Method for preparing humic acid by fractional oxidative depolymerization of lignite

Abstract

The application discloses a method for preparing humic acid by graded oxidative depolymerization of lignite, comprising the following steps: weighing lignite, adding hydrogen peroxide for oxidation pretreatment, filtering and vacuum drying to obtain preliminarily oxidized lignite, named as PTSL; using a mixed gas of ozone and oxygen to perform deep oxidation experiment on the obtained PTSL, and after the oxidation reaction is completed, centrifugal filtration is performed on the mixture, and a liquid sample is collected. The lignite is subjected to graded oxidative depolymerization in a graded oxidation mode, the weak bridge bond and side chain structure in the coal are first oxidatively broken by the green oxidant hydrogen peroxide without damaging the aromatic ring structure, then the lignite is further oxidized by the strong oxidant ozone to gradually damage the polycyclic aromatic hydrocarbon structure in the coal, and step-by-step oxidative depolymerization of the organic matter in the coal is realized. The method can improve the organic matter conversion rate and the humic acid yield of the lignite and reduce the impurity content in the humic acid.

Description

Technical Field

[0001] This invention relates to the field of comprehensive utilization technology of lignite, specifically to a method for preparing humic acid from lignite through staged oxidation and depolymerization, and particularly to a method for producing humic acid from successful lignite through staged oxidation and depolymerization using hydrogen peroxide coupled with ozone. Background Technology

[0002] my country has abundant lignite reserves. As a typical low-rank coal, it is characterized by high moisture content, high ash content, and low calorific value. How to efficiently and cleanly utilize lignite has become an urgent problem to be solved. Oxidative depolymerization is one of the most important methods for the deep processing and utilization of lignite, which can yield many high-value-added oxygen-containing organic chemicals (such as lignite wax and humic acid), which can meet various needs of the national economy.

[0003] Currently, methods to improve the yield of humic acid from lignite mainly include thermal oxidation, chemical oxidation, catalytic oxidation, and microbial oxidation. Chemical oxidation results in high impurity content and low organic matter conversion rate of lignite; microbial dissolution has a long reaction cycle, which is not conducive to large-scale production; thermal oxidation is energy-intensive and its oxidation effect is not significant; while in catalytic oxidation, coal and catalyst are difficult to separate, leading to difficulties in recycling. Therefore, it is of great significance to find an oxidation method with a short reaction cycle, low energy consumption, and the ability to improve the organic matter conversion rate and reduce impurity content of lignite. Summary of the Invention

[0004] The purpose of this invention is to provide a method for preparing humic acid from lignite through graded oxidation and depolymerization. This method can improve the organic matter conversion rate and humic acid yield of lignite, shorten the reaction cycle, and reduce energy consumption and impurity content in humic acid.

[0005] To achieve the above objectives, the present invention provides a method for preparing humic acid by staged oxidation and depolymerization of lignite, comprising the following steps:

[0006] S1. The lignite raw material is successively ground, acid-washed, and dried;

[0007] S2. Take the lignite treated in step S1, stir it with hydrogen peroxide, filter it and vacuum dry it to obtain the modified lignite, named PTSL.

[0008] S3. The PTSL obtained in step S2 is subjected to the first ozone oxidation experiment using a mixture of ozone and oxygen. After the oxidation reaction is completed, the mixture is centrifuged and filtered. The filtered solid residue is washed with solvent and then filtered again. All the filtrates in this step are combined and dried. The solid sample obtained after drying is the first oxidation product of modified lignite, which is humic acid. The solid sample obtained after washing is the first solid residue of modified lignite oxidation.

[0009] Furthermore, step S4 is included: a second ozone oxidation experiment is conducted on the modified lignite first oxidation product obtained in step S3 using a mixture of ozone and oxygen. After the oxidation reaction is completed, the mixture is centrifuged and filtered. The filtered solid residue is washed with solvent and then filtered again. All the filtrates in this step are combined and dried. The solid sample obtained after drying is the modified lignite second oxidation product, and the solid sample obtained after washing is the modified lignite second oxidation solid residue.

[0010] Further, in step S1, lignite is ground to below 40 mesh to obtain coal powder; a 3-10 mol / L dilute hydrochloric acid solution is added to the coal powder, and the mixture is treated in a water bath at 30-90℃ with stirring for 1-10 hours. After filtration and washing, the filter cake is washed with deionized water until neutral and Cl is undetectable. - The filtered cake was vacuum dried overnight at 105°C.

[0011] Preferably, in step S2, the mass fraction of hydrogen peroxide is 10-50 wt.%, the treatment temperature is 30-90℃, and the treatment time is 1-10.

[0012] Preferably, in step S2, the mass-to-volume ratio of the lignite treated in step S1 to hydrogen peroxide is 1g:10mL; the mass fraction of hydrogen peroxide is 30wt.%, the treatment temperature is 55℃, and the treatment time is 2h.

[0013] Further, the specific process of step S3 is as follows: In a typical oxidation experiment, PTSL is weighed and placed into a three-necked flask, solvent is added, and the apparatus is connected; subsequently, the oxygen cylinder is opened, and ozone generated by the ozone generator is introduced into the reactor, adjusting the oxygen inflow rate to 0.5 L / min and the ozone output rate to 20-60 m³ / min. 3 / h, a mixture of ozone and oxygen is directly introduced into a three-necked flask. The ozone concentration is detected by an ozone concentration detector. The mixture is heated and stirred at 20°C using a heat-collecting constant-temperature magnetic stirrer for 1-4 hours to allow the oxidation reaction to occur. After the reaction is complete, the mixture is centrifuged and filtered. The filtered solid residue is washed twice with solvent and then filtered again. All the filtered liquid samples are combined and dried at 105°C overnight.

[0014] Preferably, the solvent is methanol or acetic acid; the ozone output is 40m³. 3 / h, oxidation reaction occurs for 3h.

[0015] Compared with the prior art, the present invention has the following advantages:

[0016] (1) In this invention, hydrogen peroxide attacks and destroys the weak covalent bonds in coal through hydroxyl radicals, resulting in the initial degradation of the macromolecular network structure in coal. Then, the oxygen radicals generated by ozone attack the stronger bridging bonds in coal, causing them to be further cracked, resulting in the deep depolymerization of the coal macromolecular structure. Under mild conditions, the depth of oxidative depolymerization of lignite macromolecules is precisely controlled, which promotes the effective conversion of lignite macromolecules into soluble humic acid under mild conditions.

[0017] (2) This invention employs a staged oxidation method to progressively degrade lignite. Hydrogen peroxide is used under mild conditions to break down irregular bridging bonds and side-chain structures in the coal macromolecules. Next, ozone oxidation is used to further transform the solid residue, gradually destroying the polycyclic aromatic hydrocarbon structure in the coal and achieving stepwise oxidative depolymerization of the organic matter. Ozone oxidation depolymerization experiments were conducted on Shengli lignite (SL) and modified Shengli lignite (PTSL). Compared to SL, ozone oxidation depolymerization of PTSL increased the organic matter conversion rate and humic acid yield by 28.96% and 51.47%, respectively. In the cyclic experiment, the organic matter conversion rate of PTSL reached 84.48% in the second cyclic oxidation process, yielding a total dry liquid mass of 3.2519 g and a total humic acid yield of 98.81%. Attached Figure Description

[0018] Figure 1 Diagram of ozone oxidation experimental setup; In the diagram: 1. Oxygen cylinder, 2. Ozone generator, 3. Ozone concentration detector, 4. Heat-collecting constant temperature magnetic stirrer, 5. Three-necked flask, 6. Potassium iodide solution.

[0019] Figure 2 Soluble components of modified Shengli lignite ozone oxidation (a) and experimental schematic diagram (b);

[0020] Figure 3 Time gradient diagram of ozone oxidation modification of Shengli lignite;

[0021] Figure 4 Ozone gradient diagram of ozone oxidation modification of Shengli lignite;

[0022] Figure 5 Soluble components of modified Shengli lignite ozone oxidation cycle experiment (a) and experimental schematic diagram (b);

[0023] Figure 6 XRD patterns of ozone-oxidized Shengli lignite and solid residue;

[0024] Figure 7 FTIR images of ozone-oxidized depolymerized lignite, soluble components and solid residues; (a) raw coal and solid oxidation residues, (b) humic acid, (c) solvent-soluble components;

[0025] Figure 8GPC diagrams of soluble components in ozone-oxidized Shengli lignite; (a) PTSL-HA, (b) PTSL-C1-HA, (c) PTSL-C2-HA;

[0026] Figure 9 Liquid NMR spectra of soluble components of ozone-oxidized Shengli lignite; (a) 1H NMR, (b) 1C NMR.

[0027] Figure 10 XPS plots of soluble components in ozone-oxidized Shengli lignite; (ac)C 1s, (df)O 1s;

[0028] Figure 11 SEM image of ozone-oxidized and modified Shengli lignite. Detailed Implementation

[0029] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0030] The lignite used in the following implementation is Shengli lignite.

[0031] The following embodiments all adopt the following... Figure 1 The diagram shows an experimental setup for ozone oxidation modification of Shengli lignite. This setup includes an oxygen cylinder 1, an ozone generator 2, an ozone concentration detector 3, a heat-collecting constant-temperature magnetic stirrer 4, a three-necked flask 5, and an ultrasonic disruptor. The inlet of the ozone generator 2 is connected to the oxygen cylinder 1, and the outlet of the ozone generator 2 passes through the ozone concentration detector 3 and is connected to one flask opening of the three-necked flask 5. The ultrasonic probe of the ultrasonic disruptor is inserted into the other flask opening of the three-necked flask 5. One flask opening of the three-necked flask 5 serves as the exhaust outlet, and the exhaust outlet is inserted into a 2% potassium iodide (KI) solution 6 containing ozone absorber and decomposer.

[0032] Example 1

[0033] A method for preparing humic acid by staged oxidation of polylactic acid from lignite includes the following steps:

[0034] S1. Grind Shengli lignite to below 40 mesh to obtain coal powder; weigh 20g of Shengli lignite, treat with 5mol / L dilute hydrochloric acid solution in a 55℃ water bath with stirring for 2h, filter, wash the filter cake with deionized water until neutral, and no Cl can be detected. - The filtered cake was vacuum dried overnight at 105°C.

[0035] S2. Take 10g of Shengli lignite treated in step S1, stir it with 100mL of 30wt.% hydrogen peroxide (H2O2) at 55℃ for 2h, filter it and vacuum dry it overnight to obtain the modified lignite, named PTSL.

[0036] S3. In a typical oxidation experiment, weigh 1.00 g of PTSL into a three-necked flask, add 30 mL of acetic acid, and connect the apparatus. Then, open the oxygen cylinder, and introduce ozone generated by the ozone generator into the reactor. Adjust the oxygen inflow rate to 0.5 L / min and the ozone output rate to 40 mL / min. 3 / h, a mixture of ozone and oxygen gas was passed through an ozone concentration detector and directly introduced into a three-necked flask. The ozone concentration was detected by the ozone concentration detector. At room temperature (20℃), the mixture was heated and stirred using a heat-collecting constant-temperature magnetic stirrer for 3 hours to carry out the oxidation reaction. After the reaction was completed, the mixture was centrifuged and filtered. The color of the liquid sample was photographed and recorded. The solid oxidation residue was washed twice with solvent and dried overnight at 105℃ and weighed. All the filtered liquid samples were dried overnight at 105℃. The solid samples after evaporation were collected and weighed. The obtained solid sample is the modified lignite oxidation product, named PTSL-HA. The solid sample obtained after washing is the modified lignite oxidation solid residue, named PTSL-OR.

[0037] The detailed reaction conditions and related product numbers for ozone oxidation and depolymerization modification of Shengli lignite are shown in Table 1. The calculation process for organic matter conversion rate, humic acid yield, and solvent recovery rate is as follows:

[0038]

[0039] Where: m a —Initial mass of modified Shengli lignite, g;

[0040] m b —Mass of oxidized solid residue, g;

[0041] m c — Humic acid mass, g;

[0042] m d —Mass of solvent recovered after reaction, in g;

[0043] m e —Mass of feed material before reaction, in grams.

[0044] M ad and A ad —These are the air-dried moisture and ash content of raw coal, respectively, in %;

[0045] M' ad and A' ad —representing air-dried moisture and ash content of the oxidized solid residue, %;

[0046] A″ d — Ash content of humic acid on a dry basis, %;

[0047] The moisture and ash content of each sample were determined by industrial analysis. Industrial analysis of the oxidation product humic acid revealed that the ash content in the humic acid was approximately 0.5%, indicating that the ash mainly remained in the solid residue. All samples were dried at 105℃ overnight, and the industrial analysis results showed that the oxidation products contained almost no moisture.

[0048] Comparative Example 1

[0049] The difference from Example 1 is that step S2 in Example 1 is omitted, and the oxidation experiment is carried out directly with the Shengli lignite (SL) obtained in step S1.

[0050] Example 2

[0051] The difference from Example 1 is that "30 mL acetic acid" in step S3 is replaced with "30 mL methanol", while the other steps remain the same as in Example 1.

[0052] Comparative Example 2

[0053] The difference from Example 2 is that step S2 in Example 2 is omitted, and the oxidation experiment is carried out directly with the Shengli lignite (SL) obtained in step S1.

[0054] Table 1. Experimental results of ozone oxidation of different types of lignite.

[0055]

[0056] As shown in Table 1, the conversion rate and humic acid yield of untreated Shengli lignite were relatively low; the modified Shengli lignite showed a significant increase in organic matter conversion rate and humic acid yield, resulting in a substantial improvement in oxidation efficiency. Using acetic acid as a solvent, modified Shengli lignite was oxidized to obtain oxidized solid residue and humic acid as the oxidation product through ozone oxidation. The organic matter conversion rate / humic acid yield of ozone oxidation depolymerization of PTSL were 68.27% and 80.14%, respectively. However, using methanol as a solvent, the organic matter conversion rate / humic acid yield of ozone oxidation depolymerization of PTSL were 65.18% and 72.53%, respectively.

[0057] Example 3

[0058] The difference from Example 1 is that "oxidation reaction for 3 hours" in step S3 is replaced with "oxidation reaction for 1 hour", while the other steps remain the same as in Example 1.

[0059] Example 4

[0060] The difference from Example 1 is that "oxidation reaction for 3 hours" in step S3 is replaced with "oxidation reaction for 2 hours", while the other steps remain the same as in Example 1.

[0061] Example 5

[0062] The difference from Example 1 is that "oxidation reaction for 3 hours" in step S3 is replaced with "oxidation reaction for 4 hours", while the other steps remain the same as in Example 1.

[0063] like Figure 2 As shown, with increasing oxidation time, the liquid product first darkens in color, changing from reddish-brown to dark brown, and then lightens slightly. Figure 3 It can be seen that with the increase of oxidation time, the organic matter conversion rate of modified Shengli lignite gradually increases, while the humic acid yield of modified Shengli lignite first increases and then decreases. When acetic acid is used as a solvent, with the increase of oxidation time, the organic matter conversion rate / humic acid yield of lignite increases from 57.48% / 52.16% at 1 h to 68.27% / 80.14% at 3 h. Further extending the oxidation time to 4 h, the increase in organic matter conversion rate is not significant (70.55%), and the humic acid yield decreases slightly (70.41%) due to over-oxidation of the product caused by the longer oxidation time. The results indicate that the optimal oxidation time for ozone oxidation and depolymerization of modified Shengli lignite is 3 h, at which time the organic matter conversion rate and humic acid yield are higher, and the oxidation and depolymerization effect is better.

[0064] Example 6

[0065] The difference from Example 1 is that in step S3, the "ozone output is 40m" 3 Replace " / h" with "Ozone output is 20m" 3 " / h", and the other steps are consistent with those in Example 1.

[0066] Example 7

[0067] The difference from Example 1 is that in step S3, the "ozone output is 40m" 3 Replace " / h" with "Ozone output is 60m" 3 " / h", and the other steps are consistent with those in Example 1.

[0068] Comparative Example 3

[0069] The difference from Example 1 is that in step S3, the "ozone output is 40m" 3 Replace " / h" with "Ozone output is 0m" 3 " / h", and the other steps are consistent with those in Example 1.

[0070] like Figure 2 As shown, with increasing ozone levels, the liquid product first darkens in color, changing from reddish-brown to dark brown, and then lightens slightly. Figure 4It can be seen that with the increase of ozone concentration, the organic matter conversion rate of modified Shengli lignite gradually increases, while the humic acid yield of modified Shengli lignite first increases and then decreases. When acetic acid is used as a solvent, with the increase of ozone concentration, the ratio of organic matter conversion rate / humic acid yield of lignite decreases from 0% to 0%. 3 The percentage of ozone increased from 13.78% / 10.77% at a rate of 40m³ / h to [a higher percentage]. 3 At 68.27% / 80.14% of the ozone input, further increasing the ozone input slightly improved the organic matter conversion rate (70.07%), while the humic acid yield decreased significantly (52.20%) due to excessive oxidation of the product caused by the high ozone content. The results indicate that the degree of lignite oxidative depolymerization is very low in the absence of ozone. With increasing ozone input, the oxidative depolymerization effect of lignite gradually improves. The optimal ozone input for successful ozone oxidative depolymerization modification of lignite is 40 m³ / h. 3 / h, at which point the organic matter conversion rate and humic acid yield are higher, and the oxidative depolymerization effect is better.

[0071] Example 8

[0072] A method for preparing humic acid by staged oxidation of polylactic acid from lignite includes the following steps:

[0073] S1. Shengli lignite (SL) is ground to below 40 mesh to obtain coal powder; 20g of Shengli lignite is weighed and treated with 5mol / L dilute hydrochloric acid solution in a 55℃ water bath with stirring for 2h, filtered, and the filter cake is washed with deionized water until neutral and Cl is not detected. - The filtered cake was vacuum dried overnight at 105°C.

[0074] S2. Take 10g of Shengli lignite treated in step S1, stir it with 100mL of 30wt.% hydrogen peroxide (H2O2) at 55℃ for 2h, filter it and vacuum dry it overnight to obtain the modified lignite, named PTSL.

[0075] S3. A first ozone oxidation experiment was conducted on the PTSL obtained in step S2 using a mixture of ozone and oxygen. The specific procedure was as follows: In a typical oxidation experiment, 4.00 g of PTSL was weighed and placed in a three-necked flask, 50 mL of acetic acid was added, and the apparatus was connected. Subsequently, the oxygen cylinder was opened, and ozone generated by the ozone generator was introduced into the reactor. The oxygen inflow rate was adjusted to 0.5 L / min, and the ozone output rate was 40 mL / min. 3 / h, the mixture of ozone and oxygen is passed through an ozone concentration detector and then directly introduced into a three-necked flask. The ozone concentration is detected by the ozone concentration detector. At room temperature (20℃), a heat-collecting constant temperature magnetic stirrer is used for heating and stirring. The oxidation reaction takes place for 3 hours. After the reaction is completed, the mixture is centrifuged and filtered. The color of the liquid sample is photographed and recorded. The solid oxidation residue is washed twice with solvent and dried overnight at 105℃ and weighed. All the filtered liquid samples are dried overnight at 105℃. The solid samples after evaporation are collected and weighed. The obtained solid sample is the first oxidation product of modified Shengli lignite, named PTSL-C1-HA. The solid sample obtained after washing is the first oxidation solid residue of modified Shengli lignite, named PTSL-C1-OR.

[0076] S4. A second ozone oxidation experiment was conducted on PTSL-C1-OR obtained in step S3 using a mixture of ozone and oxygen. The specific process was the same as in step S3. The solid residue of the modified Shengli lignite secondary oxidation was named PTSL-C2-OR, and the product of the modified Shengli lignite secondary oxidation was named PTSL-C2-HA.

[0077] Depend on Figure 5 It can be seen that after two cycles of the experiment, the liquid color of SL lightened, changing from dark brown to light yellow. Table 2 shows that in the ozone oxidation depolymerization experiment at room temperature (20℃), the total organic matter conversion rate of PTSL gradually increased with the increase of the number of cycles. After the second cycle of oxidation, the total conversion rate of PTSL reached 80.38%, yielding a total dry liquid mass of 3.2692 g and a total humic acid yield of 91.26%. Since the low-boiling-point, small-molecule solvent-soluble components are evaporated during the liquid evaporation process, the humic acid yield obtained for PTSL in the ozone oxidation depolymerization experiment at room temperature (20℃) is greater than 91.26%.

[0078] Table 2 Results of the cyclic experiment on ozone oxidation-modified Shengli lignite

[0079]

[0080] The modified Shengli lignite, its oxidation product humic acid, and its oxidation solid residue were characterized and analyzed.

[0081] 1. Elemental analysis was performed on SL, PTSL, their oxidized solid residues, and corresponding oxidation products.

[0082] Table 3 shows that, compared to raw SL coal, PTSL modified with hydrogen oxide exhibits decreased carbon, hydrogen, and nitrogen content, while significantly increased oxygen content, H / C ratio, and O / C ratio. This indicates that the modification treatment can break weaker bridging bonds and longer side chains in lignite. After PTSL oxidation and depolymerization, compared to PTSL, the oxygen content and O / C ratio in the oxidized solid residue significantly decreased, while the carbon and hydrogen content increased. The carbon and hydrogen content in the humic acid oxidation product decreased, while the oxygen content, O / C ratio, and H / C ratio significantly increased. With increasing PTSL oxidation and depolymerization cycles, the oxygen content, H / C ratio, and O / C ratio in the oxidized solid residue decreased, while the carbon and hydrogen content increased. The carbon and hydrogen content in the humic acid oxidation product decreased, while the oxygen content and O / C ratio increased sequentially. The results indicate that modification treatment can increase the oxygen content in raw coal and enhance its oxidation and depolymerization capacity. The ozone oxidation and depolymerization process allows oxygen atoms to gradually enter the products, resulting in a relatively high number of oxygen-containing functional groups in the products.

[0083] Table 3 Elemental analysis results of different samples

[0084]

[0085]

[0086] a Subtraction method

[0087] 2. Industrial analysis of SL, PTSL, their oxidized solid residues, and corresponding oxidation products.

[0088] Table 3 shows that, compared to raw SL coal, the moisture and ash content of PTSL modified with hydrogen oxide significantly decreased, indicating that acid washing had a deashing effect during the modification process. After PTSL oxidation and depolymerization, the ash accumulated in the oxidized solid residue, while the oxidation products contained almost no ash. In the cyclic experiment, after PTSL oxidation and depolymerization, the ash content in the solid residue gradually increased, and the oxidation products contained almost no ash and less than 1%. The results indicate that the ozone oxidation process mainly decomposes and releases the organic matter in PTSL, while the ash remains in the solid residue.

[0089] Table 4. Industrial analysis results of different samples

[0090]

[0091] 3. XRD analysis of ozone-oxidized Shengli lignite and solid residue.

[0092] like Figure 6As shown in the XRD patterns, PTSL and the oxidation residue exhibit the same characteristic peaks. A distinct characteristic diffraction peak, belonging to SiO2 (the main component of coal ash), can be observed at approximately 27° 2θ. The SiO2 diffraction peak intensity of PTSL and its oxidation residue gradually increases, indicating that the ash composition is mainly present in the oxidation residue and remains unchanged during oxidation. For the cyclic experiment, the SiO2 diffraction peak intensity of PTSL and its oxidation residue gradually increases, indicating that ash gradually accumulates in the solid oxidation slag, which is consistent with the above industrial analysis and characterization results. Since the SiO2 diffraction peaks of different raw coal samples and oxidation residues are almost identical, it indicates that the ash composition does not change during oxidation and remains in the oxidation residue.

[0093] 4. FTIR analysis of ozone oxidation depolymerization modified Shengli lignite, soluble components and solid residues.

[0094] Depend on Figure 7 It can be seen that the infrared characteristic peaks of modified Shengli lignite, soluble components, and solid residues are similar at 3428 cm⁻¹. -1 OH was detected nearby at 1705 and 1725 cm⁻¹. -1 C=O(COOH) was detected nearby at 1608 cm⁻¹. -1 Aromatic C=C was detected nearby at 2922 and 2850 cm⁻¹. -1 Variable C was detected nearby al -H peak intensity. For all oxidation residues, the absorption band of COOH and the intensity of PTSL are very low. For all oxidation products, the absorption band of COOH gradually increases compared to raw coal. Similarly, the absorption band of COOH for all liquid oxidation products gradually increases compared to raw coal. The results indicate that the oxidation process leads to the oxidation of functional groups in coal to COOH.

[0095] In oxidizing solutions, the presence of a large amount of acetic acid enhances the COOH absorption band, while other absorption bands are weakened or covered. The presence of a strong COOH absorption band after drying the solid product indicates the presence of a large amount of COOH in the solid product. At 1445 cm⁻¹ -1 and 1380cm -1 The bands observed nearby are mainly due to the asymmetric deformation of CH2 groups in the bridging bonds and the symmetrical bending of CH3 groups. It is noteworthy that at 1445 cm⁻¹... -1 The peak intensity is higher than 1380 cm⁻¹ -1 The peak indicates the presence of long aliphatic side groups or methylene bridges between aromatic rings in the coal. Furthermore, at 1300-1000 cm⁻¹... -1 The intensity of the bands in the region is assigned to COC, phenolic compounds, and -OH stretching. The 900-700 cm⁻¹ range is significant across all spectra. -1The region contains bands generated by the out-of-plane bending vibrations of aromatic CH.

[0096] 5. GPC analysis of soluble components in ozone-oxidized Shengli lignite.

[0097] Depend on Figure 8 The molecular weights of the humic acid products obtained by ozone oxidation and depolymerization of PTSL were determined using gel permeation chromatography (GPC). The molecular weights of the humic acid products were mainly concentrated in the range of 250-1500 Da, with the macromolecular masses (Mw) of PTSL-HA, PTSL-C1-HA, and PTSL-C2-HA being 522, 580, and 527, respectively. The molecular weight of HA decreased significantly after ozone oxidation, indicating that ozone disrupted the structure of lignite and increased its solubility in solvents. Based on the molecular weight distribution results, the obtained products are mainly humic acid.

[0098] 6. Liquid NMR analysis of soluble components in ozone-oxidized Shengli lignite.

[0099] Liquid NMR (1H and 1C spectra) were performed on the solid obtained by PTSL ozone oxidation liquid. Figure 9 It can be seen that the liquid NMR spectra of the solid obtained by PTSL ozone oxidation of liquid and carbon are basically the same. The structure contains C-C bonds, C=C bonds, and COC bonds, as well as a large number of oxygen-containing hydroxyl and carboxyl groups. Since the detected hydrogen signals connected to the benzene ring are weak, it is inferred that the benzene ring mainly contains substituted hydroxyl and carboxyl groups, consistent with the structure of humic acids, especially fulvic acid. The corresponding results also indicate that ozone can effectively break down the macromolecular structure of lignite into solvent-soluble components, and the products contain a relatively large number of oxygen-containing groups, such as hydroxyl and carboxyl groups.

[0100] 7. XPS analysis of soluble components in ozone-oxidized Shengli lignite

[0101] XPS analysis was performed on the oxidation products of the PTSL ozone oxidation cycle experiment to observe the different chemical valence states of C and O in the samples. For example... Figure 10 As shown, for the oxidation products of the PTSL cycling experiment, with the increase of the number of cycles, the C1s plot of XPS clearly shows that the content of CC / CH bonds gradually decreases, while the content of COO- functional groups gradually increases. Simultaneously, the O1s plot of XPS clearly shows that the content of COO- functional groups gradually increases with the increase of the number of cycles. These results indicate that ozone oxidation depolymerization of PTSL can effectively cleave CO and CC bonds in lignite, and the products contain a relatively large number of carboxyl functional groups.

[0102] 8. SEM analysis of ozone oxidation-modified Shengli lignite.

[0103] like Figure 11As shown, compared to PTSL, the surface of the oxidation residue becomes looser and more porous, forming a greater number of pore structures, indicating that the organic macromolecular structure of lignite is severely damaged after ozone oxidation. In the cyclic experiment, with increasing cycle number, the dense structure of the coal sample gradually collapses, forming a richer pore structure on the surface. The solid residue after ozone oxidation has a rich pore structure and a rough surface, indicating that ozone oxidation significantly alters the surface morphology of lignite.

[0104] In summary, hydrogen peroxide attacks and breaks weak covalent bonds in coal through hydroxyl radicals, leading to the initial degradation of the coal's macromolecular network structure. Then, oxygen free radicals generated by ozone attack the stronger bridging bonds in the coal, further cleaving them and resulting in deep depolymerization of the coal's macromolecular structure. Using acetic acid as a solvent, modified Shengli lignite was oxidized to obtain oxidized solid residue and oxidation products via ozone oxidation. The organic matter conversion rate and humic acid yield of PTSL (potassium peroxide) oxidized by ozone oxidation were 68.27% and 80.14%, respectively. In the cyclic experiment, after the second cycle of oxidation, the total conversion rate of PTSL reached 80.38%, yielding a total dry liquid mass of 3.2692 g and a total humic acid yield of 91.26%. A stepwise oxidation method was used to degrade Shengli lignite step by step. Hydrogen peroxide was used under mild conditions to break weak bridging bonds and side chain structures in the coal's macromolecular structure. Then, ozone oxidation was used to achieve further deep conversion of the solid residue, gradually destroying the polycyclic aromatic hydrocarbon structure in the coal and realizing the stepwise oxidative depolymerization of the organic matter in the coal. Therefore, ozone oxidation and depolymerization modification of Shengli lignite is a promising method for the efficient extraction of humic acid from coal.

Claims

1. A method for preparing humic acid by staged oxidation and decomposition of polylactic acid from lignite, characterized in that, Includes the following steps: S1. The lignite raw material is successively ground, acid-washed, and dried; S2. Take the lignite treated in step S1, stir it with hydrogen peroxide, filter it and vacuum dry it to obtain the modified lignite, named PTSL. S3. The PTSL obtained in step S2 is subjected to the first ozone oxidation experiment using a mixture of ozone and oxygen. After the oxidation reaction is completed, the mixture is centrifuged and filtered. The filtered solid residue is washed with solvent and then filtered again. All the filtrates in this step are combined and dried. The solid sample obtained after drying is the first oxidation product of modified lignite, which is humic acid. The solid sample obtained after washing is the first solid residue of modified lignite oxidation.

2. The method for preparing humic acid from lignite through staged oxidation according to claim 1, characterized in that, The process also includes step S4: using a mixture of ozone and oxygen to conduct a second ozone oxidation experiment on the modified lignite first solid residue obtained in step S3. After the oxidation reaction is completed, the mixture is centrifuged and filtered. The filtered solid residue is washed with solvent and then filtered again. All the filtrates from this step are combined and dried. The solid sample obtained after drying is the modified lignite second oxidation product. The solid sample obtained after washing is the modified lignite second oxidation solid residue.

3. A method for preparing humic acid from lignite through staged oxidation and depolymerization according to claim 1 or 2, characterized in that, In step S1, lignite is ground to below 40 mesh to obtain coal powder; 3-10 mol / L dilute hydrochloric acid solution is added to the coal powder, and the mixture is treated in a water bath at 30-90 ℃ with stirring for 1-10 h. After filtration and washing, the filter cake is washed with deionized water until neutral and Cl is undetectable. - The filtered cake was vacuum dried overnight at 105 °C.

4. A method for preparing humic acid from lignite through staged oxidation and depolymerization according to claim 1 or 2, characterized in that, In step S2, the mass fraction of hydrogen peroxide is 10-50 wt.%, the treatment temperature is 30-90 ℃, and the treatment time is 1-10 h.

5. The method for preparing humic acid from lignite through staged oxidation according to claim 4, characterized in that, In step S2, the mass-to-volume ratio of the lignite treated in step S1 to hydrogen peroxide is 1 g: 10 mL; the mass fraction of hydrogen peroxide is 30 wt.%, the treatment temperature is 55 ℃, and the treatment time is 2 h.

6. A method for preparing humic acid from lignite through staged oxidation according to claim 1 or 2, characterized in that, The specific process of step S3 is as follows: In a typical oxidation experiment, PTSL is weighed and placed into a three-necked flask, solvent is added, and the apparatus is connected; then, the oxygen cylinder is opened, and ozone generated by the ozone generator is introduced into the reactor. The oxygen inflow rate is adjusted to 0.5 L / min, and the ozone output rate is 20-60 m³ / min. 3 / h, a mixture of ozone and oxygen is directly introduced into a three-necked flask. The ozone concentration is detected by an ozone concentration detector. The mixture is heated and stirred at 20 °C using a heat-collecting constant-temperature magnetic stirrer for 1-4 hours to allow the oxidation reaction to occur. After the reaction is complete, the mixture is centrifuged and filtered. The filtered solid residue is washed twice with solvent and then filtered again. All the filtered liquid samples are combined and dried at 105 °C overnight.

7. The method for preparing humic acid from lignite through staged oxidation and depolymerization according to claim 6, characterized in that, The solvent is methanol or acetic acid; the ozone output is 40 m³. 3 / h, oxidation reaction occurs for 3h.

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