Systems for Fiber Stock Preparation

The target of fiber preparation systems is to modify the raw materials for paper production so that the stock suits the requirements of the paper machine as well as those of the finished paper or board. Raw materials are virgin fibers from different sources such as chemical pulps from hard- or softwood or mechanical fibers like SGW, PGW, TMP or CTMP. The largest portion of raw materials today is recovered paper which is processed into secondary or recycled fibers. The recovered paper grades differ a lot as regards fiber compostion and cleanliness levels (see Vol.1). Therefore the processing systems have to take into account these differences as well as the various quality requirements of the finished stock.

4.3 Systems for Fiber Stock Preparation

Systems for Primary Fiber Preparation
The systems for primary fiber preparation are less complex than a recovered paper processing system due to the much lower level of impurities and contamination in virgin fiber pulps. Depending on the paper type produced and the availability of the various fiber types, a paper machine, or especially a multi-layer board machine, may be fed by several different fiber qualities, each of them treated in separate lines with different process steps.

Integrated paper mills have their own chemical and/or mechanical pulping plants. Here the fibers usually are not dried before they are supplied to fiber prepa¬ration. In nonintegrated mills the primary fibers are supplied dried in bales or flash dried in flakes. Therefore the fiber preparation in an integrated mill usually simply consists of refining (of mainly chemical pulp), whereas nonintegrated mills have to use more complex systems including slushing, removal of contraries and impurities ahead of deflaking and/or the refining. Fig. 5.56 shows a conventional system for virgin fiber preparation of chemical pulp.

The virgin pulp is delivered in bales, which must be placed individually on a conveyor belt. The wires holding the bales together are cut and removed by hand or automatically. The bales reach the LC-pulper via a weighing device, and slushing takes place at a consistency of 4–6 %. The high consistency (HD) cleaning stage – a hydrocyclone device – operates at the same consistency and removes heavy parti¬cles in order to protect the following deflaking and refining stages from mechan¬ical damage. The following optional intensive deflaking breaks down the remain¬ing flakes.

It also has to prevent the so-called “fish-eyes” in the final paper, which are caused by nondisintegrated fiber bundles when flash-dried pulps are used. Subsequent refining achieves the desired final fiber properties in order to meet the quality parameters of the final product. Due to the wide variety of virgin fibers, refining has to be adapted to each raw material and each paper quality target. By refining, strength properties, formation, optical properties of paper and special demands like electrical insulation or greaseproofness are influenced. The same is true for machine runnability.
Systems for Secondary Fiber Preparation
Secondary fiber preparation systems are extremely diverse. This is due to the wide variety of recovered paper grades with their different paper components and qual¬ity levels, the quantity and type of nonpaper components like fillers, debris and other detrimental substances, the varying production ranges, and especially the requirements of the paper machine and the final product. Furthermore, govern¬mental regulations regarding waste water, waste disposal and noise levels also have to be considered. On the other hand it is possible to get to an optimum paper quality at lower production costs by using recycled fibers rather than virgin fib¬ers. Systems for Graphic Paper Grades
The raw materials for graphic paper grades mainly comprise graphical post-con-sumer recovered papers and smaller portions of unprinted or printed pre-con-sumer grades. Wood-containing recovered paper is generally called deinking mate¬rial and consists of old newsprints (ONP) and old magazines (OMG). Woodfree recovered paper consists of mixed office waste (MOW) and other coated and un¬coated woodfree (CWF and UWF) printing papers. In these grades unbleached chemical fibers and mechanical fibers have to be avoided because in systems for white paper grades high demand is put on optical properties.

The product from a secondary fiber preparation plant for white grades is usually called DIP (Deinked Pulp) as there is at least one process step for ink removal integrated in the system. In most of the cases this deinking step is done by se¬lective flotation. Washers may be used for deinking in special cases when very finely dispersed printing inks (like water-based flexo inks) are present in the re¬covered paper or when not only the ink, but also the major share of the ash content needs to be removed, as is true e. g. for tissue grades.

The main parameters characterizing the quality of a DIP are brightness, stickies content, dirt specks and ash content. Depending on the application, namely the recovered paper grades used and the paper grades produced, different require¬ments are imposed on the DIP quality and thus on the system layout and efficiency.

4.3 Systems for Fiber Stock Preparation System for Wood-containing Graphic Papers
The graphic paper grade with the highest secondary fiber content today is news¬print. There are a great number of paper mills producing newsprint, improved newsprint or even SC-B as well as some LWC grades based on 100 % recycled fibers, especially in middle Europe. High grade SC-A and LWC papers may also contain up to 30 % recycled fibers. The main recovered paper source for these grades is the so-called deinking grade (European grade classification: 1.11) which represents a mix of 40–60 % ONP and 60–40 % OMG. The significant quality pa¬rameters of this raw material and of the DIP for graphic paper grades are shown in Table 4.2.

A fiber preparation system for white paper grades generally consists of the proc¬ess steps shown in Fig. 4.57.
Recovered paper is delivered in bales or loose and is fed to a conveyor belt where the wires holding the bales together are cut and automatically removed. The raw material gets to the pulping stage via a weighing device. From the measured weight the necessary amount of water for slushing and dilution, as well as the necessary amounts of deinking chemicals, are calculated and fed to the high con¬sistency (HC) pulping stage. Pulping is done at a minimum consistency of 15 % (and up to 28 % in drum pulpers) for gentle fiber-fiber friction and a low degree of contaminant break down. Operation is either batchwise in a high consistency pulper or continuous in a drum pulper. For good ink detachment, in wood-contain-ing systems pulping is done in an alkaline environment. Sodium hydroxide assists the ink detachment, sodium silicate avoids redeposition of the ink on the fibers and stabilizes hydrogen peroxide, which is added to compensate for the alkaline induced yellowing of mechanical fibers. Deinking additives like soap and/or sur¬factants can also be added in the slushing stage (see Section

Table 4.2 Quality parameters of European ONP/OMG mix and DIP for different wood-containing graphical paper grades.
Grade  DIP Brightness % ISO  DIP Dirt count > 50 mm2 m–2  mm DIP Stickiesa mm2/kg  DIP Ash content (575 °C) %
recovered paper  45–48  1500–3000  4000–8000  20–25
(Mix ONP/OMG)    
newsprint  60–62  100–200  < 200  12–16
improved newsprint  65–68  100–200  < 200  12–16
supercalendered (SC)  65–68  50–100  < 100  12–16
light weight coated  68–72  50–100  < 100  9–13

a According to TAPPI T277 (measurement with Somerville laboratory screen with 0.15 mm slotted screen plate).
After slushing the larger contaminants are removed from the suspension by coarse screening at a consistency of approximately 4.5 % in detrashing machines and/or sorting drums. Hole sizes here can vary from 4 to 16 mm. High con¬sistency cleaning removes major heavy contaminants like glass, stones or staples. The following medium consistency screening is carried out with hole baskets with a minimum hole diameter of 1.0 mm. These are especially effective for reduction of flat particles. Medium consistency means a level of 3.5 % in the first cleaning stage and dilution in the subsequent stages.

For high quality demands an intermediate consistency (IC) slot screening stage (with slot width a minimum of 0.20 mm) follows at 2 % consistency as the next step for removal of stickies and other cubic or round particles. This stage is pro¬tected against mechanical damage by a cleaning stage for sand removal which also operates at intermediate consistency.

Deinking flotation represents the “heart” of the whole deinking plant. It follows after dilution down to a low consistency (LC) of approximately 1.2 %. The main target of the flotation stage is the removal of printing inks but small lightweight particles, stickies and ash, are also removed in the deinking cells when these parti¬cles are hydrophobic. As mentioned above, the deinking chemicals like soap and/ or tenside are added either in the pulper or directly before the deinking stage, but sometimes the soap dosage is split between both stages (see Section

The multistage fine screening system operates with slot widths of 0.15 mm. If the fine screening stage is the only slot screening stage in the system, a cleaner plant ahead of it for sand removal is recommended for protection against wear.

4.3 Systems for Fiber Stock Preparation
Table 4.3 Effect of the process stages on quality parameters of wood-containing DIP.
Quality  Brightness  Dirt specks  Stickies  Ash content  Debris content  Fiber
parameters   content  contenta    design
Process stage     
pulping  detachment  detachment  saving   removal of
coarse  and dispersion  and dispersion  screen-ability   coarse
screening  of smaller  of larger    contaminants
 printing ink  printing ink   
 particles  particles   
HC cleaning      removal of
     heavy particles
MC hole    reduction   removal of flat
screening      particles
IC cleaning      removal of sand
     and debris
IC screening    reduction   removal of
flotation 1  increase by  reduction  reduction  reduction 
 removal of    
 printing inks    
HW cleaning   reduction    removal of sand,
     debris and dirt
fine screening   reduction  reduction   removal of
washing/  increase by    reductionb 
thickening/  removal of    
dewatering  printing inksb    
dispersion 1  reduction by  particle size  particle size  
 detachment  reduction  reduction  
 and dispersion    
 of smaller    
 printing ink    
flotation 2  increase by  reduction  reduction  reduction 
 removal of    
 printing inks    
dispersion 2  reduction by  particle size  particle size  
 detachment  reduction  reduction  
 and dispersion    
 of smaller    
 printing ink    

Table 4.3 (continued).
Quality parameters Process stage  Brightness  Dirt specks content  Stickies contenta  Ash content  Debris content  Fiber design
oxidative bleaching  increase (fibers)     
reductive bleaching  increase (fibers and color stripping)     
refining       increase in strenghth properties and fiber flexibility

a According to TAPPI T277. b Only, when filtrates are clarified.
Fine screening is followed by thickening of the suspension with disk filters and subsequent dewatering by wire or screw presses to a minimum consistency of 30 %. This is the precondition for dispersing. Dispersion takes place at elevated temperatures. The temperature increase is realized by steam injection, either in a heating screw or directly in the disperser. In the dispersion stage further ink de¬tachment as well as reduction of the size of dirt specks and stickies takes place.

In addition, if bleaching of the fibers is necessary to meet the brightness targets, the dispersion stage is the optimum dosing point for oxidative bleaching chem¬icals (e. g. peroxide), because of favorable consistency and high mixing efficiency (see Section 3.3). If bleaching chemicals are added, a high consistency bleaching tower for adequate retention time at 30 % consistency is installed after dispersion.

All the above described process stages can be installed in the first water loop of a wood-containing DIP system. State of the art nowadays are DIP systems with two loops, where, in the second loop, the residual ink, as well as ash, stickies and other hydrophobics are removed by a second flotation stage. The stock is then thickened again in disk filters to a consistency of approximately 12 %. For the high quality demands of SC or LWC papers, it is dewatered once more to 30 % consistency, heated and sent to a second dispersion stage for further break down of stickies, printing inks and dirt specks to sizes under the visibility limit. Optional reductive bleaching is involved either after the disk filter or, if installed, after the second dispersion stage. Chemicals used for reductive bleaching are hydrosulfite/sodium-dithionite or FAS (see Section 3.3)

Finally the stock is pumped into the storage tower. As a summary Table 4.3 shows the effect of the process stages on the quality parameters

For high grade DIP, an additional fiber design stage, consisting of a minimum one low consistency refining stage is often implemented between the storage

4.3 Systems for Fiber Stock Preparation
tower and the approach flow system of the paper machine. This is to modify the fiber characteristics in order to improve the final paper quality, namely strength and surface properties. System for Woodfree Graphic Paper Grades and Market DIP
Woodfree recovered paper, such as MOW, CWF and/or UWF, is used as raw mate¬rial to produce woodfree DIP grades for tissue, woodfree graphic paper grades or market DIP as a chemical pulp substitute. Table 4.4 shows the raw material quality parameters and the quality demands on woodfree DIPs for named applications.
A typical fiber preparation plant for woodfree DIP production basically uses nearly the same process stages as a plant for wood-containing DIP (Fig. 4.58).

The main differences from the wood-containing process in Fig. 4.57 are found in the thickening stages. In order to be able to achieve the required low ash con¬tents of 2 % in tissue production, a washing stage is used instead of the otherwise standard disk filter in both loops for thickening. The washer filtrates contain a large amount of ash and fines. These solids are removed from the filtrate in a subsequent dissolved air flotation unit (DAF) and the ash is thereby removed from the process. Of course in these applications, the yield will be significantly lower than the wood-containing DIP. For woodfree graphic papers or as a chemical pulp substitute, the DIP should not have more than 5 % ash content and a maximum of 20–40 mm2 kg–1 stickies (according to TAPPI T277, measured in a Somerville lab¬oratory screen with 0.15 mm slotted screen plate) and 100 mm2 m–2 dirt count
Table 4.4 Quality parameters of European MOW/UWF/MWF mix and DIP for different woodfree graphical paper grades.
 Brightness  Dirt Count > 50 mm  Stickiesa  Ash content (575 °C)
 % ISO  mm2 m–2  mm2 kg–1  %
recovered paper  60–70  1000–5000  5000–20 000  15–25
(Mix MOW/UWF/CWF)    
tissue  80–90  < 150  < 200  < 2
market DIP as substitute  80–90  < 100  < 20–40  < 5
of chemical pulp    

a According to TAPPI T277 (measurement with Somerville laboratory screen with 0.15 mm slotted screen plate).
(> 50 mm). This is achieved by installing at least one washing and a second disper¬sion stage [1–5]. Systems for Packaging Paper and Board Grades
For packaging paper and board grades, recovered paper is the main fiber source with a share of about 60 % worldwide. In Europe most of the mills produce packag¬ing papers based on 100 % recycled fibers. The main recovered paper qualities for the packaging grades are mixed recovered paper from households and supermar¬kets, the latter mainly being old corrugated containers (OCC). The recovered paper qualities for packaging grades contain much more debris than “white” recovered paper grades.

The recovered paper quality in general, and especially for the “brown” grades, tends to steadily decrease. At the same time the production costs, the stock quality demands concerning the paper machine runnability as well as the final product properties put high demands on the fiber preparation systems. This results in these more sophisticated than they used to be in earlier times. The major ob¬jectives in systems for packaging grades are cleanliness, strength characteristics and high yield. The importance of optical characteristics is steadily increasing as packaging material is often printed.

Many of the packaging paper and board grades are multilayer products, meaning that they consist of different layers using different fiber types, virgin and/or re¬cycled. Therefore different individual fiber preparation systems are often needed. In the case of 100 % recycled fiber based production with multilayer or multi-ply paper machine forming sections one completely separate fiber preparation plant is installed for each layer. Or, as is also state of the art, different recycled fiber quali¬ties are produced, starting with one fiber preparation line and then separating the stock flow into lines of different qualities by fractionation. These fractions can then be used either separately for each layer on the paper machine or they can be remixed in desired proportions [1, 2].

4.3 Systems for Fiber Stock Preparation
Fig. 4.59 shows a state-of-the-art fiber preparation system for packaging grades without fractionation. The recovered paper, delivered in bales, is put on a conveyor belt where the wires are cut, but not removed, and sent to a LC pulper working at a consistency of 4.5–5.5 %. In the pulper wires, strings and other spinning con¬taminants are removed by the ragger. As various other types of contaminants accu¬mulate in the pulper, an efficient detrashing system is one of the key components of a fiber preparation plant for packaging paper. The slushed stock, after high consistency cleaning, can still contain up to 20 % flakes. It is fed into a dump chest with a certain retention time, to assist the defibration of the flakes further down in the process.

After the dump chest the stock is treated in a hole screening stage at a con¬sistency of approximately 3.5 %. The design of this stage depends to a high degree on the flake content of the stock. For high flake contents, disk screens (hole sizes
2.4 mm) are recommended, at least in the second stage, as they are more robust and they have a distinctly higher deflaking potential than cylindrical screens (hole sizes 1.6 mm). The first and second stages are fed forward.

After hole screening, the flake content should not exceed 4 % to ensure a good runnability of the following cleaning and fine screening stages. Heavyweight cleaning is done after dilution of the stock and is followed by the fine screening with slot widths of 0.20–0.15 mm. Here the major part of the stickies and other contaminants is removed from the process. After that, the stock is thickened in a disk filter for separation of the water circuits of the fiber preparation and the paper machine and to reduce the volume of the following storage tower.
In the case of a fiber preparation line with fractionation (Fig. 4.60), the stock after hole screening is fractionated by screening at slot widths of 0.20 to 0.15 mm. The accept fraction represents the so-called short fiber fraction and is treated fur¬ther by removing sand through heavyweight cleaning and thickening before stor¬age. The “rejected” fraction is called the long fiber fraction and is treated much more intensively than the short fibers. This fraction contains, besides the longer fibers, an increased amount of contaminants.

Therefore it needs to be treated, as a minimum, by a heavyweight cleaning stage and a fine screening stage. If the demand on optical cleanliness is very high, a dispersion stage is used for homog¬enization. If the recovered papers potentially contain waxes, as in American OCC grades, lightweight cleaners in both fractions as well as a dispersion unit in the long fiber fraction should be installed. For applications, where the recovered fibers still show a potential for strength development, refining of the long fiber fraction is also recommended [1, 2].

Systems for Broke Treatment
Broke is an important stock source and occurs on a continuous basis as trims from the wire as wet broke and from the winders as dry broke. It can also occasionally occur as reel slab-offs, in the finishing room or during breaks in the paper ma¬chine or coating equipment. Usually all broke is fed back to the process in the approach flow system. Broke treatment starts with slushing in different pulpers. These pulpers are installed under the machine and dimensioned according to the

4.3 Systems for Fiber Stock Preparation
paper machine width like couch pit pulpers for wet broke and size-press and reel pulpers for dry broke. Alternatively, they are installed separately like winder, finish¬ing room and broke roll pulpers for dry broke (see Section 4.2.2). The design of a broke treatment system depends on the requirements of the paper machine and can contain several stages like thickening, screening, cleaning and deflaking (see Section 4.2). Sufficient buffer capacity for the broke is also important because it should be fed back in controlled portions as it has different properties from fresh stock due to drying, chemical content or, in coated paper production, to high filler content. In specialty paper production, e. g. wet-strength papers or impregnated papers, broke has either to be further treated (for example by mechanical disper¬sion, increased temperature through steam injection or the use of special chem¬icals for repulping) or be taken out of the papermaking process. In colored paper production, dry broke has to be used immediately or, if this is not possible, stored until the same color is produced again [6].

Peripheral Systems in Secondary Fiber Preparation
In secondary fiber preparation, peripheral systems are very important for the runn¬ability of the whole plant as well as for cost minimization and environmental issues. Peripheral plant components are the reject system, sludge treatment and water handling (see Fig. 4.56–4.60). Coarse rejects from the pulping section, heavy particle separation and hole screening are dewatered to a dry content of approx¬imately 60 %. If thermal treatment is involved for these rejects, metal components have to be removed and the particle size adapted to the burning process by means of shredding.

Fine rejects and sludges from the flotation stages and the DAFs are also drained to approximately 60 % dry content and either sent to energy recovery or used in the concrete or brick industry [3]. Water handling is another key element as it affects – besides costs and environmental issues – various quality parameters of the fin¬ished product such as brightness, cleanliness or odor (see Chapters 5 and 10).

Process Engineering and Automation
Today, advanced process engineering and automation are – due to the complexity of the plants – basic requirements for fiber preparation systems. Engineering pro¬vides the right connection of all the described process stages by planning the process layout and selecting adequate pump, pipe and chest designs and sizes for low energy consumption and economic investment. State of the art is a nearly chest-free stock preparation system between pulper dump tower and storage tower, where fan pumps (like in the paper machine) have widely replaced chests.

The main field of automation in a fiber stock preparation system lies in the control of those operational parameters which are important for every stage in the process e. g. consistency, flow, pressure, level, power consumption and tempera¬ture. It also includes programs in the DCS for automated start-ups and shut-downs of the different subsystems or even the whole fiber preparation system. Advanced automation concepts ensure, in the case of production (i. e. oven-dry tonnage) changes, that each subsystem is simultaneously adjusted to the new production requirements. Here a production set point is entered by the operator or automat¬ically controlled as a function of paper machine production or the level of the storage tower. The production control value for each subsystem is then calculated, whereby the losses of the individual subsystems are taken into account [7].

Another advanced approach in automation for fiber preparation systems is the introduction of quality control systems. As an example, the operator chooses the desired brightness value of the final stock in the storage tower. So-called model predictive controls calculate the necessary bleaching chemicals according to the actual conditions measured ahead of a bleaching stage(s) to control the brightness on a feed-forward basis instead of the conventional feed-back control strategy. This reduces dead times significantly and leads to more constant quality as well as to reduced costs for bleaching. An additional cost-control module for multistage bleaching processes calculates the most quality- and cost-efficient dosage of bleaching chemicals for each individual bleaching stage.