Unit 20: Paper and Pulp

Introduction

Paper is a thin, flexible material manufactured primarily from cellulose fibres derived from wood or other plant sources. It is characterised by its ability to be written on, printed, and used for packaging.

Pulp is the fibrous material obtained after separating cellulose fibres from lignin and other constituents of raw plant material. Pulp serves as the intermediate product that is further processed into paper.

Raw Materials

Wood

• Softwood (conifers): long fibres, high strength, e.g., pine, spruce.
• Hardwood (deciduous): short fibres, smooth surface, e.g., eucalyptus, birch.

Non‑Wood Fibres

• Bamboo: fast‑growing, good fibre length.
• Bagasse: sugarcane waste, abundant in tropical regions.
• Jute: bast fibre, used for specialty papers.
• Straw: wheat, rice, barley residues; lower fibre quality.

Chemicals Used in Pulping and Bleaching

• Sodium hydroxide (NaOH): alkali in Kraft pulping, breaks lignin‑carbohydrate bonds.
• Sodium sulphide (Na₂S): component of white liquor; enhances delignification.
• Sodium sulphate (Na₂SO₄): makeup chemical in Kraft process to recover NaOH and Na₂S.
• Chlorine (Cl₂) and chlorine dioxide (ClO₂): bleaching agents; remove residual lignin.
• Hydrogen peroxide (H₂O₂): eco‑friendly bleach, especially for mechanical pulps.

Sources of Raw Materials

Forests

The primary source of wood for pulping is managed forests. Sustainable forestry practices ensure continuous supply while maintaining ecological balance.

Agricultural Residues

By‑products such as bagasse (from sugarcane), straw (from cereals), and jute stalks are collected after harvest and processed into pulp, reducing waste and providing alternative fibre sources.

Recycled Paper

Post‑consumer and post‑industrial paper waste is an important secondary source. Recycling saves energy, reduces virgin fibre demand, and lowers environmental impact.

Stages in Paper Production

1. Pulping

Pulping separates cellulose fibres from lignin. Two main routes exist:

1. Chemical Pulping (Kraft Process)

   Wood chips are cooked with white liquor (a mixture of NaOH and Na₂S) at high temperature (≈150‑170 °C) and pressure.

   The delignification reaction can be simplified as:

   Lignin + 2 NaOH + Na₂S → Soluble lignin‑sulfonate + 2 NaOH + Na₂S (recovered)

   The resulting pulp is called brown stock. The Kappa number measures residual lignin:

   Kappa = (Volume of 0.1 N KMnO₄ consumed by 1 g pulp) × 10

   Lower Kappa indicates higher delignification.

2. Mechanical Pulping

   Wood logs are ground against a rotating stone or refined in a disc refiner with water. Fibres are separated largely by shear, retaining most lignin.

   Yield is high (≈90‑95 %), but the paper has lower strength and tends to yellow.

2. Bleaching

Bleaching removes residual lignin to achieve brightness. A typical multistage sequence (CEHH) includes:

• C – Chlorine dioxide (ClO₂) oxidation.
• E – Alkaline extraction (NaOH).
• H – Hypochlorite or hydrogen peroxide stage.
• H – Final peroxide stage for brightness boost.

Overall brightness (ISO Brightness) is measured as the percentage reflectance of blue light (457 nm) relative to a magnesium oxide standard.

3. Paper Making (Fourdrinier Machine)

The bleached pulp is diluted to a consistency of 0.5‑1 % and fed onto a moving wire mesh. Water drains by gravity and suction, forming a wet web.

1. Formation – Fibres interlock on the wire.
2. Pressing – The web passes through nip presses to remove more water (≈50 % solids).
3. Drying – The web contacts heated cylinders (dryers) reducing moisture to ≈5‑6 %.

4. Finishing

• Calendering – Passing the paper through heated rollers to smooth surface and improve gloss.
• Coating – Application of pigments (e.g., clay, calcium carbonate) and binders for print quality.
• Cutting – Reeling, slitting, and sheeting to final dimensions.

Flow‑Sheet Diagram for Paper Production

The following schematic illustrates the main unit operations from wood logs to finished paper roll.