Method for recovering calcium and calcium complex

Abstract

To provide a method for efficiently recovering calcium from a calcium-containing compound, and to provide a calcium complex.SOLUTION: The method for recovering calcium includes a step of preparing a calcium-containing compound and a step of bringing the calcium-containing compound into contact with a carboxylic acid having two or more and three or less carboxy groups and having one or more amino groups or hydroxy groups in water to generate a complex of calcium ions and the carboxylic acid, and can further include a step of bringing the complex into contact with an acid or a salt thereof in water to generate a calcium salt. The calcium complex contains calcium ions and a carboxylic acid having two or more and three or less carboxy groups and having one or more amino groups or hydroxy groups.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a method for recovering calcium and a calcium complex. [Background technology]

[0002] It has been known to recover calcium from calcium-containing compounds.

[0003] Examples of calcium-containing compounds include gypsum and calcium silicate. In particular, gypsum is a by-product of producing hydrofluoric acid from fluorite (mainly calcium fluoride) and sulfuric acid. In addition, phosphogypsum is produced when phosphoric acid is produced from phosphate rock, and there are many other by-product gypsums in the chemical industry, such as titanic gypsum, copper smelting gypsum, salt production gypsum, and flue gas desulfurization gypsum.

[0004] Gypsum is used in cement clinker, gypsum board, fertilizer, food additives, etc. However, in recent years, there has been little expansion into such uses, and for the effective utilization of gypsum, there is a need for efficient recovery of calcium from gypsum.

[0005] For example, a method and apparatus for treating water containing sulfate ions and calcium ions has been proposed, in which sulfate ions and calcium ions obtained from gypsum are brought into contact with an anion exchange resin consisting of at least one of OH-type, carbonate-type, and bicarbonate-type anion exchange resin to remove the sulfate ions, and then the pH is adjusted to 8 or higher in the presence of carbonate ions, and calcium ions are precipitated as calcium carbonate, recovered, and removed (see, for example, Patent Document 1).

[0006] Furthermore, in order to recover and analyze calcium and impurities in gypsum, an aqueous solution for dissolving lime has been proposed, which is an aqueous solution for dissolving gypsum and contains a chelating agent and has a pH of 4.0 or higher (see, for example, Patent Document 2). [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Laid-Open No. 2004-16996 [Patent Document 2] Japanese Patent Application Laid-Open No. 2006-292407 Summary of the Invention [Problem to be solved by the invention]

[0008] However, in the treatment method and treatment device described in Patent Document 1, it is necessary to remove sulfate ions before recovering calcium ions, which requires special equipment such as an ion exchange resin tower, complicating the treatment process and increasing the treatment cost.

[0009] Furthermore, in the gypsum dissolving aqueous solution described in Patent Document 2, an EDTA-type chelating agent such as ethylenediaminetetraacetic acid (hereinafter also referred to as EDTA) or trans-1,2-diaminohexane-N,N,N',N'-tetraacetic acid (hereinafter also referred to as CyDTA) has a high ability to form a complex with calcium ions and forms a stable complex with calcium ions, so that the dissolution rate of gypsum can be increased. However, there is a problem in that it is difficult to recover the complex as a calcium salt.

[0010] The present invention provides a method for recovering calcium, which can efficiently recover calcium from calcium-containing compounds, and a calcium complex. [Means for solving the problem]

[0011] The present invention [1] is a method for recovering calcium, comprising the steps of: preparing a calcium-containing compound; and contacting the calcium-containing compound with a carboxylic acid having two to three carboxyl groups and one or more amino or hydroxyl groups in water to form a complex of calcium ions derived from the calcium-containing compound and the carboxylic acid.

[0012] The present invention [2] is the method for recovering calcium according to the above [1], further comprising the step of producing a calcium salt by contacting the complex with an acid or a salt thereof in water.

[0013] The present invention [3] is a calcium complex containing calcium ions and a carboxylic acid having two to three carboxyl groups and one or more amino or hydroxyl groups. [Effects of the Invention]

[0014] The calcium recovery method of the present invention includes the steps of preparing a calcium-containing compound and contacting the calcium-containing compound with a carboxylic acid having two to three carboxyl groups and one or more amino or hydroxyl groups in water to form a complex between calcium ions derived from the calcium-containing compound and the carboxylic acid. Due to their complex-forming ability, the carboxylic acid promotes dissolution of calcium ions from the calcium-containing compound into water and efficiently captures the dissolved calcium ions, allowing for efficient recovery of calcium as a complex. Furthermore, because the carboxylic acid has a lower complex-forming ability than an EDTA-type chelating agent, contacting the complex with an acid or its salt in water dissociates the calcium ions in the complex from the carboxylic acid, allowing for efficient recovery of calcium as a calcium salt.

[0015] The calcium recovery method of the present invention further comprises the step of producing a calcium salt by contacting the complex with an acid or a salt thereof in water. The carboxylic acids that form the complex have lower complex-forming ability than EDTA-type chelating agents, so that by contacting the complex with an acid or a salt thereof in water, the calcium ions in the complex are dissociated from the carboxylic acids, allowing calcium to be efficiently recovered as a calcium salt.

[0016] The calcium complex of the present invention contains calcium ions and a carboxylic acid having two to three carboxyl groups and one or more amino or hydroxyl groups. The carboxylic acid that forms the complex has a lower complex-forming ability than an EDTA-type chelating agent. Therefore, by contacting the complex with an acid or a salt thereof in water, the calcium ions in the complex are dissociated from the carboxylic acid, allowing efficient recovery of calcium as a calcium salt. [Brief explanation of the drawings]

[0017] [Figure 1] 1 is a graph showing a TG-DTA curve of calcium carbonate. [Figure 2] 1 is a graph showing the TG-DTA curve of the calcium salt obtained in Example 1. [Figure 3] 1 is a graph showing the TG-DTA curve of the calcium salt obtained in Example 2. [Figure 4] 1 is a graph showing the TG-DTA curve of the calcium salt obtained in Example 3. DETAILED DESCRIPTION OF THE INVENTION

[0018] <Calcium recovery method> The calcium recovery method of the present invention includes the steps of preparing a calcium-containing compound and contacting the calcium-containing compound with carboxylic acids (hereinafter also referred to as carboxylic acids) having two to three carboxyl groups and one or more amino or hydroxyl groups in water to produce a complex of calcium ions derived from the calcium-containing compound and the carboxylic acids.

[0019] [Step of preparing calcium-containing compound] First, a calcium-containing compound is prepared. Examples of calcium-containing compounds include gypsum and calcium silicate. The main component of gypsum is calcium sulfate. Calcium sulfate is a salt that is poorly soluble in water. The route for preparing gypsum is not particularly limited, and it may be gypsum obtained as a by-product in the production of hydrofluoric acid from calcium fluoride and sulfuric acid, or gypsum obtained by other routes. The calcium-containing compound may be prepared as a suspension (slurry) or an aqueous solution.

[0020] [Step of forming a complex of calcium ions and carboxylic acids] In the step of forming a complex of calcium ions and carboxylic acids, a calcium-containing compound is contacted in water with carboxylic acids having two to three carboxy groups and one or more amino or hydroxy groups.

[0021] (carboxylic acids) The carboxylic acids used in this step are carboxylic acids and their salts having two to three carboxyl groups and one or more amino or hydroxyl groups. Due to their complex-forming ability, these carboxylic acids promote the dissolution of calcium ions from calcium-containing compounds into water and efficiently capture the dissolved calcium ions, allowing calcium to be efficiently recovered as a complex.

[0022] The number of carbon atoms in carboxylic acids having two to three carboxy groups and one or more amino groups (hereinafter also referred to as aminocarboxylic acids) is preferably 3 or more, more preferably 4 or more, and preferably 8 or less, more preferably 6 or less. The chemical formula weight of the aminocarboxylic acids is preferably 115 or more, more preferably 125 or more, even more preferably 130 or more, and preferably 275 or less, more preferably 265 or less, even more preferably 260 or less.

[0023] The number of carboxy groups in the aminocarboxylic acids is preferably 2 or 3, more preferably 2. The number of amino groups is preferably 1 or more and 3 or less, more preferably 1 or 2, and even more preferably 1. The amino group is either primary, secondary, or tertiary, preferably secondary or tertiary, and more preferably secondary.

[0024] More specifically, examples of aminocarboxylic acids include iminodiacetic acids (hereinafter also referred to as IDA) (number of carboxy groups: 2, number of amino groups: 1), nitrilotriacetic acids (hereinafter also referred to as NTA) (number of carboxy groups: 3, number of amino groups: 1), diaminopimelic acid (hereinafter also referred to as APM) (number of carboxy groups: 2, number of amino groups: 2), and ethylenediaminetriacetic acid (number of carboxy groups: 3, number of amino groups: 2).

[0025] The number of carbon atoms in carboxylic acids having two to three carboxy groups and one or more hydroxy groups (hereinafter also referred to as hydroxycarboxylic acids) is preferably 3 or more, more preferably 4 or more, and preferably 8 or less, more preferably 6 or less. The chemical formula weight of the hydroxycarboxylic acids is preferably 115 or more, more preferably 125 or more, even more preferably 130 or more, and preferably 295 or less, more preferably 285 or less, even more preferably 280 or less.

[0026] The number of carboxy groups in the hydroxycarboxylic acids is preferably 2 or 3, more preferably 3. The number of hydroxy groups is preferably 1 or more and 3 or less, more preferably 1 or 2, and even more preferably 1. The carbon adjacent to the hydroxy group is primary, secondary, or tertiary, preferably secondary or tertiary, and more preferably tertiary.

[0027] More specifically, examples of carboxylic acids having two to three carboxy groups and one or more hydroxy groups include malic acids (hereinafter also referred to as Mal.) (number of carboxy groups: 2, number of hydroxy groups: 1), citric acids (hereinafter also referred to as Cit.) (number of carboxy groups: 3, number of hydroxy groups: 1), tartaric acids (hereinafter also referred to as Tart.) (number of carboxy groups: 2, number of hydroxy groups: 2), and hydroxycitric acid (hereinafter also referred to as HCA) (number of carboxy groups: 3, number of hydroxy groups: 2).

[0028] The carboxylic acids are preferably at least one selected from the group consisting of iminodiacetic acid, citric acid, and salts thereof.

[0029] The above carboxylic acids can be used alone or in combination.

[0030] The carboxylic acids may be either free carboxylic acids or salts thereof. Carboxylic acid salts may be more expensive or more difficult to obtain than the free carboxylic acids, but they can reduce the amount of sodium hydroxide required to adjust the pH.

[0031] The carboxylic acids can also be prepared as a suspension (slurry) or an aqueous solution.

[0032] (molar equivalent ratio of carboxylic acids to calcium in calcium-containing compounds) When the calcium-containing compound is brought into contact with the carboxylic acids, the molar equivalent ratio of the carboxylic acids to the calcium in the calcium-containing compound is, from the viewpoint of promoting the formation of a calcium complex, for example, 0.5 molar equivalent ratio or more, preferably 0.8 molar equivalent ratio or more, and more preferably 1 molar equivalent ratio or more, and from the viewpoint of reducing the amount of the carboxylic acids used, it is, for example, 8 molar equivalent ratio or less, preferably 4 molar equivalent ratio or less, and more preferably 2 molar equivalent ratio or less.

[0033] (Method of contacting calcium-containing compound with carboxylic acids in water) The method for contacting the calcium-containing compound and the carboxylic acid in water is not particularly limited. However, from the viewpoint of increasing the elution of calcium ions from calcium sulfate, the main component of the calcium-containing compound, into water, it is preferable to contact at least one of the calcium-containing compound and the carboxylic acid in the form of a suspension (slurry) or an aqueous solution, and more preferably to contact both the calcium-containing compound and the carboxylic acid in the form of a suspension (slurry) or an aqueous solution. Specifically, preferred examples include contact of a suspension of the calcium-containing compound (a slurry of the calcium-containing compound) with the carboxylic acid, or contact of the calcium-containing compound with an aqueous solution of the carboxylic acid, and more preferably contact of a suspension of the calcium-containing compound (a slurry of the calcium-containing compound) with an aqueous solution of the carboxylic acid. The temperature for contacting the calcium-containing compound with the carboxylic acid in water is not particularly limited and is usually room temperature (e.g., 1 to 30°C, preferably 15 to 25°C).

[0034] (Contact time between calcium-containing compounds and carboxylic acids in water) The contact time when the calcium-containing compound is brought into contact with the carboxylic acid in water (for example, the stirring and mixing time of the calcium-containing compound and the carboxylic acid in water) is, from the viewpoint of generating a complex between calcium ions derived from the calcium-containing compound and the carboxylic acid, for example, 10 minutes or more, preferably 30 minutes or more, and from the viewpoint of suppressing dissociation of the complex, for example, 480 minutes or less, preferably 240 minutes or less, more preferably 120 minutes or less.

[0035] (pH of suspension or aqueous solution when calcium-containing compound comes into contact with carboxylic acids) The pH of the suspension or aqueous solution when the calcium-containing compound is brought into contact with the above-mentioned carboxylic acids in water is not particularly limited, but from the viewpoint of promoting dissolution of calcium ions from the calcium-containing compound into water and maintaining a high concentration of calcium complexes (calcium ion concentration) in water, the pH is preferably 4.6 or higher, more preferably 7 or higher, and even more preferably 9 or higher, and is preferably 13.5 or lower, more preferably 13 or lower.

[0036] The pH is adjusted, for example, by adjusting the pH of water when the calcium-containing compound and carboxylic acids are brought into contact. To adjust the pH of water, for example, water is prepared as an alkaline aqueous solution of a predetermined concentration. Specifically, an alkaline agent is added when preparing the aqueous solution of the calcium-containing compound and / or the aqueous solution of carboxylic acids. Examples of alkaline agents include sodium hydroxide and potassium hydroxide.

[0037] (Calcium ion and carboxylic acid complex) Complexes of calcium ions and carboxylic acids (hereinafter simply referred to as calcium complexes) are formed by carboxylic acids having two to three carboxyl groups and one or more amino or hydroxyl groups. Due to their complex-forming ability, these complexes promote the dissolution of calcium ions from calcium-containing compounds into water and efficiently capture the dissolved calcium ions, which then remain dissolved in water as calcium complexes.

[0038] (Calcium dissolution recovery rate due to calcium complex formation) In the step of producing the calcium complex, the calcium-containing compound is contacted with the carboxylic acid in water, and the calcium in the calcium-containing compound is dissolved in an aqueous solution and recovered. The calcium dissolution recovery rate (i.e., the percentage of the calcium concentration in the aqueous solution that is actually dissolved relative to the calcium concentration in the aqueous solution when all the calcium in the calcium-containing compound is dissolved) is, for example, 80% by mass or more, preferably 85% by mass or more, more preferably 90% by mass or more, and even more preferably 95% by mass or more.

[0039] (Action and effect) In the step of producing the calcium complex, the calcium-containing compound is contacted in water with a carboxylic acid having two to three carboxyl groups and one or more amino or hydroxyl groups. This allows the complex-forming ability of the carboxylic acids to promote the dissolution of calcium ions from the calcium-containing compound into water, thereby increasing the calcium dissolution recovery rate, which is obtained by dissolving calcium in the calcium-containing compound in an aqueous solution and recovering it, to, for example, 80% by mass or more, preferably 85% by mass or more, more preferably 90% by mass or more, and even more preferably 95% by mass or more.

[0040] [Step of producing calcium salt] The calcium recovery method of the present invention can further include a step of producing a calcium salt by contacting the calcium complex with an acid or a salt thereof in water.

[0041] (acid or its salt) The acid or its salt to be brought into contact with the calcium complex is not particularly limited as long as it generates a calcium salt. An anion species of the acid or its salt is effective for generating a calcium salt. From the viewpoint of precipitating and easily recovering the calcium salt, an acid or its salt that forms a poorly soluble calcium salt in aqueous solution is preferred. Examples of such an acid or its salt include inorganic acids or their salts, and organic acids or their salts. The inorganic or organic acid is preferably a weak acid. Examples of inorganic acids or their salts that form a poorly soluble calcium salt in aqueous solution include preferably carbonic acid or its salts, phosphoric acid or its salts, or silicic acid or its salts, and more preferably carbonic acid or its salts. Here, "carbonic acid and its salts" includes carbonic acid and carbonate ions derived from carbon dioxide. Examples of organic acids or their salts that form a poorly soluble calcium salt in aqueous solution include preferably oxalic acid and its salts.

[0042] The above acids or salts thereof can be used alone or in combination.

[0043] (molar equivalent ratio of the acid or its salt to calcium in the calcium complex) When the calcium complex is brought into contact with the acid or its salt, the molar equivalent ratio of the acid or its salt to calcium in the calcium complex is, from the viewpoint of promoting the production of the calcium salt, for example, 1 molar equivalent ratio or more, preferably 5 molar equivalent ratio or more, more preferably 8 molar equivalent ratio or more, and from the viewpoint of reducing the amount of the acid or its salt used, it is, for example, 20 molar equivalent ratio or less, preferably 15 molar equivalent ratio or less, more preferably 12 molar equivalent ratio or less.

[0044] (Method of contacting a calcium complex with the acid or its salt in water) The method for contacting the calcium complex with the acid or its salt in water is not particularly limited. However, from the viewpoint of efficient recovery as a calcium salt, it is preferable to add at least one of the acid and its salt to an aqueous solution in which the complex is dissolved. The acid and at least one of the salts to be added may be a solid powder, an aqueous solution, or a gas that reacts with water to produce the acid and its salt. For example, when adding at least one of carbonic acid and a carbonate to an aqueous solution in which the complex is dissolved, a solid powder of at least one of carbonic acid and a carbonate may be added to the aqueous solution, an aqueous solution of at least one of carbonic acid and a carbonate may be added to the aqueous solution, or carbon dioxide gas (carbon dioxide gas) may be blown into the aqueous solution. The temperature at which the calcium complex and the acid or its salt are contacted in water is not particularly limited and is usually room temperature (e.g., 1 to 30°C, preferably 15 to 25°C).

[0045] (Contact time between calcium complex and acid or its salt in water) The contact time when the calcium complex is brought into contact with the acid or its salt in water (e.g., the time for stirring and mixing the calcium complex with the acid or its salt in water) is, from the viewpoint of producing a calcium salt, for example, 10 minutes or more, and preferably 30 minutes or more, and from the viewpoint of preventing the contact time from becoming too long, for example, 480 minutes or less, preferably 240 minutes or less, more preferably 120 minutes or less, and even more preferably 60 minutes or less.

[0046] (pH of aqueous solution or suspension when calcium complex comes into contact with acid or its salt) In the step of producing the calcium salt, the pH of the aqueous solution or suspension when the calcium complex is brought into contact with the acid or its salt in water is not particularly limited, but from the viewpoint of efficient recovery of the calcium salt, it is preferably 4.6 or higher, more preferably 7 or higher, even more preferably 9 or higher, and is preferably 13.5 or lower, more preferably 13 or lower.

[0047] (Calcium precipitation recovery rate due to calcium salt formation) In the step of producing the calcium salt, the calcium complex is brought into contact with the acid or its salt in water, and the calcium dissolved in the aqueous solution is precipitated and recovered as a calcium salt. The calcium precipitation recovery rate is, for example, 90% by mass or more, preferably 95% by mass or more, and more preferably 100% by mass.

[0048] (Calcium dissolution and precipitation recovery rate due to formation of calcium complexes and calcium salts) The calcium dissolution / precipitation recovery rate through the step of producing the calcium complex and the step of producing the calcium salt is defined as the percentage of calcium in the calcium salt dissolved and precipitated in both steps relative to the calcium in the calcium-containing compound. Specifically, it is calculated by [(calcium dissolution recovery rate) × (calcium precipitation recovery rate)] / 100, and is, for example, 80 mass% or more, preferably 85 mass% or more, more preferably 90 mass% or more, and even more preferably 95 mass% or more.

[0049] (Action and effect) In the step of producing the calcium salt, the calcium complex is contacted with the acid or its salt in water. The moderate complex-forming ability of the carboxylic acids in the calcium complex (i.e., lower complex-forming ability than that of EDTA-type chelating agents) dissociates the calcium ions in the complex from the carboxylic acids, enabling efficient recovery of calcium as a calcium salt. Therefore, the calcium precipitation recovery rate, which is the recovery of calcium dissolved in an aqueous solution by precipitation as a calcium salt, can be increased to, for example, 90% by mass or more, preferably 95% by mass or more, and more preferably 100% by mass.

[0050] Through the process of producing the calcium complex and the process of producing the calcium salt, the calcium dissolution / precipitation recovery rate can be increased to, for example, 80% by mass or more, preferably 85% by mass or more, more preferably 90% by mass or more, and even more preferably 95% by mass or more. [Example]

[0051] Next, the present invention will be described based on examples and comparative examples, but the present invention is not limited to the following examples. Note that "parts" and "%" are by mass unless otherwise specified. Furthermore, specific numerical values ​​such as blending ratios (content ratios), physical property values, and parameters used in the following description can be substituted with the corresponding upper limit values ​​(numeric values ​​defined as "equal to or less than") or lower limit values ​​(numeric values ​​defined as "equal to or more than" or "exceeding") of the blending ratios (content ratios), physical property values, parameters, etc. described in the above "Form for Carrying Out the Invention."

[0052] Examples 1 to 3 and Comparative Examples 1 to 5 <Calcium-containing compound dissolution test> [Step of preparing calcium-containing compound] As gypsum, calcium sulfate dihydrate (CaSO4·2H2O) powder, which is its main component, was prepared.

[0053] [Step of contacting a calcium-containing compound with a carboxylic acid in water to form a complex of calcium ions and a carboxylic acid] (Preparation of aqueous solution of carboxylic acid salts) As shown in Examples 1 to 3 and Comparative Examples 1 to 5 in Tables 1 and 2, 100 mL of aqueous carboxylic acid, etc.-sodium hydroxide (NaOH) solutions containing predetermined equivalent amounts of carboxylic acid, etc. and predetermined concentrations of sodium hydroxide were prepared.

[0054] (Contact of calcium-containing compounds with carboxylic acids in water) To each of the prepared 100 mL aqueous solutions of the carboxylic acids, etc., and sodium hydroxide (NaOH), 1 g of calcium sulfate dihydrate (calcium ion concentration of 2.33 g / L when completely dissolved) was added, and the test solution was stirred using a magnetic stirrer to dissolve.

[0055] [Reference example: Dissolution test of calcium-containing compound (gypsum) only] As a reference example, a test liquid prepared by adding 1 g of calcium sulfate dihydrate to 100 mL of pure water was stirred using a magnetic stirrer to dissolve part of the calcium sulfate.

[0056] (pH measurement) At predetermined intervals from the start of stirring, 1 mL of sample liquid was collected from each test solution using a syringe, and the sample filtrate was filtered through a disposable filter and dispensed into a sample bottle. The pH of each sample filtrate collected at predetermined intervals was measured using a pH meter (LAQUAtwin compact pH meter manufactured by Horiba, Ltd.). The results are summarized in Tables 1 and 2.

[0057] (Calcium ion concentration measurement) At predetermined intervals after the start of stirring, 100 μL of each sample filtrate was added to a 10 mL volumetric flask and adjusted to volume with pure water to obtain the first dilution. 1 mL of the first dilution was added to a 10 mL volumetric flask and adjusted to volume with nitric acid aqueous solution and pure water to contain 0.3 mol / L of nitric acid to obtain the second dilution. The calcium ion concentration of the second dilution was measured using microwave plasma atomic absorption spectroscopy (MP-AES) (Agilent 4200MP-AES). To correct for changes in emission intensity, a calibration curve solution with a calcium ion concentration of 1 mg / L was measured after every three samples. The calcium ion concentration of each sample filtrate was calculated from the measured calcium ion concentration of each second dilution. The results are summarized in Tables 1 and 2.

[0058] In Tables 1 and 2, "NaOHaq" in the "Aqueous Solution Type" column indicates a sodium hydroxide solution. In the "Carboxylic Acids" column, "EDTA" indicates ethylenediaminetetraacetic acid (number of carboxyl groups: 4, number of amino groups: 2, number of hydroxyl groups: 0), "IDA" indicates iminodiacetic acid (number of carboxyl groups: 2, number of amino groups: 1, number of hydroxyl groups: 0), "Gly." indicates glycine (number of carboxyl groups: 1, number of amino groups: 1, number of hydroxyl groups: 0), "EDA" indicates ethylenediamine (number of carboxyl groups: 0, number of amino groups: 2, number of hydroxyl groups: 0), "Cit." indicates citric acid (number of carboxyl groups: 3, number of amino groups: 0, number of hydroxyl groups: 1), "Suc." indicates succinic acid (carboxyl groups: 2, number of amino groups: 0, number of hydroxyl groups: 0), and "Ac." indicates acetic acid (carboxyl groups: 1, number of amino groups: 0, number of hydroxyl groups: 0).

[0059] For the carboxylic acids used in the test, the disodium salt of EDTA (two of the four carboxyl groups are in the Na form and two are in the H form) was used, and for the carboxylic acids other than EDTA (i.e., IDA, Gly., Cit., Suc., and Ac.), the free carboxylic acids were used instead of the salts.

[0060] The amounts of the carboxylic acids and the like in Examples 1 to 3 and Comparative Examples 1 to 5 are expressed as molar equivalent ratios of the carboxylic acids and the like to calcium (Ca). Here, the number of moles of calcium in 1 g of calcium sulfate dihydrate (CaSO 2H O) is calculated using the chemical formula weight of calcium sulfate dihydrate: 172.2. 1 / 172.2=0.0058 Therefore, in Tables 1 and 2, if the carboxylic acid etc. / Ca (molar equivalent ratio) is 1, the number of moles of carboxylic acid etc. is 0.0058 mol, and if the carboxylic acid etc. / Ca (molar equivalent ratio) is 2, the number of moles of carboxylic acid etc. is 0.016 mol.

[0061] The percentage of the maximum calcium concentration in the solution in the calcium-containing compound dissolution test of Examples 1 to 3, Comparative Examples 1 to 5, or Reference Example relative to the calcium ion concentration in the aqueous solution at the time of complete dissolution (2.33 g / L) was calculated as the calcium dissolution recovery rate (mass %) due to the formation of calcium complexes, and is summarized in Tables 1 and 2.

[0062] With reference to Tables 1 and 2, the calcium dissolution recovery rate in pure water for the Reference Example was 27.0% by mass, whereas the calcium dissolution recovery rates in sodium hydroxide solutions containing EDTA (molar equivalent ratio: 1) for Comparative Example 1, IDA (molar equivalent ratio: 1) for Example 1, IDA (molar equivalent ratio: 2) for Example 2, or Cit. (molar equivalent ratio: 1) for Example 3 were essentially 100%, 90.6%, 97.9%, or 99.1% by mass, all of which were 90% by mass or higher. However, the calcium dissolution recovery rates in sodium hydroxide solutions containing Gly. (molar equivalent ratio: 2) for Comparative Example 2, EDA (molar equivalent ratio: 1) for Comparative Example 3, Suc. (molar equivalent ratio: 1) for Comparative Example 4, or Ac. (molar equivalent ratio: 1) for Comparative Example 5 were 63.1%, 33.0%, 63.9%, or 39.9% by mass, all of which were lower than 65% by mass.

[0063] <Calcium salt precipitation test> [Step of producing a calcium salt by contacting a calcium complex with an acid or its salt in water] A dissolution test of calcium sulfate dihydrate was started in the same procedure as the calcium-containing compound dissolution test for Examples 1 to 3 and Comparative Example 1 in Tables 1 and 2. 30 minutes after the start of stirring, 6.16 g of sodium carbonate (Na2CO3) was added at a molar equivalent ratio of 10 to the number of moles of calcium in the added calcium sulfate, and further stirring was continued.

[0064] [Reference example: Precipitation test of calcium-containing compounds (gypsum) only] As a reference example, a test solution prepared by adding 1 g of calcium sulfate dihydrate to 100 mL of pure water was stirred using a magnetic stirrer to dissolve some of the calcium sulfate. After 30 minutes had passed since the start of stirring, 6.16 g of sodium carbonate (Na2CO3) was added, with a molar equivalent ratio of 10 relative to the number of moles of calcium in the added calcium sulfate, and the solution was further stirred.

[0065] (pH measurement) At predetermined intervals after the start of the initial stirring, 1 mL of sample solution was taken from each test solution using a syringe, and the sample filtrate was filtered through a disposable filter and dispensed into a sample bottle. The pH of each sample filtrate was measured using the same procedure as in the calcium-containing compound dissolution test. The results are summarized in Tables 1 and 2.

[0066] (Calcium ion concentration measurement) At predetermined intervals after the start of initial stirring, 1 mL of sample solution was taken from each test solution using a syringe, and the sample filtrate was filtered through a disposable filter and dispensed into a sample bottle. The calcium ion concentration of each sample filtrate was measured using the same procedure as in the calcium-containing compound dissolution test. The results are summarized in Tables 1 and 2.

[0067] Referring to Tables 1 and 2, the concentrations of calcium ions dissolved in the pure water of the Reference Example, or the sodium hydroxide solution containing IDA (molar equivalent ratio: 1) of Example 1, IDA (molar equivalent ratio: 2) of Example 2, or Cit. (molar equivalent ratio: 1) of Example 3, all reached nearly 0 g / L 60 minutes after the start of the initial stirring (i.e., 30 minutes after the start of further stirring after the addition of sodium carbonate). Therefore, substantially 100% by mass of the calcium in the aqueous solution was precipitated as carbonate, i.e., the calcium precipitation recovery rate through the formation of calcium salts was substantially 100% by mass of the calcium dissolved in the aqueous solution.

[0068] However, calcium dissolved in the sodium hydroxide solution containing EDTA (molar equivalent ratio: 1) in Comparative Example 1 did not precipitate even after the addition of sodium carbonate, and the concentration of calcium ions dissolved in the solution did not change significantly. That is, the calcium precipitation rate due to the formation of calcium salts was essentially 0% by mass of the calcium dissolved in the aqueous solution.

[0069] Furthermore, the calcium dissolution / precipitation recovery rate due to the formation of calcium complexes and calcium salts through the calcium-containing compound dissolution test and calcium salt precipitation test (i.e., the percentage of calcium in the precipitated calcium salt relative to the calcium in the aqueous solution when all the calcium in the calcium-containing compound is dissolved) was calculated by [(calcium dissolution recovery rate) × (calcium precipitation recovery rate)] / 100. The recovery rate was 90.6% by mass for IDA (molar equivalent ratio: 1) in Example 1, 97.9% by mass for IDA (molar equivalent ratio: 2) in Example 2, and 99.1% by mass for Cit. (molar equivalent ratio: 1) in Example 3, all of which were 90% by mass or higher. However, the recovery rate was 27.0% by mass for pure water in the Reference Example, and essentially 0% by mass for EDTE (molar equivalent ratio: 1) in Comparative Example 1.

[0070] Examples 4 to 7 The calcium-containing compound dissolution test, pH measurement during the calcium-containing compound dissolution test, calcium ion concentration measurement during the calcium-containing compound dissolution test, calcium salt precipitation test, pH measurement during the calcium salt precipitation test, and calcium ion concentration measurement during the calcium salt precipitation test were conducted using the same procedures as in Example 1. However, the formulations were modified based on the information in Table 2. In Example 4, 2.40 g of potassium carbonate (K2CO3) was added at a molar equivalent ratio of 3 relative to the number of moles of calcium in calcium sulfate. In Example 5, 0.80 g of potassium carbonate (K2CO3) was added at a molar equivalent ratio of 1 relative to the number of moles of calcium in calcium sulfate. In Examples 6 and 7, 1 g of calcium silicate (calcium concentration of 2.35 g / L when fully dissolved) was used as the calcium-containing compound. In Examples 6 and 7, 2.41 g of potassium carbonate (K2CO3) was added at a molar equivalent ratio of 3 relative to the number of moles of calcium in calcium silicate.

[0071] Furthermore, the calcium dissolution recovery rate due to the formation of calcium complexes, the calcium precipitation recovery rate due to the formation of calcium salts, and the calcium dissolution and precipitation recovery rates due to the formation of calcium complexes and calcium salts were calculated according to the same procedures as in Example 1. The results are shown in Table 2.

[0072] [Table 1]

[0073] [Table 2]

[0074] [Analysis of precipitated calcium salts] (Examples 1 to 3) In the calcium salt precipitation test, stirring was stopped 24 hours after the initial stirring started, and the precipitate was collected by filtration using filter paper (ADVANTEC, No. 101). The collected precipitate (calcium salt) was sandwiched between the filter papers and dried at room temperature (25°C).

[0075] 2.5 mg of the dried precipitate was weighed onto a platinum pan, and thermogravimetry (TG) and differential thermal analysis (DTA) were performed using a simultaneous thermogravimetry and differential thermal analyzer (Rigaku ThermoPlus2 differential thermal balance) in an airflow atmosphere of 100 mL / min, at a heating rate of 10°C / min from room temperature (25°C) to 1000°C. TG and DTA were also performed on calcium carbonate (special reagent grade) as a standard sample in the same manner as above. The TG-DTA curve for calcium carbonate is shown in Figure 1, and the TG-DTA curves for the calcium salts obtained in Examples 1 to 3 are shown in Figures 2 to 4.

[0076] Table 3 also shows the mass loss rate in the TG curves when the temperature is changed from room temperature (25°C) to 600°C, 800°C, or 1000°C, and the mass loss rate between 600°C and 800°C.

[0077] Referring to Figures 1 to 4 and Table 3, the shapes of the TG-DTA curves in each figure and the TG mass loss rates (%) in each temperature range for each calcium salt in Table 3 are generally consistent, and therefore, it is believed that the calcium salts obtained in Examples 1 to 3 are all calcium carbonate.

[0078] [Table 3]

Claims

1. providing a calcium-containing compound; The method for recovering calcium includes a step of contacting the calcium-containing compound with a carboxylic acid having two to three carboxyl groups and one or more amino or hydroxyl groups in water to produce a complex of calcium ions derived from the calcium-containing compound and the carboxylic acid.

2. 2. The method for recovering calcium according to claim 1, further comprising the step of producing a calcium salt by contacting the complex with an acid or a salt thereof in water.

3. A calcium complex comprising calcium ions and a carboxylic acid having two to three carboxyl groups and one or more amino or hydroxyl groups.

Images