JP2003082600A - Method for preventing scale deposition in kraft pulp bleaching process - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for preventing scale deposition in kraft pulp- bleaching process, preventing scale deposition on various equipments in the process and having effect for removing deposited scales, preventing scale reaccumulation and the like. SOLUTION: This method for preventing scale deposition in a kraft pulp- bleaching process comprises using chlorine-free bleaching method [ECF (elemental chlorine free) bleaching method] and is characterized by adding 1- hydroxyethylidene-1,1-diphosphonic acid or its salt into a pulp slurry in the bleaching process to prevent scale deposition in equipments applied in the bleaching process.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a scale adhesion preventing method in a kraft pulp bleaching step, and more specifically, it prevents scale adhesion to various devices during the kraft pulp bleaching step, The present invention relates to a method for preventing scale adhesion in a kraft pulp bleaching process, which has excellent effects in removal and prevention of rescaling.

[0002]

2. Description of the Related Art Conventionally, in the production of bleached kraft pulp, an acid or alkali chemical is used in the bleaching step of kraft pulp to remove a coloring substance contained in pulp by a plurality of steps or to make it colorless. Bleached. In this bleaching process, calcium components, oxalic acid components, etc. contained in the raw wood of pulp are eluted into the process water and scales such as calcium carbonate and calcium oxalate are deposited, which causes various obstacles.

Also, as one of the kraft pulp bleaching processes, the introduction of an oxygen bleaching process has been attempted to reduce the load on the environment. For the purpose of further reduction, not only overseas but also in Japan, during the kraft pulp bleaching process, a bleaching method using chlorine gas to a chlorine-free bleaching method not using chlorine gas (hereinafter abbreviated as ECF bleaching method). The transition to adopting ECF (Elemental Chlorine Free) is progressing rapidly.

In general, when the ECF bleaching method is adopted, the amount of metal ions such as calcium ions carried in the latter stage (post process) tends to increase as compared with the conventional bleaching method using chlorine gas. Further, in the ECF bleaching method, when chlorine dioxide is mainly used, the oxidizing power becomes larger than that in the bleaching method using chlorine gas. In particular, when using a hardwood as a pulp raw material, lignin and hexenuronic acid present in the pulp fiber are oxidized to oxalic acid, and as a result, calcium oxalate is the main component in the apparatus used for the kraft pulp bleaching process. There is room for improvement because it becomes easier for the scale to form.

A kraft pulp bleaching method using the conventional ECF bleaching method will be described with reference to the drawings.
FIG. 2 is a schematic diagram showing an example of a kraft pulp bleaching process using the ECF bleaching method. In FIG. 2, the kraft pulp bleaching step is performed by using chlorine dioxide for D0 stage 21 and D1 stage 24, sodium hydroxide for treating E stage 22, and sodium hypochlorite for treating H stage 23. , A re-selection step 26, and a sedimentation step 27 in which a thickener with an extractor is used for treatment. In the figure, 25 is a bleaching tank, 28 is a completed chest, and 29 is a pipe for the papermaking process.

In the above-mentioned ECF bleaching method using chlorine dioxide, due to continuous operation over a long period of time, scale adheres to each device in the kraft pulp bleaching process, for example, a device such as a washer or a pump impeller, which causes various obstacles. Raise. That is, when the scale adheres to the cleaning device, it causes cleaning failure, and as a result, the bleaching chemical remains in the pulp slurry, so that it becomes necessary to increase the amount of the chemical added in the subsequent stage. If the amount of pulp slurry to be treated is reduced as a countermeasure, the bleaching treatment efficiency will be significantly reduced. In addition, when scale is attached to the pump, it becomes impossible to feed the liquid at a predetermined flow rate, which makes it difficult to maintain a constant pulp production amount. Therefore, in each apparatus of the kraft pulp bleaching process, it is necessary to regularly inspect and remove scale deposits and prevent scale deposits.

Among the devices in the kraft pulp bleaching process, for example, as a method of removing the scale adhering to the screen plate of the diffuser bleaching device, the operation of the kraft pulp bleaching process has been stopped so that the worker can diffuse the scale. A method of entering the bleaching apparatus and physically forcibly removing the attached scale using high pressure water, etc.
Alternatively, a method has been adopted in which cleaning water added with a scale removing agent is circulated in a diffuser bleaching device to elute the adhered scale over a long period of time.

However, in these methods, the operation of the kraft pulp bleaching step has to be stopped for a long time, and the pulp production amount decreases, resulting in a great loss. Especially when using a diffuser bleaching device,
Compared to the drum filter, because of its structure,
The removal of scale requires a great deal of time and labor, and has been a serious problem in the past.

In view of such a situation, in a kraft pulp bleaching process using a diffuser bleaching device, a chelating agent is added as a method for preventing scale from adhering to the screen plate of the diffuser bleaching device while continuing the operation. The method of doing has been adopted. Chelating agents have conventionally been used as scale inhibitors because they have the effect of blocking calcium ions and the like. However, since the chelating agent has poor metal selectivity and sequesters metal ions other than the intended calcium ion, a large amount of chelating agent is required to prevent adhesion of calcium-based scales, and the cost is low. In terms of aspect, it will require a lot of burden.

There is also a method of preventing the adhesion of scale by removing heavy metals by acid treatment, but this method also requires a large cost in view of the necessity of installing a dedicated device.

As described above, the method for preventing scale adhesion in the kraft pulp bleaching process using the ECF bleaching method has not yet been established as an effective countermeasure, and the ECF bleaching method which is effective at low cost is used. The establishment of a scale adhesion prevention method in the kraft pulp bleaching process has been desired.

[0012]

The object of the present invention is to prevent the scale from adhering to various devices during the kraft pulp bleaching process, and to have an excellent effect in removing the adhered scale and preventing rescaling. The present invention provides a method for preventing scale adhesion in a kraft pulp bleaching process.

[0013]

Means for Solving the Problems As a result of intensive studies to solve the above problems, the present inventors have found that 1-hydroxyethylidene-1 was added to a pulp slurry in a kraft pulp bleaching process using an ECF bleaching method. It was surprisingly found that the addition of 1,1-diphosphonic acid or a salt thereof can efficiently block calcium ions and prevent scale adhesion.

Further, even when a scale containing calcium oxalate as a main component is already attached in the bleaching apparatus, 1-hydroxyethylidene-1,1-
By adding diphosphonic acid or a salt thereof, the fact that the scale and particles forming the scale are swollen and the exfoliation of the once-adhered scale and the prevention of rescaling can be achieved was also found, and the present invention was completed.

That is, the scale adhesion preventing method in the kraft pulp bleaching step of the present invention is the scale adhesion preventing method in the kraft pulp bleaching step using the ECF bleaching method.
-Hydroxyethylidene-1,1-diphosphonic acid or a salt thereof is added to prevent scale from adhering to the apparatus used in the bleaching step.

In the present invention, the ECF bleaching method is characterized in that it is a bleaching method mainly using chlorine dioxide.

In the present invention, sulfuric acid is added to the unbleached kraft pulp slurry before the bleaching step to adjust the pH of the pulp slurry to 1 to 6.

The present invention is characterized in that it is a scale containing calcium oxalate as a main component among the scales.

In the present invention, the apparatus used in the kraft pulp bleaching step is characterized by being a dewatering and washing facility.

Further, in the present invention, the apparatus used in the kraft pulp bleaching step is a diffuser bleaching apparatus, which is characterized in that scale is prevented from adhering to the inside of the washer and the screen plate.

Further, in the present invention, the diffuser bleaching apparatus is connected with a medium-concentration pump for supplying pulp slurry, and 1-hydroxyethylidene-1,1 is contained in the pulp slurry on the suction side of the medium-concentration pump.
-Adding diphosphonic acid or a salt thereof.

Further, in the present invention, 1-hydroxyethylidene-1,1-diphosphonic acid or a salt thereof is added to the pulp slurry so as to have a concentration of 1 to 20 mg / liter.

[0023]

BEST MODE FOR CARRYING OUT THE INVENTION The method for preventing scale adhesion in the kraft pulp bleaching step of the present invention will be described in detail below.

The scale adhesion preventing method in the kraft pulp bleaching step of the present invention is a method for preventing scale adhesion in the kraft pulp bleaching step using the ECF bleaching method, and 1-hydroxyethylidene-1, as a scale adhesion preventing agent, When 1-diphosphonic acid or a salt thereof is added to a pulp slurry, calcium ions can be effectively blocked, and the scale and particles forming the scale can be swelled to easily remove the attached scale. The invention was achieved by finding a specific property.

1-Hydroxyethylidene-1,1-diphosphonic acid or a salt thereof as a scale anti-adhesive agent in the present invention has excellent anti-adhesion properties on a scale containing calcium ions, particularly a scale containing calcium oxalate as a main component. It is effective.

1-hydroxyethylidene-1,
The scale anti-adhesion agent consisting of 1-diphosphonic acid or its salt can respond to the effect of preventing scale from adhering to the diffuser bleaching equipment used in the kraft pulp bleaching process. It has an excellent effect of preventing scale from adhering to the scale on the plate.

Next, the method for preventing scale adhesion in the kraft pulp bleaching process of the present invention will be specifically described with reference to FIG. FIG. 1 is a partial schematic view of a kraft pulp bleaching process using an ECF bleaching method and a diffuser bleaching apparatus.

The processing flow in the ordinary kraft pulp bleaching process using the diffuser bleaching apparatus 2 is as follows. That is, the inlet pulp slurry 12 supplied to the feed tank 7 is the pulp slurry 1
The bleaching agent 9 added to 2 is sent from the pulp supply port B to the diffuser bleaching device 2 while being mixed by the medium concentration pump 8. Next, the pulp slurry 6 is washed and dewatered by the washing water 3 on the screen plate 1 of the washer provided on the upper part of the diffuser bleaching device 2, and is sent from the pulp outlet C to the next step as the outlet pulp slurry 11. . On the other hand, the extracted water (white water) 4 that is the dehydrated bleaching treatment liquid is extracted from the screen plate 1 and sent to the white water tank 5.

The method for preventing scale adhesion in the kraft pulp bleaching process of the present invention is performed by performing the bleaching treatment of the kraft pulp as described above, while each device in the bleaching process, for example, inside the washing machine of the diffuser bleaching device 2 and the scale in the screen plate 1. To prevent the adherence of. Further, among the scales, under conditions where calcium oxalate is likely to adhere, to the pulp slurry on the suction side of the medium concentration pump 8 that supplies the pulp slurry to the diffuser bleaching device 2, that is, from the chemical addition port A in FIG. 1-Hydroxyethylidene-1,1 as a scale adhesion preventing agent
-A method of adding diphosphonic acid or its salt 10.

In order to maximize the effect of preventing scale adhesion according to the present invention, the place of addition of 1-hydroxyethylidene-1,1-diphosphonic acid or a salt thereof as a scale adhesion preventing agent is such as calcium oxalate. It is preferably added at a point where scale is likely to be generated, that is, a point at which the pH of the pulp slurry greatly changes, for example, a point at which the liquidity changes from alkaline to acidic or vice versa. From this also, it is preferable to add 1-hydroxyethylidene-1,1-diphosphonic acid or its salt to the suction-side pulp slurry of the medium-concentration pump 8 that supplies the pulp slurry to the diffuser bleaching apparatus 2.

Further, 1-hydroxyethylidene-1,1
In order to uniformly disperse the diphosphonic acid or its salt in the pulp slurry, it is preferable to provide the feed tank 7 on the suction side of the medium concentration pump 8 with the chemical addition port A as shown in FIG.

The salt of 1-hydroxyethylidene-1,1-diphosphonic acid used in the present invention is preferably a water-soluble salt, for example, an alkali metal salt such as sodium salt or potassium salt.

In the present invention, the amount of 1-hydroxyethylidene-1,1-diphosphonic acid or a salt thereof added is such that the scale component to be adhered and 1-hydroxyethylidene-1,1-diphosphonic acid or a salt thereof used. The amount can be appropriately increased or decreased depending on the type, and is added to the pulp slurry so that the concentration is preferably 1 to 20 mg / liter, more preferably 3 to 10 mg / liter.

Here, 1 mg per pulp slurry
If the concentration is less than 1 liter / liter, a sufficient scale adhesion preventing effect cannot be obtained. On the other hand, if more than 20 mg / liter is added, the effect corresponding to the added amount cannot be obtained. Is not preferable.

In the present invention, of the scales to be adhered, 1-hydroxyethylidene-1,1-diphosphonic acid of the present invention or a salt thereof is used in the above-mentioned concentration for a scale containing calcium oxalate as a main component. If it does, it exhibits an excellent anti-adhesion effect.

In the present invention, the properties of the pulp slurry are not particularly limited when 1-hydroxyethylidene-1,1-diphosphonic acid or a salt thereof is added, but the properties of the pulp slurry are 3 to 3. 15 mass%
It is preferable that the concentration is 2 to 12, and the temperature is 45 to 95 ° C.

In the scale adhesion preventing method in the kraft pulp bleaching step of the present invention, 1-hydroxyethylidene-1,1-of the present invention is used unless the effects of the present invention are impaired.
There is no limitation on the combined use of other scale adhesion preventing agents, pitch control agents, and the like other than diphosphonic acid or salts thereof.

[0038]

EXAMPLES The present invention will be described in more detail with reference to Examples and Comparative Examples, but the present invention is not limited to these Examples. The following examples and comparative examples will be described with reference to FIG. 1 or 2.

Example 1 In a kraft pulp bleaching process employing an ECF bleaching method and a diffuser bleaching apparatus of a paper mill as shown in FIG. 2, 1-hydroxyethylidene-1,1- was added to the pulp slurry during its operation. Diphosphonic acid (hereinafter, H
Abbreviated as EDP. ) Was added to confirm the effect of preventing scale adhesion in the apparatus.

This bleaching process consists of D0 stage 21 and D1 stage 24 treated with chlorine dioxide, E stage 22 treated with sodium hydroxide, H stage 23 treated with sodium hypochlorite, reselection. Step 26 and settling separation step 2 using a thickener with an extractor
It consists of 7. Further, sulfuric acid was used for the purpose of adjusting the pH of the unbleached kraft pulp pulp slurry before the bleaching treatment.

More specifically, referring to FIG. 1 which is a partial schematic view of the D1 stage 24, the chemical addition port A on the suction side of the medium concentration pump for supplying the pulp slurry to the diffuser bleaching apparatus.
Always has a concentration of 10 with respect to the inlet pulp slurry 12.
HEDP was added to make mg / l. As a result, the HEDP was sufficiently mixed with the pulp slurry by the medium concentration pump 8 and supplied to the diffuser bleaching device 2.

In order to determine the effect of HEDP for preventing scale adhesion, the total calcium concentration TC (mg / liter) and the dissolved calcium concentration D contained in the inlet pulp slurry 12, the outlet pulp slurry 11 and the extract 4 are determined.
C (mg / liter) is measured, and these ratios DC / T
I asked for C. The obtained results are shown in Table 1 below.

Example 2 The concentration of HEDP in the inlet pulp slurry 12 was 5 m.
The test and evaluation were performed in the same manner as in Example 1 except that the amount was changed to g / liter. The obtained results are shown in Table 1 below.

Comparative Example 1 Tests and evaluations were carried out in the same manner as in Example 1 except that HEDP was not added. The obtained results are shown in Table 1 below.

[0045]

[Table 1]

From the results shown in Table 1, in Examples 1 and 2 to which HEDP was added, no change in total calcium concentration (TC) due to the difference in sample was observed. On the other hand, HEDP
In Comparative Example 1 in which the
TC of outlet pulp slurry 11 and extract 4 rather than 2
Is decreasing. It is considered that this is because the calcium component remained in the device as the pulp and the bleaching agent passed through the diffuser bleaching device, that is, the calcium component became scale and adhered to the device. From this, it is understood that the addition of HEDP can prevent the scale from adhering to the screen plate of the diffuser bleaching device.

In addition, in Example 1 and Example 2 in which HEDP was added, compared with Comparative Example 1 in which HEDP was not added, the dissolved calcium concentration (DC) in each measurement sample
Is high and DC / TC is large. From this, HEDP
It can be seen that the addition of Ca increases the dissolution amount of the calcium component and prevents the scale from adhering to the screen plate of the diffuser bleaching device.

Example 3 In the same manner as in Example 2, HEDP was added to D1 stage 24 in the kraft pulp bleaching step so as to be 5 mg / liter with respect to the pulp slurry, and oxalic acid in the D1 stage extract was added. The ion concentration was measured and its daily change was examined. The obtained results are shown in Table 2 below.

[0049]

[Table 2]

From the results shown in Table 2, the addition of HEDP
It can be seen that the oxalate ion concentration in the first-stage extract is increased, and the adhesion of scale containing oxalate ions as the main component in the kraft pulp bleaching step apparatus is suppressed.
The mechanism by which scale adhesion is prevented is not clear, but 1-hydroxyethylidene-1,1-diphosphonic acid or a salt thereof masks scale-forming components such as oxalate ions and calcium ions in some form. Therefore, it is considered that the generation of scale is suppressed.

Comparative Example 2 When the operation of the kraft pulp bleaching step was continued without adding HEDP in the same manner as in Comparative Example 1, the flow rate of the extract 4 of the diffuser bleaching apparatus 2 shown in FIG. Since the efficiency decreased, the cause was investigated, and it was found that scale adhered to the surface of the screen plate 1 provided above the diffuser bleaching device 2.

Therefore, after stopping the operation of the kraft pulp bleaching process and discharging the pulp in the diffuser bleaching device, the scale cleaning liquid (mixed liquid of aminocarboxylic acid type chelating agent and sodium hydroxide) is placed in the diffuser bleaching device. By circulating the solution, the scale was dissolved and removed to recover the flow rate of the extract 4. The scale cleaning agent is 50 g / liter and sodium hydroxide is 1 relative to the amount of cleaning liquid.
Washing was carried out with a concentration of 0 g / l. However, this method requires a large amount of scale cleaning liquid and a long cleaning time, and also requires waste liquid treatment after the completion of cleaning. The attached scale was analyzed by fluorescent X-ray and IR, and it was found that the scale was mainly composed of calcium oxalate.

Reference Example 1 The oxalate ion concentration contained in the pulp slurry of each bleaching stage was measured before and after the ECF bleaching method was adopted in the kraft pulp bleaching step. Specifically, D0 in FIG.
In the kraft pulp bleaching process before the ECF bleaching method in which the stage 21 is replaced by the C / D stage in which chlorine gas and chlorine dioxide water are treated together, the shu in the extract (dehydrated liquid) of each bleaching stage is used. The acid ion concentration was measured. Sodium hydroxide was used as the treatment agent in the E stage 22, sodium hypochlorite (hypo) in the H stage 23, and chlorine dioxide as the treatment agent in the D1 stage 24. The results obtained are shown in Table 3 below.
Also, the oxalate ion concentration in the extract (dehydrated liquid) of each bleaching stage was measured in the kraft pulp bleaching process after the ECF bleaching method was adopted by replacing the C / D stage with the D0 stage 21. The results obtained are shown in Table 3 below.

[0054]

[Table 3]

From the results shown in Table 3, in the kraft pulp bleaching step after the ECF bleaching method was adopted, the oxalate ion concentration in the extract of each bleaching stage was remarkably higher than that before the ECF bleaching method was adopted. You can see that it is increasing. Therefore, it is understood that the scale having oxalate ion as a main component is easily formed by adopting the ECF bleaching method.

[0056]

EFFECT OF THE INVENTION The method for preventing scale adhesion in the kraft pulp bleaching process of the present invention prevents scale adhesion to various devices during the kraft pulp bleaching process, removes the adhered scale, and prevents rescaling. Has excellent effect.

[Brief description of drawings]

FIG. 1 is a partial schematic diagram of a kraft pulp bleaching process using an ECF bleaching method and a diffuser bleaching apparatus.

FIG. 2 is a schematic view showing an example of a kraft pulp bleaching step employing the ECF bleaching method.

[Explanation of symbols]

1 Screen Plate 2 Diffuser Bleaching Device 3 Wash Water 4 Extraction Solution (White Water) 5 White Water Tank 6 Pulp Slurry 7 Feed Tank 8 Medium Concentration Pump 9 Bleaching Agent 10 1-Hydroxyethylidene-1,1-diphosphonic acid or its salt 11 Outlet Pulp slurry 12 Inlet pulp slurry A Chemical supply port B Pulp supply port C Pulp discharge port 21 D0 stage 22 E stage 23 H stage 24 D1 stage 25 Exposure tank 26 Reselection process 27 Settling separation process 28 Completed chest 29 Pipe to papermaking process

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Ichiro Umeda             3-4-2 Marunouchi 3-4-2, Chiyoda-ku, Tokyo             Ryo Paper Co., Ltd. (72) Inventor K. Saeki             2-10-15 Higashi-Awaji, Higashiyodogawa-ku, Osaka-shi, Osaka             Katayama Chemical Industry Research Institute Co., Ltd. (72) Inventor Keiji Kita             2-10-15 Higashi-Awaji, Higashiyodogawa-ku, Osaka-shi, Osaka             Katayama Chemical Industry Research Institute Co., Ltd. F term (reference) 4L055 AD20 AG99 AH40 EA31 EA32                       FA22

Claims (8)

[Claims] 1. A method for preventing scale adhesion in a kraft pulp bleaching step using a chlorine-free bleaching method (ECF bleaching method), wherein 1-
A method for preventing scale adhesion in a kraft pulp bleaching step, which comprises adding hydroxyethylidene-1,1-diphosphonic acid or a salt thereof to prevent the scale from adhering to an apparatus used in the bleaching step. 2. The method for preventing scale adhesion in a kraft pulp bleaching step according to claim 1, wherein the ECF bleaching method is a bleaching method mainly using chlorine dioxide. 3. A sulfuric acid is added to the unbleached kraft pulp slurry before the bleaching step to adjust the pH of the pulp slurry to 1-6.
The method for preventing scale adhesion in the bleaching process of kraft pulp according to claim 1 or 2, characterized in that: 4. The scale adhesion preventing method in the kraft pulp bleaching step according to claim 1, wherein the scale is composed mainly of calcium oxalate. 5. The apparatus used in the kraft pulp bleaching step is a dewatering and washing facility.
5. The method for preventing scale adhesion in the kraft pulp bleaching step according to any one of 4 above. 6. The apparatus used in the kraft pulp bleaching step is a diffuser bleaching apparatus, which prevents scale from adhering to the inside of the washer and the screen plate of the apparatus. A method for preventing scale adhesion in the kraft pulp bleaching step according to any one of claims. 7. The diffuser bleaching apparatus is connected with a medium-concentration pump for supplying pulp slurry, and the pulp slurry on the suction side of the medium-concentration pump contains 1
7. The method for preventing scale adhesion in the kraft pulp bleaching process according to claim 6, wherein hydroxyethylidene-1,1-diphosphonic acid or a salt thereof is added. 8. 1-hydroxyethylidene-1,1-diphosphonic acid or a salt thereof is added to the pulp slurry in an amount of 1
The method for preventing scale adhesion in the bleaching process of kraft pulp according to any one of claims 1 to 7, which is added so as to have a concentration of 20 mg / liter.