Method for controlling gypsum conversion reaction and method for producing calcium carbonate

Abstract

The present invention provides a method for controlling a gypsum conversion reaction and a method for producing calcium carbonate, wherein in a gypsum conversion reaction, in which calcium carbonate is obtained by bringing a gypsum-containing material into contact with a solution A containing a specific alkali metal salt A that has high versatility and can be applied to a wide variety of materials, that can be done with minimal handling and easy operation, one condition selected from among the pH of the reaction solution of the solution A and the gypsum-containing material, and at least one concentration selected from among the carbonate ion concentration, the hydrogen carbonate ion concentration, and the calcium ion concentration in the reaction solution, is monitored, and the end point of the reaction is determined.

Description

Method for controlling gypsum conversion reaction and method for producing calcium carbonate

[0001] The present disclosure relates to a method for controlling a gypsum conversion reaction and a method for producing calcium carbonate.

[0002] Gypsum-containing materials, particularly waste containing gypsum such as waste gypsum boards and waste recovered from flue gas desulfurization processes, are generally treated and disposed of as industrial waste, and the discarded gypsum-containing waste is landfilled in a final disposal site. However, the amount of gypsum-containing waste has been steadily increasing in recent years, and it is expected that the final disposal sites will become overwhelmed. In consideration of the environmental load, reuse of gypsum-containing waste, rather than disposal as is, has been considered (e.g., Patent Documents 1 and 2).

[0003] Patent Document 1 discloses an apparatus and method for crushing and separating waste gypsum board materials, which continuously and efficiently separate and recover high-purity gypsum and gypsum board base paper from waste gypsum board materials. Patent Document 2 also discloses a method for treating industrial waste containing gypsum, such as gypsum board, which is mainly composed of calcium sulfate, using ammonia and carbon dioxide.

[0004] JP 2011-011210 A JP 2001-000947 A

[0005] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a method for controlling a gypsum conversion reaction and a method for producing calcium carbonate, which have high versatility so as to be applicable to a wide variety of materials and can be implemented with minor and easy work.

[0006] In order to solve the above problems, the present invention provides the following method for controlling a gypsum conversion reaction: 1. A method for controlling a gypsum conversion reaction in which calcium carbonate is obtained by bringing a gypsum-containing material into contact with a solution A containing at least one alkali metal salt A selected from alkali metal carbonates and alkali metal bicarbonates, the method comprising monitoring one condition selected from the pH of a reaction liquid between the solution A and the gypsum-containing material and at least one concentration selected from the carbonate ion concentration, bicarbonate ion concentration, and calcium ion concentration in the reaction liquid, and determining an end point of the reaction.

[0007] The present invention further provides the following methods for controlling a gypsum conversion reaction as preferred embodiments. 2. The method for controlling a gypsum conversion reaction according to the above item 1, in which the gypsum-containing material is continuously or intermittently supplied to solution A containing alkali metal salt A. 3. The method for controlling a gypsum conversion reaction according to the above item 1 or 2, in which solution A containing alkali metal salt A is continuously or intermittently supplied to the gypsum-containing material. 4. The method for controlling a gypsum conversion reaction according to any one of the above items 1 to 3, in which solution A containing alkali metal salt A is a solution obtained by electrolyzing a salt solution by bipolar electrodialysis to prepare an alkaline solution, and contacting the alkaline solution with a gas containing carbon dioxide. 5. The method for controlling a gypsum conversion reaction according to the above item 4, in which the salt solution is solution B containing alkali metal salt B. 6. The method for controlling a gypsum conversion reaction according to the above item 5, in which the alkali metal salt B is at least one metal salt selected from alkali metal chlorides, alkali metal sulfates, and alkali metal nitrates. 7. 7. The method for controlling a gypsum conversion reaction according to 5 above, wherein the alkali metal of the alkali metal salt B is the same as the alkali metal of the alkali metal salt A. 8. The method for controlling a gypsum conversion reaction according to any one of 1 to 7 above, wherein the alkali metal of the alkali metal salt A is at least one metal selected from sodium and potassium.

[0008] The present invention also provides the following method for producing calcium carbonate: 9. A method for producing calcium carbonate, comprising carrying out a gypsum conversion reaction by bringing a gypsum-containing material into contact with a solution A containing at least one alkali metal salt A selected from alkali metal carbonates and alkali metal bicarbonates, wherein the gypsum conversion reaction is terminated by monitoring one condition selected from the pH of a reaction liquid between solution A and the gypsum-containing material and at least one concentration selected from the carbonate ion concentration, bicarbonate ion concentration, and calcium ion concentration in the reaction liquid, and determining an endpoint of the reaction.

[0009] According to the present invention, it is possible to provide a method for controlling a gypsum conversion reaction and a method for producing calcium carbonate, which have high versatility so as to be applicable to a wide variety of materials and can be implemented with minor and easy work.

[0010] 1 is a graph showing the results of Example 1. FIG. 2 is a graph showing the results of Example 2. FIG. 3 is a graph showing the simulation results of Example 3. FIG. 4 is a graph showing the pH of the reaction solution among the simulation results of Example 3. FIG. 5 is a graph showing the pH of the reaction solution among the simulation results of Example 4.

[0011] An embodiment of the present invention (hereinafter, sometimes referred to as "the present embodiment") will be described below. The present invention is not limited to the following embodiment, and can be implemented with any modifications within the scope that does not impair the effects of the invention. In this specification, the numerical values ​​associated with "greater than or equal to," "less than or equal to," and "to" in describing a numerical range are numerical values ​​that can be arbitrarily combined. For example, when a certain numerical range is described as "CC to DD" and "EE to FF," the numerical ranges "CC to FF" and "EE to DD" are also included.

[0012] [Method for Controlling Gypsum Conversion Reaction] The method for controlling a gypsum conversion reaction of the present embodiment is a gypsum conversion reaction in which calcium carbonate is obtained by bringing a gypsum-containing material into contact with a solution A containing at least one alkali metal salt A selected from alkali metal carbonates and alkali metal bicarbonates, and the method monitors one condition selected from the pH of a reaction liquid between the solution A and the gypsum-containing material and at least one concentration selected from the carbonate ion concentration, the bicarbonate ion concentration, and the calcium ion concentration in the reaction liquid, and determines an end point of the reaction.

[0013] The present inventors have been conducting research into the reuse of gypsum-containing materials that are discarded mainly as industrial waste, and have noticed that calcium carbonate, a valuable material, can be obtained by reacting gypsum contained in gypsum-containing materials with an alkali metal salt. They then wondered whether an alkali metal salt (e.g., alkali metal carbonate) produced as a by-product in a business could be used as the alkali metal salt.

[0014] However, the concentrations of gypsum-containing materials, and alkali metal salts that are usually produced as by-products in the form of solutions, are often unknown. For example, in the case of gypsum-containing materials, especially those that are considered waste, the amount of gypsum contained therein is often unknown. Furthermore, alkali metal salts that are replicated within a facility are usually produced as by-products in the form of solutions, but their concentrations vary and are often unknown. Examples of alkali metal salts that are produced as by-products within a facility include alkali metal carbonates and alkali metal bicarbonates, but these can occur alone or as mixtures. Furthermore, alkali metal salts such as alkali metal carbonates and alkali metal bicarbonates are not limited to those produced as by-products within a facility; those generated outside the facility may also be brought in, and in such cases, their concentrations are often unknown.

[0015] As described above, when a raw material with unknown properties is used and a reaction between a gypsum-containing material and a solution containing an alkali metal salt is performed, the concentrations of the gypsum and the alkali metal salt contained in each material must be measured and the materials must be supplied in equal amounts. It may also be necessary to identify the type of alkali metal salt. However, these operations are complicated and have low productivity.

[0016] As a method for treating gypsum-containing waste, for example, Patent Document 1 focuses on separating and recovering high-purity gypsum from gypsum board waste and gypsum board base paper. However, it does not focus on converting gypsum-containing materials into valuable resources through chemical reactions. Patent Document 2 focuses on the chemical reaction between industrial waste, such as gypsum board, and ammonia and carbon dioxide. However, the chemical reaction focused on in Patent Document 2 is extremely slow, involving placing raw materials in a container in a predetermined ratio, stirring and mixing them, sealing the container with a lid, and leaving the container for one to three months while stirring every two to three days (Patent Document 2, paragraph

[0024] ). Therefore, there is no need to identify the types of raw materials in advance or measure their concentrations.

[0017] Under these circumstances, the present inventors have focused on the pH of a reaction solution between a solution containing at least one alkali metal salt selected from alkali metal carbonates and alkali metal bicarbonates and a gypsum-containing material, and the carbonate ion concentration, bicarbonate ion concentration, and calcium ion concentration in the reaction solution, in a gypsum conversion reaction in which calcium carbonate is obtained by contacting a gypsum-containing material with a solution containing at least one alkali metal salt selected from alkali metal carbonates and alkali metal bicarbonates.The inventors have then monitored one condition selected from the pH and concentrations to determine the end point of the reaction, thereby providing a method for controlling a gypsum conversion reaction and a method for producing calcium carbonate, which are highly versatile and applicable to a wide variety of materials, and can be handled with minor and easy operations.

[0018] The method for controlling the gypsum conversion reaction of this embodiment will be described starting with the gypsum conversion reaction.

[0019] [Gypsum Conversion Reaction] The gypsum conversion reaction is a reaction in which a solution A containing at least one alkali metal salt A selected from alkali metal carbonates and alkali metal hydrogencarbonates is brought into contact with a gypsum-containing material. In this reaction, the alkali metal salt A contained in the solution A reacts with the gypsum contained in the gypsum-containing material to produce calcium carbonate.

[0020] (Gypsum-containing material) The gypsum-containing material can be any material containing gypsum, the main component of which is at least one of calcium sulfate and calcium sulfate hydrate, and can be used without particular limitation. Calcium sulfate is an anhydrous material and is called anhydrous gypsum. Examples of calcium sulfate hydrates include a dihydrate called gypsum dihydrate and a hemihydrate called gypsum hemihydrate. Regarding anhydrous materials, for example, by calcining the hydrates, soluble anhydrous gypsum (also called "type II anhydrous gypsum"), insoluble anhydrous gypsum (also called "type II anhydrous gypsum"), and even α-type, β-type, and other calcined gypsums exist, depending on the calcination method, and any of these anhydrous materials can be used. In the control method of this embodiment, any of these anhydrous materials and hydrates can be used.

[0021] Further, examples of gypsum-containing materials include products using gypsum, such as gypsum board and waste gypsum board, or waste products thereof, as well as recovered products from flue gas desulfurization treatment (also referred to as "flue gas desulfurization gypsum" and recovered products from flue gas desulfurization treatment in, for example, copper refining), and by-products produced in the production process of chemical products, such as hydrofluoric gypsum (a by-product in the process of producing hydrogen fluoride), phosphogypsum (a by-product in the process of producing wet phosphoric acid), titanic gypsum (a by-product in the process of producing titanium oxide), and activated silicate gypsum (a by-product in the process of producing activated silicic acid).

[0022] Furthermore, for example, incineration ash generated during the incineration process of municipal waste contains anhydrous gypsum. Therefore, waste such as incineration ash can also be used as a gypsum-containing material. As another waste, gypsum-containing sludge obtained in various wastewater treatment processes can also be used as a gypsum-containing material. Thus, in the carbon dioxide fixation method that is one aspect of the present invention, in addition to gypsum itself, a wide range of gypsum-containing materials can be used from various wastes and by-products.

[0023] As described above, the gypsum-containing material is a material containing gypsum, which is composed mainly of at least one of calcium sulfate and calcium sulfate hydrate. By contacting the gypsum-containing material with the solution, a reaction occurs between calcium sulfate and the alkali metal salt contained in the solution. More specifically, in the gypsum conversion reaction, the reactions shown in the following reaction formulas (1) and (2) proceed. Ma ２ CO ３ + CaSO ４ →CaCO ３ +Ma ２ SO ４ (1) 2MaHCO ３ + CaSO ４ →CaCO ３ +Ma ２ SO ４ +CO ２ +H ２ O (2) (In the above reaction formulas (1) and (2), Ma represents an alkali metal.)

[0024] The gypsum contained in the gypsum-containing material is converted into calcium carbonate by the reactions shown in the above reaction formulas (1) and (2). The resulting calcium carbonate is a valuable substance that can be used in cement compositions, mortar and concrete materials, fillers, building materials, etc. Therefore, the gypsum conversion reaction can also be considered a calcium carbonate production reaction.

[0025] As will be described in the Examples below, the reactions represented by the above reaction formulas (1) and (2) proceed rapidly before and after the addition of an alkali metal salt in an amount equal to the calcium sulfate contained in a gypsum-containing material, or when a gypsum-containing material containing calcium sulfate in an amount equal to the alkali metal salt is added. Therefore, the alkali metal salt added after the gypsum conversion reaction has progressed remains as a surplus and is disposed of. On the other hand, the gypsum-containing material added after the gypsum conversion reaction has progressed remains as a surplus and becomes an impurity when calcium carbonate is used. According to the control method of this embodiment, in the gypsum conversion reaction, which is a fast-acting reaction, the gypsum-containing material and the alkali metal salt can be brought into contact with each other in just the right amount with minor adjustments and easy operations, regardless of the properties of the various materials used.

[0026] The content of gypsum contained in a gypsum-containing material cannot be generalized because it can vary depending on whether the gypsum is present in the form of anhydrous gypsum or a hydrate (gypsum dihydrate or gypsum hemihydrate), and also depending on whether the gypsum-containing material is gypsum, a by-product, or waste. It is sufficient for a gypsum-containing material to contain a small amount of gypsum, or a large amount. Either amount can be subject to the control method of this embodiment. Typically, the content of calcium sulfate (anhydrous) is typically 1% by mass or more, 3% by mass or more, or 5% by mass or more. There is no particular upper limit; the total amount (i.e., 100% by mass) may be gypsum, and typically 97% by mass or less, 95% by mass or less, or 90% by mass or less. The types of gypsum-containing materials and the gypsum content in gypsum-containing materials are diverse and unknown, but the control method of this embodiment can be used to deal with a wide range of such gypsum-containing materials.

[0027] (Alkali Metal Salt A) In the gypsum conversion reaction, the alkali metal salt A to be reacted with the gypsum-containing material is at least one alkali metal salt selected from alkali metal carbonates and alkali metal bicarbonates (hereinafter, alkali metal carbonates and alkali metal bicarbonates are also collectively referred to as "alkali metal carbonates, etc.").

[0028] The control method of this embodiment can be applied without any particular limitation to any alkali metal salt A such as a carbonate containing the alkali metal, and is preferably used with lithium (Li), potassium (K), or sodium (Na), more preferably potassium (K), sodium (Na), and even more preferably sodium (Na). The alkali metal salt A (alkali metal carbonate, etc.) is produced in large quantities both inside and outside the plant, and the gypsum conversion reaction using the alkali metal salt A (alkali metal carbonate, etc.) proceeds rapidly, so the control method of this embodiment can be more effectively utilized.

[0029] The solution containing the alkali metal salt A (such as an alkali metal carbonate) may be, for example, a solution produced inside or outside a business premises, or a surplus solution of a produced solution. For example, a solution containing, as the alkali metal salt A, an alkali metal carbonate or the like obtained by reacting an alkali metal salt such as an alkali metal hydroxide with carbon dioxide may be used. For example, a solution obtained by reacting an alkali solution (i.e., a solution containing a salt such as an alkali metal hydroxide) with a gas containing carbon dioxide may be used.

[0030] The alkaline solution may be, for example, an aqueous solution of an alkali metal salt such as an alkali hydroxide produced in various devices such as an electrolysis device for an aqueous solution of an alkali metal salt such as an alkali metal chloride, or a hydration reaction device for a solid alkali metal hydroxide. A reaction between an aqueous solution of an alkali metal salt such as an alkali hydroxide and carbon dioxide produces a solution A containing an alkali metal salt A, which is at least one alkali metal carbonate selected from alkali metal carbonates and alkali metal hydrogencarbonates. In this case, the resulting solution A containing the alkali metal salt A is an aqueous solution, and in the control method of this embodiment, it is preferable that the solution A be an aqueous solution.

[0031] Preferred examples of the solution containing an alkali metal salt include a solution obtained by reacting an alkaline solution obtained by an electrolysis apparatus using an ion exchange membrane method with a gas containing carbon dioxide, and a solution obtained by reacting an alkaline solution obtained by bipolar membrane electrodialysis with a gas containing carbon dioxide, since the alkaline solution can be obtained with low energy. Here, the alkaline solution is the above-mentioned alkaline solution, i.e., an aqueous solution of an alkali metal salt such as an alkali metal chloride. The content of alkali metal salt A in the solution obtained by any of the above methods is often unknown. Therefore, using the solution obtained by the above method as solution A containing alkali metal salt A is preferable because it allows the control method of this embodiment to be effectively utilized.

[0032] Among the above, it is preferable to use solution A containing alkali metal salt A obtained by bipolar membrane electrodialysis, since an alkaline solution can be obtained with low energy. The method using bipolar membrane electrodialysis involves electrolyzing a salt solution with bipolar membrane electrodialysis to prepare an alkaline solution, and contacting the alkaline solution with a gas containing carbon dioxide to obtain solution A containing alkali metal salt A. The bipolar membrane electrodialysis by-produces, together with the alkaline solution, an acid solution corresponding to the alkali metal salt contained in the salt solution (hydrochloric acid when the alkali metal chloride is used).

[0033] The salt solution is preferably a solution (hereinafter also referred to as "solution B") containing a salt containing an alkali metal, such as an alkali metal chloride, alkali metal sulfate, or alkali metal nitrate (hereinafter also referred to as "alkali metal salt B"). These alkali metal salts may be used alone or in combination. In a method using bipolar membrane electrodialysis, the alkali metal of alkali metal salt B and the alkali metal of alkali metal salt A are the same. When a solution containing alkali metal salt B is used as the salt solution, electrolysis of the salt solution using bipolar membrane electrodialysis produces an alkali solution containing a hydroxide of the alkali metal of alkali metal salt B. Contact of the alkali solution with a gas containing carbon dioxide produces a carbonate or hydrogencarbonate of the alkali metal of alkali metal salt B, thereby producing solution A containing alkali metal salt A.

[0034] As described above, the content of alkali metal salt A in solution A containing alkali metal salt A is unknown, but considering the above method and the conditions under which it is obtained inside and outside the business premises, the content is usually 0.1 mol / L or more, 0.3 mol / L or more, or 0.5 mol / L or more, with the upper limit being 2 mol / L or less, 1.5 mol / L or less, 1.3 mol / L or less, or 1.0 mol / L or less. The content of alkali metal salt A in solution A containing alkali metal salt A varies widely and is unknown, but the control method of this embodiment can be used to deal with a wide range of such solutions A.

[0035] (Contact) In the control method of the present embodiment, the gypsum conversion reaction is carried out by bringing the gypsum-containing material into contact with a solution A containing an alkali metal salt A. By bringing the solution A containing the alkali metal salt A into contact with the gypsum-containing material, a reaction occurs between the alkali metal salt A and the gypsum contained in the gypsum-containing material, and calcium carbonate is obtained.

[0036] In the control method of the present embodiment, the contacting method is not particularly limited as long as it can bring the gypsum-containing material into contact with the solution A containing the alkali metal salt A. For example, the contacting can be performed while continuously or intermittently supplying the gypsum-containing material to the solution A containing the alkali metal salt A, or while continuously or intermittently supplying the solution A containing the alkali metal salt A to the gypsum-containing material.

[0037] When the contact is carried out while continuously or intermittently supplying the gypsum-containing material to solution A containing alkali metal salt A, a certain amount of solution A may be stored in a reaction tank, and the gypsum-containing material may be continuously or intermittently supplied to the reaction tank. The gypsum-containing material may be pulverized to a predetermined size and supplied, taking into consideration ease of supply, ease of reaction, etc. Examples of the gypsum-containing material include material passing through a 5 mm sieve, material passing through a 10 mm sieve, material passing through a 15 mm sieve, material passing through a 30 mm sieve, and material passing through a 50 mm sieve.

[0038] In consideration of ease of supply and control of the supply amount, the gypsum-containing material can be supplied using, for example, a powder / granular material quantitative supply device, a hopper equipped with a damper, or the like.

[0039] Furthermore, the amount of solution A stored in the reaction tank cannot be generally determined because it depends on the amount and size of the gypsum-containing material to be treated, the size of the reaction tank, and the like. However, it is preferable that the depth of solution A stored in the reaction tank is a depth that covers the entire surface of the gypsum-containing material, i.e., is equal to or greater than the size of the gypsum-containing material.

[0040] When the contact is performed while continuously or intermittently supplying the solution A containing the alkali metal salt A to the gypsum-containing material, the size of the gypsum-containing material is the same as when the contact is performed while continuously or intermittently supplying the gypsum-containing material.

[0041] The solution A can be supplied using a liquid constant volume supply device or the like, taking into consideration ease of supply and control of the supply amount.

[0042] [Determination of End Point of Reaction] The method for controlling a gypsum conversion reaction of this embodiment is characterized in that, in the gypsum conversion reaction, the pH of the reaction liquid between the solution A and the gypsum-containing material and one condition selected from at least one concentration selected from the carbonate ion concentration, the bicarbonate ion concentration, and the calcium ion concentration in the reaction liquid are monitored to determine the end point of the reaction. By monitoring one condition selected from the pH and the concentration and determining the end point of the reaction, the method has high versatility that can be applied to a wide variety of materials and can be handled with minor adjustments and easy work.

[0043] When the solution A is brought into contact with the gypsum-containing material, the pH of the reaction solution and the concentrations of carbonate ions, bicarbonate ions, and calcium ions vary with an increase in the amounts of the solution A and the gypsum-containing material in contact with each other.

[0044] (Continuous or Intermittent Supply of Gypsum-Containing Material) When contact is performed while continuously or intermittently supplying a gypsum-containing material to solution A containing alkali metal salt A, as the supply amount of the gypsum-containing material increases, the pH of the reaction solution decreases, the carbonate ion concentration and the bicarbonate ion concentration decrease, and the calcium ion concentration increases.

[0045] As shown in Example 1 described later, when the reaction starts, that is, when the supply of the gypsum-containing material starts, the pH of the reaction solution gradually decreases, and then suddenly decreases when an amount of the gypsum-containing material (more strictly, gypsum (calcium sulfate) contained in the gypsum-containing material) equivalent to the alkali metal salt A in solution A is supplied, and thereafter, even if the gypsum-containing material is supplied, almost no change in the pH of the reaction solution is observed. The gypsum-containing material supplied after the above-mentioned sudden decrease does not contribute to the reaction and remains as a surplus, which becomes an impurity when calcium carbonate is used. Therefore, in the control method of this embodiment, when the pH of the reaction solution is monitored while the gypsum-containing material is continuously or intermittently supplied, the end point of the reaction is determined to be the point at which the pH suddenly changes.

[0046] Based on the results of Example 3 described below, it is believed that the behavior of at least one concentration selected from the carbonate ion concentration, the bicarbonate ion concentration, and the calcium ion concentration in the reaction solution also changes rapidly when gypsum (calcium sulfate) contained in the gypsum-containing material is supplied. When the gypsum-containing material is supplied, the carbonate ions in the reaction solution gradually decrease when the reaction starts, and then rapidly decrease when an amount of the gypsum-containing material (more strictly, gypsum (calcium sulfate) contained in the gypsum-containing material) equivalent to the alkali metal salt A in solution A is supplied, and thereafter, almost no change is observed even when the gypsum-containing material is supplied. Furthermore, the bicarbonate ions and calcium ions in the reaction solution gradually increase when the reaction starts, and then rapidly increase when an amount of the gypsum-containing material (more strictly, gypsum (calcium sulfate) contained in the gypsum-containing material) equivalent to the alkali metal salt A in solution A is supplied, and thereafter, almost no change is observed even when the gypsum-containing material is supplied. Therefore, in the control method of the present embodiment, when at least one concentration selected from the carbonate ion concentration, the bicarbonate ion concentration, and the calcium ion concentration in the reaction solution is monitored while the gypsum-containing material is continuously or intermittently supplied, the end point of the reaction is determined to be the time point at which these concentrations suddenly change (increase or decrease).

[0047] A sudden decrease in the pH of the reaction solution and a sudden change in the various concentrations can be determined by monitoring the amount of change per unit time or per unit supply amount. The unit time and unit supply amount cannot be generalized because they can vary depending on the supply amounts of Solution A and the gypsum-containing material to be subjected to the gypsum conversion reaction, the contents of alkali metal A and gypsum, etc. However, in consideration of the ease of determining a sudden decrease in the pH of the reaction solution and a sudden change in the various concentrations, it is preferable to select the following ranges as a guide.

[0048] The unit time may be selected from the range of 15 seconds or more, 30 seconds or more, 45 seconds or more, 1 minute or more, 2 minutes or more, 3 minutes or more, and 5 minutes or more as a lower limit, and from the range of 10 minutes or less, 9 minutes or less, 8 minutes or less, and 7 minutes or less as an upper limit. Also, taking into consideration the same factors as those for the unit time, the unit supply amount may be selected from the range of 10 g or more, 25 g or more, 50 g or more, 75 g or more, and 90 g or more as a lower limit, and from the range of 300 g or less, 250 g or less, 200 g or less, 175 g or less, and 150 g or less as an upper limit.

[0049] The sudden change in the pH of the reaction solution is more specifically determined by the n-th pH (pH ｎ , n indicates the number of times and is an integer.) and the pH immediately before that (pH ｎ－１ ) and the ratio of the change in pH ｎ / pH ｎ－１ ) is 96.0% or less, 95.0% or less, 94.0% or less, 93.0% or less, or 92.0% or less, can be used as a criterion for determining whether a rapid change has occurred.

[0050] As shown in Example 1, the slope of the pH change (d(pH) / dW), which is obtained by dividing the amount of pH change (d(pH)) by the amount of gypsum-containing material supplied when the amount of change was reached (dW), can also be used as a guide for determining whether a rapid change has occurred. The slope of the pH change (d(pH) / dW) may be determined by calculating the amount of pH change (d(pH)) corresponding to the amount of gypsum-containing material (dW) as the unit supply amount, for example.

[0051] Furthermore, as shown in FIG. 1 of Example 1, when there is a point at which the amount of change in pH clearly becomes large (the point at which the amount of the gypsum-containing material added is changed from 875 g to 1000 g in FIG. 1 ), the point can be determined as the end point of the reaction without taking into consideration the rate and gradient of the amount of change in pH.

[0052] More specifically, when the concentration is carbonate ion, the sudden change in the concentration (C(X １ ) ｎ , X １is a carbonate ion, and n indicates the number of times and is an integer.) and the previous one (C(X １ ) ｎ－１ ) and the rate of change of (C(X １ ) ｎ / C(X １ ) ｎ－１ As with the pH of the reaction solution, a rapid change can be determined based on the slope of the change in concentration obtained by dividing the change in carbonate ion concentration by the amount of the gypsum-containing material supplied when the change was reached, or, if there is a point at which the change clearly becomes large, the point can be determined as the end point of the reaction without considering the rate of the change.

[0053] In the case of bicarbonate ions and calcium ions, the n-th concentration (C(X ２ ) ｎ , X ２ is a bicarbonate ion or a calcium ion, and n indicates the number of times and is an integer.) and the previous time (C(X ２ ) ｎ－１ ) and the rate of change of (C(X ２ ) ｎ / C(X ２ ) ｎ－１ ) is 105.0% or more, 110.0% or more, 115.0% or more, 120.0% or more, 125.0% or more, or 130.0% or more, can be used as a criterion for determining a rapid change. As with the pH of the reaction solution, the rapid change can also be determined based on the slope of the change in concentration obtained by dividing the amount of change in the bicarbonate ion and calcium ion concentrations by the amount of the gypsum-containing material supplied when the change was reached. Furthermore, if there is a point at which the change clearly becomes large, the point can be determined as the end point of the reaction without considering the rate of the change.

[0054] As shown in Example 1, there are cases where the above-mentioned criteria for determining a rapid change are met at multiple points, and the reaction can be determined to be at its end point. In this case, it is preferable to terminate the reaction at the first point met, but the reaction may also be terminated at the last point met.

[0055] The supply amount of the gypsum-containing material is not particularly limited, as it can vary depending on the amount of gypsum contained in the gypsum-containing material and the content of alkali metal salt A in solution A. Considering the ease of determining a sudden decrease in pH of the reaction solution and a sudden change in the above-mentioned various concentrations, when the gypsum-containing material is continuously supplied, it is preferable to adopt, as a guideline for the supply amount of the gypsum-containing material, a range from a lower limit of 1 g / min or more, 3 g / min or more, 5 g / min or more, 10 g / min or more, and 15 g / min or more, and an upper limit of 50 g / min or less, 40 g / min or less, 30 g / min or less, and 25 g / min or less, relative to a volume of 10 L of solution A. Furthermore, when the material is supplied intermittently, the supply amount should correspond to the supply rate and unit time of the continuous supply, for example, 1 g or more (corresponding to the above-mentioned "1 g / min or more"), 3 g or more (corresponding to the above-mentioned "3 g / min or more") when the unit time is 1 minute, or 2 g or more (corresponding to the above-mentioned "1 g / min or more"), 6 g or more (corresponding to the above-mentioned "3 g / min or more") when the unit time is 2 minutes.

[0056] (Continuous or Intermittent Supply of Solution A) When contact is performed while continuously or intermittently supplying solution A containing alkali metal salt A to the gypsum-containing material, an increase in the amount of the gypsum-containing material supplied increases the pH of the reaction solution, increases the carbonate ion concentration and bicarbonate ion concentration, and decreases the calcium ion concentration.

[0057] As shown in Example 2 described later, when the reaction starts, that is, when the supply of solution A begins, the pH of the reaction solution gradually increases, and then suddenly increases when a gypsum-containing material (more strictly, gypsum (calcium sulfate) contained in the gypsum-containing material) equivalent to the alkali metal salt A in solution A is supplied. When solution A is then supplied, the pH of the reaction solution gradually increases again. The pH of the reaction solution after the sudden increase increases as solution A is supplied, but this increase in pH means that the pH increases to approach the pH of solution A. For this reason, solution A supplied after the sudden increase in pH does not contribute to the reaction and remains as a surplus, and is therefore wasted. Therefore, in the control method of this embodiment, when the pH of the reaction solution is monitored while solution A is continuously or intermittently supplied, the end point of the reaction is determined to be the point at which the pH suddenly changes.

[0058] Based on the results of Example 3 described below, it is believed that the behavior of at least one concentration selected from the carbonate ion concentration, the bicarbonate ion concentration, and the calcium ion concentration in the reaction solution also changes rapidly when gypsum (calcium sulfate) contained in the gypsum-containing material is supplied. When solution A is supplied, the carbonate ions in the reaction solution gradually increase when the reaction starts, and then rapidly increase when a gypsum-containing material (more strictly, gypsum (calcium sulfate) contained in the gypsum-containing material) equivalent to the alkali metal salt A in solution A is supplied, and thereafter, almost no change is observed even when the gypsum-containing material is supplied. Furthermore, the bicarbonate ions and calcium ions in the reaction solution gradually decrease when the reaction starts, and then rapidly decrease when a gypsum-containing material (more strictly, gypsum (calcium sulfate) contained in the gypsum-containing material) equivalent to the alkali metal salt A in solution A is supplied, and thereafter, almost no change is observed even when the gypsum-containing material is supplied. Therefore, in the control method of this embodiment, when at least one concentration selected from the carbonate ion concentration, the bicarbonate ion concentration, and the calcium ion concentration in the reaction solution is monitored while supplying solution A continuously or intermittently, the end point of the reaction is determined to be the time point at which these concentrations suddenly change (increase or decrease).

[0059] When solution A is continuously or intermittently supplied, a sudden decrease in the pH of the reaction solution and a sudden change in the various concentrations can be determined in the same manner as when the gypsum-containing material is continuously or intermittently supplied.

[0060] The unit time is the same as when the gypsum-containing material is continuously or intermittently supplied. The unit supply amount may be selected from the range of 0.1 L or more, 0.2 L or more, 0.3 L or more, and 0.5 L or more as the lower limit, and from the range of 10.0 L or less, 8.0 L or less, 6.0 L or less, 5.0 L or less, and 3.5 L or less as the upper limit.

[0061] When solution A is supplied continuously or intermittently, a sudden decrease in the pH of the reaction solution and a sudden change in the various concentrations can be determined by monitoring the amount of change per unit time or per unit supply amount, as in the case of supplying the gypsum-containing material continuously or intermittently. The unit time and unit supply amount cannot be generalized because they can vary depending on the supply amounts of solution A and the gypsum-containing material supplied to the gypsum conversion reaction, the contents of alkali metal A and gypsum, etc. However, in consideration of the ease of determining a sudden decrease in the pH of the reaction solution and a sudden change in the various concentrations, the following ranges can be selected as a guide.

[0062] The sudden change in the pH of the reaction solution is more specifically determined by the n-th pH (pH ｎ , n indicates the number of times and is an integer.) and the pH immediately before that (pH ｎ－１ ) and the ratio of the change in pH ｎ / pH ｎ－１ ) is 103.0% or more, 105.0% or more, 106.0% or more, 107.0% or more, 108.0% or more, or 108.2% or more can be used as a criterion for determining whether a rapid change has occurred.

[0063] As shown in Example 2, the slope of the pH change (d(pH) / dV), obtained by dividing the amount of pH change (d(pH)) by the amount of solution A supplied when that amount of change was reached (dV), can also be used as a guide to determine whether a rapid change has occurred. The slope of the pH change (d(pH) / dV) can be determined by calculating the amount of pH change (d(pH)) corresponding to the amount of solution A supplied (dV), for example, as the unit supply amount.

[0064] As shown in Figure 2 of Example 2, if there is a point at which the gradient of the pH change clearly becomes large (the point in Figure 2 where the amount of Solution A added is changed from 10 L to 11 L), this point can be determined to be the end point of the reaction without having to consider the rate and gradient of the pH change.

[0065] More specifically, when the concentration is bicarbonate ion or calcium ion, the sudden change in the concentration (C(Y １ )ｎ , Y １ is a bicarbonate ion or a calcium ion, and n indicates the number of times and is an integer.) and the previous time (C(Y １ ) ｎ－１ ) and the rate of change of (C(Y １ ) ｎ / C(Y １ ) ｎ－１ ) is 93.0% or less, 90.0% or less, 85.0% or less, 80.0% or less, or 77.0% or less, can be used as a criterion for determining a rapid change. As with the pH of the reaction solution, the rate of change in the concentrations of bicarbonate ions and calcium ions can be determined by the slope of the rate of change in concentration, calculated by dividing the rate of change in concentration by the amount of Solution A supplied when the rate of change is reached. Alternatively, if there is a point at which the rate of change clearly increases, the point can be determined as the end point of the reaction, regardless of the rate of change.

[0066] In the case of carbonate ions, the concentration (C(Y ２ ) ｎ , Y ２ is a carbonate ion, and n indicates the number of times and is an integer.) and the previous one (C(Y ２ ) ｎ－１ ) and the rate of change of (C(Y ２ ) ｎ / C(Y ２ ) ｎ－１ ) is 105.0% or more, 110.0% or more, 115.0% or more, 120.0% or more, or 125.0% or more. As with the pH of the reaction solution, it is also possible to judge the rapid change based on the slope of the change in concentration obtained by dividing the change in carbonate ion concentration by the amount of solution A supplied when the change was reached. Alternatively, if there is a point at which the change is clearly large, the point can be determined as the end point of the reaction without considering the rate of change.

[0067] The supply amount of solution A cannot be generalized and is not particularly limited, since it can vary depending on the amount of alkali metal salt A contained in solution A and the gypsum content of the gypsum-containing material. Considering the ease of determining a sudden decrease in pH of the reaction solution and a sudden change in the above-mentioned various concentrations, when solution A is continuously supplied, it is advisable to adopt, as a guideline, a supply amount of solution A from a range of 0.1 L / min or more, 0.2 L / min or more, 0.3 L / min or more, 0.5 L / min or more, and 1.0 L / min or more as lower limits, and 10.0 L / min or less, 8.0 L / min or less, 6.0 L / min or less, and 5.0 L / min or less as upper limits, per 1 kg of the gypsum-containing material. Furthermore, when solution A is supplied intermittently, the supply amount should be set to correspond to the supply rate and unit time for the continuous supply, for example, 0.1 L or more, 0.2 L or more per minute, or 0.3 L or more, or 0.5 L or more per 2 minutes.

[0068] In the control method of this embodiment, the pH of the reaction solution may be measured continuously using an online pH meter or by sampling as needed. The concentrations of carbonate ions, bicarbonate ions, and calcium ions in the reaction solution may be measured continuously using an online TOC meter or the like or by sampling as needed.

[0069] [Method for Producing Calcium Carbonate] The method for producing calcium carbonate of the present embodiment includes carrying out a gypsum conversion reaction by bringing a gypsum-containing substance into contact with a solution A containing at least one alkali metal salt A selected from alkali metal carbonates and alkali metal bicarbonates, and monitoring the gypsum conversion reaction under one condition selected from the pH of a reaction liquid of the solution A and the gypsum-containing substance and at least one concentration selected from the carbonate ion concentration, the bicarbonate ion concentration, and the calcium ion concentration in the reaction liquid, and determining an end point of the reaction to terminate the gypsum conversion reaction.

[0070] The method for producing calcium carbonate of the present embodiment has high versatility so that it can be applied to a wide variety of materials, can be implemented with minor adjustments and easy operations, and can perform the gypsum conversion reaction in just the right amount, thereby enabling calcium carbonate to be produced efficiently.

[0071] As described above, the gypsum conversion reaction is a reaction in which a solution A containing at least one alkali metal salt A selected from alkali metal carbonates and alkali metal hydrogencarbonates is brought into contact with a gypsum-containing material, and the alkali metal salt A contained in the solution A reacts with the gypsum contained in the gypsum-containing material to produce calcium carbonate.

[0072] In the method for producing calcium carbonate of this embodiment, the gypsum conversion reaction, the alkali metal salt A containing the same, the solution A containing the gypsum, the gypsum-containing material, and the contacting method used in this reaction are as described above in the control method of this embodiment. In addition, the pH of the reaction solution, at least one concentration selected from the carbonate ion concentration, the bicarbonate ion concentration, and the calcium ion concentration in the reaction solution, and one condition selected from these are monitored to determine the end point of the reaction, as described above in the control method of this embodiment.

[0073] The present invention will now be described in detail with reference to examples, but the present invention is not limited to these examples in any way.

[0074] Example 1 A 0.55 M aqueous sodium carbonate solution was prepared as a typical sample of solution A containing an alkali metal salt A, and gypsum dihydrate (calcium sulfate dihydrate, purity: 98%, size: 100 μm full-through sieve product) was prepared as a gypsum-containing material. 11.6 L of the aqueous sodium carbonate solution was placed in a reaction tank, and while feeding gypsum dihydrate at a feed rate of 20 g / min, the pH of the reaction solution was measured every time 125 g of the gypsum-containing material was fed. The measurement results are shown in FIG. 1. FIG. 1 also shows the change in pH (d(pH) / dW).

[0075] According to Example 1, when the supply of the gypsum-containing material starts, the pH of the reaction solution immediately decreases, indicating that the gypsum conversion reaction immediately starts. Furthermore, as the supply of the gypsum-containing material continues, the pH of the reaction solution gradually decreases, and when the supply amount reaches 900 g or more and 1000 g, the pH decreases rapidly. Furthermore, the pH decreases at the same rate up to 1200 g, and even if the supply amount of the gypsum-containing material is further increased, almost no change in the pH of the reaction solution is observed.

[0076] In Example 1, the end point of the gypsum conversion reaction was when the supply amount of the gypsum-containing material reached 1000 g, i.e., when the pH reached 9.5 (pH ８ The pH of the previous 875g supply was 10.4 (pH ７ is 10.4), and the rate of change (pH ８ / pH ７ ) is 91.3%, so it can be determined that this is the end point of the reaction. Also, if the points at 1125 g and 1250 g are considered in the same way as the point at 1000 g, the rate of change (pH ９ / pH ８ ) is 95.8% (= 9.1 / 9.5), and in the case of 1250g, the rate of change (pH １０ / pH ９ ) is 95.6% (= 8.7 / 9.1), and therefore, any of these can be determined to be the end point of the reaction. Furthermore, as shown in FIG. 1 , a clear rapid decrease in pH can be confirmed when the supply amount of the gypsum-containing material is increased from 875 g to 1000 to 1125 to 1250 g. Based on this, the end point of the reaction can be determined to be any of the points at which the supply amount of the gypsum-containing material is 1000 g, 1125 g, and 1250 g. In this way, in Example 1, the end point of the reaction can be determined at multiple points. However, when actually implementing the control method of this embodiment, the reaction can be terminated at the point that is first determined to be the end point of the reaction.

[0077] (Example 2) The same solution A and gypsum-containing material as in Example 1 were used. 1,700 g of the gypsum-containing material was placed in a reaction tank, and while solution A was being supplied at a supply rate of 1.0 L / min, the pH of the reaction solution was measured every time 1 L of solution A was supplied. The measurement results are shown in FIG. 2. FIG. 2 also shows the amount of change in pH (d(pH) / dV).

[0078] According to Example 2, when the supply of Solution A started, the pH of the reaction solution immediately increased, indicating that the gypsum conversion reaction immediately started. Furthermore, as the supply of Solution A continued, the pH of the reaction solution gradually increased, and when the supply amount reached 10 L or more and reached 11 L, it increased rapidly. Furthermore, the increase slowed down until the supply amount reached 14 L. It is believed that when Solution A was further supplied, Solution A did not contribute to the reaction, and therefore the pH of the reaction solution approached the pH of Solution A.

[0079] In Example 2, the end point of the gypsum conversion reaction was when the supply amount of Solution A reached 11 L, i.e., when the pH at the time of supplying 11 L (the 11th time when the unit supply amount was 1 L) reached 10.5 (pH １１ The pH of the previous 10L supply was 9.7 (pH １０ is 9.7), and the rate of change (pH １１ / pH １０ ) is 108.2%, which can be determined as the end point of the reaction. Also, as shown in Figure 2, when the supply amount of Solution A is increased from 10 L to 11 L, a clear rapid increase in pH can be confirmed, which can also be used to determine the end point of the reaction.

[0080] Example 3 A simulation of a gypsum conversion reaction was performed under the same conditions as in Example 2 using simulation software (PHREEQC Ver. 3 (product name) manufactured by the United States Geological Survey). 1,700 g of a gypsum-containing material was placed in a reaction tank, and Solution A was supplied at a supply rate of 1.0 L / min. Simulation results for the concentrations of carbonate ions, bicarbonate ions, and calcium ions in the reaction solution were plotted every time 0.3 L was supplied. The results are shown in FIG. 3. FIG. 4 is a diagram plotting the simulation results for the pH of the reaction solution.

[0081] According to Example 3, as shown in Fig. 4, the behavior of the pH of the reaction solution is similar to that of Example 2 in which Solution A is supplied to the gypsum-containing material, and the results of the simulation are considered to be valid, including the results of the concentrations of each ion shown in Fig. 3. Therefore, it is considered that the concentrations of each ion in Example 2 behave as shown in Fig. 3 of Example 3.

[0082] Example 4 A simulation of a gypsum conversion reaction was performed under the same conditions as in Example 1 using simulation software (PHREEQC Ver. 3 (product name) manufactured by the United States Geological Survey). 11.6 L of an aqueous sodium carbonate solution was placed in a reaction tank, and while gypsum dihydrate was supplied at a supply rate of 20 g / min, the simulation results for the pH of the reaction solution were plotted every time 125 g of the gypsum-containing material was supplied. The results are shown in FIG. 5 .

[0083] According to Example 4, as shown in FIG. 5 , the behavior of the pH of the reaction solution is similar to that of Example 1 in which a gypsum-containing material is supplied to the solution A. Therefore, similar to the above-described Example 3, the simulation results are considered to be valid.

Claims

1. A method for controlling a gypsum conversion reaction in which a gypsum-containing material is contacted with a solution A containing at least one alkali metal salt A selected from alkali metal carbonates and alkali metal hydrogen carbonates to obtain calcium carbonate, the method comprising monitoring one condition selected from the pH of a reaction liquid between the solution A and the gypsum-containing material and at least one concentration selected from a carbonate ion concentration, a hydrogen carbonate ion concentration, and a calcium ion concentration in the reaction liquid, and determining an end point of the reaction.

2. The method for controlling a gypsum conversion reaction according to claim 1, wherein the gypsum-containing material is continuously or intermittently supplied to solution A containing the alkali metal salt A.

3. The method for controlling a gypsum conversion reaction according to claim 1, wherein solution A containing alkali metal salt A is supplied continuously or intermittently to the gypsum-containing material.

4. The method for controlling a gypsum conversion reaction according to claim 1, wherein the solution A containing the alkali metal salt A is a solution obtained by electrolyzing a salt solution by bipolar electrodialysis to prepare an alkaline solution, and contacting the alkaline solution with a gas containing carbon dioxide.

5. The method for controlling a gypsum conversion reaction according to claim 4, wherein the salt solution is solution B containing an alkali metal salt B.

6. The method for controlling a gypsum conversion reaction according to claim 5, wherein the alkali metal salt B is at least one metal salt selected from the group consisting of alkali metal chlorides, alkali metal sulfates and alkali metal nitrates.

7. The method for controlling a gypsum conversion reaction according to claim 5, wherein the alkali metal in the alkali metal salt B is the same as the alkali metal in the alkali metal salt A.

8. The method for controlling a gypsum conversion reaction according to claim 1, wherein the alkali metal in the alkali metal salt A is at least one metal selected from the group consisting of sodium and potassium.

9. A method for producing calcium carbonate, comprising: carrying out a gypsum conversion reaction by contacting a gypsum-containing material with a solution A containing at least one alkali metal salt A selected from alkali metal carbonates and alkali metal hydrogen carbonates; monitoring the gypsum conversion reaction under one condition selected from the pH of a reaction liquid between the solution A and the gypsum-containing material and at least one concentration selected from a carbonate ion concentration, a hydrogen carbonate ion concentration, and a calcium ion concentration in the reaction liquid; and determining an end point of the reaction to terminate the gypsum conversion reaction.

Images