Silica scale inhibitor for reverse osmosis membrane systems and method for inhibiting silica scale in reverse osmosis membrane systems

Abstract

The objective is to provide a silica scale inhibitor for reverse osmosis membrane systems that can effectively suppress silica scale formation while also suppressing the initial decrease in permeate volume of the reverse osmosis membrane. [Solution] This is a silica scale inhibitor for reverse osmosis membrane systems that contains a polymer containing structural units derived from N-vinyl cyclic lactam.

Description

Technical Field

[0001] The present invention relates to a silica scale inhibitor for a reverse osmosis membrane device and a method for inhibiting silica scale in a reverse osmosis membrane device.

Background Art

[0002] In seawater desalination plants and wastewater recovery plants, reverse osmosis membrane (RO membrane) devices are widely used to remove electrolytes and organic components. On the other hand, in Citation Document 1, a method for suppressing silica fouling of a reverse osmosis membrane system is disclosed, which comprises causing an acrylic acid-based polymer and / or a maleic acid-based polymer and a bound chlorine-based oxidizing agent and / or a bound bromine-based oxidizing agent to be present in the water to be treated. Citation Document 1 discloses that the acrylic acid-based polymer may be a copolymer of acrylic acid and acrylamidomethylpropanesulfonic acid or a terpolymer of acrylic acid, acrylamidomethylpropanesulfonic acid, and N-substituted acrylamide.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] In the reverse osmosis membrane process, problems may occur such that silica components contained in the water to be treated precipitate, causing blockage of the reverse osmosis membrane and gradually reducing the permeate water volume. Therefore, it is conceivable to add a silica scale inhibitor in the reverse osmosis membrane process. However, problems such as a decrease in the initial permeate water volume may occur due to interactions such as adsorption of the silica scale inhibitor to the reverse osmosis membrane.

[0005] Therefore, an object of the present invention is to provide a silica scale inhibitor for a reverse osmosis membrane device that can suitably suppress silica scale (amorphous silica scale) in a reverse osmosis membrane device while suppressing a decrease in the initial water permeation rate of the reverse osmosis membrane. [Means for Solving the Problems]

[0006] The present inventor conducted various studies to achieve the above object and arrived at the present invention. That is, the silica scale inhibitor for a reverse osmosis membrane device of the present disclosure is a silica scale inhibitor for a reverse osmosis membrane device containing a polymer containing a structural unit derived from N-vinyl cyclic lactam. [Effects of the Invention]

[0007] According to the present disclosure, there is provided a silica scale inhibitor for a reverse osmosis membrane device that can suitably suppress silica scale in a reverse osmosis membrane device while suppressing a decrease in the water permeation rate of the main body of the reverse osmosis membrane. [Brief Description of the Drawings]

[0008] [Figure 1] It is a diagram showing an outline of an apparatus for evaluating the RO water permeation rate decrease rate. [Modes for Carrying Out the Invention]

[0009] Hereinafter, the present disclosure will be described in detail. In addition, a combination of two or more of the individual preferred forms of the present disclosure described below is also a preferred form of the present disclosure.

[0010] [Silica Scale Inhibitor for Reverse Osmosis Membrane Device] The silica scale inhibitor for a reverse osmosis membrane device of the present disclosure contains a polymer containing a structural unit derived from N-vinyl cyclic lactam. [Polymer Containing a Structural Unit Derived from N-vinyl Cyclic Lactam] (Structural Unit Derived from N-vinyl Cyclic Lactam) In this disclosure, a polymer containing structural units derived from N-vinyl cyclic lactam (hereinafter also referred to as "the copolymer of this disclosure") contains structural units derived from N-vinyl cyclic lactam. The copolymer of this disclosure tends to have an improved silica scale suppression effect due to the inclusion of structural units derived from N-vinyl cyclic lactam. In addition, the decrease in the initial permeate rate of the reverse osmosis membrane tends to be suppressed (it is less likely to adversely affect the permeate rate of the reverse osmosis membrane). In this disclosure, a structural unit derived from an N-vinyl cyclic lactam means a structural unit having the same structure as a structure formed by the polymerization of an N-vinyl cyclic lactam. However, a structural unit derived from an N-vinyl cyclic lactam only needs to have the same structure as a structure formed by the polymerization of an N-vinyl cyclic lactam; it is not limited to structural units actually formed by the polymerization of an N-vinyl cyclic lactam. Typically, structural units derived from N-vinyl cyclic lactams include those in which at least one of the carbon-carbon unsaturated double bonds in the N-vinyl cyclic lactam is replaced by a carbon-carbon single bond.

[0011] Examples of N-vinyl cyclic lactams include, but are not limited to, N-vinyl-2-pyrrolidone, N-vinyl-2-piperidone, and N-vinyl-ε-caprolactam. The copolymers of this disclosure may contain only one structural unit derived from an N-vinyl cyclic lactam, or they may contain two or more.

[0012] (Structural units derived from hydroxyalkyl (meth)acrylate) The copolymer of this disclosure may contain structural units derived from hydroxyalkyl (meth)acrylate in addition to structural units derived from N-vinyl cyclic lactam. In the above case, the silica scale suppression effect tends to be improved. Furthermore, the decrease in the initial permeate rate of the reverse osmosis membrane tends to be suppressed (less likely to adversely affect the permeate rate of the reverse osmosis membrane). In this disclosure, a structural unit derived from hydroxyalkyl (meth)acrylate means a structural unit having the same structure as a structure formed by the polymerization of hydroxyalkyl (meth)acrylate. However, a structural unit derived from hydroxyalkyl (meth)acrylate only needs to have the same structure as a structure formed by the polymerization of hydroxyalkyl (meth)acrylate, and is not limited to a structural unit actually formed by the polymerization of hydroxyalkyl (meth)acrylate. Typically, structural units derived from hydroxyalkyl (meth)acrylates include those in which at least one of the carbon-carbon unsaturated double bonds in the hydroxyalkyl (meth)acrylate is replaced by a carbon-carbon single bond.

[0013] Examples of hydroxyalkyl (meth)acrylates include, but are not limited to, 2-hydroxyethyl acrylate, 2-hydroxyethyl methacrylate, 2-hydroxypropyl acrylate, 2-hydroxypropyl methacrylate, 4-hydroxybutyl acrylate, and 4-hydroxybutyl methacrylate. The copolymers of this disclosure do not need to contain structural units derived from hydroxyalkyl (meth)acrylate, and may contain only one type or two or more types.

[0014] (Structural units derived from other monomers) The copolymers of this disclosure may optionally contain structural units derived from monomers other than N-vinyl cyclic lactams and hydroxyalkyl (meth)acrylates (hereinafter also referred to as "other monomers"). In this disclosure, a structural unit derived from other monomers means a structural unit having the same structure as a structure formed by the polymerization of other monomers. However, a structural unit derived from other monomers only needs to have the same structure as a structure formed by the polymerization of other monomers; it is not limited to structural units actually formed by the polymerization of other monomers. Typically, structural units derived from other monomers include those in which at least one carbon-carbon unsaturated double bond in N or other monomers is replaced by a carbon-carbon single bond.

[0015] Other monomers include carboxyl group-containing monomers such as acrylic acid, methacrylic acid, alkali metal salts of acrylic acid, alkali metal salts of methacrylic acid, ammonium salts of acrylic acid, ammonium salts of methacrylic acid, maleic acid, and itaconic acid; alkyl esters of acrylic acid such as methyl acrylate and ethyl acrylate; alkyl esters of methacrylic acid such as methyl methacrylate and ethyl methacrylate; aminoalkyl esters of acrylic acid such as diethylaminoethyl acrylate; aminoalkyl esters of methacrylic acid; quaternary ammonium derivatives of aminoalkyl esters of acrylic acid; quaternary ammonium derivatives of aminoalkyl esters of methacrylic acid; quaternary ammonium compounds of diethylaminoethyl acrylate and methyl sulfate; vinylmethyl Examples include alkyl vinyl ethers such as ethers and vinyl ethyl ether; sulfonic acid group-containing monomers such as alkali metal salts of vinyl sulfonic acid, ammonium salts of vinyl sulfonic acid, styrene sulfonic acid, styrene sulfonate, allyl sulfonic acid, allyl sulfonate, methallyl sulfonic acid, and methallyl sulfonate; allyl alcohol, methallyl alcohol, isoprenol; vinyl acetate, vinyl stearate, N-vinylimidazole, N-vinylacetamide, N-vinylformamide, N-vinylcarbazole, acrylamide, methacrylamide, N-alkylacrylamide, N-methylolacrylamide, N,N-methylenebisacrylamide, glycol diacrylate, glycol dimethacrylate, divinylbenzene, and glycol diallyl ether. The copolymers of this disclosure do not have to contain structural units derived from other monomers, but may contain only one or two or more.

[0016] (For example, the composition of polymers containing structural units derived from N-vinyl cyclic lactams) The polymer of the present disclosure is not particularly limited, but preferably contains 50% by mass or more and 100% by mass or less, more preferably 70% by mass or more and 100% by mass or less, still more preferably 80% by mass or more and 100% by mass or less, and even more preferably 90% by mass or more and 100% by mass or less of the structural units derived from N-vinyl cyclic lactam based on 100% by mass of the structural units derived from all monomers. In the above case, the silica scale suppression effect tends to be improved. Also, a decrease in the initial water permeation rate of the reverse osmosis membrane is suppressed (it is difficult to have an adverse effect on the water permeation rate of the reverse osmosis membrane). Further, even in the treatment of treated water containing a large amount of aluminum or iron components, a good silica scale suppression effect tends to be exhibited.

[0017] The polymer of the present disclosure can preferably contain structural units derived from hydroxyalkyl (meth)acrylate in a preferred form. That is, a polymer containing structural units derived from N-vinyl cyclic lactam and structural units derived from hydroxyalkyl (meth)acrylate is one of the preferred forms of the present disclosure. The polymer of the present disclosure is not particularly limited, but preferably contains 50% by mass or more and 100% by mass or less, more preferably 70% by mass or more and 100% by mass or less, still more preferably 80% by mass or more and 100% by mass or less, and even more preferably 90% by mass or more and 100% by mass or less in total of the structural units derived from N-vinyl cyclic lactam and the structural units derived from hydroxyalkyl (meth)acrylate based on 100% by mass of the structural units derived from all monomers. In the above case, the silica scale suppression effect tends to be improved. Also, a decrease in the initial water permeation rate of the reverse osmosis membrane is suppressed (it is difficult to have an adverse effect on the water permeation rate of the reverse osmosis membrane). Further, even in the treatment of treated water containing a large amount of aluminum or iron components, a good silica scale suppression effect tends to be exhibited.

[0018] As a preferred form of the polymer of the present disclosure, it includes a structural unit derived from N-vinyl cyclic lactam and a structural unit derived from hydroxyalkyl (meth)acrylate, and with respect to 100% by mass of the structural units derived from all monomers, the structural unit derived from N-vinyl cyclic lactam is 50% by mass or more and less than 100% by mass, and the structural unit derived from hydroxyalkyl (meth)acrylate is more than 0 and 50% by mass or less; more preferably, the structural unit derived from N-vinyl cyclic lactam is 50% by mass or more and 95% by mass or less, and the structural unit derived from hydroxyalkyl (meth)acrylate is 5% by mass or more and 50% by mass or less; even more preferably, the polymer contains 50% by mass or more and 92% by mass or less of the structural unit derived from N-vinyl cyclic lactam and 8% by mass or more and 50% by mass or less of the structural unit derived from hydroxyalkyl (meth)acrylate. The polymer of the present disclosure preferably contains 0% by mass or more and 50% by mass or less, more preferably 0% by mass or more and 30% by mass or less, even more preferably 0% by mass or more and 20% by mass or less, and even more preferably 0% by mass or more and 10% by mass or less of the structural units derived from other monomers with respect to 100% by mass of the structural units derived from all monomers.

[0019] The polymer of the present disclosure is not particularly limited, but preferably has a weight average molecular weight of 3000 or more and 100000 or less, more preferably 5000 or more and 50000 or less, and even more preferably 8000 or more and 30000 or less. In the above cases, the silica scale suppression effect tends to improve. Also, a decrease in the initial water permeation rate of the reverse osmosis membrane is suppressed (it is difficult to adversely affect the water permeation rate of the reverse osmosis membrane). <Method for Producing a Polymer Containing a Structural Unit Derived from N-Vinyl Cyclic Lactam> The method for producing the polymer of the present disclosure is not particularly limited. The polymer of the present disclosure preferably contains N-vinyl cyclic lactam as essential, and is produced by a method including a step of polymerizing a monomer containing hydroxyalkyl (meth)acrylate and / or other monomers as desired (also referred to as a polymerization step). The polymers of this disclosure are preferably produced by polymerizing monomers, which include an N-vinyl cyclic lactam and optionally a hydroxyalkyl (meth)acrylate and / or other monomers, in the presence of a polymerization initiator. When a polymerization initiator is used, it is preferable to use one or more selected from azo polymerization initiators, water-soluble organic peroxides, persulfates, and hydrogen peroxide. The polymers of this disclosure are preferably produced by polymerizing monomers, which include an N-vinyl cyclic lactam and optionally a hydroxyalkyl (meth)acrylate and / or other monomers, in the presence of a chain transfer agent. Polymerization in the presence of a chain transfer agent allows for efficient adjustment of the molecular weight. The method for producing the polymer of this disclosure preferably includes a step of polymerizing a monomer component, which is essential for an N-vinyl cyclic lactam, in an aqueous solvent. In the present invention, the aqueous solvent refers to water or a mixed solvent containing water. The mixed solvent containing water is preferably a mixed solvent or water in which 50% by mass or more is water relative to the total solvent, and more preferably 80% by mass or more is water. It is particularly preferable to use only water. When only water is used, it is preferable in that the residue of organic solvents can be avoided. In the polymer production method of the present disclosure, the polymerization temperature is preferably 70°C or higher, more preferably 75 to 110°C, and even more preferably 80 to 105°C. The pressure within the reaction system during the polymerization process described above may be at normal pressure (atmospheric pressure), under reduced pressure, or under increased pressure. Furthermore, while the atmosphere within the reaction system during the polymerization process may be an air atmosphere, an inert atmosphere is preferred. For example, it is preferable to replace the system with an inert gas such as nitrogen before the polymerization begins. The polymer production method of this disclosure requires the polymerization step described above, but may also include a purification step, desalting step, concentration step, dilution step, drying step, etc., as needed.

[0020] <Properties of silica scale inhibitors for reverse osmosis membrane systems, etc.> The silica scale inhibitor of this disclosure may contain only the polymer of this disclosure, but may also contain other components. Examples of other components include solvents such as water, pH adjusters, inorganic salts, corrosion inhibitors, and chelating agents. The silica scale inhibitor of this disclosure may contain, for example, 1% by mass or more and 100% by mass or less of the polymer of this disclosure.

[0021] Examples of preferred forms of silica scale inhibitors are shown below [1] to [9]. [1] A silica scale inhibitor containing a polymer that includes structural units derived from N-vinyl cyclic lactam. [2] The silica scale inhibitor according to [1], wherein the polymer containing the structural unit derived from the N-vinyl cyclic lactam is one or more selected from (i) and (ii) below. (i) A polymer containing 50% by mass or more and 100% by mass or less of structural units derived from N-vinyl cyclic lactam. (ii) A polymer comprising structural units derived from N-vinyl cyclic lactam and structural units derived from hydroxyalkyl (meth)acrylate. [3] A polymer containing structural units derived from the above N-vinyl cyclic lactam, wherein the structural units derived from the N-vinyl cyclic lactam are contained in an amount of 70% to 100% by mass, preferably 80% to 100% by mass, more preferably 90% to 100% by mass, and even more preferably 95% to 100% by mass, relative to 100% by mass of the total monomers, as described in [1]. [4] A silica scale inhibitor according to [1] or [2], wherein the polymer containing structural units derived from the above N-vinyl cyclic lactam comprises structural units derived from the N-vinyl cyclic lactam and structural units derived from hydroxyalkyl (meth)acrylate in total amounts of 50% to 100% by mass, 70% to 100% by mass, 80% to 100% by mass, or 90% to 100% by mass, based on 100% by mass of structural units derived from the total monomer. [5] A polymer containing structural units derived from the above-mentioned N-vinyl cyclic lactam comprises structural units derived from the N-vinyl cyclic lactam and structural units derived from hydroxyalkyl (meth)acrylate, wherein, with a ratio of 50% to less than 100% by mass of structural units derived from the N-vinyl cyclic lactam and 0 to 50% by mass of structural units derived from hydroxyalkyl (meth)acrylate per 100% by mass of structural units derived from the total monomer; preferably, 50% to 95% by mass of structural units derived from the N-vinyl cyclic lactam and 5% to 50% by mass of structural units derived from hydroxyalkyl (meth)acrylate; more preferably, 50% to 92% by mass of structural units derived from the N-vinyl cyclic lactam and 8% to 50% by mass of structural units derived from hydroxyalkyl (meth)acrylate, comprising [1], [2], or [4]. [6] A silica scale inhibitor according to any one of [1] to [5], wherein the weight-average molecular weight of the polymer containing the structural unit derived from the above N-vinyl cyclic lactam is 3,000 or more, 100,000 or less, more preferably 5,000 or more, 50,000 or less, and even more preferably 8,000 or more, 30,000 or less. [7] A silica scale inhibitor for reverse osmosis membrane equipment according to any one of [1] to [6], comprising 1% by mass or more and 100% by mass or less of a polymer containing structural units derived from the above N-vinyl cyclic lactam. [8] A silica scale inhibitor according to any of [1] to [7], for use in reverse osmosis membrane apparatus. [9] A silica scale inhibitor according to any one of [1] to [7], for use with water to be treated by a reverse osmosis membrane apparatus. In the above case, the silica scale suppression effect tends to improve. Also, the decrease in the initial permeate rate of the reverse osmosis membrane tends to be suppressed (less likely to adversely affect the permeate rate of the reverse osmosis membrane).

[0022] [Method for suppressing silica scale in reverse osmosis membrane systems] The silica scale suppression method of this disclosure includes the steps of adding a silica scale inhibitor to water to be treated and treating the water with a reverse osmosis membrane module.

[0023] There are no particular restrictions on the type of water to be treated; for example, seawater, brackish water, industrial water, factory wastewater, sewage, etc., can be used.

[0024] The silica scale suppression method of this disclosure may include a step of pre-treating the water to be treated. The treated water obtained by pre-treating the water to be treated is also included in the water to be treated. An example of a pre-treatment step is a step of reducing impurities in the water to be treated with a reverse osmosis membrane module using a filter or the like. The pre-treatment step may include multiple steps, for example, rough filtration followed by treatment with a microfiltration membrane (MF membrane) and / or an ultrafiltration membrane (UF membrane). The pre-treatment step may include any steps such as adsorbing impurities onto an adsorbent such as activated carbon, or a settling step to allow large impurities to settle. Including a pre-treatment step is preferable because it tends to improve the efficiency of the process of treating with the reverse osmosis membrane module.

[0025] The reverse osmosis (RO) membrane used in the silica scale suppression method of this disclosure is not particularly limited, but examples include cellulose acetate-based reverse osmosis membranes and polyamide-based reverse osmosis membranes, and a polyamide-based reverse osmosis membrane is preferred. For example, an aromatic polyamide-based reverse osmosis membrane is provided, such as a reverse osmosis membrane having a porous support layer such as polysulfone formed on a nonwoven fabric such as polyester, and a polyamide layer formed by reacting m-phenylenediamine with trimesoyl chloride on top of the support layer. There are no particular restrictions on the shape of the reverse osmosis membrane, and examples include hollow fiber membranes, spiral membranes, and tubular membranes.

[0026] In this disclosure, the terms "reverse osmosis module" and "reverse osmosis apparatus" can be interpreted in their usual sense, but typically, a reverse osmosis module refers to a module containing a reverse osmosis membrane, and a reverse osmosis apparatus refers to an apparatus containing a reverse osmosis membrane.

[0027] The silica scale inhibitor used in the silica scale suppression method of this disclosure is as described in "Silica Scale Inhibitor for Reverse Osmosis Membrane Apparatus" above. The silica scale suppression method of this disclosure is more preferably used in the silica scale inhibitor described in [1] to [9] above.

[0028] There are no particular limitations on the method or timing of adding the silica scale inhibitor of this disclosure to the water to be treated. Using the silica scale inhibitor of this disclosure tends to improve the silica scale suppression effect in the reverse osmosis process. In addition, the reduction in the original permeate volume of the reverse osmosis membrane tends to be suppressed (less likely to adversely affect the permeate volume of the reverse osmosis membrane).

[0029] According to the method for suppressing silica scale in a reverse osmosis membrane apparatus (method for suppressing silica scale in a reverse osmosis membrane process) of this disclosure, it is possible to reduce the generation and growth of silica scale in the treated water processed by the reverse osmosis membrane apparatus, and to reduce the volume of silica scale in the water flow path and reverse osmosis membrane of the treated water. [Examples]

[0030] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples, nor is it restricted by these examples. [Method for evaluating the reduction rate of RO water permeability] A reverse osmosis membrane (Nitto Denko ESPA2, membrane size Φ30mm) was set in the apparatus shown in Figure 1. Ion-exchanged water was passed through the membrane at 25°C and a flow rate of 40 mL / min under a pressure of 0.75 MPa, and the flux was measured to determine the initial water permeability. A 500 mL pH 7 aqueous solution containing 100 ppm of polymer was prepared by dilution with pure water and neutralization with sodium hydroxide solution or hydrochloric acid, and a reverse osmosis membrane (Nitto Denko, ESPA2) was immersed in it for 16 hours. The membrane was removed, lightly washed with pure water, and then immersed in 500 mL of pure water for 1 hour. The membrane was then set back in the apparatus shown in Figure 1, and ion-exchanged water was passed through the membrane at a flow rate of 40 mL / min at 25°C under a pressure of 0.75 MPa. The flux was measured and defined as the water permeability after immersion. The water permeability reduction rate was defined as shown in the following formula and used for evaluation.

[0031]

number

[0032] [Evaluation of silica polymerization inhibition ability (amorphous silica scale inhibition ability)] Preparation of silica aqueous solution: 14.2 g of sodium metasilicate nonahydrate was mixed with deionized water to prepare a total of 1000 g. Preparation of sodium bicarbonate aqueous solution: 4.2 g of sodium bicarbonate was mixed with deionized water to make a total of 1000 g. 4.0 g of silica aqueous solution, 26.2 g of deionized water, 4.0 g of sodium bicarbonate aqueous solution, and 1.6 g of polymer aqueous solution diluted to 0.1% were added to a container and stirred. While stirring further, 4.2 g of 0.1 M hydrochloric acid was added. The mixture was then left to stand for 21 days (7 days for Comparative Example 3 only) while immersed in a 30°C bath. After 21 days (7 days for Comparative Example 3 only), the ionic Si concentration was measured using the molybdenum yellow method (JIS K 0101). The silica polymerization inhibition rate and the inhibition rate against the blank were calculated according to the following formulas.

[0033]

number

[0034] [Evaluation of silica polymerization inhibition ability (amorphous silica scale inhibition ability) under high aluminum conditions] Preparation of silica aqueous solution: 14.2 g of sodium metasilicate nonahydrate was mixed with deionized water to prepare a total of 1000 g. Preparation of sodium bicarbonate aqueous solution: 4.2 g of sodium bicarbonate was mixed with deionized water to make a total of 1000 g. Preparation of aluminum aqueous solution: 1.17 g of aluminum sulfate 14-18 hydrate was added to deionized water to prepare a total of 1000 g. 4.0 g of silica aqueous solution, 26.0 g of deionized water, 4.0 g of sodium bicarbonate aqueous solution, and 1.6 g of polymer aqueous solution diluted to 0.1% were added to a container and stirred. While stirring, 4.2 g of 0.1 M hydrochloric acid and 0.2 g of aluminum aqueous solution were added. The mixture was then left to stand for 3 days while immersed in a 30°C warm bath. After 3 days, the ionic Si concentration was measured using the molybdenum yellow method (JIS K 0101). The silica polymerization inhibition rate and the inhibition rate against the blank were calculated according to the following formula.

[0035]

number

[0036] [Gel Permeation Chromatography (GPC)] <Measurement conditions> The weight-average molecular weight of the copolymer was measured using the following method. Equipment: Waters Alliance (e2695) Analysis software: Waters Empower Professional + GPC option Columns used: SHODEX OHpak SB-G 6G and SB-806M (3 pieces) manufactured by Showa Denko Corporation. Detectors: Differential refractive index (RI) detector (Waters 2998), Multi-wavelength visible ultraviolet (PDA) detector (Waters 2998) Eluent: Prepared by dissolving 137.79 g of sodium dihydrogen phosphate dihydrate and 316.31 g of disodium hydrogen phosphate dodecahydrate in a mixed solvent of 17209.9 g of water and 1536 g of acetonitrile. Standard material for calibration curve creation: GL Sciences PEG standard sample (Peak top molecular weights (Mp) 194, 410, 615, 1020, 1450, 3860, 8160, 16100, 21160, 49930, 67600, 96100, 205500, 542500, 942000) Calibration curve: Prepared as a cubic equation based on the Mp and elution time of the above standard substances Flow rate: 1.0 ml / min Column temperature: 40 °C Measurement time: 45 minutes Sample solution injection volume: 50 μL (eluent solution with a sample concentration of 0.5 wt%) <GPC analysis conditions> In the obtained RI chromatogram, the flat and stable part before the exclusion limit and after the elution limit was connected with a straight line as the baseline, and the polymer part was detected and analyzed. However, when the peaks of monomers and monomer-derived impurities partially overlapped with the polymer peak and were measured, they were vertically divided at the deepest concave part of the overlapping part between them and the polymer part to separate the polymer part and the monomer part, and the molecular weight and molecular weight distribution of only the polymer part were measured. When the polymer part and the rest could not be completely separated by overlapping, they were calculated together.

[0037] [Example 1] An SUS reaction vessel equipped with a Max Blend (registered trademark of Sumitomo Heavy Industries, Ltd.) type stirring blade, a glass lid, a stirrer with a stirring seal, a nitrogen inlet tube, and a temperature sensor was charged with ion-exchanged water (420.4 parts by mass), 1.5 parts by mass of a 5% aqueous sodium hydroxide solution, and 2.5 parts by mass of sodium phosphinate monohydrate, and stirred at 200 rpm under a nitrogen stream and heated to 90 °C. As a monomer aqueous solution, a solution was prepared by adding 55.6 parts by mass of ion-exchanged water to 500 parts by mass of N-vinylpyrrolidone. As an initiator aqueous solution, a solution was prepared by adding 17.0 parts by mass of ion-exchanged water to 3.0 parts by mass of 2,2'-azobis-2-amidinopropane dihydrochloride (V-50). Nitrogen was introduced at a rate of 30 mL / min, and the monomer aqueous solution prepared above was added dropwise continuously for 360 minutes while maintaining the temperature at 90 ± 2°C. The initiator aqueous solution was also added dropwise continuously for 390 minutes. After the completion of the dropwise addition, the temperature was maintained at 90°C for 30 minutes to complete the polymerization reaction. Furthermore, an aqueous solution of polymer (1) of the present disclosure was obtained by adding 2.9 parts by mass of 88% formic acid aqueous solution (manufactured by Wako Pure Chemical Industries, Ltd.) all at once 420 minutes after the start of polymerization. The obtained aqueous solution is also referred to as the silica scale inhibitor (1) of this disclosure.

[0038] Polymer (1) of the present disclosure is a homopolymer of N-vinylpyrrolidone, and is a polymer containing 100% by mass of structural units derived from N-vinyl cyclic lactam to 100% by mass of structural units derived from the total monomer. The weight-average molecular weight of polymer (1) of the present disclosure was 21,000.

[0039] The polymer (1) of this disclosure was evaluated for its RO water permeability reduction rate, amorphous silica scale suppression ability (suppression rate against blanks), and amorphous silica scale suppression ability (suppression rate against blanks) under high-aluminum conditions. The RO water permeability reduction rate was 3%, the suppression rate against blanks was 65%, and the suppression rate against blanks under high-aluminum conditions was 60%.

[0040] [Example 2] In a glass reaction vessel equipped with a thermometer, stirrer (paddle blades), dropping funnel, nitrogen inlet tube, and reflux condenser, 90.0 parts of water were charged, the inside of the reaction vessel was purged with nitrogen while stirring, and the temperature was raised to 80°C under a nitrogen atmosphere, after which 160.0 parts of vinylpyrrolidone were added. Mixture 1, prepared by mixing 0.8 parts of (2,2'-azobis(2-methylpropionamidine) dihydrochloride (V-50) and 29.2 parts of water, and Mixture 2, prepared by mixing 0.8 parts of (2,2'-azobis(2-methylpropionamidine) dihydrochloride (V-50) and 29.2 parts of water, and Mixture 3, prepared by mixing 1.6 parts of thioglycerol and 28.4 parts of water, were simultaneously added dropwise from separate nozzles. The addition was carried out continuously at a constant rate, with Mixtures 1 and 3 being added in their entirety over 150 minutes, and Mixture 2 being added in its entirety over 210 minutes. After the addition of Mixture 2 was completed, the reaction vessel was maintained at 80°C for another 60 minutes to complete the polymerization reaction and obtain the polymer (2) of the present disclosure. The obtained aqueous solution is also referred to as the silica scale inhibitor (2) of the present disclosure.

[0041] Polymer (2) of the present disclosure is a homopolymer of N-vinylpyrrolidone, and is a polymer containing 100% by mass of structural units derived from N-vinyl cyclic lactam to 100% by mass of structural units derived from the total monomer. The weight-average molecular weight of polymer (2) of the present disclosure was 13,500.

[0042] The polymer (2) of this disclosure was evaluated for its RO water permeability reduction rate and its ability to suppress amorphous silica scale under high-aluminum conditions (suppression rate against blanks). The RO water permeability reduction rate was 10%, and the suppression rate against blanks under high-aluminum conditions was 60%.

[0043] [Example 3] In a glass reaction vessel equipped with a thermometer, a stirrer (paddle blades), a dropping funnel, a nitrogen inlet tube, and a reflux condenser, 90.0 parts of water were charged. The reaction vessel was purged with nitrogen while stirring, and the temperature was raised to 80°C under a nitrogen atmosphere. Then, mixture 1, consisting of 160.0 parts vinylpyrrolidone and 90.0 parts water, mixture 2, consisting of 0.8 parts 4,4'-azobis(4-cyanovaleric acid) (V-501) and 29.2 parts water, and mixture 3, consisting of 1.6 parts thioglycerol and 28.4 parts water, were simultaneously added dropwise from separate nozzles. The addition was carried out continuously at a constant rate, with the entire contents of mixtures 1 and 3 being added over 150 minutes, and mixture 2 over 210 minutes. After the addition of mixture 2 was completed, the reaction vessel was maintained at 80°C for another 60 minutes to complete the polymerization reaction and obtain the polymer (3) of the present disclosure. The obtained aqueous solution is also referred to as the silica scale inhibitor (3) of this disclosure.

[0044] Polymer (3) of the present disclosure is a homopolymer of N-vinylpyrrolidone, and is a polymer containing 100% by mass of structural units derived from N-vinyl cyclic lactam to 100% by mass of structural units derived from the total monomer. The weight-average molecular weight of polymer (3) of the present disclosure was 17,500.

[0045] The polymer (3) of this disclosure was evaluated for its RO water permeability reduction rate and its ability to suppress amorphous silica scale under high-aluminum conditions (suppression rate against blanks). The RO water permeability reduction rate was 7%, and the suppression rate against blanks under high-aluminum conditions was 57%. [Example 4] In a glass reaction vessel equipped with a thermometer, agitator (paddle blades), dropping funnel, nitrogen inlet tube, and reflux condenser, 43.0 parts water and 5.7 parts sodium phosphinate monohydrate were charged. The reaction vessel was purged with nitrogen while stirring, and the temperature was raised to 90°C under a nitrogen atmosphere. Mixture 1, consisting of 900.0 parts vinylpyrrolidone, 100.0 parts 2-hydroxymethacrylic acid, and 150.0 parts water, and Mixture 2, consisting of 16.1 parts 2,2'-azobis(2-methylbutyronitrile) and 145.3 parts isopropanol, were simultaneously added dropwise from separate nozzles. The addition was carried out continuously at a constant rate, with Mixture 1 being added over 120 minutes and Mixture 2 over 135 minutes. After the addition of Mixture 2 was complete, the reaction vessel was maintained at 90°C for another 105 minutes to complete the polymerization reaction. The resulting reaction solution was dried using an evaporator, and then converted back into a 50% aqueous solution to obtain the polymer (5) of the present disclosure. The obtained aqueous solution is also referred to as the silica scale inhibitor (4) of the present disclosure.

[0046] Polymer (4) of the present disclosure is a copolymer of N-vinylpyrrolidone and 2-hydroxyethyl methacrylate, and is a polymer containing 90% by mass of structural units derived from N-vinyl cyclic lactam and 10% by mass of structural units derived from hydroxyalkyl (meth)acrylate, with 100% by mass of structural units derived from the total monomer. The weight-average molecular weight of polymer (5) of the present disclosure was 12,900.

[0047] The polymer (4) of this disclosure was evaluated for its RO water permeability reduction rate and its ability to suppress amorphous silica scale (suppression rate against blank). The RO water permeability reduction rate was 0%, and the suppression rate against blank was 87%.

[0048] [Example 5] In a glass reaction vessel equipped with a thermometer, agitator (paddle blades), dropping funnel, nitrogen inlet tube, and reflux condenser, 90.0 parts of water were charged, the inside of the reaction vessel was purged with nitrogen while stirring, and the temperature was raised to 80°C under a nitrogen atmosphere, followed by the addition of 112.0 parts of vinylpyrrolidone. Mixture 1, prepared by mixing 48.0 parts of 2-hydroxymethacrylic acid and 90.0 parts of water, and Mixture 2, prepared by mixing 0.7 parts of (2,2'-azobis(2-methylpropionamidine) dihydrochloride (V-50) and 29.3 parts of water, and Mixture 3, prepared by mixing 1.5 parts of sodium phosphinate monohydrate and 28.5 parts of water, were simultaneously added dropwise from separate nozzles. The addition was carried out continuously at a constant rate, with the entire contents of Mixtures 1 and 3 being added over 150 minutes, and Mixture 2 over 210 minutes. After the addition of Mixture 2 was completed, the reaction vessel was maintained at 80°C for another 60 minutes to complete the polymerization reaction and obtain the polymer (5) of the present disclosure. The obtained aqueous solution is also referred to as the silica scale inhibitor (5) of the present disclosure.

[0049] The polymer (5) of this disclosure is a copolymer of N-vinylpyrrolidone and 2-hydroxyethyl methacrylate, and is a polymer containing 100% by mass of structural units derived from the total monomer, 70% by mass of structural units derived from N-vinyl cyclic lactam, and 30% by mass of structural units derived from hydroxyalkyl (meth)acrylate. The weight-average molecular weight of polymer (5) of this disclosure was 23,700.

[0050] The polymer (5) of this disclosure was evaluated for its RO water permeability reduction rate and its ability to suppress amorphous silica scale under high-aluminum conditions (suppression rate against blanks). The RO water permeability reduction rate was 7%, and the suppression rate against blanks under high-aluminum conditions was 56%.

[0051] [Example 6] In a glass reaction vessel equipped with a thermometer, stirrer (paddle blades), dropping funnel, nitrogen inlet tube, and reflux condenser, 90.0 parts of water were charged, the inside of the reaction vessel was purged with nitrogen while stirring, and the temperature was raised to 80°C under a nitrogen atmosphere, followed by the addition of 144.0 parts of vinylpyrrolidone. Mixture 1, prepared by mixing 16.0 parts of 2-hydroxyethyl methacrylate and 90.0 parts of water, Mixture 2, prepared by mixing 0.8 parts of (2,2'-azobis(2-methylpropionamidine) dihydrochloride (V-50) and 29.2 parts of water, and Mixture 3, prepared by mixing 1.5 parts of thioglycerol and 28.5 parts of water, were simultaneously added dropwise from separate nozzles. The addition was carried out continuously at a constant rate, with Mixtures 1 and 3 being added in their entirety over 150 minutes, and Mixture 2 over 210 minutes. After the addition of Mixture 2 was completed, the reaction vessel was maintained at 80°C for another 60 minutes to complete the polymerization reaction and obtain the polymer (6) of the present disclosure. The obtained aqueous solution is also referred to as the silica scale inhibitor (6) of the present disclosure.

[0052] The polymer (6) of this disclosure is a copolymer of N-vinylpyrrolidone and 2-hydroxyethyl methacrylate, and is a polymer containing 100% by mass of structural units derived from the total monomer, 90% by mass of structural units derived from N-vinyl cyclic lactam, and 10% by mass of structural units derived from hydroxyalkyl (meth)acrylate. The weight-average molecular weight of polymer (6) of this disclosure was 11,800.

[0053] The polymer (6) of this disclosure was evaluated for its RO water permeability reduction rate and its ability to suppress amorphous silica scale under high-aluminum conditions (suppression rate against blanks). The RO water permeability reduction rate was 6%, and the suppression rate against blanks under high-aluminum conditions was 56%.

[0054] [Comparative Example 1] A comparative copolymer (1) with a weight-average molecular weight of 7900, obtained by copolymerizing acrylic acid, t-butylacrylamide, and 2-acrylamido-2-methylpropanesulfonic acid in a molar ratio of 80:10:10, was used to evaluate the reduction in RO water permeability, amorphous silica scale suppression ability (suppression rate against blank), and amorphous silica scale suppression ability (suppression rate against blank) under high-aluminum conditions. The reduction in RO water permeability was 0%, the suppression rate against blank was 2%, and the suppression rate against blank under high-aluminum conditions was 18%.

[0055] [Comparative Example 2] A comparative copolymer (2) of sodium acrylate and ethylene oxide adduct of 3-methyl-3-buten-1-ol (average number of added moles: 50 moles) in a 20:80 (mass ratio) ratio was used to evaluate the reduction in RO water permeability, the ability to suppress amorphous silica scale (suppression rate against the blank), and the ability to suppress amorphous silica scale under high-aluminum conditions (suppression rate against the blank). The weight-average molecular weight of comparative copolymer (2) was 14,000. The reduction in RO water permeability was 36%, the suppression rate against the blank was 35%, and the suppression rate against the blank under high-aluminum conditions was 91%. [Comparative Example 3] A comparative copolymer (3) of sodium acrylate and sodium 3-allyloxy-2-hydroxy-1-propanesulfonate in a 71:29 (mass ratio) was used to evaluate the reduction in RO water permeability, the ability to suppress amorphous silica scale (suppression rate against the blank), and the ability to suppress amorphous silica scale under high-aluminum conditions (suppression rate against the blank). The weight-average molecular weight of comparative copolymer (3) was 6,000. The reduction in RO water permeability was 0%, the suppression rate against the blank was 4%, and the suppression rate against the blank under high-aluminum conditions was 0%. A lower RO permeability reduction rate indicates better performance (the initial permeability of the reverse osmosis membrane does not decrease). On the other hand, a higher amorphous silica scale suppression rate (suppression rate against blank) indicates better performance (a higher effect in suppressing amorphous silica scale). The results shown above clearly demonstrate that the silica scale inhibitor of this disclosure has excellent RO water permeability reduction rate and silica scale suppression ability. In other words, the results shown above clearly demonstrate that the silica scale inhibitor of this disclosure can suppress silica scale in a reverse osmosis membrane system while suppressing the initial decrease in permeate rate of the reverse osmosis membrane (it is less likely to adversely affect the permeate rate of the reverse osmosis membrane). Therefore, it has become clear that the silica scale inhibitor of this disclosure can be suitably used as a silica scale inhibitor for reverse osmosis membrane apparatus, a silica scale inhibitor for reverse osmosis membrane modules, or a silica scale inhibitor for reverse osmosis membrane systems. [Explanation of symbols]

[0056] 1 pump 2. Feed solution 3. Pressure Regulator 4. Pressure gauge 5 Reverse osmosis membrane (RO membrane) 6. Pressure gauge 7 Membrane permeate (pure water) 8 balance

Claims

1. A silica scale inhibitor for reverse osmosis membrane systems, containing a polymer that includes structural units derived from N-vinyl cyclic lactam.

2. The silica scale inhibitor for reverse osmosis membrane apparatus according to claim 1, wherein the polymer containing the structural unit derived from the N-vinyl cyclic lactam is one or more selected from (i) and (ii) below. (i) A polymer containing 50% by mass or more and 100% by mass or less of structural units derived from N-vinyl cyclic lactam. (ii) A polymer comprising structural units derived from N-vinyl cyclic lactam and structural units derived from hydroxyalkyl (meth)acrylate.

3. A step of adding a silica scale inhibitor to the water to be treated, The process includes treating the water to be treated with a reverse osmosis membrane module, The silica scale inhibitor contains a polymer that includes structural units derived from N-vinyl cyclic lactam. A method for suppressing silica scale in a reverse osmosis membrane system.

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