JP4887900B2 - Method for producing bleached pulp - Google Patents

Description

  The present invention relates to a process for producing bleached pulp from lignocellulosic material. More specifically, the present invention relates to a method for producing ECF (elementary chlorin-free) bleached pulp with less chlorine dioxide usage.

  Conventionally, chlorine-based bleaching chemicals such as molecular chlorine (C), hypochlorite (H), and chlorine dioxide (D) have been widely used for bleaching paper pulp, especially molecular chlorine. It has been particularly preferred because of its high action and low reactivity with cellulose that affects pulp strength. Also, in order to increase the bleaching efficiency, the sequence of C-E-H-D, C / D-E-H-E-D (C / D is a combined bleaching stage of chlorine and chlorine dioxide, E is an alkali extraction stage) As described above, pulp bleaching is generally performed in multiple stages, but it has been customary to use molecular chlorine in the first stage where the most amount of bleaching chemicals is used.

  However, since these chlorine bleaching chemicals by-produce organic chlorine compounds that are harmful to the environment during bleaching, there is an increasing trend to reduce the amount of chlorine based chemicals used. In particular, when a pulp is bleached with molecular chlorine, a lot of organic chlorine compounds are produced. Therefore, in consideration of the influence on the environment, switching to a bleaching method not using molecular chlorine is increasing. Pulp bleached without using molecular chlorine is called ECF (elementary chlorin-free) pulp, and pulp produced without using any chlorinated chemicals is called TCF (totally chlorin-free). It is recognized as a consideration-type pulp.

  In general, non-chlorine bleaching chemicals have low reaction selectivity for bleaching, and have the disadvantages of low pulp strength after bleaching and low pulp yield. Therefore, ECF using chlorine dioxide. Bleaching is the current mainstream. However, there is a movement to reduce the amount of chlorine dioxide used in ECF bleaching as much as possible, and this bleaching method is called light ECF bleaching and has begun to attract attention.

  In ECF bleaching, as a method for reducing the amount of chlorine dioxide used, ozone bleaching with a high delignification effect is generally used in the first stage. However, as described above, ozone has a problem that the amount of its use is limited because its reaction selectivity is lower than that of chlorine dioxide.

  As a method for improving the reaction selectivity of ozone, a method has long been known in which acid washing as a pretreatment is performed to remove heavy metals in pulp and to suppress the decomposition reaction of cellulose by radicals generated by the reaction between heavy metals and ozone. It has been. However, this method alone does not provide a satisfactory effect.

As another method for improving the reaction selectivity of ozone, as a pretreatment, a method of performing a chelating agent treatment (Patent Document 1), a method of performing an enzyme treatment and a chelating agent (Patent Document 2), and a method of performing a high temperature acid treatment (Patent Document 3), a method of sequentially performing a chelating agent treatment and an organic peracid treatment (Patent Document 4), a method of sequentially performing an organic peracid treatment and a chelating agent treatment (Patent Document 5), and the like are disclosed. Also disclosed are a method of coexisting a surfactant during peracid ozone bleaching (Patent Document 6), a method of using peracid and ozone in combination under controlled pH conditions (Patent Document 7), and the like.
JP 05-148785 A Japanese Patent Application Laid-Open No. 07-173785 JP 2000-290887 A Japanese Patent Laid-Open No. 08-503749 Japanese Patent Application Laid-Open No. 08-503750 Japanese Patent Application Laid-Open No. 07-119063 Japanese Translation of National Publication No. 08-511308

  An object of the present invention is to provide a method for producing ECF bleached pulp that improves the reaction selectivity of ozone bleaching, increases the reaction efficiency, and uses less chlorine dioxide in the subsequent stage.

    As a result of various studies on methods for improving reaction selectivity in ozone bleaching and increasing reaction efficiency, the present inventors have found that the state of the pulp before ozone bleaching is less metal ions, and less lignin and hexeneuronic acid. I found it effective to put it in a state. Furthermore, by changing the acid washing with sulfuric acid, which is usually performed as a pretreatment for ozone bleaching, to acid washing with an inorganic peroxy acid, the state of the pulp before ozone bleaching is reduced with less metal ions and lignin and hexeneuronic acid. Has been found to be in a state where there is less, has led to the present invention. This application includes the following inventions.

(1) Unbleached pulp obtained by digesting lignocellulosic material is bleached with alkaline oxygen, then treated with inorganic peroxyacid and / or salt thereof without adding a chelating agent, washed, and further treated with ozone A method for producing bleached pulp, comprising performing a multistage bleaching process that starts.

(2) The method for producing bleached pulp according to (1), wherein the treatment pH with the inorganic peroxyacid and / or salt thereof is 2 to 3.

(3) The method for producing bleached pulp according to (1) or (2), wherein the inorganic peroxyacid is monopersulfuric acid.

(4) The method for producing bleached pulp according to any one of (1) to (3), wherein the alkaline oxygen bleaching is performed in a plurality of reactors.

(5) The multistage bleaching process starting from the ozone treatment is characterized in that the second stage following the ozone bleaching stage is an alkali extraction stage, and a chlorine dioxide bleaching stage and an alkaline hydrogen peroxide bleaching stage are provided after the third stage. The method for producing bleached pulp according to any one of items (1) to (4).

  According to the present invention, when ozone-bleached pulp after bleaching and alkaline oxygen bleaching, pretreatment with an inorganic peroxy acid improves reaction selectivity in the ozone bleaching stage and increases reaction efficiency, resulting in a result. As a result, it is possible to reduce the amount of chlorine dioxide used in the latter stage.

  The lignocellulosic material used in the present invention is suitable for hardwoods containing a large amount of methylglucuronic acid that produces hexeneuronic acid, but may be conifers, non-woods such as bamboo and hemp, and these There is no particular limitation. As the cooking method for obtaining the pulp used in the present invention, known cooking methods such as kraft cooking, polysulfide cooking, soda cooking, alkali sulfite cooking, etc. can be used, considering pulp quality, energy efficiency, etc. Then, the kraft cooking method or polysulfide cooking is used suitably.

    For example, when kraft cooking 100% hardwood wood lignocellulose, the degree of sulfation of the kraft cooking liquor is 5 to 75%, preferably 15 to 45%, the effective alkali addition rate is 5 to 30% by weight per mass of absolutely dry wood, Preferably, 10 to 25% by mass, cooking temperature is 130 to 170 ° C., and the cooking method may be either a continuous cooking method or a batch cooking method. When a continuous cooking kettle is used, a cooking solution is added at multiple points. A cooking method may be used, and the method is not particularly limited.

  In cooking, as a cooking aid in the cooking solution to be used, known cyclic keto compounds, for example, benzoquinone, naphthoquinone, anthraquinone, anthrone, phenanthroquinone, and quinone-based alkyl substitutes such as alkyl and amino, or the quinone series 1 selected from hydroquinone compounds such as anthrahydroquinone, which is a reduced form of the compound, and a 9,10-diketohydroanthracene compound which is a stable compound obtained as an intermediate of an anthraquinone synthesis method by the Diels-Alder method A seed | species or 2 or more types may be added, The addition rate is a normal addition rate, for example, is 0.001-1.0 mass% per the absolute dry mass of a wood chip.

In the present invention, unbleached chemical pulp obtained by a known cooking method is delignified by a known alkaline oxygen bleaching method after undergoing washing, rough selection, and selection steps. As the alkaline oxygen bleaching method used in the present invention, a known medium concentration method or high concentration method can be applied as it is, but at present, a medium concentration method in which the pulp concentration is generally used at 8 to 15% by mass. Is preferred.
In the alkali oxygen bleaching method by the medium concentration method, caustic soda or oxidized kraft white liquor can be used as the alkali, and oxygen gas from oxygen or PSA (Pressure Swing Adsorption) can be used as the oxygen gas. And oxygen from VSA (Vacuum Swing Adsorption) can be used.

  The oxygen gas and alkali are added to a medium-concentration pulp slurry in a medium-concentration mixer and mixed sufficiently, and then sent to a reaction tower capable of holding a mixture of pulp, oxygen, and alkali for a certain period of time under pressure, and delignified. Is done. The oxygen gas addition rate is 0.5 to 3% by mass per mass of dry pulp, the alkali addition rate is 0.5 to 4% by mass, the reaction temperature is 80 to 120 ° C., the reaction time is 15 to 100 minutes, and the pulp concentration Is 8 to 15% by mass, and other known conditions can be applied. In the present invention, in the alkaline oxygen bleaching step, it is a preferred embodiment that the alkali oxygen bleaching is continuously performed a plurality of times, delignification is advanced as much as possible, and heavy metals are reduced. The pulp that has been subjected to alkaline oxygen bleaching is then sent to a washing step. The pulp is sent to an inorganic peroxyacid treatment stage after washing.

Examples of the inorganic peroxyacid and / or salt thereof used in the present invention include monopersulfuric acid, persulfuric acid (marshall acid), monoperphosphoric acid, perboric acid, percarbonate, peroxopolyacid, and salts thereof. Monopersulfuric acid is preferred from the standpoint of effect and economy. For example, in the case of monopersulfuric acid, it can be produced by hydrolyzing peroxydisulfuric acid, or it can be produced by mixing hydrogen peroxide and sulfuric acid at an arbitrary ratio, but the production method is particularly limited. Not what you want. It is also possible to use something like oxone is a monopersulfate double salt _{(2KHSO 5 · KHSO 4 · K} 2 SO 4). However, in consideration of economy, it is a preferred embodiment to produce and use monopersulfuric acid by mixing high concentration hydrogen peroxide and high concentration sulfuric acid.

  As a method for producing monopersulfuric acid by mixing high-concentration hydrogen peroxide and high-concentration sulfuric acid, 80% hydrogen peroxide solution having a concentration of 20% to 70% by mass, preferably 35% to 60% by mass is used. A method of dropping and mixing concentrated sulfuric acid having a mass% of 98% by mass, preferably 93% by mass to 96% by mass, is suitable. The mixing molar ratio of the sulfuric acid and hydrogen peroxide is 1: 1 to 5: 1, preferably 2: 1 to 4: 1. If both hydrogen peroxide and sulfuric acid have a low mass%, the production efficiency of monopersulfuric acid is lowered, which is not suitable. On the other hand, if the mass% is too high, the risk of ignition and the like increases, which is not suitable. Furthermore, when the mixing molar ratio of sulfuric acid and hydrogen peroxide deviates from 1: 1 to 5: 1, it is not suitable because the production efficiency of monopersulfuric acid is lowered.

  In the treatment with the inorganic peroxyacid and / or salt thereof of the present invention, the addition rate of the inorganic peroxyacid is 0.01 to 2% by mass, preferably 0.1 to 1% by mass, per mass of the absolutely dry pulp. The treatment pH is 1.5 to 6.0, preferably 2 to 3. The treatment time is 1 minute to 5 hours, preferably 10 minutes to 200 minutes. The treatment temperature is 20 ° C to 90 ° C, preferably 40 ° C to 70 ° C. The pulp concentration is 5-30%, preferably 8-15%.

  In the treatment with the inorganic peroxyacid and / or salt thereof of the present invention, the treatment pH is particularly important. In the present invention, the pH is preferably in the range of 2 to 3. When the treatment pH is in the range of 1.5 to 6.0, there is almost always delignification action, but the hexeneuronic acid decomposition action becomes maximum near pH 3, and when the pH deviates from 3, the action gradually decreases. Cellulose decomposition does not occur so much until pH 2, but it rapidly increases when the pH is lower than 2, and becomes remarkable particularly when a large amount of heavy metal is present in the pulp. Conventionally, the reaction between heavy metals and persulfuric acid has been controlled using a chelating agent, but it is sufficient even if the treatment is carried out without adding a chelating agent by controlling the treatment pH of persulfuric acid within the above range. Can be obtained. By not performing the treatment with the chelating agent, the COD load derived from the chelating agent in the waste water can be reduced, and the present invention is a preferable method from the viewpoint of the environment. Therefore, it is necessary to set the treatment pH as described above according to the amount of lignin, the amount of hexeneuronic acid, the viscosity of the pulp and the amount of heavy metals in the pulp.

  The pulp after the treatment with the inorganic peroxyacid and / or salt thereof of the present invention is always washed. In the present invention, the type and number of washing machines used in the washing stage are not particularly limited, but a press type is preferably used because of high washing efficiency. After washing, the pulp is sent to a multistage bleaching process.

  An ozone bleaching stage (Z stage) is always inserted in the first stage of the multistage bleaching process of the present invention. After the treatment with the inorganic peroxyacid and / or salt thereof, the amount of hexeneuronic acid and heavy metal in the washed pulp is reduced, so that there is an advantage that the reaction selectivity of delignification is increased in the ozone bleaching stage. Further, since both the ozone bleaching stage and the treatment with the inorganic peroxyacid and / or salt thereof are performed under acidic conditions, there is also an advantage that pH adjustment of the ozone bleaching stage is easy.

  The ozone bleaching stage of the present invention may be a single stage, or may be a so-called ozone / chlorine dioxide bleaching stage in which chlorine bleaching stage is performed after ozone bleaching, and is not particularly limited. In the ozone bleaching stage of the present invention, the ozone addition rate is 0.1 to 1% by mass per mass of the dry pulp, the reaction temperature is 20 to 80 ° C., the reaction time is 0.5 to 60 minutes, and the reaction pH is 2. It is 0-6.0, A well-known alkali and acid can be used for pH adjustment. Although it does not specifically limit regarding a pulp density | concentration, Preferably it is 3-40 mass%.

  When the first stage of the multi-stage bleaching step of the present invention is an ozone / chlorine dioxide bleaching stage, the ozone addition rate is 0.1 to 1% by mass per the dry pulp mass, and until ozone is added after adding ozone. The reaction temperature is 20 to 80 ° C., the reaction time is 0.5 to 60 minutes, and the reaction pH is 2.0 to 6.0. The subsequent addition rate of chlorine dioxide is 0.1 to 2% by mass based on the mass of the dry pulp, the reaction temperature after addition of chlorine dioxide is 30 to 80 ° C., the reaction time is 5 to 180 minutes, and the reaction pH is 2.0 to 6 0.0, and known alkalis and acids can be used for pH adjustment. Although it does not specifically limit regarding a pulp density | concentration, It is 8-15 mass% suitably from the point of operativity.

In the present invention, there is no particular limitation except that the first stage of the multi-stage bleaching process is an ozone bleaching stage, but the second stage is an alkali extraction stage, and the third and subsequent stages are a chlorine dioxide bleaching stage and an alkaline hydrogen peroxide bleaching stage. The preferred embodiment is a combination of the above. Ozone bleaching is known to produce acids such as oxalic acid and formic acid when lignin and hexeneuronic acid are decomposed. To extract and remove these, an alkali extraction stage is inserted in the second stage. Is effective. From the third stage, the chlorine dioxide stage (D) and the hydrogen peroxide stage (P), which are different in reaction mechanism from ozone, are effective. As conditions for the alkali extraction stage, the alkali addition rate is 0.5 to 3% by mass per mass of dry pulp, the reaction temperature is 60 to 120 ° C., the reaction time is 15 to 120 minutes, and the pulp concentration is 8 to 15% by mass. is there. Preferably, oxygen gas is added to the alkali extraction stage. The addition rate of oxygen gas is 0.1-3 mass% per mass of dry pulp. More preferably, hydrogen peroxide is also added. The addition rate of hydrogen peroxide is 0.05-2 mass% per mass of dry pulp.

  The hydrogen peroxide addition rate in the alkaline hydrogen peroxide bleaching stage of the present invention is 0.05 to 2% by mass per mass of the dry pulp, the reaction temperature is 60 to 120 ° C., the reaction time is 15 to 120 minutes, and the pH is 10 0.5 to 12.0, and known alkalis and acids can be used for pH adjustment. Although it does not specifically limit regarding a pulp density | concentration, It is 8-15 mass% suitably from the point of operativity.

  The chlorine dioxide addition rate in the chlorine dioxide stage of the present invention is 0.1 to 1% by mass with respect to the mass of the dry pulp, the reaction temperature is 60 to 120 ° C., the reaction time is 15 to 120 minutes, and the pH is 3.0 to 6. 0.0, and known alkalis and acids can be used for pH adjustment. Although it does not specifically limit regarding a pulp density | concentration, It is 8-15 mass% suitably from the point of operativity.

  Hereinafter, the present invention will be described more specifically with reference to examples and comparative examples. However, the present invention is not limited to these examples. In the following examples and comparative examples, unless otherwise indicated, production of monopersulfuric acid, measurement of pulp kappa number, measurement of potassium permanganate value (K value), measurement of whiteness, measurement of viscosity, The fading property was evaluated by the following methods. In addition, the addition rate of the chemical | medical agent in an Example and a comparative example shows the mass% per absolute dry pulp mass.

1. Production of monopersulfuric acid Monopersulfuric acid was produced by adding 300.24 g of commercially available 96% sulfuric acid to 68.02 g of commercially available 50% by mass hydrogen peroxide. The concentration of monopersulfuric acid produced was 415 g / l.

2. Measurement of pulp kappa number The kappa number was measured according to JIS P8211.

3. Measurement of potassium permanganate value (K value) of pulp The measurement of potassium permanganate value was performed according to JIS P 8206. Here, it was used as an index of the amount of hexeneuronic acid in the pulp.

4). Measurement of Pulp Viscosity TAPPI No. Performed according to Method 44.

5. Measurement of Pulp Whiteness After bleaching the bleached pulp, a sheet having a basis weight of 60 g / m ² was prepared according to JIS P 8209, and the whiteness of the pulp was measured according to JIS P 8148.

Example 1
900 g of mixed wood chips consisting of 70% eucalyptus wood and 30% acacia wood were collected in an absolute dry mass, liquid ratio 4, effective alkali 17% per absolute dry chip mass, sulfidity of cooking liquor 25%, cooking temperature 160 ° C Kraft cooking using a laboratory indirect heating autoclave under a cooking time of 120 minutes, and then separating the waste liquid and pulp, and selecting the pulp on a flat screen equipped with a 10-cut screen plate, the Hunter brightness 38. An unbleached 432 g of unbleached kraft pulp having 9%, copper number of 18.2, and pulp viscosity of 43.3 mPa · s was obtained.

  70.0 g of the absolute dry mass of the unbleached kraft pulp was sampled, 2.0% of caustic soda per mass of the absolute dry pulp was added, and then diluted with ion-exchanged water to adjust the pulp concentration to 10%. In an autoclave, assuming that alkaline oxygen bleaching is performed in an upflow state in a 50 m high columnar reaction vessel, 99.9% of commercially available compressed oxygen gas is injected to adjust the gauge pressure to 1 MPa, and 0 min / min. The reaction was allowed to proceed for 50 minutes while venting the gas so that the gauge pressure decreased at a rate of 0.01 MPa. After completion of the reaction, the pressure was reduced until the gauge pressure became 0.05 MPa or less, the pulp was taken out from the autoclave, washed with 7 liters of ion-exchanged water, and dehydrated. A pulp having a whiteness of 51.3%, a copper number of 9.4, and a pulp viscosity of 23.3 mPa · s was obtained.

  60 g of the kraft pulp after bleaching with alkaline oxygen was collected at an absolute dry mass, placed in a plastic bag, adjusted to a pulp concentration of 10% using ion-exchanged water, and then 0.28% monopermeate per absolute dry pulp mass. Sulfuric acid was added and monopersulfuric acid treatment was performed by immersing in a constant temperature water bath having a temperature of 60 ° C. for 60 minutes. The pH of the pulp slurry during the monopersulfuric acid treatment was 3.0. The obtained pulp was diluted to 3% with ion-exchanged water, then dehydrated and washed with a Buchner funnel. A pulp having a whiteness of 55.0%, a copper number of 8.4, and a pulp viscosity of 20.2 mPa · s was obtained.

  55 g of the pulp after monopersulfuric acid treatment was collected in an absolutely dry mass, put in a plastic bag, added ion exchange water and sulfuric acid to adjust the pulp concentration to 10% and pH 2.5, and then put into a medium concentration mixer. 0.6% ozone was added per mass of dry pulp, and the mixture was stirred and reacted at 50 ° C. for 1 minute. The obtained pulp was diluted to 3% with ion-exchanged water, then dehydrated and washed with a Buchner funnel.

  55 g of the Z-staged pulp was collected at an absolute dry mass, put in a plastic bag, and ion-exchanged water was added to adjust the pulp concentration to 10%. Then, caustic soda was 1.0% per absolute dry pulp mass, peroxidized. Add 0.3% hydrogen and mix well, then transfer to a 2 liter stainless steel indirect heating autoclave and use commercially available compressed oxygen gas with a purity of 99.9% so that the gauge pressure is 0.15 MPa. Pressurized and reacted at 70 ° C. for 20 minutes. Thereafter, the pulp slurry was taken out from the autoclave, transferred again to a plastic bag, and then immersed in a constant temperature water bath having a temperature of 70 ° C. for 70 minutes to extract the E / OP stage. The obtained pulp was diluted to 3% with ion-exchanged water, then dehydrated and washed with a Buchner funnel.

55.0 g of the pulp after the E / OP stage was collected at an absolute dry mass, put in a plastic bag, adjusted to a pulp concentration of 10% using ion-exchanged water, and then 0.25 chlorine chloride per absolute dry pulp mass. % And caustic soda 0.05% were added and immersed in a constant temperature water bath at a temperature of 70 ° C. for 180 minutes to perform D-stage bleaching. The obtained pulp was diluted to 3% with ion-exchanged water, then dehydrated and washed with a Buchner funnel. The whiteness of the obtained bleached pulp was 86.0%, and the viscosity was 16.1 mPa · s.
Table 1 shows the pH during monopersulfate treatment, the properties of the treated pulp (whiteness, kappa number, viscosity), the final D-stage chlorine dioxide addition rate, and the whiteness and viscosity of the bleached pulp.

Example 2
In Example 1, 0.6% sulfuric acid was added per mass of dry pulp during monopersulfuric acid treatment to change the pH during monopersulfuric acid treatment to 2.0, and the chlorine dioxide addition rate in the D stage was set to 0. The same operation as in Example 1 was performed except for changing to 2%. The whiteness of the pulp after monopersulfuric acid treatment was 54.7%, the kappa number was 8.6, the pulp viscosity was 19.5 mPa · s, the whiteness of the bleached pulp was 85.9%, and the viscosity was 15 It was 4 mPa · s.
Table 1 shows the pH during monopersulfate treatment, the properties of the treated pulp (whiteness, kappa number, viscosity), the final D-stage chlorine dioxide addition rate, and the whiteness and viscosity of the bleached pulp.

Example 3
In Example 1, 2.0% of sulfuric acid was added per mass of dry pulp during monopersulfuric acid treatment to change the pH during monopersulfuric acid treatment to 1.5, and the chlorine dioxide addition rate in stage D was 0.1. The same operation as in Example 1 was performed except that the ratio was changed to 3%. The whiteness of the pulp after monopersulfuric acid treatment was 54.7%, the kappa number was 8.7, the pulp viscosity was 19.3 mPa · s, the whiteness of the bleached pulp was 85.8%, and the viscosity was 15 It was 1 mPa · s.
Table 1 shows the pH during monopersulfate treatment, the properties of the treated pulp (whiteness, kappa number, viscosity), the final D-stage chlorine dioxide addition rate, and the whiteness and viscosity of the bleached pulp.

Example 4
In Example 1, 0.25% of caustic soda was added per mass of dry pulp during monopersulfuric acid treatment to change the pH during monopersulfuric acid treatment to 4.0, and the chlorine dioxide addition rate in stage D was 0.00. The same operation as in Example 1 was performed except that the ratio was changed to 3%. The whiteness of the pulp after monopersulfuric acid treatment was 55.0%, the kappa number was 8.4, the pulp viscosity was 20.9 mPa · s, the whiteness of the bleached pulp was 85.9%, and the viscosity was 16 It was 2 mPa · s.
Table 1 shows the pH during monopersulfate treatment, the properties of the treated pulp (whiteness, kappa number, viscosity), the final D-stage chlorine dioxide addition rate, and the whiteness and viscosity of the bleached pulp.

Example 5
In Example 1, 0.6% of caustic soda was added per mass of dry pulp during monopersulfuric acid treatment to change the pH during monopersulfuric acid treatment to 6.0, and the chlorine dioxide addition rate in stage D was 0.00. The same operation as in Example 1 was performed except that the ratio was changed to 35%. The whiteness of the pulp after monopersulfuric acid treatment was 55.4%, the kappa number was 8.5, the pulp viscosity was 23.0 mPa · s, the whiteness of the bleached pulp was 85.9%, and the viscosity was 15 It was 5 mPa · s.
Table 1 shows the pH during monopersulfate treatment, the properties of the treated pulp (whiteness, kappa number, viscosity), the final D-stage chlorine dioxide addition rate, and the whiteness and viscosity of the bleached pulp.

Example 6
In Example 1, the assumption of alkaline oxygen bleaching was changed to the assumption of two-stage alkaline oxygen bleaching using two towers of 25 m high tower reaction vessel in an upflow state in a tower reaction vessel of 50 m high, and 99 .9% of commercially available compressed oxygen gas was injected to make the gauge pressure 1 MPa, and the reaction was carried out for 25 minutes while venting the gas so that the gauge pressure decreased at a rate of 0.01 MPa per minute, and then 99.9% Injecting commercial compressed oxygen gas to make the gauge pressure 1 MPa, letting the gas react at a rate of 0.01 MPa per minute while reducing the gauge pressure, reacting for 25 minutes, and adding chlorine dioxide in stage D The procedure of Example 1 was repeated except that the content was changed to 0.15%. The whiteness of the pulp after alkaline oxygen bleaching was 52.5%, the kappa number was 8.9, the viscosity of the pulp was 22.0 mPa · s monopersulfuric acid, the whiteness of the pulp was 56.5%, and the kappa number 7.7, the pulp viscosity was 19.6 mPa · s, the whiteness of the bleached pulp was 86.0%, and the viscosity was 16.2 mPa · s.
Table 1 shows the pH during monopersulfate treatment, the properties of the treated pulp (whiteness, kappa number, viscosity), the final D-stage chlorine dioxide addition rate, and the whiteness and viscosity of the bleached pulp.

Example 7
900 g of mixed wood chips consisting of 70% Radiata pine wood and 30% larch wood are collected in an absolute dry mass, liquid ratio 5, effective alkali 20% per absolute dry chip mass, 25% sulfidity of cooking liquor, cooking temperature 165 ° C Kraft cooking using an experimental indirect heating autoclave under a cooking time of 120 minutes, and then separating the waste liquid from the pulp, and selecting the pulp on a flat screen equipped with a 10-cut screen plate to obtain a Hunter whiteness of 25 414 g of an unbleached kraft pulp having a dryness of 9%, a copper number of 30.8, and a pulp viscosity of 35.8 mPa · s was obtained.

  70.0 g of the absolute dry weight of the unbleached kraft pulp was sampled, 2.5% caustic soda was added per mass of the dry pulp, and then diluted with ion-exchanged water to adjust the pulp concentration to 10%. In an autoclave, assuming that alkaline oxygen bleaching is performed in an upflow state in a 50 m high columnar reaction vessel, 99.9% of commercially available compressed oxygen gas is injected to adjust the gauge pressure to 1 MPa, and 0 min / min. The reaction was allowed to proceed for 50 minutes while venting the gas so that the gauge pressure decreased at a rate of 0.01 MPa. After completion of the reaction, the pressure was reduced until the gauge pressure became 0.05 MPa or less, the pulp was taken out from the autoclave, washed with 7 liters of ion-exchanged water, and dehydrated. A pulp having a whiteness of 34.5%, a copper number of 12.6, and a pulp viscosity of 21.7 mPa · s was obtained.

  60 g of the kraft pulp after bleaching with alkaline oxygen was collected at an absolute dry mass, placed in a plastic bag, adjusted to a pulp concentration of 10% using ion-exchanged water, and then 0.28% monopermeate per absolute dry pulp mass. Sulfuric acid was added and monopersulfuric acid treatment was performed by immersing in a constant temperature water bath having a temperature of 60 ° C. for 60 minutes. The pH of the pulp slurry during the monopersulfuric acid treatment was 3.0. The obtained pulp was diluted to 3% with ion-exchanged water, then dehydrated and washed with a Buchner funnel. A pulp having a whiteness of 38.1%, a copper number of 11.6, and a pulp viscosity of 19.3 mPa · s was obtained.

  55 g of the pulp after monopersulfuric acid treatment was collected in an absolutely dry mass, put in a plastic bag, added ion exchange water and sulfuric acid to adjust the pulp concentration to 10% and pH 2.5, and then put into a medium concentration mixer. 0.3% ozone was added per mass of dry pulp, and the mixture was stirred and reacted at 50 ° C. for 1 minute. Transfer the pulp after the reaction to a plastic bag without washing, add 0.10% chlorine dioxide and caustic soda per mass of dry pulp, and stir well. It was immersed for 60 minutes and bleached in the Z / D stage. The obtained pulp was diluted to 3% with ion-exchanged water, then dehydrated and washed with a Buchner funnel.

  55 g of the pulp after the Z / D stage was collected at an absolute dry mass, put in a plastic bag, and after adding ion-exchanged water to adjust the pulp concentration to 10%, 1.0% caustic soda per absolute dry pulp mass, Add 0.3% hydrogen peroxide and mix well, then transfer to a 2 liter stainless steel indirect heating autoclave and commercially available compressed oxygen with a purity of 99.9% so that the gauge pressure is 0.15 MPa. The gas was pressurized and reacted at 70 ° C. for 20 minutes. Thereafter, the pulp slurry was taken out from the autoclave, transferred again to a plastic bag, and then immersed in a constant temperature water bath having a temperature of 70 ° C. for 70 minutes to extract the E / OP stage. The obtained pulp was diluted to 3% with ion-exchanged water, then dehydrated and washed with a Buchner funnel.

55.0 g of the pulp after the E / OP stage was collected at an absolute dry mass, put in a plastic bag, adjusted to a pulp concentration of 10% using ion-exchanged water, and then 0.25 chlorine chloride per absolute dry pulp mass. % And caustic soda 0.05% were added and immersed in a constant temperature water bath at a temperature of 70 ° C. for 180 minutes to perform D-stage bleaching. The obtained pulp was diluted to 3% with ion-exchanged water, then dehydrated and washed with a Buchner funnel. The whiteness of the obtained bleached pulp was 84.5%, and the viscosity was 16.0 mPa · s.
Table 1 shows the pH during monopersulfate treatment, the properties of the treated pulp (whiteness, kappa number, viscosity), the final D-stage chlorine dioxide addition rate, and the whiteness and viscosity of the bleached pulp.

Comparative Example 1
In Example 1, after the monopersulfuric acid treatment, the same operation as in Example 1 was performed except that the cleaning was not performed and the chlorine dioxide addition rate in the D stage was changed to 0.55%. The bleached pulp had a whiteness of 85.8% and a viscosity of 13.5 mPa · s.
Table 1 shows the final D-stage chlorine dioxide addition rate and the whiteness and viscosity of the bleached pulp.

Comparative Example 2
In Example 6, after the monopersulfuric acid treatment, the same operation as in Example 1 was performed except that the cleaning was not performed and the chlorine dioxide addition rate in the D stage was changed to 0.40%. The whiteness of the bleached pulp was 84.4%, and the viscosity was 14.1 mPa · s.
Table 1 shows the final D-stage chlorine dioxide addition rate and the whiteness and viscosity of the bleached pulp.

  As is clear from comparing Examples 1 to 5 and Comparative Example 1 and Example 7 and Comparative Example 2 in Table 1, the unbleached pulp obtained by digesting the lignocellulosic material was bleached with alkaline oxygen, and the first stage When processing in an ECF bleaching step having an ozone bleaching step and an ozone / chlorine dioxide bleaching step, an inorganic peroxyacid treatment step and a washing step are provided before the ECF bleaching step to produce bleached pulp having a desired whiteness. It can be seen that the amount of chlorine dioxide required for the reduction can be reduced. In addition, as is clear from a comparison of Examples 1-2 and 3-4, it can also be seen that the effect is enhanced by adjusting the pH of the inorganic peroxyacid treatment stage to 2-3. Further, as apparent from a comparison between Example 1 and Example 6, it can be seen that the effect is further increased by performing alkali oxygen bleaching in multiple stages.

Claims (4)

Unbleached pulp obtained by digesting lignocellulosic material is bleached with alkaline oxygen, then treated with inorganic peroxyacid at pH 1.5 to 6.0 without adding a chelating agent, washed, and further started with ozone treatment A method for producing bleached pulp, comprising performing multi-stage bleaching treatment including chlorine dioxide treatment . The method for producing bleached pulp as claimed in claim 1, wherein the treatment pH with an inorganic peroxy acid is 2-3.   The method for producing bleached pulp according to claim 1 or 2, wherein the inorganic peroxyacid is monopersulfuric acid.   The method for producing bleached pulp according to any one of claims 1 to 3, wherein the alkaline oxygen bleaching is performed in a plurality of reactors.