JP7555541B2 - Fluorine adsorbent manufacturing method and fluorine removal and recovery method - Google Patents

Description

The present invention relates to a fluorine adsorbent used to reduce or recover fluorine ions contained in wastewater, etc., and a method for removing or recovering fluorine using the same.

In the fields of metal, semiconductor, and glass manufacturing, chemicals containing fluorine are used in processes such as pickling and etching, and fluoride ions may be present in process water and wastewater.
Furthermore, among the fluorocarbon gases that have been used in large quantities as refrigerants for air conditioners and refrigerators, the production of certain fluorocarbons that have a large impact on the ozone layer, as well as alternative fluorocarbons that have a smaller impact on the ozone layer but still contribute to global warming, has been banned, and the transition to new refrigerants is underway. However, in order to prevent the stock of these refrigerants in the market from being released into the atmosphere, they must be completely decomposed using heat or other methods. The hydrogen fluoride gas generated during the decomposition is usually absorbed in water and collected in a smoke scrubber, and the absorbed water contains fluoride ions.
Fluorine is also used in a variety of other products, such as fluororesins and lubricants, and when fluorine-containing wastewater generated during the manufacturing process or product disposal is discharged, it is necessary to remove fluoride ions.

Normally, fluoride ions in wastewater are removed from the wastewater by adding powdered calcium salts such as slaked lime to produce insoluble calcium fluoride, which is then subjected to solid-liquid separation. However, the calcium fluoride produced is in the form of fine crystals. For this reason, separation is difficult using solid-liquid separation methods that utilize differences in specific gravity, such as sedimentation or centrifugation, because the difference in specific gravity with the wastewater is so slight. In addition, when performing solid-liquid separation using methods such as filtering, the filter needs to be finely coarse, which causes clogging even with a small amount of processing, making the process extremely inefficient.

One method currently used to resolve these difficulties in solid-liquid separation is to generate hydrogen fluoride microcrystals using calcium salts and add a flocculant at the same time, causing the hydrogen fluoride microcrystals to flocculate, increasing their apparent particle size and specific gravity, allowing them to be efficiently separated by settling, centrifugation, filtering, or other methods.

As flocculants, aluminum sulfate, PAC (polyaluminum chloride) or polymer flocculants are used, but polymer flocculants are generally expensive chemicals, and while aluminum sulfate and PAC are inexpensive, the recovered calcium fluoride contains significant amounts of iron salts and aluminum salts, making it difficult to reuse the calcium fluoride. As a result, large amounts of waste are generated, which not only increases disposal costs but also places a heavy burden on the environment.

For this reason, various proposals have been made to efficiently remove fluorine (see, for example, Patent Documents 1 to 4).
Patent Document 1 proposes an adsorbent composed of a composition of a hydrous oxide of a rare earth element prepared by heating and aging the hydrous oxide at 300° C. to 600° C., and a resin.

Patent Document 2 proposes magnesium oxide with a BET specific surface area of 10 m2/g or more, but states that in order to obtain magnesium oxide with a high BET specific surface area, it is calcined at 400 to 1000°C and then sieved to use granules with a particle size of 100 μm to 1 mm. In the examples, 0.2 g of magnesium oxide is added to 100 ml of an aqueous solution containing 500 ppm fluorine, and the results of adsorption for 24 hours at 25°C are shown, with the lowest fluorine content being 53 ppm.

In Patent Document 3, a durable adsorbent is provided by using a zirconium hydrate compound as a fluorine-adsorbing element compound and forming an adsorbent composed of a composition of this compound and a hydrophilic organic polymer into a specific spherical structure.

Patent document 4 proposes an adsorbent consisting of a composite granule of iron oxide hydroxide particles and/or iron oxide particles containing zirconium and an organic polymer resin.

JP 2005-28312 A JP 2005-342578 A Patent No. 4854999 Patent No. 5637708

However, the invention described in Patent Document 1 has problems such as high costs due to the use of scarce rare earth elements, the need for a high-temperature heating process, and the use of resin to maintain the shape.
In the invention described in Patent Document 2, it is calculated that 44.7 mg of F in the liquid reacted with magnesium oxide. However, this is smaller than the theoretical value of 188.7 mg when 0.2 g of magnesium oxide is completely converted to _MgF2 , and there is a problem that the utilization rate of the material is low.
The inventions described in Patent Documents 3 and 4 have a problem of high cost due to the use of expensive zirconium and a hydrophilic organic polymer.
Furthermore, Patent Documents 1 to 4 make no mention of treatment after fluorine adsorption, and there is also the problem that the adsorbent is not designed to reuse the recovered fluorine.
The present invention has been made in view of the above problems, and an object of the present invention is to provide a fluorine adsorbent which can be produced at low cost, efficiently removes and recovers fluorine from fluorine-containing wastewater, facilitates solid-liquid separation after the recovery, and enables the reuse of fluorine.

The present invention has been made to solve at least some of the problems described above, and can be realized as the following application examples. Note that the reference symbols and supplementary explanations in parentheses in this section indicate the correspondence with the examples described below in order to aid in understanding the present invention, and do not limit the present invention in any way.

[Application Example 1]
The invention described in Application Example 1 is a fluorine adsorbent comprising one or more inorganic powders made of alkaline earth metal salts molded into any shape using cellulose nanofibers as a binder.
In such a fluorine adsorbent, a composite molded body obtained by adding a small amount of cellulose nanofiber (hereinafter, also referred to as CNF) as a binder to an alkaline earth metal salt that easily reacts with fluorine to form a fluoride has both appropriate water permeability and strength sufficient to withstand collapse in a fluorine-containing aqueous solution. Therefore, by contacting this composite molded body with a fluorine-containing liquid, fluorine can be efficiently recovered.

In addition, it can be produced inexpensively because it uses common alkaline earth metal salts, does not require special firing at high temperatures, and has low manufacturing costs. Furthermore, fluorine adsorption is not physical, but rather utilizes a chemical reaction between fluorine and alkaline earth metals, which have a high affinity for fluorine, resulting in high adsorption efficiency.

In addition, since it does not break down after fluorine adsorption, it can be easily recovered and reused as fluoride.

[Application Example 2]
The fluorine adsorbent according to the second aspect of the present invention is characterized in that, in the fluorine adsorbent according to the first aspect of the present invention, the inorganic powder is one or more selected from the group consisting of magnesium salts, calcium salts, strontium salts, and barium salts.

[Application Example 3]
The fluorine adsorbent according to the third aspect of the present invention is characterized in that the inorganic powder is one or more of magnesium hydroxide, calcium hydroxide, calcium phosphate, and calcium carbonate.

The industrial method of producing hydrogen fluoride involves reacting fluorite, a natural resource, with sulfuric acid at high temperatures to recover the gaseous hydrogen fluoride that is produced. Since fluorite is composed almost entirely of calcium fluoride, by producing a fluorine adsorbent from calcium salts, the fluorine adsorbent after fluorine recovery can serve as a substitute for fluorite.

In addition, alkaline earth fluorides are used as raw materials for single crystal or halide glass, and are used in optical lenses, optical fibers, etc., taking advantage of their optical properties which are different from those of ordinary oxide-based glasses such as _SiO2 .
Furthermore, materials in which rare earth elements are added to alkaline earth fluorides have fluorescent properties and are used as luminescent materials and wavelength conversion materials.

By selecting an alkaline earth metal that has the composition required for these fluoride glasses or fluorescent materials as the inorganic powder used in the fluorine adsorbent of the present invention, the fluorine adsorbent after absorbing fluorine can be used as a raw material.

[Application Example 4]
The method for removing fluoride ions described in Application Example 4 is characterized in that the fluoride adsorbent described in any one of Application Examples 1 to 3 is brought into contact with water containing fluoride ions.

[Application Example 5]
The method for recovering fluorine described in Application Example 5 is characterized in that the fluorine adsorbent described in any one of Application Examples 1 to 3 is brought into contact with water containing fluoride ions to obtain reusable alkaline earth fluoride.

FIG. 2 is a schematic diagram showing the internal structure of a fluorine adsorbent. FIG. 2 is a graph showing the change over time in fluoride ion concentration in a fluoride-containing liquid measured in Example 1, Comparative Example 1, and Comparative Example 2. FIG. 2 is a graph showing pH changes after the molded bodies in Examples 1 to 4 and Comparative Example 1 are put into a fluorine-containing solution. FIG. 1 is a graph showing the change over time in fluoride ion concentration in a fluoride-containing liquid measured in Example 6 and Comparative Example 1. FIG. 13 is a diagram showing components in a fluorine adsorbent after calcination in Example 7.

The following describes examples of the present invention with reference to the drawings. Note that the present invention is not limited to the examples below, and various forms may be adopted as long as they fall within the technical scope of the present invention.

[Example 1]
The fluorine adsorbent 1 is prepared by molding one or more inorganic powders made of alkaline earth metal salts into a desired shape using cellulose nanofibers (CNF) as a binder.

The alkaline earth metal salt may be any salt that has low solubility in water, is easily reactive with fluorine, and produces a fluoride with low solubility in water after the reaction. It is selected from one or more of magnesium hydroxide, magnesium carbonate, magnesium phosphate, calcium hydroxide, calcium carbonate, calcium sulfate, calcium phosphate, strontium hydroxide, strontium carbonate, strontium sulfate, strontium phosphate, barium hydroxide, barium carbonate, barium sulfate, and barium phosphate, and is preferably one or more of magnesium hydroxide, magnesium carbonate, calcium hydroxide, calcium carbonate, and calcium phosphate, and more preferably one or more of magnesium hydroxide, calcium hydroxide, and calcium phosphate.

CNF is a biomass-derived material made by breaking down the cellulose contained in wood and other materials to the nano level. Various manufacturing methods have been developed, including physical crushing and chemical processing, and any method that is provided in the form of a 1-10% slurry can be selected.

(Method of manufacturing fluorine adsorbent)
The fluorine adsorbent 1 is produced by the following steps (A) to (C).
(a) Mix and stir one or more alkaline earth metal salts selected from the above-mentioned alkaline earth metal salts and the CNF slurry (mixing and stirring process).

If the viscosity is too high with only the water contained in the commercially available CNF slurry and the stirring power is insufficient, any water may be added. Any method for mixing and stirring can be used as long as it is suitable for the purpose, but it is fine to use a general stirrer, kneader, mixer, etc. that is used for slurries with relatively high viscosity.

(a) The slurry produced in the mixing and stirring process is made into a provisional molded body of any shape and size by commonly used molding methods such as casting, extrusion, injection molding, sheet molding, and papermaking (molding process). Note that in Example 1, casting molding was performed in which the slurry is placed in a mold.

(c) The provisionally molded body produced in the molding step is heated and dried under vacuum or normal pressure to obtain a molded fluorine adsorbent (drying step). The heating temperature is up to 120° C., preferably in the range of 20 to 80° C., and more preferably in the range of 40 to 60° C. In the case of drying under a vacuum, a vacuum level in the range of 10 ⁵ Pa to 10 ² Pa, preferably in the range of 10 ⁴ to 10 ³ Pa, which is called a low vacuum, is sufficient.

This is because if the drying temperature is high or the degree of vacuum is high, the molded body will lose moisture all at once in the early stages of drying, and the pores in the molded body will not be able to keep up with the volumetric expansion from water to steam, causing localized expansion and resulting in the shape of the molded body becoming distorted. On the other hand, if the drying temperature or degree of vacuum is too low, the drying time will be long and inefficient.

It was confirmed that the preform, after drying under appropriate conditions, showed almost no external changes such as expansion or contraction compared to before drying. This is thought to be because the CNF forms a strong three-dimensional network skeleton before drying.

Furthermore, it is believed that the water remaining in the molded body when the CNF network skeleton is formed becomes steam during the drying process and is discharged outside the system, leaving voids that become pores. Therefore, by controlling the amount of water remaining in the molded body before drying, it is possible to arbitrarily select the porosity in the molded body after drying.

(Features of fluorine adsorbents)
1, the molded body 5 produced by such a method can maintain a suitable strength due to the three-dimensional network of CNFs (CNF network 10) while ensuring water permeability due to its large porosity, so that when it is brought into contact with a liquid containing fluorine ions, the reaction in which the alkaline earth metal salt 20 reacts with fluorine to be converted into an alkaline earth fluoride proceeds efficiently throughout the interior of the molded body 5, and the alkaline earth fluoride after the reaction is held on the CNF network 10, so that the molded body 5 does not collapse. Therefore, the molded body 5 can be used as a fluorine adsorbent that has both strength and water permeability and allows efficient fluorine recovery while facilitating solid-liquid separation.

In addition, by contacting the fluorine adsorbent with fluorine ions equivalent to or greater than the amount of alkaline earth contained in the fluorine adsorbent, the fluorine adsorbent can be converted into an alkaline earth fluoride containing a small amount of CNF. CNF, which is an organic substance, can be relatively easily removed by calcination, making it possible to reuse it as a high-purity alkaline earth fluoride.

(Manufacturing results)
The raw materials used were calcium hydroxide slurry and CNF slurry, which were stirred and mixed and poured into a PTFE (polytetrafluoroethylene) beaker, and then dried together with the beaker. The molded body taken out after drying showed no deformation or the like.

The molded product was placed in a column through which a fluorine-containing liquid with a fluorine concentration of 65 ppm was circulated, and fluorine ions were adsorbed. The fluorine ion concentration in the fluorine-containing liquid was measured at regular intervals.

[Comparative Example 1]
A predetermined amount of powdered JIS special grade slaked lime was added while stirring to a beaker containing the same amount of fluorine-containing liquid with a fluorine concentration of 65 ppm as that circulated in Example 1, and then the mixture was held for a certain period of time while stirring.

[Comparative Example 2]
The calcium hydroxide slurry was used as the raw material, poured into a PTFE beaker, and then dried together with the beaker. The molded body after drying was brittle, but could be removed without losing its shape by carefully removing it.

Fluorine ions were adsorbed to the above molded body in the same manner as in Example 1. Figure 2 shows the change over time in the fluoride ion concentration in the fluoride-containing liquid measured in Example 1, Comparative Example 1, and Comparative Example 2. In Figure 2, the horizontal axis shows the elapsed time, and the vertical axis shows the fluoride concentration. In Figure 2, the powdered slaked lime in Comparative Example 1 is shown with a "◯", the molded body without CNF in Comparative Example 2 is shown with a "△", and the molded body with CNF in Example 1 is shown with a "□".

As shown in Figure 2, in Example 1, the time for the F concentration to decrease is longer than in Comparative Example 1, but the F concentration eventually drops to approximately the same level of 10 mg/L. In contrast, in Comparative Example 2, the F concentration does not drop completely compared to Example 1.

[Example 2]
Next, Example 2 will be described. In Example 2, calcium hydroxide slurry and CNF slurry were used as raw materials, which were mixed by stirring, and the mixed slurry was filled into 2 ml centrifugation tubes in an amount of 2 ml each, followed by centrifugal separation, removal of the supernatant, and drying the tubes at 60°C.
After drying was completed, the calcium hydroxide remaining at the bottom of the tube and the CNF mixture were integrated, and a molded body similar to that in Example 1 was produced.

[Example 3]
Next, Example 3 will be described. In Example 3, calcium hydroxide slurry and CNF slurry were used as raw materials, which were stirred and mixed to form a clay-like material, and then extruded from a die with a φ5 hole to form a cylindrical molded body, which was cut into 10 mm lengths and dried at 110°C.

[Example 4]
Next, Example 4 will be described. In Example 4, calcium hydroxide slurry and CNF slurry were used as raw materials, and the slurry obtained by stirring and mixing them was passed through a filter equipped with a φ46 filter paper, and the filtrate layer was dried together with the filter paper at 110° C. After drying, the hydrated lime and CNF were integrated on the filter paper, and a thin film layer with water permeability was formed.

[Example 5]
Next, Example 5 will be described. In Example 5, calcium hydroxide slurry and CNF slurry were used as raw materials, and the slurry obtained by stirring and mixing them was repeatedly passed through a pulp-making mesh, and the thin film of mixed slaked lime and CNF formed on the mesh was dried at 110° C. This thin film had appropriate water permeability and could be used as filter paper, and it was also possible to cut it to an appropriate size, pack it into a column, and pass treated water through it.

(Compared to conventional products)
FIG. 3 is a graph comparing the pH change after the powdered slaked lime of Comparative Example 1 was added to a fluorine-containing solution of pH 3.3, the pH change after the CNF/slaked lime composite molded body produced in a beaker of Example 1 was added to a fluorine-containing solution of pH 3.3, the pH change after the CNF/slaked lime composite molded body molded using centrifugal force of Example 2 was added to a fluorine-containing solution of pH 3.3, the pH change after the CNF/slaked lime composite molded body molded by extrusion molding of Example 3 was added to a fluorine-containing solution of pH 3.3, the pH change when a fluorine-containing solution of initial pH 3.3 was circulated through a filter paper on which a CNF/slaked lime layer was formed of Example 4, and the pH change when a fluorine-containing solution of initial pH 3.3 was circulated through a CNF/slaked lime composite molded membrane produced by papermaking of Example 5.

In FIG. 3, the vertical axis indicates the pH value, and the horizontal axis indicates the elapsed time. In FIG. 3, "◯" indicates Comparative Example 1, "△" indicates Example 1, "+" indicates Example 2, "=" indicates Example 3, "×" indicates Example 4, and "□" indicates Example 5.

Fluorine is adsorbed by this material according to the following chemical formula: Ca(OH) ₂ + 2HF → CaF ₂ + 2H ₂ O
It can be seen from this formula that as the amount of fluorine in the liquid decreases, the pH changes from acidic to neutral, and the rate of change in pH is nothing other than the rate of decrease in fluorine, so it is possible to correlate the rate of fluorine adsorption with the rate of pH change.

Therefore, the test results shown in Figure 3 show that there is a difference in the fluorine adsorption rate depending on the CNF/slaked lime manufacturing method shown in the above examples, and that ultimately, no matter which manufacturing method is used, it is possible to react with all the HF in the liquid and neutralize the liquid, in other words, it has the ability to sufficiently adsorb fluorine.

[Example 6]
Next, Example 6 will be described. In Example 6, calcium hydroxide slurry and CNF slurry were used as raw materials, and water was further added to the slurry to prepare a dilute slurry, which was then circulated by a liquid pump.
A cylindrical filter made of polypropylene (PP) and having a mesh size of 1 μm was inserted into the filter housing and placed midway along the circulation line so that the solid content in the dilute slurry could be filtered by the cylindrical filter.

Immediately after starting the circulation, a considerable amount of solid matter passed through the cylindrical filter, but as the thickness of the filtered material on the filter increased, the amount passing through decreased, and by continuing the circulation for a certain period of time, a calcium hydroxide/CNF composite layer of a certain thickness was able to be formed on the PP filtration filter.

After the circulation was completed, the liquid remaining in the filter housing was removed with compressed air, and the cylindrical filter was then removed from the filter housing and dried at 110°C. Even after drying, no peeling was observed in the calcium hydroxide/CNF composite layer on the PP filter, and the PP filter surface was coated with a constant thickness.

When the PP filter on which the calcium hydroxide/CNF composite layer was produced by the above method was inserted into a filter housing installed in the circulation line of a fluoride ion-containing liquid and allowed to adsorb fluorine, the fluoride ion concentration in the liquid decreased, similar to the molded bodies produced in Examples 1 and 2, and showed a fluoride concentration reduction rate equivalent to that of Comparative Example 1, as shown in Figure 4. Furthermore, since no collapse or peeling was observed even after the reaction with fluorine, it was confirmed that the calcium hydroxide/CNF composite layer produced by this method also functions as a fluoride adsorbent.

In FIG. 4, the horizontal axis indicates the elapsed time, and the vertical axis indicates the fluorine concentration. In FIG. 4, the powdered hydrated lime in Comparative Example 1 is indicated by a circle, and the PP filter molded body in Example 6 is indicated by a square.

(Example 7)
Next, a description will be given of Example 7. In Example 7, a fluorine adsorbent prepared in the same manner as in Example 1 was brought into contact with an aqueous solution containing fluorine ions in an amount twice the equivalent of calcium hydroxide contained in the fluorine adsorbent, to remove fluorine ions from the aqueous solution.

After the removal was completed, the fluorine adsorbent was taken out of the aqueous solution and dried at 110° C. for 24 hours to remove moisture, and then calcined in an electric furnace at 400° C. for 2 hours. The results of X-ray diffraction analysis of the fluorine adsorbent before and after calcination are shown in FIG.
As shown in FIG. 5, the diffraction peak of the fluorine adsorbent after firing (shown as "A" in FIG. 5) completely coincides with the diffraction peak of _CaF2 (a plurality of diffraction peaks shown as "B" in FIG. 5), and it was found that the product was calcium fluoride with high purity.
In FIG. 5, the vertical axis indicates the X-ray diffraction intensity, and the horizontal axis indicates the energy of the X-ray spectrum.

5... Molded body 10... CNF network 20... Alkaline earth metal salt.

Claims (5)

A fluorine adsorbent characterized by molding one or more inorganic powders made of alkaline earth metal salts into any shape using cellulose nanofibers as a binder. The fluorine adsorbent according to claim 1, characterized in that the inorganic powder is one or more of magnesium salt, calcium salt, strontium salt, and barium salt. The fluorine adsorbent according to claim 1, characterized in that the inorganic powder is one or more of magnesium hydroxide, calcium hydroxide, calcium phosphate, and calcium carbonate. A method for removing fluoride ions, comprising contacting water containing fluoride ions with the fluoride adsorbent according to any one of claims 1 to 3. A fluorine recovery method comprising contacting fluoride ion-containing water with the fluorine adsorbent according to any one of claims 1 to 3 to obtain reusable alkaline earth fluorides.