HexA in Pulping and Bleaching

FWT - KU

Presentation Outline

Hexenuronic Acid (HexA)

Properties of Hexenuronic Acids

Chemical reactivity of Hexenuronic Acids

Effects of HexA in pulping

Removal of HexA

Effects of HexA Removal on Pulp Yield

Hexenuronic Acid (HexA)

Formed by base catalyzed elimination of methanol from 4-O-methyl-D-glucurnoxylans during pulping, which are randomly distributed on both sides of xylan main chain.

Such reaction rapidly takes place during the heating phase at early stages of pulping, between 110 – 150ºC at pH 12 13

Amount of HexA in Pulps:REACTIVE COMPONENTS OF OXYGEN DELIGNIFIED KRAFT PULPS

Softwood pulp Hardwood pulp

Lignin ~ 2 % ~ 1 %

HexA < 0.5 % < 1 %

Transition metals < 0.01 % < 0.01 %

Polysaccharides ~ 97 % ~ 98 %

Hexenuronic acid groups in pulps can be quantified by selective

hydrolysis in formic acid-sodium formate buffer followed by UV

spectroscopy of the formed 2-furoic acid at 245 nm (Vuorinen et al. 1999).

Properties of Hexenuronic Acids

HeatBrightness reversion

Uncoloured

Oxidized by permanganate

Cation binding capacity

Chemical reactivity of Hexenuronic Acids

Chlorine

Hypochlorous acid

Chlorine dioxide

Ozone

Peracids

Oxygen

Alkaline peroxide

Effects of HexA in pulping

Increased consumption of bleaching chemicals

a) Chlorine Dioxide

b) Ozone

c) Peroxide (when combined with ozone bleaching)

d) Peracetic acid

Can increase kappa number

Brightness reversion

Can retain metal ions

Removal of HexAThe HexA groups generally suffer electrophilic and nucleophilic attacks,

and could be removed by strong acids.

According to Vuorinen et al., optimal HexA removal conditions were achieved using a pH 3.0 - 3.5 solution, at 85-115oC for 2 - 4h.

Acid catalyzed hydrolysis of HexA was shown to release HexA from the fiber ultimately yielding 2-furoic acid and 5-carboxy-2-furaldehdye

Removal of HexA

Peroximonosulphuric acid (Ps) is a low cost and selective agents for HexA removal.

Ps is a peracid derived from hydrogen peroxide when a hydrogen atom is replaced by a SO3H group.

In Elemental Chlorine-Free (ECF) bleaching of kraft pulp, an acid hydrolysis stage prior to final stage could lead to a 50% reduction in bleaching costs.

The elimination of HexA in kraft pulps is also beneficial to Totally Chlorine Free (TCF) pulp bleaching operations and allowed for higher brightness values to be achieved.

Effects of HexA Removal on Pulp Yield

The effect of the hot acid treatment on pulp yields remains an issue of

extreme concern.

Early research activities had noted that experimental yields for treating

kraft pulps with formic acid/sodium formate buffer (pH 3) at 100oC were

usually in the range 98 – 98.6% (Ragauskas et al.).