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
  • D21C9/1026—Other features in bleaching processes
  • D21C9/1042—Use of chelating agents
• • 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
  • D21C9/1026—Other features in bleaching processes
  • D21C9/1036—Use of compounds accelerating or improving the efficiency of the processes
• • 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
  • D21C9/16—Bleaching ; Apparatus therefor with per compounds
  • D21C9/163—Bleaching ; Apparatus therefor with per compounds with peroxides

Definitions

• the invention relates to a method of delignifying/- bleaching lignin-containing celllulose pulp.
• the cellulose pulp is produced from any known lignocellulosic material with the use of any known alkaline or acid pulping process. Also included are digestion processes that include an alkaline or neutral stage and an acid stage.
• a wide variety of ligno ⁇ cellulosic materials are available in varying quantities worldwide.
• a very common lignocellulosic material is wood, which is normally chopped into chips prior to the digestion or cooking process. Examples of different wood types are softwoods and hardwoods.
• alkaline digestion processes are the sulfate process, the polysulfide process and processes of the soda type (sodium hydroxide) in which catalysts are used, for instance some quinone compound.
• the term sulfate process includes, e.g., the use of high sulfidity, the use of counterflow cooking in which white liquor is also supplied during an advanced stage of the cook, and use of a chemical treatment of the lignocellulosic material prior to the actual sulfate cook.
• the sulfite method is an example of a predominant acid digestion process.
• the sulfite method can be divided into a single stage process, in which the pH is relatively low, and a two stage process, in which the first stage is essentially neutral and is followed by a second strongly acid stage.
• bleaching agents as oxygen gas (0), some peroxide compound (P), such as hydrogen peroxide and ozone (Z), has been proposed in recent times with regard to bleaching sulfate pulp, for instance.
• P peroxide compound
• Z hydrogen peroxide and ozone
• Other stage sequences such as 0-Z-P for instance, have also been proposed and used commercially.
• washing liquors waste liquors
• extraction (E) stages are discharged either directly to the recipient or possibly to some external cleansing measure.
• manganese is a natural constituent of the starting material, i.e. of the lignocellulosic material, e.g. in the form of wood.
• the process water used will normally contain manganese and manganese can also enter the system by the way of apparatuses used in the pulp manufacturing chain.
• a complex forming stage (Q) has been introduced into the pulp processing chain, preferably immediately prior to the peroxide bleaching stage.
• a complexing agent of the type EDTA, DTPA and NTA, among others, that is introduced at a suitable pH-value will capture any free manganese ions that are present and, above all, will convert the manganese from a bound form in the pulp to a water soluble complex bound form.
• Manganese complexes of the type Mn(EDTA) 2" or Mn(DTPA) 3" then form.
• the aforedescribed bleaching sequences that include four treatment stages will normally only result in partial bleaching of the cellulose pulp. Normally, at least one further bleaching stage is required in order to achieve full bleaching of the cellulose pulp, i.e. bleaching to a brightness of up to 90% ISO and thereabove.
• the bleaching agent in this position may be chlorine dioxide (D) or peroxide (P). In this latter case, the bleaching stage may be preceded by a complex forming stage.
• ozone (Z) in two or more bleaching stages has been proposed, in similarity with peroxide (P).
• Chlorine dioxide (D) may also be used in, for instance, double stages at the end of the bleaching sequence.
• Said patent specification recommends a pH-value above 10 during the peroxide bleaching stage.
• the Canadian Patent Specification 1 129 161 instead recommends maintaining the pH-value within the range of 1 to 7 in a similar peroxide bleaching of cellulose pulp.
• a metal ion chosen from the group tungsten, molybdenum, chromium, osmium and selenium in an amount corresponding to 0.5 to 20 mole percent calculated on moles of peroxide.
• the peroxide addition shall be 0.1-5 percent by weight, calculated on bone dry pulp.
• the present invention provides a solution to the aforesaid problem and relates to a method of delignifying/ bleaching a non-delignified/unbleached or predelignified/- prebleached lignin-containing cellulose pulp, characterized in that the cellulose pulp in one and the same stage is treated with a) complexing agent (Q); b) molybdenum-containing substance (Mo); c) a chlorine-free oxidative bleaching agent, preferably a peroxide compound (P); and in that this treatment is followed by at least one bleaching stage in which a chlorine-free oxidative bleaching agent, preferably a peroxide compound (P), is used.
• Q complexing agent
• Mo molybdenum-containing substance
• P a chlorine-free oxidative bleaching agent
• P peroxide compound
• the invention can be applied in both open and closed bleach plants.
• open bleach plants is meant that waste liquors deriving from respective treatment or bleaching stages are discharged directly to the recipient, or are passed to and collected in some pool for external cleansing of the formed optionally total volume of waste liquor.
• closed bleach pl-ants is meant that all treatment and bleaching waste liquors are handled and preferably passed in counterflow so that the waste liquor will be finally mixed with the digestion liquor, wherein the common waste liquor is evaporated and burned in a soda recovery unit. It is very much preferred to apply the invention in a closed liquid system.
• the pH of an open system for instance may be as low as 2, although it is preferred that the pH will lie within the range of 4-6.
• the treatment of the cellulose pulp in this stage has its optimum somewhere at or in the proximity of pH 5.
• tempera ⁇ ture range that cellulose pulp can be treated in this stage at both atmospheric pressure and at elevated pressure.
• a pressure range of 0.1-0.4 MPa is suitable when opting for pressurized treatment.
• Regarding the treatment time it is so that the result of the treatment improves with the time. Consequently, long treatment times are applied, wherein 20 minutes is a lowest limit at which an at least respectable result can be achieved.
• There is no upper limit from the aspect of the result achieved although from a practical and a system volume aspect an upper limit of 720 minutes, i.e. 12 hours, is stated.
• a long treatment time requires a retention vessel or tank of very large volume.
• Pulp mills for instance sulfate pulp mills, often include one or two retention vessels or buffer vessels. These vessels have a retention time in the order of six to nine hours for instance, and one such already existing vessel can be used conveniently for the aforedescribed initial treatment of the cellulose pulp.
• any complexing agent effective and known to the art can be used and added to the cellulose pulp.
• suitable complexing agents are ethylene diamine tetra- acetic acid (EDTA) and diethylene triamine pentaacetic acid (DTPA).
• EDTA ethylene diamine tetra- acetic acid
• DTPA diethylene triamine pentaacetic acid
• the complexing agent is added in an amount of 400-10 000 grams, calculated as 100% chemical for each tonne of bone dry pulp.
• a second component that shall be added to the cellulose pulp is a substance which contains the basic element molybdenum.
• Any substance which contains at least molybdenum and oxygen can be added, examples of such substances being ammonium heptamolybdate having the chemical formula (NH 4 ) 6 Mo 7 0 24 . 4H 2 0, and ammonium dimolybdate having the chemical formula (NH 4 ) 2 Mo 2 0 7 .
• the cation ammonium can, of course, be replaced with other cations, such as sodium for instance, which cation is preferred.
• Examples of other suitable molybdenum compounds are isopolymolybdates, permolybdates and permolybdic acid.
• the substance shall be added in an amount of 10- 1000 grams, calculated as molybdenum for each tonne of bone dry pulp.
• a given amount of the added substance is circulated, therewith enabling the amount charged to be kept low.
• a third component that shall be added to the cellulose pulp is the chlorine-free oxidative bleaching agent.
• bleaching agents are oxygen gas, ozone and some peroxide compound.
• a preferred bleaching agent is a peroxide compound, such as hydrogen peroxide, sodium peroxide, peroxy acetic acid, peroxosulfate, peroxodisulfate, perborate or organic peroxides.
• the bleaching agent shall be added in an amount of 3-50 kg, preferably 5-20 kg, calculated as 100% chemical for each tonne of bone dry pulp.
• the peroxide charge in this stage is normally lower than in a conventional peroxide stage, for instance.
• the pulp is preferably washed in one or more stages on any known type of washing apparatus or apparatuses, including typical presses. It is not absolutely necessary to wash the cellulose pulp at this position, although it is very much preferred as will be apparent from one of the following working examples.
• the pulp is then bleached with a chlorine-free oxidative bleaching agent in a known manner.
• bleaching agents that can be used are oxygen, ozone and peroxide also now and peroxide being the preferred bleaching agent.
• Suitable peroxides are the same as those given above, with particular preference to hydrogen peroxide.
• the hydrogen peroxide bleaching process can be carried out both with or without applying pressure.
• the alkali charge is adapted so that the pH-value of the pulp suspension at the end of the bleaching stage, i.e. the end-pH, lies in the range of 10-11.5.
• a pressure 0.2-1 MPa, preferably 0.4-0.6 MPa.
• the aforesaid bleaching stage is suitably preceded by a complex forming treatment of the cellulose pulp.
• Magnesium for instance in the form of magnesium sulfate, may otpionally be added in an amount of 0.1-10 mmole per litre of liquid, preferably 0.2-5 mmole per litre of liquid. Although not absolutely necessary, it is preferred to wash the cellu- lose pulp after this complex forming treatment.
• the initial treatment stage described above may be carried out solely on non-delignified/ unbleached cellulose pulp or on cellulose pulp that has been predelignified/prebleached.
• One alka- line oxygen-delignifying stage is by far the most suitable.
• the oxygen-gas delignification of cellulose pulp can be carried out in accordance with any known technique, including both medium consistency delignification and high consistency delignification.
• the cellulose pulp is normally washed after the oxygen-gas delignification process.
• one of the aims or targets of the present invention is to keep down the number of bleaching and treat ⁇ ment stages, it is fully possible to bleach the cellulose pulp with a chlorine-free oxidative bleaching agent in one or possibly two further stages after the aforedescribed termina- ting bleaching stage, so as to impart to the cellulose pulp a final brightness of 90% ISO and thereabove.
• additional bleaching detracts from the advantage afforded by a short bleaching sequence, it substitutes this advantage with the advantage of achieving an extremely high final brightness and/or very mild conditions during said extra bleaching stage or stages.
• a final brightness of such high magnitude can also be achieved by employing optimal parameters in said treatment stages, including a heavy charge of chlorine-free oxidative bleaching agent in the terminating stage, in accordance with the preferred embodiment of the invention.
• the application of such a complex forming stage is harmless from the aspect of a closed liquid system because of the relatively high pH of said stage, for instance a pH of 5.
• Pulp samples were taken from a birch sulfate line comprising batch wise digesters in a pulp mill, after a wash that followed a conventional alkaline oxygen-gas bleaching stage.
• the pulp was transported to the laboratory, where it was washed one further time with de-ionized water in a careful way.
• the pulp was treated in the laboratory as zero samples or zero series in the following manner:
• the complexing agent EDTA was a commercial product that contained 40% active substance.
• This stage Q x is the same for all tests 1-4.
• Tests 3 and 4 were carried out in an identical manner, with the only exception that ammonium heptamolybdate (P+Mo) having the earlier mentioned formula was added in addition to peroxide and sulfuric acid.
• the ammonium heptamolybdate was added in an amount of 300 ppm calculated solely as molybdenum. Reference is made to Table 1 below in other respects.
• the remaining stages Q 2 -E+P-P were carried out in tests 1-4 in an identical manner.
• Plastic cans having a volume of 2 litres were used in the second complex forming treatment stage Q 2 , as the pulp consistency was as low as 3%.
• the time was one hour and the temperature 60°C.
• the amount of EDTA and sulfuric acid charged and the measured pH-values are set forth in Table 1 below.
• the E+P stage was carried out at a pulp consistency of 10%. The time was two hours and the temperature 70°C. The amount of hydrogen peroxide and sodium hydroxide charged and the end-pH, residual peroxide quantities and pulp characteristics are made apparent in Table 1 below.
• the terminating peroxide bleaching stage was carried out at a pulp consistency of 12%. The time was three hours and the temperature 90°C. The amount of hydrogen peroxide and sodium hydroxide charged and the end-pH, residual peroxide quantities and pulp characteristics are made apparent in Table 1 below. The pulp samples were washed carefully with ion- exchanged water between each treatment stage. Table 1 ests Qi P resp. P + Mo
• Test 5 was carried out according to (0) - b)+c)-P and the tests 6, 7 and 8 were carried out according to (0) - a)+b)+c)-P. These three tests were thus carried out in accordance with the inventive method. Appropriate parts of the tests were actually carried out in the manner earlier described.
• the first treatment stage was carried out at a pulp consistency of 12%. The time was three hours and the temperature 95°C.
• the only difference between the three tests 6, 7 and 8 carried out in accordance with the invention resided in the addition of complex forming agent in increasing quantities and also in slightly differing amounts of sulfuric acid additions to optimize the pH-value during the treatment or bleaching stage. Other treatment parameters and measurement values determined after this treatment stage are apparent from Table 2 below. After this treatment, the pulps were washed care ⁇ fully with ion-exchanged water.
• the peroxide bleaching stage was carried out at the earlier mentioned pulp consistency and during the same length of time, whereas the temperature was lowered slightly to 90°C. The conditions were identical for all four tests, and the amounts in which hydrogen peroxide and sodium hydroxide were added and the measured values are apparent from Table 2 below. After this stage, the pulps were washed carefully with ion-exchanged water.
• a comparison between tests 7 and 8 carried out in accordance with the invention and tests 3 and 4 in Table 1 shows that the brightness of the pulps is the same, i.e. 88% ISO, whereas the viscosity of the pulps treated in accordance with the invention is slightly higher than the viscosity of the zero pulps, despite the tests carried out in accordance with the method according to the invention being restricted to three treatment stages whereas the zero pulps were treated in six stages.
• a brightness of 88% ISO is furthermore an acceptable final pulp brightness, which shows that the invention enables a fully bleached pulp to be produced with the use of only three bleaching or treatment stages.
• the pulp consistency was 10% in the first treatment stage.
• the time was three hours and the temperature 75°C.
• the amounts in which hydrogen peroxide, sulfuric acid, complexing agent and ammonium heptamolybdate were added and the measured values relating to end pH, residual peroxide, kappa number and viscosity are apparent from Table 3 below.
• the pulps were washed carefully with ion-exchanged water. In two cases, the pulps were not washed in this position.
• the subsequent peroxide bleaching stage took place at a pulp consistency of 10%.
• the time was three hours and the temperature 75°C.
• the amount of hydrogen peroxide and sodium hydroxide added and the measured values of end pH, residual peroxide, kappa number, viscosity and brightness will be apparent from Table 3 below.
• the pulps were washed carefully with ion-exchanged water after the hydrogen peroxide bleaching stage.
• the zero sample gave a kappa number of 7.9 and a viscosity of 870 cm 3 /g.
• an ammonium hepta- molybdate addition of 500 ppm, calculated solely as molyb ⁇ denum the kappa number fell to 4.6 while, surprisingly, the viscosity remained as high as 882 cm 3 /g.
• Test 15 carried out in accordance with the inven ⁇ tion differs from the other tests 10-14 carried out in accordance with the invention insomuch that the pulp was not washed after the initial treatment stage.
• the pulps are washed carefully in this position with ion-exchanged water.
• the omission of this washing stage had no effect on pulp delignification, it lowered the brightness from 84.3% ISO (test 14) to 79.2% ISO, and the viscosity from 798 cm 3 /g (test 14) to 597 cm 3 /g.
• the quality of the pulp was thus seriously impaired, which was probably because an excessively large amount of manganese ions were carried over to the final hydrogen peroxide bleaching stage.
• the omission of a pulp washing stage in said position does not have the same serious consequences under other conditions, which explains that we in this patent document have stated, that washing of the pulp in this position is only highly preferably, and not absolutely imperative.
• the pulp consistency was 12% in the first treatment stage. The time was three hours and the temperature 95°C. The amounts in which hydrogen peroxide, sulfuric acid, complexing agent and ammonium heptamolybdate, calculated solely as molybdenum were charged, and the measured values of end pH and residual peroxide will be apparent from Table 4 below. All pulps were washed carefully with ion-exchanged water after the treatment stage.
• the second stage was a complex forming stage (Q) at a pulp consistency of 3%.
• the time was one hour and the temperature 60°C.
• the amounts of EDTA and sulfuric acid charged and the measured values of pH, kappa number and viscosity will be apparent from Table 4 below.
• the pulps were washed carefully with ion-exchanged water after the mentioned treatment.
• the third and final stage was a hydrogen peroxide bleaching stage at a pulp consistency of 12%.
• the time was three hours and the temperature 90°C.
• the amounts of hydrogen peroxide and sodium hydroxide charged and the measured values of end pH, residual peroxide amount, kappa number, viscosity and brightness are apparent from Table 4 below.
• the pulps were washed carefully with ion-exchanged water after the final treatment.
• Pulp samples were taken from a pine sulfate line with a continuous digester, after a wash which followed a conventional alkaline oxygen-gas bleaching stage.
• the pulp was transported to the laboratory, where it was washed one more time carefully with de-ionized water.
• ⁇ 16.6
• viscosity 981 cm 3 /g
• brightness 34.2% ISO.
• a zero sample 24 was carried out in accordance with the sequence (0) - a) + c) -P, and the other test 25 was carried out in accordance with the sequence (0) - a) + b) + c) - P, in accordance with the invention.
• the reference (0) signifies that oxygen-gas bleaching of the pulp was carried out in the mill.
• the first treatment stage in the two tests 24 and 25 performed in the laboratory had the following in common.
• the pulp consistency was 12%, the time was three hours and the temperature 90°C.
• Hydrogen peroxide was added in an amount of 4%, calculated on bone dry pulp
• the complexing agent EDTA was added in an amount of 0.3%, calculated on bone dry pulp.
• 300 ppm of ammonium heptamolybdate was also charged in test 25 carried out in accordance with the invention, said ammonium heptamolybdate being calculated as molybdenum on bone dry pulp.
• the two pulps were then washed carefully with ion-exchanged water and their kappa numbers and viscosities were measured. The aforesaid charges and measured numerical values are apparent from Table 5 below.
• the use of the complexing agent EDTA, ammonium heptamolybdate and hydrogen peroxide in one and the same stage in accordance with the invention enables an acceptable final brightness to be achieved in solely three treatment or bleaching stages, even in the case of sulfate pulp produced from pinewood, i.e. coniferous/long fibre pulp.
• the test pulp produced in accordance with the invention has a brightness of 86% ISO, whereas the brightness of the reference pulp is only 75.9% ISO.
• a surprisingly large difference also exists with regard to the lignin content of the finally bleached pulps, expressed as kappa number. More precisely, a kappa number of 2.0 as opposed to a kappa number of 10.6. Because the viscosity of the starting pulp was relatively low, the viscosity of the finally bleached pulp produced in accordance with the invention became also relatively low.
• Tests in which the inventive method was applied were carried out in a sulfate pulp mill in which birch wood chips were digested in a number of batch digesters, to produce cellulose pulp. After the digestion, the pulp was treated normally in accordance with what is stated below. The digested pulp was screened in a closed pulp screening room and the accepted pulp was then washed on a belt washer, whereafter the pulp was delignified with oxygen- gas. Normally, 40 kg of sodium hydroxide, calculated per tonne of bone dry pulp, was added to the pulp. The tempera ⁇ ture was 110°C and the time one hour.
• the pulp was deligni- fied in one single stage in a bleaching tower, alternatively referred to as an oxygen-gas bleaching reactor, at a pulp consistency of 10% and at an oxygen-gas pressure of 0.3-0.6 MPa.
• the pulp was then washed in two series connected washing presses. After being washed, the pulp was passed to a storage tower and there stored for about six hours.
• the pulp was transferred from the storage tower to a washing filter, and thereafter to a tower in which the pulp was treated with complexing agent EDTA.
• EDTA was charged in an amount corres ⁇ ponding to 2.5 kg per tonne of bone dry pulp. 7.5 kg of sulfuric acid were also charged for each tonne of bone dry pulp, so as to maintain a pH of about 5.
• the pulp had a consistency of about 10%.
• the temperature was 73-80°C, and the time about three hours. It is important to note that all of the numerical values given shall be taken as being approximate, since they refer to mill conditions where some variations in process parameters for instance are unavoid ⁇ able.
• the process parameters can be controlled and checked much more easily in the laboratory since the treatment stages are there normally carried out intermittently, i.e. batchwise, and not continuously.
• the pulp was washed in two series connected washing filters after the complex forming treatment.
• the pulp was thereafter passed to a first peroxide bleaching stage.
• the pulp consistency was about 10%, the temperature about 85°C and the time three hours.
• 35 kg of hydrogen peroxide and 14 kg of sodium hydroxide were charged for each tonne of bone dry pulp.
• the pulp was then washed on a washing filter.
• washing liquid in counterflow only one stage calculated from the end, i.e. from the washing filter that follows the terminating hydrogen peroxide bleaching stage to the washing filter that follows the ozone bleaching stage, and thereafter to an outlet.
• fresh water was delivered to the pulp on the washing filter after the first hydrogen peroxide bleaching stage.
• the washing liquid occurrent in this position could also at times be passed to the outlet, even though the washing liquid was normally passed backwards in the process, in strict counterflow.
• the kappa number, viscosity and often the bright ⁇ ness of the pulp were determined in a number of positions, and the measured values are apparent from Table 6 below.
• the aforedescribed treatment of the forwardly moving pulp was changed at a given time point, by also adding ammonium dimolybdate in an amount of 300 ppm, calculated as molybdenum, and hydrogen peroxide in an amount of 10 kg per tonne of bone dry pulp in connection with the complex forming stage, in addition to adding EDTA and sulfuric acid in given amounts. Regarding the addition of these chemicals it was carried out in the following manner. Powdered ammonium dimolybdate was dissolved in water having a temperature of 50°C, to obtain a 5 percent solution.
• This molybdate solution and a hydrogen peroxide solution (50 percent solution) were charged in the screw of the washing filter that precedes the complex forming stage. Liquid was delivered to the washed pulp conveyed in a continuous mat in said screw to form a liquid suspension.
• the complexing agent EDTA in the form of a commercial product, i.e. a solution containing 40% active substance, and a sulfuric acid solution were delivered to the high pulp consistency pump located downstream of the mentioned washing filter and functioning to press the pulp suspension into the complex forming treatment tower.
• a change in the subsequent hydrogen peroxide bleaching stage was also made after half of the test period had passed, this change consisting in lowering the hydrogen peroxide charge to 25 kg per tonne of bone dry pulp. Treatment of the pulp was terminated with washing the pulp on the following washing filter.
• the kappa number, viscosity and often the bright- ness of the pulp were measured in several positions, the measured values being apparent from Table 6 below.
• Samples of the pulp were taken for analysis from the washing filter upstream of the complex forming treatment, from the second washing filter downstream of the complex forming treatment, and from the washing filter downstream of the hydrogen peroxide bleaching stage, among others. These pulp samples were washed carefully with ion-exchanged water in the laboratory, prior to being analyzed.
• test period had a duration of seventeen hours.
• the measured values given above have been chosen from among a large number of the values that were measured during respective time periods in the manufacture of finally bleached cellulose pulp and are representative of the results achieved.
• the measured values obtained with pulp that has been bleached in accordance with the invention have been chosen from the second half of the test period, i.e. when the hydrogen peroxide charge in P l was lowered to 25 kg per tonne of bone dry pulp.

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Cited By (7)

Citations (3)

• 1995
  • 1995-05-22 SE SE9501897A patent/SE504340E/sv not_active IP Right Cessation
• 1996
  • 1996-05-13 WO PCT/SE1996/000619 patent/WO1996037654A1/en active Application Filing
  • 1996-05-13 AU AU57850/96A patent/AU5785096A/en not_active Abandoned

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