Note: Descriptions are shown in the official language in which they were submitted.
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TREATMENT OF BLEACH PLANT FILTRATES WITH OXYGEN
THE BACKGROUND OF THE INVENTION AND PRIOR ART
The present invention refers to a method for the treatment of
cellulose fibres in a pulp stream, comprising the steps of: preparing
a pulp, subjecting the pulp to a bleaching, comprising at least one
bleaching step, by the supply of an oxidizing bleaching agent,
removing liquid from the pulp after the bleaching while obtaining a
residue liquid, and conveying at least a part flow of the residue
liquid obtained via a transport conduit directly back to the pulp
stream by means of a pump device.
It is known to bleach cellulose pulp in one or several successive
bleaching steps by the supply of an oxidizing bleaching agent, such
as hydrogen peroxide, and to remove liquid from the pulp after each
such bleaching step in such a manner that a residue liquid, the so
called filtrate, is obtained. Since this residue liquid still contains a
certain amount of the bleaching agent, it is common practice during
mechanical pulp bleaching to convey residue liquid back to a
preceding washing step or any other possible position in a counter
flow in order to utilize available peroxide or other oxidizing
bleaching agents instead of conveying it directly to the effluent
cleaning plant. In this manner the degree of utilization of the
oxidizing bleaching agent may be improved. Also in connection with
bleaching of chemical pulp, it would be desirable to be able to
convey the residue liquid or the filtrate back to any preceding
washing step and further in a counter flow relative to the pulp
stream in order to utilize the washing liquid thereby in a maximal
manner and minimize the discharge from the bleaching.
The residue liquid obtained forms, after having been conveyed in a
counter flow to the pulp flow, an effluent flow which is conveyed to
an effluent plant. The effluent flow contains oxygen consuming
organic substances, so called COD (chemical oxygen demand),
which thus must only be contained in a determined quantity of tons
per day in the effluent liquid leaving a pulp plant. Today, it is
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possible to decompose such organic substances in the effluent
plants available. Nevertheless, problems frequently occur if the
percentage of these substances suddenly for any reason is greater
than calculated, since the existing effluent plant is dimensioned to
manage a certain quantity thereof. Likewise, heavy investment
expenses are involved when the requirements from the authorities
are made more stringent and the maximal permissible level is
decreased.
It is to be pointed out that it is generally considered to be a problem,
in connection with the production of chemical pulp, to utilize the
residue liquid, or the filtrate, since it is considered to contain organic
substances consuming bleaching agents and metal ions being
harmful to the bleaching agent. If organic substances or the
percentage of metal ions is high, the bleaching result by such a
liquid has been unsatisfactory. A return of residue liquid in
connection with chemical pulp bleaching results in an increased
need of peroxide or other oxidizing bleaching agents in the
bleaching reactor for obtaining the same bleaching result.
W094/21857 discloses a method for removing metals from a filtrate
obtained in a dewatering device in a pulp stream. This document
discloses the supply of an oxidizing agent, for instance oxygen gas,
and alkali to the filtrate in a reactor. fn such a manner, the metals
dissolved in the filtrate may be percipitated as solid particles and
removed therefrom.
The Swedish patent application No 9601709-0 discloses a method
for the treatment of cellulose pulp along a pulp stream. The
cellulose pulp is bleached in several steps and a residue liquid is
separated from the pulp and returned to the pulp stream. Before
being returned, the residue liquid is brought into contact with an
oxidation gas in a oxidation reactor. According to this document, the
preferred oxidation gas is ozone.
EP-A-564443 discloses a method for the treatment of cellulose pulp
along a pulp stream comprising bleaching in several steps. A
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residue liquid is obtained after a bleaching step and returned to a
prior washing step. The residue liquid obtained is heated and
conveyed to a reactor to react with oxygen gas. Thereafter, the
residue liquid is conveyed to a heat exchanger and from there back
to the pulp stream.
SUMMARY OF THE INVENTION
The object of the present invention is to provide a method for
solving the problems mentioned above. In particular, an object is to
provide a residue liquid, in connection with pulp bleaching, which is
less harmful from an environmental point of view and which is more
appropriate for being utilized during the treatment of cellulose fibres.
This object is obtained by the method initially defined and
characterized in at said part flow of the residue liquid is treated by
oxygen by injecting oxygen into said part flow downstream of the
pump device. By such an oxygen treatment the percentage of
valuable oxidizing components in the residue liquid increases.
Thereby, the oxygen may be supplied to the residue liquid under
very simple conditions, for instance by injecting oxygen gas to the
transport conduit usually existing in a pulp plant for conveying the
residue liquid directly back to the pulp stream, i.e. no further
oxidizing promoting means, such, as reactors working at high
temperature and high pressure, are provided according to the
invention. By such a treatment of the residue liquid, possibilities are
provided for changing or rendering harmless a significant
percentage of the undesired oxygen consuming organic substances
and of the undesired harmful metal ions, which are present in the
residue liquid when it leaves the bleaching step. This is surprising
since previously the opinion was that oxygen may not react to
substances in the residue liquid during the mild conditions which
normally prevails in the liquid removed from the pulp after the
bleaching. In order to provide reactions such as inter afia a
reduction of the quantity of organic substances, the opinion
previously was that powerful oxidizing conditions were necessary,
i.e. a high pressure and a high temperature over 100°C.
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Consequently, the residue liquid treated in accordance with the
inventive method may in comparison with known technique be taken
care of in a more simple manner during the effluent cleaning and is
more suitable for being utilized for the treatment of the cellulose
fibres. Furthermore, since by the treatment according to the
invention the quality of the residue liquid is enhanced a return of the
residue liquid to the pulp stream results in an improved effect of the
residue liquid. By conveying at least a part flow of the residue liquid
obtained directly back to the pulp stream, i.e. without any
intermediate treatment or addition of substances, the method
according to the invention is characterized by a low complexity
enabling application to pulp treatment plants of today.
According to an embodiment of the invention, at least a part of said
part flow of the residue liquid may advantageously be conveyed
back to the pulp stream before the pulp reaches at least one
bleaching step. In such a manner the bleaching may be more
efficient, which results in the tact that the brightness of the pulp may
be increased or the quantity of hydrogen peroxide added may be
reduced with the brightness maintained. Moreover, at least a part of
said part flow of the residue liquid may be conveyed back to the
pulp stream downstream of a bleaching step.
According to a further embodiment of the invention, oxygen is
supplied to the residue liquid before said part flow thereof reaches
the pulp stream by injecting oxygen gas into said transport conduit.
In such a manner, the oxygen may be supplied at a relatively low
pressure, i.e. merely the pressure which appears due to the
transportation work which is necessary for transporting the residue
liquid back to the pulp stream. Thereby, the oxygen may
advantageously be supplied to the residue liquid at such a position
that the dwell time of the oxygen in the residue liquid is at least
three seconds before said part flow thereof reaches the pulp.
According to a further embodiment, said dwell time may be greater
than 10 seconds, for instance greater than 30 seconds.
1
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According to a further embodiment of the invention, oxygen is added
to said part flow of the residue liquid by injecting oxygen gas into the
pulp stream.
5 According to a further embodiment of the invention, at least a
second part flow of the residue liquid obtained is treated by oxygen
and supplied to an effluent cleaning step via at least one direct
connection, said preparing step, said bleaching step and/or said
liquid removing step.
According to a further embodiment of the invention, the oxidizing
bleaching agent comprises at least one of hydrogen peroxide,
ozone, hypochlorite, peracetic acid and chlorine dioxide. The
bleaching effect of such oxidizing bleaching agents may be
improved if the residue liquid returned has been treated by oxygen.
According to a further embodiment of the invention, the bleaching
comprises at least one first bleaching step and a second bleaching
step.
The present invention is applicable to different pulp preparing
methods and the pulp may thus comprise at least one of
chemimechanical pulp, semi-chemical pulp, mechanical pulp,
chemical pulp and secondary fibre pulp.
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention is now to be explained more closely by means
of different embodiments disclosed by way of example and with
reference to the drawings attached.
Fig 1 discloses schematically a pulp stream for the treatment of
cellulose fibres according to a first embodiment of the invention.
Fig 2 discloses schematically a pulp stream for the treatment of
cellulose fibres according to a second embodiment of the invention.
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DETAILED DESCRIPTION OF DIFFERENT EMBODIMENTS
According to the first embodiment, disclosed in Fig 1, the pulp of
cellulose fibres is prepared in a first process step 1 schematically
disclosed. This preparing step 1 may for instance comprise
digesting for preparing chemical pulp, repulping for preparing
secondary fibre pulp and refining for preparing mechanical pulp. It is
already now to be pointed out that the present invention is not
limited to any particular type of pulp may be applicable to all known
pulp types, for instance chemimechanical pulp, semi-chemical pulp,
mechanical pulp, chemical pulp, such as sulphate and sulphite pulp,
and secondary fibre pulp. The preparing step 1 schematically
disclosed may also comprise screening of the pulp for removing
greater particles therefrom.
From the preparing step 1, the pulp is transported along a pulp
stream 2 to a first washing device 3, which may comprise one or
several washing steps. The washing device 3 disclosed also
comprises a dewatering device, for instance in the form of a
dewatering press. The residue liquid obtained during the dewatering
is transported to a residue liquid tank 4.
The pulp washed and dewatered is transported from the washing
device 3 along the pulp stream 2 to a bleaching step 5 comprising a
bleaching reactor. Before the pulp reaches the bleaching reactor 5,
bleaching chemicals are added by means of a mixing device 6 and
steam is added at a steam injection device 7. The bleaching
chemicals which are utilized comprises oxidizing bleaching
chemicals, such as hydrogen peroxide, ozone, hypochlorite,
peracetic acid and chlorine dioxide. In the case of hydrogen
peroxide, alkali and silicate may also be added by means of the
mixing device 6.
The pulp bleached is thereafter transported along the pulp stream 2
to a second washing device 8, which may comprise one or several
washing steps. Also the second washing device 8 comprises a
dewatering device, for instance in the form of a dewatering press.
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The residue liquid obtained during the dewatering is transported to a
residue liquid tank 9.
After the washing in the second washing device 8, the pulp is
transported further along the pulp stream 2 to for instance further
treatment, drying, storing or to an integrated paper mill. It is to be
noted that the pulp in addition to the screening in the preparing step
1 also may be screened at several different locations along the pulp
stream 2, for instance between the first washing device 3 and the
bleaching step 5 and/or between the bleaching step 5 and the
second washing device 8.
Liquid is supplied to the pulp stream 2 in an essentially counter flow
manner against the flow direction of the pulp, i.e. relatively clean
water is added in the end of the pulp stream 2 white used water,
such as the residue liquid obtained, is added in the beginning of the
pulp stream 2. This means that in the embodiment disclosed in Fig
1, an essentially clean washing water, or for instance residue water
from an integrated paper mill, is added to the pulp stream 2 at 10,
i.e. just before the pulp reaches the second washing device 8.
Consequently, the residue liquid obtained is transported rearwardly
and may be added to the process at one or several positions, for
instance it may be added as a dilution to the second washing device
8 at 11 and/or the first washing device 3 at 12. It may also be added
to the pulp stream 2 at 13 just before the pulp reaches the bleaching
reactor 5, the pulp stream 2 at 14 after the bleaching reactor 5
and/or the preparing step 1 at 15.
A part of the residue liquid which is obtained in the residue liquid
tank 4 and 9, respectively, is conveyed back to the process, for
instance to the preparing step 1 via a transport conduit,
schematically indicated as 4a and 9a, respectively, while another
part of the residue liquid is supplied to an effluent treatment device
16 via the transport conduit 4a. The transport conduits 4a, 9a may
be realized as a number of pipes, the mere purpose of which are to
convey the residue liquid. Pump devices 4b and 9b are arranged on
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the transport conduits 4a and 9a, respectively, to provide the
necessary transportation work.
According to the present invention, the residue liquid is to be treated
by oxygen. This may be performed by adding oxygen to the residue
liquid, for instance by an oxygen gas injection directly into the
transport conduit 9a at 17 when it leaves the residue liquid tank 9.
In such a manner, all residue liquid leaving the residue liquid tank 9
will be treated by oxygen. By the oxygen injection at for instance 18
and/or 19, a more selective treatment of the residue liquid may be
performed in accordance with the particular conditions which may
prevail in a separate process. It is also to be noted that it is possible
to add the oxygen to the residue liquid after it has been supplied to
the pulp, i.e. to add the oxygen directly to the pulp by oxygen
injection directly into the pulp stream 2. At the positions disclosed
for the oxygen addition, generally a relatively low overpressure
prevails in the transport conduit 4a, 9a in which the residue liquid is
transported. This pressure corresponds to the work which is
required for transporting the residue liquid from the residue liquid
tank 9 back to the pulp stream 2 and may amount to between 1 and
6 bars {abs), for instance about 4 bars (abs). Furthermore, the
temperature of the residue liquid is, at the positions disclosed for
the addition of oxygen, relatively low and may be between 50 and
90°C, preferably between 60 and 80°C. The pH-value of the
residue
liquid at the positions disclosed for the addition of oxygen may be
between 7 and 12, preferably between 8 and 10, with regard to
basic bleaching agents and between 1 and 7 with regard to acid
bleaching agents. If the residue liquid is conveyed back to the pulp
at for instance 11 or 14, an approximate dwell time in residue liquid
before it reaches the pulp is in the order of 0,5 to 4 minutes. It is to
be noted that the dwell time may be influenced by pipe dimensions
of the transport conduits 4a, 9a, and other plant specific designs.
Fig 2 discloses a second embodiment which differs from the first
embodiment mainly in that it comprises a further bleaching step.
According to the second embodiment the pulp of cellulose fibres is
thus prepared in a first preparing step 21 in a manner corresponding
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to the one of the first embodiment. From the preparing step 21, the
pulp is transported along the pulp stream 22 to the first washing
device 23, which may comprise one or several washing steps. The
residue liquid obtained at the dewatering in the washing device 23 is
transported to the residue liquid tank 24. The pulp washed and
dewatered is transported further along the pulp stream 22 to the first
bleaching step 25. Before the pulp reaches the bleaching reactor 25
bleaching agents are supplied by means of a mixing device 26 and
steam is supplied at the steam injecting device 27. The pulp
bleached is then transported along the pulp stream 22 to the second
washing device 28, which may comprise one or several washing
steps, and the residue liquid obtained at the dewatering is
transported to the residue liquid tank 29. Thereafter, the pulp is
transported further along the pulp stream 22 to the second
bleaching step 30 which comprises a bleaching reactor. Before the
pulp reaches the second bleaching reactor 30, bleaching chemicals
are added by means of a second mixing device 31 and steam at a
second steam injecting device 32. The bleaching chemicals utilized
according to the second embodiment also comprises oxidizing
bleaching chemicals, such as hydrogen peroxide, ozone,
hypochlorite, chlorine dioxide and peracetic acid. In the case of
hydrogen peroxide alkali and silicate may also be added by means
of the second mixing device 31. It is to be noted that different
bleaching chemicals and/or different compositions of bleaching
chemicals may be utilized in the two bleaching reactors 25 and 30.
The pulp bleached is then transported along the pulp stream 22 to
the third washing device 33, which also may comprise one or
several washing steps and a dewatering device, for instance in the
form of a dewatering press. It is to be noted that according to an
alternative embodiment, the third washing device 33 may be
replaced by a pure dewatering, for instance by means of a
dewatering press. The residue liquid obtain at the dewatering is
transported to a third residue liquid tank 34. After being washed in
the third washing device 33, the pulp is transported further along the
pulp stream 22 to for instance further treatment, drying, storing or to
an integrated paper mill. By means of dewatering and return of
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residue water, the pulp concentration in the first bleaching step 25
may be between 8 and 20%, preferably about 15%, i.e. a so called
MC bleaching step and in the second bleaching step 30 between 20
and 40%, preferably about 30%, i.e. a so called HC bleaching step.
5 Aiso according to the second embodiment, the pulp may in addition
to the screening in the preparing step 21 also be screened at
several different locations along the pulp stream 22, for instance
between the first washing device 23 and the first bleaching step 25,
between the first bleaching step 25 and the second washing device
10 28 andlor between the second bleaching step 30 and the third
washing device 33.
Also according to the second embodiment, liquid is supplied to the
pulp stream 22 in an essentially counter flow manner against the
flow direction of the pulp, i.e. relatively clean water is supplied in the
end of the pulp stream while used water, such as the residue liquid
obtained, is supplied in the beginning of the pulp stream. This
means that an essentially clean washing water, for instance residue
liquid from an integrated paper mill, may be supplied to the pulp
stream 22 at 35, i.e. just before the pulp reaches the third washing
device 33. The residue liquid obtained in the third residue liquid tank
34 is thus transported rearwardly via the transport conduit 34a by
means of the pump device 34b, and may be supplied to the process
at one or several positions as exemplified below. It is to be noted
that the residue liquid also may be returned to the pulp stream at
other positions than those disclosed. The residue liquid from the
third residue liquid tank 34 may for instance be added as dilution to
the third washing device 33 at 36, the second washing device 28 at
37 andlor a first washing device 23 at 38. Moreover, the residue
liquid may be supplied to the pulp stream 22 at 39 just before the
pulp reaches the secondary bleaching reactor 30 an/or at 40 just
before the pulp reaches the first bleaching reactor 25. Likewise, it is
also possible to supply residue liquid to the pulp stream 22 at 41
after the second bleaching reactor 5 and/or at 42 after the first
bleaching reactor. The residue liquid obtained in the second residue
liquid tank 29 may for instance via the transport conduit 29a by
means of the pump device 29b be returned to the process at 43
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after the first bleaching reactor 25 or to the first residue liquid tank
24. The residue liquid obtained in the first residue liquid tank 24 may
for instance be returned to the process via the transport conduit 24a
by means of the pump device 24b to the preparing step 21. One part
of the residue liquid obtained in the first residue liquid tank 24 and
the second residue liquid tank 29 may in addition be removed from
the process to a waste treatment device 44. It is also possible to
convey at least a part of the residue liquid obtained in the third
residue liquid tank 34 directly to the effluent treatment device 44.
Moreover, Fig 2 discloses a number of examples of positions where
oxygen may be added to the residue liquid in accordance with the
present invention. Oxygen may for instance be added at 45 when
the residue liquid leaves the third residue liquid tank 34. In such a
manner all residue liquid leaving the third residue liquid tank 34 will
be treated by oxygen. By oxygen addition at for instance 46, 47, 48
and/or 49, a more selective treatment of the residue liquid may take
place in accordance with the specific circumstances which may
prevail in an separate process. In certain cases, it may also be
advantageous to treat the residue liquid from the second washing
device 28 by oxygen and to add the latter at for instance 50 or 51 to
the residue liquid flow leaving the residue liquid tank 29. Moreover,
it is to be noted that according to the second embodiment it is also
possible to add the oxygen to the residue liquid after it has been
added to the pulp, i.e. to add oxygen directly to the pulp. Also for the
second embodiment the values for the pressure, the temperature
and the pH-value defined in connection with the first embodiment
are applicable.
The present invention is not limited to the embodiments disclosed
above but may be varied and modified within the scope of the
subsequent claims.
The essentially clean washing water may also be added to the
process at other positions than at the fast washing device 8 and 33
disclosed, for instance at the associated residue liquid tank 9 and
34 respectively.
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It is to be noted that the steam supply at 7, 27, 32 is not necessary
for the method according to the invention.
One or several of the washing devices 3, 8, 23, 28 and 33 may be
replaced by mere dewatering devices, for instance dewatering
presses.
It may be dispensed with the residue liquid tanks 4, 9, 24, 29, 34,
i.e. the transport conduits or pipes 4a, 9a, 24a, 29a, 34a may extend
from the respective dewatering device directly to the pulp stream 2;
22 or to the effluent treatment device 16; 44.
The dwell time for the oxygen in the residue liquid may be increased
by for instance adding oxygen to the residue liquid in any of the
residue liquid tanks 9 and 34, respectively.
In an application of the method according to the invention, one has
obtained a higher brightness, i.e. 1,5 - 2% ISO higher brightness at
a certain hydrogen peroxide supply or about 10-13% decrease of the
hydrogen peroxide consumption at a certain brightness. Moreover,
an increase of the upper brightness limit by 1,5-2% ISO has been
noted. Finally, it has been observed that the quantity of oxygen
consuming organic substances in the effluent water has decreased
by about 10%.
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