Pulping is the process by which the bonds within the wood structure are ruptured either mechanically or chemically. Chemical pulps can be produced by either alkaline (i.e., sulphate or kraft) or acidic (i.e., sulphite) processes. The highest proportion of pulp is produced by the sulphate method, followed by mechanical (including semi-chemical, thermomechanical and mechanical) and sulphite methods (figure 1). Pulping processes differ in the yield and quality of the product, and for chemical methods, in the chemicals used and the proportion that can be recovered for reuse.

Figure 1. Worldwide pulp capacities, by pulp type

Mechanical Pulping

Mechanical pulps are produced by grinding wood against a stone or between metal plates, thereby separating the wood into individual fibres. The shearing action breaks cellulose fibres, so that the resulting pulp is weaker than chemically separated pulps. The lignin connecting cellulose to hemicellulose is not dissolved; it merely softens, allowing the fibres to be ground out of the wood matrix. The yield (proportion of original wood in pulp) is usually greater than 85%. Some mechanical pulping methods also use chemicals (i.e., the chemi-mechanical pulps); their yields are lower since they remove more of the non-cellulosic materials.

In stone groundwood pulping (SGW), the oldest and historically most common mechanical method, fibres are removed from short logs by pressing them against a rotating abrasive cylinder. In refiner mechanical pulping (RMP, figure 2), which gained popularity after it became commercially viable in the 1960s, wood chips or sawdust are fed through the centre of a disc refiner, where they are shredded into finer pieces as they are pushed out through progressively narrower bars and grooves. (In figure 2, the refiners are enclosed in the middle of the picture and their large motors are on the left. Chips are supplied though the large diameter pipes, and pulp exits the smaller ones.) A modification of RMP is thermomechanical pulping (TMP), in which the chips are steamed before and during refining, usually under pressure.

Figure 2. Refiner mechanical pulping

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One of the earliest methods of producing chemi-mechanical pulps involved pre-steaming logs before boiling them in chemical pulping liquors, then grinding them in stone grinders to produce “chemi-groundwood” pulps. Modern chemi-mechanical pulping uses disc refiners with chemical treatment (e.g., sodium bisulphite, sodium hydroxide) either prior to, during or after refining. Pulps produced in this manner are referred to either as chemi-mechanical pulps (CMP) or chemi-thermomechanical pulps (CTMP), depending on whether refining was carried out at atmospheric or elevated pressure. Specialized variations of CTMP have been developed and patented by a number of organizations.

Chemical Pulping and Recovery

Chemical pulps are produced by chemically dissolving the lignin between the wood fibres, thereby enabling the fibres to separate relatively undamaged. Because most of the non-fibrous wood components are removed in these processes, yields are usually in the order of 40 to 55%.

In chemical pulping, chips and chemicals in aqueous solution are cooked together in a pressure vessel (digester, figure 3) which can be operated on a batch or continuous basis. In batch cooking, the digester is filled with chips through a top opening, the digestion chemicals are added, and the contents cooked at elevated temperature and pressure. Once the cook is complete, the pressure is released, “blowing” the delignified pulp out of the digester and into a holding tank. The sequence is then repeated. In continuous digesting, pre-steamed chips are fed into the digester at a continuous rate. Chips and chemicals are mixed together in the impregnation zone at the top of the digester and then proceed through the upper cooking zone, the lower cooking zone, and the washing zone before being blown into the blow tank.

Figure 3. Continuous kraft digestor, with chip conveyor under construction

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The digesting chemicals are recovered in most chemical pulping operations today. The principal objectives are to recover and reconstitute digestion chemicals from the spent cooking liquor, and to recover heat energy by burning the dissolved organic material from the wood. The resulting steam and electricity supplies some, if not all, of the mill’s energy needs.

Sulphate Pulping and Recovery

The sulphate process produces a stronger, darker pulp than other methods and requires chemical recovery to compete economically. The method evolved from soda pulping (which uses only sodium hydroxide for digestion) and began to gain prominence in the industry from the 1930s to 1950s with the development of chlorine dioxide bleaching and chemical recovery processes, which also produced steam and power for the mill. The development of corrosion-proof metals, such as stainless steel, to handle the acidic and alkaline pulp mill environments also played a role.

The cooking mixture (white liquor) is sodium hydroxide (NaOH, “caustic”) and sodium sulphide (Na₂S). Modern kraft pulping is usually carried out in continuous digesters often lined with stainless steel (figure 3). The temperature of the digester is raised slowly to approximately 170°C and held at that level for approximately 3 to 4 hours. The pulp (called brown stock because of its colour) is screened to remove uncooked wood, washed to remove the spent cooking mixture (now black liquor), and sent either to the bleach plant or to the pulp machine room. Uncooked wood is either returned to the digester or sent to the power boiler to be burned.

The black liquor collected from the digester and brown stock washers contains dissolved organic material whose exact chemical composition depends on the wood species pulped and the cooking conditions. The liquor is concentrated in evaporators until it contains less than 40% water, then sprayed into the recovery boiler. The organic component is consumed as fuel, generating heat which is recovered in the upper section of the furnace as high-temperature steam. The unburned inorganic component collects at the bottom of the boiler as a molten smelt. The smelt flows out of the furnace and is dissolved in a weak caustic solution, producing “green liquor” containing primarily dissolved Na₂S and sodium carbonate (Na₂CO₃). This liquor is pumped to a recausticizing plant, where it is clarified, then reacted with slaked lime
(Ca(OH)₂), forming NaOH and calcium carbonate (CaCO₃). The white liquor is filtered and stored for subsequent use. CaCO₃ is sent to a lime kiln, where it is heated to regenerate lime (CaO).

Sulphite Pulping and Recovery

Sulphite pulping dominated the industry from the late 1800s to the mid-1900s, but the method used during this era was limited by the types of wood which could be pulped and the pollution created by discharging untreated waste cooking liquor into waterways. Newer methods have overcome many of these problems, but sulphite pulping is now a small segment of the pulp market. Although sulphite pulping usually uses acid digestion, both neutral and basic variations exist.

The cooking liquor of sulphurous acid (H₂SO₃) and bisulphite ion (HSO₃^–) is prepared on-site. Elemental sulphur is burned to produce sulphur dioxide (SO₂), which is passed up through an absorption tower that contains water and one of four alkaline bases (CaCO₃, the original sulphite base, Na₂CO₃, magnesium hydroxide (Mg(OH)₂) or ammonium hydroxide (NH₄OH)) which produce the acid and ion and control their proportions. Sulphite pulping is usually carried out in brick-lined batch digesters. To avoid unwanted reactions, the digester is heated slowly to a maximum temperature of 130 to 140°C and the chips are cooked for a long time (6 to 8 hours). As the digester pressure increases, gaseous sulphur dioxide (SO₂) is bled off and remixed with the raw cooking acid. When approximately 1 to 1.5 hours of cooking time remains, heating is discontinued and the pressure is decreased by bleeding off gas and steam. The pulp is blown into a holding tank, then washed and screened.

The spent digestion mixture, called red liquor, can be used for heat and chemical recovery for all but calcium-bisulphite-base operations. For ammonia-base sulphite pulping, the dilute red liquor is first stripped to remove residual SO₂, then concentrated and burned. The flue gas containing SO₂ is cooled and passed through an absorption tower where fresh ammonia combines with it to regenerate the cooking liquor. Finally, the liquor is filtered, fortified with fresh SO₂ and stored. The ammonia cannot be recovered because it is converted into nitrogen and water in the recovery boiler.

In magnesium-base sulphite pulping, burning the concentrated pulping liquor gives magnesium oxide (MgO) and SO₂, which are easily recovered. No smelt is produced in this process; rather MgO is collected from the flue gas and slaked with water to produce magnesium hydroxide (Mg(OH)₂). SO₂ is cooled and combined with the Mg(OH)₂ in an absorption tower to reconstitute the cooking liquor. The magnesium bisulphite (Mg(HSO₃)₂) is then fortified with fresh SO₂ and stored. Recovery of 80 to 90% of the cooking chemicals is possible.

Recovery of sodium-base sulphite cooking liquor is more complicated. Concentrated spent liquor is incinerated, and approximately 50% of the sulphur is converted into SO₂. The remainder of the sodium and sulphur is collected at the bottom of the recovery boiler as a smelt of Na₂S and Na₂CO₃. The smelt is dissolved to produce green liquor, which is converted to sodium bisulphite (NaHSO₃) in several steps. The NaHSO₃ is fortified and stored. The regeneration process produces reduced sulphur gases, in particular hydrogen sulphide (H₂S).

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Bleaching is a multi-stage process that refines and brightens raw pulp. The objective is to dissolve (chemical pulps) or modify (mechanical pulps) the brown-coloured lignin that was not removed during pulping, while maintaining the integrity of the pulp fibres. A mill produces customized pulp by varying the order, concentration and reaction time of the bleaching agents.

Each bleaching stage is defined by its bleaching agent, pH (acidity), temperature and duration (table 1). After each bleaching stage, the pulp may be washed with caustic to remove spent bleaching chemicals and dissolved lignin before it progresses to the next stage. After the last stage, the pulp is pumped through a series of screens and cleaners to remove any contaminants such as dirt or plastic. It is then concentrated and conveyed to storage.

Table 1. Bleaching agents and their conditions of use

* Concentration of fibre in water solution.

Historically, the most common bleaching sequence used to produce market-grade bleached kraft pulp is based on the five-stage CEDED process (see table 1 for definition of symbols). The first two stages of bleaching complete the delignification process and are considered extensions of pulping. Because of environmental concerns about chlorinated organics in pulp mill effluents, many mills substitute chlorine dioxide (ClO₂) for a portion of the chlorine (Cl₂) used in the first bleaching stage (C_DEDED) and use oxygen (O₂) pre-treatment during the first caustic extraction (C_DE_ODED). The current trend in Europe and North America is towards complete substitution with ClO₂ (e.g., DEDED) or elimination of both Cl₂ and ClO₂. Where ClO₂ is used, sulphur dioxide (SO₂) is added during the final washing stage as an “antichlor” to stop the ClO₂ reaction and to control the pH. Newly developed chlorine-free bleaching sequences (e.g., OAZQP, OQPZP, where Q = chelation) use enzymes, O₂, ozone (O₃), hydrogen peroxide (H₂O₂), peracids and chelating agents such as ethylene diamine tetracetic acid (EDTA). Totally chlorine-free bleaching had been adopted at eight mills worldwide by 1993. Because these newer methods eliminate the acidic bleaching steps, acid washing is a necessary addition to the initial stages of kraft bleaching to allow removal of metals bound to the cellulose.

Sulphite pulps are generally easier to bleach than kraft pulps because of their lower lignin content. Short bleaching sequences (e.g., CEH, DCEHD, P, HP, EPOP) can be used for most paper grades. For dissolving-grade sulphite pulps used in the production of rayon, cellophane and so on, both hemicellulose and lignin are removed, requiring more complex bleaching sequences (e.g., C₁C₂ECHDA). The final acid wash is both for metal control and antichlor purposes. The effluent load for dissolving-grade sulphite pulps is much greater because so much of the raw wood is consumed (typical yield 50%) and more water is used.

The term brightening is used to describe bleaching of mechanical and other high-yield pulps, because they are whitened by destroying chromophoric groups without dissolving the lignin. Brightening agents include H₂O₂ and/or sodium hydrosulphite (NaS₂O₄). Historically, zinc hydrosulphite (ZnS₂O₄) was commonly used, but has been largely eliminated because of its toxicity in effluent. Chelating agents are added before bleaching to neutralize any metal ions, thereby preventing the formation of coloured salts or the decomposition of H₂O₂. The effectiveness of mechanical pulp bleaching depends on the species of wood. Hardwoods (e.g., poplar and cottonwood) and softwoods (e.g., spruce and balsam) that are low in lignin and extractives can be bleached to a higher brightness level than the more resinous pine and cedar.

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The use of waste or recycled paper as the raw material for pulp production has increased during the last several decades, and some paper plants depend almost completely on waste paper. In some countries, waste paper is separated from other household waste at the source before it is collected. In other countries separation by grade (e.g., corrugated board, newsprint, high-grade paper, mixed) takes place in special recycling plants.

Recycled paper can be repulped in a relatively mild process which uses water and sometimes NaOH. Small metal pieces and plastics may be separated during and/or after repulping, using a debris rope, cyclones or centrifugation. Filling agents, glues and resins are removed in a cleaning stage by blowing air through the pulp slurry, sometimes with the addition of flocculating agents. The foam contains the unwanted chemicals and is removed. The pulp can be de-inked using a series of washing steps which may or may not include the use of chemicals (i.e., surfactant fatty acid derivatives) to dissolve remaining impurities, and bleaching agents to whiten the pulp. Bleaching has the disadvantage that it may reduce fibre length and therefore lessen final paper quality. The bleaching chemicals used in recycled pulp production are usually similar to those used in brightening operations for mechanical pulps. After the repulping and de-inking operations, sheet production follows in a manner very similar to that using virgin fibre pulp.

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End products of pulp and paper mills depend on the pulping process, and may include market pulp and various types of paper or paperboard products. For example, the relatively weak mechanical pulp is converted into single-use products such as newspapers and tissue. Kraft pulp is converted into multi-use paper products such as high-quality writing paper, books and grocery bags. Sulphite pulp, which is primarily cellulose, can be used in a series of diverse end-products including specialty paper, rayon, photographic film, TNT, plastics, adhesives, and even ice cream and cake mixes. Chemi-mechanical pulps are exceptionally stiff, ideal for the structural support needed for corrugated container board. The fibres in pulp from recycled paper are usually shorter, less flexible and less water permeable, and can therefore not be used for high-quality paper products. Recycled paper is therefore mainly used for the production of soft paper products like tissue paper, toilet paper, paper towelling and napkins.

To produce market pulp, the pulp slurry is usually screened once more and its consistency adjusted (4 to 10%) before it is ready for the pulp machine. The pulp is then spread onto a travelling metal screen or plastic mesh (known as the “wire”) at the “wet end” of the pulp machine, where the operator monitors the speed of the moving wire and the water content of the pulp (figure 1; the presses and the cover of the drier can be seen in the upper left; in modern mills, operators spend a great deal of time in control rooms). Water and filtrate are drawn through the wire, leaving a web of fibres. The pulp sheet is passed through a series of rotating rolls (“presses”) that squeeze out water and air until the fibre consistency is 40 to 45%. The sheet is then floated through a multi-storey sequence of hot-air dryers until the consistency is 90 to 95%. Finally, the continuous pulp sheet is cut into pieces and stacked into bales. The pulp bales are compressed, wrapped and packaged into bundles for storage and transport.

Figure 1. Wet end of pulp machine showing fibre mat on the wire.

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Although similar in principle to making pulp sheets, paper making is considerably more complex. Some mills use a variety of different pulps to optimize paper quality (e.g., a mix of hardwood, softwood, kraft, sulphite, mechanical or recycled pulps). Depending on the type of pulp used, a series of steps is necessary prior to forming the paper sheet. Generally, dried market pulp is rehydrated, while high-consistency pulp from storage is diluted. Pulp fibres may be beaten to increase the fibre-bonding area and thereby improve paper sheet strength. The pulp is then blended with “wet-end” additives (table 1) and passed through a final set of screens and cleaners. The pulp is then ready for the paper machine.

Table 1. Papermaking additives

The flow spreader and headbox distribute a thin suspension (1 to 3%) of refined pulp onto a moving wire (similar to a pulp machine, only at a much higher speed, sometimes in excess of 55 km/h) which forms the fibres into a thin felted sheet. The sheet moves through a series of press rolls to the dryer section, where a series of steam-heated rolls evaporate most of the remaining water. Hydrogen bonds between the fibres have fully developed at this stage. Finally, the paper is calendered and reeled. Calendering is the process by which the paper surface is ironed smooth and its thickness reduced. The dried, calendered paper sheet is wound onto a reel, labelled and transported to the warehouse (figure 2; note waste paper under reel, and unenclosed operator control panel). “Dry-end” additives can be added before calendering on the paper machine or in separate “off-machine” coating operations in the converting sector of the industry.

Figure 2. Dry end of a paper machine showing full paper reel and operator using air slitter to cut end.

George Astrakianakis

A variety of chemicals are used in the papermaking process to provide the paper with specific surface characteristics and sheet properties. The most commonly used additives (table 1) are typically used at the per cent level, though some such as clay and talc may contribute as much as 40% to the dry weight of certain papers. Table 1 also indicates the diversity of chemical additives which may be used for specific production purposes and products; some of these are used at very low concentrations (e.g., slimicides are added to process water in parts per million).

The process of making paperboard is similar to that of making paper or pulp. A suspension of pulp and water is dispersed onto a travelling wire, the water is removed, and the sheet dried and stored as a roll. The process differs in the way that the sheet is formed to give thickness, in the combining of multiple layers, and in the drying process. Board can be made from single or multi-layered sheets with or without a core. The sheets are usually high-quality kraft pulp (or kraft and CTMP blend), while the core is made from either a blend of semi-chemical and low-cost recycled pulp or from entirely recycled pulp and other waste material. Coatings, vapour barriers and multiple layers are added according to the end use to protect the contents from water and physical damage.

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