U.S. patent application number 10/564775 was filed with the patent office on 2007-06-14 for method for manufacturing paper and paper.
This patent application is currently assigned to UPM-Kymmene Corporation. Invention is credited to Bjorn Lax, Mikko Maijala, Roope Maijala, Matti Sipila, Paivi Solismaa.
Application Number | 20070131360 10/564775 |
Document ID | / |
Family ID | 30445173 |
Filed Date | 2007-06-14 |
United States Patent
Application |
20070131360 |
Kind Code |
A1 |
Sipila; Matti ; et
al. |
June 14, 2007 |
Method for manufacturing paper and paper
Abstract
A paper pulp used in papermaking is produced as follows: fiber
material is fed into a precipitation reactor; a reactive mineral
material is fed into the reactor; the reactive mineral material and
the fiber material are combined to form a fiber suspension in the
reactor and/or before feeding these materials into the reactor; a
gas containing a precipitant is fed into the reactor thereby
precipitating the reactive mineral material in order to form a gas
space containing the precipitant; the fiber suspension fed into the
reactor and/or formed there is dispersed into small parts into the
gas space, whereby the precipitated mineral material forms a filler
on the fibers; the treated fiber suspension is discharged from the
reactor; the filler-containing paper pulp is fed to the forming
section of the paper machine; water is removed from the paper pulp;
and the resulting paper web is dried and finished.
Inventors: |
Sipila; Matti; (Kuusankoski,
FI) ; Solismaa; Paivi; (Kuusankoski, FI) ;
Maijala; Mikko; (Espoo, FI) ; Maijala; Roope;
(Espoo, FI) ; Lax; Bjorn; (Vaasa, FI) |
Correspondence
Address: |
VENABLE LLP
P.O. BOX 34385
WASHINGTON
DC
20043-9998
US
|
Assignee: |
UPM-Kymmene Corporation
Etelaesplanadi 2
Helsinki
FI
FI-00130
FP-Pigments Oy
P.O. Box 11
Helsinki
FI
FI-021170
|
Family ID: |
30445173 |
Appl. No.: |
10/564775 |
Filed: |
July 1, 2004 |
PCT Filed: |
July 1, 2004 |
PCT NO: |
PCT/FI04/00410 |
371 Date: |
June 27, 2006 |
Current U.S.
Class: |
162/9 ; 162/158;
162/181.2; 162/183 |
Current CPC
Class: |
D21H 17/675 20130101;
D21H 17/70 20130101 |
Class at
Publication: |
162/009 ;
162/158; 162/181.2; 162/183 |
International
Class: |
D21H 17/70 20060101
D21H017/70 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 15, 2003 |
FI |
20031072 |
Jan 30, 2004 |
FI |
20040147 |
Claims
1-21. (canceled)
22. A method for manufacturing paper, comprising: producing a paper
pulp as follows: providing a precipitation reactor; providing a
fiber material comprising fibers to be used as a raw material for
the paper pulp, the fibers in the fiber material having a certain
capacity for bonding; providing a reactive mineral material;
providing a gas containing a precipitant capable of precipitating
the reactive mineral material; providing an activation zone in
front of the precipitation reactor or inside the precipitation
reactor; combining the reactive mineral material and the fiber
material to form a fiber suspension; activating the fiber
suspension in the activation zone in order to enhance the capacity
of the fibers for bonding; dispersing the fiber suspension in drops
or particles into the precipitation reactor; feeding the gas
comprising the precipitant inside the precipitation reactor;
bringing the dispersed and activated fiber suspension into contact
with the precipitant of the reactive mineral material in the
precipitation reactor in order to at least partly precipitate the
reactive mineral material; and discharging the treated fiber
suspension from the precipitation reactor; feeding the paper pulp
containing precipitated mineral material at a predetermined
consistency into a forming section of a paper machine; removing
water from the paper pulp by allowing the pulp to drain through a
water permeable forming base; and drying and finishing the paper
web thus produced in order to produce a finished paper product.
23. The method according to claim 22, wherein the reactive mineral
material is calcium hydroxide (Ca(OH).sub.2).
24. The method according to claim 22, wherein the precipitant is
carbon dioxide.
25. The method according to claim 22, wherein in order to activate
the fiber suspension the activating methods are selected from
forces exerted to the fiber suspension or chemically activating the
surfaces of the fibers comprising forming active OH-groups on the
surfaces of the fibers, the forces comprising repeated impacts,
counter impacts shearing forces, turbulence, over-pressure pulses
or under-pressure pulses which forces mechanically activate the
fibers by fibrillating or grinding the fibers or by opening the
inner parts (lumen) of the fibers to the mineral material.
26. The method according to claim 24, wherein the fiber suspension
flow flowing through the activation zone is subjected to repeated
powerful impacts and counter impacts created in the fiber
suspension flow by blades moving at 5-250 m/s.
27. The method according to claim 24, wherein in the activation
zone of the precipitation reactor, there is an impact mill-type
flow-through mixer, which has concentric cages provided with
blades, of which cages at least every other cage functions as a
rotor, and the cages adjacent to said cages function as stators or
rotors, the speed difference of the adjacent cages being 1500 m/s,
typically 50-200 m/s, and wherein the fiber suspension is fed
through the flow-through mixer and radially outwards from cage
centers, as the blades on the cages bring repeated impacts, counter
impacts, shearing forces and turbulence and/or over- and
under-pressure pulses, which together activate the fibers, to bear
on the outward flowing fiber suspension.
28. The method according to claim 27, wherein at least part of the
gas to be fed into the precipitation reactor, containing the
precipitant for precipitating the mineral material, is fed into the
reactor through the activation zone, enabling the fibers activated
in this activation zone to come into contact with said precipitant
immediately during the activation or directly after it.
29. The method according to claim 24, wherein the residence time of
the suspension containing the fiber material and the reactive
mineral material in the activation zone is under 1 second.
30. The method according to claim 22, wherein a gas containing more
than 10% of the precipitant is fed into the precipitation
reactor.
31. The method according to claim 22, wherein the gas containing
the precipitant is pure or nearly pure carbon dioxide, flue gas or
another carbon dioxide-containing gas, or contains other gas
suitable for precipitating the mineral material used, or is a
mixture of these gases, and the gas containing the precipitant is
fed into the precipitation reactor in such a manner that it will
create an over-pressure in the reactor.
32. The method according to claim 22, wherein the fiber suspension
is taken through two or more precipitation reactors connected in
series.
33. The method according to claim 22, wherein the reactive mineral
material comprises calcium hydroxide, calcium sulphate, calcium
oxide, some other reactive material suitable for the purpose and
precipitable with the precipitant and/or a mixture of these, and
wherein the reactive mineral material is chosen according the
desired quality of the product.
34. The method according to claim 22, wherein the fiber material
comprises: primary fiber obtained from chemical, mechanical,
chemimechanical, thermomechanical, semichemical or other
corresponding processes, de-inked or non-de-inked recycled fiber
obtained from newsprint, kraft paper, tissue paper, special paper
or paperboard, fiber from machine broke or other corresponding
fiber, or bleached or non-bleached fiber, ground or non-ground
fiber, dried or nondried fiber, or a mixture of these.
35. The method according to claim 22, wherein the fiber material
contains in addition to fine substances impurities and/or mineral
materials.
36. The method according to claim 22, wherein the fiber material is
fed to the precipitation reactor at a consistency of from 3 to
7%.
37. A method for manufacturing paper, comprising: producing a paper
pulp as follows: providing a precipitation reactor; providing a
fiber material comprising fibers to be used as a raw material for
the paper pulp, the fibers in the fiber material having a certain
capacity for bonding; providing a reactive mineral material;
providing a gas containing a precipitant capable for precipitating
the reactive mineral material; providing a flow-through mixer
operating by the impact mill principle in front of the
precipitation reactor or inside the precipitation reactor, the
flow-through mixer comprising concentric cages provided with blades
of which at least every other cage functions as a rotor, and the
cages adjacent to the mentioned cages function as stators or
rotors, the speed difference of the adjacent cages being 10-500
m/s; feeding apparatus for feeding the fiber material mainly into
the center of the cages; and an open outer cage that allows the
fiber suspension to flow radially outwards through the cages to
exit the cage in different directions, or an outer cage that is
provided with one or more outlets in order to discharge the fiber
suspension flowing radially outwards from the cages; combining the
reactive mineral material and the fiber material to form a fiber
suspension; activating the fiber suspension in the flow-through
mixer in order to enhance the capacity of the fibers for bonding;
dispersing the fiber suspension in drops or particles into the
precipitation reactor; feeding the gas comprising the precipitant
inside the precipitation reactor; bringing the dispersed and
activated fiber suspension into contact with the precipitant of the
reactive mineral material in the precipitation reactor in order to
at least partly precipitate the reactive mineral material; and
discharging the treated fiber suspension from the precipitation
reactor; feeding the paper pulp containing precipitated mineral
material at a predetermined consistency into a forming section of a
paper machine; removing water from the paper pulp by allowing the
pulp to drain through a water permeable forming base; and drying
and finishing the paper web thus produced in order to produce a
finished paper product.
38. The method according to claim 37, wherein the activation is
performed while the fibers are swollen due to adding of calcium
hydroxide (Ca(OH).sub.2).
39. The method according to claim 37, wherein in the paper pulp is
added at least 20 wt % of nano-sized filler particles in relation
to the total weight of the pretreated fiber.
40. The method according to claim 39, wherein the nano-sized filler
particles comprise precipitated calcium carbonate.
41. The method according to claim 37, wherein the paper web is
sized with wet end size.
42. The method according to claim 37, wherein the paper web is
calendered.
43. The method according to claim 37, wherein the paper web is
coated.
Description
[0001] The present invention relates to a method for manufacturing
paper in accordance with the independent patent claims presented
below. The invention also relates to paper.
[0002] Fillers containing mineral materials, such as natural
fine-ground calcium carbonate, precipitated calcium carbonate
(PCC), kaolin and talc are used in paper manufacture in order to
improve various paper properties, including optical and printing
properties. Furthermore, adding filler makes it possible to use a
smaller amount of fibre material in paper manufacture. The cost
savings thus achieved generally dearly exceed the costs of the
filler added.
[0003] It is thus generally considered desirable to add as much
filler as possible to the fibre suspension used in paper
manufacture. Due to factors relating to the strength properties of
the paper, however, the amount of filler added, such as calcium
carbonate, may not usually exceed 20-25%.
[0004] In order to increase the amount of calcium carbonate, it has
been proposed that a calcium-based filler be added to the fibre
suspension in the form of calcium hydroxide, and that the calcium
therein be converted into precipitated calcium carbonate by adding
carbon dioxide gas. This will cause the calcium carbonate to
precipitate and adhere both directly onto the surface and also into
the fibres, and in this way more carbonate can be added to the
paper.
[0005] Weaknesses of these prior art solutions may be considered to
be that: [0006] the precipitation reactions require a relatively
long time to take place, [0007] the precipitation reactions are
partially incomplete, [0008] the processes used are not continuous,
or [0009] the apparatuses used are not easily integrated into the
paper manufacturing process.
[0010] U.S. Pat. No. 6,471,825 suggests precipitating the calcium
hydroxide added to the fibre suspension directly onto the fibres in
calcium carbonate form. In this case it is proposed that the
suspension containing fibres and calcium hydroxide be first treated
in a disc refiner-type apparatus in order to disperse any fibre
bundles prior to feeding the carbon dioxide gas into the
suspension.
[0011] In disc refiner-type apparatuses the fibre suspension is
exposed to rough treatment that has a weakening effect on the fibre
material. After feeding the carbon dioxide into the fibre
suspension, the suspension is mixed in an auger mixer. However,
with conventional precipitation reactors equipped with blade or
auger mixers it is difficult to ensure the fast and efficient
mixing of the carbon dioxide and calcium hydroxide, thus securing
as complete as possible a reaction. In these reactors it is also
hard to achieve binding of the precipitated calcium carbonate to
the fibres.
[0012] U.S. Pat. No. 5,679,220, in turn, discloses a method wherein
the calcium hydroxide added to the fibre suspension is precipitated
onto the fibres in the form of calcium carbonate with the help of
carbon dioxide gas, while the fibre suspension flows through a
long, two-compartment, tube-like reactor with smooth interior
walls. The suspension containing calcium hydroxide is fed into the
fibre suspension at about the middle of the first compartment of
the tube-like reactor. Carbon dioxide gas is fed into the fibre
suspension both before and after feeding the suspension containing
calcium hydroxide into it. The carbon dioxide gas is fed into the
reactor through an inlet formed in the wall of the reactor with the
purpose of causing the gas to be absorbed into the suspension
flowing past inside the tube. The residence time of the fibre
suspension in the relatively long mixing reactor, exceeding 2
meters in length, is more than 1 minute.
[0013] Furthermore, methods of paper manufacture are known wherein
the fibre material is loaded with filler material, as is disclosed,
for example, in the EP application 969141. It is characteristic of
this method that the paper is calendered after the web formation.
The pretreatment of the fibre material, wherein the fibres are
provided with calcium carbonate, is not disclosed in great detail
in the publication.
[0014] Consequently, the aim of the present invention is to create
a paper manufacturing method superior to previous methods. A
further aim of the invention is to create a superior paper.
[0015] The purpose is to create a method wherein the
above-mentioned problems of the prior art have been minimised.
[0016] The purpose is thus firstly to create a method for ensuring
that the fibres and the mineral material, such as calcium hydroxide
or calcium oxide, and the precipitant chemical, such as carbon
dioxide gas, are very well mixed during the precipitation process,
and to enable the manufacture of paper from fibre material
pretreated in such a way.
[0017] The purpose is thus also to create a method that enables the
precipitation of the calcium carbonate onto or into the fibres to
start and occur in a very short time and as completely as
possible.
[0018] The purpose is hereby also to create a method that enables
the filler content of the paper to be increased compared to that
achieved by conventional practices.
[0019] The purpose is also to create a method that enables the
properties, typically the optical and strength properties of the
paper, paperboard or other corresponding product, to be influenced
in a desired manner.
[0020] The purpose is also to create a method suitable for use in
precipitating mineral material onto the fibres of very different
fibre suspensions and onto any other solid material present in the
said fibre suspension.
[0021] A further purpose is to obtain a continuous process for
manufacturing paper, paperboard or a corresponding product in web
form.
[0022] In order to achieve the above-mentioned objectives, the
method and the paper of the present invention are characterised by
what is presented in the detailed description of the invention
presented below in the independent patent claims.
[0023] The present invention relates to a method of paper
manufacture, generally comprising the following steps: [0024] (a)
the fibre material that is used as raw material for the paper pulp
is fed into the precipitation reactor; [0025] (b) a reactive
mineral material, such as calcium hydroxide Ca(OH).sub.2, is fed
into the precipitation reactor; [0026] (c) the reactive mineral
material and the fibre material are combined to form a fibre
suspension in the precipitation reactor and/or prior to feeding
these materials into the precipitation reactor; [0027] (d) the
fibre suspension is brought into contact with the precipitant of
said reactive mineral material in the precipitation reactor, in
order to precipitate the reactive mineral material within the
suspension at least partially, whereby at least some of the thus
formed precipitated mineral material is precipitated onto the
fibres within the fibre suspension, in such a manner that (d1) gas
containing a substance, such as carbon dioxide, that precipitates
the said mineral material, is fed into the precipitation reactor in
order to form a gas space containing said precipitant inside the
precipitation reactor, and (d2) the fibre suspension fed into the
precipitation reactor and/or formed therein is dispersed, or
scattered, in the form of small particles, such as drops and/or
particles containing solid matter and/or liquid, into said gas
space; [0028] (e) the fibre suspension treated in this manner is
discharged from the precipitation reactor; whereafter the paper
pulp formed by the fibre suspension is fed onto the forming section
of the paper machine and is made into paper by allowing the pulp to
drain through a permeable forming base.
[0029] Typically a gaseous precipitant is fed in a continuous gas
stream into the precipitation reactor in order to maintain the
desired gas space inside the reactor. The amount of precipitant in
the gas may be varied considerably, depending on, for example, the
source of the gaseous precipitant, its quality, and/or the desired
paper properties. The gas fed into the precipitation reactor
usually contains a precipitant, such as carbon dioxide, >5%,
typically >10%, even 100% if desired. The gas containing the
precipitant may thus be pure or nearly pure carbon dioxide, flue
gas or other suitable gas or gas mixture containing carbon dioxide.
If desired, other precipitants than carbon dioxide suitable for
precipitating the chosen reactive mineral material may naturally be
used. The gas is typically fed into the precipitation reactor in
such a manner that will create an over-pressure inside the
precipitation reactor.
[0030] The method of the present invention aims at feeding the
fibre suspension, with its liquid and solid phases, into the gas
space dispersed into extremely small parts in the form of drops
and/or particles. The fibre suspension is in this case dispersed,
according to a known or new method, into pure liquid drops; liquid
drops that contain solid matter, such as fibres and mineral
material; solid matter particles and/or solid matter particles
coated with liquid. The fibre material in the fibre suspension is
thus dispersed, at least partly, into separate fibres. The liquid
phase of the fibre suspension is dispersed on the other hand into
liquid drops mainly <10 mm, typically <1 mm. The small liquid
drops, fibres and other solid matter particles disperse into the
gas space to form an almost mist-like gas suspension, which has a
volume flow rate considerably higher than that of the fibre
suspension being fed into the reactor. This creates a large contact
area between the drops and/or particles and the surrounding gas,
which enables extremely fast and complete precipitation reactions
between the reactive mineral material to be precipitated and the
precipitant in the gas.
[0031] When applying the method of the present invention, it may
furthermore be assumed that, principally, nearly every separate
fibre is surrounded by a gas envelope, which causes fast and
efficient precipitation of the mineral material from the
surrounding liquid onto the fibre surfaces and into the fibre.
Previously the aim has been, conversely, to feed gas in fine
bubbles into a more or less viscous fibre suspension, in which
method the precipitation process is not as fast or as efficient as
with the present invention.
[0032] In applying the method of the present invention, extremely
active precipitated material areas are formed on the fibres,
through which areas the fibres presumably form inter-fibre bonds
while the precipitation reactions continue in these areas. These
bonds enhance the strength properties of the manufactured
paper.
[0033] According to a preferred embodiment of the invention an
activation zone has been formed in front of or into the
precipitation reactor, preferably at its starting point in relation
to the flow of the fibre material. In the activation zone, the
fibre suspension is exposed to forces that activate the fibres, for
example, tribomechanically or tribochemically, in a way that
enhances the capacity of the fibres to bond with each other or to
bind precipitating and/or precipitated mineral material to
themselves. Activating the fibres has a positive effect on the
strength properties of the manufactured paper.
[0034] In the activation zone of the preferred embodiment, the
fibre suspension may be both dispersed into small drops and/or
particles and activated. This activation is preferably carried out
in alkaline conditions, while the fibres are swollen due, for
example, to the addition of Ca(OH).sub.2.
[0035] In the activation zone, the fibre suspension may be
subjected, for example, to successive impacts, counter impacts,
shearing forces, turbulence, over- and under-pressure pulses or
corresponding forces that mechanically activate the fibres,
particularly their surfaces, for example by fibrillating or
grinding the fibres or opening the inner parts of the fibres
(lumen) to the mineral material. On the other hand, fibres,
especially fibre surfaces, may also be activated chemically in such
a way that active OH-groups are formed on their surfaces.
[0036] According to a preferred embodiment of the invention, the
activation may be induced, for example, in a precipitation reactor
with an activation zone provided with a flow-through mixer
operating by the so-called multi-cage impact mill principle, with
several, typically 3-8, most typically 4-6 concentric cages
equipped with blades or the like. At least every other one of the
cages acts as a rotor and the adjacent cages act as stators or
rotors moving in different directions or at different speeds. The
speed of the rotors may vary between 5 and 250 m/s. The speed
difference of adjacent cages is 10-500 m/s, typically 50-200 m/s.
Mills or mixers operating by this principle have previously been
presented, for example, in the Finnish patent publications 105669B,
105112B and in publication WO-96/18454.
[0037] In a flow-through mixer operating by the impact mill
principle, the fibre suspension is typically passed through the
mixer radially outwards from the cage centres, which enables the
blades or the like on the cages to bring impacts and counter
impacts to bear on the outward flowing fibre suspension, thus
creating shearing forces, turbulence and under- and over-pressure
pulses, which have a fibre-activating effect. A reactor operating
by the impact mill principle can efficiently treat fibre
suspensions with both high and with very low solid matter contents,
making them suitable for the precipitation phase. In a
precipitation reactor in accordance with the present invention, it
is therefore possible to precipitate mineral materials with highly
varying solid matter contents, such as 0.1-40%, typically 1-15%,
most typically 3-7% solid matter contents. Limitations are mainly
due to the pumpability of the fibre suspension in the inlet and
outlet tubes.
[0038] The adjacent cages, rotors, blades or the like of the
flow-through mixer typically move in opposite directions, which
enables effective, successive impacts in opposite directions, that
is, impacts and counter impacts, to be brought to bear on the fibre
suspension flowing through the reactor. If, on the other hand,
stationary cages, or stators, are arranged between the cages moving
in the same direction, that is, between the rotors, it is possible
to bring the impacts produced by the rotor blades and counter
impacts caused by the rotor blades colliding with the stator blades
to bear on the fibre suspension flowing through the reactor. A
similar result can be produced with rotors moving in the same
direction at highly different speeds.
[0039] The blades or the like of the rotors and stators of the
flow-through mixer can also direct the fibre suspension to proceed
radially outwards from the cage centres. As the rotor and stator
cages increase in size outwards from their centres, a pressure
difference is created between the inlet of the flow-through mixer,
or the centre, and the outlet, or the outer cage. The pressure
drops when moving outwards from the centre. The consequent pressure
difference helps the passage of the fibre suspension through the
flow-through mixer.
[0040] According to a preferred embodiment of the invention,
mechanical activation is the case, for example, when the fibre
surfaces are treated in a manner that uncovers free and reactive
fibre surfaces, to which the precipitating mineral materials can
easily adhere, or in such a manner that brings fibrils to the
surfaces of the fibres, to which fibrils the precipitating mineral
material can easily adhere. The formation of fibrils increases the
specific surface area of the fibres, enabling the fibre to bind
increasing amounts of precipitating mineral material. Some of the
fibrils formed may be detached from the fibres and thus increase
the amount of fines in the fibre suspension, which in some cases is
desirable.
[0041] According to a preferred embodiment, mechanical activation
is also the case when the fibres are treated with over- and
under-pressure pulses in such a manner that the fibres open up,
tear or so that holes are formed in them, enabling a larger amount
of the reactive mineral material in the fibre suspension to
penetrate into the fibre and to precipitate there.
[0042] According to a preferred embodiment, chemical activation is
the case, for example, when the fibre surfaces are activated in
such a manner that active chemical groups, which are able to bind
precipitating or precipitated mineral material, are formed on the
fibre surfaces. For example, active OH-groups, which are able to
form bonds with the mineral material and to bind the mineral
material to the fibres, can be created on the fibre surfaces.
[0043] According to a typical method of this invention, the fibre
material and the reactive mineral material, such as milk of lime,
Ca(OH).sub.2 are preferably combined in a fibre suspension before
introducing these materials into the precipitation reactor. The
fibre suspension containing fibre material and reactive mineral
material is typically formed by adding the reactive mineral
material to be precipitated in the form of a slurry or suspension
to the fibre material suspension. The slurry or suspension can be
mixed quickly and evenly into the fibre suspension. On the other
hand, the reactive mineral material to be precipitated may also be
added into the fibre material suspension in solid form, for
example, in the form of a powder. When the reactive mineral
material is added to the fibre material suspension prior to feeding
the suspension into the precipitation reactor, the fibres have
several minutes to absorb reactive mineral material, if desired,
and in case the mineral material is alkaline, it contributes to
swelling the fibres into an advantageous form as regards activation
and/or carbonating. In this case, the mineral material is more
easily precipitated onto and into the fibres at the beginning of
the precipitation process. If desired, the fibre material and the
mineral material may naturally be introduced into the precipitation
reactor separately, allowing them to mix only in the precipitation
reactor.
[0044] When applying the method according to this invention, the
mineral material precipitation conditions, such as the raw
material, the feed ratio of the raw material, pH, pressure and
temperature may be chosen to suit the process in question. The
solutions of this patent do not set any limitations to these
conditions.
[0045] This description refers, unless otherwise mentioned, to
[0046] a fibre material suspension, meaning a liquid based
suspension containing at least fibre material, [0047] a fibre
suspension, meaning a liquid based suspension containing at least
fibre material and a reactive mineral material necessary for
precipitation, [0048] a gas suspension, meaning a suspension formed
of fibre material, a reactive mineral material and a gaseous
precipitant, in which the fibre material and the reactive material
are fine-grained, and [0049] the treated fibre suspension, meaning
a liquid-based suspension containing at least fibre material and
precipitated mineral material particles.
[0050] The above-mentioned suspensions may naturally also contain
other materials, such as precipitated mineral particles or
non-precipitated mineral material.
[0051] In accordance with the method of the present invention, the
reactive mineral material used may be calcium hydroxide
(Ca(OH).sub.2), that is, milk of lime, or other Ca.sup.2+ ion
sources, whereby so-called precipitated calcium carbonate (PCC) can
be made to precipitate onto and/or into the fibres. The present
invention also enables the use of other similar reactive mineral
materials, such as calcium oxide or calcium sulphate, which may be
precipitated onto and bound to the fibres with a gaseous
precipitant.
[0052] The reactive mineral material used in precipitation is
chosen according to which property in the fibres, the manufactured
paper or the manufacturing process is to be improved. A mineral
material being to be precipitated into the fibre suspension,
especially into the fibres, can help to enhance paper properties,
such as whiteness, brightness, opacity, gloss, bulk, print,
printability, drainability, drying, etc.
[0053] A gaseous precipitant is preferably used as the
precipitating chemical. For example, carbon dioxide can thus be
used as a gaseous precipitant for calcium hydroxide. Hence, gas
containing carbon dioxide, such as pure or nearly pure carbon
dioxide (CO.sub.2), flue gas or other suitable gas can be fed into
the precipitation reactor. Other suitable precipitants besides
carbon dioxide may also be used.
[0054] The invention makes it possible to precipitate precipitable
reactive substances in the fibre suspension not only onto the
fibres but also onto the surfaces of other non-organic or organic
particles in the suspension. These particles may include, for
example, mineral material particles. Such particles may be titanium
oxide particles, particles of impurities, or fines from the fibres.
The method of the present invention may in this case also be used
to cover ink residues left on incompletely de-inked fibres with
precipitated calcium carbonate or a corresponding substance.
Reactive substances precipitated onto non-organic particles have
the capacity to bind the particles to fibres, in which case the
particles are retained with the fibres in the paper. On the other
hand, the mineral material precipitated onto the fibres has the
capacity to bind fibres to each other, which enhances the strength
of the paper to be manufactured.
[0055] In addition to the fibre material and the reactive mineral
material to be precipitated, the fibre suspension fed into the
precipitation reactor may contain other solids used in papermaking
or the like, such as [0056] another mineral material, such as
calcium oxide, calcium sulphate, calcium carbonate, talc, kaolin or
titanium oxide, [0057] fibre-based fines, other fines or
impurities, such as impurities detached from the fibres during
de-inking, various process rejects and/or [0058] substances used
for enhancing retention, such as starch and biocides.
[0059] Many of the above-mentioned substances can be introduced
into the paper pulp containing PCC after the reactor phase, before
the paper pulp is fed from the headbox to the forming section, onto
one moving forming base (wire) or between two moving forming bases
(wires).
[0060] The present invention is suitable for use in the
manufacturing of paper, paperboard or other corresponding pulp or
web form product made out of fibre-like material. Consequently, the
present invention is suitable for use in [0061] the manufacture of
various web form products, such as newsprint, fine paper, magazine
paper, kraft paper, tissue paper, special paper or paperboard;
[0062] the manufacture of products made out of various types of
pulp, such as chemical, mechanical, chemimechanical,
thermomechanical or semimechanical pulp, recycled fibre pulp, or a
mixture of these; [0063] the manufacture of paper made out of
various types of fibres, such as primary fibre, chemical or
mechanical fibre, bleached or unbleached fibre, ground or
non-ground fibre, dried or non-dried, inked or de-inked recycled
fibre or fibre obtained from machine broke, or in the manufacture
of paper made from a mixture of these.
[0064] Paper pulp containing filler (for example PCC) produced in
paper manufacture by the method described above is fed at an
appropriate consistency from the headbox to the paper machine
forming section, which may be a single or twin wire former (one or
two moving, forming bases that are permeable to water). In this
forming section the coherent, continuous paper web is formed out of
the paper pulp ingredients as water and the materials and any
suspended fines dissolved in the water drain through the
above-mentioned one or two forming bases. In a preferred
embodiment, the production process is a neutral-alkaline paper
manufacturing process, which means that the pH of the fibre
suspension is generally between 6.5 and 9. The fibre suspension
containing PCC obtained by pretreatment can be fed from the reactor
to the pulp chest, from where it is passed through backwater
dilution (e.g. wire pit) to the pulp processing apparatuses
(de-aeration, screens, etc.), which are located before the headbox.
Before the headbox, other substances, such as substances that
influence the structure of the paper, for example wet end sizing
agents (e.g. ASA, AKD), compatible with the neutral alkaline-paper
manufacturing process and/or auxiliary agents, for example,
retention enhancing agents, can be mixed into the treated fibre
suspension containing PCC.
[0065] It is also possible that a fibre suspension containing PCC
forms a part of the final paper pulp that determines the
composition of the paper web, which pulp has been obtained by
mixing the above-mentioned treated fibre suspension into one or
more other pulp fractions that contain other fibres.
[0066] If in pretreating the fibre material of the paper pulp, the
fibres and reactive mineral material in the form of a fine fibre
suspension are fed into the gaseous precipitant, i.e. in the
opposite way to the previous methods, the reactive mineral
material, the fibre material and the gaseous precipitant can be
mixed together with considerable ease and efficiently in terms of
precipitation.
[0067] The precipitation reactions can start immediately, and the
reactions occur rapidly on the substantially large contact surfaces
between the small fibre suspension drops and the gas. Precipitation
is easily achieved both onto the surfaces of the fibres as well as
into the fibres. By adjusting the composition of the fibre
material, the reactive mineral material and/or the gaseous
precipitant, the method and apparatus of the present invention it
is possible to control the paper properties that are obtainable,
such as strength and optical properties.
[0068] It is assumed that the more finely the fibre suspension is
dispersed, the aster and more effective the reactions.
[0069] A flow-through mixer that operates according to the impact
mill principle enables the dispersion of the fibre suspension into
a gaseous precipitant to form a mist-like gas suspension, wherein
the gas, the fibres, and the reactive mineral material to be
precipitated are mixed together extremely efficiently. The method
of the present invention enables the components of the
precipitation process to be microhomogenised to form a gas
suspension wherein the different components may react with each
other immediately. This is preferable particularly when, for
example, an activated fibre is inclined to return to a
non-activated state, that is when the fibrils or holes forming in
the fibre are inclined to dose. The mineral material in the fibre
suspension has at least partially a tendency to prevent the
reversion of the fibrils. If required, the fibre suspension may be
re-activated once or several times.
[0070] By activating the fibre material prior to the precipitation
phase and/or during the precipitation phase in a way that enhances
the capacity of the fibres to bond with each other and to bind
precipitated mineral material, it is possible to obtain a more
efficient precipitation phase and improved paper properties. Even a
single treatment in the precipitation reactor may suffice to obtain
the desired precipitation phase and the desired paper
properties.
[0071] In the following, a brief description of the invention is
given with reference to the accompanying drawings, wherein
[0072] FIG. 1 illustrates schematically and by way of an example a
vertical cross-section of a precipitation reactor in accordance
with the method of the present invention;
[0073] FIG. 2 illustrates schematically and by way of an example a
horizontal cross-section of a dispersion and activation phase
arranged in a precipitation reactor as shown in FIG. 1;
[0074] FIG. 3 illustrates schematically and by way of an example a
vertical cross-section of a second precipitation reactor in
accordance with the method of the invention;
[0075] FIG. 4 illustrates schematically and by way of an example a
horizontal cross-section of a dispersion and activation apparatus
arranged in a precipitation reactor such as the one shown in FIG.
3;
[0076] FIG. 5 illustrates schematically and by way of an example a
vertical cross-section of a precipitation reactor group in
accordance with the method of the invention;
[0077] FIG. 6 illustrates schematically and by way of an example a
vertical cross-section of a second precipitation group in
accordance with the method of the invention;
[0078] FIG. 7 illustrates schematically and by way of an example a
vertical cross-section of a third precipitation reactor group in
accordance with the method of the invention, and
[0079] FIG. 8 illustrates schematically a method for manufacturing
the paper according to the invention.
[0080] FIG. 1 illustrates a continuous precipitation reactor 10 in
accordance with the invention, which comprises a precipitation
vessel 12, dispersion and precipitation apparatus 14 arranged in
the precipitation vessel, a fibre suspension inlet tube 16, a
gaseous precipitant inlet tube 18 and a treated fibre suspension
outlet tube 20. In addition, the apparatus comprises a drive 22 and
bearings and sealing 24 between the drive 22 and the apparatus
14.
[0081] The dispersion and activation apparatus 14, of which a
horizontal cross-section is shown in FIG. 2, is a so-called
flow-through mixer, which has six concentric cages 26, 26', 26'',
28, 28', 28'' equipped with blades 26a, 26'a, 26''a, 28a, 28'a,
28''a. In apparatus 14, the fibre suspension is dispersed into
small particles, liquid drops and/or solid particles. The same
apparatus 14 is used to activate the fibres of the fibre suspension
in a way that enhances the capacity of the fibres to bond together
and to receive precipitated mineral material. The residence time in
the dispersion and activation apparatus is short, <10 s.,
typically <2 s., most typically even less than 1 s.
[0082] As the arrows in FIG. 2 indicate, the first cages 26, 26',
26'' of the dispersion apparatus serve as rotors, which in the case
shown in the drawing move counterclockwise. The other cages, 28,
28', 28'', adjacent to the first cages, also serve as rotors but,
in the case shown in the drawing, move clockwise. The cages have
been fitted with blades 26a, 26a', 26a'', 28a, 28a', 28a'', which
collide with the fibre suspension moving radially outwards through
the apparatus, subjecting it to repeated impacts and
counter-impacts. At the same time, over-pressure forms between the
adjacent blades of the rotor as the blades approach each other, and
under-pressure is formed as the blades move away from each other.
The differences in pressure create very rapid under- and
over-pressure pulses. At the same time, shearing forces and
turbulences are created in the fibre suspension passing through
apparatus 14.
[0083] The fibre suspension or fibre slurry containing fibre
material and reactive mineral material is fed through tube 16 into
the centre 30 of the dispersion and activation apparatus, from
where the fibre suspension, induced by the rotor blades and the
pressure difference between the centre and outer cage, moves
radially outwards towards the open outer edge 32 of the outermost
cage 20''. If required, the fibre suspension can also be fed into
the apparatus 14 between the cages. If desired, the fibre material
and the reactive mineral material can be fed into the dispersion
and activation apparatus 14 through separate tubes, in which case
the fibre suspension containing fibre and mineral material is not
formed until this stage.
[0084] The impacts and counter impacts, shearing forces and
turbulence as well as the over- and under-pressure pulses caused by
the rotor blades moving in opposite directions disperse the fibre
suspension into extremely fine parts, liquid drops and solid
particles, simultaneously activating the fibres, for example by
fibrillation. The activation is effective due, among other things,
to the powerful impacts and the great shearing forces exerted on
the fibre suspension. In accordance with the method of the present
invention, however, the fibre suspension is able to move along a
relatively open route through the cages and is therefore not
subjected to such grinding and fibre-breaking forces as are fibres
processed by disc or cone refiner-type methods. In accordance with
the solution of the present invention, the fibres touch the
surfaces of the rotor blades only momentarily, if at all.
[0085] In the pretreatment phase illustrated in FIGS. 1 and 2, in
accordance with the method of the invention, the gaseous
precipitant is fed into the centre 30 of the cages of the
dispersion and activation apparatus by tube 18. From this central
point, the gas flows radially outwards, forming a gas space 34
containing a gaseous precipitant in both the dispersion apparatus
and the space surrounding it in the precipitation vessel 12. The
gas is discharged from the upper part of the precipitation reactor
by tube 21. If desired, the gaseous precipitant may also be fed
into and/or between the cages of the dispersion and activation
apparatus. The precipitating reactions may already start in the gas
space of the dispersion and activation apparatus.
[0086] While being treated in the dispersion and activation
apparatus 14, the fibre suspension forms extremely fine drops and
particles that are dispersed from the apparatus 14 into the part
34' of the gas space surrounding it. The fine drops and particles
are flung out of the dispersion and activation apparatus, mainly
from the whole area of its outermost cage in a mist-like flow 36.
Outside the dispersion and activation apparatus, the precipitation
reactions may continue for a relatively long time, as the fine
drops and particles spread out over a large area in the
precipitation vessel. The treated fibre suspension settles in a
pool located at the bottom of the precipitation vessel and is
discharged from the vessel via tube 20.
[0087] The size, shape, width and height of the precipitation
vessel 12 may be selected so that the drops and particles being
flung out of the dispersion and activation apparatus have an
optimal residence time in the gas space 34' of the precipitation
vessel. The residence time of the fibre suspension can be
prolonged, for example, by adding to the height of the
precipitation vessel 12, making its shape tower-like.
[0088] The processes taking place in the precipitation reactor 10
can also be controlled by adjusting, for example, the number of
cages in the dispersion and activation apparatus, the distance
between the cages, the distance between the blades on each cage or
the blade dimensions and position.
[0089] The fibre suspension discharged from the bottom of the
precipitation vessel 12 can be circulated back into the same
precipitation reactor or fed into another reactor in order to
complete the treatment.
[0090] In FIGS. 3 and 4, which illustrate another pretreatment
phase, the precipitation reactor along with its dispersion and
activation apparatus in accordance with the method of the
invention, the same reference numbers are used as in FIGS. 1 and 2,
where appropriate. The second precipitation reactor 10 in
accordance with the present invention shown in FIG. 3 differs from
the ones shown in FIGS. 1 and 2 mainly in that the reactor
comprises a dispersion and activation apparatus 14 equipped with an
enclosed outermost cage 32 and in that the precipitation reactor
does not comprise a separate precipitation space extending beyond
the dispersion and activation apparatus. A method in accordance
with FIGS. 3 and 4 is suitable for use, for example, when it can be
assumed that the precipitation reactions have time to occur in the
desired manner already in the gas space of the dispersion and
activation apparatus.
[0091] In a dispersion apparatus such as the one in FIGS. 3 and 4,
a housing 40 surrounds the outermost cage 28'', enclosing the cage.
An outlet 42 has been formed in the housing for discharging the
treated fibre suspension from the apparatus 14. The treated fibre
suspension may be fed through the outlet 42 to further treatment or
a further process.
[0092] The reactor in FIG. 3 is suitable for use in activating the
fibre suspension also in cases where the precipitation does not
take place in this apparatus. Both the precipitation reactor in
FIG. 1 and the reactor in FIG. 3 may be arranged to form a series
of two or more reactors. FIG. 5 presents a group of three
precipitation reactors such as the one shown in FIG. 1. In this
drawing, the same reference numbers are used as in the previous
figures, where appropriate.
[0093] FIG. 5 presents three precipitation reactors 10, 10' and
10'', wherein the fibre suspension containing Ca(OH).sub.2 is
treated with CO.sub.2 gas in order to carbonate the Ca.sup.2+ ions,
i.e. in order to precipitate the CaCO.sub.3. The reactors are
connected in series in such a manner that the partly treated fibre
suspension containing fibres, precipitated carbonate and
unprecipitated calcium hydroxide from the first reactor 10 is fed
from outlet 20 to inlet 16' of the second reactor 10'. From the
second reactor 10' the treated fibre suspension is respectively fed
from outlet 20 to inlet 16'' of the third reactor 10''.
[0094] Carbon dioxide-containing gas is fed into each reactor via
tubes 18, 18' and 18''. Via inlet 18, carbon dioxide-containing gas
is introduced into the first reactor 10, starting the precipitation
(carbonating) and the formation of active carbon on the fibres
already in the dispersion and activation apparatus 14. The
precipitated calcium carbonate precipitates both onto the fibres
and onto other particles in the fibre suspension. Carbonate is also
precipitated as separate particles into the fibre suspension. The
same or other carbon dioxide-containing gas may be fed by tubes 18'
and 18'' into the second and third precipitation reactor 10' and
10'' in order to complete the precipitation reactions
(carbonation). The gas is discharged from the reactors via outlets
21, 21' and 21''.
[0095] The fibre suspension fed to the precipitation reactor 10 may
be activated before feeding it to the reactor in a separate
activation apparatus connected before the precipitation reactor 10.
The activation apparatus is preferably an impact mill-type
flow-through mixer.
[0096] FIG. 6 presents a second precipitation reactor group, which
includes two serially connected precipitation reactors 10 and 10'
according to FIG. 1. Before the first precipitation reactor 10
there is a flow-through mixer-type activation apparatus 44 mainly
structured as in FIG. 3. Inside this activation apparatus the fibre
material fed to the precipitation reactor is activated. Gaseous
precipitant is not, however, fed into the activation apparatus.
[0097] The fibre material is fed via tube 46 to the activation
apparatus 44 from above. The activated fibre material is fed
through an intermediate tank 48 to the first precipitation reactor
10. A precipitating mineral material, calcium hydroxide, may be
added to the suspension via tube 50 before the activation apparatus
44 or via tube 52 after the activation apparatus. In the
intermediate tank 48 the fibre suspension is allowed to swell for a
predetermined period of time in alkaline conditions. From the
intermediate tank, the fibre suspension, including the fibre
material and precipitating mineral material, is fed via a tube 16
from below into the dispersion and activation apparatus 14. The
gaseous precipitant 18, typically carbon dioxide, is fed along with
the fibre suspension into the apparatus 14. Gas is discharged from
the upper part of the precipitation reactor via tube 21, the gas
typically containing steam and carbon dioxide. The gas is fed for
processing in the gas washing and cooling apparatus 54. The carbon
dioxide-containing gas processed in the apparatus 54 is circulated
via the tube 18 back to the precipitation reactor 10. The treated
fibre suspension collecting in the bottom part of the precipitation
reactor is discharged from there into the outlet tube 20.
[0098] The second precipitation reactor 10' In FIG. 6 functions
mainly in the same manner as the first precipitation reactor 10.
The fibre suspension discharged from the bottom part of the first
reactor 10 into tube 20, typically including the fibre suspension,
calcium hydroxide and furthermore the precipitated calcium
carbonate, is fed via tube 16' from below into the dispersion and
activation apparatus 14' of the second reactor 10'. From the
washing and cooling apparatus 54, the carbon dioxide-containing gas
is led to the second reactor 10'. From the bottom part of the
second reactor 10', a fibre suspension which is mainly ready
treated and the desired amount of calcium carbonate has typically
precipitated onto the fibres, is discharged via tube 20', From the
upper part of the second reactor 10' gas is discharged and taken to
the gas washing and cooling machine 54 for recirculating.
[0099] FIG. 7 presents a third group of precipitation reactors,
consisting of three precipitation reactors 10, 10' and 10''
connected in series. The reactors are connected one on top of the
other, and the fibre suspension is fed into the dispersion and
activation apparatus of the reactors from above. The first reactor
10 is on top and the third reactor 10'' is at the bottom, so that
the fibre suspension flowing through the reactors goes mainly
downwards. In front of the third precipitation reactor, a separate
preactivation apparatus 44 according to FIG. 6 and an intermediate
tank 48 have been arranged.
[0100] The advantages of this invention include the following:
[0101] the fibre suspension may simultaneously be activated and
dispersed for the precipitation, [0102] the precipitation reactions
are extremely fast, effective and complete, and good results are
achieved even with one run through the precipitation reactor;
[0103] the activation process achieves strong and effective
treatment of the fibres without, however, especially breaking or
otherwise damaging the fibres; [0104] the activation process can be
regulated; [0105] extremely effective mixing of the fibre
suspension, mineral material and gas is achieved, as a result of
which each small volume unit in the fibre suspension is treated,
and precipitation takes place in each volume unit; [0106]
precipitation into the fibres can also be influenced; [0107] the
precipitation reactions bond the fibres together, and it may be
assumed that the paper strength is improved; [0108] the
precipitation reactions can cover ink residues that are left in the
fibres after de-inking; [0109] the precipitation reactions can bind
inorganic and organic particles to the fibres and therefore cause
them to be retained in the paper, and [0110] the precipitation
enables qualities such as brightness, strength and opacity of the
paper to be optimised better than before; [0111] pretreatment may
be integrated into a continuous papermaking process, where a
continuous paper pulp flow is formed from the PCC-containing fibre
suspension obtained from pretreatment, which suspension is fed from
the headbox to the forming section.
[0112] FIG. 8 presents schematically a method for papermaking. From
the pulp chest M, into which the fibre suspension is fed after the
above-described pretreatment process, the fibre material is fed via
dilution (e.g. wire pit P) to pulp preparation equipment, and from
there the paper pulp is fed to the headbox H and to the forming
section F, where the continuously progressing paper web W is
formed.
[0113] The purpose of the tests presented in the following example
is to compare the carbonation of the fibre/PCC product according to
this invention with other presented methods. The purpose is to
illustrate this invention, not to restrict its scope.
[0114] The materials used in all these tests were the same kind of
machine-round pine fibre for fine paper making, the concentration
being about 3.5%, Ca(OH).sub.2 slurry with solid matter about 17%,
and a CO.sub.2-containing gas with a similar composition.
[0115] (K1) By a method according to this invention a fibre/PCC
fibre product was formed by mixing fibre pulp containing pine fibre
and a required amount of Ca(OH).sub.2 slurry, in order to reach a
fibre/PCC ratio of 70/30 after the precipitation, and thereafter
pumping the fibre/Ca(OH).sub.2 suspension twice through the
precipitation reactor shown in FIG. 1. The fibre/Ca(OH).sub.2
suspension was then pumped according to the method of the invention
in the form of a fine suspension into the CO.sub.2-containing gas.
An excess of CO.sub.2-containing gas was then fed into the
apparatus. After this treatment the pH of the fibre/PCC product was
7.
[0116] (V1) For comparison a fibre/PCC product was formed with a
fluidising chemical mixer by pumping the fibre/Ca(OH).sub.2
suspension six times through the chemical mixer. Additionally, an
excess of CO.sub.2-containing gas was fed into the chemical mixer.
Immediately after the treatment the pH of the fibre/PCC product was
7.
[0117] (V2) For another comparison a similar precipitation as in
the test (V1) was performed, except that the chemical mixer was not
allowed to fluidise, and only an excess of CO.sub.2-containing gas
was fed into the mixer. The fibre/Ca(OH).sub.2 suspension was
pumped eight times through the chemical mixer. Immediately after
the treatment, the pH of the fibre/PCC product was 7.
[0118] (K1) The pH of the product made according to this invention
was 7, even 24 hours from production, which proves that the
carbonation was complete.
[0119] (V1) The pH of the product made according to this example
was 10 after 24 hours from production, which proves that the
carbonation was not complete, but the carbonation of the product
had to be continued for several minutes in order to complete the
carbonation reactions. Incomplete carbonation causes problems in
the chemistry at the wet end of the paper machine during
papermaking.
[0120] (V2) The pH of the product made according to this example
was 11 after 24 hours from production, which proves that the
carbonation was not complete, but the carbonation of the product
had to be continued for several minutes in order to complete the
carbonation reactions.
[0121] In all of these tests the time required for the actual
carbonation was short, but only with the method of this invention
was the carbonation complete in an extremely short time and no
further carbonation was required.
[0122] It is not the purpose to limit the scope of this invention
by the above-described descriptions and examples presented by way
of examples, but the purpose is to widely apply this invention
within the scope of the patent claims presented below. Therefore
the method of this invention can be used in a manufacturing method
for paper, paperboard or the like, in any other pretreatment of
fibre material used as a raw material in paper, paperboard or the
like, in order to activate the fibres and their surfaces, for
example, in such a manner that their capacity to bond mechanically
or chemically increases, their capacity to bind mineral material
mechanically or chemically increases, active OH-groups are formed
on their surfaces and/or that their inner parts (lumen) open up
allowing, among other things, the mineral material to be
precipitated also into the fibres. The fibre material is then
pretreated in a flow-through mixer operating by the impact mill
principle, which includes [0123] several, typically 3-8, most
typically 4-6, concentric cages provided with blades, from which at
least every other cage functions as a rotor, and the cages adjacent
to said cages function as stators or rotors, the speed difference
of the adjacent cages being 10-500 m/s, preferably 50-200 m/s,
[0124] feeding apparatus for feeding the fibre material mainly into
the centre of said cages and [0125] an open outer cage, which
allows the fibre suspension to flow radially outwards through the
cages to exit the cage in different directions, or an outer cage
provided with one or more outlets in order to discharge the fibre
suspension flowing radially outwards from the cages.
[0126] The pretreatment is performed preferably when the fibres are
swollen, for example, due to the addition of Ca(OH).sub.2. The
pretreatment of the fibres according to this invention is
especially suitable for activating the fibre material before
bringing the fibre material into contact with the reactive mineral
material, which mineral material is to be precipitated onto the
fibres. The pretreatment according to this invention is,
nevertheless, also suitable to be used in other processes in which
the aim is to pretreat the fibre material in order to achieve
similar properties required in the fibre material.
[0127] In a paper made according to this invention the filler PCC
is "hidden" inside the fibre mesh and inside the lumens, in such a
manner that it does not affect the forming of bonds or sheet
formation. This kind of paper has better strength properties than a
paper in which the PCC derives from a mineral added to the paper
pulp fibres as such.
[0128] The 80 g/m.sup.2 paper according to this invention was made
with a PCC/fibre ratio of 30/70 and the reference paper was a paper
into which the PCC was introduced as a normal additive. The paper
made according to this invention had a better tensile strength
index (19.3->30.9) and tear resistance index (7.0->9.0).
Folding strength increased from 6 to 31.
[0129] This paper also has lower porosity (the sheet is denser),
and an improved opacity and formation compared to the reference
paper. The brightness of the sheet also diminishes compared to a
paper that contains the same amount of normal filler, since part of
the filler is "hidden".
[0130] The fibre in the paper, provided with PCC and pretreated as
described above, comprises preferably at least 20 wt-% PCC of the
dry weight of the treated fibres, preferably 20-50 wt-%. Preferably
the fibre comprises at least 25 wt-% PCC, for example 25 wt-%-50
wt-% introduced onto the fibre in the manner described above. The
paper pulp may include other fibres, in which case the proportion
of PCC introduced in the above-described method of the paper pulp's
total weight diminishes correspondingly. Preferable is a paper
product in which all the raw fibre material is pretreated according
to the above-described method, in which there is at least 20 wt-%
PCC, preferably 20 wt-%-50 wt-%, most preferably at least 25 wt-%,
for example 25 wt-%-50 wt-%.
[0131] The PCC in the paper made according to the invention, which
is introduced onto the fibres by the above described method, is
nano-sized precipitated calcium carbonate that typically has a
particle size below 100 nm. By selecting the precipitation
conditions it is possible to affect the average size of the
particles and the size distribution.
[0132] It is also possible to supplement the nano-sized filler in
the pretreated fibres by adding other fillers. The additional
filler may be a normal-sized filler, for example, separately
precipitated PCC, or a chemically different filler.
[0133] The paper made according to the method of this invention may
be finished after the drying of the paper web on the paper machine
either on-line or as a separate finishing process. In order to
finish the surface, the paper web may, for example, be calendered.
A paper web made according to the above-described method may be
used, after the possible calendaring, as such as printing paper
(e.g. SC paper), or the paper web may, after the possible
calendering, be coated on-line or in a separate coating machine
(e.g. LWC paper), in which case the coating functions as a printing
surface. If the fibre material onto which the PCC is introduced in
the pretreatment is a chemical pulp, the printing paper product
formed mainly or solely from this pulp may be an uncoated fine
paper, i.e. WFU, or a coated fine paper, i.e. WFC, or a copying
paper. This invention is nevertheless not restricted to printing
papers, but can be applied to all paper products. In the patent
claims the term "paper" refers to all resilient, fibre based
products originally made in the form of a web, regardless their
grammage, including paperboard.
* * * * *