Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Method for bleaching mechanical and chemithermomechanical pulp
Technical field
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.
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
When bleaching mechanical pulp and CTMP lignin-preservative bleaching is
used. Among bleaching agents that can be used for lignin-preservative
bleaching there are
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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.
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.
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.
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
2 5 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
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(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.
Use of the reducing bleaching agent borohydride, usually sodium
borohydride, for bleaching of for example high yield pulp has also been
suggested. In the
US patent specification 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
Technical problem
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.
The solution
2 5 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
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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.
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 Blusher
(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 Blusher and the pulp
suspension is
transported through a conduit to the storage vessel by means of a pump,
located just after
the slusher.
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
2 0 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 Blusher 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
2 5 bleaching agent and to add the bleaching agent to the pulp suspension, for
example in two
or more of the stated locations.
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
30 suspension is held in a vessel and container, for instance in the form of a
Blusher and
storage vessel since even in these latter cases the pulp suspension still
moves forwards in
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the sense that it is fed into the vessel at one location and exits from said
vessel in another
location.
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. NazS204. 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/1. 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.
Pulp bleaching parametres such as temperature, time, pulp consistency, pH,
etc., are mainly determined by the conditions that prevail naturally when
producing
thermomechanical pulp (TMP), 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°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
2 5 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.
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
3 0 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.
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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.
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.
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.
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.
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.
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.
2 5 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 (faro'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. S-S%, counted on bone-dry pulp,
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temperature = 50-90°C, pulp consistency = S-38%, start-pH in the case
of hydrogen
peroxide = at least 10.
Among the ways of reinforcement with borohydride described above the use
of a pretreatment of the pulp is the absolutely preferred.
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
chemicals) 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
1 S 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°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
2 5 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.
At the subsequent peroxide bleaching of the pulp it is in some cases suitable
with addition of completion chemicals) 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
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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.
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.
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.
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.
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.
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.
2 5 hydrogen peroxide.
Advantages
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
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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.
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.
Best embodiment
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
Thermomechanical pulp was manufactured in a way, which schematically is
presented in Figure 1 in the Swedish patent application 9900816-l, 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 Blusher 14
a water
solution containing sodium dithionite in a concentration of 60 g/1 is added in
such a flow,
2 0 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°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 Figure 1 in the
patent
2 5 application mentioned and a water solution containing sodium dithionite in
a concentration
of 60 g/1 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°C, their pH-value 5.1 and their pulp
consistency 3%.
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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).
5 The pulp arrived to the laboratory was pressed to a pulp consistency of 35 %
and was
thereafter stored in a cold-storage room.
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.
At the runs of trial pulp was taken from the cold-storage room and was
10 divided into charges of 20 gram bone-dry pulp to every trial.
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°C, during a time period of 90 minutes.
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
2 5 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
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.
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Also the three bleaching trials according to the invention were carried out on
20 grams bone-dry pulp.
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°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.
In a second trial according to the invention, named P5, the pulp sample was
2 0 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
2 5 hydrogen peroxide were added to the pulp at the same time.
In a third trial according to the invention, named Pe, 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
30 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 S%.
Sulphuric acid was also added so that the pH-value in the pulp suspension was
decreased
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to the interval 6.5-7Ø 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.
Trial parameters and results achieved are clear from the Table 1 below.
Table 1
Parametres and results Trial
ReferencePr PS Pe
NaBH4 ,% 0 0.1 0.1 0.1
Na2Si03, % 3.12 3.12 3.12 3.12
NaOH, % 3.5 3.1 3.1 3.1
H202, % 4.0 4.0 4.0 4.0
Pulp consistency, % 30 30 30 30;5
Temperature, C 75 75 75 75
Time, minutes 90 90 90 80+10
End-pH 9.16 9.17 9.37
H202 residue, % 0.56 1.00 0.60 1.12
% ISO 81.6 83.6 82.4 82.2
Brightness
,
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.
2 5 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.
