U.S. patent application number 13/108521 was filed with the patent office on 2011-09-22 for method and apparatus for feeding chemicals into a process liquid flow.
This patent application is currently assigned to WETEND TECHNOLOGIES OY. Invention is credited to Jouni MATULA.
Application Number | 20110226432 13/108521 |
Document ID | / |
Family ID | 32749207 |
Filed Date | 2011-09-22 |
United States Patent
Application |
20110226432 |
Kind Code |
A1 |
MATULA; Jouni |
September 22, 2011 |
METHOD AND APPARATUS FOR FEEDING CHEMICALS INTO A PROCESS LIQUID
FLOW
Abstract
A method of mixing at least two chemicals or additives into a
process liquid flow flowing in a process liquid flow duct
including: feeding a liquid jet to the process liquid flow in the
process liquid flow duct, wherein the liquid jet is formed in a
feeding device and the liquid jet flows in a transverse direction
to a flow direction of the process liquid flow through the process
liquid flow duct; mixing the at least two chemicals or additives
together to form a mixture; feeding the mixture of the at least two
chemicals or additives into the process liquid flow duct with the
feeding liquid jet, and mixing the at least two chemicals or
additives with the feeding liquid jet in the process liquid flow
duct.
Inventors: |
MATULA; Jouni; (Savonlinna,
FI) |
Assignee: |
WETEND TECHNOLOGIES OY
Savonlinna
FI
|
Family ID: |
32749207 |
Appl. No.: |
13/108521 |
Filed: |
May 16, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11572165 |
Oct 29, 2007 |
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PCT/FI2005/000329 |
Jul 12, 2005 |
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13108521 |
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Current U.S.
Class: |
162/164.3 ;
162/158; 162/164.1; 162/166; 162/168.3; 162/175; 162/181.1;
162/181.2; 162/181.3; 162/181.5; 162/181.6; 162/181.8; 162/181.9;
162/380 |
Current CPC
Class: |
D21H 17/67 20130101;
D21H 21/16 20130101; D21H 23/02 20130101; B01F 2015/0221 20130101;
D21H 21/10 20130101; B01F 5/0057 20130101; B01F 3/0865 20130101;
B01F 5/045 20130101; B01F 5/0471 20130101; D21H 21/30 20130101;
B01F 2215/0078 20130101; B01F 5/0405 20130101 |
Class at
Publication: |
162/164.3 ;
162/158; 162/181.1; 162/164.1; 162/181.5; 162/181.6; 162/181.8;
162/181.2; 162/181.3; 162/168.3; 162/175; 162/166; 162/181.9;
162/380 |
International
Class: |
D21H 23/00 20060101
D21H023/00; D21H 17/67 20060101 D21H017/67; D21H 17/33 20060101
D21H017/33; D21H 17/68 20060101 D21H017/68; D21H 17/66 20060101
D21H017/66; D21H 17/52 20060101 D21H017/52; D21H 17/37 20060101
D21H017/37; D21H 17/29 20060101 D21H017/29 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 16, 2004 |
FI |
20040990 |
Claims
1. A method of mixing at least two chemicals or additives into a
process liquid flow flowing in a process liquid flow duct
comprising: feeding a liquid jet to the process liquid flow in the
process liquid flow duct, wherein the liquid jet is formed in a
feeding device and the liquid jet flows in a transverse direction
to a flow direction of the process liquid flow through the process
liquid flow duct; mixing the at least two chemicals or additives
together to form a mixture; feeding the mixture of the at least two
chemicals or additives into the process liquid flow duct with the
feeding liquid jet, and mixing the at least two chemicals or
additives with the feeding liquid jet in the process liquid flow
duct.
2. The method in claim 1 further comprising introducing another
chemical or additive to the feeding device before the mixing with
the feeding liquid jet and the at least two chemicals or additives
to the to the process liquid flow.
3. The method in claim 2, wherein the another chemical is mixed
with the feeding liquid substantially simultaneously with feeding
of the feeding liquid to the process liquid flow.
4. The method of claim 1 wherein said process liquid flow includes
a fiber suspension and the process liquid flow duct is in a paper
machine headbox feed duct downstream of a machine screen.
5. The method of claim 1 wherein said at least two chemicals or
additives include one or more of: fillers, binding agents, sizing
agent, optical brighteners, antifoaming agents, retention
chemicals, micro-particles, paper dyes and silicates.
6. The method in claim 1 wherein the at least two chemicals or
additives are formed of one of the following pairs of chemicals or
additives: retention chemical--filler, retention chemical--micro
particle, retention chemical--sizing agent, silicate--filler,
sizing agent--polymer, cationic polymer--anionic micro particle,
and sizing agent--starch.
7. The method in claim 1 wherein said at least two chemicals or
additives includes at least one of titanium dioxide, talc, kaolin,
calcined kaolin, calcium carbonate, PCC, magnesium carbonate,
calcium sulphate, barium sulphate, sodium silicate, aluminium
trihydrate, and magnesium hydroxide.
8. The method in claim 5 where said sizing agent includes at least
one of alkenylsuccinic anhydride, alkylketene dimer and a
polymer.
9. The method in claim 5 wherein said retention chemicals are
formed of micro-particles and polymers.
10. The method in claim 5, wherein said micro-particle is at least
one of colloidal silica, micro-polymer, bentonite, and polyethylene
oxide.
11. The method as in claim 5 wherein said retention chemical is at
least one of a cationic or an anionic acryl amid copolymer,
colloidal silica, micro-polymer, bentonite, cationic starch,
aluminum, PAC, polyethylenes, and polyamine.
12. The method as recited in claim 4 wherein a feeding point of
said at least two chemicals or additives is at a point in the
process liquid flow duct such that the mixture of said at least two
chemicals or additives and fiber suspension is substantially
homogenous at the latest when the mixture arrives from the headbox
to the wire.
13. An apparatus for feeding at least two chemicals or additives to
a process liquid flow flowing in a process liquid flow duct, the
apparatus comprising: a feeding device having a first inlet
connection to receive a feeding liquid, a second inlet connection
for a first chemical or additive; a feed connection for the feeding
liquid and the chemical or additive, a third inlet connection for a
second chemical or additive introducing the second chemical or
additive in the same space with the first chemical or additive
within the feeding device.
14. The apparatus in claim 13 wherein the feeding device includes a
feeding liquid space, a mixing liquid space and a chemical
space.
15. The apparatus in claim 13 wherein in that said feeding device
is arranged in flow communication with a paper machine headbox feed
duct downstream of a machine screen.
16. A method of mixing a plurality of chemicals or additives into a
process liquid flow flowing in a process liquid flow duct
comprising: forming a liquid jet in a feeding device which injects
the liquid jet into the process liquid flow duct transversely to a
flow direction of the process liquid flow through the process
liquid flow duct; mixing the chemicals or additives to form a
mixture; feeding the mixture directly into the process liquid flow
duct using a pressure difference formed by the feeding liquid jet
entering the process liquid flow duct, and mixing the mixture of
chemicals or additives with the feeding liquid jet in the process
liquid flow duct.
Description
RELATED APPLICATION
[0001] This invention is a divisional application of Ser. No.
11/572,165, filed 29 Oct. 2007, which is the US national phase of
international application PCT/FI2005/000329 filed 12 Jul. 2005
which designated the U.S. and claims benefit of Finnish Application
No. 20040990 filed 16 Jul. 2004, the entire contents of all of
these applications are incorporated by reference.
SUMMARY OF BACKGROUND AND INVENTION
[0002] A method and an apparatus for feeding chemicals into a
process liquid flow are disclosed herein. An application of a
preferred embodiment of the method and the apparatus of the present
invention is feeding retention chemical/chemicals together with an
additive, which may be another chemical or for example a mineral,
to paper pulp suspension flow to be fed to a paper machine. The
method and the apparatus of the invention are particularly well
applicable in feeding an additive of the paper manufacture, such as
filler, together with a retention chemical, to paper pulp
essentially simultaneously.
[0003] At first, chemicals used in the paper manufacture and their
properties will be discussed. Retention chemicals are chemical
agents the purpose of which is to bind various substances carried
by the paper pulp suspension either to each other or especially to
the fibers of the paper pulp so that the substances in question
would remain in the product to be manufactured, the so-called web
and would not be flushed away from it when the paper web is
dewatered at the wire section of the paper machine. An operating
principle of the retention chemicals is based on the electric
charge typical of the particles in paper pulp.
[0004] For example it can be thought that the typical charge of the
paper pulp fibers is negative and that of the additive or filler
used in the paper manufacture is also negative. If efforts are made
to cause these additives/fillers to remain in the paper to be
produced, the success is poor as the fibers having the same
electric charge reject these additives/fillers. Then the fibers in
a way force the substances in question to the water phase from
which they with very high probability end up in the white water
filtered out in the wire section of the paper machine.
[0005] The situation can be corrected by feeding to the paper pulp
retention chemical which has a positive specific charge and which
thus adheres both to the fibers and to the additive/filler in
question thus binding them to each other. Performed tests have
further shown that the longer the retention chemical is in contact
with for example the fibers the weaker its retention ability
becomes. This is believed to be due to the feature that the
electric charge of the fiber attracts all the time the retention
chemical molecule so that with time practically the whole molecule
rests against the fiber whereby the internal electric charge of the
retention chemical is in a way discharged to the fiber and the
substance is left without a charge or in the worst case adopts the
charge of the fiber. Naturally the additive/filler does not then
even try anymore to get in touch with the retention chemical but
stays free in the paper pulp suspension.
[0006] In the literature the retention chemicals are in most cases
understood to be cationic or anionic acryl amid copolymers. These
have been found to improve efficiently the retention of fines in
the paper formation. However, there are many alternative substances
and additives, which may also be used to improve the retention. As
an example may be mentioned, among others, combinations of two
different polymers or copolymers sometimes having even different
electric charges, used in sequence, or for example the use of a
high-mass cationic polymer in combination with an anionic micro
particle such as colloidal silica, bentonite or micro polymer,
introduced later, or the various options provided by polyethylene
oxide. The purpose of the retention chemical is, in addition to
retaining for example the fillers in the web, also to maintain
adequate tidiness of the paper machine, to provide uniform quality
in the Z direction of the web, and to ensure infiltration
ability.
[0007] The retention chemicals play a central role in the paper
manufacture and the quality of the end product. For example an
excessive dose of the retention chemical results in flocks in the
end product, which are seen as uneven quality of the product. Thus,
the aim is to dose only the necessary amount of the retention
chemical in order to achieve the goals described above, and not
more. However, some chemicals such as ASA (explained later) used as
a sizing agent require a relatively high retention chemical dose
whereby a homogenous mixing of the chemical to the paper pulp is
naturally of primary importance. ASA, which is not retained in the
fibers, is hydrolyzed during the process and the hydrolyzed ASA is
detrimental to sizing and causes agglomeration in the process. ASA
should be fed into the process by mixing it as efficiently as
possible close to the headbox. When a good retention is aimed at,
it is advantageous to dose the cationic starch in a position as
close to the headbox as possible. Cationic starch is adsorbed
unevenly to adsorbents of different type. It is adsorbed to
adsorbents with a large specific area, such as fillers and fines,
more strongly than to fibers. Yet, the influence of starch is
different in fibers and in fines. In order to avoid uneven
distribution of the starch, it should be added as dilute as
possible in a position where the mixing is good. Other substances
used as retention chemicals, which have not been mentioned yet, are
for example alum, PAC, polyethylenes and polyamines.
[0008] Thus, a common and often encountered problem with retention
chemicals is the hydrolysis, where the chemicals in question react
with water and loose their effect at a rate typical of each
chemical. Thus, if the feeding of the retention chemical to the
paper pulp could be optimized so that the chemical in question
would be in contact with water for as short a time as possible,
considerable savings could be made in chemical costs.
[0009] The retention takes place either as a mechanical or a
chemical retention where the basic idea is to change the charge of
the additional chemicals so that they would be adsorbed to the
fiber as efficiently as possible. The charge changes while the
process proceeds, which may cause dissolving of the flocks, which
have already been formed and thus result in weakening of the
efficiency of the chemical and thus overdosing. Thus, if the
feeding of the retention chemical to the paper pulp could be
optimized so that the chemical in question would be introduced in a
location as close to the headbox as possible, considerable savings
could be made in chemical costs.
[0010] When discussing fillers in paper making contexts, fine
mineral products are usually meant the size of which in most cases
is 0.5-5.0 .mu.m. The most important fillers are calcium carbonate
and kaolin. Sometimes also titanium dioxide is classified a filler
although its particle size is smaller (for example 200-300 nm) and
is priced very high compared, for instance, to calcium carbonate.
Also talc is sometimes used as a filler. It is characteristic of
most of the fillers that they are brought to the paper mill in
powder or sludge form.
[0011] A filler which has become very popular is PCC (precipitated
calcium carbonate) which is produced on site at the paper mill. PPC
consists almost fully of the calcite crystal form of calcium
carbonate. The starting material is often limestone which in most
cases is calcined to CaO. In the paper mill, water is added to the
lime in order to produce lime milk Ca(OH)2, after which carbon
dioxide CO2 is added as bubbles to the lime milk. The crystal form
of the forming PCC particles can be controlled by using different
temperatures in the manufacture. The PCC produced in the mill
usually has a weak cationic colloidal charge whereas dried PCC has
a negative (anionic) charge.
[0012] The purpose of the fillers is to fill the paper, in
particular in situations where the paper must have high brightness.
A certain type of PCC is used when a particularly high opacity and
precise thickness of the paper is desired. The use of PCC as a
filler is very much similar to that of the other calcium carbonate
products. The fact that PCC is weakly cationic while the other
minerals are anionic, must, however, be taken into account in view
of the retention. Carefully planned retention systems, however,
work with calcium carbonate fillers of both the types. In some
cases where PCC and an sizing agent, such as AKD (explained later),
are used it is recommendable to add the PCC first to the paper pulp
and after that the AKD. Then a colloidal material such as for
example starch can coat the PCC particles whereby the AKD in turn
adheres better to the starch. Other fillers used are for example
titanium dioxide, magnesium carbonate, calcium sulphate, barium
sulphate, sodium silicate, aluminium trihydrate, magnesium
hydroxide, or a combination of these.
[0013] Sizing agents, examples of which are ASA (alkenylsuccinic
anhydride=alkylene amber acid anhydride) and AKD (alkylketene
dimer) are substances designed to prevent water from being absorbed
to the paper. They are usually employed when producing paper in
neutral or alkaline conditions. The main aims in using ASA are
preventing the reactions (hydrolysis) taking place with water, even
distribution and mixing of ASA into the paper pulp, and efficient
retention to the product to be produce. The hydrolysis is prevented
by preparing the ASA emulsion only as late as possible before the
emulsion is mixed to the paper pulp. The pH of the cationic starch
solution, which is used in preparing the emulsion, is decreased for
example with alum. The purpose of the starch solution is to coat
the ASA droplets so that they would not at once contact water.
Prior art suggests adding the ASA emulsion at a position after the
vortex cleaner in the short circulation, in other words the region
preceding degassing and the headbox feed pump. Although the
cationic starch coating around the ASA droplets to some extent
contributes to the attaching of the sizing agent to the fibers, an
efficient retention system is still needed to retain the sizing
agent quickly in the web to be produced. Immediate retention is
important as the sizing agent is in any case bound to the fines and
filler and if it does not retain in the web, it ends up in the
water circulations and becomes hydrolysed. Hydrolyzed ASA in turn
can cause flocks, running problems and deterioration of sizing.
[0014] Another known sizing agent is AKD, which is alkaline and
manufactured synthetically of fatty acids. The most common form is
a wax-like solid substance, which is dispersed in small particles
in a solution containing a stabilizer. The stabilizer can be a
cationic starch or any other cationic polyelectrolyte. AKD has a
much less reactive character than ASA. When using AKD the paper
produced is hydrophobic whereby typical end products are among
other things various liquid containers and ink jet papers. The use
of AKD is particularly recommended in situations where the paper
should withstand moisture for long periods of time.
[0015] AKD is brought to the mill as a milky emulsion, whereby its
use is fairly easy. As the reactivity of AKD is weaker compared to
ASA, its use is also more flexible. Many paper manufacturers add
AKD to high consistency pulp, in other words before dilution of the
pulp to a consistency suitable for the headbox. In this way the AKD
is brought to the surface of the fibers. On the other hand, if AKD
is dosed to a pulp in a consistency suitable for the headbox it is
justified to assume that it adheres mostly to the fines. If PCC is
present it can decrease the efficiency of the sizing agent and also
with time reduce the effect of the sizing for example during
storage.
[0016] Paper manufacturers also speak about micro-particles. These
are for example colloidal silica, bentonite and some organic
compounds, which are used for the same purpose. All the
micro-particles commercially available at the moment have a
negative colloidal charge and their specific area is very large.
Micro-particles are used to improve the dewatering properties of
the fiber web. Usually they are added to the paper pulp after the
cationic polyacrylic amide or cationic starch used as the retention
chemical. In other words the polyacrylic amide or the starch is at
first allowed to flocculate the fibers and the micro-particles are
added only after that to the paper pulp. The adding usually takes
place to the headbox feed duct after the machine screen. It has
been found that the best result is obtained when the whole system
is made slightly cationic with the cationic additives before the
micro-particles are added. If the paper pulp is very anionic it
should be treated with a cationic additive such as alum,
polyaluminium chloride, polyamine or polyethyleneimine.
[0017] Further, depending on the case, very many different
chemicals, antifoaming agents, optical brighteners, dyes and
opacity pigments are used in paper machines, which aim at
influencing the properties of the end products or improving the
effect of other chemical or avoiding process problems. Examples of
these are fixatives used to bind impurities in mechanical pulp. New
pigments and their combinations influencing the paper brightness,
saving of fibers, and paper structure, etc.
[0018] When looking into the problems that at the moment have been
found in the paper manufacture and especially in the mixing of the
chemicals and other additives in it, it is best to start with the
retention chemicals as they have a central role in the whole
additive program of the paper manufacture. The worst known problem
associated with the retention chemicals has until now been the fact
that it has not been possible to mix them in an adequately
homogenous and quick way to the paper pulp. One has then been
compelled to choose, quite naturally the alternative that secures
the adequate dose of the retention chemical in the whole volume of
the paper pulp flow running to the headbox by both overdosing the
retention chemical and allowing it more time to be mixed to the
paper pulp. In other words, the retention chemical is mixed with
the pulp in most cases in the feed pump of the headbox, in the
machine screen or immediately after the machine screen, in order to
secure a flow time (=mixing time) long enough in the feed pipeline
of the headbox. This has, however, had the consequence that on the
other hand the retention chemical has lost some of its efficiency
for example for the reasons associated with the evening out of the
electric charges and chemical phenomena mentioned above and, on the
other hand, due to the overdose, there have sometimes been
complaints about the quality of the end product. It must be stated,
however, that the long mixing time and the mixing distance provided
for evening out the mixing which reduces the efficiency of the
retention chemical has to some extent compensated the chemical
overdose whereby the drawbacks have not been so imminent. Then
there is, however, the danger that even a remarkable portion of the
retention chemical is not retained in the web but becomes
hydrolyzed and ends up with the filtrate of the wire section in the
short circulation where it may for example cause precipitation.
When talking about the minimum mixing distance of the mixing
device, the distance is meant which the chemical or the
corresponding substance needs to be mixed essentially homogenously
to the pulp. With an efficient mixing device it is on the order of
1.5 to 2 seconds during which time, and also along the
corresponding distance, the chemical is homogenously mixed to the
pulp.
[0019] Further, it has been explained above how in connection with
the feeding of an additive it has been found detrimental to feed
the retention chemical to the paper pulp at a very early stage
compared with the feeding of the additives. In many cases the tests
we have performed have shown the best feeding method to be the
feeding of the retention chemical and the additives at the same
time to the paper pulp so that the retention chemical becomes at
first mixed with the additive and essentially at the same time
spreads to the paper pulp whereby in fact the entire mixing takes
place in one second or a shorter time.
[0020] Chemicals used as paper additives are usually dosed in very
small volumes. Feeding a small volume to a large volume
homogenously is not successful if as efficient mixing as possible
is not guaranteed at the feeding moment. If the mixing is poor, the
chemical gets in contact with a small portion of the pulp
suspension, only, and a remarkable portion of the pulp suspension
remains without the chemical which is seen as variations in the
properties of the end product.
[0021] Several different prior art methods and apparatus are known
for feeding both retention chemicals and among other things the
additives described above to the paper pulp. According to the
conventional paper stock manufacturing method, both the various
paper pulp fiber fractions and the additives, fillers, sizing
agents etc. required in the paper manufacture are brought to a
mixing tank in the so-called short circulation. Also a part of the
retention chemical/chemicals has/have conventionally been
introduced to the mixing tank. In the mixing tank, as also the name
suggests, the paper pulp is efficiently mixed so that both the
different fibers and the various additives are mixed homogenously
and the consistency of the suspension formed of these is adjusted
to a desirable level. From the mixing tank the paper pulp is pumped
by means of the headbox feed pump towards the head box in most
cases via vortex cleaning, gas separation and a headbox screen or
the so-called machine screen. Both the feed pump in question and
the headbox screen mix the pulp further, in other words they keep
the paper pulp as homogenous as possible. A retention chemical is
fed to the paper pulp after the headbox screen with the intention
to ensure the retention of a certain or some additive(s), filler(s)
or sizing agent(s) of the paper pulp in the paper machine wire
section.
[0022] A very weak additive retention has been found to be a
problem in the prior art short circulation process. In a test
performed the additive retention (so-called first pass retention)
was found to be in a conventional process arrangement on the order
of five percent. In other words only five percent of the additive
in the paper pulp remained in the web produced while the rest ended
in to the white water and the filtrates of the press section.
However, these filtrates are recycled to the manufacture of paper
pulp, whereby the additives, which were not retained can end up in
the paper machine but it is quite as well possible that they in
several other connections end up in the reject. In a conventional
process the additive is added to the mixing tank where also the
white water and other usable filtrates are brought and from which
the paper pulp is pumped via a vortex cleaning plant and a headbox
screen to the headbox of the paper machine. In other words both the
vortex cleaning plant and the headbox screen reject some of the
paper pulp, which always contains also some additive. Additives can
also be different in reactivity and thus they can for example be
hydrolyzed and precipitated at a point in the process, which
results both in an additive loss and problems in the process both
because of fouling and detaching of the deposits, which takes place
from time to time.
[0023] When trying to mix both the retention chemicals and the
additives homogenously to the paper pulp, an apparatus could be
used, which has proved to be very efficient in particular in the
mixing of bleaching chemicals of the pulp industry, in other words
a mechanical revolving mixer of the type described in U.S. Pat. No.
5,279,709, which could be placed in the feed pipeline of the
headbox, preferably between the machine screen and the headbox.
There are, however, a few drawbacks in the use of this apparatus.
Firstly, in order to ensure efficient and homogenous mixing,
mechanical mixers in general develop a very strong field of shear
forces, which breaks weak chemicals such as polymer chains of some
retention chemicals. Of course this problem does not exist if the
chemical to be mixed is not sensitive to shear forces. A mechanical
mixer still has other weaknesses. These are for example the high
price and the high operating costs because a mixer capable of
mixing the chemicals homogenously over the whole diameter of the
headbox feed pipe is large and it consumes a huge amount of energy
while performing the mixing action. Further, the installation of
the mechanical mixer to the pipelines and the drive motor on a
stand of its own and constructing the electrical connections
required involves a lot of work and supplies. For example
installation to an existing paper machine requires that stands are
built on the floor of the mill for the mixer and its drive. A
further requirement is that the headbox feed duct is cut and a
piece of it cut out so that flanges can be welded in the remaining
ends of the duct if reductions or expansions are not needed for the
flanges of the mixer, which would further encumber the work. The
mixer can then be installed between these flanges.
[0024] An installation work almost as complicated is required by
the use of a mixer based on the use of contoured members suggested
for some applications where in the same way a piece is cut off from
the flow duct leading to the headbox and a piece of a pipe
containing contoured members is installed it its place, the purpose
of the contoured members being to create turbulence in the paper
pulp flowing to the headbox. The apparatus in question thus
comprises a pipe replacing a part of the paper pulp feed duct,
inside of which pipe there are arranged a number of contoured
members, so-called turbulence elements. The retention chemical is
fed in connection with the elements mentioned so that turbulence
created in the flow by the contoured members is supposed to mix the
chemical evenly to the paper pulp. It is disclosed that the
apparatus is in particular used in the feeding of a two-component
retention chemical to a paper pulp flow. The apparatus may be used
also in feeding other chemical or additives to a paper pulp. In
some cases the contoured member has several feed openings all of
which can be used for feeding the same substance/chemical or some
opening for feeding another substance/chemical.
[0025] The apparatus in question is mentioned to achieve a very
gentle mixing which for example does not break the weak molecular
chains of the polymer-type retention chemicals as badly as other
prior art apparatus. The gentle nature of the chemical feed is
among other things ensured by actually not spraying the chemical to
the paper pulp flow but by just allowing them to flow to the paper
pulp flow duct at a pressure only that much higher than the process
pressure that the feed flow is in general possible. According to
our understanding, however, the practice has shown that the
turbulence created by the contoured members is in most cases too
weak to mix the chemicals homogenously to the paper pulp flow. This
is revealed among other things by the paper web being produced by
the paper machine, in the quality of which there have been found
fluctuations which cannot be explained otherwise. The reason can be
for example that it is not possible to provide in a sensible way in
the headbox feed duct a duct section containing the turbulence
elements that would be long enough. Further problems may be the
flow resistance caused by the turbulence elements, which changes
the power requirement of the headbox feed pump and possibly the
local pressure fluctuations caused by the elements, which can be
reflected up to the headbox.
[0026] In view of the total economy, the best retention chemical
mixing apparatus has been disclosed for example in U.S. Pat. No.
6,659,636 B1, the installation of which only requires drilling
fairly small holes in the wall of the flow duct. Because the
apparatus in question does not contain moving mixing elements there
is no need for separate stands for the drive motor but the
apparatus may be installed directly in support of the flow duct. As
additional apparatus for controlling the flow, valves are naturally
needed as in all chemical mixers irrespective of their type. The
operation of the apparatus in question is based on spraying by
means of a feed liquid the retention chemical to the paper pulp
flow duct through one or several nozzles located at the periphery
of the flow duct, whereby the high speed of the feed liquid causes
the retention chemical to spread in a fan-like spray throughout the
whole paper pulp volume flowing in the duct.
[0027] The method and system disclosed herein may be applied, for
example, to solve the following problems: hydrolysis of different
chemicals due to too early mixing; change of the electrical
properties of the retention chemicals due to too early mixing; high
investment costs; a mixer of its own for each chemical; each mixer
to a different location in the short circulation; large apparatus;
powerful electric motors; high operating costs caused by the
powerful electric motors; high installation costs; constructing the
stands; cutting the headbox feed duct; electrical installations
required by the drive motor of the mixer, among other things in
order to overcome the drawbacks described above the method and
apparatus to be described below has been developed, the
characteristic features of which are disclosed in the appended
patent claims.
[0028] The tests we have performed have shown that the mixing of
the chemicals or additives of the paper making industry is
successful if the mixing measures are performed in the right order
and so that, if necessary, an intermediate result of the mixing is
a homogenous mixture of paper pulp and chemicals, or at least the
end result is a homogenous mixture to be introduced to the
headbox.
[0029] For example it has been noticed that by using an apparatus,
the operating principle of which was described already above and in
the U.S. Pat. No. 6,659,636 B1, in a new way for a slightly
different purpose than before, an optimal situation can be reached
in the mixing of chemicals and additives of the paper making
industry where the volume of the chemicals to be used is remarkably
reduced at the same time as also the quality of the end product is
improved or at least remains easily at the desirable level.
[0030] Further it has been noticed in the tests performed that the
feed liquid jet typical of the apparatus in question firstly mixes
the chemical or additives fed with it homogenously to each other
already at the spraying stage so that it is justified to speak
about two almost simultaneous mixings. Firstly, the chemicals or
additives supplied with the jet are mixed both with each other and
with the solids or chemicals possibly carried by the feed liquid.
And secondly, simultaneously with the mixing in question the feed
liquid jet spreads evenly the material fed to the paper pulp
flowing to the headbox. In order to secure this, several feeding
devices are provided at the periphery of the paper pulp flow duct
if necessary. Thus for example the retention chemical and the
filler may be and advantageously is fed by means of the same
feeding device to the paper pulp. In a corresponding way, also the
sizing agent and the starch/polymer may be introduced via the same
feeding device and the retention chemical via another feeding
device a little later; in practice this distance in the paper pulp
flow direction need not be more than about two meters.
[0031] Embodiments of the method and the apparatus of the invention
disclosed herein may provide among other things for example the
following advantages: efficient and homogenous mixing of additive
to the paper pulp; quick mixing of additive and retention chemical
to each other; an essentially improved additive retention; reduced
investment, installation and operation costs, and lower chemical
costs, only to mention a few advantages.
[0032] Thus, the method and the apparatus of the invention are
applicable in all processed where various chemicals must be
introduced. As advantageous examples of the processes, among others
fiber suspension processes of paper mills, thickening processes of
various sludges, recycled fiber processes and bleaching processes
may be mentioned, and in general processes where it is necessary to
feed chemical to a filtrate, fiber suspension, sludge or a
corresponding medium.
BRIEF DESCRIPTION OF THE DRAWINGS
[0033] In the following, the method and the apparatus according to
the invention are described in more detail with reference to the
appended drawing figures, where
[0034] FIG. 1 illustrates the conventional prior art short
circulation arrangement of a paper machine,
[0035] FIGS. 2a, 2b and 2c illustrate three different variations of
a conventional prior art feeding device,
[0036] FIG. 3 illustrates a short circulation process arrangement
according to a preferred embodiment of the invention, and
[0037] FIG. 4 illustrates a feeding device according to a preferred
embodiment of the invention.
DETAILED DESCRIPTION OF THE INVENTION
[0038] According to FIG. 1, the prior art short circulation process
arrangement works so that paper pulp to be fed to a paper machine,
which is generally illustrated by a wire section 22, is diluted to
the applicable consistency in a wire pit 20 with white water from
the paper machine 22, although a separate mixing tank may also be
utilized. Other suitable liquids may be used for dilution too, if
desired, as for instance filtrate from a white water filter. Thus,
both various fiber fractions 14, which the paper to be manufactured
is desired to contain and various additives and filler 16, the use
of which both saves valuable fibers and gives the paper desired
properties such as for example brightness/opacity, gloss, moisture
resistance, etc are brought to the wire pit 20. All these and
possibly also at least a portion 18 of the retention chemicals are
mixed in the wire pit with a mixer suitable for that purpose, to
form a homogenous suspension. From the wire pit 20, the fiber
suspension is taken by means of a pump 24, which further agitates
the suspension, to a vortex cleaning plant 26 and further to a gas
separation tank 28. The gas-free fiber suspension is pumped by
means of a headbox feed pump 30, which also agitates the
suspension, to a headbox screen 32, the so-called machine screen,
which is used in addition to screening also for mixing the paper
pulp, and after which the only retention chemical 38, or if a
two-component retention chemical is used, the second component is
added to the fiber suspension by means of a feeding device 34
before the fiber suspension reaches a headbox 36 of the paper
machine 22. Retention chemical has in known arrangements been fed
also to various other positions in the short circulation between
the wire pit and the head box.
[0039] FIG. 2a illustrates an apparatus solution known per se
described in the U.S. Pat. No. 6,659,636 already mentioned above.
The feeding device 34 according to the figure is, in fact, a nozzle
comprising a casing 50 (illustrated here as being conical), flanges
52 and 54 disposed in it and preferably, but not necessarily,
placed at its opposite ends, and a conduit 56 for the retention
chemical. The feeding device 34 is connected by its flange 52 to
the feeding liquid duct and by its flange 54 to the flow duct
taking paper pulp to the headbox of the paper machine. In the
arrangement according to the figure, the casing 50 of the feeding
device 34 is converging, which by no means is absolutely necessary
either in view of the structure or the operation of the device,
from the flange 52 towards the flange 54 inside of which there is
an opening 58 of the feeding device. A purpose of the conical form
of the casing 50, or other corresponding means including adjustment
of the feed pressure of the liquid to be introduced, is to
accelerate the medium flow in the feeding device 34 so that the
velocity of the jet discharging from the feeding device 34 into the
fiber suspension flow is at least three times, but preferably about
five times the velocity of the fiber suspension flow. A velocity
difference of this kind can ensure that the liquid jet discharged
from the opening 58 penetrates quickly enough and deep enough into
the fiber suspension flow, and is mixed with the fiber suspension
essentially more homogeneously than with apparatus used before. In
the embodiment according to FIG. 2a, the retention chemical feeding
conduit 56 to the feeding device is preferably tangential in order
to ensure that the retention chemical discharging through the
opening 58 of the feeding device 34 into the fiber suspension flow
is distributed homogeneously at least over the whole periphery of
the opening 58. At the same time, tangential feeding ensures that
the retention chemical is mixed into the feed liquid under as small
shear forces as possible in order to prevent the polymer chains of
the chemical from degrading.
[0040] FIG. 2b illustrates another apparatus embodiment partly
known from the U.S. Pat. No. 6,659,636 already mentioned above.
Firstly, in the feeding device 34 of the figure, the feed liquid
inlet flange 52' has been arranged, unlike in the solution of the
patent mentioned, at the side and a feed connection 62 for the
chemical, which may be for example a retention chemical, directly
above the feed opening 58. Further, in this figure the chemical
feed connection 62 is illustrated to extend as duct 64 inside the
feeding device 34 close to the feed opening 58, by means of which
it is possible, if desired, to ensure that the retention chemical
does not contact other substances before the mixing itself.
[0041] FIG. 2c illustrates a feeding device 34 according to FIG.
2a, in fact with two additional embodiments. Firstly, inside the
feeding device 34 there is a centrally disposed hollow member 80
into which the retention chemical is supplied via the conduit 56.
In this embodiment, the member 80 essentially comprises two
rotationally symmetrical shells 82 and 84 and possibly one end wall
86 illustrated here as being conical. Further, at the end of the
member 80 on the fiber suspension flow duct side, there is
preferably an annular opening 88 provided, via which the retention
chemical is allowed to be discharged into the fiber suspension. The
retention chemical conduit 56 pierces the wall of the casing 50 of
the feeding device 34 and further leads via the annular space 90
between the casing 50 and the member 80 into the member 80 through
the outer shell 84, at the same time preferably carrying the member
80 in its place. The inner shell 82 defining the member 80 is
cylindrical and forms or includes a duct 92 which may be of two
different structures. Contrary to what has been illustrated in the
figure, the inner shell 82 may end at the level of the end wall 86
of the member 80 whereby, while the upper end of the inner shell 82
is open, some of the feed liquid flowing from the feed duct secured
to the flange 52 may be discharged to the fiber suspension flow. In
this embodiment, the retention chemical flow guided tangentially
into the member 80 turns into a spiral flow towards an annular
opening 88 of its own, via which the retention chemical is
discharged as a fan-shaped jet into the fiber suspension together
with the feed liquid discharging both from outside the opening 88
in this embodiment via the annular opening 58, and from inside the
opening 88 via a duct 92. An additional purpose of the member 80 is
to further throttle the cross-sectional flow area of the mixing
apparatus in order to ensure a sufficient velocity difference
between the retention chemical flow and the fiber suspension flow.
A second purpose of the member 80 is to enable the mixing of the
retention chemical with the feed liquid to take place essentially
at the same time as the retention chemical is fed into the fiber
suspension flow. The figure clearly shows that the retention
chemical need not necessarily be in any contact with the dilution
liquid before it is discharged through its opening 88 into the
fiber suspension flow duct.
[0042] In another embodiment illustrated in FIG. 2c, the inner pipe
92 of the member 80 is connected to the process via a flow path 94
of its own and the outer pipe of the feeding device 34, forming the
wall of the casing 50, via a flow path 96 of its own. Both flow
paths 94 and 96 have been provided with flow regulation devices 98
and 100, preferably valves, as naturally has been done also with
all the liquid connections of all the previous embodiments although
this has not been illustrated in FIGS. 2a and 2b. The flow pipe 96
functions the way already presented before, but it is now possible
to introduce into the inner pipe 92 of the member 80 e.g. either
clean water, circulation water from the paper mill, white water,
clear filtrate or some other non-clean liquid suitable for that
purpose, even fiber suspension to be fed into the headbox of the
paper machine. In other words the flow path in question is used in
feeding so-called mixing liquid to the apparatus, the liquid being
discharged to the chemical to be fed essentially at the same time
as the chemical is discharge to the feed liquid and further to the
pulp flow. It is of course possible, if it is desirable to use the
mixing liquid mentioned to dilute the chemical, to arrange the
inner pipe 92 to end at a distance from the opening 58 whereby the
mixing liquid has some time to dilute the chemical.
[0043] Further, it is possible to introduce via the flow path a
retention chemical component, if desired, especially in case of a
retention chemical containing several components. As an example, a
short-chained retention chemical might be mentioned, in case the
retention chemical is formed of a long-chained and a short-chained
chemical. In that case, the long-chained chemical is supplied
tangentially into the member 80 through the conduit 56 illustrated
earlier in FIGS. 2a and 2b.
[0044] FI patent application 2003051 illustrated a further prior
art feeding device which is to a large extent based on the basic
structure of a feeding device illustrated already in FIG. 2c. Here,
the flow paths for different liquids have been designed in a
slightly different way and particularly the structure of the feed
end of the inner duct. In this case the dilution liquid feed duct
(corresponds to the duct 92 and its feed end in FIG. 2c) is clearly
different from the ones disclosed earlier. The feed end of the duct
is actually closed but there are a number of nozzle openings
provided at the sides of the duct through which the dilution liquid
to be introduced form the duct can be discharged evenly all around
to the feed liquid flowing at a high speed outside the duct. The
basic idea in the use of the nozzle openings is to spray and mix
the chemical coming from the feed duct (corresponds to member in
FIG. 2c) outside the feed duct mentioned (corresponds to duct 92 in
FIG. 2c) efficiently to the feeding liquid coming from a duct
surrounding the chemical feed duct, just before the feeding liquid
with the substance added to it is in turn mixed efficiently and
smoothly to the paper pulp or corresponding material flowing in the
flow duct.
[0045] FI patent application no. 20031468 further discloses a prior
art feeding device which differs from the apparatus presented in
the earlier embodiments in that at the end of the feeding apparatus
flow duct, corresponding to the member 80 in FIG. 2c, facing the
paper pulp feeding duct there is a space disposed into which the
chemical to be supplied in small volumes is introduced via a
central duct. Now the chemical is thus introduced via the innermost
duct and the dilution liquid via the duct located next in the
direction towards the periphery. The space in question is defined
by the duct bringing the dilution liquid from the outside so, that
the duct in question is practically closed. The innermost duct
bringing the chemical extends close to the closed end of the
dilution liquid duct so that the chemical, while flowing from its
duct under pressure against the end of the dilution liquid, spreads
homogenously to the space where also the dilution liquid is
introduced. In this way the chemical is distributed to the dilution
liquid after which the mixture produced is discharged via the
openings provided in the side surface of the dilution liquid duct
to the feed liquid flowing outside preferably at a high speed.
Naturally also in this embodiment all the feeding device ducts
mentioned above have been provided with regulating valves so that
each flow can be adjusted independently irrespective of the other
flows.
[0046] FIG. 3 illustrates a process arrangement which the invention
tries to apply as efficiently as possible. Figure correspond
otherwise to FIG. 1 but here only fiber fractions 14 are fed to the
mixing tank/wire pit 20 and the additives of the paper manufacture
are fed by means of device 34 after the headbox screen 32. However,
it should be noted that the feeding device 34 must be understood in
a very broad way. It may also denote several apparatus disposed at
a distance from each other both in the peripheral and in the
longitudinal direction of the duct. The most essential thing,
however, is that an essential portion of the additives 38-42,
preferably all of them, are introduced to the paper pulp after the
headbox screen. Then all the added substances quickly end up onto
the paper machine wire whereby the solids fed do not have for
example an opportunity to end up in the rejects either in the
vortex cleaning or in the headbox screen. Thus, the result is a
quick response to the adjustment whereby ash control in the paper
is improved essentially.
[0047] In the following, apparatus are discussed with which the
mixing of chemicals of the type in question is successful so that
the result is a homogeneous mixing of the chemicals to the paper
pulp and also so that the chemicals have not had time for example
for harmful reactions either with each other or for instance with
water.
[0048] In principle most of the apparatus applying the method of
the invention are modifications of the feeding apparatus of the
U.S. Pat. No. 6,659,636 already mentioned earlier and also
illustrated in the above FIGS. 2a, 2b and 2c. A simple feeding
apparatus employing the method of the invention greatly resembles
the feeding device presented in FIG. 2a. However, it differs from
it to some extent, because a preferred feeding apparatus employing
the method of the invention has in addition to the retention
chemical feed connection also another feed connection for the
additive, chemical or a corresponding substance via which the
additive mentioned is brought to the feeding device. In a process
according to a preferred embodiment of the invention, so-called
filler is introduced via the feeding connection mentioned, which
may be titanium dioxide, talc, kaolin, calcined kaolin, calcium
carbonate, PCC, magnesium carbonate, calcium sulphate, barium
sulphate, sodium silicate, aluminium trihydrate, magnesium
hydroxide or a combination of these. According to another preferred
embodiment of the invention, ASA sizing agent, AKD sizing agent or
corresponding substance is fed in via the connection mentioned. In
addition to the fillers or sizing agents the process of the
invention can be used for feeding two or multi-component chemicals
to the paper pulp, such as retention chemicals, which are for
example composed of a micro particle component and a polymer
component. Micro-particles are for example colloidal silica or
bentonite. All the substances mentioned above are below generally
called additives.
[0049] The feeding connection may be positioned at a suitable
location in the casing of the feeding device, preferably
tangentially in relation to the casing either in the same or in the
opposite direction compared to the retention chemical feeding duct
already earlier in the device. On the other hand, it may also be
arranged in connection with the duct bringing feed liquid to the
feeding apparatus close to the feeding apparatus. The maximum
distance from the feeding apparatus of course depends on the
chemical introduced via the connection in question and on the
feeding liquid used. In other words if it is desirable for one
reason or another not to allow the chemical and the liquid to
contact each other before the actual mixing to the paper pulp the
chemical must be introduced as late as possible to the feeding
liquid. In practice it is, however, simplest to arrange the
connection mentioned in the casing of the feeding apparatus whereby
the feed liquid line leading to the feeding apparatus does not need
any T-connections.
[0050] When using the feeding device 34 of FIG. 2a, the feeding
liquid coming to the apparatus from above can be paper pulp taken
via a branch pipe from the flow duct leading to the headbox, white
water, or a filtrate suitable for this purpose or even clean water.
In the feeding device in question the retention chemical is fed to
the feeding liquid via the first connection and the additive
mentioned above via another connection so that in practice they can
contact each other only inside the feeding device just before the
feeding liquid jet penetrates to the paper pulp flowing in the flow
duct. The actual mixing does not take place until while spraying
both the retention chemical and the additive mentioned with the
feeding liquid to the flow duct.
[0051] The following short table illustrates the results of a test
run performed in a paper mill. In the test a conventional method of
feeding filler was compared with the feeding method according to
the invention described above
TABLE-US-00001 test 1 test 2 test 3 test 4 TiO.sub.2 feed g/ton 25
25 20 20 Opacity 91.7 91.5 92.0 92.3
[0052] Tests 1 and 2 relate to a conventional method where the
filler is fed to the paper pulp already in the wire pit or in the
headbox feed pump. Tests 3 and 4 on the other hand relate to a
method according to the invention where the filler is introduced to
the paper pulp substantially at the same time as the retention
chemical and is mixed to the paper pulp by means of a strong
feeding liquid jet close to the headbox after the machine
screen.
[0053] The table indicates that when 20 g/ton of titanium dioxide
is fed by the method of the invention, in other words 5 grams, i.e.
20 percent less than in the conventional method, the opacity
readings were in fact higher than with the conventional process
using more filler. It is also possible to calculate from the
opacity readings obtained in the tests, which filler feed amount
would, using the method of the invention, give the same average
opacity values as the ones in tests 1 and 2. The calculation
indicates that a dose of 15 g/ton is adequate, which in practice
corresponds to 40 percent smaller filler feed amount when using the
method of the invention, compared with the conventional method.
[0054] Why then does the process of the invention save the
additive, in this case titanium dioxide, i.e. the opacity pigment?
The explanation is believed to be that when earlier the filler was
dosed in the mixing tank or a corresponding member to the paper
pulp and the retention chemical either at the same time to the same
mixing tank or, as another alternative somewhere around the headbox
screen to the pulp, the retention chemical came in both cases into
contact mainly with the fibers whereby most of the retention
chemical was consumed in binding the fibers to each other and a
smaller portion of the chemical was left free for the filler. When
the retention chemical and the filler are now fed in one turbulent
jet to the paper pulp the retention chemical molecules and the
filler particles have a better chance of meeting each other. Then a
greater part of the filler particles can adhere to the retention
chemical, which in turn adheres to the fibers whereby the filler
retention as a whole improves essentially.
[0055] Another example of the superior characteristics of the
method and the apparatus of the invention compared to the prior art
technology is a test where ASA sizing agent was fed to paper pulp
both in the conventional way and according to the invention. When
the conventional method was used, about 5 percent of the ASA sizing
agent was retained at the paper machine wire section and when using
the method of the invention, the percentage was about 35. The
explanation to this phenomenon is probably the same as above.
[0056] When using the feeding device illustrated in FIG. 2b the
connection for feeding the second chemical may be located as in the
feeding device of FIG. 2a. In other words, the feeding connection
of the second chemical is located at the side of the feeding
device; via this connection the additives mentioned above or
combinations of them can be fed to the feeding device. Of course
also in this modification, a second connection for the chemical, if
the chemical allows it, can be provided already on the side of the
feeding duct attached to the flange and bringing feeding liquid to
the feeding device.
[0057] When using the structural alternatives illustrated in FIG.
2c as a starting point for using the method of the invention, the
possibilities for introducing the additional chemical/chemicals
increase. Then it is possible to arrange also in the outer wall of
the feeding device casing a connection for the additional chemical
which like in the previous embodiments can be an additive, a sizing
agent or any other chemical used in the paper manufacture which can
be fed to the feed liquid just before the headbox. In a
corresponding way, the chemical in question can be fed to the feed
liquid already before the upper flange of the feeding device.
[0058] Another alternative of introducing additional chemical is to
arrange another feed connection for another chemical inside the
inner member of the feeding device whereby two different chemicals
are brought at the same time inside the member. This can be done if
the contact between the chemicals does not disturb their reactions.
The way can very well be applied for example in feeding ASA sizing
agent and starch or ASA sizing agent and polymer solution. When
feeding these, is can even be thought that the starch or polymer
solution is fed to the ASA immediately before the ASA arrives to
the feeding device. Another possible way is to divide the inner
member for example with an axial plane in two whereby two different
chemicals can be fed irrespective of each other via the member.
[0059] A third possible way of feeding a second chemical is to
introduce it via the central and innermost duct illustrated in FIG.
2c either as a mere chemical or as mixed into a suitable
feeding/dilution liquid or a corresponding medium.
[0060] A preferred method according to the invention is illustrated
in FIG. 4 which applied the simple feeding device described in the
US patent mentioned above. The feeding device illustrated in FIG. 4
is composed in principle of a feeding device 34 according to FIG.
2b, why not also according to FIG. 2a, and a feeding connection 68
disposed upstream of it in the wall of the flow duct 70 leading to
the paper machine headbox. In other words the second chemical is
fed via the feeding connection 68 to the flow duct 70 with such a
small pressure difference that it hardly can assume its flow space
against the wall of the flow duct 70. Then the chemical flows along
the wall of the flow duct 70 where there is at a short distance
from the feed connection 68 of the second chemical provided a
feeding device 34 via which the second chemical and the feeding
liquid are sprayed to the paper pulp flowing in the flow duct.
While the second chemical flows to the strong feeding liquid jet
discharging from the feeding device 34 it spreads efficiently and
homogenously to the paper pulp quite as if it had been fed from the
feeding device 34 itself the way illustrated in the previous
figures.
[0061] Of course also a situation is thinkable, where several of
the additional chemical feed connections 68 are disposed one after
the other in the wall of the flow duct 70. In this case it should
firstly be ensured that the chemicals introduced via these
connections are such that their contact with each other even in
high concentrations is not harmful to the chemicals themselves or
to the paper pulp surrounding them at least at the center of the
flow duct. Further, the feeding point in question must be so close
to the feeding device 34 developing the feeding jet that the jet of
the feeding device is able to spread to the pulp flow all the
chemicals fed from the upstream side. In practice this means that
the mixing jet discharging from the feeding device 34 must be wider
than the area to which the chemical flow/chemical flows discharging
from the feed connection 68 have had time to spread when they reach
the feeding device 34.
[0062] On the other hand it should be noted that the apparatus of
FIG. 4 can still be simplified so that it is used to feed one
chemical, only, whereby only the so-called feeding liquid is
brought to the feeding device and the chemical or corresponding
additive is introduced upstream of the feeding device from which
the chemical flow travelling with the paper pulp flow is fed to the
paper pulp in the way described in connection with FIG. 4.
[0063] In the FIG. 4 above, only an example has been described of
the kind of a feeding apparatus that could be used in connection
with an additional chemical connection 68 provided upstream of it.
However, one must immediately remember that all the feeding devices
described above either in the figures or in text, only, are
applicable in connection with the additional chemical connection
68. The only precondition of the use of the connection is the
strong feeding liquid jet discharged from a feeding device
following it, by means of which the additional chemical coming from
the connection is caused to penetrate to the desired depth into the
paper pulp flow. Thus it is for example possible that a first
chemical is fed from the connection 68 to the paper pulp and later
a second or also a third chemical by means of the feeding
device.
[0064] Both the feeding device illustrated by using figures
illustrating prior art, and the feeding device described in FIG. 4
can be used also in the feeding of, among other things, chemicals,
such as for example retention chemicals, micro-particles, fillers,
binding agents, sizing agent, optical brighteners, paper dyes, and
silicates, to the flowing process liquid, only to mention a few
chemicals. The feeding device is thus applicable in all the
processes where these chemicals must be fed, in particular when the
chemical volume is small compared with the total volume of the
flowing suspension flow. As advantageous examples of the processes,
among others fiber suspension flows of paper mills, thickening
processes of various sludges, recycling fiber processes and
bleaching processes may be mentioned, and in general processes
where it is necessary to feed chemical, particularly in very small
amounts, to a filtrate, fiber suspension, sludge or a corresponding
medium.
[0065] In addition to the chemical combinations mentioned above,
titanium dioxide and some other suitable flocking chemical carried
by the mixing liquid to the apparatus should be mentioned as an
example of the first chemical to be fed. Another alternative is to
feed silicate as the chemical and a filler, for example titanium
dioxide, with the mixing liquid. Still a third alternative is to
feed ASA sizing agent as the chemical and bentonine in the mixing
liquid.
[0066] In the mixing device according to the invention, the feed
liquid by means of which a chemical is supplied to the process
liquid, for example to a fiber suspension, can be the same fiber
suspension, into which the chemical is to be fed. Of course also
more dilute suspensions, various filtrates or corresponding media
or mere fresh water are suitable for use as the feed liquid in the
apparatus of the publication. Thus all the liquid obtained from
another process stage that can be used in the feeding of the
chemical at the same time saves fresh water and reduces for example
the fresh water consumption of the mills.
[0067] As a summary, a situation will be described below in which
the method of the invention utilizes the feeding device described
in the U.S. Pat. No. 6,659,636 above. It is simplest to think the
feeding device in question to comprise three feeding ducts one
inside the other. The so-called mixing liquid is conventionally fed
to the paper pulp flow via the innermost of these, the chemical via
the one in the middle and the feeding liquid via the outermost
duct. Now according to the method of the invention, in addition to
the conventional chemical, chemicals can be fed for example as
follows:
[0068] the first additional chemical in the mixing liquid,
[0069] the first additional chemical in the feeding liquid,
[0070] the first additional chemical separately to the pulp flow
upstream of the feeding device the way illustrated in FIG. 4,
[0071] the first additional chemical mixed into the feeding liquid
and the second into the mixing liquid,
[0072] the first additional chemical mixed into the feeding liquid
and the second separately to the pulp flow upstream of the feeding
device the way illustrated in FIG. 4,
[0073] the first additional chemical mixed into the mixing liquid
and the second separately to the pulp flow upstream of the feeding
device the way illustrated in FIG. 4,
[0074] the first additional chemical mixed into the feeding liquid,
the second into the mixing liquid, and the third in the
conventional chemical
[0075] the first additional chemical mixed into the feeding liquid,
the second to the mixing liquid and the third separately to the
pulp flow upstream of the feeding device the way illustrated in
FIG. 4,
[0076] the first additional chemical mixed into the feeding liquid,
the second to the mixing liquid, the third in the conventional
chemical and the fourth separately to the pulp flow upstream of the
feeding device the way illustrated in FIG. 4.
[0077] In the above description it must been noted that both the
terms "in the liquid" and "mixed in the liquid" are to be given a
broad interpretation. In other words the terms cover both mixing of
the addition substance to the flow coming to the feeding device and
the feeding of the additional substance separately to the feeding
device and mixing it there into the liquid mentioned.
[0078] It has been noticed in the tests we have performed among
other things the following chemical or additive combinations when
fed with the same feeding device essentially simultaneously to the
headbox feed duct after the machine screen give a remarkably better
result than previous mixing methods: [0079] Retention
chemical--filler [0080] Retention chemical--micro-particle [0081]
Retention chemical--sizing agent [0082] Silicate--filler [0083] ASA
sizing agent--polymer
[0084] Further, it should be remembered that only examples have
been presented above of the many chemical variations, which can be
introduced to a liquid by the method and the apparatus of the
present invention. The starting point is, however, that the
chemicals fed from one and the same mixing device should preferably
be such that their contact with each other would not be harmful.
This is particularly important when the chemicals are fed via the
same flow duct of the mixing apparatus to the process liquid. This
is easiest to avoid when working with chemicals reactive in
relation to each other so that the different chemicals are taken to
the process liquid via different flow ducts of the mixing
apparatus. However, if the chemicals in question are highly
reactive with each other, it is best to feed them from separate
mixing apparatus disposed at an adequate "safety distance" from
each other.
[0085] While the invention has been described in connection with
what is presently considered to be the most practical and preferred
embodiment, it is to be understood that the invention is not to be
limited to the disclosed embodiment, but on the contrary, is
intended to cover various modifications and equivalent arrangements
included within the spirit and scope of the appended claims.
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