Paper production, a brief description of the process
Raw materials processing
The process for the production of Kraftliner and Semichemical Fluting starts with wood coming from the forest to the mill. Most of the wood is delivered to the mill in the form of pulpwood logs but also a substantial part is brought as wood chips from saw mills nearby. The pulpwood logs have to be debarked and chipped before further processing. Therefore the logs have to pass through a barking drum and a chipper.
The chips produced are screened to take away wood dust as well as oversized chips and knots. The oversized chips are reprocessed to get proper chip dimensions and brought back to the chip flow. The chips are stored in a chip pile before processing in the pulp department.
The raw material for Testliner (including White Top Testliner) and Wellenstoff recovered paper is delivered as bales, and kept on stock by grade.
With a pre-selection method, it is possible to determine the suitability of a certain recovered paper mix for the stock preparation and paper process. The stock preparation is done in accordance with a recipe for each paper grade and grammage. The grades of recovered paper are put on the conveyor to the pulper in the ratio mentioned in the recipe.
Pulping and stock preparation
Pulping and stock preparation of primary fibres
The wood chips are normally cooked to pulp for Kraftliner production by the kraft cooking process. It is a highly alkaline cooking process with caustic soda and sodium sulphide as active cooking chemicals. The cooking takes place in a digester at high pressure and a temperature of 150 -170 oC. The pulp yield is normally around 55% i.e. 1000 kg of dry wood gives 550 kg of pulp.
For Fluting production the wood chips are cooked to pulp by the Semi Chemical cooking process. It is a slightly alkaline cooking process most commonly using sodium sulphite and sodium carbonate as active cooking chemicals. The pulp yield is normally around 80%.
The spent cooking liquor is drained off and washed out from the pulp. It contains the wood substance dissolved during the cooking together with the spent cooking chemicals. The spent liquor is concentrated and burnt for steam production and recovery of cooking chemicals.
The pulp produced is defiberized in refiners, screened and washed before being sent to the paper mill.
In the paper mill the pulp is mechanically treated in beaters to improve fibre-to-fibre bonding and the strength of the paper. The pH level of the pulp slurry can be adjusted in some cases to facilitate the paper production. Functional chemicals, fillers and other pulps i.e. recovered paper pulp may also be added to give the final paper the properties required. Finally, the pulp slurry is screened and diluted before being sent to the headbox of the paper machine.
In the paper mill the pulp is mechanically treated in beaters to improve fibre-to-fibre bonding and the strength of the paper. The pH level of the pulp slurry is adjusted with acid and some additives are added to facilitate the paper production. Functional chemicals, fillers and other pulps i.e. recovered paper pulp may also be added to give the final paper the properties required. Finally the pulp slurry is screened and diluted before being sent to the head box of the paper machine.
Pulping and stock preparation of recovered fibres
In the pulper the blend of dry, baled paper is converted into a pumpable suspension of fibres. This is achieved by submerging the bales in water and agitating the mixture so that the paper is wetted and broken up to form a suspension.
Large contaminants, which do not readily break up, can be removed from the pulper using a "ragger" or "junker", e.g. large pieces of textiles, plastics etc., which are caught on the original wires of the bales.
Remaining undesirable elements in the raw material are eliminated from the pulp by screening and cleaning. After that the pulp may undergo a disperging treatment. For this treatment the pulp is dewatered by wire or screw presses and concentrated to 25-30% and heated by steam to 80 -110 oC. The hot pulp is passed through a type of deflaker (which kneads the pulp) moving through the gap between a stationary and a rotating disk with a lot of teeth.
During the treatment contaminants such as hot melt, wax, ink and coating binders loosen from the fibres and together with small fragments of paper are reduced to fine particles (“disperged”) which therefore are no longer visible. The disperging treatment also has a positive influence on certain paper characteristics and fibre bonding capability.
Pulp for recycled fibre based paper for corrugated packaging does not normally undergo a de-inking process.
The dosage of additives can take place during the stock preparation process, just before/or in the headbox, on the wire section (e.g. by spraying) or by surface treatment on the size press.
The stock passes through one or two headboxes onto the paper machine. The paper web is formed from the head box onto the wire and dewatered through the wire primarily by the action of gravity and suction/vacuum. Further dewatering by mechanical means takes place in the press section where water is taken out of the sheet by pressing between felts. The final drying takes place in the drying section of the paper machine where the sheet runs against steam heated cylinders to get its final dryness of 91-93%. The collected water is reused for diluting the thick stock coming from the stock preparation.
Semi Chemical Fluting is a paper with just one ply and therefore the paper machine has one headbox and one wire. Kraftliner is normally a two-ply product and therefore requires a paper machine with two headboxes and usually two wires. The base brown ply contains the internal machine broke pulp in addition to wood fibres from the usually integrated pulp production and could also contain recycled paper pulp. The top ply is normally wood pulp from the integrated pulp production that is more refined and cleaner to give the top surface the right characteristics and printability. For white surface grades bleached fibres are of course used for the top ply.
Testliner mostly consists of two plies of paper. Depending on the type of Testliner the fibre composition of mixes of types of recycled paper can be different in each layer. In general a better grade of mix is used for the upper layer for reasons of appearance and strength. In order to increase its strength Testliner receives a surface treatment in the size press. This involves the application of a starch solution to one or both sides of the sheet. The top ply of Testliner is given an even, mostly brown colour by colouring the mass or by means of the size press treatment. This colouring is never given to White Top Testliners. The addition of special additives (in the mass or by means of the size press) makes it possible to produce Testliners with special properties such as extra water-repellent, low-germ and anti-corrosion grades.
Recycled Fluting can be a one-ply or two-ply product. Usually a size press treatment with a starch solution is applied in-line on the paper machine in order to obtain sufficient strength and stiffness properties.
The most common surface treatment of recycled fibre based corrugated board materials is done by a size press. Essentially a size press comprises two revolving rubber covered rolls, pressed together, through which the paper web passes. In the nip formed by the rolls there is a starch solution. The paper absorbs some of this solution, is pressed between two rolls and goes into the "after dryer " section of the paper machine in order to evaporate from the paper excess water absorbed from the starch solution in the size press.
After the paper machine there is a slitter winder where the big jumbo reel from the paper machine is rewound and cut down to customer reel formats according to customer orders. Finally the reels are weighed, marked, labelled and prepared for shipment to the customer, the corrugated board industry.