Scale prevention agent and scale prevention method

Abstract

This invention provides a scale inhibitor and a scale prevention method for preventing scale formation in various processing steps using slaked lime suspension, particularly in the preparation tank for slaked lime suspension. [Solution] The scale inhibitor of the present invention is characterized by containing polyitaconic acid and / or a water-soluble salt thereof with a weight-average molecular weight of 1000 to 4000, and by adding it to a slaked lime suspension, it effectively prevents the formation and adhesion of scale. The present invention can prevent scale problems in equipment using slaked lime suspensions, which have been a problem in the past, and can bring about effects such as reduced maintenance costs and stabilization of continuous operation.

Description

Technical Field

[0001] The present invention relates to a scale inhibitor and a scale prevention method in various industrial facilities using slaked lime suspension. In particular, it relates to a scale inhibitor and a scale prevention method for solving the problems of scale adhesion and deposition on pipes, inner walls of tanks, etc. in lime slurry transfer, storage, mixing, and reaction facilities in wastewater treatment, industrial water treatment, sewage treatment, paper pulp processes, chemical industries, semiconductor processes, etc.

Background Art

[0002] In semiconductor factories and other water treatment facilities, in the process of removing fluoride ions using slaked lime suspension, there is a problem that carbon dioxide gas in the air reacts with calcium hydroxide, and calcium carbonate scale precipitates in the slaked lime suspension adjustment tank and the slurry addition line. It is known that this scale accumulates in the pipes of the device and hinders operation.

[0003] In the conventional technology, attempts have been made to remove scale by mechanical cleaning or using high-concentration chemicals. However, these methods have problems such as high labor and cost, and increasing the burden on the device and the environment. Also, in Patent Document 1, it is proposed to prevent calcium carbonate scale with slaked lime suspension containing a copolymer containing acrylic acid and 2-acrylamido-2-methylpropanesulfonic acid as constituent units and / or its water-soluble salt. However, a large addition amount is required to exhibit the scale prevention effect, and a scale inhibitor that exhibits the scale prevention effect at a lower concentration is desired for the influence on the subsequent process and reduction of environmental load.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] This invention aims to prevent the formation of calcium carbonate scale when using slaked lime suspension, and to solve the problems of adverse effects on downstream processes and environmental burden associated with the addition of large amounts of scale inhibitors, which are issues in conventional technologies. Furthermore, considering environmental and cost aspects, the invention aims to achieve efficient operation of equipment by developing a scale inhibitor that exhibits high preventative effect at low concentrations. [Means for solving the problem]

[0006] The inventors of the present invention conducted research on polymer scale inhibitors and found that polyitaconic acid and / or its water-soluble salts with a weight-average molecular weight of 1000 to 4000 exhibit a remarkable inhibitory effect against scale adhesion caused by slaked lime suspension, thereby completing the present invention.

[0007] [1] A scale inhibitor for slaked lime suspension, characterized by containing polyitaconic acid and / or a water-soluble salt thereof having a weight-average molecular weight of 1000 to 4000. [2] A method for preventing scale, characterized by adding a scale inhibitor containing polyitaconic acid and / or a water-soluble salt thereof having a weight-average molecular weight of 1000 to 4000 to a slaked lime suspension. [3] The method for preventing scale according to claim 2, characterized in that the scale inhibitor is added to the solid content of the slaked lime suspension in an amount of 100 to 10,000 ppm. [4] The scale prevention method according to claim 2 or 3, characterized in that it prevents scale from adhering to a non-heat transfer surface. [5] The method for preventing scale according to claim 2 or 3, characterized in that the target scale contains calcium carbonate. [Effects of the Invention]

[0008] By using this invention, it is possible to prevent scaling more efficiently and sustainably compared to conventional technologies, thereby supporting the long-term operation of equipment. [Modes for carrying out the invention]

[0009] The following describes embodiments for carrying out the present invention, but the present invention is not limited to these. A scale inhibitor for slaked lime suspension, characterized by containing polyitaconic acid and / or a water-soluble salt thereof having a weight-average molecular weight of 1000 to 4000.

[0010] Although the exact mechanism by which the present invention exerts its effects is not clear, the following hypothetical mechanism is considered. Polyitaconic acid has numerous carboxylic acid functional groups (-COOH or -COO-) and dissociates in water to exhibit high anionicity. By using the polyitaconic acid of the present invention and / or its water-soluble salt having a specific weight-average molecular weight, it is thought that scale formation can be effectively prevented by complex formation with calcium ions. Furthermore, even if fine particles that form the basis of scale are generated, it is thought that the polyitaconic acid of the present invention adsorbs and disperses on the surface of the fine particles, preventing aggregation of the fine particles and effectively preventing adhesion to walls and other surfaces. In addition, since polyitaconic acid and / or its water-soluble salt also possess surfactant properties, it is thought that scale adhesion is prevented by making the walls of the preparation tank and the metal surfaces of the pipes hydrophilic.

[0011] Examples of polyitaconic acid and / or its water-soluble salts used in the present invention include polyitaconic acid and alkali metal salts of polyitaconic acid, with examples of alkali metal salts being sodium salts, potassium salts, etc. Other salts include partially neutralized salts and fully neutralized salts of polyitaconic acid, such as ammonium salts and amine salts.

[0012] The method for producing polyitaconic acid according to the present invention is not particularly limited and can be obtained by known methods. For example, it can be obtained by adding an aqueous solution of alkali metal hydroxide to an aqueous solution of itaconic acid, heating, and adding a polymerization initiator dropwise to perform polymerization.

[0013] The polymerization initiators used in this invention generally include azobis compounds such as azobis-isobutyronitrile (AIBN) and azobis-2,4-dimethylvaleronitrile, persulfates such as sodium persulfate, potassium persulfate, and ammonium persulfate, hydrogen peroxide, cumene peroxide, and di-tert-butyl hydroperoxide. From the viewpoint of preventing scale formation, hydrogen peroxide and persulfates are preferred as initiators. In addition, chain transfer agents may be added as needed when adjusting the degree of dispersion. Examples of chain transfer agents include thiol-based chain transfer agents such as mercaptoethanol, 2-mercaptopropionic acid, 3-mercaptopropionic acid, and n-dodecyl mercaptan; halides such as carbon tetrachloride, methylene chloride, bromoform, and bromotrichloroethane; secondary alcohols such as isopropanol and glycerin; phosphorous acid, hypophosphorous acid, and their salts (sodium hypophosphite, potassium hypophosphite, etc.); sulfurous acid, bisulfite, dithionite, metabisulfite, sulfite, and thiosulfate (specifically, sodium bisulfite, potassium bisulfite, sodium dithionite, potassium dithionite, sodium metabisulfite, potassium metabisulfite, etc.). The amount of the chain transfer agent added is not particularly limited, but it is preferably 0.001 to 10 g per mole of itaconic acid component. The weight-average molecular weight of the obtained polyitaconic acid can be measured by high-performance liquid chromatography or the like.

[0014] The weight-average molecular weight of the polyitaconic acid and / or its water-soluble salt used in this invention is characterized by being 1000 to 4000 or less. More preferably, the weight-average molecular weight is 1000 to 3000. Excellent scale prevention effect is observed within this molecular weight range. If the weight-average molecular weight is less than 1000 or more than 4000, the scale prevention effect becomes insufficient and undesirable.

[0015] The scale inhibitor of the present invention is preferably supplied in the form of a 1 to 70% by weight aqueous solution, and particularly preferably in the form of a 1 to 50% by weight aqueous solution, in order to improve dispersibility and ensure stability.

[0016] The slaked lime suspension in this invention is not particularly limited and may be a general calcium hydroxide suspension or a calcium hydroxide suspension in which calcium hydroxide dissolved in water and calcium hydroxide dispersed and suspended in water as fine particles are present. Specifically, there are methods such as reacting quicklime (calcium oxide) with water to obtain a suspension of slaked lime (calcium hydroxide), or dispersing slaked lime in water to obtain a slaked lime suspension, and either method is acceptable. The concentration of the slaked lime suspension is determined appropriately depending on the usage conditions, but is preferably 1 to 20% by weight, and more preferably 5 to 15% by weight. Higher concentrations are undesirable because the scale prevention effect becomes difficult to achieve.

[0017] The scale targeted in this invention is calcium hydroxide in a slaked lime suspension and air This is a calcium carbonate scale, primarily composed of calcium carbonate produced by the reaction of carbon dioxide. Note that the scales considered also include those associated with calcium hydroxide, in addition to calcium carbonate.

[0018] The location where scale occurs in this invention is not particularly limited, but from the viewpoint of scale prevention effect, a location that is not a heat transfer surface is preferred. Locations where scale can occur include, for example, a preparation tank for preparing the slaked lime suspension, transfer piping for transporting the slaked lime suspension, a coagulation tank for mixing the slaked lime suspension with treated water and performing coagulation treatment, a centrifugal dewatering machine and its peripheral equipment for separating the treated water containing the coagulated material into sludge and treated water through solid-liquid separation, a storage tank for the filtrate separated by the centrifugal dewatering machine, filtrate discharge piping, and further discharge piping and peripheral equipment for performing sterilization treatment and pH adjustment on the filtrate before it is discharged into a river. Preferably, these are the preparation tank for preparing the slaked lime suspension and the transfer piping for transporting the slaked lime suspension, and most preferably, the preparation tank for preparing the slaked lime suspension.

[0019] There is no particular limitation on the addition amount of the scale inhibitor of the present invention. However, it is preferably added at 100 to 10,000 ppm, more preferably at 300 to 7,000 ppm, and most preferably at 500 to 5,000 ppm based on the solid content of calcium hydroxide in the slaked lime suspension. If it is less than 100 ppm, the scale prevention effect cannot be sufficiently exerted, and if it exceeds 10,000 ppm, economic problems will occur due to excessive addition.

[0020] There is no particular limitation on the addition location of the scale inhibitor of the present invention. However, at any stage such as during the preparation of the slaked lime suspension, during storage in a storage tank, or during injection into a transfer pipe, the required amount can be added by a chemical injection pump or the like. To particularly effectively exert the scale inhibitor of the present invention, it is preferable to add it during the preparation of the slaked lime suspension or during storage in a storage tank.

[0021] In the scale prevention method of the present invention, within a range that does not impair the effects of the present invention, other conventionally used scale inhibitors, such as ethylenediaminetetraacetic acid, polymaleic acid and its water-soluble salts, polyacrylic acid and its water-soluble salts, copolymers of acrylic acid and 2-acrylamido-2-methylpropanesulfonic acid and their water-soluble salts, maleic acid-acrylic acid copolymers and their water-soluble salts, phosphonates such as 2-phosphonobutane-1,2,4-tricarboxylic acid and 1-hydroxyethane-1,1-diphosphonic acid, etc. can be used in combination, and the present invention does not impose any restrictions.

Examples

[0022] Hereinafter, the present invention will be described more specifically by way of examples. However, the present invention is not limited to the following examples.

[0023] <Scale Prevention Test> 0.5 L of a 10% slaked lime suspension was placed in a pre-weighed 1 L glass container. A predetermined amount of the scale inhibitor described in Tables 1 and 2 was added relative to the solid content of the slaked lime suspension. Air was supplied using an air pump while stirring the slaked lime suspension. The carbon dioxide in the supplied air reacted with the slaked lime, causing scale to precipitate on the container wall (FT-IR and SEM-EDX confirmed that the scale was calcium carbonate). After 96 hours, the slaked lime suspension was removed from the container, the container was washed with pure water, and then dried in a constant temperature bath at 105°C. The weight of the container after cooling was measured, and the amount of scale precipitated was calculated from the difference in weight of the container before and after the test. The same tests as above were performed and the blank was used, except that no scale inhibitor was added. The scale prevention rate was calculated using the following formula based on the ratio of the amount of precipitation in the blank to the amount of precipitation after adding the scale inhibitor. Scale prevention rate (%) = {1 - Amount of precipitate when scale inhibitor is added (mg) / Amount of precipitate in blank (mg)} × 100

[0024] <Scale inhibitor> • Polyitaconic acid (P-IA) (Weight-average molecular weight: 1000, 2000, 4000, 5000, 15000, manufactured in-house) • Polyacrylic acid (P-AA) (weight-average molecular weight: 2000, 10000, manufactured in-house) • Copolymer of acrylic acid and 2-acrylamido-2-methylpropanesulfonic acid (1) (P-AA / AMPS(1)) (AA:AMPS weight ratio = 40:60, weight-average molecular weight 10000, manufactured in-house) • Copolymer of acrylic acid and 2-acrylamido-2-methylpropanesulfonic acid (2) (P-AA / AMPS(2)) (AA:AMPS weight ratio = 20:80, weight-average molecular weight 3000, manufactured in-house)

[0025] <Test Results> Examples and comparative examples are shown in Tables 1 and 2. When polyitaconic acid having a specific weight-average molecular weight according to the present invention was used, the scale prevention rate of calcium carbonate was 60% or higher (Examples 1-5). On the other hand, when polyitaconic acid with a weight-average molecular weight of less than 1000 or more than 4000 was used, the scale prevention rate was significantly lower compared to the examples (Comparative Examples 1-4). Furthermore, when polyacrylic acid other than that of the present invention or copolymers of acrylic acid and 2-acrylamido-2-methylpropanesulfonic acid were used, the scale prevention rate was significantly lower compared to the examples (Comparative Examples 5-8). As a reference example, Table 2 shows the test results when the amount of copolymer of acrylic acid and 2-acrylamido-2-methylpropanesulfonic acid was increased. However, to achieve the same scale prevention effect as in the examples, it was necessary to add more than 10 times the amount (Example 2 and Reference Examples 1 and 2). From the above, it was found that polyitaconic acid having a specific weight-average molecular weight according to the present invention has a unique scale prevention effect compared to polymers other than those of the present invention.

[0026] [Table 1]

[0027] [Table 2]

Claims

1. A scale inhibitor for slaked lime suspension used in a 1% to 20% by weight slaked lime suspension, characterized in that it contains polyitaconic acid and / or a water-soluble salt thereof having a weight-average molecular weight of 1000 to 4000.

2. A method for preventing scale buildup, characterized by adding a scale inhibitor containing polyitaconic acid and / or a water-soluble salt thereof having a weight-average molecular weight of 1,000 to 4,000 to a 1% to 20% by weight slaked lime suspension.

3. The method for preventing scale according to claim 2, characterized in that the scale inhibitor is added at a concentration of 100 to 10,000 ppm relative to the solid content of a 1% to 20% by weight slaked lime suspension.

4. The scale prevention method according to claim 2 or 3, characterized in that it prevents scale from adhering to a non-heat transfer surface.

5. The scaling prevention method according to claim 2 or 3, characterized in that the target scale contains calcium carbonate.

Images