A method for preparing iron phosphate from a circulating magnesium salt

Abstract

A method for preparing iron phosphate by recycling magnesium salt, comprising the following steps: mixing a purified iron source with a phosphorus salt solution prepared from high-magnesium phosphoric acid, and performing an oxidation reaction in the presence of hydrogen peroxide to obtain a first slurry; sequentially performing aging and filtering on the obtained first slurry to obtain a magnesium salt mother liquor and a first filter cake, respectively; wherein the magnesium salt mother liquor is recycled for preparing the high-magnesium phosphoric acid in the above step; slurring the first filter cake with dilute phosphoric acid to obtain a second slurry; performing high-temperature washing and aging on the obtained second slurry, and filtering to obtain a second filter cake and washing water, respectively; wherein the washing water is recycled for the purification process in the above step; sequentially performing reslurry washing, filtering, drying, crushing and calcining on the obtained second filter cake to obtain anhydrous iron phosphate. The method can recycle the phosphate, sulfate and high-concentration magnesium ions in the wastewater to produce iron phosphate with uniform crystal grains and excellent electrochemical performance.

Description

Technical Field

[0001] This invention relates to the field of ferric phosphate preparation technology, and more specifically, to a method for preparing ferric phosphate by recycling magnesium salts. Background Technology

[0002] Energy shortages and environmental pollution are two serious problems facing the world today, making the development of clean and renewable energy one of the most decisive technologies in the global economy. Lithium iron phosphate (LFP) is a lithium-ion battery electrode material with the chemical formula LiFePO4. It boasts advantages such as low cost, no pollution, and good thermal stability, making it one of the most promising cathode materials and a key focus of research and development in the field of energy storage lithium-ion batteries. However, one of the bottlenecks hindering the large-scale industrialization of LFP cathode materials is the production of iron phosphate. Under the premise of requiring stable product quality, good consistency, and electrochemical stability, cost, resources, and environmental protection have become crucial issues that cannot be ignored.

[0003] One traditional method for preparing ferric phosphate is to react pure iron with a strong oxidizing acid, or to introduce hydrogen peroxide to generate ferric iron, which is then reacted with phosphoric acid to produce ferric phosphate. For example, in "Process Synthesis of Ferric Phosphate," it is stated that when sulfuric acid is used as the strong acid, the introduced strong acid ions adhere to the surface of the resulting ferric phosphate product, making it acidic. Therefore, ammonia is often used to adjust the pH. Furthermore, when ferric phosphate is used as a raw material to prepare lithium iron phosphate, the sulfate ions in it produce sulfur dioxide upon sintering, leading to environmental pollution. For example, in "A Method for Preparing Ferric Phosphate by Iron Method" (patent application number CN202310641290.7), it is stated that reduced iron powder and phosphoric acid are used as the main raw materials. First, iron powder and dilute phosphoric acid are mixed and reacted to prepare ferrous dihydrogen phosphate (Fe(H2PO4)2). Then, an oxidant is added to oxidize and generate ferric phosphate precipitate. After filtration and drying, high-purity battery-grade ferric phosphate dihydrate is obtained. Because phosphoric acid is a weak acid, the products of its reaction with iron powder have insufficient solubility in water and will deposit on the surface of the iron powder, thus hindering the participation of iron from within the powder in the reaction and reducing the utilization rate of the iron powder. In actual production, dilute phosphoric acid is used, and an excess of phosphoric acid is required. Only when the iron powder is sufficiently pure can the filtrate and washing liquid be recycled; otherwise, the filtrate and washing liquid generated during the process need to be purified to remove impurities before recycling. However, due to the use of low-concentration solutions, the preparation efficiency of ferric phosphate is low, and energy consumption is high. Another traditional method for preparing ferric phosphate involves reacting divalent ferric salts (such as ferrous sulfate) with ammonium phosphate, sodium phosphate, phosphoric acid, ammonia or ammonium bicarbonate, hydrogen peroxide, etc. This method produces a large amount of high-concentration sulfate-containing (SO4) compounds. 2- ), phosphate (PO4) 3-Acidic wastewater containing ammonia nitrogen (NH3-N) is further neutralized with ammonia to obtain low-value byproducts ammonium salts (ammonium sulfate, sodium sulfate). The sulfate, phosphate and ammonia nitrogen in the wastewater can easily pollute the environment, and the cost of environmental treatment is high.

[0004] In summary, existing methods for producing ferric phosphate include the pure iron process and the ammonium / sodium process. The pure iron process uses reduced iron powder and phosphoric acid as the main raw materials. First, ultrapure iron powder and dilute phosphoric acid are mixed and reacted to prepare ferrous dihydrogen phosphate (Fe(H2PO4)2). Then, an oxidant is added to oxidize and generate ferric phosphate precipitate. After filtration and drying, high-purity battery-grade ferric phosphate dihydrate is obtained. This process uses low-concentration phosphoric acid as a raw material, requiring an excess of phosphoric acid. Although the filtrate and washing liquid can be recycled, the low concentration of phosphoric acid and low solid content of the slurry result in low efficiency, high energy consumption, and high cost in ferric phosphate production. The ammonium / sodium process uses ferrous sulfate, a byproduct of titanium dioxide, as the iron source and ammonium phosphate or sodium phosphate as the phosphorus source. Ferric phosphate is prepared by oxidation with hydrogen peroxide. This method is currently the mainstream process for ferric phosphate manufacturers. However, this process generates a large amount of high-concentration sulfate-containing (SO4) ions. 2- It is acidic wastewater containing ammonia nitrogen (NH3-N) or sodium ions; in production, it is produced by concentration, evaporation and crystallization, which generates a large amount of ammonium sulfate or sodium sulfate as a byproduct. This process requires a lot of energy and results in high treatment costs. Summary of the Invention

[0005] In view of this, the purpose of the present invention is to provide a method for preparing ferric phosphate by recycling magnesium salts, which can recycle phosphate, sulfate and high concentration of magnesium ions in production wastewater to produce ferric phosphate with uniform grains and excellent electrochemical performance.

[0006] This invention provides a method for preparing ferric phosphate by recycling magnesium salts, comprising the following steps:

[0007] a) After purifying the iron source, it is mixed with a phosphate solution prepared from high magnesium phosphate and subjected to an oxidation reaction in the presence of hydrogen peroxide to obtain the first slurry;

[0008] b) The first slurry obtained in step a) is aged and filtered sequentially to obtain magnesium salt mother liquor and first filter cake, respectively; wherein, the magnesium salt mother liquor is recycled for the preparation of high magnesium phosphoric acid in step a); the first filter cake is mixed with dilute phosphoric acid to obtain a second slurry;

[0009] c) The second slurry obtained in step b) is subjected to high-temperature washing and aging, and then filtered to obtain a second filter cake and wash water respectively; wherein the wash water is recycled for the purification process in step a);

[0010] d) The second filter cake obtained in step c) is successively washed with re-slurry water, filtered, dried, crushed and calcined to obtain anhydrous ferric phosphate.

[0011] Preferably, the iron source in step a) is selected from one or more of ferrous sulfate heptahydrate, ferrous nitrate, ferric nitrate, and ferric chloride; the iron source purification process specifically includes:

[0012] The iron source is dissolved in water, and a regulator is added to adjust the pH of the solution to above 3.5. After aging, the solution is filtered to obtain purified iron salt. The regulator is selected from one or more of iron powder, iron blocks, and calcium powder.

[0013] Preferably, the Fe concentration in the purified iron salt is 4wt% to 10wt%.

[0014] Preferably, the method for preparing the phosphate salt solution in step a) is as follows:

[0015] High-magnesium phosphoric acid is mixed with phosphoric acid with a concentration of 80wt% to 90wt% to obtain a magnesium dihydrogen phosphate solution, which is then diluted with water to a phosphorus concentration of 3wt% to 10wt% to obtain a phosphate salt solution.

[0016] Preferably, in step a), the mixing process involves weighing each raw material at a molar ratio of Fe:P = 0.9 to 1.2, adding hydrogen peroxide to the phosphate salt solution first, and then mixing it with the purified iron source.

[0017] Preferably, the pH value of the oxidation reaction in step a) is 1.2 to 3.0, which is adjusted by an alkaline magnesium compound; the alkaline magnesium compound is selected from one or more of magnesium oxide, magnesium carbonate, magnesium hydroxide, and magnesium dihydrogen phosphate.

[0018] Preferably, the oxidation reaction in step a) is carried out at a temperature of 50°C to 80°C for a time of 30 min to 120 min.

[0019] Preferably, the aging temperature in step b) is 50°C to 80°C and the time is 30 min to 180 min.

[0020] Preferably, the high-temperature washing and aging temperature in step c) is 80℃~110℃, and the time is 100min~180min.

[0021] Preferably, the drying temperature in step d) is 60℃~120℃ and the time is 1h~5h; the calcination temperature is 500℃~700℃ and the time is 1h~3h.

[0022] This invention provides a method for preparing ferric phosphate from recycled magnesium salts, comprising the following steps: a) purifying an iron source and mixing it with a phosphate solution prepared from high-magnesium phosphoric acid, and performing an oxidation reaction in the presence of hydrogen peroxide to obtain a first slurry; b) aging and filtering the first slurry obtained in step a) to obtain a magnesium salt mother liquor and a first filter cake, respectively; wherein the magnesium salt mother liquor is recycled for preparing high-magnesium phosphoric acid in step a); the first filter cake is slurried with dilute phosphoric acid to obtain a second slurry; c) washing and aging the second slurry obtained in step b) at high temperature, and filtering to obtain a second filter cake and wash water, respectively; wherein the wash water is recycled for the purification process in step a); d) washing, filtering, drying, pulverizing, and calcining the second filter cake obtained in step c) to obtain anhydrous ferric phosphate. Compared with the prior art, the method for preparing ferric phosphate from recycled magnesium salts provided by this invention adopts specific process steps to achieve better overall interaction, and can recycle phosphate, sulfate, and high-concentration magnesium ions in the production wastewater to produce ferric phosphate with uniform grains and excellent electrochemical performance. Attached Figure Description

[0023] Figure 1 A process flow diagram of the method for preparing ferric phosphate from recycled magnesium salts provided in an embodiment of the present invention;

[0024] Figure 2 The XRD patterns of the ferric phosphate dihydrate and anhydrous ferric phosphate samples obtained in Example 1 are shown below.

[0025] Figure 3 SEM images of the ferric phosphate dihydrate and anhydrous ferric phosphate samples obtained in Example 1;

[0026] Figure 4 SEM images of the ferric phosphate dihydrate and anhydrous ferric phosphate samples obtained in Example 2;

[0027] Figure 5 SEM images of the ferric phosphate dihydrate and anhydrous ferric phosphate samples obtained in Example 3;

[0028] Figure 6 SEM images of the ferric phosphate dihydrate and anhydrous ferric phosphate samples obtained in Example 4;

[0029] Figure 7 This is a SEM image of the anhydrous ferric phosphate sample obtained in Example 5;

[0030] Figure 8 SEM image of the anhydrous ferric phosphate sample obtained in Example 6;

[0031] Figure 9 This is a SEM image of the anhydrous ferric phosphate sample obtained in Example 7;

[0032] Figure 10SEM image of the anhydrous ferric phosphate sample obtained in Example 8;

[0033] Figure 11 SEM image of the anhydrous ferric phosphate sample obtained in Example 9;

[0034] Figure 12 This is a SEM image of the anhydrous ferric phosphate sample obtained in Example 10;

[0035] Figure 13 This is a SEM image of the anhydrous ferric phosphate sample obtained in Example 11;

[0036] Figure 14 This is a SEM image of the anhydrous ferric phosphate sample obtained in Example 12. Detailed Implementation

[0037] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0038] This invention provides a method for preparing ferric phosphate by recycling magnesium salts, comprising the following steps:

[0039] a) After purifying the iron source, it is mixed with a phosphate solution prepared from high magnesium phosphate and subjected to an oxidation reaction in the presence of hydrogen peroxide to obtain the first slurry;

[0040] b) The first slurry obtained in step a) is aged and filtered sequentially to obtain magnesium salt mother liquor and first filter cake, respectively; wherein, the magnesium salt mother liquor is recycled for the preparation of high magnesium phosphoric acid in step a); the first filter cake is mixed with dilute phosphoric acid to obtain a second slurry;

[0041] c) The second slurry obtained in step b) is subjected to high-temperature washing and aging, and then filtered to obtain a second filter cake and wash water respectively; wherein the wash water is recycled for the purification process in step a);

[0042] d) The second filter cake obtained in step c) is successively washed with re-slurry water, filtered, dried, crushed and calcined to obtain anhydrous ferric phosphate.

[0043] The purpose of this invention is to solve the production problems of large wastewater volume, high treatment cost, low production efficiency, and high energy consumption in the preparation of ferric phosphate in existing technologies. To address the environmental pollution caused by the large amount of low-value byproducts and wastewater generated in the preparation of ferric phosphate, and the high cost of wastewater treatment, this invention, based on environmental protection, resource recycling, and cost reduction and efficiency improvement, provides a method for the continuous production of battery-grade ferric phosphate using a magnesium salt process. This invention uses high-magnesium phosphate as the phosphorus source and industrial byproducts as the iron source (such as ferrous sulfate, a byproduct of titanium dioxide production). After purification and impurity removal, the iron source is fed into the synthesis process at a molar ratio n (Fe / P) of 0.90–1.20. Hydrogen peroxide is added to remove Fe... 2+ Oxidation was carried out; after synthesis and high-temperature aging, high-purity ferric phosphate wet-based filter cake was obtained by filtration. After washing, drying, crushing and calcining, high-purity ferric phosphate powder was obtained.

[0044] The present invention first purifies the iron source and mixes it with a phosphate salt solution prepared from high magnesium phosphate, and then carries out an oxidation reaction in the presence of hydrogen peroxide to obtain a first slurry.

[0045] In this invention, the iron source is preferably selected from one or more of ferrous sulfate heptahydrate, ferrous nitrate, ferric nitrate, and ferric chloride, and more preferably ferrous sulfate heptahydrate. This invention does not impose any special restrictions on the source of the iron source; any industrial production by-product well-known to those skilled in the art can be used.

[0046] In this invention, the iron source purification process is preferably as follows:

[0047] After dissolving the iron source in water, an agent is added to adjust the pH of the solution to above 3.5. After aging, the solution is filtered to obtain purified iron salt (raw material Fe salt).

[0048] In this invention, the regulator is preferably selected from one or more of iron powder, iron blocks, and calcium powder, more preferably iron powder. This invention does not impose any special restrictions on the source of the regulator; commercially available products well-known to those skilled in the art can be used.

[0049] In this invention, the Fe concentration (expressed as elemental Fe) in the purified iron salt is preferably 4 wt% to 10 wt%.

[0050] In this invention, the preferred method for preparing the phosphate salt solution is as follows:

[0051] High-magnesium phosphoric acid is mixed with phosphoric acid with a concentration of 80wt% to 90wt% to obtain a magnesium dihydrogen phosphate solution (Mg(H2PO4)2, with phosphoric acid added at a molar ratio of Mg:P = 0.5). The solution is then diluted with water to a P concentration of 3wt% to 10wt% to obtain a phosphate salt solution (raw material P salt).

[0052] In this invention, the high-magnesium phosphoric acid is preferably obtained by extracting and desulfurizing flotation phosphate tailings with sulfuric acid, specifically as a high-magnesium phosphoric acid solution. This invention utilizes high-magnesium phosphorus tailings, not only reducing resource waste and environmental pollution, but also achieving the secondary utilization of resources.

[0053] In this invention, the mixing process preferably involves weighing each raw material in a molar ratio of Fe:P = 0.9–1.2, adding hydrogen peroxide to the phosphate salt solution first, and then mixing it with the purified iron source. In this invention, the concentrations of the P and Fe salts have a relatively small impact on the reaction process, allowing production to be carried out at high solids content, thereby increasing production capacity.

[0054] In this invention, the pH value of the oxidation reaction is preferably 1.2 to 3.0 (within this range, the yield can be improved and the precipitation of metal impurities such as Mn and Mg can be prevented). The pH value is adjusted by an alkaline magnesium compound, and the amount of alkaline magnesium compound used will gradually decrease after the magnesium salt solution is circulated.

[0055] In this invention, the alkaline magnesium compound is preferably selected from one or more of magnesium oxide, magnesium carbonate, magnesium hydroxide, and magnesium dihydrogen phosphate, and more preferably magnesium oxide. This invention does not impose any special restrictions on the source of the alkaline magnesium compound; commercially available products of magnesium oxide, magnesium carbonate, magnesium hydroxide, and magnesium dihydrogen phosphate, well-known to those skilled in the art, can be used.

[0056] In this invention, the preferred temperature of the oxidation reaction is 50°C to 80°C, and the preferred time is 30 min to 120 min.

[0057] In this invention, the reaction equation for the oxidation reaction is as follows:

[0058] Mg(H2PO4)2+2FeSO4+H2O2+2H2O→2FePO4·2H2O+MgSO4+H2SO4.

[0059] After obtaining the first slurry, the present invention sequentially ages and filters the first slurry to obtain magnesium salt mother liquor and a first filter cake, respectively; wherein, the magnesium salt mother liquor is recycled for the preparation of high magnesium phosphoric acid in step a); the first filter cake is slurried with dilute phosphoric acid to obtain a second slurry. If the mother liquor is too acidic, it will affect the pH value of the synthesis reaction system during the recycling process. Therefore, calcium salt can be used for desulfurization treatment to obtain a magnesium salt solution with a suitable pH value, thereby realizing the recycling of the magnesium salt mother liquor; the mother liquor, wash water, and re-slurry wash water in the entire system are all magnesium salt solutions, and all can be returned to the system for use, with no wastewater discharge; when the magnesium salt mother liquor is recycled to a certain concentration, high-purity, high-value magnesium salt can also be produced.

[0060] In this invention, the purpose of aging is to cultivate crystals, the aging temperature is preferably 50℃~80℃, and the aging time is preferably 30min~180min.

[0061] After obtaining the second slurry, the present invention performs high-temperature washing and aging on the obtained second slurry, and after filtration, obtains a second filter cake and washing water respectively; wherein, the washing water is recycled for the purification process in step a).

[0062] In this invention, the temperature of the high-temperature washing and aging process is preferably 80℃~110℃, and the time is preferably 100min~180min.

[0063] After obtaining the second filter cake, the present invention sequentially washes, filters, dries, crushes and calcines the obtained second filter cake to obtain anhydrous ferric phosphate.

[0064] In this invention, both the re-wash water and the wash water described above can be recycled for the purification process in step a), thereby achieving resource utilization.

[0065] In this invention, the drying temperature is preferably 60℃~120℃ and the drying time is preferably 1h~5h, thereby removing free water and further pulverizing to obtain ferric phosphate dihydrate semi-finished product.

[0066] In this invention, the calcination temperature is preferably 500℃~700℃, and the calcination time is preferably 1h~3h. The calcination removes the water of crystallization and obtains a high-purity anhydrous iron phosphate powder product.

[0067] This invention provides a method for preparing ferric phosphate by recycling magnesium salts, which has the following beneficial effects:

[0068] (1) This invention creatively uses a magnesium salt cycling system to replace the traditional ammonium salt and sodium salt system to prepare iron phosphate with uniform grains and excellent electrochemical performance.

[0069] (2) The mother liquor, washing water and re-washing water generated in the preparation process of ferric phosphate are all magnesium salt solutions and can be returned to the system for use. There is no wastewater discharge, which reduces environmental pressure and lowers treatment costs.

[0070] (3) The high magnesium phosphate solution prepared by the present invention using phosphorus tailings provides some phosphorus and magnesium, realizing the secondary utilization of resources, and at the same time solving the problem of difficult tailings treatment.

[0071] This invention provides a method for preparing ferric phosphate from recycled magnesium salts, comprising the following steps: a) purifying an iron source and mixing it with a phosphate solution prepared from high-magnesium phosphoric acid, and performing an oxidation reaction in the presence of hydrogen peroxide to obtain a first slurry; b) aging and filtering the first slurry obtained in step a) to obtain a magnesium salt mother liquor and a first filter cake, respectively; wherein the magnesium salt mother liquor is recycled for preparing high-magnesium phosphoric acid in step a); the first filter cake is slurried with dilute phosphoric acid to obtain a second slurry; c) washing and aging the second slurry obtained in step b) at high temperature, and filtering to obtain a second filter cake and wash water, respectively; wherein the wash water is recycled for the purification process in step a); d) washing, filtering, drying, pulverizing, and calcining the second filter cake obtained in step c) to obtain anhydrous ferric phosphate. Compared with the prior art, the method for preparing ferric phosphate from recycled magnesium salts provided by this invention adopts specific process steps to achieve better overall interaction, and can recycle phosphate, sulfate, and high-concentration magnesium ions in the production wastewater to produce ferric phosphate with uniform grains and excellent electrochemical performance.

[0072] To further illustrate the present invention, the following embodiments provide a detailed description. Unless otherwise specified, all percentages mentioned in the following embodiments are by mass; the ferrous sulfate heptahydrate used is a byproduct of titanium dioxide production; and the high-magnesium phosphate solution used is derived from phosphate tailings.

[0073] Test materials: ferrous sulfate heptahydrate, with the following specifications:

[0074]

[0075] Test materials: magnesium salt solution, with the following specifications:

[0076]

[0077] Example 1:

[0078] (1) Preparation of Fe salt. Dissolve ferrous sulfate heptahydrate in water, add iron powder to adjust the pH of the slurry to above 3.5, age and filter to obtain purified iron salt; dilute the Fe concentration in the purified ferrous solution to 10% for testing.

[0079] (2) Preparation of P salt. Take high magnesium phosphate solution and add 85% phosphoric acid (concentration) at n(Mg:P) = 0.5 (molar ratio) to obtain magnesium dihydrogen phosphate solution. Dilute this solution with water to P = 10% and use it as P salt for later use.

[0080] (3) Weigh out the purified Fe salt from (1) (the main component is Fe). 2+ Prepare for use; calculate the required mass of P salt in (2) according to n(Fe:P)=1.2 (molar ratio) and weigh it. Calculate 1.2 times the theoretical amount required for oxidizing Fe salt and add hydrogen peroxide to P salt and stir well for use.

[0081] (4) Synthesis. The Fe salt was heated in a water bath, and the P salt was slowly added to the Fe salt. After the P salt was added, MgO slurry was added to adjust the pH of the reaction slurry to 1.2. The synthesis temperature was 50℃, and the synthesis time was 120 min. The slurry was aged at 50℃ for 180 min, and the resulting filter cake and magnesium salt mother liquor were obtained by filtration. The magnesium salt mother liquor can be reused in subsequent recycling processes.

[0082] (5) Take the synthetic filter cake obtained in (4) and slurry it with dilute phosphoric acid.

[0083] (6) High-temperature washing and aging. The slurry obtained in (5) is washed and aged at 80°C for 180 min, and then filtered to obtain a wet-based filter cake of ferric phosphate and dilute acid washing water. The dilute acid washing water can be used for subsequent recycling.

[0084] (7) Add water to the wet-based filter cake of ferric phosphate obtained in (6) and slurry it again. Wash away the residual acid on the surface and filter to obtain a high-purity wet-based filter cake of ferric phosphate. Dry the high-purity wet-based filter cake to remove free water and crush it to obtain ferric phosphate dihydrate. Calcine the ferric phosphate dihydrate at high temperature to remove the water of crystallization and obtain a high-purity anhydrous ferric phosphate powder product.

[0085] Examples 2-12: The experimental process was verified by adjusting the parameters according to the table below, while the other operation steps remained unchanged.

[0086]

[0087] Note: Example 12 is a return of the magnesium salt solution from the third cycle to the system for verification.

[0088] The key indicators of the magnesium salt mother liquor in the above embodiments are as follows:

[0089]

[0090]

[0091] The key indicators of the dilute acid washing water in each embodiment are as follows:

[0092]

[0093] The anhydrous ferric phosphate properties of each embodiment are as follows:

[0094]

[0095]

[0096] Note: The referenced standard Type I is for anhydrous ferric phosphate; ND indicates not detected.

[0097] The electrochemical performance parameters of each embodiment are as follows:

[0098]

[0099] In the above embodiments, the overall process flow, including the specific equipment used, etc., can be found in [reference needed]. Figure 1 As shown.

[0100] The synthesis mother liquor is magnesium salt mother liquor, which reacts with calcium salt in a magnesium salt storage tank (the main purpose of which is to treat SO4 in the magnesium salt mother liquor). 2- The process can separate calcium sulfate dihydrate and magnesium salt solutions; the magnesium salt solution can be transported to a high-magnesium phosphate storage tank for recycling, and the resulting CaSO4·2H2O can be used as backfill material through tailings treatment or as building material; in addition, the Mg in the magnesium salt solution... 2+ Magnesium salts can be separated after reaching a certain degree. The reaction equations involved in the above process are as follows:

[0101] H2SO4+CaCO3→CaSO4·2H2O+CO2↑;

[0102] MgSO4+Ca(H2PO4)2+2H2O→Mg(H2PO4)2+CaSO4·2H2O.

[0103] The washing water is recycled for step (1) Fe salt preparation (purification process); the washing water is also obtained during the process of filtering to obtain high-purity iron phosphate wet filter cake, and can also be recycled for step (1) Fe salt preparation (purification process) to realize resource utilization.

[0104] XRD and SEM results of the preparation of ferric phosphate in Example 1 are shown in [reference needed]. Figures 2-3 As shown; where, Figure 2 The XRD patterns of the ferric phosphate dihydrate and anhydrous ferric phosphate samples obtained in Example 1 are shown below. Figure 3 SEM images of the ferric phosphate dihydrate and anhydrous ferric phosphate samples obtained in Example 1.

[0105] Depend on Figure 2 It can be seen that the sample meets the PDF standard card 77-0094, has high crystallinity, and has basically no impurity peaks.

[0106] Depend on Figure 3 It can be seen that the sample has a regular morphology and a uniform particle size distribution.

[0107] SEM images of the preparation of ferric phosphate in Example 2 are shown below. Figure 4 As shown. By Figure 4 It can be seen that the sample has a regular morphology and a uniform particle size distribution.

[0108] SEM images of the preparation of ferric phosphate in Example 3 are shown below. Figure 5 As shown. By Figure 5 It can be seen that the sample has a regular morphology and a uniform particle size distribution.

[0109] SEM images of the preparation of ferric phosphate in Example 4 are shown below. Figure 6 As shown. By Figure 6 It can be seen that the sample has a regular morphology and a uniform particle size distribution.

[0110] SEM images of the preparation of ferric phosphate in Example 5 are shown below. Figure 7 As shown. By Figure 7 It can be seen that the sample has a regular morphology and a uniform particle size distribution.

[0111] SEM images of the preparation of ferric phosphate in Example 6 are shown below. Figure 8 As shown. By Figure 8 It can be seen that the sample has a regular morphology and a uniform particle size distribution.

[0112] SEM images of the preparation of ferric phosphate in Example 7 are shown below. Figure 9 As shown. By Figure 9 It can be seen that the sample has a regular morphology and a uniform particle size distribution.

[0113] Example 8: SEM image of the preparation of ferric phosphate (see Example 8). Figure 10 As shown. By Figure 10 It can be seen that the sample has a regular morphology and a uniform particle size distribution.

[0114] Example 9: SEM image of the preparation of ferric phosphate (see Example 9). Figure 11 As shown. By Figure 11 It can be seen that the sample has a regular morphology and a uniform particle size distribution.

[0115] Example 10: SEM image of the preparation of ferric phosphate (see Example 10). Figure 12 As shown. By Figure 12 It can be seen that the sample has a regular morphology and a uniform particle size distribution.

[0116] Example 11: SEM image of the preparation of ferric phosphate (see Example 11). Figure 13 As shown. By Figure 13 It can be seen that the sample has a regular morphology and a uniform particle size distribution.

[0117] Example 12: SEM image of the preparation of ferric phosphate (see Example 12). Figure 14 As shown. By Figure 14 It can be seen that the sample has a regular morphology and a uniform particle size distribution.

[0118] In summary, the SEM analysis of the ferric phosphate sample prepared using the method for preparing ferric phosphate using recycled magnesium salts provided by this invention is as follows:

[0119] (1) The changes in the concentration of Fe salt and magnesium dihydrogen phosphate have little effect on the indicators and morphology of the product anhydrous iron phosphate, so the concentration can be increased to improve the production capacity.

[0120] (2) Iron phosphate has a regular morphology and uniform particle size distribution. When used to prepare lithium iron phosphate, it can reduce the possibility of agglomeration, thereby improving the performance of the battery.

[0121] (3) The iron phosphate prepared by this method has the characteristics of high iron-to-phosphorus ratio, low impurity content, stable index and high product crystallinity. It exhibits excellent electrochemical performance while also taking into account compaction performance.

[0122] The above description of the disclosed embodiments enables those skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for preparing ferric phosphate by recycling magnesium salts, comprising the following steps: a) After purifying the iron source, it is mixed with a phosphate salt solution prepared by mixing high magnesium phosphate with phosphate at a concentration of 80wt%~90wt%, and an oxidation reaction is carried out in the presence of hydrogen peroxide to obtain the first slurry. b) The first slurry obtained in step a) is aged and filtered sequentially to obtain magnesium salt mother liquor and first filter cake, respectively; wherein, the magnesium salt mother liquor is recycled for the preparation of high magnesium phosphoric acid in step a); the first filter cake is mixed with dilute phosphoric acid to obtain a second slurry; c) The second slurry obtained in step b) is subjected to high-temperature washing and aging, and then filtered to obtain a second filter cake and wash water respectively; wherein the wash water is recycled for the purification process in step a); d) The second filter cake obtained in step c) is successively washed with re-slurry water, filtered, dried, crushed and calcined to obtain anhydrous ferric phosphate.

2. The method for preparing ferric phosphate by recycling magnesium salts according to claim 1, characterized in that, The iron source mentioned in step a) is selected from one or more of ferrous sulfate heptahydrate, ferrous nitrate, ferric nitrate, and ferric chloride; the iron source purification process is specifically as follows: The iron source is dissolved in water, and a regulator is added to adjust the pH of the solution to above 3.

5. After aging, the solution is filtered to obtain purified iron salt. The regulator is selected from one or more of iron powder, iron blocks, and calcium powder.

3. The method for preparing ferric phosphate by recycling magnesium salts according to claim 2, characterized in that, The Fe concentration in the purified iron salt is 4wt%~10wt%.

4. The method for preparing ferric phosphate from recycled magnesium salts according to claim 1, characterized in that, The specific method for preparing the phosphate salt solution in step a) is as follows: High-magnesium phosphoric acid is mixed with phosphoric acid with a concentration of 80wt%~90wt% to obtain a magnesium dihydrogen phosphate solution, which is then diluted with water to a phosphorus concentration of 3wt%~10wt% to obtain a phosphate salt solution.

5. The method for preparing ferric phosphate from recycled magnesium salts according to claim 1, characterized in that, In step a), the mixing process involves weighing each raw material at a molar ratio of Fe:P = 0.9~1.2, adding hydrogen peroxide to the phosphate salt solution first, and then mixing it with the purified iron source.

6. The method for preparing ferric phosphate by recycling magnesium salts according to claim 1, characterized in that, The pH value of the oxidation reaction in step a) is 1.2 to 3.0, which is adjusted by an alkaline magnesium compound; the alkaline magnesium compound is selected from one or more of magnesium oxide, magnesium carbonate, magnesium hydroxide, and magnesium dihydrogen phosphate.

7. The method for preparing ferric phosphate from recycled magnesium salts according to claim 1, characterized in that, The oxidation reaction in step a) is carried out at a temperature of 50℃~80℃ for a time of 30min~120min.

8. The method for preparing ferric phosphate from recycled magnesium salts according to claim 1, characterized in that, The aging temperature in step b) is 50℃~80℃, and the time is 30min~180min.

9. The method for preparing ferric phosphate by recycling magnesium salts according to claim 1, characterized in that, The high-temperature washing and aging process described in step c) is at a temperature of 80℃~110℃ and a time of 100min~180min.

10. The method for preparing ferric phosphate by recycling magnesium salts according to claim 1, characterized in that, The drying temperature in step d) is 60℃~120℃ and the time is 1h~5h; the calcination temperature is 500℃~700℃ and the time is 1h~3h.

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