A method for recovering calcium tungstate and sodium nitrite from tungsten wire drawing waste salt

Abstract

The application provides a method for recovering calcium tungstate and sodium nitrite from tungsten wire drawing waste salt, and relates to the technical field of industrial hazardous waste resource utilization and tungsten compound preparation. The method comprises the following steps: dissolving the tungsten wire drawing waste salt to obtain a waste salt solution; adding calcium oxide into the waste salt solution to obtain calcium tungstate solid and a mixed solution through separation; concentrating the mixed solution to a saturated state, and then performing solid-liquid separation after precipitating the solid to obtain sodium nitrite solid; and the addition amount of the calcium oxide is 80% to 99% of the theoretical addition amount of the calcium oxide. The method avoids NO x gas pollution, realizes the resource recovery of tungsten and sodium nitrite in the waste salt, and is suitable for on-site disposal and resource utilization of the waste salt in tungsten wire processing enterprises.

Description

Technical Field

[0001] This invention relates to the fields of industrial hazardous waste resource utilization and tungsten compound preparation technology, and in particular to a method for recovering calcium tungstate and sodium nitrite from waste salts from tungsten wire drawing. Background Technology

[0002] During the tungsten wire drawing process, the tip of the tungsten wire needs to be melted in a high-temperature sodium nitrite melt. This allows the tungsten to react with the sodium nitrite to form sodium tungstate, which then causes the tungsten wire to pass through the drawing die in a conical shape. As the number of melting cycles increases, sodium nitrite is continuously consumed, and sodium tungstate accumulates, leading to a decrease in melting efficiency. The waste salt discharged periodically is called tungsten wire drawing waste salt. Tungsten wire drawing waste salt is a mixture of sodium tungstate and sodium nitrite. Because it contains sodium nitrite, it is classified as hazardous waste and must be disposed of in compliance with regulations.

[0003] The current mainstream method for recycling this waste salt in the industry is as follows: the waste salt is roasted at low temperature to remove oil, then leached and boiled with alkali to allow tungsten to enter the liquid phase, followed by precipitation of tungstic acid and further preparation of ammonium paratungstate. This method only focuses on the recovery of tungsten, and its fatal flaw is that the roasting process completely decomposes sodium nitrite into sodium oxide and nitrogen oxides (NOx). x The release of NO gas not only causes severe air pollution and increases the cost of denitrification treatment, but also makes it impossible to recycle and reuse sodium nitrite, a core raw material for fiber processing. Enterprises must continuously purchase it externally, resulting in extremely low resource utilization efficiency. Some studies have attempted to directly dissolve waste salt and then precipitate tungstic acid with acid, but under strongly acidic conditions, sodium nitrite transforms into unstable nitrous acid, which also decomposes and releases NO. x This cannot solve the problem of sodium nitrite degradation.

[0004] To address the aforementioned shortcomings, the technical problem this invention aims to solve is to provide a method for simultaneously separating and recovering tungsten and sodium nitrite from waste salts generated during tungsten wire drawing without damaging the molecular structure of sodium nitrite. This method allows the recovered calcium tungstate to be used as a raw material for preparing ammonium paratungstate, and the sodium nitrite to be reused in the wire drawing process, all without generating NO throughout the entire process. x pollute. Summary of the Invention

[0005] To address the above problems, this invention provides a method for recovering calcium tungstate and sodium nitrite from waste salts used in tungsten wire drawing, comprising the following steps: Dissolve the waste salt from tungsten wire drawing to obtain a waste salt solution; Calcium oxide was added to the waste salt solution, and calcium tungstate solid and a mixed solution were obtained by separation. The mixed solution was concentrated to saturation, and after the solid precipitated, solid-liquid separation was performed to obtain sodium nitrite solid. The amount of calcium oxide added is 80% to 99% of the theoretical amount of calcium oxide to be added.

[0006] Preferably, the amount of calcium oxide added is 90% of the theoretical amount of calcium oxide to be added.

[0007] Preferably, in the waste salt solution, the weight ratio of waste salt to solvent is 1:2.5~3.5.

[0008] Preferably, the temperature for dissolving the waste salt from tungsten wire drawing is 20℃~90℃.

[0009] Preferably, after adding calcium oxide to the waste salt solution, a precipitation reaction is carried out under stirring conditions; Preferably, the precipitation reaction is carried out under at least one of the following conditions: (a) The reaction temperature is 20℃~90℃; (b) The reaction time is 0.5h to 3h.

[0010] Preferably, the concentration is carried out under at least one of the following conditions: (c) Vacuum degree -0.05MPa to -0.09MPa; and, (d) Temperature 65℃~100℃.

[0011] Preferably, the mass-volume fraction of the solid is 1.45-1.50 g / ml.

[0012] Preferably, the solid-liquid separation includes centrifugal separation or pressure filtration separation.

[0013] Preferably, the method further includes a step of drying after solid-liquid separation, wherein the drying temperature is 60℃~80℃.

[0014] Preferably, after the solid-liquid separation, a crystallization mother liquor is also obtained; The mother liquor from crystallization is returned to the step of dissolving the waste salt from tungsten wire drawing in water, and is recycled as a solvent.

[0015] The beneficial effects of this invention are: This invention abandons the traditional roasting process and adopts a method combining direct dissolution with selective precipitation of calcium oxide. Throughout the process, the molecular structure of sodium nitrite remains intact, thus preventing NO from escaping at the source. x No gaseous pollution requires a dedicated denitrification treatment device. By precisely controlling the amount of calcium oxide added to 80%~99% of the theoretical amount, the introduction of calcium hydroxide impurities by excessive calcium oxide is effectively avoided, ensuring the purity of calcium tungstate and sodium nitrite products. At the same time, dual resource recovery of tungsten and sodium nitrite from waste salt is achieved. Tungsten is recovered in the form of calcium tungstate and can be directly used as a raw material for the preparation of ammonium paratungstate. Sodium nitrite is recovered in solid form and can be reused in the wire drawing process, realizing the closed-loop recycling of sodium nitrite. This is suitable for tungsten wire processing enterprises to carry out on-site disposal and resource utilization of waste salt. Detailed Implementation

[0016] The present application will now be described in further detail with reference to embodiments. In the following description, certain specific details are included to provide a comprehensive understanding of the various disclosed embodiments. However, those skilled in the art will recognize that embodiments can be implemented without employing one or more of these specific details, but using other methods, components, materials, etc. Unless otherwise required by the present invention, the terms "comprising" and "including" should be interpreted in an open-ended, inclusive sense, meaning "including but not limited to". Throughout this specification, "an embodiment," "an embodiment," "a preferred embodiment," or "some embodiments" means that at least one embodiment includes a specific reference element, structure, or feature related to that embodiment. Therefore, the phrases "in an embodiment," "in an embodiment," "in a preferred embodiment," or "in some embodiments" appearing in different places throughout the specification do not necessarily all refer to the same embodiment. Furthermore, specific elements, structures, or features may be combined in one or more embodiments in any suitable manner.

[0017] According to a first aspect of the present invention, a method for recovering calcium tungstate and sodium nitrite from waste salt from tungsten wire drawing is provided, comprising the following steps: Dissolve the waste salt from tungsten wire drawing to obtain a waste salt solution; Calcium oxide was added to the waste salt solution, and calcium tungstate solid and a mixed solution were obtained by separation. The mixed solution was concentrated to saturation, and after the solid precipitated, solid-liquid separation was performed to obtain sodium nitrite solid. The amount of calcium oxide added is 80% to 99% of the theoretical amount of calcium oxide to be added.

[0018] In this invention, tungsten in the waste salt exists in the form of sodium tungstate, with a mass content of 15%~50%, and sodium nitrite is another major component, with a mass content of 20%~60%. Both are water-soluble salts, and traditional roasting processes would destroy the molecular structure of sodium nitrite and produce NO. x Pollution is a concern, therefore direct water dissolution to ensure both sodium nitrite and calcium oxide are completely dissolved in the solution is a prerequisite for its recovery and reuse. Calcium oxide is then added to the resulting solution. + With WO4² - The reaction produces a sparingly soluble CaWO4 precipitate, while sodium nitrite, due to its high solubility, remains in the liquid phase, thus achieving solid-liquid separation of tungsten and sodium nitrite. The amount of calcium oxide used is controlled at 80%–99% of the theoretical amount to ensure complete consumption of the added calcium oxide and prevent excess calcium oxide or calcium hydroxide residue from contaminating the calcium tungstate precipitate, thereby ensuring both tungsten precipitation rate and product purity.

[0019] WO4² is present in the waste salt solution.- and a large amount of OH - If the amount of calcium oxide added reaches or exceeds the theoretical amount, i.e., 100% or more, excess Ca²⁺ will appear in the solution. + Its relationship with OH - The addition of Ca(OH)₂ precipitate, due to the similarity in precipitate properties of both Ca(OH)₂ and CaWO₄, will severely reduce the purity of the calcium tungstate product. Furthermore, excessive calcium oxide will increase the alkalinity of the solution, affecting the quality of subsequent evaporation and crystallization of sodium nitrite. Conversely, if the addition amount is less than 80%, a large amount of WO₄²⁻ will appear in the solution. - The failure to precipitate resulted in a low tungsten recovery rate, leading to rapid accumulation of sodium tungstate in the mother liquor and increasing the burden on the mother liquor circulation system. Controlling the dosage to 80%–99% can reduce the added Ca²⁺. + Completely absorbed by WO4² during the reaction. - This method of consumption avoids the formation of calcium-based impurities at the source, ensuring the purity of the calcium tungstate product. Simultaneously, any residual sodium tungstate in the solution can be recycled back to the dissolution step via crystallization mother liquor. After multiple cycles, it is gradually separated and removed during the precipitation process, preventing the final loss of tungsten. Therefore, the dosage range of 80% to 99% represents the optimal balance between ensuring the purity of the calcium tungstate product and guaranteeing the overall tungsten recovery rate. Examples of such ranges include 80%, 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, or 99%.

[0020] The theoretical amount of calcium oxide to be added is calculated from the amount of tungstate ions in the waste salt solution. The calculation formula is as follows:

[0021] in, This indicates the theoretical mass of calcium oxide added. This indicates the mass quantity of tungstate ions.

[0022] In a preferred embodiment of the present invention, the amount of calcium oxide added is 90% of the theoretical amount of calcium oxide to be added.

[0023] In a preferred embodiment of the present invention, the weight ratio of waste salt to solvent in the waste salt solution is 1:2.5 to 3.5, for example, 1:2.5, 1:2.6, 1:2.7, 1:2.8, 1:2.9, 1:3.0, 1:3.1, 1:3.2, 1:3.3, 1:3.4 or 1:3.5.

[0024] Typical, but not limiting, solvents include water.

[0025] When the weight ratio of waste salt to solvent in tungsten wire drawing is less than 1:2.5, the total content of sodium nitrite and sodium tungstate in the waste salt is high. If the water volume is insufficient, the dissolution of both may be incomplete, especially at lower temperatures where the solubility of sodium tungstate is limited. Undissolved waste salt will mix into subsequent calcium tungstate precipitates or sodium nitrite products, affecting product quality and potentially clogging pipelines and equipment. While a ratio higher than 1:3.5 results in more complete dissolution, excessive water means that more water needs to be removed during subsequent evaporation and crystallization, significantly increasing energy consumption and processing time in the evaporation process, reducing production efficiency, and becoming uneconomical. Therefore, this weight ratio range ensures complete dissolution of the waste salt under stirring conditions without increasing subsequent evaporation energy consumption due to excessive water volume.

[0026] In a preferred embodiment of the present invention, the temperature at which the waste salt for tungsten wire drawing is dissolved is 20°C to 90°C, for example, 20°C, 30°C, 40°C, 50°C, 60°C, 70°C, 80°C or 90°C.

[0027] The dissolution temperature range of 20℃ to 90℃ ensures rapid and complete dissolution of waste salts, allows operation under normal pressure, and poses no risk to the thermal stability of sodium nitrite.

[0028] In a preferred embodiment of the present invention, waste salt from tungsten wire drawing is dissolved to obtain a waste salt solution, comprising: Waste salt from tungsten wire drawing is added to a mixing tank at a weight ratio of 1:2.5~3.5 and stirred at a temperature of 20℃~90℃ to dissolve it, thus obtaining a waste salt solution.

[0029] In a preferred embodiment of the present invention, calcium oxide is added to the waste salt solution and a precipitation reaction is carried out under stirring conditions. The precipitation reaction is carried out under at least one of the following conditions: (a) The reaction temperature is 20°C to 90°C, for example, 20°C, 30°C, 40°C, 50°C, 60°C, 70°C, 80°C or 90°C; (b) The reaction time is 0.5h to 3h, for example, 0.5h, 0.75h, 1.0h, 1.2h, 1.5h, 2.0h, 2.5h or 3.0h.

[0030] In a preferred embodiment of the present invention, calcium oxide is added to a waste salt solution to separate calcium tungstate solid and a mixed solution, comprising: Weigh out 80% to 99% of the theoretical addition amount of calcium oxide and add it to the waste salt solution. A precipitation reaction is carried out under stirring conditions: temperature 20℃ to 90℃, time 0.5h to 3h. The reaction formula is as follows:

[0031] During this process, tungsten is separated from the solution as calcium tungstate (CaWO4) precipitate, while sodium nitrite remains in the liquid phase due to its high solubility, resulting in a mixed solution. The reacted mixture slurry is then subjected to solid-liquid separation, which can be achieved by centrifugation or pressure filtration, to obtain calcium tungstate solid and a mixed solution containing sodium nitrite.

[0032] In a preferred embodiment of the present invention, the concentration is carried out under at least one of the following conditions: (c) Vacuum degree -0.05 MPa to -0.09 MPa, for example -0.05 MPa, -0.06 MPa, -0.07 MPa, -0.08 MPa or -0.09 MPa; and, (d) Temperatures of 65°C to 100°C, for example, 65°C, 70°C, 75°C, 80°C, 85°C, 90°C, 95°C and 100°C.

[0033] A vacuum of -0.05 MPa to -0.09 MPa allows water to evaporate at a relatively low temperature of 65℃ to 100℃. Negative pressure evaporation reduces heat consumption and improves evaporation efficiency. Simultaneously, the lower operating temperature effectively avoids the risk of thermal decomposition of sodium nitrite, ensuring its molecular structure remains intact. The temperature range of 65℃ to 100℃ matches the boiling point of water at this vacuum level, allowing evaporation to proceed smoothly. Furthermore, it utilizes the difference in solubility between sodium tungstate and sodium nitrite; the solubility of sodium tungstate increases with temperature at 65℃ to 100℃ (approximately 73 g / 100 g water at 20℃ and approximately 120 g / 100 g water at 80℃). The higher temperature prevents sodium tungstate from prematurely precipitating and mixing into the product during evaporation, thus retaining it as much as possible in the solution.

[0034] In a preferred embodiment of the present invention, the mass-volume fraction of the solid is 1.45-1.50 g / ml, for example, 1.45 g / ml, 1.46 g / ml, 1.47 g / ml, 1.48 g / ml, 1.49 g / ml or 1.50 g / ml.

[0035] In a preferred embodiment of the present invention, the solid-liquid separation includes centrifugal separation or pressure filtration separation.

[0036] In a preferred embodiment of the present invention, the solid-liquid separation is further followed by a drying process, wherein the drying temperature is 60°C to 80°C.

[0037] In a preferred embodiment of the present invention, after the mixed solution is evaporated and concentrated until crystals precipitate, it is post-processed to obtain sodium nitrite solid, comprising: A mixed solution containing sodium nitrite was fed into a triple-effect evaporator and concentrated under vacuum conditions of -0.05 MPa to -0.09 MPa and temperature of 65℃ to 100℃ until crystals precipitated, resulting in a solid-liquid mixture. The solid mass-volume fraction of the solid in the solid-liquid mixture was 1.45-1.50 g / ml. The solid-liquid mixture is separated into solid and liquid components, preferably by centrifugation or filtration, to obtain wet sodium nitrite crystals and mother liquor for crystallization. Sodium nitrite wet crystals were dried at 60℃~80℃ to obtain sodium nitrite solid.

[0038] In a preferred embodiment of the present invention, the crystallization mother liquor is returned to the step of dissolving waste salt in water during tungsten wire drawing and is recycled as a solvent.

[0039] In a preferred embodiment of the present invention, a method for recovering calcium tungstate and sodium nitrite from waste salt from tungsten wire drawing specifically includes: Waste salt from tungsten wire drawing was added to a mixing tank at a weight ratio of 1:2.5~3.5 and stirred at 20℃~90℃ to dissolve, obtaining a waste salt solution. After dissolution, samples were taken to analyze the WO4² content in the solution. - The concentration of ions; According to WO4² - Calculation of ion concentration and solution volume WO4² - Based on the total mass, the theoretical amount of calcium oxide to be added is calculated using the following stoichiometric formula:

[0040] in, This indicates the theoretical mass of calcium oxide added. This indicates the mass quantity of tungstate ions; Weigh out 80% to 99% of the theoretical addition amount of calcium oxide and add it to the waste salt solution. A precipitation reaction is carried out under stirring conditions: temperature 20℃ to 90℃, time 0.5h to 3h. The reaction formula is as follows:

[0041] During this process, tungsten is separated from the solution as calcium tungstate (CaWO4) precipitate, while sodium nitrite remains in the liquid phase due to its high solubility, resulting in a mixed solution. The mixture slurry after the reaction is subjected to solid-liquid separation, which can be carried out by centrifugation or pressure filtration to obtain calcium tungstate solid and a mixed solution containing sodium nitrite; A mixed solution containing sodium nitrite was fed into a triple-effect evaporator and concentrated under vacuum conditions of -0.05 MPa to -0.09 MPa and temperature of 65℃ to 100℃ until crystals precipitated, resulting in a solid-liquid mixture. The solid mass-volume fraction of the solid in the solid-liquid mixture was 1.45-1.50 g / ml. The solid-liquid mixture is separated into solid and liquid components, preferably by centrifugation or filtration, to obtain wet sodium nitrite crystals and mother liquor for crystallization. Sodium nitrite wet crystals were dried at 60℃~80℃ to obtain sodium nitrite solid.

[0042] The mother liquor from crystallization is returned to the waste salt dissolution step for recycling. The trace amounts of sodium tungstate in it gradually accumulate after multiple cycles and are eventually removed in the precipitation process.

[0043] The sodium nitrite solid prepared by this invention can be reused in the tungsten wire forming process.

[0044] Example 1 Waste salt from tungsten wire drawing was added to a mixing tank at a weight ratio of 1:3.0 and stirred at 60°C to dissolve, yielding a waste salt solution. Samples were taken from the solution to analyze the WO4²⁺ content. - Ion concentration was determined, and the theoretical amount of calcium oxide to be added was calculated based on the stoichiometric formula. 90% of the theoretical amount of calcium oxide was weighed and added to the waste salt solution. A precipitation reaction was carried out at 60°C with stirring for 1.5 hours. After the reaction, solid-liquid separation was performed to obtain solid calcium tungstate and a mixed solution containing sodium nitrite. The mixed solution was sent to a triple-effect evaporator and concentrated under a vacuum of -0.07 MPa and a temperature of 80°C until crystals precipitated (the solid mass-volume fraction was approximately 1.48 g / ml). After solid-liquid separation, wet sodium nitrite crystals were obtained and dried at 70°C to obtain solid sodium nitrite. The mother liquor from crystallization was returned to the dissolution step for recycling.

[0045] Example 2 The amount of calcium oxide added was 80% of the theoretical amount, the weight ratio of waste salt to solvent was 1:3.5, the dissolution temperature was 20℃, the precipitation reaction temperature was 20℃, the precipitation reaction time was 3.0h, the evaporation vacuum degree was -0.05MPa, the evaporation temperature was 100℃, and the rest was the same as in Example 1.

[0046] Example 3 The amount of calcium oxide added was 99% of the theoretical amount, the weight ratio of waste salt to solvent was 1:2.5, the dissolution temperature was 90℃, the precipitation reaction temperature was 90℃, the precipitation reaction time was 0.5h, the evaporation vacuum degree was -0.09MPa, the evaporation temperature was 65℃, and the rest was the same as in Example 1.

[0047] Example 4 The amount of calcium oxide added was 85% of the theoretical amount, the weight ratio of waste salt to solvent was 1:3.0, the dissolution temperature was 50℃, the precipitation reaction temperature was 50℃, the precipitation reaction time was 2.0h, the evaporation vacuum degree was -0.06MPa, the evaporation temperature was 75℃, and the rest was the same as in Example 1.

[0048] Example 5 The amount of calcium oxide added was 95% of the theoretical amount, the weight ratio of waste salt to solvent was 1:2.8, the dissolution temperature was 80℃, the precipitation reaction temperature was 70℃, the precipitation reaction time was 1.0h, the evaporation vacuum degree was -0.08MPa, the evaporation temperature was 90℃, and the rest was the same as in Example 1.

[0049] Example 6 Using the same process conditions as in Example 1, the crystallization mother liquor was returned to the dissolution step for recycling. This cycle was repeated for 5 batches, and the product quality of the 1st, 3rd, and 5th batches was tested respectively.

[0050] Example 7 The amount of calcium oxide added was 92% of the theoretical amount, and the rest was the same as in Example 1.

[0051] Comparative Example 1 Compared to Example 1, the amount of calcium oxide added was 70% of the theoretical amount, and the other conditions were exactly the same as in Example 1.

[0052] Comparative Example 2 Compared to Example 1, the amount of calcium oxide added was 100% of the theoretical amount, and all other conditions were exactly the same as in Example 1.

[0053] Comparative Example 3 Compared to Example 1, the amount of calcium oxide added was 110% of the theoretical amount, and the other conditions were exactly the same as in Example 1.

[0054] Comparative Example 4 The traditional process involves calcining the waste salt from tungsten wire drawing at low temperature (300℃, 2h) to remove oil, followed by alkaline leaching and pressure boiling to allow tungsten to enter the liquid phase. Then, tungstic acid is added to precipitate the tungsten and ammonium paratungstate is further prepared. During this process, sodium nitrite is completely decomposed in the calcination stage.

[0055] Comparative Example 5 After directly dissolving the waste salt from tungsten wire drawing, hydrochloric acid was added to adjust it to a strong acidity (pH=1~2) so that tungsten precipitates in the form of tungstic acid. After solid-liquid separation, the liquid phase was evaporated and crystallized to recover sodium nitrite. The remaining conditions were the same as in Example 1.

[0056] Performance testing: (a) Testing of calcium tungstate products 1. Purity of calcium tungstate (expressed as WO3 mass fraction) Referring to the YS / T 524-2011 standard "Synthetic Scheelite", the hydrochloric acid decomposition-tungstic acid ignition gravimetric method was used for determination: 0.5g of calcium tungstate sample was weighed into a 250ml beaker, approximately 80ml of hydrochloric acid was added, and the sample was heated at a low temperature until complete decomposition. After adding seed reagent, the sample was heated and concentrated on a low-temperature electric furnace until tungstic acid began to precipitate. 5ml of perchloric acid-nitric acid mixture was added, and heating continued until white fumes were emitted. After cooling, the mixture was diluted with hot water to 40-50ml, boiled, filtered, and the precipitate was washed with perchloric acid washing solution. The precipitate was ignited to constant weight to obtain WO3, and its mass fraction was calculated. Purity calculation formula:

[0057] 2. Tungsten recovery rate Tungsten content was determined using inductively coupled plasma atomic emission spectrometry (ICP-AES): The calcium tungstate sample was completely dissolved in concentrated hydrochloric acid and hydrogen peroxide to prepare the test solution. The tungsten content was then determined using ICP-AES, and a tungsten standard curve was plotted for quantification. The tungsten recovery rate was calculated using the following formula: Tungsten recovery rate = (in waste salt) W Total mass / of recovered calcium tungstate W (quality) × 100% (II) Testing of Sodium Nitrite Products 1. Sodium nitrite purity Referring to GB / T 633-1994 "Chemical Reagents - Sodium Nitrite", the potassium permanganate titration method was used: An appropriate amount of sample was weighed, dissolved in water, and accurately measured. The solution was then titrated with a standard potassium permanganate solution under acidic conditions. The sodium nitrite content was calculated based on the amount consumed. Purity calculation formula:

[0058] Where V is the volume of potassium permanganate standard solution consumed (ml), c is the concentration of potassium permanganate standard solution (mol / L), and M is the molar mass of sodium nitrite (69.00 g / mol).

[0059] 2. Sodium nitrite recovery rate Sodium nitrite recovery rate = (mass of recovered sodium nitrite / total mass of sodium nitrite in waste salt) × 100% IV. Test Data The composition of the waste salt raw material is: 35% Na2WO4 by mass, 45% NaNO2 by mass, and the remainder is a small amount of impurities (Na2CO3, NaCl, etc.).

[0060] Table 1

[0061] The test data in Table 1 show that, in all embodiments of the present invention, high-purity products can be stably obtained within the calcium oxide addition range of 80% to 99%. Specifically, the purity of calcium tungstate (calculated as WO3) reaches 97.5% to 99.2%, and the purity of sodium nitrite reaches 98.5% to 99.1%. Simultaneously, the tungsten recovery rate is 86.5% to 98.1%, and the sodium nitrite recovery rate is 91.8% to 93.1%, demonstrating excellent overall performance. Comparing different addition amounts reveals that when below 80%, the tungsten recovery rate drops sharply to 78.2%. While equal to or exceeding the theoretical amount can improve the tungsten recovery rate (≥99.2%), it leads to a significant decrease in the purity of calcium tungstate to 92.1% and 85.6%, and the purity of sodium nitrite to 96.3% and 94.8%, respectively. This confirms that excessive calcium oxide introduces calcium hydroxide impurities, seriously affecting product quality. Further observation of Example 7 reveals that while the tungsten recovery rate is improved compared to Example 1, the purity of both calcium tungstate and sodium nitrite is slightly lower than the corresponding values ​​in Example 1, and the sodium nitrite recovery rate is also slightly lower. This indicates that increasing the addition amount from 90% to 92%, although it can precipitate a small amount more tungsten, will slightly introduce calcium-based impurities, leading to a decrease in product purity and a slight negative impact on sodium nitrite recovery. Therefore, an addition amount of 90% is the optimal balance point, achieving the best overall balance between purity and recovery rate.

[0062] Furthermore, after five consecutive batches of mother liquor recycling, the product indicators showed almost no decline, verifying the process stability. Compared with existing technologies, although the traditional roasting process can recover tungsten, it completely decomposes sodium nitrite and produces NO. x Pollution occurs in traditional methods, such as acid precipitation of tungsten, which destroys sodium nitrite under acidic conditions, resulting in purity and recovery rates of only 82.5% and 75.6%, respectively, far lower than that of this invention. In summary, this invention achieves efficient dual recovery of tungsten and sodium nitrite while ensuring high product purity, and is environmentally friendly and has a stable process. The optimal implementation scheme is achieved by controlling the calcium oxide addition to approximately 90%.

[0063] The applicant declares that the present invention is illustrated by the above embodiments, but the present invention is not limited to the above process steps, that is, it does not mean that the present invention must rely on the above process steps to be implemented. Those skilled in the art should understand that any improvements to the present invention, equivalent substitutions of the raw materials used in the present invention, addition of auxiliary components, selection of specific methods, etc., all fall within the protection scope and disclosure scope of the present invention.

Claims

1. A method for recovering calcium tungstate and sodium nitrite from waste salt from tungsten wire drawing, characterized in that, Includes the following steps: Dissolve the waste salt from tungsten wire drawing to obtain a waste salt solution; Calcium oxide was added to the waste salt solution, and calcium tungstate solid and a mixed solution were obtained by separation. The mixed solution was concentrated to saturation, and after the solid precipitated, solid-liquid separation was performed to obtain sodium nitrite solid. The amount of calcium oxide added is 80% to 99% of the theoretical amount of calcium oxide to be added.

2. The method according to claim 1, characterized in that, The amount of calcium oxide added is 90% of the theoretical amount of calcium oxide to be added.

3. The method according to claim 1, characterized in that, In the waste salt solution, the weight ratio of waste salt to solvent is 1:2.5~3.

5.

4. The method according to claim 1, characterized in that, The temperature for dissolving the waste salt from tungsten wire drawing is 20℃~90℃.

5. The method according to claim 1, characterized in that, After adding calcium oxide to the waste salt solution, a precipitation reaction is carried out under stirring conditions; The precipitation reaction is carried out under at least one of the following conditions: (a) The reaction temperature is 20℃~90℃; and, (b) The reaction time is 0.5h to 3h.

6. The method according to claim 1, characterized in that, The concentration is carried out under at least one of the following conditions: (c) Vacuum degree -0.05MPa to -0.09MPa; and, (d) Temperature 65℃~100℃.

7. The method according to claim 1, characterized in that, The mass-volume fraction of the solid is 1.45-1.50 g / ml.

8. The method according to claim 1, characterized in that, The solid-liquid separation includes centrifugal separation or pressure filtration separation.

9. The method according to claim 1, characterized in that, The method further includes a step of drying after solid-liquid separation, wherein the drying temperature is 60℃~80℃.

10. The method according to claim 1, characterized in that, The solid-liquid separation yields a crystallization mother liquor; The mother liquor from crystallization is returned to the waste salt dissolution step of tungsten wire drawing and recycled as a solvent.

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