Note: Descriptions are shown in the official language in which they were submitted.
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Process for removal of solid non-fibrous material from pulp
The present invention relates to a process for removal of solid non-fibrous
material from an
aqueous pulp suspension and to pulp comprising less than about 50 ppm of
silica-based
material obtained by the process.
Cellulosic pulp can be obtained by various processes such as processes where
the
cellulosic fibres of wood are separated by a chemical process or processes
including
mechanical means for separating the cellulosic fibres. The former processes
are often
referred to as chemical pulping processes and the latter as mechanical pulping
processes.
As to mechanical pulping processes such processes may also comprise steps
comprising
partly separating the cellulosic fibres by chemical treatment, alternatively,
subjecting the
wood, such as wood chips, to treatment stages facilitating the separation of
cellulosic fibres
in a subsequent mechanical treatment stage. Mechanical pulping processes
comprising
additional non-mechanical process steps may be referred to as thermomechanical
pulping
processes, chemimechanical pulping processes and chemithermomechanical pulping
processes. The main constituents of wood and some non-wood materials are
cellulose,
hemicelluloses and lignin. In a chemical pulping process, wood, typically wood
chips, is
subjected to a chemical process stage where wood chips are mixed with various
chemicals
and subjected to elevated temperatures and super atmospheric pressure. During
this
process stage, often referred to as the cooking stage, and operated in large
pressure
resistant vessels (digesters) the lignin is chemically removed from the wood
material
without significantly degrading the cellulose (such as cellulose and
hemicelluloses).
Accordingly, during the cooking process the content of lignin of the wood
material is
gradually decreased down to a region depending on the usage of the pulp and
various
other factors including type of cooking process and optional bleaching
sequence. In many
applications the pulp must have a high brightness. Brightness of pulp is
highly dependent
on the amount of lignin in the pulp. A pulp with a high brightness calls for a
low amount of
lignin. As already indicated the content of lignin is gradually reduced during
the cooking
process, however, also the cellulose is degraded during said process and
becomes more
pronounced as the content of lignin diminishes. Thus, the operation and
termination of the
cooking process is significantly based on the amount of residual lignin
present in the wood
material/cellulosic fibre material and the residual lignin contain is further
reduced in the
bleaching process provided pulp with high brightness is required. After
bleaching, the pulp
may be used in a paper mill which can be integrated with the pulping (pulp)
mill.
Alternatively, the bleached pulp suspension may be dewatered and subsequently
dried to a
format facilitating transportation to the location of further processing, such
as a paper/paper
board mill. In a measure of making the pulping process and paper making
process more
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economically and environmentally viable, the processes are more and more
closed such
that spent chemicals and water are recovered and re-circulated. Water from
e.g. the wire is
to an extent re-circulated. The increasing closure of the pulping process
(pulp mill) and the
paper manufacturing process (paper mill) leads to the accumulation of solid
material in the
system such as solid silica-based material.
Thus, it is an object of the present invention to provide a process for the
removal of solid
non-fibrous material such as solid silica-based material from aqueous
suspensions
comprising cellulosic fibres.
While a share of bleached pulp is used in the manufacture of paper and tissue
bleached
pulp may also be used for the production of textiles such as rayon and
cellophane and as a
raw material for the production of cellulose derivative including carboxyalkyl
substituted
cellulose. Typically, pulps having a high content of cellulose (such as above
about 92%
based on total pulp) and, optionally also high brightness, are used for the
above mentioned
non-paper/non-tissue materials. In order to obtain a pulp with a high content
of cellulose
other non-cellulose constituents of the wood are usually removed, specifically
the
hemicellulose. The hemicellulose of wood is usually removed prior to chemical
digestion by
acid hydrolysis. Pulp with a high content of cellulose based on total pulp is
often referred to
as dissolving pulp. Many processes using pulps having a high content of
cellulose are
improved if the amount of solid non-fibre materials, specifically silica-
containing materials,
is kept low. It is beneficial to have low contents of solid silica-based
material in the pulp
while manufacturing textile fibres such as rayon.
Thus, it is an objective with the present invention to provide a process for
the removal of
solid non-fibrous material, such as solid silica-based material, from an
aqueous pulp
suspension comprising cellulosic fibres, e.g. a pulp having a high content of
cellulose,
notably a dissolving pulp.
A further object of the invention is to provide a process which is capable of
removing solid
non-fibrous material, such as silica-based material from an aqueous pulp
suspension
comprising cellulosic fibres in an energy efficient manner.
A still further object of the invention is to efficiently remove solid non-
fibrous material having
a very small particle size, such as an average particle size of less than
about 100 um.
Still further objects are apparent from the application below.
US 736464062 discloses a process for removing silica from nonwood plant
materials
involving mechanical and chemical action.
US 6190504 relates to a method in a paper machine for arrangement of water
circulation
and arrangement of a water circulation system in a paper machine.
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WO 2008/067916 Al relates to a method of treating filtrate produced in
thickening of paper
fibre suspension. One objective with the method is to reduce the loss of fine
particles which
is achieved by recycle back fine particles (13) separated from the filtrate
(F6) downstream.
The objects of the invention can be achieved by a process as defined by the
claims.
The present invention relates to a process for removal of solid non-fibrous
material from an
aqueous pulp suspension comprising providing an aqueous pulp suspension
comprising
solid non-fibrous material, removing at least part of the solid non-fibrous
material from the
aqueous pulp suspension thereby forming an aqueous pulp suspension depleted in
solid
non-fibrous material (yet further comprising residual solid non-fibrous
material) and also
typically an aqueous stream comprising at least part of the solid non-fibrous
material (may
be referred to as first stream, H1A) , dewatering the aqueous pulp suspension
depleted in
solid non-fibrous material thereby forming a web comprising cellulosic fibres
and a filtrate
comprising residual solid non-fibrous material, the filtrate typically also
comprising
cellulose-containing fibres, removing residual solid non-fibrous material from
the filtrate
thereby forming a filtrate depleted in residual solid non-fibrous material and
an (further)
aqueous stream comprising at least part of the removed residual solid non-
fibrous material
(may be referred to as second stream, H2A); and diluting the aqueous pulp
suspension
and/or the aqueous pulp suspension depleted in solid non-fibrous material with
at least part
of the filtrate depleted in residual solid non-fibrous material, whereby the
aqueous stream
comprising at least part of the removed residual solid non-fibrous material is
purged from
the system. The present invention also encompasses pulp comprising less than
about 50
ppm of silica-based material (calculated as Si02) by weight based on total
oven dry pulp
obtainable by the present process.
An embodiment of the present invention relates to a process for removal of
solid non-
fibrous material from an aqueous pulp suspension comprising providing an
aqueous pulp
suspension comprising solid non-fibrous material, removing at least part of
the solid non-
fibrous material from the aqueous pulp suspension thereby forming an aqueous
pulp
suspension depleted in solid non-fibrous material and an aqueous stream
comprising at
least part of the solid non-fibrous material (H1A), dewatering the aqueous
pulp suspension
depleted in solid non-fibrous material thereby forming a web comprising
cellulose-
containing fibres and a filtrate comprising residual solid non-fibrous
material removing at
least part of the residual solid non-fibrous material from the filtrate
thereby forming a filtrate
depleted in residual solid non-fibrous material and an aqueous stream
comprising at least
part of the removed residual solid non-fibrous material (H2A), and diluting
the aqueous pulp
suspension and/or the aqueous pulp suspension depleted in solid non-fibrous
material with
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at least part of the filtrate depleted in residual solid non-fibrous material,
whereby the
aqueous stream comprising at least part of the solid non-fibrous material
(H1A) and the
aqueous stream comprising at least part of the removed residual solid non-
fibrous material
are purged from the system.
According to a preferred embodiment, the filtrate depleted in residual solid
non-fibrous
material is used to dilute the aqueous pulp suspension prior to the removal of
solid non-
fibrous material from the aqueous pulp suspension.
According to an embodiment the invention relates to a process for removal of
solid non-
fibrous material from an aqueous pulp suspension comprising cellulose-
containing fibres,
comprising providing an aqueous pulp suspension comprising solid non-fibrous
material
and cellulose-containing fibres, a first means for removing solid material
from an aqueous
suspension (first removal means), a second means for removing solid material
from an
aqueous suspension (second removal means), means for dewatering the aqueous
pulp
suspension comprising cellulose-containing fibres, the process comprising
removing at
least part of the solid non-fibrous material from the aqueous pulp suspension
by the first
removal means thereby forming an aqueous pulp suspension depleted in solid non-
fibrous
material (but comprising residual solid non-fibrous material), dewatering the
aqueous pulp
suspension depleted in solid non-fibrous material by the dewatering means
thereby forming
a web of cellulose-containing fibres and a filtrate comprising residual solid
non-fibrous
material (and typically cellulose-containing fibres), removing at least part
of the residual
solid non-fibrous material from the filtrate by the second removal means
thereby forming a
filtrate depleted in residual solid non-fibrous material and an aqueous stream
comprising at
least part of the removed residual solid non-fibrous material; and diluting
the aqueous pulp
suspension and/or the aqueous pulp suspension depleted in solid non-fibrous
material with
at least part of the filtrate depleted in residual solid non-fibrous material,
whereby the
aqueous stream comprising at least part of the removed residual solid non-
fibrous material
is purged from the system.
Fig. 1 is a schematic view of an embodiment of the process according to the
invention.
Fig. 2 is a schematic view of a process (not according to the present
invention) where the
residual solid non-fibrous material is not removed from the filtrate.
The term aqueous pulp suspension is used to denote an aqueous cellulose-
containing fibre
suspension further comprising solid non-fibrous material. Cellulose-containing
fibres may
also be referred to as lignocelluloses-containing fibres as residual lignin
usually is present.
In the present process, and also when the process is integrated within another
fibre
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process including a pulping process or process for recycling used cellulose-
containing
fibres, the definition aqueous pulp suspension relates to the main cellulose-
containing fibre
flow. The term pulp is used to denote a cellulose-containing fibrous material
obtained from
wood, non-wood sources, recycled fibres, and mixtures thereof. According to an
embodiment of the invention pulp is obtained from wood, or recycled fibres, or
mixtures
thereof. In addition to cellulose pulp usually also contains hemicellulose,
and residual
lignin, as well as other fibrous and non-fibrous constituents albeit in a
minor amount, such
as below about 20 % w/w based on total solids (fibre and other solid
material). According to
established practice the amount of lignin on the pulp may be expressed by the
Kappa
number. Within the context of this application the kappa number is established
according to
the Standard Method SCAN-C 1:77. Preferably, the pulp is obtained from wood
such as
hardwood and softwood. As already mentioned above, the pulp may be obtained by
a
chemical or mechanical pulping process. According to an embodiment, the pulp
is obtained
by subjecting cellulose-containing material to a chemical cooking process
including the
sulphate, sulphite, soda and organosolv process, i.e. chemically digested
pulps. Chemically
digested pulp can also be referred to as a chemical pulp. The chemical cooking
process is
preferably chosen among the sulphate process and sulphite process. Thus, the
pulp may
be chosen among sulphate pulp, sulphite pulp, soda pulp, organosolv pulp, and
mixtures
thereof, more preferably the pulp may be chose among sulphate pulp and
sulphite pulp, or,
the pulp is a sulphate pulp. Cooking of softwood using the sulphate process is
preferable,
i.e. sulphate pulp obtained from softwood. According to yet another embodiment
the pulp is
obtained by the sulphate or sulphite process, the processes also comprising
subjecting
wood, suitably wood chosen among softwood and hardwood, to a chemical
treatment stage
comprising the removal of hemicellulose prior to the chemical digestion
rendering pulp with
a content of cellulose above about 85% by weight based on total oven dry pulp,
suitably
above about 90%, above about 92%. Thus, according to an embodiment the pulp
has a
content of cellulose of above about 85% by weight, preferably above about 90%,
above
about 92%, such as above about 95% by weight based on total oven dry pulp.
Pulps
obtained by a pulping process comprising chemical digestion and a stage where
hemicelluloses is removed, are usually referred to as dissolving pulps. Thus,
according to
an embodiment the pulp is a dissolving pulp.
In order to further reduce the content of lignin and increase the brightness
of the pulp after
chemical cooking process in the digester, the pulp is subjected to a bleaching
process.
Usually, the bleaching process comprises a multistage process comprising
several
bleaching stages which usually are separated by washing stages. The content of
lignin in
the pulp gradually decreases through the bleaching process, whereas the
brightness of the
pulp increases. According to an embodiment of the invention the pulp, such as
pulp having
a cellulose content as disclosed above, such as a cellulose content above
about 85%,
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above about 90% , above about 92%, has a brightness of above about 60 % (ISO
Brightness), above about 80 % ISO, suitably above about 90 % ISO. According to
a further
embodiment, the pulp is dissolving pulp, suitably with a brightness of above
about 90 %
ISO, and optionally with a content of cellulose above about 80 % by weight
based on total
oven dry pulp, such as above about 85%, or above about 90% w/w.
According to yet another embodiment the pulp has a kappa number of below about
35 such
as below about 30, suitably below about 20.
According to yet another embodiment, at least part of the cellulose-containing
fibres
comprised in the pulp of the aqueous pulp suspension may originate from
recycled paper
or board or mixtures thereof, i.e. non-virgin cellulose-containing fibres. The
pulp may also
essentially consist of cellulose-containing fibres originating from recycled
cellulose
containing products (such as paper and board). For example, the pulp may
originate from
old corrugated containers (OCC). Pulp originating from recycled paper and
board (such as
OCC) contain an amount of solid non-fibrous material such as stickies, waxes,
hot smelts.
When recycling paper and board, i.e. re-using the fibre material (cellulose-
containing
fibres), it is beneficial if non-fibrous materials are removed prior to
processing the obtained
recycled fibre material on order to produce novel paper or board. The present
method can
preferably be applied for removing solid non-fibrous materials from pulp
originating from
paper and board, recycled pulp.
The consistency of the non-diluted aqueous pulp suspension comprising solid
non-fibrous
material and cellulosic fibres is typically at least about 2.0% by weight of
oven dry matter to
total weight of pulp suspension, typically from about 2.0 % by up to about
5.0% by weight
of oven dry matter to total weight of pulp suspension, suitably from about 2.5
up to about
4.0%, and more preferably from about 2.5 up to about 3.5%. Within the context
of this
application consistency is defined as the weight percentage of oven-dry matter
accounting
for the total weight of aqueous composition, e.g. aqueous pulp suspension or
filtrate. The
consistency in this application is calculated according to TAPPI T 240 om-07.
According to
an embodiment of the present invention the present process for removal of
solid non-
fibrous material from an aqueous pulp suspension comprising cellulosic fibres
is integrated
in a process selected from the group consisting of a pulping process or a
process for
recycling fibres. According to a further embodiment the present process is
integrated in a
pulping process. The aqueous pulp suspension, the aqueous pulp suspension
depleted in
solid non-fibrous material, and the web comprising cellulose-containing fibres
represent the
(main) fibre line. When the present process is integrated within a pulping
process the
present process is commonly situated after the bleaching process and typically
after the
final pulp storage tower. According to the invention at least part of the
solid non-fibrous
material comprised in the aqueous pulp suspension is removed by subjecting the
aqueous
pulp suspension to a first means for removal of solids from an aqueous
suspension. The
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first means for removal of solids may be any means which is capable of
effectively
separating solids, specifically solid-non fibrous material, from an aqueous
pulp suspension.
Typically, the first means is capable of removing solid non-fibrous particles
which would be
retained in the web comprising cellulose-containing fibres after the
dewatering operation.
The first means for removal may be based on size exclusion e.g. filters,
screens, or
separation/removal means based on centripetal force and fluid resistance, such
as a
multistage hydrocyclone system, or any combination of means based on size
exclusion and
on centripetal force and fluid resistance. According to an embodiment the
removal means
is a process separating solid material from a liquid based on the ratio of
centripetal force to
fluid resistance, such as a multistage hydrocyclone system. Due to the
consistency of the
aqueous pulp suspension, i.e. the content of fibres in the aqueous pulp
suspension, solid
non-fibrous material having a small particles size, typically solids having an
average
particle size of less than 150 ,m, is not effectively removed by the first
means. Thus, the
aqueous pulp suspension subjected to the first removal means and depleted in
solid non-
fibrous material still contains residual solid non-fibrous material. After
dewatering the
aqueous pulp suspension, suitably on a wire, a web comprising cellulose-
containing fibres
is obtained as well as a filtrate said filtrate comprising residual solid non-
fibrous material.
The residual solid non-fibrous material mainly follows the filtrate while the
fibres are mostly
retained in the web. The consistency of the filtrate is dependent on several
factors such as
the aqueous pulp suspension and type of dewatering process. Due to the
dewatering of the
aqueous pulp suspension the consistency of the filtrate is lower or
significantly lower, than
the consistency of the aqueous pulp suspension. Suitably, the consistency of
the filtrate is
preferably less than about 0.25% by weight of oven-dry matter accounting for
the total
weight of aqueous composition, such as less than about 0.1%, less than about
0.05%, and
suitably from about 0.01% up to about 0.25% by weight, preferably from about
0.01 up to
about 0.1% by weight. According to the invention at least part of the residual
solid non-
fibrous material is removed by subjecting at least part of the filtrate to a
second means for
removing solids from an aqueous suspension. According to an embodiment the
second
means for removing solids from an aqueous suspension is a process separating
solid
material from a liquid based on the ratio of centripetal force to fluid
resistance, suitably a
multistage hydrocyclone system. As the consistency of the filtrate is low and
lower than the
consistency of the aqueous pulp suspension, the aqueous pulp suspension
optionally
diluted with the filtrate depleted in residual solid non-fibrous material, the
second means
can be designed to more efficiently remove solid non-fibrous material not
being removed by
the first means, and referred to as residual solid non-fibrous material.
Typically, the second
removal means is capable of removing solid non-fibrous particles not removed
by said first
removal means. The average particle size of the residual solid non-fibrous
material is
typically smaller than the average particle size of the solid non-fibrous
material removed by
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the first means. While the first means may effectively remove solid non-
fibrous material
having an average particle from about 500 urn, or suitably from about 200 urn,
the second
means effectively removes residual solid non-fibrous material having an
average particle
size of below about 500 urn, suitably below about 200 urn, below about 100
urn, and below
about 50 urn.
After the removal of at least part of the residual solid non-fibrous material
from the filtrate at
least part of the filtrate depleted in residual solid non-fibrous material is
used to dilute the
aqueous pulp suspension (thereby forming a diluted aqueous pulp suspension) or
the
aqueous pulp suspension depleted in solid non-fibrous material. Furthermore,
in
accordance with the invention the aqueous stream comprising at least part of
the removed
residual solid non-fibrous material is purged from the system. The aqueous
stream may
also (H1A and H2A) may also be referred to as reject, or alternatively,
aqueous reject.
According to an embodiment the aqueous stream comprising at least part of the
solid non-
fibrous material (H1A) and the aqueous stream comprising at least part of the
removed
residual solid non-fibrous material are both purged from the system. By purged
from the
system is meant that the aqueous streams comprising at least part of the solid
non-fibrous
material and/or the removed residual solid non-fibrous material is not in any
way re-cycled
back to the system. According to a further embodiment, the aqueous stream
comprising at
least part of the removed residual solid non-fibrous material (H2A) is not fed
to a location
downstream the dewatering of the aqueous pulp suspension depleted in solid non-
fibrous
material. According to yet a further embodiment the aqueous stream comprising
at least
part of the solid non-fibrous material (H1A) and also the aqueous stream
comprising at
least part of the removed residual solid non-fibrous material (H2A) are not
fed to a location
downstream the dewatering of the aqueous pulp suspension depleted in solid non-
fibrous
material._According to a preferred embodiment at least part of the filtrate
depleted in
residual solid non-fibrous material is used to dilute the aqueous pulp
suspension prior to
the diluted aqueous pulp suspension is subjected to the first removing means.
Optionally,
the filtrate depleted in residual solid non-fibrous material may, in addition
to being used as
diluents for the aqueous pulp suspension, also be added to the aqueous pulp
suspension
depleted in solid non-fibrous material. The amount/volume of filtrate used for
diluting the
aqueous pulp suspension is dependent on several factors including but not
limited to the
consistency of the aqueous pulp suspension prior to dilution and the target
consistency of
the pulp suspension just prior to dewatering. The consistency of the aqueous
pulp
suspension when subjected to the first removal means is partly dependent on
the type of
fibres (e.g. fibre length) and type of first removal means. Typically, the
consistency of the
aqueous pulp suspension when subjected to the first removal means, typically
after dilution
with the filtrate depleted in residual non-fibrous material, is the range of
from about 0.5 up
to about 3.0%, suitably from about 1.0 up to about 2.0%.
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According to an embodiment said first and second means for removing solid non-
fibrous
material relate to processes separating solid material from a liquid which is
based on the
ratio of centripetal force to fluid resistance. According to a further
embodiment, the first and
second means are multistage hydrocyclone systems. Thus, the first and second
removal
means may be defined as the first and second multistage hydrocyclone systems.
The term
multistage hydrocyclone systems encompass systems comprising several stages
where
each stage contains a multitude of hydrcyclones operated in parallel and in
forward mode.
Each stage may contain up to several hundred single hydrocyclones. Normally, a
hydrocyclone has two exits arranged along an axis and a fluid inflow
perpendicular to said
axis. Typically, the hydrocyclone comprises a cylindrical section and a
conical section
(base) where said fluid inflow is fed tangentially into the cylindrical
section. Within the fluid
(such as an aqueous pulp suspension or filtrate comprising solids, e.g. non-
fibrous
material) centrifugal forces are countered by resistance within the fluid
effecting that larger
or denser particles are transported to the wall of the hydrocyclone which
eventually exit at
the rejection side (conical side), often referred to as reject or underflow,
while the finer, or
less dense particles, remain in the liquid and exit at the overflow side of
the cylindrical
section, referred to as overflow or accept. According to a preferred
embodiment the first
and second multistage hydrocyclone systems are operated in a forward mode.
More
specifically, the aqueous pulp suspension, and filtrate comprising residual
solid non-fibrous
material, respectively, are feed to the first stage of a multistage
hydrocyclone plant
(system) operating in a forward mode creating an overflow from the first stage
hydrocyclones, i.e. an aqueous depleted in solid non-fibrous material, and a
filtrate
depleted in residual solid non-fibrous material, respectively, and an
underflow from the final
stage hydrocyclones enriched in solid non-fibrous material, i.e. aqueous
streams (H1A,
H2A) enriched in solid non-fibrous material. Forward mode operation of a
hydrocyclone
usually means that particles of higher density (such as solid non-fibrous
material) are
enriched in the underflow while particles of lower density (such as cellulosic
fibres) are
enriched in the overflow together with most of the water.
Solid non-fibrous material relates to various types of solid material not
including cellulose-
containing fibres. The solid non-fibrous material may have an average particle
size below
about 1 mm, below about 0.5 mm, below about 300 urn, suitable below 250 urn,
below
about 200 urn, below about 100 urn, preferably below about 50 urn. The process
is capable
of removing solid non-fibrous material in the colloidal range, i.e. solid non-
fibrous material
having an average particle size below about 50 urn, such as in the range from
about 1 up
to about 30 urn. The solid non-fibrous material can be any solid non-fibrous
material
including inorganic material such as silica-based material/particles and solid
non-fibrous
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material originating from re-pulping recovered paper and paper board such as
sticky
material like glue residues, adhesives and printing ink. According to an
embodiment the
non-fibrous material is solid silica-based material. Solid silica-based
material may be
defined as silica-based material. The silica-based material may originate from
the wood per
se or stem form added chemicals, such as cooking chemicals, bleaching agents,
and
chemicals used in conjunction with the paper making process such as various
silica-based
compounds, e.g. silicic acid, and silica-based polymers. Silica-based material
includes
inorganic compounds comprising silica such as silica (silicon dioxide) in
crystal or
amorphous form. The solid non-fibrous material may also be characterised as a
solid
material having a specific (or relative) density which is higher than 1,
wherein the specific
a solid material
density (SG) is defined as DS - .
a f ibrous material (cellulose fibre)
As indicated above the process can be applied to the removal of solid non-
fibrous material
form a variety of different pulps. A pulping process may be designed so that
different types
of pulp can be produced as a response to changes in the market. For example, a
pulping
process may be capable of producing a specific quality of pulp during a given
period of
time, such as market pulp for paper or paper board, and, as a response to
marker need,
(partly) switch to the production of another quality of pulp, e.g. dissolving
pulp. Many of the
processes using dissolving pulp as raw material (e.g. production of textiles
such as rayon)
are negatively influenced by the presence of solid non-fibrous material,
specifically silica-
based material. The second removal means, typically a multistage hydrocyclone
system, of
the present process removes solid non-fibrous material from the filtrate,
typically having a
low consistency. Running the second multistage hydrocyclone system at a low
consistency,
signifying that the consistency of the inflow (i.e. the filtrate) is low,
allows the multistage
hydrocyclone system to be designed so as to be able to efficiently remove fine
solid
particles, such as solid non-fibrous material and typically solid silica-based
material,
typically having an average particle size of less than 50 urn. The ability of
removing fine
solid non-fibrous material may not be needed, such as during the production of
market
pulp, and for such pulp qualities the second means for removal of solid
material can be
bypassed. In this mode of operation solid non-fibrous material is removed in
the first
removal means. Accordingly, a further object of the present invention is the
adaptability of
the process to a variety of pulp qualities.
According to the present invention the consistency of the diluted aqueous pulp
suspension,
i.e. main fibre line, subjected to the first means of removal of solids is
suitably from about
0.5 up to about 3.0%, and preferably from about 1 up to about 2%. In
conventional
processes the consistency of the main fibre line is lower, or significantly
lower, signifying
that the volume based on fibre is significantly higher for a conventional
process that the
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volume of the main fibre line (aqueous pulp suspension) of the present
process. The
reduction in consistency of the main fibre line is needed for more efficiently
removing
aggregated solids in a conventional process, but, significantly increases the
amount
volume per weight of fibre translating in a significant increase of energy
required for
processing the same amount of fibre per time compared to the present
invention. Thus, a
further advantage of the invention is that energy consumption is reduced while
at the same
time fine solid non-fibrous material is more efficiently removed from the
aqueous pulp
suspension, and hence, from the pulping process. As a result a dewatered web
comprising
cellulose-containing fibres is obtained having a low amount of solid non-
fibrous material,
such as silica-based material, comprising less than about 50 ppm, suitably
less than about
35 ppm, preferably less than 20 ppm of solid non-fibrous material, and even as
low as less
than about 10 ppm. If the solid non-fibrous material is silica-based material
the the amount
of silica-based material is calculated as SiO2.
Figure 1 shows an overview of a non-limiting embodiment of the process
according to the
invention. The main fibre line of the process is indicated by the bold line
and may be
integrated in a pulping process. Alternatively, the pulp of the aqueous pulp
suspension
(APS) may originate from recycled paper and board such as old corrugated
containers
(OCC). An aqueous pulp suspension (APS) having a consistency of around 3.0% is
diluted
to a consistency in the range of from about 1.5 to about 2.5% with the
filtrate depleted in
residual silica-based material (H2B) having a consistency of from about 0.01%
up to about
0.25%. The diluted aqueous pulp suspension (DAPS) is fed to the first stage of
a multiple
stage hydrocyclone plant (H1) operating in a forward mode creating an overflow
(H1B) from
the first stage of hydrocyclones, i.e. the aqueous suspension depleted in
silica based
material (H1B) and an underflow (H1A) from the final stage hydrocyclones
enriched in
silica-based material. Some water is purged by the underflow, thus, the
aqueous pulp
suspension depleted in silica-based material (H1B) has a slightly increased
consistency as
compared to the diluted aqueous pulp suspension. The multiple stage
hydrocyclone plant
(H1) removes part of the silica-based material having an average particle size
of suitably
down to about 500 urn, however, fine silica-based material such as silica
having an
average particle size of less than 500 urn, less than 200 urn, or less than
100 urn, herein
referred to as residual silica-based material, is still comprised in the
aqueous pulp
suspension depleted in silica-based material (H1B). The overflow (H1B), i.e.
the aqueous
pulp suspension depleted in silica-based material but still comprising
residual silica-based
material, is fed to a dewatering process which may comprise a headbox (HB) a
wire (W). In
the dewatering process a web of cellulose-containing fibres is formed together
with a filtrate
(F) (also referred to as white water) which filtrate is fed to a storage tank
(ST). Most of the
fine silica-based material (residual silica-based material) follows the
filtrate (F). At least part
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of the filtrate (F) which has a consistency of from about 0.01% up to about
0.25% is fed to
the first stage of a second multiple stage hydrocyclone plant (H2) operating
in a forward
mode creating an overflow (H2B) from the first stage hydrocyclones, i.e. a
filtrate depleted
in residual silica-based material, and an underflow (H2A) from the final stage
hydrocyclones enriched in silica-based material. The second multiple stage
hydrocyclone
plant (H2) is designed so as to effectively removing/separating the residual
fine silica-
based material from the filtrate typically having an average particle size of
less than 50 p.m.
The overflow (H2B) from the second multiple stage hydrocyclone plant (H2), is
used to
dilute the aqueous pulp suspension (APS). The web of cellulose-containing
fibres is
optionally dried. Furthermore, the aqueous pulp suspension (APS) is preferably
obtained
by diluting a pulp suspension having a consistency in the range of from 10 to
12% which
comes from the last pulp storage tank of a pulping process to a consistency of
2.5 to 3.5%.
Figure (2) shows a process where the filtrate flow over the second multiple
stage
hydrocyclone plant is by-passed. This mode of operation, which is not
according to the
invention, is preferably conducted when market pulp is produced. Hence, the
process
according to the invention is very flexible and can easily be adapted to a
variety of pulp
qualities.
