with kraft mill • initial pH usually 9-10 but decreases during the cook • carbonate/bicarbonate buffer neutralises wood acids • good liquor impregnation vital to prevent burnt centres • aim for 5-10 g/L residual sulfite and pH > 6.5 • prevents darkening by condensation, lignin redeposition & strength loss due to hydrolysis of polysaccharides

  14. NSSC pulping continued • yields typically 65 – 85% • mostly commonly for hardwoods but softwoods possible • less than half lignin originally in chips removed • then separate fibres by mechanical action • mainly used for packaging grades, esp. corrugating medium • impart good compressive strength • upper pulp yield set by onset of fibre damage during defibration

  15. Selectivity – NSAQ vs kraft

  16. Lignin reactions • retarded at near neutral pH  high temperatures • mainly restricted to phenolic units

  17. Hemicellulose reactions • Avoidance of extreme acidity or alkalinity preserves most of hemicelluloses • However glucuronic acids (0.1/xylan monomer) and acetyl groups (0.7-0.8/xylan monomer) dissolve • Acetyl groups (3-5% of hardwoods; 1-2% softwoods) • 30-45% of hemicelluloses dissolves • Formation of organic acids requires a buffer (CO32-/HCO32-) to maintain pH in 7-9 region • Also sodium acetate/acetic acid buffering from dissolved acetyl groups

  18. NSSC: hardwoods vs softwoods Reasons why more common with hardwoods: • HW lignin more reactive • HW lignin has lower average molecular weight • SW contain more lignin + require more chemical for degree of delignification • resins in coniferous woods resist dissolution in neutral conditions hindering chemical penetration

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