JPH06108274A - Treatment of boiler water - Google Patents

Abstract

PURPOSE:To effectively execute suppression of scale formation and prevention of corrosion by adding specific natural corrosion suppressing agents, sulfonic acid (salt) and unsatd. carboxylic acid polymer into boiler water. CONSTITUTION:At least >=1 kinds of the natural corrosion inhibitors selected from among tannins, lignins and sugars, the phosphoric acid (salt) and the unsatd. carboxylic acid polymer are added into the boiler water so as to attain about 20 to 500mg/l, about 5 to 100mg/l and about 5 to 100mg/l, respectively. The respective components may be respectively separately added or may be added after these components are premixed. The place of the addition may be a makeup water line or the components may be directly added to the boiler. As a result, the adhesion of the scale on metallic surfaces is prevented and the metal is isolated from an environment contg. corrosive factors by forming an insoluble adsorptive film or oxide film having excellent stability on the metal surface, thereby, the generation of the scale is suppressed and the corrosion is prevented.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for treating boiler water, and more particularly to a method for effectively preventing corrosion and scale generation in a boiler can using pure water or soft water as feed water. is there.

[0002]

2. Description of the Related Art Corrosion inhibitors such as hydrazine, sodium sulfite, tannins, lignins, and sugars, and water-soluble agents are used to suppress corrosion and scale formation in boilers that use pure water or soft water as water supply. Scale dispersants mainly composed of polymer compounds are used together with alkali agents and inorganic phosphates.

On the other hand, a water treatment agent containing a copolymer derived from a monomer having a phosphonic acid and a carboxyl group and having an ethylenically unsaturated bond, or water containing a phosphonic acid and a maleic acid-based copolymer. Polysaccharides such as lignin derivatives, tannic acids, starches as treatment agents and scale inhibitors, and water treatment methods using these derivatives in combination are disclosed in JP-A-51-146341 and JP-A-53-1, respectively.
It is disclosed in Japanese Patent No. 46279.

A metal anticorrosive agent containing a mixture of phosphonic acids and tannins or lignins as a main component is disclosed in JP-A-48-71741 in which a zinc compound, a lignin compound and a maleic acid polymer are added together. Japanese Patent Laid-Open No. 56-62974 proposes a method for preventing corrosion of underwater metal.

[0005]

In recent years, there has been a growing interest in safety and energy saving effects as well as effects on water treatment chemicals, and water treatment in boiler water systems is no exception. For the purpose of energy saving,
Along with the progress of high-concentration operation of boilers and the recovery and use of condensate, scaling and local corrosion in boiler cans caused by iron oxide introduced from makeup water and condensate are major problems.

Although hydrazine and sodium sulfite are the main corrosion inhibitors used in boiler water systems, when hydrazine is used, its toxicity may cause the generated steam or high temperature water to come into direct contact with the human body or products. It cannot be used in hospitals or food factories.

Although sodium sulfite has no problem in safety, it does not always exert sufficient effect because it produces sulfate ions that cause corrosion by reacting with oxygen. As a result of remarkably increasing the concentration and the electric conductivity, the blow amount is increased, and the energy cost is increased.

On the other hand, the corrosion inhibitors of natural products such as tannic acid have no problem of safety and problems of leading to excessive increase of dissolved solids concentration and electric conductivity of boiler water, but stable corrosion protection. It is difficult to obtain the effect, and in particular, when a scale composed mainly of iron oxide adheres to the metal surface, local corrosion due to the formation of the oxygen concentration battery occurs, and the sufficient corrosion inhibiting effect is not exhibited.

Acrylic acid polymers and maleic acid polymers are the main scale dispersants used with hardeners such as alkaline agents and inorganic phosphates in boiler water systems, but calcium hardness is mixed in the cans. It exhibits an excellent dispersing effect on calcium-based scales such as calcium carbonate and calcium phosphate, which are generated in such cases, but hardly exhibits a dispersing effect on metal oxide-based scales such as iron oxide.

Further, the water treatment method disclosed in Japanese Patent Application Laid-Open No. 51-146341 or Japanese Patent Application Laid-Open No. 53-146279 is relatively related to a method for preventing corrosion, scale and dirt in a cooling water system. Insoluble precipitate scale consisting of phosphonic acids and calcium ions that precipitates on the metal surface in cooling water with high calcium hardness is suppressed by using water-soluble polymer compounds and other scale inhibitors to simultaneously suppress corrosion and scale. I'm trying to figure out
Generally, pure water or soft water is used as the feed water, and the expected effect cannot be obtained in a boiler water system having no calcium hardness.

Further, the metal anticorrosive agent and the water treatment method proposed in Japanese Patent Laid-Open No. 48-71741 or Japanese Patent Laid-Open No. 56-62974 are the same as the above-mentioned water treatment method, specifically, in cooling water. Regarding the corrosion inhibition, the former is to improve the anticorrosion effect by mixing phosphonic acids with tannins or lignins, reduce the necessary addition concentration, and prevent water from coloring at the use concentration,
The latter is to add maleic acid polymer to the combination of zinc compound and lignin compound to improve the dispersibility of zinc and lignin compound in water, improve the anticorrosion property, and realize the treatment at a lower concentration. is there. These inventions are concerned with the anticorrosion effect, and do not mention anything about the effect related to scale control, which is another major problem of water treatment.

Further, the use of polysaccharides such as lignin derivatives, tannic acids and starch used in the above water treatment method is a scale inhibitor, which produces a reducing ability by hydrolysis under high temperature and high pressure as in a boiler water system. However, there is no suggestion of the idea of acting as a corrosion inhibitor by removing dissolved oxygen. An object of the present invention is to solve the above-mentioned drawbacks of conventional water treatment methods and to provide a water treatment method capable of effectively suppressing scale formation and preventing corrosion.

[0013]

[Means for Solving the Problems] A method for treating boiler water according to the present invention, which has been made in order to achieve the above-mentioned object, is a method in which at least one natural water selected from tannins, lignins and sugars is contained in the boiler water. It is characterized in that a physical corrosion inhibitor, phosphonic acid or a salt thereof, and an unsaturated carboxylic acid polymer are added together.

Among the three kinds of components used in the present invention, the treatment by using one kind alone or in combination of two kinds cannot obtain sufficient effects of anticorrosion and scale inhibition from the technical basis as described below.

That is, with respect to corrosion protection of metals, natural compound-based corrosion inhibitors such as tannic acid suppress corrosion by forming an insoluble film on the metal surface together with the deoxidizing action. If scale such as iron oxide adheres to the steel, local corrosion will occur due to the formation of an oxygen concentration battery, and the sufficient corrosion inhibiting effect will not be exhibited.

Regarding scale inhibition, phosphonic acid or a salt thereof exhibits a high dispersive effect due to an excellent chelating effect on calcium carbonate, especially iron oxide scale, but almost disperses on calcium phosphate scale. On the other hand, the unsaturated carboxylic acid polymer exerts an excellent dispersing effect on calcium-based scales such as calcium carbonate and calcium phosphate,
It has almost no dispersion effect on iron oxide.

In the method for treating boiler water of the present invention, by combining these three kinds of components having different characteristics, adhesion of scales including iron oxide to the metal surface is prevented,
It is intended to prevent the occurrence of local corrosion due to the formation of an oxygen concentration battery, and to prevent metal corrosion and suppress scale.

The addition amount of the above components used in the present invention is
20-500 for tannins, lignins, and sugars
mg / l preferably 50-300 mg / l, phosphonic acid or its salt 5-100 mg / l, preferably 10
50 mg / l, 5-100 m of unsaturated carboxylic acid polymer
g / l is preferably from 10 to 50 mg / l from the viewpoint of corrosion and scale inhibition effects or from the economical viewpoint.

Each of the above components may be added separately so as to have a predetermined concentration, or may be mixed in advance and added. The components may be added to the makeup water line or directly to the boiler.

The unsaturated carboxylic acid polymer used as a component of the present invention is a homopolymer of acrylic acid, maleic acid and methacrylic acid from the viewpoint of exhibiting an excellent dispersing effect on calcium carbonate and calcium phosphate scales. And / or copolymers and / or copolymers with other compounds capable of copolymerizing these monomers are preferred.

Other compounds copolymerizable with the above monomers include compounds having an ethylenic double bond, such as fumaric acid, itaconic acid, acrylamide, acrylic ester, vinyl acetate, styrene, ethylene oxide, ethylene, n. -Butylene, isobutylene, allyl sulfonic acid, vinyl sulfonic acid and styryl sulfonic acid.

The molecular weight of these polymers is from 500 to
100,000, preferably 1,000 to 20,000
Is preferable from the viewpoint of exerting an excellent scale dispersion effect.

Among the natural product compounds used as corrosion inhibitors in the present invention, tannins are hydrolyzable tannins such as gallotannin and condensable tannins such as quebracho tannin, and lignin sulfonic acid as lignins. Examples of sodium and saccharides include monosaccharides such as glucose, polysaccharides such as starch, and derivatives thereof.

In addition to calcium carbonate, phosphonic acid or its salt used as a dispersant for metal oxide scales such as iron oxide in the present invention includes aminotrimethylphosphonic acid, aminodimethylphosphonomonoethylphosphonic acid and ethylenediamine. Tetramethylphosphonic acid, diethylenetriaminepentamethylphosphonic acid, triethylenetetraaminehexamethylphosphonic acid, hydroxyethylidenediphosphonic acid, hydroxypropylidenediphosphonic acid, 1,2,4-tricarboxybutane-2-phosphonobutanetricarboxylic acid, 1, Examples thereof include 2-dicarboxybutane-2-phosphonic acid, 1,2,4-tricarboxyhexane-2-phosphonic acid and salts thereof.

In addition to the above three components used in the treatment method of the present invention, alkaline agents such as sodium hydroxide and sodium carbonate, inorganic phosphates such as sodium tertiary phosphate and sodium hexametaphosphate, glycolic acid, lactic acid, It may be used in combination with a corrosion inhibitor such as citric acid, tartaric acid, malic acid, gluconic acid, cyclohexylamine, morpholine and octadecylamine, or may be premixed before use.

[0026]

[Function] Corrosion inhibitors based on natural products such as tannic acid
As an oxygen scavenger, it removes dissolved oxygen in water, which is the main cause of corrosion, and forms an insoluble film on the metal surface to isolate the metal from the environment containing corrosion factors, thereby suppressing metal corrosion.

The phosphonic acids and unsaturated carboxylic acid polymers used as the scale dispersant are adsorbed on the surface of the scale particles to enhance the charge repulsion between the particles,
Scaling is suppressed by altering the crystal structure to prevent crystal growth.

In the treatment method of the present invention, the action of the phosphonic acid and the unsaturated carboxylic acid polymer prevents the scale such as iron oxide from adhering to the metal surface, and further includes these scale inhibitors and tannic acid. Due to the synergistic effect of the corrosion inhibitor of natural products, an insoluble adsorption film or oxide film with excellent stability is formed on the metal surface, and by separating the metal from the environment containing corrosion factors, scale inhibition and corrosion prevention are achieved. It is thought to be.

Examples of the present invention will be described below.

[0030]

Example 1 A mild steel test tube (inner diameter: 50 mm, length: 500 mm) and a mild steel test piece were installed in an experimental boiler having a capacity of 10 liters, and a corrosion test was conducted. The soft water shown in Table 1 was used as the make-up water for the boiler, and each chemical used in the treatment method of the present invention was added to the make-up water so as to have a predetermined concentration in the boiler water, and the water was supplied to the experimental boiler.

[0031]

[Table 1]

Incidentally, sodium hydroxide was added to the makeup water to adjust the pH of the boiler water to 11.0 or higher. The experimental boiler was operated under a pressure of 1 MPa and a temperature of 183 ° C. at a concentration of 10 times for 150 hours.

After the test was completed, the test tube and the test piece were taken out, and the occurrence of corrosion was observed. Further, the corrosion rate and the corrosion inhibition rate of the test piece were measured by the following equations.

[0034]

[Equation 1]

[0035]

[Equation 2]

The test results are shown in Table 2.

[0037]

[Table 2]

[0038]

Example 2 In order to evaluate the dispersibility of various scale components (calcium carbonate type, calcium phosphate type, iron hydroxide type), a dispersion test using synthetic water shown in Table 3 was conducted. Put 200 ml tall beaker with synthetic water to which each drug was added at the specified concentration and put it in a high temperature tank kept at 80 ° C.
Let stand for 0 hours, determine the concentration of scale components in the supernatant,
The scale dispersion ratio was investigated by the following formula.

[0039]

[Equation 3]

[0040]

[Table 3]

The test results are shown in Table 4.

[0042]

[Table 4]

[0043]

[Effect] The boiler water treatment method of the present invention has high safety such as the use of a natural product compound such as tannic acid as a corrosion inhibitor, and is also effective in highly concentrated operation and recovery of condensate. Since it can be sufficiently used, it can greatly contribute to the reduction of energy cost.

Furthermore, the treatment method of the present invention provides stable and excellent scale inhibition by the synergistic effect of the combined use of two kinds of scale dispersants having different dispersion characteristics and a natural product type corrosion inhibitor such as tannic acid. Anticorrosion treatment can be realized at the same time.

[Procedure amendment]

[Submission date] December 9, 1993

[Procedure Amendment 1]

[Document name to be amended] Statement

[Name of item to be corrected] 0030

[Correction method] Change

[Correction content]

[0030]

Example [Corrosion test] A mild steel test tube (inner diameter 50 mm, length 500 mm) and a mild steel test piece were installed in a 10-liter capacity experimental boiler to perform a corrosion test. The soft water shown in Table 1 was used as the make-up water for the boiler, and each chemical used in the treatment method of the present invention was added to the make-up water so as to have a predetermined concentration in the boiler water, and the water was supplied to the experimental boiler.

[Procedure Amendment 2]

[Document name to be amended] Statement

[Correction target item name] 0038

[Correction method] Change

[Correction content]

[Scale Dispersion Test] In order to evaluate the dispersibility of various scale components (calcium carbonate type, calcium phosphate type, iron hydroxide type), a dispersion test using synthetic water shown in Table 3 was performed. Put 200 ml tall beaker in synthetic water with the specified concentration of each drug, and add 80
℃ constant temperature bath at and left for 20 hours kept at obtains the scale component concentration in the supernatant was investigated scale dispersion rate by the following equation.

Claims (3)

[Claims] 1. A boiler water containing at least one natural product corrosion inhibitor selected from tannins, lignins and sugars, a phosphonic acid or a salt thereof, and an unsaturated carboxylic acid polymer. A method for treating boiler water, comprising: 2. A boiler water containing 20 to 500 mg / l of at least one natural compound selected from tannins, lignins and sugars, 5 to 100 mg / l of phosphonic acid or a salt thereof, and an unsaturated carboxylic acid. The method for treating boiler water according to claim 1, wherein the polymer is added in a combined amount of 5 to 100 mg / l. 3. The unsaturated carboxylic acid polymer is a homopolymer of acrylic acid, maleic acid, methacrylic acid and / or
The method for treating boiler water according to claim 1 or 2, wherein the method is a copolymer and / or a copolymer with another compound copolymerizable with these monomers.