Pulping and Bleaching PSE 476 Lecture #16 Sulfite Pulping

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

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.

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

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.

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).

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

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.

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.

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

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

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

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.

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

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

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

HMFs and Furfurals

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:

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