NIOBIC ACID AQUEOUS SOLUTION

Abstract

A niobic acid aqueous solution is provided having higher dispersibility than an ammonium niobate sol and having better solubility than a complex salt of niobic acid. The niobic acid aqueous solution contains 0.1 to 40 mass % of niobium in terms of Nb.sub.2O.sub.5, wherein no particles of 1.0 nm or more are detected in the particle size distribution measurement using dynamic light scattering. A method for producing the same includes adding a niobium fluoride aqueous solution containing 1 to 100 g/L of niobium in terms of Nb.sub.2O.sub.5 to an ammonia aqueous solution having an ammonia concentration of 20 to 30 mass % and reacting them, removing fluorine from the obtained reaction solution, and adding at least one selected from amines and ammonia to the obtained solution and reacting them.

Claims

1. A niobic acid aqueous solution comprising: 0.1 to 40 mass % of niobium in terms of Nb.sub.2O.sub.5, wherein no particles of 1.0 nm or more are detected in the particle size distribution measurement using dynamic light scattering.

2. The niobic acid aqueous solution according to claim 1, further comprising: a component derived from at least one selected from amines and ammonia.

3. A niobic acid aqueous solution, wherein a precipitate of Na.sub.8Nb.sub.6O.sub.19.13H.sub.2O is formed when 20 g of a sodium hydroxide aqueous solution (25° C.) at a concentration of 3 mass % is added to 20 g of the niobic acid aqueous solution (25° C.) adjusted to the concentration at which niobium is contained at 1 g per 10 g in terms of Nb.sub.2O.sub.5, while stirring for 1 minute, and then the mixture is allowed to stand while maintaining a liquid temperature at 5° C. for 20 hours.

4. A method for producing a niobic acid aqueous solution comprising three steps: (1) a step of adding a niobium fluoride aqueous solution comprising 1 to 100 g/L of niobium in terms of Nb.sub.2O.sub.5 to an ammonia aqueous solution having an ammonia concentration of 10 to 30 mass %, and reacting them to obtain a niobium-containing precipitate; (2) a step of removing fluorine from the niobium-containing precipitate obtained in the above step; and (3) a step of making the niobium-containing precipitate obtained by removing fluorine into a slurry, adding at least one selected from amines and ammonia, and reacting them.

5. The method for producing a niobic acid aqueous solution according to claim 4, wherein in the step (1), a neutralization reaction is carried out within 1 minute of addition of the niobium fluoride aqueous solution to the ammonia aqueous solution.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0020] FIG. 1 is an XRD pattern of the precipitate obtained by conducting a reactivity test with NaOH using the niobic acid aqueous solution obtained in Example 4.

[0021] FIG. 2 is an XRD pattern of the precipitate obtained by conducting a reactivity test with NaOH using the niobic acid aqueous solution obtained in Comparative Example 3.

DETAILED DESCRIPTION

[0022] Next, the present invention will be described based on embodiments. However, the present invention is not limited to the embodiments described below.

[0023] <The Present Niobic Acid Aqueous Solution>

[0024] The niobic acid aqueous solution according to one embodiment of the present invention (“the present niobic acid aqueous solution”) is a solution which contains niobium or/and niobic acid, and in which no particles of 1.0 nm or more are detected in the particle size distribution measurement using dynamic light scattering.

[0025] Although it could be confirmed that the present niobic acid aqueous solution contains niobium or/and niobic acid, the state in which it is present is still under investigation. However, while it has not been technically proven, it can be assumed that the niobic acid in the present niobic acid aqueous solution is present in water as an ion ionically bonded with an amine or/and ammonia.

[0026] In the present niobic acid aqueous solution, it is thought that while hydroxide ions are present as anions, halide ions such as fluoride ions and chloride ions are almost nonexistent, and amines or ammonia are present as cations, and therefore niobium is present as an anion such as NbO.sup.−.

[0027] Note that the niobium or/and niobic acid in the present niobic acid aqueous solution is not necessarily present in the state of Nb.sub.2O.sub.5. The content of niobium or/and niobic acid is indicated in terms of Nb.sub.2O.sub.5 based on the convention for indicating the Nb concentration.

[0028] The present niobic acid aqueous solution preferably contains 0.1 to 40 mass % niobium in terms of Nb.sub.2O.sub.5, of which a ratio of 0.5 mass % or more, and even 1 mass % or more is still more preferable, while a ratio of 30 mass % or less, and even 20 mass % or less is still more preferable.

[0029] The present niobic acid aqueous solution preferably contains a component derived from at least one selected from amines and ammonia.

[0030] Although not technically proven, it can be inferred that the ionic bonding of an amine or ammonia with niobic acid can increase its solubility in water.

[0031] The present niobic acid aqueous solution is a solution in which no particles of 1.0 nm or more are detected when the particle size distribution is measured using dynamic light scattering, and is preferably a solution in which no particles of 0.6 nm or more are detected. In other words, it is a solution in a completely dissolved state. Therefore, all of the niobate salts in the solution are water-soluble and it is clearly different from a sol.

[0032] Note that in the present invention, a liquid that contains niobic acid and in which no particles of 1.0 nm or more are detected when measured by dynamic light scattering, is referred to as a “niobic acid aqueous solution”.

[0033] Dynamic light scattering is a method in which a solution such as a suspension solution is irradiated with light such as laser light, the light scattering intensity from a group of particles in Brownian motion is measured, and the particle size and distribution are determined from the temporal variation of the intensity. Here, the measurement is performed in accordance with JIS Z 8828:2019 “Particle size analysis—Dynamic light scattering”.

[0034] In the present invention, “particles of 1.0 nm or more are detected when the particle size distribution is measured” means that a reliable value for the particle size can be measured when the particle size is measured by dynamic light scattering. “No particles of 1.0 nm or more are detected” means either that the reliable measured value for the particle size is less than 1.0 nm when the particle size is measured by dynamic light scattering, or that a reliable value for the particle size cannot be measured, such as an erroneous value being displayed.

[0035] For example, in the case of a sol containing niobic acid, the colloidal particles are dispersed in a liquid, and the particle size of the colloidal particles is 1.0 nm or more. Therefore, particles of 1.0 nm or more will be detected when the particle size distribution is measured. In contrast, if a substance derived from niobic acid is dissolved as ions in water, no particles of 1.0 nm or more will be detected when the particle size is measured using dynamic light scattering.

[0036] Note that with respect to the measurement conditions of the above dynamic light scattering method, the higher the concentration of niobium, the more likely measurable particles are contained, and therefore it is preferable to perform the measurement after adjusting niobium to be contained at a concentration of 10 mass %, 15 mass % or 20 mass % in terms of Nb.sub.2O.sub.5.

[0037] (Other Components)

[0038] The present niobic acid aqueous solution may contain components other than niobium or/and niobic acid as long as it does not interfere with the feature that no particles of 1.0 nm or more are detected when the particle size distribution is measured using dynamic light scattering.

[0039] (Reactivity with Sodium Hydroxide Aqueous Solution)

[0040] The present niobic acid aqueous solution is an aqueous solution wherein a precipitate of Na.sub.8Nb.sub.6O.sub.19.13H.sub.2O is formed when 20 g of a sodium hydroxide aqueous solution (25° C.) at a concentration of 3 mass % is added to 20 g of the niobic acid aqueous solution (25° C.) adjusted to the concentration at which niobium is contained at 1 g per 10 g in terms of Nb.sub.2O.sub.5, while stirring for 1 minute, and then the mixture is allowed to stand while maintaining a liquid temperature at 5° C. for 20 hours.

[0041] Therefore, it can be said that the present niobic acid aqueous solution is highly reactive with alkali metal salts, and that when reacted with a sodium hydroxide aqueous solution as described above, a precipitate of Na.sub.8Nb.sub.6O.sub.19.13H.sub.2O is formed.

[0042] Usually, to obtain a niobate hydrate (Na.sub.8Nb.sub.6O.sub.19.13H.sub.2O), it is necessary to react Nb hydroxide with a sodium hydroxide aqueous solution by mixing them and heating the mixture to 80° C. or more among other things, and it is therefore difficult to easily produce it. However, since the present niobic acid aqueous solution is highly reactive with alkali metal salts, it is possible to obtain a niobate hydrate (Na.sub.8Nb.sub.6O.sub.19.13H.sub.2O) simply by mixing and reacting it with a sodium hydroxide aqueous solution, followed by cooling.

[0043] Note that confirmation that the formed precipitate is a precipitate of Na.sub.8Nb.sub.6O.sub.19.13H.sub.2O can be made by, for example, identification by measuring X-ray diffraction (XRD) as follows. However, it is not limited to this method.

[0044] That is, the above formed precipitate can be measured by an X-ray diffraction measurement under the following conditions and identified as Na.sub.8Nb.sub.6O.sub.19.13H.sub.2O or not by comparing it with the XRD pattern of ICDD card No. 00-014-0370.

[0045] The X-ray diffraction measurement conditions in this case should be as follows.

[0046] Device: MiniFlex II (manufactured by Rigaku Corporation)

[0047] Measuring range (2θ): 5 to 90°

[0048] Sampling width: 0.02°

[0049] Scanning speed: 2.0°/min

[0050] X-ray: CuKα ray

[0051] Voltage: 30 kV

[0052] Current: 15 mA

[0053] Divergence slit: 1.25°

[0054] Scattering slit: 1.25°

[0055] Receiving slit: 0.3 mm

[0056] <The Present Production Method>

[0057] Next, a suitable method for producing the present niobic acid aqueous solution (referred to as “the present production method”) will be described.

[0058] An example of the present production method includes a production method including adding a niobium fluoride aqueous solution to an ammonia aqueous solution of a predetermined concentration to obtain a niobium-containing precipitate (referred to as “inverse neutralization step”), removing fluorine from the niobium-containing precipitate (referred to as “F washing step”), making the niobium-containing precipitate obtained by removing fluorine into a slurry, adding at least one selected from amines and ammonia, and reacting them to obtain the present niobic acid aqueous solution (referred to as the “water solubilization step”). However, the method for producing the present niobic acid aqueous solution is not limited to such production method.

[0059] As long as the present production method includes the above steps, other steps or other treatments can be added as appropriate.

[0060] (Inverse Neutralization Step)

[0061] In the inverse neutralization step, it is preferable to add a niobium fluoride aqueous solution to an ammonia aqueous solution of a predetermined concentration to obtain a niobium-containing precipitate. That is, it is preferable to inverse neutralize.

[0062] It is preferable to perform inverse neutralization, in which a niobium fluoride aqueous solution is added to an ammonia aqueous solution for neutralization, instead of normal neutralization, in which an ammonia aqueous solution is added to a niobium fluoride aqueous solution for neutralization.

[0063] It is assumed that the structure of niobic acid will be a structure more soluble in water by performing inverse neutralization.

[0064] The niobium fluoride aqueous solution can be prepared by reacting niobium or/and niobic oxide with hydrofluoric acid (HF) to form niobium fluoride (H.sub.2NbF.sub.7), which is then dissolved in water.

[0065] Furthermore, this niobium fluoride aqueous solution is preferably prepared by adding water (for example, pure water) to contain 1 to 100 g/L of niobium in terms of Nb.sub.2O.sub.5. In this case, if the niobium concentration is 1 g/L or more, it will result in a niobic acid compound hydrate soluble in water, and therefore the niobium concentration of the niobium fluoride aqueous solution is more preferably 1 g/L or more in terms of Nb.sub.2O.sub.5, of which 10 g/L or more, and even 20 g/L or more is still more preferable when productivity is considered. On the other hand, if the niobium concentration is 100 g/L or less, it will result in a niobic acid compound hydrate soluble in water, and therefore to synthesize a water-soluble niobic acid compound hydrate more reliably, 90 g/L or less is more preferable, of which 80 g/L or less, and even 70 g/L or less is still more preferable.

[0066] From the viewpoint of completely dissolving niobium or/and niobic oxide, the pH of the niobium fluoride aqueous solution is preferably 2 or less, of which 1 or less is still more preferable.

[0067] On the other hand, the above ammonia aqueous solution preferably has an ammonia concentration of 10 to 30 mass %.

[0068] By setting the ammonia concentration of the ammonia aqueous solution used for inverse neutralization to 10 mass % or more, it is possible to prevent Nb from remaining undissolved, and to make niobium or/and niobic acid completely soluble in water. On the other hand, if the ammonia concentration of the ammonia aqueous solution is 30 mass % or less, it is preferable since it is around a saturated aqueous solution of ammonia.

[0069] From such viewpoint, the ammonia concentration of the ammonia aqueous solution is preferably 10 mass % or more, of which 15 mass % or more, even 20 mass % or more, and even 25 mass % or more is still more preferable. On the other hand, it is preferably 30 mass % or less, of which 29 mass % or less, and even 28 mass % or less is preferable.

[0070] In the inverse neutralization step, the amount of niobium fluoride aqueous solution added with respect to the ammonia aqueous solution (NH.sub.3/Nb.sub.2O.sub.5 molar ratio) is preferably 95 to 500, of which 100 or more or 450 or less, and even 110 or more or 400 or less is still more preferable.

[0071] Moreover, the amount of niobium fluoride aqueous solution added with respect to the ammonia aqueous solution (NH.sub.3/HF molar ratio) is preferably 3.0 or more, of which 4.0 or more, and even 5.0 or more is still more preferable, from the viewpoint of the formation of a niobic acid compound that is soluble in amines and dilute ammonia water. On the other hand, from the viewpoint of cost reduction, it is preferably 100 or less, of which 50 or less, and even 40 or less is still more preferable.

[0072] Note that both the niobium fluoride aqueous solution and the ammonia aqueous solution may be at room temperature.

[0073] In the inverse neutralization step, when adding the niobium fluoride aqueous solution to the ammonia aqueous solution, the neutralization reaction is preferably carried out within 1 minute. That is, instead of adding the niobium fluoride aqueous solution gradually over time, it is preferable to carry out the neutralization reaction by charging the solution within 1 minute, for example, by charging it at once.

[0074] In this case, the addition time of the niobium fluoride aqueous solution is preferably within 1 minute, of which within 30 seconds, and even within 10 seconds is still more preferable.

[0075] (F Washing Step)

[0076] Fluorine compounds such as ammonium fluoride are present as impurities in the liquid obtained from the neutralization reaction, i.e., the niobium-containing precipitate, and it is therefore preferable to remove them.

[0077] The method for removing the fluorine compounds can be any method. For example, a method by filtration using a membrane such as reverse osmosis filtration, ultrafiltration, and microfiltration using ammonia water or pure water, as well as separation by centrifugation and other publicly known methods can be employed.

[0078] The F washing step should be performed at room temperature, and there is no need to adjust the respective temperatures.

[0079] (Water Solubilization Step)

[0080] Next, the present niobic acid aqueous solution can be obtained by making the niobium-containing precipitate obtained by removing fluorine in the above step into a slurry, adding at least one selected from amines and ammonia water to this slurry, and reacting them.

[0081] To make it into a slurry, the niobium-containing precipitate should be dispersed by adding it to a dispersion medium such as pure water.

[0082] The water solubilization step should be performed at room temperature, and there is no need to adjust the respective temperatures.

[0083] Preferable examples of the amine to be added include alkyl amines, choline ([(CH.sub.3).sub.3NCH.sub.2OH].sup.+), and choline hydroxide ([(CH.sub.3).sub.3NCH.sub.2CH.sub.2OH].sup.+OH.sup.−).

[0084] As the above alkylamines, those having 1 to 4 alkyl groups can be used. When the alkylamine has 2 to 4 alkyl groups, all 2 to 4 alkyl groups may be the same or may include different groups. As the alkyl group in the alkylamines, alkyl groups having 1 to 6 carbon atoms are preferable, of which those having 4 or less, even 3 or less, and even 2 or less are preferable from the viewpoint of solubility.

[0085] Specific examples of the above alkylamines include methylamine, dimethylamine, trimethylamine, tetramethylammonium hydroxide, ethylamine, methyl ethylamine, diethylamine, triethylamine, methyl diethylamine, dimethyl ethylamine, tetraethylammonium hydroxide, n-propylamine, di-n-propylamine, tri-n-propylamine, iso-propylamine, di-iso-propylamine, tri-iso-propylamine, n-butylamine, di-n-butylamine, tri-n-butylamine, iso-butylamine, di-iso-butylamine, tri-iso-butylamine, tert-butylamine, n-pentaamine, and n-hexaamine.

[0086] Among these, in terms of solubility, methylamine, dimethylamine, trimethylamine, tetramethylammonium hydroxide, ethylamine, methyl ethylamine, diethylamine, triethylamine, methyl diethylamine, dimethyl ethylamine and tetraethylammonium hydroxide are preferable, of which methylamine, dimethylamine, trimethylamine, and tetramethylammonium hydroxide are still more preferable. In particular, methylamine is most preferable.

[0087] If ammonia water is used, it is preferably added so as to obtain an NH.sub.3 concentration in the niobium-containing precipitate slurry of 7 mass % or less, still more preferably added so as to obtain a concentration of 6 mass % or less, and especially preferably added so as to obtain a concentration of 5 mass % or less from the viewpoint of solubility. In addition, from the same viewpoint, it is preferably added so as to obtain an NH.sub.3 concentration in the niobium-containing precipitate slurry of 0.2 mass % or more, still more preferably added so as to obtain a concentration of 0.5 mass % or more, and especially preferably added so as to obtain a concentration of 1 mass % or more.

[0088] If an amine is used, it is preferably added so as to obtain an amine concentration in the niobium-containing precipitate slurry of 15 mass % or less, still more preferably added so as to obtain a concentration of 12 mass % or less, and especially preferably added so as to obtain a concentration of 10 mass % or less from the viewpoint of solubility. In addition, from the same viewpoint, it is preferably added so as to obtain an amine concentration in the niobium-containing precipitate slurry of 0.5 mass % or more, still more preferably added so as to obtain a concentration of 1.0 mass % or more, and especially preferably added so as to obtain a concentration of 1.5 mass % or more.

[0089] After adding the amine, it is preferable to accelerate the reaction by stirring as necessary.

[0090] <Applications>

[0091] The present niobic acid aqueous solution can be used, for example, as various coating solutions.

[0092] <Vocabulary>

[0093] In the present description, the expression “X to Y” (X and Y are arbitrary numbers) includes the meaning “X or more and Y or less” as well as the meaning “preferably greater than X” or “preferably less than Y,” unless otherwise specified.

[0094] In addition, the expression “X or more” (X is an arbitrary number) or “Y or less” (Y is an arbitrary number) also includes the meaning “preferably greater than X” or “preferably less than Y”.

EXAMPLES

[0095] The present invention will be further described by the following Examples. However, the following Examples do not limit the present invention.

Example 1

[0096] 100 g of niobium pentoxide was dissolved in 200 g of 55% hydrofluoric acid aqueous solution, to which 830 mL of ion-exchanged water was added to obtain a niobium fluoride aqueous solution containing 100 g/L of niobium in terms of Nb.sub.2O.sub.5 (Nb.sub.2O.sub.5=8.84 mass %).

[0097] 200 mL of this niobium fluoride aqueous solution was added to 1 L of ammonia water (NH.sub.3 concentration: 25 mass %) in less than 1 minute (NH.sub.3/Nb.sub.2O.sub.5 molar ratio=177.9, NH.sub.3/HF molar ratio=12.2) to obtain a reaction solution (pH 11). This reaction solution was a slurry of a niobic acid compound hydrate, in other words, a slurry of a niobium-containing precipitate.

[0098] Next, the reaction solution was decanted using a centrifuge and washed until the amount of free fluorine was 100 mg/L or less to obtain a fluorine-removed niobium-containing precipitate. In this case, ammonia water was used as the washing solution.

[0099] Next, the fluorine-removed niobium-containing precipitate was diluted with pure water to obtain a slurry. A portion of this slurry was dried at 110° C. for 24 hours and then calcined at 1000° C. for 4 hours to produce Nb.sub.2O.sub.5, and the concentration of Nb.sub.2O.sub.5 in the slurry was calculated from its weight. Pure water was added to the slurry, and then a 50% dimethylamine aqueous solution was added so as to obtain a dimethylamine concentration of 7.2 mass %, to prepare a slurry with a Nb.sub.2O.sub.5 solid concentration of 24.0 mass %.

[0100] This slurry was stirred for 48 hours to obtain a niobic acid aqueous solution (sample).

[0101] The niobic acid aqueous solution (sample) had a pH of 11. In addition, when the particle size of the niobic acid aqueous solution (sample) was measured by dynamic scattering (ELSZ-2000S manufactured by Otsuka Electronics), it could not be measured. That is, no particles with a particle size of 1.0 nm or more were detected.

Example 2

[0102] 100 g of niobium pentoxide was dissolved in 200 g of 55% hydrofluoric acid aqueous solution, to which 1830 mL of ion-exchanged water was added to obtain a niobium fluoride aqueous solution containing 50 g/L of niobium in terms of Nb.sub.2O.sub.5 (Nb.sub.2O.sub.5=4.69 mass %).

[0103] 400 mL of this niobium fluoride aqueous solution was added to 1 L of ammonia water (NH.sub.3 concentration: 25 mass %) in less than 1 minute (NH.sub.3/Nb.sub.2O.sub.5 molar ratio=177.9, NH.sub.3/HF molar ratio=12.2) to obtain a reaction solution (pH 12). This reaction solution was a slurry of a niobic acid compound hydrate, in other words, a slurry of a niobium-containing precipitate.

[0104] Next, the reaction solution was decanted using a centrifuge and washed until the amount of free fluorine was 100 mg/L or less to obtain a fluorine-removed niobium-containing precipitate. In this case, ammonia water was used as the washing solution.

[0105] Next, the fluorine-removed niobium-containing precipitate was diluted with pure water to obtain a slurry. A portion of this slurry was dried at 110° C. for 24 hours and then calcined at 1000° C. for 4 hours to produce Nb.sub.2O.sub.5, and the concentration of Nb.sub.2O.sub.5 in the slurry was calculated from its weight. Pure water was added to the slurry, and then a 50% dimethylamine aqueous solution was added so as to obtain a dimethylamine concentration of 2.0 mass %, to prepare a slurry with a Nb.sub.2O.sub.5 solid concentration of 10.0 mass %.

[0106] This slurry was stirred for 48 hours to obtain a niobic acid aqueous solution (sample).

[0107] The niobic acid aqueous solution (sample) had a pH of 11. In addition, when the particle size of the niobic acid aqueous solution (sample) was measured by dynamic scattering (ELSZ-2000S manufactured by Otsuka Electronics), it could not be measured. That is, no particles with a particle size of 1.0 nm or more were detected.

Example 3

[0108] 100 g of niobium pentoxide was dissolved in 200 g of 55% hydrofluoric acid aqueous solution, to which 830 mL of ion-exchanged water was added to obtain a niobium fluoride aqueous solution containing 100 g/L of niobium in terms of Nb.sub.2O.sub.5 (Nb.sub.2O.sub.5=8.84 mass %).

[0109] 200 mL of this niobium fluoride aqueous solution was added to 2 L of ammonia water (NH.sub.3 concentration: 25 mass %) in less than 1 minute (NH.sub.3/Nb.sub.2O.sub.5 molar ratio=355.7, NH.sub.3/HF molar ratio=24.3) to obtain a reaction solution (pH 11). This reaction solution was a slurry of a niobic acid compound hydrate, in other words, a slurry of a niobium-containing precipitate.

[0110] Next, the reaction solution was decanted using a centrifuge and washed until the amount of free fluorine was 100 mg/L or less to obtain a fluorine-removed niobium-containing precipitate. In this case, ammonia water was used as the washing solution.

[0111] Next, the fluorine-removed niobium-containing precipitate was diluted with pure water to obtain a slurry. A portion of this slurry was dried at 110° C. for 24 hours and then calcined at 1000° C. for 4 hours to produce Nb.sub.2O.sub.5, and the concentration of Nb.sub.2O.sub.5 in the slurry was calculated from its weight. Pure water was added to the slurry, and then a 50% dimethylamine aqueous solution was added so as to obtain a dimethylamine concentration of 2.0 mass %, to prepare a slurry with a Nb.sub.2O.sub.5 solid concentration of 10.0 mass %.

[0112] This slurry was stirred for 48 hours to obtain a niobic acid aqueous solution (sample).

[0113] The niobic acid aqueous solution (sample) had a pH of 11. In addition, when the particle size of the niobic acid aqueous solution (sample) was measured by dynamic scattering (ELSZ-2000S manufactured by Otsuka Electronics), it could not be measured. That is, no particles with a particle size of 1.0 nm or more were detected.

Example 4

[0114] 100 g of niobium pentoxide was dissolved in 200 g of 55% hydrofluoric acid aqueous solution, to which 9800 mL of ion-exchanged water was added to obtain a niobium fluoride aqueous solution containing 10.3 g/L of niobium in terms of Nb.sub.2O.sub.5 (Nb.sub.2O.sub.5=1 mass %).

[0115] 2 L of this niobium fluoride aqueous solution was added to 2 L of ammonia water (NH.sub.3 concentration: 25 mass %) in less than 1 minute (NH.sub.3/Nb.sub.2O.sub.5 molar ratio=345.4, NH.sub.3/HF molar ratio=23.6) to obtain a reaction solution (pH 12). This reaction solution was a slurry of a niobic acid compound hydrate, in other words, a slurry of a niobium-containing precipitate.

[0116] Next, the reaction solution was decanted using a centrifuge and washed until the amount of free fluorine was 100 mg/L or less to obtain a fluorine-removed niobium-containing precipitate. In this case, ammonia water was used as the washing solution.

[0117] Next, the fluorine-removed niobium-containing precipitate was diluted with pure water to obtain a slurry. A portion of this slurry was dried at 110° C. for 24 hours and then calcined at 1000° C. for 4 hours to produce Nb.sub.2O.sub.5, and the concentration of Nb.sub.2O.sub.5 in the slurry was calculated from its weight. Pure water was added to the slurry, and then a 40% methylamine aqueous solution was added so as to obtain a methylamine concentration of 1.0 mass %, to prepare a slurry with a Nb.sub.2O.sub.5 solid concentration of 10.0 mass %.

[0118] This slurry was stirred for 48 hours to obtain a niobic acid aqueous solution (sample).

[0119] The niobic acid aqueous solution (sample) had a pH of 11. In addition, when the particle size of the niobic acid aqueous solution (sample) was measured by dynamic scattering (ELSZ-2000S manufactured by Otsuka Electronics), it could not be measured. That is, no particles with a particle size of 1.0 nm or more were detected.

Example 5

[0120] 100 g of niobium pentoxide was dissolved in 200 g of 55% hydrofluoric acid aqueous solution, to which 830 mL of ion-exchanged water was added to obtain a niobium fluoride aqueous solution containing 100 g/L of niobium in terms of Nb.sub.2O.sub.5 (Nb.sub.2O.sub.5=8.84 mass %).

[0121] 200 mL of this niobium fluoride aqueous solution was added to 2 L of ammonia water (NH.sub.3 concentration: 25 mass %) in less than 1 minute (NH.sub.3/Nb.sub.2O.sub.5 molar ratio=355.7, NH.sub.3/HF molar ratio=24.3) to obtain a reaction solution (pH 11). This reaction solution was a slurry of a niobic acid compound hydrate, in other words, a slurry of a niobium-containing precipitate.

[0122] Next, the reaction solution was decanted using a centrifuge and washed until the amount of free fluorine was 100 mg/L or less to obtain a fluorine-removed niobium-containing precipitate. In this case, ammonia water was used as the washing solution.

[0123] Next, the fluorine-removed niobium-containing precipitate was diluted with pure water to obtain a slurry. A portion of this slurry was dried at 110° C. for 24 hours and then calcined at 1000° C. for 4 hours to produce Nb.sub.2O.sub.5, and the concentration of Nb.sub.2O.sub.5 in the slurry was calculated from its weight. Pure water was added to the slurry, and then a >99% triethylamine aqueous solution was added so as to obtain a triethylamine concentration of 3.0 mass %, to prepare a slurry with a Nb.sub.2O.sub.5 solid concentration of 10.0 mass %.

[0124] The slurry was stirred for 20 days to obtain a niobic acid aqueous solution (sample).

[0125] The niobic acid aqueous solution (sample) had a pH of 11. In addition, when the particle size of the niobic acid aqueous solution (sample) was measured by dynamic scattering (ELSZ-2000S manufactured by Otsuka Electronics), it could not be measured. That is, no particles with a particle size of 1.0 nm or more were detected.

Example 6

[0126] 100 g of niobium pentoxide was dissolved in 200 g of 55% hydrofluoric acid aqueous solution, to which 830 mL of ion-exchanged water was added to obtain a niobium fluoride aqueous solution containing 100 g/L of niobium in terms of Nb.sub.2O.sub.5 (Nb.sub.2O.sub.5=8.84 mass %).

[0127] 200 mL of this niobium fluoride aqueous solution was added to 2 L of ammonia water (NH.sub.3 concentration: 25 mass %) in less than 1 minute (NH.sub.3/Nb.sub.2O.sub.5 molar ratio=355.7, NH.sub.3/HF molar ratio=24.3) to obtain a reaction solution (pH 11). This reaction solution was a slurry of a niobic acid compound hydrate, in other words, a slurry of a niobium-containing precipitate.

[0128] Next, the reaction solution was decanted using a centrifuge and washed until the amount of free fluorine was 100 mg/L or less to obtain a fluorine-removed niobium-containing precipitate. In this case, ammonia water was used as the washing solution.

[0129] Next, the fluorine-removed niobium-containing precipitate was diluted with pure water to obtain a slurry. A portion of this slurry was dried at 110° C. for 24 hours and then calcined at 1000° C. for 4 hours to produce Nb.sub.2O.sub.5, and the concentration of Nb.sub.2O.sub.5 in the slurry was calculated from its weight. 25% ammonia water and pure water were added to the slurry to prepare a slurry with a Nb.sub.2O.sub.5 solid concentration of 1.5 mass % and an ammonia concentration of 2.5 mass %.

[0130] This slurry was stirred for 48 hours to obtain a niobic acid aqueous solution (sample).

[0131] The niobic acid aqueous solution (sample) had a pH of 11. In addition, when the particle size of the niobic acid aqueous solution (sample) was measured by dynamic scattering (ELSZ-2000S manufactured by Otsuka Electronics), it could not be measured. That is, no particles with a particle size of 1.0 nm or more were detected.

Example 7

[0132] 100 g of niobium pentoxide was dissolved in 200 g of 55% hydrofluoric acid aqueous solution, to which 830 mL of ion-exchanged water was added to obtain a niobium fluoride aqueous solution containing 100 g/L of niobium in terms of Nb.sub.2O.sub.5 (Nb.sub.2O.sub.5=8.84 mass %).

[0133] 200 mL of this niobium fluoride aqueous solution was added to 2 L of ammonia water (NH.sub.3 concentration: 25 mass %) in less than 1 minute (NH.sub.3/Nb.sub.2O.sub.5 molar ratio=355.7, NH.sub.3/HF molar ratio=24.3) to obtain a reaction solution (pH 11). This reaction solution was a slurry of a niobic acid compound hydrate, in other words, a slurry of a niobium-containing precipitate.

[0134] Next, the reaction solution was decanted using a centrifuge and washed until the amount of free fluorine was 100 mg/L or less to obtain a fluorine-removed niobium-containing precipitate. In this case, ammonia water was used as the washing solution.

[0135] Next, the fluorine-removed niobium-containing precipitate was diluted with pure water to obtain a slurry. A portion of this slurry was dried at 110° C. for 24 hours and then calcined at 1000° C. for 4 hours to produce Nb.sub.2O.sub.5, and the concentration of Nb.sub.2O.sub.5 in the slurry was calculated from its weight. Pure water was added to the slurry, then a 25% TMAH (tetramethylammonium hydroxide) aqueous solution was added so as to obtain a TMAH concentration of 10.0 mass %, to prepare a slurry with a Nb.sub.2O.sub.5 solid concentration of 10.0 mass %.

[0136] The slurry was stirred for 5 days to obtain a niobic acid aqueous solution (sample).

[0137] The niobic acid aqueous solution (sample) had a pH of 11. In addition, when the particle size of the niobic acid aqueous solution (sample) was measured by dynamic scattering (ELSZ-2000S manufactured by Otsuka Electronics), it could not be measured. That is, no particles with a particle size of 1.0 nm or more were detected.

Example 8

[0138] 100 g of niobium pentoxide was dissolved in 200 g of 55% hydrofluoric acid aqueous solution, to which 830 mL of ion-exchanged water was added to obtain a niobium fluoride aqueous solution containing 100 g/L of niobium in terms of Nb.sub.2O.sub.5 (Nb.sub.2O.sub.5=8.84 mass %).

[0139] 200 mL of this niobium fluoride aqueous solution was added to 2 L of ammonia water (NH.sub.3 concentration: 25 mass %) in less than 1 minute (NH.sub.3/Nb.sub.2O.sub.5 molar ratio=355.7, NH.sub.3/HF molar ratio=24.3) to obtain a reaction solution (pH 11). This reaction solution was a slurry of a niobic acid compound hydrate, in other words, a slurry of a niobium-containing precipitate.

[0140] Next, the reaction solution was decanted using a centrifuge and washed until the amount of free fluorine was 100 mg/L or less to obtain a fluorine-removed niobium-containing precipitate. In this case, ammonia water was used as the washing solution.

[0141] Next, the fluorine-removed niobium-containing precipitate was diluted with pure water to obtain a slurry. A portion of this slurry was dried at 110° C. for 24 hours and then calcined at 1000° C. for 4 hours to produce Nb.sub.2O.sub.5, and the concentration of Nb.sub.2O.sub.5 in the slurry was calculated from its weight. Pure water was added to the slurry, then a 50% choline aqueous solution was added so as to obtain a choline concentration of 10.0 mass %, to prepare a slurry with a Nb.sub.2O.sub.5 solid concentration of 10.0 mass %.

[0142] The slurry was stirred for 5 days to obtain a niobic acid aqueous solution (sample).

[0143] The niobic acid aqueous solution (sample) had a pH ofll. In addition, when the particle size of the niobic acid aqueous solution (sample) was measured by dynamic scattering (ELSZ-2000S manufactured by Otsuka Electronics), it could not be measured. That is, no particles with a particle size of 1.0 nm or more were detected.

Example 9

[0144] 100 g of niobium pentoxide was dissolved in 200 g of 55% hydrofluoric acid aqueous solution, to which 830 mL of ion-exchanged water was added to obtain a niobium fluoride aqueous solution containing 100 g/L of niobium in terms of Nb.sub.2O.sub.5 (Nb.sub.2O.sub.5=8.84 mass %).

[0145] 200 mL of this niobium fluoride aqueous solution was added to 2 L of ammonia water (NH.sub.3 concentration: 25 mass %) in less than 1 minute (NH.sub.3/Nb.sub.2O.sub.5 molar ratio=355.7, NH.sub.3/HF molar ratio=24.3) to obtain a reaction solution (pH 11). This reaction solution was a slurry of a niobic acid compound hydrate, in other words, a slurry of a niobium-containing precipitate.

[0146] Next, the reaction solution was decanted using a centrifuge and washed until the amount of free fluorine was 100 mg/L or less to obtain a fluorine-removed niobium-containing precipitate. In this case, ammonia water was used as the washing solution.

[0147] Next, the fluorine-removed niobium-containing precipitate was diluted with pure water to obtain a slurry. A portion of this slurry was dried at 110° C. for 24 hours and then calcined at 1000° C. for 4 hours to produce Nb.sub.2O.sub.5, and the concentration of Nb.sub.2O.sub.5 in the slurry was calculated from its weight. Pure water was added to the slurry, then a 27% trimethylamine aqueous solution was added so as to obtain a trimethylamine concentration of 10.0 mass %, to prepare a slurry with a Nb.sub.2O.sub.5 solid concentration of 10.0 mass %.

[0148] The slurry was stirred for 5 days to obtain a niobic acid aqueous solution (sample).

[0149] The niobic acid aqueous solution (sample) had a pH of 11. In addition, when the particle size of the niobic acid aqueous solution (sample) was measured by dynamic scattering (ELSZ-2000S manufactured by Otsuka Electronics), it could not be measured. That is, no particles with a particle size of 1.0 nm or more were detected.

Comparative Example 1

[0150] 100 g of niobium pentoxide was dissolved in 200 g of 55% hydrofluoric acid aqueous solution, to which 830 mL of ion-exchanged water was added to obtain a niobium fluoride aqueous solution containing 100 g/L of niobium in terms of Nb.sub.2O.sub.5 (Nb.sub.2O.sub.5=8.84 mass %).

[0151] 200 mL of ammonia water (NH.sub.3 concentration: 25 mass %) was added to 200 mL of this niobium fluoride aqueous solution over 60 minutes (NH.sub.3/Nb.sub.2O.sub.5 molar ratio=355.7, NH.sub.3/HF molar ratio=24.3) to obtain a reaction solution (pH 11) (normal neutralization). This reaction solution was a slurry of a niobic acid compound hydrate, in other words, a slurry of a niobium-containing precipitate.

[0152] Next, the reaction solution was washed by Nutsche filtration using 5 C filter paper until the amount of free fluorine was 100 ppm or less to obtain a fluorine-removed niobium-containing precipitate. In this case, ammonia water was used as the washing solution.

[0153] Next, the fluorine-removed niobium-containing precipitate was diluted with pure water to obtain a slurry. A portion of this slurry was dried at 110° C. for 24 hours and then calcined at 1000° C. for 4 hours to produce Nb.sub.2O.sub.5, and the concentration of Nb.sub.2O.sub.5 in the slurry was calculated from its weight. Pure water was added to the slurry, and then a 50% dimethylamine aqueous solution was added so as to obtain a dimethylamine concentration of 2.0 mass %, to prepare a slurry with a Nb.sub.2O.sub.5 solid concentration of 10.0 mass %.

[0154] This slurry was stirred for 48 hours, but the obtained solution was not transparent and was a cloudy slurry.

[0155] This slurry had a pH of 11. In addition, this solution was diluted 100 times with pure water, and when the particle size of the niobium-containing solution (sample) was measured by dynamic scattering (ELSZ-2000S manufactured by Otsuka Electronics), the volume mean particle size was 282 nm.

Comparative Example 2

[0156] 100 g of niobium pentoxide was dissolved in 200 g of 55% hydrofluoric acid aqueous solution, to which 830 mL of ion-exchanged water was added to obtain a niobium fluoride aqueous solution containing 100 g/L of niobium in terms of Nb.sub.2O.sub.5 (Nb.sub.2O.sub.5=8.84 mass %).

[0157] 80 mL of this niobium fluoride aqueous solution was added to 390 mL of ammonia water (NH.sub.3 concentration: 5 mass %) in less than 1 minute (NH.sub.3/Nb.sub.2O.sub.5 molar ratio=34.7, NH.sub.3/HF molar ratio=2.4) to obtain a reaction solution (pH 9). This reaction solution was a slurry of a niobic acid compound hydrate, in other words, a slurry of a niobium-containing precipitate.

[0158] Next, the reaction solution was decanted using a centrifuge and washed until the amount of free fluorine was 100 mg/L or less to obtain a fluorine-removed niobium-containing precipitate. In this case, ammonia water was used as the washing solution.

[0159] Next, the fluorine-removed niobium-containing precipitate was diluted with pure water to obtain a slurry. A portion of this slurry was dried at 110° C. for 24 hours and then calcined at 1000° C. for 4 hours to produce Nb.sub.2O.sub.5, and the concentration of Nb.sub.2O.sub.5 in the slurry was calculated from its weight. Pure water was added to the slurry to prepare a slurry with a Nb.sub.2O.sub.5 solid concentration of 10.0 mass %.

[0160] To this slurry, a 50% dimethylamine aqueous solution was added so as to obtain a dimethylamine concentration of 2.0 mass %, and the mixture was stirred for 48 hours. The obtained solution was not transparent and was a cloudy slurry.

[0161] This slurry had a pH of 11. In addition, this solution was diluted 100-fold with pure water, and when the particle size of the niobium-containing solution (sample) was measured by dynamic scattering (ELSZ-2000S manufactured by Otsuka Electronics), the volume mean particle size was 290 nm.

Comparative Example 3

[0162] 100 g of niobium pentoxide was dissolved in 200 g of 55% hydrofluoric acid aqueous solution, to which 19600 mL of ion-exchanged water was added to obtain a niobium fluoride aqueous solution containing 5.1 g/L of niobium in terms of Nb.sub.2O.sub.5 (Nb.sub.2O.sub.5=0.5 mass %).

[0163] 9 L of this niobium fluoride aqueous solution was added to 5 L of ammonia water (NH.sub.3 concentration: 1 mass %) over 60 minutes (NH.sub.3/Nb.sub.2O.sub.5 molar ratio=15.5, NH.sub.3/HF molar ratio=1.1) to obtain a reaction solution (pH 8) (inverse neutralization). This reaction solution was a slurry of a niobic acid compound hydrate, in other words, a slurry of a niobium-containing precipitate.

[0164] Next, this slurry was washed using a filtration device (Microza UF: Model ACP-0013D; manufactured by Asahi Kasei Corporation) until the amount of free fluorine in the slurry was 100 mg/L or less to obtain a fluorine-removed niobium-containing precipitate.

[0165] Next, the fluorine-removed niobium-containing precipitate was diluted with pure water to obtain a slurry. A portion of this slurry was dried at 110° C. for 24 hours and then calcined at 1000° C. for 4 hours to produce Nb.sub.2O.sub.5, and the concentration of Nb.sub.2O.sub.5 in the slurry was calculated from its weight. Pure water was added to the slurry, and then a 50% dimethylamine aqueous solution was added so as to obtain a dimethylamine concentration of 2.0 mass %, to prepare a slurry with a Nb.sub.2O.sub.5 solid concentration of 10.0 mass %.

[0166] This slurry was stirred for 48 hours. The obtained solution was not transparent and was a cloudy slurry.

[0167] This slurry had a pH of 11. In addition, this solution was diluted 100-fold with pure water, and when the particle size of the niobium-containing solution (sample) was measured by dynamic scattering (ELSZ-20005 manufactured by Otsuka Electronics), the volume mean particle size was 293 nm.

[0168] <Measurement by Dynamic Light Scattering>

[0169] The particle sizes of the niobic acid aqueous solutions (samples) obtained in the Examples and Comparative Examples were measured using the dynamic light scattering particle size distribution analyzer ELSZ-20005 (manufactured by Otsuka Electronics).

[0170] Note that the detection limit of the dynamic light scattering particle size distribution analyzer used was 0.6 nm, and therefore when measurement was not possible, it was indicated as “<0.6 nm” in the table.

[0171] In addition, when particles of 1.0 nm or more were detected, colloidal particles were considered “present”, and if no particles of 1.0 nm or more were detected, it was considered that there were “no” colloidal particles.

TABLE-US-00001 TABLE 1 Examples 1 2 3 4 5 6 Neutralization Niobium g/L 100 50 100 10.3 100 100 Conditions Fluoride Aqueous Solution Nb2O5 Concentration Ammonia Mass 25 25 25 25 25 25 Concentration % Neutralization — Inverse Inverse Inverse Inverse Inverse Inverse Method Neutrali- Neutrali- Neutrali- Neutrali- Neutrali- Neutrali- zation zation zation zation zation zation Neutralization min <1 <1 <1 <1 <1 <1 Time pH after — 11 11 11 12 11 11 Neutralization Niobic Nb2O5 Mass 24.0 10.0 10.0 10.0 10.0 1.5 Acid Concentration % Aqueous Type of Ammonia Mass 2.5 Solution Dissolving % Agent and Methyl- Mass 1.0 Concentration amine % in Aqueous Dimethyl- Mass 7.2 2.0 2.0 Solution amine % Triethyl- Mass 3.0 amine % TMAH Mass % Choline Mass % Trimethyl- Mass amine Average Particle nm <0.6* <0.6* <0.6* <0.6* <0.6* <0.6* Size by Dynamic Light Scattering Presence of No No No No No No Colloidal Particles Examples Comparative Examples 7 8 9 1 2 3 Neutralization Niobium g/L 100 100 100 100 100 5.1 Conditions Fluoride Aqueous Solution Nb2O5 Concentration Ammonia Mass 25 25 25 25 5 1 Concentration % Neutralization — Inverse Inverse Inverse Inverse Inverse Inverse Method Neutrali- Neutrali- Neutrali- Neutrali- Neutrali- Neutrali- zation zation zation zation zation zation Neutralization min <1 <1 <1 60 <1 60 Time pH after — 11 11 11 11 9 8 Neutralization Niobic Nb2O5 Mass 10.0 10.0 10.0 10.0 10.0 10.0 Acid Concentration % Aqueous Type of Ammonia Mass Solution Dissolving % Agent and Methyl- Mass 5.0 Concentration amine % in Aqueous Dimethyl- Mass 2.0 2.0 Solution amine % Triethyl- Mass amine % TMAH Mass 10.0 % Choline Mass 10.0 % Trimethyl- Mass 10.0 amine % Average Particle nm <0.6* <0.6* <0.6* 282 290 293 Size by Dynamic Light Scattering Presence of No No No Present Present Present Colloidal Particles *The detection limit of the dynamic light scattering device is 0.6 nm, and no particles of 0.6 nm or more are contained, therefore measurement was not possible.

[0172] (Discussion)

[0173] The niobic acid aqueous solutions (samples) obtained in Examples 1 to 9 were solutions in which at least no particles of 1.0 nm or more were detected when the particle sizes were measured using dynamic light scattering. In other words, they were found to be completely dissolved and to contain at least no colloidal particles or/and particles larger than colloidal particles. Therefore, the niobic acid aqueous solutions (samples) obtained in Examples 1 to 9 have higher dispersibility than an ammonium niobate sol and have better solubility than a complex salt of niobic acid.

[0174] <Reactivity Test with NaOH>

[0175] If the niobium concentration of each of the niobic acid aqueous solutions (samples) obtained in Examples 1 to 9 was higher than 10 mass % in terms of Nb.sub.2O.sub.5, pure water was added to dilute the niobium concentration to 10 mass % in terms of Nb.sub.2O.sub.5, and the diluted niobic acid aqueous solutions were used as test samples for the reactivity test. On the other hand, if the niobium concentration of the niobic acid aqueous solutions (samples) was 10 mass % or less in terms of Nb.sub.2O.sub.5, no dilution with pure water was performed, and the solutions were used as is as test samples for the reactivity test.

[0176] When 20 g of 3 mass % NaOH aqueous solution (25° C.) was slowly added to 20 g of the niobic acid aqueous solution (2 g in terms of Nb.sub.2O.sub.5) as a test sample for the reactivity test over 1 minute while stirring with a magnetic stirrer, a slightly cloudy solution (25° C.) was obtained. When this cloudy solution (25° C.) was allowed to stand in a cooler while maintaining a liquid temperature of 5° C. for 20 hours, a precipitate was formed. The precipitate formed was filtered through a Nutsche filter using 5C filter paper, washed with pure water, and then vacuum dried for 15 hours by heating it in a vacuum drying oven at 60° C. (set temperature). The dried precipitate was ground in an agate mortar, and an X-ray diffraction measurement was performed on the obtained powder.

[0177] For all the niobic acid aqueous solutions (samples) obtained in any of Examples 1 to 9, this precipitate was identified as consisting of Na.sub.8Nb.sub.6O.sub.19.13H.sub.2O of ICDD card No. 00-014-0370, based on the results of X-ray diffraction measurement (XRD pattern). Note that the XRD pattern of Example 4 is shown in FIG. 1 as a representative figure.

[0178] On the other hand, similar tests were also conducted on the niobium-containing solutions (samples) of Comparative Examples 1 to 3, but the precipitate was amorphous and was not identified as Na.sub.8Nb.sub.6O.sub.19.13H.sub.2O of ICDD Card No. 00-014-0370. Note that the XRD pattern of Comparative Example 3 is shown in FIG. 2 as a representative figure.