JPH0813369A - Method for multi-stage bleaching of pulp - Google Patents

Abstract

PURPOSE:To provide a method for multi-stage bleaching of pulp having no undesirable influence on pulp in bleaching with ozone, suppressing reduction in bleaching properties, capable of effectively using ozone in a multi-stage bleaching process of pulp by adopting counter current washing method. CONSTITUTION:Washing is carried out after bleaching with oxygen, and then, bleaching with ozone is effected. The washing is carried out by counter current against the transportation direction of pulp sent from each bleaching stage. Part of washing drain from a washing machine arranged between an oxygen bleaching stage and an ozone bleaching stage is used for diluting pulp before the ozone beaching stage, and the amount of totally dissolved solid(TDS) in the washing drain and/or water in the transported pulp is monitored. The amount of the totally dissolved solid(TDS) is adjusted to a proper value, preferably <=30kg per ton based on the absolute dry weight of pulp according to the measured value.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a multi-stage bleaching method for pulp which has been subjected to oxygen bleaching and ozone bleaching, and more particularly, to prevent wasteful use of ozone and suppress deterioration of bleachability of pulp. The present invention relates to a multi-stage bleaching method for pulp that can be produced.

[0002]

2. Description of the Related Art In the current method of bleaching chemical pulp,
Since it is difficult to completely remove the coloring components in the pulp by a single step operation using only one bleaching agent, a multi-step bleaching method using a plurality of bleaching agents is generally used. On the other hand, in recent years, instead of using chlorine-based bleaching agents that cause the production of harmful organic chlorine-based compounds such as dioxins, such harmful substances are not produced, and the bleaching action is stronger than that of chlorine-based bleaching agents. Has been noted for bleaching pulp.

As a multi-stage bleaching method for pulp in which ozone is used as a bleaching agent for pulp, it has been proposed to bleach the pulp with alkali, bleach with oxygen, bleach with ozone, and finally finish bleaching. In this pulp multi-stage bleaching method, a washing machine is installed between the alkaline digestion and the oxygen bleaching and the subsequent bleaching and bleaching. By washing with this washing machine, in addition to lignin and the like decomposed and extracted from pulp by alkaline digestion and bleaching, organic decomposed products and bleaching chemicals (caustic soda in the case of oxygen) are washed out. The washing wastewater discharged from this washing machine contains soluble evaporation residual solids (TDS: Total Dissolved Solid, hereinafter referred to as TDS) mainly composed of lignin decomposition products.

As the water used for washing the pulp, a large amount of new water was used in the past, but it is discharged from the washing machine at the latter stage of the multi-stage bleaching method because of disadvantages of wastewater treatment and economical efficiency. So-called countercurrent washing is generally performed in which the washing waste water is used as dilution water for the pulp in the preceding stage.
In a multi-stage bleaching method of pulp using ozone as a bleaching agent for pulp, pulp concentration conditions of ozone bleaching in the latter stage (for example, pulp concentration of about 15% or more after washing and dehydration after oxygen bleaching) are used. The washing wastewater from the washing machines in the latter stage of the plurality of washing machines arranged between the oxygen bleaching stage and the ozone bleaching stage is diluted with the pulp in the preceding stage so that the medium concentration pulp condition is 8% to 20%). Is added as.

[0005]

However, if such countercurrent washing is performed, even if the washing is performed by a washing machine, a part of the washing wastewater is used as dilution water for pulp, so that the oxygen bleaching stage is not used. The TDS derived from the washing wastewater from the washing machine arranged between the ozone bleaching stage and the pulp is entrained in the pulp, and it is impossible to completely remove the TDS from the pulp. Unavoidable. Therefore, the TDS derived from the oxygen bleaching stage is used in the ozone bleaching stage.
Will flow in.

In the pulp multi-stage bleaching method using ozone, a washing machine is provided after the ozone bleaching stage, and the washing wastewater discharged from this washing machine is also used as dilution water for the ozone bleaching stage. . Therefore, in addition to the inflow of the TDS derived from the oxygen bleaching stage described above, the TDS derived from the ozone bleaching stage in the cleaning wastewater generated in the washer after the ozone bleaching stage also flows into the ozone bleaching stage as pulp dilution water.

In a multi-stage bleaching method for a pulp having an oxygen bleaching stage and an ozone bleaching stage, TDS derived from washing wastewater from a washing machine disposed between the oxygen bleaching stage and the ozone bleaching stage, and ozone bleaching When TDS derived from washing wastewater from a washing machine arranged after the stage is mixed in the pulp, it is not sufficiently clarified how it affects the bleaching degree of the pulp during ozone bleaching in the ozone bleaching stage. It was

On the other hand, ozone is expensive as compared with other bleaching agents and must be effectively used for bleaching pulp. However, oxygen bleaching by an oxygen bleaching stage and then ozone bleaching by an ozone bleaching stage are performed, In the multi-stage bleaching method of pulp in which the countercurrent washing is performed on the pulp transferred from each bleaching stage, the conditions for using ozone without waste were not clear.

Therefore, in the present invention, in the multi-stage bleaching method of pulp which uses ozone as a bleaching agent for pulp and adopts the countercurrent washing method, the influence of TDS derived from washing wastewater on each bleaching stage on ozone bleaching is examined. Clarified by a bleaching experiment, there is no adverse effect on the pulp produced,
It is an object of the present invention to provide a multi-stage bleaching method for pulp, which suppresses deterioration of bleaching property and can effectively use ozone during ozone bleaching.

[0010]

Means for Solving the Problems The inventors of the present invention have conducted extensive studies and found that T generated between an oxygen bleaching stage and an ozone bleaching stage rather than TDS occurring after the ozone bleaching stage.
It has been found that the inflow of DS increases the amount of ineffective consumption of ozone (due to the reaction between organic matter and ozone in TDS) during ozone bleaching in the ozone bleaching stage, hinders the increase of whiteness and causes the decrease of pulp viscosity. .

Therefore, in order to solve the above-mentioned problems, the multi-stage bleaching method of pulp of the present invention is carried out by oxygen bleaching by an oxygen bleaching stage and then ozone bleaching by an ozone bleaching stage, and the pulp transferred from each bleaching stage In a multi-stage bleaching method of pulp in which countercurrent washing is performed, (1) a part of the washing wastewater from a washing machine placed between the oxygen bleaching stage and the ozone bleaching stage is used to dilute the pulp before the ozone bleaching stage. (2) Soluble evaporation in pulp-free water flowing into the ozone bleaching stage by monitoring (2) the amount of soluble evaporation residual solids (TDS) in the washing wastewater and / or the entrained water of the transferred pulp. It is characterized in that the amount of residual solids (TDS) was adjusted to an appropriate value.

Further, it is desirable to use a part of the cleaning waste water from the washer after the ozone bleaching stage for diluting the pulp before and after the ozone bleaching stage.

It is desirable that the appropriate amount of soluble evaporation residual solids (TDS) in the pulp free water flowing into the ozone bleaching stage is adjusted to be 30 kg or less per ton dry weight of pulp. .

In the pulp bleaching method of the present invention, the amount of soluble evaporation residual solids (TDS) in the cleaning wastewater is monitored by a washing water line from the oxygen bleaching stage to the ozone bleaching stage and / or the transfer of pulp. It is desirable to use a TDS sensor provided at any position on the line.

[0015]

EXAMPLES The present invention will be described based on examples. In addition,
This embodiment is merely an example embodying the present invention, and the present invention is not limited to this.

FIG. 1 is an example of a process diagram used in the pulp multi-stage bleaching method of the present invention. Reference numeral 1 denotes an oxygen bleaching stage in which alkali-cooked pulp is introduced and oxygen bleaching is performed. Reference numeral 2 is a first washing machine, in which oxygen-bleached and diluted pulp is introduced into the washing machine 2 by a pulp transfer line and washed. 3 is a second washing machine, and this washing machine 3 is used for further washing. 4 is an ozone bleaching stage, and the pulp that has been washed as described above is ozone bleached in this ozone bleaching stage 4. Reference numeral 5 denotes a washing machine, in which the ozone-bleached and diluted pulp is introduced into the washing machine 5 and washed.

In this multi-stage bleaching method for pulp, the washing water line for passing the washing water introduced into each washing machine is
Two lines of washing water are provided, an oxygen bleaching stage system for washing the oxygen bleached pulp and an ozone bleaching stage system for washing the ozone bleached pulp. In each washing water line, the washing water is transferred so as to have a countercurrent to the pulp transfer direction. In the cleaning water line of the oxygen bleaching stage system, clean cleaning water is supplied to the cleaning machine 3, used for cleaning, and discharged as cleaning drainage. Next, a part of this cleaning waste water is supplied to the pulp transfer line between the cleaning machine 2 and the cleaning machine 3 as dilution water,
The rest of the cleaning waste water is supplied to the cleaning machine 2 as cleaning water. Similarly, a part of the cleaning wastewater used for cleaning in the cleaning machine 2 is supplied to the pulp transfer line between the oxygen bleaching stage 1 and the cleaning machine 2 as dilution water, and the rest of the cleaning wastewater is in the previous step. It is used for the alkaline cooking, etc.

In the other ozone bleaching stage cleaning water line, clean cleaning water is supplied to the cleaning machine 5 and discharged as cleaning waste water. Next, a part of this cleaning waste water is supplied to the pulp transfer line between the ozone bleaching stage 4 and the cleaning machine 5 as dilution water, and the rest of the cleaning water is partly recovered as recovered water, and the others are recovered. Washing machine 3 and ozone bleaching stage 4
It is used as dilution water in the pulp transfer line between and.
(A), (b) on the washing water line and (c), (d), (e), (f) on the pulp transfer line in FIG.
A TDS sensor for measuring the TDS amount in the washing water or the TDS amount in the pulp is arranged at a location selected from the locations shown in (g), (h), and (i).

In this multi-stage bleaching method for pulp, the effect of ozone bleaching on TDS which remains in the pulp even after passing through a washing machine placed after the oxygen bleaching stage, and the washing placed after ozone bleaching stage 4 The following experiments were conducted to investigate the effect on ozone bleaching when the wash effluent from Machine 5 was used to dilute the pulp.
That is, cleaning effluent (hereinafter referred to as oxygen bleaching effluent) collected from a plurality of washing machines arranged in the pulp transfer line from the oxygen bleaching stage 1 to the ozone bleaching stage 4 (hereinafter referred to as oxygen bleaching drainage) is added to the pulp to bleach it with ozone. The amount of each TDS in each cleaning wastewater to be added when the cleaning wastewater (hereinafter referred to as ozone bleaching wastewater) from the washer 5 arranged after the bleaching stage 4 is added to another pulp again and ozone bleaching is added. The effect of the effect on pulp bleaching degree was investigated.

In this bleaching experiment, as a target pulp to which oxygen bleaching wastewater or ozone bleaching wastewater was added, hardwood kraft pulp was alkali-cooked and oxygen-bleached pulp was used. The oxygen bleaching wastewater or ozone bleaching wastewater to be added to the pulp was filtered with a filter paper (5 type C filter) to remove suspended solids, and then the TDS in the filtrate was measured in advance. Oxygen bleaching wastewater was added to the pulp so that the TDS addition amount shown in Table 1 below was obtained. In addition, ozone bleaching wastewater is treated with TD as shown in Table 1 below for the pulp.
It was added so that the S addition amount was obtained. The amount of each TDS added is the TDS weight (k) per bone dry weight (ton).
g), and the unit is indicated by kg / BDT (BDT is an abbreviation for Born Dry Ton).

[0021]

[Table 1] Each of the pulps added with the respective washing effluents having different TDS addition amounts shown in Table 1 above had a pH of 2, a temperature of 50 ° C and a pulp concentration of 10
% Bleached with ozone. Whiteness and viscosity of the obtained pulp after the ozone bleaching treatment were measured to evaluate the bleaching property of the pulp. Further, the amount of ozone consumed before and after ozone bleaching was measured, and the amount of ozone consumed per pulp weight was defined by the following formula (1) from this and the amount of pulp used in the experiment (excess dry weight 300 g).

[0022]

[Equation 1] The relationship between the obtained ozone consumption and whiteness is shown in FIG.
The relationship between whiteness and viscosity is shown in FIG. In FIGS. 2 and 3, ■ indicates that dilution water containing no TDS was added to pulp for ozone bleaching, and ● indicates that TDS in the pulp was 10 kg / BDT when oxygen bleaching wastewater was added to the pulp. When ozone bleaching is done as above, ▲ is also TD
When ozone was bleached so that S was 30 kg / BDT, ◆ mark was when ozone bleaching was performed so that TDS was 60 kg / BDT, and ○ mark was when ozone bleaching wastewater was added to the pulp and TDS was 32 kg / The case where ozone bleaching is performed so as to obtain BDT is shown.

According to FIG. 2, it is recognized that the amount of ozone required to reach a predetermined whiteness in ozone bleaching increases as the amount of TDS added increases. Further, according to FIG. 3, it is recognized that the viscosity tends to decrease at the same whiteness as the amount of TDS added increases. From these results, when oxygen bleaching wastewater containing TDS was used as dilution water for pulp and the pulp was bleached with ozone, an increase in the TDS inflow from the dilution water to the pulp increased the ineffective consumption of ozone. , And reduce the viscosity of the pulp.

When ozone bleaching wastewater is used as the dilution water for the pulp before the ozone bleaching stage for ozone bleaching (denoted by a circle in FIGS. 2 and 3), dilution water containing no TDS is used. A slight increase in ozone consumption was observed as compared with the case where ozone was bleached by adding it to pulp (represented by ■ in FIGS. 2 and 3), but the amount of TDS in the pulp due to oxygen bleaching wastewater was 30 kg. / BDT (▲
(Represented by a mark), the ozone consumption is smaller in the case of the O mark than in the ▲ mark (see FIG. 2), although the TDS amount is almost the same.

Furthermore, from FIG. 3, when ozone bleaching wastewater was used as dilution water and pulp was ozone bleached, the T
Even if the DS amount is 32 kg / BDT, it is almost the same as the viscosity decrease when ozone-bleaching is performed by adding dilution water containing no TDS to the pulp, and it is understood that there is almost no effect on the viscosity decrease. The results of the above experiments show that in ozone bleaching, TDS particularly flowing from the oxygen bleaching wastewater significantly causes an increase in ineffective consumption of ozone and a decrease in pulp viscosity.

As a consideration of such results, since oxygen is a bleaching agent having a weaker oxidizing power than ozone, it is considered that the organic matter in TDS of the oxygen bleaching wastewater easily reacts with ozone. It is considered that the organic matter in the TDS of the ozone bleaching wastewater that has been oxidatively decomposed by is less likely to react with ozone than the organic matter in the oxygen bleaching wastewater.

Usually, after the ozone bleaching treatment, finish bleaching is further performed with hydrogen peroxide or the like. In the ordinary pulp bleaching method, the pulp after finish bleaching is required to have a whiteness of 80% or more and a viscosity of about 15 cP or more. For that purpose, the pulp after the ozone bleaching stage needs to have a whiteness of about 60 to 70% and a viscosity of 15 cP or more. When the dotted line (...) In FIG. 3 is the lower limit of required quality, it is necessary to set the amount of TDS due to oxygen bleaching wastewater to 30 kg / BDT or less in order to obtain a viscosity of about 70% and a viscosity of 15 cP or more. In addition, TDS is completely 0kg / B
It is difficult in practice to use DT.

From FIG. 2, it is understood that when the amount of TDS due to oxygen bleaching wastewater is 60 kg / BDT until the whiteness reaches 60%, the ozone consumption is about twice as much as 0 kg / BDT, but 30 kg. / 0kg / BDT for BDT
Approximately 50% increase and 10 kg / BDT increase approximately 20% and TD
It can be seen that the ineffective consumption of ozone can be significantly suppressed as the amount of S is reduced.

A preferred embodiment of the pulp multi-stage bleaching method of the present invention is carried out as follows. The alkali-cooked pulp is bleached in an oxygen bleaching stage, after which the pulp is diluted and passed through multiple washers before being further diluted and ozone bleached in an ozone bleaching stage. In this ozone bleaching step, the amount of TDS flowing into the ozone bleaching stage by the oxygen bleaching wastewater from the oxygen bleaching stage is set to 30 kg / BDT or less. The TDS inflow amount is adjusted by a TDS sensor provided at any position [(a) to (i)] shown in FIG. 1 on the washing water line from the oxygen bleaching stage to the ozone bleaching stage or the pulp transfer line. Based on the value. The pulp after ozone bleaching is further washed by a washing machine, and this washing drainage (ozone bleaching drainage) is used as dilution water for the ozone bleaching stage. The TDS in the ozone bleaching wastewater does not affect the decrease in pulp viscosity. The pulp that has passed through the washing machine after ozone bleaching is sent to the finishing bleaching step. Using finishing bleach such as chlorine dioxide, the pulp has a whiteness of 80% or more,
It is bleached with a viscosity of 15 cP or more and can meet the required quality.

In order to prevent the TDS from the oxygen bleaching stage and the oxygen bleaching effluent flowing into the ozone bleaching stage from exceeding 30 kg / BDT even if the operating conditions of the preceding stage such as cooking and oxygen bleaching change. It is necessary to control this TDS value. For example, in FIG. 1, when the TDS flowing into the ozone bleaching stage 4 becomes large, the cleaning efficiency of the cleaning machine 3 is increased by increasing the amount of clean cleaning water to the cleaning machine 3 after oxygen bleaching. TD flowing into ozone bleaching stage 4
S can be lowered. When the TDS flowing into the ozone bleaching stage 4 exceeds 30 kg / BDT, the washing machine 3
After that, the TDS can be lowered by passing it through another washing machine.

The TDS sensor for detecting the fluctuation of TDS on the line has the following characteristics:
For example, a COD (Chemical Oxygen Demand) meter or a TOC (Total Organic Carbon) meter may be used, or an automatic Kappa meter or a bleaching degree of the pulp before and after oxygen bleaching may be used. The elution amount of organic substances after oxygen bleaching may be calculated by measuring with a whiteness meter or the like. For example, the TDS sensor located at (h) in FIG. 1 can measure the TDS in the pulp accompanying water on the pulp transfer line. Further, by measuring the TDS in the pulp accompanying water with the TDS sensor located in (g) and the TDS in the cleaning wastewater with the TDS sensor located in (b), the amount of TDS at the symbol position on the pulp transfer line can be determined. It can also be calculated.

The TDS sensor may be installed in any combination at positions (c) to (i) on the pulp transfer line or at positions (a) or (b) on the wash water line, Oxygen bleaching TDS flowing into the ozone bleaching stage
Place it at a position where the amount of

[0033]

INDUSTRIAL APPLICABILITY In the present invention, TDS in ozone bleaching effluent does not affect the viscosity decrease of pulp during ozone bleaching, and TDS in oxygen bleaching effluent reduces ozone ineffective consumption and pulp viscosity decrease during ozone bleaching. Since it has been clarified that it causes the present invention, the multi-stage bleaching method of the pulp of the present invention has no adverse effect on the pulp during ozone bleaching, suppresses the decrease in bleaching property, and can effectively use ozone. it can.

[Brief description of drawings]

FIG. 1 is an example of a process diagram used in the pulp multi-stage bleaching method of the present invention.

FIG. 2 shows the relationship between ozone consumption and whiteness.

FIG. 3 shows the relationship between whiteness and viscosity.

[Explanation of symbols]

 1 Oxygen bleaching stage 2, 3, 5 Washing machine 4 Ozone bleaching stage

Claims (4)

[Claims] 1. A multi-stage bleaching method for pulp, wherein oxygen bleaching by an oxygen bleaching stage and then ozone bleaching by an ozone bleaching stage are performed, and countercurrent washing is performed on the pulp transferred from each bleaching stage. 1) A part of the washing wastewater from a washing machine arranged between the oxygen bleaching stage and the ozone bleaching stage is used for diluting the pulp before the ozone bleaching stage, and (2) the washing wastewater and / or the transfer pulp. Of the amount of soluble evaporation residue solids (TDS) in the accompanying water of (3) so that the amount of the dissolution evaporation residue solids (TDS) in the pulp free water flowing into the ozone bleaching stage becomes an appropriate value. A multi-stage bleaching method for pulp, which is characterized in that 2. A multi-stage bleaching method for pulp, wherein oxygen bleaching by an oxygen bleaching stage and then ozone bleaching by an ozone bleaching stage are performed, and countercurrent washing is performed on the pulp transferred from each bleaching stage, 1) A part of the washing wastewater from the washing machine placed between the oxygen bleaching stage and the ozone bleaching stage is used to dilute the pulp before the ozone bleaching stage, and the washing wastewater from the washing machine after the ozone bleaching stage. (2) monitoring the amount of soluble evaporation residual solids (TDS) in the washing effluent and / or the accompanying water of the transfer pulp, (3) A multistage bleaching method for pulp, characterized in that the amount of soluble evaporation residual solids (TDS) in the pulp free water flowing into the ozone bleaching stage is adjusted to an appropriate value. 3. The amount of soluble evaporation residual solids (TDS) in the pulp free water flowing into the ozone bleaching stage is adjusted so as to be not more than 30 kg per ton in absolute dry weight of pulp. 2. The multi-stage bleaching method of pulp according to 2. 4. A method for monitoring the amount of soluble evaporation residual solids (TDS) in the washing wastewater is provided at any position of a washing water line and / or a pulp transfer line from an oxygen bleaching stage to an ozone bleaching stage. The multi-stage bleaching method of pulp according to claim 1, 2 or 3, wherein the TDS sensor is provided.