Method for preparing iron phosphate by utilizing waste acid solution of aluminum product

Abstract

The invention relates to the technical field of wastewater treatment, in particular to a method for preparing iron phosphate by utilizing aluminum product waste acid liquor. The method comprises the following steps: step 1, size mixing: adding a ferric salt solution into the aluminum product waste acid solution, and fully stirring for reaction; step 2, impurity removal: adding calcium hydroxide into the filtrate, fully stirring and reacting to enable the pH value to be 2.5-3, generating a precipitate, and then standing to obtain a supernatant and a first precipitate, the supernatant being a ferrous phosphate solution; step 3, aging: adding a hydrogen peroxide solution into the ferrous phosphate solution to obtain a solid-liquid mixture, and standing the solid-liquid mixture to obtain a second precipitate; step 4, washing: washing the second precipitate by using a citric acid solution concentrated to be 3-5%, and removing the solution after washing to obtain a solid which is an iron phosphate crude product; step five, drying: drying the iron phosphate crude product to obtain an iron phosphate dihydrate product, and the method can be used for preparing sized and crystallized iron phosphate by utilizing the aluminum product waste acid solution, and has the advantage of being easy to operate.

Description

Technical Field

[0001] The invention relates to the technical field of wastewater treatment, and in particular to a method for preparing ferric phosphate by utilizing waste acid liquid from aluminum products. Background Art

[0002] In recent years, with the rapid growth of China's economy, the use of electroplating and passivation has increased. The rapid development of these industries has also spawned a large amount of industrial waste, of which waste acid accounts for a large proportion. The composition of these waste acids after surface treatment is complex and changeable, and a large proportion of them can be recycled. Therefore, it is necessary to recycle the waste acids through reasonable methods, otherwise it will have an irreversible impact on the ecological environment. Therefore, the recycling and utilization of waste acids from surface treatment will be beneficial to the economic benefits of the industry and the environmental benefits required by the government.

[0003] Most of the waste acids from surface treatment are waste phosphoric acid and waste sulfuric acid with high concentrations. If these waste acids are discharged directly into the soil without resource treatment, the nearby vegetation will wither, affecting the surrounding ecological environment and causing irreversible harm. If they are discharged directly into the nearby water system without resource treatment, the water environment will become eutrophic, affecting the ecological environment and causing damage to the water body.

[0004] Lithium iron phosphate is known as one of the most promising lithium battery cathode materials in the 21st century because of its wide source of raw materials, large specific capacity, high safety performance, long service life, and unique catalytic properties. In the 21st century, it is made into lithium batteries and widely used in various fields such as automobiles, aerospace, and mobile phones. In recent years, as the country vigorously develops the field of new energy power vehicle batteries, related research on lithium iron phosphate has also been continuously explored. At present, the precursors of lithium iron phosphate include iron oxide, iron sulfate, iron phosphate, etc. Among them, iron phosphate has a rich skeleton structure and provides both iron and phosphorus sources. Compared with other precursors, its changes in the synthetic unit cell are small, which can improve the stability of the material. Therefore, iron phosphate is the core precursor for the preparation of lithium iron phosphate.

[0005] At present, although waste solutions are recycled to prepare iron phosphate, the crystal form of the currently produced iron phosphate is mostly amorphous, and amorphous crystalline iron phosphate is not conducive to being used as a precursor material for lithium iron phosphate batteries. Summary of the invention

[0006] The purpose of the present invention is to avoid the shortcomings of the prior art and provide a method for preparing ferric phosphate using waste acid liquid from aluminum products. The method is conducive to using waste acid liquid from aluminum products to prepare shaped and crystalline ferric phosphate and has the advantage of easy operation.

[0007] To achieve the above object, the present invention provides the following technical solutions:

[0008] A method for preparing ferric phosphate using waste acid solution from aluminum products is provided, comprising the following steps:

[0009] Step 1, slurry preparation: adding an iron salt solution to the waste acid solution of aluminum products, wherein the molar ratio of the iron source to the phosphorus source in the waste acid solution of aluminum products is 1:0.9-1, stirring the mixture sufficiently for reaction, and filtering the mixture to obtain a filtrate;

[0010] Step 2, impurity removal: adding calcium hydroxide to the filtrate, stirring the reaction sufficiently to make the pH value be 2.5-3, and generating a precipitate, and then standing the solution to obtain a supernatant and a first precipitate, wherein the supernatant is a ferrous phosphate solution;

[0011] Step 3, aging: adding a hydrogen peroxide solution to the ferrous phosphate solution, wherein the molar ratio of the hydrogen peroxide solution to the ferrous phosphate solution is 1.05 to 1.50:1, to obtain a solid-liquid mixture, and allowing the solid-liquid mixture to stand to obtain a second precipitate;

[0012] Step 4, washing: washing the second precipitate with a 3% to 5% citric acid solution, removing the solution after washing, and obtaining a solid product as a crude iron phosphate;

[0013] Step 5, drying: drying the crude ferric phosphate to obtain ferric phosphate dihydrate product.

[0014] In some embodiments, the iron salt solution is ferrous sulfate with a purity of 99%.

[0015] In some embodiments, in step 1, the reaction time is 0.5 h to 4 h.

[0016] In some embodiments, in step 2, the reaction time is 10 min to 60 min, and the standing time is 1 h to 2 h;

[0017] Furthermore, after filtering the first precipitate, a filter cake is obtained, the filter cake contains calcium sulfate, aluminum hydroxide, and ferrous phosphate, and the filter cake is recovered.

[0018] In some embodiments, in step three, the reaction time is 5 min to 60 min, and the standing time is 1 h to 2 h.

[0019] In some embodiments, in step 4, the citric acid solution is washed 5 to 12 times.

[0020] In some embodiments, in step five, the drying temperature is 50°C to 70°C.

[0021] In some embodiments, in step 1, the stirring speed is 400 rpm / min to 800 rpm / min;

[0022] In step 2, the stirring speed is 400 rpm / min to 800 rpm / min.

[0023] In some embodiments, ferric phosphate is prepared using a production device, the production device comprising:

[0024] The first reactor, the second reactor and the third reactor, the first reactor is connected to the first filter plate frame at the bottom, the first reactor is connected to the second reactor, the bottom of the second reactor is connected to the third reactor, the third reactor is connected to the second filter plate frame, and the second filter plate frame is connected to the oven,

[0025] The first reactor is connected to a first feeder and a second feeder, and the second reactor is connected to a third feeder.

[0026] When using,

[0027] In step 1, the adjusted iron salt solution is added to the first feeder, the waste acid liquid of aluminum products is placed in the first reactor, and the iron salt solution is added to the waste acid liquid of aluminum products in the first reactor through the first feeder. After the reaction, the waste acid liquid is filtered by standing and the bottom material is discharged. The obtained solution is the filtrate.

[0028] In step 2, calcium hydroxide is added to the second feeder, and calcium hydroxide is added to the filtrate of the first reactor through the second feeder, and the first precipitate at the bottom of the first reactor is passed through the first filter plate frame for filtration, and the supernatant in the first reactor is passed into the second reactor;

[0029] In step 3, a hydrogen peroxide solution is added to the third feeder, and the hydrogen peroxide solution is added to the ferrous phosphate solution in the second reactor through the third feeder, and the second precipitate at the bottom of the second reactor is transferred to the third reactor.

[0030] In step 4, a citric acid solution is added to the second precipitate in the third reactor for washing. After washing, the excess solution is removed by the second filter plate frame. The obtained solid is a crude iron phosphate and is transferred to the oven for drying.

[0031] Also provided is the application of iron phosphate, wherein the iron phosphate is prepared by the above-mentioned method of preparing iron phosphate by using waste acid liquid of aluminum products, and the iron phosphate is applied in positive electrode materials of lithium ion batteries.

[0032] The beneficial effects of the method for preparing ferric phosphate using waste acid liquid of aluminum products of the present invention are as follows:

[0033] (1) The method of preparing ferric phosphate using waste acid liquid of aluminum products of the present invention uses waste acid liquid of passivated aluminum products as a phosphorus source, which, on the one hand, reduces the phosphorus concentration in the waste acid, solves the problem of waste acid discharge, is beneficial to environmental protection and simplifies the waste acid discharge process, and on the other hand, reduces the production cost of phosphate raw materials.

[0034] (2) The method of preparing ferric phosphate using waste acid liquid of aluminum products of the present invention first adds an iron salt solution to the waste acid liquid of aluminum products, and obtains a filtrate containing ferric phosphate after filtering. Then, calcium hydroxide is used to adjust the pH of the entire solution system. Since the calcium ions in the calcium hydroxide react well with the sulfate ions of the passivated waste acid liquid of aluminum products to generate solid calcium sulfate, the sulfate ions can be removed after the calcium sulfate is removed, thus laying a foundation for the waste acid liquid of aluminum products to be used as a phosphorus source, thereby overcoming the effect that the traditional use of sodium hydroxide can only adjust the pH.

[0035] (3) The method for preparing ferric phosphate using waste acid liquid of aluminum products of the present invention, after impurities are removed in step 2, precipitated calcium sulfate, aluminum hydroxide and ferrous phosphate are produced, sulfate, aluminum ions and ferrous sulfate in the waste acid liquid of aluminum products are effectively removed, and the waste acid liquid of aluminum products is further impurities removed, and the phosphate concentration in the waste acid is effectively reduced to meet the national emission standards. The obtained supernatant mainly contains ferrous ions, phosphate ions and hypophosphite ions. The ferrous phosphate solution can be used as a raw material for preparing lithium iron phosphate batteries. The ferrous phosphate solution is treated with a hydrogen peroxide solution to obtain a second precipitate containing ferric phosphate; the second precipitate is then washed with a citric acid solution to completely remove sulfate in the ferric phosphate product, which can reduce the agglomeration phenomenon between particles and control the generation of spherical ferric phosphate, effectively control the particle size between crystal grains, and facilitate the crystallization of ferric phosphate in the subsequent drying. Therefore, adding a certain concentration of citric acid for multiple washings can effectively promote the crystallization of ferric phosphate and remove excess impurities at the same time, thereby overcoming the problem that the traditional method of washing with only washing water cannot effectively remove sulfate and affects the crystallization of the finished product. Furthermore, the present invention uses a citric acid solution with a concentration of 3-5%, so as to avoid the situation that the ferric phosphate mostly forms discus when washed with low-concentration citric acid, and cannot be well crystallized into spherical ferric phosphate.

[0036] (4) In the method of preparing iron phosphate using waste acid liquid from aluminum products of the present invention, the iron phosphate product is further dried by a high-temperature device such as a muffle furnace, which promotes the formation of iron phosphate crystals and removes two molecules of water to become anhydrous iron phosphate. The crystallinity of the product meets the conditions for preparing lithium iron phosphate precursors.

[0037] (5) The method of preparing iron phosphate using waste acid liquid of aluminum products of the present invention uses waste acid of passivated aluminum products as a phosphate raw material, effectively reducing the phosphorus concentration in the waste acid and solving the problem of waste acid discharge, that is, the precursor iron phosphate that meets the performance requirements of lithium iron phosphate batteries can be prepared by using the phosphate in the waste acid of passivated aluminum products, and the phosphorus source in the waste acid can be recycled to meet the national emission requirements for discharge. Compared with the traditional treatment method of waste acid of passivated aluminum products before discharge, this method avoids the use of crystallization method, biological method, etc., not only recovers high-concentration phosphate to prepare iron phosphate, but also reduces the phosphorus source cost of the traditional method for preparing iron phosphate and simplifies the process for preparing iron phosphate, and the product properties such as crystallinity meet the standards of lithium iron phosphate battery precursors, overcomes the problem that the sludge generated by the traditional method for treating waste acid of aluminum products is difficult to recycle, and then solves the problem of phosphorus-containing sludge polluting the environment and the waste of resources due to the difficulty in recovering phosphorus in the sludge.

[0038] (5) The method of the present invention for preparing iron phosphate using waste acid liquid of aluminum products, and the method of the present invention for preparing iron phosphate using high-concentration phosphate in surface treatment waste acid as a phosphorus source, have a shorter reaction time and a lower preparation cost compared with the existing method for preparing iron phosphate; the method of the present invention for preparing iron phosphate using high-concentration phosphate in surface treatment waste acid as a phosphorus source, compared with the traditional method for preparing iron phosphate, does not produce flammable and explosive gases, and the operation is safer and more efficient. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 This is the XPS graph of the ferric phosphate dihydrate product of Experimental Example 2 of the present invention.

[0040] Figure 2 It is a comparison chart of the XPS graphs of the dihydrate iron phosphate products of Experimental Example 1 of the present invention and Comparative Example 1.

[0041] Figure 3 It is a schematic diagram of the structure of the production device of Example 2 of the present invention.

[0042] Reference numerals

[0043] 1. First reaction kettle; 2. Second reaction kettle; 3. Third reaction kettle; 4. First filter plate frame; 5. Second filter plate frame; 6. Oven; 7. First feeder; 8. Second feeder; 9. Third feeder; 10. Oven. DETAILED DESCRIPTION

[0044] The preferred embodiments of the present invention will be described in more detail below with reference to the accompanying drawings. Although the preferred embodiments of the present invention are shown in the accompanying drawings, it should be understood that the present invention can be implemented in various forms and should not be limited by the embodiments described herein. On the contrary, these embodiments are provided to make the present invention more thorough and complete, and to fully convey the scope of the present invention to those skilled in the art.

[0045] Example 1

[0046] The method for preparing ferric phosphate using waste acid solution of aluminum products disclosed in this embodiment comprises the following steps:

[0047] Step 1, slurry preparation: adding an iron salt solution to the waste acid solution of aluminum products, wherein the molar ratio of the iron source to the phosphorus source in the waste acid solution of aluminum products is 1:0.9-1, preferably 1:0.95, stirring the reaction sufficiently, and filtering to obtain a filtrate;

[0048] The phosphorus source in the waste acid liquid of aluminum products can be first obtained by ultraviolet measurement, and the iron source is adjusted according to the measured phosphorus source concentration.

[0049] Step 2, impurity removal: adding calcium hydroxide to the filtrate, stirring the reaction sufficiently to make the pH value be 2.5 to 3, preferably 2.8, and generating a precipitate, and then standing the solution to obtain a supernatant and a first precipitate, wherein the supernatant is a ferrous phosphate solution;

[0050] Step 3, aging: adding a hydrogen peroxide solution to the ferrous phosphate solution, wherein the molar ratio of the hydrogen peroxide solution to the ferrous phosphate solution is 1.05 to 1.50:1, preferably 1.2:1, to obtain a solid-liquid mixture, and allowing the solid-liquid mixture to stand to obtain a second precipitate;

[0051] Step 4, washing: washing the second precipitate with a 0.3% to 0.8% citric acid solution, removing the solution after washing, and obtaining a crude ferric phosphate solid;

[0052] Step 5, drying: drying the crude ferric phosphate to obtain ferric phosphate dihydrate product.

[0053] The iron salt solution is ferrous sulfate with a purity of 99%.

[0054] In step 1, the reaction time is 0.5 h to 4 h, preferably 3 h.

[0055] In step 2, the reaction time is 10 min to 60 min, preferably 50 min, and the standing time is 1 h to 2 h, preferably 1.5 h;

[0056] Furthermore, after filtering the first precipitate, a filter cake is obtained, the filter cake contains calcium sulfate, aluminum hydroxide, and ferrous phosphate, and the filter cake is recovered.

[0057] In step 3, the reaction time is 5 min to 60 min, preferably 40 min, and the standing time is 1 h to 2 h, preferably 1.2 h.

[0058] In step 4, the citric acid solution is washed 5 to 12 times, preferably 8 times.

[0059] In step five, the drying temperature is 50°C to 70°C, preferably 60°C.

[0060] In step 1, the stirring speed is 400 rpm / min to 800 rpm / min, preferably 700 rpm / min;

[0061] In step 2, the stirring speed is 400 rpm / min to 800 rpm / min, preferably 500 rpm / min.

[0062] Example 2

[0063] Based on Example 1, please refer to Figure 3 This embodiment discloses the use of a production device to prepare ferric phosphate, the production device comprising:

[0064] The first reactor 1, the second reactor 2 and the third reactor 3, the first reactor 1 is connected to the first filter plate frame 4 at the bottom, the first reactor 1 is connected to the second reactor 2, the bottom of the second reactor 2 is connected to the third reactor 3, the third reactor 3 is connected to the second filter plate frame 5, the second filter plate frame 5 is connected to the oven 6,

[0065] The first reactor 1 is connected to a first feeder 7 and a second feeder 8, and the second reactor is connected to a third feeder 9.

[0066] When using,

[0067] In step 1, the adjusted iron salt solution is added to the first feeder 7, the waste acid liquid of aluminum products is placed in the first reactor 1, and the iron salt solution is added to the waste acid liquid of aluminum products in the first reactor 1 through the first feeder 7. After the reaction, the waste acid liquid is filtered by standing and the bottom material is discharged. The obtained solution is the filtrate.

[0068] In step 2, calcium hydroxide is added to the second feeder 8, and calcium hydroxide is added to the filtrate of the first reactor 1 through the second feeder 8, and the first precipitate at the bottom of the first reactor 1 is passed through the first filter plate frame 4 for filtration, and the supernatant in the first reactor 1 is passed into the second reactor 2;

[0069] In step 3, a hydrogen peroxide solution is added to the third feeder 9, and the hydrogen peroxide solution is added to the ferrous phosphate solution in the second reactor 2 through the third feeder 9, and the second precipitate at the bottom of the second reactor 2 is transferred to the third reactor 3.

[0070] In step four, citric acid solution is added to the second precipitate in the third reactor 3 through the fourth feeder for washing. After washing, the excess solution is removed through the second filter plate frame 5. The obtained solid is a crude iron phosphate and is transferred to the oven 6 for drying.

[0071] The above-mentioned production device uses waste acid from passivated aluminum products as one of the raw materials, adopts a continuous feeding and discharging production method, reduces the reaction time, and the product performance meets the standards of lithium iron phosphate battery precursor materials, and effectively reduces the cost of the raw material phosphate solution. At the same time, it solves the dephosphorization of surface treatment waste acid to meet national emission standards. This method has the advantages of simple operation, high utilization rate, green environmental protection and low cost.

[0072] To further illustrate the quality of the product obtained by the method of the present invention, the following experimental examples were conducted.

[0073] Experimental Example 1

[0074] This experimental example discloses a method for preparing ferric phosphate using surface treated waste acid, comprising the following steps:

[0075] S1. Prepare an iron salt solution: add 291.9 kg of ferrous sulfate and 700.0 kg of pure water to a 1000 L iron salt storage tank and dissolve them to prepare an iron salt solution;

[0076] S2, slurry preparation: add 1000.0L of aluminum product industrial waste acid and 500.0L of iron salt solution into a 2000L reactor, stir well, filter, and obtain a feed liquid;

[0077] S3, impurity removal: adding 50 kg of calcium hydroxide to the feed liquid through a solid adder, the amount of calcium hydroxide added is 2.250 kg / min, stirring and reacting for 30 minutes, standing for 1 hour to obtain a supernatant and a precipitate, the supernatant is transported to the reactor (5) through a pipeline, and the precipitate is filtered through a plate and frame to obtain a first filter cake, which contains calcium sulfate, aluminum hydroxide, and a small amount of ferrous phosphate;

[0078] S4, aging: add 86 kg of hydrogen peroxide solution into the reactor, stir well, react for 30 min, and let stand for 1 h to obtain a precipitate;

[0079] S5, washing: the precipitate is transported to the retention kettle through a pipeline and washed with 0.5% citric acid solution, and the feed liquid is transported to the plate and frame filter through a pipeline to obtain a crude iron phosphate;

[0080] S6. Drying: The crude ferric phosphate is placed in a forced air drying oven at 60° C. to obtain ferric phosphate dihydrate product.

[0081] In the step S1, the iron source is ferrous sulfate with a purity of 99%.

[0082] In the step S1, the stirring rate is 400 rpm / min.

[0083] In step S2, the concentration of phosphoric acid in the aluminum product industrial waste acid is 0.75 mol / L, and the concentration of sulfuric acid is 0.25 mol / L.

[0084] In the step S2, the molar ratio of the iron source to the phosphorus source is 1:1.

[0085] In step S2, the stirring rate is 400 rpm / min.

[0086] In step S3, the stirring rate is 400 rpm / min.

[0087] In step S4, the molar ratio of hydrogen peroxide to the first filter cake is 1.50:1.

[0088] In step S4, the stirring rate is 400 rpm / min.

[0089] In step S5, the stirring rate is 400 rpm / min.

[0090] Experimental Example 2

[0091] This experimental example discloses a method for preparing ferric phosphate using surface treated waste acid, comprising the following steps:

[0092] S1. Prepare an iron salt solution: add 291.9 kg of ferrous sulfate and 700.0 kg of pure water to a 1000 L iron salt storage tank and dissolve them to prepare an iron salt solution;

[0093] S2, slurry preparation: add 1000.0L of aluminum product industrial waste acid and 550.0L of iron salt solution into a 2000L reactor, stir well, filter, and obtain a feed liquid;

[0094] S3, impurity removal: add 45kg of calcium hydroxide to the feed liquid through a solid adder, the amount of calcium hydroxide added is 2.100kg / min, fully stir and react for 45min, let stand for 1.5h to obtain a supernatant and a precipitate, the supernatant is transported to the reactor through a pipeline, and the precipitate is filtered through a plate and frame to obtain a first filter cake, which is calcium sulfate, aluminum hydroxide, and a small amount of ferrous phosphate;

[0095] S4, aging: add 86 kg of hydrogen peroxide solution into the reactor, stir well, react for 45 min, and let stand for 1.5 h to obtain a precipitate;

[0096] S5, washing: the precipitate is transported to the retention kettle through a pipeline and washed with a 0.6% citric acid solution, and the feed liquid is transported to a plate and frame filter through a pipeline to obtain a crude iron phosphate;

[0097] S6. Drying: The crude ferric phosphate is placed in a forced air drying oven at 70° C. to obtain ferric phosphate dihydrate product.

[0098] In the step S1, the iron source is ferrous sulfate with a purity of 99%.

[0099] In the step S1, the stirring rate is 600 rpm / min.

[0100] In step S2, the concentration of phosphoric acid in the aluminum product industrial waste acid is 0.75 mol / L, and the concentration of sulfuric acid is 0.25 mol / L.

[0101] In step S2, the molar ratio of the iron source to the phosphorus source is 1:0.95.

[0102] In step S2, the stirring rate is 600 rpm / min.

[0103] In step S3, the stirring rate is 600 rpm / min.

[0104] In step S4, the molar ratio of hydrogen peroxide to the first filter cake is 1.50:1.

[0105] In step S4, the stirring rate is 600 rpm / min.

[0106] In step S5, the stirring rate is 600 rpm / min.

[0107] Figure 1 As shown in the figure, only the characteristic peaks of Fe, P, O, and C are observed, among which C is the correction characteristic peak, indicating that the purity of the product FePO4 is high. At the same time, the At% of Fe and P in the XPS surface element analysis results are 13.78% and 16.61%, respectively, and the iron-phosphorus ratio is about 0.83.

[0108] The iron phosphate dihydrate product obtained in Experimental Example 2 was subjected to XPS testing. Figure 1 As shown in the figure, only the characteristic peaks of Fe, P, O, and C are observed, among which C is the correction characteristic peak, indicating that the purity of the product FePO4 is high. At the same time, the At% of Fe and P in the XPS surface element analysis results are 13.78% and 16.61%, respectively, and the iron-phosphorus ratio is about 0.83.

[0109] Comparative Example 1

[0110] The difference between this comparative example and experimental example 1 is that S5, washing: the precipitate is transported to a retention kettle through a pipeline to be washed with an aqueous solution, and the feed liquid is transported to a plate and frame filter through a pipeline to obtain a crude iron phosphate.

[0111] The ferric phosphate dihydrate product obtained in Experimental Example 1 and the ferric phosphate dihydrate product obtained in the comparative example were tested to obtain Figure 2 The comparison chart shown is Figure 2 It can be seen that the peak of the product of Comparative Example 1 after calcination by washing with deionized water is not obvious, indicating that the product contains certain sulfate impurities and the crystals are amorphous; after washing the product of Experimental Example 1 with citric acid solution for 5 to 10 times, the sulfate can be washed away, and the particle shape of iron sulfate is mostly fixed and crystallized spherical. After calcination, the high-purity iron phosphate product characterized by XRD is obtained.

[0112] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for preparing ferric phosphate using waste acid solution from aluminum products, characterized in that: The following steps are involved: Step 1, slurry preparation: adding an iron salt solution to the waste acid solution of aluminum products, wherein the molar ratio of the iron source to the phosphorus source in the waste acid solution of aluminum products is 1:0.9-1, stirring the mixture sufficiently for reaction, and filtering the mixture to obtain a filtrate; Step 2, impurity removal: adding calcium hydroxide to the filtrate, stirring the reaction sufficiently to make the pH value be 2.5-3, and generating a precipitate, and then standing the solution to obtain a supernatant and a first precipitate, wherein the supernatant is a ferrous phosphate solution; Step 3, aging: adding a hydrogen peroxide solution to the ferrous phosphate solution, wherein the molar ratio of the hydrogen peroxide solution to the ferrous phosphate solution is 1.05 to 1.50:1, to obtain a solid-liquid mixture, and allowing the solid-liquid mixture to stand to obtain a second precipitate; Step 4, washing: washing the second precipitate with a 3% to 5% citric acid solution, removing the solution after washing, and obtaining a solid product as a crude iron phosphate; Step 5, drying: drying the crude ferric phosphate to obtain ferric phosphate dihydrate product.

2. The method for preparing ferric phosphate using waste acid solution from aluminum products according to claim 1, characterized in that: The iron salt solution is ferrous sulfate with a purity of 99%.

3. The method for preparing ferric phosphate using waste acid solution from aluminum products according to claim 1, characterized in that: In step 1, the reaction time is 0.5h to 4h.

4. The method for preparing ferric phosphate using waste acid solution from aluminum products according to claim 1, characterized in that: In step 2, the reaction time is 10 min to 60 min, and the standing time is 1 h to 2 h; Furthermore, after filtering the first precipitate, a filter cake is obtained, the filter cake contains calcium sulfate, aluminum hydroxide, and ferrous phosphate, and the filter cake is recovered.

5. The method for preparing ferric phosphate using waste acid solution from aluminum products according to claim 1, characterized in that: In step 3, the reaction time is 5 min to 60 min, and the standing time is 1 h to 2 h.

6. The method for preparing ferric phosphate using waste acid solution from aluminum products according to claim 1, characterized in that: In step 4, the citric acid solution is washed 5 to 12 times.

7. The method for preparing ferric phosphate using waste acid solution from aluminum products according to claim 1, characterized in that: In step 5, the drying temperature is 50°C to 70°C.

8. The method for preparing ferric phosphate using waste acid solution from aluminum products according to claim 1, characterized in that: In step 1, the stirring speed is 400 rpm / min to 800 rpm / min; In step 2, the stirring speed is 400 rpm / min to 800 rpm / min.

9. The method for preparing ferric phosphate using waste acid solution from aluminum products according to claim 1, characterized in that: Ferric phosphate is prepared using a production device, the production device comprising: The first reactor, the second reactor and the third reactor, the first reactor is connected to the first filter plate frame at the bottom, the first reactor is connected to the second reactor, the bottom of the second reactor is connected to the third reactor, the third reactor is connected to the second filter plate frame, and the second filter plate frame is connected to the oven, The first reactor is connected to a first feeder and a second feeder, and the second reactor is connected to a third feeder. When using, In step 1, the adjusted iron salt solution is added to the first feeder, the waste acid liquid of aluminum products is placed in the first reactor, and the iron salt solution is added to the waste acid liquid of aluminum products in the first reactor through the first feeder. After the reaction, the waste acid liquid is filtered by standing and the bottom material is discharged. The obtained solution is the filtrate. In step 2, calcium hydroxide is added to the second feeder, and calcium hydroxide is added to the filtrate of the first reactor through the second feeder, and the first precipitate at the bottom of the first reactor is passed through the first filter plate frame for filtration, and the supernatant in the first reactor is passed into the second reactor; In step 3, a hydrogen peroxide solution is added to the third feeder, and the hydrogen peroxide solution is added to the ferrous phosphate solution in the second reactor through the third feeder, and the second precipitate at the bottom of the second reactor is transferred to the third reactor. In step 4, a citric acid solution is added to the second precipitate in the third reactor for washing. After washing, the excess solution is removed by the second filter plate frame. The obtained solid is a crude iron phosphate and is transferred to the oven for drying.

10. An application of iron phosphate, characterized in that: The iron phosphate is prepared by the method for preparing iron phosphate using waste acid solution from aluminum products as described in any one of claims 1 to 9, and the iron phosphate is used in positive electrode materials for lithium-ion batteries.

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