Method for converting nitrogen atomized aluminum surface passivation layer crystal structure into gamma-type aluminum oxide

Abstract

The invention discloses a method for converting a nitrogen atomized aluminum surface passivation layer crystal structure into gamma-type aluminum oxide. The method comprises the following steps that S1, nitrogen atomized aluminum powder with the median particle size being 1-3 microns is selected; s2, adding the selected nitrogen atomized aluminum powder into a kneading machine with heating and stirring functions; s3, a hydrogen peroxide solution with the mass fraction concentration being 30% is sprayed to the nitrogen atomized aluminum powder in the kneading machine through a spraying device; s4, an acidic aqueous solution with the PH value being 3-5 is sprayed to the nitrogen atomized aluminum powder in the kneading machine through a spraying device; s5, after spraying is completed, a top cover of the kneading machine is closed, the nitrogen atomized aluminum powder is continuously stirred at a constant speed, and nitrogen is continuously introduced into a cavity in the top of the kneading machine in the stirring process; s6, after stirring is completed, natural cooling is conducted. The technology adopted by the method is advanced and reasonable, the nitrogen atomized aluminum powder with the surface passivation layer crystal form structure being alpha type aluminum oxide is converted into the nitrogen atomized aluminum powder with the surface passivation layer crystal form structure being gamma type aluminum oxide, and the method is suitable for batch production.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of aluminum oxide powder preparation, and particularly relates to a method for converting the crystal structure of a passivation layer on the surface of nitrogen-atomized aluminum into gamma aluminum oxide. BACKGROUND

[0002] Nitrogen-atomized aluminum powder is spherical aluminum powder. The nitrogen-atomized aluminum powder has the advantages of high sphericity, few impurities, high active aluminum content, uniform particle size, narrow distribution range, and high tap density. The nitrogen-atomized aluminum powder is widely used in many fields, including national defense, aerospace, chemical industry, metallurgy, coating, photovoltaic, etc. Due to its high quality and fine spherical characteristics, the nitrogen-atomized aluminum powder is more suitable for applications with high requirements, such as high-quality fine spherical aluminum powder, aluminum powder with high active aluminum content, low oxygen content, and uniform particle size. In addition, the nitrogen-atomized aluminum powder can also be applied to the solar photovoltaic industry, high-quality coatings, and military solid rockets. The preparation process of the nitrogen-atomized aluminum powder is as follows: aluminum ingots are used as raw materials, and the aluminum powder is atomized and manufactured through a special process under the protection of a spraying medium. The spraying medium is high-purity nitrogen N2 separated from air, and the purity is greater than 99%. During the atomization production process, the trace amount of oxygen in the high-purity nitrogen reacts with the aluminum powder at a high temperature in an instant at the spraying port to produce nitrogen-atomized aluminum powder. A very thin passivation layer is produced on the surface of the nitrogen-atomized aluminum powder produced by this method. This oxidation layer is a protective layer with the crystal structure of alpha aluminum oxide on the surface passivation layer. The main function of the protective layer of alpha aluminum oxide is to prevent the aluminum powder from being re-oxidized during storage and transportation. At present, porous nitrogen-atomized aluminum powder with a specific surface area greater than 2.0 m 2 / g is required for manufacturing containers for storing nuclear waste. The nitrogen-atomized aluminum powder is required to have strong adsorption capacity, and the crystal structure of the surface passivation layer is gamma aluminum oxide. However, under normal circumstances, for safety reasons, the passivation layer of the ordinary nitrogen-atomized aluminum powder produced has the crystal structure of alpha aluminum oxide. The specific surface area of the passivation layer with the crystal structure of alpha aluminum oxide is generally about 1.6 m 2 / g, and the maximum oxygen content is not more than 0.7%. Obviously, the ordinary nitrogen-atomized aluminum powder cannot meet the material requirements for manufacturing containers for storing nuclear waste. From the production capacity of the nitrogen-atomized aluminum powder with the protective layer of the surface passivation layer with the crystal structure of alpha aluminum oxide produced by the company, it is inevitable that the nitrogen-atomized aluminum powder with the protective layer of the surface passivation layer with the crystal structure of gamma aluminum oxide will be an advanced and efficient technical means. Therefore, it is an objective need to develop a method for converting the crystal structure of the surface passivation layer of nitrogen-atomized aluminum into gamma aluminum oxide, which has advanced technology and can significantly improve the specific surface area of nitrogen-atomized aluminum powder and be mass-produced. SUMMARY

[0003] In order to solve the problems in the background art, the purpose of the present application is to provide a method for converting the crystal structure of the passivation layer on the surface of nitrogen-atomized aluminum into gamma-alumina, which is advanced in process technology, can significantly increase the specific surface area of nitrogen-atomized aluminum powder, and can be mass-produced.

[0004] The method for converting the crystal structure of the passivation layer on the surface of nitrogen-atomized aluminum into gamma-alumina according to the present application comprises the following steps: S1, selecting nitrogen-atomized aluminum powder with a median particle size of 1-3 μm, the original oxygen content of the nitrogen-atomized aluminum powder being 0.30-0.35%, and the specific surface area being 1.5-1.75 m 2 / g; S2: adding the nitrogen-atomized aluminum powder selected in step S1 into a kneader with heating and stirring functions, preheating the nitrogen-atomized aluminum powder to 80-100℃, and heating for 20-40 min; S3: spraying a hydrogen peroxide solution with a mass fraction concentration of 30% onto the nitrogen-atomized aluminum powder in the kneader using a spraying device, stirring while spraying, the mass ratio of the sprayed hydrogen peroxide solution to the nitrogen-atomized aluminum powder being 0.4-0.6:100, the spraying and stirring time being 5-6 min, the stirring speed of the kneader impeller being 20-30 r / min, and the surface temperature of the nitrogen-atomized aluminum powder being continuously controlled at 80-100℃ during the stirring process; S4: spraying an acidic aqueous solution with a pH value of 3-5 onto the nitrogen-atomized aluminum powder in the kneader using a spraying device, stirring while spraying, the mass ratio of the sprayed acidic aqueous solution to the nitrogen-atomized aluminum powder being 0.1-0.2:100, the spraying and stirring time being 8-10 min, the stirring speed of the kneader impeller being 20-30 r / min, and the surface temperature of the nitrogen-atomized aluminum powder being continuously controlled at 80-100℃ during the stirring process; S5: after the spraying is completed, closing the kneader top cover, continuously stirring the nitrogen-atomized aluminum powder at a uniform speed for 1-1.5 h, the stirring speed of the kneader impeller being 20-30 r / min, the surface temperature of the nitrogen-atomized aluminum powder being continuously controlled at 80-100℃, continuously introducing nitrogen gas into the cavity at the top of the kneader during the stirring process, and an exhaust port being provided on the kneader top cover; S6: after the stirring is completed, naturally cooling, thereby obtaining nitrogen-atomized aluminum powder with a passivation layer crystal structure of gamma-alumina on the surface.

[0005] The beneficial effects of the device are: the device sprays hydrogen peroxide solution on the nitrogen atomized aluminum powder with the surface passivation layer crystal structure of alpha type aluminum oxide according to the physical properties of the nitrogen atomized aluminum powder with the surface passivation layer crystal structure of alpha type aluminum oxide, the hydrogen peroxide solution can have a controllable oxidation reaction with the nitrogen atomized aluminum powder with the surface passivation layer crystal structure of alpha type aluminum oxide, the hydrogen peroxide solution can have an oxidation reaction with the nitrogen atomized aluminum powder with the surface passivation layer crystal structure of alpha type aluminum oxide to generate aluminum hydroxide, then the acid aqueous solution is sprayed on the aluminum hydroxide generated by the oxidation reaction, the acid aqueous solution is used for dehydrating and decomposing the aluminum hydroxide, the crystal structure of the surface passivation layer of alpha type aluminum oxide is converted, and the nitrogen atomized aluminum powder with the surface passivation layer crystal structure of alpha type aluminum oxide is dehydrated and decomposed to convert into the nitrogen atomized aluminum powder with the surface passivation layer crystal structure of gamma type aluminum oxide. 2 / g above this technical bottleneck, the specific surface area of the nitrogen atomized aluminum powder with the surface passivation layer crystal structure of gamma type aluminum oxide is significantly improved, and the chemical reaction process is easy to operate and control, the chemical reaction conversion process does not bring in additional impurity chemicals, which can effectively ensure the product quality of the nitrogen atomized aluminum powder with the surface passivation layer crystal structure of gamma type aluminum oxide after conversion, is suitable for batch production, and is easy to popularize and use. BRIEF DESCRIPTION OF DRAWINGS

[0006] Figure 1 is a front view of the present application; Figure 2 is a side view of the present application; In the figure: 1- frame, 2- kneader body, 3- stirring impeller, 4- gear transmission mechanism, 5- machine cover, 6- hydraulic opening and closing mechanism, 7- nitrogen inlet, 8- emptying port, 9- heating jacket, 10- reinforcing connecting rod, 11- heating medium inlet, 12- heating medium outlet, 13- blocking plate, 14- lifting cylinder, 15- receiving groove, 16- discharge chute, 17- screw discharge machine. DETAILED DESCRIPTION

[0007] The application will be further described in connection with the embodiments and drawings below, but not in any way limited to the application, any transformation or replacement based on the teaching of the application, all embodiments belong to the protection scope of the application. Example 1

[0008] The method for converting the crystal structure of the passivation layer on the surface of nitrogen-atomized aluminum into γ-alumina in Example 1 includes the following steps: S1, select nitrogen-atomized aluminum powder with a median particle size of 1 μm, the original oxygen content of the nitrogen-atomized aluminum powder is 0.30%, the specific surface area is 1.5 m 2 / g, the selected nitrogen-atomized aluminum powder is a nitrogen-atomized aluminum powder with a passivation layer crystal structure of α-alumina, the nitrogen-atomized aluminum powder with a passivation layer crystal structure of α-alumina is a high-hardness, high-melting-point compound with good corrosion resistance and stability, which can effectively protect the aluminum powder from the influence of the external environment and ensure its stable performance for a long time; S2: add the nitrogen-atomized aluminum powder selected in step S1 to a kneader with heating and stirring functions, preheat the nitrogen-atomized aluminum powder to 80℃, the heating time is 20 min, the kneader is heated before the nitrogen-atomized aluminum powder is added, and the nitrogen-atomized aluminum powder needs to be stirred when it is added to the kneader and heated; S3: use a spraying device to spray a hydrogen peroxide solution with a mass fraction concentration of 30% to the nitrogen-atomized aluminum powder in the kneader, stir while spraying, the mass ratio of the sprayed hydrogen peroxide solution to the nitrogen-atomized aluminum powder is 0.4:100, the stirring time is 5-6 min, the stirring speed of the kneader impeller is 20 r / min, and the surface temperature of the nitrogen-atomized aluminum powder is continuously controlled at 80℃ during the stirring process, the hydrogen peroxide solution is sprayed to the nitrogen-atomized aluminum powder with a passivation layer crystal structure of α-alumina, the hydrogen peroxide solution can have a controllable oxidation reaction with the nitrogen-atomized aluminum powder with a passivation layer crystal structure of α-alumina, by controlling the spraying amount, stirring time, and stirring speed, the hydrogen peroxide solution can react with the nitrogen-atomized aluminum powder with a passivation layer crystal structure of α-alumina to generate aluminum hydroxide, the oxidation reaction chemical equation is: 2Al + 6H2O → 2Al(OH)3↓ + 3H2↑, the spraying device used for the hydrogen peroxide solution can adopt the structure of a mobile sprayer used in the prior art; S4: spray the acid aqueous solution with a pH value of 3 to the nitrogen atomized aluminum powder in the kneader using a spraying device, and stir while spraying, the mass ratio of the acid aqueous solution sprayed to the nitrogen atomized aluminum powder is 0.1:100, the time of spraying and stirring is 8 min, the stirring speed of the kneader stirring impeller is 20 r / min, the surface temperature of the nitrogen atomized aluminum powder is continuously controlled at 80℃ during stirring, the acid aqueous solution with a pH value of 3-5 is an aqueous solution prepared by mixing an organic weak acid with water, and the organic weak acid is formic acid; spray the aluminum hydroxide generated by the oxidation reaction with the acid aqueous solution, use the acid aqueous solution to dehydrate and decompose the aluminum hydroxide, promote the crystal structure of the surface passivation layer to be transformed from the crystal structure of the α-type aluminum oxide to the crystal structure of the γ-type aluminum oxide, make the nitrogen atomized aluminum powder with the surface passivation layer of the α-type aluminum oxide dehydrate and decompose to be transformed into the nitrogen atomized aluminum powder with the surface passivation layer of the γ-type aluminum oxide, after dehydration and decomposition, the oxygen content of the nitrogen atomized aluminum powder with the surface passivation layer of the γ-type aluminum oxide is ≥1.10%, the water content is less than 1000 PPM, and the specific surface area increases to 2.1 m 2 / g or more, and the chemical reaction equation is 2Al(OH)3→3Al2O3+3H2O, and the spraying device used by the acid aqueous solution can adopt the structure of the mobile sprayer used in the prior art; S5: after spraying, close the kneader top cover, continuously and uniformly stir the nitrogen atomized aluminum powder for 1 h, the stirring speed of the kneader stirring impeller is 20 r / min, the surface temperature of the nitrogen atomized aluminum powder is continuously controlled at 80℃, and nitrogen gas is continuously introduced into the top cavity of the kneader during stirring, the purpose of introducing nitrogen gas is to achieve surface inert gas covering protection and play a role in safe production, the pressure of the introduced nitrogen gas is 0.2 Mpa, and the flow rate is 5 m 3 / h, an exhaust port is arranged on the kneader top cover, the gas generated by the dehydration and decomposition of the aluminum hydroxide is discharged from the kneader through the exhaust port, a control valve is arranged on the exhaust port, and the exhaust amount can be adjusted conveniently; S6: after stirring, naturally cool, and the nitrogen atomized aluminum powder with the surface passivation layer of the γ-type aluminum oxide is obtained.

[0009] In the above steps S4 and S5, the surface temperature of the nitrogen atomized aluminum powder in the kneader is continuously controlled at 80℃, which is mainly to increase the core temperature of the nitrogen atomized aluminum powder, so that the core temperature of the nitrogen atomized aluminum powder exceeds 130℃, and the nitrogen atomized aluminum powder with the surface passivation layer of the α-type aluminum oxide will be decomposed and transformed at a temperature higher than 130℃ to form the nitrogen atomized aluminum powder with the surface passivation layer of the γ-type aluminum oxide.

[0010] Further, in order to improve the production efficiency of nitrogen atomized aluminum powder in the kneader and ensure the quality of nitrogen atomized aluminum powder production, in step S2, the kneader with heating and stirring function includes a rack 1, a kneader body 2 and stirring impellers 3, the kneader body 2 is installed on the rack 1, the stirring impellers 3 are two groups, and the two groups of stirring impellers 3 are rotatably installed in the kneader body 2, a gear rotating mechanism 4 is installed on the rack 1 outside the kneader body 2 and is in transmission connection with the two groups of stirring impellers 3, the stirring impeller 3 is in the structure of Z-shaped stirring paddle, the Z-shaped stirring paddle includes Z-shaped paddle blades and connecting shafts installed at both ends of the Z-shaped paddle blades, the connecting shafts are rotatably installed on the kneader body 2, the gear transmission mechanism 4 is in the structure used in the prior art and includes a motor, a speed reducer, a driving gear and a driven gear and other components, the speed reducer is installed on the output shaft of the motor, the output shaft of the speed reducer is in transmission connection with the connecting shaft of one group of Z-shaped stirring paddles through a shaft coupling, the driving gear is installed on the connecting shaft of one group of Z-shaped stirring paddles, the driven gear is installed on the connecting shaft of the other group of Z-shaped stirring paddles, the driving gear and the driven gear are in meshing with each other, when the nitrogen atomized aluminum powder needs to be stirred, the motor drives the driving gear to rotate through the speed reducer, the driving gear drives the driven gear to rotate, and then the two groups of stirring impellers can be driven to rotate in opposite directions, and then the nitrogen atomized aluminum powder can be stirred, so that the nitrogen atomized aluminum powder can be fully mixed with the hydrogen peroxide solution or the acidic aqueous solution, a heating assembly is arranged outside the kneader body 2, the heating assembly can heat the kneader body 2, so that the reaction temperature of the nitrogen atomized aluminum powder in the oxidation or dehydration decomposition process reaches the set requirement, a machine cover 5 is movably arranged at the top of the kneader body 2, a hydraulic opening and closing mechanism 6 is installed on the rack 1 and is connected with the machine cover 5, the hydraulic opening and closing mechanism 6 can drive the machine cover 5 to be opened or closed, in the process of oxidation or dehydration decomposition of the nitrogen atomized aluminum powder, the hydraulic drive mechanism 6 controls the machine cover 5 to be in the opened state, when the nitrogen atomized aluminum powder is oxidized or dehydrated and reacts, nitrogen needs to be introduced for nitrogen conversion, the hydraulic drive mechanism 6 controls the machine cover 5 to be closed on the kneader body 2, the hydraulic opening and closing mechanism 6 is in the structure used in the prior art and mainly includes a connecting column, a support and a hydraulic cylinder, one end of the connecting column is fixedly installed on the surface of the machine cover 5, the support is installed outside the kneader body 2, the connecting column is rotatably installed on the support, the fixed end of the hydraulic cylinder is installed on the machine base, and the movable end of the hydraulic cylinder is hingedly connected with the other end of the connecting column, by controlling the movable end of the hydraulic cylinder to be elongated or shortened, the opening or closing of the machine cover 5 can be controlled, a nitrogen inlet 7 and a vent 8 are arranged on the machine cover 5, a pipe joint is arranged at the end of the nitrogen inlet 7, the nitrogen inlet 7 is connected with a nitrogen storage tank, nitrogen can be conveyed into the kneader body 2 through the nitrogen inlet 7, so as to ensure the conversion of the nitrogen atomized aluminum powder, and the vent 8 can timely release pressure when the pressure in the kneader body 2 is high.The nitrogen inlet 7 and the exhaust port 8 are provided with control valves, and the kneader body 1 is provided with an air pressure sensor, which is a structure used in the prior art and is directly purchased according to the requirements of use. The air pressure sensor is not shown in the figure. The air pressure sensor can monitor the air pressure condition inside the kneader body 1 in real time, ensure the accuracy and efficiency of the nitrogen protection process, provide continuous air pressure data, so that the operator can timely adjust the input amount and discharge rate of nitrogen, thereby maintaining the ideal air pressure environment, avoiding oxidation or other adverse reactions caused by improper air pressure, and also helping to save energy and reduce emissions, optimize production cost, and the bottom of the kneader body 2 is provided with an automatic discharging mechanism, which can automatically discharge from the bottom of the kneader body 2 after the nitrogen atomized aluminum powder conversion is completed, thereby improving the thoroughness and efficiency of discharging.

[0011] Preferably, in order to ensure that the heating temperature of the kneader body 2 is uniform during the aluminum powder oxidation reaction process, the heating assembly comprises a heating jacket 9 arranged at intervals outside the kneader body 2, and a plurality of reinforcing connecting rods 10 are arranged between the heating jacket 9 and the kneader body 2. The upper side of one side of the heating jacket 9 is provided with a heating medium inlet 11, and the bottom of the other side is provided with a heating medium outlet 12. The heating medium inlet 11 is used to add heating medium into the heating jacket 9. The heating medium can be steam, heat conducting oil, etc. After the heating medium enters the heating jacket 9, the heating medium flows in the heating jacket 9, which can heat the kneader body 2. With the flow of the heating medium, the heated heating medium is discharged from the heating medium outlet 12. The heating jacket 9 is provided with a temperature sensor, and the inside of the heating jacket 9 is provided with a plurality of electric heating rods. In use, the temperature sensor can timely monitor the heating temperature of the kneader body 2. If the temperature of the kneader body 2 is lower than the set standard temperature, the electric heating rod can quickly heat the kneader body 2 to improve the heating temperature of the kneader body 2 in a short time. The temperature sensor is a structure used in the prior art and is directly purchased according to the power used.

[0012] Preferably, the automatic discharging mechanism comprises a sealing plate 13, a lifting cylinder 14 and a receiving groove 15, the bottom of the kneader body 2 is processed with a strip-shaped discharging port, the receiving groove 15 is arranged outside the strip-shaped discharging port at the bottom of the kneader body 2, the sealing plate 13 is slidingly arranged in the strip-shaped discharging port, the lifting cylinder 14 is a structure used in the prior art, and a finished product can be directly purchased according to the power size used. The lifting cylinder 14 is 1-2, the fixed end of the lifting cylinder 14 is mounted on the rack 1, the movable end of the lifting cylinder 14 is fixedly connected with the sealing plate 13 after extending into the receiving groove 15, the outside of the sealing plate 13 is provided with a sealing ring, the two sides of the receiving groove 15 are provided with discharging chutes 16, the end of the discharging chute 16 is provided with a spiral conveyor 17, the end of the spiral conveyor 17 is provided with a discharging port, the spiral conveyor 17 is a structure used in the prior art, when the transformed nitrogen atomized aluminum powder needs to be discharged, the lifting cylinder 14 is controlled to drive the sealing plate 13 to move downward, the sealing of the strip-shaped discharging port by the sealing plate 13 is released, when the sealing plate 13 moves to below the discharging chute inlet 16, the lifting cylinder 14 is controlled to stop running, the nitrogen atomized aluminum powder in the kneader body 2 falls on the sealing plate 13 under the action of the stirring impeller 3, and then enters the discharging chute 16 through the sealing plate 13, and finally is discharged through the spiral discharging machine 17. The sealing plate 13 can be provided with a slope structure with a middle high and two sides low, so that the nitrogen atomized aluminum powder is conveniently arranged to enter the discharging chute 16 from the two sides of the sealing plate 13. The sealing ring arranged outside the sealing plate 13 is used to enhance the sealing performance between the sealing plate 13 and the strip-shaped discharging port, so as to avoid the escape of nitrogen from the gap between the strip-shaped discharging port and the sealing plate 13. Example 2

[0013] The method for converting the crystal structure of the passivation layer on the surface of nitrogen atomized aluminum into γ-type aluminum oxide in the embodiment 2 comprises the following steps: S1, select nitrogen atomized aluminum powder with a median particle size of 2 μm, the original oxygen content of the nitrogen atomized aluminum powder is 0.33%, the specific surface area is 1.62 m 2 / g, the selected nitrogen atomized aluminum powder is nitrogen atomized aluminum powder with α-type aluminum oxide as the crystal structure of the surface passivation layer. The nitrogen atomized aluminum powder with α-type aluminum oxide as the crystal structure of the surface passivation layer is a high-hardness, high-melting-point compound, has good corrosion resistance and stability, can effectively protect the aluminum powder from the influence of the external environment, and ensures the stable performance of the aluminum powder for a long time; S2: add the nitrogen atomized aluminum powder selected in step S1 to a kneader with heating and stirring functions, preheat the nitrogen atomized aluminum powder to 90℃, the heating time is 30 min, the kneader is heated before the nitrogen atomized aluminum powder is added, and the nitrogen atomized aluminum powder needs to be stirred when the nitrogen atomized aluminum powder is heated in the kneader; S3: Spraying the hydrogen peroxide solution with a mass fraction concentration of 30% on the nitrogen atomized aluminum powder in the kneader by using a spraying device, stirring while spraying, the mass ratio of the sprayed hydrogen peroxide solution to the nitrogen atomized aluminum powder is 0.5:100, the spraying and stirring time is 6 min, the stirring speed of the kneader stirring impeller is 25 r / min, the surface temperature of the nitrogen atomized aluminum powder is continuously controlled at 90°C during stirring, the hydrogen peroxide solution is sprayed on the nitrogen atomized aluminum powder with the surface passivation layer of α-type aluminum oxide, the hydrogen peroxide solution can have a controllable oxidation reaction with the nitrogen atomized aluminum powder with the surface passivation layer of α-type aluminum oxide, by controlling the spraying amount, stirring time, stirring speed, etc., the hydrogen peroxide solution can react with the nitrogen atomized aluminum powder with the surface passivation layer of α-type aluminum oxide to generate aluminum hydroxide, the chemical equation of the oxidation reaction is: 2Al + 6H2O → 2Al(OH)3↓ + 3H2↑, the spraying device used by the hydrogen peroxide solution can adopt the structure of the mobile sprayer used in the prior art; S4: Spraying the acidic aqueous solution with a PH value of 4 on the nitrogen atomized aluminum powder in the kneader by using a spraying device, stirring while spraying, the mass ratio of the sprayed acidic aqueous solution to the nitrogen atomized aluminum powder is 0.15:100, the spraying and stirring time is 9 min, the stirring speed of the kneader stirring impeller is 25 r / min, the surface temperature of the nitrogen atomized aluminum powder is continuously controlled at 90°C during stirring, the acidic aqueous solution with a PH value of 4 is an aqueous solution prepared by mixing an organic weak acid with water, and the organic weak acid is acetic acid; Spraying the aluminum hydroxide generated by the oxidation reaction with the acidic aqueous solution, dehydrating and decomposing the aluminum hydroxide by using the acidic aqueous solution, promoting the crystal structure transformation of the surface passivation layer of α-type aluminum oxide, and converting the nitrogen atomized aluminum powder with the surface passivation layer of α-type aluminum oxide into the nitrogen atomized aluminum powder with the surface passivation layer of γ-type aluminum oxide, after dehydration and decomposition, the oxygen content of the nitrogen atomized aluminum powder with the surface passivation layer of γ-type aluminum oxide is ≥1.10%, the water content is less than 1000 PPM, and the specific surface area increases to 2.1 m 2 / g or more, and the chemical reaction equation is 2Al(OH)3→3Al2O3+3H2O, the spraying device used by the acidic aqueous solution can adopt the structure of the mobile sprayer used in the prior art; S5: After spraying, close the kneader top cover, continuously and uniformly stir the nitrogen atomized aluminum powder for 1-1.5 h, the stirring speed of the kneader stirring impeller is 25 r / min, the surface temperature of the nitrogen atomized aluminum powder is continuously controlled at 90°C, and nitrogen gas is continuously introduced into the top cavity of the kneader during stirring, the purpose of introducing nitrogen gas is to achieve surface inert gas covering protection, and to play a role in safe production, the pressure of the introduced nitrogen gas is 0.3 Mpa, and the flow rate is 8 m 3The exhaust port is arranged on the kneader top cover, and the gas generated by the dehydration and decomposition of the aluminum hydroxide is discharged from the kneader through the exhaust port. A control valve is arranged on the exhaust port to facilitate the adjustment of the exhaust amount. S6: After the stirring is completed, the natural cooling is performed, and thus the nitrogen atomized aluminum powder with the surface passivation layer of the γ-type aluminum oxide is obtained.

[0014] In the steps S4 and S5, the surface temperature of the nitrogen atomized aluminum powder in the kneader is continuously controlled at 80℃, mainly to increase the core temperature of the nitrogen atomized aluminum powder, so that the core temperature of the nitrogen atomized aluminum powder exceeds 130℃. After the core temperature of the nitrogen atomized aluminum powder exceeds 130℃, the nitrogen atomized aluminum powder with the surface passivation layer of the α-type aluminum oxide is decomposed and converted at a temperature higher than 130℃, and the nitrogen atomized aluminum powder with the surface passivation layer of the γ-type aluminum oxide is formed.

[0015] The structure of the kneader with the heating and stirring functions used in this embodiment 2 is the same as that used in the embodiment 1, and the principle used is also the same, which is not described here. Embodiment 3

[0016] The method for converting the surface passivation layer of the nitrogen atomized aluminum to the γ-type aluminum oxide described in this embodiment 3 includes the following steps: S1: Select the nitrogen atomized aluminum powder with the median particle size of 3μm, the original oxygen content of the nitrogen atomized aluminum powder is 0.35%, and the specific surface area is 1.75m 2 / g. The selected nitrogen atomized aluminum powder is the nitrogen atomized aluminum powder with the surface passivation layer of the α-type aluminum oxide. The nitrogen atomized aluminum powder with the surface passivation layer of the α-type aluminum oxide is a high-hardness and high-melting-point compound, has good corrosion resistance and stability, can effectively protect the aluminum powder from the influence of the external environment, and ensure the stable performance of the aluminum powder for a long time. S2: Add the nitrogen atomized aluminum powder selected in the step S1 to the kneader with the heating and stirring functions, preheat the nitrogen atomized aluminum powder to 100℃, and heat for 40min. The kneader is heated before the nitrogen atomized aluminum powder is added. When the nitrogen atomized aluminum powder is added to the kneader for heating, the nitrogen atomized aluminum powder needs to be stirred. S3: spray the hydrogen peroxide solution with a mass fraction concentration of 30% to the nitrogen atomized aluminum powder in the kneader by using a spraying device, and stir while spraying, the mass ratio of the sprayed hydrogen peroxide solution to the nitrogen atomized aluminum powder is 0.6:100, the spraying and stirring time is 6 min, the stirring speed of the kneader stirring impeller is 30 r / min, the surface temperature of the nitrogen atomized aluminum powder is continuously controlled at 100°C during stirring, the hydrogen peroxide solution is sprayed to the nitrogen atomized aluminum powder with the surface passivation layer of α-type aluminum oxide, the hydrogen peroxide solution can have a controllable oxidation reaction with the nitrogen atomized aluminum powder with the surface passivation layer of α-type aluminum oxide, by controlling the spraying amount, stirring time, stirring speed, etc., the hydrogen peroxide solution can react with the nitrogen atomized aluminum powder with the surface passivation layer of α-type aluminum oxide to generate aluminum hydroxide, the chemical equation of the oxidation reaction is: 2Al + 6H2O → 2Al(OH)3↓ + 3H2↑, the spraying device used by the hydrogen peroxide solution can adopt the structure of a mobile sprayer used in the prior art; S4: spray the acidic aqueous solution with a pH value of 5 to the nitrogen atomized aluminum powder in the kneader by using a spraying device, and stir while spraying, the mass ratio of the sprayed acidic aqueous solution to the nitrogen atomized aluminum powder is 0.2:100, the spraying and stirring time is 10 min, the stirring speed of the kneader stirring impeller is 30 r / min, the surface temperature of the nitrogen atomized aluminum powder is continuously controlled at 100°C during stirring, the acidic aqueous solution with a pH value of 3-5 is an aqueous solution prepared by mixing an organic weak acid with water, and the organic weak acid is carbonic acid; the acidic aqueous solution is sprayed to the aluminum hydroxide generated by the oxidation reaction, and the acidic aqueous solution is used to dehydrate and decompose the aluminum hydroxide, so as to convert the crystal structure of the surface passivation layer from α-type aluminum oxide to γ-type aluminum oxide, and convert the nitrogen atomized aluminum powder with the surface passivation layer of α-type aluminum oxide into the nitrogen atomized aluminum powder with the surface passivation layer of γ-type aluminum oxide, after dehydration and decomposition, the oxygen content of the nitrogen atomized aluminum powder with the surface passivation layer of γ-type aluminum oxide is ≥1.10%, the water content is less than 1000 PPM, and the specific surface area increases to 2.1 m 2 / g or more, and the chemical reaction equation is: 2Al(OH)3 → 3Al2O3 + 3H2O, and the spraying device used by the acidic aqueous solution can adopt the structure of a mobile sprayer used in the prior art; S5: after the spraying is completed, the top cover of the kneader is closed, and the nitrogen atomized aluminum powder is continuously stirred at a constant speed for 1.5 h, the stirring speed of the kneader stirring impeller is 30 r / min, the surface temperature of the nitrogen atomized aluminum powder is continuously controlled at 100°C, and nitrogen gas is continuously introduced into the top cavity of the kneader during stirring, the purpose of introducing nitrogen gas is to achieve surface inert gas covering protection, and play a role in safe production, the pressure of the introduced nitrogen gas is 0.4 Mpa, and the flow rate is 10 m 3The exhaust port is arranged on the kneader top cover, and the gas generated by the dehydration and decomposition of the aluminum hydroxide is discharged from the kneader through the exhaust port. S6: After the stirring is completed, natural cooling is performed, and thus the nitrogen atomized aluminum powder with the surface passivation layer of the gamma type alumina is obtained.

[0017] In the above steps S4 and S5, the surface temperature of the nitrogen atomized aluminum powder in the kneader is continuously controlled at 80℃, mainly to increase the core temperature of the nitrogen atomized aluminum powder, so that the core temperature of the nitrogen atomized aluminum powder exceeds 130℃. After the core temperature of the nitrogen atomized aluminum powder exceeds 130℃, the nitrogen atomized aluminum powder with the surface passivation layer of the alpha type alumina will be decomposed and converted at a temperature higher than 130℃ to form the nitrogen atomized aluminum powder with the surface passivation layer of the gamma type alumina. The structure of the kneader with the heating and stirring functions used in this embodiment 3 is the same as that used in the embodiment 1, and the principle of use is also the same, which will not be described here.

[0018] In summary, the technologies used in the embodiments 1-3 are advanced and reasonable, and the chemical reaction breaking method is adopted, which not only realizes the conversion of the nitrogen atomized aluminum powder with the surface passivation layer of the alpha type alumina to the nitrogen atomized aluminum powder with the surface passivation layer of the gamma type alumina, but also breaks through the technical bottleneck that the specific surface area of the ordinary nitrogen atomized aluminum powder cannot reach 2.0m 2 / g, significantly improves the specific surface area of the nitrogen atomized aluminum powder with the surface passivation layer of the gamma type alumina, and the chemical reaction process is easy to operate and control. The chemical reaction conversion process does not bring in additional impurities, which can effectively ensure the product quality of the nitrogen atomized aluminum powder with the surface passivation layer of the gamma type alumina after conversion. The oxygen content of the nitrogen atomized aluminum powder with the surface passivation layer of the gamma type alumina prepared by the embodiments 1-3 is ≥1.10%, the moisture content is less than 1000PPM, and the specific surface area is increased to more than 2.1m 2 / g, has the advantages of good fluidity, good dispersibility, loose structure and large specific surface area, and can fully meet the use requirements of the neutron absorbing material used in the container for containing nuclear waste.

Claims

1. A method for converting the crystal structure of a passivation layer of nitrogen-atomized aluminum to gamma-alumina, comprising: The method comprises the following steps: S1, selecting nitrogen atomized aluminum powder with a median particle size of 1-3 μm, the original oxygen content of the nitrogen atomized aluminum powder being 0.30-0.35%, the specific surface area being 1.5-1.75 m 2 / g; S2: adding the nitrogen atomized aluminum powder selected in step S1 into a kneader with heating and stirring functions, preheating the nitrogen atomized aluminum powder to 80-100 DEG C, and heating for 20-40 min; S3: spraying a hydrogen peroxide solution with a mass fraction concentration of 30% into the nitrogen atomized aluminum powder in the kneader by using a spraying device, stirring while spraying, the mass ratio of the sprayed hydrogen peroxide solution to the nitrogen atomized aluminum powder being 0.4-0.6:100, the stirring time being 5-6 min, the stirring speed of the kneader stirring impeller being 20-30 r / min, and the surface temperature of the nitrogen atomized aluminum powder being continuously controlled at 80-100 DEG C during the stirring process; S4: spraying an acidic aqueous solution with a pH value of 3-5 into the nitrogen atomized aluminum powder in the kneader by using a spraying device, stirring while spraying, the mass ratio of the sprayed acidic aqueous solution to the nitrogen atomized aluminum powder being 0.1-0.2:100, the stirring time being 8-10 min, the stirring speed of the kneader stirring impeller being 20-30 r / min, and the surface temperature of the nitrogen atomized aluminum powder being continuously controlled at 80-100 DEG C during the stirring process; S5: after the spraying is completed, closing the kneader top cover, continuously uniformly stirring the nitrogen atomized aluminum powder for 1-1.5 h, the stirring speed of the kneader stirring impeller being 20-30 r / min, and the surface temperature of the nitrogen atomized aluminum powder being continuously controlled at 80-100 DEG C; and continuously introducing nitrogen into the cavity at the top of the kneader, and an exhaust port being arranged on the kneader top cover; S6: after the stirring is completed, naturally cooling, thereby obtaining the nitrogen atomized aluminum powder with a passivated layer of gamma-type aluminum oxide.

2. The method for converting the crystal structure of the passivation layer of nitrogen-atomized aluminum into γ-alumina according to claim 1, characterized in that: In step S4, the acidic aqueous solution with a pH value of 3-5 is an aqueous solution prepared by mixing an organic weak acid with water.

3. The method of claim 2, wherein the method is characterized by: The organic weak acid is any one of formic acid, acetic acid and carbonic acid.

4. The method of claim 1, wherein the method is characterized by: In step S5, the pressure of the nitrogen gas is 0.2-0.4 MPa, and the flow rate is 5-10 m 3 / h.

5. The method of claim 1, wherein the method is characterized by: In step S2, the kneader with heating and stirring functions comprises a frame (1), a kneader body (2) and stirring impellers (3), the kneader body (2) is installed on the frame (1), the two groups of stirring impellers (3) are rotatably installed in the interior of the kneader body (2), a gear rotating mechanism (4) is installed on the frame (1) outside the kneader body (2) and is in transmission connection with the two groups of stirring impellers (3), a heating assembly is arranged outside the kneader body (2), a machine cover (5) is movably arranged at the top of the kneader body (2), a hydraulic opening and closing mechanism (6) is installed on the frame (1) and is connected with the machine cover (5), a nitrogen inlet (7) and an exhaust port (8) are arranged on the machine cover (5), control valves are arranged on the nitrogen inlet (7) and the exhaust port (8), an air pressure sensor is arranged on the kneader body (1), and an automatic discharging mechanism is arranged at the bottom of the kneader body (2).

6. The method of claim 5, wherein the method is characterized by: The heating assembly comprises a heating jacket (9) arranged outside the kneader body (2) at intervals, a plurality of reinforcing connecting rods (10) are arranged between the heating jacket (9) and the kneader body (2), the upper part of one side of the heating jacket (9) is provided with a heating medium inlet (11), and the bottom of the other side is provided with a heating medium outlet (12).

7. The method of claim 6, wherein the method is characterized by: The heating jacket (9) is provided with a temperature sensor, and the inside of the heating jacket (9) is provided with a plurality of electric heating rods.

8. The method of claim 5, wherein the method is characterized by: The automatic discharging mechanism comprises a sealing plate (13), a lifting cylinder (14) and a receiving groove (15), the bottom of the kneader body (2) is processed with a strip-shaped discharging port, the receiving groove (15) is arranged outside the strip-shaped discharging port at the bottom of the kneader body (2), the sealing plate (13) is slidingly arranged in the strip-shaped discharging port, the lifting cylinder (14) is 1-2, the fixed end of the lifting cylinder (14) is mounted on the rack (1), the movable end of the lifting cylinder (14) is fixedly connected with the sealing plate (13) after extending into the receiving groove (15), the outside of the sealing plate (13) is provided with a sealing ring, the two sides of the receiving groove (15) are provided with discharging chutes (16), the end of the discharging chute (16) is mounted with a screw conveyor (17), and the end of the screw conveyor (17) is provided with a discharging port.

9. The method of claim 5, wherein the method is characterized by: The stirring impeller (3) is in the structure of a Z-shaped stirring paddle.

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