Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
11740~3
The present invention relates to cellulose pulp manuacture and in
particular to thermomechanical or chemimechanical pulp substantially free from
resin.
Bleached mechanical pulps of the kind groundwood pulp, refiner p~lp
and thermomechanical pulp ~so-called TMP) and chemically modified thermomechani-
cal pulp (so-called CTMP), i.e. pulps with yields abo~e 90 %, have been intro-
duced for hygiene products of more qualified nature, such as fluff and tissue.
For obtaining good absorption properties, a low resin content is required. For
the highest qualities, thus, the DKM extract content of the pulp must be below
Q.2 %.
This is achieved relatively easily in an open system where the pulp
in one way or the other is washed with substantial amounts of water between every
process step, and where the pulp is bleached with peroxide. When there is suf-
ficient water supply, it is even possible to work with a common backwater system,
which is quite usual for practical reasons.
The resin does not dissolve in the washing liquid, but is dispersed
in colloidal state or as coarser particles in the liquid which is preferably
alkaline. There have been serious apprehensions with respect to the possibility
of re-using the backwater to a high degree only at low resin concentrations.
T~erefore, it has been industrial practice for products of low resin contact
not to have a water system which uses less than a total of at least 15 to 20 m3
~ater ptm90. At lower water consumption levels, apparently, the low resin cont-
ent could not be maintained.
A lower water consumption, however, is of interest, especially when
waste water purification methods are concerned, but also when heat from the pro-
cess is to be recovered. Interesting results are obtained when the water con-
sumption can be reduced to below 10 m3 ptm90.
- 2 -
1~740~ 3
Counterflow washing is a well-known and widely applied method for re-
covering conce~trated waste liquors from different digestion processes. Material
dealt with in this way is almost enti~rely~dissolved substance, which per se does
not give rise to any washing-out difficulties. For sulphate pulps from wood
rich in resin, however, colloidal resin can be salted-out under certain circum~
stances and this renders washing difficult. This problem, however, generally is
solved by washing-out remaining resin in a subsequent suitably adapted multi-step
bleaching operation, provided with proper washing plants with relatively large
water consumption. A direct coupling between the technique in the manufacture
of mechanical pulps and the like and counterflow washing methods for the recovery
of waste liquors from different digestion processes has not been considered
realistic, because it has been difficult to predict how the relatively coarsely
dispersed resin will behave in mechanical pulp manufacture when being repeatedly
filtered through a pulp web.
It has now been found, however, that such a coupling is possible, pro-
vided that the conditions are adapted to the special demands of the resin in the
case of mechanical pulp.
The mechanical pulps, TMP or CTMP~ generally are defibered in slightly
acid or neutral environment, which does not bring about a stable dispersion of
2Q resin even when the mechanical processing in connection with the fibre exposure
can be ideal therefor. When, for example, peroxide bleaching in an alkaline
environment fpllows, the alkaline environment facilitates the resin dispersion.
Washing carried out between these two process steps can be of great
interest, because the lower resin content level in the peroxide solution, which
per se is favourable, reduces the risk of coagulation of resin particles and
there~y gives greater stability to the resin removal. When, however, bleaching
waste liquor is simultaneously recirculated in the bleaching step, which is of
i~74013
interest both for reducing the water consumption and for more efficiently utiliz-
ing the bleaching chemicals, salting-out concentration seems to be obtained sur-
prisingly rapidly. It is then important to relatively quickly wash the resin
thus salted-out with washing water having an ion strength which decreases succes-
sively to pure water. The washing should be carried out substantially by dis-
placement and can be driven to a high displacement degree for solved substance.
For the lowest water consumption, a washing should be carried out prior to the
peroxide step and in series with washing after the peroxide step. The washing
liquid should be moved in a consequent counterflow.
~t has been found quite surprisingly, that by such a method pulps with
acceptably low resin contents can be manufactured with very low water consumption.
According to the present invention, there is provided method of manu-
facturing a cellulose pulp substantially free from resin and in a yield above
9Q %, by mechanical defibration of wood material and subsequent bleaching, said
method being conducted in a substantially closed system corresponding to an
amount of waste water of at most 15 m3 per ton pulp, characterized in that back-
water from each of the defibration stage and the bleaching stage is maintained
separated, that excess backwater from the process is withdrawn at a washing stage
arranged after the defibrating stage, that excess backwater from the bleaching
2Q stage is used for diluting or washing of the pulp after the defibrating stage
and that the washing effect of dissolved substance amoun~s to at least 70 %.
Particular embodiments of the present invention will now be described
in detail, by way of example only, with reference to the accompanying drawings,
in which:
Plgure 1 graphically illustrates the resin content of pulp treated by
varying amounts of fresh water after chip refining in a chip refiner but before
dewatering of the resulting pulp in a press;
1~74(~ 3
Pigure 2 diagrammaticall~ illustrates steps according to the process
of the present invention; and
Pigure 3 diagrammatically illustrates a modification of the process
of Flgure 2.
Unbleached CTMP pulp was manufactured in a pilot plant by impregnating
spruce chips containing about 1.2 % resin with sodium-sulphite solution, whereby
the chips absorbed about 20 kg Na2S03/ton bone-dry chips. The chips were pre-
heated with steam to 110C and then refined in a chip refiner. The pulp con-
centration after the chip refiner was measured and found to be about 30 % by
weight. The pulp was diluted to about 4 % pulp concentration and then dewatered
~n a press to ab~ut 30 % pulp concentration. The resin content of the pulp
after the press was analysed.
Several experiments were carried out, in which the pulp after the re-
finer but before the press was diluted with water or a mixture of water and back-
water. By using an eYer decreasing amount of fresh water and an ever increasing
amount of backwater for diluting the pulp, the closing degree of the system could
be increased.
Analysis of the resin content of the pulp showed the relation illus-
trated in Figure 1 between the resln content in the pulp and the amount of fresh
2a water supplied to the system expressed in m3 fresh water/ton pulp.
By establishing material balances for resin at every experiment, it
was possible to calculate how much of the resin is found dispersed in the back-
water after refining and can be removed by dewatering and washing and, respec-
tively, how much resin cannot be removed in this way. The following results
were obtained.
1~7~01;:3
Fresh water amount added Resin amount removable by
m3/ton pulp washing or dewatering, %
0.25
0-45
0 57
0.61
The results shown in Figure 1 illustrate one of the observations, on
which the present invention is based. In the manufacture of unbleadhed CTMP
pulps, increased closing of the water system implies that an ever smaller amountof the wood resin is being solved or dispersed, and the resin content of the
pulps increases. Similar investigations in the manufacture of thermomechanical
pulps have yielded similar results. The results can be interpreted in that with
increased closing an increased salting-out o~ resin solved or dispersed at re-
fining is obtained. In the manufacture of unbleached mechanic pulps for applica-tion fields requiring a low resin content of the pulps, it has previously not
been possible to close the systems rigidly.
Pulps from the aforementioned pilot plant were then subjected to al-
kaline peroxide bleaching, which a.o. has the following objects:
- it increases the brightness of the pulp
- it lowers the resin content of the pulp.
The experiments were carried out so that unbleached TMP and CTMP pulps
after the press were diluted with fresh water or backwater from the bleaching
step. Bleaching chemicals were admixed to the pulp suspension at about 12 %
pulp concentration, whereafter the pulp suspension was passed to a bleaching
tower. After bleaching, the pulps were diluted with fresh water or backwater to
4 % pulp concentration and thereafter pressed to about 35 % pulp concentration.
The resin content of the pulp samples was determined after the press.
-- 6 --
117~013
The Table below shows some examples of pulps used ln the bleaching
experiments~ The Table also includes the chemical charges made in the bleaching
step.
Pulp Resin Presh water Resin Bleaching chemical
contentadded before content charges, kg/t pulp
before bleaching in pulp
press plant after H22 ~aOH Na EDTA
% m3/t pulp press sili-
% cate
-
CTMP 10.84 7.1 0.49 37 25 40 3.0
CTMP 20.81 23.9 0.27 25 18 50 3.8
CTMP 30.88 26.6 0.32 42 30 42 3.1
TMP 1.04 26.3 0.48 46 31 43 3.2
By establishing material and resin balances in the bleaching step it
has been possible also here to calculate how much of the ingoing resin was dis-
persed and, thus, could be washed-out and, respectively, how much resin could not
be washed-out. The pulps studied in the above Table yielded the results as
follows.
PulpPresh waterResin Resin Undispersed Temperature
added in content contentresin content in
bleaching in pulp in pulpin bleached bleaching
step and before a~ter pulp Osctep
before bleaching bleaching %
press % and pres-
m3/~t pulp ing
CTMP 15.5 0.49 0.16 0.05 70
CTMP 221.1 0.27 0.15 0.13 70
CTMP 318.4 0.32 0.15 0.13 60
TMP 23 3 0.48 0.19 0.13 85
1~7~V~3
At the bleaching, the main part of the remaining resin is solved out.
The alkaline peroxide bleaching thereby transfers the resin in such a form that
it does not precipitate out at increased closing, at least not whcn the closing
is held within reasonable limits. The water consumption at the closing experi-
ment with C'~MP pulp 1 corresponds to about 6 to 10 m3/t pulp.
The experimental investigations of the manufacture of bleached mechani-
cal pulp showed the following results.
1. In the manufacture of unbleached mechanical, thermomechanical and
chemically modified thermomechanical pulps, part of the resin in the wood is
dissolved or dispersed. The dispersed resin can be removed by washing.
2. B~ closing of the backwater system for unbleached pulp an ever smaller
part will be dispersed. The resin content of the unbleached pulp increases with
increased backwater closing.
3. Upon the bleaching of these pulps the resin is transferred in such a
~orm that the main part of the resin is dispersed. The resin has been modified
By the bleaching so that it no longer precipitates~i~tincreasing closing.
4. The resin amount not dispersible in peroxide-bleached mechanical pulp
is only 0.05 to 0.15 % of the pulp amount.
These discoveries render it possible to manufacture bleached mechanical
2a pulp ~ith a very low resin content at low water consumption, when the pulp is
washed before and after the bleaching and when the backwater is moved in a
counter~low to the pulp.
An embodiment of the i~vention is shown in Figure 2, but the invention,
of course, is not restricted to this embodiment.
The system comprises a refiner step 1, a dewatering or washing step 2,
a bleaching step 3, a washing step 4, a dewatering and/or washing step 5 and a
drying step 6 where the bleached pulp is dried.
1~7~()13
Chips are supplied to the refiner step 1 and may be untreated, steamed
and/or impregnated with chemlcals. The pulp after the refiner 1 is passed to the
~ashing step 2, which ma~ be a washlng filter or preferably a dewatering or
washing press. The pulp suspension is diluted with back~ater from the bleaching
step prior to the washing step and, if necessary, with backwater from washing
step 2.
The unbleached pulp after washing step 2 is supplied with bleaching
chemicals and diluting water, which comes from washing step 4 and is passed to
the bleaching tower 3.
After bleaching, the pulp is diluted with backwater from washing step
4 and washed in washing step 4. As displacement liquid is used backwater from
washing and dewatering step 5. The pulp is dewatered before the drier 6 in the
dewatering step 5, which-preferably can be a dewatering or washing press. The
backwater leaves the system via washing step 2. rhe washing liquid is supplied
in connection to washing step 5.
The resulting effect can be illustrated as follows. CTMP chips are
refined in step 1. The pulp concentration after the refiner is 30 %. The pulp
ls dewatered to 48 % pulp concentration in dewatering press 2. The pulp there-
after is bleached at 12 % pulp concentration, washed on the washing filter 4 and
flnally dewatered on the dewatering press 5 to 48 % pulp concentTation before
drier 6.
By means of an automatic model and the experimental results obtained,
resin balances~for the system were calculated at different closing degrees.
The following relation was obtained between the amount of backwater
leaving the system, the washing efficiency degree of resin dissolved out or dis-
persed in the bleaching step and the resin content in bleached pulp. The con-
tent of indispersible resin in the pulp after the bleaching step is assumed to
1~7~3
be 0.13 %. The Tesin content in the pulp after the refining is assumed to be
0.9 %.
Outgoing backwaterWashing efficiency Resin content
amount degree of resin in bleached pulp
m3/t pulpi dissolved out in
bleaching step % by weight
_
1 0.77 0.31
3 0.$7 0.20
o.g2 0.16
0.95 0.15
The calculation shows that it is possible by rigid closing to manufac-
ture bleached mechanical pulp with low resin content, provided that the pulp is
washed in counterflow.
Peroxide bleaching of mechanical pulp or the like ~pulps with yields
90 %) yield under normal conditions a very great amount of residue chemlcals,
~hich cannot be utilized in a simple way By tightening up the bleaching condi-
tions, but which to a more or less high degree can be utilized when the residue
solution of the chemicals after the bleaching is returned to unbleached pulp.
To carry out such return operation requires an apparatus equipment better than
used normally and a better control of the water systems. Experiments with such
return have been carried out with relatively good success, but the efficiency
degree has proved low, probably because diluting phenomena and other disturbing
effects have influenced the procedure in a relatlvely irrational way.
It has now been found entirely by surprise, that by accurately control-
ling conditions, an efficiency of almost 100 % of the bleaching with these residue
chemicals can be obtained. The prerequisite condition is that the chemicals are
returned
_ 10 .
il~4013
with only insignificant dilution, and that backwater from the refiner step, be-
fore bleach~ng, is permltted to pro~ide dilution only to a small extent. This
in its turn requires an accurately balanced backwater system, at which the back-
water of the refiner step and of the bleaching step are well separated.
The best way of separating the two process steps, wlthout risk of back-
waters being mixed together, and at the same time to provide the conditions pre-
requisite for maximum return of residue chemicals from the bleaching plant, has
proved to be the arrangement of a press immediately ~efore the bleaching plant.
Only a small amount of llquid from the preceding step then is supplied to the
bleaching plant water system, and at a limited washing water amount after the
bleaching plant only a small amount of bleaching plant backwater must be ejected.
The smaller the ejection is, the greater is the proportion of residue chemicals
to be utilized. By reduced dllution into the system also the risk of distur-
bances at the bleaching reactions of the residue chemicals is reduced.
In view of the aforesaid, the bleaching process can be designed as
ollows.
Immediately before the bleaching plant, the pulp is pressed to at least
20% dry content, possibly in combination with washing. Higher dry contents are
to be preferred, and normally at least 30 %, at maximum about 50 % should be
attained. The pulp is thereafter diluted with insignificantly diluted bleaching
plant backwater, which contains residue chemicals from the bleaching, to a pulp
concentration normal for the bleaching and possibly varying, according to circum
stances, between about 5 % and 15 %. Thereafter fresh peroxide solution and
necessary auxiliar~ chemicals are admixed in known manner to the pulp, and the
pulp is bleached during a suitable time adapted to the temperature prevailing in
the system. ~ormal times are 0.1 to 4 h, normal temperatures are 45 to 85C.
At the beginning of the bleaching, pH is high and decreases gradually to the
~1740~3
level 8 to 9. The bleaching alway~s is finished before the chemicals have been
consumed completely~, ~ecause no al~aline miscolouring should occur. In order to
obtain high brightness values, the amount of residue chemicals normally must be
rather high. The pulp thereafter is passed to a washing step with high washing
effect, for example a washing filter, where the residue solution is recovered.
In order not to dilute the residue solution, the washing must be carried out with
a small washing liquid amount, i.e. with a low dilution factor. This solution
then is returned for dilution immediately before the bleaching plant.
The method is illustrated by the following laboratory experiment and
experiments in a semi-technical pilot plant:
The effect of directly returned residue solution from peroxide bleach-
ing is illustrated in a laboratory experiment, at t~l~ch a thermomechanical pulp
at first was bleached with fresh chemicals, whereafter the residue solution was
titrated on peroxide and made stronger with fresh peroxide, sodium hydroxide,
sodium silicate and magnesium sulphate to the same total charge as in foregoing
bleaching experiments, whereafter the pulp was bleached analogous with the new
bleaching solution. This was repeated once more, and the residue solution from
the latter experiment was returned in order to control that the bleaching was not
deteriorated when the proportion of organic substance in the recirculated residue
2a solution was increased. As appears from the Tables below, the different variants
yield substantially the same result. This was obtained when the bleachings were
carried out at 12 % pulp concentration and at a high peroxide content in the resi-
due solution.
At later analogous experiments where the residue solution was returned
more diluted, and the bleaching i~s carried out at somewhat lower pulp concentra-
tion, a slightly inferior brightness was obtained for the charged peroxide
~about 2 units lower than at the highest charge).
:1174013
Conventional peroxide bleaching with resh bleachlng chemicals.
Experiment 1
Peroxide bleaching
Pulp concentration % 12
Temperature aC 40
Time min 120
Hydrogen peroxide kg/ton 40
Na~silicate kgfton 30
Mg~04 kg~ton 0-5
NaOH kg/ton 28 32 36
Initial pH 11,5 11.7 11.9
Final pH 10.3 10.5 10.6
Residue peroxide g/l 2.92 3.02 2.99
Residue peroxide kg/ton 21.4 22.1 21.9
Consumed peroxide kg~ton 18.6 17.9 18.1
Analyses:
Brightness %ISO 77.4 77.4 77.8
.
Peroxide bleaching with addition of 4.66 m3 per ton bleaching waste
liquor from experiment 1, and make up in the form of fresh chemicals.
- 13 -
117~ 3
Experiment 2
Peroxide bleaching
Pulp concentration % 12
Temperature QC 40
Time min 120
Hydrogen peroxide kg/ton )26.4+ ~13.6~=40
Na-silicat0 kg/ton 10.7+(19.3)=30
MgS04 kg~ton 0.06+(0.44)=0.5
NaOH kg/ton23+~5~ 27+(5)31+(5)
Initial pH 11.5
Flnal pH 9,9 10.1 10.3
Residue peroxide g/l 2.7g 2.74 2.69
Residue peroxide kg/ton 20.5 20.1 19.7
Consumed peroxide kg/ton 19.5 19.9 20.3
Analy~es:
Brightness %ISO .77.3 77.5 76.2
X)Make up chemicals xx~ Chemicals in residue solution
Peroxide bleaching with addition of 4.62 m3 per ton bleaching waste
liquor from experiment 2, and make up chemicals in the form of fresh chemicals.
- 14 -
1174013
Experiment 3
Peroxide bleaching
Pulp concentration % 12
Temperature C 40
Time min 120
Hydrogen peroxide kg/tonx)27 9+Xx)~12.1)=40
Na-silicate kg/ton 11.0~(19.0)=30
MgSO4 kg/ton 0.06~(0.44)=0.5
NaOH kg/ton27.7+(0.3)31.7+(0.3)35.7~(0.3)
lQ Initial pH 11.5 11.6 11.7
Final pH 9.9 10.0 10.3
Residue peroxide g~l 2.69 2.57 2.48
Residue perox~de kg/ton 19.7 18.8 18.2
Consumed peroxide kg/ton 20.3 21.2 21.8
Analyses:
Brightness %ISO 78.1 78.4 78.7
x) Make up chemicals xx) Chemicals in residue solution
The effect of washing before the peroxide bleaching for preventing the
backwater from the refiner step from disturbing the bleaching was illustrated
2Q in a laboratory experiment.
A mechanical pulp was manufactured in a pilot plant from spruce chips,
in that the spruce chips first were impregnated with sodium sulphite solution so
that the chips absorbed about 20 kg Na2S03/ton bone-dry wood. The impregnated
chips were preheated with steam and refined in a chip refiner to pulp. The
pulp after the refiner had a pulp concentration corresponding to about 25 % and
contained about 3 ton aqueous solution per ton bone-dry pulp. The aqueous solu-
1~74013
tion contained organic substance sol~ed out from the wood and anorganic substance,sodium sulphite and sodium sulphateJ from the impregnation solution. The bright-
ness of the pulp was measured and found to be 60.1 % ISO.
From the pulp obtained two samples were taken. One pulp sample was
washed well with water, so that substantially all organic and inorganic substance
was removed from the pulp. The washed and the unwashed pulp samples were bleached
in the laboratory with alkaline peroxide in known manner.
Experiments were also carried out in a semi-technical equipment in-
cluding the possibility of pressing-out the refiner step water in a press between
refining and bleaching for a chemical modified thermomechanical pulp (yield 95 to
96 %). The results of the laboratory studies interpolated to brightness 70 and,
respecti~ely, 75 % lSO are reported in the Table below.
~ashing before bleaching Unwashed Washed
Brightness %ISO 70 75 70 75
Peroxide bleaching
Pulp concentration % 12 12 12 12
Temperature C 40 40 40 40
Time min 120 120 120 120
Hydrogen peroxide kg/ton 24.0 43.5 24.0 43.5
Na-silicate kg/ton 30 30 30 30
EDTA kg/ton 3 3 3X) 3X)
MgS04 kg/ton 0.5 0.5 0.5 0.5
NaOH kg/ton 19.2 30.5 19.2 30.5
Residual peroxide kg/ton 3.0 9.0 8.0 17
Consumed peroxide kg/ton 21.0 34.5 16.0 26.5
-
x~ Added in connection with washing before bleaching
1~74013
The results from the experlments on a semi-technical scale were as
follows:
Washing before ~leaching Unwashed Washed-pressed pulp
Peroxide hleaching
Pulp concentration % 12 12
Tempera~ure C 50 50
Time min 100 100
Hydrogen peroxide kg/ton 39.3 45.5
Na-silicate kg/ton 41 42
MgS04 kg/ton 0.5 0.5
EDTA kg/ton 6.2 3.1
NaOH kg/ton 29 30
Residual peroxide kg/ton 7.3 22.6
Consumed peroxide kg/ton 32.0 22.9
Analysis:
Brightness 72.5 78.3
-
Analogous laborator~ exper~ments for pure thermomechanical pulp
resulted in the following:
Washing before bleaching Unwashed Washed
-
2aBrightness 7Q 75 70 75
Peroxide ~leaching
Hydrogen peroxide ~g~ton 25 40 25 40
NaOH kg/ton 20 28 20 28
Residue peroxide kg/ton 3.0 6.0 7.0 12.0
Consumed peroxide kg/ton 22.0 34.0 18.0 28.0
- 17 -
1~74013
The other bleaching conditions agreed with the ones shown previously.
These experiments show, that the solved wood substance from the re-
fining step consumes great peroxide amounts when pulp from the refining step is
supplied to the bleaching step without solved substance having been removed, and
they further show, that peroxide bleaching of a mechanical pulp requires great
peroxide excess for obtaining optimum bleaching.
The experimental work, thus, has yielded the results as follows:
The backwater from refining mechanical thermomechanical and
chemical modified thermomechanical pulps contains substances, which consume
peroxide and alkalî at alkaline peroxide bleaching.
- Bleaching of such mechanical pulps requires a high peroxide excess
at bleaching to high brightness values.
- The peroxide excess in the backwater can be re-utilized for bleaching
when the backwater is returned to ths bleaching step.
On the basis of these results different system couplings were studied
by means of mathematic models. It was now found possible to combine in a suit-
able manner equipment and operation conditions so, that a transfer of too much
backwater from the refining step to the bleaching step, where the backwater con-
sumes bleaching chemicals, is prevented, and that simultaneously peroxide contain-
2Q ing backwater after the bleaching step is returned with high utilization degreeto the bleaching in order thereby efficiently to utilize this peroxide.
An example of an embodiment of the invention is shown in Figure 3.
Mechanical pulp from the refining step 1 is passed to the press 2,
where backwater from the refining is washed out.
Before the bleaching step 3 fresh bleaching chemicals are added via
flow 5, and residue solution from the peroxide bleaching is added via backwater
flow.6. The pulp suspension is passed to bleaching step 3, which preferably can
1~74V13
be a bleaching tower. From the bleaching step 3 the pulp suspension is passed
to a washing step 4 where washing is carried out in an efficient washing equip-
ment, preferably a washing filter, to which washing liquid 7 is supplied. The
washing liquid 7 ma~ be pure water or backwater from subsequent operations, for
example dewatering.
Calculations have shown that at increased dry content after press 2
ever increasing amounts of chemical-consuming backwater are pressed off from therefining step, and at the same time more chemical-containing backwater 6 can be
returned to the bleaching step. By introducing a washing step 4 after the
bleaching and designing the washing step as a displacement step while simultane-ously adding washing liquid in an amount corresponding to a low dilution factor,a great part of the active chemicals remaining after the hleaching can be re-
turned to the bleaching step.
The effect obtained by a method according to Figure 1 compared with a
method, where all backwater of the refiner step is supplied to the bleaching step
and no backwater from the bleaching plant is returned, has been simulated in a
mathematic process model according to Figure 1. The pulps, viz. CTMP pulps, are
assumed having been well washed before the bleaching and require a peroxide
charge of 40 kg H202/ton pulp. The pulps are bleached to 75 % ISO. Considering
the chemical prices of to-day, being about SwCr 4:-~kg H2O2 and SwCr 0.7:-/kg
NaOH, the following chemical savings in relation to the present state of art areobtained.
By installing a press step 2 in Figure 1 the following chemical savings
at dlfferent pulp concentrations after the press step are obtained:
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1174013
Pulp concentration Chemical saving
after press step 2, % SwCr/ton pulp
32
36
44
The chemical saving in the above Table applies to a dilution factor in
washing step 4 corresponding to 1 m3/ton pulp.
By~using a normal washing filter instead of a press in position 2, a
chemical saving corresponding to about SwCr 27:-/ton pulp is obtained.
The chemical saving obtained is very great and implies a substantial
cost saving at the manufacture of bleached mechanical pulp.
As~ upper limit for the pulp concentration should be at about 50 % pulp
concentration. Above this concentration lt is difficult to dewater pulp, and the
pulp also will be difficult to defibre before the bleaching step.
The dilution factor 1 m3/ton pulp yields a somewhat better result than
2 m3/ton, which in its turn yields better results than dilution factor 3 m3/ton
pulp.
Gomplementary investigations have shown, that similar results are ob-
tained irrespective of the bleaching conditions being chosen. Correspondingbleaching studies, for example, have been carried where the pulp concentration
was varied between 5 and 15 %, the temperature in the bleaching step was varied
between 40 and 85C, and the corresponding staying time between 0.1 and 4 hours.
The invention is not restricted to the embodiments described, but can
be varied within the scope of the idea of the invention.
_ 20
