Kraft process
The Kraft process is an alkaline chemical pulping method that converts wood chips into pulp by cooking them under high temperature and pressure in a solution known as white liquor, consisting primarily of sodium hydroxide and sodium sulfide, which selectively dissolves lignin and hemicelluloses while preserving the strength of cellulose fibers to yield a strong, brownish pulp suitable for papermaking. [1][2] Invented in 1879 by Carl F. Dahl, a German chemist working in the United States, the process derives its name from the German word for "strength," reflecting the superior tensile properties of the resulting pulp compared to earlier soda pulping techniques. [3] By the early 20th century, it had become the dominant industrial method due to its tolerance for a wide range of wood species, higher pulp yield from softwoods, and the development of a closed-loop chemical recovery system that regenerates cooking liquors from spent black liquor via evaporation, combustion, and causticizing, thereby reducing operational costs and raw material needs. [4] Today, the Kraft process accounts for approximately 75% of global pulp production, generating around 170 million tonnes annually, with its scalability and energy self-sufficiency—derived from burning lignin-rich black liquor—underpinning its persistence despite environmental challenges like sulfur emissions and wastewater management. [5][4] Key innovations include the addition of anthraquinone as a catalyst to accelerate delignification and improve yield, though the core mechanism relies on the nucleophilic attack of hydrosulfide ions on lignin ether bonds, enabling efficient fiber separation without excessive carbohydrate degradation. [6] While it produces pulp with inherent color from residual lignin, requiring bleaching for white papers, its defining characteristic remains the production of high-strength fibers ideal for packaging, linerboard, and tissue, far outperforming mechanical pulping in durability but at the cost of higher chemical inputs. [7]History
Invention and Early Development
The Kraft process, a chemical pulping method using a mixture of sodium hydroxide and sodium sulfide, was invented by German chemist Carl Ferdinand Dahl in 1879 while working in Danzig, Prussia (now Gdańsk, Poland).[8] [3] Dahl adapted the existing soda pulping technique— which relied solely on sodium hydroxide—by incorporating sodium sulfate into the white liquor, enabling the in situ formation of sodium sulfide during recovery, which accelerated lignin dissolution and produced pulp fibers retaining greater tensile strength compared to soda pulp.[9] [10] This innovation addressed limitations in prior methods, such as the soda process's inefficiency with resinous softwoods, yielding pulp suitable for demanding applications like sack paper despite an initial yield penalty of approximately 45-50% versus 50-55% for soda pulping.[11] Dahl secured U.S. Patent 296,935 on April 8, 1884, for his "Process of Manufacturing Chemical Fiber-Paper Pulp," detailing the sulfate addition and its effects on pulp quality.[12] [8] Early experimentation highlighted the process's advantages in fiber integrity, with pulp exhibiting 20-30% higher tear and burst strength, attributed to reduced cellulose degradation during cooking at 160-170°C under pressure.[9] However, initial challenges included incomplete chemical recovery and odor from volatile sulfides, limiting immediate scalability. The first documented kraft pulp production occurred experimentally at the Munksjö mill in Jönköping, Sweden, around 1885, following an accidental digester blow that revealed the process's potential for strong pulp from sulfate liquor.[11] Commercial viability emerged with the establishment of the first dedicated kraft mill in Sweden in 1890, marking the onset of industrial application and gradual refinement of digestion parameters to optimize yield and purity.[9] [10] Adoption spread slowly in Europe due to capital requirements for recovery systems, but by the early 1900s, the process's efficiency with abundant softwoods positioned it for broader use, culminating in the United States' inaugural kraft mill at Roanoke Rapids, North Carolina, which produced sulfate pulp on February 26, 1909, leveraging southern pine feedstocks.[13]Commercial Adoption and Technological Advancements
The first commercial implementation of the Kraft process occurred in Sweden, where a pulp mill utilizing the technology commenced operations in 1890 following Carl F. Dahl's U.S. patent issuance in 1884.[14] Initial adoption was slow due to the dark color of the resulting pulp and challenges in chemical recovery, limiting its competitiveness against the sulfite process, which dominated pulp production by 1900.[15] Commercialization accelerated in North America starting around 1907-1908, particularly in the southern United States and Canada, where the process proved effective for pulping resinous softwoods like southern pine that resisted sulfite digestion.[16][17] By the 1920s, kraft mills proliferated in the U.S. South, with output expanding rapidly from 1915 to 1930 as demand for strong paper products grew, establishing the region as a global kraft pulp hub.[15] A pivotal technological advancement was the invention of the Tomlinson recovery boiler in the early 1930s by G.H. Tomlinson, which enabled efficient combustion of black liquor to recover cooking chemicals (sodium hydroxide and sodium sulfide) and generate steam for energy self-sufficiency.[14] Prior to this, chemical losses and high energy costs hindered scalability; the boiler's design, featuring air levels for controlled oxidation, reduced these inefficiencies, making kraft pulping economically viable for large-scale operations and contributing to its dominance over sulfite methods.[18] By the 1950s, the kraft process had become the predominant chemical pulping technology worldwide, accounting for the majority of pulp production due to its yield advantages (45-50% from wood) and versatility for bleached grades following improved delignification techniques.[19] Subsequent innovations included the transition to continuous digesters in the 1930s-1940s, which replaced batch systems to enhance throughput and uniformity, and modifications to impregnation stages for better chemical penetration into wood chips.[20] Post-1950 developments focused on energy optimization, such as direct contact evaporators for black liquor concentration and advanced washing systems to minimize freshwater use, reducing operational costs by up to 20-30% in modern mills.[14] These enhancements, driven by rising energy prices and environmental regulations, solidified kraft's position, with global capacity exceeding 130 million tons annually by the late 20th century while maintaining core alkaline chemistry unchanged since the 1880s.[20]Process Fundamentals
Raw Materials and Preparation
The primary raw material for the Kraft process consists of wood chips sourced from softwood and hardwood species, with softwoods such as pine (Pinus spp.), spruce (Picea spp.), and fir (Abies spp.) providing longer fibers suitable for strong paper products, while hardwoods like eucalyptus (Eucalyptus spp.), birch (Betula spp.), and poplar (Populus spp.) yield shorter fibers for printing and writing papers.[21][22] Wood chips may derive from whole logs or sawmill residuals, though whole log chipping predominates in hardwood mills to ensure consistent quality. Preparation commences with debarking of logs to remove bark, which harbors silica, dirt, and other impurities that could consume pulping chemicals, lower pulp yield, and degrade brightness.[2] Debarked logs, or bolts, are fed into chippers that reduce them to uniform fragments, typically measuring 20-30 mm in length, 15-30 mm in width, and 3-8 mm in thickness, facilitating even impregnation and cooking.[23] Optimal chip thickness around 3 mm enhances liquor penetration, maximizes pulp yield at 45-50%, and minimizes rejects like oversize chips greater than 8-10 mm or fines under 2 mm.[24][25] Post-chipping, screening classifies chips by size, discarding overs (thick or long pieces that resist delignification) and pins/fines (small fragments prone to overcooking and dissolution), thereby improving digester throughput and pulp uniformity.[2][25] Screened chips may undergo brief washing or atmospheric steaming to remove air and surface contaminants, preparing them for impregnation with white liquor.[2]Impregnation and Cooking
In the impregnation stage of the Kraft process, wood chips are treated with white liquor—a solution containing sodium hydroxide (NaOH) and sodium sulfide (Na₂S)—to facilitate deep penetration of the alkaline cooking chemicals into the chip structure.[26] This step typically follows pre-steaming of the chips at atmospheric pressure to displace air from voids and preheat the material, promoting capillary action and reducing resistance to liquor ingress.[27] Penetration occurs primarily through longitudinal pathways in the wood, followed by radial and tangential diffusion, with rates accelerating above 100°C as hemicelluloses begin to soften and alkali reacts mildly with extractives.[28] Effective alkali concentrations of 1–2 M are commonly used, with higher levels (e.g., 2 M) enhancing uniformity and potentially increasing pulp yield by up to 2% through better chemical distribution.[29] Poor impregnation leads to localized alkali depletion and uneven delignification, as unpenetrated regions resist breakdown during subsequent cooking.[30] The cooking stage, or digestion, builds on impregnation by elevating the temperature to 160–175°C under pressures of 7–10 bar, where the white liquor actively dissolves lignin via alkaline hydrolysis and nucleophilic attack by hydrosulfide ions (HS⁻), converting it into soluble thiolignin fragments.[31] This phase lasts 1–4 hours, controlled by the H-factor—a dimensionless parameter integrating time and temperature exponentially per the Arrhenius equation—to achieve target kappa numbers (indicating residual lignin) of 20–30 for unbleached pulp.[32] Sulfidity, defined as the ratio of Na₂S to total alkali (typically 20–30%), enhances selectivity by accelerating lignin dissolution over carbohydrate degradation, though excessive levels increase sulfide oxidation losses.[26] The process generates black liquor as a byproduct, laden with dissolved organics, which is separated post-cooking; optimal conditions minimize hemicellulose loss to preserve yield, with temperatures above 170°C risking accelerated cellulose hydrolysis.[33] Modern continuous digesters often integrate impregnation and cooking in multi-zone vessels, with countercurrent liquor flow to maintain chemical gradients and recover heat, improving efficiency over batch methods developed in the early 20th century.[34] Empirical studies confirm that impregnation-to-cooking transitions around 130–140°C ensure 75–90% liquor penetration before rapid delignification commences, correlating with reduced rejects and energy use.[34] Variations in wood species, such as hardwoods versus softwoods, necessitate adjustments; for instance, denser spruce chips demand higher alkali charges during impregnation to offset pH drops from wood acids.Pulp Separation, Washing, and Screening
Following digestion in the Kraft process, the cooked wood chips, now comprising cellulose fibers suspended in spent cooking liquor known as weak black liquor, are discharged under pressure into a blow tank. This step relieves pressure from the digester and facilitates initial gravity-based separation of the fibrous pulp, termed brown stock, from the surrounding black liquor, which contains dissolved lignin, hemicelluloses, and inorganic cooking chemicals.[26] The blow tank typically operates at atmospheric pressure, with the pulp settling to form a mat while the liquor drains or is pumped away for chemical recovery, achieving an initial pulp consistency of approximately 1-3% solids before further processing.[35] Brown stock washing follows separation to remove residual black liquor entrained in the pulp fibers, minimizing chemical loss and reducing the organic load carried forward to subsequent bleaching stages. Multi-stage washing systems, often comprising 3-5 units such as vacuum drum washers, diffuser washers, or disc filters, employ countercurrent flow of fresh or weakly contaminated water to displace dissolved organics through dilution, dewatering, and pressing mechanisms.[36] [37] Effective washing targets a black liquor solids carryover of less than 0.5-1% on oven-dry pulp basis, recovering over 95% of dissolved solids for reuse while maintaining pulp consistency at 10-15% solids exiting the washers.[38] Vacuum drum washers, for instance, form a pulp mat on a rotating drum perforated screen, applying vacuum to dewater and then flooding with wash water to extract impurities via permeation through the mat.[39] Screening of the washed brown stock pulp occurs to eliminate oversized impurities such as uncooked wood knots, shives, and debris that could impair paper quality or equipment downstream. Industrial screens feature slotted or perforated plates with apertures typically 0.15-0.25 mm wide, through which acceptable fibers pass as "accepts" while rejects are concentrated and removed for disposal or reprocessing.[40] This step, often integrated after washing in the brown stock area, operates at low consistencies (0.5-1.5%) with high dilution to ensure minimal fiber loss—usually under 0.5% of total pulp—and is followed by centrifugal cleaning to remove finer contaminants like sand or pitch.[41] Screening efficiency is enhanced by multiple passes, reducing reject rates to below 1% of input mass in modern systems.Chemical Recovery and Energy Efficiency
Black Liquor Processing
Black liquor, comprising dissolved lignin, hemicelluloses, and inorganic pulping chemicals such as sodium hydroxide and sodium sulfide, emerges from the pulp washing stage with a solids content of approximately 12-18% and represents about 45-50% of the original wood mass input in the Kraft process.[42] [43] Initial handling involves weak black liquor storage and heat recovery from washing effluents before feeding into multi-effect evaporators, which concentrate it to 65-80% solids by sequential boiling and vapor condensation, recovering up to 90% of the evaporation heat to minimize energy input.[44] [4] Final concentration stages often employ direct-contact evaporators to handle viscous, fouling-prone liquor, preventing scaling from silica and other precipitates.[45] The concentrated strong black liquor, with a higher heating value of 13-15 GJ per dry tonne primarily from organic solids, is sprayed into the furnace of a specialized recovery boiler for controlled combustion at temperatures exceeding 1000°C.[46] [47] Here, devolatilization releases combustible gases, followed by char burnout, yielding superheated steam via waterwall tubes—typically generating 3-4 tonnes of steam per tonne of black liquor solids processed—and a molten smelt of inorganic salts (mainly Na₂CO₃ and Na₂S) collected at the furnace bottom.[4] [48] A modern 1000-tonnes-per-day pulp capacity recovery boiler can thus process equivalent black liquor volumes, achieving combustion efficiencies of 65-75% and contributing over 50% of a mill's total energy needs through steam and power cogeneration.[47] [49] This processing step addresses both energy recovery and chemical conservation, with global annual production of weak black liquor estimated at 1.3 billion tonnes, underscoring its scale in the industry.[50] Challenges include emissions control for reduced sulfur compounds and particulate matter, managed via electrostatic precipitators and selective catalytic reduction, alongside strategies like liquor oxidation to enhance combustion stability and reduce volatile emissions.[4] [51]Chemical Regeneration and Reuse
The molten inorganic smelt, primarily consisting of sodium carbonate (Na₂CO₃) and sodium sulfide (Na₂S), produced in the recovery boiler is directed to a dissolving tank where it is quenched and dissolved in weak washer filtrate or water to form green liquor.[52] This green liquor, containing dissolved Na₂CO₃ and Na₂S along with minor impurities, undergoes clarification in settling tanks or pressure filters to remove insoluble dregs and suspended solids, ensuring high-quality input for subsequent steps.[52][53] Green liquor is then causticized by the addition of quicklime (CaO) in a slaker, initiating the reaction Na₂CO₃ + CaO + H₂O → 2NaOH + CaCO₃, which is exothermic and typically conducted at 102–104°C with a retention time of 15–25 minutes.[52] The slurry proceeds to a series of agitated causticizer tanks (usually 3–4) for further reaction completion, operating at similar temperatures with total retention times of 90–180 minutes, converting the majority of Na₂CO₃ to NaOH while precipitating calcium carbonate (lime mud).[52] Causticizing efficiency reaches 80–83%, limited by chemical equilibrium, yielding white liquor with total titratable alkali (TTA) around 120 g/L as Na₂O, active alkali (AA, primarily NaOH) comprising 85% of TTA, and sulfidity of approximately 25% on an AA basis.[4][52] The resulting white liquor, a mixture of NaOH and Na₂S, is separated from the lime mud via disc or drum filters or clarifiers, achieving clarity levels of 10–20 mg/L solids.[52] The lime mud is washed with water at 70–73°C to remove residual liquor, then calcined in a rotary lime kiln at temperatures exceeding 900°C, decomposing CaCO₃ to regenerate CaO and release CO₂.[52][53] This regenerated quicklime is reused in causticizing, closing the inorganic chemical cycle. Overall, the Kraft recovery process recycles approximately 97% of pulping chemicals, with makeup additions of Na₂SO₄ (for sulfur balance) and NaOH to compensate for losses from purging, leaks, and inefficiencies.[4][54] Dregs and lime mud purges manage non-process elements like silica and chloride, preventing accumulation that could impair operations.[4]Bleaching and Finishing
Delignification Methods
In the Kraft pulping process, primary delignification occurs during the cooking stage, but residual lignin in the brown stock pulp—typically measured by kappa number—necessitates further delignification during bleaching to achieve commercial brightness levels above 80% ISO.[55] This secondary delignification targets the remaining 40-60% of lignin, reducing chemical demands in subsequent brightening stages and minimizing environmental discharges like adsorbable organic halides (AOX).[56] Oxygen delignification, introduced in the 1970s and now standard pre-bleaching, uses pressurized alkaline oxygen at medium (10-12%) or high (up to 25%) consistency to selectively dissolve lignin, often reducing kappa by 50% while preserving carbohydrate yield through additives like magnesium sulfate.[57][58] Historical methods relied on chlorine gas (C-stage) for initial delignification, followed by alkaline extraction (E-stage) to remove solubilized chlorolignins, as in the CEH sequence, but these generated high dioxin levels and were phased out by the 1990s due to regulatory pressures.[59] Elemental chlorine-free (ECF) processes, dominant since the late 1990s and used for approximately 95% of bleached Kraft pulp globally, employ chlorine dioxide (D-stage) as the primary delignifying agent in multi-stage sequences like OD(EOP)D, where O denotes oxygen delignification, EOP is enhanced extraction with oxygen and peroxide, and D stages provide selective lignin oxidation with minimal AOX formation.[55] ECF achieves brightness gains of 10-15 points per D-stage at pH 3-4 and temperatures of 50-70°C, with ClO2 dosages of 10-20 kg/tonne pulp, outperforming chlorine in selectivity and effluent treatability.[60] Totally chlorine-free (TCF) methods, applied mainly to hardwood Kraft pulps since the 1990s, avoid halogens entirely, relying on oxygen, ozone (Z-stage), hydrogen peroxide (P-stage), or peracids for delignification in sequences such as O-Z-P or O-Q-PP (Q for chelation to remove metals).[61] Ozone delignification, operating at 20-40°C with 1-3 kg/tonne dosages, cleaves lignin chromophores efficiently but risks yield losses of 2-5% without protective catalysts, limiting TCF to niche markets despite zero AOX.[55] Peracetic acid pretreatments before oxygen stages have shown potential to extend delignification in softwood pulps, reducing subsequent ClO2 needs by 20-30% in hybrid approaches.[62]| Method | Key Agents | Kappa Reduction | Typical Sequence | Advantages | Limitations |
|---|---|---|---|---|---|
| Oxygen Delignification | O2, NaOH, Mg2+ | 40-60% | Pre-bleach OD | Yield protection, cost-effective | Requires high pressure (7-10 bar) |
| ECF (ClO2-based) | ClO2, O2, H2O2 | 70-90% total | OD(EOP)D1D2 | High brightness, low AOX | Residual chlorinated organics |
| TCF (Ozone/Peroxide) | O3, H2O2, O2 | 60-80% total | O-Z-P or O-PP | No chlorine derivatives | Higher energy, potential yield loss |
Modern Bleaching Technologies
Modern bleaching of kraft pulp has transitioned from elemental chlorine-based sequences, such as CEDED, to elemental chlorine-free (ECF) and totally chlorine-free (TCF) methods to mitigate the formation of persistent chlorinated organic compounds like dioxins and adsorbable organic halides (AOX).[66] [67] This shift, accelerated since the late 1980s, prioritizes chlorine dioxide (ClO₂) in ECF or oxygen-based agents in TCF, often preceded by oxygen delignification to lower the incoming kappa number and reduce overall chemical demand.[55] [66] ECF bleaching, the dominant technology accounting for approximately 75-80% of chemically bleached pulp production globally as of the early 2000s, employs multi-stage sequences like O-D-E-D-P, where O denotes oxygen delignification, D is ClO₂ bleaching, E is alkaline extraction with sodium hydroxide (NaOH), and P is peroxide reinforcement.[67] These sequences achieve pulp brightness exceeding 90% ISO while preserving fiber strength and yield better than traditional methods, with ClO₂ substituting for elemental chlorine to eliminate highly toxic congeners such as 2,3,7,8-TCDD and 2,3,7,8-TCDF.[67] [66] However, ECF still generates trace chlorinated byproducts, necessitating effluent treatment, though AOX levels are substantially reduced compared to chlorine bleaching.[67] TCF bleaching, representing about 5% of production, avoids all chlorine compounds using agents like oxygen (O), hydrogen peroxide (H₂O₂), and ozone (O₃) in sequences such as O-Q-PO or O-Z-EP, where Q indicates chelation to remove metals that decompose peroxides.[67] [55] This approach yields zero chlorinated effluents and minimal AOX, enhancing environmental sustainability, but it often results in lower pulp yield, reduced fiber strength, and higher energy demands to attain comparable brightness levels.[67] TCF adoption is more prevalent in regions like Scandinavia (up to 58% of TCF share in 2002) and select new mills in Asia and South America, driven by stringent regulations, though ECF remains preferred for its cost-effectiveness and pulp quality in high-volume applications.[67] Auxiliary technologies enhance both ECF and TCF efficiency, including enzymatic treatments with xylanases to improve bleachability by enhancing lignin accessibility, and optimized washing to minimize water use—critical amid freshwater constraints near mills.[55] Emerging sequences incorporate peroxide-oxidized manganese (POM) catalysts for targeted delignification, potentially revolutionizing chemical efficiency, though commercial scaling remains limited.[68] Overall, these modern methods balance brightness targets (typically 88-92% ISO for market pulps) with reduced environmental loads, with ECF's widespread use reflecting its superior economic viability despite TCF's purer effluent profile.[67] [55]Comparisons with Alternative Pulping Processes
Versus Sulfite and Soda Processes
The Kraft process surpasses the soda and sulfite processes in pulp strength and versatility for wood fibers, producing pulp with tensile and tear strengths approximately 100 relative units compared to 70 for sulfite and 40 for soda pulps from the same wood species.[69] This superiority stems from the inclusion of sodium sulfide in the cooking liquor, which enhances lignin dissolution and preserves cellulose integrity more effectively than soda's alkaline-only approach or sulfite's acidic bisulfite conditions.[53] As a result, Kraft pulp yields 45-55% from wood for both softwoods and hardwoods, enabling its dominance in applications like linerboard and sack paper where durability is critical.[70] In contrast, sulfite pulping excels in initial brightness and bleachability, yielding pulps with lower lignin content (kappa numbers often below 20) suitable for fine papers, but at the cost of reduced yield (typically 40-50%) due to extensive hemicellulose hydrolysis under acidic conditions.[71][72] Soda pulping, historically applied to non-woody materials like bagasse, achieves even lower yields (around 40-45% for woods) and weaker fiber bonding because of incomplete delignification without sulfide catalysis, restricting it to niche uses.[73][74] Chemical recovery efficiency further favors Kraft, where the 1933-introduced recovery boiler recycles over 95% of cooking chemicals while generating steam for energy self-sufficiency, a capability absent in soda (limited to caustic recovery with lower efficiency) and sulfite (plagued by diverse spent liquor compositions hindering combustion).[75][70] Environmentally, soda avoids sulfur-related odors and emissions inherent to Kraft's total reduced sulfur compounds, and sulfite minimizes some effluents but generates calcium sludge; however, Kraft's closed-loop recovery mitigates impacts more scalably for large-scale wood pulping.[73][74] These factors explain Kraft's rise to over 80% of global chemical pulp production by the late 20th century, displacing sulfite (now under 5%) and soda (marginal for woods).[70]| Aspect | Kraft (Sulfate) | Sulfite | Soda |
|---|---|---|---|
| Primary Chemicals | NaOH + Na₂S (alkaline) | Bisulfite salts (acidic/neutral) | NaOH (alkaline) |
| Pulp Yield (Wood) | 45-55% | 40-50% | 40-45% |
| Strength Profile | High tensile/tear | Moderate, hemicellulose loss | Low, poor delignification |
| Brightness/Bleachability | Lower initial, harder to bleach | High, easier bleaching | Variable, often dark |
| Recovery Efficiency | >95% via recovery boiler | Limited, liquor variability | Moderate, caustic-only |
| Environmental Notes | TRS odors, but recoverable energy | Sludge, lower odors | Sulfur-free, but lower overall efficiency |