Ammonium sulfate purification method

Abstract

The present invention aims to reduce impurities contained in ammonium sulfate, which is used in a variety of fields, including fertilizer, pharmaceuticals, and industrial applications. The objective of the present invention is to neutralize impurities under mild conditions at atmospheric pressure and below 100°C, without using an autoclave or a heater capable of heating above 100°C. [Solution] Ammonium sulfate can be purified by adding an oxidizing agent to ammonium sulfate under normal pressure. It is preferable to use hydrogen peroxide as the oxidizing agent, and after adding the oxidizing agent, heat the mixture to 30 to 100°C. It is also preferable to add the oxidizing agent in an amount of 0.5 to 5 equivalents of the number of moles of iron(II) calculated by the following method (1). Method (1): Add 20 to 2000 ppm of iron ions from a soluble iron(III) salt to ammonium sulfate, and after one hour or more, quantify the iron(II) produced by the iron-phenanthroline method.

Description

[Technical Field]

[0001] The present invention relates to a method for purifying ammonium sulfate, and more particularly to a method for easily neutralizing reactive species that can cause reduction reactions among impurities contained in ammonium sulfate without requiring labor-intensive and costly procedures such as recrystallization or membrane filtration. [Background technology]

[0002] Ammonium sulfate is obtained as a by-product in the smelting process and in the production process of ε-caprolactam, a raw material for 6,6-nylon, and is used in a variety of fields, including fertilizers, pharmaceuticals, and industrial applications. Ammonium sulfate crystals obtained by crude purification through water concentration and granulation of an ammonium sulfate aqueous solution generally contain inorganic and organic impurities derived from the production process, such as polyamines, ammonium nitrate, heavy metal ions, etc. Among these, ammonium sulfate obtained in the production process of ε-caprolactam contains a particularly large amount of polyamines, which can cause coloration in some cases. Ammonium sulfate for agricultural purposes can often be used even at low purity, but for industrial ammonium sulfate, even small amounts of impurities can be problematic depending on the intended use. Methods for producing polymer dispersions in brine for industrial applications such as wastewater treatment, sludge dewatering flocculants, and papermaking chemicals are disclosed in JP 62-15251 A, JP 62-20511 A, and JP 2007-16086 A. Ammonium sulfate is an essential raw material for producing these polymer dispersions in brine to obtain high-molecular-weight, low-viscosity products. Some impurities in ammonium sulfate cause polymerization reaction delays and low-molecular-weight products. It is generally known that the addition of masking agents can neutralize polymerization inhibition in the case of heavy metal ions, but no other methods have been established. From the perspective of stable production and maintaining the quality of polymerized products, it is necessary to minimize the amount of impurities. To improve the efficiency of the crystallization and granulation processes required to obtain ammonium sulfate at a purity higher than that required for industrial use, such as in reagents, a method for pretreating impurities is also needed. For example, Patent Document 1 proposes a method for removing impurities from ammonium sulfate by adding a reducing agent to an ammonium sulfate solution under pressure and heating it to a temperature of 200 to 350°C. However, this method requires an autoclave or a 280°C heater, making it unfeasible in factories that do not have such equipment. Patent Document 2 proposes a method for treating a Beckmann rearrangement mixture in an ammonium sulfate aqueous solution. This method involves separating ammonium sulfate from a lactam-containing organic liquid and then purifying the ammonium sulfate solution by crystallization, but it is not intended to reduce trace impurities in ammonium sulfate.

[0003] [Patent Document 1] Japanese Patent Application Publication No. 4-228419 [Patent Document 2] Special Publication No. 2003-507294 Summary of the Invention [Problem to be solved by the invention]

[0004] The present invention aims to reduce impurities contained in ammonium sulfate, which is used in a variety of fields, including fertilizer, pharmaceuticals, and industrial applications. In particular, the present invention aims to develop a method for neutralizing impurities in ammonium sulfate that cause polymerization failure and reaction delay when producing polymers dispersed in saltwater, which are used as flocculants for wastewater treatment and sludge dewatering, and as papermaking chemicals. The neutralization method is carried out under mild conditions at atmospheric pressure and at temperatures below 100°C, without the need for an autoclave or a heater capable of heating above 100°C. [Means for solving the problem]

[0005] The present inventors have conducted extensive research to solve the problems inherent in the prior art and have found that the addition of an oxidizing agent can neutralize reactive species and reduce impurities. In particular, the content of reactive species that can cause reduction reactions can be determined by adding an iron (III) salt, and then an oxidizing agent in an amount sufficient to determine the determined value can be added to neutralize the reactive species, which is an efficient method. [Effects of the Invention]

[0006] The present invention is characterized by not removing all impurities from ammonium sulfate, but by adding an oxidizing agent to primarily remove reactive species that may undergo reduction reactions. Furthermore, we discovered that the reactive species can be quantified by utilizing the reduction of added iron(III) to iron(II) due to the reducing power of impurities. This quantitative value allows the addition of the minimum necessary amount of oxidizing agent, thereby avoiding the risk of excess oxidizing agent remaining in ammonium sulfate and becoming a new impurity. Purification can be performed at temperatures below 100°C under relatively mild conditions. Therefore, the introduction of an autoclave or an ignition heater is unnecessary, allowing for low-cost implementation. When the ammonium sulfate of the present invention is used in the production of polymers dispersed in brine for use as flocculants for sludge dewatering and papermaking chemicals, polymerization failure and reaction delay do not occur. Furthermore, the present invention is effective as a pretreatment for purification processes such as crystallization and granulation when obtaining high-purity ammonium sulfate crystals. DETAILED DESCRIPTION OF THE INVENTION

[0007] In the present invention, when ammonium sulfate is solid, it is dissolved and then an oxidizing agent is added. Before adding the oxidizing agent, it is preferable to quantify the reactive species in ammonium sulfate that can cause a reduction reaction.

[0008] Ammonium sulfate is dissolved at a concentration in the range of 1 to 42% by mass. If the concentration is less than 1% by mass, the accuracy of the quantitative value of reactive species that can cause a reduction reaction decreases, and if the concentration exceeds 42% by mass, ammonium sulfate crystals will precipitate when the solution is at room temperature, making the solution heterogeneous.

[0009] The quantity of reactive species capable of causing a reduction reaction is determined by adding a soluble iron(III) salt. Iron(III) salts that can be used include iron chloride, iron sulfate, iron nitrate, iron phosphate, and their hydrates. Iron chloride hexahydrate is preferred because it is inexpensive and dissolves in demineralized water. These salts are dissolved in a concentration of 1 to 10% by mass. If the dissolved concentration is less than 1% by mass, the amount added to the ammonium sulfate solution increases, while if it exceeds 10% by mass, dissolution takes time. Acid may be added during dissolution, if necessary.

[0010] When quantifying reactive species that can cause reduction reactions, iron ions in iron (III) salts can be added in the range of 20 to 2000 ppm relative to ammonium sulfate. If the amount is less than 20 ppm, it becomes difficult to quantify all reactive species in ammonium sulfate, and if the amount is more than 2000 ppm, the ammonium sulfate solution becomes colored, interfering with the analysis, so the range of 20 to 2000 ppm is desirable.

[0011] After mixing the iron(III) salt solution and ammonium sulfate solution, let it stand until the reduction reaction from iron(III) ions to iron(II) ions has progressed. If it is left standing for less than one hour, the reaction may not proceed sufficiently, and if it is left standing for a long time, side reactions other than the reduction reaction may occur. A leaving time of 1 to 3 hours is desirable.

[0012] The amount of iron(III) ions reduced to iron(II) ions is determined by the iron-phenanthroline method. Phenanthroline can be added either as a solid or as a solution. The amount of phenanthroline added should be at least 32 times the mass of the iron(III) ions.

[0013] After adding phenanthroline to the ammonium sulfate solution, measure the absorbance at a wavelength of 510 nm using an ultraviolet-visible absorption spectrum after 3 minutes. If necessary, the sample may be diluted with demineralized water so that the measured value falls within the range of 0.1 to 1.0.

[0014] Using a calibration curve prepared in advance, the amount (moles) of iron (II) generated by reduction with reactive species in the ammonium sulfate solution is calculated. From the above, the number of moles of iron (II) can be determined by the following method (1). Method (1): Add 20 to 2000 ppm of iron ions from a soluble iron(III) salt to ammonium sulfate, and after one hour or more, quantify the iron(II) produced by the iron-phenanthroline method.

[0015] It is preferable to add 0.5 to 5 equivalents of the oxidizing agent to the ammonium sulfate solution based on the calculated amount (moles) of iron (II). If the amount is less than 0.5 equivalents, it is not possible to neutralize all reactive species, and if the amount is 5 or more equivalents, the oxidizing agent will remain in the ammonium sulfate solution, which may inhibit polymerization, cause crystallization, or result in impurities during granulation. 0.5 to 4 equivalents is preferred.

[0016] Considering that decomposition products remain in the ammonium sulfate solution, it is preferable that the oxidizing agent to be added does not contain heavy metals. Candidates include peroxides such as hydrogen peroxide, ammonium persulfate, and peracetic acid, as well as hypochlorous acid. From the viewpoint that the decomposition products are components of the ammonium sulfate solution, hydrogen peroxide or ammonium persulfate is preferred.

[0017] After adding an oxidizing agent to the ammonium sulfate solution, it is preferable to heat it in the range of 30 to 100°C. If the temperature is lower than 30°C, the acid oxidation-reduction reaction efficiency decreases as the temperature drops, and sufficient treatment effect cannot be obtained. If the temperature exceeds 100°C, equipment capable of heating, such as an oil bath or steam, is required. A temperature of 40 to 100°C is preferable.

[0018] If heating is required, continue for at least one hour after the solution temperature has stabilized. If the heating time is less than one hour, unreacted chemical species may remain.

[0019] The ammonium sulfate obtained by the purification method of the present invention can be used directly in a polymerization reaction. Alternatively, it may be used in further purification processes such as crystallization and granulation. The polymerization reaction can be used, particularly, in the production of polymer dispersions in brine, which are used as flocculants for wastewater treatment, sludge dewatering, and papermaking chemicals. Polymer dispersions in brine can be produced by known methods such as those disclosed in JP-A-62-15251, JP-A-62-20511, and JP-A-2007-16086. While ammonium sulfate is an essential raw material in the production of these polymer dispersions in brine, some impurities in ammonium sulfate can cause polymerization failure, reaction delay, and low molecular weight products. When used as a flocculant for wastewater treatment, sludge dewatering, or papermaking chemicals, the viscosity of a 0.5% by mass salt solution can be used as an indicator of molecular weight; a viscosity of 10 mPa·s or higher is preferred, with a viscosity of 15 mPa·s or higher being more preferred. By using ammonium sulfate purified by the purification method of the present invention, it is possible to produce a polymer dispersed in salt water without causing polymerization failure, reaction delay, or low molecular weight. The viscosity of a 0.5% by mass salt water solution is the viscosity when the polymer is dissolved in a 2% by mass ammonium sulfate aqueous solution to a polymer concentration of 0.5% by mass, and is measured using a Brookfield viscometer with a No. 1 rotor at 60 rpm. A Brookfield viscometer such as a TVB-10M from Toki Sangyo Co., Ltd. is used. [Example]

[0020] The method for purifying ammonium sulfate according to the present invention will be specifically described below, but the present invention is not limited to the following examples.

[0021] (Measurement of the amount of reactive species in ammonium sulfate) 2.10 g of commercially available industrial ammonium sulfate 1 (Industrial Grade 1) was weighed and dissolved in 2.90 g of demineralized water to obtain 5.00 g of 42% by weight ammonium sulfate solution (specific gravity = 1.2250). 0.05 g of 2% by weight ferric chloride hexahydrate solution was added to the ammonium sulfate solution and allowed to stand at room temperature for 1 hour, protected from light. 1.00 mL of 0.15% by weight phenanthroline solution was added to the ammonium sulfate solution and allowed to stand for 3 minutes. The solution was placed in a PMMA (acrylic) cell with a 1.0 cm optical path length, and the absorbance at 510 nm was measured using a UV-Vis spectrophotometer (JASCO V-650). Using the molar extinction coefficient (ε = 11785) from a previously prepared calibration curve, the number of moles of iron(III) ions reduced by reactive species in the ammonium sulfate solution was calculated using the following equation (1). The results are shown in Table 1. Similar measurements were also carried out on reagent ammonium sulfate (Kanto Chemical, special grade reagent) and a blank in which the amount of ammonium sulfate was replaced with demineralized water. The results are shown in Table 1. Formula (1): Amount of reduced iron(III) ions (moles per ammonium sulfate) = [Absorbance × (volume of ammonium sulfate solution + volume of iron(III) solution) × specific gravity (mL) + volume of phenanthroline solution (mL)] / [ε (L mol -1 ·cm -1 ) × cell length (cm) × amount of ammonium sulfate (g) × 1000

[0022] 0.21 g of commercially available industrial ammonium sulfate 2 (industrial grade 2) was dissolved in 4.79 g of demineralized water to obtain 5.00 g of 42% by weight ammonium sulfate solution (specific gravity = 1.0397). 0.05 g of 2% by weight ferric chloride hexahydrate solution was added to the ammonium sulfate solution and allowed to stand at room temperature for 1 hour, protected from light. 1.00 mL of 0.15% by weight phenanthroline solution was added to the ammonium sulfate solution and allowed to stand for 3 minutes. The solution was placed in a PMMA (acrylic) cell with a 1.0 cm optical path length, and the absorbance at 510 nm was measured using a UV-Vis spectrophotometer (JASCO V-650). The molar extinction coefficient (ε = 11785) was used to calculate the number of moles of iron(III) ions reduced by reactive species in ammonium sulfate using equation (1) based on a previously prepared calibration curve. The results of this measurement are shown in Table 1.

[0023] (Table 1) TIFF2026040825000001.tif2459

[0024] Example 1 To the industrial grade ammonium sulfate 1 for which the reactive species were measured, 4 equivalents of hydrogen peroxide per mole of reduced iron (III) ions were added, and the mixture was heated at 50°C for 6 hours. Every 2 and 6 hours, the amount of reactive species lost and the amount of hydrogen peroxide consumed were measured. The results are shown in Table 2 as Example 1-1. Similar tests were also conducted by changing the amount of ammonium sulfate, the amount of hydrogen peroxide added, and the temperature. The results are shown in Table 2 as Examples 1-2 to 1-4.

[0025] Comparative Example 1: The same amount of hydrogen peroxide as in Example 1-2 was added to an ammonium sulfate solution as a reagent and demineralized water of the same weight as the ammonium sulfate solution, and the amount of reactive species reduced and the amount of hydrogen peroxide consumed were measured every 2 and 6 hours. Similar tests were also conducted by changing the amount of hydrogen peroxide added and the temperature. These results are shown in Table 2 as Comparative Examples 1-1 to 1-4.

[0026] The amount of hydrogen peroxide (moles relative to ammonium sulfate) in Example 1 and Comparative Example 1 was calculated as follows. After 2 and 6 hours, 20.00 g of the 42% by mass ammonium sulfate solution containing added hydrogen peroxide was weighed, and 0.005 M potassium permanganate solution was added dropwise. The point at which the solution turned pale pink was used as the endpoint and the quantitative value was calculated. The blank titration amount was measured, and the amount of hydrogen peroxide was calculated using the following formula (2). Formula (2): Amount of hydrogen peroxide (moles per ammonium sulfate) = [(titration volume (mL) - blank titration volume (mL)) x 0.005 x 5] / (2 x 1000 x ammonium sulfate (g))

[0027] (Table 2) TIFF2026040825000002.tif75160

[0028] As shown in Table 2, when reactive species are present in ammonium sulfate, the amount of reactive species can be significantly reduced by adding hydrogen peroxide. Heating to 50°C after adding hydrogen peroxide was more effective than heating to 20°C.

[0029] Example 2 A polymer dispersion in salt water was produced by a known method using ammonium sulfate as the salt. The ammonium sulfate used was the same as in Example 1-1, which had been treated at 50°C for 2 hours after adding hydrogen peroxide. The production method is shown below. A reaction vessel equipped with a stirrer and a temperature control device was charged with 175.49 g of a 50 mass% aqueous solution of acrylamide (abbreviated as AAM), 75.20 g of an 80 mass% aqueous solution of acryloyloxyethyltrimethylammonium chloride (abbreviated as DMQ), 2.62 g of an 80 mass% aqueous solution of N-(2-acryloyloxyethyl)-N-benzyl-N,N-dimethylammonium chloride (abbreviated as DMABC), 71.25 g of a 20 mass% aqueous solution of polyacryloyloxyethyltrimethylammonium chloride, 4.05 g of itaconic acid, 7.06 g of an 85 mass% solution of glycerin, and 658.15 g of a 42 mass% aqueous solution of ammonium sulfate, and the contents were mixed and completely dissolved. The temperature inside the vessel was maintained at 36°C, and after 30 minutes of nitrogen substitution, 0.012 g of 2,2'-azobis[2-(2-imidazolin-2-yl)propane] dihydrochloride (VA-044, Fujifilm Wako Pure Chemical Industries, Ltd.) was added. Polymerization was carried out for 24 hours at a reaction temperature of 36±2°C to obtain a polymer dispersed in saltwater (AAM / DMQ / DMABC = 79.5 / 20 / 0.5 mol% copolymer). The reaction rate was also measured 3, 6, and 20 hours after the addition of the polymerization initiator. These results are shown in Table 3 as Example 2-1. Similarly, polymer dispersed in saltwater was produced under the same production conditions using ammonium sulfate industrial grade 1 and 2, which were treated for 2 hours with different amounts of hydrogen peroxide and treatment temperatures. These results are shown in Table 3 as Examples 2-2 to 2-8.

[0030] Comparative Example 2 Polymers dispersed in salt water were produced under the same conditions as in Example 2-1 using ammonium sulfate for industrial use 1 and 2 that had not been treated with hydrogen peroxide. The results are shown in Table 3 as Comparative Examples 2-1 and 2-2.

[0031] (Table 3) TIFF2026040825000003.tif77159Product viscosity: Viscosity (mPa·s) of the product measured at 25°C. Viscosity of aqueous salt solution: Viscosity (mPa·s) measured at 25°C when the polymer is dissolved in a 2% by mass aqueous ammonium sulfate solution to a concentration of 0.5% by mass.

[0032] The hydrogen peroxide required to neutralize reactive species in aqueous ammonium sulfate solution was found to be particularly effective at 1 to 4 equivalents relative to the reactive species. Heating conditions after the addition of hydrogen peroxide showed an improvement, with an increase in reaction rate compared to the comparative example, even at room temperature. However, the reaction rate was higher at 3 and 6 hours compared to treatment at 40 and 50°C, which may lead to gelation during polymerization and insufficient viscosity of the polymer salt solution. Based on these results, the treatment temperature after the addition of hydrogen peroxide should be in the range of 15 to 100°C, with 30°C or higher being preferred, and 40°C or higher being even more preferred.

[0033] The method for purifying ammonium sulfate according to the present invention provides a method for easily neutralizing reactive species that can cause reduction reactions among impurities contained in ammonium sulfate without labor-intensive and costly procedures such as recrystallization or membrane filtration. When the ammonium sulfate according to the present invention is used in the production of brine dispersions or water-in-oil emulsions used in wastewater treatment, sludge dewatering flocculants, and papermaking chemicals, polymerization failure and reaction delay do not occur. Furthermore, the present invention is effective as a pretreatment for purification processes such as crystallization and granulation when obtaining high-purity ammonium sulfate crystals, and has extremely high industrial utility value.

Claims

1. A method for purifying ammonium sulfate, comprising adding an oxidizing agent to ammonium sulfate under atmospheric pressure.

2. The method for purifying ammonium sulfate according to claim 1, wherein the amount of the oxidizing agent added is 0.5 to 5 equivalents of the number of moles of iron (II) calculated by the following method (1): Method (1): Add 20 to 2000 ppm of iron ions in a soluble iron (III) salt to ammonium sulfate, and after one hour or more, quantify the iron (II) produced by the iron-phenanthroline method.

3. 2. The method for purifying ammonium sulfate according to claim 1, wherein the oxidizing agent in the purification method is hydrogen peroxide.

4. The method for purifying ammonium sulfate according to claim 1, characterized in that after adding the oxidizing agent according to claim 1, the mixture is heated to 30 to 100°C.

5. A method for producing a polymer dispersed in salt water, comprising using ammonium sulfate purified by the purification method of claim 1.

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