Method for preparing fuel grade ultrafine iron powder by hydrogen-based reduction of iron powder

Abstract

This invention discloses a method for preparing fuel-grade ultrafine iron powder from hydrogen-based reduced iron powder, belonging to the field of hydrogen metallurgy technology. The method involves hydrogen-reduced reduced iron powder under an inert atmosphere with an oxygen volume fraction not exceeding 2%, followed by closed-loop feeding, airflow milling, turbine classification, and surface passivation treatment. Nitrogen is used as the milling medium, the milling pressure is 0.4~0.8MPa, and the output D50 is 5~30μm. The classification span (D90-D10) / D50 is not greater than 2.0, and coarse particles are returned to the mill. Passivation is carried out at 40~80℃ and an oxygen volume fraction of 0.5%~2.0%, forming a 5~20nm Fe3O4 passivation layer. The passivated powder is then conveyed and inertized under closed-loop conditions with oxygen volume fraction and temperature interlocking. This invention solves the problems of spontaneous combustion and agglomeration of ultrafine iron powder while maintaining the metallization rate. The product's spontaneous combustion temperature is not lower than 180℃, making it suitable for blending or full combustion in iron powder boilers.

Description

Technical Field

[0001] This invention relates to the field of hydrogen metallurgy and metal fuel preparation technology, specifically to a method for preparing fuel-grade ultrafine iron powder by using hydrogen-based reduced iron powder as raw material, through ultrafine grinding and controllable surface passivation. Background Technology

[0002] Hydrogen-based direct reduction is one of the important technological pathways for achieving deep decarbonization in the steel industry. Reduced iron powder produced using hydrogen as a reducing agent is free of impurities such as carbon and sulfur, and has a high metallization rate. Besides being used as a raw material for steelmaking, it can also be used as a zero-carbon fuel: the product of iron powder combustion is solid iron oxide, which can be regenerated into metallic iron through green hydrogen reduction, thus forming a metal fuel cycle that does not emit carbon dioxide. Using iron powder for co-firing or full combustion in power plant boilers is one of the potential solutions for long-term energy storage and industrial thermal decarbonization.

[0003] To ensure stable ignition and complete combustion of iron powder in a boiler, its particle size needs to be refined to the tens of micrometers level, maintaining a narrow particle size distribution. However, the particle size of industrial hydrogen-based reduced iron powder is typically 50-150 μm, which presents challenges such as difficulty in ignition, low burnout rate, and poor flame stability when directly fed into the furnace. After ultrafine grinding, the specific surface area of ​​the reduced iron powder increases dramatically. The fresh metal surface has extremely high reactivity, rapidly oxidizing and releasing heat in air, and spontaneously combusting at room temperature. This poses a serious risk of dust explosion and fire during grinding, conveying, storage, and packaging. Furthermore, the significant inter-particle forces in ultrafine iron powder make it prone to agglomeration, leading to poor flowability and uneven feeding.

[0004] Existing methods for stabilizing ultrafine iron powder mainly include phosphating, chromate passivation, organic coating, and oil immersion. While these methods can suppress spontaneous combustion to some extent, they introduce impurities such as phosphorus, chromium, and carbon into the iron powder. This reduces the metallization rate and calorific value of the product, and introduces new pollutants into the combustion products, contradicting the original intention of zero-carbon recycling of metal fuels. Furthermore, the organic coating layer is prone to aging and failure during long-term storage. In addition, existing pulverization and grading processes do not adequately control the oxygen and water content in the pulverizing medium, leading to excessive oxidation of the newly formed surface during pulverization, resulting in a decrease in the metallization rate and poor batch consistency.

[0005] Therefore, there is an urgent need for a method to prepare fuel-grade ultrafine iron powder that can simultaneously solve the problems of spontaneous combustion and agglomeration of ultrafine iron powder while maintaining a high metallization rate, and is suitable for large-scale continuous production. Summary of the Invention Technical problems to be solved

[0006] The technical problem to be solved by this invention is to provide a method for preparing fuel-grade ultrafine iron powder from hydrogen-based reduced iron powder. Under the premise of refining the reduced iron powder to a D50 of 5~30μm and maintaining a metallization rate of not less than 90%, the method overcomes the problems of easy self-ignition, easy agglomeration, and poor flowability of ultrafine iron powder, so that the product meets the requirements of iron powder boilers for blending or full combustion in terms of safety, flowability, and combustion performance. Technical solution

[0007] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0008] A method for preparing fuel-grade ultrafine iron powder from hydrogen-based reduced iron powder includes the following steps: S1 Closed feeding: Under an inert protective atmosphere with an oxygen volume fraction of not more than 2%, the reduced iron powder obtained by hydrogen-based reduction is fed into the closed feeding unit. The particle size of the reduced iron powder is 50~150μm and the metallization rate is not less than 92%. S2 Airflow milling: Reduced iron powder is fed into an airflow mill, using nitrogen as the milling medium, and is subjected to ultrafine milling under the conditions of milling pressure of 0.4~0.8MPa and classifier speed of 2000~6000r / min to obtain powder with D50 of 5~30μm; S3 Turbine Classification: The pulverized powder is fed into a turbine classifier for classification. The particle size distribution range (D90-D10) / D50 is controlled to be no greater than 2.0. The coarse particles classified are returned to step S2. S4 Surface passivation: The graded powder is fed into a passivator, and a nitrogen-oxygen mixture with an oxygen volume fraction of 0.5% to 2.0% is introduced at 40 to 80°C and held for 3 to 10 minutes to generate a passivation layer with a thickness of 5 to 20 nm and Fe3O4 as the main component on the particle surface. S5 Closed Conveying and Inerting Packaging: The passivated powder is conveyed and inerted under closed conditions with an oxygen volume fraction not exceeding 2% and a temperature not exceeding 60℃.

[0009] Preferably, the nitrogen working medium of the air jet mill in step S2 is recycled after being treated for dust removal, deoxygenation and dehydration, and the oxygen volume fraction in the recycled nitrogen is not higher than 0.5% and the dew point is not higher than -20°C.

[0010] Preferably, steps S2 and S3 constitute a closed-loop crushing and grading circuit. The controller adjusts the speed of the grading wheel and the feeding rate according to the D50 deviation measured by the online particle size analyzer, so that the output D50 is maintained at 8~20μm.

[0011] Preferably, the passivator in step S4 is a fluidized bed passivator with an apparent gas velocity of 0.1~0.3m / s. Cooling water is introduced into the passivator jacket to maintain the bed temperature at 40~80℃. A drying process is also provided before step S4 to ensure that the moisture content of the powder entering the passivator is not higher than 0.1%. The drying heat source is low-temperature nitrogen gas after heat exchange.

[0012] Preferably, in step S5, the conveying and packaging unit is equipped with an oxygen volume fraction sensor and a temperature sensor, and is interlocked with the nitrogen replenishment pipeline: when the oxygen volume fraction exceeds 2% or the temperature exceeds 60°C, the conveying is cut off and nitrogen is introduced into the unit; the inerting packaging adopts a vacuuming and nitrogen-filling replacement method, and the oxygen volume fraction in the packaging bag is not higher than 1%.

[0013] Preferably, the dust-laden exhaust gas emitted by the entire system is recovered in two stages: a cyclone separator and a cartridge dust collector, with a total iron powder recovery rate of not less than 99%.

[0014] Preferably, the obtained fuel-grade ultrafine iron powder has a BET specific surface area of ​​0.05~0.5m² / g, a loose packing density of 0.8~1.8g / cm³, a metallization rate of not less than 90%, and an auto-ignition temperature of not less than 180℃. Beneficial effects

[0015] Compared with the prior art, the present invention has the following beneficial effects: 1. Full-process inert protection: Crushing, grading, conveying and packaging are all carried out under nitrogen protection with an oxygen volume fraction of no more than 2%. The crushing working medium is recycled after dust removal, oxygen removal and water removal. The circulating nitrogen oxygen volume fraction is no more than 0.5% and the dew point is no more than -20℃, which effectively inhibits the oxidation of the newly formed surface during the crushing process. The metallization rate of the product is no less than 90%, and the metallization rate loss compared with the raw material is no more than 2 percentage points. 2. Controllable particle size and narrow distribution: The air jet mill and turbine classifier form a closed loop. The speed of the classifier wheel and the feed rate are adjusted in a closed loop by an online particle size analyzer and controller. The output D50 is stably controlled at 8~20μm and the particle size distribution span is no more than 2.0, which is conducive to improving the combustion rate and burnout rate and stabilizing the flame. 3. Passivation does not introduce heterogeneous elements: Low-temperature, short-time, controllable passivation is used to generate a 5-20nm Fe3O4 passivation layer in situ on the particle surface, without introducing impurities such as phosphorus, chromium, and carbon. The passivation product itself is an iron oxide, which does not affect the material balance and zero-carbon properties of the metal fuel cycle. The product's auto-ignition temperature is not lower than 180℃, and the safety of storage and transportation is significantly improved. 4. Inhibit agglomeration and improve flowability: The passivation layer blocks direct metal contact between particles. Combined with low-temperature nitrogen drying (moisture content not higher than 0.1%), the product has a loose density of 0.8~1.8g / cm³, good flowability, and is convenient for pneumatic conveying and boiler feeding. 5. Process safety and controllability: The conveying and packaging units are equipped with dual parameter interlocking of oxygen volume fraction and temperature. If the limit is exceeded, the conveying will be cut off and nitrogen will be automatically filled. After inerting packaging, the oxygen volume fraction in the bag will not be higher than 1%, eliminating the risk of combustion and explosion from both process and equipment aspects. 6. High resource utilization: Dust-laden exhaust gas is recovered in two stages through a cyclone separator and a cartridge dust collector, with a total iron powder recovery rate of no less than 99%. Nitrogen is recycled, resulting in low operating costs and suitability for large-scale continuous production. Attached Figure Description

[0016] Figure 1 This is a schematic diagram of the process flow for preparing fuel-grade ultrafine iron powder using hydrogen-based reduced iron powder according to the present invention.

[0017] Explanation of reference numerals in the attached drawings: 1-Closed feeding unit; 2-Air jet mill; 3-Turbine classifier; 4-Fluidized bed passivator; 5-Closed conveying and inerting packaging unit; 6-Cyclone separator; 7-Cartridge dust collector; 8-Nitrogen purification and circulation unit; 9-Online particle size analyzer; 10-Controller; 11-Oxygen volume fraction sensor; 12-Temperature sensor. Detailed Implementation

[0018] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. The following embodiments are only for illustrating the present invention and are not intended to limit the scope of protection of the present invention; all equivalent substitutions or improvements made without departing from the essence of the technical solution of the present invention fall within the scope of protection of the present invention.

[0019] The raw materials used in each embodiment and comparative example were reduced iron powder obtained by hydrogen-based vertical shaft furnace reduction, with a particle size of 50~150μm, a metallization rate of 93.5%, and a moisture content of 0.08%; operations without specific conditions were carried out according to conventional conditions in the art.

[0020] Detection methods: The metallization rate was calculated after determining the metallic iron content by potassium dichromate titration; the auto-ignition temperature was determined by thermogravimetric-differential thermal analysis in air atmosphere at a heating rate of 5℃ / min, with the exothermic peak onset temperature as the auto-ignition temperature; the BET specific surface area was determined by low-temperature nitrogen adsorption method; the loose packing density was determined by standard funnel method; and the passivation layer thickness was determined by observing the particle cross-section using transmission electron microscopy. Example 1 (1) Closed feeding: Under a nitrogen atmosphere with an oxygen volume fraction of 1.2%, the reduced iron powder is fed into the air jet mill (2) through the closed feeding unit (1). (2) Airflow pulverization: Circulating nitrogen gas that has been treated to remove dust, oxygen and water is used as the pulverizing medium. The circulating nitrogen gas has an oxygen volume fraction of 0.3%, a dew point of -32℃, a pulverizing pressure of 0.6MPa, and a classifier speed of 4000r / min. (3) Turbine classification: The output D50 is 12.5μm, and the particle size distribution span (D90-D10) / D50 is 1.6. The coarse particles from the classification are returned to the air jet mill (2); the online particle size analyzer (9) feeds back the measured D50 to the controller (10), and the controller (10) adjusts the speed of the classifying wheel and the feeding rate to stabilize the output D50 at 12~13μm; (4) Surface passivation: After the powder is dried to a moisture content of 0.05% by low temperature nitrogen, it enters the fluidized bed passivator (4). The apparent gas velocity is 0.2 m / s. The jacket cooling water keeps the bed temperature at 60℃. A nitrogen-oxygen mixture with an oxygen volume fraction of 1.0% is introduced and held for 6 min. (5) Closed transport and inerting packaging: The oxygen volume fraction is 1.5% and the temperature is 45℃. The packaging is carried out in a closed manner and the packaging is carried out by vacuuming and nitrogen replacement. The oxygen volume fraction in the packaging bag is 0.6%. The interlocking thresholds of the oxygen volume fraction sensor (11) and the temperature sensor (12) are set to 2% and 60℃, respectively. The resulting product has the following characteristics: D50 of 12.5 μm, passivation layer thickness of 12 nm, BET specific surface area of ​​0.21 m² / g, loose packing density of 1.35 g / cm³, metallization rate of 91.8%, and auto-ignition temperature of 215℃; the total iron powder recovery rate of the entire system is 99.3%. Example 2

[0021] The process steps of Example 1 were implemented, with the following differences: crushing pressure 0.4 MPa, classifying wheel speed 2000 r / min; passivation temperature 40℃, oxygen volume fraction of nitrogen-oxygen mixture 0.5%, and residence time 10 min.

[0022] The resulting product has the following characteristics: D50 of 26.4 μm, particle size distribution span of 1.8, passivation layer thickness of 5 nm, BET specific surface area of ​​0.06 m² / g, loose packing density of 1.76 g / cm³, metallization rate of 92.4%, and auto-ignition temperature of 186℃; the total iron powder recovery rate of the entire system is 99.1%. Example 3

[0023] The process steps of Example 1 were implemented, with the following differences: crushing pressure 0.8 MPa, classifying wheel speed 6000 r / min; passivation temperature 80℃, oxygen volume fraction of nitrogen-oxygen mixture 2.0%, residence time 3 min.

[0024] The resulting product has the following characteristics: D50 of 8.5 μm, particle size distribution span of 1.9, passivation layer thickness of 19 nm, BET specific surface area of ​​0.46 m² / g, loose packing density of 0.86 g / cm³, metallization rate of 90.3%, and auto-ignition temperature of 238℃; the total iron powder recovery rate of the entire system is 99.5%. Comparative Example 1

[0025] The process steps of Example 1 are implemented, except that the surface passivation in step (4) is cancelled, and the material is directly conveyed and packaged in a sealed manner after crushing and grading.

[0026] The resulting product has a D50 of 12.8 μm, a BET specific surface area of ​​0.24 m² / g, a loose density of 1.28 g / cm³, and a metallization rate of 92.0%. However, its auto-ignition temperature is only 62℃, and it clumps after being stored at room temperature for 72 hours after packaging, failing to meet the safety requirements for storage and transportation. Comparative Example 2

[0027] The process steps of Example 1 were implemented, except that: a nitrogen-oxygen mixture with an oxygen volume fraction of 5.0% was introduced during passivation, the passivation temperature was 80°C, and the residence time was 15 minutes.

[0028] The resulting product has a passivation layer thickness of 46nm and an auto-ignition temperature of 265℃, but the metallization rate drops to 82.6%, which is lower than the requirement of no less than 90% for fuel-grade iron powder, and the calorific value of combustion decreases significantly. Comparative Example 3

[0029] The process steps of Example 1 are implemented, except that the working fluid for air jet milling is changed to nitrogen gas that has not been deoxygenated or dehydrated, with an oxygen volume fraction of 3.5% and a dew point of -5°C.

[0030] The resulting product had a D50 of 13.2 μm, exhibited significant powder agglomeration, a metallization rate reduced to 84.1%, an auto-ignition temperature of 190℃, and a total iron powder recovery rate of 97.2%. Table 1. Performance comparison of Examples 1-3 and Comparative Examples 1-3 project D50 / μm span passivation layer thickness / nm BET / (m² / g) Loose bulk density (g / cm³) Metallization rate / % Autoignition temperature / °C Recovery rate / % Example 1 12.5 1.6 12 0.21 1.35 91.8 215 99.3 Example 2 26.4 1.8 5 0.06 1.76 92.4 186 99.1 Example 3 8.5 1.9 19 0.46 0.86 90.3 238 99.5 Comparative Example 1 12.8 1.7 0 (Not passivated) 0.24 1.28 92.0 62 99.3 Comparative Example 2 12.6 1.7 46 0.19 1.31 82.6 265 99.2 Comparative Example 3 13.2 1.8 12 0.22 1.33 84.1 190 97.2

[0031] As shown in Table 1, Examples 1-3 maintained a metallization rate of no less than 90% while having an auto-ignition temperature of no less than 180°C, thus meeting both combustion performance and storage and transportation safety requirements. Comparative Example 1, without passivation, had an auto-ignition temperature of only 62°C. Comparative Example 2 was excessively passivated, resulting in a metallization rate of 82.6%. Comparative Example 3 had excessive oxygen and water content in the pulverizing medium, leading to a metallization rate of 84.1% and agglomeration. These results demonstrate that the range of process parameters in this invention is a necessary condition for achieving its technical effects.

Claims

1. A method for preparing fuel-grade ultrafine iron powder from hydrogen-based reduced iron powder, characterized in that, Includes the following steps: S1 Closed feeding: Under an inert protective atmosphere with an oxygen volume fraction of not more than 2%, the reduced iron powder obtained by hydrogen-based reduction is fed into the closed feeding unit. The particle size of the reduced iron powder is 50~150μm and the metallization rate is not less than 92%. S2 Airflow milling: Reduced iron powder is fed into an airflow mill, using nitrogen as the milling medium, and is subjected to ultrafine milling under the conditions of milling pressure of 0.4~0.8MPa and classifier speed of 2000~6000r / min to obtain powder with D50 of 5~30μm; S3 Turbine Classification: The pulverized powder is fed into a turbine classifier for classification. The particle size distribution range (D90-D10) / D50 is controlled to be no greater than 2.

0. The coarse particles classified are returned to step S2. S4 Surface passivation: The graded powder is fed into a passivator, and a nitrogen-oxygen mixture with an oxygen volume fraction of 0.5% to 2.0% is introduced at 40 to 80°C and held for 3 to 10 minutes to generate a passivation layer with a thickness of 5 to 20 nm and the main component of Fe3O4 on the particle surface. S5 Closed Conveying and Inerting Packaging: The passivated powder is conveyed and inerted under closed conditions with an oxygen volume fraction not exceeding 2% and a temperature not exceeding 60℃.

2. The method according to claim 1, characterized in that, In step S2, the nitrogen working medium of the air jet mill is recycled after being treated for dust removal, deoxygenation and dehydration. The oxygen volume fraction in the recycled nitrogen is not higher than 0.5% and the dew point is not higher than -20℃.

3. The method according to claim 1, characterized in that, Steps S2 and S3 constitute a closed-loop crushing and classification circuit. The controller adjusts the speed of the classifying wheel and the feeding rate according to the D50 deviation measured by the online particle size analyzer, so that the output D50 is maintained at 8~20μm.

4. The method according to claim 1, characterized in that, The passivator mentioned in step S4 is a fluidized bed passivator with an apparent gas velocity of 0.1~0.3m / s. Cooling water is introduced into the passivator jacket to maintain the bed temperature at 40~80℃.

5. The method according to claim 1, characterized in that, A drying process is also provided before step S4 to ensure that the moisture content of the powder entering the passivator is not higher than 0.1%. The drying heat source is low-temperature nitrogen gas after heat exchange.

6. The method according to claim 1, characterized in that, In step S5, the conveying and packaging unit is equipped with an oxygen volume fraction sensor and a temperature sensor, and is interlocked with the nitrogen replenishment pipeline: when the oxygen volume fraction exceeds 2% or the temperature exceeds 60°C, the conveying is cut off and nitrogen is introduced into the unit; the inerting packaging adopts a vacuuming and nitrogen-filling replacement method, and the oxygen volume fraction in the packaging bag is not higher than 1%.

7. The method according to claim 1, characterized in that, The dust-laden exhaust gas emitted by the entire system is recovered in two stages: a cyclone separator and a cartridge dust collector, with a total iron powder recovery rate of no less than 99%.

8. The method according to any one of claims 1 to 7, characterized in that, The obtained fuel-grade ultrafine iron powder has a BET specific surface area of ​​0.05~0.5m² / g, a loose packing density of 0.8~1.8g / cm³, a metallization rate of not less than 90%, and an auto-ignition temperature of not less than 180℃.

9. A fuel-grade ultrafine iron powder, characterized in that, It is prepared by the method according to any one of claims 1 to 8.

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