Ultrapure coal preparation method based on catalytic hot-melt extraction method

Abstract

The invention discloses an ultra-pure coal preparation method based on a catalytic hot-melt extraction method. The method comprises the following steps: (1) grinding a coal sample to obtain coal powder; (2) uniformly mixing the pyrite catalyst with pulverized coal to obtain a coal / catalyst mixture; (3) mixing the coal / catalyst mixture with a composite extraction solvent, and extracting; wherein the composite extraction solvent comprises methylnaphthalene oil and methanol; (4) carrying out solid-liquid separation on the extracted product to respectively obtain extraction residues and extraction liquid; and (5) distilling to remove the extraction solvent in the extraction liquid, and cleaning the obtained solid product to obtain the ultra-pure coal. Through cooperation of catalysis and hot melting extraction, the extraction effect is good, and the yield of the ultra-pure coal is high.

Description

Technical Field

[0001] The present invention belongs to the field of carbon material synthesis, and particularly relates to a method for preparing ultra-pure coal based on catalytic hot solvent extraction method. Background Art

[0002] Coal is an organic macromolecular substance rich in aromatic structures, and various coal-based new materials can be synthesized from coal. Among them, coal-based activated carbon is a carbon material with a porous structure. Due to its advantages such as a large specific surface area, many nano-pore diameters, and rich pore channels, it has currently attracted extensive attention from researchers in the field. With the continuous development of supercapacitor technology, conventional coal-based activated carbon can no longer meet the growing demand. Affected by coals with different degrees of metamorphism and origins, the composition and content of minerals in coal are different. The electrochemical performance of activated carbon for supercapacitors will be affected by the minerals in coal. Therefore, reducing the ash content of activated carbon has become a current research hotspot, which requires us to start from the preparation of ultra-pure coal (index requirement: organic matter content > 99.9%).

[0003] Currently, the methods for preparing ultra-pure coal mainly include physical methods and chemical methods. Among them, the physical method has the advantages of simple process, low cost, and green and pollution-free, and can be easily operated. The physical method is mainly gravity separation, which can reduce the ash content of coal to a certain extent, but it cannot meet the ash content standard of ultra-pure coal. The chemical method mainly uses strong acids and strong bases to react with the minerals in coal to form soluble salts or gases, so as to achieve the purpose of removing minerals in coal. However, the cost of strong acids and strong bases is relatively high and they will corrode the equipment.

[0004] CN117866682A discloses a method for extracting and preparing ultra-low ash coal with coal as the raw material. Specifically, it belongs to the hot solvent extraction method (or hot solution extraction method), mainly using N-methylpyrrolidone with solubility at high temperature to penetrate into the interior of the molecules of the coal structure and dissolve the molecules in the coal, so as to achieve the goal of reducing the ash content of coal, and has great application prospects. However, its main disadvantage is that the yield of hot solution extraction is relatively low. In addition, its extraction solvent is N-methylpyrrolidone, and the cost is relatively high.

[0005] Therefore, it is necessary to develop a new method for preparing ultra-pure coal to overcome its disadvantages while retaining the advantages of the hot solvent extraction method as much as possible. Summary of the Invention

[0006] The purpose of the present invention is to provide a method for preparing ultra-pure coal based on catalytic hot solvent extraction method to solve the problem of relatively low yield of the conventional hot solution extraction method.

[0007] To achieve the above invention purpose, the present invention adopts the following technical solutions:

[0008] A method for preparing ultra-pure coal based on catalytic thermal dissolution extraction, the preparation method comprising the following steps:

[0009] (1) Grinding and pulverizing a coal sample to obtain pulverized coal;

[0010] (2) Mixing a catalyst with the pulverized coal obtained in step (1) evenly to obtain a coal / catalyst mixture; wherein, the catalyst is pyrite;

[0011] (3) After mixing the coal / catalyst mixture obtained in step (2) with a composite extraction solvent, placing it in a high-temperature reactor and performing extraction in an inert protective atmosphere; wherein, the composite extraction solvent comprises methylnaphthalene oil (also known as methylnaphthalene) and methanol;

[0012] (4) Separating the solid and liquid of the product after extraction in step (3) to obtain extraction residue and extraction liquid respectively;

[0013] (5) Distilling to remove the extraction solvent in the extraction liquid and washing the residual extraction solvent on the obtained solid product to obtain ultra-pure coal.

[0014] In step (1) of the present invention, the coal sample is ground and pulverized to obtain pulverized coal. It can be understood in the art that through grinding and pulverizing, it is beneficial to fully dissociate the minerals and organic matter in the coal for subsequent catalysis / extraction; preferably, in step (1), the coal sample is ground to below 200 mesh (Tyler standard sieve, the same below), that is, it can pass through 200 mesh sieve, 250 mesh sieve, 300 mesh sieve or 400 mesh sieve, preferably ground to below 400 mesh, so that the organic matter and minerals in the coal are more fully dissociated.

[0015] In step (1) of the present invention, a planetary ball mill can be used for grinding and pulverizing the coal sample. Specifically, it is well-known in the art. For example, the ball milling method can be wet, and the dispersant used can be ethanol; the pulp concentration range of the dispersant mixed with the coal sample can be 25%-50%, preferably 25%-54%, such as 30%; the ball milling time can be 0.5-4h, preferably 0.5-3h, such as 2h.

[0016] In a preferred embodiment, in step (1), before the grinding treatment, the coal sample can also be preliminarily separated by a gravity separation method (the principle is to separate the light components and heavy components according to the density difference between the organic matter and minerals in the coal and fish out the light components floating on the liquid surface), so as to remove at least part of the inorganic minerals, and the obtained light component product is washed and dried as the coal sample for subsequent grinding treatment. Specifically, in step (1), when separating the coal sample by the gravity separation method, the coal sample with a particle size greater than 0.5mm, such as 3mm, 4mm or 5mm, can be added to a density of 1.10-1.50 kg / cm 3For example, 1.20 kg / cm 3 , 1.30 kg / cm 3 or 1.40 kg / cm 3 , stir and separate in a heavy medium suspension, take the floating light component product, wash, dry and obtain a coal sample.

[0017] In step (2) of the present invention, the pyrite (i.e., FeS₂) catalyst is mixed evenly with the pulverized coal obtained in step (1); it is understood in the art that the catalyst and the pulverized coal can be mixed in powder form. For example, when mixing, the coal sample and the catalyst are dry-mixed in a ball mill to mix evenly, and at the same time the catalyst is also pulverized (for example, pulverized to a size comparable to that of the pulverized coal, such as below 200 mesh), which is beneficial to exert the catalytic activity on the surface of the catalyst; of course, the catalyst can also be pulverized first and then evenly mixed with the pulverized coal.

[0018] Preferably, in step (2), when mixing, the mass ratio of the coal sample to the catalyst is (10 - 30):1, preferably (15 - 25):1, such as 12:1, 18:1, 20:1, 22:1 or 28:1; it can be understood in the art that too much catalyst is not conducive to cost reduction.

[0019] In step (3) of the present invention, the coal / catalyst mixture is mixed with a composite extraction solvent and then extracted. The atmosphere during extraction should be such that it does not have a negative impact on extraction. For example, extraction is carried out in an inert protective atmosphere; among them, through the cooperation of catalysis and hot dissolution extraction, the extraction effect is good and the yield of ultra-pure coal is high.

[0020] In step (3) of the present invention, the composite extraction solvent includes methylnaphthalene oil and methanol. Preferably, the mass ratio of methylnaphthalene oil to methanol is (5 - 15):1, more preferably (8 - 14):1, such as 10:1 or 12:1, and the two cooperate with each other to improve the extraction effect.

[0021] Preferably, in step (3), the extraction temperature is 300 - 410 °C, preferably 320 - 400 °C, such as 340, 350, 360 or 380 °C.

[0022] Preferably, in step (3), the solid-liquid mass ratio is controlled to be 1:(5 - 15), preferably 1:(8 - 12), such as 1:10. It is understood in the art that too much extraction liquid is not conducive to cost reduction, and vice versa, it affects the extraction effect.

[0023] In a preferred embodiment, the extraction is carried out under the rotation of the stirring paddle provided in the high-temperature reactor to improve the extraction effect / efficiency; in some embodiments, the rotation speed of the stirring paddle can be 150-300 rpm, such as 190-230 rpm, 200 rpm, and the extraction time can be 0.5-4 h, such as 1-3.5 h, 2 h or 3 h.

[0024] In step (4) of the present invention, the product after extraction in the high-temperature reactor is subjected to solid-liquid separation to obtain extraction residue and extraction liquid respectively; the specific solid-liquid separation method can be the commonly used solid-liquid separation methods in the art, which are well known in the art. For example, high-speed centrifugation can be used for solid-liquid separation. The rotation speed of the centrifuge can be 3000-5500 rpm, such as 3500-5000 rpm, 4000 rpm, and the centrifugation time can be 5-15 min, such as 8-12 min, 10 min.

[0025] In step (5) of the present invention, the extraction liquid is distilled to remove the extraction solvent therein. For example, the extraction solvent is recovered by vacuum distillation. The heating temperature during distillation can be 100-150 °C, preferably 120-150 °C, such as 140 °C.

[0026] In step (5) of the present invention, considering that the solid product obtained by distillation may still have residual extraction solvent, the obtained solid product can be further washed to remove the residual extraction solvent to obtain ultra-clean coal.

[0027] In some embodiments, in step (5), during washing, alcohol can be used to ultrasonically treat, centrifuge and wash the obtained solid product until the solution is colorless to obtain an ultra-clean coal sample; specifically, the solid product obtained by distillation can be added to an ethanol solution and ultrasonically treated. The sample is ultrasonically treated at a power of 80-200 W, such as 100 W, in an ultrasonic device, and the ultrasonic treatment time can be 5-10 min; then the ultrasonically treated suspension is placed in a high-speed centrifuge for centrifugation and separation, and the rotation speed is controlled at 3000-5500 rpm, preferably 3500-5000 rpm, such as 4000 rpm. This process is repeated until the solution is colorless, and finally an ultra-clean coal sample is obtained.

[0028] In the preparation method of the present invention, the types of raw coal for preparing the coal sample can be various, such as lignite, long-flame coal, coking coal or anthracite.

[0029] Compared with the prior art, the present invention has the following advantages:

[0030] The present invention purifies the organic matter in coal by a physical-chemical combination method, using pyrite as a catalyst for preparing ultra-pure coal by thermal dissolution extraction method. Under high temperature and high pressure, it can promote the cleavage and depolymerization of covalent bonds such as ether oxygen bonds. At the same time, the cleavage and depolymerization of the molecular structure in coal are beneficial to the dissolution of small molecules in coal by the mixed extractant. Especially when combined with methanol solvent, it can break hydrogen bonds and non-covalent bonds. It can act as a nucleophile to attack the oxygen bridge bonds of coal molecules, causing further bond cleavage. The cooperating methylnaphthalene solvent has a strong dissolving ability for polar substances in coal molecules. Therefore, by utilizing their synergistic effect, the dissolution of organic matter can be significantly promoted, which will greatly improve the extraction yield and quality. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 It is a process flow diagram of an embodiment of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0032] In order to enable those skilled in the art to better understand the solution of the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without making creative efforts shall fall within the protection scope of the present invention.

[0033] In the ranges disclosed herein, the endpoints and any values are not limited to the exact ranges or values. These ranges or values should be understood to include values close to these ranges or values, such as values within ±10% of the endpoint values. For numerical ranges, between the endpoint values of each range, between the endpoint values of each range and individual point values, and between individual point values, they can be combined with each other to obtain one or more new numerical ranges, and these numerical ranges should be regarded as specifically disclosed herein. Without conflict, the embodiments in the present application and the features in the embodiments can be combined with each other.

[0034] It should be noted that the terms "including" and "having" and any variations thereof are intended to cover non-exclusive inclusion. For example, a process, method, system, product, or device that includes a series of steps or units does not necessarily have to be limited to those steps or units clearly listed, but may include other steps or units not clearly listed or inherent to these processes, methods, products, or devices.

[0035] In the following examples / comparative examples, the coal samples used are all coal samples dried at 105 °C, and in the following examples / comparative examples, the relevant dosage ratios are all mass ratios.

[0036] Example 1

[0037] A method for preparing ultra-pure coal based on catalytic thermal dissolution extraction method, comprising the following steps:

[0038] (1) Grind and pulverize a coal sample (lignite) to less than 200 mesh to obtain coal powder;

[0039] (2) Mix the pyrite catalyst evenly with the coal powder obtained in step (1) to obtain a coal / catalyst mixture; wherein, the mass ratio of the coal sample to the catalyst is 20:1;

[0040] (3) After mixing the coal / catalyst mixture obtained in step (2) with a composite extraction solvent, place it in a high-temperature reaction kettle and carry out extraction in a nitrogen atmosphere; wherein, the composite extraction solvent is methylnaphthalene oil and methanol with a dosage ratio of 10:1; control the solid-liquid ratio to be 1:10 during extraction, and the extraction temperature is 360 °C; the stirring paddle speed in the high-temperature reaction kettle is 200 rpm, and the extraction time is 2 h;

[0041] (4) Use a high-speed centrifuge (centrifuge speed is 4000 rpm, time is 10 min) to carry out solid-liquid separation on the product after extraction in step (3) to obtain extraction residue and extraction liquid respectively;

[0042] (5) Remove the extraction solvent in the extraction liquid by vacuum distillation at 120 °C and wash the residual extraction solvent on the obtained solid product to obtain ultra-pure coal. The results are shown in Table 1.

[0043] Example 2

[0044] A method for preparing ultra-pure coal based on catalytic thermal dissolution extraction method, comprising the following steps:

[0045] (1) Grind and pulverize a coal sample (long-flame coal) to less than 200 mesh to obtain coal powder;

[0046] (2) Mix the pyrite catalyst evenly with the coal powder obtained in step (1) to obtain a coal / catalyst mixture; wherein, the mass ratio of the coal sample to the catalyst is 30:1;

[0047] (3) After mixing the coal / catalyst mixture obtained in step (2) with a composite extraction solvent, place it in a high-temperature reaction kettle and carry out extraction in a nitrogen atmosphere; wherein, the composite extraction solvent is methylnaphthalene oil and methanol with a dosage ratio of 5:1; control the solid-liquid ratio to be 1:15 during extraction, and the extraction temperature is 300 °C; the stirring paddle speed in the high-temperature reaction kettle is 150 rpm, and the extraction time is 4 h;

[0048] (4) Use a high-speed centrifuge (centrifuge speed is 4000 rpm, time is 10 min) to carry out solid-liquid separation on the product after extraction in step (3) to obtain extraction residue and extraction liquid respectively;

[0049] (5) The extraction solvent in the extract was removed by vacuum distillation at 120 °C, and the residual extraction solvent on the obtained solid product was washed to obtain ultra-clean coal. The results are shown in Table 1.

[0050] Example 3

[0051] A method for preparing ultra-clean coal based on catalytic thermal solution extraction method, comprising the following steps:

[0052] (1) The coal sample (coking coal) was ground and crushed to less than 400 mesh to obtain coal powder.

[0053] (2) The pyrite catalyst was mixed evenly with the coal powder obtained in step (1) to obtain a coal / catalyst mixture; wherein, the mass ratio of the coal sample to the catalyst was 10:1.

[0054] (3) After the coal / catalyst mixture obtained in step (2) was mixed with the composite extraction solvent, it was placed in a high-temperature reaction kettle and extracted in a nitrogen atmosphere; wherein, the composite extraction solvent was methylnaphthalene oil and methanol with a dosage ratio of 15:1; during extraction, the solid-liquid ratio was controlled at 1:5, and the extraction temperature was 410 °C; the stirring paddle speed in the high-temperature reaction kettle was 300 rpm, and the extraction time was 2 h.

[0055] (4) The product after extraction in step (3) was separated by solid-liquid separation using a high-speed centrifuge (the centrifuge speed was 3000-5000 rpm and the time was 5-15 min) to obtain extraction residue and extract respectively.

[0056] (5) The extraction solvent in the extract was removed by vacuum distillation at 100-150 °C, and the residual extraction solvent on the obtained solid product was washed to obtain ultra-clean coal. The results are shown in Table 1.

[0057] Example 4

[0058] A method for preparing ultra-clean coal based on catalytic thermal solution extraction method, comprising the following steps:

[0059] (1) The coal sample (lignite) was separated by gravity separation. The coal sample with a particle size greater than 0.5 mm was added to a heavy medium suspension with a density of 1.10-1.50 kg / cm 3 and stirred and separated; then the obtained light component product was ground and crushed to less than 200 mesh to obtain coal powder.

[0060] (2) The pyrite catalyst was mixed evenly with the coal powder obtained in step (1) to obtain a coal / catalyst mixture; wherein, the mass ratio of the coal sample to the catalyst was 15:1.

[0061] (3) Mix the coal / catalyst mixture obtained in step (2) with a composite extraction solvent, and then place it in a high-temperature reactor for extraction under a nitrogen atmosphere. The composite extraction solvent is methylnaphthalene oil and methanol with a dosage ratio of 12:1. During extraction, control the solid-liquid ratio at 1:8 and the extraction temperature at 380 °C. The stirring paddle in the high-temperature reactor rotates at a speed of 300 rpm, and the extraction time is 1 h.

[0062] (4) Use a high-speed centrifuge (centrifuge speed: 4000 rpm, time: 10 min) to perform solid-liquid separation on the product after extraction in step (3) to obtain extraction residue and extract respectively.

[0063] (5) Remove the extraction solvent from the extract at 120 °C under reduced pressure and wash the residual extraction solvent on the obtained solid product to obtain ultra-clean coal. The results are shown in Table 1.

[0064] Example 5

[0065] A method for preparing ultra-clean coal based on catalytic thermal dissolution extraction method, comprising the following steps:

[0066] (1) Use the gravity separation method to separate the coal sample (lignite). Add the coal sample with a particle size greater than 0.5 mm to a heavy medium suspension with a density of 1.10 - 1.50 kg / cm 3 and stir and separate. Then grind the obtained light component product to less than 400 mesh to obtain coal powder.

[0067] (2) Mix the pyrite catalyst evenly with the coal powder obtained in step (1) to obtain a coal / catalyst mixture. The mass ratio of the coal sample to the catalyst is 25:1.

[0068] (3) Mix the coal / catalyst mixture obtained in step (2) with a composite extraction solvent, and then place it in a high-temperature reactor for extraction under a nitrogen atmosphere. The composite extraction solvent is methylnaphthalene oil and methanol with a dosage ratio of 8:1. During extraction, control the solid-liquid ratio at 1:12 and the extraction temperature at 320 °C. The stirring paddle in the high-temperature reactor rotates at a speed of 250 rpm, and the extraction time is 2 h.

[0069] (4) Use a high-speed centrifuge (centrifuge speed: 4000 rpm, time: 10 min) to perform solid-liquid separation on the product after extraction in step (3) to obtain extraction residue and extract respectively.

[0070] (5) Remove the extraction solvent from the extract at 120 °C under reduced pressure and wash the residual extraction solvent on the obtained solid product to obtain ultra-clean coal. The results are shown in Table 1.

[0071] Comparative Example 1

[0072] The difference from Example 1 is that instead of adding a catalyst, the coal sample in step (1) is directly mixed with the composite extraction solvent; the rest is the same as in Example 1. The results are shown in Table 1.

[0073] Comparative Example 2

[0074] The difference from Example 1 is that the methanol in step (3) is replaced with an equal mass of methylnaphthalene oil; the rest is the same as in Example 1. The results are shown in Table 1.

[0075] Comparative Example 3

[0076] The difference from Example 1 is that the methylnaphthalene oil in step (3) is replaced with an equal mass of methanol; the rest is the same as in Example 1. The results are shown in Table 1.

[0077] Comparative Example 4

[0078] The difference from Comparative Example 1 is that the composite extraction solvent is replaced with an equal mass of N-methylpyrrolidone; the rest is the same as in Comparative Example 1. The results are shown in Table 1.

[0079] Table 1

[0080] Example / Comparative Example Yield / wt. % Ash content / wt. % Example 1 90.4 0.06 Example 2 85.2 0.05 Example 3 92.1 0.09 Example 4 90.5 0.06 Example 5 88.7 0.05 Comparative Example 1 70.3 0.06 Comparative Example 2 60.7 0.03 Comparative Example 3 20.3 0.05 Comparative Example 4 88.4 0.07

[0081] Among them, the yield of ultra-pure coal is calculated by dividing the mass of the solid product obtained by extraction by the mass of the organic matter in the raw coal.

[0082] As can be seen from the above Example 1 / Comparative Example 1, due to the lack of a catalyst, the coal structure in Comparative Example 1 was not fully opened, chemical bonds were broken, and the solvent could not fully dissolve the organic matter, resulting in a decrease in the yield;

[0083] As can be seen from the above Example 1 / Comparative Example 2, due to the lack of a catalyst, the coal structure in Comparative Example 2 was not fully opened, chemical bonds were broken, and the solvent could not fully dissolve the organic matter, resulting in a decrease in the yield;

[0084] As can be seen from the above Example 1 / Comparative Example 3, due to the lack of a catalyst, the coal structure in Comparative Example 1 was not fully opened, chemical bonds were broken, and the solvent could not fully dissolve the organic matter, resulting in a decrease in the yield;

[0085] In Comparative Example 4, the extraction solvent is N-methylpyrrolidone. Compared with the composite extraction solvent used in the present invention, since N-methylpyrrolidone lacks nitrogen-containing polar substances and polycyclic rings. In addition, the lack of nucleophilic oxygen atoms in methanol makes it impossible to attack the carbon atoms connected to the bridging oxygen bonds in coal, and the coal molecular structure cannot be fully cracked, resulting in a decrease in the yield.

Claims

1. A method for preparing ultra-pure coal based on catalytic thermal dissolution extraction method, characterized in that, The preparation method includes the following steps: (1) Grind and crush the coal sample to obtain coal powder; (2) Mix the catalyst evenly with the coal powder obtained in step (1) to obtain a coal / catalyst mixture; wherein, the catalyst is pyrite; (3) After mixing the coal / catalyst mixture obtained in step (2) with the composite extraction solvent, place it in a high-temperature reaction kettle and carry out extraction in an inert protective atmosphere; wherein, the composite extraction solvent includes methylnaphthalene oil and methanol; (4) Separate the solid and liquid of the product after extraction in step (3) to obtain extraction residue and extraction liquid respectively; (5) Distill off the extraction solvent in the extraction liquid and wash the residual extraction solvent on the obtained solid product to obtain ultra-clean coal.

2. The preparation method according to claim 1, wherein In step (1), before the grinding treatment, first use the gravity separation method to preliminarily separate the coal sample to remove at least part of the inorganic minerals therein, and wash and dry the separated light component product as the coal sample for subsequent grinding treatment.

3. The preparation method according to claim 2, characterized in that, In step (1), when separating the coal sample by the re-selection method, the coal sample with a particle size greater than 0.5 mm is added to a heavy medium suspension with a density of 1.10 - 1.50 kg / cm 3 and stirred and separated.

4. The preparation method according to any one of claims 1-3, characterized in that, In step (1), grind the coal sample to less than 200 mesh, preferably less than 400 mesh, so that the organic matter and minerals in the coal are fully dissociated.

5. The preparation method according to any one of claims 1-4, characterized in that, In step (2), when mixing, the mass ratio of the coal sample to the catalyst is (10-30):

1.

6. The preparation method according to any one of claims 1-5, characterized in that, In step (3), during extraction, control the solid-liquid mass ratio to be 1:(5-15), the extraction temperature to be 300-410 °C, and the mass ratio of methylnaphthalene oil to methanol in the composite extraction agent to be (5-15):

1.

7. The preparation method according to any one of claims 1-6, characterized in that, In step (3), carry out extraction under the rotation of the stirring paddle in the high-temperature reaction kettle, the rotation speed of the stirring paddle is 150-300 rpm, and the extraction time is 1-4 h.

8. The preparation method according to any one of claims 1-7, characterized in that, In step (4), use a high-speed centrifuge for solid-liquid separation, the rotation speed of the centrifuge is 3000-5000 rpm, and the time is 5-15 min.

9. The preparation method according to any one of claims 1-8, characterized in that, In step (5), pour the upper-layer liquid obtained by centrifugation into a round-bottom flask for vacuum distillation, distill out the solvent for recovery, and control the heating temperature to be 100-150 °C.

10. The preparation method according to claims 1-9, characterized in that, In step (5), during washing, use alcohol to ultrasonically treat, centrifuge, and wash the obtained solid product until the solution is colorless to obtain an ultra-clean coal sample.

Images