Method for producing sodium tungstate

Abstract

To provide a production method that heats and melts a tungsten raw material together with a sodium salt, produces a molten treated product of a sodium tungstate, and obtains a sodium tungstate aqueous solution, which has little impurities such as sulfide ions and niobic acid ions.SOLUTION: A method for producing a sodium tungstate that heats and melts a tungsten raw material together with a sodium salt raw material, and obtains a molten treated product containing a sodium tungstate as a main component, which controls the sodium salt raw material containing a sulfur component so that an Na / W molar ratio of the tungsten raw material and the sodium salt raw material containing the sulfur component is within a range of 3.0 or more and 5.0 or less, and an S / W molar ratio is within a range of 0.05 or more and 0.30 or less, and suppresses generation of a sulfide, and heats and melts the tungsten raw material containing niobium while co-existing at least any one ion raw material of a metallic iron, ferrous sulfate and ferric sulfate.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a manufacturing method capable of obtaining an aqueous sodium tungstate solution with few impurities from a tungsten raw material.

Background Art

[0002] In recent years, tungsten has been used not only as a cemented carbide material for cutting tools, but also as various materials such as electrode materials, wiring materials, and tungsten catalysts, and its demand has been increasing year by year. However, the resources of tungsten raw materials are limited, and in order to achieve a stable supply, it is required to recover tungsten from tungsten scraps and use it effectively.

[0003] As a method for recovering tungsten from tungsten scraps containing tungsten carbide and metallic tungsten, an alkaline agent containing a sodium salt such as sodium hydroxide or sodium sulfate is added to the tungsten scraps and heated for reaction to convert the tungsten in the scraps into sodium tungstate, and this melt-treated product is dissolved in water to recover it as an aqueous sodium tungstate solution. The recovered aqueous sodium tungstate solution is purified through various purification steps into ammonium paratungstate, tungsten oxide, metallic tungsten, and tungsten carbide, and then reused.

[0004] A method of heating and melting tungsten scraps together with the above-mentioned alkaline raw material containing a sodium salt to obtain sodium tungstate, etc., for example, Patent Document 1 describes a method of performing an oxidation treatment of tungsten using a molten salt composed of 60 to 90% by weight of NaOH and 10 to 40% by weight of Na 2 SO 4 : A method of performing an oxidation treatment of tungsten using a molten salt composed of 60 to 90% by weight of NaOH and 10 to 40% by weight of Na

[0005] Also, Patent Document 2 discloses that a raw material containing tungsten as a sub-component is composed of 60 to 95% by mass of NaOH and Na 2 SO 4: It is described that a heating reaction is carried out together with a molten salt solution containing 5 to 40% by mass, the generated molten liquid decomposition product is dissolved in water, and an aqueous suspension containing a solution composed of a mixture of a sodium compound and an alkali metalate containing tungsten or the like, a solid metal phase composed of a metal group such as Co, and a solid phase composed of a hydroxide or oxide hydrate such as Al is obtained. It is described that the solution (aqueous sodium tungstate solution) composed of the mixture of the sodium compound and the alkali metalate is separated by filtration.

[0006] Further, Patent Document 3 describes a method of reacting cemented carbide scraps containing tungsten and cobalt with a molten salt added with a metal oxide containing sodium sulfate to obtain sodium tungstate. In this method, an oxide containing iron is used as the metal oxide, and 10% by weight or more and less than 30% by weight of the weight of the cemented carbide scraps is added, and 70% by weight or more and 100% by weight or less of the weight of the cemented carbide scraps is added with sodium sulfate.

Prior Art Documents

Patent Documents

[0007]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0008] The melting treatment methods described in Patent Documents 1 to 3 are highly reactive and efficient as a method for simultaneously causing oxidation of tungsten and conversion to soluble salts. On the other hand, since this method is very highly reactive, trace elements other than tungsten in tungsten scraps and elements in additive agents also cause secondary reactions, and there is also a problem that the concentration of elements other than tungsten and the impurity concentration in the sodium tungstate aqueous solution, which is the recovered product after melting treatment, are very likely to increase.

[0009] For example, when sulfur is contained such as sodium sulfate, in the presence of oxygen such as in an air atmosphere, tungsten carbide reacts with sodium sulfate to form sodium tungstate. At this time, if the addition amount of sodium sulfate is excessive, a large amount of sodium sulfide (Na 2 S) is generated secondarily. When a melting treatment product containing such a large amount of Na 2 S is dissolved in water, it becomes a sodium tungstate aqueous solution in which a large amount of sulfide ions (S 2- ) as impurities are dissolved.

[0010] Also, for example, tungsten scraps may contain titanium (Ti), niobium (Nb), tantalum (Ta), etc. as additives for cemented carbide tools and coating materials in addition to tungsten. These elements easily react with alkalis and, through an oxidation reaction, form soluble sodium titanate (Na 2 Ti 5 O 11 ), sodium niobate (Na 3 NbO 4 ), sodium tantalate (Na 3 TaO 4 ), etc. When a melting treatment product containing these compounds is dissolved in water, titanate ions (HTiO 3 - ), niobate ions (NbO 3 - ), tantalate ions (TaO 3 -) will turn into an aqueous sodium tungstate solution with a large amount of dissolved . In particular, niobium is the most easily soluble, so there was a problem that the aqueous sodium tungstate solution containing a large amount of niobium as an impurity would be formed.

[0011] The methods of Patent Documents 1 and 2 do not sufficiently address the influence of such sulfide ions. In particular, in the method of Patent Document 1, when using a molten salt with a high sodium sulfate content, the influence of sulfide ions becomes significant. Furthermore, since there is no means for separating impurities such as niobium, an aqueous sodium tungstate solution with a high niobium concentration will be formed. Also, in the method of Patent Document 2, it is described that a solution composed of a mixture of a sodium compound obtained by dissolving the molten liquid decomposition product in water and an alkali metalate containing tungsten etc., and a solid metal phase etc. are separated by filtration. However, since niobium forms soluble salts and dissolves in the aqueous solution, such a filtration separation method cannot separate and remove the dissolved niobium as an impurity.

[0012] In the method of Patent Document 3, as a countermeasure method for sulfide ions using metal oxides, it is shown that the reaction of sodium sulfide and metal oxides in a molten salt suppresses the generation of sulfide ions. However, in the method of Patent Document 3, since a large amount of sodium sulfate of 70% by weight or more is added to the scrap raw material, excessive sodium sulfide is generated. To oxidize this excessive sodium sulfide, an even larger amount of metal oxides is added. The treatment load is high and the removal of niobium is not sufficient.

Means for Solving the Problems

[0013] The manufacturing method of the present invention relates to a manufacturing method capable of obtaining an aqueous sodium tungstate solution with few sulfide impurities in the molten product when heating and melting a tungsten raw material together with a sodium salt raw material containing a sulfur component to produce a molten product mainly composed of sodium tungstate, and when dissolving the molten product in water, there are remarkably few impurities such as sulfide ions. Preferably, it also relates to a manufacturing method capable of obtaining an aqueous sodium tungstate solution with remarkably few impurities such as niobium together with sulfide.

[0014] The manufacturing method of the present invention is a method for manufacturing sodium tungstate having the following configuration. 〔1〕In a method for manufacturing 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, for the sodium salt raw material containing a sulfur component, the Na / W molar ratio by the tungsten raw material and the sodium salt raw material containing the sulfur component is controlled in the range of 3.0 or more to 5.0 or less, and the S / W molar ratio is controlled in the range of 0.05 or more to 0.30 or less to suppress the generation of sulfides. A method for manufacturing sodium tungstate. 〔2〕The method for manufacturing sodium tungstate according to [1] above, wherein the sodium salt raw material containing the sulfur component is sodium sulfate, or a combination of sodium sulfate and sodium hydroxide, or a combination of sulfuric acid and sodium hydroxide. 〔3〕For a tungsten raw material containing niobium, the Na / W molar ratio is controlled in the range of 3.0 to 5.0 and the S / W molar ratio is controlled in the range of 0.05 to 0.30, and at least one iron raw material of metallic iron or iron sulfate (ferrous sulfate to ferric sulfate) is coexisted with the sodium salt raw material containing the sulfur component to immobilize the niobium as an insoluble solid. The method for manufacturing sodium tungstate according to any one of [1] or [2] above. 〔4〕The method for manufacturing sodium tungstate according to any one of [1] to [3] above, wherein the molten product is dissolved in water to obtain an aqueous sodium tungstate solution. The production method of sodium tungstate described in the above [4], wherein the oxidation-reduction potential of the aqueous sodium tungstate solution obtained by dissolving the above molten product in water is -200 mV (vs. Ag / AgCl) or higher.

[0015] 〔Specific description〕 The method of the present invention is 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. For the sodium salt raw material containing a sulfur component, the Na / W molar ratio of the above tungsten raw material and the sodium salt raw material containing the sulfur component is controlled in the range of 3.0 or more to 5.0 or less, and the S / W molar ratio is controlled in the range of 0.05 or more to 0.30 or less to suppress the formation of sulfides.

[0016] By heating and melting the tungsten raw material together with the sodium salt raw material, a molten product mainly composed of sodium tungstate is obtained. As the sodium salt, a sodium salt raw material containing sodium sulfate can be used. Also, a sodium salt raw material containing sodium hydroxide together with sodium sulfate can be used. Further, sulfuric acid may be added to these sodium salt raw materials. For example, adding sulfuric acid to sodium hydroxide gives the same effect as using sodium sulfate. The heating temperature for heating and melting is preferably 700°C or higher to 1000°C or lower, and more preferably 850°C or higher to 1000°C or lower. If it is less than 700°C, the progress of the reaction is insufficient, and if it exceeds 1000°C, there is a concern about corrosion and deterioration of equipment and devices due to the high-temperature environment.

[0017] The above heating and melting gives a molten product mainly composed of 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, as shown in the following formulas [1][2][3], tungsten carbide reacts with sodium hydroxide and sodium sulfate to form sodium tungstate.

[0018] 2WC(s) + 4NaOH(s) + 5O 2 → 2Na 2 WO 4 (s) + 2H 2 O(g) + 2CO 2 (g) ··· [Equation 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)·[Equation 2] 2WC(s) + 3Na 2 SO 4 (s) → 2Na 2 WO 4 (s) + Na 2 S(s) + 2CO(g) + 2SO(g) ··[Equation 3]

[0019] In the above-mentioned heat melting, when sodium sulfate is used as the sodium salt raw material, in an atmosphere where sufficient oxygen exists with respect to the added sulfur content, as shown in [Equation 2], sodium tungstate is generated and sodium sulfate is decomposed to produce sodium oxide and sulfur dioxide, and sulfur dioxide gasifies and escapes from the system. On the other hand, when an excessive amount of sodium sulfate is present, as shown in [Equation 3], sodium sulfide and sulfur monoxide are secondarily generated along with the production of sodium tungstate. Sulfur monoxide gasifies and escapes from the system, but sodium sulfide remains in the molten product. When such a molten 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 as impurities. The same thing also occurs 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 using sodium sulfate, those using a combination of sodium sulfate and sodium hydroxide, or those using a combination of sulfuric acid and sodium hydroxide are all referred to as sodium salt raw materials containing a sulfur component.

[0020] In the heat melting using a sodium salt raw material containing such a sulfur component, it has been found that the generation of sodium sulfide is suppressed by controlling the Na / W molar ratio and the S / W molar ratio with a tungsten raw material and a sodium salt raw material containing a sulfur component. The production method of the present invention is based on the above finding, and can significantly reduce the generation of sodium sulfide, and thus obtain an aqueous sodium tungstate solution that hardly contains sulfide ions as impurities, which is a production method of sodium tungstate.

[0021] Specifically, the production method of the present invention is a method for producing sodium tungstate in which, in the heat melting using a sodium salt raw material containing a sulfur component, the Na / W molar ratio with a tungsten raw material and the sodium salt raw material containing the sulfur component is controlled within a range of 3.0 or more to 5.0 or less, and the S / W molar ratio is controlled within a range of 0.05 or more to 0.30 or less to suppress the generation of sulfides.

[0022] 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, so the reactions of [Equation 1][Equation 2] do not proceed sufficiently. On the other hand, if the Na / W molar ratio is 3.0 or more and the S / W molar ratio is 0.05 or more, the reactions of [Equation 1][Equation 2] proceed sufficiently. In this production reaction of sodium tungstate, it has been found that if the reaction of [Equation 1] mainly proceeds, the reaction of [Equation 2] may be limited. To promote the reaction of [Equation 1], sodium hydroxide may be used as the sodium salt raw material. Therefore, when sodium sulfate is used as the sodium salt raw material containing a sulfur component, a sodium salt raw material containing sodium hydroxide together with sodium sulfate may be used.

[0023] If the amount ratio of the sodium salt raw material containing a sulfur component and the tungsten raw material is an S amount such that the S / W molar ratio is less than 0.30, sodium sulfide (Na 2 2S) hardly generates, and even if it locally generates partially, the generated amount is extremely small. Moreover, the generated sodium sulfide (Na 2 2S) easily undergoes air oxidation to sodium sulfate (Na2 SO 4 ) is oxidized or, as shown in the following [Formula 5], when the melt-treated product is dissolved in water, it reacts with metal hydroxides generated from the metals in the scrap in the water-dissolved suspension to form a stable solid phase of sulfide and is immobilized as an insoluble solid. Therefore, almost no sulfide ions are generated in the liquid.

[0024] Na 2 S + 2O 2 → Na 2 SO 4 (s) [Formula 4] Na 2 S + M(OH) 2 (s) → MS(s) + 2NaOH [Formula 5]

[0025] On the other hand, in the case of an S amount exceeding the S / W molar ratio = 0.30, the progress of [Formula 3] becomes remarkable and a large amount of sodium sulfide is contained in the melt-treated product. Therefore, when this is dissolved in water, an aqueous solution containing a large amount of sulfide ions is obtained, which is not preferable. Also, in this case, even if the oxidation reaction proceeds sufficiently and almost no sodium sulfide is generated, sulfur derived from sodium sulfate etc. will be contained in the melt-treated product in the form of sulfate. When this is dissolved in water to obtain an aqueous solution of sodium tungstate, sulfate ions (SO 4 2- ) will elute in the form of.

[0026] An aqueous solution containing an excessive amount of sulfate ions competes with the ionic species of tungsten, which is an anion, in the subsequent tungsten purification process, for example, an ion exchange purification process or a solvent extraction process, reducing the efficiency of the tungsten purification process. Also, problems such as sulfur being mixed into the tungsten product due to being associated with the tungsten purification reaction occur. Therefore, an S / W molar ratio = 0.30 or less is preferable.

[0027] Also, when the Na / W molar ratio exceeds 5.0, the amount of sodium salt raw material used is excessive, resulting in increased costs. Also, there is a lot of scattering of sodium salt during heating and melting, and there are concerns about corrosion of the equipment etc., so it is not preferable.

[0028] Regarding an aqueous sodium tungstate solution obtained by dissolving a melt-treated product in water, sulfide ions (S 2- ) contained in the solution can be grasped by the oxidation-reduction potential of the aqueous solution. If the oxidation-reduction potential is -200 mV (vs Ag / AgCl) or higher, there is almost no sulfide ion. When the oxidation-reduction potential is lower than -200 mV (vs Ag / AgCl), since the amount of sulfur added to the heat melting reaction is excessive, the solution tends to be strongly reducing and sulfide ions are generated. In order not to make the solution overly reducing, it is preferable to suppress the excessive progress of the formation of sodium sulfide as shown in [Equation 3].

[0029] When continuously performing the melting treatment and the treatment of dissolving the melt-treated product in water to recover the aqueous sodium tungstate solution, if it is possible to continuously measure the oxidation-reduction potential of the obtained aqueous sodium tungstate solution and confirm a tendency or behavior that the oxidation-reduction potential is likely 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 at the time of raw material input. Thereby, the oxidation-reduction state of the obtained aqueous sodium tungstate solution can be controlled to have an oxidizing tendency.

[0030] When the oxidation-reduction potential drops below -200 mV (vs Ag / AgCl) and the liquid property becomes reducing, it is advisable to adjust the oxidation-reduction potential to -200 mV (vs Ag / AgCl) or higher by appropriately mixing a sufficiently highly oxidizing aqueous sodium tungstate solution obtained by reducing the amount of sodium sulfate input and then dissolving and recovering the melt-treated product in water. Also, when the oxidation-reduction potential drops significantly below -200 mV (vs Ag / AgCl) and the liquid property becomes strongly reducing, and it becomes difficult to adjust it to -200 mV (vs Ag / AgCl) or higher only by mixing an oxidizing aqueous sodium tungstate solution, it is advisable to adjust the oxidation-reduction potential to -200 mV (vs Ag / AgCl) or higher by a known method such as adding an oxidizing agent or blowing in oxygen.

[0031] If the redox potential is adjusted to -200 mV (vs Ag / AgCl) or higher, the dissolved sulfide ions can be oxidized and converted into sulfate ions etc. In the subsequent purification process, sulfate ions are less likely to cause adverse effects such as precipitate purification, blockage, and processing interference compared to sulfide ions. Therefore, it is desirable to convert the sulfide ions generated in the sodium tungstate aqueous solution into sulfate ions.

[0032] Some tungsten raw materials contain niobium. For example, cemented carbide tool scraps or alloy scraps made of tungsten carbide containing niobium can be used as tungsten raw materials. When tungsten scraps containing niobium carbide are heated and melted together with a sodium salt raw material as the tungsten raw material, usually, as shown in the following formula [6], the niobium carbide contained in the tungsten raw material forms sodium-soluble salts such as sodium niobate (Na 3 NbO 4 ). When this melted product is dissolved in water and recovered as an aqueous solution of sodium tungstate, as shown in the following formula [7], the soluble salt niobate ion (NbO 3 - ) dissolves and becomes an impurity in the liquid.

[0033] 2NbC(s) + 6NaOH(s) + 15 / 2O 2 → 2Na 3 NbO 4 (s) + 3H 2 O(g) + 2CO 2 (g) ···[Equation 6] Na 3 NbO 4 + H 2 O → NbO 3 - (aq) + 3Na(aq) + 2OH - (aq) ···[Equation 7]

[0034] However, in the above heating and melting, if at least one iron raw material such as metallic iron or iron sulfate (ferrous sulfate or ferric sulfate) coexists, as shown in the following formulas [8][9]

[10] , niobium carbide reacts with iron to form iron niobate (FeNbO3 ) was found to be formed. Further, when the coexisting iron raw material undergoes a solid-phase change to iron oxide, niobium is incorporated into the crystal structure of the iron oxide and insolubilized.

[0035] NbC(s)+Fe(s)+5 / 2O 2 → FeNbO 3 (s)+CO 2 ···[Equation 8] NbC(s)+FeSO 4 (s)+2NaOH+2O 2 → FeNbO 3 (s)+Na 2 SO 4 +H 2 O+CO 2 ··· [Equation 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 ···[Equation 10]

[0036] The iron raw material to be used is at least one of metallic iron and iron sulfate (ferrous sulfate or ferric sulfate). Iron oxide (Fe 2 O 3 ) is not preferred. Since iron oxide is very stable, the formation of iron niobate (FeNbO 3 ) is insufficient, and hardly any niobium is incorporated into the crystal structure during the solid-phase change to iron oxide, resulting in insufficient immobilization of niobium.

[0037] Also, when using iron sulfate as the iron raw material, iron sulfate reacts with sodium hydroxide of the sodium salt raw material to generate sodium sulfate, so the amount of sodium sulfate used as part of the sodium salt raw material can be reduced. Also, when using metallic iron, it is economical because raw materials containing inexpensive metallic iron such as iron powder and iron scrap can be used.

[0038] The usage amount of the iron raw material is preferably an amount such that, with respect to the tungsten raw material having a niobium content of 0.01 to 5.0% by mass, the iron amount is 1.0% by mass or more and less than 10.0% 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 becomes 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 the contact between the sodium salt raw material and the tungsten raw material, and the melting oxidation reaction of tungsten, which is the main reaction, becomes insufficient, so it is not preferable.

Advantages of the Invention

[0039] In the methods of Patent Documents 1 and 2, in the alkali melting of the tungsten raw material using sodium hydroxide and sodium sulfate, since up to 40% by weight of sodium sulfate is used, a large amount of sodium sulfide is contained in the melt-treated product, and there is a possibility that a large amount of sulfide ions are contained in the aqueous sodium tungstate solution obtained by dissolving this in water. Further, in the method of Patent Document 3, a large amount of sodium sulfate is added (70% by mass or more and 100% by mass or less of the scrap weight), and for this reason, a large amount of metal oxide is added to oxidize the Na 2 S generated, increasing the burden of the treatment process and also increasing the treatment cost.

[0040] On the other hand, in the production method of the present invention, since the usage amount of sodium sulfate is controlled by the Na / W molar ratio and the Na / W molar ratio, the melt-treated product obtained by heating and melting contains almost no sodium sulfide or the like, and an aqueous sodium tungstate solution containing almost no sulfide ions can be obtained.

[0041] Furthermore, the amount of sulfur can be controlled by measuring the oxidation-reduction potential of the aqueous sodium tungstate solution so that sulfide ions are not generated, and there is no need to use a high-cost metal oxide as in the method of Patent Document 3. Also, in the method of the present invention, almost no sulfide ions are generated in each treatment stage, so sudden generation of hydrogen sulfide gas does not occur, the safety of the treatment process is high, and safety facilities such as advanced odor countermeasures and exhaust gas facilities can also be reduced.

[0042] When the tungsten raw material contains niobium or the like, in the alkali fusion of Patent Documents 1 and 2, niobium forms a sodium salt soluble in water, so niobium and tungsten cannot be selectively separated from the fusion product. Further, the metal oxide (such as iron oxide) used in the method of Patent Document 3 oxidizes the excessively generated sodium sulfide, and moreover, niobium cannot be insolubilized with iron oxide.

[0043] On the other hand, in the method of the present invention, in the heat fusion of a tungsten raw material containing niobium, the Na / W molar ratio and the Na / W molar ratio are controlled to suppress the generation of sodium sulfide and the like, and further, by coexisting metallic iron or iron sulfate, a niobium stable solid phase is generated, and the dissolution of niobium can be suppressed when the fusion-treated product is dissolved in water. Therefore, an aqueous sodium tungstate solution containing almost no niobate ions together with sulfide ions can be recovered.

[0044] According to the production method of the present invention, an aqueous sodium tungstate solution containing almost no impurities such as sulfide ions and niobate ions can be obtained. Further, tungsten chemical species with a low impurity concentration, such as ammonium paratungstate, can be recovered by a tungsten purification step, and a high-purity tungsten product can be obtained. In the tungsten purification step, for example, ion exchange treatment or solvent extraction treatment is applied. However, if an aqueous sodium tungstate solution with a high impurity concentration is used, there is a risk that ion exchange resins and organic solvents will be contaminated and deteriorated by impurities. However, such contamination and deterioration can be avoided with the sodium tungstate produced by the method of the present invention, and the protection of purification equipment and the reduction of running costs can be achieved.

Embodiments for Carrying Out the Invention

[0045] Hereinafter, examples of the present invention are shown together with comparative examples. 〔Example 1〕 As a tungsten raw material containing niobium, cemented carbide scrap containing 0.6% by mass of niobium and 89% by mass of tungsten was used. Approximately 50 g of this tungsten raw material was placed in a reaction vessel, and metallic iron powder was added to achieve an iron content of 2.0% by mass based on the weight of the tungsten raw material. Further, a sodium salt raw material composed of sodium hydroxide and sodium sulfate was added such that the Na / W molar ratio became 2.5, 3.0, 3.5, 4.0 and the S / W molar ratio became 0, 0.05, 0.1, 0.2, 0.3, 0.4, 0.5 with respect to the tungsten amount in the tungsten raw material. The reaction vessel containing the above tungsten raw material, the above sodium salt raw material, and the above metallic iron powder was heated to 850 °C, held for 5 hours, and then naturally cooled. After the temperature drop, the molten product in the reaction vessel was taken out and suspended (primary water dissolution) using four times the amount of pure water with respect to the tungsten amount in the tungsten raw material to obtain a primary water dissolution slurry. This primary water dissolution slurry was mixed and stirred for 1 hour, the oxidation-reduction potential (ORP: mV vs. Ag / AgCl) was measured, and then filtered using a 1.0 μm membrane filter to obtain a primary recovered liquid and a primary dehydrated residue. Furthermore, the primary dehydrated residue was suspended again using the same amount of pure water as the amount of water used for the primary water dissolution to obtain a secondary water dissolution slurry. After this secondary water dissolution slurry was mixed and stirred for 1 hour, it was filtered through a 1.0 μm membrane filter to obtain a secondary recovered liquid and a secondary dehydrated residue. The secondary dehydrated residue was further dried at 105 °C for 24 hours to obtain a dried residue. When the dried residue contained unreacted tungsten raw material, it was separated by sieving and recovered as a solid residue. The dried residue after recovering the solid residue was recovered as a powder residue. The dissolved element concentrations of the primary recovered liquid and the secondary recovered liquid were measured by ICP-AES. The contained element concentrations of the solid residue and the powder residue were measured by chemical analysis treatment (ICP-AES after complete alkali dissolution with nitrate). Also, the W recovery rate was determined. The W recovery rate was calculated as a percentage using the following formula. The results are shown in Table 1. W recovery rate = (amount of W in the primary recovered liquid and the secondary recovered liquid) / (total amount of W in the primary recovered liquid, the secondary recovered liquid, the solid residue, and the powder residue)

[0046] As shown in Table 1, in the range of S / W = 0.05 - 0.3 (No.2 - No.5) and in the range of Na / W = 3.0 - 4.0 (No.B - No.D), the redox potential of the primary water-soluble slurry is -200 mV (vs. Ag / AgCl) or higher (-198 mV or higher) in all cases, and sulfide ions are not contained (S 2- : 0 ppm). Furthermore, the W recovery rate is 90% or higher in all cases. Regarding the secondary recovery solution, the concentration of sulfide ions was 0 ppm in all cases. On the other hand, for the sample (No.1 of No.A) that does not satisfy any of the above conditions, the amount of Na and S is insufficient, and the W recovery is as low as 68%. Also, for No.6 and No.7 of sample No.A, since the amount of S is excessive, the above redox potential is -522 mV or lower, and there are many sulfide ions.

[0047] In addition, in all samples, since metallic iron powder is coexisted and melted to satisfy 2.0 mass% of Fe with respect to the weight of the tungsten raw material, the Nb immobilization rate is 94 - 95%, and sufficient insolubilization of Nb is achieved. The Nb immobilization rate is obtained as the percentage of the following formula. Nb immobilization rate = (amount of Nb in solid residue and powder residue) / (total amount of Nb in primary recovery solution, secondary recovery solution, solid residue, and powder residue)

[0048]

Table 1

[0049] 〔Example 2〕 As the tungsten raw material containing niobium, a cemented carbide scrap containing 0.7 mass% of niobium and 85 mass% of tungsten was used. Approximately 50 g of this tungsten raw material was placed in a reaction vessel, metallic iron powder was added to achieve 2.0 mass% of iron with respect to the weight of the tungsten raw material, and further, the Na / W molar ratio was 2.5, 3.0, 3.5, 4.0 and the S / W molar ratio was 0.05, 0.1, 0.2, 0.3, 0.4 with respect to the amount of tungsten in the tungsten raw material. The treatment was the same as in Example 1 except that a sodium salt raw material composed of sodium hydroxide and sodium sulfate was added. The results are shown in Table 2.

[0050] As shown in Table 2, in the range of S / W = 0.05 to 0.3 (No. 10 to No. 13) and in the range of Na / W = 3.0 to 4.0 (No. B to No. D), except for the sample of No. 13 of No. B, the redox potential of the primary water-soluble slurry is -200 mV (vs. Ag / AgCl) or higher (-187 mV or higher), and sulfide ions are not contained (S 2- : 0 ppm). Furthermore, the W recovery rate is 95% or higher in all cases. Regarding the secondary recovery solution, the concentration of sulfide ions was also 0 ppm in all cases. On the other hand, for the sample (No. 1 of No. A) that does not satisfy any of the above conditions, the amount of Na is insufficient and the W recovery is less than 90%. Also, for sample No. 14, since the amount of S is excessive, the above redox potential is -582 mV or lower and there are many sulfide ions. In addition, in any of the samples, since metallic iron powder is coexisted and melted and reacted so as to satisfy the Fe amount of 2.0 mass% with respect to the weight of the tungsten raw material, the Nb immobilization rate is 93 to 95%, and sufficient insolubilization of Nb is achieved.

[0051]

Table 2

Claims

1. In a method for producing sodium tungstate, wherein 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, for the 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 in the range of 3.0 or more to 5.0 or less, and the S / W molar ratio is controlled in the range of 0.05 or more to 0.30 or less to suppress the formation of sulfides. A method for producing sodium tungstate.

2. 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.

3. For the tungsten raw material containing niobium, the Na / W molar ratio is controlled in the range of 3.0 to 5.0 and the S / W molar ratio is controlled in the range of 0.05 to 0.30, and at least one of metallic iron or iron sulfate (ferrous sulfate or ferric sulfate) is coexisted with the sodium salt raw material to immobilize the niobium as an insoluble solid. The method for producing sodium tungstate according to any one of claim 1 or claim 2.

4. The method for producing sodium tungstate according to any one of claims 1 to 3, wherein the molten product is dissolved in water to obtain an aqueous sodium tungstate solution.

5. The method for producing sodium tungstate according to claim 4, wherein the oxidation-reduction potential of the aqueous sodium tungstate solution obtained by dissolving the molten product in water is -200 mV (vs. Ag / Agcl) or more.

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