Coagulant, coagulant manufacturing method and fluorine in waste water removal method

Abstract

To provide a new technology capable of sufficiently reducing fluorine concentration in waste water when removing fluorine in waste water using a calcium salt.SOLUTION: A coagulant for treating calcium fluoride containing ettringite coagulates calcium fluoride in waste water containing calcium fluoride.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present disclosure relates to a flocculant used in the treatment of fluorine in wastewater. [Background technology]

[0002] In the fields of metal, semiconductor, and glass manufacturing, chemicals containing fluorine are used in processes such as pickling and etching, and the wastewater from these processes may contain fluoride ions. In addition, hydrogen fluoride gas generated during the decomposition and treatment of alternative fluorocarbons is usually absorbed in water and collected in a smoke scrubber, and the absorbed water contains fluoride ions. Fluorine is also used in products in various fields, such as fluororesins and lubricants.

[0003] The fluoride ions in the wastewater can be removed by adding powdered calcium salts such as slaked lime to generate insoluble calcium fluoride, which is then separated into solid and liquid form. However, this method may not be able to sufficiently reduce the fluorine concentration, and clogging caused by calcium fluoride, which is in the form of fine crystals, can easily become a problem.

[0004] As a method for solving the difficulty of solid-liquid separation, a coagulant is added together with the calcium salt to coagulate the resulting calcium fluoride microcrystals. In this method, the apparent particle size and specific gravity of calcium fluoride are increased, allowing efficient separation by settling, centrifugation, filtering, etc. As the coagulant, aluminum sulfate, PAC (polyaluminum chloride), polymer coagulants, etc. are used.

[0005] As a method for solving the difficulty of solid-liquid separation, Patent Document 1 proposes a fluorine adsorbent in which lime-based powder is molded into bodies of several mm or more using a cellulose-based binder, and the bodies can be taken out as calcium fluoride molded bodies after fluoride recovery from a fluoride-containing solution. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Patent Publication No. 2022-085501 Summary of the Invention [Problem to be solved by the invention]

[0007] The problem of solid-liquid separation can be reduced by flocculating calcium fluoride. When fluoride ions in wastewater are removed from the wastewater by reacting with calcium, insoluble calcium fluoride is produced according to the following formula (1), so theoretically it is possible to reduce the fluorine concentration to the solubility of calcium fluoride. However, in practice, it is virtually difficult to completely proceed with the reaction of formula (1) due to various reasons, so there are cases where a sufficient reduction in the fluorine concentration in wastewater cannot be achieved. Ca 2+ +2F - →CaF2↓ (1)

[0008] In such a case, the supernatant after the coagulation treatment needs to have its fluoride ion concentration reduced by using an anion exchange resin, a chelating resin, etc. However, such a method has the problem of being very costly.

[0009] An object of the present disclosure is to propose a novel technology capable of sufficiently reducing the fluorine concentration in wastewater when fluorine in the wastewater is removed using a calcium salt. [Means for solving the problem]

[0010] One aspect of the present disclosure is a flocculant for flocculating calcium fluoride in wastewater containing calcium fluoride, the flocculant including ettringite for treating calcium fluoride.

[0011] With such a configuration, it is possible to realize the aggregation of calcium fluoride by the aggregation effect of ettringite and the adsorption of fluorine in the wastewater by ettringite, and not only to promote solid-liquid separation by the aggregation of calcium fluoride but also to reduce the fluoride ion concentration in the wastewater. Therefore, by using the flocculant of the present disclosure, it is possible to highly remove fluorine from wastewater containing calcium fluoride.

[0012] Another aspect of the present disclosure is a method for removing fluorine from wastewater, comprising: adding a flocculant containing ettringite to the wastewater containing calcium fluoride. In this method, the flocculation of calcium fluoride and the adsorption of fluoride ions in the wastewater are realized by the flocculating agent containing ettringite, so that fluorine in the wastewater can be removed to a high extent.

[0013] The above-mentioned fluoride removal method may include a step of producing calcium fluoride by adding a calcium salt to the wastewater containing fluoride ions, the calcium salt being capable of reacting with fluoride ions to produce calcium fluoride, prior to adding the coagulant to the wastewater.

[0014] With this method, by adding a calcium salt to wastewater containing fluoride ions, most of the fluorine can be converted into calcium fluoride, and then the calcium fluoride can be coagulated by a coagulant and the fluoride ions in the wastewater can be removed by adsorption.

[0015] Yet another aspect of the present disclosure is a method for producing a flocculant for calcium fluoride treatment, the method comprising stirring a mixture containing a calcium-containing compound, an aluminum-containing compound, and a sulfuric acid-containing compound in water, filtering the stirred mixture, and then drying the filtered mixture.

[0016] Such a production method can produce the flocculant according to one aspect of the present disclosure. The drying step may include a step of drying at 70° C. or higher. The drying step may include a step of drying at 100° C. or higher. [Brief description of the drawings]

[0017] [Figure 1] Photographs showing the sedimentation state using the flocculants of Examples 1 and 2 and aluminum sulfate. [Diagram 2] Photographs showing the sedimentation state using the flocculants of Examples 1 and 3 and aluminum sulfate. [Diagram 3] 1 is a photograph showing the state of precipitation when the flocculants of Examples 4 to 6 were added to the test solution at 0.2%. [Figure 4] 1 is a photograph showing the sedimentation state when the flocculants of Examples 4 to 6 were added to the test solution at 0.5%. [Diagram 5] 1 is a photograph showing the sedimentation state when the flocculants of Examples 4 to 6 were added to the test solution at 1.0%. [Figure 6] 1 is a graph showing the results of XRD measurement of the flocculants of Examples 4 to 6. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] Hereinafter, an embodiment of the present disclosure will be described with reference to the drawings. [1. Embodiment] [1-1. Composition of flocculant] The flocculant disclosed herein is a flocculant for treating fluorine in wastewater and contains ettringite. Ettringite is a mineral expressed as 3CaO·Al2O3·3CaSO4·32H2O and is one of the components contained in cement.

[0019] At least a part of the flocculant of the present disclosure is composed of ettringite. Among the components constituting the flocculant, there is no particular limitation other than ettringite. For example, aluminum, calcium, sulfate ions (sulfates), etc., which do not constitute ettringite, may be contained in the flocculant. Naturally, components other than those mentioned above may also be contained.

[0020] Ettringite has a function of aggregating calcium fluoride in wastewater and an ability to adsorb fluoride ions in the wastewater, and therefore can promote separation of calcium fluoride from the wastewater and reduce the concentration of fluoride ions in the wastewater.

[0021] The term "wastewater" as used herein broadly refers to water that contains fluoride ions or calcium fluoride. In other words, wastewater is not necessarily limited to water discharged from factories, etc., but broadly includes water generated during a specific process, water that is reused, etc. In addition, "for fluoride treatment" means that the wastewater is used to remove fluoride ions and calcium fluoride contained in the wastewater.

[0022] [1-2. Manufacturing of flocculants] The flocculant of the present disclosure can be manufactured using as raw materials (i) a compound containing calcium, (ii) a compound containing aluminum, and (iii) a compound containing sulfuric acid. A compound containing two or more of calcium, aluminum, and sulfate may be used as a raw material. Specific examples of each compound will be described later.

[0023] In a mixture of these compounds, the molar ratio is Ca:Al:SO4 2- The part where the ratio is 6:2:3 is the part that can be synthesized as ettringite. Of course, the mixing ratio in the material is not limited to the above ratio.

[0024] An example of the production method includes a process of mixing the above-mentioned compounds, a process of mixing and stirring the mixture in room temperature water, a filtration process, a process of drying the filtered residue, and a process of pulverizing the dried mixture. The room temperature water can be, for example, in the range of 0°C to 30°C. This temperature range is convenient because it does not require special temperature adjustment in the mixing and stirring process.

[0025] The drying temperature can be, for example, 40°C or higher, but even if it is dried at a high temperature of 70°C or higher, it can exhibit a high fluorine removal function. The drying time can be, for example, 12 hours or more, but it may be continued until a constant weight is reached at that drying temperature. Since it is known that the skeleton of ettringite is maintained up to 150°C (Kinichi Kira, Yoshio Makino, Yoshihiko Murata, "Dehydration and Rehydration of Ettringite", Gypsum & Lime No. 170 (1981)), the drying temperature may be 150°C or lower. Drying at 100°C or higher can shorten the drying time.

[0026] Various known methods for producing ettringite can be adopted for producing the flocculant of the present disclosure. For example, Japanese Patent No. 6591343 discloses a method for producing amorphous ettringite by dissolving slaked lime in an aqueous aluminum sulfate solution to obtain a slurry containing ettringite, and then performing a solid-liquid separation and drying process. Regarding the characteristics of ettringite, it is known that ettringite produced from blast furnace slag decomposes at high temperatures, but when synthetic ettringite decomposed by 94% at 90°C in dry air is held in pure water at 25°C, about 60% of ettringite is regenerated (Aya Harashima, Kimihisa Ito, "Conditions for producing ettringite by reaction of blast furnace slag with alkaline aqueous solution" (Tetsu-to-Hagane Vol. 101 (2015) No. 11)).

[0027] (Production Example 1) Aluminum sulfate (Al2(SO4)3) and quicklime (CaO) or hydrated lime (Ca(OH)2) are suspended in water and mixed for a certain period of time, then filtered. The filtered residue is dried and then crushed.

[0028] By adding an amount of calcium (Ca) in quicklime or slaked lime that is 6 times the molar ratio of aluminum sulfate, a flocculant containing a high proportion of ettringite can be obtained. By adding an amount of calcium that is 10 times or less the molar ratio of aluminum sulfate, a flocculant containing a sufficient proportion of ettringite can be obtained. Here, if the amount of calcium is excessive relative to aluminum sulfate, the amount of insoluble slaked lime that does not contribute to the ettringite formation reaction increases, and the amount of precipitate increases. On the other hand, if the amount of calcium is too little, the function as a flocculant can be maintained, but the proportion of ettringite formed decreases. In order to ensure a sufficient proportion of ettringite, it is preferable that the amount of calcium is 4 times or more relative to aluminum sulfate, and more preferably 5 times or more. In order to suppress an increase in precipitates unrelated to flocculation, the above ratio is preferably 8 times or less, and more preferably 7 times or less.

[0029] Also, waste containing aluminum sulfate as a main component may be used as a raw material. In this case, the amount of the waste may be adjusted so that the concentration of other components contained in the waste in the supernatant water after use as a flocculant is within a range that does not violate emission regulations. Of course, industrial aluminum sulfate used as an additive for commercially available aluminum sulfate for water treatment, acid paper, or fire extinguishing agents may also be used.

[0030] In addition, calcium compounds other than quicklime and slaked lime can be used as the Ca source as long as they do not leave harmful substances in the supernatant after use. Of course, waste materials containing calcium salts can also be used.

[0031] (Production Example 2) Slag generated during the manufacture of steel and other products, which contains calcium oxide (CaO), silicon oxide (SiO2), and aluminum oxide (Al2O3), is used as the raw material to make up for the deficiency in sulfuric acid (SO4 2- ) can be added to obtain a flocculant containing ettringite.

[0032] The lower limit of the sulfuric acid content in the coagulant is the same molar amount of sulfuric acid as the aluminum oxide contained in the slag, and the upper limit is the total molar amount of sulfuric acid of the aluminum oxide and calcium oxide. If the amount of sulfuric acid is equal to or greater than the molar amount of aluminum oxide, a sufficient amount of ettringite is generated. If the amount of sulfuric acid is equal to or less than the total molar amount of aluminum oxide and calcium oxide, the pH is prevented from shifting to the acidic side when the coagulant is added to water, and a sufficient coagulation effect can be achieved. If the amount of sulfuric acid is equal to or less than the total molar amount of aluminum oxide and calcium oxide, any sulfuric acid in excess of the amount required to generate ettringite reacts with the calcium content and exists as gypsum (CaSO4), but this does not adversely affect the coagulation or fluoride removal capabilities. As in Production Example 1, sulfuric acid and slag in the amounts in the above ranges are suspended in water, mixed for a certain period of time, filtered, and the residue is dried and pulverized to obtain a flocculant.

[0033] (Production Example 3) The third method for obtaining the flocculant according to the present disclosure is to use multiple substances containing Ca, Al, and SO4 as raw materials.

[0034] Examples of calcium sources include metallic calcium, limestone CaCO3, quicklime CaO, slaked lime Ca(OH)2, and calcium salts such as gypsum CaSO4 and calcium chloride CaCl2. The Al source may be metallic Al, aluminum hydroxide, aluminum oxide, or an aluminum salt such as aluminum sulfate, aluminum nitrate, or aluminum chloride. Sources of SO4 include sulfuric acid, H2SO4, or sulfate salts such as sodium sulfate or ammonium sulfate.

[0035] These substances can be made from not only industrially produced reagents and raw materials, but also by-products and waste materials from factories, etc. For example, waste gypsum (calcium sulfate) and aluminum sludge, a by-product of aluminum production, can be used.

[0036] These raw materials are mixed and then stirred in water at room temperature for a certain period of time to obtain a precipitate containing ettringite. The precipitate is subjected to solid-liquid separation by a method such as filtration, and excess water is removed by drying to obtain the flocculant of the present disclosure. When the raw materials are mixed, a mixture containing at least three times as much Ca as the amount of Al contained in the raw materials by molar ratio and containing no more than the molar amount of Al and Ca combined is used. 2- The ratio of each raw material may be adjusted so as to obtain the above.

[0037] [1-3. Example] [Example 1] 58.46g of commercially available aluminum sulfate (aluminum sulfate, manufactured by Kyokuyo Co., Ltd.) was mixed with 41.24g of JIS special slaked lime (special slaked lime as specified in JIS standard R9001:2006 industrial lime, the same applies below) together with water, and the mixture was stirred overnight. After filtration and drying at 120°C, the weight of the filtrate was 88.13g. This was mechanically crushed to obtain 78.46g of flocculant 1.

[0038] 1.63 g of this flocculant was added to 25 ml of hydrofluoric acid solution with a fluorine concentration of 465 mg / L, and then it was shaken well and left to stand. After 10 minutes, the supernatant was colorless and transparent, and the fluorine concentration was 3.6 mg / L. This confirmed that flocculant 1 has flocculation effect and fluorine adsorption effect.

[0039] [Example 2] 10.08 g of steel slag having the composition shown in Table 1 was mixed with 12.6 g of concentrated sulfuric acid and water, and the mixture was stirred overnight. After filtration and drying at 120°C, the weight of the residue was 14.6 g. The residue was ground in a mortar until it passed through a 1 mm sieve, and flocculant 2 was obtained.

[0040] [Table 1]

[0041] [Example 3] 20g of dried aluminum hydroxide sludge discharged from an aluminum manufacturer and 54g of JIS special grade slaked lime were mixed in water, 38g of concentrated sulfuric acid was added, and the mixture was stirred overnight. After filtration and drying at 120°C, the weight of the filtrate was 98g. This was ground in a mortar until it passed through a 1mm sieve, and flocculant 3 was obtained.

[0042] [Example 4] 600g of aluminum sulfate and 444g of JIS special grade hydrated lime were mixed with water overnight, and the residue was dried at 40°C to prepare a sample (flocculant 4). In addition, 600 g of aluminum sulfate and 444 g of JIS special grade slaked lime were mixed with water overnight, and the filtrate was dried at a temperature of 70°C to prepare a sample (flocculant 5). In addition, 600 g of aluminum sulfate and 444 g of JIS special grade slaked lime were mixed with water overnight, and the filtrate was dried at a temperature of 120°C to prepare a sample (flocculant 6).

[0043] [1-4. Evaluation test] [Evaluation test 1] 0.35g of slaked lime was added to a hydrofluoric acid solution adjusted to a concentration of 435mg / L, and then stirred for 1 hour. 50ml of this solution was measured into each cylindrical resin container with a diameter of 30mm and a height of 80mm. The fluorine concentration after the slaked lime treatment was 52.5mg / L.

[0044] To this solution, 2.4 g each of Flocculant 1 and Flocculant 2 prepared in Example 1 and Example 2, and also commercially available aluminum sulfate were added, and after shaking well, the solution was left to stand for 2 hours. The state of settling after 1.5 hours is shown in Figure 1. The fluorine concentration in the supernatant after 2 hours was measured, and the results are shown in Table 2.

[0045] [Table 2]

[0046] In the case of no flocculant, sediment was observed, but the supernatant remained cloudy after 1.5 hours. No change was observed in the fluorine concentration. In the cases of adding flocculant 1, flocculant 2, and commercial aluminum sulfate, the supernatant became almost colorless and transparent in all three cases, but while aluminum sulfate did not show a decrease in the fluorine concentration, flocculant 1 and flocculant 2 (Examples 1 and 2) both showed a significant decrease in the fluorine concentration.

[0047] [Evaluation test 2] Hydrofluoric acid was diluted to prepare 400 L of an aqueous solution with a fluorine concentration of approximately 1000 mg / L. 25 kg of fluorine adsorbent (cellulose / slaked lime composite molding, manufactured by Ueda Lime Manufacturing) was added to the aqueous solution, and the fluorine in the liquid was collected as calcium fluoride. Most of the calcium fluoride produced remained on the fluorine adsorbent, but small particles that separated from the adsorbent did not settle but remained in the liquid, causing the supernatant liquid to become cloudy. The fluorine concentration in this supernatant was collected and measured, finding it to be 183 mg / L.

[0048] To 50 ml of this supernatant, 1.5 g of commercially available aluminum sulfate and each of the flocculants produced in Examples 1 and 3 were added, and the mixture was vigorously shaken and then allowed to stand for 2 hours. The sedimentation state when this mixture was left to stand is shown in Figure 2. The concentration in the supernatant after 2 hours was measured, and the results are shown in Table 3.

[0049] [Table 3]

[0050] The supernatant of both flocculants 1 and 3 was colorless and transparent, demonstrating their flocculating effect, but most of the aluminum sulfate added dissolved.Also, aluminum sulfate was effective in reducing the fluoride concentration by about one-third, from 183mg / L to about 50mg / L, but flocculants 1 and 3 reduced it to about 3mg / L, demonstrating that they have a much higher fluoride removal capacity than aluminum sulfate.

[0051] [Evaluation test 3] As in Evaluation Test 2, the fluorine adsorbent (manufactured by Ueda Lime Manufacturing Co., Ltd.) used in Evaluation Test 2 was added to an aqueous solution with a fluorine concentration of about 1000 mg / L, and the fluorine in the solution was recovered as calcium fluoride. Then, the supernatant liquid with a fluorine concentration of 183 mg / L was recovered.

[0052] The sedimentation state when a fixed amount of each of flocculants 4 to 6 from Example 4 was added to 50 ml of this supernatant is shown in Figures 3 to 5. Figure 3 shows the sedimentation state when 0.2% of the flocculant was added to the test liquid. Figure 4 shows the sedimentation state when 0.5% of the flocculant was added to the liquid. Figure 5 shows the sedimentation state when 1.0% of the flocculant was added to the liquid. Table 4 shows the results of measuring the fluoride concentration in the supernatant 18 hours after the start of the coagulation treatment.

[0053] [Table 4]

[0054] It can be seen that there is almost no difference in the settling state due to the drying temperature. Also, although there is a slight difference in the settling rate depending on the addition rate, the difference is not significant. It can be seen that the fluoride removal effect also showed a large reduction from the initial concentration of 183 mg / L at all drying temperatures and dosage amounts, but when comparing fluoride concentrations at dosage amounts of 0.5% or more at drying temperatures of 70°C to 120°C, the fluoride concentration values ​​were slightly lower than those at dosage amounts of 0.5% or more at 120°C. In particular, it can be seen that the value for the 120°C dried sample with a dosage of 0.5% or more was below the fluoride wastewater standard.

[0055] <Confirmation of the presence of ettringite> XRD measurement was performed on flocculants 4 to 6 produced in Example 4. The results are shown in Figure 6. From the graph shown in Figure 6, it was confirmed that the ettringite phase was contained. Looking at the crystal peak near 2θ = 9°, which indicates the ettringite phase, it can be seen that a small amount of ettringite is observed at 40°C, and ettringite is predominant at 70°C to 120°C.

[0056] From the X-ray diffraction (XRD) at each drying temperature, it was found that the ettringite phase was contained even when drying was performed at 120°C in the drying process. The flocculant according to the present disclosure does not need to be amorphous, and as long as it contains ettringite even in part, it exhibits flocculation and fluorine adsorption performance. Therefore, drying can be performed at any temperature between 60°C and 120°C that is usually used industrially.

[0057] The preferred drying temperature is 70°C or higher, which provides a high fluorine removal function. In particular, when the amount of flocculant added exceeds 0.5%, the higher the drying temperature is (for example, at 100°C or higher), the higher the fluorine removal function is. If the drying temperature is 100°C or higher, the drying time can be shortened. Therefore, the drying temperature may be 100°C or higher. In addition, the drying temperature may be 150°C or lower, which can maintain the ettringite skeleton.

[0058] [1-5. Effects] By performing fluorine removal treatment from wastewater using flocculants 1 to 6 of the present disclosure, a much higher level of fluorine removal could be achieved compared to the case where no flocculant was used or the case where aluminum sulfate was used as the flocculant.

[0059] In addition, when flocculants 1, 3, and 6 were used, the fluorine concentration could be lowered to less than 8 mg / L. Since the solubility of calcium fluoride in water is 16 mg / L, 7.8 mg / L of fluoride in terms of fluoride ions remains in the liquid, which is slightly lower than the wastewater standard of 8 mg / L. However, even if calcium salt is actually added in an amount several times the fluorine equivalent, the fluorine concentration can only be reduced to about several tens of mg / L. With the flocculant of the present disclosure, the fluorine concentration in the wastewater can be reduced to an extremely low value by utilizing adsorption to ettringite.

[0060] In addition, since the calcium fluoride produced is in the form of fine crystals, it is practically difficult to separate the calcium fluoride from the wastewater by solid-liquid separation methods such as sedimentation or centrifugation, which utilize the difference in specific gravity, because the difference in specific gravity between the wastewater and the calcium fluoride is small, and the separation takes a long time. In addition, when performing solid-liquid separation by filtering or other methods, the filter needs to be fine, so clogging occurs even with a small amount of processing, making the efficiency very poor. However, the flocculant of the present disclosure can promote solid-liquid separation by flocculating calcium fluoride and can suppress clogging of the filter.

[0061] [2. Other embodiments] Although the embodiments of the present disclosure have been described above, it goes without saying that the present disclosure is in no way limited to the above-described embodiments, and can take various forms as long as they fall within the technical scope of the present disclosure.

[0062] For example, the configuration in which a calcium salt is added to wastewater to generate calcium fluoride (or together with the addition of the calcium salt) and then a flocculant is added has been exemplified, but the flocculant may be added without adding the calcium salt. Even in this case, the fluoride ion concentration can be reduced by adsorption by ettringite.

Claims

1. A flocculant for flocculating calcium fluoride in wastewater containing calcium fluoride, comprising: Flocculants for calcium fluoride processing, including ettringite.

2. A method for removing fluorine from wastewater, comprising: adding a flocculant containing ettringite to the wastewater containing calcium fluoride.

3. The method for removing fluorine from wastewater according to claim 2, a step of adding a calcium salt to the wastewater containing fluoride ions to produce calcium fluoride upon reaction with fluoride ions, to produce calcium fluoride, before adding the coagulant to the wastewater.

4. A method for producing a flocculant for calcium fluoride treatment, comprising the steps of: Stirring a mixture containing a calcium-containing compound, an aluminum-containing compound, and a sulfuric acid-containing compound in water; filtering and then drying the stirred mixture; A method for producing a flocculant, comprising:

5. The method for producing a flocculant according to claim 4, wherein the drying step includes a step of drying at 70° C. or higher.

6. The method for producing a flocculant according to claim 4, wherein the drying step includes a step of drying at 100° C. or higher.

Images