Method for removing chromium from aluminum mud through in-situ transformation in chromium salt production

Abstract

The invention discloses a method for removing chromium from aluminum mud through in-situ transformation in chromium salt production, belongs to the field of resource utilization of chromium-containing aluminum mud, and solves the problems of harsh process conditions, low recovery rate and difficulty in removing chromium in a utilization method of the chromium-containing aluminum mud. The method comprises the following steps: adding a sodium chromate alkaline solution into a reaction kettle, adding the sodium chromate solution from the top of the reaction kettle through spraying, 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 at the same time to realize continuous production, filtering and separating a product, and washing with water to obtain chromium-containing aluminum mud; and carrying out three times of in-situ transformation washing on the chromium-containing aluminum mud by using a sodium hydroxide solution to obtain the chromium-removed aluminum mud. Through the in-situ chemical conversion method, the alkali reacts with the chromium-containing aluminum mud, hexavalent chromium in the aluminum mud is deeply removed, the qualified product aluminum hydroxide is prepared, and the defects that a traditional method is complex in procedure, low in product quality, long in production period and the like are overcome.

Description

Technical Field

[0001] The invention belongs to the field of resource utilization of chromium-containing aluminum mud, and particularly relates to a method for in-situ transformation and chromium removal of aluminum mud in chromium salt production. Background Art

[0002] Chromium-containing aluminum sludge is the primary solid waste generated during the aluminum removal process in chromium salt production. It not only occupies land and wastes resources, but also seriously pollutes the environment. Over the years, researchers at home and abroad have conducted extensive research on the comprehensive utilization of chromium-containing aluminum sludge, including using aluminum sludge to produce tanning agents and cement, removing aluminum through chemical neutralization with phosphoric acid, removing chromium with high-temperature roasting, removing chromium with strong flocculants, and removing chromium with reducing agents. However, some of these methods require harsh process conditions and high equipment investment, while others have low recovery rates and highly variable product quality, making them unsuitable for industrial application.

[0003] On the other hand, a considerable portion of Na2CrO4 in chromium-containing aluminum mud exists in the form of combined and crystalline states, making it difficult to remove. This has always been the difficulty in removing chromium from aluminum mud. Summary of the Invention

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

[0005] The technical solution of the present invention is: a method for in-situ transformation and chromium removal of aluminum mud in chromium salt production, comprising the following steps: S1. Add sodium chromate alkaline solution (i.e. yellow brine) into the reactor, and simultaneously add sodium chromate solution (i.e. pre-acidified clear solution) from the top of the reactor by spraying, and stir continuously to maintain the reaction temperature at 80-90°C; S2. Always maintain the pH value of the reaction slurry at 9-9.5, while continuously discharging 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, using sodium hydroxide solution to perform an in-situ transformation washing on the chromium-aluminum mud, the transformation time is not less than 6 hours; S4. Using sodium hydroxide solution, perform a second stage of in-situ transformation washing on the chromium-aluminum sludge after the first stage of in-situ transformation washing, with the transformation time being not less than 6 hours; S5. The chromium-containing aluminum sludge after the second-stage in-situ conversion washing is subjected to a third-stage in-situ conversion washing using a sodium hydroxide solution for a conversion time of not less than 6 hours to obtain a chromium-removed aluminum sludge. The chromium-removed aluminum sludge is washed with water and then 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 heated at 10 m 3 / h~12m 3 / h flow rate into the reactor.

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

[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 the second-stage in-situ transformation washing to remove chromium, and then the obtained dilute sodium hydroxide solution is evaporated and used for the third-stage in-situ transformation washing.

[0011] The beneficial effects of the present invention are: The present invention proposes an innovative process for in-situ transformation and chromium removal of aluminum mud aimed at resource utilization of hazardous waste containing chromium aluminum mud. Through an in-situ chemical conversion method, the chromium-containing aluminum mud discharged from a chromium salt plant is taken as the treatment object. Starting from the aluminum removal process of sodium chromate alkaline liquid, a method is developed to remove toxic hexavalent chromium salts from the chromium-containing aluminum mud and recycle them. At the same time, alkali is used to react with the chromium-containing aluminum mud to deeply remove the hexavalent chromium in the aluminum mud, thereby producing a qualified product, aluminum hydroxide.

[0012] This invention utilizes a novel yellow brine neutralization and aluminum removal process, avoiding the drawbacks of traditional methods such as complex procedures, low product quality, and long production cycles. Through transformation and washing alone, hexavalent chromium can be deeply removed from aluminum sludge to produce aluminum hydroxide. The sodium chromate content in the aluminum hydroxide sludge can be reduced to below 1%. This aluminum hydroxide sludge can be directly used in the production of aluminum hydroxide or cryolite. The invention utilizes pre-acidified clear liquid for chromium removal, enabling continuous production. This invention proposes a novel process for chromium removal from aluminum sludge in chromium salt production, reducing production costs and significantly improving environmental protection.

[0013] The method of the present invention breaks through the bottleneck of high energy consumption and high secondary pollution risk of traditional aluminum mud treatment technology. It has the characteristics of short process, low cost and environmental friendliness. It can be widely used in the treatment of chromium-containing aluminum mud generated in electrolytic aluminum, electroplating, leather making and other industries. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 It is a process flow chart of the present invention. DETAILED DESCRIPTION

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

[0016] (1) Laboratory experiments Example 1 A method for in-situ transformation and chromium removal of aluminum mud in chromium salt production comprises the following steps: S1, add sodium chromate alkaline solution (i.e. yellow brine) at 10m 3 / h flow rate into the reactor, and at the same time, 2m sodium chromate solution with a pH value of 5 to 6 (i.e., pre-acidified clear solution) 3 / h flow rate from the top of the reactor by spraying, and constantly stirring, stirring speed of 40r / min, maintaining the reaction temperature at 80℃, so that the aluminum in the yellow brine is precipitated as Al(OH)3; S2. Use a pH meter for online detection to always maintain the pH value of the reaction slurry at 9, while continuously discharging 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, using sodium hydroxide solution to perform an in-situ transformation washing on the chromium-aluminum mud, the concentration of the sodium hydroxide solution is 30g / L, and the transformation time is controlled within 6 hours; S4, using sodium hydroxide solution to perform a second stage of in-situ transformation washing on the chromium-aluminum mud after the first stage of in-situ transformation washing, the concentration of the sodium hydroxide solution is 30g / L, and the transformation time is controlled within 6 hours; S5. The chromium-containing aluminum sludge after the second-stage in-situ conversion washing is subjected to a third-stage in-situ conversion washing using a sodium hydroxide solution at a concentration of 30 g / L and a conversion time of 6 hours to obtain a chromium-removed aluminum sludge. The chromium-removed aluminum sludge is washed with water and then dried to obtain chromium-removed aluminum hydroxide.

[0017] The following Examples 2-6 used pH as a variable, and the other parameters were 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, it can be seen that when the pH value is 9-9.5, the particle size of the obtained aluminum mud is larger, the specific surface area is smaller, and the sedimentation is fastest, so the optimal pH value is determined to be 9-9.5.

[0019] In the following Examples 7-9, the optimal reaction temperature was determined using temperature as a variable and the other parameters were the same as in Example 1. The experimental results are shown in Table 2.

[0020] According to the particle size analysis data in Table 2, it can be seen that when the reaction temperature is 80~90℃, the particle size of the obtained aluminum mud is larger and the specific surface area is smaller, so the optimal reaction temperature is determined to be 80~90℃.

[0021] The following Examples 10-18 used the number of sodium hydroxide solution washes, wash time, and sodium hydroxide solution concentration as variables, with the remaining parameters being the same as in Example 1, to investigate the effects of these factors on wash depth, with the goal of reducing costs and increasing efficiency. The experimental results are shown in Table 3.

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

[0023] (2) Pilot test Based on the above experimental data, pilot test equipment was built in the workshop and the pilot test was completed.

[0024] Example 19 A method for in-situ transformation and chromium removal of aluminum mud in chromium salt production comprises the following steps: S1, add sodium chromate alkaline solution (i.e. yellow brine) at 12m 3 / h flow rate into the reactor, and at the same time, 3m sodium chromate solution with a pH value of 5 to 6 (i.e., pre-acidified clear solution) 3 / h flow rate from the top of the reactor by spraying, and constantly stirring, stirring speed of 40r / min, maintaining the reaction temperature at 86℃, so that the aluminum in the yellow brine is precipitated as Al(OH)3; S2. Using a pH meter for online testing, 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 and then washed with water to obtain chromium-aluminum mud; S3, using sodium hydroxide solution to perform an in-situ transformation washing on the chromium-aluminum mud, the concentration of the sodium hydroxide solution is 30g / L, and the transformation time is controlled within 6 hours; S4, using sodium hydroxide solution to perform a second stage of in-situ transformation washing on the chromium-aluminum mud after the first stage of in-situ transformation washing, the concentration of the sodium hydroxide solution is 30g / L, and the transformation time is controlled within 6 hours; S5. The chromium-containing aluminum sludge after the second-stage in-situ conversion washing is subjected to a third-stage in-situ conversion washing using a sodium hydroxide solution at a concentration of 30 g / L and a conversion time of 6 hours to obtain a chromium-removed aluminum sludge. The chromium-removed aluminum sludge is washed with water and then dried to obtain chromium-removed aluminum hydroxide.

[0025] The following Examples 20-25 were tested at different pH values ​​and reaction temperatures, with the remaining parameters being the same as those in Example 19. The experimental results are shown in Table 4.

[0026] According to the data in Table 4, it can be seen that the pilot test results are consistent with the laboratory test results. When the temperature is controlled at 80~90℃ and the pH value is controlled at 9~9.5, chromium-containing aluminum mud with coarse particles, relatively uniform particle size distribution and low chromium content can be obtained.

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

[0028] According to the data in Table 5, it can be seen that the pilot test results are consistent with the laboratory test results. Using 30 g / L sodium hydroxide solution as the alkaline washing solution, an aluminum hydroxide product with a hexavalent chromium content of 5 ppm can be obtained after three washings.

[0029] (3) Project implementation results According to the experimental results, the process route of using pre-acidification liquid as neutralizing liquid to neutralize yellow brine to produce chromium-containing aluminum mud with good filtration performance and low chromium content, and then washing it three times with sodium hydroxide solution to produce qualified aluminum hydroxide is feasible and achieves the expected experimental purpose.

[0030] The chromium content of the aluminum hydroxide sludge obtained by the present invention after chromium removal, calculated as sodium chromate, is reduced to below 5 ppm. This aluminum hydroxide sludge can be directly used in the production of aluminum hydroxide or cryolite. The present invention utilizes pre-acidified supernatant to remove chromium, enabling continuous production. This method provides a new process for removing chromium from aluminum sludge in chromium salt production, reducing production costs and achieving significant environmental benefits.

Claims

1. A method for in-situ transformation and chromium removal of aluminum mud in chromium salt production, characterized in that The following steps are involved: S1. Add sodium chromate alkaline solution into the reactor, and at the same time add sodium chromate solution from the top of the reactor by spraying, and stir continuously, maintaining the reaction temperature at 80-90°C; S2. Always maintain the pH value of the reaction slurry at 9-9.5, while continuously discharging 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, using sodium hydroxide solution to perform an in-situ transformation washing on the chromium-aluminum mud, the transformation time is not less than 6 hours; S4. Using sodium hydroxide solution, perform a second stage of in-situ transformation washing on the chromium-aluminum sludge after the first stage of in-situ transformation washing, with the transformation time being not less than 6 hours; S5. Using sodium hydroxide solution, the chromium-containing aluminum mud after the second-stage in-situ transformation washing is subjected to a third-stage in-situ transformation washing, with the transformation time being no less than 6 hours, to obtain the aluminum mud after the chromium is removed.

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

3. The method for in-situ transformation and chromium removal of aluminum mud in chromium salt production according to claim 2, characterized in that: In step S1, sodium chromate solution is prepared at 2 m 3 / h~3m 3 / h flow rate of spraying addition.

4. The method for in-situ transformation and chromium removal of aluminum mud 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 transformation and chromium removal of aluminum mud 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. The method for in-situ transformation and chromium removal of aluminum mud in chromium salt production according to any one of claims 1 to 5, characterized in that: After the second stage in-situ transformation washing, barium salt is added to the chromium-containing sodium hydroxide solution to remove chromium, and then the obtained dilute sodium hydroxide solution is evaporated and used for the third stage in-situ transformation washing.

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