Sodium carbonate manufacturing method

Abstract

To improve the production efficiency of sodium carbonate by industrially and stably producing sodium carbonate with low content of coarse and fine powder, and to provide a simple and economical sodium carbonate production method.SOLUTION: This invention provides a method for producing sodium carbonate by contacting carbon dioxide with an aqueous sodium hydroxide solution, wherein the pH of the aqueous solution containing sodium carbonate after contacting carbon dioxide with the aqueous sodium hydroxide solution is adjusted to 13.0 to 14.5 to precipitate a sodium carbonate monohydrate, then the sodium carbonate monohydrate is obtained by solid-liquid separation, the sodium carbonate monohydrate is then dispersed in water or an aqueous solution containing sodium carbonate to obtain a fluid dispersion of sodium carbonate monohydrate, and the dispersion is then brought into contact with carbon dioxide.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a method for producing sodium carbonate. More specifically, the present invention relates to a method for producing sodium carbonate that can industrially and stably produce sodium carbonate with a low content of fine powder and coarse powder. [Background technology]

[0002] Sodium carbonate is a compound that is widely used as an alkaline agent in various industries, a raw material for glass, and the like (see Patent Document 1). Sodium carbonate is produced, for example, by the ammonia-soda process. That is, in the ammonia-soda process, limestone is heated to generate carbon dioxide gas, which is brought into contact with an aqueous solution containing ammonia and table salt (carbonation step) to obtain a slurry containing sodium bicarbonate. Next, the sodium bicarbonate is separated from the slurry containing sodium bicarbonate into solid and liquid, and the obtained sodium bicarbonate is heated to produce sodium carbonate.

[0003] The sodium carbonate obtained by the above production method is so-called light ash, which has a small particle size and a low bulk density. This light ash can be difficult to handle depending on the application. Therefore, by spraying water on the light ash, sodium carbonate monohydrate with a large particle size and bulk density is produced, and the sodium carbonate monohydrate is dried, sodium carbonate with a large particle size and bulk density (so-called dense ash) can be produced.

[0004] Alternatively, sodium carbonate can be first dissolved in water to prepare an aqueous solution containing sodium carbonate, and then the water in the aqueous solution can be evaporated to precipitate sodium carbonate monohydrate with a large particle size and bulk density. Sodium carbonate monohydrate isolated by solid-liquid separation can then be dried to produce sodium carbonate with a large particle size and bulk density (so-called dense ash).

[0005] As described above, it is known that the particle size, bulk density, etc. of sodium carbonate monohydrate affect the particle size, bulk density, etc. of the finally obtained sodium carbonate. It is also known that fine powder and coarse powder precipitate during precipitation of sodium carbonate monohydrate. Therefore, various production methods have been proposed for reducing the precipitation of fine powder during precipitation of sodium carbonate monohydrate. Specifically, a method has been proposed for producing sodium carbonate monohydrate with a large particle size and bulk density by contacting an aqueous sodium hydroxide solution with carbon dioxide gas to produce sodium carbonate monohydrate at a predetermined temperature and adjusting the concentration of sodium hydroxide in the aqueous sodium hydroxide solution to fall within a predetermined range (see Patent Document 2). [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Special Publication No. 46-033215 [Patent Document 2] Special Publication No. 46-012291 [Patent Document 3] Special Publication No. 55-049007 Summary of the Invention [Problem to be solved by the invention]

[0007] Although the method disclosed in Patent Document 2 above can produce sodium carbonate monohydrate with a large particle size and bulk density, it was found that, because a liquid containing sodium hydroxide adheres to the sodium carbonate monohydrate obtained by solid-liquid separation, agglomerated particles of sodium carbonate are by-produced when the sodium carbonate is dried to produce sodium carbonate, and corrosion occurs on the inner surface of the dryer, leaving room for improvement in terms of industrially stable production. Meanwhile, a method has also been proposed in which sodium carbonate monohydrate to which sodium hydroxide has adhered is fluidized-dried using a carbon dioxide-containing gas, and fine sodium carbonate monohydrate is separated and removed, and then the sodium carbonate monohydrate is calcined to obtain sodium carbonate (see Patent Document 3).

[0008] However, the method disclosed in Patent Document 3 still leaves room for improvement in that clogging of the fluidized bed dryer is likely to occur due to fine sodium carbonate monohydrate, which is a by-product of the adhering sodium hydroxide and carbon dioxide, and that multiple drying steps are required.

[0009] That is, an object of the present invention is to provide a simple, economical method for producing sodium carbonate by industrially and stably producing sodium carbonate having a small content of coarse powder and fine powder, thereby improving the production efficiency of sodium carbonate. [Means for solving the problem]

[0010] In view of the above problems, the present inventors have conducted extensive research. As a result, it has been found that in the reaction of sodium hydroxide solution with carbon dioxide, a solid precipitates until the reaction proceeds to a certain extent, and this solid is sodium carbonate monohydrate with little coarse or fine powder. However, it has also been found that if the reaction proceeds further, the precipitated solid redissolves and shrinks.

[0011] Therefore, the inventors have investigated methods for precipitating sodium carbonate monohydrate while suppressing redissolution in solution, and have found that sodium carbonate monohydrate having a large particle size and bulk density can be produced by adjusting the pH of an aqueous solution containing sodium carbonate monohydrate after contacting carbon dioxide with an aqueous sodium hydroxide solution within a predetermined range. Furthermore, the inventors have found that even when the obtained sodium carbonate monohydrate is separated by solid-liquid separation, sodium hydroxide contained in the aqueous solution that was not separated into the sodium carbonate monohydrate crystals remains, but that sodium hydroxide can be removed by preparing a dispersion of the sodium carbonate monohydrate in water or an aqueous sodium carbonate solution and contacting the dispersion with carbon dioxide, which has led to the completion of the present invention.

[0012] That is, the present invention is a method for producing sodium carbonate by contacting carbon dioxide with an aqueous sodium hydroxide solution, 35 to 108 degrees CelsiusAfter the contact, the pH of the aqueous solution containing sodium carbonate is adjusted to 13.0 to 14.5 to precipitate sodium carbonate monohydrate, and then the sodium carbonate monohydrate is obtained by solid-liquid separation. The obtained sodium carbonate monohydrate is then dispersed in water or an aqueous solution containing sodium carbonate. Sodium carbonate monohydrate is 20 to 50% by mass A method for producing sodium carbonate, comprising obtaining a dispersion of sodium carbonate monohydrate and then contacting the dispersion with carbon dioxide.

[0013] The present invention can preferably adopt the following aspects.

[0014] (1) Obtaining sodium carbonate monohydrate from the dispersion of sodium carbonate monohydrate that has been contacted with carbon dioxide by solid-liquid separation, and then drying the obtained sodium carbonate monohydrate to obtain sodium carbonate. (2) The concentration of the aqueous sodium hydroxide solution is higher than 25% by mass. (3) The solid-liquid separation is performed using a wet cyclone. (4) The liquid separated by the solid-liquid separation is used as an aqueous sodium hydroxide solution to be contacted with carbon dioxide. (5) The carbon dioxide to be brought into contact with the aqueous sodium hydroxide solution is exhaust gas containing carbon dioxide. (6) The carbon dioxide-containing exhaust gas in (5) above is exhaust gas generated in the ammonia-soda process. [Effects of the Invention]

[0015] The present invention is characterized by precipitating sodium carbonate monohydrate by adjusting the pH of the aqueous solution after contacting carbon dioxide with an aqueous sodium hydroxide solution within a predetermined range, and then subjecting the precipitated sodium carbonate monohydrate to solid-liquid separation, dispersing it in water or an aqueous solution containing sodium carbonate, and contacting the dispersion with carbon dioxide. This production method makes it possible to obtain sodium carbonate monohydrate with a low content of coarse and fine particles. The obtained sodium carbonate monohydrate can then be dried to obtain sodium carbonate (dense ash) with a low content of coarse and fine particles.

[0016] The sodium carbonate monohydrate separated by solid-liquid separation is dispersed in water or an aqueous solution of sodium carbonate to obtain a dispersion of sodium carbonate monohydrate, and then the dispersion is brought into contact with carbon dioxide. This makes it possible to remove sodium hydroxide remaining in the aqueous solution that was not separated from the separated sodium carbonate monohydrate, thereby preventing the by-production of agglomerated sodium carbonate particles and corrosion of a dryer when the obtained sodium carbonate monohydrate is dried to produce anhydrous sodium carbonate.

[0017] As described above, the sodium carbonate obtained by the production method of the present invention contains little coarse or fine powder, and therefore, operations such as sieving to remove these powders are not required. This enables the production process to be shortened, and dense ash sodium carbonate to be produced efficiently, thereby greatly increasing the industrial applicability of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0018] As described above, the present invention is characterized by precipitating sodium carbonate monohydrate by adjusting the pH of the aqueous solution after contacting carbon dioxide with an aqueous sodium hydroxide solution within a predetermined range, and then subjecting the precipitated sodium carbonate monohydrate to solid-liquid separation, dispersing it in water or an aqueous solution containing sodium carbonate, and contacting the dispersion with carbon dioxide. This production method makes it possible to obtain sodium carbonate with a low content of coarse powder and fine powder. In the present invention, "coarse powder" refers to the powder remaining on a sieve with a mesh size of 1000 μm when sodium carbonate is sieved. Furthermore, "fine powder" in the present invention refers to the powder that passes through a sieve with a mesh size of 125 μm when sodium carbonate is sieved.

[0019] Although the details of why the above-described production method of the present invention can produce sodium carbonate with a low content of coarse and fine particles are unclear, the present inventors speculate as follows. Specifically, after the reaction of carbon dioxide with an aqueous sodium hydroxide solution, the reaction is carried out under conditions in which the sodium hydroxide is completely consumed, resulting in the production of an aqueous sodium carbonate solution. Sodium carbonate monohydrate can be precipitated from the resulting aqueous sodium carbonate solution by distilling off water through heating. However, it is speculated that the low pH of this aqueous solution results in the production of coarse and fine particles. On the other hand, in the reaction in which carbon dioxide gas is blown into an aqueous sodium hydroxide solution, the pH of the solution is high until the reaction has progressed to a certain extent, resulting in the precipitation of sodium carbonate monohydrate with a low content of coarse and fine particles. The solubility of sodium carbonate monohydrate in an aqueous solution varies significantly depending on the pH of the aqueous solution. It is speculated that as the pH of the aqueous solution decreases as the reaction progresses, the precipitated sodium carbonate monohydrate redissolves in the solution, resulting in a reduction in particle size.

[0020] In the production method of the present invention, it is believed that dissolution of sodium carbonate monohydrate can be suppressed by precipitating sodium hydroxide monohydrate while retaining sodium hydroxide by adjusting the pH of the aqueous solution to a predetermined alkaline range. Additionally, it is believed that separating the precipitated sodium carbonate monohydrate from the aqueous solution containing residual sodium hydroxide by solid-liquid separation can separate sodium carbonate monohydrate and sodium hydroxide. Furthermore, it is believed that dispersing the separated sodium carbonate monohydrate in water or an aqueous solution containing sodium carbonate can minimize pH changes in the aqueous solution due to reaction with carbon dioxide, thereby suppressing dissolution of sodium carbonate monohydrate. Therefore, it is believed that sodium carbonate monohydrate with a low content of coarse and fine particles can be obtained, and that drying this sodium carbonate monohydrate can yield sodium carbonate with a low content of coarse and fine particles.

[0021] Furthermore, sodium hydroxide adhering to the separated sodium carbonate monohydrate is removed by the reaction with carbon dioxide, and it is presumed that this can suppress the by-production of sodium carbonate agglomerated particles and corrosion inside a dryer when sodium carbonate is produced by drying the sodium carbonate monohydrate separated by solid-liquid separation from the dispersion after contact with carbon dioxide.

[0022] In this specification, unless otherwise specified, the expression "A to B" for numerical values ​​A and B means "A or more and B or less." In such an expression, when a unit is assigned only to numerical value B, the unit also applies to numerical value A. The method for producing sodium carbonate of the present invention will be described in detail below.

[0023] (carbon dioxide) The carbon dioxide used in the production method of the present invention is not particularly limited and can be in any industrially applicable form. Specific examples include gases containing carbon dioxide (so-called carbon dioxide gas). Incidentally, carbon dioxide-containing exhaust gases may also be used. Examples of carbon dioxide-containing exhaust gases include exhaust gases emitted from various processes. Specific examples include exhaust gases generated when fossil fuels such as oil and coal are burned in thermal power plants or combustion boilers, exhaust gases emitted in the ammonia-soda process, and exhaust gases generated when sludge is burned. Generally, the exhaust gases generated when fossil fuels such as oil and coal are burned in thermal power plants or combustion boilers are controlled within a certain range of combustion conditions, and the amount of carbon dioxide in the exhaust gases is often within a certain range. On the other hand, the amount of carbon dioxide generated in exhaust gases generated when sludge is burned varies depending on the type of sludge, and the amount of carbon dioxide may vary depending on the combustion operation. Furthermore, in the ammonia-soda process, calcium chloride, ammonium chloride, and other substances are produced in addition to sodium carbonate, and the amount of carbon dioxide emitted varies depending on the balance of their production volumes. In the production method of the present invention, any of the above carbon dioxide-containing exhaust gases can be suitably used. In particular, the production method of the present invention is suitable for use with such exhaust gases, since it is possible to produce sodium hydroxide while suppressing residual sodium hydroxide even when the amount of carbon dioxide emitted fluctuates.

[0024] The exhaust gas may contain nitrogen oxides (NOx), sulfur oxides (SOx), and heavy metals, but these impurities may be removed in advance by known methods before being subjected to the production method of the present invention.

[0025] (sodium hydroxide solution) The aqueous sodium hydroxide solution used in the production method of the present invention is not particularly limited, and aqueous solutions of various concentrations can be used. As described above, the solubility of sodium carbonate monohydrate is affected by the concentration, pH, etc. of the aqueous solution. Therefore, from the viewpoint of increasing the amount of precipitated sodium carbonate monohydrate, the concentration of sodium hydroxide in the aqueous sodium hydroxide solution is more preferably in the range of more than 25% by mass, and particularly preferably in the range of 48 to 60% by mass.

[0026] (Contact between carbon dioxide and aqueous sodium hydroxide solution) In the production method of the present invention, carbon dioxide is brought into contact with an aqueous sodium hydroxide solution. This contact causes a reaction between the carbon dioxide and sodium hydroxide, resulting in a slurry containing sodium carbonate monohydrate. Known methods can be used to bring the carbon dioxide and aqueous sodium hydroxide solution into contact in the production method of the present invention. Specifically, a method can be used in which a gas containing carbon dioxide is blown into a vessel filled with an aqueous sodium hydroxide solution, and the gas is brought into contact with the aqueous sodium hydroxide solution.

[0027] In the production method of the present invention, the temperature at which carbon dioxide is brought into contact with the aqueous sodium hydroxide solution may be any temperature at which sodium carbonate monohydrate is produced, and is typically within the range of 35 to 108°C. If the temperature is lower than 35°C, sodium carbonate heptahydrate tends to precipitate, while if the temperature is higher than 108°C, anhydrous sodium carbonate tends to precipitate. In either case, it tends to be difficult to control the particle size of the precipitated solid. From the viewpoint of efficiently precipitating the sodium carbonate monohydrate, the temperature at which carbon dioxide is brought into contact with the aqueous sodium hydroxide solution is preferably within the range of 60 to 106°C, and particularly preferably within the range of 80 to 106°C.

[0028] Furthermore, in order to increase the amount of precipitated sodium carbonate monohydrate, the aqueous solution may be heated and concentrated to remove water during contact between the carbon dioxide and the aqueous sodium hydroxide solution.

[0029] (pH control of aqueous solutions containing sodium carbonate) In the production method of the present invention, it is necessary to adjust the pH of the aqueous solution containing sodium carbonate after contacting carbon dioxide with the aqueous sodium hydroxide solution to 13.0 to 14.5. As described above, by adjusting the pH to within this range, it is possible to precipitate sodium carbonate monohydrate with a low content of coarse powder and fine powder. From the viewpoint of efficiently precipitating the sodium carbonate monohydrate, the pH of the aqueous solution containing sodium carbonate is preferably 14.0 to 14.4, and particularly preferably 14.2 to 14.3.

[0030] The method for adjusting the pH of the aqueous solution containing sodium carbonate to 13.0 to 14.5 is not particularly limited, and can be carried out, for example, by adjusting the amounts of carbon dioxide and sodium hydroxide to be contacted.

[0031] (Solid-liquid separation of sodium carbonate monohydrate) The production method of the present invention described above can produce a slurry containing sodium carbonate monohydrate. The obtained sodium carbonate monohydrate can be separated by known methods such as a wet cyclone or a filter press. Although sodium hydroxide remains in the slurry, most of the sodium hydroxide can be separated by solid-liquid separation.

[0032] It is preferable to carry out solid-liquid separation using a wet cyclone, as this has a high effect of reducing the water content in the separated sodium carbonate monohydrate and has a simple structure.

[0033] The solution after solid-liquid separation contains sodium carbonate and sodium hydroxide dissolved in the solution, and therefore, the solution may be used as an aqueous sodium hydroxide solution to be contacted with carbon dioxide.

[0034] (Contact of separated sodium carbonate monohydrate with carbon dioxide) The sodium carbonate monohydrate isolated by the production method of the present invention contains approximately 0.5 to 5.0 mass% residual sodium hydroxide. When this sodium carbonate monohydrate is dried directly to obtain anhydrous sodium carbonate, there is a risk of by-production of agglomerated sodium carbonate particles and corrosion due to exposure to high temperatures inside the dryer. Therefore, in the production method of the present invention, the separated sodium carbonate monohydrate is dispersed in water or an aqueous solution containing sodium carbonate to prepare a dispersion of sodium carbonate monohydrate, and carbon dioxide is brought into contact with the dispersion to react the sodium hydroxide with carbon dioxide. By bringing the dispersion into contact with carbon dioxide, the sodium hydroxide in the dispersion reacts with carbon dioxide to produce sodium carbonate monohydrate, thereby removing the sodium hydroxide.

[0035] The amount of water used for the dispersion is preferably as small as possible to dissolve the precipitate of sodium carbonate monohydrate. Furthermore, in order to inhibit dissolution of sodium carbonate monohydrate in the dispersion, it is preferable to dissolve sodium carbonate in water in advance and use an aqueous solution containing sodium carbonate as the dispersion medium. The concentration of sodium carbonate in the aqueous solution containing sodium carbonate can be determined appropriately depending on the production conditions, but is usually in the range of 25.0 to 30.5% by mass.

[0036] The concentration of the sodium carbonate monohydrate slurry used in producing the sodium carbonate monohydrate dispersion may be appropriately set within a range of 20 to 50% by mass from the viewpoints of efficient removal of sodium hydroxide and the solubility of sodium carbonate monohydrate. A specific example of a method for contacting a sodium carbonate monohydrate dispersion with carbon dioxide is to blow a carbon dioxide-containing gas into a container filled with the dispersion and bring the gas into contact with an aqueous sodium hydroxide solution. The carbon dioxide used for contact should be in an amount sufficient to consume the sodium hydroxide in the dispersion. When the dispersion medium is water, a portion of the sodium carbonate monohydrate dissolves in water, and when the dispersion medium is an aqueous solution containing sodium carbonate, sodium carbonate dissolves in the dispersion medium. Therefore, when the sodium hydroxide in the dispersion is consumed, the pH of the dispersion becomes the pH of the aqueous sodium carbonate solution (11.9 to 12.2). Therefore, the pH of the dispersion can be checked, and the contact with carbon dioxide can be continued until the pH reaches that of the aqueous sodium carbonate solution.

[0037] The temperature during contact is not particularly limited, and it is usually sufficient to carry out the contact at a temperature in the range of 35 to 108° C. Furthermore, in order to increase the amount of precipitated sodium carbonate monohydrate, the aqueous solution may be heated and concentrated to distill off water.

[0038] The sodium carbonate monohydrate obtained after removing sodium hydroxide can be isolated by a known method such as a filter press. The isolated sodium carbonate monohydrate contains approximately 1 to 10% by mass of water. By drying the isolated sodium carbonate monohydrate using a steam tube dryer or the like, the water content is removed and anhydrous sodium carbonate can be obtained. This production method allows sodium carbonate with a low content of coarse powder or fine powder to be obtained. Furthermore, the solution after isolation contains sodium carbonate dissolved in the solution. Therefore, the solution may be used as a dispersion of sodium carbonate monohydrate.

[0039] The drying temperature in the drying step may be set to a temperature sufficient to produce an anhydrous product, and may be appropriately set within the range of 150 to 180°C. The drying time may also be set to a time sufficient to produce an anhydrous product, and when drying is performed within the above temperature range, 0.5 to 2.0 hours is usually sufficient. When sodium carbonate monohydrate is dried using a dryer such as a steam tube dryer and steam is used as a heat source, the steam discharged from the dryer can be used as a heat source when the exhaust gas containing carbon dioxide is brought into contact with the aqueous sodium hydroxide solution, or when the aqueous sodium carbonate solution and dispersion are heated and concentrated to produce sodium carbonate monohydrate.

[0040] (Sodium carbonate obtained by the production method of the present invention) The production method of the present invention can produce sodium carbonate with a low content of coarse powder and fine powder. Specifically, the obtained sodium carbonate has a bulk density of 1.1 to 1.3 kg / L as measured according to JIS K1201-1, and contains 3% by mass or less of coarse powder remaining on a sieve with 1 mm openings and 6% by mass or less of fine powder passing through a sieve with 125 μm openings. [Example]

[0041] The present invention will be described in more detail below by way of examples, but the present invention is not limited to these examples. The physical properties of the sodium carbonate obtained in the following examples and comparative examples were evaluated by the following methods.

[0042] <Physical property evaluation method> The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples. In the following examples and comparative examples, an aqueous solution containing sodium carbonate The physical properties of the sodium carbonate and the corrosion of the dryer were evaluated by the following methods.

[0043] <Method for analyzing aqueous solutions containing sodium carbonate> pH Measurement was performed using a pH meter (Mettler Toledo SevenCompact S220). -Analysis of aqueous solutions (sodium carbonate concentration, sodium bicarbonate concentration, sodium hydroxide concentration) 5 g of the resulting solution was accurately weighed into a 100 mL conical beaker, and several drops of phenolphthalein were added. While stirring well, the solution was titrated with 1 mol / L hydrochloric acid until the pink color disappeared. Several drops of methyl orange were added to the solution, and while stirring well, the solution was titrated with 1 mol / L hydrochloric acid until the yellow color turned orange. The concentrations of sodium carbonate, sodium bicarbonate, and sodium hydroxide were calculated from the titration volume.

[0044] <Physical property evaluation method> ·Particle size distribution 100 g of the obtained sodium carbonate was shaken for 5 minutes in a low-tap sieve shaker (without tap) and sieved. The sieve sizes were as shown below, and the mass of sodium carbonate remaining on the sieves was measured. The average particle size was calculated by taking the weighted average of the mass of sodium carbonate remaining on each sieve. Sieves used in the sieve shaker: 1000 μm, 500 μm, 250 μm, 180 μm, 150 μm, 125 μm, and receiver Bulk density The measurement was carried out according to the method described in JIS K1201-1.

[0045] <Dryer corrosion evaluation method> The sodium carbonate monohydrate obtained after centrifugation in each example and comparative example was dried at 180°C for a residence time of 1.0 hour using a rotary dryer made of SUS304. The dryer was operated using a continuous input / output method for 8 hours, and the corrosion state before and after use was confirmed visually and by penetrant testing.

[0046] Example 1 A sodium hydroxide solution adjusted to 48% by mass was fed into a reaction tank (10 L) at a rate of 1200 g / h, and a carbon dioxide-containing gas (carbon dioxide concentration 15% by volume) was fed at a rate of 3000 NL / h. The agitator rotation speed was set to 500 rpm. The reaction temperature was set to 80°C, and sodium hydroxide and carbon dioxide were reacted to precipitate sodium carbonate monohydrate. The resulting slurry containing sodium carbonate monohydrate was withdrawn from the reaction tank at a rate of 2400 g / h. The pH of the slurry containing sodium carbonate monohydrate was 14.1. Sodium carbonate monohydrate was separated from the slurry using a wet cyclone. The separated sodium carbonate monohydrate had a water content of 31% by mass and a sodium hydroxide content of 1.5% by mass.

[0047] 1200 g of the sodium carbonate monohydrate obtained by the hydrocyclone was dispersed in 1400 g of a dispersion liquid in which the sodium carbonate concentration was adjusted to 30% by mass, and the dispersion was reacted with a gas containing carbon dioxide (carbon dioxide concentration 15% by volume) to remove the remaining sodium hydroxide. The pH of the slurry after sodium hydroxide removal was 12.0.

[0048] The resulting sodium carbonate monohydrate slurry was then separated into sodium carbonate monohydrate using a centrifuge, and the sodium carbonate monohydrate crystals were placed in a rotary dryer and dried at 180°C, adjusting the crystals' residence time in the dryer to 1.0 hour, to yield 750g of anhydrous sodium carbonate crystals. Evaluation of the resulting crystals revealed a coarse powder content of 2.2% by mass, a fine powder content of 4.5% by mass, and a bulk density of 1.25 kg / L. No corrosion was observed inside the dryer after the crystals were removed.

[0049] Examples 2 to 4 Sodium carbonate (anhydrous) was produced under the same conditions as in Example 1, except for the conditions shown in Table 1. Table 1 shows the results of evaluation of the physical properties of the obtained sodium carbonate crystals and the results of evaluation of dryer corrosion.

[0050] [Table 1]

[0051] Comparative Examples 1-2 A reaction between an aqueous sodium hydroxide solution and carbon dioxide was carried out under the conditions shown in Table 1. Sodium carbonate was obtained in the same manner as in Example 1, except that the slurry containing sodium carbonate monohydrate extracted from the reaction vessel was directly separated into sodium carbonate monohydrate using a centrifuge, and the sodium carbonate monohydrate crystals were dried in a rotary dryer. The results of evaluation of the physical properties of the obtained sodium carbonate crystals are shown in Table 1.

Claims

1. A method for producing sodium carbonate by contacting carbon dioxide with an aqueous sodium hydroxide solution, comprising the steps of: the carbon dioxide is contacted with the aqueous sodium hydroxide solution at 35 to 108°C, and then the pH of the aqueous sodium carbonate solution is adjusted to 13.0 to 14.5 to precipitate sodium carbonate monohydrate; Then, sodium carbonate monohydrate is obtained by solid-liquid separation, The method for producing sodium carbonate comprises dispersing the obtained sodium carbonate monohydrate in water or an aqueous solution containing sodium carbonate to obtain a sodium carbonate monohydrate dispersion containing 20 to 50 mass % of sodium carbonate monohydrate, and then contacting the dispersion with carbon dioxide.

2. obtaining sodium carbonate monohydrate by solid-liquid separation from the dispersion of sodium carbonate monohydrate that has been contacted with carbon dioxide; 2. The method for producing sodium carbonate according to claim 1, further comprising drying the resulting sodium carbonate monohydrate to obtain sodium carbonate.

3. 2. The method according to claim 1, wherein the concentration of the aqueous sodium hydroxide solution is higher than 25% by mass.

4. 2. The method for producing sodium carbonate according to claim 1, wherein the solid-liquid separation is performed using a wet cyclone.

5. 2. The method for producing sodium carbonate according to claim 1, wherein the liquid separated by the solid-liquid separation is used as the aqueous sodium hydroxide solution to be contacted with carbon dioxide.

6. 2. The method for producing sodium carbonate according to claim 1, wherein the carbon dioxide brought into contact with the aqueous sodium hydroxide solution is a carbon dioxide-containing exhaust gas.

7. 7. The method for producing sodium carbonate according to claim 6, wherein the exhaust gas containing carbon dioxide is an exhaust gas generated in an ammonia-soda process.

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