JP3358760B2 - Multi-stage bleaching of pulp - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a multistage bleaching method for pulp which has been subjected to oxygen bleaching and ozone bleaching, and more particularly to a method for preventing wasteful use of ozone and suppressing a decrease in pulp bleachability. The present invention relates to a multistage bleaching method for pulp.

[0002]

2. Description of the Related Art In a current chemical pulp bleaching method,
Since it is difficult to completely remove the coloring components in the pulp only by a single-stage operation using one type of bleach, a multi-stage bleaching method using a plurality of bleaches is generally used. On the other hand, in place of the use of chlorine bleach or the like which causes the formation of harmful organic chlorine compounds such as dioxin in recent years, such harmful substances are not generated, and the bleaching action of ozone is stronger than that of chlorine bleach etc. Has been noted for its application in pulp bleaching.

As a multi-stage bleaching method for pulp using this ozone as a pulp bleaching agent, it has been proposed to carry out alkaline bleaching of pulp, oxygen bleaching, further bleaching with ozone, and finally finish bleaching. In this multi-stage bleaching method of pulp, a washing machine is provided between alkaline digestion and oxygen bleaching, and between subsequent bleaching and bleaching. By this washing machine, organic decomposition products and bleaching chemicals (such as caustic soda in the case of oxygen) are washed out in addition to lignin and the like decomposed and extracted from the pulp by alkali digestion and bleaching. Washing wastewater discharged from the washing machine contains soluble evaporated residual solids (TDS: Total Dissolved Solids, 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 has been used in the past, but it is discharged from a downstream washing machine in the multi-stage bleaching method because of disadvantages and economics of wastewater treatment. So-called countercurrent washing is generally performed in which washing wastewater is used as dilution water for pulp in the preceding stage.
In the multi-stage bleaching method of pulp using ozone as a pulp bleaching agent, pulp having a pulp concentration of about 15% or more after being washed and dehydrated after oxygen bleaching has a pulp concentration condition of a subsequent ozone bleaching (for example, The washing wastewater from the washing machine at the rear of the plurality of washing machines disposed between the oxygen bleaching stage and the ozone bleaching stage is diluted with the dilution water to the pulp at the preceding stage so as to conform to the medium bleaching condition. Is added as

[0005]

However, if such a countercurrent washing is performed, even if the washing is performed by a washing machine, a part of the washing wastewater is used as pulp dilution water. The TDS derived from the washing wastewater from the washing machine arranged before the ozone bleaching stage accompanies the pulp, and it is impossible to completely remove the TDS from the pulp. Inevitable. For this reason, TDS derived from the oxygen bleaching stage is added to the ozone bleaching stage.
Will flow in.

In the multistage bleaching method for pulp using ozone, a washing machine is provided after the ozone bleaching stage, and the washing wastewater discharged from the 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, the TDS derived from the ozone bleaching stage in the washing wastewater generated by the washing machine after the ozone bleaching stage also flows into the ozone bleaching stage as pulp dilution water.

[0007] In a multistage bleaching method for 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 have been used. It has not been fully clarified how the TDS derived from the washing wastewater from the washing machine disposed after the stage has mixed into the pulp, which has an effect on the bleaching degree of the pulp during ozone bleaching in the ozone bleaching stage. Was.

On the other hand, ozone is more expensive than other bleaching agents and must be used effectively for pulp bleaching. Oxygen bleaching by an oxygen bleaching stage and then ozone bleaching by an ozone bleaching stage are carried out. In a multi-stage bleaching method for pulp in which countercurrent washing is performed on pulp transferred from each bleaching stage, conditions for using ozone without wasting were not clear.

Accordingly, the present invention relates to a multi-stage bleaching method for pulp using ozone as a pulp bleaching agent and employing a countercurrent washing method, in which the influence of TDS derived from the washing wastewater in each of these bleaching stages on ozone bleaching is considered. Bleaching experiments reveal that there is no adverse effect on the pulp produced,
It is an object of the present invention to provide a multi-stage pulp bleaching method capable of suppressing the decrease in bleachability and effectively using ozone during ozone bleaching.

[0010]

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

Therefore, in order to solve the above-mentioned problems, the multi-stage bleaching method of the pulp of the present invention performs oxygen bleaching by an oxygen bleaching stage and then ozone bleaching by an ozone bleaching stage. In the multi-stage bleaching method for pulp in which countercurrent washing is performed, (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. (2) monitoring the amount of soluble evaporation residual solids (TDS) in the wash effluent and / or in the entrained water of the transferred pulp; and (3) soluble evaporation in the pulp free water entering the ozone bleaching stage. The amount of solid residue (TDS) is pulp absolute dry weight
The amount is adjusted to be 30 kg or less per ton in terms of quantity .

It is desirable to use a part of the washing wastewater from the washing machine after the ozone bleaching stage for diluting the pulp before and after the ozone bleaching stage.

[0013]

[0014] In the pulp bleaching method of the present invention, the method of monitoring the amount of soluble evaporation residual solids (TDS) in the washing effluent comprises a washing water line and / or pulp transfer from the oxygen bleaching stage to the ozone bleaching stage. It is desirable to perform this by a TDS sensor provided at an arbitrary position on the line.

[0015]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described based on embodiments. In addition,
This embodiment is merely an example that embodies the present invention, and the present invention is not limited to this.

FIG. 1 is an example of a process diagram used in the multistage bleaching method for pulp of the present invention. Reference numeral 1 denotes an oxygen bleaching stage, into which alkali-digested pulp is introduced and oxygen bleached. Reference numeral 2 denotes 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. Reference numeral 3 denotes a second washing machine, which performs further washing. Reference numeral 4 denotes an ozone bleaching stage, and the washed pulp is ozone bleached in the ozone bleaching stage 4. Reference numeral 5 denotes a washing machine, and pulp diluted with ozone bleach is introduced into the washing machine 5 and washed.

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

In the other washing water line of the ozone bleaching stage system, clean washing water is supplied to the washing machine 5 and discharged as washing wastewater. Next, a part of the washing wastewater is supplied to the pulp transfer line between the ozone bleaching stage 4 and the washing machine 5 as dilution water, and the remaining part of the washing water is partially recovered as recovered water, and the rest is recovered. Washing machine 3 and ozone bleaching stage 4
Is used as dilution water in the pulp transfer line.
(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 amount of TDS in the wash water or the amount of TDS in the pulp is disposed at a location selected from the locations shown in (g), (h), and (i).

In this multi-stage bleaching method for pulp, the effect of TDS still remaining on the pulp in ozone bleaching even after passing through the washing machine arranged after the oxygen bleaching stage, and the washing arranged after the ozone bleaching stage 4 The following experiment was conducted to examine the effect on ozone bleaching when the washing wastewater from the machine 5 was used for pulp dilution.
That is, ozone bleaching is performed by adding washing wastewater (hereinafter, referred to as oxygen bleaching wastewater) collected from a plurality of washing machines disposed in the pulp transfer line from the oxygen bleaching stage 1 to the ozone bleaching stage 4 to the pulp, The amount of each TDS in each washing wastewater to be added when washing wastewater (hereinafter referred to as ozone bleaching wastewater) from the washing machine 5 disposed after the bleaching stage 4 is added to another pulp again and ozone bleached. The effect of pulp on the degree of pulp bleaching was investigated.

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

[0021]

[Table 1] Each pulp to which each washing wastewater having a different TDS addition amount shown in Table 1 was added was subjected to pH 2, temperature 50 ° C., and pulp concentration 10
% Ozone bleaching. The whiteness and viscosity of the obtained ozone bleached pulp were measured to evaluate the bleaching properties 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 as the following equation (1) based on the measured ozone amount and the pulp amount (absolute dry weight: 300 g) used in the experiment.

[0022]

(Equation 1) FIG. 2 shows the relationship between the obtained ozone consumption and whiteness,
FIG. 3 shows the relationship between whiteness and viscosity. In FIGS. 2 and 3, the symbol Δ indicates ozone bleaching when diluting water containing no TDS was added to the pulp, and the symbol ● indicates that TDS in the pulp was 10 kg / BDT when oxygen bleaching wastewater was added to the pulp. When ozone bleaching was performed,
When ozone bleaching was performed so that S became 30 kg / BDT, o indicates ozone bleaching when the TDS became 60 kg / BDT, and o indicates ozone bleaching wastewater was added to pulp to obtain a TDS of 32 kg / BDT. A case where ozone bleaching is performed so as to obtain BDT is shown.

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

When ozone bleaching wastewater is used as a pulp dilution water before the ozone bleaching stage and ozone bleached (indicated by a circle in FIGS. 2 and 3), the dilution water containing no TDS is used. A slight increase in ozone consumption is observed as compared with the case where ozone bleaching is performed by adding to the pulp (indicated by a mark in FIGS. 2 and 3), but the amount of TDS in the pulp due to oxygen bleaching wastewater is 30 kg. / BDT (▲
It can be seen that although the TDS amount is almost equal to (shown by the mark), the ozone consumption is smaller in the mark (▲) than in the mark (▲).

Further, from FIG. 3, when ozone bleaching wastewater is used as dilution water to ozone bleach pulp, the T
Even when the DS amount is 32 kg / BDT, the decrease in viscosity is almost the same as that when ozone bleaching is performed by adding dilution water containing no TDS to the pulp, and it can be seen that there is almost no effect on the decrease in viscosity. The results of the above experiments show that in ozone bleaching, especially TDS flowing from the oxygen bleaching wastewater significantly increases the ineffective consumption of ozone and lowers the viscosity of pulp.

As a consideration of such a result, it is considered that since oxygen is a bleaching agent having a lower oxidizing power than ozone, the organic matter in the TDS of the oxygen bleaching wastewater easily reacts with ozone. It is considered that the organic matter in the TDS of the ozone bleaching effluent oxidized and decomposed is less likely to react with ozone than the organic matter in the oxygen bleaching effluent.

Usually, after the ozone bleaching treatment, finish bleaching is performed with hydrogen peroxide or the like. In a usual pulp bleaching method, 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, it is necessary that the pulp at the stage after the ozone bleaching stage has a whiteness of about 60 to 70% and a viscosity of 15 cP or more. If the dotted line (...) in FIG. 3 is the lower limit of the required quality, the amount of TDS by oxygen bleaching wastewater needs to be 30 kg / BDT or less in order to achieve a whiteness of about 70% and a viscosity of 15 cP or more. In addition, TDS is completely 0kg / B
It is difficult in practice to make DT.

From FIG. 2, it can be seen that when the amount of TDS by the oxygen bleaching wastewater is 60 kg / BDT until the whiteness reaches 60%, the ozone consumption is about twice that of 0 kg / BDT. 0kg / BDT for / BDT
About 10% increase for 10kg / BDT and about 20% increase in TD
It can be seen that the ineffective consumption of ozone can be greatly suppressed with the reduction of the S amount.

A preferred embodiment of the pulp multi-stage bleaching method of the present invention is carried out as follows. The alkaline cooked pulp is bleached in an oxygen bleaching stage, after which the pulp is diluted and passed through a plurality of washing machines, then further diluted and ozone bleached in an ozone bleaching stage. In this ozone bleaching step, the flow rate of TDS into the ozone bleaching stage due to oxygen bleaching wastewater from the oxygen bleaching stage is set to 30 kg / BDT or less. The adjustment of the TDS inflow amount is performed by measuring with a TDS sensor provided at an arbitrary position [(a) to (i)] shown in FIG. 1 of the washing water line from the oxygen bleaching stage to the ozone bleaching stage and the pulp transfer line. Perform based on values. The pulp after ozone bleaching is further washed by a washing machine, and this washing wastewater (ozone bleaching wastewater) is used as dilution water in the ozone bleaching stage. The TDS in the ozone bleaching effluent does not affect the viscosity of the pulp. After ozone bleaching, the pulp that has passed through the washing machine is sent to the finish bleaching step. Using a finish 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 satisfy required quality.

In order to ensure that the TDS from the oxygen bleaching stage and the oxygen bleaching wastewater flowing into the ozone bleaching stage does not exceed 30 kg / BDT even if the preceding operating conditions such as digestion 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 washing efficiency of the washing machine 3 is increased by increasing the amount of clean washing water to the washing machine 3 after oxygen bleaching, TD flowing into ozone bleaching stage 4
S can be reduced. When the TDS flowing into the ozone bleaching stage 4 exceeds 30 kg / BDT, the washing machine 3
After that, it can be passed through another washing machine to lower the TDS.

The TDS sensor for detecting the fluctuation of TDS on the line has a function of measuring the amount of dissolved organic substances.
For example, a COD (Chemical Oxygen Demand; Chemical Oxygen Demand) meter or TOC (Total Organic Carbon; Total Organic Carbon Amount) meter may be used. The elution amount of organic matter after oxygen bleaching may be calculated by measuring with a whiteness meter or the like. For example, the TDS in the pulp entrained water on the pulp transfer line can be measured by the TDS sensor located at (h) in FIG. Also, by measuring the TDS in the pulp entrained water with the TDS sensor located in (g) and the TDS in the washing 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 at any of the positions (c) to (i) on the pulp transfer line or at the position (a) or (b) on the washing water line. Oxygen bleaching TDS flowing into the ozone bleaching stage
To a position where the amount of

[0033]

According to the present invention, the TDS in the ozone bleaching wastewater does not affect the decrease in pulp viscosity during ozone bleaching, and the TDS in the oxygen bleaching wastewater reduces the ineffective consumption of ozone and the decrease in pulp viscosity during ozone bleaching. Since the present invention was constituted and the present invention was constituted, the multistage bleaching method of the pulp of the present invention has no adverse effect on the pulp at the time of ozone bleaching, suppresses the decrease in bleachability, and makes it possible to use ozone effectively. it can.

[Brief description of the drawings]

FIG. 1 is an example of a process diagram used in the multistage bleaching method for pulp 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

──────────────────────────────────────────────────続 き Continuation of front page (56) References JP-A-6-101187 (JP, A) JP-A-53-19407 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) D21C 1/00-11/14

Claims (3)

(57) [Claims] 1. A multi-stage pulp bleaching method in which oxygen bleaching in an oxygen bleaching stage and then ozone bleaching in an ozone bleaching stage are performed, and pulp transferred from each bleaching stage is subjected to countercurrent washing. 1) Part of the washing wastewater from the washing machine disposed between the oxygen bleaching stage and the ozone bleaching stage is used for diluting pulp before the ozone bleaching stage. (2) Pulp in the washing wastewater and / or transferred pulp (3) The amount of soluble evaporation residue solids (TDS) in the pulp free water flowing into the ozone bleaching stage is determined by the absolute pulp weight of the pulp. G
A multistage bleaching method for pulp, characterized in that the weight is adjusted to 30 kg or less per pulp. 2. A multi-stage pulp bleaching method in which oxygen bleaching in an oxygen bleaching stage and then ozone bleaching in an ozone bleaching stage are performed, and pulp transferred from each bleaching stage is subjected to countercurrent washing. 1) A part of the washing wastewater from the washing machine disposed between the oxygen bleaching stage and the ozone bleaching stage is used for diluting 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 wastewater and / or the entrained water of the transferred pulp; (3) The amount of soluble evaporation residual solids (TDS) in the pulp free water flowing into the ozone bleaching stage is determined by the absolute pulp dry weight.
A multistage bleaching method for pulp, characterized in that the weight is adjusted to 30 kg or less per pulp. 3. The method of monitoring the amount of soluble evaporation residual solids (TDS) in the washing wastewater, the method comprising the steps of: providing a washing water line and / or a pulp transfer line from an oxygen bleaching stage to an ozone bleaching stage; 3. The multi-stage bleaching method for pulp according to claim 1, wherein the bleaching is performed by a TDS sensor provided.