CN110615437A - Comprehensive utilization method of lignite - Google Patents

Abstract

The invention discloses a method for comprehensive utilization of lignite. In the method, lignite is pulverized into powder, cleaned and dried in the sun; humic acid in lignite is extracted by using KOH and urea as a composite extractant; and then, through chemical activation, the KOH activates and remodels the pores of the extraction residue, and at the same time uses urea as a nitrogen source to modify the porous carbon with nitrogen doping to prepare the negative electrode material of N-doped porous carbon composite lithium battery; in addition, the waste liquid in the extraction and preparation process Neutralization treatment to make nitrogen, phosphorus and potassium compound fertilizer; the invention creates a new green process method for comprehensive utilization of lignite, through which humic acid and N-doped porous carbon composite lithium battery negative electrode can be produced materials; in addition, the waste liquid generated in the whole process is also prepared into nitrogen, phosphorus and potassium compound fertilizers; thereby realizing the maximum conversion of lignite into high-quality products, basically achieving zero emissions.

Description

A method for comprehensive utilization of lignite

technical field

The invention relates to the field of lignite processing and utilization, in particular to a method for comprehensive utilization of lignite.

Background technique

China is rich in lignite resources, accounting for more than 55% of the total coal reserves; Inner Mongolia Autonomous Region has the largest lignite reserves, accounting for 77% of the national lignite resources; among them, the total humic acid content in Chifeng lignite in Inner Mongolia is 31.6%, and free humic acid The content is 24.9%, the ash is 14.53%, and the water is 27.8%. Humic acid is mainly a type of organic polymer formed and accumulated by animal and plant remains, through microbial decomposition, transformation and a series of geophysical and chemical reactions. Widely present in carbonaceous sedimentary rocks such as water, soil, peat, lignite, weathered coal, and shale; the structure of humic acid is rich in active oxygen-containing functional groups such as carboxyl, phenolic hydroxyl, carbonyl, sulfonic acid, and methoxy , has an important impact on its acidity, ion exchange, colloidal properties and complexation properties; it is widely used in various fields such as agriculture, forestry, animal husbandry, petroleum, chemical industry, building materials, medicine and health, environmental protection, etc.; especially now promotes the construction of ecological agriculture , pollution-free agricultural production, green food, pollution-free environmental protection, etc., make "humic acid" highly respected; lignite is rich in humic acid, which is the main resource for large-scale utilization of humic acid; and soil humic acid In comparison, lignite humic acid has higher carbon content, lower nitrogen content, and higher hydrogen content, indicating better chemical activity and substitution performance; domestic and foreign researchers have used alkali-dissolved acid analysis, acid Humic acid in lignite is extracted by extractant method and microbial dissolution method, and the extraction rate of humic acid reaches more than 80%. In the residue after humic acid extraction, at least 25% of the organic matter has not been fully utilized; If these organic matter are carbonized and activated to prepare N-doped porous carbon composite lithium battery negative electrode materials, it will be of great significance to realize the clean, efficient and high value-added utilization of lignite; currently, there is no research and report on this aspect in the existing technology .

Therefore, there is an urgent need in this field to develop a comprehensive utilization clean production process that can convert lignite into high-quality products to the greatest extent.

Contents of the invention

The purpose of the present invention is to provide a method for comprehensive utilization of lignite, which is to pulverize lignite into powder, clean and dry; use KOH and urea as composite extractants to extract humic acid in lignite; then, through chemical activation, realize KOH activates and remodels the pores of the extraction residue, and at the same time uses urea as a nitrogen source to modify the porous carbon with nitrogen doping to prepare N-doped porous carbon composite lithium battery anode materials; in addition, the waste liquid in the extraction and preparation process Neutralize and process to make compound fertilizers of nitrogen, phosphorus and potassium, and convert lignite into high-quality products to the greatest extent through this method.

The technical scheme adopted in the present invention is as follows: a method for comprehensive utilization of lignite, comprising the following steps:

S1: preparing lignite raw material: pulverizing lignite into powder, cleaning with distilled water, and drying to obtain clean lignite raw material;

S2: KOH and urea alkaline extraction: Weigh the clean lignite raw material obtained in step S1 and mix it with urea. The mass ratio of lignite raw material to urea is 1:0.5～1:2.5; The solid-liquid ratio (g:mL) of the solution is 1:5-1:30; the mass fraction of the KOH solution is 0.5%-4.5%; after that, the prepared solid-liquid mixture is placed at a temperature of 70-100°C , cook for 70-100min; after cooling, stir the reaction with a magnetic stirrer, and then centrifuge with a centrifuge to separate the residue and extract;

S3: Preparation of N-doped porous carbon composite lithium battery anode material:

S3.1: put the residue obtained in step S2 into a tube furnace with nitrogen as a protective gas, and carbonize and activate at a temperature of 410-720°C for 0.4-2.2 hours to obtain a carbonized activated product;

S3.2: Soak the activated carbonization product prepared in step S3.1 with a dilute acid solution, filter it with suction, wash it with deionized water, and dry it in a blast drying oven to obtain an N-doped porous carbon composite lithium battery negative electrode material ;

In step S3.2, the dilute acid solution in which the activated carbonization product is soaked is a nitric acid solution or a phosphoric acid solution with a concentration of 0.2-3.0 mol/L;

S4: Preparation of humic acid:

The extract solution obtained in step S2 is subjected to acid analysis with a dilute acid solution to make the pH value of the solution less than 4, and the obtained precipitate is filtered and vacuum-dried to obtain humic acid;

The dilute acid solution used in the acid analysis process in step S4 is a phosphoric acid solution or a nitric acid solution with a concentration of 0.3-2.5 mol/L.

Further, the lignite used in step S1 of the present invention is lignite from the Chifeng area of Inner Mongolia.

In a preferred embodiment, nitric acid solution is used in the dilute acid solution soaked in the carbonization activation product in step S3.2 of the present invention, and the dilute acid solution used in the acid analysis process in step S4 is phosphoric acid solution; then step S2, S3, The waste liquid produced in S4 is collected together for neutralization to obtain compound fertilizers of nitrogen, phosphorus and potassium.

In another preferred embodiment, the dilute acid solution soaking the carbonization activation product in step S3.2 of the present invention adopts phosphoric acid solution, and the dilute acid solution used in the acid analysis process in step S4 is nitric acid solution; then step S2, S3 , and the waste liquid produced in S4 are collected together for neutralization to obtain compound fertilizers of nitrogen, phosphorus and potassium.

The beneficial effects of the present invention are: the present invention provides a method for comprehensive utilization of lignite, which pulverizes lignite into powder, cleans it, and dries it; uses KOH and urea as composite extractants to extract humic acid in lignite; Then, through chemical activation, KOH activates and remodels the pores of the extraction residue, and at the same time uses urea as a nitrogen source to modify the porous carbon with nitrogen doping to prepare N-doped porous carbon composite lithium battery negative electrode material; in addition, the extracted And the waste liquid in the preparation process is neutralized and processed to make nitrogen, phosphorus and potassium compound fertilizers.

The invention creates a new green process method for comprehensive utilization of lignite, through which humic acid and N-doped porous carbon composite lithium battery negative electrode materials can be prepared; in addition, the waste liquid produced in the whole process is also prepared Nitrogen, phosphorus, and potassium compound fertilizers; thereby realizing the maximum conversion of lignite into high-quality products, and basically achieving zero emissions.

Description of drawings

Fig. 1 is a schematic diagram of the process flow of the present invention.

Fig. 2 is an SEM image of the N-doped porous carbon composite lithium battery negative electrode material prepared by the method in Example 1 of the present invention.

Fig. 3 is a cycle performance diagram of the N-doped porous carbon composite lithium battery negative electrode material prepared by the method of Example 1 of the present invention.

Detailed ways

In order to enable those skilled in the art to better understand the technical solution of the present invention, the present invention will be further described in detail below in conjunction with the accompanying drawings and embodiments. The following description is only used to illustrate the technical solution of the present invention and not to limit it.

As shown in Figure 1, a method for comprehensive utilization of lignite, the method pulverizes lignite into powder, cleans it, and dries it in the sun; uses KOH and urea as a composite extractant to extract humic acid in lignite; then, through chemical activation , to realize KOH activation and remodeling of the pores of the extraction residue, and at the same time use urea as a nitrogen source to carry out nitrogen doping modification on porous carbon to prepare N-doped porous carbon composite lithium battery negative electrode material; in addition, the extraction and preparation process The waste liquid is neutralized and processed to make compound fertilizers of nitrogen, phosphorus and potassium, and the lignite is converted into high-quality products to the greatest extent through the method.

A method for comprehensive utilization of lignite, the method comprising the following steps:

S1: Preparation of lignite raw material: pulverize lignite into powder, clean it with distilled water, and dry it to obtain clean lignite raw material; the lignite used in the method of the present invention is preferably lignite from the Chifeng area of Inner Mongolia; it can be removed by repeated washing with distilled water debris mixed in lignite and surface silt;

S2: KOH and urea alkaline extraction: Weigh the clean lignite raw material obtained in step S1 and mix it with urea. The mass ratio of lignite raw material to urea is 1:0.5～1:2.5; The solid-liquid ratio (g:mL) of the solution is 1:5-1:30; the mass fraction of the KOH solution is 0.5%-4.5%; after that, the prepared solid-liquid mixture is placed at a temperature of 70-100°C , cooking for 70 to 100 min; after cooling, stir the reaction with a magnetic stirrer, then centrifuge with a centrifuge to separate the residue and extract; after this step, humic acid is separated from the lignite particles and dissolved in the extract, Porous cavities are formed on the surface, which increases its specific surface area. At this time, the residue has a certain adsorption capacity, and the pores on the surface provide an embeddable area for the adsorbed ions.

First, the N-doped porous carbon composite lithium battery negative electrode material is prepared by using the separated residue, and the specific steps are as follows:

S3: Preparation of N-doped porous carbon composite lithium battery anode material:

S3.1: put the residue obtained in step S2 into a tube furnace with nitrogen as a protective gas, and carbonize and activate at a temperature of 410-720°C for 0.4-2.2 hours to obtain a carbonized activated product;

S3.2: Soak the activated carbonization product prepared in step S3.1 with a dilute acid solution, filter it with suction, wash it with deionized water, and dry it in a blast drying oven to obtain an N-doped porous carbon composite lithium battery negative electrode material ;

In step S3.2, the dilute acid solution in which the activated carbonization product is soaked is a nitric acid solution or a phosphoric acid solution with a concentration of 0.2-3.0 mol/L;

At the same time, humic acid can also be prepared by using the separated extract; the specific steps are as follows:

S4: Preparation of humic acid:

The extract solution obtained in step S2 is subjected to acid analysis with a dilute acid solution to make the pH value of the solution less than 4, and the obtained precipitate is filtered and vacuum-dried to obtain humic acid;

The dilute acid solution used in the acid analysis process in step S4 is a phosphoric acid solution or a nitric acid solution with a concentration of 0.3-2.5 mol/L.

In order to further maximize the conversion of lignite into high-quality products and reduce emissions; in this process plan, the waste liquid generated by the above processes is also collected together for the preparation of nitrogen, phosphorus, and potassium compound fertilizers. The specific steps are as follows:

S5: Preparation of nitrogen, phosphorus and potassium compound fertilizers:

Collect the waste liquids produced in the above steps S2, S3, and S4 together for neutralization to obtain nitrogen, phosphorus, and potassium compound fertilizers; When the acid solution adopts nitric acid solution, the dilute acid solution used in the acid analysis process in step S4 is a phosphoric acid solution; or when the dilute acid solution soaked in the carbonization activation product in step S3. The dilute acid solution is nitric acid solution.

The main advantages of the present invention are: create a new green process method for comprehensive utilization of lignite, humic acid, N-doped porous carbon composite lithium battery negative electrode material can be produced through this process method; The generated waste liquid is prepared into compound fertilizers of nitrogen, phosphorus and potassium; thereby realizing the maximum conversion of lignite into high-quality products and basically realizing zero discharge.

The present invention is further described below in conjunction with specific examples. However, it should be understood that these examples are only used to illustrate the present invention and not to limit the scope of the present invention. The test methods for which specific conditions are not indicated in the following examples are generally in accordance with conventional conditions, or in accordance with the conditions suggested by the manufacturer.

Example 1

S1: Prepare lignite raw material: use lignite from the Chifeng area of Inner Mongolia, pulverize the lignite into 60-mesh powder, clean it with distilled water, and dry it to obtain a clean lignite raw material;

S2: Alkaline extraction of KOH and urea: Weigh the clean lignite raw material obtained in step S1 and mix it with urea. The mass ratio of lignite raw material and urea is 1:1; The ratio (g:mL) was 1:20; the mass fraction of the KOH solution was 2%; after that, the prepared solid-liquid mixture was boiled in a water bath at 90°C for 90 minutes; it was taken out and cooled and stirred with a magnetic stirrer for reaction, and then Centrifuge with a centrifuge to separate the residue and extract;

S3: Preparation of N-doped porous carbon composite lithium battery anode material:

S3.1: put the residue obtained in step S2 into a tube furnace with nitrogen as a protective gas, and carbonize and activate at a temperature of 600°C for 1 hour;

S3.2: Soak the carbonized activated product prepared in step S3.1 with a dilute nitric acid solution with a concentration of 2.0 mol/L, filter it with suction, wash it with deionized water, and dry it in a blast drying oven to obtain N-doped Porous carbon composite lithium battery anode material;

S4: Preparation of humic acid:

The extract solution obtained in step S2 is subjected to acid analysis with 1.5 mol/L phosphoric acid solution, so that the pH value of the solution reaches 2, and the obtained precipitate is filtered to obtain humic acid after vacuum drying;

S5: Preparation of nitrogen, phosphorus and potassium compound fertilizers:

The waste liquids generated in steps S2, S3, and S4 are collected together for neutralization treatment to obtain nitrogen, phosphorus, and potassium compound fertilizers.

Figure 2 is the SEM image of the N-doped porous carbon composite lithium battery negative electrode material prepared in this example, where (a) is a low-magnification SEM image, (b) is a high-magnification SEM image, and (c) is an EDS surface Scanning picture; From Figures (a) and (b), it can be seen that the material has an obvious pore structure with a large specific surface area, and more pores can alleviate the volume expansion during lithium ion deintercalation and intercalation, and prevent Pulverization; it can accelerate the transmission of lithium ions and improve the conductivity of the material; in addition, the larger specific surface area provides more attachment sites for lithium ions, increasing the specific capacity of the material. In addition, it can be seen from Figure (c) that the nitrogen element is uniformly distributed in the material, which can improve the electrical conductivity of the material, thereby improving its electrochemical performance.

As shown in Figure 3, it is the cycle performance figure of the N-doped porous carbon composite lithium battery negative electrode material that the method for the embodiment of the present invention 1 makes; The traditional graphite lithium battery negative electrode has only the specific capacity of 375mA/g, and the present embodiment makes The specific capacity of the material reaches 1072mA/g, which is nearly three times that of the negative electrode of the existing graphite lithium battery.

The following table 1 is the chemical composition analysis table of the nitrogen, phosphorus, potassium compound fertilizer that the present embodiment makes; The chemical composition content of the nitrogen, phosphorus and potassium compound fertilizers obtained is different.

The chemical composition of the nitrogen, phosphorus and potassium compound fertilizer prepared by the embodiment 1 of table 1

*The content of other ingredients is less than 0.01%.

Example 2

S1: Prepare lignite raw material: use lignite from the Chifeng area of Inner Mongolia, pulverize the lignite into 60-mesh powder, clean it with distilled water, and dry it to obtain a clean lignite raw material;

S2: Alkaline extraction of KOH and urea: Weigh the clean lignite raw material obtained in step S1 and mix it with urea. The mass ratio of lignite raw material to urea is 1:2; The ratio (g:mL) was 1:20; the mass fraction of the KOH solution was 1%; after that, the prepared solid-liquid mixture was boiled in a water bath at 80°C for 70 minutes; it was taken out and cooled and stirred with a magnetic stirrer for reaction, and then Centrifuge with a centrifuge to separate the residue and extract;

S3: Preparation of N-doped porous carbon composite lithium battery anode material:

S3.1: put the residue obtained in step S2 into a tube furnace with nitrogen as a protective gas, and carbonize and activate at a temperature of 600°C for 1 hour;

S3.2: Soak the carbonized activation product prepared in step S3.1 with a phosphoric acid solution with a concentration of 3.0 mol/L, filter it with suction, wash it with deionized water, and dry it in a blast drying oven to obtain N-doped porous Carbon composite lithium battery anode material;

S4: Preparation of humic acid:

The extract solution obtained in step S2 is subjected to acid analysis with 2.0mol/L nitric acid solution, so that the pH value of the solution reaches 1.5, and the obtained precipitate is filtered to obtain humic acid after vacuum drying;

S5: Preparation of nitrogen, phosphorus and potassium compound fertilizers:

The waste liquids generated in steps S2, S3, and S4 are collected together for neutralization treatment to obtain nitrogen, phosphorus, and potassium compound fertilizers.

The pore structure of the N-doped porous carbon composite lithium battery negative electrode material obtained in this example is basically the same as in Example 1, and its electrical conductivity is similar; different.

Example 3

S1: preparing lignite raw material: pulverizing lignite into powder, cleaning with distilled water, and drying to obtain clean lignite raw material;

S2: Alkaline extraction of KOH and urea: Weigh the clean lignite raw material obtained in step S1 and mix it with urea. The mass ratio of lignite raw material to urea is 1:0.5; The ratio (g:mL) was 1:5; the mass fraction of the KOH solution was 0.5%; after that, the prepared solid-liquid mixture was cooked for 70 minutes at a temperature of 70°C; after cooling, the reaction was stirred with a magnetic stirrer, and then Centrifuge with a centrifuge to separate the residue and extract.

First, the N-doped porous carbon composite lithium battery negative electrode material is prepared by using the separated residue, and the specific steps are as follows:

S3: Preparation of N-doped porous carbon composite lithium battery anode material:

S3.1: put the residue obtained in step S2 into a tube furnace with nitrogen as a protective gas, and carbonize and activate at a temperature of 410° C. for 0.4 h to obtain a carbonized activated product;

S3.2: Soak the carbonized activated product prepared in step S3.1 with nitric acid solution, filter it with suction, wash it with deionized water, and dry it in a blast drying oven to obtain an N-doped porous carbon composite lithium battery negative electrode material;

In step S3.2, the concentration of the nitric acid solution soaking the carbonized activation product is 0.2mol/L;

At the same time, humic acid can also be prepared by using the separated extract; the specific steps are as follows:

S4: Preparation of humic acid:

The extract solution obtained in step S2 is subjected to acid analysis with a phosphoric acid solution, so that the pH value of the solution reaches 2, and the obtained precipitate is filtered and vacuum-dried to obtain humic acid;

The concentration of the phosphoric acid solution used in the acid analysis process in step S4 is 0.3 mol/L.

Example 4

S1: preparing lignite raw material: pulverizing lignite into powder, cleaning with distilled water, and drying to obtain clean lignite raw material;

S2: KOH and urea alkali extraction: Weigh the clean lignite raw material obtained in step S1 and mix it with urea. The mass ratio of lignite raw material to urea is 1:2.5; The ratio (g:mL) was 1:30; the mass fraction of the KOH solution was 4.5%; after that, the prepared solid-liquid mixture was cooked for 100 min at a temperature of 100°C; after cooling, the reaction was stirred with a magnetic stirrer, and then Centrifuge with a centrifuge to separate the residue and extract.

First, the N-doped porous carbon composite lithium battery negative electrode material is prepared by using the separated residue, and the specific steps are as follows:

S3: Preparation of N-doped porous carbon composite lithium battery anode material:

S3.1: put the residue obtained in step S2 into a tube furnace with nitrogen as a protective gas, and carbonize and activate at a temperature of 720° C. for 2.2 hours to obtain a carbonized activated product;

S3.2: Soak the carbonized activated product prepared in step S3.1 with nitric acid solution, filter it with suction, wash it with deionized water, and dry it in a blast drying oven to obtain an N-doped porous carbon composite lithium battery negative electrode material;

In step S3.2, the concentration of the nitric acid solution soaking the carbonized activation product is 3.0mol/L;

At the same time, humic acid can also be prepared by using the separated extract; the specific steps are as follows:

S4: Preparation of humic acid:

The extract solution obtained in step S2 is subjected to acid analysis with a phosphoric acid solution, so that the pH value of the solution reaches 2, and the obtained precipitate is filtered and vacuum-dried to obtain humic acid;

The concentration of the phosphoric acid solution used in the acid analysis process in step S4 is 2.5mol/L.

Among the above-mentioned examples, Example 1 is the optimal scheme obtained through comparative experiments. In this scheme, the extraction rate of humic acid is the largest in the experiment, thereby forming more holes on the surface of lignite particles, and the specific capacity is also largest in the experiment.

Although the present invention has been described in detail with reference to the aforementioned examples, those skilled in the art can still modify the technical solutions described in the aforementioned embodiments, or perform equivalent replacements for some of the technical features. Within the spirit and principles of the invention, any modifications, equivalent replacements, improvements, etc., shall be included within the protection scope of the present invention.

Claims (5)

1. A method for comprehensively utilizing lignite is characterized by comprising the following steps:

s1: preparing a lignite raw material: pulverizing lignite into powder, cleaning the powder by using distilled water, and drying the powder in the sun to prepare a clean lignite raw material;

s2: alkali extraction of KOH and urea: weighing the clean lignite raw material obtained by the processing of the step S1, and mixing the clean lignite raw material with urea, wherein the mass ratio of the lignite raw material to the urea is 1: 0.5-1: 2.5; mixing the mixture with a KOH solution to prepare a mixture, wherein the solid-to-liquid ratio (g: mL) of the mixture to the KOH solution is 1: 5-1: 30; the mass fraction of the KOH solution is 0.5 to 4.5 percent; then, cooking the prepared solid-liquid mixture for 70-100 min at the temperature of 70-100 ℃; cooling, stirring with a magnetic stirrer for reaction, and centrifuging with a centrifuge to separate residue and extractive solution;

s3: preparing an N-doped porous carbon composite lithium battery negative electrode material:

s3.1: putting the residue obtained in the step S2 into a tubular furnace with nitrogen as protective gas, and carbonizing and activating at the temperature of 410-720 ℃ for 0.4-2.2 h to obtain a carbonized and activated product;

s3.2: soaking the carbonized and activated product prepared in the step S3.1 in a dilute acid solution, performing suction filtration, washing with deionized water, and drying in a forced air drying oven to obtain the N-doped porous carbon composite lithium battery cathode material;

in the step S3.2, the dilute acid solution for soaking the carbonized and activated product is a nitric acid solution or a phosphoric acid solution, and the concentration is 0.2-3.0 mol/L;

s4: preparing humic acid:

carrying out acid precipitation on the extracting solution obtained in the step S2 by using a dilute acid solution to ensure that the pH value of the solution is less than 4, and filtering the obtained precipitate to obtain humic acid after vacuum drying;

the diluted acid solution used in the acid precipitation process in the step S4 is a phosphoric acid solution or a nitric acid solution, and the concentration is 0.3-2.5 mol/L.

2. The comprehensive lignite utilization method according to claim 1, wherein the comprehensive lignite utilization method comprises the following steps: the lignite used in the step S1 is lignite in the red peak area of inner mongolian city.

3. The comprehensive lignite utilization method according to claim 1 or 2, wherein the comprehensive lignite utilization method comprises the following steps: the dilute acid solution for soaking the carbonization activation product in the step S3.2 is a nitric acid solution, and the dilute acid solution used in the acidification process in the step S4 is a phosphoric acid solution; then the waste liquid generated in the steps S2, S3 and S4 is collected together for neutralization treatment to obtain the nitrogen, phosphorus and potassium compound fertilizer.

4. The comprehensive lignite utilization method according to claim 1 or 2, wherein the comprehensive lignite utilization method comprises the following steps: the dilute acid solution for soaking the carbonization activation product in the step S3.2 is phosphoric acid solution, and the dilute acid solution used in the acidification process in the step S4 is nitric acid solution; then the waste liquid generated in the steps S2, S3 and S4 is collected together for neutralization treatment to obtain the nitrogen, phosphorus and potassium compound fertilizer.

5. The comprehensive lignite utilization method according to claim 2, wherein the comprehensive lignite utilization method comprises the following steps:

in step S1, the lignite is pulverized into 60 mesh powder;

in the step S2, the mass ratio of the lignite raw material to the urea is 1: 1; the solid-to-liquid ratio (g: mL) of the mixture to the KOH solution is 1: 20; the mass fraction of the KOH solution is 2 percent; then, the prepared solid-liquid mixture is cooked for 90min in a water bath with the temperature of 90 ℃;

in the step S3.1, under the condition that the temperature is 600 ℃, carbonization and activation are carried out for 1 h;

in the step S3.2, soaking the substrate by using a dilute nitric acid solution with the concentration of 2.0 mol/L;

in step S4, the solution is acidified with 1.5mol/L phosphoric acid solution to reach pH 2, and the precipitate obtained by filtration is dried in vacuum to obtain humic acid.