A method for determining the content of tungsten trioxide in a tungsten sample

Abstract

The application provides a method for determining the content of tungsten trioxide in a tungsten sample, and belongs to the technical field of non-ferrous metal chemical analysis. The pH value of the oxidation product obtained through the oxidation reaction is strictly controlled in the weak acid interval of 1.2-1.8, so that the precipitation of tungstic acid is ensured, and the tungstic acid is prevented from being dissolved again due to the too low pH value and from being gelled due to the too high pH value, and the precipitation recovery rate is greatly improved; the cinchonine + pulp synergistic precipitation system is used to destroy the fine colloidal particles of the tungstic acid; and the acid-base back titration method is used to improve the accuracy of the determination.

Description

Technical Field

[0001] This invention belongs to the field of non-ferrous metal chemical analysis technology, specifically relating to a method for determining the tungsten trioxide content in a tungsten sample. Background Technology

[0002] Tungsten, as a strategic non-ferrous metal, is widely used in metallurgy, machinery, military industry, new materials and other fields. In the process of tungsten ore mining, tungsten alloy production and resource utilization of tungsten-containing waste, accurate determination of tungsten trioxide content is the core link of quality control and process optimization.

[0003] Currently, the thiocyanate spectrophotometric method is commonly used for the quantitative detection of tungsten trioxide. However, this method has drawbacks such as a narrow linear range and weak anti-interference capabilities. Furthermore, spectrophotometry relies on spectrophotometers, limiting rapid on-site detection, and high-content tungsten samples exhibit significant dilution errors. Therefore, improving the method to enhance the accuracy of the determination has become a pressing technical challenge in this field. Summary of the Invention

[0004] The purpose of this invention is to provide a method for determining the tungsten trioxide content in tungsten samples. The method provided by this invention has high accuracy and reproducibility, and can rapidly determine the tungsten trioxide content.

[0005] To achieve the above-mentioned objectives, the present invention provides the following technical solution: This invention provides a method for determining the tungsten trioxide content in a tungsten sample, comprising the following steps: (1) Dissolve the tungsten sample, alkali and solvent together to obtain a solution; (2) The solution obtained in step (1) is mixed with an oxidant to carry out an oxidation reaction to obtain an oxidation product; the pH value of the oxidation product is 1.2~1.8; (3) The oxidation product obtained in step (2), the cinnamon solution and the pulp are mixed and subjected to a precipitation reaction. After separation, the precipitate is obtained. (4) Mix the precipitate, strong alkaline solution and water obtained in step (3) to dissolve them and obtain the test solution; the strong alkaline solution includes sodium hydroxide solution and / or potassium hydroxide solution; (5) Using phenolphthalein as an indicator, the test solution obtained in step (4) is titrated with hydrochloric acid and / or sulfuric acid solution, and then the content of tungsten trioxide is calculated.

[0006] Preferably, the alkali in step (1) includes sodium hydroxide; the mass ratio of the tungsten sample to the alkali is 1:(2~3).

[0007] Preferably, the solvent in step (1) includes carbon dioxide-free distilled water, and the ratio of the mass of the tungsten sample to the volume of the solvent is 0.1 g: (50~80) mL.

[0008] Preferably, the dissolution temperature in step (1) is 80~90℃ and the dissolution time is 30~60min.

[0009] Preferably, the oxidant in step (2) is a nitric acid solution.

[0010] Preferably, the oxidation reaction temperature in step (2) is 90~110℃.

[0011] Preferably, the ratio of the mass of the tungsten sample in step (1) to the molar amount of cinnamon in the cinnamon solution in step (3) is (0.1~0.2) g : (0.003~0.005) mol.

[0012] Preferably, the precipitation reaction in step (3) is carried out at room temperature and the precipitation reaction time is ≥30 min.

[0013] Preferably, in step (4), before mixing the precipitate, strong alkaline solution, and water, the precipitate is washed with potassium nitrate solution until it is neutral.

[0014] Preferably, the concentration of the strong alkali solution in step (4) is 0.1~0.2 mol / L.

[0015] This invention provides a method for determining the tungsten trioxide content in a tungsten sample, comprising the following steps: mixing and dissolving a tungsten sample, an alkali, and a solvent to obtain a solution; mixing the solution with an oxidant to conduct an oxidation reaction to obtain an oxidation product; the pH value of the oxidation product is 1.2-1.8; mixing the oxidation product, a cinquercetin solution, and pulp to conduct a precipitation reaction, and separating the precipitate; mixing the precipitate, a strong alkali solution, and water to dissolve the precipitate to obtain a test solution; the strong alkali solution includes sodium hydroxide solution and / or potassium hydroxide solution; titrating the test solution with hydrochloric acid and / or sulfuric acid solution using phenolphthalein as an indicator, and then calculating the tungsten trioxide content. This invention strictly controls the pH value of the oxidation product obtained from the oxidation reaction within the weakly acidic range of 1.2-1.8, ensuring stable precipitation of tungstic acid while avoiding excessively low pH values ​​leading to redissolution of tungstic acid and excessively high pH values ​​causing colloid formation, thus significantly improving the precipitation recovery rate; employing a cinquercetin + pulp synergistic precipitation system to break down fine colloidal particles of tungstic acid; and combining this with acid-base back titration to improve the accuracy of the determination. Attached Figure Description

[0016] Figure 1 This is a schematic diagram illustrating the effect of pH on tungsten trioxide content. Figure 2 This is a schematic diagram illustrating the effect of settling time on tungsten trioxide content. Figure 3 This is a schematic diagram showing the effect of the amount of cinkonin solution on the tungsten trioxide content. Detailed Implementation

[0017] This invention provides a method for determining the tungsten trioxide content in a tungsten sample, comprising the following steps: (1) Dissolve the tungsten sample, alkali and solvent together to obtain a solution; (2) The solution obtained in step (1) is mixed with an oxidant to carry out an oxidation reaction to obtain an oxidation product; the pH value of the oxidation product is 1.2~1.8; (3) The oxidation product obtained in step (2), the cinnamon solution and the pulp are mixed and subjected to a precipitation reaction. After separation, the precipitate is obtained. (4) Mix the precipitate, strong alkaline solution and water obtained in step (3) to dissolve them and obtain the test solution; the strong alkaline solution includes sodium hydroxide solution and / or potassium hydroxide solution; (5) Using phenolphthalein as an indicator, the test solution obtained in step (4) is titrated with hydrochloric acid and / or sulfuric acid solution, and then the content of tungsten trioxide is calculated.

[0018] The method provided by this invention is applicable to any tungsten sample containing tungsten oxide. This invention does not impose any special limitations on the source and composition of the tungsten sample; any tungsten sample well-known in the art can be used.

[0019] Unless otherwise specified, the present invention does not have any special limitations on the source of each raw material, and commercially available products or well-known preparation methods known to those skilled in the art can be used for preparation.

[0020] This invention involves mixing a tungsten sample, an alkali, and a solvent, then dissolving the mixture to obtain a solution. The invention utilizes a reaction between the alkali and the tungsten sample under heating conditions to generate soluble sodium tungstate, achieving complete transfer of tungsten from the solid sample to the aqueous solution. Adding two drops of phenolphthalein indicator to the resulting solution produces a deep red color.

[0021] In this invention, the alkali preferably includes sodium hydroxide; the mass ratio of the tungsten sample to the alkali is preferably 1:(2~3). The use of sodium hydroxide in this invention improves the accuracy of the determination. Using other substances, such as sodium peroxide, can lead to lower results because sodium peroxide reacts with carbon dioxide in the air at high temperatures to form sodium carbonate, which affects the subsequent hydrochloric acid titration.

[0022] In this invention, the solvent preferably includes carbon dioxide-free distilled water; the mass ratio of the tungsten sample to the volume of the solvent is preferably 0.1 g : (50~80) mL. As one embodiment, the mass ratio of the tungsten sample to the volume of the solvent can be 0.1 g : 60 mL or 0.1 g : 70 mL.

[0023] The present invention does not have any special limitations on the operation of mixing the tungsten sample, alkali and solvent, and any technical solution for preparing the mixture well known to those skilled in the art can be used.

[0024] In this invention, the preferred dissolution temperature is 80-90°C; the preferred dissolution time is 30-60 minutes. As one embodiment, the dissolution temperature can be 85°C; the dissolution time can be 40 minutes, 45 minutes, or 50 minutes.

[0025] After obtaining the solution, the present invention mixes the solution with an oxidizing agent to carry out an oxidation reaction to obtain an oxidation product.

[0026] In this invention, the oxidant is preferably a nitric acid solution; the mass concentration of the nitric acid solution is preferably 65-70%; and the nitric acid solution is preferably added dropwise. In this invention, nitric acid is added dropwise to an alkaline sodium tungstate solution to gradually neutralize the alkalinity of the solution. The tungstate ions react with H+ in an acidic environment. + The combination produces a yellow tungstic acid precipitate; at the same time, nitric acid, as a strong oxidizing agent, can completely oxidize the low-valent tungsten present in the sample to hexavalent tungsten, avoiding the possibility that the low-valent tungsten cannot be completely precipitated, which would lead to a lower result.

[0027] As one implementation method, the mass concentration of the nitric acid solution can be 68%.

[0028] The present invention does not have a special limitation on the amount of nitric acid solution used, as long as the pH value of the oxidation product meets the requirements.

[0029] The present invention does not impose any special limitations on the dripping operation; any operation known to those skilled in the art can be used.

[0030] In this invention, the preferred temperature for the oxidation reaction is 90-110°C. As one embodiment, the temperature can be 100°C. This invention does not impose a specific time limit on the oxidation reaction, as long as the pH value of the oxidation product meets the requirements.

[0031] In one embodiment, the oxidation reaction of the solution and oxidant can be carried out by adding the oxidant dropwise to the solution, each drop being 0.2 mL, stirring and heating for 1-2 minutes after each addition, until the red color of the solution completely disappears; then, the oxidant is added in increments of 0.05-0.1 mL, and heating and stirring are continued until a large amount of bright yellow tungstic acid precipitate is formed in the system until the pH value is 1.2-1.8, at which point the addition of the oxidant is stopped, and heating is continued for 5 minutes.

[0032] In this invention, the pH value of the oxidation product is 1.2 to 1.8. As one embodiment, the pH value of the oxidation product can be 1.3, 1.4, 1.5, 1.6, or 1.7. In this invention, the pH value of the oxidation product is the pH value of the supernatant obtained from the oxidation reaction. This invention strictly controls the pH value of the oxidation product to 1.2 to 1.8, maintaining an acidic environment throughout the process to prevent the loss of tungstic acid through hydrolysis.

[0033] As one implementation method, the pH value of the oxidation products can be detected using 0.5~2.0 precision pH test paper or a portable pH meter.

[0034] After obtaining the oxidation product, the present invention mixes the oxidation product, cinkonin solution, and pulp, and carries out a precipitation reaction, obtaining the precipitate after separation. In the present invention, cinkonin is a nitrogen-containing organic alkaloid that can form an insoluble tungstic acid-cinkonin association precipitate with tungstic acid in a weakly acidic environment; the addition of pulp can adsorb the precipitate particles and avoid filtration loss.

[0035] In this invention, the oxidation product is preferably diluted before use; the diluent used for dilution is preferably distilled water; and the dilution is preferably carried out under boiling conditions. This invention does not have a particular limitation on the amount of diluent used, and any operation well-known to those skilled in the art can be used. This invention does not have a particular limitation on the dilution temperature; boiling is sufficient. The dilution method used in this invention can reduce the probability of subsequent co-precipitation of impurities.

[0036] As one implementation method, the dilution can be to a concentration of 100 mL.

[0037] In this invention, the preferred ratio of the mass of the tungsten sample to the molar amount of cinchonine in the cinchonine solution is (0.1~0.2) g : (0.003~0.005) mol, more preferably (0.1~0.2) g : 0.004 mol. This invention does not impose a specific limitation on the concentration of the cinchonine solution, as long as the ratio of the mass of the tungsten sample to the molar amount of cinchonine in the cinchonine solution is within the above range. As one embodiment, the concentration of the cinchonine solution can be 125 g / L; the cinchonine solution can be prepared by dissolving 12.5 g of cinchonine in 100 mL of hydrochloric acid solution (1+1).

[0038] In this invention, the cinchonine solution is preferably added in batches. This invention does not impose any special limitations on the batch addition operation; any operation well-known to those skilled in the art can be used.

[0039] In this invention, the pulp is preferably prepared from filter paper; when the mass of the tungsten sample is 0.1g, it is preferably 1 / 8 of the total area of ​​the filter paper; the diameter of the entire filter paper is preferably 11cm.

[0040] This invention does not impose any special limitation on the moisture content of the pulp; any pulp well-known to those skilled in the art can be used. This invention also does not impose any special limitation on the operation of preparing the pulp using filter paper; any operation well-known to those skilled in the art can be used. Furthermore, this invention does not impose any special limitation on the source of the filter paper; any filter paper well-known to those skilled in the art can be used. As one embodiment, the filter paper can be a new type of quantitative filter paper.

[0041] The present invention does not impose any special limitations on the operation of mixing the oxidation product, cincorine solution and pulp; any technical solution for preparing the mixture well known to those skilled in the art can be used.

[0042] In this invention, the temperature of the precipitation reaction is preferably room temperature; the time of the precipitation reaction is preferably ≥30 min; and the precipitation reaction is preferably carried out under static conditions.

[0043] The present invention does not impose any special limitations on the separation operation; any operation well known to those skilled in the art can be used to obtain the filter residue.

[0044] After obtaining the precipitate, the present invention mixes the precipitate, a strong alkaline solution, and water to dissolve it, thereby obtaining the test solution.

[0045] In this invention, the precipitate is preferably washed with potassium nitrate solution until neutral before mixing with the strong alkaline solution and water; the neutrality is preferably tested using methyl orange indicator. This invention uses a neutral potassium nitrate solution for washing, replacing traditional pure water washing, effectively inhibiting tungstic acid dissolution through the salt effect, thus providing double protection against precipitate loss.

[0046] In one embodiment, the concentration of the potassium nitrate solution can be 0.2 mol / L; the concentration of the methyl orange indicator can be 1 g / L.

[0047] In this invention, the strong alkali solution includes sodium hydroxide solution and / or potassium hydroxide solution; the concentration of the strong alkali solution is preferably 0.1~0.2 mol / L. This invention does not have a specific limitation on the amount of the strong alkali solution used, as long as it is sufficient to completely dissolve the precipitate.

[0048] As one implementation method, the solute in the strong alkaline solution can be carbon dioxide-free distilled water.

[0049] In this invention, the water is preferably free of carbon dioxide. There is no particular limitation on the amount of water used; any amount well-known to those skilled in the art can be used.

[0050] The present invention does not have any special limitations on the operation of mixing the precipitate, strong alkaline solution and water, and any technical solution for preparing the mixture well known to those skilled in the art can be used.

[0051] After obtaining the test solution, this invention uses phenolphthalein as an indicator and titrates the test solution with hydrochloric acid and / or sulfuric acid solution to calculate the tungsten trioxide content. Alternatively, this invention uses phenolphthalein as an indicator and titrates excess strong alkaline solution with hydrochloric acid and / or sulfuric acid solution until the red color disappears and does not return within 1 minute, which is the endpoint.

[0052] In this invention, the concentrations of the hydrochloric acid and sulfuric acid solutions are preferably 0.1~0.2 mol / L. This invention does not impose any particular limitation on the amount of hydrochloric acid and / or sulfuric acid solution used; titration is sufficient until the red color disappears and does not reappear within 1 minute.

[0053] As one implementation, the solute in the hydrochloric acid and sulfuric acid solutions can be carbon dioxide-free distilled water.

[0054] The present invention does not impose any particular limitation on the amount of phenolphthalein used; any amount known to those skilled in the art can be used. As one embodiment, the amount of phenolphthalein used can be 1-2 drops per 0.1g tungsten sample; the concentration of phenolphthalein can be 10g / L.

[0055] In this invention, the tungsten trioxide content is preferably calculated using the following formula: ; In the formula, c1 is the molar concentration of the strong alkali solution, mol / L; c2 is the molar concentration of the hydrochloric acid and / or sulfuric acid solution, mol / L; V1 is the volume of the strong alkali solution, mL; V2 is the volume of hydrochloric acid and / or sulfuric acid solution consumed, mL; M(WO3) is the molar mass of tungsten trioxide; and m is the mass of the tungsten sample, g.

[0056] The method provided by this invention uses readily available reagents, conventional equipment, and standardized operation, and has high detection accuracy and reproducibility, enabling rapid determination of tungsten content.

[0057] Compared with the prior art, the present invention has the following outstanding advantages: Precise pH control to suppress hydrolysis: During the oxidation stage, the pH of the system is strictly controlled within the weakly acidic range of 1.2 to 1.8, which ensures the stable precipitation of tungstic acid and avoids the redissolution of tungstic acid due to excessively low pH and the colloidalization caused by excessively high pH, ​​thus greatly improving the precipitation recovery rate. Optimized sedimentation system to eliminate filter loss: A synergistic sedimentation system of cinnarizine and pulp is adopted to break down fine colloidal particles of tungstic acid; Washing with a neutral salt solution replaces traditional pure water washing, effectively inhibiting the dissolution of tungstic acid by utilizing the salt effect, providing double protection against precipitate loss. Eliminating interference from impurities and ensuring stable titration endpoints: The alkaline solution is prepared using decarbonated distilled water throughout the process, which solves the problems of titration endpoint drift and repeated color changes, resulting in good reproducibility of the test.

[0058] The technical solutions of this invention will be clearly and completely described below with reference to the embodiments thereof. Obviously, the described embodiments are only a part of the embodiments of this invention, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0059] Testing equipment: electronic analytical balance, electric furnace, volumetric flask, pipette, burette, beaker, precision pH test paper or portable pH meter.

[0060] Reagents: solid sodium hydroxide, cinnarizine solid, potassium nitrate solid, hydrochloric acid, nitric acid, phenolphthalein, methyl orange. All reagents used were of analytical grade (AR). The water used was carbon dioxide-free distilled water.

[0061] Solution preparation: 0.1 mol / L sodium hydroxide standard titration solution, 0.1 mol / L hydrochloric acid standard titration solution.

[0062] Example 1 Method for determining the tungsten trioxide content in tungsten oxide samples (WO3 content ≥ 99.0%, tungsten trioxide analytical grade, using Kemio reagent): Weigh 0.2000g of tungsten oxide sample and 0.4g of NaOH solid into a 250mL Erlenmeyer flask, add 100mL of carbon dioxide-free distilled water, heat to 85℃ for 45min to dissolve, and obtain a solution. Oxidation was achieved by adding 68% nitric acid solution dropwise to the solution in batches, with each drop being 0.2 mL. After each addition, the solution was stirred and heated at 80°C for 1 min until the red color of the solution completely disappeared. Subsequently, nitric acid solution was added in batches of 0.05 mL, and the solution was stirred at 80°C until the precipitate turned completely yellow and the pH value was 1.4. The addition of nitric acid solution was then stopped, and the solution was heated for another 5 min to obtain the oxidation product. Then, hot, carbon dioxide-free distilled water was added and boiled to dilute to 100 mL. Subsequently, 10 mL of 125 g / L cinkonium chloride solution was added in batches, followed by the addition of pulp. The mixture was allowed to stand at room temperature for 35 minutes, then filtered. The precipitate was washed with 0.2 mol / L potassium nitrate solution until neutral (tested with methyl orange indicator), yielding the precipitate. The mass ratio of the tungsten oxide sample to the molar amount of cinkonium chloride in the cinkonium chloride solution was 0.2 g:0.004 mol. The pulp was prepared using filter paper. When the mass of the tungsten oxide sample was 0.1 g, 1 / 8 of the entire filter paper area was used. The diameter of the entire filter paper was 11 cm. The filter paper was Nova quantitative filter paper. Return the precipitate and filter paper to the original conical flask, add 25 mL of 0.1 mol / L sodium hydroxide standard titration solution, and add 10 mL of carbon dioxide-free water to dissolve the precipitate and obtain the test solution. Using phenolphthalein as an indicator, excess sodium hydroxide was titrated with 0.1 mol / L hydrochloric acid standard titration solution. The solution was added dropwise with stirring at a uniform rate, and the titration continued until the red color disappeared and did not return within 1 minute, which was the endpoint. Calculate the tungsten trioxide content using the following formula: .

[0063] The method of Example 1 was repeated 4 times, and the content of tungsten trioxide is shown in Table 1.

[0064] Table 1. Tungsten trioxide content obtained from Example 1 repeated 4 times.

[0065] As can be seen from Table 1, after the method of this embodiment was repeated 4 times, the RSD of the tungsten trioxide content in the tungsten sample was <5%, indicating that the method can accurately measure the tungsten trioxide content in tungsten oxide.

[0066] Based on Example 1, the pH value of the oxidation product was changed by adjusting the amount of nitric acid (1.0, 1.2, 1.4, 1.6, 1.8, 2.2, 2.6, 3.0). A schematic diagram illustrating the effect of pH value on tungsten trioxide content is shown below. Figure 1 As shown.

[0067] from Figure 1 It can be seen that a pH value between 1.2 and 1.8 is more conducive to the precipitation of tungstic acid.

[0068] Based on Example 1, the settling time was adjusted (10~50 min). The schematic diagram of the effect of settling time on tungsten trioxide content is shown below. Figure 2 As shown.

[0069] from Figure 2 It can be seen that tungsten trioxide precipitates completely after standing for 30 minutes.

[0070] Based on Example 1, the amount of cinkonin solution was adjusted (2, 4, 6, 8, 10, 12 mL). A schematic diagram illustrating the effect of cinkonin solution dosage on tungsten trioxide content is shown below. Figure 3 As shown.

[0071] from Figure 3 It can be seen that when 10 mL of cinnarizine solution is added, tungsten trioxide is fully precipitated.

[0072] Example 2 Based on Example 1, only the tungsten oxide sample was modified to silver tungsten scrap (sample with unknown content), while other conditions remained unchanged.

[0073] The tungsten trioxide content determined in Example 2 is shown in Table 2.

[0074] Table 2 shows the tungsten trioxide content obtained from three repetitions of Example 2.

[0075] Example 3 Based on Example 1, only the tungsten oxide sample was modified to silver tungsten scrap (sample with unknown content), while other conditions remained unchanged.

[0076] The tungsten trioxide content determined in Example 3 is shown in Table 3.

[0077] Table 3 shows the tungsten trioxide content obtained from three repetitions of Example 3.

[0078] As can be seen from the above embodiments, the method provided by the present invention has high detection accuracy and reproducibility, and can quickly determine the tungsten trioxide content.

[0079] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for determining the tungsten trioxide content in a tungsten sample, comprising the following steps: (1) Dissolve the tungsten sample, alkali, and solvent together to obtain a solution; (2) The solution obtained in step (1) is mixed with an oxidant to carry out an oxidation reaction to obtain an oxidation product; the pH value of the oxidation product is 1.2~1.8; (3) The oxidation product obtained in step (2), the cinnamon solution and the pulp are mixed and subjected to a precipitation reaction. After separation, the precipitate is obtained. (4) Mix the precipitate, strong alkaline solution and water obtained in step (3) to dissolve them and obtain the test solution; The strong alkaline solution includes sodium hydroxide solution and / or potassium hydroxide solution; (5) Using phenolphthalein as an indicator, the test solution obtained in step (4) is titrated with hydrochloric acid and / or sulfuric acid solution, and then the content of tungsten trioxide is calculated.

2. The method according to claim 1, characterized in that, The alkali in step (1) includes sodium hydroxide; the mass ratio of the tungsten sample to the alkali is 1:(2~3).

3. The method according to claim 1, characterized in that, The solvent in step (1) includes carbon dioxide-free distilled water, and the ratio of the mass of the tungsten sample to the volume of the solvent is 0.1 g: (50~80) mL.

4. The method according to claim 1, characterized in that, The dissolution temperature in step (1) is 80~90℃, and the dissolution time is 30~60min.

5. The method according to claim 1, characterized in that, In step (2), the oxidant is a nitric acid solution.

6. The method according to claim 1, characterized in that, The oxidation reaction in step (2) is carried out at a temperature of 90~110℃.

7. The method according to claim 1, characterized in that, The ratio of the mass of the tungsten sample in step (1) to the molar amount of cinnamon in the cinnamon solution in step (3) is (0.1~0.2) g : (0.003~0.005) mol.

8. The method according to claim 1, characterized in that, In step (3), the precipitation reaction is carried out at room temperature for a time ≥ 30 min.

9. The method according to claim 1, characterized in that, In step (4), before mixing the precipitate, strong alkaline solution, and water, the precipitate is washed with potassium nitrate solution until it becomes neutral.

10. The method according to claim 1, characterized in that, The concentration of the strong alkali solution in step (4) is 0.1~0.2 mol / L.

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