JP7688841B2 - Method for producing sodium tungstate - Google Patents

Description

The present invention relates to a manufacturing method that can obtain an aqueous solution of sodium tungstate with few impurities from a tungsten raw material.

In recent years, tungsten has been used not only as a cemented carbide material for cutting tools, but also as a variety of other materials, including electronic materials such as electrode materials and wiring materials, and tungsten catalysts, and the demand for it is increasing year by year. However, raw tungsten resources are limited, and in order to ensure a stable supply, there is a need to recover tungsten from tungsten scrap and use it effectively.

A known method for recovering tungsten from tungsten scrap containing tungsten carbide or metallic tungsten is to add an alkaline agent containing a sodium salt such as sodium hydroxide or sodium sulfate to the tungsten scrap and heat it to react, converting the tungsten in the scrap to sodium tungstate, and dissolving this melt in water to recover it as an aqueous sodium tungstate solution. The recovered aqueous sodium tungstate solution is refined through various purification processes into ammonium paratungstate, tungsten oxide, metallic tungsten, and tungsten carbide for reuse.

As a method for obtaining sodium tungstate or the like by heating and melting tungsten scrap together with an alkali raw material containing the above-mentioned sodium salt, for example, Patent Document 1 describes a method for performing an oxidation treatment of tungsten using a molten salt consisting of 60 to 90% by weight of NaOH and 10 to 40% by weight of Na ₂ SO ₄ .

Patent Document 2 describes a method in which a raw material containing tungsten as a minor component is heated and reacted with a salt melt containing 60 to 95 mass % NaOH and 5 to 40 mass % Na ₂ SO ₄ , the resulting molten liquid decomposition product is dissolved in water, and an aqueous suspension containing a solution consisting of a mixture of a sodium compound and an alkali metalate containing tungsten and the like, a solid metal phase consisting of a metal group such as Co, and a solid phase consisting of a hydroxide or oxide hydrate of Al and the like is obtained, and the solution consisting of the mixture of the sodium compound and the alkali metalate (aqueous sodium tungstate solution) is separated by filtration.

Patent Document 3 describes a method of obtaining sodium tungstate by reacting cemented carbide scrap containing tungsten and cobalt with a molten salt to which a metal oxide containing sodium sulfate has been added. In this method, it is described that an oxide containing iron is used as the metal oxide, and is added in an amount of 10% by weight or more and less than 30% by weight of the cemented carbide scrap, and that sodium sulfate is added in an amount of 70% by weight or more and 100% by weight or less of the cemented carbide scrap.

Patent No. 3958790 Patent No. 5550336 Patent No. 6018958

The melting treatment methods described in Patent Documents 1 to 3 are highly reactive and are efficient methods for simultaneously oxidizing tungsten and converting it into a soluble salt. On the other hand, because this method is so highly reactive, minor elements other than tungsten in the tungsten scrap and elements in the additives also undergo secondary reactions, which creates the problem that the concentrations of elements other than tungsten and impurities in the sodium tungstate aqueous solution recovered after the melting treatment are likely to increase significantly.

For example, when sulfur is contained, such as sodium sulfate, tungsten carbide and sodium sulfate react with each other to produce sodium tungstate in the presence of oxygen, such as in the air. If an excessive amount of sodium sulfate is added, a large amount of sodium sulfide (Na ₂ S) is generated as a by-product. If such a melt-treated product containing a large amount of Na ₂ S is dissolved in water, the resulting solution becomes a sodium tungstate aqueous solution containing a large amount of sulfide ions (S ^2- ), which are impurities.

For example, tungsten scrap may contain titanium (Ti), niobium (Nb), tantalum (Ta), etc., as additives or coating materials for cemented carbide tools. These elements react easily with alkali, and undergo oxidation reactions to produce soluble sodium titanate (Na ₂ Ti ₅ O ₁₁ ), sodium niobate (Na ₃ NbO ₄ ), sodium tantalate (Na ₃ TaO ₄ ), etc. When a melt-processed product containing these compounds is dissolved in water, it becomes a sodium tungstate aqueous solution containing large amounts of titanate ions (HTiO ₃ ^- ), niobate ions (NbO ₃ ^- ), and tantalate ions (TaO ₃ ^- ). In particular, niobium is the most soluble, so there is a problem that the resulting sodium tungstate aqueous solution contains a large amount of niobium, which is an impurity.

The methods of Patent Documents 1 and 2 do not adequately address the effects of sulfide ions. In particular, in the method of Patent Document 1, the effects of sulfide ions become significant when a molten salt with a high sodium sulfate content is used. Furthermore, since no means for separating impurities such as niobium is provided, the resulting aqueous solution of sodium tungstate has a high niobium concentration. In addition, the method of Patent Document 2 describes filtering and separating a solution consisting of a mixture of a sodium compound and an alkali metalate containing tungsten, etc., obtained by dissolving the decomposed product of the molten liquid in water, and a solid metal phase, etc. However, since niobium forms soluble salts and is dissolved in the aqueous solution, such a filtering and separation method cannot separate and remove niobium dissolved as an impurity.

The method of Patent Document 3 shows that as a countermeasure against sulfide ions using metal oxides, sodium sulfide and metal oxides are reacted in a molten salt to suppress the generation of sulfide ions. However, the method of Patent Document 3 adds a large amount of sodium sulfate (70% by weight or more) to the scrap raw material, which generates excess sodium sulfide, and in order to oxidize this excess sodium sulfide, an even larger amount of metal oxide is added, which imposes a high processing load and does not sufficiently remove niobium.

The manufacturing method of the present invention relates to a manufacturing method in which, when a tungsten raw material is heated and melted together with a sodium salt raw material containing a sulfur component to produce a melt treated product mainly composed of sodium tungstate, the melt treated product contains few sulfide impurities, and when the melt treated product is dissolved in water, an aqueous solution of sodium tungstate containing significantly fewer impurities such as sulfide ions can be obtained, and preferably, an aqueous solution of sodium tungstate containing significantly fewer impurities such as niobium together with sulfides can be obtained.

The manufacturing method of the present invention is a method for manufacturing sodium tungstate having the following configuration.
[1] A method for producing sodium tungstate, in which a tungsten raw material is heated and melted together with a sodium salt raw material to obtain a melt treated product mainly composed of sodium tungstate, the method comprising controlling, for a sodium salt raw material containing a sulfur component, a Na/W molar ratio in the range of 3.0 or more and 5.0 or less and a S/W molar ratio in the range of 0.05 or more and 0.30 or less, to suppress production of sulfides, using the tungsten raw material and the sodium salt raw material containing the sulfur component.
[2] The method for producing sodium tungstate according to the above item [1], wherein the sodium salt raw material containing a sulfur component is sodium sulfate, or a combination of sodium sulfate and sodium hydroxide, or a combination of sulfuric acid and sodium hydroxide.
[3] A method for producing sodium tungstate as described in either of [1] or [2] above, in which a tungsten raw material containing niobium is controlled so that the Na/W molar ratio is in the range of 3.0 to 5.0 and the S/W molar ratio is in the range of 0.05 to 0.30, and at least one iron raw material of metallic iron or iron sulfate (ferrous sulfate or ferric sulfate) is made to coexist with the sodium salt raw material containing a sulfur component, thereby immobilizing the niobium as an insoluble solid.
[4] A method for producing sodium tungstate according to any one of [1] to [3] above, comprising dissolving the melt-treated product in water to obtain an aqueous sodium tungstate solution.
[5] The method for producing sodium tungstate according to the above item [4], wherein the oxidation-reduction potential of an aqueous sodium tungstate solution obtained by dissolving the melt-treated product in water is -200 mV (vs. Ag/AgCl) or more.

[Specific explanation]
The method of the present invention is a method for producing sodium tungstate in which a tungsten raw material and a sodium salt raw material are heated and melted together to obtain a molten treated product containing sodium tungstate as a main component, and in which, for a sodium salt raw material containing a sulfur component, the Na/W molar ratio between the tungsten raw material and the sodium salt raw material containing the sulfur component is controlled in a range of 3.0 or more and 5.0 or less, and the S/W molar ratio is controlled in a range of 0.05 or more and 0.30 or less to suppress production of sulfides.

A melt-processed product containing sodium tungstate as the main component can be obtained by heating and melting a tungsten raw material together with a sodium salt raw material. As the sodium salt, a sodium salt raw material containing sodium sulfate can be used. A sodium salt raw material containing sodium hydroxide together with sodium sulfate can also be used. Sulfuric acid may also be added to these sodium salt raw materials. For example, the addition of sulfuric acid to sodium hydroxide produces the same effect as when sodium sulfate is used. The heating temperature for the heat melting is preferably 700°C or higher and 1000°C or lower, and preferably 850°C or higher and 1000°C or lower. If the temperature is lower than 700°C, the reaction does not proceed sufficiently, and if the temperature exceeds 1000°C, there is a concern that the high temperature environment will cause corrosion and deterioration of facilities and equipment.

The above-mentioned heating and melting process produces a melt-processed product whose main component is sodium tungstate. For example, when tungsten carbide is used as the tungsten raw material and heated and melted together with a sodium salt raw material containing sodium hydroxide or sodium sulfate, the tungsten carbide reacts with the sodium hydroxide and sodium sulfate to produce sodium tungstate, as shown in the following formula [1][2][3].

2WC(s)+4NaOH(s)+5O ₂ → 2Na ₂ WO ₄ (s)+2H ₂ O(g)+2CO ₂ (g) ... [Formula 1]
2WC(s)+3Na ₂ SO ₄ (s)+3/2O ₂ (g) → 2Na ₂ WO ₄ (s)+Na ₂ O(s)+2CO ₂ (g)+3SO ₂ (g)・[Formula 2]
2WC(s) + 3Na ₂ SO ₄ (s) → 2Na ₂ WO ₄ (s) + Na ₂ S(s) + 2CO(g) + 2SO(g) ・・[Formula 3]

In the above-mentioned heating and melting, when sodium sulfate is used as the sodium salt raw material, in an atmosphere where there is sufficient oxygen relative to the added sulfur content, as shown in [Formula 2], sodium sulfate is decomposed to produce sodium oxide and sulfur dioxide along with the production of sodium tungstate, and the sulfur dioxide is gasified and escapes from the system. On the other hand, when sodium sulfate is present in excess, as shown in [Formula 3], sodium tungstate is produced and sodium sulfide and sulfur monoxide are produced as a by-product. Sulfur monoxide is gasified and escapes from the system, but sodium sulfide remains in the melted product. When such a melted product containing a large amount of sodium sulfide is dissolved in water, it becomes an aqueous solution of sodium tungstate containing a large amount of sulfide ions, which are impurities. The same thing happens when sodium sulfate and sodium hydroxide are used in combination as the sodium salt raw material, or when sulfuric acid and sodium hydroxide are used in combination. Hereinafter, those that use sodium sulfate, those that use sodium sulfate and sodium hydroxide in combination, or those that use sulfuric acid and sodium hydroxide in combination are all referred to as sodium salt raw materials containing sulfur components.

It has been discovered that in the heating and melting of such a sodium salt raw material containing sulfur components, the generation of sodium sulfide can be suppressed by controlling the Na/W molar ratio and the S/W molar ratio of the tungsten raw material and the sodium salt raw material containing sulfur components. The manufacturing method of the present invention is based on the above findings, and is a method for manufacturing sodium tungstate that significantly reduces the generation of sodium sulfide and therefore can obtain an aqueous sodium tungstate solution that contains almost no sulfide ions as an impurity.

Specifically, the manufacturing method of the present invention is a method for manufacturing sodium tungstate in which, during heating and melting using a sodium salt raw material containing a sulfur component, the Na/W molar ratio of the tungsten raw material and the sodium salt raw material containing the sulfur component is controlled to a range of 3.0 to 5.0 and the S/W molar ratio is controlled to a range of 0.05 to 0.30, thereby suppressing the generation of sulfides.

When the Na/W molar ratio is less than 3.0 or the S/W molar ratio is less than 0.05, the sodium salt raw material is insufficient, and the reactions of [Formula 1] and [Formula 2] do not proceed sufficiently. On the other hand, when the Na/W molar ratio is 3.0 or more and the S/W molar ratio is 0.05 or more, the reactions of [Formula 1] and [Formula 2] proceed sufficiently. In this reaction to produce sodium tungstate, it has been found that as long as the reaction of [Formula 1] mainly proceeds, the reaction of [Formula 2] can be limited. To proceed with the reaction of [Formula 1], sodium hydroxide can be used as the sodium salt raw material, so when sodium sulfate is used as the sodium salt raw material containing a sulfur component, a sodium salt raw material containing sodium hydroxide can be used together with sodium sulfate.

If the ratio of the amount of sodium salt raw material containing sulfur components to the amount of tungsten raw material is such that the S/W molar ratio is less than 0.30, the sodium sulfide (Na ₂ S) shown in [Formula 3] is hardly generated, and even if it is generated locally, the amount generated is extremely small. Moreover, in the melting process, the generated sodium sulfide (Na ₂ S) is easily oxidized to sodium sulfate (Na ₂ SO ₄ ) by air oxidation as shown in the following [Formula 4], or, when the melting treatment product is dissolved in water, it reacts with metal hydroxides generated from the metal in the scrap in the water-dissolved suspension and becomes a stable solid phase of sulfide, and is fixed as an insoluble solid as shown in the following [Formula 5]. Therefore, almost no sulfide ions are generated in the liquid.

_Na2S + _2O2 → _Na2SO4 (s) [Equation ₄ ]
Na ₂ S + M(OH) ₂ (s) → MS(s) + 2NaOH [Formula 5]

On the other hand, when the S content exceeds the S/W molar ratio of 0.30, the progress of [Formula 3] becomes significant, and the melt-treated product contains a large amount of sodium sulfide, which is undesirable since dissolving it in water results in an aqueous solution containing a large amount of sulfide ions. In this case, even if the oxidation reaction proceeds sufficiently to produce almost no sodium sulfide, sulfur derived from sodium sulfate and the like will be contained in the melt-treated product in the form of sulfate salts. When this is dissolved in water to obtain an aqueous solution of sodium tungstate, it will elute in the form of sulfate ions (SO ₄ ^2- ) in the aqueous solution.

An aqueous solution containing an excess amount of sulfate ions competes with anionic tungsten ions in subsequent tungsten refining processes, such as the ion exchange refining process and solvent extraction process, reducing the efficiency of the tungsten refining process, and also causes problems such as sulfur contamination of tungsten products due to its association with the tungsten refining reaction. Therefore, a S/W molar ratio of 0.30 or less is preferable.

Furthermore, if the Na/W molar ratio exceeds 5.0, the amount of sodium salt raw material used will be excessive, resulting in increased costs. In addition, there will be a lot of sodium salt scattering during heating and melting, which can cause concerns about corrosion of the equipment, making this undesirable.

The sulfide ions (S ^2- ) contained in the aqueous sodium tungstate solution obtained by dissolving the melt-treated material in water can be determined by the redox potential of the solution. If the redox potential is -200 mV (vs Ag/AgCl) or higher, there are almost no sulfide ions. If the redox potential is lower than -200 mV (vs Ag/AgCl), the amount of sulfur added to the heating and melting reaction is excessive, so the solution tends to be significantly reducing, and sulfide ions are generated. In order to prevent the solution from becoming excessively reducing, it is preferable to suppress the production of sodium sulfide from proceeding excessively, as shown in [Equation 3].

When performing continuous melting and dissolving the melted product in water to recover an aqueous sodium tungstate solution, it is advisable to continuously measure the redox potential of the resulting aqueous sodium tungstate solution, and if a tendency or behavior is observed for the redox potential to fall below -200 mV (vs Ag/AgCl), it is advisable to control the input amount so as to reduce the amount of sodium sulfate, etc., added to the raw material. This allows the redox state of the resulting aqueous sodium tungstate solution to be controlled to have an oxidizing tendency.

When the redox potential falls below -200 mV (vs Ag/AgCl) and the liquid becomes reducing, it is advisable to adjust the redox potential to -200 mV (vs Ag/AgCl) or higher by appropriately mixing an aqueous solution of sodium tungstate having a sufficiently high oxidizing property, which is obtained by melting the solution obtained by reducing the amount of sodium sulfate input, dissolving the melt-treated product in water, and recovering the melt-treated product.
Furthermore, when the redox potential is significantly lower than −200 mV (vs Ag/AgCl) and the liquid property is significantly inclined toward reduction, making it difficult to adjust the redox potential to −200 mV (vs Ag/AgCl) or higher by simply mixing an oxidizing aqueous solution of sodium tungstate, it is advisable to adjust the redox potential to −200 mV (vs Ag/AgCl) or higher by a known method such as adding an oxidizing agent or blowing in oxygen.

By adjusting the redox potential to -200 mV (vs Ag/AgCl) or higher, the dissolved sulfide ions can be oxidized and converted to sulfate ions and other ions. Since sulfate ions are less likely to cause adverse effects such as precipitate purification, blockage, and process interference in the subsequent purification process than sulfide ions, it is desirable to convert the sulfide ions that have formed in the sodium tungstate aqueous solution to sulfate ions.

Some tungsten raw materials contain niobium. For example, scraps of cemented carbide tools made of tungsten carbide containing niobium and alloy scraps can be used as tungsten raw materials. When tungsten scrap containing niobium carbide is heated and melted together with a sodium salt raw material as a tungsten raw material, the niobium carbide contained in the tungsten raw material usually forms a sodium soluble salt such as sodium niobate (Na ₃ NbO ₄ ), as shown in the following formula [6]. When this melted product is dissolved in water and recovered as an aqueous sodium tungstate solution, the niobate ion (NbO ₃ ^- ), which is a soluble salt, dissolves and becomes an impurity in the liquid, as shown in the following formula [7].

2NbC(s)+6NaOH(s)+15/2O ₂ → 2Na ₃ NbO ₄ (s)+3H ₂ O(g)+2CO ₂ (g) ・・・[Formula 6]
Na ₃ NbO ₄ +H ₂ O → NbO ₃ ^- (aq)+3Na(aq)+2OH ^- (aq) ...[Formula 7]

However, it was found that, in the above-mentioned heating and melting, when at least one iron raw material, namely metallic iron or iron sulfate (ferrous sulfate or ferric sulfate), is present, niobium carbide reacts with iron to form water-insoluble iron niobate (FeNbO ₃ ), as shown in the following formula [8][9][10]. It was also found that, when the above-mentioned coexisting iron raw material changes into a solid phase of iron oxide, niobium is incorporated into the crystal structure of the iron oxide, making it insoluble.

NbC(s)+Fe(s)+5/ _2O2 → _FeNbO3 (s)+ _CO2 ...[Equation 8]
NbC(s)+FeSO ₄ (s)+2NaOH+2O ₂ → FeNbO ₃ (s)+Na ₂ SO ₄ +H ₂ O+CO ₂ ... [Formula 9]
2NbC(s)+Fe ₂ (SO ₄ ) ₃ (s)+6NaOH+7/2O ₂ → 2FeNbO ₃ (s)+3Na ₂ SO ₄ +3H ₂ O+2CO ₂ ...[Formula 10]

The iron raw material used is at least one of metallic iron and iron sulfate (ferrous sulfate or ferric sulfate). Iron oxide ( _Fe2O3 ) is not preferred. Iron oxide is very stable, so the production of iron niobate ( _FeNbO3 ) is insufficient, and niobium is hardly incorporated into _the crystal structure during the solid phase change to iron oxide, resulting in insufficient immobilization of niobium.

In addition, when ferrous sulfate is used as the iron raw material, the ferrous sulfate reacts with sodium hydroxide, a sodium salt raw material, to produce sodium sulfate, so the amount of sodium sulfate used as part of the sodium salt raw material can be reduced. In addition, when metallic iron is used, it is economical because it is possible to use raw materials containing inexpensive metallic iron, such as iron powder and iron scrap.

The amount of the iron raw material used is preferably 1.0% by mass or more and less than 10.0% by mass of iron relative to a tungsten raw material with a niobium content of 0.01 to 5.0% by mass. If the amount of the iron raw material is less than 1.0% by mass, the amount of iron is too small and the immobilization of niobium is insufficient. If the amount of the iron raw material is 10.0% by mass or more, a large amount of iron oxide is generated, which inhibits contact between the sodium salt raw material and the tungsten raw material and makes the main reaction, the melting and oxidation reaction of tungsten, insufficient, which is not preferable.

In the methods of Patent Documents 1 and 2, sodium sulfate is used at a maximum of 40% by weight in the alkaline melting of the tungsten raw material using sodium hydroxide and sodium sulfate, so that a large amount of sodium sulfide is contained in the melting treatment product, and the aqueous sodium tungstate solution obtained by dissolving this in water may contain a large amount of sulfide ions. In addition, in the method of Patent Document 3, a large amount of sodium sulfate is added (70% by mass or more to 100% by mass or less of the scrap weight), and a large amount of metal oxide is added to oxidize the large amount of Na ₂ S generated as a result, which increases the burden of the treatment process and the treatment cost.

On the other hand, in the manufacturing method of the present invention, the amount of sodium sulfate used is controlled by the Na/W molar ratio and the Na/W molar ratio, so the melt-processed product obtained by heating and melting contains almost no sodium sulfide, etc., and an aqueous sodium tungstate solution containing almost no sulfide ions can be obtained.

Furthermore, by measuring the oxidation-reduction potential of the sodium tungstate aqueous solution, the amount of sulfur can be controlled so that sulfide ions are not generated, and there is no need to use expensive metal oxides as in the method of Patent Document 3. In addition, since the method of the present invention generates almost no sulfide ions at each treatment stage, there is no sudden generation of hydrogen sulfide gas, making the treatment process safer and reducing the need for advanced safety equipment such as odor control and exhaust gas facilities.

When the tungsten raw material contains niobium, the alkali fusion of Patent Documents 1 and 2 produces sodium salts that are soluble in water, so niobium and tungsten cannot be selectively separated from the fusion product. In addition, the metal oxide (iron oxide, etc.) used in the method of Patent Document 3 is used to oxidize the excess sodium sulfide produced, and iron oxide cannot make niobium insoluble.

On the other hand, in the method of the present invention, when a tungsten raw material containing niobium is heated and melted, the Na/W molar ratio and Na/W molar ratio are controlled to suppress the production of sodium sulfide, etc., and furthermore, a stable niobium solid phase is generated by allowing metallic iron or iron sulfate to coexist, and the dissolution of niobium can be suppressed when the melted product is dissolved in water, so that an aqueous sodium tungstate solution containing almost no niobate ions can be recovered along with sulfide ions.

According to the manufacturing method of the present invention, it is possible to obtain an aqueous solution of sodium tungstate that contains almost no impurities such as sulfide ions and niobate ions. In addition, the tungsten refining process can recover tungsten species with low impurity concentrations, such as ammonium paratungstate, and a high-purity tungsten product can be obtained. In the tungsten refining process, for example, ion exchange processing or solvent extraction processing is applied, and when an aqueous solution of sodium tungstate with a high impurity concentration is used, there is a risk that the ion exchange resin or organic solvent may be contaminated and deteriorated by the impurities. However, with the sodium tungstate manufactured by the method of the present invention, such contamination and deterioration can be avoided, and it is possible to protect the refining equipment and reduce running costs.

Examples of the present invention will be described below together with comparative examples.
Example 1
As the tungsten raw material containing niobium, cemented carbide tool scrap containing 0.6 mass% niobium and 89 mass% tungsten was used. About 50 g of this tungsten raw material was placed in a reaction vessel, metallic iron powder was added so that the amount of iron was 2.0 mass% relative to the weight of the tungsten raw material, and sodium salt raw materials consisting of sodium hydroxide and sodium sulfate were added so that the Na/W molar ratios were 2.5, 3.0, 3.5, and 4.0 and the S/W molar ratios were 0, 0.05, 0.1, 0.2, 0.3, 0.4, and 0.5 relative to the amount of tungsten in the tungsten raw material.
The reaction vessel containing the tungsten raw material, the sodium salt raw material, and the metallic iron powder was heated to 850° C. and held for 5 hours, and then naturally cooled. After cooling, the melted product in the reaction vessel was removed and suspended (primary water dissolution) in pure water in an amount four times the amount of tungsten in the tungsten raw material to obtain a primary water-dissolved slurry. This primary water-dissolved slurry was mixed and stirred for 1 hour to measure the oxidation-reduction potential (ORP: mV vs. Ag/AgCl), and then filtered using a 1.0 μm membrane filter to obtain a primary recovery liquid and a primary dehydration residue.
Furthermore, the primary dehydrated residue was suspended again using the same amount of pure water as that used in the primary water dissolution to obtain a secondary water-dissolved slurry. This secondary water-dissolved slurry was mixed and stirred for 1 hour, and then filtered through a 1.0 μm membrane filter to obtain a secondary recovery liquid and a secondary dehydrated residue. The secondary dehydrated residue was further dried at 105° C. for 24 hours to obtain a dry residue, and if the dry residue contained unmelted tungsten raw material, it was separated by sieving and recovered as a solid residue. The dry residue after the solid residue was recovered was recovered as a powder residue. The primary and secondary recovered liquids were measured for dissolved element concentration by ICP-AES. The solid and powder residues were measured for contained element concentration by chemical analysis (ICP-AES after total alkali dissolution by nitrate). The W recovery rate was also calculated. The W recovery rate was calculated as a percentage according to the following formula. The results are shown in Table 1.
W recovery rate=(W amount in the primary recovered liquid and the secondary recovered liquid)/(total W amount in the primary recovered liquid, the secondary recovered liquid, the solid residue, and the powder residue)

As shown in Table 1, in the range of S/W = 0.05 to 0.3 (No. 2 to No. 5) and Na/W = 3.0 to 4.0 (No. B to No. D), the oxidation-reduction potential of the primary water-dissolved slurry is -200 mV (vs. Ag/AgCl) or more (-198 mV or more) and does not contain sulfide ions (S ^2- : 0 ppm). Furthermore, the W recovery rate is 90% or more. The sulfide ion concentration of the secondary recovered liquid was also 0 ppm in all cases. On the other hand, the sample that does not satisfy any of the above conditions (No. 1 of No. A) has insufficient amounts of Na and S, and the W recovery is low at 68%. Moreover, No. 6 and No. 7 of sample No. A have excessive amounts of S, so the oxidation-reduction potential is -522 mV or less and there are many sulfide ions.

In all samples, the tungsten raw material was melted and reacted with metallic iron powder so that the amount of Fe was 2.0 mass% relative to the weight of the tungsten raw material, so the Nb fixation rate was 94 to 95%, and sufficient insolubilization of Nb was achieved. The Nb fixation rate is calculated as a percentage using the following formula.
Nb fixation rate = (Nb amount in solid residue and powder residue) / (total Nb amount in primary recovered liquid, secondary recovered liquid, solid residue, and powder residue)

Example 2
As the tungsten raw material containing niobium, cemented carbide tool scrap containing 0.7 mass% niobium and 85 mass% tungsten was used. About 50 g of this tungsten raw material was placed in a reaction vessel, and metallic iron powder was added so that the iron content was 2.0 mass% relative to the weight of the tungsten raw material. Furthermore, sodium salt raw material consisting of sodium hydroxide and sodium sulfate was added so that the Na/W molar ratio was 2.5, 3.0, 3.5, and 4.0 relative to the tungsten content of the tungsten raw material, and the S/W molar ratio was 0.05, 0.1, 0.2, 0.3, and 0.4, respectively. The treatment was performed in the same manner as in Example 1. The results are shown in Table 2.

As shown in Table 2, in the range of S/W = 0.05 to 0.3 (No. 10 to No. 13) and Na/W = 3.0 to 4.0 (No. B to No. D), except for sample No. 13 of No. B, the oxidation-reduction potential of the primary water-dissolved slurry is -200 mV (vs. Ag/AgCl) or more (-187 mV or more) and does not contain sulfide ions (S ^2- : 0 ppm). Furthermore, the W recovery rate is 95% or more. The sulfide ion concentration of the secondary recovered liquid was also 0 ppm in all cases. On the other hand, the sample (No. 1 of No. A) that does not satisfy any of the above conditions has a shortage of Na and a W recovery of less than 90%. Moreover, sample No. 14 has an excessive amount of S, so the oxidation-reduction potential is -582 mV or less and there is a lot of sulfide ions.
In addition, in all samples, the melting reaction was carried out in the presence of metallic iron powder so that the amount of Fe was 2.0 mass% relative to the weight of the tungsten raw material, so that the Nb fixation rate was 93 to 95%, and sufficient insolubilization of Nb was achieved.

Claims (5)

A method for producing sodium tungstate in which a tungsten raw material is heated and melted together with a sodium salt raw material to obtain a molten product mainly composed of sodium tungstate, the method controlling the Na/W molar ratio of the tungsten raw material and the sodium salt raw material containing sulfur components to a range of 3.0 to 5.0 and the S/W molar ratio to a range of 0.05 to 0.30, thereby suppressing the generation of sulfides. The method for producing sodium tungstate according to claim 1, wherein the sodium salt raw material containing a sulfur component is sodium sulfate, or a combination of sodium sulfate and sodium hydroxide, or a combination of sulfuric acid and sodium hydroxide. A method for producing sodium tungstate according to claim 1 or claim 2, in which the Na/W molar ratio of a tungsten raw material containing niobium is controlled to a range of 3.0 to 5.0 and the S/W molar ratio is controlled to a range of 0.05 to 0.30, and at least one iron raw material of metallic iron or iron sulfate (ferrous sulfate or ferric sulfate) is allowed to coexist with the sodium salt raw material, thereby immobilizing the niobium as an insoluble solid. A method for producing sodium tungstate according to any one of claims 1 to 3, in which the molten product is dissolved in water to obtain an aqueous solution of sodium tungstate. The method for producing sodium tungstate described in claim 4, wherein the oxidation-reduction potential of the aqueous sodium tungstate solution obtained by dissolving the above melt-treated product in water is -200 mV (vs. Ag/Agcl) or higher.