Method for extracting alkali metal ion

Abstract

To provide a method for extracting alkali metal ions, capable of selectively extracting alkali metal ions from a cement hardened body.SOLUTION: A method for extracting alkali metal ions from a cement hardened body comprises an extraction step of extracting alkali metal ions from a cement hardened body with water having a pH of 6.5 or more and 11 or less. Since only the alkali metal ions can be extracted from the cement hardened body, calcium remains in the form of calcium carbonate in the cement hardened body, which can maintain the strength of the cement hardened body before and after the extraction step.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for extracting alkali metal ions, and more particularly to a method for extracting alkali metal ions from a hardened cement paste. [Background technology]

[0002] Concrete is a hardened cement mixture made by solidifying aggregates such as sand and gravel with water, and is widely used as a material in construction and civil engineering works.

[0003] One of the causes of concrete deterioration is alkali-aggregate reaction, a phenomenon in which the alkaline aqueous solution contained in concrete reacts with specific components of aggregate (gravel or sand), causing abnormal expansion and subsequent cracking.

[0004] Alkali-aggregate reaction is not a particular problem when the concrete contains small amounts of alkali metals such as Na and K. However, in recent years, methods have been studied in which concrete is impregnated with an aqueous solution of alkali metal carbonate to store carbon in the concrete. In such cases, alkali metals are present in high concentrations in the concrete, which becomes a problem.

[0005] Alkali metals in concrete can also be removed by, for example, reversing the current flow in the re-alkalinization method. Patent Document 1 discloses a method in which the steel material inside the concrete serves as the anode, a solution is held between the surface of the concrete and a sheet placed on the surface, an electrode is inserted into the solution to serve as the cathode, and an electric current is passed between the anode and cathode to electrochemically permeate the anions in the solution from the surface of the concrete to the steel material inside the concrete.

[0006] According to the invention of Patent Document 1, anions in the solution can migrate from the surface of the concrete to the steel material side inside the concrete, while cations can be expelled from inside the concrete into the solution outside the concrete. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Laid-Open No. 2015-227578 [Non-patent literature]

[0008] [Non-Patent Document 1] Shigemitsu Hatanaka et al., "Committee Report: Research Committee on Establishment of Construction Standards and Quality Assurance Resistance for Performance-Based Design-Compatible Porous Concrete," Proceedings of the Japan Concrete Institute, Vol. 37, No. 1, 2015 Summary of the Invention [Problem to be solved by the invention]

[0009] As described above, according to the invention of Patent Document 1, cations (including alkali metal ions) can be discharged from inside the concrete into a solution outside the concrete. However, since not only alkali metal ions but also calcium ions in the concrete are discharged as cations, there is a problem in that calcium carbonate in the concrete disintegrates.

[0010] In view of the above problems, an object of the present invention is to provide a method for selectively extracting alkali metal ions from a hardened cement paste. [Means for solving the problem]

[0011] The present inventors have conducted extensive research to achieve the above object, and as a result have found that alkali metal ions in a hardened cement paste can be selectively extracted by extracting the hardened cement paste with water having a predetermined pH, thereby completing the present invention.

[0012] That is, to achieve the above object, the invention described in claim 1 is a method for extracting alkali metal ions from a hardened cement body, characterized by having an extraction step of extracting alkali metal ions from the hardened cement body using water having a pH of 6.5 or more and a pH of 11.0 or less.

[0013] According to this invention, only alkali metal ions can be extracted from the hardened cement body, so calcium remains in the hardened cement body in the form of calcium carbonate, allowing the strength of the hardened cement body to be maintained before and after the extraction step.

[0014] It is also preferable to have a supplying step, which is carried out before the extraction step, of supplying an aqueous solution of an alkali metal carbonate to the cement composition or the hardened cement product.

[0015] Furthermore, it is preferable that the hardened cement body has a porous structure that allows water to pass through.

[0016] Moreover, it is preferred that the alkali metal ion is a potassium ion. [Effects of the Invention]

[0017] According to the method for extracting alkali metal ions of the present invention, only alkali metal ions can be extracted from a hardened cement body, so that calcium remains in the hardened cement body in the form of calcium carbonate, allowing the strength of the hardened cement body to be maintained before and after the extraction step.

[0018] Furthermore, since the concentration of alkali metal ions in the hardened cement body decreases after the extraction step, the risk of alkali-aggregate reaction is also reduced, and therefore, the effect of preventing deterioration of the hardened cement body is expected. [Brief explanation of the drawings]

[0019] [Figure 1] FIG. 1 is a flow chart showing a method for extracting alkali metal ions from a hardened cement paste according to the present invention. [Figure 2]FIG. 1 is a schematic diagram illustrating a permeability test for determining the permeability coefficient of a hardened cement paste. [Figure 3] FIG. 2 is a schematic diagram showing an example of the continuous extraction step (S110) of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0020] As shown in FIG. 1, the method of the present invention for extracting alkali metal ions from a hardened cement paste includes an optional supplying step (S100) and an extraction step (S110).

[0021] First, a case where only the extraction step (S110) is performed on the hardened cement paste will be described.

[0022] A hardened cement product is a hardened product of a cement composition containing cement, water, and aggregate. Hardened cement products are not limited to cement compositions immediately after hardening, but also include recycled aggregate (concrete aggregate produced by crushing, grinding, sorting, etc. using concrete blocks generated during the demolition of concrete structures as raw materials), concrete debris (concrete rubble generated at construction sites, etc.), concrete waste (waste such as concrete chips and defective products generated during the manufacturing process of concrete products, etc.), and cement-based powder (cement-based powder such as recycled fine powder generated during the manufacturing of recycled aggregate).

[0023] Alkali metal ions are cations of elements belonging to Group 1 of the periodic table. From the viewpoint that they are causative substances of alkali-aggregate reaction, alkali metal ions are selected from the group consisting of sodium ions and potassium ions. Furthermore, among the alkali metal carbonates supplied in the supply step described below, potassium carbonate has a higher solubility in water than sodium carbonate, and therefore potassium is more likely to remain in the hardened cement paste. Therefore, it is preferable that the alkali metal ion extracted is potassium ion. [Extraction process (S110)] In this step, alkali metal ions are extracted from the hardened cement body with water having a pH of 6.5 or higher and 11.0 or lower, preferably with water having a pH of 7.0 or higher and 8.0 or lower. A pH of less than 6.5 is undesirable because Ca in the hardened cement body will leach out, while a pH of more than 11.0 is undesirable because crystallization of the alkali metal carbonate generated in the hardened cement body will not proceed if the optional supply step (S100) described below is included.

[0024] The water may be any water having a pH in the range of 6.5 to 11.0, but for the purpose of extraction, it is preferable that the water contains no alkali metal or contains only a small amount of alkali metal. In the present invention, a small amount of alkali metal in the water means, for example, that the alkali metal concentration in the water is 100 mg / L or less, preferably 50 mg / L or less, and particularly preferably 20 mg / L or less.

[0025] In the present invention, examples of water for extraction include distilled water, ion-exchanged water, tap water, river water, and lake water. Water for extraction includes not only water at room temperature, but also cold water and hot water.

[0026] The pH of the water can be adjusted using any known and commonly used pH buffer solution, but even in this case, the amount of alkali metal in the water after pH adjustment should be within a range that can be considered small.

[0027] The extraction may be carried out batchwise or continuously.

[0028] In the case of a batch system, for example, the hardened cement body is placed in a tank large enough to contain it, and the water is poured into the tank until the entire hardened cement body is submerged. The hardened cement body is then immersed in the water for a time sufficient to extract the alkali metal ions, and finally the water in the tank is drained out to complete the extraction.

[0029] During the immersion, it is preferable to agitate the water in the tank to improve the extraction efficiency of alkali metal ions, and it is also preferable to carry out the immersion under reduced pressure to promote the penetration of the water into the hardened cement body.

[0030] In the case of the continuous method, for example, the hardened cement body is placed in a tank large enough to contain it, and water is poured into the tank until the entire hardened cement body is submerged. Thereafter, water is poured into the tank at a constant rate while simultaneously draining the water from the tank at the same constant rate from another drain outlet attached to the tank.

[0031] In addition, from the viewpoint of efficiently extracting alkali metal ions from within the hardened cement body, the hardened cement body preferably has a porous structure that allows water to pass through.

[0032] Examples of hardened cement products with a porous structure that allows water to pass through include so-called porous concrete, which has continuous voids inside, and hardened cement products in which fibrous materials are introduced into a cement composition, mixed, and hardened, resulting in water and air paths formed around the fibrous materials.

[0033] The manufacturing method for porous concrete is disclosed in the "Committee Report: Research Committee on Establishing Construction Standards and Quality Assurance Resistance for Performance-Design-Compatible Porous Concrete" (Hatanaka Shigemitsu et al., Proceedings of the Annual Conference of the Concrete Engineering Society, Vol. 37, No. 1, 2015). Unlike regular concrete, porous concrete uses aggregate of a single particle size, or aggregate with a lower fine powder content than the standard value of JIS A 5005, and is manufactured with a significantly smaller unit water content than regular concrete.

[0034] The ability of hardened cement to pass water can be expressed by the coefficient of permeability.

[0035] The measurement test for the hydraulic conductivity will be explained with reference to FIG.

[0036] 2 is a schematic diagram illustrating a permeability test for determining the permeability coefficient of a hardened cement paste. As shown in the figure, an intake pipe 112 of a vacuum trap 110 (e.g., an acrylic small vacuum container (300 type), manufactured by AS ONE Corporation) is inserted into a water tank 100, and the bottom surface Xa of a specimen X is attached to the end of the intake pipe 112 on the water tank side. The specimen X is inserted into a polyvinyl chloride pipe (inner dimension φ10 cm) and the two are joined with a waterproof adhesive. Therefore, the side surface Xb of the specimen X is the outer surface of the polyvinyl chloride pipe, and the bottom surface Xa and top surface Xc of the specimen X are open surfaces.

[0037] A vacuum pump P (e.g., a belt-driven oil rotary vacuum pump, manufactured by Yamato Scientific Co., Ltd.) is provided in the exhaust pipe 114 of the vacuum trap 110, and the suction pressure of the vacuum pump P generates a negative pressure in the space of the vacuum trap 110, which causes water in the water tank 100 to permeate into the test piece X from the upper surface Xc of the test piece X, and the water that has permeated the test piece X is trapped in the vacuum trap 110.

[0038] The permeability test is performed by attaching the specimen X to the end of the intake pipe 112 on the water tank side and starting the operation of the vacuum pump P for a period of time t. The suction pressure of the vacuum pump P is set to a maximum negative pressure of 0.08 MPa, and the pressure is adjusted according to the permeability. The permeability is the amount of water trapped in the vacuum trap 110 (water amount Q in equation (2) described later). The permeability coefficient can then be calculated from the test results as follows:

[0039] In other words, when the length L of the saturated sample is constant, a proportional relationship (proportionality constant k) exists between the hydraulic gradient i and the flow velocity v, and this relationship is Darcy's law. The apparent flow velocity v of water flowing from the high water level to the low water level in the sample is given by Equation (1):

[0040]

number

[0041]

number

[0042] The permeability coefficient of ordinary concrete is 1 x 10 -9 cm / s~1×10 -12 The permeability coefficient of hardened cement, which has a porous structure that allows water to pass through, is 1×10 -6 cm / s or more, and 1×10 -5 It is preferable that the speed is at least cm / s.

[0043] Furthermore, as will be described later, when performing column-type extraction, it is preferable from the viewpoint of extraction efficiency that the water not pass through the hardened cement immediately but pass through the hardened cement over a certain period of time. Therefore, when performing column-type extraction, the hydraulic conductivity of the hardened cement, which has a porous structure that allows water to pass through, is 1 x 10 -2 Preferably less than 1×10 cm / s -3 cm / s or less 1×10 -4 It is particularly preferable that the speed is at least cm / s.

[0044] 3 is a schematic diagram showing an example of the continuous extraction step (S110). As shown in the figure, in this example, a cylindrical column C was filled with a cylindrical hardened cement product E having an outer diameter equal to the inner diameter of the column C up to about the middle of the column C.

[0045] The lower end of column C is connected to the upper end of drain pipe D, which drains water from the column, and its lower end protrudes into trap T. Above trap T, one end of exhaust pipe F, which exhausts air from trap T, protrudes into trap T, and a pump P is provided at the other end of exhaust pipe F.

[0046] Therefore, by pouring water into column C from above and starting pump P, negative pressure is created in trap T and exhaust pipe F, and the water poured onto hardened cement body E is pulled into the hardened cement body E, passes through the hardened cement body E, and accumulates in trap T (this is the extraction process (S110)).

[0047] Therefore, according to the present invention, in which only the extraction step (S110) is performed on the hardened cement body, only alkali metal ions can be extracted from the hardened cement body, and calcium remains in the form of calcium carbonate in the hardened cement body, allowing the strength of the hardened cement body to be maintained before and after the extraction step (S110).

[0048] The reason why only alkali metal ions can be selectively extracted from hardened cement paste in this way is that the solubility of calcium carbonate is very low, at 0.013 g / L, in water in the pH range assumed by the present invention (at 25°C), while the solubility of alkali metal carbonates is very high (for example, the solubility of potassium carbonate is five orders of magnitude higher in the same pH range at 20°C).

[0049] Furthermore, since the concentration of alkali metal ions in the hardened cement body is reduced after the extraction step (S110), the risk of alkali-aggregate reaction is also reduced, and therefore, the effect of preventing deterioration of the hardened cement body is expected.

[0050] In the example of the continuous extraction step (S110) in FIG. 3, the column C is filled to about the middle with a cylindrical hardened cement product E having an outer diameter equal to the inner diameter of the column C, but it is not essential that the outer diameters of the column C and the hardened cement product E be the same. For example, cement residue or cement-based powder may be filled into the column C. In this case, it is preferable to fill the column C with as few gaps as possible from the viewpoint of extraction efficiency.

[0051] Next, a case where the supplying step (S100) is performed prior to the extracting step (S110) will be described.

[0052] While hardened cement emits large amounts of CO2 during its manufacturing process, it has the ability to absorb and fix CO2 during its use, creating a need for it as a CO2 storage destination. This process supplies alkali metal carbonate as a CO2 source to the hardened cement or cement composition. [Supply process (S100)] That is, this step is a supplying step, which is carried out before the extraction step (S110), of supplying an aqueous solution of an alkali metal carbonate to the cement composition or hardened cement body.

[0053] The alkali metal carbonate is at least one selected from the group consisting of Na2CO3, NaHCO3, KHCO3, and K2CO3. Among them, from the viewpoint of solubility in water, at least one selected from the group consisting of NaHCO3 and K2CO3 is preferred, and K2CO3 is particularly preferred.

[0054] The cement composition is a composition containing cement, water, and optionally aggregate. Cement is an inorganic binder that hardens when mixed with water, and hydraulic cement is used in the present invention. As the hydraulic cement, a simple cement such as Portland cement (JIS R5210), hydraulic lime, Roman cement, or natural cement may be used, or a mixed cement such as lime-blended cement or mixed Portland cement (JIS R5211, R5212, R5213) may be used.

[0055] In the present invention, the water added to the cement is not limited to pure water, but distilled water, ion-exchanged water, tap water, river water, and lake water can also be used.

[0056] Aggregate is an optional component that is generally used in the production of concrete and is added to suppress heat generation due to the hydration reaction of the cement composition, suppress shrinkage, and reduce the amount of cement used to reduce costs.Aggregate is divided into coarse aggregate and fine aggregate, and coarse aggregate is one that retains 85% or more by mass on a 5 mm mesh sieve, while fine aggregate is one that passes through a 5 mm mesh sieve and 100% by mass on a 10 mm mesh sieve.

[0057] Examples of aggregate materials include river sand, mountain sand, sea sand, blast furnace slag, and copper slag.

[0058] It may also contain optional admixtures such as air entraining agents (air entraining agents), water reducing agents (air entraining water reducing agents, water reducing agents, high performance air entraining water reducing agents, etc.), superplasticizers, setting and hardening regulators, quick-setting admixtures, rust inhibitors, waterproofing agents, etc. In particular, from the viewpoint of improving the hydraulic conductivity of the hardened cement body obtained after hardening, it is preferable to contain one or more of air entraining agents, air entraining water reducing agents, and high performance air entraining water reducing agents.

[0059] Furthermore, to improve the hydraulic conductivity of the hardened cement paste, fibrous materials can be added. Any fibrous material can be used as long as it can create water and air paths around the fibrous material when hardened, and can be selected from a variety of materials, such as vegetable fibers, glass fibers, metal fibers, carbon fibers, and polymeric material fibers.

[0060] When supplying an aqueous solution of alkali metal carbonate to a cement composition, it can be mixed with the water to be added to the cement to form mixing water (hereinafter also referred to as internal supply), or the cement composition is poured into a mold and the aqueous solution of alkali metal carbonate is mixed with the curing water used for underwater curing (hereinafter also referred to as external supply). Depending on the target amount of CO2 to be pooled in the hardened cement body after hardening, only internal supply, only external supply, or both may be used.

[0061] The aqueous solution of alkali metal carbonate is supplied to the hardened cement body in the same manner as the external supply described above. That is, the hardened cement body is immersed in the aqueous solution of alkali metal carbonate. The aqueous solution of alkali metal carbonate may also be supplied to the hardened cement body by spraying the aqueous solution of alkali metal carbonate onto the hardened cement body.

[0062] The alkali metal carbonate supplied in this step reacts with calcium ions in the cement composition or hardened cement paste to form CaCO3, which is pooled in the hardened cement paste (the end of this is the supply step (S100)).

[0063] The next extraction step (S110) is the same as described above, so the description thereof will be omitted.

[0064] Therefore, according to the alkali metal ion extraction method of the present invention, which includes the supply step (S100) and the extraction step (S110), not only can a large amount of CO2 be pooled in the hardened cement body as CaCO3, but also the alkali metal ions that have been absorbed in excess into the hardened cement body in the supply step (S100) can be discharged in the extraction step (S110) and reused in the supply step (S100).

[0065] In particular, while K2CO3 is an excellent CO2 source due to its high solubility in water, potassium is expensive and there has been a strong demand for its reuse. However, the alkali metal ion extraction method of the present invention makes it possible to store a large amount of CO2 in the hardened cement body and reuse the potassium ions by using K2CO3 and extracting potassium ions.

[0066] Furthermore, because the interior of hardened cement is generally strongly alkaline, the CaCO3 produced by reaction within the hardened cement immediately after the supply step (S100) in this strongly alkaline environment is amorphous and not fixed within the hardened cement. However, by passing water with a pH of 6.5 to 11.0 through the hardened cement, the pH of the hardened cement decreases, causing the CaCO3 to crystallize. This is thought to have the effect of stabilizing carbon by incorporating it into the cement. [Example]

[0067] EXAMPLES The present invention will be specifically explained below by showing examples, but the present invention is not limited to these examples. 1. Fixation test of CO2 in potassium carbonate into cement A solution containing alkoxysilane was hydrolyzed to prepare a sol of the SiO2 component, and a calcium hydroxide (Ca(OH)2) solution, which was the second component, was added to produce a calcium silicate hydrate gel (CSH gel). The SiO2 sol was prepared by adding ethanol and ion-exchanged water (IEW) to tetraethoxysilane ((C2H5O)4Si, TEOS) manufactured by Kanto Chemical Co., Ltd., and adding 15% diluted hydrochloric acid as an acid catalyst. The molar ratio of TEOS, ethanol, ion-exchanged water, and hydrochloric acid was 1:4:2:0.02. This SiO2 sol was mixed with an aqueous solution of calcium hydroxide to achieve the specified Ca / Si ratio, and the mixture was stirred in a fluororesin Erlenmeyer flask for two days. N2 gas was sealed in the flask to prevent carbonation of the sample. The mixture was then separated into solid and liquid phases by suction filtration, yielding a CSH gel.

[0068] A 50% by mass aqueous solution of potassium carbonate (K2CO3) was added to this CSH gel (C / S ratio = 0.8) so that the powder ratio W / P (g / mL) was 6, and the gel was immersed at room temperature of 20°C for one day to allow the CSH gel and K2CO3 to react sufficiently. Note that the C / S ratio refers to the CaO / SiO2 molar ratio.

[0069] Next, the resulting reaction solution was centrifuged at 3000 rpm for 10 minutes to separate the solid and liquid, yielding liquid phase 1 and solid phase 2. Isopropyl alcohol was added to the resulting solid phase 2, and the mixture was further centrifuged at 3000 rpm for 10 minutes to separate it again into solid phase 3 and liquid phase 4. Five grams of this solid phase 3 was sampled and dried at 105°C for 24 hours in a nitrogen-filled drying oven to yield solid phase 4.

[0070] The obtained solid phase 4 was identified by XRD test. The XRD test was performed using a tabletop XRD instrument, Aeris, manufactured by PANalytica. The measurement conditions were: X-ray source Cu-Kα, tube voltage 40 kV, tube current 15 mA, scan range 2θ = 5 to 70°, step width 0.02°, scan rate 2° / min. Quantitative analysis of the crystalline and amorphous phases was performed using the external standard method, with corundum (α-Al2O3) used as the external standard.

[0071] The test results showed that calcium carbonate hydrate was obtained by the reaction of CSH gel with potassium carbonate.

[0072] Therefore, for example, by supplying a potassium carbonate aqueous solution (aqueous solution of alkali metal carbonate) to a hardened cement body as a CO2 source, the carbonic acid in this potassium carbonate aqueous solution can be pooled in the hardened cement body in the form of calcium carbonate hydrate, and then by passing water through this hardened cement body, it was found that only the excess potassium carbonate can be extracted.

Claims

1. A method for extracting alkali metal ions from a hardened cement body, comprising an extraction step of extracting alkali metal ions from the hardened cement body with water having a pH of 6.5 or more and 11.0 or less.

2. 2. The method for extracting alkali metal ions according to claim 1, further comprising a supplying step of supplying an aqueous solution of alkali metal carbonate to the cement composition or hardened cement product, which is carried out before the extraction step.

3. 2. The method for extracting alkali metal ions according to claim 1, wherein the hardened cement body has a porous structure that allows water to pass through.

4. 4. The method for extracting alkali metal ions according to claim 1, wherein the alkali metal ions are potassium ions.

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