Sample pretreatment method for chromite microelement analysis

Abstract

The invention relates to a sample pretreatment method for chromite microelement analysis. The method comprises the following steps: firstly, treating a sample to specific particle size distribution (Dv (50) lt, 5 [mu] m, Dv (90) lt, 20 [mu] m) through physical accurate grinding, and then adopting an improved two-step acid dissolution and digestion process: firstly, destroying main crystal lattices of minerals by using mixed acid, then, carrying out targeted dissolution on a residual insoluble phase by using aqua regia, and finally, carrying out chlorine removal treatment to obtain a solution to be detected. Through the synergistic effect of physical accurate grinding and secondary acid dissolution, almost complete decomposition of chromite is achieved, especially the extraction efficiency of key indissolvable elements such as Sc and Zn is improved by 20% or above, the accuracy and precision of ICP-MS analysis results are remarkably improved, and the problem that microelement measured value systematicness is low due to incomplete dissolution in a traditional method is solved.

Description

Technical Field

[0001] This invention relates to the fields of geological analytical chemistry and instrumental analysis technology, specifically to a method for pretreatment of trace element samples of sparingly soluble minerals, particularly chromite, and especially a method combining physical grinding and improved chemical digestion. Background Technology

[0002] Chromite is a dark-colored metallic mineral with the general chemical formula (Mg, Fe)(Al,Cr, Fe)₂O₄, primarily composed of oxides of iron, magnesium, and chromium. This mineral plays a crucial role in understanding geological processes and demonstrating its economic value. Widely found in the mantle and crustal rocks of the lithosphere, chromite contains important information about the formation and evolution of primordial magma. Furthermore, chromite is the main mineral forming chromite deposits and is the most important and currently the only source of metallic chromium; chromium, as a key metallic element, is widely used in the production of stainless steel, high-temperature alloys, and other high-performance materials.

[0003] Trace elements (such as Sc) in chromite provide crucial information for revealing the composition of the parent magma, crystallization temperature and pressure conditions, oxygen fugacity, and subsequent alteration processes. Therefore, obtaining accurate trace element data is essential. Currently, the main techniques for testing trace elements in chromite include neutron activation analysis (INAA), laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), and chemical digestion methods (such as alkali fusion and acid dissolution). Among these, INAA eliminates the need for chemical digestion, avoids pretreatment contamination, and is free from matrix effects. However, it relies on nuclear reactor facilities, has a long analysis cycle, and completely loses spatial resolution. LA-ICP-MS has significant advantages in in-situ micro-area analysis, but it suffers from matrix effects. When using silicate glass standards for calibration and quality control, matrix differences may affect data accuracy. Alkali fusion (such as Na2O2 or NaOH) in chemical digestion methods forcibly decomposes insoluble minerals at high temperatures, but the high proportion of flux leads to severe sample dilution, decreased sensitivity, high reagent background, and the high-salt matrix can easily clog the instrument. Acid dissolution methods (such as HF and HNO3) are relatively conventional benchmark analysis methods, offering simple pretreatment and low reagent blanks. However, chromite is a physically and chemically stable, insoluble mineral, posing a significant challenge to chemical digestion sample preparation. Traditional acid dissolution methods (such as using HF-HNO3) often fail to completely decompose the dense crystal structure of chromite, easily leaving acid-resistant mineral residues. This results in systematically low determinations of trace elements (such as Sc and Zn), failing to accurately reflect their total content.

[0004] Chromite, as an important metallic mineral, uses its trace element (Sc, V, Co, Ni, Zn, Ga, etc.) composition as a crucial geochemical indicator for retrieving parent magma composition, crystallization temperature and pressure conditions, oxygen fugacity, and subsequent alteration history. Therefore, obtaining accurate trace element data is essential. However, the dense crystal structure and complex trace element occurrence states of chromite often present challenges to traditional acid dissolution methods, leading to incomplete decomposition and lower-than-expected results.

[0005] Currently, the main techniques for testing trace elements in chromite include neutron activation analysis (INAA), laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), and chemical digestion-solution injection ICP-MS. While INAA does not require digestion, it relies on a nuclear reactor, has a long analysis cycle, and lacks spatial resolution. LA-ICP-MS allows for in-situ micro-area analysis, but it suffers from matrix effects and requires well-matched standards. Chemical digestion ICP-MS, as a benchmark analysis method, offers relatively simple pretreatment and low reagent blanks, and is considered a reliable means of obtaining accurate total elemental data.

[0006] However, chromite is a sparingly soluble mineral with extremely stable physical and chemical properties, posing a significant challenge to chemical digestion methods for sample preparation. Traditional acid dissolution methods (such as those using HF-HNO3) often fail to completely decompose the dense crystal structure of chromite, easily leaving acid-resistant mineral residues. This results in systematically lower than expected values ​​for trace elements (such as Sc and Zn), failing to accurately reflect their total content. Furthermore, the digestion process of traditional methods is relatively simple, with insufficient control over side reactions that may cause elemental hydrolysis or precipitation, affecting the accuracy and precision of the data.

[0007] Therefore, there is an urgent need in this field to develop a sample pretreatment method that can thoroughly and completely decompose chromite and achieve efficient and stable leaching of all trace elements therein. Summary of the Invention

[0008] To overcome the shortcomings of existing technologies, this invention provides a sample pretreatment method for the precise analysis of trace elements in chromite. By using a physical grinding-modified chemical digestion method, it is possible to achieve near-complete dissolution of six trace elements, namely Sc, V, Co, Ni, Zn, and Ga, in chromite, significantly improving the dissolution efficiency of key trace elements such as Sc and Zn, thereby enhancing the accuracy and precision of ICP-MS analysis results.

[0009] In a first aspect, the present invention provides a sample pretreatment method for trace element analysis of chromite, comprising the following steps:

[0010] 1) Physical grinding: The chromite sample is coarsely ground and then finely ground to make Dv(50)<5μm and Dv(90)<20μm, and the particle size distribution spacing (SPAN)<4.5.

[0011] Furthermore, after fine grinding, the sample's volume average particle size D[4,3] < 8 μm, surface area average particle size D[3,2] < 4 μm, and specific surface area of ​​2000-3000 m². 2 / kg.

[0012] Furthermore, the coarse grinding particle size is 200-300 mesh.

[0013] Furthermore, the finely ground particle size is 800-1000 mesh.

[0014] Furthermore, the fine grinding is performed using a planetary ball mill.

[0015] The inventors discovered that using a combination of coarse and fine grinding effectively achieves particle refinement and homogenization, ensuring good reactivity and uniformity of the sample during subsequent digestion. The most significant change is a substantial increase in the sample's specific surface area. Since specific surface area is a key factor controlling the solid-liquid or solid-gas reaction interface, it greatly enhances the sample's reactivity and accelerates the mineral dissolution process.

[0016] 2) Single acid digestion: Add hydrofluoric acid (HF) and nitric acid (HNO3) to the finely ground sample, carry out a single heating reaction, and after completion, cool and evaporate to dryness.

[0017] Furthermore, the volume ratio of HF to HNO3 is preferably 1:3 to 1:8, more preferably 1:6.

[0018] Furthermore, the concentration of the chromite sample is 3-5 mg / ml.

[0019] Furthermore, the initial heating temperature is 180-200℃, and the heating time is 40-50 hours.

[0020] Furthermore, during a single acid dissolution process, the strong complexing ability of HF and the strong oxidizing property of HNO3 are utilized to initially disrupt the main crystal structure of chromite, releasing the trace elements contained therein.

[0021] Furthermore, the evaporation temperature is 140-160℃.

[0022] Furthermore, the acid dissolution and digestion process is carried out in a closed container.

[0023] 3) Secondary acid dissolution and digestion: Add aqua regia to the residue after the first digestion and evaporation, carry out a second heating reaction, and after the reaction is completed, cool and evaporate to dryness.

[0024] This step utilizes the strong oxidizing properties of aqua regia and its high concentration of Cl. -Its strong complexing effect effectively dissolves the insoluble phase remaining after the initial digestion, releasing elements such as Co, Ni, and Zn, and stabilizing ions in the solution.

[0025] Furthermore, the aqua regia is prepared by mixing concentrated nitric acid and concentrated hydrochloric acid in a volume ratio of 1:3.

[0026] The amount of aqua regia added is needed to dissolve the residue; preferably, the mass-volume ratio of residue to aqua regia is 1:5-10 mg / ml.

[0027] Furthermore, the secondary heating temperature is 130-150℃, and the heating time is 40-50 hours.

[0028] Furthermore, the evaporation temperature is 120-130℃.

[0029] Furthermore, in the above steps, the concentration of hydrofluoric acid is 40-48%, and the concentration of nitric acid is 65-68%.

[0030] This invention employs a two-step digestion reaction: "first crystal breaking, second extraction." The first acid dissolution (HF-HNO3 system) is a solid-state lattice destruction process. Its core task is to disrupt the dense main lattice structure of chromite. This process requires relatively high energy (180-200℃) to drive HF to break strong chemical bonds such as chromium-oxygen and aluminum-oxygen bonds, representing the most challenging stage. The subsequent second aqua regia treatment transforms the main reaction mechanism from lattice destruction to the oxidation-complexation dissolution of the exposed insoluble phase, resulting in a liquid-phase complexation reaction. The temperature is set within the range of 130-150℃ because: 1) Cl... - With Co 2+ Ni 2+ Zn 2+ Complexation reactions involving target metal ions are mostly exothermic or equilibrium reactions. For exothermic reactions, compared to higher acidolysis temperatures (180-200℃), temperatures of 130-150℃ are more conducive to shifting the reaction towards the product (i.e., the complex) (Le Chatelier's principle). Therefore, this temperature range provides an environment for appropriate cooling, which is more conducive to increasing the equilibrium constant of the complexation reaction, thereby thermodynamically promoting the transfer of metal ions from the solid phase to the liquid phase and ensuring their stable existence in the solution, thus improving the actual dissolution efficiency of the target element and the stability of the solution; 2) The components of aqua regia (HCl, HNO3) have a certain degree of volatility. Within this specific temperature range, their volatilization and decomposition during long-term reactions (40-50 hours) can be effectively suppressed, ensuring that the system maintains sufficient Cl. - Concentration and a necessary oxidizing environment are required to achieve continuous and thorough action on the residual phase; 3) This temperature range is suitable for the strong oxidizing properties of aqua regia and Cl... -Its complexing ability provides an optimal window of action, enabling efficient targeting of specific poorly soluble substances while avoiding sample loss due to side reactions or localized violent reactions that may be caused by excessively high temperatures.

[0031] Thus, it can be seen that in the process from "macroscopic structural destruction" to "microscopic targeted dissolution", this invention significantly improves the release efficiency, completeness and dissolution stability of various trace elements in chromite through the precise synergy of physical parameters (temperature) and chemical system (acid combination).

[0032] 4) Chlorine removal: After secondary acid dissolution and digestion, the product is dried by heating with nitric acid.

[0033] Furthermore, the amount of nitric acid added is sufficient to dissolve the sample.

[0034] Furthermore, to completely remove residual chloride ions (to prevent corrosion of the instrument and interference from polyatomic ions), concentrated nitric acid was used for heating and drying.

[0035] Furthermore, it also includes step 5) volume adjustment: add the chlorine-removed sample to the internal standard solution, heat to dissolve, cool, and adjust the volume to obtain a clear and transparent test solution, which can be directly used for ICP-MS detection.

[0036] Furthermore, the internal standard solution is a dilute nitric acid solution containing Rh, wherein the concentration of Rh is 200 ng / mL.

[0037] In some embodiments of the present invention, after chlorination, 1 mL of dilute nitric acid solution containing Rh internal standard (200 ng / mL, containing 2% HNO3), 2 mL of HNO3 and 3 mL of deionized water are added to the sample, and the mixture is heated in an oven at 150°C for 8 hours to ensure complete redissolution and homogeneity. After cooling, the volume is adjusted to obtain a clear and transparent test solution.

[0038] In a second aspect, the present invention provides a test solution for ICP-MS determination of trace elements in chromite, wherein the test solution is prepared by the pretreatment method described in any one of the first aspects.

[0039] Furthermore, the test solution is a clear, transparent, and homogeneous solution, free of visible solid particles, and contains chloride ions (Cl...). - The concentration was effectively removed to a level that did not affect ICP-MS determination.

[0040] Furthermore, the test solution contains ions of Sc, V, Co, Ni, Zn, and Ga elements, and their concentrations accurately reflect the original content in the chromite sample.

[0041] Furthermore, the recovery rates of Sc and Zn elements are no less than 95%.

[0042] A third aspect of the present invention provides the application of the pretreatment method described in the first aspect in the field of geochemical research or analytical detection.

[0043] Furthermore, the application specifically includes its use in the determination of chromite standard reference values. The pretreatment method described above is used to process candidate chromite standard references, and their trace element content is determined using ICP-MS to obtain accurate standard values.

[0044] And / or, in the study of mineral deposit genesis. The aforementioned pretreatment method is used to accurately determine the trace element composition of chromite deposits from different locations or types, providing reliable geochemical data for inverting the composition of the parent magma, crystallization temperature and pressure conditions, and mineralization processes.

[0045] And / or, in the application of quality control in analytical laboratories. The aforementioned pretreatment method is used as a benchmark method for the analysis of trace elements in sparingly soluble minerals to validate the accuracy of other rapid analytical methods (such as LA-ICP-MS).

[0046] The beneficial effects of this invention are:

[0047] 1. This invention employs a physical grinding method, which grinds chromite samples to 800-1000 mesh, increasing their specific surface area by more than 40% and significantly homogenizing the particle size distribution. This fundamentally increases the acid-ore reaction interface, greatly accelerates the dissolution kinetics process, and lays a solid foundation for subsequent chemical decomposition, thus being the primary guarantee for achieving efficient dissolution.

[0048] 2. This invention employs an improved chemical digestion method by introducing a secondary digestion step using aqua regia, forming a targeted, staged dissolution mechanism. This fully leverages the strong oxidizing properties of aqua regia and the complexing ability of chloride ions, precisely targeting the insoluble phases exposed after the initial digestion, thereby completely releasing the trace elements contained within. This solves the core problem of insufficient decomposition of stubborn mineral phases in traditional methods. Simultaneously, by optimizing the aqua regia digestion temperature range, efficient and targeted dissolution of insoluble metal ions is achieved, improving the dissolution efficiency of metal ions while enhancing experimental safety and stability.

[0049] 3. This invention utilizes the synergistic effect of physical grinding and improved chemical digestion. First, by grinding the sample to a specific particle size (Dv(50) < 5 μm, Dv(90) < 20 μm) and significantly increasing the specific surface area, not only is the reaction interface of the acid solution greatly increased, accelerating the dissolution kinetics, but the stubborn insoluble phase inside the mineral is also pre-crushed, creating optimal physical conditions for subsequent chemical decomposition. Second, this invention employs an improved two-step acid dissolution method to specifically achieve "crystal breaking first, then extraction": the first high-temperature acid dissolution HF-HNO3 treatment is responsible for destroying the dense main crystal structure, while the second medium-temperature aqua regia treatment, with its strong oxidizing properties and chloride ion complexing ability, accurately dissolves the insoluble phase remaining after the first digestion. Through the synergistic effect of physical grinding and improved chemical digestion, the digestion solution becomes clear and transparent. At the same time, the extraction efficiency of key sparingly soluble elements such as Sc and Zn is systematically improved by more than 20% (the extraction efficiency of Sc and Zn is improved by 21% and 44% respectively), which fundamentally solves the problem of systematically low element determination values ​​caused by incomplete dissolution in traditional methods.

[0050] 4. This invention reveals the differentiated dissolution patterns of different trace elements, which not only verifies the effectiveness of the method in extracting sparingly soluble elements, but also provides methodological support for understanding the occurrence state of elements, demonstrating its important value in the field of geochemical analysis. Attached Figure Description

[0051] Figure 1 The particle size distribution diagrams are of the coarsely ground sample from Example 2 and the finely ground sample from Example 1.

[0052] Figure 2 The images shown are experimental pictures of different experimental stages in Examples 1-4. Among them, A is a picture of the state after the first acid dissolution in Example 1, B is a picture of the state after the first acid dissolution in Example 2, C is a picture of the state after the second acid hydrolysis in Example 1, D is a picture of the state after the second acid hydrolysis in Example 2, E is a picture of the solution after the addition of internal standard in Example 1, F is a picture of the solution after the addition of internal standard in Example 2, G is a picture of the solution after the first acid dissolution and digestion in Example 3, I is a picture of the solution after the addition of internal standard and heating in Example 3, H is a picture of the solution after the first acid dissolution and digestion in Example 4, and J is a picture of the final solution in Example 4.

[0053] Figure 3 The ICP-MS test results for each element in Examples 1-4 are shown below. Red represents Example 1 (fine grinding + improvement), green represents Example 2 (coarse grinding + improvement), purple represents Example 3 (fine grinding + conventional), and blue represents Example 4 (coarse grinding + conventional).

[0054] Figure 4 This is a comparison of the results of Example 1 with the INAA trace element values ​​in the literature. Detailed Implementation

[0055] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation of the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0056] The reagents and raw materials used in the embodiments of this invention are all commercially available.

[0057] The hydrofluoric acid (HF) and nitric acid (HNO3) reagents used in the embodiments of this invention are all commercially available concentrated hydrofluoric acid and concentrated nitric acid products purified by sub-boiling distillation to achieve ultra-high purity. The concentration of hydrofluoric acid is 40-48%, and the concentration of nitric acid is 65-68%.

[0058] Example 1

[0059] A sample pretreatment method for trace element analysis of chromite includes the following steps:

[0060] 1) Physical grinding: The pod-shaped chromite standard material GBW07292 (GPt-5) was ground to 200 mesh using an agate mortar, and then further ground to 800 mesh using a planetary ball mill. The results showed that Dv(50) = 3.75 μm, Dv(90) = 16.0 μm, and the particle size distribution span (SPAN) was 4.008.

[0061] 2) Single acid digestion: Weigh 10 mg of sample powder, add 0.5 mL of sub-boiling HF and 3 mL of sub-boiling HNO3. Tightly seal the sample dissolution vessel, place it in a stainless steel sleeve, and heat in an oven at 190℃ for 48 hours. After complete cooling, remove the vessel; black particles were observed in the solution. Figure 2 (A) Place the sample dissolving cup on a heating plate at 150°C to evaporate to dryness.

[0062] 3) Secondary acid digestion: Add 10 mL of aqua regia (HNO3:HCl = 1:3) to the product from the first acid digestion and evaporation, then heat at 140℃ for 48 hours. After complete cooling, remove the solution; it should become clear. Figure 2 (C), place the sample dissolving cup on a heating plate at 120℃ to evaporate to dryness.

[0063] 4) Chlorine removal: After evaporation to dryness, add 0.6 mL of sub-boiling HNO3 and evaporate to dryness at 150℃. Repeat this step twice. Then add 1 mL of Rh internal standard solution (200 ng / mL, containing 2% HNO3), 2 mL of HNO3, and 3 mL of deionized water. Seal tightly and heat in an oven at 150℃ for 8 hours. After cooling, shake well; the solution should be clear and transparent. Figure 2(E), transfer 0.4 mL of the supernatant and dilute to 8 mL to obtain a clear and transparent solution.

[0064] 5) The obtained test solution was analyzed by ICP-MS.

[0065] Example 2 (Coarse grinding + Improvement)

[0066] 1) Physical coarse grinding: Grind the pod-shaped chromite standard material GPt-5 to 200 mesh using an agate mortar.

[0067] Steps 2)-5) are the same as in Example 1.

[0068] Example 3 (Fine grinding + conventional grinding)

[0069] The secondary acid dissolution and digestion step in Example 1 is omitted:

[0070] 1) Physical grinding: Grind the pod-shaped chromite standard material GPt-5 to 200 mesh using an agate mortar, and then continue grinding to 800 mesh using a planetary ball mill.

[0071] 2) Single acid digestion: Weigh 10 mg of sample powder, add 0.5 mL of sub-boiling HF and 3 mL of sub-boiling HNO3. Tightly seal the dissolution vessel, place it in a stainless steel sleeve, and heat in an oven at 190℃ for 48 hours. After complete cooling, remove the vessel and place it on a heating plate at 150℃ to evaporate to dryness.

[0072] 3) Chlorine removal and volume adjustment: After evaporation to dryness, add 0.6 mL of sub-boiling HNO3 and evaporate to dryness at 150℃. Then add 1 mL of Rh internal standard solution (200 ng / mL, containing 2% HNO3), 2 mL of HNO3 and 3 mL of deionized water, seal tightly and heat in an oven at 150℃ for 8 hours. After cooling, shake well, transfer 0.4 mL of the supernatant and adjust the volume to 8 mL to obtain a clear and transparent solution.

[0073] 4) The obtained test solution was analyzed by ICP-MS.

[0074] Example 4 (coarse grinding + conventional grinding)

[0075] 1) Physical coarse grinding: Grind the pod-shaped chromite standard material GPt-5 to 200 mesh using an agate mortar.

[0076] 2) Single acid digestion: Weigh 10 mg of sample powder, add 0.5 mL of sub-boiling HF and 3 mL of sub-boiling HNO3. Tightly seal the dissolution vessel, place it in a stainless steel sleeve, and heat in an oven at 190℃ for 48 hours. After complete cooling, remove the vessel and place it on a heating plate (150℃) to evaporate to dryness.

[0077] 3) Chlorine removal and volume adjustment: After evaporation to dryness, add 0.6 mL of sub-boiling HNO3 and evaporate to dryness (150℃). Then add 1 mL of Rh internal standard solution (200 ng / mL, containing 2% HNO3), 2 mL of HNO3 and 3 mL of deionized water, seal tightly and heat in an oven at 150℃ for 8 hours. After cooling, shake well, transfer 0.4 mL of the supernatant and adjust the volume to 8 mL to obtain a clear and transparent solution.

[0078] 4) The obtained test solution was analyzed by ICP-MS.

[0079] Example 5

[0080] The other steps are the same as in Example 1, except that the secondary acid dissolution and digestion temperature is higher.

[0081] 2) Secondary acid digestion: Add 10 mL of aqua regia (HNO3:HCl = 1:3) to the product from the first acid digestion and evaporation, and heat again at 190℃ for 48 hours. After complete cooling, remove the sample and place the sample cup on a heating plate (120℃) to evaporate to dryness.

[0082] Example 6 (Fine grinding - particle size not up to standard)

[0083] 1) Physical treatment: Grind the same batch of GPt-5 samples to 200 mesh using an agate mortar and pestle, and then continue grinding to 600 mesh using a planetary ball mill.

[0084] Steps 2) to 5) are exactly the same as in Example 1.

[0085] Test Example 1: Particle Size Distribution of the Sample

[0086] The determination was performed using a Malvern Mastersizer 3000+ Ultra laser particle size analyzer equipped with a wet sample introduction unit (Hydro LV). An appropriate amount of sample was weighed, added to pure water, and sonicated (5-10 minutes) to fully disperse the particle clusters. The dispersed suspension was then added dropwise to the running Hydro LV sampler until the laser shading stabilized within the ideal measurement range of 10-15%. All operations were performed according to the instrument's built-in standardized operating procedures (SOPs). The final result was the average of three measurements. Key parameters included volume average particle size D[4,3], surface average particle size D[3,2], and characteristic particle sizes Dv(10), Dv(50), and Dv(90).

[0087] The finely ground samples obtained in step 1) of Example 1, the coarsely ground samples obtained in step 1) of Example 2, and the coarsely ground samples obtained in step 1) of Example 6 were subjected to particle size analysis, and the results are shown in Table 1. Among them, Dv(50) and Dv(90) are the cumulative median diameter and 90% particle size of the volume datum, respectively, used to characterize that the central particle size of the sample tends to be distributed at the coarse particle end; SPAN is the particle size distribution span, which is equal to [Dv(90) - Dv(10)] / Dv(50), used to evaluate the concentration and uniformity of particle size distribution; D[3,2] is the surface area average diameter, which reflects the size of the particle specific surface area and directly affects the reaction interface; D[4,3] is the volume average diameter, which is more sensitive to the presence of large particles in the sample.

[0088] Table 1 Comparison of particle size of samples

[0089]

[0090] The data in Table 1 for Examples 1 and 2 show that fine grinding systematically optimized the physical properties of the samples: compared with coarsely ground samples, the median particle size Dv(50) of finely ground samples decreased significantly by about 40%, indicating that the overall particle size was significantly refined and the median particle size was effectively reduced; the coarse particle end index Dv(90) decreased significantly by about 55%, proving that fine grinding effectively removed the coarse particle tail; the particle size distribution span SPAN value decreased to 4.008, indicating that the particle size distribution was more concentrated and uniform; while the specific surface area increased sharply by about 49%, which fundamentally enhanced the sample reactivity; D[3,2] (surface area average diameter) is sensitive to fine particles, and its reduction directly confirms the increase in specific surface area. D[4,3] (volume average diameter) is sensitive to large particles, and its reduction (from 13.2 to 6.9) is much greater than the reduction of Dv(50) (from 6.32 to 3.75), which strongly proves that fine grinding has a particularly outstanding effect on crushing the "coarse particle tail" (i.e., large particles). These changes together create a better reaction interface and mass transfer conditions for the subsequent acid dissolution process.

[0091] The evolution of the particle size distribution curve further confirms the effectiveness of fine grinding. Figure 1 The unground sample of Example 2 exhibited a broad distribution plateau at the coarse particle end (20-100 μm), while the plateau of the finely ground sample of Example 1 essentially disappeared, with the main peak migrating towards the finer particles and becoming sharper. This clearly demonstrates that fine grinding not only effectively pulverized the coarse particle components but also achieved an overall transformation of the particle size distribution towards a finer and more concentrated range.

[0092] Data from Examples 1 and 6 show that although Example 6 underwent 600-mesh fine grinding, its Dv(50) and Dv(90) both exceeded the suitable range, and the SPAN value also indicated poor distribution uniformity. While its specific surface area was better than the coarse-ground sample, it was still lower than the standard fine-ground sample. The physical state of Example 6 was between that of coarse grinding and standard fine grinding, failing to create optimal reaction conditions for subsequent acid leaching. Even using the exact same improved two-step acid dissolution process, it was difficult to achieve complete decomposition of chromite and efficient extraction of trace elements, highlighting the necessity of synergistic effects between physical parameters and chemical steps in this invention.

[0093] Test Example 2: Black particulate residue

[0094] Observe the solution state of different embodiments after key processing steps ( Figure 2 The significant impact of the treatment method on the dissolution effect can be clearly seen.

[0095] The dissolution process in Example 1 (fine grinding + improved secondary digestion method) shows a trend of continuous optimization: black particles are present in the solution after the first acid digestion. Figure 2 (A); After secondary digestion with aqua regia, the clarity of the solution was significantly improved ( Figure 2 (C); After adding the internal standard and heating for the third time, the solution reached a clear and transparent state (C). Figure 2 E).

[0096] Example 2 (coarse grinding + improved secondary digestion method) although it underwent the same secondary acid dissolution and digestion process, the dissolution effect was worse than that of Example 1: the number of black particles in the solution after the first acid dissolution and digestion was comparable to that of Example 1. Figure 2 (B); After secondary digestion, obvious black particles still remained in the solution ( Figure 2 (D); After adding the internal standard and heating for the third time, obvious black particles were still present in the solution. Figure 2 (F).

[0097] In the traditional single-stage digestion method system, Example 3 (fine grinding + traditional single-stage digestion method) showed that the solution after a single acid dissolution digestion contained black particles ( Figure 2 After adding the internal standard and heating, the number of particles decreased relatively, and the solution became clearer. Figure 2 I). In Example 4 (coarse grinding + traditional single-stage digestion method), under the same treatment conditions, the solution after a single acid digestion contained black particles (I). Figure 2 The final solution still contained a significant number of black particles (H). Figure 2 J).

[0098] The experimental results clearly show that fine grinding of the sample significantly improves the dissolution efficiency of chromite, while the improved chemical secondary digestion method effectively reduces insoluble residues. When fine grinding is combined with the improved secondary digestion method, the best synergistic effect is achieved, resulting in the most thorough sample decomposition.

[0099] Furthermore, in Example 6, the number of black particles was significantly greater than in Example 1 after the first acid dissolution, and the solution remained turbid after the second acid dissolution. The final volumetric solution had poor clarity, which also demonstrates the importance of fine grinding conditions.

[0100] Test Example 3: Trace Element Content Detection

[0101] The median contents of six trace elements (Sc, V, Co, Ni, Zn, and Ga) in the solutions of each experimental group were systematically analyzed using ICP-MS. The results are as follows: Figure 3 As shown in Figure 1 and Table 2.

[0102] Table 2 Trace element content detection

[0103]

[0104] Based on the data and results analysis in Table 2, the synergistic effect of fine grinding and improved digestion method in Example 1 showed a significant advantage in trace element extraction. The trace element content distribution characteristics of each treatment group are as follows:

[0105] Scandium (Sc): Example 1 (fine grinding + improved method) showed the highest content (7.18 μg / g), while Example 4 (coarse grinding + conventional method) showed the lowest (2.23 μg / g). The extraction efficiency was still limited when fine grinding or the improved method was used alone (as in Examples 2 and 3), indicating that Sc may mainly reside in the sparingly soluble mineral phase, requiring the synergistic effect of physical fine grinding to break down the encapsulation structure and an improved acid hydrolysis system (strong oxidation and complexation with aqua regia) for effective release.

[0106] Vanadium (V): Example 1 showed the highest content (736.3 μg / g), which was significantly better than other single process combinations. This is because fine grinding greatly increases the specific surface area, while the aqua regia oxidation and chlorine removal treatment in the improved digestion method effectively prevents V from hydrolyzing and precipitating during digestion, thus demonstrating the dual synergistic effect of physical refining and chemical stabilization.

[0107] Co (Cobalt), Zn (Zinc), and Ga (Ga): The highest contents of all three were observed in Example 1 (Co 176.09 μg / g, Zn 389.52 μg / g, Ga 21.58 μg / g). However, under the same digestion method, the improvement brought by fine grinding was less than that of the improved digestion method, indicating that the extraction of these elements is mainly dominated by the chemical digestion step. The strong oxidizing properties and high Cl⁻ environment of aqua regia can effectively dissolve insoluble phases and form stable chloride complexes, while fine grinding assists this chemical process by increasing the reaction interface.

[0108] Ni (Ni): The median value in Example 1 was the highest (1230.31 μg / g), and the differences between groups were relatively small, indicating that Ni may mainly exist in acid-soluble form in chromium spinel. Most of it can be released by conventional acid digestion, and fine grinding and improved methods further enhance the recovery stability.

[0109] Furthermore, data from Example 6 shows that although it employed the same "improved" digestion method as Examples 1 and 2, the extraction efficiency of all elements decreased to some extent due to insufficient fine grinding. In particular, the extraction yields of elements such as Sc and Zn, which rely on physical breaking of encapsulation, were low, demonstrating that substandard fine grinding cannot create sufficient reaction conditions for subsequent chemical dissolution. This result proves the synergistic necessity of the fine grinding physical parameters and the improved chemical digestion steps in this invention; both must be satisfied simultaneously to achieve optimal extraction of trace elements from chromite.

[0110] In summary, Example 1 (fine grinding + improved digestion method) achieved the highest recovery rate in the extraction of six trace elements: Sc, V, Co, Ni, Zn and Ga, highlighting the synergistic effect of physical fine grinding and improved chemical digestion: fine grinding improves the mineral reaction interface, while the improved acid digestion system enhances the dissolution and stability of sparingly soluble elements, thereby achieving more comprehensive and stable recovery of trace elements.

[0111] Furthermore, the relevant experimental data from Examples 5-6 were examined to investigate the extraction efficiency of each element during the secondary acid hydrolysis process, as shown in Table 3.

[0112] Table 3 Trace element content detection

[0113]

[0114] Comparing the data from Examples 1 and 5 reveals that increasing the secondary acid dissolution temperature from the optimized 140℃ to 190℃ resulted in a decrease in the extraction efficiency of all six target trace elements. This confirms the necessity of strictly controlling the secondary digestion temperature within the range of 130-150℃ in this invention.

[0115] 1) Impact on the equilibrium and stability of the complexation reaction: The core role of secondary acid dissolution (aqua regia system) is based on Cl... -With metal ions (such as Co) 2+ Ni 2+ Zn 2+ ,Sc 3+ (e.g.,) forms stable chloride complexes, thereby "extracting" them from the solid phase to the liquid phase and storing them stably. This complexation process is mostly exothermic. Therefore, at higher temperatures (190℃), the equilibrium shifts towards the endothermic direction (i.e., the direction of complex dissociation), which is unfavorable for the formation and stability of complexes. This leads to a significant decrease in the recovery rate of metal elements, especially those with a strong tendency to form chloride complexes, such as Zn (decreased by 11.5%) and Sc (decreased by 5.3%).

[0116] 2) High temperatures exacerbate reagent volatilization and system deactivation: Both HCl and HNO3 in aqua regia are volatile. During the prolonged digestion process, high temperatures significantly accelerate acid volatilization, which not only leads to the deactivation of HCl but also... - A decrease in concentration weakens the complexing ability and causes the oxidation potential of the system to decay prematurely, making it impossible to continuously and thoroughly oxidize and dissolve the remaining insoluble phase.

[0117] Based on the above data, this invention constructs an efficient and stable trace element analysis system for chromite by combining physical grinding with improved chemical digestion technology. Firstly, at the physical level, grinding (to 800-1000 mesh) not only significantly reduces the sample particle size (Dv(50) < 5μm) but also greatly increases the specific surface area (by approximately 49%), creating optimal mass transfer conditions and reaction interfaces for subsequent chemical reactions. Secondly, at the chemical level, the improved "two-step gradient temperature acid dissolution" process achieves step-by-step targeted action. The first high-temperature (180-200℃) acid dissolution (HF-HNO3 system) focuses on disrupting the dense main crystal lattice structure; the second medium-temperature (130-150℃) extraction (aqua regia system) utilizes its strong oxidizing properties and Cl-... - Its strong complexing ability precisely dissolves residual insoluble phases, thereby releasing the encapsulated trace elements. This ultimately overcame two major technical bottlenecks: the dense crystal structure of chromite and the persistent state of trace element occurrence.

[0118] The results showed that the digestion solution was clear and transparent, which directly proved that the dissolution was complete. The extraction efficiency of six trace elements, especially key elements such as Sc and Zn, was improved by more than 20%. The accuracy and precision of the determination results of each element were high, which effectively solved the systematic bias problem of traditional methods.

[0119] Test Example 3 Method Verification

[0120] To verify the reliability of the pretreatment method of this invention, the contents of trace elements such as Sc, V, Co, Ni, and Zn in the chromite standard material were normalized and analyzed in this test example, using the INAA recommended values ​​studied by Akhter et al. (2016 Akhter, R., Shirai, N., Ebihara, M. (2016). Chemical characterization of a chromitite reference sample GPt-5 using INAA and ICP-MS. Geochemical Journal, 50(2), 179-185). The results are as follows. Figure 4 As shown in Table 4.

[0121] Table 4. Verification of Trace Element Content

[0122]

[0123] Note: The original INAA paper does not provide a specific recommended value or uncertainty for Ga, and this is not reflected in the table.

[0124] Depend on Figure 4 As shown in Table 4, the measured values ​​of six trace elements (Sc, V, Co, Ni, Zn, and Ga) obtained by Example 1 of this invention (fine grinding + improved digestion) generally show high agreement with the INAA recommended values. The ICP / INAA ratios of V and Co are close to 1 (1.07, 0.97, and 1.06, respectively), indicating that the extraction efficiency of this method for these elements is comparable to the INAA standard method, and the results are reliable. The measured value of Sc (7.18 μg / g) is 21% higher than the INAA recommended value (5.91 μg / g), with an ICP / INAA ratio of 1.21; the measured value of Zn (389.52 μg / g) is significantly higher than the recommended value (271 μg / g), with a ratio as high as 1.44. This demonstrates that the synergistic strategy of physical grinding (increasing specific surface area and breaking mineral encapsulation) and improved secondary acid dissolution (enhanced oxidation-complexation using aqua regia system) employed in this invention can more thoroughly release Sc and Zn contained in the insoluble phase or tightly encapsulated in chromite, achieving more complete dissolution. The measured value of Ni (1230.31 μg / g) is slightly lower than the INAA recommended value (1320 μg / g), with a ratio of 0.93, still within the acceptable range of consistency. The measured value of Co is almost consistent with the recommended value (ratio 0.97), indicating that the extraction of Co by this method has also reached a near-complete level. The relative standard deviation (RSD) of the determination of V, Co, Ni, and Zn by this method is between 2.22% and 3.81%, showing good analytical precision.

[0125] The above results not only verify the effectiveness of the pretreatment method used, but also reveal the geochemical differences in the leaching behavior of different trace elements, providing a methodological basis and theoretical reference for the full extraction of trace elements from chromite and other refractory minerals.

Claims

1. A sample pretreatment method for trace element analysis of chromite, characterized in that, Includes the following steps: 1) Physical grinding: The chromite sample is coarsely ground and finely ground to make the median particle size Dv(50) < 5 μm, the 90% particle size of the volume reference Dv(90) < 20 μm, and the particle size distribution spacing SPAN < 4.

5. 2) Single acid digestion: Add a mixture of hydrofluoric acid and nitric acid to the finely ground sample, carry out a single heating reaction, and after completion, cool and evaporate to dryness; 3) Secondary acid dissolution and digestion: Add aqua regia to the residue after the first digestion and evaporation, carry out a second heating reaction, and after the reaction is completed, cool and evaporate to dryness; 4) Chlorine removal: After secondary acid dissolution and digestion, the product is dried by heating with nitric acid.

2. The pretreatment method according to claim 1, characterized in that, In step 1), the coarse grinding particle size is 200-300 mesh; the fine grinding particle size is 800-1000 mesh. The sample had a volume average particle size D[4,3] < 8 μm and a surface area average particle size D[3,2] < 4 μm; the specific surface area was 2000-3000 m². 2 / kg.

3. The pretreatment method according to claim 1, characterized in that, In step 2), the volume ratio of HF to HNO3 in the mixed acid is 1:3 to 1:8; the concentration of chromite sample in the mixed acid is 3-5 mg / ml.

4. The pretreatment method according to claim 1, characterized in that, In step 2), the heating temperature is 180-200℃ and the heating time is 40-50 hours; the drying temperature is 140-160℃.

5. The pretreatment method according to claim 1, characterized in that, In step 3), the temperature of the secondary heating is 130-150℃, and the heating time is 40-50 hours; the drying temperature is 120-130℃.

6. The pretreatment method according to claim 1, characterized in that, In step 4), nitric acid is used for evaporation to dryness at a temperature of 140-160℃.

7. The pretreatment method according to claim 1, characterized in that, It also includes step 5) volume adjustment: add the chlorine-removed sample to the internal standard solution, heat to dissolve, cool, and adjust the volume to obtain a clear and transparent test solution.

8. The pretreatment method according to claim 1, characterized in that, It also includes step 5), in which the internal standard solution is a nitric acid solution containing Rh.

9. The pretreatment method according to claim 8, characterized in that, In step 5), the test solution contains ions of Sc, V, Co, Ni, Zn and Ga, and the recovery rate of Sc and Zn is not less than 95%.

10. The pretreatment method according to any one of claims 1-9, characterized in that, The pretreatment method is used for geochemical research or analytical testing.

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