Note: Descriptions are shown in the official language in which they were submitted.
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METHOD AND APPARATUS FOR FEEDING CHEMICALS INTO A PROCESS
LIQUID FLOW
The present invention relates to a method and an apparatus for feeding
chemicals into
a process liquid flow. 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
simultaneousiy.
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-calied 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.
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.
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
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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.
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.
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 an adhesive 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
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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.
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 alone.
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 achieved in chemical costs, alone.
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 pm. 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 the price
very high
compared for instance to caicium 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.
A filler which has become very popular, i.e. PCC (precipitated calcium
carbonate) in
turn 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 lime stone
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
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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.
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
adhesive, 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.
Adhesives, examples of which are ASA (alkenylsuccinic anhydride = alcylene
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 produced. 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 adhesive to the fibers, an efficient
retention system is
still needed to retain the adhesive quickly in the web to be produced.
Immediate
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retention is important as the adhesive 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.
5
Another known adhesive 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.
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 adhesive and also with
time reduce
the effect of the adhesion for example during storage.
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,
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polyamine or polyethyleneimine.
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 and/or improving the effect
of other
chemical and/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.
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
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order of 1.5 - 2 seconds during which time, and also along the corresponding
distance,
the chemical is homogenously mixed to the pulp.
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.
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.
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, adhesives 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. I most cases 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 adhesive/s of the paper pulp in the
paper machine
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wire section.
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 up 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.
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 US patent 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
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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.
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 substancelchemical.
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 practise 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
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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
5 headbox.
In view of the total economy, the best retention chemical mixing apparatus has
been
disclosed for example in US patent 6,659,636 BI, the installation of which
only requires
drilling fairly small holes in the wall of the flow duct. Because the
apparatus in question
10 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.
The purpose of the present invention is to solve for example 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
o a mixer of its own for each chemical
o each mixer to a different location in the short circulation
o large apparatus
o powerful electric motors
- high operating costs caused by the powerful electric motors,
- high installation costs
o constructing the stands
o cutting the headbox feed duct
o 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
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of which are disclosed in the appended patent claims.
The tests we have performed have shown that the mixing of the chemicals and/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.
For example it has been noticed that by using an apparatus, the operating
principle of
which was described already above and in the US patent 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.
Further it has been noticed in the tests performed that the feed liquid jet
typical of the
apparatus in question firstly mixes the chemical and/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 and/or additives
supplied
with the jet are mixed both with each other and with the solids and/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 apparatus 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 apparatus to the paper pulp. In a corresponding way, also the adhesive
and
the starch/polymer may be introduced via the same feeding apparatus and the
retention chemical via another feeding apparatus a little later; in practice
this distance
in the paper pulp flow direction need not be more than about two meters.
The method and the apparatus of the invention 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
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- reduced investment, installation and operation costs, and
- lower chemical costs
only to mention a few advantages.
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.
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
Figure 1 illustrates the conventional prior art short circulation arrangment
of a paper
machine,
Figures 2a, 2b and 2c illustrate three different variations of a conventional
prior art
feeding apparatus,
Figure 3 illustrates a short circulation process arrangement according to a
preferred
embodiment of the invention, and
Figure 4 illustrates a feeding apparatus according to a preferred embodiment
of the
invention.
According to figure 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
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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
apparatus 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.
Figure 2a illustrates an apparatus solution known per se described in the US
patent
6,659,636 already mentioned above. The feeding apparatus 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 apparatus 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 apparatus 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 apparatus. 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 apparatus 34 so
that the
velocity of the jet discharging from the feeding apparatus 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
figure 2a, the retention chemical feeding conduit 56 to the feeding apparatus
34 is
preferably tangential in order to ensure that the retention chemical
discharging through
the opening 58 of the feeding apparatus 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
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14
chemical from degrading.
Figure 2b illustrates another apparatus embodiment partly known from the US
patent
6,659,636 already mentioned above. Firstly, in the feeding apparatus 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
apparatus 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.
Figure 2c illustrates a feeding apparatus 34 according to figure 2a, in fact
with two
additional embodiments. Firstly, inside the feeding apparatus 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 apparatus 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
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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
5 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.
In another embodiment illustrated in figure 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
10 apparatus 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 figures 2a
and 2b.
The flow pipe 96 functions the way already presented before, but it is now
possible to
15 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.
Further, it is possibie to introduce via the flow path 94 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 eariier in figures 2a and 2b.
Fl patent application 2003051 illustrated a further prior art feeding
apparatus which is
to a large extent based on the basic structure of a feeding apparatus
illustrated already
in figure 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
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the dilution liquid feed duct (corresponds to the duct 92 and its feed end in
figure 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 from 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 80 in figure 2c) outside the feed duct
mentioned
(corresponds to duct 92 in figure 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.
Fl 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 figure 2c,
facing
the paper pulp feeding duct there is disposed a space 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 apparatus ducts mentioned
above
have been provided with regulating valves so that each flow can be adjusted
independently irrespective of the other flows.
Figure 3 illustrates a process arrangement which the invention tries to apply
as
efficiently as possible. Figure 3 correspond otherwise to figure 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 apparatus 34 after the headbox screen 32.
However,
it should be noted that the apparatus 34 must be understood in a very broad
way. It
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17
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.
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.
In principle most of the apparatus applying the method of the invention are
modifications of the feeding apparatus of the US patent 6,659,636 already
mentioned
earlier and also illustrated 'in the above figures 2a, 2b and 2c. A simple
feeding
apparatus employing the method of the invention greatly resembles the feeding
device
presented in figure 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 adhesive, AKD adhesive or corresponding
substance is fed in via the connection mentioned. In addition to the fillers
or adhesives
the process of the invention can be used for feeding two or multicomponent
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 silicon or bentonite. All the substances mentioned above are below
generally
called additives.
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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.
When using the feeding apparatus 34 of figure 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 apparatus 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 practise they can contact each other only inside the feeding
apparatus 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.
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
test I test 2 test 3 test 4
Ti02 feed g/ton 25 25 20 20
Opacity 91.7 91.5 92.0 92.3
Tests I 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
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pulp by means of a strong feeding liquid jet close to the headbox after the
machine
screen.
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 practise corresponds to a 40 percent
smaller
filler feed amount when using the method of the invention, compared with the
conventional method.
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.
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 adhesive
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 adhesive 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.
When using the feeding device illustrated in figure 2b the connection for
feeding the
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second chemical may be located as in the feeding apparatus of figure 2a. In
other
words, the feeding connection of the second chemical is located at the side of
the
feeding apparatus; via this connection the additives mentioned above or
combinations
of them can be fed to the feeding apparatus. Of course also in this
modification, a
5 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 apparatus.
When using the structural alternatives illustrated in figure 2c as a starting
point for
10 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, an adhesive or any other chemical
used in
the paper manufacture which can be fed to the feed liquid just before the
headbox. In a
15 corresponding way, the chemical in question can be fed to the feed liquid
already
before the upper flange of the feeding apparatus.
Another alternative of introducing additional chemical is to arrange another
feed
connection for another chemical inside the inner member of the feeding
apparatus
20 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 adhesive and starch or
ASA
adhesive 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.
A third possible way of feeding a second chemical is to introduce it via the
central and
innermost duct illustrated in figure 2c either as a mere chemical or as mixed
into a
suitable feeding/dilution liquid or a corresponding medium.
A preferred method according to the invention is illustrated in figure 4 which
applied the
simple feeding device described in the US patent mentioned above. The feeding
apparatus illustrated in figure 4 is composed in principle of a feeding
apparatus 34
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21
according to figure 2b, why not also according to figure 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 apparatus 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 it spreads efficiently and homognously to the paper pulp quite as if it
had been
fed from the feeding apparatus 34 itself the way illustrated in the previous
figures.
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
apparatus 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
practise this
means that the mixing jet discharching from the feed apparatus 34 must be
wider than
the area to which the chemical flow/chemical flows discharging from the feed
opening
68 have had time to spread when they reach the feeding apparatus 34.
On the other hand it should be noted that the apparatus of figure 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 apparatus and the chemical or
corresponding
additive is introduced upstream of the feeding apparatus from which the
chemical flow
travelling with the paper pulp flow is fed to the paper pulp in the way
described in
connection with figure 4.
In the figure 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
apparatus described above either in the figures or in text, only, are
applicable in
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connection with the additional chemical connection. The only precondition of
the use of
the connection is the strong feeding liquid jet discharged from a feeding
apparatus
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 apparatus.
Both the feeding apparatus illustrated by using figures illustrating prior
art, and the
feeding apparatus described in figure 4 can be used also in the feeding of,
among
other things, chemicals, such as for example retention chemicals, micro
particles,
fillers, binding agents, adhesive, optical brighteners, paper dyes, and
silicates, to the
flowing process liquid, only to mention a few chemicals. The feed 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.
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 adhesive as the chemical and
bentonite in the
mixing liquid.
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.
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As a summary, a situation will be described below in which the method of the
invention
utilizes the feeding device described in the US patent 6,659,636 above. It is
simplest to
think the feeding apparatus 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:
1. the first additional chemicalin the mixing liquid,
2. the first additional chemical in the feeding liquid,
3. the first additional chemical separately to the pulp flow upstream of the
feeding
apparatus the way illustrated in figure 4,
4. the first additional chemical mixed into the feeding liquid and the second
into
the mixing liquid,
5. the first additional chemical mixed into the feeding liquid and the second
separately to the pulp flow upstream of the feeding apparatus the way
illustrated in figure 4,
6. the first additional chemical mixed into the mixing liquid and the second
separately to the pulp flow upstream of the feeding apparatus the way
illustrated in figure 4,
7. the first additional chemical mixed into the feeding liquid, the second
into the
mixing liquid, and the third in the conventional chemical
8. 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
apparatus the way illustrated in figure 4,
9. 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 apparatus the way illustrated in figure
4.
In the above description it must been noted that both the terms "..in the
liquid" and
"mixed in the ... liquid" must be given a broad interpretation. In other words
the terms
cover both mixing of the addition substance to the flow coming to the feeding
apparatus
and the feeding of the additional substance separately to the feeding
apparatus and
mixing it there into the liquid mentioned.
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It has been noticed in the tests we have performed among other things the
following
chemical and/or additive combinations when fed with the same feeding apparatus
essentially simultaneously to the headbox feed duct after the machine screen
give a
remarkably better result than previous mixing methods:
Retention chemical - filler
Retention chemical - micro particle
Retention chemical - adhesive
Silicate - filler
ASA adhesive - polymer
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.
