Chemical fibre separation and bleaching
At the chemical fibre separation facility at Iggesund mill long-fibre pulp from spruce and pine and short-fibre pulp from birch are produced in two parallel pulping and bleaching lines. This optimises fibre utilisation and facilitates efficient chemical and energy recovery, with the consequent environmental benefits. The wood chips produced in the chippers are fed into the large vertical vessels known as “continuous digesters”. As the name suggests, these machines operate 24 hours a day, with raw material being fed in at the top and processed pulp being extracted from the base. During this part of the process, the fibre is subjected to caustic soda and sodium sulphide at a temperature of 170 °C. This separates most of the lignin from the cellulose. The cellulose fibre or “pulp”, as it is now called, is then subjected to a number of washing and screening processes which separate even more of the lignin from the pulp. The residual liquor is taken to a special reprocessing system, where over 99% of the chemicals are extracted and returned to the digester. The dissolved lignin is evaporated and used as a substitute for fuel oil. This reprocessing system actually generates more energy, in the form of heat and steam, than is consumed by the whole pulp mill thus adding to the economy of the total system.
To achieve the pure white characteristics of fully bleached pulp, the “digested” brownish pulp is passed to the bleaching process. This modern facility uses a sophisticated combination of delignifying, washing and bleaching process steps. It starts in the digester with isothermic cooking that extends the delignification. After careful washing and screening oxygen is used to remove more lignin (oxygen bleaching). Final bleaching is done by using hydrogen peroxide and chlorine dioxide (no chlorine gas). This has facilitated a marked reduction of emissions which impact the environment. Once all the lignin has been removed, the pulp is transferred to storage tanks, ready for use in the boardmaking process.
The process of chemical defibration and bleaching. Click to enlarge image.
Cross section of a continous digester. Click to enlarge image.
At the Iggesund mill the two types of pulp are supplied directly from the pulp mill to the board machines to produce a homogeneous white multi-ply board. The pulp mill is equipped with advanced measurement and control systems. These systems together with well trained operators are important factors in maintaining uniformity and a high level of quality. The pulp characteristics are customised to suit the board mill.
The production of folding box board at the Workington mill involves combining inner plies of bleached refiner mechanical pulp made on site with outer plies of bleached chemical pulp. The chemical pulp, some of which is produced at Iggesund, is brought in as dried sheets. The fibres are reconstituted in water suspension in a hydrapulper – a large chamber which breaks up the sheets to release the fibres.
Additives are used to assist the fibres during the formation process and add other functions. Starch and size are added to improve fibre strength and water repellency.
Retention-aiding chemicals help to retain the fibres and additives in the process during sheet formation.
The use of pure bleached cellulose fibres for manufacture of the solid bleached Invercote product range produces outstanding aesthetic and functional qualities with taste and odour neutrality.
The combination of mechanical and chemical fibre in different layers in the folding box board Incada gives high bulk, stiffness and smooth and bright surface qualities.
The paperboard machine
The basic features of the paperboard making machines at Workington and Iggesund are very similar, so the following description applies to both sites except where otherwise specified.
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1. Multi-ply Forming
In contrast to paper, Iggesund paperboard is built up in several layers, or plies. Fibres are supplied via inlets (“headboxes”) at the wet end of the paperboard machine. Concentration at the wet end is 0.3% fibres and 99.7% water. A low fibre concentration is essential in order to obtain as uniform a distribution of fibres as possible in each layer. The first layer is formed on a plastic wire and the water drains downwards. The subsequent layers are stabilised on two upper wires and water drainage is done both upwards and downwards depending on wire and position. In the wet state, the layers of fibre consolidate easily.
Precision in the distribution of fibres and the consolidation of the fibre layers is a basic condition for qualities such as flatness, smoothness, strength and good creasing properties.
When the paperboard web reaches the press section, water content has dropped to 80–85%. The press section is important for achieving the correct consolidation of the fibre layers. Sandwiched between two fabrics (felts), the paperboard web is pressed between hard rolls. The water is effectively removed so that moisture content in the paperboard at the end of the press section is 60–65%.
Here, qualities such as flexibility, stiffness and runnability are ensured.
The drying section allows optimal control over the drying process. The paperboard web passes over steam-heated, polished cylinders which gradually reduce the moisture. A sophisticated system controls the temperature of the cylinders to ensure that the web is under a controlled tension during the drying process.
The drying section establishes a uniform moisture profile, flatness and stability.
4. Surface Sizing
Here a pigmented starch solution is applied to one or both sides to prepare the paperboard for coating. Surface sizing binds the fibres to the surface, making the paperboard more uniform and dense.
It is here that the paperboard’s future printing performance is determined, as well as surface strength, stiffness and smoothness.
The paperboard is nipped between steel rolls to further increase surface smoothness. This controls the paperboard’s thickness and density but especially its smoothness.
6. Surface Coating
The liquid, white-pigmented coating is applied and smoothed out over the surface with a blade on either one or both sides depending on the product. Each layer is dried independently by infra-red and hot air dryers.
The surface coating section allows paperboard to be coated twice on both sides to provide a high degree of whiteness, smoothness and gloss. Coating also determines the surface’s ink and varnish receptivity.
7. Calendering and Burnishing
The final gloss of the surface is achieved by gloss calendering in a gloss calender or brush polisher. In the gloss calender the paperboard web passes between a heated hard steel roll and a soft polymer roll. In the brush polisher the paperboard is polished by rotating brushes.
These processes give a uniform, smooth surface – which is essential for good printing and varnishing.
The paperboard web is reeled onto a large steel core, together weighing 30–40 tonnes depending on the product. Each finished reel of paperboard is given a unique identification code which allows the product to be traced all the way back to the raw materials.
9. On-line Measurement and Control
The web passes thousands of measuring points from which data is transmitted to the central control system. Optical on-line measurement is carried out on the moving web to check thickness, grammage, coating weight, moisture content, whiteness and gloss. The resulting regulation and control capabilities are a prerequisite for uniform, high quality.
10. Machine Glazing
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The Workington board machine has a machine glazing cylinder located in the drying section. This cylinder is called an MG or Yankee cylinder. The MG cylinder has a large diameter of over 6 m. It is steam heated and has a mirror-like polished steel surface.
The paperboard web, containing about 40% moisture, adheres to this surface. The web dries in contact with the rotating cylinder until the moisture content is insufficient to maintain adhesion. The surface then leaves the cylinder, retaining the high level of smoothness imparted to it by the polished steel, and continues the drying process on smaller cylinders.
The method gives a smooth surface without loss of thickness, hence maintaining good stiffness properties for the given grammage.
Winding and Slitting
The large reels from the board machine are divided into more manageable reels of smaller diameter and width. The width depends on whether the board is required as stock for subsequent sheeting or whether it is for a specific customer order. The latter type of reels are sent to the wrapping section for careful wrapping and labelling.
Sheeters cut and count the board for specific customer orders or standard stock sizes. The sheeted paperboard can also be supplied with tabs inserted at specified intervals. Wrapped reams, consisting of typically 100 sheets each, can be requested in various formats. Invercote G is available in a linen embossed version in all grammages.
Finally, in-house label identification is replaced with a customer label giving information about the order and product. The manufacturing identity can be traced from the information on the customer label.
- Accurate dimensions, squarely cut sheets.
- No edge wander on reels.
- Dust free edges cleanly cut.
- Accurate sheet counting.
- Clear labelling with information which can be used to trace the manufacturing and finishing origin of the paperboard.
The finished product is wrapped mechanically. Reels are stretch wrapped in polyethylene film together with end caps of corrugated board. Pallets are wrapped with polyethylene which is heat shrunk to provide watertight protection. It is important not to remove the moisture resistant wrapping until the board is in temperature equilibrium with the environment in which it will be printed and converted.
Measurement and control
In our processes we ensure that we produce high quality products which are uniform within a particular order and consistent between repeat makings for the same purpose.
In this area the skill of the operator is vitally important but it is also necessary that operators have access to instruments and computerised control systems. The better the operator can follow what is happening at many different points, the sooner corrective action can be taken either automatically or by the operator. This also means that the machine is stabilised much more quickly after product changes, thereby reducing waste and improving quality. Operators are also provided with trend information at intervals of minutes, hours, shifts or months as required. Critical points of the process are automatically scanned at set intervals.
On the board machine, grammage, moisture content and thickness profiles are constantly scanned and linked automatically to process features such as the rate of flow of pulp, steam pressures and steam distribution to the driers. Coating weight and gloss can be scanned on-line, as can web tension, with alarm indicators in the event of a web break. Sophisticated video monitoring systems find holes and other defects in the web, and if there is a web break the operator can quickly localise the cause.
- Uniform product quality within the order.
- Consistent product quality between orders.
Extrusion coating and lamination
At the Strömsbruk plant, which is located near Iggesund, paperboard from Iggesund and Workington is extrusion coated with a range of plastic materials, and extrusion laminated with foil or film.
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The paperboard is loaded into an unwinding position.
2. Surface Treatment
The paperboard surface is pre-treated with an electrical corona discharge. The plastic film can be treated with ozone. These methods assist the adhesion of the plastic to the paperboard.
Good adhesion of plastic to board at melt temperatures, which avoids odour and taste problems from oxidised plastic.
3. Extrusion Coating
Plastic granules such as polyethylene (PE), polypropylene (PP) and polyethylene terephthalate (PET) are converted by pressure and heat to the molten state in the barrel of the extruder. The molten plastic passes through a narrow slit in the automatically controlled die and on-to the surface of the paperboard. The control of temperature is critical. The plastic surface is immediately pressed against the chilled face of a steel roll, the surface of which, in combination with the paperboard characteristics, controls the finish of the plastic surface. Reverse- side coatings have an NSO (Non-Set-Off) finish and print-side coatings usually have a gloss finish.
Uniform coating thickness profile. Specified grammages of plastic coating. Low residues of substances affecting odour and taste. Tailored barrier function. Specified surface finish – gloss or NSO (Non-Set-Off) to achieve good printability and runnability in the converting operations. Single-sided plastic coating. Double-sided plastic coating. Alubarrier Alufoil lamination.
Extrusion lamination machine. Click to enlarge image.
4. Extrusion Lamination
Immediately after the initial plastic coating is applied an unwind station is available. Foil or film can be fed from this position into the nip between the molten plastic film and the chill roll such that the plastic initially performs the functions of an adhesive.
Plastic functions as an adhesive with improved surface smoothness and additional barrier properties.
5. Corona Treatment
Print-side plastic coatings are subjected to corona treatment to achieve good ink wetting. One-sided plastic coatings are also corona treated to improve the sealing characteristics and permit gluing with emulsion adhesives.
Ink wettability. Heat sealability. Gluing with emulsion adhesives.
The paperboard is wound onto large steel cores (drums) in batches of between 1 and 3 tonnes depending on the product. Each drum is given a unique in-house identification code.
Customer orders can be traced to point of manufacture.
Glue lamination machine. Click to enlarge image.
The glue lamination machine has four unwind stands.
IR (infra-red) driers are used to control the shape of the sheets.
Water-based adhesives are used to glue the board webs.
10. Press Nip
After gluing, the webs are pressed together.
11. Sheeting and Stacking
The glued board is sheeted and stacked on pallets in line.
Finishing and Packing
After conversion at Strömsbruk a number of finishing and packing operations are carried out in a similar manner as at the paperboard mills.