Note: Descriptions are shown in the official language in which they were submitted.
Wo 94/15016 215 1812 PCT/US93/12125
IMPROVED PULP BLEACHING APPARATUS AND METHOD
This invention relates to the making of pulp for paper products and, more
particularly to an irnproved method and apparatus for adding chemicals and/or
adjusting temperature in a bleaching operation.
RAcKGROuND OF THE INVENTION
~h~mic~l removal of lignin from wood pulp is a well known process which
is constantly being studied and incrementally improved. For high quality paper, it is
important that delignification (or bleaching) be as complete as practicably ~tt~in~ble.
One of the earliest methods for removal of residual lignin rem~ining in the
pulp after it has passed through a digesting stage involves the use of chlorine as the
bleachin~ chemical for the first bleaching stages. Apart from the corrosive tendency
of chlorine, it is difficult to remove from the pulp liquor and may contribute to
harmful emissions from the mill. Accordingly, a great deal of effort has been
concentrated on attempting to replace chlorine based blç~ching reagents using a variety
of alternative processes, including oxygen bl~.achin~, which produce more benignbyproducts. Oxygen ble~ching is performed on pulp of high and medium consistency.
ln addition, if chlorine is totally elimin~te~l, oxygen bleaching may be followed by an
ozone bleaching process. For ozone to be most effective and to minimi7e pulp
degradation, it is best to achieve maximum bleaching before the ozone stage by high
consistency oxygen delignification, for example.
It is very difficult to add or extract chemicals and/or heat in a high
consistency (27%-45%) pulp. Sodium hydroxide, or some other ~lk~lin~. compound, is
required by the oxygen delignification process to adjust the pH of the pulp. Uniform
distribution of temperature and of the alkali added is especially difficult to achieve in
high consistency pulp.
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Lowering the consistency of the pulp, to obtain a more uniform distribution of the
~hemic~l, wastes expensive chemicals because dewatering of the pulp removes
dissolved chemicals as well and transfers a portion upstream in the process along with
extracted liquor.
After oxygen delignification, the pulp may be exposed to an ozone
b1ça~hing step. Ozone bleaching has markedly different requirements from those of
the oxygen b1e~ching process. For example, oxygen b1ea~hing is most efficient at high
temperatures (approximately 190 degrees F~ and high pH (8-9), while ozone blPaching
is most efficient at low pH and a generally low temperature (120 degrees to 140
degrees F). Thus, the pulp must be acidified and cooled, with the same concerns for
economy and efficiency, after exiting the oxygen de1ignific~tion stage.
The foregoing illustrates limitations known to exist in present day bleaching
methods and apparatus. Thus, it is apparent that it would be advantageous to provide
an alternative directed to overcoming one or more of the limit~tions set forth above.
Accordingly, a suitable alternative is provided including features and advantages more
fully described hereinafter.
SUMMARY OF THE INVENTION
In one aspect of the invention, this is accomplished by a method for
uniformly adjusting chemical content and/or temperature of a wood pulp slurry in a
pulp blea~hing system by introducing pulp into a liquid filled wash press which has
provisions for displacement washing and for extraction of liquid from the pulp. Liquid
is extracted from the pulp to form a pulp mat in a first chamber, and the pulp mat is
conveyed into a second chamber. A heated or cooled chemical solution is introduced
into the second chamber in an amount and at a concentration and temperature
sufficient to perform displacement washing and to bring the pulp mat to a
predetermined temperature and chemical content. Liquid is displaced from the pulp
WO 94/15016 ~ 1 ~ 18 ~ 2 PCT/US93/12125
mat by extracting liquid introduced with the pulp while simultaneously replacing the
liquid with liquid from the second chamber to an extent required to yield pulp at a
desired temperature and a desired chemical content, after which, liquid extraction is
continued in order to bring pulp consistency to a desired level for discharge from the
5 washing press.
The foregoing and other aspects will become appalent from the following
detailed description of the invention when considered in conjunction with the
acco.llpallying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a sch~.m~ic diagram showing a washing press used to add
chemicals, to increase consistency of the pulp, and/or to adjust temperature of the pulp
at the end of a bleaching stage;
Fig. 2 is another schematic diagram of the washing press as applied in
another bl~aching stage in a pulp making process.
15DETAILED ~ESCRIPTlON
Fig. 1 shows a washing press in which the chemical content and
tempera~ul~ of a pulp slurry is uniforrnly adjusted while the consistency of the slurry is
increased. This press and the method of its operation promote uniform mixing of the
chemical added with the pulp slurry, uniformity of pulp temperature, and minimum20 waste of the chemical
Washing press 100 contains two identical rolls 62, mounted on shafts 62b,
all within a tank 64. Tank 64 has an arcuately shaped nip wedge 63 to decrease
volume of chamber II of tank 64 and to train pulp into the nip between rolls 62. The
press further comprises two baffles 66, substantially separating chambers, I and II.
WO 94/15016 PCT/US93112125
2~s~
Streams 10, 15, and 17 combine in mix chest 60 to form stream 70 and to
enter tank 64 in the chamber I between rolls.62 and baffles 66. Streams 14 are also
shown as entering tank 64 in the chambers II between baffles 66 and tank 64.
In operation, the streams 10 enter mix chest 60 where the consistency is
reduced by the addition of liquor from streams 15 and 17. Stream 15 contains a part
of the effluent stream 11 from rolls 62. Stream 14 is a solution of chemical 16, like
sodium hydroxide, mixed with wash liquor from stream 19. A portion of wash liquor
19 is preferably mixed with chemical 16 to produce a chemical solution stream 14which is then supplied to tank 64. The chemical material which is supplied to
chamber II of stream 14 is of a sufficient strength and amount to provide the output
pulp 13 from washing press 100 with a sufficient amount of chemical to satisfy
downstream bleaching operations. For the particular example of alkali addition, the
downstream operation would be an oxygen stage. It is well known that, for oxygen to
react with lignin, typically 1-4% by weight of caustic soda (sodium hydroxide) has to
be added to the pulp.
In operation, an approximately 4% consistency pulp stream is provided from
chest 60 as feed stock to both sides of the tank 64 along with the chemical solution
stream 14. Consistencies are used for illustrative purposes to indicate the desirability,
at various process stages, of lesser or greater consistencies. Rolls 62 are counter-
rotating and will produce an output pulp mat 13 having a consistency of approximately
28%, or greater - a high consistency pulp mat.
The functions within tank 64 take place under differential pressure with
pressurized feed stock, stream 70 from chest 60 and stream 14 injected into tank 64,
for liquid extraction and displacement washing of the feed pulp slurry. Liquid
extraction is driven by pressurized slurry in chambers I and Il losing liquor to the
relatively low pressure within perforated rolls 62. Properly applied, pressure or
vacuum will drive the operation with equal effectiveness.
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Bleachin~ systems of the prior art, have introduced the required chemicals,
e.g., alkali materials for a subsequent oxygen delignification stage, either into mix
chest 60, or into the high consistency output stream 13 of the extraction press. While
addition of chemical to a mix chest before the extraction press promotes good mixing
5 of the chemical with the pulp with relatively low power cost (because only a 4%
consistency solution resides within the mix chest 60), it leads to an excessive loss of
chemic~l; because some of the chemical is lost upstream with extracted liquor. The
recycle line 21 will contain about 50% of the lost chemical. If chemicals are added to
the high consistency pulp output stream, there are no losses of chemical, but it is very
10 difficult to obtain a uniform chemical distribution in the pulp. This results in non-
uniform bleacching results. In short, supplying the required chemical to the lowconsistency pulp at the inlet of the extraction press, or supplying the chemical to the
output stream thereof, has two different but undesirable consequences.
This invention follows neither prior art approach but instead concentrates on
15 where and how the pulp and chemicals are combined for the best results. As applied
in this invention, baffles 66 are mounted on pivot points 66a and have mechanical
positioning devices 66b which establish a "no load" baffle position. In operation of
wash press 100, a pulp mat forms on rolls 62 by liquor extraction through the porous
roll s~ ces. Compaction baffles 66 gently iron the pulp mat but, more importantly,
20 separate mat formation chamber I from chemical and temperature adjusting solution
cont~ined in chamber II.
The space between the periphery of roll 62 and baffle 66 defines chamber I,
into which is directed a preferably pressurized stream 70 of pulp slurry. Chamber II is
defined between baffle 66, tank 64, and the arcuately shaped nip wedge 63. A
25 chemical cont~ining input stream 14, pressurized as is stream 70, is introduced into
chamber II and comes in contact with the pulp mat as the mat emerges from Chamber
I (behind baffles 66). The chemical containing solution of Chamber II begins to
displace liquor from the pulp mat, after the pulp mat passes the end of baffles 66, and
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2~ 6
continues to do so until the pulp mat enters the nip between rolls 62 and above nip
wedge 63.
The pulp slurry stream 70 is directed into the first chamber I which, at its throat,
would typically have a width of, approximately 20 to 80 millimeters, while the
5 distance between baffles 66 and rollers 62, at the juncture of chambers I and II, would
be on the order of 6 millimeters. Thus chamber I exhibits a continuously decreasing
width as the pulp mat advances to the end of baffles 66. The chemical con~ining
stream 14 is introduced into chamber II, which is separated from chamber I by the
compaction baflles 66. The stream 14 enters chamber II, joins with the pulp mat as it
10 emerges from chamber I, and displaces liquor from the pulp mat until the mat enters
the nip between rolls 62 just above nip wedge 63.
After the respective rolls 62 have advanced sufficiently, the mat is
discharged in stream 13 at high consistency (approximately 28%), at the requiredtemperature, and having the required chemical content uniformly distributed therein.
Note that drainage of the chemical solution through the perforated peflphe~y
of the rolls 62 is minimi7ed, if not totally avoided, by exposing the mat and the
chemical to one another only after the mat has already been formed by extraction of a
substantial fraction of the pulp liquor and compacted by baffles 66. Thus, there is
negligible dilution of the chemical solution, during displacement of pulp liquor by the
20 chemical solution, due to the less-than-one ratio of the volume of chemical solution
added to the liquor rem~ining in the mat as it leaves the press. Also, there is some
resistance to penetration of chemical solution through the compacted mat.
Slurry stream 70 is fed into washing press 100 at a pressure between 3 psi
and 15 psi. This pressure is generally uniform throughout the press, except in the
2s washing chamber II, where it is usually higher, and continually drives the liquid
extraction due to the pressure differential through the surfaces of rolls 62. As the pulp
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2151812
layers formed on rolls 62 approach the nip of rolls 62, the mechanical pressure on the
pulp mat increases significantly due to the convergence of nip wedge 63 and rolls 62
and because the rolls 62 are urged against each other at very high pressure.
J
The liquor forced by hydrostatic pressure through the perforations into the
5 interior of the rolls is drained away to seal tank 11. The loss of chemical material in
the present process and apparatus is significantly reduced because only a small part of
the original pulp liquor has to be displaced by the chemical solution. The balance of
the liquor has been removed prior to contact of the pulp mat with the chemical
solution. By this process, most of the liquor is removed from the pulp during
10 form~tion of the pulp mat in chamber I and is not available in chamber II forintelmixi~lg with the chemical solution and for dilution thereof. The amount of
chemical introduced into chamber II of wash press 100 is generally around 2.5%
sodium hydroxide if the high conci.ctency pulp discharge stream 13 is to be introduced
into an oxygen reactor.
In Fig. 2, the 28% con~i~tency slurry from output stream 13 of the washing
press enters an oxygen reactor 82. The pulp slurry effluent 86 from oxygen reactor
82, together with liquors flowing from lines 96 and 95, all of them feeding into mix
chest 60 and from there into inlet box 88, create a 4% con~i~tency pulp slurry which
has to be acidified and cooled in prel)alalion for ozone reactor 114. Again, a washing
press is shown at 90, and it will drain liquor through line 92 into seal tank 6.
Ozone blea~.hing requires a lower temperature for reaction than does oxygen
bleaching and requires an acidic pulp, as opposed to an alkali pulp. Thus, acid (from a
source not shown) is conveyed via conduit 140 to blend with wash liquor from conduit
120, which may be fresh water or liquor from a succee~ing stage and which is passed
25 through a heat exchanger 98 for cooling as required. The combined flow from
conduits 120 and 140 is provided, via conduit 102, to a washing press 90, identical in
operation and structure to that which has been already described.
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~ s~ 8
This time washing press 90 produces a very high consistency pulp slurry at
approximately 35-45% consistency. Also the chemical solution, in this case acidic, as
before is provided in one chamber, while the slurry is provided in another chamber.
After a mat of pulp has been formed on the rolls in the first chamber, the mat is
5 exposed to the chemical solution in the second chamber where the chemical solution
displaces the liquor from the mat to adjust the pH and the temperature mat. After
passing through the nip of the rolls the output slurry is conveyed to an ozone reactor
114.
It should be noted that the wash liquor enters the washing press 90 in such
10 a manner that only that portion which is to be combined with the acid, passes through
heat exchanger 98, while the majori~y of the flow bypasses the heat exchanger as well
as the aci-lifir~tion step and goes directly to mix chest 60.
A compaLison of the apparatus and techniques used to produce a relatively
high consistency (28%) pulp mat with a relatively uniforrn alkali distribution
15 throughout the pulp, at a relatively high temperature for the oxygen blPaching process,
with the apparatus and technique used for producing a high consistency (approximately
35%) pulp mat with a relatively uniform acid distribution at relatively lower
temperatures for the ozone ble~ching process, shows that all objectives are achieved
and the same economies are obtained in both examples by the same methods and
20 appar~tus.
