A process for preparing high-purity sodium tungstate by deep impurity removal of a high-impurity sodium tungstate solution

Abstract

The application discloses a process for preparing high-purity sodium tungstate by deeply removing impurities from a high-impurity sodium tungstate solution, and belongs to the technical field of tungsten hydrometallurgy. The process comprises the following steps: pre-oxidizing a crude sodium tungstate solution, adjusting the pH of the solution to 9-10, adding magnesium salt and aluminum salt to perform first precipitation and solid-liquid separation, and obtaining a first purified solution; adjusting the pH of the first purified solution to 7-8, adding a complexing agent to perform complexing purification and solid-liquid separation, and obtaining a purified solution from which impurities are removed; selectively adsorbing and removing impurities from the purified solution from which impurities are removed to obtain a deeply purified solution, then performing concentration through a nanofiltration membrane system to obtain a sodium tungstate concentrated solution; and finally performing crystallization, solid-liquid separation and drying to obtain a high-purity sodium tungstate product. Through the multiple steps of pre-oxidation, gradient-pH step-by-step precipitation for removing impurities, selective adsorption, membrane concentration, crystallization and drying, one-time deep purification of the high-impurity sodium tungstate solution is realized, the product has high purity, high tungsten recovery rate and low energy consumption, and is suitable for industrial application.

Description

Technical Field

[0001] This invention relates to the field of tungsten hydrometallurgy, and in particular to a process for deep purification of high-purity sodium tungstate solution to prepare high-purity sodium tungstate. Background Technology

[0002] Sodium tungstate is an important intermediate in the preparation of ammonium paratungstate (APT), metallic tungsten powder, and tungsten chemicals. Currently, the industrial process mainly uses alkaline pressure leaching to process tungsten concentrate or waste tungsten materials to obtain crude sodium tungstate solution. This solution typically contains high concentrations of impurities such as phosphorus (P), arsenic (As), silicon (Si), molybdenum (Mo), and tin (Sn). The presence of these impurities not only affects the quality of subsequent APT products (e.g., leading to the formation of yellow or blue tungsten), but also forms eutectics or inclusions during the crystallization process, reducing the purity of the sodium tungstate product.

[0003] In existing technologies, the classic magnesium salt precipitation method is the main means of removing P, As, and Si. However, this method usually requires high-temperature boiling (>90℃), which consumes a lot of energy. In addition, for high-concentration impurity solutions, it is easy to form colloidal precipitates, which makes filtration difficult and results in serious tungsten entrainment loss (often up to 3-6%). Although the ion exchange method can effectively transform and remove impurities, it is only suitable for low-concentration solutions (WO3<25g / L) and generates a large amount of high-salt wastewater, making it impossible to directly prepare high-purity sodium tungstate products. The solvent extraction method is carried out under acidic conditions, which requires the sodium tungstate solution to be acidified and transformed first. The process is long and the selective separation effect on molybdenum and tin is limited.

[0004] For solutions with high impurities, especially leaching solutions from waste cemented carbide recycling or low-grade ores, existing technologies often struggle to remove impurities to below the ppm level. While the one-step magnesium-aluminum salt precipitation method disclosed in Chinese patent CN1029678C can remove the main impurities such as phosphorus, arsenic, and silicon, this method operates under a single pH condition. The hydrolysis and precipitation behaviors of different impurity ions compete, leading to incomplete impurity removal, particularly for solutions with high impurity concentrations. Furthermore, it cannot effectively remove impurities such as molybdenum and tin, which have similar chemical properties to tungsten, limiting the purity of the resulting product. Although Chinese patent CN1276104C introduces pre-oxidation and iron salt adsorption, this method requires high-temperature boiling for magnesium salt precipitation, resulting in high energy consumption. The introduction of iron salts also increases the burden of subsequent iron removal, making the process relatively complex. Chinese patent CN111424170B uses advanced oxidation combined with heated magnesium salt precipitation to treat sodium tungstate solutions, but its final target product is ammonium paratungstate, indicating a different process path. It also does not involve the selective deep removal of specific impurities such as molybdenum and tin.

[0005] Therefore, there is a lack of existing technologies that can simultaneously achieve deep impurity removal, high tungsten recovery rate, low energy consumption, and a compact process for purifying high-impurity sodium tungstate solutions. In particular, existing technologies have failed to provide an effective solution for the technical requirement of simultaneously removing multiple impurities to the ppm level at high impurity concentrations. Summary of the Invention

[0006] To address the problems of incomplete impurity removal, high tungsten loss rate, inability to selectively remove molybdenum and tin impurities, and high energy consumption in existing technologies, this invention proposes a process for deep purification of high-impurity sodium tungstate solutions to prepare high-purity sodium tungstate. First, a pre-oxidation treatment converts difficult-to-precipitate trivalent arsenic into easily precipitated pentavalent arsenic, laying the foundation for subsequent chemical precipitation to remove arsenic. Then, a gradient pH stepwise precipitation strategy is employed. At pH 9-10, magnesium and aluminum salts are used to precipitate and remove phosphorus, arsenic, and silicon, the main impurities. Next, at pH 7-8, a complexing agent is used to complex and mask residual cations, preventing them from forming eutectic during subsequent crystallization. Finally, modified silica gel loaded with specific functional groups is used to selectively adsorb and remove molybdenum and tin, which have similar chemical properties to tungsten, through coordination chelation. These steps work together synergistically to achieve one-time deep purification of high-impurity sodium tungstate solutions in a single process, obtaining high-purity sodium tungstate products with WO3 purity ≥99.9% and key impurities below 10 ppm, while maintaining a tungsten direct recovery rate ≥99.2%.

[0007] The technical solution of the present invention is as follows: This invention provides a process for deep purification of high-purity sodium tungstate solution to prepare high-purity sodium tungstate, comprising the following steps: S1. Pre-oxidation treatment: An oxidant is added to the crude sodium tungstate solution and mixed to react, oxidizing the trivalent arsenic in the solution to pentavalent arsenic; S2, Gradient pH Stepwise Precipitation for Impurity Removal: First, adjust the pH of the solution treated in step S1 to 9-10, add magnesium salt and aluminum salt for the first precipitation and solid-liquid separation to obtain a first purified solution; then adjust the pH of the first purified solution to 7-8, add a complexing agent for complexation purification and solid-liquid separation to obtain a purified solution with impurities removed. S3. Selective adsorption: The purified solution is passed through an adsorption column filled with modified silica gel adsorbent to selectively adsorb and remove molybdenum and / or tin impurities from the solution, thereby obtaining a deeply purified solution. S4. Membrane concentration: The deep purified liquid is concentrated through a nanofiltration membrane system to obtain sodium tungstate concentrate; S5. Crystallization and drying: The sodium tungstate concentrate is crystallized, separated from its solid state, and dried to obtain a high-purity sodium tungstate product.

[0008] Preferably, in step S1, the oxidant is selected from at least one of hydrogen peroxide and sodium hypochlorite solution; the amount of oxidant added is 3-18 times the amount of oxidant theoretically required to oxidize all trivalent arsenic in crude sodium tungstate solution to pentavalent arsenic; and the reaction temperature is 50-70°C.

[0009] Preferably, the concentration of the hydrogen peroxide is 28-32 wt%.

[0010] Preferably, the effective chlorine content of the sodium hypochlorite solution is 5-15 wt%.

[0011] As a further description of the present invention, arsenic in sodium tungstate solution is typically expressed as As 3+ And As 5+ Two valence states coexist. As 3 + Magnesium salt precipitates (such as Mg3(AsO3)2) have a much higher solubility than As. 5+ Magnesium salt precipitates (Mg3(AsO4)2), and precipitation is incomplete under alkaline conditions, easily leading to arsenic residue. In step S1 above, the crude sodium tungstate solution undergoes pre-oxidation treatment to remove As... 3+ Convert all to As 5+ This ensures that the gradient pH stepwise precipitation and impurity removal in the subsequent S2 step achieves a high level of arsenic removal efficiency, avoiding product defects caused by arsenic residue.

[0012] Preferably, in step S2, the magnesium salt is selected from at least one of magnesium chloride and magnesium sulfate, and the aluminum salt is selected from at least one of aluminum sulfate and aluminum chloride; the amount of magnesium salt added is based on a Mg / (P+As) molar ratio of (1-1.5):1, and the amount of aluminum salt added is based on an Al / Si molar ratio of (0.2-1.2):1.

[0013] Preferably, in step S2, the complexing agent is selected from at least one of disodium ethylenediaminetetraacetate and aminotrimethylenephosphonic acid, and the amount of complexing agent added is 0.1-0.3g of complexing agent per 1 liter of crude sodium tungstate solution.

[0014] Preferably, sulfuric acid or hydrochloric acid is used to adjust the pH of the solution in step S2.

[0015] More preferably, in step S2, a 10-20 wt% dilute sulfuric acid solution is used to adjust the pH of the solution treated in step S1 to 9-10; and a 5-10 wt% dilute sulfuric acid solution is used to adjust the pH of the primary purification solution to 7-8.

[0016] More preferably, S2 is: Gradient pH stepwise precipitation for impurity removal: First, adjust the pH of the solution treated in step S1 to 9-10, add magnesium salt and aluminum salt, mix and stir for 50-70 minutes, and then filter to obtain a primary purified solution; then adjust the pH of the primary purified solution to 7-8, add a complexing agent, mix and stir for 25-35 minutes, and then filter to obtain a purified solution with impurities removed.

[0017] Further explanation of the present invention: The above-mentioned S2 process involves gradient pH stepwise precipitation to remove impurities. Utilizing the differences in ion hydrolysis precipitation characteristics, adsorption co-precipitation, and complexation masking effects under different pH conditions, various impurities are removed in stages and directionally. The first precipitation is carried out at pH 9-10, where magnesium and aluminum ions are fully hydrolyzed to generate Mg(OH)2 and Al(OH)3 colloids. On one hand, this chemical precipitation generates sparingly soluble salts such as magnesium ammonium phosphate and magnesium arsenate to remove the main phosphorus and arsenic impurities. On the other hand, the adsorption co-precipitation of Al(OH)3 colloids captures silicate ions and suspended particles, achieving significant removal of the main impurities. Subsequently, the pH is lowered to 7-8, and a complexing agent is added for complexation purification. At this point, the complexing agent is in its optimal complexing activity range, capable of forming stable water-soluble complexes with the residual trace amounts of calcium, magnesium, iron, and aluminum cations after the first precipitation. This prevents them from re-hydrolyzing and precipitating during subsequent concentration and crystallization, and from forming eutectic or inclusion bodies with sodium tungstate, thereby further improving product purity. This invention employs gradient pH stepwise precipitation to remove impurities after solution pre-oxidation treatment, avoiding incomplete removal caused by precipitation competition at a single pH level, and achieving phased and targeted removal of the main impurities through chemical precipitation and complexation masking of residual ions.

[0018] Preferably, in step S3, the modified silica gel adsorbent is an adsorbent with phenylphosphonic acid or salicylaldehyde oxime functional groups loaded on a silica gel carrier.

[0019] Preferably, the adsorption column operating temperature in S3 is 25-40℃, and the empty column flow rate is 1-2 BV / h.

[0020] Preferably, the adsorption column has an inner diameter of 40-60 mm and a column height of 400-600 mm.

[0021] As a further explanation of the present invention, molybdenum in alkaline solution exists as molybdate (MoO4). 2- It exists in the form of ) and its chemical properties are similar to those of tungstate (WO4). 2- Tin is extremely similar to magnesium and aluminum salts, and its selective separation is difficult using precipitation methods and complexing agents. In alkaline solutions, tin partially exists as stannate (Sn(OH)6). 2-Tungsten dissolves in a form that is difficult to completely remove even by precipitation. The modified silica gel adsorbent used in S3 above has phenylphosphonic acid or salicylaldehyde oxime functional groups grafted onto its surface. These groups have specific coordination chelation and ion exchange selectivity for molybdate and stannate, but have extremely weak adsorption affinity for tungstate, thus achieving highly selective deep removal without affecting the tungsten recovery rate.

[0022] Preferably, the operating pressure of the nanofiltration membrane system in S4 is 1.0-2.0 MPa, the molecular weight cutoff of the nanofiltration membrane is 200-400 Da, and the concentration factor is 3-5 times.

[0023] Preferably, the vacuum degree of crystallization in S5 is 0.08-0.095 MPa, and the crystallization temperature is 60-75℃.

[0024] Preferably, the drying temperature in step S5 is 100-110℃ and the drying time is 2-4 hours.

[0025] Preferably, the crude sodium tungstate solution contains WO3 concentration of 100-200 g / L, total impurity content of P, As, and Si of 1-15 g / L, Mo content of 0.1-1 g / L, and Sn content of 0.15-0.5 g / L.

[0026] Preferably, the crude sodium tungstate solution contains As 3+ The content accounts for 35-50% of the total As content.

[0027] Preferably, the high-purity sodium tungstate product has a WO3 purity ≥ 99.90%, P, As, and Si contents ≤ 10 ppm, and Mo content ≤ 5 ppm.

[0028] The present invention has the following beneficial effects: 1. Compared with the prior art, the present invention achieves simultaneous and deep removal of multiple impurities in high-impurity sodium tungstate solution through a multi-stage synergistic process of pre-oxidation, gradient pH stepwise precipitation and selective adsorption, while maintaining an extremely high tungsten recovery rate. Specifically, the pre-oxidation step oxidizes trivalent arsenic to pentavalent arsenic, creating the necessary conditions for subsequent magnesium salt precipitation to remove arsenic, overcoming the problem of incomplete arsenic removal caused by the deviation of the pre-oxidation objective in existing technologies. Gradient pH stepwise precipitation first removes phosphorus, arsenic, and silicon as the main impurities by precipitating with magnesium and aluminum salts at pH 9-10, and then uses a complexing agent to mask residual cations at pH 7-8, avoiding the competitive reactions and limited complexing capacity problems caused by adding all reagents at once at a single pH, significantly improving precipitation and purification efficiency. The modified silica gel selective adsorption utilizes the coordination chelation between the loaded phenylphosphonic acid or salicylaldehyde oxime functional groups and molybdenum and tin ions, achieving selective and deep removal of molybdenum and tin impurities with chemical properties very similar to tungsten, while almost completely avoiding tungsten adsorption, thus avoiding tungsten loss caused by the large amount of tungsten-containing waste residue generated in traditional sulfide precipitation methods. The above steps work together synergistically, ensuring that the content of key impurities in the final product is below ppm, the product purity meets the requirements of high-end tungsten chemical raw materials, and the direct tungsten recovery rate is significantly higher than existing technologies.

[0029] 2. This invention employs a nanofiltration membrane system for pre-concentration of the deeply purified liquid at room temperature, replacing the traditional multi-effect evaporation process and significantly reducing energy consumption in subsequent crystallization steps. Simultaneously, the nanofiltration membrane's retention effect further removes residual small-molecule organic impurities, improving the overall efficiency of the process. This invention performs vacuum evaporation crystallization under negative pressure, lowering the crystallization temperature and effectively avoiding eutectic and dissolution of impurities at high temperatures. The resulting sodium tungstate product is a high-purity, white, flaky crystal with stable quality. Compared to existing purification methods that are lengthy, energy-intensive, or produce insufficient product purity, this invention's process is highly efficient and energy-saving, with mild operating conditions. It achieves deep impurity removal while also considering economic and environmental benefits, demonstrating promising prospects for industrial application. Attached Figure Description

[0030] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the structures shown in these drawings without creative effort.

[0031] Figure 1 The process flow diagram for preparing high-purity sodium tungstate by deep impurity removal from high-impurity sodium tungstate solution according to the present invention is shown.

[0032] The realization of the objective, functional features and advantages of the present invention will be further explained in conjunction with the embodiments and with reference to the accompanying drawings. Detailed Implementation

[0033] The technical solutions described below in conjunction with the embodiments will be clearly and completely described. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0034] This invention provides a process for deep purification of high-purity sodium tungstate solution to prepare high-purity sodium tungstate, comprising the following steps: S1. Pre-oxidation treatment: An oxidant is added to the crude sodium tungstate solution and mixed to react, oxidizing the trivalent arsenic in the solution to pentavalent arsenic; S2, Gradient pH Stepwise Precipitation for Impurity Removal: First, adjust the pH of the solution treated in step S1 to 9-10, add magnesium salt and aluminum salt for the first precipitation and solid-liquid separation to obtain a first purified solution; then adjust the pH of the first purified solution to 7-8, add a complexing agent for complexation purification and solid-liquid separation to obtain a purified solution with impurities removed. S3. Selective adsorption: The purified solution is passed through an adsorption column filled with modified silica gel adsorbent to selectively adsorb and remove molybdenum and / or tin impurities from the solution, thereby obtaining a deeply purified solution. S4. Membrane concentration: The deep purified liquid is concentrated through a nanofiltration membrane system to obtain sodium tungstate concentrate; S5. Crystallization and drying: The sodium tungstate concentrate is crystallized, separated from its solid state, and dried to obtain a high-purity sodium tungstate product.

[0035] Preferably, in step S1, the oxidant is selected from at least one of hydrogen peroxide and sodium hypochlorite solution; the amount of oxidant added is 3-18 times the amount of oxidant theoretically required to oxidize all trivalent arsenic in crude sodium tungstate solution to pentavalent arsenic; and the reaction temperature is 50-70°C.

[0036] Preferably, the concentration of the hydrogen peroxide is 28-32 wt%.

[0037] Preferably, the effective chlorine content of the sodium hypochlorite solution is 5-15 wt%.

[0038] Preferably, in step S2, the magnesium salt is selected from at least one of magnesium chloride and magnesium sulfate, and the aluminum salt is selected from at least one of aluminum sulfate and aluminum chloride; the amount of magnesium salt added is based on a Mg / (P+As) molar ratio of (1-1.5):1, and the amount of aluminum salt added is based on an Al / Si molar ratio of (0.2-1.2):1.

[0039] Preferably, in step S2, the complexing agent is selected from at least one of disodium ethylenediaminetetraacetate and aminotrimethylenephosphonic acid, and the amount of complexing agent added is 0.1-0.3g of complexing agent per 1 liter of crude sodium tungstate solution.

[0040] Preferably, sulfuric acid or hydrochloric acid is used to adjust the pH of the solution in step S2.

[0041] More preferably, in step S2, a 10-20 wt% dilute sulfuric acid solution is used to adjust the pH of the solution treated in step S1 to 9-10; and a 5-10 wt% dilute sulfuric acid solution is used to adjust the pH of the primary purification solution to 7-8.

[0042] More preferably, S2 is: Gradient pH stepwise precipitation for impurity removal: First, adjust the pH of the solution treated in step S1 to 9-10, add magnesium salt and aluminum salt, mix and stir for 50-70 minutes, and then filter to obtain a primary purified solution; then adjust the pH of the primary purified solution to 7-8, add a complexing agent, mix and stir for 25-35 minutes, and then filter to obtain a purified solution with impurities removed.

[0043] Preferably, in step S3, the modified silica gel adsorbent is an adsorbent with phenylphosphonic acid or salicylaldehyde oxime functional groups loaded on a silica gel carrier.

[0044] Preferably, the adsorption column operating temperature in S3 is 25-40℃, and the empty column flow rate is 1-2 BV / h.

[0045] Preferably, the adsorption column has an inner diameter of 40-60 mm and a column height of 400-600 mm.

[0046] Preferably, the operating pressure of the nanofiltration membrane system in S4 is 1.0-2.0 MPa, the molecular weight cutoff of the nanofiltration membrane is 200-400 Da, and the concentration factor is 3-5 times.

[0047] Preferably, the vacuum degree of crystallization in S5 is 0.08-0.095 MPa, and the crystallization temperature is 60-75℃.

[0048] Preferably, the drying temperature in step S5 is 100-110℃ and the drying time is 2-4 hours.

[0049] Preferably, the crude sodium tungstate solution contains WO3 concentration of 100-200 g / L, total impurity content of P, As, and Si of 1-15 g / L, Mo content of 0.1-1 g / L, and Sn content of 0.15-0.5 g / L.

[0050] Preferably, the crude sodium tungstate solution contains As 3+ The content accounts for 35-50% of the total As content.

[0051] Preferably, the high-purity sodium tungstate product has a WO3 purity ≥ 99.90%, P, As, and Si contents ≤ 10 ppm, and Mo content ≤ 5 ppm.

[0052] It should be noted that the magnesium salts and aluminum salts described in this invention include their anhydrous forms and various hydrate forms. The crystalline hydrates used in the examples are merely illustrative and do not constitute a limitation on the scope of protection.

[0053] The technical solution of the present invention will be further described below with reference to specific embodiments.

[0054] Example 1

[0055] A process for deep impurity removal from high-impurity sodium tungstate solution to prepare high-purity sodium tungstate includes the following steps: S1. Pre-oxidation treatment: Take 1000 mL of crude sodium tungstate solution obtained from alkaline leaching of waste cemented carbide. Its composition is: WO3 150 g / L, P 2.5 g / L, As 1.8 g / L, Si 3.0 g / L, Mo 0.5 g / L, Sn 0.2 g / L, of which As in the crude sodium tungstate solution... 3+ The content accounts for 40% of the total As content; the above crude sodium tungstate solution is heated to 60°C, and 15 mL of 30 wt% hydrogen peroxide is added and mixed for 40 min to fully oxidize the trivalent arsenic in the solution to pentavalent arsenic; S2. Gradient pH stepwise precipitation for impurity removal: Adjust the pH of the pre-oxidized solution to 9.5 with 10wt% dilute sulfuric acid, add 25g MgCl2·6H2O and 10g Al2(SO4)3·18H2O, stir for 60min and filter to obtain the first purified solution; then adjust the pH of the first purified solution to 7.5 with 10wt% dilute sulfuric acid, add 0.2g disodium ethylenediaminetetraacetate, stir for 30min and filter to obtain the purified solution after impurity removal; S3. Selective adsorption: The purified solution is passed through an adsorption column (Φ50×500 mm) filled with phenylphosphonic acid modified silica gel adsorbent at a flow rate of 1.5 BV / h. The operating temperature is controlled at 30℃. Molybdenum and tin impurities in the solution are selectively adsorbed and removed to obtain a deeply purified solution. S4. Membrane Concentration: The deep-purified liquid is treated by a nanofiltration membrane system with a molecular weight cutoff of 200 Da and an operating pressure of 1.5 MPa, and concentrated to 1 / 4 of the original volume to obtain sodium tungstate concentrate. S5. Crystallization and drying: The sodium tungstate concentrate is vacuum evaporated and crystallized at a vacuum degree of 0.09 MPa and a temperature of 65°C. After centrifugation, the crystals are collected and dried at 105°C for 3 hours to obtain high-purity sodium tungstate product.

[0056] The high-purity sodium tungstate product obtained in this embodiment was tested: The high-purity sodium tungstate product obtained in this example was analyzed for trace elements using inductively coupled plasma optical emission spectrometry (ICP-OES, Agilent 5800). 1.0000 g of the product was dissolved in ultrapure water and diluted to 100 mL. The impurity element content was then tested. The impurity content (ppm) in the product was calculated using the following formula: Impurity content (ppm) = [Test concentration (mg / L) × Volume (L)] / Sample weight (g) × 1000 The results of impurity content testing are shown in Table 1 below: Table 1

[0057] The high-purity sodium tungstate product obtained in this embodiment was tested and found to be a white flaky crystal. The purity of WO3 was determined to be 99.92% by the quinoline tungstate gravimetric method, with P content of 8.6 ppm, As content of 7.2 ppm, Si content of 9.5 ppm, Mo content of 3.8 ppm, and Sn not detected. The direct tungsten recovery rate was 99.3%.

[0058] In this embodiment, the contents of key impurities P, As, Si, Mo, and Sn in the high-purity sodium tungstate product are all below 10 ppm, with Mo < 5 ppm and Sn not detected, thus meeting the standard for high-purity sodium tungstate.

[0059] Example 2

[0060] A process for deep impurity removal from high-impurity sodium tungstate solution to prepare high-purity sodium tungstate includes the following steps: S1. Pre-oxidation treatment: Take 1000 mL of crude sodium tungstate solution obtained from alkaline leaching of waste cemented carbide. Its composition is: WO3 100 g / L, P 1.0 g / L, As 1.2 g / L, Si 2.5 g / L, Mo 0.1 g / L, Sn 0.15 g / L, wherein the crude sodium tungstate solution contains As... 3+ The content accounts for 35% of the total As content; the above crude sodium tungstate solution is heated to 50°C, and 10 mL of 30 wt% hydrogen peroxide is added and mixed for 35 min to fully oxidize the trivalent arsenic in the solution to pentavalent arsenic; S2, Gradient pH Stepwise Precipitation for Impurity Removal: Adjust the pH of the pre-oxidized solution to 9 with 10wt% dilute sulfuric acid, add 12g MgCl2·6H2O and 6g Al2(SO4)3·18H2O, stir for 50min and filter to obtain the first purified solution; then adjust the pH of the first purified solution to 7 with 10wt% dilute sulfuric acid, add 0.1g disodium ethylenediaminetetraacetate, stir for 25min and filter to obtain the purified solution with impurities removed; S3. Selective adsorption: The purified solution is passed through an adsorption column (Φ40×400 mm) filled with phenylphosphonic acid modified silica gel adsorbent at a flow rate of 1.0 BV / h. The operating temperature is controlled at 25℃. Molybdenum and tin impurities in the solution are selectively adsorbed and removed to obtain a deeply purified solution. S4. Membrane Concentration: The deep-purified liquid is treated by a nanofiltration membrane system with a molecular weight cutoff of 200 Da and an operating pressure of 1.0 MPa, and concentrated to 1 / 3 of the original volume to obtain sodium tungstate concentrate. S5. Crystallization and drying: The sodium tungstate concentrate is vacuum evaporated and crystallized at a vacuum degree of 0.08 MPa and a temperature of 60℃. After centrifugation, the crystals are collected and dried at 105℃ for 3 hours to obtain high-purity sodium tungstate product.

[0061] According to the detection method in Example 1, the high-purity sodium tungstate product obtained in this example is a white flaky crystal with a WO3 purity of 99.90%, a P content of 8 ppm, an As content of 7 ppm, a Si content of 9 ppm, a Mo content of 4 ppm, and no Sn detected; the tungsten direct recovery rate is 99.5%.

[0062] Example 3

[0063] A process for deep impurity removal from high-impurity sodium tungstate solution to prepare high-purity sodium tungstate includes the following steps: S1. Pre-oxidation treatment: Take 1000 mL of crude sodium tungstate solution obtained from alkaline leaching of waste cemented carbide. Its composition is: WO3 200 g / L, P 5.0 g / L, As 4.5 g / L, Si 5.0 g / L, Mo 1.0 g / L, Sn 0.5 g / L, where the crude sodium tungstate solution contains As... 3+ The content accounts for 50% of the total As content; the above crude sodium tungstate solution is heated to 70°C, and 80 mL of sodium hypochlorite solution with an effective chlorine content of 10 wt% is added and mixed and reacted for 50 min to fully oxidize the trivalent arsenic in the solution to pentavalent arsenic; S2, Gradient pH Stepwise Precipitation for Impurity Removal: Adjust the pH of the pre-oxidized solution to 10 with 10wt% dilute sulfuric acid, add 55g MgCl2·6H2O and 15g AlCl3, stir for 70min and filter to obtain the first purified solution; then adjust the pH of the first purified solution to 8 with 10wt% dilute sulfuric acid, add 0.3g aminotrimethylenephosphonic acid, stir for 35min and filter to obtain the purified solution with impurities removed; S3. Selective adsorption: The purified solution is passed through an adsorption column (Φ60×600 mm) filled with phenylphosphonic acid modified silica gel adsorbent at a flow rate of 2.0 BV / h. The operating temperature is controlled at 40℃. Molybdenum and tin impurities in the solution are selectively adsorbed and removed to obtain a deeply purified solution. S4. Membrane Concentration: The deep-purified liquid is treated by a nanofiltration membrane system with a molecular weight cutoff of 400 Da and an operating pressure of 2.0 MPa, and concentrated to 1 / 5 of the original volume to obtain sodium tungstate concentrate. S5. Crystallization and drying: The sodium tungstate concentrate is vacuum evaporated and crystallized at a vacuum degree of 0.095 MPa and a temperature of 75°C. After centrifugation, the crystals are collected and dried at 105°C for 3 hours to obtain high-purity sodium tungstate product.

[0064] According to the detection method in Example 1, the high-purity sodium tungstate product obtained in this example is a white flaky crystal with a WO3 purity of 99.91%, a P content of 9 ppm, an As content of 8 ppm, a Si content of 10 ppm, a Mo content of 5 ppm, and no Sn detected; the direct tungsten recovery rate is 99.2%.

[0065] Comparative Example 1 The difference between this comparative example and Example 1 is that the crude sodium tungstate is not pre-oxidized in step S1. Specifically, it includes the following steps: S1. Raw material preparation: Take 1000 mL of crude sodium tungstate solution obtained by alkaline boiling and leaching of waste cemented carbide. Its composition is: WO3 150 g / L, P 2.5 g / L, As 1.8 g / L, Si 3.0 g / L, Mo 0.5 g / L, Sn 0.2 g / L, of which As in the crude sodium tungstate solution... 3+ The content of As accounts for 40% of the total As content; S2. Gradient pH stepwise precipitation for impurity removal: Adjust the pH of the above crude sodium tungstate solution to 9.5 with 10wt% dilute sulfuric acid, add 25g MgCl2·6H2O and 10g Al2(SO4)3·18H2O, stir for 60min and filter to obtain the first purified solution; then adjust the pH of the first purified solution to 7.5 with 10wt% dilute sulfuric acid, add 0.2g disodium ethylenediaminetetraacetate, stir for 30min and filter to obtain the purified solution after impurity removal; S3-S5 are operated in the same manner as in Example 1.

[0066] According to the detection method in Example 1, the high-purity sodium tungstate product obtained in this comparative example was found to be a slightly yellow crystal. The product had a WO3 purity of 98.76%, an As content of 85 ppm, a P content of 12 ppm, a Si content of 15 ppm, a Mo content of 6 ppm, and no Sn detected. The tungsten direct recovery rate was 98.5%.

[0067] Compared to Example 1, this comparative example lacks a pre-oxidation treatment step, and the As in the solution... 3+ Not oxidized to As 5+ As 3 + The precipitation effect at pH 9.5 is far inferior to that of As.5+ The magnesium arsenite produced had relatively high solubility, resulting in incomplete precipitation and a significantly higher residual arsenic content than in Example 1. Furthermore, the product was slightly yellow, indicating that impurities affected its appearance. This comparative example demonstrates that pre-oxidation treatment is crucial for deep arsenic removal.

[0068] Comparative Example 2 The difference between this comparative example and Example 1 is that step S2 does not use gradient pH stepwise precipitation for impurity removal, but instead uses single pH precipitation for impurity removal. Specifically, S2 is as follows: Single pH precipitation for impurity removal: Adjust the pH of the pre-oxidized solution to 9.5 with 10wt% dilute sulfuric acid, add 25g MgCl2·6H2O, 10g Al2(SO4)3·18H2O and 0.2g disodium ethylenediaminetetraacetate, stir for 60min and filter to obtain the purified solution.

[0069] According to the detection method in Example 1, the high-purity sodium tungstate product obtained in this comparative example was found to be a white crystal with a slightly cloudy appearance. The product had a WO3 purity of 99.15%, a P content of 25 ppm, an As content of 18 ppm, a Si content of 35 ppm, a Mo content of 8 ppm, and no Sn detected. The tungsten direct recovery rate was 97.8%.

[0070] Compared to Example 1, this comparative example uses a single pH precipitation method for impurity removal. Under a single pH of 9.5, although magnesium and aluminum salts showed good precipitation effects on phosphorus, arsenic, and silicon, the complexing ability of disodium ethylenediaminetetraacetate (EDTA) was not optimal at this pH, and its stability constants with different metal ions were significantly affected by pH. Furthermore, the simultaneous addition of magnesium, aluminum, and EDTA complexing agents led to competitive reactions among the ions, resulting in incomplete precipitation of some impurities. In contrast, Example 1 employed a gradient pH stepwise precipitation method, first efficiently removing the main impurities of phosphorus, arsenic, and silicon at pH 9-10, and then deeply purifying residual ions at pH 7-8 using the optimal complexing range of the complexing agent. Therefore, the impurity removal effect was far superior to the single pH operation. Moreover, the precipitate particles formed under a single pH were finer and more difficult to filter, leading to increased tungsten entrainment loss and a decreased direct recovery rate. This comparative example demonstrates that gradient pH stepwise precipitation is crucial for deep impurity removal and reducing tungsten loss.

[0071] Comparative Example 3 The only difference between this comparative example and Example 1 is that a complexing agent is not added in step S2. Specifically, S2 is as follows: Gradient pH stepwise precipitation for impurity removal: Adjust the pH of the pre-oxidized solution to 9.5 with 10wt% dilute sulfuric acid, add 25g MgCl2·6H2O and 10g Al2(SO4)3·18H2O, stir for 60min and filter to obtain the first purified solution; then adjust the pH of the first purified solution to 7.5 with 10wt% dilute sulfuric acid, stir for 30min and filter to obtain the purified solution with impurities removed.

[0072] According to the detection method in Example 1, the high-purity sodium tungstate product obtained in this comparative example was found to be a white crystal with a WO3 purity of 99.35%, a P content of 20 ppm, an As content of 15 ppm, a Si content of 22 ppm, a Mo content of 7 ppm, and no Sn detected; the direct tungsten recovery rate was 99.0%.

[0073] Compared to Example 1, this comparative example did not add a complexing agent for complexation purification in the stepwise pH gradient precipitation process. After the first precipitation with magnesium and aluminum salts at pH 9.5, trace amounts of soluble cations such as calcium, magnesium, iron, and aluminum remained in the solution. Without a complexing agent, these cations would re-hydrolyze and precipitate during subsequent pH adjustment or concentration crystallization, forming eutectic or inclusions with sodium tungstate, affecting product purity. Adding a complexing agent (disodium ethylenediaminetetraacetate) forms stable water-soluble complexes with these residual cations, preventing precipitation and allowing them to be discharged with the permeate or mother liquor during subsequent filtration and concentration. This comparative example demonstrates that adding a complexing agent in the stepwise pH gradient precipitation process plays a crucial role in preventing eutectic formation and improving product purity.

[0074] Comparative Example 4 The only difference between this comparative example and Example 1 is the pH value of the solution in step S2, which is as follows: Gradient pH stepwise precipitation for impurity removal: Adjust the pH of the pre-oxidized solution to 8.0 with 10wt% dilute sulfuric acid, add 25g MgCl2·6H2O and 10g Al2(SO4)3·18H2O, stir for 60min and filter to obtain the first purified solution; then adjust the pH of the first purified solution to 5.5 with 10wt% dilute sulfuric acid, add 0.2g disodium ethylenediaminetetraacetate, stir for 30min and filter to obtain the purified solution with impurities removed.

[0075] According to the detection method in Example 1, the high-purity sodium tungstate product obtained in this comparative example was found to be a white crystal with a WO3 purity of 98.90%, a P content of 45 ppm, an As content of 32 ppm, a Si content of 55 ppm, a Mo content of 9 ppm, and no Sn detected; the tungsten direct recovery rate was 96.5%.

[0076] Compared to Example 1, in this comparative example, the pH of the pre-oxidized solution was adjusted to 8.0 in the gradient pH stepwise precipitation purification step, which is lower than the 9-10 range specified in this invention. At this pH, the hydrolysis of magnesium and aluminum ions is incomplete, resulting in insufficient amounts of magnesium hydroxide and aluminum hydroxide colloids. The chemical precipitation and adsorption co-precipitation capabilities for phosphorus, arsenic, and silicon are significantly reduced. In particular, the magnesium salt precipitation of phosphorus and arsenic requires relatively strong alkaline conditions to be fully formed. Subsequently, the pH of the primary purification solution was adjusted to 5.5, which is lower than the 7-8 range specified in this invention. Under these acidic conditions, some of the precipitated aluminum hydroxide will dissolve back due to acid dissolution, releasing the impurities that were just adsorbed and removed. Simultaneously, at low pH, some silicic acid may polymerize to form more difficult-to-remove colloidal silicon, and there is a risk of tungsten acid precipitation, leading to a significant decrease in the direct tungsten recovery rate. This comparative example demonstrates that the pH range specified in the gradient pH stepwise precipitation purification step of this invention is crucial for achieving deep purification and high tungsten recovery.

[0077] Comparative Example 5 The difference between this comparative example and Example 1 is that step S3 selective adsorption is omitted, and the purified liquid is directly processed by a nanofiltration membrane system to obtain sodium tungstate concentrate. The remaining steps and parameters are the same as in Example 1.

[0078] According to the detection method in Example 1, the high-purity sodium tungstate product obtained in this comparative example was found to be a white crystal with a WO3 purity of 99.45%, a P content of 12 ppm, an As content of 10 ppm, a Si content of 18 ppm, a Mo content of 120 ppm, and a Sn content of 35 ppm; the tungsten direct recovery rate was 99.4%.

[0079] Compared to Example 1, the selective adsorption step is omitted in this comparative example. Step S2, gradient pH stepwise precipitation for impurity removal, has limited effectiveness in removing molybdenum and tin. In alkaline solution, molybdenum is removed as molybdate (MoO4). 2- It exists in the form of ) and its chemical properties are similar to those of tungstate (WO4). 2- Tin is extremely similar to magnesium and aluminum salts, and its selective separation is difficult using precipitation methods and complexing agents. In alkaline solutions, tin partially exists as stannate (Sn(OH)6). 2- The molybdenum and tin impurities dissolve in a soluble form, and precipitation is insufficient to completely remove them. After omitting the selective adsorption step, the residual molybdenum and tin levels are significantly higher than in Example 1, failing to meet the requirements for high-purity products. This comparative example demonstrates that the selective adsorption step of the modified silica gel is crucial for the deep removal of molybdenum and tin impurities.

[0080] Comparative Example 6 The difference between this comparative example and Example 1 is that conventional unmodified silica gel adsorption is used in step S3 selective adsorption. The purified liquid is passed through an adsorption column filled with silica gel adsorbent to obtain a deeply purified liquid. The remaining steps and parameters are the same as in Example 1.

[0081] According to the detection method in Example 1, the high-purity sodium tungstate product obtained in this comparative example was found to be a white crystal with a WO3 purity of 99.60%, a P content of 9 ppm, an As content of 7 ppm, a Si content of 9 ppm, a Mo content of 45 ppm, and a Sn content of 15 ppm; the tungsten direct recovery rate was 99.5%.

[0082] Compared to Example 1, this comparative example uses conventional unmodified silica gel for adsorption in step S3. Compared to phenylphosphonic acid-modified silica gel, ordinary silica gel only has silanol groups on its surface and mainly adsorbs impurities through physical adsorption and weak hydrogen bonding, exhibiting poor selectivity and low adsorption capacity for molybdate and stannate. In contrast, the phosphonic acid groups grafted onto the surface of the phenylphosphonic acid-modified silica gel have a strong coordination chelating effect on molybdenum and tin, achieving highly selective deep removal. This comparative example demonstrates that modified silica gel loaded with organophosphonic acid or oxime functional groups has a significantly better selective adsorption capacity for molybdenum and tin than conventional unmodified silica gel.

[0083] The above examples and comparative test data fully demonstrate that there is a significant synergistic effect among the steps of the present invention, such as pre-oxidation, gradient pH stepwise precipitation for impurity removal (including the addition of complexing agents), and selective adsorption of modified silica gel. The absence of any step or deviation of parameters will lead to a significant deterioration in the impurity removal effect or the direct tungsten recovery rate, thus verifying the necessity and superiority of the process of the present invention.

[0084] In summary, this invention provides a process for deep purification of high-purity sodium tungstate solution to prepare high-purity sodium tungstate. This process converts trivalent arsenic in the solution to pentavalent arsenic through pre-oxidation treatment, creating favorable conditions for subsequent arsenic precipitation. Then, a gradient pH stepwise precipitation purification strategy is employed. First, under pH 9-10 conditions, magnesium and aluminum salts are used to precipitate and remove the main impurities of phosphorus, arsenic, and silicon. Next, under pH 7-8 conditions, a complexing agent is used to complex and mask residual cations, preventing them from forming eutectics or inclusions during subsequent crystallization. Subsequently, a modified silica gel adsorbent loaded with phenylphosphonic acid or salicylaldehyde oxime functional groups is used to selectively remove molybdenum and tin impurities. Finally, the product is concentrated via nanofiltration membrane, vacuum evaporation crystallization, and drying to obtain high-purity sodium tungstate. This invention achieves simultaneous and deep removal of multiple impurities from high-impurity sodium tungstate solutions through the multi-stage synergistic effect of pre-oxidation treatment, gradient pH stepwise precipitation for impurity removal, and selective adsorption, while maintaining a high tungsten recovery rate. The process is highly efficient and energy-saving, with mild operating conditions and low energy consumption. The resulting sodium tungstate product has high purity and stable quality, and has good prospects for industrial application.

[0085] The above description is merely a preferred embodiment of the present invention and does not limit the patent scope of the present invention. Any equivalent structural transformations made using the contents of the present invention under the inventive concept of the present invention, or direct / indirect applications in other related technical fields, are included within the patent protection scope of the present invention.

Claims

1. A process for deep purification of high-purity sodium tungstate solution to prepare high-purity sodium tungstate, characterized in that, Includes the following steps: S1. Pre-oxidation treatment: An oxidant is added to the crude sodium tungstate solution and mixed to react, oxidizing the trivalent arsenic in the solution to pentavalent arsenic; S2, Gradient pH Stepwise Precipitation for Impurity Removal: First, adjust the pH of the solution treated in step S1 to 9-10, add magnesium salt and aluminum salt for the first precipitation and solid-liquid separation to obtain a first purified solution; then adjust the pH of the first purified solution to 7-8, add a complexing agent for complexation purification and solid-liquid separation to obtain a purified solution with impurities removed. S3. Selective adsorption: The purified solution is passed through an adsorption column filled with modified silica gel adsorbent to selectively adsorb and remove molybdenum and / or tin impurities in the solution, thereby obtaining a deeply purified solution. S4. Membrane concentration: The deep purified liquid is concentrated through a nanofiltration membrane system to obtain sodium tungstate concentrate; S5. Crystallization and drying: The sodium tungstate concentrate is crystallized, separated from its solid state, and dried to obtain a high-purity sodium tungstate product.

2. The process according to claim 1, characterized in that, In step S1, the oxidant is selected from at least one of hydrogen peroxide and sodium hypochlorite solution; the amount of oxidant added is 3-18 times the amount of oxidant theoretically required to oxidize all trivalent arsenic in crude sodium tungstate solution to pentavalent arsenic; and the reaction temperature is 50-70℃.

3. The process according to claim 1, characterized in that, In S2, the magnesium salt is selected from at least one of magnesium chloride and magnesium sulfate, and the aluminum salt is selected from at least one of aluminum sulfate and aluminum chloride; the amount of magnesium salt added is based on the Mg / (P+As) molar ratio of (1-1.5):1, and the amount of aluminum salt added is based on the Al / Si molar ratio of (0.2-1.2):

1.

4. The process according to claim 1, characterized in that, In step S2, the complexing agent is selected from at least one of disodium ethylenediaminetetraacetate and aminotrimethylenephosphonic acid, and the amount of complexing agent added is 0.1-0.3g of complexing agent per 1 liter of crude sodium tungstate solution.

5. The process according to claim 1, characterized in that, In step S3, the modified silica adsorbent is an adsorbent on a silica carrier loaded with phenylphosphonic acid or salicylaldehyde oxime functional groups.

6. The process according to claim 1, characterized in that, The adsorption column in S3 operates at a temperature of 25-40℃ and has an empty column flow rate of 1-2 BV / h.

7. The process according to claim 1, characterized in that, The operating pressure of the nanofiltration membrane system in S4 is 1.0-2.0 MPa, the molecular weight cutoff of the nanofiltration membrane is 200-400 Da, and the concentration factor is 3-5 times.

8. The process according to claim 1, characterized in that, The vacuum degree for crystallization in S5 is 0.08-0.095 MPa, and the crystallization temperature is 60-75℃.

9. The process according to claim 1, characterized in that, The crude sodium tungstate solution contains WO3 at a concentration of 100-200 g / L, P, As, and Si impurities at a total content of 1-15 g / L, Mo at a content of 0.1-1 g / L, and Sn at a content of 0.15-0.5 g / L.

10. The process according to claim 1, characterized in that, The high-purity sodium tungstate product has a WO3 purity ≥ 99.90%, P, As, and Si contents ≤ 10 ppm, and Mo content ≤ 5 ppm.

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