Holik H. (ed.) Handbook of Paper and Board

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The fillings in a kneading disperger are much coarser than in a disk disperger. The

rotor/stator spacing is about 10 mm, the peripheral speed is 5–15 m/s. The design

may be 1-shaft or 2-shaft. Energy input for kneading dispergers is usually between

30 and 80 kW/t, in special cases up to 120 kW/t. They can be operated unheated at

normal process temperatures without noticeably decreasing freeness of the

stock.

Control of the transferred dispersing energy for the kneading disperger is by

adjusting the entering stock consistency. For the disk disperger energy control is

by adjusting the spacing of the rotor/stator elements.

Fig. 4.52 Kneading disperger with fillings (source: Voith).

Fig. 4.53 Disk disperger fillings (source: Voith).

4 Stock Preparation

192

The effects of the two disperger types in general are very similar with some differ-

ing tendencies:

• the disk disperger may be advantageous when good sticky and dirt speck reduc-

tion is required

• the kneading disperger is recommended when high porosity and bulk of the

finished product are a priority.

4.2.11

Mixing and Storing

Mixing has to ensure that all individual component flows entering a mixing appa-

ratus have been uniformly distributed in the exiting suspension flow. When mix-

ing is done in a chest its whole volume has to be agitated continuously. This

requires relatively high energy input.

Mixing of suspensions in chests is usually done at stock consistencies of 3 to

5%. The geometry of mixing chests should have an approximate 1:1 up to 1:1.6

diameter to height ratio for minimum energy demand and good mixing effect. The

suspension is agitated by a chest mixing propeller similar to a ship’s propeller as

shown in Fig. 4.54. Specific energy demand is 0.2 to 0.5 kW m

–3

, depending on the

size and geometry of the chest as well as on the stock type and consistency.

During storage of a suspension, demixing of water and fibers and other compo-

nents may occur at low and medium consistencies. This has to be avoided. Stirring

only a small part of the suspension at regular intervals at different positions in the

storing chest is sufficient to hinder or correct separation and reflocculation. Thus

the energy demand is limited.

Vertically suspended stirrer shafts with pairs of propeller blades at several levels

of the shaft are used to prevent demixing. In larger diameter storing chests the

shaft rotates epicyclically. The kind of equipment in Fig. 4.55 is applied at smaller

and medium storage volumes. The energy demand is about 0.02 to 0.1 kW m

–3

Fig. 4.54 Chest mixing propeller with

horizontal axis (source: Voith).

4.2 Main Unit Processes and Equipment

193

For large storage volumes towers are used with reduced diameters at the bottom.

Only this lower part is agitated by a mixing propeller. In this case stock can also be

stored at higher consistencies, even above 10%.

In storing high consistency stock of about 10 to 15%, the crucial requirement is

the continuous removal of the stock. For that the stock is diluted close to the outlet.

Here demixing is usually not a problem.

4.3

Systems for Fiber Stock Preparation

Andrea Stetter

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.

Fig. 4.55 Storage chest with a vertical stirrer shaft

to prevent demixing (source: Voith).

4 Stock Preparation

194

4.3.1

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

Fig. 4.56 Fiber preparation system for chemical pulp (source:

Voith).

4.3 Systems for Fiber Stock Preparation

195

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.

4.3.2

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.

4.3.2.1 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 effi-

ciency.

4 Stock Preparation196

4.3.2.1.1 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 3.6.7.2).

Table 4.2 Quality parameters of European ONP/OMG mix and

DIP for different wood-containing graphical paper grades.

Grade DIP

Brightness

DIP

Dirt count > 50 mm

DIP

Stickies

DIP

Ash content (575 °C)

% ISO mm

–2

/kg %

recovered paper

(Mix ONP/OMG)

45–48 1500–3000 4000–8000 20–25

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

(LWC)

68–72 50–100 < 100 9–13

According to TAPPI T277 (measurement with Somerville laboratory

screen with 0.15 mm slotted screen plate).

4.3 Systems for Fiber Stock Preparation 197

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 3.6.7.2).

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.

Fig. 4.57 Fiber preparation system for wood-containing

deinked pulp production (source: Voith).

4 Stock Preparation

198

Table 4.3 Effect of the process stages on quality parameters of wood-containing DIP.

Quality

parameters

Process stage

Brightness Dirt specks

content

Stickies

content

Ash content Debris content Fiber

design

pulping

coarse

screening

detachment

and dispersion

of smaller

printing ink

particles

detachment

and dispersion

of larger

printing ink

particles

saving

screen-ability

removal of

coarse

contaminants

HC cleaning removal of

heavy particles

MC hole

screening

reduction removal of flat

particles

IC cleaning removal of sand

and debris

IC screening reduction removal of

cubical/round

particles

flotation 1 increase by

removal of

printing inks

reduction reduction reduction

HW cleaning reduction removal of sand,

debris and dirt

specks

fine screening reduction reduction removal of

cubical/round

particles

washing/

thickening/

dewatering

increase by

removal of

printing inks

reduction

dispersion 1 reduction by

detachment

and dispersion

of smaller

printing ink

particles

particle size

reduction

particle size

reduction

flotation 2 increase by

removal of

printing inks

reduction reduction reduction

dispersion 2 reduction by

detachment

and dispersion

of smaller

printing ink

particles

particle size

reduction

particle size

reduction

4.3 Systems for Fiber Stock Preparation 199

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 disper-

sion.

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 [1–5].

For high grade DIP, an additional fiber design stage, consisting of a minimum

one low consistency refining stage is often implemented between the storage

Table 4.3 (continued).

Quality

parameters

Process stage

Brightness Dirt specks

content

Stickies

content

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

According to TAPPI T277. b Only, when filtrates are clarified.

4 Stock Preparation200

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.

4.3.2.1.2 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 mm

–1

stickies (according to TAPPI T277, measured in a Somerville lab-

oratory screen with 0.15 mm slotted screen plate) and 100 mm

–2

dirt count

Fig. 4.58 Fiber preparation system for woodfree deinked pulp

production (source: Voith).

4.3 Systems for Fiber Stock Preparation

201