Method for preparing fertilizer grade ammonium sulfate by deep purification of copper smelting waste acid

Abstract

The application discloses a method for preparing fertilizer-grade ammonium sulfate by deeply purifying copper smelting waste acid. The copper smelting waste acid is doped, the pH value is adjusted, and copper and zinc are extracted in sequence; the pH value of the obtained raffinate is adjusted, bismuth oxide is added for dechlorination by precipitation; the pH value of the obtained liquid after dechlorination is adjusted, iron source, crystal seeds of crandallite and oxidizing agent are added, and arsenic is precipitated by aeration oxidation; the pH value of the obtained liquid after arsenic precipitation is adjusted, and iron is hydrolyzed to generate a precipitate; lanthanum sulfate is added to the liquid after iron precipitation to precipitate and remove fluorine; the obtained liquid after fluorine removal is contacted with an adsorbent to remove residual metal ions; the obtained purified liquid is concentrated, crystallized, separated and dried, and the target product is obtained. The method realizes the resource recycling of valuable metals and the deep removal of harmful elements in the treatment of copper smelting waste acid by constructing a multi-level collaborative purification system. The pH value is gradient-controlled in the whole process by an ammonia-sulfuric acid system, no foreign impurities are introduced, and the target product components are directly converted, so that the fertilizer-grade ammonium sulfate product meeting the standard is prepared.

Description

Technical Field

[0001] This invention belongs to the field of non-ferrous metallurgical waste resource utilization technology, specifically relating to a method for deep purification of copper smelting waste acid to prepare fertilizer-grade ammonium sulfate. Background Technology

[0002] During copper smelting, the flue gas scrubbing and purification process generates a large amount of waste acid. This waste acid is characterized by high acidity and high arsenic content, and also contains Cu, Zn, F, Cl, and small amounts of Fe, Sb, Mg, Mn, Pb, Cd, Hg, and other multi-component impurities. Due to the complex forms of arsenic, fluorine, chlorine, cadmium, mercury, and heterocations present in this waste acid, to utilize it as a resource and produce fertilizer-grade ammonium sulfate products that meet the requirements of GB / T 535-2020 "Fertilizer-Grade Ammonium Sulfate," deep purification of the waste acid is necessary to ensure that the limits of harmful substances meet the product standard requirements. However, existing treatment methods are insufficient for simultaneously and deeply removing harmful impurities from this complex water quality to compliant levels.

[0003] In the prior art, patent CN110482740B discloses a method for treating waste acid. This method involves neutralizing waste acid from steel sintering desulfurization and denitrification to a pH of 3-5 with liquid ammonia, adding a heavy metal scavenging agent, polyferric iron, and PAM to remove heavy metals, and then extracting ammonium chloride and ammonium sulfate through temperature-controlled crystallization after filtration via a ceramic membrane. This method treats waste acid from steel sintering desulfurization and denitrification, whose water quality characteristics differ significantly from those of copper smelting waste acid. Therefore, its process route is difficult to directly apply to the deep purification of copper smelting waste acid.

[0004] Patent CN218833877U discloses a waste acid liquid ammonium sulfate recovery device, which uses waste acid liquid to react with ammonia gas to produce ammonium sulfate. Patent CN121850011A discloses a method for preparing silica and ammonium sulfate from titanium dioxide waste acid after precision filtration and low-temperature crystallization for pre-iron removal. Although the above technologies are all product-oriented towards ammonium sulfate, they treat general waste acid or titanium dioxide waste acid respectively, and neither has carried out synergistic purification research on the complex water quality of copper smelting waste acid, which contains high concentrations of arsenic, fluorine, chlorine, and multiple heavy metals.

[0005] Furthermore, the lime (iron salt) neutralization method, sulfide precipitation method, or their combined routes, as well as membrane separation / ion exchange method, used in the treatment of waste acid from copper smelting also have their own shortcomings: The lime (iron salt) neutralization method neutralizes acidity and precipitates heavy metals and arsenic by adding lime or limestone. This process produces a large amount of slag, in which arsenic mainly exists as calcium arsenic or iron arsenic compounds, which are prone to secondary leaching and exhibit poor arsenic stability. Simultaneously, the large amount of calcium introduced... 2+ This results in heterocation contamination in subsequent ammonium sulfate products.

[0006] The sulfide precipitation method utilizes the reaction of sulfides with heavy metal ions to form insoluble sulfide precipitates. The precipitate is highly toxic arsenic sulfide, a hazardous waste that readily releases toxic H2S gas under acidic conditions, posing a significant environmental safety hazard. Furthermore, the precipitation process easily leads to non-selective co-precipitation of valuable metals such as Cu and Zn, hindering the separate recovery of these metals.

[0007] Membrane separation / ion exchange methods suffer from severe membrane fouling and resin poisoning under conditions of high acidity and high impurities, resulting in poor operational stability and high maintenance costs. Furthermore, their ability to remove anionic impurities such as As, F, and Cl is limited, making it difficult to meet the hazardous substance limits for fertilizer-grade ammonium sulfate products. In addition, this technology typically produces regenerated sulfuric acid, not fertilizer-grade ammonium sulfate, failing to achieve the direct resource conversion of waste acid.

[0008] In summary, existing technologies for treating waste acid from copper smelting and for producing ammonium sulfate from waste acid have significant shortcomings. Developing a method to achieve deep purification of waste acid from copper smelting and directly produce fertilizer-grade ammonium sulfate conforming to GB / T 535-2020 standards would have significant environmental benefits and resource utilization value. Summary of the Invention

[0009] The purpose of this invention is to provide a method for preparing fertilizer-grade ammonium sulfate through deep purification of waste acid from copper smelting.

[0010] The objective of this invention is achieved as follows: the method for preparing fertilizer-grade ammonium sulfate through deep purification of copper smelting waste acid includes the following steps: Step 1: Remove impurities from the copper smelting sludge, adjust the pH value, and extract copper and zinc sequentially to obtain the raffinate; Step 2: Adjust the pH of the raffinate obtained in Step 1, add bismuth oxide, precipitate and dechlorinate, and separate the solid and liquid to obtain the dechlorinated liquid; Step 3: Adjust the pH value of the dechlorinated liquid obtained in Step 2, add iron source, scorched onion seed crystals and oxidant, and supplement with aeration. The arsenic is precipitated by oxidizing the seed crystals, and the solid-liquid separation is used to obtain the arsenic-precipitated liquid. Step 4: Adjust the pH value of the arsenic precipitation solution obtained in Step 3 to hydrolyze the iron to form a precipitate, and then separate the solid and liquid to obtain the iron precipitation solution. Step 5: Adjust the pH value of the liquid after iron precipitation obtained in Step 4, add lanthanum sulfate, precipitate and defluorinate, and separate the solid and liquid to obtain the defluorinated liquid; Step 6: Contact the defluorinated liquid obtained in Step 5 with the adsorbent to remove residual metal ions and obtain a purified liquid; Step 7: Concentrate the purified liquid obtained in Step 6, crystallize it, separate the solid and liquid, and dry it to obtain the target fertilizer-grade ammonium sulfate.

[0011] Compared with the prior art, the technical solution described in this invention has the following advantages: 1. The method described in this invention utilizes the significant differences in the morphology of impurity ions such as copper, zinc, iron, and arsenic at different pH ranges to perform sequential extraction and precipitation separation, thereby achieving efficient recovery of valuable metals and stepwise removal of impurities.

[0012] 2. The method described in this invention induces arsenic precipitation through seed crystals. The added arsenic crystals provide ready-made epitaxial growth crystal faces. At the set reaction temperature, heterogeneous nucleation and directional growth can be induced on the surface of the seed crystals to obtain arsenic precipitates with high crystallinity, low specific surface area and strong stability.

[0013] 3. The dechlorination process selected in the method of the present invention allows the precipitation reaction to take place under strong acid conditions, which, together with the acidity remaining in the extraction section, forms a concentration gradient from high acid extraction to strong acid precipitation. This eliminates the need for an additional alkali adjustment process, thus avoiding the waste caused by adjusting the pH value.

[0014] 4. The method described in this invention treats chlorine and fluorine anionic impurities in the system in steps, taking advantage of the difference between the strong affinity of bismuth ions for chloride ions and the strong affinity of lanthanum ions for fluorine ions, thereby achieving the sequential and in-depth removal of chlorine and fluorine impurities.

[0015] 5. The method described in this invention uses ammonia and sulfuric acid as pH adjusters throughout the entire process. The neutralization reaction does not introduce any exogenous impurity ions, but rather converts them into part of the product, greatly reducing the amount of waste salt that needs to be treated separately.

[0016] 6. The chelating adsorption resin selected by the method of the present invention has a very high selective adsorption coefficient for residual metal ions under weakly acidic conditions, but has almost no chelating adsorption effect on ammonium ions, thus retaining trace metal residues from the previous steps and ensuring that the purified liquid meets the purity standard of fertilizer-grade ammonium sulfate in terms of metal ion content.

[0017] 7. The back-extraction solution generated in the process described in this invention can be further electrowinning or crystallized to recover Cu and Zn, realizing resource value-added and solving the problem of resource loss of valuable metals.

[0018] In summary, this technical solution, through the construction of a multi-level synergistic purification system, achieves the resource recovery of valuable metals and the deep removal of harmful elements in the treatment of waste acid from copper smelting. Simultaneously, the entire process utilizes an ammonia-sulfuric acid system to achieve gradient pH control, preventing the introduction of exogenous impurities and directly converting them into the target product components, forming a closed-loop process. It produces fertilizer-grade ammonium sulfate products that meet the GB / T 535-2020 standard, improving upon the problems of large slag production and diverse hazardous waste types associated with conventional purification routes. This provides a new technical route for the resource recovery of copper smelting and has significant potential for widespread application. Attached Figure Description

[0019] Figure 1This is a process flow diagram of the technical solution described in this invention. Detailed Implementation

[0020] The present invention will be further described below, but this is not intended to limit the invention in any way. Any modifications or substitutions made based on the teachings of the present invention shall fall within the scope of protection of the present invention.

[0021] The method for preparing fertilizer-grade ammonium sulfate through deep purification of copper smelting waste acid according to the present invention includes the following steps: Step 1: Remove impurities from the copper smelting waste acid, adjust the pH value, and extract copper and zinc sequentially to obtain raffinate; back-extract the loaded organic phase to recover copper and zinc respectively; return the back-extracted organic phase to the extraction section for recycling. Step 2: Adjust the pH of the raffinate obtained in Step 1, add bismuth oxide, precipitate and dechlorinate, and separate the solid and liquid to obtain the dechlorinated liquid; Step 3: Adjust the pH value of the dechlorinated liquid obtained in Step 2, add iron source, scorched onion seed crystals and oxidant, and supplement with aeration. The arsenic is precipitated by oxidizing the seed crystals, and the solid-liquid separation is used to obtain the arsenic-precipitated liquid. Step 4: Adjust the pH value of the arsenic precipitation solution obtained in Step 3 to cause iron to hydrolyze and form a precipitate. Separate the solid and liquid to obtain the iron precipitation solution and iron slag. Step 5: Adjust the pH value of the liquid after iron precipitation obtained in Step 4, add lanthanum sulfate, precipitate and defluorinate, and separate the solid and liquid to obtain the defluorinated liquid; Step 6: Contact the defluorinated liquid obtained in Step 5 with the adsorbent to remove residual metal ions and obtain a purified liquid; Step 7: Concentrate the purified liquid obtained in Step 6, crystallize it, separate the solid and liquid, and dry it to obtain the target fertilizer-grade ammonium sulfate.

[0022] The method may also include: Step 8: Regenerate the BiOCl and LaF3 precipitates with sodium hydroxide solution to recover Bi. 3+ / La 3+ BiOCl is regenerated into Bi2O3 by alkali conversion, and then washed with water to remove entrained Na. + Then, it is reused in step 2; LaF3 is regenerated into La(OH)3 by alkali conversion, and then washed with water to remove Na. + Then, it is dissolved in sulfuric acid to convert it into a lanthanum sulfate solution, which is then reused in step 5.

[0023] Step 9: Dispose of the iron-containing arsenic slag, namely crystalline arsenic stone and the iron slag obtained in Step 4, in accordance with the regulations for arsenic-containing hazardous waste. Arsenic stone has high crystallinity and low leaching toxicity, and its storage, transportation and landfill safety is significantly better than that of amorphous ferric arsenate slag and calcium arsenic slag.

[0024] In step 1, the impurity removal involves removing suspended solids from copper smelting waste acid through sedimentation and filtration.

[0025] In step 1, the copper extraction involves adjusting the pH of the copper smelting waste acid to 1.5-2.5 with ammonia water, and then extracting copper using an aldoxime extractant under conventional extraction conditions.

[0026] In step 1, the extraction of zinc involves adjusting the pH of the raffinate after copper extraction to 2.0-3.5 with ammonia water, and then extracting zinc with phosphoric acid or phosphonic acid extractants under conventional extraction conditions.

[0027] In step 1, the back-extraction of the supported organic phase is to back-extract the supported organic phase with sulfuric acid.

[0028] In step 2, the precipitation dechlorination is carried out by adjusting the pH of the raffinate obtained in step 1 to ≤1 with sulfuric acid, adding bismuth oxide (Bi2O3) at a Bi / Cl molar ratio of 1 to 1.5:1, stirring and reacting to remove chloride ions as bismuth oxychloride (BiOCl) precipitation.

[0029] In step 3, the oxidative precipitation of arsenic involves adjusting the pH of the dechlorinated liquid obtained in step 2 to 1.0–2.0 using ammonia or sulfuric acid, adding an iron source to make the Fe / As molar ratio 1.5–2.0:1, adding 10–20 g / L of arsenic seed crystals, and adding an oxidant at 1.2–1.5 times the total molar amount of arsenic. Under aeration conditions, the mixture is stirred and reacted at 90–95°C for 12–24 hours to remove arsenic by precipitation of crystalline arsenic (FeAsO4·2H2O).

[0030] The iron source is ferric sulfate or ferrous sulfate.

[0031] The oxidant is hydrogen peroxide.

[0032] In step 4, the iron hydrolysis to form a precipitate involves adjusting the pH of the arsenic-precipitated solution obtained in step 3 to 3.5–4.0 with ammonia water, and stirring the reaction to allow the iron ions (Fe) to form a precipitate. 3+ Hydrolysis produces ferric hydroxide (Fe(OH)3) precipitate.

[0033] In step 5, the precipitation defluorination involves adjusting the pH of the iron precipitation solution obtained in step 4 to 3.5-4.5 with ammonia water, adding lanthanum sulfate (La2(SO4)3) to the iron precipitation solution according to a La / F molar ratio of 0.4-0.6:1, and stirring the reaction to precipitate and remove fluoride ions as lanthanum fluoride (LaF3).

[0034] In step 6, the adsorbent is a chelating ion exchange resin bed. Preferably, it is an iminodiacetic acid-based chelating resin bed with a height-to-diameter ratio of 4:1. The flow rate of the defluorinated liquid is set at 6 BV / h, and the operating temperature is 30°C.

[0035] In step 7, the concentration is carried out using a reduced pressure method to concentrate the purified liquid to an ammonium sulfate mass fraction of 46-50%. The drying is carried out using a fluidized bed at 90°C.

[0036] Example 1

[0037] Raw material: copper smelting waste acid, which, after testing, contains 100 g / L H2SO4, 3.0 g / L As, 23 mg / L Cu, 67 mg / L Zn, 1.25 g / L F, 2.34 g / L Cl, 80 mg / L Fe, 5 mg / L Cd, and 0.5 mg / L Hg.

[0038] Step 1: Extraction of copper and zinc: The copper smelting waste acid is subjected to sedimentation and filtration to remove suspended solids. The pH value of the filtered copper smelting waste acid is adjusted to 2.0 with ammonia water. Copper is extracted with an aldoxime extractant (LIX984) under conventional extraction conditions.

[0039] The pH of the raffinate after copper extraction was adjusted to 3.0 with ammonia water, and zinc was extracted with phosphonic acid extractant (P507) under conventional extraction conditions to obtain the raffinate.

[0040] The loaded organic phase is back-extracted with sulfuric acid to recover copper and zinc, respectively; the back-extracted organic phase is returned to the extraction section for recycling.

[0041] Step 2: Dechlorination: Adjust the pH of the raffinate obtained in Step 1 to 0.5 with sulfuric acid, add bismuth oxide (Bi2O3) at a Bi / Cl molar ratio of 1.2:1, stir and react at 80℃ for 60 min, and separate the solid and liquid to obtain the dechlorinated liquid and bismuth oxychloride (BiOCl) precipitate.

[0042] Step 3 Arsenic removal: Adjust the pH of the dechlorinated solution obtained in Step 2 to 1.5 with ammonia water, add ferric sulfate to make the Fe / As molar ratio 2.0:1, add 15 g / L of arsenic seed crystals, add hydrogen peroxide at 1.3 times the total molar amount of arsenic, and stir the reaction at 90℃ for 16 h under aeration conditions. After solid-liquid separation, the arsenic-precipitated solution and crystalline arsenic (FeAsO4·2H2O) precipitate are obtained.

[0043] Step 4: Iron precipitation: Adjust the pH of the arsenic-precipitated solution obtained in Step 3 to 3.8 with ammonia water, and stir the reaction at 60℃ for 30 minutes to allow iron ions (Fe) to precipitate. 3+ Hydrolysis produces ferric hydroxide (Fe(OH)3) precipitate, and solid-liquid separation yields the precipitated liquid and precipitated slag.

[0044] Step 5: Defluorination: Adjust the pH of the iron precipitation solution obtained in Step 4 to 4.0 with ammonia water. Add lanthanum sulfate (La2(SO4)3) to the iron precipitation solution according to the La / F molar ratio of 0.5:1. Stir and react at 30℃ for 30 min. Separate the solid and liquid to obtain the defluorinated solution and lanthanum fluoride (LaF3) precipitate.

[0045] Step 6: Removal of residual metals: The defluorinated liquid obtained in step 5 is brought into contact with an iminodiacetic acid chelating resin bed with a height-to-diameter ratio of 4:1. The flow rate of the defluorinated liquid is 6 BV / h, and the operating temperature is 30℃. Residual metal ions are removed to obtain a purified liquid.

[0046] Step 7 Evaporation and Crystallization: The purified liquid obtained in Step 6 is concentrated to an ammonium sulfate mass fraction of 48% under a vacuum of -0.06 MPa and a temperature of 70°C. It is then cooled to 30°C for crystallization, separated by centrifugation, and dried in a fluidized bed at 90°C to obtain the target fertilizer-grade ammonium sulfate.

[0047] Step 8: Reagent Regeneration: The BiOCl and LaF3 precipitates were regenerated by alkaline washing with 8% sodium hydroxide solution at 80°C for 60 min. BiOCl was regenerated to Bi2O3 by alkaline conversion, and then washed with water to remove entrained Na. + Then, it is reused in step 2; LaF3 is regenerated into La(OH)3 by alkali conversion, and then washed with water to remove Na. + Then, it is dissolved in sulfuric acid to convert it into a lanthanum sulfate solution, which is then reused in step 5.

[0048] Step 9: Slag Disposal: The iron- and arsenic-containing slag, namely crystalline arsenic stone and the iron slag obtained in Step 4, shall be disposed of in accordance with the regulations for hazardous waste containing arsenic.

[0049] Example 2

[0050] Raw materials: Same as in Example 1.

[0051] In this embodiment, only the Fe / As molar ratio in step 3 is adjusted to 1.5; the remaining steps and process conditions are the same as in Example 1.

[0052] Example 3

[0053] Raw materials: Same as in Example 1.

[0054] In this embodiment, only the Fe / As molar ratio in step 3 is adjusted to 1.75; the remaining steps and process conditions are the same as in Example 1.

[0055] Example 4

[0056] Raw materials: Same as in Example 1.

[0057] In this embodiment, only the La / F molar ratio in step 5 is adjusted to 0.6:1, and the remaining steps and process conditions are the same as in embodiment 1.

[0058] Example 5

[0059] Raw materials: Same as in Example 1.

[0060] In this embodiment, only the Bi / Cl molar ratio in step 2 is adjusted to 1.5:1, and the remaining steps and process conditions are the same as in embodiment 1.

[0061] The experimental results of Examples 1-5 are shown in Table 1. The results show that, through the above specific implementation methods, it is possible to achieve deep purification of copper smelting waste acid and prepare fertilizer-grade ammonium sulfate that meets the requirements of GB / T535-2020.

[0062] Table 1. Elemental content of fertilizer-grade ammonium sulfate prepared in Examples 1-5

[0063] The technical principle of the technical solution described in this invention is as follows: 1. Arsenic occurrence forms and extraction selectivity: Under strong acidic conditions, arsenic exists in neutral molecular / anionic forms (As(III) is mainly in the form of H3AsO3, and As(V) is mainly in the form of H3AsO4). With increasing pH, some arsenic dissociates into H2AsO4. - ,HAsO4² - The mechanism of copper extraction by chelation with aldoxime and zinc extraction by phosphate / phosphonic acid cation exchange is mutually exclusive, thus achieving "recovery of copper and zinc and arsenic in the residual extract".

[0064] 2. The dual function of ammonia and the impurity-free nature of sulfuric acid reversion: Ammonia, as a neutralizing acid, also serves as an ammonium source, replacing lime / liquid alkali and avoiding Ca from the source. 2+ / Na + It enters the ammonium sulfate crystals; ammonia is a weak base, which can achieve fine segmented adjustment of pH value; the sulfuric acid-sulfate ions used when adjusting the pH are components of the ammonium sulfate product and do not introduce new impurities.

[0065] 3. Principle of chlorine removal: BiOCl has extremely low solubility; under acidic conditions, Cl... - with Bi 3+ It can still stably generate bismuth oxychloride (BiOCl) precipitate, thereby achieving the precipitation and removal of chloride ions.

[0066] 4. Arsenic removal and solidification principle of stinky onion stone: Under certain conditions, As(V) reacts with Fe... 3+ The combination produces crystalline arsenic (FeAsO4·2H2O), which has extremely low solubility and thermodynamic stability. Its leaching toxicity is significantly lower than that of amorphous ferric arsenate and calcium arsenic slag, enabling long-term stable solidification of arsenic.

[0067] 5. Principle of fluoride removal: Lanthanum sulfate is a soluble lanthanum salt; La in the solution... 3+ With F -The combination can form a LaF3 precipitate.

Claims

1. A method for preparing fertilizer-grade ammonium sulfate through deep purification of waste acid from copper smelting, characterized in that, Includes the following steps: Step 1: Remove impurities from the copper smelting sludge, adjust the pH value, and extract copper and zinc sequentially to obtain the raffinate; Step 2: Adjust the pH of the raffinate obtained in Step 1, add bismuth oxide, precipitate and dechlorinate, and separate the solid and liquid to obtain the dechlorinated liquid; Step 3: Adjust the pH value of the dechlorinated liquid obtained in Step 2, add iron source, scorched onion seed crystals and oxidant, and supplement with aeration. The arsenic is precipitated by oxidizing the seed crystals, and the solid-liquid separation is used to obtain the arsenic-precipitated liquid. Step 4: Adjust the pH value of the arsenic precipitation solution obtained in Step 3 to hydrolyze the iron to form a precipitate, and then separate the solid and liquid to obtain the iron precipitation solution. Step 5: Adjust the pH value of the liquid after iron precipitation obtained in Step 4, add lanthanum sulfate, precipitate and defluorinate, and separate the solid and liquid to obtain the defluorinated liquid; Step 6: Contact the defluorinated liquid obtained in Step 5 with the adsorbent to remove residual metal ions and obtain a purified liquid; Step 7: Concentrate the purified liquid obtained in Step 6, crystallize it, separate the solid and liquid, and dry it to obtain the target fertilizer-grade ammonium sulfate.

2. The method for preparing fertilizer-grade ammonium sulfate through deep purification of copper smelting waste acid according to claim 1, characterized in that, In step 1, the copper extraction involves adjusting the pH of the copper smelting waste acid to 1.5-2.5 with ammonia water, and then extracting copper using an aldoxime extractant under conventional extraction conditions.

3. The method for preparing fertilizer-grade ammonium sulfate through deep purification of copper smelting waste acid according to claim 1, characterized in that, In step 1, the extraction of zinc involves adjusting the pH of the raffinate after copper extraction to 2.0-3.5 with ammonia water, and then extracting zinc with phosphoric acid or phosphonic acid extractants under conventional extraction conditions.

4. The method for preparing fertilizer-grade ammonium sulfate through deep purification of copper smelting waste acid according to claim 1, characterized in that, In step 2, the precipitation dechlorination involves adjusting the pH of the raffinate obtained in step 1 to ≤1 with sulfuric acid, adding bismuth oxide at a Bi / Cl molar ratio of 1 to 1.5:1, stirring the reaction, and removing chloride ions as bismuth oxychloride precipitate.

5. The method for preparing fertilizer-grade ammonium sulfate through deep purification of copper smelting waste acid according to claim 1, characterized in that, In step 3, the oxidative precipitation of arsenic involves adjusting the pH of the dechlorinated liquid obtained in step 2 to 1.0–2.0 using ammonia or sulfuric acid, adding an iron source to make the Fe / As molar ratio 1.5–2.0:1, adding 10–20 g / L of arsenic seed crystals, and adding an oxidant at 1.2–1.5 times the total molar amount of arsenic. Under aeration conditions, the mixture is stirred and reacted at 90–95°C for 12–24 hours to remove arsenic by precipitating it as crystalline arsenic.

6. The method for preparing fertilizer-grade ammonium sulfate through deep purification of copper smelting waste acid according to claim 1, characterized in that, In step 4, the iron hydrolysis to form a precipitate is achieved by adjusting the pH of the arsenic-precipitated solution obtained in step 3 to 3.5-4.0 with ammonia water, and stirring the reaction to hydrolyze the iron ions to form ferric hydroxide precipitate.

7. The method for preparing fertilizer-grade ammonium sulfate through deep purification of copper smelting waste acid according to claim 1, characterized in that, In step 5, the precipitation defluorination involves adjusting the pH of the iron precipitation solution obtained in step 4 to 3.5-4.5 with ammonia water, adding lanthanum sulfate to the iron precipitation solution according to a La / F molar ratio of 0.4-0.6:1, and stirring the reaction to remove fluoride ions as lanthanum fluoride precipitate.

8. The method for preparing fertilizer-grade ammonium sulfate through deep purification of copper smelting waste acid according to claim 1, characterized in that, In step 6, the adsorbent is a chelating ion exchange resin bed.

9. The method for preparing fertilizer-grade ammonium sulfate through deep purification of copper smelting waste acid according to claim 8, characterized in that, The adsorbent is an iminodiacetic acid chelating resin bed.

10. The method for preparing fertilizer-grade ammonium sulfate through deep purification of copper smelting waste acid according to claim 1, characterized in that, In step 7, the concentration is carried out by vacuum distillation, concentrating the purified liquid to a mass fraction of 46-50% ammonium sulfate.

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