Method and system for lignite extraction of humic acid

Abstract

This application proposes a method and system for extracting humic acid from lignite, comprising the following steps: Step S1, lignite is pre-activated with hydrogen peroxide solution to obtain a first reaction product; Step S2, the first reaction product is reacted with a first nitric acid solution to obtain a preliminary oxidation product; Step S3, the preliminary oxidation product is reacted with a second nitric acid solution to complete the oxidation and obtain humic acid; the concentration of the first nitric acid solution is lower than that of the second nitric acid solution, and the reaction pressure in step S2 is lower than that in step S3. The method for extracting humic acid from lignite proposed in this application employs a two-stage nitric acid oxidation reaction. By optimizing the two-stage reaction process, NOx is fully released, reducing its impact on the reaction and thus improving the humic acid yield.

Description

Technical Field

[0001] This invention relates to the field of coal chemical industry, and more specifically, to a method and system for extracting humic acid from lignite. Background Technology

[0002] Humic acid is a heterogeneous, amorphous, macromolecular organic weak acid mixture rich in various active functional groups such as carboxyl, phenolic hydroxyl, and carbonyl groups, formed through microbial decomposition and synthesis, as well as a series of geophysical and chemical interactions. It is widely used in agriculture, horticulture, animal husbandry, and aquaculture. Lignite, as a low-rank coal, is rich in humic acid and is an ideal raw material for its extraction.

[0003] Currently, nitric acid is commonly used as an oxidant to extract humic acid from lignite, as its strong oxidizing properties increase the oxygen content after degradation. However, using nitric acid for oxidation presents the following problems:

[0004] ① A large amount of NOx is generated during the oxidation of nitric acid. The NOx coats the lignite, limiting the further progress of the reaction; and the large amount of NOx foam accumulation restricts the feed into the reactor.

[0005] ②Nitric acid, as a strong oxidizing agent, may over-oxidize humic acid, leading to a decline in product performance;

[0006] ③ The yield of humic acid is low, and the amount of nitric acid used is large, which increases the process cost.

[0007] Current technologies propose using hydrogen peroxide in combination with nitric acid to oxidize lignite, but the oxidation of nitric acid is not controlled, the reaction is too violent, and the problems caused by the large amount of NOx produced by the nitric acid reaction cannot be solved, and the quality of humic acid products cannot be guaranteed.

[0008] Patent CN107383390A proposes a process for extracting humic acid from coal, including the following steps: a) lignite crushing; b) oxidative degradation: first, add hydrogen peroxide with a mass concentration of 15-25%, the ratio of hydrogen peroxide added to lignite powder mass is 0.01-0.35:1, then add nitric acid solution with a mass concentration of 30-60%, the ratio of nitric acid solution added to lignite powder mass is 0.4-0.6:1.

[0009] Patent CN106279717A discloses a method for preparing humic acid with a high extraction rate from low-rank coal, including the following steps: Step 1) First, the dried low-rank coal is pulverized to a fineness of 40 mesh or higher and matured overnight using dilute acid or alkali; Step 2) The matured low-rank coal is mixed with hydrogen peroxide, water, and catalyst in a mass ratio of 1:(0.1~1):(2~5):(0.01~0.1), and reacted at 30~150℃ for 0.5~3 hours, followed by solid-liquid separation. The solid residue is obtained, and then mixed acid and water are added to the solid residue at a mass ratio of 1:(0.1~1):3. The reaction is continued for 0.5~3 hours to obtain crude humic acid. In step 3), 0.5~5% of the mass of alkali metal fluoride salt is added to the crude humic acid. After ultrasonic stirring for 1~60 minutes, 1~10% of the mass of alkali is added to the solid residue. The reaction is continued for 0.5~3 hours. After solid-liquid separation, the solution is concentrated to obtain high-quality, high-purity humic acid product.

[0010] In view of the above technical problems, this invention is hereby introduced. Summary of the Invention

[0011] The main objective of this invention is to provide a method and system for extracting humic acid from lignite, which achieves effective control of the nitric acid oxidation reaction through a two-stage reaction, thereby increasing the nitric acid yield and reducing nitric acid consumption.

[0012] To achieve the above objectives, this application proposes a method for extracting humic acid from lignite, comprising the following steps: Step S1, lignite is pre-activated with hydrogen peroxide solution, and a first reaction product is obtained by separation; Step S2, the first reaction product is reacted with a first nitric acid solution, and a preliminary oxidation product is obtained by separation; Step S3, the preliminary oxidation product is reacted with a second nitric acid solution to complete the oxidation and obtain humic acid; the concentration of the first nitric acid solution is lower than the concentration of the second nitric acid solution, and the reaction pressure in step S2 is lower than the reaction pressure in step S3.

[0013] Preferably, the reaction pressure in step S2 is 0.01–0.08 MPa, and the reaction time is 3–5 hours.

[0014] Preferably, the reaction pressure in step S3 is 0.1–0.5 MPa, and the reaction time is 0.5–1 hour.

[0015] Preferably, the concentration of the first nitric acid solution used in step S2 is 10–20 wt%.

[0016] Preferably, the concentration of the second nitric acid solution used in step S3 is 20–40 wt%.

[0017] Preferably, at least a portion of the NOx obtained in step S2 is returned to participate in the reaction in step S1.

[0018] Preferably, the volume ratio of NOx to hydrogen peroxide solution returned in step S2 is 0.02 to 0.1:1.

[0019] Preferably, the concentration of the hydrogen peroxide solution used in step S1 is 10-20 wt%, and the mass ratio of the hydrogen peroxide solution to lignite is 0.1-0.4:1.

[0020] In addition, this application also proposes a system for extracting humic acid from lignite, comprising a first reaction unit, a buffer reaction unit, and a third reaction unit connected in series. The concentration of the first nitric acid solution in the buffer reaction unit is lower than the concentration of the second nitric acid solution in the third reaction unit, and the reaction pressure in the buffer reaction unit is lower than the reaction pressure in the third reaction unit.

[0021] Preferably, the top of the buffer reaction unit is connected to the bottom of the first reaction unit, and the NOx gas released in the buffer reaction unit is injected from the bottom of the first reaction unit.

[0022] Preferably, it also includes a buffer device, which is connected to the buffer reaction unit to absorb the discharged NOx gas.

[0023] Preferably, the buffer device is connected to the first reaction unit, and the buffer device feeds a portion of the NOx gas into the first reaction unit.

[0024] In summary, the method and system for extracting humic acid from lignite proposed in this embodiment achieve the following technical effects:

[0025] 1. A two-stage nitric acid oxidation reaction is adopted. By optimizing the two-stage reaction process, NOx is fully released, reducing its impact on the reaction and thus improving the yield of humic acid.

[0026] 2. The first stage of nitric acid oxidation reaction utilizes a slightly negative pressure environment to promote the precipitation of NOx produced by the reaction from the solution, reducing the encapsulation of lignite by foam; a low nitric acid concentration is used to slow down the reaction rate and avoid the large accumulation of NOx gas; by extending the reaction time, NOx is fully released.

[0027] 3. The third reaction unit increases the concentration of nitric acid to further react the lignite that did not react sufficiently in the buffer reaction unit.

[0028] 4. By injecting the NOx generated in the buffer reaction unit into the first reaction unit, the oxidation capacity of the hydrogen peroxide solution is improved, thereby further increasing the yield of humic acid.

[0029] 5. Using hydrogen peroxide as a pre-oxidant avoids reaction with the active metals in lignite. Attached Figure Description

[0030] The accompanying drawings, which form part of this specification, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an undue limitation of the invention. In the drawings:

[0031] Figure 1 The process flow diagram of the lignite extraction method of this application is shown.

[0032] Figure 2 The basic components of the lignite extraction humic acid system of this application are shown.

[0033] The above figures include the following reference numerals:

[0034] R1, first reaction unit; R2, buffer reaction unit; R3, third reaction unit; H1, buffer device. Detailed Implementation

[0035] It should be noted that, unless otherwise specified, the embodiments and features described in the present invention can be combined with each other. The present invention will now be described in detail with reference to the accompanying drawings and embodiments.

[0036] The present invention will be further described in detail below with reference to specific embodiments. These embodiments should not be construed as limiting the scope of protection claimed by the present invention. The term "comprising" indicates the presence of a feature, but does not exclude the presence or addition of one or more other features. The terms "lateral," "upper," "lower," "front," "rear," "left," "right," "top," "bottom," "inner," and "outer," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings, and are only for the purpose of description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of the present invention. In addition, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0037] In this description, it should be noted that, unless otherwise explicitly specified and limited, the terms "installation," "connection," and "linking" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal connection of two components. Those skilled in the art can understand the specific meaning of these terms in this invention based on the specific circumstances. Furthermore, in the description of this invention, unless otherwise stated, "a plurality of" means two or more.

[0038] This application proposes a method and system for extracting humic acid from lignite, combined with the appendix. Figure 1 and attached Figure 2As shown, the extraction system includes a first reaction unit R1, a buffer reaction unit R2, and a third reaction unit R3 connected in series. The first reaction unit is used to pre-oxidize the lignite, and the buffer reaction unit R2 and the third reaction unit R3 are used to oxidize the pre-oxidized lignite step by step.

[0039] Specifically, in the first reaction unit R1, hydrogen peroxide is used as the oxidant to break the aromatic rings in the coal, converting quinone groups into carboxyl groups and increasing the acidic functional groups in the lignite. Using hydrogen peroxide as a pre-oxidant, compared to nitric acid solution, avoids reacting with the active metals in the lignite, preventing the introduction of nitro groups and eliminating NOx emissions, thus reducing process costs. The hydrogen peroxide concentration in the first-stage reaction unit is 10–20 wt%, and the mass ratio of hydrogen peroxide to lignite is 0.5–1:1.

[0040] This application involves further oxidizing lignite, which has undergone pre-oxidation with hydrogen peroxide, in a first nitric acid solution. During this nitric acid oxidation process, a large amount of NOx gas is generated. If this NOx gas cannot be discharged in time, it will form a large amount of foam, which will coat the surface of the lignite and inhibit the reaction. Furthermore, the large amount of foam will limit the feed rate into the reactor, reducing the system's reaction efficiency.

[0041] Based on the problems existing in the nitric acid reaction, this application proposes a two-stage nitric acid oxidation reaction, which improves the reaction efficiency by controlling and optimizing the process of the two-stage reaction.

[0042] Specifically, buffer reaction unit R2 and the third reaction unit R3 use nitric acid of different concentrations as oxidants. The concentration of nitric acid in buffer reaction unit R2 is lower than that in the third reaction unit R3, and the reaction pressure in buffer reaction unit R2 is lower than that in the third reaction unit R3. Preferably, the concentration of nitric acid in the buffer reaction unit is 10–20 wt%, the pressure is 0.01–0.08 MPa, the reaction temperature is 60–90°C, and the reaction time is 3–5 hours. The pressure in the third reaction unit is 0.1–0.5 MPa, the concentration of nitric acid in the third reaction unit is 20–40%, the reaction temperature is 60–90°C, and the reaction time is 0.5–1 hour.

[0043] In the buffer reaction unit, a slightly negative pressure environment promotes the precipitation of NOx generated during the reaction from the solution, reducing foam and minimizing its encapsulation of lignite, thereby improving the conversion rate. Furthermore, extending the reaction time allows for the full release of NOx, further enhancing the conversion rate. Additionally, reducing the nitric acid concentration in the buffer reaction unit slows the reaction rate, preventing the large-scale accumulation of NOx gas, and also reduces the amount of high-concentration nitric acid solution used. This creates a relatively slow oxidation reaction within the buffer reaction unit, ensuring the full release of NOx.

[0044] Compared to the buffer reaction unit, the third reaction unit increases the concentration of nitric acid to further react the lignite that was not fully reacted in the buffer reaction unit, thereby improving the overall conversion rate. This results in a relatively short but intense oxidation reaction in the third reaction unit.

[0045] This application optimizes the process conditions of the buffer reaction unit and the third reaction unit, ensuring that the oxidation reaction is primarily carried out in the buffer reaction unit, with the conversion rate in the buffer reaction unit accounting for at least 80% of the total conversion; the remaining conversion occurs in the third reaction unit. Since most of the conversion occurs in the buffer reaction unit, the reaction intensity in the third reaction unit is reduced, and the amount of NOx generated is significantly less than in the buffer reaction unit. The generated NOx has a smaller impact on the reaction, therefore, depressurization is unnecessary.

[0046] Furthermore, this application proposes to return at least a portion of the NOx obtained from the buffer reaction unit R2 to the first reaction unit R1. This reduces the internal pressure of the buffer reaction unit R2, and the NOx entering the first reaction unit R1 reacts with the hydrogen peroxide solution to generate a trace amount of nitric acid solution, thereby increasing the oxidizing power of the first reaction unit and addressing the problem of low oxidizing power of hydrogen peroxide. However, the amount of NOx entering the first reaction unit R1 needs to be strictly controlled to avoid interference with the hydrogen peroxide oxidation reaction. Preferably, the volume ratio of returned NOx to hydrogen peroxide solution is 0.02–0.1:1. Additionally, the NOx in the buffer reaction unit R2 is introduced from the top of the reactor in the buffer reaction unit R2 into the bottom of the reactor in the first reaction unit R1, and the bottom of the reactor in the first reaction unit R1 is equipped with a nozzle (not shown in the figure) to promote the reaction between NOx and the hydrogen peroxide solution by spraying in NOx.

[0047] In addition, the reactor in the buffer reaction unit is a reaction vessel, which is equipped with a gas-liquid separation component to separate the NOx gas released under a slight negative pressure environment. The reaction system also includes a buffer device H1, which is connected to the buffer reaction unit R2 to absorb the discharged NOx gas. The buffer device H1 is also connected to the first reaction unit R1, and a small amount of NOx is introduced into the first reaction unit through the buffer device H1.

[0048] This application uses Baoqing lignite as the extraction raw material, with a humic acid content of approximately 30-50%. The extraction method and system proposed in this application are not only applicable to Baoqing lignite, but also to other types of lignite with similar compositions, and are not specifically limited here.

[0049] The solution will be further explained below with reference to specific embodiments.

[0050] Example 1:

[0051] Each of the first reaction unit R1, the buffer reaction unit R2, and the third reaction unit R3 includes at least one reactor, selected as a reaction vessel. The three reaction units utilize a first reaction vessel, a second reaction vessel, and a third reaction vessel, respectively. The concentration of the hydrogen peroxide solution in the first-stage reaction vessel is 15 wt%; the concentration of the nitric acid solution in the second-stage reaction vessel is 12 wt%; and the concentration of the nitric acid solution in the third-stage reaction vessel is 30 wt%.

[0052] Lignite was pretreated by drying and pulverizing to reduce the moisture content to 10-20% and the particle size to less than 5 mm. 100 g of lignite powder and 50 g of hydrogen peroxide solution were fed into a first-stage reactor. The reaction temperature in the first-stage reactor was 70°C and the pressure was 0.6 MPa. After reacting for 3 hours, the mixture was cooled to room temperature and filtered to obtain the first reaction product.

[0053] Take 50g of the first reaction product and send it into the second-stage reactor. The second-stage reactor is filled with 50g of 12wt% nitric acid solution. Maintain the temperature of the second-stage reactor at 70℃ and the pressure at 0.05MPa. React for 5 hours, cool to room temperature, and filter to obtain the preliminary oxidation product.

[0054] 40g of the initial oxidation product was fed into the third-stage reactor, which was filled with 50g of 30wt% nitric acid solution. The temperature of the third-stage reactor was maintained at 70℃ and the pressure at 0.5MPa. The reaction was carried out for 1 hour, and the final product was obtained by filtration. The yield was calculated based on the weight of humic acid obtained.

[0055] Example 2:

[0056] As a comparative example, the buffer reaction unit is omitted in this embodiment. The concentration of hydrogen peroxide solution in the first-stage reactor is 15 wt%; the concentration of nitric acid solution in the third-stage reactor is 30 wt%.

[0057] Lignite was pretreated by drying and pulverizing to reduce the moisture content to 10-20% and the particle size to less than 5 mm. 100 g of lignite powder and 50 g of hydrogen peroxide solution were fed into a first-stage reactor. The reaction temperature in the first-stage reactor was 70°C and the pressure was 0.6 MPa. After reacting for 3 hours, the mixture was cooled to room temperature and filtered to obtain the first reaction product.

[0058] 50g of the first reaction product was fed into a third-stage reactor, which was then filled with 100g of a 30wt% nitric acid solution. The temperature of the third-stage reactor was maintained at 70℃ and the pressure at 0.5MPa for 6 hours. The final product was obtained by filtration. The yield was calculated based on the weight of humic acid obtained.

[0059] Example 3:

[0060] Each of the first reaction unit R1, the buffer reaction unit R2, and the third reaction unit R3 includes at least one reactor, selected as a reaction vessel. The three reaction units utilize a first reaction vessel, a second reaction vessel, and a third reaction vessel, respectively. The concentration of the hydrogen peroxide solution in the first-stage reaction vessel is 15 wt%; the concentration of the nitric acid solution in the second-stage reaction vessel is 20 wt%; and the concentration of the nitric acid solution in the third-stage reaction vessel is 30 wt%.

[0061] Lignite was pretreated by drying and pulverizing to reduce the moisture content to 10-20% and the particle size to less than 5 mm. 100 g of lignite powder and 50 g of hydrogen peroxide solution were fed into a first-stage reactor. The reaction temperature in the first-stage reactor was 70°C and the pressure was 0.6 MPa. After reacting for 3 hours, the mixture was cooled to room temperature and filtered to obtain the first reaction product.

[0062] Take 50g of the first reaction product and send it into the second-stage reactor. The second-stage reactor is filled with 50g of 20wt% nitric acid solution. Maintain the temperature of the second-stage reactor at 70℃ and the pressure at 0.1MPa. React for 5 hours. After cooling to room temperature, filter to obtain the preliminary oxidation product.

[0063] 40g of the initial oxidation product was fed into the third-stage reactor, which was filled with 50g of 30wt% nitric acid solution. The temperature of the third-stage reactor was maintained at 70℃ and the pressure at 0.5MPa. The reaction was carried out for 1 hour, and the final product was obtained by filtration. The yield was calculated based on the weight of humic acid obtained.

[0064] Example 4:

[0065] Each of the first reaction unit R1, the buffer reaction unit R2, and the third reaction unit R3 includes at least one reactor, selected as a reaction vessel. The three reaction units utilize a first reaction vessel, a second reaction vessel, and a third reaction vessel, respectively. The concentration of the hydrogen peroxide solution in the first-stage reaction vessel is 15 wt%; the concentration of the nitric acid solution in the second-stage reaction vessel is 25 wt%; and the concentration of the nitric acid solution in the third-stage reaction vessel is 30 wt%.

[0066] Lignite was pretreated by drying and pulverizing to reduce the moisture content to 10-20% and the particle size to less than 5 mm. 100 g of lignite powder and 50 g of hydrogen peroxide solution were fed into a first-stage reactor. The reaction temperature in the first-stage reactor was 70°C and the pressure was 0.6 MPa. After reacting for 3 hours, the mixture was cooled to room temperature and filtered to obtain the first reaction product.

[0067] Take 50g of the first reaction product and send it into the second-stage reactor. The second-stage reactor is filled with 50g of 20wt% nitric acid solution. Maintain the temperature of the second-stage reactor at 70℃ and the pressure at 0.06MPa. React for 5 hours. After cooling to room temperature, filter to obtain the preliminary oxidation product.

[0068] 40g of the initial oxidation product was fed into the third-stage reactor, which was filled with 50g of 30wt% nitric acid solution. The temperature of the third-stage reactor was maintained at 70℃ and the pressure at 0.5MPa. The reaction was carried out for 1 hour, and the final product was obtained by filtration. The yield was calculated based on the weight of humic acid obtained.

[0069] Example 5:

[0070] Each of the first reaction unit R1, the buffer reaction unit R2, and the third reaction unit R3 includes at least one reactor, which is selected as a reaction vessel. The three reaction units respectively employ a first reaction vessel, a second reaction vessel, and a third reaction vessel. The concentration of hydrogen peroxide solution in the first-stage reaction vessel is 15 wt%; the concentration of nitric acid solution in the second-stage reaction vessel is 10 wt%; and the concentration of nitric acid solution in the third-stage reaction vessel is 30 wt%.

[0071] Lignite was pretreated by drying and pulverizing to reduce the moisture content to 10-20% and the particle size to less than 5 mm. 100 g of lignite powder and 50 g of hydrogen peroxide solution were fed into a first-stage reactor. The reaction temperature in the first-stage reactor was 70°C and the pressure was 0.6 MPa. After reacting for 3 hours, the mixture was cooled to room temperature and filtered to obtain the first reaction product.

[0072] Take 50g of the first reaction product and send it into the second-stage reactor. The second-stage reactor is filled with 50g of 20wt% nitric acid solution. Maintain the temperature of the second-stage reactor at 70℃ and the pressure at 0.06MPa. React for 5 hours. After cooling to room temperature, filter to obtain the preliminary oxidation product.

[0073] 40g of the initial oxidation product was fed into the third-stage reactor, which was filled with 50g of 30wt% nitric acid solution. The temperature of the third-stage reactor was maintained at 70℃ and the pressure at 0.5MPa. The reaction was carried out for 1 hour, and the final product was obtained by filtration. The yield was calculated based on the weight of humic acid obtained.

[0074] Example 6:

[0075] Each of the first reaction unit R1, the buffer reaction unit R2, and the third reaction unit R3 includes at least one reactor, selected as a reaction vessel. The three reaction units respectively employ a first-stage reaction vessel, a second-stage reaction vessel, and a third-stage reaction vessel. The concentration of the hydrogen peroxide solution in the first-stage reaction vessel is 15 wt%; the concentration of the nitric acid solution in the second-stage reaction vessel is 12 wt%; and the concentration of the nitric acid solution in the third-stage reaction vessel is 30 wt%. Additionally, in this embodiment, a trace amount of NOx gas is introduced into the first-stage reaction vessel.

[0076] Lignite was pretreated by drying and pulverizing to achieve a moisture content of 10-20% and a particle size of less than 5 mm. 100 g of lignite powder and 50 g of hydrogen peroxide solution were fed into the first-stage reactor, and 3 ml of NOx gas was introduced into the bottom of the reactor. The reaction temperature in the first-stage reactor was maintained at 70°C and the pressure at 0.6 MPa. After reacting for 3 hours, the mixture was cooled to room temperature and filtered to obtain the first reaction product.

[0077] Take 50g of the first reaction product and send it into the second-stage reactor. The second-stage reactor is filled with 50g of 20wt% nitric acid solution. Maintain the temperature of the second-stage reactor at 70℃ and the pressure at 0.06MPa. React for 5 hours. After cooling to room temperature, filter to obtain the preliminary oxidation product.

[0078] 40g of the initial oxidation product was fed into the third-stage reactor, which was filled with 50g of 30wt% nitric acid solution. The temperature of the third-stage reactor was maintained at 70℃ and the pressure at 0.5MPa. The reaction was carried out for 1 hour, and the final product was obtained by filtration. The yield was calculated based on the weight of humic acid obtained.

[0079] Table 1. Humic acid yields measured in different embodiments.

[0080]

[0081] By comparing the humic acid conversion rates of different embodiments, it can be seen that setting up a two-stage oxidation reaction and adjusting the process conditions of the two-stage reaction can improve the conversion rate of humic acid; adding a small amount of NOx to the hydrogen peroxide pre-oxidation reaction can further improve the conversion rate of humic acid.

[0082] Specifically, combining Example 1 and Example 2, under the same reaction time, although the concentration of nitric acid was maintained at a high level in Example 2, the final humic acid conversion rate was significantly lower than that in Example 1. During the reaction, a large amount of NOx gas was observed to be generated in the third-stage reactor in Example 2. Due to the excessive pressure in the third-stage reactor, the NOx gas could not be released, forming foam accumulation, thereby inhibiting the further progress of the reaction.

[0083] Based on Examples 1 and 3, when a slight negative pressure condition is not formed in the second-stage reactor, the generated NOx gas cannot be released and exists in the form of foam, inhibiting the progress of the second-stage reaction.

[0084] Based on Examples 1, 4, and 5, a relatively ideal yield can be obtained when the concentration of the nitric acid solution in the second-stage reactor is in the range of 8% to 25%, while maintaining a slight negative pressure in the reactor. However, considering process cost and safety, the temperature of the nitric acid solution is locked at 20%.

[0085] Combining Examples 1 and 6, adding an appropriate amount of NOx to the first-stage reactor can improve the oxidation capacity of hydrogen peroxide without affecting the reactor pressure, thereby increasing the yield of humic acid.

[0086] In summary, the method and system for extracting humic acid from lignite proposed in this embodiment achieve the following technical effects:

[0087] 1. A two-stage nitric acid oxidation reaction is adopted. By optimizing the two-stage reaction process, NOx is fully released, reducing its impact on the reaction and thus improving the yield of humic acid.

[0088] 2. The first stage of nitric acid oxidation reaction utilizes a slightly negative pressure environment to promote the precipitation of NOx produced by the reaction from the solution, reducing the encapsulation of lignite by foam; a low nitric acid concentration is used to slow down the reaction rate and avoid the large accumulation of NOx gas; by extending the reaction time, NOx is fully released.

[0089] 3. The third reaction unit increases the concentration of nitric acid to further react the lignite that did not react sufficiently in the buffer reaction unit.

[0090] 4. By injecting the NOx generated in the buffer reaction unit into the first reaction unit, the oxidation capacity of the hydrogen peroxide solution is improved, thereby further increasing the yield of humic acid.

[0091] 5. Using hydrogen peroxide as a pre-oxidant avoids reaction with the active metals in lignite.

[0092] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for extracting humic acid from lignite, characterized in that, Includes the following steps: Step S1: Lignite is pre-activated with hydrogen peroxide solution, and the first reaction product is obtained by separation. Step S2: The first reaction product is reacted with the first nitric acid solution to separate and obtain the preliminary oxidation product; Step S3: The preliminary oxidation product reacts with the second nitric acid solution to complete the oxidation and obtain humic acid; The concentration of the first nitric acid solution is lower than that of the second nitric acid solution, and the reaction pressure in step S2 is lower than that in step S3. In step S2, the reaction pressure is 0.01–0.08 MPa and the reaction time is 3–5 hours. In step S3, the reaction pressure is 0.1–0.5 MPa and the reaction time is 0.5–1 hour. The concentration of the first nitric acid solution used in step S2 is 10–20 wt%, and the concentration of the second nitric acid solution used in step S3 is 20–40 wt%.

2. The method for extracting humic acid from lignite according to claim 1, characterized in that, At least a portion of the NOx obtained in step S2 is returned to participate in the reaction in step S1.

3. The method for extracting humic acid from lignite according to claim 2, characterized in that, The volume ratio of NOx returned in step S2 to the hydrogen peroxide solution is 0.02 to 0.1:

1.

4. The method for extracting humic acid from lignite according to claim 1, characterized in that, The concentration of the hydrogen peroxide solution used in step S1 is 10-20 wt%, and the mass ratio of the hydrogen peroxide solution to the lignite is 0.1-0.4:1.

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