Step-by-step chemical extraction method based on research on occurrence state of rare earth elements in thermal change coal

Abstract

The invention discloses a step-by-step chemical extraction method based on research on occurrence states of rare earth elements in thermal change coal, which comprises the following specific steps of: indirectly quantifying the occurrence states of the rare earth elements in the coal by adopting a plurality of reagents (H2O-CH3COONH4-HCl-HNO3-H2O2-HF) through a six-step step-by-step chemical extraction system; the three states include a water soluble state, an ion exchange state, a carbonate binding state, a sulfide binding state, an organic binding state and an aluminosilicate binding state. According to the method, an indirect method, namely a step-by-step chemical extraction experiment, for researching the occurrence state of the rare earth elements in the thermal change coal is researched, so that the geochemical behaviors of the rare earth elements in the thermal change coal can be deeply understood, and reference is provided for the research method of the occurrence state of the rare earth elements in the coal; and scientific basis can be provided for recovery and utilization of rare earth elements, and the method has important environmental and economic significance.

Description

Technical Field

[0001] The invention relates to the technical field of element extraction and state research, and more particularly to a step-by-step chemical extraction method based on the study of the occurrence state of rare earth elements in thermally altered coal. Background Art

[0002] Rare earth elements (REY) are widely used as geochemical indicators to study the sedimentary environment and later changes of ore deposits due to their consistency and predictability in geochemical processes. In addition, the excellent magnetic, optical and electrical properties of rare earth elements make them play a key role in traditional industries such as military, petrochemical, metallurgy, and emerging fields such as permanent magnets, photovoltaic films, and catalysts. Therefore, REY has become an important strategic resource to promote the upgrading of traditional industries, the development of high-tech industries, and the advancement of national defense technology, and its value is irreplaceable.

[0003] The study of the occurrence state of REY in coal is very important. It not only provides us with valuable information on peat deposition, coal diagenesis, the formation of coal-bearing basins and the evolution of regional geological background, but also has an important impact on coal mining, coal preparation, combustion and comprehensive utilization in practical applications. In particular, the occurrence mode of REY in coal and coal ash is crucial for the development of methods and technologies for extracting REY from coal or coal ash.

[0004] There are many methods for studying the occurrence state of elements in coal. The specific analysis methods are mainly divided into two categories: indirect methods and direct methods. The indirect methods include statistical analysis (mainly including correlation analysis, cluster analysis and principal component analysis), float and sink method (or density separation) and step-by-step chemical extraction method; the direct methods include optical microscopy, proton induced X-ray emission (PIXE), high sensitivity and high resolution ion probe (SHRIMP), transmission electron microscopy (TEM), scanning electron microscopy combined with energy dispersive X-ray spectroscopy (SEM-EDS), electron probe analysis (EMPA), X-ray photoelectron spectroscopy (XPS), laser ablation-inductively coupled plasma mass spectrometry (LA-ICP-MS) and time-of-flight secondary ion mass spectrometry (TOF-SIMS).

[0005] Researchers' understanding of the occurrence state of REY in coal relies heavily on indirect methods, such as multivariate statistical analysis and step-by-step chemical extraction experiments. Based on these analytical methods, elements that are weakly or uncorrelated with ash yield, retained in residual organic matter during step-by-step chemical extraction, and enriched in light components during density separation are generally considered to have organic affinity. However, these methods also have some drawbacks. For example, the correlation analysis method in multivariate statistical methods may sometimes lead to erroneous understanding and judgment. In some cases, some elements have a good correlation with ash yield, which may not indicate that they are present in minerals. In addition, during step-by-step chemical extraction and density separation experiments, some elements are present in tiny mineral particles. Because they are wrapped in organic matter and enriched in organic light components, or because they cannot be leached out by acid during step-by-step chemical extraction, they may be mistakenly considered to be present in organic matter.

[0006] Researchers still do not have a clear understanding of the occurrence state of rare earth elements in thermally altered coal, the research methods are not systematic and comprehensive, and the opinions vary greatly. In particular, in the indirect step-by-step chemical extraction experiments, the extraction methods vary, and the total extraction rate of each form of some elements does not reach 80%-120%, and the reliability of the experimental data needs to be considered. Therefore, how to improve the step-by-step extraction to ensure that the extraction rate provides a reliable basis for scientific research is an urgent problem to be solved by technicians in this field. Summary of the invention

[0007] In view of this, the present invention improves the indirect method for studying the occurrence state of REY in thermally altered coal, that is, step-by-step chemical extraction, to effectively ensure the total extraction rate of each form and improve the reliability of the data.

[0008] To achieve the above object, the present invention adopts the following technical solution:

[0009] A step-by-step chemical extraction method based on the study of the occurrence state of rare earth elements in thermally altered coal comprises the following steps:

[0010] Step 1: Water-soluble extraction

[0011] Weigh the coal sample after air-drying, impurity removal, grinding and sieving, disperse it in deionized water, seal it and place it on a shaker for shaking. After the treatment is completed, cool it to room temperature, filter it and separate it. Add deionized water to the filtrate to make up the volume for ICP-MS measurement;

[0012] Step 2: Ion exchange extraction

[0013] The solid residue from step 1 was dried, cooled to room temperature and then dispersed in CH 3 COONH 4 After sealing, place on a shaker for shaking treatment. After treatment, cool to room temperature and then filter to separate. Add deionized water to the filtrate to make up to volume for ICP-MS measurement.

[0014] Step 3: Carbonate-bound extraction

[0015] The solid residue in step 2 was dried, cooled to room temperature, dispersed in dilute HCl, sealed, and placed on a shaker for shaking. After the treatment, it was cooled to room temperature, then filtered and separated. Deionized water was added to the filtrate to make up the volume for ICP-MS measurement;

[0016] Step 4: Sulfide-bound extraction

[0017] The solid residue in step 3 was dried, cooled to room temperature and dispersed in dilute HNO 3 After sealing, heat on a hot plate for digestion. After digestion, cool naturally to room temperature, filter and separate. Add deionized water to the filtrate to make up to volume for ICP-MS measurement.

[0018] Step 5: Organically bound state extraction

[0019] The solid residue in step 4 was dried, cooled to room temperature and dispersed in dilute HNO 3 After sealing, place on a shaker for shaking. After the treatment is completed, cool to room temperature and add H 2 O 2 Afterwards, seal the lid and set air holes on the lid. Then heat the lid in a water bath for extraction. After the extraction is completed, cool the lid to room temperature and add CH 3 COONH 4 The solution was extracted in a water bath, and then cooled to room temperature naturally after the extraction was completed. Then, it was separated by suction filtration, and the filtrate was fixed to volume with deionized water for ICP-MS measurement.

[0020] Step 6: Aluminosilicate Bound State Extraction

[0021] Take the solid residue from step 5, dry it, weigh it, and then add dilute HNO 3 After the digestion is completed, the digestion solution is transferred to a PFA sample cup and placed on a hot plate at 180°C to drive the acid to the size of soybean grains. Then, 5 mL of HNO diluted 1 times is added. 3 , cover and extract for 4 h. After cooling, dilute to volume with deionized water for ICP-MS testing.

[0022] Preferably, the sieving in step 1 is through a 200 mesh sieve.

[0023] Preferably, the mass volume ratio of the coal sample to deionized water in step one is 1 g:30 mL; the shaking table treatment parameters in steps one to three and five are: 18-22°C, 200 r / min, and treatment for 12 hours.

[0024] Preferably, the drying temperature in step 2 to step 5 is 50° C. and the drying time is 6 hours.

[0025] Preferably, in step 2, CH 3 COONH 4 The solution concentration is 1 mol / L, CH 3 COONH 4 The volume ratio of the solution to the deionized water in step 1 is 1:1.

[0026] Preferably, the concentration of the dilute HCl in step three is 3%, and the volume ratio of the dilute HCl to the deionized water in step one is 1:1.

[0027] Preferably, the dilute HNO in step 4 3 Concentration is 5%, dilute HNO 3 The volume ratio with the deionized water in step 1 is 1:1; the heating temperature of the hot plate is 180°C, and the heating digestion time is 30 minutes.

[0028] Preferably, the dilute HNO in step 5 3 The concentration is 20%, dilute HNO 3 The volume ratio of H to the deionized water in step 1 is 1:6; 2 O 2 With HNO 3 The volume ratio is 3:1, and H 2 O 2 The water bath temperature was 85°C and the extraction time was 5h; CH 3 COONH 4 The concentration is 1 mol / L, CH 3 COONH 4 With HNO 3 The volume ratio is 1:1, and CH 3 COONH 4 The water bath temperature was 85°C and the extraction time was 5 h.

[0029] Preferably, in step 6, HNO is added during the digestion and extraction process based on the weight of the solid residue. 3 The volume to sample mass ratio was 100 mL:1 g, and the volume ratio of HNO 3 :HF=5:2,HNO 3 :H 2 O=1:1;

[0030] The super microwave digestion program is:

[0031] The basic load of the digestion tank consists of 330 mL of deionized water, 30 mL of 30% H 2 O 2 and 2 mL 98% HO 2 SO 4 The composition, initial nitrogen pressure was 50 bar, the maximum temperature was 240 °C, and the microwave digestion procedure was as follows:

[0032]

[0033]

[0034] Preferably, in the filtration process in steps 1 to 5, the container is washed with deionized water and the washing liquid is filtered together.

[0035] It can be seen from the above technical solution that, compared with the prior art, the present invention discloses a step-by-step chemical extraction method based on the study of the occurrence state of rare earth elements in thermally altered coal, which has the following beneficial effects:

[0036] The present invention studies an indirect method for studying the occurrence state of rare earth elements in thermally altered coal - a step-by-step chemical extraction experiment. It not only helps to deeply understand the geochemical behavior of rare earth elements in thermally altered coal, but also provides a reference for the research methods of the occurrence state of rare earth elements in coal, and also provides a scientific basis for the recovery and utilization of rare earth elements, which has important environmental and economic significance. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on the provided drawings without paying creative work.

[0038] Figure 1 This is a step-by-step chemical extraction experiment flow chart of the present invention;

[0039] Figure 2 The extraction rate of rare earth elements from the coal sample according to the method of Example 1;

[0040] Figure 3 This is a graph showing the proportion of rare earth elements in the coal sample by chemical extraction step by step according to the method of Example 1;

[0041] Figure 4 The extraction rate of rare earth elements from the coal sample of Comparative Example 1;

[0042] Figure 5 This is a diagram of the organic matter situation of the flotation coal sample in comparative example 2. DETAILED DESCRIPTION

[0043] The technical solution of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0044] Example 1

[0045] Instruments and drugs required for the test:

[0046] Balance: accurate to 1.0000g;

[0047] Polytetrafluoroethylene (PFA) sample cup;

[0048] 150mL conical flask; filtration device;

[0049] 40% HF(GR), 65% HNO after sub-boiling distillation purification by acid purification system 3 (GR), 36% HCl (GR), 98% H 2 SO 4 (GR); 30%H 2 O 2 (GR); CH 3 COONH 4 (MOS); deionized water;

[0050] Nylon filter membrane with a diameter of 50 mm and a pore size of 0.45 μm;

[0051] Sample pretreatment: The collected coal samples are first air-dried naturally, and then the impurities such as calcite are removed by a cutting machine. After crushing, the coal samples are ground into fine particles with an agate mortar and sieved through a 200-mesh sieve.

[0052] Specific steps:

[0053] Step 1 (water-soluble state): weigh 1g±0.0005g of coal sample into a 150mL conical flask (number the conical flask before weighing and record its weight for subsequent calculation of the sample mass involved in each form extraction), add 30mL of deionized water, seal it with a sealing film, place it in a shaker, 20±2℃, 200r / min, and shake for 12h; then take out the sample, cool it to room temperature, and filter it for separation (number the filtration device before filtration, and record the weight of the conical flask at the bottom, so as to read the fixed volume weight after filtration); and wash the conical flask three times with 5mL of deionized water, filter the three washing liquids together into the extract, and use a washing bottle to squeeze out a small amount of deionized water to rinse the liquid splashed on the inner wall of the device during the filtration process, and try to ensure that all the extracted elements are transferred to the filtration liquid; add deionized water to the filtered clear liquid to make it up to 50mL for ICP-MS testing;

[0054] Note: One blank sample and one parallel sample should be made at the same time during the experiment;

[0055] Step 2 (ion exchange state): The solid residue in step 1 was placed in an oven at 50°C for about 6 hours to ensure complete drying. After cooling to room temperature, all samples were scraped into the corresponding conical flask with a weighing spoon, weighed and recorded; 30 mL 1 mol / L CH 3 COONH 4 , the steps of oscillation, filtration and constant volume are the same as the first step;

[0056] Step 3 (carbonate bound state): the solid residue in step 2 is placed in an oven at 50°C for about 6 hours to ensure complete drying. After cooling to room temperature, all the samples are scraped into the corresponding conical flask with a weighing spoon, weighed and recorded; 30 mL of dilute HCl (3%) is added, and the shaking, filtration and volume adjustment steps are the same as in step 1;

[0057] Step 4 (sulfide-bound state): The solid residue in step 3 is placed in an oven at 50°C for about 6 hours to ensure complete drying. After cooling to room temperature, use a weighing spoon to scrape all the samples into the corresponding conical flask, weigh and record; add 30 mL of dilute HNO 3 (5%), sealed with sealing film, and then placed on a hot plate for digestion at 180°C for 30 minutes (turn off the hot plate after digestion, and remove the conical flask from the hot plate after cooling to room temperature to prevent the sealing film from being broken by pressure due to sudden temperature drop or the conical flask from rupturing); filtration and volume determination steps are the same as in the first step;

[0058] Step 5 (organic binding state): The solid residue in step 4 is placed in an oven at 50°C for about 6 hours to ensure complete drying. After cooling to room temperature, use a weighing spoon to scrape all the samples into the corresponding conical flask, weigh and record; add 5 mL of dilute HNO 3 (20%), seal with sealing film, place in a shaker, 20±2℃, 200r / min, shake for 12h; then take out the sample, cool to room temperature, add 15mL, seal with sealing film and pierce some holes with a syringe, extract in a water bath at 85℃ for 5h; cool to room temperature, add 5mL 1mol / LCH 3 COONH 4 , extract in a water bath at 85°C for 5h; after cooling to room temperature, filter and adjust volume in the same way as in the first step;

[0059] Step 6 (aluminosilicate bound state): Dry the solid residue in step 5 at 50 °C for about 6 h to ensure complete drying. Weigh 0.05 g ± 0.0005 g of sample into a TFM digestion tube. Add 5 mL of HNO 3 and 2 mL HF, and digested using Milestone's UltraClave microwave digester according to the set program, and then the digestion solution was completely transferred to a 15 mL polytetrafluoroethylene (PFA) sample cup, and the acid was driven to the size of soybeans at 180 ° C on a hot plate, and then 5 mL of 1-fold diluted HNO3 , cover and extract for 4 h, cool, and make up to 50 mL with deionized water for ICP-MS testing.

[0060] Super microwave digestion program settings:

[0061] The basic load of the digestion tank consists of 330 mL of deionized water, 30 mL of H 2 O 2 and 2 mL H 2 SO 4 The initial nitrogen pressure was set to 50 bar and the maximum temperature was set to 240 °C. The microwave digestion program is shown in the table below.

[0062] step Time / h:min:s Temperature / ℃ Pressure / bar Power / W 1 00：12：00 60 100 1000 2 00：20：00 125 100 1000 3 00：08：00 160 130 1000 4 00：15：00 240 160 1200 5 00：60：00 240 160 1000 Cooling time 00：60：00

[0063] Coal quality data and vitrinite random reflectance (R o,ran ) are shown in Table 1. According to GB / T15224.1-2018 standard, the ash content (A d ) ranges from very low ash coal to very high ash coal. The average ash content of raw coal (LD7-7, LD7-8) not affected by magma is 14.01%, which is classified as low ash coal. The A content of thermally altered coal (LD7-1 to L7-6) affected by magma is d The value fluctuates greatly, ranging from 9.07% to 46.79%, with an average value of 21.20%. ad ) has an average value of 1.92%, while the M ad The volatile matter of raw coal (V daf ) has an average value of 34.51%, while the V daf It ranges from 7.69% to 20.03%, with an average of 13.95%.

[0064] According to the international coal classification standard ISO 11760, the R o,ran The average value is 0.86%, which is classified as medium-rank bituminous coal; the R o,ran The value range is 1.39% to 3.83%, with an average value of 2.47%. The coal rank transitions from medium-rank bituminous coal to high-rank anthracite.

[0065] According to GB / T 15224.2-2021 standard, the total sulfur content (S t,d ) averaged 0.2%, which is classified as extra-low sulfur coal. In contrast, the total sulfur content (S t,d ) ranged from 0.35% to 3.57%, with an average of 1.14%.

[0066] Table 1 Industrial analysis and elemental analysis of coal samples

[0067] Sample No. <![CDATA[M ad / %]]> <![CDATA[A d / %]]> <![CDATA[V daf / %]]> <![CDATA[R o,ran / %]]> Coal rank <![CDATA[S t,d / %]]> LD7-1 2.08 46.79 10.77 3.83 High-grade anthracite 3.57 LD7-2 1.86 31.33 7.69 2.64 High-grade anthracite 1.15 LD7-3 0.79 9.07 15.29 2.30 High-grade anthracite 0.66 LD7-4 0.71 16.63 20.03 1.98 Medium rank bituminous coal 0.35 LD7-5 1.45 11.66 9.98 2.70 High-grade anthracite 0.58 LD7-6 0.77 11.71 19.95 1.39 Medium rank bituminous coal 0.52 LD7-7 2.05 14.00 34.92 0.85 Medium rank bituminous coal 0.25 LD7-8 1.78 14.01 34.11 0.87 Medium rank bituminous coal 0.15

[0068] The type and concentration of the extractant according to the method of Example 1 are adjusted, such as Figure 2 , 3 As shown, the six-step extraction rate of 15 kinds of rare earth elements in the experimental coal samples is basically between 80% and 120%, and through the analysis of coal quality and organic binding state, the present invention not only ensures the normal extraction rate, but also greatly increases the distinction between organic binding state and aluminosilicate binding state, and more accurately extracts rare earth elements associated with organic components in coal. The organic binding state of rare earth elements in the two raw coal samples not affected by magma is higher than that in thermally altered coal, and medium-heavy rare earth elements show stronger organic affinity than light rare earths.

[0069] Comparative Example 1

[0070] The difference from Example 1 is that in the fifth step, 5% dilute HNO 3 As an extractant.

[0071] The results are as follows Figure 4 As shown in the figure, this method seems to fail to extract the organically bound rare earth elements as completely as possible. And this method has certain limitations, and it cannot ensure that the total extraction rate of rare earth elements in all samples is within the ideal range of 80%-120%. In addition, this method also has uncertainty in distinguishing organically bound and aluminosilicate-bound rare earth elements, which will affect the accurate assessment of the occurrence state of rare earth elements in coal.

[0072] Comparative Example 2

[0073] Density separation method, using chloroform (CHCl 3 )Specific gravity liquid (1.47g / cm 3 ) The organic matter is floated out, and the upper and lower layers of the flotation liquid are extracted to obtain the organic bound state and the carbonate bound state.

[0074] The density of clay minerals is generally 2.4-2.6g / cm 3 The density of pyrite is 4.9-5.2g / cm 3 Therefore, bromoform (2.89 g / cm 3 ) Separate clay minerals and pyrite to obtain aluminosilicate-bound states and carbonate-bound states.

[0075] However, it is not possible to completely float out a specific component based on density, and errors are inevitable. Experiments have found that density separation is not suitable for all coal samples, especially thermally altered coal samples. Due to the complexity of its coal components, stratification may not occur, or organic matter loss caused by coal metamorphism may even lead to CHCl 3 The upper layer cannot float out organic matter, as follows Figure 5 shown.

[0076] In this specification, each embodiment is described in a progressive manner, and each embodiment focuses on the differences from other embodiments, and the same and similar parts between the embodiments can be referred to each other. The above description of the disclosed embodiments enables professionals and technicians in this field to implement or use the present invention. Various modifications to these embodiments will be obvious to professionals and technicians in this field, and the general principles defined herein can be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention will not be limited to these embodiments shown in this article, but will comply with the widest range consistent with the principles and novel features disclosed herein.

Claims

1. A step-by-step chemical extraction method based on the study of the occurrence state of rare earth elements in thermally altered coal, characterized in that: The following steps are involved: Step 1: Water-soluble extraction Weigh the coal sample after air-drying, impurity removal, grinding and sieving, disperse it in deionized water, seal it and place it on a shaker for shaking. After the treatment is completed, cool it to room temperature, filter it and separate it. Add deionized water to the filtrate to make up the volume for ICP-MS measurement; Step 2: Ion exchange extraction The solid residue from step 1 was dried, cooled to room temperature, dispersed in an ammonium acetate (CH3COONH4) solution, sealed, and placed on a shaker for shaking. After the treatment, it was cooled to room temperature, then filtered and separated. The filtrate was fixed to volume with deionized water for ICP-MS measurement. Step 3: Carbonate-bound extraction The solid residue in step 2 is dried, cooled to room temperature, dispersed in dilute hydrochloric acid (HCl), sealed, and placed on a shaker for shaking. After the treatment is completed, it is cooled to room temperature, then filtered and separated, and the filtrate is added with deionized water to make up to volume for ICP-MS measurement; Step 4: Sulfide-bound extraction The solid residue in step 3 was dried, cooled to room temperature and dispersed in dilute nitric acid (HNO3), sealed and placed on a hot plate for heating and digestion, cooled naturally to room temperature after digestion, and then filtered and separated, and the filtrate was fixed to volume with deionized water for ICP-MS measurement; Step 5: Organically bound state extraction The solid residue in step 4 was dried, cooled to room temperature and dispersed in dilute HNO3, sealed and placed on a shaker for shaking, cooled to room temperature after treatment, sealed after adding hydrogen peroxide (H2O2) and provided with pores on the seal, then extracted by heating in a water bath, cooled to room temperature after extraction, added with CH3COONH4 solution, continued water bath extraction, cooled naturally to room temperature after extraction, then filtered and separated, and the filtrate was fixed to volume with deionized water for ICP-MS measurement; Step 6: Aluminosilicate Bound State Extraction Take the solid residue in step five, dry it and weigh it, then add HNO3 and HF and perform super microwave digestion. After digestion, transfer the digestion solution to the PFA dissolution cup, place it on a hot plate at 180℃ to drive the acid to the size of soybean grains, then add 5mL of 1-fold diluted HNO3, cover and extract for 4h, cool and make up to volume with deionized water for ICP-MS testing.

2. A step-by-step chemical extraction method based on the study of the occurrence state of rare earth elements in thermally altered coal according to claim 1, characterized in that: The sieving in step 1 is through a 200 mesh sieve.

3. A step-by-step chemical extraction method based on the study of the occurrence state of rare earth elements in thermally altered coal according to claim 1, characterized in that: The ratio of the coal sample mass to the volume of deionized water in step 1 is 1 g:30 mL; the shaking table treatment parameters in steps 1 to 3 and 5 are: 18-22° C., 200 r / min, and treatment for 12 h.

4. A step-by-step chemical extraction method based on the study of the occurrence state of rare earth elements in thermally altered coal according to claim 1, characterized in that: The drying temperature in step 2 to step 5 is 50° C. and the time is 6 hours.

5. The step-by-step chemical extraction method based on the study of the occurrence state of rare earth elements in thermally altered coal according to claim 1 is characterized in that: The concentration of the CH3COONH4 solution in step 2 is 1 mol / L, and the volume ratio of the CH3COONH4 solution to the deionized water in step 1 is 1:

1.

6. The step-by-step chemical extraction method based on the study of the occurrence state of rare earth elements in thermally altered coal according to claim 1 is characterized in that: The concentration of the dilute HCl in step three is 3%, and the volume ratio of the dilute HCl to the deionized water in step one is 1:

1.

7. A step-by-step chemical extraction method based on the study of the occurrence state of rare earth elements in thermally altered coal according to claim 1, characterized in that: The concentration of the dilute HNO3 in step 4 is 5%, and the volume ratio of the dilute HNO3 to the deionized water in step 1 is 1:1; the heating temperature of the hot plate is 180°C, and the heating digestion time is 30 minutes.

8. The step-by-step chemical extraction method based on the study of the occurrence state of rare earth elements in thermally altered coal according to claim 1, characterized in that: The concentration of dilute HNO3 in step five is 20%, and the volume ratio of dilute HNO3 to deionized water in step one is 1:6; the volume ratio of H2O2 to HNO3 is 3:1, the water bath temperature is 85°C after adding H2O2, and the extraction time is 5h; the concentration of CH3COONH4 is 1 mol / L, the volume ratio of CH3COONH4 to dilute HNO3 is 1:1, the water bath temperature is 85°C after adding CH3COONH4, and the extraction time is 5h.

9. The step-by-step chemical extraction method based on the study of the occurrence state of rare earth elements in thermally altered coal according to claim 1, characterized in that: In step 6, based on the weight of the solid residue, the volume ratio of HNO3 added during digestion and extraction to the sample mass was 100 mL:1 g, and the volume ratios of HNO3:HF were 5:2 and HNO3:H2O were 1:1; The super microwave digestion program is: The basic load of the digestion tank consisted of 330 mL of deionized water, 30 mL of 30% H2O2, and 2 mL of 98% sulfuric acid (H2SO4). The initial nitrogen pressure was 50 bar, the maximum temperature was 240 °C, and the microwave digestion program was as follows:

10. The step-by-step chemical extraction method based on the study of the occurrence state of rare earth elements in thermally altered coal according to claim 1, characterized in that: In the filtration process from step 1 to step 5, the container is washed with deionized water and the washing liquid is filtered together.

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