JP2003213582A - Method for treating pulp - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To firstly provide an improved method for an acid treatment process for bleaching pulp, to secondarily provide an improved pulp bleaching method for obtaining bleached pulp having average viscosity and strength in the market while further reducing the amount of bleaching chemical used in a bleaching process and to thirdly provide a bleaching process for reducing the amount of AOX produced in the whole waste water of the bleaching process, promoting recovery of bleaching waste water and further closing the waste water by effectively carrying out the improved acid treatment and decreasing use of chlorine-based chemical. <P>SOLUTION: In the acid treatment process for bleaching chemical pulp, at least one kind of a salt of sulfurous acid and an alkali metal and/or a salt of sulfurous acid and an alkaline earth metal is added to the process besides the addition of an acid. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a pulp treating method for bleaching chemical pulp, which comprises adding a salt and / or a salt of sulfurous acid and an alkali metal in the acid treating step of the chemical pulp. The present invention relates to a method for treating pulp, which comprises adding at least one kind of sulfite salt among salts of sulfurous acid and alkaline earth metal.

[0002]

2. Description of the Related Art While attention has been paid to the effects of substances discharged from a bleaching process of a pulp and paper mill on the environment, conventional bleaching methods mainly using chlorine and / or chlorine-based chemicals have
Chlorine-free ECF bleaching and even more advanced TCF bleaching without chlorine-based chemicals are becoming the mainstream worldwide.
However, it is mainly used in ECF bleaching and TCF bleaching,
For example, most chemicals such as chlorine dioxide, hydrogen peroxide, oxygen and ozone are more expensive than conventional bleaching chemicals such as chlorine and sodium hypochlorite, resulting in higher processing costs for ECF bleaching and TCF bleaching. There's a problem. This prevents the spread of ECF bleaching and TCF bleaching. Under these circumstances, it is required to develop a pulp bleaching method that can further reduce the amount of bleaching chemicals used in ECF bleaching and TCF bleaching and can reduce the bleaching cost.

Against this background, acid treatment is known as one of the methods for reducing the amount of bleaching chemicals used. For example, in Japanese Patent Publication No. 10-508346, a suspension of cellulose pulp produced by a sulfate method or an alkali method is heated and treated at a temperature of about 85 to 150 ° C. at a pH of about 2 to 5 to give cellulose. At least about 50 of hexenuronic acid in pulp
%, And the Kappa number of the pulp is reduced by 2 to 9 units.

In the chemical pulp cooking process, hexenuronic acid is produced from methylglucuronic acid in hemicellulose. This hexenuronic acid has the property of pushing up the Kappa number and wasting the electrophilically reacting bleaching chemicals such as chlorine, chlorine dioxide, ozone and hydrogen peroxide. The acid treatment is a treatment method of decomposing hexenuronic acid in pulp by keeping the pulp under acidic and high temperature conditions for several hours.

Therefore, when hexenuronic acid is decomposed and removed by the acid treatment, the bleaching efficiency in the subsequent stage is improved and the bleaching chemical is reduced. Also, when using a chlorine-based chemical in the latter stage,
Since acid treatment can reduce chlorine-based chemicals, the concentration of chlorine ions in the effluent of the bleaching process as a whole will be low, and it will be possible to reduce the amount of AOX generated, promote the collection of bleaching effluent, and even close effluent.

However, the conventional acid treatment at a high temperature has a problem that not only the decomposition of hexenuronic acid but also the hydrolysis of cellulose and other polysaccharides causes a decrease in pulp viscosity, which causes a decrease in pulp viscosity. Development of suppressed acid treatment technology is desired.

[0007]

SUMMARY OF THE INVENTION A first object of the present invention is to provide an improved process of acid treatment for bleaching pulp. A second object is to provide an improved pulp bleaching method, which is capable of obtaining a bleached pulp having a viscosity and strength that are common in the market, while further reducing the amount of bleaching chemicals used in the bleaching step. Furthermore, the third problem is to carry out the improved acid treatment more effectively and to reduce the use of chlorine-based chemicals, thereby reducing the amount of AOX generated in the bleaching process wastewater as a whole, promoting the recovery of the bleaching wastewater, and It is to provide a bleaching process that enables closed drainage.

[0008]

According to the present invention, these problems are solved in the acid treatment step for bleaching chemical pulp, in addition to the addition of acid, a salt of sulfurous acid and an alkali metal and / or a sulfurous acid. This can be solved by a method for treating pulp, which is characterized by adding at least one kind of sulfite salt among salts with alkaline earth metals.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION The chemical pulp applied to the present invention includes those produced by the soda method, sulfite method and kraft method. In addition to conifers and hardwoods, so-called non-wood materials such as kenaf and straw can be applied as the material. Further, regarding the craft method, MCC, EMCC, ITC, Lo-solid method, etc. are known as modified methods, but are not limited to these methods, and those with or without oxygen delignification treatment are also known. Either can be applied.

The acid treatment to which the present invention can be applied may be any acid treatment for the purpose of decomposing and removing hexenuronic acid in pulp, and examples of the acid type include sulfuric acid, nitric acid,
Mineral acids such as hydrochloric acid, and organic acids such as formic acid, acetic acid can be used.

The sulfite used in the present invention is a salt of sulfite and a metal, and the metal is an alkali metal such as lithium, sodium and potassium and an alkaline earth metal such as magnesium, calcium, strontium and barium. This sulfite can be used alone or 2
You can use more than one type.

The concentration of the pulp to be subjected to the acid treatment to which the sulfite of the present invention is added is preferably in the range of about 1 to 30% by weight of solid content, and preferably about 3 to 15% by weight of solid content. If the pulp concentration is less than 3% by weight, the amount of acid used and the amount of steam required for heating increase, which is not preferable from the viewpoint of treatment cost. On the other hand, if it exceeds 15% by weight, the amount of free water decreases and the reactivity with acid decreases, which is not preferable.

The pH of the acid treatment containing sulfite is 1.2 to
The range of 3.7 is good, and the pH is preferably 1.8 to 3.5. When the pH is less than 1.8, more acid than the amount of acid required for decomposing hexenuronic acid is required, which is not preferable from the viewpoint of treatment cost. On the other hand, if the pH exceeds 3.5, the rate of hydrolysis of hexeuronic acid by the acid becomes slow, which is not preferable.

The temperature in the acid treatment with the addition of sulfite is 80-
The temperature is preferably 110 ° C, preferably 85 to 95 ° C. When the temperature is lower than 85 ° C., the rate of hydrolysis of hexeuronic acid by the acid becomes slow, which is not preferable. Also, 95
When the temperature is higher than 0 ° C, the pressure in the acid-treating reaction tower rises and the reaction becomes difficult to control, which is not preferable.

The amount of sulfite added is preferably in the range of 0.0001 to 2 kg per ton of absolutely dry pulp. If it is less than 0.0001 kg / t, the decrease in pulp viscosity due to acid treatment cannot be sufficiently suppressed. On the other hand, since the effect of suppressing the decrease in viscosity due to the addition of sulfite is leveled off around 2 kg / t, the addition of sulfite exceeding 2 kg / t is meaningless and is not preferable from the viewpoint of treatment cost.

The sulfuric acid and the sulfite may be added to the pulp suspension by adding the sulfite first and then the sulfuric acid, or simultaneously. Alternatively, sulfuric acid and sulfite may be mixed in advance and added to the pulp suspension.

The details of the mechanism for suppressing the decrease in pulp viscosity by the addition of sulfite are unknown, but the present inventors presume as follows. Originally, the acid treatment is intended to heat-treat pulp under acidic conditions to selectively decompose hexenuronic acid present in the xylan side chain, but at the same time also hydrolyze cellulose and hemicellulose, causing a decrease in viscosity. I will end up. It is speculated that the addition of sulfite during the acid treatment stabilizes the structure of the polysaccharide in the pulp fiber, and thus can suppress the decrease in pulp viscosity.

The fact that the structure of the polysaccharide can be stabilized by adding sulfite leads to more efficient and effective acid treatment, which is conventionally limited in its application due to the decrease in pulp viscosity. . In other words, this also leads to the promotion of bleaching efficiency in the latter stage, and by further reducing the amount of chlorine-based chemicals used, the amount of AOX generated in the bleaching process wastewater as a whole, the promotion of bleaching wastewater collection, and further Enables closed drainage.

[0019]

EXAMPLES The present invention will be described in more detail with reference to the following examples, but the present invention is not limited to these examples. The physical properties of pulp were measured by the following test methods.・ Kappa number (KN number): JIS P8211: 1998 Viscosity: J.TAPPI No.44-84

[0020]

Example 1 After cooking hardwood chips in a continuous digester,
Oxygen delignification was performed to obtain a pulp having a kappa number of 11.3, a Hunter whiteness of 45.7% and a pulp viscosity of 24.4 cP (this pulp was also used in the following Example 2, Comparative Examples 1 and 2). The pulp after this oxygen delignification was added with 0.8% by weight of sulfuric acid.
(Vs. dry pulp), sodium sulfite addition amount 0.05 solids wt% (vs. dry pulp), pulp concentration 10 solids wt%,
The acid treatment was carried out at a reaction temperature of 95 ° C and a reaction time of 90 to 270 minutes.
The bleaching conditions and the results at this time are shown in Example 1 of Table 1.

[0021]

Comparative Example 1 The same pulp after oxygen delignification as in Example 1 was treated with a sulfuric acid addition rate of 0.7% by weight (versus dry pulp), a pulp concentration of 10% solids by weight, a reaction temperature of 95 ° C., and a reaction time of 90 to 270 minutes. Acid treatment was performed. The bleaching conditions and the results at this time are shown in Comparative Example 1 in Table 1.

[Table 1]

[0022]

Example 2 The oxygen delignified pulp shown in Example 1 was prepared by adding 0.8% by weight of sulfuric acid (to dry pulp), 0.05% by weight of sodium sulfite (to dry pulp), and 10 solids of pulp. Min% by weight, reaction temperature 95 ℃, reaction time 210
Acid treatment was performed in minutes. At this time, KN value is 6.5, viscosity is 23.7c
It was P. The acid-treated pulp is washed, and subsequently, the ozone addition rate is 0.5% by weight (versus dry pulp), the pulp concentration is 10% solids by weight, the reaction temperature is 50 ° C., the reaction time is 10 minutes,
Ozone bleaching was performed at treatment pH 3.0. After washing again, the amount of sodium hydroxide added is 0.9% solids by weight (versus dry pulp), the pulp concentration is 10% solids by weight, reaction temperature is 80 ° C, reaction time is 60
Min, alkali extraction was performed at treatment pH 11.8. The zero-span breaking length at this time was 11.7 km. Wash the pulp after alkali extraction, followed by chlorine dioxide addition rate 0.5 wt
%, Pulp concentration 10% solids by weight, reaction temperature 70 ° C., reaction time 180 minutes, chlorine dioxide bleaching was performed. The whiteness at this time is 8
The viscosity was 4.6%, the viscosity was 17.6 cP, and the zero-span breaking length was 11.7 km. The bleaching conditions and the results at this time are shown in Example 2 in Table 2.

[0023]

Comparative Example 2 The pulp shown in Example 1 was mixed with sulfuric acid at a rate of 0.
The acid treatment was carried out at 7% by weight (versus dry pulp), a pulp concentration of 10% solids by weight, a reaction temperature of 95 ° C. and a reaction time of 210 minutes.
At this time, the KN value was 6.6 and the viscosity was 19.8 cP. The pulp after this acid treatment is washed, and then the ozone addition rate is increased.
Ozone bleaching was carried out at 0.5% by weight (versus dry pulp), pulp concentration of 10% solids by weight, reaction temperature of 50 ° C., reaction time of 10 minutes, and treatment pH of 3.0. After washing again, the amount of sodium hydroxide added was 0.
Alkali extraction was performed at 9 solids weight% (versus dry pulp), pulp concentration 10 solids weight%, reaction temperature 80 ° C, reaction time 60 minutes, treatment pH 11.8. Zero-span breaking length at this time is 11.4km.
Met. The pulp after alkali extraction was washed, and subsequently, chlorine dioxide bleaching was carried out at a chlorine dioxide addition rate of 0.5% by weight, a pulp concentration of 10% solids by weight, a reaction temperature of 70 ° C. and a reaction time of 180 minutes. At this time, the whiteness was 84.4%, the viscosity was 16.3 cP, and the zero-span breaking length was 11.1 km. The bleaching conditions and the results at this time are shown in Comparative Example 2 in Table 2.

[Table 2]

From the results in Table 1, the viscosity was plotted against the KN value, and FIG. 1 was prepared. In the comparison with the same KN value,
It is clear that the pulp viscosity obtained when sodium sulfite is added as in Example 1 is higher than that in the case where sodium sulfite is not added as in Comparative Example 1.

[Figure 1]

FIG. 2 was prepared from the results of Table 2 after bleaching.
Figure 2-1 shows a comparison of the zero-span breaking lengths of bleached pulp.
A comparison of the bleached pulp viscosities is shown in Figure 2-2. It is clear that the pulp obtained by adding sodium sulfite as in Example 2 has a higher zero-span breaking length and higher viscosity than the pulp obtained by not adding sodium sulfite as in Comparative Example 2. is there.

[Fig. 2]

[0026]

INDUSTRIAL APPLICABILITY In the acid treatment step for bleaching chemical pulp, in addition to the addition of acid, at least one kind of salt of sulfurous acid and alkali metal and / or salt of sulfurous acid and alkaline earth metal is used. By adding sulfite, hexenuronic acid can be decomposed and removed while suppressing the decrease in the viscosity and strength of pulp. This makes it possible to reduce the amount of bleaching chemicals used in the subsequent stage. When bleaching with chlorine-based chemicals is performed after acid treatment, the amount of chlorine-based chemicals used can be further reduced, reducing the amount of AOX generated in the bleaching process wastewater as a whole, promoting the collection of bleaching wastewater, and even closing the wastewater. Make more possible.

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Claims (2)

[Claims] 1. In an acid treatment step for bleaching chemical pulp, in addition to addition of an acid, at least one kind of salt of sulfurous acid and alkali metal and / or salt of sulfurous acid and alkaline earth metal is used. A method for treating pulp, which comprises adding sulfite. 2. The method of treating pulp according to claim 1, wherein the acid added in the acid treatment step of the chemical pulp is sulfuric acid, and the sulfite salt added is sodium sulfite.