1. Pulping and Bleaching PSE 476
Lecture #16
Sulfite Pulping

2. Sulfite Pulping Agenda
Conditions
Cooking Liquor
Preparation
Factors Affecting Pulping
Lignin Reactions
Carbohydrate Reactions

3. Sulfite Pulping
Unlike alkaline pulping, the purpose of sulfite pulping is not heavy lignin fragmentation.
Acid sulfite
Under acidic conditions, the object is to solubilize lignin through the addition of hydrophillic sulfonate groups (sulfonation)
Neutral sulfite
Most often neutral sulfite pulping is used in association with mechanical pulping (Neutral Sulfite Semi Chemical-NSSC). The purpose here is mainly to “soften” the lignin.
Alkaline sulfite
The goal is sulfonation and fragmentation.

4. Sulfite Pulping Conditions
Long cooking time is necessary to prevent diffusion of SO2 ahead of the base (avoid lignin condensation, “black cook”); weak pulp; low hemis; acid hydrolysis; tissue paper
Medium grades paper: newsprint and writing paper
Good stiffness pulp, high yield, corrugating medium
Very similar pulps to kraft pulping in terms of yield, brightness, bleachability and strength

5. Sulfite Pulping Sulfur Oxides Equilibriums
Sulfur dioxide exists in different forms depending upon solution pH. This is demonstrated by the chemical equilibrium's shown below and in figure in the next slide.

6. Effect of pH on Sulfur Dioxide Species
SO2
HSO3-
SO32-
Ca pH=2 to stay in the solution,
Mg pH=4,
Na and NH4+ strongly alkaline without precipitation (any pH).

7. Acid Sulfite Cooking Liquor Production
Molten sulfur is burned with O2 at 1100°C to form SO2
Controlled regulation of O2 – over oxidation production of SO3
The SO2 is cooled to 200°C and adsorbed into water forming SO2• H2O often referred to as H2SO3
A base is added to the SO2
Ca, Na, NH3, Mg
Liquor is fortified with additional SO2 from digester (relief) in pressure system
Brow or a red liquor is the waste liquor
Reference

8. Sulfite Pulping Variables
Wood species
Many softwoods (pines,etc) and hardwoods are difficult to pulp particularly under acid conditions.
General chip characteristics
Size, Moisture, contaminants, etc.-very important to have a uniform liquor penetration, if not chips will char not enough base to supply the buffering action
Cooking liquor
Liquor/wood
Digester variables
Packing, venting, heating, etc.

9. Sulfite Pulping Factors Affecting Cooking Rate
pH, Rate 
SO2 Concentration, Rate
Temperature (most important), Rate 
Base (from slowest to fastest)
Ca<Na<Mg<NH4
Other factors influence choice of base
Costs: Mg>Na>NH4, Ca
Recovery systems: calcium cannot typically be recovered
NH4 pulps have lower brightness.

10. Sulfite Pulping Lignin Reactions
At high temperatures a proton
attacks the benzylic oxygen
Intermediate-
benzylic carbocation
Three Major Reactions
Sulfonation
Sulfonation on a carbon (majority) and g carbon increases the polarity of lignin and solubilize lignin
Bond Cleavage*
Acid: a-O-4 linkage only linkage affected.
Neutral/Alkaline: b-O-4, a-O-4 linkages cleaved.
+ ROH
* This reaction does not reduce MW or improve delignification

11. NSSC pulping Under neutral sulfite conditions:
Sulfonation reactions are limited to β-O-4 ethers with free phenolic hydroxyls only
The net result of these reactions is light alpha-sulfonation and some beta-ether cleavage
Lignin is "softened" and most remains in the pulp (higher temperatures it "gives" like thick molassas and spreads out amongst the fibers)
When made into a sheet and cooled down, the lignin hardens again resulting in a stiff sheet
This is the basis of cardboard type products

12. Sulfite Pulping Lignin Reactions
Condensation
These reactions are very prevalent in acid sulfite pulping. The MW of the lignin is significantly increased. This is a detriment to lignin removal.
β-1
β-6

13. Sulfite Pulping Lignin Reactions ( What Really Happens)
The unmentioned reaction.
Six years ago it determined that there was basically no free a-O-4 linkages in lignin. This means that the proposed mechanisms for the reduction in lignin molecular weight were incorrect.
A forgotten proposed mechanism from Nakano seems to explain this phenomena. In this mechanism, lignin molecules are linked through lignin-carbohydrate-lignin linkages. During acid sulfite pulping, the carbohydrate is cleaved through acid hydrolysis thus lowering the lignin molecular weight.

14. Sulfite Pulping Acid Reactions of Carbohydrates
Cellulose: cleavage of glycosidic linkages slow because of macromolecular structure: crystalline nature.
Xylans:
Arabinofuranosyl linkages very acid labile
Uronic acid linkages somewhat stable
Acetyl groups easily cleaved
Galactoglucomannans:
a-D-(1®6) galactosidic linkage very acid labile
Acetyl Groups easily cleaved; this increases resistance of glucomannan linkages to cleavage
Glucomannan linkages more stable than xylan

15. Sulfite Pulping Acid Reactions of Carbohydrates
Cellulose Yield
Cellulose DP
Xylose
Galactose
Mannose
Arabinose
Carbohydrate degradation phenomena during sulfite cooking of Spruce

16. Softwood Xylans
4--D-Xly-14--D-Xly-14--D-Xly-14--D-Xly4--D-Xly 
4-O-Me--D-Glc    
-L-Araf  2 2 2 1

17. HMFs and Furfurals

18. Sulfite Pulping Reactions of Monosaccharides
Monosaccharides react with bisulfite to form aldonic acids under high temperature. Bisulfite is converted to thiosulfate, thus severe lignin condensation (black cook).
In an acid sulfite cook, typically 10-20% of the sugars are converted.
Thiosulfate responsible for condensation reactions.
Formation of thiosulfate is self catalyzing:

19. Sulfite versus kraft Not used why:
New bleaching techniques made efficient bleaching of kraft pulp possible
More pollution compared to kraft and high SO2 loss to atmosphere
Applicable to small amount of wood species because of limited capabilities to dissolve extractives (spruce, balsam fir, and hemlock, aspen, poplar, birch, maple)
Long cooking process
Weaker pulp
Poor chemical recovery