Method for producing humic acid by hydrothermal carbonization

Abstract

The application relates to the field of agroforestry solid waste resource technology, and discloses a method for producing humic acid through hydrothermal carbonization. In the method, straw raw materials are mixed with water, uniformly soaked, and then frozen and expanded; after thawing, a hydrothermal carbonization reaction is carried out; after the reaction, liquid products and hydrothermal carbon are separated; the hydrothermal carbon is extracted by using a mixed extraction liquid of sodium pyrophosphate and sodium hydroxide to obtain an extraction liquid; the liquid products and the alkaline extraction liquid are respectively acid-precipitated to obtain humic acid. The method improves the structural depolymerization degree of cellulose and lignin in the straw through a freezing expansion and dissociation process, strengthens the hydrothermal carbonization and humification efficiency, simultaneously improves the humic acid content of the liquid products and the hydrothermal carbon, and further improves the overall humic acid yield and stability, thereby providing a reference for the high-value utilization of straw resources in cold regions, reducing production costs, and reducing environmental pollution.

Description

Technical Field

[0001] This application relates to the field of agricultural and forestry solid waste resource utilization technology, specifically to a method for producing humic acid through hydrothermal carbonization. Background Technology

[0002] Humic acid is an important organic substance widely used in agriculture, environmental protection, and industry. Traditional methods for preparing humic acid mainly rely on extraction from natural humus or mineral-derived humic acid such as lignite, which suffers from limited resources and low efficiency. In recent years, thermochemical technologies such as hydrothermal carbonization have been widely used in biomass conversion due to their high efficiency and environmental friendliness. The resulting solid product, hydrothermal carbon, can be used to prepare carbon-based materials, soil conditioners, and solid fuels, while the liquid product is used for the recovery of key nitrogen and phosphorus elements or the synthesis of other platform compounds such as furfural.

[0003] In humic acid preparation, the hydrothermal decomposition process generally requires the partial breakdown of the lignocellulose tri-element structure of the biomass itself, followed by a series of dehydration and decarboxylation reactions to form small molecules, which then condense with organic acids to form large humic acids. Existing hydrothermal carbonization technologies mainly form solid hydrothermal char through two main pathways: one is the condensation of intermediate products; the other is a solid-solid conversion of the recalcitrant portion through a process similar to dry pyrolysis. This conversion method allows for thorough carbonization of the raw material, but the correlation between the degree of carbonization and the degree of humification has not yet been confirmed. Therefore, if the initial physical structure of the raw material can be dissociated through the first method, it is beneficial for humic acid formation, thereby increasing the yield of humic acid. In fact, the decomposition and carbonization behavior of lignin in the hydrothermal process follows the same pattern: one is the degradation and condensation of subcritical water-soluble lignin monomers, and the other is the molecular rearrangement of recalcitrant lignin, which is similar to low-temperature pyrolysis. However, lignin usually begins to degrade and transform at temperatures above 260°C. The second pathway accounts for a very small proportion in the low-temperature hydrothermal carbonization process. Low-temperature humification is usually difficult to break down the lignocellulose structure and release lignin. Summary of the Invention

[0004] In view of this, the purpose of this application is to provide a method for producing humic acid by hydrothermal carbonization, which can enhance the efficiency of hydrothermal carbonization, thereby increasing the overall yield of humic acid while reducing energy consumption.

[0005] To solve the aforementioned technical problems / achieve the aforementioned objectives, or at least partially solve the aforementioned technical problems / achieve the aforementioned objectives, this application provides a method for hydrothermal carbonization to produce humic acid, comprising:

[0006] S1. Mix the straw raw material with water, soak it, and then freeze it to expand.

[0007] S2. The straw is thawed and then subjected to a hydrothermal carbonization reaction;

[0008] S3. After the hydrothermal carbonization reaction, the liquid product and hydrothermal carbon are separated; the hydrothermal carbon is extracted with an extractant to obtain an alkaline extract.

[0009] S4. The liquid product and the alkaline extract are respectively precipitated with acid to obtain humic acid.

[0010] Optionally, the solid-liquid ratio of the straw to water is 1:8 to 1:12.

[0011] Optionally, the freeze expansion includes freezing at a temperature not higher than -20°C for not less than 12 hours.

[0012] Optionally, the conditions for the hydrothermal carbonization reaction include: a temperature of 180–220°C, a time of 1–24 h, and a filling ratio of 1:4–1:6.

[0013] Optionally, the extractant includes sodium pyrophosphate and sodium hydroxide.

[0014] Optionally, the extraction is performed at 80–100°C for 1–2 hours.

[0015] Optionally, S4 includes:

[0016] The liquid product and alkaline extract were respectively acidified and adjusted to a pH of 2-4. After standing, humic acid was precipitated, and the supernatant was removed.

[0017] Optionally, it also includes:

[0018] After dissolving the precipitated humic acid in deionized water, repeat the S4 acidification precipitation process to remove residual inorganic salts and impurities.

[0019] Optionally, the process further includes adding an alkali to assist in humification during the hydrothermal carbonization reaction in S2. More preferably, the mass ratio of the alkali to the straw raw material is not less than 1:10.

[0020] This application improves the structural depolymerization of cellulose and lignin in straw through a freeze expansion dissociation process, enhances the efficiency of hydrothermal carbonization and humification, and increases the humic acid content of liquid products and hydrothermal carbon, thereby improving the overall yield and stability of total humic acid. This provides a reference for the high-value utilization of straw resources in cold regions, reduces production costs, and reduces environmental pollution. Attached image description:

[0021] The accompanying drawings, which form part of this application, are used to provide a further understanding of this application. The illustrative embodiments of this application and their descriptions are used to explain this application and do not constitute an undue limitation of this application.

[0022] Figure 1The results show the hydrothermal carbon yield of the examples and comparative examples; wherein, Example 1: freeze-thaw - HTC (hydrothermal carbonization), Example 2: freeze-thaw - HTC + 1:5KOH, Comparative Example 1: HTC, Comparative Example 2: HTC + 1:5KOH, Comparative Example 3: HTC + 1:10KOH, Comparative Example 4: -50℃ - HTC + 1:5KOH, Comparative Example 5: HTC - 12h;

[0023] Figure 2 The results show the concentration of humic acid in the liquid products and the content of humic acid in hydrothermal carbon for the examples and comparative examples; wherein, Example 1: freeze-thaw - HTC (hydrothermal carbonization), Example 2: freeze-thaw - HTC + 1:5KOH, Comparative Example 1: HTC, Comparative Example 2: HTC + 1:5KOH, Comparative Example 3: HTC + 1:10KOH, Comparative Example 4: -50℃ - HTC + 1:5KOH, Comparative Example 5: HTC - 12h;

[0024] Figure 3 The results show the mass of liquid humic acid (50 mL) and hydrothermal carbonized humic acid of Examples 1-2 and Comparative Examples 1-2; wherein, Example 1: freeze-thaw - HTC (hydrothermal carbonization), Example 2: freeze-thaw - HTC + 1:5 KOH, Comparative Example 1: HTC, Comparative Example 2: HTC + 1:5 KOH;

[0025] Figure 4 The results show the total humic acid mass of Examples 1-2 and Comparative Examples 1-2; wherein, Example 1: freeze-thaw-HTC (hydrothermal carbonization), Example 2: freeze-thaw-HTC+1:5KOH, Comparative Example 1: HTC, Comparative Example 2: HTC+1:5KOH;

[0026] Figure 5 The results show the total humic acid enhancement rate compared to Example 1 and Comparative Example 1, and Example 2 and Comparative Example 2. Detailed implementation method:

[0027] This application discloses a method for hydrothermal carbonization to produce humic acid. Those skilled in the art can refer to the content of this document and appropriately modify the process parameters to achieve the desired result. It should be particularly noted that all similar substitutions and modifications are obvious to those skilled in the art and are considered to be included in this application. The products, applications, and preparation methods of this application have been described through preferred embodiments. Those skilled in the art can clearly modify or appropriately change and combine the methods described herein without departing from the content, spirit, and scope of this application to realize and apply the technology of this application.

[0028] For typical agricultural waste like straw, existing composting methods require overcoming its high water content to achieve humification. Furthermore, the lignin fiber structure of straw is highly stable, and pretreatment technologies such as mechanical crushing can only reduce its size at the micrometer scale, offering limited dissociation between lignin, cellulose, and hemicellulose, thus affecting the hydrothermal humification process. To address these issues, this application utilizes freeze-thaw expansion to disrupt the hydrogen bond structure between lignin and cellulose in straw, reducing crystallinity. During hydrothermal carbonization, sugars are more easily converted into organic acids and small-molecule intermediates, increasing the degree of humification and consequently raising the content of humic acid in the hydrothermal carbon and aqueous phase products. Moreover, freeze-thaw expansion in this application has very low energy consumption, especially in cold regions, where the freezing process can be carried out outdoors with almost no additional energy required for straw pretreatment. Additionally, the freeze-thaw expansion treatment intensity is moderate, effectively disrupting certain lignin and cellulose structures. During the hydrothermal process, the raw material can more easily hydrolyze, releasing sugars or lignin monomers, avoiding the problem of high hydrothermal carbonization and high carbon content, which leads to low humic acid yield, as is often the case with direct solid-solid conversion.

[0029] This application provides a method for producing humic acid through hydrothermal carbonization, comprising:

[0030] S1. Mix the straw raw material with water, soak it, and then freeze it to expand.

[0031] S2. The straw is thawed and then subjected to a hydrothermal carbonization reaction;

[0032] S3. After the hydrothermal carbonization reaction, the liquid product and hydrothermal carbon are separated; the hydrothermal carbon is extracted with an extractant to obtain an alkaline extract.

[0033] S4. The liquid product and the alkaline extract are respectively precipitated with acid to obtain humic acid.

[0034] In some embodiments of this application, the straw is dried and pulverized before the freeze-thaw expansion treatment to facilitate subsequent freeze-thaw expansion and hydrothermal carbonization. Preferably, the pulverization is performed to a particle size of less than 400 mesh.

[0035] In some embodiments of this application, the solid-liquid ratio of straw to water in step S1 is 1:8 to 1:12 (g / mL), for example, 1:8, 1:9, 1:10, 1:11, 1:12, or any ratio between two of these. Within this ratio range, direct hydrothermal reaction after freeze-thaw expansion can yield a higher content of humic acid. Insufficient water affects the freeze-thaw expansion effect, while excessive water leads to a decrease in hydrothermal char production, thereby reducing the humic acid content. Simultaneously, the concentration of humic acid in the liquid product will also decrease.

[0036] In some embodiments of this application, the freezing includes freezing at a temperature not higher than -20°C for at least 12 hours. Lower freezing temperatures can improve the efficiency of hydrothermal carbonization to produce humic acid, but this efficiency improvement becomes less significant after a certain temperature is reached. For example, the overall increase in humic acid production is not significant when freezing at -50°C compared to -20°C. Therefore, the freezing temperature is preferably -20 to -30°C or any value within this range. In other embodiments of this application, the freezing time is 12 to 24 hours, and the specific time can be adjusted appropriately according to the freezing temperature, for example, 12h, 13h, 14h, 15h, 16h, 17h, 18h, 19h, 20h, 21h, 22h, 23h, 24h or any value between two of these.

[0037] In some embodiments of this application, the conditions for the hydrothermal carbonization reaction include: a temperature of 180–220°C, a time of 1–24 h, and a filling ratio of 1:4–1:6. The hydrothermal carbonization temperature can be selected from 180°C, 190°C, 200°C, 210°C, 220°C, or any value between two of these values. The hydrothermal carbonization time can be selected from 1 h, 2 h, 3 h, 4 h, 5 h, 10 h, 15 h, 20 h, 24 h, or any value between two of these values. Longer hydrothermal carbonization times result in higher humic acid yields until no further increase, but also lead to higher energy consumption and longer processing times. The method described in this application achieves a relatively high humic acid yield in approximately 2 hours, reducing the reaction time and significantly improving the efficiency of humic acid production compared to the excessively long reaction times in existing technologies.

[0038] In some embodiments of this application, after the hydrothermal carbonization reaction is completed, the liquid product and hydrothermal char can be separated by centrifugation or pressure filtration, both of which contain humic acid. The liquid product can be directly acidified to precipitate humic acid, for example, by adjusting the pH to 2-4 with hydrochloric acid and allowing it to stand for 1-8 hours. Before acidification and precipitation, the hydrothermal char needs to be extracted with an extractant to obtain an alkaline extract. The extractant includes sodium pyrophosphate and sodium hydroxide, both preferably at a concentration of 0.1 mol / L, and the liquid-to-solid ratio of the extractant to the hydrothermal char is 30 mL: 1 g. In other embodiments of this application, the hydrothermal char is first dried and pulverized before extraction. The extraction temperature is 80-100°C, and the extraction time is 1-2 hours.

[0039] In some embodiments of this application, it further includes:

[0040] After dissolving the precipitated humic acid in deionized water, repeat the S4 acidification precipitation process to remove residual inorganic salts and impurities.

[0041] In some embodiments of this application, the addition of an alkali to assist humification during the hydrothermal carbonization reaction in S2, such as the addition of strong alkalis like potassium hydroxide or sodium hydroxide, is also included. This application achieves a 6.1% increase in total humic acid yield through freeze-thaw expansion pretreatment; with the addition of a strong alkali, the increase can be further increased to 7.8%.

[0042] In other embodiments of this application, the mass ratio of the alkali to the straw raw material is not less than 1:10, more preferably 1:10 to 1:5, such as 1:10, 1:9, 1:8, 1:7, 1:6, 1:5, or any value between the two. A mass ratio of alkali to straw raw material higher than 1:5, meaning an increasing amount of alkali, can further increase the humic acid content, but this can lead to increased costs and equipment corrosion in actual production.

[0043] In the comparative experiments provided in this application, unless otherwise specified, all experimental conditions and materials remain consistent to ensure comparability. Furthermore, all materials used in this application are commercially available.

[0044] The following is a further description of the hydrothermal carbonization method for producing humic acid provided in this application.

[0045] Example 1:

[0046] The obtained rice straw was dried at 60℃ and pulverized to a particle size below 400 mesh. 5g of rice straw powder was mixed with 50mL of deionized water (solid-liquid mass ratio 1:10), thoroughly stirred, and then frozen at -20℃ for 24 hours. The frozen sample was fully thawed at room temperature and directly transferred to a hydrothermal reactor. The freeze-thawed rice straw and liquid mixture was transferred directly to a 500mL hydrothermal reactor without any further treatment. The filling ratio was 1.1:5. The stirring speed of the electromagnetic stirrer was 200RPM / min, and the heating rate was 3.5℃ / min. Hydrothermal carbonization was carried out at 200℃ for 2 hours. After the reaction was complete, the reactor was allowed to cool naturally at room temperature.

[0047] After the hydrothermal carbonization reaction was completed, solid-liquid separation was performed by centrifuging at 10,000 rpm to obtain liquid product and hydrothermal carbon. After the hydrothermal carbon was thoroughly washed with water, the solid hydrothermal carbon was dried at 60°C for 48 hours in a vacuum drying oven until constant weight was achieved. The solid carbon was then weighed and the humic acid content was analyzed.

[0048] Solid hydrothermal char was vacuum dried and then thoroughly ground into powder in an agate mortar. The hydrothermal char powder was then extracted in a sodium pyrophosphate / NaOH (0.1 mol / L each) extractant solution at a liquid-to-solid ratio of 30 mL / g at 100°C in a water bath for 120 min. After extraction, the remaining residue was separated by centrifugation or pressure filtration to obtain an alkaline extract. The liquid product can be directly subjected to further acidification precipitation and purification.

[0049] Hydrochloric acid was slowly added to the extract or liquid product to adjust the pH to 2, causing humic acid to precipitate due to decreased solubility. After standing for approximately 8 hours, the mixture was filtered, and the precipitate (crude humic acid) was collected. The precipitated humic acid was dissolved in deionized water, and the acidification and precipitation process was repeated to remove residual inorganic salts and impurities. Finally, the humic acid precipitate was freeze-dried under vacuum to obtain high-purity humic acid powder. The purity of the final product was >90%, and the humic acid content was determined by titration.

[0050] In this embodiment, the overall volume of rice straw increased by 8.7% after freezing and expanding with water. The solid yield of hydrothermal char was 61.4%, the humic acid content in the hydrothermal char reached 26.3%, and the humic acid concentration in the aqueous phase (liquid product, the same below) was approximately 11.2 g / L.

[0051] Example 2:

[0052] The size control of the rice straw and the freeze-thaw treatment process are the same as in Example 1. The freeze-thawed rice straw and liquid mixture are transferred directly to a hydrothermal reactor without any further treatment. The raw material filling ratio is 1.1:5. The difference from Example 1 is that KOH is added in this example to assist hydrothermal humification (KOH / straw raw material ratio is 1:5). The hydrothermal carbonization process is the same as in Example 1. The extraction and purification process of humic acid from the hydrothermal char and liquid products is the same as in Example 1.

[0053] In this embodiment, the overall volume increase of rice straw after freezing and expanding with water is consistent with that in Example 1. However, after KOH-assisted humification, the solid yield of hydrothermal char is 65.4%, the humic acid content in the hydrothermal char is 23.4%, and the humic acid concentration in the aqueous phase is approximately 28.2 g / L, which is a significant increase. More humic acid in the hydrothermal char is released into the liquid product under the action of KOH.

[0054] As shown in Table 1 below, the addition of alkali KOH reduces the carbon content and increases the oxygen content in hydrothermal carbon. This explains the increase in humic acid concentration in the liquid product, and the humic acid content in hydrothermal carbon also decreases accordingly.

[0055] Table 1. Hydrothermal carbon elemental composition obtained from freeze-thaw treatment-HTC and conventional HTC processes.

[0056]

[0057] Comparative Example 1:

[0058] The obtained rice straw was dried at 60℃ and pulverized to a particle size below 400 mesh. 50 mL of deionized water and 5 g of rice straw powder (solid-liquid mass ratio 1:10) were directly transferred into the hydrothermal reactor without any pretreatment. No other components were added during the hydrothermal process. The hydrothermal carbonization and solid-liquid separation processes were the same as in Example 1. The extraction and purification processes for humic acid in the hydrothermal carbon and liquid products were also the same as in Example 1.

[0059] In this comparative example, the hydrothermal char yield was 61.4%, and the humic acid content of the hydrothermal char was 25.3%. The humic acid concentration in the aqueous phase was approximately 10 g / L. Both the hydrothermal char and the humic acid content in the aqueous phase were significantly lower than in Examples 1 and 2. This indicates that freeze-thaw treatment and alkali-assisted hydrothermal carbonization can promote the humification process of rice straw.

[0060] Comparative Example 2:

[0061] The obtained rice straw was dried at 60℃ and pulverized to a particle size below 400 mesh. 50 mL of deionized water and 5 g of rice straw powder (solid-liquid mass ratio 1:10) were directly transferred to the hydrothermal reactor without any pretreatment. The raw material filling ratio was 1.1:5. KOH was added to assist hydrothermal humification (KOH / straw raw material ratio 1:5). The stirring speed of the electromagnetic stirrer was 200 RPM / min, and the heating rate was 3.5℃ / min. The hydrothermal carbonization reaction was carried out when the temperature reached 200℃, and the residence time was 2 hours. After the reaction was completed, the reactor was allowed to cool naturally at room temperature. After the hydrothermal carbonization reaction, solid-liquid separation was performed using a centrifuge at 10000 rpm to obtain liquid product and hydrothermal carbon. The hydrothermal carbon was thoroughly washed with water, and the solid hydrothermal carbon was dried at 60℃ in a vacuum drying oven for 48 hours until constant weight. Weighing and humic acid content analysis were then performed. The extraction and purification process for humic acid from hydrothermal char and liquid products is the same as in Example 1.

[0062] In this embodiment, the hydrothermal carbon yield was 71.8%, the humic acid content of the hydrothermal carbon was 20.4%, and the humic acid concentration in the aqueous phase was approximately 26.2 g / L. This comparative example did not undergo freeze-thaw expansion. Compared with Example 2, although the hydrothermal carbon yield was slightly increased, the humic acid content in both the hydrothermal carbon and the aqueous phase decreased. Overall, the humic acid yield was lower than that of the hydrothermal carbonization products after freeze-thaw treatment.

[0063] Comparative Example 3:

[0064] The amount and size of rice straw used were controlled in the same manner as in Example 1. The raw materials were transferred directly into the hydrothermal reactor without any pretreatment. The filling ratio of the raw materials was 1.1:5. A lower concentration of KOH was added during hydrothermal carbonization to assist hydrothermal humification (KOH / straw raw material ratio was 1:10). The hydrothermal carbonization process and the extraction and analysis of humic acid were the same as in Comparative Example 2.

[0065] In this comparative example, the hydrothermal carbon yield was 66.2%, and the humic acid content of the hydrothermal carbon was 20.2%. The humic acid concentration in the aqueous phase was approximately 23.8 g / L. Compared with the higher concentration of KOH assisted hydrothermal carbonization (compared to Comparative Example 2), the humic acid content in both the solid and liquid phase products decreased with the lower concentration of KOH.

[0066] Comparative Example 4:

[0067] The size control of rice straw was the same as in Example 1. 5g of rice straw (solid-liquid mass ratio 1:10) was mixed with 50mL of deionized water, thoroughly stirred, and then frozen at -50℃ for 24 hours. The frozen sample was then fully thawed at room temperature. The frozen-thawed rice straw and liquid mixture was transferred directly to a hydrothermal reactor without any further treatment. The raw material filling ratio was 1.1:5. KOH was added to assist hydrothermal humification (KOH / straw raw material ratio was 1:5). The specific process of hydrothermal carbonization, and the extraction and purification process of humic acid content in the hydrothermal carbon and aqueous products, were the same as in Example 1.

[0068] In this comparative example, the overall volume increase of rice straw and water after freezing and expansion at lower temperatures was approximately 9%, not significantly different from that at -20℃. However, the solid yield of hydrothermal char after KOH-assisted humification was 64.3%, and the humic acid content in the hydrothermal char was 23.2%. The humic acid concentration in the aqueous phase was approximately 29.1 g / L. It can be seen that at lower freeze-thaw temperatures, the yield of hydrothermal char produced by KOH-assisted hydrothermal humification, as well as the increase in humic acid content in both the solid and liquid phases, is not significant. Considering energy consumption factors, this is not the primary consideration. The preferred freezing temperature is -20 to -30℃, but the analysis results of humic acid in the product without the freeze-thaw process are significantly different.

[0069] Comparative Example 5:

[0070] The obtained rice straw was dried at 60℃ and pulverized to a particle size below 400 mesh. 50 mL of deionized water and 5 g of rice straw powder (solid-liquid mass ratio 1:10) were directly transferred into the hydrothermal reactor without any pretreatment. No other components were added during the hydrothermal process. The residence temperature for hydrothermal carbonization was the same as in Example 1, but the residence time was increased to 12 hours, and the solid-liquid separation process was the same as in Example 1. The extraction and purification process for humic acid in the hydrothermal carbon and aqueous products was the same as in Example 1.

[0071] In this comparative example, the hydrothermal carbon yield was 60.8%, the humic acid content of the hydrothermal carbon was 20.3%, and the humic acid concentration in the aqueous phase was approximately 18.1 g / L. The humic acid content in both the hydrothermal carbon and the aqueous phase was significantly lower than in Examples 1 and 2, indicating that both freeze-thaw treatment and alkali-assisted hydrothermal carbonization can promote the humification process of rice straw.

[0072] Experimental example:

[0073] Based on the test results of various embodiments and comparative examples, the following summary analysis is presented: Figure 1 ;

[0074] according to Figure 1 The yield results of hydrothermal char showed that the yields of each example and comparative example were generally between 60% and 72%, but the humic acid content of the hydrothermal char varied among the groups.

[0075] according to Figure 2 The results of the humic acid content in hydrothermal carbon and liquid products show that, compared with the HTC (hydrothermal carbonization) group, the humic acid content in both hydrothermal carbon and liquid products in the freeze-thaw-HTC group is increased to a certain extent; compared with the HTC+1:5KOH group, the humic acid content in both hydrothermal carbon and liquid products in the freeze-thaw-HTC+1:5KOH group is also increased to a certain extent.

[0076] Based on the combined mass of hydrothermal carbon humic acid and the mass of liquid product humic acid (per 50 mL), calculations show that the overall humic acid content increased after freeze-thaw cycles. (See attached data.) Figures 3 to 5 ;

[0077] The total humic acid content in the freeze-thaw-HTC group was 1.36741 g, compared to 1.27671 g in the HTC (hydrothermal carbonization) group, representing an increase of approximately 7.104%. The total humic acid content in the freeze-thaw-HTC+1:5KOH group was 2.17518 g, compared to 2.04236 g in the HTC+1:5KOH group, representing an increase of approximately 6.503%. This indicates that freeze-thaw treatment combined with hydrothermal carbonization technology, and the use of a strong alkali during hydrothermal carbonization, can significantly increase the humic acid content in rice straw within a short time (approximately 2 hours).

[0078] The above description is only a preferred embodiment of this application. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of this application, and these improvements and modifications should also be considered within the scope of protection of this application.

Claims

1. A method for producing humic acid through hydrothermal carbonization, characterized in that, include: S1. Mix the straw raw material with water, soak it, and then freeze it at -30~-20℃ for 12~24h; the solid-liquid ratio of the straw to water is 1g:10mL; S2. After thawing the straw, a hydrothermal carbonization reaction is carried out. An alkali is added to assist in the humification process during the hydrothermal carbonization reaction. The mass ratio of alkali to straw raw material is 1:10 to 1:

5. The conditions for the hydrothermal carbonization reaction include: a temperature of 200°C and a time of 2 hours. S3. After the hydrothermal carbonization reaction, the liquid product and hydrothermal carbon are separated; the hydrothermal carbon is first dried and pulverized, and then extracted with an extractant at 80-100℃ for 1-2 hours to obtain an alkaline extract; the extractant includes sodium pyrophosphate and sodium hydroxide, the concentration of sodium pyrophosphate and sodium hydroxide is 0.1mol / L, and the liquid-solid ratio of the extractant to the hydrothermal carbon is 30mL:1g; S4. The liquid product and the alkaline extract are respectively precipitated with acid to obtain humic acid; after dissolving the precipitated humic acid with deionized water, the acidification and precipitation process of S4 is repeated to remove residual inorganic salts and impurities.

2. The method according to claim 1, characterized in that, S4 includes: The liquid product and alkaline extract were respectively acidified and adjusted to pH 2-4, allowed to stand, and after humic acid precipitated, the supernatant was removed.

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