Method and system for extracting humic acid from lignite

Abstract

The invention provides a method and system for extracting humic acid from lignite, and the method comprises the following steps: S1, pre-activating lignite and a hydrogen peroxide solution to obtain a first reaction product; s2, the first reaction product reacts with a first nitric acid solution, and a preliminary oxidation product is obtained; s3, the preliminary oxidation product reacts with a second nitric acid solution, oxidation is completed, and humic acid is obtained; the concentration of the first nitric acid solution is lower than that of the second nitric acid solution, and the reaction pressure in the step S2 is lower than that in the step S3. According to the method for extracting the humic acid from the lignite, the two-stage nitric acid oxidation reaction is adopted, NOx is fully released by optimizing the two-stage reaction process, and the influence on the reaction is reduced, so that the humic acid yield is increased.

Description

Technical Field

[0001] The invention relates to the field of coal chemical industry, and in particular to a method and a system for extracting humic acid from lignite. Background Art

[0002] Humic acid is a carrier of plants and animals. It is a kind of heterogeneous-aliphatic-aromatic amorphous macromolecular organic weak acid mixture rich in carboxyl, phenolic hydroxyl, carbonyl and other active functional groups formed through a series of interactions between microorganisms and geophysics and chemistry. It has a wide range of applications in agriculture, gardening, animal husbandry, breeding and other fields. Lignite is a low-rank coal with rich humic acid content, which is an ideal raw material for extracting humic acid.

[0003] At present, nitric acid is usually used as an oxidant to extract humic acid from lignite. The strong oxidizing property of nitric acid increases the oxygen content after degradation. However, the use of nitric acid oxidation has the following problems:

[0004] ① A large amount of NOx is produced during the nitric acid oxidation process. NOx wraps around the lignite, limiting the further progress of the reaction; and a large amount of NOx foam accumulates, limiting the feed to the reactor.

[0005] ② As a strong oxidant, nitric acid may over-oxidize humic acid, resulting in a decrease in product performance;

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

[0007] At present, the existing technology proposes to oxidize lignite by using hydrogen peroxide combined with nitric acid, but the nitric acid oxidation is not controlled, the reaction is too violent, and a series of problems caused by the large amount of NOx produced by the nitric acid reaction cannot be solved, and the quality of the humic acid product cannot be guaranteed.

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

[0009] Patent CN106279717A discloses a method for preparing humic acid with high extraction rate from low-rank coal, comprising the following steps: Step 1) firstly crush the dried low-rank coal to above 40 meshes, and ripen it overnight with dilute acid or alkali; Step 2) mix the ripened low-rank coal with hydrogen peroxide, water, and catalyst in a mass ratio of 1: (0.1-1): (2-5): (0.01-0.1), react at 30-150°C for 0.5-3 hours, and separate the solid and liquid. A solid residue is obtained, and then mixed acid and water are added to the solid residue at a mass ratio of solid residue: mixed acid: water of 1: (0.1-1): 3, and the reaction is continued for 0.5-3 hours to obtain a crude humic acid product; step 3) 0.5-5% of the mass of an alkali metal fluoride salt is added to the crude humic acid product, and after ultrasonic stirring for 1-60 minutes, 1-10% of the mass of a base is added to continue the reaction for 0.5-3 hours, and then the solid-liquid separation is carried out, and the solution is concentrated to obtain a high-quality, high-purity humic acid product.

[0010] In view of the above technical problems, the present invention is specially introduced. Summary of the invention

[0011] The main purpose of the present invention is to provide a method and system for extracting humic acid from lignite, which can effectively control the nitric acid oxidation reaction through a two-stage reaction, improve the nitric acid yield, and reduce the nitric acid consumption.

[0012] In order to achieve the above-mentioned purpose, the present application proposes a method for extracting humic acid from lignite, comprising the following steps: step S1, pre-activating the lignite with a hydrogen peroxide solution, and separating to obtain a first reaction product; step S2, reacting the first reaction product with a first nitric acid solution, and separating to obtain a preliminary oxidation product; step S3, reacting the preliminary oxidation product with a second nitric acid solution, completing oxidation, and obtaining 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, in step S2, the reaction pressure is 0.01 to 0.08 MPa, and the reaction time is 3 to 5 hours.

[0014] Preferably, in step S3, the reaction pressure 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 returned in step S2 to the hydrogen peroxide solution is 0.02-0.1:1.

[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 the lignite is 0.1-0.4:1.

[0020] In addition, the present 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 in sequence, wherein 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 precipitated in the buffer reaction unit is sprayed into 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 exhausted NOx gas.

[0023] Preferably, the buffer device is connected to the first reaction unit, and the buffer device feeds part 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 process of the two-stage reaction, NOx is fully released, the impact on the reaction is reduced, and thus the humic acid yield is increased.

[0026] 2. The first stage of nitric acid oxidation reaction uses a slightly negative pressure environment to cause the NOx produced by the reaction to precipitate from the solution, reducing the foam wrapping around the lignite; using low nitric acid concentration to reduce the reaction rate and avoid 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 with the lignite that has not reacted sufficiently in the buffer reaction unit.

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

[0029] 5. Using hydrogen peroxide as a pre-oxidant avoids reaction with active metals in lignite. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] The accompanying drawings constituting a part of the present invention are used to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation of the present invention. In the accompanying drawings:

[0031] Figure 1 The process flow of the method for extracting humic acid from lignite of the present application is shown;

[0032] Figure 2 The basic composition of the lignite humic acid extraction system of the present application is shown.

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

[0034] R1, first reaction unit; R2, buffer reaction unit; R3, third reaction unit; H1, cache device. DETAILED DESCRIPTION

[0035] It should be noted that, in the absence of conflict, the embodiments of the present invention and the features in the embodiments can be combined with each other. The present invention will be described in detail below with reference to the accompanying drawings and in combination with the embodiments.

[0036] The present invention is further described in detail below in conjunction with specific embodiments, and these embodiments cannot be understood as limiting the scope of protection claimed by the present invention. The term "including" when used indicates the existence of a feature, but does not exclude the existence or addition of one or more other features; the orientation or position relationship indicated by the terms "lateral", "upper", "lower", "front", "rear", "left", "right", "top", "bottom", "inside", "outside", etc. is based on the orientation or position relationship shown in the drawings, which is only for the convenience of description, and does not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation on the present invention; in addition, the terms "first" and "second" are only used for descriptive purposes, and cannot be understood as indicating or implying relative importance.

[0037] In the description, it should be noted that, unless otherwise clearly specified and limited, the terms "installed", "connected", and "connected" should be understood in a broad sense. For example, it can be a fixed connection, a detachable connection, or an integral connection; it can be directly connected, or indirectly connected through an intermediate medium, or it can be the internal communication of two components. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances. In addition, in the description of the present invention, unless otherwise specified, "plurality" means two or more.

[0038] This application proposes a method and system for extracting humic acid from lignite. 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 in sequence. 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: the first reaction unit R1 uses hydrogen peroxide as an oxidant to break the aromatic ring in the coal, convert the quinone group into a carboxyl group, and increase the acid functional group in the lignite. Compared with nitric acid solution, hydrogen peroxide is used as a pre-oxidant. Hydrogen peroxide will not react with the active metals in the lignite, avoid the introduction of nitro groups, and will not produce NOx waste gas, reducing process costs. The hydrogen peroxide concentration of the first-stage reaction unit is 10-20wt%, and the mass ratio of hydrogen peroxide to lignite is 0.5-1:1.

[0040] In the present application, the lignite pre-oxidized with hydrogen peroxide is further oxidized in the first nitric acid solution. A large amount of NOx gas will be generated during the nitric acid oxidation process. If the NOx gas cannot be discharged in time, a large amount of foam will be formed, which will wrap around the surface of the lignite and inhibit the reaction. In addition, a large amount of foam will limit the feed amount in the reactor and reduce the reaction efficiency of the system.

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

[0042] Specifically, the buffer reaction unit R2 and the third reaction unit R3 use nitric acid of different concentrations as an oxidant, the concentration of nitric acid in the buffer reaction unit R2 is lower than that in the third reaction unit R3, and the reaction pressure in the 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 10-20wt%, the pressure is 0.01-0.08Mpa, the reaction temperature is 60-90°C, and the reaction time is 3-5 hours. The pressure of the third reaction unit is 0.1-0.5Mpa, 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, the slightly negative pressure environment is used to cause the NOx produced by the reaction to precipitate from the solution, reduce foam, and reduce the wrapping of the foam on the lignite, thereby improving the conversion rate; and by extending the reaction time, the NOx is fully released, and the conversion rate is further improved by releasing the NOx produced by the reaction. In addition, by reducing the nitric acid concentration in the buffer reaction unit, the reaction speed is reduced, a large amount of NOx gas is avoided, and the amount of high-concentration nitric acid solution is reduced. In this way, a relatively slow oxidation reaction is formed in the buffer reaction unit, allowing NOx to be fully released.

[0044] Compared with the buffer reaction unit, the third reaction unit increases the concentration of nitric acid to further react the lignite that has not fully reacted in the buffer reaction unit, thereby increasing the overall conversion rate. In this way, a relatively short and violent oxidation reaction is formed in the third reaction unit.

[0045] The present application optimizes the process conditions of the buffer reaction unit and the third reaction unit so that the oxidation reaction is basically carried out in the buffer reaction unit, and the conversion rate of the buffer reaction unit accounts for at least 80% of the total conversion rate; the remaining conversion occurs in the third reaction unit. Since most of the conversion occurs in the buffer reaction unit, the intensity of the reaction in the third reaction unit is reduced, the amount of NOx generated is significantly less than that in the buffer reaction unit, and the generated NOx has little effect on the reaction, so there is no need for pressure reduction treatment.

[0046] In addition, the present application also proposes to return at least part of the NOx obtained by the buffer reaction unit R2 to the first reaction unit R1. On the one hand, the internal pressure of the buffer reaction unit R2 can be reduced. On the other hand, NOx enters the first reaction unit R1 and can react with the hydrogen peroxide solution in the first reaction unit R1 to generate a trace amount of nitric acid solution, thereby improving the oxidizability of the first reaction unit and solving the problem of low oxidizability of hydrogen peroxide. However, it is necessary to strictly control the amount of NOx entering the first reaction unit R1 to avoid NOx interfering with the hydrogen peroxide oxidation reaction. Preferably, the volume ratio of the returned NOx to the hydrogen peroxide solution is 0.02 to 0.1:1. In addition, the NOx in the buffer reaction unit R2 is passed from the top of the buffer reaction unit R2 reactor to the bottom of the reactor in the first reaction unit R1, and a nozzle (not shown in the figure) is provided at the bottom of the reactor in the first reaction unit R1 to promote the reaction of NOx and the hydrogen peroxide solution by spraying NOx.

[0047] In addition, the reactor for the reaction in the buffer reaction unit is a reactor, and a gas-liquid separation component is provided inside the reactor to separate the NOx gas precipitated under a slightly negative pressure environment. A buffer device H1 is also provided in the reaction system, and the buffer device H1 is connected to the buffer reaction unit R2 to absorb the discharged NOx gas. The buffer device H1 is connected to the first reaction unit R1, and the buffer device H1 introduces a trace amount of NOx into the first reaction unit.

[0048] This application uses Baoqing lignite as the extraction raw material, and its humic acid content accounts for about 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 components, which are not specifically limited here.

[0049] The scheme is further described below in conjunction with specific embodiments.

[0050] Embodiment 1:

[0051] The first reaction unit R1, the buffer reaction unit R2 and the third reaction unit R3 each include at least one reactor, the reactor type is a reactor, and the three reaction units respectively use a first reactor, a second reactor and a third reactor. The concentration of the hydrogen peroxide solution in the first reactor is 15wt%; the concentration of the nitric acid solution in the second reactor is 12wt%; the concentration of the nitric acid solution in the third reactor is 30wt%.

[0052] The lignite is dried and crushed for pretreatment, so that the moisture content of the lignite is 10-20% and the particle size is less than 5 mm. 100 g of lignite powder and 50 g of hydrogen peroxide solution are sent to the first-stage reactor, the reaction temperature in the first-stage reactor is 70°C and the pressure is 0.6 MPa. After reacting for 3 hours, the temperature is lowered 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. The temperature of the second-stage reactor is maintained at 70°C and the pressure is 0.05Mpa. The reaction is carried out for 5 hours. After cooling to room temperature, it is filtered to obtain a preliminary oxidation product.

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

[0055] Embodiment 2:

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

[0057] The lignite is dried and crushed for pretreatment, so that the moisture content of the lignite is 10-20% and the particle size is less than 5 mm. 100 g of lignite powder and 50 g of hydrogen peroxide solution are sent to the first-stage reactor, the reaction temperature in the first-stage reactor is 70°C and the pressure is 0.6 MPa. After reacting for 3 hours, the temperature is lowered to room temperature and filtered to obtain the first reaction product.

[0058] 50g of the first reaction product was sent to the third-stage reactor, and 100g of 30wt% nitric acid solution was filled in the third-stage reactor. The temperature of the third-stage reactor was kept at 70°C and the pressure was 0.5Mpa. The reaction was continued for 6 hours, and the final product was obtained by filtration. The yield was calculated based on the weight of the obtained humic acid.

[0059] Embodiment 3:

[0060] The first reaction unit R1, the buffer reaction unit R2 and the third reaction unit R3 each include at least one reactor, the reactor type is a reactor, and the three reaction units respectively use a first reactor, a second reactor and a third reactor. The concentration of the hydrogen peroxide solution in the first reactor is 15wt%; the concentration of the nitric acid solution in the second reactor is 20wt%; the concentration of the nitric acid solution in the third reactor is 30wt%.

[0061] The lignite is dried and crushed for pretreatment, so that the moisture content of the lignite is 10-20% and the particle size is less than 5 mm. 100 g of lignite powder and 50 g of hydrogen peroxide solution are sent to the first-stage reactor, the reaction temperature in the first-stage reactor is 70°C and the pressure is 0.6 MPa. After reacting for 3 hours, the temperature is lowered 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. The temperature of the second-stage reactor is maintained at 70°C and the pressure is 0.1Mpa. The reaction is carried out for 5 hours. After cooling to room temperature, it is filtered to obtain a preliminary oxidation product.

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

[0064] Embodiment 4:

[0065] The first reaction unit R1, the buffer reaction unit R2 and the third reaction unit R3 each include at least one reactor, the reactor type is a reactor, and the three reaction units respectively use a first reactor, a second reactor and a third reactor. The concentration of the hydrogen peroxide solution in the first reactor is 15wt%; the concentration of the nitric acid solution in the second reactor is 25wt%; the concentration of the nitric acid solution in the third reactor is 30wt%.

[0066] The lignite is dried and crushed for pretreatment, so that the moisture content of the lignite is 10-20% and the particle size is less than 5 mm. 100 g of lignite powder and 50 g of hydrogen peroxide solution are sent to the first-stage reactor, the reaction temperature in the first-stage reactor is 70°C and the pressure is 0.6 MPa. After reacting for 3 hours, the temperature is lowered 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. The temperature of the second-stage reactor is maintained at 70°C and the pressure is 0.06Mpa. The reaction is carried out for 5 hours. After cooling to room temperature, it is filtered to obtain a preliminary oxidation product.

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

[0069] Embodiment 5:

[0070] The first reaction unit R1, the buffer reaction unit R2 and the third reaction unit R3 each include at least one reactor, the reactor type is a reactor, and the three reaction units respectively use a first reactor, a second reactor and a third reactor. The concentration of the hydrogen peroxide solution in the first reactor is 15wt%; the concentration of the nitric acid solution in the second reactor is 10wt%; the concentration of the nitric acid solution in the third reactor is 30wt%.

[0071] The lignite is dried and crushed for pretreatment, so that the moisture content of the lignite is 10-20% and the particle size is less than 5 mm. 100 g of lignite powder and 50 g of hydrogen peroxide solution are sent to the first-stage reactor, the reaction temperature in the first-stage reactor is 70°C and the pressure is 0.6 MPa. After reacting for 3 hours, the temperature is lowered 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. The temperature of the second-stage reactor is maintained at 70°C and the pressure is 0.06Mpa. The reaction is carried out for 5 hours. After cooling to room temperature, it is filtered to obtain a preliminary oxidation product.

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

[0074] Embodiment 6:

[0075] The first reaction unit R1, the buffer reaction unit R2 and the third reaction unit R3 each include at least one reactor, the reactor type is a reactor, and the three reaction units respectively use a first-stage reactor, a second-stage reactor and a third-stage reactor. The concentration of the hydrogen peroxide solution in the first-stage reactor is 15wt%; the concentration of the nitric acid solution in the second-stage reactor is 12wt%; the concentration of the nitric acid solution in the third-stage reactor is 30wt%. In addition, in this embodiment, a trace amount of NOx gas is introduced into the first-stage reactor.

[0076] The lignite is dried and crushed for pretreatment, so that the moisture content of the lignite is 10-20% and the particle size is less than 5mm. 100g of lignite powder and 50g of hydrogen peroxide solution are sent to the first-stage reactor, and 3ml of NOx gas is introduced into the bottom of the first-stage reactor. The reaction temperature in the first-stage reactor is maintained at 70°C and the pressure is 0.6MPa. After reacting for 3 hours, the reaction is 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. The temperature of the second-stage reactor is maintained at 70°C and the pressure is 0.06Mpa. The reaction is carried out for 5 hours. After cooling to room temperature, it is filtered to obtain a preliminary oxidation product.

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

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

[0080]

[0081] By comparing the humic acid difference rate values ​​of different embodiments, it can be seen that the conversion rate of humic acid can be improved by setting a two-stage oxidation reaction and adjusting the process conditions of the two-stage reaction; the conversion rate of humic acid can be further improved by appropriately introducing a small amount of NOx into the hydrogen peroxide pre-oxidation reaction.

[0082] Specifically, in combination with Example 1 and Example 2, under the same reaction time, although the high concentration of nitric acid was maintained in Example 2, the final humic acid conversion rate was significantly lower than that in Example 1. During the reaction, it was observed that a large amount of NOx gas was 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 precipitated, and foam accumulation was formed, thereby inhibiting the further progress of the reaction.

[0083] In combination with Example 1 and Example 3, when the slightly negative pressure condition is not formed in the second-stage reactor, the generated NOx gas still cannot be released and exists in the form of foam, inhibiting the progress of the second-stage reaction.

[0084] Combining Example 1 with Examples 4 and 5, it can be seen that when the second-stage reactor is under a slightly negative pressure, the concentration of the nitric acid solution in the second-stage reactor is in the range of 8-25%, and a relatively ideal yield can be obtained. However, considering the process cost and process safety, the temperature of the nitric acid solution is locked at 20%.

[0085] In combination with Example 1 and Example 6, adding an appropriate amount of NOx in 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 process of the two-stage reaction, NOx is fully released, the impact on the reaction is reduced, and thus the humic acid yield is increased.

[0088] 2. The first stage of nitric acid oxidation reaction uses a slightly negative pressure environment to cause the NOx produced by the reaction to precipitate from the solution, reducing the foam wrapping around the lignite; using low nitric acid concentration to reduce the reaction rate and avoid 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 with the lignite that has not reacted sufficiently in the buffer reaction unit.

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

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

[0092] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for extracting humic acid from lignite, characterized in that: The following steps are involved: Step S1, pre-activating lignite and a hydrogen peroxide solution, and separating to obtain a first reaction product; Step S2, the first reaction product is reacted with a first nitric acid solution to separate and obtain a preliminary oxidation product; Step S3, the preliminary oxidation product reacts with a second nitric acid solution to complete 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.

2. The method for extracting humic acid from lignite according to claim 1, characterized in that: In step S2, the reaction pressure is 0.01-0.08 MPa, and the reaction time is 3-5 hours.

3. The method for extracting humic acid from lignite according to claim 2, characterized in that: In step S3, the reaction pressure is 0.1-0.5 MPa, and the reaction time is 0.5-1 hour.

4. The method for extracting humic acid from lignite according to any one of claims 1 to 3, characterized in that: The concentration of the first nitric acid solution used in step S2 is 10-20 wt %.

5. The method for extracting humic acid from lignite according to any one of claims 1 to 3, characterized in that: The concentration of the second nitric acid solution used in step S3 is 20-40 wt %.

6. The method for extracting humic acid from lignite according to any one of claims 1 to 3, characterized in that: At least part of the NOx obtained in step S2 is returned to participate in the reaction in step S1.

7. The method for extracting humic acid from lignite according to claim 6, characterized in that: The volume ratio of the NOx returned in step S2 to the hydrogen peroxide solution is 0.02-0.1:

1.

8. The method for extracting humic acid from lignite according to any one of claims 1 to 3, 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.

9. A system for extracting humic acid from lignite, characterized in that: The invention comprises a first reaction unit (R1), a buffer reaction unit (R2) and a third reaction unit (R3) which are connected in series in sequence, wherein the concentration of the first nitric acid solution in the buffer reaction unit (R2) is lower than the concentration of the second nitric acid solution in the third reaction unit (R3), and the reaction pressure in the buffer reaction unit (R2) is lower than the reaction pressure in the third reaction unit (R3).

10. The system for extracting humic acid according to claim 9, characterized in that: The top of the buffer reaction unit (R2) is connected to the bottom of the first reaction unit (R1), and the NOx gas precipitated in the buffer reaction unit (R2) is sprayed into the bottom of the first reaction unit (R1).

11. The system for extracting humic acid according to claim 9 or 10, characterized in that: It also includes a buffer device (H1), which is connected to the buffer reaction unit (R2) and absorbs the exhausted NOx gas.

12. The system for extracting humic acid according to claim 11, characterized in that: The buffer device (H1) is connected to the first reaction unit (R1), and the buffer device (H1) feeds part of the NOx gas into the first reaction unit (R1).

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