A method for preparing biochar-supported nano zero-valent iron by using biochar-supported nano zero-valent iron and hydrogen reduction

Abstract

The present invention discloses a method for preparing biochar supported nano zero-valent iron by hydrogen reduction of biochar supported nano zero-valent iron. The iron oxide mineral is subjected to ultra-fine wet grinding treatment, and the obtained nano-scale mineral slurry is loaded onto the biomass raw material by impregnation method, and then pre-carbonized and hydrogen-reduced to obtain biochar supported nano zero-valent iron composed of highly dispersed nano zero-valent iron stably loaded on the biochar in an inlaid form. The special loading method of nano zero-valent iron can effectively prevent its agglomeration and shedding during use, and can maintain its high reaction activity. In addition, the preparation method of biomass char supported nano zero-valent iron directly uses iron oxide mineral and biomass as raw materials, with low cost, and avoids the use of inorganic acids, sodium borohydride reducing agents and harmful organic reagents, etc., which is safe, environmentally friendly and can be produced on a large scale.

Description

Technical Field

[0001] The present invention relates to a biochar-supported nano zero-valent iron, and also relates to a method for preparing the supported nano zero-valent iron by coupling hydrogen reduction of iron oxide with biomass pyrolysis, belonging to the field of preparation of functional iron powder. Technical Background

[0002] The zero-valent iron technology is commonly used to treat heavy metal ions with high valence and halogenated organic pollutants. Its characteristics such as low cost and high efficiency have been widely recognized and valued. In recent years, it has developed rapidly in China and several industrial experiments have been carried out. From the actual use situation, large-scale zero-valent iron is suitable for the degradation of heavy metal pollution, while higher-activity nano zero-valent iron (nZVI) is required for the treatment of more difficult organic pollution. Nano zero-valent iron has a higher electron-donating ability and can quickly react to degrade pollutants. Therefore, it is easy to oxidize and agglomerate and inactivate in the environment, and its stability is poor. Therefore, for nano-level zero-valent iron products, coating or loading methods are often used to ensure their use performance. The loading method mostly uses cheap and environmentally compatible activated carbon, alumina, diatomite, etc., and is often loaded on the carrier during the liquid-phase reduction process of iron. It is difficult to ensure the bonding strength, especially it is easy to fall off during the adsorption process of zero-valent iron, reducing the use effect and even causing secondary pollution.

[0003] Currently, nano zero-valent iron is mainly synthesized by chemical methods. In these methods, inorganic acids, borohydride reducing agents, and organic reagents are usually required, which have problems such as strict control, high danger, and high cost, restricting the large-scale production of nano zero-valent iron. Summary of the Invention

[0004] Aiming at the problems existing in the prior art, the first object of the present invention is to provide a biochar-supported nano zero-valent iron, which is composed of highly dispersed nano zero-valent iron stably loaded on the biochar in an inlaid form. This loading method can effectively prevent the agglomeration and shedding of nano zero-valent iron during use and can maintain its high reaction activity.

[0005] The second object of the present invention is to provide a method for preparing biochar-supported nano zero-valent iron. This method directly uses iron oxide minerals and biomass as raw materials, with low cost, and avoids the use of inorganic acids, sodium borohydride reducing agents, and harmful organic reagents, etc., which is safe, environmentally friendly, and can be mass-produced.

[0006] In order to achieve the above technical objectives, the present invention provides a preparation method of biomass carbon supported nano zero-valent iron, which comprises the following steps: 1) Ultrafine wet grinding treatment is carried out on iron oxide minerals to obtain nano-scale mineral slurries; 2) Biomass raw materials are immersed in the nano-scale mineral slurries to obtain biomass loaded with minerals; 3) The biomass loaded with minerals is first pre-carbonized under a protective atmosphere and then reduced under a hydrogen-containing atmosphere to obtain biochar supported zero-valent iron.

[0007] The present invention uses iron oxide minerals and biomass raw materials as direct raw materials to realize the generation of biochar through a process combining ultrafine grinding, loading, pre-carbonization and hydrogen reduction, as well as the in-situ generation, highly dispersed and stable loading of nano zero-valent iron on the surface of biomass carbon. Taking iron oxide minerals as the direct raw materials of nano zero-valent iron, it is necessary to carry out ultrafine grinding on iron oxide minerals. The obtained nano-scale iron oxide minerals serve as the precursors of nano zero-valent iron. However, nano-scale iron oxide minerals are prone to agglomeration during the hydrogen reduction process, and the binding stability between nano-scale iron oxide minerals and biomass raw materials is poor, making it difficult to achieve the stable loading of nano zero-valent iron on biomass carbon. These are the two primary technical problems to be solved. The present invention first realizes the loading of iron oxide minerals on biomass raw materials through an impregnation method. Utilizing the characteristics that the iron oxide minerals activated by ultrafine grinding have a large specific surface area and a positively charged surface, they can be fully adsorbed in biomass raw materials, enabling relatively uniform dispersion of iron oxide minerals. On this basis, low-temperature pre-carbonization is carried out first, which is very crucial for the stable loading of iron oxide minerals on biomass carbon. Based on the catalytic effect of nano iron oxides on the pyrolytic carbonization of biomass raw materials, during the pre-carbonization process, the organic matter around the nano iron oxides is preferentially carbonized, forming a "semi-encapsulation" form around the nano iron oxides, thereby realizing the effective fixation of nano iron oxides. Thus, during the further high-temperature hydrogen reduction process, nano iron is in-situ generated from nano iron oxides, avoiding agglomeration. At the same time, nano iron particles are embedded and loaded on the surface of the generated biomass carbon material, and the stability is much higher than other loading forms. At the same time, this embedded loading, different from coating, can still expose nano iron particles, fully exerting their activity and not affecting their performance.

[0008] As a preferred solution, the iron grade of the iron oxide minerals is not less than 60%. The iron grade of the iron oxides in the present invention does not require to be too high and can contain beneficial metal elements such as nickel and manganese. Preferred iron oxide minerals can be various natural iron oxide ores, such as hematite, magnetite, etc., or secondary iron-containing resources such as iron scale.

[0009] As a preferred embodiment, the conditions for the ultrafine wet grinding treatment are as follows: the solid-liquid ratio is 1 g: 5-10 mL, the motor speed is 1000-3000 r / min, and the size of the grinding beads is 0.05-0.2 mm. Ultrafine grinding can be carried out using a high-energy ball milling device and / or a nanosand milling device. As a preferred embodiment, during the ultrafine wet grinding treatment, water and / or ethanol are used as the medium, and at the same time, sodium hexametaphosphate and / or polyvinylpyrrolidone with a mass of 0.1-0.5% of the iron oxide-containing mineral are added as dispersants. Under the preferred ultrafine grinding conditions, nanoscale iron oxide-containing minerals can be obtained. By selecting a suitable dispersant during the ultrafine grinding process, the nanoscale iron oxide-containing minerals can be more evenly dispersed, and a uniform slurry can be obtained. The motor speed is further preferably 2000-3000 r / min.

[0010] As a preferred embodiment, the biomass raw material includes woody biomass or herbaceous biomass, such as sawdust, bamboo, straw, or coconut shells, etc., which are agricultural or processing waste.

[0011] As a preferred embodiment, the impregnation adopts the equal-volume impregnation method: the biomass raw material is placed in the nanoscale mineral slurry for impregnation and then dried, and the impregnation and drying are repeated multiple times until the iron mass content loaded on the biomass raw material reaches 5-10%. Through the equal-volume impregnation method, not only can the nanoscale minerals be evenly loaded in the biomass raw material, but also the loading amount of the nanoscale minerals can be effectively controlled. The concentration of the nanoscale mineral slurry can be adjusted as needed by appropriate drying or supplementing pure water. The nanoscale mineral slurry can be pretreated by ultrasonic waves to improve the dispersibility of the nanoscale minerals. During the equal-volume impregnation process, the nanoscale slurry is added according to the measured saturated absorption amount of the biomass and mixed with the biomass raw material for impregnation. The single impregnation time is 0.5-1 h, and the set loading concentration is achieved by repeating this process multiple times.

[0012] As a preferred embodiment, the conditions for pre-carbonization are as follows: the temperature is 350-450 °C, and the time is 1-2 h. Pre-carbonization adopts a low-temperature carbonization process. The purpose of this process is not to completely carbonize the biomass raw material. The key purpose is to fix the nanoscale iron oxide-containing minerals evenly adsorbed on the biomass raw material. Utilizing the characteristic that the nanoscale iron oxide-containing minerals can rapidly catalyze the dehydrogenation and deoxygenation of the organic matter on or around their surfaces at a relatively low temperature to achieve carbonization, a "semi-encapsulated" structure is formed, thereby effectively fixing the nanoscale iron oxide-containing minerals on the biomass surface. If the pre-carbonization temperature is lower than the preferred temperature range, it is difficult to react effectively. When the temperature is higher than the preferred temperature, the biomass itself undergoes rapid pyrolysis, thereby destroying the loading effect. If only one-step high-temperature carbonization reduction is used, the loading effect will also deteriorate. Pre-carbonization is carried out under a protective atmosphere. The protective atmosphere can be an inert atmosphere, such as argon, or nitrogen, or a pyrolysis gas can be used as the protective atmosphere after vacuum pumping.

[0013] As a preferred embodiment, the reduction conditions are as follows: the temperature is 600 - 800 °C, and the time is 1 - 2 h. The end point of reduction controls the iron metallization rate > 85%. For the hydrogen reduction process of iron oxide, the higher the temperature, the higher the reduction efficiency, but it is easier to cause the aggregation of the generated nano-zero-valent iron. In the present invention, the temperature is preferably 600 - 800 °C, mainly because after the low-temperature pre-carbonization treatment, due to the effective fixation of nano-scale iron oxide-containing minerals, using a high temperature of 800 °C will not cause particle aggregation, so that rapid production of zero-valent iron can be achieved in a higher temperature range. Moreover, for the conventional uses of nano-iron, the requirement for its reduction degree is not high, and there is no obvious difference in the effect when it is above 85%. In addition, the reduction temperature also affects the activity of nano-zero-valent iron. The biochar-supported nano-zero-valent iron formed at a lower temperature has a better adsorption effect, and the biochar-supported nano-zero-valent iron generated at a higher temperature has better strength and is more suitable for exerting catalytic effects in harsh environments such as high temperature. However, when the reduction temperature is higher, the biomass carbon basically does not have its own adsorption and other functions.

[0014] As a preferred embodiment, the hydrogen volume content in the hydrogen-containing atmosphere is above 60%. The hydrogen-containing atmosphere refers to a main atmosphere of hydrogen, with the hydrogen ratio above 60%. The higher the hydrogen content, the faster the reduction rate. The source of the hydrogen-containing atmosphere can be externally supplied or obtained by recycling and processing the pyrolysis gas of biomass. After hydrogen reduction, the supported zero-valent iron has higher activity. In addition to externally supplying hydrogen, the pyrolysis gas during the carbonization process is also a good source of hydrogen.

[0015] The pre-carbonization and hydrogen reduction of the present invention can be carried out in a sealed furnace body such as a horizontal, vertical tube furnace or box furnace.

[0016] The biochar-supported zero-valent iron obtained in the present invention can be directly sold and used as a product. It can also be further subjected to high-temperature treatment to achieve carbonization and activation, thereby optimizing the product performance.

[0017] The present invention also provides a biochar-supported nano-zero-valent iron obtained by the method.

[0018] The biochar-supported nano-zero-valent iron of the present invention has a specific composite structure. The nano-zero-valent iron is highly dispersed, and most of the nano-zero-valent iron is loaded on the biochar in an inlaid form, greatly improving the loading stability of the nano-zero-valent iron, effectively preventing the aggregation and shedding of the nano-zero-valent iron during use, and maintaining a high reaction activity.

[0019] Compared with the prior art, the beneficial technical effects brought by the technical solution of the present invention:

[0020] 1) The present invention uses readily available iron oxide minerals and biomass raw materials as direct raw materials to produce biomass carbon-supported nano-zero-valent iron. The raw materials have a wide source and low cost, and the process can achieve large-scale production. Moreover, hydrogen-based gases are mainly used as reducing agents in production, which is green and pollution-free, does not increase carbon emissions, and avoids the use of toxic and harmful liquid reducing agents, resulting in lower costs.

[0021] 2) The present invention realizes the stable loading of ultrafine iron oxides on the biomass carbon matrix material through a pre-carbonization method, which can effectively reduce the agglomeration phenomenon between fine particles. The subsequent reduction at a higher temperature will not cause malignant bonding and agglomeration, which can greatly improve the production rate of the process, and the dispersion degree of zero-valent iron in the product is also better.

[0022] 3) With the catalytic action of iron oxides during the preliminary carbonization process, the present invention realizes the embedded loading on the biomass matrix, with higher loading strength, which can more effectively prevent the oxidation and shedding of zero-valent iron during use, and the product has better use effects. Brief Description of the Drawings

[0023] Figure 1 It is the electron microscope image of the biochar-supported nano-zero-valent iron prepared in Example 1; it can be seen from the figure that the nano-zero-valent iron shows embedded loading, the nano-zero-valent iron is evenly distributed, and the dispersion degree is high.

[0024] Figure 2 It is the electron microscope image of the biomass-supported nano-zero-valent iron prepared in Comparative Example 3; it can be seen from the figure that the loading effect of the product prepared by the mixed loading method is poor, and large agglomeration regions often appear.

[0025] Figure 3 It is the electron microscope image of the biochar-supported nano-zero-valent iron prepared in Comparative Example 4; it can be seen from the figure that flocculent Fe-C substances appear in the product after high-temperature carbon reduction, and the activity is low. Detailed Description of the Invention

[0026] To better understand the present invention, the content of the present invention is more clearly elaborated and analyzed using specific examples. However, the claims of the present invention are not limited to the following example methods and conditions. Other examples obtained by those skilled in the relevant art without creative labor are within the protection scope of the present invention.

[0027] Example 1

[0028] The raw material selected is low-carbon steel phosphorus as the iron source, and its iron grade is 70.4%, including some impurities such as silicon, manganese, sulfur, and phosphorus. Corn stalks are selected as biomass raw materials. After drying, they are ground into powder. The powder is often in the form of long strips with a particle size of 100-200 mesh. A rod-pin nano sand mill is used in the ultrafine grinding process of the iron source. The specific material selection and process parameters are: zirconia ball mill diameter 0.2mm, material-ball ratio 1:1, water as solvent, PVP as dispersant, PVP dosage is 0.3% of the mass of ultrapure iron concentrate, wet grinding solid-liquid ratio 1:5, rotation speed 2700r / min, grinding time 4h. The grinding result is that the iron concentrate particles are refined to D 50 109nm, D 90 233nm.

[0029] The saturated water absorption of each batch of biomass raw materials was determined, and an appropriate amount of water was calculated and added to the slurry according to the solid-liquid ratio difference to control the solid-liquid mass ratio to 1:10 and fully dispersed by ultrasound. Then, the slurry was evenly mixed with the biomass raw materials, adsorbed for 0.5 hours and then dried. After repeating the above impregnation operation 4 times, a crude product with an iron loading of about 15% was obtained. The raw materials were carbonized at 450°C in a tubular reactor for 1 hour, and the subsequent products were reduced by introducing cracking gas simulation gas, in which the hydrogen ratio was 70%, the reduction temperature was 800°C, and the reduction was carried out for 1 hour. At this time, the metallization was about 87%, and the final product, biochar loaded with nano zero-valent iron, was obtained.

[0030] Electron microscope image of biomass carbon loaded with nano-zero-valent iron Figure 1 : Under the conditions of this embodiment, nano-zero-valent iron with good dispersion effect and high binding strength (semi-embedded load) was obtained, and as can be seen in the figure, the particle size of the nano-zero-valent iron is generally below 200 nm.

[0031] Example 2

[0032] The raw material is common ultra-pure iron concentrate with an iron grade of 71.5% and almost no impurities. The biomass raw material is a mixture of bamboo powder and willow powder, which is easier to crush than straw raw materials and has a particle size of 150 to 300 meshes.

[0033] The pin-type nano sand mill was used in the ultrafine grinding process. The specific material selection and process parameters were as follows: the diameter of the zirconium oxide ball mill was 0.1 mm, the material-ball ratio was 1:1, anhydrous ethanol was used as the solvent, sodium hexametaphosphate was used as the dispersant, the dosage was 0.4% of the mass of the ultrapure iron concentrate, the wet grinding solid-liquid ratio was 1:10, the speed was 2700 r / min, and the grinding time was 6 h. The grinding result was that the iron concentrate particles were refined to D 50 80nm.

[0034] The saturated ethanol adsorption capacity of the mixed biomass raw materials was measured, and its adsorption capacity was greater than that in Example 1. An appropriate amount of ethanol was added to control the solid-liquid mass ratio of 1:20, and ultrasonic dispersion was carried out to disperse it evenly. Then, it was mixed evenly with the biomass raw materials, adsorbed for 1 h, and then dried. The above impregnation operation was repeated three times to obtain a crude product with a loading amount of about 5%. The raw materials were carbonized in a tubular reaction furnace at 350 °C for 1 h, and the subsequent product was reduced with pure hydrogen, in which the hydrogen ratio was 70%, the reduction temperature was 600 °C, and the reduction time was 2 h. At this time, the metallization was about 94%, and the final product of supported nano-zero-valent iron was obtained.

[0035] Comparative Example 1

[0036] The only difference from Example 1 is that:

[0037] During the carbonization process, the biomass raw materials loaded with minerals were carbonized in a tubular reaction furnace at 300 °C for 1 h. This pre-carbonization temperature was relatively low, and it was impossible to achieve preferential carbonization around the ultrafine iron oxides to form an embedded structure. As a result, the in-situ reduced nano-zero-valent iron could not form an embedded structure in the "semi-wrapped" form. During the subsequent high-temperature reduction stage, the volatiles of the biomass escaped too quickly, which would damage the matrix structure. Under this condition, the reduced nano-zero-valent iron showed obvious agglomeration phenomena.

[0038] Comparative Example 2

[0039] The only difference from Example 1 is that:

[0040] During the reduction process, the biomass raw materials loaded with minerals were directly reduced by introducing pyrolysis gas simulated gas, in which the hydrogen ratio was 70%, the reduction temperature was 800 °C, and the reduction time was 1 h. If the low-temperature carbonization was not carried out first, although the nano-iron oxides were successfully loaded, the loading strength was not high. Similar to Comparative Example 1, after directly entering the high-temperature stage, the biomass structure was severely damaged, and the morphology of the nano-zero-valent iron deteriorated.

[0041] Comparative Example 3

[0042] The only difference from Example 1 is that:

[0043] The loading was completed by simple liquid-phase mixing. The biochar raw materials were mechanically mixed with the ultrafine iron oxide slurry (10% by iron measurement), and then dried. After simplifying the loading process, the distribution of the ultrafine iron oxide particles was uneven, and local agglomeration often occurred.

[0044] Comparative Example 4

[0045] The only difference from Example 1 is that:

[0046] During the reduction process, hydrogen is not introduced, and only the reducibility of biomass carbon itself is used to reduce iron oxide at high temperature. The pre-carbonized product is directly heated for reduction, with a reduction temperature of 900 °C and the reduction time extended to 2 h. After the reduction is completed, there are a large number of Fe-C compounds on the surface of the zero-valent iron reduced by carbon in the product, which appear as flocculent substances under the electron microscope. Some zero-valent iron particles are almost completely trapped in the biomass carbon structure. If no additional activation is carried out subsequently, the activity of the material is low and the effect is poor.

Claims

1. A preparation method of biochar-supported nano zero-valent iron, characterized in that: It includes the following steps: 1) Ultra-finely wet grind the iron oxide mineral to obtain a nano-scale mineral slurry; 2) Immerse the biomass raw material in the nano-scale mineral slurry to obtain the mineral-loaded biomass; the immersion adopts the equal-volume immersion method: immerse the biomass raw material in the nano-scale mineral slurry and then dry it, and repeat the immersion and drying multiple times until the iron mass content loaded on the biomass raw material reaches 5-10%; 3) First pre-carbonize the mineral-loaded biomass under a protective atmosphere, and then reduce it under a hydrogen-containing atmosphere to obtain biochar-supported nano-zero-valent iron; the conditions for the pre-carbonization are: temperature is 350-450 °C, time is 1-2 h; the conditions for the reduction are: temperature is 600-800 °C, time is 1-2 h.

2. The preparation method of a biochar-supported nano zero-valent iron according to claim 1, characterized in that: The iron grade of the iron oxide mineral is not less than 60%.

3. The preparation method of a biochar-supported nano zero-valent iron according to claim 1 or 2, characterized in that: The conditions for the ultra-fine wet grinding treatment are: solid-liquid ratio is 1 g:5-10 mL, motor speed is 1000-3000 r / min, and the size of the grinding beads is 0.05-0.2 mm.

4. The preparation method of a biochar-supported nano zero-valent iron according to claim 3, characterized in that: During the ultra-fine wet grinding treatment, water and / or ethanol are used as the medium, and at the same time, 0.1-0.5% of sodium hexametaphosphate and / or polyvinylpyrrolidone based on the mass of the iron oxide mineral are added as dispersants.

5. The preparation method of a biochar-supported nano zero-valent iron according to claim 1, characterized in that: The biomass raw material includes woody biomass and / or herbaceous biomass.

6. The preparation method of a biochar-supported nano zero-valent iron according to claim 1, characterized in that: The hydrogen volume content in the hydrogen-containing atmosphere is above 60%.

7. A biochar-supported nano zero-valent iron, characterized in that: Obtained by the preparation method according to any one of claims 1-6.

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