JP2000234286A - Cleaning of scale in pulp digester and its peripheral equipment - Google Patents

Abstract

PROBLEM TO BE SOLVED: To clean scales in a pulp digester, etc., with a slight influence exercised over surrounding environment without a risk of corrosion of an apparatus by using an aqueous solution of a specific acidic amino acid derivative as a cleaning liquid. SOLUTION: An alkaline aqueous solution at pH >=9 containing >=5 wt.% of one or more kinds of an acidic amino acid derivative represented by the formula (n is 1 or 2), its alkali metal salt, an ammonium salt and an amine salt thereof, e.g. N,N-dicarboxymethylaspartic acid (salt) and/or N,N- dicarboxymethylglutamic acid (salt) is used as a cleaning liquid for cleaning scales in a digester or peripheral equipment thereof in a process for producing a kraft pulp.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a pulp digester in a kraft pulp manufacturing process and a method for washing and removing scale attached to peripheral equipment.

[0002]

2. Description of the Related Art Kraft pulp is made by pulping wood chips as a raw material in a cooking liquor containing caustic soda and sodium sulfide at a high temperature and a high pressure to elute lignin contained in the wood. It is manufactured by doing. The cooking liquor is used by diluting white liquor, and in some cases, black liquor, and the cooking temperature is about 170 ° C. White liquor is an aqueous solution containing schematic caustic soda 55~100g / L, sodium sulfide 18~45g / L, of sodium carbonate 10 to 30 g / L (both Na ₂ O approximately). However, during this cooking step, calcium ions may elute from the wood chips, react with the carbonate ions in the cooking liquor, and form calcium carbonate in the system and precipitate. Further, the phosphorus component contained in the wood chips becomes phosphate ions in the cooking liquor, and this reacts with calcium to generate and precipitate calcium phosphate. In this way, calcium carbonate, calcium phosphate, and a combination thereof are attached as so-called scales in the digester and the subsequent tank piping. Scales impede the flow of liquid and chips, causing problems such as reduced productivity and non-uniform pulp quality.If they adhere to the heating section or strainer section, they will cause a major problem in operation, such as reduced thermal efficiency. Becomes For this reason, the operation which removes the scale adhered to the digester is usually performed 3 to 6 times a year.

Conventionally, acid washing using hydrochloric acid or sulfamic acid has been widely performed as a method for removing scale attached to a digester. However, in this method, while the process fluid is strongly alkaline, an acid is used for cleaning.Therefore, it is necessary to drain the process liquid and then perform acid cleaning when switching between the operation and the cleaning of the process. Since the washing liquid is an acid, it was necessary to lower the temperature of the digester to around 50 ° C. in order to reduce equipment corrosion due to the acid. For this reason, there have been problems such as troublesome cleaning, prolonged process downtime, and concern about corrosion of the device due to acid.

[0004] Therefore, a method that can reduce the trouble of cleaning work and the danger of equipment corrosion has long been desired. A method of removing scale using an aqueous solution containing a chelating agent such as sodium ethylenediaminetetraacetate (Japanese Patent Laid-Open No. 1-248686).
Gazette) was proposed. In this method, since the cleaning solution is alkaline, the amount of work required for use is significantly reduced, and the conventional problems such as the labor required for cleaning, the downtime of the process, and the corrosion of the device by acid are considerably improved.

On the other hand, aspartic acid is used as a target for dirt generated when alkaline earth metal salts and organic components such as calcium ions and magnesium ions contained in an aqueous system such as industrial cooling water are deposited in a process and grow on a scale. A method using diacetates and glutamic acid diacetates (Japanese Patent Application Laid-Open No. 9-221697) has been proposed, which is directed to calcium carbonate scales generated in a mild environment such as a cooling water system. High temperature and high pressure as in the pulp digester and its peripheral equipment, high temperature (about 170 ° C.), high pressure (10 kg / cm ² ), high alkali (p
H13), and there was no study on the scale complexed by the presence of calcium phosphate as well as calcium carbonate.

[0006]

The use of ethylenediaminetetraacetic acid for the scale generated in the pulp digester and its peripheral equipment was considered to be an effective method, but ethylenediaminetetraacetic acid is not biodegradable. Is bad,
When the wastewater containing these is treated with activated sludge or the like, there is a problem that the treatment requires a great deal of labor and cost. Particularly in recent years, consideration of the surrounding environment is an important management item, and a cleaning method with good biodegradability has been strongly desired.

Accordingly, an object of the present invention is to provide a method for cleaning scale adhered to the inside of a pulp digester and its peripheral equipment in a kraft pulp manufacturing process, since it is excellent not only in scale cleanability but also in biodegradability. It is another object of the present invention to provide a method for cleaning and removing scale using a cleaning liquid which is easy to treat waste water, has little influence on the surrounding environment even if it flows into the natural environment as it is, and has no danger of equipment corrosion.

[0008]

DISCLOSURE OF THE INVENTION The present inventors have intensively studied the cleaning effect of scale and the biodegradability of the cleaning liquid with respect to the cleaning of scale attached to a pulp digester and peripheral equipment in a kraft pulp manufacturing process. As a result, they have found that an alkaline aqueous solution of a specific acidic amino acid derivative is suitable for this purpose, and have made the present invention based on this finding.

That is, the present invention provides a compound represented by the general formula (I):

Embedded image (Wherein n represents 1 or 2), containing one or more of an alkali metal salt, an ammonium salt and an amine salt thereof, and having a pH of 9 or more. This is a scale cleaning method for a pulp digester and its peripheral equipment using a certain alkaline aqueous solution as a cleaning liquid.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail.

The present invention provides an acidic amino acid derivative represented by the general formula (I), one or more of an alkali metal salt, an ammonium salt and an amine salt thereof, and a pH of 9 or more. Is used as a washing liquid. It is known that this acidic amino acid derivative has excellent chelating ability and good biodegradability.

In the general formula (I), n is 1 or 2, and when n is 1, N, N-dicarboxymethylaspartic acid (hereinafter referred to as "aspartic acid diacetic acid"), n
Is 2, N, N-dicarboxymethylglutamic acid (hereinafter referred to as "glutamic acid diacetate").

Aspartic acid diacetate and glutamic acid diacetate have a left-handed form (L-) and a right-handed form (D-) due to their chemical structures.
In the present invention, any of these optical isomers may be used, or a mixture of both may be used.

The method for preparing the washing solution of the present invention is not particularly limited. For example, an aqueous solution of aspartic acid diacetate, glutamic acid diacetate or a salt thereof is prepared, and an alkali source is added thereto or prepared in advance. It is obtained by dissolving aspartic acid diacetate, glutamic acid diacetate or a salt thereof in the obtained alkaline aqueous solution. Aspartic acid diacetate and glutamic acid diacetate may be used in the form of a free acid or in the form of salts. The type of the salt is not particularly limited as long as it does not hinder the water solubility. For example, alkali metal salts, ammonium salts, amine salts and the like are used. Aspartic diacetate and glutamic diacetate have a pH of 9 even when a free acid is used.
In the above alkaline aqueous solution, some or all of the four carboxyl groups in the molecule exist as salts.

The aspartic acid diacetate and glutamic acid diacetate may be mono-, di-, tri-, or tetra-salts, or may be of any degree of neutralization.

Aspartic acid diacetate according to the present invention,
The alkali metal salts of glutamic acid diacetate are sodium salts, potassium salts, and lithium salts, and the amine salts are salts with aliphatic amines such as methylamine, dimethylamine, ethylamine, and propylamine. These salts may be a single salt, a mixture of two or more salts,
Further, a complex salt of a sodium salt and a potassium salt may be used. However, practically, the alkali metal salt is most preferably a sodium salt. The alkali source of the aqueous alkali solution is
Sodium hydroxide, potassium hydroxide, lithium hydroxide,
And aqueous solutions of ammonia and amines. These alkali sources may be one or two
Mixtures of more than one species are acceptable. However, practically, sodium hydroxide is most preferred.

The alkaline aqueous solution containing aspartic acid diacetate and / or glutamic acid diacetate or a salt thereof according to the present invention has a pH of 9 or more, preferably 10 or more. If the pH is lower than 9, the desired scale cleaning effect is poor.

The concentration of aspartic acid diacetate and / or glutamic acid diacetate present in the washing solution of the present invention is not particularly limited. However, as the concentration increases, the scale dissolution rate increases, so that 5% by weight is used. It is preferable to use the above. The chemical equivalent of aspartic acid diacetate and / or glutamic acid diacetate or a salt thereof to be contained in the aqueous solution used for scale removal is at least equivalent to the chemical equivalent of calcium or magnesium contained in the target scale. Is an essential condition, and preferably at least twice the amount.

In order to clean the scale of the pulp digester and its peripheral equipment using the cleaning liquid of the present invention, the cleaning liquid of the present invention is added to the process liquid while the operation of the process is continued, or the operation of the process is performed. After stopping the operation and draining the process liquid,
This can be achieved by filling the washing liquid of the present invention as it is and circulating the washing liquid in the system. The use temperature of the cleaning liquid of the present invention is 50 to 160 ° C. However, the higher the temperature, the higher the scale removal efficiency. Therefore, the cleaning liquid is used in the vicinity of the temperature during normal operation. As described above, the scale removing method according to the present invention can significantly reduce the labor required for cleaning.

The period required for scale cleaning in the present invention is not uniformly determined because it varies depending on the method of applying the cleaning solution, the degree of scale adhering to the process, the type of scale, and other process conditions. It takes about one day to stop the process and circulate the cleaning liquid. Also,
The cleaning solution of the present invention can be added to the process solution during the operation without stopping the process.

Incidentally, as long as the object of the present invention is achieved, other optional components such as a surfactant are mixed with the cleaning solution,
Or, there is no limitation on the use in combination.

The waste water containing aspartic acid diacetate, glutamic acid diacetate or a salt thereof, or a complex compound thereof,
When treated with activated sludge or the like, its biodegradability is very good, and the treatment cost is significantly reduced as compared with conventional ethylenediaminetetraacetic acid salts.

Now, the present invention will be described in further detail with reference to Examples.

[0024]

EXAMPLES Example 1 Using scales collected from the wall of a continuous kraft pulp digester, the dissolving power of the scales with various test liquids was compared.

[Test Scale] The sampled scale was a composite scale having the following ash composition.

[Test Method] 100 mL of the test solution was placed in a 300 mL Erlenmeyer flask, and 200 mg of the above scale adjusted to a particle size of 20 to 50 mesh was added to the test aqueous solution. Bioshaker (BR300 manufactured by Taiyo Kagaku Kogyo Co., Ltd.)
In L), the test solution was brought into contact with the scale while shaking at 100 rpm. No. 3 hours after contact. The undissolved scale was collected by filtration using 6 filter papers. The weight of the undissolved scale was measured, and the scale dissolution rate was determined by the following equation.

[0027]

(Equation 1)

[Test Results] The results obtained are shown in Table 1. As to the scale dissolving ability, the aqueous solution of tetrasodium aspartate diacetate and the aqueous solution of glutamic acid diacetate are the same as those of the conventional disodium ethylenediaminetetraacetate. And hydrochloric acid.

[0029]

[Table 1]

Example 2 The effect was compared by changing the pH of the cleaning solution.

[Test method] Using the same scale as in Example 1, the scale dissolving effect of aqueous solutions of tetrasodium aspartate diacetate, tetrasodium glutamic acid diacetate, and disodium ethylenediaminetetraacetate at various pH values Was examined. The pH was adjusted with sodium hydroxide and hydrochloric acid. The test method was the same as in Example 1, with a temperature of 150 ° C. and a contact time of 3 hours.
The weight of the undissolved scale was measured in the same manner as in Example 1,
The scale dissolution rate was determined.

[Test Results] As shown in Table 2, the results are as shown in Table 2.
The scale dissolving effect of the aqueous solution of glutamic acid diacetate tetrasodium salt was exhibited at pH 9 or more, and the scale dissolving effect was almost the same as that of the conventional aqueous solution of disodium ethylenediaminetetraacetate.

[0033]

[Table 2]

Example 3 The effect of corrosion on a stainless steel plate generally used as a coating material for the inner surface of a pulp digester was investigated.

[Test method] Test liquid 3 was placed in a 350 mL wide-mouth bottle.
Of each of the stainless steels (S
US304) test piece was immersed and continuously stirred.
After 50 hours, corrosion products were removed with a nylon scourer, washed with water and dried, and the weight loss of the test piece was measured.
The corrosion rate was determined by the following equation.

[0036]

(Equation 2)

[Test Results] The results obtained are shown in Table 3.
From these results, it can be seen that the aqueous solution of tetrasodium aspartate diacetate and the aqueous solution of glutamic acid diacetate tetrasodium have almost no corrosion effect on stainless steel in the range of 50 to 150 ° C.

[0038]

[Table 3]

Example 4 The biodegradability of an aqueous solution of tetrasodium aspartate diacetate, an aqueous solution of glutamic acid diacetate and an aqueous solution of disodium ethylenediaminetetraacetate was compared.

[Test Method] Aspartic acid diacetate tetrasodium salt, glutamic acid diacetate tetrasodium salt and ethylenediaminetetraacetic acid disodium salt were each subjected to COD
_A solution diluted with soft water so as to have a _cr of 0.1% was prepared. 10 g of sludge collected from a wastewater treatment plant of a paper mill was put into a 1 L beaker, and 1 L of an aqueous solution of tetrasodium aspartate diacetate, tetrasodium glutamic acid diacetate, and disodium ethylenediaminetetraacetate prepared above were added. Air was continuously fed from a thin glass tube and continuously aerated for 48 hours. Every 12 hours, 20 ml of the supernatant of a 1 L beaker was sampled, filtered through No. 6 filter paper, and subjected to COD _cr measurement based on JIS K0102.

[Test Results] Table 4 shows changes over time of COD _{cr in} the test liquid. From this result, it can be seen that tetrasodium aspartate diacetate and tetrasodium glutamic acid diacetate are much more biodegradable than disodium ethylenediaminetetraacetate.

[0042]

[Table 4]

[0043]

According to the present invention, an alkaline aqueous solution of tetrasodium aspartate diacetate and an alkaline aqueous solution of glutamic acid diacetate can be used as long as they exhibit excellent washing effects on scales adhering to a pulp digester and its peripheral equipment. It is much better in terms of biodegradability than the conventionally used disodium ethylenediaminetetraacetate, and its wastewater treatment is easy. Low and no danger of equipment corrosion. However, it can be said that the pulp and paper industry has significant benefits such as increasing the operation rate of the pulp manufacturing process, enabling safe operation, and contributing to the protection of the surrounding environment.

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Claims (1)

[Claims] 1. A compound of the general formula (I) (Wherein n represents 1 or 2), containing one or more of an alkali metal salt, an ammonium salt and an amine salt thereof, and having a pH of 9 or more. A scale cleaning method for a pulp digester and its peripheral equipment, wherein a certain alkaline aqueous solution is used as a cleaning liquid.