Raw Material Preparation
Wood serves as the main raw material for pulp production, although alternative materials can also be utilized The pulp manufacturing process begins with the preparation of raw materials, which involves several steps such as debarking, chipping, chip screening, and handling Additionally, processes like chip storage and depithing are essential, particularly when using materials like bagasse.
Wood enters pulp and paper mills as logs or chips, processed in the woodyard, which operates independently of the pulping method When logs arrive, they undergo several steps, including cutting to length at the slasher, debarking, chipping, and screening, before being conveyed to storage The resulting wood chips, whether from logs or purchased, are stored on-site in large piles, where they are screened for size, cleaned, and prepared for further processing.
Mechanical pulping processes, such as stone groundwood pulping, primarily utilize roundwood, but most operations depend on wood chips For optimal efficiency and pulp quality, a uniform chip size—typically 20 mm in length and 4 mm in thickness—is essential The chips are screened to eliminate those that are too large or small; oversized chips are re-cut while smaller ones are often burned with bark or sold for other uses Non-wood fibers are processed according to their specific composition to reduce fiber degradation and enhance pulp yield, and these raw materials are generally managed in bales.
The debarking process produces two primary products: chips, which serve as the main product, and bark, considered a by-product Bark is often utilized as fuel for energy production in burners or can be sold off-site for various applications Additionally, several debarking methods are currently in use (Ressel 2006).
2 Basic Overview of Pulp and Paper Manufacturing Process
After debarking, logs are chipped using a radial chipper to reduce their size and produce uniform chips, which is crucial for minimizing raw material consumption and enhancing energy efficiency The subsequent screening process separates improperly chipped long-size chips and removes sawdust, a valuable by-product that can be utilized as fuel Optimizing this screening is essential for improving pulp quality and reducing environmental impact, although it may require increased raw material consumption Finally, the processed chips are transported to the pulping stage using various types of conveyors.
Storage facilities play a crucial role in managing both raw materials, such as wood, and the chips produced from them Proper storage conditions are essential, especially when these materials are intended for transportation Ensuring optimal storage not only preserves the quality of the materials but also facilitates efficient logistics.
Pulping
Chemical Pulping
Chemical pulping, including processes such as kraft, soda, and sulphite, entails the treatment of raw materials like wood chips with aqueous chemical solutions under high temperature and pressure to extract pulp fibers This method efficiently produces chemical pulp through the cooking of these raw materials using the kraft and sulphite processes.
Kraft pulping is the predominant method for producing virgin fiber in the United States, representing over 80% of the nation's total pulp production.
The Kraft process is the dominant method in the pulp industry, accounting for 91% of chemical pulping and 75% of total pulp production, primarily due to its advantages in chemical recovery and pulp strength This process utilizes an alkaline cooking liquor, consisting of sodium hydroxide and sodium sulfide, to digest wood chips in a digester After cooking, the mixture is discharged into a blow tank, where the softened chips disintegrate into pulp The pulp is then separated from the spent cooking liquor in a series of brown stock washers Various pulp grades are produced, with unbleached grades, which retain more lignin and are used for packaging, characterized by a dark brown color In contrast, bleached pulp grades, which are used for white papers, represent nearly half of Kraft production but yield less pulp The effectiveness of the Kraft process has further improved with the introduction of modified cooking technologies.
Pulp grades Raw material End product use
Sulfi te pulp Softwoods and hardwoods
Kraft sulfate pulp Softwoods and hardwoods
Bleached-printing and writing papers, paperboard, unbleached-heavy packaging papers, paperboard
Dissolving pulp Softwoods and hardwoods
Viscose rayon, cellophane, acetate fi bers, and fi lm
Cold-caustic process Softwoods and hardwoods
Newsprint and groundwood printing papers
Neutral sulfi te process Hardwoods Newsprint and groundwood printing papers
Stone groundwood Mainly softwoods Corrugating medium
Refi ner mechanical (RMP) Mainly softwoods Newsprint and groundwood printing papers Thermomechanical (TMP) Mainly softwoods Newsprint and groundwood printing papers Chemi-mechanical (CTMP) Mainly softwoods Newsprint, Fine Papers
Since the early 1980s, the pulp and paper manufacturing process has evolved significantly, with three generations of modified kraft pulping techniques emerging Notable examples include MCC, ITC, and Compact Cooking for continuous cooking, as well as Cold-blow, Superbatch/RDH, and Continuous Batch Cooking (CBC) for batch cooking technology This advancement is a result of ongoing research and development efforts in the industry.
The sulphite pulping process utilizes an acidic cooking liquor composed of sulfurous acid and bisulphite ions, created by absorbing cooled sulfur dioxide gas in water with bases like magnesium, ammonia, sodium, or calcium This method effectively degrades lignin bonds in wood fibers, resulting in sulphite pulps that are less colored and easier to bleach than kraft pulps, although they possess lower strength The efficiency of the sulphite process is influenced by the type of wood and the absence of bark, leading to a decline in its use compared to kraft pulping Sulphite pulps are produced in various grades, with bleached grades being predominant, and typically yield between 40-50%, though bleached grades tend toward the lower end This process is particularly sensitive to the characteristics of wood species, making it less suitable for resinous softwoods, tannin-rich hardwoods, or materials containing bark Ultimately, the sulphite process produces bright pulp that can be easily bleached and refined for papermaking applications.
The sulphite process offers greater flexibility than the kraft process, which relies on a uniform method using highly alkaline cooking liquor In Europe, the predominant sulphite pulping method is magnesium sulphite, although some mills utilize sodium as a base Both magnesium and sodium bases facilitate chemical recovery, and the lignosulphonates produced in the cooking liquor can serve as a valuable raw material for various chemical products.
Mechanical Pulping
Mechanical pulp is categorized into three types: groundwood pulp, refining pulp, and chemi-mechanical pulp Both grinding and refining processes involve raising the temperature to soften lignin, which helps break the bonds between fibers Groundwood pulp is particularly advantageous due to its high brightness (≥85% ISO post-bleaching), excellent light scattering, and bulk properties, enabling the production of lightweight papers.
The groundwood process allows for the use of hardwoods, such as aspen, to enhance brightness and smoothness, maintaining groundwood pulp's status as the quality leader in magazine papers (Arppe 2001) Thermomechanical pulping (TMP), the most significant refining mechanical pulping process, utilizes high-temperature steaming to soften lignin and expose cellulosic surfaces for better bonding TMP pulps are typically stronger than groundwood pulps, allowing for reduced chemical pulp usage in newsprint and magazine papers While softwoods are the primary raw material for TMP due to their superior pulp strength, hardwoods tend to yield weaker pulps However, hardwood TMP pulps, known for their high cleanliness and scattering coefficient, are primarily used as filler-grade pulp The addition of chemicals like hydrogen sulfite enhances fiber liberation during refining, resulting in chemithermomechanical pulp (CTMP) that demonstrates good strength properties even with hardwood fibers, provided the sulfonation conditions are optimal Although mechanical pulps are weaker and less expensive to produce—costing about 50% of chemical pulp—they achieve yields between 85-95%, making them a significant component of the pulp market.
Mechanical paper is expected to solidify its role as a key fiber source for high-end graphic papers, accounting for 20% of all virgin fiber material To meet the increasing demand for high-quality pulp in the future, a balanced use of both softwood and hardwood as raw materials will be essential.
The future of mechanical pulp faces significant challenges primarily due to its high specific energy consumption, particularly in Thermomechanical Pulping (TMP) processes, which require substantially more energy than groundwood methods Additionally, the rising utilization of recovered fiber is likely to impede the growth of mechanical pulp production volumes.
Semi-chemical Pulping
Semi-chemical pulping is an efficient process that combines chemical and mechanical methods to extract pulp fibers from wood chips Initially, the chips are softened in a digester using chemicals, steam, and heat After this, mechanical techniques finalize the pulping process, followed by washing the pulp to eliminate cooking liquor chemicals and organic compounds To improve machinability, the resulting virgin pulp is blended with 20–35% recovered fiber, such as double-lined kraft clippings, or repulped secondary fiber, like old corrugated containers The chemical components and washing steps in semi-chemical pulping closely resemble those found in the kraft pulping process.
The pulp and paper manufacturing process primarily involves the semi-chemical pulping method, which utilizes either a nonsulfur or neutral sulphite semi-chemical (NSSC) process In the nonsulfur method, wood chips are cooked using sodium carbonate or a combination of sodium carbonate and sodium hydroxide, while the NSSC process employs a sodium-based sulphite cooking liquor Semichemical pulps, derived mainly from hardwoods, achieve yields between 65% and 85%, with an average of around 75% The NSSC process is crucial, as it uses a buffered sodium sulphite solution for partial chemical pulping, followed by mechanical defibration in disc refiners, which weakens the fibers through the sulfonation of lignin NSSC pulp is ideal for unbleached products requiring high strength and stiffness, such as corrugating medium and grease-proof papers Additionally, NSSC pulping is often integrated with kraft mills for efficient chemical recovery, where the sulphite spent liquor contributes essential components for the kraft process Despite advancements in kraft mill recovery efficiencies reducing the need for NSSC make-up, semichemical pulps remain significant, accounting for 3.9% of all virgin fiber material.
Secondary Fibre Pulping
Recovered paper is a vital source of fiber for paper-making, accounting for nearly 50% of the raw material used The recycling process involves rewetting and pulping recycled paper or paperboard, primarily through mechanical methods, while inks and contaminants are removed via chemical deinking and mechanical separation Recycled fibers possess different physical properties compared to virgin wood pulp fibers due to the drying and rewetting process Some mills can operate without effluent discharge by utilizing closed water cycles and biological treatment systems, particularly for products that can tolerate some contamination, such as packaging and construction papers However, about 30-40% of the raw material processed in recycling plants results in sludge that requires solid waste management Non-deinked recycled paper is suitable for applications like corrugated board and carton board, while deinking processes enhance brightness and cleanliness for higher-quality applications.
Deinking processes can achieve brightness levels comparable to newsprint, magazine paper, and tissue, but they often result in lower pulp yields, typically around 60-70% of the recovered paper Consequently, 30-40% of the input material may end up in the white water, necessitating additional internal treatment before the wastewater can be safely discharged.
Dissolving Kraft and Sulphite Pulping Processes
Dissolving kraft and sulphite pulping processes are employed to create highly bleached and purified wood pulp, which is ideal for manufacturing products like rayon, viscose, acetate, and cellophane (EPA 2002).
Non-wood Pulping
Non-wood sources account for approximately 6% of the global fiber supply for papermaking, primarily derived from agricultural fibers like straw, bamboo, bagasse, and annual crops such as kenaf While non-wood fibers are more expensive to collect and process in regions with adequate wood supplies, they are extensively used in areas of Asia and Africa where wood is scarce These fibers, which are typically shorter and similar to hardwood fibers, are suitable for various paper grades, including writing paper, newsprint, and corrugated board Non-wood fibers generally require different cooking processes, often utilizing soda cooking instead of Kraft cooking, which leads to lower concentrations of dissolved organics and chemicals in the spent liquors However, non-wood pulping plants are usually smaller, producing less than 100,000 tons of pulp annually, resulting in limited chemical recovery and higher waste emissions compared to larger Kraft mills Additionally, the higher silica content in non-wood fibers poses challenges in chemical recovery and negatively impacts paper quality, leading to increased scaling in evaporators and reduced efficiency in lime conversion processes.
The pulp and paper manufacturing process involves significant environmental considerations, particularly regarding non-wood pulping facilities, which tend to release larger amounts of lime mud and require greater quantities of lime or limestone as a supplement In the United States, the production of non-wood fiber pulp is relatively uncommon, as noted by the EPA in 2001.
Pulp Washing
Pulp washing aims to produce pulp free from unwanted solubles, typically achieved by replacing contaminated liquor with clean water In contemporary pulp mills, this process may involve the displacement of one liquor type with another Additionally, washing equipment must facilitate the effective separation of chemical regimes or temperature levels between different fiber-line process steps The benefits of pulp washing are significant, contributing to overall process efficiency and product quality.
– Minimizing the chemical loss from the cooking liquor cycle
– Maximizing recovery of organic substances for further processing or incineration – Reducing the environmental impact of fi breline operations
– Limiting the carry-over between process stages
– Maximizing the re-use of chemicals and the energy conservation within a single bleaching stage
– Obtaining a clean fi nal pulp product
Pulp washing should ideally utilize minimal wash water to conserve fresh water resources and reduce the burden on downstream processing areas handling the wash filtrate This process often involves a compromise between achieving clean pulp and the volume of wash water used Pulp washing operations are typically conducted during brownstock washing, in the bleach plant, and may also occur in digesting and on dewatering machines (Smook 1992; Krotscheck 2006).
After pulp production, pulp is processed to remove impurities, such as uncooked chips, and recycles any residual cooking liquor via the pulp washing process (Smook
Pulp processing varies based on the method used, such as chemical or sulphite, and involves several steps to remove impurities, including screening, defibering, and deknotting To enhance product uniformity, pulp may be thickened by reducing water content and blended at an additional cost For long-term storage, drying is essential to prevent fungal or bacterial growth The residual cooking liquor from chemical pulping is washed away using specific pulp washers, with efficient washing being crucial to maximize the recovery of cooking liquor and minimize its carryover into the bleach plant, as excess liquor can lead to increased consumption of bleaching chemicals.
Hemicelluloses present in the liquor can bind to bleaching chemicals, leading to higher consumption of these chemicals Furthermore, these organic compounds serve as precursors to chlorinated organic compounds, such as dioxins and furans, which raises the likelihood of their formation.
Rotary vacuum washing is the most prevalent washing technology, typically performed in two to four sequential units Other methods include diffusion washers, rotary pressure washers, horizontal belt filters, wash presses, and dilution/extraction washers Pulp screening effectively eliminates oversized particles like bark fragments and uncooked chips In open screen rooms, the wastewater generated is treated before discharge, while in closed loop systems, it is reused in pulping operations and eventually enters the mill's chemical recovery system After screening, centrifugal cleaning, also known as liquid cyclone or hydro-cyclone, removes dense contaminants such as sand and dirt Rejects from the screening process are either repulped or disposed of as solid waste.
The primary goal of brown stock washing is to efficiently eliminate dissolved solids from the pulp while minimizing water usage Residual dissolved solids can hinder subsequent bleaching and papermaking processes, leading to increased operational costs Additionally, these solids reduce heat recovery in the recovery furnace and necessitate the addition of makeup chemicals to the liquor system to compensate for the lost substances (Gullichsen 2000).
Achieving high washing efficiencies in pulp mills often requires a balance between the amount of wash water used and the desired efficiency Excessive wash water can lead to increased costs due to the need for evaporation before the liquor is burned in the recovery furnace, making it a significant bottleneck in operations By reducing wash water usage, mills can lower steam costs associated with evaporation In the dilution/extraction washing process, pulp slurry is mixed with weak wash liquor or fresh water, followed by thickening the pulp through filtering or pressing to extract the liquor This process must be repeated multiple times to ensure effective washing of the pulp.
Displacement washing involves replacing the liquor in the pulp with weaker wash liquor or clean water, aiming to prevent mixing at the interface However, some mixing is unavoidable, leading to residual original liquor in the pulp and wash liquor flowing through it The efficiency of this washing process relies on the extent of this mixing, as well as the rates of desorption and diffusion of dissolved solids and chemicals from the pulp fibers.
Pulp washing equipment operates primarily on two fundamental principles: displacement washing and dilution/extraction In the digester washing zone, displacement washing is employed, while rotary vacuum washers combine both dilution/extraction and displacement washing methods Typically, pulp washing systems incorporate multiple washing stages to enhance efficiency The optimal washing performance is attained through the strategic use of these techniques.
The pulp and paper manufacturing process traditionally required significant amounts of fresh water at each stage, but this method is now largely avoided due to high water consumption Instead, the countercurrent washing system is commonly employed, where the pulp is washed with the cleanest available water in the final stage before exiting the system The drained water from this stage is then recycled back through the earlier stages, flowing in the opposite direction to the pulp.
Pulp Screening, Cleaning and Fractionation
Screening pulp is essential for eliminating oversized and undesirable particles, ensuring that only high-quality fibers are used in papermaking This process enhances the suitability of the screened pulp for specific paper or board products, ultimately improving the final product's quality.
In the pulp production process, the largest oversized particles, known as knots, are defined as uncooked wood fragments These knots are typically removed prior to washing and fine screening In low-yield pulps, they can either be broken down in refiners or fiberizers, or eliminated using specialized coarse screens referred to as knotters.
Fine screening primarily aims to eliminate shives, which are small fiber bundles that remain unseparated during chemical pulping or mechanical processing Additionally, chop, another type of oversized wood particle, poses a challenge, especially in hardwood pulping, as it primarily stems from irregularly shaped hardwood vessels and cells Unlike shives, chop particles are shorter and more rigid Collectively referred to as debris, shives, chop, and other unwanted materials can adversely affect the papermaking process and the quality of the final paper product.
Bleaching
Bleaching of pulp is primarily aimed at enhancing its brightness for use in high-quality paper products, such as printing grades and tissue papers This chemical process transforms pulp to achieve a whiter, brighter, softer, and more absorbent quality compared to unbleached pulps Bleached pulps are essential for applications requiring high purity and minimal yellowing, like printing and writing papers, while unbleached pulp is typically utilized for boxboard, linerboard, and grocery bags The effectiveness of bleaching varies based on the type of fiber and pulping process used, as well as the intended qualities of the final product The lignin content in pulp significantly influences its bleaching potential; pulps with high lignin content, such as mechanical or semi-chemical pulps, are challenging to bleach fully and often necessitate substantial chemical inputs Over-bleaching these types can lead to a reduction in pulp yield due to fiber degradation.
24 destruction Chemical pulps can be bleached to a greater extent due to their low
Bleach plants typically discharge effluent to external treatment rather than recirculating it into the chemical recovery system due to the high lignin content (10%) This discharge is necessary because recirculating the effluent would lead to an accumulation of chlorides and other undesirable inorganic elements, resulting in potential issues such as corrosion and scaling within the recovery system.
Chemical pulping offers significant benefits, such as reducing fiber bundles, shives, and bark fragments, which enhances pulp cleanliness Bleaching plays a crucial role in preventing paper yellowing by removing residual lignin from unbleached pulp, while also eliminating resin and other extractives that improve absorbency—an essential quality for tissue paper For reconstituted cellulose products like rayon and cellulose derivatives, bleaching effectively purifies the pulp by removing hemicelluloses and wood extractives Achieving high brightness is often necessary for product quality, making it a secondary characteristic rather than the primary benefit Therefore, advocating for lower brightness bleaching based solely on product requirements oversimplifies the process.
Bleaching significantly alters the papermaking properties of chemical pulps by removing residual lignin, which enhances fiber flexibility and strength However, decreased hemicellulose content can lower the swelling potential and bonding ability of the fibers Excessive bleaching conditions may damage fibers, resulting in reduced paper strength The primary goal of bleaching is to eliminate lignin and achieve the desired brightness in pulp This process involves multiple stages of delignification and extraction, with the potential addition of oxygen or hydrogen peroxide to enhance effectiveness Since its inception, chemical Kraft bleaching has evolved from a simple hypochlorite treatment to a complex multi-stage process utilizing various chemicals, including chlorine, chlorine dioxide, hydrogen peroxide, and ozone Modern bleaching techniques have progressed beyond the traditional CEHDED sequence, incorporating diverse combinations of chlorine-containing and non-chlorine chemicals.
The introduction of chlorine and chlorine dioxide in the 1930s and early 1940s significantly enhanced the efficiency of the bleaching process Chlorine, being more reactive and selective than hypochlorite, minimized damage to cellulose and other carbohydrate components in wood, resulting in higher pulp strength While it did not brighten the pulp as effectively as hypochlorite, it effectively degraded lignin, enabling its removal along with the spent liquor.
The pulp and paper manufacturing process involves an alkaline extraction that produces brownish Kraft pulp, which requires additional bleaching to enhance brightness, leading to the development of a multi-stage process The introduction of chlorine dioxide, a more effective brightening agent than hypochlorite, significantly improved the efficiency of the Kraft process Between the 1970s and 1990s, both incremental and radical innovations further enhanced process efficiency while minimizing environmental impacts Key advancements included oxygen delignification, modified cooking methods, improved operational controls, and optimized chemical mixing, all contributing to better economics and a notable reduction in wastewater production.
In addition, higher chlorine dioxide substitution, brought down signifi cantly the generation and release of harmful chlorinated organic compounds
Due to concerns about chlorinated compounds like dioxins, furans, and chloroform, there has been a significant shift away from using these substances in the bleaching process In this process, bleaching chemicals are introduced to the pulp in stages within bleaching towers, and spent chemicals are subsequently removed in washers between each stage The effluent from the washers is collected in seal tanks, where it can either be reused as wash water in other stages or directed to wastewater treatment.
Bleaching mechanical pulp employs lignin-saving methods, differing fundamentally from chemical pulp bleaching, which focuses on lignin removal This process transforms chromophoric groups in lignin polymers into colorless forms, enhancing the pulp's brightness while minimizing losses in dry solids and overall yield However, the brightness achieved is not permanent, leading to yellowing over time, making bleached mechanical pulp more suitable for applications like newsprint and magazine paper rather than books or archival materials The bleaching is typically performed in one to two stages, guided by the desired final brightness, using either reductive or oxidative agents Reductive bleaching with sodium dithionite minimally affects yield but can cause corrosion in downstream metallic components, necessitating the use of metal chelating agents like EDTA or DTPA to mitigate this issue In contrast, oxidative bleaching with hydrogen peroxide results in a slight yield reduction and enhances the pulp's strength and water absorption capabilities To prevent discoloration and peroxide decomposition due to heavy metal ions, chelating agents are added prior to bleaching Incorporating a washing stage between pulping and bleaching can effectively reduce problematic metals, thereby decreasing the need for chelating agents and improving overall process efficiency.
26 the effectiveness of the applied peroxide The bleached pulp is acidifi ed with sulfu- ric acid or sulfur dioxide to a pH of 5–6.
Chemical Recovery
Black Liquor Concentration
Residual weak black liquor from the pulping process is a dilute solution containing approximately 12–15% solids, consisting of wood lignins, organic materials, oxidized inorganic compounds like sodium sulfate and sodium carbonate, as well as white liquor, which includes sodium sulfide and sodium hydroxide To enhance the solids content to around 50%, the weak black liquor is initially processed through a series of multiple-effect evaporators.
Strong black liquor from multiple-effect evaporators can either be oxidized in the black liquor oxidation system or routed to a nondirect contact evaporator, known as a concentrator Oxidizing the black liquor prior to evaporation in a direct contact evaporator helps minimize emissions of odorous total reduced sulfur compounds, which are released when the liquor interacts with hot flue gases from the recovery furnace Typically, the solids content of the black liquor after the final evaporator or concentrator averages between 65% and 68% In contrast, the soda chemical recovery process, akin to the kraft process, does not utilize black liquor oxidation systems, as it is a nonsulfur process that avoids total reduced sulfur emissions.
Recovery Furnace
Concentrated black liquor is injected into the recovery furnace, where its organic compounds undergo combustion, transforming sodium sulfate into sodium sulfide This process utilizes black liquor with a substantial energy content, ranging from 5,800 to 6,600 British thermal units per pound.
2 Basic Overview of Pulp and Paper Manufacturing Process
The recovery process generates steam from dry solids, which is essential for various operations, including cooking wood chips, heating and evaporating black liquor, preheating combustion air, and drying pulp or paper products This process steam is often enhanced with power boilers that burn fossil fuels and/or wood Particulate matter, mainly sodium sulfate, is captured from the hot flue gases using an electrostatic precipitator (ESP) and is subsequently added to the black liquor for firing in the recovery furnace, along with any necessary additional sodium sulfate.
In the process of firing black liquor, "saltcake" may be introduced, leading to the formation of molten inorganic salts known as "smelt" that accumulate in the furnace's char bed This smelt is subsequently extracted and dissolved in weak wash water within the smelt dissolving tank, resulting in a solution of carbonate salts called "green liquor." Green liquor primarily consists of sodium sulfide and sodium carbonate, along with insoluble unburned carbon and inorganic impurities known as dregs, which are eliminated through a series of clarification tanks.
Causticizing and Calcining
In the causticizing area, decanted green liquor undergoes a transformation where sodium carbonate is converted into sodium hydroxide through the addition of lime (calcium oxide) Initially, the green liquor is directed to a slaker tank, where calcium oxide from the lime kiln reacts with water, resulting in the formation of calcium hydroxide.
In the causticizing process, liquor flows from the slaker through a series of agitated tanks known as causticizers, where calcium hydroxide reacts with sodium carbonate to produce sodium hydroxide and calcium carbonate The resulting causticizing product is then directed to the white liquor clarifier, which effectively removes the calcium carbonate precipitate.
Lime mud is processed in a mud washer to eliminate remaining sodium traces The resulting mud is dried and calcined in a lime kiln to create reburned lime, which is then returned to the slaker The mud washer's filtrate, referred to as weak wash, is utilized in the SDT for dissolving recovery furnace smelt Additionally, the white liquor obtained from the clarifier is recycled back to the digesters in the mill's pulping area (Arpalahti et al 2000).
Stock Preparation and Papermaking
Stock preparation is a critical process that tailors pulp properties to meet specific product requirements Optimizing stock preparation involves balancing the efficiency and reliability of papermaking with the quality of the final product The goal of fiber stock preparation systems is to modify incoming raw materials to ensure that the finished stock meets both the operational needs of the paper machine and the quality standards for the produced paper or board.
There are 28 types of virgin pulps and recovered paper grades available in bales, loose material, or as suspensions in integrated mills, with the finished stock being a suspension of defined quality based on fiber mixture, additives, and impurities This quality is crucial for the runnability of paper machines and the final quality of paper and board Stock preparation involves several steps that vary based on the raw stock and desired quality, with operations including dispersion, refining, and metering and blending of fiber and additives New concepts in virgin pulp stock preparation have been introduced by Kadant Lamort Pulpers are essential for dispersing dry pulp into water, creating a slush or slurry, where accelerated and decelerated stock generates hydrodynamic shear forces that loosen fibers and reduce flakes to individual fibers.
Pulp produced in a mill without mechanical treatment is inadequate for most paper grades, as unbeaten virgin pulp results in low strength, bulkiness, and a rough surface To achieve high-quality paper, it is essential for the fibers to be matted into a uniform sheet and to form strong bonds at their contact points Beating and refining processes are crucial for enhancing these properties.
Table 2.3 Unit processes in stock preparation
Slushing and deflaking are essential processes for transforming raw fiber materials into a manageable suspension of individual fibers The primary goal of slushing is to create a pumpable mixture that allows for effective coarse separation, while deflaking can be performed as needed to refine the fiber consistency.
In the case of recovered paper, ink particles and other nonpaper particles should be detached from the fi bers
Screening To separate particles from the suspension which differ in size, shape and deformability from the fi bers
Fractionation To separate fi ber fractions from each other according to defi ned criteria such as size or deformability of the fi bers
Centrifugal cleaning To separate particles from the suspension which differ in specifi c gravity, size and shape from the fi bers
Refi ning To modify the morphology and surface characteristics of the fi bers
Selective fl otation To separate particles from the suspension which differ in surface properties (hydrophobicity) from the fi bers
Nonselective fl otation To separate fi ne and dissolved solids from water
Bleaching To impart yellowed or brown fi bers with the required brightness and luminance
Washing To separate fi ne solid particles from suspension (solid/solid separation)
Dewatering To separate water and solids
Dispersing To reduce the size of dirt specks and stickies (visibility, fl oatability), to detach ink particles from fi bers Based on Holik ( 2006 )
2 Basic Overview of Pulp and Paper Manufacturing Process are the processes by which the undesirable characteristics are changed (Baker
Mechanical treatment is crucial in preparing papermaking fibers, with "beating" referring to batch processing in a Hollander beater and "refining" involving continuous processing through refiners The refining process enhances fiber bonding capabilities while preserving individual fiber strength and minimizing drainage resistance, tailored to the specific requirements of different paper grades Additionally, the morphological properties of various fibers influence their response to refining, making it essential to consider fiber type during this process.
Pulp refining enhances most strength properties of paper due to increased fiber-fiber bonding, but it negatively impacts tear strength, which is heavily reliant on the strength of individual fibers Beyond a certain refining point, the limiting factor shifts from fiber bonding to the strength of the fibers themselves, leading to a decline in various strength properties While refining increases fiber flexibility and results in denser paper, it simultaneously decreases bulk, opacity, and porosity Mechanical and hydraulic forces are applied to modify fiber characteristics, with shear stresses arising from rolling, twisting, and tension between bars, and normal stresses resulting from bending and crushing actions on fiber clumps During the refining process, fibers are subjected to tensile, compressive, shear, and bending forces, responding in three distinct ways.
– Fibres develop new surfaces externally through fi brillation and internally through fi bre wall delamination
Fibres undergo deformation, leading to alterations in their geometric shape and the alignment of their fibrils along their length This process typically results in the flattening or collapsing of the fibres Additionally, changes in fibre curl and kinks may occur, either being induced or straightened On a smaller scale, phenomena such as dislocations, crimps, and microcompressions are either introduced or reduced.
– Fibres break, resulting in changes in length distribution and a decrease in mean- fi bre length A small amount of fi bre wall material also dissolves
The changes in pulp fibers are simultaneous and largely irreversible, influenced by factors such as fiber morphology, temperature, chemical environment, and treatment conditions These conditions are determined by equipment design and operational variables, including consistency, intensity, and treatment amount Each type of pulp responds uniquely to these conditions, and not all fibers within the pulp receive uniform treatment Additionally, chemical additives like resins, dyes, pigments, fillers, and sizing agents can be incorporated to enhance properties such as wet strength, color, optical qualities, and sheet control.
30 penetration of liquids and to improve printing properties (Krogerus 2007 ; Bajpai
2004 ; Hodgson 1997 ; Davison 1992 ; Neimo 2000 ; Roberts 1996 , 1997 ) The com- monly used additives are shown in Table 2.4
Following stock preparation, the subsequent phase involves shaping the slurry into the intended paper type at the paper machine A typical papermaking process is illustrated in Fig 2.2, which outlines two main processes essential to papermaking.
In wet end operations, the cleaned and bleached pulp is formed into wet paper sheets
In the dry end operations, those wet sheets are dried and various surface treat- ments are applied to the paper
Table 2.4 Common pulp stock additives Acids and bases:
Control of pH Sizing agents:
Water repellent Dry strength additives: Strength and stiffness (starch) Wet strength additives: Linking of fi bers (polymers) Fillers:
Gloss, brightness, and opacity (kaolin, TiO 2 )
Reduce foam and entrained air (surfactants)
Fig 2.2 A fl ow diagram for a typical papermaking process
2 Basic Overview of Pulp and Paper Manufacturing Process
The traditional Fourdrinier machine remains prevalent in the paper industry, but many grades have transitioned to twin wire machines, gap formers, and hybrid formers for enhanced efficiency (Ishiguro 1987; Buck 2006; Atkins 2005; Lund 1999; Malashenko and Karlsson 2000) Twin wire formers have become the leading design, where the fiber suspension is processed between two wires moving at the same speed, allowing for drainage from one or both sides Various types of twin wire formers exist, including gap formers that inject diluted stock directly into the space between the wires, as well as hybrid formers that combine features of both Fourdrinier and twin wire systems While multiple paper types can be produced using different forming devices, there has been a recent focus on manufacturing two and three ply papers and liners using multi-Fourdrinier wet ends Regardless of the forming method, the wet paper web undergoes pressing to mechanically extract water, followed by evaporation on multiple drying cylinders to further reduce moisture content.
The Fourdrinier paper making machine consists of three main sections: the forming section, the press section, and the dryer section In the forming section, a paper slurry containing 0.5–1.0% fiber is pumped into a head box, where it flows onto a moving wire belt, allowing water to drain and create a wet, weak paper This paper is then pressed, heated, and dried to produce a continuous roll or "web" ready for further finishing The head box is crucial for delivering a uniform slurry to the forming wire, utilizing designs that induce turbulence while preventing cross currents to maintain stock uniformity The simplest design, the gravity-fed head box, effectively uses height and weight differences to manage pulp flow at rates up to 400 ft/min with an eight-inch stock depth.
Fig 2.3 Details of papermaking process
To achieve optimal production speeds of up to 4,000 ft/min, stock must be fed under pressure in specialized machines These systems utilize hydraulic head boxes to force stock through conical injectors, a perforated plate, and a horizontally split apron and slice The heights of both the apron and slice, which regulate the pulp jet, can be independently adjusted using hydraulic controls.
The pulp that flows onto the forming wire contains approximately 0.5–1.0% fibers, primarily suspended in water As water is removed from the slurry, the fibers settle on the surface of a traveling wire, creating a wet paper mat The key goal of the forming section is the controlled removal of water Initially, gravity facilitated drainage through a brass forming wire with 60–70 meshes per inch, but as production speeds increased, more efficient methods emerged Today, a fine polymer screen with about 65 meshes per inch is used, allowing for better control over drainage and agitation as the slurry passes over table rolls, foils, and suction boxes Water jets positioned along the edges of the forming wire help define the web's width, known as the deckle edge The first fibers that form on the wire are oriented in the machine's direction, creating the wire side of the paper.
To ensure optimal tear resistance and surface properties in paper production, it is crucial to prevent the fibers in the slurry from aligning in the same direction Relying solely on gravity for dehydration necessitates operating the machine at low speeds to address this orientation issue Alternatively, quickly removing water while the fibers remain agitated from the headbox effects can enhance the quality of the final product.
The initial stage of de-watering involves a bank of table rolls, which have evolved from a series of small solid rollers to larger units These modern table rolls serve as the primary method for water removal As the rolls rotate in contact with the covered wire, a vacuum is created between them, effectively extracting water from the web.
With increasing speeds the table rolls cause problems with paper uniformity and are not able to remove enough water before the presses Foils have replaced most, if
Couch roller Pickup roller Wet Press Section Dryer Section Calender Section
Bottom felt, Felt Dryer Felt Felt Dryer Heated dryer Top felt Felt dryer
Fig 2.4 Schematic of Fourdrinier paper machine