Method for in-situ transformation and chromium removal of aluminum mud in chromium salt production

Abstract

The application discloses a method for in-situ transformation and chromium removal of aluminum mud in chromium salt production, and belongs to the field of chromium-containing aluminum mud resource utilization, and solves the problems of harsh process conditions, low recovery rate and difficult chromium removal in the utilization method of the chromium-containing aluminum mud.The method comprises the following steps: adding a sodium chromate alkaline solution into a reaction kettle, spraying a sodium chromate solution from the top of the reaction kettle, continuously stirring, and keeping the reaction temperature at 80-90 DEG C; keeping the pH value of the reaction slurry at 9-9.5, continuously discharging from the bottom of the reaction kettle, realizing continuous production, washing the product after filtration and separation, and obtaining the chromium-containing aluminum mud; and washing the chromium-containing aluminum mud three times by using a sodium hydroxide solution to obtain the aluminum mud after chromium removal.The application utilizes the reaction between the alkali and the chromium-containing aluminum mud by using the in-situ chemical conversion method, deeply removes the hexavalent chromium in the aluminum mud, and prepares qualified aluminum hydroxide, so that the defects of the traditional method, such as complicated process, low product quality and long production cycle, are avoided.

Description

Technical Field

[0001] This invention belongs to the field of resource utilization of chromium-containing aluminum mud, specifically involving a method for in-situ transformation and chromium removal of aluminum mud during chromium salt production. Background Technology

[0002] Chromium-containing aluminum sludge is a major solid waste generated during the aluminum removal process in chromium salt production. It not only occupies land and wastes resources but also severely pollutes the environment. For many years, researchers both domestically and internationally have conducted extensive work on the comprehensive utilization of chromium-containing aluminum sludge, including using it to produce tanning agents and cement, chemical neutralization with phosphoric acid for aluminum removal, high-temperature roasting for chromium removal, strong flocculants for chromium removal, and reducing agents for chromium removal. However, some of these methods have demanding process conditions and high equipment investment, while others have low recovery rates and large fluctuations in product quality, making them unsuitable for industrial application.

[0003] On the other hand, a considerable portion of Na2CrO4 in chromium-containing aluminum sludge exists in bound and crystalline forms, making it difficult to remove. This has always been the main challenge in removing chromium from aluminum sludge. Summary of the Invention

[0004] The purpose of this invention is to provide a method for in-situ transformation and chromium removal from aluminum sludge during chromium salt production, in order to solve the problems of harsh process conditions, low recovery rate, and difficulty in removing chromium in the utilization of chromium-containing aluminum sludge.

[0005] The technical solution of this invention is: a method for in-situ transformation and chromium removal from aluminum sludge during chromium salt production, comprising the following steps: S1. Add the sodium chromate alkaline solution (i.e., yellow brine) to the reactor, and at the same time add the sodium chromate solution (i.e., pre-acidified clear solution) from the top of the reactor by spraying, while stirring continuously to maintain the reaction temperature at 80-90℃. S2. The pH value of the reaction slurry is always maintained at 9-9.5. At the same time, the material is continuously discharged from the bottom of the reactor to achieve continuous production. The product is filtered and separated and then washed with water to obtain chromium-aluminum mud. S3. Use sodium hydroxide solution to perform an in-situ transformation washing of chromium-containing aluminum mud for a transformation time of not less than 6 hours. S4. Use sodium hydroxide solution to perform a second-stage in-situ transformation washing of the chromium-containing aluminum mud after the first-stage in-situ transformation washing, with a transformation time of not less than 6 hours. S5. The chromium-containing aluminum sludge after the two-stage in-situ transformation washing is subjected to a three-stage in-situ transformation washing using sodium hydroxide solution. The transformation time is no less than 6 hours, yielding chromium-removed aluminum sludge. The chromium-removed aluminum sludge is then washed with water and dried to obtain chromium-removed aluminum hydroxide.

[0006] As a further improvement of the present invention, in step S1, the sodium chromate alkaline solution is at 10m 3 / h~12m 3The solution is added to the reactor at a flow rate of / h.

[0007] As a further improvement of the present invention, in step S1, the sodium chromate solution is at 2m 3 / h~3m 3 Spraying is added at a flow rate of / h.

[0008] As a further improvement of the present invention, in step S1, the stirring speed is 40 r / min.

[0009] As a further improvement of the present invention, in steps S3, S4, and S5, the concentration of the sodium hydroxide solution is 20-30 g / L.

[0010] As a further improvement of the present invention, barium salt is added to the chromium-containing sodium hydroxide solution obtained after two-stage in-situ transformation washing to remove chromium, and then the resulting dilute sodium hydroxide solution is evaporated and used for three-stage in-situ transformation washing.

[0011] The beneficial effects of this invention are: This invention proposes an innovative in-situ chromium removal process for the resource utilization of hazardous waste containing chromium aluminum sludge. Using an in-situ chemical conversion method, this invention treats chromium-containing aluminum sludge discharged from chromium salt plants. Starting with the sodium chromate alkaline solution aluminum removal process, a method has been developed to remove and recycle toxic hexavalent chromium salts from the chromium-containing aluminum sludge. Simultaneously, the reaction of alkali with the chromium-containing aluminum sludge deeply removes hexavalent chromium, yielding qualified aluminum hydroxide.

[0012] This invention employs a novel process for neutralizing and removing aluminum from aluminum sludge using yellow brine, avoiding the drawbacks of traditional methods such as complex procedures, low product quality, and long production cycles. It achieves deep removal of hexavalent chromium from aluminum sludge through only transformation and washing, yielding aluminum hydroxide product. This reduces the sodium chromate content in the aluminum hydroxide sludge to below 1%, allowing it to be directly used in the production of aluminum hydroxide or cryolite. This invention utilizes pre-acidification and cleaning solutions for chromium removal, enabling continuous production. This invention proposes a new approach to chromium removal from aluminum sludge during chromium salt production, reducing production costs and offering significant environmental benefits.

[0013] The method of this invention breaks through the bottlenecks of high energy consumption and high risk of secondary pollution in traditional aluminum sludge treatment technology. It has the characteristics of short process, low cost and environmental friendliness, and can be widely used in the treatment of chromium-containing aluminum sludge generated in industries such as electrolytic aluminum, electroplating, and leather making. Attached Figure Description

[0014] Figure 1 This is a process flow diagram of the present invention. Detailed Implementation

[0015] The present invention will be further described in detail below with reference to specific embodiments.

[0016] (I) Laboratory Experiments Example 1 A method for in-situ chromium removal from aluminum sludge during chromium salt production includes the following steps: S1. Add sodium chromate alkaline solution (i.e., yellow brine) at 10m 3 The solution is added to the reactor at a flow rate of / h, and at the same time, 2m of sodium chromate solution (i.e., pre-acidified clear solution) with a pH of 5-6 is added. 3 The solution is added from the top of the reactor by spraying at a flow rate of / h, and the mixture is continuously stirred at a speed of 40r / min. The reaction temperature is maintained at 80℃, so that the aluminum in the yellow brine precipitates out as Al(OH)3. S2. Using a pH meter for online monitoring, the pH value of the reaction slurry is always maintained at 9. At the same time, the material is continuously discharged from the bottom of the reactor to achieve continuous production. The product is filtered and separated, then washed with water to obtain chromium-aluminum mud. S3. Use sodium hydroxide solution to perform an in-situ transformation washing of chromium-containing aluminum mud. The concentration of sodium hydroxide solution is 30 g / L, and the transformation time is controlled at 6 hours. S4. Use sodium hydroxide solution to perform a second-stage in-situ transformation washing on the chromium-containing aluminum mud after the first-stage in-situ transformation washing. The concentration of sodium hydroxide solution is 30 g / L, and the transformation time is controlled at 6 hours. S5. The chromium-containing aluminum sludge after the two-stage in-situ transformation washing was subjected to a three-stage in-situ transformation washing using sodium hydroxide solution. The concentration of the sodium hydroxide solution was 30 g / L, and the transformation time was controlled at 6 hours to obtain chromium-removed aluminum sludge. The chromium-removed aluminum sludge was washed with water and then dried to obtain chromium-removed aluminum hydroxide.

[0017] Examples 2-6 below use pH as the variable, with other parameters the same as in Example 1, to determine the optimal neutralization pH. The experimental results are shown in Table 1.

[0018] According to the particle size analysis data in Table 1, when the pH value is 9~9.5, the resulting aluminum sludge has a larger particle size, a smaller specific surface area, and settles the fastest. Therefore, the optimal pH value is determined to be 9~9.5.

[0019] Examples 7-9 below use temperature as the variable, with other parameters the same as in Example 1, to determine the optimal reaction temperature. The experimental results are shown in Table 2.

[0020] According to the particle size analysis data in Table 2, when the reaction temperature is 80~90℃, the resulting aluminum sludge has a larger particle size and a smaller specific surface area. Therefore, the optimal reaction temperature is determined to be 80~90℃.

[0021] Examples 10-18 below use the number of washes and washing time with sodium hydroxide solution, as well as the concentration of sodium hydroxide solution, as variables, with other parameters the same as in Example 1, to investigate the effects of these factors on washing depth, in order to achieve the goal of cost reduction and efficiency improvement. The experimental results are shown in Table 3.

[0022] According to the experimental data in Table 3, controlling the sodium hydroxide concentration to 30 g / L and the washing time to 6 hours, aluminum hydroxide with a hexavalent chromium content of 5 ppm can be obtained after three alkaline washes. Longer washing times will increase production costs.

[0023] (II) Pilot-scale experiment Based on the above experimental data, pilot-scale experimental equipment was built in the workshop to complete the pilot-scale experiment.

[0024] Example 19 A method for in-situ chromium removal from aluminum sludge during chromium salt production includes the following steps: S1. Add sodium chromate alkaline solution (i.e., yellow brine) at 12m 3 The solution is added to the reactor at a flow rate of / h, and simultaneously, 3m³ of sodium chromate solution (i.e., pre-acidified clear solution) with a pH of 5-6 is added. 3 The solution is added from the top of the reactor by spraying at a flow rate of / h, and the mixture is continuously stirred at a speed of 40r / min. The reaction temperature is maintained at 86℃, so that the aluminum in the yellow brine precipitates out as Al(OH)3. S2. Using a pH meter for online monitoring, the pH value of the reaction slurry is always maintained at 9.21. At the same time, the material is continuously discharged from the bottom of the reactor to achieve continuous production. The product is filtered and separated, then washed with water to obtain chromium-aluminum mud. S3. Use sodium hydroxide solution to perform an in-situ transformation washing of chromium-containing aluminum mud. The concentration of sodium hydroxide solution is 30 g / L, and the transformation time is controlled at 6 hours. S4. Use sodium hydroxide solution to perform a second-stage in-situ transformation washing on the chromium-containing aluminum mud after the first-stage in-situ transformation washing. The concentration of sodium hydroxide solution is 30 g / L, and the transformation time is controlled at 6 hours. S5. The chromium-containing aluminum sludge after the two-stage in-situ transformation washing was subjected to a three-stage in-situ transformation washing using sodium hydroxide solution. The concentration of the sodium hydroxide solution was 30 g / L, and the transformation time was controlled at 6 hours to obtain chromium-removed aluminum sludge. The chromium-removed aluminum sludge was washed with water and then dried to obtain chromium-removed aluminum hydroxide.

[0025] Examples 20-25 below were conducted with different pH values ​​and reaction temperatures, with other parameters the same as in Example 19. The experimental results are shown in Table 4.

[0026] As can be seen from the data in Table 4, the results of the pilot-scale experiment are consistent with those of the laboratory experiment. When the temperature is controlled at 80~90℃ and the pH value is controlled at 9~9.5, chromium-containing aluminum mud with large particles, relatively uniform particle size distribution and low chromium content can be obtained.

[0027] Examples 26-30 below were conducted with in-situ transformation washing experiments at different temperatures, with other parameters the same as in Example 19. The experimental results are shown in Table 5.

[0028] As can be seen from the data in Table 5, the pilot-scale experimental results are consistent with the laboratory experimental results. Using 30 g / L sodium hydroxide solution as the alkaline washing solution, aluminum hydroxide product with a hexavalent chromium content of 5 ppm can be obtained after three washings.

[0029] (III) Project Implementation Results Based on the experimental results, the process of using pre-acidified liquid as a neutralizing solution to neutralize yellow brine to produce chromium-containing aluminum mud with good filtration performance and low chromium content, followed by three washings with sodium hydroxide solution to produce qualified aluminum hydroxide, is feasible and achieves the expected experimental objectives.

[0030] The chromium content (calculated as sodium chromate) in the aluminum hydroxide sludge obtained by the method of this invention is reduced to below 5 ppm in the aluminum hydroxide sludge after chromium removal. This aluminum hydroxide sludge can be directly used in the production of aluminum hydroxide or cryolite. This invention utilizes pre-acidification cleaning solution for chromium removal, enabling continuous production. It proposes a new process for chromium removal from aluminum sludge in chromium salt production, reducing production costs and demonstrating significant environmental benefits.

Claims

1. A method for in-situ chromium removal from aluminum sludge during chromium salt production, characterized in that... Includes the following steps: S1. Add the alkaline sodium chromate solution to the reactor, and simultaneously add the sodium chromate solution from the top of the reactor by spraying, while continuously stirring to maintain the reaction temperature at 80-90℃. S2. The pH value of the reaction slurry is always maintained at 9-9.

5. At the same time, the material is continuously discharged from the bottom of the reactor to achieve continuous production. The product is filtered and separated and then washed with water to obtain chromium-aluminum mud. S3. Use sodium hydroxide solution to perform an in-situ transformation washing of chromium-containing aluminum mud for a period of not less than 6 hours. S4. Use sodium hydroxide solution to perform a second-stage in-situ transformation washing of the chromium-containing aluminum mud after the first-stage in-situ transformation washing, with a transformation time of not less than 6 hours. S5. Use sodium hydroxide solution to perform a three-stage in-situ transformation washing on the chromium-containing aluminum sludge after the two-stage in-situ transformation washing. The transformation time shall not be less than 6 hours to obtain aluminum sludge after chromium removal.

2. The method for in-situ chromium removal from aluminum sludge in chromium salt production according to claim 1, characterized in that: In step S1, the sodium chromate alkaline solution is diluted to 10m... 3 / h~12m 3 The solution is added to the reactor at a flow rate of / h.

3. The method for in-situ chromium removal from aluminum sludge in chromium salt production according to claim 2, characterized in that: In step S1, the sodium chromate solution is dispensed at 2m 3 / h~3m 3 Spraying is added at a flow rate of / h.

4. The method for in-situ chromium removal from aluminum sludge in chromium salt production according to claim 1, characterized in that: In step S1, the stirring speed is 40 r / min.

5. The method for in-situ chromium removal from aluminum sludge in chromium salt production according to claim 1, characterized in that: In steps S3, S4, and S5, the concentration of the sodium hydroxide solution is 20-30 g / L.

6. A method for in-situ chromium removal from aluminum sludge in chromium salt production according to any one of claims 1-5, characterized in that: After two stages of in-situ transformation washing, barium salt is added to the chromium-containing sodium hydroxide solution to remove chromium. Then, the resulting dilute sodium hydroxide solution is evaporated and used for three stages of in-situ transformation washing.

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