SK Dispersion and Kneading

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8 CTP/PTS Advanced Training Course on Deinking                                                     Grenoble, May 29-30-31, 2007



1.  INTRODUCTION............................................................................................................ 3
  2.1. High Speed Dispersion............................................................................................ 5
    2.1.1. Principle ........................................................................................................... 5
    2.1.2. Theoretical approach........................................................................................ 7
    2.1.3. Miles and May theory applied to dispersing...................................................... 8
    2.1.4. CTP approach.................................................................................................. 9
  2.2. Low Speed Kneading .............................................................................................10
3. OPERATING CONDITIONS..........................................................................................11
4. DISPERSION ................................................................................................................12
  4.1. Asphalt ...................................................................................................................12
  4.2. Hot Melt Contaminants...........................................................................................12
    4.2.1. Waxed papers and boards ..............................................................................13
    4.2.2. Hot melt glues for book bindings and container sealing...................................13
  4.3. Stickies...................................................................................................................14
  4.4. Residual ink and specks.........................................................................................15
5. INK FRAGMENTATION AND DETACHMENT ..............................................................17
  5.1. Hot dispersion between 2 deinking stages .............................................................18
    5.1.1. Low speed kneader between 2 deinking stages ..............................................18
    5.1.2. High-speed disperser between 2 deinking stages ...........................................19
    5.1.3. Comparison of high-speed disperser and low speed kneader between 2
    deinking stages .............................................................................................................22
  5.2. Dispersion before deinking .....................................................................................23
  5.3. Two hot dispersion stages......................................................................................26
6. BLEACHING .................................................................................................................28
  6.1. Use of high-speed disperser as mixer ....................................................................28
  6.2. Bleaching in low-speed kneader and high-speed disperser ....................................28
    6.2.1. Peroxide bleaching..........................................................................................28
    6.2.2. Reducing bleaching.........................................................................................29
  6.3. Destroying catalase................................................................................................29
7. MICROBIOLOGICAL DECONTAMINATION .................................................................30
8. FIBRE PROPERTIES....................................................................................................31
  8.1. High-speed dispersers ...........................................................................................31
  8.2. Low-speed kneaders ..............................................................................................33
  8.3. Comparison of the effects of low-speed kneader and high-speed disperser on fibre
9. Summary.......................................................................................................................34
10.   References ................................................................................................................35

8 CTP/PTS Advanced Training Course on Deinking                              Grenoble, May 29-30-31, 2007

         1. INTRODUCTION
Early work on the dispersion of contaminants began in 1946 by a group of American paper mills with
the intent of recycling kraft bitumen paper. It led to the development of a number of processes. The
most common “thermo-mechanical” processes, or asphalt dispersion, consisted of high temperature
(150° C) treatment in a device such as a disk type refiner [1][2]. The pressurized disk refiner
processes have experienced considerable industrial development since the 1960's in contrast to the
chemical or purely thermal processes [3] which have remained largely unused.

In mills recycling packaging papers and boards, hot dispersion is used to disperse thermofusible
contaminants, such as waxes, hot melt and bitumen in order to avoid problems of spots (especially on
hot plates during the converting of liners) and sticking between sheets. Dispersion or/and kneading
equipment is found in most modern mills recycling packaging papers. It may be applied to the whole
pulp or only on the long fibre fraction. However, in some applications, due to the progress in fine slot
screening, the use of hot dispersion is questioned.

Hot dispersion in deinking plants has been utilized since 1978. Dispersion homogenizes the stock
[4][5], residual ink particles which have not been detached from the fibers (such as inks used for offset
newspapers) are so finely dispersed as to no longer appear as undesirable specks, i.e., to say that
smaller than 40-60 µm, not to be seen by the naked eye. Dispersion or kneading has also been
proposed to simultaneously disperse specks and as a high consistency mixer for bleaching the pulp
[6]. Several papers proposing a hot dispersion stage between two flotation stages [7][8][9], or between
a flotation stage and a washing stage [11] were presented during the 1989 EUCEPA symposium in
Ljubljana. Since the end of the 1980’s, dispersion has become a basic treatment in multi-loop deinking
processes. The high treatment temperatures also decrease the bacterial content of the pulp, leading to
decrease in microbes in the final pulp. Dispersion and/or kneading equipment is now included in all
modern deinking facilities.

The main applications of dispersion and kneading are as follow:

• Dispersion of hot-melt contaminants, stickies, specks, residual ink.
• Reduce dirt specks below the visibility limit & distribute them finely or make them floatable
• Break down stickies & distribute them finely or make them floatable.
• Distribute wax very finely.
• Treat fibre thermally to increase bulk.
• Detachment of ink prior to deinking, or removal of residual ink prior to post-deinking.
• Bleaching: thermal pretreatment, mixing chemicals or use as bleaching reactor.
• Microbiological decontamination: elevated temperatures in the presence of hydrogen peroxide
  destroy bacteria and fungi.
• Changes in fiber properties: depending on device and operating conditions.

8 CTP/PTS Advanced Training Course on Deinking                                           Grenoble, May 29-30-31, 2007

         2. PRINCIPLES  OF                            DISPERSION                    AND              KNEADING
Hot dispersion consists of mechanical treatment performed generally at high temperature; high
consistency and using appropriate techniques to transfer energy to the pulp. It must be however noted
here that dispersion does not remove contaminants associated with the incoming fibres (ink, toner
particle, stickes..); but what it does is to break down these large size particles due to high shearing
forces applied on the fibres & the contaminants, the result is fine dispersion of these non wanted
particles. It can be however possible that due to high level of forces applied on high consistency pulp,
the unwanted particle gets detached from the fibres and get removed in the subsequent processes.

Key components of a dispersing station are
thickener, the plug crew & the steam-
preheater to raise the temperature. The
dispersion system can normally be operated
in both atmospheric as well as pressurised
conditions, the latter offering advantages to
ink & sticky dispersion in some grades. Plug
screw facilitates the pulp flow to the
preheater and helps avoiding pressure drop
and steam losses. Fig. 1 provides a look at
the complete dispersing system with its
various sub-parts.

The table below shows the various
applications of dispersion & kneading
operations   are   linked   for property
enhancement or contaminant removal for                Fig. 1      Dispersion system with its full assembly consisting
white and brown paper grades.                                  of a dewatering screw, heating screw, & disperser.


                                                                                      Test Liner

                                                                                                              (top liner)
                                           SC paper




Task of dispersion
Dirt specks & stickies
Wax dispersion

Coating grits dispersion

Ink/toner detachment
Bleaching agents/ stock

Strength improvement
Bulk Increase
Tab. I            Dispersion requirement for paper grades produced from recycled fibres[10]

Two principal technologies are present for dispersing operations: high-speed dispersion and low
speed dispersion, also called low speed kneading. Machinery suppliers, with a few rare exceptions,
supply devices using one or the other technology. Consequently, these two technologies have often
been contrasted. A few papers make comparisons of the two technologies [10][12][13][14][15].

8 CTP/PTS Advanced Training Course on Deinking                                  Grenoble, May 29-30-31, 2007

      2.1. High Speed Dispersion

        2.1.1. Principle

High-speed dispersion corresponds to a very short mechanical treatment (less than one second)
applied to a small mass of fibrous material with a strong shearing effect due to a narrow disc gap and
high rotational speeds. Speeds range from 1000–3000 rpm, but are generally around 1200–1800 rpm.
The dispersion effect results mainly from the impact of contaminants or ink against the tackle surface.

High-speed dispersion (disc dispersers) has been developed through the technology transfer from
pulper-refiners for mechanical and thermal mechanical pulp (Asplund-Defibrator, Beloit, Krima-
Cellwood, Andritz-Sprout-Bauer, Kvaerner-Hymac, etc.) with bar discs, and by the adaptation of high
concentration deflakers with toothed discs (Voith-Sulzer, Krima-Cellwood). A high-speed disperser is
illustrated in Fig. 2.

The unit is comprised of stator and a rotor disc. Stock
is fed into the centre and due to centrifugal forces, the
pulp moves radially and comes into contact with stator      Thoothed fillings
and rotor edges. Contacts with rotor accelerate the
pulp travel while contacts with stator slows it down.
These impacts induce a velocity differential resulting
in shear forces and dispersion. Two types of plates
are commonly used:
     - pyramidal design (intermeshing toothed
         pattern). The pulp is forced radially through
         the small chanels created between the teeth
         on opposing plates,
     - refiner bar (fine or coarse bars). The pulp is          Fig. 2    High-speed disperser (Voith-Sulzer)
         forced through the high shear zone between
         rotor and stator plates

Some high-speed dispersers, (i.e., Krima-Cellwood or Voith-Sulzer) can be equipped with different
types of teeth or bars; others are designed to operate at medium consistency (15%). The use of
refiners operating at low consistency (5%) has also been proposed for the dispersion of specks [16].

Sunds Defibrator [17] has recently proposed a conical high-speed disperser. Compared to a
conventional flat disc high-speed disperser at a constant outer radius, the conical design is said to
provide a larger dispersion area. This increases the likelihood of an ink or sticky particle to
experiencing multiple impacts and fragmenting into smaller particles.

Metso paper [18] also manufacturers conical dispersers. They mention that the same conical disperser
can be utilized for viscous or mechanical fibre development by using different fillings (toothed bar
fillings or straight bar fillings)

According to Heimonen [19], conical high-speed dispersion technology in comparison to disc high-
speed disperser allows to have a pulp flow through the plates more uniform, i.e. the effect of
centrifugal force on the pulp flow is decreased and used for fibre treatment. Compared to the same
diameter disc, conical filling gives larger treatment area, which means that the dispersing energy can
be fed to the pulp more gently through a higher number of impacts into impurities. In multi-disc high-
speed disperser, the treatment area is increased but the control of plate gap, and hence the control of
dispersion uniformity is much more complex.

8 CTP/PTS Advanced Training Course on Deinking                              Grenoble, May 29-30-31, 2007

                                   Fig. 3   Principle of ConiDisc [20]

Aikawa [20] has developed a completely new type disperser called ‘ConiDisc Dipserser’ (Fig. 3). This
typical disperser has inner conical and outer disc combination fillings. The working area of the conical
part is much more larger than the disc plate
pattern. This results in longer retention times
in the working zone, hence increasing the
impacts between fibre and bar fillings. The
disc plate outside the conical part serves as a
valve to fill in the conical part with stock.

The adjustment of power consumption in
high-speed disperser can be achieved by gap
adjustment or by dilution at the inlet of the
disperser. Regarding gap adjustment, the
temperature of dispersing is one of the
parameter that needs to be taken into account
in terms of dispersing efficiency (Fig. 4).
Indeed, high temperature permits the
narrower filling gap for a given motor load. As
a result, the impurities receive strong
dispersion power and are dispersed to very
small pieces [20].                                 Fig. 4   Power consumption in Conical disperser as a
                                                             function of gap and temperature [20]

8 CTP/PTS Advanced Training Course on Deinking                                          Grenoble, May 29-30-31, 2007

           2.1.2. Theoretical approach
The easiest way to characterize mechanical treatments is to consider the specific energy consumption
(energy per unit of dry material expressed in kWh/T). This way has the advantage to give directly
economic consideration but it cannot be used directly to characterize the phenomena involved during
mechanical treatment. Several approaches coming from refining theory has been proposed to
describe the phenomena involved during high-speed dispersing treatment.

      •   Brecht approach
Brecht [21] developed a theory to characterize refining operation. It assumes that refining treatment
occurs at the edges of refiner bars with pulp response to refining being identical whatever the device
considered (size, type). He defined the average specific edge load as the effective power input per
total edge length per second. For barred plate patterns, the total edge length per second (LB) and the
average specific edge load (SELB) are given by the following equations:

For bar-                                                   LB        Total edge length per second (m/s)
                                                           w         Rotational speed (rpm)
              LB =    ⋅ Z RB ⋅ Z SB ⋅ l B                  ZRB       Number of bars on rotor
                   60                                      ZSB
                                               where                 Number of bars on stator
                      P − PNL
              SELB = A                                     lB        Average length of a bar (m)
                         LB                                SELB      Average specific edge load (Ws/m)
                                                           PA        Average total power (W)
                                                           PNL       No load power at given refiner speed (W)

Ruzinsky et al. in 2004 [22][23] applied the specific edge load theory to characterize the magnitude of
force involved during high-speed dispersing operations and the specific energy to measure the extent
of dispersion. It was assumed that the treatment (ink detachment, particle comminution, etc.) occurs
primarily at the edges of dispersing elements. It was also assumed that the magnitude of force applied
at the edge of high-speed disperser element is the critical factor determining ink detachment.

In the case of pyramidal plates, the total edge
was calculated by summing the lengths of the
leading (active) edges of the intermeshing
pyramids. To perform calculation, they
number the rows of pyramids beginning at the
centre of disperser as illustrated in Fig. 5. The
calculation assumes that the first rotor row
intermeshes between the first and second
rows on the stator. The pyramid dimensions
change with radial position due to the bevel of
the base plate, and is included in the
calculation. The total edge length per second
and the average specific edge load for                         Fig. 5 Diagram of rotor and stator plates used by
pyramidal plates are given by:                                  Ruzinsky et al. [22] – Numbers represent the position
                                                                     of pyramidal rows from the centre of the disc

For pyramidal-plates:                                     LP      Total edge length per second (m/s)
                                                          w       Rotational speed (rpm)
                                                          N RP    Number of pyramid in row i on the rotor
          w 
                         [(        ) (         )
                                                                  Number of pyramid in row i on the stator
            ⋅ ∑ N RP l PI N SP + l PO N SP1
                                         i+                 i
LP =                i   i    i      i                     N SP
          60  i =1                                        i      Average length in inner pyramidal edge in row i (m)
                                                   where l PI
                                                         l PO     Average length of outer pyramidal edge interacting
      P − PNL                                                     in row i
SELP = A                                                  n       Total number of rows of pyramid on a plate
         LP                                               SELP    Average specific edge load (Ws/m)
                                                          PA      Average total power (W)
                                                          PNL     No load power at given refiner speed (W)

8 CTP/PTS Advanced Training Course on Deinking                                              Grenoble, May 29-30-31, 2007

CTP high-speed disperser was characterized by this approach. A comparison with data reported by
Ruzinsky et al. is given in the following table.

                                                                                                      SEL               LB
Disperser type               Plate pattern                   Source                   energy
                                                                                      KWh/T           Ws/m        m/s
Lab refiner                  Pyramidal plates                Ruzinsky et al. [22]      6-36         0.03-0.27 3 900-5 600
Lab refiner                  Barred plates                   Ruzinsky et al. [24]     50-280        0.02-0.20    49 000
                             Toothed plates                  McCarthy [13]            40-100         9-14 *             -
Industrial                                                   Ruzinsky et al. [22]
                             Toothed plates                                            35-63            -               -
disperser                                                    **
                             CTP       plates        (from                                                         5 500-10
CTP disperser                                                Calculated               40-220        0.26-0.92
                             Bonpertuis)                                                                              200
*: estimated value by Ruzinsky et al. [22] based on units usually operating at Cm between 25 and 30% with a temperature
between 77 and 90° and rotor speed between 1200 an d 1888 rpm
**: dispersers used for SOW/MOW or ONP/OMG
           Tab. II                 Data related to various high-speed disperser type according to Brecht approach

               2.1.3. Miles and May theory applied to dispersing

In 2001, Ruzinsky et al. [24] characterized high-speed disperser by applying the theory of Miles and
May [25][26] that allows to distinguish the number of bar impacts and the specific energy per bar
impact imposed during the high-speed dispersing stage. This theory calculates the radial velocity of
pulp moving through the refiner allowing the residence time and the number of bar impacts imposed to
be calculated. For their test conditions, the following equations were used:

For bar-plates:                                                E       Specific energy consumption (J/kg)
E = n⋅e                                                        n       Number of impacts imparted to a fibre
                                                               e       Specific energy per impact (J/kg/impact)
                N ⋅h⋅a⋅ E ⋅c         r 
n=    r
           ⋅                     ⋅ ln 2                        r     Radial coefficient of friction
      t        2 ⋅ w ⋅ (r2 − r1 )  r1 
                                              where            t     Tangential coefficient of friction
                                                               N       Average number of bars per unit length of arc
                2 ⋅ w ⋅ (r2 − r1 )
                                                               h       Number of rotating discs in refiner
e=    t
                                 r                           a       Constant of friction
               N ⋅ h ⋅ a ⋅ c ⋅ ln 2 
                                 r                           r2      Outer radius of dispersing zone (m)
                                  1                          r1      Inner radius of dispersing zone (m)
                                                               w       Rotational speed (rad/s)

As mentioned by Ruzinsky et al. [22] in 2004, the Miles and May theory was developed for bar-plates
and cannot be used for pyramidal plates in its present form.

Comparison between refining and dispersing is also reported in the following table in terms of specific
energy consumption, number of impact and specific energy per impact.

                                                                       E (MJ/kg)      n (impacts)       e (J/kg.impact)
Laboratory refiner (Sprout Waldrom 12") [24]                           0.17-1.31       300-3600               250-610
Sprout-Bauer 36-1CP Refiner [26]                                           4.3        8100-16700              260-530
Typical disperser [13]                                                 0.14-0.36          n.d.                  n.d.
          Tab. III                Comparison of operating parameters for refiners and high-speed dispersers [24]

8 CTP/PTS Advanced Training Course on Deinking                                        Grenoble, May 29-30-31, 2007

            2.1.4. CTP approach

Another approach consists in considering volume energy consumption. This idea has been
successfully applied to have overall and composite information for pulping phenomena [27]: the
knowledge of volume energy consumption allows determining defibering level whatever the pulper
device (LC pulper, Helico pulper or drum pulper), the consistency and the pulping time.

Based on this idea, the following equations can be written:
                                                                      Em      Specific energy consumption (kWh/T)
      E                                        E                      E       Energy consumption (kWh)
Em =                         and         Ev =          where          Mpulp   Mass of o.d. pulp treated (T)
     M pulp                                   Vtot                    Ev      Volume energy (kWh/m )
                                                                      Vtot    Total volume of pulp treated (m )
By combining the two equations,
     M pulp
Ev =        ⋅ Em
If we suppose that the density of pulp suspension corresponds to water density (ρ=1000 kg/m =1
T/m ), then
            1                                                 Em         Specific energy consumption (kWh/T)
Vtot =             ⋅ M tot                                    Cm         Mass consistency expressed as a fraction (-)
         ρ water                                              Mpulp      Mass of o.d. pulp treated (T)
         M pulp              M pulp                     where Ev         Volume energy (kWh/m )

Ev =               ⋅ Em =             ⋅ Em = Cm ⋅ Em          Vtot                                      3
                                                                         Total volume of treated pulp (m )
          Vtot               M tot
                                                                         Water density (1000 kg/m )

Note that the expression of this estimated volume energy is present in the equation given by Miles and
May to describe the number of impact imparted to a fibre. Besides, the other parameters are constant
in the present study. In other terms, the approach of Miles and May can be summarized by the
estimated volume approach.

The different phenomena that are controlled during the dispersing stage can be viewed as 'solid'
fragmentation. The fragmentation phenomena can then be described by two main parameters:
    - The forces involved during pulping. The overall forces could be described by energy
        consideration even if we are not able to determine the energy applied to solid particles [28]. By
        using energy consideration, it should be possible to take into account both the intensity of the
        mechanical forces and the retention time inside the disperser.
    - The 'solid' particle strength. Cohesive forces that linked the 'solid' particles each other could
        describe it. The cohesive forces could be affected by external parameters such as
        temperature or physico-chemical parameters.

Application of this approach will be given in p. 20. [29]

8 CTP/PTS Advanced Training Course on Deinking                                                  Grenoble, May 29-30-31, 2007

          2.2. Low Speed Kneading
Low speed kneading imparts a rather prolonged mechanical treatment (some minutes) to a large mass
of fibrous material with a moderate shearing effect. This action is related to the relatively wide interbar
clearance and slow rotation. Rotational speeds are normally 100–200 rpm, with a few exceptions at
higher speed. The dispersion effect results mainly from fibre-to-fibre friction (rubbing action). In
general, low-speed kneaders are devices in which the pulp is fed in by a screw and held by a
discharge door as it is transferred under pressure between rows of fingers on a shaft and others on
the stator wall.

Kneading technology for low speed dispersion consists of equipment with a single–shaft (Erwepa,
Voith-Sulzer, Lamort-Fiberprep, Maule), or two shafts (Shinhama, Modomekan-Ahlstrom-Kamyr),
specially developed for hot dispersion or resulting from technology transfer from stock mixers (Micar
Black-Clawson). A typical single shaft kneader is illustrated in Fig. 6.
Various designs are used for double shaft kneaders.

                                     A) Inlet of raw stock
      A                              B) Shaft with feed screw
                                     and rotating kneading cogs
                                     C) Body with stationary cogs



      D) Steam inlet
      E) Air-operating trap
      F) Outlet of processed stock                                  F

 Fig. 6         Single shaft low-speed kneader (Kādant-                 Fig. 7   Double shaft low-speed kneader (Shinhama)

The Shinhama low-speed kneader, illustrated in Fig. 7, has two counter-rotating shafts, one turning at
95 rpm, and the other rotating slightly faster at 110 rpm [30]. The device previously called
"Frotapulper" (Modemekan-Kamyr), and currently referred to as "MDR Kneader," (Alhlstrom-Kamyr)
looks like a low-speed kneader, but the two screws counter-rotate synchronously at a speed of 900-
1800 rpm (in the range of the of high-speed dispersers). The device called "Micar" (Black-Clawson)
runs at intermediate speed (400-500 rpm) [12].

The discharge door controls the volume of pulp in the device. Stock moves through the device at a
rather low speed. Differences in stock velocity are created inside the machine. The stock near the
rotor is moving at a higher velocity than the stock near the stator wall. In the double shaft kneader, the
rotors turn in opposite directions causing a shearing effect when the stock changes its direction of
movement). These differences in velocity induce the fiber-to-fiber friction and the dispersion effect.

A gentle kneader including dewatering equipment has been proposed [31] and is illustrated in Fig. 8. It
consits of 3 cylinders. At the first half of each cylinder, a screw boots the stock forward and at the
latter half, the stock proceeds through kneading blades. When the stock proceeds from one cylinder to
another, the rotation of shaft alters vice versa. In the third cylinder chemicals can be introduced. The 3
cylinders allow increasing the kneading time. Each cylinder is equipped with a single motor permitting
an independent control of each of them so that it is possible to adjust the unit to a variation of stock
and/or target quality of the pulp to be produced. In this kind of device, no steam introduction is
required as the temperature can reach spontaneous 50 to 90° due to the friction involved in the
kneading parts. The energy consumption is between 40 and 80 kWh/T. The main difference with the
other low-speed kneader is the presence of dewatering zone in the first and second cylinder. This

8 CTP/PTS Advanced Training Course on Deinking                             Grenoble, May 29-30-31, 2007

dewatering area permits a free water in the stock to be drained away that is claimed to remove by
washing the small particles generated by the kneading mechanical action. However, it must be
brought to the fore that the inlet consistency is 20% and the outlet about 30% inducing a "low washing
effect". The principle of this device is quite similar to the Bivis from Clextral.

                        Fig. 8   Gentle Kneading process from Taizen Co, Japan

The dispersion stage is implemented after a thickening stage. Various devices including the double
wire thickener and screw press are used to concentrate the stock.

A heating screw can be implemented between the thickening stage and the dispersion unit. It is
required in high-speed dispersion when steam is introduced to increase the pulp temperatures to the
appropriate level. Voith Paper also proposed a new type of high-speed disperser where the pulp is
directly heated in it [32]. In low speed kneading, steam can be introduced at the low-speed kneader
itself, and a heating screw is not generally required.

The dispersion consistency is generally 25-30%, with some devices operating at up to 35-40%.
Depending upon the pulp characteristics, some devices are designed to operate at medium

The running temperature depends on the requirements of the dispersion and varies within a large
range. Some low-speed kneaders are operated without any steam input for deinking applications. In
this application, the increase of temperature (up to 40-60 ° is related to mechanical energy
dissipation. Most applications use steam to raise the dispersion temperature to approximately 90°
Some low-speed kneaders and high-speed dispersers are designed as pressurized units, which can

8 CTP/PTS Advanced Training Course on Deinking                               Grenoble, May 29-30-31, 2007

operate at temperatures up to 150° C. These conditi ons were required in board mills for asphalt

For both types, the energy consumption is in the range of 35–100 kWh/t, with typical industrial values
in the range 60-80 kWh/t. Various techniques can be used to adjust the energy consumption
depending on the device used. These approaches include adjusting the pressure on the discharge
door, gap between rotor and stator, inlet consistency, etc.

In the deinking plant, dispersion can be combined with bleaching. Bleaching chemicals can be
introduced at the heating screw or at the inlet of the dispersion unit, which can be used as mixer or
bleaching reactor. Depending on the operating temperature and the device used, retention time may
or not be required after the dispersion unit.

Generally the pulp discharged from a low-speed kneader does not require dilution and bleaching is
conducted at the kneading consistency. The pulp can be discharged at operating consistency in some
high-speed dispersers but generally a dilution is required for discharge. In this case storage for
bleaching extension is not possible.

Various types of contaminants can be dispersed by using high-speed dispersers or low-speed
kneaders. These include bitumen, waxes, stickies, specks, residual ink, and hot melt contaminants
such as bookbinding or container sealing glues. Dispersion of wet–strength paper is also reported

It must be brought to the fore that dispersing treatment induces fragmentation of the particles so that
they cannot be seen. However, even if the pulp seems to be cleaner, these contaminants are still
present in the pulp but they are not visible. If they are not removed during the next step of the process,
some problems could appear in some cases during papermaking and during the final use of the

      4.1. Asphalt
Dispersion of asphalt was the first use of dispersion systems. Asphalt dispersion requires high
temperature, therefore a pressurized high-speed disperser should be used in order to perform the
dispersion at approximately 150° C. These processes have been developed for a long time in North
America [1][2].

Precautions must be taken in order to avoid excessive losses in mechanical properties due to the high
temperature. For recovered paper mixtures containing 2% of bitumen-containing paper, it has been
suggested, that the pulp is preheated up to 85° C a nd processed through a frotapulper at 120-130
kWh/t [33].

Some drawbacks regarding problems of picking of asphalt on the wire and dryers and the relative cost
of the treatment have been reported and alternative treatments including fine slotted screening and
reverse cleaning have been proposed [34].

      4.2. Hot Melt Contaminants
Hot dispersion has been used for control of hot melt contaminants. The temperature rise decreases
the viscosity of hot melt products and their internal cohesion, therefore the dispersion under shear
force becomes easier. The required temperature for dispersion depends of the type of material.
Bitumen requires a very high temperature (pressurized devices should be used), hot melt adhesives

8 CTP/PTS Advanced Training Course on Deinking                                                  Grenoble, May 29-30-31, 2007

utilized for bookbinding or package sealing have lower softening temperature and can be dispersed by
non-pressurized devices . Wax has an even lower softening temperature and is easily dispersed.

          4.2.1. Waxed papers and boards

The recyclability of waxed papers is being questioned. The inclusion of waxed papers and boards in
the recycled pulp leads to the production of paper containing a considerable number of translucent

A group of industrialists including papermakers, suppliers of wax, and converters has asked the
Centre Technique du Papier (CTP) to develop or adapt technologies for recycling the various grades
of papers and boards treated with conventional waxes. The main objective of the project was to
develop a process to recycle waxed papers without downgrading. All grades were considered,
including white waxed papers and brown waxed board containers. The main conclusions were
published in a previous paper [36].
In recycling plants using a hot dispersion                     BURST INDEX
stage, a spot-free pulp can be produced      kPa.m²/g
when recycling waxed OCC, however,             4

hot dispersion has a negative effect on      3.5
physical properties. An additional light
post refining treatment can restore the        3

initial level of strength properties, but if 2.5
the same energy is applied to a pulp
containing no waxed OCC, better results        2

can be achieved (Fig. 9).                    1.5
                                                                    Chest        Screw press        Kneader      Post refining
Hot dispersion of waxed OCC also                                                         %waxed OCC
promotes strong hydrophobicity in the
                                                                                     0     2   12     100
pulp fibers, so that a dramatic increase in
the “drop test” is observed. Post refining                                    C,
                                                               Kneading at 95 ° 100 kWh/t after coarse screening and
does not alter this aspect.                                                         thickening
                                               Fig. 9 Burst index after various treatments of pulps
White waxed papers (which are generally           produced by repulping mixtures containing various
wet strength papers) can be recycled in a                     amounts of waxed OCC.
conventional flotation deinking plant
provided the pulping conditions are
modified. A recycled pulp with characteristics similar to that of wood-free deinked pulp can be

          4.2.2. Hot melt glues for book bindings and container sealing

The small hot melt glue particles which have been                                 Dispersion consistency (10-14 %). Power
disintegrated and not removed by cleaning and screening                                  consumption 54-71 kWh/T
can be dispersed in non-pressurized low-speed kneaders                            Dispersion temperature        Hot melt speck
or high-speed dispersers working at temperature close to                               (inlet/outlet)             reduction
100° C. The visual aspect of the paper is improved, and
                                                                                           25/32 °                   21.6 %
the translucent specks disappear after dispersion.
                                                                                           25/68 °                    43 %
Results are reported in Tab. IV regarding the dispersion                                         C
                                                                                           55/90 °                    92 %
temperature required for hot melt and wax introduced in a
                                                                                  Tab. IV Effect of Temperature on Hot-
stock from clean corrugated clippings in the medium
                                                                                              Melt Dispersion [35]
consistency in the Black Clawson low-speed kneader [35].

  Notice that new generation hot melt glues proposed and starting to be used for bookbinding have higher softening
temperature. They are designed to be resistant to the shearing forces in the pulper and to be completely removed by coarse
screening, they will be probably more difficult to disperse, but due to the high screening removal ability this characteristic is no
more required.

8 CTP/PTS Advanced Training Course on Deinking                                 Grenoble, May 29-30-31, 2007

      4.3. Stickies

For more efficient dispersion of stickies, high temperatures                  Data set Data set    Data set
are suggested by American equipment suppliers [37].                              1        2           3
Trials performed with a pressurized high-speed disperser               C
                                                                     90°        93.4      90.7       92.1
(deflaker type) gave the results shown in Tab. V.
                                                                    105°        95.8      96.1       98.0
Mannes has operated a high-speed disperser and low-                C
                                                                120°      95.4     96.1        97.5
speed kneader in parallel in their pilot plant to determine     Tab. V Percent reduction in stickies
the ability of the two machines to disperse stickies in                           [37]
industrial stocks [38]. Both machines are able to achieve
reduction in stickies. Stickies area was reduced by 65-90%
with a high-speed disperser. Results obtained with the low-speed kneader were lower and varied over
a wider range. The recommendation was: “Effective and reliable stickies treatment is only possible
with high-speed dispersers.”

The incidence of retention time inside high-
speed disperser through changes in feed flow
has been reported by Kanazwa and Fujita
[20]: the lower the productivity (i.e. the higher
the retention time in the high-speed
disperser), the higher the increase in stickies
fragmentation (increase in very small stickies
number as reported in Fig. 10). In the same
study, they reported the effect of dispersing
temperature: the higher the dispersing
temperature, the narrower filling gap for a
given power load and the better the dispersing
effect on stickies due to stronger power
imparted to the particles.

When comparing the low-speed kneader and
the high-speed disperser in a carton board
mill, Stemmer [39] reported that high-speed         Fig. 10 Incidence of retention time through productivity
disperser    induces    higher    cuts   of                 changes on micro-stickies generation [20]

High consistencies and high temperatures are recommended for effective dispersion. The
effectiveness of dispersion increases with the circumferential speed. Speeds of 50-60 m/s represent
the optimum level.

In the framework of an EU project (FOREST 1991-1993) entitled “Use of Surface and Rheological
Properties in Stickies Removal and Control”, conducted with Pira (UK), PTS (Germany) and TNO (The
Netherlands), CTP was responsible for studying the changes in the shape of stickies in order to
improve their removal efficiency [40][41]. Various model stickies were used in the study: acrylic and
styrene-butadiene rubber (SBR) labels and acrylic tapes. The trials were performed in the CTP pilot
plant facilities, which features a Lamort low-speed kneader.

Kneading induces fragmentation of the stickies in a large particle size range. The fragmentation
increases dramatically when kneading temperature is increased from 60° to 90° C. Increasing the
specific energy also increases stickies fragmentation, but the effect is lower.

8 CTP/PTS Advanced Training Course on Deinking                                   Grenoble, May 29-30-31, 2007

Kneading modifies the shape of the stickies            RESIDUAL STICKIES AREA (mm²/g)
causing them to become more spherical.            30
The shape modification is particularly                        S1 : Screening 1              K : Kneading
                                                              S2 : Screening 2              CL : Cleaning
significant (including small particles) when      25
                                                         S1                       S1        FL : Flotation
kneading is performed at high temperature.
This change in shape increases the removal        20                                   S1
                                                                 S1                    +
efficiency by screening, but also increases                   S1 +                     K
                                                  15                                        +
cleaning and flotation effectiveness due to                   + CL                          K
                                                                        S1                       S1
the reduction in stickies size (Fig. 11).                     S2
                                                                         +                  +         S1
                                                  10                                              +    +
                                                                        FL                  S2   K
The final recommendations of this study                                                               K
                                                                                                  +    +
were to implement screening, but also              5                                             CL   FL
flotation and centrifugal cleaning after
kneading in deinking plants including a post-      0
deinking stage in order to enhance stickies         Fig. 11 Effect of kneading on the removal of stickies
removal.                                                  (from acrylic tapes) by screening, cleaning and
                                                                           flotation [41]

The first observation dealing with change in shape of contaminants by kneading was reported in 1976
based on experience with a frotapulper: “plastic is not defibrated but rolled into spiral fashioned
shapes which can be later removed by flat screening...” [42]. Similar observations (stickies particles
observed to roll up into balls rather than be fragmented) have also been reported when using a twin-
shaft kneader operating without steam [43][44]. Industrial experiences have also been reported. They
confirmed that the shape of the stickies changes from amorphous masses to spherical particles. It was
also observed that ink can become a part of the sticky balls. The average stickies particle size after
kneading is significantly reduced while their number is dramatically increased. It is considered
(contradictory to the CTP observations reported previously) that screens and flotation are not efficient
in the removal of these micro stickies [45].
We can try to explain these different behaviors of stickies when subjected to the action of a high-
speed disperser or low-speed kneader. The dispersion is the consequence of stretching the particles
beyond a level inducing break-up; it becomes easier due to the increase in temperature, which causes
softening of the stickies particle and reduction of its internal cohesion. The viscoelasticity of the
stickies particle is the key point. The low-speed kneader induces more rubbing action and its rather
low speed stretches the sticky, allowing the particle to conform and become more spherical. In
contrast, the high-speed disperser produces more impacts and quickly stretches the particle beyond
its breaking point leading to break-up of the particle and higher dispersion.

      4.4. Residual ink and specks
The use of hot dispersion was proposed in the 1970's in deinking plant to improve the visual aspects
of deinked pulp produced from offset print on uncoated paper (i.e., offset newspaper printed with ink
containing a high amount of self-setting binder). Hot dispersion was implemented at the end of the
process in order to reduce the size of large ink particles remaining attached to the fibres which
produced a mottled appearance [4][5][46].

The drawback of this application was a brightness loss, and therefore, post-deinking stages have been
proposed. They will be discussed in the next section.

Specks are black or colored visible particles. In deinked pulp they appear when ink has not been
broken up by pulping into particles small enough to be undetectable to the naked eye. Different values
of this size limit are from 40 to 60 microns and more, depending on the contrast.

The main origins of specks (also called dirts) are [47][48]:
   • Recovered paper from household collection may contain varnished printed papers (mainly
       with UV-cured varnishes). High gloss covers of magazines are the main source of UV
       varnishes and specks.
   • Recovered office paper contains toner printed papers (laser printed papers or photocopies).
   • Very old offset inks, heat set offset on SC papers, etc…

8 CTP/PTS Advanced Training Course on Deinking                                  Grenoble, May 29-30-31, 2007

High-quality print on coated papers with inks hardened by drying can also cause specks, which result
from binding between the pigment and the coating material. Recovered papers, which produce these
kinds of specks, are also glossy magazines. Papers printed by unconventional processes such as UV-
cured inks [49] or some grades of modern digital print also induce speck formation.

A large recycling study of papers focusing on speckled deinked pulp and UV-varnished papers
particularly, has been performed in the Centre Technique du Papier [50].

First, the specks from various origins have been     99.9        Kneading efficiency (%)
compared regarding their difficulty to be
dispersed by kneading. Trials were performed                        Resistant coating
at the CTP deinking pilot plant with a low speed       99
kneader, which is a hot dispersion unit (TL0                                          UV varnish
from Lamort, 20 kg/hr). Results from resistant
coated papers and UV varnished printed papers          90
are reported in Fig. 12 [50].
                                                                                  Energy consumption (kWh/t)
Low energy is sufficient for dispersion of specks       0
from resistant coated papers. With treatment at            40     60    80 100 120 140 160 180
current industrial energy consumption (60            Pulp from unprinted wastepaper contaminated with 4
kWh/t) the pulp looks clean to the naked eye.        % of a pulp produced by pulping UV varnished printed
Higher energy consumption is necessary for           papers or 10 % of a pulp produced from water resistant
                                                     coated papers (offset printing). Influence of energy
dispersal of specks from UV varnished printed
                                                     consumption. Pilot plant trials (pulp consistency 32 %,
papers. Good dispersion can be obtained at           temperature 90° C). Efficiency is calculated as spec k
high temperature and with high energy                area reduction (specks larger than 100 µm).
consumption in a slow speed kneader.
                                                          Fig. 12 Hot dispersion of specks in a low speed
                                                                            kneader [50].
The work has been focused on UV-varnished papers; kneading and dispersing have been investigated
as part of this study. Trials have been performed in pilot plants of machinery suppliers in order to
optimize the dispersion of visible specks. Pulp for trials was produced at CTP by blending a pulp from
unprinted papers with 2% contaminated pulp. Contaminated pulp contained UV-varnished printed
papers pulped for 20 min at 14% consistency and 50° C with deinking chemicals. Fig. 13 to Fig. 15
show results from treatment of clean pulp mixed with 2% UV-varnish contaminated pulp (Unprinted
wastepaper including 2 % of UV varnished printed papers. Particles larger than 100 µm were considered.
Efficiency is calculated as speck area reduction).

Both low-speed kneaders and high-speed dispersers are efficient in reducing the size of large specks
as shown Fig. 13 [51].

Influence of energy consumption, pulp                99.9        Dispersion efficiency (%)
consistency and temperature on results in trials
performed with an industrial low-speed kneader                       Kneading
are shown in Fig. 14. Increasing pulp                  99
consistency and temperature improves the
dispersion efficiency of specks from UV                                                    Dispersing
varnishes. Fig. 15 shows the results of trials         90
performed with a high-speed disperser for
dispersion of specks from UV varnishes [52].                                      Energy consumption (kWh/t)
Dispersion is more efficient at high temperature        0
and, according to the machinery supplier, the               40      60    80    100     120   140   160    180
implementation of a post deinking stage is
                                                     CTP pilot plant trials on industrial devices. Specks from
recommended for removal of speck particles,
                                                       UV varnishes, only specks larger than 300µm are
which have been broken in smaller, floatable                                 considered.
                                                            Fig. 13 Dispersion efficiency versus energy
                                                                          consumption [51]

8 CTP/PTS Advanced Training Course on Deinking                                     Grenoble, May 29-30-31, 2007

99.9       Kneading efficiency (%)                       99.9       Dispersion efficiency (%)
                 90° 37 %
  99                                                      99
                                60° 37 %                                                                (95°C)
  90                                                      90
                                60 and 90°C, 29 %                                                    (65°C)
                            Energy consumption (kWh/t)                                Energy consumption (kWh/t)
      0                                                    0
         40     60    80   100 120 140 160 180                40    60     80   100 120 140 160 180
      Influence    of   energy   consumption, pulp          Influence   of    energy    consumption and
      consistency and temperature                           temperature. Pulp consistency 30%.
 Fig. 14 Hot dispersion efficiency of specks from UV     Fig. 15 Hot dispersion of specks from UV varnishes
         varnishes in a low-speed kneader [52]                       in a high-speed disperser [52]

In Fig. 14 and Fig. 15 only particles larger than 100 µm were considered. Efficiency is calculated as
speck area reduction.

Voith-Sulzer is supplying both high-speed dispersers and low speed kneaders. They have compared
the dispersion of specks for various raw materials [53]. Both devices have advantages and their
efficiencies increase with specific energy consumption. The efficiencies of the two machines also
increase slightly as temperature increases. With a high-speed disperser, better dispersion of specks
(in a deinked pulp after a single flotation stage) from mixtures ONP/OMG is achieved, while the low
speed kneader presents advantages with laser inks and UV varnishes.

Dispersion of specks by using a refiner operating at low consistency has been proposed. With energy
consumption of 70 kWh/t, 50% of the dirts larger than 50 µm are dispersed. Larger particles (>200 µm)
are more efficiently dispersed (70%), and dispersion efficiency increases with increasing energy input


The dispersion of ink particles to improve the visual           Number of ink particles / mg pulp
aspects of deinked pulp brings about a loss of                  7 000
brightness. Speck size is reduced by hot                        6 000                            before kneading
dispersion, but smaller ink particles (not visible by           5 000
the naked eye) are also broken up. The increase in                                               after kneading
                                                                4 000
the number of small ink particles is responsible for
the brightness loss. Data regarding reduction of ink            3 000
particle size by kneading are reported in Fig. 16.              2 000
                                                                1 000
This brightness loss can be very important if                       0
kneading is performed (without chemical) on a                       1.0 1.4 2.0 2.8 4.0 5.6 8.0 11 16 23 32
poorly deinked pulp [11]. Brightness losses of                         Average size of ink particles (µm)
deinked pulp containing various amounts of               Fig. 16 Reduction of ink particle size by kneading
residual ink are reported in Fig. 17. It can be seen           (industrial plant, 70 kWh/t, 35 %, woodfree
that only a part of the brightness loss can be                              deinked pulp) [54]
recovered by a hyperwashing stage, indicating that
a portion of the dispersed ink has been irreversibly redeposited onto the fibers.

8 CTP/PTS Advanced Training Course on Deinking                                                                             Grenoble, May 29-30-31, 2007

      Brightness (%)                  ENTIRE PULP                                        Brightness (%)            HYPERWASHED PULP
                                                                            - 1.2   88
            Before kneading   After kneading                                                    Before kneading      After kneading
76                                                            - 3.5                                                                                       - 0.4
72                                             - 4.7
                              - 4.5                                                                                                             - 0.6
68                                                                                                                                - 1.7

64                                                                                  82
                                                                                                                  - 3.8
              - 7.8                                                                 80           - 4.2

52                                                                                  78
                           Increased amount of residual ink                                                  Increased amount of residual ink

      Furnish: woodfree printed paper, conventional deinking (pulping 14 %, with soda, sodium silicate, hydrogen
      peroxide and soap, flotation). Same pulp deinked by using 0, 1, 2, 3 and 4 aeration stages.
                      Fig. 17 Comparison of brightness loss of poorly deinked pulp and well deinked pulp [11]

           5.1. Hot dispersion between 2 deinking stages
The efficient detachment of residual ink by high-speed dispersers and low speed kneaders has led to
the development of multi-loop deinking processes. Although some mills [55][56] are operating post-
flotation after a dispersion stage without chemical and produce a DIP for newsprint which meets the
brightness requirement, the advantages of combining hot dispersion and peroxide bleaching have
been widely described and higher brightness is attained.

              5.1.1. Low speed kneader between 2 deinking stages

CTP has cooperated with a deinking mill to develop a low investment cost, two loop deinking system
that is still in operation [11][54]. The basic concept is that a peroxide bleaching stage restores the
brightness loss caused by kneading. Post-deinking is performed by washing (and flotation of the wash
water which is reused for the forward dilution of the washed pulp). The alkaline conditions after
bleaching are suitable for bleaching.
The brightness gain obtained by                Brightness (%)   Before washing After washing
kneading and peroxide bleaching is
higher than the sum of the                  84

brightness loss during kneading and         82

brightness gained by washing the            80
bleached (and unkneaded pulp).              78
These results are reported in Fig. 18       76
[57].                                       74
Additional trials have compared                                       70
various operating conditions and                                             deinked pulp         kneaded pulp            bleached pulp          kneaded and
concluded that the best approach is                                                                                       (no kneading)         bleached pulp
to introduce hydrogen peroxide at
the inlet of the low-speed kneader
[58][59].                                                                  Conventional alkaline deinking of woodfree wastepaper,
                                                                           washing by inclined screw press, pH 10, inlet consistency
A lot of results demonstrating the
benefit of using hot dispersion in      Fig. 18 Brightness changes by kneading, bleaching and washing
combination with peroxide bleaching                             post deinking [57]
before post flotation have been
published during the past ten years.
It is not possible to cite all of them, but the following examples illustrate the benefits of this

8 CTP/PTS Advanced Training Course on Deinking                                   Grenoble, May 29-30-31, 2007

Results of a joint study examining optimum             brightness, ISO
kneading conditions for peroxide bleaching         65 -
                                                          outlet post flotation with chemicals
and post flotation have been published by
                                                   60 -
Lamort and Interox [60][61]. Fig. 19 illustrates
that kneading in the presence of bleaching                                               outlet dispersion
                                                   55 -                                   with chemicals
chemicals is an efficient pretreatment step for
flotation post-deinking.                           50 -
The use of the Black Clawson high-speed 45 -            outlet dispersion
                                                          no chemicals
disperser (single shaft device which works at
medium rotating speed of 400-500 rpm) 40 -
between a washing deinking stage and a              0 10 20 30 40 50 60 70 80 90 100
flotation stage has been proposed [62] when                    specific energy, kWh/t
deinking a mixture of impact CPO, white          Fig. 19 Energy optimization with 1 % H2O2 in the high-
ledger and office waste. The dispersion                           speed disperser (70°C) [61]
induces a reduction of ink particle size,
                                                          Chemicals : 0.7 % NaOH, 3 % silicate.
thereby improving flotation removal for
                                                 Brightness at the high-speed disperser inlet : 52.2 % ISO
particles in the range of 80-300 µm.
However, the removal of smaller particles (4-40 µm) decreases, so it is suggested that these particles
should be removed by a washing stage prior to dispersion.

          5.1.2. High-speed disperser between 2 deinking stages

      •   Implementation of dispersing stage between two deinking loops
Most of modern deinking mills use a dispersion stage between two flotation stages. As indicated in the
introduction, during the 1989 EUCEPA Symposium in Ljubljana several papers by machinery suppliers
were presented proposing a hot dispersion stage between two flotation stages [7][8]. An example of a
mill running with these conditions was also presented [9]. Dispersion or/and kneading equipment is
now included in all modern deinking plants.

Since that symposium, machinery suppliers have published several papers recommending the use of
a high-speed disperser between two flotation stages, and have proposed the introduction of peroxide
bleaching chemicals into the heating screw before the high-speed disperser (even if few mills use
chemicals in the high-speed disperser), particularly for production of higher quality DIP than for use for
newsprint, such as that used for the production of SC papers. A few examples are given.

To improve the quality of a DIP produced from 60% ONP and 40% OMG, dispersion and post flotation
has been investigated. The dispersion with hydrogen peroxide induces a brightness gain of 4.2 points,
while dispersion without peroxide results in a brightness loss of 2.7 points. The post-flotation
performed after dispersion/bleaching achieves a brightness increase of 5.8 as compared to only 3.1
points for undispersed pulp. The total brightness gain (inlet dispersion/ outlet post flotation) was 10.3

When deinking ledger containing various hard chemically nondispersible inks, the positions before
flotation and between two flotation stages have been examined. Based on measurements of ink
particle size, it is recommended to have a first flotation stage to remove the fine ink particles even
though a large portion of the visible specks are not removed. These visible specks are reduced to
microscopic size by dispersion and are then removed by post flotation [65].

According to a study performed by Selder et al. [64], North American newspapers require the least
amount of stock preparation process equipment for recycling since it is relatively easy to remove ink
from the fibres (mainly due to differences in ink composition). For the American newspapers they have
tested, it requires no dispersing treatment for better deinkability, which is why the dispersing treatment
in some US deinking lines has been shut down. Besides, it appears that storage time has little effect
on high rub-off US and Canadian papers, but significantly affects the rub-off characteristics of
European newspapers and strongly affects the Asian newspapers (that are characterized by high
content of alkyl resins and vegetable oil, resulting on oxidative bonding reactions between binder, ink
and substrate). Besides, Asian countries import a lot of their raw material from Europe and North
America inducing more difficulty to detach inks (this furnish must travel along the globe resulting in
natural and thermal aging). Besides, the usage of colour ink is higher in local newsprint for the Asia-
Pacific region as mentioned by Haynes [66]). The presence of dispersing treatment for European and
Asian newspapers is required to have a good deinking line efficiency.

8 CTP/PTS Advanced Training Course on Deinking                                                         Grenoble, May 29-30-31, 2007

      •    Optimisation of dispersing stage through an overall approach [29]

For wood containing grade [29]:

During this study, 60% ONP / 40% OMG mixture has been 100% artificially aged in an oven at 60°       C
during 3 days that corresponds to very harsh conditions responsible for poor ink detachment, high
speck content, high ink fragmentation and poor ink removal. This raw material has been subjected to
conventional first deinking loop and then fed to the dispersing stage at different consistencies, with
and without the introduction of peroxide bleaching chemicals. The main result is reported in Fig. 20 as
a function of estimated volume energy described in p. 9. The zero volume energy application
corresponds to the pulp that have been thickened and submitted to post-flotation without dispersing

According to all the properties determined, it appears that the most significant effects of high-speed
dispersion between two deinking loops are:
    - Ink fragmentation phenomenon that is responsible for a decrease in ink removal efficiency and
        therefore in final brightness. It is therefore necessary to decrease as much as possible the
        energy applied during this stage.
    - Ink detachment occurs as soon as high-speed dispersing treatment is applied. However, there
        is no significant improvement if the volume energy is higher than 10 kWh/m . There is
        certainly a possibility for energy saving regarding this parameter.
    - Speck fragmentation requires more energy, but there is no significant improvement if the
        estimated volume energy is higher than 20 kWh/m .
    - As soon as the estimated volume energy is greater than 20 kWh/m , fibre degradation starts
        to be significant.

                                                  250                                                              600
                                                                 Ink content on post-flotaed pulp
                  ERIC HyW, Speck contamination


                                                                                                                         ERIC post-flotation
                                                                                          Ink detachement          300

                                                                         Speck content after post flotation
                                                    0                                                              0
                                                        0       20          40           60                   80
                                                             Volume energy estimated by Cm.Em
                                                            High-speed dispersing at 22% in neutral condition
                                                            High-speed dispersing at 33% in neutral condition
                                                            High-speed dispersing at 33% in alkaline condition
          Fig. 20 Incidence of high-speed dispersing treatment on ink detachment, ink removal and final speck
                                          contamination after post-flotation [29]

8 CTP/PTS Advanced Training Course on Deinking                                                    Grenoble, May 29-30-31, 2007

The main phenomena are                Estimated volume energy (kWh/m3)                           10                20
illustrated in the following table
and allow us to determine the                         Decrease in ink removal
strategy to be applied. The

tendencies are reported as a                          Ink detachment
function of estimated volume
energy as it allows describing                        Speck fragmentation
the phenomena and the
corresponding specific energy                         Fibre degradation

consumption          for       two
                                      Cm = 20 % Specific energy (kWh/t)                          50               100
consistencies are reported.
In order to control the Cm = 30% Specific energy (kWh/t)                      33                  66
dispersing stage, a reduction in
energy applied is required in                  Fig. 21 Summary of the effect induced by disperser
order to reduce ink fragmentation and to reduce the negative impact on flotation efficiency. However, it
is necessary to apply a sufficient energy level to induce ink detachment (but a stagnant value is
reached at 10 kWh/m ) as well as reduction in speck contamination (stagnation levels commence at
             3                                                  3
20 kWh/m ). Note that for energy upper than 20 kWh/m , fibre degradation becomes significant.
Dispersing parameters should therefore take into account these antagonists phenomena according to
the inlet of this stage:
   - If cleanliness is good after the first deinking loop, it is not necessary to put high energy level
       during dispersing
   - If cleanliness is "poor" (generally due to raw material composition variations), the energy to be
       applied must be adapted to the target of the mill in order to reduce the drawbacks.
   - In any case, ERIC and speck measurements at the inlet (and/or at the outlet) of the high-speed
       disperser is required for the regulation of dispersing running parameters.

For wood free grade:
An example of the results obtained during mill        100

trials (woodfree deinking line for market pulp) is     90

                                                               Residual specks or stickies (%)

represented in Fig. 22, where decreased                80                                            Fine

specks (after post-flotation) and macrostickies        70                                            Mechanical

                                                                                                                                 (tear x tensile)1/2
are represented as a function of specific energy       60                                                          22

consumption. The fines content and the                 50

                                                                                                                         Fine content (%)
compromise between tear and tensile                    40                                                          18

properties are reported in the same figure.            30

The dispersing treatment induces:                      20                                                          14

   - Fines generation with consequences on             10

        process yield (part will be removed             0                                                          10

        during flotation and/or washing) and on           0      20          40         60          80
                                                                   Specific Energy consumption (kWh/T)

        mechanical properties. It was observed
        that an increase in mechanical forces Fig. 22 Incidence of energy during high-speed dispersing
        applied (through a decrease in gap             between two deinking loops (Industrial measurements
        responsible for an increase in specific                       after post-flotation) [67]
        energy consumption) induced higher fine generation. It can be advanced that this fine
        generation is much more pronounced when specific energy is over 75 kWh/T.
   - Specific energy above 75 kWh/T, does not cause more fragmentation of macro-stickies.
   - Increase in speck removal during post-flotation because they are fragmented during dispersing,
        to the size desired for removal by flotation.
   - Increased mechanical properties (expressed by [tear*tensile] ) due to the refining effect,
        although a plateau is reached when energy consumption is the highest.
Practical consequences of such an analysis are some possible energy savings and reduction in
sludge amounts. Indeed as the above figures clearly illustrates, energy can be economised in post-
refining after the dispersing treatment. It appears that energy of >75 kWh/T is excessive and does not
significantly improve mechanical or optical properties, or the dispersion of stickies. There are two main
drawbacks in using excessive energy. Firstly there is an additional energy of no clear benefit and
secondly this leads to an increase in fine generation (and therefore increase in process losses).
Optimising dispersing by decreasing the energy consumed can be recommended when this treatment
is applied before the deinking loop, particularly when washing is present (fines removal occurs mainly
during this stage).

8 CTP/PTS Advanced Training Course on Deinking                                      Grenoble, May 29-30-31, 2007

       •     Implementation of fractionation between 2 deinking loops

Recent publications [68][69] lead to the implementation of fractionation between the two deinking
loops and to implement the dispersing treatment only on the long fraction. This process option allows
to increase the capacity of existing deinking line and to reduce the specific energy consumption. The
main drawbacks that have been reported concern mainly the specks and the macrostickies as only a
part of the pulp is submitted to the mechanical treatment. This process configuration has been
implemented in a mill [68].

             5.1.3. Comparison of high-speed disperser and low speed kneader between 2
                     deinking stages
A detailed comparison of a low speed kneader and high-speed disperser, operating at various
conditions (energy, temperature) and in the presence of various chemicals has been performed in
CTP. The furnish was a wood-containing mixture comprised of printed papers that were difficult to
deink such as old offset newspaper and heatset offset-printed SC papers [59]. Some of the most
interesting results obtained when running at high energy (90-10 kWh/t) and temperature (70-90° C)
are reported in Tab. VI and Tab. VII.

Low speed kneading without chemicals produces strong ink fragmentation (lower brightness and
higher ERIC value on the entire pulp, particularly when kneading is performed in the presence of a
large amount of ink, i.e., after a first neutral deinking stage). The presence of peroxide bleaching
chemicals improves ink detachment and/or reduces ink redeposition (lower ERIC value on
hyperwashed pulp).

High-speed dispersion is efficient for breaking up specks (particularly when the speck content is high,
i.e., after a first neutral deinking stage) and detaching large ink particles from long fibers (high
brightness and low ERIC value of hyperwashed pulp) even if performed without chemicals.

                                             Dispersion     with      bleaching Dispersion without       chemicals
Before                                       chemicals (after post-flotation)   (after post-flotation)
                                             L.S. kneader     H.S. disperser    L.S. kneader      H.S. disperser

      57.3      Brightness % (entire pulp)        64.6              60.3              54.1               57.9

                Fiber brightness %
      55.0                                        62.5              60.5              56.2               58.7
                (hyperwashed pulp)
                Total ink (ppm) (ERIC on
       306                                         120               230               359               260
                entire pulp)
                Attached ink (ERIC on
       259                                         94                118               170               149
                hyperwashed pulp)

      9477      Black specks (mm²/m²)             1858              2935              2497               2228

  Tab. VI             Kneading and dispersion after a first deinking in alkaline conditions (pulping 15 %, 40°C,
                    15 min, 1 % caustic soda, 2,5 % sodium silicate, 1 % hydrogen peroxide, 0.6 % soap)

                                             Dispersion     with      bleaching Dispersion without       chemicals
Before                                       chemicals (after post-flotation)   (after post-flotation)
                                             L.S. kneader     H.S. disperser    L.S. kneader      H.S. disperser
      49.2      Brightness % (entire pulp)        59.1              58.5              44.8               49.5
                Fiber      brightness %
      48.9                                        59.2              60.0              48.2               52.1
                (hyperwashed pulp)
                Total ink (ppm) (ERIC on
       476                                         272               309               614               416
                entire pulp)
                Attached ink (ERIC on
       387                                         197               172               348               248
                hyperwashed pulp)
      18341     Black specks (mm²/m²)             7018              4976              6882               3895

 Tab. VII            Kneading and dispersion after a first deinking in neutral conditions (pulping 15 %, 40°C, 15
                                                   min, 0.5 % surfactant)

8 CTP/PTS Advanced Training Course on Deinking                               Grenoble, May 29-30-31, 2007

Both high-speed dispersion and low speed kneading improve ink detachment and subsequent post
flotation efficiency, especially if performed in the presence of bleaching chemicals. When hot
dispersion is performed after a first deinking stage, the best results after post flotation are obtained
when using a low speed kneader (higher brightness: +4%, lower ERIC value: 120 vs. 230 ppm). This
difference is mainly due to a better detachment of residual ink from cellulosic fines and fillers, while a
similar ink detachment from long fibers has been obtained.

A comparison study between a high-speed disperser and a low speed kneader (Shinhama) has also
been performed with wood free recovered papers [70], however the comparison was not done with
strictly identical recovered papers and moreover the deinking processes tested were different: for the
high-speed disperser, the later was performed after a thickening stage before any deinking whereas
the kneading stage was performed after a first washing stage. The comparison becomes therefore not
very significant.

      5.2. Dispersion before deinking
Dispersion has also been proposed for use before deinking. For producing DIP for use in tissue from
ledger, computer paper and coated grades, it has been suggested a high-speed disperser be used
prior to deinking. This approach can promote the release of ink and coating from the fibers to get
smaller particles more easily removed by flotation or washing. This treatment is not recommended for
newsprint stock in order to avoid a grayish appearance [46].

For deinking of laser printed papers the use of a high-speed disperser has also been proposed [71].
The treatment caused a reduction in the number of visible toner ink particles but the results were not
good enough so flotation was necessary. Flotation of non-dispersed pulp was unsuccessful. However,
flotation after dispersion yielded good results.

In order to obtain better contaminant removal, a process with a separate stage for removal of
contaminants and deinking has been proposed. It includes pulping, cleaning and fine screening at low
temperature without chemicals in order to remove non-ink contraries before they are degraded into
small particles. A second alkaline stage is used to release and remove ink. A hot dispersion stage at
high temperature with peroxide bleaching chemicals is used for ink detachment and bleaching prior to
flotation deinking [72].

The use of a twin-shaft kneader running without steam after pulping and coarse screening and before
the first flotation stage is the basic part of the “Pacific Rim Deinking Technology" [30][73]. This
technology is recommended mainly for deinking mixed office papers containing computer printout, and
copy papers with laser and electrostatic inks, but it also allows the use of ONP as part of the furnish
[30]. In old mills the low-speed kneader can be implemented before a soaking tower designated to
complete the separation of ink from the fibers [30]. The use of two kneading stages, including one
before deinking, is also proposed [74].
This technology has been suggested for deinking mixed office paper containing laser print or mixtures
containing other difficult to deink print, such as UV-cured and heavy inks and varnishes [43][44]. The
twin shaft kneader works in “cold” conditions (i.e., without steam introduction; the power consumption
of 50-60 kWh/t produces a temperature increase from 22 ° up to 44-47 °                C). It has been
recommended for reduction of speck size and to prepare them for removal by flotation.

The kneader discharge temperature should be below the glass transition temperature to prevent the
toner inks from smearing back on to the fibers. In an effort to replace an agglomeration technology, a
process based on kneading and flotation has been implemented in a mill recycling office waste. The
final pulp has lower final dirt counts. The agglomeration technology is considered as a valid
technology but with more risk than kneader/flotation technology [75].

The following analogy has been forwarded by Ferguson and McBride [43] in order to explain why
kneading at low temperature is more efficient: “think of a piece of chewing gum stuck on a shirt- it is
easier to remove the gum with an ice cube and pull it intact from the cloth rather than use a steam iron
and smear the gum over the cloth.”

The effect of dispersion/bleaching with hydrogen peroxide in the high-speed disperser (Frotapulper
which is a “high speed kneader”) on ink detachment has also been studied by Dubreuil et al. [76].
They showed that with a wood-free furnish (mainly sorted office paper), high-speed disperser

8 CTP/PTS Advanced Training Course on Deinking                                                      Grenoble, May 29-30-31, 2007

bleaching with H2O2 induces a higher brightness gain during flotation. Lower flotation losses (and
lower ash removal) were observed when dispersion was performed with peroxide bleaching chemicals
than occurred with flotation performed after dispersion without chemical. The presence of sodium
silicate during flotation may have been responsible for this effect.

A more fundamental study has been carried out by Ruzinsky et al. [24] (the approach from refining
theory is also described in page 8). ONP was pulped in laboratory Helico pulper during 40 min. The
reslushed pulp pass then through a laboratory high-speed disperser for several consistencies (12-
30%) and for various gap (0.51 to 1.14 mm). The following points have been observed in their
laboratory refiner:
    - ink detachment increased with increased specific energy application until 2.3 MJ/kg
    - At higher energy application (when 30% consistency was reached), strong ink fragmentation
         was observed as well as ink redeposition on the fibres.
    - An increase in number of impact imparted to a fibre induces an increase in ink redeposition
         whereas an increase in specific energy per impact facilitates the ink detachment.
With similar experiences, Ruzinsky et al.[22] by using the Brecht approach (page 7), demonstrates
that ink detachment increased as the intensity of the treatment increased when measured using the
specific edge load. Besides, the effect of rotor speed was also brought to the fore:
    - dispersing at 2500 rpm decreased ink content on fibres up to approximately 40 kWh/T
    - dispersing at 1750 rpm decreased ERIC at lower rate
The effect of rotor speed was then explained by a decrease in treatment intensity (0.08 W.s/m at 1750
rpm compared to 0.12 W.s/m at 2500 rpm). A rise in treatment intensity to 0.12 W.s/m improve the ink
detachment whereas higher treatment intensity (>0.12 W.s/m) did not show additional effect.
For both approach, no indication is given regarding the ink removal by flotation.

The effect of dispersing or kneading
                                                          65       Blank with dispersion but floated                      7000
just after pulping+screening has

                                                                                                                                 Specks area, mm²/m²
been investigated by CTP during                           63
                                          Brightness, %

the last years [59][77]. The                              61                                                              5000
parameters studied included                                                                                               4000
    - The          raw       material                     59
        composition:           wood                       57
        containing and wood free
        recovered papers,
                                                          55                                                              1000
    - The        temperature,    the                      53                                                              0
        chemical introduced, the                               0        10     20      30     40                     50
        energy input in the high-
        speed disperser or the low-                                     Dispersing consistency, %
        speed kneader,                                               brightness     HW brightness      specks area
    - Unconventional         running                      30%.old offset heat set on SC – 30% ONP – 40% OMG
        conditions for kneading,                          Brightness measurement on entire pulp and hyperwashed
    - The consistency.                                    pulp pad – specks on handsheet
                                         Fig. 23 Brightness and cleanliness of the pulp after high speed
According to the results reported, it         dispersion at different inlet consistency and flotation [77]
appears       that      the      best
brightness/speck compromise giving
also the best ink detachment / ink redeposition compromise after dispersion and flotation, is obtained
for a low consistency dispersing.

8 CTP/PTS Advanced Training Course on Deinking                                                 Grenoble, May 29-30-31, 2007

In other terms, the "optimal"
                                                            500   Blank without dispersion but floated                  8000
sequence            would         be

                                            E.R.I.C., ppm
[Pulping+screening+dispersion+flota                         400

                                                                                                                               Specks area,
tion] without thickening. Compared

to similar sequence but with a                                                                                          4000
thickening to perform dispersing at                         200
high consistency (25-45% range), a                                                                                      2000
large final brightness decrease can
be observed (it can reach 5 to 10%                            0                                                         0
after flotation step): 64% if                                     0        10       20       30          40        50
dispersion is performed at 3% but
only 59% and even 54% if                                               Dispersing consistency, %
dispersing is performed at 25 and                                        E.R.I.C.    HW E.R.I.C.     Specks area
45% respectively. Even if dispersing
at 3% is able to decrease the speck         30%.old offset heat set on SC – 30% ONP – 40% OMG
                                            Residual ink content (ERIC) on entire pulp and hyperwashed
contamination, it represents higher
                                            pulp pad – specks on handsheet
contamination (2 to 3 times higher)
than more conventional dispersing           Fig. 24 Residual ink and cleanliness of the pulp after high speed
consistency.                                      dispersion at different inlet consistency and flotation [77]
For kneading before the first deinking loop, it can only be performed at high consistency (requirement
for such a technology) and induces a large ink fragmentation as well as a large ink redeposition
leading to poor efficiency of the deinking step. Better results are obtained if the kneading treatment is
performed with conventional chemistry used for peroxide bleaching (soda, silicate and peroxide).
However, such a solution is not sufficient to reach acceptable deinked pulp.

Some unconventional kneading conditions (longer kneading time and/or higher energy input with or
without peroxide) have been investigated. However, the ink detachment improvement by a kneading
treatment before any deinking cannot be obtained neither by an increase in kneading time or in
kneading energy input. Indeed, the ink amount in the pulp is too large and the compromise between
ink detachment and ink redeposition is not in favour of the ink detachment (even if the forces present
in such a device are able to improve the ink detachment, the ink fragmentation is more pronounced
and is responsible for higher ink redeposition). The ink detachment from fines elements by kneading
with alkaline bleaching liquor is thus possible only after a first deinking stage [59] (in that case, the
removal of ink particles reduces the potential risk of redeposition caused by a too high ink content and
a too high ink fragmentation).

Finally, if a high quality deinked pulp is required keeping inside the fine elements, the first deinking
loop with a previous low consistency dispersing treatment could be followed by a low-speed kneader
with peroxide in order to detach the ink particles from the fines before their removal in a post flotation

Trials have been also reported regarding the influence of dispersing and kneading conditions for a
wood free recovered papers that contains 50% laser prints and 50% photocopies [77]. The wood free
recovered papers have been pulped in Helico pulper in neutral condition including 0.15% of Rhoditec
1000. The dispersing treatment is then performed just after screening at 30% consistency and 90
kWh/T. The influence of alkaline bleaching liquor as well as temperature have been investigated:
    - The best "final ink content / speck contamination " compromise is reached for high speed
        dispersion performed after the peroxide bleaching when dispersing temperature is below the
        softening temperature of the toners,
    - Good efficiencies are also obtained with the low-speed kneader in presence of alkaline
        bleaching liquor, still below the softening temperature of the toners.
    - The detrimental effect of high temperature is more pronounced for kneader than for high-
        speed disperser. At high temperature, the kneading treatment leads to very bad results both in
        terms of brightness and cleanliness of the pulp
The negative impact of high temperature observed during this study confirms results published by
Ferguson et al. [78].

More recently, full pilot plant simulation of deinking lines for tissue mill (wood free grade) including 2
deinking loops combining each of them flotation and washing steps have been presented [79] (more
details are given in the paper devoted to Pulping and Ink detachment). Two options were investigated:

8 CTP/PTS Advanced Training Course on Deinking                                    Grenoble, May 29-30-31, 2007

      - [Pulper][screening][High-speed dispersing][1 deinking loop][Thickening][second deinking
        loop] with high speed treatment performed at 3% consistency, 45°C and 90 kWh/T
    - Pulper][screening][1       deinking loop][Thickening+steam][High-speed dispersing][second
        deinking loop] with high speed treatment performed at 30% consistency, 70° and 90 kWh/T
The implementation of high-speed dispersing stage at low consistency allowed to improve the ink
detachment and to reduce the speck contamination so that the final pulp was equivalent between the
two options proposed.

For wood-free grade, Ruzinsky et al. [34] still using Brecht approach has determined the efficiency of
various pyramidal and barred plate patterns for high-speed dispersing performed directly after pulping
of printed toner. The efficiency was determined in terms of specks (>113 µm) and small particles (by
ERIC measurements). The following results were brought to the fore:
    - Speck contamination decreased exponentially with specific energy consumed, with dispersion
         more efficient when carried out at higher consistency.
    - Toner particle size reduction was attributed to the magnitude of the forces applied to the
         particles and their probability to be affected by the mechanical forces.
    - The pyramidal plates induced a mider range of forces, resulting in a greater particle size
         distribution than the barred plates that develop more uniform plate gaps.
The Brecht theory was applied to describe high-speed dispersion by assuming that all treatments were
done by the edges of the bars/pyramids. A Specific Edge Load (SEL) of 0.1 W.s/m was found to give
the best reduction in particle size for barred patterns. However the knowledge of the SEL did not
reconcile the action of the different plate patterns and did not explain the behaviour of pyramidal
pattern. One more time, no data are given regarding the possible effect of such treatment on ink
removal efficiencies.

Regarding the implementation of mechanical treatment before any deinking sequence, the most
recent published research works performed in this area [68][80] consist in introducing fractionation and
to have optimized separate treatments on each fraction including mechanically actions on the long
fraction rich in specks and attached inks. A presentation on this aspect will be done during the present
ATC by Lascar and Borrego [80].

          5.3. Two hot dispersion stages
The use of low-speed kneaders and high-speed dispersers is widely developed in Japan [74]. These
devices are considered as useful for both ONP
                                                                           Brightness    Ink particles Deinkability
and OWP (recovered office                                     Two steps
                                                                                      diameter average
paper)     deinking.    High-                                                  (%)           (µm)          (%)

speed      dispersers     are                                     One step     84.4           28.1          92
recommended for stripping
                                                                               88.8           14.5          97
ink from cellulose fibers and
kneaders are used to adjust
ink particle size. In some
cases (deinking of toner-
                                   0.1       1          10        100
printed      papers),     two
                                           Particle diameter, µm
kneading steps seem to be
necessary before flotation              Fig. 25 The effect of kneading (toner-printed paper) [74]
to adjust ink particle size
distribution (Fig. 25).
Modern mills designed for the production of wood-containing DIP for SC or LWC paper or wood-free
DIP for market pulp are more and more complex and can include three (and sometimes four) loops
and flow sheets including two dispersion stages are often proposed.

Regarding multi-loop deinking plants including two dispersion stages, the main questions are “Which
devices? In what order implement them? In which conditions to operate them?”

Some Japanese deinking plants include two kneading stages. The use of two twin-shaft kneaders
working in “cold” conditions is proposed for double-loop flotation deinking [43][81]. These processes
are proposed to deink furnish containing a large part of toner printed papers.

8 CTP/PTS Advanced Training Course on Deinking                                Grenoble, May 29-30-31, 2007

For the production of SC paper from European household waste collection (mixture ONP/OMG) it has
been proposed a three loop process including two dispersion stages (with bleaching chemicals)
located after flotation stages and a final low consistency refining stage [82].

The operations are very sophisticated in mills producing market deinked pulp from office paper. Some
of them in North America [83][84][85] and also in Europe [86][87] include two dispersion stages before
the flotation stages.

It has also been suggested to combine the advantages of high-speed disperser and low-speed

Voith-Sulzer recommends positioning the high-speed disperser in the first dispersion stage due to its
more efficient sticky reduction and the low-speed kneader in the second stage because it treats the
fibers more gently. The low-speed kneader can be operated at low temperature, thereby preserving
the integrity of the fibers [53].

Some North American mills producing market deinked pulp from office paper, operate a kneading
stage at the beginning of the process before flotation, and dispersion after this first flotation stage. An
additional deinking stage by flotation or washing follows the dispersion stage [83][84][85].

Regarding our results, we recommend the opposite in order to achieve higher brightness and lower
specks content. The high-speed disperser should be operated early in the process in order to break up
larger specks and increase their removal by the flotation stage. Alternatively, this first loop can be
operated at non-alkaline conditions for the removal of waterbased inks. After a first flotation stage and
the removal of a significant amount of ink, a low-speed kneader should be operated at a rather high
temperature (90° C), coupled with the benefit of pe roxide bleaching chemicals. The residual ink
attached on fine fibers can be detached and removed by post-flotation [88].

8 CTP/PTS Advanced Training Course on Deinking                                        Grenoble, May 29-30-31, 2007

      6. BLEACHING
In the deinking plant, the hot dispersion stage is often combined with bleaching. High-speed
dispersers and low-speed kneaders are efficient mixers and can be used to mix bleaching chemicals
before a bleaching tower. Due to the high consistency and the high temperature required for the
treatments, low-speed kneaders and high-speed dispersers have been proposed for bleaching
reactors (the first relevant papers on this subject were published in the 1970's [6][89]). The high
temperature can also be used as an efficient pretreatment to destroy catalase.

      6.1. Use of high-speed disperser as mixer
High-speed dispersers have been proposed as mixers for peroxide bleaching with a short retention
time (the bleaching time can be reduced due to the high consistency and high temperature) in a pipe
(retention time 15–30 min) or a tower after dispersing [8][46].
It has also been proposed to use hydrosulphite in the high-speed disperser. The high-speed disperser
is considered to be an instant bleach mixer, which allows short bleaching times and does not require
air [46].

      6.2. Bleaching in low-speed kneader and high-speed disperser

        6.2.1. Peroxide bleaching                                                                        deinked pulp
                                                                                                         kneaded pulp
Low-speed kneaders are good mixers and the high                    Brightness (%)                        bleached pulp
temperature is favorable for peroxide bleaching.              80
Results of various pilot plant and industrial CTP             75
trials are reported in Fig. 26. Brightness losses
during kneading and brightness gains by peroxide
bleaching are shown. In a single loop deinking                65
process with a kneading stage at the end of the               60
process to reduce specks and improve the visual               55
aspect of the pulp, the brightness loss induced by            50       sorted         upper     magazine      woodfree
kneading can be restored by peroxide bleaching                       household        grade                     waste
either in the low-speed kneader or in conventional                   wastepaper     magazine                    paper
                                                                   (CTP pilot plant) (mill 1)     (mill 2)     (mill 3)
bleaching equipment after kneading.
                                                                     Bleaching in a kneader     Bleaching after kneading
As shown previously, brightness gains become
                                                           Fig. 26 Peroxide bleaching in a low-speed
significant if a post-deinking is implemented after
                                                               kneader and after kneading CTP Results
kneading and bleaching and particularly in the case
of bleaching in the low-speed kneader. Although a higher ink concentration in pulp before kneading
induces a higher brightness loss, kneading in the presence of bleaching chemicals induces a
brightness gain, which is not dependent on the amount of residual ink (Fig. 27).

In deinking plants,         Brightness (% ISO)                           Brightness (% ISO)
the concentration of    70                          after flotation   75
                                                                              no chemicals
carbonate ions can                                                    70      with chemicals       +6.1
reach levels that can   60                         after disperser                   +5.6           D(1)
be detrimental for                                                    65              D(1)
bleaching         with  50                                                                    D(0)
peroxide in a hot                                                     60
high-speed              40                                            55
disperser. The effect       0      5     10      15      20        25               5             20
of neutralization with              Flotation time (min)          (a)            Flotation time (min)       (b)
carbon         dioxide
should therefore be        Fig. 27 (a) Brightness after flotation and dispersion versus flotation time, (b)
carefully monitored.                           Bleaching with 1 % hydrogen peroxide [61]:
According to Süss et al. [90], it can be applied if any excess of carbonate is precipated either by
heating in the presence of magnesium or calcium salts. Preheating of the pulp in order to precipitate
carbonate ions as CaCO3 or MgCO3 allows then an unaffected application of peroxide in the high-
speed disperser.

8 CTP/PTS Advanced Training Course on Deinking                                      Grenoble, May 29-30-31, 2007

        6.2.2. Reducing bleaching

The high temperature in the high-speed disperser and rapid reaction rate of reducing chemicals have
been taken into account in order to develop bleaching sequences with short reaction times. FAS at a
low application rate (0.2%) but at high temperature is as efficient as a higher application rate (0.6%) at
50° C [91]. This supports its use in high-speed dis persers without retention time.

Sodium hydrosulphite and FAS bleaching                    Brightness gain
have been tested in a mill and compared to           4.0 -
                                                           Hydrosulphite                               FAS
hydrosulphite bleaching in a tower [92]. These
                                                     3.0 -
two chemicals (when used in a high-speed
disperser) are more efficient than hydrogen          2.0 -
peroxide which needs a longer storage time
than the reducing agents. A brightness loss of       1.0 -
1.4 points was observed without bleaching
and a brightness gain of 3.5 to 4.5 was                0-
obtained by using FAS or sodium
                                                   - 1.0 -
hydrosulphite in the high-speed disperser
(Fig. 28). These results show only 1 to 1.5%                 without bleaching
                                                   - 2.0 -
brightness less than those obtained by a
                                                         1          2      3        4      5     6     7     8
conventional hydrosulphite bleaching in a                                                              C
                                                           Consistency 28/ 30 % - temperature : 90 - 95°
tower [92].
                                                     Fig. 28 Results of reducing bleaching in a high-speed
                                                             disperser versus cost in DM/100 kg [92]
      6.3. Destroying catalase

Some difficulties related to bleaching of deinked pulps are related to the presence of catalase in the
pulp. Catalase, which decomposes hydrogen peroxide, is an enzyme produced by microorganisms.
Destroying the catalase enzyme by a chemical treatment of sodium hypochlorite or by a thermal
treatment has been described in a previous paper [93] where industrial deinked pulps were treated in
the CTP pilot plant. These results have demonstrated significant improvements in brightness gain as
well as an important reduction in peroxide decomposition as shown by higher levels of residual

The thermal treatment is preferred because of the                 Brightness gain (%)
environmental concerns related to the use of a                6
chlorinated product and because it can often be               5
implemented without any additional cost. In fact, the         4
pulp is generally thickened up to a high consistency          3
before the hot dispersion stage. If the temperature of                                                  H 2O2
the pulp is increased to 80° C, the catalase enzyme           2                                        applied
will be destroyed and further dilution of the pulp with       1                                           5%
the alkaline peroxide bleach liquor will bring down           0                                           2%
the temperature of the pulp to the optimum value for              Residual H 2O 2 (%)
                                                              3                                           1%
storage in the bleaching tower. Therefore, it is
important to heat the pulp first in order to destroy the      2
catalase and then introduce the alkaline peroxide             1
bleach liquor.                                                0
                                                                   Blank     55°C       65°C   75°C
Testing results presented in Fig. 29 were obtained in                        79         106    108
the laboratory on a recycled pulp contaminated with               Bleaching experiments carried in the
catalase and treated under various conditions with                laboratory after cooling down the pulp
an industrial low-speed kneader [94].                             samples. Recovered paper furnish: upper
                                                                  grade magazines
Independent of the brightness loss related to the       Fig. 29 Effect of heat treatment during kneading
residual ink fragmentation during the hot dispersion           on the peroxide bleaching response of a
stage, it is clearly shown that the thermal treatment        wastepaper pulp contaminated with catalase
increases the brightness gain. The results indicate                              [94]
that a charge of 1.0% H2O2 (100%) applied to the
treated pulp can achieve a higher brightness gain than a charge of 5.0% H2O2 (100 %) applied to the
untreated pulp. Furthermore, higher levels of residual peroxide can be observed due to the lower
decomposition of peroxide.

8 CTP/PTS Advanced Training Course on Deinking                                    Grenoble, May 29-30-31, 2007

Hot dispersion at very high temperature is often prescribed as a means of microbiological
decontamination. Hot dispersion at temperatures lower than 100° C in combination with hydrogen
peroxide can be used as an efficient microbiological decontamination treatment.

An extensive study related to microbiological decontamination of pulp and paper produced from
recovered paper is on-going in the CTP. The initial results which include decontamination in a non-
pressurized low-speed kneader are encouraging and have been published [95].

Various recovered paper furnishes from household waste collection systems were used to investigate
microbiological decontamination processes. Such highly contaminated waste papers were intentionally
chosen to demonstrate the potential of decontaminating processes, although they are not used as raw
materials for the production of food-contact papers and boards. The trials were carried out at the
Recycling Pilot Plant and on the Pilot Paper Machine of the CTP.

Microbiological contamination was measured using the conventional tryptone-glucose agar plate
method. Various decontamination processes were investigated, either at the pulp preparation stage, or
directly on the paper machine. Hydrogen peroxide was used at various stages of the process. The
strongest decontaminating effect was observed in the bleaching stage of deinked process, and in all
cases, when peroxide was added to the pulp just before high consistency hot kneading.

The combination of kneading and hydrogen peroxide          c.f.u./g (1E+8 initial pulp microbiological contamination)
has also been evaluated when recycling paper grades        1,000,000,000
for which the use of hydrogen peroxide is not
conventional (Fig. 30). The raw material tested was a
mixture of household packages and old corrugated                100,000
containers. Peroxide was introduced in the low-speed
kneader without caustic or silicate, and the kneading             1,000
temperature was 90-95° C.

The results are clear with a mixture used to produce                  0
                                                                        0            1              2             3
paper for corrugating. The introduction of 1.5%
                                                                            Kneading peroxide introduction rate (%)
hydrogen peroxide in the low-speed kneader at 90 °C
brings about total microbiological decontamination.

An updated version has been recently published in          Fig. 30 Microbiological decontamination by using
order to take into account new EU regulation in food           hydrogen peroxide in a low-speed kneader [95]
contact [96]. The total microbiological
decontamination can be obtained by introducing peroxide in the low-speed kneader. A short thermal
treatment at high temperature (130° during 5 min) is efficient but require specific equipment and will
induce some detrimental consequences on the pulp properties and on recycling circuits.

These results are interesting, but such treatments cannot always be implemented. Sometimes the
pulp is fractionated and only the long fiber fraction is treated in the low-speed kneader. The pulps are
remixed so decontaminating only half of the pulp would make no sense. On the other hand, high
consistency pulp at the low-speed kneader outlet is diluted with process water in the machine chest.
This means that the pulp will be contaminated with the microorganisms present in the water and
microbial growth in paper machine circuits is well documented. For this reason, we have also
investigated treatments directly on the paper machine. Trials have been performed in the CTP pilot
plant and then in a large industrial machine producing testliner. Complete decontamination was
obtained with comparatively low amounts of peroxide in the size-press color of the paper machine.

8 CTP/PTS Advanced Training Course on Deinking                                                                                                                                          Grenoble, May 29-30-31, 2007

                                          8. FIBRE PROPERTIES
Dispersion can induce changes in pulp and fiber properties due to the effects of mechanical forces
and temperature. These changes have been described in a few papers. Some examples are given

                                          8.1. High-speed dispersers
High-speed dispersers are                                                                       Stock : household waste paper
claimed      to   have      no
detrimental effects on pulp

                                                                                                                                             Breaking length (km)

                                                                                                                                                                                                 Tear (mN)
                                                                                                52 -                                                                4.1 -                                    830 -
properties, as illustrated by
                                                                               Freeness (°
                                                                                                50 -                                                                3.9 -                                    810 -
Fig. 31 [97] and Fig. 32 [65].
                                                                                                                                                                    3.7 -
                                                                                                48 -                                                                                                         790 -
During     trials   using    a                                                                                                                                      3.5 -
Cellwood dispersing unit for                                                                    46 -                                                                                                         770 -
                                                                                                                                                                    3.3 -
deinking laser printed papers
                                                                                                44 -                                                                3.1 -                                    750 -
within the temperature range
90-130° C, an increase in                                                                       42 -                                                                2.9 -                                    730 -
tear       strength       with                                                                          a b c d e                                                            a b c d e                                a b c d e
unchanged breaking length
was observed [71].                                                                                      a   wire press inlet                                                       d    after disperger
                                                                                                        b   after heating screw                                                         30 % cons. in discharge
In the conical high-speed                                                                               c   after disperger                                                        e    after disperger - dilution
disperser     proposed     by                                                                               5 % cons. in discharge                                                      to 5 % cons. in chest
Sunds Defibrator [33], the                                                                              Fig. 31 The influence of dispersion on pulp properties
effect of energy consumption                                                                                      (Sulzer Escher-Wyss Disperger)([97]
and temperature on the
efficiency of dispersing has                                                                    140 -
been also studied. The                                                                                                                                                                                                     feed
results are reported in Fig.                                                                    120 -                                                                                                                      after SDR
33 and Fig. 34. Whereas a
                                                                                                100 -
                                                                                PROPERTIES, %

high dispersing temperature
is favorable for dispersion of                                                                   80 -
specks (Fig. 33), it may be
detrimental for the strength                                                                     60 -
development (Fig. 34).
                                                                                                 40 -

                                                                                                 20 -

                                                                                                            freeness     burst breaking   tear                                                wet                  fines         specks
                                                                                                                       strength length strength                                               TEA
                                                                                                        Fig. 32 The influence of dispersion on pulp properties
                                                                                                                         (Beloit Diskperser) [65]

                                    100                                                                                                 46
  Speck area reduction > 50µm (%)

                                                                                                                                                                       70°       90°C        C
                                    80                                                                                                  44
                                                                                                                        Tensile index

                                    60                                                                                                  42

                                    40                                                                                                  40

                                    20                                                                                                  38
                                                         70°          C
                                                                    90°                   C

                                     0                                                                                                  36
                                          0   20    40         60         80                      100        120                                   0                        20     40       60                80           100       120
                                                   Energy consumption (kWh/T)                                                                                                    Energy consumption (kWh/T)

                                     Fig. 33 The influence of dispersion on speck                                       Fig. 34 The influence of dispersion on tensile index
                                              reduction [17] – ONP/ OMG                                                                   [17] – ONP/ OMG

8 CTP/PTS Advanced Training Course on Deinking                              Grenoble, May 29-30-31, 2007

A comparison between conical high-speed disperser and conical refiner has been presented by Metso
paper [98]. The main differences between the two apparatus are
    - The inlet consistency (at least 30% for dispersing versus 3-4% for refining)
    - The temperature (90-95°C for dispersing versus 40-4 5°C for refining)
    - The periphery rotor speed (more than double in high-speed disperser)
    - The pattern of the plates
The two apparatus commonly works at the same energy inlet (40-80 kWh/T) but they are not
equivalent in terms of effects. For DIP line, they reported the following points:
    - Dispersing consumes more energy than does refining when the refining degree is considered.
        The refiner creates more fines (and higher fibre length reduction), which speeds up freeness
    - High-speed disperser induces a better speck contamination reduction than refiner. This is
        attributed to more fibre abrasive treatment and higher number of impact in the high-speed
    - Hot melts are dispersed in dispersing treatment whereas their amount increased after refining
        (this was explained by the different temperature process and by the fact that these hot-melts
        can agglomerate during refining!).
    - Refiner develops slightly better tensile strength than dispersing in relation to energy
        application during the treatment. However, if the tensile index is plotted as a function of
        freeness, the slight difference is reduced.
For OCC lines, they reported the following points:
    - The refiner and high-speed disperser behave in a similar way to DIP pulps,
    - The tear index is slightly reduced in the refiner whereas it is slightly increased after the
        dispersing treatment,
    - The Scott Bond (that gives information relative to interfibre bonds in the paper thickness) is
        increased for both processes,

In a conical refiner for Tissue deinking line, the mechanical properties can be improved: tensile index
can be improved from 38.5 to 46.5 N.m²/g and tear index from 8.9 to 9.3 mN.m²/g [19]. This leads also
to energy saving for post refining.

For ONP/OMG deinking for SC paper quality, Le Ny and Messmer [99] compared the performance of
high consistency and low consistency refining versus high-speed dispersing on the development of
recycled fibres properties. The main differences are
    - Consistency between 3% for LC refining and 30% for HC refining and HC high-speed
    - Temperature: 50°C for refining versus 70° for high -speed dispersing
    - Energy input: 20-90 kWh/T for LC refining, 65 kWh/T for HC refining and 110 kWh/T for high-
        speed dispersing
    - Peroxide bleaching was performed in the case of high-speed dispersing
The main results are then the following:
    - Refining on SC paper quality is positive (surface smoothness, gloss, porosity and tensile) with
        the exception on tear, bulk and scattering coefficient; The HC refining effect on paper structure
        is not so pronounced but allows to improve tensile index with limited detrimental effect on tear.
    - High-speed dispersing has a negative effect on paper surface properties but high energy
        could be promising for paper strength development. Note that this treatment was also
        performed at higher temperature in presence of peroxide bleaching chemicals.
    - The fibre properties developed during these treatments can explain the differences observed
        in terms of paper strength properties. Refining performance has been characterized as fibre
        peeling and cutting, external fibrillation, fine generation, curl and kinks removal (HC refining
        induces lower fibre cutting and allows also to develop internal fibrillation that is expected to
        contribute to fibre flexibility).

8 CTP/PTS Advanced Training Course on Deinking                                          Grenoble, May 29-30-31, 2007

         8.2. Low-speed kneaders
CTP has checked the changes in fibre properties during kneading for wood-containing and wood-free
deinked pulps. With wood-free DIP a slight decrease (about 10%) in mechanical properties (burst and
tensile strengths) is induced by kneading, but this loss is totally restored by peroxide bleaching [54].

Tab. VIII shows results obtained with a mixture of news and magazines from household wastes in the
CTP deinking pilot plant [57].

The trials on wood-containing pulp were performed with a higher temperature than the trials on wood-
free DIP and the losses in mechanical properties were slightly higher. The bleaching stage (performed
in the low-speed kneader for these trials) partly restores these properties. An increase in burst and
tensile indexes was observed during the post-deinking stage. The combination of kneading, bleaching,
and post-deinking improves brightness, cleanliness and mechanical properties of the deinked pulp.

       Furnish: mixture ONP/OMG. Kneading : outlet temperature 95° energy consumption 100 kWh/t. Peroxide
                    bleaching with 1 % caustic soda. 2.5 % sodium silicate, 1.5 % hydrogen peroxide
                                                 Kneading                Bleaching           Post deinking
      Pulp properties
                                           Inlet          Outlet        in kneader      Flotation     Washing
      °                                        70            68             73             77             71
      Sheet density (kg / m )                  583           567            587            613           606
      Tensile index (N.m/g)                    44             37             40             47            48
      Burst index (kPa.m²/g)                   2.9            2.4           2.6            3.1            3.1
      Stretch-to-break (%)                     2.3            2.6           2.8            2.6            3.1
      Brightness ISO (%)                      60.5           57.5           61.0           67.0          66.5
      Relative specks count                    100            37             46             21            26
           Tab. VIII             Effect of kneading, peroxide bleaching and post-deinking on pulp properties.

Ferguson and McBride [43] have reported that the pulp after kneading (twin-shaft kneader) combined
with washing and flotation did not show significant property changes. There was no freeness change
across the process, and no significant changes in bursting strength and breaking length were found.

         8.3. Comparison of the effects of low-speed kneader and high-speed
              disperser on fibre properties
According to McKinney [100], dispersion and kneading have a substantial impact on the physical
properties of fibers, partly due to their impact on curl. Dispersion reduces curl, whereas kneading
increases curl. The two processes appeared to have quite different effects on the mechanical
properties such as freeness. But the physical effect of the low-speed kneader on the fibers is
disguised by the change in properties due to the increase in curl whereas, with dispersion, the effects
are additive. Indeed, as curl is reduced, dispersion apparently has a greater impact on physical
properties (Tab. IX).

                  Tab. IX              Effect of dispersion on fibre properties ([100] McKinney, 1996)
                                        Single shaft kneading                      High-speed dispersion
                                   Before kneading    After kneading          Before dispersion After dispersion
Freeness (CSF, ml)                       177                172                     145                97
Bulk (cm / g)                            1.92               2.02                     1.9               1.8
Breaking length (km)                     3.18               2.87                     3.8              3.93

Voith-Sulzer (which supplies the two types of devices) considers that the high-speed disperser can
produce a pulp with better fiber properties while low-speed kneader should be recommended to
increase bulk [53].

8 CTP/PTS Advanced Training Course on Deinking                                Grenoble, May 29-30-31, 2007

      9. Summary
Dispersion treatment is used primarily as a means to improve appearance characteristics by
disguising the presence of contaminants. When quality levels achieved by cleaning and screening are
not adequate for a particular product, additional techniques can be used, which can include dispersion
on the entire pulp or on a part of it, after fractionation. The aim of a dispersing treatment is to disperse
contaminants to such an extent that their detrimental effects are reduced.

Dispersion at high temperature was used initially to disperse bitumen when recycling OCC, and later
to disperse other contaminants such as wax and hot melt glues.

For white grades, hot-dispersion was proposed to disperse hot melt contaminants and to improve the
visual aspect by dispersing residual ink and specks.

In high-speed disperser, all contaminants are reduced in size. They were then claimed to protect
paper machine from stickies. However, because dispersion does not remove contaminants, but
disperses them into the pulp, it can create some detrimental effects such as deposits on the paper
machine. As a consequence of the progress made in cleaning and screening efficiency, hot dispersion
has been questioned in the paper and board recycling units.

In brown grade recycling, high-speed disperser is the most popular technique used. The intensity of its
action is determined by the design of the disc, the energy input (generally between 40 and 80 kWh/T),
temperature, residence time and the gap between the discs. Narrow disc gaps, low temperature and
high residence time increase the dispersing intensity (that allow good dispersion to be achieved) but
also increase the freeness loss.

For deinking application, the future of hot dispersion is mainly related to the improvement of the
deinking efficiency. Both high-speed dispersers and low-speed kneaders are proposed for the
improvement in ink detachment, mainly before post-flotation. Various high-speed dispersing and low-
speed kneading technologies are proposed and the choice of the most effective type is the subject of
debate. A combination of the various technologies in order to combine the advantages of each is also

Actually, a regain of interest appears on the development of paper mechanical properties of the pulp
during these stages. Under typical high-speed dispersing conditions, burst and tensile properties are
enhanced, similar to low consistency refining, but without the freeness loss caused by cutting in
refiner. Under high temperature conditions, fibre cutting is further limited in high-speed disperser.

High-speed dispersion and low-speed kneading can introduce curl into the treated fibres. Low-speed
kneader induces more higher fibre curl, mainly because of higher residence time. This affects then
tear strength, dimensional stability, bulk, etc.

New applications, such as microbiological decontamination, by using chemicals in a low-speed
kneader, have also been proposed.

8 CTP/PTS Advanced Training Course on Deinking                                    Grenoble, May 29-30-31, 2007

        10. References
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8 CTP/PTS Advanced Training Course on Deinking                                       Grenoble, May 29-30-31, 2007

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