Pulp and Paper Industry

Pulp Making Process

Pulp making process Wood is mostly composed of cellulose fibres which gives tensile (stretching) strength and lignin matrix provides compressive strength. Tensile (stretching) strength Compressive strength

Cellulose is a linear poly(β-glucose) and pure cellulose is white color.

Lignin is aromatic, highly crosslinked, and causes paper to yellow on aging due to oxidation. The first step is pulping where the wood is broken down into fibres by either mechanical or chemical pulping (Kraft pulping).

Chemical pulping or Kraft process dissolves and removes lignin matrix from cellulose fibre. In the Kraft pulp process, the active cooking chemicals (white liquor) are NaOH and Na2S. In addition, the pulp remains brown and usually bleaching is needed (to make white pulp).

From this chemical pulping process, longer fibres can be produced so stronger paper can be made, but this process has lower yield of cellulose (about 50%) and more expensive. Besides that, the pulp remains brown and usually bleaching is needed; non-bleaching pulp is used to made brown paper bags or bleaching pulp is used to made high quality white paper.

Bleaching destroys residual lignin to reduce yellowing Bleaching destroys residual lignin to reduce yellowing. Traditionally, Cl2 is used but now replaced by ClO2 for Elemental Chlorine Free process. Furthermore, there is another bleaching agent which is called 'Totally Chlorine Free' process which uses O2/O3/H2O2.

Paper Making Process

Paper making process Additives (such as clay, CaCO3, TiO2) are added into pulp to improve optical and physical properties (denser, brighter, softer and smoother)

The pulp is diluted to 1% solids aqueous suspension and sprayed onto a moving screen. Then, it dewatered by gravity and suction to 20% solids (2). After that, paper then pressed and dried over a steam-filled cylinders to a few percent water (3).

The final step in making paper is surface coatings to improve gloss, printing resolution and strength. The coating typically consists of an aqueous suspension of pigment (e.g. clay or TiO2). Finally, paper is calendered or compressed between hot rollers to allign clay platelets, improving gloss.

Calendering (to smooth the paper surface)