U.S. patent application number 10/203202 was filed with the patent office on 2003-07-03 for method for bleaching mechanical and chemithermomechanical pulp g.
Invention is credited to Ni, Yonghao, Wang, Shuyu, Zhang, Eric Yijing.
Application Number | 20030121625 10/203202 |
Document ID | / |
Family ID | 20278409 |
Filed Date | 2003-07-03 |
United States Patent
Application |
20030121625 |
Kind Code |
A1 |
Zhang, Eric Yijing ; et
al. |
July 3, 2003 |
Method for bleaching mechanical and chemithermomechanical pulp
g
Abstract
The present invention relates to a method for bleaching
mechanical and chemithermomechanical pulp including that an
advancing pulp suspension obtained after that the fibres are laid
free is prebleached by adding reductive bleaching agent to the pulp
suspension in a location just after the fibres are laid free and
that the bleaching is carried out under given conditions in the
form of high temperature and minimized oxygen access in respect of
said adding location and immediately downstream of said location,
characterized in, that the in the described manner prebleached pulp
is subjected to at least one further bleaching treatment including
that the pulp is bleached with an oxidizing bleaching agent,
preferably peroxide bleaching agent, reinforced with
borohydride.
Inventors: |
Zhang, Eric Yijing;
(Norrtalje, SE) ; Ni, Yonghao; (Fredericton,
CA) ; Wang, Shuyu; (Fredericton, CA) |
Correspondence
Address: |
JOSEPH PETRYSZAK
P O BOX 116
BRUCETON MILLS W
VA
26525
|
Family ID: |
20278409 |
Appl. No.: |
10/203202 |
Filed: |
November 18, 2002 |
PCT Filed: |
February 8, 2001 |
PCT NO: |
PCT/SE01/00260 |
Current U.S.
Class: |
162/24 ; 162/37;
162/78; 162/80; 162/83; 162/90 |
Current CPC
Class: |
D21C 9/163 20130101 |
Class at
Publication: |
162/24 ; 162/37;
162/78; 162/80; 162/83; 162/90 |
International
Class: |
D21C 003/02; D21C
003/04; D21C 009/10; D21C 009/16 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 11, 2000 |
SE |
0000431-7 |
Claims
1. Method for bleaching mechanical and chemithermomechanical pulp
including that an advancing pulp suspension obtained after that the
fibres are laid free is prebleached by adding reductive bleaching
agent to the pulp suspension in a location just after the fibres
are laid free and that the bleaching is carried out under given
conditions in the form of high temperature and minimized oxygen
access in respect of said adding location and immediately
downstream of said location, characterized in that the in the
described manner prebleached pulp is subjected to at least one
further bleaching treatment including that the pulp is bleached
with an oxidizing bleaching agent, preferably peroxide bleaching
agent, reinforced with borohydride.
2. Method according to claim 1, characterized in, that borohydride
is added to the pulp and is permitted to react with this during a
short period of time in the form of a pretreatment before the pulp
is bleached with peroxide bleaching agent.
3. Method according to claim 1, characterized in, that borohydride
is added to the pulp at the beginning of the peroxide bleaching
treatment.
4. Method according to claim 1 characterized in that borohydride is
added to the pulp at the end of the peroxide bleaching
treatment.
5. Method according to claims 1-4, characterized in, that the
prebleached pulp is bleached with peroxide bleaching agent and
thereafter with peroxide bleaching agent reinforced with
borohydride.
6. Method according to claim 5, characterized in, that the first
peroxide bleaching stage is carried out at medium consistency and
that the second peroxide bleaching stage is carried out at high
pulp consistency and that spent liquor containing residue peroxide
from the second bleaching stage is returned and mixed into the pulp
before or in connection with the first peroxide bleaching
stage.
7. Method according to claims 1-6, characterized in, that to the
pulp is delivered at least one of the chemicals alkali, water glass
(sodium silicate) and magnesium compound as completion to
borohydride.
8. Method according to claims 1-7, characterized in, that to the
pulp is delivered at least one of the chemicals alkali, water glass
(sodium silicate) and magnesium compound as completion to peroxide
bleaching agent.
9. Method according to claim 1, characterized in, that the
reductive bleaching agent is sodium dithionite.
10. Method according to claims 1-9, characterized in, that the
borohydride is sodium borohydride, preferably in the form of the
commodity Borol.
11. Method according to claims 1-10 characterized in, that the
peroxide bleaching agent is hydrogen peroxide.
Description
TECHNICAL FIELD
[0001] The present invention relates to a method for bleaching
mechanical and chemithermomechanical pulp (CTMP). Within the group
mechanical pulps there are two dominating types of pulps, namely
groundwood pulp (SGW) and thermomechanical pulp (TMP). In the
manufacture of groundwood pulp roundwood logs are usually cut into
lengths about one meter, which during water addition are pressed
against a rotating grindstone. In the manufacture of
thermomechanical pulp roundwood logs are usually chopped into a
great amount of small wood pieces, named chips, and the true
free-laying of the fibres is carried out by means of one or more
refiners. Large parts of the chemithermomechanical pulp
manufacturing process are similar to the thermomechanical pulp
manufacturing process. The main difference is that one in a
prestage treats the lignocellulose material, normally wood chips,
with a sodium sulphite solution at a certain temperature and during
a certain period of time. Consequently, the pulp yield will usually
be one or some percent lower than in the case of thermomechanical
pulp.
[0002] Any lignocellulose material whatsoever can be used as
starting material for the pulps mentioned. Examples of such
materials are bamboo, straw, bagasse, kenaf and wood. Wood is the
preferred starting material, and both hardwood and softwood can be
beneficially used, either separately or in combination.
BACKGROUND ART
[0003] When bleaching mechanical pulp and CTMP lignin-preservative
bleaching is used. Among bleaching agents that can be used for
lignin-preservative bleaching there are as well reducing bleaching
agents as oxidizing bleaching agents. Within the first mentioned
group dithionite, usually sodium dithionite, also called
hydrosulfite, is the most common from a commercial point of view.
Within the last mentioned group hydrogen peroxide is the most
common from a commercial point of view.
[0004] The absolutely most common bleaching agent in the described
connection seems to be hydrogen peroxide. The big advantage with
hydrogen peroxide is that this agent is a very effective, i.e. a
strongly brightening, bleaching agent. However, as a rule the
bleaching with hydrogen peroxide demands that a separate bleaching
tower and also other bleaching equipment have to be used, which
leads to that the fixed costs at the hydrogen peroxide bleaching
will be and are high.
[0005] Dithionite, usually sodium dithionite, is often not so
effective from a bleaching point of view as hydrogen peroxide, but
has the advantage, that it besides the use of a bleaching tower
also can be added directly to the pulp suspension, for example in a
storage tower or earlier in the pulp manufacturing process, for
example just after the free-laying of the fibres, without the use
of bleaching towers and other bleaching equipment. The last
mentioned means a decrease of the fixed costs. This with adding a
reducing bleaching agent, for example sodium dithionite, directly
to the advancing pulp suspension just after the free-laying of the
fibres is known from the Swedish patent application 9900816-1.
[0006] Transition metals, particularly iron and manganese, are
detrimental to the bleaching of mechanical pulp and CTMP with, e.g.
as well hydrogen peroxide as dithionite. The presence of mangane
ions in significant quantities is particularly serious when
bleaching of such pulp with hydrogen peroxide. An increasing amount
of manganese ions leads to an increasing decomposition of the added
amount of hydrogen peroxide which means, that the part of the
hydrogen peroxide which is decomposed comes not to a use for
brightening the pulp. Furthermore the decomposed products are in
themselves harmful, because they form chromophore groups in the
pulp, which works against that which is tried to attain, namely a
brightening of the pulp. When bleaching of such a pulp with
dithionite there are the iron ions, which are particularly
dangerous. These transition metals are normally removed from or
neutralized in the pulp and the pulp suspension by complex-binding
the transition metals with a complexing agent, for instance in the
form of ethylene diamine tetraacetic acid (EDTA) and/or diethylene
triamine pentaacetic acid (DTPA). It has also been suggested that a
reducing chemical such as sodium hydrosulphite or sodium sulphite
for instance, is added to the pulp suspension in addition to a
complexing agent. Success has also been achieved by treating the
starting material, usually wood chips, with a complexing agent
solely or with both of the aforesaid chemicals.
[0007] Use of the reducing bleaching agent borohydride, usually
sodium borohydride, for bleaching of for example high yield pulp
has also been suggested. In the U.S. Pat. No. 3,100,732 it is
learned out about use of alkali metal borohydride and a peroxide
bleaching agent in the same stage. Two ways of carrying out such a
bleaching of the pulp are described. According to one of the ways a
common bleaching agent solution is prepared which is added to the
pulp, i.e. sodium borohydride is dissolved in a solution of
peroxide, which also can contain water glass (sodium silicate),
magnesium sulfate and sodium hydroxide. According to the other way
a sodium borohydride solution is added to the pulp at the end of
the peroxide bleaching stage in order to use the residue peroxide
in an effective way at the finish of the bleaching stage. According
to both ways an increase in brightness was obtained in comparison
with that solely peroxide was used as a bleaching agent. A drawback
with borohydride is their high price.
DISCLOSURE OF THE INVENTION
[0008] Technical Problem
[0009] In the manufacture of bleached mechanical pulp like TMP, for
example from fresh Scandinavian spruce wood, it is today normal to
achieve a brightness of 80% ISO and possibly one or two percent
units above that. To achieve pulp brightnesses that are higher than
those just mentioned, for example up against 85% ISO, is not
possible with the known technique.
[0010] The Solution
[0011] The present invention provides a solution to this problem
and relates to a method for bleaching mechanical and
chemithermomechanical pulp including that an advancing pulp
suspension obtained after that the fibres are laid free is
prebleached by adding reductive bleaching agent to the pulp
suspension in a location just after the fibres are laid free and
that the bleaching is carried out under given conditions in the
form of high temperature and minimized oxygen access in respect of
said adding location and immediately downstream of said adding
location, characterized in, that the in the described manner
prebleached pulp is subjected to at least one further bleaching
treatment including that the pulp is bleached with an oxidizing
bleaching agent, preferably peroxide bleaching agent, reinforced
with borohydride.
[0012] Among the earlier enumerated pulps TMP is a dominating pulp
type from a quantitative point of view. In the manufacture of such
a pulp the process for laying the fibres free is usually carried
out either in one refiner or in two subsequent refiners. Subsequent
to the refiner or subsequent to respectively refiner a steam
separator is situated, ususally a cyclone of some kind, through
which the obtained pulp suspension is brought to pass. The pulp
suspension is thereafter transported usually to a slusher (which
however is not absolutely necessary), for example by means of a
screw conveyor, and further on to a storage vessel (latency chest)
and therefrom to a screening department and from there to a
bleaching treatment. In certain TMP-mills there is a slusher and
the pulp suspension is transported through a conduit to the storage
vessel by means of a pump, located just after the slusher.
[0013] There are a number of alternative locations for adding the
reducing bleaching agent, usually in the form of a water solution,
to the advancing pulp suspension. The bleaching agent can, for
example, be added to the pulp suspension in the earlier mentioned
screw conveyor. One other adding location and a preferred one is
the earlier mentioned pump. One possibly obtains a superior
distribution of the bleaching agent in the pulp suspension as a
result of adding the bleaching agent in that pump. White water is
normally delivered to the slusher and the bleaching agent can be
introduced in that white water, which later on is introduced into
the pulp suspension. It is also possible, to introduce the
bleaching agent directly into the slusher. It is of course possible
to divide the charge of bleaching agent and to add the bleaching
agent to the pulp suspension, for example in two or more of the
stated locations.
[0014] The phrase "the advancing pulp suspension" used in the
foregoing and also in the main claim shall be given a wide meaning.
This phrase shall not solely be seen to mean when the pulp
suspension flows forwards in a conduit or pipe, but also when the
pulp suspension is held in a vessel and container, for instance in
the form of a slusher and storage vessel since even in these latter
cases the pulp suspension still moves forwards in the sense that it
is fed into the vessel at one location and exits from said vessel
in another location.
[0015] Any known reducing bleaching agent can be used. Examples of
such bleaching agents are dithionite (which is sometimes called
hydrosulfite and which is preferred), borohydride, hydrazine and
formamidine sulfuric acid. Dithionite is commercially available
primarily as sodium dithionite, i.e. Na.sub.2S.sub.2O.sub.4. The
bleaching agent concerned is introduced into the pulp suspension
primarily in the form of an aqueous solution, the concentration of
which will suitably lie within the range of 20-120 g/l. The amount
of bleaching agent added will depend, among other things, on the
difficulty in bleaching the pulp in question and how big increase
in brightness of the pulp which is desired in the prebleaching in
comparison with the increase in brightness of the pulp totally.
[0016] Pulp bleaching parametres such as temperature, time, pulp
consistency, pH, etc., are mainly determined by the conditions that
prevail naturally when producing thermomechanical pulp (TWP), as in
the described case. At the aforesaid locations at which the
bleaching agent is added to the pulp suspension, the temperature
will, of course, be very high, e.g. 80-95.degree. C., and the
consistency of the pulp normally low, e.g. 2-4%. The bleaching time
will be short as a result of this very high temperature among other
things, and will probably range from a time span of some seconds up
to some minutes. The bleaching time will probably also depend
partly on the rate at which the pulp suspension flows at the
location where the bleaching agent is added. The pH-value will
naturally lie within the range of 4-7. In some cases, it may be
advisable to adjust the pH-value by adding either an acid or an
alkali to the pulp suspension at the location concerned. When using
dithionite as a bleaching agent, the pH-value should lie from 4.5
and upwards, in order to achieve an optimal bleaching result.
Although a pH-value as high as 8.5 can be used for bleaching
purposes, a pH of this magnitude is less suitable for other
reasons.
[0017] The amount of oxygen in the pulp suspension shall at the
addition of the bleaching agent be as low as possible, preferably
zero. The pulp suspension is protected from air contact and air
dispersion in a considerable degree from the location of laying the
fibres free and forward some length in the chain of pulp treatment.
Besides that the temperature of the pulp suspension is very high in
that area, which also contributes to an effectively bleaching of
the pulp.
[0018] Manufacture of CTMP is very similar to manufacture of
thermomechanical pulp, which has been described briefly above and
the locations for addition of the bleaching agent at prebleaching
of such a pulp coincident to a high degree with the above
described.
[0019] When manufacturing groundwood pulp the pulp suspension is
collected in a grinder pit after that the fibres are laid free,
wherefrom the pulp suspension is transported forward. Suitable
locations for addition of the bleaching agent are the grinding pit
and just subsequent to that. It is especially suitable to carry out
the bleaching in question in a system pressurized with steam, i.e.
at pressure grinding.
[0020] This prebleaching of pulp is described in detail in our
Swedish patent application No. 9900816-1. In this patent
application it is also described how the reject pulp flow is taken
care of and preferably is bleached before the reject pulp flow is
mixed into the main pulp flow.
[0021] Subsequent to screening (usually) and possibly passing a
buffer vessel and/or a storage tower the pulp is subjected to at
least one further bleaching treatment.
[0022] Either directly or after the bleaching stage laying
therebetween, the pulp is bleached with an oxidizing bleaching
agent, preferably peroxide bleaching agent, reinforced with
borohydride.
[0023] In the determination reinforced with borohydride is included
as well that the pulp is treated with borohydride in a pretreatment
stage, i.e. before the true peroxide bleaching stage, as that
borohydride is added to the pulp some time during the true peroxide
bleaching stage, including that the borohydride is added to the
pulp suspension together with the peroxide bleaching agent and that
the borohydride is added to the pulp suspension at the end of the
peroxide bleaching stage.
[0024] The peroxide bleaching stage can in itself be conventional
(except a possible addition of borohydride) and any known peroxide
bleaching agent can be used. Examples of such are hydrogen
peroxide, sodium peroxide, peracetic acid, performic acid and
peroxosulphuric acid (Caro's acid). Hydrogen peroxide is the
preferred peroxide bleaching agent. The bleaching time is at least
30 minutes, preferably at least 60 minutes and can, for example,
reach 2 hours, i.e. 120 minutes. The parametres otherwise are;
charge of peroxide, counted as hydrogen peroxide,=0.5-5%, counted
on bone-dry pulp, temperature=50-90.degree. C., pulp
consistency=5-38%, start-pH in the case of hydrogen peroxide=at
least 10.
[0025] Among the ways of reinforcement with borohydride described
above the use of a pretreatment of the pulp is the absolutely
preferred.
[0026] Any known alkali metal borohydride can be added to the pulp.
The most common is sodium borohydride. This can be added as such to
the pulp. In practice the commodity Borol, which is a water
solution containing 12% sodium borohydride and 40% sodium
hydroxide, is the most used. It is an advantage to use one or more
completion chemical(s) to the borohydride during the pretreatment
stage. In the case the commodity Borol is used as a borohydride
source it is not always necessary to add an alkali, e.g. sodium
hydroxide, to the pulp, in view of that the commodity contains a
considerable amount of sodium hydroxide. However, it is usual to
add variable amounts of alkali to the pulp, for example together
with the commodity Borol. In the case that sodium borohydride as
such is added to the pulp it is necessary with an addition of
alkali also; e.g. sodium hydroxide. Further suitable completion
chemicals are water glass (sodium silicate) and some magnesium
compound, as magnesium sulfate. The pretreatment of the pulp with
at least borohydride takes place during a short period of time, for
example at least 15 seconds and at most 500 seconds. Optimal result
is obtained within the interval 60 to 180 seconds. The time period
in question is counted from when the borohydride is added to the
pulp up to that the peroxide bleaching agent is added to the pulp
in the subsequent peroxide bleaching stage. It is suitable that the
borohydride, counted as sodium borohydride, is added to the pulp in
an amount of 0.05-0.3%, counted on bone-dry pulp. Suitable
temperature is 20-95.degree. C. The pulp consistency is not
directly critical, but it is preferred with from medium consistency
and higher, i.e. from 6% to 38%. While alkali always is present
during the pretreatment of the pulp via the addition of the
commodity Borol or via a direct addition or via both the ways the
pretreatment will be carried out under alkaline conditions, for
example within the pH-interval 9-13.
[0027] At the subsequent peroxide bleaching of the pulp it is in
some cases suitable with addition of completion chemical(s) and in
some case also necessary. If it in the preferred case with hydrogen
peroxide as bleaching agent is not added any extra alkali or too
little alkali in the pretreating stage it is necessary with an
addition of alkali, e.g. sodium hydroxide, in the peroxide
bleaching stage. It can be good with an addition of water glass and
some magnesium compound also, especially if these chemicals are not
added at all or are added in a too low amount to the pulp in the
pretreatment stage.
[0028] The treatment of the pulp described above, i.e. this
especially technique of bleaching is in detail described in a
parallel patent application, with the same filing date as the
present patent application. That Swedish patent application has the
number 0000425-9.
[0029] It is possible (as earlier stated) to mix the borohydride
and the peroxide bleaching agent and any completion chemicals in
one and the same water solution forming one bleaching liquor which
is added to the pulp suspension leading to a bleaching treatment of
the pulp suspension in correspondence with earlier described
peroxide bleaching stage.
[0030] Furthermore it is possible to add to the pulp suspension
borohydride and any completion chemical at the end of, for example
a conventional peroxide bleaching stage, for example, when the
peroxide bleaching has been run in 100-120 minutes.
[0031] The charge of borohydride, counted as sodium borohydride,
can also in these two cases lie within the interval 0.05-0.3%,
counted on bone-dry pulp.
[0032] As earlier stated the pulp suspension just before the
bleaching with peroxide bleaching agent reinforced with borohydride
can be bleached in some other way in one or more stage(s).
According to a preferred embodiment of the invention the in the
earlier described way prebleached pulp is bleached with peroxide
bleaching agent at medium consistency, while the subsequent
peroxide bleaching stage reinforced with borohydride is carried out
at high pulp consistency and furthermore the spent bleaching liquor
from the high consistency bleaching stage, which contains residue
peroxide like residue hydrogen peroxide, shall be delivered
backwards in the pulp refining process and be mixed into the pulp
just before or in the beginning of the medium consistency bleaching
stage with e.g. hydrogen peroxide.
[0033] Advantages
[0034] By bleaching mechanical pulp and CTMP in the described way,
i.e. in accordance with the invention, it is possible to achieve a
brightness of the pulp of up against 85% ISO. The use of usually
three different bleaching agents leads, however, to an increase in
the price of the bleaching process seen in total, in comparison
with a conventional use of one bleaching agent in usually one
stage, but one has then to remember, that the market is always
asking for an increased brightness of the pulps of the described
kinds and consequently also for increased brightness of paper
manufactured from such pulps. This demand has as a natural result,
that the market is prepared to pay substantially for that which is
demanded.
[0035] It is also so, that even if three different bleaching agents
are used in the method according to the invention leading to a
substantial cost for the bleaching agents the bleaching process
seen in total is most cost effectively, among other things of the
reason that the inventive different stages are associated with low
fixed costs.
[0036] Best Embodiment
[0037] Since the invention already has been described in detail and
even in a very detailed way with reference to two other patent
documents that limitation is done here, that only one working
example will be presented.
EXAMPLE 1
[0038] Thermomechanical pulp was manufactured in a way, which
schematically is presented in FIG. 1 in the Swedish patent
application 9900816-1, to which is referred once more. The starting
material for the pulp manufacturing was considerable fresh, barked
spruce logs of Scandinavian origin. To the advancing pulp
suspension in location 14 and more precisely in the pump (not shown
in the Figure) just after the slusher 14 a water solution
containing sodium dithionite in a concentration of 60 g/l is added
in such a flow, that the charge of the bleaching agent was 6 kg per
tonne of bone-dry pulp. The temperature of the pulp suspension in
that location was 88.degree. C., their pH-value 4.6 and their pulp
consistency 3%. The pulp prebleached in that way was screened in
the screening department 19 so that an accept pulp flow and a
reject pulp flow were obtained. The reject pulp flow was treated in
accordance with that presented in FIG. 1 in the patent application
mentioned and a water solution containing sodium dithionite in a
concentration of 60 g/l was added to the reject pulp suspension
flow in the pump (not shown in the Figure) situated just after the
slusher 31 in such a flow, that the charge of the bleaching agent
was 6 kg per tonne dry pulp also in that location. The temperature
of the pulp suspension in that location was 85.degree. C., their
pH-value 5.1 and their pulp consistency 3%.
[0039] On the disc filter 21 pulp was picked out for further
transportation to the laboratory. In that location the pulp, i.e.
the prebleached pulp, had a brightness of 71% ISO and a metal
content, determined according to standard analysis methods, of 12.1
ppm (or mg per kg bone-dry pulp) manganese (Mn), 11.2 ppm iron (Fe)
and 3.8 ppm copper (Cu). The pulp arrived to the laboratory was
pressed to a pulp consistency of 35% and was thereafter stored in a
cold-storage room.
[0040] Four trials were done with this pulp in the laboratory, one
reference trial and three trials where the method according to the
invention was simulated.
[0041] At the runs of trial pulp was taken from the cold-storage
room and was divided into charges of 20 gram bone-dry pulp to every
trial.
[0042] At the reference trial the pulp was subjected to a
conventional hydrogen peroxid bleaching in the way that the 20
grams of pulp was divided finely by hand so that the pulp become
fluffed. Thereafter the pulp was put down into a plastic bag. To
this plastic bag a certain amount of a water solution containing
hydrogen peroxide in a charge of 4%, counted on bone-dry pulp,
sodium hydroxide in a charge of 3.5%, counted on bone-dry pulp, and
water glass (sodium silicate) in a charge of 3.12%, counted on
bone-dry pulp was delivered. After the addition of the mentioned
water solution the content of the plastic bag, i.e. the pulp
sample, was kneaded by hand so that the chemicals were distributed
equal in the pulp amount stated. The pulp consistency was 30% after
mixing the chemicals into the pulp. Thereafter the plastic bag with
its content was placed into a water bath, with a temperature of
75.degree. C., during a time period of 90 minutes.
[0043] The bleaching was interrupted by picking the plastic bag up
from the water bath and its content was moved to a glass vessel, in
which the pulp sample was diluted with deionized water down to a
pulp consistency of 0.6%. A sulphuric acid solution was also added
so that the pH-value of the pulp suspension was decreased to 5.
During the dilution of the pulp sample and before the addition of
sulphuric acid a specimen of the spent bleaching liquor was taken
out in which the end-pH and the content of hydrogen peroxide
residue were determined according to conventional analysis technic.
It was manufactured a sheet by hand of the pulp sample according to
SCAN-CM 11:75 on which 30 the brightness was determined according
to the analysis method SCAN-P 3:93. All the brightness values
mentioned in this working example are measured in the just
described way.
[0044] Also the three bleaching trials according to the invention
were carried out on 20 grams bone-dry pulp.
[0045] In one of these trials, named Pr, at first a water solution
containing sodium borohydride and sodium hydroxide in the form of
the commodity Borol in an amount regarding sodium borohydride of
0.1%, counted on bone-dry pulp, plus an extra amount of sodium
hydroxide of 1.55%, counted on bone-dry pulp, and water glass in an
amount of 3.12%, counted on bone-dry pulp, was added to the pulp
sample in the plastic bag. After thoroughly dividing of the
chemicals by means of hand kneading the sealed plastic bag was
placed in the earlier mentioned water bath, which held a
temperature of 75.degree. C. After 2 minutes the plastic bag was
picked up from the water bath and opened and a water solution
containing hydrogen peroxide in an amount of 4%, counted on bone
dry pulp, and sodium hydroxide in an amount of 1.55%, counted on
bone-dry pulp, was delivered. After this addition of the water
solution the pulp had a pulp consistency of 30%. The plastic bag
was sealed and again placed in the water bath mentioned and was
kept there for a time period of 90 minutes. Also in this trial the
handling of the pulp sample was continued in the way described
above. The characterizing thing for this trial was obviously that
the sodium borohydride was added to the pulp in a pretreatment
stage before that the true hydrogen peroxide bleaching of the pulp
was carried out.
[0046] In a second trial according to the invention, named PS the
pulp sample was treated in accordance with the reference trial with
the important difference, that the pulp beside the in that trial
mentioned chemicals also was fed with the commodity Borol, so that
the charge of sodium borohydride was 0.1%, counted on bone-dry
pulp. Furthermore the direct charge of sodium hydroxide was
decreased to 3.1%, counted on bone-dry pulp. The characterizing
thing for this trial was obviously, that the sodium borohydride and
the hydrogen peroxide were added to the pulp at the same time.
[0047] In a third trial according to the invention, named P.sub.s,
the pulp sample was treated in accordance with the reference trial
with the important difference, that after 80 minutes of the
hydrogen peroxide bleaching of the pulp the plastic bag was picked
up from the water bath and opened. To the bag the commodity Borol
was delivered so, that sodium borohydride was added to the pulp in
an amount of 0.1%, counted on bone-dry pulp and furthermore
deionized water was added so that the pulp consistency was
decreased to 5%.
[0048] Sulphuric acid was also added so that the pH-value in the
pulp suspension was decreased to the interval 6.5-7.0. The plastic
bag was sealed and the chemicals were equilized within the pulp
sample by means of hand kneading. Thereafter the plastic bag was
again placed in the water bath during a time period of 10 minutes.
Also in this trial the direct charge of sodium hydroxide was
decreased to 3.1%, counted on bone-dry pulp. The characterizing
thing for this trial was obviously, that the sodium borohydride was
added to the pulp just at the end of the hydrogen peroxide
bleaching.
[0049] Trial parameters and results achieved are clear from the
Table 1 below.
1 TABLE 1 Trial Parametres and results Reference P.sub.r P.sub.s
P.sub.e NaBH.sub.4, % 0 0.1 0.1 0.1 Na.sub.2SiO.sub.3, % 3.12 3.12
3.12 3.12 NaOH, % 3.5 3.1 3.1 3.1 H.sub.2O.sub.2, % 4.0 4.0 4.0 4.0
Pulp consistency, % 30 30 30 30;5 Temperature, .degree. C. 75 75 75
75 Time, minutes 90 90 90 80 + 10 End-pH 9.16 9.17 9.37 --
H.sub.2O.sub.2 residue, % 0.56 1.00 0.60 1.12 Brightness, % ISO
81.6 83.6 82.4 82.2
[0050] As illustrated all the pulp samples which have been bleached
according to the invention are given brightnesses, which are higher
than the brightness of the reference pulp. In the trial where an
explicit pretreatment stage of the pulp with borohydride has been
used the increase in brightness is especially pronounced and more
exactly 2% ISO.
[0051] Furthermore it has afterwards been verified that at the
occasion when prebleached pulp was picked out in the TMP-mill and
was delivered to the laboratory it happened to be so that the
brightness of the initial pulp, i.e just after the second refiner
stage, was unusually low, which leads to a lower end brightness of
the pulp than what under other circumstances should be the
case.
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