|
Overview of the Manufacturing Process for Paper and Board
Papermaking today includes, in principle, the same process steps as applied for centuries: preparation of the fiber material, sheet or web forming, pressing, dry¬ing, sizing and smoothing. However, in the last two centuries much of the detail has changed. Each process step has undergone – and still undergoes today – in¬tensive research and development work to meet economic and ecological require¬ments. All links in the chain between fiber and end user contribute to this pro¬gress. The chain does not only include the paper producing industry itself and its suppliers such as the machine and chemical industry, but also the paper industry’s customers and related industries, e. g. printing-houses, printing ink and printing machine suppliers and the manufacturers of corrugated board.
R&D focus has been on economic and environmental aspects such as
. • reduction in consumption of raw material, energy and water as well as noise reduction
. • high machine runnability and long lifetime of machinery and its components
• improvement of paper and board quality with respect to improvement of con¬verting quality
which has led to results of high practical value such as
. • better understanding and consequent control of the whole process in a narrow band
. • reduction in fiber consumption by reducing basis weight at the same quality level and practical value . • increased ratio of recycled fibers in graphic paper production, with up to 100 % for newsprint and a growing ratio in high grades such as supercalendered (SC) and light weight coated (LWC) papers
. • fillers and coatings replacing part of the expensive fiber material and improving quality
. • new coating and calendering technologies
. • higher safety in Yankee dryer and suction press roll operation
. • new methods of material design for fighting wear of machine components
.• minimum number of personnel involved in the paper and board production process.
.R&D work is supported by modern tools and sciences e. g.
. • process analysis using advanced measuring and analysis techniques
. • process simulation and advanced control techniques
. • morphological characterization of fibers for papermaking
. • chemistry developing functional and process chemicals
. • finite element method (FEM) and computational fluid dynamics (CFD)
. • visualization techniques in the micro- or nano-ranges, video documentation and analysis
.• material sciences including plasma ions implantation into the base material at low temperatures.
.The papermaking process (Fig. 1.2) starts with the delivery of the raw material of the stock components. These are
.• fibers such as
.– virgin pulps (chemical or mechanical) which are usually supplied in bales or, in special cases, as a suspension when both pulp and paper are manufactured at the same location (integrated processing)
.– recovered paper in bales or as loose material
. • fillers and pigments
. • chemical additives
. • coating colors when coated paper is produced. All these components have to be adequately prepared for optimum use in the papermaking process steps. The additives may be delivered ready for use or may have to be finally prepared according to the requirements in the mill. Fiber stock preparation includes several unit operations depending on the furnish and the purpose. Stock preparation of virgin fiber pulp needs less machinery and energy than the preparation of recovered paper which, however, is the cheaper raw mate¬rial. Fiber stock preparation ends at the paper machine chest. Here stock of high consistency is preferred to minimize carry-over of chemicals and contaminants.
Stock preparation is followed by the approach flow system connecting stock preparation with the paper machine. Its main tasks are
• to dose exactly and mix uniformly all the different components of the final sus¬pension to be delivered to the paper machine • to supply a continuous suspension flow of constant consistency, quality and flow rate at constant pressure to the headbox of the paper machine.
The approach flow system ends at the distributor of the headbox.
The task of the paper machine is to produce paper or board of the quality re¬quired by the end user – or by the intermediate process steps such as converting or printing. The paper and board properties have to be uniform in machine direction (MD) as well as in cross machine direction (CD). Further, the paper machine has to make the best use of the quality potential of the entering stock. The paper machine includes
. • the headbox distributing the suspension across the machine width onto the wire
. • the wire section where the suspension is formed into an endless web by dewa¬tering
. • the press section pressing water out of the web by mechanical means
. • the dryer section where the residual water is evaporated • often a sizing unit where starch, or pigments are transferred onto the web
. • sometimes a coating section where coating color is applied to the web
. • the calender to finally smooth the paper or board surface. The paper manufacturing process ends with the paper web being reeled at the reeler at full width.
By tradition and technical feasibility, coating and supercalendering for surface quality improvement have been off-line processes. Today both are increasingly integrated into the paper machine. The final activities in paper and board produc¬tion are slitting of the full width reels into smaller rolls at the winder followed by packaging the rolls for shipment. Paper broken during the manufacturing process has to be recycled and fibers are recovered from the white water of the paper machine in a saveall. White water is fed back from the paper machine to the approach flow system and stock prepara¬tion. Fresh water is supplied to the paper machine.
Along the paper production line stock consistency varies according to the re¬quirements of the unit operations. Unfortunately the terms low, medium and high consistency relate to different consistency numbers depending on the actual unit operation (Table 1.1). It is also important to note whether the number gives the overall consistency including fibers and fillers or just fibers.
|
