JP2020121310A - Recovery method of hydrofluoric acid and nitric acid - Google Patents

Abstract

To provide: a method for efficiently recovering both of hydrofluoric acid and nitric acid that are valuables from a nitrohydrofluoric acid waste containing a large amount of hydrosilicofluoric acid in high yield even in an industrial scale implementation; and further a method for producing a hydrofluoric acid salt and silica from hydrosilicofluoric acid.SOLUTION: The recovery method of hydrofluoric acid and nitric acid comprises: contacting an aqueous nitrohydrofluoric acid solution containing a large amount of hydrosilicofluoric acid with a nitrate salt to precipitate a hydrosilicofluoric acid salt; separating the hydrosilicofluoric acid salt by solid-liquid separation; supplying a residual liquid to a distillation column; and separating hydrofluoric acid and nitric acid in the presence of sulfuric acid to recover each of them.SELECTED DRAWING: Figure 1

Description

The present invention relates to a method for separating and recovering hydrofluoric acid and nitric acid from a hydrofluoric nitric acid aqueous solution (hydrofluoric acid/nitric acid mixed solution) containing hydrosilicofluoric acid, which is represented by a semiconductor cleaning waste liquid. Recovers hydrofluoric acid and nitric acid from the hydrosilicofluoric acid immobilization reaction filtrate by a distillation method, and simultaneously separates silica and hydrofluoric acid from the immobilization reaction hydrofluorosilicic acid salt. It relates to a method of collecting.

In recent years, a large amount of hydrofluoric nitric acid has been used for cleaning and processing silicon in the industrial field where silicon is used as a base material for semiconductors and solar cells. Along with this, the waste liquid of hydrofluoric and nitric acid is increasing, and from the viewpoint of environmental problems and resource recovery, development of a method for recovering hydrofluoric acid and nitric acid using the waste liquid as a raw material is required.

JP, 6-57466, A JP, 2007-254254, A JP, 9-302483, A JP, 2008-57043, A JP, 2005-324164, A

In order to perform processing such as the above-mentioned cleaning and etching of silicon, hydrofluoric acid (HF) with a concentration of about 50 wt% and nitric acid (HNO ₃ ) with a concentration of about 70 wt% are mixed and adjusted to an appropriate concentration. Nitric acid is used. The used hydrofluoric nitric acid waste liquid contains a large amount of hydrosilicofluoric acid (H ₂ SiF ₆ ) generated by the reaction of hydrofluoric nitric acid and silicon. Specifically, the used hydrofluoric/nitric acid waste liquid is formed of hydrofluoric acid, nitric acid, and a hydrosilicofluoric acid aqueous solution.

The present invention recovers both hydrofluoric acid and nitric acid at the reusable initial concentrations (about 50 wt% and about 70 wt%, respectively) from such waste liquid as a raw material, and also recovers hydrosilicofluoric acid. It relates to a method of converting into a usable form. Here, a typical composition of hydrofluoric nitric acid used for cleaning and processing silicon is 30 wt% of hydrofluoric acid and 28 wt% of nitric acid. To prepare this composition, hydrofluoric acid having a concentration of about 50 wt% is used. And nitric acid of about 70 wt% are required, and the present invention aims to recover such concentrations of hydrofluoric acid and nitric acid. According to the conventional technique, it is difficult to separate and collect each from a mixed solution of hydrofluoric acid and nitric acid that does not contain hydrofluoric acid. For example, Patent Document 1 discloses a recovery method by a distillation method, but "in the three-component system of nitric acid-hydrofluoric acid-water, an azeotropic mixture is formed, and each acid cannot be concentrated to a desired concentration. It is described that each acid cannot be separated and collected with high purity. On the other hand, Patent Document 2 discloses a method in which a gas-liquid equilibrium of each component is changed by adding a metal salt to a hydrofluoric nitric acid solution, and hydrofluoric acid is separated by a distillation method. The produced hydrofluoric acid has a low purity, and nitric acid cannot be recovered. Furthermore, in the hydrofluoric nitric acid solution containing hydrosilicofluoric acid, which is the subject of the present invention, the three components of hydrofluoric acid, nitric acid, and hydrosilicofluoric acid interfere with each other to form a complicated gas-liquid equilibrium state, so that distillation separation is performed. Becomes even more difficult, and even in the methods disclosed in Patent Documents 3 and 4, only nitric acid is recovered. On the other hand, Patent Document 5 discloses a method of extracting acetic acid, nitric acid, and hydrofluoric acid in this order using an acetate ester and an orthophosphate ester of an alcohol having 8 to 12 carbon atoms as an extractant. Since a large amount of water is used in the stripping and collecting step of the components, and nitric acid is extracted with orthophosphoric acid ester which is an organic substance, there is a risk of contact with nitric acid, and it is hard to say that it is a good idea in a commercial process.

An object of the present invention is to efficiently recover both hydrofluoric acid and nitric acid, which are valuable resources, in high purity from a hydrofluoric nitric acid waste liquid containing a large amount of hydrosilicofluoric acid, even on an industrial scale. An object of the present invention is to provide a method for producing hydrofluoric acid salt and silica from hydrosilicofluoric acid.

As a result of earnest studies, the present inventors have found that when concentrated sulfuric acid is added to a mixed solution of hydrofluoric acid and nitric acid, the gas-liquid equilibrium of hydrofluoric acid and nitric acid is significantly changed, and the gas phase composition is on the hydrofluoric acid side. It was found that the gas-liquid equilibrium greatly affects the amount of sulfuric acid added.

According to the present invention, first, hydrosilicofluoric acid in a hydrofluoric nitric acid waste liquid is brought into contact with a nitrate (referred to as an immobilization reaction), precipitated and removed from the waste liquid as a solid, and the remaining hydrofluoric acid/nitric acid mixture is purified. Distillation made it possible to obtain high-concentration hydrofluoric acid and high-concentration nitric acid.

It is a figure which shows the one aspect|mode of implementation of this invention. It is a figure which shows the one aspect|mode of implementation of this invention.

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail with reference to the embodiments and exemplifications (Figs. 1 and 2), but the present invention is not limited to the embodiments and exemplifications shown below, and the gist of the present invention It can be implemented by arbitrarily changing it without departing from the scope.
<raw material>
The content of hydrosilicofluoric acid, hydrofluoric acid, nitric acid and water in the hydrofluoric nitric acid aqueous solution containing hydrofluorosilicic acid, which is a raw material of the present invention, is not particularly limited, but is as follows. The hydrosilicofluoric acid content is preferably 1 to 30 wt %, and more preferably 5 to 15 wt %. Hydrofluoric acid is preferably 5 to 40 wt%, more preferably 20 to 40 wt%. The content of nitric acid is preferably 10 to 50 wt%, more preferably 20 to 40 wt%. The water content is preferably 30 to 65 wt %, and more preferably 35 to 45 wt %. Further, components other than the above may be contained in a content that does not substantially affect the separation and recovery process of the present invention.

<Immobilization of hydrosilicofluoric acid>
When a hydrofluoric nitric acid aqueous solution containing hydrosilicofluoric acid is directly subjected to distillation, hydrofluoric acid, hydrosilicofluoric acid, and water form an azeotropic composition, so that separation by distillation becomes extremely difficult. In the present invention, since hydrosilicofluoric acid is previously removed, it is converted into a sparingly soluble hydrofluoric acid salt with a nitrate (immobilization reaction), precipitated as a solid, and separated and recovered. That is, by contacting a hydrofluoric nitric acid aqueous solution containing hydrosilicofluoric acid with a nitrate, as shown in the following formula (1), hydrosilicofluoric acid is reacted with a nitrate to produce a hydrosilicofluoride and nitric acid. (Formula (1) gives an example of potassium nitrate).

H ₂ SiF ₆ + 2KNO ₃ → K ₂ SiF ₆ ↓ + 2HNO ₃・・・(1)
Examples of the nitrate include potassium nitrate, sodium nitrate, calcium nitrate, barium nitrate, and the like, and it is preferable to use potassium nitrate having the lowest solubility of hydrofluorosilicic acid salt formed.
The amount of the nitrate to be brought into contact with the hydrofluoric nitric acid aqueous solution containing hydrosilicofluoric acid in the present invention is not particularly limited, but it is preferably 2 mol or more, and more preferably 1 mol of hydrosilicofluoric acid. 3 mol or more is desirable. It is economically preferable that excess nitrate is separated from the nitric acid by distillation in the subsequent step (fourth distillation column) and reused again.

The mode of contact between the hydrofluoric nitric acid aqueous solution containing hydrosilicofluoric acid and the nitrate is not particularly limited, but in the case of the batch method, a method of dropping the hydrofluoric nitric acid aqueous solution containing hydrofluoric nitric acid under the liquid is preferable. .. More preferably, it is a method of dropping a hydrofluoric nitric acid aqueous solution containing hydrosilicofluoric acid while stirring the nitrate aqueous solution. The reactor is preferably a jacketed stirring tank for temperature control. ((1) in Figure 1).

The temperature of the contact between the aqueous solution of hydrofluoric nitric acid containing hydrosilicofluoric acid and the nitrate shows a slight endothermic reaction, so that it is economically desirable to react at 35° C. or lower. The hydrofluorosilicic acid salt deposited by contact between the hydrofluoric nitric acid aqueous solution containing hydrosilicofluoric acid and the nitrate is separated into a hydrosilicofluoric acid salt and a liquid containing hydrofluoric acid and nitric acid by solid-liquid separation. Examples of the solid-liquid separation method include filtration and centrifugation ((2) in FIG. 1). The filtrate obtained by the solid-liquid separation is sent to the first distillation column (FIG. 1) in the distillation separation step. On the other hand, the solid hydrosilicofluoride is dried if necessary ((3) in FIG. 1) and supplied to the next step of producing silica and hydrofluoride (FIG. 2).

<Distillation separation process Figure 1>
Next, the step of distilling and separating hydrofluoric acid will be described (FIG. 1).
Since the filtrate obtained by separating the hydrosilicofluoride salt (filtrate A in FIG. 1) contains hydrofluoric acid, nitric acid and an excess of nitrate, the filtrate is supplied to the first distillation column with a reflux condenser. By doing so, about 30 wt% hydrofluoric acid containing several percent of nitric acid can be obtained from the top of the tower. The theoretical plate number of the distillation column is preferably 10 or more, and more preferably 20 to 40. The reflux ratio is preferably 1 or more, more preferably 3 or more. The distillation method may be a batch method or a continuous method, but in the batch method, distillation is performed while appropriately changing the reflux ratio.

The distillate (column top liquid) distilled in the first distillation column is mixed with sulfuric acid and supplied to the second distillation column, so that the concentration of 50 wt% or more containing nitric acid at a concentration of 50 wt% or more from the top of the column. Hydrofluoric acid (the object of the present invention) can be distilled off. The theoretical plate number of the distillation column is preferably 10 or more, and more preferably 20 to 40. The reflux ratio is preferably 1 or more, more preferably 3 or more. The concentration of sulfuric acid supplied to the distillation column after the addition of sulfuric acid is preferably 45 wt% or more, more preferably 60 wt% or more. The concentration of hydrofluoric acid in the gas phase greatly shifts to the side of hydrofluoric acid depending on the concentration of sulfuric acid in the feed liquid of the second distillation column. It can be distilled at an acid concentration. The reason why sulfuric acid is not added to the first distillation column is that the unreacted nitrate is contained in the residual liquid after the hydrosilicofluoride separation, and therefore the following reaction (2) occurs when sulfuric acid is added ( (Examples of potassium salts are shown), nitrates and sulfuric acid are lost, and sulfate accumulates in the system, leading to operational troubles, so sulfuric acid is added to the system where there is no nitrate (top distillation column top liquid). Doing so is economically advantageous.

KNO ₃ + H ₂ SO ₄ → KHSO ₄ + HNO ₃・・・(2)
Since the bottom liquid of the second distillation column contains hydrofluoric acid, nitric acid and sulfuric acid, the diluted hydrofluoric acid and dilute nitric acid are recovered from the top of the column by supplying to the fifth distillation column with a reflux device. After partly purging as required, it is recycled to the hydrosilicofluoric acid immobilization reaction ((1) in FIG. 1). The theoretical plate number of the distillation column is preferably 10 or more, and more preferably 20 to 40. The reflux ratio is preferably 1 or more, more preferably 3 or more.

The bottom liquid of the fifth distillation column contains dilute sulfuric acid, is supplied to a concentration tank ((4) in FIG. 1) to distill off water, and 75 wt% or more concentrated sulfuric acid is discharged from the bottom of the tank. It is recycled to the feed liquid of the second distillation column. The pressure for dilute sulfuric acid concentration is preferably 6.7 kPa or less, more preferably 3.3 kPa or less. The heating temperature is preferably 100° C. or higher, more preferably 150° C. or higher. The sulfuric acid concentration at the bottom of the tank is preferably 75 wt% or more, more preferably 85 wt% or more.

As the type of concentrating tank, a general evaporator such as a natural circulation type, a calandria type, or a forced circulation type can be used. On the other hand, the bottom liquid of the first distillation column contains low-concentration hydrofluoric acid and excess nitrate in addition to nitric acid.By supplying it to the third distillation column, dilute nitric acid is distilled from the top of the column. Then, after partly purging as required, it is recycled to the immobilization reaction of hydrosilicofluoric acid ((1) in FIG. 1). The theoretical plate number of the distillation column is preferably 10 or more, and more preferably 20 to 40. The reflux ratio is preferably 1 or more, more preferably 3 or more.

The bottom liquid of the third distillation column contains high-concentration nitric acid and excess nitrate, and by supplying the nitric acid to the fourth distillation column, 65 wt% or more of nitric acid can be distilled from the top of the column (the present invention. Object). The bottom liquid of the fourth distillation column contains an excess amount of nitrate and a very small amount of nitric acid, and if necessary, a part of it may be purged to the hydrofluoric acid immobilization reaction ((1) in FIG. 1). Be recycled. The theoretical plate number of the distillation column is preferably 10 or more, and more preferably 20 to 40. The reflux ratio is preferably 1 or more, more preferably 3 or more.

<Silica and hydrofluoric acid salt manufacturing process FIG. 2>
This step is a processing step for decomposing the hydrosilicofluoride salt obtained by the immobilization reaction into silica and hydrofluoric acid salt, which are more industrially useful, and making them easier to reuse. Note that for economic reasons, a process of reusing the hydrosilicofluoride as it is conceivable. In this step, as exemplified by the formulas (3) and (4), the hydrofluorosilicic acid salt is decomposed with an alkaline substance (an example of a potassium salt is shown).

K ₂ SiF + 2K ₂ CO ₃ → SiO ₂ + 6KF + 2CO ₂ ↑ ・・・・(3)
K ₂ SiF ₆ + 4KOH → SiO ₂ + 6KF + 2H ₂ O ··· (4)
As the alkaline substance, various metal carbonates and hydroxides can be used. Among them, Na and K salts are preferable, and further, K salt, which has high water solubility of the hydrofluoride salt to be generated, is preferable.
The reaction molar ratio of alkali/hydrosilicofluoride is preferably 4.00 to 4.25, more preferably 4.05 to 4.20. If the molar ratio exceeds the upper limit, the following reaction (4) occurs, water-soluble silicate is formed, silica is not obtained, and the quality of hydrofluoride is affected.

SiO ₂ + 2KOH → K ₂ SiO ₃ + H ₂ O ・・・(4)
As the decomposition reactor, a jacketed stirring tank for temperature control is preferred ((5) in FIG. 2). The contact method is preferably a method in which an aqueous alkali solution is dropped into a hydrosilicofluoride slurry. In the opposite case or when the molar ratio exceeds the upper limit, the phenomenon of silica sticking occurs and silica cannot be taken out. The water slurry concentration of hydrosilicofluoride is preferably 30 wt% or less, more preferably 25 wt% or less. If the slurry concentration of hydrofluorosilicic acid salt is high, agglomeration and sticking of silica occur, and it becomes difficult to take out silica from the reactor. The reaction time is preferably 1 hour or longer, more preferably 2 hours or longer. The alkali concentration is preferably 50 wt% or less.

The solid-liquid separation method of the precipitated crude silica is preferably filtration or centrifugation ((6) in FIG. 2). On the other hand, since the hydrofluoric acid salt is attached to the crude silica on the solid side, it is supplied to the hot water suspension washing tank ((7) in FIG. 2) and, after suspension washing with hot water, By performing solid-liquid separation by filtration or centrifugation ((8) in FIG. 2), the hydrofluoric acid salt can be dissolved and removed, and high-purity silica can be obtained. The hot water suspension washing temperature is preferably 80°C or higher, more preferably 95°C or higher. The silica slurry concentration during suspension washing is preferably 20 wt% or less, and more preferably 10 wt% or less. The suspension washing time is preferably 0.5 hours or more, more preferably 1 hour or more. A stirring tank with a jacket for temperature control is preferable as the suspension washing device ((7) in FIG. 2).

The hydrofluoric acid salt-containing filtrate thus obtained is supplied to a vacuum concentration crystallization tank ((9) in FIG. 2) to evaporate water (concentration) to precipitate hydrofluoric acid salt. Therefore, a high-purity hydrogen fluoride salt can be obtained by solid-liquid separation by filtration or centrifugation ((10) in FIG. 2) and then drying. The vacuum concentration crystallization tank is preferably a jacketed stirring tank for temperature control ((9) in FIG. 2). In concentrating hydrofluoric acid salts, it is important not to raise the internal temperature above the control temperature. If the temperature is higher than the control temperature, the hydrofluoric acid salt may aggregate and not only form a slurry, but also stick. For that purpose, it is important to keep the internal temperature below the upper limit value by appropriately adjusting the pressure while monitoring the internal temperature under reduced pressure. The control temperature is 80 to 100°C, preferably 85 to 95°C. Particularly, in the case of potassium fluoride, the solubility in water is as large as about 50 wt %, and crystal precipitation does not occur unless it is concentrated to 60 wt% or more. When this happens, the boiling point rises with concentration, the internal temperature rises, and the cohesion and fixation of potassium fluoride occurs. In order to avoid the sticking phenomenon, it is necessary to prevent the internal temperature from rising by concentrating in a reduced pressure system. The pressure is preferably 26.7 kPa or less, more preferably 13.3 kPa or less.

The concentration of potassium fluoride after concentration is preferably 65 to 80 wt%, more preferably 70 to 75 wt%. The filtration temperature is preferably 40 to 70°C, and more preferably 50 to 60°C. Since the filtrate after solid-liquid separation of potassium fluoride contains a high concentration of potassium fluoride, it is recycled into the filtrate obtained by solid-liquid separation of crude silica, and the concentrated solution in a vacuum concentration crystallization tank (see FIG. 2). (9)).

(Experimental example 1)
A fluororesin reactor equipped with a stirrer was charged with 184 g of a 40 wt% potassium nitrate aqueous solution and maintained at a temperature of 30° C. with stirring. Next, 500 g of a hydrofluoric nitric acid waste liquid having a weight composition of HF/HNO ₃ /H ₂ SiF ₆ /water=25/25/7/43 was placed in a fluororesin dropping funnel and dropped into the reactor while maintaining the internal temperature at 30°C. , Reacted with potassium nitrate. Potassium nitrate corresponds to 3 times the molar number of hydrosilicofluoric acid. The potassium hydrosilicofluoride salt deposited at the same time as the dropping was filtered, washed and collected. The amount of recovered potassium hydrosilicofluoride was 53 g, of which the content analysis values of fluorine, silicon, and potassium were 51.9 wt%, 12.7 wt%, and 35.5 wt%, respectively. The value was close to the composition value. The fluorine and nitric acid concentrations were measured by an ion chromatograph (ICS-1500) manufactured by Dionix, and the potassium and silicon concentrations were measured by an ICP light emitting device.

<Measurement conditions>
Ion chromatograph analysis column: IonPacAG23(4mm×50mm), IonPacAS23(4mm×250mm)
Column temperature: 30℃, suppressor: ARRS-300 4mm,
Eluent: 4.5 mmol/L Na2CO3, 0.8 mmol/NaHCO3 Flow rate: 1.0 ml/min
Detector: Electrical conductivity detector
ICP emission analyzer
Agilent ICP VISTAPRO, power: 1.2kw
Measurement wavelength Si: 251.611n K: 766.491nm

(Experimental example 2)
The same composition as the liquid (weight composition HF/HNO ₃ /KNO ₃ /water=19.8/24.6/3.9/51.6) of the filtrate composition after filtering off the potassium hydrofluoride in Experimental Example 1 of the first rectification column 1500 g of the liquid was prepared, and distillation was carried out using a batch type atmospheric pressure rectification column (diameter 26 mm x height 1200 mmH, packing: fluororesin Raschig ring) equipped with a fluororesin stirring tank and a reflux condenser. did. The column bottom temperature was 115 to 120° C., the column top pressure was 101.3 kPa at atmospheric pressure, and the reflux ratio was 3. The mixed liquid composition of the distillate at a distillation rate of 63% from the top of the prepared liquid was HF/HNO ₃ /water=28.4/6.7/64.9 by weight. The liquid composition remaining at the bottom of the rectification column at this time was HF/HNO ₃ /KNO ₃ /water=5.1/55.3/29.0/10.6 by weight. At this point the distillation was complete.

(Experimental example 3)
Second distillation column 600 g of a liquid having the same composition as the distillate obtained in Experimental Example 2 was prepared, 540 g of 98 wt% sulfuric acid was added to the liquid, and a rectification column having the same shape as in Experimental Example 2 was used. Distillation was carried out at a normal pressure, a bottom temperature of 126 to 167° C., and a reflux ratio of 3.0. With respect to the prepared liquid amount (600 g), the distillate weight composition at the time when the distillation rate from the top of the column was 10% (60 g) was HF/HNO ₃ /water=75/10/15. Further, the distillation was continued, and the weight composition of the distillate mixed at a distillation rate of 45% (269 g) was HF/HNO ₃ /water=55/10/35 (hydrofluoric acid concentration 55 wt%). This distillate is hydrofluoric acid containing 10 wt% of nitric acid, but when mixed with concentrated nitric acid, it has sufficient concentration and purity to be reused as hydrofluoric nitric acid having a concentration used for cleaning and processing silicon substrates ( Object of invention). On the other hand, the weight composition of the residual liquid at the bottom of the rectification column at a distillation rate of 45% was HF/HNO ₃ /H ₂ SO ₄ /water=3.4/3.2/61.2/32.2.

(Experimental example 4)
In a rectification column having the same shape as in Experimental Example 2, 600 g of a liquid having the same composition as the distillate obtained in Experimental Example 2 of the second distillation column was prepared, and 270 g of 98 wt% sulfuric acid was added. Distillation was carried out at normal pressure, a bottom temperature of 117 to 164° C., and a reflux ratio of 3.0. With respect to the prepared liquid amount (600 g), the distillate weight composition at the time of the distillation rate from the column top being 10% (60 g) was HF/HNO ₃ /water=64/8/28. Further, the distillation was continued, and the weight composition of the distillate mixed at a distillation rate of 33% (198 g) was HF/HNO ₃ /water=50/4/46. This distillate also has such a concentration and purity that it can be reused as hydrofluoric nitric acid. On the other hand, the weight composition of the residual liquid at the bottom of the rectification column at a distillation rate of 33% was HF/HNO ₃ /H ₂ SO ₄ /water=9.5/4.5/46/40.

(Experimental example 5)
Second distillation column 600 g of a liquid having the same composition as the distillate obtained in Experimental Example 2 was prepared, sulfuric acid was not added, and a rectification column having the same shape as in Experimental Example 2 was used under normal pressure and Distillation was performed at a bottom temperature of 111 to 112°C and a reflux ratio of 3.0. With respect to the adjusted liquid (600 g), the distillate composition from the top of the column to the time when the distillation rate was 25% (150 g) was HF/HNO ₃ /water=12.0/0.3/86.7. Distillation was further continued, and the distillate composition at a distillate rate of 40% (240 g) was HF/HNO ₃ /water=20.6/0.3/79.0.
The hydrofluoric acid concentration of this distillate is low, and is not suitable for reuse as hydrofluoric nitric acid.

(Experimental example 6)
The same weight composition (HF/HNO ₃ /KNO ₃ /water=5.1/55.3/29.0/10.6) as the residual liquid at the bottom of the distillation column obtained after performing the treatment of the rectification column of the third rectification column experimental example 2. ) Was adjusted to 1500 g, and distillation was carried out in a rectification column having the same shape as in Experimental Example 2 at atmospheric pressure, a column bottom temperature of 119 to 123° C., and a reflux ratio of 3.0. The weight composition of the distillate from the top of the tower is HF/HNO ₃ /water=14/53/33 with a mixed solution (540 g) up to a distillation rate of 36%, and the bottom residue composition is the weight composition HF/HNO ₃ /KNO ₃ /Water=0.4/64.0/16.6/19.0.

(Experimental example 7)
The same weight composition (HF/HNO ₃ /KNO ₃ /water=0.4/64.0/16.6/19.0) as in the distillation still residue obtained in Experimental Example 6 of the fourth rectification column was adjusted to 960 g, and the same as in Example 2. Distillation was carried out in a rectification column having a shape at atmospheric pressure, a column bottom temperature of 123 to 126° C., and a reflux ratio of 3. The weight composition of 468 g of the mixed liquid up to the distillation rate of 56% from the column top was HF/HNO ₃ /water=0.6/67/32.4 (67% nitric acid). This is the object of the present invention, and the original hydrofluoric nitric acid aqueous solution can be prepared by mixing with 52 wt% of hydrofluoric acid.

(Experimental example 8)
Equal to the weight composition (HF/HNO ₃ /H ₂ SO ₄ /water=3.4/3.2/61.2/32.2) of the bottom liquid of the distillation column after the distillation in the rectification column treatment of the fifth rectification column experimental example 3 1155 g of a liquid having the composition of was prepared and subjected to simple distillation at a temperature of 123 to 126° C. and a pressure of 33.3 kPa. The distillate weight composition at a distillation rate of 55% (246 g) calculated from the amount excluding sulfuric acid from the charged amount was HF/HNO ₃ /water=16/13/71. The weight composition of the residue at that time was HF/HNO ₃ /H ₂ SO ₄ /water=0.014/0.20/75/24.8.

(Experimental example 9)
It has the same composition as the weight composition (HF/HNO ₃ /H ₂ SO ₄ /water=0.014/0.20/75/24.8) of the liquid remaining at the bottom of the column after the completion of the distillation by the rectification column treatment in the sulfuric acid concentration experiment example 8. The liquid was adjusted to 500 g and charged into a flask, and water was distilled off under a reduced pressure of 150 to 199° C. and a pressure of 3.3 kPa. The temperature inside the kettle increased as the water was distilled off, and a sulfuric acid concentration of 92 wt% could be obtained when the temperature inside the kettle reached 199°C.

Preparation of Silica from Potassium Hydrofluorosilicate 88.0 g of potassium hydrofluorofluoride obtained in the same manner as in Experimental Example 1 and 350 g of water were placed in a 500 ml flask equipped with a stirrer and made into a slurry state, and the temperature was raised to 100°C. Then, 184 g of 50 wt% potassium hydroxide aqueous solution (KOH/K ₂ SiF ₆ molar ratio: 4.10) was added dropwise over 30 minutes. After the dropwise addition was completed, the reaction was continued at an internal temperature of 100° C. for 3 hours. The internal temperature was cooled to room temperature, and silica was obtained by vacuum filtration. 45 g of the obtained hydrous silica was put into 300 g of water at 100° C., and after water suspension washing was carried out for 30 minutes, it was collected by hot filtration at 100° C. to obtain 23 g as a dried product. As a result of analyzing silicon and oxygen of the recovered silica, the silicon content was 46.3 wt% and the oxygen content was 53.2 wt %, which were almost theoretical values. In addition, fluorine in the silica was 500 wtppm or less, potassium was 0.5 wt% or less, and high purity silica having a purity of 99 wt% or more and an average particle diameter of about 25 μm could be recovered. Silicon was analyzed by an ICP light emitting device, and oxygen was analyzed by an inert gas melting method. The average particle diameter was measured by a laser diffraction/scattering method.

<Analysis conditions>
Fluorine: Ion chromatograph (according to the above analysis method)
Potassium: ICP light emitting device (according to the above analysis method)
Oxygen: Inert gas melting method
Model LECO-TC-436AR, Detector: Infrared absorption detector Average particle size: Dispersing solvent: Ethanol, Measuring range: 0.12 to 704 μm

(Example 10)
Potassium fluoride obtained in Example 9 of hydrofluoric potassium (hydrofluoric acid potassium KF) solution were charged to a 500ml glass reactor equipped with a stirrer, fluoride under reduced pressure system at a bath temperature of 90 ° C. below the temperature Water was distilled off until the concentration of potassium iodide was 60 wt% or more. Since potassium fluoride crystals are precipitated when the concentration is increased, potassium fluoride was obtained by filtration in the temperature range of 40°C to 60°C.

It was dried under reduced pressure at a temperature of 100° C. to obtain dry potassium fluoride. When the water content was measured by the Karl Fischer method, it was 0.5% or less, and it was found that the hydrous potassium fluoride was an anhydride. Analysis of fluorine and potassium concentrations showed values close to theoretical values. Furthermore, when the amount of silicon as an impurity was analyzed, it was 400 wtppm or less, and 99 wt% or more of high-purity potassium fluoride could be recovered.

(Experimental example 11)
The apparatus used the rectification column in Experimental Example 2. 600 g of the distillate composition obtained in Experimental Example 2 was adjusted and distilled at a reflux ratio of 3.0 in a normal pressure system. The weight composition of the distillate at a distillation rate of 8% (48 g) was HF/HNO ₃ /water=1/0.5/98.5. The distillate weight composition which was continuously distilled and was mixed at a distillation rate of 24% (144 g) was HF/HNO ₃ /water=6/0.3/35/93.7, and the distillation was mixed at the time of a distillation rate of 24%. The liquid weight composition is HF/HNO ₃ /water=12/0.3/87.7, and the distillate weight composition mixed at the time of the distillation rate of 32% (192 g) is HF/HNO ₃ /H ₂ SO ₄ /water=17/. It was 3/82.7. When the distillation was further continued and the distillation rate was 40%, the weight composition of the distillate mixture was HF/HNO ₃ /water=21/0.3/78.7, and high-concentration hydrofluoric acid could not be obtained.

(Experimental example 12)
88.0 g of potassium hydrofluorosilicic acid salt obtained by the same method as in Experimental Example 1 and 350 g of water were charged into a 500 ml flask equipped with a stirrer to prepare a slurry state, and after raising the temperature to 100° C., 224 g of 50 wt% potassium hydroxide aqueous solution ( KOH/K ₂ SiF ₆ molar ratio: 5.00) was added dropwise over 30 minutes. After the dropping was completed, the precipitated silica began to dissolve, and after 30 minutes, it could not be completely dissolved and recovered.

(Experimental example 13)
Into a 500 ml flask equipped with a stirrer, 88.0 g of potassium hydrosilicofluoride salt obtained by the same method as in Experimental Example 1 was charged into a 500 ml flask with a stirrer, and the temperature was raised to 100° C., and then 202 g of 50 wt% potassium hydroxide aqueous solution. (KOH/K ₂ SiF ₆ molar ratio: 4.50) was added dropwise over 30 minutes. After completion of the dropping, a part of the precipitated silica began to dissolve, and the remaining silica was agglomerated and fixed on the flask wall and could not be taken out and could not be recovered.

(Experimental example 14)
88.0 g of potassium hydrofluorosilicic acid salt obtained by the same method as in Experimental Example 1 and 132 g of water were charged into a 500 ml flask equipped with a stirrer to make a slurry concentration of 40 wt% and heated to 100° C., and then 50 wt% hydroxylated. 184 g of potassium aqueous solution (KOH/K ₂ SiF ₆ molar ratio: 4.10) was added dropwise over 30 minutes. After the completion of the dropping, the silica was aggregated and fixed in the flask and could not be collected and recovered.

(Experimental example 15)
A 500 ml flask equipped with a stirrer was charged with 184 g of 50 wt% potassium hydroxide aqueous solution and 132 g of water to form a uniform solution, and after raising the temperature to 100° C., 88 g of potassium hydrosilicofluoride salt obtained by the same method as in Experimental Example 1 was used for 30 minutes. (KOH/K ₂ SiF ₆ molar ratio: 4.10). Immediately after the addition, silica was not generated, and after the completion of the addition, the silica was agglomerated and fixed in the flask and could not be taken out and recovered.

(Experimental example 16)
An aqueous potassium fluoride solution obtained by the same method as in Experimental Example 9 was charged into a 500 ml glass reactor equipped with a stirrer, and water was added at a bath temperature of 150° C. under a normal pressure system until the potassium fluoride concentration reached 60 wt% or more. Distillation was carried out. When the concentration increased, potassium fluoride did not form a slurry and adhered to the stirring blade and the flask wall, and could not be taken out and recovered.

(Experimental Example 17)
An aqueous potassium fluoride solution obtained in the same manner as in Experimental Example 9 was charged into a 500 ml glass reactor equipped with a stirrer, and the bath temperature was 135° C. and the pressure was 42.6 kPa (320 Torr) under reduced pressure at an internal temperature of 110 to 120° C. Water was distilled off until the concentration of potassium iodide was 60 wt% or more. Although potassium fluoride was deposited, it partially adhered to the flask wall and was not easy to collect.

Claims (4)

A method for recovering hydrofluoric acid and nitric acid, which comprises supplying a mixture of hydrofluoric acid and nitric acid to a distillation column, separating hydrofluoric acid and nitric acid in the presence of sulfuric acid, and recovering each. A method for producing hydrofluoric acid, comprising the step of supplying a mixture of hydrofluoric acid and nitric acid to a distillation column and separating hydrofluoric acid and nitric acid in the presence of sulfuric acid. A method for producing nitric acid, comprising the step of supplying a mixture of hydrofluoric acid and nitric acid to a distillation column and separating hydrofluoric acid and nitric acid in the presence of sulfuric acid. A hydrofluoric nitric acid aqueous solution containing hydrosilicofluoric acid is brought into contact with a nitrate to precipitate a hydrofluorosilicic acid salt, and the hydrofluorosilicic acid salt is separated by solid-liquid separation, and the obtained residual liquid is supplied to a distillation column. , A method for recovering hydrofluoric acid and nitric acid, in which hydrofluoric acid and nitric acid are separated in the presence of sulfuric acid and recovered respectively.

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