Pulping processes: mechanical, kraft and chemical recovery

In short

Pulping is the separation of cellulosic fibres from the lignin and hemicellulose matrix of wood or agro-residues, either by mechanical energy, by chemical dissolution of lignin, or by a combination of both. The kraft (sulphate) process makes about four-fifths of the world's chemical pulp because of its tolerance of all raw materials, high pulp strength and, decisively, its closed chemical recovery cycle that regenerates cooking chemicals and generates steam and power. Indian mills work with bamboo, eucalyptus, casuarina, subabul and large volumes of wheat straw and bagasse, so pulping selection, alkali charge and recovery of silica-rich liquors are live engineering problems. The topic carries heavy weight in pulp and paper technology papers, in Chemical Engineering GATE-type questions on mass and energy balances, and in technical rounds at ITC PSPD, JK Paper, West Coast Paper Mills, TNPL, Century Pulp & Paper, Seshasayee and Andritz/Valmet.

Explained

Raw material and the chemistry that pulping must defeat

• Wood composition (dry basis, typical): cellulose 40-45%, hemicellulose 25-35%, lignin 18-25% (hardwood) or 25-32% (softwood), extractives 2-8%, ash under 1%.
• Agro-residues: wheat straw and bagasse have less lignin (15-20%) but 3-14% ash with high silica, which is why Indian straw mills struggle with recovery boilers and lime mud.
• Lignin is a three-dimensional phenylpropane polymer, a thermosetting adhesive in the middle lamella; delignification means cleaving its ether (β-O-4) and carbon-carbon linkages so fibres separate without being cut.
• Fibre dimensions decide end use: softwood 2.5-4.0 mm long (strength, sack kraft), bamboo 2.0-3.5 mm, hardwood/eucalyptus 0.7-1.2 mm (smoothness, printing papers), straw 1.0-1.5 mm with many fines.
• Wood preparation: debarking (drum/ring), chipping to 15-25 mm × 3-5 mm, screening to remove overthick chips (poor liquor penetration) and pin chips/fines (overcooked, lose yield), then chip washing and conveying to the chip silo.

Mechanical pulping

• Principle: lignin is softened by heat and friction (glass transition about 100-140 °C, lowered by water) and fibres are torn apart. Almost all the wood is retained, so yield is 90-97%, but the pulp has low strength, high opacity, high bulk and it yellows on light exposure.
• Stone groundwood (SGW): debarked bolts pressed against a revolving grindstone with shower water; pressurised version is PGW. Freeness about 70-100 mL CSF, SEC 1,100-1,500 kWh/t.
• Refiner mechanical pulp (RMP): chips refined at atmospheric pressure between discs with bar-and-groove plates; longer fibres than SGW, stronger pulp.
• Thermomechanical pulp (TMP): chips presteamed at 110-130 °C (about 1.5-3 bar) and refined under pressure in the first stage; lignin softens, so fibres peel instead of fracturing. Tear and tensile strength much higher than SGW; SEC 1,800-2,500 kWh/t - the process is an electricity consumer, which is the main objection to it in India.
• Chemithermomechanical pulp (CTMP): chips impregnated with sodium sulphite/NaOH (1-4% on wood) before steaming and refining; yield 85-94%, lower energy, better strength and brightness; used for fluff pulp and board plies.
• Semichemical: NSSC (neutral sulphite semichemical, Na₂SO₃ + Na₂CO₃ buffer, 70-80% yield) gives stiff hardwood pulp for corrugating medium.
• Energy and heat recovery: about 90% of refining energy appears as steam; modern TMP lines recover clean steam through reboilers for paper drying.
• Limitations: lignin stays in the pulp, so the pulp is not bleachable to high brightness without loss (only lignin-preserving bleaching is used), and the paper is not permanent - newsprint yellows.

Chemical pulping: processes and comparison

• Objective: dissolve lignin selectively, keep cellulose. Chemical pulp yield is only 42-50%, but strength is far higher and the pulp bleaches to 88-90% ISO brightness.
• Kraft (sulphate): alkaline, NaOH + Na₂S. Tolerates resinous and mixed woods, fast, strongest pulp, and the only process with a mature recovery cycle. Drawbacks: dark unbleached pulp (high bleaching demand), lower yield than sulphite, and odorous TRS gases (H₂S, methyl mercaptan, dimethyl sulphide/disulphide).
• Soda: NaOH alone, sometimes with anthraquinone (AQ) as a redox catalyst that protects carbohydrates and accelerates delignification. Widely used in Indian agro-based mills because sulphur-free liquor eases odour and some recovery problems; pulp is weaker than kraft.
• Sulphite: acid, neutral or alkaline sulphite with Ca, Mg, Na or NH₄ base. Gives bright, easily bleached pulp of lower strength; calcium base has no recovery, magnefite (Mg) base recovers well. Largely displaced; survives for dissolving pulp (rayon grade).
• Mechanism of kraft delignification: hydroxide and especially hydrosulphide (HS⁻) ions attack phenolic β-aryl ether units, forming quinone methides and soluble lignin fragments (thiolignin). HS⁻ is the reason kraft is faster and more selective than soda.
• Three kinetic phases: *initial* (up to about 150 °C, extractives and some lignin, diffusion-limited), *bulk* (150-170 °C, about 90% of lignin removed, first-order in lignin), *residual* (slow, carbohydrate-destructive - stopping here protects yield).
• Carbohydrate losses: peeling reactions on cellulose/hemicellulose chain ends, which stop by stabilising reactions; AQ and polysulphide reduce peeling.

Kraft cooking: liquors, variables and equipment

• White liquor: fresh cooking liquor, NaOH + Na₂S. Black liquor: spent liquor with dissolved lignin and spent chemical. Green liquor: smelt dissolved in water (Na₂CO₃ + Na₂S). All compositions are reported as Na₂O (or as NaOH) - the basis must be stated.
• Definitions (as Na₂O unless stated): Active alkali = NaOH + Na₂S; Effective alkali = NaOH + ½Na₂S; Total titratable alkali (TTA) = NaOH + Na₂S + Na₂CO₃; Sulphidity = Na₂S/(NaOH + Na₂S) × 100; Causticity = NaOH/(NaOH + Na₂CO₃) × 100.
• Typical kraft conditions: active alkali 14-18% on oven-dry wood (higher for bamboo/softwood), sulphidity 25-30%, liquor-to-wood ratio 3.5:1 to 4.5:1, cooking temperature 165-175 °C, time to temperature 60-90 min, time at temperature 60-120 min.
• Batch digester: vertical pressure vessel, 100-400 m³, indirect heating via forced circulation and heater; cycle = chip filling, liquor charge, ramp, cook, blow to blow tank at 7-9 bar (fibres separate by sudden pressure drop). Displacement (cold blow / RDH / SuperBatch) systems displace hot black liquor to accumulators, cutting steam use 30-40% and improving pulp strength.
• Continuous digester (Kamyr type): chip feed via low-pressure and high-pressure feeders, impregnation, co-current then counter-current cooking, Hi-heat washing and cold blow at the bottom; needs steady chip quality but has lower steam and labour cost.
• Modified cooking (MCC/EMCC/ITC, Lo-Solids): level out alkali profile, keep low dissolved-lignin concentration and low temperature at the end - deeper delignification (kappa 18-22 softwood) at the same viscosity, so less bleaching chemical.
• Additives: AQ, polysulphide (raise yield 2-4%), surfactants for pitch.
• Blow heat recovery, turpentine recovery (softwood) and tall oil (soap) skimming from black liquor are standard by-product streams.
• Brown stock washing on vacuum drum washers, wash presses or diffusers: countercurrent, 3-4 stages; measured by dilution factor and displacement ratio. Poor washing carries alkali to bleaching (chemical loss) and soda to the mill (pollution).
• Screening and centricleaning remove shives, knots (knotter rejects re-cooked), and sand; then unbleached pulp goes to bleaching or to paper machine as brown grades.

Chemical recovery cycle

The recovery island is the economic heart of a kraft mill: it recovers over 95% of cooking chemical, burns lignin for steam and power, and keeps effluent load down.

1. Multiple-effect evaporation: weak black liquor 13-18% solids concentrated to 50-55% in 5-7 effects (falling film, often lamella type), steam economy roughly (n − 1) kg water per kg steam; concentrators take it to 72-85% (high solids firing). Soap skimming between effects; methanol-bearing foul condensate stripped.
2. Black liquor at 65-75% solids behaves as a fuel with HHV about 13.5-15 MJ/kg dry solids; every tonne of pulp yields roughly 1.5-1.8 t dry solids.
3. Recovery boiler (Tomlinson): liquor sprayed into the furnace; stages are drying, pyrolysis, char burning in a reducing char bed (where Na₂SO₄ + 2C → Na₂S + 2CO₂, giving reduction efficiency 90-95%), and oxidation in the upper furnace with secondary/tertiary air. Produces high-pressure steam (typically 65-110 bar, 480-510 °C) for a turbo-generator, and smelt (Na₂CO₃ + Na₂S) tapped through spouts.
4. Hazards: a smelt-water explosion is the defining risk - water leaking into the furnace flashes violently; BLRBAC-style emergency shutdown procedure (ESP) is standard practice. Boiler design, registration and inspection in India now follow the Boilers Act, 2025 and the Indian Boiler Regulations, 2026 (in force October 2026), which supersede the IBR 1950.
5. Dissolving tank: smelt dissolved in weak wash → green liquor; green liquor clarified/filtered, dregs to the dregs filter.
6. Slaking and causticizing: CaO + H₂O → Ca(OH)₂ (slaker, grit removal), then Ca(OH)₂ + Na₂CO₃ → 2NaOH + CaCO₃ in causticizers at 95-100 °C; causticizing efficiency 78-85% (equilibrium-limited).
7. White liquor clarification/pressure filtration returns liquor to the digester; lime mud (CaCO₃) is washed, filtered on a precoat or disc filter and calcined.
8. Lime kiln: rotary kiln at 1,000-1,200 °C, CaCO₃ → CaO + CO₂, fuel oil/gas/petcoke or biomass-derived fuel; makeup lime and purge of non-process elements (NPE) close the loop.
9. Non-process elements (Si, K, Cl, Al, Mg, P) accumulate in the loop; silica from straw and bagasse forms unfilterable green liquor and scales evaporators - the central reason agro-residue mills in India either run small-scale recovery or adopt silica removal/desilication before evaporation.
10. Energy picture: a modern integrated kraft mill is a net exporter of electricity; black liquor is India's largest industrial biomass fuel after bagasse.

Environment, standards and Indian practice

• CPCB classifies pulp and paper as a Red category / highly polluting industry; large integrated mills with chemical recovery must have continuous (online) effluent and emission monitoring connected to CPCB/SPCB servers under the Water Act, 1974 and Air Act, 1981, with consent to operate from the State Pollution Control Board. Discharge standards are set under the Environment (Protection) Rules and differ for large pulp-and-paper, agro-residue-based and RCF-based (waste-paper) mills - quote the limit for the specific mill category, not a single number.
• Key pollution indicators: BOD, COD, AOX, colour, SS, and TRS for odour; kraft mills also report soda loss (as Na₂SO₄ kg/t pulp).
• Unbleached kraft pulping itself discharges little if washing and recovery are tight - most load historically came from poor washing and absence of recovery in small mills.
• Testing and specification: pulp and paper tests follow IS/ISO methods adopted by BIS (TAPPI and SCAN methods are also used in mills); kappa number is determined by the standard permanganate method (TAPPI T 236/ISO 302), and pulp sampling and conditioning at 23 °C/50% RH per the standard atmosphere method. Where an old IS number may have been revised or withdrawn, cite the test by subject.
• Labour, safety and welfare in mills are governed by the four Labour Codes in force from 21 November 2025, with the OSH&WC Code, 2020 replacing the former Factories Act, 1948 for safety, hazardous-process notification, pressure-vessel and confined-space duties.
• Indian raw-material reality: no large softwood resource, so mills rely on farm-forestry eucalyptus/subabul/casuarina (agro-forestry plantations supported by mills themselves), bamboo, and hardwood mixes; waste paper and agro-residue fill the gap and import dependence on pulp and waste paper remains high.

Key points

• Pulping - separating fibres from lignin by mechanical, chemical or hybrid means; yield and strength trade off against each other.
• Mechanical yield - 90-97%, high opacity and bulk, low strength, non-permanent (yellows); energy-intensive.
• TMP vs SGW - presteaming softens lignin, so TMP fibres are longer and stronger but SEC is higher (1,800-2,500 vs 1,100-1,500 kWh/t).
• CTMP - sulphite pretreatment before refining; lower energy, higher strength, 85-94% yield.
• Chemical yield - 42-50%; kraft gives the strongest pulp and bleaches to 88-90% ISO.
• Kraft chemicals - NaOH + Na₂S; HS⁻ accelerates and makes delignification selective.
• Active alkali - NaOH + Na₂S as Na₂O; effective alkali = NaOH + ½Na₂S; sulphidity = Na₂S/(NaOH + Na₂S).
• Cooking variables - active alkali 14-18% on wood, sulphidity 25-30%, L:W 3.5-4.5, 165-175 °C.
• H-factor - single variable combining time and temperature; used to control batch cooks to a target kappa.
• Kappa number - measure of residual lignin; lignin % ≈ 0.15 × kappa; softwood 25-35, hardwood 14-20 unbleached.
• Delignification phases - initial, bulk (90% of lignin) and residual (carbohydrate-destructive).
• Modified cooking - levelled alkali profile and low end-temperature give lower kappa at the same viscosity.
• Brown stock washing - countercurrent; judged by dilution factor and displacement ratio; controls soda loss.
• Recovery cycle - evaporation → recovery boiler → dissolving → causticizing → lime kiln; over 95% chemical recovery.
• Reduction efficiency - Na₂S/(Na₂S + Na₂SO₄) in smelt, 90-95%; causticizing efficiency 78-85%.
• Black liquor fuel value - 13.5-15 MJ/kg dry solids; fired at 65-85% solids; mill becomes a power exporter.
• Smelt-water explosion - the critical recovery boiler hazard; emergency shutdown procedure is standard practice.
• Silica and NPEs - the barrier to recovery in straw/bagasse mills; lime mud and green liquor become unfilterable.

Formulas and examples

Example 1 - White liquor analysis. A white liquor contains NaOH 95 g/L and Na₂S 30 g/L, both expressed as Na₂O.

• AA = 95 + 30 = 125 g/L as Na₂O.
• EA = 95 + 15 = 110 g/L as Na₂O.
• Sulphidity = 30/125 × 100 = 24%.
• As NaOH basis: AA = 125 × 1.290 = 161.3 g/L as NaOH.
• If a cook needs AA of 16% on o.d. wood for 40,000 kg of wood: alkali required = 0.16 × 40,000 = 6,400 kg as Na₂O → liquor volume = 6,400/0.125 = 51,200 L of white liquor.
• If L:W is 4.0, total liquor = 160,000 L; chips at 50% moisture carry 40,000 L of water, so black-liquor/weak-liquor make-up = 160,000 − 51,200 − 40,000 = 68,800 L.

Example 2 - H-factor of a batch cook. Relative reaction rate k = exp(43.2 − 16,113/T).

• At 170 °C (443.15 K): 16,113/443.15 = 36.360; exponent = 43.2 − 36.360 = 6.840; k = e^6.840 ≈ 933.
• At 160 °C (433.15 K): 16,113/433.15 = 37.199; exponent = 6.001; k ≈ 404.
• Cook held 1.5 h at 170 °C → H from the hold = 933 × 1.5 ≈ 1,400.
• A ramp approximated as 0.75 h at an average effective rate of 404 adds ≈ 303, so total H ≈ 1,700, a typical softwood kraft H-factor. Doubling the hold time at 170 °C would raise H to about 3,100 and over-cook the charge, so temperature and time are traded against each other at constant H.

Example 3 - Yield, kappa and alkali from a digester balance. 100 t/day o.d. eucalyptus chips cooked to 47% yield, kappa 17.

• Pulp = 47 t/day o.d.
• Dissolved organics + inorganics leaving as black liquor solids ≈ 53 t organic + alkali consumed. With AA 15% on wood = 15 t as Na₂O, total black liquor dry solids ≈ 65-70 t/day (roughly 1.4-1.5 t solids per t pulp - low because eucalyptus is a high-yield hardwood).
• Residual lignin in pulp = 0.15 × 17 = 2.55% → 47 × 0.0255 = 1.2 t/day of lignin remains for bleaching to remove.
• Heat available if solids have HHV 14 MJ/kg: 67,000 kg × 14 = 938 GJ/day ≈ 10.9 MW thermal.

Example 4 - Causticizing and reduction efficiency. Green liquor: Na₂S 35 g/L, Na₂CO₃ 95 g/L, Na₂SO₄ 3 g/L (all as Na₂O). After causticizing: NaOH 98 g/L, Na₂CO₃ 18 g/L.

• Causticizing efficiency = 98/(98 + 18) × 100 = 84.5%.
• Reduction efficiency = 35/(35 + 3) × 100 = 92.1%.
• Lime demand: Na₂CO₃ converted = 95 − 18 = 77 g/L as Na₂O = 77/62 = 1.242 mol/L → CaO needed = 1.242 × 56 = 69.6 g/L of green liquor, plus excess.

Example 5 - Evaporation duty. Weak black liquor at 15% solids is concentrated to 70% solids; basis 1 t dry solids.

• Feed mass = 1/0.15 = 6.667 t; product = 1/0.70 = 1.429 t → water evaporated = 5.238 t per t solids.
• A six-effect evaporator with steam economy 5.0 needs 5.238/5.0 = 1.05 t live steam per t solids.
• For a 500 t/day pulp mill producing 1.6 t solids per t pulp = 800 t solids/day: water evaporated = 4,190 t/day and live steam ≈ 838 t/day (about 35 t/h).

Where it is used

• Integrated pulp and paper mills: ITC PSPD (Bhadrachalam), JK Paper (Rayagada, Songadh), West Coast Paper Mills (Dandeli), Century Pulp & Paper (Lalkuan), Seshasayee Paper (Erode), Star Paper (Saharanpur) - kraft/soda-AQ cooking with full chemical recovery.
• Agro-residue mills: TNPL (Karur) uses bagasse with a dedicated recovery line; wheat-straw mills in Punjab, Haryana and Uttar Pradesh use soda pulping and face silica-related recovery limits.
• Mechanical pulping and newsprint: Nepa Limited and newsprint lines use groundwood/TMP; CTMP supplies fluff and board plies for packaging growth.
• Job roles: pulp mill/digester shift engineer, recovery boiler and evaporator engineer, causticizing and lime kiln operations, process control engineer (kappa and H-factor control loops), environment and effluent engineer, R&D at CPPRI Saharanpur and IIT Roorkee (Paper Technology, Saharanpur).
• Equipment and technology suppliers: Andritz, Valmet, Thermax and Indian fabricators supplying digesters, recovery boilers, lime kilns and washing lines - commissioning and service engineering roles.
• Allied products: lignosulphonates and kraft lignin, tall oil and turpentine, dissolving pulp for viscose rayon (Grasim/Birla Cellulose), and power export from black liquor-fired cogeneration.

Interview questions

• Q: Why does the kraft process dominate despite its low yield? A: Because it accepts any raw material, cooks fast, gives the strongest pulp, and its chemical recovery cycle returns over 95% of the cooking chemical while burning lignin for steam and power, which makes it economically superior to higher-yield processes.
• Q: What exactly does Na₂S do in kraft cooking? A: It hydrolyses to give HS⁻, a strong nucleophile that cleaves phenolic β-aryl ether bonds in lignin faster and more selectively than OH⁻ alone, so delignification is quicker with less carbohydrate damage than soda pulping.
• Q: Differentiate active, effective and total titratable alkali. A: Active alkali is NaOH + Na₂S; effective alkali is NaOH + ½Na₂S (the half accounts for the hydrolysis of Na₂S yielding one OH⁻); TTA adds Na₂CO₃, which is inactive in cooking.
• Q: What is the H-factor used for? A: It combines cooking time and temperature into one number using the Arrhenius rate of delignification, so a cook disturbed in temperature can be corrected in time to reach the same kappa.
• Q: What is kappa number and how does it relate to lignin? A: It is the volume in mL of 0.1 N KMnO₄ consumed per gram of oven-dry pulp under standard conditions; lignin content in per cent is roughly 0.15 × kappa.
• Q: Why is TMP stronger than stone groundwood? A: Presteaming at 110-130 °C softens lignin above its glass transition, so fibres separate at the middle lamella and stay long and intact instead of being fractured.
• Q: Name the four main unit operations of the kraft recovery cycle in order. A: Multiple-effect evaporation of black liquor, combustion in the recovery boiler to give smelt and steam, dissolving and clarification to green liquor, and causticizing with reburnt lime plus the lime kiln to regenerate CaO.
• Q: What is reduction efficiency and where is it measured? A: Na₂S/(Na₂S + Na₂SO₄) in the smelt or green liquor, expressed as Na₂O; it measures how well the char bed reduced sulphate to sulphide, typically 90-95%.
• Q: Why can causticizing efficiency never reach 100%? A: The reaction Ca(OH)₂ + Na₂CO₃ ⇌ 2NaOH + CaCO₃ is equilibrium-limited and NaOH itself suppresses conversion, so practical efficiency is 78-85%.
• Q: Why is recovery difficult for wheat straw and bagasse pulping in India? A: Their high silica content gives viscous, scaling black liquor and unfilterable green liquor and lime mud, which fouls evaporators and causticizing, so many agro mills operate without full recovery or need desilication.
• Q: What causes the characteristic kraft mill odour and how is it controlled? A: Total reduced sulphur compounds - H₂S, methyl mercaptan, dimethyl sulphide and disulphide - collected as concentrated and dilute non-condensable gases and incinerated in the lime kiln, recovery boiler or a dedicated furnace.
• Q: What is the most serious hazard of a recovery boiler? A: A smelt-water explosion if water contacts molten smelt; mills run strict tube-leak detection and an emergency shutdown procedure, with boiler design and inspection under the Boilers Act, 2025 and Indian Boiler Regulations, 2026.

Common mistakes

• Mixing the reporting basis - quoting alkali as Na₂O in one line and NaOH in the next; always convert with 1 g Na₂O ≡ 1.290 g NaOH.
• Computing effective alkali as NaOH + Na₂S, forgetting the one-half factor on Na₂S.
• Forgetting that chip moisture is part of the liquor-to-wood ratio, which makes the calculated white liquor volume wrong.
• Treating the H-factor as linear in temperature - it is exponential; 10 °C roughly doubles the rate.
• Confusing green, white and black liquor, or saying the recovery boiler produces white liquor (it produces smelt → green liquor).
• Claiming mechanical pulp can be bleached to full brightness like chemical pulp - only lignin-preserving bleaching applies, and the paper still yellows.
• Quoting a single effluent limit for all paper mills; standards are category-specific (large integrated, agro-residue, waste-paper based), and claiming mandatory status for a scheme without checking it.
• Ignoring yield when comparing processes, or quoting chemical pulp yield on bleached basis without saying so.

Related topics

Pulp bleaching and recycled fibre processing · Papermaking: stock preparation, the paper machine and paper testing · Wood and fibre chemistry (cellulose, hemicellulose, lignin) · Mass and energy balances in process industries · Evaporation and multiple-effect evaporators · Industrial effluent treatment and pollution control in process industries