A carbon-based hydrogen storage material and its preparation method using industrial waste salt to synergistically activate biomass.

Abstract

This invention discloses a carbon-based hydrogen storage material and its preparation method using industrial waste salt to synergistically activate biomass, belonging to the technical field of carbon-based hydrogen storage materials. The carbon-based hydrogen storage material has a hierarchical pore structure of macropores-mesopores-micropores, with a specific surface area of ​​800–2500 m² / g. The preparation method includes: mixing biomass raw materials with industrial waste salt in a certain proportion, adding a binder, pressing into shape, and then performing salt-sealing pyrolysis under an inert atmosphere. The resulting pyrolysis product is washed with water to remove waste salt and dried to obtain the final product. The industrial waste salt is by-product salt from the chlor-alkali industry, seawater desalination waste salt, or industrial mixed salt. The alkali metal chlorides contained therein melt during pyrolysis to form a liquid-phase reaction medium, and impurity metal ions catalyze the carbonization and pore formation of the biomass. This invention utilizes industrial waste salt to replace pure chemical activators, realizing the resource utilization of waste materials, and preparing a high specific surface area carbon-based hydrogen storage material with a hierarchical pore structure in one step. The waste salt is recyclable, low-cost, and environmentally friendly.

Description

Technical Field

[0001] This invention relates to the field of carbon-based hydrogen storage materials technology, specifically to a carbon-based hydrogen storage material and its preparation method using industrial waste salt to synergistically activate biomass. Background Technology

[0002] Carbon-based hydrogen storage materials have become an important research focus in solid-state hydrogen storage technology due to their advantages such as large specific surface area, high chemical stability, and fast hydrogen adsorption / desorption kinetics. Biomass, as a renewable zero-carbon resource, is an advantageous carbon source for the preparation of carbon-based hydrogen storage materials. However, biochar obtained from direct biomass pyrolysis has a low specific surface area and underdeveloped pore structure, making it difficult to meet the requirements for hydrogen adsorption. Therefore, it is necessary to activate biochar to develop its pore structure.

[0003] Currently, commonly used activation methods include physical activation (CO2, H2O) and chemical activation (KOH, NaOH, ZnCl2, H3PO4, etc.). Chemical activation, especially KOH activation, can prepare activated carbon with ultra-high specific surface area, but it has the following problems: (1) Chemical reagents such as KOH are expensive and highly corrosive, requiring high-end equipment; (2) After activation, a large amount of acid washing is required to remove residual alkali, generating a large amount of acidic waste liquid, which has a heavy environmental burden; (3) The activation process relies solely on chemical etching, and the ability to regulate pore structure is limited. Although physical activation is relatively environmentally friendly, the specific surface area of ​​the resulting biochar is usually low, making it difficult to meet the requirements of high hydrogen storage capacity.

[0004] Meanwhile, my country produces a massive amount of industrial waste salt, mainly including by-product salt from the chlor-alkali industry, waste salt from seawater desalination, and waste salt from pesticide intermediates, with annual discharges exceeding ten million tons. Currently, this waste salt is mostly disposed of through landfill or ocean discharge, which not only occupies land resources but also poses a potential threat to the ecological environment. The main components of this waste salt are NaCl and KCl, while also containing impurity ions such as Ca²⁺, Mg²⁺, and Fe³⁺. If this waste salt could be used as an activator in the preparation of biomass carbon materials, it would not only solve the waste salt disposal problem but also reduce the preparation cost of carbon-based hydrogen storage materials, achieving "waste treatment with waste."

[0005] However, directly mixing waste salt with biomass for pyrolysis presents the following technical challenges: (1) impurity metal ions in the waste salt may volatilize or form low-melting-point eutectics during pyrolysis, making uniform distribution difficult; (2) insufficient contact between salt and biomass results in limited activation; and (3) a lack of systematic methods for recycling activated salt. Therefore, developing a method for efficiently activating biomass using industrial waste salt to prepare high specific surface area carbon-based hydrogen storage materials is of significant practical importance. Summary of the Invention

[0006] Technical problems to be solved

[0007] To address the shortcomings of existing technologies, this invention provides a carbon-based hydrogen storage material and a method for preparing it by synergistic activation of biomass using industrial waste salt. This method solves the problems of high cost of chemical activators, serious waste liquid pollution, and difficulty in resource utilization of waste salt in existing technologies.

[0008] Technical solution

[0009] To achieve the above objectives, the present invention provides the following technical solution:

[0010] First, the present invention provides a carbon-based hydrogen storage material having a hierarchical porous structure, comprising macropores, mesopores, and micropores, wherein the macropore diameter is 0.5–5 μm, the mesopore diameter is 2–50 nm, and the micropore diameter is less than 2 nm; the specific surface area of ​​the carbon-based hydrogen storage material is 800–2500 m² / g, the microporosity is not less than 60%, the total pore volume is 0.5–1.5 cm³ / g, and the average pore size is 1.0–3.5 nm.

[0011] Preferably, the carbon-based hydrogen storage material has an ash content of less than 2 wt.% and an oxygen content of 5 to 15 wt.%.

[0012] Preferably, the hydrogen adsorption capacity of the carbon-based hydrogen storage material is 4.0 to 6.5 wt.% at -196°C and 3 MPa, and 0.3 to 0.8 wt.% at 25°C and 5 MPa.

[0013] Secondly, the present invention provides a method for preparing the above-mentioned carbon-based hydrogen storage material, comprising the following steps: mixing biomass raw materials and industrial waste salt at a mass ratio of 1:(1-5), adding a binder, pressing and molding, and then performing salt-sealing pyrolysis at 600-1000℃ for 0.5-4 h under an inert atmosphere. The obtained pyrolysis product is washed with water to remove waste salt, and dried to obtain the carbon-based hydrogen storage material. The industrial waste salt is by-product salt of chlor-alkali industry, waste salt of seawater desalination, or industrial mixed salt, and the alkali metal chloride contained therein is NaCl and / or KCl, and the impurity metal ions contained therein include one or more of Ca²⁺, Mg²⁺, and Fe³⁺.

[0014] Preferably, the biomass raw material is one or more of agricultural and forestry waste, food processing waste, or urban greening waste, including corn stalks, rice husks, sawdust, fruit shells, sugarcane bagasse, coffee grounds, and empty bunches of oil palm fruit.

[0015] Preferably, the binder is one or more of starch, dextrin, polyvinyl alcohol, and lignin sulfonate, and the amount added is 1% to 10% of the total mass of biomass and waste salt.

[0016] Preferably, the pressing pressure is 5-30 MPa, and the formed blank is blocky, columnar, or granular with a size of 5-50 mm.

[0017] Preferably, the salt-sealed pyrolysis adopts a two-stage heating program: first, the temperature is raised to 300-500℃ at 5-15℃ / min and held for 0.5-1 h to pre-carbonize the biomass and release volatiles; then, the temperature is raised to 600-1000℃ at 3-10℃ / min and held for 0.5-3 h for high-temperature activation.

[0018] Preferably, the washing is performed by washing with deionized water or industrial recycled water 1 to 5 times until the conductivity of the washing solution is lower than 500 μS / cm; the drying is performed by drying at 80 to 120°C for 4 to 12 hours.

[0019] Preferably, the waste salt removed by water washing is recovered after evaporation and crystallization or spray drying and reused for the activation of the next batch of biomass, with a recycling frequency of 2 to 5 times.

[0020] Beneficial effects

[0021] This invention provides a carbon-based hydrogen storage material and a method for preparing it by synergistic activation of biomass using industrial waste salt. It has the following beneficial effects:

[0022] 1. This invention is the first to propose using industrial waste salts (chlor-alkali by-product salts, seawater desalination waste salts, etc.) as biomass activators, replacing traditional high-cost, highly corrosive chemical reagents such as KOH and NaOH. The alkali metal chlorides (NaCl, KCl) in the waste salts melt at high temperatures to form a liquid-phase reaction medium, uniformly coating biomass particles and promoting deep activation of the carbon framework. Naturally present impurity ions such as Ca²⁺, Mg²⁺, and Fe³⁺ in the waste salts play a dual role in catalyzing graphitization and pore formation during pyrolysis, synergistically increasing the specific surface area and microporosity of carbon materials. This invention achieves high-value utilization of industrial waste salts, turning waste into treasure, and significantly reducing the preparation cost of carbon-based hydrogen storage materials.

[0023] 2. This invention employs an integrated process of "compression molding - salt-sealing pyrolysis - water washing and desalination". Compression molding ensures close contact between biomass and waste salt, preventing material from flying during pyrolysis. During salt-sealing pyrolysis, the molten salt forms a closed liquid-phase environment, encapsulating the biomass carbon skeleton. This inhibits excessive shrinkage and graphitization of the carbon structure, while the salt acts as a hard template, leaving macropores and mesopores after water washing, forming a hierarchical pore structure of macropores-mesopores-micropores. Compared to simple chemical activation, the hierarchical pore structure of this invention is more conducive to the adsorption and storage of hydrogen in different pressure ranges.

[0024] 3. This invention enables the recycling of waste salt. The waste salt removed by water washing can be recycled for the activation of the next batch of biomass after simple evaporation and crystallization or spray drying. The activation effect shows no significant decline after 2-5 cycles. This not only further reduces raw material costs but also avoids environmental pollution caused by waste salt discharge, aligning with the concepts of green chemistry and circular economy.

[0025] 4. The biomass raw materials used in this invention are widely available and inexpensive, including agricultural, forestry, and food processing waste such as corn stalks, rice husks, sawdust, fruit shells, and coffee grounds. The entire preparation process does not require the use of strong acids or alkalis; the washing wastewater is mainly neutral brine, which is easy to treat or reuse, making it environmentally friendly. The resulting carbon-based hydrogen storage material has a specific surface area of ​​800–2500 m² / g, a hydrogen storage capacity of 4.0–6.5 wt.% at -196℃ and 3 MPa, and a hydrogen storage capacity of 0.3–0.8 wt.% at 25℃ and 5 MPa, exhibiting performance superior to or close to that of traditional KOH-activated samples. Attached Figure Description

[0026] Figure 1 This is a process flow diagram of the preparation method of the present invention;

[0027] Figure 2 The nitrogen adsorption-desorption isotherm and pore size distribution of the carbon-based hydrogen storage material prepared in Example 1 of this invention are shown.

[0028] Figure 3 This is a comparison chart of the low-temperature hydrogen storage capacity of Example 1 and Comparative Example 1 (activated with pure KOH).

[0029] Figure 4 This is a comparison chart of the room temperature hydrogen storage capacity of Example 1 and Comparative Example 2 (direct pyrolysis without activation) of the present invention;

[0030] Figure 5 This is a SEM image of the carbon-based hydrogen storage material prepared in Example 1 of the present invention, showing a hierarchical porous structure. Detailed Implementation

[0031] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0032] Example 1

[0033] This embodiment provides a carbon-based hydrogen storage material and a method for preparing it by synergistic activation of biomass using industrial waste salt.

[0034] Step 1: Raw material preparation

[0035] Biomass raw materials: Corn stalks are crushed and sieved to obtain stalk powder with a particle size of 0.5-2 mm, which is then dried at 105℃ for 12 h for later use.

[0036] Industrial waste salt: taken from a by-product of a chlor-alkali plant, its main components are NaCl (approximately 85% by mass), KCl (approximately 8%), CaSO4 (approximately 3%), MgCl2 (approximately 2%), FeCl3 (approximately 0.5%), and other trace impurities. The waste salt does not require purification and can be used directly.

[0037] Binder: Industrial grade starch.

[0038] Step 2: Mixing and Shaping

[0039] Weigh out 500 g of dried straw powder, 1500 g of industrial waste salt (biomass:waste salt mass ratio = 1:3), and 40 g of starch (accounting for 2% of the total mass). Put the three into a twin-screw mixer and mix for 15 min to make the material uniform. Put the mixture into a mold and press it into a block shape of 50 mm × 50 mm × 30 mm under a pressure of 15 MPa.

[0040] Step 3: Salt-sealed pyrolysis

[0041] The block-shaped billet was placed in a corundum boat and then placed in a tube furnace. After purging with nitrogen (1 L / min) for 20 min, the temperature was increased to 400℃ at a rate of 10℃ / min and held for 1 h to pre-carbonize the straw and release volatiles. Then, the temperature was increased to 850℃ at a rate of 5℃ / min and held for 2 h for high-temperature activation. During pyrolysis, the waste salt melted to form a liquid phase, encapsulating the carbon skeleton. After pyrolysis, the material was naturally cooled to room temperature under a nitrogen atmosphere.

[0042] Step 4: Washing, desalting, and drying

[0043] The pyrolysis product was removed and placed in a beaker. Deionized water (solid-liquid ratio 1:10) was added, and the mixture was stirred and washed at 60°C for 30 min, followed by filtration. The washing was repeated three times until the conductivity of the filtrate was below 500 μS / cm. The washed solid was dried at 105°C for 8 h to obtain a carbon-based hydrogen storage material, designated CS-850-3.

[0044] Step 5: Waste Salt Recycling

[0045] The washing filtrates were combined and evaporated at 105°C to crystallize the waste salt solids for use in the next batch of activation.

[0046] Product characterization:

[0047] Specific surface area and pore structure were determined using nitrogen adsorption-desorption (77K). Results showed that the BET specific surface area of ​​CS-850-3 was 2150 m² / g, the microporous specific surface area was 1850 m² / g (microporosity 86%), the total pore volume was 1.12 cm³ / g, and the average pore size was 1.92 nm. The pore size distribution curve showed the presence of micropores (0.5–2 nm), mesopores (2–10 nm), and a small number of macropores (0.5–2 μm), exhibiting a hierarchical pore structure. Scanning electron microscopy revealed circular pores (originating from a salt crystal template) with a size of 1–3 μm distributed on the material surface, with the pore walls filled with nanoscale pores.

[0048] Elemental analysis: Carbon content is 86.5 wt.%, oxygen content is 11.2 wt.%, hydrogen content is 0.8 wt.%, and ash content is 1.5 wt.%.

[0049] Hydrogen storage performance testing: A Sieverts testing instrument was used. Low-temperature (-196℃) hydrogen storage test: At 3 MPa, the hydrogen adsorption capacity of CS-850-3 was 6.1 wt.%; at 5 MPa, it was 6.5 wt.%. Room temperature (25℃) hydrogen storage test: At 5 MPa, the hydrogen adsorption capacity was 0.61 wt.%.

[0050] Example 2

[0051] This embodiment is basically the same as Embodiment 1, except that the biomass raw material is changed to rice husk, the mass ratio of biomass to waste salt is 1:2, the pyrolysis temperature is 750℃, and the holding time is 1.5 h. The resulting carbon-based hydrogen storage material has a specific surface area of ​​1820 m² / g, a microporosity of 82%, a hydrogen storage capacity of 5.2 wt.% at -196℃ and 3 MPa, and a hydrogen storage capacity of 0.45 wt.% at room temperature and 5 MPa.

[0052] Example 3

[0053] This embodiment is basically the same as Embodiment 1, except that: industrial waste salt is replaced with seawater desalination waste salt (mainly NaCl, containing Mg²⁺, Ca²⁺, SO₄²⁻, etc.), biomass is coffee grounds, the biomass to waste salt mass ratio is 1:4, and the pyrolysis temperature is 900℃. The resulting carbon-based hydrogen storage material has a specific surface area of ​​2380 m² / g, a microporosity of 88%, a hydrogen storage capacity of 6.3 wt.% at -196℃ and 3 MPa, and a hydrogen storage capacity of 0.68 wt.% at room temperature and 5 MPa.

[0054] Example 4

[0055] This embodiment examines the recycling effect of waste salt. The waste salt recovered in Example 1 was used for the activation of a second batch of straw, and carbon materials were prepared under the same conditions. Cycle 1: Specific surface area 2120 m² / g, hydrogen storage 6.0 wt.%; Cycle 2: Specific surface area 2080 m² / g, hydrogen storage 5.9 wt.%; Cycle 3: Specific surface area 1950 m² / g, hydrogen storage 5.6 wt.%; Cycle 4: Specific surface area 1800 m² / g, hydrogen storage 5.2 wt.%; Cycle 5: Specific surface area 1650 m² / g, hydrogen storage 4.8 wt.%. It is evident that the waste salt can be recycled three times while maintaining a good activation effect.

[0056] Comparative Example 1 (activated with pure KOH)

[0057] Corn stalks were carbonized and then mixed with KOH at a mass ratio of 1:4. The mixture was activated at 850℃ for 2 h, followed by acid washing, water washing, and drying. The resulting sample had a specific surface area of ​​2560 m² / g, a hydrogen storage capacity of 6.0 wt.% at -196℃ and 3 MPa, and a hydrogen storage capacity of 0.52 wt.% at room temperature and 5 MPa. The method of this invention (Example 1) achieves comparable hydrogen storage performance without using KOH, eliminates the need for acid washing, and produces neutral brine as wastewater.

[0058] Comparative Example 2 (Direct Pyrolysis Without Activation)

[0059] Direct pyrolysis of corn stalks at 850℃ for 2 h (without adding waste salt) yielded biochar with a specific surface area of ​​only 180 m² / g and a hydrogen storage capacity of only 1.2 wt.% at -196℃ and 3 MPa.

[0060] Comparative Example 3 (Simple pyrolysis of waste salt and biomass, without pressing into shape)

[0061] Corn stalks and waste salt were mixed in a 1:3 ratio but not pressed into shape, and then directly pyrolyzed at 850℃. During pyrolysis, the material experienced severe dispersion, resulting in a low product yield (only 35%) and uneven distribution of the waste salt. The resulting carbon material had a specific surface area of ​​only 650 m² / g and a hydrogen storage capacity of 2.8 wt.%. This indicates that pressing and the salt-sealing environment are crucial for the activation effect.

[0062] Performance Comparison Summary

[0063] sample Specific surface area (m² / g) Microporosity (%) Hydrogen storage capacity at -196℃ / 3MPa (wt.%) Hydrogen storage capacity at 25℃ / 5MPa (wt.%) Activator cost Waste liquid type Example 1 2150 86 6.1 0.61 Extremely low (waste salt) neutral saline Comparative Example 1 (KOH) 2560 92 6.0 0.52 high acidic waste liquid Comparative Example 2 (without activation) 180 — 1.2 0.08 — — Comparative Example 3 (without compression molding) 650 — 2.8 0.22 Low neutral saline

[0064] Mechanism analysis

[0065] The formation of the hierarchical porous structure and its high specific surface area characteristics of the carbon-based hydrogen storage material of this invention are the result of the synergistic effect of multiple components in industrial waste salt. The mechanism of action is analyzed as follows:

[0066] (1) The molten template effect of alkali metal chlorides: NaCl (melting point 801℃) and KCl (melting point 770℃) in industrial waste salt melt at pyrolysis temperatures (600~1000℃) to form a low-viscosity liquid phase. This liquid phase uniformly wets and coats the biomass particles, acting as a physical barrier to inhibit excessive shrinkage and graphitization of the carbon skeleton, and as a "hard template" to occupy space. After washing away the salt, macropores (0.5~5 μm) and mesopores (2~50 nm) are left at the original salt crystal locations, forming channels for rapid hydrogen diffusion.

[0067] (2) Catalytic pore-forming effect of impurity metal ions: Impurity ions such as Ca²⁺, Mg²⁺, and Fe³⁺ naturally present in waste salts are embedded in defect sites of the carbon lattice during pyrolysis. These metal ions have unoccupied d orbitals, which can interact electronically with carbon atoms, catalyzing the selective breaking of carbon-carbon bonds and promoting the formation of micropores (<2 nm). At the same time, Fe³⁺ is reduced to nano-Fe particles by carbon at high temperature, further catalyzing the disordering of carbon and increasing defect density and microporosity.

[0068] (3) Confinement effect of salt-sealed environment: Press molding creates a closed structure of "salt-encased carbon" between waste salt and biomass. During pyrolysis, the molten salt encapsulates the carbon skeleton, restricting the rapid escape of volatiles and causing secondary reactions of volatiles within the carbon skeleton, further etching the pores. At the same time, the high heat capacity of the molten salt makes the system heat up more uniformly, avoiding local overheating that could lead to pore collapse.

[0069] The synergistic effect of these three factors ultimately forms a hierarchical porous structure: macropores serve as hydrogen diffusion channels, mesopores as hydrogen buffers, and micropores as hydrogen adsorption sites. This allows the material to primarily utilize micropore adsorption in the low-temperature region (-196℃), supplemented by mesopore filling in the high-pressure region, and rely on the synergistic adsorption of oxygen-containing functional groups and micropores in the room-temperature region. Compared to simple KOH activation, the method of this invention utilizes the natural components in waste salt to achieve equivalent or even better pore-forming effects, and it eliminates the need for acid washing, making it environmentally friendly.

[0070] Industrial applicability

[0071] The method provided by this invention uses agricultural and forestry waste as biomass raw materials, which are widely available and inexpensive; industrial waste salt is a byproduct of industries such as chlor-alkali and seawater desalination, and can be used directly without purification; the process equipment includes conventional tubular furnaces, tablet presses, and mixers, which are easy to scale up industrially; the washing wastewater is neutral brine, which can be recycled by evaporation and crystallization, achieving zero discharge. Based on an annual production of 1000 tons of carbon-based hydrogen storage material, compared with the traditional KOH activation method, it can save approximately 5 million yuan / year in activator costs and reduce acidic waste liquid discharge by approximately 8000 tons / year, demonstrating significant economic and environmental benefits.

[0072] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A carbon-based hydrogen storage material, characterized in that, It has a hierarchical pore structure, including macropores, mesopores and micropores, wherein the macropore diameter is 0.5-5 μm, the mesopore diameter is 2-50 nm, and the micropore diameter is less than 2 nm; the specific surface area of ​​the carbon-based hydrogen storage material is 800-2500 m² / g, the microporosity is not less than 60%, the total pore volume is 0.5-1.5 cm³ / g, and the average pore size is 1.0-3.5 nm.

2. The carbon-based hydrogen storage material according to claim 1, characterized in that, The carbon-based hydrogen storage material has an ash content of less than 2 wt.% and an oxygen content of 5–15 wt.%.

3. The carbon-based hydrogen storage material according to claim 1, characterized in that, The carbon-based hydrogen storage material has a hydrogen adsorption capacity of 4.0–6.5 wt.% at -196℃ and 3 MPa, and a hydrogen adsorption capacity of 0.3–0.8 wt.% at 25℃ and 5 MPa.

4. A method for preparing the carbon-based hydrogen storage material according to any one of claims 1-3, characterized in that, Includes the following steps: Biomass raw materials and industrial waste salt are mixed at a mass ratio of 1:(1-5), a binder is added, and the mixture is pressed into shape. Then, it is subjected to salt-sealing pyrolysis at 600-1000℃ for 0.5-4 h under an inert atmosphere. The resulting pyrolysis product is washed with water to remove the waste salt, and then dried to obtain the carbon-based hydrogen storage material. The industrial waste salt is by-product salt from the chlor-alkali industry, waste salt from seawater desalination, or mixed industrial salt. The alkali metal chlorides it contains are NaCl and / or KCl, and the impurity metal ions it contains include one or more of Ca²⁺, Mg²⁺, and Fe³⁺.

5. The preparation method according to claim 4, characterized in that, The biomass raw materials are one or more of agricultural and forestry waste, food processing waste, or urban greening waste, including corn stalks, rice husks, sawdust, fruit shells, sugarcane bagasse, coffee grounds, and empty bunches of oil palm fruit.

6. The preparation method according to claim 4, characterized in that, The binder is one or more of starch, dextrin, polyvinyl alcohol, and lignin sulfonate, and the amount added is 1% to 10% of the total mass of biomass and waste salt.

7. The preparation method according to claim 4, characterized in that, The pressing pressure is 5–30 MPa, and the formed blank is in the form of blocks, columns or granules with a size of 5–50 mm.

8. The preparation method according to claim 4, characterized in that, The salt-sealed pyrolysis adopts a two-stage heating program: first, the temperature is raised to 300-500℃ at 5-15℃ / min and held for 0.5-1 h to pre-carbonize the biomass and release volatiles; then, the temperature is raised to 600-1000℃ at 3-10℃ / min and held for 0.5-3 h for high-temperature activation.

9. The preparation method according to claim 4, characterized in that, The washing process involves washing with deionized water or industrial recycled water 1 to 5 times until the conductivity of the washing solution is below 500 μS / cm; the drying process involves drying at 80 to 120°C for 4 to 12 hours.

10. The preparation method according to claim 4, characterized in that, The waste salt removed by water washing is recovered after evaporation and crystallization or spray drying and reused for the activation of the next batch of biomass, with a recycling frequency of 2 to 5 times.

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