NL1011698C2 - Removal of fluoride from waste water comprises using a fluid bed crystallizer with a support, adding a water-soluble sodium and aluminum reagents to form cryolite and removing the treated water - Google Patents

Abstract

Crystallization process for removing fluoride from waste water comprises introducing the water to a fluid bed crystallizer with a support, adding a water-soluble sodium reagent and a water-soluble aluminum reagent to give cryolite (Na3AlF6), the mol. ratio of Al:F being 0.8-1.6 and of Na:F being above 0.5, and removing the treated water.

Description

Crystallization method for removing fluoride from wastewater

The present invention relates to a crystallization process for removing fluoride from wastewater in an economical and efficient manner.

In a variety of industries including semiconductor, chlorofluorocarbon (CFC) and glass production, a large amount of fluoride-containing wastewater with a high concentration of fluoride is produced. Thus, many researchers have attempted to remove fluoride from the fluoride-containing wastewater.

10 Jansen, in U.S. Pat.

5,106,509 describe a crystallization method for removing fluoride from wastewater in a fluidized bed reactor. The method includes adding CaCl 2 to wastewater to react calcium ions and fluoride in the wastewater to form calcium fluoride crystals. Compared to the coagulation / precipitation method currently used in factories, the advantages of Jansen's method are that the waste sludge is reduced and can be reused. Since calcium fluoride has a very low solubility, such a crystallization process can effectively remove a large amount of fluoride. However, another consequence of the low solubility is that calcium fluoride will quickly supersaturate in some places, generating fine particles that can plug the pipes. For this reason, in a practical application, wastewater removed from the factory should be diluted with a high concentration of fluoride to a concentration below 500 mg F / l. In order to house such a large amount of diluted wastewater, the cost of the facility and the land to accommodate the facility increases.

Japanese Patent No. 8-11232 has described a coagulation process for removing fluoride from wastewater. The method involves mixing fluoride-containing wastewater with aluminum and sodium ions to form cryolite coagulate as a means of removing fluoride. The residual fluoride contained in the treated wastewater can be further removed. A water-soluble aluminum salt is added to the treated water at pH 7 to form aluminum hydroxide [Al (OH) 3] flakes which can adsorb residual fluoride in the treated waste water to form a co-precipitate. The coprecipitate can be made basic or acidic, thus forming an aluminum ion source for the coagulation process to remove fluoride from wastewater according to the procedures described above.

In Japanese Patent No. 8-11232, the cryolite is in a form of coagulation. The coagulated sludge contains approximately 60-80% water, which is very difficult to dehydrate.

In addition, the coagulated sludge is present in a very large volume and has many impurities; thus it is not easily reused.

An object of the present invention is to solve the above problems and to provide an economical and efficient crystallization method for removing fluoride from wastewater.

To accomplish this goal, the crystallization process for removing fluoride from wastewater comprises the following steps of: (a) introducing fluoride-containing wastewater into a supported fluidized bed crystallizer; (b) adding a water-soluble sodium reagent and a water-soluble aluminum reagent to the fluidized bed crystallizer to form crystallized cryolite (Na3AlFg) on the support capable of removing the fluoride in the wastewater, the molar ratio of aluminum to fluorine is between 0.8-1: 6 and the molar ratio of sodium to fluorine is greater than 0.5; and (c) removing the treated wastewater from the fluidized bed crystallizer to yield a primary treated water.

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The present invention will be more fully understood from the detailed description given below and the accompanying drawings, which are given by way of illustration only and are not, therefore, intended to limit the present invention.

Fig. 1 is a schematic diagram showing the crystallization process for removing fluoride from wastewater according to Example 1 of the present invention.

FIG. 2 is a schematic diagram showing the crystallization process for removing fluoride from wastewater according to Example 2 of the present invention.

Fig. 3 shows the X-ray diffraction diagram of the cryolite crystal obtained in the fluidized bed crystallizer of the present invention.

The present invention provides a crystallization method for removing fluoride from wastewater. Fluoride-containing waste water is introduced into a fluidized bed crystallizer provided with a carrier. Then, a water-soluble sodium reagent and a water-soluble aluminum reagent are added to the fluidized bed crystallizer. To form cryolite (Na3AlFg), the amount of sodium reagent and aluminum reagent added should be such that the molar ratio of aluminum to fluorine is between 0.8-1: 6, and the molar ratio of sodium to fluorine is greater than 0.5. By such ratios, cryolite (Na3AlFg) will be crystallized on the support with which the fluoride in the waste water can be removed. Finally, the treated wastewater is removed from the fluorinated bed crystallizer to yield a primary treated water.

In order to improve the quality of the primary treated water, it can be further treated. A water-soluble aluminum reagent is added to the primary treated water. Then, the primary treated water is adjusted to a pH of about 7 by an hydroxide of an alkali metal to form an insoluble aluminum hydroxide flake. The aluminum hydroxide flake will adsorb the 1011698 4 fluoride in the primary treated water to form a co-precipitate, allowing the residual fluoride in the primary treated water to be further removed. Finally, the obtained treated waste water is separated from the coprecipitate to yield a secondary treated water.

In order to obtain treated water of a desired quality, the secondary treated water can be further treated. The treatment procedures include adding a water-soluble aluminum reagent to the treated water and adjusting the pH to form insoluble aluminum hydroxide to adsorb the fluoride to form the coprecipitate. Such treatment procedures can be repeated if desired until the finally treated water reaches a desired concentration.

The co-precipitate of aluminum hydroxide and fluoride as described above can be reused to serve as the aluminum source for the crystallization process. Specifically, the co-precipitate is adjusted to a pH of less than 3 or more than 11, whereby the coprecipitate is dissolved to form aluminum ions. Such aluminum ions can serve as the water-soluble aluminum reagent that is added to a fluidized bed crystallizer to undergo a further crystallization process to remove fluoride.

In order to form as much cryolite crystal as possible, to remove as much fluoride as possible during crystallization, the supported large particle cryolite can be removed from the fluidized bed crystallizer and replaced with a fresh support to form additional cryolite crystallization on the carriers. The water-soluble aluminum reagents suitable for use in the present invention include aluminum chloride, aluminum sulfate and aluminum nitrate.

The water-soluble sodium reagents suitable for use in the present invention include sodium chloride, sodium hydroxide, sodium sulfate and sodium nitrate.

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In U.S. Patent No. 5,106,509, a calcium reagent is used to react with fluoride in the wastewater to form calcium chloride crystal. In the present invention, a sodium reagent and an aluminum-5 reagent are added to react with fluoride in the waste water to form cryolite crystal. The price of the cryolite crystal is ten times that of the calcium chloride, so it is more valuable to recycle and reuse the cryolite crystal. In addition, the aluminum reagent used in the present invention can be recycled for reuse, while the calcium reagent used in US Patent No. 5,106,509 cannot. For treating the wastewater containing a high concentration of fluoride, since cryolite has a high solubility (about 100-200 mg F "/ l), it is very simple to ensure that most cryolite formed is in the is in the form of crystals, and fine particles are not formed even though the wastewater is not diluted.The crystallized cryolite ratio reaches about 91-20 97%, and the device and energy cost in the present invention is about one-tenth of that in U.S. Patent No. 5,106,509, so the process of the present invention is much more efficient.

In Japanese Patent No. 8-11232, the cryolite formed is in the form of a coagulate. The coagulated sludge contains approximately 60-80% water, which is very difficult to dewater. In addition, the coagulated sludge is present in a very large volume and has many impurities; thus it is not easily reused. In the present invention, the cryolite formed is in the form of crystals, having a diameter of 1 to 2 mm. Such crystals contain approximately less than 10% water, and are therefore very easy to dewater. Similarly, because they have a small volume, they are very easy to transport. Furthermore, the crystal has a very high purity (higher than 90%), and is therefore suitable for reuse. In addition, the fluidized bed crystallizer support has a high surface area to react with fluoride and chemicals; thus it is more efficient.

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Example 1

Reference to Fig. 1, which is a schematic diagram showing the crystallization process for removing fluoride from wastewater according to Example 1 of the present invention.

A fluidized bed crystallizer (FBC) was filled with water and provided with an appropriate amount of carrier (25). Circulated water (26) was introduced into the FBC to fluidize the support. Acid fluoride-containing wastewater 10 (11) at a concentration of 5,500 mg F ~ / l was introduced into the pH adjusting kettle and adjusted to pH 5 with NaOH (12). The modified wastewater (13) was then introduced into the FBC provided with a support (25) at a flow rate of 10 ml / min. An aluminum reagent (14) with a pH of 3.8 and an aluminum ion content of 1,250 mg / l was introduced into the FBC at a flow rate of 9 ml / min. The molar ratio of aluminum to fluorine was between 0.8-1: 6, and the molar ratio of sodium to fluorine was greater than 0.5, so that the reaction of the aluminum reagent, NaOH, and the fluoride would cause cryolite (Na3AlF6) crystallize on the support. After reacting for a period of time, cryolite (27) with a large particle size was removed at the bottom of the bed and replaced with a new carrier (25) with a smaller particle size. In the crystalizer, the fluoride surface load was 10.5 kg F "/ m2-h, and the pH was about 3.5.

The primary treated water (17) obtained had a fluoride concentration of 73 mg F / l, which was introduced into the precipitation kettle. 7.9 g of aluminum sulfate (18) were added for one liter of primary treated water. The mixture was stirred and 45% aqueous sodium hydroxide (19) was added to adjust the mixture to a pH of about 7.0. At that time, the water-soluble aluminum ion was reacted to form a soluble aluminum hydroxide flake.

The mixture was stirred for 15 minutes to allow the aluminum hydroxide to absorb flake fluoride to form a white co-precipitate (21). The secondary treated water (20) withdrawn from the precipitation kettle had a fluoride concentration of 1.4 mg F ~ / l, which can be directly removed or further treated to yield a lower fluoride concentration.

Example 2 The co-precipitate (21) obtained from Example 1 can practically be recycled before use. With reference to FIG. 2, the white co-precipitate (41) of aluminum hydroxide and fluoride obtained from Example 1 was added to the alumina salt-dissolving kettle and a small amount of aluminum sulfate was added. Concentrated sulfuric acid (42) was added to the aluminum salt dissolving kettle to bring the pH of the mixture to 1.8 so that aluminum hydroxide flake was dissolved in aluminum salt. At that time, the mixture in the aluminum salt dissolving kettle 15 had an aluminum concentration of 1.350 mg / L and a fluoride concentration of 100 mg / L.

A fluidized bed crystallizer (FBC) was filled with water and provided with an appropriate amount of a carrier (45). Circulated water (46) was introduced into the FBC to fluidize the support. Acid fluoride-containing wastewater (31) at a concentration of 5,500 mg F / l was introduced into the pH adjusting kettle and adjusted with NaOH (32) to a pH of 12.4. The modified wastewater (33) was then introduced into the FBC equipped with the support 25 (45) at a flow rate of 10 ml / min. The soluble aluminum salt reagent (44) obtained from the aluminum-dissolving shells as mentioned above, with an aluminum concentration of 1,350 mg / l and a fluoride concentration of 100 mg / l, was introduced into the FBC at a flow rate of 10 ml / min. The amount of aluminum and sodium in the FBC was adjusted so that the molar ratio of aluminum to fluorine was about 1: 6, and the molar ratio of sodium to fluorine was greater than 0.5, so that the reaction of the aluminum reagent, NaOH, and the fluoride would form crystallized cryolite 35 (Na3AlFg) on the support. After reacting for a period of time, the large particle size crystallized cryolite (47) was removed at the bottom of the bed and replaced with a small particle size support (45). In the crystallizer, the fluoride loading surface was 10.7 kg F / m2-h, and the pH was about 3.1.

The primary treated water (37) obtained had a fluoride concentration of 260 mg F "/ l which was introduced into the precipitation kettle. For one liter of the primary treated water, 7.9 g aluminum sulfate (38) was added. The mixture was stirred and 45% aqueous sodium hydroxide (39) was added to adjust the mixture to a pH of about 7.0 At that time, the water-soluble aluminum ion was reacted to yield insoluble aluminum hydroxide flake The mixture was stirred for 15 minutes to allow the aluminum hydroxide flake to adsorb fluoride to form a white co-precipitate (41) The secondary treated water (40) removed from the precipitation kettle had a fluoride concentration of 9 mg F / L.

This proves that the soluble salt reagent obtained from the co-precipitate of aluminum hydroxide and fluoride is useful for use in wastewater treatment. In addition, the co-precipitate (41) can be added to the aluminum salt dissolving kettle to be treated as above to serve as the source of the soluble salt reagent for wastewater treatment.

The results of Examples 1 and 2 are set forth in Table 1.

25

Table 1 30 V Concen. van Concen. of the Concen. of the ratio of the alumina supplied from the supplied alumium water. the pH was adjusted from FBC crystallized 35 r modified salt reagent l treated wastewater fluoride the water ee (mg / 1) (mg / 1) (mg / 1) (%) (mg / 1) 4 0 d F Na Al F Na Al F Na F Al 1 5,500 6,000 1,250 - 73 71 17.5 97 1.4 2 1,350 9,500 1,350 100 3,600 260 225 125 91 9 0.9 conc. is short for concentration.

45 Ft indicates the concentration of total fluoride.

Fs indicates the concentration of the soluble fluoride.

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In addition, the cryolite crystal obtained from the fluidized bed crystallizer was further advanced to qualitative analysis and quantitative analysis.

5 1. Qualitative analysis;

The white crystal obtained in the present invention was analyzed with an X-ray diffraction analyzer (XRD). Figure 3 is the XRD diagram of the white crystal obtained in the present invention. Table 2 shows the 20, d, and I / I values of the ten stronger peaks in the XRD diagram for the standard cryolite and crystal obtained from the present invention. From fig. 3 and the data of Table 2, it can be seen that the data of the standard cryolite is very close to that of the crystal obtained according to the present invention, indicating that the crystal obtained according to the present invention is cryolite.

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Table 2

The comparison of the XRD diagram between the standard cryolite and the crystal obtained in the present invention

Sample 20 d I / I-, (deg) (A) 58.7 1.5710 35 10 57.8 1.5 950 14 53.2 1.7090 20

Standard 46.7 1.9430 95 Cryolite 39.7 2,269 25 38.8 2,321 25 32.6 2,748 100 32.0 2,797 25 22.9 3,886 65 19.5 4,540 55 58.8 1.5 702 44 15 57.9 1.5 912 24

Crystal 53.5 1.7119 18 obtained 46.8 1.9379 100 in the 39.9 2.2595 25 present 38.5 2.3362 49 20 invention 32.5 2.7557 99 32.0 2.7915 35 22.8 3, 8894 80 19.6 4.5 228 57 2. Quantitative analysis:

The white crystal obtained in the present invention was dissolved with an acid to analyze the concentration of F, Al and Na. The dissolved crystal solution had a concentration of 6,900 mg F / L, 1,800 mg Al / 1 and 4,200 mg Na / 1. That is, the molar ratio of F: Al: Na was 6: 1.1: 3. The molar ratio of F: Al: Na of standard crystal is also 6: 1.1: 3. As a result, it can be concluded that it white crystal obtained in the present invention is cryolite.

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Claims (8)

A crystallization method for removing fluoride from wastewater, the method comprising the steps of: (a) introducing fluoride-containing wastewater into a fluidized bed crystallizer provided with a carrier; (b) adding a water-soluble sodium reagent and a water-soluble aluminum reagent to the fluidized bed crystallizer to form crystallized cryolite (Na3AlFg) on the support capable of removing the fluorine in the wastewater, the molar ratio from aluminum to fluorine is between 0.8-1: 6 and the molar ratio of sodium to fluorine is greater than 0.5; and (c) removing the treated wastewater from the fluidized bed crystallizer to yield a primary treated water. The method of claim 1, the method further comprising: (d) adding a water-soluble aluminum reagent 20 to the primary treated water; (e) adjusting the primary treated water to a pH of about 7 by an alkali metal hydroxide to form insoluble aluminum hydroxide so that aluminum hydroxide adsorbs the fluoride in the primary treated water to form a coprecipitate, where can be further removed by the residual fluoride in the primary treated water; and (f) separating the treated wastewater obtained from step (e) from the co-precipitate to yield a secondary treated water. The method of claim 2, the method further comprising: (g) adding a water-soluble aluminum reagent to the secondary treated water; (H) adjusting the secondary treated water to a pH of about 7 by a hydroxide of an alkali metal 101 1698 to form insoluble aluminum hydroxide so that aluminum hydroxide adsorbs the fluoride into the secondary treated water to form a coprecipitate, whereby the residual fluoride in the secondary treated water can be further removed; and (i) separating the treated wastewater obtained from step (h) from the coprecipitate to yield a tertiary treated water. The method of claim 1, wherein the method after step (b) and before step (c) further comprises: (b1) removing the crystallized cryolite from the fluidized bed crystallizer; and (b2) adding a fresh support to the fluidized bed crystallizer to form crystallized cryolite on the support. The method of claim 1, wherein the water-soluble aluminum reagent in step (b) is selected from the group consisting of aluminum chloride, aluminum sulfate and aluminum nitrate. The method of claim 1, wherein the water-soluble sodium reagent in step (b) is selected from the group consisting of sodium chloride, sodium hydroxide, sodium sulfate and sodium nitrate. 7. Crystallization method for removing fluoride from wastewater, the method comprising the steps of: (a) introducing the fluoride-containing wastewater into a fluidized bed crystallizer provided with a carrier; (B) adding a water-soluble sodium reagent and a water-soluble aluminum reagent into the fluidized bed crystallizer to form crystallized cryolite (Na 2 AlFg) on the support capable of removing the fluoride in the wastewater, whereby the 35 aluminum to fluorine molar ratio is between 0.8-1: 6, and the sodium to fluorine molar ratio is greater than 0.5; and (c) separating the treated waste water from the supported crystalline cryolite to yield a primary treated water, whereby the water-soluble aluminum reagent is obtained by adjusting the pH of the coprecipitate obtained from step (e) from claim 2 to less than 3. 8. Crystallization method for removing fluoride from wastewater, the method comprising the steps of: (a) introducing the fluoride-containing wastewater into a fluidized bed crystallizer equipped with a carrier; (b) adding a water-soluble sodium reagent and a water-soluble aluminum reagent into the fluidized bed crystallizer to form crystallized cryolite (Na-jAlFg) on the support capable of removing the fluorine in the wastewater, the molar ratio of aluminum to fluorine is between 0.8-1: 6, and the molar ratio of sodium to fluorine is greater than 0.5; and (c) removing the treated wastewater from the crystallized cryolite to provide a primary treated water, whereby the water-soluble aluminum reagent is obtained by adjusting the pH of the coprecipitate obtained from step (e) of claim 2 to more than 11. 1011698