CA2031850C - Method of bleaching pulp - Google Patents
Method of bleaching pulpInfo
- Publication number
- CA2031850C CA2031850C CA002031850A CA2031850A CA2031850C CA 2031850 C CA2031850 C CA 2031850C CA 002031850 A CA002031850 A CA 002031850A CA 2031850 A CA2031850 A CA 2031850A CA 2031850 C CA2031850 C CA 2031850C
- Authority
- CA
- Canada
- Prior art keywords
- ozone
- recited
- pulp
- chlorine dioxide
- bleaching
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired - Lifetime
Links
Classifications
-
- D—TEXTILES; PAPER
- D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
- D21C—PRODUCTION OF CELLULOSE BY REMOVING NON-CELLULOSE SUBSTANCES FROM CELLULOSE-CONTAINING MATERIALS; REGENERATION OF PULPING LIQUORS; APPARATUS THEREFOR
- D21C9/00—After-treatment of cellulose pulp, e.g. of wood pulp, or cotton linters ; Treatment of dilute or dewatered pulp or process improvement taking place after obtaining the raw cellulosic material and not provided for elsewhere
- D21C9/10—Bleaching ; Apparatus therefor
- D21C9/147—Bleaching ; Apparatus therefor with oxygen or its allotropic modifications
- D21C9/153—Bleaching ; Apparatus therefor with oxygen or its allotropic modifications with ozone
-
- D—TEXTILES; PAPER
- D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
- D21C—PRODUCTION OF CELLULOSE BY REMOVING NON-CELLULOSE SUBSTANCES FROM CELLULOSE-CONTAINING MATERIALS; REGENERATION OF PULPING LIQUORS; APPARATUS THEREFOR
- D21C9/00—After-treatment of cellulose pulp, e.g. of wood pulp, or cotton linters ; Treatment of dilute or dewatered pulp or process improvement taking place after obtaining the raw cellulosic material and not provided for elsewhere
- D21C9/10—Bleaching ; Apparatus therefor
- D21C9/12—Bleaching ; Apparatus therefor with halogens or halogen-containing compounds
- D21C9/14—Bleaching ; Apparatus therefor with halogens or halogen-containing compounds with ClO2 or chlorites
- D21C9/144—Bleaching ; Apparatus therefor with halogens or halogen-containing compounds with ClO2 or chlorites with ClO2/Cl2 and other bleaching agents in a multistage process
Abstract
A method of bleaching pulp of cellulosic fibrous material in the production of chemical pulp in a bleach plant is described wherein the entering pulp is fed continuously in a bleaching line thereof. The method comprises the step of bleaching the pulp with chlorine dioxide and ozone in one and the same stage which is started with chlorine dioxide and followed by ozone. Chlorine dioxide and ozone are added at points separated from each other and each at at least one point in the bleaching line.
Said stage is free from intermediate washing between the points where chlorine dioxide and ozone are added.
Said stage is free from intermediate washing between the points where chlorine dioxide and ozone are added.
Description
2031~
METHOD OF BLEACHING PULP
Field and Background of the Invention The present invention relates to a method of bleaching pulp of cellulosic fibrous material in the production of chemical pulp in a bleach plant wherein the entering pulp is fed continuously in a bleaching line thereof and bleached with a plurality of bleaching agents comprising chlorine dioxide and ozone.
Chlorine is one of the bleaching agents most frequently used for bleaching pulp of cellulosic fibrous material, particularly chemical pulp. However, chlorine, which forms chlorinated organic compounds, is considered detrimental to the environmental so that there is increasing resistance to the use of chlorine. This has resulted in lower limit values for the emission of chlorinated organic substances, quantified as kg adsorbable organic halogen ( AOX ) per ton pulp. Until the turn of the century a level of 1.0-2.0 kg AOX per ton dry pulp will probably be accepted by the environment conservation authorities. Thereafter, and possibly earlier if technically feasible, one may calculate with levels of below 1.0 kg AOX per ton pulp.
A number of different methods have been proposed in order to fulfil the increasingly stringent requirements placed on the use of chlorine, such as reducing the lignin content in the pulp prior to chlorine bleaching by prolonged digestion and by carrying out an initial delignification with oxygen gas. Other methods include reducing the quantity of chlorine added or replacing chlorine by chlorine dioxide.
It is also known that, from the environmental aspect, ozone is greatly superior to the co~m~rcially used bleaching agents chlorine and chlorine dioxide for bleaching cellulosic pulp. However, in view of the high quality demands placed on the pulp, particularly brightness, purity and strength, it is presently impossible to entirely omit chlorine dioxide.
However, the commercial use of ozone has hitherto not been possible due to the high costs for producing ozone and of the considerably less selective nature of ozone when conventional methods of bleaching are used. By "low selectivity" is meant that, in addition to removing lignin, the ozone also noticeably breaks down the cellulose. The kappa number of a pulp is a measure of the lignin content, whereas its viscosity is a measure of the average chain length of the cellulose and an indication of its strength. For a bleached pulp of softwood with a brightness of 90 IS0 and good strength properties, the viscosity ought to be above 800 SCAN units (dm /kg).
When a pulp bleached with oxygen gas is further bleached with chlorine dioxide and/or chlorine, four or five separate stages are necessary for the bleached pulp to achieve a brightness which fulfils market demand, i.e. approximately 90 IS0. By "separate stages" it is meant that each of the stages is preceded and followed by a wash of the pulp. The equipment for washing the pulp is what requires the highest capital investment in a bleaching plant.
SummarY of the Invention The present invention aims to provide an improved method of bleaching pulp of cellulosic fibrous material which considerably reduces adsorbable organic halogen and which is more efficient with respect to the bleaching processes previously used.
A
~.~....
The invention also aims to provide an improved method of bleaching pulp of cellulosic fibrous material which reduces the use of chlorine and chlorine dioxide.
The invention further aims to provide an improved method of bleaching pulp of cellulosic fibrous material using a fewer number of bleaching stages in order to obtain a bleached pulp with brightness and strength common to the market.
The invention also aims to increase the selectivity of the delignification so that sufficiently high viscosity can be achieved after the ozone treatment to enable the pulp to be subjected to additional separate bleaching stages in order to achieve brightness common to the market.
The method of present invention concerns bleaching pulp of cellulosic fibrous material in the production of chemical pulp in a bleach plant wherein the entering pulp is fed continuously in a bleaching line thereof. The method comprises the step of bleaching the pulp with chlorine dioxide and ozone in one and the same stage, said stage being started with chlorine dioxide and followed by ozone, the chlorine dioxide and ozone being added at points separated from each other and each at at least one point in said bleaching line, said one and the same stage being free from intermediate washing between said points for the addition of chlorine dioxide and ozone In a preferred embodiment of the invention ozone is added at one or more points in said stage which can be defined as initial stage. A mixer is used at each such point.
It is suitable for the ozone to be added when from 10%
to 99% of the chlorine dioxide has reacted.
20~1850 The ozone is added with the aid of a carrier gas containing about 2-13 per cent by weight ozone. Preferably, the pulp is treated with ozone at a pressure in excess of 1 bar over-pressure. The sequence bleaching of pulp with chlorine dioxide and ozone is performed at a temperature of 25-70C, preferably 45-65C.
The treatments with chlorine dioxide and ozone are followed by one or two alkali extractions, at least one of which may be reinforced with oxygen gas and/or hydrogen peroxide, the first alkali extraction being performed either in sequence with the chlorine dioxide and ozone treatments without intermediate washing or in a separate stage preceded by washing.
The pulp is finally bleached with chlorine dioxide in one or more stages, usually two stages.
The supplied and treated pulp is preferably of medium consistency, about 6-15~, and suitably consists of oxygen-delignified pulp which has been digested continuously in accordance with a modified process designated MCC or in accordance with a standard process. The pulp may also be treated with ozone.
In preferred embodiments chlorine dioxide is added in the one and same stage in an amount of 5-60 kg/air dry metric ton (ADMT), particularly 10-30 kg/ADMT calculated as active chlorine.
Preferably the ozone is added in an amount of 1-2 kg/ADMT, particularly 3-10 kg/ADMT.
When alkali extraction is performed in a separate stage preceded by washing the chlorine dioxide is added for the ultimate bleaching in an amount of 10-50 kg/ADMT particularly 15-35 kg/ADMT
calculated as active chlorine.
20318S~
4a Brief DescriPtion of the Drawings The invention will be described further in the following, with reference to the drawings in which Figure 1 shows schematically a bleaching plant for carrying out one embodiment of the method according to the invention, and Figure 2 shows another bleaching plant for carrying out a second embodiment of the method according to the invention.
Description of Illustrated Embodiments The bleaching plant shown in Figure 1 comprises a supply tank 1, a flash tank 2, a first upstream bleaching tower 3 with diffuser washing equipment at the top, a downtube 4 and a second upstream bleaching tower 5 with diffuser washing equipment at the top. Pulp of medium consistency (about 6-15~) is fed from the supply tank 1 to the flash tank 2 through a pipe 6 and then from the flash tank 2 to the bleaching tower 3 through a pipe 7. The bleaching tower 3 is connected to the downtube 4 by a pipe 8 and the downtube being connected to the bottom of the bleaching tower 5 by a pipe 9 containing a suitable pump 10 arranged close to the downtube 4. The bleached pulp is removed from the top of the bleaching tower 5 through a pipe 11. Washing liquid is supplied to the top of the bleaching towers 3 and 5 through pipes 12 and 13, respectively. Carrier gas, such as oxygen gas, nitrogen gas or mixtures thereof, is removed from the top of the flash tank 2 through a pipe 14 and conducted to a suitable equipment for possible recovery.
The pulp is fed from the supply tank 1 through a discharger 15 disposed at the bottom of the tank and from the flash tank 2 through a similar discharger 16 disposed at the bottom of the tank. The dischargers 15, 16 are specially designed for discharging pulp of medium consistency. Furthermore, pipe 6 contains two mixers 17, 18, disposed one after the other, i.e. apparatus to mix treating agent homogeneously into the pulp passing therethrough. The mixer 17 is connected to the mixer 18 located downstream by means of a pipe portion 6a of predetermined length. A similar mixer 19 is disposed in the pipe 7 leading from the flash tank 2 to the ~irst bleaching tower 3. Each mixer 17, 18, 19 comprises 2~31~0 fluidizing means to ensure that the various treating agents are vigorously and homogeneously mixed into the pulp. Such an intensive mixer may advantageously comprise a "Kamyr MC mixer", i.e. a mixer specially designed to provide highly efficient mixing of treating agent into pulp of medium consistency.
Close to the outlet from the supply tank 1, if necessary, an acidification agent such as sulphuric acid is added via a pipe 20 in order to acidify the pulp to a value below pH 5, e.g. pH 2-4. Chlorine dioxide is supplied through a pipe 21 to the first mixer 17 with high mixing effect, whereas a carrier gas containing ozone is supplied through a pipe 22 to the following second mixer 18 with high mixing effect. An alkaline agent such as sodium hydroxide is added through a pipe 23 to the outlet from the flash tank 2 and oxygen gas may also be added at the same point through a pipe 24 if desired. Furthermore, a pipe 25 is connected to the third mixer 19 with high mixing effect for the supply of hydrogen peroxide, and another pipe 26 for the supply of steam under high pressure to the pulp. At the outlet from the downtube 4 the pump 10 is provided with a pipe 27 for the supply of chlorine dioxide. A reaction vessel 28 may suitably be disposed in the pipe portion 6b between the second mixer 22 and the flash tank, this vessel being so dimensioned that the pulp passing therethrough remains therein for about 0.1-10 minutes, preferably 0.5-5 minutes. If the pipe portion 6b is sufficiently long to allow the pulp to remain therein for the required time, the reaction vessel 28 may be omitted.
The pulp acidified to a pH value of 4, for instance, is homogeneously mixed in the first mixer 17 with chlorine dioxide and is then, after a relatively short period of time, mixed with carrier gas containing ozone. This brief period of time may be from 10 seconds up to a 7 2~31850 few or several minutes, e.g. 10 minutes. It is suitable for 10% up to 99% of the chlorine dioxide to have reacted with the pulp before ozone in gaseous form is added. With current technology for the production of ozone, the carrier gas may contain about 2-13 per cent by weight ozone, however, with better methods in the future it should be possible to increase the portion of ozone, e.g.
up to about 20 per cent by weight. The amount of ozone supplied can be controlled by the choice of a suitable gas pressure in that the higher the gas pressure is when it is mixed into the pulp, the more ozone can be mixed in. Since none of the carrier gas is consumed, this is removed through pipe 14 for possible recovery. After the flash tank 2 sodium hydroxide is added to neutralize the pulp and increase the pH value up to about 11-12. A small quantity of oxygen gas may possibly be added to oxidize the easily oxidized compounds in the pulp so that the hydrogen peroxide will be utilized more efficient. A
small quantity of the last-mentioned bleaching agent is added at the third mixer 19. High-pressure steam may also be added thereto in sufficient amount to increase the temperature of the pulp to about 60-90C. Until this point where the high-pressure steam is added, and particularly before the flash tank 2, the temperature of the pulp is set at a relatively low level of about 25-70C, preferably 45-65C. An alkali extraction, reinforced by oxygen gas and peroxide, occurs immediately at the various additions after the flash tank 2 and up to the washing equipment in the bleaching tower 3 and is thus interrupted at the top of the bleaching tower 3 by the pulp being washed with a washing liquid. The washed pulp is then bleached with chlorine dioxide in the bleaching tower 5, this compound being added in a predetermined amount at the pump 10 located upstream. The bleaching process described thus consists of a bleaching sequence with only two bleaching stages, with a washing stage between them carried , . . .
out in the top part of the bl~ching tower 3. The bleaching sequence can thus be illustrated as follows:
( DZEOP )D .
Figure 2 shows a modified bleaching plant which with respect to that according to Figure 1 has been supplemented for carrying out a washing stage between the ozone treatment and the alkali extraction. The parts and elements substantially equivalent in the two figures are denoted with the same reference numbers and are therefore not further explained. As can be seen in Figure 2, the discharger 16 of the flash tank 2 is provided with a pipe 29 connected to a washing apparatus 30 of diffuser type.
The pipe 29 contains a pump 31 to supply pulp of medium consistency to the washing apparatus 30, which is supplied with washing liquid via a pipe 32. The pulp is carried by a pipe 33 from the washing apparatus 30 to a downtube 34 connected to the bottom of the bleaching tower 3 by a pipe 7. The pipe 7 contains a suitable pump 35 disposed close to the downtube 34. Two pipes 23, 24 connected to the pump 35 supply this with an alkaline agent such as sodium hydroxide, and oxygen gas, respectively, if desired. The pipe 7 also includes a mixer 19 with high mixing effect. The mixer has a pipe 25 for the supply of hydrogen peroxide and a pipe 26 for the supply of steam to the pulp if desired. In the alternative embodiment of the bleaching process according to the invention, performed using the bleaching plant shown in Figure 2, a washing stage is thus introduced before the alkali addition. The bleaching sequence thus consists of three bleaching stages, i.e. (DZ)(EOP)D. This embodiment entails higher investment costs but the consumption of sodium hydroxide is lower with respect to the embodiment first described.
The following example illustrates the invention futher and shows its unexpected results in relation to two 9 20~1850 comparative tests which were performed under equivalent conditions.
Example Three tests of bleaching processes were performed by simulating on a laboratory scale a bleaching plant substantially in accordance with Figure 1, but supplemented for treatment with chlorine dioxide in a further final bleaching stage. In test 2, however, no chlorine dioxide was added at the first mixer (which was bypassed). In test 3 the first chlorine dioxide treatment was performed as a conventional prebleaching stage.
A standard sulphate pulp of softwood was used for the tests, the pulp having been bleached with oxygen gas and washed in conventional manner. The sulphate pulp fed into the supply tank 1 had a kappa number of 18, a viscosity of 1020 dm3/kg and a consistency of 10%. This consistency was maintained throughout the bleaching process in all three tests.
Three tests were carried out according to the folloing bleaching sequences.
Test 1 Test 2 Test 3 Bleaching sequence (DZEOP) DD ( ZEOP) DD D ( EOP ) DD
Number of bleaching stages 3 3 4 The first bleaching stage with chlorine dioxide (D) in test 3 was performed, as stated, in conventional manner at a pH of 3-4, a temperature of 60C and for a period of 60 minutes, followed by a washing stage and a bleaching stage with alkali extraction reinforced by oxygen gas and peroxide (EOP). The sulphate pulp had the same ~0~1850 consistency (10%) as in the other two tests as mentioned above.
In the ozone treatments according to test 1 and test 2, the temperature of the sulphate pulp fed from the supply tank was in both cases 28C. In test 1 the pulp was vigorously mixed with chlorine dioxide in the first stage (DZEOP) in accordance with the present invention. The carrier gas cont~;n;ng ozone was supplied at a pressure of 4.8 bar. The pH value of the sulphate pulp was between 3.5 and 4 and was controlled by the addition of sulphuric acid at the outlet from supply tank 1. The ozone treatment took place for a period of 5 minutes and was interrupted by the addition of alkali when substantially all ozone had had time to react.
The alkali extraction reinforced by oxygen gas and peroxide was performed in all three tests at a temperature of 70C and for a period of 60 minutes, during which time the pressure was reduced from 3 bar to atmospheric pressure. The alkali extraction was interrupted by supplying washing liquid to the upper part of the bleaching tower 3, and then two bleaching stages using chlorine dioxide were performed at the previously set temperature, 70C, for a period of 180 minutes in each case and the same for the three tests.
Dosage of the various chemicals, final pH value at the alkali extractions and the partial and final results of the three tests can be seen in the following table. In the table and in the rest of the description the initials ADMT signify "Air Dry Metric Ton", i.e. ton of air dry pulp, and AOX "Adsorbable Organic Halogens".
.. ,, . , , .. ,, , . . . . , , . . ~ .
TABLE
Test 1 Test 2 Test 3 Bleaching sequence (DZEOP)DD (ZEOP)DD D(EOP)DD
. Stage 1 Stage 1 _ _ _ _ C102, kg/ADMT 20 - 40 Stage 1 0.3, kg/ADMT 10 15 - Stage 2 NaOH, kg/ADMT 35 20 20 2, kg/dm 3 3 3 H202, kg/dm 3 Kappa number 1.9 4.1 2.5 Viscosity, dm3/kg 810 720 980 Stage 2 Stage 2 Stage 3 C102, kg/ADMT 20 45 35 Stage 3 Stage 3 Stage 4 C102, kg/ADMT 10 10 10 Brightness, %ISO89.7 88.9 89.7 Viscosity, dm3/kg810 720 930 Total C102, kg/ADMT 50 55 85 Total AOX, kg/ADMTca 0.1 - 1.5 .. . ..
12 2û31850 It is clear from the table that a 4-stage bleaching sequence D(EOP)DD according to test 3 requires an addition of chlorine dioxide, calculated as active chlorine, in an amount of 85 kg/ADMT pulp in order to achieve a brightness of about 90 ISO. The amount of AOX
in the outlet from the bleaGhing stage was 1.5 kg/ADMT.
It is also clear that a 3-stage bleaching sequence (ZEOP)DD according to test 2, with an initial ozone treatment in the first stage and a total addition of chlorine dioxide, calculated as active chlorine, of 55 kg/ADMT pulp, gives an even lower brightness (88.9 ISO), and in particular, the viscosity is at an unacceptably low level, resulting in poor strength properties. Owing to these low values it was of no interest to measure AOX.
Finally the table shows that a 3-stage bleaching sequence (DZEOP)DD according to test 1 (the invention), with a subsequent ozone treatment in the first stage in ; mm~; ate conjunction with the continuous addition of chlorine dioxide, surprisingly results in a sufficiently high brightness level and a viscosity within the acceptable limit (over 800 dm3/kg). The selectivity of the delignification has thus been improved. It is also noted that test 1 results in a surprisingly low quantity of AOX, about 0.1 kg/ADMT. The reasons for this extremely favourable result from the environmental aspect have not been fully understood but a probable explanation might be that the ozone breaks down a larger proportion of the amount of AOX initially formed in the chlorine dioxide treatment.
In the example described above an oxygen-delignified sulphate pulp of softwood was used, which had been continuously digested in accordance with a standard method and which thus required two separate chlorine dioxide stages. For an oxygen-delignified sulphate pulp 13 ~03185~
of softwood which has been continuously digested in accordance with a modified process designated MCC
(Modified Continuous Cooking), and which has a kappa number of 12-14 and a viscosity of 1000-1100 dm3/kg, it is usually sufficient to perform only a final bleaching stage with chlorine dioxide in order to achieve brightness common to the market. Pulp prepared from hardwood is also usable. The invention can of course also be applied to sulphate or sulphite pulps which have not been oxygen-delignified.
METHOD OF BLEACHING PULP
Field and Background of the Invention The present invention relates to a method of bleaching pulp of cellulosic fibrous material in the production of chemical pulp in a bleach plant wherein the entering pulp is fed continuously in a bleaching line thereof and bleached with a plurality of bleaching agents comprising chlorine dioxide and ozone.
Chlorine is one of the bleaching agents most frequently used for bleaching pulp of cellulosic fibrous material, particularly chemical pulp. However, chlorine, which forms chlorinated organic compounds, is considered detrimental to the environmental so that there is increasing resistance to the use of chlorine. This has resulted in lower limit values for the emission of chlorinated organic substances, quantified as kg adsorbable organic halogen ( AOX ) per ton pulp. Until the turn of the century a level of 1.0-2.0 kg AOX per ton dry pulp will probably be accepted by the environment conservation authorities. Thereafter, and possibly earlier if technically feasible, one may calculate with levels of below 1.0 kg AOX per ton pulp.
A number of different methods have been proposed in order to fulfil the increasingly stringent requirements placed on the use of chlorine, such as reducing the lignin content in the pulp prior to chlorine bleaching by prolonged digestion and by carrying out an initial delignification with oxygen gas. Other methods include reducing the quantity of chlorine added or replacing chlorine by chlorine dioxide.
It is also known that, from the environmental aspect, ozone is greatly superior to the co~m~rcially used bleaching agents chlorine and chlorine dioxide for bleaching cellulosic pulp. However, in view of the high quality demands placed on the pulp, particularly brightness, purity and strength, it is presently impossible to entirely omit chlorine dioxide.
However, the commercial use of ozone has hitherto not been possible due to the high costs for producing ozone and of the considerably less selective nature of ozone when conventional methods of bleaching are used. By "low selectivity" is meant that, in addition to removing lignin, the ozone also noticeably breaks down the cellulose. The kappa number of a pulp is a measure of the lignin content, whereas its viscosity is a measure of the average chain length of the cellulose and an indication of its strength. For a bleached pulp of softwood with a brightness of 90 IS0 and good strength properties, the viscosity ought to be above 800 SCAN units (dm /kg).
When a pulp bleached with oxygen gas is further bleached with chlorine dioxide and/or chlorine, four or five separate stages are necessary for the bleached pulp to achieve a brightness which fulfils market demand, i.e. approximately 90 IS0. By "separate stages" it is meant that each of the stages is preceded and followed by a wash of the pulp. The equipment for washing the pulp is what requires the highest capital investment in a bleaching plant.
SummarY of the Invention The present invention aims to provide an improved method of bleaching pulp of cellulosic fibrous material which considerably reduces adsorbable organic halogen and which is more efficient with respect to the bleaching processes previously used.
A
~.~....
The invention also aims to provide an improved method of bleaching pulp of cellulosic fibrous material which reduces the use of chlorine and chlorine dioxide.
The invention further aims to provide an improved method of bleaching pulp of cellulosic fibrous material using a fewer number of bleaching stages in order to obtain a bleached pulp with brightness and strength common to the market.
The invention also aims to increase the selectivity of the delignification so that sufficiently high viscosity can be achieved after the ozone treatment to enable the pulp to be subjected to additional separate bleaching stages in order to achieve brightness common to the market.
The method of present invention concerns bleaching pulp of cellulosic fibrous material in the production of chemical pulp in a bleach plant wherein the entering pulp is fed continuously in a bleaching line thereof. The method comprises the step of bleaching the pulp with chlorine dioxide and ozone in one and the same stage, said stage being started with chlorine dioxide and followed by ozone, the chlorine dioxide and ozone being added at points separated from each other and each at at least one point in said bleaching line, said one and the same stage being free from intermediate washing between said points for the addition of chlorine dioxide and ozone In a preferred embodiment of the invention ozone is added at one or more points in said stage which can be defined as initial stage. A mixer is used at each such point.
It is suitable for the ozone to be added when from 10%
to 99% of the chlorine dioxide has reacted.
20~1850 The ozone is added with the aid of a carrier gas containing about 2-13 per cent by weight ozone. Preferably, the pulp is treated with ozone at a pressure in excess of 1 bar over-pressure. The sequence bleaching of pulp with chlorine dioxide and ozone is performed at a temperature of 25-70C, preferably 45-65C.
The treatments with chlorine dioxide and ozone are followed by one or two alkali extractions, at least one of which may be reinforced with oxygen gas and/or hydrogen peroxide, the first alkali extraction being performed either in sequence with the chlorine dioxide and ozone treatments without intermediate washing or in a separate stage preceded by washing.
The pulp is finally bleached with chlorine dioxide in one or more stages, usually two stages.
The supplied and treated pulp is preferably of medium consistency, about 6-15~, and suitably consists of oxygen-delignified pulp which has been digested continuously in accordance with a modified process designated MCC or in accordance with a standard process. The pulp may also be treated with ozone.
In preferred embodiments chlorine dioxide is added in the one and same stage in an amount of 5-60 kg/air dry metric ton (ADMT), particularly 10-30 kg/ADMT calculated as active chlorine.
Preferably the ozone is added in an amount of 1-2 kg/ADMT, particularly 3-10 kg/ADMT.
When alkali extraction is performed in a separate stage preceded by washing the chlorine dioxide is added for the ultimate bleaching in an amount of 10-50 kg/ADMT particularly 15-35 kg/ADMT
calculated as active chlorine.
20318S~
4a Brief DescriPtion of the Drawings The invention will be described further in the following, with reference to the drawings in which Figure 1 shows schematically a bleaching plant for carrying out one embodiment of the method according to the invention, and Figure 2 shows another bleaching plant for carrying out a second embodiment of the method according to the invention.
Description of Illustrated Embodiments The bleaching plant shown in Figure 1 comprises a supply tank 1, a flash tank 2, a first upstream bleaching tower 3 with diffuser washing equipment at the top, a downtube 4 and a second upstream bleaching tower 5 with diffuser washing equipment at the top. Pulp of medium consistency (about 6-15~) is fed from the supply tank 1 to the flash tank 2 through a pipe 6 and then from the flash tank 2 to the bleaching tower 3 through a pipe 7. The bleaching tower 3 is connected to the downtube 4 by a pipe 8 and the downtube being connected to the bottom of the bleaching tower 5 by a pipe 9 containing a suitable pump 10 arranged close to the downtube 4. The bleached pulp is removed from the top of the bleaching tower 5 through a pipe 11. Washing liquid is supplied to the top of the bleaching towers 3 and 5 through pipes 12 and 13, respectively. Carrier gas, such as oxygen gas, nitrogen gas or mixtures thereof, is removed from the top of the flash tank 2 through a pipe 14 and conducted to a suitable equipment for possible recovery.
The pulp is fed from the supply tank 1 through a discharger 15 disposed at the bottom of the tank and from the flash tank 2 through a similar discharger 16 disposed at the bottom of the tank. The dischargers 15, 16 are specially designed for discharging pulp of medium consistency. Furthermore, pipe 6 contains two mixers 17, 18, disposed one after the other, i.e. apparatus to mix treating agent homogeneously into the pulp passing therethrough. The mixer 17 is connected to the mixer 18 located downstream by means of a pipe portion 6a of predetermined length. A similar mixer 19 is disposed in the pipe 7 leading from the flash tank 2 to the ~irst bleaching tower 3. Each mixer 17, 18, 19 comprises 2~31~0 fluidizing means to ensure that the various treating agents are vigorously and homogeneously mixed into the pulp. Such an intensive mixer may advantageously comprise a "Kamyr MC mixer", i.e. a mixer specially designed to provide highly efficient mixing of treating agent into pulp of medium consistency.
Close to the outlet from the supply tank 1, if necessary, an acidification agent such as sulphuric acid is added via a pipe 20 in order to acidify the pulp to a value below pH 5, e.g. pH 2-4. Chlorine dioxide is supplied through a pipe 21 to the first mixer 17 with high mixing effect, whereas a carrier gas containing ozone is supplied through a pipe 22 to the following second mixer 18 with high mixing effect. An alkaline agent such as sodium hydroxide is added through a pipe 23 to the outlet from the flash tank 2 and oxygen gas may also be added at the same point through a pipe 24 if desired. Furthermore, a pipe 25 is connected to the third mixer 19 with high mixing effect for the supply of hydrogen peroxide, and another pipe 26 for the supply of steam under high pressure to the pulp. At the outlet from the downtube 4 the pump 10 is provided with a pipe 27 for the supply of chlorine dioxide. A reaction vessel 28 may suitably be disposed in the pipe portion 6b between the second mixer 22 and the flash tank, this vessel being so dimensioned that the pulp passing therethrough remains therein for about 0.1-10 minutes, preferably 0.5-5 minutes. If the pipe portion 6b is sufficiently long to allow the pulp to remain therein for the required time, the reaction vessel 28 may be omitted.
The pulp acidified to a pH value of 4, for instance, is homogeneously mixed in the first mixer 17 with chlorine dioxide and is then, after a relatively short period of time, mixed with carrier gas containing ozone. This brief period of time may be from 10 seconds up to a 7 2~31850 few or several minutes, e.g. 10 minutes. It is suitable for 10% up to 99% of the chlorine dioxide to have reacted with the pulp before ozone in gaseous form is added. With current technology for the production of ozone, the carrier gas may contain about 2-13 per cent by weight ozone, however, with better methods in the future it should be possible to increase the portion of ozone, e.g.
up to about 20 per cent by weight. The amount of ozone supplied can be controlled by the choice of a suitable gas pressure in that the higher the gas pressure is when it is mixed into the pulp, the more ozone can be mixed in. Since none of the carrier gas is consumed, this is removed through pipe 14 for possible recovery. After the flash tank 2 sodium hydroxide is added to neutralize the pulp and increase the pH value up to about 11-12. A small quantity of oxygen gas may possibly be added to oxidize the easily oxidized compounds in the pulp so that the hydrogen peroxide will be utilized more efficient. A
small quantity of the last-mentioned bleaching agent is added at the third mixer 19. High-pressure steam may also be added thereto in sufficient amount to increase the temperature of the pulp to about 60-90C. Until this point where the high-pressure steam is added, and particularly before the flash tank 2, the temperature of the pulp is set at a relatively low level of about 25-70C, preferably 45-65C. An alkali extraction, reinforced by oxygen gas and peroxide, occurs immediately at the various additions after the flash tank 2 and up to the washing equipment in the bleaching tower 3 and is thus interrupted at the top of the bleaching tower 3 by the pulp being washed with a washing liquid. The washed pulp is then bleached with chlorine dioxide in the bleaching tower 5, this compound being added in a predetermined amount at the pump 10 located upstream. The bleaching process described thus consists of a bleaching sequence with only two bleaching stages, with a washing stage between them carried , . . .
out in the top part of the bl~ching tower 3. The bleaching sequence can thus be illustrated as follows:
( DZEOP )D .
Figure 2 shows a modified bleaching plant which with respect to that according to Figure 1 has been supplemented for carrying out a washing stage between the ozone treatment and the alkali extraction. The parts and elements substantially equivalent in the two figures are denoted with the same reference numbers and are therefore not further explained. As can be seen in Figure 2, the discharger 16 of the flash tank 2 is provided with a pipe 29 connected to a washing apparatus 30 of diffuser type.
The pipe 29 contains a pump 31 to supply pulp of medium consistency to the washing apparatus 30, which is supplied with washing liquid via a pipe 32. The pulp is carried by a pipe 33 from the washing apparatus 30 to a downtube 34 connected to the bottom of the bleaching tower 3 by a pipe 7. The pipe 7 contains a suitable pump 35 disposed close to the downtube 34. Two pipes 23, 24 connected to the pump 35 supply this with an alkaline agent such as sodium hydroxide, and oxygen gas, respectively, if desired. The pipe 7 also includes a mixer 19 with high mixing effect. The mixer has a pipe 25 for the supply of hydrogen peroxide and a pipe 26 for the supply of steam to the pulp if desired. In the alternative embodiment of the bleaching process according to the invention, performed using the bleaching plant shown in Figure 2, a washing stage is thus introduced before the alkali addition. The bleaching sequence thus consists of three bleaching stages, i.e. (DZ)(EOP)D. This embodiment entails higher investment costs but the consumption of sodium hydroxide is lower with respect to the embodiment first described.
The following example illustrates the invention futher and shows its unexpected results in relation to two 9 20~1850 comparative tests which were performed under equivalent conditions.
Example Three tests of bleaching processes were performed by simulating on a laboratory scale a bleaching plant substantially in accordance with Figure 1, but supplemented for treatment with chlorine dioxide in a further final bleaching stage. In test 2, however, no chlorine dioxide was added at the first mixer (which was bypassed). In test 3 the first chlorine dioxide treatment was performed as a conventional prebleaching stage.
A standard sulphate pulp of softwood was used for the tests, the pulp having been bleached with oxygen gas and washed in conventional manner. The sulphate pulp fed into the supply tank 1 had a kappa number of 18, a viscosity of 1020 dm3/kg and a consistency of 10%. This consistency was maintained throughout the bleaching process in all three tests.
Three tests were carried out according to the folloing bleaching sequences.
Test 1 Test 2 Test 3 Bleaching sequence (DZEOP) DD ( ZEOP) DD D ( EOP ) DD
Number of bleaching stages 3 3 4 The first bleaching stage with chlorine dioxide (D) in test 3 was performed, as stated, in conventional manner at a pH of 3-4, a temperature of 60C and for a period of 60 minutes, followed by a washing stage and a bleaching stage with alkali extraction reinforced by oxygen gas and peroxide (EOP). The sulphate pulp had the same ~0~1850 consistency (10%) as in the other two tests as mentioned above.
In the ozone treatments according to test 1 and test 2, the temperature of the sulphate pulp fed from the supply tank was in both cases 28C. In test 1 the pulp was vigorously mixed with chlorine dioxide in the first stage (DZEOP) in accordance with the present invention. The carrier gas cont~;n;ng ozone was supplied at a pressure of 4.8 bar. The pH value of the sulphate pulp was between 3.5 and 4 and was controlled by the addition of sulphuric acid at the outlet from supply tank 1. The ozone treatment took place for a period of 5 minutes and was interrupted by the addition of alkali when substantially all ozone had had time to react.
The alkali extraction reinforced by oxygen gas and peroxide was performed in all three tests at a temperature of 70C and for a period of 60 minutes, during which time the pressure was reduced from 3 bar to atmospheric pressure. The alkali extraction was interrupted by supplying washing liquid to the upper part of the bleaching tower 3, and then two bleaching stages using chlorine dioxide were performed at the previously set temperature, 70C, for a period of 180 minutes in each case and the same for the three tests.
Dosage of the various chemicals, final pH value at the alkali extractions and the partial and final results of the three tests can be seen in the following table. In the table and in the rest of the description the initials ADMT signify "Air Dry Metric Ton", i.e. ton of air dry pulp, and AOX "Adsorbable Organic Halogens".
.. ,, . , , .. ,, , . . . . , , . . ~ .
TABLE
Test 1 Test 2 Test 3 Bleaching sequence (DZEOP)DD (ZEOP)DD D(EOP)DD
. Stage 1 Stage 1 _ _ _ _ C102, kg/ADMT 20 - 40 Stage 1 0.3, kg/ADMT 10 15 - Stage 2 NaOH, kg/ADMT 35 20 20 2, kg/dm 3 3 3 H202, kg/dm 3 Kappa number 1.9 4.1 2.5 Viscosity, dm3/kg 810 720 980 Stage 2 Stage 2 Stage 3 C102, kg/ADMT 20 45 35 Stage 3 Stage 3 Stage 4 C102, kg/ADMT 10 10 10 Brightness, %ISO89.7 88.9 89.7 Viscosity, dm3/kg810 720 930 Total C102, kg/ADMT 50 55 85 Total AOX, kg/ADMTca 0.1 - 1.5 .. . ..
12 2û31850 It is clear from the table that a 4-stage bleaching sequence D(EOP)DD according to test 3 requires an addition of chlorine dioxide, calculated as active chlorine, in an amount of 85 kg/ADMT pulp in order to achieve a brightness of about 90 ISO. The amount of AOX
in the outlet from the bleaGhing stage was 1.5 kg/ADMT.
It is also clear that a 3-stage bleaching sequence (ZEOP)DD according to test 2, with an initial ozone treatment in the first stage and a total addition of chlorine dioxide, calculated as active chlorine, of 55 kg/ADMT pulp, gives an even lower brightness (88.9 ISO), and in particular, the viscosity is at an unacceptably low level, resulting in poor strength properties. Owing to these low values it was of no interest to measure AOX.
Finally the table shows that a 3-stage bleaching sequence (DZEOP)DD according to test 1 (the invention), with a subsequent ozone treatment in the first stage in ; mm~; ate conjunction with the continuous addition of chlorine dioxide, surprisingly results in a sufficiently high brightness level and a viscosity within the acceptable limit (over 800 dm3/kg). The selectivity of the delignification has thus been improved. It is also noted that test 1 results in a surprisingly low quantity of AOX, about 0.1 kg/ADMT. The reasons for this extremely favourable result from the environmental aspect have not been fully understood but a probable explanation might be that the ozone breaks down a larger proportion of the amount of AOX initially formed in the chlorine dioxide treatment.
In the example described above an oxygen-delignified sulphate pulp of softwood was used, which had been continuously digested in accordance with a standard method and which thus required two separate chlorine dioxide stages. For an oxygen-delignified sulphate pulp 13 ~03185~
of softwood which has been continuously digested in accordance with a modified process designated MCC
(Modified Continuous Cooking), and which has a kappa number of 12-14 and a viscosity of 1000-1100 dm3/kg, it is usually sufficient to perform only a final bleaching stage with chlorine dioxide in order to achieve brightness common to the market. Pulp prepared from hardwood is also usable. The invention can of course also be applied to sulphate or sulphite pulps which have not been oxygen-delignified.
Claims (20)
1. A method of bleaching pulp of cellulosic fibrous material in the production of chemical pulp in a bleach plant wherein the entering pulp is fed continuously in a bleaching line thereof, comprising the step of bleaching the pulp with chlorine dioxide and ozone in one and the same stage, said stage being started with chlorine dioxide and followed by ozone, the chlorine dioxide and ozone being added at points separated from each other and each at at least one point in said bleaching line, said one and the same stage being free from intermediate washing between said points for the addition of chlorine dioxide and ozone.
2. A method as recited in claim 1 wherein ozone is added at two or more points in said bleaching line in said one and the same stage.
3. A method as recited in claim 1 wherein ozone is added after the last one of two or more separate points for the addition of chlorine dioxide.
4. A method as recited in claim 1 wherein the ozone is added when from 10% to 99% of the chlorine dioxide has reacted.
5. A method as recited in claim 1 wherein the ozone is added by means of a carrier gas containing about 2-13 per cent by weight ozone.
6. A method as recited in claim 1 wherein the pulp is treated with ozone at a pressure in excess of 1 bar overpressure.
7. A method as recited in claim 1 wherein said consecutive bleaching with chlorine dioxide and ozone is performed at a temperature of 25-70°C.
8. A method as recited in claim 1 wherein said consecutive bleaching with chlorine dioxide and ozone is performed at a temperature of 45-65°C.
9. A method as recited in claim 1 wherein said treatments with chlorine dioxide and ozone are followed by one or two alkali extractions, at least one of which is reinforced with oxygen gas or hydrogen peroxide.
10. A method as recited in claim 9 wherein said alkali extraction is performed in sequence with said chlorine dioxide and ozone treatments without intermediate washing.
11. A method as recited in claim 9 wherein said alkali extraction is performed in a separate stage preceded by washing.
12. A method as recited in claim 1 wherein the pulp is ultimately bleached with chlorine dioxide in one or more stages.
13. A method as recited in claim 1 wherein the entering pulp consists of oxygen-delignified pulp which has been cooked continuously in accordance with a modified process designated MCC
or in accordance with a standard process.
or in accordance with a standard process.
14. A method as recited in claim 1 wherein the entering and treated pulp is of medium consistency, about 6-15%.
15. A method as recited in claim 1 wherein chlorine dioxide is added in said one and the same stage in an amount of 5-60 kg/ADMT calculated as active chlorine.
16. A method as recited in claim 1 wherein chlorine dioxide is added in said one and the same stage in an amount of 10-30 kg/ADMT calculated as active chlorine.
17. A method as recited in claim 1 wherein ozone is added in an amount of 1-20 kg/ADMT.
18. A method as recited in claim 1 wherein ozone is added in an amount of 3-10 kg/ADMT.
19. A method as recited in claim 12 wherein chlorine dioxide is added for the ultimate bleaching in an amount of 10-50 kg/ADMT
calculated as active chlorine.
calculated as active chlorine.
20. A method as recited in claim 12 wherein chlorine dioxide is added for the ultimate bleaching in an amount of 15-35 kg/ADMT
calculated as active chlorine.
calculated as active chlorine.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| SE8904404-4 | 1989-12-29 | ||
| SE8904404A SE467261B (en) | 1989-12-29 | 1989-12-29 | WHITING CELLULOSAMASSA WITH CHLORIDE Dioxide AND OZONE IN ONE AND SAME STEP |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| CA2031850A1 CA2031850A1 (en) | 1991-06-30 |
| CA2031850C true CA2031850C (en) | 1995-11-21 |
Family
ID=20377905
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA002031850A Expired - Lifetime CA2031850C (en) | 1989-12-29 | 1990-12-10 | Method of bleaching pulp |
Country Status (9)
| Country | Link |
|---|---|
| JP (1) | JP2934512B2 (en) |
| AT (1) | AT395444B (en) |
| BR (1) | BR9006579A (en) |
| CA (1) | CA2031850C (en) |
| DE (1) | DE4039294C2 (en) |
| FI (1) | FI95292C (en) |
| FR (1) | FR2656633B1 (en) |
| NO (1) | NO176060C (en) |
| SE (1) | SE467261B (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN110656527A (en) * | 2019-09-12 | 2020-01-07 | 华南理工大学 | Method and device for efficiently bleaching medium-consistency paper pulp by using ozone and chlorine dioxide in synergy mode |
Families Citing this family (5)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US5322598A (en) * | 1990-02-06 | 1994-06-21 | Olin Corporation | Chlorine dioxide generation using inert load of sodium perchlorate |
| US6174409B1 (en) | 1997-09-19 | 2001-01-16 | American Air Liquide Inc. | Method to improve final bleached pulp strength properties by adjusting the CI02:03 ration within a single (D/Z) stage of the bleaching process |
| SE522593C2 (en) * | 1999-07-06 | 2004-02-24 | Kvaerner Pulping Tech | Oxygen gas delignification system and method of pulp of lignocellulosic material |
| GB0322598D0 (en) | 2003-09-26 | 2003-10-29 | Victrex Mfg Ltd | Polymeric material |
| JP5987223B2 (en) * | 2012-03-06 | 2016-09-07 | 愛媛県 | Method for producing cellulose solution, method for producing cellulose, method for saccharification of cellulose |
Family Cites Families (5)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| DE1298875B (en) * | 1965-01-09 | 1969-07-03 | Gunnar Alexander Dipl Ing | Process for bleaching cellulose in a chlorine bleach bath and with ozone |
| US4080249A (en) * | 1976-06-02 | 1978-03-21 | International Paper Company | Delignification and bleaching of a lignocellulosic pulp slurry with ozone |
| US4902381A (en) * | 1988-12-09 | 1990-02-20 | Kamyr, Inc. | Method of bleaching pulp with ozone-chlorine mixtures |
| US4959124A (en) * | 1989-05-05 | 1990-09-25 | International Paper Company | Method of bleaching kraft pulp in a DZED sequence |
| FI89516B (en) * | 1989-05-10 | 1993-06-30 | Ahlstroem Oy | Foerfarande Foer blekning av cellulosamassa med Otson |
-
1989
- 1989-12-29 SE SE8904404A patent/SE467261B/en unknown
-
1990
- 1990-12-09 DE DE4039294A patent/DE4039294C2/en not_active Expired - Fee Related
- 1990-12-10 CA CA002031850A patent/CA2031850C/en not_active Expired - Lifetime
- 1990-12-12 AT AT0251890A patent/AT395444B/en not_active IP Right Cessation
- 1990-12-26 BR BR909006579A patent/BR9006579A/en not_active IP Right Cessation
- 1990-12-27 NO NO905598A patent/NO176060C/en unknown
- 1990-12-27 FI FI906401A patent/FI95292C/en active IP Right Grant
- 1990-12-28 FR FR9016499A patent/FR2656633B1/en not_active Expired - Fee Related
- 1990-12-28 JP JP2415578A patent/JP2934512B2/en not_active Expired - Fee Related
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN110656527A (en) * | 2019-09-12 | 2020-01-07 | 华南理工大学 | Method and device for efficiently bleaching medium-consistency paper pulp by using ozone and chlorine dioxide in synergy mode |
Also Published As
| Publication number | Publication date |
|---|---|
| FR2656633A1 (en) | 1991-07-05 |
| FR2656633B1 (en) | 1996-06-07 |
| NO905598D0 (en) | 1990-12-27 |
| SE467261B (en) | 1992-06-22 |
| NO176060B (en) | 1994-10-17 |
| AT395444B (en) | 1992-12-28 |
| SE8904404L (en) | 1991-06-30 |
| JPH04263687A (en) | 1992-09-18 |
| DE4039294C2 (en) | 1993-11-11 |
| SE8904404D0 (en) | 1989-12-29 |
| NO176060C (en) | 1995-01-25 |
| NO905598L (en) | 1991-07-01 |
| FI906401A0 (en) | 1990-12-27 |
| CA2031850A1 (en) | 1991-06-30 |
| BR9006579A (en) | 1991-10-01 |
| FI95292C (en) | 1996-01-10 |
| ATA251890A (en) | 1992-05-15 |
| FI906401A (en) | 1991-06-30 |
| JP2934512B2 (en) | 1999-08-16 |
| DE4039294A1 (en) | 1991-07-04 |
| FI95292B (en) | 1995-09-29 |
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| EEER | Examination request | ||
| MKLA | Lapsed | ||
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Effective date: 20121202 |