Method for rapidly measuring solid solution metal impurities and surface free metal impurities of titanium dioxide primary product

Abstract

The invention discloses a method for rapidly measuring solid solution metal impurities and surface free metal impurities of a titanium dioxide primary product, and belongs to the technical field of titanium dioxide production. In order to realize separation and detection of internal solid solution and surface free metal impurities in a titanium dioxide primary product, the invention provides a method for rapidly measuring the solid solution and surface free metal impurities in the titanium dioxide primary product, which comprises the following steps: quantitatively detecting the metal impurities in the titanium dioxide primary product through ICP-OES or ICP-MS, and recording the total concentration as c1; after the titanium dioxide primary product with the mass marked as m1 reacts with the dilute acid aqueous solution, weighing as m2, quantitatively detecting metal impurities, and marking the internal solid solution concentration as c2; the content of internal solid solution and surface free metal impurities in the titanium dioxide primary product is obtained. According to the method, selective separation of the internal solid solution and the surface free metal impurities is realized by utilizing the dissolution difference between the titanium dioxide solid solution and free metal impurities and the acid, and accurate quantitative analysis of the solid solution and free metal impurities is realized.

Description

Technical Field

[0001] The present invention belongs to the technical field of titanium dioxide production, and particularly relates to a method for rapidly measuring the dissolved metal impurities and surface free metal impurities in the crude titanium dioxide product. Background Art

[0002] Titanium dioxide (TiO2) with excellent optical properties, chemical stability and hiding power makes it one of the most representative products among white pigments. However, the production process of titanium dioxide is complex, involving multiple chemical reactions and physical treatment steps. The presence of metal impurities not only affects the crystal grain size and crystal grain morphology of titanium dioxide, but also has a negative impact on its hue, optical properties, dispersibility and durability. Therefore, the content of metal impurity elements is one of the key factors affecting product quality. In the production process of chloride process titanium dioxide, the metal impurity elements in the crude titanium dioxide product (i.e., titanium dioxide without post-treatment) mainly come from raw materials, production equipment and introduction during the process. These metal impurity elements (such as Fe, Cr, etc.) may exist in two forms: one is dissolved inside the titanium dioxide particles, which may come from titanium tetrachloride raw materials and the oxidation reaction process; the other is free on the particle surface, which may come from the damage of production equipment during the production and transportation process. The dissolved metal impurities will affect the crystal structure and optical properties of titanium dioxide, while the surface free metal impurities may affect its dispersibility and stability. Therefore, measuring the content of metal impurities in different states in the crude titanium dioxide product is beneficial to clarify the sources of the main metal impurities in the crude titanium dioxide product, and effectively control and reduce the content of metal impurities in titanium dioxide in a targeted manner.

[0003] However, in the actual production process, the content of metal impurities in the crude titanium dioxide product is usually low, especially the content of key metal impurities such as Fe and Cr is often below 100 ppm. The existence state of such low-concentration metal impurities in titanium dioxide particles is difficult to accurately detect by conventional analytical detection means (such as X-ray diffraction XRD, high-resolution transmission electron microscopy HR-TEM, etc.). Therefore, the existing measurement methods cannot meet the accurate measurement requirements of the content of dissolved metal impurities inside and surface free metal impurities in the crude titanium dioxide product in factory production. At present, there are few patent literature reports on the detection of metal impurities in titanium dioxide at home and abroad. Most of the existing detection methods focus on the determination of the total amount of metal impurities in titanium dioxide, and there is no relevant report on the measurement method for metal impurities in different existence states (dissolved or free) in the crude titanium dioxide product. This technical blank makes it difficult for production enterprises to accurately evaluate the content of metal impurities from different sources and their impact on product quality in actual production, so that they cannot optimize the production process and improve product quality in a targeted manner.

[0004] Therefore, developing a method for quickly and accurately measuring the content of dissolved metal impurities and surface free metal impurities in crude titanium dioxide has become a technical problem that urgently needs to be solved in the field of titanium dioxide production. Summary of the Invention

[0005] In order to realize the separation and detection of the content of internal dissolved metal impurities and surface free metal impurities in crude titanium dioxide particles, the present invention utilizes the difference in the dissolution of acids by the metal impurities dissolved in titanium dioxide and the metal impurities free outside titanium dioxide to selectively separate the surface free metal impurities and the internal dissolved metal impurities. By strictly controlling the purity of the reagents added throughout the sample treatment process, the risk of introducing new metal impurities is avoided, and the ICP-OES or ICP-MS technology is used to realize the quantitative analysis of the metal impurities in the crude product and the pickled crude product.

[0006] The present invention provides a method for quickly measuring the dissolved metal impurities and surface free metal impurities in crude titanium dioxide, which includes the following steps:

[0007] A. Take the crude titanium dioxide sample and send it to ICP-OES or ICP-MS for quantitative detection of metal impurities. The total concentration of the internal dissolution and surface free of the obtained metal impurities is recorded as c1;

[0008] B. Take the crude titanium dioxide sample and mix it with a dilute acid aqueous solution at a mass-to-volume ratio of 1 g: 4.5 - 5.5 mL, and stir and react at room temperature to obtain a reaction solution. In step B, the mass of the taken crude titanium dioxide sample is recorded as m1; the dilute acid aqueous solution is at least one of a nitric acid aqueous solution with a mass concentration of 10% - 30%, a sulfuric acid aqueous solution with a mass concentration of 20% - 40%, or a hydrochloric acid aqueous solution with a mass concentration of 10% - 20%;

[0009] C. Perform solid-liquid separation and washing on the reaction solution obtained in step B. After drying the obtained powder, weigh it, and record the mass as m2; send it to ICP-OES or ICP-MS for quantitative detection of metal impurities under the same conditions as in step A, and record the internal dissolution concentration of the obtained metal impurities as c2;

[0010] D. Analyze the content of the internal dissolved metal impurities and surface free metal impurities in the crude titanium dioxide according to the detection results; the total content of a certain metal impurity in the crude titanium dioxide is m1·c1, the internal dissolution content of the metal impurity in the crude titanium dioxide is m2·c2, and the surface free content of the metal impurity in the crude titanium dioxide is m1·c1 - m2·c2;

[0011] The metal impurities are one or more of Fe, Cu, Ni, Al, Cr, or V.

[0012] Preferably, in the above method, in step B, the dilute acid aqueous solution is a nitric acid aqueous solution with a mass concentration of 15% - 20%.

[0013] Among them, in the above method, in step B, when preparing the nitric acid aqueous solution, the purity of the nitric acid used is above the purity specification of G2, the total gold impurity is less than 1 ppb, and the total gold impurity of the water used is less than 1 ppb.

[0014] Among them, in the above method, in step B, the reaction time is 8 to 24 hours.

[0015] Among them, in the above method, in step C, the drying temperature is 110 to 130 °C.

[0016] Among them, in the above method, in step C, the drying time is 6 to 8 hours.

[0017] Among them, in the above method, in step D, when m2·c2≈m1·c1, it indicates that the metal impurity in the titanium white preliminary product is mainly the solid-solution metal impurity inside the powder particles; when m2·c2≈0, it indicates that the metal impurity in the titanium white preliminary product is mainly the free metal impurity on the surface of the powder particles; when 0<m2·c2<m1·c1, it indicates that part of the metal impurity in the titanium white preliminary product is the solid-solution metal impurity inside the powder particles and part is the free metal impurity on the surface of the powder particles.

[0018] In the present invention, the preparation of the test samples of the titanium white preliminary product and the reaction solution can adopt the conventional methods in the art. For example, the following methods.

[0019] ICP-OES / ICP-MS test sample preparation method for titanium dioxide:

[0020] Accurately weigh 0.1000 g of the titanium white preliminary product sample (or a sample of other weight, and adjust the corresponding amounts of other materials accordingly) into a 100 mL beaker, add 4 mL of concentrated H2SO4 and 1.6 g of (NH4)2SO4, heat the mixture at about 250 °C for 30 min until the sample is dissolved clearly, cool and transfer it to a 50 mL volumetric flask, dilute it to the mark with ultrapure water, and then further dilute it until the SO2 4- concentration in the solution is about 0.15%. Under the optimized experimental conditions, use the ICP-OES / ICP-MS method to determine the impurity element content, and conduct a blank experiment at the same time. Among them, (NH4)2SO4, H2SO4, and HNO3 are all of superior grade purity. All utensils are soaked in 20% (volume fraction) HNO3 for 6 to 8 h, and then rinsed 3 times with ultrapure water (resistivity ≥18 MΩ) and reserved.

[0021] ICP-MS test sample preparation method for the reaction solution after acid dissolution:

[0022] Weigh 10 ml of the reaction solution sample into a centrifuge tube, centrifuge at 6000 rpm for 5 min, accurately weigh 0.5000 g of the supernatant into a 50 mL volumetric flask, and then dilute it to the mark with ultrapure water. Under the optimized experimental conditions, use ICP-MS method to determine the content of impurity elements, and conduct a blank experiment at the same time.

[0023] In the present invention, when using ICP-OES / ICP-MS method to determine the content of metal impurity elements or Ti content, general means in the art can be adopted for the detection instrument, detection parameters, etc., without adjustment.

[0024] Advantages of the present invention:

[0025] The present invention utilizes the dissolution differences between the metal impurities dissolved in titanium dioxide and the metal impurities free outside titanium dioxide with nitric acid, etc., controls the acid used for pickling, its concentration and pickling time, realizes the selective separation of surface free metal impurities and internal dissolved metal impurities, combines the strict control of the purity of the added reagents throughout the sample treatment process, avoids the risk of introducing new metal impurities, and through ICP-OES or ICP-MS, realizes the precise quantitative analysis of dissolved and free gold impurities.

[0026] The method of the present invention has the advantages of simple operation, fast analysis speed, accurate results, etc., provides an effective technical detection means for titanium dioxide production, is used to evaluate production stability, optimize production processes and improve product quality, provides a scientific basis for the analysis and control of the sources of metal impurities in the titanium dioxide production process, and has high application value and popularization prospects. Specific embodiments

[0027] Specifically, a method for quickly measuring the dissolved metal impurities and surface free metal impurities of titanium dioxide primary products includes the following steps:

[0028] A. Take the titanium dioxide primary product, send it to ICP-OES or ICP-MS for quantitative detection of metal impurities, and record the total concentration of internal dissolution and surface free of the obtained metal impurities as c1 (the unit is generally ppm);

[0029] B. Take the titanium dioxide primary product, mix it with a dilute acid aqueous solution at a mass-to-volume ratio of 1 g: 4.5 - 5.5 mL, stir and react at room temperature to obtain a reaction solution; in step B, the mass of the taken titanium dioxide primary product is recorded as m1 (the unit is generally g); the dilute acid aqueous solution is at least one of a nitric acid aqueous solution with a mass concentration of 10% - 30%, a sulfuric acid aqueous solution with a mass concentration of 20% - 40%, or a hydrochloric acid aqueous solution with a mass concentration of 10% - 20%.

[0030] C. Carry out solid-liquid separation and washing on the reaction solution obtained in step B. After drying the obtained powder, weigh it, and record the mass as m2 (the unit is generally g); send it to ICP-OES or ICP-MS for quantitative detection of metal impurities under the same conditions as in step A, and record the internal solid solution concentration of the obtained metal impurities as c2 (the unit is generally ppm);

[0031] D. According to the detection results, analyze the content of internal solid solution metal impurities and surface free metal impurities in the crude titanium dioxide product; the total content of a certain metal impurity in the crude titanium dioxide product is m1·c1 (the unit is generally μg), the internal solid solution content of the metal impurity in the crude titanium dioxide product is m2·c2 (the unit is generally μg), and the surface free content of the metal impurity in the crude titanium dioxide product is m1·c1 - m2·c2 (the unit is generally μg);

[0032] The metal impurity is one or more of Fe, Cu, Ni, Al, Cr or V.

[0033] In step B of the present invention, in order to avoid introducing new metal impurities and affecting the accuracy of the detection results, when preparing the nitric acid aqueous solution, the purity of the nitric acid used is above the purity specification of G2, the total metal impurities are less than 1 ppb, and the total metal impurities of the water used are less than 1 ppb. When preparing the sulfuric acid aqueous solution and the hydrochloric acid aqueous solution, the requirements for the sulfuric acid and nitric acid used are the same.

[0034] In the present invention, treating titanium dioxide with a nitric acid aqueous solution with a mass concentration of 10% - 30%, a sulfuric acid aqueous solution with a mass concentration of 20% - 40%, or a hydrochloric acid aqueous solution with a mass concentration of 10% - 20% will not dissolve the titanium dioxide and cannot destroy the stable crystal structure of TiO2, but can dissolve substances such as surface-free Fe, Cu, Ni, Al, etc. Therefore, the present invention can selectively dissolve the surface-free metal impurities in the crude titanium dioxide product by using at least one of 10% - 30% nitric acid, 20% - 40% sulfuric acid, or 10% - 20% hydrochloric acid. Preferably, in step B, the dilute acid aqueous solution is a nitric acid aqueous solution with a mass concentration of 15% - 20%.

[0035] In step B of the present invention, the reaction time is 8 - 24 hours.

[0036] In step C of the present invention, the drying temperature is 110 - 130 °C; the drying time is 6 - 8 hours.

[0037] The content of metal impurities in titanium dioxide particles is relatively low, basically at the ppm level. In this case, pickling with diluted acid can effectively wash away the surface gold impurities. If concentrated acids (such as hydrochloric acid, nitric acid, sulfuric acid, etc.) are used to dissolve the gold impurities, titanium dioxide will also be dissolved. Therefore, by using the change value of the total gold impurities in titanium dioxide before and after dilute acid hydrolysis in the present invention, the contents of free gold impurities on the surface of titanium dioxide particles and solid-solution gold impurities inside are calculated, which further provides strong evidence and support for the analysis of the source of gold impurities in actual production and measures to reduce gold impurities.

[0038] In step D of the present invention, when m2·c2≈m1·c1, it indicates that the metal impurity in the crude titanium dioxide product is mainly the solid-solution metal impurity inside the powder particles, and its main sources are the raw materials and processes of the oxidation reaction. The source of raw material gold impurities can be reduced by optimizing the source of titanium raw materials, or the gold impurity pollution of the equipment can be reduced by changing the equipment material or regularly cleaning the equipment; when m2·c2≈0, it indicates that the metal impurity in the crude titanium dioxide product is mainly the free metal impurity free on the surface of the powder particles, and its main source is the transmission process after the oxidation reaction rather than the oxidation reaction process. This type of metal impurity can be removed by the pickling process; when 0<m2·c2<m1·c1, it indicates that part of the metal impurity in the crude titanium dioxide product is the solid-solution metal impurity inside the powder particles, and part is the free metal impurity free on the surface of the powder particles. Part of the metal impurity can be removed by the pickling process.

[0039] In the present invention, to avoid introducing new metal impurities and affecting the accuracy of the detection results, the total gold impurities of the reagents (such as water, etc.) used are required to be less than 1 ppb. For the devices used, such as flasks, magnetic stirrers, solid-liquid separation devices, etc., nitric acid cleaning is generally required.

[0040] The method of the present invention can obtain information on the existence state of metal impurities on the surface of crude titanium dioxide particles, clarify the main sources of metal impurities, provide a scientific basis for the analysis and control of the sources of metal impurities in the titanium dioxide production process, and provide important support for the stability and improvement of the quality of titanium dioxide products.

[0041] The present invention will be further described in detail below through examples, but the protection scope of the present invention is not limited to the scope of the examples described.

[0042] Test Example 1: Screening of acid concentration and acid dissolution time

[0043] Accurately weigh 5 groups of 8.0000 g of No. 4 crude titanium dioxide products, place them in a round-bottom flask, add a magnetic stirrer, and add 40 mL of high-purity dilute nitric acid solutions with concentrations of 5, 15, and 35% respectively. Stir and react at room temperature for 3, 12, and 24 hours respectively to obtain the reaction solutions. Detect the Ti content and Fe content of the reaction solutions. The experimental results are shown in Tables 1 and 2.

[0044] Table 1 Results of acid concentration

[0045] Number Concentration of nitric acid, % Acid dissolution time, h Ti content in the reaction solution, ppm Fe content in the reaction solution, ppm 1 5 12 ＜0.1 7 2 15 12 ＜0.1 11 3 35 12 8 11

[0046] Table 2 Acid Dissolution Time Results

[0047] Number Concentration of nitric acid, % Acid dissolution time, h Ti content in the reaction solution, ppm Fe content in the reaction solution, ppm 4 15 3 ＜0.1 5 2 15 12 ＜0.1 11 5 15 24 ＜0.1 11

[0048] It can be seen from Table 1 and Table 2 that a dilute nitric acid solution with an appropriate concentration can achieve the complete dissolution of surface free metal impurities without dissolving titanium dioxide, and the Ti content in the reaction solution is close to 0. At the same time, by appropriately extending the acid dissolution time, the complete dissolution of surface free metal impurities can be ensured, guaranteeing the accuracy of the test results. Therefore, in the present invention, an aqueous nitric acid solution with a mass concentration of 10% - 30% and a similar dilute acid aqueous solution are used, and the acid dissolution reaction time is 8 - 24 hours.

[0049] Example 1

[0050] Detect the content of dissolved Fe and surface free Fe in the first-grade titanium dioxide sample No. 1.

[0051] (1) Weigh 5 g of the first-grade titanium dioxide sample No. 1 and send it to ICP-OES for Fe quantitative detection. The test result is 110 ppm;

[0052] (2) Clean instruments such as the flask with nitric acid solution in advance. Accurately weigh 8.2550 g of the first-grade titanium dioxide sample No. 1, place it in a round-bottom flask, add a magnetic stirrer, and add 40 mL of 15% high-purity dilute nitric acid solution. Stir and react at room temperature for 12 hours;

[0053] (3) Use a glass Buchner funnel washed with nitric acid to perform vacuum filtration on the reaction solution obtained in step (2). After the acid solution is filtered out, wash the powder with pure water. After drying the powder obtained by filtration at 120 °C for 6 hours, weigh the mass of the pickled powder as 8.2543 g and send it to ICP-OES for Fe quantitative detection; the test result is 22 ppm;

[0054] (4) It can be calculated from the experimental results that the total Fe content of the first-grade titanium dioxide sample No. 1 is 8.2550 × 110 ppm = 908.0500 μg; the remaining Fe amount of the first-grade titanium dioxide sample No. 1 after nitric acid washing is 8.2543 × 22 ppm = 181.5946 μg. Then the content of dissolved metal impurities inside the first-grade titanium dioxide sample No. 1 is 181.5946 μg, and the content of surface free metal impurities on the powder is 726.4554 μg. It can be seen from the data that most of the Fe in the first-grade titanium dioxide sample No. 1 is free on the surface of the powder particles, and a small part of Fe is dissolved inside the powder particles. The Fe in the first-grade titanium dioxide sample No. 1 mainly comes from the transfer process after the oxidation reaction, which may be introduced by the corrosion of equipment pipelines; most of the metal impurities can be removed through the pickling process.

[0055] (5) Collect the pickling waste liquid obtained in step (3), record its mass as 85 g, send the acid solution to ICP-OES for quantitative detection of Fe and Ti. The test results are 8.5 ppm and <0.1 ppm respectively. It is calculated that Fe is 722.5 μg, which is basically consistent with the free Fe content obtained in step (4); and there is no free Ti in the acid solution, indicating that the titanium dioxide particles themselves are not dissolved and there will be no solid solution overflow. At the same time, continue to react the titanium dioxide powder obtained in step (3) with 15% high-purity dilute nitric acid solution at room temperature for 12 hours. After solid-liquid separation, the Fe content in the acid solution is measured to be <0.1 ppm. It can be seen that the free Fe on the surface of No. 1 titanium dioxide powder has been completely dissolved, and the undissolved titanium dioxide and internal solid solution gold impurities remain.

[0056] Example 2

[0057] Detect the content of solid solution Fe and surface free Fe in the initial product of No. 2 titanium white.

[0058] (1) Weigh 5 g of the initial product of No. 2 titanium white and send it to ICP-OES for quantitative detection of Fe. The test result is 9 ppm;

[0059] (2) Clean the instruments such as the flask with nitric acid solution in advance. Accurately weigh 8.5112 g of the initial product of No. 2 titanium white, place it in a round-bottom flask, add a magnetic stirrer, and add 43 mL of 15% high-purity dilute nitric acid solution. Stir and react at room temperature for 14 hours;

[0060] (3) Use a glass Buchner funnel washed with nitric acid to carry out vacuum filtration on the reaction solution obtained in (2). After the acid solution is filtered, wash the powder with pure water; dry the powder obtained by filtration at 120 °C for 6 hours, weigh the mass of the pickled powder as 8.5111 g, and send it to ICP-OES for quantitative detection of Fe; the test result is 8 ppm;

[0061] (4) It can be calculated from the experimental results that the total Fe content of the initial product of No. 2 titanium white is 8.5112 × 9 ppm = 76.6008 μg; the remaining Fe amount of the initial product of No. 2 titanium white after nitric acid washing is 8.5111 × 8 ppm = 68.0888 μg. Then the content of solid solution metal impurities inside the initial product of No. 2 titanium white is 68.0888 μg, and the content of free metal impurities on the powder surface is 8.5120 μg. From the data, it can be seen that most of the Fe in the initial product of No. 2 titanium white is solid-dissolved inside the powder particles, and a small part of Fe is free on the surface of the powder particles. The Fe in the initial product of No. 2 titanium white mainly comes from raw materials such as TiCl4 or the oxidation reaction process; the Fe in the initial product of No. 2 titanium white cannot be removed by the pickling process.

[0062] Example 3

[0063] Detect the content of solid solution Cu, Ni and surface free Cu, Ni in the initial product of No. 3 titanium white.

[0064] (1) Weigh 5 g of the initial titanium white product No. 3 and send it to ICP-OES for quantitative detection of Cu and Ni. The test results are 8 ppm and 3 ppm respectively;

[0065] (2) Clean the instruments such as the flask with nitric acid solution in advance. Accurately weigh 9.4672 g of the initial titanium white product No. 3 and place it in a round-bottom flask. Add a magnetic stirrer and 50 mL of 20% high-purity dilute nitric acid solution. Stir and react at room temperature for 16 hours;

[0066] (3) Use a glass Buchner funnel cleaned with nitric acid to carry out vacuum filtration on the reaction solution obtained in (2). After the acid solution is filtered out, wash the powder with pure water; dry the powder obtained by filtration at 120 °C for 6 hours, then weigh the mass of the pickled powder as 9.4660 g and send it to ICP-OES for quantitative detection of Cu and Ni; the test results are 6 ppm and 3 ppm;

[0067] (4) It can be calculated from the experimental results that the total Cu content of the initial titanium white product No. 3 is 9.4672 × 8 ppm = 75.7376 μg; the total Ni content is 9.4672 × 3 ppm = 28.4016 μg; the remaining amount of Cu in the initial titanium white product No. 3 after nitric acid washing is 9.4660 × 6 ppm = 56.7960 μg; the remaining amount of Ni is 9.4660 × 3 ppm = 28.3980 μg. Then the content of Cu metal impurities dissolved in the initial titanium white product No. 3 is 56.7960 μg; the content of Ni metal impurities is 28.3980 μg, the content of free Cu impurities on the powder surface is 18.9416 μg; the content of Ni impurities is 0.0036 μg. It can be seen from the data that most of the Cu in the initial titanium white product No. 3 is dissolved in the powder particles, and a small part of the Cu is free on the powder surface; most of the Ni in the initial titanium white product No. 3 is basically dissolved in the powder particles. The Cu and Ni in the initial titanium white product No. 3 mainly come from raw materials such as TiCl4 or the oxidation reaction process; the Cu and Ni in the initial titanium white product No. 3 cannot be effectively removed through the pickling process.

Claims

1. A method for rapidly measuring the dissolved metal impurities and surface free metal impurities in the crude titanium dioxide product, characterized in that: It includes the following steps: A. Take the crude titanium dioxide product and send it to ICP-OES or ICP-MS for quantitative detection of metal impurities. The total concentration of internal solid solution and surface free of the obtained metal impurities is denoted as c1; B. Take the crude titanium dioxide product and mix it with a dilute acid aqueous solution at a mass-to-volume ratio of 1 g: 4.5 - 5.5 mL, and stir and react at room temperature to obtain a reaction solution. In step B, the mass of the taken crude titanium dioxide product is denoted as m1; the dilute acid aqueous solution is at least one of a nitric acid aqueous solution with a mass concentration of 10% - 30%, a sulfuric acid aqueous solution with a mass concentration of 20% - 40%, or a hydrochloric acid aqueous solution with a mass concentration of 10% - 20%; C. Perform solid-liquid separation and washing on the reaction solution obtained in step B. After drying the obtained powder, weigh it, and the mass is denoted as m2; send it to ICP-OES or ICP-MS for quantitative detection of metal impurities under the same conditions as in step A, and the internal solid solution concentration of the obtained metal impurities is denoted as c2; D. According to the detection results, analyze the content of internal solid solution metal impurities and surface free metal impurities in the crude titanium dioxide product; the total content of a certain metal impurity in the crude titanium dioxide product is m1·c1, the internal solid solution content of the metal impurity in the crude titanium dioxide product is m2·c2, and the surface free content of the metal impurity in the crude titanium dioxide product is m1·c1 - m2·c2; The metal impurities are one or more of Fe, Cu, Ni, Al, Cr, or V.

2. The method according to claim 1, wherein: In step B, the dilute acid aqueous solution is a nitric acid aqueous solution with a mass concentration of 15% - 20%.

3. The method according to claim 2, wherein: In step B, when preparing the nitric acid aqueous solution, the purity of the used nitric acid is above the G2 purity specification, the total metal impurities are less than 1 ppb, and the total metal impurities of the used water are less than 1 ppb.

4. The method according to claim 1, wherein: In step B, the reaction time is 8 - 24 hours.

5. The method according to claim 1, characterized in that: In step C, the drying temperature is 110 - 130 °C.

6. The method according to claim 1, characterized in that: In step C, the drying time is 6 - 8 hours.

7. The method according to any one of claims 1 to 6, characterized in that: In step D, when m2·c2 ≈ m1·c1, it indicates that the metal impurity in the crude titanium dioxide product is mainly the internal solid solution metal impurity in the powder particles; when m2·c2 ≈ 0, it indicates that the metal impurity in the crude titanium dioxide product is mainly the free metal impurity on the surface of the powder particles; when 0 < m2·c2 < m1·c1, it indicates that part of the metal impurity in the crude titanium dioxide product is the internal solid solution metal impurity in the powder particles and part is the free metal impurity on the surface of the powder particles.