A system and method for using it to remove aluminum nitride by secondary aluminum ash washing

Abstract

The present invention discloses a system and a method for removing aluminum nitride from secondary aluminum ash. The system and combined process of air stripping and hydrothermal treatment of secondary aluminum ash are adopted. The air stripping improves the decomposition efficiency of aluminum nitride and can also strip ammonia for absorption and utilization, thereby removing most of the aluminum nitride in the secondary aluminum ash and significantly improving the removal rate of aluminum nitride in the secondary aluminum ash, achieving the process purpose of harmless treatment of secondary aluminum ash and realizing resource utilization of secondary aluminum ash. Compared with the traditional alkaline leaching method, the method of the present invention has a faster removal rate and a higher removal rate of aluminum nitride, improves the resource efficiency of secondary aluminum ash, saves processing costs, and solves the shortcomings of high water consumption, high energy consumption, and low aluminum nitride removal rate in the traditional wet method for removing aluminum nitride.

Description

1. Technical field

[0002] The invention relates to a pretreatment method for removing aluminum nitride by secondary aluminum ash water washing, and in particular to a system for removing aluminum nitride by secondary aluminum ash water washing and a use method thereof. 2. Background technology

[0004] Aluminum ash is a production waste generated in the process of molten salt electrolytic alumina and casting processing in the aluminum industry. According to the different aluminum content in aluminum ash, aluminum ash is divided into primary aluminum ash and secondary aluminum ash. Secondary aluminum ash is the waste generated after aluminum is recovered from primary aluminum ash by frying or pressing. The aluminum content in secondary aluminum ash is lower than that in primary aluminum ash, and it contains more harmful impurities, making it difficult to handle.

[0005] Every ton of aluminum alloy produced will produce 100-200 kg of aluminum ash. In recent years, my country's aluminum industry has developed rapidly, and the annual aluminum ash emissions from the production of aluminum alloys are more than 4 million tons. In the latest "National List of Hazardous Wastes (2021 Edition)", "aluminum ash slag produced on the melt surface during the transfer, refining, alloying, and casting of electrolytic aluminum and aluminum liquid, and salt slag and secondary aluminum ash produced during the aluminum recovery process" are clearly listed as hazardous wastes. Therefore, traditional landfill, stacking and other treatment methods no longer meet national regulatory requirements. At the same time, secondary aluminum ash contains impurities such as aluminum nitride, fluoride, and soluble salts, which have certain leaching toxicity and are very harmful to soil and groundwater. The treatment of secondary aluminum ash requires an efficient and mature process to achieve lightweight, harmless, and resource-based aluminum ash.

[0006] At present, the main methods for processing and recycling secondary aluminum ash include recycling alumina, preparing flocculants, and preparing building materials. Due to its high aluminum content in secondary aluminum ash, recycling alumina is the key to the resource utilization of aluminum ash. There are mainly pyrometallurgical processes and wet processes. The pyrometallurgical process is to fully mix the electrolytic aluminum ash with sodium carbonate, and after high-temperature sintering, solid sodium aluminate is obtained; water or dilute alkaline solution is added to leach, sodium aluminate solution is obtained, a purifier is added to remove impurities, carbon dioxide is introduced for carbon separation, and aluminum hydroxide solid is precipitated. The wet process is divided into acid leaching and alkaline leaching. The leaching rate of the acid leaching method is relatively high, but other metal impurities in the aluminum ash will also react with the acid, and the purity of the metal ions in the filtrate is low, requiring subsequent impurity removal. The large amount of waste acid produced by the acid leaching method also needs to be subsequently treated. The filtrate obtained by the alkaline leaching method will not be interfered by other metal ions, but there are disadvantages such as low leaching rate of aluminum ions, large water consumption, and high energy consumption.

[0007] Secondary aluminum ash not only takes up a lot of space, but also releases pungent odors when piled up for a long time. It is reactive, polluting the environment and wasting a lot of resources. To achieve the harmless and resource-based treatment of secondary aluminum ash, the removal of aluminum nitride is an essential key process. Only after sufficient denitrification treatment can secondary aluminum ash be used for subsequent resource utilization. (III) Summary of the invention

[0009] The purpose of the present invention is to provide a system and method for using secondary aluminum ash to remove aluminum nitride by water washing, which adopts a system and a combined process for air stripping and hydrothermal treatment of secondary aluminum ash. While air stripping improves the decomposition efficiency of aluminum nitride, it can also strip ammonia for absorption and utilization, remove most of the aluminum nitride in the secondary aluminum ash, significantly improve the removal rate of aluminum nitride in the secondary aluminum ash, achieve the process purpose of harmless treatment of secondary aluminum ash, and realize the resource utilization of secondary aluminum ash. Compared with the traditional alkaline leaching method, the method of the present invention has a faster removal rate and a higher removal rate of aluminum nitride, improves the resource efficiency of secondary aluminum ash, saves processing costs, and solves the shortcomings of high water consumption, high energy consumption, and low aluminum nitride removal rate in the traditional wet method for removing aluminum nitride.

[0010] The technical solution adopted by the present invention is:

[0011] The present invention provides a secondary aluminum ash water washing system for removing aluminum nitride, the secondary aluminum ash water washing system for removing aluminum nitride comprising a blower (1), a heat exchanger (2), a reaction tank (3), an absorption tower (4), and a water pump (5); the heat exchanger (2) is provided with an air inlet (21) at the bottom, a steam inlet (22) at the top, a hot air outlet (23) at the upper end of the side, and a condensate outlet (24) at the lower end of the side; the reaction tank (3) is provided with a hot air inlet (31) at the bottom side, a secondary aluminum ash inlet (32) at the top, an alkali feeding port (33), a tap water inlet (34), an exhaust gas outlet (35), a slurry outlet (36) at the side, and an air distributor (37) and a stirrer (38) inside; the absorption tower (4) is provided with an exhaust gas inlet (41) at the bottom end of the side, a sulfuric acid injection port (33), and a tap water inlet (34) at the bottom. The heat exchanger (2) has an inlet (47) and a tap water injection port (48), an ammonia water outlet (46) at the bottom, an exhaust port (42) at the top, a circulating water inlet (44) at the top of the side opposite to the exhaust gas inlet, a circulating water outlet (43) at the bottom, and a spray device (45) in communication with the circulating water inlet at the upper end; the blower (1) is in communication with the air inlet (21) of the heat exchanger (2), the hot air outlet (23) of the heat exchanger (2) is in communication with the hot air inlet (31) of the reaction tank (3); the exhaust gas outlet (35) of the reaction tank (3) is in communication with the exhaust gas inlet (41) of the absorption tower (4); the circulating water outlet (43) of the absorption tower (4) is in communication with the circulating water inlet (44) via a water pump (5), and the circulating water inlet (44) is in communication with the spray device (45).

[0012] Preferably, the blower is a Roots blower; the heat exchanger is a steam-to-gas heat exchanger that heats the air by steam.

[0013] Preferably, an air distributor (37) and an agitator (38) are provided inside the reaction tank (3) to ensure that the entering secondary aluminum ash, water and air are fully mixed and reacted.

[0014] Preferably, the absorption tower is a multi-stage absorption tower composed of 1 or 2-6 absorption towers connected in series, the exhaust gas inlet of the next absorption tower is connected to the exhaust port of the previous absorption tower; the ammonia outlet of the next absorption tower is connected to the tap water inlet of the previous absorption tower, and the circulating water outlet of each absorption tower is connected to the circulating water inlet through a water pump. The last absorption tower is fed with tap water and dilute sulfuric acid to control the pH of the circulating water of the last absorption tower, and the circulating water of the tower is discharged separately and regularly; the penultimate absorption tower is fed with tap water regularly to maintain the ammonia concentration in the circulating water of the first absorption tower.

[0015] The present invention also provides a method for removing aluminum nitride from secondary aluminum ash by using the secondary aluminum ash water washing and aluminum nitride removal system. The method comprises: placing a reaction tank (3) in a water bath, passing tap water from a tap water inlet (34) into the reaction tank (3), turning on a stirrer (38) of the reaction tank (3), then adding secondary aluminum ash from a secondary aluminum ash inlet (32) into the reaction tank (3), then adding alkali from an alkali feeding port (33) into the reaction tank (3), and mixing the aluminum ash and water under the action of the stirrer (38). The aluminum nitride begins to decompose and forms an aluminum ash slurry. After the secondary aluminum ash is added, the blower (1) is immediately turned on to send air from the air inlet (21) to the heat exchanger (2) to exchange heat with the steam introduced from the steam inlet (22). The heated air enters the reaction tank (3) through the hot air inlet (31) of the reaction tank (3), is distributed by the air distributor (37), and reacts with the secondary aluminum ash slurry. The waste gas generated is discharged from the waste gas outlet (35) of the reaction tank (3) and passes through the absorption tower (4). ) enters the exhaust gas inlet (41) of the absorption tower (4) into which tap water and dilute sulfuric acid have been previously injected. The water pump (5) is turned on to circulate the tap water and dilute sulfuric acid solution in the absorption tower (4) to react with the exhaust gas discharged from the reaction tank (3) in a circulating spraying manner to generate gas and absorption liquid. The completely absorbed gas is discharged from the exhaust port (42) of the absorption tower (4). The ammonia nitrogen concentration of the absorption liquid in the absorption tower (4) is sampled and tested. When the ammonia nitrogen mass concentration is higher than 15%, it is discharged through the ammonia water outlet (46). A portion of the absorption liquid is discharged and ammonia water is recovered, and tap water and dilute sulfuric acid solution are added to maintain the tap water volume and pH 1-4 in the absorption tower (4); the ammonia nitrogen concentration of the aluminum ash slurry in the reaction tank (3) is monitored. When the ammonia nitrogen concentration in the reaction tank 3 is less than 100 mg / L and no longer increases significantly, the aluminum nitride is completely hydrolyzed, and the aluminum ash slurry in the reaction tank (3) is discharged from the slurry outlet (36) and filtered to obtain a filtrate and a filter cake. The filter cake is washed and dried, and then the aluminum ash is recovered to remove the aluminum nitride. The filtrate is evaporated and concentrated to recover aluminum and salt by-products.

[0016] Preferably, the alkali includes calcium oxide, calcium hydroxide, and sodium carbonate. The mass ratio of the alkali to the secondary aluminum ash is 0.1-1.0:1, preferably 0.2:1.

[0017] Preferably, the volume of tap water added to the reaction tank is 1-10 L / kg aluminum ash, preferably 5 L / kg, based on the mass of secondary aluminum ash. During the reaction, tap water in the reaction tank is replenished in time according to the liquid level in the reaction tank to ensure sufficient water and uniform mixing.

[0018] Preferably, the volume ratio of air to steam in the heat exchanger is 1:0.02-0.5, preferably 1:0.2.

[0019] Preferably, the hot air is continuously introduced into the reaction tank (preferably at a rate of 100-1000 L / h) to maintain the reaction temperature in the reaction tank at 60-85°C, and the air stripping rate per liter of water washing liquid is 0.5 L·h -1 .

[0020] Preferably, the reaction temperature in the reaction tank is 60-85°C, preferably a constant temperature reaction of 80°C.

[0021] Preferably, the absorption tower (4) is a multi-stage absorption tower consisting of 2 to 6 absorption towers, the exhaust gas inlet of the latter absorption tower is connected to the exhaust port of the former absorption tower, and the exhaust gas passes through the multi-stage absorption towers in sequence; the ammonia water outlet of the latter absorption tower is connected to the tap water inlet of the previous absorption tower, and the circulating water outlet of each absorption tower is connected to the circulating water inlet through a water pump; tap water and dilute sulfuric acid are added to the last absorption tower to control the pH value of the circulating water in the absorption tower to 1-4, the circulating water outlet of the last absorption tower is regularly discharged to recover ammonium sulfate, and the ammonia water discharged from the remaining absorption towers flows by gravity to the tap water inlet of the previous absorption tower; the ammonia nitrogen concentration in the first absorption tower is detected, and when the ammonia nitrogen mass concentration in the first absorption tower is higher than 15%, part of the absorption liquid is discharged through the ammonia water outlet to recover the ammonia water, and tap water is added to the penultimate absorption tower to maintain the water volume in the first absorption tower.

[0022] The present invention removes aluminum nitride in secondary aluminum ash by hot air stripping in coordination with calcification hydrothermal reaction, that is, secondary aluminum ash is mixed with water and alkali is added in a reaction tank, aluminum nitride in the aluminum ash is removed by a hydrothermal method, and ammonia generated by the hydrothermal method is stripped and collected by hot air stripping at the same time, and then the washing liquid after hot air stripping and hydrothermal treatment is filtered, the filter cake is washed and dried, and the aluminum ash with aluminum nitride removed is recovered, and the filtrate is evaporated and concentrated to recover aluminum and salt by-products.

[0023] Compared with the prior art, the beneficial effects of the present invention are mainly reflected in: the present invention innovatively adopts a hot air stripping and hydrothermal method to treat secondary aluminum ash, which not only improves the decomposition rate of aluminum nitride and the removal rate of aluminum nitride, but also can simultaneously recover the ammonia generated by the decomposition of aluminum nitride. The hydrothermal method of adding calcium oxide is used in advance to decompose aluminum nitride and fix aluminate, avoiding the process of difficult-to-treat wastewater. The process of the present invention is simpler, uses less water (the ash-water ratio is controlled at 1:10), and requires a simple catalyst. It is an efficient and environmentally friendly method for treating secondary aluminum ash. The temperature required for water washing in the existing technology requires more than 100°C, and the high-temperature slurry is ball-milled for hydrolysis, which is complex and energy-intensive. The ash-water ratio is 1:15 or lower, and multiple catalysts are required to work together. See references (Institute of Process Engineering, Chinese Academy of Sciences, Luoyang Tianrui Environmental Protection Technology Co., Ltd. A method for deep hydrolysis of aluminum nitride in aluminum ash: CN201810074432.5[P]. 2018-06-29.) and (Hangzhou Zhuzhen Technology Co., Ltd. A system for secondary aluminum ash harmlessness and full element resource utilization: CN202121729323.6[P]. 2022-04-01.) (IV) Description of the drawings

[0025] Figure 1 This is a schematic diagram of the system for washing and removing aluminum nitride from secondary aluminum ash according to the present invention, wherein 1 is a blower; 2 is a heat exchanger, 21 is an air inlet, 22 is a steam inlet, 23 is a hot air outlet, and 24 is a condensed water outlet; 3 is a reaction tank, 31 is a hot air inlet, 32 is a secondary aluminum ash inlet, 33 is an alkali feeding port, 34 is a tap water inlet, 35 is an exhaust gas outlet, 36 is a slurry outlet, 37 is an air distributor, and 38 is an agitator; 4 is an absorption tower, 41 is an exhaust gas inlet, 42 is an exhaust port, 43 is a circulating water outlet, 44 is a circulating water inlet, 45 is a spray device, 46 is an ammonia water outlet, 47 is a sulfuric acid injection port, and 48 is a tap water injection port; 5 is a water pump. (V) Specific embodiments

[0027] The present invention is further described below in conjunction with specific embodiments, but the protection scope of the present invention is not limited thereto:

[0028] The secondary aluminum ash processed by the embodiment of the present invention comes from molten salt slag produced on the surface during aluminum electrolytic casting, and is then recovered through ash roasting, and the aluminum nitride content is 13.7%.

[0029] Example 1: Constant temperature air dehydration hydrothermal method

[0030] 1. Secondary aluminum ash washing system to remove aluminum nitride

[0031] Reference Figure 1The secondary aluminum ash water washing and aluminum nitride removal system includes a blower 1, a heat exchanger 2, a reaction tank 3, an absorption tower 4, and a water pump 5; the heat exchanger 2 is provided with an air inlet 21 at the bottom, a steam inlet 22 at the top, a hot air outlet 23 at the upper end of the side, and a condensed water outlet 24 at the lower end of the side; the reaction tank 3 is provided with a hot air inlet 31 at the bottom side, a secondary aluminum ash inlet 32, an alkali feeding port 33, and a tap water inlet 34 at the top, an exhaust gas outlet 35 and a slurry outlet 36 at the side, and an air distributor 37 and an agitator 38 inside; the absorption tower 4 is provided with an exhaust gas inlet 41 at the bottom end of the side, a sulfuric acid injection port 47 and a tap water injection port 48 at the top, an ammonia outlet 46 at the bottom, an exhaust port 42 at the top, a circulating water inlet 44 at the top of the side opposite to the exhaust gas inlet, a circulating water outlet 43 at the bottom, and a spray device 45 connected to the circulating water inlet at the upper end of the inside.

[0032] The blower 1 is connected to the air inlet 21 of the heat exchanger 2, and the hot air outlet 23 of the heat exchanger 2 is connected to the hot air inlet 31 of the reaction tank 3; the hot air inlet 31 of the reaction tank 3 is connected to the air distributor 37, and the exhaust gas outlet 35 is connected to the exhaust gas inlet 41 of the absorption tower 4; the circulating water outlet 43 of the absorption tower 4 is connected to the circulating water inlet 44 through the water pump 5.

[0033] The effective volume of the reaction tank 3 is 500 mL. The effective volume of the absorption tower is 200 mL.

[0034] 2. Removal of aluminum nitride from secondary aluminum ash

[0035] 250 mL of tap water is introduced into the reaction tank 3 through the tap water inlet 34, and the reaction tank 3 is placed in a water bath at 80°C, the agitator 38 of the reaction tank 3 is turned on, and then 50 g of secondary aluminum ash and 10 g of calcium oxide are added into the reaction tank 3 through the secondary aluminum ash inlet 32 ​​and the alkali feeding port 33. The aluminum ash and water react under the action of the agitator 38, and aluminum nitride begins to decompose to form an aluminum ash slurry; after the aluminum ash is added, the blower 1 is immediately turned on, and air is sent into the heat exchanger 2 through the air inlet 21, and the air is mixed with the steam introduced through the steam inlet 22 at a volume ratio of 1:0.2 for heat exchange, and the generated condensed water is discharged through the condensed water outlet 24, and the heated air is discharged through the hot air outlet 23, and then enters the reaction tank 3 through the hot air inlet 31 of the reaction tank 3 at a speed of 200 L / h, and the hot air is continuously introduced to maintain the temperature in the reaction tank at 80°C. After being distributed by the air distributor 37, it reacts with the secondary aluminum ash slurry, and the generated waste gas is discharged from the reaction tank The waste gas is discharged from the exhaust gas outlet 35 of the tank 3 and enters the absorption tower 4 filled with 0.2L tap water and 0.2L 5% mass concentration dilute sulfuric acid aqueous solution (dilute sulfuric acid is used to control the pH value to 4) through the exhaust gas inlet 41 of the absorption tower 4. The water pump 5 is turned on to discharge the tap water and dilute sulfuric acid solution of the absorption tower 4 from the circulating water outlet 43 and circulate into the spray device 45 of the absorption tower 4 from the circulating water inlet 44 through the water pump 5. The waste gas is sprayed cyclically to react with the waste gas in the absorption tower 4 to produce absorption liquid and absorbed waste gas. The spray flow rate of the absorption liquid circulation and the volume ratio of the introduced waste gas are 1-2:1000. The completely absorbed waste gas is discharged from the exhaust port 42 of the absorption tower 4. The ammonia nitrogen concentration of the absorption liquid in the absorption tower 4 is sampled and tested. When the ammonia nitrogen mass concentration is higher than 15%, part of the absorption liquid is discharged through the ammonia water outlet 46 to recover the ammonia water, and then tap water is added to maintain the ammonia nitrogen mass concentration in the absorption tower 4 less than 15%, and the mass concentration of 5% dilute sulfuric acid aqueous solution is added to control the pH value to 4. The ammonia nitrogen concentration in the aluminum ash slurry in the reaction tank 3 is measured by sampling. When the ammonia nitrogen content is less than 100 mg / L and no longer increases significantly, it can be considered that the secondary aluminum ash and water in the reaction tank 3 have reacted fully and the aluminum nitride has been completely decomposed. The completely degraded slurry is discharged from the slurry outlet 36 and filtered to obtain a filtrate and a filter cake. The filter cake can be used as a cement raw material or a water flocculant raw material after being washed with clean water and dried. The filtrate is evaporated and concentrated to dryness to recover aluminum and salt by-products.

[0036] The aluminum ash slurry in the reaction tank 3 and the absorption liquid in the absorption tower 4 were taken at 4, 8, 12 and 24 hours of reaction, respectively. The total ammonia nitrogen content of the aluminum ash slurry and the absorption liquid was detected to estimate the amount of aluminum nitride hydrolysis, and the aluminum nitride removal rate was further estimated. The results are shown in Table 1.

[0037] Table 1 Removal rate of aluminum nitride in secondary aluminum ash

[0038]

[0039] Comparative Example 1: Constant Temperature Stirring Hydrothermal Method

[0040] A reaction vessel was placed on a constant temperature electromagnetic stirrer, 50 g of secondary aluminum ash and 250 ml of deionized water were added to the reaction vessel, the reaction temperature was controlled at 80 ° C, and the gas generated in the reaction vessel was passed into a gas absorption bottle containing 200 ml of 5% dilute sulfuric acid aqueous solution. The experiment was repeated, and the experimental time was controlled at 4 hours, 8 hours, 12 hours, and 24 hours. After stopping the experiment, the ammonia nitrogen content in the reaction vessel and the absorption bottle was detected, and the ammonia generated by the hydrolysis of aluminum nitride was calculated to estimate the hydrolysis amount of aluminum nitride. The results are shown in Table 1. It can be seen from the data in Table 1 that after adding the air stripping process in Example 1, the hydrolysis rate and hydrolysis removal rate of aluminum nitride in the secondary aluminum ash are higher than those of the traditional hydrothermal hydrolysis method in Comparative Example 1.

[0041] Embodiment 2,

[0042] The calcium oxide in Example 1 was changed to 5g, and other operations were the same. The results are shown in Table 1.

[0043] When the mass ratio of secondary aluminum ash and calcium oxide is 5:1, the hydrolysis rate and hydrolysis removal rate of aluminum nitride in secondary aluminum ash can be significantly improved.

[0044] Embodiment 3,

[0045] Temperature experiment: The temperature in Example 1 was controlled at 20°C, 40°C, 60°C, and 80°C, respectively, and other operations were the same, and the experimental time was controlled at 24 hours. After stopping the experiment, the ammonia nitrogen content in the reaction tank 3 and the absorption tower 4 was detected, and the ammonia gas generated by the hydrolysis of aluminum nitride was calculated to estimate the hydrolysis amount of aluminum nitride. The results are shown in Table 2.

[0046] Table 2. Secondary aluminum ash removal rate at different temperatures

[0047]

[0048] Comparative Example 3

[0049] The temperature in Comparative Example 1 was controlled at 20°C, 40°C, 60°C, and 80°C, respectively, and other operations were the same, and the experimental time was controlled at 24 hours. After stopping the experiment, the ammonia nitrogen content in the reaction container and the absorption bottle was detected, and the ammonia gas generated by the hydrolysis of aluminum nitride was calculated to estimate the hydrolysis amount of aluminum nitride. The results are shown in Table 2.

Claims

1. A method for removing aluminum nitride from secondary aluminum ash, characterized in that: The method adopts a secondary aluminum ash water washing system for removing aluminum nitride, and the secondary aluminum ash water washing system for removing aluminum nitride comprises a blower (1), a heat exchanger (2), a reaction tank (3), an absorption tower (4), and a water pump (5); the heat exchanger (2) is provided with an air inlet (21) at the bottom, a steam inlet (22) at the top, a hot air outlet (23) at the upper end of the side, and a condensate outlet (24) at the lower end of the side; the reaction tank (3) is provided with a hot air inlet (31) at the bottom side, a secondary aluminum ash inlet (32) at the top, an alkali feeding port (33), a tap water inlet (34), an exhaust gas outlet (35), a slurry outlet (36) at the side, and an air distributor (37) and a stirrer (38) inside; the absorption tower (4) is provided with an exhaust gas inlet (41), a sulfuric acid inlet (42) at the bottom end of the side, and a condensate outlet (24) at the lower end of the side. The air blower (1) is connected to the air inlet (21) of the heat exchanger (2), the hot air outlet (23) of the heat exchanger (2) is connected to the hot air inlet (31) of the reaction tank (3); the waste gas outlet (35) of the reaction tank (3) is connected to the waste gas inlet (41) of the absorption tower (4); the circulating water outlet (43) of the absorption tower (4) is connected to the circulating water inlet (44) through the water pump (5), and the circulating water inlet (44) is connected to the spray device (45); The method comprises: placing a reaction tank (3) in a water bath, passing tap water into the reaction tank (3) from a tap water inlet (34), turning on a stirrer (38) of the reaction tank (3), then adding secondary aluminum ash into the reaction tank (3) from a secondary aluminum ash inlet (32), then adding alkali into the reaction tank (3) from an alkali feeding port (33), and reacting the aluminum ash and water under the action of the stirrer (38), at which time aluminum nitride begins to decompose to form an aluminum ash slurry; after the secondary aluminum ash is added, The blower (1) is immediately turned on to send air from the air inlet (21) to the heat exchanger (2) to exchange heat with the steam introduced from the steam inlet (22). The heated air enters the reaction tank (3) through the hot air inlet (31) of the reaction tank (3), is distributed by the air distributor (37), and reacts with the secondary aluminum ash slurry. The generated waste gas is discharged from the waste gas outlet (35) of the reaction tank (3), and enters the absorption tower (4) into which tap water and dilute sulfuric acid are previously injected through the waste gas inlet (41) of the absorption tower (4). The water pump (5) is turned on to circulate the tap water and dilute sulfuric acid solution in the absorption tower (4) to react with the waste gas discharged from the reaction tank (3) in a circulating spraying manner to generate gas and absorption liquid. The gas after complete absorption is discharged from the exhaust port (42) of the absorption tower (4). The ammonia nitrogen concentration of the absorption liquid in the absorption tower (4) is sampled and tested. When the ammonia nitrogen mass concentration is higher than 15%, part of the absorption liquid is discharged through the ammonia water outlet (46). The ammonia water is recovered, and tap water and dilute sulfuric acid solution are added to maintain the tap water volume and pH value of 1-4 in the absorption tower (4); the ammonia nitrogen concentration of the aluminum ash slurry in the reaction tank (3) is monitored. When the ammonia nitrogen concentration is less than 100 mg / L and no longer increases significantly, the aluminum nitride is completely hydrolyzed, and the aluminum ash slurry in the reaction tank (3) is discharged from the slurry outlet (36) and filtered to obtain a filtrate and a filter cake. The filter cake is washed and dried to recover the aluminum ash from which the aluminum nitride is removed, and the filtrate is evaporated and concentrated to recover aluminum and salt by-products.

2. The method according to claim 1, characterized in that The blower is a Roots blower; the heat exchanger is a steam-to-gas heat exchanger, which heats the air by steam.

3. The method according to claim 1, characterized in that The absorption tower is a multi-stage absorption tower composed of 2-6 absorption towers connected in series, the exhaust gas inlet of the rear absorption tower is connected to the exhaust port of the front absorption tower; the ammonia water outlet of the rear absorption tower is connected to the tap water inlet of the upper absorption tower, and the circulating water outlet of each absorption tower is connected to the circulating water inlet through a water pump.

4. The method according to claim 1, characterized in that The alkali includes calcium oxide, calcium hydroxide and sodium carbonate; the mass ratio of the alkali to secondary aluminum ash is 0.1-1.0:

1.

5. The method according to claim 1, characterized in that The volume of tap water added to the reaction tank (3) is 1-10 L / kg of aluminum ash based on the mass of secondary aluminum ash.

6. The method according to claim 1, characterized in that The volume ratio of air to steam in the heat exchanger is 1:0.02-0.

5.

7. The method according to claim 1, characterized in that The heated air is continuously introduced into the reaction tank (3) to maintain the reaction temperature in the reaction tank at 60-85°C.

8. The method according to claim 1, characterized in that The absorption tower (4) is a multi-stage absorption tower composed of 2 to 6 absorption towers connected in series in sequence, the exhaust gas inlet of the latter absorption tower is connected to the exhaust port of the former absorption tower, and the exhaust gas passes through the multi-stage absorption tower in sequence; the ammonia water outlet of the latter absorption tower is connected to the tap water inlet of the previous absorption tower, and the circulating water outlet of each absorption tower is connected to the circulating water inlet through a water pump; the last absorption tower is supplemented with tap water and dilute sulfuric acid to control the pH value of the circulating water in the absorption tower to be 1-4, the circulating water outlet of the last absorption tower is regularly discharged to recover ammonium sulfate, and the ammonia water discharged from the remaining absorption towers flows by gravity to the tap water inlet of the previous absorption tower; the ammonia nitrogen concentration in the first absorption tower is detected, and when the ammonia nitrogen mass concentration in the first absorption tower is higher than 15%, part of the absorption liquid is discharged through the ammonia water outlet to recover the ammonia water, and tap water is supplemented in the penultimate absorption tower to maintain the water volume in the first absorption tower.

9. The method according to claim 8, characterized in that The ratio of the spray flow rate of the water pump circulation to the volume of the exhaust gas introduced is 1~3:1000.

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