Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
~6805~
SPECIFICATION
Cellulose pulp is usually bleached in order to remove "residual
lignin" from the pulp, i.e. lignin which may not have been dissolved out
during the first delignification treatment stages carried out during digestion
5 of the raw lignocellulosic material in accordance with sulphite, su,lphate,
oxygen-gas and alkali, or other cooking methods. Since residual lignin is
dissolved, the bleaching can be referred to as a delignification stage. Bleachlng
also is carried out in order to increase the brightness of the pulp. In pulp
bleaching processes carried out on a commercial scale, the pulp is treateà
10 stepwise with one or more actlve chemicals for delignifying and/or bleaching
cellulose pulp, including chlorine, chlorine dioxide, hypochlorite, sodium
hydroxide, hydrogen peroxide, dithionite, sulphite, and, more recently,
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oxg-gen gas and aLkali.
The cost of the active chemicals required for bleaching and/or
15 delignification is very high, and constitutes an important proportion of the
cost of the manufacture of bleached cellulose pulp. A sufficient amount of
active chemical must be used, but not too much, since the quality of the end
product is affected by the quantity of bleaching chemicals supplied. Conse-
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quently, it is important that the correct quantity of active chemicals be charged
20 to the system. However, the correct amount is not easy to determine, becausethe residual lignin content of the cellulosic material varies, and so does the
amount of chemicals required. The relatively long bleaching times required for ~-
effective bleaching and for deligniflcation alsoincreases the difficulty of control.
The ~ asures taken to protect the environment against harmful
25 and contaminating discharges have increased the retention oi impurities and ~-
washing residues in the li~uor in which the pulp is transported to the bleaching
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section. Variations in the extent to which the lignocellulosic material is
~vashed and digested are reflected in variations in the total lignin in the
pulp transported for bleaching, and in the distribution of the lignin dissolved
in the suspending liquor and lignin bound to the cellulose fibers. In addition
5 to dissolved lignin, the liquor also contains chemicals such as sulphide,
thiosulphate and organic sulphur compounds that are reactive with the active
chemicals used for bleaching and/or delignification, and that also consume such
active chemicals. The amount o~ delignifying and/or bleaching chemicals that
react with such substances and with the lignin bound to the cellulose fibers is
10 of course lost for bleaching and/or delignification, and this is reflected hl an
incomplete délignification and/or bleaching, or worse. It is therefore necessary
to take this into account in determining the amount of active chemicals added
The art has accordingly utilized one of the following alternatives: -~
a~ The addition of actlve chemicals to the pulp suspension is based
on the content of residual chemicals hl the pulp suspension, determined ;~-
sùbsequent to the bleaching. The amount of residual chemicals, e.g. active
chlorine compounds dissolved in the residual bleaching liquid, is determined
manually~ at predetermined time intervals. Owhng to the variations in the
properties of the pulp suspension, and the relatively long residence ~time of the
20 pulp in the bleaching section, it is necessary to charge the bleaching chemicals
to the system hl a slight excess, in order to ensure a satisfactory bleaching~
which has a negative effect on the quality of the pulp, and increases bleaching
costs unnecessarily.
b) The addition of active chemicals to the pulp suspension is based
25 on the content of residual chemicals in the pulp suspensl~n determ~ned ;shortly
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after the bleaching process has begun. The addition of active chemicals is so
made that the measurement value by the analytic method being used is main-
tained constant at the point at which the measurementis taken (so-called set-
point control). The measurement value can be, for example, a redox potential,
5 a polarographic analysis value, or a signal obtained by optical means. The
desired set-point value can be adjusted as desired on the basis of manually
effected residual-chemical analyses. In this method, the amount of active
chemicals added to the system can be changed more rapidly than in the method
described under a), but the method is deficient, inasmuch as it assumes the
lQ measurement value reflects the amount of active chemicals consumed by the
pulp, but it actually does not. The measurement value is in fact inaccurate
to the extent that the active chemicals are also consumed by nonpulp substances
reactive therewith and dissolved in the liquid phase of the pulp suspension.
c) The amount of active chemicals added to the pulp suspension is
15 changed in accordance with set-point values in the manner described in b) above,
but in this case the set-point value is changed, as required, on the basis of
values obtained from an automatic analysis of the residual-chemical content
after the termination of the bleaching reaction. This is a combination of a) and
b)~ but although the result obtained hereby is an improvement on these
20 alternatives taken alone, the method does not eliminate all the disadvantages
encountered with the method b) above. Furthermore, because of the relatively
long residence time of the pulp in the bleaching tower, only relatively slow
variations in the amount of chemicals required to be charged to the system can
be compensated for by changing the metering of chemicalsO
.
25 N~e of these methods takeF into accolmt the fact that the amount
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of active chemicals consumed is not solely dependent upon the
lignin content of the cellulose pl~lp, but also is dependent on
dissolved lignin, i.e. the washing losses which have not been
recovered and passed to an evaporation and combustion section,
together with inorganic compounds that consume active chemicals,
such as chlorine.
In one particular aspect the present invention provides
a process for controlling the addition of delignifying and/or
bleaching chemical for delignifying and/or bleaching cellulose
pulp suspended in an aqueous liquor containing spent chemical
other than the cellulose pulp that is reactive with such
delignifying and/or bleaching chemical, which comprises
determining firstly prior to the addition of delignifying and/or
bleaching chemical the amount of delignifying and/or bleaching
chemical consumed by the chemical reactive with the delignifying ;
and/or bleaching chemical in the liquor in the absence of the
cellulose pulp; adding a known amount of the delignifying and/or
bleaching chemical to the cellulose pulp suspension, and then,
during the delignification and/or bleaching after a predetermined
time interval following addition of the delignifying and/or
bleaching chemical to the cellulose pulp suspension, determining -
secondly in the absence of cellulose pulp fibers the residual
amount of delignifying and/or bleaching chemical in the liquor;
from these first and second determinations and the known amount
of delignifying and/or bleaching chemical determining the amount
of delignifying and/or bleaching chemical consumed by the pulp; --
and then carrying out the delignification and/or bleaching with
an amount of delignifying and/or bleaching chemical ad~usted
according to the amount of delignifying and/or bleaching chemical
consumed by the pulp.
Two analyses are necessary for this determination, one
taken before and one after the addition of the active chemical
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to the pulp suspension.
An important feature of the method according to the
invention is that the amount of active chemical is determined
a predetermined time interval after the active chemical has
been charged to the liquor. This time interval can be rather
short and is within the ran8e from about 30 seconds to about
ten hours.
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Since a certaill quantity of the chemical is immediately consumed, sampling
of the suspending pulp liquor for the purpose of obtaining this measurement
value can be effected immediately, i.e. within a few seconds, after charging
the active chemical to the liquor. It is advantageous to take the measurement
5 after a predetermined time interval during the actual bleaching stage, when it
is observed that the bleaching and/or delignification reaction is taking placeO
Thus, the analytical determination can be effected at the beginning of the
bleaching and/or delignification stage, or during the stage. The determination
can also be made at the end of the stage, particularly if the reaction time in the
lO bleaching stage is shorte~ than half an hour. It is particularly convenient to
- take the sample during the initial stage~, i.e. within the first 1j2 hour of the
bleaching and/or delignification stage, preferably within the flrst twenty
minutes, in order to obtain the correct value as quickly as possible, and thus
control the addition of active chemical to the pulp suspension at the start.
15 However, when the active chemical is not completely consumed during the
bleaching and/.or delignification stage, a determination can be made of the
quantity of residual, unconsumed active chemical at the end of the stage.
By "bleaching and/or delignification stage" is meant a bleaching or ~ -
deligniflcation process in which one or more bleaching and/.or delignifying
20 chemicals is supplied to the pulp, and permitted to react therewith, resulting
in consumption of bleaching and/or deligniflcation chemi~al~. Another form of
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i06~054
process to wllich the invention can be applied is a lignin extraction stage, in
a pul~ bleaching process, in which stage the pulp is treated with an aL~aline
solution to dissolve lignin from the pulp. Ln this instance, in accordance with
the invention, there is taken a first sample to determine the a~ali requirement
5 of the pulp-suspending liquor for the lignin extraction, and a second sample,
subsequent to the alkali addition, to determine the amount of a~ali consumed
by the pulp, after which the measurement values are used to control the
quantity of a~kali introduced to the system.
The method according to the invention makes it possible to control ~ ~ -
10 the delignification in a way to equalize variations in the lignin content of the
treated pulp.
The method according to the invention is particularly suited for
use wi~ lignocellulosic material such as wood which has been digested by means ;
of chemical processes, such as sulphite, sulphate, oxygen gas/alkali, bisulphite,
15 and soda cooking processes. The method can also be applied to pulps obtained
by semichemical, mechanical, and thermomechanical processes. The method
according to the invention is particularly suitable for use with chemical pulps
havinga~lignin content corresponding to a Kappa number within the range from
about 100 to about 5, suitably from about 40 to about 9 and preferably from
20 about 35 to about 10, the ~ethod being preferably applied in an introductory
bleaching stage, wherein in addition to obtaining an improved brightness, a
continued delignification of the pulp is also obtainedO When the method of the
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1068~54
invention is applied in conjunction with the manufacture of semichemical,
mechanical, and thermomechanical pulps, so-called high-yield pulp~, the
brightness-improving reaction is more pronounced than the delignifying effect.
The method according to the invention can also be applied to any
5 multistage bleaching and/or delignification processes, for example a bleaching
stage,in which different bleaching chemicalst such as chlorine and chlorine
dio~cide, are used in sequential phases in the same treatment stage, without
intermediate alkaline extraction or washing processes. The method according
to the invention can also be applied while simultaneously using a plurality of
10 active chemicals, for example, bleaching stages in which mixtures of chlor~
and chlorine dioxide are used as the bleaching agents. 'I
Figure 1 represents a flow sheet showing application of the invention
to a bleaching and/or delignification stage in a pulp manufacturing plant using
for example chlorine as the active chemical.
Figure 2a is a graph showing the calorimetrically measured
potentials in millivolts plotted against titrimetric chlorine consumption in
the analysis of a sulphate pulp-suspending liquor of Example 1.
Figure ~b is a graph showing the calorimetrically measured
potentials in millivolts plotted again9t titrimetric chlorine consumption in
20 the analysis of a sulphite pulp-suspending liquor of Example 1.
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Figure 3 is a graph showing the calormietrically measured
potentials in millivolts plotted against titrimetric chlorine consumption in
the analysis of a sulphite pulp-suspending liquor of Example 2.
In the process shown in Fig_e , a pulp suspension 1, arriving
5 from a cooking-screening stage from a sulphate, sulphite, oxygen/alkali or
- other pulp manufacturing processes or from an earlier bleaching and/or
delignification stage, was passed to a sampler 2 in which the liquor sample
` stream 3 was taken from the pulp suspension and filtered to remove pulp
fibers. The sample stream 3 was passed to an analyzer 4, in which the
10 amount of chlorine consumed by the liquor was determined. The pulp
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suspension was mixed with a stream of chlorine 6 in a bleaching stage 5. ~ ~
After the chlorine had been added to the pulp suspension, the suspension ~ -
was permitted to react with the chlorine for, for example, twenty minutes,
and was then passed to the next bleaching stage. After this time interval .
15 after the addition of chlorine, the pulp suspension was passed to a second
.
sampler 8 in which the liquid sample 9 was taken from the pulp suspension
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and filtered to remove pulp fibers. The time at which the second sample -
is taken may vary, however, between a few seconds after the chlorine has ~ -
:
been mixed with the pulp suspension, to the end of the stage, when the
20 suspension leaves the bleaching stage 5.
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The sample stream 9 was pa~secl to an analyzer 10, in which the
quantity of residual chlorine in the liquid is determined. On the basis of the
results obtained in the analyzers 4 and 10, it is possible to calculate the
quantity of chlorine consumed by lignin bound to the cellulose in the pulp
5 fibers, and the amount of chlorine consumed by chlorine-reactive substances
in the suspending liquor. These calculations may bé made electronically, in
the computer 13, to which the measuring results are transmitted electrically
via lines 11 and 12. On the basis oE the results calculated by the computer 13,
the flow af chlorine 6 to the bleaching stage 5 is regulated by regulating means
10 14.
In addition to controlling the flow of chlorine to the bleaching stage
by means of the signals obtained from the analyzers 4 and 10, the signals from
the computer 13 may also be used to control the addition of bleaching agents to
subsequent stages. By making two analyses, one prior to the addition of
15 bleaching chemicals and one during the bleaching stage, it is possible to
calculate the amount of bleaching agent actually required by the pulp, prior
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to passing said pulp to the bleaching stage. Thus, there is obtained a lignin-
flow analyzer or a Kappa number analyzer, which effects a measurement
on the whole of the pulp flow passed to the bleaching stage, and provides
20 continuous measuring ~ values.
This represents a great advantage over samples which are taken
manually. The bleaching chemicals are dispensed to the pulp suspension in
accordance with the Kappa number or the chlorine number of the input pulp,
the number being a measurement of the lignin content, and related to active
25 chemical consumption. The method according to the invention enables for the
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first time an analysis of the lignin flow to be made continuously, accurately
and directly on the flow oE material with which it is desired to control the amount
of bleaching agent charged thereto. ~1other important advantage obtained with
the method according to the inYention is that the ana~ysis made prior to the
5 bleaching stage illustrates the efficiency of the wa~te pulping liquor recovery
system~ ~n addition hereto it is possible to take into account the amount of
bleaching chemicals consumed by substances dissolved in the suspending
liquor at exactly the right moment, since the sample for this analysis is taken -
before bleaching chemicals are added to the pulp suspension.
The analysis of the amount of bleaching chemicals consumed by
the suspending liquor can be carried out in a number of ways, which are - ~ `
known and form no part of the instant invention. - ~ -
One convenient method is to supply bleaching chemicals to a sample
of suspending liquor in a known quantity, and in an excess quantity, e . g. in the
15 form of chlorine water, and then, after a predetermined time interval,
determine the ez~cess of bleaching chemical in known matter, for examply be
iodine titration, polarographic measurement, measurement of the redo~ ;
potential, measurement of the conductivity or pH, photometry, coulornetry,
or the like. It is particularly convenient, however, to permit the flow 3 to
20 react with an excess of bleaching agent, and to determine the amount of heat
developed thereby. This method is designated the calorimetric method The
calorimetric determination may also be effect with a reagent other than chlorine
water, for example hypochlorous acid, hypochlorite, chlorine dioxide or
hydrogen peroxide. Despite the fact that the substances dissolved in the
25 suspending liquor comprise a mixture of a large ~umber of mainly unknown
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~ 068054
substances, it ~las surprisingly beerl found that the heat developed during the
calorimetric determinatioll can be accepted as a reproducible measurement of
the amo~ult of bleaching agent consumecl by the suspending liquor.
Analysis of the content of bleaching agent in the pulp suspension
5 subsequent to addition of bleaching agent thereto and subsequent to the
commencement of the bleaclling reactions can be carried out in a number of
ways. The most common methods include redox potential measurement,
polarographic measurement, measurement of the conductivity or pH of the
sample, and manual or automatic iodine titratioll on the content of residual ;~
active bleaching agent. It is desirable that the analysis be Rffected continuously,
and that it be specific for the bleaching agent to be analyzed ~ ;
It has been found particularly advantageous to tal~e a fiber-free
sample of the suspending liquor 9, and to react the liquor with a suitable
reagent, and to use the heat developed thereby as a reproducible measurement
of the residual content of bleaching agent. By the selection of a suitable reagent,
an analysis can be made in this way, for example on sodium hydroxide1 hypo-
chlorite, chlorine, chlorine dioxide and hydrogen peroxide. With certain
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bleaching processes, mixtures of bleaching agents are used, and, for example,
chlorine and chlorine dioxide can be used in mixture. By the selection of a ~O
suitable reagent, it i~ possible to determine calorimetrically the total amount
of active chlorine, or to determine the chlorine content and the chlorine
dioxide content separately. Thus, according to the invention it is also possible
to obtain specific information on how the content of one of the bleaching
chemicals changes during the reaction sequence.
The invention is exemplified by the following E~amples, in which
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E~ample 1 illustrates the analytical process for determining
the chlorine consumption of the suspending liquor, Example 2
illustrates how the content of residual chlorine is determined
after a predetermined time interval, and Example 3 illustrate~
the ~anner in which the flow of chlorine to an introductory
chlorine stage is controlled. Example 4 illustrates the manner
in which a chemical addition comprising the sequential addition
of chlorine and chlorine dioxide is controlled. Example 5
illustrates the control of an alkaline charge to an alkaline
delignification or lignin extraction ~tage, in a bleaching - - -
sequence.
Example 1 - -
The determination of the chlorine consumption of liquor from
a pulp suspension.
From the pulp suspensions in sulphite and in sulphate
pulping liquors entering a first bleaching Stage as at 1 in
Figure 1 were taken a large number of samples 3 of Fi~ure 1
of the liquor at regular intervals over an extended period of
time. The chlorine conumption of the liquor samples was
determined both titrimetrically and calorime~rically. `
The titrimetric analysis was effected in the following
manner: 50 ml of the liquor sample was reacted with 10 ml of
a saturated chlorine solution, approximately 5 g/l Cl~. After
S minutes there was added an excess of potassium iodide, which
had been acidified to pH 1. The iodine thus formed was titrated
with sodium thiosulphate. Blanks were made in the same manner
but with 50 ml distilled water instead of sample liquor.
The calorimetric analysis was carried out as follows:
The fibre-free liquor sample in a continuous stream was mixed
with a stream of saturated aqueous chlorine solution. The
reaction mixture was then passed into a coil reactor, and the
heat of reaction developed therein was measured as a potential
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in millivolts by a number o series-connected thermoelements. The potential
obtained from the thermoelements provided a relationship with the titrimet-
rically-determilled chlorine consumption, which was linear over a wide range.
The relatiollship between the calorimetrically and titrimetrically determined
analytic values is illustrated in Figures 2a and 2b, in which the calorimetric
_
potentials are plotted on the Y-axis, and the corresponding titrimetric
chlorine-consumption values are plotted on the X-axis. Figure 2a is for
sulphate liquors, while Flgure 2b is for sulphite liquors. As will be seen
from the Figures_, the relationship is linear (correlation coefficient 0.995),
and hence the calorimetric values can ~elia~ly be used for continuously
determining the amount of chlorine consumed by the liquid. L
Example 2
Anal_sis of residual chlorine in the bleaching of cellulose pulp. ~ -
A large number of samples were taken from the bleaching stage
15 of a sulphite plant l5 minutes after the pulp suspension and chlorine were mixed . ~ ~
Fibre-free liquor samples were takenj and analyzed calorimetrically and ~ -
manually using an iodometric titration process. The iodometric titration was
effected in a conventional manner, by reacting the sample with acidified
potassium iodide solution. The liberated iodine was then titrated with sodium
thiosulphate, whereafter the residual chlorine content was calculated. In the
calorimetric analysis of the chlorine content, a æeries of calibration solutions
was prepared by adding 0. 01, 0. 05, 0.10, and 0.15 gh Cl2 to a liq~r sample 9
according to Figure 1, from which sample all chlorine had been stripped offO
An exact determination of the chlorine content of these calibration solutions
25 was obtained by iodometric titration. In the calorimetric calibration, a stream
of calibration solution was mixed with a stream of reagent solution comprising
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~06~054
O. 2''~ pot~ssium iodide with 0.2'YC sulphuric acid. The reaction heat developed
therewith was measured to provide a pbtential by means of thermoelements.
As the measurements were continued, the calibration solutions were replaced
with a continuQus stream of sample 9 according to Figure 1. In Figure 3, ~ -
5 obtained calorimetric values from several samples obtained at different
sampling times are plotted on the Y-axis, and the corresponding titrimetric
residual-chlorine contents are plotted on the X-axis. Figure 3 illustrates the
reLationshipbetween the calorimetric values, in millivolts, and the manually
titrimetric values of the residual chlorine content, in gh. As will be seen
10 from the Figure, a practically linear relationship is obtained between the
calorimetric values and the titrated values. Thus, the amount of residual
chlorine can be advantageously determined by means of a continuous
calorimetric determining process.
Exam~le 3
15 Controlling the flow of chlorine to an introductory ~lorine stage.
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The tests were carried out on a suspension of unbleached pine
sulphate pulp in pulping liquor, flowing to the introductory bleaching stage of a
sulphate plant. The pulp was charged to the bleaching stage at a flow rate of
12, 000 l/min, a pulp consistency of 2.5~c, and a temperature of 20C. The
20 flow of chlorine to the introductory chlorine bleaching stage was controlled in
the following manner:
As a control, the flow of chlorine to the stage was first controlled
in accordance with the conventional set-point control process. Then, chlorine
was charged to the introductory chlorinating bleaching stage in accordance
25 with the method of the invention.
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During the hvo run~) samples were taken manually in order to
determine the K~ppa nu~nber, the chlorine consumption, and the consistency
of the influent pulp. The resiclual lignin content of the pulp was determined
subsequent to the chlorination and alkaline extraction stages.
In the conventional set-point control method, using a redox potential
measuring process, the set-point value with respect to the ac~ve chlorine
content was held constant at 0.145 g/l, and the analytic measurements were
taken after 75 seconds of reaction time between the chlorine and lignin.
In the method in accordance with the invention, the analytic
measurements were taken after the same time interval, and the set-point
value was adjusted manually every 5 minutes, with the guidance of the following
empirical equation: .
SET-POlNT VALU~ 145 (I ~ 0.00300 (FC~2 - 12 x KF -20.8)3 ~ -
15 in which KF is the chlorine consumption of the material in the incoming
liquid (g/ 1) and FCl2 is the relevant flow of chlorine ~g/min).
The results obtained are given in Table I b-lo-:
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Table I
Control Method
Conventional According to
Set-Po~nt Method the invention
Consistency of pulp entering
thebleachingstage, ~ 2~50 4 0.01 2.50~ 0.0
Kappa number, unbleached
pulp 33.2 ~ 3.9 32.9 ~ 4.0
Chlorine consumption of
suspending liquor entering
the bleaching stage, gh o. 10 ' 0.12 0. 20 ~ 0.10
Kappa number subsequent
to chlorination and alkaline --. -
extraction 6.15 ' 0.71 6.16~ 0.10
As will be seen from the Table, the variations in Kappa number
( = lignin content) of the pulp flowing to the introductory chlorinatingstage were
of approximately the same order of magnitude in both runs (+ 3. 9 vs. ~ 4. 0, .
respectively). In the method according to the invention, the amount of chlorine
consumed by the suspending liquor passing to the bleaching stage was twice as
20 large as that consumed in t~e conventional set-point method, which is represent-
ative of the variations which normally occur in pulp lplants, and which can be
attributed to the variations in washing efficiency.
As seen from the Table, the distribution range in Kappa number
obtained with the conventional set-point control method was about 7 times -f~ ~;
25 greater than in the method according to the invention (' Ot.71 vs. ' OolO~
respectively)r When the amount of lchlorine consumed varies to such an e~tent
that it may be double that expected, the inadequacy of the control made possible
by the conventional set-point control method becomes particularly marked, and ~ `
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as a result of the inaclequate c ontrol, the variations in the lig~nin content of the
chlorinated pulp are consider~ble. On the other haild, in the method according
to the invelltion, the control is better, and as a result the Kappa number is
relatively uniform, the variations in Kappa number being evened out.
Example 4
Controlling the addition of chemicals in a sequential chlorine and_hlorine
dioxide bleaching stage.
Unbleached pine sulphate pulp suspension at a pulp consistency of
3~c was flowed to a chlorine dio~idetchlorine bleaching stage of a sulphate plant
at a flow rate of 18, 000 l/min.
The bleaching stage was a reaction stage with sequential addition of ;
firstly chlorine dioxide, and secondly chlorine, without intermediate washing oraLkaline extraction.
The flow of chlorine dioxide and chlorine to the introductory -
bleaching stage was controlled in the following manner.
During the first test period, chlorine dioxide was added to the
pulp suspension in the first stage, and chlorine in the second stage, using
conventional control methods .
During the second test period, chlorine dioxide and chlorine were
added in respective stages using the control method of the invention.
During the tests, the influent pulp suspension was analyzed with
respect to itæ consistency, Kappa number, and the chlorine consumption of ~
the liquor in which the pulp was suspended. - ;
Subsequent to the introductory chlorine dioxide/chlorine stages,
the pulp was extracted with alkali ~ a laboratory scale to remove lignin, prior
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to analyzing the lignill content of the pulp a~ter the bleaching stage.
In the conventional control method, chlorine dioxide was added in
the first stage in direct proportion to the production level, which during the
test corresponded to an addition of llr 1 kg active chlorine/min . The chlorine
was charged using a conventional set-point control method. The content of
active chlorine was held constant at 0.130 g active chlorine/l, when measuring
after 30 seconds reaction time.
In the control method accordhlg to the invention, chlorine dioxide was
charged to the first stage in direct proportion to the production level, which
corresponded to an addition of 11.1 kg active chlorine/min. The content of
residual chlorine dioxide was determined as active chlorine after 10 seconds
reaction time between chlorine dioxide and lignin. The chlorine charge was
adjusted in accordance with the invention manually every 5 minutes, in accordance
with the following empirical equation:
Fcl2 3 93 (Fcl ~2 - 18 C + 18 KF) where
FCl àn~. Fclo represent the chlorine and chlorine dioxide flow,respectively,
in kg active chlorine/minute, C is the residual chlorine-dioxide content
determined as active chlorine after 10 seconds reaction time in grams per
liter .
KF is the amount of bleaching chemicals used in the entering
suspending liquor in grams of active chlor~e per liter.
The method according to the invention was applied in conjunction
with the measurement of the active çhlorine requirement of the suspending
liquor of the pulp suspension fed to a bleaching stage comprising two sequentialbleaching stages. The ~alysis results from the first bleaching stage were used
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to con~rol the ad~lition of chemicals to the subsequent bleaching stage. Thus,
the first bleacl~ g stage was used as a lignin-flow allalyzer. The results-
obt~inecl are sho vn in Table II below.
Table II
Control Method
Conventional accordingto
Set-Point Method the invention
Consistency of pulp entering
the bleaching stage, ~ 3.05 ~ 0.15 3.00 ' 0.16
10Kappa number unbleached
pulp 31.3 ' 3.4 32.1 ' 3.6
Chlor ine consumed in the
suspending liquor entering
the bleaching stage ( g
active chlorine/l) 0.13 ~ 0.08 0.16 _ 0.09
Kappa number subsequent
to chlorination and alkaline
extraction - 5.90~ 0.73 5.95 ' 0.14
~s seen from the Table, the distribution range in the Kappa number
I ~0 obtained was ~ 0~73 in the conventional control method, and only ~ 0.14 in the
method of the invention. The results show that in the method of the invention,
the chlorine charge to the second bleaching stage can b`e according to the -
requirements adjusted, which is not possible in the conventional set-point , ;;
control method, since it utili~es feed-back control, and the chemical charge
~5 is regulated by maintaining a constant residual chlorine content at the measuring
position. If there is direct control of the chemical charge to the second
bleaching stage, as in the method of the invention, the chemical supply can be
utilized more effectively. ~ this way, the costs of chemicals can be-reduced.
,.
. .
19 ~,
1068054
Ex_ple 5
Cont olli g the alkali harge t~ an alk~ c:-~r~ctioll st ge in a bleaching
sequence .
__.
Unbleached pine sulphate at a pulp consistency of 3~/c was flowed at
5 a rate OI 18, 000 l/min to the introcluctory bleaching stage, a conventional
chlorine stage, in which chlorine was added by the method in accordance with ;;
the invention as described in Example 3, but with corrections for the larger
pulp flow. Subsequent to passing the chlorine stage, the pulp was washed, and
extracted with aqueous alkali (NaOHj solution to remo~e lignir~ ~ The flow of aqueous
10 alXali solution to the alkaline extraction stage following the introductory chlorine
stage was experim~ntally controlled in accordance with a conv~ticnal method i~
and in accordance with a feed-forward control method according to the invention,
with the guidance of the amount of chlorine consumed by the substances dissolved ,~i ;
in the in~oming liquid, and the chlorine flow to the chlorine stage.
During a first test period, aqueous alkali solution was charged to
the aLkali stage in a conventional manner, according to pH values determined
manually after the chlorine stage. The pH after this stage should exceed
10.5, if complete lignin dissolution is to take place. A higher pH means that
excessive alkali has been charged. It was found necessary, however, to
20 maintain the pH at an average of 11.0 after the extraction stage, in order to
prevent the pH from falling below 10. 5 at any time of said stage, because of
the long residence time (60 minutes) in the extraction stage.
During a second test period, which was effected immediately after
the first test period, aqueous alkali solution was charged to the alkali stage
25 using the method according to the invention. The amount of chlorine corlsumed
1068054
by substances dissolved in t}E liquor cntering the bleachin~ stage was determined.
Further, a set-point contr(>l was made ~l respect of the chlorine charge to the
introductory chlorine stage. The value obtained when the chlorine flow was
reduced with the product of the pulp flow and the consumption of chlorine by
5 the incoming liquor was used to determined the lignin flow. The alkali flow
to the subsequent alkali extraction stage was controlled by means of a feed- ~
forward control process, with the guidance of the following empirical equations ~-
~alkali (t + 60) = Fcl (t) 0. 429 - KF (t).5.04
in which
0 F alkaii (t + 60) = the alkali flow in kg/min to the a~aline extraction stagewith a time displacement of 60 min from the chlorine charge~
to the chlorine stage,
FCl (t) = the chlorine flow to the introductory chlorine stage in kg/min
KF (t) = the amount of bleaching chemical, chlorine, consumed by the
entering suspending liquor in grams chlorine~
The test showed that in the method of the invention the alkali
charge was more precise, so that the pH was held at an average of 1OD 7 during
the whole of the test period, without the pH at any time falling below 10. 5 . The
Example shows how the invention can be applied to provide a more exact
20 alkali charge than was previously possible, thereby saving considerable
quai~tities of alkali. When the average pH was lowered from 11. 0 to 10. 7,
as obtained in accordance with the invention, an approximately 100~C reduction
in aLkali consumption was obtained.
In the Examples given aboye, calorimetric measuring techniques
25 have been used in the analyses made prior to and subsequent to or during the
bleaching stage. It will be readily understood, however, that other measuring
methods may also be used. One such method is the so-called chemiluminescence -
21
106~3~S4
methodJ in ~Y}liCh the bleachillg chemicals in the suspending liquor are reactedwith a reagent whilst emitting registerable light - this being a rapid and sensi-
tive method which is particularly suitable for measuring low contents of active
chlorine. Redox potential and polarographic measuring techniques can also be
5 used, since they involve no reagent consumption.
The advantage afforded by the calorimetric measuring method,
however, is that it is very accurate, as is evident from the curves in Figure 2a
and 2b, and in Figure 3. Since the reaction heat is measured, this metho~
affords the advantage that a large number of different bleaching agents can be
10 used for analytical purposes, for example, such agents as chlorine, chlorine
dioxide, hypochlorite, hydrogen peroxide and chlorite. The calorimetric
measuring technique may also be us ed to advantage for determining aLkali in
those bleaching processes in which alkali, such as sodium hydroxide, is in-
cluded ~s an active component, e.g. alkaline oxygen-gas bleaching, and in
alkali extraction processes~ One advantage of this is that it is possible to use
the same type of instrument for all of these substances.
