JP7690909B2 - Method for purifying sodium tungstate aqueous solution - Google Patents

Description

The present invention relates to a purification 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 materials such as electronic materials such as electrode materials and wiring materials, and tungsten catalysts, and the demand for it has been 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 compound such as sodium hydroxide, sodium sulfate, or sodium nitrate to the tungsten scrap and heat it to react, converting the tungsten in the scrap to sodium tungstate, and dissolving this heat-treated material 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.

Tungsten-containing scrap, which is the raw material for sodium tungstate, may contain elements other than tungsten, such as titanium and niobium. In the reaction process between the scrap and a sodium compound, these elements form sodium titanate and sodium niobate, just like tungsten, and since these coexist with sodium tungstate, they may be included as impurities in the sodium tungstate aqueous solution. Niobium in particular is highly soluble and is an element that is particularly likely to be mixed into the sodium tungstate aqueous solution.

In addition, sulfide ions and the like may be generated as by-products during the manufacturing process of the sodium tungstate aqueous solution and may be dissolved in the sodium tungstate aqueous solution. Alternatively, if a chemical containing sulfide ions is used as an additive in the manufacturing process, sulfide ions may also be dissolved in the sodium tungstate aqueous solution.

The titanate ions, niobate ions, and sulfide ions contained in sodium tungstate aqueous solutions are difficult to separate and remove using ion exchange or solvent extraction methods, and titanium, niobium, and sulfur are likely to be mixed into the final tungsten product as impurities. Alternatively, some of the impurities may undergo air oxidation or hydrolysis and re-precipitate after a certain amount of time has passed in the solution. Precipitates formed with such a time lag can lead to the formation of scale and equipment blockages, causing serious problems in facility operation. In addition, if reprecipitation occurs in the later stages of the refining process, these impurity-containing precipitates will distribute as solid particles to various tungsten products, causing a problem of reduced product quality.

Methods for separating tungsten and niobium by a wet process, or methods for purifying an aqueous sodium tungstate solution are known, for example, from Patent Documents 1 to 3.
Patent Document 1 describes a method in which a sodium hydroxide solution is added to glass scrap raw material containing tungsten, phosphorus, and niobium, and the raw material is heated at 50° C. or higher to dissolve and remove phosphorus and tungsten in the raw material. However, in this method, when a sodium hydroxide solution is added and heated in the first alkali fusion treatment, a part of niobium is dissolved in the solution in addition to tungsten, resulting in a solution in which tungsten and niobium coexist. In Patent Document 1, the alkali fusion slurry is separated into solid and liquid to recover an alkali fusion cake containing high-quality niobium, but the method for separating tungsten and niobium from the solid-liquid separated solution is unknown, and it is not possible to recover a sodium tungstate aqueous solution with a low concentration of impurities such as niobium.

In Patent Document 2, for a tungstate (sodium tungstate) aqueous solution contaminated with tantalum, niobium, etc., in the first step, carbon dioxide gas is blown in to adjust the pH to 8 to 10 to precipitate some of the impurities in the liquid, and then the impurities are partially removed by solid-liquid separation; in the second step, the liquid obtained after the solid-liquid separation is purified by ion exchange using an anion exchanger (OH-type anion-based ion exchange resin) (the liquid obtained after the solid-liquid separation is passed through a column for adsorption treatment, and further elution treatment is performed using an aqueous sodium hydroxide solution); and in the third step, _CO2 and an aqueous NaOH solution are recovered from the solution by membrane electrolysis.

However, the method of Patent Document 2 has many treatment stages, and the treatment time and treatment cost are likely to be high. In addition, in terms of removal efficiency, in the first stage alone, the proportion of niobium in the solution that precipitates is partial, and the removal efficiency is low, and dissolved niobium is likely to move to the second stage. Furthermore, in the second stage, the sodium tungstate aqueous solution is purified by ion exchange treatment, but in reality, niobium forms oxoanions, such as NbO ₃ ^- , in the solution in the same way as tungsten, and tends to show chemical properties similar to those of tungsten oxoanions WO ₄ ^2- , so that selective separation of niobium and tungsten is likely to be insufficient in ion exchange treatment using an anion exchanger (OH-type anion-based ion exchange resin). Furthermore, the precipitate formed in the first stage contains tungsten as well as Al, Ti, Ta, and Nb, and the content of tungsten is about 15% by weight, and the loss of tungsten in the first stage is large.

Patent Document 3 discloses a method of adding a divalent iron compound to a sodium tungstate solution containing Cr and V, and then adding an acid to adjust the pH of the solution to 8 to 13, thereby precipitating and removing Cr and V. However, with this method, it is unclear whether impurity elements other than Cr and V contained in the sodium tungstate solution can be separated and removed, and simply adding a divalent iron compound and adjusting the pH may not be sufficient to separate and remove elements other than Cr and V.

Patent No. 5431780 Japanese Patent Application Publication No. 08-231225 Patent No. 5761500

The present invention provides a method for purifying an aqueous solution of sodium tungstate that solves the above problems of conventional treatment methods, and provides a purification method that has a simple treatment process yet is highly effective in removing niobium, titanium, and sulfide ions.

(1) A method for removing impurity ions from an aqueous sodium tungstate solution containing at least any one of niobium, titanium, and sulfide ions as an impurity ion, the method comprising the steps of adding a divalent iron compound and a trivalent iron compound to the aqueous sodium tungstate solution, adjusting the pH of the aqueous solution to 9 to 12 to form a precipitate, and filtering out the precipitate to remove the impurity ions.
(2) A method for purifying a sodium tungstate aqueous solution described in [1] above, in which the ratio of the added amounts of divalent iron compound [Fe(II)] and trivalent iron compound [Fe(III)] is 10 to 90 mass% Fe(II) and 10 to 90 mass% Fe(III), in terms of the mass ratio of the amount of iron contained.
(3) The method for purifying a sodium tungstate aqueous solution according to the above item [1] or [2], wherein the sodium tungstate aqueous solution containing impurity ions is obtained by heating tungsten-containing scrap together with a sodium compound to produce a heat-treated product, and dissolving the heat-treated product in water.
(4) The method for purifying a sodium tungstate aqueous solution according to the above item [1] or [2], wherein the sodium tungstate aqueous solution containing impurity ions is an aqueous sodium tungstate solution obtained by dissolving tungsten-containing electrode material scrap, or a tungsten concentrate obtained by separating components other than tungsten from tungsten-containing electrode material scrap, in an aqueous solution containing a sodium compound.

[Specific explanation]
The purification method of the present invention will be specifically described below. Note that % is % by mass.
The purification method of the present invention is a method for removing impurity ions from an aqueous sodium tungstate solution containing at least any one of niobium, titanium, and sulfide ions as impurity ions, the method comprising the steps of adding a divalent iron compound and a trivalent iron compound to the aqueous sodium tungstate solution, adjusting the pH of the aqueous solution to 9 to 12 to generate a complex compound precipitate of divalent iron and trivalent iron and incorporating the impurity ions into the precipitate, and filtering out the precipitate to remove the impurity ions.

In the refining method of the present invention, the aqueous sodium tungstate solution containing at least any one of niobium, titanium, and sulfide ions as impurity ions is an aqueous sodium tungstate solution obtained by reacting tungsten-containing scrap, such as tungsten-containing super tool scrap, alloy scrap, or electrode material scrap, with a sodium compound.

Specifically, for example, cemented carbide tool scrap containing 0.6% niobium, 1.0% titanium, and 89% tungsten is used as the tungsten-containing raw material, which is placed in a reaction vessel, to which sodium hydroxide and sodium sulfate are added as alkaline agents, and the raw material is heated at 850°C for 5 hours and then naturally cooled. After the temperature has dropped, the heat-treated product in the reaction vessel is removed, and a 10-fold amount of pure water is added relative to the amount of tungsten in the tungsten raw material to perform a water-dissolution treatment to produce a water-dissolved slurry. This water-dissolved slurry contains 90 g/L of tungstic acid (WO ₃ ), 7,250 mg/L of sulfide ions (S ^2- ), 110 mg/L of titanium, and 64 mg/L of niobium.

The sodium tungstate aqueous solution obtained from electrode material scrap or tungsten concentrate derived from electrode material scrap can be, for example, an aqueous sodium tungstate solution obtained by dissolving electrode material scrap containing tungsten in an aqueous solution containing a sodium compound, or an aqueous sodium tungstate solution obtained by leaching the electrode material scrap with acid, filtering the filtered residue (tungsten concentrate), and dissolving the filtered residue (tungsten concentrate) in an aqueous solution containing sodium.

A divalent iron compound and a trivalent iron compound are added to an aqueous solution of sodium tungstate containing the above impurity ions, the pH of the aqueous solution is adjusted to 9 to 12 to produce a complex compound precipitate of divalent iron and trivalent iron, the above impurity ions are incorporated into the precipitate, and the precipitate is filtered to remove the impurity ions.

By adding both divalent iron compound [Fe(II)] and trivalent iron compound [Fe(III)] to an aqueous solution of sodium tungstate, a complex compound of divalent iron and trivalent iron, represented by magnetite [Fe ₃ O ₄ (Fe ^(II) Fe ^(III) ₂ O ₄ )], is produced as shown in the following formula (1). During the solid phase formation process, magnetite incorporates dissolved metal ions and other substances into its crystal structure, enabling it to efficiently remove impurities from the solution.
Fe ²⁺ +2Fe ³⁺ + 4H ₂ O → Fe ₃ O ₄ (Fe ^(II) Fe ^(III) ₂ O ₄ ) + 8H ⁺ ... (1)

Furthermore, if the liquid contains anion species such as sulfate ions (SO ₄ ^2- ), carbonate ions (CO ₃ ^2- ), or chloride ions (Cl ^- ) in addition to tungstic acid, layered double hydroxides (such as green rust) are formed between these anion species and divalent iron or trivalent iron, as shown in the following equations (2) and (3).

4Fe ²⁺ +2Fe ³⁺ +14H ₂ O+SO ₄ ^2- → [Fe ^(II) ₄ Fe ^(III) ₂ (OH) ₁₂ ][SO ₄・2H ₂ O]+12H ⁺ ...(2)
4Fe ²⁺ +2Fe ³⁺ +14H ₂ O+CO ₃ ^2- → [Fe ^(II) ₄ Fe ^(III) ₂ (OH) ₁₂ ][CO ₃・2H ₂ O]+12H ⁺ ...(3)
3Fe ²⁺ +Fe ³⁺ +8H ₂ O Cl ^- → [Fe ^(II) ₃ Fe ^(III) (OH) ₈ ][Cl・2H ₂ O]+6H ⁺ ...(4)

These layered double hydroxides can efficiently incorporate dissolved anions in the liquid between the layers of their structure through ion exchange reactions, and as a result can efficiently remove impurities from the liquid. For example, niobate ions (NbO ₃ ^- ) and titanate ions (HTiO ₃ ^- ) in the liquid are incorporated into the solid through ion exchange reactions in part of the layered double hydroxide, as shown in the following equations (5) and (6).

[Fe ^(II) ₄ Fe ^(III) ₂ (OH) ₁₂ ][SO ₄ ]・2H ₂ O+2NbO ₃ ⁻ = [Fe ^(II) ₄ Fe ^(III) ₂ (OH) ₁₂ ][(NbO ₃ ) ₂ ]・2H ₂ O+SO ₄ ² −... (5)
[Fe ^(II) ₄ Fe ^(III) ₂ (OH) ₁₂ ][SO ₄ ]・2H ₂ O+2HTiO ₃ ^- = [Fe ^(II) ₄ Fe ^(III) ₂ (OH) ₁₂ ][(HTiO ₃ ) ₂ ]・2H ₂ O+SO ₄ ² −...(6)

These layered double hydroxides themselves are also oxidized in solution to change into complex compounds such as magnetite, and in this process, impurity ions in the solution are incorporated into the crystal structure, making it possible to remove the impurity ions. These complex compounds incorporate various anion species in the solution, such as niobate ions, titanate ions, sulfide ions, tantalate ions, and molybdate ions, and therefore also have the effect of removing these impurity ions.

On the other hand, when a divalent iron compound or a trivalent iron compound is added alone, iron hydroxide (II) [Fe(OH) ₂ ] or iron hydroxide (III) [Fe(OH) ₃ ] is generated as shown in the following formulas (7) and (8). This iron hydroxide (III) [Fe(OH) ₃ ] is highly effective at adsorbing the above impurity ions, but at the same time, it is very likely to adsorb tungstate ions, so it is not possible to selectively separate tungstate ions from the above impurity ions. In addition, the amount of precipitate generated is large and the volume is likely to be bulky.

Fe ²⁺ +2OH ^- → Fe(OH) ₂ (s) ・・・(7)
Fe ³⁺ +3OH ^- → Fe(OH) ₃ (s) ・・・(8)

The generated iron hydroxide (II) [Fe(OH) ₂ ] is gradually oxidized by oxygen in the solution or oxygen supplied from the atmosphere at the solution surface, and eventually changes into a solid phase such as magnetite or green rust after a long time, but the reaction is very slow, so the oxidation reaction does not proceed sufficiently in the reaction time of a general treatment process, for example, a treatment time of about one hour, and the effect of removing impurity ions is lower than when a divalent iron compound and a trivalent iron compound are used in combination. Furthermore, since the reactivity of sulfide ions with divalent iron is low, if sulfide ions are present in the liquid, the effect of removing sulfide ions by divalent iron compounds alone is often insufficient.

As the divalent iron compound and trivalent iron compound to be added to the sodium tungstate aqueous solution, for example, iron (II) sulfate powder, iron (III) sulfate powder, iron (II) chloride powder, iron (III) chloride powder, or a solution in which these compounds are dissolved can be used. In addition, as the solution, a polyiron sulfate solution, an etching solution or its waste liquid, an acid pickling waste liquid of an iron-based material, etc. may be used. The ratio of the divalent iron compound and the trivalent iron compound to be added is preferably 10 to 90 mass% of the divalent iron compound and 10 to 90 mass% of the trivalent iron compound in terms of the mass ratio of the iron contained. It is desirable to appropriately adjust the above ratio within this range while checking the tendency of the impurity concentration in the solution. For example, as the ratio of the trivalent iron compound increases, the effect of removing sulfide ions tends to be promoted. On the other hand, as the ratio of the divalent iron compound increases, the amount of precipitate generated decreases, so that the load of solid-liquid separation and the cost of treating the precipitate can be reduced.

Divalent iron compounds and trivalent iron compounds are added, and the pH of the aqueous solution is adjusted to 9-12 to produce a complex compound precipitate of divalent iron and trivalent iron. If the pH exceeds 12, the effect of removing impurities such as niobium is likely to decrease. On the other hand, if the pH is less than 9, the tungstate ions in the solution also precipitate, resulting in a loss of tungsten recovery. A pH range of 9 to 12 is preferable. If the pH of the aqueous solution is outside the above range, an acid or alkali is added to adjust the pH to the above range, and if the pH of the aqueous solution is within the above range when the divalent iron compounds and trivalent iron compounds are added, there is no need to add an acid or alkali.

The above complex compound precipitate is sufficiently formed even at room temperature of 25°C, but the formation of the precipitate can be promoted by heating to, for example, 60°C. If the reaction temperature exceeds 70°C, the temperature burden on the equipment and facilities will increase, so the heating temperature is preferably 70°C or lower. On the other hand, if the reaction temperature is below 20°C, dissolution of the additives and formation of the precipitate will be slow, and the processing efficiency will decrease, so the solution temperature is preferably 20°C or higher.

The aqueous sodium tungstate solution to which the divalent iron compound and the trivalent iron compound are added may be in the form of a slurry in which other solid particulate matter is suspended. For example, an aqueous sodium tungstate solution obtained by reacting scrap raw materials such as cemented carbide tools and tungsten alloy products with a sodium compound may contain dissolved niobium, titanium, and other metals contained in these raw materials as oxoanions, and may also contain suspended solid particles of oxides and hydroxides of metal species such as cobalt, nickel, copper, and iron contained in these raw materials.

In addition, the aqueous sodium tungstate solution obtained by reacting a solution containing a sodium compound with a tungsten concentrate such as a battery material or electrode material discharged from a recycling process for battery materials or electrode materials, or a solid residue or sludge containing tungsten recovered after separating and removing components other than tungsten from these battery materials or electrode materials, may contain suspended solid particles of oxides or hydroxides of metal species contained in the electrode material, or solid particles of carbon contained in the electrode material. These suspended particles of metal oxides, metal hydroxides, carbon, etc. do not affect the formation of the above-mentioned complex compound precipitate or the incorporation of dissolved ions such as niobium, titanium, and sulfide ions into the precipitate, so the aqueous solution may be a slurry containing these suspended particles. It may also be an aqueous solution from which the above-mentioned suspended particles have been removed by solid-liquid separation.

The purification method of the present invention makes it difficult for tungsten to precipitate from an aqueous solution of sodium tungstate containing niobate ions, titanate ions, and sulfide ions, thereby making it difficult for recovery losses of tungsten to occur, and selectively incorporates the above impurities into a complex compound precipitate, which can be effectively removed by solid-liquid separation.

The purification method of the present invention can easily separate and remove niobium, titanium, and sulfide ions by producing a complex compound of divalent iron compounds and trivalent iron compounds. For example, in Patent Document 2, the pH is adjusted by blowing in carbon dioxide gas, the resulting precipitate is subjected to solid-liquid separation, and then an aqueous sodium tungstate solution is purified by multiple steps, such as adsorption of impurities by ion exchange resin and elution of tungsten from the ion exchange resin. On the other hand, the purification method of the present invention completes the purification process only by adding divalent iron compounds and trivalent iron compounds, adjusting the pH, and performing solid-liquid separation of the resulting precipitate, so that an aqueous sodium tungstate solution with a significantly lower amount of impurities can be obtained quickly and at low cost.

The method of Patent Document 3 uses a divalent iron compound alone, so it is less effective at removing niobate ions, titanate ions, and sulfide ions, but the purification method of the present invention produces a complex compound of a divalent iron compound and a trivalent iron compound, so it is more effective at removing niobate ions, titanate ions, and sulfide ions contained in the sodium tungstate aqueous solution.

In addition, in the method using only trivalent iron compounds, a large amount of precipitate is generated, which is bulky and places a heavy burden on the treatment of the precipitate. Tungsten is also mixed into the precipitate, which is likely to result in a loss of tungsten recovery. On the other hand, the refining method of the present invention produces a composite compound of divalent iron compounds and trivalent iron compounds, which produces a smaller amount of precipitate than when trivalent iron compounds are used alone, reducing treatment costs and increasing the recovery rate of tungsten.

The purified aqueous sodium tungstate solution is further subjected to advanced purification processes such as ion exchange and solvent extraction. If the impurities in the aqueous solution are not sufficiently removed during this process, the ion exchange resin and organic solvent may be contaminated or deteriorated by the impurities. On the other hand, the aqueous sodium tungstate solution purified by the method of the present invention contains significantly fewer impurities, so the risk of such contamination and deterioration can be greatly reduced, and it is possible to protect the purification equipment and reduce running costs.

Examples of the present invention are shown below. The concentrations of various elements in the solution were measured by ICP-AES. The tungsten residual rate (W residual rate) was shown as an evaluation index of tungsten recovery loss. The tungsten residual rate was calculated based on the following formula.
W residual rate [%] = _WO3 concentration in recovered liquid [g/L] / _WO3 concentration before treatment [g/L] × 100

Example 1
To an aqueous solution of sodium tungstate (WO ₃ : 90 g/L) containing 7250 mg/L of sulfide ions (S ^2- ), 110 mg/L of titanium, 64 mg/L of niobium, and 12 g/L of solid particles mainly composed of cobalt hydroxide, iron (II) sulfate powder and iron (III) sulfate powder were added so that the iron (II) concentration in the aqueous solution [Fe(II)] became 2.2 g/L and the iron (III) concentration [Fe(III)] became 2.2 g/L, and the solution was stirred for 1 hour at room temperature of 25°C, after which the pH was measured (pH 9.6) to produce a precipitate. This was filtered to obtain a recovered liquid and a dehydrated residue (Sample No. A1).
In addition, the same treatment as for sample No. 1 was carried out except that the liquid temperature during stirring for 1 hour was heated to 60° C., the pH was measured (pH 9.4), and a precipitate was formed. This was filtered to obtain a recovered liquid and a dehydrated residue (sample No. A2).
The _WO3 concentration, ^S2- concentration, titanium and niobium concentrations of these recovered solutions were measured. The surface of the dehydrated residue was thoroughly washed with pure water, and then dried at 105°C for 24 hours to recover the residue, which was then weighed. The results are shown in Table 1.

Comparative Example 1
A sodium tungstate aqueous solution similar to that in Example 1 was treated in the same manner as in Example 1 except that iron (II) sulfate powder was added so that the iron (II) concentration in the aqueous solution was 2.2 g/L or 4.4 g/L, or iron (III) sulfate powder was added so that the iron (III) concentration in the aqueous solution was 2.2 g/L or 4.4 g/L, the treatment temperature was set to 25°C or 60°C, and the pH was set to 8.0 to 12.0 using an aqueous sodium hydroxide solution or sulfuric acid, to generate a precipitate, which was then filtered to obtain a recovered solution and a dehydrated residue. The _WO3 concentration, ^S2- concentration, titanium and niobium concentrations of these recovered solutions were measured in the same manner as in Example 1, and the weight of the treated residue was measured. The results are shown in Table 1 (Comparative Samples No. B1 to B10).

As shown in Table 1, in both samples A1 and A2 of the present invention, the sulfide ion concentration, titanium concentration, and niobium concentration of the recovered liquid (purified liquid) are sufficiently reduced. In addition, the decrease in the _WO3 concentration in the recovered liquid is small, and the residual rate of tungsten is 96% or more.
On the other hand, in the comparative samples B1 to B8, only one of iron sulfate (II) or iron sulfate (III) is added, so the sulfide ion concentration, titanium concentration, and niobium concentration in the recovery solution are high, and the removal of these is insufficient. In addition, in the samples B4 and B8 in which iron sulfate (III) is added alone at an amount of 4.4 g/L, the tungsten residual rate drops to 91%, the tungsten recovery loss is large, and the residue amount is 34 g/L and 37 g/L, respectively, which is larger than when the same amount of Fe (II) and Fe (III) are added together. Furthermore, in the comparative samples B9 and B10 with a pH of 8.0, impurities are sufficiently removed, but the WO ₃ concentration in the recovery solution is significantly reduced, and the tungsten recovery loss is significant. As shown in Table 1, it was confirmed that by using a divalent iron compound and a trivalent iron compound in combination and forming a composite compound precipitate at a liquid pH of 9 to 10, an aqueous sodium tungstate solution having a low impurity concentration can be recovered while suppressing the recovery loss of tungsten.

Example 2
For a sodium tungstate aqueous solution (WO ₃ : 270 g/L) containing 8690 mg/L of sulfide ions (S ^2- ), 160 mg/L of titanium, 126 mg/L of niobium, and 34 g/L of solid particles mainly composed of cobalt hydroxide suspended therein, iron sulfate (II) powder and iron sulfate (III) powder were added so that the [Fe (II) concentration] and [Fe (III) concentration] in the aqueous solution were in the ratios shown in Table 2, and the pH of the aqueous solution was adjusted to 12.0 using an aqueous sodium hydroxide solution or sulfuric acid, and the solution was kept at room temperature of 25°C and stirred and mixed for 1 hour. The rest of the procedure was the same as in Example 1. The results are shown in Table 2. The addition ratios of Fe (II) and Fe (III) are as follows:
Fe(II): 0.44 g/L and Fe(III): 1.76 g/L → Fe(II): 20%, Fe(III): 80%
Fe(II): 0.73 g/L and Fe(III): 1.47 g/L → Fe(II): 33%, Fe(III): 67%
Fe(II): 1.10 g/L and Fe(III): 1.10 g/L → Fe(II): 50%, Fe(III): 50%
Fe(II): 1.47 g/L and Fe(III): 0.73 g/L → Fe(II): 67%, Fe(III): 33%
Fe(II): 1.76 g/L and Fe(III): 0.44 g/L → Fe(II): 80%, Fe(III): 20%

Comparative Example 2
The same treatment as in Example 2 was carried out except that the pH of the above aqueous solution was adjusted to 13.0. The results are shown in Table 2.
As shown in Table 2, in the samples A10 to A14 of the present invention, the sulfide ion concentration in the recovered solution was reduced to 1 mg/L or less, the titanium concentration was reduced to 8 mg/L or less, and the niobium concentration was reduced to 29 mg/L or less, and impurities were sufficiently removed. In addition, the _WO3 concentration in the recovered solution was 266 g/L or more, and there was almost no recovery loss of tungsten. On the other hand, in the comparative samples B10 to B14, the _WO3 concentration in the recovered solution was 267 g/L or more, and there was almost no recovery loss of tungsten, but large amounts of sulfide ions, titanium, and niobium remained in the recovered solution, and the removal of impurities was insufficient.
Thus, from the results in Table 2, when both a divalent iron compound and a trivalent iron compound are added, it is preferable to adjust the pH to 12.0 or less.

Example 3
The treatment was carried out in the same manner as in Example 2, except that the reaction temperature was raised to 60° C. The results are shown in Table 3.
As shown in Table 3, in the samples A20 to A24 of the present invention, the sulfide ion concentration in the recovered solution was reduced to 15 mg/L or less, the titanium concentration was reduced to 6 mg/L or less, and the niobium concentration was reduced to 25 mg/L or less, and the titanium and niobium concentrations were lower than those in Example 2. In addition, the _WO3 concentration in the recovered solution was 266 g/L or more, and there was almost no loss in the recovery of tungsten. It can be confirmed that the removal effect of titanium and niobium is higher when the treatment temperature is 60°C.

Example 4
The treatment was carried out in the same manner as in Example 2, except that the reaction temperature was raised to 60° C. and the pH was set to 9.0, 10.0, and 11.0. The results are shown in Table 4.
As shown in Table 4, in the samples A30 to A35 of the present invention, the sulfide ion concentration in the recovered solution was reduced to 9 mg/L or less, the titanium concentration was reduced to 6 mg/L or less, and the niobium concentration was reduced to 24 mg/L or less. In addition, the _WO3 concentration in the recovered solution was 265 g/L or more, and there was almost no recovery loss of tungsten.
From the results of Tables 3 and 4, it was confirmed that an aqueous sodium tungstate solution with a significantly low impurity concentration can be recovered by adding both a divalent iron compound and a trivalent iron compound, adjusting the pH to 9.0 to 12.0, and reacting under heating.

Claims (4)

A purification method for removing impurity ions from an aqueous sodium tungstate solution containing at least one of niobium, titanium, and sulfide ions as impurity ions, the method comprising adding a divalent iron compound and a trivalent iron compound to the aqueous sodium tungstate solution, adjusting the pH of the aqueous solution to 9 to 12 to form a precipitate, and filtering out the precipitate to remove the impurity ions. The method for purifying an aqueous sodium tungstate solution according to claim 1, wherein the ratio of the amount of divalent iron compound [Fe(II)] and the amount of trivalent iron compound [Fe(III)] added is 10-90% by mass of Fe(II) and 10-90% by mass of Fe(III), in terms of the mass ratio of the amount of iron contained. The method for purifying an aqueous sodium tungstate solution according to claim 1 or 2, wherein the aqueous sodium tungstate solution containing the impurity ions is an aqueous sodium tungstate solution obtained by dissolving in water a heat-treated product produced by heating tungsten-containing scrap together with a sodium salt. 3. The method for purifying a sodium tungstate aqueous solution according to claim 1 or 2, wherein the sodium tungstate aqueous solution containing impurity ions is an aqueous sodium tungstate aqueous solution obtained by dissolving tungsten-containing electrode material scrap or a tungsten concentrate obtained by separating components other than tungsten from tungsten-containing electrode material scrap, in an aqueous solution containing a sodium compound.