Pulping is the first chemical-and-mechanical conversion step in any pulp and paper mill, separating cellulose fibres from lignin, hemicellulose and other wood components so they can be reformed into paper. The two dominant routes are chemical pulping (kraft, sulphite, soda) and mechanical pulping (TMP, RMP, SGW, PGW), with several hybrid routes (CTMP, BCTMP, semi-chemical) sitting between them. Globally, chemical pulping accounts for over 70% of virgin-wood pulp output and mechanical pulping the balance, per FAO Forestry Production Yearbook 2025.
Quick answer: Chemical pulping (kraft, sulphite) cooks wood chips with sodium hydroxide and sodium sulphide white liquor in a digester to dissolve lignin, leaving clean cellulose fibres at 40-55% pulp yield. Mechanical pulping uses refiner plates or grindstones to physically separate fibres at 90-95% yield, but the retained lignin makes paper weaker and yellowing-prone (TAPPI 2026). Chemical pulp dominates premium packaging, writing-printing and tissue grades; mechanical pulp dominates newsprint and low-cost board (FAO 2025; CPPRI 2026).
Key Takeaways
- Chemical pulping (kraft, sulphite): yield 40-55%, strong bleachable fibres, kappa 8-30 before bleaching.
- Mechanical pulping (TMP, SGW, RMP): yield 90-95%, weaker fibres, electricity-intensive at 1,800-2,400 kWh per tonne.
- Kraft is a net energy producer; mechanical is a net energy consumer.
- Hybrid routes (CTMP, BCTMP) bridge yield and strength for printing, tissue and food-grade board.
- Decision rule for mill operators: kraft for premium packaging and writing-printing; mechanical for newsprint and low-cost board; blended for everything in between.
What is Pulping in Paper Making?
In a pulp and paper mill, pulping is the unit operation that converts raw lignocellulosic material (typically softwood, hardwood, bagasse, wheat straw or recovered paper) into a slurry of individual fibres at a controlled freeness, pulp consistency, brightness and kappa number. Without pulping, the cellulose remains bound to lignin and cannot be reformed into a sheet. Three commercial types of pulping exist in industrial use today:
- Chemical pulping: dissolves lignin with hot alkali or acid liquors. Kraft pulp is the dominant variant (over 80% of chemical pulp output globally per FAO 2025), followed by sulphite, soda, and organosolv routes.
- Mechanical pulping: physically tears wood into fibres using refiners, grindstones, or pressurised thermomechanical refining. The lignin remains in place.
- Chemi-mechanical pulping: a hybrid that pretreats wood with mild chemicals before refining, partly softening lignin and improving fibre quality at higher yield than chemical pulp. CTMP and BCTMP are the most common variants (CEPI 2026).
Mill operators select the route based on the target paper grade, raw-material availability, energy economics, and the local effluent regulatory regime. Each route produces fibre with measurably different kappa number, freeness, tensile index, and brightness profile, all of which a paper machine operator must accommodate downstream.
Chemical Pulping: Process, Chemistry and Mill Economics
Chemical pulping cooks wood chips with white liquor in a continuous or batch digester at 160-175 degrees C for 1-3 hours, depending on grade. The lignin dissolves into the spent cooking liquor (black liquor), which is then recovered, evaporated, and burned in a recovery boiler, regenerating the cooking chemicals and producing steam plus electricity. This recovery loop is what makes kraft pulping cost-competitive at integrated scale (TAPPI Recovery Boiler Manual 2026).
Types of Chemical Pulping
- Kraft (sulphate) process: dominant route. White liquor: sodium hydroxide + sodium sulphide. Produces strong, dark-brown unbleached pulp that bleaches well to high brightness. Roughly 85% of world chemical wood pulp comes from kraft (FAO 2025).
- Sulphite process: uses sulphurous acid plus a base (Mg, Ca, Na, NH4). Produces lighter-colour pulp with easier bleaching but lower strength than kraft. Used for specialty grades, dissolving pulp, and some hygiene applications (CEPI 2026).
- Soda and modified soda: alkaline cook without sulphide. Used for non-wood fibres (bagasse, straw) common in India and parts of Asia.
- Organosolv and emerging green routes: solvent-based chemical pulping still mainly at pilot scale; potential for lower-impact operations.
Advantages of Chemical Pulping
- Highest tensile strength and burst index per gram of fibre: typically 2-3 times mechanical pulp at equivalent basis weight (TAPPI fibre strength reference 2026).
- Bleachable to high ISO brightness (>90) for premium printing and tissue grades.
- Resistant to yellowing and light-induced degradation because residual lignin is very low (kappa typically 8-30 before bleaching).
- Net energy producer when integrated with a recovery boiler: modern kraft mills export 0.5-1.5 GJ/t of paper to the grid (CEPI 2026).
- Self-sufficient on cooking chemicals once the recovery loop is steady state.
Disadvantages of Chemical Pulping
- Lower yield: 40-55% of dry wood ends up as pulp; the rest leaves as dissolved lignin and extractives (TAPPI 2026).
- High capital intensity: a greenfield 200,000 TPA kraft line is in the USD 350-500 million range per RISI 2026 capex benchmarks.
- Sulphur-bearing emissions (TRS gases) require dedicated odour control; AOX in effluent must meet local norms (US EPA NESHAP; EU IED; India CPCB Charter on CREP 2023).
- Longer time-to-payback than mechanical or recycled-fibre routes.
Mechanical Pulping: Process, Variants and When It Wins
What is mechanical pulping? The mechanical pulping process fibrillates wood logs or chips by force, either with rotating grindstones (SGW), pressurised disk refiners (TMP), or pre-steamed and chemically softened chips (CTMP). The lignin remains largely in place, which is why mechanical pulp is sometimes called high-yield pulp: 90-95% of the dry wood ends up in the sheet (FAO 2025).
Types of Mechanical Pulping
- SGW (Stone Groundwood): oldest commercial route. Logs are pressed against a rotating grindstone. SGW pulp is short-fibre and suited for newsprint and lower-grade board.
- RMP (Refiner Mechanical Pulp): wood chips refined in disk refiners under atmospheric pressure. Longer fibres than SGW.
- TMP (Thermomechanical Pulp): chips pre-steamed at 120-130 degrees C then refined under pressure. Better strength and brightness than RMP. TMP pulp is the dominant mechanical route today (RISI 2026).
- PGW (Pressurised Groundwood): improved SGW under pressure; intermediate between SGW and TMP.
Advantages of Mechanical Pulping
- High yield: 90-95%: roughly twice the wood-to-fibre conversion of chemical pulping (FAO 2025).
- Lower capex: 40-60% of a kraft line of equivalent capacity per RISI 2026 benchmarks.
- Good opacity and bulk: preferred for printing grades where bulk and ink absorption matter (TAPPI 2026).
- No chemical recovery infrastructure required.
Disadvantages of Mechanical Pulping
- Weaker paper: tensile strength is 30-50% of equivalent kraft basis weight (TAPPI fibre strength reference 2026).
- Yellows under UV exposure because lignin remains in the fibre.
- Electricity intensive: refiner energy is 1,800-2,400 kWh per tonne for TMP, making mechanical mills net energy consumers (IEA 2026 industry data).
- Cannot bleach to as high a brightness as chemical pulp; ceiling around 75-80 ISO with peroxide bleaching (CEPI 2026).
CTMP, BCTMP and Chemi-Mechanical Pulping: The Hybrid Route
Chemi-mechanical pulping bridges chemical and mechanical routes. Wood chips are pretreated with sulphite, sodium hydroxide, hydrogen peroxide or alkaline peroxide to soften lignin and partially separate fibres before refining. The result is pulp with mechanical-pulp yield (75-90%) but chemical-pulp-like brightness and strength at the upper end.
- CTMP (Chemi-Thermomechanical Pulp): chips pretreated with sodium sulphite then refined under pressure. Yield around 90%. Used for printing and writing grades, food-grade board, and tissue (TAPPI 2026).
- BCTMP (Bleached CTMP): CTMP pulp plus peroxide bleaching. Brightness up to 85 ISO. Mainly used for premium tissue and lightweight coated grades.
- APMP (Alkaline Peroxide Mechanical Pulp): alkaline-peroxide pretreatment instead of sulphite. Strong process for hardwoods.
- Semi-chemical (NSSC): Neutral Sulphite Semi-Chemical cooking for corrugating medium. Yield 70-80%. Standard furnish for fluting in corrugated board mills (FAO 2025).
Mill economics for CTMP and BCTMP sit between kraft and TMP: refiner energy is lower than pure TMP, but the partial chemical recovery loop is more complex than mechanical pulping. For mills targeting LWC, SC, and premium board grades, the hybrid route is often the best fit (CEPI 2026).
Bio-Pulping: The Emerging Third Route
Bio-pulping uses lignin-degrading white-rot fungi (Ceriporiopsis subvermispora and similar species) to pretreat wood chips for 2-4 weeks before either mechanical or chemical pulping. Pilot trials at the USDA Forest Products Laboratory and follow-up studies show electricity savings of 30-40% for TMP and modest yield improvements at chemical pulping (US Forest Service Research, "Evaluating chemical-, mechanical-, and bio-pulping life cycle assessment", 2024). Bio-pulping is not yet at commercial mill scale but is actively researched at IPPTA-affiliated institutions in India and by several European specialty mills.
Energy & Economics: Chemical vs Mechanical at the Mill Level
The economic case for each route changes with grid electricity price, wood availability, and effluent permits. Industry benchmarks for greenfield mill operators in 2025:
Three implications mill operators draw from these benchmarks:
- Kraft pays back capex through downstream pulp strength + energy export to the grid; the mill is in effect a co-generation plant.
- TMP pays back through low capex and good fit with locally available softwood: but only economically where electricity is cheap (under USD 0.06/kWh historically; threshold has shifted upward as grid prices have risen 2023-2025).
- RCF / DIP (recycled) is the cheapest virgin-fibre substitute and dominates new capacity announcements in Asia per RISI 2026.
Which Pulping Route Should You Choose? Decision Matrix for Mill Operators
The right pulping route is dictated almost entirely by the target paper grade and the freight-adjusted economics of the mill location. Mill engineers map paper grade to pulping route as follows:
In practice, most modern integrated mills run a blended furnish: virgin kraft for strength and brightness, mechanical or recycled fibre for cost and opacity. Integrated kraft mills (ITC Bhadrachalam, JK Paper, West Coast Paper, International Paper, Stora Enso, Smurfit Westrock, Suzano, UPM-Kymmene) operate primarily on chemical pulping with secondary recycled-fibre lines, while smaller newsprint and low-cost packaging mills lean on mechanical and recycled-fibre furnishes (IPMA 2026; CEPI 2026).
Comparison Snapshot: Chemical vs Mechanical Pulping
At-a-glance comparison of the two dominant routes:
Where Chemical Pulp Goes: Common End-Uses
- Premium writing and printing papers (copier paper, offset paper, fine paper).
- Kraft packaging, corrugated liner, food-grade carton board.
- Tissue and hygiene paper (with or without recycled blend).
- Banknotes and security papers (sulphite kraft blends).
- Specialty filter, electrical insulation, and laminate base paper.
- Dissolving pulp for viscose, lyocell and cellulosic textile fibre.
Where Mechanical Pulp Goes: Common End-Uses
- Newsprint (still 30-40% mechanical in modern mills) and directory paper.
- LWC and SC magazine paper (blended with chemical kraft for printability).
- Tissue and toilet paper (low-grade segments using CTMP).
- Cores, board backing, and low-cost packaging where strength is not critical.
- Moulded fibre products and some egg-tray applications.
Sources & Further Reading
All statistics, ratios and process values cited above are verifiable against the following industry authorities. Links open in a new tab.
- TAPPI (Technical Association of Pulp & Paper Industry): pulping standards, recovery boiler manual, fibre strength references. tappi.org/publications-standards
- FAO Forestry Production Yearbook 2025: global pulp output, method splits, virgin-fibre data. fao.org/faostat (Forestry)
- CEPI (Confederation of European Paper Industries): Key Statistics, sustainability factsheets, recovery boiler benchmarks. cepi.org/statistics
- IEA (International Energy Agency): pulp and paper industry energy intensity. iea.org/energy-system/industry/paper
- CPPRI (Central Pulp & Paper Research Institute, India): capacity and production data. cppri.res.in
- IPMA (Indian Paper Manufacturers Association): annual industry outlook 2025. ipma.co.in
- IPPTA (Indian Pulp & Paper Technical Association): review of traditional pulping methods 2025. ippta.co
- US Forest Service Research: evaluating chemical-, mechanical-, and bio-pulping life-cycle assessment. research.fs.usda.gov/treesearch/38599
- RISI / Fastmarkets: pulp and paper capex benchmarks 2024 (commercial; subscription). fastmarkets.com/risi
- US EPA NESHAP (pulp and paper effluent and air emission limits): epa.gov/...pulp-and-paper-production-NESHAP
- CPCB India (Central Pollution Control Board, Charter on CREP for pulp and paper): cpcb.nic.in
Continue reading on worldpapermill.com: explore essential raw materials for paper making, the growth of the Indian paper industry, the end-to-end paper production process, and the pulp bleaching process.
Both chemical and mechanical pulping start with the same wood-preparation step. For the woodyard sequence, see our chipping process in paper making guide.
