Method for recycling waste sodium hydroxide solution

Abstract

To provide a method for efficiently regenerating wastewater solutions of sodium hydroxide. [Solution] A method for regenerating a waste sodium hydroxide aqueous solution containing aluminum, comprising the steps of: mixing and reacting an aqueous slurry containing a calcium compound with the waste sodium hydroxide aqueous solution to form a reaction solution containing a supernatant liquid and a solid reaction residue; and subjecting the reaction solution to a solid-liquid separation treatment to remove the reaction residue, thereby recovering the supernatant liquid as a regenerated sodium hydroxide aqueous solution.

Description

Technical Field

[0001] The present invention relates to a method for regenerating a sodium hydroxide waste aqueous solution.

Background Art

[0002] For tools (processing tools) for processing metallic aluminum or aluminum alloys (hereinafter sometimes referred to as "aluminum materials"), aluminum materials may adhere during processing. Since the adhered aluminum materials cause problems during the next use of the processing tool, the processing tool is immersed in an aqueous solution of sodium hydroxide (caustic soda). The adhered aluminum materials are dissolved and removed in the sodium hydroxide aqueous solution.

[0003] In addition, the sodium hydroxide aqueous solution is also used for treating objects other than processing tools, for example, products containing aluminum materials. Specifically, for example, it is used for alkali etching of aluminum processed products such as aluminum sashes and plates. Further, it is conceivable to use a sodium hydroxide aqueous solution when dissolving aluminum-containing waste materials such as waste batteries, aluminum alloy scraps, and aluminum-containing sludge, or aluminum waste materials.

[0004] In such a treatment using a sodium hydroxide aqueous solution, each time the treatment is repeated, the aluminum concentration in the aqueous solution increases. On the other hand, when the aluminum concentration in the aqueous solution increases, various problems occur. For example, when treating a processing tool, an aqueous solution with a high aluminum concentration has a low aluminum dissolution rate. Therefore, the reaction time required for removing aluminum materials becomes long, and there is a risk of delay in the aluminum material processing process. Further, when treating a product, if the aluminum concentration in the aqueous solution is high, precipitated aluminum hydroxide may adhere to the product, which may cause defective products. Therefore, although the sodium hydroxide aqueous solution with a high aluminum concentration has been discarded as a waste aqueous solution, there is a problem that the waste liquid treatment is costly.

[0005] To solve these problems, a technique has been proposed to regenerate sodium hydroxide (caustic soda) by adding a calcium compound to an aqueous solution to remove aluminum from the solution. For example, Patent Document 1 discloses a method for regenerating caustic soda wastewater containing aluminum, characterized by adding a calcium compound at a concentration of 300 g / l or less per mole of aluminum, reacting it at 80 to 120°C, and precipitating and separating the aluminum as a compound mainly composed of 3CaO·Al2O3·6H2O (Claim 1 of Patent Document 1). [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Special Publication No. 54-4720 [Overview of the project] [Problems that the invention aims to solve]

[0007] However, the technology proposed in Patent Document 1 had the problem of requiring the reaction temperature to be raised to 80°C or higher, and also requiring a long reaction time. As a result, the energy cost required for the reaction increased, and there was a problem of limited design flexibility when manufacturing the reaction apparatus. Furthermore, it was necessary to keep the concentration of sodium hydroxide (caustic soda) in the aqueous solution low (300 g / l or less), which caused inconveniences in actual processing.

[0008] In light of these problems, the inventors conducted thorough research. As a result, they found that when regenerating a sodium hydroxide wastewater solution containing aluminum, using an aqueous slurry containing a calcium compound and reacting this slurry with the sodium hydroxide wastewater solution allows for effective removal of aluminum from the sodium hydroxide wastewater solution, even at low temperatures and for short periods of time. Furthermore, they found that aluminum can be effectively removed even from aqueous solutions with high sodium hydroxide concentrations, and as a result, the sodium hydroxide wastewater solution can be efficiently regenerated.

[0009] This invention was completed based on such findings, and aims to provide a method for efficiently regenerating sodium hydroxide wastewater solutions. [Means for solving the problem]

[0010] The present invention encompasses the following embodiments (1) to (9). In this specification, the expression "~" includes the numerical values ​​at both ends. That is, "X~Y" is synonymous with "X or more and Y or less".

[0011] (1) A method for regenerating a waste aqueous solution of sodium hydroxide containing aluminum, A step of mixing and reacting an aqueous slurry containing a calcium compound with the waste aqueous solution of sodium hydroxide to form a reaction solution containing a supernatant liquid and a solid reaction residue, and A method comprising the step of subjecting the reaction solution to a solid-liquid separation treatment to remove the solid reaction residue, thereby recovering the supernatant liquid as a regenerated sodium hydroxide aqueous solution.

[0012] (2) The method of (1) above, wherein the amount of calcium contained in the reaction solution is 1.5 mol or more and 2.0 mol or less per mol of aluminum.

[0013] (3) The method of (1) or (2) above, wherein the calcium compound is either or both of calcium hydroxide (Ca(OH)2; slaked lime) and calcium oxide (CaO; quicklime).

[0014] (4) Any of the methods (1) to (3) above, wherein the calcium compound is calcium hydroxide (slaked lime; Ca(OH)2) and the concentration of the calcium compound in the aqueous slurry is 950 g / L or less.

[0015] (5) Any of the methods (1) to (3) above, wherein the calcium compound is calcium oxide (quicklime; CaO) and the concentration of the calcium compound in the aqueous slurry is 300 g / L or less.

[0016] (6) The method according to any one of (1) to (5) above, wherein the reaction between the aqueous slurry and the sodium hydroxide waste aqueous solution is carried out at a temperature of 0°C or higher and 80°C or lower.

[0017] (7) The method according to any one of (1) to (6) above, wherein the reaction between the aqueous slurry and the sodium hydroxide waste aqueous solution is carried out for 1 hour or longer and 24 hours or shorter.

[0018] (8) The method according to any one of (1) to (7) above, further comprising a step of mixing a calcium compound and a regenerated sodium hydroxide aqueous solution to prepare the aqueous slurry.

[0019] (9) When the concentration of aluminum and the concentration of sodium hydroxide in the sodium hydroxide waste aqueous solution are defined as the first Al concentration and the first NaOH concentration, respectively, and the concentration of aluminum and the concentration of sodium hydroxide in the regenerated sodium hydroxide aqueous solution are defined as the second Al concentration and the second NaOH concentration, respectively, the aluminum removal rate determined according to the following formula (1) is 70% or higher. The method according to any one of (1) to (8) above.

[0020]

Number

Advantages of the Invention

[0021] According to the present invention, there is provided a method capable of efficiently regenerating a sodium hydroxide waste aqueous solution.

Embodiments for Carrying out the Invention

[0022] Specific embodiments of the present invention (hereinafter referred to as "the present embodiment") will be described below. However, the present invention is not limited to the following embodiments, and various modifications are possible without changing the gist of the present invention. Further, in this specification, as long as technical consistency can be achieved, any combination of preferred aspects can be adopted. For example, one and the other of preferred numerical ranges can be arbitrarily combined.

[0023] <<1. Method for regenerating sodium hydroxide waste aqueous solution>> The present embodiment relates to a method for regenerating a sodium hydroxide waste aqueous solution containing aluminum. This method includes the following steps: a step of mixing and reacting an aqueous slurry containing a calcium compound and a sodium hydroxide waste aqueous solution to form a reaction solution containing a supernatant liquid and a solid reaction residue (reaction step), and a step of subjecting the obtained reaction solution to solid-liquid separation treatment to remove the solid reaction residue, thereby recovering the supernatant liquid as a regenerated sodium hydroxide aqueous solution (separation and recovery step). Details of the regeneration method will be described below.

[0024] <Sodium hydroxide waste aqueous solution> The sodium hydroxide waste aqueous solution (hereinafter sometimes referred to as "waste aqueous solution" or "unregenerated waste aqueous solution") is an aqueous solution of sodium hydroxide (NaOH) containing dissolved aluminum (Al). The waste aqueous solution includes, but is not limited to, for example, etching waste liquid, extrusion die cleaning waste liquid, and tool cleaning waste liquid for cutting aluminum materials. Further, it may contain components contained in aluminum materials such as silicon (Si), iron (Fe), and copper (Cu) as impurities. In the method of the present embodiment, a pretreatment for removing impurity components may be performed on the waste aqueous solution.

[0025] The concentration of sodium hydroxide (NaOH) in the wastewater solution is not particularly limited. However, by moderately increasing the NaOH concentration, the NaOH concentration of the final recycled sodium hydroxide solution can be increased. Therefore, the amount of aluminum dissolved in the recycled sodium hydroxide solution can be increased. On the other hand, if the NaOH concentration in the wastewater solution is excessively high, the viscosity will increase, which may lead to heterogeneity of the slurry obtained by mixing with calcium compounds. The NaOH concentration in the wastewater solution is preferably 600 g / L or less, and more preferably 450 g / L or less. The lower limit of the NaOH concentration is not limited. For example, it may be 10 g / L or more, or 20 g / L or more.

[0026] The aluminum (Al) concentration in the wastewater solution is not particularly limited. For example, the Al concentration may be between 5 g / L and 150 g / L, or between 10 g / L and 125 g / L. If the Al concentration is excessively high, the wastewater solution may be diluted so that the Al concentration falls within the above range. Alternatively, the Al concentration may be adjusted by performing a multi-step reaction.

[0027] <Reaction Process> In the reaction step, an aqueous slurry containing a calcium compound is mixed and reacted with a sodium hydroxide wastewater solution to form a reaction solution containing a supernatant and a solid reaction residue. The aqueous slurry containing the calcium compound is a slurry (suspension) containing an aqueous liquid medium and a calcium compound dispersed in this aqueous liquid medium. By adding the calcium compound in slurry form in this way, aluminum can be effectively removed from the wastewater solution even at low temperatures and for a short time. Furthermore, aluminum removal is possible even in wastewater solutions with high sodium hydroxide concentrations. For example, wastewater solutions used for cleaning extrusion dies contain high concentrations of sodium hydroxide (e.g., 400 g / L or more). According to the method of this embodiment, even wastewater solutions containing high concentrations of sodium hydroxide can be regenerated. Moreover, by using a calcium compound in slurry form, wastewater solutions containing high concentrations of impurities such as silicon (Si) can also be regenerated. Therefore, efficient regeneration of wastewater solutions becomes possible.

[0028] In contrast, conventional methods of adding powdered calcium compounds dry result in low reactivity between the calcium compound and the wastewater solution. Therefore, high-temperature and long-duration reaction treatment is required. For example, if the wastewater solution contains a high concentration of aluminum, increasing the amount of calcium compound added will increase the slurry concentration. As a result, the uniformity of the calcium compound in the solution decreases, and the reactivity decreases. Also, if a large amount of silicon is dissolved in the wastewater solution, the uniformity of the calcium compound decreases even at low slurry concentrations, and the reactivity decreases.

[0029] The efficient regeneration of wastewater solutions is possible by using calcium compounds in slurry form. While this should not be interpreted restrictively, the following mechanism is considered: When calcium compounds are mixed with sodium hydroxide wastewater, a reaction occurs, producing calcium aluminate. This reaction proceeds rapidly, starting immediately after mixing. When a calcium agent (solid calcium compound) is used and added to wastewater, the reaction proceeds before it can uniformly disperse in the wastewater, resulting in the solid calcium compound forming large aggregates. These aggregates have a central portion that does not contribute to the reaction, thus reducing reactivity. In contrast, dispersing the calcium compound in a liquid before adding it to the wastewater suppresses the formation of such aggregates, thus reducing losses.

[0030] The aqueous slurry is prepared from a calcium compound and an aqueous liquid medium. The calcium compound is not particularly limited as long as it is an inorganic compound containing calcium. Examples include calcium hydroxide (Ca(OH)2; slaked lime), calcium oxide (CaO; quicklime), calcium sulfate (CaSO4; gypsum), calcium carbonate (CaCO3), calcium chloride (CaCl2), and their hydrates. Preferably, the calcium compound is either or both of calcium hydroxide (Ca(OH)2; slaked lime) and calcium oxide (CaO; quicklime). By using Ca(OH)2 and / or CaO, calcium aluminate, for which effective utilization methods have been thoroughly considered, can be obtained as a reaction product (reaction residue). Therefore, it becomes possible to effectively utilize the reaction residue remaining after the separation and recovery process.

[0031] The aqueous liquid medium is a medium whose main component is water. The aqueous liquid medium may be water such as distilled water or deionized water. In this case, a slurry can be easily prepared. The aqueous liquid medium may also be an aqueous solution containing sodium hydroxide (NaOH). In this case, a decrease in the NaOH concentration of the waste aqueous solution to which the slurry is added and the regenerated sodium hydroxide aqueous solution obtained at the end can be prevented. Alternatively, the aqueous liquid medium may be a regenerated sodium hydroxide aqueous solution obtained in a previous treatment. In this case, a decrease in the NaOH concentration of the regenerated sodium hydroxide aqueous solution can be prevented, and the regenerated sodium hydroxide aqueous solution can be recycled. Therefore, raw material costs can be reduced and efficient regeneration treatment can be achieved. Preferably, the regeneration method further includes a step of mixing the previously obtained regenerated sodium hydroxide aqueous solution with a calcium compound to prepare the aqueous slurry.

[0032] Aqueous slurry can be prepared by adding a calcium compound and an aqueous liquid medium to a container. The calcium compound and aqueous liquid medium may be stirred during this process.

[0033] The concentration of the calcium compound in the slurry is not particularly limited. However, a moderately high concentration can more effectively promote the reaction between the calcium compound and the wastewater solution, as described later. On the other hand, if the concentration is excessively high, there is a risk of calcium compound aggregation. If the calcium compound aggregates, the reactivity will worsen, and the separation of the reaction residue in the subsequent separation and recovery process will become difficult. From the viewpoint of maintaining good reactivity while facilitating the separation of the reaction residue, the concentration of the calcium compound in the slurry is preferably 20 g / L to 1000 g / L. In particular, when the calcium compound is calcium hydroxide, the concentration of the calcium compound in the slurry is preferably 50 g / L to 950 g / L. Also, when the calcium compound is calcium oxide, the concentration of the calcium compound in the slurry is preferably 40 g / L to 300 g / L.

[0034] Next, the prepared aqueous slurry is mixed with the sodium hydroxide waste solution. This causes the reaction shown in equation (A) below to proceed from right to left. That is, sodium aluminate (NaAlO2) and calcium ions (Ca 2+ ) reacts, causing calcium aluminate hydrate (CaO·Al2O3·6H2O) to precipitate, along with sodium ions (Na + ) generates.

[0035] [ka]

[0036] Here, the precipitated calcium aluminate is dispersed in the aqueous solution. Therefore, the reaction yields a suspended liquid (reaction solution). The reaction solution contains sodium ions (Na + The reaction solution contains a supernatant liquid containing ) and a solid reaction residue mainly consisting of calcium aluminate (CaO·Al2O3) or its hydrate dispersed in the supernatant liquid. Therefore, in the subsequent separation and recovery process, the supernatant liquid and the solid reaction residue can be separated by applying a solid-liquid separation treatment to the reaction solution.

[0037] In the reaction step, the aqueous slurry and the sodium hydroxide wastewater solution should be mixed and reacted. The aqueous slurry may be added to the wastewater solution, or the aqueous slurry may be added to the wastewater solution. Alternatively, the aqueous slurry and wastewater solution may be added to a container simultaneously and mixed in place.

[0038] The mixing ratio of the aqueous slurry containing the calcium compound to the wastewater solution is not particularly limited. For example, the calcium compound may be mixed so that the amount of calcium (Ca) per mole of aluminum (Al) in the reaction solution, which is a mixture of the wastewater solution and the aqueous slurry, is preferably 1.0 mol to 3.0 mol, more preferably 1.5 mol to 2.0 mol. Alternatively, the calcium compound may be mixed so that the amount of calcium (Ca) per mole of aluminum (Al) in the wastewater solution is preferably 1.0 mol to 3.0 mol, more preferably 1.5 mol to 2.0 mol. Alternatively, if both the wastewater solution and the slurry solvent (aqueous liquid medium) contain aluminum (Al), the calcium compound may be mixed so that the amount of calcium (Ca) per mole of aluminum (Al) in the mixture (reaction solution) is preferably 1.0 mol to 3.0 mol, more preferably 1.5 mol to 2.0 mol. By moderately increasing the amount of Ca, the reactivity is improved and the removal of aluminum proceeds more effectively. On the other hand, if the amount of calcium is excessively high, a large amount of unreacted calcium compounds will remain, which can be difficult to remove. By keeping the amount of calcium at an appropriate level, it is possible to reduce the amount of unreacted calcium compounds that remain.

[0039] As long as the reaction of equation (A) above proceeds, the reaction temperature is not particularly limited. However, the method of this embodiment has the effect of effectively removing aluminum from wastewater even with a low temperature and short reaction time. Furthermore, lowering the reaction temperature has the effect of increasing the design flexibility of the reaction apparatus from the viewpoint of heat resistance. It is preferable to carry out the reaction between the aqueous slurry and the wastewater at a temperature of 0°C to 80°C, more preferably at a temperature of 40°C to 80°C, and even more preferably at a temperature of 60°C to 80°C.

[0040] As long as the reaction of equation (A) above proceeds, the reaction time is not particularly limited. However, if the reaction time is too short, the reaction may not proceed sufficiently, and the removal of aluminum from the wastewater solution may be insufficient. On the other hand, if the reaction time is too long, the cycle time will be long, and efficient aluminum removal will be hindered. It is preferable to carry out the reaction between the aqueous slurry and the wastewater solution for 1 hour or more and 24 hours or less, more preferably for 1 hour or more and 6 hours or less, and even more preferably for 1 hour or more and 4 hours or less.

[0041] <Separation and Recovery Process> In the separation and recovery process, the obtained reaction solution is subjected to solid-liquid separation treatment to remove solid reaction residue. The supernatant is then recovered as a recycled sodium hydroxide aqueous solution. As described above, the reaction solution contains sodium ions (Na + The reaction solution contains a supernatant liquid containing ) and a solid reaction residue mainly consisting of calcium aluminate (CaO·Al2O3) or its hydrate. Therefore, in the separation and recovery process, the supernatant liquid and the solid reaction residue can be separated by applying a solid-liquid separation treatment to the reaction solution. Since aluminum is mainly distributed into the reaction residue, a supernatant liquid (recycled sodium hydroxide aqueous solution) with a reduced amount of aluminum can be obtained.

[0042] The means of solid-liquid separation are not limited as long as the reaction residue can be removed. Examples include gravity filtration, pressure filtration, and centrifugation. Of these, centrifugation is particularly preferred. Because the amount of liquid in the reaction residue can be reduced, the supernatant liquid can be recovered and the reaction residue can be effectively utilized. In addition, powdered reaction residue can be recovered because it can be scraped off with a scraper. Furthermore, it has the advantage of allowing for efficient washing of the reaction residue with water. However, the means of solid-liquid separation are not limited to centrifugation, and other methods such as gravity filtration and pressure filtration may also be used.

[0043] In this way, the reaction solution can be separated into supernatant and reaction residue. The recovered supernatant (recycled sodium hydroxide aqueous solution) has a reduced amount of aluminum compared to the waste sodium hydroxide aqueous solution. Specifically, the aluminum removal rate is preferably 70% or more, more preferably 75% or more, and even more preferably 80% or more. Here, the aluminum removal rate is an indicator of the proportion of aluminum removed from the aqueous solution by the regeneration treatment, and is determined according to the following equation (1). In the following equation (1), the first Al concentration and the first NaOH concentration are the concentrations of aluminum and sodium hydroxide in the waste sodium hydroxide aqueous solution used for the regeneration treatment, respectively, and the second Al concentration and the second NaOH concentration are the concentrations of aluminum and sodium hydroxide in the regenerated sodium hydroxide aqueous solution obtained by the regeneration treatment, respectively.

[0044]

number

[0045] Because the recycled sodium hydroxide aqueous solution has a reduced aluminum content, it has a high aluminum dissolution rate. Therefore, it can be suitably used for applications such as removing aluminum adhering to processing tools. On the other hand, the reaction residue, which mainly contains calcium aluminate (CaO·Al2O3), can be recovered and utilized.

[0046] According to the regeneration method of this embodiment, sodium hydroxide wastewater can be efficiently regenerated. For example, aluminum can be effectively removed from the wastewater even at low temperatures and for short periods of time. Furthermore, aluminum can be effectively removed even from aqueous solutions with high sodium hydroxide concentrations. However, the method of this embodiment is not limited to methods that perform reaction treatment at low temperatures and for short periods of time, or methods that treat aqueous solutions with high sodium hydroxide concentrations. As long as the requirements based on this embodiment are satisfied, efficient regeneration of sodium hydroxide wastewater can be achieved. [Examples]

[0047] The present invention will be described in more detail using the following examples and comparative examples. However, the present invention is not limited to the following examples.

[0048] [Example 1 (Comparative Example)] In Example 1, a sodium hydroxide wastewater solution (unrecycled wastewater solution) was treated using powdered slaked lime (Ca(OH)2).

[0049] First, a sodium hydroxide wastewater solution (unrecycled wastewater solution) was prepared. Analysis of this unrecycled wastewater solution revealed that the NaOH concentration was 286.5 g / L and the Al concentration was 93.3 g / L. It also contained Si at a concentration of 530 mg / L.

[0050] Next, 250 mL of unrecycled wastewater solution was placed in a SUS reaction vessel, and the solution was heated while being stirred. After confirming that the solution temperature reached 60°C, 96.08 g (1.3 mol) of slaked lime was added to the solution (unrecycled wastewater solution) to prepare the reaction mixture. In this case, slaked lime with a D50 of approximately 6 μm was used. The amount of Ca relative to Al (1 mol) in the reaction mixture was 1.5 mol. The slaked lime appeared to clump together and was difficult to disperse in the solution.

[0051] The heating of the reaction mixture was stopped 4 hours after the addition of slaked lime. The entire reaction mixture was then filtered by suction. The NaOH concentration of the filtrate was 327.5 g / L, and the Al concentration was 39.2 g / L. The Al removal rate was somewhat low at 63.2%.

[0052] [Example 2] In Example 2, a slurry was prepared from distilled water and slaked lime (Ca(OH)2), and this slurry was used to treat a sodium hydroxide wastewater solution (unrecycled wastewater solution).

[0053] First, a sodium hydroxide wastewater solution (unrecycled wastewater solution) was prepared and analyzed. The NaOH concentration was 280.0 g / L, and the Al concentration was 91.3 g / L. It also contained Si at a concentration of 530 mg / L.

[0054] Next, a slaked lime-containing slurry was prepared, and an unrecycled wastewater solution was added to the prepared slurry. Specifically, slaked lime (94.02 g, 1.27 mol) and distilled water (250 mL) were placed in a SUS reaction vessel and stirred to prepare the slurry. In this case, slaked lime with a D50 of approximately 6 μm was used. The ratio of slaked lime to distilled water was 376.1 g / L. The slurry was heated while stirring, and after confirming that the slurry temperature reached 60°C, the unrecycled wastewater solution (250 mL) was added dropwise to the slurry over 30 minutes using a polypipette to prepare the reaction solution. The amount of Ca relative to Al (1 mol) in the reaction solution was 1.5 mol.

[0055] The heating of the reaction mixture was stopped 4 hours after the start of dropwise addition. The entire volume of the reaction mixture was filtered by suction. The NaOH concentration of the filtrate was 156.9 g / L, and the Al concentration was 8.5 g / L. The Al removal rate was a good 83.4%.

[0056] [Example 3] In Example 3, a slurry was prepared using a sodium hydroxide wastewater solution (recycled wastewater solution) that had been regenerated in a previous treatment, and slaked lime (Ca(OH)2). Then, this slurry was used to treat another sodium hydroxide wastewater solution (unregenerated wastewater solution).

[0057] First, we prepared the sodium hydroxide wastewater solution (unrecycled wastewater solution) to be treated this time. Analysis of this unrecycled wastewater solution revealed that the NaOH concentration was 277.0 g / L and the Al concentration was 90.8 g / L. It also contained Si at a concentration of 530 mg / L. Separately, we prepared a sodium hydroxide wastewater solution that had been recycled in a previous treatment (recycled wastewater solution). The NaOH concentration of this recycled wastewater solution was 310.7 g / L and the Al concentration was 36.3 g / L.

[0058] Next, a slaked lime-containing slurry was prepared, and unrecycled wastewater solution was added to the prepared slurry. Specifically, slaked lime (52.35 g, 0.71 mol) and recycled wastewater solution (100 mL) were placed in a SUS reaction vessel and stirred to prepare the slurry. The ratio of slaked lime to recycled wastewater solution was 523.5 g / L. The slurry was heated while stirring, and after confirming that the slurry temperature reached 60°C, the unrecycled wastewater solution (100 mL) was added dropwise to the slurry over 17 minutes to prepare the reaction solution. The amount of Ca relative to Al (1 mol) in the reaction solution was 1.5 mol.

[0059] The heating of the reaction mixture was stopped 4 hours after the start of dropwise addition. The entire volume of the reaction mixture was filtered by suction. The NaOH concentration of the filtrate was 335.1 g / L, and the Al concentration was 16.3 g / L. The Al removal rate was a good 77.5%.

[0060] [Example 4] In Example 4, three batches of recycled sodium hydroxide wastewater (recycled wastewater) from a previous treatment were used, and a slurry was prepared from these three batches of recycled sodium hydroxide wastewater and slaked lime (Ca(OH)2). This slurry was then used to treat another sodium hydroxide wastewater (unrecycled wastewater). The unrecycled wastewater used was a solution containing a high concentration (300 g / L or more) of sodium hydroxide (NaOH).

[0061] First, we prepared the sodium hydroxide wastewater solution (unrecycled wastewater solution) to be treated this time. Analysis of this unrecycled wastewater solution revealed that the NaOH concentration was 354.3 g / L and the Al concentration was 77.0 g / L. Separately, we prepared three batches of sodium hydroxide wastewater solution (recycled wastewater solution) that had been recycled in a previous treatment, and mixed these three batches of recycled wastewater solution (80 mL, 10 mL, and 10 mL) to prepare a mixed recycled wastewater solution (100 mL). The NaOH concentration of the mixed recycled wastewater solution was 368.2 g / L and the Al concentration was 19.7 g / L.

[0062] Next, a slaked lime-containing slurry was prepared, and unrecycled wastewater solution was added to the prepared slurry. Specifically, slaked lime (39.83 g, 0.54 mol) and mixed recycled wastewater solution (100 mL) were placed in a SUS reaction vessel and stirred to prepare the slurry. The ratio of slaked lime to mixed recycled wastewater solution was 398.3 g / L. The slurry was heated while stirring, and after confirming that the slurry temperature reached 60°C, 10 mL of unrecycled wastewater solution (100 mL) was added to the slurry every minute to prepare the reaction solution. The amount of Ca relative to Al (1 mol) in the reaction solution was 1.5 mol.

[0063] The heating of the reaction mixture was stopped 4 hours after the start of dropwise addition. The entire volume of the reaction mixture was filtered by suction. The NaOH concentration of the filtrate was 404.9 g / L, and the Al concentration was 14.3 g / L. The Al removal rate was a good 73.6%.

[0064] [Example 5] In Example 5, a slurry was prepared using a sodium hydroxide wastewater solution (recycled wastewater solution) that had been regenerated in a previous treatment, and quicklime (CaO). This slurry was then used to treat another sodium hydroxide wastewater solution (unregenerated wastewater solution).

[0065] First, we prepared the sodium hydroxide wastewater solution to be treated this time (unrecycled wastewater solution). Analysis of this unrecycled wastewater solution revealed that the NaOH concentration was 284.3 g / L and the Al concentration was 76.1 g / L. Separately, we prepared a sodium hydroxide wastewater solution that had been recycled in a previous treatment (recycled wastewater solution). The NaOH concentration of this recycled wastewater solution was 335.2 g / L and the Al concentration was 14.2 g / L.

[0066] Next, a quicklime-containing slurry was prepared, and unrecycled wastewater solution was added to the prepared slurry. Specifically, quicklime (28.12 g, 0.50 mol) and recycled wastewater solution (100 mL) were placed in a SUS reaction vessel and stirred to prepare the slurry. The ratio of quicklime added to the recycled wastewater solution was 281.2 g / L. The slurry was heated while stirring, and after confirming that the slurry temperature reached 60°C, 10 mL of unrecycled wastewater solution (100 mL) was added to the slurry every minute to prepare the reaction solution. The amount of Ca relative to Al (1 mol) in the reaction solution was 1.5 mol.

[0067] The heating of the reaction mixture was stopped 4 hours after the start of dropwise addition. The entire volume of the reaction mixture was filtered by suction. The NaOH concentration of the filtrate was 359.3 g / L, and the Al concentration was 8.8 g / L. The Al removal rate was 83.2%.

[0068] From the above results, it can be understood that this embodiment provides a method for efficiently regenerating sodium hydroxide wastewater solution.

Claims

1. A method for regenerating a waste aqueous solution of sodium hydroxide containing aluminum, A step of mixing and reacting an aqueous slurry containing a calcium compound with the waste aqueous solution of sodium hydroxide to form a reaction solution containing a supernatant liquid and a solid reaction residue, and A method comprising the step of subjecting the reaction solution to a solid-liquid separation treatment to remove the solid reaction residue, thereby recovering the supernatant liquid as a regenerated sodium hydroxide aqueous solution.

2. The method according to claim 1, wherein the amount of calcium contained in the reaction solution is 1.5 mol or more and 2.0 mol or less per mol of aluminum.

3. The calcium compound is calcium hydroxide (Ca(OH) ２ The method according to claim 1 or 2, wherein the material is either one or both of slaked lime and calcium oxide (CaO; quicklime).

4. The calcium compound is calcium hydroxide (slaked lime; Ca(OH) ２ The method according to claim 1 or 2, wherein the concentration of the calcium compound contained in the aqueous slurry is 950 g / L or less.

5. The method according to claim 1 or 2, wherein the calcium compound is calcium oxide (quicklime; CaO), and the concentration of the calcium compound in the aqueous slurry is 300 g / L or less.

6. The method according to claim 1 or 2, wherein the reaction between the aqueous slurry and the wastewater sodium hydroxide solution is carried out at a temperature of 0°C to 80°C.

7. The method according to claim 1 or 2, wherein the reaction between the aqueous slurry and the waste aqueous sodium hydroxide solution is carried out for 1 hour or more and 24 hours or less.

8. The method according to claim 1 or 2, further comprising the step of preparing the aqueous slurry by mixing a previously obtained recycled sodium hydroxide aqueous solution with a calcium compound.

9. The method according to claim 1 or 2, wherein when the concentrations of aluminum and sodium hydroxide in the waste sodium hydroxide aqueous solution are defined as the first Al concentration and the first NaOH concentration, respectively, and the concentrations of aluminum and sodium hydroxide in the recycled sodium hydroxide aqueous solution are defined as the second Al concentration and the second NaOH concentration, respectively, the aluminum removal rate, which can be determined according to the following formula (1), is 70% or more. [Math 1]

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