A fly ash circulating gradient pickling pretreatment method

Abstract

The present application relates to solid waste treatment technical field, disclose a kind of fly ash circulation gradient pickling pretreatment method.The method is carried out three-stage gradient pickling to dechlorination fly ash.First-stage pickling is carried out by leaching solution to zinc in fly ash and separates zinc precipitate.Second-stage pickling is carried out to copper and separates copper.Three-stage pickling is carried out to lead and cadmium and separates lead and cadmium.The method selectively separates zinc and copper with higher value, improves the efficiency of fly ash metal resource utilization.The method significantly reduces the content of heavy metals in fly ash, significantly improves the quality of detoxification fly ash.The method also uses benzothiazole thiohydantoin as second-stage pickling solution, benzothiazole thiohydantoin and citric acid-tea saponin system produce synergistic effect, can inhibit iron and aluminum dissolution and significantly improve copper dissolution rate, can significantly improve copper product purity.

Description

Technical Field

[0001] This invention relates to the field of solid waste treatment technology, and in particular to a method for pretreatment of fly ash by circulating gradient acid washing. Background Technology

[0002] Incineration fly ash contains a large amount of metal elements and chloride salts. If these components are reused or landfilled directly without treatment, they will severely pollute the environment and reduce the performance of reused products. Therefore, detoxification treatment is necessary before applying incineration fly ash. Traditional fly ash detoxification methods involve water washing, which removes chlorine and heavy metals from the fly ash through leaching, significantly reducing the content of metal elements and chloride salts. For example, CN116371893B discloses a system and method for enhancing the dechlorination effect of fly ash water washing and reducing the toxicity of heavy metal leaching. This technical solution uses an automatic control system to control the water washing and phosphoric acid washing processes of the fly ash, improving chloride leaching efficiency, reducing the concentration of heavy metals in the leaching solution, and increasing the purity of chloride recovery.

[0003] After in-depth research, this invention found that while the aforementioned technical solutions reduce heavy metal leaching from fly ash by controlling the endpoint pH and precipitate the leached heavy metals as phosphates through phosphoric acid washing, they do not effectively utilize the heavy metals in the fly ash. Fly ash typically contains high levels of zinc, lead, copper, and cadmium, which pose a high risk of leaching in subsequent applications. Therefore, it is necessary to minimize the content of these heavy metals in fly ash. Furthermore, zinc and copper have high secondary utilization value. Therefore, providing a technical solution that accelerates the effective leaching and efficient separation and recovery of zinc, copper, lead, and cadmium is of great significance for improving the resource utilization of incinerated waste fly ash. Summary of the Invention

[0004] The purpose of this invention is to improve the leaching of zinc, lead, copper, and cadmium from fly ash and to efficiently separate and recover zinc, copper, lead, and cadmium.

[0005] This invention employs a three-stage gradient acid washing process for dechlorinated fly ash. The first-stage acid washing leaches zinc from the fly ash using a leaching solution, followed by zinc precipitation and separation. The second-stage acid washing leaches and separates copper. The third-stage acid washing leaches and separates lead and cadmium. This method selectively separates the more valuable zinc and copper, improving the efficiency of fly ash metal resource utilization. This method significantly reduces the heavy metal content in fly ash and significantly improves the quality of detoxified fly ash. Furthermore, this method utilizes benzothiazolylthiourea as the second-stage acid washing solution. Benzothiazolylthiourea synergistically interacts with the citric acid-tea saponin system, inhibiting iron and aluminum leaching and significantly increasing copper leaching rate, thereby significantly improving the purity of copper products.

[0006] The specific technical solution of this invention is as follows: A method for pretreatment of fly ash by circulating gradient acid washing includes the following steps: (1) Add the dechlorinated fly ash to the leaching solution for primary pickling to produce primary filter residue and primary leaching solution. Zinc product is obtained by zinc precipitation in the primary pickling solution. (2) The primary filter residue is added to the secondary pickling solution for secondary pickling to produce secondary filter residue and secondary leaching solution. The secondary pickling solution is benzothiazolyl thiourea and leaching solution. The secondary pickling solution is electrolyzed to produce copper products. (3) The secondary filter residue is added to the tertiary pickling solution for tertiary pickling to produce detoxified fly ash, dilute tail liquid and metal concentrate; the tertiary pickling solution is EDTA solution and leaching solution, the metal concentrate is used to precipitate lead and cadmium to obtain lead and cadmium products, the dilute tail liquid is returned to the secondary pickling solution, and the leaching solution is citric acid and tea saponin.

[0007] As a preferred option, the conditions for primary pickling include: pH 5.2-5.5, and reaction at room temperature for 30-40 minutes.

[0008] As a preferred method, the zinc precipitation conditions are as follows: zinc powder is added, stirred and reacted, and filtered to obtain filtrate and crude copper slag. Carbonate is added to the filtrate and stirred and reacted at 35~45℃ for 20~30 min.

[0009] As a preferred option, the conditions for secondary acid washing include: pH 3~4.5, stirring reaction for 30~40 min, and a molar ratio of benzothiazolylthiourea to copper in fly ash of 2~3:1.

[0010] As a preferred option, the conditions for tertiary pickling include: pH 5.5-6.5, reaction at room temperature for 30-40 min, and a molar ratio of EDTA solution to lead and cadmium in fly ash of 1-2:1.

[0011] Preferred conditions for lead-cadmium precipitation include: adjusting the pH to 8-8.5, slowly adding sodium sulfide solution and stirring for 20-30 minutes.

[0012] As a preferred method, crude copper slag is added to citric acid and benzothiazolyl thiourea to separate the acid hydrolysate, which is then electrolyzed to produce copper products.

[0013] Preferably, the electrolytic treatment conditions include: stainless steel cathode, inert graphite anode, cell voltage of 2.2~2.8V, and current density of 100~120A / m. 2 .

[0014] Preferably, the concentration of citric acid is 110~120g / L.

[0015] Preferably, the concentration of tea saponin is 7–9 g / L.

[0016] This invention provides a pretreatment method for fly ash cyclic gradient acid washing. This method employs a three-stage acid washing process to separate zinc, copper, lead, and cadmium from incinerator fly ash, significantly reducing the heavy metal content in the detoxified fly ash. It also yields valuable zinc, copper, and lead-cadmium products, with the copper product exhibiting high purity, significantly improving the resource utilization rate of heavy metals in fly ash. This invention uses citric acid-tea saponin as the leaching solution. In the first-stage acid washing process, the citric acid-tea saponin system preferentially leaches zinc from the fly ash while avoiding the dissolution of calcium, aluminum, and other components, thus preferentially leaching and separating zinc. In the second-stage acid washing process, this invention uses a thiourea chelating agent to selectively complex copper in the fly ash. In the third-stage acid washing process, the broad-spectrum chelating agent EDTA is selected to complex lead, cadmium, and other metal elements in the fly ash, significantly reducing the heavy metal content in the detoxified fly ash and improving the application quality of the detoxified fly ash.

[0017] This invention, using thiourea chelating agents, discovered that conventional sulfur-based chelating agents dissolve iron along with copper, resulting in iron impurities in the copper product and reduced purity. Therefore, this invention further optimized the thiourea chelating agent, ultimately finding that the copper product from benzothiazolyl thiourea contained very few iron impurities and had high purity. Furthermore, this invention also discovered a synergistic effect between benzothiazolyl thiourea and citric acid-tea saponin. The hydrophobic benzene ring structure of benzothiazolyl thiourea can generate strong intermolecular interactions with the hydrophobic triterpenoid ends of tea saponin, forming an enriched adsorption layer on the surface of fly ash particles. This enhances the contact efficiency between the active component and the copper mineral sites and further blocks the dissolution of impurity metals such as iron and aluminum, achieving highly selective copper leaching.

[0018] Compared with the prior art, this application has the following technical effects: (1) The three-stage acid washing process separated zinc, copper, lead and lead, which are present in high amounts in incinerated waste fly ash, significantly reduced the heavy metal content in detoxified fly ash, and significantly improved the utilization rate of heavy metal resources in fly ash. (2) The copper in fly ash was leached with benzothiazolyl thiourea and citric acid-tea saponin with high selectivity, which significantly improved the copper leaching rate in fly ash and significantly improved the purity of copper products. Detailed Implementation

[0019] The present invention will be further described below with reference to embodiments.

[0020] To better understand the content of this invention, further explanation is provided below with reference to specific embodiments. It should be understood that the following embodiments are for illustrative purposes only and are not intended to limit the scope of this invention.

[0021] Example 1:

[0022] A method for pretreatment of fly ash by circulating gradient acid washing includes the following steps: (1) The incinerated waste fly ash is washed with water to obtain dechlorinated fly ash, and the solid-liquid ratio of the water washing treatment is 1:3; (2) Add the dechlorinated fly ash and leaching solution (citric acid-tea saponin leaching solution, the concentration of citric acid is 115g / L and the concentration of tea saponin is 8g / L) to the first-stage pickling tank of the multi-stage countercurrent pickling device at a solid-liquid ratio of 1:3. Control the pH to 5.2~5.5 and carry out the first-stage pickling treatment at room temperature for 35 minutes. After the treatment is completed, filter the solution to produce the first-stage filter residue and the first-stage leaching solution. (2) Add zinc powder to the first-stage leaching solution and stir for 20 min. Filter to obtain filtrate and crude copper slag. Add crude copper slag to citric acid and benzothiazolyl thiourea to separate the acid hydrolysate. Slowly add sodium carbonate solution to the filtrate and stir for 25 min at 40°C. Filter to obtain zinc product (zinc carbonate). (3) Add the primary filter residue and the secondary pickling solution (benzothiazolyl thiourea and citric acid-tea saponin leaching solution, with the molar ratio of benzothiazolyl thiourea to copper in fly ash being 2.5:1) to the secondary pickling tank of the multi-stage countercurrent pickling device at a solid-liquid ratio of 1:3. Control the pH to 3~4.5 and stir for 35 minutes. After the treatment is completed, filter the solution to produce secondary filter residue and secondary leaching solution. (4) The secondary leaching solution and acid hydrolysis solution are subjected to activated carbon adsorption to remove organic impurities and prepare an electrolyte. The electrolyte is then injected into an electrolysis device to carry out an electrolysis reaction to produce copper products. The parameters of the electrolysis device are: stainless steel cathode, inert graphite anode, cell voltage 2.5V, and current density 110A / m. 2 ; (5) Add the secondary filter residue and the tertiary pickling solution (EDTA solution and citric acid-tea saponin extract, with the molar ratio of EDTA solution to lead and cadmium in fly ash being 1.5:1) to the tertiary pickling tank of the multi-stage countercurrent pickling device at a solid-liquid ratio of 1:3. Control the pH to 5.5~6.5 and react at room temperature for 35 minutes. After the treatment is completed, filter the residue to produce detoxified fly ash, dilute tail liquid and concentrated metal liquid. The dilute tail liquid is returned to the secondary pickling solution. (6) Use sodium hydroxide solution to adjust the pH of the metal concentrate to 8~8.5, then slowly add sodium sulfide solution and stir for 25 minutes, then filter to obtain lead-cadmium products (lead sulfide and cadmium sulfide).

[0023] Example 2:

[0024] A method for pretreatment of fly ash by circulating gradient acid washing includes the following steps: (1) The incinerated waste fly ash is washed with water to obtain dechlorinated fly ash, and the solid-liquid ratio of the water washing treatment is 1:3; (2) Add the dechlorinated fly ash and leaching solution (citric acid-tea saponin leaching solution, the concentration of citric acid is 110g / L and the concentration of tea saponin is 7g / L) to the first-stage pickling tank of the multi-stage countercurrent pickling device at a solid-liquid ratio of 1:3. Control the pH to 5.2~5.5 and carry out the first-stage pickling treatment at room temperature for 30 minutes. After the treatment is completed, filter the solution to produce the first-stage filter residue and the first-stage leaching solution. (2) Add zinc powder to the first-stage leaching solution and stir for 20 min. Filter to obtain filtrate and crude copper slag. Add crude copper slag to citric acid and benzothiazolyl thiourea to separate the acid hydrolysate. Slowly add sodium carbonate solution to the filtrate and stir for 20 min at 35°C. Filter to obtain zinc product (zinc carbonate). (3) Add the primary filter residue and the secondary pickling solution (benzothiazolyl thiourea and citric acid-tea saponin leaching solution, with a molar ratio of benzothiazolyl thiourea to copper in fly ash of 2:1) to the secondary pickling tank of the multi-stage countercurrent pickling device at a solid-liquid ratio of 1:3. Control the pH to 3~4.5 and stir for 30 minutes. After the treatment is completed, filter the solution to produce secondary filter residue and secondary leaching solution. (4) The secondary leaching solution and acid hydrolysis solution are subjected to activated carbon adsorption to remove organic impurities and prepare an electrolyte. The electrolyte is then injected into an electrolysis device to carry out an electrolysis reaction to produce copper products. The parameters of the electrolysis device are: stainless steel cathode, inert graphite anode, cell voltage 2.2V, and current density 100A / m. 2 ; (5) Add the secondary filter residue and the tertiary pickling solution (EDTA solution and citric acid-tea saponin extract, with the molar ratio of EDTA solution to lead and cadmium in fly ash being 1:1) to the tertiary pickling tank of the multi-stage countercurrent pickling device at a solid-liquid ratio of 1:3. Control the pH to 5.5~6.5 and react at room temperature for 30 minutes. After the treatment is completed, filter the solution to produce detoxified fly ash, dilute tail liquid and concentrated metal solution. The dilute tail liquid is returned to the secondary pickling solution. (6) Use sodium hydroxide solution to adjust the pH of the metal concentrate to 8~8.5, then slowly add sodium sulfide solution and stir for 20 minutes, then filter to obtain lead-cadmium products (lead sulfide and cadmium sulfide).

[0025] Example 3:

[0026] A method for pretreatment of fly ash by circulating gradient acid washing includes the following steps: (1) The incinerated waste fly ash is washed with water to obtain dechlorinated fly ash, and the solid-liquid ratio of the water washing treatment is 1:3; (2) Add the dechlorinated fly ash and leaching solution (citric acid-tea saponin leaching solution, the concentration of citric acid is 120g / L and the concentration of tea saponin is 9g / L) to the first-stage pickling tank of the multi-stage countercurrent pickling device at a solid-liquid ratio of 1:3. Control the pH to 5.2~5.5 and carry out the first-stage pickling treatment at room temperature for 40 minutes. After the treatment is completed, filter the solution to produce the first-stage filter residue and the first-stage leaching solution. (2) Add zinc powder to the first-stage leaching solution and stir for 20 min. Filter to obtain filtrate and crude copper slag. Add crude copper slag to citric acid and benzothiazolyl thiourea to separate the acid hydrolysate. Slowly add sodium carbonate solution to the filtrate and stir for 30 min at 45°C. Filter to obtain zinc product (zinc carbonate). (3) Add the primary filter residue and the secondary pickling solution (benzothiazolyl thiourea and citric acid-tea saponin leaching solution, with a solid-liquid ratio of 1:3) to the secondary pickling tank of the multi-stage countercurrent pickling device, control the pH to 3~4.5 and stir for 40 min. After the treatment is completed, filter the solution to produce secondary filter residue and secondary leaching solution. (4) The secondary leaching solution and acid hydrolysis solution are subjected to activated carbon adsorption to remove organic impurities and prepare an electrolyte. The electrolyte is then injected into an electrolysis device to carry out an electrolysis reaction to produce copper products. The parameters of the electrolysis device are: cathode stainless steel plate, anode inert graphite, cell voltage 2.8V, current density 120A / m 2 ; (5) Add the secondary filter residue and the tertiary pickling solution (EDTA solution and citric acid-tea saponin extract, with the molar ratio of EDTA solution to lead and cadmium in fly ash being 2:1) to the tertiary pickling tank of the multi-stage countercurrent pickling device at a solid-liquid ratio of 1:3. Control the pH to 5.5~6.5 and react at room temperature for 40 minutes. After the treatment is completed, filter the solution to produce detoxified fly ash, dilute tail liquid and concentrated metal solution. The dilute tail liquid is returned to the secondary pickling solution. (6) Use sodium hydroxide solution to adjust the pH of the metal concentrate to 8~8.5, then slowly add sodium sulfide solution and stir for 30 minutes, then filter to obtain lead-cadmium products (lead sulfide and cadmium sulfide).

[0027] Comparative Example 1: The difference between Comparative Example 1 and Example 1 is that the dechlorinated fly ash was not acid-washed, while all other conditions were the same as in Example 1.

[0028] Comparative Example 2: The difference between Comparative Example 2 and Example 1 is that thiourea is used instead of benzothiazolylthiourea, and the following steps are included: (1) The incinerated waste fly ash is washed with water to obtain dechlorinated fly ash, and the solid-liquid ratio of the water washing treatment is 1:3; (2) Add the dechlorinated fly ash and leaching solution (citric acid-tea saponin leaching solution, the concentration of citric acid is 115g / L and the concentration of tea saponin is 8g / L) to the first-stage pickling tank of the multi-stage countercurrent pickling device at a solid-liquid ratio of 1:3. Control the pH to 5.2~5.5 and carry out the first-stage pickling treatment at room temperature for 35 minutes. After the treatment is completed, filter the solution to produce the first-stage filter residue and the first-stage leaching solution. (2) Add zinc powder to the first-stage leaching solution and stir for 20 min. Filter to obtain filtrate and crude copper slag. Add crude copper slag to citric acid and benzothiazolyl thiourea to separate the acid hydrolysate. Slowly add sodium carbonate solution to the filtrate and stir for 25 min at 40°C. Filter to obtain zinc product (zinc carbonate). (3) Add the primary filter residue and the secondary pickling solution (thiourea and citric acid-tea saponin leaching solution, with a molar ratio of thiourea to copper in fly ash of 2.5:1) to the secondary pickling tank of the multi-stage countercurrent pickling device at a solid-liquid ratio of 1:3. Control the pH to 3~4.5 and stir for 35 minutes. After the treatment is completed, filter the solution to produce secondary filter residue and secondary leaching solution. (4) The secondary leaching solution and acid hydrolysis solution are subjected to activated carbon adsorption to remove organic impurities and prepare an electrolyte. The electrolyte is then injected into an electrolysis device to carry out an electrolysis reaction to produce copper products. The parameters of the electrolysis device are: stainless steel cathode, inert graphite anode, cell voltage 2.5V, and current density 110A / m. 2 ; (5) Add the secondary filter residue and the tertiary pickling solution (EDTA solution and citric acid-tea saponin extract, with the molar ratio of EDTA solution to lead and cadmium in fly ash being 1.5:1) to the tertiary pickling tank of the multi-stage countercurrent pickling device at a solid-liquid ratio of 1:3. Control the pH to 5.5~6.5 and react at room temperature for 35 minutes. After the treatment is completed, filter the residue to produce detoxified fly ash, dilute tail liquid and concentrated metal liquid. The dilute tail liquid is returned to the secondary pickling solution. (6) Use sodium hydroxide solution to adjust the pH of the metal concentrate to 8~8.5, then slowly add sodium sulfide solution and stir for 25 minutes, then filter to obtain lead-cadmium products (lead sulfide and cadmium sulfide).

[0029] Comparative Example 3: The difference between Comparative Example 3 and Example 1 is that p-tolylthiourea is used instead of benzothiazolylthiourea, and the following steps are included: (1) The incinerated waste fly ash is washed with water to obtain dechlorinated fly ash, and the solid-liquid ratio of the water washing treatment is 1:3; (2) Add the dechlorinated fly ash and leaching solution (citric acid-tea saponin leaching solution, the concentration of citric acid is 115g / L and the concentration of tea saponin is 8g / L) to the first-stage pickling tank of the multi-stage countercurrent pickling device at a solid-liquid ratio of 1:3. Control the pH to 5.2~5.5 and carry out the first-stage pickling treatment at room temperature for 35 minutes. After the treatment is completed, filter the solution to produce the first-stage filter residue and the first-stage leaching solution. (2) Add zinc powder to the first-stage leaching solution and stir for 20 min. Filter to obtain filtrate and crude copper slag. Add crude copper slag to citric acid and benzothiazolyl thiourea to separate the acid hydrolysate. Slowly add sodium carbonate solution to the filtrate and stir for 25 min at 40°C. Filter to obtain zinc product (zinc carbonate). (3) Add the primary filter residue and the secondary pickling solution (toluene-thiourea and citric acid-tea saponin leaching solution, with the molar ratio of toluene-thiourea to copper in fly ash being 2.5:1) to the secondary pickling tank of the multi-stage countercurrent pickling device at a solid-liquid ratio of 1:3. Control the pH to 3~4.5 and stir for 35 minutes. After the treatment is completed, filter the solution to produce secondary filter residue and secondary leaching solution. (4) The secondary leaching solution and acid hydrolysis solution are subjected to activated carbon adsorption to remove organic impurities and prepare an electrolyte. The electrolyte is then injected into an electrolysis device to carry out an electrolysis reaction to produce copper products. The parameters of the electrolysis device are: stainless steel cathode, inert graphite anode, cell voltage 2.5V, and current density 110A / m. 2 ; (5) Add the secondary filter residue and the tertiary pickling solution (EDTA solution and citric acid-tea saponin extract, with the molar ratio of EDTA solution to lead and cadmium in fly ash being 1.5:1) to the tertiary pickling tank of the multi-stage countercurrent pickling device at a solid-liquid ratio of 1:3. Control the pH to 5.5~6.5 and react at room temperature for 35 minutes. After the treatment is completed, filter the residue to produce detoxified fly ash, dilute tail liquid and concentrated metal liquid. The dilute tail liquid is returned to the secondary pickling solution. (6) Use sodium hydroxide solution to adjust the pH of the metal concentrate to 8~8.5, then slowly add sodium sulfide solution and stir for 25 minutes, then filter to obtain lead-cadmium products (lead sulfide and cadmium sulfide).

[0030] Comparative Example 4: The difference between Comparative Example 4 and Example 1 is that methylthiourea is used instead of benzothiazolylthiourea, and the following steps are included: (1) The incinerated waste fly ash is washed with water to obtain dechlorinated fly ash, and the solid-liquid ratio of the water washing treatment is 1:3; (2) Add the dechlorinated fly ash and leaching solution (citric acid-tea saponin leaching solution, the concentration of citric acid is 115g / L and the concentration of tea saponin is 8g / L) to the first-stage pickling tank of the multi-stage countercurrent pickling device at a solid-liquid ratio of 1:3. Control the pH to 5.2~5.5 and carry out the first-stage pickling treatment at room temperature for 35 minutes. After the treatment is completed, filter the solution to produce the first-stage filter residue and the first-stage leaching solution. (2) Add zinc powder to the first-stage leaching solution and stir for 20 min. Filter to obtain filtrate and crude copper slag. Add crude copper slag to citric acid and benzothiazolyl thiourea to separate the acid hydrolysate. Slowly add sodium carbonate solution to the filtrate and stir for 25 min at 40°C. Filter to obtain zinc product (zinc carbonate). (3) Add the primary filter residue and the secondary pickling solution (methylthiourea and citric acid-tea saponin leaching solution, with the molar ratio of methylthiourea to copper in fly ash being 2.5:1) to the secondary pickling tank of the multi-stage countercurrent pickling device at a solid-liquid ratio of 1:3. Control the pH to 3~4.5 and stir for 35 minutes. After the treatment is completed, filter the solution to produce secondary filter residue and secondary leaching solution. (4) The secondary leaching solution and acid hydrolysis solution are subjected to activated carbon adsorption to remove organic impurities and prepare an electrolyte. The electrolyte is then injected into an electrolysis device to carry out an electrolysis reaction to produce copper products. The parameters of the electrolysis device are: stainless steel cathode, inert graphite anode, cell voltage 2.5V, and current density 110A / m. 2 ; (5) Add the secondary filter residue and the tertiary pickling solution (EDTA solution and citric acid-tea saponin extract, with the molar ratio of EDTA solution to lead and cadmium in fly ash being 1.5:1) to the tertiary pickling tank of the multi-stage countercurrent pickling device at a solid-liquid ratio of 1:3. Control the pH to 5.5~6.5 and react at room temperature for 35 minutes. After the treatment is completed, filter the residue to produce detoxified fly ash, dilute tail liquid and concentrated metal liquid. The dilute tail liquid is returned to the secondary pickling solution. (6) Use sodium hydroxide solution to adjust the pH of the metal concentrate to 8~8.5, then slowly add sodium sulfide solution and stir for 25 minutes, then filter to obtain lead-cadmium products (lead sulfide and cadmium sulfide).

[0031] Comparative Example 5: The difference between Comparative Example 5 and Example 1 is that no tea saponin was added to the secondary pickling solution, and the process included the following steps: (1) The incinerated waste fly ash is washed with water to obtain dechlorinated fly ash, and the solid-liquid ratio of the water washing treatment is 1:3; (2) Add the dechlorinated fly ash and leaching solution (citric acid-tea saponin leaching solution, the concentration of citric acid is 115g / L and the concentration of tea saponin is 8g / L) to the first-stage pickling tank of the multi-stage countercurrent pickling device at a solid-liquid ratio of 1:3. Control the pH to 5.2~5.5 and carry out the first-stage pickling treatment at room temperature for 35 minutes. After the treatment is completed, filter the solution to produce the first-stage filter residue and the first-stage leaching solution. (2) Add zinc powder to the first-stage leaching solution and stir for 20 min. Filter to obtain filtrate and crude copper slag. Add crude copper slag to citric acid and benzothiazolyl thiourea to separate the acid hydrolysate. Slowly add sodium carbonate solution to the filtrate and stir for 25 min at 40°C. Filter to obtain zinc product (zinc carbonate). (3) Add the primary filter residue and the secondary pickling solution (benzothiazolyl thiourea and citric acid leaching solution, with a citric acid concentration of 115 g / L and a molar ratio of benzothiazolyl thiourea to copper in fly ash of 2.5:1) to the secondary pickling tank of the multi-stage countercurrent pickling device at a solid-liquid ratio of 1:3. Control the pH to 3~4.5 and stir for 35 min. After the treatment is completed, filter the solution to produce secondary filter residue and secondary leaching solution. (4) The secondary leaching solution and acid hydrolysis solution are subjected to activated carbon adsorption to remove organic impurities and prepare an electrolyte. The electrolyte is then injected into an electrolysis device to carry out an electrolysis reaction to produce copper products. The parameters of the electrolysis device are: stainless steel cathode, inert graphite anode, cell voltage 2.5V, and current density 110A / m.2 ; (5) Add the secondary filter residue and the tertiary pickling solution (EDTA solution and citric acid-tea saponin extract, with the molar ratio of EDTA solution to lead and cadmium in fly ash being 1.5:1) to the tertiary pickling tank of the multi-stage countercurrent pickling device at a solid-liquid ratio of 1:3. Control the pH to 5.5~6.5 and react at room temperature for 35 minutes. After the treatment is completed, filter the residue to produce detoxified fly ash, dilute tail liquid and concentrated metal liquid. The dilute tail liquid is returned to the secondary pickling solution. (6) Use sodium hydroxide solution to adjust the pH of the metal concentrate to 8~8.5, then slowly add sodium sulfide solution and stir for 25 minutes, then filter to obtain lead-cadmium products (lead sulfide and cadmium sulfide).

[0032] Example of detection: The chemical composition, metal content, and copper product purity of the dechlorinated fly ash prepared in Examples 1-3 and Comparative Examples 1-5 were tested. The metal content was tested in accordance with the published content of "GB / T 176-2017 Cement Chemical Analysis Methods" and "HJ 781-2016 Determination of Metal Elements in Solid Waste by Graphite Furnace Atomic Absorption Spectrophotometry". The purity of copper products was tested in accordance with the content published in GB / T 467-2010 Cathode Copper; The test results are shown in Table 1; Table 1 Test Results

[0033] As shown in Table 1, compared with Comparative Example 1 (without acid washing), the contents of zinc, copper, lead, and cadmium in the dechlorination fly ash of Examples 1-3 were significantly reduced after three-stage acid washing. Calculations showed that the recovery rates of copper reached 92.26%, zinc 90.32%, cadmium 84.39%, and lead 90.32%. The purity of the obtained copper product reached 99.42%. These results indicate that the method of the present invention can significantly reduce the content of heavy metal elements in dechlorination fly ash, and the detoxified fly ash has high aluminum, silicon, and calcium content, significantly improving the quality of the detoxified fly ash. This method can also recover the separated zinc, copper, lead, and cadmium, significantly improving resource utilization efficiency.

[0034] In Comparative Examples 2, 3, and 4, thiourea, tolylthiourea (phenylthiourea), and methylthiourea (alkylthiourea) were selected as copper chelating agents, respectively. Compared to Comparative Examples 2, 3, and 4, the copper products obtained from thiourea, tolylthiourea, and methylthiourea in Example 1 had significantly lower purity than those obtained from benzothiazolylthiourea. Analysis revealed that thiourea, tolylthiourea, and methylthiourea not only chelate copper but also iron, resulting in iron impurities in the electrolyzed copper products. In contrast, benzothiazolylthiourea exhibited high selectivity for copper but low selectivity for iron, thus significantly reducing the iron content in the copper products. These results indicate that using benzothiazolylthiourea can significantly improve the purity of copper products and significantly increase the value of the recycled products.

[0035] Comparative Example 5 did not use tea saponin. Compared with Example 1, the purity of the copper product in Comparative Example 5 was significantly reduced, while the leaching of iron and aluminum was significantly increased, resulting in a significant decrease in the purity of the copper product. These results indicate that tea saponin and benzothiazolylthiourea have a synergistic effect, significantly inhibiting the leaching of iron and aluminum from fly ash and significantly improving the purity of the copper product.

[0036] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Any simple modifications, alterations, and equivalent transformations made to the above embodiments based on the technical essence of the present invention shall still fall within the protection scope of the present invention.

Claims

1. A method for pretreatment of fly ash by circulating gradient acid washing, characterized in that, Includes the following steps: (1) Add the dechlorinated fly ash to the leaching solution for primary pickling to produce primary filter residue and primary leaching solution. Zinc product is obtained by zinc precipitation in the primary pickling solution. (2) The primary filter residue is added to the secondary pickling solution for secondary pickling to produce secondary filter residue and secondary leaching solution. The secondary pickling solution is benzothiazolyl thiourea and leaching solution. The secondary pickling solution is electrolyzed to produce copper products. (3) The secondary filter residue is added to the tertiary pickling solution for tertiary pickling to produce detoxified fly ash, dilute tail liquid and metal concentrate; the tertiary pickling solution is EDTA solution and leaching solution, the metal concentrate is used to precipitate lead and cadmium to obtain lead and cadmium products, the dilute tail liquid is returned to the secondary pickling solution, and the leaching solution is citric acid and tea saponin.

2. The method according to claim 1, characterized in that, The conditions for primary pickling include: pH 5.2-5.5, and reaction at room temperature for 30-40 minutes.

3. The method according to claim 1, characterized in that, The conditions for zinc precipitation are as follows: zinc powder is added, stirred and reacted, and filtered to obtain filtrate and crude copper slag. Carbonate is added to the filtrate and stirred and reacted at 35~45℃ for 20~30 min.

4. The method according to claim 1, characterized in that, The conditions for secondary acid washing include: pH 3~4.5, stirring reaction for 30~40 min, and a molar ratio of benzothiazolylthiourea to copper in fly ash of 2~3:

1.

5. The method according to claim 1, characterized in that, The conditions for tertiary pickling include: pH 5.5-6.5, reaction at room temperature for 30-40 min, and a molar ratio of EDTA solution to lead and cadmium in fly ash of 1-2:

1.

6. The method according to claim 1, characterized in that, The conditions for lead-cadmium precipitation include: adjusting the pH to 8-8.5, slowly adding sodium sulfide solution and stirring for 20-30 minutes.

7. The method according to claim 3, characterized in that, Crude copper slag is added to citric acid and benzothiazolyl thiourea to separate the acid hydrolysate, which is then electrolyzed to produce copper products.

8. The method according to claim 1 or 7, characterized in that, The conditions for electrolytic treatment include: stainless steel cathode, inert graphite anode, cell voltage of 2.2~2.8V, and current density of 100~120A / m. 2 .

9. The method according to claim 1, characterized in that, The concentration of citric acid is 110~120g / L.

10. The method according to claim 1, characterized in that, The concentration of tea saponin is 7–9 g / L.

Images