JP2008184719A - Method for reusing waste water from acid treatment step in production process of kraft pulp which has acid treatment step before bleaching step - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for reusing waste water discharged from an acid treatment step and efficiently carrying out reduction of chromaticity of waste water discharged to the environment in addition to saving of an acid treatment chemical and reduction of amount of water used or discharged, in a process for producing kraft pulp having the acid treatment step before bleaching step. <P>SOLUTION: In production process of craft pulp having an acid treatment step S5 before bleaching steps S7-S13, waste water treated in the acid treatment step is recovered and the recovered waste water is subjected to oxidation treatment S20 and the waste water is added to at least either of a washing machine S6, a dehydration machine S6 before or after the acid treating step and the acid treatment step S5 to recycle the waste water in the acid treatment process. On this occasion, it is recommended that oxidation treatment of waste water treated in the acid treatment step is carried out by using a gas containing at least ozone. In this case, it is preferable on efficient utilization of ozone to utilize discharged ozone-containing gas for oxidation of acid treatment waste water when using ozone in bleaching step. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a method for recycling wastewater discharged in the process of producing kraft pulp having an acid treatment step before the bleaching step, and more specifically, elemental chlorine and hypochlorite represented by the ECF bleaching method. The present invention relates to a method for reusing acid treatment process wastewater in the manufacture of unused kraft pulp.

  Conventionally, bleached kraft pulp is made by combining bleaching agents consisting of oxidizing agents and reducing agents such as chlorine, hypochlorite, chlorine dioxide, oxygen and hydrogen peroxide after cooking and oxygen delignification. In recent years, however, environmental problems have been highlighted, and chlorine-free typified by the so-called ECF bleaching method that does not use elemental chlorine or hypochlorite as a method to prevent the generation of organic chlorine compounds and chloroform. A bleaching method has been adopted.

  The hexeneuronic acid produced in the kraft cooking process in an alkaline environment consumes bleaching agents such as chlorine dioxide and ozone used in ECF bleaching, and increases costs, or causes scale to be generated in a selection facility. Furthermore, in the ECF bleaching, especially in the chlorine dioxide method, the problem of yellowing of the kraft pulp after bleaching is considered to be the causative substance of yellowing by acid treatment with sulfuric acid etc. in the previous step of the ECF bleaching step. A method of decomposing and removing xenuronic acid is being adopted (for example, Japanese Patent Application Laid-Open No. 2004-339628).

  On the other hand, since a large amount of water is used in each papermaking process at a paper mill, it is necessary to reduce the environmental impact of the drainage, especially in a factory with a kraft pulp manufacturing process that emits a lot of COD and BOD. There are also simple sedimentation treatment (removal of contaminants such as fine fibers), biological treatment (removal of BOD by microorganisms in the aeration layer), and agglomeration sedimentation treatment (aggregation of microorganisms and suspended suspended matter by aggregating chemicals for separation and removal) In combination with sand filtration, etc., processing is performed in multiple stages to reduce pollutants such as COD, BOD, and SS.

  In addition, part of the treated wastewater is reused in pulp washing, waste paper treatment processes, etc. within a range that does not affect the product, thereby reducing the amount of water used, that is, the amount of wastewater, and reducing the environmental load.

  Although the pollutants such as COD in the wastewater have been reduced by the above method, it is difficult to remove the specific coloring components of the wastewater from the kraft pulp manufacturing process, and these are not subject to legal regulations. There is a demand for removal (decomposition) of colored components due to emotional problems with factory wastewater by residents and fishermen.

  Accordingly, in parallel with the development of kraft pulp in the ECF bleaching method, the following method has been proposed as a method for solving the problems of wastewater coloring and COD load reduction.

  Japanese Patent Application Laid-Open No. 2005-219021 adds slaked lime and ferrous chloride to acid wastewater (drainage such as chlorine dioxide bleaching stage), alkaline wastewater (drainage such as alkali extraction stage) or mixed wastewater of ECF bleaching process In addition, a method for adsorbing and separating colored components together with COD components has been proposed.

  According to this method, the burden on the later comprehensive wastewater treatment is reduced and the coloration of the discharged wastewater is also reduced, but not only the chemical costs of slaked lime and ferrous chloride are incurred, but the separated solids are made from paper. Although it is usually incinerated or landfilled as sludge, incineration requires supplementary combustion, but the volume can be reduced by incineration, but the incinerated ash after incineration requires landfill disposal and reuse as an inorganic substance Even so, there is a problem that the inherent ferrous chloride causes a decrease in whiteness and hinders reuse.

  Japanese Patent Laid-Open No. 2005-334873 proposes a method of using incinerated ash generated in the factory for wastewater in the ECF bleaching process as a method for solving the cost increase of wastewater treatment chemicals by the above method.

  However, the quality of incinerated ash is not stable due to fluctuations in each process of papermaking, for example, changes in the product type of papermaking. Furthermore, in order to prevent elution of metallic substances in the ash, the pH of the wastewater to be treated is 8 or more. In order to finally separate the ash, the amount of dehydration and combustion treatment increases, and the ash is melted by repeated combustion, which causes problems in terms of processing cost and reuse. .

  Japanese Patent Laid-Open No. 2004-50124 (Nippon Paper Industries) proposes and suggests the following method.

  1. Ozone exhaust gas discharged from the ozone bleaching stage of the KP bleaching process is used.

  2. Ozone exhaust gas is added to the wastewater containing the wastewater from the KP bleaching process. Wastewater containing wastewater from the KP bleaching process is wastewater mixed with other process wastewater such as chipyard wastewater and papermaking process wastewater.

  3. Treatment is performed under the conditions of ozone injection rate x residence time of 500 to 2000 mg / min / L.

  4). A treatment device that has one or more reaction layers, injects waste water from the top and diffuses ozone exhaust gas from the bottom.

  5. It is preferable that the waste water contains waste water from the alkali extraction stage.

(This is because the ozone absorption efficiency is improved when the drainage is alkaline with a pH of 10.4 to 11.3.)
When the pH is low, an alkaline agent such as caustic soda is added and adjusted.

  6). The ozone concentration in the ozone exhaust gas is preferably 1000 to 10,000 ppm. If it is less than 1000 ppm, the efficiency is poor, and if it is 10,000 ppm or more, ozone bleaching is inappropriate.

  7. When ozone exhaust gas is passed to waste water, it is preferable to aerate with an underwater aerator or the like in order to promote contact.

  According to this method, the load on the apparatus for decomposing ozone in the exhaust gas in the ozone bleaching process can be reduced or stopped, and further, there is no increase in waste, However, it is not sufficient in terms of process control by using pH control and pH adjusting chemicals, economy, color reduction efficiency, and wastewater reuse.

  That is, as described above, the cost increase of chlorine dioxide, ozone, etc., which are ECF bleaching chemicals, the suppression of scale generation in the selection equipment, and the causative substance of the yellowing problem of kraft pulp after bleaching in the ECF bleaching method using chlorine dioxide In order to selectively decompose hexeneuronic acid, a pretreatment method using an acid such as sulfuric acid has been adopted in the step before bleaching.

  The pulp slurry that has been subjected to the acid treatment is sent to the bleaching step, which is the next step. Before that, the pulp slurry is washed and dehydrated by a displacement press device or the like in order to remove substances decomposed by the acid treatment.

  This waste water contains acidic chemicals added in the acid treatment step and exhibits an acidity of about pH 2 to 4. According to the method disclosed in the aforementioned Japanese Patent Application Laid-Open No. 2004-50124, ozone In order to utilize the ozone-containing exhaust gas discharged in the bleaching step, it is not effective unless the pH is adjusted to the alkali side by adding alkali chemicals or alkali drainage from the alkali extraction stage.

  However, adjusting the pH by adding alkaline chemicals not only makes process control complicated, but also increases chemical costs, so it is not possible to selectively treat only acid-treated wastewater and discharge it. It is actually mixed with other wastewater and treated with ozone-containing gas, etc., and comprehensive wastewater treatment is performed by sedimentation separation / biological treatment, coagulation sedimentation treatment, etc. The treated wastewater is the same as before Reused only for certain purposes.

JP-A-2005-219021 JP 2005-334873 A JP, 2004-50124, A The present inventor made the subject the method of collect | recovering the waste_water | drain of the acid treatment process in a kraft pulp manufacturing process, and repeatedly using the acidic substance contained in an acid treatment process, As a result of earnest research It came to make this invention.

  That is, as a result of analyzing pollutants in the bleaching process wastewater mainly composed of alkali extraction process wastewater and the acid treatment process wastewater, the former wastewater is mainly composed of COD components having a molecular weight of about 3000 whereas the latter. Found that the main component is a molecular weight <150.

  As a result of further investigation, it was found that the former component was decomposed lignin, and the latter component was formic acid, 2-furonic acid, and 5-formyl-2-furonic acid from which hexeneuronic acid was decomposed. .

  Therefore, in the conventional method shown in the cited document, the pollutant derived from hexeneuronic acid, which is the colored substance of the wastewater, cannot be selectively treated, and the chromaticity of the wastewater cannot be reduced efficiently. In addition, it was found that acid treatment water containing unreacted acid treatment chemicals was not utilized and the amount of waste water was not reduced.

  The present invention has been made on the basis of the above knowledge, and at least in the manufacturing process of kraft pulp having an acid treatment step before the bleaching step, the waste water discharged from the acid treatment step can be reused, and the acid treatment. The object is to provide a method for efficiently reducing the chromaticity of wastewater discharged into the environment in addition to reducing chemicals and reducing the amount of water used and wastewater.

  As a means for achieving the above-mentioned problems, the present invention collects the wastewater from the acid treatment step in the kraft pulp manufacturing process having an acid treatment step before the bleaching step, and oxidizes the collected wastewater. The basic feature is that it is added to at least one of a washing machine, a dehydrator, and an acid treatment process before and after the acid treatment process, and the acid treatment process waste water is reused.

  In that case, it is recommended that the oxidation treatment of the acid treatment process waste water is performed using a gas containing at least ozone. In this case, when ozone is used in the bleaching step, it is preferable to use the discharged ozone-containing gas for the oxidation of the acid treatment waste water in terms of efficient use of ozone. In this case, the ozone concentration of the ozone-containing gas that is discharged without contributing to bleaching in the bleaching step and is used for oxidation of the acid treatment wastewater is preferably 100 to 1000 ppm. If it is less than 100 ppm, the oxidation treatment efficiency of the acid treatment wastewater becomes low, and 1000 ppm In the case of exceeding, it indicates that ozone is not working effectively in the ozone bleaching process. An ozone concentration of about 100 to 1000 ppm is sufficient. The reason is presumed that trace amounts of metals such as copper and manganese contained in the acid treatment wastewater act catalytically, decompose ozone and effectively produce active oxygen, and promote reaction with colored substances. .

  For the acid treatment wastewater in the case of carrying out the present invention against 500 to 2000 mg · min / L of the product of ozone injection rate x residence time for ECF bleaching alkaline wastewater in JP-A-2004-50124 The product of the ozone injection rate x residence time of may be as low as 300 to 1300 mg · min / L, which is an economical treatment method.

  Of course, the ozone is not limited to the ozone-containing gas discharged from the bleaching step, but may be ozone gas produced by another ozone generator. Such ozone gas or ozone-containing exhaust gas is preferably supplied from the lower part of the gas-liquid contact reaction tank, and acid treatment waste water is supplied from the upper part to cause an oxidation reaction.

  Furthermore, the amount of wastewater oxidized by the above method is preferably 25 to 95% of the amount of wastewater from the acid treatment process.

  The reason is that the wastewater of the acid treatment process is oxidized and used in the preceding and following steps of the acid treatment process and / or in the acid treatment own process. When the amount of discharged wastewater increases and exceeds 95%, metal materials such as iron, copper, and manganese during oxidation treatment increase, and scales in piping and screens around the acid treatment process may be generated. This is because it adversely affects productivity.

  Preferably, it is 40 to 90%.

  According to the present invention, by treating the wastewater from the acid treatment step alone, the wastewater can be reused at least in the acid treatment step, the amount of newly added acid and water can be reduced, and the amount of wastewater in other wastewater treatment. The load can be reduced, and the chromaticity of the waste water after treatment can be reduced.

  Next, the best mode for carrying out the present invention will be described with reference to the accompanying drawings. FIG. 1 shows a pulp manufacturing process by a chlorine dioxide ECF bleaching method, and FIG. 2 shows an ozone ECF bleaching method. The pulp manufacturing process is shown.

First, the common part of each pulp manufacturing process shown in FIG. 1 and FIG. 2 will be described. In these pulp manufacturing processes, the cooking process S ₁ , S ₄₁ , the washing process S ₂ , S ₄₂ of the papermaking raw material (lignocellulose material). Unbleached pulp is produced through the alkaline oxygen lignin steps S ₃ and S ₄₃ and the washing steps S ₄ and S ₄₄ , and the unbleached pulp is further subjected to acid treatment (sulfuric acid treatment in these embodiments) steps S ₅ and S _45. and washing or via dehydration stage S _6, S ₄₆ are sent to the bleaching step of the next step. The acid-treated wastewater is collected in a separate stock tank.

Steps S _{1 to} S6 and S _{41 to} S ₄₆ described above are common parts in the pulp manufacturing process shown in FIGS. 1 and 2, and the subsequent processes for each pulp manufacturing process shown in FIGS. Will be explained.

First, the pulp manufacturing process by the chlorine dioxide ECF bleaching shown in FIG. 1 will be described. In the pulp manufacturing process shown in FIG. 1, after the acid treatment (sulfuric acid treatment) step S ₅ and the washing or dehydrating stage S ₆ of unbleached pulp, chlorine dioxide The pulp is bleached by the ECF bleaching method.

The pulp bleaching process by the chlorine dioxide ECF bleaching method will be described in sequence. In the embodiment shown in FIG. 1, D ₀ stage (first stage of chlorine dioxide) S ₇ , washing stage S ₈ , E ₀ stage (alkali extraction containing oxygen). stage) S _9, the cleaning stage S _10, P stage or D ₁ stage (D ₁ stage peroxide oxygen P stage or chlorine dioxide) S _11, the cleaning stage S _12, D ₂ stage or P stages (chlorine dioxide D Bleached pulp is produced through _two stages or hydrogen peroxide P stage) S ₁₃ and washing stage S ₁₄ . Then, in this embodiment of FIG. 1, step S ₂₀ for oxidization treatment (ozone treatment) effluent cleaning or dewatering stage S ₆ is carried out acid treatment of the unbleached pulp after (sulfate process) Step S ₅ is performed, the oxidizing the treated waste water (oxidation effluent) is, but is intended to be supplied as cleaning water or dilution water for several steps carried out acid treatment (sulfuric acid treatment) step S ₅ before and after the unbleached pulp, specifically In the embodiment of FIG. 1, the wastewater (oxidation wastewater) W ₀ oxidized in the wastewater oxidation treatment (ozone treatment) step S ₂₀ is the acid treatment dilution in the unbleached pulp acid treatment step S ₅ . water W _1, dilution water W _3, the cleaning stage after the D ₀ stage S ₇ S ₈ in the wash water W _2, D ₀ stage S ₇ of the washing machine in the washing or dehydration step S ₆ after the acid treatment step S ₅ the dilution water W _5, the cleaning stage S ₁₀ after the E ₀ stage S ₉ in the washing water W _4, E ₀ stage S ₉ in In dilution water W _7, cleaning water W _8, D ₂ stage or P stage S ₁₃ in the washing stage S ₁₂ after the P stage or D ₁ stage S ₁₁ in kicking cleaning water W _6, P stage or D ₁ stage S ₁₁ It is used as a dilution water W ₉ and the like.

On the other hand, when describing the bleaching sequence in the embodiment shown in FIG. 2 (pulp manufacturing process using ozone ECF bleaching method), in the embodiment shown FIG. 2, acid treatment (sulfuric acid treatment) of the unbleached pulp step S ₄₅ and the cleaning or dewatering stage after the S _46, bleaching of pulp with ozone ECF bleaching process is carried out.

The pulp bleaching process by the ozone ECF bleaching method will be described in sequence. In the embodiment shown in FIG. 2, the Z stage (first stage of ozone) S ₄₇ , the washing stage S ₄₈ (may be omitted), the E ₀ stage ( alkaline extraction stage) S ₄₉ containing oxygen, washing stage S _50, P stage or D ₁ stage (D ₁ stage P-stage or chlorine dioxide in hydrogen peroxide) S _51, the cleaning stage S _52, D ₂ stage or P stage (P stage of D ₂ stage or hydrogen peroxide chlorine dioxide) S _53, the bleached pulp is produced by conducting a washing step S _54.

Then, in the embodiment of FIG. 2, step S ₆₀ for oxidization treatment (ozone treatment) effluent cleaning or dewatering stage S ₄₆ Acid treatment of unbleached pulp is carried out after the (sulfuric acid treatment) step S ₄₅ is performed, the oxidizing the treated waste water (oxidation effluent) is, but is intended to be supplied as cleaning water or dilution water for several steps to be performed before and after unbleached acid treatment of pulp (sulfate process) step S _45, specifically In the embodiment of FIG. 2, the wastewater (oxidation wastewater) W ₄₀ oxidized in the wastewater oxidation treatment (ozone treatment) step S ₆₀ is the acid treatment dilution in the unbleached pulp acid treatment step S ₄₅ . Water W ₄₁ , Washing water W ₄₂ after washing or dehydrating stage S ₄₆ after the acid treatment step S ₄₅ , Diluted water W _{43 at} D ₀ stage S ₄₇ , Z stage (first stage of ozone) S ₄₇ cleaning water W ₄₄ (cleaning step S ₄₈ in the washing stage S ₄₈ is sometimes omitted , Dilution water W ₄₇ in dilution water W _45, cleaning water W _46, P stage or D ₁ stage S ₅₁ in the washing stage S ₅₀ after the E ₀ stage S ₄₉ in E ₀ stage S _9, the P stage or D ₁ is used as a diluting water W ₄₉ or the like in the dilution water W _48, D ₂ stage or P stage S ₅₃ in the washing stage S ₅₂ after step S _51.

1 and 2, the ozone used in the wastewater oxidation (ozone treatment) steps S ₂₀ and S ₆₀ may be ozone gas generated by an appropriate ozone generator. good, instead of the ozone gas generated by an ozone generator, such as described above (or in combination with such ozone gas), the unreacted ozone-containing gas recovered from the pulp bleaching step (S ₄₇ to S ₅₁₎ Can also be used.

  Specifically, the gas containing unreacted ozone generated in the ozone stage is recovered with a fan. Moreover, it is good to collect | recover the waste_water | drain containing the unreacted ozone in a pulp slurry through a filter before an alkali extraction stage.

  In this case, the oxidation treatment tank is provided with a central pipe to which an aeration nozzle or a bubbler is attached at the bottom of the container, and supplies the recovered gas containing unreacted ozone into the drainage.

  The recovered wastewater containing unreacted ozone passes through the suction side of the pump that sends the acid treatment wastewater to the oxidation treatment tank, is mixed in the acid treatment wastewater, and is supplied from the upper part of the oxidation treatment tank.

  Oxidized acid treatment waste water is extracted from the bottom of the oxidation treatment tank with a pump and used as washing water for washing machines / dehydrators before and after the acid treatment process or as dilution water for the acid treatment process.

  It should be noted that the pH of the acid treatment wastewater that is oxidized in the oxidation treatment tank is not controlled.

As described above, in the pulp manufacturing process of each embodiment shown in FIGS. 1 and 2, the washing or dehydration stages S ₆ and S ₄₆ performed after the acid treatment (sulfuric acid treatment) steps S ₅ and S _{45 of} unbleached pulp are performed. step S _20, S ₆₀ to drain the oxidation treatment (ozone treatment) is performed, the oxidation-treated waste water (oxidation treatment wastewater) acid treatment of the unbleached pulp (sulfate process) step S _5, S ₄₅ rows back and forth It is supplied as washing water or dilution water for some of the processes, which can reduce the amount of newly added acid and water, and reduce the amount of wastewater and load in some other wastewater treatment processes. In addition, the chromaticity of the waste water after treatment can be reduced.

[Example]
Examples of the present invention will be described below, but the present invention is not limited to these examples.

COD measurement Measured according to JISK-0102 “Oxygen consumption by potassium permanganate at 100 ° C. (CODMn)”. Further, the COD reduction rate was calculated by the following formula 1.

Reduction rate (%) = 100 × [(before treatment−after treatment) / before treatment] (Equation 1)
Measurement of chromaticity Drainage chromaticity was measured based on the following papers. The chromaticity reduction rate was calculated in the same manner as in the COD reduction rate (Formula 1).

[Papers]
Prominent people: Lee Sangye, Ryuichiro Kondo, Katsumi Sakai, Tomoaki Nishida, Yoshimasa Takahara Paper title: Treatment of kraft pulp bleaching effluent with lignin-degrading bacteria (Part 1)
Decolorization of kraft pulp bleaching waste liquor by lignin-degrading bacteria, IZU-154 strain Material: Journal of the Wood Society Vol.39, No.4, pages 470-478, published in 1993
Samples of pulp Unbleached hardwood kraft pulp (LUKP) after oxygen delignification was sampled at the actual production site, not washed, and stored in a refrigerator at 5 ° C. until testing. (Pulp concentration: 32%; Copper number 10.2)
Creating drainage 1. Acid treatment (stage A) wastewater 200g (absolute amount) of LUKP was diluted to 11% concentration slurry with ion-exchanged water, adjusted to pH 3.0 with 4N sulfuric acid, and again with ion-exchanged water. The pulp concentration was adjusted to 10%. Acid treatment is carried out under conditions of 90 ° C. for 5 hours, pulp and acid treatment waste water are separated by filter paper, and the acid treatment waste water is treated with ozone.

The acid-treated pulp was washed with ion exchange water (2 L). The acid treatment was carried out in two batches, and the washed pulp was mixed and bleached in an ECF bleaching sequence of D ₀ -E-P-D ₁ to prepare wastewaters in the D ₀ and E stages. (D ₀ : first stage of chlorine dioxide; E: alkali extraction stage; P: hydrogen peroxide stage; D ₁ : last stage of chlorine dioxide)
2. ECF Bleached Acid (D ₀ Stage) Wastewater LUKP (350 g, absolute dry amount; kappa number: 5.4) after acid treatment was subjected to D ₀ stage under the following conditions. Pulp concentration: 4%; Temperature: 60 ° C .; Reaction time: 25 minutes; Chlorine dioxide addition rate: 0.67%.

D ₀ stage pulp, the drainage of the pulp and D ₀ stage were separated by filter paper, the waste water is treated with ozone. The pulp after D ₀ stage was washed with ion-exchanged water (2 L), and the next alkali extraction stage was performed.

3. ECF bleached alkali (E stage) drainage D Pu pulp after stage ₀ is subjected to alkali extraction under conditions of pulp concentration: 12%; temperature: 75 ° C; reaction time: 55 minutes; caustic soda addition rate: 1.3% It was.

In the E-stage pulp, the pulp, E-stage and waste water are separated by filter paper, and the waste water is treated with ozone. Pulp after E stage, and washed with deionized water (2L), and bleaching with the following P-stage and D ₁ stage.

4). P-stage and D-stage ₁ bleaching E-stage pulp: pulp concentration: 12%; temperature: 75 ° C; reaction time: 65 minutes; hydrogen peroxide addition rate: 0.3%; caustic soda addition rate: 0.4 P stage was performed under the condition of%. The P-stage pulp was washed with ion-exchanged water (2 L) using a bifunnel funnel. Thereafter, D ₁ stage of pulp concentration: 12%; temperature: 72 ° C .; reaction time: 105 minutes; hydrogen peroxide addition rate: 0.24% was performed and washed with ion-exchanged water (2 L). The final ISO brightness was 86.0%. In addition, the waste water of the P stage and the D ₁ stage was not collected.

Characteristics of collected wastewater Table 1 shows the characteristics of each wastewater. Among the wastewater used, ECF bleaching alkaline wastewater had high pH and chromaticity, and acid wastewater had the lowest COD and chromaticity, and the acid treatment wastewater had the highest COD.

Since there is no residual ozone gas in the actual operation of wastewater ozone treatment, 8% ozone gas was produced by the ozone generator of PSA Ozonizer SGA-01A-PSA4 manufactured by Sumitomo Precision Industries, and the wastewater was ozonized. That is, a gas washing bottle (1 L) was placed in a constant temperature bath at 45 ° C., drainage (250 ml) was put, and ozone was injected. Samples of waste water were collected at an ozone treatment time of 30 seconds, 1 minute, 2 minutes, 3 minutes, and 4 minutes, and COD and chromaticity were measured. Ozone was absorbed in 10% potassium iodide and titrated with 0.1N thiosulfuric acid to determine the ozone concentration.

[result]
The COD reduction rate by ozone treatment of each waste water is shown in FIG. The ozone concentration indicated on the X-axis corresponds to an ozone treatment time of 0 seconds, 30 seconds, 1 minute, 2 minutes, 3 minutes, and 4 minutes. In the case where the treatment time is 1 minute (ozone concentration: 200 mg / L) or less, the COD reduction rates of the A stage waste water, the D ₀ stage waste water, and the D ₀ stage and E stage mixed waste water are almost the same, but the treatment time is 1 The COD reduction rate of stage A wastewater is excellent at any treatment time (ozone concentration) between ˜4 minutes (ozone concentration: 200-1050 mg / L), and the COD reduction rate at the point of ozone treatment time of 4 minutes is It was a high value of 23%. On the other hand, the COD reduction rate by ozone treatment of ECF bleaching alkali (E stage) wastewater was inferior.

The chromaticity reduction rate by ozone treatment of each waste water is shown in FIG. Similar to the COD reduction rate, the chromaticity reduction rate of the A-stage drainage was excellent. At the ozonation time of 4 minutes, the color of the A-stage drainage was 50%, 37%, and 41% of the chromaticity reduction rates of the ECF bleaching D _0- stage drainage, E-stage drainage, and D _0- stage and E-stage mixed wastewater. The degree of reduction was the highest value of 70%.

  Since acid-treated wastewater is sufficiently improved in COD and chromaticity after ozone treatment, it can be reused in the acid treatment system (washing water before and after the process, washing water from the dehydrator or dilution water to the A-stage reactor). Become.

It is a process flow figure in the pulp manufacturing process (pulp manufacturing process by the chlorine dioxide ECF bleaching method) concerning a 1st embodiment of the present invention. It is a process flow figure in the pulp manufacturing process (pulp manufacturing process by ozone ECF bleaching method) concerning a 2nd embodiment of the invention in this application. It is a graph which shows the COD reduction rate by the ozone treatment of each waste_water | drain in the pulp manufacturing process concerning the Example of this invention. It is a graph which shows the chromaticity reduction rate of each waste_water | drain by the ozone treatment of each waste_water | drain in the pulp manufacturing process concerning the Example of this invention.

Claims (4)

In the manufacturing process of kraft pulp having an acid treatment step before the bleaching step,
The wastewater from the acid treatment step is collected, the collected wastewater is oxidized, and added to at least one of a washing machine, a dehydrator, and an acid treatment step before and after the acid treatment step. Reuse method of wastewater from acid treatment process in manufacturing process.   The method for reusing acid treatment process wastewater in a kraft pulp manufacturing process according to claim 1, wherein the oxidation treatment of the acid treatment process wastewater is performed using a gas containing at least ozone.   In the manufacturing process of the kraft pulp according to claim 1 or 2, wherein the oxidation treatment is performed using the exhaust gas discharged in the ozone bleaching step of kraft pulp and / or the generated ozone gas from the ozone generator. Reuse method of acid treatment process wastewater.   The method for reusing acid treatment process wastewater in the production process of kraft pulp according to any one of claims 1 to 3, wherein a wastewater amount of 25 to 100% of the acid treatment process wastewater amount is oxidized.

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