Sodium carbonate manufacturing method

Abstract

To provide a method for producing sodium carbonate that can be produced industrially stably using an exhaust gas containing carbon dioxide.SOLUTION: A method for producing sodium carbonate by contacting an exhaust gas containing carbon dioxide with an aqueous solution of sodium hydroxide is provided, which is characterized by setting the pH of the aqueous solution containing sodium carbonate to 11.4 to 12.4 after contacting the exhaust gas containing the carbon dioxide with the aqueous solution of sodium hydroxide. In the production method of the present invention, it is preferable that the exhaust gas containing the carbon dioxide is an exhaust gas generated in an ammonia-soda process.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 stably using exhaust gas containing carbon dioxide. [Background technology]

[0002] In recent years, carbon dioxide gas emissions released into the atmosphere have become a problem, and various studies have been conducted to reduce carbon dioxide gas emissions. For example, a method has been proposed in which sodium hydroxide, produced by electrolysis of table salt or the like, is reacted with carbon dioxide in exhaust gas from a boiler or the like to produce sodium carbonate and sodium bicarbonate, and carbon dioxide gas is then recovered from the exhaust gas (see Patent Documents 1 and 2).

[0003] Sodium carbonate is a compound used as an alkaline agent in various industries, a raw material for glass, and the like. Sodium carbonate is produced by the ammonia-soda process (see Patent Document 3). Specifically, in the ammonia-soda process, limestone is heated to generate carbon dioxide gas, which is then brought into contact with an aqueous solution containing ammonia and table salt (carbonation step) to obtain a slurry containing sodium bicarbonate. The sodium bicarbonate is then subjected to solid-liquid separation from the sodium bicarbonate-containing slurry, and the resulting sodium bicarbonate is heated to produce sodium carbonate. After solid-liquid separation of sodium bicarbonate from the aqueous solution containing sodium bicarbonate, the mother liquor contains carbon dioxide and ammonia. The carbon dioxide gas and ammonia can be recovered by distilling the mother liquor. Furthermore, unreacted carbon dioxide gas is also produced in the carbonation step, and exhaust gas containing carbon dioxide is also emitted during the production of sodium carbonate by the ammonia-soda process. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Special Publication No. 2008-538738 [Patent Document 2] Patent No. 6110103 [Patent Document 3] Special Publication No. 46-033215 Summary of the Invention [Problem to be solved by the invention]

[0005] As described above, exhaust gas containing carbon dioxide is emitted during production in the ammonia-soda process. Therefore, in an attempt to effectively utilize the carbon dioxide in the exhaust gas emitted from the ammonia-soda process, the inventors of the present application attempted to obtain an aqueous solution containing sodium carbonate by reacting the exhaust gas with sodium hydroxide, and then isolate the sodium carbonate from the aqueous solution containing sodium carbonate. However, they discovered that solid matter was generated in an apparatus for obtaining the aqueous solution containing sodium carbonate by reacting the exhaust gas with sodium hydroxide, causing clogging and making operation difficult, and that corrosion sometimes occurred on the inner surface of a dryer when drying the isolated sodium carbonate.

[0006] That is, an object of the present invention is to provide a method for producing sodium carbonate that can be industrially and stably produced using exhaust gas containing carbon dioxide. [Means for solving the problem]

[0007] In view of the above problems, the present inventors have conducted extensive research and have conducted detailed studies on a method for recovering sodium carbonate from an aqueous solution obtained by reacting the above-mentioned exhaust gas containing carbon dioxide with sodium hydroxide.

[0008] They discovered a phenomenon in which solid matter was generated in an apparatus for reacting the exhaust gas with sodium hydroxide to obtain an aqueous solution containing sodium carbonate, causing clogging and making operation difficult.

[0009] Various hydrates of sodium carbonate are known, and the one obtained at the boiling point (approximately 105°C) at atmospheric pressure is the monohydrate. Therefore, we attempted to produce anhydrous sodium carbonate by precipitating sodium carbonate monohydrate by evaporating a portion of the water from the aqueous solution, and then recovering and drying the monohydrate. As a result, we found that corrosion of the dryer occurs when sodium carbonate monohydrate containing sodium hydroxide is dried to produce anhydrous sodium carbonate. We also discovered that the amount of carbon dioxide in the exhaust gas emitted from the ammonia-soda process changes over time, and that when the amount of carbon dioxide in the exhaust gas is low, sodium hydroxide becomes excessive, resulting in residual sodium hydroxide.

[0010] Based on the above findings, the inventors investigated the contact conditions between exhaust gas containing carbon dioxide and an aqueous sodium hydroxide solution. As a result, they found that by adjusting the pH of the aqueous sodium carbonate solution after contacting the exhaust gas with sodium hydroxide, sodium bicarbonate, which has low solubility in the aqueous solution, is prevented from being produced in an amount exceeding its solubility and precipitating as a solid, thereby suppressing the phenomenon of solids being generated and clogging in an apparatus for producing an aqueous sodium carbonate solution by reacting the exhaust gas with sodium hydroxide, suppressing the amount of sodium hydroxide remaining in the aqueous solution, precipitating sodium carbonate monohydrate from the aqueous solution, and drying the isolated sodium carbonate monohydrate to produce anhydrous sodium carbonate, thereby suppressing corrosion of a dryer, which led to the completion of the present invention.

[0011] That is, the present invention provides: Generated in the ammonia-soda process This is a production method for obtaining sodium carbonate by bringing exhaust gas containing carbon dioxide into contact with an aqueous sodium hydroxide solution, characterized in that the pH of the aqueous sodium carbonate solution obtained after contacting the exhaust gas containing carbon dioxide with the aqueous sodium hydroxide solution is adjusted to 11.4 to 12.4.

[0012] The present invention can preferably adopt the following aspects.

[0013] (1) The aqueous solution containing sodium carbonate further contains sodium bicarbonate, and the content of the sodium bicarbonate is 0.01 to 1.0% by mass with respect to the aqueous solution containing sodium carbonate. (2) The exhaust gas containing carbon dioxide is an exhaust gas generated in an ammonia-soda process. (3) The exhaust gas containing carbon dioxide is brought into contact with the aqueous sodium hydroxide solution using a gas-liquid contactor. (4) In the above (3), the carbon dioxide concentration in the carbon dioxide-containing exhaust gas supplied to the gas-liquid contactor is measured, and the supply amount of the exhaust gas is adjusted so that the pH of the aqueous solution containing sodium carbonate discharged from the gas-liquid contactor is 11.4 to 12.4. (5) In the above (3), the carbon dioxide concentration in the carbon dioxide-containing exhaust gas supplied to the gas-liquid contactor is measured, and the supply amount of the sodium hydroxide aqueous solution is adjusted so that the pH of the aqueous solution containing sodium carbonate discharged from the gas-liquid contactor is 11.4 to 12.4. (6) Obtaining an aqueous solution containing sodium carbonate by any one of the above-described production methods of the present invention, then precipitating sodium carbonate monohydrate from the obtained aqueous solution containing sodium carbonate, then obtaining sodium carbonate monohydrate by solid-liquid separation, and then drying the obtained sodium carbonate monohydrate to obtain sodium carbonate. [Effects of the Invention]

[0014] The present invention is characterized by adjusting the pH of the aqueous sodium carbonate solution obtained after contacting exhaust gas containing carbon dioxide with an aqueous sodium hydroxide solution to a range of 11.4 to 12.4. This adjustment prevents the formation of sodium bicarbonate in excess of its solubility in the resulting aqueous sodium carbonate solution and also prevents residual sodium hydroxide. This prevents the generation of solids and clogging in an apparatus that reacts the exhaust gas with sodium hydroxide to obtain an aqueous sodium carbonate solution, and also prevents corrosion of a dryer when sodium carbonate monohydrate is precipitated from the aqueous solution and isolated and dried to produce anhydrous sodium carbonate. As a result, sodium carbonate can be produced industrially and stably using exhaust gas containing carbon dioxide. The production method of the present invention is applicable to any exhaust gas containing carbon dioxide, such as exhaust gas from a boiler or the like, or exhaust gas emitted from an ammonia-soda process. The production method of the present invention is particularly effective when the amount of carbon dioxide in exhaust gas, such as exhaust gas emitted from an ammonia-soda process, may fluctuate over time. INDUSTRIAL APPLICABILITY As described above, the production method of the present invention can effectively utilize carbon dioxide contained in exhaust gas to industrially and stably produce sodium carbonate, and has extremely high industrial applicability. DETAILED DESCRIPTION OF THE INVENTION

[0015] The present invention is characterized in that the pH of the aqueous solution containing sodium carbonate obtained after contacting exhaust gas containing carbon dioxide with an aqueous sodium hydroxide solution is adjusted to a range of 11.4 to 12.4. Adjusting the pH in this manner makes it possible to prevent sodium bicarbonate from being produced in an amount exceeding its solubility and to prevent sodium hydroxide from remaining in the resulting aqueous solution containing sodium carbonate. This prevents the generation of solids and clogging in an apparatus that reacts the exhaust gas with sodium hydroxide to obtain an aqueous solution containing sodium carbonate, and prevents corrosion of a dryer when sodium carbonate monohydrate is precipitated from the aqueous solution and the isolated sodium carbonate monohydrate is dried to produce anhydrous sodium carbonate.

[0016] Although the details of why the manufacturing method of the present invention can prevent corrosion of a dryer are unclear, the inventors speculate as follows. Specifically, when sodium hydroxide is contained in an aqueous solution containing sodium carbonate, sodium carbonate monohydrate is precipitated. Therefore, when the monohydrate is isolated by solid-liquid separation, most of the sodium hydroxide remains in the aqueous solution and is therefore almost entirely removed. However, some of the aqueous solution adheres to the crystals and is sent to the drying process. The resulting sodium carbonate monohydrate is dried at a high temperature of 150°C or higher to separate the water of hydration and obtain anhydrous sodium carbonate. However, as moisture is removed during drying, the concentration of sodium hydroxide in the aqueous solution adhering to the crystals increases rapidly. Since stainless steel is generally used as the internal material of a dryer, it is speculated that contact with a high-concentration aqueous sodium hydroxide solution at high temperatures causes corrosion inside the dryer.

[0017] On the other hand, in the present invention, the pH of the aqueous solution obtained when exhaust gas containing carbon dioxide is brought into contact with an aqueous sodium hydroxide solution is set to a range of 11.4 to 12.4. By setting the pH within this range, it is presumed that no sodium hydroxide remains in the aqueous solution, and therefore corrosion during subsequent drying can be prevented. The method for producing sodium carbonate of the present invention will be described in detail below.

[0018] (exhaust gas containing carbon dioxide) In the production method of the present invention, the carbon dioxide-containing exhaust gas may be 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 amount of carbon dioxide in exhaust gases generated when fossil fuels such as oil and coal are burned in thermal power plants or combustion boilers is often within a certain range, as the combustion conditions in the thermal power plants or combustion boilers are controlled 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 fluctuate depending on the combustion operation. Furthermore, in the ammonia-soda process, calcium chloride, ammonium chloride, and the like are produced in addition to sodium carbonate, and the amount of carbon dioxide emitted varies depending on the balance of their production amounts. Any of the above carbon dioxide-containing exhaust gases can be suitably used in the production method of the present invention. In particular, the production method of the present invention is capable of producing sodium hydroxide while suppressing residual sodium hydroxide even when the amount of carbon dioxide emitted fluctuates, and is therefore suitable for application to such exhaust gases.

[0019] 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.

[0020] (Exhaust gas emitted from the ammonia-soda process) As described above, in the ammonia-soda process, carbon dioxide gas generated by heating limestone is contacted with an aqueous solution containing ammonia and salt (carbonation step) to obtain an aqueous solution containing sodium bicarbonate. Sodium bicarbonate is then separated from the aqueous solution containing sodium bicarbonate and the resulting sodium bicarbonate is heated to produce sodium carbonate. The mother liquor obtained after solid-liquid separation of sodium bicarbonate from the aqueous solution containing sodium bicarbonate contains carbon dioxide and ammonia, and the carbon dioxide gas and ammonia can be separated by distillation. Unreacted carbon dioxide gas is also produced in the carbonation step, and carbon dioxide-containing exhaust gas is also emitted during the production of sodium carbonate by the ammonia-soda process. Carbon dioxide gas is also dissolved in the liquid used to wash the sodium bicarbonate after solid-liquid separation, and carbon dioxide gas can be separated from these liquids by distillation.

[0021] Furthermore, in the ammonia-soda process, calcium chloride, ammonium chloride, and the like are produced in addition to sodium carbonate, and the amount of carbon dioxide emitted varies depending on the balance of the production amounts of these substances. Furthermore, the amount of carbon dioxide in the exhaust gas emitted by the various processes described above also varies. Therefore, due to these factors, the amount of carbon dioxide in the carbon dioxide-containing exhaust gas emitted by the ammonia-soda process may vary significantly; specifically, the carbon dioxide concentration may vary within a range of 10 to 15% by volume over a period of several hours to about half a day. In the production method of the present invention, the carbon dioxide-containing exhaust gas emitted by the various processes of the ammonia-soda process described above can be suitably used.

[0022] (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 below, when sodium carbonate monohydrate is obtained from the resulting aqueous solution containing sodium carbonate, water is distilled off from the aqueous solution. Therefore, from an industrial perspective, it is preferable that the concentration of sodium hydroxide in the aqueous sodium hydroxide solution be as high as possible. On the other hand, if the concentration is too high, solids may be generated when the aqueous sodium hydroxide solution is contacted with exhaust gas containing carbon dioxide, causing clogging in the gas-liquid contactor, which may make operation control difficult. Therefore, the concentration of sodium hydroxide in the aqueous sodium hydroxide solution is preferably in the range of 20 to 24% by mass, more preferably in the range of 22 to 24% by mass, and particularly preferably in the range of 23 to 24% by mass.

[0023] (Contact of exhaust gas containing carbon dioxide with aqueous sodium hydroxide solution) In the production method of the present invention, exhaust gas containing carbon dioxide is contacted with an aqueous sodium hydroxide solution. This contact causes a reaction between carbon dioxide and sodium hydroxide, resulting in the production of an aqueous sodium carbonate solution. Known methods can be used to contact the exhaust gas containing carbon dioxide with the aqueous sodium hydroxide solution in the production method of the present invention. Specific examples include a method in which exhaust gas containing carbon dioxide is blown into a vessel filled with aqueous sodium hydroxide solution to contact the exhaust gas with the aqueous sodium hydroxide solution, and a method in which the exhaust gas containing carbon dioxide and the aqueous sodium hydroxide solution are supplied to a gas-liquid contactor and contacted therewith. Examples of gas-liquid contactors include scrubbers, wetted-wall columns, plate columns, and packed columns. Among these methods, a method using a packed column is preferred from the viewpoint of simplicity of the equipment. Furthermore, the method of contacting the exhaust gas with the aqueous sodium hydroxide solution in a packed column is not particularly limited. Examples include a method in which both the exhaust gas containing carbon dioxide and the aqueous sodium hydroxide solution are supplied from the top or bottom of the packed column, or a method in which the exhaust gas and the aqueous sodium hydroxide solution are supplied from the top and bottom of the packed column, respectively. Among these methods, it is particularly preferred, from the viewpoint of the efficiency of the gas-liquid contact reaction, to supply an aqueous sodium hydroxide solution from the upper part of the packed tower, supply an exhaust gas containing carbon dioxide from the lower part of the packed tower, and discharge the resulting aqueous solution containing sodium carbonate from the lower part of the packed tower.

[0024] (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 obtained by contacting exhaust gas containing carbon dioxide with an aqueous sodium hydroxide solution to 11.4 to 12.4. As described above, by adjusting the pH to this range, no sodium hydroxide remains in the aqueous solution, thereby preventing corrosion during subsequent drying. The pH of the aqueous solution containing sodium carbonate is preferably 11.6 to 12.2, and particularly preferably 11.8 to 12.0, in order to accommodate fluctuations in the concentration of carbon dioxide contained in the exhaust gas.

[0025] The method for adjusting the pH of the aqueous solution containing sodium carbonate to 11.4 to 12.4 is not particularly limited, and can be either a method of adjusting the amounts of carbon dioxide and sodium hydroxide to be contacted, or a method of contacting a carbon dioxide-containing exhaust gas with an aqueous sodium hydroxide solution to obtain an aqueous solution containing sodium carbonate, and then checking the pH of the aqueous solution and adjusting it to the above range. Among these methods, the method of adjusting the amounts of carbon dioxide and sodium hydroxide to be contacted is preferred because it allows for continuous industrial production of an aqueous solution containing sodium carbonate. In theory, carbon dioxide and sodium hydroxide react with each other at a ratio of 2 moles of sodium hydroxide to 1 mole of carbon dioxide to produce sodium carbonate. Therefore, the pH can be adjusted to the above range by increasing the molar amount of carbon dioxide to more than twice the molar amount of sodium hydroxide. If the amount of carbon dioxide is greater than the amount of sodium hydroxide, the excess carbon dioxide will also produce sodium bicarbonate. When sodium carbonate monohydrate is precipitated from an aqueous solution containing sodium carbonate, as described below, the presence of a trace amount of sodium bicarbonate does not significantly change the particle size of the precipitated sodium carbonate monohydrate. The amount of sodium hydrogencarbonate contained in the aqueous solution containing sodium carbonate is preferably 0.01 to 1.0% by mass relative to the aqueous solution containing sodium carbonate.

[0026] When the exhaust gas containing carbon dioxide is brought into contact with the aqueous sodium hydroxide solution in the gas-liquid contactor, the supply amounts of the exhaust gas containing carbon dioxide and the aqueous sodium hydroxide solution may be appropriately determined taking into consideration the contact efficiency and reaction rate of the gas-liquid contact.

[0027] As described above, the amount of carbon dioxide in exhaust gases such as exhaust gases containing carbon dioxide discharged in the ammonia-soda method may vary greatly. In such cases, the carbon dioxide concentration in the exhaust gas containing carbon dioxide may be measured, and the amount of the exhaust gas containing carbon dioxide or the amount of the aqueous sodium hydroxide solution to be supplied may be determined appropriately in accordance with the carbon dioxide concentration of the exhaust gas.

[0028] Alternatively, a predetermined amount of exhaust gas containing carbon dioxide may be brought into contact with an aqueous sodium hydroxide solution to obtain an aqueous solution containing sodium carbonate, and the pH of the aqueous solution may then be checked and adjusted to fall within the above range. If the pH of the aqueous solution is too low, sodium hydroxide or the like may be added, and if the pH is too high, sodium hydrogencarbonate or the like may be added to control the pH within the above range.

[0029] (Other conditions) The temperature at which the exhaust gas containing carbon dioxide is brought into contact with the aqueous sodium hydroxide solution may be any temperature that allows efficient reaction between carbon dioxide and sodium hydroxide, and may generally be set appropriately within the range of 30 to 70°C.

[0030] (Production of sodium carbonate monohydrate) In the aqueous solution containing sodium carbonate obtained by the production method of the present invention, sodium carbonate is dissolved. As described above, sodium carbonate forms various hydrates, and the stability in solution and solubility in water vary depending on the hydrate. In particular, the monohydrate is easy to control to a desired particle size, and anhydrous sodium carbonate can be produced by isolating the monohydrate and then drying the monohydrate.

[0031] The sodium carbonate concentration in the aqueous solution containing sodium carbonate is about 27 to 29% by mass, and sodium carbonate monohydrate can be obtained by concentrating the aqueous solution. Evaporative concentration is a suitable method for concentrating the aqueous solution. A temperature of 80 to 108°C during evaporation is usually sufficient. The amount of water to be concentrated can be determined appropriately taking into account the amount of sodium carbonate monohydrate to be produced, but it is sufficient to concentrate the solution until the concentration of sodium carbonate monohydrate in the slurry reaches about 20 to 40% by mass.

[0032] The method for concentrating the aqueous solution containing sodium carbonate by evaporation to obtain sodium carbonate monohydrate is not particularly limited, and either a batch method in which a predetermined amount of the aqueous solution is charged into a concentrator, a predetermined amount of water is evaporated and concentrated, the entire amount is then withdrawn, and the precipitated sodium carbonate monohydrate is isolated, or a continuous method in which the aqueous solution containing sodium carbonate is supplied to a concentrator and the slurry containing sodium carbonate monohydrate produced in the concentrator is continuously withdrawn, can be employed. When the slurry containing sodium carbonate monohydrate is obtained by a continuous method, the supply of the aqueous solution containing sodium carbonate and the withdrawal of the slurry containing sodium carbonate monohydrate produced in the concentrator can be adjusted so that the residence time of the aqueous solution containing sodium carbonate supplied to the concentrator is 2 to 5 hours, from the viewpoint of stably obtaining the monohydrate having a large particle size.

[0033] Sodium carbonate monohydrate can be precipitated using the above method. The resulting sodium carbonate monohydrate can be isolated by known methods 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. The drying temperature in the drying process may be set to a temperature sufficient to produce an anhydrous sodium carbonate, and may be appropriately set within the range of 150 to 180°C. The drying time may also be sufficient to produce an anhydrous sodium carbonate. When drying within the above temperature range, 0.5 to 2.0 hours is usually sufficient. When drying sodium carbonate monohydrate using a dryer such as a steam tube dryer, using steam as a heat source, the steam discharged from the dryer can be used as a heat source for contacting the carbon dioxide-containing exhaust gas with the aqueous sodium hydroxide solution or for producing sodium carbonate monohydrate. [Example]

[0034] 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, the physical properties of the aqueous solution containing sodium carbonate and sodium carbonate, and the corrosion of the dryer were evaluated by the following methods.

[0035] <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 an aqueous solution containing sodium carbonate 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. A few 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.

[0036] <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.

[0037] <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.

[0038] Example 1 Packed tower (0.5m 3 Aqueous sodium hydroxide solution adjusted to 24% by mass was fed into the top of the column at a rate of 23.6 kg / h, and exhaust gas containing carbon dioxide discharged from the soda process (soda process exhaust gas, carbon dioxide concentration 12.0% by volume) was fed into the bottom of the column at a rate of 14.7 Nm 3 The sodium hydroxide solution and the soda process exhaust gas were brought into contact with each other inside the packed tower, and the unabsorbed soda process exhaust gas was released from the top of the packed tower, while the aqueous solution containing sodium carbonate was extracted from the bottom. The carbon dioxide concentration in the soda process exhaust gas remained almost constant during the contact.

[0039] An aqueous sodium hydroxide solution was introduced from the top of the column, and when the introduction of exhaust gas from the bottom of the column and the discharge of the aqueous sodium carbonate solution from the bottom of the column became stable, the aqueous sodium carbonate solution extracted from the bottom of the column was analyzed and found to have a pH of 11.9, a sodium carbonate concentration of 27.9% by mass, and a sodium hydrogencarbonate concentration of 0.3% by mass. No sodium hydroxide was detected in the aqueous solution.

[0040] The aqueous solution containing sodium carbonate extracted from the bottom of the column was heated and concentrated at approximately 106°C in a crystallization tank to obtain a slurry containing sodium carbonate monohydrate crystals, which was then centrifuged to obtain sodium carbonate monohydrate crystals. The sodium carbonate monohydrate crystals were then placed in a rotary dryer, and the residence time of the crystals in the dryer was adjusted to 1.0 hour, followed by drying at 180°C to obtain sodium carbonate anhydrous crystals. The physical properties of the obtained crystals were evaluated, revealing an average particle size of 390 μm and a bulk density of 1.25 kg / L. No corrosion was observed inside the dryer after the crystals were removed.

[0041] Comparative Example 1 The aqueous sodium hydroxide solution was contacted with the exhaust gas from the soda process in the same manner as in Example 1, except for the conditions shown in Table 1. Analysis of the aqueous solution containing sodium carbonate extracted from the bottom of the tower revealed that the pH was 12.7 and the sodium carbonate concentration was 27.7 mass%. The aqueous solution also contained 0.3 mass% sodium hydroxide. Using this aqueous solution containing sodium carbonate, sodium carbonate crystals were obtained in the same manner as in Example 1. The results of evaluation of the physical properties of the obtained sodium carbonate crystals are shown in Table 1. Corrosion was confirmed inside the dryer after the crystals were removed.

[0042] [Table 1]

[0043] Example 2 100 kg of the aqueous solution containing sodium carbonate obtained in Comparative Example 1 was transferred to a tank, and 1.5 kg of sodium bicarbonate was added and dissolved with stirring to adjust the pH to 12.1. Analysis of the aqueous solution containing sodium carbonate after preparation revealed that the sodium carbonate concentration was 28.1% by mass and the sodium bicarbonate concentration was 0.1% by mass. No sodium hydroxide was detected in the aqueous solution.

[0044] The aqueous solution was used to obtain crystals of sodium carbonate in the same manner as in Example 1. The obtained crystals were evaluated for their physical properties, and as a result, they had an average particle size of 390 μm and a bulk density of 1.20 kg / L. No corrosion was observed inside the dryer after the crystals were removed.

[0045] Examples 3 to 5 The aqueous sodium hydroxide solution was contacted with the soda process exhaust gas in the same manner as in Example 1, except for the conditions shown in Table 1. The analytical results of the aqueous sodium carbonate solution extracted from the bottom of the column are shown in Table 1. Using this aqueous sodium carbonate solution, sodium carbonate crystals were obtained in the same manner as in Example 1. The results of evaluation of the physical properties of the obtained sodium carbonate crystals and the results of evaluation of dryer corrosion are shown in Table 1.

[0046] Example 6 The same procedure as in Example 1 was repeated, except that operation was performed under conditions in which the carbon dioxide concentration contained in the soda process exhaust gas gradually changed. The carbon dioxide concentration contained in the soda process exhaust gas was stable at 12.0% by volume at the start of operation, but gradually increased with fluctuations in the soda process load, stabilizing at 14.8% by volume after 6 hours. By gradually reducing the flow rate of the soda process exhaust gas in accordance with the increase in carbon dioxide concentration during operation of the packed tower, the pH of the aqueous solution discharged from the tower bottom did not deviate from the target range. Table 2 shows the analytical results of the aqueous solution containing sodium carbonate extracted from the tower bottom. Sodium carbonate crystals were obtained using the obtained aqueous solution containing sodium carbonate in the same manner as in Example 1. Table 2 shows the results of evaluation of the physical properties of the obtained sodium carbonate crystals and the results of the dryer corrosion evaluation.

[0047] [Table 2]

[0048] Example 7, Comparative Example 2 The aqueous sodium hydroxide solution was contacted with the soda process exhaust gas in the same manner as in Example 6, except for the conditions shown in Table 2. Table 2 shows the analytical results of the aqueous sodium carbonate solution extracted from the bottom of the column. This aqueous sodium carbonate solution was used to obtain sodium carbonate crystals in the same manner as in Example 2. Table 2 shows the results of evaluating the physical properties of the obtained sodium carbonate crystals and the results of evaluating the dryer corrosion. In Comparative Example 2, solids gradually formed inside the packed column during operation, causing the packed column to clog, so the operation was stopped and no further operations were performed. Table 2 shows the analytical results of the aqueous sodium carbonate solution discharged from the bottom of the column at the end of the operation.

Claims

1. A method for producing sodium carbonate by contacting exhaust gas containing carbon dioxide generated in an ammonia-soda process with an aqueous sodium hydroxide solution, comprising: A manufacturing method characterized in that the pH of the aqueous solution containing sodium carbonate after contacting the exhaust gas containing carbon dioxide with the aqueous sodium hydroxide solution is adjusted to 11.4 to 12.

4.

2. 2. The production method according to claim 1, wherein the aqueous solution containing sodium carbonate further contains sodium bicarbonate, and the content of the sodium bicarbonate is 0.01 to 1.0% by mass based on the amount of the aqueous solution containing sodium carbonate.

3. 3. The method according to claim 1, wherein the exhaust gas containing carbon dioxide is contacted with the aqueous sodium hydroxide solution in a gas-liquid contactor.

4. The method according to claim 3, wherein a concentration of carbon dioxide in the carbon dioxide-containing exhaust gas supplied to the gas-liquid contactor is measured, and an amount of the exhaust gas supplied is adjusted so that the pH of the aqueous solution containing sodium carbonate discharged from the gas-liquid contactor is 11.4 to 12.

4.

5. The production method according to claim 3, characterized in that a concentration of carbon dioxide in the carbon dioxide-containing exhaust gas supplied to the gas-liquid contactor is measured, and the supply amount of the sodium hydroxide aqueous solution is adjusted so that the pH of the aqueous solution containing sodium carbonate discharged from the gas-liquid contactor is 11.4 to 12.

4.

6. An aqueous solution containing sodium carbonate is obtained by the production method according to any one of claims 1 to 5, Next, sodium carbonate monohydrate is precipitated from the obtained aqueous solution containing sodium carbonate, Then, sodium carbonate monohydrate is obtained by solid-liquid separation, The method for producing sodium carbonate comprises drying the obtained sodium carbonate monohydrate to obtain sodium carbonate.

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