JPH04316690A - Bleaching of lignocellulose material - Google Patents

Abstract

PURPOSE:To decrease the amount of chlorine-based bleaching chemicals and prevent the lowering of the viscosity of pulp by applying a specific pretreatment to a pulp composed of a lignocellulose material before oxygen bleaching, thereby effectively removing lignin from the pulp. CONSTITUTION:A pulp derived from a lignocellulose material is mixed with a nitrite and an acid and left standing preferably at 60-90 deg.C for 30min to 3hr to effect the pretreatment of the pulp. The pretreated pulp is subjected to oxygen bleaching. The nitrite is preferably sodium nitrite or potassium nitrite and the acid is preferably nitric acid, sulfuric acid or acetic acid. Preferably, the aqueous solution of the nitrite and the acid are separately added in two stages in the pretreatment.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention relates to a method for bleaching lignocellulosic materials, and more specifically, a method for bleaching lignocellulosic materials that further advances delignification in oxygen bleaching and reduces the decrease in pulp viscosity. It is related to.

0002

BACKGROUND OF THE INVENTION Examples of pulp produced by cooking lignocellulose material with alkali include kraft pulp, polysulfide pulp, and soda pulp. In order to use these pulps for many purposes, such as papermaking raw materials, it is necessary to bleach them. For example, when bleaching unbleached kraft pulp, in order to remove residual lignin, which is a substance that causes coloration, oxygen bleaching first removes residual lignin by 40 to 60%.
degree removal, followed by chlorine, chlorine dioxide, hypochlorite,
Bleaching agents and bleaching aids such as oxygen, hydrogen peroxide, and sodium hydroxide are used to further remove residual lignin. When producing high-strength bleached kraft pulp as a papermaking raw material, a bleaching method is used that minimizes the decrease in pulp viscosity in order to maintain the high strength of the pulp fiber itself. Such bleaching treatment methods include oxygen bleaching treatment (O), chlorine and chlorine dioxide treatment (C+D), alkaline extraction treatment (E), oxygen-enhanced alkaline extraction treatment (Eo), and hypochlorite treatment. Process (
H), chlorine dioxide treatment (D), hydrogen peroxide treatment (P)
), O→(C+D)→E→H→D, O→(C+D)→Eo→
D→E→D or O→(C+D)→Eo→H→P→D
The processing order is as follows. The bleaching effluent from oxygen bleaching is usually recycled as a cleaning solution for the pulp after cooking, and is ultimately burned together with the cooking effluent to recover the ash and convert it into inorganic chemicals such as sodium hydroxide. will be played. However, since the bleaching wastewater discharged from the chlorine-based treatment stage contains chlorine ions, it cannot be recovered by combustion like the oxygen bleaching wastewater mentioned above. being processed. For this reason, in order to reduce the amount of chlorine ions and organic chlorine compounds in the bleaching liquid discharged from the chlorine treatment stage, it is necessary to reduce the amount of chlorine chemicals used. As a countermeasure to this problem, it is possible to reduce the amount of chlorine-based chemicals used by further delignifying the pulp in the oxygen bleaching process that is performed before the chlorine-based process. If delignification is attempted to proceed further by increasing the pressure, treatment temperature, etc., the strength of the pulp fibers and the viscosity of the pulp will decrease. Note that since there is a positive correlation between the strength and viscosity of pulp fibers, the viscosity of pulp is widely used as an index of the strength of pulp fibers.

[0003] Therefore, in oxygen bleaching, in order to promote delignification and at the same time suppress the decrease in pulp viscosity, several so-called pretreatment oxygen bleaching methods have been proposed in which pulp is pretreated before oxygen bleaching. ing. for example,
Samuelson proposed a method of pretreating pulp made from lignocellulosic material with nitrogen dioxide, followed by oxygen bleaching (Japanese Patent Publication No. 63).
-50464, Tokuko Sho 63-50465, Tokuko Sho 63-
50466, Special Publication No. 63-50467, Special Publication No. 63-6
0158, Special Publication No. 64-2715). In Samuelson's method, as a pretreatment, pulp with high or medium concentration is first treated in the gas phase with nitrogen dioxide (gas) to oxidize the remaining lignin, and then modified by further nitration.
After adding sodium nitrate to regenerate nitrogen dioxide and performing the above reaction again, oxygen bleaching is performed (Journal of Pulp and Paper Science (J. Pulp Sci.) Vol. 13, No. 5) , J150
(1989), Tappi Journal (Tappi J
．． Vol. 73, No. 2, p. 141 (1990)). Also,
There is also a pilot plant named Prenox in which highly concentrated pulp is pretreated with nitrogen dioxide similar to the Samuelson method described above. Furthermore, the pulp produced from lignocellulosic materials is pretreated with acids such as sulfuric acid, hydrochloric acid, and acetic acid, followed by washing with water to remove metal ions that are responsible for promoting pulp viscosity, and then oxygen bleaching. There is also a method of delignification by
Vol. 8, No. 12, p. 1179 (1984), Pulp & Paper, Vol. 43,
No. 9, p. 148 (1969), Pulp & Paper Magazine of Canada (Pulp & Pap
er Mag. Can. ), Volume 73, No. 12, 102
Page (1972)).

0004

[Problems to be Solved by the Invention] However, Samuelson's method of pre-treating pulp with nitrogen dioxide is difficult to uniformly react to the pulp because it is treated in the gas phase. The problem is that it is small. In particular, in order to treat high-concentration pulp with gaseous nitrogen dioxide, as in the Plenox pilot plant, equipment to compress the pulp and equipment to crush (fluff) the pulp are required, both of which are not commercially viable. The current situation is that this has not been done. In addition, the above-mentioned method of pre-treating pulp with acids such as sulfuric acid, hydrochloric acid, acetic acid, etc. has little effect, and when delignification progresses in oxygen bleaching, the viscosity of the pulp decreases significantly. is not widespread either. The present invention solves the conventional problems as described above, and in order to reduce the amount of chlorine-based chemicals used in the pulp bleaching process, the present invention further advances delignification in the oxygen bleaching process and does not reduce the viscosity of the pulp. It is an object of the present invention to provide a method for bleaching lignocellulosic materials.

[0005]

[Means for Solving the Problems] As a result of intensive studies, the inventors of the present invention were able to solve the above-mentioned conventional problems. That is, the present invention provides a method for delignifying pulp produced from lignocellulosic material by oxygen bleaching, in which the pulp is first pretreated by adding nitrite and acid, and then oxygen bleaching is performed. The present invention provides a method for bleaching lignocellulosic materials.

[0006] The present invention will be explained in more detail below. The bleaching method of the present invention is characterized by the addition of nitrite and acid as a pretreatment to the pulp produced from lignocellulosic material, followed by oxygen bleaching. Unlike conventional methods, pretreatment is not performed using gaseous nitrogen dioxide. What is lignocellulose material here?
It is a type of composite cellulose in which cellulose and lignin are tightly bound physically or chemically, and representative examples include wood, bamboo, hemp, straw, bagasse, etc. (Chemicals that can be used for pretreatment) Pulp that can be used for pretreatment includes, for example, pulp prepared by cooking lignocellulose material with an alkali. There are no particular limitations on the nitrite that can be used in the method of the present invention. For example, commercially available sodium nitrite, potassium nitrite, etc. can be mentioned. Furthermore, a nitrite aqueous solution prepared by absorbing NO2 or NO into a sodium carbonate or sodium hydroxide aqueous solution can also be used. This is preferable since it is possible to effectively utilize NO2 and NO generated in the exhaust gas of a boiler or the like in a pulp bleaching factory or by burning bleaching waste liquid. When these nitrites are added to pulp, they are usually added as an aqueous solution so that they are uniformly mixed with the pulp.

[0007] As the acid, both inorganic acids and organic acids can be used, examples of which include nitric acid, sulfuric acid, hydrochloric acid, and acetic acid. Preferred combinations include sodium nitrite and nitric acid, sulfuric acid and acetic acid, potassium nitrite and nitric acid, sulfuric acid, and acetic acid for subsequent combustion recovery. Moreover, the above-mentioned different inorganic acids and organic acids can also be used simultaneously.

(Amount of chemical added) The amount of nitrite added is determined based on the dry weight of the pulp to be treated.
2-) 0.1 to 10% by weight, preferably 0.5 to 10% by weight
It is 5% by weight, more preferably 1.5 to 3% by weight. The amount of acid added is 0 based on the dry weight of the pulp to be treated.
．． The amount is 1 to 10% by weight, preferably 0.5 to 5% by weight, and more preferably 2 to 4% by weight. The acid is preferably added as a dilute acid with a concentration of about 10%. The weight of acid in this case means the weight of hydrochloric acid, sulfuric acid, etc. in the dilute acid.

(Pretreatment) Pretreatment is performed using the chemicals and amounts added as described above. The concentration of the pulp at the time of pretreatment, that is, the dry weight of the pulp relative to the total weight of the water-containing pulp, is a medium concentration, for example, 5 to 17%, preferably 8 to 14%.
, is good. If the pulp concentration is lower than 5%, the processing amount will increase and the treatment efficiency will decrease, and if it is higher than 17%, the reaction will become non-uniform, which is not preferable. In order to uniformly carry out the reaction in the pretreatment and to maximize this effect, it is preferable that the nitrite aqueous solution and the acid are added separately in two stages and mixed with the pulp. The nitrite and acid can be added in either order. That is, ■ Preparing an aqueous solution of nitrite and/or an aqueous solution of nitrogen oxides such as NO2 and NO dissolved in an alkali, adding this to the pulp and mixing it, and then adding a separately prepared acid to the pulp. and mix. Conversely, ■ the prepared acid is added to the pulp and mixed, and then a previously prepared aqueous solution of nitrite and/or an aqueous solution of nitrogen oxides such as NO2 and NO dissolved in alkali is added to the pulp. Any of the following methods may be used. In this way, better results can be obtained than using a solution in which nitrite is added to an acid or a solution in which an aqueous nitrite solution and an acid are mixed. Further, it is not preferable to prepare an acidic nitrite solution in advance because NO2-, which is an effective component in the pretreatment, is decomposed. The pulp to which either the nitrite aqueous solution or the acid has been added is subsequently mixed, using a mixer for medium consistency pulp processing such as MC mixer manufactured by Kamiya Co., Ltd., for example. Next, add one of the other chemicals and mix as above, at 40-95°C, preferably at 6°C.
The pulp is maintained at 0 to 90°C for 20 minutes to 5 hours, preferably 30 minutes to 3 hours, and the pulp is washed with water or the like. The preprocessing is completed as described above.

Good results can be obtained by adding oxygen in this pretreatment. Oxygen may be added at any stage of the pretreatment. The addition method may be any method, but examples include a method of pressurizing oxygen or oxygen-enriched air into a pulp mixing mixer. The amount added is 0.1 to 6%.

[0011] Furthermore, after the pretreatment is completed, an alkali can be added to the pulp without squeezing or washing the pulp, followed by oxygen bleaching. This is preferable because it eliminates the need for equipment used for squeezing and washing the pulp. Various alkalis can be used as the alkali in this case, such as sodium hydroxide, potassium hydroxide, and the like. The amount added is 0.1 to 15% by weight, preferably 1 to 5% by weight, based on the dry weight of the pulp upon completion of pretreatment.

(Oxygen bleaching) After the pulp has been pretreated as described above, oxygen bleaching is performed. Oxygen bleaching can be performed by conventional methods. For example, it can be carried out by a medium concentration oxygen bleaching method or a high concentration oxygen bleaching method, which are industrially put into practical use.

[0013]

[Examples] The present invention will be explained below with reference to Examples. In the examples shown below, the kappa number of the pulp was measured according to the TAPPI test method T236cm-85, and the viscosity of the pulp was measured according to the TAPPI test method T230om-89. The degree of delignification D (%) is expressed as follows, where Ku is the kappa number of untreated pulp and Ko is the kappa number of pulp after oxygen bleaching.

[0014]

[Math. 1] D (%) = 100 x (Ku-Ko) ÷ Ku

0
015]. In addition, the kappa number of untreated beechwood kraft pulp is 20.9, the viscosity is 54.8,
The kappa value of untreated Scots pine kraft pulp is 30.
．． 9. The viscosity was 32.6.

Example 1 (Pretreatment) A total of 134 ml of water was added to 20 g (dry weight) of beech wood kraft pulp to make the pulp concentration 13%, and the mixture was placed in a Teflon bag, and NO2- ions were added to the pulp. 3 ml of 1.5% by weight based on dry weight
A 5% by weight aqueous sodium nitrite solution was added and mixed with the pulp. Furthermore, 4 ml of a 10% nitric acid aqueous solution was added to this so that the amount of nitric acid was 2% by weight based on the dry weight of the pulp, and mixed with the pulp. The final pulp density is 12%
And so. Pulp in a Teflon bag is kept at 60℃ in a constant temperature bath.
Pretreatment was carried out by heating the sample to a temperature of 100.degree. C. and holding it for 30 minutes. After pretreatment, the pulp was washed with water. (Oxygen bleaching) Prepare 4 g of pretreated pulp to a pulp concentration of 30%, and add 2, 3 and 5% by weight of sodium hydroxide and 1% by weight of magnesium sulfate, respectively, based on the dry weight of the pulp. 1, 1.5 and 2.5 ml of normal sodium hydroxide aqueous solution and 0.4 ml of 10% magnesium sulfate aqueous solution were added and mixed with the pulp. The final pulp density was 22%. The pulp was heated to 120°C in a reaction vessel with an oxygen pressure of 3.5 kg/cm2, and the reaction vessel was slowly rotated (15 times/min) for 30 minutes.
Oxygen bleaching was performed by holding for a minute. After oxygen bleaching, the pulp was washed with water and its kappa number and viscosity were measured. The results are shown in Table 1.

Comparative Example 1 Beechwood kraft pulp without pretreatment was oxygen bleached in the same manner as in Example 1, and the kappa number and viscosity of the pulp were measured. The results are shown in Table 1.

Comparative Example 2 A total of 60 ml of water was added to 20 g (dry weight) of beech wood kraft pulp to make the pulp concentration 25%, fluffed using a rotary blade, and placed in a reaction vessel with a capacity of 2 liters. The air inside was removed. To this, 0.28 ml of liquid nitrogen dioxide equivalent to 2% of the dry weight of the pulp was added completely vaporized, the pulp was shaken, and after 5 minutes air was introduced into the container, and the pulp was heated for another 5 minutes. Pretreatment was performed by shaking. The pretreatment temperature was 25°C. After the pretreatment, the pulp was washed with water, oxygen bleached in the same manner as in Example 1, and the kappa number and viscosity of the pulp were measured. The results are shown in Table 1.

Comparative Example 3 A total of 143 ml of water was added to 20 g (dry weight) of beech wood kraft pulp, and 4 ml of a 10% nitric acid aqueous solution was added to this so that the nitric acid amount was 2% by weight based on the dry weight of the pulp. and mixed with pulp. The final pulp concentration was 12%. The pulp in a Teflon bag was heated to 60° C. in a constant temperature bath and held for 30 minutes for pretreatment. After the pretreatment, the pulp was washed with water, oxygen bleached in the same manner as in Example 1, and the kappa number and viscosity of the pulp were measured. The results are shown in Table 1.

[0020]

[Table 1]

Example 2 (Pretreatment) A total of 134 ml of water was added to 20 g (dry weight) of Scots pine kraft pulp, and the pulp concentration was adjusted to 1.
3% and put it in a Teflon bag, so that NO2- ion is 3% by weight based on the dry weight of the pulp.
Add 6 ml of 15% by weight aqueous sodium nitrite solution,
mixed with pulp. Furthermore, 8 ml of a 10% nitric acid aqueous solution was added to this so that the amount of nitric acid was 4% by weight based on the dry weight of the pulp, and mixed with the pulp. The final pulp concentration was 12%. The pulp in a Teflon bag was heated to 90° C. in a constant temperature bath and held for 3 hours for pretreatment. After the pretreatment, the pulp was washed with water, oxygen bleached in the same manner as in Example 1, and the kappa number and viscosity of the pulp were measured. The results are shown in Table 2.

Example 3 A total of 134 ml of water was added to 20 g (dry weight) of Scots pine kraft pulp to make the pulp concentration 13%, and the mixture was placed in a Teflon bag, and NO2- ions were added to the dry weight of the pulp. 6 ml of 1 ml to give 3% by weight
A 5% by weight aqueous sodium nitrite solution was added and mixed with the pulp. Furthermore, 8 ml of a 10% nitric acid aqueous solution was added to this so that the amount of nitric acid was 4% by weight based on the dry weight of the pulp, and mixed with the pulp. The final pulp density is 12%
And so. Next, in a Teflon bag at 25℃, 80℃ at 1 atm.
0 ml of oxygen was sealed. Furthermore, this was heated to 90°C in a constant temperature bath.
Pretreatment was carried out by heating to ℃ and holding for 3 hours. After the pretreatment, the pulp was washed with water, oxygen bleached in the same manner as in Example 1, and the kappa number and viscosity of the pulp were measured. The results are shown in Table 2.

Example 4 A total of 134 ml of water was added to 20 g (dry weight) of Scots pine kraft pulp to make the pulp concentration 13%, and the mixture was placed in a Teflon bag, and NO2- ions were added to the dry weight of the pulp. 6 ml of 1 ml to give 3% by weight
A 5% by weight aqueous sodium nitrite solution was added and mixed with the pulp. Furthermore, 8 ml of a 10% nitric acid aqueous solution was added to this so that the amount of nitric acid was 4% by weight based on the dry weight of the pulp, and mixed with the pulp. The final pulp density is 12%
And so. Next, in a Teflon bag at 25℃, 80℃ at 1 atm.
0 ml of oxygen was sealed. Furthermore, this was heated to 90°C in a constant temperature bath.
Pretreatment was carried out by heating to ℃ and holding for 3 hours. Immediately after the pretreatment, 7.5% of a 2N aqueous sodium hydroxide solution was added to the pulp in a Teflon bag so that the amount of sodium hydroxide was 3 or 5% by weight and the amount of magnesium sulfate was 1% by weight based on the dry weight of the pulp. 5 or 12.5 ml and 2 ml of 10% magnesium sulfate aqueous solution,
mixed with pulp. The final pulp density was 11%. The pulp is heated in a reaction vessel with an oxygen pressure of 3.5 kg/cm2.
Oxygen bleaching was performed by heating to 20°C and holding for 30 minutes. After oxygen bleaching, the pulp was washed with water and its kappa number and viscosity were measured. The results are shown in Table 2.

Comparative Example 4 A kraft pulp made of Scots pine without pretreatment was oxygen bleached in the same manner as in Example 1, and the kappa number and viscosity of the pulp were measured. The results are shown in Table 2.

Comparative Example 5 A total of 60 ml of water was added to 20 g (dry weight) of Scots pine kraft pulp to give a pulp concentration of 25%, which was fluffed using a rotary blade and placed in a reaction vessel with a capacity of 2 liters. The air inside was removed. To this, 0.28 ml of liquid nitrogen dioxide equivalent to 2% of the dry weight of the pulp was added completely vaporized, the pulp was shaken, and after 5 minutes air was introduced into the container, and the pulp was heated for another 5 minutes. Pretreatment was performed by shaking. The pretreatment temperature was 25°C. After the pretreatment, the pulp was washed with water, oxygen bleached in the same manner as in Example 1, and the kappa number and viscosity of the pulp were measured. The results are shown in Table 2.

[0026]

[Table 2]

As can be seen from Tables 1 and 2, the method of the present invention can be applied to both hardwood and softwood pulps. Furthermore, it has been clearly shown that the method of the present invention is superior to conventional methods of treatment with nitrogen dioxide and methods of cleaning with acid. Furthermore, as can be seen from Table 2, it can be seen that adding oxygen to the pretreatment showed better results. Furthermore, it can be seen that better results are obtained when the pretreated pulp is not squeezed or washed.

[0028]

Effects of the Invention In the method of the present invention, in particular, since a liquid is used as the delignification chemical, the pulp and the chemical can react more uniformly than in the conventional method using gaseous nitrogen dioxide. The lignin contained in the pulp produced from the material can be effectively removed. As a result, the amount of chlorine bleaching chemicals used can be reduced, and pulp fibers with high whiteness and strength can be produced. Furthermore, the nitrite and acid used in the method of the present invention can be burned and recovered together with the oxygen bleaching effluent and cooking effluent.

Claims (5)

[Claims] 1. A method for delignifying pulp produced from lignocellulosic material by oxygen bleaching, comprising first pretreating the pulp by adding nitrite and acid to the pulp, followed by oxygen bleaching. Characterized by
Method for bleaching lignocellulosic materials. 2. The method according to claim 1, wherein the nitrite and acid are added separately to the pulp in two stages. 3. A method according to claim 1, wherein oxygen is added to the pulp pretreatment. 4. The method according to claim 1, wherein after the pretreatment of the pulp, oxygen bleaching is performed by adding an alkali to the pulp without squeezing and/or washing the pulp. 5. The amount of nitrite added is 0.1 to 0.1 as nitrite ions (NO2−) based on the dry weight of the pulp.
10% by weight, and the amount of acid added is 0.1 to 10% by weight,
Processing temperature is 40 to 95°C, processing time is 20 minutes to 5 hours,
5. The method according to claim 1, wherein the pretreatment is carried out at a pulp concentration of 5 to 17%.