Commercial quantities are available in a wide range of particle sizes and brightness levels, the finer and brighter grades being used in high- quality papers. The use of china clay provides a smooth receptive surface that easily accepts printing ink, and although not as opaque as some more expensive fillers, it is satisfactory for many types of paper. Chalk Naturally occurring chalk tends to be of coarser particle size than china clay; hence, it increases matt surface to the paper.
However, as it reacts with the acidic alum used in conventional sizing, its use as a filler is restricted. Another source of calcium carbonate is from the alkali recovery stage in the sulfate process. Titanium Dioxide Titanium dioxide with its high refractive index provides excellent opacity and brightness to paper, especially useful for thin bible papers, laminate base papers, and waxed papers.
Titanium dioxide is also used in combination with other fillers, such as china clay, whereby in order to reduce costs, the proportion of titanium dioxide is kept to a minimum. Other Additives Other additives used are slimicides, antifoaming agents, filler retention aids, pitch control agents, and wet-strength agents.
Sizing Agents Chemicals that are used in the process of sizing are known as sizing agents. The process of sizing can be accomplished in two principal ways: 1 internal sizing and 2 surface sizing. Internal sizing plays an important role in controlling the absorption and penetration of liquid such as water and ink into the paper, paperboard, and sheet material.
The purpose of internal sizing is to inhibit penetration of liquids into the internal structure of paper. Internal sizes are introduced at the wet end of the papermaking system, usually as colloidal suspensions, which are retained in the fiber network during sheet formation.
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 treatments are applied to the paper. The traditional Fourdrinier machine is still widely used but for many paper grades has been replaced with twin-wire machines or gap formers and hybrid formers. Twin-wire formers have become the state-of-the-art design.
In twin-wire formers, the fiber suspension is led between two wires operating at the same speed, and is drained through one or both sides. There are different types of twin-wire formers.
In gap formers, the diluted stock is injected directly into the gap between the two wires, and combinations of Foudrinier and twin wires hybrid formers. Multiply papers can be made on a variety of formers, but recently two and three ply papers and liners are being made on multi-Fourdrinier wet ends.
Whatever the forming device, the wet paper web is passed through presses to remove as much water as possible by mechanical means. More moisture is removed by evaporation on multiple drying cylinders. The Fourdrinier papermaking machine is composed of three main sections: the forming section, the press section, and the drier section..
A paper slurry consisting of about 0. Once on the belt, the water is removed by draining and suction, leaving the fibers to form a very wet and weak paper. Forming The forming section of the Fourdrinier constitutes what is called the wet end of the machine. This section consists of the head box, the forming wire, foils, suction boxes, couch roller, breast roller, and dandy roll. Pulp is pumped from the machine box through the screens and cleaners to the head box.
The purpose of the head box is to deliver a uniform slurry to the forming wire. It is therefore transferred to a traveling woolen felt, which supports it through the first of a series of presses whose function is to remove more water by squeezing and at the same time make the sheet denser and smoother. Two or three presses are used in series, and the paper may go directly through, or it may pass under one press and be reversed through its rolls so that the two sides of the sheet may be more nearly alike.
The top roll of the press stands vertically over the lower roll, and it is connected with compound levers and weights, which permit regulation of the pressure applied and a maximum pressure much greater than that supplied merely by the weight of the top roll. Each press has a separate felt to carry the web, and just before the web reaches each set of rolls, the felt often passes over a suction box to aid in water removal.
All felts are kept taut by a series of stretch rolls as they return to the point at which they picked up the paper web. Transferring the web from the couch to the first felt is done when starting by cutting a narrow strip by means of the squirt on the wire and blowing it onto the felt by an air blast; in slow machines it may be done by picking it off the couch by hand and lifting it onto the felt. At each press the web sticks to the top roll and has to be transferred to the next felt by hand or by air blast.
A paper machine drier is a cast iron drum with closed ends, very carefully made so that it may be in good running balance and supplied with a steam inlet and a device to remove condensed water continuously and without loss of pressure. The outer surface is turned and polished as smooth as possible. Driers are usually mounted in two rows, one above the other, but staggered, so that an upper drier is over the space between its two neighbors in the bottom row. At the back, end of each drier is a gear, which meshes with the gears on two driers in the row above or below, so that all turn at the same speed.
A row of driers is usually broken into two banks with approximately an equal number of drums, and each bank is driven independently of the other. Finishing After the drying section, the web is subjected to several finishing steps prior to shipping it as a final product. The web can be sized, giving the paper surface resistance, or if other properties are needed, the web can be surface-coated.
The web can also be supercalendered, giving the surface a very smooth, uniform surface. In the final stages, the web is rewound and slit into two or more rolls and if needed sheeted. Sizing Sizing imparts resistance to liquids on the paper surface, a property necessary for paper used for writing or printing. Without external sizing, ink would bleed and feather.
External or surface sizing can be performed either on the paper machine or on a stand-alone unit. Machine sizing can be performed either by running the web through a size vat or by running the web through a size press. In the case of the size vat, the web, after exiting the drier section, is directed down into a vat and through another set of drying cans. Size presses are located after between the two drier sections and apply a coat of sizing by transference from rollers, and the metering is accomplished by the nip.
The most common types of sizing consist of pigments and starches, although animal glue and glycerin can also be used. Functional properties can be for protection from liquids,oils, gases, and chemicals, improve adhesion characteristics, improve wear, or some other property.
Coatings can be classified as aqueous, solvent, high solids, or extrusion. Aqueous coatings, used for commodity papers, contain water-soluble binders and are applied as a liquid. Common aqueous binders are casein, starch, protein, acrylics, and polyvinyl acetates. Solvent coatings are used in situations where the binders are not soluble in water and are used with specialty papers.
High-solid and extrusion coatings are used for specialized papers, where chemical, gas, or liquid resistance is necessary. High-solid coatings are applied as a coating of monomers and are polymerized by UV or electron curing.
Extrusion coatings are applied as a molten film of wax or polymer. Supercalendering, Cockling, and Embossing After the chemical processes have been completed, physical processes, such as supercalendering, cockling, and embossing, can be used to create the desired surface texture to the paper. Supercalendering uses friction and pressure to create a very smooth and glossy paper surface.
The supercalender consists of a stack of rollers having surfaces alternating between steel and cotton in construction. There is enough pressure between the steel and cotton rollers to slightly compress the cotton surface causing a drag. The difference in surface speed on either side of the nip creates friction, which polishes the paper surface. The cockle finish on many bond writing papers is created by the vat, sizing the web, then subjecting it to highvelocity air driers under high tension, and then under low tension.
The finished paper is usually heavily sized and has the characteristic rattle associated with high-quality bond paper. Embossing is achieved by running the web through an off-line press, where it is subjected to an engraved cylinder. The concept is similar to the dandy roll, but because the paper fibers cannot be redistributed, the surface of the paper is raised or depressed. Slitting, Sheeting, and Shipping Once the paper roll machine log is reeled from the paper machine, it is removed and transferred to a rereeler or a machine winder.
A rereeler unreels the web from the mandrels to create a full log. During this process any defects can be removed and the web spliced. A machine winder is similar to the rereeler, but is able to slit the web into multiple, narrower rolls. These rolls can be further finished by supercalendering, embossing, etc. If the finished product is sheeted paper, the rewound rolls are transferred to machines known as cutters.
The cutters can slit the web to form multiple narrower webs and cut across the web creating sheets. The paper rolls are placed onto a stand at one end of the machine.
As the web unwinds, it can be slit either adjusting the web width or creating several parallel webs. After the slitters, the web travels under a revolving knife, which cuts the web into sheets. After being cut, the sheets are jogged through an online inspection system, which checks caliper and dimensions. If the sheet does not conform, it drops down into a sheeter for recycling as broke.
After the cutters, the paper stacks are placed into guillotine trimmers, where the edges receive their final trim. Chemical recovery is a crucial component of the chemical pulping process: it recovers process chemicals from the spent cooking liquor for reuse. The chemical recovery process has important financial and environmental benefits for pulp and paper mills.
Environmental benefits include the recycle of process chemicals and lack of resultant discharges to the environment. The stepwise progression of chemical reactions has been refined; for example, black liquor gasification processes are now in use in an experimental phase. The schematic diagram of the kraft pulping process and the corresponding chemical and energy recovery process.. The primary operations of the kraft recovery process are concentration of black liquor by evaporation.
Combustion of strong black liquor to give the recovered inorganic chemicals in the form of smelt. The smelt, sodium sulfide, and sodium carbonate, dissolved in water, give green liquor.
Causticizing sodium carbonate to sodium hydroxide, using calcium hydroxide that is recovered as sodium carbonate. Recovery of by-products such as tall oil, energy, and turpentine. Regeneration of calcium carbonate to calcium hydroxide in a limekiln. Adams TN Lime reburning. Pulp and Paper Manufacture, 3rd ed. Alliance for Environmental Technology Trends in world bleached chemical pulp production: —, USA.
Annergren G and Lundqvist F Continuous Kraft Cooking: Research and Applications. Anonymous a. Pilao develops a new concept of refining. Anonymous b. Series of conflo refiners complete. Paperi ja Puu, 70 5 : Aoshima K New refiner and disperser providing a principle of conical refiner Double Conifiner and ConiDisc. White liquor preparation. Fapet Oy, Helsinki, p.
Arppe M Mechanical pulp: has it got a future or will it be discontinued? Int Papwirtsch, 45— Atkins J The forming section: beyond the fourdrinier. Bajpai P Emerging Technologies in Sizing. Bajpai P a. Environmentally Benign Approaches for Pulp Bleaching. Elsevier Science B. Bajpai P b.
Technological Developments in Refining. Chemical Recovery in Pulp and Paper Making. Baker CF Refining Technology, Baker C ed. Pira International, Leatherhead, UK, pp. Advances in the practicalities of refining. Biermann CJ a. Wood and fiber fundamentals. Handbook of Pulping and Papermaking. Academic Press, San Diego, p. Biermann CJ b. Pulping fundamentals. Biermann CJ c. Kraft spent liquor recovery. Biermann, CJ d. Refining and pulp characterization.
Handbook of Pulping and Papermaking, 2nd ed. Biermann CJ e. Stock preparation and additives for papermaking. Biermann CJ f. Paper manufacture. Bristow JA What is ISO brightness. Tappi J, 77 5 : — Buck RJ Fourdrinier: principles and practices. Casey JP a. Chemical pulping: a perspective. Tappi J, 66 1 : — Casey JP b. Mechanical and chemi-mechanical pulping: a perspective. Tappi J, 66 6 : 95— Davison RW Internal sizing.
Pulp and Paper Manufacture, Vol. Bleaching agents survey. Kirk-Othmer Encyclopedia of Chemical Technology, 4th ed. Wiley, New York, p. Fredette MC Pulp bleaching: principles and practice. Tappi Press, Atlanta, p. Gabl H Papillon: a new refining concept. Gerald K Raw material for pulp. Handbook of Pulp, Sixta H ed. KgaA, Germany, pp. Gullichsen J Fiber line operations. Fapet Oy, Helsinki, Finland, p. Hodgson KT Overview of sizing. Ishiguro K Paper machine. Jpn Tappi, 41 10 : 44— Krogerus B Papermaking additives.
Lankford A New refiner development. Latta JL Surface sizing I: overview and chemistry. Ljokkoi R Pulp screening applications. Papermaking Science and Technology, Vol. Fapet Oy, Helsinki, Finland, pp. A— A Lumiainen J The Conflo refiner—A new concept for LC-refining.
Refining of chemical pulp. Lund A The paper machine years Nord. Pappershistorisk Tidskr, 27 2 : 9— New and innovative internal sizing strategies for the sizing of PC containing fine paper. Proceedings Papermakers Conf. Malashenko A and Karlsson M Twin wire forming—an overview. Malinen R and Fuhrmann A Recent trends in bleaching of chemical pulp. Pap Puu, 77 3 : 78— Continuous cooking applications, papermaking science and technology 6.
McDonald S Advances in kraft pulping. The logs can also be fed horizontally to a disk mounted at the proper angle. Generally, the horizontal feed provides better control but is less suitable for scrap wood pieces. Off-size chips adversely affect the processing and quality of pulp. Acceptable-size chips are usually isolated from fines and oversized pieces by passing the chips over multistage vibratory screens. Conventional screening segregates chips only on the basis of chip length.
More recently, the greater importance of chip thickness has been recognized, and a few recently designed screens now segregate according to this parameter. Within mill areas, most chips are transported on belts or in pipes, using an airveying system. Chips are readily handled by air over distances of — m, but power consumption is high and chip damage can be significant.
By contrast, a belt conveyor system has a much higher initial cost. Other systems such as chain and screw conveyors are also used to move chips, but usually for relatively short distances. Bucket elevators are used for vertical movement.
Chip storage is widely utilized primarily because chips are more economical to handle than logs. Some disadvantages are apparent, for example, blowing of fines and airborne contamination, but it has been only recently that the significant loss of wood substance from respiration, chemical reactions, and microorganism activity has been quantified.
Considerable research has already been carried out to find a suitable chip preservative treatment, but so far, a totally effective, economical, and environmentally safe method has not been identified. In the meantime, it makes good sense to provide a ground barrier of concrete or asphalt before building a chip pile to reduce dirt contamination and inhibit the mobility of ground organisms.
Wind-blown concentrations of fines should be avoided because they reduce the dissipation of heat that builds up in the pile from various causes. Thermal degradation and even spontaneous combustion can result from localized heat buildup.
Optimum chip handling depends partly on pulping requirements. Because loss of extractives is high for the first 2 months of outside storage, all chips for sulfite pulping should go to storage to reduce resin problems. If by-product recovery is important as for some kraft pulping operations , fresh chips should bypass storage wherever possible to maximize yield. A number of reclaiming methods are in use.
Older installations employ a belt or chain conveyor along the side of the pile, which is fed by a bulldozer that pushes chips down the side of the pile onto the conveyor. Modern installations work automatically, some employing augers or chain conveyors on rotating platforms at the base of the pile. With respect to a given wood source, the quality of chips is measured by uniformity of size i.
Oversized chips represent a handling problem and are the main cause of screen rejects in chemical pulping. Size reduction of the oversize fraction is difficult to accomplish without generation of fines. Pin chips and especially fines and rotten wood cause lower yields and strengths in the resultant pulps and contribute to liquor circulation problems during cooking of chemical pulps.
Bark mainly represents a dirt problem, especially in mechanical and sulfite pulping. The kraft pulping process is much more tolerant of bark because most bark particles are soluble in the alkaline liquor. Figure 2. This breaks the bonds between the fibers. Moreover, the ground-wood process also offers the possibility of using hardwood to achieve even higher levels of brightness and smoothness. Ground-wood pulp has been the quality leader in magazine papers, and it is predicted that this situation will remain unchanged.
The most important refiner mechanical pulping process today is thermomechanical pulping TMP. TMP pulps are generally stronger than groundwood pulps, thus enabling a lower furnish of reinforcing chemical pulp for newsprint and magazine papers.
TMP is also used as a furnish in printing papers, paperboard, and tissue paper. Softwoods are the main raw material used for TMP because hardwoods give rather poor pulp strength properties. This can be explained by the fact that hardwood fibers do not form fibrils during refining but separate into short, rigid debris. Thus, hardwood TMP pulps, characterized by a high-cleanness, high-scattering coefficient, are mainly used as filler-grade pulp.
The application of chemicals such as hydrogen sulfite prior to refining causes partial sulfonation of middle lamella lignin. The better swelling properties and the lower glass transition temperature of lignin result in easier liberation of the fibers in subsequent refining. The chemithermomechanical pulps show good strength properties, even when using hardwood as a fiber source, and provided that the reaction conditions are appropriate to result in high degrees of sulfonation.
It is foreseen that mechanical paper will consolidate its position as one major fiber supply for high-end graphic papers. The growing demand on pulp quality in the future can only be achieved by the parallel use of softwood and hardwood as a raw material. The largest threat to the future of mechanical pulp is its high specific energy consumption. In this respect, TMP processes are most affected due to their considerably higher energy demand than groundwood processes. Moreover, the increasing use of recovered fiber will put pressure on the growth in mechanical pulp volumes.
Semichemical Pulping Semichemical pulping processes are characterized by a mild chemical treatment preceded by a mechanical refining step.
The most important semichemical process is the neutral sulfite semichemical NSSC process, in which chips undergo partial chemical pulping using a buffered sodium sulfite solution, and are then treated in disk refiners to complete the fiber separation.
The sulfonation of mainly middle lamella lignin causes a partial dissolution so that the fibers are weakened for the subsequent mechanical defibration. NSSC pulp is used for unbleached products where good strength and stiffness are particularly important; examples include corrugating medium, grease-proof papers, and bond papers.
NSSC pulping is often integrated into a kraft mill to facilitate chemical recovery by a so- called crossrecovery, where the sulfite-spent liquor is processed together with the kraft liquor.
The sulfite-spent liquor then provides the necessary makeup Na, S for the kraft process. However, with the greatly improving recovery efficiency of modern kraft mills, the NSSC makeup is no longer needed so that high-yield kraft pulping develops as a serious alternative to NSSC cooking.
Semichemical pulp is still an important product category, however, and accounts for 3. Chemical Pulping Chemical pulping dissolves the lignin and other materials of the interfiber matrix material, and also most of the lignin that is in the fiber walls. This enables the fibers to bond together in the papermaking process by hydrogen bond formation between their cellulosic surfaces. Chemical pulps are made by cooking digesting the raw materials, using the kraft sulfate and sulfite processes.
Kraft Process The kraft process produces a variety of pulps used mainly for packaging and highstrength papers and board. Wood chips are cooked with caustic soda to produce brown stock, which is then washed with water to remove cooking black liquor for the recovery of chemicals and energy. Sulfite Process This process uses different chemicals to attack and remove lignin.
Compared to kraft pulps, sulfite pulps are brighter and bleached more easily, but are weaker. Sulfite pulps are produced in several grades, but bleached grades dominate production. Compared to the kraft process, this operation has the disadvantage of being more sensitive to species characteristics.
The sulfite process is usually intolerant of resinous softwoods, tannin-containing hardwoods, and any furnish containing bark. The sulfite process produces bright pulp, which is easy to bleach to full brightness, and produces higher yield of bleached pulp, which is easier to refine for papermaking applications. Pulps are processed in a wide variety of ways, depending on the method that generated them.
Some pulp processing steps that remove pulp impurities include screening, defibering, and deknotting. Pulp may also be thickened by removing a portion of the water.
At additional cost, pulp may be blended to ensure product uniformity. If pulp is to be stored for long periods, drying steps are necessary to prevent fungal or bacterial growth. Efficient washing is critical to maximize return of cooking liquor to chemical recovery and to minimize carryover of cooking liquor known as washing loss into the bleach plant because excess cooking liquor increases consumption of bleaching chemicals.
In addition, these organic compounds function as precursors to chlorinated organic compounds, increasing the probability of their formation. Brown Stock Washing The objective of brown stock washing is to remove the maximum amount of liquordissolved solids from the pulp while using as little wash water as possible.
The dissolved solids left in the pulp after washing will interfere with later bleaching and papermaking and will increase costs of these processes. The loss of liquor solids due to solids left in the pulp means that less heat can be recovered in the recovery furnace. Also, makeup chemicals must be added to the liquor system to account for lost chemicals. Screening Screening of the pulp is done to remove oversized and unwanted particles from good papermaking fibers so that the screened pulp is more suitable for the paper or board product in which it will be used.
The biggest oversized particles in pulp are knots. Knots can be defined as uncooked wood particles. The knots are removed before washing and fine screening. The most important of these is to increase the brightness of the pulp so that it can be used in paper products such as printing grades and tissue papers.
For chemical pulps, an important benefit is the reduction of fiber bundles and shives as well as the removal of bark fragments. This improves the cleanliness of the pulp. Bleaching also eliminates the problem of yellowing of paper in light, as it removes the residual lignin in the unbleached pulp.
Resin and other extractives present in unbleached chemical pulps are also removed during bleaching, and this improves the absorbency, which is an important property for tissue paper grades. In the manufacture of pulp for reconstituted cellulose such as rayon and for cellulose derivatives such as cellulose acetate, all wood components other than cellulose must be removed. In this situation, bleaching is an effective purification process for removing hemicelluloses and wood extractives as well as lignin.
To achieve some of these product improvements, it is often necessary to bleach to high brightness. Thus, high brightness may, in fact, be a secondary characteristic of the final product and not the primary benefit.
It is therefore simplistic to suggest that bleaching to lower brightness should be practiced based on the reasoning that not all products require high brightness. The stock in the pulper is accelerated and decelerated repeatedly, and hydrodynamic shear forces are produced by the severe velocity gradients. The resulting forces serve to loosen fibers and reduce any flakes into individual fibers. Paper made from unbeaten virgin pulp has a low strength, is bulky, and has a rough surface. In good-quality paper, the fibers must be matted into a uniform sheet and must develop strong bonds at the points of contact.
Beating and refining are the processes by which the undesirable characteristics are changed. The pulp components are supplied from a high-density storage tower.
Therefore, a series of controlled dilution steps and mixing stages are necessary to achieve a uniform consistency. Accurate proportioning of pulps and additives into a blend is the major task of this stage. Many chemical additives are added to the stock at different points before the paper machine. Routine additions at the beater or paper machine wet-end stage include sizing agents, mineral fillers, starch, and associated products and dyes. China Clay China clay has the benefit of being chemically inert and therefore can be used in its natural state with any type of sizing agent, acidic or alkaline.
Commercial quantities are available in a wide range of particle sizes and brightness levels, the finer and brighter grades being used in high- quality papers. The use of china clay provides a smooth receptive surface that easily accepts printing ink, and although not as opaque as some more expensive fillers, it is satisfactory for many types of paper. Chalk Naturally occurring chalk tends to be of coarser particle size than china clay; hence, it increases matt surface to the paper.
However, as it reacts with the acidic alum used in conventional sizing, its use as a filler is restricted. Another source of calcium carbonate is from the alkali recovery stage in the sulfate process. Titanium Dioxide Titanium dioxide with its high refractive index provides excellent opacity and brightness to paper, especially useful for thin bible papers, laminate base papers, and waxed papers. Titanium dioxide is also used in combination with other fillers, such as china clay, whereby in order to reduce costs, the proportion of titanium dioxide is kept to a minimum.
Other Additives Other additives used are slimicides, antifoaming agents, filler retention aids, pitch control agents, and wet-strength agents. Sizing Agents Chemicals that are used in the process of sizing are known as sizing agents.
The process of sizing can be accomplished in two principal ways: 1 internal sizing and 2 surface sizing. Internal sizing plays an important role in controlling the absorption and penetration of liquid such as water and ink into the paper, paperboard, and sheet material. The purpose of internal sizing is to inhibit penetration of liquids into the internal structure of paper.
Internal sizes are introduced at the wet end of the papermaking system, usually as colloidal suspensions, which are retained in the fiber network during sheet formation.
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 treatments are applied to the paper. The traditional Fourdrinier machine is still widely used but for many paper grades has been replaced with twin-wire machines or gap formers and hybrid formers.
Twin-wire formers have become the state-of-the-art design. In twin-wire formers, the fiber suspension is led between two wires operating at the same speed, and is drained through one or both sides. There are different types of twin-wire formers. In gap formers, the diluted stock is injected directly into the gap between the two wires, and combinations of Foudrinier and twin wires hybrid formers. Multiply papers can be made on a variety of formers, but recently two and three ply papers and liners are being made on multi-Fourdrinier wet ends.
Whatever the forming device, the wet paper web is passed through presses to remove as much water as possible by mechanical means. More moisture is removed by evaporation on multiple drying cylinders. The Fourdrinier papermaking machine is composed of three main sections: the forming section, the press section, and the drier section.. A paper slurry consisting of about 0. Once on the belt, the water is removed by draining and suction, leaving the fibers to form a very wet and weak paper.
Forming The forming section of the Fourdrinier constitutes what is called the wet end of the machine. This section consists of the head box, the forming wire, foils, suction boxes, couch roller, breast roller, and dandy roll. Pulp is pumped from the machine box through the screens and cleaners to the head box. The purpose of the head box is to deliver a uniform slurry to the forming wire.
It is therefore transferred to a traveling woolen felt, which supports it through the first of a series of presses whose function is to remove more water by squeezing and at the same time make the sheet denser and smoother. Two or three presses are used in series, and the paper may go directly through, or it may pass under one press and be reversed through its rolls so that the two sides of the sheet may be more nearly alike.
The top roll of the press stands vertically over the lower roll, and it is connected with compound levers and weights, which permit regulation of the pressure applied and a maximum pressure much greater than that supplied merely by the weight of the top roll. Each press has a separate felt to carry the web, and just before the web reaches each set of rolls, the felt often passes over a suction box to aid in water removal.
All felts are kept taut by a series of stretch rolls as they return to the point at which they picked up the paper web. Transferring the web from the couch to the first felt is done when starting by cutting a narrow strip by means of the squirt on the wire and blowing it onto the felt by an air blast; in slow machines it may be done by picking it off the couch by hand and lifting it onto the felt.
At each press the web sticks to the top roll and has to be transferred to the next felt by hand or by air blast. A paper machine drier is a cast iron drum with closed ends, very carefully made so that it may be in good running balance and supplied with a steam inlet and a device to remove condensed water continuously and without loss of pressure.
The outer surface is turned and polished as smooth as possible. Driers are usually mounted in two rows, one above the other, but staggered, so that an upper drier is over the space between its two neighbors in the bottom row. At the back, end of each drier is a gear, which meshes with the gears on two driers in the row above or below, so that all turn at the same speed. A row of driers is usually broken into two banks with approximately an equal number of drums, and each bank is driven independently of the other.
Finishing After the drying section, the web is subjected to several finishing steps prior to shipping it as a final product. The web can be sized, giving the paper surface resistance, or if other properties are needed, the web can be surface-coated. The web can also be supercalendered, giving the surface a very smooth, uniform surface.
In the final stages, the web is rewound and slit into two or more rolls and if needed sheeted. Sizing Sizing imparts resistance to liquids on the paper surface, a property necessary for paper used for writing or printing. Without external sizing, ink would bleed and feather.
External or surface sizing can be performed either on the paper machine or on a stand-alone unit. Machine sizing can be performed either by running the web through a size vat or by running the web through a size press. In the case of the size vat, the web, after exiting the drier section, is directed down into a vat and through another set of drying cans.
Size presses are located after between the two drier sections and apply a coat of sizing by transference from rollers, and the metering is accomplished by the nip. The most common types of sizing consist of pigments and starches, although animal glue and glycerin can also be used. Functional properties can be for protection from liquids,oils, gases, and chemicals, improve adhesion characteristics, improve wear, or some other property.
Coatings can be classified as aqueous, solvent, high solids, or extrusion. Aqueous coatings, used for commodity papers, contain water-soluble binders and are applied as a liquid. Common aqueous binders are casein, starch, protein, acrylics, and polyvinyl acetates. Solvent coatings are used in situations where the binders are not soluble in water and are used with specialty papers. High-solid and extrusion coatings are used for specialized papers, where chemical, gas, or liquid resistance is necessary.
High-solid coatings are applied as a coating of monomers and are polymerized by UV or electron curing. Extrusion coatings are applied as a molten film of wax or polymer. Supercalendering, Cockling, and Embossing After the chemical processes have been completed, physical processes, such as supercalendering, cockling, and embossing, can be used to create the desired surface texture to the paper.
Supercalendering uses friction and pressure to create a very smooth and glossy paper surface. The supercalender consists of a stack of rollers having surfaces alternating between steel and cotton in construction.
There is enough pressure between the steel and cotton rollers to slightly compress the cotton surface causing a drag. The difference in surface speed on either side of the nip creates friction, which polishes the paper surface. The cockle finish on many bond writing papers is created by the vat, sizing the web, then subjecting it to highvelocity air driers under high tension, and then under low tension.
The finished paper is usually heavily sized and has the characteristic rattle associated with high-quality bond paper. Embossing is achieved by running the web through an off-line press, where it is subjected to an engraved cylinder. The concept is similar to the dandy roll, but because the paper fibers cannot be redistributed, the surface of the paper is raised or depressed. Slitting, Sheeting, and Shipping Once the paper roll machine log is reeled from the paper machine, it is removed and transferred to a rereeler or a machine winder.
A rereeler unreels the web from the mandrels to create a full log. During this process any defects can be removed and the web spliced. A machine winder is similar to the rereeler, but is able to slit the web into multiple, narrower rolls.
These rolls can be further finished by supercalendering, embossing, etc. If the finished product is sheeted paper, the rewound rolls are transferred to machines known as cutters.
The cutters can slit the web to form multiple narrower webs and cut across the web creating sheets. The paper rolls are placed onto a stand at one end of the machine. As the web unwinds, it can be slit either adjusting the web width or creating several parallel webs.
After the slitters, the web travels under a revolving knife, which cuts the web into sheets. After being cut, the sheets are jogged through an online inspection system, which checks caliper and dimensions. If the sheet does not conform, it drops down into a sheeter for recycling as broke.
After the cutters, the paper stacks are placed into guillotine trimmers, where the edges receive their final trim. Chemical recovery is a crucial component of the chemical pulping process: it recovers process chemicals from the spent cooking liquor for reuse. The chemical recovery process has important financial and environmental benefits for pulp and paper mills.
Environmental benefits include the recycle of process chemicals and lack of resultant discharges to the environment. The stepwise progression of chemical reactions has been refined; for example, black liquor gasification processes are now in use in an experimental phase.
The schematic diagram of the kraft pulping process and the corresponding chemical and energy recovery process.. The primary operations of the kraft recovery process are concentration of black liquor by evaporation. Combustion of strong black liquor to give the recovered inorganic chemicals in the form of smelt. The smelt, sodium sulfide, and sodium carbonate, dissolved in water, give green liquor.
Causticizing sodium carbonate to sodium hydroxide, using calcium hydroxide that is recovered as sodium carbonate. Recovery of by-products such as tall oil, energy, and turpentine. Regeneration of calcium carbonate to calcium hydroxide in a limekiln. Adams TN Lime reburning. Pulp and Paper Manufacture, 3rd ed. Alliance for Environmental Technology Trends in world bleached chemical pulp production: —, USA.
Annergren G and Lundqvist F Continuous Kraft Cooking: Research and Applications. Anonymous a.
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