Paper and Pulp

Every page of this book began as a tree, a stalk of babiyo grass or an old newspaper. Paper is a thin mat of cellulose fibres felted together from a water suspension; making it is essentially one chemical problem — how to strip away the lignin that glues the fibres together in a plant, without destroying the cellulose itself. This unit follows that problem from the forest to the reel.

20.1 Introduction

The fibre is cellulose, (C₆H₁₀O₅)ₙ, a linear polymer of β-glucose units with n running into the thousands. Along with it, plant cell walls contain:

Component	Nature	Wanted in paper?
Cellulose, 40–50 %	long β-1,4-glucose chains; strong, white, insoluble	yes — it is the fibre
Hemicellulose, 20–30 %	short branched sugar polymers	partly — helps fibres bond
Lignin, 18–30 %	cross-linked aromatic polymer, brown, binds fibres	no — it is stiff and yellows in light
Extractives, 2–8 %	resins, fats, tannins	no — they make pitch spots

Paper was invented in China by Ts'ai Lun in about 105 AD from rags and bark, and Nepal has its own old tradition: lokta paper, hand-made from the inner bark of the Himalayan Daphne bholua shrub, has carried scriptures and lal purja land certificates for centuries because it resists insects and lasts for generations.

20.2 Raw materials and their sources

Class	Examples	Source
Softwood (long fibre, 3–5 mm)	pine, fir, spruce	conifer plantations; gives strength
Hardwood (short fibre, 1–2 mm)	sal, poplar, eucalyptus, birch	forests; gives smoothness and bulk
Non-wood fibre	babiyo (sabai) grass, rice and wheat straw, bagasse, jute, bamboo, kans grass	Terai farms and river banks — the main Nepali source
Bast fibre	lokta bark, rags, cotton linters	Himalayan shrubs, textile waste
Recycled fibre	waste paper, cartons	urban waste — now over half of world furnish
Chemicals	NaOH, Na₂S, Na₂SO₄, lime, Cl₂/ClO₂, H₂O₂	imported in Nepal
Fillers and sizes	china clay, CaCO₃, TiO₂, talc, rosin, alum, starch	mineral suppliers

A rule of thumb: about 2.2 tonnes of oven-dry wood yield one tonne of bleached chemical pulp, because roughly half of the wood substance is dissolved away.

20.3 Stages in production of paper

(a) Preparation of the wood. Logs are debarked, chipped to pieces about 20 mm × 20 mm × 4 mm and screened, so that the cooking liquor can penetrate quickly and evenly.

(b) Pulping. The chips are broken down into individual fibres.

	Mechanical pulping	Chemical pulping
How	logs ground against a wet revolving stone, or chips refined between discs	chips cooked with chemicals in a digester
Lignin	left in the pulp	dissolved out
Yield	90–95 %	45–55 %
Fibre strength	low (fibres are cut)	high
Colour stability	poor — newsprint yellows	good
Used for	newsprint, cheap magazine paper	printing, writing, packaging paper

The important chemical route is the sulphate or Kraft process (Kraft is German for "strength"). Chips are cooked with white liquor — a solution of sodium hydroxide and sodium sulphide — at about 170 °C and 7–8 atm for 2–4 hours. The alkali breaks the ether links of lignin and the HS⁻ ion attacks it further, so the lignin dissolves as soluble sodium salts, leaving the cellulose fibres intact. The spent liquor, now dark brown, is called black liquor.

The older sulphite process cooks the chips instead with calcium or magnesium bisulphite solution containing free SO₂, which converts lignin into soluble lignosulphonic acids:

2SO₂ + CaCO₃ + H₂O → Ca(HSO₃)₂ + CO₂↑

It gives a paler pulp that is easier to bleach, but a weaker one, and its waste liquor is hard to recover; most mills have moved to the Kraft process.

(c) Washing and screening. The pulp is washed free of black liquor on rotary vacuum washers and screened to remove knots, uncooked chips, sand and grit.

(d) Bleaching. Unbleached Kraft pulp is brown (it is the paper of cement sacks). The residual lignin is removed in stages with oxygen, chlorine dioxide and hydrogen peroxide. Chlorine dioxide is generated on site because it cannot be stored:

2NaClO₃ + 4HCl → 2ClO₂↑ + Cl₂↑ + 2NaCl + 2H₂O

(e) Beating (refining) and stock preparation. The wet pulp is passed between rotating bars, which fray and swell the fibre walls so that more –OH groups are exposed and the fibres bond better. Then additives go in:

Additive	Purpose
Rosin soap + alum, Al₂(SO₄)₃	sizing — deposits aluminium resinate on the fibres so ink does not spread
China clay, CaCO₃, TiO₂, talc	loading — fills the gaps, giving opacity, whiteness and smoothness
Starch, cellulose gums	dry strength
Dyes, optical brighteners	colour, whiteness

6C₁₉H₂₉COONa + Al₂(SO₄)₃ → 2(C₁₉H₂₉COO)₃Al + 3Na₂SO₄

(f) Sheet formation on the paper machine. A very dilute stock, about 0.5 % fibre in water, flows from the headbox onto a moving endless wire mesh. Water drains through the wire and is sucked away, leaving a wet web of about 20 % solids; press rolls squeeze it to 40 %; steam-heated drying cylinders take it to 94–95 %; calender rolls press it smooth and the sheet is wound on a reel and slit.

20.4 Flow sheet for paper production

20.5 Quality of paper

Paper is sold and specified by measurable properties, not by appearance alone.

Property	Meaning and unit	Typical value
Grammage (GSM)	mass per unit area, g m⁻²	newsprint 45, photocopy 70–80, card 200+
Caliper	thickness, μm	100 μm for 80 gsm
Bulk	caliper ÷ grammage, cm³ g⁻¹	1.2–1.5
Brightness	% of blue light reflected	newsprint 58, copier paper 90+
Opacity	% of light stopped; raised by TiO₂ and clay	> 90 % for printing paper
Tensile, burst and tear strength	resistance to pulling, bursting, tearing	high in long-fibre Kraft
Sizing (Cobb value)	water absorbed in 60 s, g m⁻²	low for writing paper
Ash content	% of mineral filler left on ignition	5–20 %
pH	acid-free paper has pH 7–9	archival paper is alkaline
Smoothness and finish	calendered, machine or glazed	—

Paper and pulp in Nepal

Nepal's one large integrated mill, Bhrikuti Pulp and Paper at Gaindakot in Nawalparasi, was built in the mid-1980s with Chinese assistance and used babiyo (sabai) grass and paddy straw rather than wood. It supplied much of the country's paper, was privatised in 1992 and closed in 2011, and Nepal now imports most of its paper from India, China and Indonesia. The mills still working are small ones that recycle waste paper, together with several hundred cottage units in the hill districts producing lokta paper for notebooks, gift wrap and export. The attraction of Nepal's non-wood fibres is clear from the composition chart: less lignin means a milder cook, less chemical and less effluent — but they are bulky, seasonal and low in ash-free fibre, which is why a mill must sit close to its fields.

Chapter summary

• Paper is a mat of cellulose fibres, (C₆H₁₀O₅)ₙ, bonded by hydrogen bonds; pulping is the removal of lignin, which is brown, stiff and light-sensitive.
• Wood is roughly 45 % cellulose, 25 % hemicellulose and 25 % lignin; non-wood fibres such as babiyo grass and straw contain much less lignin.
• Raw materials: softwood (long fibre, strength), hardwood (short fibre, smoothness), non-wood fibre, rags and lokta bark, and recycled waste paper.
• Mechanical pulping gives 90–95 % yield but weak, yellowing pulp; chemical pulping gives 45–55 % yield of strong, stable fibre.
• Kraft (sulphate) process: cook chips with white liquor (NaOH + Na₂S) at 170 °C for 2–4 h; the sulphite process uses Ca(HSO₃)₂ with free SO₂.
• Chemical recovery: burn black liquor (Na₂SO₄ + 2C → Na₂S + 2CO₂↑), dissolve the smelt to green liquor, causticise it (Na₂CO₃ + Ca(OH)₂ → 2NaOH + CaCO₃↓) and reburn the lime mud (CaCO₃ → CaO + CO₂↑; CaO + H₂O → Ca(OH)₂).
• Stages: chipping → pulping → washing and screening → bleaching (ECF with ClO₂, not Cl₂) → beating, sizing with rosin + alum and loading with clay → sheet formation on the Fourdrinier wire → pressing, drying, calendering, reeling.
• Quality is measured by grammage (g m⁻²), caliper, brightness, opacity, tensile, burst and tear strength, sizing, ash content and pH; acid-free paper lasts longest.
• Bhrikuti Pulp and Paper, Gaindakot, worked on babiyo grass and straw from the mid-1980s until 2011; Nepal's living paper tradition is hand-made lokta paper.

Practice questions

Group A — Multiple choice (1 mark each)

1. The chief chemical constituent of paper is [1]aligninbcellulosecstarchdhemicellulose
2. The white liquor used in the Kraft process contains [1]aNaOH and Na₂SbCa(HSO₃)₂ and SO₂cNaOCl and NaOHdNaClO₃ and HCl
3. Lignin must be removed from pulp mainly because it [1]ais too expensivebdissolves the cellulosecis brown and makes the paper yellow in lightdprevents drying
4. Causticisation of green liquor is represented by [1]aCaCO₃ → CaO + CO₂bNa₂CO₃ + Ca(OH)₂ → 2NaOH + CaCO₃cNa₂SO₄ + 2C → Na₂S + 2CO₂d2SO₂ + CaCO₃ + H₂O → Ca(HSO₃)₂ + CO₂
5. Rosin and alum are added to pulp for [1]ableachingbloadingcsizingddrying
6. The grammage of ordinary photocopy paper is about [1]a10 g m⁻²b45 g m⁻²c80 g m⁻²d300 g m⁻²

Answers to Group A

1. (b) — the fibre itself is cellulose.

2. (a) — sodium hydroxide with sodium sulphide.

3. (c) — residual lignin oxidises in light and yellows the sheet.

4. (b) — lime converts carbonate back to hydroxide.

5. (c) — aluminium resinate stops ink spreading.

6. (c) — 70–80 g m⁻² is the usual office grade.

Group B — Short answer (5 marks each)

1. What are pulp and paper? Name the raw materials used for papermaking in Nepal and state one advantage of non-wood fibre. [5]
2. Compare mechanical and chemical pulping under yield, strength, colour stability and use. [5]
3. Describe the Kraft process, giving the composition of white liquor, the cooking conditions and the equations of chemical recovery. [5]
4. What is the purpose of (i) bleaching, (ii) beating, (iii) sizing and (iv) loading? Name one chemical used in each. [5]
5. List six properties by which the quality of paper is judged and explain why acid-free paper lasts longer. [5]
6. 4.0 tonnes of oven-dry straw contains 40 % cellulose, 28 % hemicellulose and 17 % lignin. If cooking removes all the lignin and one-quarter of the hemicellulose, calculate the mass of pulp and the percentage yield. [5]

Answers to Group B

1. Pulp is the fibrous cellulose mass got by breaking plant material down mechanically or chemically; paper is the sheet formed when dilute pulp is drained on a wire, pressed and dried. Nepali raw materials: babiyo (sabai) grass, rice and wheat straw, bagasse, bamboo, lokta bark and waste paper. Non-wood fibre contains much less lignin, so it needs a milder cook, less chemical and less energy.

2. See the table in §20.3: mechanical 90–95 % yield, weak short fibres, yellows in light, used for newsprint; chemical 45–55 % yield, strong fibres, colour-stable, used for printing, writing and packaging paper.

3. Chips are cooked with white liquor (NaOH + Na₂S) at about 170 °C and 7–8 atm for 2–4 hours; lignin dissolves, leaving cellulose. Recovery: Na₂SO₄ + 2C → Na₂S + 2CO₂↑ in the furnace; Na₂CO₃ + Ca(OH)₂ → 2NaOH + CaCO₃↓ in the causticiser; CaCO₃ --Δ--> CaO + CO₂↑ and CaO + H₂O → Ca(OH)₂ in the lime kiln.

4. (i) Bleaching removes residual lignin and whitens the pulp — ClO₂ or H₂O₂. (ii) Beating frays and swells the fibres so they bond — mechanical, no chemical. (iii) Sizing makes the sheet resist ink spread — rosin with alum, Al₂(SO₄)₃. (iv) Loading fills the gaps for opacity and smoothness — china clay, CaCO₃ or TiO₂.

5. Grammage, caliper, brightness, opacity, strength (tensile, burst, tear), sizing or Cobb value, ash content, pH, smoothness. Acid-free paper (pH 7–9) is not slowly hydrolysed at its glycosidic links, so the cellulose chains keep their length and the sheet does not become brown and brittle.

6. Cellulose retained = 0.40 × 4000 = 1600 kg. Hemicellulose retained = ¾ × 0.28 × 4000 = 840 kg. Lignin retained = 0. Pulp = 1600 + 840 = 2440 kg; yield = 2440/4000 × 100 = 61 %.

Group C — Long answer (8 marks each)

1. Describe the manufacture of paper from wood by the Kraft process with a flow-sheet diagram, giving the stages from chipping to reeling and the equations of chemical recovery. [8]
2. (a) What is meant by the grammage of paper? A reel of paper 1.2 m wide and 4000 m long weighs 336 kg. Find its grammage. [4](b) Explain how bleaching is carried out in a modern mill and why elemental chlorine has been abandoned. [4]

Answers to Group C

1. Logs are debarked and chipped; the chips are cooked in a digester with white liquor (NaOH + Na₂S) at 170 °C for 2–4 h, which dissolves the lignin; the pulp is blown, washed and screened; it is bleached with O₂, ClO₂ and H₂O₂; beaten, sized with rosin–alum and loaded with china clay; formed into a web on the Fourdrinier wire from a 0.5 % suspension; pressed, dried on steam cylinders, calendered, reeled and cut. The black liquor is evaporated and burnt (Na₂SO₄ + 2C → Na₂S + 2CO₂↑), the smelt dissolved to green liquor and causticised (Na₂CO₃ + Ca(OH)₂ → 2NaOH + CaCO₃↓), and the lime mud reburnt (CaCO₃ → CaO + CO₂↑; CaO + H₂O → Ca(OH)₂). Draw the flow sheet of §20.4.

2. (a) Grammage is the mass of one square metre of the paper, in g m⁻². Area = 1.2 × 4000 = 4800 m²; mass = 336 kg = 336 000 g. Grammage = 336 000 / 4800 = 70 g m⁻². (b) Bleaching is done in stages: an oxygen stage removes about half the residual lignin cheaply, then chlorine dioxide (made on site by 2NaClO₃ + 4HCl → 2ClO₂↑ + Cl₂↑ + 2NaCl + 2H₂O) oxidises the rest, with alkaline extraction and hydrogen peroxide stages between. Elemental chlorine was abandoned because it chlorinates lignin to dioxins and other toxic, persistent organochlorines that pass into the river; ECF and TCF sequences avoid them.