Note: Descriptions are shown in the official language in which they were submitted.
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BACKGROUND OF THE INVENTION
18 The invention relates to both electrochemical bleaching of lignocelIulosic materials
particularly wood and nonwood pulps and more particularly to wood pulps prepared by
20 standard pulping methods, especially chemical pulping methods, more specifically to
activation of lignocellulose brightening which occurs during such electrochemical
2 2 bleaching process, and to product prepared thereby and processes for their use.
Chemical pulp is prepared by treating lignocellulosic material with various ~ pulping
24 chemicals ~ to render soluble the major portion of the lignin portion of the material. The
most common chemical pulp is prepared from wood chips and non-wood bast and core by
26 the ~ Kraft ~ process. The resulting pulps, while quite strong, and are highly coloured
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probably due to a large amount of residual lignin. ~ White ~ papers are prepared from such
2 pulps and from other chemical pulps by further delignifying to reduce the Kappa number
below 10 and then by bleaching which primarily comprises oxidation and/or reduction of
4 a large number of chromophores in the residual lignin. The usual way this is accomplished
is by treatment with hydrogen peroxide, chlorine dioxide and/or other oxidative or
6 reductive chemicals, which changes the chromophores to noncolored products.
Recently other oxidative processes employing materials such as peracetic acid, sodium
8 perborate, ozone and bleach activators have been suggested as alternatives to reduce and
replace the need for chlorine dioxide in the bleaching of pulp. For a number of reasons,
10 well known to those in the art, sodium perborate and peracetic acid have proven to be of
particular interest and bleaching sequences employing peracetic acid which are intended
12 to reduce the use of chlorine dioxide are in commercial use. Sodium perborate is a good
oxidant at high temperatures whereas peracetic acid is a stronger oxidant even at lower
14 temperatures. At high temperature prolonged time of reaction (temperatures greater than
90~ and long bleaching time of about 2 h or more) of sodium perborate with alkali first to
16 generate hydrogen peroxide and then efficient brightening reaction of generated hydrogen
peroxide with chromophore is difficult to attain without degradation of cellulose.
18 Peracetic acid generation using sodium perborate and bleach activators is a well-known
art and the process is in commercial operation in the detergent industry. Main advantage
2 0 of using bleach activators is to reduce bleaching temperature to as low as 25~ or even lower
by generating active bleaching chemicals in a solution containing sodium perborate,
22 hydrogen peroxide and an activator. This reduction of the severity of the conditions
employed in bleaching reduces tendency towards cellulose degradation and increases
2 4 efficiency of pulp brightening. Several such activators are known . They are acetyl
derivatives of nitrogenous organic compounds such as tetraacetylglycolurile, diacetyl
26 dimethylurea, tetraacetylethylenediamine, triazine derivatives as well as some glucose
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acetyl compounds such as pentaacetyl glucose. These catalysts are expensive and required
2 in very high amount if they are used in a conventional bleaching process such as by
adding in combination with other oxidants such as sodium perborate and hydrogen
4 peroxide.
One way to improve the efficiency of these bleach activators is the use of
6 electrochemical treatment of the peracetic acid generating chemicals such as sodium
carbonate, borate and such activators in an electrochemical cell containing pulp to bleach.
8 Electrochemical generation of oxidants or other ~ electron carriers ~ in situ or in a closed
cycle process in pulp bleaching and even in pulping process for lignocellulosic material
have been experimented with in the past but, as far as is known, with little or no practical
success and these processes have never been used commercially.
12 Electrochemically generated compounds such as hypochlorite, hydrogen peroxide and
the like have been shown to react with chromophore to provide significant gain in
14 brightness of commercial interest provided the bleaching reaction has been carried out in a
separate process and generally not under the same conditions that provide efficient
16 generation of those chemicals electrochemically. Therefore, all known pulp bleaching
processes are essentially ~ two-step ~ process in which the first step is essentially efficient
18 generation of those oxidising chemicals separately in an electrochemical cell under their
optimum yield condition and the final step is use of those chemicals in a separate process
where no electrolysis is in effect and where the reaction conditions such as temperature,
time, chemical composition etc. are different from that used for generation of these
2 2 oxidising chemicals electrochemically. The prior art has not recognised the importance of
in-situ bleaching of pulp in an electrochemical cell under the same condition that generates
2 4 those oxidising bleaching chemicals electrochemically. Oxidative bleaching may, therefore,
be conducted under milder temperature condition and at a shorter time cycle than are
2 6 presently employed in the conventional process.
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Even under the milder conditions of electrochemically generated peracetic acid
2 bleaching, loss of active bleaching chemical such as perhydroxyl ion occurs because of
deposition of pulp on the electrode surface and pulp bleached with a consistency of more
4 than 1 part of dry pulp per 100 g of electrolyte solution will have a poor brightening effect
even inferior to that of a similar pulp bleached by conventional processes with the same
6 hydrogen peroxide or peracetic acid level.
It has been found however that under the conditions of electrochemically generated
8 peracetic acid with bleach activator where the temperature is lower than that of
conventional hydrogen peroxide or peracetic acid bleaching, a change in the design of the
conventional electrochemical cell particularly cathode design and more particularly by
introducing a rotatable cathode to creat constant agitation during electrolysis is effective in
1 2 preventing such a loss of active bleaching chemical such as perhydroxyl ions and enables
improved pulp bleaching to high ISO brightness even with pulp consistency above 3 parts
14 per 100 parts of electrolyte solution.
CITATION OF RELEVANT ART
A. Electrochemical Generation of Oxidants References
The most pertinent publications in this area of which applicants are aware are two papers
2 0 from Netherlands, one paper from the USA, one patent each from Bulgaria and the USA,
and one paper authored by us. These are P.M. v.d. Wiel et al., Electrochimica Acta, U.K.,
2 2 1971, vol. 16, pp 1217 to 1226, ~ Electrolysis of a carbonate-borate solution with a platinum
anode- I. Current efficiency at perborate concentration of zero ~; P.M. v.d. Wiel et al.,
24 Electrochimica Acta, U.K., 1971, vol. 16, pp 1227 to 1234, ~ Electrolysis of a carbonate-
borate solution with a platinum anode-II. Relation between current efficiency and
26 perborate concentration~; J. L. Culbertson and W.C. Teach, Trans. of Electrochemical
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Society; 1942, vol. 81, pp. 191-98; ~ Perborate formation at a rotating anode ~; and M. M.
2 Sain et al., Canadian Journal of Chemical Engineering; 1997, vol. 75 (1) pp. 62.
4 In first three of these documents electrochemical generation of sodium perborate and
hydrogen peroxide is taught. Wiel et al. specifically teach that the perborate formation is
6 efficient at low temperature and the solubility of perborate at low temperature is limited
which results in crystalline deposition of sodium perborate in electrolytic solution on
8 prolonged electrolysis. It also taught that the current efficiency of the electrolytic formation
of perborate decreases with increasing concentration of hydrogen peroxide in the solution.
10 Weil et al. did not work on electrochemical bleaching of pulp. Thus, no teaching or
suggestion is provided by these authors that generated perborate can be effectively
1 2 consumed in the electrolytic cell by bleaching pulp in-situ which would further increase
rate of perborate formation in the electrolytic cell.
1 4 U.S. patent number 1,081,191 to Arndt discloses an addition of sodium carbonate to the
electrolyte to improve the current efficiency of perborate formation. Additional Bulgarian
1 6 Patent number 1,444,399 to Kiselev at al. disclosed that an addition of ammonium
carbonate or bicarbonate or hydroxide to the electrolyte used in the production of sodium
1 8 perborate, increases the efficiency of perborate generation. Again none of these two patents
teach the art of bleaching pulp electrochemically in-situ in the perborate-generating cell by
20 oxidising chromophores of lignin present in the pulp. These references strictly concerned
with generating more perborate in the electrolytic cell with lower energy consumption
2 2 rather than using perborate in the cell for in situ oxidation of chromophores.
In one of our recent publication co-authored by M. Parenteau we have shown that
24 addition of an activator such as tetraacetylethylenediamine in a perborate solution can
efficiently bleach a mechanical pulp. It has been found that perborate reacted with
26 tetraacetyl ethylenediamine to generate peracetic acid which was an effective bleaching
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agent for thermomechanical pulp. This paper did not recognise the necessity of generating
2 a more effective and single step bleaching process by generating peracetic acid or peracetic
acid generating chemical in an electrolytic cell. Thus no proposition or recommendation
4 was made in this paper to bleach pulp in situ in a system which produces perborate and
peracetic acid electrochemically respectively without and with tetraacetyl
6 ethylenediamine.
Applicants are also aware of the following publications and patents:
8 M. Traube, Chem. Ber., 1982, vol. 15, pp 2434; C. Oloman, U.S. Patent number 3,969,201
(1976); J. Blajej et al., Chem. Zvesti, 1976, Vol. 30, pp 611; A.P. Watkinson, J. Appl.
1 0 Electrochemistry, 1979, vol. 9, pp. 117. All of them worked on the generation of hydrogen
peroxide in caustic soda via the cathodic reduction of oxygen. The references teach only
12 that the hydrogen peroxide generated in situ electrochemically can be substituted for
hydrogen peroxide generated externally and supplied as an aqueous solution. No
14 bleaching was suggested.
~).F. Dong and A.L. Clifford in U.S. patent number 4,891,107 (1990) disclosed an1 6 improved electrochemical method to generate hydrogen peroxide using a packed bed of
~ composite chips ~ as cathode material. This disclosure teaches the possibility to improve
18 efficiency of peroxide formation compared to a chemical method to prepare peroxide. The
method also teaches by reference a method to prepare peroxide at the user site, e.g., near
2 0 pulp and paper mill, under ambient conditions by a catalytic process with anthaquinone.
In situ electrochemical bleaching of pulp and use of activator therein are not mentioned.
22 U.S. Pat. No. 4,541,989 (1985) by P.C. Foller discloses the use of an air cathode to
generate ozone by direct anodic oxidation of water. This reference does not concern in-situ
24 bleaching of pulp in an electrochemical cell. In one technical article published in J. Appl.
Electrochemistry vol. 17, (1987) pp. 773-778 S. Stucki et al. generated ozone
2 6 electrochemically in ultra pure water which was then used by fine chemicals and
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pharmaceutical industries. This ultra pure water requirement is very difficult to achieve in
2 an in situ bleaching process where pulp usually contains various impurities.
It is readily apparent that of all the above literature and patents, only the above cited
4 Arndt and Weil et al. references are really relevant and these do not teach or suggest the
simultaneous use of electrochemical cell to bleach pulp during in situ generation of sodium
6 perborate.
B. References related to bleach activator
1 0 What is probably the reference of most interest in this group is European pat. No. EP 0
437 329 Al (1992) which teaches the use of various peroxyacid bleach precursors in
1 2 lowering the washing temperature in detergent applications. Tetraacetylethylenediamine
(TAED) is one of the bleach precursors used to improve low temperature efficiency for
14 washing. However, this reference does not concern the electrochemical generation of
perborate and in situ generation of peracetic acid in an electrochemical cell by adding
16 TAED. Moreover, no suggestion or recommendation has been made in this teaching on the
in situ bleaching that pulp using generated peracetic acid by adding TAED in an
1 8 electrochemical cell.
Another article, which considered the use of different bleach activators on the generation
of peracetic acid, is published in J. Prakt. Chem. Vol. 334 (1992) pp. 293-297. Authors
reported that both sodium perborate and hydrogen peroxide can generate peracetic acid
2 2 by reacting with tetraacetyl at a relatively low temperature and the activator efficiency is
maximum between 10 to 30 min of addition of the activator. This reference is strictly
24 teaches the generation of peracetic acid by reacting TAED with sodium perborate in
aqueous alkali. No bleaching of pulp was suggested and the teaching does not concern
2 6 with electrochemical generation of peracetic acid.
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Applicants consider that given the substantial differences between electrochemically
2 generated peracetic acid or any other strong oxidant promoted in situ bleaching of pulp
from an aqueous mixture of carbonate-borate or carbonate-borate-TAED and conventional
4 perborate, peroxide or peracetic acid bleaching that above references singly or in
combination provide no teaching enabling one to carry out bleaching of pulp in a6 electrochemical cell in a temperature range between 5 to 90~C.
SUMMARY OF THE PRESENT INVENTION
The invention provides a process for brightening of lignocellulosic material which
comprises treating of lignocellulosic material in situ in an electrochemical cell with an
electrochemically generated oxidant from a mixture of carbonate-borate or carbonate-
12 borate-TAED at a temperature range between 5 to 90~ Celsius (C.) and a current density
between 0.2 to 6.0 kA/m2 and a pulp consistency between 0.5 to 10%.
14 In another embodiment of this invention the bleaching process provides a mean to mix
pulp in the cell by rotating the cathode of the said electrochemical process.
16 The tangible embodiments produced by the process aspect of the present invention
possess the inherent physical characteristics of being relatively bright pulps when tested by
18 the standard brightness methods, and of having comparable pulp brightness bleached by
hydrogen peroxide by conventional method under the conditions employed in prior art
2 0 processes, thus being useable for all standard uses of lignocellulosic pulp based paper.
Special mention is made of embodiments of the invention wherein the method has
2 2 advantage over peroxide and chlorine dioxide bleaching processes for industrial pollution
control.
24 Special mention was also made of the embodiments of the invention wherein the
lignocellulosic materials are wood pulp and non wood pulp, wherein the pulps were
26 delignified to an extent that their kappa number were at least below 10 either by a
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conventional alkali extraction, oxygen and chlorine dioxide delignification processes or by
2 an electrochemical delignification process and of embodiments wherein the pH in the
electrochemical cell is between 8.5 to 12, preferably from pH 9 to about pH 11.
4 The said electrochemical bleaching process showed promise for an improvement in pulp
bleaching efficiency by multiple stage bleaching of pulp by recycling the electrolyte
6 solution.
The in situ bleaching of delignified pulp carried out by electrolysing a carbonate-borate
8 solution and a carbonate-borate-TAED solution containing 10-50 g/L of borax, 50 -150
g/L of sodium carbonate and 0.5 to 2 moles of TAED/ mole of generated oxidant inpresence or absence of 1 g/L of magnesium silicate within a temperature range 5 to 90~ C,
current density 0.2 to 6 kA/m2 and electrolysis time 30 to 180 min is within the scope of
12 this invention.
DESCRIPTION OF THE DRAWING
The drawing FIGURE 1 is a schematic representation of a preferred apparatus
configuration for the practice of the invention.
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DESCRIPTION OF THE PREFERRED EMBODIMENTS
The pulp used in the present invention was delignified Kraft pulp from nonwood and
6 wood origins. The process of the invention is an unique single-stage process for bleaching
of delignified pulps by generating an oxidising agent or a combination of more than one
8 oxidising agents in an electrochemical cell. The manner of practising the process of the
invention will be now described with reference to the drawing, employing as an
illustration of a preferred embodiment thereof, namely the bleaching of softwood Kraft
pulp lla in a preferred form of apparatus to be described in detail hereinafter. Referring
12 now to the drawing, to practice the process of the invention, the delignified lignocellulosic
material 11, hardwood, softwood and nonwood pulps and more conveniently softwood14 pulp lla prepared by a conventional Kraft process and then delignified by a conventional
process (oxygen and chlorine dioxide) or by an electrochemical process (alkaline solution
16 of ferricyanide) to a kappa number less than 5 and a cellulose viscosity not less than 12 cP
and then optionally pre-keated with a chelating agent, conveniently diethlyenetriamine-
18 pentaacetic acid, may be suspended in a solution 12 containing sodium carbonate 12a ,
borax 12b and sodium bicarbonate 12c, conveniently 100 g/L carbonate, 30 g/L borax and
2 o 25 g/L bicarbonate sufficient to provide 10 to 40 mmol/L of hydrogen peroxide
equivalence of perborate with or without an amount of tetraacetyl ethylenediamine
2 2 (TAED) 12d sufficient to provide 1-20 mmol/L of peracetic acid during electrolysis.
Sodium perborate and peracetic acid may be generated in-situ by passing a current 9,
2 4 conveniently about 6 amperes in the anode compartment 3 of an electrolysis cell 1, which
may be conveniently separated from cathode compartment 2, by a pulp suspension
26 containing an amount of pulp 11, conveniently 5 parts par 100 parts of solution 12 which
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may be prevented from deposition on to the anode and cathode surfaces by rotating the
2 cathode compartment with a constant speed. The electrolysis may be continuously carried
out in pulp suspension for a short period of time, conveniently about 3 hours to produce a
4 pulp of about 76 to 86 IS0% brightness and a viscosity of about 8 to 14 cP. The spent
solution recovered from the bleached pulp suspension 11 may be recirculated to the
6 electrolysis cell with added amount of pulp to readjust the consistency. In anode
compartment 3, in addition to perborate being generated, at electrolysis temperature above
8 40~C without the addition of TAED and at electrolysis temperature below 50~C with the
addition of TAED, perborate in electrolyte solution 12 may further form perhydroxyl ion
and bleach pulp more effectively. The resulting pulp, if desired, may be formed directly
into paper, or it may be further bleached by any conventional bleach sequence.
12 As used herein the term ~ a bleach effective amount of electrolytes ~ meansconcentrations of carbonate, borax and bicarbonate in solution are about 50 to 150 g/L, 0 to
14 45 g/L of borax and 0 to 50 g/L of bicarbonate, preferably 100 g/L carbonate, 30 g/L borax
and 25 g/L bicarbonate.
16 As used herein and appended in the claims the term ~ a bleach activation effective
amount of TAED~ means that a concentration of TAED 12d in solution of from about 0.1
18 to 2 moles of TAED per one mole of hydrogen peroxide equivalence of perborate
generated in the system, preferably from about 0.2 mole to about 0.5 mole per mole of
2 0 hydrogen peroxide equivalence.
As used herein the term ~ rotative cathode ~ means a cathode which helps mixing of
22 pulp in the electrochemical cell by optimising the speed of rotation and will facilitate
bleaching with pulp concentration above 1% by weight of electrolyte solution 12.24 The pH of the electrolyte solution 12 may vary from about 9 to about 11, preferably
from about 9.5 to 11. The temperature at which the process may be carried out is critical
26 and will depend on whether an activator is present or not. It will also depend on the
CA 022149~7 1997-11-0~
required pulp viscosity and brightness but conveniently should be less than the 90~ to
2 105~C. at which conventional peroxide bleaching stages are normally carried out. When
TAED is employed as the bleach activator, temperatures of from about 15~ C. to about 40~
4 C. are preferred.
One of the skill in the art will understand that the time required for the reaction will
6 also depend upon the type of pulp, and the extent of prior delignification and bleaching.
One of the skill of the art will be able to select a desired reaction period to optimise
8 bleaching while minimising cellulose depolymerization employing ISO brightness and
viscosity determinations already standard in the industry.
10The concentration of pulp or other lignocellulosic material in the slurry is particularly
critical and is largely limited by the difficulty of handling, mixing and diffusing reagents
12 through pulp slurries which are too concentrated and the large volume and low
production rate involved with too dilute slurries. Normally pulp concentrations from
14about 1 to 12%, preferably from about 3 to about 8% is preferred because of the ease of
mixing slurries in these preferred consistency range in a rotative cathode type apparatus.
16 The particular configuration of the apparatus employed to handle concentrated pulp in
the practice of the invention is not particularly critical and may be any of the prior art
18 devices. Particularly preferred, however, is a device comprising of an electrochemical cell 1
with a rotative stainless steel cathode 2 and a platinum anode 3 placed at the bottom of the
2 o cell. The cell is equipped with a slurry pump 5 which allows pulp at high concentration to
recirculate into the cell for efficient mixing and diffusion of electrolyte solution 12. The
2 2 recirculation pump is connected to the cell by a stainless steel tube and a valve 7 is used to
control the recirculation. It is preferred to use the cell without pump 5 up to a pulp
24 concentration of 5% and above this concentration the usage of pump 5 for recirculation is
preferred.
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In operation, electrolyte solution containing the carbonate 12a, borax 12b and bicarbonate
2 12c in the desired concentrations may be introduced into the system. Passing an electric
current from EMF source 9 carried by wires through electrochemical cell 1 produces
4 perborate solution still containing delignified pulp under agitation effected by rotative
cathode 2. After a sufficient electrolysis time to allow the electrolysis reaction and
6 simultaneous reaction with delignified pulp, the solution with pulp is taken out through
the pump 5 to a tank where the pulp is separated from electrolyte by filtration and
8 electrolyte was recirculated back to the cell 1. This process is generally practised for a pulp
concentration below 7% by weight of electrolyte solution 12. For pulp above 6% by weight
10 of electrolyte solution the preferred process is a continuous recirculation of pulp slurry
through pump 5 during the time of electrolysis at a given mass flow, adjusted by valve 7,
12 sufficient enough to keep a constant volume of electrolyte solution 12 in cell 1.
In another embodiment, electrolyte solution containing the bleach activator 12d in
14 desired concentration may be introduced into the system. The point of addition of bleach
activator is critical. The bleach activator can be added directly into the electrolyte at the
16 beginning of the electrolysis, more conveniently after a certain electrolysis time sufficient
enough to generate 10 to 30 mmol/L of peroxide equivalence of perborate. The amount of
18 bleach activator can be added at one time. Preferably they can be added in two to three
steps with a time sequence sufficient enough to confirm no residual activator in electrolyte
2 0 and also that there is a sufficient amount of perborate to react with the bleach activator for
generating peracetic acid or an intermediate oxidant effective for delignified Kraft pulp
2 2 bleaching.
The EMF required for the process of the invention as determined by the potential of the
24 anode with reference to a standard calomel electrode may vary from about +0.5 to +10 V
with about + 6.0 V being preferred. The cell current automatically adjusts to oxidise all
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species passing through anode compartment which are reactive at the electrical potential
2 selected particularly carbonate which is highly reactive in this potential range.
The current density applied in the electrode in the present invention is also critical and
4 is largely limited by cost of production of perborate. At high current density the bleaching
is more effective. Normally current density of from about 0.2 kA/m2 to about 6 kA/m2,
6 ~referably from about 2 kA/m2 to about 4 kA/m2 are preferred because of the cost of
production and bleaching efficiency.
ISO brightness referred to herein is a measure of brightness of pulp and is determined
according to TAPPI standard.
Pulp ~viscosity~ or ~viscosity ~ referred to herein is a measure of the degree of
12 polymerisation of cellulose in the pulp. It is determined according to TAPPI standard T230
os-76. Decreasing pulp viscosity reflects an increasing degree of cellulose destruction via
1 4 depolymerization.
16 The following examples further illustrate the best mode contemplated by the inventors for
the practice of their invention.
EXAMPLE 1
Portions of delignified softwood Kraft pulp (8 g, kappa less than 3, viscosity 11 cP and
ISO brightness 74.3%, b~ 8.8) are treated at various temperatures to reach the ISO
2 2 brightness' (about 90 min. total) by agitating them with a rotative cathode in an
electrochemical cell containing 250 ml of electrolyte solution comprising of 30 g/L of
24 borax, 100 g/L of sodium carbonate and 25 g/L of sodium bicarbonate. The solution was
subjected to 3 ampere of current for initial 30 min. and 9 ampere of current for the last 60
26 min. The temperature of the initial 30-min. was always kept constant at 8~C to generate
CA 022149~7 1997-11-0~ 15
perborate and the temperature of the last 60 min. time was varied from 8~ to 65~C to
2 improve brightening effect. No activator was added to the system.
Pro~rty 8~C 25~C 50~C 65~C 80~C
ISO Brightness, % 77 78 80 82 83.3
b~ 8.1 7.9 7.3 4.6 4.3
Voltage,V 9.2 9.4 9.1 9.4 10.6
Energy Consumed, kWh/kg 11.511.8 11.3 11.8 14.9
dry pulp
EXAMPLE 2
Portions of delignified softwood Kraft pulp (8 g, kappa less than 3, viscosity 10.3 cP and
14 ISO brightness 76%, b~ 8.4) are treated at various temperatures to reach the ISO
brightness' shown by agitating them with a rotative cathode in an electrochemical cell
16 containing 250 mL of electrolyte solution comprising of 30 g/L of borax, 100 g/L of
sodium carbonate and 25 g/L of sodium bicarbonate. The solution was subjected to 10
1 8ampere of current for 150 minNo activator was added to the system.
Propert~ 40~C 50~C 65~C 80~C
ISOBrightness, % 84.985.9 83.8 84.8
b~ 3.7 3.2 4.2 4.5
Voltage,V 9.2 9.4 9.1 9.4
Energy Consumed, kWh/kg 23.123.7 24.9 25.3
dry pulp
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16
EXAMPLE 3
2 Portions of delignified softwood Kraft pulp (8 g, kappa less than 3, viscosity 11 cP and
ISO brightness 74.3%, b* 8.8) are treated under conditions analogous to those described in
4 Example 1. The pulp solution containing the electrolyte was subjected to 3 ampere of
current for 30 min. at 8~C and then the current was increased to 9 ampere at 65~C for
6 electrolysis and bleaching to be carried out in-situ for various time intervals to reach the
ISO brightness'. No activator was added to the system.
Property 30 min 60 min 90 min 120 min
ISO Brightness, % 81.9 82.3 83.1 83.6
b* 5.9 5.3 4.7 4.4
Voltage,V 9.2 9.3 9.1 9.4
Energy Consumed, kWh/kg 8.2 11.7 17.1 21.9
dry pulp
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EXAMPLE 4
2 Portions of delignified softwood Kraft pulp (8 g, kappa less than 3, viscosity 11 cP and
ISO brightness 74.3%, b~ 8.8) are treated under conditions analogous to those described in
4 Example 1 for various supplied current. The pulp solution containing the electrolyte was
subjected to 3 ampere of current for 30 min. at 8~C and then electrolysis and in-situ
6 bleaching was carried out for various amount of supplied current at 65~C for additional 60
min to obtain ISO brightness values shown. No activator was added to the system.
Property 3A 6A 9A
ISO Brightness, % 79.8 82.1 82.7
b~ 6.8 5.7 4.9
Voltage,V 4.3 6.5 9.1
Energy Consumed, kWh/kg 2.7 5.1 11. 6
dry pulp
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EXAMPLE 5
Electrolysis cell as described in Figure 1 containing 250 ml of electrolyte solution
4 comprising of 30 g/L of borax, 100 g/L of sodium carbonate and 25 g/L of sodium
bicarbonate in absence of pulp was agitated with a rotative cathode as described in
6 Example 1 to generate perborate for initial 30 min. When the concentration of perborate in
equivalence of 20 mmole/L of hydrogen peroxide was generated an activator TAED was
8 added to the electrolyte in two different concentrations as given here to generate peracetic
acid electrochemically. The amount of hydrogen peroxide equivalence of perborate and
10 peracetic acid generated in-situ in the system is shown. Current applied was 3 ampere and
electrolysis was carried out for 180 min at 10~C.
Property Without With 15With 30
TAED mmole/ mmole/L
L TAED TAED
H202 equivalence of 42 26 24
perborate, mmol/L
Peracetic acid, mmol/L 0 5 17
Voltage,V 4.3 6.5 9.1
Energy Consumed, kWh/kg 2.7 5.1 11. 6
dry pulp
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19
EXAMPLE 6
2 Portions of delignified softwood Kraft pulp (8 g, kappa less than 3, viscosity 10.3 cP and
ISO brightness 76%, b* 8.4) are treated under conditions analogous to Example 2 except
4 the temperatures are 40~ and 50~C and 30 mmol/L TAED was added after 30 min of initial
electrolysis to reach the ISO brightness' (about 150 min. total) as shown.
Property without TAED with TAED
40~C 50~C 40~C 50~C
ISO Brightness, % 84.9 85.2 86.2 86.5
b* 3.7 3.2 2.8 2.6
Voltage,V 9.2 9.4 11.2 11.5
Energy Consumed, kWh/kg 28.7 29.5 32.7 33.5
dry pulp
EXAMPLE 7
Portions of delignified softwood Kraft pulp (8 g, kappa less than 3, viscosity 10.3 cP and
ISO brightness 76%, b* 8.4) are treated under conditions analogous to Example 2 except
the temperature is 50~C and two different concentrations of TAED were added after 30
min of initial electrolysis to reach the ISO brightness' (about 150 min. total) as shown.
Property No 15 30
TAED mmole/L mmole/L
TAED TAED
ISOBrightness, % 85.2 85.8 86.5
b* 3.2 2.9 2.6
Voltage,V 9.4 11.2 11.5
Energy Consumed, kWh/kg 29.5 33.1 33.5
dry pulp
CA 022149~7 1997-11-0~
EXAMPLE 8
2 Variable quantity of delignified softwood Kraft pulp ( kappa less than 3, viscosity 10.3 cP
and ISO brightness 76%, b* 8.4) are treated under conditions analogous to Example 2
4 except the temperature is 50~C to reach the ISO brightness' (about 150 min. total) as
shown.
Dry pulp, g 2.6 5.3 8.0 15.0
Pulp consistency,% 1 2 3 6
ISO Brightness, % 84.2 84.7 85.1 84.9
b* 3.9 3.6 2.9 3.2
Voltage,V 9.4 9.7 9.6 10.1
Energy Consumed, kWh/kg 29.5 32.1 33.2 35.2
dry pulp
EXAMPLE 9
Electrolyte remained after bleaching is filtered, recuperated and reused as electrolyte in
the said electrochemical cell with portions of delignified softwood Kraft pulp (8 g, kappa
less than 3, viscosity 10.3 cP and ISO brightness 76%, b* 8.4) under conditions analogous to
Example 1 except the temperature of the bleaching step was 65~C and the ISO brightness
reached is shown.
Electrolyte recycling ISO Brightnessgain b* Change in
number Brightness, % b*
82.9 8.9 4.7 -3.8
2 81.2 7.2 5.0 -3.4
3 81.9 7.9 4.6 -3.8
4 80.9 6.9 4.9 -3.5
CA 022149~7 1997-11-0~
2 EXAMPLE 10
4 Portions of the delignified softwood Kraft pulp (10 g, kappa less than 3, viscosity 10.3 cP
and ISO brightness 76%, b* 8.4) is bleached conventionally with hydrogen peroxide in a
6 conventional bleaching process where the pulp is mixed with 100 ml solution containing
variable amount of hydrogen peroxide, 2.5% sodium silicate and 0.5% MgSO4. Bleaching is
8 carried out at 90~C for 150 min and the brightness values are shown.
Hydrogenperoxide,wt% 1 1.5 2.0
Brightness, ISO % 79.5 80.6 81.3
b* 6.4 5.5 4.9
Viscosity, cP 8.9 8.1 7.8
