Method for measuring content of sulfur element in rare earth enrichment

Abstract

The invention provides a method for measuring the content of a sulfur element in a rare earth enriched product, which comprises the following steps: (1) adding an acid solution into the rare earth enriched product to obtain a pretreatment solution; (2) sequentially adding a first oxidizing agent and a second oxidizing agent into the pretreatment solution to obtain a solution to be detected; (3) measuring the content of the sulfur element in the solution to be measured; the step (1) and the step (2) are both carried out under a heating condition. According to the method, the acid liquor is matched with the first oxidizing agent and the second oxidizing agent for use, the rare earth enrichment is fully dissolved and oxidized, conversion from full-value sulfur to + 6-valence sulfur is achieved, and the accuracy of a measurement result is improved.

Description

Technical Field

[0001] The present invention relates to the field of chemical technology, specifically to the field of element content determination, and particularly to a method for determining the sulfur element content in rare earth concentrates. Background Art

[0002] Rare earths have excellent magnetic, optical, and electrical properties that cannot be replaced, playing a huge role in improving material properties and increasing product varieties. They are widely used in fields such as metallurgy, military, petrochemical, glass ceramics, agriculture, and new materials. With the development of industrial technology and science and technology, new rare earth materials emerge in an endless stream, and the related detection requirements for rare earths are also continuously improving. The determination of sulfur elements in rare earth concentrates has important industrial, environmental, and quality control significance. In the prior art, there are mainly three methods for determining the sulfur element content. One is to directly determine using a carbon-sulfur analyzer, but it is for solid samples. Another is to use ion chromatography to determine sulfate ions. This method requires high-temperature calcination of the sample to convert sulfur elements into gas, then receiving with an aqueous solution and adding an oxidant to convert all sulfur oxides into sulfate ions, and measuring the sulfur element content based on the content of sulfate ions. The process is complex and the energy consumption cost is high. The third method is to use inductively coupled plasma emission spectrometry. This method is convenient and fast, but the accuracy of sulfur element determination needs to be improved.

[0003] CN101639443A discloses a method for quickly and accurately determining the sulfur element content in fluorite. By heating the fluorite sample in a mixed acid of hydrochloric acid, nitric acid, hydrofluoric acid, and perchloric acid until perchloric acid fumes are emitted and the volume of the solution is 0.5 - 1 mL, the fluorite sample is completely converted into easily soluble salts, then adding hydrochloric acid, heating until the salts are completely dissolved, cooling and making up the volume, and measuring the sulfur element content. This method does not pay attention to the influence of sulfur element valence on the measurement result.

[0004] CN106338551A discloses an analysis method for different forms of sulfur element content in a reforming catalyst, including feeding the reforming catalyst sample into a combustion furnace, fully burning in oxygen at 900 - 1200 °C, absorbing the gas generated by combustion with a strong oxidant solution to obtain an absorption solution, and measuring the sulfate ion content in the absorption solution by ion chromatography, and thus obtaining the total sulfur content in the reforming catalyst. However, this method requires calcination of the reforming catalyst at high temperature, with high energy consumption.

[0005] In view of the technical problems existing in the prior art, it is of great significance to develop a determination method that can accurately determine the sulfur element content in rare earth concentrates, with low cost and simple process. Summary of the Invention

[0006] Aiming at the deficiencies of the existing technology, the purpose of the present invention is to provide a method for determining the sulfur element content in rare earth concentrates. By using an acid solution in combination with a first oxidant and a second oxidant, the present invention fully dissolves and oxidizes rare earth concentrates, realizes the complete dissolution of sulfur elements in rare earth concentrates, and oxidizes all sulfur elements to the +6 valence state to match the +6 valence sulfur in the sulfur standard solution, avoiding test errors caused by differences in test signal responses of sulfur elements in different valence states and improving the accuracy of test results. The determination method provided by the present invention has high accuracy, is simple to operate, and realizes the full oxidation of sulfur elements without calcination, significantly reducing the determination cost.

[0007] To achieve the object of this invention, the following technical solutions are adopted:

[0008] In the first aspect, the present invention provides a method for determining the sulfur element content in rare earth concentrates, and the determination method includes:

[0009] (1) Add an acid solution to rare earth concentrates to obtain a pretreatment solution;

[0010] (2) Add a first oxidant and a second oxidant to the pretreatment solution in sequence to obtain a test solution;

[0011] (3) Determine the sulfur element content in the test solution;

[0012] Both step (1) and step (2) are carried out under heating conditions.

[0013] By using an acid solution in combination with a first oxidant and a second oxidant, the present invention fully dissolves and oxidizes rare earth concentrates. Adding a first oxidant to the pretreatment solution further dissolves and oxidizes rare earth concentrates, and then adding a second oxidant realizes the complete dissolution of sulfur elements in rare earth concentrates and oxidizes all sulfur elements to the +6 valence state to match the +6 valence sulfur in the sulfur standard solution, avoiding inaccurate test results caused by differences in the atomization effects of sulfur in different valence states.

[0014] Preferably, before adding the acid solution to the rare earth concentrates in step (1), the rare earth concentrates are sequentially subjected to a first drying and a second drying, and the temperature of the first drying is lower than that of the second drying.

[0015] In the present invention, the purpose of the first drying of rare earth concentrates at a lower temperature is to slowly evaporate the moisture in rare earth concentrates without taking away sample components, and the second drying is carried out at a higher temperature to further dry the moisture, ensuring accurate weighing and further ensuring the accuracy of sulfur element content determination.

[0016] Preferably, the temperature of the first drying is 60°C - 80°C, for example, it can be 60°C, 62°C, 64°C, 66°C, 68°C, 70°C, 72°C, 74°C, 76°C, 78°C or 80°C, including but not limited to the listed values, and other unlisted values within the numerical range are equally applicable.

[0017] Preferably, the time of the first drying is 24h - 48h, for example, it can be 24h, 26h, 28h, 30h, 32h, 34h, 36h, 38h, 40h, 42h, 44h, 46h or 48h, including but not limited to the listed values, and other unlisted values within the numerical range are equally applicable.

[0018] Preferably, the temperature of the second drying is 105°C - 110°C, for example, it can be 105°C, 106°C, 107°C, 108°C, 109°C or 110°C, including but not limited to the listed values, and other unlisted values within the numerical range are equally applicable.

[0019] Preferably, the time of the second drying is 1h - 2h, for example, it can be 1h, 1.1h, 1.2h, 1.3h, 1.4h, 1.5h, 1.6h, 1.7h, 1.8h, 1.9h or 2h, including but not limited to the listed values, and other unlisted values within the numerical range are equally applicable.

[0020] Preferably, after the first drying and before the second drying, it further includes grinding the rare earth concentrate after the first drying.

[0021] In the present invention, the rare earth concentrate after the first drying is ground and then subjected to the second drying to ensure more sufficient second drying, further drying the moisture, and improving the accuracy of the final determination result of the sulfur element content.

[0022] In the present invention, both step (1) and step (2) are carried out under heating conditions, which is beneficial to the progress of the dissolution and oxidation reactions. If the heating temperature is too high, the reaction is violent, the water evaporates quickly, resulting in sulfur element loss. If the heating temperature is too low, it is not conducive to sample dissolution and the oxidation reaction is incomplete.

[0023] Preferably, the temperature of the heating is 220°C - 240°C, for example, it can be 220°C, 222°C, 224°C, 226°C, 228°C, 230°C, 232°C, 234°C, 236°C, 238°C or 240°C, including but not limited to the listed values, and other unlisted values within the numerical range are equally applicable.

[0024] Preferably, the acid solution in step (1) includes hydrochloric acid.

[0025] In the present invention, hydrochloric acid is selected to digest the rare earth concentrate, which is more conducive to the dissolution of the rare earth concentrate.

[0026] Preferably, the concentration of the acid solution in step (1) is 10wt%-25wt%, for example, it can be 10wt%, 12wt%, 14wt%, 16wt%, 18wt%, 20wt%, 22wt%, 24wt% or 25wt%, including but not limited to the listed values, and other unlisted values within the numerical range are equally applicable.

[0027] Preferably, the solid-liquid ratio of the rare earth concentrate to the acid solution in step (1) is 3g / L-30g / L, for example, it can be 3g / L, 6g / L, 9g / L, 12g / L, 15g / L, 18g / L, 21g / L, 24g / L, 27g / L or 30g / L, including but not limited to the listed values, and other unlisted values within the numerical range are equally applicable.

[0028] Preferably, the first oxidant in step (2) includes hydrogen peroxide solution.

[0029] In the present invention, using hydrogen peroxide solution as the first oxidant can not only promote the conversion of sulfur in different valence states in the pretreatment solution to sulfur in +6 valence state, but also promote the dissolution of the insoluble rare earth concentrate, achieve the complete dissolution of the rare earth concentrate, and improve the accuracy of sulfur element content determination.

[0030] Preferably, the concentration of hydrogen peroxide in the hydrogen peroxide solution in step (2) is 25wt%-35wt%, for example, it can be 25wt%, 27wt%, 29wt%, 30wt%, 31wt%, 33wt% or 35wt%, including but not limited to the listed values, and other unlisted values within the numerical range are equally applicable.

[0031] Preferably, the second oxidant in step (2) includes perchloric acid solution.

[0032] Preferably, the concentration of perchloric acid in the perchloric acid solution in step (2) is 65wt%-75wt%, for example, it can be 65, 67, 69wt%, 70wt%, 71wt%, 73wt% or 75wt%, including but not limited to the listed values, and other unlisted values within the numerical range are equally applicable.

[0033] Preferably, based on the mass of the sample to be treated, the volume of the first oxidant added in step (2) is 0.05g / mL-0.3g / mL, for example, it can be 0.05g / mL, 0.1g / mL, 0.15g / mL, 0.2g / mL, 0.25g / mL or 0.3g / mL, including but not limited to the listed values, and other unlisted values within the numerical range are equally applicable.

[0034] Preferably, based on the mass of the sample to be processed, the volume of the second oxidant added in step (2) is 0.2 g / mL - 1.2 g / mL. For example, it can be 0.2 g / mL, 0.3 g / mL, 0.4 g / mL, 0.5 g / mL, 0.6 g / mL, 0.7 g / mL, 0.8 g / mL, 0.9 g / mL, 1 g / mL, 1.1 g / mL or 1.2 g / mL, including but not limited to the listed values, and other unlisted values within the numerical range are equally applicable.

[0035] Preferably, before adding the first oxidant and the second oxidant to the pretreatment solution in step (2), it further includes evaporating the pretreatment solution obtained in step (1) to 15% - 50% of the original volume. For example, it can be 15%, 20%, 25%, 30%, 35%, 40%, 45% or 50%, including but not limited to the listed values, and other unlisted values within the numerical range are equally applicable.

[0036] Preferably, after adding the first oxidant to the preliminary dissolution solution in step (2), after the reaction until no gas escapes, then add the second oxidant.

[0037] Preferably, the method for determining the sulfur element content in the test solution in step (3) includes quantitative determination using an inductively coupled plasma emission spectrometer.

[0038] The determination method provided by the present invention can achieve the conversion of all valence states of sulfur elements to +6 valence sulfur to match the +6 valence sulfur in the sulfur standard solution, avoiding inaccurate test results caused by differences in the atomization effects of sulfur in different valence states.

[0039] Before quantitative determination using an inductively coupled plasma optical emission spectrometer in the present invention, it further includes volumetrically diluting the test solution obtained in step (2), and then using an inductively coupled plasma optical emission spectrometer to detect the sulfur element content in the volumetrically diluted test solution. The observation direction is axial, the power is 1100W - 1300W, for example, it can be 1100W, 1150W, 1200W, 1250W or 1300W, the flow rate of the test liquid is 0.5mL / min - 3.00mL / min, for example, it can be 0.5mL / min, 1mL / min, 1.5mL / min, 2mL / min, 2.5mL / min or 3mL / min, the atomizing gas flow rate is 0.2 - 0.8L / min, for example, it can be 0.2L / min, 0.3L / min, 0.4L / min, 0.5L / min, 0.6L / min, 0.7L / min or 0.8L / min, the plasma gas flow rate is 10.00L / min - 15.00L / min, for example, it can be 10L / min, 11L / min, 12L / min, 13L / min, 14L / min or 15L / min, the auxiliary gas flow rate is 0.20 - 0.80L / min, for example, it can be 0.20L / min, 0.30L / min, 0.40L / min, 0.50L / min, 0.60L / min, 0.70L / min or 0.80L / min. The sulfur content is quantitatively determined by the standard curve method, and the sulfur concentrations of the standard curve points are 0mg / L, 10mg / L, 50mg / L, and 100mg / L respectively.

[0040] Preferably, the analysis wavelength of sulfur in the inductively coupled plasma optical emission spectrometer is 180.672nm.

[0041] In the determination method defined in the present invention, sulfur has a high emission intensity, good sensitivity, and higher accuracy at a wavelength of 180.672nm.

[0042] In the sulfur element determination method provided by the present invention, the oxidation and atomization rules of the sulfur element valence can also be applied to all applications using inductively coupled plasma optical emission spectrometry to determine the sulfur element content.

[0043] Compared with the prior art, the present invention has the following beneficial effects:

[0044] (1) By using the acid solution in combination with the first oxidant and the second oxidant, the present invention fully dissolves and oxidizes the rare earth concentrate, realizes the complete dissolution of sulfur elements in the rare earth concentrate, and oxidizes all sulfur elements to the +6 valence state to match the +6 valence sulfur in the sulfur standard solution, avoiding the test error caused by the difference in test signal response of sulfur elements in different valence states, and significantly improving the accuracy of sulfur element content determination.

[0045] (2) The determination method provided by the present invention has high accuracy, is simple to operate, and realizes the full oxidation of sulfur elements without calcination, significantly reducing the determination cost. Detailed implementation manners

[0046] The technical solution of the present invention will be further described below through specific implementation manners. Those skilled in the art should understand that the embodiments are only for helping to understand the present invention and should not be regarded as specific limitations on the present invention.

[0047] The reference value of the sulfur content in the rare earth concentrate used in the specific implementation manner of the present invention is 2.49%.

[0048] Example 1

[0049] This example provides a method for determining sulfur elements in rare earth concentrates, and the determination method includes:

[0050] (1) Bake the rare earth concentrate in an oven at 80 °C for 24 h, then grind and crush it to prevent caking, and then place it in an oven at 105 °C for 1 h and take it out. Weigh 0.2021 g and pour it into a 100 mL conical flask. Measure 20 mL of hydrochloric acid solution with a concentration of 12 wt% with a measuring cylinder and pour it into the conical flask along the wall of the conical flask in a circular motion. Place the conical flask on a heating plate, set the heating plate temperature to 220 °C, and heat to obtain a pretreatment solution.

[0051] (2) Evaporate the pretreatment solution in step (1) to about 5 mL, add 1 mL of hydrogen peroxide, mix evenly, and after no reaction bubbles, add 0.4 ml of perchloric acid, mix evenly and heat until there is no reaction, then stop heating to obtain a solution to be measured.

[0052] (3) Add pure water to the solution to be measured to 20 mL, mix evenly and filter. After the filtration of the solution to be measured is completed, rinse the conical flask three times with pure water and pour it into the filter paper for filtration together. Then rinse the filter paper three times with pure water, and pour all the above filtrates into a volumetric flask and make up the volume to 100 mL.

[0053] Use an inductively coupled plasma emission spectrometer to detect the diluted solution to be measured. Among them, the analysis wavelength of sulfur is 180.672 nm, the power is 1200 W, the observation direction is axial, the peristaltic pump speed is 1.00 mL / min, the atomizing gas flow rate is 0.50 L / min, the plasma gas flow rate is 12.00 L / min, the auxiliary gas flow rate is 0.50 L / min, and the standard curve method is used for quantitative determination of sulfur content. The sulfur concentrations of the standard curve points are 0 mg / L, 10 mg / L, 50 mg / L, and 100 mg / L respectively.

[0054] Example 2

[0055] This embodiment provides a method for determining sulfur in rare earth concentrates, and the determination method includes:

[0056] (1) Bake the rare earth concentrates in an oven at 60 °C for 48 h, then grind and crush to prevent caking, and then place them in an oven at 110 °C for 1 h and take them out. Weigh 0.2993 g and pour it into a 100 mL conical flask. Measure 10 mL of hydrochloric acid solution with a concentration of 25 wt% using a measuring cylinder and pour it into the conical flask along the wall of the conical flask in a circular motion. Place the conical flask on a heating plate, set the heating plate temperature to 240 °C, and heat to obtain a pretreatment solution.

[0057] (2) Evaporate the pretreatment solution in step (1) to about 5 mL, add 1 mL of hydrogen peroxide, mix evenly, and after no reaction bubbles, add 0.25 mL of perchloric acid, mix evenly and heat until there is no reaction, then stop heating to obtain a test solution.

[0058] (3) Add pure water to the test solution to 20 mL, mix evenly and filter. After the filtration of the test solution is completed, rinse the conical flask three times with pure water and pour it into the filter paper for filtration together. Then rinse the filter paper three times with pure water, and pour all the above filtrates into a volumetric flask and make up the volume to 100 mL.

[0059] Use the inductively coupled plasma emission spectrometer method to detect the test solution after volume fixing. The analysis wavelength of sulfur is 180.672 nm, the power is 1200 W, the observation direction is axial, the peristaltic pump speed is 1.00 mL / min, the atomizing gas flow rate is 0.50 L / min, the plasma gas flow rate is 12.00 L / min, the auxiliary gas flow rate is 0.50 L / min, and the standard curve method is used to quantitatively determine the sulfur content. The sulfur concentrations of the standard curve points are 0 mg / L, 10 mg / L, 50 mg / L, and 100 mg / L respectively.

[0060] Example 3

[0061] This embodiment provides a method for determining sulfur in rare earth concentrates, and the determination method includes:

[0062] (1) Bake the rare earth concentrates in an oven at the first drying temperature of 70 °C for 30 h, then grind and crush to prevent caking, and then place them in an oven at 108 °C for 1.5 h and take them out. Weigh 0.1006 g and pour it into a 100 mL conical flask. Measure 30 mL of hydrochloric acid solution with a concentration of 10 wt% using a measuring cylinder and pour it into the conical flask along the wall of the conical flask in a circular motion. Place the conical flask on a heating plate, set the heating plate temperature to 230 °C, and heat to obtain a pretreatment solution.

[0063] (2) Evaporate the pretreatment solution in step (1) to about 5 mL, add 2 mL of hydrogen peroxide, mix evenly. After no reaction bubbles are observed, add 0.5 mL of perchloric acid, mix evenly and heat until no reaction occurs, then stop heating to obtain the solution to be tested.

[0064] (3) Add pure water to the solution to be tested to make it 20 mL, mix evenly and filter. After the filtration of the solution to be tested is completed, rinse the conical flask three times with pure water and pour it into the filter paper for filtration together. Then rinse the filter paper three times with pure water, pour all the above filtrates into a volumetric flask, and make the volume up to 100 mL.

[0065] Use the inductively coupled plasma emission spectrometer method to detect the volume-fixed solution to be tested. Among them, the analysis wavelength of sulfur is 180.672 nm, the power is 1200 W, the observation direction is axial, the peristaltic pump speed is 1.00 mL / min, the atomizing gas flow rate is 0.50 L / min, the plasma gas flow rate is 12.00 L / min, the auxiliary gas flow rate is 0.50 L / min. The standard curve method is used to quantitatively determine the sulfur content. The sulfur concentrations of the standard curve points are 0 mg / L, 10 mg / L, 50 mg / L, and 100 mg / L respectively.

[0066] Comparative Example 1

[0067] This comparative example provides a method for determining sulfur elements in rare earth concentrates. Except that hydrogen peroxide is not added in step (2), the rest are the same as in Example 1.

[0068] Comparative Example 2

[0069] This comparative example provides a method for determining sulfur elements in rare earth concentrates. Except that hydrogen peroxide is not added in step (2), the rest are the same as in Example 2.

[0070] Comparative Example 3

[0071] This comparative example provides a method for determining sulfur elements in rare earth concentrates. Except that perchloric acid is not added in step (2), the rest are the same as in Example 1.

[0072] Comparative Example 4

[0073] This comparative example provides a method for determining sulfur elements in rare earth concentrates. Except that perchloric acid is not added in step (2), the rest are the same as in Example 2.

[0074] Comparative Example 5

[0075] This comparative example provides a method for determining sulfur elements in rare earth concentrates. Except that the analysis wavelength of sulfur in step (3) is 182.565 nm, the rest are the same as in Example 1.

[0076] Comparative Example 6

[0077] This comparative example provides a method for determining sulfur elements in rare earth concentrates. Except that the analysis wavelength of sulfur in step (3) is 182.565 nm, the rest are the same as in Example 2.

[0078] Performance test:

[0079] Using the methods for determining sulfur elements provided in all the examples and comparative examples of the present invention, the sulfur content in rare earth concentrates produced in a certain mining area was detected. The detection results are shown in Table 1, where the sampling amount is the mass of the rare earth concentrate in step (1), the concentration refers to the concentration of sulfur elements in the measured solution after constant volume detected by the inductively coupled plasma emission spectrometer in step (3), and the sulfur content is calculated according to the formula where V represents the final constant volume of the measured solution in liters, and the recovery rate = sulfur content / sulfur content reference value.

[0080] Table 1

[0081]

[0082]

[0083] According to the measurement results of Examples 1 to 3, by using the acid solution in combination with the first oxidant and the second oxidant, the rare earth concentrate is fully dissolved and oxidized, realizing the complete dissolution of sulfur elements in the rare earth concentrate, and oxidizing all sulfur elements to the +6 valence state, which matches the +6 valence sulfur in the sulfur standard solution, improving the accuracy of the measurement.

[0084] According to the measurement results of Examples 1 and 2 and Comparative Examples 1 and 2, if the first oxidant peroxide is not used for oxidation but perchloric acid is directly added, the rare earth concentrate is not completely dissolved and sulfur elements are lost, resulting in a low measurement result.

[0085] According to the measurement results of Examples 1 and 2 and Comparative Examples 3 and 4, if the second oxidant perchloric acid is not added for further oxidation after the oxidation by the first oxidant peroxide, the measurement result is low because the valence state of sulfur elements cannot be completely oxidized to the +6 valence state.

[0086] According to the measurement results of Examples 1 and 2 and Comparative Examples 5 and 6, when the analysis wavelength of sulfur elements is selected as 180.672 nm, the luminous intensity is high and the accuracy is good.

[0087] The applicant declares that the above description is only a specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily thought of by those skilled in the art within the technical scope disclosed by the present invention fall within the protection scope and the disclosure scope of the present invention.

Claims

1. A method for determining the sulfur content in rare earth enriched materials, characterized in that: The determination method comprises: (1) adding an acid solution to a rare earth enriched product to obtain a pre-treated solution; (2) adding a first oxidant and a second oxidant to the pretreatment solution in sequence to obtain a test solution; (3) Determine the sulfur content in the solution to be tested; Both step (1) and step (2) are carried out under heating conditions.

2. The measuring method according to claim 1, characterized in that Before adding the acid solution to the rare earth enriched material in step (1), the step further includes sequentially performing a first drying and a second drying on the rare earth enriched material, wherein the temperature of the first drying is lower than the temperature of the second drying; Preferably, the first drying temperature is 60°C-80°C; Preferably, the first drying time is 24h-48h; Preferably, the second drying temperature is 105°C-110°C; Preferably, the second drying time is 1h-2h; Preferably, after the first drying and before the second drying, the method further comprises grinding the rare earth enriched product after the first drying.

3. The measuring method according to claim 1 or 2, characterized in that The heating temperature is 220°C-240°C.

4. The assay method according to any one of claims 1 to 3, characterized in that The acid solution in step (1) comprises hydrochloric acid; Preferably, the concentration of the acid solution in step (1) is 10wt%-25wt%; Preferably, in step (1), the solid-to-liquid ratio of the rare earth concentrate to the acid solution is 3 g / L-30 g / L.

5. The assay method according to any one of claims 1 to 4, characterized in that Step (2) the first oxidant comprises a hydrogen peroxide solution; Preferably, the concentration of hydrogen peroxide in the hydrogen peroxide solution in step (2) is 25wt%-35wt%; Preferably, in step (2), the second oxidant comprises a perchloric acid solution; Preferably, the concentration of perchloric acid in the perchloric acid solution in step (2) is 65wt%-75wt%.

6. The assay method according to claim 5, wherein Based on the mass of the rare earth enriched material, the volume of the first oxidant added in step (2) is 0.05 g / mL-0.3 g / mL; Preferably, based on the mass of the rare earth concentrate, the volume of the second oxidant added in step (2) is 0.2 g / mL-1.2 g / mL.

7. The assay method according to any one of claims 1 to 6, characterized in that Before adding the first oxidant and the second oxidant to the pretreatment liquid in sequence in step (2), the step also includes evaporating the pretreatment liquid obtained in step (1) to 15%-50% of the original volume.

8. The assay method according to any one of claims 1 to 7, characterized in that Step (2) After adding the first oxidant to the pretreatment liquid, the second oxidant is added after the reaction is completed until no gas escapes.

9. The assay method according to any one of claims 1 to 8, characterized in that The method of determining the sulfur content in the solution to be tested in step (3) includes using an inductively coupled plasma emission spectrometer for quantitative determination.

10. The assay method according to any one of claims 1 to 9, characterized in that The analysis wavelength of sulfur in the inductively coupled plasma emission spectrometer is 180.672 nm.

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