JP5255861B2 - Synthetic fluorite recovery method and recovery device - Google Patents

Description

  The present invention relates to fluorine-containing wastewater containing hydrosilicofluoric acid, for example, (1) wastewater from an apparatus related to electronic component production such as production of flat panel displays (FPD) such as liquid crystal panels and plasma panels, and solar cell production. Generated by washing with water the wastewater pretreated from the wastewater, and (2) gas generated by decomposing PFC gas used for electronic parts manufacturing such as FPD manufacturing and solar cell manufacturing with a detoxifying device. Wastewater or wastewater pretreated from the wastewater, (3) wastewater produced as a by-product when hydrogen fluoride is produced by decomposing fluorite in a hydrogen fluoride production facility, or wastewater pretreated from the wastewater, (4) phosphoric acid Purity of 97% or more suitable for reuse as a raw material for hydrogen fluoride by treating wastewater produced as a by-product when phosphoric acid is produced by decomposing fluorine apatite at a production facility, or wastewater pretreated from the wastewater, etc. Average particle size 5-10 [mu] m, preferably to synthetic fluorite recovery method and the recovery apparatus for recovering fluorine as 10~50μm synthetic fluorite.

  Reduce environmental load caused by neutral salt, waste acid and waste alkali contained in wastewater discharged as by-products or waste from various processes, and recover and recycle useful components in by-products or waste. Developing available technologies is a major challenge in various plants. In particular, when the amount of waste generated is large and the useful component in the waste is a depleted resource, it becomes an urgent and important issue.

  For example, fluorine is present in the form of fluoride ions in fluorine-containing wastewater generated from a semiconductor manufacturing apparatus, and the content of impurities such as silicon is extremely small. For this reason, fluorine can be recycled and has already been implemented. For example, in Patent Document 1, synthetic fluorite having a purity and particle size suitable for hydrogen fluoride production is recovered from fluorine-containing wastewater having a high fluorine concentration by performing a crystallization operation under hydrochloric acid acidic conditions at pH 2 or lower. A method is disclosed. Patent Document 2 discloses a method for efficiently producing hydrofluoric acid from the synthetic fluorite recovered by the method described in Patent Document 1 with energy saving.

  In addition, the rinse water with a low concentration of fluorine generated from the semiconductor manufacturing apparatus and the diluted cleaning water generated from the detoxification apparatus have a low fluorine concentration. Therefore, the method described in Patent Document 1 uses an apparatus per unit fluorine amount. However, by using the electrodialysis apparatus and method described in Patent Document 3 to separate and concentrate fluorine to obtain fluorine-containing wastewater having an increased fluorine concentration, the method of Patent Document 1 is used. Thus, synthetic fluorite having a particle size and purity suitable for hydrogen fluoride production can be recovered.

  In other words, fluorine-containing wastewater containing a small amount of silicon generated from a semiconductor manufacturing apparatus can already be recycled for both high-concentration and low-concentration wastewater.

In recent years, the amount of fluorine-containing wastewater containing a large amount of silicon and wastewater mainly composed of hydrofluoric acid (H ₂ SiF ₆ ) has increased. For example, in the manufacturing process of FPDs and solar cells, a large amount of fluorine-containing wastewater containing hydrosilicofluoric acid is generated. The form of fluorine is silicofluoride ion, and the content of silicon is larger than that of fluorine. In particular, the generation amount of fluorine-containing wastewater containing silicon in the form of hydrosilicofluoric acid has been increasing rapidly from the FPD production apparatus in recent years due to the large screen and mass production. Also, from the solar cell manufacturing equipment, fluorine-containing wastewater containing hydrofluoric acid is increasing as the production volume increases.

Further, hydrofluoric acid is generated as a by-product from the production process of hydrogen fluoride and phosphoric acid. Fluorite or fluorapatite contains silicon (Si) as an impurity, and silicon (Si) reacts with hydrogen fluoride (HF) and reacts with silicon tetrafluoride (HF) during the sulfuric acid treatment in the manufacturing process. SiF ₄ ) is produced and reacts with water to produce hydrofluoric acid as a by-product.

This hydrosilicofluoric acid is used as a surface treatment agent for quartz glass and as an additive for plating baths, but its use and amount are limited, and since it contains silicon, it is a useful substance such as fluorine. Is difficult to recycle, and so far there have been few examples of commercial implementation. This is because there is the following problem when trying to recover hydrofluoric acid and recovering the generated fluorine in the form of a useful fluorine compound such as calcium fluoride (CaF ₂ ).

(1) Decomposition of hydrosilicofluoric acid and CaF ₂ crystallization by a calcium compound are complicated in dissolution / precipitation phenomenon, and it is difficult to operate and control silica and CaF ₂ stably.
(2) The silicon produced by decomposition tends to aggregate into gel-like silica (SiO ₂ ), and it is difficult to separate calcium fluoride and gel-like silica to be recovered.
(3) Since the recovered calcium fluoride has a small particle size and is in powder form, when it is used for the production of hydrogen fluoride, dust is generated inside the hydrogen fluoride production apparatus, which hinders the production of hydrogen fluoride.
(4) Silicon is mixed into the recovered calcium fluoride as an impurity, reducing the purity of the recovered material and reducing the yield during the production of hydrogen fluoride, and at the same time, a large amount of hydrofluoric acid is by-produced. There arises a problem that the processing is required again.
(5) If recovery is made in the form of magnesium fluoride (MgF ₂ ) instead of calcium fluoride, there is no production apparatus for producing hydrogen fluoride using magnesium fluoride as a raw material, and there is no use of magnesium fluoride There is no practical use.

For this reason, hydrosilicofluoric acid is mostly treated by the coagulation precipitation method using calcium hydroxide (Ca (OH) ₂ ), and this treatment makes it impossible to reuse silica and CaF _{2 which} are not easy to handle. There was a problem that a large amount of mixed sludge was generated. Part of this fluorine-derived sludge is reused as cement raw material, roadbed material, etc., but there are problems with the strength of reusable products, elution of components, etc. difficult. For this reason, most of the sludge has been disposed of in landfills, but in Japan, landfill disposal sites are limited, and it is difficult to locate new disposal sites. In plants that generate fluorine-containing wastewater containing hydrofluoric acid, securing a disposal destination for sludge is becoming a serious problem.

  On the other hand, fluorite is a depleted resource with uneven reserves and a resource that is strongly desired to be recycled. More than half of fluorite production comes from China. Fluorite includes (1) raw materials for fluorine chemicals such as hydrogen fluoride, chlorofluorocarbons, and fluorine resins, (2) fluxes during steel refining, (3) emulsions and fluxes in glass and enamels, etc. Used in In particular, fluorite used in Japan is often used as a raw material for fluorine chemicals starting from hydrofluoric acid, and acid grade fluorite having a purity of 97% or higher is required. For this reason, it is highly dependent on Chinese fluorite with few impurities. The particle size of fluorite suitable for hydrofluoric acid production is generally in the range of 5 to 100 μm.

  However, recently, the prices of fluorite and hydrogen fluoride have soared, and there has been a problem that it is difficult to secure the amount of hydrogen fluoride whose demand is increasing. This is due to the rapid increase in demand for hydrogen fluoride in China and the establishment of fluorite export quotas through resource conservation policies.

  Thus, from the viewpoint of disposal problems and securing resources, fluorine is recovered in a useful form such as synthetic fluorite (calcium fluoride crystals) from fluorine-containing wastewater containing hydrofluoric acid that has been disposed of as sludge. Recycling to secure resources and, as a result, reducing sludge is becoming increasingly important. In addition, synthetic fluorite with low purity, particle size is too small or too large compared to natural fluorite has been recycled by mixing with natural fluorite in the past. Synthetic fluorite with low purity, particle size is too small or too large due to a decrease in imports of natural fluorite from China and a large amount of hydrofluoric acid solution to be recovered Then, it is thought that it becomes difficult to use it mixed with natural fluorite.

  Therefore, in the future, fluorine will be recovered from the fluorine-containing wastewater containing hydrofluoric acid as synthetic fluorite having a high purity equal to or higher than that of natural fluorite and having a particle size suitable for hydrogen fluoride production. It is necessary to be able to produce hydrogen fluoride only with the recovered synthetic fluorite in order to continuously recycle fluorine-containing wastewater containing hydrofluoric acid. Further, it is more preferable if the process of producing hydrogen fluoride with synthetic fluorite is energy-saving and efficient as compared with the process of producing hydrogen fluoride from conventional natural fluorite.

Thus, there is a demand for a recycling method in which hydrofluoric acid is reacted with a calcium compound to produce synthetic fluorite (CaF ₂ ) and used as a raw material for hydrofluoric acid. Development has been done.

  Various methods are known that allow the production of hydrogen fluoride, the most important starting material in fluorine chemistry, by recovering calcium fluoride from fluorine-containing wastewater rich in hydrosilicofluoric acid. However, at present, there is no method that is widely used commercially. Conventional techniques for recovering useful substances such as fluorine from fluorine-containing wastewater containing hydrosilicofluoric acid include the following.

In Patent Literature 4, calcium carbonate (CaCO ₃ ) is prepared by adjusting a hydrofluoric acid solution having a concentration of 2.5 to 6% to a pH of 3.5 to 6.7 within a range of 1.7 to 54.4 ° C. And a method for separating and recovering CaF ₂ precipitated from an aqueous solution containing colloidal silica by the reaction with benzene. However, this method has a disadvantage that 4 to 7% of SiO ₂ is contained in CaF ₂ .

In Patent Document 5, first, an equivalent amount of calcium carbonate of 85% or less is added to a hydrosilicic acid solution of 4% or less to separate and recover high-purity CaF ₂ produced from an aqueous solution containing colloidal silica. A method is disclosed in which a silica containing no fluorine is obtained by separating the solid content containing CaF ₂ by adding calcium carbonate to a pH of 7 to 7.3. In this method, CaF ₂ having a SiO ₂ content of 1.5% or less is obtained, but since the reaction rate is slow, there is a disadvantage that the particle size of the obtained CaF ₂ is small. There is also a restriction that the reaction time is too taken long hydrated silica would lower the purity of the CaF ₂ was coprecipitated with CaF _2.

In Patent Document 6, a calcium carbonate suspension obtained by adding three times or more of water to calcium carbonate is kept at 0 to 40 ° C., and in the first stage, an equivalent or higher hydrofluoric acid solution has a pH of 2 to 3. In the second stage, by adding calcium carbonate and adjusting the pH to 4-6, the sol-like water-soluble SiO ₂ produced by the reaction is synthesized with a low content of silica. A method for separating from stone is disclosed. However, the synthetic fluorite obtained by this method can only have a purity of about 85%, contains about 11% calcium carbonate and about 3% silica as impurities, and is not suitable as a raw material for hydrofluoric acid. It is believed that there is.

In Patent Document 7, a hydrofluoric acid solution is added to a premixed aqueous solution of quicklime (CaO) and sodium hydroxide while maintaining a pH of 11 or higher, and precipitated CaF ₂ is recovered by filtration. A method is disclosed. However, the synthetic fluorite obtained by this method has only a purity of about 85%, contains nearly 10% of silica as an impurity, and is considered to be inappropriate for a raw material of hydrofluoric acid.

In Patent Document 8, magnesium fluoride (MgF ₂ ) produced by adding magnesium sulfate while coagulating or centrifuging the fluoride-containing wastewater containing silicate ions at a pH of 3 to 7 is fluorinated. A method for recovering magnesium fluoride sludge having a high magnesium content is disclosed. However, this method has a problem that the recovery rate becomes large because the production rate of magnesium fluoride is slow. In addition, when trying to produce hydrogen fluoride using magnesium fluoride obtained by this method as a raw material, the fluidity in the hydrogen fluoride production apparatus differs greatly from the case of using fluorite as a raw material. Hydrogen fluoride production equipment cannot be used. At present, there is no hydrogen fluoride production method and apparatus using magnesium fluoride as a raw material, and therefore there is a problem that even if recovered as magnesium fluoride, it cannot be recycled.

  Patent Document 9 discloses a method of obtaining a mixed slurry of 100 μm or more large single crystal sodium fluoride and 5 μm or less silica particles produced by neutralizing a concentrated silicofluoric acid solution. In this method, since fluorine is recovered as sodium fluoride crystals, it cannot be recycled as a raw material for producing hydrogen fluoride using a conventional hydrogen fluoride production apparatus.

  In Patent Document 10, a sodium compound is added to silicic hydrofluoric acid wastewater to control the pH to 7-9, and in a state where sodium fluoride is saturated, the reaction is performed at the boiling point to produce amorphous silica. Later, diluting water was added to dissolve the sodium fluoride crystals, the amorphous silica and the sodium fluoride solution were separated, and sulfuric acid was added to the sodium fluoride crystals obtained by evaporating and concentrating the separated sodium fluoride aqueous solution. A method for recovering hydrogen fluoride is disclosed. In this method, it is necessary to heat the solution to the boiling point, and since fluorine is recovered as sodium fluoride crystals, it is recycled as a raw material when producing hydrogen fluoride using conventional hydrogen fluoride production equipment. I can't do it.

  As described above, in the prior art, synthetic fluorite recovered by adding a calcium compound to fluorine-containing wastewater containing hydrofluoric acid has low purity such as containing silicon as an impurity and has a small particle size. In order to use it as a raw material for hydrogen fluoride production, there is a restriction that it must be used in a small amount mixed with natural fluorite.

  In addition, Chinese fluorite, which is suitable as a raw material for hydrogen fluoride, has a significant reduction in imports to Japan due to restrictions on China's export license limits, and hydrofluoric acid discharged from various industries. Since it is expected that fluorine-containing wastewater containing water will increase rapidly, it is considered that synthetic fluorite recovered using conventional techniques cannot be mixed with natural fluorite and used.

  Because of the above problems, it is possible to produce hydrogen fluoride from fluorine-containing wastewater containing hydrofluoric acid using only the recovered synthetic fluorite with high purity and appropriate particle size. Development of a technique for recovering as a possible high-grade synthetic fluorite has been desired.

JP 2005-206405 A JP 2007-112683 A JP 2007-14827 A U.S. Pat. No. 2,780,521 U.S. Pat. No. 2,780,523 U.S. Pat. No. 3,907,978 US Pat. No. 5,910,297 JP 2006-61754 A Japanese Patent Publication No. 50-18477 U.S. Pat. No. 4,308,244

  The present invention has been made in view of the above circumstances. Fluorination using a conventional hydrogen fluoride production apparatus using only recovered synthetic fluorite by treating fluorine-containing wastewater containing hydrofluoric acid. Synthetic fluorite recovery that enables recovery of high-purity synthetic fluorite with an appropriate particle size that does not become dust, that is, a purity of 97% or more and an average particle size of 5 to 100 μm, which enables production of hydrogen It is an object to provide a method and a recovery device.

To achieve the above object, synthetic fluorite recovery method of the present invention is a synthetic fluorite recovery method for recovering fluorine as synthetic fluorite from fluorine-containing waste water containing hydrosilicofluoric acid, the fluorine-containing waste water and adding step of adding a stabilizer, wherein after the adding step, the mixed fluorine-containing waste water and a sodium compound to decompose the hydrosilicofluoric acid, silica mixture is mainly the insoluble silica and sodium fluoride solution A neutralization decomposition step for producing a slurry, a separation step for separating insoluble silica from the silica slurry produced in the neutralization decomposition step to obtain silica separation water, and supplying a calcium compound to the silica separation water, A crystallization step for producing calcium fluoride having a purity of 97% or more and an average particle diameter of 5 to 100 μm.

When a sodium compound such as sodium hydroxide or sodium carbonate is added to the hydrosilicofluoric acid solution and the pH is controlled to 7 to 10, hydrosilicic acid is neutralized and decomposed to form insoluble silica and aqueous sodium fluoride solution. A silica slurry mainly composed of the mixture is produced. For example, in the case where the target is 1.5% -F hydrosilicofluoric acid (0.37% -Si), silica obtained by separating insoluble silica from neutralized and decomposed silica slurry. The silica concentration in the separated water is about 100 to 1000 mg-Si / L in combination with SS property and solubility. Although it is higher than the concentration of silica contained in fluorine wastewater generated in a general semiconductor factory, most of Si can be removed. And for silica separation water (pH is neutral to weakly alkaline) obtained as supernatant water, for example, by performing a crystallization operation in which a calcium compound is added under conditions of a water temperature of 30 to 80 ° C. and / or pH 2 or less, Synthetic fluorite having an average particle size of 5 to 100 μm and a purity of 97% or more can be recovered. In addition, the recovered synthetic fluorite has a Si content of 1% -SiO ₂ or less, similar to acid grade natural fluorite.

In the neutralization decomposition step, it is preferable to add diluting water that prevents the precipitation of sodium fluoride crystals contained in the silica slurry or dissolves the sodium fluoride crystals.
When the concentration of sodium fluoride is high and precipitates are formed exceeding the solubility, precipitation of sodium fluoride crystals is prevented, or a diluting water that dissolves sodium fluoride crystals is added to obtain an aqueous solution of sodium fluoride. I can do it. Dilution water is not required when the concentration of sodium fluoride is low and below solubility. The dilution water to be added is preferably the minimum water amount from the viewpoint of not increasing the amount of drainage.

  The pH control range in the neutralization decomposition step is 7 to 10, and the sodium compound is preferably sodium carbonate and / or sodium hydroxide. It is preferable that the water temperature in the crystallization step is 30 to 80 ° C. and / or the pH is 2 or less.

  It is preferable to perform a pH adjustment step of adjusting the pH of the silica separation water between the separation step and before the crystallization step. It is preferable to further include a fluorite recovery step of dehydrating and recovering calcium fluoride obtained in the crystallization step as synthetic fluorite.

Before SL stabilizers are, for example, at least one oxidizing agent selected from such hydrogen peroxide or ozone.

  Until now, the insoluble silica produced during the neutralization and decomposition of hydrofluoric acid has been unstable, and as time passes, the silica particles agglomerate and gelation proceeds. Operation may be difficult. In addition, insoluble silica that does not settle easily remains in the supernatant water, which may adversely affect the subsequent crystallization process, or the recovered synthetic fluorite may have a higher Si content than natural fluorite. there were. By adding a flocculant such as a polymer flocculant in the separation step, the dehydration operation and the supernatant water can be improved. However, the effect of the flocculant causes residual Si to enter the crystallization step when the concentration of raw water is high. There was a case to have an influence. In contrast, by adding an oxidizing agent such as hydrogen peroxide as a stabilizer in advance to the hydrofluoric acid solution, it is possible to easily and surely stabilize the insoluble silica in the neutralization decomposition process, Clear water is obtained.

  The desorbed water obtained by the dehydration operation in the separation step may be returned to the neutralization decomposition step, or may be introduced into the crystallization step or the pH adjustment step.

The synthetic fluorite recovery apparatus of the present invention is a synthetic fluorite recovery apparatus for recovering fluorine as synthetic fluorite from fluorine-containing wastewater containing hydrofluoric acid, and a stabilizer for adding a stabilizer to the fluorine-containing wastewater. and addition device, by mixing the fluorine-containing waste water and a sodium compound to decompose the hydrosilicofluoric acid, and neutralization cracker mixtures of insoluble silica and sodium fluoride solution to produce a silica slurry of mainly the A separation device that separates insoluble silica from the silica slurry produced by the neutralization cracking device to obtain silica separation water, and a calcium compound is supplied to the silica separation water, with a purity of 97% or more and an average particle size of 5 It has a crystallizer that produces ~ 100 μm calcium fluoride.

  According to the synthetic fluorite recovery method and recovery apparatus of the present invention, fluorine can be recovered as high-purity synthetic fluorite having an appropriate particle size as a raw material for hydrogen fluoride from fluorine-containing wastewater containing hydrofluoric acid. . Moreover, the recovered synthetic fluorite can be expected to be used for producing hydrogen fluoride with energy saving.

  Specific examples of embodiments of the present invention will be described below with reference to the drawings. In the following examples, the same or corresponding members are denoted by the same reference numerals, and redundant description is omitted.

  FIG. 1 is a flowchart showing a synthetic fluorite recovery method according to an embodiment of the present invention, and FIG. 2 is a silica separation water (upper) by performing an addition step, a neutralization decomposition step and a separation step in the flowchart shown in FIG. It is a schematic diagram which shows the outline | summary of the synthetic fluorite collection | recovery apparatus which processes until it obtains (clear water). As shown in FIG. 2, the synthetic fluorite recovery apparatus that performs the treatment until silica separation water (supernatant water) is obtained includes the stabilizer addition apparatus 10 that performs the addition process shown in FIG. 1 and the neutralization decomposition process that is shown in FIG. 1, a separation device 30 that performs the separation step shown in FIG. 1, and a pH adjustment device 60 that performs the pH adjustment step shown in FIG. 1.

  The stabilizer addition device 10 is a mixing tank 11, a raw water introduction pipe 12 that introduces fluorine-containing wastewater containing hydrofluoric acid as raw water into the mixing tank 11, and a stabilizer added to the raw water in the mixing tank 11. And a motor 15 that rotates a stirring blade 14 that stirs the raw water in the mixing tank 11. The mixing tank 11 is connected to one end of a raw water extraction pipe 16 for adding the stabilizer in the mixing tank 11 and extracting the raw water stirred by the stirring blade 14 from the mixing tank 11. A pump 17 is interposed between the two.

  This stabilizer addition device 10 adds a stabilizer to raw water (fluorine-containing wastewater containing hydrofluoric acid) in order to further improve the solid-liquid separation of insoluble silica produced in the following neutralization decomposition step. It is for performing the addition process shown in FIG. If the solid-liquid separation state of the insoluble silica produced in the following neutralization decomposition process is good or sufficient settling time of undissolved silica can be secured, the addition process is omitted without providing a stabilizer addition device. can do. That is, as shown in FIGS. 3 to 5, the raw water introduction pipe 12 may be connected to the neutralization decomposition tank 21 of the neutralization decomposition apparatus 20 so that the raw water is directly introduced into the neutralization decomposition tank 21. .

  As the stabilizer, an oxidizing agent is effective. There are no particular restrictions on the type of oxidizing agent, and known oxidizing agents such as hydrogen peroxide, ozone and hypochlorous acid can be used. In particular, hydrogen peroxide is suitable because it is liquid and easy to handle, and its drainage properties do not change after the reaction.

  When a stabilizer is not added to the raw water, insoluble silica (silica floc) generated by natural oxidation generated in the following neutralization decomposition step may be in a gel state. At this time, (1) the sedimentation rate of insoluble silica is small, (2) the concentration of insoluble silica in the concentrated silica slurry obtained after sedimentation is low and the compactness is poor, and (3) it is difficult to dehydrate the concentrated silica slurry. 4) There is an adverse effect that gel-like insoluble silica remains in the silica separation water (supernatant water) to increase the SS concentration, and (5) insoluble silica adheres to the wall surface and blocks the water channel.

  On the other hand, by adding a stabilizer to the raw water and preventing gelation when silica is insolubilized to form particulate silica, (1) the sedimentation rate of insoluble silica is increased, (2) after sedimentation Increase insoluble silica concentration and improve compaction, (3) facilitate dehydration of concentrated silica slurry, (4) reduce residual SS concentration in silica separation water (supernatant water), (5) wall surface of insoluble silica The effect of preventing the blockage of the water channel due to the decrease in adhesion to the water is obtained.

The reason why stable insoluble silica and clear supernatant water can be obtained by adding a stabilizer to the raw water is not clear, but the surface of silicon (Si) generated in the neutralization decomposition process is promptly chemically oxidized. By forming silica (SiO ₂ ) particles having a dense chemical oxide film, aggregation and gelation of the silica particles are suppressed, and as a result, silica particles with good sedimentation are formed and solid-liquid separation is easy. I think.

The amount of hydrogen peroxide added as a stabilizer does not necessarily need to be added in an amount equivalent to the silicon load caused by the decomposition of hydrofluoric acid shown in the following chemical formula 1. The amount of hydrogen peroxide added as a stabilizer is affected by the neutralization decomposition conditions such as the concentration of hydrofluoric acid, neutralization decomposition pH, and the concentration of impurities contained, but generally, the amount of hydrogen peroxide is about 0.1. Since an effect is acquired at 05-1, it is preferable.
Si + 2H ₂ O ₂ → SiO ₂ + 2H ₂ O (Chemical Formula 1)

  As shown in FIG. 2, the neutralization and decomposition apparatus 20 includes a neutralization and decomposition tank 21 in which the other end of the raw water extraction pipe 16 is connected to the upper part, and an alkaline agent in the raw water introduced into the neutralization and decomposition tank 21. An alkaline agent supply unit 22 for supplying, a dilution water supply unit 23 for supplying dilution water into the neutralization decomposition tank 21, a motor 25 for rotating a stirring blade 24 for stirring the liquid in the neutralization decomposition tank 21, A pH measuring device 26 for measuring the pH of the liquid (silica slurry) in the sum decomposition tank 21 is provided. The neutralization decomposition increase 21 is connected to one end of a silica slurry extraction tube 27 for extracting the liquid (silica slurry) in the neutralization decomposition increase 21. The pH measuring device 26 and the alkaline agent supply unit 22 constitute a pH concentration adjusting unit that adjusts the pH of the liquid (silica slurry) in the neutralization decomposition tank 21 to, for example, 7 to 10.

This neutralization decomposition apparatus 20 is for performing the neutralization decomposition process shown in FIG. 1 in which hydrosilicic acid is decomposed to produce a silica slurry mainly composed of a mixture of insoluble silica and aqueous sodium fluoride. is there. In this neutralization decomposition step, as shown in the following chemical formulas 2 and 3, hydrofluoric acid is decomposed by addition of a sodium compound such as sodium carbonate or sodium hydroxide, and silicon becomes insoluble silica and precipitates. Becomes an aqueous solution of sodium fluoride. The pH range in which this neutralization decomposition reaction occurs is 7-10.
H ₂ SiF ₆ + 3Na ₂ CO ₃ → 6NaF + SiO ₂ ↓ + 3CO ₂ ↑ + H ₂ O (Chemical Formula 2)
H ₂ SiF ₆ + 6NaOH → 6NaF + SiO ₂ ↓ + 4H ₂ O (Chemical formula 3)

FIG. 7 shows the results of measuring the relationship between the pH and the filtrate Si concentration when neutralizing titration of a hydrosilicofluoric acid aqueous solution (concentration 4.3% -F) with a sodium hydroxide solution. In FIG. 7, the influence of dilution due to the addition of sodium hydroxide solution is corrected. From FIG. 7, when the pH is in the range of 1 to 7, the Si concentration of the filtrate rapidly decreases as the pH increases, and hydrosilicic acid is decomposed to form an insoluble silicon compound (Na ₂ SiF ₆ , pH 6 below pH 6). From the above, it is understood that silica is produced, and the silica concentration of the filtrate filtered through a filter paper having a pore diameter of 1 μm is lowered. Further, it is confirmed that the filtrate silica concentration starts to increase when the pH exceeds 9, and increases rapidly when the pH is 10 or more. In this case, an aqueous solution of sodium fluoride having a filtrate silica concentration of 1000 mg-Si / L or less in a pH range of 7 to 10, particularly in a pH of 7.5 to 9.5, having a filtrate silica concentration of 500 mg-Si / L or less. was gotten.

In the case where the aqueous sodium fluoride solution is obtained as the supernatant water for the precipitation separation operation, the concentration of silica considered to be soluble is about 100 to 500 mg-Si / L. The silica concentration floating in the SS state is about 100 to 500 mg-SiO ₂ / L (47 to 230 mg-Si / L), and is floating in the SS state when no stabilizer is added to the raw water. silica concentration is 100-1000 mg-SiO ₂ / about L.

  The solubility of sodium fluoride (NaF) is about 40 g-NaF / L (18 g-F / L) at room temperature, and when hydrofluoric acid richer than this is neutralized and decomposed, crystals of sodium fluoride are obtained. Precipitates in the silica slurry. In this case, by adding dilution water, precipitation of sodium fluoride crystals can be prevented or sodium fluoride crystals can be dissolved. In this example, the dilution water is supplied from the dilution water supply pipe 23 into the neutralization decomposition tank 21, but it may be added as an aqueous solution of a sodium compound or previously added to the fluorine-containing waste water. Any method can be taken. Further, the NaF precipitate may be dissolved by washing water when the concentrated silica slurry is dehydrated and washed.

  As shown in FIG. 3, a fluorine concentration meter 28 is provided on the downstream side of the separation device 30, and a valve provided in the dilution water supply pipe 23 is provided on the basis of the fluoride ion concentration measured by the fluorine concentration meter 28. The supply amount of dilution water may be controlled by automatically controlling opening and closing. FIG. 3 shows an example of control using the motor 29. This fluorine concentration meter 28 may be used in combination with that provided for controlling the injection amount of the calcium compound in the following crystallization step.

  The reaction for decomposing hydrosilicofluoric acid is preferably carried out in the range of pH 7 to 10, preferably 7.5 to 9.5, in which the sodium compound is slightly excessive and the silica solubility is low. That is, when the pH is 7 or less, the sodium compound is insufficient and hydrofluoric acid is not completely decomposed. Further, the solubility of the silica produced by decomposition is slightly higher than that at pH 7 or higher, which is not preferable because the silica concentration in the aqueous sodium fluoride solution is increased. In addition, since the generated insoluble silica tends to become fine particles or gel, there is a problem that the sedimentation rate is reduced, the sedimentation / separation apparatus becomes large, or the solid-liquid separation becomes difficult. Neutralization and decomposition process having a configuration in which the pH of 6 to 7 is temporarily maintained, for example, a hydrofluoric acid-containing wastewater and a small amount of alkali are mixed in a pipe or a tank to reach a pH of 6 to 7. Is not preferred because of the above problems. The same applies to the case of mixing dilution water added to make the fluorine concentration below NaF solubility. A configuration in which wastewater containing hydrofluoric acid is directly supplied to a neutralization decomposition tank maintained at a pH of 7 to 10, preferably pH 7.5 to 9.5 is preferable. A pH of 10 or more is not preferable because the solubility of silica becomes extremely high, and the silica concentration in the aqueous sodium fluoride solution is increased.

  Since the properties of insoluble silica depend on the pH conditions at the time of neutralization decomposition, in order to obtain particles of insoluble silica having stable properties, the pH is kept within the above range, and the pH is locally increased. Is preferably supplied while stirring so that the fluorine-containing wastewater and the sodium compound are uniformly mixed. Further, when the pH of the neutralization separation tank 21 is more strictly controlled, as shown in FIG. 8, a concentrated sodium compound aqueous solution is added to adjust the pH to 6 or less, and then a dilute sodium compound aqueous solution is immediately added. Therefore, it is preferable to finely adjust to a desired pH.

  The temperature of the neutralization decomposition reaction system is not particularly limited and can be operated in the range from room temperature to the boiling point, but 20 to 40 ° C. is efficient and preferable. Below 20 ° C, the decomposition rate of hydrofluoric acid is slow and the reaction system becomes large. Above 40 ° C., a solution heating device and exhaust gas cooling / cleaning device are required. The speed of the neutralization decomposition reaction varies depending on the neutralization decomposition conditions such as the water temperature, but is longer than the time used for normal pH adjustment because it involves a decomposition reaction. The reaction time is 15 to 120 minutes, preferably 60 to 90 minutes.

  The sodium compound to be added is not particularly limited, such as generally used sodium hydroxide, but sodium carbonate is more preferable. Sodium carbonate is easier and more convenient to control the pH of the solution than other sodium compounds, and the resulting insoluble silica tends to be more particulate, making solid-liquid separation easier. Because.

  As shown in FIG. 2, the separation device 30 draws out the silica separation water as the supernatant water from the precipitation tank 31 connected to the other end of the silica slurry drawing pipe 27 for drawing out the silica slurry. A silica separation water extraction pipe 32 and a sludge extraction pipe 33 for extracting sludge (concentrated silica slurry) collected at the bottom of the settling tank 31 are provided. And the sludge extraction pipe | tube 33 is connected to the dehydrator 34, and the sludge extracted through the sludge extraction pipe 33 is dehydrated by the dehydrator 34 and discharged as a silica cake. The dehydrator 34 is connected to a desorbed water supply pipe 35 that supplies desorbed water to the downstream side of the separation apparatus 30 and a desorbed water supply pipe 36 that supplies to the upstream side of the neutralization decomposition apparatus 20. In FIG. 2, both the desorbed water supply pipe 35 and the desorbed water supply pipe 36 are illustrated, but either one may be provided.

  The separation device 30 is for performing the separation step shown in FIG. 1 for separating insoluble silica from the silica slurry produced in the neutralization decomposition step. In the separation step, insoluble silica is separated from the silica slurry as a concentrated silica slurry by solid-liquid separation operation using the precipitation tank 31. Further, the concentrated silica slurry is dehydrated by a dehydrating operation by the dehydrator 34, and the concentrated silica slurry is discharged as a silica cake having a low moisture content. As the method used for the solid-liquid separation operation and the dehydration operation, known operations can be used without restriction. For example, a method of obtaining a clear supernatant water and a silica cake by combining a sedimentation operation and a dehydration operation using a filter press or a centrifugal dehydrator is preferably used. It is also possible to perform the solid-liquid separation operation and the dehydration operation in one apparatus, for example, a method in which silica slurry is directly applied to the dehydration apparatus.

  In the solid-liquid separation operation by the separation device, a flocculant such as an inorganic flocculant or a polymer flocculant may be used in order to increase efficiency. As the flocculant that can be used, existing flocculants can be used without particular limitation.

  For example, as shown in FIG. 4, a second separation device 30a is provided in addition to the separation device 30 shown in FIG. 2, and a mixing tank 41 and a coagulation tank 42 are arranged between the separation devices 30 and 30a. good. In the second separation device 30a, the same members as those in the separation device 30 are denoted by the same numerals followed by “a”. In the mixing tank 41, a flocculant supply unit 43 that supplies a flocculant into the mixing tank 41, a pH adjuster supply part 44 that supplies a pH adjuster into the mixing tank 41, and a rotation that mixes the liquid in the mixing tank 41. A motor 46 that rotates the blade 45 and a pH measuring device 48 that measures the pH of the liquid in the mixing tank 41 are provided. The coagulation tank 42 is provided with a motor 50 that rotates a rotary blade 49 that mixes the liquid in the coagulation tank 42.

  In the case of the example shown in FIG. 4, the solid-liquid separation operation is divided into two stages. On the other hand, a flocculant is added and mixed, and the insoluble silica is further removed by sedimentation operation by the second separation device 30a. This example is desirable because the Si load to be separated can be reduced, and the amount of the flocculant used can be reduced. The agglomeration tank 42 is for coarsening the agglomeration floc. For example, when the aggregation flocs have a very high sedimentation property, the agglomeration tank may be omitted.

  Further, as shown in FIG. 5, a filter 51 may be installed in the silica separation water extraction pipe 32, and the concentration of insoluble silica after solid-liquid separation may be reduced by a filtration operation using a filter medium or a membrane. . This filtration operation may be provided before the pH adjustment step or after the pH adjustment step. When it is provided after the pH adjustment step, it is preferable in that the substance produced by the change in pH can be filtered.

  In this example, a desorption water supply pipe 35 for supplying desorption water to the downstream side of the separation device 30 is connected to the dehydrator 34, and the desorption water obtained by the dehydration operation is mixed with supernatant water (silica separation water). It is supplied as raw water for the crystallization process. In addition, when the Si concentration derived from insoluble silica remaining in the desorbed water is high and adversely affects the Si content or crystallization behavior of the synthetic fluorite obtained in the crystallization step, as shown in FIG. It is desirable to connect the desorbed water to the dehydrator 34 with a desorbed water supply pipe 36 that supplies the desorbed water to the upstream side of the neutralization decomposition apparatus 20 and return the desorbed water to the neutralization decomposition step. Of course, it is also possible to reduce the SS concentration in the desorbed water by using a dehydration aid together with the dehydration.

  Sodium fluoride derived from the mother liquor is adhered to the silica cake obtained by the dehydration operation. When adhesion of sodium fluoride hinders disposal or recycling of the silica cake, sodium fluoride can be removed by repeating washing and dehydration.

As shown in FIG. 2, the pH adjusting device 60 includes a pH adjusting tank 61 and a pH adjusting agent supply unit 62 that supplies the pH adjusting agent to the pH adjusting tank 61. The pH adjusting device 60 is for performing the pH adjusting step shown in FIG. 1 for adjusting the crystallization tank pH in the crystallization step to be more acidic, and the crystallization tank pH in the crystallization step is more acidic. Omitted if no adjustment is required. In the pH adjusting step by the pH adjusting device 60, an acid (pH adjusting agent) such as hydrochloric acid or nitric acid is added to the aqueous sodium fluoride solution obtained in the solid-liquid separation step. By this operation, the pH in the crystallization tank 74 described below is further lowered, the solubility of CaF ₂ is increased, the generation of fine crystal nuclei in crystallization is suppressed, and the average particle size is increased. In addition, the solubility of calcium compounds that are impurities in wastewater, such as calcium phosphate, is increased, and the phenomenon of precipitation together with calcium fluoride can be reduced. As a result, a CaF ₂ crystal having a low phosphorus content can be obtained.

In addition, even when heavy metal ions that form hydroxides and precipitate under neutral to alkaline conditions are contained as impurities in the wastewater, they do not form hydroxides due to acidification of the pH, but as ions Since it is dissolved, calcium fluoride particles having a lower heavy metal ion concentration can be obtained. Preferably, by setting the pH to 2 or less, these effects can be expressed more highly.
NaF + HCl → HF + NaCl (Chemical formula 4)

  As the acid to be used, a strong acid that can be expected to have a high effect is more preferable, and an aqueous hydrochloric acid solution is particularly preferable. When nitric acid is used, it is necessary to perform nitrogen treatment in wastewater treatment in the subsequent process. When sulfuric acid is used, calcium sulfate hydrate is generated during the subsequent crystallization step, and is contained as an impurity of the recovered fluorite. This calcium sulfate hydrate is not preferable because moisture is released in the hydrogen fluoride production apparatus during the production of hydrofluoric acid and the corrosive environment is deteriorated.

  FIG. 6 is a schematic diagram showing an outline of the synthetic fluorite recovery apparatus from the flowchart shown in FIG. 1 until the process of recovering the synthetic fluorite by performing the crystallization process and the fluorite recovery process for the silica separation water. is there. As shown in FIG. 6, the synthetic fluorite recovery apparatus that performs the crystallization process and the fluorite recovery process for the silica separation water to recover the synthetic fluorite recovers the crystallization apparatus that performs the crystallization process shown in FIG. 70 and a recovery device 100 for performing the fluorite recovery process shown in FIG.

  The crystallizer 70 divides a raw water tank 71 for storing silica separation water as raw water, a calcium chloride tank 72 for storing calcium chloride therein, and a separation zone using the density difference between the supernatant water and calcium fluoride particles. A crystallization tank 74 having a partition wall 73 therein, a raw water supply pipe 76 having a pump 75 for supplying raw water (silica separation water) in the raw water tank 71 to the crystallization tank 74, and a chlorination in the calcium chloride tank 72. A calcium chloride supply pipe 78 having a pump 77 for supplying calcium to the crystallization tank 74 is provided. The raw water supply pipe 76 is provided with a flow meter 79 and a fluorine concentration meter 80, and outputs from the flow meter 79 and the fluorine concentration meter 80 are input to the calculator 81, and calcium chloride is output from the calculator 81 as a result. The pump 77 installed in the supply pipe 78 is configured to be controlled.

  The crystallization tank 74 is provided with a heater 82 as a temperature adjusting unit for heating the internal liquid to, for example, 30 to 80 ° C., and a motor 84 for rotating the stirring blade 83 for stirring the liquid. The installation location of the heater 82 is not limited to the inside of the crystallization tank, but may be in the circulation pipe 86 or upstream of the raw water supply pipe. As a method for raising the temperature of the heater, known methods such as an electric method and a heat exchange method can be adopted. A calcium fluoride extraction pipe 85 for extracting the calcium fluoride slurry in the crystallization tank 74 is connected to the bottom of the crystallization tank 74, and the calcium fluoride extraction pipe 85 is connected to the calcium fluoride extraction pipe 85. A circulation pipe 86 that branches and returns to the crystallization tank 74 is connected. A pump 87 is installed inside the circulation pipe 86.

The circulation pipe 86 and the calcium fluoride extraction pipe 85 may be provided separately, and each pipe may be directly connected to the crystallization tank. Further, a calcium fluoride extraction pipe 85 may be provided by branching from a circulation pipe 86 directly connected to the crystallization tank. When the circulation pipe is directly connected to the crystallization tank, the circulating slurry may be introduced from a portion other than the bottom, such as a side portion of the crystallization tank. Moreover, you may return a circulating slurry to the arbitrary parts of a crystallization tank.
When the calcium fluoride extraction pipe 85 is directly connected to the crystallization tank, the slurry may be allowed to flow from a portion other than the bottom, such as a side portion of the crystallization tank.

  Further, the upper portion of the crystallization tank 74 is connected to a treated water drawing pipe 88 for drawing the supernatant water in the crystallization tank 74 as treated water. The other end of the treated water drawing pipe 88 is connected to the treated water tank. 89. The treated water stored in the treated water tank 89 is sent to a subsequent wastewater treatment through a treated water drain pipe 91 having a pump 90.

The crystallizer 70 is formed in the crystallizer 74 while stirring so that the silica separation water (raw water) and the calcium chloride aqueous solution are uniformly mixed under acidic conditions of hydrochloric acid having a pH of 2 or less or a water temperature of 30 to 80 ° C. The crystallization step shown in FIG. 1 is carried out, in which calcium fluoride (CaF ₂ ) is produced by simultaneous introduction into the solution. The reaction at this time is shown in Reaction Formula 5.
2HF + CaCl ₂ → CaF ₂ ↓ + 2HCl (Chemical formula 5)

Here, by raising the water temperature to 30 to 80 ° C., the solubility of CaF ₂ is increased, and the dissolution of fine crystals and the coarsening of crystals are promoted. When the water temperature is 30 ° C. or lower, fine crystal nuclei are remarkably generated and the crystal does not become coarse. When the water temperature is 80 ° C. or higher, the solubility of CaF ₂ is too large and the recovery rate of CaF ₂ is small.

  The addition rate of calcium chloride is preferably 1.0 to 1.3 with respect to the equivalent amount. In this example, a fluorine concentration meter 80 is provided, and the fluorine concentration of the sodium fluoride aqueous solution is monitored by, for example, a known ion electrode method, thereby adjusting the amount of calcium chloride added to follow the water quality fluctuations. Stabilize. That is, in this example, the addition amount of calcium chloride is calculated from the fluorine concentration and flow rate values of the raw water (silica separation water), and the flow rate of the pump 77 provided in the calcium chloride supply pipe 78 is controlled.

Instead of the calcium chloride aqueous solution, other calcium compounds such as calcium carbonate may be used. When calcium carbonate is used instead of the calcium chloride aqueous solution, the dissolution and decarboxylation of CaCO ₃ are promoted by acidifying the pH of the reaction system.

  By such an operation, fluorine can be recovered as calcium fluoride having a purity of 97% or more and an average particle diameter of 5 to 100 μm. This is the same purity and average particle size as natural fluorite and can be used as a substitute for natural fluorite.

  In the crystallization tank 74, a separation zone using the density difference between the supernatant water and the calcium fluoride particles is provided by partition walls 73 to suppress the outflow of the calcium fluoride particles to the supernatant water side. The recovery rate of calcium can be increased. As a method of extracting the calcium fluoride slurry from the crystallization tank 74, a part of the calcium fluoride slurry may be extracted continuously or intermittently, or the entire amount of calcium fluoride slurry may be extracted batchwise. good. In this example, the supernatant water (treated water) and the calcium fluoride slurry are taken out separately. However, the whole slurry is taken out as a calcium fluoride slurry without taking out as the supernatant water. May be separated into solid and liquid by a dehydrator.

  In addition, by appropriately selecting the crystal growth time, it is possible to obtain calcium fluoride having an average particle size of 10 to 50 μm, which is a particle size more suitable for hydrogen fluoride production, and a relatively narrow particle size distribution. .

  Hydrogen fluoride is produced by drying fluorite and then mixing and heating with anhydrous sulfuric acid in a heating furnace. If the particle size of the fluorite is 5 μm or less, it becomes dusty during drying and mixing, making it difficult to handle. Therefore, the average particle size is more preferably 10 μm or more. On the other hand, if the particle size of fluorite exceeds 50 μm, it is necessary to lengthen the residence time required for the reaction in the heating furnace or increase the heating temperature, and the energy consumption increases. For this reason, the average particle diameter of fluorite suitable for hydrogen fluoride production is preferably 10 to 50 μm, and more preferably 20 to 40 μm.

  Since natural fluorite is produced by crushing rocks, it is characterized by a wide particle size distribution. In order to prevent dust during pulverization and hydrogen fluoride production, the average particle size of fluorite is generally set to 50 μm or more. So far, when producing hydrogen fluoride using natural fluorite as a raw material, It was necessary to increase the temperature and lengthen the residence time, which consumed much energy. By making the average particle size of the recovered fluorite smaller than that of natural fluorite, hydrogen fluoride can be produced with energy saving.

  As shown in FIG. 6, the recovery device 100 includes a dehydrator 101 connected to the calcium fluoride drawing pipe 85 and a wash water supply pipe 102 that supplies wash water to the dehydrator 101. A drain pipe 103 of 101 is connected to the treated water tank 89.

This recovery device 100 is a synthetic fluorite whose water content is lowered by washing and removing hydrochloric acid and remaining hydrogen fluoride from water from the calcium fluoride (synthetic fluorite) slurry obtained in the crystallization step and dehydrating it. The crystallization recovery process shown in FIG. 1 is performed. As washing water, industrial water, city water, etc. can be used conveniently. As the dehydrator 101, an existing dehydrator such as a centrifugal dehydrator or a filter press can be used. However, the centrifugal dehydrator is simple and preferable. Further, the water content of fluorite (CaF ₂ crystal) recovered after dehydration and washing is preferably about 3 to 15%. If the water content is 3% or less, dust may be generated during transportation and storage of the synthetic fluorite, which is not preferable. A water content of 15% or more is not preferable because it increases the weight during transportation and increases the load in the fluorite drying process before the production of hydrogen fluoride.

  The following examples illustrate the invention more specifically. The following description of the examples illustrates specific examples of the present invention, and the present invention is not limited to these descriptions.

Example 1
Silica separation treatment was performed using the apparatus shown in FIG. 2 (excluding the pH adjusting apparatus). Hydrogen peroxide was added in advance to hydrofluoric acid (1.7% -F, 0.42% -Si) so as to have an equivalent amount of Chemical Formula 1, and neutralized and decomposed using 40% -NaOH. The results of measuring the SS content of the supernatant water and the filtrate silica concentration are shown in Experiment 1-1 of Table 1. The dewatered water from the dehydrator was returned to the neutralization decomposition tank. In addition, with the apparatus shown in FIG. 3, the SS content of the supernatant water and the concentration of filtrate silica are measured when hydrosilicic acid is neutralized and decomposed with sodium hydroxide and precipitated and separated without adding hydrogen peroxide. The results are shown in Experiment 1-2 in Table 1 (both the influence of dilution is corrected). Filtrate silica concentration is measured as an alternative to soluble silica concentration.

  From Table 1, when hydrogen peroxide was added (Experiment 1-1), the SS content in the filtrate decreased, and the supernatant water (sodium fluoride aqueous solution) with higher transparency and lower residual silica concentration. was gotten. Only 9% of Si remained in the supernatant water. A Si removal rate of 91% was obtained. Further, even when hydrogen peroxide was not added (Experiment 1-2), an Si removal rate of 81% was obtained. In any case, the silica could be solid-liquid separated without the appearance of gel silica.

Subsequently, the supernatant water (silica separation liquid) obtained by the test 1-1 was adjusted to pH 3, and a crystallization operation was performed using the apparatus shown in FIG. In addition, the specifications of the device, the properties of the supplied water, the operating conditions and the results are shown below. From this, it was confirmed that crystals having a crystal purity of> 97%, an average particle diameter of 5 to 100 μm, and a Si content <1% -SiO ₂ could be recovered.

<Device specifications>
Effective volume: 14L
・ Material: Container: Heat-resistant PVC
: Stirring blade: Wetted part is Teflon (registered trademark)
・ Circulation pump: Diaphragm pump ・ Heater: Electric heater

<Characteristics of supply water>
-Use supernatant water: Created in Test 1-1-pH adjustment conditions: 3

<Operating conditions>
-Seed crystal: Average particle size: 22 μm
: Amount added: 1400 g
-Water temperature: Set water temperature 50 ° C (water temperature in the tank during operation 50-55 ° C)
・ Residence time: 4h
・ Ca addition amount: Equivalent ratio

<Result>
Crystallization tank pH: 1-2
Fluorine recovery rate: 77%
CaF ₂ purity: 98%
Si content of CaF ₂ : 0.2% (0.4% -SiO ₂ )
CaF ₂ average particle size: 26 μm

(Example 2)
The supernatant water (silica separation water) obtained in Test 1-1 of Example 1 was adjusted to pH 5, and the crystallization operation was performed under the same apparatus and operating conditions as in Example 1. Even in this case, crystals having a crystal purity of> 97%, an average particle size of 5 to 100 μm, and a Si content of <1% -SiO ₂ were obtained. There was a tendency for the fluorine recovery to increase. From this, it was confirmed that CaF ₂ crystals suitable for HF production can be recovered even when the pH of the crystallization tank exceeds 2.

<Device specifications>
Same as Example 1 <Supply water>
-Use supernatant water: Created in Test 1-1-pH adjustment conditions: 5
<Operating conditions>
Same as Example 1

<Result>
Crystallization tank pH: 2-3
Fluorine recovery rate: 83%
CaF ₂ purity: 97%
Si content of CaF ₂ : 0.3% (0.6% -SiO ₂ )
CaF ₂ average particle size: 24 μm

(Example 3)
For supernatant water (silica separation water) obtained in Test 1-2 of Example 1 with a slightly high silica concentration, the pH is adjusted to 5 and the crystallization operation is performed under the same apparatus and operating conditions as in Example 1. went. As a result, although the average grain size was slightly reduced and the Si content was increased, crystals with crystal purity> 97%, average grain size 5 to 100 μm, and Si content <1% -SiO ₂ were obtained. Even when the addition step was not provided before the neutralization decomposition step, CaF ₂ crystals suitable for HF production could be recovered.

<Device specifications>
Same as Example 1 <Supply water>
・ Used supernatant water: Created in Test 1-2 ・ pH adjustment conditions: 5
<Operating conditions>
Same as Example 1

<Result>
Crystallization tank pH: 2-3
Fluorine recovery rate: 73%
CaF ₂ purity: 97%
Si content of CaF ₂ : 0.4% (0.8% -SiO ₂ )
CaF ₂ average particle diameter: 22 μm

(Comparative example)
The crystallization operation was performed on the hydrosilicofluoric acid aqueous solution (concentration 1.7% -F) itself. The apparatus and operating conditions were the same as in Example 1. As a result, the crystals obtained after the operation hardly increased from 1400 g charged, and could not be recovered as CaF ₂ .

Example 4
Silica hydrofluoric acid wastewater (1.5% -F) containing 0.05% phosphorus was subjected to silica separation treatment under the operating conditions shown in Test 1-2. A feed solution for supplying a crystallization operation to a dehydrated filtrate (desorbed water) obtained by suction filtration of the sedimented slurry after sedimentation separation using a filter paper having a pore size of 5 μm and mixing with supernatant water; did. The Si concentration including the solubility and SS property of this liquid was 400 mg-Si / L. From this, it was confirmed that CaF ₂ having a phosphorus content <0.1% was obtained even when phosphorus was included.

<Device specifications>
Same as Example 1 <Supply water>
-Use supernatant water: Created in this example-pH adjustment conditions: 3
<Operating conditions>
Same as Example 1

<Result>
Crystallization tank pH: 1-2
CaF ₂ purity: 97%
Si content of CaF ₂ : 0.3% (0.6% -SiO ₂ )
P content of CaF ₂ : <0.1% (as P ₂ O ₅ )
CaF ₂ average particle size: 26 μm

(Comparative Example 2)
In Example 4, the crystallization tank pH was 7 to 8 under the condition where the pH of the feed water was 7, and the phosphorus content of CaF ₂ increased to 1% (as P ₂ O ₅ ). From this, it was confirmed that when the crystallization was performed in the neutral region, the phosphorus content increased.

It is a flowchart which shows the synthetic fluorite collection | recovery method of embodiment of this invention. It is a schematic diagram which shows the outline | summary of the synthetic | combination fluorite collection | recovery apparatus which performs a process until the silica separation water (supernatant water) is obtained by performing the addition process, the neutralization decomposition process, and the separation process in the flowchart shown in FIG. It is a figure which shows the modification of FIG. It is a figure which shows the other modification of FIG. It is a figure which shows the further another modification of FIG. It is a schematic diagram which shows the outline | summary of the synthetic | combination fluorite collection | recovery apparatus until it performs the process of collect | recovering synthetic fluorite by performing the crystallization process and the fluorite collection process with respect to the silica separation water in the flowchart shown in FIG. It is a graph which shows the result of having measured the relationship between pH and filtrate Si density | concentration when neutralizing titration of the hydrosilicofluoric acid aqueous solution with a sodium hydroxide solution. It is a figure which shows the graph shown in FIG. 7 with the outline | summary in the case of implementing a neutralization process by 2 steps | paragraphs.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Stabilizer addition apparatus 11 Mixing tank 12 Raw water introduction pipe 20 Neutralization decomposition apparatus 21 Neutralization decomposition tank 22 Alkaline agent supply part 23 Dilution water supply part 26 pH measuring device 28 Fluorine concentration meter 30 Separation apparatus 31 Precipitation tank 34 Dehydrator 41 Mixing tank 42 Coagulation tank 43 Coagulant supply part 44 pH adjuster supply part 47 pH measuring instrument 51 Filter 60 pH adjuster 61 pH adjuster tank 62 pH adjuster supply part 70 Crystallizer 71 Raw water tank 72 Calcium chloride tank 74 Crystal Analysis tank 79 Flow meter 80 Fluorine concentration meter 81 Calculator 86 Circulating pipe 100 Recovery device 101 Dehydrator 102 Washing water supply pipe

Claims (15)

A synthetic fluorite recovery method for recovering fluorine as synthetic fluorite from fluorine-containing wastewater containing hydrosilicofluoric acid,
An addition step of adding a stabilizer to the fluorine-containing wastewater;
After said adding step, a mixture of the fluorine-containing waste water and a sodium compound to decompose the hydrosilicofluoric acid, and neutralization decomposition step the mixture of insoluble silica and sodium fluoride solution to produce a silica slurry of mainly ,
A separation step of separating insoluble silica from the silica slurry produced in the neutralization decomposition step to obtain silica separation water;
A synthetic fluorite recovery method comprising a crystallization step of supplying calcium compound to the silica separation water to produce calcium fluoride having a purity of 97% or more and an average particle size of 5 to 100 μm.   The method for recovering synthetic fluorite according to claim 1, wherein dilution water is added in the neutralization decomposition step.   The method for recovering synthetic fluorite according to claim 1 or 2, wherein a pH control range in the neutralization decomposition step is 7 to 10, and the sodium compound is sodium carbonate and / or sodium hydroxide.   The synthetic fluorite recovery method according to any one of claims 1 to 3, wherein the crystallization step is performed at a water temperature of 30 to 80 ° C and / or pH 2 or less.   The synthetic fluorite recovery method according to any one of claims 1 to 4, wherein a pH adjustment step of adjusting the pH of the silica separation water is performed between the separation step and before the crystallization step.   6. The synthetic fluorite recovery method according to claim 1, further comprising a fluorite recovery step of dehydrating and recovering the calcium fluoride slurry obtained in the crystallization step as synthetic fluorite. The synthetic fluorite recovery method according to any one of claims 1 to 6, wherein the stabilizer is an oxidizing agent. The separation step includes a dehydration operation of the silica slurry, and the desorbed water obtained by the dehydration operation is returned to the neutralization decomposition step or introduced into the crystallization step or the pH adjustment step. The synthetic fluorite recovery method according to any one of 1 to 7 . A synthetic fluorite recovery device for recovering fluorine as synthetic fluorite from fluorine-containing wastewater containing hydrosilicofluoric acid,
A stabilizer addition device for adding a stabilizer to the fluorine-containing waste water;
And neutralization cracking unit the fluorine-containing waste water and sodium compound are mixed to decompose the hydrosilicofluoric acid, a mixture of insoluble silica and sodium fluoride solution to produce a silica slurry of mainly
A separation device that separates insoluble silica from the silica slurry produced by the neutralization decomposition device to obtain silica separation water;
A synthetic fluorite recovery apparatus, comprising a crystallizer for supplying calcium compound to the silica separation water to produce calcium fluoride having a purity of 97% or more and an average particle diameter of 5 to 100 μm. The synthetic fluorite recovery apparatus according to claim 9 , wherein the neutralization decomposition apparatus includes a dilution water supply unit that supplies dilution water for dissolving sodium fluoride crystals contained in the silica slurry. The synthetic fluorite recovery apparatus according to claim 9 or 10 , wherein the neutralization decomposition apparatus includes a pH adjustment unit that adjusts the pH to a range of 7 to 10. The said crystallizer has at least one of the water temperature adjustment part which adjusts water temperature to 30-80 degreeC, and the pH adjustment part which adjusts pH to 2 or less, The Claim 9 thru | or 11 characterized by the above-mentioned. Synthetic fluorite recovery device. The synthetic fluorite recovery apparatus according to any one of claims 9 to 12 , further comprising a pH adjuster for adjusting the pH of the silica separation water. The synthetic fluorite collecting apparatus according to any one of claims 9 to 13 , further comprising a collecting device for dehydrating and collecting the calcium fluoride slurry obtained by the crystallizer as synthetic fluorite. The synthetic fluorite recovery apparatus according to claim 9, wherein the stabilizer is an oxidizing agent.

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