Method for reducing alkali metals contained in calcium carbonate and method for producing calcium carbonate

Abstract

Provided are: a method for reducing alkali metals contained in calcium carbonate, the method comprising a step for bringing a gypsum-containing material, which is capable of reducing alkali metals contained in calcium carbonate and is obtained using gypsum and an alkali metal salt, into contact with a solution containing an alkali metal carbonate and / or an alkali metal bicarbonate to obtain calcium carbonate, and in which ultrasonic irradiation is performed when bringing the gypsum-containing material into contact with the solution; and a method for producing high-purity calcium carbonate.

Description

Method for reducing alkali metal contained in calcium carbonate and method for producing calcium carbonate

[0001] The present disclosure relates to a method for reducing alkali metal contained in calcium carbonate and a method for producing calcium carbonate.

[0002] In recent years, interest in global warming has increased, and reduction of carbon dioxide emissions into the atmosphere has been demanded. In various facilities such as power plants, incinerators, cement plants, ironworks, and factory facilities, studies have been conducted on recovering exhaust gas containing carbon dioxide generated by operation without releasing it into the atmosphere.

[0003] For example, Patent Document 1 discloses a carbon dioxide fixation method and a method for producing calcium carbonate, including: a first step of bringing a first solution containing an alkali metal hydroxide into contact with a gas containing carbon dioxide to generate a second solution containing at least one alkali metal salt selected from alkali metal carbonates and alkali metal bicarbonates; and a second step of bringing the second solution into contact with a gypsum-containing material to generate calcium carbonate.

[0004] Japanese Unexamined Patent Application Publication No. 2023-051282

[0005] An object of the present invention is to provide a method for reducing alkali metal, which can reduce alkali metal contained in calcium carbonate obtained by using gypsum and an alkali metal salt, and a method for producing high-purity calcium carbonate.

[0006] In order to solve the above problems, the present invention provides the following method for reducing alkali metal contained in calcium carbonate. 1. A method for reducing alkali metal contained in calcium carbonate, comprising a step of obtaining calcium carbonate by bringing a gypsum-containing material into contact with a solution containing at least one alkali metal salt selected from alkali metal carbonates and alkali metal bicarbonates, wherein ultrasonic irradiation is performed during the contact.

[0007] Furthermore, the present invention provides the following preferred embodiments for reducing alkali metals contained in calcium carbonate: 2. The alkali metal reduction method according to 1 above, wherein a solution containing the alkali metal salt is supplied to the gypsum-containing material. 3. The alkali metal reduction method according to 2 above, wherein a solution containing the alkali metal salt is supplied continuously or intermittently. 4. The alkali metal reduction method according to 1 above, wherein the gypsum-containing material is immersed in a solution containing the alkali metal salt. 5. The alkali metal reduction method according to any one of 1 to 4 above, wherein the alkali metal salt content in the solution is 0.01 M or more and 1.5 M or less. 6. The alkali metal reduction method according to any one of 1 to 5 above, wherein the gypsum-containing material is waste gypsum board.

[0008] Furthermore, in order to solve the above problems, the present invention provides the following method for producing calcium carbonate: 7. A method for producing calcium carbonate comprising the step of contacting a gypsum-containing material with a solution containing at least one alkali metal salt, which is an alkali metal carbonate or an alkali metal bicarbonate, wherein ultrasonic irradiation is performed during the contact.

[0009] Furthermore, the present invention provides the following methods for producing calcium carbonate as preferred embodiments: 8. The method for producing calcium carbonate according to 7, wherein a solution containing the alkali metal salt is supplied to the gypsum-containing material. 9. The method for producing calcium carbonate according to 8, wherein a solution containing the alkali metal salt is supplied continuously or intermittently. 10. The method for producing calcium carbonate according to 7, wherein the gypsum-containing material is immersed in a solution containing the alkali metal salt. 11. The method for producing calcium carbonate according to any one of 7 to 10, wherein the alkali metal salt content in the solution is 0.01 M or more and 1.5 M or less. 12. The method for producing calcium carbonate according to any one of 7 to 11, wherein the gypsum-containing material is waste gypsum board.

[0010] According to the present invention, it is possible to provide a method for reducing alkali metals contained in calcium carbonate obtained using gypsum and alkali metal salts, as well as a method for producing high-purity calcium carbonate.

[0011] The following describes embodiments of the present invention (which may be referred to as "these embodiments"). The present invention is not limited to the following embodiments and can be modified and implemented as such without hindering the effects of the invention. In this specification, the numerical range notation "AA to BB" means "AA or greater and BB or less". In this specification, the numerical values ​​related to "greater than or equal to", "less than or equal to", and "~" in the description of numerical ranges can be any combination. For example, if a certain numerical range is described as "CC to DD" and "EE to FF", then numerical ranges such as "CC to FF" and "EE to DD" are also included.

[0012] [Method for reducing alkali metals contained in calcium carbonate] The method for reducing alkali metals contained in calcium carbonate according to this embodiment includes the step of obtaining calcium carbonate by contacting a gypsum-containing material with a solution containing at least one alkali metal salt, which is an alkali metal carbonate or an alkali metal bicarbonate, and performing ultrasonic irradiation when making the contact.

[0013] The technology described in Patent Document 1 is a method for recovering carbon dioxide contained in exhaust gas generated in the above-mentioned various facilities, and is also extremely useful as a method for producing calcium carbonate as a valuable material. On the other hand, the higher the purity of the calcium carbonate, the more desirable it is, and the performance requirements for purity are increasing year by year. The method described in Patent Document 1 uses alkali metal salts. However, recent research has shown that alkali metals derived from alkali metal salts may be included as impurities in the calcium carbonate. Therefore, when the performance requirements for calcium carbonate are high, it has become necessary to further improve the properties of the calcium carbonate obtained by the method described in Patent Document 1. Thus, in a situation where the performance requirements are becoming stricter year by year, there is a need to develop a method that can meet even the most stringent performance requirements.

[0014] While continuing further development, the inventors discovered that when using waste materials such as discarded gypsum board, alkali metals tend to be included as impurities, leading to a decrease in the purity of calcium carbonate. Conventionally, discarded gypsum board is crushed after the paper material on the surface is removed and used as a ground improvement material or cement auxiliary material, but problems such as the generation of hydrogen sulfide can occur, so most of it is discarded without being recycled. Meanwhile, the amount of discarded gypsum board is expected to increase significantly year by year, and there is a need to establish effective methods for utilizing discarded gypsum board. Against this backdrop, reducing the amount of alkali metals included as impurities in calcium carbonate, even when using a wider range of gypsum-containing materials including waste materials such as discarded gypsum board, is extremely useful not only for reducing carbon dioxide emissions but also for the effective utilization of waste materials.

[0015] The inventors furthered their research on reaction environments for reacting gypsum-containing materials with alkali metal salts by contacting the gypsum-containing material with a solution containing at least one of alkali metal salts, namely alkali metal carbonate and alkali metal bicarbonate. As a result, they found that when the reaction between the gypsum-containing material and the alkali metal salt, more specifically the reaction between the gypsum contained in the gypsum-containing material (i.e., calcium sulfate) and the alkali metal salt, was carried out under an ultrasonic irradiation environment, not only was the reaction accelerated, but the amount of alkali metal contained in the resulting calcium carbonate could also be reduced.

[0016] (Gypsum-containing materials) Any material containing gypsum can be used without particular restrictions. Examples include gypsum boards, waste gypsum boards, and other products or waste materials using gypsum; flue gas desulfurization treatment recovery materials (also called "flue gas desulfurization gypsum," which are recovery materials from flue gas desulfurization treatment in copper refining, for example); hydrofluoric acid gypsum (a by-product in the process of producing hydrogen fluoride); phosphate gypsum (a by-product in the process of producing wet phosphoric acid); titanium gypsum (a by-product in the process of producing titanium oxide); activated silicate gypsum (a by-product in the process of producing activated silica); and other by-products produced in the manufacturing process of chemical products.

[0017] Considering the characteristic of this embodiment's method for reducing alkali metals in calcium carbonate (hereinafter also simply referred to as the "alkali metal reduction method"), which can be used with any containing material that includes gypsum, it is preferable to use waste materials such as waste gypsum board. Waste gypsum board is a type of waste that is generated in large quantities when buildings are rebuilt. As mentioned above, waste gypsum board is one of the resources that has not been recycled due to problems such as the generation of hydrogen sulfide. The ability to produce valuable materials such as calcium carbonate from such waste not only has the advantage of reducing waste materials, but also has extremely great economic benefits.

[0018] For ease of handling, the average particle size of the gypsum-containing material should be 5 mm or less, 1.0 mm or less, and 0.5 mm or less. Furthermore, when used as a pre-mixed liquid (slurry), the particle size of the gypsum-containing material should be approximately 500 μm or less to ensure a stable mixture (slurry). In this specification, the average particle size of the gypsum-containing material is measured by sieving. A rotary tap type automatic sieving machine is used with standard sieves conforming to the provisions of JIS Z 8801:2019. The sieves are stacked in order from smallest to largest mesh size, and the sample remaining on each sieve is weighed. The particle size at which the cumulative total reaches 50% is taken as the average particle size.

[0019] To achieve the above particle size, the gypsum-containing material can be pre-ground using a pulverizer such as a roller mill. Furthermore, when using waste gypsum board as the gypsum-containing material, it is preferable to remove any wallpaper or other surface coverings beforehand.

[0020] Furthermore, when using a gypsum-containing material as a mixed liquid (slurry), water is preferred as the liquid used in combination with the gypsum-containing material. The content of gypsum-containing material (solids) in the mixed liquid (slurry) is preferably 0.1 g / ml or more, more preferably 0.5 g / ml or more, even more preferably 0.8 g / ml or more, with an upper limit of preferably 2.0 g / ml or less, more preferably 2.0 g / ml or less, and even more preferably 1.5 g / ml or less. When the content of gypsum-containing material (solids) in the mixed liquid (slurry) is within the above range, contact between the gypsum-containing material and the solution containing the alkali metal salt can be achieved more efficiently, and as a result, the reaction between gypsum (calcium sulfate) and the alkali metal salt can be promoted.

[0021] (Solution containing alkali metal salt) The alkali metal solution used in the alkali metal reduction method of this embodiment is a solution containing at least one of alkali metal salts, such as alkali metal carbonate and alkali metal bicarbonate.

[0022] Preferably, the alkali metals in alkali metal salts are lithium, sodium, and potassium, and more preferably sodium and potassium. These alkali metals can be used individually or in combination. Considering ease of handling, it is preferable to use only one alkali metal.

[0023] Regarding alkali metal carbonates and alkali metal bicarbonates, any alkali metal salt can be used, but from the viewpoint of more efficiently promoting the reaction between gypsum (calcium sulfate) and the alkali metal salt, it is preferable to use alkali metal carbonates. Preferred alkali metal carbonates include lithium carbonate, sodium carbonate, and potassium carbonate, and more preferably sodium carbonate and potassium carbonate.

[0024] Alkali metal carbonates can be used alone, in which case one alkali metal carbonate or multiple alkali metal carbonates may be used. The same applies when using alkali metal bicarbonates. Alkali metal carbonates and alkali metal bicarbonates may also be used in combination, in which case one alkali metal carbonate, one alkali metal bicarbonate, multiple alkali metal carbonates, or multiple alkali metal bicarbonates may be used. Considering ease of handling, it is preferable to use one alkali metal carbonate and one alkali metal bicarbonate.

[0025] The alkali metal salt content in the solution containing the alkali metal salt is preferably 0.01 M or more, more preferably 0.05 M or more, even more preferably 0.1 M or more, and even more preferably 0.2 M or more, with an upper limit of preferably 1.5 M or less, more preferably 1.4 M or less, and even more preferably 1.3 M or less. When the alkali metal salt content is within the above range, contact between the gypsum-containing material and the alkali metal salt-containing solution can be achieved more efficiently, and as a result, the reaction between gypsum (calcium sulfate) and the alkali metal salt can be promoted. Furthermore, ultrasonic irradiation, which will be described later, becomes easier, and the amount of wastewater can be reduced.

[0026] (Contact between gypsum-containing material and a solution containing alkali metal salts) The method for reducing alkali metal salts in this embodiment involves contacting a gypsum-containing material with a solution containing at least one of alkali metal salts, alkali metal carbonate and alkali metal bicarbonate. By contacting the gypsum-containing material with the solution containing alkali metal salts, calcium carbonate is obtained through a reaction between the gypsum (calcium sulfate) contained in the gypsum-containing material and the alkali metal salts. Furthermore, by performing ultrasonic irradiation when contacting the gypsum-containing material with the solution containing alkali metal salts, the alkali metals contained in the calcium carbonate can be reduced, and high-purity calcium carbonate can be obtained.

[0027] When gypsum-containing material comes into contact with a solution containing alkali metal salts, a reaction between gypsum (calcium sulfate) and alkali metal salts proceeds. By applying ultrasonic irradiation during this process, not only is the reaction accelerated, but alkali metals that tend to remain as impurities embedded in the calcium carbonate produced by the reaction can be removed, resulting in the efficient acquisition of high-purity calcium carbonate.

[0028] The supply amounts of the gypsum-containing material and the alkali metal salt-containing solution should be determined such that the ratio of moles of gypsum in the gypsum-containing material to moles of alkali metal salt in the alkali metal salt-containing solution is 1:1. Here, the above ratio of 1:1 does not have to be strictly 1:1, but may have an error of ±5% or less, for example, it is acceptable to supply a larger amount of alkali metal salt-containing solution, such as 1:1 to 1.05.

[0029] When the entire volume of the solution containing the gypsum-containing material and the alkali metal salt is brought into contact with the fluid, the content of the alkali metal salt relative to the liquid component (water) in the fluid is preferably 0.01 M or more, more preferably 0.05 M or more, even more preferably 0.1 M or more, and even more preferably 0.2 M or more, with an upper limit of preferably 1.5 M or less, more preferably 1.4 M or less, and even more preferably 1.1 M or less.

[0030] Regarding the method of contacting the gypsum-containing material with the solution containing the alkali metal salt, there are no particular restrictions as long as these materials can be brought into contact. For example, (a) supplying the solution containing the alkali metal salt to the gypsum-containing material, and (b) immersing the gypsum-containing material in the solution containing the alkali metal salt are preferred.

[0031] (Method (a)) When Method (a) described above is adopted, the supply of the solution containing the alkali metal salt can be carried out continuously or intermittently. When the supply of the solution containing the alkali metal salt is carried out continuously, the solution should be supplied without interruption, and the required amount may be supplied all at once (also referred to as "bulk supply"), or a fixed amount may be supplied continuously over a fixed period of time. When a fixed amount is supplied continuously over a fixed period of time, for example, the supply rate is preferably such that the entire amount can be supplied in 1 minute or more and 30 minutes or less, more preferably such that the entire amount can be supplied in 3 minutes or more and 20 minutes or less, and even more preferably such that the entire amount can be supplied in 5 minutes or more and 10 minutes or less. By setting the supply rate of the solution containing the alkali metal salt within the above range, contact between the gypsum-containing material and the solution containing the alkali metal salt is facilitated, and as a result, the reaction between gypsum (calcium sulfate) and the alkali metal salt can be promoted. It also facilitates ultrasonic irradiation, which will be described later.

[0032] When supplying a solution containing an alkali metal salt intermittently, for example, a certain amount may be supplied, followed by a period of time, and this process may be repeated (also referred to as "intermittent supply"). The supply may be stopped once the required amount has been supplied. So-called "dropping" is also included in intermittent supply. In this case, the certain amount and the period of time can be appropriately determined according to the amount of gypsum-containing material and solution to be in contact with, and there is no general rule. However, for example, the certain amount is preferably 1 / 30 to 1 / 2 of the required supply amount, more preferably 1 / 20 to 1 / 3 of the required supply amount, and even more preferably 1 / 15 to 1 / 5 of the required supply amount. Similarly, the period of time is also not a general rule, but it is preferably 1 minute to 15 minutes, more preferably 2 minutes to 10 minutes, and even more preferably 3 minutes to 7 minutes. By setting the certain amount and period of time within the above ranges, contact between the gypsum-containing material and the solution containing the alkali metal salt becomes easier, and as a result, the reaction between gypsum (calcium sulfate) and the alkali metal salt can be promoted. It also facilitates ultrasonic irradiation, which will be described later.

[0033] When employing method (a), the gypsum-containing material may be supplied with the alkali metal salt solution in its original state, or it may be supplied in a mixed solution (slurry) state, with the latter being preferable. By keeping the gypsum-containing material in a mixed solution (slurry) state, ultrasonic irradiation described later becomes easier, and the alkali metal salt solution can more easily come into even contact with the entire surface of the gypsum-containing material, thereby promoting the reaction more efficiently. From a similar viewpoint, it is preferable to supply the alkali metal salt solution intermittently.

[0034] (Method (b)) When Method (b) is adopted, the gypsum-containing material can be immersed in the alkali metal salt solution by supplying it continuously or intermittently, similar to the supply of the alkali metal salt solution in Method (a) above. When the gypsum-containing material is supplied continuously, it is sufficient to supply it without interruption. The required amount may be supplied all at once (also referred to as "bulk supply"), or a fixed amount may be supplied continuously over a fixed period of time. When a fixed amount is supplied continuously over a fixed period of time, for example, a supply rate that allows the entire amount to be supplied in 1 minute or more and 30 minutes or less is preferable, a rate that allows the entire amount to be supplied in 3 minutes or more and 20 minutes or less is more preferable, and a rate that allows the entire amount to be supplied in 5 minutes or more and 10 minutes or less is even more preferable. By setting the supply rate of the gypsum-containing material within the above range, contact between the gypsum-containing material and the alkali metal salt solution becomes easier, and as a result, the reaction between gypsum (calcium sulfate) and alkali metal salt can be promoted. It also makes it easier to perform ultrasonic irradiation, which will be described later.

[0035] When supplying gypsum-containing material intermittently, for example, a certain amount may be supplied, followed by a period of time, and this process may be repeated (also referred to as "intermittent supply"). The supply may be stopped once the required amount has been supplied. So-called "dropping" is also included in intermittent supply. In this case, the certain amount and the period of time can be appropriately determined according to the amount of gypsum-containing material and solution to be in contact with, and there is no general rule. However, for example, the certain amount is preferably 1 / 30 to 1 / 2 of the required supply amount, more preferably 1 / 20 to 1 / 3, and even more preferably 1 / 15 to 1 / 5. Similarly, the period of time is also not a general rule, but it is preferably 1 minute to 15 minutes, more preferably 2 minutes to 10 minutes, and even more preferably 3 minutes to 7 minutes. By setting the certain amount and period of time within the above ranges, contact between the gypsum-containing material and the solution containing the alkali metal salt becomes easier, and as a result, the reaction between gypsum (calcium sulfate) and the alkali metal salt can be promoted. It also facilitates ultrasonic irradiation, which will be described later.

[0036] When employing method (b), the gypsum-containing material may be supplied directly to the solution containing the alkali metal salt and immersed, or it may be supplied and immersed after being in a mixed solution (slurry) state, with the latter being preferable. By keeping the gypsum-containing material in a mixed solution (slurry) state, ultrasonic irradiation described later becomes easier, and the entire surface of the gypsum-containing material can come into even contact with the solution containing the alkali metal salt, thus promoting the reaction more efficiently. From a similar viewpoint, it is preferable to supply the gypsum-containing material intermittently.

[0037] (Ultrasonic Irradiation) The alkali metal reduction method of this embodiment involves applying ultrasonic irradiation when bringing the gypsum-containing material into contact with a solution containing an alkali metal salt. High-purity calcium carbonate can be obtained by applying ultrasonic irradiation. As long as ultrasound is applied during the contact, there are no particular restrictions on the method of ultrasonic irradiation. For example, a reaction vessel for bringing the gypsum-containing material and the solution containing an alkali metal salt into contact can be placed in a tank capable of ultrasonic irradiation, and ultrasonic irradiation can be performed from outside the reaction vessel. Alternatively, an ultrasonic irradiator can be placed inside the reaction vessel, and ultrasonic irradiation can be performed from inside the reaction vessel.

[0038] The ultrasonic waves used for ultrasonic irradiation can be adjusted by controlling the oscillation frequency and output power. While the irradiation conditions, such as oscillation frequency and output power, vary depending on the amount of gypsum-containing material and alkali metal salt solution in contact with the ultrasonic waves, it is not possible to generalize. However, the oscillation frequency is preferably 15 kHz or higher, more preferably 20 kHz or higher, even more preferably 25 kHz or higher, and even more preferably 30 kHz or higher. The upper limit is preferably 70 kHz or lower, more preferably 65 kHz or lower, even more preferably 60 kHz or lower, and even more preferably 50 kHz or lower. The output power is preferably 10 W or higher, more preferably 30 W or higher, even more preferably 50 W or higher. The upper limit is preferably 150 W or lower, more preferably 125 W or lower, and even more preferably 100 W or lower. When the ultrasonic conditions are within the above range, high-purity calcium carbonate can be obtained more efficiently.

[0039] Ultrasonic irradiation may be performed when the gypsum-containing material and the solution containing the alkali metal salt are brought into contact. The irradiation may be performed temporarily or intermittently while the materials are in contact, or it may be performed continuously throughout the entire contact period. From the viewpoint of obtaining high-purity calcium carbonate, continuous irradiation throughout the entire contact period is preferable.

[0040] (Other Steps) The method for reducing alkali metals according to the present embodiment may include a drying step after the contact of the gypsum-containing material with the solution containing an alkali metal salt. The fluid obtained through the contact of the gypsum-containing material with the solution containing an alkali metal salt is a fluid containing water and the like in addition to calcium carbonate. When calcium carbonate is used as a powder, the fluid containing the calcium carbonate may be dried.

[0041] The drying method is not particularly limited as long as water can be removed from the fluid containing calcium carbonate. Examples thereof include a method by solid-liquid separation such as decantation and filtration, a method by heating, and a method by drying under reduced pressure (vacuum drying). When drying by heating, the drying temperature is preferably, for example, 35°C or higher and 160°C or lower.

[0042] (Properties of Calcium Carbonate) In the calcium carbonate obtained by the method for reducing alkali metals according to the present embodiment, the content of alkali metals derived from the alkali metal salt used when converting gypsum (calcium sulfate) contained in the gypsum-containing material into calcium carbonate is reduced, resulting in high-purity calcium carbonate. Specifically, the content of impurity alkali metals (in terms of alkali metal oxide) is 1.00% by mass or less, 0.90% by mass or less, 0.85% by mass or less, 0.80% by mass or less, 0.75% by mass or less, or 0.70% by mass or less.

[0043] In addition, in the calcium carbonate obtained by the method for reducing alkali metals according to the present embodiment, the content of sulfur compounds derived from gypsum (calcium sulfate) contained in the gypsum-containing material is also reduced, resulting in high-purity calcium carbonate. Specifically, the content of gypsum-derived sulfur compounds as impurities (as SO ３ conversion) is 0.80% by mass or less, 0.75% by mass or less, 0.70% by mass or less, 0.65% by mass or less, 0.60% by mass or less, or 0.55% by mass or less. As described above, according to the method for reducing alkali metals of the present embodiment, not only the content of alkali metals contained in calcium carbonate can be reduced, but also the content of sulfur compounds can be reduced, and high-purity calcium carbonate can be obtained.

[0044] Here, the content of impurity alkali metals (in terms of alkali metal oxide) and the content of sulfur compounds (as SO３ conversion) is carried out as follows: calcium carbonate obtained by the method for reducing alkali metals of the present embodiment is dissolved in an acid such as hydrochloric acid to prepare a measurement sample. The content of alkali metals is measured by atomic absorption spectrometry using an atomic absorption spectrophotometer, and sulfur oxides are quantified based on a wet chemical analysis method, specifically by a gravimetric analysis method using barium chloride (also referred to as barium chloride gravimetric method). The measurement can also be performed by simple analysis using fluorescent X-ray.

[0045] Furthermore, the average particle diameter of calcium carbonate obtained by the method for reducing alkali metals of the present embodiment is 6.0 µm or less, 5.5 µm or less, 5.0 µm or less, 4.5 µm or less, 4.4 µm or less, 4.0 µm or less, or 3.5 µm or less. As described above, calcium carbonate obtained by the method for reducing alkali metals of the present embodiment has a small average particle diameter. This is considered to be attributed to ultrasonic irradiation performed when bringing the gypsum-containing material into contact with the solution containing the alkali metal salt, as can be seen from the results of Examples and Comparative Examples described later. Here, the average particle diameter of calcium carbonate is the average particle diameter (D50) at a cumulative frequency of 50% in a volume-based particle size distribution measured by a laser diffraction scattering particle size distribution measurement method.

[0046] [Method for Producing Calcium Calcium] The method for producing calcium carbonate according to the present embodiment is a method for producing calcium carbonate, comprising a step of bringing a gypsum-containing material into contact with a solution containing at least one alkali metal salt selected from the group consisting of alkali metal carbonates and alkali metal bicarbonates to obtain calcium carbonate, wherein ultrasonic irradiation is performed during the contact.

[0047] In the calcium carbonate production method of this embodiment, the process of obtaining calcium carbonate by contacting a gypsum-containing material with a solution containing at least one alkali metal salt, such as an alkali metal carbonate or an alkali metal bicarbonate, and the application of ultrasonic irradiation during contact, can be directly applied as described in the alkali metal reduction method of this embodiment. Furthermore, the properties of the calcium carbonate obtained by the calcium carbonate production method of this embodiment can be directly applied as those of the calcium carbonate obtained in the alkali metal reduction method of this embodiment.

[0048] The present invention will be described in detail below with reference to examples, but the present invention is not limited in any way to these examples.

[0049] (Measurement of alkali metal and sulfur compound content) Approximately 1 g of calcium carbonate obtained in the examples and comparative examples was taken as a calcium carbonate sample, and 40 mL of water and hydrochloric acid (1:1) were added to the calcium carbonate sample to prepare the measurement sample. The alkali metal content (in alkali metal oxide equivalent) of the measurement sample was measured using flame atomic absorption spectrometry with an atomic absorption spectrophotometer ("7J1-8010 (model number)", manufactured by Hitachi High-Tech Science Corporation) (the wavelengths used for atomic absorption were Na: 589.0 nm, K: 766.5 nm, and the lamp current value was 10 mA). The sulfur compound content (SO ３ The conversion was measured based on the barium chloride gravimetric method.

[0050] (Measurement of average particle size) For the calcium carbonate obtained in the examples and comparative examples, the average particle size (D50) at a cumulative frequency of 50% in the volume-based particle size distribution measured using a laser diffraction scattering particle size distribution analyzer (Microtrac MT2000, manufactured by Nikkiso Co., Ltd.) was defined as the average particle size.

[0051] (Example 1) A glass container (capacity: 500 cc) was placed in the cleaning tank of a tabletop ultrasonic cleaner ("US-101 (model number)", manufactured by SND Corporation), and waste gypsum board (17.2 g (0.1 mol) of gypsum (calcium sulfate hydrate)) was placed inside. The ultrasonic generator (frequency: 38 kHz, output: 55 W) was started. A sodium carbonate aqueous solution, prepared by dissolving 10.6 g (0.1 mol) of sodium carbonate in 83.8 ml of water, was supplied all at once (bulk supply) to adjust the sodium carbonate content to 1.0 M based on the total amount of gypsum-containing material and sodium carbonate aqueous solution (see "Alkali Metal Salt Content" in the table). After supplying the sodium carbonate aqueous solution all at once (bulk supply) to bring the gypsum-containing material and sodium carbonate aqueous solution into contact, it was left for 60 minutes, and then the fluid in the container was dried at 40°C to obtain calcium carbonate powder. The alkali metal and sulfur compound content and average particle size of the obtained calcium carbonate were measured using the method described above. The results are shown in Table 1-1.

[0052] (Examples 2 and 3) Calcium carbonate powder was obtained in the same manner as in Example 1, except that the sodium carbonate content, based on the total amount of gypsum-containing material and sodium carbonate aqueous solution, was set to the concentration shown in Table 1-1. The alkali metal and sulfur compound content and average particle size of the obtained calcium carbonate were measured using the method described above. The results are shown in Table 1-1.

[0053] (Examples 4 and 5) Except that the frequency and output of the ultrasonic generator and the sodium carbonate content (indicated as "alkali metal salt content" in the table) based on the total amount of gypsum-containing material and sodium carbonate aqueous solution were set to the conditions shown in Table 1-1, the calcium carbonate powders of Examples 4 and 5 were obtained in the same manner as in Example 1. The alkali metal and sulfur compound content and the average particle size of the obtained calcium carbonate were measured using the method described above. The results are shown in Table 1-1.

[0054] (Example 6) In Example 1, the solution containing the alkali metal salt was divided into 10 equal parts (referred to as solutions 1 to 10), and solution 1 was supplied, followed by solution 2 after 5 minutes, and this process was repeated until solution 10 was supplied (intermittent supply). Calcium carbonate powder was obtained in the same manner as in Example 1. The alkali metal and sulfur compound content and average particle size of the obtained calcium carbonate were measured using the method described above. The results are shown in Table 1-1.

[0055] (Examples 7 and 8) Calcium carbonate powder was obtained in the same manner as in Example 6, except that the sodium carbonate content (indicated as "alkali metal salt content" in the table) based on the total amount of gypsum-containing material and sodium carbonate aqueous solution was set to the concentration shown in Table 1-1. The alkali metal and sulfur compound content and average particle size of the obtained calcium carbonate were measured using the method described above. The results are shown in Table 1-1.

[0056] (Examples 9 and 10) Except for the frequency and output of the ultrasonic generator and the sodium carbonate content (indicated as "alkali metal salt content" in the table) relative to the total amount of gypsum-containing material and sodium carbonate aqueous solution being the conditions shown in Table 1-1, calcium carbonate powders of Examples 9 and 10 were obtained in the same manner as in Example 6. The alkali metal and sulfur compound content and average particle size of the obtained calcium carbonate were measured using the method described above. The results are shown in Table 1-1.

[0057]

[0058] (Comparative Examples 1-6) In Example 1, the sodium carbonate content (indicated as "alkali metal salt content" in the table) was set to the concentration shown in Table 1-2, based on the total amount of gypsum-containing material and sodium carbonate aqueous solution. Calcium carbonate powder was obtained in the same manner as in Example 1, except that ultrasonic irradiation was not performed. The alkali metal and sulfur compound content, as well as the average particle size, of the obtained calcium carbonate were measured using the method described above. The results are shown in Table 1-2.

[0059]

[0060] (Examples 11-13) Calcium carbonate powder was obtained in the same manner as in Example 1, except that the alkali metal salt was changed from sodium carbonate to potassium carbonate, and the sodium carbonate content (indicated as "alkali metal salt content" in the table) based on the total amount of gypsum-containing material and sodium carbonate aqueous solution was set to the concentration shown in Table 2-1. The alkali metal and sulfur compound content and average particle size of the obtained calcium carbonate were measured using the method described above. The results are shown in Table 2-1.

[0061] (Examples 14-16) Calcium carbonate powder was obtained in the same manner as in Example 6, except that the alkali metal salt was changed from sodium carbonate to potassium carbonate, and the sodium carbonate content (indicated as "alkali metal salt content" in the table) based on the total amount of gypsum-containing material and sodium carbonate aqueous solution was set to the concentration shown in Table 2-1. The alkali metal and sulfur compound content and average particle size of the obtained calcium carbonate were measured using the method described above. The results are shown in Table 2-1.

[0062] (Comparative Examples 7-9) In Example 1, the alkali metal salt was changed from sodium carbonate to potassium carbonate, the sodium carbonate content (indicated as "alkali metal salt content" in the table) based on the total amount of gypsum-containing material and sodium carbonate aqueous solution was set to the concentration shown in Table 2-2, and ultrasonic irradiation was not performed. Calcium carbonate powder was obtained in the same manner as in Example 1. The alkali metal and sulfur compound content, as well as the average particle size, of the obtained calcium carbonate were measured using the method described above. The results are shown in Table 2-2.

[0063]

[0064]

[0065] From the results of the examples, it was confirmed that the alkali metal reduction method and calcium carbonate production method of this embodiment can reduce the alkali metal content in the obtained calcium carbonate, and also reduce the sulfur compound content. Furthermore, it was confirmed that the average particle size (D50) of the obtained calcium carbonate was small, at 3.1 μm and 3.5 μm when sodium carbonate was used as the alkali metal salt, and at 4.0 μm and 6.9 μm when potassium carbonate was used.

[0066] In contrast, the results from Comparative Examples 1 to 6 confirmed that when ultrasonic irradiation was not performed and sodium carbonate was used as the alkali metal salt, the alkali metal content in the resulting calcium carbonate was higher compared to when ultrasonic irradiation was performed. Here, it was confirmed that the alkali metal and sulfur compound content in the resulting calcium carbonate increased or decreased depending on the amount of sodium carbonate (indicated as "alkali metal salt content" in the table) relative to the total amount of gypsum-containing material and sodium carbonate aqueous solution, although no consistent trend was observed. Therefore, the comparison of the alkali metal content in the calcium carbonate between the examples and comparative examples can be specifically confirmed by comparing, for example, Example 1 and Comparative Example 2, Example 2 and Comparative Example 4, and Example 3 and Comparative Example 5, where the alkali metal salt content and solution supply method are the same as shown in the table. Furthermore, the results from Comparative Examples 1 to 6 confirmed that the sulfur compound content was also high, and it was confirmed that the calcium carbonate obtained in Comparative Examples 1 to 6 could not be said to be of high purity.

[0067] Furthermore, the results from Comparative Examples 7 to 9 showed that even when ultrasonic irradiation was not performed and potassium carbonate was used as the alkali metal salt, the same results as those obtained when sodium carbonate was used were obtained.

Claims

1. A method for reducing alkali metals contained in calcium carbonate, comprising the step of contacting a gypsum-containing material with a solution containing at least one alkali metal salt, such as an alkali metal carbonate or an alkali metal bicarbonate, to obtain calcium carbonate, wherein ultrasonic irradiation is performed during the contact.

2. The method for reducing alkali metals according to claim 1, wherein the solution containing the alkali metal salt is supplied to the gypsum-containing material.

3. The method for reducing alkali metals according to claim 2, wherein the solution containing the alkali metal salt is supplied continuously or intermittently.

4. The method for reducing alkali metals according to claim 1, wherein the gypsum-containing material is immersed in the solution containing the alkali metal salt.

5. The method for reducing alkali metals according to claim 1, wherein the content of alkali metal salts in the solution is 0.01 M or more and 1.5 M or less.

6. The method for reducing alkali metals according to claim 1, wherein the gypsum-containing material is waste gypsum board.

7. A method for producing calcium carbonate, comprising the step of contacting a gypsum-containing material with a solution containing at least one alkali metal salt, such as an alkali metal carbonate or an alkali metal bicarbonate, wherein ultrasonic irradiation is performed during the contact.

8. The method for producing calcium carbonate according to claim 7, wherein the solution containing the alkali metal salt is supplied to the gypsum-containing material.

9. The method for producing calcium carbonate according to claim 8, wherein the solution containing the alkali metal salt is supplied continuously or intermittently.

10. A method for producing calcium carbonate according to claim 7, wherein the gypsum-containing material is immersed in a solution containing the alkali metal salt.

11. The method for producing calcium carbonate according to claim 7, wherein the content of alkali metal salt in the solution is 0.01 M or more and 1.5 M or less.

12. The method for producing calcium carbonate according to claim 7, wherein the gypsum-containing material is waste gypsum board.

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